Sewage sludge combustion

In the current review paper, various issues related to the combustion of se wage sludge are discussed. After briefly explaining the formation and treat ment of sewage sludge, current and future sludge production are discussed. Thereafter, the four sludge disposal methods which are currently used, i.e. recycling in agriculture, landfilling, dumping into sea and incineration, are examined, and the future trend presented showing the increasing role of sludge incineration. Thereafter, technologies for thermal processing of se wage sludge are presented. They are discussed in three groups, i.e. mono-co mbustion, co-combustion and alternative processes. Various mono-combustion incinerators, including multiple hearth, fluidized bed and smelting furnace s are briefly discussed, whereas for co-combustion, attention has been give n to co-combustion with coals in pulverized and fluidized bed coal combusto rs, as well as co-incineration with municipal solid wastes in various furna ces. Where possible, data from large scale plants are presented. Currently being discussed in the sludge disposal cycles are the alternative thermal p rocesses to sludge combustion. These include wet oxidation, pyrolysis, oil from sludge processes, and combinations of pyrolysis, combustion and gasifi cation processes. Some of these alternative technologies are also briefly d iscussed. An important aspect during thermal processing of sewage sludge is its combustion mechanisms. Compared to coals, sewage sludge has very high contents of moisture and volatile matter which can affect the combustion pr ocess. The importance of the drying and devolatilization processes for sewa ge sludge combustion is thus examined. In a special case, the release and c ombustion of the volatiles during sludge combustion in fluidized bed combus tors is analysed, and some information concerning the combustion of sludge char is presented. Another important issue of sludge combustion is the emis sions of pollutants gases as well as the handling of solid by-products. Of concern include the heavy metals, mercury, dioxins and furans, acid gases, as well as NOx and N2O. These are also briefly discussed. A peculiar charac teristic of sewage sludge is its high content of nitrogen, and attention ha s been given to see how this affects the N2O and NOx emissions. In a specia l case, emission performance of large scale combustors of sewage sludge is presented. (C) 1998 Elsevier Science 1998. All rights reserved.