Catalytic gasification and assessment of Dimethyl ether synthesis from Victorian brown coal

Abstract

This is the first ever study assessing the possibility of dimethyl ether (DME) production through gasification of Victorian brown coal. This project involves gasification of Victorian brown coal and catalyst development for syngas to DME conversion process. Victoria has large reserves of brown coal, 430 billion tonnes at current estimate. Use of Victorian brown coal is currently limited mostly to mine-mouth power generation because of high moisture content of the as-mined coal and high reactivity of the dried coal; both these properties make Victorian brown coal, raw or dried, unexportable. Gasification based alternative processing paths can provide export market for brown coal derived products, and more energy efficient application of brown coal. Syngas from Victorian brown coal can be catalytically converted into DME with higher energy efficiency and at potentially lower CO₂ emission. DME is a non-toxic, non-carcinogenic and non-corrosive compound. In addition, it has wide application as a fuel in cars, gas turbines, fuel cells and household applications. A process simulation for as-mined Victorian brown coal to DME was performed using ASPEN Plus. The simulation study shaped the experimental matrix as it provided a realistic range of operating conditions (e.g. gasification temperature and syngas H₂ to CO ratio). CO₂ Gasification kinetics for raw parent coal as well as demineralised and catalyst-loaded (Ca, Fe) coals were studied using a thermogravimetric analyser. Pyrolysis and gasification of the coal was performed in an entrained flow reactor (EFR) and the solid, liquid and gaseous products were characterised. DME synthesis experiments were performed in a high pressure fixed-bed reactor, using commercial and developed catalysts, and synthetic syngas consisting H₂ and CO. A 3² factorial experimental design was used to optimise catalyst composition and syngas ratio (H₂ to CO). The developed catalysts were prepared based on the information generated from preliminary experiments with commercial catalysts. Physical mixing and coprecipitation-impregnation methods were used for the preparation of bi-functional DME synthesis catalysts. Performance (CO conversion, DME yield and DME selectivity) of the developed catalysts was compared with that of commercial catalysts. Effects of sulphur poisoning on CO-conversion, DME yield and DME selectivity were also studied. Process simulation using ASPEN plus showed that the low temperature gasification at 900 °C can produce syngas with appropriate H₂ to CO ratio. The ratio was found to be 0.81 at the gasifier outlet (before the recycle stream) and 1.41 at the DME reactor inlet (after the recycle stream). The overall process efficiency was found to be ∼ 32% after considering the energy penalty for CO₂ separation, higher than the power generation efficiency of 28% (without CO₂ separation). Two kinetic models (Grain model and random pore model) were used to find the intrinsic CO₂ gasification kinetics. Random pore model predicted the experimental results better than the grain model. The activation energy for char-CO₂ gasification was ∼189 kJ/mol. Ca-loaded coal char showed better gasification reactivity. However, addition of iron did not show any improvement. The results indicate that the effect of minerals become insignificant at 1000 °C or above and catalytic gasification showed be carried out below this temperature. EFR studies showed that the tar yield rapidly decreased as the gasification temperature was increased. The residence time and gasification temperature in the EFR were not enough for complete carbon conversion. In situ synchrotron radiation X-ray diffraction on methanol and DME synthesis catalysts showed rapid catalyst deactivation at temperatures above 300 °C, resulted from phase mobility and thermal sintering. The extent of deactivation was higher for the bi-functional DME catalyst compared to the methanol synthesis catalyst. Regression analysis on the yield data, obtained using commercial catalysts, showed that a H₂ to CO ratio of 1.45 and a catalyst consisting 58% methanol synthesis component results maximum DME yield. Among the four developed catalysts (DSC-1, DSC-2, DSC-M, DSC-1-PRE), three catalysts (except DSC-1-PRE) showed performance similar or better than the commercial catalyst mixture M1A1. CO conversion was between 67-70% for the DSC-1 catalyst, best among the developed catalysts, compared to 58-60% conversion for the M1A1 catalyst. DME yield was 36-40% and 35-38% for the DSC-1 and M1A1, respectively. A 10 hour exposure of the catalyst to 103 ppm H₂S showed at least 12% reduction in conversion and yield, indicating rapid deactivation in the catalyst activity. All the results were at least duplicated, and triplicated in most of the cases. The obtained results positively indicate that the conversion of syngas from Victorian brown coal to DME is a feasible option.