Carbothermal interaction between Cu-based oxygen carrier and ash minerals in the chemical-looping gasification of coal and biomass
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摘要: 从反应温度、灰的种类和灰的比例三个方面，对煤和生物质化学链气化过程中铜基载氧体与灰中矿物的碳热反应过程进行了研究；通过往复切换固定床的氧化还原气氛模拟化学链气化的循环过程，利用XRD和SEM-EDS等表征手段并结合热力学计算对产物进行分析。结果表明，灰中Fe2O3和Al2O3易与CuO/Cu2O反应形成CuAl2O4、Cu2Fe2O4和CuFe2O4等尖晶石结构的物质，而CaO能够通过阻碍Cu-Al和Cu-Si复合化合物的形成来缓解铜基载氧体的烧结。温度升高促使CuO极易与CaSiO3和MgSiO3等硅酸盐矿物发生固-固反应，生成CaCuSi2O6和CuMgSi2O6等而降低铜基载氧体的反应活性。随着灰分比例的增加，Ca2+和Fe3+等离子富集所生成的Ca2Fe9O13易与SiO2发生反应生成高熔点的CaFeSi2O6三相共熔体，与铜基载氧体共熔并覆盖在载氧体表面，阻碍其释氧性能。Abstract: The carbothermal interaction between Cu-based oxygen carrier and ash minerals in the chemical-looping gasification of coal and biomass were investigated experimentally by considering three factors of reaction temperature, type of ash and ash content. The chemical-looping gasification was simulated by reciprocally switching the redox atmosphere of the fixed bed and the products were characterized by XRD and SEM-EDS and analyzed by thermodynamic calculation. The results show that Fe2O3 and Al2O3 in the coal ash can easily react with CuO/Cu2O, forming complexes such as CuAl2O4, Cu2Fe2O4 and CuFe2O4, which are difficult to reduce. However, CaO can alleviate the sintering of Cu-based oxygen carriers by hindering the formation of Cu-Al and Cu-Si complexes. The increase of reaction temperature promotes the solid-solid reaction of CuO with silicate minerals such as CaSiO3 and MgSiO3, producing CaCuSi2O6 and CuMgSi2O6 and reducing the reactivity of Cu-based oxygen carriers. With the increase of ash content, Ca2Fe9O13 generated from Ca2+ and Fe3+ can react with SiO2, forming three-phase eutectic CaFeSi2O6 with a high-melting point, which co-fuses with Cu-based oxygen carrier and covers the surface of the oxygen carrier, leading to a decrease in the oxygen release performance.
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Key words:
- mineral carbothermal reaction /
- Cu-based oxygen carriers /
- ash /
- chemical-looping gasification /
- coal /
- biomass
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图 1 四种灰原样的XRD谱图
Figure 1 XRD patterns of four kinds of ash
Q: SiO2; H: Fe2O3; Ca: Al2O3; C: CaCO3; Ar: MgAl2O4; Sy: KCl; S: Al2SiO5; Pe: MgO
图 3 不同灰组分与CuO之间主要反应ΔG0-T图
Figure 3 ΔG0-T diagram of main reactions between ash components and CuO
图 4 CuO 10次循环后XRD谱图及SEM照片
Figure 4 XRD patterns and SEM image of CuO after 10 cycles reaction CP: Cu2O; CO: CuO; Cu: Cu
图 5 CuO与四种灰的混合物(4:1)在900 ℃下不同循环次数后的XRD谱图
Figure 5 XRD patterns of CuO and four kinds of ashs (4:1) at different cycles at 900 ℃
(a): CuO+GY ash; (b): CuO+SM ash; (c): CuO+DC ash; (d): CuO+LJ ash CP: Cu2O; Q: SiO2; CS: CaSiO3; CF: Cu2Fe2O4; CO: CuO; CI: Ca2Fe9O13; CF5: Ca2FeAlO5; CA: CuAl2O4; CCS: CaCuSi2O6; CA8: CaAl2Si2O8; CC: CaCuO2; V: CuFeS2; M: Fe3O4; CF2: CuFe2O4; CM: CuMgSi2O6; He: CaFeSi2O6; D: CaMgSi2O6; DS: Ca0.8Mg1.2(SiO3)2; CA2: CaAl2O4; MC: (Mg0.03Ca0.97) ·CO3
图 6 四种灰不同循环次数产物晶相变化
Figure 6 Variation of mineral phases of CuO and four kinds of ash after different cycles
(a): CuO +GY ash; (b): CuO +SM ash; (c): CuO+DC ash; (d): CuO +LJ ash
图 7 CuO与辣椒杆灰(4:1)在不同温度下的XRD谱图及其晶相变化
Figure 7 XRD patterns and variation of mineral phases of CuO+ LJ ash (1:1) at different temperatures
CO: CuO; CP: Cu2O; CI: Ca2Fe9O13; DS: Ca0.8Mg1.2(SiO3)2; D: CaMgSi2O6; MC: (Mg0.03Ca0.97)·CO3; CC: CaCuO2; CM: CuMgSi2O6; CM2: Cu2Mg5SiO22(OH)2; He: CaFeSi2O6; CCS: CaCuSi2O6
图 8 CuO与不同比例辣椒杆灰(9:1/1:1)在900 ℃下循环的XRD谱图及其晶相变化
Figure 8 XRD patterns and variation of mineral phases of CuO and different ratios of LJ ash (9:1/1:1) at 900 ℃
CI2: CaFe2O4; CP: Cu2O; CO: CuO; Mo: Ca(Mg0.88Fe0.12)SiO4; He: CaFeSi2O6; CC: CaCuO2; CA: CuAl2O4; CI: Ca2Fe9O13; D: CaMgSi2O6; Mo2: Ca(Mg0.93Fe0.07)SiO4; CM: CuMgSi2O6; CM2: Cu2Mg5SiO22(OH)2; CCS: CaCuSi2O6; CF: Cu2Fe2O4; CA2: CaAl2O4
图 9 新鲜CuO载氧体(a)和CuO+灰(b)循环反应前SEM照片
Figure 9 SEM images of fresh Cu-based oxygen carrier (a) and CuO+ ash (b) before reaction
图 10 循环反应后复合铜化合物的SEM照片及对应点的能谱图
Figure 10 SEM images and EDS analysis of composite copper compound
(a): CuO+LJ ash after 10 cycles at 900 ℃; (b): CuO+GY ash after 10 cycles at 900 ℃; (c): CuO+SM ash after 10 cycles at 900 ℃; (d): CuO+DC ash after 10 cycles at 900 ℃
图 11 CuO与矿物固相反应机理示意图
Figure 11 Proposed mechanism for the solid reaction between CuO and minerals
表 1 四种灰化学组分分析
Table 1 Chemical composition of four kinds of ash
Type of ash Content w/% SiO2 Al2O3 Fe2O3 CaO MgO Na2O SO3 K2O P2O5 others GY 51.43 18.9 11 3.46 0.98 3.77 3.08 0.42 5.18 1.76 LJ 10.93 6.59 2.13 20.89 15.18 2.42 11.09 22.68 7.94 0.1 DC 57.29 3.87 1.05 7.9 1.67 3.14 1.42 19.15 4.26 0.24 SM 7.62 3.77 2.08 59.14 5.13 0.95 2.72 12.65 5.68 0.27 notes: four kinds of ashes, viz., GY, LJ, DC and SM were obtained from Guanyun coal, chili stick, rice straw and apple tree timber, respectively, by cineration at 600-800 ℃ 
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表 2 主要反应及其吉布斯-亥姆霍兹方程
Table 2 Main reactions and the corresponding equations to get the Gibbs-Helmholtz energy
No. Reaction equation ΔG0/(kJ·mol-1) 1 Al2O3+CuO=CuAl2O4 24.11152-0.0208T 2 Fe2O3+CuO=CuFe2O4 13.389-0.01672T 3 Fe2O3+Cu2O=2CuFeO2 -30.125+0.002637T 4 SiO2+CuO=CuSiO3 -13.179+0.007572T 5 CaO+CuO=CaCuO2 41.148-0.04236T 6 CaO+SiO2=CaSiO3 -82.007-0.006235T 7 CaO+Al2O3=CaAl2O4 -13.398-0.02333T 8 CO(g)+2CuO=Cu2O+CO2(g) -141.545-0.02385T 9 CO(g)+CuO=Cu+CO2(g) -127.11-0.006623T 10 CO(g)+Cu2O=2Cu+CO2(g) -112.675+0.0106T 11 CO(g)+CuAl2O4=Cu+Al2O3+CO2(g) -115.749+0.01415T 12 CO(g)+CuFe2O4=Cu+Fe2O3+ CO2(g) -94.499+0.010096T 13 CO(g)+2CuFeO2=2Cu+Fe2O3+CO2(g) -82.55+0.007966T 14 CO (g)+CuSiO3=Cu+SiO2+CO2(g) -71.589+0.006486T 15 CO (g)+CaCuO2 = Cu+CaO+CO2(g) -123.358+0.03574T
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表 3 EDS元素组成
Table 3 Element composition of the composite copper compounds at the points 1-4 shown in Figure 10
1# 2# 3# 4# element w/% watom/% element w/% watom/% element w/% watom/% element w/% watom/% O 23.25 57.6 O 32.63 63 O 18.91 50.2 O 34.6 66.61 Si 1.16 1.64 Al 17.03 19.5 Fe 25.15 19.1 Si 12.37 13.56 K 1.24 1.64 Cu 21.45 10.4 Cu 37.97 25.2 Cu 26.04 12.53 Ca 22.81 22.6 Ca 9.28 7.17 Ca 2.89 3.07 Ca 9.41 7.25 Cu 40.28 16.9 Si 1.69 2, 56
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