Gasification of Australian Coals

Four Australian coals were trialled in the Siemens 5 MWth pilot scale gasifier in Freiberg, Germany, in August 2007. A summary report of these trials was issued by the former CCSD, which gave an overview of the results of the trials and some preliminary links between existing, laboratory-scale gasification data and coal behaviour under pilot-scale conditions. This report provides a more detailed analysis and interpretation of these gasification trials, as well as providing analysis and characterisation of process streams from the gasifier (in particular feed coals, slags, and other process solids).

Whilst all coals achieved similar ranges of conversion over the conditions used, there was some differentiation of the coals in terms of cold gas efficiency and in terms of the O:C ratio required to achieve suitable slag formation and conversion. The slag behaviour of the coals varied from poor to excellent.

These differences in conversion and slag behaviour were consistent with our expectation of coal behaviour based on characterisation using laboratory facilities (e.g. Pressurised Entrained Flow Reactor, slag viscometer, etc). These consistencies demonstrate the relevance of the suite of 'coal test procedures' developed to date as tools for estimating the performance of coals under entrained flow gasification conditions.

Model estimates and measurements of slag viscosity behaviour of slags made from the coals in the laboratory were shown to be good indicators of the potential slag flow behaviour under entrained flow gasification conditions. One exception to this was the fluxed coal sample-the fluxed mineral matter for this coal did not melt and flow to the extent suggested by the laboratory tests. Analysis of the structure of the chars made from this coal, however, suggest that this is related to the dense, closed nature of chars formed, which prevents interaction of the mineral matter and the slag under entrained flow conditions.

Coals that had good volatile yields and char reactivities in laboratory studies, with high conversion and good gas quality as indicated by PEFR tests, were easier to gasify than those that had low volatile yields and poor char reactivity. This meant that they could achieve good conversion levels at relatively low temperatures, leading to good cold gas efficiencies. The results of the trials, however, demonstrated the importance of considering conversion properties and slag behaviour properties in combination when assessing the potential gasification behaviour of a coal. Good 'reactivity' coals with mineral matter that requires a high temperature to melt and flow will not produce good quality syngas compared with a coal that has good 'reactivity' properties and mineral matter that melts and flows at low temperatures. These interactions were evident from the cold gas efficiency of the two 'best' coals (in terms of their conversion reactivity) but which differed significantly in the temperature required for good slag behaviour.  

Another valuable aspect of trialling coals in a pilot scale gasifier is the access to realistic process streams. Samples of slag and 'process water solids' (solids that were removed from the gas by the quench water spray) were collected from each set of conditions. The process water solids are essentially fly ash (coal mineral matter that has not melted) and 'fly slag' (mineral matter that has melted but not deposited on the wall of the gasifier), containing unconverted carbon in accordance with the conversion efficiency of the coal.

It was shown that the composition of the mineral matter of the feed coal is not necessarily consistent with the composition of the process water solids or the slag formed on the wall of the gasifier. There is a degree of partitioning of mineral matter components between the fly ash and the slag; there is also the potential to have the composition of the slag formed altered by the slag deposited form previous feeds to the gasifier. These changes in composition have been shown to be significant enough to alter the viscosity behaviour of the slag tapped from the gasifier, and therefore should be considered when assessing the potential of a coal's mineral matter for use in slagging gasifiers.

Key goals of the gasification trials campaign were to provide access to realistic samples of slag and process solids, and to provide the means by which data and knowledge from existing (laboratory-based) tools and techniques could be applied and tested against realistic gasification data. The work presented in this report clearly achieves these goals by providing new insights into coal conversion to syngas and slag, and by clearly demonstrating the relevance of laboratory and PEFR studies of coal gasification performance to our understanding of coal behaviour under entrained flow gasification conditions.