Published: December 1987
The separation of carbon dioxide from gaseous mixtures is an essential part of natural gas production and of some chemical operations such as the manufacture of ammonia and hydrogen. Methane-rich natural gas from underground wells is usually contaminated with acid gases, mainly CO2 and to a lesser degree, sulfur-containing components such as H2S and COS. To produce fuel gas of acceptable quality it is necessary to separate these acid gases. In NH3 and H2 production, CO2 is an unavoidable by-product of the syngas generation step (regardless of whether the route used is natural gas steam reforming, hydrocarbon partial oxidation, or coal gasification), which must be separated before further downstream processing is possible.
Virtually all commercial processes for CO2 separation are based on absorption in liquid solvents. The solvents used may be categorized into two types--chemical solvents (such as aqueous solutions of monoethanolamine or potassium carbonate, where the mechanism of absorption is via a reversible chemical reaction) or physical solvents (such as methanol used in Rectisol® or dimethyl ethers of polyethylene glycols used in Selexoll® , where the absorption of CO, and other acid gases is without chemical reactions). When significant quantities of sulfur-bearing acid gases (H25 and COS) are present with CO2 (as from the partial oxidation of vacuum residuum or coal gasification) the solvents used must selectively split the sulfur-bearing gases from the CO2 to facilitate sulfur recovery. (Both the Rectisol® and Selexoll® processes achieve this.)
The substantial worldwide, solid/liquid CO2 industry is primarily based on the availability of CO2 as a by-product of other operations. Besides NH3 and H2 plants, which have comprised the main sources of supply, other sources include fermentation ethanol and ethylene oxide plants. These sources furnish the high purity CO2 needed for the solid/liquid segment.
The gaseous CO2 market segment has been very small if we exclude the manufacture of urea and of sodium carbonates--where the source of supply has always been captive. However, following the sharp rise in oil prices during the late seventies/early eighties, a new major use for gaseous CO2 emerged in the United States. This is enhanced oil recovery (EOR) by the "miscible-gas flooding" technique. Since a lower purity CO2 product can be used for this application and because of the large volumes needed, the original interest was centered on sources such as natural gas processing and natural CO2 wells (such as those in Colorado, Mississippi, New Mexico, and Wyoming). Although these two sources provide the lowest cost CO2 there was also a growing interest in the potentially large quantities of CO2, available in alternative sources such as power plant flue gases and the gaseous effluent from cement plants.
In this report we examine the technology and economics of CO2 separation from natural gas fired power plant flue gases. Two processes are evaluated--one based on the Dow Gas Specl® FT-1 technology (which uses proprietary monoethanolamine solutions containing corrosion inhibitors) and the other based on Benfield technology (which uses K2CO3solutions). For comparison we also examine the economics of CO2 separation from NH3 syngas mixtures with these two types of solvents. The monoethanolamine solvent process is based on Union Carbide Corporation�s AAGl® (Advanced Amine Guard) technology, and the K2CO3 process is based on Benfield�s LoHeatl® technology. We also present an outline of the CO2 industry status with particular reference to the United States, Western Europe, and Japan, and a technical review of CO2 separation processes.