Selective non-catalytic reduction (SNCR) is a method to lessen
nitrogen oxide Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:
Charge-neutral
*Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide
*Nitrogen dioxide (), nitrogen(IV) oxide
* Nitrogen trioxide (), or n ...
emissions in conventional
power plants that burn
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
,
waste and
coal. The process involves injecting either
ammonia or
urea into the firebox of the boiler at a location where the
flue gas is between to react with the nitrogen oxides formed in the combustion process. The resulting product of the chemical
redox reaction is molecular
nitrogen (N
2),
carbon dioxide (CO
2), and
water (H
2O).
The conversion of noxious NO
x to innocuous N
2 is described by the following simplified equation:
:4 NO + 4 NH
3 + O
2 → 4 N
2 + 6 H
2O
When urea is used, the pre-reaction occurs to first convert it to ammonia:
:NH
2CONH
2 + H
2O → 2 NH
3 + CO
2
Being a solid, urea is easier to handle and store than the more dangerous
ammonia (NH
3), so it is the reactant of choice.
The reaction requires a sufficient reaction time within a certain temperature range, typically , to be effective. At lower temperatures the NO and the ammonia do not react. Ammonia that has not reacted is called ammonia slip and is undesirable, as the ammonia can react with other combustion species, such as
sulfur trioxide
Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
(SO
3), to form
ammonium salts.
At temperatures above 1093 °C ammonia oxidizes:
:4 NH
3 + 5 O
2 -> 4 NO + 6 H
2O
In that case NO is produced instead of being removed.
A further complication is mixing. In general, more NO will form in the center of the reaction vessel and less near the walls, as the walls are cooler than the center. Thus, more ammonia must find its way to the center and less near the walls, otherwise NO in the center meets insufficient ammonia for reduction and excess ammonia near the walls slips through.
Although in theory selective non-catalytic reduction can achieve the same efficiency of about 90% as
selective catalytic reduction (SCR), the practical constraints of temperature, time, and mixing often lead to worse results in practice. However, selective non-catalytic reduction has an economical advantage over selective catalytic reduction, as the cost of the catalyst is not there.
References
External links
How SNCR Works
See also
Selective catalytic reduction
{{DEFAULTSORT:Selective Non-Catalytic Reduction
NOx control