Study and Modeling of the Reduction of Sulfur Dioxide, Nitrogen Oxides and Hydrogen Chloride by Dry Injection Technologies

Abstract: The potential and mechanism to reduce acid gases, such as SO2, NOx and HCl by dry Ca-based sorbents have been studied to improve the efficiencies of the process and sorbent utilization. Several natural limestones were tested for SO2 removal. Calcium conversion as high as 45% was achieved within 0.3 s at 1000°C, 1000 ppm SO2 and Ca/S=1. A SO2 removal efficiency of 95% was reached at Ca/S=2. Two models for estimating the sulfation of CaO at high temperature are presented. Short-residence-time sulfation is described by a pore size distribution model and long-residence-time sulfation by a particle expansion model. The pore size distribution model explains the effects of particle size, pore size distribution and partial pressure of SO2, suggesting these three factors be the most important for CaO conversion. For particles larger than 1-2 mm in furnace sorbent injection, pore diameters of 50-300Å are desirable. When large particles or long residence times are used, as in fluidized bed combustion, the particle expansion model shows the particle size and the sorbent type be the main factors affecting the reaction. By using the selected limestone and additives the simultaneous SO2/NOx removal was also measured. Urea gave higher NOx removal efficiency than several other ammonium salts. By urea-limestone sorbent, 90% SO2 removal and 80% NOx removal were obtained at Ca/S and N/ NOx ratios both equal to 2. The N2O formed was less than 10 ppm at 2 s residence time. A reductive path from NOx to N2 is proposed to explain the experimental results. The spent sorbents from SO2 reduction processes were found to have further capacity for HCl removal at 150-600°C. After being calcined and slaked, they even showed the same reactivity as pure Ca(OH)2. The kinetics of the reaction was studied with a shrinking core model. The multiple sorbent utilization can reach up to 80% for acid gases reduction.