How do we decarbonize planes?
An overview of biomass-to-kerosene processes
I made a video summary of what I explained here
Every time I get on a plane, my favourite part is the moment of lift-off. The plane accelerating down the runway, everyone in anticipation, as the powerful engines thrust us up into the air, leaving everything behind.
These moments are memorable to me as I sit in awe of this impeccable engineering in action. I also feel a little guilty, as I know that this plane is contributing to the ever-growing problem of climate change.
This is why I spent the last 2 months trying to understand how we can decarbonize our flying experiences. This paper summarizes what I’ve learned about renewable aviation fuels, an important technology in the fight for a greener future.
Abstract
The aviation industry has the aspiration of achieving 0 emissions by 2050 by investing in renewable alternatives that can make their planes fly. Yet there are many problems that arise from the renewable alternatives we have. For example, green hydrogen is prohibitively costly, lithium-ion batteries can’t survive the 10 to 20-hour flights demanded of them and biofuels have a high freezing point making them unsuitable for high altitudes. That leaves us with one of the most promising renewable alternatives, which is synthetic aviation fuels also known as renewable fuels. Renewable fuels are manufactured using biomass and converting it into hydrocarbons that are then processed to make an equivalent aviation fuel (aviation fuel = kerosene). This review paper will explain different ways researchers are trying to convert biomass to make renewable kerosene.
Outline Of Paper
- Introduction
2. State Of The Industry
2.1 Why Kerosene is the fuel of choice in planes
2.2 Parameters for a good fuel
3. ASTM-approved aviation biomass-to-kerosene processes
3.1 Fischer Tropsch
3.2 HEFA
3.3 Synthesised iso-paraffins (SIP)
3.4 Alcohol to jet (ATJ)
3.5 HH-SPK
3.6 CHJ
4. Potential biomass-to-kerosene processes pending ASTM approval
4.1 Pyrolisis oil-derived fuels
5. Conclusion
1. Introduction
The global aviation industry produces around 2.1% of all human-induced carbon dioxide (CO2) emissions. That's around 1 billion tons of CO2 a year. Currently, the aviation industry seems to be the hardest industry to disrupt in a renewable way. Why is this? Is it because the aviation industry isn’t trying to innovate?
On the contrary over the past few years, huge aviation companies such as Boeing and Airbus have been trying to move towards a sustainable way of flying. They’ve had to place their bets on 4 alternatives: lithium-ion batteries, hydrogen, biofuels and renewable fuels.
Lithium-ion batteries are weight sensitive, meaning the more batteries you stack to have more energy density and go longer distances, the more weight you accumulate. This means that to have the 5 to 20-hour flights demanded of these battery packs you would need way fewer passengers and way less cargo for it to be aerodynamically stable. This is why most electric planes are more like helicopters than Boeing 747s.
Hydrogen has been touted as an ideal solution as it could potentially solve all the problems in aviation as well as many others, such as steel production. Hydrogen can be separated into two types of hydrogen. Blue hydrogen and green hydrogen.
Blue hydrogen uses a process called steam reforming to heat up water and separate the hydrogen atoms from the oxygen atoms in a relatively cheap way. But, there is a catch, steam reforming uses fossil fuels.
So then you are left with green hydrogen, which uses electrolysis to produce hydrogen without fossil fuels. But green hydrogen is only 70% energy efficient and very costly.
Biofuels are another popular alternative, not only in the aviation space but in the trucking industry as well. However, biofuels are Fatty acid methyl esters (FAME) which have several problems when it comes to aviation including:
- Higher freezing point: FAME freezes at 1˚C which makes it unusable in planes where you are flying at high altitudes
- Higher pH: You have to create new types of engines as the pH of Fatty acid methyl esters is more acidic which means they can rust and corrode engines meant for kerosene.
- Lower energy density: FAME has a lower energy density than kerosene, which means it can only be blended in small proportions.
That leaves us with one other alternative. Renewable fuels (also known as synthetic aviation fuels), unlike any other alternative I just mentioned, are able to fit with all of the existing infrastructure put in place by the aviation industry.
Renewable fuel is a term for a fuel that has the same chemical formula as regular kerosene but was produced from biomass. Biomass takes carbon out of the air when it grows which offsets the emissions when it is burned.
This is amazing on so many levels! This means (in theory) that you could replace all of the kerosene used today in planes with synthetic aviation fuels.
From what we know now let’s jump into the world of synthetic aviation fuels looking at different processes that can convert biomass to kerosene!
2. State of the Industry
Aviation fuel, as most of us call it, is actually just a distillate of petroleum that we call Kerosene. Kerosene is a combustible mix of different paraffinic hydrocarbons that are used in jet engines and sometimes rocket engines. These hydrocarbons usually have a carbon number range of 10 to 16 carbon atoms.
The chemical composition of kerosene is fairly complex, it is a mixture of paraffins (55.2%), naphthenes (40.9%), and aromatic hydrocarbons (3.9%). Paraffins are saturated hydrocarbons (alkanes), naphthenes are cycloalkanes (closed-loop alkanes) and aromatic compounds are ones with benzene rings.
2.1 Why Kerosene is the fuel of choice in planes
Commercial planes usually use 3 types of fuels. Jet A-1, Jet A and Jet B fuels. Jet A-1 is used all around the world, it has a low freezing point of -47 Celsius. Jet A fuel is cheaper than Jet A-1 and is used mostly in the United States as it has a higher freezing point of -40 Celsius. Jet B fuel is used in Canada and has a freezing point of -47 degrees Celsius.
All this is to say that these 3 fuels are just a variation of kerosene. So why is kerosene the fuel of choice in the aviation industry?
Low freezing point: Planes fly at extremely high altitudes, meaning they spend a lot of time in sub-zero temperatures. As a result, planes need to use fuel with a low freezing point so the fuel functions properly without solidifying during the flight.
The long chain-like structure of kerosene’s molecules makes it hard for them to stick together and form a solid. Instead, the molecules are able to move around more freely even when they start to get cold, which prevents them from sticking together and forming a solid.
Highly flammable: The high flammability of kerosene is due to the high energy content of its molecular bonds. When kerosene is exposed to a source of heat or a spark, the bonds between the carbon and hydrogen atoms break apart. This releases a large amount of energy in the form of heat and light, which results in a flame.
The longer carbon chains in kerosene molecules contain more bonds between carbon and hydrogen atoms, which means more energy is released when these bonds are broken. This makes kerosene more reactive and more likely to ignite.
Low viscosity: Viscosity is a measure of a fluid's resistance to flow. Liquids with high viscosity are thick, sticky and gluey, this is not ideal as you don’t want your fuel to clog up your engine. Kerosene maintains a low viscosity during flights thanks to its low freezing point.
The more complex a chemical structure is the more friction and the higher viscosity it will have. Kerosene’s molecular structure is composed of relatively simple chains of alkane hydrocarbons.
High flash point: Kerosene has a high flash point, which means it requires a higher temperature to ignite than some other liquid fuels. This makes it a safer fuel option in the event of an accident or crash.
2.2 Parameters for a good aviation fuel
Now that we know why kerosene is the most utilized fuel in the aviation sector now let’s look at what are the parameters we need to meet to compete with kerosene. We will rate our biomass to kerosene processes based on the following parameters…
Low-temperature fluidity: Low-temperature fluidity is one of the most important characteristics of bio jet fuels. Lower temperature fluidity refers to the ability of a fluid to flow at low temperatures without becoming too thick or viscous. We can measure temperature fluidity using a pour point, meaning the lowest temperature at which a fluid will flow when it is tilted in a test container.
Freezing point: As mentioned before, bio-kerosene has to have a freezing point in the range of -40 to -47 degrees Celsius.
Kinematic viscosity: The kinematic viscosity is another parameter that is commonly used to characterize the low-temperature fluidity of aviation fuel. Kinematic viscosity is defined as the viscosity of a liquid divided by its density at the same temperature. For aviation fuels the kinematic velocity has to be below -20 degrees Celsius
Thermal oxidation stability: The biofuel has to not oxidize over high-temperature stress in the presence of oxygen. When jet fuel oxidizes, it undergoes a chemical reaction that releases energy in the form of heat and light, which can cause fires and explosions. Overall, preventing jet fuel from oxidizing is critical for ensuring safe and reliable air travel.
Cetane number: Derived Cetane Number (DCN) is a representative for the efficiency of fuel. A higher DCN means a shorter ignition delay time, which allows fuels to combust more completely. Consequently, better combustion performance with more power and fewer harmful emissions can be achieved. Kerosene has a cetane number in the range of 40 to 52, usually around 49.
Cost: You could have 10/10 in all of these parameters but if your renewable fuel costs more than that of kerosene, no one is going to take it. Currently, the price of kerosene is around $2.5 (US dollars) per Gallon.
Now that we know what parameters to fit, let’s look at different biomass-to-kerosene processes to determine which ones are the most efficient and affordable.
3. ASTM-approved aviation biomass-to-kerosene processes
The ASTM (American Society for Testing and Materials) over the past few years has approved 7 renewable fuel processes in a maximum of 50% blends with kerosene. Let’s look at these 7 processes and see how they match up with each other. We will start from the first to be approved, Fischer-Tropsch (FT-SPK), in 2009 to the last to be approved, catalytic hydro thermolysis to jet (CHJ), in 2020.
3.1 Fischer Trospch
During the Second World War, the Germans were facing a big problem. They were rich in coal and biomass but low on petroleum. Since no one would sell them petroleum, how were they going to fuel their tanks and jets to fight this war?
Thankfully or should I say disastrously for the Allies, a few years back, 2 german chemists named Franz Fischer and Hans Tropsch discovered a way to create fuels out of coal, biomass or natural gas. These fuels were used by the Nazis as ersatz (replacement) fuels. Fischer Tropsch fuel production accounted for an estimated 9% of German war production of fuels and 25% of automobile fuel.
Fischer Tropsch is a process that converts syngas (CO), derived from the gasification of some sort of feedstock, into liquid hydrocarbons using hydrogen at high temperatures in the presence of a catalyst.
The process can be defined in 3 steps: gasification and syngas cleaning, Fischer Tropsch reaction, catalytic cracking.
Step 1: Gasification and Syngas cleaning
Gasification is a process where you heat up biomass or any carbonaceous feedstock to produce syngas or synthetic gas (CO). This is done by reacting the feedstock material at high temperatures (typically >700 °C) and controlling the amount of oxygen and steam in the reactor to prevent combustion.
The process begins with the loading of biomass into the gasifier. This can be represented by the formula here:
The gasifier produces a gas rich in carbon monoxide and hydrogen. This is then separated into syngas, hydrogen, and other constituents. The syngas is then cooled and cleaned to remove impurities such as sulphur, nitrogen, and ash. This is done using a series of scrubbers and filters.
Here’s a helpful diagram with the components of this process.
Now that we have cleaned our syngas from particulates and sulfuric acid, our syngas is ready to liquefy.
Step 2: Fischer Tropsch reaction
The Fischer Tropsch process liquefies the syngas into long paraffinic hydrocarbons by heating up the syngas in the presence of hydrogen and a catalyst. The catalyst is typically a mixture of iron, cobalt, or nickel. We can also use some of the hydrogen produced from the gasification process to lower the amount of expensive green hydrogen you need to use.
The formula for the Fischer Tropsch process can be defined by:
Step 3: Catalytic cracking
Now that we have our long hydrocarbon chains we use a process called catalytic cracking to get the final product in the carbon range we want (10 to 16 for kerosene).
Catalytic cracking is a chemical process used to break down large hydrocarbon molecules into smaller ones. The hydrocarbon feedstock is heated up at high temperatures and then mixed with a catalyst, usually zeolite.
To Summarize: In that 3 step process, we went from biomass to syngas to long hydrocarbon liquid chains to kerosene-length paraffinic chains.
Pros:
- Any type of feedstock can be converted to kerosene
- Gasification produces many useful products including green hydrogen
Cons:
- Uses green hydrogen
3.2 HEFA
In the HEFA process also known as the HVO process, vegetable oil is hydrotreated to create kerosene. Vegetable oils are Fatty acid methyl esters (FAME). As said previously these are long chains of H and C atoms with oxygen atoms and double bonds between them.
To convert these vegetable oils to jet fuel we must first clean the oils to remove impurities. Then, we remove the oxygen molecules from the oil and convert any olefins to paraffins by reacting the oils with hydrogen in a process called hydrodeoxygenation and hydrotreating.
This process uses hydrogen and a catalyst at high temperatures and pressures to make the hydrogen bond with the fatty acid methyl esters. This saturates the FAME hydrocarbons, getting rid of the carbon double bonds and the oxygen.
The removal of the oxygen atoms raises the heat of combustion of the fuel and the removal of the olefins increases the thermal and oxidative stability of the fuel.
After the fatty acid methyl esters are converted to long alkane chains. A second reaction cracks the paraffins with carbon numbers in the jet range. This is usually done with catalytic cracking.
Pros:
- Creates high-quality kerosene
Cons:
- Uses a lot of expensive green hydrogen
- Limited feedstock (can only use certain types of vegetable oils)
3.3 Synthesized iso-paraffins (SIP)
Synthesized iso-paraffins (SIP) is a process that produces kerosene from the hydroprocessing of farnesene. This process was certified for up to 10% blends for jet fuel in 2015.
Farnesene is a chemical product derived from sugarcane biomass through fermentation. It is an alkene with the chemical formula of C15H24.
SIP is the process of fermenting sugarcane into this farnesene product in order to hydrogenate it and turn it into kerosene. This hydrogenation process (similar to the HEFA process described above) uses hydrogen and a catalyst to saturate the farnesene molecule with hydrogen thus breaking down the carbon double bonds and obtaining paraffinic hydrocarbons.
The results thus far, yield a highly-viscous hydrocarbon similar to kerosene but with poorer combustion qualities. This is why only up to 10% of this fuel is allowed to be blended with current jet fuels.
Pros:
- Farnesene is derived from sugarcane and lignocellulosic biomass, which is readily available
Cons:
- Only used in 10% blends
- Highly viscous
- Uses green hydrogen
3.4 Alcohol to jet (ATJ)
ATJ is a process where you convert alcohol feedstocks into hydrocarbon fuels. There are two approved processes that use 2 different types of feedstocks. The first process was approved in 2016 and converts isobutanol into a kerosene blend of 50%. The second process was approved in 2018 and converts ethanol into kerosene, blends of max 50%. It can potentially also use different types of feedstocks sugarcane, lignocellulosic biomass or even fermented microbes.
We’ll explore both alcohols (isobutanol and ethanol) and then look at how they are converted into pure hydrocarbon fuels.
Isobutanol is an organic compound with the formula (CH3)2CHCH2OH. It is an isomer, meaning one of its hydrogens is replaced with the hydroxyl (OH) functional group. Meaning it has oxygen which is not wanted in our end product.
Ethanol is an isomer where one of the ethane’s hydrogen atoms is replaced with a hydroxyl group (OH). This too has oxygen which destabilizes the fuel. Ethanol is usually produced from corn and its formula can be written as C2H6O or C2H5OH. Represented by the image below…
The ATJ process can convert both of these alcohols into pure hydrocarbons (hydrocarbons without oxygen) with the carbon numbers required for kerosene in 4 steps dehydration, oligomerization, hydro deoxygenation, and fractionation.
Step 1: Dehydration
The definition of dehydration is a chemical reaction that removes water molecules. We take a mixture of isobutanol and water or ethanol and water and remove the water molecules. This leaves us with pure isobutanol or ethanol molecules.
Step 2: Oligomerization
Oligomerization is a chemical process that involves combining multiple smaller molecules, called monomers, to form larger molecules, called oligomers.
We take the ethanol or isobutanol molecules after dehydration (which are monomers) and we combine these monomers, oligomerizing them into longer chain hydrocarbons usually by using a catalyst. After this process, you have longer chain hydrocarbons.
Step 3: Hydro deoxygenation
After this, these longer-chain hydrocarbons are hydro deoxygenated into long paraffinic hydrocarbons without oxygen. This happens through the use of a catalyst and hydrogen at high temperatures and pressures.
Step 4: Fractionation
We fracture (through catalytic cracking) the hydrocarbon chains into lighter hydrocarbons in the carbon number range we want for kerosene. This gives us a kerosene product that is ready to be used for fuel.
Pros:
- Has two ASTM-approved feedstock processes
- Produces high yields of kerosene
Cons:
- Uses a lot of green hydrogen
3.5 Hydroprocessed hydrocarbons-synthesized isoparaffinic kerosene (HH-SPK)
HH-SPK is a process that involves the hydroprocessing of a specific type of algae called Botryococcus braunii. The HH-SPK process involves several stages, including biomass cultivation, harvesting, lipid extraction, and hydroprocessing.
The first stage of the HH-SPK process is biomass cultivation, which involves growing the Botryococcus braunii algae in a suitable environment. This is normally done in a closed bioreactor.
The second stage of the HH-SPK process is harvesting, which involves taking the cultivated algae and cleaning it.
The third stage of the HH-SPK process is lipid extraction, which involves separating the lipids from the harvested algae. The extracted lipids are then purified to remove any impurities or contaminants that could affect the hydroprocessing stage.
The fourth stage of the HH-SPK process is hydroprocessing, which involves converting the bio-derived hydrocarbons from the algae into pure hydrocarbon fuel.
The resulting HH-SPK fuel is a high-quality, drop-in replacement for conventional jet fuel that meets the same performance and safety requirements.
Pros:
- Uses algae which takes a lot of carbon out of the air
Cons:
- Uses green hydrogen
- Uses a specific type of algae and can’t use a wide range of feedstocks
3.6 Catalytic Hydrothermolysis Jet fuel (CHJ)
The CHJ process is an innovative approach to converting triglycerides (fats) into kerosene. In the CHJ process, fatty acid waste oils are combined with preheated feed water at very high temperatures and high pressures (supercritical conditions) to convert the triglycerides to non-ester renewable fuels.
This process consists of several steps, including preconditioning, catalytic hydro thermolysis conversion, and post-refining.
Step 1: Preconditioning
We begin with a triglyceride which is an ester derived from glycerol and three fatty acids. It can be represented by this…
The preconditioning step involves modifying the triglycerides by using a series of reactions including conjugation and cyclization. The aim of this step is to improve the overall efficiency of the process, as preconditioned triglycerides give better yields of jet fuel.
Conjugation involves the linkage of 2 fatty acids together to improve stability by alternating the single bonds of the fatty acids with the double bonds.
Cyclization involves the process of forming a cyclic molecule from an open chain forming a closed loop or ring to achieve greater stability. In this context, the triglyceride chains after being conjugated form a closed-loop and form cyclic chains.
Now that these 2 reactions are done we’ve preconditioned the triglycerides so that they can be turned into more efficient kerosene yields.
Step 2: Hydro thermolysis conversion
In this step, the preconditioned triglycerides are put into a reactor and mixed with water. The fats and water are heated at extreme temperatures and pressures to reach a supercritical state, where they are subjected to hydrothermal treatment using a catalyst.
Hydro thermolysis conversion breaks down the triglycerides into smaller molecules, such as fatty acids, glycerol, and other byproducts, which can then be further processed into synthetic kerosene.
Step 3: Hydrotreatment and fractionation
Hydrotreatment is the final step in the CHJ process and involves taking the byproducts from the hydro thermolysis conversion reaction and hydrotreating it (using hydrogen, a catalyst and heat) to create long hydrocarbon chains.
The heavy hydrocarbon chains are then fractionated (using catalytic cracking) into lighter hydrocarbon chains in the carbon range needed for kerosene.
Pros:
- High yields of high-quality kerosene
- Produces hydrogen
- Feedstock variability
Cons:
- Uses a lot of energy to heat up at supercritical conditions, which can be a reason for the high cost
- Uses green hydrogen
4. Potential biomass-to-kerosene processes pending ASTM approval
There are also new up-and-coming biomass-to-kerosene processes (pending ASTM approval) that may be used in the aviation industry. Here’s one that I thought was promising.
4.1 Pyrolysis oil to kerosene
Pyrolysis is a new and innovative way of heating up biomass to produce 3 products: syngas, bio-char and bio-oil. Biochar is useful for agriculture and syngas is useful (as mentioned before) for kerosene production.
Most researchers are looking at how they can convert the third product (bio-oil) into kerosene through hydrogenation. This would be useful as pyrolysis can produce hydrogen. The representation of this is shown here:
Pros:
- Produces hydrogen
- Produces many useful byproducts including synthetic gas and biochar
- It can use a wide variety of feedstocks
Cons:
- Uses green hydrogen
- Not yet approved for aviation fuel blends
5. Conclusion
In conclusion, there are many different ways of producing kerosene in a renewable way through biomass. This is very exciting as biomass to kerosene processes can fit with already-made infrastructure, potentially reducing emissions globally by 2 to 3%. Though this is great on paper there are a lot of problems that I’ve noticed when it comes to these processes.
First and foremost, most of these processes make more costly kerosene than regular kerosene derived from crude oil. There are many factors that explain this, but the biggest one I’ve noticed by writing this article is that all of these ASTM-approved processes use green hydrogen which is costly. Though, some of these processes such as Fischer Tropsch and CHJ produce a smaller amount of green hydrogen through their reactions, which therefore offsets some of the cost.
The second problem I’ve noticed with renewable fuels is supplying biomass feedstock for kerosene. Many of these processes rely on one or two feedstocks which can create supply chain issues. This is the case with the HH-SPK process which only uses a certain type of algae and the ATJ which only uses certain types of alcohols (isobutanol and ethanol). Though the CHJ and Fischer Trospch processes have more flexibility when it comes to feedstock options.
Third, it has been difficult to determine the actual quality of the kerosene produced. Is it able to compete with normal kerosene? This question was not answered in most of the research papers I read, but the answer that seemed to be eluded to was: “not yet”. In order to assess the quality of the kerosene and the feasibility of the process, more insight into the costs and cetane numbers for each of these fuels would be required. This information is required to conclude on which process is the best.
I’m excited that there is a large amount of research being done to decarbonize planes by transforming biomass into renewable kerosene, even though I still feel like we might be a ways away from a “guilt-free” lift off.
Sources:
- https://figshare.com/articles/journal_contribution/Preparation_of_jet_engine_range_fuel_from_biomass_pyrolysis_oil_through_hydrogenation_and_its_comparison_with_aviation_kerosene/7583891
- https://www.aidic.it/cet/12/29/192.pdf
- https://onlinelibrary.wiley.com/doi/abs/10.1002/ceat.201400648
- https://www.sciencedirect.com/science/article/abs/pii/S0306261914008769
- https://www.sciencedirect.com/science/article/abs/pii/S1364032121007954
- https://www.iata.org/contentassets/d13875e9ed784f75bac90f000760e998/saf-technical-certifications.pdf
- https://www.greencarcongress.com/2020/02/2020-0201-astmchj.html
- https://www.faa.gov/about/office_org/headquarters_offices/apl/research/aircraft_technology/cleen/reports/media/PW_ARA_Final.pdf
- https://www.sciencedirect.com/science/article/abs/pii/S0016236118317927
- https://chemistry-europe.onlinelibrary.wiley.com/doi/am-pdf/10.1002/cssc.201801690
- https://medium.com/@tobias_gm/how-to-make-a-diesel-farm-9b216112a533
- https://en.wikipedia.org/wiki/Fatty_acid_methyl_ester
- https://en.wikipedia.org/wiki/Fatty_acid_ester
- https://en.wikipedia.org/wiki/Ester
- https://en.wikipedia.org/wiki/Organyl_group
- https://www.youtube.com/watch?v=3rqmBK1VA_I
- https://www.youtube.com/watch?v=m9jM8lWxrAE
- https://www.youtube.com/watch?v=uIYmNO-sR9o
- https://www.youtube.com/watch?v=uIYmNO-sR9o
- https://study.com/academy/lesson/alkanol-alkanoic-acid-functional-groups-properties.html
- https://www.youtube.com/watch?v=4JZOtWM20dI
- https://www.youtube.com/watch?v=5FlEcQ9-XQA
- https://en.wikipedia.org/wiki/Alcohol_(chemistry)
- https://en.wikipedia.org/wiki/Methanol
- https://www.youtube.com/watch?v=LSeGb1yh7PE&t=2s
- https://www.rix.co.uk/blog/why-do-planes-use-kerosene/#:~:text=Kerosene
- https://www.youtube.com/watch?v=8pN09kPMH0k&t=84s
- https://www.youtube.com/watch?v=C_BK7PRugK4&list=PLx7ERghZ6LoMJmpjadDGR5-Hs2Y5taPgO&index=30
- https://www.youtube.com/playlist?list=PLX2gX-ftPVXVTDoY2anCtOULy5fjsfpx7
- https://en.wikipedia.org/wiki/Decarboxylation
- https://en.wikipedia.org/wiki/Hydrolysis
- https://www.britannica.com/science/isomerization
- https://en.wikipedia.org/wiki/Dehydration_reaction
- https://en.wikipedia.org/wiki/Gasification
- https://en.wikipedia.org/wiki/Fischer–Tropsch_process
- https://en.wikipedia.org/wiki/Isobutanol
- https://www.youtube.com/watch?v=Qeal3nbjLOo
- https://en.wikipedia.org/wiki/Oligomer
- https://www.ars.usda.gov/northeast-area/wyndmoor-pa/eastern-regional-research-center/docs/biomass-pyrolysis-research-1/what-is-pyrolysis/#:~:text=Pyrolysis%20is%20the%20heating%20of,strong%20bio%2Dpolymers%20mentioned%20above.
