High temperature, entrained flow gasification of Victorian brown coals and rhenish lignites
2017-02-24T00:24:32Z (GMT) by
The use of low rank coals in coal-to-products processes is of particular interest for countries with significant domestic resources, including Australia and Germany. Significant advances have been made in coal-derived syngas generation, and high temperature, entrained flow gasification of high rank coals is currently state of the art. Despite their advantageous properties, low rank coals such as Victorian brown coals and Rhenish lignites have yet to be thoroughly investigated for coal-to-products processes via high temperature entrained flow gasification. This is the first publicly available investigation on entrained flow gasification of Victorian brown coals. In this research, a series of interrelated studies of the fuel-dependent processes which occur during entrained flow gasification of these fuels was performed. To facilitate the high temperature part of the investigation, a fit-for-purpose apparatus was designed, implemented and utilised to assess the suitability, behaviour and potential process issues related to the entrained flow gasification of Victorian brown coals and Rhenish lignites. Fundamental kinetics analysis was performed by thermogravimetry of Victorian brown coal chars generated under rapid, entrained flow pyrolysis conditions and indicated that a fuel-specific optimum pyrolysis temperature exists under which the most reactive char for a particular coal is produced. Kinetic data were produced for each coal relating to the rate limiting, heterogeneous char gasification reactions, and the major factors influencing char gasification reactivity with CO₂ and steam were shown to be char morphology and catalytic inorganic constituents, respectively. The release and reaction mechanisms of volatile inorganic species from Victorian and Rhenish coals under CO₂ and steam gasification conditions were investigated by molecular beam mass spectrometry. The mechanisms under different atmospheres were shown to be distinct but related, and separate pyrolysis and gasification phases were observed. The majority of volatilisation occurred during the pyrolysis phase, and was independent of the gasification atmosphere. The NaCl – NaOH and H₂S – COS equilibria showed opposite trends under CO₂ and steam atmospheres. A competitive, cyclic, mechanism involving Na, Ca, S, Si and Al was derived, which results in permanent capture of various species depending on the absence, presence and local partial pressure of the gasification reagent. Low temperature (< 1000 °C) entrained flow gasification of a Victorian brown coal resulted in an increase in syngas yield and decrease in hydrocarbon contaminants with increasing temperature, indicating that tar-free product gas may be expected at higher temperatures. The low temperature assessment confirmed the need for high temperature trials as the char conversion was low (< 55 %) and the syngas contaminant levels high at the maximum possible conditions of 1000 °C and 6 s residence time. A fuel-flexible, high temperature, entrained flow reactor - The HELENA - was therefore designed and commissioned and a comprehensive matrix of pyrolysis and gasification trials conducted using Victorian brown coal and Rhenish lignite. Based on the results of this research, the high temperature, entrained flow gasification of these fuels was determined to be technically feasible, and some important operational considerations were highlighted. High reactivity chars were produced at all temperatures up to 1400 °C, tar generation from all coals was negligible at high temperatures, and char conversions of up to 99% were achieved. Efficient carbon conversion, high throughput and tar-free syngas may therefore be expected under industrial conditions. The maximum char reactivity for the Victorian coal was observed at 1200 °C, confirming that there exists a fuel-dependant optimum gasification temperature and highlighting the potential for use of Victorian brown coals in moderate temperature, entrained flow apparatus with suitable ash handling systems. The Victorian coal was more reactive than the Rhenish coal under all conditions, possibly due to the low ash melting temperature of the Rhenish coal which can impede gas transfer to reactive carbon sites within the char structure. Slight agglomeration of the Rhenish coal was also observed at low conversion, and significant particle size reduction for both fuels was observed for high conversion experiments. Ammonia was not detected under the conditions tested. However, HCN, H₂S and COS contaminants increased with increasing temperature to ppmv levels at 1400 °C. Through this research, a comprehensive assessment of the high temperature, entrained flow gasification behaviour of Victorian brown coal and Rhenish lignite has been performed using proven experimental techniques under conditions of industrial interest. The generated data provide optimums, limits and trends which are applicable and comparable in their own right, and may also be used as direct inputs to process modelling, design and optimisation of large scale applications. Furthermore, the design and implementation of the HELENA serves both current and future requirements for a dedicated, fuel-flexible, high temperature apparatus to facilitate the continuation of research in this field.