The effects of coal ash on sulfur transformation during secondary reactions of coal pyrolysis
摘要: 煤灰作为热载体的热解二次反应中存在煤灰和挥发分的交互反应，势必影响到热解产物的分布特性。为此，本研究将在固定床反应器上研究二次反应过程中煤灰对硫迁移特性的影响规律。实验结果表明，小龙潭煤灰在二次反应过程中的低温阶段抑制了H2S和COS的析出，主要归因于煤灰中Fe2O3对含硫气体的固定；然而在高温阶段（≥ 800 ℃）促进了H2S和COS的析出，这是由于煤灰中硫酸钙与热解煤气发生还原反应生成了大量的H2S和COS。为进一步揭示煤灰在高温下的作用机制，研究了煤灰主要组分Fe2O3和CaSO4的影响规律，发现800 ℃时添加Fe2O3显著降低了煤气中H2S和COS的析出量，而CaSO4促进了H2S和COS的析出，证明了煤灰对含硫气体的影响规律与其所含的Fe2O3和CaSO4密切相关。Abstract: During the coal pyrolysis with the coal ash as heat carrier, the interactions of coal ash and volatile matters could occur in the secondary reactions, which would affect the final sulfur-containing product yields of volatile matter. The objective of this paper was to reveal the effects of coal ash on the sulfur transformation during the secondary reactions. The results showed that the presence of XLT ash during secondary reactions inhibited H2S and COS release at low temperatures due to the sulfur fixation of Fe2O3 existed in the ash. However, it enhanced H2S and COS release at high temperatures, which was mainly caused by the formation of sulfur-containing gases through the reactions between CaSO4 and reducing gas (i.e. H2, CO, and CH4). The influences of Fe2O3 and CaSO4 were also investigated to reveal the mechanism of the influence of coal ash, and it was found that H2S and COS yields were reduced with the addition of Fe2O3, but those two gases were slightly increased by the presence of CaSO4 at 800 ℃, indicating that Fe2O3 and CaSO4 played important roles in the interactions between the coal ash and sulfur-containing gases.
表 1 煤质分析
Table 1 Main characteristics of XLT coal
Sample Proximate analysis wd/% Ultimate analysis wd/% Sulfur Forms analysis wd/% M V A C H N O* St Ss Sp So* XLT 4.80 39.61 19.36 54.38 3.72 1.15 18.27 2.28 0.05 0.57 1.66 SHM 2.93 26.81 20.76 59.63 3.45 0.82 14.43 0.68 0.02 0.39 0.27 M: moisture; V: volatile, A: ash; St: total sulfur; Sp: pyritic sulfur; Ss: sulfate sulfur; So: organic sulfur, O*, So*: by difference; O* = 100−M−A−C−H−N−S, So* = St−Ss−Sp
表 2 煤灰成分分析
Table 2 Analysis of ash compositions in raw coals
Sample Composition w/% SiO2 Fe2O3 Al2O3 CaO MgO K2O Na2O SO3 othersa XLT 22.79 11.68 9.71 23.15 4.05 0.66 0.13 27.12 2.94 SHM 33.76 5.45 47.27 6.46 0.95 0.32 0.15 4.07 1.57 a: by difference
表 3 煤灰中CaSO4在二次反应过程的分解率
Table 3 CaSO4 decomposition rate during secondary reactions
600 ℃ 800 ℃ Blank sample Decomposition rate/% 0.9 47.2 0.4 Note：the difference between the black and normal tests is as follows: during the front part of flat-temperature zone (Figure 2), no substance is placed during blank test, while the coal is placed during normal tests.
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