A pollution-free coal based electric power plant – Futuristic view and Skepticism:

A pollution-free coal based electric power plant – Futuristic view and Skepticism:

Coal is the workhorse of global electric power sector and is used to generate more than half of the electricity world consumes. It is also world’s most abundant fossil fuel, with supplies projected to last almost 250 years or more. As coal-fired power plants generally produce the lowest-cost electricity and coal is abundant, most of the country’s economic and energy security depend on the continued use of the fuel. But most disadvantage part of this fuel is: coal is an inherently dirty fuel; it contains more pollutants than oil or gas, and burning coal or any fossil fuel releases the pollutants into the atmosphere. Fossil-fuel combustion also releases carbon dioxide, the primary greenhouse gas that causes global warming.

A. One way to clean the electric power generation system is to separate out carbon at the point of combustion and capture and isolate it in a process known as sequestration. Sequestration techniques now under study range from injecting CO2 underground or deep into the ocean to having forests absorb the gas. One objective of the, much talked about, ‘FutureGen’ project is to explore the feasibility of several of these techniques. High cost of sequestration technology is the most intriguing factor, at present. As per the estimation of DOE (Department of Energy of Federal Govt. of US) the cost of sequestration using existing technologies is in the range of $100 to $300/ton of carbon emissions. A goal of the FutureGen program (FutureGen's developers - an alliance of a dozen big power and coal companies such as American Electric Power Inc., BHP Billiton, Consol Energy Inc., Foundation Coal Corp., Peabody Energy Corp. and Rio Tinto Energy etc.) is to employ advanced technologies to reduce that figure to $10 or less by 2015.

B. Facts about the program – a discussion:

(i) Coal gasification is a mature technology in the chemicals industry, which uses the process to create feedstock for ammonia and other chemicals and fine chemicals.

(ii) For generating electricity, this technology has not yet been fully used by industry. In fact, the most economical way to make hydrogen is from methane natural gas.

(iii) In case of extracting hydrogen from coal, 30% of fuel’s latent energy is lost.

(iv) It is expected to initially capture at least 90% of the CO2 that system produces; adding that advanced technologies could eventually achieve nearly 100% capture.

(v) Once captured, the CO2 will be injected deep underground.

(vi) Potential graves include saline aquifers thousands of feet below the surface, depleted oil or gas reservoirs, and unmineable coal seams or basalt formations.

(vii) Once buried, the greenhouse gas would have little chance of escaping into the atmosphere.

C. Skepticism about the program – a discussion:

(i) It's easier to eliminate the pollutants in coal such as nitrogen oxides (NOX) and sulfur dioxide (SO2) at the front end than at the back, where they end up dispersed in flue gas.

(ii) The plan is to clean SO2 and NOx from the coal gases and convert them to byproducts such as fertilizers and soil enhancers.

(iii) Mercury also would be removed, and CO2 would be captured and sequestered in deep, underground geologic formations.

(iv) People are coming to the realization that making sequestration work is going to be very expensive and efficiency of removal of SO2 and NOx.

(v) There was no guarantee of carbon sequestration technology that rock formations, destined for the carbon, would seal-in the offending material.

(vi) We may have to spend billions and billions of dollars chasing technology that, even when perfected, is not nearly as perfect as the renewable energy (such as wind, solar, geothermal) in creating energy that gives us the added benefit of saving our environment.

‘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%.