Industrial boilers may soon do more than generate heat: they could help eliminate a difficult industrial waste stream. New research suggests that carefully controlled amounts of organic cleaning wastewater can be blended with bituminous coal and thermally destroyed in coal-fired boilers without immediately disrupting combustion. The finding offers a potentially inexpensive route for managing wastewater produced when industrial equipment is chemically cleaned—but it also reveals a critical limit. Small additions may make coal easier to ignite, while excessive amounts can weaken combustion and delay complete burnout.
The wastewater comes from the chemical cleaning of boilers, pipelines, heat exchangers, and other equipment where scale, corrosion products, and organic deposits accumulate. Although cleaning restores equipment performance, it produces a complex liquid waste containing organic compounds, ammonia nitrogen, dissolved salts, metal ions, and substantial moisture. Conventional treatment can be technically demanding and expensive, particularly when compounds such as ethylenediaminetetraacetic acid, or EDTA, are present. Because coal-fired boilers already operate at high temperatures, researchers are investigating whether they can serve as existing thermal-destruction systems for this challenging waste.
In a study published in Energy & Environment Nexus, researchers from Southeast University examined how organic cleaning wastewater changes the ignition, mass-loss behavior, burnout, and reaction kinetics of bituminous coal. They prepared coal blends containing 1%, 3%, 5%, and 10% wastewater by weight and analyzed them using non-isothermal thermogravimetric analysis. This technique continuously measures changes in sample mass as temperature rises at controlled heating rates, allowing scientists to identify when ignition begins, how rapidly volatile and fixed carbon components react, and when combustion is completed.
The results showed that low and moderate wastewater additions could significantly reduce the temperature required to ignite the coal. At a heating rate of 10 °C per minute, untreated coal ignited at approximately 411.6 °C. When wastewater was added, the ignition temperature fell as low as 390.6 °C. The researchers attribute this shift to the combined influence of oxygen-containing organic compounds and inorganic species, particularly iron and sodium. These components may promote early oxidation reactions or assist in the breakdown of oxygen-containing functional groups on the coal surface, creating a more reactive environment during the initial stages of heating.
The strongest kinetic improvement occurred at a 5% wastewater ratio. For untreated coal, the average apparent activation energy was calculated at 131.68 kilojoules per mole. In the 5% blend, it dropped to 115.92 kilojoules per mole, indicating that less energy was needed to initiate the dominant combustion reactions. The 10% blend showed an intermediate value of 122.77 kilojoules per mole. Apparent activation energy is not a direct measurement of one isolated chemical reaction; rather, it summarizes the energy barrier associated with the overall reaction pathway observed under the experimental conditions. Even so, the trend suggests that moderate wastewater loading can improve the early reactivity of coal.
The apparent benefit, however, did not continue indefinitely. As the wastewater proportion increased, the maximum and average mass-loss rates generally declined, and overall combustion performance fell by approximately 4% to 15% under several test conditions. Moisture in the wastewater absorbs heat during evaporation, reducing the energy available for oxidation. Its dissolved salts and mineral matter also dilute the combustible fraction of the blend. As heating proceeds, inorganic residues may accumulate around coal particles and form a denser layer, restricting oxygen transport to the particle surface and slowing the final burnout stage.
The 10% blend made this inhibitory effect especially visible. At heating rates of 20 and 40 °C per minute, the burnout temperature increased, meaning that the coal-wastewater mixture required a higher temperature to complete combustion. This behavior reflects the competing mechanisms inside the heated particle. Organic compounds and metal species may accelerate initial oxidation, but water evaporation, fuel dilution, and ash-related diffusion resistance can dominate later. The study therefore presents wastewater not as a universally beneficial combustion additive, but as a chemically complex material whose effect depends strongly on concentration and operating conditions.
“Our results show that organic cleaning wastewater does not simply promote or suppress coal combustion,” corresponding author Yaji Huang said. “Its effects depend strongly on the blending ratio and result from a balance between catalytic substances and components that absorb heat or restrict oxygen transfer.” According to the researchers, a moderate addition may provide a practical compromise between easier ignition and stable combustion, whereas excessive loading should be avoided. The 5% blend delivered the lowest average activation energy among the tested mixtures, but that result does not by itself establish an optimal operating ratio for a commercial boiler.
The findings could open a new pathway for industrial waste management by combining wastewater disposal with an existing energy infrastructure. In principle, high-temperature combustion could destroy hazardous organic compounds while reducing the need for a separate treatment facility. Yet the laboratory evidence is only an initial step. Full-scale trials must determine how the wastewater affects nitrogen oxide and other pollutant emissions, ash composition, slagging, fouling, boiler corrosion, and the long-term reliability of fuel-feeding systems. The researchers also emphasize the need to verify whether all organic contaminants are destroyed and whether metals or salts become concentrated in the resulting ash. Until those questions are answered, co-firing should be viewed as a promising but tightly controlled engineering option rather than a ready-made solution.
Subject of Research: Combustion behavior and reaction kinetics of bituminous coal blended with organic cleaning wastewater.
Article Title: Combustion characteristics and thermokinetics of coal blended with organic cleaning wastewater
News Publication Date: 30 June 2026
Web References: Energy & Environment Nexus: https://doi.org/10.48130/een-0026-0012
References: Zhang J, Huang Y, Qiu Y, Jiang X, Zhang L, et al. 2026. “Combustion characteristics and thermokinetics of coal blended with organic cleaning wastewater.” Energy & Environment Nexus 2: e018. DOI: 10.48130/een-0026-0012
Image Credits: Jun Zhang, Yaji Huang, Yizhuo Qiu, Xinyi Jiang, Lanpeng Zhang and Hao Shi
Keywords
Coal combustion, organic cleaning wastewater, wastewater treatment, thermokinetics, apparent activation energy, bituminous coal, thermogravimetric analysis, industrial boilers, EDTA, co-disposal, combustion kinetics, energy and environment
