(134f) Process Design for the Conversion of Lignocellulosic Biomass into Sustainable Aviation Fuel

Over the past century, global energy demand has followed an exponential growth trajectory. Key drivers include the expanding world population (now exceeding 7 billion) and rising welfare levels. Currently, the majority of our energy comes from fossil fuels such as crude oil, natural gas, and coal. However, the combination of high energy demand and limited fossil fuel supply has led to oil prices surpassing $100 per barrel. Unfortunately, heavy reliance on fossil fuels has also contributed to elevated CO₂ levels in the atmosphere. In recent decades, there has been growing interest in renewable energy sources such as solar, wind, and geothermal energy, as well as the utilization of biomass. Solar and wind energy are expected to gradually replace fossil fuels for heating and electricity generation. However, transitioning away from gasoline, diesel, and jet fuel in the transportation sector poses a greater challenge. While green electricity can be convenient for short-distance transport, it is not feasible for air travel or heavy long-distance transportation. Consequently, a portion of the transportation fuel sector will likely need renewable carbon-based fuels. Biomass-based renewable transportation fuels offer an attractive solution for this purpose.

At present, first-generation biofuels, such as bioethanol derived from sugar/starch crops and biodiesel obtained from oil-bearing seeds, have been successfully commercialized and are readily available in the market. However, ethical concerns related to the competition between food and fuel have hindered the widespread adoption of these initial biofuels. Consequently, there is growing interest in utilizing lignocellulosic biomass (also known as woody biomass) as a non-food carbon resource. Examples of lignocellulosic biomass include wood and agricultural residues like straw, grass, nuts, and corn stover. The precise composition of lignocellulosic biomass varies depending on the source, but it consistently contains three main components. First, cellulose (approximately 40% by weight): A high-molecular-weight polymer composed of glucose monomers. Its chemical formula can be represented as (C₆H₁₀O₅)_n, where n ranges from 500 to 4000. Cellulose forms bundled fibers that provide strength to plants and trees. Second, hemicellulose (around 25% by weight) comprising both C₆ and C₅ sugars, hemicellulose has a significantly lower average molecular weight than cellulose. It acts as a binding agent, gluing together cellulose bundles. Third, lignin (approximately 20% by weight) forms a three-dimensional network of highly branched and substituted aromatic molecules. It is covalently bound to the hemicellulose fraction .

Airlines have committed to achieving carbon-neutral growth in international commercial aviation starting in 2021. U.S. airlines have set an ambitious goal to reduce carbon dioxide (CO₂) emissions by 50% by 2050, compared the levels recorded in 2005. Although U.S. airlines have already improved efficiency by 130% since 1978, further enhancements in aircraft and engine efficiency alone may not suffice. To meet the 2050 target, they will need fuels with a lower carbon footprint, commonly known as sustainable aviation fuel (SAF). One of the challenges in providing sustainable aviation fuel (SAF) lies in the expansive and growing jet fuel market. Another significant challenge lies in the current cost of sustainable aviation fuel (SAF), which exceeds that of petroleum-based Jet A fuel. Given that fuel constituents 20%-30% of an airline’s operating expenses, this price disparity poses a hurdle. However, research and development (R&D) efforts can help reduce SAF costs. Unlike light-duty vehicles, where electrification is a viable option, the low energy density of even the best batteries severely limits opportunities for electrification in aviation. While progress is being made for smaller aircraft, airlines currently have no alternative but to rely on SAF to operate within a GHG-emission-constrained future.

Jet fuels consist of various components, including n-alkanes, iso-alkanes, cycloalkanes, and aromatics. Aromatics do not burn as cleanly as alkanes, result in higher particulate emissions and have lower specific energy. The n-alkanes are acceptable, they don’t meet fluidity and handling properties well. The iso-alkanes possess high specific energy, good thermal stability, and low freezing points. Cycloalkanes brings complementary value to iso-alkanes by providing similar benefits to aromatics, enable fuels to meet density requirements and may potentially offer seal-swelling capacity similar to that provided by aromatics. Combined, iso-alkanes and cycloalkanes can enhance fuel properties by enabling high specific energy and energy density, minimize emission characteristics. This fuel properties could lead to increased range, higher payload capacity, or fuel savings.

Various technologies are available for converting lignocellulosic biomass into transportation fuel. In general, the technologies for converting lignocellulosic biomass into transportation fuel can be categorized into thermochemical processes (gasification, pyrolysis, and liquefaction) and low-temperature processes (hydrolysis, fermentation, and anaerobic digestion). Biomass gasification aims to produce syngas, primarily composed of CO and H₂. Syngas serves as a valuable precursor for liquid fuels, such as those produced via the Fischer-Tropsch process. This work focuses on transforming lignocellulosic biomass into liquid transportation fuels through a process involving fast pyrolysis, followed by catalytic hydrotreatment and refining. During fast pyrolysis, lignocellulosic biomass is rapidly heated in the absence of oxygen to high temperatures (typically around 500°C). Biomass breaks down into vapors, aerosols, and some char and non-condensable gases. To prevent repolymerization of vapor components, the residence time of the vapors is kept short (around 2 seconds or less). The resulting vapors are then rapidly cooled to maximize the yield of a dark brown, viscous liquid known as “fast pyrolysis oil” (or bio-oil). This bio-oil can serve as a green combustion fuel or as an intermediate product for further conversions (such as hydrotreatment). It essentially represents liquefied lignocellulosic biomass. Advantages include easier transport compared to the original solid biomass and a lower ash content.

Catalytic hydrotreatment is a process in which a liquid feed reacts with hydrogen and a catalyst at elevated temperature and pressure. When applied to fast pyrolysis oil, the primary goal is to remove reactive oxygenates from the oil, enhancing its stability and reducing acidity. This hydrotreatment typically occurs in batch, slurry, or packed bed reactors. Achieving significant reductions in oxygen content requires relatively long reaction times in batch reactors (typically around 4 hours) or low space velocities in continuous reactors. Initially, catalytic hydrotreatment of fast pyrolysis oil was likened to hydrodesulfurization (HDS), a well-established process used to reduce Sulphur content in oil refinery streams. The main objective of hydrotreatment is to remove heteroatoms (such as Oxygen (O) in the case of fast pyrolysis oil or Sulphur (S) in fossil feeds) to form water and H₂S, respectively. The fast pyrolysis of biomass is already being commercialized, while the upgrading via bio-oil hydrotreating to transportation fuels has only been demonstrated in the laboratory and on a small engineering development scale. Based on the experiment results, we can explain through the simulation of the process design of pyrolysis oil upgrading via hydrotreating section is revised to incorporate the most recent improvements: a low-temperature stabilizer reactor has been added ahead of the one-stage hydrotreaters to obtain the sustainable aviation fuel.