The air fuel ratio, and hence the equivalence ratio, can be estimated from measured flow rates of air and fuel. It can also be computed using the measured O2 percentage in the exhaust for lean mixtures. Using O2 percentage data the equivalence ratio of the exhaust stream was approximated by:

Vfiue ~ 1 — 4.76 * O2, <Pfiue < 1-0 (Annamalai and Puri, 2007)

The equation above assumes that all the fuel has been gasified. If large particles are not gasified, the O2 percentage will increase. This will cause the у based on exhaust gases to decrease. Figure 3.21 plots the yfue computed from flue gas analysis versus the ifw computed from air and fuel flow rates. It is seen that yfue is less than ynow. This indicates that the burnt fraction (BF) BF is less than 1.0. Also, note that the ifw requires knowledge of the fuel flow rate. Due to limitations of the feeder, only average flow rates of solids could be measured. Figure 3.22 presents the exhaust equivalence ratio for WYO and WYO:DB blended fuels. Ideally, the data points would follow the ideal line (yflue = ynow). The real data points lie within the experimental uncertainty of each other. This indicates that the values are valid. In all future plots, the у represents the equivalence ratio based on measured air flow rates and the calibrated fuel flow rate.