Regulations and investments in new technologies for biofuel production as an alternative to fossil fuels have boosted in the last years due to concerns about climate change and anthropogenic CO2 emissions. A valuable supply chain of biomass for biofuel production is one the most significant factors, in order to support the market’s demand. A promising feedstock that is widely available and considered a residue can be Namibia’s encroacher bush (EB). Namibia is facing an environmental crisis with bush encroachment, where EB takes over grasslands, reducing biodiversity and groundwater availability. A thermochemical process that can take advantage of EB can be Hydrothermal Liquefaction (HTL). With HTL, biomass feedstocks are liquefied under hot compressed water, producing bio-oil (BO), biochar (BC), an aqueous phase (AP) and gases. Catalytic HTL is a more attractive pathway, due to the increased BO yield and quality. To this day, there has not been a study investigating the potential of catalytic HTL with EB, thus the present MSc Thesis will attempt to close that knowledge gap and provide state-of-theart insight.

To achieve that, Acacia Mellifera from Namibia was tested in sub-critical HTL conditions. In particular two campaigns were formulated with two different goals. The first one, focused on the selection of a suitable catalyst among 4 different categories of catalysts (zeolites, alkaline earth metals, lanthanides and transition metals). The catalysts performance was evaluated by comparing
the Energy Recovery (ER) under same operational conditions. Then, the catalyst with the highest ER was used in the second experimental campaign using a Desing of Experiments approach. This approach had the goal to optimize HTL reaction conditions -temperature, residence time and catalyst loading- for maximizing BO yield and energy content. Central Composite Design (CCD) of experiments was used, with the parameters ranges being 250-340oC, 5-60mins and 0-10wt%, respectively. Selected BO and BC samples from both campaigns were then characterized using various methods.

The HTL experiments with EB revealed that BO from EB could be produced. The highest ER obtained from the catalyst screening campaign was 41.1% . Main organic compounds found in all the BOs were phenolic derivatives, alicyclic ketones and fatty carboxylic acids. Using the best performing catalyst based one ER, the CCD model indicated that the optimum conditions for maximizing BO yield were 340oC, 60 minutes and 5wt%, which yielded 27.0wt% BO. However, the 330oC - 60 minutes - 7.5wt% point gave both the highest yield and highest ER, 28.5wt% and 46.2% respectively. The CCD also reduced the O content in the BO samples by 54%. Finally, BC samples showed fuel characteristics similar to lignite and low concentration of heavy metals, making them legitimate alternatives for solid fuels or soil amendment.

The global temperature rise has pushed governments more into finding ways to reduce CO2 emissions, by increasing the use of renewable fuels. Lignocellulosic biomass (wood, forest residues, agricultural residues) is a renewable fuel that has still not been utilized to its full potential. That is because it lacks properties such as homogeneity, high volumetric energy density and low moisture that its main fossil fuel competitor -coal- has. Therefore, a type of pre-treatment is needed for these disadvantages. Torrefaction, a process of heating biomass (200-300 oC) in an inert environment (no combustion), provides a product with improved physicochemical properties: reduced hydrophobicity, easier grindability, homogeneity, reduced microbial activity and increased calorific value. However, it is not enough to torrefy the biomass, as it needs to be densified in order to be utilized. With the process of pelletizing, a type of densification, torrefied biomass becomes compact and can be considered as bio-coal. While pelletizing is influenced by many parameters (moisture content, die pressure, torrefaction temperature etc), herbaceous types of biomass such as wheat straw, hay, reeds and grass, still cannot produce quality pellets without the addition of a binder. Typical organic binders such as starch, lignin and sawdust can either be expensive or biologically degrade in storage conditions. That is why addition of plastic binder should be considered. The aim of this report was to investigate how quality torrefied pellets from herbaceous biomass can be produced with the addition of plastic binder in a commercial capacity. Wheat straw and woodchips were used as feedstock, while polyethylene resin was the plastic binder. The biomasses were torrefied at 240 oC and 270 oC on the Torrgreen facilities, via a pilot-scale packed bed reactor, which recycles the volatiles from torrefaction for inertization and pelletized with a 100 kg/h pellet mill. For the torrefied wheat straw, a design of experiments was formed, to investigate how 4 parameters for pelletizing (torrefaction temperature, moisture content, plastic binder addition and pellet diameter) improved mechanical durability. Results indicated that 5% of plastic improved durability and pellet formation in total, while higher torrefaction temperature weakens the pellets and aggravates pellet formation. The highest durability achieved was 90.3%. Torrefied wood chips were pelletized for reference and showed low durability, mostly due to inability of the pelletizer. Water immersion tests on the straw pellets showed that higher torrefaction temperature increased hydrophobicity a lot, while plastics also had a positive effect. Comparison of the straw pellets produced with conventional wood pellets revealed that the former lack in mechanical durability and are high in ash content, making them incompliant with current standards. Overall, inclusion of plastics in pelletizing of torrefied herbaceous biomass, demonstrated very positive results and improved their quality, while making the operation smoother, but for scaling up and keeping the whole process sustainable a polyethylene (or other plastic) waste stream should be utilized.