Gasification Behaviour of Australian Coals

Entrained-flow gasification has been identified as the most efficient and environmentally-friendly technology for producing low cost electricity (together with other fuel and chemical options) from coal and other carbonaceous feedstocks. This project addresses coal and power industry needs to understand the suitability and performance of Australian bituminous coals for use in the leading new Integrated Gasification Combined Cycle (IGCC) power generation technologies based on entrained flow slagging gasifiers. These technologies will provide the basis for the transition to future coal based power systems capable of operating with near zero emissions.

The results discussed in this report comprise gasification conversion data for a commercially realistic suite of Australian coals under conditions that are relevant to entrained-flow gasification technologies. Gasification performance parameters such as conversion levels, gas quality indices, char structural parameters and gas compositions are measured as a function of coal type, reaction stoichiometry and residence times. These performance parameters are measured and analysed to further the understanding of high pressure gasification processes, as well as begin to identify criteria that will be used in test procedures which will be required to assess coals for use in entrained-flow gasification.

Optimum gasification performance (high conversion levels with maximum calorific value of syngas) is obtained in a specific range of oxygen:carbon ratios. For the coals tested in this report, this optimum stoichiometry was approximately 90-100% (of theoretical oxygen required for carbon conversion to CO). The effects of changing this ratio on coal conversion were similar for all coals, irrespective of their rank. Gasification efficiency (a measure of the conversion of the energy in the coal to energy in the gas) was best for coals with large volatile yields and/or high char reactivities. This observation, however, is also related to the amount of conversion achieved by these coals under these experimental conditions and thus is not solely an indication of the coal's effect on syngas quality. Planned tests at higher temperatures will achieve higher conversion levels of char and thus provide a more realistic indication of coal quality effects on syngas calorific values.

Volatile matter release strongly affected the levels of conversion achievable by a coal at 1100°C (the temperature used for most of the experiments used in this work). Even at these low temperatures, however, char reactivity plays an important role. Based on preliminary high-temperature experiments it is expected that as reaction temperatures are increased, the relative importance of char reactivity will increase. It is also likely that under true gasification conditions, the most significant coal properties affecting gasification conversion performance will be volatile yields and the interaction of char reactivity and char structure.

All fourteen coals in the CCSD Reference Coal Bank have been examined in this first major 'rangefinding' research project with the pressurised entrained-flow reactor (PEFR) facility. This work provides the first high temperature, high pressure coal conversion data for Australian black coals under gasification conditions. It begins to draw together existing bench-scale data (in particular on coal pyrolysis and char reactivity) to allow the development of test criteria for entrained flow gasification. Future work at higher temperatures, and with controlled environments of gasification reactants, will begin to deconvolute some of the important and complex reactions which is required for the development of reliable gasification conversion models applicable to Australian coals. It will also enable the development of procedures suitable for more routine application and for testing of coals for use in entrained flow gasification.