‘Carbon sequestration’ - Greatest challenge of clean coal technology (‘FutureGen’ project) to deliver "zero emissions" in reality:

‘Carbon sequestration’ - Greatest challenge of clean coal technology (‘FutureGen’ project) to deliver "zero emissions" in reality:

A present trend of clean coal technology is moving rapidly towards a very interesting phase, realizing efficiency improvements of coal. In fact, this clean coal technology together with the use of natural gas and renewables such as wind will not provide the deep cuts in greenhouse gas emissions necessary to meet future national targets. Naturally, a plant to produce hydrogen from coal and sequester emissions will be the world’s zero emission coal-fired plant – as envisaged for ‘FutureGen’ project.

As discussed earlier, the clean coal technology field is moving in the direction of coal gasification with a second stage so as to produce a concentrated and pressurized carbon dioxide stream followed by its separation and geological storage. This technology has the potential to provide what may be called "zero emissions" - in reality, extremely low emissions of the conventional coal pollutants, and as low-as-engineered carbon dioxide emissions.

A. The greatest challenge now is to sequester emissions by carbon capture and geological storage technology. The technology, known as carbon sequestration, has attracted global attention from industries and governments that are eager to capture and bottle up the gas that can linger in the atmosphere for decades.

B. Carbon capture and sequestration begins with the separation and capture of CO₂ from power plant flue gas and other stationary CO₂ sources. At present, this process is costly and energy intensive, accounting for the majority of the cost of sequestration. However, analysis shows the potential for cost reductions of 30–45 percent for CO₂ capture. Post-combustion, pre-combustion, and oxy-combustion capture systems being developed are expected to be capable of capturing more than 90 percent of flue gas CO₂.

C. The primary function of carbon sequestration research and development (R&D) objectives are:

(1) Lowering the cost and energy penalty associated with CO₂ capture from large point sources; and

(2) Improving the understanding of factors affecting CO₂ storage permanence, capacity, and safety in geologic formations and terrestrial ecosystems.

D. After capturing of carbon the next step is to sequester (store) the CO_2; which has mainly two processes - (i) The primary means for carbon storage are injecting CO₂ into geologic formations or (ii) using terrestrial applications.

(i) Geologic sequestration involves taking the CO₂ that has been captured from power plants and other stationary sources and storing it in deep underground geologic formations in such a way that CO₂ will remain permanently stored. Geologic formations such as oil and gas reservoirs, unmineable coal seams, and underground saline formations are potential options for storing CO₂. Storage in basalt formations and organic rich shales is also being investigated.

(ii) Another form of sequestration is ‘terrestrial sequestration’, which involves the net removal of CO₂ from the atmosphere by plants and microorganisms that use CO₂ in their natural cycles. Terrestrial sequestration requires the development of technologies to quantify with a high degree of precision and reliability the amount of carbon stored in a given ecosystem.

E. Any carbon sequestration program should involve (a) Core R&D, and (b) Demonstration & Deployment.

(a) Core R&D – Core R&D accomplished through laboratory and pilot-scale research, develops new technologies and systems for reducing greenhouse gas emissions from industrial sources. Core R&D integrates basic research and computational sciences to study advanced materials and energy systems. It focuses on few major areas for technology development: (i) CO₂ Capture, (ii) Carbon Storage, (iii) Monitoring, Mitigation, and Verification, (iv) Non-CO₂ Greenhouse Gas Control, and (v) Breakthrough Concepts.

(b) Demonstration & Deployment – It speeds the development of new technologies through commercial opportunities and collaboration with Govt. departments. Core R&D scientists also learn practical lessons from these demonstration projects and are helpful to develop further technology solutions and innovations.

As mentioned above, this system along with use of natural gas and renewable energy sources such as wind, solar etc., will be advantageous in order to mitigate to a great extent in greenhouse gas emissions necessary to meet future national targets. Many countries see "zero emissions" coal technology as a core element of its future energy supply in a carbon-constrained world. They have program to develop and demonstrate the technology and have commercial designs for plants with an electricity cost of only 10% greater than conventional coal plants available by 2012. Australia is very well endowed with carbon dioxide storage sites near major carbon dioxide sources, but as elsewhere, demonstration plants will be needed to gain public acceptance and show that the storage is permanent. In general, "zero emissions" technology seems to have the potential for low avoided cost for greenhouse gas emissions.

‘FutureGen’ project - to design, build and operate a nearly emission-free coal-based electricity and hydrogen:

‘FutureGen’ project - to design, build and operate a nearly emission-free coal-based electricity and hydrogen:

The clean coal technology field is moving very rapidly in the direction of coal gasification with a second stage so as to produce a concentrated and pressurised carbon dioxide stream followed by its separation and geological storage. At present the high cost of carbon capture and storage renders the option uneconomic. But a lot of work is being done by many of the research institutes, to improve the economic viability of this system.

More recently department of energy (DOE) of Federal Govt. of the USA has announced ‘FutureGen’ project to design, build and operate a nearly emission-free coal-based electricity and hydrogen production plant. It will use cutting-edge technologies to generate electricity while capturing and permanently storing carbon dioxide deep beneath the earth. The integration of these technologies is what makes FutureGen unique. Researchers and industry have made great progress advancing technologies for coal gasification, electricity generation, emissions control, carbon dioxide capture and storage, and hydrogen production. But these technologies have yet to be put together and tested at a single plant - an essential step for technical and commercial viability.

Therefore, the FutureGen initiative would have comprised a coal gasification plant with additional water-shift reactor, to produce hydrogen and carbon dioxide. About one million tones of CO₂ would then be separated by membrane technology and sequestered geologically. The hydrogen would have been be burned in a power generating plant and in fuel cells. The project was designed to validate the technical feasibility and economic viability of near-zero emission coal-based generation. Construction of FutureGen was due to start in 2009, for operation in 2012.

Coal gasification processes –

(a) In conventional plants coal, often pulverised, is burned with excess air (to give complete combustion), resulting in very dilute carbon dioxide at the rate of 800 to 1200 g/kWh.

(b) Gasification converts the coal to burnable gas with the maximum amount of potential energy from the coal being in the gas.

(c) In Integrated Gasification Combined Cycle (IGCC) the first gasification step is pyrolysis, from 400°C up, where the coal in the absence of oxygen rapidly gives carbon-rich char and hydrogen-rich volatiles.

(d) In the second step the char is gasified from 700°C up to yield gas, leaving ash. With oxygen feed, the gas is not diluted with nitrogen.

(e) The key reactions today are C + O₂ to CO, and the water gas reaction: C + H₂O (steam) to CO & H₂ - syngas, which reaction is endothermic.

(f) In gasification, including that using oxygen, the O₂ supply is much less than required for full combustion, so as to yield CO and H₂.

(g) The hydrogen has a heat value of 121 MJ/kg - about five times that of the coal, so it is a very energy-dense fuel.

(h) However, the air separation plant to produce oxygen consumes up to 20% of the gross power of the whole IGCC plant system.

(i) This syngas can then be burned in a gas turbine, the exhaust gas from which can then be used to raise steam for a steam turbine, hence the "combined cycle" in IGCC.

(j) To achieve a much fuller clean coal technology in the future, the water-shift reaction will become a key part of the process so that:

(i) C + O₂ gives CO, and

(ii) C + H₂O gives CO & H₂, then the

(iii) CO + H₂O gives CO₂ & H₂ (the water-shift reaction).

(k) The products are then concentrated CO₂ which can be captured, and hydrogen. (There is also some hydrogen from the coal pyrolysis), which is the final fuel for the gas turbine.

(k) Overall thermal efficiency for oxygen-blown coal gasification, including carbon dioxide capture and sequestration, is about 73%.

(l) Using the hydrogen in a gas turbine for electricity generation is efficient, so the overall system has long-term potential to achieve an efficiency of up to 60%.