1. CO2 recovery technology
(1) CO2 recovery technology (for natural gas, syngas, and flue gas)
CO2 separation/recovery is widely prevalent in the natural gas and syngas fields and has been carried out for decades. The CO2 in natural gas is not only useless because it decreases the caloric value of natural gas, but also because it creates problems at LNG/ethane recovery plants by solidifying into dry ice. CO2 is hence removed to prevent such problems.
In a plant where natural gas or naphtha is reformed to manufacture H2, CO2 is separated after being converted from the CO produced with hydrogen as syngas. In an ammonia/urea plant, CO2 is separated from the gas mixture of H2, N2, and CO2 to produce urea, using H2/N2-derived synthetic ammonia and separated CO2.
Previously, however, CO2 separation/recovery from flue gas was not in such large demand, finding limited applications only in food processing and dry ice. When CO2 is separated from natural gas or syngas, the separation is relatively easy because of the high pressure of the gas. To the contrary, CO2 separation/recovery from flue gas is difficult in many technological respects due to the very low pressure of flue gas as well as the presence of oxygen, SOx, NOx, and soot and dust in the flue gas.
(2) Necessity to recover CO2 from fixed sources
Most fossil fuels (oil, natural gas, and coal) in the world are used as fuel for boilers, gas turbines, and internal combustion engines, releasing CO2 into the atmosphere as exhaust gas. As a result, it is alleged that the atmospheric concentration of CO2 has increased, resulting in global warming, which can only be prevented through the mitigation of CO2 emissions. There are, however, many difficulties in recovering and sequestering CO2 from movable bodies such as automobiles and vessels, naturally rendering it easier to recover CO2 from stationary sources, like boilers, gas turbines, etc.
(3) Characteristics and superiority of technology to recover CO2 from exhaust gas
The Kansai Electric Power Co., Inc. and Mitsubishi Heavy Industries, Ltd. began a joint R&D program in 1990 to recover CO2 from the exhaust gas of power plants and other facilities as a global-warming countermeasure. First, they assessed the conventional "monoethanolamine" (MEA) liquid absorbent-based technology considered at that time a CO2 recovery process that could save the largest amount of energy. It is a process developed by the former Dow Chemical Co. and later assigned to Fluor Daniel, Inc. This MEA-based technology was found disadvantageous for use in large plants as a measure against global warming because of such problems as the large amount of energy required for CO2 recovery and the great loss in the liquid absorbent due to its rapid degradation. Kansai Electric Power and Mitsubishi Heavy Industries started with basic research to explore a new liquid absorbent, resulting in the successful development of a novel energy-saving liquid absorbent less prone to degradation. It has already been put into practical use for the manufacture of urea in Malaysia.
2. CO2 sequestration technology
Geological sequestration and ocean sequestration are being widely studied and a commercial project of the former has already begun. Geological CO2 sequestration is being carried out, using the Enhanced Oil Recovery (EOR) method or the coal seam sequestration-accompanied Coal Bed Methane Recovery method. Aquifer sequestration and sequestration into closed oil/gas fields are options as well, if the only objective is CO2 sequestration. Figure 1 shows a conceptual view of CO2 recovery-EOR combination.
CO2-EOR commercialization started in the 1970’s mainly in the United States, enhancing oil production by approximately 200,000 barrels/day. Outside the United States, Canada, Turkey, and Hungary also utilize the CO2 recovery-EOR combination.
Underground aquifers are widely distributed on the earth wherever sedimentary layers are located. Even in Japan, where aquifers are scarce and small in structure because of Japan’s volcanic and earthquake history, surveys are underway for possible CO2 sequestration sites. If a geological underground layer has cavities, CO2 can be sequestered in these cavities, which indigenously contain water (mainly salt water), by pumping in CO2, thereby displacing the water. This is already underway in Norway. For Japan, Norway-like CO2 sequestration into aquifers distributed over continental shelves are considered the most realistic.
Other than into aquifers, CO2 can also be sequestered into closed oil/gas fields where production has already been terminated. Closed oil/gas fields once were active oil/gas fields because their geological structures did not permit oil/gas leaks. Therefore, such closed oil/gas fields are considered secure CO2 sequestration sites.
Photo 1 Conceptual view:
CO2 recovery from power plant flue gas for EOR
Source: New Energy and Industrial Technology Development Organization (NEDO), Japan Coal Energy Center (JCOAL)
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