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A Longer Discussion of Combine Fires

A combination of personal experience and university research


Combine harvesters have always carried some level of fire risk.   The machines use multiple mechanical processes to cut, transport, and thresh large volumes of dry crop material.   Additional mechanical processes separate chaff, dust, and straw from grain and then transport the grain and expel the residue.   Electrical systems, hydraulic systems, fuel systems, and numerous mechanical parts provide combinations and conditions that can result in a fire.  Under the right circumstances the occurrence of fires on combine harvesters can become very common.  In some crops these conditions occur almost every year and fires become a dreaded expectation for the harvest season.

Combine Harvester Evolution

Manufacturers continuously change and improve designs of harvesters to meet the needs of their users.   This evolution means larger machines with greater capacity and automation.  The changes have included higher power levels, new materials, and greater use of electronics, hydraulics, and controls.  While these changes don’t necessarily mean more fires, they can result in more catastrophic fires.  Older machines were made primarily of steel.  Belts and chains transmitted rotary power and hydraulics were used for some heavy lift functions.   Electronic systems were relatively simple.  Fires were not necessarily uncommon, but there was not as much combustible material on the machine to sustain them to the point where the machine was lost.   I once operated a combine in soybeans with a fire burning inside it.  My father had used a welder earlier in the day to add a bead to some of the teeth on the worn rasp bars of the cylinder.   When I came to relieve him, he told me that there was a smoldering fire behind some of those rasp bars.  He had tried, unsuccessfully, to put it out with his water jug.  He said that it didn’t seem to bother the machine.  So, I combined into the night with an occasional plume of orange sparks exploding from the front of the feederhouse.   Don’t ever do that!  My father was fearless, and I was young and foolish.   The point is, that machine had little for the fire to feed on but soybean dust and steel.  A combine purchased today uses hydraulic systems much more extensively.   That means rubber hoses, and very likely, oil films in many areas of the machine.  Many of the hydraulic circuits are electronically controlled to manage machine functions automatically.  Automation requires sensors, so the wiring harness now reaches every part of the machine with its conductors running inside plastic sheaths.   The shields and cowlings on modern combines are often plastic or fiberglass.   A smoldering fire burning in crop residue does not have to go far to find a synthetic material that really wants to burn.   And then your neighbor sees a column of black smoke on the horizon and wonders what’s happening in your field?

Sources of Ignition

Bearings:         When one of the many bearings on a harvester fails the resulting friction and heat can easily ignite adjacent crop residue.   Overheated bearings are certainly not unusual, but they can be managed with careful monitoring and maintenance.  Off-season maintenance to replace old or worn bearings will help protect from this ignition source and prevent down time during harvest as well.  A cheap new tool that can help identify potential problems before they ignite is a non-contact infrared thermometer.  These are now available at very low cost with a laser aiming capability.   Bearings can be scanned after operating a machine to quickly look for hot components that may be approaching failure.  While waiting for a truck or cart to unload into, an operator can walk around the combine and simply point the thermometer at bearings, and potentially catch failing parts early.

Trapped Crop Material:         We were taught as children that fires could be started by rubbing two sticks together.  The combine is very good at rubbing things together.  It is possible for tough crop stems to become wrapped around a shaft where friction can then cause them to ignite.

Electrical Short Circuits:        Agricultural machines are now designed with a much higher level of automation, control, and measurement.   For example, a combine may now incorporate systems for automatic guidance, automatic header height control, yield monitoring, loss sensing, and other automated functions.   Many other functions have variable speed control that use sensors to determine speed, and electro-hydraulic valves to vary hydraulic flow and change speed.  Engine systems designed to reduce pollution rely heavily on sensors and controls.  All these innovations require wires for signals and power.    Although the designers protect wires with flexible sheaths, it is possible for wires to chafe with vibration over time and eventually cause a short circuit.  A short can cause sparks or heat and ignite adjacent crop dust.   The plastic sheath that protected the wires is then a source of synthetic fuel to aid the fire as it begins to spread.

Rocks:             Rocks can be a cause of sparks for ignition.   A rock picked up by a ground-hugging header, or a pickup reel, can cause sparks on the head or in the feederhouse.   Combines have rock traps to prevent most rocks or stones from passing through the threshing and separation systems, but no system is perfect.   Any place where a rock, steel, and power are combined can cause sparks that can create a smolder in adjacent crop residues.

Static Discharge:        Static is a topic that may generate strong opinions.   It is unquestioned that combines can generate static charges.  Friction with dry material will cause a static charge to build on one surface relative to another.   Materials, such as the plastic and fiberglass shields, on current model combines are more prone to static buildup than a mild steel shield.   It is also unquestionable that static can discharge through a spark, and sparks can ignite smolders.   Many combine operators drag a chain in attempts to discharge to ground the static charge built up on the machine.  While this is not harmful or costly, it is unlikely to affect the number of fires occurring on a machine.   Research was conducted in laboratories at South Dakota State University with sunflower residue taken from a combine surface and a lab device to generate a controlled discharge of sparks.   In that work we were unable to ignite a smolder in dry residue with the discharge of a single static spark.  Not until 20 sparks per second were sent to the same point through a layer of dry sunflower residue was it possible to create a smolder.   Static discharge will occur across a gap from a point of high charge to a point with low charge when the charge level becomes greater than the resistance of the air between the two points. That tends to be where the two points or surfaces are close together, just as your fingertip is the point where the spark jumps to when it comes close to the door knob in the winter.   If static discharge was causing fires regularly on a combine it might be expected that the same locations would see smolders again and again, as static would build up and discharge from these same points where the spark can easily jump.  Instead, smolders tend to occur at distributed locations.  While it would be unwise to say that static can never cause a fire to start on a combine, this phenomenon is not the cause of the frequent smoldering fires that occur in some crops.

The Engine Exhaust System:              Engine exhaust systems have changed dramatically over the last 25 years.   Diesel engines prior to that time might have had a turbocharger to extract some waste power from the exhaust stream to pump more air into the engine intake.  This allows more fuel to be added and more engine power to be obtained from the same basic engine.  That goal of extracting more power from the same engine has led to higher levels of turbocharger use and greater fueling levels, which means more waste heat going out through the exhaust system.   The manufacturer specifications for the engine used in one common model combine from a few years ago indicated that the exhaust gas temperature for that engine would be 967° F when it was operated at rated capacity.   At the same time an aftermarket company was selling a “chip”, or alternate program, to control that same engine to extract more power.  This means fueling the engine at a higher rate.  That chip manufacturer indicated that the exhaust gas temperature should be monitored and not allowed to exceed 1300° F!    Cast iron will begin to glow dull red near that temperature.  

 The EPA limits on diesel exhaust to improve air quality have changed exhaust systems further.   These limits went from Tier I through their current specification at Tier IV B.   The rules require engine designers to limit the output of two pollutants - particulates and NOx.   Particulates are solid particles of incompletely combusted fuel that we see as black diesel smoke.   The NOx is an invisible gas that forms at peak temperatures and pressures in combustion.  The systems to reduce or capture these pollutants differ with manufacturer.  Nearly all new diesels used in agriculture now use a system called Selective Catalytic Reduction (SCR) to convert the NOx to N2, which is not a pollutant.  SCR uses a urea-water solution called diesel exhaust fluid (DEF) as a part of its process.  The particulate pollution, or black smoke, is trapped in a second system called a Diesel Particulate Filter (DPF) that filters it much like dust in an air filter.   This soot eventually begins to plug the filter, so sensors monitor the pressure required to push the exhaust through.  When the filter becomes plugged, the engine computer will call for it to be “regenerated”.   This means essentially burning the soot off the face of the filter matrix with heat. Diesel Particulate Filters can get very hot when regenerating and can be a source of ignition for organic dust particles.   Other engine producers have developed approaches that manage the combustion process in a way that produces very little soot and does not require a DPF.   Usually that means high combustion temperatures and pressures and results in more production of the NOx pollutant.  Since this gas is handled by the SCR system and Diesel Exhaust Fluid, these engines only need one system for conversion of pollutants.  However, they may produce very hot exhaust, and very hot exhaust systems.  Even without complicated exhaust management systems it is possible for the exhaust system to get very hot.  If hot enough, the exhaust system can ignite dusts that are in contact with it for even a brief time.

Combine Fires:   University Research and Volatile Crops

The South Dakota Oilseeds Council in 2011 asked engineers at South Dakota State University (SDSU) to study the problem of endemic fires on combines in sunflowers to understand why this occurs so frequently.   After visiting with a number of producers to understand their experience, the engineers conducted a number of tests to document the conditions that cause fires with sunflowers.

Hotplate Test:            Sunflower residues as well as corn and soybean residues were tested on an instrumented hotplate.   Dust was placed on the surface of an aluminum plate while the temperature of the plate was gradually increased from below.  The experiment revealed that sunflower plant residues would ignite at lower temperatures than corn or soybean plant residues.  Sunflower dust allowed to sit on the hot plate would eventually begin to spontaneously rise in temperature as it began to smolder.  This would occur at temperatures as low as 500 degrees F.  The tests also revealed that the smaller the particles the lower the ignition temperature on the hot surface.

Dust Size and Porosity:          Researchers measured the size of sunflower dust particles in samples taken from the surfaces of a combine.  These particles tended to be very small bits of plant material.   Laboratory tests to measure surface area showed these particles to be very porous with large surface areas.  Think of these particles as a sponge with its many voids, cavities, and pockets. This characteristic makes the particles very chemically reactive, which is another way of saying they were highly combustible. 

Plant Components:                During sunflower harvest a portion of the plant stem goes through the combine in addition to the flower head.  These plant components are subjected to the violence of the threshing system in the combine and broken into small pieces.  Sample sunflower plants were broken into components of heads, stem pieces, and the stem core, or pith.  These different components were ground to fine particles in the laboratory.  The chemical composition of each sample was then examined through gas chromatography.   A sample of sunflower field dust, taken from the surfaces of a combine, was also examined with the gas chromatograph.   What this test revealed was that the dust that built up on the combine surfaces closely matched the samples of the pith from the center of the plant stem.  This whitish material is very light in weight.  Once it is dry, as in the late fall after frost, it will become very crumbly or friable.   When it goes through the combine it is ground to dust that is so light that it takes a long time to settle to the ground.  The dust cloud that surrounds a combine in sunflowers is mostly made up of this material.  That same dust may also carry an electrical charge so that it is attracted to the opposingly charged surfaces of the combine, explaining why dust builds up so quickly with this crop.  These small, lightweight particles are also too small to be trapped by the screen at the radiator intake, so the dust goes through the radiators and is blown across the engine.

Field Measurements:          A CASE 8120 model combine was instrumented in the field to measure conditions on the exhaust system.   This model does not have any of the exhaust aftertreatment systems that later models have.  Thermocouples for measurement of temperature were placed on various points on the exhaust components.  Temperatures on the inlet of the turbocharger were as high as 800° F when air was not blowing on the turbo.   These temperatures are high enough to easily ignite the volatile dust particles from the sunflower stem pith.

Ignition Scenario:        The research conducted at SDSU suggests the following explanation of the chronic fires in sunflower harvest.    After a hard frost in the fall the crop can warm up and dry down.  The whole plant, but particularly the pith inside the stem, can become very dry and friable.   The harvester breaks down this material as it passes through the rotor and concave.  The fine lightweight dust hangs in the air around the machine and is drawn through the radiators.   The dust then passes across the hot exhaust components.   If these components are above some threshold temperature, the dust that impacts the turbocharger can ignite on contact.  The dust may not even have to make contact to ignite if conditions are volatile enough.  The burning dust particles are now airborne and swirling through the engine compartment.  They can pass to the left side of the machine where the air blast dives under the side shields where there are more dusty surfaces.   Any time a spark lands on a dusty surface it can start a smolder.   These areas can also relocate burning embers from one surface to another in the windy conditions.   Producers’ experience suggests that an engine load threshold exists that causes this sequence to begin.   Below the threshold the exhaust system is below the rapid ignition temperature, but at engine loads above the threshold sparks are immediately produced at the turbocharger or other exhaust components and fires will occur very frequently if not immediately.

Improvements to Reduce Fire Risks

Many innovations have been tried to prevent the fires that plague the harvest of volatile crops.  Most help, but don’t completely stop the problem.

Chimneys:       The chimney, or snorkel, is a riser tube of any sort that forces the air taken in by the engine radiator system to come from higher up.   If the air passing over the engine has no dust in it, then the fire risk is greatly reduced.  The air above the combine will have less dust than the air alongside the machine.   A chimney can help prevent fires but has some practical challenges.  The chimney can only be so tall before it becomes a hazard itself.   Wind can sweep dust from behind the combine and overtake the chimney and defeat it.   It is also possible for swirling dust to enter the engine compartment and reach the exhaust area without going through the radiators.

Shield Removal:         Some producers remove all the shields from the sides of the combine with the goal of allowing the wind to blow dust off of surfaces and keep the machine cleaner.  This won’t prevent ignition of dusts at the exhaust system but will reduce the number of dust laden surfaces where airborne embers can start a smolder.

Coatings:        Some producers have wrapped the exhaust components with layers of ceramic fabric tape.  This material does not transmit heat easily and can keep the heat inside the exhaust component while the surface of the tape is lower in temperature.  As with the other fixes, this can help to a limited extent.  The ceramic tape does change the surface temperature.   However, it is difficult to wrap all parts of the system thoroughly to completely isolate the hot components.   Also, the tape provides a surface with small pockets and crevices for dust to lodge in.  The air blast from the radiator is less likely to clear these of dust than the bare surface of the exhaust component.  Dust that sits in a trapped location on the taped surface may cook to the ignition temperature as seen in the laboratory tests and begin to smolder.  Once smoldering, the burning dust can release and relocate on the combine.

A Comprehensive Solution

Any solution that completely eliminates the problem of chronic smolders in volatile crops must separate the plant dusts from the hottest engine components.  It must do this under all harvest conditions, all weather conditions, and all engine loads.   The system developed and tested at South Dakota State University achieves this.   An enclosure around the turbocharger, exhaust manifold, and exhaust pipe separates the hot zone from the dusty environment.   A supply of clean filtered air is pumped into the enclosure.   Openings in the enclosure allow air to flow around the exhaust components and out of the enclosure.   Positive air pressure in the clean air box prevents dusty air from entering the enclosure around the hot zone.   While temperatures on the surface of the hottest exhaust components may reach 800 degrees or more, the temperature of the enclosure surface, and the air coming out of it rarely exceed 200 degrees.  Fire risk is no longer linked to engine load, and the full capacity of the engine and the combine can be applied to the crop.   Fire risk is reduced to the same levels as exist in non-volatile crops such as wheat or corn.   Fire can still start with a hot bearing or an electrical short, but ignition no longer occurs at the enclosed exhaust components.  This solution requires the addition of the exhaust enclosure and the hardware to clean and move air, but it is a comprehensive solution to the problem of volatile organic dust igniting at the exhaust components.  It has been demonstrated to prevent fires at all engine loads in sunflowers for 8 years.  Although developed to solve the chronic problem in sunflowers, it will also protect machines in other volatile crops.  It has now been purchased and successfully used to prevent fires while cutting soybeans and safflower.

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