Biodiesel

Biodiesel is a renewable energy fuel produced from animal fats and vegetable oils by lipid transesterification. Chemically, it is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats. It is produced by a chemical process (transesterification) which removes glycerol from the oil.

Biodiesel is non-flammable and non-explosive (flash point 150C for biodiesel and 64C for petrodiesel) . It is biodegradable and nontoxic.

Table of contents
1 History
2 Production
3 Oil preparation
4 Biodiesel recipe
5 Fuel quality, standards and properties
6 Properties
7 Availability
8 External links

History

Transesterification of a vegetable oil was conducted as early as 1853, by scientists E. Duffy and J. Patrick , making it before the first diesel engine became functional.

At Augsburg, Germany on August 10, 1893, Rudolf Diesel's prime model, a single 10-foot iron cylinder with a flywheel at its base, ran on its own power for the first time. Because of this, the International Biodiesel Day is on August 10 every year.

Rudolf Diesel demonstrated his engine at the World Fair in Paris, France, 1898. This engine stood as an example of Diesel's vision because it was powered by peanut oil - a biofuel. He thought that the utilization of a biomass fuel was the real future of his engine.

In a 1912 speech, Rudolf Diesel said "the use of vegetable oils for engine fuels may seem insignificant today, but such oils may become, in the course of time, as important as petroleum and the coal-tar products of the present time."

During the 1920s diesel engine manufacturers created a major challenge for the biofuel industry. Diesel engines were altered to utilize the lower viscosity of the fossil fuel (petrodiesel) rather than a biomass fuel (vegetable oil). The petroleum industries were growing and establishing themselves during this period.

Some see a conspiracy theory here and argue that the business tactics and the wealth that many of these "oil tycoons" already possessed greatly influenced the development of all engines and machinery. The alteration was the first step in the elimination of the production infrastructure for biomass fuels. Some see this as the first step in forcing the concept of biomass as a potential fuel base into obscurity, erasing the possibilities from the public awareness.

However, others have pointed out that fossil diesel is simply cheaper than vegetable oil, and that no conspiracy is necessary to explain the move toward fossil fuels.

Production

Biodiesel can be produced from biolipids. This is:

A 1998 joint study by the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA) found that biodiesel yields 3.2 units of fuel product energy for every unit of fossil fuel energy consumed.

According to a study written by Drs. Van Dyne and Raymer for the Tennessee Valley Authority, although the average US farm requires 33 litres (8.75 gallons) of fuel to cultivate 1 acre of land, rapeseed produces an average of 420 l (110 gallons) of oil per acre. The average yield of high-yield rapeseed fields is 550 l (145 gallons) per acre. Unfortunately by themselves these statistics are not enough to show whether such a change makes economic sense. Additional factors including the fuel equivalent of the energy required for processing, the yield of fuel from raw oil, the return on cultivating food and the relative cost of biodiesel versus petrodiesel must be taken into account.

The issue is economic: one of the exceptions Nassau Senior noted to the idea that machines aren't harmful to wages is, where the machines themselves make demands on resources that would have gone into food production. So the important question isn't whether biodiesel can be produced as whether that it is the most efficient use of resources, and the expense of biodiesel in comparison to traditional forms of diesel suggests that the answer is no.

For third world countries, biodiesel sources that use marginal land could make more sense, e.g. honge nuts grown along roads. Biodiesel can use waste vegetable oil, but alternative uses for WVO compete with its use as biodiesel, making large scale conversion of WVO to biodiesel more expensive than petrodiesel.

The direct source of the energy content of biodiesel is solar energy captured by plants during photosynthesis. The website biodiesel.co.uk discusses the positive energy balance of biodiesel:

When straw was left in the field, biodiesel production was strongly energy positive, yielding 1 GJ biodiesel for every 0.561 GJ of energy input (a yield/cost ratio of 1.78).

When straw was burned as fuel and oilseed rapemeal was used as a fertilizer, the yield/cost ratio for biodiesel production was even better (3.71). In other words, for every unit of energy input to produce biodiesel, the output was 3.71 units (the difference of 2.71 units would be from solar energy).

The production of biodiesel processors is measured in metric tonnes/year with a specific gravity of 0.89

Biodiesel is becoming of interest to companies interested in commercial scale production as well as the more usual home brew biodiesel user and the user of straight vegetable oil or waste vegetable oil in diesel engines. Homemade biodiesel processors are many and varied.

Oil preparation

Biodiesel processor machines, need the vegetable oil to have some specific properties:

  • Suspended particles lower than 1% (mass/mass) and than 5 micrometers. Because of this, the following are necessary:
    • Filtration to 5 micrometers.
    • Washing with hot water.
    • Decantation.
    • Heating of the oil.
    • Second decantation.
  • Anhydrous (waterless). Because of this, the final step of preparation, after the second decantation is drying.
  • Easy solubility in the alcohol to use.

Biodiesel recipe

Like a recipe for making a cake, a biodiesel recipe specifies quantity of every ingredient required, and the steps for combining and processing them to make biodiesel fuel.

The most common recipe uses waste vegetable oil (WVO), alcohol (methanol or ethanol) and sodium hydroxide (caustic soda), to produce biodiesel and glycerol. To produce 1 t of biodiesel:

  • One needs 1 t of biolipids (animal or vegetable oil) and 0,1 t of methanol.
  • One receives 0,1 t of glycerol.

The most common steps are:

  1. Preparation: cleaning/heating biolipid (i.e. WVO). With wet oil, you will obtain soap with the biodiesel, the conversion index from vegetable oil to biodiesel will be smaller and you will obtain an excess of triglycerides.
  2. Titration of WVO sample. Optimal pH for Biodiesel is 7 (neutral), the same as distilled water (and most tap water). Some fat has a high level of free fatty acids which require an acid esterification (to obtain an pH lower than 3) before the alkaline transesterification.
  3. Mixing the bioalcohol (methanol or ethanol) and catalyst (sodium hydroxide) in exact amounts, to produce methoxide
  4. Combining at 50C methoxide with the biolipids.
  5. Separation:
    1. Of biodiesel and glycerol (by decantation, centrifugation...).
    2. Removing of alcohol (by distillation).
  6. Biodiesel purification: separation from the biodiesel of the wastes (catalyst and soap): washing and drying the biodiesel.
  7. Disposing of the wastes.

There are three basic routes to biodiesel production from biolipids (biological oils and fats):

  • Base catalyzed transesterification of the biolipid.
  • Direct acid catalyzed transesterification of the biolipid.
  • Conversion of the biolipid to its fatty acids and then to biodiesel.

Almost all biodiesel is produced using base catalyzed transesterification as it is the most economical process requiring only low temperatures and pressures and producing a 98% conversion yield.For this reason only this process will be described bellow.

Transestrification is crucial for producing biodiesel from biolipids. The transesterification process is the reaction of a triglyceride (fat/oil) with an bioalcohol to form esters and glycerol.

Reaction

The reaction may be shown

CH2COOR1
|
CHCOOR1 + 3 CH3OH <------------> (CH2OH)2CH-OH + 3 CH3COO-R1
|
CH2COOR1

Since we are dealing with nature, the alkyl group on the triglycerides are probably different, so it would actually be more like

                 
CH2OC=OR1
|
CHOC=OR2 + 3 CH3OH <------------> (CH2OH)2CH-OH + CH3COO-R1 + CH3COO-R2 + CH3OC=O-R1
|
CH2COOR3

Triglyceride + methanol <-------> Glycerol + Esters

R1, R2, R3 : Alkyl group.

During the esterification process, the triglyceride is reacted with alcohol in the presence of a catalyst, usually a strong alkaline (NaOH, KOH or sodium silicate). The main reason for doing a titration to produce biodiesel, is to find out how much alkaline is needed to insure a complete transesterfication. Empirically 6.25 g / l NaOH produces a very useable fuel. One uses about 6g NaOH when the WVO is light in colour and about 7g NaOH when it is dark in colour.

The alcohol reacts with the fatty acids to form the mono-alkyl ester (or biodiesel) and crude glycerol. The reaction between the biolipid (fat or oil) and the alcohol is a reversible reaction so the alcohol must be added in excess to drive the reaction towards the right and ensure complete conversion.

Base catalysed Mechanism

You want to mix the base (KOH,NaOH) with the alcohol to make a reactive anion

KOH + ROH -> RO- + H2O

KOH and NaOH are strong bases, so the reaction equilibrium is far to the right.

The ROH needs to be very dry. Any water in the alcohol will reduce the amount of RO- that gets formed.

The RO- is a reactive guy, so you must be very careful with this stuff. Often in chemistry alcohols are mixed with KOH to make a "base bath" for cleaning glass. It actually dissolves the surface of the glass, so be really careful with this stuff.

Once the RO- group is formed, it is added to the triglyceride. The Sn2 reaction that follows replaces the alkyl group on the tricglyceride in a series of reactions.

The carbon on the ester of the triglyceride has a slight positive charge, and the oxygens have a slight negative charge, most of which is located on the oxygen in the double bond. This charge is what attracts the RO- to the reaction site

   
                       R1
    backside attack    |
RO- -----------------> C=O
                       |
                       O-CH2-CH-CH2-O-C=O
                             |        |
                             O-C=O    R3
                               |
                               R2

This yields a transition state that has a pair of electrons from the C=O bond now located on the oxygen that was in the C=O bond.

  R1
  |
RO-C-O- (pair of electrons)
  |
  O-CH2-CH-CH2-O-C=O
        |        |
        O-C=O    R3
          |
          R2

These electrons then fall back to the carbon and push off the glycol forming the ester.

  R1
  |
RO-C=O
+  
  -O-CH2-CH-CH2-O-C=O
         |        |
         O-C=O    R3
           |
           R2

Then 2 more RO's react via this mechanism at the other 2 C=O groups. This type of reaction has several limiting factors. RO- has to fit in the space where there is a slight positive charge on the C=O. So MeO- works well because it is small. As the R on RO- gets bigger, reaction rates decrease. This effect is called steric hinderance. That is why methanol and ethanol are typically used.

There are several competing reactions, so care must be taken to ensure the desired reaction pathway occurs. Most methods do this by using an excess of RO-.

The acid catalysed method is a slight variance, but is also affected by steric hinderance.

Process

The reaction mix is kept just above the boiling point of the alcohol (around 70 C) to speed up the reaction and the reaction takes place. Recommended reaction time varies from 1 to 8 hours, and some systems recommend the reaction take place at room temperature.Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters.

  • The glycerin phase is much more dense than biodiesel phase and the two can be gravity separated with glycerin simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.

  • Once the glycerin and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation. In others systems, the alcohol is removed and the mixture neutralized before the glycerin and esters have been separated. In either case, the alcohol is recovered using distillation equipment and is re-used. Care must be taken to ensure no water accumulates in the recovered alcohol stream.

  • The glycerin by-product contains unused catalyst and soaps that are neutralized with an acid and sent to storage as crude glycerin (water and alcohol are removed later, chiefly using evaporation, to produce 80-88% pure glycerin).

  • Once separated from the glycerin, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.

Fuel quality, standards and properties

Biodiesel is a clear amber-yellow liquid with a viscosity similar to petrodiesel (the industry term for diesel produced from petroleum).

Pure Biodiesel (BD100) can be used in any petroleum diesel engine.Biodiesel has the disadvantage of degrading rubber gaskets and hoses in older vehicles (prior to 1992), but not in newer vehicles. When mixed with petrodiesel, biodiesel can be used at any concentration and is commonly referred to according to its "BD factor": 20% biodiesel is called BD20.

Properties necessary for biodiesel to ensure trouble-free operation in diesel engines are:

  • Complete reaction.
  • Removal of glycerin.
  • Removal of catalyst.
  • Removal of alcohol.
  • Absence of Free fatty acids.

The basic industrial tests includes gas chromatography that verifies only the really more important variables (glycerides,...). More complete testings cost more.

The international standard for biodiesel is ISO 14214.

Properties

  • Because biodiesel is an effective solvent, it also cleans the fuel system. Fuel filters catch petrodiesel particulates when biodiesel is used, so clogging is not an issue once the precipitates have been removed (In a study at a U.S. military base, a biodiesel blend was used as a replacement for heating oil at housing on the base. Due to the solvent power of biodiesel, residues that had been present in fuel tanks for decades were dissolved. The particulate component of the residues caused repeated clogging of fuel strainers, requiring repeated replacement, cleaning, and in some cases installation of higher capacity filters. Due to the relatively smaller surface area and service life of fuel tanks in motor vehicles and mobile equipment, filter clogging is less prevalent but still a factor to be considered).
  • Biodiesel reduces emissions carbon monoxide (CO) by approximately 50%. It does produce as much carbon dioxide as regular diesel.
  • Biodiesel contain less aromatic hydrocarbons: benzofluoranthene - 56%; Benzopyrenes - 71%.
  • It also eliminates sulfur emissions (SO2), because biodiesel doesnt include sulfur.
  • Reduces by as much as 65% the emission of particulates (small particles).
  • Diesel vehicles burning BD100 can utilize modern catalytic converters to eliminate NOx emissions, which are similar to petrodiesel.
  • It has a higher cetane rating (less knocking) than petrodiesel

Availability

Biodiesel is commercially available in most oilseed-producing states in the U.S At this time, it is considerably more expensive than fossil diesel. Many farmers who raise oilseeds use a biodiesel blend in tractors and equipment as a matter of policy, to foster production of biodiesel and raise public awareness. Similarly, some agribusinesses and others with ties to oilseed farming use biodiesel for public relations reasons. In 2003 some tax credits are available in the U.S. for using biodiesel. In 2002 almost 3.5 million gallons of commercially produced biodiesel were sold in the U.S., up from less than 0.1 million gallons in 1998. Due to increasing pollution control requirements and tax relief, the U.S. market is expected to grow to 1 or 2 billion gallons by 2010. The price of biodiesel has come down from an average $3.50/gallon in 1997 to $1.85 a gallon in 2002. However this is still higher than petrodiesel which averaged about $0.85 a gallon in 2002 before road tax is added.

See also: batch, bioalcohol, bubble wash, environmental economics, energy balance, ethylester biodiesel, how to make Biodiesel, hydrogen car, renewable energy, SVO, WVO.

External links




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