Introduction

With respect to Cubico’s separate application for a Battery Energy Storage System (BESS) we consulted an experienced Chemical Engineer (ex-Fellow of the I.Chem.E.) and put together an article explaining the key hazards. This research was triggered after reading about the many fires which have been reported, one of which we discussed in this post Huge Fire at Moss Landing Sends out Plumes of Toxic Smoke. In particular we looked at the hazards which can arise from fires and thermal runaway. In the interests of full disclosure, we used Chat-GPT to gather the detailed data and estimates for threats to life.

Lithium-ion battery fires in large-scale battery energy storage systems (BESS) are a significant safety concern, particularly when a thermal runaway occurs.

The key toxic gases emitted are Carbon Monoxide, Hydrogen Fluoride, Phosphorus Pentafluoride and Acrolein. All can be extremely hazardous to health at relatively low concentrations. Fires also emit large quantities of flammable gases such as Hydrogen, Methane, Ethylene and Propylene. Combustion will also release fine particulates which cause respiratory and cardiovascular issues.

Below is a detailed explanation of what happens during such events and the associated hazards.

What is Thermal Runaway in Lithium Batteries?

Thermal runaway is a self-sustaining reaction where a battery’s temperature rises uncontrollably due to internal or external factors. It occurs when:

1. Overheating, short-circuits, or damage cause internal temperatures to rise.
2. The electrolyte and electrodes decompose, releasing heat.
3. The generated heat accelerates further chemical reactions, releasing more heat and flammable gases, creating a feedback loop.

In large-scale systems, a single cell in a battery pack can trigger thermal runaway, which propagates to neighboring cells (thermal propagation) and leads to a chain reaction.

Why Are These Fires Nearly Impossible to Put Out?

1. Oxygen Supply from the Battery:
   • Lithium-ion batteries contain oxygen-rich compounds in their cathodes (like lithium nickel manganese cobalt oxides or lithium iron phosphate). During thermal runaway, these compounds release oxygen.
   • This internal oxygen supply sustains combustion, making traditional fire suppression methods (like water or foam) ineffective because the fire doesn’t rely on atmospheric oxygen.
2. High Temperatures:
   • The reaction generates extreme heat (up to 1,000–2,000°C), which can ignite adjacent materials and cause further escalation.
3. Re-ignition:
   • Even if extinguished temporarily, the intense heat and remaining reactive materials can re-ignite the fire.
4. Gas and Explosion Risks:
   • The accumulation of flammable gases (e.g., hydrogen, methane, carbon monoxide) in a confined space increases the risk of explosions, further complicating fire suppression efforts.

Gases Emitted During Thermal Runaway

Thermal runaway produces a toxic and flammable cocktail of gases. Key emissions include:

1. Flammable Gases:
   • Hydrogen (H₂): Highly flammable, can cause explosions.
   • Methane (CH₄): A potent greenhouse gas and highly flammable.
   • Ethylene (C₂H₄) and Propylene (C₃H₆): Hydrocarbons that contribute to fire intensity.
2. Toxic Gases:
   • Carbon Monoxide (CO): Highly toxic, can cause asphyxiation.
   • Hydrogen Fluoride (HF): Extremely hazardous to health, causes severe respiratory damage, burns, and systemic toxicity.
   • Phosphorus Pentafluoride (PF₅): Highly reactive and toxic, resulting from electrolyte decomposition.
   • Acrolein (C₃H₄O): A respiratory irritant that can cause severe pulmonary damage.
3. Particulate Matter:
   • Fine particulates released during combustion can enter the lungs, causing respiratory and cardiovascular issues.

Impact on the Surrounding Area

1. Extent of Gas Dispersion:
   • Toxic gases can travel significant distances, depending on wind speed, temperature, and atmospheric conditions.
   • Gas clouds from BESS fires have been reported to affect areas up to several kilometers downwind.
2. Health Hazards:

   Immediate Health Effects. Inhalation of toxic gases (described above) can cause:

   • Respiratory distress.
   • Severe burns to mucous membranes and airways.
   • Acute poisoning leading to death at high concentrations.

   Long-Term Health Effects:

   • Prolonged exposure to low levels of HF and particulates can result in chronic respiratory diseases and systemic toxicity (e.g., kidney and bone damage).
   • First Responders: Emergency personnel are particularly at risk as a result of direct exposure during firefighting.

Challenges for Emergency Response

1. Detection and Monitoring:
   • Toxic gases like HF are difficult to detect without specialised equipment.
   • First responders require protective gear (self-contained breathing apparatus, acid-resistant suits) and real-time gas monitoring systems.
2. Environmental Impact:
   • Toxic gas clouds and combustion residues can contaminate soil, air, and water.
   • Cleanup and remediation can take months, with significant environmental consequences.
3. Containment Zones:
   • In case of a fire, authorities often establish large exclusion zones (up to several kilometers) to protect the public from toxic gas exposure.

Prevention and Mitigation Strategies

1. Battery Design:
   • Using safer chemistries (e.g., lithium iron phosphate) that are less prone to thermal runaway.
   • Incorporating robust thermal management systems to prevent overheating.
2. Fire Suppression Systems:
   • Advanced systems like inert gas flooding or aerosol-based suppressants can help limit fire spread but may not completely extinguish the fire.
3. Monitoring and Early Detection:
   • Thermal cameras and gas sensors can detect early signs of thermal runaway and trigger alarms.
4. Emergency Planning:
   • Establishing clear protocols for evacuation, containment, and public communication during incidents.

Summary of Toxicity and Hazards

The following table shows what the Permissible Exposure Limits (PELs) and the Immediately Dangerous to Life or Health (IDLH) limits are for thermal runaway. We also include an estimate for the amount of toxic gases that might be emitted over a 12 hour period for an 85MW BESS (a badly managed situation could last 24 hours).

Substance	IDLH (ppm)	PEL (ppm)	Est Vol	Health Hazards
Carbon Monoxide	1,200	50	85,000–170,000 m³	Hypoxia, loss of consciousness, and death.
Hydrogen Fluoride	30	3	6.8–68 kg (~5.7–57 m³)	Severe burns, respiratory damage, systemic toxicity, and cardiac arrest due to hypocalcemia.
Phosphorus Pentafluoride	Not established	Not established	6.8–34 kg (~4.6–23 m³)	Hydrolyses to HF, causing corrosive damage and systemic fluoride toxicity.
Acrolein	2	0.1	1,000–2,000 kg (~810–1,620 m³)	Severe respiratory and eye irritation, pulmonary oedema, chronic lung damage, and potential carcinogen.

Key Takeaways

1. Carbon Monoxide (CO): A silent killer with rapid onset of hypoxia.
2. Hydrogen Fluoride (HF): One of the most dangerous chemicals, with severe corrosive and systemic effects even at low concentrations.
3. Phosphorus Pentafluoride (PF₅): Extremely hazardous due to HF formation.
4. Acrolein (C₃H₄O): A potent irritant and lung toxin with both acute and chronic risks.

All these substances require careful handling, proper protective equipment, and real-time monitoring to avoid severe health consequences.

Conclusion

Lithium-ion battery fires in large-scale storage facilities pose a severe hazard due to their self-sustaining nature, the release of flammable and toxic gases, and the difficulty of extinguishing them. These events can impact large areas, endanger human health, and strain emergency response systems. Proactive safety measures, robust system design, and thorough emergency planning are essential to mitigate risks.

These hazards are all the more important to consider given the adjacency to the main Rochdale Road, Turn Village, farms and a school.