CO2 Capture Pathways
As shown in the following diagram, there are several pathways for the capture of CO2 from carbon-based energy conversion systems.
Chemical processes, such as upstream natural gas conditioning and the manufacturing of ammonia/urea and hydrogen, separate nearly pure CO2 streams from the process. However in absence of any incentive, the unused CO2 is vented to atmosphere.
Cement manufacturing requires large quantities of fuel to drive high temperature, energy-intensive reactions associated with the calcination of the limestone – that is, calcium carbonate being converted to calcium oxide with the evolution of CO2.
Globally, the emissions from the cement industry account for 6% of total emissions of CO2 from stationary sources. In 2002, GHG emissions from cement production contributed an estimated 10.2 Mt (or 1.4%) to Canada's national GHG emission total.
At present, CO2 is not captured from cement plants, but possibilities do exist. The concentration of CO2 in the flue gases is between 15-30% by volume and this makes the process amenable to CO2 capture.
Iron and steel production
Like cement manufacturing discussed above, the concentration of CO2 from steel plants is higher than gas- or coal-fired power plants due to process related emissions. According to recent estimates, using available capture technologies, this sector alone can help reduce global CO2 emissions by 4% (Gielen, 2003)
Large quantities of hydrogen are widely used in petroleum refining, ammonia synthesis and in the upgrading of raw bitumen extracted from the oil sands in the Western Canadian Sedimentary Basin (WCSB). Production of refined petroleum products from oil sand bitumen requires 5-10 times the amount of hydrogen compared to conventional crude. With the projected expansion of oil sands operations in WCSB, hydrogen demand for the oil sand sector alone is likely to quadruple to 56 m3/day by 2010 (Keith, 2002; Thambimuthu, 2003). This will be equivalent to 20% of current world production of H2 for refining applications. This scenario is likely to place Alberta as the world’s largest concentration of hydrogen plants and possibly an attractive opportunity for low cost CO2 capture.
The refinery is essentially a carbon/hydrogen manipulator, tailoring and reshaping molecules and boiling ranges to meet the production needs of particular fuels. All emissions from the refinery itself originate from the feedstocks used. These feedstocks are mainly crude oil(s) to be processed plus other imported feedstocks such as natural gas for steam or hydrogen plants.
CO2 emissions from refineries are dominated by fossil fuel-fired heaters (~44%). In practice, refineries have a large number of process heaters within the plant area. These heaters emit flue gases with CO2 concentrations of 4-14% depending upon the fuel used. This makes CO2 capture difficult, extremely expensive or even impractical. However, there is the potential for capture of CO2 produced from the power generation (~13%), the hydrogen production (~20%) and utilities (~13%) facilities within the refinery complex. These facilities are responsible for more than half the total refinery CO2 emissions.
Electric Power Generation
For electric power generation, there are essentially three pathways for CO2 capture. They are: post combustion, pre-combustion, and oxy-fuel combustion.
CO2 is the end product from the combustion of any carbonaceous fuel. Capture of CO2 in the downstream of a combustion unit is referred as the post-combustion capture system. Conventional air-fired process heaters and industrial and utility boilers fit into this category. In these systems, the fossil fuels are combusted in excess air, resulting in a flue gas stream which contains low concentrations of CO2 (10-15 v/v% for modern coal fired power plants and 5-8 v/v% for natural gas fired plants). In some cases, such as cement kilns and blast furnaces where flue gases contain process related CO2 as well as fuel related CO2, the CO2 concentration in the flue gases may vary from 14-33%. CO2 from the post combustion flue gases can be captured by a variety of techniques such as absorption by amines, membrane separation and cryogenic separation. With the current state of technology, only absorption and to some extent membranes are considered to be economically viable technologies.
The term "polygeneration" is used to describe operations which generate multiple products from the same feed fuel of feedstock, in this case coal. As described previously, the gasification of coal produces what is called syngas, which consists mainly of carbon monoxide and hydrogen. The syngas can be used like natural gas to efficiently generate electricity through an integrated gas turbine/steam turbine combined cycle (IGCC). Syngas can also be used as a feedstock for manufacturing chemicals and synthetic fuels, e.g., ammonia and methanol. Through the Fischer-Tropsch process, syngas can be converted into gasoline or diesel fuel.
Thus, a polygeneration system can be more efficient in the conversion of energy and can produce useable, high value products from what otherwise would have been waste streams.
In the pre-combustion capture system, carbonaceous fuels are converted to syngas (from synthesis gas - a mixture of predominantly carbon monoxide, CO, and hydrogen, H2) through gasification, partial oxidation or steam reforming.
Next, the CO is converted to CO2 through the water gas shift conversion process:
CO + H20 --> CO2 + H2
The concentration of CO2 in this stream is around 25-40% and the total pressure is typically in the range of 2.5 - 5 MPa. Thus, the partial pressure of CO2 in the pre-combustion process is very high compared to conventional combustion systems. This makes it easier to separate the CO2 by techniques such as scrubbing through physical solvents. The CO2 can then be used or disposed of.
The H2 can be used as a feedstock in a chemical plant, or it can be combusted in a gas turbine to produce electricity. Because the CO2 is captured prior to the utilization of the hydrogen product, the system is classified as a “pre-combustion” CO2 capture procedure.
The atmospheric oxy-fuel process
As oxy-fuel combustion inherently produces a flue gas stream that is concentrated in CO2, the CO2 can readily be captured without the use of post combustion solvents. Therefore, oxy-fuel technology is neither a "pre-combustion" nor a "post-combustion" process. Consequently, it is generally classified as a "combustion" process.
Oxy-fuel combustion is widely used in glass and metal industries where very high temperatures are required for the process. Compared to the glass and metal industries, oxy-fuel combustion for power generation and other large industrial boilers is a relatively a new approach.
The approach consists of separating O2 from air and using the O2 as the oxidant for fossil fuel combustion. With the nitrogen removed (78% of the air) from the combustion process, the result is a highly concentrated flue gas stream composed mainly of CO2 (> 80 v/v%) water vapour, and other minor amounts of contaminants, such as particulates, NOx, SOx and trace elements. The contaminants can be easily removed, to further concentrate the CO2, through physical gas purification techniques, such as cryogenic separation.
Since the combustion takes place in an O2/CO2 environment, this variant of the oxy-fuel technology is sometimes called O2/CO2 recycle combustion.
Several variants of oxy-fuel combustion systems have been proposed and are under development for retrofit or for new applications in power plants. One variant is the O2/CO2 recycle process. In this process, CO2 is recycled to the combustor to control the flame temperature. This is currently the most advanced oxy-fuel combustion approach and has attracted the interest of industry for applications in conventional furnaces, process heaters and power plants.
Pressurized Oxy-fuel Combustion
With pressurized oxy-fuel combustion, the increased system pressure enables use of gas-to-liquid steam-hydroscrubbing to collect and remove pollutants and to recover latent heat from water entrained or produced in the combustion process. The pressurized oxy-fuel approach enables CO2 to be recovered as a pressurized liquid through direct condensation, and it delivers the captured CO2 product as compressed liquid or solid (dry ice) ready for beneficial use or sequestration.
Increasing the pressure of combustion shifts the temperature at which water, CO2, mercury and acid gases condense. The elevated pressure and condensation temperature process conditions enhance the heat transfer, mass transfer and liquid vapor equilibrium regimes which are suited to capture of pollutants and CO2. Elevating the pressure enables the use of the phenomenon of nucleate condensation in a heat exchanger that simultaneously recovers heat and condenses and captures pollutants.
Pressurized oxy-fuel combustion eliminates energy lost in the exhaust nitrogen by eliminating the nitrogen and recovers the latent heat of vaporization of both the produced and entrained water. The process enables the condensing heat exchanger to collect particulates, acid gases and mercury into a condensed phase that is smaller than the volume of gas treated by conventional atmospheric pressure flue gas clean-up systems.
For more information on CO2 capture pathways and clean coal technologies click here.
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