Thermodynamic study of uranium in the process of coal thermal utilization
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摘要: 铀元素是一种放射性核素，也是一种重要的战略资源。中国部分地区煤炭中U含量异常富集，在高温转化后煤中U大量富集于固体产物中，会产生放射性风险。为了控制煤中U的释放并对产物中U进行资源化利用，探究和掌握煤中铀在热利用过程中的迁移转化规律十分必要。本研究基于热力学平衡原理，计算分析了干河煤、小龙潭煤和胜利煤中铀在热解、气化、燃烧过程中的形态分布以及钙基添加剂对U迁移转化的影响，以期为后续的实验研究提供理论指导。结果表明，煤中铀在热解、气化、燃烧条件下的存在形态各异，UO3(g)是各工况下产生的唯一的气相产物，高温、低压、强氧化性环境都会使UO3(g)的生成量增大。将铀固定在铀酸钙中能明显降低铀的挥发，使用钙基添加剂后干河煤在燃烧过程中铀酸钙的生成量明显增加，其中，以CaO的作用效果最为显著。Abstract: Uranium is a kind of radionuclide and also an important strategic resource. In some areas of China, the content of U in coal is extremely enriched. After high temperature transformation, U in coal can be enriched in solid products, which may cause radioactivity risk. In order to control the release of U and transform U in coal products into resource, it is necessary to investigate the migration law of uranium during thermal utilization of coal. Based on the thermodynamic equilibrium principle, the morphology distribution of uranium was calculated and analyzed in the processes of pyrolysis, gasification and combustion of Ganhe coal, Xiaolongtan coal and Shengli coal. Also the influence of calcium based additives on U migration was analyzed to provide theoretical guidance for subsequent experimental studies. The results show that uranium has different forms in pyrolysis, gasification and combustion processes; UO3(g) is the only gas phase product produced under different working conditions, and higher temperature, lower pressure and stronger oxidation environment can increase the formation amount of UO3(g); volatilization of uranium is significantly week when uranium is fixed to form calcium uranate. Especially, the amount of calcium uranate in Ganhe coal during combustion is increased obviously after the use of calcium based additives and the effect of CaO is the most significant.
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Key words:
- uranium /
- form distribution /
- migration and transformation /
- combustion /
- gasification /
- pyrolysis
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图 1 热解过程中铀的存在形态
Figure 1 Equilibrium composition of uranium species during the pyrolysis process
(a): GH; (b): XLT; (c): SL
图 2 干河煤在CO2气化过程中各工况下的含铀化合物的含量
Figure 2 Amount of equilibrium composition of uranium species of GH at different working conditions during the gasification process with CO2
(a): UO2; (b): UO2.33(B); (c): UO2(U); (d): UO3(g)
图 3 三种煤在H2O气化过程中各工况下UOS的含量
Figure 3 Amount of UOS of the three kinds coals
(a): GH; (b): XLT; (c): SL
图 4 干河煤在不同过氧系数的空气中燃烧各形态铀的百分比
Figure 4 Percentage of various forms of uranium of GH burned in air with different peroxide coefficients
(a): α = 0.8; (b): α = 1.2; (c): α = 1.5
图 5 小龙潭煤在不同过氧系数的空气中燃烧各形态铀的百分比
Figure 5 Percentage of various forms of uranium of XLT burned in air with different peroxide coefficients
(a): α = 0.8; (b): α = 1.2; (c): α = 1.5
图 6 胜利煤在不同过氧系数的空气中燃烧各形态铀的百分比
Figure 6 Percentage of various forms of uranium of GH burned in air with different peroxide coefficients
(a): α = 0.8; (b): α = 1.2; (c): α = 1.5
图 7 使用添加剂后干河煤在燃烧时的CaO·UO3含量
Figure 7 CaO·UO3 content of GH in combustion after using additives
表 1 煤样煤质分析（空气干燥基）
Table 1 Proximate and ultimate analysis of feed coal (air dry)
Sample Proximate w/% Ultimate analysis w/% U/(mg·kg−1) M A V FC C H O N S XLT 17.32 10.20 39.80 32.68 49.93 3.33 16.92 1.14 1.12 9.69 GH 1.36 7.72 43.09 47.83 42.83 2.06 11.44 0.58 8.48 179.00 SL 10.46 41.96 25.87 21.21 33.31 2.78 10.36 0.59 0.54 1.72 
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表 2 各煤种815 ℃灰化学成分数据
Table 2 Chemical composition of feed coal at 815 ℃
Sample Composition w/% SiO2 Al2O3 CaO Fe2O3 K2O MgO Na2O SO3 TiO2 P2O5 XLT 33.97 10.88 29.48 11.67 0.43 2.80 0.08 9.72 0.70 0.27 GH 52.13 25.41 3.89 5.49 2.55 1.53 1.55 6.21 0.54 0.70 SL 61.07 22.05 5.88 2.05 1.10 2.67 2.33 1.51 1.16 0.18
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表 3 计算所考虑的含铀化合物
Table 3 List of species containing uranium considered in this work
Phase Uranium compounds Gas U，UOx，UClx，UFx，UOFx，UAl2Cl10，UO2Cl2，UO2F，UO2F2，US，US2，(US)2 Liquid UO2SO4，UO2SO3，UO2(NO3)2，UO2CO3，UO2(CH3COO)2，U，UOx，UClx，UFx，UOFx，USx，UNx，UPx，UCx，UOClx，UO2Clx，U2O2Cl5，(UO2)2Cl3，U2O5Cl5，U5O12Cl，UAlx，UFe2，UH3，USix，UPx，(NH4)3UO2F5，NH4(UO2)2F5·nH2O，NaUF6，UClxF4−x，U2O3F6，U3O5F8，BaO·UO2 Solid BaO·nUO3，Ba(UO2)2(PO4)2，CaO·UO3，Ca(UO2)2(PO4)2，Fe(UO2)2(PO4)2，H2(UO2)2(PO4)2，K2UO4，K2(UO2)2(PO4)2，MgUO4，Mg(UO2)2(PO4)2，Na2O·nUO3，NaUO3，NaUO2(CH3COO)3，Na2(UO2)2(PO4)2，SrUO4，Sr(UO2)2(PO4)2，UO2CO3，UO2SO3，UOS，UPO5，UP2O7，U(SO3)2，USiO4 note：UOx, UClx and UFx represent compounds corresponding to different values of x,e.g.:UOx—UO，UO2，UO3，U2O2，U2O3，U2O4
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表 4 1000 kg煤在不同系数下燃烧所需空气量
Table 4 Amount of air required for 1000 kg coal to burn at different peroxide coefficients
Sample Gas Amount of air reguired/kmol α=0.8 α=1.2 α=1.5 GH O2 31.94 47.91 59.89 N2 127.55 191.43 239.34 XLT O2 36.65 54.97 68.72 N2 136.12 204.18 255.23 SL O2 25.32 37.87 47.34 N2 94.98 142.46 178.08
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