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

Authors: Tumuluru, Jaya Shankar

Submitted to: Biofuels, Bioproducts, & Biorefining (Biofpr)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2023
Publication Date: 6/29/2023
Citation: Tumuluru, J. 2023. High moisture pelleting of corn stover at pilot and commercial scale: Impact of moisture content, L/D ratio, and hammer mill screen size on pellet quality and energy consumption. Biofuels, Bioproducts, & Biorefining (Biofpr). https://doi.org/10.1002/bbb.2519.
DOI: https://doi.org/10.1002/bbb.2519

Interpretive Summary: Second-generation biorefineries utilize agricultural and forest residues, municipal solid and industrial wastes, and other low-cost carbon resources as feedstocks. The physical properties of these biomass feedstocks, such as low bulk density, cause storage, handling, and transportation challenges for the biorefineries. These challenges include fires in storage facilities due to spontaneous combustion and difficulties in feeding the feedstocks during biofuels production. Densifying the biomass into pellets or briquettes helps to overcome these challenges, but current methods are costly, due to drying, and produce greenhouse gas emissions. High moisture pelleting that eliminates the high-temperature drying step was tested at pilot and commercial scale. The quality of the pellets produced met the international pellet quality standards and the pellets were produced with 60 % lower energy compared to drying of high-moisture biomass feedstock in high-temperature rotary dryers before pelleting. This work could lead to more economically and environmentally feasible biofuels production from renewable biomass feedstocks.

Technical Abstract: The high-moisture pelleting process is an economic alternative that eliminates capital and energy-intensive drying step in pellet production. In this study, high-moisture pelleting of corn stover was tested in pilot and commercial-scale systems. For the pilot-scale studies, corn stover was ground in a hammer mill fitted with a 6.35-mm screen, reconditioned to adjust the moisture levels between 14 and29.6 % (w.b.), and pelleted by varying length-to-diameter (5-9) ratio of the pellet die. For commercial-scale testing, the high-moisture corn stover bales (24-26 %, w.b.) were processed in stage-1 & 2 grinders and pelleted. Pilot-scale studies indicated that higher moisture of 29% (w.b.) and lower L/D ratio of 5 produced lower bulk density and durable pellets (<500 kg/m3, <95 %). The response surface models developed explained the process adequately based R2 values. The analysis of variance showed a strong interaction between the process variables and pellet properties and energy consumption. Commercial-scale testing produced pellets with a density >650 kg/m3 and durability >98 % using high-moisture corn-stover bales. A bigger hammer mill screen size (11.11 mm) grind for pelleting reduced the density (675 to 575 kg/m3) but not the durability (about 98 %). The energy consumption of the commercial-scale unit operations for the high-moisture pelleting process was in the range of 98-124 kWh/ton, which is 64-72 % of less energy than for conventional biomass drying (350 kWh/ton) from 30 % (w.b.) to 10 % (w.b.) moisture content. Further, the pellets produced by the high moisture pelleting process met ISO and PFI international standards.