jueves, 10 de enero de 2013

The problems of biodiesel: chemical differentiation with petrodiesel

Dear readers,
This is the first post on the practical problems of using biodiesel in our engines. Written by a chemical engineer with decades of experience, is a very technical post but also very enlightening little problem addressed: the imperfect substitutability of conventional diesel (petrodiesel) for biodiesel.


When I think of biodiesel, I am reminded of the phrase "Nothing new under the sun", and already in 1893 the German engineer Rudolf Diesel was operated for the first time a single cylinder engine fueled with none other than peanut oil.

Its vision was amazing because in a speech in 1912 he said: "the use of vegetable oils as engine fuel may seem insignificant today, but over time will be important as a substitute for oil and coal."

Despite this beginning, vegetable oils were replaced quickly by hydrocarbons obtained from the distillation of petroleum because of its lower cost and better technical properties and advantages. The fraction of oil used as fuel for diesel engines, is what we call diesel (gas oil, diesel) in honor of the inventor of the motor.

Following the oil crisis of the 70s of last century, was revived again the use of vegetable oils as a starting point to develop diesel fuels.

Solve problems that produced the direct use of vegetable oils as fuel in diesel engines had seen were mainly:

High viscosity (difficult to spray the combustion chamber)
Low Volatility (hinders the formation of air-fuel mixture)
High gelling temperature (in winter gels formed below 10-15 ° C)
Incomplete combustion tendency to form carbon residues and rubbery polymers in the combustion chamber

These problems are due to chemically vegetable oils are triglycerides, ie esters of various fatty acids and glycerol or glycerin (propanetriol).

Each molecule consists of three fatty acids which may be identical or different and one molecule of glycerol, high molecular weight is responsible for the low volatility and high viscosity.

The fatty acids, hydrocarbon oils are mainly linear chains from C4 to C30 and an acid group terminated. The chains can be saturated (eg, stearic acid) and unsaturated with one (eg oleic acid), two (eg linoleic) and up to three double bonds (linolenic pe).

Fatty acids having many similarities with the hydrocarbon constituents of petroleum diesel (petrodiesel), it was thought that separating the glycerin and changing it by a short chain mono-alcohol (methanol, ethanol) have a behavior very similar to petrodiesel.

Biodiesel developed Thus, the hitherto most widely used process for its manufacture is the transesterification in the breaking-acid glycerol triester and replaced it by a monoalcohol (methanol, ethanol) in the presence of a catalyst typically an alkali strong (soda, potash) the result is the formation of three molecules of Ester (Methyl-Ethyl) and glycerin.

                           1 3 1 3

To favor the reaction yield, it must use an excess of alcohol over the stoichiometric reaction and hot make almost to the boiling point of the alcohol.

Still remaining end a certain amount of unreacted triglycerides, diglycerides (only moves an acid), monoglycerides (moving only two acids) together with glycerin, excess alcohol and catalyst residues esters.

The reaction should be as moisture-free as possible from the presence of water causes the formation of sodium or potassium soaps of fatty acids.

Order to minimize the formation of mono and diglycerides the shorter chain alcohol best performance is obtained when less steric hindrance against triglyceride, so more alcohol used is methanol.

After the reaction must proceed to operations of separation of all the impurities and byproducts of the reaction, a microfiltration and the addition of additives to improve properties of viscosity, freezing point and antioxidants. Store in dry places and deposits specifically designed for this type of fuel.

The methyl esters of fatty acids, containing in its molecule and two oxygen atoms in the fatty acid chains of double bonds (unsaturation) have a very different behavior of petrodiesel hydrocarbons.

The presence of oxygen gives a more polar and therefore a higher viscosity, higher density, a higher degree of gelation temperature, lower energy content per unit volume (7%), increased formation of nitrogen oxides in the combustion, greater propensity to form acidic compounds, greater power attacking and softening solvent and rubber gaskets in contact. In deposits have contained vehicles only petrodiesel, biodiesel putting, dissolve all impurities that were in the funds and conduits, blocking filters and injectors. It is more hygroscopic and absorbs humidity.

The double bond is prone to oxidation, form peroxides and acid compounds, causing the formation of polymers and lacquers in the combustion chamber and on soft metal corrosion (copper, tin, etc.).

As is more easily oxidized, the life time is limited and although biodiesel is kept in suitable tanks and additives are added, lasts no more than about 6 to 10 months.

Another serious problem is that in the presence of moisture, are easily formed precipitating colonies of anaerobic bacteria in sediments gummy deposits and funds in the lines, blocking filters easily.

As advantages with greater power for lubrication and reduced formation of carbon monoxide (CO) in the combustion, lower emissions and hydrocarbon free of aromatic and cycloaliphatic compounds. Low carbonaceous microparticle formation in the exhaust gases and essentially free of sulfur. High biodegradability.

Good to trigger, it has a high cetane number (C16 Hydrocarbons) in diesel would amount to octane in gasoline case

The fatty acid methyl esters or FAME for short Ingés (Fatty Acid Methyl Ester) may be used alone (100%) the oil type called "B100" in some vehicles or blended with petroleum diesel. Usually using a 5% (diesel type "B5")

The oils used as a source of biodiesel are rapeseed and sunflower in temperate countries (Europe) soybean oil and palm oil in tropical countries.

Average yields in processed oil are:

Rapeseed oil: 954 Lt / Ha
Soybean Oil: 922 "
Sunflower Oil: 767 "
Palm Oil: 4570 "

He is working hard on trying to get oils from microalgae cultivation, but not yet at the stage of pilot plant.

Apart from the transesterification method, other methods of modifying both vegetable and animal oils which can obtain different hydrocarbon composition capable of use as fuel for diesel engines, but all energy consumers are as described.

For example, to avoid problems of unsaturation and also select the fatty acids that we consider optimum chain to give a good yield and a high cetane number, for example could distill the FAME in a tower under high vacuum and select the interval distillation optimal for diesel engines, or you could do the acid or alkaline hydrolysis of oils and fatty acids obtained separate, distill and then esterificarlos taste.

To avoid oxidation could hydrogenate unsaturated fatty acids.

Trials have been made for "cracking" of oils as if it were oil but gives a cloud of undesirable molecules apart from low performance in hydrocarbon separation nonviable.

Any of these processes consumes a lot of energy "obscene" in heating, cooling, agitation, filtration, evaporation, treatment of liquid, gaseous, solid, etc..

We should also mention that methanol is obtained from natural gas by partial combustion in the presence of water (synthesis gas) at elevated temperature and pressure and with special copper catalysts.

Chemically, by being able to do what you do want, otherwise energy is the prohibitive cost of any work in addition to the amount of polluting byproducts.

But, if we look at biodiesel as energy carrier for use in special vehicles, ambulances, police, agricultural tractors, etc, you can assume the low current TRE biodiesel and cost.

Given that global consumption in 2007 was petrodiesel MMTn 1157 and assuming an average yield of oil production of 1 ton / ha to produce the oil needed to transform it into biodiesel consumed during this year, it would take about 1200 mm has.

Here is where you see as the absurdity of believing that they can keep the current consumption of biodiesel replacing diesel. Considering all the energy consumed in the cultivation and harvesting of seeds with fertilizers and insecticides and herbicides, and then all the energy consumed in the process of obtaining the FAME will see that the rate of energy return (ERR) is as much 2 -3, well below the 10 admissible as renewable.

Arable. The planet is estimated to have about 5017 MMHA of arable land, 38.5% of the land area, of which grasses take most, so it is estimated that agricultural uses less than a third, 1,530 MMHA only, ie, 0.22 hectares for each of the 7,000 million inhabitants, and this surface is decreasing each year by climate change and increased desertification and other causes.

Primary energy. Consumption in 2006 was 11,500 Mtoe (million tonnes of oil equivalent) with 33% oil, 21% natural gas, 26% coal and 1% away from solar and wind energy.

Cereals. World production in 2006 was of 1,995 Mt (million tonnes) of which 598 were of wheat, 30% corn 693, 35% rice 420, 21%, and the remaining 284 Mt of millet, sorghum and other . Human food consumed 48%, 29% industrial and 23% was for animal feed.

Oilseeds. The production of these seeds in 2006 was 390 Mt, soy being 219, (56%) rapeseed 49, (13%) 42 cotton seed, sunflower Mt 30 and 50 seeds remaining other less known .

Biofuels. Adding the grains and oilseeds have of 2,385 Mt of which we can get an estimated 20% of ethanol or biodiesel, only 500 Mt, ie a theoretical 4% of the energy currently consumed. And that's assuming that people do not empleáramos 48% on food, other than animal feed and industrial needs.

But considering that this production of grain-grain or oilseed-half is destined for human consumption, the rest would cover only 2% of current global energy needs.

Today's agriculture is based on very high consumption of fossil energy in its different operations: preparation, plowing, sowing, fertilizers, irrigation, pesticides, harvesting, transportation, storage and so on that we wish to substitute biofuels.

Besides this agricultural model is deficient in terms of energy efficiency, that is, for a unit of energy, a calorie-spent more than 10 in the works, transport and handling of this forced and industrial agriculture.

The fuel energy is based on the difference in potential energy of the bond, usually regarding carbon-hydrogen bond potential energy once oxidized, CO2 and H2O. Therefore, to obtain molecules with high chemical bonding potential, it is necessary to provide an amount of energy much greater than the difference of binding energies, meaning that all chemical bonds manipulate it entails a very high energy consumption.

Another thing is that the Wise Nature by atmospheric CO2, H2O rain, and metal ions from the soil and the wonderful catalyst chlorophyll in plants, by photon energy captured from sunlight, is able to break bonds O = C = O and HOH to form hydrocarbon chains CH3-CH2-CH2-.... n, with functional groups, acid, ester, alcohol, ketone, aldehyde, incorporation into the molecule of N, S, etc. and mechano these parts and with other enzymes build the complexity of living beings. The solar photon energy accumulated in these links is then released which when oxidized - and re-combustion chemical stability of the starting components. It is a typical example of the mountain and the valley, the CH bond is at the top of the mountain, and once its potential energy released in the oxidation, returns to the valley where most comfortable (law of minimum energy)

Finally, agriculture is not merely the result of photosynthesis, sunlight, water and nutrients, which give crops each year. Oilseed rape, palm soya, etc. make your function at your leisure and even though we do not want to multiply and accelerate production by a factor of a million, since fossil energy-oil, natural gas and coal, which adds 80% of global energy consumption has the same source, but is the result of photosynthesis produced and accumulated over hundreds of millions of years.

In one hundred years, we have consumed what nature has accumulated 100 million each year or "spend" energy capital accumulated over 1 million years! and now pretend that Nature accelerates 1 million times, to continue to spend the same in the current absurd BAU.

Any crop has a yield that is determined by the nature and we can modify but previously providing energy increases and we want to get well, to get better harvests energy will require an effort that so far has been at the expense of the "inexhaustible" fossil energy . Threatened this energy so far has saved us in its unstoppable decline once the Peak Oil, is to replace some agricultural products: cereals and oilseeds are getting precisely with the fossil fuels which are intended to replace, plus environmental degradation that involves the whole process, nutrient depletion, soil and water, thus closing the circle of the impossible.

Maybe we should investigate is to improve the original design to use Mr Diesel engine oils directly, we would avoid chemically manipulate the oil, but the energy problem would remain the same. We can accelerate the process of photosynthesis and the land available for cultivation is limited and unthinkable to maintain current BAU.


Source: http://crashoil.blogspot.com.ar/2012/01/los-problemas-del-biodiesel.html

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