Summary

Most ethanol made from plants at present is made from corn. However for every unit of energy used to produce it, the process yields only 1.4 units of energy. A profit, but not much.

Sugar cane in Brazil has a profit of 8X. Agave in the arid regions of Mexico, and sugar palms from the tropics, sugar beet from the cold areas, are promising much better energy profits than for corn.

Energy from cellulose covered in the next page is promising to be very profitable. Same for seaweed.

The numbers vary with: the region, the climate, the soils, the accounting method, and the story teller. It is impossible to get a definitive answer to which is the best process.

Every process claims to be better than corn.

Biofuels - Ethanol from sugar

Biofuels are fuels made from any life form. Here we will divide them into alcohols and oils. Bio-char will be included with oil.

Producing ethanol by fermenting sugar or starch is ancient technology. (man's most ancient chemical reaction, after fire and cooking)  Nowadays we are developing ways of extracting the sugar from more difficult sources such as cellulose and the alignate in seaweed.

Biofuels face tremendous problems in replacing oil from underground. It is unlikely they will be able to. They require too much agricultural land. At present the total biomass harvested from crops, wood, and waste, is equivalent to 20% of our liquid fuel energy use. It is unlikely we will be able to increase this dramatically. 

There are many different processes being researched and trialed. It is impossible to tell which is the most suitable as everyone is out to sell their process and will sometimes, shall we say, present their project in the best light.

Octane number (RON) is a measure of how much the fuel can be compressed before igniting. This allows higher compression ratio. Higher compression means higher temperature. Higher temperature means higher efficiency.

Click on images for source and more information.

(Note: there is quite a variation in reported octane numbers.)

Alcohol fuels - Wikipedia

Glucose, starch and cellulose

The sun’s energy falling on chlorophyll  combine carbon dioxide with water, producing carbohydrate, mainly glucose. The name says it: carbo-hydrate.

A plant will often store the glucose as a more stable for starch. The glucose is stacked in a simple regular pattern.. Starch is easy to ferment. 

To make cellulose, the plant stacks glucose so it alternates. This forms crystals and is very stable. Cellulase enzymmes  can break it down into sugar for fermentation

.

Glucose - is a 6 carbon sugar - easy to ferment

An excellent summary of carbohydrates can be found at:

http://www.scientificpsychic.com/fitness/carbohydrates.html

   Starch - glucose stacked same side up - easy to ferment

   Cellulose - glucose stacked alternately - must be pretreated then split with cellulase enzymes to make glucose available

Sugar for ethanol

Sugar for ethanol comes from: sugar cane, sugar beet, Sugar palm, and agave.

The debate is whether food crops should be used for fuel. The fuel option is keeping the food prices high so there is commercial and political pressure in this debate.

The other question is over how much energy is used to produce the ethanol.

Sugar Cane

Sugar cane grows in most tropical countries, but Brazil has taken the production of ethanol much more seriously than anywhere else. Ethanol is 25% of all petrol, but this varies according to the crop yields. Because the cane waste fibre, bagasse, is used to power the sugar and distillation processes, the production of this ethanol has a very low greenhouse footprint.

Sugar Beet

One third of the world's sugar comes from sugar beet. Most of it is grown in the colder parts of 
Europe, Russia, and North America. When the beet is first harvested the sugar is quite high. However within a very short time the plant converts this to starch for long term storage.

So the beet must be processed as soon as possible, meaning the mill must operate only in the season and be idle the rest of the year. Cane sugar has the same problem. This capital lying idle increases the cost.

There is a new beet called “energy beet” and is being used for ethanol. There are plans to produce ethanol in UK. Although the beet mill will run only during the season, the distillery would run 365 days a year.

Ref...

Sugar palm

Sugar palm covers several species of palm producing sugar. 

Arenga pinnata (syn. A. saccharifera)
Borassus
Caryota urens
Cocos nucifera

Nypa fruiticans or Nipah is a palm classified as a mangrove growing in SE Asia and northern Australia

Nipah has a very high sugar-rich sap yield. According to one study (Hamilton and Murphy 1988), nipah sap can produce 6,480-15,600 liters of ethanol per hectare, compared to 3,350-6,700 liters/hectare from sugarcane and 2,000 litres/Hectr/yr for corn.

sugar palm case study Indonesia

Ethanol from sugar palm

Agave

Agave is a native of Mexico and is used to produce tequila and rope. Agave-derived ethanol has been shown to have the potential to produce good yields on hot, dry land with minimal environmental impact.

"Our enhanced Agave tequilana weber variety plants have a sugar content of 27º to 38º Brix, [which is] 3-times sugarcane's and weigh around 300 kilos (660 pounds) each individual. But we've got plants from another agave species that weigh 1.2 metric tons each with sugar content around 25º Brix, double that of sugar cane. This means that annually per acre, we could easily get 5,000 gallons of distilled ethanol and fifty metric tons of dry biomass, with 33 metric tons of cellulose content."

Professor Remigio Madrigal Lugo Autonomous University of Chapingo.

Agave in Australia, 

global potential for agave, 

Ethanol from Agave

Different varieties of Agave

Sugar cane

Sugar beet

Sugar palm

Sugar palm - Nypa fruiticans

Agave

Ethanol from starch

The plant stores sugar as starch, cellulose, or hemicellulose. These are covered in the next two pages.

Ethanol in USA