Friday, 27 January 2012

Smouldering fire of large compost heap in Havelock

After Hurricane Irene made landfall on Aug 27, 2011, the city of Havelock in North Carolina (USA) decided to shred all damaged trees and vegetation and collect them in a large heap. The resulting biomass stockpile was ~4000 m^3 in volume, the equivalent capacity of a 16 m cube. The idea was to produce plenty of free compost for the city.

But as the heap dried and decomposed, on its way to become compost, it also started to self-heat and a smouldering fire was initiated without any external source. All organic porous media is known to self-heat under the right environmental conditions of low moisture, poor ventilation and large stockpiles, and these were met in the Havelock heap. Havelock News reports that the fire was first detected on Jan 17, 2012, probably becuase of large and visible off-gassing. This means that the first hotspots formed several weeks before that, maybe during early Fall. After it was detected, the Fire Havelock Fire and Rescue Department and the US Forest Service were called in. They dosed large amounts of water and spread the heap material over a wider area to quench the fire. And it worked, so far. But hotspots deep into the heap are going to be very difficult to cool down completely.
A bulldozer spreads out the pile of smouldering biomass in Havelock. Photo by Drew C. Wilson/Havelock News.
In this poster that I presented recently, I explained self-heating as follows:
"Self-heating refers to the tendency of certain reactive solids in oxidative atmospheres to spontaneous exothermic reactions at low or ambient temperatures. This is a well known problem for industries transporting and storing porous carbon-rich materials [eg, peat, biochar, coal, char and most organic powders]. Initially, small amounts of heat are released and accumulate during longer times when heat losses are low (eg, large stockpiles, high ambient temperatures). This results in a sustained increase of temperature without any external heat source. Above a certain temperature, the process self-accelerates and leads to thermal run away. For example, Semenov’s Ignition Theory describes mathematically the process as controlled by heat generation and heat losses"
Mititating Actions
The main culprit in the case of the Havelock heap seems to be its large size. In other to prevent self-heating events, stockpiles must be kept below a certain critical size. The fact that the fire was detected in winter time when ambient temperatures are low (between -1 and 10 C), testifies to the strong reactivity of the biomass heap. But there are other contributing factors. In order to mitigate the risk of self-heating fires, heaps ought to be designed complying with the following principles:  

*Stockpile size: As the size of the pile is made smaller, heat losses increase and the risk of self-heating is reduced. The maximum safe stockpile size is given by the ambient temperature and reactivity of the material.  *Wetting: Material with large moisture contents do not ignite.  
*Ventilation: Add design features that enhance natural ventilation and cooling.
*Inertation: Reduce reactivity by mixing the biomass with inert material like sand (see here).

Where these observed in this case?