Zhuojin Pipe Fitting Co., LtdExperimental Comparison Between Aircraft Fuel Tank Inerting Processes Using NEA and MIG
Literature Overview
Published in Chinese Journal of Aeronautics (Vol. 31, No. 7, 2018, pp. 1515–1524), this study by Lei Shao and colleagues from Nanjing University of Aeronautics and Astronautics presents a rigorous experimental comparison between two aircraft fuel tank inerting technologies: Nitrogen-Enriched Air (NEA) used in the On-Board Inert Gas Generation System (OBIGGS) and Mixed Inert Gas (MIG) comprising CO2, N2, and O2 produced by the Green On-Board Inert Gas Generation System (GOBIGGS).
Background and Motivation
Aircraft fuel tank fires represent one of the most catastrophic failure modes in commercial aviation. The flammable envelope in the fuel ullage space is defined by the oxygen concentration threshold, which for typical jet fuels (JP-8, Jet A-1) lies between approximately 8% and 14% oxygen by volume. Inerting systems reduce the oxygen concentration below the limiting oxygen concentration (LOC) to prevent ignition. The two dominant approaches differ fundamentally in their inert gas composition and generation methodology.
OBIGGS uses a cryogenic air separation unit to produce NEA, which is air with a reduced oxygen fraction (typically 2–4% O2). GOBIGGS employs flameless catalytic combustion to consume oxygen, producing a gas mixture rich in CO2 (typically 12–15%) with residual N2 and O2. The critical difference lies in the solubility characteristics: CO2 is nearly 20 times more soluble in jet fuels than N2, which has profound implications for the inerting performance when fuel scrubbing (fuel surface treatment) is employed.
Experimental Methodology
The authors constructed a dedicated inerting test bench to systematically compare the inerting capabilities of NEA and MIG under both ullage washing and fuel scrubbing modes. The test bench simulates the ullage and fuel volumes of a typical commercial aircraft wing tank with controlled gas injection, oxygen concentration monitoring, and dissolved oxygen measurement capabilities.
Two inerting strategies were evaluated:
- Ullage washing: Injection of inert gas into the ullage space above the fuel surface.
- Fuel scrubbing: Direct injection of inert gas into the fuel to strip dissolved oxygen.
| Inerting Mode | Inert Gas Type | Ullage O2 Reduction | Fuel Dissolved O2 | Relative Performance |
|---|---|---|---|---|
| Ullage washing | NEA | Gradual decline | Moderate reduction | Comparable to MIG |
| Ullage washing | MIG | Gradual decline | Moderate reduction | Comparable to NEA |
| Fuel scrubbing | NEA | Effective reduction | Low residual O2 | Superior |
| Fuel scrubbing | MIG | Effective reduction | Higher residual O2 | Inferior |
Key Technical Findings
The most significant finding is that ullage washing performance is nearly identical for both NEA and MIG, regardless of the inert gas composition. This suggests that the primary inerting mechanism in ullage washing is dilution of the gas-phase oxygen concentration, and the specific composition of the inert gas (whether N2-rich or CO2-rich) has minimal influence on the equilibrium oxygen level achieved in the ullage.
However, the fuel scrubbing results reveal a critical disparity: MIG scrubbing consistently results in higher ullage and dissolved oxygen concentrations compared to NEA scrubbing. This counterintuitive result can be explained by the high solubility of CO2 in jet fuel. When MIG is injected into the fuel, a significant fraction of the CO2 dissolves into the liquid phase, reducing the effective volume of inert gas available for oxygen displacement. The dissolved CO2 also establishes a different equilibrium condition for oxygen solubility, potentially increasing the dissolved oxygen concentration in the fuel.
The practical implication is that MIG is suitable for ullage washing but inferior to NEA for fuel scrubbing applications. This finding has direct consequences for the design and certification of GOBIGGS systems, which must be limited to ullage inerting or must incorporate additional measures to compensate for the scrubbing deficiency.
Engineering Practice Implications
For aircraft manufacturers and maintenance organizations, this study provides critical data for inerting system selection and qualification:
- GOBIGGS systems should be configured for ullage washing only, as the MIG composition is unsuitable for fuel scrubbing.
- OBIGGS systems remain the preferred choice for applications requiring both ullage washing and fuel scrubbing, as NEA provides consistent performance across both modes.
- Hybrid approaches combining GOBIGGS for ullage inerting with supplemental NEA for scrubbing may offer a cost-effective solution, though the added complexity must be weighed against the performance benefit.
From a safety analysis perspective, the dissolved oxygen concentration in the fuel is a critical parameter for assessing fire risk during fuel transfer operations. The higher dissolved oxygen levels associated with MIG scrubbing increase the flammability risk during fueling and defueling, which must be accounted for in the aircraft's safety assessment per DNV-ST-F101 or equivalent airworthiness standards.
Study Insights and Reflections
The elegance of this study lies in its straightforward experimental design that reveals a counterintuitive result with significant engineering consequences. The 20-fold difference in CO2 versus N2 solubility in jet fuel is a well-known physicochemical property, yet its implications for fuel tank inerting had not been systematically quantified prior to this work.
I find the distinction between ullage washing and fuel scrubbing particularly important for engineering practice. Many inerting system specifications treat these two modes as interchangeable, but this study demonstrates that the choice of inert gas composition has differential effects depending on the mode of operation. This has implications for the certification basis of inerting systems under regulations such as EASA CS-25.753 and FAA FAR 25.853.
The study also highlights the importance of dissolved oxygen monitoring in fuel tank safety. Traditional inerting system verification focuses on ullage oxygen concentration, but the fuel scrubbing results demonstrate that dissolved oxygen in the liquid phase is equally critical and may not be adequately controlled by NEA or MIG alone.
Reference Value and Outlook
This paper is essential reading for engineers involved in aircraft fuel system design, inerting system certification, and safety assessment. The experimental data provide a quantitative basis for comparing NEA and MIG inerting performance, which is lacking in the current regulatory framework.
Future research should address the long-term effects of CO2 dissolution in jet fuel on fuel stability, compatibility with tank materials (aluminum 2024-T3, carbon fiber composites), and the interaction between dissolved CO2 and trace contaminants such as water and microbial growth products. The environmental benefits of GOBIGGS—reduced weight, lower energy consumption, and elimination of cryogenic equipment—make it an attractive technology, but the fuel scrubbing limitation must be overcome before it can fully replace OBIGGS in new aircraft designs.