Wednesday, July 15, 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.

Courtesy  Marcial

Srikanth Nyshadham