| China has raised the requirements for efficient clean coal use along with the increasing stringency of pollutant emission regulations and the gradual aggravation of environmental problems due to coal burning. The coal use situation has transited from simple combustion to coal-based energy and chemical industry systems, so as to make efficient, eco-friendly and economic use of coal. The mainstream clean coal utilization techniques are based on the coal conversion into gas and liquid fuels or chemical products. Due to technical limitations, the coal conversion produces abundant high-coal pulverized semi-coke, which reduces the coal use efficiency. Moreover, the pulverized semi-coke formed after processing is less combustible and unsuitable for secondary use through conventional combustion methods. In this work, based on preliminary study on preheated combustion, a preheated combustion system suitable for pulverized semi-coke is built and used to investigate the preheated combustion and nitrogen conversion of pulverized semi-coke, with the aim to achieve efficient pulverized semi-coke combustion with low NOx emissions.In the modified experimental system, the circulating fluidized bed (CFB) used in preheating is a structure without an air distributor and is used to preheat pulverized semi-coke without addition of inert bed material. During the preheating, a part of pulverized semi-coke is separated by a cyclone into returning semi-coke, while the temperature inside CFB is stabilized through the circulation of returning semi-coke. Thus, the circulation of material formed from a part of pulverized semi-coke is the key that maintains the CFB running stably and continuously. During the preheating, the partial burning of the returning semi-coke and pulverized semi-coke could maintain energy balance in the CFB, and is supported by the high-temperature returning semi-coke as an ignition heat source.During the preheating of pulverized semi-coke or Datong Coal, increasing the preheat temperature or particularly the equivalent ratio of the primary air (λCFB) could both promote the conversion of fuel nitrogen. In particular, the rise of λCFB allows more fuel to burn in the CFB. Thus, more coal is consumed with the emission of fuel nitrogen, and meanwhile, the reducing atmosphere in the CFB prevents the released nitrogen from being oxidized into NOX. Therefore, the increase of λCFB within appropriate range would efficiently improve the advanced nitrogen removal in the preheating process.In the preheating of pulverized semi-coke and Datong Coal, the conversion of fuel nitrogen changes in a trend different from both specific surface area and pore volume. During the preheating of Datong Coal, the conversion of fuel nitrogen is finished in two steps:conversion of volatile nitrogen and conversion of char nitrogen. Meanwhile, the peaks on the curves of pore area and pore volume appear after the separation of volatile matter. It is indicated that the separation of volatile matter could efficiently increase the pore area and pore volume. After that, with the increase of λCFB, the distribution curves of pore area and pore volume gradually move downwards, suggesting the increase of λCFB leads to the partial pore collapse and complicates the separation of char nitrogen. During the preheating of pulverized semi-coke, the volatile matter nitrogen and char nitrogen are not separated obviously, as the conversion of fuel nitrogen is basically synchronized with the conversion of char and volatile matter. Thus, with the rise of λCFB, the curves of pore area and pore volume gradually move upwards. It is suggested that the increase of λCFB leads to the expansion of pore structures, providing space for and facilitating the conversion of char nitrogen.Comparison of Datong Coal, pulverized semi-coke and QingHe sewage sludge during preheating shows that the distribution of fuel nitrogen between volatile matter and fixed carbon decides the fuel nitrogen conversion during preheating. During the preheating of Datong Coal or QingHe sewage sludge, the volatile matter is first converted. The conversion rate of fuel nitrogen from clean river sludge is significantly higher compared with Datong Coal, because its fuel nitrogen is mainly distributed in the volatile matter. Thus, during the preheating of high-volatile fuels, the distribution of fuel nitrogen between volatile matter and char is the first factor that affects the fuel nitrogen during preheating.During the preheating of pulverized semi-coke, the types of N-containing groups in the pulverized semi-coke decide that the fuel nitrogen is converted and released in the forms of NH3 and N2. The H-containing groups in the pulverized semi-coke supply H for the formation of NH3. During the combustion, the NO concentration peak appearing at 100 mm along the down-fired combustor (DFC) is mainly due to the oxidation of NH3 coming from the syngas. The formation of such NO can be restricted through the structural optimization of the secondary air nozzle. The NO concentration peak appearing at 900 mm along the DFC is related to the direct oxidation of fuel nitrogen coming from the incomplete burning of pulverized semi-coke after the tertiary air being ventilated.During the preheated combustion of pulverized semi-coke, appropriate selection of λCFB and preheat temperature would facilitate the fuel nitrogen conversion during the preheating and reduce the entry of NH3 into the DFC. During the combustion, appropriate distribution of secondary air can efficiently avoid the appearance local oxidized zones, prevent NH3 from contacting with oxygen to form NO, promote the conversion from NH3 and coke to N2, and thus effectively reduce the formation of NO. At the late stage of fuel nitrogen release, the formation of NO can be further reduced through the adjustment of tertiary air distribution. It is found that parameter optimization helps to reduce the NOX emission to 60 mg/m3(@6%O2) and improve the combustion efficiency to 97.74%, which indicates the achievement of efficient clean combustion of pulverized semi-coke.Preheat combustion and nitrogen conversion experiments with Datong Coal and pulverized semi-coke show that during the preheating, the temperature distribution inside the CFB is related with the fuel properties. Specifically, the decrease of volatile content would reduce the temperature difference in the CFB, makes the temperature distribution uniform, and also leads to the appearance of after-burning in the returner. During the combustion, the heat value of fuel significantly affects the temperature distribution in the DFC. As for fine char from gasification, the temperature at the late combustion stage significantly drops, and the very low combustion temperature complicates the separation of nitrogen. The collocation of secondary air nozzle in the initial burning area has a smaller effect on the final NOx emissions. The preheating of Datong Coal produces far more NH3 compared with the other two fuels. The very high NH3 concentration leads to very high NO2 or NO concentration in the initial burning area, thus complicating the subsequent NOx emission. Meanwhile, the nitrogen in the preheated Datong Coal is gradually released during the combustion, further complicating the control of NOx Thus, for these types of fuels, the nitrogen distribution between volatile matter and coke should be clarified, the pre-burning conversion rate of fuel nitrogen should be improved, and the optimized air distribution structures are needed in the whole burning area. |
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