Review of transition paths for coal-fired power plants

0 Introduction

The history of energy utilization has been dominated by fossil energy, which pollutes the environment.The increasing worldwide awareness and consensus has led to a transition from traditional fossil fuels to renewable energy.The move toward a renewable future lies on the change of the global energy market from fossil-based energy to clean energy, which is expected by the second half of this century.Ultimately, a significant and indispensable goal to enable the transition is to limit greenhouse gas (GHG) emissions [1].In addition, a lower-carbon economy will be a significant step toward meeting the need for climate stability [2].

Energy policy experts from several fields have researched transitions in developed countries.For example,reference [3] conducted an early investigation into the biomass transition from coal to oil in the United States(US), which was completed in decades, considerably faster than the other countries [4-5].Contextual factors have significantly influenced the change in European countries,including infrastructure, industrial and domestic energy demand, and government policies [6-7].Energy transition has led to a significant decrease in the cost of energy services and an increase in their utilization [2, 8].

A significant portion of the traditional power supply comes from coal-fired power plants [9].However, this type of plant has many adverse environmental and health effects,such as air and water pollution.The burning of coal emits into the atmosphere far more CO2 per unit of heat energy than the combustion of other fossil fuels.There is increasing concern regarding the potential effects of global warming,which could be caused in part by increases in atmospheric CO2 [10].This study contributes to the literature by analyzing different policy projections and their assumptions to reduce carbon emissions from coal-fired power plants,organized by some major countries in the energy sector.We focused on different clean transformation pathways for coalfired power plants, such as to evaluate which one is more efficient for current environmental concerns.In our review,we attempted to find a solution to the major concerns from an environmental perspective, such as what are the feasible,efficient, and significant transition pathways of coal power plants to achieve a carbon-free habitable environment.In addition, we provide a discussion on various initiatives adopted by different governments in the world.

The remainder of the paper is structured as follows:Section II presents the historical background of primary energy transition, related works on the transition of coalfired power, and policies of different countries to achieve the carbon-free goal.Section III analyzes the five most important and efficient coal-fired power plant transition paths that can be taken by different governments to achieve the net-zero emissions target by 2050.Section IV presents suggestions for future coal-fired plant transitions.Finally,conclusions are presented in Section V.

1 Energy transition paths in the world

1.1 History of the primary energy transition

Following the transition from wood to coal and then to oil and gas, the third largest transition from oil and gas to renewable energy will occur in the future.The world’s first coal-fired power plant was designed in France in 1875.The advancement of human civilization accelerated the growth of the coal industry, and by the 1780s, coal had surpassed wood as the most important source of primary energy.This was the first conversion, from wood to coal.Internal combustion technology produced a significant increase in the need for both oil and natural gas resources in 1886.Advancements in geological theory, drilling, completion,and refinery technologies have led to a significant increase in oil and gas production.As a result, the contribution of oil and gas in the primary energy mix steadily increased to more than 50% in 1965, and these energy resources surpassed coal as the world’s largest energy source, which was the second transition, from coal to oil and gas.With the continued rise in economic and social demand for energy and the emergence of a low-carbon society, the third major transition from conventional fossil fuels to non-fossil energy has become unavoidable.

The growth of ecological and environmental concerns about coal, oil, and other fossil fuel resources has increased the awareness of the issues related to these resources.Coal, other forms extensively used, carbon resources, and the resulting fog are primary reasons for the early 1900s atmospheric problems in London and current air pollution issues in China and India.The increasing environmental consciousness creates a high demand for sustainable energy.Natural gas and renewable resources have occupied a larger share of the primary energy mix.By 2025, more than a fifth of the global energy demand will be supplied by oil, gas,coal, and new resources.However, new energy resources are far from having a significant role.

1.2 Three key tendencies in primary energy transition

In terms of energy resource types, production methods,and consumption methods, the global energy growth has changed from high- carbon to low-carbon, high technology,and even from one-time to multiple utilization.

(1) Energy types: from high-carbon to low-carbon energy sources, or from fossil to non-fossil energy sources.Wind power, hydropower, nuclear energy, and solar energy emit virtually no CO2.Harmful byproducts and carbon emissions produced by various forms of energy sources have been reduced as a result of the transformation from coal to hydrocarbons and hydrocarbons to new energy sources, meeting the needs for a green growth.

(2) Production methods: from original to technical production.As per the general pattern of energy production,primitive humans obtained wood directly from nature, and the value of engineering technology grew from coal mining to oilfield development.Technological advancements in nuclear, wind, solar, and other emerging energy technologies are time-consuming.The significance of technology often emphasizes the production of every form of oil.Oil well production, horizontal drilling, and hydraulic fracking enabled the successful utilization of numerous low-yielding wells by vertical drilling.The use of staged fracking in horizontal wells has sparked a “shale oil and gas revolution” in the energy industry in recent years.

(3) Energy utilization: starting from direct use and continuing to various forms of energy transformation.Before the Industrial Revolution, only wood was used for heating, which was replaced by coal with the development of the steam engine in 1769 and the internal combustion engine in 1875, leading to a new power production.The discovery of the electromagnetic induction by Faraday in 1831 ushered the age of power use for electrification.

1.3 Energy consumption and production by different energy sources

Currently, fossil fuels (coal, natural gas, and petroleum),renewable energy sources, and nuclear energy are the primary sources of energy.Electricity generation is a potential energy source produced via primary energy sources.Over the last few centuries, the availability of energy has changed the course of human history.New energy sources have been utilized, -first fossil fuels,then nuclear, hydropower, and now other renewable technologies-and the amount produced and consumed has increased.This section focuses on the amount of energy consumed, including total energy and electricity consumption, per capita consumption of countries, and changes of energy consumption over time.

(1) Fossil fuel

The massive increase in energy demand in emerging economies, combined with the approaching ecological carrying capacity of the environment, has forced humans to choose between various energy resources.This decision would have an impact on and transform the new global system of fossil energy use.The demand for energy in the US, Europe, and other developed nations remains stable,although the demand in emerging economies in the Asia-Pacific region is rapidly increasing.Global energy use has moved from a three-way division between North America,Europe, and the Asia-Pacific region to a polarization between the East and the West.World fossil energy consumption approached 1297908 kWh in 2014, with North America, Europe, and the Asia-Pacific region accounting for 21.3%, 20.1%, and 43.1%, respectively.As the primary energy use has increased, the demand for fossil fuels such as natural gas, oil, and coal has remained high.Oil usage has annually increased, whereas consumption of other nonrenewable fuels has fluctuated, with natural gas briefly replacing coal as the second-most-used fuel in 2015 [11].Natural gas demand is projected to increase globally due to lower natural gas prices in the US over the last two decades.Coal is the most widespread fossil energy source on the planet [12], and China accounted for more than half of the global coal output in 2019.

(2) Renewable energy

Power generation has become a major target for the use of renewable energy and is expected to continue in the future.With ongoing scientific and technological advancements in the production and utilization of wind,solar, and other renewable resources, Europe, Asia-Pacific region, and North America have initially emerged as three major new energy-generating regions.In 2019, a wind power capacity of 60.4 GW was installed globally,representing an improvement of 19% over the amount in the previous year.China and the US continue to be the world’s largest onshore wind market, accounting for more than 60%of the new capacity in 2019.Furthermore, according to the International Renewable Energy Agency, global gridconnected solar power reached 580.1 GW at the end of 2019,with 3.4 GW of off-grid photovoltaic (PV) capacity.Asia has the world’s largest share of PV capacity, with 330.1 GW of combined installed capacity, followed by China (205.7 GW),Japan (61.8 GW), and India (34.8 GW) [14].

(3) Hydropower

The world’s hydropower technology is maturing, and the growth of the industry primarily influences water resource delivery.The Asia-Pacific region, Europe, North America,and Latin America are the main manufacturing regions.In 2019, the global hydropower installed capacity was 1308 GW.China, Brazil, the US, Canada, and India are the top five countries for hydropower production, with an installed capacity of 356.4 GW, 109.1 GW, 102.8 GW, 82.4 GW,and 50.1 GW, respectively [13].Different governments such as China, the US, and Canada have encouraged the development of hydropower.

(4) Nuclear power

Global nuclear power production has decelerated because of the Fukushima nuclear disaster.Europe and North America are the two main manufacturing areas.At the end of December 2019, the global operating nuclear power capacity was 392.1 GW(e), comprising 443 operational nuclear power reactors in 30 countries.Nuclear power generated 2586.2 TWh of emission-free, low-carbon baseload electricity in 2019 [14].This corresponded to approximately 10% of the overall global electricity and nearly one-third of the low-carbon electricity generation.

The interactive graph in Fig.1 shows the change in the global energy supply.Historically, CO2 emissions from fossil fuels started to increase in 1900 owing to the unlimited use of fossil fuels.Fig.2 shows the annual CO2 emissions by country from 1785 to 2017.The introduction of a low-carbon economy has become a common concern in the context of climate change and global warming.The primary goal of a low-carbon economy is to reduce anthropogenic CO2 emissions and mitigate climate change.

Fig.1 Global primary energy consumption and transitions,1800–2019 from BP Statistical Review of World Energy

Fig.2 Annual CO2 emission by different counties from 1785 to 2017 from the Global Carbon Project and the Carbon Dioxide Information Analysis Centre

The combustion of fossil fuels for energy generation,heating, transportation, and industry produces most of the global air pollution.Moreover, phasing out fossil fuel energy requires a significant transformation of current policy frameworks.It should begin with aligning energy policies with the 2030 agenda goals and then prioritize technologies to cost-effectively deliver the fossil energy transition.It is essential to encourage public and private investment in fossil-free energy to create an enabling environment for renewable energy transition.

1.4 Related literature study

The increasing use of coal depends on the development of technological advances such as to mitigate the environmental problems associated with the coal use,thus promoting economic growth [15-17].The negative environmental impacts of coal mining and coal combustion,including the disposal of coal by-products, are well documented in the scientific literature.A number of processes have been used in coal-fired power plants to improve the efficiency and environmental acceptability of coal mining, processing, and use, and others are under development [18].These methods are referred to as clean coal technologies.They do not reduce emissions to zero or close to zero.Therefore, it may be more appropriate to refer to as clean coal technologies [19-20].

It shows how clean coal technologies reduce pollution and waste and increase the percentage of energy recovered from each ton of coal in reference [20-21].A variety of physical and chemical treatments can be used in the preor post-combustion stages, which can be broadly classified as processes related to (1) combustion efficiency or (2)emission reduction.

Coal gasification is a process in which coal reacts with a certain amount of oxygen to produce syngas or synthesis gas,a cleaner gaseous fuel [22].This process reduces SO2, NOx,and mercury emissions, resulting in an improved cleaner environment [23].The hydrogen gas produced can be used for power generation or as a transportation fuel [24-27].

An integrated gasification combined cycle (IGCC) is a system that attempts to maximally extract energy from burned fuel.Using IGCC, solid or liquid fuel is mixed to produce electricity.The technology used in combinedcycle power plants is comparable to that used in modern natural gas power plants [28].These technologies have been installed in a large number of new coal-fired power plants worldwide.In addition, IGCC power plants are claimed to have the lowest levels of air pollutant emissions (NOx, SOx,CO, and PM10) of coal-fired power plants in the world [29].Carbon capture, usage, and storage (CCUS) is an emerging technology that has a considerable potential to reduce emissions compared to other clean coal solutions [30].

The combination of clean coal technology with a sociotechnical transition will lead to a reasonable reduction in carbon emissions from coal-fired power plants.Voegele[31] introduced a socio-technical transition path in their research work, which is a multi-level perspective (MLP)approach.Additionally, technologies expected to become critical in the future have also been studied, such as hydrogen and battery vehicles [22].MLP approaches [32-34]are frequently used to study transformation processes, such as the transition from horse-drawn carriages to automobiles[35] or the transition from sailing ships to steamships [36].Examples include hydrogen and battery cars [37].Verbong[38-39] and Rosenbloom [40] examined decarbonization pathways by focusing on the entire energy system rather than a single technology.

Our study examines how the currently dominant technology becomes obsolete and the various transition paths.Although the MLP has proven to be an effective tool for studying socio-technical transitions to coal-fired power plants in the medium term, it has also been subject to constructive criticism.Fundamental politics and power relations remain unexplored.One reason is that MLP is heavily based on structuration theory, which does not adequately account for collective actors because of its focus on knowing actors, recursive action, and reflexivity [41-42].However, collective actors such as industry associations,government agencies, social organizations, and specific groups may determine field-level regulations and resources through surveillance.By considering these aspects in Section 3, we present five efficient transformation options to help achieve the ultimate climate goal.

1.5 Policies of different countries to achieve the carbon free goal

The reduction of GHGs emission is important to reduce the dangerous temperature fluctuations of the earth, tackle climate change, and guarantee the global temperature increase goal within 2 °C by 2100 [43].By 2015, the Paris Agreement established the goal of global net zero emissions of CO2.A growing number of governments have established strategies to achieve a carbon-free future.Currently, over 100 countries have joined an alliance to reach the net-zero emissions goal.Fig.3 represents the current status of the Paris Agreement by goal status and target date, considering the 26 most mentioned countries worldwide.

Fig.3 Progress of different countries based on Paris agreement

Countries such as Norway, Finland, Sweden, and Uruguay have set a target to achieve net-zero emissions by 2030.Their present status is still under peer review.Hungary, France, Canada, and the United Kingdom (UK)have signed a cooperative agreement to realize net zero emissions by 2050.These countries have attempted to achieve more significant emission reductions for the next decade and have scheduled to close their last remaining coal power plants by 2025 and increase renewable energy share.China has announced its commitment to become carbon-neutral before 2060 [17].Fig.4 illustrates the rates of carbon emissions reduction of nine countries committed to reducing emissions by 2050.The goals, policies, and measures of six of these countries are discussed in detail in the following sections.

Fig.4 CO2 reduction targets for different countries

The number of countries that have committed to reach net zero has increased, but even if they achieve this goal,there will be 22 billion tons of CO2 in the atmosphere by 2050, resulting in a temperature increase of approximately 2.1 °C by 2100, according to the International Energy Agency (IEA) in its “Net Zero by 2050” report.Considering these alarming issues, some important countries have announced different policies in their various sectors, such as the residential and service, industrial, energy, and transport sectors to maintain the temperature at a tolerable level.In this section, we discuss some significant policy issues for the six major countries.

(1) France

The French government has set goals to reach carbon neutrality by 2050 and reduce non-renewable energy usage by 60% before 2030.France plans to reduce emissions primarily from residential and service sectors, the industrial sector, the energy industry, and the transport sector [44].For the residential and service sectors, the government has adopted the following primary measures: new energy efficiency regulation, sustainable development tax credit,zero percent ecological loan, renovation of government buildings, and social housing renovation.

For the transport sector, the following primary measures have been adopted: development of alternative modes of transport, private vehicle emission reduction, automobile bonus-malus, development of biofuels, and kilometers ecotax for heavy vehicles.

For the industry sector, measures include the review of directives establishing a ceiling and quota exchange system.The renewable heat fund has stimulated proposals for the construction of biomass power plants and offered funds to encourage industries to reduce GHG emissions [45].

Various measures have been adopted for the energy sector, including energy-saving certificates, eco-design directive implementation, renewable energy production,carbon tax, biomass, geothermal power, wind power, solar power, and hydroelectric power promotion [46].

(2) Germany

Germany adopted its climate change strategy, Climate Action 2050, in November 2016 [47].This law sets targets for individual industries to reduce emissions by 2030.According to the German government targets, the energy sector must limit its GHG emissions to 175 MtCO2e by 2030, a reduction of 62% below 1990 levels.Electricity generation must be completely decarbonized by 2050.The energy sector met its 2020 target (280 MtCO2e) in 2019 (254 MtCO2e) [48].

The German Climate Act also specifies that new emissions allowances for the years after 2030 will be determined in 2025 [49].The CO2 emissions of German are still mainly from coal-fired power plants that generate electricity and heat [50].A multi-stakeholder “coal commission” was established in January 2019 to eliminate coal-fired power generation in Germany [51].The commission has established the phasing out coal by 2038.Projections show that under the German coal phase-out law, coal-fired power plants will release approximately 2 GtCO2e by 2038.Half of Germany’s carbon budget will be within the 1.5 °C limits of global warming for compensating the impacted regions [51-52].Germany has set a target to reduce carbon emissions by 51% by 2030 compared to 1990 levels.In this regard, researchers have developed joint projects with the government and the industry [53].

The current policy projections consist of two scenarios:one is based on an update of the forecast report published in March 2020 from the Federal Environment Agency,which includes all policies agreed by January 29, 2020, and those from the Climate Action Program.The other scenario,prepared in March 2020 on behalf of the German Ministry of Economic Affairs and Energy, quantifies the initiatives of the Climate Action Program [54].

(3) United Kingdom

UK GHG emissions will be nearly net-zero by 2035,following the proposed climate change action plans [55].The UK plans to reduce emissions primarily from domestic,agricultural, land use, waste management, industrial sector,energy supply, and transport.

The transport sector will explore policies on achieving an all-electric transportation system [56].In February 2020, the UK government issued a consultation seeking comments on moving the prohibition on new petrol and diesel automobiles for five years, from 2040 to 2035, or sooner if a more rapid transition is viable [57].The Road to Zero Strategy, published in 2018, comprises a £106 million investment in the development of low- and zero-emission cars in the UK [58].

The UK electricity sector has rapidly decarbonized in recent years.This is mainly due to a sharp decline in coalbased electricity generation, which accounted for only 2.5% of the total generation in 2019, offset by an increase in production from renewables and natural gas [59].By decommissioning fossil fuel production and government support for renewables, the energy sector has accounted for approximately half of the total reduction in carbon emissions in the UK since 1990.The industrial strategy of the UK is based on developing intelligent energy generation to renovate the power grid, redesign construction techniques to increase efficiency, and reduce industrial emissions [60].

(4) United States

The US has a long-term plan to achieve net-zero emissions by the mid-century [61].Policies have been adopted to decarbonize different sectors.

The US government has adopted primary measures regarding the industry sector, including electrification,hydrogen, material efficiency, longevity, replacement of fluorinated gases, methane capture and destruction,carbon tax, and industry energy efficiency standards.In addition, the electric vehicle (EV) sales mandate, carbonfree electricity standards, building electrification, and electrolysis of hydrogen are also included [62].

Regarding the power sector, carbon-free electricity standard complementary power sector policies have been adopted.Carbon emission-free standards or renewable portfolio standards are the most effective carbon-free electricity standards.Increasing the power transmission scale to facilitate power allocation is the first complementary policy to these standards [29].The second complementary policy is to expand grid flexibility, which includes peakload regulation and demand-side response.The federal government will implement incentives to encourage utilities to increase their electric building components.

Fundamentally, the US provides two policy corridors for coal transition: the federal policy corridor and the state policy corridor.Since 2015, the federal policy corridor has provided federal financing to enhance financial transition in coal regions [63].In the absence of a comprehensive national policy framework to address the implications of coal transition, some Western states have implemented legislations to address the effects of the coal industry decline.State policies depict a variety of policy channels and interventions to address the economic and social aspects of coal transformation.New Mexico’s recent law includes $30 million for restitution and $40 million for three transition programs, one of which is designated exclusively for afflicted indigenous communities.This will provide more support for coal transition in the future.

(5) Japan

Japan, as the fifth largest CO2 emitter in the world,has sought to reduce GHG emissions to zero and become a carbon-neutral country by 2050 [64].In addition to promoting renewable generation, it is still necessary to promote new coal-fired generation.Japan has also sought to increase its nuclear power [65].The transition from fossil fuels relies on the mass manufacturing of largecapacity batteries, allowing for excess electricity storage.Japan is preparing its transition to a hydrogen-based energy economy.Green hydrogen is a fuel with no emissions,whose main by-product is water.A new high-efficiency framework has been developed to ensure the phase-out of inefficient coal-fired power plants by 2030 as part of the energy strategy.In this regard, Japan’s coal-fired power generation is expected to reduce from 31% of its energy mix in the fiscal year 2019-20 to 26% in the fiscal year 2030-31, through regulatory measures and guidance [66].

(6) China

China has announced its carbon-neutral goal by 2060 and aims to reach peak emissions within the next decade [34-36].Regarding the power sector, achieving the necessary reductions implies a significant shift to renewable energy use for electricity, enhanced grid flexibility, power system restructuring, and technologies for carbon taxing for coal-fired generation at a large scale.In addition, the industry should advance and implement novel decarbonization technologies for electricity and heat generation.It is essential to improve precise monitoring and energy management services.

There should be a transition to EVs in the transport sector, assisted by public policies, battery development, and scaling-up of charging facilities.Commercial application of hydrogen fuel is also essential.Further reduction of pollution must be accomplished by treating waste incineration and enhancing carbon sink capability.

China has implemented several programs to reduce coal consumption and its impact.From 2013 to 2015, China developed a plan to reach peak CO2 emissions until 2030,while reducing the contribution of non-fossil sources as its primary energy to 20% [67-69].These targets were incorporated into China's nationally defined Paris Agreement emissions reduction commitments.More comprehensive methods and policies can also be adopted for faster transition,such as forming a task force, using instrument schemes,and coupling transition plans with equitable utilization of employees and their communities [70].

Based on our review, we found that some major countries worldwide have adopted various transformation steps for coal power plants.These countries do not approve the construction of new coal power plants for electricity generation.In addition, they have established two measures to control the existing coal plants.One is a coal retirement mechanism (CRM) to obtain and eliminate existing coalfired power plants within 10-15 years, rather than the 30-40 years expected lifetime.The other is a sustainable energy transformation mechanism (SETM) that provides technical skills and financial assistance to replace obsolete and proposed coal plants with a mix of energy conservation,renewable energy, storage, and probably gas as a bridge fuel.These countries have attempted to change their current rising coal-fired electricity production pattern and introduce policies to accelerate the phase-out of coal from electricity generation.They have developed low-carbon and carbonneutral power generation technologies to remove fossil fuel consumption from electricity generation by approximately the mid-century.

Coal-fired power plant transition policies can be followed by adequate clean energy (renewable energy)phase-in plans.Accomplishing the long-term temperature target of the Paris Agreement and reducing the use of fossil fuels provides multiple advantages and avoids negative consequences.In the next section, we discuss the different transition paths of coal-fired power plants, which can help attain a low-carbon emission goal.

2 Transition paths of coal-fired power plants

Several recent studies have focused on sustainability transitions.The MLP is a prime replacement for the mesotheory, whose theories aim to explain such transition processes [71].The key concept of MLP is that transitions occur through relations between three different levels:niche, landscape, and regime [72-73].The MLP argues that the top-down pressures of the landscape and bottom-up innovations of many new niches contribute to the instability of existing regimes, giving niches opportunities to develop and replace the existing regime.The strategic niche management (SNM) literature, which is closely related to the MLP [74] and evolves simultaneously, has emerged as a solution for expanding technology policies to promote the creation of technical niches through experimental policy instruments, potentially encouraging transitions to new regimes linking niche development and expanded niche market size [75-76].

Although MLP has proved to be a useful mediumterm mechanism for examining socio-technical transitions to sustainability, it has also been subjected to constructive criticism [42].The majority of critiques have focused on the lack of actionability, regime operationalization,epistemology, descriptive style, and methods.By addressing these concerns in our research, we can provide five critical and efficient transformation approaches that will aid in the achievement of the ultimate climate target.

2.1 Replacement with different renewables

Renewable energies are derived from natural processes such as sunlight, air, water flow, biological processes, and geothermal energy flows.The implementation of CRM is required to obtain and resign existing coal-fired generation within 10-15 years rather than the predicted lifespan of 30 years.In addition, SETM provides technical expertise and financial support to remove the retired and planned coal plants with a mixture of energy efficiency, renewable sources and storage, and possibly gas as a bridge [77].The use of CRM funds for green energy can be specifically addressed in SETM.

As shown in Table 1, the gap of renewable generation policies between countries is significant owing to various time limitations.The variety of clean energy sources and the various degrees of economic and social growth further contribute to a strategic difference in implementing typical scenarios, such as Denmark’s emphasis on the transition from coal power development to wind power.Denmark is a frontrunner in renewable energy because of the country’s abundance of offshore wind [78].The level of social recognition in the country is also significant for the development of clean energy[79].In general, investment in renewable energy is low.Fig.5 shows that clean energy investment has been relatively resilient, but it is still far below the levels necessary to attain a lasting decrease in global emissions.

Fig.5 Global investment in clean energy and efficiency and share in total investment.Source: IEA 2020

Table 1 Summary of renewable energy policies of some countries

Overview Wind power Solar power EV Energy storage Germany 65% renewable sources by 2030 55% to 60% of renewable energy by 2035 98 GW by 2030 10 million EVs by 2030 24 GW by 2030 United Kingdom 50% renewable sources by 2030 One-third of the nation’s electricity by 2030 54 GW by 2035 Ban the sale of all petrol and diesel cars by 2040 30 GW by 2050 Denmark 55% renewable sources by 2030 50% of electricity by 2020 3400 GW by 2030 1 million electric or hybrid vehicles by 2030 Australia 50% renewable sources by 2030 No data Large scale solar 8 MW,Smaller solar 26 GW, Midscale solar 2 GW by 2030 Half the new cars sold in Australia in 2035 3 GW by 2030 China 35% renewable sources by 2030 26% of electricity by 2030 35% of electricity by 2030 40% of all vehicles by 2030 12.5 GW will be deployed by 2024 India 175 GW of clean energy capacity by the end of 2022 60 GW by 2022 100 GW by 2022 No data The market will reach 70 GW by 2022

Although China’s energy production has a significant contribution from coal-fired generation, the country has the capability and financial means to accelerate its shift toward renewable generation.China plans to reduce the share of the nation’s energy supply to 44% by 2035.However,emissions from coal-fired generation will still be higher than the worldwide emissions target set by the Intergovernmental Panel on Climate Change (IPCC) [80].The outstanding commitment of the other G20 economies to withdraw coal could stimulate China to contribute to meeting the IPCC’s 2030 and 2050 goals.

In contrast, many lower-income countries, despite significant changes in renewable energy prices and the storage and distribution of electricity, lack the financial and technological resources to rapidly scale up renewable energy [45-46].Developing countries face political resistance, including legal restrictions in the transition from coal to other energy sources, such as their industrialized counterparts.The CRM/SETM will help prevent political opposition, strengthen trust in addressing energy security,and reduce the economic effect on communities affected and workers.In the long run, the CRM aims to close only 50% of existing coal electricity facilities, with a 10-15 year transitional phase.As confidence increases, this number is also expected to increase.By taking more robust action on the energy transition, countries such as China and Japan,which work on constructing and financing new coal-fired plants, will have more opportunities for their corporations to export renewable energy solutions.

2.2 Changing the fuel

Canada, the US, Japan, India, and China have attempted to convert their coal-fired power plants to natural gas and biomass power plants.However, it is controversial whether these conversions are beneficial to the environment [81].Reasons include regional renewable energy targets, national incentives and strict environmental regulations, consumer demand, and an economic climate that also makes coal less attractive.Although the conversion costs are significant,plants, water supplies, and power grids have already been implemented.Retrofitting an existing plant is often less expensive than installing complicated emission controls to maintain an aging coal plant during operation.

The majority of existing conversion projects consist of conversions to biomass fuel sources [82].Available biomass for energy includes agricultural food and feed crops, agricultural crop wastes and residues, herbaceous and woody energy crops, aquatic plants, wood wastes and residues, and other solid wastes.When wood is used as fuel, it releases the same amount of CO2 as burning fossil fuels.Some coal-fired power plants are reported to have been transformed from coal to natural gas, although such transformations are recommended to be substitutes rather than retrofits.The electricity industry can theoretically switch to natural gas by retrofitting existing coal-fired units to burn natural gas or close the coal plants and build new gas-fired plants.Several environmental impacts of biomass are better understood than those of natural gas.The combustion of natural gas produces nearly half the amount of CO2 emissions as coal, emits reduced nitrogen oxides and particulates, and generates virtually no SO2 and mercury emissions.The low concentrations of these air emissions indicate that natural gas does not contribute significantly to smog and acid rain formation.

Different countries have implemented different measures to convert existing coal-fired generation to burn other fuel types.For example, the Dover Municipal Power Plant in the US has been converted to a natural gas plant.In the UK,the Drax power station obtained state aid to be converted from coal to biomass, which costs £700 M.The European Commission has approved a €550 million state aid scheme to support the conversion of the Avedore power station in Denmark into a biomass plant.China and India have also proposed a plan to convert the coal unit of a giant plant to another fuel source to reduce carbon emissions.

2.3 Operation with ancillary service

Owing to the increasing penetration of various intermittent renewable energy technologies, enhanced cycling of fossil fuel plants is needed to meet additional energy demands and meet other ancillary services.It has been reported that increasing the integration of wind and solar energy increases the cost of maintaining the electricity supply [83].Energy-conserving technologies of renewable energy and energy storage systems are offered by power producers to reduce the costs of operation and maintenance[52-53].

Energy storage benefits extend beyond energy balancing.Power storage improves energy grid security and stability by offering a wide variety of ancillary services required for modern generation and distribution.According to the US National Energy Board, auxiliary services are needed to support the transmission of electricity from the seller to the purchaser, given the obligations of the control areas and the transmission of utilities within those control areas, to maintain the reliable operation of the interconnected power system.Energy storage vendors can provide a variety of ancillary services, such as regulations and reserves.

Regulation services are used to address unexpected changes in electricity production and consumption.Instead of starting and stopping thermal units, energy storage can be used in these short time frames to balance the grid by charging during the overproduction of electricity and discharging when the electricity supply does not meet the demand.Load following is an hour-t++o-hour regulation to meet the increasing demand for energy in the mornings and the declining demand in the evening hours.The functions include: 1) peaking - regulating the generation of power to meet peak requirements; 2) ramping, which ensures an equilibrium of power output within seconds to minutes after the generation of significant power fluctuations (e.g.,intermittent wind, passing of clouds over solar panels);3) frequency regulation - regulate electrical generation to accommodate demand only for a brief time.

Reserve service providers generate additional electricity used only to compensate for power outages and maintenance activities.Backup generators are an ideal technology for providing contingency power in the market.In this regard,there are three solutions: 1) spinning reserve - online generation, which is synchronized to the grid and can respond immediately within 10 minutes to generation and distribution outages; 2) non-spinning reserve - generation,which can reach full output within a 10 minute period,but does not achieve it immediately; 3) backup supply -generation of backups within 1 h for spinning and nonspinning reserves available.

Several power storage technologies provide both regulation and reserve options, but their best applications depend on the storage space and response time [81].The currently available methods are illustrated as follows.1) Battery: the properties of batteries depend on their chemistry and size.The fast response time and operation when separated from the grid makes batteries useful for regulation and energy reserve storage applications.2) Compressed air energy derived from renewable sources is used to compress the air and store it in underground caverns.Air is extended to turn the turbine and generate power to produce electricity.Compressed air storage facilities cycle daily to prevent damage from the cycling of compression and generation equipment; hence, they are not optimal for rapidly regulating electricity.3) Flywheels:flywheels store rotational energy by maintaining the angular velocity.Flywheel systems are suitable for shortterm savings, and their fast response times make them an excellent choice for regulation services.4) Pumped Hydro:the potential energy is converted into electricity when water goes down to spin a turbine.Pumped hydro storage systems are very well suited for energy reserves but are not viable for grid regulation.

2.4 Carbon trading

Carbon trading imposes limits on GHG emissions from power plants, industrial plants, and aviation [84].Companies receive allowances to optimally cover their total CO2 emissions for a year, which allows them to emit a certain amount of CO2.Therefore, companies can trade allowances mainly on the global market; they can purchase allowances from companies that are no longer using them or from auctions held to sell all allowances that are no longer needed.This trading strategy relies on reducing pollution in areas where this can be achieved with lower costs [85].The European Union indicates that the emissions trading system (ETS) has been active in reducing GHG emissions.This system has enabled a reduction of 29% of emissions in the European Union since it was implemented.China has implemented significant policies to address carbon emissions [84, 86, 87], limiting the share of coal-energy use,which accounted for 58% of the overall energy consumption in 2020.If China continues its decreasing coal production strategy, the CO2 emissions may continue decrease in the future.However, if the coal abatement is ceased, the CO2 emission in 2030 will be much higher than the 2015 level,as shown in Fig.6.

Fig.6 Continuous coal abatement in China using carbon trading policy

2.5 Carbon Capture, Utilization, and Storage(CCUS)

Handling the current coal-fired power fleet to minimize emissions is crucial for the world’s transition to clean energy.Pollution will be minimized by better retrofitting plants with CCUS, retiring obsolete plants before they start aging, and retiring inefficient plants at the end of their expected lifetimes.Any newly installed coal capacity would make an effective transition to clean energy more difficult to achieve.

Governments should apply a more positive approach to carbon capture [88].Carbon capture requires immediate removal of CO2 from the GHG emission system before emission.There are three essential methods of CO2 capture[89-90]: 1) post-combustion capture, which consists of capturing CO2 from the flue gas during combustion;2) pre-combustion capture by obtaining the synthesis gas (a combination of CO2 and hydrogen gas) from the fuel before combustion using a chemical process, such as gasification or reforming, and in sequence capturing the CO2 from that mixture; 3) oxyfuel capture, by utilizing(almost) pure oxygen to burn the fuel such that flue gas has a high concentration of CO2, which allows a relatively direct isolation.

When collected, CO2 needs to be purified to eliminate contaminants such as oxygen and water [91].The critical stages of the three carbon capture methods are shown in Fig.7.After collecting a more significant stream of CO2, it can be processed or used in an industrial process.The former process is called carbon capture and storage (CCS), and the latter technique is called carbon capture and use (CCU).

Fig.7 Basic principle of carbon capture

In carbon sequestration for carbon storage, three storage methods can be utilized: geological, ocean, or underwater.For CO2 utilization, many methods such as mineral carbonation can be used as a chemical feedstock for the manufacture of chemicals such as methanol [84], [92].The CCS and CCU methods are very expensive and difficult to incorporate into an already-functioning power generation,and deep storage of carbon underground or in the oceans may result in environmental hazards [86], [93], [94].Similar to other investments, the manufacturing costs of CCS or CCU increase with the size of the production unit [95].

On the one hand, supporting a CCS/CCU system would lead to the use of CO2; on the other hand, the system would consume a significant amount of energy.Both routes require financing.

3 Suggestions for the future coal-fired power plants transition

From the experience of the last decade, the world will need to increase carbon-free energy by at least 15 times over previous rates.Carbon-emitting technology should be immediately discontinued and banned [96].Over the past ten years, fossil fuel consumption has increased by nearly 150 million tons of oil equivalent (mtoe).In 2018, the record rise of 114 mtoe in carbon-free energy was offset by an increase of more than 275 mtoe in fossil fuels.Thus, the world's rising carbon-free energy production is additive, not a replacement for fossil fuels.

3.1 Principles and comprehensive measures

A successful and feasible strategy for the phaseout of a coal power station can be implemented in China, India, and other countries based on three principles [97]:

(1) No new coal plants

The effective execution of the 2 °C and 1.5 °C compatible coal phaseout strategies proposed depends on the current halt of new constructions of traditional coal plants in the world.Deciding not to build planned or underconstruction plants will prevent substantial investments from stalling and allow existing plants to phase out at an acceptable rate.

(2) Complete phase-out of the laggards

Close a group of existing units in a reasonably short period of time, based on the following criteria: age and efficiency.

(3) Guaranteed lifetime

The remaining units can have progressively and responsibly decreased hourly commitment (time commitment).The suggested pathways include the gradual retirement of older coal-fired power plants and the gradual construction of new privately built power plants powered by natural gas or renewables to meet the growing availability of renewable energy.The phase-out focuses on several evaluation parameters, such as grid energy stability,mitigating regional economic disparity, and delivering actual health benefits to people.

Achieving this ambitious and workable target will enable good partnerships and new government policies.Such a phase-out would entail near-term consultations and decisions on the introduction of a set of comprehensive measures [64-65].

(4) Financial support for transition

The provision of financial aid and other compensation mechanisms can encourage the reduced generation of electricity [98].With a guaranteed lifespan, most conventional coal-fired power plants will work on a 2 °C path over a projected 30 year life cycle and repay initial investments by operating for 20 years on a 1.5 °C pathway.Under this approach, the critical factors of possible economic effects on plants are shortened hours of service and no further operating period extension.When coal plants are run for limited hours to support massive renewable energy during periods of low demand, a limited-duration subsidy or load changing price can, in some circumstances,be acceptable.As a suggestion, any financial incentive scheme should be limited to already operating plants to prevent new production.

(5) Continued market reform

Ongoing power sector reforms and switching to marketbased dispatch mechanisms are vital components of this transition.Market-based dispatch supports accelerated renewable generation by allowing the most cost-competitive resources to be prioritized.In addition, it would remove the hidden protections that have shielded the coal plants from unfavorable market conditions and policy signals that would otherwise have shut them down.

(6) Grid planning and modernization

In countries like China and India, significant energy supplies are concentrated in their northern and western parts, and far from the extremely energy-demanding regions of the east coast [97-98].Support for a substantial number of sporadic energy sources (wind and solar) would require comprehensive grid maintenance and planning[99].Modernizing grid transmission and delivery over the long term, developing next-generation storage and other scalable technologies [100-101], and introducing demandside management technologies are promising strategies that require extra investment and resources from both the public and private sectors.

3.2 Efficient benefits and valuable suggestions

We analyzed some significant literatures on coal plant transition and identified some benefits based on the recommended transition pathways from coal to renewablebased energy efficient systems, which are as follows.

(1) Clean and affordable energy access.There are many renewable energy options, both on traditional grids and on decentralized microgrids or off-grid, providing access to all communities.Renewable energy can be distributed quickly and in places that are not connected to the grid,which is critical for those who have no access to electricity.Renewables in rural communities provide electricity while reducing poverty and deprivation in these areas, allowing for educational opportunities and longer study hours with access to light [102].

(2) Employment.Renewable energy creates job prospects that are critical for post-COVID-19 restoration.It provides economic and employment opportunities, and construction and maintenance can be labor-intensive,resulting in local job creation.The employment benefits from renewable energy outnumber the jobs lost during the transformation from fossil fuels.It is reported that 7.49 jobs are generated per $1 million invested in renewables,whereas this number is 2.65 jobs for the same investment in fossil fuels [103].

(3) Energy safety and inertness.Numerous countries in the region depend on fossil fuel imports, because their domestic supply is insufficient to meet their demand [104].The installation of renewable energy will replace fossil imports, provide security [105-106], and reduce price fluctuations.

(4) Emission trading.This could reduce global warming by implementing a carbon trading transition route.This is a source of income for developing countries, and pressures companies to be more environmentally conscious.

At the end of this section, we provide some valuable suggestions for achieving the carbon-free goal.First,national governments need to pursue best practices such as phasing out fossil fuels, pricing carbon, supporting renewable energy, and encouraging investment relocation.Second, national governments should shift to transition strategies, set targets, and develop long-term plans.Third, policymakers should focus on specific pathways to anticipate the transition and avoid further stranded properties.Fourth, the government must provide both financial support and capacity-building.Developed nations should focus their support for finance and other growth on the need for transition investments, leveraging corporate investors to finance the transformation of the coal energy system to clean energy.Fifth, regional and global collaborations should be built, in line with the goals of the Paris Agreement and involving stakeholders, the private sector, and civil society.International cooperation and coordination will help accelerate the implementation of recommended practices and jointly achieve key benchmark targets.Sixth, strive to improve cross-border electricity grid interconnections.Cross-border power grid integration is a critical area for enhanced regional cooperation [107].Seventh, various major financial institutions should work together to promote sustainable financing, transparent policies, and accountability.Development banks have a critical role in the investment flows of the Asia-Pacific and European regions.They are crucial for accessing private sector finance and improving the economy, particularly in a political environment where investment is still considered as a significant risk due to political uncertainties and other obstacles.Private sector involvement is required to achieve the ultimate goal.

4 Conclusion

Coal power plants are one of the world’s primary sources of electricity production, and are significant contributors to rising CO2 emissions worldwide.In this review, we analyzed some essential and efficient transformation pathways of coal power plants aimed to achieve a carbonfree environment.The path to global coal transformation requires significant changes in the global energy sector.First, an increase in energy efficiency and reduction of the overall energy supply to lower levels are necessary.Second,the world’s energy supply from coal-fired power plants has been decarbonized.By 2050, the share of renewable energy worldwide will increase significantly, leading to dramatic changes in the energy sources of the world.Fossil fuel consumption will decrease by 21% by 2030, and 66%of this decrease will come from renewables.New coal-fired power plants should not be built, and the decommissioning of existing coal-fired power plants should be accelerated.Traditional heat generators must become more adaptable.Flexibility should be improved by retrofitting hardware elements, lowering the minimum loads, shortening the startup times, and increasing the ramp speeds.By 2050, fossil fuel consumption will decrease by two-thirds of the current levels.

In this study, the most important and efficient transition paths of coal-fired power plants were analyzed.First, by implementing a sustainable energy transition mechanism and a CRM, countries can accelerate the transition from thermal energy to renewable energy while creating new jobs, improving public health, and changing the tendency of carbon emissions.Second, adapting the fuel of the existing coal-fired power plants with different biomass and natural gas sources instead of coal can reduce the carbon emissions.Third, additional ancillary services such as the use of electricity storage technologies, including batterypumped hydro, compressed air, and flywheel, can be another transition path.Fourth, the ETS limits gross GHG emissions and encourages low-emitting industries to sell excess emission allowances to higher-emitting industries.Finally, CCUS is one of the most promising technological solutions for achieving large-scale low-carbon fossil fuel use, although it is considerably capital intensive.

Our study also discussed some future proposals for the conversion of coal power plants according to principles,various strategies adopted by governments, and their comprehensive measures.A successful and feasible strategy for coal power plant phase-out can be implemented through three principles: no approval of new coal-fired power plants, immediate phase-out of laggard plants, and guaranteed lifetime for the necessary plants.Some broad measures can be adopted to support these principles, such as financial support for transition, further market reforms,and modern grid planning.The primary contributions of this paper to thermal power transition are in three aspects: 1) consideration of various measures taken by different countries for power plant transition, 2) analysis of efficient transition paths of coal-fired power plants, and 3) suggestions for the future transition of coal-fired power plants.

This study analyzed the transformation path of coal power based on low-carbon energy, advanced technology,and environmental impact, and relevant suggestions were proposed.However, considering carbon trading and CCUS technology, more research is needed in the future for a more detailed scheme.

Acknowledgement

This work was supported by Global Energy Internet Group Co., Ltd Science and Technology Project(SGGEIG00JYJS2000046) and by National Natural Science Foundation of China (51977123).

Declaration of Competing Interest

The authors have no conflicts of interest to declare.