Over the years, many researchers and experts have been drawn to the potential of torrefying biomass materials to produce a superior solid biofuel. This process enhances durability, improves grindability, increases bulk density and calorific value, and ultimately results in higher energy density compared to the untreated biomass.

During the torrefaction process, the researchers heat the original biomass to temperatures ranging from 180°C to 350°C. This heating partially devolatilizes the biomass, leading to a decrease in solid mass. The solid product retains the initial energy content of the feedstock biomass. The torrefied biomass has a higher energy density than the original biomass when mass loss exceeds energy content loss, which makes it more attractive for application in the transportation sector.

Torrefaction is an additional step in the chain of biomass processing. However, recirculating the heat from gas combustion results in savings in drying, as well as in other steps in the biomass usage chain – such as logistics, storage, handling, milling, and combustion – offsetting the associated capital and operating expenses, along with conversion losses. Hence, biofuel production through torrefaction of biomass is an appealing option for carbon net zero energy generation. 

Current Research into Torrefied Biomass for Biofuels:

Researchers from the Faculty of Engineering Technology, University of Twente, The Netherlands, compared fast pyrolysis experiments for biofuel production from raw and torrefied woody biomass feedstocks, using a 500g/h entrained down flow reactor. The researchers used hardwood (ash wood), softwood (spruce wood), and mixed waste wood as the feedstocks. They torrefied these feedstocks using the Torbed® method, which involves a directly heated moving bed, at various temperatures ranging from 250°C to 300°C. The effect of pelletising was also analysed for the hardwood feedstock, comparing torrefied chips and torrefied pellets. In comparison to bio-oils from raw feedstock, the bio-oils derived from tests with torrefied feedstock exhibited generally better oxygen and heating value qualities.

Hardwood pellets torrefied at 265°C with a residence time of 45 minutes produced bio-oils with the highest quality with respect to oxygen mass fraction and higher heating values, with the former decreasing from 45.7% to 37.2% and the latter increasing from 19.1MJ/kg to 23.1MJ/kg, compared to the non-torrefied bio-oil from hardwood feedstock. But the bio-oil yield suffered significantly as a result of this temperature and duration, falling from 44% for raw feedstock to an average of 31% for torrefied feedstock. However, a chain analysis revealed that a torrefaction pre-treatment can be more appealing on an energy basis than a typical rapid pyrolysis process with a deoxygenation upgrading phase.

Maximizing Torrefied Biomass Energy Density: Torrefaction Advancements:

Researchers in the Department of Mechanical Engineering, Kasetsart University, Thailand, examined the torrefaction performance and characteristics of sugarcane bagasse in an inert environment. The researchers conducted investigations into the effects of torrefaction temperature and duration on mass yield, enhancement factor, energy yield, and chemical-physical properties. The researchers used proximate and ultimate analyses, lignocellulose components, thermogravimetric analysis, and Fourier Transform Infrared (FTIR) spectroscopy to characterize the torrefied biomass.

The study found that the torrefaction temperature had an impact on the chemical and physical qualities, while the effects of residence time were insignificant. Raising the torrefaction temperature up to 275°C led to an increase in the energy density by 20% and decreases in the mass yield and energy yield from 80% to 54% and 89% to 69%, respectively. The researchers found that the torrefaction process altered the original biomass by degrading carbohydrates and increasing lignin, while also decreasing hemicellulose. The final solid biofuel was found to be similar in quality to that of low-rank coal and lignite.

Researchers at Iowa State University’s School of Agricultural and Biosystems Engineering in US investigated torrefaction process parameters in an effort to increase the energy density of biomass and decrease its moisture content. The researchers used three distinct temperatures (200°C, 250°C, and 300°C), three different reaction periods (10 minutes, 20 minutes, and 30 minutes), and three different moisture content levels (30%, 45%, and 50%) to torrefy the biomass made from corn stover. The researchers studied the resulting solid, liquid, and gaseous biofuels, and they determined the mass and energy balance of the reactions. After torrefaction, they found that the energy density increased by 2-19%, while the mass decreased by 3-45%. Additionally, the energy yield decreased by 1-35%. The initial biomass moisture content, mass loss, and energy loss after torrefaction were proportional with one another.

Optimal Biochar Production from Ground Coffee Residue via Torrefaction:

Ground Coffee Residue (GCR) underwent torrefaction in the Department of Environmental Health at Valaya Alongkorn Rajabhat University, Thailand, to produce biochar under both nitrogen and carbon dioxide atmospheres.The researchers conducted several experiments in which they varied the process temperature between 200°C and 300°C, altered the residence time between 30 minutes and 60 minutes, and varied the sweeping gas flow between 50 ml/min and 250 ml/min. The researchers found that the optimum condition for producing biochar from GCR was high temperature torrefaction at 300°C with a residence time of 30 minutes under a low carbon dioxide flow rate of 50 ml/min.

The higher heating value of this biochar was 31.1MJ/kg, with the energy yield equal to 48% with H/C and O/C atomic ratios equal to 0.94 and 0.14 respectively. These attributes are comparable with sub-bituminous coal. Torrefaction resulted in significant losses of oxygen and hydrogen, according to calculations for decarbonisation, dehydrogenation, and deoxygenation. Furthermore, the influence of sweeping gas had only a minimal impact on the qualities of the produced biochar with temperature being most important, according to an investigation of the thermal breakdown properties and surface chemical functional groups.

Optimizing Corncob Biomass Torrefaction for Biofuel Production:

Researchers at the Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China, torrefied corncob biomass under nitrogen and carbon dioxide atmospheres at temperatures ranging from 220°C to 300°C. The researchers created solid biofuels and achieved mass yields ranging from 69% to 95%, along with higher heating values ranging from 16.5 MJ/kg to 24.8 MJ/kg for the specified range of temperatures, when conducting tests within a nitrogen atmosphere. Torrefaction adopting a carbon dioxide atmosphere over the same range of temperatures produced solid biofuels with mass yields of 67-95% and higher heating values of 16.7-24.1MJ/kg. Hemicelluloses were not detected during torrefaction under high temperature conditions.

However, as the researchers increased temperatures, they observed an increase in carbon concentrations of the torrefied biomass, accompanied by a reduction in hydrogen and oxygen concentrations. Combustion of the produced biofuels showed that the burnout temperature of the solid biofuels decreased at higher reaction temperatures with higher torrefaction temperatures resulting in shorter combustion times of the produced biofuel overall. After conducting all the testing, the researchers concluded that torrefaction at 260°C using a carbon dioxide atmosphere was the most optimal condition for the production of biofuel from biomass.

The Future of Biofuel Production from Torrefied Biomass:

The market for torrefaction of biomass to solid biofuels is set to grow in the upcoming decade. The major factors driving this growth are the increasing demand for biofuels as an alternative energy source, and stringent environmental regulations imposed by governments across the globe. Environmental regulations on coal-fired powerplants have led to increased consumption of biomass, since the carbon dioxide produced is within a carbon cycle as compared to fossil-based solid fuels. Furthermore, biofuels produced from biomass through torrefaction tend to emit less sulphur dioxide compared to traditional solid fossil fuels.

However, as the usage of biofuels become more common, they create a negative impact risk due to potential shortages in sustainable biomass availability in the long-term. To control this risk and prevent negative effects from moving up the supply chain, there is a need for global uniform standards and sustainability criteria. While there is no silver bullet for a scalable net zero emission fuel, currently available biofuels can offer the flexibility of an interim drop-in solution or act as a net zero pilot fuel for other net zero emission fuels under development.