Experimental Study On Circulating Fluidized Bed Combustion Of Gasification Fine Char

Fluidized bed gasification technology is one of the main clean coal technologies. Subjected to coal gasification reaction rate and other factors, carbon losses by the fine char from coal gasification out of the tail of fluidized bed gasification boiler is high, resulting in low carbon conversion of coal gasification process. The comprehensive utilization of fine char from coal gasification has become the technology barrier which constraints the further development of coal fluidized bed gasification technology. Reburning the fine char from coal gasification could improve the coal utilization efficiency. However, due to the ultrafine particle size, high ash content, low volatile content (almost zero), poor combustion reaction activities, high ignition and burnout temperature and other factors, the high efficiency combustion of fine char from coal gasification could be hardly achieved in normal pulverized coal boilers or circulating fluidized bed boilers. In this paper, the combustion characteristics of fine char from coal gasification and transformation characteristics of sulfur and nitrogen were systematically studied. The main conclusions are listed as follows:The combustion characteristics of the fine char from coal gasification is dependent on the microstructure and chemical composition. As the fine char has experienced high temperature gasification process, the pore structure and surface morphology have changed. In this paper, fine char from coal gasification produced by circulating fluidized bed gasification furnace in Liaocheng and Suqian were taken as research objects. Laser particle size analysis and low temperature nitrogen adsorption technology were used to test the particle size distribution, specific surface area, pore size distribution and other physicochemical characteristics of two kinds of fine char from coal gasification samples. Thermogravimetric analysis was used to test the combustion characteristics. The combustion characteristic parameters and the activation energy were acquired, and the influence of different sample source and particle size on the combustion characteristics was investigated. The results show that, in comparison of two fine char samples and their corresponding raw coal samples, the combustion characteristics of raw coal are significantly better than their corresponding fine char from coal gasification. Because of the difference between the position of sampling, Liaocheng fine char contains a part of fine ash separated by bag precipitator. The particle size of Liaocheng fine char is smaller than that of Suqian fine char, and the pore volume of Liaocheng fine char is 2.3 times of that of Suqian fine char. Liaocheng fly ash has more developed pore structure and a greater specific surface area. The combustion characteristics of Liaocheng fine char are better than those of Suqian fine char. Besides the different composition, greater specific surface area and more developed pore structure could also improve the ignition and burnout characteristics of Liaocheng fine char.Fine char from coal gasification has high sulfur and nitrogen contents. To master the transformation mechanism of sulfur and nitrogen during re-burning process is essential for pollutant emission control. X-ray photoelectron spectroscopy (XPS) was used to analyze the occurrence of sulfur and nitrogen in raw coals and their corresponding fine char from coal gasification, and the content and occurrence change of sulfur and nitrogen from coal to fly ash during the gasification process in the circulating fluidized bed boiler. Thermogravimetric mass spectrometry (TG-MS) was used to detect SO2 and NO formation characteristics during the combustion process of fine char from coal gasification. The results show that, compared with raw coal, fine char from coal gasification contains sulfur and nitrogen with higher binding energy, and the release temperature of SO2 and NO during the combustion process increases. Sulfide sulfur and sulfate sulfur in fine char from coal gasification is reduced, while thiophene sulfur and sulfoxide sulfur increases. The content of all forms of nitrogen decreases, but the release rates differ from each other.Based on the physicochemical characteristics, combustion characteristics and pollutant emission characteristics, the circulation fluidized bed combustion test was conducted to improve the combustion efficiency and control the NO and SO2 emissions of fine char from coal gasification by changing the position of the secondary air outlet, the share of secondary air and the flow velocity, etc. Meanwhile, the combustion characteristics of fine char from coal gasification with different raw coal sources were comparatively studied. The results show that both the two kinds of fine char from coal gasification could achieve stable combustion in the circulating fluidized bed, and the combustion efficiency exceeds 98%. Reasonable arrangement of the secondary air, higher share of the air in feed legs and longer residence time could improve the combustion efficiency. Increasing the separation efficiency of the cyclone could also improve the combustion efficiency.