Composting Fires

burningfacility

The fire that burned for nearly a month at a huge composting facility in Southeast Austin will cost the city about $9 million, mostly to remove as many as 12,000 truckloads of charred debris, according to recent estimates.

The fire started February 25th in a large pile of wood chips, transforming the neat windrows of yard trimmings and sludge at the Hornsby Bend Biosolids Management Plant into something looking like primeval planet Earth, with a haze of smoke hanging steadily over an ashy landscape.

It took until late March for the fire department and a contractor to extinguish the fires.  The city council this week approved paying $3 million for that contractor.

The Austin American Statesman reported that Austin Water Utility plans to ask for $6 million to finish the clean-up.

The subject of fires at composting facilities is a bad news/good news situation. The bad news is that fires are more common than we realize. Ask a group of facility operators if they have had to deal with a fire, and the majority will quietly admit they have. The good news is that we don’t realize that fires are fairly common at composting facilities.

Nevertheless, a fire is a serious matter. A minor one threatens to attract public inquiry about the risks and nuisances of composting activities at a particular site and within the compost industry generally. A major fire threatens a multimillion dollar investment and presents a potential danger to workers and firefighters.

A suspected cause of the fire is spontaneous combustion, which is among the more common — yet also one of the more mysterious. However, fires at composting facilities also have developed from other causes; including lightning strikes, heat from equipment, sparks from welding activities, wildfires and arson.

Fires due to lightning, sparks, wildfires and arson can be thought of as surface fires because the fire usually starts and spreads along the exterior. Surface fires are more easily detected and controlled than internal fires, as occurs with spontaneous combustion.

Spontaneous combustion may be the most frequent cause of fires at compost facilities. It happens when materials self-heat to a temperature high enough to cause them to ignite. No external energy source is needed. The temperature increases because more heat is generated internally than lost to the surrounding environment. As spontaneous combustion progresses through the steps in the chain, there is less and less time to react and halt it.

Key conditions that lead to spontaneous combustion are biological activity, relatively dry materials or dry pockets, large well insulated piles or vessels, limited air flow, and time for temperature to build up. In addition, there may be other contributing factors such as short circuiting of air flow, a non-uniform mix of materials, poor moisture distribution, difficulty in knowing temperatures throughout a pile, and sometimes a lapse or oversight in monitoring.

The problem with large piles and vessels is that they are difficult to monitor. Most thermometers and temperature sensors cannot penetrate deep enough or cover enough locations to detect all potential hot spots in a large pile or vessel. In-vessel systems often employ numerous temperature sensors, but compost is such a good insulator that a very hot pocket of material can escape the detection of a sensor just a few feet away.

The nature of composting and organics recycling activities presents ample opportunities for a fire to develop. Large, undisturbed piles of partially dry decomposing materials pose the greatest risks. Problems occur when regular monitoring is reduced or interrupted.

Let’s hope no compost pile is left un-monitored in your back yard.

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