Experiments On The Biomass Pyrolysis And Char Combustion During The Anaerobic Gasification Process

There is an increasing attention on the high-efficiency and low-pollution near zero emission system due to the energy crisis and greenhouse effect. Adopting iste biomass as feedstock, the zero emission system can produce not only high purity hydrogen but also high concentration CO2 that easier to be stored and utilized. Biomass anaerobic gasification based on the methodology of CaO absorbent gasification is the core unit for biomass near zero emission system. This article gives experimental study of both biomass pyrolysis in presence of abundant CaO sorbents and biomass char oxygen-enriched combustion at elevated pressures. The results are expected to provide reference material for the utilization of biomass anaerobic gasification in future.Adopting thermogravimetric Fourier transform infrared (TG-FTIR) analysis and X-ray diffraction (XRD) technology, sawdust atmospheric pyrolysis in presence of abundant CaO sorbents is studied. Effects of CaO amounts and heating rates on weight loss process and volatile evolution for sawdust atmospheric pyrolysis are analyzed. Results shows that CaO sorbents can transform the mechanism of sawdust pyrolysis. At lower temperatures, CaO absorb CO2 and H2O in the volatiles, forming CaCO3 and Ca(OH)2, respectively. The produced CaCO3 and Ca(OH)2 then decompose in higher temperatures. The effect of CaO on sawdust pyrolysis is increased with increasing CaO amounts. Moreover, the evolutions of tar species such as CO, CH4, carbonyl groups, aromatics and phenols are also reduced. Additionally, the pyrolysis reaction moves to higher temperatures and the char yield is enhanced with increasing heating rates.Utilizing a Thermax 500 pressurized thermogravimetric analyzer (PTGA) combined with Gas chromatographic (GC) and XRD technology, effect of pressure on sawdust pyrolysis in presence of abundant CaO sorbents is examined. Pressure conditions influence the pyrolysis apparently. Firstly, the pyrolysis reaction moves to higher temperatures and the char yield is enhanced with increasing pressures. Secondly, The CaO carbonation reaction is enhanced and the decomposition temperature of CaCO3 is elevated at pressurized conditions, which are favorable for CO2 capture in the gasifier. Further, residence time of tars species is prolonged at pressurized conditions due to the restrained diffusion. As a result, high pressure pyrolysis is more favorable for tar reduction and decomposition.Using the PTGA and scanning electron microscopy (SEM) analysis, effects of char production and combustion conditions on biomass char pressurized oxygen-enriched combustion are both surveyed. Sawdust chars produced at pressurized conditions or in presence of CaO sorbents have worse reactivity. The ignition temperature increase and the value of combustion stability become lower. With increasing oxygen volume concentrations at steady total pressure, the combustion reaction move to lower temperatures, meanwhile, the ignition temperature decrease and the value of combustion stability become higher. At a steady oxygen volume concentration, the same conclusions can be obtained as above with increasing total pressures from 0.1 MPa to 1.3MPa. Moreover, increasing total pressure at a steady oxygen partial pressure will make the combustion reaction moving towards higher temperature as the oxygen volume concentration decreasing. Correspondingly, the ignition temperature increase and the value of combustion stability become lower. Oxygen volume concentration has larger influence on char pressurized oxygen-enriched combustion than the total pressure.