Spontaneous combustion in compost piles is an unfortunately common occurrence at large composting facilities in North America. With more large composting facilities being built to manage organics, and more air permit conditions aimed at preventing fires, compost pile fire prevention is more important than ever. Fortunately, the science of spontaneous combustion is well established, and there are many practical prevention and detection strategies for composters to use.
J. Armstrong (1973) and the Canadian Department of Forestry Research Institute explain that spontaneous combustion in compost piles is due to a “combination of mechanisms, with biological reactions providing an initial temperature increase great enough to start a physiochemical process which [can] raise the temperature to the ignition point.”
Thermophilic bacteria produce the initial biological reactions which generate heat in the piles. Their respiration process, known as bio-oxidation, turn bio-available carbon into carbon dioxide, water and heat. Compost piles that are well insulated and rich in bio-available carbon sources can easily reach and sustain temperatures of 85 or 90 deg C (185F to 195F) without sufficient cooling mechanisms.
While these temperatures are high enough to begin to inhibit and eventually kill off thermophilic bacteria, the hot temperatures can also initiate chemical reactions which drive more heat generation in the feedstock mix and consume the available oxygen. Without sufficient oxygen, these chemical reactions can produce pyrolytic (flammable) gases. These gases may accumulate and then ignite into sustained, smoldering fires. When these smoldering fires are exposed to excess oxygen, they can turn into open flames which readily consume the carbon-rich, compostable material.
As with all exothermic chemical reactions, the total energy (heat) released is dependent on the volume and concentration of the reactants. J. G. Quintierere et. al. (2012) demonstrated an “increasing tendency for spontaneous ignition with increasing concentration (of reactants)”. The accumulation rate of gaseous reactants which fuel compost fires is quite slow, as we know from observing the weeks between pile construction and combustion. In his 1973 paper, J. Armstrong cites an experiment where researchers had to artificially maintain compost temperatures at 130C (266F) for 14 days with no air changes (aeration) to generate enough reactants to achieve spontaneous combustion in a laboratory setting. In other words, a compost pile must be left effectively unventilated for weeks to accumulate enough reactant gases for spontaneous combustion to become possible.
While there are other factors that facilitate spontaneous combustions, which we will touch on later, it’s the condition of having essentially no air-changes in the pile pore space that is a key ingredient in compost pile fires. Even very low flow-rate aeration systems will fully replace the pore space air with fresh ambient air many times a day. ECS systems are designed to deliver uniform aeration, which helps to ensure that reactant gases do not present a fire risk.
We have calculated the number of daily air changes that can be achieved with average aeration rates in a low-flow ECS ASP system, which is designed to deliver air uniformly though a composting mass:
| Pile Aeration Rate (cfm/cy) | 0.6 |
| Porosity of Mix (%)* | 40 |
| Air Changes per Hour (ACH) | 2.2 |
| Air Changes per Day (ACD) | 53 |
* “Porosity” refers to the empty spaces within the pile that are not occupied by solid feedstock particles or water. It has an inverse relationship with bulk density. The lower the bulk density the higher the porosity, and vice versa.
So, knowing what we do about the risk factors for spontaneous combustion, how do we apply this to prevent fires in compost piles?
A low airflow-rate system can provide enough daily air changes to improve safety, but a medium to high air-flow rate aeration system can help provide enough cooling to prevent temperatures from remaining above 65C (150F) for more than a few days. Since we know it takes both heat and time to generate enough reactants to ignite a pile fire, investing in a medium to high airflow system for your active compost phase can help ensure that pile temperature remain within the bounds of both safety and optimal compost biology.
By providing enough air to microbes, your compost pile also benefits from robust colonies of aerobic bacteria which stabilize the feedstock material as they consume (bio-oxidize) it. With improved rates of bio-oxidation, you can expect that your compost will stabilize relatively quickly. Stable composts are much less likely to heat up in curing or maturation piles, which are typically un-aerated.
Well-established Best Management Practices (BMPs) for good composting outcomes also help prevent spontaneous combustion conditions. Providing a starting moisture between 50-65% ensures that there is sufficient moisture for evaporative cooling, and a mix bulk density between 750-950 lbs/cubic yard should offer adequate porosity for low-resistance air changes. Operators should aim to homogenize their mix as best is possible, evenly distributing nutrients, moisture, and bulking agents to prevent overly dry and/or anaerobic pockets.
Generally, an ASP with a BMP-compliant feedstock mix can be safely built to maximum heights of 10-11’. Material in an ASP will be prone to drying and settling over time, rendering the aeration system less effective at moving air, and the dry material more primed for spontaneous combustion. Material should not be left undisturbed in an ASP for more than 4 weeks. Instead, operators should break down the ASP, mix the material to break up the compacted areas (which will also release any reactant gases) and introduce moisture as needed to achieve stability and air pollution goals. This material can then be aerated in a secondary composting phase or cured in an un-aerated pile.
Un-aerated static piles and windrows, especially those with active feedstocks, should be kept under 8’ to maximize surface area and be turned regularly to release any reactant gases that do accumulate in the anoxic interiors of the piles.
Once built, proactive monitoring of un-aerated piles can help prevent large and damaging pile fires. Weekly visual monitoring for hot spots (best during cool times of day so that water vapors exiting hot spots are most obvious) is a simple method that any operation can employ immediately, but not the only one that should be used. Temperature probes should be inserted in and around visually located hotspots. If temperatures above 170F are discovered, the spots should be flagged and measured two or more times per week. If temperatures exceed 180F the pile should be broken down to release the accumulation of reactive gases. Operators should never walk on top of hot spots in a pile.
Incoming feedstocks should not be stored undisturbed for more than 2 weeks and bulking agents, like wood chips, present a fire risk when stored in tall piles (>20 feet) for prolonged periods of time.
Curing piles should ideally be composed of material that has already achieved a degree of stability (Solvita 6+) to prevent biogenic re-heating. Curing piles with poorly-to-moderately stabilized material and overs should be stored in smaller piles, monitored for hot spots, and moved/turned proactively every 4-12 weeks depending on level of stability. In piles of curing material, wood chips, and overs that have not been stabilized in the active composting process (achieved a Solvita 6+) there is a risk that exposure to intermittent wetting, like rain, can cause biogenic heating that increases the combustion risk of spontaneous combustion. For this reason, it is advisable that monitoring protocols be increased after periods of rain.
For aerations systems with biofilters, there is also a risk of spontaneous combustion in the biofilter media. The same conditions that result in pile fires can occur in any biofilter media because biofilters also rely on aerobic bacteria to help consume odorous gases that are captured in the media. Older biofilters, or biofilters constructed with easily degradable substrates, risk becoming channelized as the material compacts in certain areas. When this occurs, air does not move uniformly through the biofilter and areas of the media which do not get airflow can accumulate reactant gases. Additionally, surface wetting is important for biofilter performance, but when the moisture of the media is inconsistent, there is a greater for combustible, dry pockets to develop.
For a biofilter that has been built to a BMP spec, the combination of regular visual inspections, temperature tracking with embedded probes, and uniform irrigation can greatly diminish the risk of spontaneous combustion. When a biofilter has reached its maximum service age (generally between 3-5 years), it should be replaced. More information on biofilters here.
Once started, compost pile fires can be dangerous, expensive, and time consuming to put out. And the PR harms associated with them can often be insurmountable. For these reasons, an ounce of prevention is always worth more than the pound of cure. Aeration systems that are well-configured and intelligently engineered can provide more than ample air during the active composting phase, encouraging rapid stabilization and evaporative cooling. These stabilized feedstocks then present a much lower risk of fire in the curing and storage phases, saving a compost operation time, money, space, and anxiety.
ECS is dedicated to delivering the safest and most reliable aeration systems in the industry. Get in touch to learn more about how an ECS aeration system can support your processing goals, or how our in-house process experts can help improve the safety and efficiency of your operation.
For more reading about how to manage an active compost fire, please refer to Biocycle‘s excellent series on the topic.
