A comprehensive comparative study on the energy application of chars produced from different biomass feedstocks via hydrothermal conversion, pyrolysis, and torrefaction

Understanding the suitability of different conversion technologies for different types of biomass feedstocks is crucial in delivering the full valorisation of different types of biomasses. This is novel research which presents an extensive comparative study on how three different thermal conversion technologies (torrefaction, pyrolysis, and semi-continuous hydrothermal conversion) and process interdependencies are influenced by different feedstocks (Rapeseed (RS), Whitewood (WW), Seaweed Laminaria digitata (LD))) for the optimisation of char (hydrochar/biochar) formation and their associated bioenergy applications. A wide range of processing conditions was analysed to optimise char formation and potential applications of these chars in energy production were extensively investigated. Based on the evaluation of char structures, hydrothermal conversion could be an applicable method for char production from WW and RS. The char yield of WW is in the range of 30-50 wt% at the early stage of hydrothermal carbonisation (HTC, 235 degrees C). Increasing temperature (>265 degrees C) decreased char yield but produced a higher HHV char (similar to 30 kJ/g). Approximately 90 wt% of LD dissolved into the water at low temperatures (<200 degrees C) during hydrothermal conversion, leaving small amounts of char with a significant ash content (similar to 50 wt%). During pyrolysis, RS and WW gradually decomposed and produced char with yield of similar to 35-40 wt% at 400 degrees C consisting of a high lignin content with a HHV of > 34 kJ/g. Similarly, LD decomposed gradually with a char yield of 45 wt% at 400 degrees C, but with a low HHV (similar to 15 kJ/g) and high ash content (20 wt%). WW had relatively high char yield of similar to 60 wt% during pyrolysis at 250 degrees C, with a HHV of 25 kJ/g. Although RS had a high char yield (similar to 75 wt%) with a high HHV (>30 kJ/g), the chars still contained a significant amount of volatiles. The WW char from these three thermal conversion technologies, and RS chars produced by pyrolysis and hydrothermal conversion, could have a potential application in bioenergy production. However, the ash content and low HHV make LD unsuitable for bioenergy applications.

Automated summary

This study compares the effects of three different thermal conversion technologies on different biomass feedstocks and explores their potential bioenergy applications. The results indicate that hydrothermal conversion is suitable for char production from Whitewood and Rapeseed, while Seaweed Laminaria digitata is not suitable for bioenergy applications due to its high ash content and low higher heating value.