In the developed world, the lion’s share of CO2 is recovered from plants which produce ethanol (fermentation), reformer operations (in oil refineries), natural underground wells and to a smaller degree, ethylene oxide, titanium dioxide and even flue gas.
The most rapidly growing sector of this supply group has undoubtedly been fermentation, however this industry at large is in a state of reorganisation and development from grain-based to cellulosic, sugarcane, sorghum and algae.
The sugarcane-based plants found in places such as Brazil and Asia will continue, since this is a competitive and readily acceptable means of producing ethanol; and the CO2 by product will also continue to grow.
In terms of recovery, when we go to the developing world, where CO2 usage is usually limited to soft drink manufacturing and some cylinder gas use, often the sourcing is via combustion plants.
These burn a variety of fuels such as diesel to produce a flue gas, and from this flue gas CO2 is recovered via MEA, and downstream the rest of the plant (after recovery and concentration via the solvent) is constructed very much like many of the chemical and natural source operations.
Today, the cheapest method of producing CO2 could possibly be from natural wells, which if sourced from high pressure (sometimes near 2,000 psig) often eliminate the need for feed compression – thus saving on power and capital cost. Separately, if these sources have a significant amount of heavier hydrocarbons or even methane, then catalytic oxidation may be required; which would also eliminate many of the impurities found in other feed gas sources beyond natural wells.
Specific to various chemical feedstock sources, would be special metallurgy when considering ethylene oxide, and wet mill-grain-based ethanol sources (not usually constructed any longer) are more problematic than the greater number of dry mill operations.
In terms of world scale size, a plant would be from 400 to 800 tonnes in capacity, and these plants make for a better cost per tonne compared to something significantly smaller, particularly when such plants are in the more price competitive developed markets with lower selling prices for the commodity.
Furthermore, when a plant is recovering and producing for a niche market, such as enhanced oil recovery (EOR) the purification is often removed, and the downstream cost could then include a much higher pipeline investment – and other niche markets may not have any downstream investment past the CO2 plant.
The same would occur in the case of projects considering sequestration – no need for purification, and perhaps a great deal of compression for downhole transfer and storage of CO2 into an aquifer.
The CO2 liquefaction/purification process is essentially rather simple, (a source other than flue gas, or downstream of the MEA recovery plant). One needs to knock out the water, compress the feed gas, and use a series of desuperheaters in process, water wash column in some cases, and absorbers, dryers, and ammonia refrigeration, which involves (ammonia) compression.
The process further entails CO2 condensers, and a stripper column; then from the CO2 subcooler to liquid storage. If dry ice is made, this is via (pressurised) liquid CO2 flashed to atmospheric pressure, then pressed; and often using vapor recovery.
New plants involve more efficient screw compressors and controls for optimal efficiency, and often unmanned operation during part of the time. The CO2 industry will see biofuels develop, and newer cellulosic and biogas sources will become a major feedstock in the term ahead.
About the author
Sam A. Rushing is a chemist and President of Advanced Cryogenics, Ltd., a major CO2 consulting firm, providing all forms of consulting expertise from technical to business and market related.
Tel: 305 825 2597