OCR Text |
Show ratio, temperature, and residence tirre in the rebuming zone) the NOx concentrations can typically be reduced by 50-70%. In the third stage the overfue air (OFA) is injected at a lower temperature to complete combustion. The chemical rrechanism of rebuming was investigated in many publications, for instance [2-4]. Addition of the reburning fuel leads to its rapid oxidation to CO and hydrogen. The reburning fuel provides a fuel-rich mixture with certain concentrations of carbon-containing radicals: CH3, CH2, CH, HCCO, etc. ll1ese active species can convert NO into N2 via formation of intermediate products, such as HCN, NH3, NH2, NH, etc. The OFA added on the last stage of the process oxidizes existing CO, H2, HCN, and NH3• Reburning is currently a commercial NOx control technology for boilers and furnaces. In recent demonstrations in three industrial boilers, 60-70% NO removal was achieved [5]. Since fossil fuels contain nitrogen, several studies have been conducted to evaluate influence of N-compounds on rebuming efficiency. Wendt and Mereb [3] used anunonia doped natural gas reburning to evaluate the effect of nitrogen content in the rebuming fuel. The presence of nitrogen produced higher levels of HCN, NH3 and NO (Total Fuel Nitrogen, TFN) within the first 0.4 s, but at greater residence times the level of TFN was about the same as without ammonia. Rudiger et aI. [6] reported a positive effect of nitrogen in the reburning fuel on NO reduction. EER recently developed the Advanced Reburning (AR) process [7], a synergetic integration of basic reburning and N-agent injection which is an adaptation of SNCR. With AR, the N-agent is injected along with the OFA and the reburning system is adjusted to optimize the NOx reduction due to the N-agent. By adjusting the rebuming fuel injection rate to achieve near stoichiometric conditions (instead of the fuel rich conditions nonnally used for rebuming), the CO level is controlled and the temperature window is considerably broadened and deepened. The reburning fuel is reduced to about 10% which has considerable economic benefits (the increrrental cost of gas for gas reburning and the cost of the coal pulverization equipment for coal rebuming). While the NOx control due to reburning is reduced somewhat, this reduction is more than offset by significant enhancement of the N-agent NOx control The AR process can achieve about 80% NOx control with only 10% reburning fuel. This paper examines further improvements over basic reburning and the AR processes for high efficiency and low cost NOx control in coal fired utility boilers. The Second Generation Advanced Reburning (SGAR) systems are also based on hybrid reburning/SNCR schemes. They incorporate injection of a N-agent into the reburning zone and enhancement of the effect of N-agents by using 2 |