Pyrolysis characteristics of corn cob and release rule of gas products

Abstract: Shortage of fossil fuels and environmental pollution become increasingly severe with the rapid economic development. As the only renewable energy which can be directly converted to gas, liquid and solid fuels, biomass has aroused growing attention all over the world. Corn is one of the main crops in China. Corn cob is the main agricultural waste produced in process of maize production, and the corn cob biomass contains a lot of biodegradable organic matter. Thermo-chemical conversion is an efficient means of biomass energy conversion. It can convert the organic matter of corn cob into many forms of energy, such as gas, liquid, solid, and other biomass products at high temperature. Pyrolysis is the most basic process of thermal chemical conversion. The characteristics of pyrolysis are important tool which can express the influence of pyrolysis parameters on raw material conversion rate. In order to fully grasp the pyrolysis characteristics of corn cob and the release law of gas-phase products with temperature change in the thermal decomposition process in different working conditions, and to deeply understand the pyrolysis behavior of corn cob and its reaction mechanism, simultaneous thermogravimetry-mass spectrometry (TG-MS) was used to investigate the pyrolysis behavior and kinetics of corn cob under nitrogen atmosphere. The pyrolysis behavior of corn cob was comparatively studied at different heating rates (5, 10, 20℃/min), different particle sizes (74, 154, 280, 450 μm) and different carrier gas flow rates (30, 60, 90 mL/min). It was found that the non-isothermal weight loss process of the samples was composed of dehydration, preheating pyrolysis, volatile matter separation and carbonization. The temperature interval of 210-405℃ was the main floating zone. There were two obvious peaks in corn cob's weight loss rate curves. The release laws of small molecule gas products (CO, CO2, CH4, O2, H2 and H2O) were studied by mass spectrometry analysis. The pyrolysis characteristics index was calculated as well, showing that the higher the heating rate, the quicker the pyrolysis reaction. The maximum pyrolysis rate and the index increased with the rise of heating rate. The peak corresponding to the maximum pyrolysis rate moved to higher temperature. The peak temperature of maximum pyrolysis rate varied along with the change of particle size weakly. But the relationship between the maximum weight loss rate and particle size was not obvious. Within the scope of particle size less than 450 μm, the total pyrolytic weight loss of sample increased with the rise of particle size. The process of pyrolysis was mainly affected by particle internal heat and the mass transfer for the sample of 154-450 μm. Over 500℃, it showed a strong exothermic reaction. The heat release increased with the rise of particle size. But the pyrolysis was mainly controlled by the rate of intrinsic reaction kinetics for the sample with particle size less than 154 μm. The effect of carrier gas flow rate on the pyrolysis was negligible especially for the pyrolysis reaction rate. The kinetic parameters were calculated by the Coats-Redfern method, indicating that the volatile matter separation stage of corn cob pyrolysis could be described in the single first-order reaction. This research has guiding significance for the optimization of the parameter of thermal chemical conversion process and for improving the yield and quality of fuel products. Moreover, it can also be used to provide reference for designing and developing some efficient biomass energy conversion devices.