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Carbon Capture from Burning Coal - What's New? Why is it so Difficult?
One of the biggest challenges worldwide is to reduce, capture and store the carbon dioxide emitted from burning coal for power generation.
Coal is, and is likely to remain, the major energy source worldwide for the foreseeable future. Humans have the capacity to only act when it is too late, or almost too late, and to be reluctant to make major phase shifts to new technology. It is better to patch or adapt the old methods of generating power rather than making the fundamental changes.
Carbon capture and storage is a classic example of this - coal can be burnt as before, as long as we capture and remove the carbon dioxide from the smoke stacks and lock it away in somewhere safe.
Where are we Now with Carbon Capture and Storage Technologies?
This topic can be subdivided into four sub-topics:
- Modification of the coal before it is burnt
- Removal of nitrogen from the air supplied to burn the coal
- Capturing the carbon
- Storing the carbon
1. Modification of the coal before it is burnt (Pre-combustion processes)
The so called 'Pre-combustion' process involves gasifying the fuel under pressure to form a mixture of H2 and CO2 known as synthesis gas (“syngas”), prior to combustion.
The fuel produced is hydrogen. CO2 concentrations are typically 35-40%. This technology already widely applied in fertilizer, chemical, gaseous fuel (H2, CH4), and power production. The resulting CO2 need to be removed before combustion takes place.
Various polymeric and hollow fibre materials are already used to separate CO2 from natural gas at the well head. Membranes that are permeable to hydrogen generally essentially work as molecular sieves, with pore sizes small enough to only allow hydrogen molecules to pass through.
These can be based on carbon nanotubes a zeolite matrix or other substrates. Other options use the high affinity of vanadium and palladium metals towards hydrogen for the filtering.
Membranes for CO2 filtering generally work by relying on selective binding of the membrane material itself.
Multi walled carbon nanotubes and ion-exchange materials such as zeolites, silica-zirconia and have been tried for these membranes.
Research and Development
- Duke Energy in the US is developing a coal-fired plant in Edwardsport, Indiana, that will transform the coal into a gas composed of a mixture of carbon monoxide and hydrogen.
- The Taylorville Energy Center has a plant with a process to transform coal into natural gas and then to burn the natural gas.
- Tenaska, is developing a plant that will convert coal into methane. Depending on market conditions, the plant will either burn the methane in a combined-cycle power plant, or send the methane to a nearby natural gas pipeline for use in the grid.
The ZECA Process is one of the upcoming power generation technologies of the future with a baseline efficiency is 68.9%. The ZECA Process is characterised by the following chemical equations ( s = solid; g = gas ):
1. C (s) + 2H2 (g) => CH4 (g) + Heat
2. CH4 (g) + 2H2O (g) + CaO (s) => CaCO3 (s) + 4H2 (g)
3. 2H2 (g) + O2 (g) => 2H2O (g) + heat + electricity
4. CaCO3 (s) + Heat=> CaO (s) + CO2 (g) (regenerates the CaO)
The process begins when coal is gasified using hydrogen, in the Gasification Reactor, which also releases light hydrocarbons and methane which are then passed to the next reactor - the carbonation reactor. There the gases react with steam and yield carbon dioxide which is absorbed by calcium oxide (lime) converting it to calcium carbonate. The calcium oxide is regenerated using the waste heat from the fuel cell. For every mole of Carbon, from coal that is gasified, 2 moles of hydrogen gas are consumed and 4 moles of hydrogen gas are generated in the Carbonation Reactor. There is a net gain of 2 moles of hydrogen gas per mole of Carbon consumed.
2. Removal of Nitrogen from the air supplied to burn the coal (oxy-fuel combustion)
There have been major developments in this area with the US Energy Department recently announcing a major shift in its strategy towards this idea. The government announced it would contribute $737 million to modify an obsolete plant in Meredosia, Illinois.
The newly designed plant would be fed pure oxygen. When the coal was burnt the emission would be almost pure carbon dioxide. It appears that it is much easier to do it this way than use air for the combustion and have to remove the 20% carbon from the emitted exhaust emitted from conventional plants which is 80% Nitrogen.
The waste carbon dioxide from the plant would be piped about 170 miles east to Mattoon and dumped underground. This project was substituted for a plant in Mattoon, Illinois, which would have converted coal into gaseous hydrocarbon, filter out the carbon and in turn burning the hydrogen. Some of the oxygen will be provided by the energy company Air Liquide, which uses conventional cryogenic techniques, cooling the air to very low temperatures until the oxygen turns to a liquid at 297 degrees below zero (Fahrenheit).
The energy and costs required to generate large volume of oxygen that has always been considered a fault with this approach.
Newer membrane technologies (discussed above) are being examined which would be much cheaper. Various Polymeric and hollow fibre materials are already used to separate CO2 from natural gas at the well head.
Chemical Looping Combusion - which uses a metal oxide to bind and carry the oxygen. The particles of metal oxide react in a fluidized bed combustor with a liquid, solid or gaseous fuel. This produces solid metal particles and a mixture of water vapour and carbon dioxide. The water vapour is cooled and condensed, leaving behind the carbon dioxide, which can be piped away and sequestered. The solid metal particles are transferred to another fluidized bed where they combine with air, generating heat and re-generating the metal oxide particles which are in turn returned to the fluidized bed combustor. Another type of chemical looping is 'calcium looping', which uses calcination of a CaO based carrier as a means of capturing CO2.
3. Capturing the carbon from smoke stacks (post combustion capture)
- Amine based solvents - Currently, carbon dioxide is removed on a large scale by absorption of carbon dioxide into various amine-based solvents. To scrub the CO2 the cooled flue gas is bubbled through the solvent in the absorber. CO2 is bound to the solvent in the absorber. The flue gas is then washed with water to remove any solvent droplets before leaving the absorber. The solvent containing the bound CO2 is then transferred to the top of a stripper which removes the CO2 and returns the solvent to be recycled back to the stack. The CO2 is piped away for sequestration. Various modified amine-based solvents are being trialled, which are highly effective for CO2 capture.
- Another long-term option is carbon capture directly from the air using hydroxides.
- Other options involve conversion into baking soda.
- Aqueous Ammonia - This process is based on an ammonium carbonate solution with which CO2 reacts to form the bicarbonate from which the CO2 can be extracted and piped away. A new cooled-ammonia process will require only about 10 percent of the power of the plant. In this process, the flue gas is cooled down to about 5 degrees C, which increases the concentration of carbon dioxide and condenses out the water. The water is drained away along with other unwanted substances such as sulfur dioxide. The remaining flue gas is almost pure CO2, which can be absorbed by the ammonia.
- Various solid adsorbents are being trialled, which bind carbon dioxide on their surfaces. These include porous crystalline materials called metal-organic frameworks (MOFs) and solid amine–based adsorbents. These solid-amine adsorbents have amine polymers attached to a silica substrate.
- Hot Carbonate Absorption Process (CAP) uses carbonate salt (sodium and potassium carbonate) as a solvent for CO2 capture.
4. Storing the carbon
- Geological sequestration - The method of geo-sequestration or geological storage involves pumping carbon dioxide down into underground formations such as gas fields, oil fields, saline aquifer formations, unminable coal seams, and underground saline-filled basalt formations. Various physical, chemical and geo-chemical trapping mechanisms prevent the CO2 from leaking back to the surface. Old mines and caverns, that are commonly used to store natural gas, are not considered because of a lack of storage safety.
- Other options are liquid storage deep in the ocean, and solid storage after reacting the CO2 with metal oxides and producing stable carbonates.
- At the Australian CO2CRC demonstration project in Western Victoria about 100,000 tonnes of carbon dioxide is being injected and stored in a depleted natural gas reservoir 2km underground. More than 50,000 tonnes have been stored there so far, without any signs of leakage.
- Cement production is another option for capturing CO2. Globally, about 5% of CO2 emissions is current produced during the manufacture of cement. The process of turning carbon into cement involves using sea water, which is split via electrolysis to make solutions of sodium hydroxide and hydrochloric acid. The acid is then reacted with silicate rocks, generating magnesium chloride and sand. The sodium hydroxide solution can be used to trap the carbon dioxide issuing from industrial smokestacks, producing sodium bicarbonate, which when added back to sea water rich in magnesium and calcium ions, leaves a precipitate of calcium carbonate and magnesium carbonate. These two by-products can be used in making cement.
- Biological Carbon Sinks - A carbon sink is an artificial or natural store that builds-up and stores carbon-based chemical compounds, including carbon dioxide itself, for an indefinite period. The major natural sinks for carbon are: Absorption of carbon dioxide by the oceans via physicochemical and biological processes; Photosynthesis by terrestrial plants in forests and vegetation and Storage in Soils and biomass.
- Conversion of the CO2 to bicarbonates (using limestone) or in solid clathrate hydrates which already existing on the ocean floor.
© 2010 Dr. John Anderson