Fires in Mulch Piles - Advice and Experience from the Industry - Findings of a Preliminary Survey

By: Robert Rynk, Agricultural Engineering, State University of New York (SUNY) Cobleskill and Richard Buggeln, Center for Industrial Services, University of Tennessee

Introduction
During the latter months of 2008, with much assistance from Amerimulch, we conducted a survey to better explain the factors that lead to fires at mulch and compost production facilities. We also wanted to learn how to successfully deal with a fire when one does occur. In short, we are searching for the “Do’s and Don’ts” of fire prevention and response.

Spontaneous combustion (SC), either certainly or probably, accounts for three-quarters of the fires documented by the survey. It is also the most difficult type of fire to detect and fight. Other suspected causes include cigarettes, heat from grinders, arson and hot incoming feedstock.

What causes Spontaneous Combustion?
SC is not, in itself, a mystery. It is the result of a chain of chemical reactions that release heat at a self-accelerating pace. With moist organic materials, the initial heat-producing chemical reactions are largely biological, e.g., assisted by microorganisms. As the temperature surpasses 180 F, the biology quits and purely chemical (“abiotic”) processes take over. This chemical reaction chain can raise temperatures high enough to cause the material to ignite and burn. Burning produces even more heat, which causes neighboring materials to ignite. If enough oxygen is present, the burning shows itself as flames. If oxygen is lacking, a smoldering burn persists, inside the material. The smoldering mass expands until it either reaches daylight or daylight reaches it, e.g., by opening up a pile or turning on a blower. Then, the flames appear.

We know the general factors that lead to SC at mulch and compost facilities. If you pardon the pun, a “sure fire” way to achieve SC is to build a large pile of moderately dry organic materials and then leave the pile undisturbed. A fire will eventually occur. It may take weeks or it may take months depending on internal and external conditions. Conversely, a fire can be avoided if there is enough moisture to evaporate away the heat generated, or if someone breaks the reaction chain by disturbing the pile (thus releasing the accumulated heat). Bone-dry materials have a low chance to spontaneously heat because the lack of moisture prevents the initial biological heating. However, a good rain may jump-start the biological process. The scary part of this scenario is that a fire may begin within a small moist section of an otherwise very dry pile.

Possible SC Factors
We are uncertain about the specific factors that determine the time frame, severity and nature of mulch fires. Consider the comments from three experienced mulch producers who participated in the survey:

“Piles over 60 days [old], especially when cold and windy are the most prone to burning”
“Every fire that we have had occurred when the outside temperature changed to cold [and when there was] a low [cloud] ceiling pushing downward.”
“Fires at our mulch piles normally occur in springtime, Mar-May (longest period of storage); windy days; on weekends or night, in piles that are 15 ft. or taller and driven on; normally on the side [from] which the wind is coming; seems to occur after a period of rain; are worse with wood mulch vs. bark mulch; normally start at the bottom of the pile …”

These observations suggest some of the potential factors, interactions and contradictions associated with SC in bulk organic materials. These kinds of observations prompted our survey. Can we sort out and/or rank factors such as pile height, compaction, material type, climate and weather parameters in respect to their importance in promoting or preventing SC events? In some cases the responses to survey questions were “suggestive”, in other cases no clarity emerged.

We received responses from 42 mulch-producing facilities widely disbursed within the U.S. and Canada. Of the 42 responses ten reported never experiencing a fire and provided no other data. The collective information from the other 32 facilities provides enlightening advice but still leaves some of our questions unanswered.

Highlights of the survey’s findings

Feedstocks: Nearly all of the facilities are primarily mulch-producing businesses. About 12 facilities handle materials that can be categorized as “green” yard trimmings, e.g., brush, land clearing debris leaves, etc.; 9 facilities handle primarily dry wood (pallets, construction debris); and 7 process mostly bark.

Timing: It is more likely that the time of year that a fire occurs has more to do with how long the feedstocks have been held than the season.

Discovery: In about 65% of the cases, the fire was discovered by the facility staff member, e.g. operator, manager, security, rather than a neighbor, passer-by or police officer. The most frequent way that the fire showed itself was by its flames (37%). Smoke (30%) and a burning smell (20%) were also common first-signs. The prominence of flaming fires begs the question: Did these facilities have an undiscovered smoldering fire for days, weeks or even months prior to the emergence of flames?

Weather: While fire victims often blame a recent rain or passing weather front for initiating a fire, the survey results do not point to a smoking weather gun (again, pardon the pun). The responses show no pattern in regard to ambient temperature, humidity and wind conditions.

Pile Size: Most of the piles that caught fire can be considered large (20+ ft. tall), well insulated and difficult to monitor. Such large piles are apparently the norm for mulch production. Furthermore, most of these piles are static (no turning, no movement). Typically, the piles that burned were in place four to six months before the fire was discovered, although in two cases fires occurred after only one month.

Pile Compaction: Some facility operators take false comfort in the thought that large compacted piles are oxygen-starved and thus not prone to fire. 70% of the facilities experiencing spontaneous combustion report they intentionally compact the piles by driving on them with a wheel loader or bulldozer. (Since any pile higher than 20 ft. probably needs to be driven upon to achieve its height, “compacted piles” likely comprise 85% of the cases). Why does fire occur even in the absence of oxygen? First, it is difficult to completely eliminate oxygen from a freestanding pile of bulky materials; there will be a natural “chimney effect”. Some oxygen diffuses or flows in to feed heat-releasing processes. Second, even if no oxygen is present, certain heat releasing chemical reactions still take place and raise the temperature of the surrounding material. Third, a large pile size, plus compaction, makes the pile more energy dense and reduces heat loss to the ambient environment. In summary, a large pile can heat and burn even when compacted, and possibly especially when compacted.

Fire Fighting: Collectively, the survey responses endorsed the practice of fighting a spontaneous combustion fire with a wheel loader, excavator or bulldozer along with a fire hose. It was clear that the burning or hot material in the pile needs to be removed before it can be extinguished with water. It is ineffective to merely spray water on a burning pile, unless a “surface fire” was caused by cigarettes, equipment heat and sparks.

If you would like a compilation of the survey results, please contact Amerimulch at jenniferm@amerimulch.com or either of us at rynkrf@cobleskill.edu or richard.buggeln@tennessee.edu . We hope that this survey becomes a first step in a more comprehensive and widespread effort to explain, prevent and respond to fires at mulch and composting facilities.