Is recovery of coal waste from impoundments a bridge to net-zero future?

Context of waste coal recovery

Decarbonisation of the energy and industrial sectors is essential to meeting the Paris Agreement targets that suggested keeping the global mean temperature below 2°C and undertaking efforts to limit it to 1.5°C above pre-industrial levels [1]. This is because these sectors were responsible for 63% of global CO2 emissions from fossil fuels in 2018 [2]. Achieving such emissions reduction target requires a wide deployment of novel clean technologies that will enable these sectors to remain competitive at near-zero or even negative CO2 emissions.

Despite the recent decarbonisation movement and subsequent reduction in global coal consumption by 7% between 2018-2020, coal demand is expected to rebound by up to 2.6% in 2021 [3]. It is expected that coal will remain an important fuel in power generation at least until the 2030s, especially in countries whose energy sectors heavily rely on coal-fired power plants, such as China, India, and Poland. In the long term, the prospect of using coal for power generation is week. Without carbon capture and storage, coal-fired power generation must be phased out to achieve the net-zero power sector. However, it is often ignored that coal will have a role to play in enabling our transition to a net-zero economy. Namely, the production of iron and steel heavily relies on the supply of metallurgical coal. For example, approximately 150 (250) tonnes of metallurgical coal is necessary to produce steel for a single onshore (offshore) wind turbine [4]. Therefore, despite our activities to reduce the reliance on fossil fuels, we will still require a supply of coal to support our construction, steel making and cement industries.

Our continuous reliance on coal, however, poses significant environmental challenges. Namely, the development of new coal mines will add to already substantial methane emissions of 40 million tonnes per annum. Each tonne of coal extracted from a new coal mine results in 4 kg of CH4 emitted into the atmosphere [5]. With the global warming potential being 28-36 times that of CO2 over 100 years, methane emissions are a significantly higher contributor towards global warming [6]. Therefore, further coal supply for industrial production needs to be re-invented and alternative coal sources need to be explored.

Decades of coal exploration, mining and processing has led to a significant amount of coal tailings and waste being produced. It has been estimated that the waste produced during coal production corresponds to up to 40% of the total amount of material extracted [7]. It is estimated that the amount of coal fines and tailings reached approximately 30 billion tonnes in Top10 coal producing countries in 2014, with approximately 1 billion tonnes being generated by these countries per annum [8]. Recovery and reuse of this waste by-product can not only supply a significant fraction of the industrial coal demand, but also can open a route to land remediation and restoration.

How waste coal can be processed?

The waste coal processing technology developed by Changeover Technologies (aka. MetaForm?) aims to produce a pelletised coal product from waste material found in the coal waste impoundments. The process comprises four distinctive phases, as shown in Figure 1.   

Figure 1: Block flow diagram of coal processing technology by Changeover Technologies

In the first step of the considered process, a coal-rich slurry is extracted from the waste coal impoundment using front-end loaders and pit trucks. The extracted slurry usually comprises 20-25% suspended solid coal waste. Therefore, before the coal waste can be pelletised, it is further processed in the washing unit that uses a combination of scalping screen, 2-stage froth cells and a centrifuge to reduce the moisture content from 75–80% to 20–25%. The washed fines are then transported for treatment with the formula patented by Changeover Technologies and subsequently fed into the densifying unit. In the densifying unit, the formula binds the fines at a molecular level, producing smooth-faced pellets. The produced pellets are then transported to a cold curing unit, where these are exposed to air for an extended period. At this stage, the pellets attain their final strength and water resistance. Moreover, the moisture content is further reduced to 5-18%, depending on the atmospheric conditions. The entire process takes place at atmospheric temperature and pressure. Therefore, the only energy requirement for the process is associated with the electricity required to drive the machinery. 

How do waste coal pellets compare with conventional coal?

Our assessment of the coal processing technology has revealed that its cradle-to-gate global warming potential (GWP) would be 2.73 kgCO2,eq/GJ if the electricity consumed by the process is supplied via the natural gas combined cycle power plant. For the process that produces pellets at 50 t/hr, this corresponds to 3,923.73 kgCO2,eq emitted into the atmosphere over the period of 100 years. To better appreciate the distribution of these emissions, the contribution of each process stage to the total GWP is presented in Figure 2. The extraction and washing stages account for most of the GWP (71.2%), as their operation results in the GWP of 1.94 kgCO2,eq/GJ. This is associated with the diesel extraction (1.68 kgCO2,eq/GJ) and subsequent combustion (0.25 kgCO2,eq/GJ). The coal processing technology developed by Changeover Technologies results in the GWP of 0.78 kgCO2,eq/GJ (28.8%). These emissions stem solely from the electricity required to drive the process equipment.

Figure 2: Distribution of global warming potential for the coal processing technology under hierarchist (GWP100a) perspective

The main aim of the coal processing technology is to reduce the environmental impact of the coal supply to the industries that utilise coal as part of their operation, such as the steel or cement industry. Therefore, its environmental performance needs to be compared with the coal supply from the conventional coal mine. The cradle-to-gate GWP100a of the conventional coal supply in the US was estimated to be 12.76 kgCO2,eq/GJ (Figure 3), assuming the average calorific value of the bituminous coal is 28.83 GJ/t. These emissions mostly stem from the coal mine operation associated with its diesel, residual oil and electricity requirement, and residual methane emissions (i.e. 4 kg CH4 per tonne of coal extracted) specified in the NREL database [9]. For the supply of 50 t/hr of bituminous coal, the conventional mine will result in CO2, CO and CH4 emissions of 18186.1 kgCO2, 557.8 kgCO and 376.6 kgCH4, respectively, over the period of 100 years. It is equivalent to the absolute GWP of 18,392.3 kgCO2,eq under the hierarchist perspective.

Comparing the GWP of the conventional coal process and the coal processing technology, including the harvesting and washing steps, it can be observed that the latter has the potential to reduce the GWP associated with coal supply by up to 78.7%. It is a significant reduction, considering that the operation of the coal processing technology is still driven by diesel and natural gas.

Figure 3: Comparison of global warming potential for pellet coal and conventional coal considering individualist (GWP20a), hierarchist (GWP100a) and egalitarian (GWP500a) perspectives

Benefits of using coal pellets to industrial decarbonisation: A steelmaking case study

To understand the benefits of using the coal pellets produced in the MetaForm process in the industrial processes, the global warming potential of the steelmaking plant is assessed (Figure 4).

Based on the ultimate analysis of the waste coal pellets and conventional coal provided by Changeover Technologies, the CO2 emissions associated with coal use in the steel plant were estimated to be 90.1 kgCO2,eq/GJ and 89.8 kgCO2,eq/GJ, in line with the data reported in the literature[10]. Transport is not considered at this stage. The analysis has shown that the overall GWP of steelmaking will be 92.82 kgCO2,eq/GJ when the coal is supplied from the coal processing technology. This is 9.5% less than the GWP for the conventional process (102.56 kgCO2,eq/GJ). This implies that decarbonisation of the supply chain can bring a meaningful reduction in industrial CO2 emissions, even before these processes are fully decarbonised.

Figure 4: Comparison of the global warming potential for pellet coal and conventional coal application in steelmaking plant under hierarchist (GWP100a) perspective

The steelmaking industry needs to be fully decarbonised, as it is crucial to achieving the sustainable growth of our economy. Even though the operation of renewable energy sources does not result in CO2 emissions, the manufacturing of their components and foundations requires steel. For example, approximately 150 (onshore) to 250 (offshore) tonnes of coal is required to produce the steel required for manufacturing a single wind turbine. Complete substitution of the conventional coal used in the metallurgical processes with the waste coal pellets can reduce the specific CO2 equivalent emissions by 9.5%, from 1937.8 kgCO2eq/tsteel to 1754.0 kgCO2eq/tsteel. Such a reduction will contribute to the production of net-zero wind turbines (Table 1). Importantly, a partial substitution of, for example, 40% waste coal pellets in the coke oven will result in a 3.8% reduction in the lifetime CO2 equivalent emissions of steelmaking, considering both coal processing and coal use.

Table 1: Comparison of lifetime emissions for onshore and offshore wind turbines made with steel that was produced with waste coal pellets and conventional coal

Perspective for waste coal recovery

Achieving our decarbonisation targets requires collective action by all sectors of our economy. A full transition to a net-zero economy, where most of the electricity is supplied via renewables and nuclear sources, and the process industries are decarbonised with a mix of energy efficiency, carbon capture and hydrogen, is our ultimate goal. However, we do need to act now to meet the emission reduction targets by 2050. Ultimately, we need to stop using fossil fuels by (mid)-2030s and no new mines should be built as of now. The problem is, some industries, such as the steelmaking industry, rely on the supply of coal as a part of their manufacturing process. Recovery of the waste coal from existing coal impoundments can offer a viable way to avoid the need for opening new coal mines, with the added benefit of reduced global warming potential per tonne of coal supplied. If we replace just 1% of the coal use in the global power, cement and steel industry with waste coal pellets, we will reduce our annual CO2 emissions by 35.5 Mt. This is equivalent to planting 3.5 billion trees over a year. We will also be able to reclaim up to 3,650 km2 of land globally that can be repurposed for forests or property developments.

References

[1] UN. Adoption of the Paris Agreement, United Nations Framework Convention on Climate Change; Paris, 2015.

[2] IEA. CO2 Emissions from Fuel Combustion - Highlights; International Energy Agency: Paris, France, 2017.

[3] IEA. Coal 2020 – Analysis and Forecast to 2025; International Energy Agency: Paris, France, 2021.

[4] Clemente, J. The One Market That’s Sure To Help Coal; Forbes, 2018.

[5] NREL. U.S. Life Cycle Inventory Database; National Energy Technology Laboratory, 2012.

[6] Evans, S. ‘Profound shifts’ underway in energy system, says IEA World Energy Outlook; Carbon Brief, 2019.

[7] Lutynski, A.; Suponik, T.; Lutynski, M. Investigation of Coal Slurry Properties Deposited in Impoundments Located in the Upper Silesian Coal Basin. Physicochem. Probl. Miner. Process. 2013, 49 (1).

[8] CoalTech. Waste Coal Fines - Market Overview, 2021.

[9] NREL. U.S. Life Cycle Inventory Database; National Energy Technology Laboratory, 2012.

[10] Juhrich, K. CO2 Emission Factors for Fossil Fuels; German Environment Agency, 2016.