Biomass To Ethanol Process

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Conversion of biomass to ethanol

A brief overview of the conversion of biomass to ethanol follows. It includes definitions of various technical terms and a summary of significant relevant issues and concepts.


Contents

Biofuels

Biofuels are liquid or gaseous fuels made from plant matter and residues such as agricultural crops, municipal wastes and agricultural and forestry by-products. Biofuels most usually refers to liquid fuels used in the transportation sector but also can refer to fuel used for direct combustion for electricity production.

Possible avenues for producing bio-fuels from biomass are

  • ethanol production through microbial fermentation,
  • extraction of oils from crops,
  • pyrolysis and
  • gasification of biomass.[1]

Biofuels are generally dependent on a combination of the following government policies:

  • taxation based policies;
  • agriculture based policies / subsidies;
  • fuel mandates.

Price of raw materials is volatile and represents 60-75% of total production cost for bio-ethanol. Production technology from sugar/starch crops is relatively mature so production cost decreases are not likely to come from that area.

Advantages of biofuels: made from biomass, a renewable feedstock and one of the most abundant reproducible resource on earth; contribute to greenhouse gas emission reduction by replacing fossil fuels.

Biofuels may be broadly divided into bioethanols and biodiesels." [2]pg 2276

Image:CharacteristicsAltFuels.GIF

ZFacts Characteristics of ethanol [3]

Bioethanol

Bioethanol is ethanol produced from a source that was recently alive. Bioethanol can be used as a replacement for petroleum fuel, and interest in its use is increasing due to limited oil reserves, greenhouse gas emission impact on climate change and also a wish to promote of domestic rural economies. with the consequent impact on economic and environment benefits. It is the most widely used biofuel globally.

Biorefining

is the sustainable processing of biomass into a spectrum of bio-based products: food, feed, chemicals, materials and bioenergy:biofuels, power and/or heat [4]

Bioethanol feedstocks

Feedstocks for bioethanol can be divided into three major groups:

(1) sucrose-containing feedstocks e.g. sugar cane, sugar beet,sweet sorghum and fruits,

(2) starchy materials (e.g. corn, milo, wheat, rice, potatoes, cassava, sweet potatoes and barley), and

(3) lignocellulosic biomass e.g. wood, straw, and grasses [2]

  • Lignocellulosic or cellulosic biomass refers to biomass that is not food or feed, and the non-food component of traditional agricultural crops such as rice straw and corn stover. "[5]


Flowchart for the production of bioethanol from lignocellulosic materials [2] Image:FlowchartBiomassToBioethanol.GIF

Feedstocks - issues to be considered

"For a given production line, the comparison of the feedstocks includes several issues: (1) chemical composition of the biomass, (2) cultivation practices, (3) availability of land and land use practices, (4) use of resources, (5) energy balance, (6) emission of greenhouse gases, acidifying gases and ozone depletion gases, (7) absorption of minerals to water and soil, (8) injection of pesticides, (9) soil erosion, (10) contribution to biodiversity and landscape value losses, (11) farm-gate price of the biomass, (12) logistic cost (transport and storage of the biomass), (13) direct economic value of the feedstocks taking into account the coproducts, (14) creation or maintain of employment, and (15) water requirements and water availability"[2]

Insert Table 5 "Bioethanol pathways from different raw materials"[2] pg 2279 Image:ComparisonCropsToBioethanol.GIF

Biofuel production

"Second generation biofuels are produced on two main technology platforms:

Biochemical: conversion of cellulose and hemicellulose to sugars followed by fermentation to alcohol fuels

Thermochemical: gasification to syngas and synthesis to fuels. They aim to reduct costs, broaden range of feedstocks and hence allow increase in scale" [6]

Biomass

Bioenergy is renewable energy made from any organic material from plants or animals. Sources of bioenergy are called "biomass," and include agricultural and forestry residues, municipal solid wastes, industrial wastes, and terrestrial and aquatic crops grown solely for energy purposes.[7]

Table 1 Routes for converting biomass into energy products and services [8]

Issues affecting commercial viability of biomass:

  • "Low energy content combined with high moisture and low bulk density make long distance transport uneconomical - max 200km"[9]
  • Low bulk density also necessitates the use of relatively large equipment for handling, storage and conversion. [9]
  • Variability of biomass properties mean conversion systems must be customised for different fuel classes.[9]


Federal statute (7 USC 7624 § 303) biomass definition

Fermentation

"Fermentation is a biochemical process of fermentation which converts glucose sugar (C6H12O6) to alcohol (C2H5OH) and carbon dioxide gas (CO2). The reactions are driven by microorganisms such as yeast. 

    C6H12O6 -> 2C2H5OH + 2CO2
     Sugar       Alcohol  Carbon dioxide gas
     Glucose    Ethyl alcohol
"[2] pg 2279

Hydrolysis of the cellulosic biomass is achieved by acid or enzymatic hydrolysis or both.

Pre-hydrolysis with dilute or concentrated acid hydrolysis is commonly used to make the cellulose more accessible to hydrolysis by enzymes.

Achievement of economically viable ethanol yield is dependent on optimizing sugar recovery from the hydrolysis process and cost-effectively recovering the acid for recycling.

thermodynamic

upload figure 1 from Biomass Conversion to Biofuels and Biochemicals pg 2

Image:Biomass pathways.GIF[10]


Value and economics of alternatives affected by volatility in global prices of conventional fuels - oil and gas.

Issues identified include food crop competition; land availability; scale of operations; transportation; distribution;development of sustainable strategy independent of government policy changes (re subsidies and price supports)[11]

Conversion processes

Agricultural biomass to ethanol - biochemical processing

"Starch is a high yield feedstock for bioethanol production, but its hydrolysis is required to produce bioethanol by fermentation [77]. Starch is a biopolymer, defined as a homopolymer consisting only one monomer, D-glucose [78]. To produce bio-ethanol from starch it is necessary to break down the chains of this carbohydrate for obtaining glucose syrup, which can be converted into bio-ethanol by yeasts. This type of feedstock is the most utilized for bioethanol production in North America and Europe. Corn and wheat are mainly employed with these purposes [74]. The United States is predominantly a producer of bio-ethanol derived from corn, and production is concentrated in Midwestern states with abundant corn supplies [79]. Corn-based bio-ethanol production in most of the countries assessed is limited, especially compared to the United States. Only Canada reported explicit plans for significant future development of corn-based bio-ethanol, although China has"[2] pg 2276

Hydrolysis of lignocelluloses followed by fermentation is much more complicated than just fermentation of sugar. In hydrolysis the cellulosic part of the biomass is converted to sugars, and fer mentation converts these sugars to bio-ethanol. To increase the yield of hydrolysis, a pretreatment step is needed that softens the biomass and breaks down cell structures to a large extent [85]. A successful pretreatment must meet the following requirements [98]: (1) improve formation of sugars or the ability to subsequently form sugars by hydrolysis, (2) avoid the degradation or loss of carbohydrate; (3) avoid the formation of by-products inhibitory to the subsequent hydrolysis and fermentation processes, and (4) be cost-effective. Hydrolysis without preceding pretreatment[2] pg 2278

Agricultural biomass to ethanol - thermodynamic processing

Cellulosic ethanol

Cellulosic biorefineries in USA are now at pilot stage of trialling the use of corn fibre, cobs and stalks to produce ethanol [12] Woody biomass to ethanol - biochemical processing

Fig 2 pg 2279 - flow chart lignocellulosic to bioethanol [2] pg 2279

"Bioconversion of lignocellosics to bio-ethanol is difficult due to: (1) the resistant nature of biomass to breakdown, (2) the variety of sugars which are released when the hemicellulose and cellulose polymers of which they are comprised are broken and the need to find or genetically engineer organisms to efficiently ferment these sugars, and (3) costs for collection and storage of low density lignocellosic feedstocks. ........ Processing of lignocellulosics to bio-ethanol consists of four major unit operations: (1) pretreatment,(2) hydrolysis, (3) fermentation, and (4) product separation/distillation.[2] pg 2278

Some work has been done on genetically engineering microorganisms to optimise the hydrolysis and fermentation steps. Improving efficiency and cost of ethanol production from woody biomass would reduce the competition for starchy feedstocks such as corn which are also used in food products.

Woody biomass to ethanol - thermodynamic processing

Thermal conversion of wood to ethanol involves combustion, carbonisation and gasification technologies.[13] Combustion may be direct as in direct burning of wood, or indirect as in burning wood in a boiler to provide steam for industrial purposes.


Further reading

[14]

References

  1. ↑ Bio-char sequestration in terrestrial ecosystems: A review pg 412
  2. ↑ ZFacts Characteristics of ethanol | http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=73d5bd86-5a94-4053-a413-6661e1f03426
  3. ↑ Biorefineries: adding value to the sustainable utilisation of biomass pg 7| http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=4a6c5468-cf4e-48c1-aa04-ee42f5986680
  4. ↑ ETAAC Final report
  5. ↑ Overview and outlook for biofuels | http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=c02a6f56-aeb5-4b72-9407-d1390b4c2196
  6. ↑ US Department of Energy[http://www1.eere.energy.gov/biomass/biomass_basics_faqs.html
  7. ↑ Benefits of Bioenergy pg 6 |
  8. ↑ 9.0 9.1 9.2 Strategies for combustion of agricultural biomass fuels | http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=69082d42-6060-43dd-b5b1-980390ce5079
  9. ↑ Biomass conversion to biofuels and biochemicals |http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=51a3fbd7-df13-464d-9988-806d5e5b435a
  10. ↑ Biomass Conversion to Biofuels and Biochemicals
  11. ↑ Overview and outlook for biofuels
  12. ↑ The potential for bioenergy production from Australian forests, its contribution to national greenhouse targets and recent developments in conversion processes | http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=333857c0-f234-4eda-94cb-a2c596bb658f
  13. ↑ Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels Next Generation Hydrocarbon Biorefineries | http://www.moxy.com.au/index.php/moxy-library/metadata?option=com_alfresco&view=file&id=1&uuid=4276adc6-85d3-43c0-8116-c57d05e77516]
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