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'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
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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. The Global CCS Institute established in 2009 and based in Australia aims “to accelerate the development, demonstration and deployment of carbon capture and storage (CCS), a vital technology to tackle climate change and provide energy security.”
In mid-2010 the IEA published a report saying that CCS was challenging, and quoting $26 billion committed in the previous two years to CCS projects.
In mid-2016 the Global CCS Institute said that there were 15 large-scale CCS projects in operation, with a further seven under construction. The total CO2 capture capacity of these 22 projects is around 40 Mt/yr. There are another six large-scale CCS projects at the most advanced ('define') stage of development planning, with a total CO2 capture capacity of around 6 Mt/yr. A further 12 large-scale CCS projects are in earlier stages ('identify' and 'evaluate') of development planning and have a total CO2 capture capacity of around 25 Mt/yr.
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.
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 but see demonstration project sections below.
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
There are three main categories of geological storage for CO2: oil and gas replacement – notably enhanced oil recovery (EOR); coal seam storage; and deep saline aquifers. The first can have direct economic benefit offsetting the cost. To 2014, 55 million tonnes of CO2 had been sequestered with monitoring. At the end of 2016, 17 large-scale operational projects had a total potential capture rate of 30 Mt CO2 per year, but only one-quarter of the captured CO2 was being stored with appropriate monitoring and verification, according to the IEA's Energy Technology Perspectives. Most of this CO2 is from natural gas processing.
The UK-based CCS Forum reported in February 2016 that in order to meet the Paris Agreement it is expected that 120-160 Gt of CO2 needs to be stored from now until 2050. The theoretical storage capacity is estimated at approximately 11,000 Gt of CO2 with 1,000 Gt provided by oil and gas reservoirs and 9,000-10,000 Gt provided by deep saline aquifers. In addition, there is significant potential capacity in unmineable coal seams. At the 120-160 Gt by 2050 level, there is enough storage capacity for global CCS needs to be met well beyond the next century.
Enhanced oil recovery
Captured carbon dioxide gas can be put to good use, even on a commercial basis, for enhanced oil recovery (EOR), and a majority of CCS projects are oriented thus. 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. The CO2 acts to reduce the viscosity of the oil, enhancing its flow to recovery wells. It is then separated and re-injected.
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.
Chevron’s Rangely project in the Rocky Mountain area injects 3 million tonnes of CO2 per year supplied by pipeline for EOR in sandstone formations 1800 m deep.
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