In-line microwave reflection measurement technique for determining moisture content of biomass materials

Authors: JULRAT, SAKOL - Oak Ridge Institute For Science And Education (ORISE); Trabelsi, Samir

Submitted to: Biosystems Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2019
Publication Date: 10/22/2019
Citation: Julrat, S., Trabelsi, S. 2019. In-line microwave reflection measurement technique for determining moisture content of biomass materials. Biosystems Engineering. https://doi.org/10.1016/j.biosystemseng.2019.09.013.
DOI: https://doi.org/10.1016/j.biosystemseng.2019.09.013

Interpretive Summary: Moisture content is a critical quality factor for biomass material, sawdust for example, in the process of converting it into pellets for convenient and efficient handling and transport. In fact, moisture content influences other properties of biomass materials such as frictional characteristics, density and strength of the resulting pellets. These parameters affect the pelleting process with respect to energy consumption, production capacity and operating and maintenance costs. Normally, the raw biomass material will be dried to about 10% (wet basis) before being considered for pelleting. During the pelleting processes, the moisture content is raised to the optimum value, about 15% for example, for controlling the pelleting process. A moisture-sensing technique that can be integrated into the production line and provide real-time, accurate and low-cost measurements of moisture content would be very useful. An in-line microwave reflection measurement technique for determining moisture content of biomass material (sawdust) moving past the sensor on a conveyer was designed, assembled and studied in this research. The system consisted of four open-ended rectangular substrate-integrated waveguide sensors mounted on the conveyor walls, three RF-switches and a 3.0 GHz low-cost six-port reflectometer for the reflection measurements from the sensors. The dielectric properties of the biomass as a function of moisture content and bulk density were extracted by using the six-port measurement technique. The density-independent algorithm for moisture content determination was developed based on the power readings from the six-port circuit at conveyor speeds of 0.15, 0.23 and 0.35 meter per second. Finally, the predicted moisture contents were compared with those determined by the standard oven-drying method with standard errors of calibration of 1.75%, 1.70% and 1.28% moisture content for conveyor speeds of 0.15, 0.23 and 0.35 meters per second, respectively. The predicted dielectric constant and dielectric loss factor divided by the bulk density increased linearly with moisture content for all sensors. The bulk density variation can be reduced by choosing a proper conveyor speed, and position and arrangement of the sensors on the conveyor. The predicted moisture contents agreed well with oven-moisture-content determinations. The best predicted biomass moisture content by the system sensors, for moisture ranging from 8.5% to 37.7% at room temperature (23C), was found at the highest conveyor speed. The proposed measurement technique is suitable for in-line nondestructive determination of moisture content in biomass materials and can provide essential moisture information for biomass pelleting and other processes.

Technical Abstract: An in-line microwave reflection measurement technique for determining moisture content of biomass material (sawdust) moving past the sensor on a conveyer is described in this paper. The proposed system consists of four open-ended rectangular substrate-integrated waveguide (ORSIW) sensors mounted on the conveyor walls, three RF-switches and a 3.0 GHz six-port reflectometer for the reflection measurement from the sensors. The dielectric permittivity of the biomass as a function of moisture content and bulk density can be extracted by using the six-port measurement technique. The density-independent algorithm for moisture content determination was developed based on the power readings from the six-port circuit at conveyor speeds of 0.15, 0.23 and 0.35 m s-1. Finally, the predicted moisture contents were compared with those determined by the standard oven-drying method with standard errors of calibration (SEC) of 1.747%, 1.704% and 1.281% for conveyor speeds of 0.15, 0.23 and 0.35 m s-1, respectively.