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Comparison of Processes for Separation and Sequestration of CO2 from the Combustion of Coal

Stuart W. Churchill1, Michelle Czarniak2, Elisa Lau2, Candice McLeod2, and William B. Retallick3. (1) Chemical & Biomolecular Engineering, The University of Pennsylvania, 220 South 33rd St, 311A Towne Bldg., Philadelphia, PA 19104, (2) Chemical and Biomolecular Engineering Department, The University of Pennsylvania, 220 South 33rd St, 311A Towne Bldg., Philadelphia, PA 19104, (3) W. B. Retallick Associates, 1422 Johnny's Way, West Chester, PA 19380

     CO2 is a major “greenhouse” gas and combustion of carbonaceous fuels is one of the primary sources of its addition to the atmosphere.  The sequestration of CO2 from combustion or other sources is not currently prescribed by law and has no direct economic advantages but a recent study by the National Academy of Engineering has listed it among the principal technological challenges of the 21st century. This presentation examines the technical feasibility and economic costs of several processes for the recovery, separation, and sequestration of CO2 from the combustion of coal. Our analysis is based on bituminous coal, which, despite widespread opposition, is generally predicted to be a major source of energy for the generation of electricity in the next several decades. The Illinois Basin, which underlies Illinois, western Indiana, and western Kentucky was arbitrarily chosen for this analysis because (1) it has both large and accessible holdings of bituminous coal, (2) includes a number of geological formations suitable for sequestration, and (3) produces 255 million metric tons of CO2 per year (11.7% of total US emissions), 92% of which are from coal-fired power plants. (The other major sources include petroleum refineries, cement plants, and miscellaneous heavy industries.)

     The possible venues for sequestration include algae, the ocean depths, depleted oil and gas reservoirs, unmineable coal seams, and deep saline formations. Algae are most productive users of CO2 in the plant world, but the potential capacity and lifetime of this form of sequestration is difficult to estimate at the present. The algae can also be burned, which produces thermal energy with no net production of CO2. However, before it can be burned the algae must be dried, which is very energy-consumptive. Because of a lack of the necessary technical information or industrial experience this alternative was not examined in detail. The choice of the Illinois Basin as a site eliminated the possibility of sequestration in the ocean. The oil and gas reservoirs, the coal seams, and the saline formations have an estimated capacity of 282 million metric tons, 282 million metric tons, and 4.14 billion metric tons of CO2, respectively. Insertion of CO2 in the oil and gas reservoirs may enhance their recovery and in coal seams may replace methane that can be recovered, but the fractional offsets of the cost of sequestration appear to be negligibly small. In saline formations, the CO2 dissolves in part and may react in part to form carbonates, but the latter do not appear to have any economic potential. If all the electricity produced in the Illinois Basin were produced by the combustion of coal, and if the corresponding production of CO2 was all sequestered in the oil and gas reserves there they would be saturated in 1.12 years. The coal seams have an almost equal capacity, while the saline formations have a capacity of 16.2 years. This total of only 18.4 years is discouraging at first sight, but it is the extreme case and provides a window of opportunity in time to seek other solutions. Because of its greater capacity, attention was focused on sequestration in the saline formations.

     For the quantitative calculations of the combustion of coal and of the separation and sequestration of CO2, “Old Ben 26” coal and a conventional furnace and turbine capable of producing 500MW of electricity at an overall efficiency of 36% and operated at 65% capacity on an annual basis, were chosen as representative. The corresponding annual requirement of coal and production of CO2 were calculated to be 1.0 and 2.47 million metric tons, respectively. These quantities amount to approximately 1% of those for the whole basin. The flue gas produced by the combustion of this low-sulfur bituminous coal contains a significant amount of ash, CO, and N2, and traces of NOx and SO2. The ash is postulated to be removed by electrostatic precipitation, and the NOx and SO2 catalytically. However, these costs were not assigned to that of sequestration because they are required in its absence. The concentration of CO was based on the use of 25% excess air. Gaseous N2 is not ordinarily considered a pollutant but it is in terms of the sequestration of CO2. It can be almost totally avoided in the stack gas by burning the coal with oxygen rather than air, but at the price of a whole set of problems of its own, including the high cost of oxygen and the necessity of a massive recirculation of flue gas, or of some other expediency, to reduce the flame temperature. This alternative was not examined in detail. Therefore, the separation of CO2 from the flue gas was considered to be a necessary part of the process of sequestration.

     Three processes for separation of the CO2 from the resulting “clean” flue gas were examined, namely absorption in an aqueous solution of monoethylamine (MEA), adsorption by and reaction with calcium oxide  (CaO) , and adsorption by and reaction with magnesium oxide(MgO). Because the use of aqueous MEA as an absorbent for various chemicals is a well-developed technology, it was adopted as the base case. The use of CaO as an adsorbent and reactant is well developed but in the case of CO2 it has a serious shortcoming, namely the temperature of 1175K required for the decomposition of the calcium carbonate (Ca2CO3) that is formed. Magnesium oxide (MgO) was considered as a possible alternative to CaO because the decomposition of magnesium carbonate (Mg2CO3) occurs at the much lower temperature of 700K, but  the physical handling of powdered MgO was a real concern. The discovery of a DOE report by Breault and Reasbeck that suggested the impregnation of MgO on solid granules of alumina (Al2O3 ) resulted in its reconsideration, and our detailed calculations revealed the resulting process to be more economical than those utilizing MEA or CaO. These calculations were based on the recovery of 90% of the CO2  in 245 kg/s of cleansed stack gas being fed from a pipeline at 311K and slightly above 1bar.

     The MEA system is conventional except for its size, consisting of 12 identical 18-tray columns, 4.27m in diameter and 15.85m high. On the other hand the MgO system has two unique characteristics: the  preparation of the impregnated tablets and the utilization of six identical packed-bed adsorber/reactor/desorbers, 5.0m in diameter, 6.3m high, and operated in parallel in batch cycles at different stages. To produce the combined MgO /Al2O3 granules,  alumina tablets 5mm in diameter are soaked for up to 3 days in an aqueous solution of 20-80 gm/l  of magnesium hydroxide. The water of hydration is then driven off by heating, resulting in a mass ratio of MgO/Al2O3 of 1.2. The total requirement for MgO supported on alumina in the six adsorbers was estimated to be 990Mg with an annual replacement of 10% due to attrition.  Clean stack gas at 384K and 1.5bar is compressed to 14.6bar before entering a packed bed in the first step of the cycle. The adsorption and reaction are very exothermic and recirculation and cooling of a portion of the exiting gas by heat exchange water is necessary to avoid an excessive temperature. The recirculation to feed ratio is 12/5. The stream exits the adsorber/reactor at 475K with only 10% of the CO2 remaining, 90% having been converted to M2C O3. The desorption is very endothermic and the same of rate of recycling, in this case with heating by exchange with flue gas is necessary to maintain a sufficient temperature.

     Before sequestration the CO2 must be in the supercritical phase, which requires a pressure of 73.3bars at 311K. The cost of this step was based on three-stage centrifugal compression to 95.2bar with intercooling and over a distance of 167 km in a 10-in (0.254-m) pipe. The use of holding tanks in the event of a temporary shutdown of the compressors was examined but concluded to be impractical.

     The estimated costs of separation and sequestration of CO2 using the MEA and MgO/Al2O3 processes for the indicated conditions (a 500MW combustor/generator in the Illinois Basin) are 




Operating cost


Capital Investment



$ per metric ton of CO2

¢ per KWH






















     Based on the preceding analyses, it is concluded that separation using MgO is superior to that using MEA in every respect except operational experience. The fixed and variable costs are both much less and the plant equipment is more compact. Even for this process, the overall costs for capture, separation and sequestration are huge and no easy means of their reduction is apparent. Because it produces no income,  this process of sequestration can be justified only for environmental reasons or legal requirements.