Compost pile fire hazard

 

Compost pile fires at composting facilities are very common. For compost producers, this is a serious problem, as it can lead to inquiries or even the closure of composting and mulching activities due to the danger to human life and the risk of major fires.

 

Causes of compost pile fire

 

Compost pile fire occurs due to spontaneous combustion. Spontaneous combustion in compost happens when the heat produced in the pile exceeds the rate of heat loss.

How spontaneous combustion occurs in compost piles

 1. Microbial activity generates heat

 

Organic materials like compost, mulch, and decaying plants contain moisture and nutrients that support microbial decomposition. As microbes break down organic matter, they release heat—especially during the active composting stage. In large piles, this heat can accumulate faster than it dissipates, especially if the pile is compacted or poorly ventilated.

 

 2. Insulation traps heat

 

Materials like compost or mulch are good insulators, so the heat produced internally cannot easily escape. When internal temperatures rise above 70°C (158°F), the risk of spontaneous combustion increases.

3. Dry zones and oxygen supply

 

In some areas of the pile, especially near the surface, the material may dry out while still being exposed to oxygen. This combination can promote oxidation of organic compounds or even self-heating reactions.

 

 Presence of oils or fats

 

Certain agricultural materials (e.g., linseed, canola, cottonseed, or oily plant matter) are more prone to oxidative self-heating. These oils can undergo slow oxidation, generating heat even without microbial action.

 

What are the heat sources in compost pile fires?

 

Compost piles generate heat from three primary sources, each contributing at different stages of decomposition:

1. Biotic heat – Respiration by living cells and microorganisms

 

Still-living plant cells continue to respire after vegetation is cut, releasing heat via the citric acid cycle until they die from lack of moisture or high temperature (25°C–50°C). Microorganisms (bacteria and fungi) then feed on the released sugars and metabolites, producing additional heat through microbial respiration. Source: Buggeln & Rynk, 2002

 

 2. Abiotic heat – Chemical oxidation of plant compounds

 

Lipids (e.g., linolenic, oleic, palmitic acids) in plant cells oxidize in the presence of oxygen, generating heat without biological activity. This auto-oxidation is detectable around 80°C and is accelerated by moisture, similar to rust formation.

        Source: Buggeln & Rynk, 2002

 

 3. Pyrolytic heat – Thermal decomposition in low-oxygen zones

 

In oxygen-starved parts of the pile, slow pyrolysis of cellulose and lignin occurs at 65°C–70°C, producing gases and char. These gases can react with oxygen to produce more heat. Pyrolysis also lowers pH via carbonic acid formation, accelerating further decomposition and heat release. Sources: Buggeln & Rynk, 2002; Gustafsson, 2013

These combined heat sources can drive compost temperatures high enough to risk spontaneous combustion if not managed properly.

 

How does spontaneous combustion occur in compost piles as a chain reaction?

 

Spontaneous combustion (SC) in compost piles is a chain reaction that results from the sequential build up of biotic, abiotic, and pyrolytic heat sources.

As outlined in the previous explanation of heat sources:

• Biotic heat (from respiration of plant cells and microbes) initiates the process.
• When temperatures exceed ~80°C (175°F), abiotic oxidation of dry organic compounds takes over.
• In low-oxygen pockets, pyrolysis begins, further raising temperatures.

This chain reaction is self-accelerating:

• Each stage raises the temperature, enabling the next.
• Reaction rates double with every 10°C increase, meaning heat builds exponentially—16× faster at 100°C than at 60°C.
• If cooling mechanisms like moisture evaporation, aeration, or turning fail, temperatures can rapidly reach 150–200°C (300–400°F), resulting in smoldering or flaming fires.

In essence, spontaneous combustion is not caused by a single factor, but by a progressive interplay of all three heat sources, where early intervention is critical to avoid ignition.
 Sources: Rynk, 2000a; Alberta, 2006

 

Compost pile fire prevention strategies

 

Before moving into the prevention strategies it’s very important to know about the root causes of compost pile spontaneous combustion. Spontaneous combustion (SC) is primarily caused by a combination of biological activity, dry conditions, and poor pile management.

Larger, well-insulated piles—especially as they dry—are more prone to heat buildup due to their insulating properties. Storage piles (unprocessed feedstocks, curing, or finished compost) are more at risk than active composting piles because they are typically less monitored and rely on passive rather than forced aeration.

Critical risk factors include:

 

• Limited airflow and reduced free air space
• Poor moisture distribution (with 25%–40% being the danger zone)
• Nonuniform material mix
• Inadequate temperature monitoring

 

Quick Tip: How to detect fire risk in compost piles

 

Check compost piles weekly for heat vents—look for steam or discoloration on the surface, especially in the early morning. Vents on compost piles can signal internal heat build up—a potential fire risk. These openings allow heat to escape and oxygen to enter, which can fuel spontaneous combustion. Spotting vents early and checking their temperature helps detect fire danger before it spreads. (Naylor, 2004). Use a temperature probe in these spots. If temperatures are near or above 93°C (200°F), stop aeration and remove the material to prevent fire.

 

How to extinguish a compost pile fire hazard

 

Break down the pile and spread the material in thin layers (6–8 inches) to release heat, then soak it with water immediately. Simply spraying water on top won’t work—it doesn’t reach the burning core (Rynk, 2000b).

 

References

Alberta Agri-Facts. “Fire-Compost and Organic Matter”, Agdex 086-9, April 2006.

Buggeln, R. and R. Rynk. “Self-Heating in Yard Trimmings: Conditions Leading to Spontaneous Combustion,” Compost Science and Utilization, (2002), Vol. 10, No. 2, p. 162-182.

Rynk, R. “Fires at composting facilities: Handling and extinguishing fires. Part II.” BioCycle 41 (2000b): 58-62.

Rynk, R. “Fires at Composting Facilities: Causes and Conditions”, BioCycle 41 (2000a): 54-57.

Naylor, L. “Fire Prevention at Composting, Mulch Facilities”, BioCycle 45 (2004): 30-33 at https://www.biocycle.net/fire-prevention-at-composting-mulch-facilities/