Pollutant emissions released during sewage sludge combustion in a bubbling fluidized bed reactor

The combustion of dry sewage sludge particles in a bubbling fluidized bed was studied in detail, analyzing the composition of the exhaust gases by means of a FTIR equipment. The operating conditions of the fluidized bed, i.e., the bed temperature and mass flow rate of fluidizing air, were varied to quantify their effect on the exhaust gas composition. The bed material was also varied, using sepiolite, silica sand and braunite particles, to evaluate the effect of different bed densities on the pollutant emissions. The results obtained for the combustion of sewage sludge particles in the different fluidized beds tested were compared to combustion tests run for the same operating conditions and bed materials using Cynara cardunculus L. as a fuel. Pollutant emissions derived from sludge combustion are much higher than those obtained from combustion of Cynara. The operating conditions also affect the emissions, e.g., the concentration of CO in the exhaust fumes decreased substantially when increasing bed temperature and air flow rate. The bed density has an effect on the combustion efficiency of sludge, obtaining higher efficiencies in low-density beds for high temperature and air flow rates, while the efficiency was increased in high-density beds for low bed temperature and fluidizing air flow rate. The effects of ash accumulation and agglomerates formation were also analyzed.