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

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.