热解温度对回转窑玉米秸秆热解产物理化特性的影响

Evaluation on pyrolysis characteristics of straw in rotary kiln

针对北方农业秸秆废弃物产量巨大且无法全部还田导致丢弃和露天焚烧现象激增等问题,该文通过搭建小型回转窑生物质热解装置考察不同热解温度下秸秆热解特性,分析主要产物的产率、元素组成等理化特性指标。结果表明:回转窑内热解温度的增加提高了热解液相产物产率和热解水产率,焦油产率呈先增加后降低趋势。与此同时,热解气总体积逐渐增加,H2含量和CH4含量也有所提高,生物炭产率和热值有所降低。当热解温度从400℃增加至700℃时,焦油产率从12.21%增加至21.70%;当温度进一步增加至800℃时,焦油产率降低至20.13%;相应的焦油热值从400℃时的19 974.0 kJ/kg逐渐增加到800℃时的21 710.0 kJ/kg。高热解温度加快热解过程中的热传递,加剧生物质大分子所含的羟基、羰基等含氧官能团的分解并促进挥发物的产生,进而提高了热解液体产物、热解水和焦油产率。过高的加热温度会加剧挥发分的二次反应,降低焦油产率;更多的含氧杂环结构会随着热解温度提高逐渐分解,因而焦油热值逐渐增加。生物炭产率随着温度增加逐渐降低,生物炭pH值和C/N比均逐渐增加,在兼顾生物炭产率和应用于炭基肥制备所需理化性质的同时需充分考虑热解温度影响。

Hebei is a largely agricultural province in northern China,and the crop straw amount was 58.42 million in 2017.Pyrolysis technology is an important way to realize the utilization of crop straw resources.The biochar produced also plays an important role in soil improvement,heavy metal adsorption,and water purification,especially it can regulate soil pH value and effectively alleviate soil acidification and alkalinity and other organic matter content.Pyrolysis gas can be used for heating in the north and alleviating the pollution caused by the use of a large number of inferior loose coal in rural areas.Pyrolysis liquids and wood vinegar can be used as fuels or chemical raw materials.Therefore,the pyrolysis technology has received extensive attention from experts at home and abroad.To investigate the pyrolysis characteristics of corn straw under different temperature conditions and analyze the physical and chemical properties of pyrolysis products,this paper proposed a rotary kiln.The results showed that increasing pyrolysis temperature raised the yields of pyrolysis liquid products and water,but decreased the yield and high heating value(HHV)of biochar.The tar yield increased first and then decreased with the rising of temperature.When the pyrolysis temperature increased from 400 to 700℃,the tar yield rose from 12.21%to 21.70%,when the temperature increased to 800℃,the tar yield reduced to 20.13%.The tar HHV escalated from 19 974.0 to 21 710.0 kJ/kg with the increase in pyrolysis temperature from 400 to 800℃.More oxygen-containing heterocyclic structures such as hydroxyl and carbonyl groups gradually decomposed with the increase of pyrolysis temperature,thus raising the tar HHV.However,the excessively high temperature exacerbated the secondary reaction of volatiles,and therefore reduced the tar yield.The pyrolysis gas volume,H2 and CH4 content increased with the rising of temperature,but the biochar yield and HHV decreased,while biochar pH value and C/N ratio presented an increasing trend.The increase of pyrolysis temperature gradually increased the C/N ratio of biochar,and the corresponding biochar H/O decreased,which was mainly due to the escape of a large amount of volatiles during pyrolysis.When the pyrolysis temperature increased from 400 to 800℃,the C/N ratio of biochar increased from 21.02 to 28.60.When the pyrolysis temperature was 600℃,the C/N ratio of biochar was 24.85,which was suitable for composting process and production of carbon-based fertilizer.The results of this study provide a reference for biochar application in agricultural production and composting of agricultural wastes.