Combustion characteristics of coals using a drop-tube furnace q M. Cloke * , E. Lester, A.W. Thompson Coal Technology Research Group, School of Chemical, Mineral and Environmental Engineering, University of Nottingham, Nottingham NG7 2RD, UK Received 31 August 2001; revised 23 October 2001; accepted 24 October 2001; available online 5 December 2001 Abstract Fourteen coals were selected for char re®ring tests using a drop-tube furnace DTF) in order to compare with previous tests on a 1 MW combustion facility. Each coal was sieved into two size fractions 53±75 and 106±125 mm) and characterised using proximate analysis and conventional petrographic tests as well as a test for % unreactives using image-analysis. The coal fractions were then pyrolysed at 1300 8C for 200 ms in 1 vol% oxygen in nitrogen. The reactivity, morphology and surface area of the chars were evaluated using thermal, optical and adsorption techniques. Each char fraction was then passed through the DTF at 1300 8C using a residence time of 600 ms and a furnace atmosphere of 5 vol% oxygen in nitrogen to evaluate burnout propensity. The characteristics of the coals, the chars and the residues after re®ring were compared to determine whether any links exist between burnout, intermediate char products and coal composition. The link between % unreactives and burnout was con®rmed for high volatile bituminous coals. Results obtained from two low volatile coals con®rmed that their burnout was better than predicted from their properties. For the S. American Guasare) coal poorer than expected burnout was obtained, as in previous work, but only for the larger size fraction. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Re®ring; Morphology; Bituminous coals 1. Introduction There has been a signi®cant increase in the use of imported coals for power generation both in the UK and other parts of theworld.Thishasledtoanincreasedneedforanunderstand- ing of the behaviour of coals, often from very different geolo- gical origins, during pulverised fuel pf) combustion and for characterisation techniques to assess the behaviour of these coals. In particular, the burnout characteristics of coals can be dif®cult to assess from standard proximate and ultimate analysis data. The link between petrographic composition and burnout has been investigated [1]. Vitrinite has been shown to burn readily [2], although the rate of burnout will normally decrease as its re¯ectance increases [3]. Inertinite macerals have often been associated with burnout problems [4]. However, in recent years low re¯ectance inertinite mate- rial has been shown to devolatilise and form open char struc- tures [5,6]. Since many coals from the Southern Hemisphere contain high levels of inertinite, a thorough understanding of how the macerals behave in a combustion ¯ame is essential. Inthislaboratory,atechniquehasbeendeveloped[7]which predicts burnout behaviour of coals, including coal blends, from a grey-scale histogram produced using image-analysis andhasbeenevaluatedagainstcombustiontrialsofcoalsfrom different parts of the world on a 1 MW combustion rig. In general, a linear relationship was found between an `unreac- tives' parameter derived from the histogram and the burnout behaviour of the coals tested. However, two particular coals gaveirregularresults.Guasare,aSouthAmericancoalburned worse than expected, leading to high carbon-in-ash values. This could be partly explained by poor grinding characteris- tics, where the pf contained higher than normal quantities of particles greater than 200 mm, but initial tests on a drop-tube furnaceDTF)indicatedthatotherfactorscouldberesponsible forthiscoal'scombustionbehaviour.Ontheotherhand,Poca- hontas, a high rank bituminous North American coal burned muchbetterthanexpected.Thishasimplicationswherehigher rank coals are to be combusted, perhaps in a blend. Theaimoftheworkdescribedinthispaperwastocarryout aseriesofcombustiontestsonaDTFtofurtherinvestigatethe anomalous behaviour of the above two coals. Included in the testswereanumberofotherS.Hemispherecoalstogetherwith two standard UK combustion coals for comparative purposes. 2. Experimental 2.1. Coals Fourteen coals used in this study were supplied as pf by Fuel 81 2002) 727±735 0016-2361/02/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0016-236101)00199-5 www.fuel®rst.com * Corresponding author. Tel.: 144-115-951-4169; fax: 144-115-951- 4115. E-mail address: michael.cloke@nottingham.ac.uk M. Cloke). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com