Making biodiesel is a straightforward chemical process: you react vegetable oil or animal fat with methanol in the presence of a catalyst, producing a clean-burning diesel substitute and glycerin as a byproduct. The standard ratio is roughly 100 pounds of oil to 10 pounds of methanol, yielding about 100 pounds of biodiesel and 10 pounds of glycerin. The whole reaction can be done in a home workshop with basic equipment, though getting a quality result requires attention to ratios, temperatures, and purification.
The Core Chemical Reaction
Biodiesel production relies on a reaction called transesterification. In plain terms, you mix oil or fat with an alcohol (almost always methanol) and a chemical catalyst that kicks off the reaction. The catalyst breaks apart the fat molecules and lets the methanol attach to them, creating new molecules called methyl esters. That’s your biodiesel. The leftover backbone of the original fat molecule becomes glycerin, which sinks to the bottom because it’s heavier.
The catalyst is typically sodium hydroxide (lye) or potassium hydroxide. Sodium hydroxide is cheaper and widely available, while potassium hydroxide dissolves more easily in methanol and produces a softer glycerin byproduct that’s easier to work with. Both get the job done.
What You Need for a Small Batch
For a basic batch, you’ll need:
- Oil or fat: New or used vegetable oil, such as soybean, canola, or sunflower oil. Used fryer oil from restaurants is the most common free feedstock, but it requires extra preparation.
- Methanol: About 10% of the oil’s weight. For one liter of oil, that’s roughly 200 milliliters of methanol.
- Catalyst: Sodium hydroxide (about 3.5 grams per liter of new oil) or potassium hydroxide (about 7 grams per liter of new oil).
- A reactor vessel: A stainless steel or HDPE container with a way to stir the mixture and control temperature. Many home producers use modified water heaters or stainless steel tanks with electric heating elements.
You’ll also need safety equipment. Methanol is toxic and flammable, absorbed through the skin and lungs. Sodium hydroxide causes chemical burns on contact. Work outdoors or in a well-ventilated space, and wear chemical-resistant gloves, safety goggles, and a respirator rated for organic vapors.
Preparing Used Cooking Oil
New oil can go straight into the reaction. Used cooking oil needs filtering and testing first. Strain it through a fine mesh or cloth to remove food particles, then heat it to about 120°F (49°C) to drive off any water. Water in the oil causes the catalyst to produce soap instead of biodiesel, ruining the batch.
Used oil also contains free fatty acids from repeated heating. You can test for these with a simple titration: dissolve one gram of catalyst in one liter of isopropyl alcohol, then slowly add it to a small sample of your oil mixed with a pH indicator until it changes color. The amount of extra catalyst needed to neutralize those acids tells you how much additional catalyst to add beyond the baseline amount. This step is essential for used oil. Skip it and you’ll end up with a container of soapy sludge.
Running the Reaction
First, prepare the methoxide solution by dissolving your measured catalyst into the methanol. Do this in a sealed container, because methanol fumes are hazardous and the reaction generates heat. Stir until the catalyst is fully dissolved.
Heat your oil to about 130°F (55°C) in the reactor. Pour in the methoxide solution and stir vigorously, around 500 rpm if you’re using a mechanical mixer. The reaction needs consistent agitation to ensure the methanol and oil stay in contact. Keep the temperature steady and continue mixing for 60 to 90 minutes.
Once mixing stops, let the batch settle for at least 8 hours, ideally overnight. Two distinct layers will form. The darker, heavier glycerin settles to the bottom, while the lighter biodiesel rises to the top. Drain the glycerin from the bottom of the tank. What remains is crude biodiesel that still contains traces of soap, methanol, catalyst, and glycerin.
Purifying the Fuel
Crude biodiesel must be cleaned before it’s safe for an engine. The two main approaches are water washing and dry washing.
Water Washing
This is the most common method for home producers. Gently mist warm water (about 100°F) over the surface of the biodiesel and let it settle. The water pulls dissolved soap, residual glycerin, and leftover methanol out of the fuel as it sinks to the bottom. Drain the water layer and repeat three to five times until the wash water comes out clear. After the final wash, heat the biodiesel to about 150°F (65°C) and hold it there for 15 to 20 minutes to evaporate any remaining water.
Dry Washing
Dry washing uses adsorbent materials instead of water. Products like Magnesol (a magnesium silicate powder) or ion-exchange resins attract and trap polar contaminants like soap and glycerin. You mix the powder into the biodiesel, stir, then filter it out. This method produces no wastewater and requires less energy, but it’s generally less thorough than water washing. It also doesn’t remove residual methanol, so you may still need a heat treatment. Activated carbon has shown strong results in some testing, outperforming both water washing and silica gel at stripping impurities.
Testing Your Finished Product
Commercial biodiesel in the U.S. must meet the ASTM D6751 standard. While home producers aren’t regulated the same way, these benchmarks tell you whether your fuel is engine-safe:
- Flash point: At least 93°C (200°F). If your biodiesel ignites at a lower temperature, it still contains methanol. Heat it longer to drive off the alcohol.
- Water and sediment: No more than 0.05% by volume. Cloudy fuel or visible particles mean more washing or filtering is needed.
- Total glycerin: No more than 0.24% by mass. Excess glycerin clogs injectors and leaves carbon deposits inside the engine.
A simple home test is the “3/27 conversion test.” Mix 3 milliliters of your biodiesel with 27 milliliters of methanol. If the biodiesel dissolves completely and the solution stays clear, the conversion rate is high. If it turns cloudy or separates, unconverted oil remains and you may need to run a second reaction on the batch.
What To Do With the Glycerin
Each batch produces roughly 10% of its starting oil weight in crude glycerin. This byproduct contains residual methanol and catalyst, so it shouldn’t be dumped directly down a drain or into the yard. According to the EPA, wash water from biodiesel production is generally safe for municipal sewer systems, but raw glycerin can gel and clog pipes.
The safest options for small producers: bring crude glycerin to your local wastewater treatment facility, or let it evaporate in a well-ventilated area to remove the methanol first. Many home producers heat the glycerin to about 150°F to boil off methanol, then use the cleaned glycerin to make soap. If none of these work, check whether your local household hazardous waste facility will accept it. Landfills technically can take it under household hazardous waste exemptions, but most prefer not to accept liquid waste.
Using Biodiesel in Your Engine
Pure biodiesel (called B100) works in most diesel engines, but blending it with petroleum diesel is more common and more forgiving. A B20 blend (20% biodiesel, 80% petroleum diesel) is the most widely used ratio. Testing by the National Renewable Energy Laboratory found that B20 blends caused no significant changes to the dimensions, volume, or strength of common fuel system rubber components after 500 hours of exposure, performing essentially the same as straight diesel.
Higher concentrations deserve more caution. B100 is a solvent, and in older engines it can loosen deposits from fuel lines and tanks, sending debris into filters. Plan to change your fuel filter more frequently during the first few tanks. Older vehicles (roughly pre-1994) may have natural rubber seals and hoses that degrade over time with straight biodiesel. Replacing them with synthetic rubber components solves the issue.
Cold weather also matters. Biodiesel gels at higher temperatures than petroleum diesel. B100 made from soybean oil can start to thicken around 32°F (0°C). Blending with petroleum diesel lowers the gel point, which is another reason B20 is the practical sweet spot for year-round use in most climates.