Bioconversion of biomass to mixed alcohol fuels

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The bioconversion of biomass to mixed alcohol fuels can be accomplished using the MixAlco process. Through bioconversion of biomass to a mixed alcohol fuel, more energy from the biomass will end up as liquid fuels than in converting biomass to ethanol by yeast fermentation.

Contents

The process involves a biological/chemical method for converting any biodegradable material (e.g., urban wastes, such as municipal solid waste, biodegradable waste, and sewage sludge, agricultural residues such as corn stover, sugarcane bagasse, cotton gin trash, manure) into useful chemicals, such as carboxylic acids (e.g., acetic, propionic, butyric acid), ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone) and biofuels, such as a mixture of primary alcohols (e.g., ethanol, propanol, n-butanol) and/or a mixture of secondary alcohols (e.g., isopropanol, 2-butanol, 3-pentanol). Because of the many products that can be economically produced, this process is a true biorefinery. [1] [2] [3]

Pilot Plant (College Station, Texas) MixAlco Pilot Plant.JPG
Pilot Plant (College Station, Texas)

The process uses a mixed culture of naturally occurring microorganisms found in natural habitats such as the rumen of cattle, termite guts, and marine and terrestrial swamps to anaerobically digest biomass into a mixture of carboxylic acids produced during the acidogenic and acetogenic stages of anaerobic digestion, however with the inhibition of the methanogenic final stage. The more popular methods for production of ethanol and cellulosic ethanol use enzymes that must be isolated first to be added to the biomass and thus convert the starch or cellulose into simple sugars, followed then by yeast fermentation into ethanol. This process does not need the addition of such enzymes as these microorganisms make their own. [4]

As the microorganisms anaerobically digest the biomass and convert it into a mixture of carboxylic acids, the pH must be controlled. This is done by the addition of a buffering agent (e.g., ammonium bicarbonate, calcium carbonate), thus yielding a mixture of carboxylate salts. Methanogenesis, being the natural final stage of anaerobic digestion, is inhibited by the presence of the ammonium ions or by the addition of an inhibitor (e.g., iodoform). The resulting fermentation broth contains the produced carboxylate salts that must be dewatered. This is achieved efficiently by vapor-compression evaporation. Further chemical refining of the dewatered fermentation broth may then take place depending on the final chemical or biofuel product desired.

The condensed distilled water from the vapor-compression evaporation system is recycled back to the fermentation. On the other hand, if raw sewage or other waste water with high BOD in need of treatment is used as the water for the fermentation, the condensed distilled water from the evaporation can be recycled back to the city or to the original source of the high-BOD waste water. Thus, this process can also serve as a water treatment facility, while producing valuable chemicals or biofuels.

Because the system uses a mixed culture of microorganisms, besides not needing any enzyme addition, the fermentation requires no sterility or aseptic conditions, making this front step in the process more economical than in more popular methods for the production of cellulosic ethanol. These savings in the front end of the process, where volumes are large, allows flexibility for further chemical transformations after dewatering, where volumes are small.

Carboxylic acids

Carboxylic acids can be regenerated from the carboxylate salts using a process known as "acid springing". This process makes use of a high-molecular-weight tertiary amine (e.g., trioctylamine), which is switched with the cation (e.g., ammonium or calcium). The resulting amine carboxylate can then be thermally decomposed into the amine itself, which is recycled, and the corresponding carboxylic acid. In this way, theoretically, no chemicals are consumed or wastes produced during this step. [5]

Ketones

There are two methods for making ketones. The first one consists on thermally converting calcium carboxylate salts into the corresponding ketones. This was a common method for making acetone from calcium acetate during World War I. [6] The other method for making ketones consists on converting the vaporized carboxylic acids on a catalytic bed of zirconium oxide. [7]

Alcohols

Primary alcohols

The undigested residue from the fermentation may be used in gasification to make hydrogen (H2). This H2 can then be used to hydrogenolyze the esters over a catalyst (e.g., copper chromite), [8] which are produced by esterifying either the ammonium carboxylate salts (e.g., ammonium acetate, propionate, butyrate) or the carboxylic acids (e.g., acetic, propionic, butyric acid) with a high-molecular-weight alcohol (e.g., hexanol, heptanol). [9] From the hydrogenolysis, the final products are the high-molecular-weight alcohol, which is recycled back to the esterification, and the corresponding primary alcohols (e.g., ethanol, propanol, butanol).

Secondary alcohols

The secondary alcohols (e.g., isopropanol, 2-butanol, 3-pentanol) are obtained by hydrogenating over a catalyst (e.g., Raney nickel) the corresponding ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone). [10]

Drop-in biofuels

The primary or secondary alcohols obtained as described above may undergo conversion to drop-in biofuels, fuels which are compatible with current fossil fuel infrastructure such as biogasoline, green diesel and bio-jet fuel. Such is done by subjecting the alcohols to dehydration followed by oligomerization using zeolite catalysts in a manner similar to the methanex process, which used to produce gasoline from methanol in New Zealand. [11]

Acetic acid versus ethanol

Cellulosic-ethanol manufacturing plants are bound to be net exporters of electricity because a large portion of the lignocellulosic biomass, namely lignin, remains undigested and it must be burned, thus producing electricity for the plant and excess electricity for the grid. As the market grows and this technology becomes more widespread, coupling the liquid fuel and the electricity markets will become more and more difficult.[ citation needed ]

Acetic acid, unlike ethanol, is biologically produced from simple sugars without the production of carbon dioxide:

C6H12O6     2 CH3CH2OH   +   2 CO2
(Biological production of ethanol)
C6H12O6     3 CH3COOH
(Biological production of acetic acid)

Because of this, on a mass basis, the yields will be higher than in ethanol fermentation. If then, the undigested residue (mostly lignin) is used to produce hydrogen by gasification, it is ensured that more energy from the biomass will end up as liquid fuels rather than excess heat/electricity. [12]

3 CH3COOH   +   6 H2     3 CH3CH2OH   +   3 H2O
(Hydrogenation of acetic acid)
C6H12O6 (from cellulose)   +   6 H2 (from lignin)      3 CH3CH2OH   +   3 H2O
(Overall reaction)

A more comprehensive description of the economics of each of the fuels is given on the pages alcohol fuel and ethanol fuel, more information about the economics of various systems can be found on the central page biofuel.

Stage of development

The system has been in development since 1991, moving from the laboratory scale (10 g/day) to the pilot scale (200 lb/day) in 2001. A small demonstration-scale plant (5 ton/day) has been constructed and is under operation and a 220 ton/day demonstration plant is expected in 2012.

See also

Related Research Articles

Alcohol (chemistry) Type of organic compound

In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl functional group (−OH) bound to a saturated carbon atom. The term alcohol originally referred to the primary alcohol ethanol, which is used as a drug and is the main alcohol present in alcoholic drinks. An important class of alcohols, of which methanol and ethanol are the simplest members, includes all compounds for which the general formula is CnH2n+1OH. Simple monoalcohols that are the subject of this article include primary, secondary and tertiary alcohols.

Carboxylic acid Organic compound

A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

Ester Chemical compounds consisting of a carbonyl adjacent to an ether linkage

An ester is a chemical compound derived from an acid in which at least one –OH hydroxyl group is replaced by an –O– alkyl (alkoxy) group, as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are fatty acid esters of glycerol; they are important in biology, being one of the main classes of lipids and comprising the bulk of animal fats and vegetable oils.

Butanol (also called butyl alcohol) is a four-carbon alcohol with a formula of C4H9OH, which occurs in five isomeric structures (four structural isomers), from a straight-chain primary alcohol to a branched-chain tertiary alcohol; all are a butyl or isobutyl group linked to a hydroxyl group (sometimes represented as BuOH, n-BuOH, i-BuOH, and t-BuOH). These are n-butanol, 2 stereoisomers of sec-butanol, isobutanol and tert-butanol. Butanol is primarily used as a solvent and as an intermediate in chemical synthesis, and may be used as a fuel. Biologically produced butanol is called biobutanol, which may be n-butanol or isobutanol.

Biofuel Type of biological fuel produced from biomass from which energy is derived

Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. Since biomass can be used as a fuel directly, some people use the words biomass and biofuel interchangeably. However, the word biofuel is usually reserved for liquid or gaseous fuels, used for transportation. The U.S. Energy Information Administration (EIA) follows this naming practice.

Acetate Chemical compound

An acetate is a salt formed by the combination of acetic acid with a base. "Acetate" also describes the conjugate base or ion typically found in aqueous solution and written with the chemical formula C
2
H
3
O
2
. The neutral molecules formed by the combination of the acetate ion and a positive ion are also commonly called "acetates". The simplest of these is hydrogen acetate with corresponding salts, esters, and the polyatomic anion CH
3
CO
2
, or CH
3
COO
.

Ethanol fermentation Biological process that produces ethanol and carbon dioxide as by-products

Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish where it provides energy when oxygen is scarce.

Cellulosic ethanol is ethanol produced from cellulose rather than from the plant's seeds or fruit. It can be produced from grasses, wood, algae, or other plants. It is generally discussed for use as a biofuel. The carbon dioxide that plants absorb as they grow offsets some of the carbon dioxide emitted when ethanol made from them is burned, so cellulosic ethanol fuel has the potential to have a lower carbon footprint than fossil fuels.

Biorefinery

A biorefinery is a refinery that converts biomass to energy and other beneficial byproducts. The International Energy Agency Bioenergy Task 42 defined biorefining as "the sustainable processing of biomass into a spectrum of bio-based products and bioenergy ". As refineries, biorefineries can provide multiple chemicals by fractioning an initial raw material (biomass) into multiple intermediates that can be further converted into value-added products. Each refining phase is also referred to as a "cascading phase". The use of biomass as feedstock can provide a benefit by reducing the impacts on the environment, as lower pollutants emissions and reduction in the emissions of hazard products. In addition, biorefineries are intended to achieve the following goals:

  1. Supply the current fuels and chemical building blocks
  2. Supply new building blocks for the production of novel materials with disruptive characteristics
  3. Creation of new jobs, including rural areas
  4. Valorization of waste
  5. Achieve the ultimate goal of reducing GHG emissions
Alcohol fuel

Various alcohols are used as fuel for internal combustion engines. The first four aliphatic alcohols are of interest as fuels because they can be synthesized chemically or biologically, and they have characteristics which allow them to be used in internal combustion engines. The general chemical formula for alcohol fuel is CnH2n+1OH.

1-Butanol Chemical compound

1-Butanol, also known as butan-1-ol or n-butanol, is a primary alcohol with the chemical formula C4H9OH and a linear structure. Isomers of 1-butanol are isobutanol, butan-2-ol and tert-butanol. The unmodified term butanol usually refers to the straight chain isomer.

Lignocellulosic biomass

Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of biofuels. It is composed of two kinds of carbohydrate polymers, cellulose and hemicellulose, and an aromatic-rich polymer called lignin. Each has distinct chemical behavior. Being a composite of three very different components makes the processing of lignocellulose challenging. The highly evolved resistance to degradation or even separation is referred to as recalcitrance. Overcoming this recalcitrance to produce useful, high value products requires a combination of heat, chemicals, enzymes, and microorganisms. These carbohydrate-containing polymers contain different sugar monomers and they are covalently bound to lignin. Lignocellulosic biomass can be broadly classified into virgin biomass, waste biomass, and energy crops. Virgin biomass includes all naturally occurring terrestrial plants such as trees, bushes and grass. Waste biomass is produced as a low value byproduct of various industrial sectors such as agriculture and forestry. Energy crops are crops with high yield of lignocellulosic biomass produced to serve as a raw material for production of second generation biofuel; examples include switchgrass and Elephant grass.

Butanol fuel Fuel for internal combustion engines

Butanol may be used as a fuel in an internal combustion engine. It is more similar to gasoline than it is to ethanol. A C4-hydrocarbon, butanol is a drop-in fuel and thus works in vehicles designed for use with gasoline without modification. Both n-butanol and isobutanol have been studied as possible fuels. Both can be produced from biomass as well as from fossil fuels. The chemical properties depend on the isomer, not on the production method.

Fermentation Metabolic process

Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.

Acetone–butanol–ethanol fermentation Chemical process

Acetone–butanol–ethanol (ABE) fermentation, also known as the Weizmann process, is a process that uses bacterial fermentation to produce acetone, n-butanol, and ethanol from carbohydrates such as starch and glucose. It was developed by chemist Chaim Weizmann and was the primary process used to produce acetone, which was needed to make cordite, a substance essential for the British war industry during World War I.

Second-generation biofuels, also known as advanced biofuels, are fuels that can be manufactured from various types of non-food biomass. Biomass in this context means plant materials and animal waste used especially as a source of fuel.

Biogasoline or biopetrol is a type of gasoline produced from biomass such as algae. Like traditionally produced gasoline, it is made up of hydrocarbons with 6 (hexane) to 12 (dodecane) carbon atoms per molecule and can be used in internal-combustion engines. Biogasoline is chemically different from biobutanol and bioethanol, as these are alcohols, not hydrocarbons.

Syngas fermentation, also known as synthesis gas fermentation, is a microbial process. In this process, a mixture of hydrogen, carbon monoxide, and carbon dioxide, known as syngas, is used as carbon and energy sources, and then converted into fuel and chemicals by microorganisms.

Acetic acid Colorless and faint organic acid found in vinegar

Acetic acid, systematically named ethanoic acid, is an acidic, colourless liquid and organic compound with the chemical formula CH3COOH. Vinegar is at least 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water and other trace elements.

In industrial paper-making processes, organosolv is a pulping technique that uses an organic solvent to solubilise lignin and hemicellulose. It has been considered in the context of both pulp and paper manufacture and biorefining for subsequent conversion of cellulose to fuel ethanol. The process was invented by Theodor Kleinert in 1968 as an environmentally benign alternative to kraft pulping.

References

  1. Advanced Biomass Refinery – Third-Generation 2007 (video) Archived 1 July 2010 at the Wayback Machine
  2. EPA Presentation on the process [ permanent dead link ]
  3. Application
  4. F.K. Agbogbo, M.T. Holtzapple (23 August 2005). "Fixed-bed fermentation of rice straw and chicken manure using a mixed culture of marine mesophilic microorganisms". Bioresource Technology. 98 (8): 1586–1595. doi:10.1016/j.biortech.2006.06.021. PMID   16962320.
  5. Williamson, S.A. 2000. Conversion of carboxylate salts to carboxylic acids via reactive distillation. M.S. Thesis
  6. Yeh, H. 2002. Pyrolytic decomposition of carboxylate salts. M.S. thesis
  7. Ingram, D. 2002. Ketonization of acetic acid. B.S. student report.
  8. Bradley, M.W., Harris, N., Turner, K. 1982. Process for Hydrogenolysis of Carboxylic Acid Esters WO 82/03854, Nov. 2011
  9. Preparation of esters by reaction of ammonium salts with alcohols.
  10. Aldrett-Lee, S. 2000. Catalytic hydrogenation of liquid ketones with emphasis on gas–liquid mass transfer. PhD dissertation
  11. "Production of gasoline from methanol" (PDF). Archived from the original (PDF) on 1 December 2012. Retrieved 27 November 2010.
  12. Eggeman, T., Verser, D., and Weber, E. (2005), An Indirect Route for Ethanol Production US Department of Energy