Microwave Technology Aids Moisture, Density Measurements

Industrial inline and at-line methods measure core and surface moisture of wood pellets.
By Paul Jadot | September 13, 2014

The accurate measurement of the moisture of wood pellets is of crucial importance in nearly every stage of the manufacturing process. Wood pellets can’t be too moist or too dry, and bulk density has to be on target. Instantaneous and accurate automated moisture measurement systems that allow continuous adjustments to the production process guarantee superior product quality, reduce raw material waste, diminish customer complaints, save energy and ensure adherence to industry norms.

The role of the quality assurance staff of a wood pellet plant is to provide verifiable and real-time data to the manufacturing department to enable it to meet or exceed product specification targets at the lowest possible cost. Systems that largely rely on manual or semimanual measurement data collection methods cannot sufficiently reduce the variability of the production inputs in a timely manner. Also, those tend to be labor-intensive.

Poor adherence to product specifications may result in rejected lots. When pellets are too moist, the product quality is inferior and production speeds cannot be optimized. When too dry, the pellet press may clog. Quality suffers when adjustments to equipment such as dryers or conveyer speeds are based on inaccurate, incomplete or historical data, or when moisture data collection is only done in the final prebagging phase of the process. Measuring the moisture of the sawdust/wood particles at the beginning of the production process and of the pellets at the end of the line are two key quality control inputs.

Industrial users and retail customers expect product performance to meet the advertised specifications of the manufacturer, and they purchase pellets on a weight basis. High-moisture products have a lower Btu value per unit-weight and are more expensive to transport than comparable, drier pellets. Pellets with varying density, and thus uneven Btu output, will create difficulties for large and small users, alike. For example, owners of home furnaces with minimal automation may have difficulties controlling heat output. It goes without saying that customer complaints and product returns are a drain on company resources.  

It is also important to note that energy consumption is reduced when drying results closely track product moisture targets. Dryers are an integral part of pellet processing lines, and automatic adjustments to the drying process (in conjunction with automated control devices) can only be made when continuous high-speed moisture measurement systems are used.

For instantaneous, true-core moisture and density measurements in the wood pellet industry, there are three types of moisture measurement systems or devices: inline systems, at-line instruments and bypass systems. 

Inline systems combine high-speed moisture sensors with fast processors and data analyzers that compute and present density-independent moisture measurements and product temperatures in real time. The processors are networkable and provide an array of outputs to enable the direct control of devices using analogue or digital outputs to computerized control systems. They also provide outputs for data analysis and storage. Low-powered resonance microwave sensors are particularly effective in measuring core moisture independently of mass, at acquisition frequencies exceeding several thousand times per second.

At-line and laboratory instruments using resonance microwave technology provide extremely accurate and instantaneous measurement results for both moisture and density of pellets on a sample basis. They are often used as a verification tool of the accuracy of the inline measurement systems. They are also used to support product development and testing.

Bypass systems are particularly effective in accurately measuring bulk density of wood pellets in addition to moisture. Here, a tubular resonator microwave sensor is used whose microwaves penetrate the collected sample. The processor is programmable to control valves to divert pellets from the main conveyer or chute into the bypass feed tubes to fill the cavity (e.g. pneumatically or by way of augers). After the instantaneous measurement is completed, the system releases the sample back into the main flow. The full cycle of filling the sensor, measuring and releasing pellets takes just over one minute. The sample collection frequency is entirely programmable. Bypass sensors are used primarily to accurately determine bulk density, a proxy for Btu output. Analogue outputs are available to control devices that can impact density, such as pellet presses.

Inline systems based on the commonly used infrared light (NIR) are not able to measure core moisture of pellets because NIR light only reacts to surface moisture.  In addition, color and ambient light affect the signal. Low-powered resonance microwave technology overcomes those limitations by completely penetrating the wood pellets. Additionally, the sensor chamber functions as a resonator, with walls designed to reflect the microwaves, creating a standing wave pattern when the sensor is empty (at resonance).
Microwave resonance technology is based on the interaction between the microwaves and water molecules in the wood pellets. During this process, the sensor, such as a high-speed planar sensor, builds up a weak microwave field and constantly analyzes the resonance frequency of the sensor system by scanning the microwave frequency. If pellets are moving into the microwave field, a shift in resonance frequency occurs and its amplitude is dampened. The relationship between frequency shift and dampening in this context is a density-independent measure of the water content in percent and thus compensates for any density effects. This ratio constitutes what is called the microwave moisture value, which in turn represents the core moisture. The absolute moisture content in the material (percent of the product mass) is calculated using an initial calibration against a standard reference method such as a drying oven, rapid moisture analyzer or Karl-Fischer titration.

The patented resonator method developed by TEWS Elektronik, Hamburg, Germany, is based on measuring attenuation and frequency shifts as two separate parameters. This permits the measurement of the moisture independently from the mass or density of a product. For more than 25 years, TEWS Elektronik has supplied precision moisture-measuring systems to the wood products industry. MebTec Technology Inc. is Tews' North American agent.

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Measurement Locations

• Wood strands can be measured before and after the dryer to measure and control the drying process using an inline planar sensor, installed at an angle or flush with the chute’s sidewall. Such installations require the sensor mounting in a hinged flap or a removable panel to facilitate easy inspection and cleaning.

• Temporary storage of wood particles in a silo before moistening allows for the continuous moisture measurement using a high-speed planar sensor, located near the bottom of the silo to ensure moisture readings when the silo is only partially full. The sensor has to be mounted in a hinged door or a removable panel to facilitate easy nullification, inspection and cleaning.

• After moistening of the sawdust or wood particles, an inline sensor measures moisture continuously. The sensor should be angled or flush with the chute’s side wall, when inclined.

• Between the hammer mill and the dryer and press are locations where continuous inline measurements are important and a sensor is easy to install. The installation requires an angled mount to ensure the coverage of the sensor with moving products.

• At the bagging station, the final moisture and density can be inspected for final quality control inspection. Here, the opportunity exists to measure the moisture at-line using laboratory equipment and inline using planar sensors. In this location, bypass sensors are of particular relevance because they can measure bulk density of the final pellets and provide outputs to the pellet presses to control the density.


Author: Paul Jadot
President, MebTec Technology Inc.
[email protected]


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