Control of NOx from combustion of high nitrogen content fuels (e.g., coal) can be understood by
the net stoichiometric ratio. Control of the NOx from combustion of low nitrogen fuels (such as
gas and oil) can be seen as lean versus rich fuel/air ratios. Either way, this technique avoids the
ideal stoichiometric ratio because this is the ratio that produces higher temperatures that generate
higher concentrations of thermal NOx.
Combustion temperature may be reduced by: (1) using fuel rich mixtures to limit the amount of
oxygen available; (2) using fuel lean mixtures to limit temperature by diluting energy input;
(3) injecting cooled oxygen-depleted flue gas into the combustion air to dilute energy;
(4) injecting cooled flue gas with added fuel; or (5) injecting water or steam. Low-NOx burners
are based partially on this principle.
The basic technique is to reduce the temperature of
combustion products with an excess of fuel, air, flue gas, or steam. This method keeps the vast
majority of nitrogen from becoming ionized (i.e., getting a non-zero valence).
Method 2. Reducing Residence Time -- Reducing residence time at high combustion
temperatures can be done by ignition or injection timing with internal combustion engines. It can
also be done in boilers by restricting the flame to a short region in which the combustion air
becomes flue gas. This is immediately followed by injection of fuel, steam, more combustion
air, or recirculating flue gas. This short residence time at peak temperature keeps the vast
majority of nitrogen from becoming ionized. This bears no relationship to total residence time
of a flue gas in a boiler.
Method 3. Chemical Reduction of NOx – This technique provides a chemically reducing (i.e.,
reversal of oxidization) substance to remove oxygen from nitrogen oxides. Examples include
Selective Catalytic Reduction (SCR) which uses ammonia, Selective Non-Catalytic Reduction
(SNCR) which use ammonia or urea, and Fuel Reburning (FR). Non-thermal plasma, an
emerging technology, when used with a reducing agent, chemically reduces NOx. All of these
technologies attempt to chemically reduce the valence level of nitrogen to zero after the valence
has become higher.
Some low-NOx burners also are based partially on this principle.
Method 4. Oxidation of NOx -- This technique intentionally raises the valence of the nitrogen
ion to allow water to absorb it (i.e., it is based on the greater solubility of NOx at higher valence).
This is accomplished either by using a catalyst, injecting hydrogen peroxide, creating ozone
within the air flow, or injecting ozone into the air flow. Non-thermal plasma, when used without
a reducing agent, can be used to oxidize NOx. A scrubber must be added to the process to
emissions to the atmosphere. Any resultant nitric acid can be either neutralized by
the scrubber liquid and then sold (usually as a calcium or ammonia salt), or collected as nitric
acid to sell to customers.
Method 5. Removal of nitrogen from combustion -- This is accomplished by removing
nitrogen as a reactant either by: (1) using oxygen instead of air in the combustion process; or
(2) using ultra-low nitrogen content fuel to form less fuel NOx. Eliminating nitrogen by using
oxygen tends to produce a rather intense flame that must be subsequently and suitably diluted.
Although Method 2 can lower the temperature quickly to avoid forming excessive NOx, it cannot