The Influence of Material Flow on Pellet Mill Performance

Wood dust is a byproduct of all wood manufacturing. Thus, all wood processors have dust management for safety reasons in common. For a dust explosion to happen, all that is required is a single spark from a hot surface or an electrical device. When the dust disperses and mixes with atmospheric oxygen, ignition in an enclosed or contained area causes an explosion. However, it is often not the first reaction that is devastating, but the much larger amount of dust aroused by the blast wave. This can ultimately lead to a chain reaction that can potentially destroy a whole plant. If there is a risk of accumulating wood dust, the main hazard control measures are good housekeeping, well maintained equipment to reduce any ignition risk, and existing controls to reduce the effects of an explosion, such as vents or dust collectors.
 
The Effects of Wood Dust
Most pellet producers are well aware of explosion risks. However, wood dust has an enormous effect on the pelleting process and plant performance, too. For instance, a high amount of wood dust leads to increased maintenance on filters and a heightened risk for filter fires. Additionally, when raw material management in the silo is not considering the wood dust properly, the material flow is inhomogeneous. An uneven material mix will lead to high variances in power consumption and pellet quality. A German wood pellet plant with four pellet mills (4-ton-per-hour capacity each) recently upgraded its pelleting process with a dry hammer mill with a higher capacity. With the installation, the plant has been able to reach a higher material flow. The dust content is greater than 50%. With such a large amount of small particles and wood dust, the effective press channel length for a high-quality residential heating ENplus wood pellet is just 35 millimeters (mm), or 1.378 inches. The dry hammer mill sifters have a size of 5.5 to 6 mm and the pellet mills are producing approximately 3.6 tons of wood pellets at 420 amps. The power consumption of 75 kilowatt-hours per ton (kWh/t) is very high, compared to the industry average for this equipment of 55 to 60 kWh/t. Potential reasons for the high power consumption include particle size, incoming material humidity, and the dwell time in the ripening bunker. In general, the material should ripen for 10 to 15 minutes and have a moisture content of 10 to 12% when entering the pellet mill.

At a status quo assessment that we witnessed, the pellet producer had been operating the pellet mills with caution. The plant was not running at full capacity. The motors of the pellet mill were operating at 75% of maximum current (motor power in kW x2 -10%). Due to the low load, the product in the die longer, which causes overrunning of material. Additionally, the key performance indicator (KPI) power consumption measured in kWh/t was metered at the flapper scale. The ideal measurement for the power consumption and the according kWh/t KPI is directly behind the press, including debris and dust from abrasion. The pellet mill should be operated with an open pellet chute for one minute at 70 to 80% load and collect the pellets, dust and debris. The weighed material translates into kWh/t. The performance is also measurable by the feeder screw speed.

The plant desires to improve on performance toward nameplate capacity. Our recommendations from the assessment are:

• Correct measurement of KPIs, especially power consumption.

• Increase load toward current maximum, while adjusting effective press channel length and other parameters.

• Improve material management in the silo for a more even mixture.
In the following months, we will help increase production toward nameplate capacity while also ensuring other KPIs are met. Power consumption is especially of importance. Now more than ever, prices for power are increasing significantly.

In conclusion, the amount of dust in the process has an impact on quality and pelletizing performance. Get in touch with us to discuss performance optimization in pellet production.


Author: Holger Streetz
Chief Operating Officer, Bathan AG  
[email protected]