Evaluation on pyrolysis characteristics of straw in rotary kiln

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. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.