What Can Be Done to Recycle These Tons of Ash?
When certain fuels are combusted, it leaves a fine, powdery substance byproduct called ash. Two common forms of this type of waste are coal ash and wood ash.
Of the two, coal ash can be the most problematic because it can take several forms and is the most difficult to recycle. In contrast, wood ash can be reintroduced into the soil as a fertilizer.
What is Wood Ash?
Wood ash is a waste product remaining from the combustion of biomass (as opposed to pyrolysis, which results in biochar). Its chemical composition varies depending on factors such as the type of wood, combustion, and temperature. However, it generally contains large amounts of plant nutrient, specifically calcium—particularly calcite, calcium oxide, and calcium manganate. This can make it highly valuable as a fertilizer to increase crop yields if used properly.
Wood ash also contains toxic ingredients, such as mercury, lead, and arsenic.
In small amounts, wood ash is good for gardens, having a liming effect and delivering potassium, calcium and magnesium to the soil, alongside various trace elements.
Still, adding wood ash to the soil is generally beneficial by increasing its pH, neutralizing acid, and increasing the soil’s cation exchange capacity (CEC). This in turn improves the ability of soils to retain nutrients. In small amounts, wood ash is good for gardens, having a liming effect and delivering potassium, calcium, and magnesium to the soil alongside various trace elements.
Wood Ash’s Impacts on People, Animals, and the Environment
The International Energy Agency (IEA) estimated in 2019 that 60% of total energy use in sub-Saharan Africa is from burning solid biomass—it’s almost three-quarters if South Africa is excluded—primarily due to using inefficient, “three-stone” cookstoves to prepare meals, when more fuel-efficient options are available. The IEA further estimated in 2022 that more than 80% of the population in sub-Saharan Africa relies on biomass for residential energy.
All this wood ash waste could be plowed into farmlands or used in urban agriculture projects to improve local food production. Instead, a common mode of wood ash disposal is taking it to poorly regulated dumps.
Research Outreach reported in their November 2020 paper that the sub-Saharan region produces around nineteen megatons of ash every year. This waste contains hundreds of tons of arsenic, cadmium, chromium, more than a kiloton of mercury, and three kilotons of lead. Communities in the countries of Rwanda, Burundi, Uganda, Nigeria, and Guinea-Bissau could see as much as 4,000 kg (8,818 lbs) of wood ash dumped on one square kilometer of land every year. Wood ash also adds to air pollution, and “[c]onsumption of these kinds of pollutants is known to lead to permanent loss of cognition, disability, reduced lifespan and even death,” the Research Outreach report said.
An improvement in the design and construction of stoves, resulting in more efficient combustion, could reduce emissions and the amount of ash produced. However, the most important factor is how the ash is disposed of in terms of collection, transportation, treatment, and recycling or disposal.
[T]he most important factor is how the ash is disposed of in terms of collection, transportation, treatment, and recycling or disposal.
According to a 2022 Canadian paper published in the journal, Environmental Reviews, there is mounting evidence that wood ash can be used to help counter the loss of nutrients in calcium-deficient soils and also boost forestry productivity.
There is also evidence to suggest that wood ash from wildfires entering the ocean may increase phytoplankton growth at certain times of the year when there is a nutrient deficiency.
What is Coal Ash?
Coal ash results from the combustion of coal in coal-fired power plants. There are four specific forms of coal ash—fly ash, bottom ash, boiler slag, and flue gas desulphurization material. As with wood ash, it contains large amounts of calcite, calcium oxide, and calcium manganate.
Fly ash is a fine, powdery material that mostly consists of silica, unburned carbon, and other inorganic substances. Bottom ash is primarily made of silica, ferric oxide, and alumina—its particles are too large to be lifted up through the smokestacks and remain in the bottom of the coal furnaces. Boiler slag is a smooth, molten form of bottom ash that becomes porous after cooling with water. Flue gas desulphurization material is left over from the process of reducing sulfur dioxide emissions from a coal-fired boiler by adding a source of calcium. This typically results in calcium sulfite or calcium sulfate (calcium-based desulfurization ash) or a dry, powdered material containing sulfites and sulfates.
Coal Ash’s Impacts on People and the Environment
According to the IEA, world coal production reached 6,122 Mtce (million tons of coal equivalent) in 2022. This large, persistent, global demand for coal for energy is viewed as a serious threat to the environment and human health, as well as the source of vast amounts of coal waste.
Coal combustion residuals (CCR) contain many toxic substances, including mercury, lead, chromium, and arsenic. Advocacy groups like the Environmental Integrity Project warn that these substances are still leaching into the environment, while the Physicians for Social Responsibility reported in 2009 the adverse health effects of coal pollution on the respiratory, cardiovascular, and neurological systems, including lung cancer, cardiac arrhythmia, and ischemic stroke. In 2018, the physicians’ group linked toxicants from coal ash to kidney disease, reproductive issues, and gastrointestinal issues.
[T]he Physicians for Social Responsibility reported … the adverse effects of coal pollution on the respiratory, cardiovascular, and neurological systems, including lung cancer, cardiac arrhythmia, and ischemic stroke, [and] … linked toxicants from coal ash to kidney disease, reproductive issues, and gastrointestinal issues.
Unregulated or improper management of coal ash and waste presents significant risks to health and the environment. Coal ash impoundments (storage locations) are known to be susceptible to catastrophic failure—such as in Tennessee (2008) and North Carolina (2014)—resulting in dangerous pollution of the local environment. In the former, about 5.4 million cubic yards of coal ash spilled into Swan Pond Embayment and three sloughs, while in the latter, 27 million gallons of coal ash wastewater and 30,000–39,000 tons of coal ash spilled into the Dan River.
Management and Disposal of Coal Ash
Coal ash and its byproducts can be recycled into products such as concrete (from fly ash) or wallboard (from gypsum), helping to reduce greenhouse gas emissions and the costs of disposal. Incorporating coal ash in some products may also improve their strength and durability. For example, the use of fly ash in fresh and hardened Portland cement concrete can improve workability, decrease water demand, increase ultimate strength, and reduce permeability. Municipal bottom ash may also be used for cement applications, such as for aerated concrete. There is also research to extract rare earth elements from coal ash.
Coal ash and its byproducts can be recycled into products such as concrete (from fly ash) or wallboard (from gypsum), helping to reduce greenhouse gas emissions and the costs of disposal.
Power plants dispose of coal ash in different ways. Some plants retain it in pools or ponds on the surface called impoundments, or send it to landfills. Other plants discharge it into nearby water courses under a water discharge permit.
In the US, the American Coal Ash Association released its 2021 survey results that indicated decreased production of all combustion coal products. Beneficial reuse accounted for over one-third of the amount of coal ash produced, with the rest sent to a landfill or kept in impoundments. The Environmental Protection Agency is currently getting comments on a proposed rule that would expand regulations to inactive surface impoundments at inactive facilities.
In Europe, The Netherlands reuses almost all of its fly ash for building materials production; its landfills are reserved for cases in which recovery or incineration of waste are not possible.
Germany has used fly ash to refill and reclaim depleted lignite mines, with other usage for soil beneficiation, surface recultivation, and production of cement and concrete. In addition, German company Zaak Technologies GmbH implemented a Smart Sand pilot plant through a 2016–2020 project in which they utilized fly ash to produce artificial sand.
Meanwhile, in the Czech Republic, a project called the Eden Silesia project seeks to convert a former coal mining region into greenhouses, a research center, and tourist attractions. A feasibility study for this was started in 2022.
A ‘Plateau’ on Coal?
According to the Boom and Bust Coal 2023 report, the US retired the most power from coal—13.5 GW—in 2022. In contrast, China increased its coal usage by 26.8 GW, which completely offset all coal plant retirements from the rest of the world.
Meanwhile, Peru and the United Arab Emirates joined four other countries that have phased out coal (Austria, Belgium, Sweden, and Portugal).
Global hard coal production increased in 2022 to 7.9 Gt, and the IEA said it is now expecting a “decade-long plateau” of coal supply and demand, based on the European Association for Coal and Lignite’s first market report in 2023.
If transitioning away from coal is truly the global goal, countries need to work together toward finding alternative energy sources and keeping their commitments.
*Robin Whitlock is an England-based freelance journalist specializing in environmental issues, climate change, and renewable energy, with a variety of other professional interests, including green transportation.