Enhanced NOx Reduction with SO2 Capture under Air-Staged Conditions by Calcium Magnesium Acetate in an Oil-Fired Tunnel Furnace

Abstract

The technique of combustion modification by air staging (over-fire air) for the control of NOx emissions is currently implemented in many coal-fired power stations. This paper presents results from a new process involving the injection of calcium magnesium acetate (CMA), which can reduce SO2 and at the same time enhance NOx reductions above those achievable by air staging alone. The experiments were performed in a 3.5m long, horizontal tunnel furnace with an internal diameter of 500 mm operated at 80 kWth by firing gas-oil. The organic content of CMA behaves like a fuel, and the Ca content calcines principally to CaO for acid gas capture in the furnace at temperatures greater than 1000 °C. The solubility of CMA in water means that concentrated solutions can be sprayed into the furnace as a fine mist, giving the possibility of intimate mixing with combustion gases. The concentration of fuel nitrogen in the fuel could be easily modified by varying the amount of dopant (quinoline) injected into the oil feed to simulate typical levels of NOx emission. SO2 concentrations were set by injecting SO2 gas into the combustion air. NOx reduction studies were performed at staging levels which created near-burner zone stoichiometries (λnbz) of 1.18 (3% O2 dry) at 0% staging to 0.64 at 46% staging. Over this range of staging levels, the injection of CMA improved the reduction of NOx by a further 25−35% for an initial NOx level of 450 ppm at an overall stoichiometry of λ1 = 1.18. The effect of CMA on NOx reduction was more apparent at lower levels of staging because of higher initial NOx levels. The near-burner zone (nbz) stoichiometry was 0.64 at this condition. SO2 reductions were studied up to a Ca/S ratio of 2.25, where reductions in the region of 80% were achieved for initial levels of 940 ppm.