Biofuels

BiofuelsBiofuels have beenused since the dawnof human civilization

Not always in a way that we would be proud of today...

But, with the exception of such shameful episodes, biofuels played animportant positive role in human lives for uncountable generations….

Biofuels were used for cooking,... … for heating human dwellings,

e.g., caves…

…or for making light.

Oil lamp ( olive oil, or whale oil )

Bee waxWooden torches

In the XX-th century simple biofuels like firewood were “running out of favor” – they were used mostly in leisure activities, e.g., for barbecuing, in “family time” at a fireplace, etc.

However, firewood is now“returning with vengeance”,as wood pellets.Single-home heating instal-lations using firewood insuch form are fully auto- matic and require littlemaintenance.The wood-pellet industryin the US is growing fast. Advantages: it’s a non-polluting fuels, does not add “new” CO2 to the atmosphere (rather, it “recycles” the naturalCO2), and there is much wood available for making pellets.

However, today we most often talk about biofuels as ofpotential alternatives to fossil fuels used in transportation.

It’s not a new concept – in the XIX Century many American trainswere running on firewood.

Yet, today we need “bio-alternatives” to liquid petroleum-based fuels, such as gasoline, diesel, and jet fuel.

A promising alternative to gasoline is bio-ethanol.Bio-ethanol has been used by humans since ancienttimes, but rather for “fueling” people (by drinkingbeer, wine, whisky…), not cars. Essentially, almostall ethanol (CH3CH2OH) ever made by humans was “bio”, because it was obtained from natural sugarsmade by plants, through a fermentation process involving yeast (single-cell living microorganisms).

Gasoline engines can run on ethanol or ethanol-gasoline mixtures after a small modification of thefuel-injection system (up to 15% of ethanol, no modification is even needed). The energy content of ethanol is ~2/3 of that of gasoline.

A real “bio-ethanol paradise” is Brazil. About 50% of their carsrun on ethanol, not on gasoline.

Brazilians make their ethanol from sugar cane – a plant that grows verywell in their hot climate. The processis pretty straightforward: the “syrup”extracted from the canes is fermentedby yeast. The product contains ~15%of CH3CH2OH. Then, pure ethanol is extracted from it by distillation.

Brazilian biofuel: 6inputenergy Total

contentEnergy

Unfortunately, sugar cane does not grow well in the United States…

However, ethanol can also be obtained from starch.Starch molecules are made up of many sugar mole-cules, linked into a long “chain”. There are many starch-producing plants that grow well in the US:wheat, barley, potato, corn (a.k.a. maize). The latteris the best “starch-producer” of all of them.

But yeast cannot convert starch to ethanoldirectly. Therefore, a two-step process is needed:

Amylose (starch) molecule: 500 or more sugar molecules are linked toform a long chain:

The two-step process:(a)First, the amylose chain has to be broken down into

individual sugar molecules by special enzymes . Fortunately, the enzyme is easy to obtain – there is plenty of it in sprouting barley seeds. The recipe has been known for centuries, and widely used, e.g., by whisky distillers (whisky is made from starch!)(b)Then, the sugars are fermented by yeast, the same way as in the Brazilian process.

However, the entire process is much more energy-consumingthan that used in Brazil. The energy content of US bioethanolis only about two times higher than the net energy input. Somecritics claim that the ratio is even less than two.

Another liquid biofuel that currently recieves strong interestis “biodiesel”.

“Normal” diesel fuel consist of hydrocarbon molecules (general chemical formula: CnH2n+2 ) with n = 12-16.

Almost all fats produced byplants or animals have thesame general structure:namely, three “fatty acidmolecules” are attachedto a single glycerolmolecule.

A single “fatty acid chain”contains a similar number of carbon atoms as an averagehydrocarbon molecule in diesel fuel.

The viscosity of some vegetable oils is low enough for using them directly as a fuel for a Diesel engine.

In fact, in the first public demonstration of theengine – at the Paris World Exhibition in theyear of 1900 – Dr. Rudolf Diesel used peanutoil as a fuel.

However, the viscosity is still “a bit too high”.In cold weather, the oil “thickens”, and the engine cannot be started.

Some ingenious “biodiesel fans” install specialheaters that heat the oil up to about 180 F. Then they can run, e.g., on leftover oil theycan buy for cents from restaurants.

Dr. Rudolf Diesel

The first engineprototype

However, a better solution is to de-attach the fatty acidchains from the glycerol molecule. Single chains arechemically quite similar to the diesel fuel hydrocarbon chains – except that they are acids, meaning that theyare pretty corrosive substances.

Fortunately, their acidity can be easily neutralized by combining each chain with a molecule of methyl alcoholCH3OH, in a process called “esterification”. The physicalproperties and the energy content of the “fatty acid methylesters” obtained in that way are very similar to those of a“normal” diesel fuel.

This is how biodiesel is being made at industrial scale – but the estrification apparatus is so simple that one can install such a device in a garage. Many such “estrificators” have been built by OSU student.

Canola, an oil-seed plant that grows well in the US, yieldsabout 120 gallons of oil par acre.

The ratio value

is usually given as 3 – 4, so it is significantly better than thatof the US-made bioethanol.

process production in theinput energy Totalbiodiesel based-canola ofcontent Energy

“Cellulosic ethanol” – it may be an attractive option forthe future.

Sugar in sugar cane, and starch in corn can be thought of as substances containing “stored solar energy”. In fact,plants use solar energy in the process of combining atmo-spheric carbon dioxide (CO2) and water (H2O) to producesimple sugars (glucose, fructose) in a process known asphotosynthesis. Those molecules can be further convertedinto larger sugar molecules (e.g., of sucrose, a.k.a. “tablesugar”). In sugar cane, the process stops at sucrose, whichis “yeast-fermentable”; in corn, the process of adding sugarmolecules continues, yielding large starch molecules, whichhave to be broken down by enzymes to make them “yeast-fermantable”. Yet, most of the solar energy used by plants goes forsynthesizing CELLULOSE.

Cellulose is the main “scaffolding material” of which plantsmake their leaves, stems, branches, trunks….Interestingly, the basic “building blocs” for making celluloseare the very same simple sugars that sugar cane convertsinto sucrose, and corn into starch. Cellulose molecules,however, are much larger and sturdier than those of starch.

Cellulose molecules can be broken down into yeast-fermen-table sugars. Such a process has been pioneered at indus-trial scale by German firms in the closing years of the XIXCentury. The technology was also used in the US in themid-XX century, but it was not financially viable andtherefore the production was discontinued.

In the corn plant, much more photosynthesis products areconverted to cellulose than to starch!

In the existing technology of making corn bioethanol, thecellulose is simply wasted.

However, if viable technologies of converting cellulose tosugar were developed, much more bioethanol could beobtained from the same amount of “biomass”!

Actually, corn wouldn’t even be needed any longer – thereare other plants that yield muchmore cellulose per acrethan corn, and are much less “demanding” – one such plant is switchgrass that can be grownalmost everywhere in the US.

Bioethanol will be the topic of one of the “students’ presentation” in this course, and therefore we willnot continue the discussion here.

There are many new exciting developments in the cellulosic ethanol area – you will learn about them from that presentation!

On the other hand, there are opinions that convertingplant cellulose to ethanol is not the best way of utilizingit. It is argued that the amount of usable energy obtainedfrom cellulose can be much higher if other technologiesare used, not involving fermentation. Please read thediscussion in this article from the renowned SCIENCE magazine.