Transformation of alkali and alkaline earth metallic species during pyrolysis and CO2 gasification of rice straw char
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摘要: 研究了稻秆焦炭中碱金属与碱土金属（AAEMs）在N2热解和CO2气化气氛下的迁移过程。通过对不同热处理时间的固相样品分析，得到了两种气氛下AAEMs的迁移规律，并讨论了CO2气化气氛对AAEMs迁移的影响机理。在两种气氛下，K的释放比例都随热处理时间延长先快速增加，然后缓慢增加，而Ca和Mg的释放比例都很低。气化前期K的释放比例高于热解，气化后期K的释放比例与热解几乎相同。热解时，焦炭中酸溶K和Ca的比例先降低然后维持稳定，而酸溶Mg的比例几乎不变。气化时，酸溶K的比例先缓慢降低，然后迅速降低；酸溶Ca和Mg的比例则先增加后迅速降低。气化前期，酸溶AAEMs的比例要高于热解相同时间的焦炭样品；气化后期，酸溶AAEMs的比例则明显低于热解焦炭样品。CO2通过与焦炭有机结构反应，促进了char-K的释放，提高了K的释放比例，也促进了难溶的有机结合的AAEMs分解为酸溶AAEMs；在焦炭气化后期，焦炭中的Si会与AAEMs反应生成难溶硅酸盐。Abstract: The release of alkali and alkaline earth metallic species (AAEMs) can cause serious technical problems during biomass thermal utilization. The release characteristics of AAEMs from rice straw char during pyrolysis in N2 and CO2 gasification at 900℃ were investigated using a fixed-bed reactor. The release and transformation results of AAEMs under both atmospheres were obtained based on analysis of solid residues sampled after different residence time, and effect of CO2 atmosphere on AAEMs migration was discussed. Under both pyrolysis and gasification conditions, K release ratio initially increased rapidly and the growth rate reduced with time. At early stage of gasification, K release ratio is higher than that after pyrolysis for the same time, while the K release ratio is almost same at late stage of gasification and pyrolysis. Little amounts of Ca and Mg were released during both pyrolysis and gasification. During pyrolysis, fractions of acid-soluble K and Ca first decreased and then remained nearly constant, while the fraction of acid-soluble Mg always remained constant. During gasification, the fraction of acid-soluble K first decreased slowly and then decreased rapidly, while that of Ca and Mg increased first and then reduced with time. At early stage of gasification, the fractions of acid-soluble AAEMs were higher than that after pyrolysis. At last stage of gasification, significant higher fractions of insoluble AAEMs were formed. CO2 atmosphere facilitated release of K by enhancing decomposition and release of char-K. Insoluble char-associated AAEMs are transformed to be acid-soluble due to consumption of char matrix. At the end of gasification, significant insoluble AAEM-silicates compounds were formed.
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Key words:
- AAEMs /
- biomass char /
- pyrolysis /
- gasification
1) 本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18725813) -
图 4 初始焦炭中AAEMs释放比例随时间的变化
Figure 4 Release ratios of AAEMs as a function of reaction time
图 5 热处理后固相样品中酸溶AAEMs的比例
Figure 5 Ratios of acid-soluble AAEMs in solid phase after thermal conversion
图 6 热解与气化过程中初始焦炭中的释放量
Figure 6 Release amounts of K and Cl during pyrolysis and CO2 gasification process
表 1 稻秆和初始焦炭成分分析
Table 1 Composition of rice straw and raw char
Proximate analysis wd /% Straw Char A 12.27 39.20 V 69.94 5.47 FC 17.79 55.33 Ultimate analysis wdaf/ % C 47.92 78.75 H 6.27 0.49 O (calculated by the difference) 44.41 18.15 N 1.17 2.00 S 0.23 0.61 Main ash-forming elements Si 3.43 11.97 Al 0.02 0.07 K 2.46 7.07 Na 0.05 0.16 Ca 0.58 1.84 Mg 0.18 0.55 Cl 1.10 1.85 P 0.12 0.34 
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表 2 焦炭样品的气化时间与转化率
Table 2 char samples with different gasification time and corresponding char conversion
Time t/min 4 7 10.5 16 32 49 Char conversion x/% 20 40 60 80 98 100
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