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铁尾矿是工业选矿后产生的固体废物,通常在尾矿库中进行露天堆放处置,除了长期占用大量土地外,还容易产生含有大量重金属的废水,对环境造成污染[1]。以粤北大宝山尾矿库为例,参照《GBT 3838-2002地表水环境质量标准》[2],尾矿区废水中大量重金属元素超标,其中Cu平均含量是Ⅴ类水体标准的6倍,平均Mn含量是标准的40倍[3]。
铁尾矿的还原磁化焙烧则是回收铁元素的有效技术之一。其中气基还原的主要还原剂为CO,具有高反应效率、还原更充分均匀的特点,但工业CO和H2的价格昂贵,不适合大量生产[4]。自然界中的生物质简单易得,在300~400 ℃下热解即可产生大量CO,在高温条件下更可生成一定量的H2,在还原焙烧铁尾矿从而生产磁铁矿上有巨大的应用潜力[5]。TAN等[6]利用N2为载气在650 ℃下对榴莲壳进行无氧热解,在GC-TCD检测中发现热解气中含有大约50%的CO2与35%的CO;AL[7]利用快速热解技术,在680 ℃的条件下对甘蔗渣进行热解,最终得到的还原气中含有45.3%的H2和20.5%的CO;MALIUTINA等[8]在600~900 ℃的条件下分别对小球微藻和棕榈壳进行了闪速热解,在800 ℃的条件下,微藻产生的热解气中含有44.33%的H2,而棕榈壳在900 ℃下产生的热解气中H2的含量为26.62%。
以往研究大多关注生物质热解后各组分的含量,针对如何提高热解气中还原性气体占比的研究较少(本研究中还原性气体含量指CO与H2总量),也少有将动力学机理纳入热解气的生成探讨中。本研究针对广东省韶关地区大量种植的杉树与青竹进行单因素试验,热重分析与红外光谱分析,考察不同反应条件下杉树木屑与竹屑的热解产气情况,并与热解动力学协同研究,探讨这两种生物质的产气机理与一般规律。
木屑生物质热解制备高还原性气体机理
Mechanism of pyrolysis of sawdust biomass to produce highly reducing gas
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摘要: 针对气基磁化焙烧还原铁尾矿成本昂贵的问题,基于生物质廉价易得且具有较好的还原性气体制备潜力的特点进行产气机理探究。通过热解实验研究了温度、含水率、Na2CO3催化剂添加量对杉树木屑与竹屑热解制备还原性气体的影响,利用TG-IR探究二者的产气特征,并结合动力学分析确定杉树木屑与竹屑的热解机理函数与活化能。结果表明,生物质在较低含水率(40%以下)的条件下提高热解温度,有利于还原气的制备;当热解温度为700 ℃、Na2CO3添加量为4%时,烘干的杉树木屑热解气中还原性气体占比约为76%,竹屑则约为66%;二者的还原性气体均集中产生于热解第二阶段,其热解过程符合反应级数模型;经计算,杉树木屑的活化能为118.32~135.43 kJ·mol−1,竹屑的活化能为112.39~118.75 kJ·mol-1。本研究结果可为生物质热解制备还原性气体提供技术指导。Abstract: Due to the high cost for reducing iron tailings by gas-based magnetization roasting, this paper explored the mechanism of gas production, which was based on the cheap, easily obtaining, and good potential for reducing gas production characteristics of biomass. Through single-factor experiments, the effects of temperature, water content, and Na2CO3 catalyst addition on the fir sawdust and bamboo sawdust were explored. Besides, the gas production characteristics between these two sawdust were investigated by TG-IR. By kinetic analysis, the pyrolysis mechanism function and activation energy of fir sawdust and bamboo sawdust were determined. The results indicated that the moisture content below 40% was conducive to reduce gas production. When the pyrolysis temperature was 700 ℃ and the addition amount of Na2CO3 was 4%, the proportion of reducing gas in the pyrolysis gas of dried fir sawdust was about 76%, and that of bamboo sawdust was about 66%. Both of the reducing gases are concentrated in the second stage of pyrolysis, which can be better described by the reaction series model. By calculated, the activation energy of Chinese fir sawdust was 118.32~135.43 kJ·mol-1; while the activation energy of bamboo sawdust was 112.39~118.75 kJ·mol-1. The results can provide technical guidance for the preparation of reductive gases by biomass pyrolysis.
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Key words:
- fir sawdust /
- bamboo sawdust /
- pyrolysis /
- reducing gas /
- kinetics
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表 1 样品的元素分析、工业分析与成分分析
Table 1. Ultimate, proximate and material analysis of samples
%(质量分数) 样品 元素分析 工业分析 成分分析 C H O N S Mad Aad Vad FCad 多糖 木质素 杉木 48.60 6.20 44.84 0.26 0.03 8.12 0.40 75.23 16.25 53.97 34.70 竹屑 47.46 6.62 45.66 0.24 0.03 6.64 1.56 75.97 15.83 65.26 25.70 表 2 无模型法计算得到的热解动力学参数
Table 2. pyrolysis kinetic parameters by model-free method
无模型法 杉树木屑 竹屑 α R2 E/ kJ·mol−1 α R2 E/ kJ·mol−1 KAS法 − 0.996 79 118.32 − 0.999 59 117.17 FWO法 0.1 0.998 01 118.48 0.1 0.987 24 91.88 0.2 0.998 66 123.12 0.2 0.986 96 101.10 0.3 0.998 28 127.01 0.3 0.988 65 114.80 0.4 0.998 75 130.49 0.4 0.987 14 123.21 0.5 0.998 06 131.92 0.5 0.986 06 126.01 0.6 0.998 37 134.87 0.6 0.989 52 130.81 0.7 0.998 15 147.02 0.7 0.988 41 137.05 平均 − 130.42 平均 − 117.84 Friedman法 0.1 0.997 81 119.69 0.1 0.985 78 91.84 0.2 0.998 53 124.37 0.2 0.985 58 101.36 0.3 0.998 11 128.31 0.3 0.987 57 115.64 0.4 0.998 62 131.86 0.4 0.985 98 124.35 0.5 0.997 87 133.28 0.5 0.984 82 127.19 0.6 0.998 21 136.30 0.6 0.988 59 132.17 0.7 0.997 98 148.99 0.7 0.987 43 138.67 平均 − 131.83 平均 − 118.75 表 3 不同反应级数的热解动力学参数
Table 3. Pyrolysis kinetic karameters of different reactions orders
升温速率/(K·min−1) 杉树木屑 竹屑 G(α) R2 E/ kJ·mol−1 A/s−1 G(α) R2 E/ kJ·mol−1 A/s−1 10 F5.6 0.957 57 137.87 3.66×1011 F4.59 0.945 29 122.05 2.60×1010 20 F5.23 0.951 07 139.25 2.25×1011 F3.79 0.933 25 116.42 3.38×109 40 F3.94 0.925 52 129.18 1.00×1010 F2.35 0.916 18 98.71 2.48×107 -
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