Daniel Roberts

The effects of pressure on coal reactions during pulverised coal combustion and gasification

Abstract Advanced clean coal technologies, e.g. power generation from integrated gasification combined cycle (IGCC) and pressurised fluidised bed combustor, have attracted increased interest from the scientific and technological communities over the last few decades. Pressures up to 40xa0atm have been applied to these technologies, which inherently result in an increase in coal throughput, a reduction in pollutant emissions and an enhancement in the intensity of reaction. Therefore, fundamental understanding of the effect of operating pressure on coal reactions is essential to the development of these technologies. In this paper, the pressure effect on a variety of aspects of coal reactions reported in the open literature has been reviewed. Major emphasis of the paper is given to experimental observations, although some theoretical modelling is reviewed. The pressure has been found to significantly influence the volatiles yield and coal swelling during devolatilisation, hence the structure and morphology of the char generated. More char particles of high porosity are formed at higher pressures. Char structure appears to play a significant role in burnout of residual char and ash formation. In general, at higher pressures, coal particles burn quicker and form finer ash particles. Increasing reactant pressure enhances char combustion and gasification reaction rate, which can be understood by an adsorption–desorption mechanism. These factors have been applied to the understanding of a practical high-pressure gasifier. Most of the work published has been at the lower temperatures (typically

Modeling char combustion: The influence of parent coal petrography and pyrolysis pressure on the structure and intrinsic reactivity of its char

Chars were made from four Australian coals of varying vitrinite content at pressures of 5, 10, and 15 atm. The morphology of the chars was correlated with the petrography of the parent coal. The intrinsic reaction rates of the chars at high pressures were measured, and no systematic effect of pyrolysis pressure or maceral concentration was found. It is concluded that observed variations in conversion rates under process conditions are likely to be due to char structural properties and not a result of variation in the intrinsic reactivity of the carbon in the chars. Consequently, this paper presents a char structural submodel that is integrated into an existing char combustion model to account for the combustion behavior of char particles of different morphologies. The char morphology used in the model was predicted using the developed correlation with parent coal petrography, so that a petrographic analysis as well as the proximate and ultimate analyses is required for model input. Validation of the model shows that chars produced at high pressure with a high percentage of cenospherical types burn more rapidly under process conditions than those at low pressure, with model predictions matching measurements. It is suggested that incorporating the char structural submodel into the existing char combustion model improves its predictability.

Trace Element Partitioning and Leaching in Solids Derived from Gasification of Australian Coals

Trace element concentrations vary between coals from ppb to ppm levels and can depend on the rank of the coal and its geological origins. During gasification, some of the trace elements are volatilised at high temperatures and may condense and deposit in cooler downstream parts of the system or in quench water streams. Some species may appear in condensed phases such as slag or fly ash. Changes in the trace element concentrations in the slag and flyash from that of the parent coal are expected due to the reactions occurring at high temperatures and the different chemical activity of the trace element phases in the slag, fly ash, and syngas. Four Australian coals were used in an entrained flow gasification test program conducted in the Siemens 5 MWth gasification test facility. Solid samples were collected from different points in the gasification process during each test. Compositions of these samples were analysed and the distribution of trace elements was studied. The elements can be classified as follows, according to their tendency to appear in the slag and fly ash: Partitioned between slag and fly ash: Cu, W, Mo, Cd, Bi, Zn, Sn, Sb Partially volatile and depleted from either slag or fly ash: Be, Th, Sc, Y, Li, Mn, Ni, Sr, Ba Highly volatile (i.e. were not observed in either slag or fly ash): As, Se, B, Hg, F, Pb, V. Comparison of these experimental results with equilibrium calculations of trace element appearance in the condensed phases suggests that the modelling approach is suitable only for certain elements. For several of the trace elements of significance in this study, kinetic factors have to be considered in conjunction with thermodynamic modelling. The leaching behaviour of the trace elements in the slag was also studied. This work shows very low leachability for most of the trace elements except Zn and Sb, which, due to their relatively high volatility, reported to the slag samples in very low concentrations. f 2011 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 28 October 2010; Received in revised form 26 January 2011; Accepted 14 March 2011