反应条件对水稻秸秆慢速热解产物的影响与评价

    Effect and evaluation of reaction conditions on the slow pyrolysis products of rice straw

    • 摘要: 秸秆炭化还田是培肥地力和土壤固碳的重要途径。该研究采用慢速热解试验平台,研究了热解温度 (450、500、550、600和650 ℃) 和停留时间 (30、40、50和60 min) 对水稻秸秆热解产物理化性质(以还田利用指标为主)的影响,同时分析了不同生产条件下的产品收率和能量分布。试验结果表明,热解温度为450~650 ℃时制备的水稻秸秆炭O/C均低于0.2,H/C均低于0.7,且随着热解温度升高和停留时间的增加,O/C和H/C呈现明显减小趋势;随着热解温度升高,水稻秸秆炭的比表面积、电导率和pH值均呈上升趋势,其值分别为4.5~83.4 m2/g、688~1 059 μs/cm和9.8~10.5;阳离子交换量在43.7~71.1 cmol /kg之间无规律波动;随着反应条件变化,水稻秸秆炭的比表面积、电导率和pH值具有较强的相关性,比表面积与pH值相关系数达到0.83,pH值与电导率相关系数为0.66,比表面积和电导率相关系数为0.54。随着热解温度的升高,炭产率降低,热解气产率增加,热解气中H2、CH4等可燃气组分富集,热值增加,最大可达到15.74 MJ/m3;热解温度为445~650 ℃变化时,水稻秸秆炭能量收率为45.2%~53.8%,热解气能量收率为11.6%~19.1%。该研究为水稻秸秆炭化还田轻简化热解设备开发提供了基础支撑。

       

      Abstract: Straw pyrolysis co-production has been one of the most important ways to realize the efficient utilization of straw at present, particularly for the high high-quality biochar and combustible gas. Much efforts have has been made on the important data support for the development of straw pyrolysis carbonization technology. However, it is still lacking on the straw charcoal return to field utilization for the carbon quality under different pyrolysis conditions, especially the correlation analysis for the various physical and chemical properties of straw charcoal. In this study, a systematic investigation was carried out to clarify the effects of pyrolysis temperature and residence time on the physicochemical properties of rice straw pyrolysis products (mainly the return to field utilization index). The correlation analysis was also performed on the specific surface area of rice straw carbon, electrical conductivity, pH value, and cation exchange capacity. At the same time, the yield and energy distribution of products were explored under different experimental conditions. A simple and practical platform was established for the slow pyrolysis of rice straw and the collection of all components of carbon, gas and liquid products. An analysis was made to determine the influence of pyrolysis temperature (450, 500, 550, 600, and 650 ℃) and residence time (30, 40, 50, and 60 min) on the pyrolysis products of rice straw using the slow pyrolysis co-production. The results show that the pyrolysis temperature was posed the an outstanding effect on the product characteristics and yield, while only a little effect was observed in the residence time. When the pyrolysis temperature increased from 450 ℃ to 650 ℃, the carbon yield of rice straw gradually decreased from 40.8% to 35.1%, with a decrease of 5.7 percentage points. The calorific value also decreased slightly. The pyrolysis gas yield increased from 18.3% to 20.7%, with an increase of 2.4 percentage points. The gas calorific value increased from 10.3 to 15.1 MJ/Nm3, with an increase of 4.8 MJ/Nm3, due mainly to the increase in the yield of alkane and olefin olefin-enriched gases, such as H2 and CH4. The pyrolysis temperature was reduced the volatiles of rice straw carbon for the high content of fixed carbon. The O/C and H/C of rice straw carbon that produced in the range of 450-650 ℃ were lower than 0.2, indicating the better stability. There was a strong correlation between the specific surface area, conductivity, and pH value of rice straw carbon. Specifically, the correlation coefficients between the specific surface area and pH value, pH value and conductivity, and the specific surface area and conductivity were 0.83, 0.66, and 0.54, respectively. An increasing trend was found in the specific surface area, conductivity, and pH value of rice straw carbon with the increase of in temperature. The highest specific surface area, the highest electrical conductivity, and the highest pH were 83.4 m2/g, 1 059 μS/cm, and 10.5, respectively. There was no significant change in the cation exchange capacity in the temperature range. The average cation exchange capacity was 56.4 cmol/kg. The energy distribution of rice straw pyrolysis products also changed with the increase of in temperature. The more energy that contained in raw materials was transferred to the gas products. The carbon energy yields of rice straw were 53.8% and 45.2% at the temperatures of 450 ℃and 650 ℃, respectively, whereas, the pyrolysis gas energy yields were 11.6% and 19.1%, respectively. The finding can provide the basic data support to the simplified pyrolysis equipment for the rice straw.

       

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