General Concepts from the Risk Assessment and Hazard Identification of HTL-Derived Bio-Oil: A Case Study of the MARINES Project ^†

Abstract

This study evaluates the risk assessment and hazard identification of hydrothermal liquefaction (HTL)-derived bio-oil from the MARINES project, which converts military organic waste into fuel. The high oxygen content (35–50 wt%), acidic pH (2–4), and viscosity (10–1000 cP) of bio-oils pose unique challenges, including oxidative polymerization, corrosion, and micro-explosions during combustion. Key hazards include storage instability, particulate emissions (20–30% higher than diesel), and aquatic toxicity (LC50 < 10 mg/L for phenolics). Mitigation strategies such as inert gas blanketing, preheating, and spill containment are proposed. While offering renewable fuel potential, HTL bio-oil demands rigorous safety protocols for military/industrial deployment, warranting further experimental validation.

1. Introduction

The MARINES (Military wAstes Recycled INto fuElS) project converts organic residues into bio-oil for military fuel applications via hydrothermal liquefaction (HTL), posing risks such as combustion, environmental contamination, and atmospheric dispersion during accidents. This study qualitatively assesses HTL bio-oil hazards, emphasizing its unique chemical composition (high oxygen content, viscosity, and aerosolization) and associated risks in storage, combustion, and release. Key challenges include polymerization instability, micro-explosions, and aquatic toxicity. While quantitative modeling (e.g., EPIcode) is reserved for future work, this analysis establishes foundational safety insights into military/industrial applications, advocating risk mitigation strategies and further experimental validation.

2. Bio-Oil Properties and Risks

HTL-derived bio-oil is produced through the thermochemical conversion of diverse organic feedstocks, including food waste (20–30% of input), plastics (10–15%), and paper products (15%). This heterogeneous origin results in a chemically complex fuel that differs fundamentally from petroleum derivatives [1,2,3,4]. The fuel’s composition typically includes 35–50 wt% oxygenated hydrocarbons, 15–40% water, 5–15% phenolic compounds, and 8–12% ketones and aldehydes [3,4]. These components collectively create both opportunities and challenges for military applications.

The high oxygen content that distinguishes bio-oil from conventional fuels has effects on its properties and handling requirements. This oxygen content manifests primarily as carboxylic acids, alcohols, and aldehydes, giving the fuel an acidic pH between 2 and 4, that may accelerate corrosion in carbon steel containers at rates of 0.1–0.3 mm/year, necessitating specialized materials like 316L stainless steel or polymer-lined tanks [5,6]. The oxygen content also reduces the fuel’s energy density to 16–25 MJ/kg, compared to 45 MJ/kg for diesel, requiring the careful consideration of mission logistics. Viscosity represents another critical differentiator, with bio-oil typically measuring 10–1000 cP at 40 °C versus just 2–4 cP for diesel oil [7]. This elevated viscosity, caused by lignin-derived oligomers and polymeric fractions, creates challenges for fuel injection systems. The problem that compounds face over time is due to oxidative polymerization, where oxygen-driven reactions increase viscosity by 5–10% monthly and produce insoluble sludge that accounts for 0.5–2% of the total volume [7,8]. These stability issues are exacerbated when blending petroleum fuels, often causing phase separation that damages the equipment.

Combustion characteristics present further complexities. The 15–40% water content reduces NOx emissions by 30–50% compared to diesel but also promotes micro-explosions during combustion [3,4,5,6]. These occur when entrapped water vaporizes rapidly, fragmenting fuel droplets and creating unstable burning conditions. Incomplete combustion is common, yielding 20–30% higher particulate emissions than conventional fuels. Environmental behavior adds another layer of concern, as the phenolic compounds of bio-oil demonstrate acute toxicity to aquatic life (with lethal concentration LC50 values below 10 mg/L for many fish species) and tend to form persistent emulsions that complicate the spill response [5,6,7,8].

Table 1. Physicochemical characteristics and risk-relevant properties of HTL-derived bio-oil, diesel fuel No. 4, and residual fuel oil [3,4,5,6,7,8].

Factor	Bio-Oil (MARINES Project)	Diesel Fuel No. 4 (68476-31-3)	Fuel Oil Residual (68476-33-5, 68334-30-5)
Source	Hydrothermal liquefaction (HTL)	Crude oil distillation	Residual fraction from petroleum refining
Viscosity	High	Moderate	Very high
Sulfur Content	Low	Varies (up to 0.5%)	High (up to 3.5%)
Flashpoint	~70–100 °C	~60 °C	>100 °C
Environmental Risk	Moderate (oxygenated compounds)	High (pollutants and CO2 emissions)	High (persistent in spills)

provides a detailed comparison of these properties compared to conventional fuels.

3. Hazard Identification

The hazard assessment of HTL-derived bio-oil encompasses three critical dimensions, each presenting unique challenges for military fuel applications. First, storage and handling risks are examined, focusing on the fuel’s instability due to oxidative polymerization, the necessity of inert gas blanketing to prevent autooxidation, and the potential for hazardous mist formation during transfer operations [5,6]. Next, combustion and explosion hazards were analyzed, particularly the risks of micro-explosions from water content, incomplete combustion leading to elevated particulate emissions, and the formation of explosive vapor–air mixtures under fluctuating temperatures [6,7]. Finally, environmental and health implications were addressed, including the persistence of bio-oil emulsions in aquatic systems, the toxicity of phenolic compounds to marine life, and airborne dispersion risks from spills that may threaten both ecosystems and personnel safety [6,7]. The following provides a comprehensive framework for understanding the risks associated with bio-oil deployment.

3.1. Storage and Handling Risks

The storage and handling of HTL bio-oil present multiple technical challenges. Oxidative polymerization stands as the primary degradation mechanism, where atmospheric oxygen reacts with fuel components to form high-molecular-weight compounds [3,4,5,6]. This process increases the viscosity by 5–10% monthly under typical storage conditions, eventually producing insoluble sludge that accounts for 0.5–2% of total volume. The sludge readily fouls filters and injectors, potentially disabling critical fuel systems.

Material compatibility issues compound these challenges. The fuel’s carboxylic acid content (typically 3–8 wt%) corrodes carbon steel at 0.1–0.3 mm/year, requiring alternative materials like 316L stainless steel. Even with proper materials, storage tanks demand inert gas blanketing (typically nitrogen at >99% purity) to prevent oxidative degradation [6]. During transfer operations, bio-oil’s low surface tension promotes mist formation, generating respirable droplets of 1–10 μm in size. These mists pose both health risks (exceeding OSHA’s permissible exposure limit of 10 mg/m³ without controls) and explosion hazards when combined with the fuel’s propensity for static charge accumulation [3,4,5,6].

Mitigation requires a multi-layered approach. Storage tanks must incorporate conductivity controls (resistivity < 10⁸ Ω·m) with verified grounding systems. The monthly monitoring of viscosity and sediment accumulation provides an early warning of stability issues, while engineered controls like vapor recovery systems and explosion-proof pumps address mist and ignition risks [6]. These measures collectively extend viable storage duration from weeks to several months, though eventual fuel upgrading remains necessary for long-term use [8].

3.2. Combustion and Explosion Hazards

Bio-oil combustion introduces unique risks that demand careful engineering controls. The micro-explosion phenomenon represents perhaps the most distinctive challenge; it occurs when the fuel’s 15–40% water content rapidly vaporizes during combustion. As droplets are heated to 200–300 °C, entrapped water turns to steam, and localized pressure spikes of 2–5 bar are created that fragment the droplet [4,5,8]. This violent process destabilizes flames, reducing combustion efficiency by 10–20% and potentially damaging engine components through the erratic heat flux.

Incomplete combustion presents another major concern, primarily resulting from the poor atomization of the viscous fuel. The resulting soot formation typically reaches 20–40 g/kg of the fuel burned (substantially higher than diesel oil), leading to the rapid fouling of exhaust systems [6,7,8]. These operational challenges coincide with significant safety risks from the fuel’s vapor behavior. When heated above its flash point (70–100 °C), bio-oil generates flammable vapors that reach their lower explosive limit (LEL) at a concentration of just 1.2–1.8 vol% in the air.

Effective risk management requires integrated solutions. Preheating fuel to (80 ± 5) °C significantly improves atomization while remaining safely below most autoignition temperatures (200–400 °C). Combustion systems must incorporate flame arrestors and continuous LEL monitoring, particularly in confined spaces like tank farms or shipboard fuel rooms. Advanced burner designs that enhance turbulence and residence time can mitigate soot formation, though particulate emissions will likely remain higher than petroleum fuels without post-combustion treatment [7].

3.3. Environmental and Health Risks

The environmental profile of HTL bio-oil presents a paradox: while derived from renewable feedstock and lower in sulfur than fossil fuels, its spill behavior and toxicity raise significant concerns. In aquatic environments, bio-oil forms stable emulsions that persist 5–10 times longer than equivalent diesel spills [4,5,6]. These emulsions adhere tenaciously to sediments and vegetation, complicating cleanup efforts. The fuel’s phenolic compounds exhibit particular environmental persistence, with bioaccumulation factors (BCFs) of 200–500 in fish species, posing risks to higher trophic levels, including humans.

Airborne exposure risks emerge during both routine operations and accident scenarios. Spills that occur under windy conditions or elevated temperatures generate inhalable droplets smaller than 5 μm, which are capable of penetrating deep lung tissue. The threshold limit value (TLV) for oil mists at 10 mg/m³ as an 8-h time-weighted average can be exceeded quickly in enclosed spaces without proper ventilation [6]. Chronic exposure is linked to respiratory irritation and more serious pulmonary effects.

Mitigation strategies focus on primary prevention through engineered controls like double-walled tanks, secondary containment using booms and absorbents, and tertiary remediation via enhanced bioremediation techniques [2,3,7,8].

4. Conclusions

The MARINES project demonstrates HTL-derived bio-oil’s potential as a renewable military fuel despite significant challenges. The key findings identify three critical hazard areas: (i) storage instability from oxidative polymerization requiring inert gas blanketing (N₂) and corrosion-resistant materials; (ii) combustion risks (including micro-explosions and incomplete combustion needing optimized atomization); and (iii) environmental concerns from persistent emulsions and phenolic toxicity demanding rapid spill containment.

While the lower sulfur content of bio-oil reduces SO₂ emissions versus conventional fuels, its successful integration requires mitigation strategies like advanced monitoring for storage stability, preheating/emulsification for combustion control, and biodegradable dispersants for spills. Future work should prioritize modeling techniques and experimental validation to refine safety protocols for military deployment.