A study into the use of torrefied biomass in a fixed bed biomass combustor
Date of Award
Doctor of Philosophy
School of Engineering
Yasir M. Al-Abdeli
Biomass is a widely accepted renewable fuel with the potential to substitute conventional solid fossil fuels in thermal power generation due to its abundant availability and CO2 neutrality (if sustainably sourced). For the direct utilisation of biomass, combustion stands as the widely used thermochemical conversion process to generate heat and power. However, several drawbacks such as its low water resistivity, structural heterogeneity, high oxygen content, and poor grindability limit its utilisation as a convenient solid fuel. In this regard, pre‐treatment of biomass, i.e. torrefaction, a thermochemical process conducted at a temperature range between 200-300 °C, is considered a promising option to enhance biomass fuel properties for energy applications. The available literature on torrefaction thermal pre-treatment presented the extent of improvement in fuel quality at various operating conditions using non-pelletised biomass. However, the use of pelletised biomass torrefied under partially oxidative conditions and its direct combustion behavior or kinetic analyses remained unnoticed.
This research focused on the non-conventional production, characterisation and usage of torrefied densified biomass with the methods more representative to industrial scale, i.e. carrier gas, heating medium, and utilisation of real-time biomass combustion flue gases for torrefaction. Both experiment and kinetic modelling were carried out to build a deeper understanding of the thermal treatment of biomass fuel.
At first, the potential of the densified biomass used in this study was examined, considering three main factors: torrefaction temperature, torrefaction time and pellet size. Results reveal that the effects of pellet size on torrefied solid fuel’s properties appear to be relatively stronger at shorter torrefaction time (30 min) compared to 60 min. Small pellets exhibited better fuel properties, however, at the expense of greater mass loss than larger pellets.
Secondly, experimental work coupled with kinetic modelling was carried out to compare the torrefaction performance and combustion kinetics under both non-oxidative and oxidative atmospheres. Results show that solid biofuel torrefied under partially oxidative atmospheres has superior chemical functionality (e.g., less oxygenates and higher thermal stability) but comparable physical properties to inert atmospheres. A lower combustion activation energy for oxidative torrefied biomass was obtained compared to non-oxidative torrefaction.
Thirdly, the utilisation of real-time low-grade combustion waste heat and flue gases to produce torrefied biomass pellets in a fixed-bed reactor was also investigated. The fuel quality, hydrophobicity, chemical functionality inorganics contents, and combustion kinetics of the flue gas torrefied (FGT) biomass fuel were thoroughly analysed and compared with the torrefied pellets produced under partially oxidative torrefaction (POT) conditions using electrical heaters. Results revealed that the presence of carbon dioxide (and other combustion byproducts) in the flue gases did not adversely affect the quality of the torrefied biomass. In addition, low concentrations of alkali and alkaline earth metals (AAEM) were observed in FGT biomass compared to POT biomass fuel. Furthermore, it was also observed that despite known decomposition temperature ranges of individual biomass constituents, torrefaction performance indicators (mass loss, hydrophobicity, thermal stability) are significantly influenced by the heating method used for torrefaction.
Finally, the densified feedstock torrefied in oxygen concentration (O2: 5 vol%) typical of those in biomass combustion flue gases was combusted in actual reactor conditions (non-uniformities, temperature gradients) while using packed-bed geometry. The direct combustion behaviour with a particular focus on gaseous emissions of raw, blended (25% torrefied), and torrefied (100%) pellet fuels in a batch-type combustor was analysed at varying primary airflows. The results indicate that the fuels torrefied under partially oxidative conditions burned 45% faster, attained higher packed-bed temperatures (1382 °C) and exhaust gas temperatures (657 °C) than raw biomass (bed: 1128 °C, exhaust: 574 °C) at similar airflow. Additionally, 100% torrefied pellets emitted 38% less NOx compared to raw biomass pellets.
The outcome of this work provides valuable contributions that have direct relevance to sustainable and efficient torrefaction of renewable biomass. In particular, this work suggests the viability of using biomass torrefied under partially-oxidative conditions, incorporating the use of waste process heat and flue gases, to reducing the cost associated with conventional torrefaction.
Access to this thesis has been embargoed until 11th May 2025.
Riaz, S. (2023). A study into the use of torrefied biomass in a fixed bed biomass combustor. https://doi.org/10.25958/e4a4-xh77