Pilot-Scale Pelleting Tests on High-Moisture Pine, Switchgrass, and Their Blends: Impact on Pellet Physical Properties, Chemical Composition, and Heating Values

Abstract

In this study, we evaluated the pelleting characteristics of southern yellow pine (SYP), switchgrass (SG), and their blends for thermochemical conversion processes, such as pyrolysis and gasification. Using a pilot-scale ring-die pellet mill, we specifically assessed the impact of blend moisture, length-to-diameter (L/D) ratio in the pellet die, and ratio of pine to SG on the physico-chemical properties of the resulting pellets. We found that an increase in pine content by 25–50% marginally affected the bulk density; however, it also led to an increase in calorific value by 7% and a decrease in ash content by 72%. A moisture content of 25% (wet basis) and an L/D ratio of 5 resulted in poor pellet durability at <90% and bulk density values of <500 kg/m3, but increasing the L/D ratio to 9 and lowering the moisture content to 20% (w.b.) improved the pellet durability to >90% and the bulk density to >500 kg/m3. Blends with ≥50% pine content resulted in lower energy consumption, while a lower L/D ratio resulted in higher pelleting energy. Based on these findings, we successfully demonstrated the high-moisture pelleting of 2.5 ton of pine top residues blended with SG at 60:40 and 50:50 ratios. The quality of the pellets was monitored off-line and at-line by near infrared (NIR) spectroscopy. Multivariate models constructed by combining the NIR data and the pelleting process variables could successfully predict the pine content (R2 = 0.99), higher heating value (R2 = 0.98), ash (R2 = 0.95), durability (R2 = 0.94), and bulk density (R2 = 0.86) of the pellets. Thus, we established how blending and densification of SYP and SG biomass could improve feedstock specifications and that NIR spectroscopy can effectively monitor the pellet properties during the high-moisture pelleting process.

High‐moisture pelleting of corn stover using pilot‐ and commercial‐scale systems: Impact of moisture content, L/D ratio and hammer mill screen size on pellet quality and energy consumption

“…During the high-moisture pelleting process, the final moisture content in the pellet depends on the initial feedstock moisture content. 9,20,23,24,26,30,31 For example, the moisture loss during pelleting feedstock at about 30% (w.b.) was about 10% (w.b.…”

“…23,24 Recent high-moisture pelleting studies on the flat die and pilot-scale pellet mills using switchgrass, pine and blends of switchgrass and pine indicated the same trend where the moisture loss increased with an increase in the feedstock moisture content and L/D ratio of the pellet die. 26,30 One possible reason for the moisture loss in the feedstock can be due to moisture flash-off when the biomass passes through a constricted die hole under elevated temperature and pressure. 20…”

“…As highlighted by many authors, the general trend observed was that higher feedstock moistures produced lower unit and bulk density pellets. 23,24,26,28,30 The bulk and unit densities of the pellets produced at 13. 7).…”

“…Most of the work on high-moisture pelleting of corn stover and other feedstocks was conducted on a labscale, flat-die pellet mill. 20,[23][24][25][26][27][28][29] A recent study by Tumuluru et al 30 reported promising results in high-moisture pelleting of pine and switchgrass and their blends using a pilot-scale ring-die pellet mill (1 ton/h). In the study, the authors successfully demonstrated the high-moisture pelleting of 2.5 tons at different pine and switchgrass blend ratios.…”

“…scale ring-die pellet mill, where higher moisture and lower L/D ratios produced pellets of pine, switchgrass and their blends with the reduced unit and bulk density values. 26,30 Tumuluru 20 reasoned that the lower unit and bulk density pellets produced from high-moisture feedstock could be due to the diametric expansion of the pellets.…”