ABSTRACT
SCR-NH3 Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671 21.4 SCR-HC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679
The acronym SCR (Selective Catalytic Reduction) is commonly used to indicate the selective reduction of nitrogen oxides (NO, NO2, or N2O) in the presence of gaseous oxygen and a reducing agent, either inorganic (ammonia principally) or organic (saturated or unsaturated hydrocarbons, oxygenated hydrocarbons such as methanol, or nitrogen-containing chemicals such as urea). Originally, the SCR concept was used to indicate not only the abatement of NOx emissions from stationary sources using ammonia as the selective reducing agent (SCR-NH3) and
“DK3029_C021” — #2
metal oxide catalysts such as vanadia supported on titania [1], but also the selective reduction of NOx with hydrocarbons (SCR-HC) or urea (SCR-urea) in the exhaust emissions from lean-burn or gasoline engines [2-4] as well as the selective reduction of N2Owith hydrocarbons in the tail-gas from nitric acid plants [5]. While the SCR process with ammonia for the reduction of NOx emissions from stationary sources is a well-established commercial process to treat emissions of industrial and utility plants (gas-, oil-, and coal-fired applications), industrial and municipal waste incinerators, chemical plants (HNO3 tail-gases, FCC regenerators, facilities for the manufacture of explosives), and in glass, steel, and cement industries [6,7], the use of SCRwith hydrocarbons or urea (for both stationary and mobile sources) has not yet been commercially established. However, SCR with ammonia will soon be introduced in heavy-duty diesel powered commercial vehicles [8,9]. Note that in this case urea is just a precursor that is hydrolyzed on site (on a first catalyst layer) to generate ammonia and CO2. On-board storage as well as distribution infrastructure (tank refillers) of urea is simpler and safer using an aqueous solution of urea rather than ammonia (compressed tanks must be used, and ammonia itself is a toxic chemical), compensating for the higher costs related to vaporization and mixing of urea as well as the necessity of an additional catalyst layer for the hydrolysis of the urea. Another case where urea is preferred instead of ammonia, as the selective reducing agent, is when urea is already available, such as in boiler services for hothouses.
