Biopower
Reburning with Wood Fuels for NOx Mitigation Reburning is a combustion modification technology based on the principle that hydrocarbon fragments (CH) can react with Nitrogen Oxides (NOx). Reburning is accomplished by secondary fuel injection downstream of the fuel-lean primary combustion zone or a furnace. The second stage or reburning zone is usually operated at an overall fuel-rich condition, allowing a significant fraction of the primary NOx to be reduced to N2 and other nitrogenous species. In the third zone, additional air is introduced to establish overall fuel-lean conditions and allow for the burnout of remaining fuel fragments. Reburning studies with coal and natural gas have shown NOx emission reductions of 50-60% with about 15% of the heat input coming from the reburn fuel. In contrast, experimental results have shown NOx reductions as high as 70% using approximately 10-15% wood heat input. The stoichiometric ratio in the reburn zone was the single most important variable affecting NOx reduction. The highest reductions were found at a reburn stoichiometric ratio of 0.85. One additional benefit of using wood instead of natural gas for reburning—it is difficult to mix natural gas into the products of the primary combustion zone since the gas must be injected from the wall, at relatively low flows. Wood particles, which must be transported to the furnace by a carrier medium (likely candidates are air or flue gas), would have a ballistic effect upon entering the furnace that would enhance cross-stream mixing compared to natural gas.
Source: Brouwer, J., N.S. Harding, M.P. Heap, J.S. Lighty, and D.W. Pershing, 1997, An Evaluation of Wood Reburning for NOx Reduction from Stationary Sources, final report to the DOE/TVA Southeastern Regional Biomass Energy Program, Muscle Shoals, AL, Contract No. TV-92271 (available at www.bioenergyupdate.com). |
