Oxy-combustion is considered as one of the CO2 capture options for the coal-fired power plants. The CO2-rich flue gas from such a plant will have to be compressed and purified for sequestrtion. Praxair is developing a near zero emissions technology that will produce CO2 virtually free of trace impurities. One key element of this technology is the removal of SOx and NOx from the flue gas as a result of the combustion of sulfur and nitrogen compounds contained in coal. The technology developed uses activated carbon to catalytically convert the SOx and NOx to their superior oxides SO3 and NO2 which are further adsorbed on the activated carbon material and removed from the gas phase. This process takes advantage of compression requirements of the CO2 purification process and is conducted at elevated pressures. High pressure enhances the oxidation and adsorption process. Once the activated carbon became saturated with the contaminants it can be easily regenerated by simply washing it with water. During this stage the adsorbed oxides react with water to form the corresponding acids H2SO4, H2SO3, HNO3 and HNO2. In this manner the SOx and NOx contaminants are captured in the liquid phase. The waste water can be minimized by the recirculation of the wasted acid solution. The simplicity and good performance of this technology make it attractive and easy to implement.
Experimental Testing and Scale-Up
Comprehensive experimental investigation was conducted in a single bed batch unit containing activated carbon. Tests with synthetic flue gas with SOx only, NOx only and mixtures of SOx and NOx assessed the process performance for a wide range of operating conditions. The effect of concentration level, ratio of SOx/ NOx, water concentration, temperature and pressure was investigated. For this purpose break-through tests for several adsorption-regeneration cycles were conducted. Process performance in terms of material capacity to adsorb a certain amount of contaminants, and the efficiency of each contaminant removal were determined.
The findings obtained from bench-unit experimental investigation created the bases for the first stage of scale-up. A dual bed continuous unit for the purification of 0.125 TPD CO2 has been designed and it is being built. The continuous unit consists of two fixed activated carbon beds operating alternatively in adsorption and regeneration mode in order to ensure continuous purification of a synthetic flue gas stream. The main objectives of the continuous unit test program are to assess material longevity and long term process performance. At the same time it has been sought to optimize cycle steps duration.
Results and Conclusions
The single bed bench unit tests with synthetic flue gas at elevated pressures and ambient temperatures showed that activated carbon is able to remove SOx and NOx when fed individually or together. Investigation of various operating conditions showed that the process has better performance at ambient temperature (~20 oC) compared to temperatures above ambient. The presence of moisture had a beneficial effect on the retention of SOx. Higher operating pressure (220 psig vs. 50 psig) significantly improved the process performance, especially for NOx removal. The process achieved simultaneous removal of SOx and NOx from flue gas with the removal efficiency of > 99 % for SOx and > 96 % for NOx.
The present paper will discuss in detail the influence of feed composition and operating conditions on SOx /NOx removal using activated carbon as adsorbent. Further, the results from the continuous unit and recommendations for the scale-up of the technology will be presented.
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