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Show NOx REDUCTION TECHNIQUES FOR BIOMASS FUELS In order to significantly reduce the amount of N O x reduction from biomass fuels, it is necessary to target the "Fuel N O x " conversion, since this results in the largest single contribution to the N O x . One proven method for reducing the amount of bound nitrogen that converts to N O x is to introduce the biomass into a primary combustion zone that is oxygen deficient, or sub-stoichiometric. In this way the nitrogen that is liberated from the fuel has little or no free oxygen to bond with and therefore mostly recombines to N2. The balance of the air required to complete the combustion is introduced further downstream in a secondary combustion zone. This guarantees complete burnout of the fuel and reduces the temperature of the combustion products. LOW NOx SANDERDUST FIRING IN A PACKAGED BOILER In the early 1990's, a medium density fiberboard plant decided to replace its aging Dutch oven boiler with a new Nebraska "A" type packaged boiler. The availability and need for disposal of the sanderdust produced by the plant resulted in the decision to use sanderdust as the primary fuel to fire the boiler, with natural gas as a backup. The boiler was designed to produce 45,352 kg/hr (100,000 lb/hr) of steam on gas or oil firing, but was de-rated by the site to produce 27,211 kg/hr (60,000 lb/hr) of steam on biomass fuel. The permit application to the state air quality authority resulted in a permit that would require the boiler to meet emission levels of 0.7 lb/MBtu. The plant produced sanderdust from two different board processes, resulting in differing nitrogen levels of 0.59% and 2.35%. The burner employed was a Dual Air Zone (DAZ) gas burner with an annular scroll to introduce the sanderdust, see Figure 1. In this register, the biomass fuel is introduced between two counter-rotating air streams to provide rapid turbulent mixing of fuel and combustion air. The design employed was to provide only 8 0 % of the stoichiometric air (i.e., fuel rich) through the burner. A separate system of adjustable direction air ports in the boiler front wall was used to introduce the secondary combustion air, which amounted to 4 0 % of the stoichiometric air. The burner was sized for a total firing rate of 22.5 M W (77 MBtu/hr). Upon installation, the burner was tested for baseline N O x emissions without the use of any air staging, putting 1 2 0 % stoichiometric air through the burner only. This testing resulted in N O x numbers below what was expected, and low enough to meet permit values without staging. However, it was still desirable to test the system to determine the best NOx achievable. When the system Figure 1: Sander Dust Scroll Burner 3 |