Chapter 3

THERMAL and CATALYTIC

INCINERATORS
Donald R. van der Vaart
James J. Spivey
Research Triangle Institute
Research Triangle Park, NC 27709

William M. Vatavuk
Innovative Strategies and Economics Group, OAQPS
Albert H. Wehe
Policy Planning and Standards Group, OAQPS
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711

December 1995

Contents

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3.2 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3.2.1 Thermal Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

3.2.1.1 Direct Flame Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

3.2.1.2 Recuperative Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

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 3.2.1.3 Regenerative Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

3.2.2 Catalytic Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

3.2.2.1 Fixed-Bed Catalytic Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

3.2.2.2 Fluid-Bed Catalytic Incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

3.2.3 Other Considerations: Packaged versus Field-Erected Units, Auxiliary Equipment

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.2.3.1 Packaged vs. Field-Erected Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.2.3.2 Acid Gas Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.2.3.3 Heat Exchangers (Preheaters and Other Waste Energy Recovery Units) . 3-15

3.2.3.4 Other Auxiliary Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

3.2.4 Technology Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

3.3 General Treatment of Material and Energy Balances . . . . . . . . . . . . . . . . . . . . . . . . 3-17

3.4 Design Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18

3.4.1 Steps Common to Thermal and Catalytic Units . . . . . . . . . . . . . . . . . . . . . . . 3-19

3.4.2 Steps Specific to Thermal Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23

3.4.3 Steps Specific to Catalytic Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-29

3.5 Cost Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-34

3.5.1 Estimating Total Capital Investment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-34

3.5.1.1 Equipment Costs, EC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-34

3.5.1.2 Installation Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-37

3.5.2 Estimating Total Annual Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-41

3.5.2.1 Direct Annual Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-41

Appendix 3A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-50

Appendix 3B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-54

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-59

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3.1 Introduction
Incineration, like carbon adsorption, is one of the best known methods of industrial gas waste
disposal. Unlike carbon adsorption, however, incineration is an ultimate disposal method in that
the objectionable combustible compounds in the waste gas are converted rather than collected.
On the other hand, carbon adsorption allows recovery, of organic compounds which may have
more value as chemicals than just their heating value. A major advantage of incineration is that
virtually any gaseous organic stream can be incinerated safely and cleanly, provided proper
engineering design is used.

The particular application of both thermal and catalytic incineration to gaseous waste
streams containing volatile organic compounds (VOCs) is discussed here. The U.S.
Environmental Protection Agency defines any organic compound to be a VOC unless it is
specifically determined not to be a VOC. Indeed, a number of organics (e.g., methane) are
specified as not being VOCs. Although both VOC and non-VOC organic compounds are
combustible and are therefore important in the design of the incinerator, this distinction is
important since it is the control of VOCs that is regulated.

3.2 Process Description

Seldom is the waste stream to be combusted a single organic compound. Rather, it is common
to have a complex mixture of organic compounds. This mixture is typically analyzed for
carbon, hydrogen, oxygen, and other elements; and an empirical formula is developed which
represents the mixture. Combustion of such a mixture of organic compounds containing carbon,
hydrogen, and oxygen is described by the overall exothermic reaction:

y z y
C xH yO z x O2 xCO2 HO (3.1)
4 2 2 2

The complete combustion products CO2 and H2O are relatively innocuous, making incineration
an attractive waste disposal method. When chlorinated sulfur-containing compounds are present
in the mixture, the products of complete combustion include the acid components HCl or SO 2,
respectively, in addition to H2O and CO2. In general, these streams would require removal of
the acid components by a scrubber unit, which could greatly affect the cost of the incineration
system. (The sizing and costing of these scrubbers is covered in the "Wet Scrubbers" chapter
of this Manual.)

The heart of an incinerator system is a combustion chamber in which the VOC-containing

waste stream is burned. Since the inlet waste gas stream temperature is generally much lower
than that required for combustion, energy must be supplied to the incinerator to raise the waste
gas temperature. Seldom, however, is the energy released by the combustion of the total
organics (VOCs and others) in the waste gas stream sufficient to raise its own temperature to
the desired levels, so that auxiliary fuel (e.g., natural gas) must be added.

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 The combustion of the waste gases may be accomplished in a thermal incinerator or in a
catalytic incinerator. In the catalytic incinerator a catalyst is used to increase the rate of the
combustion reaction, allowing the combustion to occur at lower temperatures. Because the
catalytic process operates at a lower temperature than the thermal process, less auxiliary fuel
may be required in the catalytic process to preheat the waste gas.

Auxiliary fuel requirements may also be decreased, and energy efficiency improved, by
providing heat exchange between selected inlet streams and the effluent stream. The effluent
stream containing the products of combustion, along with any inerts that may have been present
in or added to the inlet streams, can be used to preheat the incoming waste stream, auxiliary air,
or both via a "primary", or recuperative, heat exchanger. It is useful to define the fractional
energy recovery by the preheater, or primary, heat exchanger as follows:

Fractional Energy actually recovered flue gas

Energy Maximum energy recoverable if flue gas approaches 3.2
Recovery lowest temperature available to heat exchanger

The energy actually recovered, the numerator of Equation 3.2, is the increase in sensible heat
of the gas, i.e., waste gas or waste gas plus dilution air, being heated. The maximum energy
recoverable would be the decrease in sensible heat of the flue gas, if it were cooled to the
temperature of the incoming waste gas. While this maximum energy recovery would be
attained only with a very large heat exchanger, the concept of fractional energy recovery is
useful in expressing the extent of the improvement in energy efficiency using a "primary" heat
exchanger.

Energy efficiency can be further improved by placing another ("secondary") exchanger

downstream of the primary exchanger to recover additional energy from the effluent stream
(e.g., to generate low pressure process steam or hot water). However, secondary energy
recovery is generally not used, unless there is a specific on site use for the steam or hot water.

The majority of industrial gases that contain VOCs are dilute mixtures of combustible gases
in air. In some applications, such as air oxidation processes, the waste gas stream is very
deficient in oxygen. Depending on the oxygen content of the waste stream, auxiliary air may
be required to combust the total organic content of the waste gas as well as any auxiliary fuel
that has been used.

The concentration of combustible gas in the waste gas stream plays an integral role in the
design and operation of an incinerator. From a cost standpoint, the amount of air in excess of
the stoichiometric amounts should be minimized. For safety reasons, however, any mixture
within the flammability limits, on either the fuel-rich or fuel-lean side of the stoichiometric
mixture, presents an unacceptable fire hazard as a feed stream to the incinerator. The lower, or
fuel-lean, explosive limit (LEL) of a given organic compound defines the minimum
concentration of that compound in air that can produce more energy than is needed to raise its
own temperature to the ignition point (i.e., ignite). Similarly, the upper, or fuel-rich, explosive

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 limit (UEL) represents the highest concentration of the organic in air that is ignitable. In the
latter case, air is limiting the reaction. Both the LEL and the UEL are measured at ambient
conditions. Empirically, it has been found that mixtures of hydrocarbons in air at their LEL
have a heating value of approximately 50 Btu/scf.

Since the majority of industrial waste gases that contain VOCs are dilute mixtures of
combustible gases in air, their heating value is low and their oxygen content exceeds that
required to combust both the waste organics (VOCs and others) and the auxiliary fuel. If a
waste gas above 50 percent LEL (about 25 Btu/scf) is encountered, it must be diluted to satisfy
fire insurance regulations. Generally, the streams are brought to below 25 percent LEL,
although concentrations from 25 percent to 50 percent are permitted provided the waste stream
is continuously monitored by LEL monitors. Because air is the usual diluent gas, care must be
taken with preheating the diluted stream so that it remains below about 1200 F. (See discussion
below on preheating.) A table showing LEL, UEL, and heats of combustion for selected organic
compounds is given in Appendix 3A.

The goal of any incineration system is to control the amount of VOCs released to the
environment. Performance of a control device such as an incinerator can be described by a
control efficiency, defined according to the following equation:

Inlet mass rate VOC Outlet mass rate VOC

Control Eff., % × 3.3
Inlet mass rate VOC

It is important to note, however, that incomplete combustion of the inlet VOCs could result in
the formation of other VOCs not originally present. For example, the incomplete oxidation of
dichloroethane can yield vinyl chloride. Both of these compounds are VOCs. The definition
given in Equation 3.3 would still be meaningful, however, as long as the newly formed VOC
(e.g., vinyl chloride) is detected. This situation necessitates the complete chemical analysis of
the inlet and outlet gas streams to confirm compliance with State and Federal regulations.

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 Figure 3.1: Thermal Incinerator - General Case

Performance of an incinerator can also be measured solely by the outlet VOC concentration,
usually in ppmv.

There are a number of different incinerator designs. These designs can be broadly classified
as thermal systems and catalytic systems. Thermal systems may be direct flame incinerators
with no energy recovery, flame incinerators with a recuperative heat exchanger, or regenerative
systems which operate in a cyclic mode to achieve high energy recovery. Catalytic systems
include fixed-bed (packed-bed or monolith) systems and fluid-bed systems, both of which
provide for energy recovery. The following sections discuss design aspects of these systems.

3.2.1 Thermal Incinerators

The heart of the thermal incinerator is a nozzle-stabilized flame maintained by a combination
of auxiliary fuel, waste gas compounds and supplemental air added when necessary (see Figure
3.1). Upon passing through the flame, the waste gas is heated from its inlet temperature (e.g.,
100 F) to its ignition temperature. The ignition temperature varies for different compounds and
is usually determined empirically. It is the temperature at which the combustion reaction rate
(and consequently the energy production rate) exceeds the rate of heat losses, thereby raising
the temperature of the gases to some higher value. Thus, any organic/air mixture will ignite if
its temperature is raised to a sufficiently high level.

The organic-containing mixture ignites at some temperature between the preheat

temperature and the reaction temperature. That is, ignition, as defined in this section, occurs
at some point during the heating of a waste stream as it passes through the nozzle-stabilized
flame regardless of its concentration. The mixture continues to react as it flows through the
combustion chamber.

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 The required level of VOC control of the waste gas that must be achieved within the time
that it spends in the thermal combustion chamber dictates the reactor temperature. The shorter
the residence time, the higher the reactor temperature must be. The nominal residence time of
the reacting waste gas in the combustion chamber is defined as the combustion chamber volume
divided by the volumetric flow rate of the gas. Most thermal units are designed to provide no
more than 1 second of residence time to the waste gas with typical temperatures of 1,200 to
2,000 F. Once the unit is designed and built, the residence time is not easily changed, so that
the required reaction temperature becomes a function of the particular gaseous species and the
desired level of control. Table 3.1 illustrates the variability in (theoretical) reactor temperatures
that is required to destroy 99.99 percent of the inlet mass of various noxious compounds with
excess air for a 1-second reactor residence time [l].

Table 3.1: Theoretical Reactor Temperatures Required for 99.99 Percent Destruction by
Thermal Incineration for a 1-Second Residence Time*

Compound Temperature, F
acrylonitrile 1,344
allyl chloride 1,276
benzene 1,350
chlorobenzene 1,407
1,2-dichloroethane 1,368
methyl chloride 1,596
toluene 1,341
vinyl chloride 1,369
*Reference [1]

These temperatures cannot be calculated a priori, although incinerator vendors can provide
guidelines based on their extensive experience. In practice, most streams are mixtures of
compounds, thereby further complicating the prediction of this temperature. Other studies
[2,3,4], which are based on actual field test data, show that commercial incinerators should
generally be run at 1600 F with a nominal residence time of 0.75 seconds to ensure 98%
destruction of non-halogenated organics. In some States the reactor temperature and residence
time of the unit are specified rather than attempting to measure actual levels of VOC control.
The selected temperature must be maintained for the full, selected residence time for
combustion to be complete.
These three studies also conclude that mixing is a critical factor in determining the
destruction efficiency. Even though it cannot be measured, mixing is a factor of equal or even
greater importance than other parameters, such as temperature. The most feasible and efficient
way to improve the mixing in an incinerator is to adjust it after start-up. The 98% control level
discussed in the previous paragraph presumes such an adjustment.

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 Ultimately, once the unit is built, it is the responsibility of the user to operate and maintain
the incinerator to insure compliance with applicable regulations.

3.2.1.1 Direct Flame Incinerators

Many configurations of thermal incinerators exist with the same goal— to raise the VOC-
containing stream to the desired reaction temperature and hold it there for the given reaction
time to achieve the required destruction efficiency. The simplest example of such a system is
the direct flame incinerator. With reference to Figure 3.1, the direct flame incinerator is
comprised only of the combustion chamber. The waste gas preheater and the secondary energy
recovery heat exchanger are energy recovery devices and are not included as part of the direct
flame incinerator.

3.2.1.2 Recuperative Incinerators

Recuperative incinerators have improved energy efficiency as a result of placing heat

exchangers in the hot outlet gas streams. With reference to Figure 3.1, the recuperative
incinerator is comprised of the combustion chamber, the waste gas preheater, and, if
appropriate, the secondary, energy recovery heat exchanger.

Primary Energy Recovery (Preheating Inlet Streams) Considerable fuel savings can be
realized by using the exit (product) gas to preheat the incoming feed stream, combustion air, or
both via a heat exchanger, as shown in Figure 3.1 in the so-called "recuperative" incinerator.
These heat exchangers can recover up to 70% of the energy (enthalpy) in the product gas.

The two types of heat exchangers most commonly used are plate-to-plate and shell-and-
tube. Plate-to-plate exchangers offer high efficiency energy recovery at lower cost than shell-
and-tube designs. Also, because of their modular configuration, plate-to-plate units can be built
to achieve a variety of efficiencies. But when gas temperatures exceed 1000 F, shell-and-tube
exchangers usually have lower purchase costs than plate-to-plate designs. Moreover, shell-and-
tube exchangers offer better long-term structural reliability than plate-to-plate units.[5] In any
case, because most incinerators installed are packaged units, the design (and cost) of the
recuperative heat exchangers have already been incorporated.

Most heat exchangers are not designed to withstand high temperatures, so that most of the
energy needed to reach ignition is supplied by the combustion of fuel in the combustion
chamber and only moderate preheat temperatures are sought in practice (<1200 F).

Secondary Energy Recovery (Additional Waste Energy Recovery) It should be noted,

however, that at least some of the energy added by auxiliary fuel in the traditional thermal units
(but not recovered in preheating the feed stream) can still be recovered. Additional heat
exchangers can be added to provide process heat in the form of low pressure steam or hot water
for on-site application. Obviously, an in-plant use for such low level energy is needed to realize
these savings.

The need for this higher level of energy recovery will be dependent upon the plant site. The
additional heat exchanger is often provided by the incineration unit vendor. The cost of this

3-9
additional heat exchanger may be estimated via standard heat exchanger correlations and should
be added to the costs estimated using the cost correlations in this chapter.

3.2.1.3 Regenerative Incinerators

A distinction in thermal incinerators can now be made on the basis of this limitation in the
preheat temperature. The traditional approach to energy recovery in the units (shown
schematically in Figure 3.1) still requires a significant amount of auxiliary fuel to be burned in
the combustion chamber when the waste gas heating values are too low to sustain the desired
reaction temperature at the moderate preheat temperature employed. Additional savings can,
under these conditions, be realized in units with more complete transfer of exit-stream energy.
This is the concept behind the so-called excess-enthalpy or regenerable burner systems. These
systems use direct contact heat exchangers constructed of a ceramic material that can tolerate
the high temperatures needed to achieve ignition of the waste stream.

The operation of the regenerative system is illustrated in Figure 3.2. The inlet gas first
passes through a hot ceramic bed thereby heating the stream (and cooling the bed) to its ignition
temperature. If the desired temperature is not attainable, a small amount of auxiliary fuel is
added in the combustion chamber. The hot gases then react (releasing energy) in the combustion
chamber and while passing through another ceramic bed. thereby heating it to the combustion
chamber outlet temperature. The process flows are then switched, now feeding the inlet stream
to the hot bed. This cyclic process affords very high energy recovery (up to 95%).

The higher capital costs associated with these high-performance heat exchangers and
combustion chambers may be offset by the increased auxiliary fuel savings to make such a
system economical. The costs of these regenerative units will be given separately in the cost
correlations presented in Section 3.5. Regenerative incinerators are not packaged units but are
field-erected only. Accordingly, the costs given in Section 3.5 for regenerative units are for
field-erected units.

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3-11
Figure 3.2: Regenerable-type Thermal Incinerator

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 Figure 3.3: Catalytic Incinerator

3.2.2 Catalytic Incinerators

Catalytic incinerators employ a bed of active material (catalyst) that facilitates the overall
combustion reaction given in Equation 3.1. The catalyst has the effect of increasing the reaction
rate, enabling conversion at lower reaction temperatures than in thermal incinerator units.
Nevertheless, the waste stream must be preheated to a temperature sufficiently high (usually
from 300 to 900 F) to initiate the oxidation reactions. The waste stream is preheated either
directly in a preheater combustion chamber or indirectly by heat exchange with the incinerator's
effluent or other process heat or both (Figure 3.3). The preheated gas stream is then passed over
the catalyst bed. The chemical reaction (combustion) between the oxygen in the gas stream and
the gaseous pollutants takes place at the catalyst surface. Catalytic incineration can, in
principle, be used to destroy essentially any oxidizable compound in an air stream. However,
there are practical limits to the types of compounds that can be oxidized due to the poisoning
effect some species have on the catalyst. These limits are described below. In addition, most
configurations require a low heating value of the inlet gas and a particulate content which is less
than some small value.

Until recently, the use of catalytic oxidation for control of gaseous pollutants has really
been restricted to organic compounds containing only carbon, hydrogen and oxygen. Gases
containing compounds with chlorine, sulfur, and other atoms that may deactivate the supported
noble metal catalysts often used for VOC control were not suitably controlled by catalytic
oxidation systems. Catalysts now exist, however, that are tolerant of such compounds. Most
of these catalysts are single or mixed metal oxides, often supported by a mechanically strong
carrier such as -alumina. Perhaps most of the development of poison-tolerant catalysts has
focused on the oxidation of chlorine-containing VOCs. These compounds are widely used as
solvents and degreasers and are often the subject of concern in VOC control. Catalysts such as
chromia/alumina [6,7], cobalt oxide [8], and copper oxide/manganese oxide [9] have been used
for oxidation of gases containing chlorinated compounds. Platinum-based catalysts are active
for oxidation of sulfur containing VOCs, although they are rapidly deactivated by the presence
of chlorine. Compounds containing atoms such as lead, arsenic, and phosphorous should, in
general, be considered poisons for most oxidation catalysts. Nevertheless, their concentration
may be sufficiently low so that the rate of deactivation and therefore, the catalyst replacement
costs, could be low enough to consider catalytic oxidation.

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Table 3.2: Catalyst Temperatures Required for Oxidizing 80% of Inlet VOC to CO2, F for
Two Catalysts

Compound Temperature, F
CO3O4 Pt - Honeycomb
acrolein 382 294
n-butanol 413 440
n-propylamine 460 489
toluene 476 373
n-butyric acid 517 451
1,1,1-trichloroethane 661 >>661
dimethyl sulfide — 512

As was the case for thermal units, it is impossible to predict a priori the temperature and
residence time (i.e., inverse space velocity) needed to obtain a given level of conversion of a
VOC mixture in a catalytic oxidation system. For example, Table 3.2 from Pope et al. [8]
shows the temperature needed for 80% conversion of a number of VOCs over two oxidation
catalysts in a specific reactor design. This table shows that the temperature required for this
level of conversion of different VOCs on a given catalyst and of the same VOC on different
catalysts can vary significantly.

Particulate matter, including dissolved minerals in aerosols, can rapidly blind the pores of
catalysts and deactivate them over time. Because essentially all the active surface of the
catalyst is contained in relatively small pores, the particulate matter need not be large to blind
the catalyst. No general guidelines exist as to particulate concentration and particulate size that
can be tolerated by catalysts because the pore size and volume of catalysts vary greatly.

The volumetric gas flow rate and the concentration of combustibles in the gas flowing to the
catalytic incinerator should be constant for optimal operation. Large fluctuations in the flow
rate will cause the conversion of the VOCs to fluctuate also. Changes in the concentration or
type of organics in the gas stream can also affect the overall conversion of the VOC
contaminants. These changes in flow rate, organics concentration, and chemical composition
are generally the result of upsets in the manufacturing process generating the waste stream. It
may be uneconomical to change the process for the sake of making the operation of the catalytic
incinerator feasible. In such cases, thermal incinerators (discussed earlier in this chapter) or
carbon adsorption (discussed in Chapter 4 of this Manual) should be evaluated as alternative
control technology.

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 The method of contacting the VOC-containing stream with the catalyst serves to distinguish
catalytic incineration systems. Both fixed-bed and fluid-bed systems are used.

3.2.2.1 Fixed-Bed Catalytic Incinerators

Fixed-bed catalytic incinerators may use a monolith catalyst or a packed-bed catalyst. Each of
these is discussed below.

Monolith Catalyst Incinerators The most widespread method of contacting the VOC-
containing stream with the catalyst is the catalyst monolith. In this scheme the catalyst is a
porous solid block containing parallel, non-intersecting channels aligned in the direction of the
gas flow. Monoliths offer the advantages of minimal attrition due to thermal expansion/
contraction during startup/shutdown and low overall pressure drop.

Packed-Bed Catalytic Incinerators A second contacting scheme is a simple packed-bed in

which catalyst particles are supported either in a tube or in shallow trays through which the
gases pass. The first scheme is not in widespread use due to its inherently high pressure drop,
compared to a monolith, and the breaking of catalyst particles due to thermal expansion when
the confined catalyst bed is heated/cooled during startup/shutdown. However, the tray type
arrangement, where the catalyst is pelletized is used by several industries (e.g., heat-set web-
offset printing). Pelletized catalyst is advantageous where large amounts of such contaminants
as phosphorous or silicon compounds are present.[10]

3.2.2.2 Fluid-Bed Catalytic Incinerators

A third contacting pattern between the gas and catalyst is a fluid-bed. Fluid-beds have the
advantage of very high mass transfer rates, although the overall pressure drop is somewhat
higher than for a monolith. An additional advantage of fluid-beds is a high bed-side heat
transfer as compared to a normal gas heat transfer coefficient. This higher heat transfer rate to
heat transfer tubes immersed in the bed allows higher heat release rates per unit volume of gas
processed and therefore may allow waste gas with higher heating values to be processed without
exceeding maximum permissible temperatures in the catalyst bed. In these reactors the gas
phase temperature rise from gas inlet to gas outlet is low, depending on the extent of heat
transfer through imbedded heat transfer surfaces. The catalyst temperatures depend on the rate
of reaction occurring at the catalyst surface and the rate of heat exchange between the catalyst
and imbedded heat transfer surfaces.

As a general rule, fluid-bed systems are more tolerant of particulates in the gas stream than
either fixed-bed or monolithic catalysts. This is due to the constant abrasion of the fluidized
catalyst pellets, which helps remove these particulates from the exterior of the catalysts in a
continuous manner.

A disadvantage of a fluid-bed is the gradual loss of catalyst by attrition. Attrition-resistant

catalysts have been developed to overcome this disadvantage.[11]

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3.2.3 Other Considerations: Packaged versus Field-Erected Units,
Auxiliary Equipment

3.2.3.1 Packaged vs. Field-Erected Units

With the exception of regenerative incinerators, the equipment cost correlations included in this
chapter are for packaged units only. They are not valid for field-erected units. For regenerative
incinerators, the correlations are valid for field-erected units only. Packaged units are units that
have been shop fabricated and contain all elements necessary for operation, except for
connection to facilities at the site, e.g., utilities. The elements include the combustion chamber,
preheater, instrumentation, fan, and the necessary structural steel, piping, and electrical
equipment. This equipment is assembled and mounted on a "skid" to facilitate installation on
a foundation at the plant site. Tie-in to the local emission source is not part of the packaged
unit. Units are usually sized to handle flow rates of <20,000 scfm, but can be built to
accommodate flow rates up to 50,000 scfm. The cost correlations in this chapter are valid to
50,000 scfm for packaged units, except for fluid-bed units which are valid to 25,000 scfm.

Conversely, field-erected units may be built to any desired size. The combustion chamber,
preheater, and other equipment items are designed and fabricated individually, and assembled
at the site. However, both the equipment and installation costs of field-erected units are
typically higher than those for equivalent-sized packaged units because the factors that improve
efficiency of shop-fabrication, such as uniform working environment, availability of tools and
equipment, and more efficient work scheduling, are generally not available in the field.

3.2.3.2 Acid Gas Scrubbers

The final outlet stream of any incineration system may contain certain pollutants that must be
removed. The combustion of sulfur-containing compounds results in SO 2, while chlorinated
compounds yield Cl2 and HCl in the product stream. These acid gases must be removed from
the gas stream if they are present at significant concentrations (regulations for limits on these
gases vary from state to state). This removal can be effected in, for instance, a packed-bed gas
absorber (vertical scrubber) in which the flue gas is contacted with a caustic scrubbing liquid.
For fluid-bed catalytic reactors, venturi scrubbers are often used because they provide for
particulate removal as well as acid gas scrubbing. In most cases adding a scrubber or absorber
significantly increases the cost of the incineration unit, sometimes by a factor of two. Costing
of absorbers is discussed in the "Gas Absorbers" chapter (Chapter 9) of this Manual.

If chlorinated VOCs are present in the waste gas, heat exchangers may require special
materials of construction. This added expense is not included in the costing procedures outlined
in this chapter.

3.2.3.3 Heat Exchangers (Preheaters and Other Waste Energy Recovery Units)

For the thermal and catalytic units having some degree of energy recovery, the cost of the
primary heat exchanger is included in the cost, and its design is usually done by the incineration
unit vendor. The cost correlations presented in this chapter include units both with and without

3-16
energy recovery. Secondary energy recovery, if desired, requires an additional heat exchanger,
which is also often provided by the incineration unit vendor. Costing procedures for secondary
energy recovery are not included in this chapter.

3.2.3.4 Other Auxiliary Equipment

Additional auxiliary equipment such as hoods, ductwork, precoolers, cyclones, fans, motors, and
stacks are addressed separately in other chapters of this Manual.

3.2.4 Technology Comparison

Both the thermal and catalytic incineration systems are designed to provide VOC control
through combustion at a level in compliance with applicable state and federal requirements.
Given the wide range of options available, however, it is obvious that not every incinerator will
fulfill these requirements at the same cost. This section presents a first step toward deciding
how best to deal with VOC emission abatement using incinerators considering some qualitative
factors pertinent to the types of incinerators described in this chapter. It is the intent of the
remainder of Chapter 3 to provide a method by which the cost of VOC control for a particular
application can be calculated.

3-17
 Table 3.3: Principal VOC Incinerator Technologies
4444444444444444444444444444444444444444444444444444444444444444444444

Thermal Systems

Direct Flame Incinerator

Recuperative Incinerator (Direct Flame with Recuperative Heat

Exchanger)

Regenerative Incinerator Operating in a Cyclic Mode

Catalytic Systems

Fixed-Bed

– Monolith

– Packed-Bed

Fluid-Bed

4444444444444444444444444444444444444444444444444444444444444444444444

A summary of the principal types of incinerators is presented in Table 3.3. From the earlier
discussions, the following factors relating to the presence of contaminants should be considered
by potential users [12]:

The fouling of the catalyst in a catalytic system is a possibility. Poisons to the system
include heavy metals, phosphorous, sulfur and most halogens, although catalysts have
been developed that are chlorine resistant.

The possibility of process upsets that could release any of the above poisons or cause
fluctuations in the heating value to the incinerator would favor a thermal system.

Except for the No.2 grade, fuel oil should not be considered as auxiliary fuel to a
catalytic system due to the sulfur and vanadium it may contain.[10]

All of the above factors would serve to increase the operating expense of a catalytic unit through
replacement costs of the catalyst. An additional factor relates to relative energy efficiency of
the various types of incinerators:

3-18
 Thermal units generally require more auxiliary fuel than catalytic units and operate at
temperatures that are roughly 1000 F higher. This difference in fuel requirement
increases as the heating value of the waste stream decreases.

In general, a trade-off exists between the higher capital costs of catalytic incinerators and
the higher operating costs of thermal incinerators. This difference will be illustrated by a design
example presented in Section 3.4 which treats both technologies.

3.3 General Treatment of Material and Energy Balances

In the sizing and costing of the incinerator and the calculation of the auxiliary fuel requirements,
it is necessary to make material and energy balances around the entire incinerator unit and
around certain parts of the unit, such as the combustion chamber or the preheater. This section
presents a general approach to making these balances.

These balances are based on the law of conservation of mass and energy. They can be stated
in general equation form as

In - Out + Generation = Accumulation (3.4)

Because the incineration process is a steady-state process, the accumulation term is zero and the
equation becomes

In - Out + Generation = 0

For mass balances it is useful to restrict the balances to be made on the mass of each atomic
species so that for mass balances the generation term becomes zero. However, because the
combustion reaction liberates energy, the energy balances around equipment where combustion
takes place would include a generation term. Thus, the simplified equations are

In - Out = 0 , for steady-state mass balances (3.5)

In - Out + Generation = 0 , for steady-state energy balances (3.6)

For the incineration process the two terms In and Out are generally mass terms (for a mass
balance) of the form,

where
= density (mass per unit volume)
Q = volumetric flow rate (volume per unit time)

3-19
or sensible heat terms (for an energy balance) of the form,

QCp(T - Tref)

where
Cp = heat capacity
T = temperature

The reference temperature, Tref, is often taken to be zero or the temperature of a

convenient stream, e.g., the inlet gas stream, in whatever units T is in, so the T ref term may not
appear in the equations. When the reference temperature is taken as zero, the sensible heat
terms become

QCpT.

Energy losses, HL, are also part of the Out term and, for the incinerator process, are taken here
to be 10% of the total energy input to the incinerator.

For the incineration process, the generation term for energy balances accounts for the
energy released through the combustion reactions. This term is generally of the form

Q(- hc)

where
(- hc) = heat of combustion.

3.4 Design Procedures

The following procedure is designed to provide parameters for use in developing a study cost
estimate (accuracy ± 30%). The principal parameters of interest are

flue gas flow rate, upon which all the equipment cost correlations are based.

auxiliary fuel requirement, which is important in estimating annual operating costs.

For applications which involve control of waste gas streams that are dilute mixtures of VOCs
in air (>20% oxygen in the waste gas stream), the flue gas flow rate is greater than the inlet
waste gas flow rate by the amount of auxiliary fuel and the increase in the moles of gas as a
result of the combustion reaction. Because these two factors usually cause only small increases
in flow rate, a number of simplifying assumptions can be made in the design calculations. For

3-20
applications where diluent air must be used to adjust the combustible concentration in the waste
gas to 25% LEL and where auxiliary fuel and auxiliary combustion air are needed, more
complete mass and energy balances must be made.

The design procedure illustrated below is for waste gas streams that are dilute mixtures of
VOCs in air (>20% oxygen in the waste gas stream). In this discussion the design procedure
will be illustrated by a sample problem that will be solved step-by-step.

3.4.1 Steps Common to Thermal and Catalytic Units

Step 1 - Establish design specifications The first step in the design procedure is to determine
the specifications of the incinerator and the waste gas to be processed. The following
parameters of the waste gas stream at the emission source must be available:

Volumetric flow rate, scfm—Standard conditions are normally 77 F and 1 atm. pressure

Temperature

Oxygen content

Chemical composition of the combustibles

Inerts content

Heating value— In cases the heating value may act as a surrogate for the chemical
composition of the combustibles. This is particularly true for dilute mixtures of
combustibles in air.

Particulate content—The particulate content is important if catalytic incinerators are to

be coated. An upstream filter may suffice if particulate content is too high. Fluid-bed
catalytic incinerators can tolerate higher particulate contents than fixed-bed catalytic
incinerators.

The following parameters must be specified for the incinerator:

Desired control efficiency—This efficiency should be based on requirements dictated

by relevant state and federal regulations.

Combustion chamber outlet temperature—This temperature may also be based on

requirements of a regulation or on recommendations developed during regulatory
development.

Desired percent energy recovery—The desired percent energy recovery should be the
result of a process optimization in which costs of incinerators with several different
levels of energy recovery are estimated and the minimum cost design selected. The

3-21
 Table 3.4: Specification of Sample Problem
Variable Value
Preheater Inlet Waste Gas Vol Flow Rate, Qwi scfm 20,000
Preheater Inlet Waste Gas Temp., Twi, F 100
Composition
Benzene Content, ppmv 1000
Methyl Chloride Content, ppmv 1000
Air Content Balance
Particulate Content Negligible
Moisture Content Negligible
Desired Control Efficiency, % 98
Desired Percent Energy Recovery, HR% 70

tradeoff is between the capital cost of the energy recovery equipment and the operating
(fuel) cost.

Specifications for the sample problem are given in Table 3.4.

Step 2 - Verify that the oxygen content of the waste gas exceeds 20% There must be
sufficient oxygen in the waste gas to support the combustion of the waste organics (including
VOCs) and the auxiliary fuel, if auxiliary fuel is needed. It may be necessary to add auxiliary
air if the oxygen content is less than about 20%. This example is based on streams that contain
>20% oxygen, as shown below:

1000 1000
Content, Vol. % 100.0 × 100 × 100
10 6
10 6 3.7
99.8%

Oxygen Content, % Air Content × 0.209

3.8
20.86%

Step 3 - Calculate the LEL and the Percent of the LEL of the gas mixture Note: If the waste
stream contains a significant amount of inerts in addition to the nitrogen associated with the
oxygen in air, the calculation of LEL (and UEL) loses meaning since the LEL (and UEL) is
measured in mixtures of organic with air only. A complete chemical analysis is necessary to
complete the design procedure in such a case.

3-22
 The example chosen here is typical, in that there is more than one VOC component in the
gas stream. An approximate method to calculate the LEL of a mixture of compounds, LEL mix,
is given by Grelecki [13] as

1
n xj
LELmix
j 1
n 3.9
( x i) × LELj
i 1

where
xi = volume fraction of combustible component i
LELj = lower explosive limits of combustible component j (ppmv)
n = number of combustible components in mixture

For the example case,

n
6
xi (1,000 1,000) × 10
i 1 3.10
6
2,000 × 10

From standard references [14] or from Appendix 3A,

LELBz = 14,000ppnv fir benzene

LELMC = 82,5000 ppmv for methyl chloride

1
1000 1000
LELmix
2,000 × 14,000 2,000 × 82,500 3.11
23,938 ppmv

total combustible conc. in mixture

LELmix × 100 3.12
LELmix

2,000
× 100 8.4% 3.13
23,938

3-23
 The percent LEL of the mixture is therefore 8.4%. Because this is well below 25%, no
dilution air is needed in this example. If the mixture had been above 25% LEL, sufficient
dilution air would have been needed to bring the concentration of the mixture to less than 25%
to satisfy fire insurance regulations.

Step 4 - Calculate the volumetric heat of combustion of the waste gas streams, ( - hcw),
Btu/scf The energy content of the gas stream, expressed in terms of the heat of combustion, is
calculated as follows:

n
( hc ) ( hc ) xi 3.14
w i
i 1

where

( hc ) heat of combustion of the waste stream (Btu/scf)

w

( hc ) volumetric heat of combustion of component i at 25 C (Btu/scf)

i

xi volume fraction of component i in the waste gas

n number of combustible components in the waste gas

The heat of combustion that should be used in these calculations is the "lower" heat of
combustion, i.e., with gaseous water, rather than liquid water, as a reaction product since water
leaves the incinerator in the vapor state. From Appendix 3A or standard references [14,15] with
appropriate conversion of units, the volumetric heat of combustion at 25 C for the two
components is calculated to be as follows:

( hc ) 3,475 Btu / scf for benzene

Bz

( hc ) 705 Btu / scf for methyl chloride

MC

The compositions specified earlier as ppmv are converted to volume fractions as follows:

6 3
xBz 1,000 ppmv × 10 10 for benzene
6 3
xMC 1,000 ppmv × 10 10 for methyl chloride

Using these values of heat of combustion and composition, the heat of combustion of the
waste gas stream per standard cubic foot of incoming gas is

3-24
 ( hc ) (3,475) (10 3) (705) (10 3)
w 3.15
4.18 Btu / scf

Assuming the waste gas is principally air, with a molecular weight of 28.97 and a
corresponding density of 0.0739 lb/scf, the heat of combustion per pound of incoming waste gas
is
( hc ) 56.6 Btu /lb
w

The negative heat of combustion, by convention, denotes an exothermic reaction. Also by

convention, if one refers to heat of reaction rather than heat of combustion, then a positive value
denotes an exothermic reaction.

Empirically, it has been found that 50 Btu/scf roughly corresponds to the LEL of organic/air
mixtures. Insurance codes require a value below 25% LEL, which corresponds to about 13
Btu/scf. However, if LEL sensors and monitors are installed, one can incinerate a waste gas
with a combustible organic content between 25 and 50% LEL, which corresponds to 13 to 25
Btu/scf.

For catalytic applications the heat of combustion must normally be less than 10 Btu/scf (for
VOCs in air) to avoid excessively high temperatures in the catalyst bed. This is, of course, only
an approximate guideline and may vary from system to system.

After Step 4, determination of the (- hcw) design procedure for thermal and catalytic
incinerators is discussed separately, beginning with Step 5 for each type of incinerator.

3.4.2 Steps Specific to Thermal Units

Figure 3.1 shows a generic thermal incinerator with the appropriate streams labeled.

Step 5t - Establish the temperature at which the incinerator will operate As mentioned in
Section 3.2.1, both the reactor temperature and residence time of the waste gas in the reactor
determine the level of VOC destruction. In general, state and local regulations specify the
required level of destruction that the customer must meet. In this example a destruction
efficiency of 98 percent is specified. Studies by Mascone [2,3,4] show that this destruction
efficiency can be met in a thermal incinerator operated at a temperature, T fi , of 1,600 F and a
residence time of 0.75 second. (Note: This higher efficiency level is the minimum achievable
by any new properly designed and operated incinerator. Many incinerators can achieve
destruction efficiencies of 99% or higher.)

3-25
Step 6t - Calculate the waste gas temperature at the exit of the preheater The extent of the
heat exchange to be carried out in the preheater is the result of a technical and economic
optimization procedure that is not illustrated in this example. As the VOC stream temperature
leaving the heat exchanger, Two, increases, the auxiliary fuel requirement decreases, but at the
expense of a larger heat exchanger. However, there are several important limits on T wo. First,
Two must not be close to the ignition temperature of the organic-containing gas to prevent
damaging temperature excursions inside the heat exchanger should the gas ignite. Second, for
gases containing halogens, sulfur, and phosphorous (or other acid-forming atoms), the flue gas
temperature after the heat exchanger, Tfo, must not drop below the acid dew point. Both
limitations limit the amount of heat exchange and thus the maximum value of Two. The
calculation of the acid dew point is not simple. It is recommended that vendor guidance be
sought to ensure that the dew point is not reached. Condensation of acid gases will result in
corrosion of many of the metals used in heat exchangers. As an example, fuel sulfur contents
of 1 to 2 percent can give acid dew points of about 200 to 270 F. Increasing the sulfur content
to 4 percent can raise the dew to about 290 F. Chlorine and phosphorous have a much smaller
effect on acid dew elevation.

With the following assumptions, one can estimate T wo using equation 3.2, the definition of
fractional energy, recovery for a heat exchanger.

The fractional energy recovery is specified.

The amount of auxiliary fuel, Qaf, and auxiliary combustion air, Qa, are small relative
to the waste gas, Q w, so that the mass flow rates of gases, wQ w and fQ f, on both sides
of the preheater are approximately the same, or

w Qw f Qf

The heat capacities of the gases on both sides of the preheater are approximately the
same, regardless of composition. This is true for waste streams which are dilute
mixtures of organics in air, the properties of the streams changing only slightly on
combustion.

The mean heat capacities above the reference temperature of the gases on both sides of
the preheater are approximately the same regardless of temperature.

With these assumptions, the equation for fractional energy recovery for a heat exchanger
becomes
Tw Tw
Fractional Energy Recovery o i
3.16
Tfi Tw
i

3-26
For this example with a fractional energy recovery of 0.70, an incinerator operating temperature,
Tfi, of 1600 F, and a waste gas inlet temperature, Twi, of 100 F, the waste gas temperature at
the end of the preheater becomes

Tw 1,150 F
o

The temperature of the exhaust gas, Tfo, can be determined by an energy balance on the
preheater, which, with the same assumptions as used in deriving Equation 3.16 regarding the
mass flow rates and average heat capacities of the gases involved, results in the following
equation:

Tf Tf Tw Tw
i o o i

i.e., the temperature rise in the waste gas is approximately equal to the temperature decrease in
the flue gas with which it is exchanged. For this example, this results in

Tf 550 F
o

This value of Tfo should be well above the acid dew point of the flue gas stream.

It should be remembered that T wo should be well below the ignition temperature of the VOC
stream to prevent unwanted temperature excursions in the preheater. This must be verified even
if the stream is well below the LEL because flammability limits can be expanded by raising the
reactant stream temperature. A sufficiently high preheat temperature, Two, could initiate
reaction (with heat release) in the preheater. This would ordinarily be detrimental to the
materials of construction in the heat exchanger. The one exception is the thermal incinerator
of the regenerable type described in Section 3.2. The 95-percent energy recovery, obtainable
in regenerable systems would result in this example in a Two of 1,525 F. The significant
reaction rate that would occur at this temperature in the ceramic packing of the heat
exchanger/reactor is by design.

Step 7t - Calculate the auxiliary fuel requirement, Q af Auxiliary fuel will almost invariably
be needed for startup of the unit. However, at steady state, if the energy released by combustion
of the organics present in the waste stream is sufficient to maintain the reactor temperature
(1,600 F in the example), only a small amount of auxiliary fuel (< 5% of the total energy input)
is needed to stabilize the flame. In most cases, however, more fuel than just this stabilizing fuel
will be required to maintain the reactor temperature.

3-27
 With the following assumptions, one can estimate Qaf using a mass and energy balance
around the combustion chamber and following the principles discussed in Section 3.3, with
reference to Figure 3.1.

The reference temperature, Tref, is taken as the inlet temperature of the auxiliary fuel, T af.

No auxiliary air, Qa, is required.

Energy losses, HL, are assumed to be 10% of the total energy input to the incinerator
above ambient conditions.[16,17] Thus, if the reference temperature is near ambient
conditions,

HL 0.1 fiQfiCpm (Tfi Tref) 3.17

fi

The heat capacities of the waste gases entering and leaving the combustion chamber are
approximately the same, regardless of composition. This is true for waste streams which
are dilute mixtures of organics in air, the properties of the streams changing only slightly
on combustion.

The mean heat capacities above the reference temperature of the waste gases entering
and leaving the combustion chamber are approximately the same regardless of
temperature. Thus the mean heat capacity for the waste gas stream entering or leaving
the combustion chamber should be evaluated at the average of Two and Tfi. For air this
assumption introduces an error of, at most, 5% over the temperatures of interest.

With these assumptions, the mass and energy balance around the combustion chamber
reduces to the following equation:

wi
Qw [Cpm (1.1Tf Tw 0.1Tref) ( hc )]
Q
af af
i air i o wo
3.18
( hc ) 1.1Cpm (Tfi Tref)
af air

Input data for this equation are summarized below:

The waste stream is essentially air so that

wo wi
0.0739 lb / scf, air at 77 F, 1 atm.

3-28
 Table 3.5: Summary of Example Problem Variable Valuation
Tref = 77 F

j, Qj, Cpmj, Tj,

Stream Subscript, j lb/scf scfm Btu/lb F F
IN - Sensible heat
Auxiliary Air a na* na* na* na*
Auxiliary Fuel af 0.0408 167 ** 77
Waste Gas wo 0.0739 20,000 0.255 1,150

OUT - Sensible Heat

Waste Stream fi 0.0739 20,167 0.255 1,600

(- hc), waste gas = 56.6 Btu/lb

(- hc), auxiliary fuel = 21,502 Btu/lb
*Not applicable.
**Not used because reference temperature is taken equal to auxiliary fuel temperature.

Cpmair = 0.255Btu/lb F, the mean heat capacity of air between 77 F and 1,375 F (the
average temperature of the waste gas entering and leaving the
combustion chamber)

Other input data to Equation 3.18 include:

Qw Qw 20,000 scfm
o i

( hc ) 21,502 Btu/lb, for methane

af

T af Tref 77 F, assume ambient conditions

af
0.0408 lb/ft 3, methane at 77 F ,1 atm.
T fi 1,600 F, Step 5t
Tw 1,150 F, Step 6t
o

( hc ) 56.6 Btu/lb. Step4

wo

substituting the above values into Equation 3.18 results in:

Qaf = 167 scfm

3-29
 Table 3.6: Terms in Energy Balance Around Combustor—Example Problem

Value,
Stream Subscript, j Btu/min
IN - Sensible Heat, jQjCpmj (Ti - Tref)
Auxiliary Air a 0
Waste Gas wo 404,403
OUT - Sensible Heat, jQjCpmj (Ti - Tref)
Waste Stream fi 578,796
OUT - Losses
10% of total energy input 57,880
GENERATION -
Heat of Combustion, jQj (- hej)
Waste Gas wo 83,655
Auxiliary Fuel af 146,506

The values of the parameters in the energy balance are summarized in Table 3.5.

It is instructive to examine the magnitude of the various terms in the energy balance around
the combustor for the sample problem. This is done in Table 3.6. The energy balance shown
does not quite add to zero due to round-off-error and simplifying assumptions. Table 3.6 shows
that the largest inlet term is the sensible heat of the incoming waste gas. The heat of combustion
of the organics contained in the waste gas stream is somewhat smaller than that of the auxiliary
methane because of the relatively small amount of organics in the waste gas stream. The largest
term in the outlet stream is the sensible heat of the outgoing waste stream. The overall energy
losses are based on an assumption, but are relatively small. Because the sensible heat contents
of the entering and leaving waste stream are so large, it is apparent that energy recovery is an
important factor in achieving energy efficiency. In fact, with zero energy recovery in the
sample problem, the auxiliary fuel requirements would be 605 scfm, about four times the energy
requirements based on 70% energy recovery.

Step 8t - Verify that the auxiliary fuel requirement is sufficient to stabilize the burner
flame Only a small amount of auxiliary fuel (< 5% of the total energy input) is needed to
stabilize the burner flame. In general, more fuel than just this stabilizing fuel will be required
to maintain the reactor temperature. It is wise to verify that the auxiliary fuel requirement
calculated in Step 7t is sufficient for stabilization. If it is insufficient, then a minimum amount
of auxiliary fuel must be used, and the amount of energy recovery, specified earlier must be
reduced to avoid exceeding the specified temperature at which the incinerator will operate (Step
5t).

This check is made by calculating 5% of the total energy input to the incinerator and
comparing it with the auxiliary fuel energy input. The total energy input is given as follows:

3-30
 Total Energy Input QfiCpm (Tfi
fi
Tref) 3.19
fi

Auxiliary Fuel Energy Input af

Qaf ( hc ) 3.20
af

The auxiliary fuel used in the design, Q af, should be the larger of 5% of the total energy input
(28,900 Btu/min.) and the auxiliary fuel energy input (146,500 Btu/min.). The auxiliary fuel
used easily meets this criterion.

Step 9t - Calculate the total volumetric flow rate of gas through the incinerator, Q fi The
total volumetric flow rate of gas leaving the incinerator is referred to as the flue gas flow rate,
Qfi, and is the gas rate on which the incinerator sizing and cost correlations are based. The flue
gas flow rate measured at the standard conditions of 77 F and 1 atmosphere, where the increase
in volumetric throughput due to an increase in the number of moles of gas as a result of
combustion is neglected, is the sum of the inlet streams to the incinerator.
Qfi Qw Qa Qaf
o

20,000 0 167
20,167 scfm

This result conforms with the assumptions stated in Step 6t, i.e., the mass (and volume) flow
rates on both sides of the preheater are approximately equal. Finally, it must be emphasized that
steps 5t to 9t apply to thermal recuperative incinerators, only. To calculate the auxiliary fuel
requirements for other types of thermal incinerators (e.g., regenerative), a different procedure
must be used. (See Appendix 3B.)

3.4.3 Steps Specific to Catalytic Units

Figure 3.3 shows a generic catalytic incinerator with the appropriate streams labeled. The
approach used in the calculations on the catalytic incinerator is somewhat different than that
used in the thermal incinerator. This difference arises because of additional constraints which
are placed on the catalytic incinerator. These constraints are as follows:

The desired catalyst bed outlet temperature is typically 700 to 900 F. The maximum
temperature to which the catalyst bed can be exposed continuously is limited to about
1,200 F. Therefore, the combustible content of the waste gas is limited, and the amount
of heat exchange that occurs in the primary heat exchanger may be limited.

The inlet temperature to the catalyst bed itself must be above the catalytic ignition
temperature required to give the desired destruction efficiency in the incinerator.
Therefore, the combustible content of the waste gas is further limited to that which,

3-31
 when combusted, will raise the temperature in the catalyst bed no more than the T
between the required reactor bed inlet temperature, and the desired reactor bed outlet
temperature.

Auxiliary fuel, in combination with the preheat from the primary heat exchanger, is used
to preheat the waste gas to the reactor inlet temperature. A minimum amount of
auxiliary fuel (< 5% of the total energy input) must be used to stabilize the flame in the
preheat combustion chamber. This has the effect of further limiting the combustible
content of the waste gas stream and the amount of heat exchange permissible in the
primary heat exchanger.

The steps outlined below represent one approach to recognizing these constraints and
incorporating them into the calculation procedures.

Step 5c - Establish the desired outlet temperature of the catalyst bed, T fi The energy
released by the oxidation of the VOCs in the catalyst bed will raise the temperature of the gases
by an amount, T, as the gases pass through the catalyst bed. An outlet temperature from the
catalyst, and thus from the reactor, must be specified that will ensure the desired level of
destruction of the VOC stream. As in thermal incinerators, this value varies from compound
to compound and also varies from catalyst to catalyst. Final design of the incinerator should be
done by firms with experience in incinerator design. Guidelines given by Combustion
Engineering [12] indicate that values from 300 to 900 F result in destruction efficiencies
between 90 and 95 percent. To prevent deactivation of the catalyst a maximum bed temperature
of 1,200 F should not be exceeded. In the example problem the catalyst outlet temperature, T fi,
is selected to be 900 F.

Step 6c - Calculate the waste gas temperature at the exit of the preheater (primary) heat
exchanger The waste gas temperature at the exit of the primary heat exchanger is estimated
in the same manner as for the thermal incinerator. The equation for fractional energy recovery
Equation 3.16, is used, with the same assumptions as used for the thermal incinerator. For the
example problem with a fractional energy recovery of 0.70, a catalyst bed outlet temperature,
Tfi, of 900 F, and a waste gas inlet temperature, Twi, of 100 F, the gas temperature at the exit
of the preheater becomes
Tw 660 F
o

The same considerations regarding the closeness of the temperature of the exhaust gas, T fa,
to its dew point apply to the catalytic incinerator as they did to the thermal incinerator.

Step 7c - Calculate the auxiliary fuel requirement, Q af The auxiliary fuel requirement, Q af,
is calculated by making mass and energy balances around the preheater combustion chamber
and the catalyst chamber. The auxiliary fuel requirement calculated in this manner must be
checked to insure that it falls within the constraints imposed by design considerations of the
catalytic incinerator. These constraints are as follows:

3-32
 The auxiliary fuel requirement must be positive. A negative fuel requirement indicates
that the heat of combustion of the waste gas, (- h c), is too high for the fractional energy
recovery in the primary heat exchanger that was selected.

The auxiliary fuel amount must be high enough to provide a stable flame in the
preheater combustion chamber (See Step 8c below).

An energy balance around the preheater combustion chamber and the catalyst chamber,
taken together, results in Equation 3.18, the same equation used in the thermal incinerator
calculations. The input data for Equation 3.18 for the catalytic incinerator example problem are
summarized below:

The waste stream is essentially air so that

wo = wi = 0.0739 lb/scf, air at 77 F, 1 atm

Cpmair = 0.248 Btu/lb F, the mean heat capacity of air between 77 F and 780 F (the
average of the preheater exit and catalyst bed outlet
temperatures)

Other input data to Equation 3.18 include

Qw Qw 20,000 scfm
o i

( hc ) 21,502 Btu/lb, for methane

af

T af 77 F, assumeambientconditions

af
0.0408 lb/ft 3, methane at 77 F, 1atm.
T fi 900 F, from Step 5c
Tw 660 F, from Step 6c
o

( hc ) 56.6 Btu/lb, from Step 4

w

Substituting the above values into Equation 3.18 results in

Qaf = 40 scfm

If the outlet temperature of the catalyst bed, Tfi, is 800 F, then Qaf, decreases to -6.7 scfm.
In other words, no auxiliary fuel would, theoretically, be required at this bed temperature.
However, as discussed above in Step 8t, a certain quantity of auxiliary fuel would be required
to maintain burner stability.

At 70% energy recovery and 900 F outlet catalyst bed temperature, a waste gas with a heat
of combustion, (- hcw ), of about 79.9 Btu/lb would cause the auxiliary fuel requirement, Qaf, to
o
become negative, indicating the catalyst bed would exceed 900 F. At 70% energy recovery and
800 F outlet catalyst bed temperature, this same result occurs with a (- h cw ) of 52.7 Btu/lb.
o

3-33
Both of these heats of combustion are relatively low for typical waste gases. These results are,
of course, dependent on the assumption of energy losses from the combustion chamber. The
lower the energy losses, the lower the allowable waste gas heat of combustion before
overheating occurs in the catalyst bed.

Step 8c - Verify that the auxiliary fuel requirement is sufficient to stabilize the burner
flame Only a small amount of auxiliary fuel (< 5% of the total energy input) is needed to
stabilize the burner flame. In general, more fuel than just this stabilizing fuel will be required
to maintain the reactor temperature. It is wise to verify that the auxiliary fuel requirement
calculated in Step 7c is sufficient for stabilization. If it is insufficient, then a minimum amount
of auxiliary fuel must be used and the amount of energy recovery specified earlier must be
reduced to avoid exceeding the specified temperature at which the incinerator will operate (Step
5c).

This check is made in the same manner as that in Step 8t of the thermal incinerator
calculation. The results of this check indicate that the auxiliary fuel requirement is more than
sufficient to stabilize the burner flame.

Step 9c - Estimate the inlet temperature to t he catalyst bed, T ri The inlet temperature to the
catalyst bed must be calculated to ensure that the inlet temperature is above that necessary to
ignite the combustible organic compounds in the catalyst that was selected for use.

The inlet temperature to the catalyst bed, T ri, should be such that, when the temperature rise
through the catalyst bed, T, is added to it, the resulting temperature is T fi, 900 F. Thus,

T Tf Tr (3.21)
i i

The value of T is determined by an energy balance around the preheater portion of the
combustor. The preheater is required to heat the gases up to the catalyst bed inlet temperature
using auxiliary fuel.1 This energy balance is made with the assumptions made earlier in
deriving Equation 3.18 and further assuming that only auxiliary fuel is combusted in the
preheater portion. The resulting equation is very similar to Equation 3.18 except that (1) the
terms with an fi subscript become terms with ri subscripts to denote a catalytic reactor inlet
stream rather than a combustor outlet (flue gas inlet to the primary heat exchanger) and (2) the
term for combustion of the waste gas organics does not appear. The resulting equation is as
follows:

1
At equilibrium, the temperature of the catalyst bed is maintained without requiring auxiliary fuel.

3-34
 wo
Qw [Cpm (1.1Tr Tw 0.1Tref)]
o air i o
Q
af af 3.22
( hc ) 1.1Cpm (Tr Tref)
af air i

This equation may be rearranged to solve for T ri explicitly. This produces an equation that
is somewhat complex and non-intuitive.

af
Qaf [( hc ) 1.1Cpm Tref ] w oQ w Cpm
(Tw 0.1Tref)
af air o
3.23
o air
Tr
i
1.1Cpm ( af
Qaf wo
Qw )
air o

After substituting the example problem parameters into Equation 3.23, we obtain a value
for Tri of 693 F. Based on ignition temperatures shown in Table 3.2, this reactor inlet
temperature should be satisfactory. Prior to a more definitive design, the ignition temperatures
for the specific chemicals should be verified.

The temperature rise across the catalyst bed is thus (900 - 693) or 207 F. These
temperatures are somewhat sensitive to the assumption for energy losses from the combustor.
The assumption for energy losses is perhaps somewhat conservative, i.e., it causes a larger Q af
to be estimated than would a less conservative assumption, and becomes more conservative as
the combustor size and insulation are increased.

Step 10c - Calculate the total volumetric flow rate of gas through the incinerator, Q fi The
total volumetric flow rate of gas leaving the incinerator is referred to as the flue gas flow rate,
Qfi, and is the gas rate on which the incinerator sizing and cost correlations are based. The flue
gas flow rate measured at the standard conditions of 77 F and 1 atmosphere, where the increase
in volumetric throughput due to an increase in the number of moles of gas as a result of
combustion is neglected, is the sum of the inlet streams to the incinerator.

Qf Qw Qa Qaf
i o

20,000 0 40
20,040 scfm

Step 11c - Calculate the volume of catalyst i n the catalyst bed If the volumetric flow rate of
gas through the catalyst bed, Q fi, and the nominal residence time (reciprocal space velocity) in
the catalyst bed are known, then the volume of catalyst can be estimated. There exists complex
set of relationships between the catalyst volume and geometry, overall pressure drop across the
catalyst, conversion of the oxidizable components in the gas, gas temperature, and the reaction
rate. These relationships are dependent on the catalyst and the type of compound being
oxidized. It is beyond the scope if this Manual to discuss these relationships, even in an

3-35
approximate way. For the purposes of cost estimation, the space velocity, in reciprocal time
units, necessary to achieve the required level of destruction can be used to approximate the
catalyst volume
requirement. The space velocity is defined as

Qfi
Vcat

where
Vcat = Overall bulk volume of the catalyst bed, including interparticle voids (ft 3)

By petro-chemical industry convention, the space velocity is computed at the conditions of 60 F

(not 77 F) and 1 atm. The volumetric flow rate, Q fi must be corrected to these conditions. The
proper space velocity to achieve a desired level of conversion is based on experimental data for
the system involved. For precious metal monolithic catalysts, the space velocity generally lies
between 10,000 h-1 and 60,000 h-1. (Base metal catalysts operate at lower space velocities,
ranging from 5,000 to 15,000 h-1.)[10]

For the example, using a space velocity of 30,000 h-1 or 500 min-1, and using Q fi at 60 F,

60 460
Qfi at 60 F 20,040
77 460

19,400 ft 3 / min

19,400 ft 3 / min
Vcat
1
500 min
39 ft 3

There are a number of catalyst bed parameters, such as catalyst configuration and bed
design, that are not significant for study type cost estimates. Accordingly, design of these
factors is not discussed here.

3-36
 Table 3.7: Scope of Cost Correlations

Total (Flue) Gas

Incinerator Type Flowrate, scfm Figure Number
Thermal - Recuperative 500a-50,000 3.4
Thermal - Regenerative 10,000-100,000 3.5
Fixed-Bed Catalytic 2,000-50,000 3.6
Fluid-Bed Catalytic 2,000-25,000 3.7
a
Although Figure 3.4 covers the 1,000 to 50,000 scfm range, the correlation is valid for the 500 to 50,000 scfm
range.

3.5 Cost Analysis

This section presents procedures and data for estimating capital and annual costs for four types
of incinerators:(1) thermal-recuperative, (2) thermal regenerative, (3) fixed-bed catalytic, and
(4) fluid-bed catalytic.

3.5.1 Estimating Total Capital Investment

Total capital investment, TCI, includes the equipment cost, EC, for the incinerator itself, the
cost of auxiliary equipment (e.g., ductwork), all direct and indirect installation costs, and costs
for buildings, site preparation, offsite facilities, land, and working capital. However, the last
five costs usually do not apply to incinerators. (See Chapter 2 of this Manual for a detailed
description of the elements comprising the TCI)

3.5.1.1 Equipment Costs, EC

As discussed in Section 3.2.3, the equipment costs, EC, given in this chapter apply to packaged
incinerators, except for regenerative incinerators. For regenerative incinerators, the costs apply
to field-erected units. The EC typically includes all flange-to-flange equipment needed to
oxidize the waste gas, including the auxiliary burners, combustion chamber, catalyst, primary
heat exchanger (except for the "zero heat recovery" cases), weathertight housing and insulation,
fan, flow and temperature control systems, a short stack, and structural supports. Smaller units,
e.g., typically less than 20,000 scfm, are typically preassembled skid-mounted [18]. The various
available incineration systems are presented in four groups delineated according to their
similarity of design. These groups are outlined in Table 3.7. With the exception of regenerative
thermal and fluid-bed catalytic incinerators, the maximum size for which costs are given is
50,000 scfm. Although larger units of each technology can be built, applications are rare at flow
rates above 50,000 scfm. Regenerative thermal incinerator costs are provided for flow rates
from 10,000 to 100,000 scfm. Fluid-bed catalytic incinerator costs are provided for flow rates
from 2,000 to 25,000 scfm.

The cost curves are least-squares regressions of cost data provided by different vendors. It
must be kept in mind that even for a given incineration technology, design and manufacturing

3-37
procedures vary from vendor to vendor, so that costs may vary. As always, once the study
estimate is completed, it is recommended that more than one vendor be solicited for a more
detailed cost estimate.

The additional expense of acid gas clean-up or particulate control is not treated in this
section. The equipment cost of a gas absorber to remove any acid gases formed in the
incinerator can be quite large, sometimes exceeding the equipment cost of the incinerator itself
even for simple packed tower scrubbers [19]. For more complex absorbers that include venturi
scrubbers instead of, or in addition to, packed beds, the cost of the scrubber alone may be up to
4 times that of the incinerator [11]. These more complex absorbers are sometimes necessary
when particulates, in addition to acid gases, must be removed from the flue gas. (Note: Chapter
9 of the Manual provides data and procedures for sizing and costing gas absorbers.)

Thermal Incinerators Among the thermal units, the direct flame (0% energy recovery) and
recuperative systems are treated together because the various levels of energy recovery are
achieved simply by adding heat exchanger surface area. Costs for these units were provided by
several vendors [12,20,21]. The EC of these units are given as a function of total volumetric
throughput, Qtot, in scfm. "Qtot", is the total volume of the gaseous compounds exiting the
combustion chamber; it is identical to the term, "Q fi," used in Figures 3.1 and 3.3. This includes
the combustion products, nitrogen, unburned fuel and organics, and other constituents. (See
Figure 3.4). Note that costs are given free on board (F.O.B.) in April

3-38
Figure 3.4. Equipment Costs of Thermal Incinerators, Recuperative

3-39
 1988 dollars* . Based on a least-squares regression analysis, a log-log relationship between
throughput and EC was found for a given level of energy recovery (HR) over the flow rate range
from 500 to 50,000 scfm. These relationships are as follows:

0.2355
EC 10294Qtot HR 0%
(3.24)
0.2609
EC 13149Qtot HR 35% (3.25)
0.2502
EC 17056Qtot HR 50% (3.26)
EC
0.2500
21342Qtot HR 70% (3.27)

The regenerative (or excess enthalpy) systems provide up to 95 percent heat recovery at the
expense of higher capital costs. Their unique design [22,23], which combines the heat
exchanger and reactor, is substantially different from traditional thermal units and is therefore
treated separately in Figure 3.5. The ECs of these systems are given as an approximately linear
function of total flow rate over a 10,000 to 100,000 scfm range by the following equation:

EC 2.204 × 105 11.57 Qtot 3.28

Again, the higher capital costs of these units can be substantially offset by the substantial
savings in auxiliary fuel costs.

Catalytic Incinerator The EC for a catalytic incinerator is a function of the type of catalyst
contacting pattern used and the total gas flow rate, Qtot, for a given level of energy recovery.
There are three types of contacting configurations used in catalytic systems: fixed-bed, catalytic
monolith, and fluid-bed. The EC for the first two are generally comparable and are given in
Figure 3.6. The data provided by several vendors [12,20,21,24] exhibited curvilinear
relationships with Qtot for each of the energy recovery rates. Least squares regressions of the
data yielded the following correlations for total flow rates between 2,000 and 50,000 scfm:

0.5471
EC 1105Qtot HR 0% (3.29)
0.4189
EC 3623Qtot HR 35% (3.30)
0.5575
EC 1215Qtot HR 50% (3.31)
EC
0.5527
1443Qtot HR 70% (3.32)

*For information on escalating these and the other incinerator prices to more current dollars, refer to the
EPA report Escalation Indexes for Air Pollution Control Costs and updates thereto, all of which are installed
on the OAQPS Technology Transfer Network (CTC Bulletin Board).

3-40
Figure 3.5. Equipment Costs of Thermal Incinerators, Regenerative
3-41
 Fluid-bed catalytic incinerators afford certain advantages over fixed-bed catalyst units in
that they tolerate waste streams with (1) higher heating values, (2) particulate contents, and (3)
chlorinated species. For this enhanced flexibility of feed streams, a higher capital cost is
incurred, as indicated by the EC shown in Figure 3.7. The data shown were provided by vendors
[11,19] and exhibited a linear relationship over the range of flow rates from 2,000 to 25,000
scfm. They can be approximated by the following equations:

EC 8.48 x 104 13.2Qtot HR 0% (3.33)

4
EC 8.84 x 10 14.6Qtot HR 35% (3.34)
4
EC 8.66 x 10 15.8Qtot HR 50% (3.35)
EC 8.39 x 104 19.2Qtot HR 70% (3.36)

3-42
3-43
Figure 3.6. Equipment Costs of Catalytic Incinerators, Fixed-Bed

3-44
 A comparison of the thermal, catalytic fixed-bed, and catalytic fluid-bed systems with 50
percent energy recovery is shown in Figure 3.8.

3.5.1.2 Installation Costs

As explained in Chapter 2, the purchased equipment cost, PEC, is calculated by taking the sum
of the EC and the cost of auxiliary equipment (e.g., ductwork), taxes, freight, and
instrumentation. Average values of direct and indirect installation factors [25] to be applied to
the PEC are given in Table 3.8 for both recuperative thermal and fixed- and fluid-bed catalytic
incinerators.

Table 3.9 shows the itemized installation costs that are obtained when these installation factors
are applied to the PECs for the example incinerators. Depending on the site conditions, the
installation costs for a given incinerator could deviate significantly from costs generated by
these average factors. Vatavuk and Neveril [25] provide some guidelines for adjusting the
average installation factors to account for other-than-average installation conditions. For units
handling total gas flow rates lower than 20,000 scfm the installation costs are minimal,
amounting normally to only utility tie-ins (electrical and, if necessary, combustion or dilution
air). The installation costs for these smaller incinerators would be 20 to 25 % of the PEC.
Smaller units may be installed on the roofs of manufacturing buildings rather than at ground
level. In such cases the installation factors could be as high as (or higher than) the factors
shown in Table 3.8, even though the units would be "packaged".

3.5.2 Estimating Total Annual Cost

The total annual cost (TAC) is the sum of the direct and indirect annual costs. The TAC for
both example systems is given in Table 3.10, alone with suggested factors for calculating them.

3.5.2.1 Direct Annual Costs

Direct annual costs for incinerators include labor (operating and supervisory), maintenance
(labor and materials), fuel, electricity, and (in catalytic units) replacement catalyst. For thermal
and catalytic units, the fuel usage rate is calculated as shown in Sections 3.4.2 and 3.4.3,
respectively where natural gas (methane) is assumed to be the fuel. (Other fuels could be used
f o r t h e r m a l u n i t s . )

3-45
Figure 3.7. Equipment Costs of Catalytic Incinerators, Fluid-Bed

3-46
Figure 3.8: Equipment Costs Comparison of Incinerator Types

3-47
 Table 3.8. Capital Cost Factors for Thermal and Catalytic Incineratorsa

Cost Item Factor

Direct Costs
Purchased equipment costs
Incinerator (EC) + auxiliary equipmentb As estimated, A
Instrumentationc 0.10 A
Sales taxes 0.03 A
Freight 0.05 A
Purchased equipment cost, PEC B = 1.18 A
Direct installation costs
Foundations & supports 0.08 B
Handling & erection 0.14 B
Electrical 0.04 B
Piping 0.02 B
Insulation for ductworkd 0.01 B
Painting 0.01 B
Direct installation cost 0.30 B
Site preparation As required, SP
Buildings As required, Bldg

Total Direct Cost, DC 1.30 B + SP + Bldg.

Indirect Costs (installation)
Engineering 0.10 B
Construction and field expenses 0.05 B
Contractor fees 0.10 B
Start-up 0.02 B
Performance test 0.01 B
Contingencies 0.03 B
Total Indirect Cost, IC 0.31 B

Total Capital Investment = DC + IC 1.61 B + SP + Bldg.

a
Reference [25]
b
Ductwork and any other equipment normally not included with unit furnished by incinerator vendor.
c
Instrumentation and controls often furnished with the incinerator, and those often included in the EC.
d
If ductwork dimensions have been established, cost may be estimated based on $10 to $12/ft2 of surface for fluid
application. (Alternatively, refer to Chapter 10 of this Manual.
Fan housings and stacks may also be insulated.

3-48
3-49
Table 3.9. Capital Costs for Thermal and Catalytic Incinerators
Example Problem

3-50
 Table 3.10. Annual Costs for Thermal and Catalytic Incinerators
Example Problem

Cost Item Suggested Factor Unit Costa Thermal Fluid-Bed

Catalyst

Direct Annual Costsb, DC

Operating Labor
Operator 0.5 h/shift $12.95/h 6,480 6,480
Supervisor 15% of operator — 972 972
Operating Materials —
Maintenance
Labor 0.5 h/shift $14.25/h 7,130 7,130
Material 100% of — 7,130 7,130
maint. labor
Catalyst replacement 100% of catalyst $650/ft3 for 0 15,100
replaced ea. 2 yr. metal oxide
Utilities
Natural Gas — $3.30/kft3 264,500 63,400
Electricity — $0.059/kWh 36,500 44,200
Total DC $321,200 $144,400
Indirect Annual Costs, IC
Overhead 60% of sum of — 13,000 13,000
operating, supr.,
& maint. labor &
maint. materials.
Administrative charges 2% TCI — 9,650 17,800
Property taxes 1% TCI — 4,830 8,900
Insurance 1% TCI — 4,830 8,900
Capital recoveryc CRF [TCI- 1.08 — 68,800 122,700
(Cat. Cost)]
$101,100 $171,300

Total Annual Cost (rounded) $422,000 $316,000

a
1988 dollars
b
Assumes 8.000 h/yr
c
The capital recovery cost factor, CRF, is a function of the catalyst or equipment life (typically, 2 and 10 years, respectively)
and the opportunity cost of the capital (i.e., interest rate). For example, for a 10-year equipment life and a 7% interest rate,
CRF = 0.1424.

3-51
 Table 3.11. Typical Pressure Drop Across Selected Equipment

Equipment Type Energy Recovery, % P, in. H 2O

Thermal Incinerators 0 4
Catalytic Fixed-bed Incinerators 0 6
Catalytic Fluid-bed Incinerators 0 6-10
Heat Exchangers 35 4
" " 50 8
" " 70 15

The electricity costs are primarily associated with the fan needed to move the gas through
the incinerator. The power (in kilowatts) needed to move a given inlet volumetric flow rate of
air (Qwi per Sections 3.4.2 and 3.4.3) at a total flange-to-flange pressure drop of P inches of
water and combined motor/fan efficiency, , is adapted from Equation 2.7, as follows:
1.17 × 10 4 Qw P
Powerfan i
(3.37)

Fan efficiencies vary from 40 to 70 percent [15] while motor efficiencies are usually 90 percent.

The total pressure drop across an incinerator system depends on the number and types of
equipment elements included in the system and on design considerations. The estimation of
actual pressure drop requirements involves complex calculations based on the specific system's
waste gas and flue gas conditions and equipment used. For the purposes of this section,
however, the approximate values shown in Table 3.11 can be used.

For the example cases, we will assume 8,000 hours per year operation and a 60% efficiency
for the fan and motor together. Using pressure drops of 4 and 8 inches of water, respectively,
for the thermal and fluid-bed catalytic incinerators *, and adding the pressure drop of 15 inches
of water for 70% heat recovery, the fan power requirements can be calculated as follows:

Thermal Incinerator**
1.17 × 10 4 (20,900 acfm) (19 inches water)
Powerfan
0.60
77.4 kW

*A fluid-bed catalytic incinerator is used because the waste gas contains a chlorinated compound which
would poison the catalyst in a fixed-bed incinerator.

**Computed from inlet waste-gas flow rate (20,000 scfm) at preheater inlet temperature (100 F).

3-52
 Catalytic Incinerator
1.17 × 10 4 (20,900 acfm) (23 inches water)
Powerfan
0.60
93.7 kW

The annual electricity costs would be the products of these usages, the annual operating
hours, and the electricity cost ($/kWh), or:
Electricity Cost (Thermal) 77.4 kW × 8,000 hours / yr × $0.059 /kWh
$36,500 per yr
Electricity Cost (Catalytic) 93.7 kW × 8,000 hours / yr × $0.059 / kWh
$44,200 per yr

The catalyst replacement costs and scheduling are highly variable and depend on the nature
of the catalyst, the amount of "poisons" and particulates in the gas stream (including the
auxiliary fuel), the temperature history of the catalyst, and the design of the unit. It is
impossible to predict the costs in a general sense. However, noble metal monolith catalysts
operating on pure hydrocarbon gases in air will last longer than fluid-bed base metal catalysts
operating on chlorinated hydrocarbons in air. Noble metal catalysts are also more expensive
than base metal oxide catalysts. The catalyst life for many field units is from 1 to 4 years. The
cost, in April 1988 dollars, of the replacement catalyst must be obtained from the vendor, but
it may be estimated at $3,000/ft3 for noble metal catalysts and $650/ft 3 for base metal oxide
catalysts. For the example case, the catalyst is a base metal oxide because the waste gas
contains a chlorinated compound. We will assume a two year catalyst life. Knowing that the
catalyst volume is 39 ft3(Section 3.4.3) and using a cost of $650/ft3 and a capital recovery factor
of 0.5531 (2-year life at a 7% interest rate), the annual expense for catalyst replacement is
$
Annual Catalyst Replacement Cost 39 ft3 × 650 × 0.5531 × 1.08
ft3
$15,100 per year

(The " 1.08" factor covers the freight and sales tax for the replacement catalyst.)

To calculate the fuel or electricity annual cost, multiply the fuel usage rate (scfm) or the
electricity usage rate (kW) by the total hours per year of operation (e.g., 333 d/yr x 24 h/d =
8,000 h/yr) and by the appropriate unit cost (e.g., $/scfm for fuel and $/kWh for electricity).

For the example cases, the fuel costs can be calculated from the fuel usage rates and the
natural gas unit cost of $0.00330/scf. For the thermal incinerator example, the annual fuel cost
is calculated as follows:

3-53
 $ scf min hr
Annual Fuel 0.00330 × 167 × 60 × 8,000
scf min hr yr
Cost, Thermal $264,500 per year

For the catalytic incinerator example. the annual fuel cost is found similarly:

Annual Fuel Cost, Catalytic = $63,400 per year

Operating and maintenance labor are estimated as 0.5 hours per 8-hour shift each,
supervisory labor at 15% of operating labor, and maintenance material as 100% of maintenance
labor.

3.5.2.2 Indirect Annual Costs

The indirect (fixed) annual costs include capital recovery, overhead, and property taxes,
insurance, and administrative (G&A) charges. The last three of these can be estimated at 1%,
1%, and 2% of the total capital investment, respectively. The system capital recovery cost is
based on an estimated 10-year equipment life. (See Section 2 for a thorough discussion of the
capital recovery cost and the variables that determine it.) The system capital recovery cost is
the product of the system capital recovery factor (CRF) and the total capital investment (TCI)
less the purchased cost of the catalyst (Ccat x 1.08 where the 1.08 is for freight and sales tax).
These values calculated for the example cases are given in Table 3.10.

3.5.3 Cost Comparison for Example Case

The example VOC stream defined in Section 3.4.1 serves to illustrate some typical
characteristics of thermal and catalytic systems. The total annual costs shown in Table 3.10
show that the catalytic system's auxiliary fuel costs are significantly lower than those of the
thermal unit. The disparity is enough to offset the higher capital costs of the catalytic
incinerator over the assumed 10-year lifetime of the units. Two factors that should be noted in
the comparison of these two systems are (1) the 98 percent level of destruction met by the
thermal incinerator may be difficult to reach by the catalytic system (this may be important in
some cases), and (2) the example waste stream is of particularly low heating value (4 Btu/scf)
which favors the catalytic system due to the lower auxiliary fuel requirements.

3.6 Acknowledgements

The authors gratefully acknowledge the following companies for contributing data to this
chapter:

Peabody Engineering (Stamford, CT)

3-54
Combustion Engineering - Air Preheater, Inc. (Wellsville, NY)

TEC Systems, Inc. (DePere, WI)

Air Research, Inc. (ARI) (Palatine, IL)

Energy Development Associates (EDA) (Itasca, IL)

Pillar Technologies, Inc. (Hartland, WI)

Huntington Energy Systems, Inc. (Union, NJ)

Regenerative Environmental Equipment Co. (REECO) (Morris Plains, NJ)

Englehard Corp. (Edison, NJ)

3-55
Appendix 3A

Properties of Selected
Compounds

3-56
Table 3.12. Limits of Flammability of Combustible Organic Compounds in Air at
Atmospheric Pressure, Room Temperature*

Molecular LELa, UELb,

Compound Weight vol. % vol. %
Methane 16.04 5.00 15.00
Ethane 30.07 3.00 12.50
Propane 44.09 2.12 9.35
Butane 58.12 1.86 8.41
Pentane 72.15 1.40 7.80
Hexane 86.17 1.18 7.40
Octane 114.23 0.95
Nonane 128.25 0.83
Decane 142.28 0.77
Ethylene 28.05 2.75 28.60
Propylene 42.08 2.00 11.10
Acetylene 26.04 2.50 80.00
Cyclohexane 84.16 1.26 7.75
Benzene 78.11 1.40 7.10
Toluene 92.13 1.27 6.75
*Reference [14]
a
Lower Explosive Limit
b
Upper Explosive Limit

3-57
 Table 3.13: Molar Heat Capabilities of Gases at Zero Pressure*

Cp a bT cT 2 dT 3 ; T in K
T2
CpdT
T1
Cpm
(T2 T1)
Cp in calories/g moles oK Btu/lb mole oR

Temperature
2 6 9
Compound a b x 10 c x 10 d x 10 Range, K

Methane 4.750 1.200 0.3030 -2.630 273-1500

Ethane 1.648 4.124 -1.530 1.740 273-1500
Propane -0.966 7.279 -3.755 7.580 273-1500
Butane 0.945 8.873 -4.380 8.360 273-1500
Pentane 1.618 10.85 -5.365 10.10 273-1500
Hexane 1.657 13.19 -6.844 13.78 273-1500

Cyclopentane -12.957 13.087 -7.447 16.41 273-1500

Cyclohexane -15.935 16.454 -9.203 19.27 273-1500

Benzene -8.650 11.578 -7.540 18.54 273-1500

Toluene -8.213 13.357 -8.230 19.20 273-1500

Nitrogen 6.903 -0.03753 0.1930 -0.6861 273-1800

Oxygen 6.085 0.3631 -0.1709 0.3133 273-1800
Air 6.713 0.04697 0.1147 -0.4696 273-1800
Carbon dioxide 5.316 1.4285 -0.8362 1.784 273-1800

W444444444444444444444444444444444444444444444444444444444444444444
* Reference [26]

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 Table 3.14 Heats of Combustion of Selected Gaseous Organic Compounds, - hc, at 25 C
and constant pressure to form gaseous water and carbon dioxide.*

Molecular - hc
Compound Weight cal/g. Btu/lb

Methane 16.04 11,953.6 21,502

Ethane 30.07 11,349.6 20,416
Propane 44.09 11,079.2 19,929
Butane 58.12 10,932.3 19,665
Pentane 72.15 10,839.7 19,499
Hexane 86.17 10,780.0 19,391
Octane 114.23 10,737.2 19,256
Nonane 128.25 10,680.0 19,211
Decane 142.28 10,659.7 19,175

Ethylene 28.05 11,271.7 20,276

Propylene 42.08 10,942.3 19,683

Cyclopentane 70.13 10,563.1 19,001

Cyclohexane 84.16 10,476.7 18,846

Benzene 78.11 9,698.4 17,446

Toluene 92.13 9,784.7 17,601

W44444444444444444444444444444444444444444444444444444444444444444444
*Reference [15]

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Appendix 3B

Design Procedure for Non-Recuperative

Thermal Incinerators

3-60
 Not all thermal incinerators are equipped with recuperative heat exchangers to transfer
energy from the flue gas stream to the incoming waste gas stream. These non-recuperative units
use other mechanisms to recovery flue gas energy. One of these types is the regenerative
incinerator. As discussed in Section 3.2.1.3, a regenerative incinerator accomplishes energy
recovery by conveying the flue gas through a ceramic bed which captures a portion of the
stream's enthalpy. After a switching mechanism is engaged, the incoming waste gas passes
through this hot bed and is warmed to its ignition temperature. This process is illustrated in
Figure 3.2.

While we can determine the stream inlet and outlet temperatures for a recuperative heat
exchanger fairly accurately, we cannot always do so for a regenerative incinerator bed. For one
thing, these beds do not behave like typical heat exchangers. The bed temperature profiles are
often difficult to predict. More importantly, because regenerative incinerators do not operate
at steady state conditions, the temperatures within the beds and many other parts of the unit vary
with time. For that reason, it is more convenient to view the entire regenerative incinerator as
a "black box" into which waste gas and auxiliary fuel flow and from which flue gas emanates.
Around this black box we may make mass and energy balances. In this way, we need not make
any assumptions about what occurs inside the incinerator regarding temperatures, flowrates, or
other stream parameters.

However, to determine the auxiliary fuel requirement for regenerative incinerators via the
procedure shown in this appendix we have to make two key assumptions, viz.: (1) the
temperatures and flowrates of all streams entering and leaving the incinerator are at steady state
and (2) the combustion temperature (and by inference, the heat loss fraction) are constant as
well. The other assumptions will be addressed in the following design steps:

Steps 1 to 4: These are the same as those for thermal recuperative and catalytic incinerators.
(See Section 3.4.1.)

Step 5t - Establish the incinerator operating temperature: Because their designs are more
resistant to thermal stresses and because they can achieve very high heat recoveries,
regenerative incinerators are usually operated at higher temperatures than recuperative units.
Consequently, higher VOC destruction efficiencies are achieved. Operating temperatures of
1800 to 2000 F are typical.

Step 6t - Calculate the waste gas temperature at the exit of the preheater: As explained
above, regenerative incinerators do not employ preheaters. The preheating is done by and
within the ceramic beds. Moreover, because the mass and energy balances are made around the
entire unit, we do not need to know the temperature of the preheated waste gas to calculate the
auxiliary fuel requirement.

Step 7t - Calculate the auxiliary fuel requirement, Q af: Because a regenerative incinerator
recovers nearly all of the energy from the combustion (flue) gas, its auxiliary fuel requirement
is usually lower than that for a recuperative incinerator. However, as discussed above, this fuel
requirement is determined via mass and energy balances taken around the entire unit, not just
the combustion chamber. Consider the following diagram:

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 Flue gas
T
* (fo)
*
R
+)))))))))))))))))))))))))),
* *
* Incinerator unit *
Aux. fuel S)))))Q * (any type) * S)))) Waste

(af) * * gas
* * (wi)
.))))))))))))))))))))))))))-

Taking mass and energy balances around the incinerator, we obtain:

Mass balance:

Mass in = Mass out

Mass fuel + Mass waste gas = Mass flue gas

Qaf +
af wi Qwi = fo Qfo (3B-1)

Energy balance:

Next, we take an energy balance around the incinerator unit:

Energy in - Energy out + Energy generated = 0

The terms of the energy balance equation are the inlet waste gas and outlet flue gas
enthalpies (Hwi and Hfo, respectively), the energy loss (HL), and the waste gas VOC and fuel
(natural gas) heat contents (Hcwi and Hcaf, in turn):

Hwi - (Hfo + HL) + (Hcwi + Hcaf) = 0 (3B-2)

The variables comprising each of the terms in this energy balance equation are listed in
Table 3.6. They are:

Hwi = wi QwiCpmwi(Twi - Tref)

Hfo = fo QfoCpmfo(Tfo - Tref)

HL = QfiCpmfi(Tfi - Tref)
fi

Hcwi = wi Qwi(- hcwi)

Hcaf = af Qaf(- hcaf)

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 where: = energy loss from combustion chamber (fractional)
Tfi = combustion temperature ( F)

We next substitute these variables into eq. (3B-2) and solve for the fuel mass rate ( Q af).
af
When doing so, we make the following assumptions:

The streams flowing to and from the incinerator are at steady state conditions.

The auxiliary air requirements are zero.

The ambient, reference, and fuel inlet temperatures are equal (77 F). (This assumption
results in the inlet fuel stream having a zero enthalpy.)

The heat capacities of the gas streams to and from the unit are approximately the same,
regardless of composition.

The mean heat capacities of the streams above the reference temperature (77 F) are
approximately equal, regardless of temperature. Further, the mean heat capacity of the
waste gas/flue gas stream entering/leaving the incinerator is evaluated at the average of the
inlet (Twi) and combustion (Tfi) temperatures. That is, Cpmwi = Cpmfi = Cpmfo =Cpm.

When we do all this, we get the following expression:

wi QwiCpm(Twi - Tref) - [ foQfoCpm(Tfo - Tref) + fiQfiCpm(Tfi - Tref)] +

(Energy in) (Energy out)

[ wi Qwi(- hcwi) + afQaf(- hcaf)] = 0

(Energy generated)

Substitution for Qfo per eq. 3B-1 above yields:

fo

{ wi QwiCpm(Twi-Tref)} - { Cpm( af Qaf + wi Qwi)(Tfi-Tref) +

Cpm( af Qaf + wi Qwi)(Tfo-Tref)} + { wi Qwi(- hcwi) + Qaf(- hcaf)} = 0

af

Finally, solving for Qaf, the auxiliary fuel mass rate (lb/min):
af

wi Qwi{Cpm[ (Tfi - Tref) + (Tfo - Twi)] - (- hcwi)}

af Qaf = ))))))))))))))))))))))))))))) (3B-3)
{(- hcaf) - Cpm[ (Tfi - Tref) + (Tfo - Tref)]}

Equation (3B-3) provides the auxiliary fuel requirement for any type of thermal incinerator,
as it is independent of any intermediate variables, such as the temperature of the preheated
waste gas. Clearly, this equation can be used with regenerative incinerators, as long as the
above-stated assumptions hold.

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 The heat loss fraction ( ) will vary according to the incinerator type, how the incinerator
components are configured in the unit, the construction materials, the type and amount of
insulation, and other factors. For instance, for recuperative incinerators, is approximately
0.10. The for regenerative incinerators is considerably lower, however. There are two reasons
for this. First, the components of a regenerative incinerator—combustion chamber, ceramic
beds, etc.—are housed in a single enclosure, while in a recuperative incinerator the combustion
chamber, heat exchanger, and interconnecting ductwork are housed separately, thus offering
more heat transfer area. Second, because regenerative units are lined with ceramic, they are
better insulated than recuperative incinerators.

To gain an estimate of this heat loss fraction, we contacted two regenerative incinerator
vendors. [27,28] Based on the heat loss data that they supplied, we calculated values ranging
from 0.002 to 0.015 (0.2 to 1.5% ). These values varied according to the incinerator
configuration (vertical or horizontal), the waste gas flow rate, the ambient temperature, and the
wind speed.

Step 8t - Verify that the auxiliary fuel requirement is sufficient to stabilize the burner
flame: As explained in Section 3.4.2, only a small amount (< 5% of the total energy input) is
needed to stabilize the burner flame. With recuperative incinerators, the auxiliary fuel
requirement is usually much larger than the burner stabilization requirement, so that this
constraint rarely comes into play. With regenerative incinerators, however, the auxiliary fuel
requirement may be as low or lower than the fuel needed to stabilize the burner. Therefore, it
is important to compare these two requirements. This comparison is made via equations 3.19
and 3.20. If the auxiliary fuel is less, the minimum fuel requirement would be set at 5% of the
total energy input.

Step 9t - Calculate the flue gas volumetric flow rate, Q fi: As with thermal recuperative
incinerators, the regenerative incinerator flue gas flow rate is the rate used to size and cost the
unit. Measured at standard conditions (1 atmosphere and 77 F), Qfi is the sum of the inlet
waste gas (Qwi) and fuel (Qaf) flow rates. But since Qaf for regenerative units is small compared
to Qwi, the waste gas and flue gas flows should be virtually identical.

3-64
References

[1] Prudent Practices for Disposal of Chemicals from Laboratories, National Academy
Press, Washington, D.C., 1983.

[2] Memo from Mascone, D.C., EPA, OAQPS, to Farmer, J. R., OAQPS, EPA, June 1 1 ,
1980, Thermal Incinerator Performance for NSPS.

[3] Memo from Mascone, D.C., EPA, OAQPS, to Farmer, J. R., OAQPS, EPA, July 22,
1980, Thermal Incinerator Performance for NSPS, Addendum.

[4] Memo from Mascone, D.C., EPA, OAQPS, to Farmer, J. R., OAQPS, EPA, August 22,
1980, Thermal Incinerators and Flares.

[5] Letter from Thomas Schmidt (ARI International, Palatine, IL) to William M. Vatavuk
(EPA, OAQPS, Research Triangle Park, NC), August 16, 1989.

[6] Weldon, J. and S. M. Senkan, Combustion Sci. Technol., 1986, 47.

[7] Manning, P., Hazard Waste, 1984, 1(1).

[8] Pope, D., Walker, D. S., Moss, R. L., Atmos. Environ., 1976, 10.

[9] Musick, J. K., and F. W. Williams, Ind. Eng. Chem. Prod. Res. Dev., 1974, 13(3).

[10] Letter from Robert M. Yarrington (Englehard Corporation, Edison, NJ) to William M.
Vatavuk (EPA, OAQPS, Research Triangle Park, NC), August 14, 1989.

[11] Personal Communication from Bill Sheffer (ARI, Inc., Palatine, IL) to Donald R. van
der Vaart (RTI, Research Triangle Park, NC), March 30, 1988.

[12] Personal Communication from Ralph Stettenbenz (Combustion Engineering, Air

Preheater, Inc., Wellsville, NY) to Donald R. van der Vaart (RTI, Research Triangle
Park, NC), March 23, 1988.

[13] Grelecki, C., Fundamentals of Fire and Explosion Hazards Evaluation, AIChE Today
Series, New York, 1976.

3-65
[14] Weast, R.C. (ed.), CRC Handbook of Chemistry and Physics, 49th ed., CRC Press,
Cleveland, Ohio, 1968.

[15] Perry, R. H. and C. H. Chilton(eds.), Chemical Engineers Handbook, 5th ed., McGraw-
Hill, New York, 1973.

[16] Personal Communication from Robert Yarrington (Englehard Corp., Edison, NJ) to
William M. Vatavuk (EPA, OAQPS, Research Triangle Park, NC), June 6, 1989.

[17] Personal Communication from Thomas Schmidt (ARI International, Palatine, IL) to
William M. Vatavuk (EPA, OAQPS, Research Triangle Park, NC), June 7, 1989.

[18] Githens, R. E. and D. M. Sowards, Catalytic Oxidation of Hydrocarbon Fumes, PB-299

132, National Technical Information Service, Springfield, VA.

[19] Personal Communication from Andrew Jones (Energy Development Associates, Itasca,
IL) to Donald R. van der Vaart (RTI, Research Triangle Park, NC), March 4, 1988.

[20] Personal Communication from C. L. Bumford (Peabody Engineering, Stamford, CT) to

Donald R. van der Vaart (RTI, Research Triangle Park, NC), March 28, 1988.

[21] Personal Communication from C. M. Martinson (TEC Systems, DePere, WI) to Donald
R. van der Vaart (RTI, Research Triangle Park, NC), March 28, 1988.

[22] Personal Communication from Ronald J. Renko (Huntington Energy Systems, Inc.,
Union, NJ) to Donald R. van der Vaart (RTI, Research Triangle Park, NC), March 16,
1988.

[23] Personal Communication from James H. Mueller (Regenerative Environmental

Equipment Co., Inc., Morris Plains, NJ) to Donald R. van der Vaart (RTI, Research
Triangle Park, NC), January 13, 1988.

[24] Personal Communication from Robert Hablewitz (Pillar Technologies, Hartland, WI)
to Donald R. van der Vaart (RTI, Research Triangle Park, NC), March 20, 1988.

[25] Vatavuk, W. M. and R. Neveril, "Estimating Costs of Air Pollution Control Systems,
Part II: Factors for Estimating Capital and Operating Costs", Chemical Engineering,
November 3, 1980, pp. 157-162.

[26] Kobe, K. A. and associates, "Thermochemistry for the Petrochemical Industry",

Petroleum Refiner, Jan. 1949 through Nov. 1954.

[27] Letter from William M. Vatavuk (EPA, OAQPS, Research Triangle Park, NC) to Gerald
Schrubba (Salem Engelhard, South Lyon, MI), September 22, 1992.

3-66
[28] Letter from William M. Vatavuk (EPA, OAQPS, Research Triangle Park, NC) to Rod
Pennington (REECO/Research Cottrell,
Somerville, NJ), September 22, 1992.

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