The results on gas analysis obtained from MS for a typical experiment (ER = 4.24 and S:F = 0.35) are shown in Figure 3.47a as a function of the time. The data on gas composition have a cyclic dynamic behavior in the vicinity of an average value. However, at first glimpse, it appears that the average is almost constant during the experimental period. Figure 3.47a shows the mole fraction of N2, H2, CO2, CO, CH4, and C2H6 (on a dry basis) along with the average mole fraction and the standard deviation (STDEV) of the data. The data on H2 present the major standard deviation

(3.2) about of the average value of 18.62% whereas the data on CH4, CO2, and C2H6 show a lower standard deviation and the data on CO shows a standard deviation of 1.53. As discussed

earlier, in general for the set of experiments discussed in this paper, the composition value of the gases analyzed fluctuated within ±15% of the average value.

As discussed before, at constant S:F, increasing the ER decreases the O2 supplied with the air at the bottom that implies decreasing Tpeak in the combustion zone. Then, as the temperature is lowered, the reaction C + O2 ^ CO2 is favored. CO2 increases at lower temperatures. More production of CO2 implies consumption of more O2 via CO2, thus, less O2 is consumed via CO and hence less CO is produced (Fig. 3.47b). Also, at constant S:F, increased ER increases the steam-air ratio (S:A), which implies decreased air supplied and hence the combustion of char takes place in a H2O-rich mixture which favors the heterogeneous reaction of char with H2O to produce H2. The rate of H2 and CO produced by the heterogeneous reaction C + H2O ^ CO + H2 becomes important when the reaction occurs at low O2. On the other hand, the concentrations of CH4 and C2H6 were lower (0.43 < CH4 < 1.75 and 0.2 < C2H6 < 0.7) as compared with those of other gases and were almost not affected by the ER.

The effect of the ER and S:F ratio on the concentrations of H2, CO, and CO2 are presented in Figure 3.48a, Figure 3.48b, and Figure 3.48c. At constant ER, higher S:F ratios signify more steam available to react with char to produce CO and H2 (steam char reaction) in the high temperature reducing zone immediately above the combustion zone (i. e. O2 deficient) near the bottom of the bed. The CO produced by the steam reforming reaction reacts with the surplus steam (shift reaction) in the upper zone (reduction) to produce more H2 and CO2; hence, more C atoms contained in the DB result in CO2. It is evident from the graphs of Figure 3.48b that lower ERs

have a lower effect on the CO production compared to higher ERs. Also, the results show that at constant ER, changing the S:F ratio affects the production of H2 more than the production of CO. For instance, at ER = 1.59 changing the S:F from 0.35 to 0.8 increases the production of H2 by 57.5% but decreases the production of CO by only 26.2% (Figs. 3.48a, b). Since the decrease in CO% is less than the increase in H2% then there must be a heterogeneous steam char reaction resulting in production of H2. This is also evident from the lowered Tpeak. Under the operating conditions discussed (1.59 <ER < 6.36 and 0.35 < S:F < 0.80), the CO ranged from ~4.77 to ~ 11.73%, H2 from 13.48 to 25.45%, CO2 from 11 to 25.2%, CH4 from 0.43 to 1.73%, and C2H6 from 0.2 to 0.69%.

3.14.4.2.4 Gas composition results with enriched air and CO2:O2 mixture Figure 3.49 shows the gas composition obtained for enriched air gasification with ER = 2.1. The percentage of carbon dioxide produced increased with increased oxygen percentage due to higher oxygen concentration in the incoming gasification medium. It was accompanied by a decrease in carbon monoxide and an increased production of hydrogen.

Figure 3.50 shows the comparison between the gas composition at 21% O2 obtained for the gasification with air and carbon dioxide at ER = 4.2.

Since carbon dioxide replaced nitrogen in the air the gases produced during gasification had a higher percentage of carbon dioxide, which possibly includes carbon dioxide produced during gasification as well as the carbon dioxide coming in as the gasifying medium. In addition, the heating value of the gases produced using carbon dioxide as the gasifying medium was higher when compared to that of air gasification having nitrogen.