Carbon Dioxide (CO2) emissions from coal-fired power stations are reduced by approximately 3% for every 1% improvement in burning efficiency. In this context, the retrofitting and/or replacement of existing coal-fired power plant with new-technology generation capacity can deliver a greater greenhouse benefit than some renewable technologies. This requires the introduction of “Clean Coal” technologies – a term variously used to describe a range of technologies from high-efficiency generation systems to the ultimate, zero emission power production.
[Coal] Coal will remain a substantial part of the energy mix in the foreseeable future as new technologies improve efficiencies and reduce emissions. Power station thermal efficiency has a major effect on greenhouse gas production such that improving efficiency results in a reduction in emissions and coal consumption. A 1% improvement in power station efficiency delivers a 3% reduction in CO2 emissions.
There is another strong case for taking the improved efficiency path using coal for power generation. Renewable energy resources are not sufficiently reliable to ensure security of electricity supply. For example, wind turbines typically operate a low percentage of the time – being at the mercy of the vagaries of nature – the wind blowing. For instance, installed wind generation capacity (10,543MW) in Germany during 2002, only produced 2.5% of the potential power due to poor wind conditions. To replace a 300MW coal station would require 600 windmills, assuming 100% utilisation. Bearing in mind utilisation is < 30%, then nearly 2,000 windmills would be required to theoretically deliver 300MW. Whether 600 or 2,000, windmills leave a much larger footprint than a coal-fired power station.
There is a strong case to say that new-generation, higher-efficiency power stations, replacing older stations, will deliver as good CO2 reductions at far less cost and a much smaller footprint than windmills.
While there is a place for wind generation, adding windmills to the system can reduce the cost effectiveness of conventional generation, as wind generation still needs conventional power station back up in “calm” times. The inevitable conclusion is that the solution is greater efficiency in coal generation, even retrofitting, rather than renewables for the sake of renewables.
Improvements beyond 40% efficiency for conventional pulverised fuel (pf) stations can be achieved with advanced combustion chamber technology with “Supercritical” (40-45% efficiency) and “Ultrasupercritical” (>45% efficiency) systems. The impact of efficiency is large on greenhouse gas reduction. For instance, a supercritical plant replacing Muja A & B in Collie would reduce greenhouse emission by around 50%. Advanced materials for pf plants could raise efficiencies to nearly 50%. In addition, gains can be made by co-firing with biomass, such as sawmill waste, to reduce emissions by over 10% without loss of power station efficiency.
In the longer-term, the Integrated Gasification Combined Cycle (IGCC) offers one of the most efficient and “greenhouse friendly” coal-based electricity generation processes. Here coal is gasified into fuel that is subsequently burnt in a combined cycle turbine plant with overall thermal efficiency of 44-45%. One advantage of gasification is the ability to use low-quality fuels and fuels with environmental stigmas – high in metals, nitrogen and sulphur - and it greatly simplifies CO2 recovery, particularly with the use of pressurised, oxygen-blown gasification systems. Similar O2/CO2 combustion technology is applicable to conventional power stations with CO2 concentrated to >95% by displacing air with O2.
Gas turbines are currently considered an extremely attractive option because of higher efficiencies and lower consequent emissions, advantages in size options, compactness, capital cost, time to construct and rapid start up. The combined cycle system, producing an overall thermal efficiency of 45-50%, is nearly the least greenhouse gas intensive of the fossil fuel technologies.
Utilisation of biomass in Integrated Combined Cycle systems is a popular concept as biomass is deemed a form of renewable energy. Gasification of the biomass, through drying and heating the material, produces a fuel gas suitable for combustion in a boiler or gas turbine.
Ultimately, zero emission technology may be the saviour of fossil fuels. Geological disposal of CO2 by reinjection into expired gas fields is already done. An alternative is to liquefy the CO2 and pump it to a sea floor depth of at least 1km where it should remain having formed a crystal hydrate with seawater.
The capture and storage of CO2 is likely essential for Australia with around 40% of our anthropogenic emissions currently coming from coal-fired power generation. Costs correspond to an increase in electricity price of up to 25%, comparing favourably with renewable energy alternatives.
Removal of CO2 by chemical reaction to new species in absorption technologies is gaining favour. The ZECA (Zero Emission Coal Alliance) program in Canada applies absorption in the development of a novel technology for electricity generation and/or hydrogen from coal.
Flue Gas Recycling is a promising zero emissions technology for existing pf stations and requires no major infrastructure changes. The process employs combustion in an O2 rich environment, and is potentially much less expensive than other CO2 removal options.
The cost of CO2 avoidance in coal-fired generation may be high but seems clearly warranted while costs are lower than most technologies utilising renewable energy sources.
Renewable energy sources (hydro, wind, solar, biomass, wave and tidal) and technology can provide some generation capacity but the current costs are often not viable (except hydro) and would be unable to meet demand.