Fires and accidents at biodiesel plants have heightened everyone’s awareness of the need to emphasize safety in plant operations. One chronic problem is fires that start mysteriously, with no apparent cause. Adding to the mystery is that many of these facility fires occur at night, when very few, if any, workers are present to point a finger at the source. Spontaneous combustion is a likely source of many of these fires.

Researchers in the USDA-sponsored Biodiesel Education Program at the University of Idaho are studying the nature of spontaneous combustion of materials used to clean up biodiesel spills. We intend to isolate the ignition point of certain biodiesel fuels, the mechanisms leading to self-ignition, and recommend procedures to reduce the risk of spontaneous combustion in biodiesel production facilities.

Spontaneous combustion is a bit of a nebulous phenomenon in the biodiesel industry, although it is a well-known problem with petrol fuel. Materials soaked in natural oils with a high number of double-bonded carbon atoms—such as many feedstocks used to make biodiesel—oxidize quickly. The oxidation process releases heat that causes an increase in core temperature. The materials begin to smolder and eventually self-ignite. Biodiesel researchers think they know the recipe for this reaction in theory, but there is very little research available that has studied its detailed cause or provided industry safety practices. 

In comparison, biodiesel producers are well aware of the risks of methanol use. They work with it every day and have a lot of respect for its volatile nature. Most safety information available for biodiesel production concentrates on reducing fire exposure to methanol, and rightly so. It is important to note that methanol is often just the fuel for the fire, and not the cause.

The problem with analyzing the cause of fires that result in explosion or intense heat is finding the cause of the blaze. Could the fire have started with a spark or a flame? An often overlooked possibility is spontaneous combustion from improper disposal of material used to clean up biodiesel oil spills.  

A relatively recent fire where the cause has been attributed to spontaneous combustion took place in Nagoya, Japan, in August 2005. A facility worker simply placed a work rag in a cardboard box. The rag and box were consumed in the fire, but enough of the rag was left to determine that trace amounts of fatty acid methyl esters were present in the rag. Since the rag was found within the point of origin of the fire, officials speculated that the source of the fire was the spontaneous ignition of the rag.

 
Researchers at the University of Idaho have demonstrated that spontaneous combustion may pose a serious threat to biodiesel production facilities. We replicated a scenario of spontaneous combustion in our facility, using sawdust soaked in a mixture of biodiesel and boiled linseed oil for the initial experiments. This mixture is susceptible to quick oxidation due to a high degree of unsaturation. The core temperature of the mix increased steadily over time. Within two hours, the material began to visibly smolder; within eight hours, the material ignited and was consumed by fire. At the time when a visible flame appeared, the material had reached a core temperature of more than 400 degrees Celsius. Our observations lead us to believe that the probability of spontaneous combustion is associated with the level of rancidity of the biodiesel, the characteristics of the absorbent materials, the ease with which air could move through the material, and the ambient conditions in the storage environment. We are investigating what combination of these variables must be present to lead to spontaneous combustion, and what changes in the combination of these variables can prevent it. 

Production facility workers must be diligent in the disposal of materials used to clean up equipment and biodiesel spills. Rags used to clean up biodiesel should be disposed of in a separate, fire-safe, enclosed container, or laid out individually to dry. Biodiesel-soaked absorbent materials such as sawdust, Magnesol or Oil-Dri should not be composted. A recent incident of a compost fire caused by spontaneous combustion of oil-soaked Magnesol smoldered for weeks. In fact, some landfills refuse to accept these materials.

As we conduct more research into the causes of spontaneous combustion of biodiesel-soaked materials, we will post our findings, photographs and video on our Biodiesel Education Program website, www.BiodieselEducation.org.

Author: Jon Van Gerpen
Biological and Agricultural Engineering Department Head,
University of Idaho
208-885-7891
jonvg@uidaho.edu