Published: November 11, 2011 r2011 American Chemical Society 118 dx.doi.org/10.1021/ef2011518 | Energy Fuels 2012, 26, 118–129 ARTICLE pubs.acs.org/EF Control of Agglomeration and Defluidization during Fluidized-Bed Combustion of South Australian Low-Rank Coals Philip J. van Eyk, Adam Kosminski, and Peter J. Ashman* South Australian Coal Research Laboratory, Centre for Energy Technology, School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: South Australian low-rank coals have high sulfur and high sodium contents, which cause operational problems when the coals are combusted. Fluidized-bed combustion (FBC) of these coals allows for efficient combustion and for convenient sulfur removal by the addition of in-bed sorbents, such as limestone or dolomite. However, the presence of sodium may result in operational problems for FBC because sodium compounds, such as sodium sulfate, which is present in the coal ash, may cause the bed particles to become “sticky” and lead to a loss of bed fluidization. Combustion experiments have been performed in a laboratory- scale fluidized-bed combustor for two South Australian low-rank coals: Kingston and Lochiel. This work was undertaken to compare the behavior of these two coals and to allow for comparisons to previous experience gained using Lochiel coal in both laboratory experiments and pilot-plant operation. Kaolinite-rich clay additives were used in these experiments in an attempt to alleviate the problems associated with sodium present in coals. The effect of refreshing and removing the bed material without interruption of the combustion process was also studied experimentally. Kingston coal showed better performance in the FBC process than Lochiel coal. The ash layer formed from FBC of Kingston coal was found to be less sticky than that formed by Lochiel coal, resulting in longer defluidization times for Kingston coal than for Lochiel coal when no clay additives were used. Kingston coal was able to be combusted at 850 °C with the addition of clay at 5% of the total feed rate and with the addition/removal of bed material at 5% of the total feed rate. Analysis showed that the sodium from the coal had reacted with the kaolinite in the clay to form nepheline, a high- melting-point solid compound, which thus restricted the formation of liquid sodium sulfates in the bed. The results of this study show good agreement with the results of previous studies that showed that the addition of kaolinite-rich clays led to problem-free combustion of Lochiel in both small- and pilot-scale operations at 800 °C. 1. INTRODUCTION The large reserves of South Australian lignite coals have the potential to be an economical and abundant source of fuel for power generation; however, they have unusually high contents of moisture (>50%), sulfur, sodium, and chlorine, and because of this, their use is difficult. These coals are currently not used for power generation in conventional pulverized-coal-fired furnaces because of the likelihood of unacceptable levels of heat-exchanger fouling and unreasonably high emissions of sulfur oxides. Fluidized-bed combustion (FBC) is a well-established process that offers a potential solution to these problems. FBC is well- known to be capable of combusting a wide range of solid fuels at temperatures below the ash fusion point of these materials. Kingston coal from the south east of South Australia is now being considered as a fuel for a coal-fired boiler using FBC technology. The combustion and gasification of South Australian lignites, particularly Lochiel and Bowmans coals, have been investigated by the former State power generator Electricity Trust of South Australia (ETSA) and The University of Adelaide as part of the Cooperative Research Centre for New Technologies for Power Generation from Low Rank Coals and the Cooperative Research Centre for Clean Power from Lignite. 1À4 Kingston coal and also Lochiel coal attracted serious attention as possible energy sources for power generation during the 1980s. Extensive research resulted in characterizing the behavior of Lochiel and Kingston coals during combustion in a pilot-scale pulverized-coal process; however, problems because of fouling of heat-transfer surfaces and the sub- stantial emissions of SO 2 proved to be intractable. 1,4À6 Further pilot-scale work focusing on Lochiel coal indicated that, under restricted conditions, the FBC process was able to successfully overcome these problems. 4 A disadvantage of FBC is that agglomerates may form because of the formation of low-melting point inorganic compounds in the fuel ash, and this may lead to the entire bed defluidizing because of the presence of large agglomerates. 7 Defluidization is a major inhibitor to the use of fluidized-bed technology for these coals. Controlling agglomeration and subsequent defluidization is thus critical for any commercial fluidized-bed process. Many investigations have been performed with the aim of understand- ing the high-temperature defluidization phenomena in fluidized beds in the combustion or gasification of coal, 8À15 combustion of petroleum coke, 16,17 combustion or gasification of biomass, 18À27 and co-combustion of biomass and coal. 28 In general, the mecha- nism of agglomeration and subsequent defluidization of an inert bed material during FBC is summarized as follows: 29À33 (1) Inorganic transformations within the fuel particle result in the Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: November 11, 2011