Effects of holding time and particle size on physicochemical properties and energy conversion of pyrolysis product conponent of rice straw and cotton stalk

The development of pyrolysis technology has great significances for using crop residue resources and relieving energy crisis. Physicochemical properties and energy conversion of gases and biochar obtained from the pyrolysis with different particle sizes of rice straw and cotton stalk were investigated under different holding time. The results showed that the biochar yields firstly decreased from 0 to 90 min and then slightly increased, and the gas yields gradually increased. With the increase of holding time, the proportions of CO2 and CO in bio-gas decreased, while the proportions of CH4, CnHm and H2 in bio-gas increased. With the increase of holding time, the contents of the ash, fixed carbon, C, pH value and electrical conductivity with different particle sizes of biochar increased from 0 to 120 min, while the contents of volatile matter, H, O and N of the biochar decreased. The higher heating values of gases and biochar with different particle sizes of crop residues gradually increased with the increasing holding time, and the energy conversion efficiency in the gas increased, however, the energy conversion efficiency in the biochar decreased. The carbon conversion efficiency in the biochar was significantly higher than that in gas (P<0.05). 1.5%-5.4% energy of crop residue was remained in the bio-gas, and 50%-57% energy of crop residue was remained in the biochar. The biochar produced at the holding time of 90 min had good carbonization degree and can be used as fuels. Different crop residues and different particle sizes had great significant influences on the physicochemical properties of pyrolysis gases and biochar (P<0.05). Comparing the three different particle sizes of crop residues, the biochar yield of the coarse grinding crop residue was the highest and the tar yield was the lowest, while the biochar yield and gas yield of the ultrafine grinding crop residue were the lowest and the tar yield was the highest. The proportions of CO and CH4, higher heating value and the energy conversion efficiency of the pyrolysis gases of the fine grinding crop residue were the highest. The contents of volatile matter, H, O, N and the carbon conversion efficiency of the coarse grinding crop residue biochar were the highest, and the contents of S, electrical conductivity and energy conversion efficiency of the fine grinding crop residue biochar were the highest, while the contents of ash, C, pH value of the ultrafine power crop residue biochar were the highest. Comparing the different types of crop residues, the gas yield, carbon conversion efficiency, higher heating value and energy conversion efficiency of the pyrolysis gases of rice straw were higher than those of cotton stalk, while the proportions of CO2, CH4, CnHm and H2 produced by the pyrolysis of cotton stalk were higher than those of rice straw. The ash content and electrical conductivity of rice straw biochar were higher than those of cotton stalk biochar, while the volatile matter, fixed carbon, pH value, C, H, N, S, carbon conversion efficiency, higher heating value and energy conversion efficiency of the cotton stalk were higher than those of rice straw biochar. The maximum higher heating values of gases and biochar of rice straw and cotton stalk were 7.33, 6.62 MJ/m3 and 20.22, 23.58 MJ/kg, respectively. Cotton stalk is more suitable for pyrolysis carbonization, and the biochar of cotton stalk is more suitable to be applied as fuels.