Recall that O2 in the exhaust is an indicator of ф used in experimentation. Thien (2002) derived an expression for the burnt fraction of a solid fuel approximated as:

BF « * 1 — —

ф XO2,A

where BF is the burnt fraction, ф is the measured equivalence ratio from flow rates, XO2 is the mole fraction of oxygen in the exhaust gases (dry basis), and XO2A is the mole fraction of oxygen in the ambient air (dry basis). This equation can be used for rich or lean mixtures. When BF is very high, near unity, it implies that all of the fuel was combusted. Note that BF is larger than 1 for some of the extremely lean experiments. These values demonstrate the limitations of equation

(3.2) as well as experimental uncertainties including fuel compositions. As is to be expected, BF decreased with increasing equivalence ratio. In rich combustion, insufficient air was provided to completely oxidize all fuel carbon to CO2, leaving unburned fuel in the ash. This caused the BF to be less than 1.

Figure 3.27 and Figure 3.28 present the BF for TXL and TXL:DB blended fuels and WYO and WYO:DB blended fuels, respectively. Even in the very rich combustion (ф = 1.2), approximately 83% of the fuel was burnt.