Wood chips are a versatile material used across numerous industries, from energy production to horticulture. They are a manufactured product resulting from the mechanical reduction of larger timber, not simply wood waste. The quality and uniformity of these small pieces of wood determine their ultimate application and value.
Defining Wood Chips and Their Origin
A wood chip is a small, uniformly sized fragment of wood produced by cutting or chipping larger pieces of timber. These fragments are engineered for specific purposes, unlike sawdust or shavings, which are general byproducts of sawing or planing. Chip size typically ranges from 10 to 45 millimeters, depending on the intended use.
Sources include virgin wood from dedicated cutting, logging residues like branches and crowns, and residues from sawmill operations and untreated wood off-cuts. The distinction between chips made from clean wood and those from waste streams is important for quality. Clean wood, such as debarked logs, yields a higher-quality product suitable for pulp and paper. Waste materials like stumps, roots, or recycled wood are often directed toward biomass fuel or landscaping. The type of tree, whether hardwood or softwood, also influences the chip’s properties.
How Wood Chips Are Manufactured
Raw wood is transformed into chips using specialized machinery called wood chippers. These industrial machines use powerful cutting actions to rapidly reduce large pieces of wood into consistent fragments. The goal of this process is to achieve a uniform particle size, which is essential for efficient handling and processing.
Two main types of chippers dominate the industry: disc chippers and drum chippers. Disc chippers use a heavy, spinning steel disc with knives mounted on its face, slicing the wood against an anvil. This design produces uniform chips preferred for applications demanding high dimensional control, such as paper pulping. Drum chippers use a horizontal rotating cylinder with blades that continuously shear the wood as it is pulled into the machine. Drum chippers are better suited for processing larger, more irregular raw materials, such as bulky logs or fibrous brush. After chipping, the material is often passed through a screening process to separate oversized fragments and fine particles, ensuring the final product meets size specifications.
Classifying Wood Chips by Size and Quality
Wood chips are classified according to internationally recognized standards that define their physical and chemical properties and determine their commercial grade. Particle size distribution is a fundamental metric, separating chips into classes based on the percentage of material that falls within a main fraction. Standards also limit the amount of fine (undersized) and coarse (oversized) material. For instance, the P31.5 size class specifies that a minimum percentage of the material must be between 3.15 millimeters and 31.5 millimeters.
Moisture content is another factor, as it significantly impacts the material’s weight, storage stability, and energy value. Freshly felled trees can have a moisture content ranging from 40% to 60%. Chips destined for energy use are often dried to a maximum of 35% moisture content (M35) or lower for smaller boilers. High moisture levels increase the risk of mold growth and biological activity during storage, which degrades quality.
Purity and species also influence the chip’s final grade. High-quality chips, such as Grade A (A1 and A2), are sourced mainly from clean stem wood and feature very low levels of bark and ash. Ash content is important for biomass fuel, as high levels can cause operational problems in heating systems.
Primary Uses of Wood Chips
Wood chips are utilized across three major industrial and domestic sectors, capitalizing on their renewable nature. One significant application is in the energy sector, where they function as a biomass solid fuel. Wood chips are combusted in industrial boilers and power plants to generate heat and electricity, offering an alternative to fossil fuels.
The pulp and paper industry represents another core use, relying on wood chips as the raw material for cellulose fiber production. High-quality chips with uniform size and low contamination are processed to separate the cellulose fibers, which are then used to manufacture paper, cardboard, and other wood-based composites.
In horticulture and landscaping, wood chips are widely employed as organic mulch. When spread over garden beds or pathways, they help suppress weed growth, regulate soil temperature, and reduce water evaporation from the soil. As the chips slowly decompose, they also contribute organic matter, improving soil structure and fertility.
Minor Uses
Minor uses for wood chips include:
- Animal bedding.
- Material for playground surfaces.
- A carbon-rich component in composting operations.