Bench-Scale Process Evaluation of Reburning and Sorbent Injection for In-Furnace NOx/SOx Reduction

摘要

The report gives results of combining reburning with the injection of calcium-based sorbents to investigate the potential for combined NOx and SOx reduction. Reburning, applied to pulverized-coal-fired utility boilers, involves injecting a secondary fuel above the main firing zone to produce a reducing zone where NOx is converted to molecular nitrogen. Overfire air is added above the reburn reducing zone to complete the combustion. Bench-scale evaluations of reburning, carried out in a plug-flow furnace at 23 kWt, have indicated that NOx reductions of up to 70% can be achieved depending on a number of process variables, dominant of which are the initial NOx level that is to be reduced, the reburning fuel type (pulverized coal type or natural gas), and the stoichiometry, residence time, and temperature in the reducing zone. Combining the two processes indicates that the optimum sorbent injection location in a reburning system is with the burnout air because of the increased reactivity of the calcined sorbent. Dolomite consistently produces higher capture than calcitic limestone for a wide spectrum of combustion conditions. It is possible to relate sulfur capture directly to sorbent reactivity as characterized by surface area after calcination. Sulfur capture in a coal-fired system is similar to that obtained with doped propane. Sulfur capture is most dependent on injection temperature.