Page 319 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

296 10 Post-combustion Air Emission Control

Meanwhile, the SCR catalyst also catalyze the oxidation of SO 2 to SO 3 , which
may cause corrosion problems downstream at lower temperatures where water is
condensed and sulfuric acid is formed.
Like many catalytic reactions, catalyst degradation is a big concern here as well.
Ammonium sulfate can also be formed in the SCR unit which causes corrosive
deposits. Therefore, SCR is not recommended for coal-fired plants when the sulfur
content in the fuel is above 0.75 %-wt.
The SCR degradation is also caused by catalyst poisoning by As and other trace
elements, loss of active catalyst by evaporation, in addition to the corrosion and
erosion and the buildup of solid deposits in the catalyst structure. In order to
minimize the particle deposition in SCR, the flue gas is preferably passed down-
ward through a series of 2–4 layers of catalyst beds in the SCR unit. This allows the
fly ash particles to settle down by gravitational settling.
There is typical a loss of 10–20 % of the initial efficiency within 1–1.5 years of
operation. Therefore, a minimum catalyst life of 2 years is currently considered
acceptable. This frequently replacement leads to a major economic challenge to
SCR because typically SCR catalysts take 30–40 % of the cost of the entire SRC
system.

10.4.2 SNCR

Like many air pollution control technologies, there is always a balance between the
economic costs and the effectiveness. Similarly selective non-catalytic NO reduc-
tion (SNCR) process offers less efficiency but often also less cost for NO x reduction.
In a SNCR process, also referred as the Thermal De-NO x process, ammonia is
added to the flue gas where the temperature is about 900 °C. At this temperature,
nitric oxide is converted to molecular nitrogen where water is a by-product. The
step reactions can be described as follows [2, 3].

NH 3 þ OH ! NH 2 þ H 2 O ð10:22Þ
NH 2 þ NO ! NNH þ OH ! N 2 þ H 2 O ð10:23Þ

NH 2 þ NO ! N 2 þ H þ OH ð10:24Þ

The presence of O and OH radicals in the flue gas is necessary for ammonia to
decompose to amino radicals (NH i ), which reacts with nitric oxide to produce N 2
and water.
SNCR is only practical within a narrow temperature range from 850 to 1000 °C
thus making it very temperature sensitive. The optimum temperature for the SNCR
process is 950 °C. At a higher temperature, NH 3 starts to react to nitric oxide, while