Structure and oxidation reactivity of char: Effects of pyrolysis heating rate and pressure
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摘要: 以锡盟褐煤（L）、西山烟煤（B）和玉米秸秆（C）为原料考察了热解压力和升温速率对焦结构及氧化反应性的影响。利用两段式加压固定床反应器，在终温900℃，升温速率5℃/min和200℃/min以及压力0.1-2.0 MPa的热解条件下分别得到了慢速热解焦（SC）和快速热解焦（FC），对焦进行了比表面积、表面形貌和芳香度表征，并且采用等温热重法对焦的氧化反应性进行了分析。结果表明，热解压力和升温速率影响挥发分的停留时间和释放速率，进而影响焦的产率和性质。三种原料的热解行为不同，热解压力和升温速率对焦的产率及焦结构和反应性的影响表现出不同的特点。三种原料快速热解焦产率都低于慢速热解焦产率，且焦产率都随着压力的升高而略微上升。L-FC和B-FC的比表面积分别大于L-SC和B-SC的比表面积。C-FC的比表面积却小于C-SC的比表面积。FC的表面要比SC的表面更为粗糙。B-FC的芳香度小于B-SC的芳香度，但是在加压热解条件下L-SC和C-SC的芳香度反而分别比L-FC和C-FC的低。高压慢速热解的焦氧化反应性较差。玉米秸秆焦的氧化反应性与矿物质的催化密切相关。热解过程中升温速率和压力会影响玉米秸秆焦中矿物质的含量和分布，这也是玉米秸秆焦的氧化反应性明显高于煤焦的氧化反应性的主要原因。Abstract: Two different ranks of coals (Ximeng lignite, L and Xishan bituminous coal, B) and biomass (cornstalk, C) were selected to investigate effects of heating rate and pressure on structure and oxidation reactivity of chars from pyrolysis. The chars were prepared in a two-step pressurized fixed bed reactor at 900℃ from slow pyrolysis and fast pyrolysis under a range of pressures (0.1-2.0 MPa), which were marked as SC and FC, respectively. Specific surface area, surface morphology, and aromaticity of chars were characterized. Isothermal thermogravimetry was performed to study oxidation reactivity of chars. Results indicate that the char yields of three samples are distinctly varied and pyrolysis pressure and heating rate influence them through different residence time and diffusion rate of volatiles varying with different raw materials. The yields of chars from fast pyrolysis are all lower than those from slow pyrolysis and they increase slightly with increasing pyrolysis pressure from atmospheric pressure to 2.0 MPa. The specific surface areas of L-FC and B-FC are larger than that of L-SC and B-SC. However, the specific surface area of C-FC is smaller than that of C-SC. The morphology of FC is all rougher than that of SC. The aromaticity of B-SC is higher than that of B-FC, while that of L-SC and C-SC is lower than that of L-FC and C-FC derived from pressurized pyrolysis. The evolution of char structure at low heating rate and high pressure results generally in poor reactivity of char. The reactivity of cornstalk char is much higher than that of coal char, which should be correlated to the dispersion and concentration of inorganic matters in the char which is affected by heating rate and pressure.
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Key words:
- heating rate /
- pyrolysis pressure /
- specific surface area /
- surface morphology /
- aromaticity /
- oxidation reactivity
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Figure 4 SEM micrographs of SC and FC of three raw materials prepared under different pressures
note:L-SC-0.1 represents the L-SC prepared under 0.1 MPa, 5 nm, the other analogy
Table 1 Proximate and ultimate analyses of raw materials
Sample Proximate analysis w/% Ultimate analysis wdaf /% Mad Ad Vdaf C H N S O* L 10.97 19.75 46.63 69.48 4.33 1.37 1.09 23.73 B 0.41 7.47 16.18 91.14 4.39 1.39 1.57 1.51 C 5.79 2.27 81.04 48.99 5.84 0.72 0.08 44.37 note: d is dry basis; ad is air dry basis; daf is dry and ash-free basis; *: by difference 
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Table 2 Char sample IDs and preparation conditions
Sample ID Raw material Pyrolysis type Pyrolysis pressure p /MPa L-SC lignite slow pyrolysis 0.1-2.0 L-FC lignite fast pyrolysis 0.1-2.0 B-SC bituminous slow pyrolysis 0.1-2.0 B-FC bituminous fast pyrolysis 0.1-2.0 C-SC cornstalk slow pyrolysis 0.1-2.0 C-FC cornstalk fast pyrolysis 0.1-2.0
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Table 3 Ash composition analysis of raw materials
Sample Composition w/% SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 K2O Na2O P2O5 L 57.28 12.63 5.21 7.00 2.94 0.70 9.00 1.80 1.44 0.36 B 28.57 41.21 2.70 0.54 0.22 1.23 0.50 0.52 1.43 0.03 C 40.99 1.47 2.09 3.31 2.54 0.07 3.55 30.72 0.95 4.13
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