Abstract
The combustion performance of a bio-solid waste from a wastewater treatment plant was studied in terms of efficiency and gaseous emissions, by carrying out experiments in a bubbling fluid bed unit and was compared with that of an agricultural waste. The effects of excess air and fuel feeding rate were investigated and a factorial design and modelling was developed in order to provide optimal values of process variables for minimizing emissions. The results showed that sewage sludge fuel burned at a much lower temperature than the woody fuel. CO emissions were low, SO2 emissions from the agricultural waste negligible, while those of sewage sludge exceeded legislation limits when excess air was below 50%. An increase in excess air from 30 to 50%, or of fuel feeding rate from 0.48 to 0.6kg/h, resulted in higher NOx emissions from the woody fuel, exceeding emission guidelines. Concerning SO2 and NOx emissions from sewage sludge fuel, the effect of feeding rate was the same, however that of excess air was opposite, reducing their levels in flue gases. Combustion efficiency for both fuels was high, between 98.5 and 99.6%. Factorial design analysis and modeling indicated that for the woody fuel the optimum emission performance was accomplished when the feeding rate was 0.48 kg/h and the excess air 30%. For the sewage sludge fuel the optimum combustion and emission performance was achieved when the feeding rate was 0.72 kg/h and the excess air 50%.
Highlights
The ever increasing quantities of urban wastes, which are generated by every social activity, are becoming matter of public concern, due to environmental and human health impacts
Both fuels were rich in volatile matter, the higher content of ash and lower of carbon for sewage sludge resulted in a lower calorific value
Olive stone and sewage sludge behaved differently. Both fuels reached a maximum temperature just 30 mm above the air diffuser within the bed, due to volatiles combustion, olive stone having higher carbon content and calorific value whereas lower content of ash (Table 1), burned at a much higher temperature, 849◦C against 787◦C for sewage sludge, which decreased rapidly toward the end of the expanded bed (>250 mm), as the heat transfer rate between the dense phase and the freeboard was higher in this region
Summary
The ever increasing quantities of urban wastes, which are generated by every social activity, are becoming matter of public concern, due to environmental and human health impacts.
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