It’s hard to believe. But, the grass trimmings in the compost bin can actually catch fire spontaneously, and become a fire hazard to your house and neighbors.
What can cause a compost pile to self-ignite?
Compost piles can self-ignite when the heat generated during decomposition is trapped inside a dry, insulated, and compacted pile with small particle size and pockets that are low in oxygen. Hot, windy weather conditions can serve as catalysts for a small pile of yard trimmings that are only 10 inches (0.25m) tall to self-combust.
I’ll explain how several conditions can combine to cause a chain reaction leading to the spontaneous combustion of a compost pile. I will also go through the precautions that can be taken to prevent it.
Can compost piles self-combust?
Home compost bins with yard trimmings in the backyard can self-ignite and burst into flames, although it is not very common.
In May 2012, a house of two floors in the southwestern suburb of London in the United Kingdom was burnt down after a compost bin in the backyard self-ignited when the owners were away (Gayle, 2012, May 29). The bin with dry grass cuttings was believed to have burst into flames as the compost pile has not been turned for a while and was overheated under the hot weather. The fire quickly engulfed the house and even spread to the neighbors.
Research has also shown that yard trimmings in backyards can also self-combust when under certain conditions (Buggeln & Rynk, 2002). More on the conditions later in this article.
Spontaneous combustion of compost piles at composting facilities is more common and widely reported. For example, on August 14th, 2017, a 20ft (6.1m) tall by 70ft (21.3m) wide compost pile self-ignited at a compost facility in Houston, Texas. Within 30 minutes of combustion, the entire pile was engulfed in flames.
In fact, apart from compost, many other types of organic materials have been found to be susceptible to spontaneous combustion as well (Kayser & Boyars 1975). They include hay, grains, animal fibers (wool), alfalfa meal, beans, coal, coconut fiber, cotton, feeds and fertilizers (manure), wood chips, etc.
What causes compost to self-ignite?
1. High biological activity
For spontaneous combustion to happen, the first event that must happen is high microbial activities during the decomposition of organic matter. In the last stage (thermophilic stage) of composting, biotic heat is generated by the microbes during the decomposition of living plant matter and the temperature can reach up to 160 °F (70 °C).
From that point onwards, the temperature will typically drop because the heat kills the microbes, effectively ending the microbial activities. However, when other conditions exist, the temperature will continue to accumulate.
2. Low moisture content
Even after the cessation of microbial activities, the core temperature of a compost pile will continue to climb above 160°F (70°C) if there is low moisture content in the compost pile.
This is because water plays an important role in releasing heat when it evaporates. A low moisture content would thus mean that heat cannot be released, causing heat to build up inside the compost.
Research has shown that compost with a low moisture level of only 20-34% can accumulate so much heat that will reach a temperature of between 160 – 194 °F (70-90 °C). When other conditions exist, spontaneous combustion can easily occur.
3. Insulated pile
An insulated pile retains heat and prevents the heat generated in the decomposition from escaping to the outside.
When the buildup of heat is faster than the escape of heat from the surface of the compost pile, the internal temperature can soar quickly to the point of self-combustion.
Large piles in particular can hold a lot of heat, especially in the core, and thus require more frequent turning.
4. Windy environment
A windy environment can act as a catalyst to cause high temperatures to keep climbing despite the cessation of biological decomposition.
When air flows into the compost and oxygen reacts with plant chemicals, another type of heat (abiotic heat) can be generated, causing the temperature to keep climbing. When other conditions such as low moisture content also exist, a compost pile can self-ignite.
5. Absence of oxygen
When the organic substance is heated in an environment that is not only low in moisture but also low in oxygen, pyrolysis occurs where intense heat is generated by increments of 41-45 °F (5-7 °C) per minute.
This often happens when the compost pile is compacted because it has not been turned. It also happens when such conditions occur only in certain pockets.
During pyrolysis, organic matter self-ignited quickly and the temperature quickly shoots up to 932°F (500 °C). Methane gas and carbon monoxide are also generated in the process. When pockets of methane gas are met with exceedingly hot, dry materials, it can erupt into a fire or even an explosion even in small compost bins.
6. Small particle size
When the conditions for pyrolysis already exist for it to occur and when the compost particle size is small, under 2 mm in diameter, the heat generated during pyrolysis is much more explosive by increments of 572 °F (300 °C) per minute (Garcia-Perez et al., 2011).
7. Hot weather
Once the conditions are met for heat to build up to a critical range, a small change in the external temperature can trigger a dramatic temperature rise. A study has shown that even a small basket of Eucalyptus leaves and wood could self-ignite with just a 36-37 °F (2 to 3 °C) rise in the ambient temperature.
High surrounding temperature also encourages water to evaporate, thus lowering the moisture content in the pile and making it hard for heat to escape.
Does size matter?
Spontaneous combustion can happen even with small piles, and the size of the pile only matters in relation to the type of materials in the compost.
Studies have been performed to find the “critical pile size”, or the minimum dimensions of a pile of material for spontaneous combustion to take place under ambient conditions.
The smallest critical pile size is only 0.5 ft (0.14 m) in diameter for a pile of cotton gauze with linseed oil (Gross & Robertson, 1958), 10 inches (0.25cm) in width for a pile of leaves, stems, seeds, and bits of cotton (Buggeln & Rynk, 2002), 13 ft (3.9m) in width for cubes of Eucalyptus leaves (Jones & Raj, 1988), 159 ft (48.5m) for cubes of wood (Jones & Raj, 1988), and 167 ft (51 m) in diameter for spheres of dry hay (Bowes, 1984).
| Compost content | Critical pile size for self-ignition |
| Cotton gauze with linseed oil | 0.5 ft (0.14m) |
| Leaves, stems, seeds, and bits of cotton | 10 inches (0.25m) |
| Eucalyptus leaves | 13 ft (3.9m) |
| Wood | 159 ft (48.5m) |
| Hay | 167 ft (51m) |
While a larger pile can self-ignite under a lower ambient temperature, smaller compost piles can also self-ignite when under a higher ambient temperature.
How long does a compost pile stay hot?
Generally, it can stay hot for a few days. But how long a compost pile stays at any given temperature depends on a variety of factors, primarily the moisture content, frequency of turning the pile, size of the compost pile, exposure to oxygen, and carbon to nitrogen ratio.
How to tell when the compost gets too hot?
There are a couple of ways to determine if your compost pile is becoming too hot. One way is to use a compost thermometer. This can be a great tool to monitor your pile on a daily or weekly basis. If your compost pile is getting above 160 °F (71.1 °C), then your compost pile is becoming too hot to sustain the proper organic material decomposition function of soil microbes. At this temperature and higher, oxygen is being used up by microbes faster than it can be replenished from the external environment, creating an anaerobic environment.
You may also be able to tell if your compost pile is too hot by looking for the presence of prominent white, ashy anaerobic bacteria on the pile. This presence on your pile would indicate the core temperature may be too hot for the balanced functioning of healthy soil microbes.
It’s important to note that compost piles frequently steam as soil microbes heat the environment and break down organic matter. This steaming is a natural and normal occurrence that is of no concern as long as the pile is being adequately turned and kept moist.
How to cool down compost?
Turning your pile can actually help heat up or cool down the compost, depending on the conditions of the pile. If the soil microbes are fairly inactive due to a lack of oxygen, turning the compost will allow oxygen to permeate the pile, kick-starting the soil microbe activity and raising the temperature over a few days.
However, if the compost pile is becoming too hot from high microbe activity and the insulative properties of the pile, then turning the compost and exposing the inner sections to the ambient air temperature will help immediately cool down the pile.
Another way to cool down a compost pile is by adding more carbon (also known as “browns”), such as dry leaves, tree branches. Nitrogen is crucial to the cellular growth of soil microbes. If there is too much nitrogen in the carbon-nitrogen ratio of the compost pile, soil microbe communities will become too active and numerous, creating high core temperatures through the organic matter decomposition process.
If you suspect this is why your compost pile is too hot, cool it down by turning and adding in carbon-rich materials such as dry leaves and wood chips.
How to prevent the compost from catching fire
To prevent the compost from catching fire, you can:
- Monitor the temperature of your compost constantly. Keep a compost thermometer with a long probe inserted into the core of your compost. This is necessary especially if your compost pile is close to your house.
- Turn the pile as soon as possible after it reached 160 °F (71 °C) or higher. This is to prevent the pile from compacting and forming pockets that are low in oxygen which would lead to an explosive release of heat under pyrolysis.
- Mix in coarse bulky substances such as branches, and wood chips to increase aeration.
- If the temperature exceeds 160 °F (71 °C), sprinkle water to cool down.
- Prevent the pile from becoming too dry. Make sure that the compost pile is moist to the touch, without being drenched. The best moisture level is 60-65 %, which is if you squeeze a ball of compost, the compost stays in the shape of a ball without falling apart and there is no water dripping. Overly dry piles contain more combustible materials.
- Consistently turning the pile, at least 5 times within a 15-day period, to help release heat and prevent parts of the compost from becoming anaerobic.
- Place your compost pile, container, or bin at a safe distance from structures, fencing, and decking. If a fire does occur, there is a reduced likelihood that it will quickly spread. If your compost pile is inside, such as in a garage, make sure the pile is small enough to not produce significant thermal mass. It’s also even more imperative to consistently monitor the core temperature and regularly expose the inner portions of the pile to the much cooler outside air temperature.
Conclusion
The internal temperature of a compost pile soars and ultimately explodes into flames when a host of conditions are met.
The level of moisture and oxygen inside the compost pile are crucial factors for reaching the critical temperature for spontaneous combustion. If the particle size is small and the day is hot and windy, even small compost piles can erupt into flames on their own.
To prevent it from happening, the temperature of the compost must be monitored and proper compost management practices must be implemented.
Happy gardening!
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References
Clawson, B. (2022). Compost piles will warm up and steam in the winter. Michigan State University Extension. January 21.
Bowes, P.C. (1984). Self-heating: evaluating and controlling the hazards. vi+500pp. Elsevier Science Publishers. Amsterdam
Buggeln, R. & Rynk, R. (2002). “Self-Heating in Yard Trimmings: Conditions Leading to Spontaneous Combustion,” Compost Science and Utilization, (2002), Vol. 10, No. 2, p. 162-182.
Compost Physics. Cornell Composting.
Compost Fundamentals: Compost Needs – Temperature. Compost Fundamentals.
Garcia-Perez, M., Chen, S., Zhou, S., Wang, Z., Lian, J., Lee Johnson, R., Liaw SS., Das, O. (2011). Methods for Producing Biochar and Advanced Biofuels in Washington State – Part 1: Literature Review of Pyrolysis Reactors. Ecology Publication. Number 11-07-017.
Gayle, D. (2012, May 30). Home gutted by fire after compost bin spontaneously combusts in hot weather. Mail Online.
Gross, D. & Robertson, A. F. (1958). Self-ignition temperatures of materials from kinetic-reaction data. Journal Res. Nat. Bureau Stds., 61:413-417
Jones, J. C. & Raj, S. C. (1988). The self-heating and ignition of vegetation debris. Fuel 57:1208-1210
Miller, R. A. (2007b, March 23). Composting And Greenhouse Gas Emissions: A Producer’s Perspective. BioCycle.
NMSU: Bernalillo County Master Composters: Spontaneous Combustion of Compost. (n.d.). College of Agricultural, Consumer, and Environmental Sciences.
Rodale Institute (2019). Turning Compost by Temperature. August 26.
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