Process optimization for straws pyrolysis and analysis of biochar physiochemical properties

Abstract: Biochars are solid products from the pyrolysis of crop residues, and their physicochemical properties vary with the pyrolysis conditions and crop residue types. It is of great significance to optimize the pyrolysis process and to regulate the physicochemical properties for high-value utilization of biochars. In this study, rice straw, wheat straw, corn stover, rape stalk and cotton stalk were taken as raw materials. And the orthogonal experiment was designed for considering the carbonization temperature, holding time and heating rate. Then the comprehensive scoring method was used to optimize the pyrolysis process. At last, the physicochemical properties of biochar under optimal technological conditions were analyzed. The results showed that the main order of the quality factors affecting physiochemical properties was carbonization temperature, holding time and heating rate. The optimum combination of the five kinds of straw carbonization process parameters was that the carbonization temperature was 500℃, the holding time was 30 min, and the heating rate was 10℃·min-1. The yields of biochars under the optimal conditions were 37.38% for rice straw, 34.42% for wheat straw, 34.39% for corn stover, 32.74% for rape stalk and 37.87% for cotton stalk, respectively. Thermal analysis showed that most of cellulose, hemicellulose and lignin in the straws were decomposed when the carbonization temperature exceeding 500℃ and the biochars showed the higher stability under optimal conditions. Due to the difference of straw compositions, the residue content of five kinds of straw had the significant differences. Under the optimal conditions, the mass fraction of fixed carbon was more than 40% and the C element was greater than 53%. The mass fractions of the N and K were 0.7% - 2.5% and 3.41% - 6.81%, respectively. The H/C and O/C ratios of the biochars were lower than those of crop residues. The ratio of H/C was lower than 0.6 in the corn stover, rape stalk and cotton stalk biochar. Therefore the biochars can be used as soil additives or carbon sequestration materials. The mass fraction of P, K, Na, Ca and Mg of the biochars were more than those of original feedstocks. Especially, the mass fraction of K was from 3.41% to 6.81%. Therefore, the biochars could be used to improve soil nutrient levels and promote plant growth, particularly in potassium-deficient soil. Furthermore, oxygenic functional groups such as -COOH and -OH are found on the surface of biochars obtained under the optimal conditions by Fourier transform infrared spectra analysis, and the species of groups were similar. The BET surface area of rice straw biochar obtained under optimal conditions was 10.35 m2/g. The N2 adsorption and desorption isotherms of the rice biochar belong to the type Ⅳ of isotherms. The average diameter of pores in rice straw biochar was calculated by the Barrett-Joyner- Halenda model. It showed that some meso-pores and macro-pores with diameters were greater than 3 nm. The biochar surface had oxygen-containing functional groups and mesoporous structures, which had the potential of fertilizer utilization. The results could provide some theoretical basis for the efficient utilization of crop residue biochars.