The Big Sandy coal-fired power plant in Kentucky burns through 90 railroad cars of coal every day
Coal is an extremely important fuel and will remain so. According to the U.S. Department of Energy, In August 2012, some 23% of U.S. primary energy needs were met by coal and 39% of electricity is generated from coal, up from 32% in April 2012.
Coal is the world's most abundant and widely distributed fossil fuel source. The International Energy Agency (IEA) expects a 43% increase in its use from 2000 to 2020. About 70% of world steel production depends on coal feedstock.
There are 398 coal-fired power plants in operation in the United States as of August 2012. The vast majority of these plants are located east of the Mississippi River (Click Image To Enlarge)
However, burning coal produces about 12 billion tons of carbon dioxide each year which is released to the atmosphere, about 70% of this being from power generation. Other estimates put carbon dioxide emissions from power generation at one third of the world total of over 28 billion tonnes of CO2 emissions.
Development of new "clean coal" technologies is addressing this problem so that the world's enormous resources of coal can be utilised for future generations without contributing to global warming. Much of the challenge is in commercialising the technology so that coal use remains economically competitive despite the cost of achieving low, and eventually "near-zero", emissions.
As many coal-fired power stations approach retirement, their replacement gives much scope for 'cleaner' electricity. Alongside nuclear power and harnessing renewable energy sources, one hope for this is via "clean coal" technologies, such as are now starting to receive substantial R&D funding.
Managing wastes from coal
Burning coal, such as for power generation, gives rise to a variety of wastes which must be controlled or at least accounted for. So-called "clean coal" technologies are a variety of evolving responses to late 20th century environmental concerns, including that of global warming due to carbon dioxide releases to the atmosphere. However, many of the elements have in fact been applied for many years, and they will be only briefly mentioned here:
- Coal cleaning by 'washing' has been standard practice in developed countries for some time. It reduces emissions of ash and sulfur dioxide when the coal is burned.
- Electrostatic precipitators and fabric filters can remove 99% of the fly ash from the flue gases - these technologies are in widespread use.
- Flue gas desulfurisation reduces the output of sulfur dioxide to the atmosphere by up to 97%, the task depending on the level of sulfur in the coal and the extent of the reduction. It is widely used where needed in developed countries.
- Low-NOx burners allow coal-fired plants to reduce nitrogen oxide emissions by up to 40%. Coupled with re-burning techniques NOx can be reduced 70% and selective catalytic reduction can clean up 90% of NOx emissions.
- Increased efficiency of plant - up to 46% thermal efficiency now (and 50% expected in future) means that newer plants create less emissions per kWh than older ones. See Table 1.
- Advanced technologies such as Integrated Gasification Combined Cycle (IGCC) and Pressurised Fluidised Bed Combustion (PFBC) enable higher thermal efficiencies still - up to 50% in the future.
- Ultra-clean coal (UCC) from new processing technologies which reduce ash below 0.25% and sulfur to very low levels mean that pulverised coal might be used as fuel for very large marine engines, in place of heavy fuel oil. There are at least two UCC technologies under development. Wastes from UCC are likely to be a problem.
- Gasification, including underground coal gasification (UCG) in situ, uses steam and oxygen to turn the coal into carbon monoxide and hydrogen.
- Sequestration refers to disposal of liquid carbon dioxide, once captured, into deep geological strata.
Some of these impose operating costs and energy efficiency loss without concomitant benefit to the operator, though external costs will almost certainly be increasingly factored in through carbon taxes or similar which will change the economics of burning coal.
However, waste products can be used productively. In 1999 the EU used half of its coal fly ash and bottom ash in building materials (where fly ash can replace cement), and it used 87% of the gypsum from flue gas desulfurisation.
Carbon dioxide from burning coal is the main focus of attention today, since it is implicated in global warming, and the Kyoto Protocol requires that emissions decline, notwithstanding increasing energy demand.
Carbon Capture and Storage or Sequestration (CCS) technologies are in the forefront of measures to enjoy “clean coal”. CCS involves two distinct aspects: capture, and storage.
The energy penalty of CCS is generally put at 20-30% of electrical output, though since no full commercial systems are yet in operation, this is yet to be confirmed. US and European figures below suggest a small or even negligible proportion.
Table 1. Coal-fired power generation, thermal efficiency
Click Image To Enlarge
Capture & separation of CO2
A number of means exist to capture carbon dioxide from gas streams, but they have not yet been optimised for the scale required in coal-burning power plants. The focus in the past has often been on obtaining pure CO2 for industrial purposes rather than reducing CO2 levels in power plant emissions.
Where there is carbon dioxide mixed with methane from natural gas wells, its separation is well proven. Several processes are used, including hot potassium carbonate which is energy-intensive and requires a large plant, a monoethanolamine process which yields high-purity carbon dioxide, amine scrubbing, and membrane processes.
Development of CCS for coal combustion has lost momentum in the last few years, partly due to uncertainty regarding carbon emission prices.
In mid 2010 the IEA published a report says CCS was challenging, and quoting $26 billion committed in the previous two years to CCS projects. There were 80 large-scale integrated CCS projects under way, 5 of them operating. It said that “notable efforts" were being made and "increased action”, but "rapid progress is now required" if CCS is to be deployed by 2020.
Post-combustion capture
Capture of carbon dioxide from flue gas streams following combustion in air is much more difficult and expensive than from natural gas streams, as the carbon dioxide concentration is only about 14% at best, with nitrogen most of the rest, and the flue gas is hot. The main process treats carbon dioxide like any other pollutant, and as flue gases are passed through an amine solution the CO2 is absorbed. It can later be released by heating the solution. This amine scrubbing process is also used for taking CO2 out of natural gas. There is a significant energy cost involved. For new power plants this is quoted as 20-25% of plant output, due both to reduced plant efficiency and the energy requirements of the actual process.
No commercial-scale power plants are operating with this process yet. At the new 1300 MWe Mountaineer power plant in West Virginia, less than 2% of the plant's off-gas is being treated for CO2 recovery, using chilled amine technology. This has been successful. Subject to federal grants, there are plans to capture and sequester 20% of the plant's CO2, some 1.8 million tonnes CO2 per year.
Oxyfuel combustion
Where coal is burned in oxygen rather than air, it means that the flue gas is mostly CO2 and hence it can more readily be captured by amine scrubbing - at about half the cost of capture from conventional plants. A number of oxyfuel systems are operational in the USA and elsewhere, and the FutureGen 2 project involves oxy-combustion. Such a plant has an air separation unit, a boiler island, and a compression and purification unit for final flue gas.
The Integrated Gasification Combined Cycle (IGCC) plant is a means of using coal and steam to produce hydrogen and carbon monoxide (CO) from the coal and these are then burned in a gas turbine with secondary steam turbine (ie combined cycle) to produce electricity. If the IGCC gasifier is fed with oxygen rather than air, the flue gas contains highly-concentrated CO2 which can readily be captured post-combustion as above.
In China, the first phase of Huaneng Group’s $1.5 billion GreenGen project is a 250 MWe oxyfuel IGCC power plant burning hydrogen and carbon monoxide is due to commence operation by mid 2012. A second phase involves a pilot plant to produce electricity from hydrogen, as below. Phase 3 will be a 400 MWe commercial plant with CCS.
Pre-combustion capture
Further development of the IGCC process will add a shift reactor to oxidise the CO with water so that the gas stream is basically just hydrogen and carbon dioxide, with some nitrogen. The CO2 with some H2S & Hg impurities are separated before combustion (with about 85% CO2 recovery) and the hydrogen alone becomes the fuel for electricity generation (or other uses) while the concentrated pressurised carbon dioxide is readily disposed of. (The H2S is oxidised to water and sulfur, which is saleable.) No commercial-scale power plants are operating with this process yet.
Currently IGCC plants typically have a 45% thermal efficiency.
Capture of carbon dioxide from coal gasification is already achieved at low marginal cost in some plants. One (albeit where the high capital cost has been largely written off) is the Great Plains Synfuels Plant in North Dakota, where 6 million tonnes of lignite is gasified each year to produce clean synthetic natural gas.
Oxy-fuel technology has potential for retrofit to existing pulverised coal plants, which are the backbone of electricity generation in many countries.
In China, the major utility China Datang Corp is teaming with Alstom to build two demonstration CCS projects. A 350 MWe coal-fired plant at Daqing, Heilongjiang province, will be equipped with Alstom's oxy-firing technology, and a 1000 MWe coal-fired plant at Dongying, Shandong province, will use an Alstom's post-combustion capture technology, either chilled ammonia or advanced amines. The two projects are expected to be operational in 2015 and each capture over one million tonnes of CO2 per year, which would be about 40% of output from Daqing and 15% from Dongying, though Alstom says that the actual levels of capture and storage have not yet been defined and will be in the scope of the first feasibility studies of the respective projects. Adjacent oilfields will be used for sequestration, enabling enhanced oil recovery.
Storage & sequestration of carbon dioxide
Captured carbon dioxide gas can be put to good use, even on a commercial basis, for enhanced oil recovery (EOR). This is well demonstrated in West Texas, and today over 5800 km of pipelines connect oilfields to a number of carbon dioxide sources in the USA.
Despite the improving efficiency of coal-fired power stations, CO2 emissions remain a problem.
Carbon capture and storage (CCS) involves capturing the carbon dioxide, preventing the greenhouse gas entering the atmosphere, and storing it deep underground.
1. CO2 pumped into disused coal fields displaces methane which can be used as fuel
2. CO2 can be pumped into and stored safely in saline aquifers
3. CO2 pumped into oil fields helps maintain pressure, making extraction easier
At the Great Plains Synfuels Plant, North Dakota, some 13,000 tonnes per day of carbon dioxide gas is captured and 5000 t of this is piped 320 km into Canada for enhanced oil recovery. This Weyburn oilfield sequesters about 85 cubic metres of carbon dioxide per barrel of oil produced, a total of 19 million tonnes over the project's 20-year life. The first phase of its operation has been judged a success.
Overall in USA, over 6200 km of pipelines transport up to 72 million tons of CO2 per year that the oil industry uses in enhanced oil recovery, 55 Mt from natural sources, 17 Mt anthropogenic. This produces 281,000 barrels of domestic oil per day, or 6% of US crude oil production. The EOR industry has captured, transported, and injected large volumes of CO2 for oil recovery over four decades with no major accidents, serious injuries or fatalities. Present EOR technology has the potential to recover at least an additional 26 billion barrels of US oil, and improved technology could double this, while sequestering over 20 billion tonnes of CO2. The USA in 2011 set up a National Enhanced Oil Recovery Initiative (NEORI) to help realize CO2-EOR’s full potential as a national energy security, economic and environmental strategy. Its central recommendation is for a production tax credit for CO2 capture and sequestration with EOR.
The world's first industrial-scale CO2 storage was at Norway's Sleipner gas field in the North Sea, where about one million tonnes per year of compressed liquid CO2 separated from methane is injected into a deep reservoir (saline aquifer) about a kilometre below the sea bed and remains safely in place. The US$ 80 million incremental cost of the sequestration project was paid back in 18 months on the basis of carbon tax savings at $50/tonne. (The natural gas contains 9% CO2 which must be reduced before sale or export.) The overall Utsira sandstone formation there, about one kilometre below the sea bed, is said to be capable of storing 600 billion tonnes of CO2. In 2007 the Snohvit project joined Sleipner in CCS there.
Injecting carbon dioxide into deep, unmineable coal seams where it is adsorbed to displace methane (coal seam gas, effectively: natural gas) is another potential use or disposal strategy. Currently the economics of enhanced coal bed methane extraction are not as favourable as enhanced oil recovery, but the potential is large as coal seam gas is increasingly tapped.
While the scale of envisaged need for CO2 disposal far exceeds today's uses, they do demonstrate the practicality. Safety and permanence of disposition are key considerations in sequestration.
Research on geosequestration is ongoing in sevaral parts of the world. The main potential appears to be deep saline aquifers and depleted oil and gas fields. In both, the CO2 is expected to remain as a supercritical gas for thousands of years, with some dissolving.
Large-scale storage of CO2 from power generation will require an extensive pipeline network in densely populated areas. This has safety implications.
Given that rock strata have held CO2 and methane for millions of years there seems no reason that carefully-chosen chosen ones cannot hold sequestered CO2. However, the eruption of a million tonnes of CO2 from Lake Nyos in Cameroon in 1986 asphyxiated 1700 people, so the consequences of major release of heavier-than-air gas are potentially serious.
Gasification processes
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.
Gasification converts the coal to burnable gas with the maximum amount of potential energy from the coal being in the gas.
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.
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.
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.
In gasification, including that using oxygen, the O2 supply is much less than required for full combustion, so as to yield CO and H2. 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. However, the air separation plant to produce oxygen consumes up to 20% of the gross power of the whole IGCC plant system. 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.
Coal gasification plants are favoured by some because they are flexible and have high levels of efficiency. The gas can be used to power electricity generators, or it can be used elsewhere, i.e. in transportation or the chemical industry.
1. Coal burnt to produce syngas
2. Syngas burnt in combustor
3. Hot gas drives gas turbines |
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4. Cooling gas heats water
5. Steam drives steam turbines
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Economics, R&D
The World Coal Institute noted that in 2003 the high cost of carbon capture and storage (estimates of US$ 150-220 per tonne of carbon, $40-60/t CO2 - 3.5 to 5.5 c/kWh relative to coal burned at 35% thermal efficiency) made the option uneconomic. But a lot of work is being done to improve the economic viability of it, and the US Dept of Energy (DOE) was funding R&D with a view to reducing the cost of carbon sequestered to US$ 10/tC (equivalent to 0.25 c/kWh) or less by 2008, and by 2012 to reduce the cost of carbon capture and sequestration to a 10% increment on electricity generation costs. These targets now seem very unrealistic.
A 2000 US study put the cost of CO2 capture for IGCC plants at 1.7 c/kWh, with an energy penalty 14.6% and a cost of avoided CO2 of $26/t ($96/t C). By 2010 this was expected to improve to 1.0 c/kWh, 9% energy penalty and avoided CO2 cost of $18/t ($66/t C), but these numbers now seem unduly optimistic.
Figures from IPCC Mitigation working group in 2005 for IGCC put capture and sequestration cost at 1.0-3.2 c/kWh, thus increasing electricity cost for IGCC by 21-78% to 5.5 to 9.1 c/kWh. The energy penalty in that was 14-25% and the mitigation cost $14-53/t CO2 ($51-200/tC) avoided. These figures included up to $5 per tonne CO2 for transport and up to $8.30 /t CO2 for geological sequestration.
In 2009 the OECD’s International Energy Agency (IEA) estimated for CCS $40-90/t CO2 but foresees $35-60/t by 2030, and McKinseys estimated EUR 60-90/t reducing to EUR 30-45/t after 2030.
Present trends
The clean coal technology field is moving 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. 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.
This has come about as a result of the realisation that efficiency improvements, 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.
The US DOE sees "zero emissions" coal technology as a core element of its future energy supply in a carbon-constrained world. It had an ambitious 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, but this is at least postponed.
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.
Natural gas as alternative fuel
There are many advocates for the use of natural gas as an alternative to coal for electricity generation, on the grounds that it emits much less CO2 per kWh generated. This is true on almost any basis of comparison, but it ignores the global warming potential of leaked natural gas, and the CO2 emissions in transporting it as LNG (up to one third of the energy is consumed in transport). Leakage of 3% of the natural gas will bring it into approximate parity with coal-fired electricity in terms of global warming effect.
There is a range of ways of using natural gas primarily for power generation:
Central Heat and Power (CHP) - Typically burn in a combined cycle gas turbine (CCGT) for electricity, using exhaust gas to heat steam boiler to make more electricity, and finally using "the exhaust stream to heat buildings or other purposes. Thermodynamic efficiencies of 80% for this have been reported.
Combined cycle gas turbine – On its own, the best efficiency is GE's H series, which claims 60% efficiency.
Direct gas turbine - high 30's% efficiency, or straight steam boiler with about 40% efficiency (now obsolete).
All of these have potential for CCS. Methane when burned gives CO2 and water, the latter is easily separated. With high efficiencies the nitrogen proportion should be less that that with low efficiency, such as most coal.
COMMENTARY: Here's what we know so far about coal as a fuel and clean coal technology:
- Coal is a vital fuel in most parts of the world.
- Coal is the second most enironmentally polluting fuel by CO2 emissions.
- The U.S. ranks No 2 just below China in world consumption of coal.
- Burning coal without adding to global carbon dioxide levels is a major technological challenge which is being addressed.
- The most promising "clean coal" technology involves using the coal to make hydrogen from water, then burying the resultant carbon dioxide by-product and burning the hydrogen.
- The greatest challenge is bringing the cost of this down sufficiently for "clean coal" to compete with nuclear power on the basis of near-zero emissions for base-load power.
Each year energy-related carbon dioxide emissions account for more than 80 percent of greenhouse gas emissions in the United States. According to the Energy Information Association, that adds up to over 5,814 million metric tons (MMT) of carbon in 2008 alone. The Obama administration recognizes that this is not sustainable and that’s why we’ve actively sought to not only drive innovation in the renewable energy sector but also curb the emissions produced by fossil fuels such as coal, petroleum and natural gas through the development of carbon capture and storage (CCS) technologies.
In February 2010, President Obama went so far as to issue a challenge to the federal government: come up with a plan to achieve widespread, cost-effective deployment of carbon capture and storage within 10 years, with a goal of bringing five to 10 commercial demonstration projects online by 2016. This was accompanied by the creation of an Interagency Task Force on Carbon Capture and Storage, co-chaired by the U.S. Environmental Protection Agency (EPA) and the Department of Energy (DOE), which yesterday delivered a series of recommendations to the President.
Their findings, which reflect input from 14 federal agencies and departments as well as hundreds of stakeholders and CCS experts, state that CCS is viable, concluding that there are no insurmountable technical, legal, institutional, or other barriers to the deployment of CCS technology. The report also concludes that CCS can play an important role in domestic greenhouse gas (GHG) emissions reductions while preserving the option of using coal and other abundant domestic fossil energy resources.
They also noted that widespread cost-effective deployment of CCS will occur only if the technology is commercially available at economically competitive prices and we have supportive national policy frameworks, such as a cap on carbon. Already, the United States has made the largest government investment in carbon capture and storage of any nation in history and these investments are being matched by private capital. The Department of Energy is currently pursuing multiple demonstration projects using close to $4 billion in federal funds, matched by more than $7 billion in private investments, which will begin to pave the way for widespread deployment of advanced CCS technologies within a decade.
In fact in August 2010, Secretary Chu announced the selection of 15 projects to develop technologies aimed at safely and economically storing carbon dioxide in geologic formations. Funded with $21.3 million over three years, the 15 projects will complement existing DOE initiatives to help develop the technology and infrastructure to implement large-scale CO2 storage in different geologic formations across the Nation.
This announcement, in tandem with the Task Force’s set of recommendations, marks an important milestone in our efforts to mitigate the growing atmospheric CO2 emissions from human sources. These actions bring us one step closer to achieving a cleaner, greener economy.
The full report and the presidential memorandum establishing the task force can be found on the Department's site for the Interagency Task Force on Carbon Capture and Storage.
Courtesy of an article dated August 2012 written by the World Nuclear Association and an article dated November 28, 2005 appearing in BBC News
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