| Lignites or brown coals which account for nearly13%of the coal reserves are playingan increasingly important role in electricity production in China. However, the utilization oflignite has been limited by its drawbacks of low combustion efficiency due to high moisturecontent and low calorific value, as well as the high spontaneous combustion tendencies as aresult of high volatile matter content. Due to the growing demand for lignite and theincreasing coal consumption year by year, there is scope to develop methods by whichlignite can be utilized more efficiently. Thermally evaporative upgrading of lignite is such amethod that has received extensive attentions worldwide.The Shengli (SL) lignite was thermally upgraded in a fluidized-bed reactor attemperatures of200-500oC under N2, or simulated flue gas (SFG), or organic (ORG)atmosphere. The combustion behaviors and physical-chemical structures of raw andupgraded coals were characterized by thermogravimetric analyzer, Fourier transforminfrared spectroscopy (FTIR), N2adsorption isotherm, and scanning electron microscopy(SEM) respectively. X-ray photoelectron spectroscopy (XPS), ASTM D2492-02andsequential leaching method were used to investigated the different sulfur and mercuryforms.The result showed that highly qualified coals with high low heating value (LHV), lowmoisture content, low self-ignition tendency and high synergistic removal efficiency ofsulfur and mercury have been obtained when SL lignite was upgraded in the fluidized-bedreactor. As temperature increased, the char yield and average pore diameter of upgradedcoals gradually decreased, while the LHV, moisture removal, sulfur removal, mercuryremoval, ignition temperature (Tig), Car/(Car+C=O), Car/Hal, and surface area graduallyincreased.The changes of the physical-chemical structure had great effect on the spontaneouscombustion characteristics. With the decreasing of the oxygen-containing functional groupsand average pore diameter, the Tigincreased. The Tigof the lignite upgraded under SFG andORG atmospheres increased linearly with the decrease of the average pore diameter, whilethe value of Car/(Car+C=O) had an opposite correlation with Tig. Organic sulfur and organic mercury was the primary sulfur and mercury forms of SLlignite. The removal of total sulfur via thermal upgrading, reached to about60%, wasattributed to the evolution of organic sulfur. XPS results showed that aromatic, thiophene,and aliphatic sulfur were the major organic sulfur forms in SL lignite, and the relativeabundance was estimated as40:30:30%, respectively. Aliphatic sulfur was the mostunstable organic sulfur and began to release at200oC. Aromatic sulfur began to release at300oC. Thiophene sulfur was the most stable and could not be removed even if thetemperature at500oC under different environments. Exchangenale-Hg began to release at200oC, and sulfide-Hg rapidly decomposed at400oC.The oxygen in SFG and the ethanol in ORG atmosphere had the selective oxidation ofcarbon, sulfur and mercury. Thermal decomposition was the primary approach for theevolution of the organic sulfur during thermal upgrading. The addition of oxygen andethanol in the environments showed different capabilities for selective decomposition oforganic sulfur and mercury forms in coal. There was strong synergistic removal correlationbetween the remove of sulfur and mercury. A high linear relationship betweendevolatilization and sulfur removal, and an exponential relationship betweendevolatilization and mercury removal, and an exponential relationship between sulfurremoval and mercury removal were obtained during thermal upgrading process. |
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