IEA Clean Coal Centre

What are clean coal technologies?

Up until the late 20th Century the term 'clean coal' was used colloquially to describe what was otherwise known as 'smokeless coal'.  The use of anthracite and high-grade bituminous coals resulted in significant reductions in the production of smoke and associated particles from burning coal for primarily domestic applications.

 The earliest use of the term with its modern meaning can be traced back to U.S. Senate Bill 911 in April, 1987:

'The term clean coal technology means any technology…deployed at a new or existing facility which will achieve significant reductions in air emissions of sulphur dioxide or oxides of nitrogen associated with the utilisation of coal in the generation of electricity.'

The scope of clean coal technologies has expanded in recent years. The term now includes any technology which reduces emissions of SOx, NOx, mercury, particulates or other pollutants arising from the use of coal. It includes efficiency measures which will reduce emissions of CO2 and carbon capture and sequestration technologies.

·                     Sulphur oxides (SOx)

In the 1970s, the focus was on sulphur and nitrogen oxides. When released from coal-fired plant, these combine with water vapour in the air to form dilute acids that can fall to earth as ‘acid rain’. Initially, some coals were cleaned or washed prior to combustion to help minimise sulphur oxide emissions. However, although these techniques can remove much of the mineral matter (ash) present, typically, they only remove about a quarter of the sulphur.  In the UK, environmental legislation introduced during the 1970s helped drive forward the development and application of more effective systems for removing sulphur species from coal-fired power plants.

Although the first commercial sulphur control systems were developed and installed in Britain during the 1930s (at Battersea and Fulham in London and Swansea in Wales), it was not until much later that ‘flue gas desulphurisation (FGD) units’ were applied to coal-fired plants. The first commercial-scale units were installed in several parts of the world during the late 1960s. This was followed by a major burst of activity during the 1970s. In the USA and elsewhere, this was driven by the introduction of environmental legislation. Since then, a wide range of technologies have been developed to control sulphur emissions.

FGD units are essentially large towers in which aqueous mixtures of lime or limestone sorbents are sprayed through the plant’s flue gas, absorbing the sulphur species (largely as SO2) present. FGD units or ‘scrubbers’ can reduce sulphur emissions by 90% or more. Today, many coal-fired plants deploy some form of sulphur control system. Their numbers continue to grow and it has become standard practice to install some form of FGD unit to both existing and new coal-fired plant in many countries. 

·                     Nitrogen oxides (NOx)

Nitrogen oxides produced when coal is burnt contribute to acid rain and can also form harmful levels of ozone and reduce visibility.

Starting in the mid 1960s, a succession of ‘low NOx burner’ designs were developed, a process that continued throughout the 1970s and 80s. Engineers developed and tested new types of coal burners that fired coal in stages and regulated the amount of oxygen in the stages where combustion temperatures were the highest (‘staged combustion’). This led to the development of special low NOx burners, capable of reducing NOx levels by 40 to 60%. These have since been refined further and retrofitted to many power stations throughout the world; the numbers are growing. For instance, around three quarter of all coal-fired power plants in the USA now employ low NOx burners. There are also a number of ‘combustion modifications’ that have been developed. These include a number of techniques (Overfire Air, Reburning, Flue Gas Recirculation, and Operational Modifications) that may be used on their own or in conjunction with other emissions control systems.

In recent years, increasingly stringent environmental legislation in many parts of the world has driven efforts to reduce NOx levels even further. This has led to the development and application of newer systems such as Selective Catalytic Reduction (SCR), where flue gases are treated with anhydrous ammonia, which converts NOx present into harmless nitrogen and water. SCR systems for large stationary sources, such as utility boilers and large industrial facilities, were first commercialised in Japan as a response to stringent air quality standards and emission regulations brought into force in the 1970s. In Europe, some countries (such as Germany) introduced NOx emission regulations during the 1980s. At present, hundreds of SCR systems are operating successfully in Japan, Europe, the USA and several Asian countries.

Some power plants also inject ammonia (or urea) directly into the coal furnace. This is termed Selective Non-Catalytic Reduction (SNCR). Both types of system are capable of significantly reducing levels of NOx released to the atmosphere. In Western Europe, SNCR systems have been used commercially in coal-fired power plants since the end of the 1980s, and in the USA, since the early 1990s.

·                     Particulates

The first electrostatic precipitator (ESP) was introduced in 1923 to reduce soot emissions. This used electrical fields to remove particulate matter from a boiler's flue gas, much in the way that static electricity causes dust to cling to certain types of materials. During the 1930s, several units were installed on German power plants, reducing emissions by 75-80%. Post-war, in many countries where coal use was significant, it gradually became standard practice to fit ESP systems to power plants.  Later, ‘baghouses’ were introduced; these operate like large-scale vacuum cleaners, capturing dust particles in felt or woven fabric bags. Both baghouses and ESPs are capable of capturing 99% or more of the particulates in the flue gas. Today, many coal-burning power plants throughout the world employ one or even both of these devices.

 ·                     Mercury

Mercury is only present in minute quantities in coal, but when released during combustion, it enters the environment, potentially creating a human health problem.  In some cases, FGD units, ESPs or baghouses can help reduce mercury emissions, although their effectiveness is not universal. At the moment, new ways to minimise mercury emissions are being developed. Several approaches are being pursued. In some cases, existing pollution control devices are being modified, and in others, novel systems based on the application of special sorbents (such as activated carbon) are being trialled. 

• Carbon capture and storage (CCS)

Carbon dioxide (CO2) emissions are the latest in the line of challenges that coal has faced over the years and many new techniques for its control are currently being developed and tested. These encompass different approaches aimed at mitigating global warming by capturing carbon dioxide from large point sources such as power plants, and subsequently storing it instead of releasing it into the atmosphere. A key to successful carbon capture and storage will be to find affordable ways to separate carbon dioxide from the flue gases of coal-fired plants. Development efforts over the past decade have led to a new family of promising carbon capture technologies. These include techniques that potentially can be applied to conventional combustion systems such as post-combustion capture, as well as to more advanced systems such as oxyfuel combustion and Integrated Gasification Combined Cycle (IGCC) plants. 

Post-combustion capture

One of the most established methods in the chemical industry for CO2 capture from gases at low pressure is solvent scrubbing using chemically active agents that are regenerated by heating. Aqueous solutions of amines (such as MEA and MDEA) have been proven for use on other gases and are gradually being demonstrated for use for CO2 capture from power station flue gas. In the USA, there are several coal-fired plants that use amine-based scrubbers to capture CO2 from their flue gases and use it for food processing, freezing, beverage production and chilling purposes. However, a plant fitted with a CO2 scrubber will have a reduced thermal efficiency because of the energy needed for solvent regeneration and for liquefaction and compression of the CO2. There are many projects underway around the world, developing and testing new technologies and solvents that could reduce this energy penalty.

Oxyfuel combustion

Oxyfuel combustion involves the combustion of coal in a mixture of oxygen and recycled flue gas instead of air. The CO2-rich gases from the boiler are cooled, condensate removed, the recycle stream returned and the balance of CO2 taken away for storage. The system could be aplied to PCC or CFBC plants. Oxyfuel combustion for power generation has reached the scale of 30-40 MWt pilot plants. Coal-based projects are operating or being developed in Australia, China, Denmark, Germany, Italy, Japan, Spain, the UK and the USA. In Germany, Vattenfall started up a 30 MWt  pilot plant in 2008, and in Australia, an existing PCC power plant at Callide is being retrofitted with oxyfuel technology.

 • Integrated Gasification Combined Cycle (IGCC)

Coal gasification is a process by which coal is converted into a fuel gas rich in hydrogen and carbon monoxide. In the early 1990s  European and US governments and other agencies provided financial support for four medium-sized IGCC demonstration projects. Three of the plants are In Europe: 
  • The 284 MW Willem Alexander plant at Buggenum, The Netherlands – started up in 1994-95.
  • The 400 MW Vresova IGCC plant in the Czech Republic – started up in 1996.
  • The 335 MW Puertollano IGCC plant in Spain – came on line in 1998.
 All three are still operating. In the USA, the US DOE Clean Coal Technology Program provided cost-sharing funding for the country’s first large scale demonstration plants. These were the 296 MW Wabash River plant in Indiana (startup 1995), and the 313 MW Polk Power Station in Florida (startup 1996), and the 107 MW Pinon Pine plant in Nevada. The first two are still in use.  Coal-based systems are now being offered by several major manufacturers, and standardised plant reference designs are now available. Worldwide, numerous projects are at different stages in their development.

•          Development of advanced combustion systems

For power generation, systems based on pulverised coal combustion (PCC) using subcritical steam conditions form the largest segment of the power industry. However, more efficient alternatives have been developed and are used increasingly. These include: 

Supercritical (SC) and ultrasupercritical (USC) PCC 

Supercritical power plants are not new, the first were built in the late 1950s and early 1960s in the United States. Technical problems experienced by these early plants have been overcome and hundreds have been built around the world. Many more are planned or under construction. Increasingly sophisticated materials of construction continue to be developed, allowing for increased steam conditions and hence, greater plant efficiency. Most major coal-consuming nations now operate some SC/USC-based power plants. 

 

Fluidised bed combustion (FBC) 
Development of FBC systems began during the 1960s. The first units were used in smaller industrial plants and for local heating systems although the technology is now much further advanced. There are several main variants of FBC technology (predominantly bubbling and circulating systems). Worldwide, hundreds of both types, ranging from small scale commercial units up to utility-scale, are now in operation.

These technologies have reduced the emission of pollutants resulting from the use of coal and the advanced combustion systems have increased the efficiency of coal combustion, thus reducing the emissions of all pollutants per kW of energy produced.

The IEA Clean Coal Centre has published a large number of reports on all these subjects. Please visit our bookshop.


 
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