Effect of operating conditions on release and transformation of sodium during CFB gasification of Zhundong coal
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摘要: 为了指导CFB气化炉运行，研究了气化温度、停留时间及气化剂类型对准东煤中钠的挥发和转化的影响，相关研究依托固定床反应器开展，并结合Factsage软件模拟计算。实验结果表明，气化温度是钠挥发与转化的显著性影响因素。温度对钠挥发的促进作用归因于挥发的强化及石灰/偏高岭土的竞争反应。同时，高温可以促进钠霞石和熔渣的形成。熔渣形成的阈值大约950 ℃。气化过程中钠的挥发可以归结为两个阶段：煤热解及半焦气化。在煤热解阶段，部分有机态和水溶性钠被挥发，剩余的钠与煤焦或矿物质结合；在半焦气化阶段，与半焦结合态钠（CM-Na）随着煤焦气化而挥发。由于NaOH形成及熔融温度降低，水蒸气对钠的挥发呈现促进作用；相反，氧气和氮气则表现出抑制作用。前者归因于难挥发Na2SO4形成，而后者是因为煤焦的化学成键和物理束缚。Abstract: To provide some useful suggestions to the operation of circulating fluidized bed (CFB) gasifier, the effect of gasification temperature, residence time and agent on the release and transformation of sodium was studied by using a fixed bed reactor combined with Factsage software. The results indicated that gasification temperature was the significant factor to the release and transformation of sodium. For the promoting effect of sodium release, it was ascribed to the intense of sodium volatilization and competitive reaction between lime and meta-kaolin. Meanwhile, the high temperature promoted the formation of nepheline and slag. The threshold temperature of latter was near 950 °C. It was interesting to find that the release of sodium could be divided into two stages: coal pyrolysis and char gasification. In coal pyrolysis, part of organic and water-soluble sodium was released. The remainder either combined with char structure, or reacted with minerals. In char gasification, Sodium, combined with char structure, was released along with char gasification. Due to the decrease of melting temperature and the formation of NaOH, steam showed a promoting effect on the sodium release. Oppositely, oxygen and nitrogen presented an inhibiting effect. The former was ascribed to the formation of Na2SO4, while the latter was caused by the chemical binding and physical wrapping effect of char.
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Key words:
- Zhundong coal /
- gasification /
- sodium /
- release and transformation
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Figure 2 Distribution of sodium species in coal
Na-W: water-soluble sodium; Na-A/W: ammonium-acetate-soluble but water-insoluble sodium; Na-I: ammonium-acetate-insoluble sodium
Figure 3 XRD pattern of low-temperature ash C: Calcite (CaCO3); K: Kaolinite (Al2Si2O5(OH)4); N: Nitratine (NaNO3); Q: Quartz (SiO2); S: Siderite (FeCO3)
Figure 5 XRD patterns of gasification residues
1: hematite (Fe2O3); 2: lime (CaO); 3: larnite (Ca2SiO4); 4: nepheline (NaAlSiO4); 5: srebrodolskite (Ca2Fe2O5); 6: magnetite (Fe3O4); 7: calcite (CaCO3); 8: gehlenite (Ca2Al2SiO7)
Figure 8 Effect of residence time on the carbon conversion and sodium release (gasification agent is 60%H2O + 40%N2; gasification temperature is 950 ℃)
Figure 10 XRD patterns of residues under different gasification agent
1: hematite (Fe2O3); 2: lime (CaO); 3: larnite (Ca2SiO4); 4: nepheline (NaAlSiO4); 5: srebrodolskite (Ca2Fe2O5); 6: magnetite (Fe3O4); 7: calcite (CaCO3); 8: gehlenite (Ca2Al2SiO7); 9: sodium sulfate (Na2SO4); 10: quartz (SiO2)
Figure 11 Schematic diagram of transformation behaviors of sodium species during coal steam gasification
Table 1 Proximate and ultimate analysis
Proximate analysis w/% Ultimate analysis wdaf/% Mad Ad Vdaf C H Oa N St 10.65 3.82 28.93 82.06 3.47 13.25 0.87 0.35 ad: air-dried base; d: dry base; daf: dry and ash free base; a: by difference 
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Table 2 Ash chemical compositions
Content w/% SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 K2O Na2O P2O5 27.02 13.97 13.10 20.28 4.61 0.63 7.94 0.20 12.21 0.05
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