Page 170 - Handbook of Energy Engineering Calculations

21,520/(730 × 23,879) = 0.00124. By substituting in the equation in step 1,
we have Q = 58.4 (W )(O). This is an important equation because it relates
g
the oxygen consumption from the exhaust gases to the burner fuel

consumption.

4. Find the fuel input to the HRSG
The fuel input is given by W + h + Q = (W + W )(h ), where h and h g2
g
f
g
g1
g1
g2
are the enthalpies of the exhaust gas before and after the fuel burner,
respectively; W = fuel input, lb/h (kg/h); Q = fuel input in Btu/h (kJ/h).
f
The relation above requires enthalpies of the gases before and after the
burner, which entails detailed combustion calculations. However, considering
that the mass of fuel is a small fraction of the total gas flow through the
HRSG, the fuel flow can be neglected. Using a specific heat for the gases of

0.31 Btu/lb°F (1297.9 J/kg K), we have, Q = 150,000(0.31)(1575 − 950) = 29
6
× 10 Btu/h (8.49 kW).
6
The percent of oxygen by volume, O = (29 × 10 )/(58.4 × 150,000) = 3.32
percent. That is, only 3.32 percent oxygen by volume is consumed and we
still have 15.00 − 3.32 = 11.68 percent left in the flue gases. Thus, more fuel
can be fired and the gases will not run out of oxygen for combustion.
Typically, the final oxygen content of the gases can go as low as 2 to 3

percent using 3 percent final oxygen, the amount of fuel that can be fired =
(150,000)(58.4)(15 − 3) = 105 MM Btu/h (110.8 MM J/h). It can be shown
through an HRSG simulation program (contact the author for more
information) that all of the fuel energy goes into steam. Thus, if the unfired

HRSG were generating 23,000 lb/h (10,442 kg/h) of steam with an energy
absorption of 23 MM Btu/h (24.3 MM J/h), approximately, the amount of
steam that can be generated by firing fuel in the HRSG = 23 + 105 = 128
MM Btu/h (135 MM J/h), or 128,000 lb/h (58,112 kg/h) of steam. This is

close to a firing temperature of 3000 to 3100°F (1648 to 1704°C).

Related Calculations. Using the methods given in Section 1 of this
handbook, one may make detailed combustion calculations and obtain a flue-
gas analysis after combustion. Then compute the enthalpies of the exhaust
gas before and after the burner. Using this approach, you can check the

burner duty more accurately than using the gas specific-heat method