Fluidized bed torrefaction of biomass pellets: A comparison between oxidative and inert atmosphere

Dry torrefaction and continuous thermochemical conversion for upgrading agroforestry waste into eco-friendly energy carriers: Current progress and future prospect

Agroforestry Waste (AW) is seen as a carbon neutral resource. However, the poor quality of AW reduced its potential application value. Even more unfortunately, chlorine in AW led to the formation of organic pollutants such as dioxins under higher temperatures. Alkali and alkaline earth metals (AAEMs) in ash may deepen the reaction degree. Co-pretreatment of dry torrefaction and de-ashing followed by thermochemical conversion is a promising technology, which can improve raw material quality, inhibit the release of organic pollutants and transform AW into eco-friendly energy carriers. In order to better understand the process, theoretical basis such as the structural characteristics, thermal properties and separation methods of structural components of AW are described in detail. In addition, dry torrefaction related reactors, process parameters, kinetic analysis models as well as the evaluation methods of torrefaction degree and environmental impact are systematically reviewed. The problem of ash accumulation caused by dry torrefaction can be well solved by de-ashing pretreatment. This paper provides a comprehensive discussion on the role of the two- and three-stage conversion technologies around dry torrefacion, de-ashing pretreatment and thermochemical conversion in products quality enhancement. Finally, the existing technical challenges, including suppression of gaseous pollutant release, harmless treatment and reuse of torrefaction liquid product (TPL) and reduction of torrefaction operating costs, are summarized and evaluated. The future research directions, such as vitrification of the reused TPL (after de-ashing or acid catalysis) and integration of oxidative torrefaction with thermochemical conversion technologies, are proposed.

A comprehensive review on immobilized microbes - biochar and their environmental remediation: Mechanism, challenges and future perspectives

The environment worldwide has been contaminated by toxic pollutants and chemicals through anthropogenic activities, industrial growth, and urbanization. Microbial remediation is seen to be superior compared to conventional remediation due to its low cost, selectivity towards particular metal ions, and high efficiency. One key strategy in enhancing microbial remediation is employing an immobilization technique with biochar as a carrier. This review provides a comprehensive summary of sources and toxic health effects of hazardous water pollutants on human health and the environment. Biochar enhances the growth and proliferation of contaminant-degrading microbes. The combined activity of biochar and microbes in eliminating the contaminants has gained the researcher's interest. Biochar demonstrates its biocompatibility by fostering microbial populations, the release of enzymes, and protecting the microbes from the acute toxicity of surrounding contaminants. The current review complies with the immobilization technique and remediation mechanisms of microbes in pollutant removal. This review also emphasizes the combined utilization, environmental adaptability, and the potential of the combined effect of immobilized microbes and biochar in the remediation of contaminants. Challenges and future outlooks are urged to commercialize the immobilized microbes-biochar interaction mechanism for environmental remediation.

Flue gas torrefaction of distilled spirit lees and the effects on the combustion and nitrogen oxide emission

Flue gas torrefaction (FGT) integrated with combustion was introduced for the clean treatment of distilled spirit lees (DSL). The effects of temperature, residence time, and volumetric flow rate of FGTs were investigated. The improvement in the physicochemical and combustion characteristics of the torrefied DSL and the reaction mechanisms were clarified by a tube furnace and the TG-MS device. The results showed that FGT could effectively improve the properties of DSL. With increasing temperature, residence time, and volumetric flow rate, the mass and energy yields decreased. FGT showed positive effects on the removal of free and bonding water, as well as the enrichment of lignin. FGT effectively inhibited the release of NO_x. The overall emission of NO_x was reduced by 57.3%. Additionally, the cost of DSL drying and denitrification could be greatly reduced by FGT. This study provided a practical treatment for DSL and new insight into FGT.

Synergetic Co-Production of Beer Colouring Agent and Solid Fuel from Brewers’ Spent Grain in the Circular Economy Perspective

Brewers’ Spent Grain is a by-product of the brewing process, with potential applications for energy purposes. This paper presents the results of an investigation aiming at valorization of this residue by torrefaction, making product for two purposes: a solid fuel that could be used for generation of heat for the brewery and a colouring agent that could replace colouring malt for the production of dark beers. Decreased consumption of malt for such purposes would have a positive influence on the sustainability of brewing. Torrefaction was performed at temperatures ranging between 180 °C and 300 °C, with a residence time between 20 and 60 min. For the most severe torrefaction conditions (300 °C, 60 min), the higher heating value of torrefied BSG reached 25 MJ/kg. However, the best beer colouring properties were achieved for mild torrefaction conditions, i.e., 180 °C for 60 min and 210 °C for 40 min, reaching European Brewery Convention colours of 145 and 159, respectively. From the solid fuel properties perspective, the improvements offered by torrefaction in such mild conditions were modest. Overall, the obtained results suggest some trade-off between the optimum colouring properties and optimum solid fuel properties that need to be considered when such dual-purpose torrefaction of BSG for brewery purposes is implemented.

Kinetic Study of the Isothermal Degradation of Pine Sawdust during Torrefaction Process

The reaction kinetics of solid fuel is a critical aspect of energy production because its energy component is determined during the process. The overall fuel quality is also evaluated to account for a defined energy need. In this study, a two-step first-order reaction mechanism was used to model the rapid mass loss of pine sawdust (PSD) during torrefaction using a thermogravimetric analyzer (Q600 SDT). The kinetic analysis was carried in a MATLAB environment using MATLAB R2020b software. Five temperature regimes including 220, 240, 260, 280, and 300 °C and a retention time of 2 h were used to study the mechanism of the solid fuel reaction. Similarly, a combined demarcation time (i.e., estimating the time that demarcates the first stage and the second stage) and iteration technique was used to determine the actual kinetic parameters describing the fuel's mass loss during the torrefaction process. The fuel's kinetic parameters were estimated, while the developed kinetic model for the process was validated using the experimental data. The solid and gas distributions of the components in the reaction mechanism were also reported. The first stage of the degradation process was characterized by the rapid mass loss evident at the start of the torrefaction process. In contrast, the second stage was characterized by the slower mass loss phase, which follows the first stage. The activation energies for the first and second stages were 10.29 and 141.28 kJ/mol, respectively, to form the solids. The developed model was reliable in predicting the mass loss of the PSD. The biochar produced from the torrefaction process contained high amounts of the intermediate product that may benefit energy production. However, the final biochar formed at the end of the process increased with the increase in torrefaction severity (i.e., increase in temperature and time).

Torrefaction subsequent to pelletization: Characterization and analysis of furfural residue and sawdust pellets

Torrefaction integrated with pelletization has gained increasingly interest as it enhances the characteristics of fuel pellets (e.g. hydrophobicity and energy density). In current study, torrefaction of furfural residue pellets (FRPs) and sawdust pellets (SPs) was performed by employing tubular reactor furnace, and quality of pellets was compared. The characteristics of both types of pellets were significantly improved with increasing torrefaction temperature from 200 °C to 300 °C and residence time from 15 min to 30 min. The highest lower heating value of 23.78 MJ/kg and energy density ratio (1.27) for torrefied furfural residue pellets (TFRPs) and 26.76 MJ/kg and 1.46 for torrefied sawdust pellets (TSPs) were achieved at 300 °C and 120 min. Increasing torrefaction temperature and residence time, the volumetric energy densities of TFRPs increased from 25.69 (at 200 °C and 15 min) to 27.59 kJ/m³ (at 300 °C and 120 min), while those of TSPs correspondingly decreased from 20.81 to 16.69 kJ/m³. The highest true densities (i.e. 2.40 and 1.85 g/cm³) and porosities (i.e. 52 and 65 v %) of TFRPs and TSPs were achieved at 300 °C and 120 min, much higher than those of un-torrefied pellets. Moisture uptake of TFRPs and TSPs at 300 °C were only 1.4 wt% and 2.0-2.8 wt%, respectively, showing strong water-resistant ability. The crystallinity of cellulose in FRPs was found higher than that of SPs, while the crystallinity of cellulose in TFRPs was found lower than that of TSPs at same process conditions. FTIR showed that O-H bond was destroyed after torrefaction for both FRP and SP.

Biochar production and applications in agro and forestry systems: A review

Biochar is a product of biomass thermochemical conversion. Its yield and quality vary significantly with the production technology and process parameters, which also affect its performance in agro and forestry systems. In this review, biochar production technologies including slow pyrolysis, fast pyrolysis, gasification, and torrefaction were compared. The yield of biochar was found to decrease with faster heating rate or more oxygen available. The benefits of biochar application to agro and forestry systems were discussed. Improvements in soil health, plant growth, carbon sequestration, and greenhouse gas mitigation are apparent in many cases, but opposite results do exist, indicating that the beneficial aspect of biochar are limited to particular conditions such as the type of biochar used, the rate of application, soil type, climate, and crop species. Limitations of current studies and future research needed on biochar are also discussed. Specifically, the relationships among biochar production technologies, biochar properties, and biochar performance in agro and forestry systems must be better understood.

Thermochemical Conversion of Olive Oil Industry Waste: Circular Economy through Energy Recovery

The demand for new sources of energy is one of the main quests for humans. At the same time, there is a growing need to eliminate or recover a set of industrial or agroforestry waste sources. In this context, several options may be of interest, especially given the amounts produced and environmental impacts caused. Olive pomace can be considered one of these options. Portugal, as one of the most prominent producers of olive oil, therefore, also faces the problem of dealing with the waste of the olive oil industry. Olive pomace energy recovery is a subject referenced in many different studies and reports since long ago. However, traditional forms of recovery, such as direct combustion, did not prove to be the best solution, mainly due to its fuel properties and other characteristics, which cause difficulties in its storage and transportation as well. Torrefaction and pyrolysis can contribute to a volume reduction, optimizing storage and transportation. In this preliminary study, were carried out torrefaction and pyrolysis tests on olive pomace samples, processed at 300 °C, 400 °C, and 500 °C, followed by laboratory characterization of the materials. It was verified an improvement in the energy content of the materials, demonstrating that there is potential for the use of these thermochemical conversion technologies for the energy recovery of olive pomace.

Bio-oils from vacuum ablative pyrolysis of torrefied tobacco residues

Thermochemical conversion of tobacco residues to value-added bio-fuels and chemicals via fast pyrolysis, in combination with torrefaction pretreatment, in a rotating blade ablative reactor under vacuum conditions.