Clean Coal technology profile

(Note: This technology report is for conventional energy. Conventional energy technologies are not covered by EuroREX. This profile is intended to provide an indication of the level of detail, the type of information, the contacts, etc. that are available at EuroREX's Level 3 information for all RENEWABLE ENERGY TECHNOLOGIES!)

Introduction

A significant amount of world wide electricity production comes from coal combustion. In Europe, about 35% of electricity comes from solid fuels. The majority of the coal is burnt in pulverised fuel (PF) boilers heating water to drive steam turbines. However, in today's terms, these PF turbines are relatively inefficient with high levels of emissions. These disadvantages have driven research into efficiency improvements (better turbines, higher boiler temperatures), alternative methods of combusting coal (gasification, fluidised bed combustion), co-firing with other fuels, and even the conversion of coal into alternative types of fuel (eg. liquefaction, pyrolysis). This report analyses the technological changes that have occurred within the combustion process itself to improve the economic, efficiency and environmental performance of electricity from coal combustion.

Clean coal technologies are not a new concept, as an understanding of the theory behind these applications has been in the public domain for decades. However, the process of transferring the theory into practical commercial applications has proved to be difficult and still proves to be the main stumbling block behind their lack of widespread implementation. There are many reasons behind this problem, the main ones of which are covered in this report.

Present status

Importance of Coal

There are two aspects to the importance of clean coal. The first is coal itself. Coal is the most abundant of the fossil fuels and therefore has a long term future after natural gas and oil reserves have been exhausted. Therefore, the long term prospects for clean coal are good. However, this assumption is based upon the concept that the reserves are accessible. The European (deep mine) coal industry is shrinking rapidly and is already a tiny proportion of even the industry 20 years ago. If this decline continues until there is almost no remaining European deep mine industry, many of these current reserves will become inaccessible and cease to be a fuel source.

The second aspect is the technology itself. Coal is a major world wide energy source for electricity generation owing to its abundance of supply, and it will continue to be so for many years. However, coal-fired generation will only achieve its full potential if clean efficient plant is developed and installed. Therefore, it is imperative that the best technological options for clean coal are developed as soon as possible in preparation for its resurgence as the technology of choice.

Competition

Again, economic competition is split into two components for coal as in the fuel itself and the technology. In comparison with a few decades ago, coal is now a cheap plentiful source of fuel, especially from the open-cast mines in South Africa, Australia, Asia and Latin America. In a European context, coal is relatively expensive, mainly owing to the currently low gas prices. Europe has a large natural gas resource, but this is expected to start depleting within the next couple of decades. Once gas prices increase, as is foreseen, coal will again become an economic option for electricity generation.

On the other hand, a major threat to the coal industry is the prospect of the introduction of green taxes, such as a carbon tax. This will penalise coal and make it relatively uneconomic in comparison with most other competing fuels.

With respect to the technologies involved, clean coal is still an uneconomic option. Demonstration plant needs to be constructed, so that experience can be gained of the theoretical concepts associated with some parts of the generating process. The consequence over time will be an established range of technologies that are reliable and which can compete against alternatives.

A changing industry

Technological development in the industry is a continual process. There are many factors influencing leaps in technological progress, each of which lead to improvements in various aspects of clean coal combustion. The forces behind technological change are listed below.

Economics

In Europe, growth in electricity demand has been relatively low over the last two decades. Therefore, the factor behind new plant has tended to be as a replacement for retired power stations, unless there are other circumstances causing changes in the market. There are two major factors that affect the economics of clean coal plant. One, fuel costs, is uncontrollable, while the other, investment costs, can be directly affected through research and development.

If clean coal is to become the first choice in new plant construction, it has to become economic against the alternative technologies such as traditional coal-fired pulverised fuel steam turbines and gas-fired combined cycle gas turbines.

Fuel price

Another economic factor against clean coal is the price of the fuel itself. Deep mined coal in Europe is expensive in comparison to other fossil fuels, so the various indigenous coal industries have only survived owing to government support, be it in the form of a direct subsidy or by guaranteeing prices with the utilities. On the other hand, cheap imports are available, so this is not necessarily a factor against clean coal technology. However, despite the world market price for imported coal being relatively stable, this can be greatly affected by shipping costs, which is an added risk.

Competition

Competition between various generating technologies is intense in Europe. The future for coal-fired generation is clean coal. Therefore, as Europe has broad experience in coal burning technologies, intense research is being carried out to make clean coal competitive against alternative technologies, especially with respect to the export market to Asia and the rest of the world.

Environment

Coal is a relatively dirty fuel compared to gas, renewables, etc. This is in terms of carbon dioxide emissions (global warming), sulphur dioxide and nitrogen oxide emissions (acid rain) and ash disposal (heavy metals to landfill). Therefore, high levels of research are being carried out to minimise the environmental effects of coal burn, especially in terms of acid rain mitigation. Efficiency improvements will lead to reduced CO₂ emissions per unit of electricity produced.

Private ownership of the utilities has significantly changed the culture of these organisations. Over the last few decades, investment policy has become much more financially based. This is currently a factor against clean coal. On the other hand, this gives the component manufacturers the opportunity to invest in clean coal plant themselves, which frees them from the constraint of having a single/few electricity supplier(s) within the region. However, despite no clean coal plant having yet been built in Europe as a result of privatisation, there is scope for this to happen.

Liberalisation of electricity markets

This change towards third party access is more associated with choice to the consumer. However, this allows the concept of product differentiation to be introduced. It is possible that clean coal electricity can be marketed as a premium source of electricity which will encourage the development of this new market. Further research would be necessary in this area to investigate the possibility of clean coal electricity as a niche market and to ascertain the type of niche that this technology can hold.

Security of energy supply

Coal is Europe's largest indigenous energy source, so its use in the past was encouraged as a means of minimising energy imports. However, owing to changes in the world coal trade and cheap imports from other countries, the majority of the European coal industry has closed down during the last fifty years. This has led to a perceived decrease in the importance of coal as a fuel.

A second major impact on the industry has been the termination of the ban on the use of natural gas in electricity generation. As cheap natural gas supplies are plentiful in Europe, there is currently little need for new clean coal plant. However, in the medium to long term, coal is expected to achieve a renaissance as natural gas becomes a scarcer commodity.

Technology leadership and exports

The main rationale behind maintaining a thriving clean coal technology industry is the export market to developing countries, especially in Asia. For Asia, an increase in the market for clean coal is expected over the next few decades. By being technology leaders, European organisations can benefit from their expertise in these markets. The net result is that these exports can be used to develop and test new processes in preparation for penetrating the European electricity generation market itself.

Key players in industry

The players forcing change in clean coal technology are both external and internal to the industry.

The main external influence comes from international and national government funds. For example, the European Commission is a major source of funds encouraging research, development and demonstration of clean coal plant. This is making Europe, with the USA and Japan, a world leader in clean coal technologies, which provides an excellent basis for exports to the rest of the world.

Alternatively, government can force change through regulations and legislation. However, as the majority of the relevant regulations refer to emissions, coal combustion is penalised owing to its high emissions. On the other hand, laws can be introduced requiring long term security of fuel supply. This would help to secure the majority of the remaining deep-mined coal production in Europe, which is currently under threat from cheap imports and competing fuels.

Another key type of player is the utilities. Their aim is to provide the most economic electricity to the consumer within any restrictions set by government. If clean coal developments can make it the best option in certain locations, the long term future for coal combustion in Europe will be assured. Additionally, the faster that technological progress can be made, the better are the prospects for clean coal in the rest of the world.

The final key players are the manufacturers and installers of clean coal technology plant and components. They have a vested interest in the rapid integration of the plant into a first choice technology. Their current markets are in the developing world where experience can be obtained into the operating characteristics of this plant. It is vital that this experience can be gained so that continuing improvements in efficiencies and emissions from clean coal combustion can be made.

Technology Forecast

• Supercritical and ultra-supercritical coal combustion
• Pressurised and atmospheric fluidised bed combustion (PFBC/AFBC)
• Integrated gasification combined cycle (IGCC)
• Integrated gasification humid air turbine (IGHAT)
• Direct coal fired combined cycle (DCCC)

Technology Performance

The technical performance of each of these options is shown in table 1.

Table 1 - Technical performance of clean coal technologies (2000)

Source: DGXII

Note: ¹2005

Supercritical coal

Conventional supercritical coal-fired steam power plant employing elevated live steam parameters have already been built in past decades. Austenitic materials had to be applied for components operating in the high-temperature at that time. Recent developments in the material properties permit the use of ferritic/martensitic steel in present state-of-the-art hard-coal fired power plants and steam parameters of 221 bar and 540°C. R&D activities for ultra super critical plant are aiming at higher steam parameters of 248 bar and 566°C. For steam generator components, pipes, and turbine components, nickel based materials and methods of manufacturing the named components from these materials have to be developed. However, ultra super critical is still at the development stage, and demonstration plants of this technology can not be expected before 2005-2015.

PFBC

PFBC can be considered as commercially available and applicable for a large variety of coals. The pressurised combustion reduces the space requirements and the fluidised bed can be designed to control emissions of SO_X (by admixing limestone) and NO_X (by air splitting and temperature control in the furnace) below the environmental standards without cost intensive flue gas cleaning. Using cyclones and candle filters, the flue gas particles can be reduced by 99.9 % before entering the gas turbines, which provides sufficient protection of the turbines and complies with most requirements for emission control.

IGCC

Gasification of solid fuels and heavy oils is a well proven and commercially established technology. Together with gas cleaning and combustion it offers the possibility of power generation at very high efficiencies, taking in this manner full advantage of the achievements on gas turbines and combined cycles technology. Decisive characteristics of integrated gasification cycles are the gasification processes (entrained flow, fluidised bed and fixed/moving bed) and oxidant (oxygen- or air-blown). Demonstration plants with an installed capacity of 200-300MW are under construction. The commercial availability of larger units is not expected before 2005.

IGHAT

In conventional gas turbines, more than a third of the power generated in the expander is absorbed to compress the working fluid. This loss of exergy is increased by the requirement for an excess air flow to allow cooling of the turbine components. Humid air turbines (HAT) allow for lowering this inconvenient disadvantage in two ways: first, they use intercoolers on a multistage compressor, therefore reducing the power requirement of compression and producing low temperature heat. Secondly, moisture is added to the compressed air (20-40%). Low grade heat is used to produce hot water. This heat is recovered from the compressor intercooler, aftercooler and turbine exhaust.

When HAT is integrated with a coal gasifier (IGHAT), there is a large amount of heat available from the gasifier and other processes (including air separation plant for oxygen blown gasifiers). The hot water is brought into contact with the compressed air in a counter current saturator. Humid air leaving the saturator may have a moisture content of 20% (natural gas HAT) and 40% (IGHAT). This is a key stage of the HAT cycle, since the saturator is a multistage column and the heat exchange approaches reversibility. The humid air is further heated against the hot turbine exhaust in the recuperator. The variable boiling point working fluid of steam and air avoids the large temperature divergence between the water and turbine exhaust in a gas turbine combined cycle. Thus, the moisture addition increases the work output form the turbine, while the intercooling reduces the compressor work requirement. This combines to increase the net power output. The IGHAT cycle can achieve efficiencies higher than those of conventional combined cycles, and is especially adapted for base-load electricity production. An additional feature of this cycle is that, given the high moisture content of the exhaust gas, it is particularly well-suited for district heating.

DCCC

The research on direct coal firing in gas turbines has been carried out for over forty years. Since 1986, Westinghouse and Textron have run a project at Morgantown Energy Technology Centre of the US DoE, with federal funding. The initial difficulties were related to the severe effects of coal ash on turbine blade path components (corrosion, erosion and deposition). Latter-day effort on direct coal firing (DCF) has concentrated on developing high pressure (12-16 bar) slagging coal combustors which allow removal of ash as a liquid prior to entering the turbine. The hot gas clean up must take place above the ash melting temperature (1400-1600ºC) and high pressure (at least 18 bar). This high temperature provides additional gains in exergy with respect to other advanced coal cycles, such PFBC or IGCC. The molten ashes accumulate on the edge of the slagging chamber by a centrifugal effect. Nevertheless, there is an efficiency penalty with respect to gas-fuelled natural cycles: the foreseeable efficiency ranks around 44%, and, in general, may be estimated as the efficiency of natural gas combined cycle minus a penalty of around 6 percentage points. Its estimated costs would be twice those of gas-fuelled combined cycle.

It should be kept in mind that DCFCC is not yet a proven technology, its state of development being still in the laboratory phase. Apart from the slagging combustion system and the liquid ash and contaminants removal, all other components of DCFCC (gas turbine, steam cycle, etc.) are commercially available technologies.

A study carried out by Westinghouse/Gilbert-Commonwealth Inc. in 1990 investigated the economics of direct coal fired gas turbines fitted with slagging combustion technology. The study concluded that the cost of electricity from this plant type could be 11-18% cheaper than a comparable 220 MW_e pulverised coal boiler fitted with FGD.

Economic Performance

The economic data of these five technologies is outlined in table 2.

Table 2 - Clean coal technology economic data (2000-2030)

Source: DGXII

Note: ¹2005

Research and development in clean coal has two effects for the technology. Ultimately, it has the effect of making it more economic, with the major issues being:

• operating efficiencies
• emissions (sulphur dioxide, nitrogen oxides, particulates)
• ash management and disposal
• control of the combustion process

Secondly, R&D is used to build demonstration plant so that the theories can be tested. The lessons learnt from full-scale plant are fundamental as part of the demonstration process.

In terms of R&D management, there is also the question of combining all of these separate issues for the completed plant, as different areas of research can impinge upon the studies being performed in a different section of the electricity generating process.

Therefore, in order to achieve the best benefits and value, it is vital that research and development is co-ordinated so that all results can be integrated to provide a valuable product.

Market Forecasts

In Europe, the current market penetration of clean coal technologies is extremely small. There are only a few sites, the main ones being in the Netherlands and Spain. However, only super-critical has actually penetrated the market, as the others are demonstration plants.

However, on a world wide basis, there are several clean coal generating stations in the developing world where there is rapid growth in electricity demand, particularly in countries such as China. It is these areas where significant growth in market penetration will be achieved. As experience is gained and the technologies become more economic, the potential for significant penetration in Europe will increase during the next decade.

Barriers to future markets

There are many factors affecting the almost general lack of clean coal plant installation within the European Union. However, there are two factors that have combined to dominate all of the others. The first was the lifting of the EU directive banning natural gas use in power stations, which was predominantly prompted by the realisation that gas was being wasted by flaring off to get to the underlying oil, rather than by the move towards liberalisation. The second factor was the availability of efficient and relatively cheap Combined Cycle Gas Turbine (CCGT) technology. Consequently, even cheap standard coal plant cannot compete in direct terms with the combined cycle gas turbines currently being constructed.

The other major barrier to the widespread introduction of clean coal is environmental impact. Even the relatively low emissions from clean coal plant are greater than those from natural gas-fired electricity generation. With climate change commitments requiring the reduction of overall carbon dioxide emissions, the clean coal technologies may only become an option where they replace a 'dirtier' technology, which is old coal or oil plant.

On a world-wide basis, coal will remain a major primary fuel for at least the next two decades. Countries with a large indigenous coal resource and a rapidly growing demand for electricity, such as China and India, will inevitably build significant new coal-fired electricity generating capacity. In these circumstances, however, competition arises from traditional pulverised fuel plant which is cheap and an established technology. This issue cannot be underestimated.

Technology Support

As the European electricity market becomes more liberalised, clean coal plant will only penetrate to significant levels if it becomes economic against alternative fuels and technologies. This can be achieved by subsidising the capital cost of the plant or by subsidising plant operation. Plant operation can be subsidised by receiving a premium price for electricity generated (similar in principle to the UK Non Fossil Fuel Obligation) or by reducing operating costs (tax credits or fuel subsidies - for example, the former German KohlPfennig, Spanish electricity subsidies, etc)

The alternative means of stimulating demand is to force it. This is achieved through regulations requiring the implementation of clean coal plant, or by preventing the market penetration of competing options.

Conclusions