The problem of fouling of exchange surfaces in heat recovery steam generators

An energetic recovery from combustion processes requires to face all problems related to the characteristics of gas originated by combustion products. Generally speaking, the heat recovery takes place through thermal exchange and the exchange surfaces inter react with gas whose characteristics involve a series of phenomena which, besides compromising the plant integrity (for instance corrosion, etc.), become responsible for a modification of the heat exchange characteristics of the surfaces themselves. The present work analyzes the influence of surface fouling for the heat recovery steam generator upon the performances of energetic recovery and mainly upon the production of steam and electric energy. First of all, an evaluation of experimental results is made, derived from operative data of plants typical of the sector, in order to create a quantitative basis to model the phenomenon. Starting from the obtained data and introducing suitable hypotheses, a model of calculation is presented allowing for an evaluation of the change with time of the final temperature of the heat exchanger. Such a model, on the assumption of a change in the fouling thickness, depends linearly upon time and allows for an evaluation on how the heat exchange coefficient might vary. As a consequence, the time rate of the overall heat exchange is evaluated in order to be allowed to identify suitable control systems. To this purpose, cleaning procedures of the exchange surface are hypothesized, trying to define the optimized time intervals for the treatment of the surfaces keeping in mind the fact that in general these interventions become responsible for a halt or for a consistent reduction of the plant efficiency, therefore strongly penalizing the production level. In this way, operative conditions emerge which, as a function of certain bonds (for instance time required for cleaning, etc.) allow to optimize the parameters for a proper running of the plant. At last, the possibility is analyzed to introduce into the steam generator systems able to abate the particulate of the gas (besides those already existing, operating directly onto the surfaces), trying to identify their most convenient localization and their effective applicability. As a conclusion, this work gives an updated scenario of all the aspects of fouling of exchange surfaces for systems relying on unconventional fuels, trying to identify operative and running modalities allowing for a reduction of the detrimental impact.