Amidst the climate crisis arising from the escalating emissions generated by coal-fired power plants, some proponents contend that modifying our approach towards coal plants, rather than their complete retirement, would solve the climate issues. In this context, the term “clean coal technology” has emerged. However, a critical question arises: does clean coal technology represent the definitive solution for addressing climate change, or is it merely an endeavour to create a greenwashing perception?
The environmental and health impacts of coal emissions from the power sector underscore the urgency of transitioning to cleaner and more sustainable energy sources to mitigate climate change. In addition to shifting to renewable sources, there have been discussions emphasizing the importance of not vilifying coal but acknowledging it as an energy source that necessitates the implementation of modern technologies to minimize its environmental impact. This approach holds promise as it enables the continued utilization of coal-based power generation, which still is a highly cost-effective option for electricity generation in numerous countries, including Indonesia.
Discussions surrounding the concept of “Clean Coal Technology” or CCT emerged several decades ago, with its origins dating back to the 1960s when flue gas desulfurization technology was introduced, aimed to reduce the acidity levels in flue gas, which is responsible for causing acid rain [4]. In subsequent years, particularly in the 1970s and 1980s, the discourse shifted towards mitigating the adverse health impacts and visibility of the emissions by removing particulate matter and nitrogen oxide from coal combustion [4]. After implementing these technological measures, effectively reducing sulfur and particulate matter, the term “clean coal” regained prominence in the late 20th century. This renewed attention stemmed from the growing recognition of the need to address carbon dioxide (CO2) emissions from coal-fired power plants, which have been identified as a significant contributor to global warming. Now, the technological avenues that aim to reduce carbon dioxide emissions are taken based on three techniques: increasing the boiler efficiency, changing the physical properties of coal, and integrating carbon capture and storage.
This article aims to explore various types of modern-day clean coal technologies. It provides a case and compares these technologies with other electricity generation methods. The objective is to assess whether clean coal technology holds promise in facilitating the transition towards a more renewable future or is merely a form of greenwashing, lacking the necessary technological advancements required to mitigate the impact of global warming effectively.
Technological Avenues on Boiler Efficiency
In traditional coal-fired power plants, combustion involves directly burning pulverized coal (solid form) to generate heat. The heat converts water into high-pressure steam in a boiler, which drives a steam turbine connected to a generator. The boiler plays a crucial role in determining the efficiency of coal-fired power plant technology, as it functions as a heat transfer medium that facilitates the conversion of energy from burning coal to water.
Most coal-fired power plants worldwide currently employ subcritical boiler technology [3]. Subcritical technology allows for steam production below its critical point (200 bar) and temperatures of 540 degrees Celsius [6]. Subcritical boilers are the most mature technology in the market, and their maturity enables them to have a lower cost than other boiler technologies. However, the drawback of subcritical technology lies in its low efficiency, resulting in higher coal consumption per unit of electricity produced and contributing to higher emissions. “Clean coal technology”, in this case aims to enhance boiler efficiency in coal-fired power plants by implementing supercritical/ultra-supercritical boilers. These boilers enable steam production at pressures above 221.2 to 374 bar, with temperatures above 540 degrees Celsius for supercritical boilers and above 660 degrees Celsius for ultra-supercritical boilers [6]. By operating at these higher parameters, supercritical and ultra-supercritical boilers achieve better efficiency compared to subcritical boilers in coal-fired power plants.
The improved efficiency of supercritical and ultra-supercritical boilers also reduces carbon dioxide emissions per unit of electricity produced. Citing studies from Steen in 2000 [8] and Kutani et al. in 2015 [5]. It is confirmed that the supercritical and the ultra-supercritical achieve lower emissions per kWh of electricity produced, ranging from 8 to 13%, compared to the less efficient subcritical counterparts.
Furthermore, the implementation of supercritical and ultra-supercritical boilers allows for an increase in flexibility parameters. Conventional coal-fired power plants typically operate as baseload units, inflexible in adjusting their electricity output to meet demand. However, with supercritical and ultra-supercritical boilers, coal-fired power plants can enhance their flexibility parameters, such as the ramp rate [6]. This enables them to adjust their electricity output better to match demand. This aspect is crucial as the integration of variable renewable energy sources, such as solar and wind power, increases, which will demand additional dispatchable power plants to help stabilize the grid by matching supply and demand.
Ultra-supercritical coal technology involves working with extremely high temperatures and pressures. Managing and controlling these complex conditions necessitate advanced control systems and sophisticated instrumentation to ensure safe and efficient operation. In addition, finding suitable materials to maintain structural integrity and resist corrosion at such high temperatures & extreme conditions is a significant challenge. The selection and development of appropriate alloys and coatings are crucial to ensure ultra-supercritical coal plants’ long-term reliability and performance [2].
Technological Avenues on Changing Physical Properties of Coal
While in the typical coal-fired power plants, the coal is burned in a pulverized solid form, coal gasification involves the conversion of solid coal into a gaseous form, called syngas, to transform coal into a cleaner burning fuel that exhibits higher efficiency. The Integrated Gasification Combined Cycle (IGCC) is one technology that can encompass that. IGCC technology comprises a dual-stage process called coal gasification and combined cycle power generation. Coal undergoes a transformative process to yield syngas, an artificial gas composition during coal gasification, facilitating the conversion of coal into its gaseous constituents, primarily carbon monoxide (CO) and hydrogen (H2) [6]. The gasification step also aids in the elimination of impurities, including sulfur and ash, resulting in a cleaner gas product.
Following purification, the purified syngas is utilized as a fuel source within a combustion turbine, akin to a gas turbine. Combusting the syngas within the turbine produces high-pressure and high-temperature gases that induce turbine rotation, generating electricity. The exhaust gases emitted from the combustion turbine are then directed to a heat recovery steam generator (HRSG). In this component, the residual heat from the exhaust gases is harnessed to produce steam, which drives a steam turbine connected to an electric generator, leading to additional electricity production. Incorporating this combined cycle configuration augments the system’s overall energy efficiency, it is noted that the IGCC currently observes similar nameplate efficiency to the highly efficient ultra-supercritical technology with the potential of surpassing its efficiency by 2050 [6].
Furthermore, through a comparative analysis of the greenhouse gas emissions presented in Table 3, it becomes evident that the IGCC technology exhibits the lowest magnitude of carbon emissions compared to alternative pulverised coal technologies. Additionally, the IGCC technology, owing to its purification process, demonstrates reduced levels of particulate matter and NOx gases when contrasted with its counterparts utilizing pulverised coal [6].
Despite the promises, IGCC systems are more complex and more technically awkward to build and operate than standard coal-fired systems. There are still some barriers to be overcome before it can be regarded as a standardized utility option [2], one of them is cost. Currently, the levelised cost of electricity for IGCC is 8.7 USD cents per kWh, which is 35% higher than the widely used subcritical coal-fired power plants with a cost of 6.4 USD cents per kWh [3]. Furthermore, the investment cost for IGCC is higher compared to subcritical technology. The capital cost to construct a subcritical coal plant is 1.65 USD/Million per MW, while for IGCC, it is 45% higher at 2.4 million USD per MW [6].
Technological Avenues on Coal + CCS
Finally, an example of what is considered a clean coal technology involves the integration of carbon capture and storage (CCS) with the coal-fired plant technologies described. Carbon capture and storage is a collection of technologies that aim to extract carbon dioxide emissions from a gas mixture and subsequently store the captured carbon dioxide in an underground facility, thus preventing its release into the environment and mitigating potential environmental harm. It is claimed that CCS technology has the ability to remove 85 to 90% of all carbon dioxide emissions of coal-fired power plants [9].
Two carbon capture methods are employed in the context of Coal-Fired Power Plants (CFPP). For pulverized coal plants (subcritical to ultra-supercritical), post-combustion capture is typically utilized to extract carbon dioxide from the combustion flue gas. Conversely, the alternative method, pre-combustion capture, commonly integrated into Integrated Gasification Combined Cycle (IGCC) systems, is employed to extract carbon dioxide from the flue gas prior to the combustion process.
However, one of the major drawbacks of the CCS implementation of coal-fired power plant is the energy penalty incurred during carbon dioxide extraction/separation process from the flue gas or syngas. This energy requirement is unavoidable and is sourced from the plant itself. According to Hammond’s in 2014, the integration of CCS in coal power generation results in an annual energy penalty ranging from 8% to 16% of the total energy produced. Reducing energy efficiency necessitates the plant to consume more fuel to generate electricity, leading to increased fuel expenditure [1]. Consequently, this impacts the plant’s Levelised Cost of Electricity (LCOE), which can result in an LCOE increase ranging from 27% to 84% [3].
In addition to the considerable LCOE, integrating capture systems into power plants entails significant additional capital expenses, which vary significantly across different technologies. In the case of IGCC plants, incorporating carbon capture systems necessitates an initial investment that is 47% higher compared to non-CCS counterparts. Conversely, in the case of supercritical coal plants, integrating CCS technology requires a substantial 139% increase in investment costs compared to those without carbon capture systems [6].
The take:
Based on three technological avenues of modern-day clean coal technology, its evident that it holds the potential for reducing carbon dioxide emissions. However, it is important to acknowledge that none of the aforementioned technologies guarantees complete carbon dioxide removal. Consequently, it can be inferred that clean coal is not the same as green coal, indicating that coal-fired power plants do have some negative environmental impact during their operation.
Hence, one may question whether clean coal could be considered a form of greenwashing. This interpretation solely depends on the reader’s assessment of the information presented in this article. Nonetheless, as mitigation to combat climate change should be undertaken with many technological avenues, it is thus more crucial not to perceive coal as green technology or a definitive solution to climate change but rather as a temporary solution that can be improved through the aforementioned technological avenues to facilitate the shift towards renewable energy sources.
Author: Aditya Perdana
Sources :
Hammond, G. Spargo, J, 2014. The Prospects for Coal-Fired Power Plants with Carbon Capture and Storage: A UK Perspective. Energy Conversion and Management 86 (2014) 476-489
IEA Clean Coal Centre. 2019. Technology Readiness of Advanced Coal-Based Power Generation Systems.
2023. Making Energy Transition Succeed-A 2023’s Update on The Levelized Cost of Electricity and Levelised Cost of Storage in Indonesia. Jakarta : Institute for Essential Services Reform
Kuchta, D. 2022. What is Clean Coal Technology? Overview, History, Carbon Emissions. https://www.treehugger.com/what-is-clean-coal-technology-5200812 . Accessed on 27th May 2023
Kutani, I. and V. Anbumozhi (2015), ‘Comparison of Technologies’, in The Macroeconomic Impact of Coal-Fired Power Plants. ERIA Research Project Report 2014-43, Jakarta: ERIA, pp.15-25
MEMR, DEA. 2021. Technology Data for The Indonesian Power Sector : Catalogue for Generation and Storage of Electricity. Jakarta. Ministry of Energy and Mineral Resources.
2022. Combustion Types of Operational Global Coal Power Stations by Plant Age 2022. https://www.statista.com/statistics/859116/global-coal-plants-combustion-types-by-age/ . Acessed on 27th May 2023.
Steen, M. 2000. Greenhouse Gas Emissions from Fossil Fuel Fired Power Generation Systems. Joint Research Centre, European Commission-EU.
2021. CCS Technology Equipped to Exceed 90 per cent capture. https://ccsknowledge.com/blog/ccs-technology-equipped-to-exceed-90-per-cent-capture . Accessed on 27th May 2023.
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