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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

See Part 2 >