How to Make a Diesel Farm

A technological analysis of how carbon-neutral diesel is made.

On the electric vehicle supply chain:

“Put very simply, all the world’s cell production combined represents well under 10% of what we will need in 10 years,” Mr. Scaringe said last week, while giving reporters a tour of the company’s plant in Normal, Ill. “Meaning, 90% to 95% of the supply chain does not exist,” he added.
- The Wall Street Journal

That was RJ Scaringe, CEO of the electric vehicle company Rivian, talking about the big supply chain issues for batteries in electric cars. With every car maker on the planet setting targets for their electric car sales, and not enough battery materials to go around, the supply crunch for batteries threatens to leave many companies behind.

This shortage will be felt even more strongly when trying to electrify trucks and lorries, which account for ~5% of global greenhouse gas emissions (GGE). The Tesla Semi will require ~8X more batteries than a passenger electric vehicle like the Model 3 — putting even more strain on battery materials supply. This and many other challenges faced with electrifying trucks (charger networks, lifetime of current diesel trucks, etc.) make other solutions look more appealing.

The ideal scenario would be if we could change nothing, and these emissions would just stop. That’s the promise of renewable diesel.

This article will cover:

1. What renewable diesel (RD) is and is not
2. What needs to be true to have a perfect petrodiesel replacement
3. Status quo: how RD is manufactured, and the differences with each process.

Renewable diesel (unlike biodiesel) is a carbon neutral drop-in replacement for petrodiesel

When people think of alternative ways to power on the road heavy-duty vehicles, three thoughts probably come to mind: batteries, hydrogen, or biofuels. We’ve already discussed the downsides of batteries, and I will get to hydrogen later in this writing. Biofuels are a good buzzword, but most don’t know why they haven’t taken over the trucking industry like they were “supposed” to.

2006: Claims about a “biofuel revolution” that never came true.

Since our concern is diesel, let’s look at biodiesel. Why don’t all trucks run on biodiesel ?— and let’s even take cost out of the equation.

One can look at the properties of biodiesel for an easy answer. Biodiesel is a fatty acid methyl esters (FAME), not a hydrocarbon. The molecular formula for FAME is:

whereas the molecular formula for hydrocarbons such as alkanes or alkenes are CnH2n+2 and CnH2n respectively. The visual difference between these are shown here:

Butane, a type of alkane.

Ethylene, a type of alkene.

A fatty acid ester. Notice the red atoms; those are oxygen.

The core difference between FAME and hydrocarbons is that there are oxygen atoms in FAME. The simple fact of having a few oxygen molecules instead of hydrogen bonded to some carbons on the carbon chain changes the properties of the molecule due to polarity.

Some notable changes of properties include:

• Higher freezing point. Biodiesel freezes at 1˚C which makes it unusable on its own in the winter.
• The high freezing point not only makes it more difficult to use, but more difficult to transport because it cannot be used in the regular pipelines that petrodiesel uses.
• Energy density 9% lower than petrodiesel.
• Higher reactivity due to its polarity which makes it more prone to react with materials around it. Because of its reactivity, biodiesel will react with metals like zinc, copper, tin, and lead which speeds up the process of corrosion of these metals. This can damage the inside of the combustion engine.

For these reasons, biodiesel is not fully replaceable in the trucks used today; it can only be used in a blend with petrodiesel up to 20%, after which the harmful properties begin to take effect.

Photo by Rhys Moult on Unsplash

Renewable diesel is an encapsulating term for fuel that has the same chemical formula as regular diesel, but which was produced with recycled carbon.

By recycled carbon, I mean taking carbon out of the air and using it to produce the fuel — thereby not generating any extra emissions when being burned. This “recycled carbon” is usually sourced from biomass.

In terms of what needs to be true performance-wise for the fuel to be drop-in replaceable and useful, certain properties are important to optimize:

• Energy density (MJ/kg)
• Cold properties (freezing point)
• Cetane number → cetane number is a measure of the ignition quality of a fuel in a reciprocating internal combustion engine (for our purposes, a diesel engine). The higher the cetane number of a fuel the shorter is the ignition delay and the higher the thermal efficiency of this engine equalling a lower fuel consumption.
• Stability → like I discussed before, the oxygen atoms play a big role in the stability or instability of the fuel. Not only does this affect the engine and transportation pipes, but the reactivity impacts how the fuel reacts with its ambient environment (during storage for example). Low stability = fast aging of the fuel, which would make your fuel have an expiration date — no good!
• Sulphur content (≤ 0.2% for EN 590 requirements; Europe’s diesel fuel standard)
• Ash and metal content
• Filterability (tendency to block filter due to absorption of solid particulates)
• Odour
• Lubricity

Any form of renewable diesel must meet the ASTM D975 fuel standards (United States) and the EN 590 diesel fuel standards (European Union) for it to be replaceable in today’s trucks.

Instead of going one by one and knowing exactly what requirements need to be satisfied for each property, the most easy/simple way of making sure all requirements are met is by making renewable fuel the exact same chemical composition as petrodiesel, but using any process that recycles carbon from the air.

Then what is diesel, exactly?

Petroleum-derived diesel is composed of about 75% saturated hydrocarbons (alkanes), and 25% aromatic hydrocarbons. These molecules are a mixture of carbon chains that typically contain between 9 and 25 carbon atoms: the average chemical formula for diesel fuel is C₁₂H₂₃. This mixture of molecules is what we are trying to replicate.

Aromatic hydrocarbons: derivatives of benzene.

Today: how renewable diesel is manufactured.

There currently exists 6 pathways to creating renewable diesel.

1. Traditional hydrotreating
2. Biological sugar upgrading
3. Catalytic conversion of sugars
4. Gasification
5. Pyrolysis
6. Hydrothermal processing

#1: Traditional hydrotreating

Traditional hydrotreating refers to converting triglycerides into hydrocarbons using hydrogen. Triglycerides are the main constituents of vegetable fat, their chemical structure looks like this:

Unsaturated triglyceride. 3 glycerols on the left and then fatty acid tails (some unsaturated).

The first step in hydrotreating is to convert the solid feedstock (crops) into oils, this is done by cold-pressing whatever seed was farmed. Different crops will have different weight percentages of triglycerides in them.

Table of fatty acids and vegetable sources. Ratio X:Y = Carbon atoms: C-C double bonds.

Next, the oils are de-oxygenated through 3 different reactions that occur simultaneously: decarbonylation, decarboxylation, and hydrodeoxygenation. The conditions for these reactions are high temperatures (280–450˚C) and high pressures (1–5MPa). Catalysts are often used, but there are so many possibilities that it’s not worth mentioning.

All of these reactions use hydrogen, and if you’re using these processes to create renewable fuel, that means green hydrogen (hydrogen created without emitting CO2) is required.

Once the oxygen from the oil has been removed, the hydrotreated oil (oil with impurities removed by hydrogen) is nothing but straight chains of hydrocarbons (100% alkanes); the mix of molecules does not have the different types of branched molecules that regular fuel does (required for properties like lower freezing point, cetane number, etcetera).

To produce the branched hydrocarbons, the molecules undergo isomerization — heat applied with acidic catalysts to transform some of the straight alkanes into branched isomers.

Example of isomerization.

The output from this process is many different fuels including gasoline, diesel and jet fuel.

The traditional hydrotreating process in full.

Pros:

• The infrastructure needed (like machines for separation and hydroisomerization) for this process has existed a long time and has been optimized because it is used in the petroleum refining industry.
• We know it is scalable because it has been done before with the oil industry.

Cons:

• Requires hydrogen.

#2 Biological sugar upgrading

Biological sugar upgrading is when a biological organism (usually bacteria) consumes sugar, uses it in a self-sustaining reaction and releases another chemical as a byproduct of the reaction. The input for this process can be any form of biomass.

Steps:

1. Biomass is size-reduced and preprocessed into a uniform feedstock.
2. The feedstock is treated with sulfuric acid at mild conditions to liberate the cellulose + hemicellulose sugars (easier to break down with enzymes) and break down the biomass cell walls.
3. The pH level is raised to the pH needed by the enzymes to survive and enzymatic hydrolysis is performed in bioreactors. Enzymatic hydrolysis is a chemical reaction where enzymes break the cellulose chains into smaller sugars (glucose and xylose) which are easier for microbes to ferment into biofuels.
   Definition of fermentation: Fermentation is a metabolic (life-sustaining) process that produces chemical changes in organic substrates through the action of enzymes.
   Basically, microbes receive organic chemicals as input and output different organic chemicals.
4. Fermentation, the microbes digest the sugars and secrete useful compounds.
5. The products are separated by boiling point.

If that was confusing, here’s an excellent 3-minute video by the BioEnergy YouTube channel:

I have seen a few analyses that claim that the microbes can ferment pure hydrocarbons (not biofuels), yet there is not substantive literature on this. I will investigate more if this is a true claim, because that would mean that this process requires no hydrogen at all, which could significantly reduce the cost of renewable diesel.

Pros:

• Potential for genetic modification to make the perfect process.
• Can be done without hydrogen (?)

Cons:

• Barriers with scalability and throughput speed → fermentation usually takes 3 days.
• Requires bioreactors to keep the organisms alive. This is less straightforward than machines like gasifiers, which don’t have has many constraints because there are no living organisms.

#3 Catalytic conversion of sugars

Nearly the same process as biological sugar upgrading except instead of biologically upgrading the sugars, the latter undergo non-biological catalytic reactions.

There are very few resources on this process, but from what I understand, liquid biomass sugars are reacted with water to generate hydrogen and a range of hydrocarbon molecules — most of the hydrocarbon molecules are small because the bigger they are, the harder they become to produce.

Pros:

• Can be done without hydrogen (?)

Cons:

• Hard to produce large hydrocarbon molecules. (Large hydrocarbons = above 6-carbon chain)

#4 Gasification

Gasification is a way to go from biomass almost directly to fuels such as diesel.

The inputs to the gasifier are biomass, oxygen, and steam. The output is what is called “syngas” or “synthetic gas”; a mixture of hydrogen gas, CO2, CO, CH4, hydrogen sulphide, and carbonyl sulphide.

Simplified gasification reaction. Endothermic reaction.

What is special about gasification is the mixture of products that it produces; these products are useful for multiple different applications (see graphic).

When trying to make diesel, the Fischer-Tropsch reaction is the best way to do it after gasification, and what is great is that gasification produces hydrogen for you to use in the reaction (lowering the amount of expensive green hydrogen you need to buy or produce).

Fischer-Tropsch process.

Fischer-Tropsch produces hydrocarbons ranging from methane to high molecular alkanes and alkenes. This is perfect for making diesel since diesel is a mix of low molecular weight and high molecular weight alkanes and alkenes.

Pros:

• Self-production of hydrogen that can be used to make diesel.

Cons:

• Probably not enough hydrogen produced in the reaction to make diesel without buying hydrogen.

#5 Pyrolysis

Pyrolysis is one of the technologies available to convert biomass to an intermediate liquid product that can be refined to drop-in hydrocarbon biofuels, oxygenated fuel additives and petrochemical replacements. Pyrolysis is the heating of an organic material, such as biomass, in the absence of oxygen. Biomass pyrolysis is usually conducted at or above 500 °C, providing enough heat to deconstruct the strong bio-polymers mentioned above.

Because no oxygen is present combustion does not occur, rather the biomass thermally decomposes into combustible gases and bio-char. Most of these combustible gases can be condensed into a combustible liquid, called pyrolysis oil (bio-oil), though there are some permanent gases (CO­2, CO, H2, light hydrocarbons), some of which can be combusted to provide the heat for the process. Thus, pyrolysis of biomass produces three products: one liquid, bio-oil, one solid, bio-char and one gaseous, syngas.

The proportion of these products depends on several factors including the composition of the feedstock and process parameters. However, all things being equal, the yield of bio-oil is optimized when the pyrolysis temperature is around 500 °C and the heating rate is high (1000 °C/s) fast pyrolysis conditions. Under these conditions, bio-oil yields of 60–70 weight % of can be achieved from a typical biomass feedstock, with 15–25 weight % yields of bio-char. The remaining 10–15 weight % is syngas. Processes that use slower heating rates are called slow pyrolysis and bio-char is usually the major product of such processes. The pyrolysis process can be self-sustained, as combustion of the syngas and a portion of bio-oil or bio-char can provide all the necessary energy to drive the reaction.

- Source

One of many ways to use all the products from a pyrolysis machine. Source is the same for the text above.

As so well-explained above, pyrolysis converts biomass into 3 products: biochar (solid), bio-oil (liquid) and syngas (gaseous). One important caveat for pyrolysis is that the inputted biomass must be dry.

Biochar is a useful product for agriculture; it increases the soil fertility and stays in the ground for thousands of years unlike nitrous oxide from nitrogen fertilizers. And synthetic gas can be used for diesel production (as mentioned before), natural gas production, sustaining the pyrolysis reaction through heat, etc.

Bio-oil on its own is not useful, as it is a high oxygen-containing oil with many other impurities. Like I mentioned earlier, the oxygen has a significant impact on important properties:

The most striking difference could be the high water of bio-oil (20–30% for bio-oil and only 0.32 wt% for heavy fossil fuels), which lowers the heating value of bio-oil (13 MJ/kg for bio-oil and 40.63 MJ/kg for heavy fossil fuels).

Source

Pyrolysis bio-oil

Most direct applications of bio-oil involve using it alongside fossil fuels, basically like a participation trophy. For example, using bio-oil at 10–30% in boilers designed for fossil fuels and then rightfully claiming that emissions were reduced.

To derive any real value from bio-oil, it must be transformed into…

• Regular oil through hydrotreating. This requires a lot of hydrogen. Once the bio-oil is de-oxygenated, the different length hydrocarbons can be distilled to get desired fuels.
• Hydrogen through reacting bio-oil with steam in the presence of a catalyst. This is perhaps the best use of pyrolysis bio-oil, as one could imagine producing hydrogen with one gallon of bio-oil and then using that hydrogen to hydrotreat a different gallon of bio-oil to make fuel. One of the advantages of producing hydrogen from bio-oil is that bio-oil’s density is much higher than that of biomass, making it easier to transport. Hydrogen is an extremely difficult product to transport and store, so the transportation logistics of bio-oil beat another process like gasification where the hydrogen is produced immediately.

Steam reforming of bio-oil.

• Chemicals through distillation of bio-oil at high temperatures, extraction of a chemical by solvent and then hydrotreating that chemical to transform into whatever chemical you want.

Pros:

• Produces multiple useful products.
• Can use liquid product to produce hydrogen.

Cons:

• Requires hydrogen to react liquid product into useful fuels.

#6 Hydrothermal processing

Also called hydrothermal liquefaction (HTL), it is similar to pyrolysis but can process wet feedstocks, making it useful for wet crops like algae (so you don’t waste energy on dewatering). HTL produces a bio-oil called bio-crude that has twice the energy density of pyrolysis oil because it has significantly less oxygen. This makes it a more appealing oil to hydrotreat.

HTL produces a solid and liquid, but makes significantly more liquid than solid. The solid produced is HTL char, and unlike biochar which is very useful, I could not find any commercial applications of HTL char.

Pros:

• No energy wasted on de-watering if using wet feedstock.

Cons:

• Requires hydrogen for transforming product into useful fuel.
• Only one useful product unlike pyrolysis.

Bioreactors

Next step towards RD

The key insight from looking into the six processes of manufacturing renewable diesel (traditional hydrotreating, biological sugar upgrading, catalytic conversion of sugars, gasification, pyrolysis, HTL) is that 4/6 of them depend on a source of clean hydrogen; a fuel that shows no signs of ever becoming cheap enough to use.

Of the two existing processes that don’t require hydrogen, the research laying out these processes is hard to find. However, the development of this technology is crucial in order to decarbonize the heavy-duty trucking industry.

Fermentation and catalytic conversion of sugars is what I’ll be looking into next, and hopefully I’ll come to some insight that will “take the load off” of the battery supply chain.

Thanks for reading. If you’re interested in more, you can visit tobiasgm.com to find more of my work or subscribe with this link to receive monthly updates on my projects 🙂.