Biochars Made in the Field Using Coupled Oxygen-Limiting and Mist-Spraying Technique and Their Properties

Liang XIAO; Jing WEI; Guo-dong YUAN; Dong-xue BI; Jie WANG; Li-rong FENG; Guan-hua SHEN

doi:10.13718/j.cnki.xdzk.2019.06.003

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Biochars have a range of functions from improving soil properties and remediating contaminated soils to reducing greenhouse gas emission from soil and increasing carbon content in soil. Their large-scale application, however, has been constrained by high costs of production and transportation. To solve this problem, a novel method has been developed for biochar production in the field with a coupled oxygen-limiting and mist-spraying technique. Agricultural and forest residues are placed in a soil trough and compacted layer by layer, a steel square tube with multiple holes is imbedded in the biomass, and an iron mesh is put on the top of it. The tube has the function of adjusting ventilation and oxygen supply to maintain flame temperature during biomass burning; and the mesh allows for the formation of a water film to limit oxygen supply. In field operations, cotton stalk or Salicaceae in the trough is ignited at one direction, whereas reed straw is ignited at multiple positions, for biomass carbonization, followed by water mist spraying layer by layer for a quick stop of carbonization. The biochars thus obtained have a yield of about 30% and are relatively homogeneous. They have abundant carboxyl group (0.71-1.43 mol/kg) and phenolic hydroxyl group (0.43-1.09 mol/kg) and a specific surface area of 45.5-83.2 m²/g. Being able to produce good-quality biochars directly in the field at a low cost, this method makes feasible agricultural and environmental application of biochars.

Corresponding author: Guo-dong YUAN ;

Received Date: 11/09/2018

Available Online: 20/06/2019

Key words:

oxygen-limiting carbonization in field /
mist-spraying technique /
biochar production /
surface functional group /
specific surface area

Abstract: Biochars have a range of functions from improving soil properties and remediating contaminated soils to reducing greenhouse gas emission from soil and increasing carbon content in soil. Their large-scale application, however, has been constrained by high costs of production and transportation. To solve this problem, a novel method has been developed for biochar production in the field with a coupled oxygen-limiting and mist-spraying technique. Agricultural and forest residues are placed in a soil trough and compacted layer by layer, a steel square tube with multiple holes is imbedded in the biomass, and an iron mesh is put on the top of it. The tube has the function of adjusting ventilation and oxygen supply to maintain flame temperature during biomass burning; and the mesh allows for the formation of a water film to limit oxygen supply. In field operations, cotton stalk or Salicaceae in the trough is ignited at one direction, whereas reed straw is ignited at multiple positions, for biomass carbonization, followed by water mist spraying layer by layer for a quick stop of carbonization. The biochars thus obtained have a yield of about 30% and are relatively homogeneous. They have abundant carboxyl group (0.71-1.43 mol/kg) and phenolic hydroxyl group (0.43-1.09 mol/kg) and a specific surface area of 45.5-83.2 m²/g. Being able to produce good-quality biochars directly in the field at a low cost, this method makes feasible agricultural and environmental application of biochars.

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当前，我国农田已不堪重负，良田沃土退化、土壤污染、耕地地力下降、养分贫瘠等问题凸显，存在严重的生态环境和食品安全隐患.为应对这些问题，近年来，诸如护耕、休耕、农田涵养水源以及土壤增碳等计划逐步启动和实施^[1-4].在土壤增碳方面，被学术界誉为“黑色黄金”的生物炭，已成为调理土壤健康的环境功能材料.它是由生物质在缺氧条件下经高温裂解生成的孔状固体物质，有较大的比表面积、较多的含氧活性基团及一些植物营养元素^[5]；其不仅具有改善土壤性质、提升土壤肥力、减轻土壤污染等农艺和生态功能^[6-10]，也是增加土壤碳汇、减少N₂O和CH₄等温室气体排放的技术选项之一^[11-12].

《全国农业可持续发展规划(2015-2030年)》首次将“生物炭改良土壤”明确写入扶持政策范畴，与秸秆还田、深耕深松、积造施用有机肥和种植绿肥同列^[13].可见秸秆炭化还田已具备市场化条件，建设生物炭产业是践行秸秆炭化还田理念的体现.但是目前生物炭产业在我国尚处于培育阶段，其制造多见于采用热裂解技术和水热炭化法在实验室和小型炭炉中进行^[14-17]，应用实例也多见于科研用途，未能在农业生产中大规模推广使用，这主要是因为面临着使用成本过高这一瓶颈问题^[18-19].若能在田间直接将秸秆转化为生物炭，并应用于退化土壤的改良与污染环境的修复，不仅能够节省秸秆收集与前处理、制炭场地与设备运维、成品运输等环节费用，大幅降低制炭成本，又能够为提升地力、净化土壤、固碳减排等做出新的有益贡献^[20].

本研究以黄河三角洲地区常见的农林生物质(棉花秸秆、芦苇、竹柳)为原料，探究将其在田间直接转化为生物炭的技术，以期获得性能稳定的生物炭产品.目的在于有效地降低生物炭的制备和使用成本，并为其制备和应用提供新的技术选项和思路——田间原位炭化还田.

3. 生物炭的表征与比较分析

表 1展示了农林生物质在田间经“限氧喷雾”技术制备的生物炭产品的基本理化性质，它们都呈现出了碱性pH值(7.99~9.21)，这与传统用高温裂解装置制备的生物炭相似.同时，采用田间“限氧喷雾”技术制备的生物炭也表现出了较高的成炭率(26.3%~34.1%)，保存了较多的营养元素N(0.44%~0.63%).更为重要的是，获得的生物炭产品的比表面积大(45.5~83.2 m²/g)，含有丰富的表面官能团：羧基量介于0.71~1.43 mol/kg，酚羟基量介于0.43~1.09 mol/kg.因此，它们具备吸附剂的基本属性和较大的阳离子交换能力.本研究制炭过程中成炭温度较为稳定，说明采用该技术制备的生物炭质地较为均一.此外，生物质炭的成炭率在30%左右，也就是说，与秸秆直接焚烧相比减少了约1/3的CO₂排放量.生物炭的基本理化性质分析结果表明，采用田间“限氧喷雾”技术制备生成的炭质材料是不同于秸秆直接燃烧生成的草木灰的.该方法的关键意义在于能够为生物炭的制备提供新的思路和技术选项，也首次证明了可以采用简单易行、成本低廉的方法在田间制备出生物炭.

4. 结论与展望

我国理论生物质资源大约为50亿t/年，其中农作物秸秆10亿多t^[20]，其可再生性和有害物质含量低等属性使得生物质还田具有营养元素和有机质循环利用等生态学意义，但由于影响下季作物生长及病虫害增加等原因，秸秆还田实际上不容易推广，而秸秆炭化还田可提供新的技术选项；此外，随着新时代生态文明建设步伐的加快，城市园林废弃物处理问题也面临挑战.

本文提出的在田间制备生物炭的技术，主要通过“限氧喷雾”过程实现生物炭的制备.其中，竹柳和棉秆等木质素类物料采用单向引燃、逐次喷雾成炭的方式进行，芦苇等纤维素类物料采用多位点引燃、逐层喷雾成炭的方式进行.该方法成炭率高，且操作简单、易学，制成的生物炭产品性能稳定，含有丰富的表面官能团和较大的比表面积.

原材料的收集、运输及炭生产设备的购置与运行费用对生物炭生产商是一笔不少的开支(每吨生物炭成本大于1 000元)，而炭成品的运输费进一步提高了生物炭的成本.高成本无疑限制了生物炭的规模化使用.田间“限氧喷雾”制炭技术突破了传统高温裂解技术需要建立工厂和炭炉来制备生物质炭的局限，规避了原材料和生物质炭产品的运输费用、省去了仓储成本、不需要昂贵的设备，制炭效率大幅提升且不再受场地的限制.该技术有望让农民在自己的地里用自家的农林生物质废弃物制备出低成本的生物质炭(200元/t)，用于当地农业生产和土壤改良，做到就地取材、就地制备、就地使用，帮助实现提升地力、减轻污染等国家战略目标，也可将生物炭产品外销，变废为宝，增加农村就业，提高农民收入，服务乡村振兴战略.

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Biochars Made in the Field Using Coupled Oxygen-Limiting and Mist-Spraying Technique and Their Properties

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