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A new technology (called here, spray-and-scattered-bubble technology) based on preozonation was designed and tested for simultaneous removal of SO 2 and NOx from power plant flue gas. It combines the advantages of the common spray tower and the jet bubble reactor, in which the flue gas experiences an initial SO 2 /NOx removal in the spray zone and then undergoes further removal in the bubble zone. Factors that affect the simultaneous removal of SO 2 /NOx were investigated through lab-scale experiments, by varying the O 3 /NO molar ratio, liquid/gas ratio and the immersion depth. The results showed the removal of SO 2 and NOx can be significantly improved as compared to a separate spray column or bubble reactor, by as much as 17%, for the spray column and 18% for the bubble reactor for NOx and 11% for the spray column, and 13% for the bubble reactor for SO 2 , for liquid/gas ratio of 4 dm 3 /m 3 or immersion depth of 100 mm. The O 3 /NO molar ratio had little effect on the SO 2 removal, but it strongly affected the removal efficiency of NOx especially when it was less than 1.0. Both the liquid/gas ratio and immersion depth demonstrated a positive correlation with the removal efficiency. However, a balance must be maintained between efficiency and economics, since the liquid/gas ratio directly influences the performance and number of the circulating pumps, and the depth is closely related to the flue gas pressure drop, and both factors affect energy requirements. To further confirm its industrial feasibility, a 30 h test using real coal-fired flue gas was conducted in a pilot-scale experimental facility (flue gas volume of 5000 Nm 3 /h). Increasing SO 2 concentration in flue gas can promote the removal efficiency of NOx, but the SO 2 removal was almost complete under all conditions tested. Finally, taking a 300 MW unit as an example, the total energy cost of this new technology is estimated as being 10% lower than that of the common spray tower technology, based on an analysis using Aspen Plus™, with the largest difference reflected in the energy requirements of the circulating pumps and the ozonizer. Over all, the new technology offers the joint advantages of reducing emissions and saving energy.

The simultaneous calcination/sulfation (SCS) reaction is the realistic reaction process for limestone use in CFB boilers. A SCS reaction model based on the randomly-overlapped pore concept, which takes into consideration the calcination of CaCO3, the sulfation of CaO and the sintering effect simultaneously, was developed. The results of this model fit well with the results from the thermogravimetric analyzer (TGA) tests and, thus this model was used to study the characteristics of the SCS reaction. The SCS reaction consists of a mass-loss stage and a mass-growth stage, and the two stages are seperated by a minimum mass point. The mass-loss stage is dominated by the calcination of CaCO3, while the mass-growth stage is dominated by the sulfation of CaO. The minimum mass point is a balance point of the mass change caused by the two reactions. The calcination reaction occurred in a layer of the particle. As the calcination reaction progresses, the reaction front moves inward and a CaO layer is formed. The SO2 in the calcination atmosphere can react with the CaO layer and produce CaSO4. The CaSO4 can fill the pores of the CaO layer and narrow the pore width, increase the CO2 diffusion resistance and consequently slow the calcination reaction. The sulfation reaction becomes slower as the reaction progresses. There was an upper limit to the sulfation conversion, which is much higher in the outer layer of the particle. For a typical particle with a radius of 200 μm, the sulfation reaction ceases in the inner part (0-150 μm) of the particle due to the exhaustion of SO2, while in the outer part of the particle (150-200 μm), the decrease of the sulfation rate is caused by the simultaneous decline of the reaction surface area, surface Ca 2+ ion concentration and SO2 concentration.

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Nitrogen dioxide (NO 2 ) has been attracting a lot of attention because of its toxicity and potential in contaminating land, air, and water. However, research on NO 2 release behavior under oxy−fuel conditions is still insufficient. This paper aims to explore the formation and reduction mechanisms of NO 2 during oxy−fuel combustion of char by using both experimental and density functional theory (DFT) methods. Results showed that with the increase of temperature, the energy barrier of the NO oxidation reaction increases, while that of the NO 2 reduction reaction decreases. These facts lead to a higher peak concentration of NO and lower NO 2 emissions. Because of a larger effect of temperature on the rate constant of the NO 2 reduction reaction than that of the NO oxidation reaction, the NO 2 concentration decreases dramatically and rapidly even if the temperature increases only slightly. As the temperature increases to above 1273 K, the equilibrium constant of the NO oxidation reaction falls to below 10 5 . That is, the oxidation of NO to NO 2 is incomplete and reversible over the temperature range of 1273−1673 K. By contrast, the reduction of NO 2 to NO is complete and irreversible over this temperature range. As a result, almost no NO 2 is observed at high temperatures. In the presence of H 2 O, DFT calculations show that the water-gas-shift reaction is less important in contrast to the H 2 O−carbon reaction, which is consistent with previous research. In this case, NOx emissions under an O 2 /CO 2 /H 2 O atmosphere are lower than those under an O 2 /CO 2 atmosphere. Overall, combined experiments with DFT calculations offer a new approach to study the microcosmic mechanisms of NO 2 formation and reduction under oxy−fuel conditions during isothermal combustion of char.