ABSTRACT
NOx emissions from artificial sources have increased to 100 million tons yearly. Emissions are increasing primarily in developing countries. In several countries, including the United States, Japan, and Germany, emissions have started to decrease (Fig. 1) but are still at high levels resulting in acid rain and photochemical smog. Since nearly all the NO, is derived from fossil-fuel combustion, combustion modification to reduce NO, by 20 to 50% has become popular in many countries. For further NOx abatement, ammonia has been used to convert NOx to N2 by the following reactions:
In 1991, more than 500,000 tons of ammonia was consumed for NOx reduction. Much larger amounts of ammonia may be consumed in future as the NOx abatement will become more and more important in many countries. To remove 10% of the total NOx by selective catalytic reduction (SCR), 7 million tons of ammonia will be consumed annually. Urea has also been used to reduce NOx. Processes to convert NO, in flue gases to nitrogen fertilizers have also been tested in several countries. If these become commercially feasible, large amounts of nitrogen fertilizer may be produced from NO,. In the present chapter we outline NO, removal processes that consume ammonia and also those that yield nitrogen fertilizers as a by-product.
