Coal fired power plants form part of the generation portfolio in most countries, but have the disadvantage of relatively high levels of carbon dioxide emissions. But a number of solutions – both short and long term – can put coal on a level with other technologies.
Les King, Mitsui Babcock, UK
In a balanced energy generation portfolio comprising nuclear, coal, gas and renewables, coal has a number of attractive features:
Figure 1. In Europe, new supercritical coal fired plant of Mitsui Babcock design such as those at Hemweg and Meri Pori have a design cycle efficiency of over 43 per cent
- It is easy to store and transport and can be sourced from diverse stable suppliers worldwide
- Pulverised coal fired power stations offer unique load carrying flexibility, particularly useful in meeting peak demand, and in compensating for the intermittency of renewables
- Coal fired generation (including emission control equipment to the latest stringent standards) is the lowest cost option for electricity generation (typically 1.6p/kWh for existing plant, 2-2.5p/kWh for a new plant).
Against these advantages, coal suffers from the disadvantage of having the highest level of carbon dioxide (CO2) emissions, close to 1 t of CO2 per MWh generated for the majority of plant installed around the world. This is twice the level associated with modern gas fired generation. For coal to have an environmentally acceptable future, CO2 emissions from new and existing coal fired plant need to be reduced to as low a level as possible. A future with renewables and nuclear alone – both essentially CO2 free – is unlikely to be sustainable.
There are two complementary solutions to the reduction of CO2 emissions from coal fired power plants.
In the short term, major improvements in CO2 emissions from pulverised coal fired plant are possible by:
- Improvements in generation efficiency (giving a reduction in CO2 emissions per megawatt of electricity generated), either via new plant or upgrade of existing plant
- By substituting a fraction of the coal with biomass (biomass co-firing), biomass being CO2 neutral. Modern coal fired boiler designs are capable of accommodating up to 20 per cent biomass co-firing, with a corresponding reduction in CO2 emissions
- Use of advanced concepts in plant integration.
Simultaneous adoption of all three improvements outlined above would reduce CO2 emissions from pulverised coal fired plant by 50-60 per cent to a level comparable to a modern gas fired plant.
A longer-term solution is CO2 capture and geological storage (CCS). This involves a chain of technologies for CO2 capture, transportation and storage.
There are three basic options for CO2 capture, namely:
- Post-combustion capture, in which the CO2 is separated from flue gases
- Pre-combustion capture, in which the CO2 is captured prior to combustion generally by a shift reaction to convert fuel gases to CO2 and hydrogen
- Oxyfuel firing, in which fuel is burnt in an oxygen/CO2 mixture, thus producing a CO2-rich flue gas that is easier to capture.
Mitsui Babcock carried out successful trials of oxyfuel firing of coal in the mid-1990s on its test rigs in Renfrew. More recently it has joined two new European Union-funded Framework 6 projects, ENCAP (oxyfuel firing) and Castor (amine scrubbing) and is involved in several techno-economic design and feasibility studies on CO2 capture in conjunction with advanced supercritical boilers (ASC).
Early results from these studies indicate that ASC with CO2 capture by amine scrubbing or oxyfuel firing will be competitive with gasification and pre-combustion capture, and, significantly, give an electricity cost inclusive of CO2 capture and storage that is less than the cost of electricity from renewables.
Ideally, CO2 capture technologies should be capable of being retrofitted to existing plant as well as being used with new plant.
New build designs
In Europe, new supercritical coal fired plant of Mitsui Babcock design such as those at Hemweg and Meri Pori have a design cycle efficiency of over 43 per cent (steam conditions up to 259 bar, 568°C). For new build in China, Mitsui Babcock is supplying supercritical boiler plant of 42 per cent net efficiency (steam conditions up to 259 bar, 571°C) and can offer today ‘best available technology’ advanced supercritical plants capable of 45 or 46 per cent net efficiency (steam conditions up to 295 bar, 595°C). This efficiency increase compared to subcritical designs (e.g. from 38 to 45 per cent) translates directly into a 20 per cent reduction in CO2 emissions.
Figure 2. The materials employed for the ASC design are the best commercially available, utilizing P91 and T91 ferritic steels
For new coal fired power plant, Mitsui Babcock believes that advanced supercritical boiler/turbine plants offer the highest efficiency and lowest CO2 emissions per megawatt. Comparative studies by the International Energy Agency (IEA) and a number of utilities have shown that such plants, fitted with state-of-the-art emission control equipment, generally give the lowest cost-of-electricity, highest availability and lowest specific CO2 emissions for a wide range of coals.
Competing clean coal technologies such as integrated gasification combined cycle (IGCC) systems or circulating fluidized bed (CFB) boilers have a complementary role to play where particularly high sulphur or high ash coals are to be used, albeit with slightly lower efficiency and higher costs.
In the UK, the existing coal fired fleet is exclusively of the natural circulation subcritical design (steam conditions up to 169 bar, 568°C) with overall ‘as designed’ cycle efficiencies in the range 36 to 38 per cent, equating to the nominal 1 t of CO2 per MWh generated. The majority of the fleet is at, or past, its original design life, and coupled with the economics associated with the UK power generation market in the last few years, most plant currently operate at up to a couple of percentage points lower than the design cycle efficiency.
For these existing plant, reductions of up to 20 per cent in CO2 emissions (and fuel consumption) are possible by retrofitting ASCs and modifying the steam turbines within the existing power station building and support steelwork. Further reductions of 10 to 20 per cent are possible by use of CO2 neutral biomass (typically 5-10 per cent by pre-blending biomass with coal and a further ten per cent through the direct injection of biomass).
The Mitsui Babcock ASC design is equally suited to retrofits and new build. It is a two-pass design, similar in overall shape and size to the two-pass natural circulation boiler it replaces. The best available technology incorporates Mitsui Babcock’s low mass flux vertical tube once through furnace with the advantages of a positive flow characteristic, reduced weight and faster response. The materials employed are the best commercially available, utilising P91 and T91 ferritic steels and with the option of an austenitic steel final superheater and reheater – all widely employed in current new build and retrofit situations. No exotic alloys are required.
For an advanced supercritical boiler/turbine retrofit on a 600 MW unit, it has been estimated that a 12 month outage would be necessary. The cost of such a retrofit (approximately £150/kWe or $270/kWe) is such that the resultant marginal cost of CO2 abatement can be less than £6/t of CO2. This compares very favourably with the costs of abatement for renewable technologies, nuclear or CCS. Given that the basic ‘fuel and wires’ infrastructure is in place in the retrofit scenario, implementation timescales are also reduced compared to the new build alternative. Additionally there are benefits in improved plant flexibility, load response and availability of a supercritical plant compared to subcritical designs. Retrofit (and indeed new plant) designs can also be ‘capture ready’, i.e. designed to allow subsequent CO2 capture by amine scrubbing or oxyfuel firing.
On 9 August 2004, the UK Department of Trade and Industry (DTI) launched a public consultation on the content of its Carbon Abatement Strategy. It is planning to publish the new strategy by the end of 2004. The Carbon Abatement Strategy is a follow-up to the Energy White Paper and will define what the government will do to promote and support CO2 abatement from fossil fuels. The proposed strategy covers both efficiency improvements of existing power generation technologies in the short term and more radical carbon capture and storage technologies for the longer term. It is considered that the latter will be needed to meet the UK government’s CO2 reduction targets from 2020 onwards and that the efficiency improvements are a necessary precursor.
Given the uncertainties being widely expressed about the achievement of the government’s targets for carbon abatement, increasing gas prices and the perceived loss in confidence that the European Emissions Trading System (ETS) will drive reductions in CO2 emissions, Mitsui Babcock believes that the government should be placing more emphasis on the short term elements of the Carbon Abatement Strategy, including best available technology ASC retrofits, with incentives to achieve CO2 reductions from coal and gas fired plant at least on a par with those for renewables.