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Combining renewable energy with coal

World Coal,

Despite efforts to diversify the energy mix, coal remains an important energy sources for many economies. Dr Stephen Mills, IEA Clean Coal Centre, discusses the benefits of combining renewable energy sources with coal.

Global energy demand to increase

There are strong incentives to develop improved sources of energy. By 2040, the world’s population will have reached nearly nine billion. By then, global energy demand will be around a third greater than current levels. Oil, natural gas and coal will continue to be the most widely used fossil fuels, accounting for up to 80% of total global energy consumption.

Despite efforts to diversify, coal remains important for many economies. Since 2000, apart from renewables, it has been the fastest-growing global energy source. It is the second source of primary energy after oil, and provides more than 30% of global primary energy needs. Around 42% of the world’s electricity comes from coal-fired power plants. For the past 12 years, year-on-year, global coal production has increased. During this time, consumption has risen by nearly 60%, increasing from 4.6 Gt in 2000, to around 7.2 Gt in 2010.

Alongside the ever-increasing use of fossil fuels of all types, there has also been a marked increase in the uptake of renewable energies. These now represent a rapidly growing share of energy supply in many economies. Of these technologies, the biggest contributions have been made by wind power and biomass. The latter can take many forms although the present report concentrates mainly on the potential of forest residues. In some parts of the world, considerable quantities are potentially available although, at the moment, these are often a largely underexploited resource.

When combined with coal, biomass can provide a number of advantages. However, its use on a large, commercial scale has the potential to expose a number of issues. For example, the volumes to be harvested and handled can be substantial, some forms may be subject to limited or seasonable availability, various pre-treatments may be needed, and so on. Inevitably, this adds complexity and cost to energy production.

Advantages of cogasification

Gasification is an effective, versatile technology that can be used to produce energy from coal and biomass. Combining the two via cogasification can be beneficial. Cogasifying biomass in large coal gasifiers can achieve high efficiencies and improve process economics through the greater economies of scale that can be tapped into, and it can help smooth out fluctuations in biomass availability and variable properties. Combining biomass and coal in this way can be useful, both environmentally and economically, as it may be possible to capitalise on the advantages of each feedstock, and overcome some of their individual drawbacks.

A major aim of this study was to examine different systems that combine coal use with various other technologies to create efficient and clean energy-producing systems. In a number of countries, hybrid concepts for the production of SNG, electricity and/or heat, and liquid transport fuels have either been proposed or are in the process of being developed or trialled. A number are based on the cogasification of coal and biomass in various ways.

Adding renewables to the energy mix

However, as well as incorporating biomass, some propose to take this a step further by adding yet another element of renewable energy to the system, generally by incorporating electricity generated by intermittent renewables (such as wind and solar power). A major drawback with wind and solar power is their intermittency. Consequently, times of peak output may not correspond with periods of high electricity demand, and vice versa. At times, there can be significant amounts of surplus unwanted energy available, particularly from wind farms. This can be quite a widespread phenomenon, and the usual solution is to take wind turbines off line.

However, rather than waste this electricity, it would clearly be beneficial to put it to good use. One way would be to use it to electrolyse water, producing hydrogen and oxygen. Both gases have the potential to be component parts of such hybrid systems, and there are various schemes where the hydrogen could be fed into the syngas from a gasification system, used in fuel cells, used directly as a transport fuel, or combusted in gas turbines to generate electricity. Similarly, oxygen could be used for a number of industrial applications, or fed to a coal/biomass gasifier or an oxyfuel combustion plant to generate electricity. Different concepts and schemes combining gasification, intermittent renewables and electrolysis are currently being examined. If the economics can be made to work, several look promising. The number of such projects is growing, and some also aim to incorporate carbon capture and storage. For example, an ongoing project in Germany is combining coal-based power generation with aspects of carbon capture and wind generated electricity with trials of advanced electrolyser technology. Success could encourage increased uptake of, for instance, advanced electrolysis, as a component part of different coal/renewables systems. Examples of promising technologies and their current status are discussed in the study.

Projects and conclusions

Some hybrid systems are at early stages in their development or have been undertaken at a very small size, hence extrapolating to commercial scale and obtaining firm process costs remains problematic. For a variety of reasons, not all schemes proposed are considered to be technically and/or economically viable. However, from the former standpoint, some appear to be much more robust. Ongoing improvements in, for instance, gasifier and electrolyser design could encourage further development of such systems for energy production. Where hydrogen and/or oxygen production forms part of such schemes, reductions in the cost of electricity supplied by renewable energy sources (such as wind and solar) would also be beneficial in making electrolysis more cost effective.

A number of projects are further advanced than others, with development programmes well under way. Some component parts (such as the cogasification stage) have now been well established, and others are under development or being trialled (such as the commercial scale demonstration of hydrogen production from wind power, and testing of advanced electrolysers). A number of such proposed hybrid systems show potential although in the medium term, assuming outstanding technical and economic issues can be resolved fully, most seem likely to be applied initially to niche markets, or to find application under specific, favourable circumstances.

Dr Stephen Mills has reported on a range of clean coal-related topics that have focused on advanced power generation, life cycle analysis, technology roadmapping, and issues associated with the growing use of renewable energies.

The full report is available from the IEA Clean Coal Centre Bookshop.

Adapted to house style by Katie Woodward

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