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April 2006




Fermenting waste fruit to fuel ethanol

By Dr Garth Cambray

For hundreds of years the lawn around a house represented a sort of fuel tank - you parked your transport - a horse - outside your house and it ate your grass keeping a nice trimmed lawn around your house. Horses like short grass, so lawns that are short - ie closest to the house, are kept nicely trimmed. Within the last 70 or so years this changed as horses were phased out, and more and more people had to use mechanical devices to get rid of the excess grass outside their houses. Most of these use fossil fuels. This series of articles shows you how you can use waste fruit, possibly from your garden, to make your own fuel and cut your lawn sustainably (or make ethanol to power a generator or fuel cell system to make electricity for your house!).

Why make your own lawnmower fuel? This might sound like a silly project, but consider, if there are half a billion lawns in the world with an average size of 100m2, which are mowed 12 times a year, these lawns represent a fuel consumption of about 1.4 million tons. Assuming there are half a billion gardens on Earth with three fruit trees that produce 100kg each of fruit that lies and rots below the tree, if this fruit was fermented to ethanol and used to power lawnmowers, we would reduce the number of fossil fuel tons used by half.

What will you need to make your own petrol (gasoline) lawnmower run on homemade ethanol fuel?

Well first we need to understand fermentation, specifically alcoholic fermentation. Yeast are simple single celled micro organisms that can convert sugar, by reacting it with oxygen, to carbon dioxide and water pretty much the same way we do. In the absence of oxygen, yeasts can convert sugars to alcohol and carbon dioxide, and continue living until the make so much alcohol they die (about 12 to 13%). Humans can also survive for short periods in low oxygen environments, but we have the disadvantage of producing lactic acid instead of ethanol as a by product. The lactic acid damages cellular function, so, too long without air and you die.

C6H12O6  ->  2CH3CH2OH + 2CO2

The above formula shows how a molecule of sugar on the left is converted to two molecules each of ethanol and carbon dioxide. This is a very simple reaction and you would predict that if you had 10 kilograms of fruit sugar to ferment, that you should be able to get about 5.1 kg of ethanol fuel by fermenting the sugar. Just now we will spoil that dream.

From the equation we can also see that oxygen needs to be kept out so that the yeast does not go back and consume that alcohol. So, we need a container. A plastic barrel is quite effective as shown in the picture. The lid keeps the yeast out of contact with the atmosphere. This container is a fermentor - a vessel in which fermentation happens - in other words you now posses a 'bioreactor' that biotechnological device which allows a biological system to produce a product such as ethanol in a controlled contained environment.

Fruit can be simply mashed with a pole and added to the drum. Don't worry about straining it now - the fruit will decompose as it ferments and release more juice than if you separate the fruit fibre from the juice. You are making alcohol for your engine not for your own consumption, so a little bit of wood alcohol getting into the product will not hurt anybody!

Next you need to add some yeast to the fruit juice mix. Fruits such as pineapples have so much wild yeast on them that you don't need to bother with this step. For other fruits such as apples and plums, a little added yeast will help you get alcohol rather than other by products. If possible get an ethanol tolerant yeast, normally these are available from wine or beer maker supply stores.

A hydrometer will give you an indication of how much sugar you have in your juice when you start. As the fermentation goes along, it will show you that there is less and less sugar left - when it tells you there is none, proceed to the next article which shows you how to make a solar still to separate the fuel alcohol from all the other gunk in the fermentor (link to solar still article).

Earlier we discussed the fact that in a perfect world 1kg of sugar would give us 0.51 kg of fuel alcohol, based on the reaction that produces alcohol. This is called a theoretical yield. In reality, your yield of alcohol will be lower as the yeast, like all living organisms, will behave both logically and erratically. Some of the sugar will be used to make more yeast - reproductive activity - which is, as we all know, energy intensive. Some will be used to make mistakes - in beer one often gets a butter scotch flavour when the yeast made a mistake. All in all, this adds up to the fact that you will get a yield of somewhere between 0.3 and 0.4 grams of ethanol per gram of sugar.

Do not leave your fermented raw fuel undistilled - other microbes will get in there and rapidly convert it to all manner of other compounds, most of which will hurt your engine. You need to extract the alcohol as fast as possible.

In pictures:

A drum such as this one can hold 220 litres of fruit mash - don't fill it all the way as it will overflow and make a mess when the fermentation begins. Rather start with about 200 litres. 200 litres of fruit juice with 100 grams a litre of sugar will give you about 8kg of fuel - enough to fill your lawnmower tank 9 times.

Once fermentation begins, the fruit rapidly takes on an unsightly appearance!

See the next article on how to make your own solar still.

More information:


Related articles:

Solar Cells take Stellar leap

Off the grid for 5 years

Separating your alcohol from your fermented fruit

Making your own reflux column from a piece of old irrigation pipe or some tin cans


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