Microwave sensing of moisture in flowing biomass pellets

Production of pelleted biomass for fuel is an emerging industry in the United States. Moisture content is a primary quality attribute of pelleted biomass materials because it is critical in binding, storage, combustion, and the pricing of pelleted biomass. To produce pellets of high quality, moisture content must be tightly controlled. A microwave system was designed for moisture sensing in flowing bulk material and used to determine feasibility of sensing moisture content in pelleted biomass from measurement of the dielectric properties at microwave frequencies. Samples of pelleted biomass derived from peanut hulls and pine sawdust were used for moisture content determination. Moisture contents of pine sawdust pellets ranged from 5.4%-9.9% (wet basis), and the range for peanut hull pellets was 8.9%-14.5%. At each moisture content, three different material flow rates were tested, and moisture content predictions were compared to those obtained with static measurement. Moisture content of flowing material was predicted by using a permittivity-based density-independent moisture calibration function. Root-mean-square deviations were computed for comparisons between reference moisture content, and predicted moisture contents for both static and flowing materials. Results showed that predicted moisture contents under static and flowing conditions were comparable.