Nitrogen oxides are among the most commonly found pollutants. They contribute to the formation of acid rain and are the primary cause of the presence of ground level ozone. They also contribute to particulate formation, and their geographical impact can be extensive as they are easily displaced by winds.
Where Nitrogen Oxides are found
All combustion and manufacturing processes. NOx are generated by any high temperature combustion process where both oxygen and nitrogen are present.
Natural endothermic reactions
These nitrogen oxides are formed as heat from the combustion causes endothermic reactions between nitrogen and oxygen. Nitrogen monoxide (NO) is generally the main component, accounting for 90% of NOx.
Natural gas-fired combustion processes generate NOx as the nitrogen in the air reacts with oxygen. Coal and oil-fired combustion processes, as well as waste and biomass combustion also generate NOx as a result of the oxidation of the nitrogen contained in the fuel.
Legislation for Nitrogen Oxides
For European Waste to Energy facilities, the Emission Limit Value range for NOx is 50 to 150 mg/Nm³. In urban areas, due to the cumulative effect of NOx from vehicle traffic, lower ELVs are often imposed.
SOLVAir® solution for Nitrogen Oxides
The SOLVAir® Solution consists in the Dry Sorbent Injection (DSI) of sodium-based products for cleaning flue gases, which avoids the reheating of flue gases when tail-end catalytic DeNOx systems are used.
A conjunction with SO2 mitigation
When used as a dry sorbent injected upstream of a filter, sodium-based products readily react with acid gases. In some cases, 10-20% of NOx mitigation occurs in conjunction with the removal of SO2.
Limiting ammonium sulfate deposits
NOx reduction is often conducted by installing a Selective Catalytic Reduction (SCR) system, which uses ammonia and a catalyst to convert NOx into N2. The SCR needs to operate at quite high temperatures in order to avoid the deposition on the catalyst layer of ammonium (bi)sulfate. But as sodium bicarbonate has the highest reactivity toward SO2, it leaves a very low level of remaining pollutant, thus minimizing the risk of formation and deposit of ammonium (bi)sulfate.
Increased energy efficiency
Furthermore, the extended operating temperature range of bicarbonate-based sorbents enables to avoid the cooling of the flue gases which is required for other treatment options.
The sum of these facts allows the use of the catalytic DeNOx device at the same temperature as the bag filter, saving the energy needed to reheat the gases and avoiding related CO2 emission. Additionally, another step of energy recovery of the heat from the clean gases can be installed. Finally, SOLVAir® bicarbonate based products are also compatible with non-catalytic DeNOx systems (SNCR), which inject ammonia or urea solutions in the combustion zone.
