waste vegetable oil

Waste Vegetable Oil (WVO) is vegetable oil that has become unfit for food preparation. The common causes of this degradation are:

Chemical degradation, including

oxidation.
hydrogenation.

Accumulation of contaminants.

Its uses include:

Animal feed additive.
Fuel for waste-to-energy plants.
Use as a biofuel, especially:

As feedstock for the production of biodiesel by transesterification.
As fuel for diesel engines modified to accept unprocessed vegetable oil as straight vegetable oil (SVO) fuel.

As feedstock for the production of soap.

Animal feed

As of 2003, the use of waste vegetable oil in animal feed is no longer permitted in the European Union, but continues in the United States.

Use in diesel engines

One of the first demonstration diesel engines ran on peanut oil. However, modern diesel engines are designed to run on petrodiesel. They run poorly on unmodified vegetable oil, with a risk of damage. There are two solutions to this, both of them technically successful and growing in popularity:

Convert the oil to biodiesel.
Convert the engine to run on straight vegetable oil.

Biodiesel

Biodiesel has similar characteristics to petrodiesel, notably its viscosity. It can be produced by transesterification of WVO. Since the early 1990s, the use of waste vegetable oil to produce biodiesel has been growing steadily. Biodiesel and petrodiesel may be mixed in the same fuel tank.

Straight vegetable oil

The other approach is to convert the engine. The most common conversion simply provides a way of heating the fuel to reduce its viscosity. Such engines often have two fuel tanks, one for petrodiesel, biodiesel or a mixture of the two, for use in starting the engine, the other for the vegetable oil which is the main fuel. After starting and running for a short time to allow the fuel heating system to come into operation, the fuel supply is swapped to the heated vegetable oil feed. WVO to be used in these engines requires only a hot water wash before use.

The quantities involved

As of 2000, the United States were producing in excess of 11 billion liters (0.011 km³) of waste vegetable oil annually, mainly from industrial deep fryers in potato processing plants, snack food factories and fast food restaurants. Waste vegetable oil has a stable market value of approximately USD 0.40 per gallon (USD 117.58 per metric tonne) as of 2003, enough to make collection economically viable.