Study On The Combustion And Flashover Characteristics Of Biomass Boiler

With the development of society and economy, the demand of humanities for energy increases year by year while the utilization of traditional fossil fuel is confronted with the resource depletion and environment pollution problems. Biomass is renewable energy of low pollution and high production which draws people’s attention gradually, and it is widely used currently. Thus, it is imperative to investigate the issue of biomass energy. Meanwhile, some accidents happened in the process of biomass energy utilization due to the biomass characteristics and biomass utilization equipment research is not known completely, for instance, copy the traditional method of making utilization of coal mechanically and resulted in the low combustion efficiency and the flashover phenomenon. Hence, it is essential to investigate the law of biomass combustion and the flashover mechanism based on the characteristics of biomass.The fuel characteristics of biomass briquette in Guangdong area were investigated by experiment methods, such as ultimate analysis, proximate analysis, heating value analysis and thermos-gravimetric analysis. The results indicated that the average C, N, S element contents in the biomass were 46.82%, 0.42%, and 0.124% respectively which less than coal were. Besides, the contents of fixed carbon was less than coal either while the volatile ration was higher than coal. The combustion process of biomass could be divided into three stages, namely water evaporation stage, volatile release stage and fixed carbon combustion stage. The activation energy of biomass briquette 1 and coal was 110 k J/mol and 176 k J/mol respectively, the ignition temperature was 540 K(biomass) and 710 K(coal) according to tangent method which illustrated that biomass is easier to be ignited than coal, and this point should take into consideration when it comes to boiler design. In the thermos-gravimetric experiments, the biomass is reacted more intensive with high proportion of O2. The volatile release rate is proportion to the temperature rising rate. By comparing the TG carves of different grain size of raw, 40 mesh size, and 80 mesh size, no significant difference was found.The combustion simulation of a typical biomass grate furnace in Guangdong was carried out by applying the fluid dynamics-based incinerator code(FLIC) and Fluent. The solid phase temperature distribution along the bed length, the spatial temperature profile and the compositions of the flue gas were obtained. The results show that there are mainly three stages of furnace combustion of biomass, including the dehydration and drying(0-0.5 m), fast pyrolysis and combustion of volatiles(0.3-1.7 m), and finally the combustion of fixed carbon(0.6-2.2 m). A comparison of TG curves of the numerical simulation to the TG analysis demonstrated that the two curves exhibited consistently staged variations, including dehydration and drying, fast pyrolysis and combustion of volatiles, and the burnout of residual carbon due to the different heating conditions. The combustion profiles of flue gas, including the temperature distribution of gas phase, the velocity of flue gas, the concentration of the various components and the velocity vector diagram were obtained by Fluent.The stability of combustion system of biomass boiler was studied with the catastrophe theory in combination with the measured and calculated data, the result demonstrates that swallowtail catastrophe conforms to the flashover mechanism. With building energy equation, conducting dimensionless treatment and carrying out the swallowtail catastrophe research methods, the instability point is obtained, the gained critical flashover temperature of hot smoke layer is 686.71℃ which is similar to studies of predecessors. Then changing the parameters values for the sake of calculating different hot gas layer temperatures. The higher the hot gas layer temperature is, the flashover tend to be more hazardous. The research reveals that increasing the initial temperature of air or aggrandizing area of fire grate(larger than 3.36 m2) contribute to increase flashover temperatures. However, augment of vapor heat and heat transfer area have the opposite effects.The research is beneficial to provide theoretic basis to the high efficiency and security combustion of biomass boiler in southern China to some extent.