Decoding Wood Chip Quality Standards: A Deep Dive into G-Class and M-Class Specifications

In the biomass industry of 2026, precision is the difference between a high-performing energy plant and a mechanical nightmare. As boilers become more automated and efficient, the demand for standardized fuel has never been higher. To ensure system compatibility, the industry relies on the ISO 17225-4 standard, which categorizes wood chips based on two critical factors: Particle Size (G-Class) and Moisture Content (M-Class).

Understanding these codes is vital for procurement officers, plant engineers, and suppliers alike.

Decoding Wood Chip Quality Standards: A Deep Dive into G-Class and M-Class Specifications

1. Particle Size: The G-Class Categories

The "G-Class" determines how well the fuel will flow through a system’s mechanical feeders. If chips are too small, they can restrict airflow; if they are too large, they will jam the screw conveyors.

Grade	Main Fraction (at least 60%)	Max Length	Typical Application
G30	2.8mm to 16mm	< 150mm	Small-scale domestic or commercial boilers.
G50	5.6mm to 31.5mm	< 200mm	Medium to large industrial boilers.
G100	11.2mm to 63mm	< 250mm	Large-scale power plants and district heating.

Note: In 2026, "fine" particles (dust) are strictly limited, as high dust content increases the risk of respiratory issues and explosive dust clouds in storage.

2. Moisture Content: The M-Class Categories

Moisture is the most significant variable in wood chip quality. It dictates the Net Calorific Value (NCV)—essentially, how much heat you get for every ton you buy.

• M20 (Kiln-Dried / Seasoned): Moisture $< 20\%$. These chips are premium and provide immediate, high-intensity heat. They are ideal for systems without advanced flue gas recovery.

• M30 (Air-Dried): Moisture $20\% - 30\%$. The "sweet spot" for most industrial biomass boilers, balancing cost and efficiency.

• M40 - M50 (Green Chips): Moisture $> 40\%$. Often sourced directly from forest residues. These require specialized "wet-fuel" boilers equipped with moving grates to dry the fuel during the combustion process.

Buying "wet" chips may seem cheaper per ton, but you are effectively paying for the transport of water, which provides zero energy.

3. Beyond Size and Water: Ash and Contaminants

While G and M classes are the primary focus, professional 2026 grading also considers:

• Ash Content (A-Class): Higher ash content (often from bark or soil contamination) leads to "clinkering"—the formation of glassy deposits that can fuse to boiler grates.

• Energy Density: Usually measured in $kWh/m^3$. Because wood chips are bulky, understanding how much energy fits in a single delivery truck is essential for logistical planning.

4. Why Grading Matters for Your Equipment

Every boiler manufacturer specifies a narrow "operating window" for fuel.

• The Jamming Risk: Using G50 chips in a G30-rated screw feeder will likely cause a mechanical shear pin to break, halting production.

• The Thermal Risk: Feeding M50 (wet) chips into a boiler designed for M20 (dry) chips will cause the firebox temperature to drop, leading to incomplete combustion and excessive smoke/emissions.

5. Certification and Quality Assurance

In 2026, simply claiming a grade is no longer enough. Reliable suppliers now provide batch certificates and often participate in certification schemes like ENplus® (for wood chips) or local sustainable forestry certifications. These ensure that the chips are not only the right size but also free from chemical contaminants like lead or plastics found in recycled wood.

Conclusion

Decoding wood chip standards is about more than just terminology; it is about protecting your investment. By matching the correct G-Class and M-Class to your specific boiler technology, you ensure maximum uptime, lower maintenance costs, and the highest possible return on your green energy investment.