Wood Pellets Moisture Measurement

Stages of Wood Pellet Production

Wood Pellets are the most common type of pellet fuel and are made from compacted sawdust and related industrial waste from the milling of lumber, manufacture of wood products and furniture, and construction. In recent years, consumers have turned to wood pellets and other biomass fuel sources as an alternative to oil.

Wood pellet moisture control is one of the major challenges faced by pellet manufacturers. Being proactive with moisture measurement and control can result in tremendous cost savings in the manufacturing process and help eliminate the chance of packaging wet or low-quality product. Process Sensors’ on-line moisture sensors provide continuous, non-contact moisture measurement throughout the manufacturing process for closed-loop control throughout the wood pellet manufacturing process.

Sawdust preparation

Moisture levels of incoming sawdust are tested at the pellet plant upon receipt to determine the level of drying necessary before pellet production can begin. The moisture content gives the proportion of evaporable water to the total weight of the material. The optimal moisture content of the sawdust is 10-15%.

Sawdust with high moisture content should be dried since wet sawdust requires more power to be hammered than dry sawdust, and wet sawdust may impede the screening process by clogging or smearing the screen. Sawdust with lower moisture content, such as sawdust from hardwood flooring plants, can skip the initial drying procedure.

Screening

At large commercial plants, sawdust is screened to remove stones, plastics, metals, or other hard materials that may damage the equipment. Stones and stone-like hard materials are removed by a stone trap when the sawdust passes over the screen while metals are picked up by a magnet. Controlling the moisture content of the sawdust prior to the screening process ensures equipment remains unclogged and that maximum debris is removed before hammering. For smaller plants with less capital, this step is not necessary.

Hammering

Hammering is the next step in the wood pellet manufacturing process prior to the formation of the pellets themselves. In this stage, the incoming raw materials are homogenized to an even size. In the piles of sawdust, there might be wood lumps, dead knots, etc. which need to be hammered so that they can pass the die holes of the sawdust pellet machines.

Pelletizing

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Next, the sawdust goes into the pelletizing chamber of the sawdust pellet machine where it is heated up to 120-130℃ at high pressure. At this temperature, the lignin naturally present within the sawdust is plasticized and acts as a natural “glue” to bind the particles together to help to form the pellets. The pellets are molded by being pressed through the pellet dies of the sawdust pellet machine and then are cut off by the cutter at the desired length. The quality of the finished pellets is determined in large part to achieving consistent and optimal moisture content of the sawdust prior to pelletizing to ensure the lignin bonds effectively.

Cooling & Packaging

After the pelletizer, the pellets are hot, soft, and susceptible to deformation and must be cooled to become rigid. The final wood pellet moisture content after the cooler can be as low as 6% and may increase up to 8-10 % by absorbing moisture from the surrounding air. A moisture sensor mounted at this stage provides real-time data to optimize the cooling process.

At large commercial factories, the pellets undergo one final screening before packaging to separate well-formed pellets from other fine content. Once cooled and screened, the wood pellets are packaged and ready for transport.

Where Should You Install an On-Line Moisture Sensor?

Moisture is a major consideration throughout the wood pellet manufacturing process to meet finished pellet quality goals and increase profits. Below are some of the locations where our customers have used our sensors to monitor and control moisture in wood pellet production

• Incoming sawdust prior to the dryer
• The main advantage at this measurement location is to aid the loader operator in balancing the input moisture load to the dryer. Incoming sawdust is generally stored outside where it is subject to the environment, meaning some is wet and some is dry. Large amounts of wet material can overload the dryer. The loader operator needs a large readout at the input hopper so he/she can intelligently select wet or dry material to balance the input moisture.
• The output of the dryer
• This provides quality control on the dryer output and allows the product to be binned by moisture content.
• The output of individual bins
• Measurement at this location helps with accurate blending.
• Blended product as it enters the pellet press
• This critical measuring point monitors sawdust entering the pellet press which must be within a tight moisture range to produce a high-quality finished pellet. Instantaneous moisture readings from an in-line moisture meter are helpful to the press operator, and 4-20 mA output can be sent to a PLC which in turn can control the Pellet Press drive parameters in real-time.
• Laboratory or offline measurement
• For grab samples, moisture results can be obtained in 10 seconds with a benchtop moisture analyzer.
• Incoming hog fuel
• Some pellets plants use hog fuel for the boiler feed. An NIR on-line moisture sensor provides real-time data for improved boiler control.

An MCT460 General Purpose On-Line NIR Sensor is typically mounted within 6 to 16” of the product looking directly at the sawdust. When needed, the sensor can be mounted on the exterior of a vessel and measure product moisture through a glass window or utilize a snorkel sampler to grab samples when the product flow is light or discontinuous. Depending on the environmental conditions at the plant, air purge and cooling systems are available for installations in exceptionally dusty or hot conditions. The MCT460-WP’s analog outputs are normally connected to the control room allowing plant operators and managers to monitor their process and log historical data.

For quick, accurate, and reliable sample testing, the QuikCheck Benchtop Analyzer can be used in the laboratory for precise moisture measurement of grab samples in under 10 seconds.

Value and Quality

Moisture measurement and control is important in all phases of wood pellet manufacturing. Homes and businesses throughout the world are looking at wood pellet fuel as an alternative or supplementary source for heat. Wood pellets yield high BTUs at an economical price. Cord wood that you might burn in a fireplace or fire pit generally has moisture from 20 to 60% and is relatively inefficient in energy conversion. Wood pellets, on the other hand, offer high BTU energy yields and are easier to burn, store and transport.

Want to Learn More About How We Can Help You Achieve Your Quality and Profit Goals?

We have worked with dozens of wood pellets plants, both large and small, to help them optimize their process and save additional time and labor in all stages of pellet manufacturing. Get in touch with us to tell us a little bit about your production process and let's find out if we have a solution that can help improve your bottom line.