Surfactant-Tweaked Salt Blend Slows Coal Spontaneous Combustion

Coal spontaneous combustion remains one of the most persistent hazards in mining and coal storage, and a new study published in Scientific Reports offers a deceptively simple upgrade to a classic prevention strategy. Researchers led by Jiuling Zhang of North China University of Science and Technology, working with colleagues at China University of Mining and Technology, modified a sodium chloride-based coal inhibitor with a common anionic surfactant, sodium dodecyl sulfate (SDS), and demonstrated that the combination dramatically improves the way treated coal handles moisture and oxygen. The work, published on 18 October 2026, identifies 20% NaCl plus 0.15% SDS as the optimal formulation through a rigorous multi-criteria evaluation.

The underlying chemistry is grounded in how physical inhibitors interrupt the coal–oxygen interactions that drive low-temperature oxidation. Sodium salts absorb and retain moisture, keeping coal damp and thereby limiting the contact between oxygen and reactive coal surfaces. But the approach has a well-known weakness: sodium salt solutions do not always wet and spread effectively on coal, which is inherently hydrophobic in many ranks. Poor wetting means the inhibitor cannot distribute evenly, leaving patches of coal unprotected and undermining the moisture-regulation strategy the salt is meant to provide.

SDS addresses that weakness directly. As an anionic surfactant, it lowers the surface tension of the NaCl solution and improves the interfacial wettability between the liquid and the coal surface. Contact-angle measurements in the study showed that SDS promoted wetting, spreading, and effective contact of the NaCl solution on coal, allowing the hygroscopic and hydration effects of the salt to operate across a much larger treated area. In effect, the surfactant acts as a delivery enhancer, ensuring the active inhibitory component reaches the coal surface where it matters most.

The quantitative results are striking. Coal samples treated with 20% NaCl plus 0.15% SDS exhibited an average water absorption of 15.45 grams during the moisture absorption stage, a 99% increase compared with samples treated with 20% NaCl alone. During the moisture desorption stage, the same formulation produced an average mass loss of only 4.76 grams, representing a 59% decrease relative to NaCl-only treatment. In practical terms, the composite inhibitor lets coal take up roughly twice as much moisture and then hold onto it far more stubbornly, keeping the coal in a state that resists oxidation for longer.

Those moisture effects translate into measurable suppression of the oxidation process itself. The study reports that oxygen consumption was effectively reduced and carbon monoxide generation was effectively suppressed in treated samples. Both metrics are critical indicators of coal spontaneous combustion risk: oxygen uptake reflects the rate at which coal reacts with air, while CO output is a widely used early-warning signal that low-temperature oxidation is accelerating. By damping both, the composite inhibitor attacks the problem at its chemical source rather than merely masking symptoms.

Spectroscopic evidence adds another layer of understanding. Fourier-transform infrared (FTIR) results showed that different inhibitor treatments altered the relative functional-group composition and chemical environment of the coal. This suggests the treatment does not simply sit on the surface as a passive barrier but interacts with the coal’s molecular structure, changing which functional groups are present and how they behave. Combined with the contact-angle data confirming improved coal–liquid interfacial wettability, the findings point to a mechanism in which SDS enables better physical distribution while NaCl provides sustained moisture retention, together weakening coal–oxygen interactions and retarding low-temperature oxidation.

To move beyond single-parameter comparisons, the team employed the entropy weight method, a quantitative multi-criteria decision-making approach that assigns objective weights to different performance indicators based on the variability of the data. This comprehensive evaluation identified 20% NaCl plus 0.15% SDS as the optimal formulation among the tested combinations. The method is notable because it avoids subjective weighting, letting the experimental data itself determine which formulation performs best across moisture absorption, mass retention, oxygen consumption, and CO suppression simultaneously.

The practical implications for mining safety and coal storage are significant. Coal spontaneous combustion causes substantial economic losses and environmental damage worldwide, and existing prevention methods range from inert gas injection to grouting and chemical inhibitors. Sodium-salt-based physical inhibitors are attractive because they are inexpensive, non-toxic, and easy to apply, but their effectiveness has been limited by wetting and spreading performance on coal surfaces. The SDS enhancement addresses that specific limitation with a surfactant that is itself widely available and low-cost, making the composite formulation a realistic candidate for field deployment rather than a laboratory curiosity.

The research was sponsored by the National Natural Science Foundation of China under grants 51504077 and 51404086, and by the Natural Science Foundation of Hebei Province under grant E2022209101. The authors declare no competing interests, and the article is published open access under a Creative Commons licence, allowing broad dissemination of the findings.

Looking ahead, the study provides a reference framework for formulation optimization and parameter design of surfactant-enhanced sodium-salt inhibitors. By demonstrating that a small concentration of SDS — just 0.15% — can nearly double the moisture uptake and cut desorption-related mass loss by more than half compared with NaCl alone, the work suggests that targeted interfacial chemistry, rather than simply increasing inhibitor concentration, may be the key to unlocking better coal spontaneous combustion prevention. As coal remains a major global energy source, improvements in how it is stored and handled carry real consequences for safety, economics, and environmental protection alike.

Subject of Research: Surfactant-enhanced sodium salt inhibitors for preventing coal spontaneous combustion

Article Title: Inhibitory characteristics of a surfactant-enhanced sodium salt inhibitor for coal spontaneous combustion

Article References: Zhang, J., Chuai, Y., Li, J., & Huo, J. (2026). Inhibitory characteristics of a surfactant-enhanced sodium salt inhibitor for coal spontaneous combustion. Scientific Reports. https://doi.org/10.1038/s41598-026-75706-7

Image Credits: AI Generated

DOI: 10.1038/s41598-026-75706-7

Keywords: coal spontaneous combustion, sodium chloride inhibitor, sodium dodecyl sulfate, SDS surfactant, coal wettability, moisture retention, low-temperature oxidation, entropy weight method, FTIR spectroscopy, contact angle, mining safety, coal storage