Upgrading Efficiency of World's Coal Fleet to Reduce CO2 Emissions

By Ian Barnes, Associate, IEA Clean Coal Centre for Cornerstone

Coal remains an important source of energy for the world, particularly for power generation. During the last decade the demand for coal has grown rapidly, as has the demand for gas, oil, nuclear, and renewable energy sources. Various projections for future growth in energy demand suggest that this trend will continue, dominated by coal use in the emerging economies, particularly China and India. Continuing pressure to cut CO2 emissions to mitigate the effects of climate change, specifically to limit the average rise in global temperature to between 2°C and 3°C, will require halving (from current levels) CO2 emissions by 2050.

To contribute to this goal, emissions from coal-fired power generation will need to be reduced by around 90% over this period: Cuts this deep will require carbon capture and storage (CCS). In the International Energy Agency (IEA) 450 ppm CO2 climate change scenario, around 3400 large-scale CCS plants must be operating globally by 2050 to abate the required amount of CO2 emissions.1 At the same time, the growing need for energy, and its economic production and supply to the end user, must remain central considerations in power plant construction and operation.

In 2012, the IEA concluded that, in general, larger, more efficient, and hence younger coal-fired power plants are most suited for economic CCS retrofit. However, the agency also found that only around 29% of the existing installed global coal-fired fleet could be retrofitted with CCS. Furthermore, on average, the efficiency of existing global coal-fired capacity is comparatively low, at about 33% (net HHV basis for all loads, all coals, and all steam conditions)A, although the recent establishment of large tranches of modern plants, particularly in China, is raising this figure. This article examines the first step in the decarbonization of the coal-fired electricity sector: increasing power plant efficiency.

Recently the IEA CCC published a study evaluating how improving coal-fired power plant efficiency would reduce CO2 emissions. For all nations evaluated, increasing the efficiency of the fleet of coal-fired power plants offered considerable CO2 emission reduction benefits, although variability was observed in the time frame in which such benefits could be realized.

REALIZING DECARBONIZATION THROUGH EFFICIENCY GAINS

Operating at lower efficiency means that relatively large amounts of coal must be used to produce each unit of electricity. As coal consumption rises, so do the levels of CO2 and other emissions. Upgrading existing plants and building new high-efficiency, low-emissions (HELE) coal-fired power plants addresses climate change concerns in two important ways. In the near term, emissions can be reduced by upgrading existing plants or building new HELE plants. Such plants emit almost 20% less CO2 than a subcritical unit operating at a similar load. Over the longer term, HELE plants can further facilitate emission reductions because coal-fired plants operating at the highest efficiencies are also the most appropriate option for CCS retrofit. For these reasons, there is considerable global interest in HELE technologies. Figure 1 illustrates the impact of employing progressively more effective HELE technologies and CCS on CO2 abatement (presented in terms of LHV at full load with hard coal).


FIGURE 1. Reducing CO2 emissions from pulverized coal-fired power generation

The terms subcritical, supercritical, ultra-supercritical (USC), and advanced ultra-supercritical (AUSC) describe the steam conditions by which electricity is generated in a thermal power plant. HELE technologies center on improvements to the steam cycle, allowing for higher steam temperatures and pressures and the consequent improvement in the steam cycle efficiency. A switch from subcritical to current USC steam conditions raises efficiency by around four to six percentage points. Historically, the majority of pulverized coal-fired plants were based on subcritical steam-cycle technology, but supercritical technology is now widespread, largely due to improvements in boiler tube materials. Table 1 summarizes the differences in operating pressures and temperatures for various types of coal-fired plants currently in operation. Although the definitions of supercritical and USC vary from country to country, the ranges cited in the table are used frequently.

Supercritical plants can be found in 18 countries and are now the norm for new plants in industrialized nations; USC steam cycles are now the state of the art. A current coal-fired plant operating with a high-efficiency USC steam cycle not only has improved efficiency, but is also more reliable and has a longer life expectancy.

Whereas the first supercritical units were relatively small (typically less than 400 MWe), larger units of up to 1100 MWe are now being built based on USC technology (such as the Neurath USC lignite-fired plant in Germany) and even larger units are planned.

Developments in AUSC steam cycles are expected to continue this trend. AUSC coal-fired plants are designed with an inlet steam temperature to the turbine of 700–760°C. Average metal temperatures of the final superheater and final reheater could be higher, up to about 815°C. Nickel-based alloy materials are needed to meet this demanding requirement. Various research programs are underway to develop AUSC plants. If successful, a commercial AUSC-based plant would be expected to achieve efficiencies in the range of 45–52% (LHV [net], hard coal). A plant operating at 48% efficiency (HHV) would emit up to 28% less CO2 than a subcritical plant, and up to 10% less than a corresponding USC plant. Commercial AUSC plants could be widely available by 2025, with the first units coming online in the near future.

To illustrate the potential of HELE technologies, Figure 2 summarizes the impact of different steam-cycle conditions on an 800-MWe power station boiler burning hard coal and operating at an 80% capacity factor. Such a unit would generate 6 TWh of electricity annually and emit the quantities of CO2 shown in the figure, depending on its steam-cycle conditions and corresponding efficiency. Thus, replacing a unit of this type operating with a subcritical steam cycle with a unit based on AUSC technology (under development) would result in savings of CO2 in the region of 30%.


FIGURE 2. The impact of HELE technologies on CO2 emissions

COAL FLEETS IN DIFFERENT COUNTRIES

Across nations, a legacy of using coal to produce electricity has given rise to coal fleets of differing age and efficiencies. Countries with a long history of using coal to generate power tend to have mature coal fleets that are maintained and upgraded with replacement components and new plants when necessary. Newer coal users tend to have younger coal fleets, in some cases based on the best available technology. These two extremes are well illustrated by comparing the coal fleet profiles of Russia and South Korea (Figures 3 and 4, respectively). Russia’s fleet is older, and thus consists of mostly subcritical plants, whereas South Korea’s recently built and rapidly growing fleet is made up primarily of supercritical and USC plants.

FIGURE 3. Russian coal-fired power fleet by year of construction and steam-cycle conditions
*Planned or under construction


FIGURE 4. South Korean coal-fired power fleet by year of construction and steam-cycle conditions
*Planned or under construction

The IEA CCC recently examined the potential of HELE coal-fired power to reduce CO2 emissions; the principal coal-consuming nations were studied: Australia, China, Germany, India, Japan, Poland, Russia, South Africa, South Korea, and the U.S. Notably, the coal-fired power fleets of these countries vary in age and efficiency, and have different local conditions and policies that affect the possible scope for implementing HELE technologies.

The coal fleet profile of each country to meet future electricity demand was assessed under three scenarios: continuing electricity generation based on the existing fleet and retiring and replacing older plants on the basis of a 50-year or 25-year plant life. The potential impact of HELE upgrades on CO2 emissions was quantified and costs of implementation were estimated. Industry norms were used for unit efficiency and availability and current assumptions on capture rates from CCS retrofitted to HELE plants were assumed.

HELE UPGRADES IN THE LARGEST EMERGING ECONOMIES

As China and India represent the largest emerging economies and both rely heavily on coal, the key findings for the Chinese and Indian studies are summarized below.

China

The Chinese coal-based fleet is the largest in the world, as are the associated CO2 emissions. These plants account for approximately 41% of the global coal-fired capacity and are responsible for approximately 37% of global CO2 emissions from coal through the production of electricity.2 China’s coal-based fleet—with a median age of less than 20 years—is by far the youngest currently in operation. In addition, a significant number of the newer plants employ supercritical or USC steam conditions.

By the end of 2013, China’s total electricity capacity was 1247 GW. With a reported coal-fired power generation capacity of over 786 GW and an annual total generation of 3947 TWh (2013 data),3 China is the world’s largest producer of power from coal. Predictions on the role of coal in China’s future energy requirements generally agree that coal will continue to be a very significant contributor to the country’s energy needs, although estimates of the relative importance of coal with respect to other primary energy sources differ. China is actively seeking to diversify its electricity supplies. The electricity capacities from other energy sources currently stand at 22% for hydroelectric, ~8% for other renewables (led by wind at ~6% and solar at ~2%), 6% for natural gas, and 1% for nuclear power. Although power from these sources is growing, they still account for a relatively small share of China’s energy generation profile, with coal still responsible for about 70% of electricity generation.

The Chinese government has set a target to raise non-fossil fuel energy consumption to 11.4% of the total energy mix by 2015 as part of its 12th Five-Year Plan. The U.S. Energy Information Administration (EIA) projects coal’s share of the total energy mix to fall to 59% by 2035 due to anticipated higher energy efficiencies and China’s goal to reduce its carbon intensity.4 Still, absolute coal consumption is expected to double over this period, reflecting the large growth in total energy consumption.

China is the premier example of a country benefitting from an actively pursued HELE upgrade policy. By utilizing state-of-the-art USC plants for new and replacement capacity, and with the retirement of older, less efficient units, CO2 emissions are projected to rise less steeply than the increase in demand for coal-based electricity; emissions are projected to reach 6136 Mt in 2040. If China continues to adopt the best technology and retire older units on a roughly 25-year timescale, a largely AUSC-based coal fleet would see projected CO2 emissions actually fall between 2035 and 2040; in this case the CO2 emissions are projected to be 5153 Mt in 2040 (a 16% reduction over the base case scenario), despite a continuing increase in demand. If the most effective CO2 abatement pathway is followed (25-year plant retirement, AUSC upgrades after 2025, CCS installation) emissions could fall to 750 Mt in 2040 (see Figure 5). Although the analysis presented here does not incorporate China’s recent announcement to peak coal utilization by 2020, such a policy approach would certainly require continued aggressive deployment of HELE coal-fired power plants.


FIGURE 5. China’s coal-based power fleet composition and CO2 emissions under a plan to retire plants
after 25 years of operation, from 2015–2040

India

India has the third largest coal-fired power plant fleet installed in a single country. The Indian coal fleet contributes approximately 6% of the global coal-fired capacity with approximately 8% of global CO2 emissions from coal through the production of electricity.2 India has a relatively high share of smaller units (i.e., <400 MWe) and many of India’s power plants burn high-ash coal (up to 50%). The majority of the Indian coal-fired power plant fleet is based on subcritical technology, although some recently built plants have incorporated supercritical steam cycles. Overall, the fleet is relatively young and a very large portfolio of supercritical plants is reported as planned or under construction, which will make India the second fastest growing user of coal for electricity generation (after China) by 2020.1

India’s 12th Five-Year Plan (2012–2017) sets a goal that 50–60% of new coal-fired plants must use supercritical technology, although observers suggest that significantly less is likely to be achieved. Early indications of India’s longer-term policy direction suggest that the 13th Five-Year Plan (2017–2022) will stipulate that all new coal-fired plants must be at least supercritical, thus no new subcritical plants would be allowed.5

India is a rapidly developing country with considerable energy poverty and rapidly growing energy demand. Growth in coal-based energy demand is projected to extend to 2040, with no sign of leveling off. If new capacity is based on the best available HELE technologies and older plants are retired after 25 years and replaced with HELE units, CO2 emissions will first flatten out and then decline, despite increasing demand: 764 Mt in 2015 to 1063 Mt in 2040; a 39% increase (see Figure 6). With implementation of CCS, emissions could be reduced much more rapidly.


FIGURE 6. India’s coal-based power fleet composition and CO2 emissions if subcritical plants
were retired after 25 years of operation, from 2015–2040

KEY CONCLUSIONS

The results of the IEA CCC study reveal trends for the major coal-consuming countries. Some trends are specific and depend on the profile of the respective coal fleet and the prospects for growth or decline in coal-sourced electricity, while other trends are more generally applicable. A few key conclusions can be garnered from the larger IEA CCC analysis:

  • Countries experiencing a prolonged period of growth necessitating additional power capacity and having a relatively new coal fleet are characterized by rising CO2 emissions, but these are projected to be offset by the use of AUSC over USC plants for new builds (e.g., China and India).
  • Countries experiencing a prolonged period of growth necessitating additional capacity and having a more mature coal fleet are characterized by rising CO2 emissions, but these are projected to be offset by the use of AUSC over USC (e.g., South Africa), particularly when older plants are retired and replaced by AUSC units.
  • Countries experiencing a prolonged period of growth necessitating additional capacity and having an old, relatively inefficient coal fleet see falling levels of CO2 emissions, even with growth in electricity demand (e.g., Poland and Russia).
  • Countries experiencing relatively low to moderate levels of growth and having an efficient coal fleet do not see significant reductions in CO2 emissions until 2040 when some older plants are projected to retire (e.g., South Korea).
  • As an existing coal fleet transitions to a HELE composition it becomes smaller with respect to installed capacity. This potentially benefits the siting and replacement of plants, particularly in countries where planning regulations are demanding and time consuming.
  • The greatest gains are seen when plant life is limited to 25 years (an evolving practice in China) rather than 40 years or more (common in OECD countries). Policies and incentives to encourage shorter timescale plant renewal would enhance CO2 savings.
  • When CCS readiness is considered, in all cases, the 25-year plant life scenario represents the best option for CCS deployment as all coal fleets transition to a high HELE composition quickly and enjoy maximum CO2 abatement as any remaining lower efficiency capacity is retired. This is particularly evident in the Indian case where the effects of rapidly increasing electricity demand are attenuated by a combination of HELE and CCS technologies.
  • Economics will govern the decision to replace plants unless policies and incentives drive the selection toward HELE technologies.

HELE plant upgrades can be considered a “no regret” option for coal-fired power plant owners and operators. A current state-of-the-art coal-fired plant operating with a high-efficiency USC steam cycle will be more efficient, more reliable, and have a longer life expectancy than its older subcritical counterparts. Most significantly, it will emit almost 20% less CO2 compared to a subcritical unit operating under similar load. In the near future, developments in AUSC steam cycles promise to continue this trend: A plant operating at 48% efficiency would emit up to 28% less CO2 than a subcritical plant, and up to 10% less than a corresponding USC plant. In addition, when CCS is available it will likely be applied to higher efficiency plants, making HELE a first step toward deep carbon emission reductions.

It is hoped that this study has provided an overview of what might be achieved in the major coal-using countries through an aggressive uptake of HELE technologies and the role they can play in reducing CO2 emissions. Deeper analysis by the IEA CCC is planned on a country-by-country basis to provide policy makers and planners with a local perspective on how HELE implementation can reduce emissions.


Steam turbines at the ultra-supercritical Waigaoqiao No. 3 (Shanghai) (photo courtesy of IEA CCC)

 

This article is republished by permission from cornerstonemag.net.  All rights reserved.

The content included in Cornerstone is based on the opinion of the authors, and does not necessarily reflect the views of the World Coal Association or its members.

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REFERENCES

  1. International Energy Agency (IEA). (2012). Technology roadmap: High-efficiency, low-emissions coal-fired power generation, www.iea.org/publications/freepublications/publication/TechnologyRoadmapHighEfficiencyLowEmissionsCoalFiredPowerGeneration_Updated.pdf
  2. IEA. (2010). CO2 emissions from fuel combustion, www.oecd-ilibrary.org/energy/co2-emissions-from-fuel-combustion-2010_9789264096134-en
  3. China Electricity Council. (2014). Generation, english.cec.org.cn/No.117.index.htm
  4. Energy Information Administration (EIA). (2013). Annual energy outlook 2013, www.eia.gov/forecasts/archive/aeo13/
  5. George, T. (2014). Private communication. Second Secretary Energy & Resource Security, British High Commission, New Delhi, India.

This article is based on an IEA CCC report, “Upgrading the Efficiency of the World’s Coal Fleet to Reduce CO2 Emissions”, by Ian Barnes, CCC/237, 99 pp, July 2014. The report is available for download from the IEA Clean Coal Centre Bookshop: bookshop.iea-coal.org; the author can be reached at ianbarnes@hatterrall.com

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