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MAKING BIODIESEL
THE BEST WAY TO LEARN HOW TO MAKE YOUR OWN BACKYARD BIODIESEL IS
TO START WITH A ONE-LITER BATCH. BY ROB ELAM
It’s easy to make a small batch of
biodiesel that will work in any diesel
engine. You don’t need any special
equipment — an old juice bottle will
serve as the “reactor” vessel — and
on such a small scale you can quickly
refine your technique and perform
further experiments. After a few liters’
worth of experience, you’ll know if
you’ve been bitten by the biodiesel bug.
The principle behind biodieseling is to
take vegetable oil (either new or used),
and process it into a fuel that’s thin
enough to spray from a regular diesel
engine’s fuel-injection system. This is
done chemically, by converting the oil
into two types of compounds: biodiesel,
which shares the original oil’s combustibility, and glycerin, which retains the
oil’s thick, viscous properties. Drain
away the glycerin, and you’re left with
a fuel that you can pour into any diesel
vehicle with no further modification.
Once you get to the far side of the
learning curve, making biodiesel is very
much like cooking. In fact, a commercial
biodiesel production plant shares more
in common with a large-scale bakery
than a petroleum refinery. There’s
organic chemistry involved in baking
a cake, but most bakers wouldn’t consider themselves organic chemists.
BIODIESEL CHEMISTRY
Vegetable oil is a triglyceride, which means that its molecule
consists of a glycerin “backbone” with three fatty acids attached,
forming a shape like a capital letter E. To make biodiesel, we add
lye and methanol. The highly caustic lye breaks the three fatty acid
branches off of the glycerin backbone. These free fatty acids then
bond with the methanol, which turns them into fatty acid methyl
esters — otherwise known as biodiesel. The freed glycerin, which
is heavier, sinks to the bottom, leaving the fuel (and lye) on top.
Wash the lye out of the upper layer, and you have pure biodiesel.
But it’s not that simple. With some triglyceride molecules, only
one or two fatty-acid branches break off, which leaves mono- or
di-glyceride molecules (shaped like capital Ts or Fs), rather than
free glycerin. At the same time, mixing methanol and lye produces
some water — and oil, water, and lye mixed together make soap.
With all of these incomplete and competing chemical reactions,
your batch will inevitably contain soap, water, leftover lye, methanol,
and mono- and di-glycerides, along with the nice biodiesel and glycerin. Mono- and di-glycerides are emulsifiers, so they prevent mixed
liquids from separating, making it harder to extract biodiesel. The
picture gets even muddier when you use waste vegetable oil rather
than pure oil, since it contains free fatty acids, water, and countless
random contaminants from all those French fries.
These by-products are bad for an engine, potentially causing
micro-abrasions that damage fuel injectors or clog fuel filters. But
you can remove them by washing or cooking the biodiesel in various
ways, or by processing the incompletely converted biodiesel again,
as if it were vegetable oil. In extreme cases, you’ll end up with a
thick, soapy mass that never separates. All biodieselers wind up
with a batch of this glop sooner or later. Fortunately, you can use it
to make a good, grease-cutting soap — which is something that all
biodiesel homebrewers need to have on hand.
