How to Convert Waste Wood to Biofuel Feedstock

Size reduction solutions to two of the biggest challenges when converting waste wood to biofuels

Throughout our 80+ year history, wood processing has been one of our cornerstone industries. Among our largest customers are forest products companies, lumber yards, makers and recyclers of pallets, furniture manufacturers, truss plants and more. Each with unique products, but all with one common issue: wood waste. The level of waste varies depending on the industry.  An interesting example is forest products companies. When processing logs for lumber, up to 40% of the tree is deemed scrap in terms of its lumber value.

Traditionally, these by-products were considered waste, and at best, used as onsite boiler fuel or sold at a minimal profit to local buyers for such uses as landscape mulch and  animal bedding.  Enter biofuels.  Amid growing environmental concerns and the desire to reduce dependence on fossil fuel, the emphasis on biofuel creation continues to increase.  As a result, wood has emerged as an ideal feed stock. Now the wood waste once seen as a disposal problem is now a valuable product.

describe the imageBut here is the hitch – each of the potential biofuel end products have specific material characteristics that are critical to their efficiency. As a result, there are a number of challenges inherent in using a non-uniform scrap material as biofuel feed stock. In two specific cases, hammer mills are instrumental in taking wood waste from the sawmill floor to the biofuel market.

To illustrate this point, let’s use the example of converting wood waste to pellets and briquettes.

Challenge No 1:Particle size and uniformity

These are critical factors in the preparation of wood to biofuel feedstock. On average, most biofuels require a consistent feedstock particle size of -1/8”.  Typical waste wood as is much larger and non-uniform.

Hammer Mill Solution:

The first and most obvious consideration is size reduction. Can hogged wood scrap, bark, pallets, furniture scrap, etc be ground to a size that is appropriate for biofuel production? The answer is of course, yes. The solution lies in selecting the correct wood grinder for your production goals.

To determine this, the following must be considered:

Size and type of in-feed material

Desired finished particle size

Hourly production goals

Answering these questions will determine not only what wood grinder is most suitable for your application, but also whether your goals require a one or two stage grinding process.

For example, if your waste material is hogged wood scrap and your goal is a -1/8” for pelletizing, a finish grinding industrial hammer mill would be most appropriate.  From there, your production goals determine the size and style of the recommended mill.

Conversely, if your waste material is pallets and your goal -1/8” for pelletizing, you will require a two step process.  A slow speed ram fed grinder is ideal for the initial grinding of the whole pallets. However, this type of mill is not suitable for the fine grinding required for the optimal finished particle size for pelletizing. A secondary grind in a finish grinding industrial hammer mill will be necessary.  Typically, the material will be pneumatically drawn from the ram fed grinder, across magnets to remove all nails, and then though the finish grinder. The upside of this is that this pneumatic component can substantially increase the throughput rate and convey the finished product to storage.

Screen selection for the hammer mill is the second component in determining finished particle size. Referring again to our original three considerations, the desired end particle size will dictate the size of the openings on the perforated screen covering the discharge of the hammer mill.

For example:

Finished particle size of 1/8” will require a screen size of  1/4″ or smaller.

The material will remain in the grinding chamber of the hammer mill until it is reduced to a size that will pass through the screen.

Challenge No 2: Material moisture content

The majority of biofuel applications require a moisture content of ≤10%. However, it is not uncommon for waste wood, such as bark or green wood chips, to have ≥50% moisture content. Not only is the moisture content too high for fuel efficiency, it is also too high for a wood grinder to reduce the wood to a uniform particle size suitable for pelletizing.

Hammer Mill Solution:

Under these circumstances, the solution is often a three phase process where hammer mills play a pivotal role. The following is an example of how one of our customers addressed this challenge. Their goal was to convert wood waste with an average 40 to 50% moisture content to suitable feed stock for pelletizing and briquetting.

An industrial wood grinder was used to pre-grind the green wood down to a uniform ½” particle size recommended for optimal drying in the rotary drier. The drier would reduce the moisture content down to the required ≤10%.   Finally, the material is fed into a second industrial wood grinder where it is ground to its finished particle size, ideal for the customer’s pellet and briquette production.

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