In May 2018, Purdue University announced it had received a $1.8 million grant to study ways to break down lignocellulosic biomass into a slurry. According to Purdue, what prompted this grant is that lignocellulosic biomass at ethanol plants too often plugs the augers used to convey it, which forces the plants to shut down while maintenance technicians repair or clear the augers.
That the federal government is investing millions of dollars in research to overcome this problem highlights the importance of this hurdle in producing ethanol from lignocellulosic biomass. It also raises the question of why research money is being spent on this issue when lignocellulosic ethanol plants could avoid the problem by moving biomass with a drag conveyor instead of an auger.
Why are Ethanol Producers Using Augers?
The reason why lignocellulosic-biomass ethanol producers are using augers is guesswork. They may be using the augers to move mash into the cookers. Or perhaps they’re using them elsewhere. Likewise, it’s guesswork as to why the augers are plugging up.
The augers might be plugging because they exacerbate the lignocellulosic material’s natural tendency to compact, ball up, or not flow well. As the auger moves the material, the material is continually being lifted, dropped, and tumbled. This agitation may increase the material’s likelihood that it packs in gaps and decrease its ability to flow well. And if the material is stringy, it will wrap around the screws, further decreasing efficiency and flowability. Whether due to agitation or natural tendency, if lignocellulosic material doesn’t flow well, the augers moving it need to be loaded to a lower capacity, say 1/3 to 1/2 full.
On a similar note, if the fibrous material in the auger is longer than the pitch—the distance from flight to flight—the material will simply never flow well.
Maybe ethanol producers are using augers because they consulted with “experts” who pointed out that conveyors (specifically, belt conveyors used in the agricultural industry) could not move material at a steep-enough angle to fit the space restrictions at the plant (agricultural belt conveyors don’t allow for conveyance of material at an angle greater than the material’s angle of repose; the material will fall back down the conveyor if the angle is too great). Thus, the ethanol producers went with augers. Experts whom these ethanol producers might also have suggested augers because they cost less than conveyors.
It’s well known, however, that as an auger’s angle of inclination increases, the machine runs less efficiently and can move less biomass. So, maybe ethanol producers are simply asking too much of their augers, and this is causing them to plug. Augers typically run about 2/3 to 3/4 full. When they are loaded to 100 percent capacity, the material tends to get stuck in the gap between the blades and hanger bearings where the screw supports are located.
Overfilling an auger can also occur when the pitch at the inlet wasn’t designed correctly. When the flow rate into and through the auger is incorrect, it can lead to overloading.
What else might cause trouble for these augers is the augers’ purpose. If the augers these ethanol producers use are meant for agricultural use, they’re going to break down more often than augers built for industrial purposes. Industrial equipment is more durable than agricultural products, as it’s meant for daily, year-round use, whereas agricultural equipment only has to withstand seasonal wear and tear.
Unplugging the Solution
Rather than investing millions of research dollars to make lignocellulosic material flow better through augers, it seems ethanol producers could overcome their difficulties by using different equipment. If they’re using augers to move mash into the cookers, they could instead use pumps. Otherwise, they could use enclosed drag conveyors. Ethanol plants have successfully used drag conveyors for many years to move both wet and dry material. Indeed, screw conveyors typically aren’t found in ethanol plants at all.
Using a drag conveyor provides the following benefits against an auger. These conveyors:
- Can move material at steep angles—up to vertical
- Can be equipped with clawed paddles, which break up material instead of compacting it
- Last two to three times longer than augers
- Require less power to operate than augers
- Don’t lose capacity as their angle of incline increases
- Have no hanger bearings around which material can pack
Sometimes the answer to production problems is to invest in research. This doesn’t seem like one of those instances. Replacing augers with different conveyance systems would solve the problem of plugged augers. And it doesn’t require millions of dollars from the federal government.
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