(43a) Modelling Acid Gas Reactions In Air Products Sour Compression Process

Oxyfuel combustion in a coal-fired power station produces a raw CO₂ product containing contaminants such as water vapor plus oxygen, nitrogen and argon derived from the excess oxygen for combustion, impurities in the oxygen used, and any air leakage into the system.  There are also acid gases present, such as SO₃, SO₂, HCl and NO_X produced as byproducts of combustion.  The overall process for purifying the CO₂ takes the impure CO₂ from the power boiler and cools it to condense water vapor, remove traces of ash and dissolve soluble gases such as SO₃ and HCl.  The cooled, raw CO₂ is then compressed to about 30 bar and the CO₂ is purified by partial liquefaction and phase separation. 

In 2006, Air Products first presented reactions that gave a pathway for SO₂ to be removed as H₂SO₄ and NO and NO₂ to be removed as HNO₃ during the compression of the raw CO₂ from the Oxyfuel combustion process.  This proprietary process is now widely known as Air Products’ sour compression process.  Then, in 2008, we presented initial results from the Oxycoal-UK project in which these reactions were studied experimentally to provide the important reaction kinetic information at elevated pressure that was so far missing from the literature.  This experimental work was carried out at lab scale in Imperial College London using synthetic flue gas, and then using actual flue gas via a sidestream from Doosan Babcock’s 160kW coal-fired oxyfuel rig.  From these experimental results, it was clear that the main reaction pathways are viable and the rates of reaction sufficient to produce the desired results:  SO_x and NO_x removal by compression and contact with water.  And, in 2010, we presented further results on experimental work on improving our understanding of the sour compression process.

Simultaneous with the above efforts, Air Products has pursued two opportunities to demonstrate the sour compression process at larger scale: 1) a U.S. Department of Energy (DOE) project to design and construct a proprietary CO₂ purification system for slip stream pilot-scale tests at the site of a 15 MW_th tangentially-fired (T-fired) oxycombustion test facility; and 2) a slip stream pilot plant to demonstrate proprietary Oxyfuel purification technology at Vattenfall's 30 MWth wall-fired Oxyfuel boiler research and development facility in Schwarze Pumpe, Germany.  Both projects allow us to better understand the sour compression reactions, using real Oxyfuel-derived raw CO₂, with a vapor-liquid contacting device equivalent to that forseen for full scale plants. 

At present, our pursuit of larger demonstration plant projects for the Oxyfuel CO₂ purification technology relies, in part, on further enhancing our understanding of the sour compression process.  To that end, in late 2010 Air Products commissioned a purpose built lab that is being used for studying the sour compression reactions in more detail than previous experimental work allowed.  To date, the experimental program has explored the effect of key process variables, including SO₂ and NO_x concentrations, pressure, residence time, and humidity; through experiments run under several different gas-liquid contacting configurations.  Analysis of these data has allowed us to better elucidate the fundamental reaction network and to develop a robust kinetic model that is being utilized for our design work on several commercial scale demo plant projects. 

In this paper, we will present improvements we have made in our understanding of the reaction network and in our process model, as a result of the recent lab work.