Used cooking oil is the easiest biodiesel feedstock to find, but it must be filtered through cloth, settled so the free water can be drained off the bottom, gently dried, and titrated before it is reacted, because its water and free fatty acid are what spoil batches. Drying by heating a wet oil to the parent’s 120°C is dangerous, since water trapped beneath hot oil flashes to steam and throws oil out of the pot.
Part of Biodiesel Production
Waste cooking oil is fryer and kitchen oil that has been used and discarded, collected as a biodiesel feedstock.
Dark And Smelly Is Fine; Wet And Acidic Is Not
The parent says dark colour and a rancid smell do not matter, but the worse the oil smells, the more lye it will need because of its free fatty acid. Those are the two enemies: free fatty acid, which turns lye into soap (Free Fatty Acid Problem), and water, which does the same and causes soap in the batch.
What You Need
- Cloth for filtering (the parent: any cotton fabric) and a clean bucket
- A tall settling container with a valve at the bottom, or a way to siphon off the lower layer
- A deep pot you fill only halfway, for drying
- A thermometer reading above 100°C
- Titration kit (Oil Preparation & Titration)
Method
Step 1: Strain out the food bits through cloth while the oil is still warm and thin, then again when cool.
Step 2: Settle for several days in a tall container. Water is denser than oil, so it collects below. Drain or siphon the water and any sludge off the bottom and discard it. This removes most of the free water without heating anything.
Step 3: Dry what is left, carefully. The parent heats the oil to 120°C briefly to drive off water. Water boils at 100°C, and any droplets sitting under the oil flash into steam and eject hot oil from the pot, so: use a deep pot filled no more than halfway, outdoors, heat slowly with stirring, keep your face and hands back, and stop heating as soon as the spitting stops. Keep heat below about 110°C rather than going to 120°C unless the oil is already clear.
Step 4: Titrate a sample to find the extra lye the free fatty acid will use.
What The Oil Was Doing Before
Cooking oil is food (9 kcal per gram; see Feedstock Sources), and it burns in a lamp (Fuel Sources names rendered fat and vegetable oil as safe lamp fuels) or goes into soap. “Waste” is the status of the surplus, not of all of it. In a food-short community, fry once more, then lamp or soap, and make fuel only from the rest.
Common Mistakes
- Skipping the settle. Heating oil with a water layer under it is where the splash accidents happen.
- Overfilling the drying pot, so spitting oil reaches the flame.
- Reusing a pot or drum that held fuel or chemicals.
- Drying with an open flame under a pot full of oil. Oil itself ignites if overheated; use a thermometer.
Hot oil
Oil at 100°C or more burns deeply and spits violently over water. Never pour water onto hot oil or onto an oil fire; smother it with a lid or dry sand. Keep a lid and sand within reach, children away, and work outdoors.
What’s Next
- Learn next: Oil Preparation & Titration, the test that sets the lye dose
- Combine with: Animal Fats, the other waste-stream feedstock