OCR Text |
Show catalytic reactor is required to achieve about 8 0 % N 0 X control, e.g. in-duct catalyst. The process can overcome the limitations of the S N C R process, such as low N O x reduction efficiency and ammonia slip, and substantially reduce S C R costs. NO Adsorption The NOXSO process utilizes NOx adsorption in the fluidized bed of sodium carbonate - y-alumina particles and includes two steps: adsorption and regeneration of the sorbent. U p to about 9 0 % N 0 X and SO2 can be simultaneously removed. The spent sorbent is regenerated by heating (which produces a concentrated N O x stream) and treatment with natural gas to reduce the sodium sulfate to S 0 2 and H 2 S for subsequent formation of sulfur via the Claus process. The stream of N 0 X is recycled back to the line supplying the combustion air to the boiler. Several similar adsorption methods are currently under development. They utilize the same adsorption-regeneration-recycle principle and use different metal oxides that have high N O x adsorption capacity, and catalytic activity in some cases. These methods can be applied to stationary combustion sources and to start-up N O x reduction from mobil sources. Advanced Reburning (AR) A R methods incorporate synergistic combinations of reburning and N-agent injection and capable of achieving about 9 0 % N 0 X control. In AR, N-agents (ammonia or urea) can be injected in the reburning zone, along with the overfire air or downstream in the burnout zone. Several mineral alkali compounds such as sodium carbonate, which is inexpensive and readily available, can significantly promote the efficiency of N-agent. Other non-sodium promoters are currently under investigation. Several A R variants have been tested at pilot scale with utilization of alternative and waste fuels. These fuels already include volatile nitrogen- and alkali-containing compounds and therefore can achieve higher efficiency N O x reduction. Promoted SNCR Process As mentioned above for A R , small additives of soluble inorganic promoters can be added to the N-agents and improve the effectiveness of S N C R N 0 X control, widen the temperature window of the process, and minimize ammonia slip. The effect of promoters is based on formation of additional hydroxyl radicals via chain processes that involve interaction of alkali atoms, oxides, and hydroxides. Since the promoters are inexpensive, easy to handle (they can be added to aqueous solutions to N-agents), and effective in very small amounts, promoted S N C R can provide higher efficiency N 0 X reduction at costs comparable to unpromoted S N C R . It was also discovered that the promoters could reduce N 2 O emissions, the major drawback in application of urea as the N-agent. Non-Flame Low-NOx Alternatives Catalytic combustion has been known for a long time as a low-temperature combustion technique in which the fuel is oxidized on the surface of a catalyst at a low temperature, resulting in ultra low N 0 X formation. Another example of non-flame fuel oxidation is gasification, i.e. the partial oxidation of solid fuels in a restricted supply of oxygen. In gasification reactor, the fuel is converted into a gaseous product that can be utilized in gas turbines or direct combustion with significantly lower N 0 X emissions. 3 |