1. Sources

The present Combustion File is part of the cluster of CFs produced within the literature survey phase of the industrial sponsored research and development project PowerFlam1 and is confidential to the participants registered for that project.

This CF is specifically concerned with the research project:

Werther, J. and Ogada, T. (1999): Sewage sludge combustion, which is published within The Journal of Progress in Energy and Combustion Science 25 p.55-116.

2. Background

·         This combustion file is concerned with the provision of combustion related data to the sponsors.

·         In this html file the source of the data is summarised in section 4 below, in order to give the reader a general overview of the way the experiment was carried out.

·         The research project in which this CF is based, does not present fuel and related data, therefore, they are not presented in individual Microsoft Excel Worksheets.

·         All credits and sources, and where necessary, instructions/advice for data use, are presented in this html file.

3. Abstract

In the current review paper, various issues related to the combustion of sewage sludge are discussed. After briefly explaining the formation and treatment 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, land filling, 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 sewage sludge are presented. They are discussed in three groups, i.e. mono-combustion, co-combustion, and alternative processes. Various mono-combustion incinerators, including multiple hearth, fluidised bed and smelting furnaces are briefly discussed, whereas for co-combustion, attention has been given to co-combustion with coals in pulverized and fluidised bed coal combustors, as well as co-incineration with municipal solid wastes in various furnaces. Where possible, data from large-scale plants are presented. Currently being discussed in the sludge disposal cycles are the alternative thermal processes to sludge combustion. These include wet oxidation, pyrolysis, oil from sludge processes, and combinations of pyrolysis, combustion, and gasification processes. Some of these alternative technologies are also briefly discussed. 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 process. The importance of the drying and devolatilisation processes for sewage sludge combustion is thus examined. In a special case, the release and combustion of the volatiles during sludge combustion in fluidised bed combustors is analysed, and some information concerning the combustion of sludge char is presented. Another important issue of sludge combustion is the emissions 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 characteristic of sewage sludge is its high content of nitrogen, and attention has been given to see how this affects the N2O and NOx emissions. In a special case, emission performance of large-scale combustors of sewage sludge is presented.

4. Synopsis