| Coal is expected to keep its important position as a major world energy source for a long term because of the relative abundance of its reserves compared to those of natural gas and, in particular, petroleum. Coal plays a particularly vital role in power generation, currently accounting for about 40% of the world’s electricity production; with this situation expected to continue at least by 2050. In addition to the major combustible components, coal contains many toxic elements in trace amounts (<1000 ppm), such as mercury, arsenic and chromium. Since large quantities of coal are consumed each year, the pollutant emissions from coal utilization may cause serious risks to ecosystem. As a result, there are an increasing number of international and national programmes and action plans concerned with reducing trace element emissions.Integrated gasification combined cycles (IGCC) may become the preferred way of using coal for power generation because of the promise of high efficiency and minimal environmental impact. The hot raw product gas leaving the gasifier in IGCC power plants carries a complement of toxic and/or corrosive trace elements. In order to improve thermal efficiency and reduce capital and operating costs, without adversely affecting environmental performance, it will be necessary to develop hot gas cleaning technology to purify the coal-derived gas streams at much higher temperatures. However, more volatile elements such as mercury and arsenic, may be present totally, or partially, in the vapour phase and more liable to pass to the turbine at higher temperatures during gasification. Therefore, there is a need to investigate the possibility of removing trace compounds from raw coal gas at elevated temperatures. Solid non-carbon based sorbents appears to offer a potential solution here.The aim of the present study was to investigate the emission and the control of trace elements during gasification. The first stage was mainly on the volatility and speciation of various trace elements as well as clorine, fluorine and rare elements, during high temperatures pyrolysis and gasification of four typical Chinese coals commonly used in gasification stations and two high toxic concent coals. The analysis methods in the study were various involving TGA, INAA, CVAAS, XRF, ICP-MS, CVAFS, XRD, CCSEM and FSEM-EDS. Experiments were conducted on bench-scale fixed bed, drop tube furnace or entrain flow reactor under atmospheric/high pressure at final temperatures from 400 to 1500℃, respectively. The gas-phase mercury was analyzed by an atomic fluorescence mercury analyzer according to the Ontario Hydro Method. It was observed that most of the volatile elements release from coals at high temperatures, such as mercury, arsenic, selenium and so on. However, some non-volatile elements still remained in the coal ash or captured on fine particles, such as chromium and rare elements. The volatility of volatile and some semi-volatile elements increased with temperature monotonically with temperature over 30 min. Pressure and holding time at ending temperatures had positive effects on volatility of trace elements in these two groups, however, the influence were not that obviously as temperature did. For gas-phase mercury, elemental mercury was the dominant species at most of the temperatures and holding times examined. It was also observed that high temperatures and long times enhanced mercury oxidation. The maximum value of the ratio of Hg2+/HgT was achieved during the high temperature steam gasification.Comparative studies on mercury emission under different conditions indicated that the volatility and speciation of mercury may correlate with the halogen concentration in the coals. However, no obvious correlation between Hg2+/HgT and basic oxides or acidic oxides in the ash was observed. Thermodynamic equilibrium calculations based on the principle of Gibbs free energy minimisation had been run, to predict speciation of mercury during high temperature pyrolysis and gasificaiton. The results were in coincidence with the experimental results.The second stage of the study focused on the removal of mercuy released during coal gasification, by contacting with suitable sorbents. A new high-temperature, mineral, non-carbon based dispersed sorbent derived from paper recycling products has been shown to capture mercury at high temperatures in excess of 600℃. The sorbent was consisted of kaolinite/calcite/lime mixtures. Experiments have been conducted on chemi-sorption of elemental mercury in air on a packed bed. The sorption occurs at temperatures between 600-1100℃and requires activation of the minerals contained within the sorbents. Mercury capture was dominated by temperature and capture on sorbents over long time scales. The capture shows a maximum effectiveness at 1000℃and increases monotonically with temperature. The presence of oxygen was also required. Freshly activated sorbent was the most effective, and deactivation of sorbents occurs at high temperatures with long pre-exposure times. This activation was suspected to involve a solid-solid reaction between intimately mixed calcium oxide and silica that were both contained within the sorbent. Deactivation occurs at temperatures higher than 1000℃, and this was due to melting of the substrate and pore closure. The situation in packed beds was complicated because the bed also shrinks, thus allowing channeling and by-passing, and consequent ambiguities in determining sorbent saturation. Sorbent A had significantly greater capacity for mercury sorption than did Sorbent B, for all temperatures and exposure time examined. The effect of SiO2 on poor Sorbent B was much larger than sorbent A. This study described results of a research program at the University of Utah that focuses on mercury sorption in packed beds. Companion work had focused on mercury sorption in disperse phase flow reactors. |
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