Preventing spontaneous combustion in grain, hay, and nut hull storage

Understanding self-heating fire risks in agricultural storage

Organic materials like grain, hay and tree nut hulls are commonly stored at agricultural and processing operations. These materials can create serious fire risk, especially to buildings and storage facilities, when they contain too much moisture or begin to deteriorate.

When moisture is present, materials can begin to heat up on their own. This process is called self-heating and can lead to smoldering or spontaneous combustion. When self-heating occurs in or near buildings, a small hot spot can quickly spread and cause major damage.

What causes self-heating?

Self-heating happens when organic materials break down due to moisture and biological deterioration.

• Bacteria and fungi grow in materials with excess moisture
• This biological deterioration produces heat
• Heat becomes trapped inside piles or bales

Why this matters

Fires caused by self-heating are often called spontaneous combustion fires. These fires are a known risk in agricultural and processing operations and are recognized in fire prevention guidance such as NFPA 61. They often start deep inside stored materials, where they are hard to detect.

Risks of spontaneous combustion fires:

Potential impacts to facilities

When materials are stored in buildings or against walls, fires can extend to:

What materials are prone to self-heating?

Some materials carry a higher risk of self-heating and spontaneous combustion:

• Compost
• Out-of-condition or wet grain 
• Baled hay, especially when stored too wet 
• Tree nut hulls such as almonds and pistachios 
• Oilseeds and oilseed products such as sunflowers, soybeans or cottonseed

These materials often act as both the fuel and ignition source, increasing the potential for fire to spread within a facility.

Risk increases when materials are stored in large piles or in areas with limited airflow, where heat can become trapped.

Warning signs of self-heating

Early signs of self-heating include:

• Steam or vapor coming from piles
• Musty or sour odors
• Warm or hot spots when checking materials
• Mold or darkened areas

If smoke is visible, the smoldering fire may already be accelerating.

How to reduce the risk

Focus on a few key controls:

Control moisture

• Properly condition materials before placing into storage
• Avoid storing wet or rain-damaged materials
• Check moisture levels during storage
• Protect stored materials from moisture ingress.

Maintain airflow

• Ensure airflow through storage piles or turn piles on a regular cadence to dissipate heat

Manage pile size

• Keep piles at a manageable size
• Avoid dense, tightly packed storage

Monitor for heat

• Check materials regularly. 
• Use temperature monitoring (PDF) tools when available
• Watch for changes in temperature, humidity, wind or other weather conditions
• Act quickly if heat is detected to help prevent fire spread

Key takeaway

Self-heating organic materials are a hidden fire risk. These self-igniting fires can start without warning and spread quickly to buildings and equipment.

Managing moisture, storage pile arrangements, and monitoring practices can significantly reduce the risk and help protect your operation.

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