3432
Ind. Eng. Chem. Res. 2005, 44, 3432-3441
Combustion of Dried Sewage Sludge in a Fluidized-Bed Reactor M. Hartman,* K. Svoboda, M. Pohorˇ ely´ , and O. Trnka Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6-Suchdol, Czech Republic
Fluidized-bed combustors are capable of destroying effectively a variety of organic wastes such as sewage sludge. Because of its very high content of water, mechanically dewatered sewage sludge cannot be incinerated on its own. Peculiar characteristics of dried sewage sludge include its very high proportion of volatile matter and its high contents of the fuel-bound nitrogen and ash. The experimental apparatus consisted of three fundamental parts: an electrically heated reactor, 0.98-m height and 0.0936-m diameter; a facility for the continuous withdrawal and analysis of gas samples, and a feeder of solids. Predried sewage sludge particles were injected near the base of a shallow, air-fluidized, bubbling bed of ceramsite under different operating conditions. The study comprised an experimental program of steady-state tests for providing a full picture concerning the influence of the main operating variables (i.e., bed temperature, freeboard temperature, and excess air) on the most significant gaseous emissions (i.e., CO, NOx, and N2O). Further, this work also explores the significant influence of the freeboard (i.e., the region above a dense, bubbling bed) on the overall combustor performance. Of concern are also elements such as As, Cd, Hg, Cr, Ni, P, Pb, and Zn and their partitioning between the bottom (bed) ash and the fly (cyclone) ash. Introduction Treatment of municipal wastewater inevitably leads to the generation of huge volumes of thin sewage sludge, which is sometimes referred to as municipal biosolids waste. Sewage sludge is viewed as an aqueous, more or less diluted suspension of a wide variety of colloids as well as particular matter. Various harmful pollutants, such as salts, organic contaminants, and heavy metals, concentrate in this refuse. Processing and disposal of its extremely large amounts present one of the most environmentally challenging issues of the wastewater treating process.1-5 All indications suggest that the sludge production will continue increasing, and a feasible solution to the disposal of the expected enormous quantities of sludge must be found. With respect to nitrogen, phosphorus, potassium, magnesium, organics, and other nutrients contained in sludge, it appears that on hygienization this material might be utilized as a soil conditioner or fertilizer. However, particularly in the case of industrial regions, municipal sludge is likely to be contaminated with heavy metals and is not generally allowed to be applied in agriculture. Moreover, in addition to heavy metals, the presence of pathogens and persistent organics in sludge cannot be ruled out. Thus, the use of sludge in agriculture can only be considered under very wellcontrolled conditions. The disposal of dewatered sludge in sanitary landfills is not very satisfactory either, and the new EU legislation will not make it possible to dispose of any waste containing more than 5% organic carbon after 2005.6 An old, hardly acceptable disposal methodsdumping the sludge into the sea or other surface waterswas banned a few years ago. The current trend indicates an ever-increasing interest in sludge thermal processes, particularly in sludge * To whom correspondence should be addressed. Tel.: +420 220390254. Fax: +420 220920661. E-mail:
[email protected].
incineration. Such operations usually involve the total or partial conversion of organic solids into oxidized end products. Sludge incineration offers some advantages not found in other alternatives, including a large reduction of the sludge volume to a small amount of stabilized ash, which accounts for only 10% of the volume of mechanically dewatered sludge, and the thermal destruction of pathogens as well as toxic compounds. Also, the calorific (heating) value of dry sludge is not far from that of lignite, and therefore this energy potential can be recovered. On the other hand, there are a number of issues that cannot be overlooked. For example, the net heating value of mechanically dewatered sludge is not sufficient (
