The CO2 emission in kg/GJ of heat released is of extreme importance in ascertaining GWP of fuel particularly when fuels are switched. Based on the Boie equation, Figure 3.14 plots the CO2 in g/MJ (or kg/GJ) for C-H-O fuels. It is clear that biomass has similar CO2 emissions compared to coals even though it has less C% compared to coals but the heat value of biomass is correspondingly lower!

20 —

0 —————————————————— 1—————————————-

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

H/C

Figure 3.14. CO2 emission in g/MJ (or kg/GJ) for C-H-O fuels. Multiply ordinate by 2.326 to get lb per mm BTU.

3.2.3 Air flow rate and multi-fuels firing

Similar O2% in exhaust implies excess air% remain similar for most solid fuels (Lawrence, 2007). Since thermal output = HHVO2 x stoichiometric O2 flowrate = HHVair x stoichiometric air flow rate = HHVair x actual air flow rate/(1 + x/100) where x is% excess air; thus when actual air flow rate is maintained the same, then one may switch the fuel and adjust the fuel flow rate such that same O2% is maintained which ensures similar thermal output. In automobiles, when alternate fuels are used for combustion, same thermal energy input is assured when air flow is maintained the same and fuel flow is adjusted such that same O2% is maintained in exhaust.

3.2.4 CO2 and fuel substitution

Since HHVO2 is constant for most fuels, then for given thermal input, the O2 moles consumed remain the same; a fuel with higher RQ produces more CO2 for the same thermal heat input! The reader is referred to Chapter 2 in this book on the basics of thermal energy conversion, stoichiometry, air fuel ratio and flame temperatures.