A small piece of wood, a big impact – why chip quality matters

Chips are produced through a multi-stage process that turns logs into uniform, cookable particles for the digester. This process includes everything from raw material selection and storage to debarking, chipping, screening, and reject handling, and finally feeding the chips into the pulping process.

Chip quality is a combination of physical, chemical, and biological properties that ultimately determine how the chips behave during the cooking process.

“High-quality chips have the desired geometry and are even in size. They maintain consistent moisture, stable density, and have minimal bark content. However, there’s no universal ‘perfect chip’, as each mill’s equipment, raw material, and cooking process define what high quality means for them – and that makes it all very interesting,” says Magnus Wilhelmsson, Senior Product Manager, Cooking, at Valmet.

Chip quality is a sum of numerous parameters

The factors behind chip quality can be divided into four categories: raw material, wood supply management, wood handling process, and operative issues.

“The species, diameter, age, and moisture content of the raw material all have an impact on quality, and so does wood supply management, including storage and logistics. Chipping equipment naturally plays a key role in securing the geometry and size distribution of the chips. But what is even more important is the impact of operative issues on chip quality – process stability and equipment condition, to name a few. Chip quality is a complex topic and, as such, very challenging to control,” says Ari Havu, R&D Manager, Woodhandling, at Valmet.

How’s it cooking?

The reason why chip quality matters is the impact it has on the cooking process. High-quality chips benefit pulping operations in terms of higher yield, less reject, lower kappa level, less kappa variation, lower chemical consumption, and higher pulp strength. Besides affecting pulp quality, chip quality also influences the operational stability of the entire process.

“You could say that in this case, size truly matters. An uneven chip size distribution reduces yield: fines and pin chips overcook, degrading cellulose and weakening fiber strength, while oversized chips undercook, increasing the mill's reject levels by producing more knots and shives. Uneven chip size also causes segregation in the digester, affecting chip bed porosity and leading to uneven liquor flow and channeling. And since mills always ‘cook for the worst’, poor chip uniformity inevitably raises chemical and energy consumption”, Wilhelmsson explains.

Improving raw material efficiency saves money

About 60% of the total annual cost in a chemical pulp mill is related to the raw material. In a mechanical pulp mill, the same figure is 40%. 

“As such, raw material cost is the single largest expense at a pulp mill, and even a one-percent improvement in raw material efficiency saves a significant amount of money. Raw material efficiency can be enhanced through improving chip quality – the more uniform the chip size distribution, the less rejects,” reminds Minna Vepsä, Product Sales Manager, Pulp Drying, Baling, Wood & Biomass Handling, at Valmet.

Future is built on data

Even today, many mills do not systematically measure chip quality. They rely on occasional lab samples, making it difficult to diagnose process deviations. All three Valmet experts believe that data collection and automation will be among the central future improvements at pulp mills.

“The most important advice is to start measuring and collecting data – such as log diameter and chip size distribution, and moisture – right away. It’s the first step toward knowledge-based process control. Later, chip data can be connected with cooking and maintenance data to provide even more insight for process improvement and a valuable tool for problem-solving – not to mention the influence it has on overall lifecycle management of the mill,” says Ari Havu.