Biomass Fuel Processing

Biomass Fuel Processing seems certain to play a role in our Sustainable Energy Future. Learn more about Biomass Fuel Processing here.

Direct Fired Wood Burners
The most common feedstock for direct fired wood burners is 3″ (76 mm) minus ground wood. Low moisture and dirt content are desirable however many burners can operate with high moisture wood (50%) at lower energy output. Minimum fines may also be desirable for maximum boiler efficiency depending on the boiler type. Urban Wood waste and forest residuals are common feedstock. Horizontal grinders are the most economical machine type to reduce wood for this application.

3-inch minus wood ground with a Peterson horizontal grinder

Short Fibre Applications
A growing number of specialized applications for biomass fuel require short fiber dry wood. These uses include the wood pellet industry, suspension burners (common in coal power plants) and certain gasification processes. The pellet industry needs a feedstock with a fiber length no longer than the pellet width. This is typically 0.25″ (6 mm) for household pellets and larger for industrial pellets. The feedstock needs to be dried to 5-8% moisture before the pellets are extruded.

Suspension burners also require short fiber wood, preferably less than 5 mm. The moisture level should be less than 20%. Some gasification processes also require low moisture feedstock although a short fiber length is not a requirement for many gasification processes.

There are a variety of processes that can result in a short fiber, dry finished product. The best process will depend on the type of feedstock and the initial moisture content. If the feedstock is primarily solid wood with high moisture content, a good process is to first chip and then dry the wood. The dry chips are then resized to the final fiber length specifi¬cation in a hammermill. The drying process works best with a consistent chip thickness. The fines may be screened out before drying since they can be entrained in the drying air stream. The fines can also be used for fuel in the dryer. The dry chips are more brittle so they are more readily reduced in the hammermill. Starting the process with short fiber chips (8-10 mm) will make the hammermill much more productive than longer fiber feedstock.

Short Fiber Chips produced by a Peterson 5900

Disc and Drum Chippers
Disc and drum chippers are the common machines used to reduce wood into short fiber chips. Disc chippers are the main type of chipper used to produce high quality wood chips for the pulp and paper industry. Disc chippers provide a more consistent chipping angle which results in more uniform chip thickness and length. The anvil gap, knife extension, counter knife angle, and attack angle can all be readily adjusted with a disc chipper to produce the highest quality chips. Drum chippers typically do not have the same number of parameters that can be fine turned to optimize the chip quality and minimize fines.

Peterson 5900 Chipper in Action
A typical chip length specification for the pulp industry is between 0.62″ (16 mm) and 1.12″ (28 mm). Modifications must be made to the disc, counter knife and chipping speed to produce the shorter chips (< 10 mm) that are preferred as feedstock for wood pellets and suspension burners. Peterson has developed and tested a sort chip configuration for the 5900 chipper and 5000H delimber, debarker chipper. Production with these machines will vary with wood species and tree size however 50-70 tons/hour will be typical in a wide range of feedstock. The fuel consumption will range from 0.2 – 0.5 gal./ton (0.76 – 1.9 liters/ton) depending on the wood moisture level. Fuel consumption of 0.2 gal/ton (0.76 liters/ton) will be typical for fresh wood.

Advantages of Debarking
Low bark content is required for household pellets in order to minimize ash. Removing the bark also cleans the wood, eliminating sand and dirt that can reduce the service life of chipper components and pelletizing dies. It may be advantageous to debark the wood to save on chipping and pelletizing costs even if a low ash final product is not needed. The bark that is removed can be used for fuel in the drying process or sold as high value landscape bark.

Peterson can pair the 4800 debarker with the 5900 chipper to achieve a low ash wood pellet. The 5000H delimber, debarker, chipper can provide even lower bark and ash content all within a single machine.
Peterson 4800 Flail paired with a Peterson 5900 Chipper

Dry Wood Processing
If the wood has dried from moisture loss after felling or from an infestation such as a pine beetle attack, chipping costs will be substantially higher than with fresh wood. With dry wood, a horizontal grinder may be the best choice for the primary reduction. Tests with Southern Pine show that the fuel consumption is twice as high chipping wood that has dried six months (low 40’s% moisture content.) compared to chipping fresh wood. Dry wood will fracture well in a horizontal grinder. Peterson is able to fine tune the rotor speed, bit type and grate openings to optimize the product size. If a short fiber product is required, the small grate openings can be used.

Sourced and published by Henry Sapiecha 19th JULY 2009

Posted on July 18th, 2009 by Editor
Filed under: WOOD