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.
Highlights
Various woody and herbaceous biomass sources—such as sugarcane, corn stover, dedicated bioenergy crops, forest, and agricultural residues—could be used for biofuels production (National Renewable Energy Laboratory, 2008)
Demonstration of high-moisture pelleting using 2.5 ton of 2-in. (50.8 mm) and 6-in. (152.4 mm) southern yellow pine (SYP) tops blended with SG at 50:50 and 60:40 ratio successfully produced pellets with a bulk density between 473 and 586 kg/m3 and durability between 91 and 93%
Partial least square regression models (PLSR), obtained by integrating the pelleting process data with at-line and offline NIR principal component scores, were successfully developed to predict the pellet pine ratio, ash content, HHV, durability, and bulk density with coefficients of determination (R2) between 0.86 and 0.997
Summary
Various woody and herbaceous biomass sources—such as sugarcane, corn stover, dedicated bioenergy crops, forest, and agricultural residues—could be used for biofuels production (National Renewable Energy Laboratory, 2008). The Renewable Fuel Standard (RFS) mandates that cellulosic biofuel should slowly displace transportation fuels and reach 16 billion gallons by 2022 (U.S Environmental Protection Agency, 2018), which requires the annual processing of approximately 1,000–1,200 million tons of biomass. Perennial grasses, such as switchgrass (SG), have the potential to be environmentally beneficial, high-yielding sources of cellulosic feedstock. The significant advantages of SG are that it can grow on marginal lands that are not being used currently to grow food crops (Hartman et al, 2011; Feng et al, 2017), it mitigates nutrient pollution from fertilizer, provides flood control, increases yield when used in a crop rotation, creates wildlife habitats, prevents soil erosion, and sequesters carbon with its extensive root system (Blanco-Canqui et al, 2017)
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