Effect Of Flux In The Gasification Process And Its Mechanism

Great attentions have been focused on the high efficiency utilization of poor quality coal with high ash fusion temperature since the primary energy is now in a state of short supply. Recently coal gasification technology is developing towards thigh temperature and pressure, slag-tap gasification, which bring opportunities for the utilization of poor quality coal. The ash fusion temperature (AFT) of coal should be lower about 100℃than the operation temperature of the slag-tap gasification technology in order to make sure normal runing of the slagging system. The poor quality coal reserves which AFT is higher than 1500℃account for about 50% of the coal gross reserves and those coals are not suitable for slag-tap gasification. So it is important work to decrease the AFT of those coals to meet the demand of the slag-tap gasification technology. If this goal can be realized, lots poor quality of coal can feed in gasifier, not only increase the coal utilization efficiency but also alleviate the energy crisis in some extent.For the above purpose, the major oxides that exist in the coal ash were regarded as the research objects for the present study. The mechanism that the fluxing agents could decrease the AFT were obtained through the analysis of the ternary phase diagram of SiO2-Al2O3-CaO and the simulation ash experiment. Combined TG-DSC and XRD, the consistent mechanism of decreasing the AFT could be further explained. Based on the mentioned mechanism, the following conclusion, which was applicable in both simulation and real ash, could be drawn as a general rule to explain why the AFT was decreased when fluxing agents were added, the contents of the major oxides were adjusted and the ternary or quaternary mixture system which have low melting point were formed. In the simulation ash and real coal ash experimental process, two most effective fluxing agents which could guarantee the AFT in an idea temperature interval were got. In order to investigate the impacts that fluxing agents imposed on the coal char reactivity and the producer gas formation rule, the STA449 F3 simultaneous thermal analyzer was used in this experiment. Some significant results were obtained.1) Combinating XRD and TG-DSC analysis, it was found some high melting minerals such as quartz and mullite can be formed when the temperature was higher than 1000℃and lead to a high ash fusion temperature, And the low eutectic minerals are formed by adding fluxing agents during the coal ash heating process which was the reason the AFT of coal can be decreased remarkably.2) Adjusting acid-base ratio of coal ash to 0.82-1.50 by adding fluxing agent or silica-alumina ratio to 2.15-2.60 by blending biomass the AFT of coal can come down the required temperature of slag-tap gasification which was under 1350℃.3) The ash melting point of coal which is higher than 1500℃was decreased evidently by adding any one of the fluxing agents of Na2O, CaO or Fe2O3, and the fluxing effect was in the order of Na2O> CaO> Fe2O3. In addition, the AFT of coal was also decreased remarkably when both CaO and Fe2O3 added in the coal ash due to some synergy effects between each other and the optimum mass ratio of CaO and Fe2O3 is 3:1.4) It was found that coal char reactivity was enhanced after fluxing agent used to reduce the AFT of coal, and the catalytic effect of Na-based fluxing agent was superior to multi-fluxing agent containg CaO and Fe2O3 with a mass ratio of 3:1. In addition, the coal char reactivity of two coals order was not changed when multi-fluxing agent was added and Na-based fluxing agent had limited function in changing the reactivity of different rank coal. 5) It was found that the H2/CO molar ratio of gas all demonstrated downward trends with the gasification temperature increased for Dongshan and Xishan raw Coal as well as the two coals with fluxing agents. The composite fluxing agent was fit for high temperature gasification and Na-based fluxing agent did for low temperature gasification. And the two kinds fluxing agents all made the gas molar ratio of H2/CO change obviously with operation temperature up and the function of the latter one was more evident which could satisfy the synthetic requirements for different chemical products.