Reduction and Control Technologies
This involves the injection of natural gas after primary fuel combustion. It is typically used only on large utility power plant boilers firing coal or residual oil. It is a three-stage combustion process based on fuel staging. The main burner zone is operated at a fuel-lean condition with about 90% of the required fuel input. At the point where primary combustion is complete, the balance of the fuel, the “reburn fuel” is injected. This creates a chemically reducing zone which decomposes most of the NO and the chemical constituents likely to form NO. Tertiary air is then blended in slowly to complete the combustion process.
Natural gas reburning can be an effective and economic means of reducing NOx emissions from large industrial and electric utility boilers. A reburn application which entails the injection of natural gas into a coal-fired boiler above the primary combustion zone- representing 15 to 20 percent of the total fuel mix can produce NOx reductions in the 50 to 70 percent range.