Spontaneous combustion in goaf areas remains a persistent and serious safety concern in coal mining operations. As mining depths increase and extraction intensity rises, the inherent propensity of coal seams to self-ignite becomes more pronounced. Among the various fire prevention technologies available, nitrogen injection has gained widespread acceptance due to its effective inerting performance and manageable operating costs. Moving nitrogen generation equipment from the surface to underground locations-specifically, employing mobile nitrogen generators in coal mines-is increasingly being adopted by operations seeking more responsive solutions.
Equipment Principle and Technical Configuration
An underground mobile nitrogen generator is a self-contained system capable of separating nitrogen from compressed air and delivering high-purity nitrogen directly at the working site. Most mobile units currently meeting coal mine explosion-proof requirements utilize Pressure Swing Adsorption (PSA) with carbon molecular sieves. The underlying mechanism is straightforward: under elevated pressure, the molecular sieve material adsorbs oxygen molecules more rapidly than nitrogen. Through alternating pressurization (adsorption) and depressurization (desorption), the system continuously produces nitrogen with consistent purity.
A complete installation typically comprises three core components: a mining-duty explosion-proof air compressor, a compressed air pretreatment unit, and the nitrogen generation module. These are mounted on flatbed mine cars, allowing the entire assembly to be towed along underground roadways. This configuration enables placement in close proximity to the goaf or target inerting zone, substantially reducing the nitrogen delivery distance from the source.
Why Choose Underground Mobile Nitrogen Generation
The conventional approach-generating nitrogen on the surface and piping it down long boreholes or shafts-delivers large volumes but introduces significant drawbacks: high capital expenditure for pipeline networks, substantial maintenance burdens, and inevitable pressure losses or leakage along extended routes. When working faces advance rapidly or mining panels transition frequently, fixed surface systems often lack the agility to keep pace.
Mobile underground systems address these constraints with distinct advantages:
- Rapid inerting response: With the generator positioned nearby, nitrogen reaches the goaf almost immediately after production, offering timely inerting-particularly valuable for coal seams with short spontaneous combustion periods.
- Elimination of dedicated nitrogen pipelines: The system can utilize existing mine compressed air lines or short temporary connections, reducing infrastructure costs.
- Flexible relocation: The equipment can be towed along as the working face progresses, allowing a single unit to serve multiple faces over its service life, improving capital utilization.
Key Technical Considerations for Field Application
The effectiveness of nitrogen-based fire prevention hinges on maintaining oxygen concentrations in the goaf oxidation zone consistently below the critical level for coal self-heating. China's Coal Mine Safety Regulations mandate a nitrogen purity of no less than 97% for injection. In practice, several operational factors require careful attention:
- Injection mode: For active fire suppression, continuous nitrogen injection is recommended to quickly establish an inert atmosphere. For routine fire prevention, intermittent injection is often more practical-suspending flow once oxygen levels drop below the safety threshold and resuming when they rise back to warning levels. This approach balances protective effect with energy consumption.
- Injection point placement: The nitrogen discharge outlet should be positioned on the intake side of the goaf and progressively buried at planned intervals as the face advances, ensuring the nitrogen stream effectively blankets the oxidation zone. The spacing between injection points should be determined based on "three-zone" monitoring data (the heat dissipation zone, oxidation zone, and asphyxiation zone).
- Flow rate and safety verification: The required nitrogen injection rate must be calculated considering goaf air leakage and initial oxygen concentration. Equally important, the oxygen level in the return airway must be verified to remain above 19%-a fundamental safety requirement for personnel working in the area.
Equipment Selection Guidelines
The technical standard "Technical Specifications for Nitrogen Fire Prevention in Coal Mines" stipulates that underground mobile nitrogen units must have dimensions compatible with mine cages and flatbed cars, and all electrical components must carry certified explosion-proof ratings. Currently available systems offer nitrogen production capacities ranging from 200 to 2,000 m³/h, covering the needs of various face sizes. When selecting equipment, priority should be given to operational reliability in the hot, humid, and dusty underground environment, as well as the level of automation and the system's interface capability with mine-wide monitoring networks. Shenger Gas maintains a long-standing focus on gas separation technology for industrial safety applications and continues to provide reliable technical support to the coal mining sector.
Underground mobile nitrogen generators relocate the nitrogen source directly to the point of use, delivering a flexible and timely technical solution for goaf fire prevention and suppression. With a reasonable capital outlay, they effectively balance nitrogen supply capacity with on-site adaptability-particularly advantageous in complex extraction conditions. As equipment continues to evolve toward higher flow rates, greater purity, and intelligent control, its role within the broader coal mine fire protection framework will only grow more significant.
