Effect of Pressure on Ash Formation during Pulverised Coal Combustion and Gasification

In this paper, effects of the operating pressure on ash formation reported in the open literature have been reviewed. In particular, the recent significant advances achieved at the Cooperative Research Centre for Coal in Sustainable Development (CCSD) in Australia are highlighted. The operating pressure significantly influences the size and the chemistry of ash generated through its effects on the structure of chars generated. Previous work has shown that pressure has marked impacts on the volatile yield new work has shown that it also impacts particle swelling behaviour during devolatilisation and hence the resulting char structure and morphology. Char particles generated at elevated pressures have high porosity. These char particles experience more extensive fragmentation during the combustion and gasification, which leads to finer ash particles compared to that formed at low pressures. At high pressure, the char particles appear to burn faster. The char structure also determines the ash liberation for the different combustion stages. The PSD of ash is less sensitive to pressure during gasification process. Ongoing research in the CCSD is determining the chemistry of the char and ash generated in a Pressurised Entrained Flow Reactor (PEFR), and has developed a mechanistic model to predict char structure and hence ash properties. Introduction Advanced clean coal technologies, including pressurised fluidised bed combustion (PFBC) and integrated gasification combined cycle (IGCC) have attracted increasing technological and scientific interests over the last decades [1]. These technologies provide several advantages over the conventional coal firing processes, including an increase in coal throughput, a reduction in pollutant emission and an enhancement in the intensity of reactions [1,2]. The recent previous work on coal pyrolysis [3-9], coal swelling [10-14] and char reactivities [7,15-31], has revealed that the operating pressure has marked impacts on coal swelling during devolatilisation, and that char reactivity is enhanced at high pressure. However, ash formation at elevated pressures has not been well understood. Studies of mineral matter transformation are motivated by the concerns with performance of the utility boilers [32,33], such as fouling and slagging. The ash formed during the gasification process has been found to have a major effect on IGCC system design and operation, slag formation and tapping, ash deposition in gas circuit, heat exchange passes and fly ash collection equipment [1]. Fundamental knowledge of ash formation at elevated pressures is therefore essential to the development of these technologies and to the improvement of the performance of the pressurised reactors. General mechanism of ash formation has been summarized in the literature [33-35]. Many factors are known to control the chemistry of the final ash particles during pulverised coal combustion and gasification, including mineral content, mineral distribution among pulverized coal particles, coal particle size distribution, char structure and burnout mechanism, physical properties of the ash, char fragmentation [32]. It should be pointed out that strong association exists between ash formation and the char structure related behaviour [36,47], such as char fragmentation [32,37,38] and the coalescence of the included minerals [39]. Therefore, any factors that influence char structure may impact the ash formation and chemistry. Significant advances in the understanding of effects of the operating pressure on the ash formation has been achieved through the projects undertaken at the Cooperative Research Centre for Coal in Sustainable Development (CCSD) of Australia. In particular, it has been found that the operating pressure significantly influences the ash formation mechanism through its effect on chemistry of chars formed during the devolatilisation. In this paper, the effects of the pressure on the devolatilisation behaviour of coal and the chemistry of the final ash have been summarised. Effects of pressure on the deveolatilisation of coal Impacts of the operating pressure on pyrolysis behaviour of coal have been extensively investigated[3-9], and documented [40-43]. In general, pressure significantly influences the volatile matter yields, coal particle swelling and the structure of the resulting char residues. This further influences the char reaction rate [36] and the ash formation mechanism [47]. Volatile matter yields A pronounced reduction of the total weight loss and tar yields at elevated pressures and temperatures has been observed using wire mesh reactors or entrained flow reactors. The early investigations carried out using Pittsburgh bituminous coal [44] showed that the total volatile matter yield decreased with increasing the operating pressure, and the effect was more distinguishable at high temperature. Measurements on Pittsburgh No.8 coal by Suuberg et al [45] revealed that as pressure increases the total volatile matter and tar yields decreases whilst total gas production increases, as shown in Fig 1. The published data at various conditions regarding the pressure effect in the open literature have been summarised in Fig 2 [42].