PCC puts our proprietary technology and combustion processes know-how to work selecting the optimum burner(s), system configuration, process design, and operating conditions to meet the specified NOx emission limits. All NOx generation methods – “thermal NOx formation”, “chemical NOx formation” and “prompt NOx formation” are evaluated when determining whether low NOx burners, staged air systems, staged fuel systems and/or multi-stage combustion systems are required to achieve low NOx emissions.
What if fuel-bound nitrogen is present?
In certain situations, combustion techniques, process changes and modifying operating conditions might not meet the specified NOx emission limits. In these case, NOx emissions can be met by adding one of two post combustion NOx reduction processes:
When necessary, PCC incorporates the appropriate post combustion NOx reduction technology (SNCR or SCR) into our thermal oxidizer systems. We custom design and supply the complete SNCR or SCR package. We can also design and supply “stand alone” SNCR or SCR systems for new or existing combustion systems.
Here is what PCC’s typical scope of supply includes:
Table 1 – Comparison of Typical SNCR and SCR Systems
|NOx Reduction Efficiency||40-75%||60-90%|
|Reactant||Ammonia or Urea||Ammonia or Urea|
|Waste Disposal||None||Spent catalyst|
|Thermal Efficiency Debit||0 – 0.3%||0%|
|Energy Consumption||Low||High I.D. fan|
|Capital Investment Costs||Low||High|
|Maintenance||Low||3 to 5 years
(typical catalyst life)
|Ammonia/NOx (Molar Ratio)||1.0 – 1.5||0.8 to 1.2|
|Urea/NOx (Molar Ratio)||0.5 – 0.75||Not Applicable|
|Ammonia Slip||5 to 20 ppmvd||5 to 10 ppmvd|
|Mechanical Draft||Not Required||Required|
Ammonia-based process details.
In this SNCR process, ammonia vapor carried by an air stream or steam is injected into the flue gas at the appropriate temperature zone, 870°-1200°C (1600°-2200°F), effecting a reduction of NOx to nitrogen and water.
The injection of ammonia into flue gas leads to a complexity of intermediate chain branching reactions. The following two simplified chemical equations summarize the overall process:
Equation 1 – 2NO + 4NH3 + 2O2 = 3N2 + 6H2O
Equation 2 – 4NH3 + 5O2 = 4NO + 6H2O
Equation 1 is the NOx reduction reaction which occurs in the 870°-1200°C (1600°-2200°F) temperature range by the injection of ammonia alone. NOx reduction effectiveness can be enhanced down to 700°C (1300°F), by injection of hydrogen (H2 along with NH3). However, as indicated by Equation 2, the injection of NH3 into high temperature flue gas results in increased NOx formation and is thus counterproductive.
For initial NOx levels of 200 ppmvd or less, NH3/NOx molar ratios of about 1.5 are commonly used.
Urea-based process details.
This SNCR process uses urea, CO (NH2)2 as a reducing agent. It injects an aqueous urea solution into the path of the NOx laden combustion products. The urea thermally decomposes to produce chemical species which react with NOx to form nitrogen, carbon dioxide, and water.
Equation 3 – CO (NH2)2 + 2NO + 1/2 O2 = 2N2 + CO2 + 2H2O
Equation 4 – 4NH3 + 5O2 = 4NO + 6H2O
In Equation 3, it follows that the stoichiometric molar rate of urea relative to NO in the combustion products is 0.5, since one mole of urea potentially has two moles of NH2 available to react with NO. The urea injection process for NOx control is also temperature sensitive. The urea solution, therefore, must be injected in the temperature range of 870°-1200°C (1600°-2200°F).
Factors that influence NOx reduction efficiency.
The NOx reduction efficiency of both SNCR processes depends on the following factors:
The selective catalytic reduction process removes nitrogen oxides (NOx) from flue gases by injecting ammonia (NH3) into the flue gas and passing the well mixed gases through a catalyst bed. NOx reacts with NH3 in the presence of the catalyst to produce nitrogen (N2) and water (H2O) as shown in the following equations.
Equation 5 – 4NO + 4NH3 + O2 = 4N2 + 6H2O
Equation 6 – 6NO + 4NH3 = 5N2 + 6H2O
Equation 7 – 2NO2 + 4NH3 + O2 = 3N2 + 6H2O
Equation 8 – 6NO2 + 8NH3 = 7N2 + 12H2O
Equation 9 – NO + NO2 + 2NH3 = 2N2 + 3H2O
Note that the first reaction for refinery applications generally dominates since 90% of the NOx is NO. A wide variety of available catalysts can operate at flue gas temperature windows ranging from 165°–600°C (325°–1100°F). High NOx reduction efficiencies can be achieved if the parameters such as residence time, space velocity, and the correct temperature window are controlled.
The common ratio for SCR technology is NH3/ NOx molar ratio of 1.0.