If you spend much time on the road you will almost certainly have seen a colourful truck carrying ingredients for McDonalds with the slogan, “We recycle our used cooking oil into Biodiesel for our trucks”. Can it be true? Is there enough chip fat in McDonald’s fryers to fuel a truck fleet? And how do they do it?
Well, there is a lot. McDonald’s use approximately 12 million litres of cooking oil a year in the UK. Obviously a lot of that adheres to the food and is sold, but they collect 4 million litres of it and return it for processing. Not surprisingly they have a well worked out supply chain and system for getting the oil back.
McDonald’s oil is supplied by ADM Pura, and delivered by Martin Brower a specialist haulier. Martin Brower provide a waste oil tank called OSCAR (Oil Storage Container Assisting Restaurants), when a burger bar changes its oil the waste is decanted into OSCAR and when Martin Brower deliver the next batch of new oil they empty OSCAR and take away the waste. The actual processing of the oil into biodiesel is carried out by Agri Energy in Liverpool.
The science bit
Biodiesel isn’t the same as petroleum diesel, but it has to be close enough to perform satisfactorily. Crucially it has to have a very similar viscosity to avoid clogging the fuel system. Cooking oil is a lot thicker than diesel. All vegetable oils molecules are Triglycerides, that is they are made of a single glycerol molecule with three fatty acid molecules attached. Different oils are made of different fatty acids, that’s what makes the difference between a supermarket own brand generic cooking oil and a premium olive oil. But they are all based on glycerol, and glycerol is a thick syrupy type of alcohol.
To make a thinner oil better suited to diesel engines the glycerol is swapped for a lighter alcohol, methanol, in a process known as transesterification. If you use nice clean fresh oil then the usual method is to mix methanol and oil together with some sodium hydroxide (caustic soda) or another alkali as a catalyst. Heat speeds things up, and some processors use high pressure too. Dirty old chip fat isn’t quite so easy
Do you want FFAs with that?
Heat, and particularly steam, breaks down triglycerides. Dropping moist food into hot oil is guaranteed to degrade the oil. First one of the fatty acid molecules splits off leaving a diglyceride and a ‘free fatty acid’ or FFA. Later the diglyceride can degrade further to a monoglyceride. It’s this process that causes used oil to start to smell and taste bad, at which point of course reputable restaurants change it.
As if that wasn’t bad enough, FFAs in hot oil can attach to the ends of other fatty acids still forming part of a triglyceride creating longer chains, a process known as polymerisation. This is what creates the brown ‘varnish’ that can build up around the edges of fryers. By the time Agri Energy get the oil from McDonalds it contains mono and diglycerides, triglyceride polymers, food particles and water. Simple transesterification would not work well with a feedstock like that.
The Agri energy plant is quite small, larger biodiesel plants using virgin oils often use a continuous process, but small plants process the oil in batches. Size apart, batch processing is a better way of dealing with used oils from different sources because the operation can be adjusted to suit each. It also allows McDonalds to boast that they are using their oil to power their trucks, they wouldn’t want any old KFC or Burger King oil.
The first stage is to filter the out the food particles, then the oil is reacted with methanol using sulphuric acid catalyst. The acid encourages the esterification of FFAs; they attach themselves to methanol molecules, converting them to Fatty Acid Methyl Esters, commonly known as FAME or to most drivers as Biodiesel. The acid is neutralised and the resulting water removed before more methanol is added, this time with potassium methylate catalyst, to transesterify the oil.
The third stage resembles a miniature version of the distillation columns used in oil refineries. This is necessary to remove the long chain biodiesel created from polymerised oil. If not removed, this heavier oil would tend to clog the fuel system. The finished oil conforms to the European standard for biodiesel, EN 14214.
From the four million litres they collect, Agri Energy produce 3.4 million litres of biodiesel. That’s a lot, but it’s not enough for a big truck fleet, the biodiesel is blended with conventional diesel for delivery to McDonalds’ distribution centres. The blend ratio varies according to the time of year. All oils thicken in cold weather, even conventional diesel has to have a ‘winterizing’ additive to keep it liquid during the winter. Biodiesel thickens up at a relatively high temperature, so rather than add lots of expensive additive, operators prefer to reduce the blend ratio. McDonalds use various blends throughout the year going from B30 (30 per cent biodiesel) in the winter to B50 and B70 as the weather improves and right up to B100 through the summer period.
Burning burger fat
Fryers are not the only source of oil, McDonalds flip a lot of burgers and each one leaks fat as it cooks. The fat drips into collection trays, and that too makes its way to Liverpool. It is possible to process this into biodiesel too, but the amount of work needed to produce a decent product from such unpromising material makes it uneconomic. Agri Energy have a better use for it, they process it into a cruder, heavier oil that they burn to provide heat for all their processing. Neat.