View source for Bioconversion of biomass to mixed alcohol fuels

{{Short description|Biochemical process}}
{{Use dmy dates|date=October 2014}}
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.

The process involves a biological/chemical method for converting any [[biodegradation|biodegradable]] material (e.g., urban wastes, such as [[municipal solid waste]], [[biodegradable waste]], and [[sewage treatment|sewage sludge]], agricultural residues such as [[corn stover]], sugarcane [[bagasse]], [[cotton gin]] trash, [[manure]]) into useful chemicals, such as [[carboxylic acids]] (e.g., [[Acetic acid|acetic]], [[Propionic acid|propionic]], [[butyric acid]]), [[ketones]] (e.g., acetone, [[methyl ethyl ketone]], [[diethyl ketone]]) and [[biofuels]], such as a mixture of primary alcohols (e.g., ethanol, [[Propan-1-ol|propanol]], [[n-Butanol|''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]].<ref>[http://soilcrop.tamu.edu/podcasts/seminars/MarkHoltzapple/9-5-07-640.wmv Advanced Biomass Refinery – Third-Generation 2007 (video)] {{webarchive|url=https://web.archive.org/web/20100701101942/http://soilcrop.tamu.edu//podcasts/seminars/MarkHoltzapple/9-5-07-640.wmv |date=1 July 2010 }}</ref><ref>[http://www.epa.gov/Region6/6pd/pd-u-sw/wte_ftworth/cafo/holtzapple.pdf EPA Presentation on the process]{{dead link|date=November 2016 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{Cite web |title=Application of the Mixalco Process to in-Situ Conversion of Dairy Manure and Chipped Yard Waste for Production of Fuels and Chemicals |url=https://aiche.confex.com/aiche/2005/techprogram/P14000.HTM |access-date=2022-12-29 |website=aiche.confex.com}}</ref>
[[Image:MixAlco Pilot Plant.JPG|right|thumbnail|500px|Pilot Plant ([[College Station, Texas|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 [[anaerobic digestion|anaerobically digest]] biomass into a mixture of carboxylic acids produced during the [[acidogenesis|acidogenic]] and [[acetogenesis|acetogenic]] stages of [[anaerobic digestion]], however with the inhibition of the [[methanogenesis|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.<ref>{{cite journal
  | author = F.K. Agbogbo, M.T. Holtzapple
  | title = Fixed-bed fermentation of rice straw and chicken manure using a mixed culture of marine mesophilic microorganisms.
  | journal = Bioresource Technology
  | volume = 98
  | issue = 8
  | pages = 1586–1595
  | date = 23 August 2005
  | url =
  | doi = 10.1016/j.biortech.2006.06.021
  | pmid = 16962320}}</ref>

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 [[Biochemical oxygen demand|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==
{{Details|Carboxylic acid}}
[[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.<ref>Williamson, S.A. 2000. Conversion of carboxylate salts to carboxylic acids via reactive distillation. M.S. Thesis</ref>

==Ketones==
{{Details|Ketone}}
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]].<ref>Yeh, H. 2002. Pyrolytic decomposition of carboxylate salts. M.S. thesis</ref> The other method for making ketones consists on converting the vaporized carboxylic acids on a [[catalyst|catalytic bed]] of [[zirconium oxide]].<ref>Ingram, D. 2002. Ketonization of acetic acid. B.S. student report.</ref>

==Alcohols==
{{Details|Alcohol fuel}}

===Primary alcohols===

The undigested residue from the fermentation may be used in [[gasification]] to make [[hydrogen]] (H<sub>2</sub>). This H<sub>2</sub> can then be used to [[hydrogenolysis|hydrogenolyze]] the [[esters]] over a catalyst (e.g., copper chromite),<ref>{{Cite patent|number=CA1229096A|title=Process for hydrogenolysis of carboxylic acid esters|gdate=1987-11-10|invent1=Bradley|invent2=Harris|invent3=Turner|inventor1-first=Michael W.|inventor2-first=Norman|inventor3-first=Keith|url=https://patents.google.com/patent/CA1229096A/en}}</ref> 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 (disambiguation)|heptanol]]).<ref>{{Cite journal |last1=Filachione |first1=E. M. |last2=Costello |first2=E. J. |last3=Fisher |first3=C. H. |date=1951 |title=Preparation of Esters by Reaction of Ammonium Salts with Alcohols |url=https://pubs.acs.org/doi/abs/10.1021/ja01155a075 |journal=Journal of the American Chemical Society |language=en |volume=73 |issue=11 |pages=5265–5267 |doi=10.1021/ja01155a075 |bibcode=1951JAChS..73.5265F |issn=0002-7863|url-access=subscription }}</ref> 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 [[hydrogenation|hydrogenating]] over a catalyst (e.g., Raney nickel) the corresponding ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone).<ref>Aldrett-Lee, S. 2000. [https://www.proquest.com/openview/e1a84d44f8e9d8d211e314973c0dcbcb/1?pq-origsite=gscholar&cbl=18750&diss=y Catalytic hydrogenation of liquid ketones with emphasis on gas–liquid mass transfer]. PhD dissertation</ref>

==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.<ref>{{Cite web |url=http://nzic.org.nz/ChemProcesses/energy/7D.pdf |title=Production of gasoline from methanol. |access-date=27 November 2010 |archive-url=https://web.archive.org/web/20121201041305/http://nzic.org.nz/ChemProcesses/energy/7D.pdf |archive-date=1 December 2012 |url-status=dead |df=dmy-all }}</ref>

==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 market]]s will become more and more difficult.{{fact|date=October 2014}}

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

<div style="text-align: center; margin: 1em 10%; border: 1px solid">
C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>  &nbsp; &nbsp; → &nbsp; &nbsp; 2 CH<sub>3</sub>CH<sub>2</sub>OH &nbsp; + &nbsp; 2 CO<sub>2</sub> {{center|(Biological production of ethanol)}}
</div>

<div style="text-align: center; margin: 1em 10%; border: 1px solid">
C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>  &nbsp; &nbsp; → &nbsp; &nbsp; 3 CH<sub>3</sub>COOH {{center|(Biological production of [[acetic acid]])}}
</div>

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.<ref>{{Cite journal |last1=Eggeman |first1=T. |last2=Verser |first2=D. |last3=Weber |first3=E. |date=2005-04-29 |title=An Indirect Route for Ethanol Production |url=http://www.osti.gov/servlets/purl/841137-u4bFGI/ |language=en |pages=841137 |doi=10.2172/841137}}</ref>

<div style="text-align: center; margin: 1em 10%; border: 1px solid">
  3 CH<sub>3</sub>COOH &nbsp; + &nbsp; 6 H<sub>2</sub>  &nbsp; &nbsp; → &nbsp; &nbsp; 3 CH<sub>3</sub>CH<sub>2</sub>OH &nbsp; + &nbsp; 3 H<sub>2</sub>O {{center|(Hydrogenation of acetic acid)}}
</div>

<div style="text-align: center; margin: 1em 10%; border: 1px solid">
C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> (from [[cellulose]]) &nbsp; + &nbsp; 6 H<sub>2</sub> (from lignin)  &nbsp; &nbsp; → &nbsp; &nbsp; 3 CH<sub>3</sub>CH<sub>2</sub>OH &nbsp; + &nbsp; 3 H<sub>2</sub>O {{center|(Overall reaction)}}
</div>

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&nbsp;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.{{Citation needed|date=December 2022}}

== See also ==
{{Portal|Energy|Renewable energy}}
*[[Anaerobic digestion]]
*[[Bioreactor]]
*[[Mechanical biological treatment]]

==References ==
{{reflist|colwidth=30em}}

{{Bioenergy}}

{{DEFAULTSORT:Bioconversion Of Biomass To Mixed Alcohol Fuels}}
[[Category:Anaerobic digestion]]
[[Category:Biodegradable waste management]]
[[Category:Alcohol fuels]]
[[Category:Waste treatment technology]]
[[Category:Biomass]]