ECN publication
Combustion characteristics of low quality battle coals: mineral matter transformations
Published by: Publication date:
ECN 1999
ECN report number: Document type:
ECN-C--99-035 ECN publication
Number of pages: Full text:
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The results of an experimental study on the ash behaviour of selected lowquality battle coals for pf-fired boilers are presented. The study was run parallel to a test program supported within the International Energy Agency Coal Combustion Sciences Program annex 2 part 4 addressing the combustion behaviour of these coals. A suite of coals and blends, varying in rank from medium-volatile bituminous to subbituminous, was selected in consultation with the Dutch electricity producers and extensively analysed, focusing on ash composition. Relatively high levels of the elements Ca, Mg, Fe, Na and K were found in these coals. The speciation of the inorganic constituents in terms of mineral composition and size distribution was determined by means of Computer Controlled Scanning Electron Microscopy (CCSEM). Single coals as well as binary blends were fired in a 2.5 MWth boiler furnace simulator, and ash and deposit samples were collected for off-line analysis and evaluation. The results have been expressed and ranked in terms of slagging and fouling propensity for each coal, and have been compared with the outcome of a considerable number of commonly used predictive ash deposition indices. Observations of significant slagging or fouling were in many cases related to the presence of alkali and alkaline earth metals, which were often ill-predicted due to the occurrence of mineral interactions. Selected tests were performed in a laboratory-scale combustion simulator under well-defined (low-NOx) conditions to elaborate further on the prevailing ash deposition mechanisms. The observed mechanisms were generalised into three classes in order to allow a certain extension of the conclusions towards new, similar fuels. From these mechanisms guidelines were derived for coal blending aiming to reduce ash deposition problems. The methodology of lab-scale fuel testing to evaluate in advance their potential to induce or enhance typical ash-related problems - here slagging and fouling - is currently being extended to an integral assessment of ash behaviour and ash quality for utilisation options. This report includes the initial development of a procedure involving lab-scale fly ash production and miniaturised leaching testing for advance evaluation of fly ash quality. 5 refs.

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