OCR Text |
Show T w o novel low-NOx alternatives to direct combustion are currently under development: flameless oxidation and unmixed combustion. If flue gas is recirculated into the combustion air or fuel, at some point the flame can become unstable, lift off and finally blow out at temperatures below self-ignition. It has been found, however, that if the furnace temperature and the flue gas recirculation rates are sufficiently high, the fuel can react steadily in the flameless oxidation mode. T o operate in this regime, the combustion reactor should be heated to temperatures higher than about 1100 K and the recirculation rate should be high enough to avoid the flame regime. Under these conditions, formation of thermal N O can be completely suppressed. Unmixed combustion, a revolutionary novel firing method, fundamentally changes basic combustion equipment and results in formation of less than 0.1 p p m NOx . Unmixed combustion proceeds in a reactor in which a metal is alternately exposed to air and a gaseous fuel. A s the metal is alternately oxidized and reduced, the fuel is correspondingly oxidized without direct contact with air. The resulting process has excellent heat transfer to solids and very low pollutant emissions. Unmixed combustion has been already demonstrated with a number of different fuels, including natural gas, propane, carbon monoxide, hydrogen, and diesel oil, and in a variety of applications, including production of hydrogen via "unmixed reforming". NO Oxidation with Subsequent Removal of Oxidation Products Since NO2 is much more reactive than NO, the NO-to-N02 oxidation followed by combined NO2/SO2 removal from combustion flue gas in a scrubber is an alternative strategy for N O x control. A large number of wet and dry scrubbing systems for SO2 control are already in place or planned for the near future, and the possibility of controlling N O x can become an important feature of the scrubber. Several chemicals were proposed and tested as N O oxidizers in flue gas. They include ozone, oxides of chlorine, methanol, yellow phosphorus, hydrocarbons, and hydrogen peroxide. Sodium-based, calcium-based (with additives of sodium compounds), and ammonia-based scrubbing systems have the potential to be applied for effective NO2-SO2 removal. Further oxidation of N O 2 and SO2 to a mixture of sulfuric and nitric acids or their salts (fertilizers) is also considered. N O oxidation techniques can be applied at relatively low temperatures as a tail-end method in combination with upstream technologies, such as reburning or S N C R. Reduction of N?Q Nitrous oxide is formed in the temperature range of 1000-1300 K. There are two major combustion related processes resulting in high N 2 O emissions: fluidized bed combustion and injection of urea in the S N C R process. Available methods of N 2 O control include optimization of fluidized bed and S N C R reactor design. Novel developing methods of N 2 O removal include afterburning/reburning and addition of alkali or other mineral compounds. Afterburning removes N 2 O via the reaction with hydrogen atoms: N 2 O + H -» N2 + O H . Alkali compounds can directly react with N 2 O via reactions such as N a + N 2 O -> N2 + NaO. Plasma and Electromagnetic Methods These methods can reduce nitrogen oxide emissions by injection of plasma-generated radicals into flue gas or by applying high frequency electromagnetic waves to facilitate the reactions of nitrogen oxides on the surface of char particles. Though the plasma methods are typically suffer from high-energy consumption and formation of by-products, they can find specific applications in a variety of industries. Both high temperature and low temperature plasma methods are under investigation. 4 |