Numerical Simulation And Experimental Investigations On The Combustion Of Low-Volatile Coals And Their Blends

Thermal power generating capacity fueled with low-volatile coal (LVC) accounts for a sizeable proportion in China. Currently, practical applications suffer commonly from the problems of low combustion efficiency, the excess emission of pollutants, slagging et al.. Therefore, the study of the combustion of high efficiency and low pollution emission for LVC is still strongly needed and is of great significance in China. The combustion of the pulverized coal in utility boiler is a complex physical and chemical process. In order to achieve its safe, efficient and clean combustion, based on having fully grasped the coal combustion characteristics, the appropriate methods of combustion, reasonable size and structure of the furnace and burner layout, the optimization of operating conditions and new technology must be used. Following this line of thought, the researches of the efficient and clean combustion of LVC and their blends were conducted on several aspects of the combustion kinetics, the combustion simulation of coal blends, and novel stable combustion technology for LVC, and the characteristics of burnout, NOx emission and slagging for LVC boilers, with a view to obtaining the valuable research results for industrial applications.First, the kinetic parameters of chemical reaction, apparent pre-exponential factor A and activation energy E, are taken as variables changed with burnout degree, and they can be obtained from TG and DTG curves of thermo-gravimetric analysis (TGA) of a coal at two heating rates. A thermo-gravimetric analyzer and a drop tube furnace (DTF) were used to investigate the characteristics of combustion kinetic parameters changed with burnout degree of a LVC, with focus on the rationality of the kinetic parameters E(α) and A(α) describing the deactivation during the late burnout stage of coal char. The results show that the combustion kinetic parameters E(α) and A(α) can reasonably predict the low reactivity in the late burnout stage of the pulverized coal. The burnout process of the burnout degree bigger than 0.9 of this coal can be appropriate calculated. The calculated relative error of the burnout degree compared with experimental data is 4.98% in maximum, and decreases with the increase of the burnout degree. E(α) and A(α) will be used as the basic input data for the numerical simulation of the combustion process of pulverized coal. Secondly, the numerical simulation of the combustion processes of a LVC blend were carried out on a 300 MW tangentially coal-fired boiler. At first, the combustion process employing inferior bituminous coal and anthracite as fuel, injected at different floor levels, was simulated in two ways. One way was calculation by the two-mixture fraction/probability density function (PDF) approach, assuming the simultaneous presence of two kinds of coal of different property. Another way was using the single-mixture fraction/probability density function method, assuming a single coal blend of weighted average property. Simulation and actual test measured results show that the temperature and the oxygen concentration distributions in the furnace are non-uniform, but symmetric across corners and that the two-mixture fraction / PDF calculation approach is more in conformity with the actual combustion process of coal blends. Then, in order to improve the boiler performances in security and economics, refractory coverage was added on the water-cool wall region near the burns and a new mode of blended coal combustion was adopted. Numerical simulation shows that this program can effectively enhance the boiler combustion efficiency. On this basis, combustion experiments were performed to determine the reasonable operational parameters, such as the velocities of the primary and tertiary air, the distribution of the secondary air, excess air ratio, the operational mode of pulverizing system et al.. These provide the reliable directions for the optimum operation of the studied boiler.Then, the characteristics of burnout, NOx emission and slagging for LVC boilers were studied with W-flame boilers as examples. The plans for the reforming and experiments of the over fire air for W-flame boiler (OFAW)were then designed. The innovative use of the advantages of the adequate margin in air quantity and high pressure of the primary was proposed. So, some excess primary air treated as OFAW was sent into the furnace to ensure the air quantity and velocity of OFAW. The experimental results show that this method can reduce the carbon content in the fly ash and NOx emission, but the quantity is small because of the percentage of OFAW of less than 5%. The numerical simulations of slagging characteristics in three W-flame boilers were performed for the first time using the slagging models coupled with the gas-solid two phase flows combustion models. Combined with the operating conditions, the slagging position, extent and reasons were deep analyzed, the inhibition methods of slagging were presented. The results show that the slagging characteristics of W-flame boilers are inherent and determined by the aerodynamic characteristics inside the furnace. stopping side burners, reducing boiler load and using coals with low slagging tendency can effectively suppress the furnace slagging. OFAW has little affection on the the characteristics of slagging of W-flame boilers. Finally, based on the researches above, the exploratory discussion and primary investigations on the novel stable combustion technology of LVC were performaed. The mechanism of the heat absorbing, temperature increasing and ignition of the coal/air mixture flow and the mechanism for stabilizing combustion flame were deep analyzed. Some new interpretations were presented. On this basis, a kind of novel adjustable self-adaptive pulverized coal main burner and a combustion method of separating coarse and fine pulverized coal are being developed. The structure and working principle of the main burner were introduced in details. The combustion characteristics of the main burner which was applied to a swirl burner were simulated on a 125 KW down-fired furnace. The results show that an increase in the swirl strength of the outer primary air can effectively improve the ignition and burnout characteristics of LVC with the secondary air swirl intensity of 0.67. The results above may provide useful guidance for the research and development of the adjustable, self-adaptive main burner and combustion technology of separating coarse and fine pulverized coal.