Page 169 - Handbook of Energy Engineering Calculations

the fuel input and oxygen consumed, using the gas specific-heat method.

Calculation Procedure:

1. Determine the air equivalent in the exhaust gases
In gas-turbine–based cogeneration/combined-cycle projects, the HRSG may
be fired to generate more steam than that produced by the gas-turbine exhaust
gases. Typically, the gas-turbine exhaust gas contains 14 to 15 percent

oxygen by volume. So the question arises: How much fuel can be fired to
generate more steam? Would the oxygen in the exhaust gases run out if we
fired to a desired temperature? These questions are addressed in this

procedure.
If 0 percent oxygen is available in W lb/h (kg/h) of exhaust gases, the air-
g
equivalent W in lb/h (kg/h) is given by: W = 100(W )(32O )/[23(100)(29.5)]
a
x
g
a
= 0.0417 W (O). In this relation, we are converting the oxygen from a volume
g
basis to a weight basis by multiplying by its molecular weight of 32 and
dividing by the molecular weight of the exhaust gases, namely 29.5. Then
multiplying by (100/23) gives the air equivalent as air contains 23 percent by
weight of oxygen.

2. Relate the air required with the fuel fired using the MM Btu (kJ) method

Each MM Btu (kJ) of fuel fired (HHV basis) requires a certain amount of air,
A. If Q = amount of fuel fired in the turbine exhaust gases on a LHV basis
(calculations for turbine exhaust gases fuel input are done on a low-heating-

value basis), then the fuel fired in lb/h (kJ/h) = W = Q/LHV.
f
6
The heat input on an HHV basis = W (HHV)/(10 ) = (Q/LHV)(HHV)/10 6
f
Btu/h (kJ/h). Air required lb/h (kg/h) = (Q/LHV)(HHV)(A), using the MM
Btu, where A = amount of air required, lb (kg) per MM Btu (kJ) fired. The
above quantity = air available in the exhaust gases, W = 0.0417 W (O).
a
g

3. Simplify the gas relations further
6
From the data in step 2, (Q/LHV)(HHV)(A)/10 = 0.0417 W (O). For natural
g
gas and fuel oils it can be shown that (LHV/A HHV) = 0.00124. For
x
example, LHV of methane = 21,520 Btu/lb (50,055.5 kJ/kg); HHV = 23,879
Btu/lb (55,542.6 kJ/kg), and A = 730 lb (331.4 kg). Hence, (LHV/A HHV) =
x