AIR EMISSION FROM SLUDGE FLUIDIZED BED INCINERATION SYSTEM

Fluidized bed incineration has been gaining more attention over the last few years as municipalities and industries focus on reducing sludge volumes and producing class A sludge. A concern surrounding incineration has to do with air emission of pollutants from the combustion process. In general, the emissions can be divided into three types: heavy metals, particulates, and gaseous pollutants (including nitrogen oxides, hydrocarbons, carbon monoxides). Air emissions recorded at several Infilco Degremont sludge incineration plants using typical air pollution control system (venturi scrubber and/or wet ESP or dry ESP) are presented in this paper. With the ultimate goal of creating a database of recorded emissions for all air pollutants from sludge fluidized bed incineration plant, this paper will be a useful tool for engineers and regulators when designing air pollution control systems and establishing air emission requirements.Analysis of the recorded data shows that removal efficiency of a given metal increases with its concentration in the air pollution control system inlet gas (or its concentration in the incoming sludge). Mercury and selenium had the lowest removal efficiencies. Removal efficiencies of metals other than mercury and selenium (including arsenic, cadmium, chromium, copper, nickel, lead, zinc, and beryllium) were in excess of 98%. While strong linear relations can be established between particulate emission and emission of cadmium, zinc, copper and lead, the relations are less evident with arsenic, chromium, nickel, selenium, beryllium and mercury.When total particulate emission levels of thirteen plants were compared to US EPA requirement (0.65 gr/kg dry feed), all thirteen plants had emission levels much lower than requirement. When a wet ESP is installed in the downstream of the venturi scrubber followed by a cooling tray, emission of particulates is reduced by an order of magnitude (0.015 gr/kg dry feed versus 0.175 gr/kg dry feed).Because of the high turbulence, long gas residence time, and homogeneous distribution of fuel and temperature in the fluidized beds, emissions of gaseous pollutants from a sludge fluidized bed incinerator are typically low compared to the US EPA requirements.The average emission of carbon monoxides recorded from twelve plants was 7.7 ppmv (ranging from a 0.4 ppmv to 22.5 ppmv) compared to US EPA requirement of 100 ppmv. The emission of total hydrocarbons (as propane) collected from eight plants ranged from 0.5 to 5.45 ppmv with an average of 1.8 ppmv, compared to 100 ppmv required by US EPA.The average emission of nitrogen oxides (recorded as NO2) of nine plants was 33.1 ppmv, ranging from a minimum of 0.97 ppmv to a maximum of 69.36 ppmv corrected to 7% O2. Although, US EPA does not have any requirement on nitrogen oxides, the emissions from all of the nine plants comply with local requirements.