Fossil-Fueled Power Plants - Energy and the Environment - page 111

unburnt carbon burns up and less NO x is formed, according to the right-hand side of Figure 5.12.
The net result is complete burn-out of the fuel and less NO x formation, circumventing the peak
NO x formation of Figure 5.12.
The retrofitting of boilers with LNB is relatively easy and inexpensive to accomplish. The
incremental cost of electricity production using LNB is only 2-3%. In fact, most fossil-fueled
power plants and industrial boilers have installed LNBs. The problem is that LNB can reduce NO x
formation only by 30-55% compared to regular burners. Because of the prevailing problems of acid
deposition and high ozone concentrations in urban-industrial regions of continents, the pressure is
mounting on operators of fossil-fueled power plants and industrial boilers to further reduce NO x
emissions by more effective means than LNB.

After Combustion

Selective Catalytic Reduction. In a selective catalytic reduction (SCR) process either ammonia or
urea is injected into a catalytic reactor through which the flue gas flows. The following reaction
takes place when ammonia is injected:

4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O (5.8)

Thus, NO is reduced by ammonia and ammonia is oxidized by NO and O 2 to form elemental
nitrogen. The catalyst is a mixture of titanium and vanadium oxides dispersed on a honeycomb
structure. The SCR reactor is placed between the economizer and air preheater sections of the boiler,
where the flue gas temperature is 300-400 ◦ C. The reaction is 80-90% complete; thus, 10-20%
of the NO x escapes through the smoke stack, and 10-20% of the unreacted ammonia also escapes.
This is called ammonia slip . While ammonia is a toxic gas, by the time the flue gas plume disperses
to the ground, its concentration is not considered harmful.
The catalyst is easily “poisoned,” especially if the flue gas contains fly ash and sulfur oxides.
(Note that the catalytic reactor is upstream from the ESP and scrubbers.) Therefore, the catalyst
requires frequent replacement, which is one of the major cost elements in operating a SCR. For
SCR a major reconstruction is necessary, because the catalytic reactor must be inserted between the
economizer and air preheater sections of the boiler. The incremental cost of electricity production
in a coal-fired power plant using SCR is about 5-10%.

Selective Noncatalytic Reduction. A reduction of NO can be accomplished at a higher temperature
without a catalyst by using a process of selective noncatalytic reduction (SNCR). In this process,
urea is used instead of ammonia. An aqueous solution of urea is injected into the superheater
section of the boiler, where the temperature is about 900-1000 ◦ C, which is sufficiently high for
the reaction to proceed to near completion:

4NO + 4CO ( NH 2 ) 2 ( aq ) + O 2 → 4N 2 + 4CO 2 + 2H 2 O (5.9)

Many operators of electric power plants prefer this option because it needs no catalyst. However,
urea is more expensive than ammonia. This option is also more amenable to retrofitting of existing
plants because the urea injector can be mounted directly onto the wall of the boiler.