Ethanol For Fuel

Ethanol has powered automobiles for more than a century. Ford designed his model T for ethanol as a fuel. Gradually petroleum took over and became the dominating transport fuel. In the seventies Brazil adopted an ethanol strategy and today motor fuel grade ethanol (MFGE) is an extremely fast growing market worldwide.

Bioethanol is renewable, because it is made from glucose created in green plants by the sun, the so called photosynthesis:

(1) 6CO₂ + 6H₂O + sunshine → C₆H₁₂O₆ + 6O₂
Sun energy transforms carbon dioxide (CO₂) into glucose (C₆H₁₂O₆). This glucose is transformed once again to ethanol (C₂H₆O) by classic yeast fermentation:
(2) C₆H₁₂O₆ → 2C₂H₆O + 2CO₂ + heat
Heat is released calling for cooling of the fermentation vessels and heat is released again when the ethanol is burned in the combustion engine:
(3) C₂H₆O + 3O₂ → 2CO₂ + 3H₂O + heat

The three equations clearly demonstrate why bioethanol is said to be CO₂-neutral. Only the CO₂ - absorbed during the photosynthesis - and no more - is released by the engine.

The equations also explain why bioethanol is considered a form of solar energy. All the CO₂ and water (H₂O) absorbed by the green plants are released again. All the three processes - photosynthesis, fermentation and burning - do (all together), is in fact, to turn sun light into heat.

Bioethanol is friendly to the environment. As a liquid it is a convenient form of solar energy and may replace gasoline or part of it in modern transportation.

Conventional motors tolerate mixtures of as much as 10% ethanol in the petrol (E10). Newer motors are built to tolerate up to 85% ethanol (E85) and we will see more motors able to run on pure ethanol (E100) in the future.

Ethanol contains 34% less energy per unit volume than gasoline and consequently a lower mileage.
The higher octane rating of ethanol (129), however, allows for design of high-compression ethanol-only vehicles with improved energy utilization. Such engines will make the same mileage per unit volume as gasoline engines do today.

Materials. Bioethanol is manufactured by yeast fermentation of sugar. Any carbohydrate or biomass may be turned into glucose sugar and used as raw material. At the present state of art only sugar and starch in agricultural crops like sugar cane, maize and cassava is used. In future even celluloses in these crops will be turned into glucose increasing yield. The ethanol plant may also process the waste from starch wet milling as well as fresh crops.

Efficient process.

Cellulolytic enzymes are still too expensive to make it feasible to ferment the cellulose directly to ethanol. In the meantime we turn the hidden energy in the cellulosic parts into bio-gas powering the whole plant.
Equatorial regions are rich in sunshine and all that sunshine may be captured in cassava - well adapted to a hot climate. We developed at process extracting the solar energy and makes bioethanol direct from fresh harvested cassava.

Cassava roots are harvested all year round and transported to the factory on a daily basis and processed within 24 hours after harvest.

Dirt and foreign material is removed in a washing stage. Then the roots are chopped and grated. Transferred to large rotating conical screens the starch content is extracted as crude starch slurry leaving the cellulosic residue as a wet pulp.

The starch slurry is concentrated in a battery of hydrocyclones.

Glucose intermediate.

Liquefaction: The crude starch slurry is mixed with α-amylase and additives and cooked with steam in a jet-cooker and continuously hydrolyzed to a solution of starch sugars.

Saccharification: Enzymatically the blend of these starch sugars are all cut into smaller sugar molecules - an optimal feed for the yeast.

This feed boosts the fermentation section yielding a thin alcohol solution.

Distillation of fermented solutions is well-known since ancient times to increase the alcohol content. The alcohol (ethanol) is concentrated by distillation as hydrated ethanol, an azeotrope comprising 95.6% ethanol and 4.4% water.

Although ethanol-powered and "flex" vehicles utilize such azeotropic distillate, the market still demands anhydrous alcohol. Hydrated ethanol is immiscible with gasoline and the water has therefore to be removed somehow. We use molecular sieves for the purpose.

A molecular sieve contains materials with tiny pores of a precise and uniform size that will absorb gasses. Molecules small enough to pass through the pores - the water - are adsorbed while larger molecules - the ethanol - are not. A molecular sieve can adsorb water up to 22% of its own weight. When saturated it is regenerated and can be used repeatedly.

By-Products

The spent pulp is a valuable by-product when turned into biogas. So is the mash residue, the vinasse - often dried and sold as "Distiller's Dried Solubles" whenever a market exists.

Second equation, page one, shows formation of substantial amounts of carbon dioxide (CO₂) during fermentation. The concentration in the exhaust is high and the carbon dioxide is easily trapped and compressed into a marketable product. Carbonized beverage is a traditional application for carbon dioxide.
Oil production is slowing as many wells mature, and carbon dioxide pumped down the hole can loosen up and release the remaining oil. Combined Heat and Power Plant (CHP) The biogas digester processes the by-products - pulp and vinasse.

The biogas typically powers a gas engine based Combined Heat and Power Plant (CHP) providing more or less energy self-sufficiency. The cogenerated steam and electricity powers the whole factory. The electricity produced in the CHP plant is CO₂ neutral, contrary to the electricity produced from fossil fuels and the wasted heat is put to good use in the still.

The future will bring about more integrated alcohol and biogas plants.

Market

The main outlet for bio-ethanol is as automobile fuel. Total world production reached 50 billion litres in 2007 and still increasing. Fossil oil is coming to an end within a foreseeable future, and for transport a liquid replacement is required. There is possible competition from other products like methanol, but they have to be derived from the same feedstocks - and ethanol is by far the most engineered product today.

Another emerging application still in the cradle but with a growing potential is alcohol for Direct-ethanol fuel cells or DEFCs converting ethanol direct to electricity. The technique resembles that of Direct-methanol fuel cells or DMFCs with small portable units already on the market.

An alcohol stove burns pure and does not require a chimney. A popular consumer item - just hang it on a nail.

adapted from www.starch.dk