Furnace design and
operation
Pollutant formation
Pollutant formation
The combustion of fossil fuels results in the
emission of flue gases into the atmosphere and
in the case of some solid and liquid fuels in ash
The primary products of the combustion
process, carbon dioxide and water vapour, are
no in themselves toxic, but contribute to the
‘greenhouse effect’ in the atmosphere
Impurities in the fuel, combustion air and
processed materials all give rise to unwanted
emissions or pollutants
Pollutant formation
Most fuels consist of carbon and hydrogen with small
quantities of sulphur, chlorine, phosphorous and
nitrogen etc. together with traces of metals. Gaseous
pollutant chemistry can be reduced to
C + O2 CO2
CO2 + C 2CO
2C + O2 2CO
N2 + O2 2NO
2NO + O2 2N
Cl2 + H2 2HCl
S + O2 SO2
2H2S + 3O2 2H2O + 2SO2
Pollutant formation
With the exception of water vapour, all of the compounds on the
right hand side of the equations are designated pollutants.
Most of these reactions are reversible and depend on the
concentration of the species and the temperature for their
equilibrium state.
High temperature and/or the presence of oxygen drive most of
these reactions to the right.
In addition to the above reactions, there are some compounds,
formed during intermediate reactions, which may persist in the flue
gases under certain combustion conditions. Typical of these are
volatile organic compounds (VOC’s), dioxins and furans.
Pollutant formation
A further source of emissions comes from the mineral matter
contained primarily in solid fuels but also in heavy fuel oil fractions
and some waste liquids.
These minerals are usually converted to metallic oxides and emitted
with the flue gases as fly ash, or deposited in the furnace as
residual ash.
Pollutant formation
Carbon, nitrogen and sulphur each have more than one oxide
The two oxides of carbon have a special importance, since the
concentration of CO present in the final flue gas is a very good
indicator of combustion performance, whilst CO2 has been identified
as a primary cause of global warming.
The oxides of nitrogen are precursors in the formation of
atmospheric smog, and together with the oxides of sulphur give rise
to acid rain.
Carbon monoxide
Carbon monoxide CO is highly toxic and is the result of
incomplete combustion
There is always some residual CO in furnace flue gases
because mixing processes are not perfect
CO should be limited to a few parts/million (ppm),
normally in the range of 10 - 50 ppm. Higher values
indicate poor combustion conditions
CO is the first step in the oxidation of carbon. Oxidation
of CO to CO2 is a ‘slow’ reaction, but is catalysed by the
presence of water vapour
Nitrogen oxides
Oxides of nitrogen formed during combustion include
N2O, NO, NO2 and N2O4, and are termed collectively as
NOx emissions. NO is usually the primary form
generated in a flame, subsequent oxidation or reduction
producing the other oxides
There are three routes whereby NOx is formed in a
flame
Thermal NOx
Fuel NOx
Prompt NOx
Nitrogen oxides
Thermal NOx - formed from atmospheric
nitrogen and oxygen and is a function of combustion
temperature
Fuel NOx - formed from nitrogen in the
fuel and is a function of fuel composition
Prompt NOx - formed from atmospheric
nitrogen and fuel and is a function of mixture
stoichiometry
Thermal NOx
Thermal NOx is formed by the combination of atmospheric nitrogen
and oxygen at very high temperatures
It is formed by a mechanism known as the Zeldovich couple
k1f
O N2 NO N
k1b
10.1
k 2f
N O2 NO O
k 2b
10.2
The extended Zeldovich mechanism includes
N + OH NO + H
The amount formed depends exponentially on the temperature and
local [O2] concentration
Thermal NOx formation is not significant at temperatures below
1550oC
Thermal NOx
Fuel NOx
Most fuels, other than gas, contain nitrogen bound as an organic
compound in the structure
When the fuel is burnt this organic nitrogen becomes converted
into a range of cyanide and amine species, which are subsequently
oxidised to NOx, depending mainly on the local oxygen availability
This mechanism is only weakly dependent on temperature
On heating, the nitrogen present in the fuel will divide between the
char and volatile components, with typically 20% of the nitrogen in
the char and 80% in the gaseous phase, the latter both as the light
fractions and tars
the mechanisms involved are not fully understood at present
Fuel NOx
Nitrogen bearing fuel structures
Simplified schematic for NOx formation from fuel nitrogen
Prompt NOx
Involves the fixation of nitrogen by hydrocarbon compounds in fuel
rich areas of the flame and is formed in the early portion of the
flame
Accounts for the first 15-20 ppm of NOx formed during combustion
The nitrogen is initially fixed by hydrocarbon fractions
CH + N2 HCN + N
C + N2 CN + N
and the resulting nitrogen atom is then oxidised
N + O2 NO + O
N + OH NO + H
Predicting NOx
The previous descriptions of NOx formation via the thermal,
prompt and fuel routes are only a précis of processes which are
extremely complex and not yet well understood
Some of the currently available models are capable of predicting
the trends of NOx formation with changes in flame conditions and
fuel type, but the reliability of predictions is poor and sometimes
little better than orders of magnitude
Currently, the most reliable methods of estimating NOx emissions
from full-scale flames is by direct comparison or scale-up from test
flame data
Sulphur oxides
Sulphur compounds are emitted where sulphur-bearing fuel (oil,
coal etc.) is burnt or where sulphur-bearing materials are processed
The reaction kinetics are such that it generally the case that all
sulphur is oxidised (SOx), except under severe reducing conditions
when H2S is formed
Since most furnaces operate most efficiently at reasonably low
excess air levels sulphur dioxide, SO2, is the primary oxide of
sulphur found in the flue gases
SO2 is acidic and corrosive, and causes major problems with metal
and refractory corrosion in furnaces and in the surrounding
environment
Sulphur oxides
At high oxygen concentrations and moderate temperatures sulphur
trioxide, SO3, tends to be be formed, as well as in the presence of
heavy metals
SO3 is even more corrosive than SO2 and leads to sulphuric acid
formation with the water from combustion, at the so-called acid
dew point
It causes severe corrosion in flue gas ducting, air heaters and
economisers of oil fired boilers, and with unburnt carbon leads to
acid smuts
A significant amount of the SO2 discharged to atmosphere in the
flue gas is further oxidised to SO3 on mixing with air, and then
combines with water vapour to create a persistent visible plume
(i.e. acid mist)
SOx in the atmosphere
Products of intermediate
combustion (PIC’s)
Under certain conditions some of the intermediate chemicals
formed during the combustion process are released as products of
incomplete combustion or PICs, although not all of them arise in
flue gas emissions as a direct result of the inefficient combustion of
fuel in the furnace
These can be divided into three main groups
Volatile organic compounds (VOC’s)
Poly-cyclic aromatic hydrocarbons (PAH’s)
Poly-chlorinated bi-phenols, dioxins and furans
VOC’s
These are compounds containing hydrogen, carbon and oxygen
VOC emissions arise primarily from internal combustion engines
They contribute to the formation of low level ozone, a respiratory
irritant, and are noted for their generally offensive smell
Expensive techniques are available for controlling VOC emissions
such as incineration, condensation, absorption, or catalytic
combustion
They do not normally result from the combustion process in furnace
operations, but are driven off from the raw feed in plants where
feedstock materials, including solid fuels, containing organics are
being processed
They are emitted to atmosphere in processes such as drying, milling
or preheating, where the temperature is too low for them to burn
PAH’s
The simplest compound in this category is naphthalene, which
contains two benzene rings. Naphthalene can occur in the
combustion products of some petroleum-derived fuels
Other examples of PAH occurring in combustion processes include
anthracene and phenanthrene (three rings), pyrene (four rings)
Most of the PAHs are known to have serious effects on health so
attention has to be given to their formation even in trace amounts.
One of the earliest reported identifications of a substance believed
to cause cancer was soot, in 1775, and it is now known that PAH
deposited on soot is largely responsible for the carcinogenic activity
PAH’s
Sources of PAH’s include
Unburnt petroleum based fuels - PAH’s are an inherent
constituent of crude oil
Fuel synthesis from heating fuels under non-combusting
conditions - e.g heating acetylene to 700oC yields ~6.5%
pyrene and smaller quantities of other PAH
Coal pyrolysis - formed from the volatile fuel components
PAH’s have been measured in the flue gases of natural gas fired
equipment - this may be the result of hydrogenation of soot
particles
PCB’s, PCDD’s and PCDF’s
Another class of PIC pollutants includes the carbon-hydrogen-
oxygen-halogen compounds, They fall into three main categories
PCB’s - polychlorinated biphenyls - family of 209 compounds based on
two partially chlorinated benzene rings linked by a single bond
PCDD’s - polychlorinated dibenzene-para-dioxins, or dioxins for short
PCDF’s - polychlorinated dibenzofurans, or furans for short
PCB’s PCDD’s and PCDF’s
These pollutants have been shown to be extremely toxic to some
laboratory animals, but their effect on humans is not fully
understood
Human exposure is generally from the soil via the food chain,
especially via eggs and meat
They are of particular importance as a pollutant, owing to their
chemical and physical stability. They do not readily degrade in the
environment and have a tendency to bio-accumulate
They have been identified as occurring in most human and animal
tissue, although their toxicity is highly dependent on both the
degree of chlorination and isomeric arrangement
From some 419 identified types of dioxin related compounds, only
29 are currently considered to have significant toxicity, with
2,3,7,8-T4CDD being the most toxic.
Toxicity
The toxicity of pollutants is defined by the toxic equivalence
factor
The toxic equivalence factor is an assign value, describing how toxic
each dioxin and dioxin-like compound is compared to the most toxic
members of the category: 2,3,7,8- T4CDD and 1,2,3,7,8-PnCDD
To account for how emissions vary in toxicity, we use a weighted
value called the toxic equivalent (TEQ)
Sources of PCB’s etc.
PCB’s, dioxins and furans can form in any combustion process
where chlorine and fluorine are present
They are most commonly found in waste incineration and coal
combustion processes
The chemistry of their formation is not fully understood, but as with
PAHs, synthesis rather than pyrolysis is more likely to be the route
of the formation of these pollutants in the combustion process.
Pre-existing PCB’s, dioxins and furans that have been generated from other
sources and deposited from the atmosphere onto the fuel and/or feedstock
being volatilised at low temperature and released into the flue gases
Gas phase formation from chloro-phenol groups inherent in the fuel in the
temperature range 500-700oC.
Reaction of benzene type ring structure in the fuel (e.g. lignin) with chlorine
in the temperature range 500-700oC.
Heterogeneous catalytic formation promoted by heavy metals present in the
fly ash in the temperature range 150-400oC. (often termed ‘de-novo’
synthesis)
Sources of PCB’s etc.
Current wisdom suggests that the ‘de-novo’ synthesis route is the
most likely primary source from combustion as the temperature
window corresponds to that used by most high efficiency flue gas
cleaning equipment e.g. electrostatic precipitators, bag filters
In the most industrialised societies, the greatest measured sources
of dioxins and furans are the combustion of coal and municipal
waste incinerators, both of which have significant quantities of
metallic ashes
In more primitive societies they arise from inefficient combustion of
wood, etc in open fires
Particulate emissions
Particulate emissions arise from ash in the fuel (coal and oil), and in
the case of direct contact processes also from the feedstock and/or
product (e.g. cement and lime manufacture, alumina calcination,
ferrous and non-ferrous metals production and glass manufacture)
Combustible particulates, such as soot and sub-micron unburnt
carbon, may also be emitted from combustion of gaseous
hydrocarbons as the products of incomplete combustion (PIC’s)
Particulate emissions from combustion of fuels fall into three main
categories
soot
ash
non-combustible volatiles
Soot
Soot may form in all combustion systems, but it is especially
common in those using diffusion flames
Most furnaces operate using this type of flame as it has the
advantages of a large stability range
The time taken for soot to form is in the order of milliseconds
It is formed from fuel in the gaseous phase, burning in the absence
of sufficient oxygen to produce heavy hydrocarbons, which in turn
condense as liquids, and then become solid as soot.
Soot formation
Soot is formed in a number of stages
an initial stage in which the first particles, solid or liquid are formed
coalescent collision of these species to form larger spherical particles
surface growth on species formed in stages 1 and/or 2 from the
complex gas phase hydrocarbon soup of the flame
carbonisation of the spherical particles after which further collisions will
result in chain structures
surface growth onto the chain structures
Stage 1 corresponds to nucleation, stages 2 and 3 to the formation
of the primary spheres and stages 4 and 5 to the formation of the
characteristic soot structure
Soot formation
Agglomeration
Temperature ~1000oK
Condensing
Hydrocarbon
aromatics
fuel
Dehydr
ogenati
on Increasing temperature
and reducing time
Carbon
Atomic particles
carbon
Temperature >3500oK
Soot combustion
The critical amount of oxygen required to suppress soot formation
is less than the stoichiometric quantity for complete combustion,
but considerably more that required for carbon to exist in
equilibrium
For aromatic hydrocarbons, the stoichiometry lies between 0.75 and
0.8, whilst for aliphatic hydrocarbons with 6 to 16 carbon atoms it is
between 0.67 and 0.74
Soot combustion
0.2
0.18
0.16
0.14
Burning time seconds
0.12
0.1
0.08
0.06
0.04
0.02
0
1600 1700 1800 1900 2000 2100 2200 2300 2400
Particle temperature K
Soot burning times for 500 Å particle at an oxygen partial pressure of 0.05 bar
Ash
Fuel ash is the residual incombustible material remaining after the
combustion of char particles
The mineral matter in the original fuel is usually a combination of
adventitious or excluded (i.e. discrete mineral particles) and
inherent or included (i.e. chemically bonded into the organic fuel
structure) material
Mineral matter contains some or all of the following elements, Al,
Ba, Ca, Cr, Cu, Fe, Hg, K, Mg, Mn, Na, Ni, Sn, Si, Sr and Ti as salts,
or complex organo-compounds
As char combustion proceeds, the carbon matrix is oxidised and the
mineral inclusions melt. As more of the carbon surface is removed,
the molten mineral matter agglomerates with other molten ash
particles
Ash formation
The ash melt is primarily an Al-Si-Fe-Ca-O complex
The physical appearance of ash particles is dependent primarily on
the specific fuel properties and combustion temperature, and falls
into one of three structures
solid fused ash particles
fused cenosphere containing some blowholes
loose lacy structures
Solid fused ash particle Fused cenosphere Lacy ash structures
with blowholes
Ash formation
Excluded minerals
Pyrite1100C
fusionClays1300C
Quartz1550C
expansion
H2O
CO2
Cenospheres
Framhiodal
pyrite
fragmentation
Clay mineral
halloysite
Ash formation
Included minerals and organically bound cations
fragmentation
swelling
5-30μm
Na
K
70μm S heterogeneous
25% ash Surface enrichment
vaporisation Mg condensation
non-swelling homogeneous
SiO nucleation
MgO coalescence
0.02 – 0.2μm
coalescence
30μm
shedding of larger discrete
mineral grains expansion
CO quench
coalescence of smaller minerals
and organically bound inorganics 10 - 90μm
disintegration
< 30μm
Non-combusting volatiles
Inorganic alkaline metal species, such as sodium and potassium
organic acid salts in the fuel are readily vaporised in the flame as
hydroxides and chlorides at temperatures above 950oC
In the cooler parts of the furnace, these alkali metals re-condense
reacting with mainly sulphur dioxide to form sub-micron sulphate
particles (<1100oC), or dissolve into the surface of silicate particles
(1200-1600oC)
These sulphates will also react with the aluminosilicates in the ash
at temperatures above 750oC to form cementaceous compounds,
giving rise to the build up of severe hard ash deposits on surfaces
in the furnace and downstream equipment and ductwork
Fouling index
Prediction of the fouling tendency using the analysis of the ash is
expressed as a Fouling Index
Na2O K 2O Fe 2O3 CaO MgO
Fouling Index x%Na 2O in ash
SiO2 Al 2O3 TiO
The fouling potential classified by the Index value are
Low <0.2
Medium 0.2 – 0.5
High 0.5 – 1.0
Severe >1.0
Deposition on coal fired boiler tubes
after 700hrs operation
Control of emissions
The operation of almost any furnace will be subject to either
regulatory or voluntary environmental control by local, national and
sometimes international bodies
In many cases, the furnace is considered as part of a wider
industrial site operation, the’ bubble’ principle and regulation is
based on the emissions at the periphery of the site, rather than on
individual unit operations
Traditionally monitoring of furnace operations were reported
regularly to a designated agency with local knowledge to meet local
standards
Newer controls are based on integrated pollution and prevention
legislation covering whole industry sectors, both nationally and
internationally. Thus factors such as energy efficiency, noise, raw
materials, health and safety, site remediation etc. are covered
within the legislation, as well as the monitoring of emissions
Prevention and abatement
Of the emissions considered
some may be controlled by adjusting equipment design and operation
to avoid their formation (viz. pre-flame and in-flame techniques)
some may be chemically modified or destroyed within the furnace (viz.
reburning, and catalytic reduction)
some may only be captured by treating the flue gases prior to
discharge to atmosphere (viz. end–of-pipe treatments)
It is important when considering abatement techniques to take a
holistic view, as some of the available technological solutions may
just ‘shift’ the pollutant problem to another part of the process
Some pollutants, such as sulphur oxides and ash, will be the
inevitable result of the burning process, whilst others, such as CO,
VOC’s, PAH and NOx, can be minimised or avoided by design
and/or operational changes
Pre-flame control
Any process that improves the quality of the fuel, usually termed as
beneficiation, will reduce emissions by removing potential pollutants
and/or by improving the thermal efficiency
Pre-flame treatment of the fuel is an effective method of controlling
the impact of pollutants such as ash and sulphur
Washing coal at the pithead to remove excluded mineral matter - slag
heaps
Froth floatation to deep wash coal, and also reclaim from slag heaps
The slag is either returned to the mine or used for cement making
There are no commercially viable processes for the removal of sulphur
from coal, but desulphurisation of fuel oil is a well-established
technology within the normal commercial refinery process, and is
required to avoid catalyst poisoning in the cracking and reforming
processes
Pre-flame control
A ‘poor’ fuel can be improved by gasification or liquifaction. These
processes alter the chemical or physical state of the initial fuel
source, but are expensive and not generally economic
It is inevitably the case that the more beneficiated the fuel, the
higher its cost, however all such processes (viz. liquefaction,
gasification, catalytic cracking) tend to produce an expensive high
quality fuel stock, and a cheap residual, which is usually almost
pure carbon with a significant proportion of the original impurities
In-flame control
We have discussed the importance of mixing and turbulence in the
combustion process, and concluded that for most industrial flames,
these processes control the rate of combustion
Research efforts through the first part of the 20th century
developed ground breaking designs in combustion
Whittle - jet engine
Swirling flames - smaller, more efficient furnaces
These techniques ‘intensify’ the combustion process by promoting
higher turbulence and mixing rates
This results in increased NOx formation
The latter part of the 20th century has been spent trying to develop
flames that minimise pollution
NOx control
A wide range of ‘Low NOx’ combustion technologies have been
developed
flue gas recirculation
combustion staging
reburn
oxy-fuel
NOx control requires control of one or more of the parameters
which effect NOx generation in a flame
flame temperature
residence time of hot gases in the flame
oxygen availability - particularly in the hottest regions of the flame
The choice of which combination of these parameters is best to
target depends upon the fuel and the combustion system. For
example, control of the temperature has limited effectiveness if the
dominant source of NOx derives from the nitrogen in the fuel
NOx control
It is important to note that each of these parameters is
interdependent, fluctuating and plant specific.
For example, changing the oxygen availability will change the
temperatures in the flame. Unfortunately the reaction rates
associated with the oxidation of the fuel, and notably of CO, also
depend upon the same parameters. That is, if the residence time,
the oxygen availability and the temperature are reduced, the
formation rates of both NO and of CO2 will be reduced. This gives
rise to a dilemma, in that that most changes to a system which
reduce NO also increase CO emissions
Pre-mixed systems (e.g. internal combustion engines and some
gas turbines) can operate with low NOx in the very lean region
while diffusion flames cannot
NOx control
An example of the trade-off between
Effect of stoichiometry on NOx emissions
NO and CO emissions from an internal
for diffusion and premixed flames
combustion engine, which operates as
pre-mixed flame
Reducing thermal NOx
Reducing the flame temperature will reduce NOx formation by the
thermal route
The addition of an inert species - most commonly achieved by flue gas
recirculation (FGR)
Fuel and air staging
Increasing the flame emissivity - by changing fuel or modifying the
mixing
Reducing the amount of air preheat - poor option as it also reduces the
efficiency
Increasing the amount of air preheat – this requires a redesign of the
furnace system, and process operation to use very high air preheating
with delayed fuel mixing to give an even temperature environment in
the furnace, Variously termed as flameless oxidation (FLOX), high
temperature air combustion (HTAC), high efficiency combustion (HEC)
or excess enthalpy combustion (EEC)
Flue gas recirculation
Some of the warm flue gases
are returned to an injection
point behind the flame front,
and added into the flame
can achieve a 60% - 70% NOx
reductions in boilers with 25% -
35 mass% of the flue gas
recirculated
Low NOx burners
Most of the ‘low NOx’ burner technologies that are commercially
available use combustion staging to create fuel rich and fuel lean
zones
The zones can either be created within individual flames, as in low
NOx burners or, in the case of multi-burner systems, can be
generated within different regions of the furnace
Low NOx burners
It is a general requirement
for a low NOx burner that all
of the combustion air is
controlled, so it is not
possible to fit low NOx
burners to furnaces where
the secondary air is
uncontrolled e.g. rotary kiln
In multi-burner furnaces Aerodynamically air staged burner
burner biasing may be used
to create the required zones,
whereby some burners are
operated in a fuel rich mode
and others are operated with
additional excess air to
compensate
Reburn
Even greater reduction of fuel
NOx can be achieved using the
‘reburn’ technique, which has
three stages of combustion
The first stage operates with
normal amounts of excess air
and will therefore produce
normal amounts of NOx
The hot products of combustion
are fed directly into a zone of
secondary fuel-rich combustion by adding more fuel (usually natural
gas) where the fuel-rich conditions drive the NO reaction scheme back
to the formation of molecular nitrogen
The final stage occurs with a low mixing intensity as air is added to
complete the combustion
FLOX
Combustion air is preheated to <1000oC by flue gases
The fuel and preheated air/flue gases are injected through separated
ports and mixing and combustion proceed evenly throughout the
furnace enclosure
There are no significant peak temperatures and the furnace
corresponds closely to the well-stirred model
Usually use two regenerative burners to achieve air preheating
Oxy-fuel
Thermal NOx can be reduced by removing the source of nitrogen,
i.e. by burning in an oxygen rich atmosphere
Experimental studies have shown that the amount of thermal NOx
formed in a gas flame is reduced once the oxygen content is >60%
Oxy-fuel flames have significantly higher flame temperatures and
lower flue gas volumes, and the furnace must be specifically
designed to meet these changes
Inert (flue gas or water) gas recycling can be employed to reduce
the flame temperature and increase flue gas volumes as a retrofit
option for existing furnaces
This is an expensive reduction technique, and is only justifiable if
there are other process or operational benefits
In-flame control
The essence of most in-flame emissions control technologies is to
design a burner and furnace system that does just enough to meet
the process demands, whilst satisfying emissions legislation
If the combustion process is ‘too’ good then NOx regulations are
likely to be exceeded, whilst if the performance is not quite good
enough then PIC’s will be the inevitable consequence
Such performance can be achieved on a fully instrumented and
monitored test furnace, but to translate this to a production furnace
requires equivalent monitoring instrumentation, accurate control of
the fuel, air, etc. inputs, and flexibility in the burner design to
perform over the required operating range
End-of-pipe control
End-of-pipe control techniques and equipment are primarily
concerned with the capture of sulphur oxides and particulates,
although there are also techniques available for the reduction of
NOx and CO2 capture
Sulphur oxides are captured either by wet or dry scrubbing
Particulates emissions can be minimised by the use of dust control
equipment (cyclones, electrostatic precipitators and bag filters)
NOx control techniques involve the injection of ammonia or urea into
the flue gases to reduce NOx to nitrogen under controlled temperature
conditions
Sequestration, which is the liquefaction of CO2 from the flue gases, is a
technically proven method for CO2 capture
Sulphur oxides
Wet scrubbers, generally consisting of a water spray tower through
which the flue gases are passed, are highly efficient in the removal
of SOx, as these gasses are extremely hygroscopic. They will also
remove a significant quantity of NOx from the flue gases, sufficient
to meet most regulatory requirements
The resulting effluent water from the spray tower is itself a hazardous
acidic waste
Dry scrubbing is carried out using either granular limestone, or lime
as an absorbent
in-flight techniques where the absorbent is ‘blown or sprayed’’ into the
flue gases
fixed bed absorption, where the flue gases pass through a packed bed
of the absorbent
Particulates
The design of dust
extraction equipment is
well established
Cyclone - captures >10
micron particles
Electrostatic precipitator
- capture >2-5 microns
depending on gas
condition
Baghouse filters -
collection down to sub-
micron particles
depending on filter
medium and
temperature
Particulates
Emissions limits can be set according to local, national, international
or industry specific standards. A typical value would be 50mg/Nm3
flue gases (6%O2 equivalent)
Further limits may refer to PM10 (sub 10 micron) and PM2.5 (sub 2.5
micron) values, as these are related to respiratory retention
Toxic metal emissions are contained
in the smaller particles, or are
condensed on to particles
(viz. mercury)
NOx
Injection of ammonia or urea into the flue gases to reduce NOx to
nitrogen under controlled temperature conditions
This may be carried out by passing the treated flue gases
over a catalyst at a controlled temperature (selective catalytic
reduction, or SCR)
or within a carefully controlled time and temperature window in the
flue gas ducting (selective non-catalytic reduction or SNCR)
Both these techniques involve considerable additional cost for the
ammonia or urea, and the SCR route has the additional cost of the
catalyst, which is subject to poisoning and fouling
Carbon dioxide
Carbon dioxide has been identified as the primary greenhouse gas
responsible for climate change. Although VOC’s, and in particular
methane, are an order of magnitude more damaging to the
environment, their much lower emission rate reduces their impact,
relative to CO2
Sequestration, which is the liquefaction of CO2 from the flue gases,
is a technically proven method for CO2 capture, although no
commercial installations have been built
Flue gas
inerts e.g. N2
N2
Wet flue gas Dry flue gas
recycle recycle
Air O2
Air separation
unit Furnace Dust Condenser CO2
Process filter Liquifaction
CO2 rich
flue gas
Fuel
Dust Acidic waste CO2
water
