The most common feedstock for direct fired wood boilers is 3" (76 mm) minus ground wood. Low moisture and low ash content are desirable. Dry wood has a higher net heating value. Dirt in the wood may cause boiler damage and increase the amount of ash to be disposed. Minimum fines (particles less than 1/8" (3 mm)) 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. They are more tolerant than chippers to any contamination in the feedstock.
Short Fiber 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. Ideally the wood fibers are 2-3 mm in length so the die wear is minimized and the fibers are still long enough to provide strength to the pellet. The feedstock needs to be dried to 5-8% moisture before the pellets are extruded.
Suspension burners common at coal power plants also require short fiber wood, preferably less than 3 mm in length. The moisture level for suspension burners should be less than 20%. Some gasification processes also require low moisture and short fiber feedstock. 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 final product sizing required. If the feedstock is primarily solid wood, a good process is to first chip and then dry the wood if necessary. The chips are then re-sized in a hammermill or refiner to the final fiber length.
The drying process works best with a thin chip with a consistent thickness. Dry chips are more brittle so they are more readily reduced in a hammermill. Short fiber chips will make the hammermill or refiner more productive than a longer fiber feedstock. Refiners can work with wet chips.
Disc and Drum Chippers
Disc and drum chippers are common machines used to reduce wood into short fiber micro 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 tuned to optimize the chip quality.
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 6-9 mm micro chips. .
Peterson has developed a micro chip configuration for the 5900 chipper and 5000H delimber, debarker chipper. Production with these machines will vary with wood species, chip length and tree size. Typical production with a four pocket disc making 6-7 mm) length chips will be 50 tons per hour. Production making 9-10 mm chips with a four pocket disc will be about 70 tons/hour. The fuel consumption will range from 0.2 - 0.5 gallon/ton (0.76 - 1.9 liters/ton) depending on the wood species, size and moisture level. Fuel consumption of 0.2 gallon/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 4800E debarker with the 5900 chipper to achieve a low ash micro chip. The 5000H delimber, debarker, chipper can provide even lower bark and ash content all within a single machine.
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. Tests with Southern Pine show that the energy consumption is twice as high chipping wood that has dried six months (low 40's% moisture content) compared to chipping fresh wood. Chipping knife cost will also be much higher with dry wood. The lower moisture level in the wood however will result in a higher net heating value and drying costs will be reduced.
If the boiler can accept 3" (76 mm) minus feedstock, a horizontal grinder is the lowest cost reduction solution for dry wood. Peterson is able to fine tune the rotor speed, bit type and grate openings in horizontal grinders to optimize the product size and minimize energy consumption.