Effect of Drying Temperature On the Fuel Value of Wastewater Sludge

Abstract

Two operational problems that plague the multiple hearth furnace (MHF) are the frequent need for supplementary fuel to maintain the incineration process, and emission of volatile compounds from the incinerator. Sludge is exposed to elevated drying temperatures prior to active combustion in the MHF and many of the volatile emissions are a result of thermal degradation of organic matter within the sludge. This loss of volatile matter from the sludge prior to its combustion represents a direct loss of fuel from the MHF. The objective of this research is to determine the effect of drying temperature on the final heating value of sludge. This objective is accomplished through the determination of higher heating values (HHV) by oxygen bomb calorimetry of sludge samples heated to temperatures between ambient and 400°C prior to the calorimetric measurements. Results from laboratory work performed for this research indicate that for temperatures in excess of 105°C, the HHV of dried sludge is dependent upon the temperature to which it is exposed prior to the measurement of the heat value. Sludges exposed to temperatures above 105°C lose enough volatile matter to have measurably lower HHVs than those sludges exposed to temperatures below 105°C. Since sludge biomass is thermally unstable at temperatures as low as 105°C, application of this research to the MHF reveals that the countercurrent flow of sludge and exhaust gases within the MHF is a fundamental handicap to the efficient combustion of sludge in this type of incinerator.