Coal Flexibility Of Two 125 MW Tangentially Anthracite-Fired Boilers

Many utility boilers have to burn the off-designed coal because of rapid development of electric power industry in China. It brings negative affection to these furnaces, such as combustion instability, efficiency reduction, slagging and et al. Therefore, the study of enhancing the security and economic performance of the boilers with off-designed coals as fuel and broadening the coal flexibility is important and strongly needed. At present, 100MW units of furnaces burning coals, accounts for a sizeable proportion. Hence, the researches on these kinds of boilers are of great significance. This dissertation systematically studies the coal flexibility of two 125 MW tangentially anthracite-fired boilers burning low-volatile anthracites and their blends by using laboratory tests, numerical simulations, retrofit of the instruments and industrial experiments. An expert system of coal blending and combustion optimization is established to guide the coal blending and combustion operation, in order to improve security and efficiency.Basic information on 8 types of coal, mainly Vietnamese anthracite, and several proportion of their blends, such as coal properties, characteristics of combustion and slagging is obtained. With the above information, the optimum proportion of several coal blends of anthracite can also be analyzed, by using Fussy mathematics method and multiple evaluation model. Combustion study on mixed coals with optimum proportion of several coal blends of anthracite operating under nearly practical condition is also operated. The result showed that, Vietnamese anthracite mixed with Yangquan coal performs better in improving ignition characteristics, and can benefits in decreasing carbon remain in fly ash and slaggering when mixed with Dongbei lean coal, while mixed with Xinan coal, it can greatly enhance the ignition and burnout ability. The optimum proportion of different coals mixed with Vietnamese anthracite can be summarized as follows, 2.5-3:1 for Yangquan coal, 2-2.5:1 for Dongbei lean coal, 2-2.5:1 for Xinan coal. The results give suggestions to the power plants for coal purchase, coal blending and combustion operation.The present work also studies the impacts of air distribution mode, primary air velocity and excess air ratio on the variation of aerodynamic field, temperature field, radiation flux, coal burnout degree et al, by two-mixture-fraction approach numerical simulation. From above simulations, optimizing information can be obtained. Pagoda type distribution air strategy and larger excess air ratio will enhance the burnout rate. Lower primary air velocity enhances combustion stability; Appropriate increase in the lower layer secondary air will reduce the combustibles in ash. Based on the analysis of adaptability of the equipment, some of the instruments of the G420/13.7-M419 industrial furnace was improved to enhance its performance, including (1)increasing refractory belt by 64 m2 (2) replacing pulverized coal feeder, and the output increases from 6t/h to 7.5t/h (3) transferring a coarse pulverized coal separator to classifier with double axis .(4) enlarging the area of spout of the burner and flame stabilizing teeth. Following the combustion optimizing information and the numerical simulation results, combustion optimization tests on several loads are performed, and induce abundant information for optimization. With these improvement, (1) combustion stability improved obviously, no serious slagging and superheating were observed, and it showed safety of burning anthracite blends. (2)when burning coal of high quality, pagoda type distribution air mode with 3-4% of oxygen content of exit gas will be favor of decrease in carbon in fly ash and improving combusting efficiency, which conforms the numerical simulation result. (3)the carbon content of fly ash reduces by 13%,from 21% to 8%, the boiler efficiency increases by 4-6%, and the standard coal consumption reduces by 20g/kw.h. Then for DG420/13.7-â…¡2 furnace, improvement scheme includes: (1) pagoda type distribution air mode is recommended above load of 90 MW (2) constricted air distribution mode is better below 90MW.Integrating the work above, we exploit an expert system for combustion optimization, base on the method of grey-relation decision-making, to guide blended coals combustion. The collection of coal properties, operation conditions and boiler efficiency is modeled using the BP artificial neural network. Four typical parameters, volatile content, calorific value, secondary air distribution and oxygen fraction in flue gas, are used as inputs, while boiler efficiency is output. Secondary air distribution is characterized by the ratio of upper and lower secondary wind valve opening R. A simple optimizing strategy is introduced based on above model: Optimization of secondary air distribution and oxygen content in waste gas can be obtained by volatile content and calorific heat, hence optimization system can be realized. The result shows that, for SG420/13.7-M419 furnace, when burning Vietnamese anthracite mixed with Yangquan coal, using the date calculating from the expert system, compared to the previous combustion condition with the efficiency of 86%, boiler efficiency gains high progress, increasing to 89.91%, and combustibles in slag is 3.72%. As a result, the system can guide blended coal combustion and optimize the combustion operation strategy.