Selective Catalytic Reduction – In SCR a catalyst
vessel is installed downstream of the furnace. Ammonia
is injected into the flue gas before it passes over the
fixed-bed catalyst. The catalyst promotes a reaction
between NOx and ammonia to form nitrogen and water.
NOx reductions as high as 90% are achievable, but
careful design and operation are necessary to keep
ammonia emissions (referred to as ammonia slip) to
a concentration of a few ppm.