OCR Text |
Show due to excess air has the opposite effect and thus CO in the gas appears to remain insensitive to the increase in excess air. The CO % falls off with the increase in excess air mostly due to the dilution effect. The efficiency ratios for gasification and oxidation are shown in Fig. 6. As excess air is increased, the oxidation efficiency increases for excess air up to 25%; beyond which the efficiency appears to fall off. 4.2.3 Effect of Temperature Fig. 7 shows the results for the effects of bed temperature. As o o temperature is increased from 615 C (0% heater setting) to 700 C (70% heater setting), the O2 in the gas first decreases indicating more combustion and then increases again. The H2 and hydrocarbon contents in the gas though small, seem to increase slightly. The slight increase may be due to pyrolysis of fine material deposited on the exhaust pipes exiting from the bed. (It should be noted that pyrolysis for manure starts around 200°C, see Fig. 1). It should further be noted that the sum of C02and CO increases as the bed temperature increases. Beyond 650 C, there seems to be no apparent gain in the sum of C02and CO . Bailey (1968) reports that the CO for coal fired FBC can be as high as 1% when bed temperature is at 700° C and the CO content decreases to 0.1% as bed temperature is increased to 800 C. Coal constitutes only up to 30-40% volatile matter whereas the volatile matter of manure consists of up to 80%. Since volatiles rapidly oxidize to CO, and CO oxidation rate is slow, higher CO content is expected for manure fired FBC. If fouling is not a problem, the future experiments will be conducted at higher bed 2.4.22 |