PROCEDURE FOR CALCULATING POWER PLANT EMISSIONS
By: Marcial T. Ocampo
September 17, 2009
Calculating power plant emissions (mg SOX / Nm3, mg NOX / Nm3, mg CO / Nm3 or
as ppmv SOX, ppmv NOX, ppmv CO) is sometimes difficult for ordinary power plant
engineers. Carbon dioxide emissions are usually given in % by volume dry or as kg
CO2 per kWh generation.
Well, your favorite energy technology expert has developed a spreadsheet model (in
MS Excel) for calculating in a breeze the common power plant emissions.
SO2 and CO2 emissions are readily calculated from fuel analysis (by mass for solid
and liquid fuels, by volume for gaseous fuels) and typically measured by Orsat or
other gravimetric methods.
On the other hand, NOX and CO are result of thermal dissociation of N2 into NOx at
high combustion temparature while CO is the result of incomplete combustion due to
insufficient O2 or inadequate fuel and air mixing. Hence, NOX and CO are measured
using instruments and later computed as mg NOX / Nm3 or mg CO / Nm3). (Nm3
means normal cubic meter of gas).
The basic steps followed by my model are as follows:
Basic steps:
1)
Input natural gas (fuel) analysis: % volume (same as % mol), molecular
weights:
e.g. H2, CH4, C2H6, C3H8 ... CO2, S, O2, N2, H2O moisture, ash.
2)
Convert % volume to ultimate analysis % mass or weight (%C, %H2, % S, %
O2, %N2, %H2O moisture, ash)
3)
From the combustion equations;
C + O2 = CO2
S + O2 = S02
H2 + 1/2 O2 = H20
calculate the stoichiometric O2 in mols and lbs and that of N2 from air
analysis.
4)
Using the excess %O2 wet basis measurement in the flue gas, the level of
excess air ratio is computed. Use this excess air ratio to compute actual
excess mass of O2 and N2 to compute the total flue gas (fuel + air).
5)
Convert the mass to mols using their molecular weights and convert to %
volume. The computed %O2 must tally with the measured %O2 wet basis.
6)
Compute dry flue gas (total flue gas - H2O moisture) and calculate %O2 dry
basis.
7)
Input the generator output MWe (gross), fuel firing rate (m3/s), fuel density
(kg/m3), fuel lower heating value (LHV in kJ/kg). Compute firing rate in
kg/s, gross heat rate of power plant in kJ/kWh and thermal efficiency in %
LHV basis.
8)
Using 100 lbs of fuel as basis, convert the mass of each flue gas component
into kg/s and kg/MWh.
9)
Input the emission measurements in mg/Nm3 and kg/h of CO, NO2, SO2 and
dust. Convert to kg/s and kg/MWh.
10)
Due to partial combustion to CO and formation of NO2, the previous results
for CO2, O2 and N2 has to be corrected for C, O and N that should have gone
to CO2, O2 and N2 had there been complete combustion to CO2 and no
dissociation of O2 and N2:
C + 1/2 O2 = CO
1/2 N2 + O2 = NO2
11)
After correcting the kg/MWh of CO2, O2 and N2, we will have finally the
corrected flue gas composition consisting of CO2, CO, NO2, SO2, O2, N2, H2O
and dust. Using their molecular weights, the kg/MWh is converted to
mol/MWh and % mol or % volume. Compute the resulting analysis in wet
basis and % O2 measured dry basis.
12)
Finally, correct the measured mg/Nm3 or ppmv to 6% O2 reference from the
%O2 measured dry basis.
(X, O2 ref) = (X, meas) * (21 - O2 ref) / (21 - O2 meas)
where: X = CO, NO2, SO2, dust, etc.
The relationship between mg/Nm3 and ppmv (ppm volume) is defined by the
molecular weight of the component X and the standard molal gas volume of
0.022413 m3/mol:
(mg/Nm3 of X) = (ppmv of X) / 1000 x (M.W. of X) / (0.022413
m3/mol)
or (ppmv of X) = (mg/Nm3 of X) * 1000 * (0.022413 m3/mol) /
(M.W. of X)
where:
ppmv = parts per million (volume)
1000 = (1,000,000 / million) / (1,000 mg/g)
M.W. of X = molecular weight of X, g/mol
Nm3 = normal cubic meter (at STP - 0 deg C, 1 atm)
13)
The corrected emissions at 6% O2 is then compared with the Clean Air Act
standards and with other world standards for comparison. It can also be
compared with the supplier/contractor's guaranteed emission performance.
14)
Compute the year-to-date or annual power generation in MWh and multiply
with the average emission in kg/MWh of CO2, CO, NO2, SO2 and particulates
(dust). Convert the annual emissions to metric tons per year. Compare with
allowable emission levels for the particular area.
I would like to invite you and your company to continue supporting this blog thru
the DONATE button. You may order my power generation technology articles and
project financial models thru the ENERGY DATA page. Thanks!
Marcial T. Ocampo
(Friendly note: All content written by Engr. Marcial T. Ocampo are copyrighted and may not be
redistributed in any way or form.)
A spreadsheet for calculating solid fuel, liquid fuel and gaseous fuels power plant
emission has been prepared and my be obtained thru the ENERGY DATA page of this
website. It includes the following basic steps:
A) SELECT TYPE OF FUEL
Select type of fuel by entering 1
as
received
1
0
Coal
0
Carbon
0
0
0
26.10
32.41
37.47
41.51
44.74
47.33
Hydrogen
1.80
2.24
2.58
2.86
3.09
3.26
Sulfur
0.65
0.81
0.94
1.04
1.12
1.19
NOX
0.00
0.00
0.00
0.00
0.00
0.00
CO
0.00
0.00
0.00
0.00
0.00
0.00
Oxygen
11.25
13.97
16.15
17.89
19.29
20.40
Nitrogen
Water
Vapor
0.23
0.29
0.33
0.37
0.39
0.42
54.75
43.80
35.04
28.03
22.42
17.94
Ash
Total
Btu / lb
5.22
6.48
7.49
8.30
8.95
9.47
100.00
4794
100.00
5966
100.00
6903
100.00
7653
100.00
8253
100.00
8733
B) CALCULATE THEORETICAL OXYGEN REQUIREMENT
From % wt
No.
Substance
Formula
Table 2 - Calculation of Combustion Products and Theoretical Oxygen Requirements - Molar Basis (Babcock & Wilcox, p. 9-4
Moles
Gross
Net
Molecular Analysis
per
Analysis
HV
HV
Combustion Molecular
Oxygen
Theoritical Air
100 lb
Weight
% wt
fuel
% vol
btu/lb
btu/lb
Products
Weight
Required
O2 (m
1
Carbon
C
12.0110
26.10
2.1730
33.51
14093
14093
CO2
44.0098
1.00
2
Hydrogen
H2
2.0159
1.80
0.8929
13.77
61095
51625
H2O
18.0153
0.50
29
Sulfur
S
32.0660
0.65
0.0204
0.31
NOX
NO2
46.0055
0.00
0.0000
0.00
CO
CO
28.0104
0.00
0.0000
0.00
3
Oxygen
O2
31.9988
11.25
0.3516
4
Nitrogen
N2
28.0134
0.23
0.0082
Water Vapor
H20
18.0153
54.75
3.0389
46.86
32
Ash
TOTAL
3980
3980
SO2
64.0648
4347
4347
NO2
46.0055
1.00
CO
28.0104
-0.50
5.42
O2
31.9988
-1.00
0.13
N2 (fuel)
28.0134
0.00
H2O
18.0153
0.00
N2 (air)
28.1610
5.22
15.4203
100.00
6.4850
GIVEN
100.00
4804
4634
4,000
4634
1.00
per lb
fuel
C) CALCULATE FLUE GAS ANALYSIS USING ASSUMED EXCESS AIR RATIO (OR
CALCULATE FROM MEASURED EXCESS O2 - this requires stoichiometric
calculations using fuel analysis, molecular weights and ambient air analysis)
D) CONVERT PPMV TO MG PER Nm3 USING MOLECULAR WEIGHT AND STANDARD
MOLAL VOLUME OF GAS AT STANDARD TEMPERATURE AND PRESSURE (1 ATM, 0 DEG
CELSIUS)
As Measured
Flug Gas
Emission vs
DENR
CO2
H2O
SO2
(COMPUTED)
NO2
(MEASURED)
CO
(MEASURED)
O2
N2 (fuel)
H2O
N2 (air)
wet gas
dry gas
6
Reference
DENR
lb mole/
% vol
% vol
ppm V
mg / Nm3
ppm V
mg / Nm3
mg / Nm3
100 lb
(WET)
(DRY)
(DRY)
(DRY)
(DRY)
(DRY)
(DRY)
2.173
0.893
12.81842
5.26721
16.68917
0.020
0.12034
0.15668
245
700
210
601
700
0.000
0.00000
0.00000
487
1,000
418
858
1000
400
500
343
429
500
0.458
0.008
3.039
10.361
16.952
13.020
2.69969
0.04843
17.92599
61.11992
100.00000
3.51491
0.06306
79.57619
100.00000
84.38%
% Sulfur Removal
ppmv = (% vol / 100) * 10^6 = % vol * 10^4
X, O2 ref = X * (21 - O2 ref) / (21 - O2 meas) where O2 ref = 6%
ppmv X = (mg/Nm3 X) * 10^3 * (0.0224 m3/mol) / (MW X)
mg/Nm3 X = (ppmv X) / 10^3 * (MW X) / (0.0224 m3/mol)