By Stephen Mills, Senior Consultant IEA Clean Coal Centre for Cornerstone
The world is undoubtedly hungry for energy and this hunger is growing. There are strong incentives to develop improved sources of energy. By 2040, the world’s population will have reached nearly nine billion.1 All of these people will need to be housed, fed, and have the opportunity to make a living; this inevitably means that much more energy is going to be needed. By 2040, global energy demand will be about a third greater than current levels.2 Oil, natural gas, and coal will continue to be used widely, although in some situations, the increasing use of renewable energy sources may reduce the amount of fossil fuels currently used. Regardless, on a global basis, coal will continue to play a major role. This will be particularly true in some of the emerging economies where growing industrialization and urbanization continue to relentlessly drive electricity demand upward.
At the moment, over 1.2 billion people lack access to any electricity, and another two billion are considered to have inadequate access. A key goal of the 2010 Copenhagen Accord is to provide energy to these underserved populations. There may be few energy source options available—in some countries, coal is the only economically available bulk source capable of providing reliable energy. Although its use is set to decline in some developed economies, coal will continue to be used widely and in considerable quantities. For over a decade, global coal consumption has risen steadily; in some non-OECD countries, in particular, both production and consumption have increased dramatically. During this time, consumption has risen by nearly 60%, from 4.6 Gt in 2000 to about 7.8 Gt in 2012.3 Despite efforts to diversify, coal remains vitally important for many economies. Since 2000, apart from renewables, it has been the fastest-growing global energy source. It’s the second source of primary energy after oil, and provides more than 30% of global primary energy needs.
The biggest individual coal reserves are in the U.S., Russia, China, Australia, and India. In all of these countries, coal is used to generate large percentages of electricity. In several, it also provides important economic benefits as it is exported to other power-hungry economies. At the moment, coal’s principal use remains electricity generation; coal-fired power plants produce 41–42% of the world’s electricity. In the coming years, electricity will continue to be provided by many different generating technologies, but the projected combinations are highly site-specific. The IEA World Energy Outlook (2012) suggests that, for the foreseeable future, power production from most sources will continue to increase (Figure 1).4 In many countries, coal and renewable energy systems are being deployed at greater percentages and, thus, there is increased interest in how to optimally integrate these systems. In fact, there are a significant number of opportunities.
FIGURE 1. Global power generation mix4
AN ODD PARTNERSHIP?
With the ever-increasing use of all types of fossil fuels, there has also been a marked increase in the uptake of renewable energy sources. In many economies, these now represent a rapidly growing share of electricity supply; Table 1 shows the top regions and countries at the end of 2012.
In 2013 renewables made up more than 26% of global generating capacity; in 2013 they produced 22% of the world’s electricity. Global renewable power capacity continues to increase. In 2013, hydropower and solar PV each accounted for about 33% of new renewable capacity, followed by wind at about 29%.5
Several driving forces support the growth in renewables. All developed nations rely heavily on an adequate and accessible supply of electricity and, for a long time, demand has continued to rise in nearly every country. However, in recent years, concerns over issues such as the depletion of energy resources and global climate change have been heightened. The preferred response of many western governments has been a supply-side strategy—namely, to raise the share of renewables (especially renewables other than hydropower) in the energy mix toward 20% and beyond. To date, wind power has emerged as the most competitive and widely deployed renewable energy, although levels of solar power are also growing steadily. Renewable energy technologies such as wind and solar have obvious features that make their use attractive. Although initial capital costs for renewables-based systems can be high, operating costs can be low; emissions generated during day-to-day operation are effectively zero.
Especially in faster-growing energy markets, these renewable energy systems are not replacing existing or even new coal-fired power plants. Renewables and coal-fired power generation are growing simultaneously. Therefore, it is worth exploring the many options for combining these very different forms of energy in the most cost-effective, environmentally conscious, and efficient means possible. A growing number of hybrid coal-renewables systems have been proposed or are being developed around the world, several of which could offer significant potential.
Coal and Biomass
Combining biomass with coal is a prime example of combining renewables and coal. Such a combination is already deployed fairly widely in the form of cofiring biomass in large conventional coal-fired power plants. Around the world, a growing number of power plants regularly replace a portion of their coal feed with suitably treated biomass. More than 150 coal-fired power plants now have experience with cofiring biomass or waste fuels, at least on a trial basis. There are ~40 pulverized coal combustion (PCC) plants that cofire biomass on a commercial basis, with an average of 3% energy input from biomass.6
Biomass comes in many forms and can be sourced from dedicated energy crops (such as switchgrass and miscanthus), short-rotation timber, agricultural crops and wastes, or forestry residues. When combined with coal, biomass can provide a number of advantages. However, its use on a large commercial scale could create 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, and various pre-treatments may be needed. Inevitably, such challenges can add complexity and cost to energy production.
Co-utilization of coal and biomass need not be limited to co-combustion in existing power plants—there are a number of other possibilities such as co-gasification. Coal gasification is a well-established versatile technology. Combining these two different feedstocks can be beneficial. For instance, facilities that co-gasify biomass in large coal gasifiers can achieve high efficiencies and improve process economics through greater economies of scale compared to a biomass-only facility. Such a combination can also help reduce the impact of fluctuations in biomass availability and its variable properties. Combining biomass and coal in this way can be useful, both environmentally and economically, as it may be possible to capitalize on the advantages of each feedstock, and overcome some of their individual drawbacks. Biomass can have an impact on CO2 emissions from a combustion or gasification process. Replacing part of the coal feed with biomass (assuming that it has been produced on a sustainable basis) can effectively reduce the overall amount of CO2 emitted. Potentially, the addition of carbon capture and storage (CCS) technology could result in a carbon-neutral or even carbon-negative process. Globally, considerable quantities of biomass are potentially available—in many countries, biomass remains an underexploited resource.
Similar to many conventional coal-fired power plants, several commercial-scale, coal-fueled, integrated gasification combined cycle (IGCC) plants in operation have at least trialed combining biomass with their coal feed, and several proposed IGCC projects aim to do the same. For instance, a planned IGCC and chemicals production plant (with CCS) at Kędzierzyn in Poland will co-gasify coal and biomass.7 To date, useful operational experience in co-gasifying has been gained with all major gasifier variants (entrained flow, fluidized bed, and fixed bed systems). Different types of coal have been co-gasified successfully with a wide range of materials, many of which are wastes that would have otherwise ended up in landfills or, at least, created disposal problems.
Co-utilizing coal and biomass is not limited to power generation. 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 tested. Coal/biomass co-gasification features in some of these. 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).
Coal, Wind, Solar, and Geothermal
Wind power has become the most widely deployed renewable energy. In 2013, global capacity hit a new high of 318 GW. In that year, China alone installed more than 16 GW; by 2020, the IEA projects the country will more than double its wind power capacity from the present level of 90 GW to around 200 GW.8 For comparison, the European Union countries have a combined ~90 GW of installed capacity. In 2013, wind surpassed nuclear to become the number three source of energy after coal and hydropower in China.9 Reportedly, this is part of the greatest push for renewable energy that the world has ever seen.10
Most major wind and solar facilities do not operate in isolation. Generally, they feed electricity into existing power grids or networks. Often, such grids are fed by a variety of different types of power plants—there may be various combinations of coal- and gas-fired power plants, some hydro, and possibly nuclear. The grid makeup and ratio between plant types is never the same, as these factors differ from country to country based on the local circumstances. On the face of it, the addition of a large amount of wind power into a grid, for example, is a positive development. However, a large input from intermittent sources into existing power systems can upset grid stability and have major impacts, particularly on how thermal power plants within the system operate. Many coal- and gas-fired power plants no longer exclusively provide baseload power, but are now required to operate on a more flexible basis. Many are increasingly switched off and on, or ramped up and down, much more frequently than they were designed to be. Inevitably, this is guaranteed to throw up a number of issues—significantly increasing wear and tear on plant components, reducing the operating efficiency of units not designed for variable operation, and impairing the effectiveness of emission control systems. Ideally, such important impacts should be taken into consideration and factored into any energy-producing scheme, but this is particularly true in cases where coupling intermittent renewables with conventional thermal power plants is being proposed.
Clearly, the most significant drawback with wind and solar power is their intermittency. Consequently, periods of peak power output often do not correspond with periods of high electricity demand, and vice versa. At times, there can be significant amounts of surplus unwanted electricity available, particularly from wind farms. This can be quite a widespread phenomenon, and the usual solution is to take wind turbines offline. However, rather than “waste” this electricity, it would be much more beneficial to find an effective means of using it. One option is to use electricity not needed to fill demand to electrolyze water, producing hydrogen and oxygen. Both gases have the potential to be component parts of hybrid energy systems and there are various schemes that propose feeding the hydrogen into syngas from gasification systems, use it in fuel cells or directly as a transport fuel, or combust it in gas turbines to generate electricity.
Similarly, the oxygen could be used for a host of commercial and industrial applications, or fed to a coal/biomass gasifier or an oxy-fuel combustion plant to generate electricity. Different concepts and schemes combining gasification, intermittent renewables, and electrolysis are currently being examined. Some aim to incorporate carbon capture and storage. For example, an on-going project in Germany is combining coal-based power generation with aspects of carbon capture and wind-generated electricity with trials of advanced electrolyzer technology (to produce hydrogen and oxygen from water).11 Success could encourage increased uptake of, for instance, electrolysis, as a component part of various coal/renewables systems. Assuming that the economics can be made to work, several schemes look promising.
Another ongoing project in Germany is expected to lead to significant improvements in the overall efficiency of the electrolysis process: E.On’s power-to-gas project at Falkenhagen. This technology utilizes multiple electrolyzers driven by excess electricity from a nearby wind farm to provide the power to produce hydrogen and oxygen. Output from the region’s wind farms frequently exceeds demand, so instead of taking the turbines offline when this happens, some of the electricity is now being fed to the electrolyzers. In this case, the hydrogen produced is being injected into the local natural gas grid, which acts as a large storage system. Effectively, it’s a clever way of storing renewable energy.
There is also an opportunity to integrate coal-fired power plants with renewable sources of thermal energy, such as geothermal or solar thermal. The benefit of this type of integrated hybrid system is that the renewable source of energy can take advantage of the existing infrastructure of the coal-fired power plant, such as the steam cycle, connection to the grid, and transformers. Generally, this makes the economics much more attractive compared to a stand-alone renewable plant. Obviously, the availability of the renewable resource at the coal-fired power plant site is a prerequisite for such hybrid systems to be successful.
Hybrid thermal systems operate by using heat from renewable energy to increase the temperature of the coal-fired power plant boiler feedwater. This increases the efficiency of the power plant, effectively displacing some coal for renewable energy (or using the same amount of coal and producing more electricity). Such thermal hybrid projects may be the most cost-effective option for large-scale use of solar thermal and geothermal energy, although, to be employed, this approach must be recognized under renewable energy incentives. In the future, there may also be an opportunity for renewable sources of energy to provide the thermal load required for carbon capture and storage, thus significantly reducing the overall impact to the power plant and contributing to large-scale reductions in greenhouse gas emissions.
Currently, around 15 hybrid solar thermal plants, including those on coal- and natural gas-fired power plants, are being developed, with a total capacity of 460 MW.12 Thermal hybrid projects based on unconventional geothermal resources are at an earlier stage of development and the field will require additional research prior to large-scale demonstrations.13
Some 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 of the different schemes being considered appear to be technically and/or economically viable. However, some do appear to be more robust. On-going developments (in, for instance, gasifier and electrolyzer design) should improve cost competiveness. Where hydrogen and/or oxygen production forms part of a hybrid energy scheme, reductions in the cost of electricity provided by renewable energy sources (such as wind and solar) would also be beneficial in making electrolysis more cost effective. Some examples of on-going hybrid projects are given in Table 2. Although some are currently focused only on biomass, potentially different elements from these processes could also be incorporated into systems fueled by coal/biomass combinations.
A number of projects are more advanced than others, with development programs well underway. Some components (such as co-gasification) have now been well established, and others are under development or being trialed (such as the commercial-scale demonstration of hydrogen production from wind power and testing of advanced electrolyzers). A number of proposed hybrid systems show potential—although in the near to 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, favorable circumstances.
Set against a background of growing global population and rising energy demand, there is a pressing need to come up with new, cost-effective, clean, reliable energy systems. To help tackle this, many hybrid energy schemes have been proposed, some more practical than others. Despite efforts by many countries to diversify their fuel mix, fossil fuels such as coal will continue to provide a significant part of the world’s energy for the foreseeable future. For a number of reasons, where possible, it makes sense to look at coupling coal use with renewable energy sources. Each power-producing system has its own pros and cons, but combining these different systems in creative ways may offer the possibility of overcoming some of these shortcomings. With this in mind, various energy production concepts that propose combining a number of different technologies with coal are being developed around the world.
To be a practical proposition, as with all power-producing systems, any hybrid scheme needs to be clean, workable, and economically sound. Based on work carried out recently by the IEA Clean Coal Centre, some hybrid systems appear to be viable and have potential.14,15 Although coal and renewable energy sources might appear to be strange bedfellows, it’s not unrealistic to suppose that in the coming years we could see increased deployment of combinations of the world’s two fastest-growing energy sources becoming a reality.
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