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Research and Design of a mobile grass pellet mill

February 7, 2011
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Hudson Valley Grass Energy has developed a fully mobile mill capable of producing two tons of grass pellets per hour. Implementing this mobile unit is the central goal of our company, although we are also working to further research and marketing of grass pellets. Intensive field R&D over the last year and a half has brought the mill, which sits on a 25-foot trailer, to operation today. This article will share with you a few significant improvements that were realized throughout our journey.

Material transport and storage, some of you may already be familiar with the fact that ground hay does not flow and has serious bridging problems. We’ve tackled this issue by applying constant mechanical agitation using augers. Augers transport raw material between all of the components on our trailer. We spent months building an effective mixing bin that would act as a buffer between the hammer and pellet mills. I believe that it was our fifth iteration that finally worked. This buffer zone compensates for the variable output of the hammer mill and is key in ensuring a constant feed rate to the pellet mill.

Earth team volunteer John and Operator Toby building custom buffer bin.

Testing various methods for feeding the hammer mill, Mike O’Dell

Feed rate, many mill operators will attest to the fact that pellet mills need a constant feed rate to run at optimal efficiency. However, we noticed that even with the feed rate constant our mill responds differently to each raw material. This was problematic in that some batches handled fine while others overloaded the mill or wouldn’t pellet at all. This makes sense considering that grasses vary so much in physical composition. Therefore, in addition to keeping a feed rate in the mill constant, it is also vital to be able to adjust this rate to make up for varying material density and composition. Ultimately our method has become to start feeding the material in slowly then ramp up the rate until the die warms up and reaches its “sweet spot”.

Steam, adding steam to our operation has also helped us to pelletize a range of material characteristics. With steam you can accept hays that are otherwise too dry or require higher temperatures to activate their natural binders (which is often a problem with switchgrass). Additionally, steam is beneficial to the die because it reduces wear and helps it to reach and maintain optimal temperature. Pellets produced with steam often are higher quality with less fines and greater hardness.

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