|The composting process benefits from the ability of the mix of ingredients and microbes to be self-heating. This property drives compost drying, physical degradation, and especially the regulated requirements of pathogen kill and vector attraction reduction. All the operator has to do to kick off the heating cycle is furnish the compost biology with appropriate environmental and nutritional requirements. In turn, this vigorous biology loses about a third of the energy produced as heat - and it is this heat we count on to make the composting process successful. But it is also this heat that can get operators in trouble.|
Microbially generated heat - or what I call a “Biological Fire” - is the match that can lead to spontaneous combustion, a chemical fire with smoking embers, and at worst, flames. While surface fires nearly always are caused by human or external situations, spontaneous combustion is the result of failing to control the internal pile temperature. In both cases, the source of this energy is oxidation of organic matter, or volatile solids. Water, carbon dioxide, energy and other gases are given off, leaving a residue. In the case of the composting process, waste energy is generated as heat, and the residue is compost.
For spontaneous combustion to occur, heat from both biological oxidation and chemical oxidation are needed. The biology of the process can bring the temperature up through 55°C to assure pathogen kill, but will continue to rise into the 70°C to 80°C range, where chemical oxidation takes over as the predominant energy source and biological death occurs. Unless immediate action reduces this temperature, a compost fire is very likely. In short, both biological and chemical oxidation - combined with retention of the heat in a pile - are required for spontaneous combustion.
Let's take a brief look at the theoretical heat generated by the Biological Fire. In an aerated static pile experiment I did last year, we prepared a compostable mix of biosolids and shredded yard trimmings, and connected it to an induced aeration system in which the exhaust side of the blower discharged to a biofilter. This pile consisted of 400 wet tons of the mix at 50 percent dry solids. That equated to 200 tons of water and 200 tons of dry matter, of which 70 percent was volatile solids. Eighteen percent, or about 25 tons, of the volatile solids were considered biodegradable. Some sources suggest that oxidation of volatile solids, whether chemical or biological, generates about 10,000 BTUs per pound. Using this number, the 400 tons of compost mix has the potential to generate half a billion BTUs. This is not an insignificant number, but neither is 400 tons an insignificant mass.
A FEW ENERGY DEFINITIONS
Before going any further, three terms need to be defined: BTU, temperature versus heat energy, and heat capacity. One BTU is the quantity of energy required to heat one pound of water one degree Fahrenheit. In other words, when you pick up a pint of beer, and hold it in your hand for a bit, by the time that beer has been warmed 1°F, you have transferred 1 BTU of energy to the beer. That's a BTU.
Next are temperature and heat. Temperature is a sensory measurement, how it “feels,” and can be measured with a thermometer. Heat, on the other hand, is based on how much work, such as warming compost, can be done - the “quantity” of energy. For example, a match flame is really hot, but a bathtub full of warm water has a lot more heat energy if you are trying to warm up your body.
Heat capacity is the amount, or quantity, of energy per unit mass that a material will “soak up” before its temperature changes. For example, water needs to absorb one BTU to experience a temperature change of 1°F. In contrast, when organic matter soaks up one BTU, its temperature will rise 4°F. This is one reason why dry clothes warm up faster than damp or wet clothes. And, of course, why overly wet compost piles may be slow to warm up, an important consideration during cold weather when heat demands to achieve regulatory-required temperatures are greatest.
The next step is to connect this energy or thermodynamics information to the composting process. Perhaps 75 percent of the half billion BTUs in the 400 ton compost mix example used above will be released in the first two weeks of composting. It takes about 500,000 BTUs to raise the temperature of the 400 tons of compost 1°F. Putting this together suggests that if the heat is released uniformly and no heat is lost from the pile (an unlikely scenario), a theoretical temperature change of about 54°F per day for two weeks is calculated. So there is plenty of heat available to get the pile into the danger zone. But few piles get so hot so fast. One reason of course is that the temperature achieved would quickly and completely sterilize the pile of compost thus eliminating any heat generated by microbes. Other factors also will minimize temperature increases.
In reality, this heat is lost from compost piles in a variety of ways. Two important losses come through pile aeration. Heat generated within the pile evaporates water. In our example, about 70 million BTUs may be absorbed by evaporation. Second, either natural or mechanical ventilation carries heat evaporated moisture out of the pile, releasing it to the environment. Without this ventilation, excessive heat buildup is possible.
RECIPE FOR A COMPOST FIRE
So what situation(s) can lead to a fire? Here's what can happen with a low moisture, large pile with little air exchange, combined with water getting into the pile in a place where there is enough air to support biological activity and chemical oxidation, but not enough to cool the pile.
An old, dry compost pile, or a pile of overs screened out of the finished product, is a case in point. Water seeping into the dry compost can restart microbial activity and initiate reheating. A “macropore” or crack from the hot spot to the surface often develops into a vent, or chimney. Air movement up through this vent draws more oxygen into the hot spot where heat is being generated, rapidly escalating the transition from a biological fire to smoke and glowing embers. Appearance of this hot, humid air at the surface can be an important indicator that heating is taking place inside the pile.
Vents can be identified in the cooler times of the day when the condensing mist from the vent shows up most easily. As the mist emerges from the pile, condensation on the surface discolors the compost around the vent. Sometimes, mushrooms may be growing there. Walk the top of your piles weekly and look for these vents. Insert your temperature probe right down into the vent to look for excessive temperatures as an early warning sign.
Probing the vent will give us an indication of the hottest temperatures within the pile. While detecting an internal temperature of 80°C to 90°C does not guarantee a compost fire, probability of a fire rapidly escalates at this temperature.
In some cases, composting facilities cure compost or mulch in tall piles (e.g., up to 18-feet in height). I recall a composting facility where the pile of curing compost nearly reached the ceiling of a storage building and was pressing against a concrete pushwall and steel siding in the back. This pile of finished, curing compost generated enough heat to blister the paint on the exterior of the steel siding. In a situation like this, heated air from inside the pile looks for the path of least resistance. That path is usually along the compost/concrete interface (or a short circuit), and up to the top of the pile. As a prevention strategy, I recommended that they drill several openings in the concrete push wall. These openings were large enough in diameter to easily insert a stainless steel temperature probe into the compost on a regular basis to monitor temperature.
RULES FOR FIRE PREVENTION
Rule #1. Set up a meeting with your local fire department. Discuss compost fires, and agree on guidelines on how to handle compost fires once they begin. Have the correct fire fighting gear on site. If your site is remote, a runoff pond can serve as source of water for the composting process as well as for fire fighting. (Editor's Note: An article in BioCycle - “Preventing Fires In Grinding Equipment,” November 2003 - had a number of valuable fire prevention tips.)
Rule #2. Assure adequate ventilation of the pile to release heat and increase evaporation of water, a heat absorbing process. Ventilation can be achieved by turning the pile or using a mechanical aeration system. Ventilation can also be improved by constructing narrower, shallower windrows or piles, generally less than 6-feet deep.
Rule #3. Avoid pile depths greater than 12-feet, and watch for vents in deep piles. Use these vents to monitor internal pile temperatures.
Rule #4. Locate the hot spot before it turns into a fire. Monitor temperature of all piles on a weekly basis, seeking out the hottest spot in the pile. For this proactive monitoring, we are totally uninterested in the average pile temperature (a useless bit of information at this point). We need to know the hottest spot in the pile.
Rule #5. If you have a fire, it needs to be located in the pile. That is usually accomplished by very carefully using a large wheel loader to open up the pile. A fire hose should be available as the loader removes material to spray directly onto burning embers - or a burning loader. The fire department or an in-house fire brigade should be on stand-by as the pile is opened. Don't underestimate the damage - physical or political - a smokey fire can do.
Rule #6. As Smokey the Bear knows best, only you can prevent [compost] fires. Prevention is the only adequate solution to avoiding dangerous and expensive fires at compost facilities.
Lew Naylor is an environmental scientist with Black & Veatch in Gaithersburg, Maryland.He has worked with compost and organic residuals for 30 years. This Compost Operators Forum is based on his presentation at the Texas Recycling & Sustainability Summit in September 2004 in San Antonio, Texas.