A Comprehensive Study on NOx Emissions in Fluidized-bed Combustion

• Main Authors: Pin-Wei Li, 李品慰
• Other Authors: Chien-Song Chyang
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/anvhbm

博士 === 中原大學 === 化學工程研究所 === 107 === To understand NOx emission characteristics in fluidized-bed combustion (FBC) comprehensively, formation mechanism, reduction mechanism, and reduction technologies of NOx are integrated in this study. The effect of fuel characteristics on NOx formation and conversion is also discussed. Furthermore, despite the relatively low NOx emissions in FBC, it may not be able to meet the stricter emission standards in the near future. The current NOx emission standards in America, European Union (EU) and China are organized in this study and compared with those in Taiwan, and it can be found that for large coal-fired power plants, the strictest emission limit is 30 ppm legislated by Taiwan, followed by America (44 ppm), China (122 ppm), and lastly by EU (73 ppm). According to the formation mechanism, the production of thermal NO in FBC conditions is less than 0.01 ppm, which is negligible compared with other sources. Fuel-N can be divided into char-N and volatile-N. The heterogeneous oxidation of char-N is the main formation path of NO, while most of volatile-N is converted to HCN and NH3 first before oxidation, and its conversion depends on temperature, fuel structure, heating rate, etc. Although NOx formation in combustion process is inevitable, NOx concentrations are able to be decreased by the reduction reactions, including the reactions of NO with char, CO, and NH3. The heterogeneous reduction of NO and char is the key factor influencing the final emissions, and N2 is formed by the reaction between C(N) and NO. CO may reduce NO catalytically, but its concentration should be higher than 1% to reach significant effects. NO reduction by NH3 can be divided into selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Even though the de-NOx efficiency of SCR is high, the costs and the need for space are not practical for existing combustion units. In the SNCR process, NH3 or urea is injected into the zone with an appropriately high temperature in the absence of a catalyst bed, and the de-NOx efficiency is claimed to be 60 – 80%. In comparison to the end-of-pipe treatment of flue gas, Reducing NOx by combustion control is much cost-effective, so the effects of temperature, excess oxygen, staged combustion, and flue gas recirculation (FGR) on NOx reduction are investigated in this study. The result indicates that controlling bed temperature by water spray is more effective in NOx reduction than by heat exchanger tubes, but the possibility of back-end corrosion should be taken into consideration. NOx emission decreases 8 – 29 ppm per 10% decrease of the excess oxygen, and the impact would be weakened at lower temperatures. In FBC, the application of staged combustion can reduce NOx emissions by 20 – 60%, and the reduction is mainly from the increased residence time by lowering the flow rate of primary gas instead of the reducing atmosphere generated by the decrease of oxygen concentration. Although it is suggested that approximate 30% of NOx emission can be reduced by means of FGR, its effect in this study, however, is unclear, which may be attributed to the fuel characteristics and operating conditions. Fuel characteristic is regarded as the key factor for NOx emissions. The effects of nitrogen content, elemental ratios of fuels, the distribution between char-N and volatile-N as well as the fuel-N structure on the emission and conversion of NOx are investigated in this study, and the correlation between them is also analyzed. The result indicates that the fuel-N conversion decreases exponentially with nitrogen content, but there is no clear tendency for emission concentrations. The correlation between CH/N ratio and fuel-N conversion at 800 °C is superior to that of O/N ratio and H/N ratio. The conversion of volatile-N to NOx is evidently higher than that of char-N, indicating that nitrogen in volatiles is prone to emit NOx. The effect of fuel-N structure on NOx emission is significant, and it may be related to the thermal stability of aromatic rings and the structure of branch trains. In summary, the application of combustion control is able to reduce NOx emission to 100 ppm approximately for most of fuels, but its efficacy is rather limited for those with higher emission values, implying that the inherent characteristics in fuel are key impact factors. SNCR or SCR should be adopted in case of lower emission limits.