Development of a Binder from Hydrolyzed Specified Risk Materials for Pelletization of Torrefied Wood

• Author / Creator

  Zhu, Chengyong

• Torrefaction is a thermal pretreatment of woody biomass in a temperature range of 200-300 ℃ without air or oxygen, which leads to a heating value for the woody biomass that is closer to that of coal, and also improves hydrophobicity. Torrefied wood pellets could improve co-firing rate with coal in coal-fired power plants or be used as substitute for coal. However, a densification technique like pelletization is needed to increase the bulk density of torrefied wood to decrease costs associated with handling, transportation, and storage. It is difficult to pelletize torrefied wood because the natural binder for wood pellets, lignin, undergoes structural changes during torrefaction. Therefore, an external binder is needed. The objective of this research was to develop a protein-based binder from specified risk materials (SRM) that could be used for pelletization of torrefied wood. SRM are a proteinaceous by-product from cattle tissues where prions are most likely to concentrate. About 300,000 tonnes of SRM are landfilled or incinerated annually in North America. As an alternative, SRM can be thermally hydrolyzed, allowing peptides recovery. Prior to using peptides as a binder for pelletization of torrefied wood, several challenges need to be addressed: a suitable binding strength of the peptides must be achieved, chlorine and salts must be removed, and an industrially relevant method for drying SRM hydrolysates must be developed. During this study, the freeze-drying step currently used for processing of SRM hydrolysates at laboratory scale was replaced with spray drying, a much more industrially feasible drying option. The available carboxylic and primary amine groups were estimated and found to be statistically similar to those of peptides obtained from freeze drying. Chlorine in freeze-dried and spray dried peptides was assessed at 1.8 % and 1.3 %, respectively. A washing step was then introduced before thermal hydrolysis to reduce the chlorine content in resulting peptides. It was found that the prewashing step could reduce chlorine levels to 0.436 %, and could also lower levels of some salts. The use of unmodified peptides did not improve the durability of torrefied wood pellets at a 0-2 % binder level. This clearly indicated the necessity to improve the binding strength of peptides by proper chemical modification and/or chemical crosslinking. Polyamidoamine epichlorohydrin (PAE) was chosen as a crosslinker, as previous work in the Bressler lab showed that peptides-PAE was a successful plywood adhesive. At 1 % peptides-PAE (25 % PAE) binder level and 28 % moisture, the durability of torrefied wood pellets was 81.7 ± 2.0 %. An increase of peptides-PAE (25 % PAE) to 2 % significantly improved the durability to 88.6 ± 1.0 %. Increasing the PAE percentage from 23 % to 25 % did not improve the durability of pellets. However, under the same pelletization conditions, the peptides-PAE binder resulted in pellets of much higher durability than that of the control sodium lignosulphonate. The lowest moisture content using the pilot scale pelletizer in this research for pelletization was 27-28 %. This was too high from the industrial perspective as cracks or pores could be created in pellets during drying and moisture removal, which could result in pellets with low durability. In order to lower the moisture content for pelletization, a different single pellet press pelletization system was used. It was found that the durability of single pellets with 3 % peptides (prewashed & spray dried)-PAE (33 % PAE) was significantly better than that of pellets without binder, at 10 % moisture content.This research showed that using of peptides-PAE as a binder could improve durability of resulting torrefied wood pellets. Additionally it demonstrated that spray drying could replace freeze drying and be adapted into SRM hydrolysates processing protocol, washing SRM before thermal hydrolysis could reduce chlorine content of resulting peptides, and moisture content for pelletization could be lowered to 10 % using a single pellet press.

• Subjects / Keywords

  • Binder; Specified Risk Materials; Pelletization; Torrefied wood

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-wh3h-sj55

• License

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