Combine Fire Research Progresses
One day last fall, while ag engineer Dan Humburg was perched atop a combine during sunflower harvest in central South Dakota, the grower operating the machine stopped at the end of the field and asked, “Do you smell that?”
The South Dakota State University researcher could detect a burnt residue smell in the air that the grower had indicated. “He told me that’s the smell he gets just before he detects a fire someplace. He had just pushed the combine near capacity to generate higher temperature readings for our monitors. He’s so sensitive to it and knows exactly what to smell for to be on guard,” Humburg explains.
Humburg realized that farmers have to rely on intuition and a keen sense of smell to avoid combine fires. He hopes the work he and his fellow researchers at South Dakota State University have been conducting can provide farmers solid solutions to avoid the dangers and property loss due to harvest fires. He’s also hoping to fix the problem at the source before the farmer has to count on his nose to detect a problem.
A team of researchers within the SDSU Agricultural and Biosystems Engineering Department, backed by funding from the South Dakota Oilseed Council, began their work in July of 2011. They set out to first investigate the primary question: Is it the sunflower dust, or is it the machinery?
The study’s three-part objective is to (1) understand the basic characteristics of sunflower dust in the lab, (2) see it in action in the field and how it interacts with different areas of the combine, and (3) bring the data together to suggest potential engineering solutions that could serve to change or interrupt one or more of the factors present when a harvest fire starts.
The National Sunflower Association first introduced this research project in the August/September 2011 issue (go to article) of The Sunflower. At that time, SDSU biosystems engineer Zhen Grong Gu and grad student Joe Polin were busy in the lab conducting tests to characterize the physical and chemical properties of sunflower dust that contribute to combine fires. Dust used for lab testing initially was generated from stalks gathered from an unharvested field planted in 2010.
First, the dust was mechanically separated into different fractions using a stack of sieves and a sieve shaker. This was done to isolate the finer dust particles that are most easily suspended in air. The sunflower plant parts (head, stalk pith and stalk outer layer) were also segregated, milled and analyzed to better understand the origin of the dust that settles on combines during harvest. All samples had the same moisture content and bulk temperature prior to testing.
Another lab test entailed using a hot plate to determine ignition points of the various dust particles. A thermocouple was centrally located in the dust sample layer and recorded temperature changing profile during continuous heating.
The finest dust collected was from the head of the sunflower plant. This dust was placed on a hot plate at temperatures of 260° C and 250° C (i.e., about 500 and 482° F). The test at 260° showed a peak, which indicates combustion, compared to the 250° test, which did not show a significant peak or combustion. Essentially, this difference is used to estimate the ignition point.
Temperature spikes and ignition points were also tested on sunflower versus corn dust. According to the hot plate tests, the sunflower head dust has a lower minimum ignition temperature than corn dust at every similar particle size. The lower ignition point indicates that sunflower head dust is more easily ignited than corn stover and can also be ignited at temperatures when corn stover won’t ignite.
Additional lab testing was conducted to evaluate and compare the ignition byprod¬ucts of field dust with samples gathered dur¬ing the 2011 harvest. It was concluded that the inner stalk (pith) is the main source of dust found on a combine. In addition to the hot plate test, a variety of other characterizing tests were conducted. For example, the researchers learned that sunflower dust begins to volatize at 428° F. The team thinks that leads to the pre-ignition smell that many farmers notice just before fires or smoldering problems begin.
In addition to the lab tests on sunflower dust, Humburg and his colleagues spent time taking measurements on producers’ combines during the sunflower harvest in central South Dakota. Kevin Dalsted, an SDSU ag engineering professor who specializes in machine systems, is a collaborator on the project and joined Humburg in the fields last fall. The pair observed the machinery, took temperature measurements and recorded weather statistics.
“For me, the most interesting part was information gathered atop the engine compartment, taking temperatures with a hand-held infrared measuring device near the machine’s exhaust manifold area,” Humburg notes. “That particular producer was using some modifications with ceramic heat tape wrapped around the exhaust manifold to reduce the amount of heat radiating from the manifold and pass the heat down to the muffler. So it wasn’t a typical situation, perhaps. But we learned that area reached a temperature of 600° F after a short run without reaching maximum engine capacity.”
While the team strongly suspects the focal point should be on the exhaust manifold, they are exploring different areas to not overlook potential problems throughout the machine. For instance, during this year’s harvest they look to return to the field to repeat some tests to solve certain problems they had gathering data last year, as well as include new areas of interest.
“We were able to learn some from the soybean issues as well last year in this region of the country,” Humburg adds. “Extraordinary occurrence of fires during both sunflower and soybean harvest last year occurred in areas of southeast South Dakota, northwest Iowa, southwest Minnesota and northeast Nebraska. There are commonalities among the situations. What we see happening more frequently in sunflower was also happening in the soybean fields on specific days.”
Anyone who has participated in harvest of any crop knows that the environment can be very chaotic when it comes to the air flow around the machine. Wind speed and direction can make a difference, which often varies greatly. Air blast from the radiator fan is interacting with the wind, blowing dust and debris to all areas of the machine, so it’s very difficult to control the distribution of the chaff.
To try and get a handle on this, additional testing will be done in the lab to simulate the environment right around the exhaust environment to quantify exactly what temperature it takes to possibly ignite dust flying adjacent to a hot surface. “Currently, we are in the process of building a device to generate a continuously suspended dust particle cloud through a tube furnace,” Dalsted explains. “The process will simulate the exposure of airborne dust to very hot surfaces and will document the temperatures needed to ignite the dust in the air stream. We may also be able to extend this test to observe the behavior of ignited embers landing on dusty surfaces.”
In addition, unanswered questions remain surrounding the static electricity on combines and what role that plays. “We have a lot of producers who say they think this might be a source or a contributing factor to the fires occurring,” adds Humburg. “I’ve had some industry people say they cannot make it happen in their testing. If we can’t get it to happen in the lab, it would indicate that while it might happen in the field, it’s very rare. If we can make it hap-pen in the lab, then we may have another issue to address.”
According to Dalsted, the team has also designed and ordered a machine to generate energy through static sparks. “We will apply this to both dust layers and suspended dust clouds in open systems — not confined space — to examine the energy needed to ignite dust particles or dust layers. These tests could help to determine if it is possible to ignite dust particles or dust layers with static discharges, and, if so, under what conditions this might occur.”
Despite multiple areas of concern when it comes to “hot spots” on a combine, the pair continues to focus on the exhaust manifold in the lab as well. “With the change over to the newer models of combines, we went from a naturally aspirated engine to a turbo-charged system to keep up with the demand from the producer for more power from the engine. That bumps up the temperature in the area of the exhaust between the turbo charger and the engine,” Humburg explains.
Sunflower growers know to back off slightly and not to push the machine to capacity because of the propensity for fires. Many also have made modifications to their machines to help alleviate certain issues that lead to fire breakouts. The SDSU research team is also testing possible alterations or additions to equipment in the field that would be aiming to change the behavior of the ignition sources. Humburg refers to the combine chimney designed by the North Dakota farmer featured in last year’s August/Sept. 2011 issue of The Sunflower. The chimney apparatus, attached to the air intake system, extends up to draw clean air into the radiator and keep the engine clean.
“We are looking at how we can preclude fires in that area of the exhaust manifold in varied circumstances. The chimney, for example, under good, common circumstances, is a modification that can go a long way in preventing fires,” Humburg observes. “It’s not an absolute solution because in some circumstances it could get overwhelmed — and that’s some of what we saw in this area last year. These farmers weren’t using chimneys per se; but their machines that ordinarily don’t catch fire, did have excessive occurrences last year.”
“We have set up a salvaged engine exhaust manifold turbo system in our shop and will use a propane-fired set-up to achieve nominal operating temperatures”, Dalsted explains. “We should be able to evaluate and model the heat transfer around this hot surface. This should help us to better understand the potential dust ignition in the combine engine compartment area. We will also consider potential engineering solutions to hot surface-induced problems experienced during the sunflower harvest.”
Another area of focus for the research team, when they return to the field this fall, will be the air flow around the combine. Air cleaners on the machines have a pre-cleaner section that screens out the coarser dust and separates it before it goes in to the fine paper filter that filters the finer dust before it reaches the air cleaner. As it builds up, it gets sucked out of the pre-cleaner and discharged or blown out at the latter part of the muffler. Ideally, that exhaust has cooled to a point where it’s no longer a source for ignition; but if the machine is running hot, it might be a problem area.
The SDSU researchers question whether there’s a chance that some of those coarser particles of sunflower dust are being ignited and blown upward through the muffler and over the top of the machine. Ordinarily, that dust is carried off by wind or extinguishes itself before it lands anywhere that matters. But if wind conditions are adverse and it lands back on the machine, could it be a problem? It may be there is no way for that dust to be hot enough, but it should be ruled out as a source for fires.
“This year, we plan to capture some of that dust that’s going past the pre-cleaner and coming out the muffler and see if it’s
hot enough to be a problem. This is just a theory, but we’d like to take a closer look at it,” Humburg notes. It’s another speculation that the researchers hope to iron out in the field tests this year.
The numerous unanswered questions illustrated here serve to shed light onto just how complicated the issue of combine fires really can be — not only for producers, but for researchers attempting to understand it and offer solutions. Ongoing research seeks the answers.
— Sonia Mullally
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