Aspects of fouling in case of heat exchangers with polluted gas

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.This paper presents and discusses various aspects of fouling in case of heavily polluted flue gas (and/or off-gas) coming especially from incinerators/waste-to-energy systems. A long-term experience and know-how from this area as well as continuing research and development brings new insights into manufacturing of equipment and their operating. Polluted off-gas causes high propensity to fouling and necessity of consequent cleaning. In some cases, it is not possible to utilize a conventional approach. Introductory part of the paper provides a description of various types of units for thermal processing of wastes (including sludge and contaminated biomass). Fouling is classified according to various cases of industrial applications connected with easy/difficult cleaning. It is shown how fouling can influence geometry of heat exchangers and their selection. In the following part, several industrial cases are shown taking into account aspects like: • solid particles (ash and flying ash) in the gas, and adapting design according to their concentration; • species contained in flue gas which can chemically react and create fouling deposits on heat transfer surfaces; • potential corrosion between the fouling layer and heat transfer surface caused by local temperature decrease. The above aspects are clearly illustrated through industrial applications as follows: • heat recovery system of unit for the thermal treatment of sludge coming from pulp and paper production; • boiler systems in incineration plants and chemical industry plants. There are various methods to reduce fouling. We have utilized very efficient approach combining intuitive design and sophisticated tools based on CFD (Computational Fluid Dynamics). However, fouling cannot be eliminated completely therefore various efficient methods (mostly tailor-made ones) are utilized (like common mechanical cleaning, air guns, controlled local explosion) for cleaning of surfaces. There is also an effort to develop a mathematical model for fouling prediction, and selection of the most economically acceptable systems connected with current research and development in the field. However, it is difficult to validate the models. Fouling in the field described in the paper is in fact a “never ending story”.