Fire risk when harvesting pulses

Phil Bowden, Pulse Australia, NSW

Harvesting season in Australian cropping areas is the most stressful time for farmers as they glean finished crops. Ideally, harvest occurs under hot dry conditions, but the risk of fire is extreme and a fire can damage crops, machinery and property, not to mention the lives of the community as well.

The proximity of flammable material to heat sources such as exhaust manifolds and turbochargers with high ambient temperature, low humidity and windy conditions in the paddock make this an explosive situation, even under ideal conditions.

To make matters worse there are crops with residue characteristics that promote this risk of fire. These include chickpeas, lentils and sunflowers of the mainstream crops, and pyrethrum as a specialty crop.

Hazard and risk of fire

For a fire to start we can look at the Triangle of Fire to understand the mechanism. Three things are needed for a fire; suitable fuel, oxygen (air) and a heated ignition source in close proximity occurring simultaneously. In the context of harvest operations we have all these in abundance.

Heated ignition sources can be in different areas, including:

• around the engine bay of the header such as the exhaust manifold or the tubocharger where temperatures can get up to 650°C
• mechanical failures from bearings
• sparks from electrical short circuit
• striking metal fences or rocks to cause a spark
• static electricity from moving parts or operators clothing
• foreign objects that are taken into the header

Fuel sources can include:

• the dried standing crop itself
• crop residue that has been chopped finely to create an ideal fuel for instant ignition.
• flammable fuels and oils used in the header.
• dust that carries crop residue

Oxygen (air) is being blown around inside and out of the header.

A review of the causes of header fires (Quick 2010) concluded that 75% of the fires started in the engine bay and others from failed bearings, electrical problems and rock strikes. Static electricity is often blamed, but the evidence shows that this is a minor risk, even though it is common to get a build up from many parts of a header. The energy to ignite crop residue is not enough in a static electrical discharge.

Increased risk with pulses

Many of the pulse crops come with an increased fire risk because of certain characteristics of the residues. For example, chickpeas have high concentrations of malic and oxalic acid exuded from the fine leaf hairs, which causes the residues to stick together inside the header. This can lead to buildup of fine particles that are extremely combustible.

Lentils also have a very fine residue that can stick to the header and to dust particles. In addition lentils bear the pods very close to the ground so the pick up comb needs to be lowered and can easily strike a rock, causing sparks to ignite the fine powder.

Peas, lupins and faba beans also have fine residue that powders easily and create higher risk of ignition. Advice from Canada suggests that diseased crop leaves are a higher risk of fire at harvest.

All of these pulse crop residues may well have much lower ignition temperatures than cereal crops. Sunflower residue was found to have 30°C < lower ignition temperature in a US study (Polin et al. 2013) and it has a similar reputation to many of these pulse crops.

When harvesting these crops more regular clean downs with a high pressure air compressor are required. In lentils or chickpeas this may be as often as each stop at the chaser bin.

Reducing the risk of header fires

There are several important things that reduce the risk of fires.

• Operate only when the conditions are favourable under the Grain Harvesting Code of Practice (see below).
• Diligent, regular clean down of residues especially with lentils, chickpeas and peas crops.
• Check under guards and covers for build up of dust and chaff.
• Check bearings and moving parts for hot spots (use a hand held digital thermometer).
• Check electrical system for worn cables (esp for rodent damage).
• Check fuel and hydraulic lines for leaks.
• Use a drag chain to avoid build up of static electricity.
• Use the battery isolation switch when the header is not in use.
• Locate fire fighting gear close by.
• Train all staff in using firefighting equipment.

Use of fire suppression systems on the header

There are several systems that can be put on the header to prevent fires or to deal with fire if it starts.

Fire Knock Out will drench the engine bay in fire retardant using a self actuating switch.

Fire Prevention Shield reduces the temperature of the components in the engine bay by drawing air from the cooling fan through a heat exchanger, charging it to higher pressure to clean residues from around the muffler. This effectively reduces residues and temperature to lower the risk of fire.

References

1. The Fire Triangle Wikipedia
2. Quick (2010) Harvester Fires Final Report GRDC
3. Polin, Gu, Humburg & Dalsted (2013) Sunflower dust properties that contribute to increased fire risk during harvest and bio-refinery operations. Industrial Crops and Products Vol 50 p227-31
4. Reducing harvester fire risk: Back Pocket Guide GRDC