Furnace design and
operation
Defining efficiency
Defining efficiency
The thermal efficiency of any process, or
component of the process is
Electrical, heat,
embodied chemical
Useful energy out
hth =
Total energy in
Fuel, electricity,
preheated air,
preheated feed
System efficiency
The system thermal efficiency is the product of a
number of contributing efficiencies, thus for
example in a boiler it would be derived as
hsys = hcycle x hcomb x hht
Thermodynamic Combustion Heat transfer
cycle efficiency efficiency efficiency
Factors that influence
efficiency
Process thermodynamics
Type and properties of the fuel
Physical properties of the material processed
Aerodynamics of the system
Burner design
Slagging, fouling, etc. effects on heat transfer
Combustion efficiency
Combustion efficiency is a measure of the
completeness of oxidation of the combustible
components of the fuel (carbon, hydrogen,
sulphur)
Smoke or carbon monoxide are the most
common signs of incomplete combustion
Combustion of carbon
complete oxidation of carbon
C + O2 CO2 + 394 kJ.mol-1
incomplete combustion of carbon
C + ½O2 CO + 221 kJ. mol-1
the burnout of carbon monoxide is a slow
reaction, but is enhanced in the presence of
hydrogen (water)
Combustion of carbon
CO in the flue gases are the result of
incomplete combustion
Incomplete combustion is usually the result of
poor fuel/air mixing owing to the
aerodynamics of the system or the burner
design
CO represents an energy waste
CO is toxic and is an air pollutant
CO>600ppm can be lethal
Combustion of hydrogen
Fuel hydrogen is preferentially oxidized to
water
Gross calorific value
572 kJ.mol-1 (water condensed)
H + O2 H2O +
484 kJ.mol-1 (water as steam)
Nett calorific value
Stoichiometry
The chemical balances for the combustion of
carbon and hydrogen show that there is an
exact quantity of oxygen required for the
reaction to be complete. This is termed the
stoichiometric oxygen requirement
For any given fuel, there is an equivalent
stoichiometric oxygen or air requirement
Any additional oxygen or air , termed excess air,
is theoretically un-necessary and potentially
reduced the system efficiency
Example
Fuel oil with the following ultimate analysis (%w/w basis) is to be
burnt with 15% excess air in a furnace
Element % w/w
Carbon 84.6
Hydrogen 10.9
Sulphur 0.7
Oxygen 3.8
Total 100.0
Assuming complete combustion, determine:
i) the mass of air required to burn 1 kg fuel oil,
(ii) the mass of flue gas produced from the combustion of 1 kg
fuel oil,
(iii) the flue gas composition (%v/v)
(iv) the volume of flue gas produced (at 300 oC and 1 atm) from 1
kg oil
Example (cont)
where F, A, and G are in kg. The composition of air is assumed to be 23.3% w/w
Oxygen and 76.7% w/w Nitrogen.
Basis for the calculation:- 1 kg fuel oil
Total mass balance gives F + A = G
or, as F=1 1 + A = G
Complete combustion involves the following reactions:
C + O2 CO2
2H + 0.5O2 H2O
S + O2 SO2
The flue gas will consist of:
carbon dioxide c kmol
water w kmol
sulphur dioxide s kmol
nitrogen n kmol
excess oxygen y kmol
Example (cont)
Consider now the steady-state mass balance for individual atomic species:
Mass IN = Mass OUT
Carbon (kg): 0.846 = 12c thus c = 0.0705 kmol
Hydrogen (kg): 0.109 = 2w thus w = 0.0545 kmol
Sulphur (kg): 0.007 = 32s thus s = 0.00022 kmol
Nitrogen (kg): A x 0.767 = 28n
Oxygen (kg): A x 0.233 + 0.038 = 32c + 16w + 32s + 32y = 3.135 +32y
Thus A x 0.233 = 3.097 +32y
Thus the stoichiometric oxygen requirement = 3.097 kg
Total oxygen supplied = (3.097 + 32y) kg
As the excess air required is 15%
3.097 x 1.15 = 3.097 + 32y
Thus y = 0.0145
Example (cont)
The answers are then worked out from the derived relationships as follows:
(i) the mass of air required for 1 kg fuel oil: A = 15.29 kg
(ii) the mass of flue gas produced from 1 kg fuel oil: G = 15.29 + 1 = 16.29
(iii) the flue gas composition (%v/v):
Flue gas component kmol % mol (= % v/v)
CO2 0.0705 12.62
H 2O 0.0545 9.76
SO2 0.0002 0.03
N2 0.4188 74.99
O2 0.0145 2.60
Total 0.5585 100.00
(iv) the volume of flue gas produced (at 300 C and 1 atm) from 1 kg oil:
The gas laws state that PV = nRT
where R (universal gas constant) =0.03697 m3.atm.K-1.kmol-1
Thus V= (0.5585 x 0.03697 x (273+300))/1.0 = 12 m3
Heat transfer efficiency
Flame heat transfer by radiation depends on
both the type of fuel and the air/fuel mixing
System efficiency depends on total radiant
and convective heat transfer
Burner design influences both flame radiation
and fuel burnout
Hence two combustion systems can have the
same combustion efficiency, but result in a
different system efficiency
Flame radiation
natural gas flame heavy fuel oil flame
IFRF No.1 furnace
System thermal efficiency
For a furnace in steady state operation
hf = hc + hs + hg
Heat Useful Furnace Heat lost
supplied heat to structure in flue
by fuel product losses gases
Heat from fuel
The heat produced from the fuel depends on its type,
and the burner design. For 100% combustion efficiency
hf = qf x CV
Heat Fuel Calorific
supplied flowrate value of
by fuel the fuel
Useful heat
The useful heat is the sum of the sensible heat and any
thermodynamic heat transferred to the product
hc = qc x (Cpc x {Tc -Ta} + hr)
Useful heat Product Mean specific Temperature Heat required
to product flowrate heat of difference of to change
product product physical or
between Tc between inlet chemical state
and Ta and exit of product
For simple heating processes the thermal efficiency of
the process is hc/hf, whilst for processing furnaces it is
hr/hf
Structure losses
Heat is lost through the furnace walls by conduction,
and then by convection and radiation at the outside
surface
hs = Ai x k x (Ti -To)
Furnace Inside area Thermal Temperature difference
structure of furnace conductivity between inside and
losses walls of walls outside of walls
hs = Ao x (h x (To - Ta} + σ x ε x {To4 - Ta4})
Outside area Convective Temperature difference Emissive power
of furnace htc between outside walls of outside
walls and ambient surface
Flue gas losses
Heat is lost as sensible and latent heat in the flue gases
hg = qg x Cpg x (Tg -Ta) + qgwater x hfg
Heat lost in Flue gas Mean Temperature Flue gas Latent heat
flue gases flowrate specific heat difference water of
of flue gas exit flue gas vapour evaporation
between Tg and ambient flowrate of water
and Ta
If the net calorific value is used in calculating hf, the latent heat is
discounted
If the flue gases contain particles of dust, then these must also be
included
Excess air increases the value of qg and hence reduces the
efficiency
Other energy inputs
In addition to the steady state energy balance,
further thermal input is required to heat the
furnace structure up to its operating
temperature. This is called the no-load or
standing heat loss and includes hs
Monitoring the furnace performance can
quantify all these energy values
Furnace performance chart
Furnace performance data
50
45
40
35
Thermal input MW
30
25
20
15
10
0
0 20 40 60 80 100
production rate tonne/hr
Furnace performance chart
Furnace performance data
50
45
40
Useful heat
35 Sensible heat loss in product
Thermal input MW
Flue gas heat loss
30 Standing heat loss
25
20
15
10
0
0 20 40 60 80 100
production rate tonne/hr
Optimum system efficiency
Furnaces are designed to meet a specific load or
duty, and should be most efficient at that load
Running the furnace at lower that full
throughput increases the ratio of the standing
losses relative to the useful heat, making it less
efficient
Increasing the throughput above the design
flow results in reduced product quality (either
not hot enough or under-processed), again
making it less efficient
Optimum system efficiency
Five zone pusher furnace performance
3.5 35
3 30
Specific fuel consumption MJ/kg
2.5 25
Thermal efficiency %
2 20
1.5 15
1 10
0.5 5
0 0
0 50 100 150 200 250 300 350 400 450 500
Production rate kg steel/hr m 2 surface area
Specific fuel consumption Thermal efficiency
Drivers to improve
efficiency
The cost of operating a furnace is based on
Capital cost, depreciation and interest charges
Fuel and electricity costs
Maintenance and repair costs
Labour costs
Environmental costs and charges (where relevant)
In times of high interest rates the capital costs and
depreciation charges dominate
High labour costs drive up both labour and maintenance
charges
Steadily rising energy costs seem likely to continue, or
even accelerate, driven by the demand for oil exceeding
the available supply
Drivers to improve
efficiency
Unless there is a major discovery of easily won
oil or the link between CO2 emissions and
climate change is disproved, both unlikely
scenarios in the medium term, fuel, electricity
and environmental charges will be the major
furnace operating costs of the future, hence
requiring designers to achieve much higher
efficiencies than in the past
