Don't be fooled. <
'Clean Coal' Technologies, Carbon Capture & Sequestration
(Updated November 2018)
Coal is used extensively as a fuel in most parts of the world.
Burning coal produces over 14 billion tonnes of carbon dioxide each year.
Attempting to use coal without adding to atmospheric carbon dioxide levels is a major technological challenge.
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.
There is typically at least a 20% energy penalty involved in 'clean coal' processes.
World R&D on CCS exceeded $1 billion per year over 2009 to 2013, then fell sharply.
The term 'clean coal' is increasingly being used for supercritical coal-fired plants without CCS, on the basis that CO2 emissions are less than for older plants, but are still much greater than for nuclear or renewables.
Some 27% of primary energy needs are met by coal and 38% of electricity is generated from coal. About 70% of world steel production depends on coal feedstock. Coal is the world's most abundant and widely distributed fossil fuel source. However, each year burning coal produces over 14 billion tonnes of carbon dioxide (CO2), which is released to the atmosphere, most of this being from power generation.
Development of new 'clean coal' technologies is attempting to address 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 would remain economically competitive despite the cost of achieving low, and eventually 'near-zero', emissions. The technologies are both costly and energy-intensive.
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 carbon capture and sequestration, also called carbon capture and storage (both abbreviated as CCS) or carbon capture, use and storage (CCUS). It involves the geological storage of CO2, typically 2-3 km deep, as a permanent solution. However in its Energy Technology Perspectives 2014 the International Energy Agency (IEA) notes: “CCS is advancing slowly, due to high costs and lack of political and financial commitment.” In its 2016 version of the same report, the IEA reported that there were 17 large-scale CCS projects operating globally.
Consequently the term 'clean coal' is increasingly being used for supercritical and ultra-supercritical coal-fired plants without CCS, running at 42-48% thermal efficiency. These are also known as high-efficiency low-emission (HELE) plants. The capital cost of ultra-supercritical (USC) HELE technology is 20-30% greater than a subcritical unit, but the higher efficiency reduces emissions and fuel costs to about 75% of subcritical plants. A supercritical steam generator operates at very high temperature (about 600°C) and pressures (above 22 MPa), where liquid and gas phases of water are no longer distinct. In Japan and South Korea about 70% of coal-fired power comes from supercritical and ultra-supercritical plants.
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.
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
country Technology Efficiency Projected efficiency with CCS
Australia Black ultra-supercritical WC 43% 33%
Black supercritical WC 41%
Black supercritical AC 39%
own ultra-supercritical WC 35% 27%
own supercritical WC 33%
own supercritical AC 31%
Belgium Black supercritical 45%
China Black supercritical 46%
Czech Republic own PCC 43% 38%
own ICGG 45% 43%
Germany Black PCC 46% 38%
own PCC 45% 37%
Japan, Korea Black PCC 41%
Russia Black ultra-supercritical PCC 47% 37%
Black supercritical PCC 42%
South Africa Black supercritical PCC 39%
USA Black PCC & IGCC 39% 39%
USA (EPRI) Black supercritical PCC 41%
OECD Projected Costs of Generating Electricity 2010, Tables 3.3.
PCC= pulverised coal combustion, AC= air-cooled, WC= water-cooled.
Capture & separation of CO2
See Part 2 >
view the rest of the comments →
22151555? ago
Part 7 >
Rest of world
In China, the first phase of Huaneng Group’s $1.5 billion GreenGen project – a 250 MWe oxyfuel IGCC power plant burning syngas (mainly hydrogen and carbon monoxide) from coal feed – commenced operation at Tianjin in 2012 and has been fully operational since 2014. The second phase involves a pilot plant which draws about 7% of the syngas from the IGCC power plant, shifts CO and water to CO2 and H2, then separates the CO2 from the H2 after desulphurisation, and produces electricity from hydrogen. The 60,000 to 100,000 tpa CO2 is used for enhanced oil recovery. Phase 3 will be a 400 MWe commercial IGCC plant with CCS to capture up to 2 million tonnes of CO2 per year, from about 2020.
The Sinopec Shengli power plant CCS project is planned to come online in 2018, with post-combustion capture and 1 Mt/yr CO2 used for EOR.
The Uthmaniyah project in the Eastern Province of Saudi Arabia commissioned in 2015 will capture around 800,000 tonnes of CO2 per year from the Hawiyah natural gas liquids recovery plant to be injected for enhanced oil recovery (EOR) at the Ghawar oilfield.
In Australia the $240 million Callide Oxyfuel project in Queensland aims to demonstrate oxyfuel capture technology retrofitted to a 30 MW unit of an existing coal-fired power plant and to research how it might be applied to new power stations. The plant was commissioned in 2012 and was to run for an extended test period until November 2014. By mid-2013 the project had demonstrated CO2 capture rates from the oxyfuel flue gas stream to the CO2 capture plant in excess of 85%, and produced a high quality CO2 product suitable for geological storage. The project achieved more than 10,000 hours of oxy-combustion and more than 5,000 hours of carbon capture from Callide A. The plant was then decommissioned. CS Energy led the project and is working closely with an international team of partners including IHI Corporation (Japan), J-Power (Japan), Mitsui & Company (Japan), and Xstrata Coal.
Also in Australia the $150 million Delta Post Combustion Capture project hosted at Delta’s 1320 MWe Vales Point coal-fired power station in NSW aimed to demonstrate capture and sequestration of 100,000 t/yr of CO2 by 2015. However, after massive losses the plant was sold for a token sum in November 2015, with no mention of the CCS project.
Both Australian projects were funded by federal and state governments and the coal industry.
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, mostly CO, 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.
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:
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.
Overall thermal efficiency for oxygen-blown coal gasification, including carbon dioxide capture and sequestration, is about 73%. 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%.
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.
Sources:
International Energy Agency, World Energy Outlook 2018
Prime Minister's Science Engineering and Innovation Council, Australia 2002, Beyond Kyoto report
David Cain & staff, Rio Tinto, pers. comm.
Smith, D 2002, CO2 capture articles in Modern Power Systems, Nov-Dec 2002
World Coal Institute, publications on Clean Coal Technologies
Australian Coal Association (integrated into the Minerals Council of Australia in 2013)
COAL21 Fund
World Coal Institute, Sustainable Entrepreneurship: the Way Forward for the Coal Industry
International Energy Agency 2002, Key World Energy Statistics
International Energy Agency 2002, Solutions for 21st Century – Zero emissions technologies for fossil fuels
US DOE 27/2/03 announcement
US DOE NETL 21/3/03, Carbon sequestration – technology roadmap and program plan.
Gale, J., Geological storage of CO2: What do we know, where are the gaps and what more needs to be done?, Energy, Vol. 29, issue 9, pages 1329-1338 (2004)
US DOE Clean Coal Research
National Enhanced Oil Recovery Initiative (NEORI)
Michel J.H., Lost hopes for CCS – added urgency for renewable energy, Air Pollution & Climate Secretariat, Air Pollution and Climate Series 28, June 2013
International Energy Agency, Energy Technology Perspectives 2016 & 2017
Royal Academy of Engineering, CCS Forum Report, 10-12 February 2016
Carbon capture and storage a global priority, Engerati (3 August 2016)
A pathway to zero emissions from coal, World Coal Association website
END