The term “green wood” refers to wood recently harvested or cleared from forested areas, containing all or most of its original moisture. For this reason, green firewood requires curing to effectively burn, as water content reduces the heat it produces. As opposed to wood that’s already been seasoned or dried, it’s unstable, often warping or otherwise changing shape as it dries. Because of its lack of structural integrity, green wood doesn’t work well for most building projects.
While green wood mainly requires time to cure or dry properly, green wood waste is another matter. On the surface, it would seem of little use, just a timber industry waste byproduct. However, some of the properties of green wood waste make it more useful for certain applications, as it’s a more straightforward substance with which to work.
What is Green Wood Waste?
Green wood waste includes offcuts from forest harvesting activities, such as sawmill residue or clearing done to manage forests to prevent forest fires. It can also include mixed organic waste from urban and suburban areas, such as tree limbs and leaves, grass cuttings, cleared brush, and other plant matter. As it can be obtained cheaply or freely, green wood waste presents itself as an economically viable material when used astutely.
With wood becoming an increasingly lucrative commodity, industrial use of such wood waste offers an additional revenue source for many businesses. A market for green wood waste exists for such things as paper pulp, wood composite materials, animal bedding, compost, mulch, and use as fuel in generating energy. Wood waste can also be used to treat sewage sludge to enrich the soil for agricultural purposes. Because of its bulk, however, wood waste cannot be cost-effectively transported over long distances.
Two Alabama cities, Anniston and Decatur, have developed successful programs for recycling woody material. Anniston collects yard trimmings and other wood waste from individual citizens, businesses, and city workers. Decatur collects stumps, grass clippings, and yard trimmings, along with other waste wood. The cities produce green wood chips, compost to enrich soil, and chipped wood used to fuel boilers.
Uses for Green Wood Waste
Most green wood waste gets ground into sawdust, making it easier to use for certain commercial applications, and is sometimes used as an additive to make soil more fertile. Large amounts of green hardwood sawdust are used for smoking meat, while green softwood sawdust is burned in some areas for domestic heating. Some landfills use it to cover trash daily.
Some end products of green wood waste include:
- Biomass for biodiesel, ethanol, and other renewable biofuels
- Briquette production
- Covering for playgrounds
- Paper mulch
- Wood pellet production for industrial boilers, home heating, or thermal power generation
Because green wood waste often contains contaminants that increase its ash content, however, it’s considered less efficient for producing biomass fuels and can sometimes cause problems with equipment during processing. Moisture levels affect energy content, with cleaned and dried hardwood producing about 8500 BTUs/lb. of energy, while green wood produces approximately 4000 BTUs/lb. Yet partly due to sizable federal tax credits, green wood waste is still economically feasible to use in biomass fuel production, as it has up to 90% less ash than coal. Because of this, energy produced emits far less carbon dioxide, so though less efficient, it’s a greener alternative to fossil fuels.
Green wood waste not ground into sawdust also has its uses. Green wood waste chips traditionally are used for making paper pulp and as organic mulch for landscaping, with neither application requiring much control on the material’s moisture content. Such wood chips can also be used for preventing erosion, suppressing weed growth, controlling dust, and other landscaping uses.
Recently, however, green wood waste chips have become popular in the production of biomass fuel. In fact, they are used increasingly in specially designed furnaces. This requires identifying optimal moisture levels for the wood chips so that they work most efficiently. Modern technology has also allowed them to be used for making fuel from biomass.
Processing Green Wood with Hammermills
Hammermills are very effective for breaking down and grinding green wood waste or any other fibrous material and are integral equipment in much of the wood-processing industry. Hammermills are used in biofuel, pellet production, and particleboard production because of their flexible configurations, which allow for optimal material processing.
To make biofuel from waste wood, particles must consistently be reduced to at least 1/8 inch (3 mm). However, moisture content must be under 15% for hammer mills to grind wood to that size properly. To get around this requirement, manufacturers must follow certain steps.
Steps for reducing green wood with hammermills:
- Pre-grind wood with mill outfitted with larger hammers that are 1 inch (25 mm), or a screen that is ¾ inch (19 mm). This results in ½ inch (12 mm) finished particle size, which allows the particles to dry more quickly.
- Use a drier to reduce moisture content from over 50% to at least 10%.
- Finish grinding the dried wood with a second hammermill that’s outfitted with a thinner hammer and finer screen to reach the
The same procedure for reducing green wood waste for pellets or briquettes used for fuel should be followed to make biofuel, though moisture content needs to be less than 10%. Hammermills are robust by design and provide manufacturers with a reliable way to reduce wood fibers efficiently, with specific features designed to mill green wood. This includes grinding even the most fibrous, stringy, and wet bark scrap types of green wood waste.
Raw material to make particle board consists of wood chips, planer shavings, and sawdust. Particleboard manufacturers can either generate the material on-site or have them shipped. If made on-site, mills need to debark logs and saw them to the correct length before breaking them down for chipping. Hammermills or other machinery further reduce them. Once milled, vibrating or gyratory screens or air classifiers separate the material by size, shape, and density. This removes the finer material, separating surface from core material, which is then stored in bins.
Manufacturers produce particle boards in densities that range from about 37 lbs. per cubic foot (590 kg per cubic meter) to over 800 kg per cubic meter (50 lbs. per cubic foot). The particleboard is then formed into panels, though manufacturers also make molded particleboard for pallets, furniture, or doors.
The first product ever developed by the Prater Pulverizer Company – the former incarnation of Prater Industries – was a hammermill. Since 1925, the company sold thousands of hammermills to various industries, refining and advancing the machinery’s technology to adapt it to new milling applications. Prater continues to design machines that deal with the most demanding processing challenges, using its expertise in various industries, innovative designs, and technological advancements to increase their value.
Prater continues to provide solutions for some of the most demanding processing challenges with our innovative designs, industry expertise, and advancements that provide a shorter wait for your business’ return on investment.
Some benefits of Prater’s hammermills include:
- Can bridge the gap between fine grinders and hammermills.
- Capable of 24/7 non-stop operation for extended periods.
- Lower requirements for power and maintenance.
- Provides uniform grinding with less build-up of heat.
- Reduces downtime.
- Runs more quietly and efficiently than other hammermills.
- Shaft and bearing arrangements offer quieter and smoother operation.
Prater’s hammermills have been designed for applications that include reducing wood chips, wood shavings, and other wood waste. Contact us directly to find out how we can assist with green waste reduction.