Modeling and parametric analysis of nitrogen and sulfur oxide removal from oxy-combustion flue gas using a single column absorber

Abstract

Oxy-coal combustion has great potential as one of the major CO[subscript 2] capture technologies for power generation from coal. In oxy-coal combustion, the oxygen source is a high concentration oxygen stream and the product flue gas consists primarily of CO[subscript 2] and H[subscript 2]O with contaminants like nitrogen oxides (NO[subscript X]), sulfur oxides (SO[subscript X]) and non-condensable gases like argon, oxygen and nitrogen. NO[subscript X] and SO[subscript X] removal can be achieved via traditional selective catalytic reduction (SCR) and flue gas desulfurization (FGD). These traditional methods however result in relatively high capital cost and energy requirement and face complex material handling challenges. White et al. proposed a different approach to NO[subscript X]/SO[subscript X] removal based on the nitric acid and lead-chamber chemistry process (White et al., 2010). This two-column design utilizes an intermediate and a high-pressure reactive absorption column connected in series to respectively remove SO[subscript X] and NO[subscript X] from the high CO[subscript 2]-concentration flue gas. In this study, we propose a modification to this two-column process that achieves the complete removal of SO[subscript X] and NO[subscript X] from the CO[subscript 2] stream in a single column. We demonstrate by means of pressure sensitivity studies that this new design can meet the same separation targets as the two-column process in fewer column stages and half the feed water requirement by exploiting the pressure dependence of the rate determining NO oxidation reaction. Furthermore, we make use of parametric studies to analyze the dependence of NO[subscript X]/SO[subscript X] removal on key design and operating parameters for the proposed system: pressure, vapor hold-upper stage and water flow rate. Results show that the process is strongly pressure dependent, with a 3-order of magnitude decrease in required residence time when the operating pressure is varied from 4 bars to 30 bars. Vapor holdup volume and feed water flow rate have a significant impact on NO[subscript X]/SO[subscript X] removal up to a point – about 20 m[superscript 3] and 2 kg/s respectively for the case analyzed. Beyond these values, column performance shows substantially less sensitivity to increasing holdup volume or water flow rate. The analysis presented in this paper also shows that recycling bottoms liquid can reduce the feed water requirement by up to 40% without significantly affecting the exit gas purity.