OCR Text |
Show chemical separation or by other techniques. N -agents and promoters can be also fonned from the rebuming fuel directly in the rebuming zone with a time delay which is defined by the rates of their release or fonnation from the rebuming fuel. The location of OFA injectors is an important factor which affects process efficiency. Depending on temperature profiles, the residence time in the rebuming zone should be kept as long as possible to complete the NOx reduction process. However, too low OFA injection temperature will increase concentrations of CO and other combustibles in flue gas. Maximum NO removal with N-agent injection into the rebuming zone can be achieved with OFA injection at about 1300 K. This temperature is high enough to prevent CO emissions and low enough to provide higher NO removal. Arand et al. [13] and Brogan [14] presented results on NO reduction by interaction with urea and anunonia, respectively, under fuel-rich combustion. They considered the presence of Nagents in the fuel-rich main combustion zone. Rebuming provides fIring most of the fuel under fuellean conditions and only small portion of the fuel is fIred under fuel-rich conditions in relatively small part of the furnace. This process can substantially reduce corrosion and burner deposits simultaneously providing high efficiency NOx control. CONCLUSIONS The results of this study demonstrate a significant influence of nitrogen- and sodium-containing compounds on NO removal via rebuming. Delayed appearance of N -agents in the gas mixture provide the time which is necessary for depletion of the oxygen from the main combustion zone by the reburning fuel, thus preventing oxidation of the N-agents into NO. Sodium compounds, such as sodium carbonate, promote the effect of N-agents. The promotion effect can be explained by reactions of additional active radicals fonned in the reburning zone via interaction of sodium compounds with water molecules. The following factors primarily control process effIciency: stoichiometric ratio in the reburning zone, delay time between injection of the rebuming fuel and formation of NHi radicals in the reburning zone, concentration of N-agent, and OFA injection location. About 88% NO reduction was achieved by injection of only 10% natural gas as reburning fuel (SR2=0.99) at 1600 K followed by 0.5 s delayed injection of aqueous ammonia or urea (NSR=1.5) with sodium carbonate (15 ppm in gas mixture) and injection of OF A at 1150-1300 K. Under similar 10 |