| China's energy structure that being rich in coal,meager in oil and poor in gas determines that coal remains to be one of China's main energy source for a long time.The coal resources in Zhundong Area,Xinjiang is of great strategic significance to China's energy supply due to its abundance reserve and cost-effective mining.However,the high alkali content in coal leads to serious fouling problems during combustion,which endangers safety operation of boilers and brings economic loss.This issue greatly limits the large-scale application of Zhundong coal in coal-fired boiler.At present,there is no effective anti-fouling strategy for purely burning Zhundong coal.Therefore,developing anti-fouling technology during burning 100% pure Zhundong coal is essential and necessary.The development of such technologies would improve the economic value of Zhundong Coal,and promote the economic growth of Zhundong and Xinjiang.The work presented in this Thesis firstly discussed the fouling mechanisms and influencing factors during combustion of Zhundong coal.This was followed by reviewing the existing anti-fouling strategies.Moreover,based on the existing research results in the laboratory and the analysis into the fouling mechanisms and the anti-fouling advantages during combustion in circulating fluidized bed,an idea of parallel circulating fluidized bed graded combustion heat transfer was put forward.The core strategy of this idea was to decrease the furnace outlet temperature to the temperature at which the alkali metal is less likely to evaporate,fouling problem in the convective heat transfer surfaces is thus avoided.Based on this idea,a 75 t/h parallel circulating fluidized bed(PCFB)burning 100% pure Zhundong coal was designed with its relevant thermal calculation,gas-solid flow characteristics between the parallel circulating fluidized beds conducted.The gas-solid flow characteristics and their influencing factors between the circulating fluidized beds on a self-designed parallel circulating fluidized bed cold-mold test bed were systematically studied by using the optical fiber gas concentration and gas-solid rate measurement system.The key gas-solid flow and control parameters that required for the design and development of a “pure burning Zhundong Coal parallel circulating fluidized bed system” were obtained.The main work and innovation in the present work are listed as follows:A parallel circulating fluidized bed(PCFB)graded combustion heat transfer system for burning pure Zhundong coal was proposed for the first time.The high temperature heat transfer surface in the convective tail flue in conventional CFB would be arranged in the PCFB furnace,and at the same time the furnace outlet temperature would be reduced.Therefore the fouling tendency on heat transfer surfaces during combustion of Zhundong coal would be decreased and long-term stable operation of the boiler could be achieved.In the present work,a set of cold test system for the gas-solid flow characteristics that required for the proposed technical scheme was designed,and the cold experimental study and analysis into the gas-solid flow characteristics especially the bed material overflow characteristics and the factors that influencing the heating surface of the furnace were studied by using the optical fiber gas concentration and particle velocity measurement system.The experimental results showed that superficial velocity of main-fluidized bed,the height of the overflow wall,material height,superficial velocity of “vice-fluidized bed one” and “vice-fluidized bed two” could influence the material overflow characteristics.The results also proved the feasibility of this anti-fouling scheme in terms of the gas-solid flow and heat exchange balance in PCFB for burning 100% pure Zhundong coal. |
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