Why Your Biomass Pellets Have Cracks: 5 Common Causes & Fixes

Finding cracks on the surface of your newly produced biomass pellets is more than a cosmetic issue—it's a clear sign that something is off in your production process. Cracked pellets are weaker, produce more dust, and can break down during handling and transport, reducing their market value and burning efficiency. Understanding why these cracks appear is the first step to producing strong, durable pellets.

The quality of your final pellet is directly tied to the balance between your raw material, machine settings, and component condition. Let's examine the five most common reasons behind cracked biomass pellets and how to address each one.

1. Oversized Raw Material Particles

The size of your feedstock matters more than you might think. If the raw material contains particles that are too large, they cannot properly interlock and bind together during the intense compression in the pelletizing chamber. These large pieces create weak points and internal stress lines. As the pellet is extruded and cools, this stress releases, causing visible cracks on the pellet surface. The solution is to ensure consistent, properly sized raw material through effective grinding before pelleting.

2. A Severely Worn-Out Die

The die is the heart of your pellet mill, and its condition dictates pellet quality. Over time, the inner surface of the die holes wears down, becoming smoother and larger. This wear and tear reduces the compression pressure and friction needed to form a solid pellet. The material passes through too easily, resulting in a loosely structured pellet that lacks integrity and is prone to cracking. Regularly inspecting and replacing worn dies is non-negotiable for quality control.

3. Incorrect Raw Material Moisture Content

Moisture acts as a natural binder in the pelleting process. Straying too far from the optimal range—typically around 12-15% for most biomass—directly causes cracking.

Too High Moisture: Excess moisture turns to steam during compression. As this steam escapes after extrusion, it can create longitudinal vertical cracks or striations along the pellet's length.

Too Low Moisture: Insufficient moisture means there isn't enough natural lignin plasticity to bind the particles together. This leads to poor formation and often results in horizontal cracks or a crumbly pellet that falls apart.

4. Mismatched Die Compression Ratio

The compression ratio of your die must be compatible with the density of your raw material. Using a die with a compression ratio that is too high for a soft material creates excessive, sudden pressure that can over-compress and then fracture the pellet structure. Conversely, a ratio that is too low for a dense material won't apply enough pressure to form a solid, crack-free pellet. Selecting the correct die specification for your feedstock is crucial.

5. Incorrect Gap Between Die and Rollers

The gap between the die and the rollers is a critical mechanical setting. If this gap is too large, the rollers cannot effectively grip the material and press it firmly into the die holes. The resulting pellet is formed under insufficient pressure, making it weak, spongy, and highly susceptible to cracking immediately after production. This gap must be adjusted to the manufacturer's precise specification for your specific machine and material.

Conclusion: Achieve Smooth, Durable Pellets

Cracked pellets are a solvable problem. By systematically checking these five key areas—raw material size, die condition, moisture content, compression ratio, and roller gap—you can diagnose the root cause. The goal is a perfect harmony between your prepared feedstock and your machine's configuration. Paying attention to these details will lead to stronger, smoother, more professional pellets that hold together perfectly and command a better price.