Recently a group from the University of Colorado (Boulder) analysed the world’s power stations and declared that just 5% of the world’s fossil fuelled stations could be responsible for 73% of all emissions coming from the power sector (Grant and others, 2021 ). The paper ‘Reducing CO2 emissions by targeting the world’s hyper-polluting power plants’ identified the locations of some of some the largest stationary emitters of GHGs in the power sector.
Within the paper, there is a plethora of statistical methods and references, but one of the main items is a list of the top ten emitters in the world. It is of little surprise that the short list is populated by extremely large coal-fired plants. However, the surprise came when six of the ten plants in the list were higher efficiency supercritical (SC) and ultrasupercritical (USC) plants. Most are also equipped with a suite of pollutant controls to reduce or almost eliminate the release of particulate matter (dust), SOx, and NOx. Without clarity on the individual assumptions used for each plant it is difficult to assess the accuracy of the calculations, especially as the average utilisation of fossil fuel plants worldwide is significantly below their ideal design conditions.
It is therefore worth exploring a couple of questions provoked by the Colorado paper: can so few plants generate such a large proportion of the emissions? And is there an another way to make meaningful cuts in GHG emissions from the fossil fuel fleet that does not require the immediate closure of these power plants?
Nomenclature is important here, where Grant and others (2021) count numbers of plants, whereas perhaps a more logical approach would be to consider megawatt (MW) capacity. This is because a single plant can comprise many separate units of varying MW capacity built with different technologies and running with varying CO2 intensities. This is particularly the case with some of the large plants listed.
An alternative assessment of the latest 2021 S&P World Electric Power Plant (WEPP) database by the ICSC found that its own “5% list” of the largest emitting plants could account for 50-60% of fossil generating capacity when converted to installed MWe generating capacity. Similarly, the “5% list” accounted for roughly the same proportion of emissions . These large plants are likely to be running at higher loads than smaller ones, but delivering more power more effectively. Alternatively, plants may be operating flexibly to balance out the peaks and troughs of electricity generated from variable renewable energy, namely wind and solar.
In India, the largest plants tend to operate at plant load factors (PLF) of 90% or more, yet the national average has been fallen to around 50%. The Sasan UMPP is listed as one of the major ‘polluters’, but before seeking closure of this plant, consider that it serves seven states in India with a potential outreach of 17 million people. What are the immediate alternatives? Running such a plant of this scale is critical to providing better access to power, better access to clean water supplies, street lighting, and so on.
The Japanese plants that were identified in the ‘5% list’ comes as a surprise given Japan has been operating the most efficient coal fleet in the world for years. However, 3 of the 5 units at Hekinan are around 30 years old so are nearing the end of their working lives. Japan’s fossil fuel plants, particularly coal, are responsible for energy security in Japan following the loss of nuclear power plants since 2011. Today, nuclear output is 80% below levels seen just prior to the Fukushima incident. So, some coal-fired plants may be operating at high loads to fill this gap – illustrating the strategic role of fossil-fuelled power plants.
In 2018-19, coal plants worldwide operated at an average utilisation of just 53%, surpassed only by geothermal and nuclear power. Gas-fired power plants operated at 47% and oil at 21%. These levels were seen in almost every region, including Europe and North America (ICSC estimates based on IEA WEO 2020). This reveals a little more about why and how large amounts of emissions appear to come from relatively few coal plants. As a commodity, international coal prices tend to fluctuate at much lower amplitudes than gas and oil, and so countries dependent on fuel imports have greater energy security with a broader fuel mix, rather than being dependent on just a few volatile ones.
The higher proportion of emissions coming from large coal power plants with higher operating loads is logical compared with smaller scale coal plants running at lower loads and other fossil fuels. There are thousands of plants in the rest of the fleet that do not appear in the ‘5% list’. Of these, gas could account for 50-60% of the capacity, while coal accounts for 30% and oil at 10-20%. In future, gas plants may take on a larger role as countries look to scale back power production from coal while coal plants shift to a more flexible role to supplement variable renewable power.
Some large plants in Asia are big emitters – but are also more modern and efficient than anything seen in Europe or North America
Calculating CO2 emissions from the power sector also requires accurate assumptions for unit-level fuel consumption, electricity output, plant heat rates (efficiency), and CO2 emission factors for different technologies. This is no easy task due to the dearth of comprehensive performance data for the world’s power plants.
We can estimate the overall emissions of the world’s fossil fuel fleet based on past records of fuel consumption and electrical output. According to ICSC estimates, in 2018, the global coal fleet averaged 34.9% (net LHV) efficiency, oil products averaged 36.2%, and gas was 43.1%. The global average for gas plants falls short of the 60% (net) efficiency expected of the latest combined cycle gas turbines (CCGT) on account of half the world’s gas fleet being single or open cycle gas turbines (GT) or internal combustion engines (IC). Similarly, the average coal plant performs below the current technical potential of >49% (net) efficiency.
However, the efficiency of the coal power fleet is rising steadily. Since 2000, China’s coal fleet efficiency has increased from 32% to 39% in 2018. Given the size of the coal power fleet in China, this achievement is impressive. Efficiency gains have also been seen in Poland, India and Germany, while ageing fleets in countries like the UK have continued to decline in terms of both efficiency and output.
In recent years, the building of large efficient coal plants in China has replaced massive numbers of ageing, small, and highly polluting plants. This strategy helped to halve China’s carbon intensity from 1.2 kgCO2/US$ in 2005 to 0.7 kg CO2/US$ in 2018 . Within the last 15 years, China has built the world’s largest coal fleet using some of the largest generating units, each exceeding 1000 MW. Almost all these massive units are SC or USC. No new units of this scale use subcritical technology anywhere in the world. So, it seems counter-intuitive that large stations should be labelled as ‘polluters’ when these same high efficiency plants are equipped with some of the latest emission control systems. China’s emission standards for coal plants far out-do those seen in many parts of Europe. These large, modern units emit less CO2 and use less fuel per kWh, and therefore reduce emissions and resource usage for the same amount of electrical output. So perhaps the target for improving local air quality and GHG emissions from the many smaller plants should not go overlooked at the expense of larger, cleaner plants?
Alternative pathways for fossil fuelled plants
If the bulk of the problem of CO2 emissions lies with fossil-fuelled power plants, perhaps, so does the solution. Grant and others (2021) offer pragmatic solutions that avoid a hasty closure and potential costly replacement of such large generating assets. These changes could and should happen very quickly. They include:
- increasing the operating efficiency of plants to reduce CO2 emission by 25%;
- and retrofitting carbon capture and storage (CCS) to slash greenhouse gases by almost 50% (Grant and others, 2021).
On the first point, a 25% reduction in CO2 can be achieved by replacing a subcritical unit emitting 1100 gCO2/kWh (35% net efficiency) with an USC unit, which emits closer to 800 gCO2/kWh (>44% net efficiency). Individual units can also be upgraded to varying extents to improve efficiency. Existing subcritical plants can be modified; for example steam turbine retrofits can achieve 5% points of efficiency gain, which could mean an emissions reduction of 10-20% depending on the efficiency of the original unit. The performance of existing SC and USC plants that operate at around 44% (net) can also be improved by various means to reach efficiencies of up to 49%. In Western and Central Europe, most coal plants are likely to be closed within the next fifteen years, but for the rest of the world, and especially in Asia. opportunities to push the efficiency of coal plants close to 50% (net) are available where the fleet is younger.
One of the most important outcomes from Grant and others (2021) is the need for CCS, which they estimate can reduce emissions by an estimated 50% from coal plants. The origins of the 50% quoted by Grant and others (2021) is unclear, but it seems very conservative. On a unit basis, CCS can theoretically cut CO2 emissions by more than 90%, even accounting for the extra energy required to operate the CO2 capture system. CCS is already proven in North American coal plants and momentum is gathering in Europe to create hubs to capture CO2 from clusters of point sources. However, to date CCS has only been fitted on small coal-fired units in North America of 120-240MW each project capturing 1.0-1.6 MtCO2/y. Scaling CCS up to large plants would create huge business opportunities to decarbonise and secure cleaner energy across the world. Therefore, financial and regulatory incentives to capture and store CO2 permanently should be facilitated by policymakers as part of creating a credible pathway to achieve net-zero emissions.
Grant and others (2021) have done some important work to draw attention to the problem and provided a new insight into what needs to be done urgently, but it is imperative that the message is not interpreted simplistically. Policymakers should be watchful of targeting the closure of what appears to be a small number of plants, which in reality, includes a significant amount of modern and high-efficiency generating capacity that supply millions of people with reliable, affordable supplies of electricity. Investing in decarbonising solutions to existing fossil-fuel stations is part of a prudent and effective pathway to meeting climate goals without jeopardising the strategic value offered by large thermal generating assets.
1 https://iopscience.iop.org/article/10.1088/1748-9326/ac13f1
2 The results will differ from Grant and others (2021) due to variations in assumptions, for example in 2018, the world average electrical efficiency for coal plants was 34.9% (net), 36.2% (net) for oil, and 43.1% for both CCGT and non-CCGT gas capacity; utilisations for the world fleet was 53.1% coal; 46.7% gas; 21.4% oil
