Hammer Mill Maintenance: Keys to Performance

Size reduction and homogenization is a key parameter to a pellet mill’s steady, high output.
By Holger Streetz | February 22, 2023

Hammer mills prepare woody biomass for the pelleting process. It is vital to pay close attention to the hammer, as it has an important influence on performance.  Hammer mills turn heterogeneous material into homogeneous material. Grinding with hammer mills is done in feed mills, food processing, plastic waste and biomass operations. There are many applications for ground woody biomass, such as insulation material, animal bedding, packaging accessories and filling agents.

A typical pellet plant has a two-stage size reduction. Wet or green wood hammer mills are designed for wet grinding of shavings and chips in preparation for the drying process. These hammer mills can have a throughput of up to 60 tons per hour. Dry hammer mills further reduce the size of pre-dried woody particles for pelleting. Typical dry hammer mills in wood pellet preparation have a throughput of 9 to 11 tons per hour using a 600 horsepower motor, but performance characteristics depend on the raw material quality.

High-quality raw material, such as clean sawdust from a manufacturer’s own sawmill, is so clean that hammers will virtually last forever. Pictures from a German wood pellet supplier with its own sawmill shows very low wear (Image 1), whereas lower-quality raw material, including bark, decreases hammer mill lifetime dramatically. It is obvious that softwood is less aggressive on hammers than hardwood. Other quality factors are metals and dirt in the woody biomass. We have encountered large bicycle parts in eastern European raw material batches, as well as nails and screws in German industrial wood. With very sandy soil, sand can be deposited into the wood. Even with high-quality raw material, the resin content has a significant effect on performance.

The performance of a hammer mill is mainly determined by the sieve perforation size and the number of hammers. The more hammers and the wider the holes, the higher the output. Additionally, the distance between hammers and sieves has an influence. With rough grinding, the distance between hammers and sieves is relatively larger than with fine grinding. With a wider distance, the fiber is longer and larger. This is not ideal for wood pelleting, as too many large particles lead to overrunning rollers and clogging of the die (Image 2). Thus, the focus of the maintenance plan should be on material quality and the wear of hammers. In general, the hammers should be changed when the plating is worn off.  Reality, however, is sometimes a lot different. With low attention to the hammer mill, completely worn hammers can occur (Image 3), resulting in particles that are too large, and subsequently having a negative effect on pellet mill performance.

Author: Holger Streetz
COO, Bathan AG
[email protected]