SNCR
Encyclopedia
Selective Non-Catalytic Reduction (SNCR) is a method to lessen nitrogen oxide
emissions in conventional power plants that burn biomass
, 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 760 and 1093 °C (1,400 and 1,999.4 F) to react with the nitrogen oxides formed in the combustion process. The resulting product of the chemical redox
reaction is elemental nitrogen
(N2), carbon dioxide (CO2), and water
(H2O).
Urea (NH2CONH2) is easier to handle and store than the more dangerous ammonia (NH3). In the process it reacts like ammonia:
The reduction happens according to (simplified)
The reaction mechanism itself involves NH2 radical
s that attach to NO and then decompose.
The reaction requires a sufficient reaction time within a certain temperature range, typically 760 to 1093 °C (1,400 to 1,999.4 F), to be effective. At lower temperatures the NO and the ammonia don't 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
(SO3), to form ammonium salts
.
At temperatures above 1093 °C ammonia decomposes:
In that case NO is created instead of removed.
A further complication is mixing. In general, more NO will form in the center 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.
Though in theory selective non-catalytic reduction can achieve the same efficiency of about 90 % as selective catalytic reduction (SCR)
, these 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.
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
emissions in conventional power plants that burn biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
, waste
Incineration
Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. Incineration and other high temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into ash, flue gas, and...
and coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
. The process involves injecting either ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
or urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
into the firebox of the boiler at a location where the flue gas
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
is between 760 and 1093 °C (1,400 and 1,999.4 F) to react with the nitrogen oxides formed in the combustion process. The resulting product of the chemical redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reaction is elemental nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N2), carbon dioxide (CO2), and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
(H2O).
Urea (NH2CONH2) is easier to handle and store than the more dangerous ammonia (NH3). In the process it reacts like ammonia:
- NH2CONH2 + H2O -> 2NH3 + CO2
The reduction happens according to (simplified)
- 4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O
The reaction mechanism itself involves NH2 radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
s that attach to NO and then decompose.
The reaction requires a sufficient reaction time within a certain temperature range, typically 760 to 1093 °C (1,400 to 1,999.4 F), to be effective. At lower temperatures the NO and the ammonia don't 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
Sulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...
(SO3), to form ammonium salts
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
.
At temperatures above 1093 °C ammonia decomposes:
- 4 NH3 + 5 O2 -> 4 NO + 6 H2O
In that case NO is created instead of removed.
A further complication is mixing. In general, more NO will form in the center 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.
Though in theory selective non-catalytic reduction can achieve the same efficiency of about 90 % as selective catalytic reduction (SCR)
Selective catalytic reduction
Selective catalytic reduction is a means of converting nitrogen oxides, also referred to as with the aid of a catalyst into diatomic nitrogen , , and water, . A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed...
, these 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.