| The widespread use of biomass fuels, as a renewable energy, could meet the demand of all kinds of energy and reduce carbon dioxide emissions. Combustion is the most simple and direct technology nowadays available for biomass utilization. But its lower burning temperature and energy utilization efficiency has long limited its wide application. Biomass fuel was broken into microns named as biomass micron fuel (BMF). It is necessary to develop BMF’ efficient combustion technology for high temperature, which can further promote the industrialization of biomass application and greenhouse gas reductions.In this dissertation, BMF(sawdust) and its biochar was used as raw material. In order to further improve the cyclone combustion temperature, the biochar with the higher heating value was added to co-combustion with BMF, and then the air was preheated to simulate flue gas recycling technology. At the same time, the combustion kinetics model is set up in order to further establish a comprehensive combustion mathematical model. The following work was carried out in this dissertation:(1) Biomass has high volatile and its ignition and burnout performance is good, but its energy density is low. Biochar contains higher fixed carbon content and calorific value than raw biomass, but its ignition performance is worse due to lower volatile than raw biomass. Thus, biomass and biochar seem to have important potential to compensate the disadvantages coming from each other. Biochar may contribute to the calorific value of biomass, and biomass may promote early ignition of biochar.(2) The pyrolysis and combustion characteristics of BMF were obtained at different heating rates in a thermo gravimetric analyzer. The results showed Comprehensive reaction activity increased and the maximum reaction rate increased linearly with the increase of heating rate. A mechanism based on three independent parallel reactions has been used to model the pyrolysis process of hemicellulose, cellulose and lignin. Combustion process can be divided into two distinct stages, with first stage coinciding with pyrolysis process and the second one concerning lignin char combustion with higher activation energy. The fitting curve of TG obtained from nonlinear regression method is coincident with experiment curves, which confirmed the presumed reaction mechanism does exist during the process of BMF pyrolysis and combustion.(3) The co-combustion of biomass and biochar was investigated by thermo gravimetric analysis. The results showed that synergy exists between the two components and better combustibility was feasible by co-firing biochar with biomass. On the basis of comprehensive thermal analysis, the effects of equivalence ration (ER), particle size, moisture ratio of BMF and biochar blending ratio on combustion performance were studied. The results showed the optimal value of ER and biochar blending ratio obtained was respectively1.2and20%. Smaller particles (below0.177mm) and lower moisture ratio ((below8.1%) results in not only better combustion performances but also economy cost. Combustion efficiency of BMF was higher than that of the biomass briquette. The harmful gas content, such as NOx and SO2, was less in flue gas.(4) Moreover, a pilot study of cement calcining with BMF demonstrated the maximum temperature can be raised to above1360℃through the air preheating. The maximum temperature could meet the demand of cement calcining. However, the combustion efficiency was relatively low for this kiln due to heat losses of flue gas. As for components, calcined raw meal was basically similar to industrial cement clinker. It was feasible to calcine cement by the technology. But the energy evaluation of the system showed that the low combustion efficiency of the device was mainly caused by the exhaust heat loss.(5) To reduce exhaust heat loss, the recycled flue gas was used to preheat air, which can improve the biomass combustion temperature and energy utilization efficiency.Therefore, the flue gas recycling technology for biomass combustion was basically investigated via TG-DTA, TG-MS and GC. By using TG-DTA, the effect of the O2/CO2atmosphere and air (under the conditions of different oxygen concentration) on the biomass ignition mode and the combustion characteristic was analysed. The results showed that with the increasing oxygen concentration, the ignition mode of BMF from joint fire was transformed into homogeneous ignition mode; Maximum combustion rate increased, the ignition advanced and burning time was shortened under O2/CO2atmosphere. At the same time, CO2atmosphere helped to inhibit NOx generation. Morever, under O2/CO2the combustion initial stage-CO2gasification experiments of BMF were carried out in tubular fixed-bed reactor. The results showed that higher temperature, smaller size and CO2atmosphere favored gasification process. Gasification produced more combustible gas and less biochar than pyrolysis atmosphere. CO2atmosphere promoted the whole combustion process, which was expected to increase biomass combustion temperature and reduce the pollution of the environment under O2/CO2atmosphere. |
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