Formation of hydrogen cyanide and ammonia during the gasification of sewage sludge and bituminous coal

HCN and NH3 released during the gasification of sewage sludge have been measured during a program of tests with a laboratory-scale spouted-bed gasifier. The data have been compared with results from gasification tests with coal. The effect of altering the bed temperature has been investigated, and the results have been related to reactions involving gaseous N species known to occur in the gasifier. The effect of steam addition on the HCN release has been examined. It has been found that the HCN concentrations in the exit gas increase with the operating temperature, which is thought to indicate increased formation as a primary product of the decomposition of the fuel-N compounds. Increasing the height of the char bed caused a significant reduction in the HCN concentration at the exit, as this promoted the decomposition of HCN to NH3. Steam addition caused a rise in the HCN concentration during tests with sewage sludge and a similar effect had previously been reported on the NH3 concentration during tests with coal. The NH3 concentration decreased with increasing temperature, and this is thought to reflect the increased rate of the equilibration of NH3 in the gas phase to form N-2 and H-2.