Kinetics characteristics of pressurized pyrolysis and gasification of pine powder

Abstract: Gasification is one of the most important biomass energy utilization techniques, which is clean and efficient. The pressurized biomass gasification has the advantages of large capacity, high efficiency and low cost compared with the atmospheric gasification of biomass. Meanwhile, thermogravimetric analyzer is a widely used for studying biomass gasification. The researches on atmospheric gasification in thermogravimetric analyzer have been widely reported, but there are a few papers about the pressurized gasification. In this work, the processes of pressurized pyrolysis and CO2 gasification of pine powder were performed in a pressurized thermogravimetric analyzer, which pressures were set as 0, 0.3, 0.6 and 0.9 MPa. The pressure of thermogravimetric analyzer was controlled by a pressure electromagnetic valve which could be operated by computer. Air was removed firstly by a vacuum air pump in order to avoid the influence of the air, and then N2 and CO2 were pumped in. When the pressure in the thermogravimetric analyzer reached the setting pressure, the temperature was increased to 1400℃ at the heating rate of 10℃/min. The Malek method was used for inferring the most probable mechanisms of pressurized pyrolysis and gasification, and the effects of pressure on pyrolysis and gasification process as well as their kinetics characteristics were investigated. The results showed that pressure had a significant effect on pyrolysis/gasification of pine powder. The release of volatile matter was inhibited and the maximum weight loss decreased from 73.80% at 0 MPa to 71.40% at 0.9 MPa with the increase of pressure under the N2 atmosphere. According to the results of Malek method, the pressurized pyrolysis was divided into two stages, and the secondary reaction mechanism and the random successive nucleation growth were proved to be the most probable mechanisms for the first and the second stage of pressurized pyrolysis, respectively. The activation energy in the first stage of pressurized pyrolysis increased from 41.15 kJ/mol at 0 MPa to 52.41 kJ/mol at 0.9 MPa. Under CO2 atmosphere, the high pressure inhibited the release of volatile matter in pyrolysis stage which also happened under N2 atmosphere but promoted the char gasification in the gasification stage. The maximum weight loss rate increased from 0.101%/min to 0.162%/min and their corresponding temperatures decreased during the process of char gasification. Furthermore, the two-dimensional diffusion (cylindrical symmetry) was proved to be the most probable mechanism for pressurized CO2 gasification. The activation energy of char gasification increased from 201.94 kJ/mol at 0 MPa to 230.73 kJ/mol at 0.9 MPa. Meanwhile, the frequency factor rose from 4.55E+06 to 7.04E+07 min-1. Additionally, when the value of carbon conversion was in the range of 0.6-0.95, the reaction rate of CO2 gasification gradually increased with the increase of pressure, and the finish temperature of the char reaction shown a downward trend. The results provide a theoretical basis for the efficient and clean utilization of biomass.