Advanced fossil power generation
THE TECHNOLOGY
Most pulverised coal steam plants operate with sub-critical steam parameters and efficiencies between 32-40%. Supercritical plants with steam conditions of 540°C and 300 bar have been in commercial operation for a number of years, with efficiencies of 40-45%. When best technologies are used, as in ‘advanced supercritical’ plants, with steam conditions up to 600°C and 300 bar, net efficiencies of 46-49% should be achieved. These technologies require stronger and more corrosion resistant steels, but potential efficiency savings offset their extra cost.
The next step for the use of coal, under development since the 1990s, is ‘ultrasupercritical’ (USC) power plants. Currently, steam conditions of 600°C and 300 bar can be achieved, with efficiencies of 45% and higher, for bituminous coal fired power plants. Future USC plants are planned which use 700°C and 350 bar or higher and yield net efficiencies of about 50-55%.
In 1998, a major group of power industry players started a 17-year demonstration project, supported by the EC THERMIE programme, called the ‘Advanced (700C) PF Power Plant’. The main aim is to make the jump from the use of steels to nickelbased superalloys for boilers to achieve the highest temperatures and increased efficiency (50-55%). However, it is not known when a 700°C steam coal power plant will become a commercial reality.
Many of the technology objectives and actions for advanced fossil fuel power generation are the same as for the development of carbon capture and storage (CCS). These include the establishment of an R&D programme that will address fossil fuel conversion technologies aimed at improving power plant efficiency in all main fossil fuel power generation routes to better compensate for the efficiency penalty imposed by CO2 capture.
ONGOING RESEARCH
Diversifying fuel can have a marked impact on power plant efficiency, while minimising fluctuations in fuel price and availability. Combined cycle plants can achieve thermal efficiencies of 50-60%, for example, through co-firing with natural gas or biomass. Biomass has been used widely in amounts up to 5% of energy input, with minimum impact on efficiency and extra preparation costs. Fluidised bed systems, which suspend solid fuels on upward-blowing jets of air during the combustion process, have been exploited extensively in Nordic countries. Meanwhile, Integrated Gasification Combined Cycle (IGCC) technology has been successfully demonstrated on a large scale. IGCC turns coal into gas (syngas) and then removes and recycles impurities, while recuperating excess heat. An obstacle to the greater use of this technology is still its high cost, especially when combined with CO2 capture.
Further into the future, IGCC with hybrid fuel cells, gas turbines and steam turbines could possibly reach 60% efficiency, with zero emissions. For both new and retro-fitting of existing PC combustion plants, oxyfuel combustion is a promising option, using pure oxygen rather than air, as it minimises the cost of carbon capture. Polygeneration from coal is another option, as it not only recuperates both electricity and heat, but also chemical feedstocks and alternative fuels for transport.
R&D investments
Public and private investment in carbon capture and storage R&D in 2007 was about €280 million.
Technology objectives and actions
The portfolio of fossil fuel plants that will be deployed in the future will only be compatible with the European goal for more sustainable energy production if the following conditions are met: high CO2 prices are maintained; carbon capture and storage technology (CCS) is developed and enabled; medium or high fossil fuel prices prevail.
THE INDUSTRY
All energy forecasts show that fossil fuels will remain the main fuel for electricity generation in the medium and long term, because of extensive coal reserves and their distribution across politically stable regions. Fossil fuels will therefore likely retain at least a 40 – 50 % share of power generation in 2030. The use of coal will probably increase in the future because it is cheap and abundant, while CO2 capture and storage would reduce CO2 emissions significantly.
Savings in CO2 emissions, of the order of 23.6%, can be achieved by major retro-fitting of old sub-critical power plants with supercritical steam cycles. Similarly, adding reheating stages, increasing the number of feed heaters, increasing the final feed water temperature and generally improving housekeeping by reducing leakages and heat losses, would further increase efficiency by 4-5 percentage points.
Co-firing with biomass also reduces CO2 emissions, although technical challenges associated with fuel feeding, fouling and corrosion limit the amount of biomass that can added without compromising operating reliability. The next step, involving carbon capture and storage (CCS) at a demonstration scale beyond 2015, will demand the highest power generation efficiency in order to compensate for the inevitable energy cost associated with CO2 capture processes.
BARRIERS
The cost of CO2 emissions within the European Emissions Trading Scheme (ETS) is likely to have a substantial impact on the cost of electricity production, hence the need to maximise efficiency. But, without major refitting, installed boiler and turbine designs will limit the scope for improvements in efficiency of existing coal-fired power plants. Fossil CO2 emissions can be reduced by co-firing with biomass. The barriers to direct co-firing with biomass are very low, as only the fuel-feed systems need to be changed significantly.
Meanwhile, co-firing of waste poses both a legal barrier and a technical challenge. Under the European Waste Framework Directive (WFD), waste combustion may only take place in a plant that conforms to the European Waste Incineration Directive. However, millions of tonnes of Solid Recovered Fuel (SRF) are produced from municipal solid waste each year and at least some could be used in power plants without adverse effects. The main challenge is to reclassify SRF as a “product” rather than waste.
The main technology challenge by far on the immediate horizon is the introduction of CCS. Meanwhile, the uncertain reliability of ultra-supercritical steam cycle plants is a disincentive to investment.
NEEDS
The main factor in minimising barriers to investment in, and operation of power plants is a stable economic climate. This means greater stability of investment costs than over the last 3 - 5 years, and a stable CO2 price when the European Emissions Trading Scheme is in full operation. A regulatory market framework needs to be developed, along with appropriate policies to promote financial stability of the energy market. It will also be necessary to look at the financing and regulation of CO2 transport and storage infrastructure, on both the European and the Member State level, to enable the power generation sector to plan its capacity and fuel supplies for the future.
INSTALLED CAPACITY
The power generation sector in the EU is mature and has been thriving in a relatively undisturbed commercial environment until quite recently. The current fleet of power plants with an operating capacity of 400 GW is expected to decline to 65 GW by 2030, as existing power plants are retired. Between 510 GW and 635 GW of new fossil fuel power plant capacity will need to be constructed in the EU by 2030 to meet the rising demand for electricity and to replace retiring power plants. An example of what is needed from new fossil power plants is the Nordjyllandsvæket USC combined heat and power plant near Aalborg (Denmark), that has been achieving 47 % electrical efficiency with an output of 410 MW and steam conditions of 582°C and 290 bar. The plant started operation in 1998 and benefits from the use of seawater to provide cooling for the turbine.
