Biofuels in Gasoline Engines - 2009

8/8/2019 Biofuels in Gasoline Engines - 2009

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POTENTIAL CONTRIBUTIONAND

RESEARCH CHALLENGES FORUTILISING

B IOFUELS IN GASOLINE ENGINES

TO 2030

Adam A Marsh

0404304

15th February 2009


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POTENTIAL CONTRIBUTIONAND

RESEARCH CHALLENGES FORUTILISING

B IOFUELS IN GASOLINE ENGINES

TO 2030

Adam A Marsh

0404304

15th February 2009

MSC SUSTAINABLE ENERGYAND ENVIRONMENT

CARDIFF SCHOOL OF ENGINEERING

CARDIFF UNIVERSITY


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ABSTRACT

Development of biofuels has seen an unprecedented increase in the last

decade, however, it is warned that too-rapid development will lead to

catastrophic mistakes being made, namely to local environments, habitat

destruction and concerns surrounding the fuel for food debate. This report

looks at some of the future potential contributions and research challenges

for the use of biofuels in gasoline, spark ignition engines.

It was found that gasoline fuel replacements to date are limited to alcohols,

and future research is concerned with the production of higher alcohols such

as butanol. The fermentation process is under constant development with the

genetic engineering of bacteria and fungi to increase yields, ferment more

complicated sugars and produce higher alcohols than ethanol.

Future research must aim to peruse the creation of a biofuel replicating

gasoline to ensure its success by enabling seamless integration with existing

infrastructure and spark ignition engines.


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CONTENTS PAGE

1.INTRODUCTION 2

2.FUELS 4

2.1GASOLINE 4

2.2ETHANOL 4

2.3BUTANOL 7

2.4BIOGASOLINE 8

3.M ICROORGANISMS 9

4.PROCESSES 12

4.1FERMENTATION 12

4.2ACETONE-BUTANOL-ETHANOL FERMENTATION 13

4.3SACCHARIFICATION 14

4.4EXPLOSIONS 14

4.5HYDROLYSIS 15

4.6FISCHER-TROPSCH SYNTHESIS 16

5.CONCLUSION 18

APPENDX I

APPENDIX II

REFERENCES

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1.INTRODUCTION

Transport is a key asset to the way we live in the modern world. With globalised

economies, international business and the desire to travel, the need for fuels to

provide the energy needed is ever increasing. With rising prices and depletingreserves of fossil fuels, national desire for security of fuel supply and

sustainability, coupled with the increased awareness of greenhouse gas effects

from fossil fuel combustion, the transport sector is in a critical state to meet

requirements set by governmental legislations and demands of their peers. Road

vehicles account for 93% of the UKs transport greenhouse gasses and are

heavily dependant on fossil fuels.1 Transport can reduce its consumption of fossil

fuels by utilising more energy efficient vehicles, improving public transportsystems and changing the fuels that are used. It would appear that the British

government are concentrating on the latter. In 2005, the Renewable Transport

Fuel Obligation (RTFO) was passed, requiring 5% of total fuel sales to be from

renewable sources by 2010/11, which was expected to be almost entirely from

biofuels.1 Alongside legislation, the government have included tax reductions for

biofuels to aid their market position.

Projections of fossil fuel use into 2030 indicate a gentle turning point around the

year 2020.2 The demand for fuels for transport is expected to continue to rise,

with some of the slack taken up with biofuels, figure 1.1. Bioethanol and

biodiesel are the primary transport biofuels of late and have received rapid

development of industrial scale production and quantity as the substitution of

biofuels for petroleum based fuels has become an important factor in energy

strategies worldwide. In 2007, 12.5 billion gallons of bioethanol was produced,62% of which was produced in the US and Brazil.3 The current interest in

biofuels stems from governments needing to secure energy supplies whilst the

cost of traditional fossil fuels is rising, making alternative fuels closer to being

economically viable, together with recent concerns about greenhouse gas

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emissions and global warming. Biofuels have the potential to be carbon neutral

as it is assumed that each mole of carbon dioxide released during combustion

has been sequestered from the atmosphere during growth of the biomass,

resulting in no net increase in levels of atmospheric carbon dioxide.

Figure 1.1 - Exx onMobil Projections of World petroleum Supply to 203 0 (D.L. Greene,

2007)

Development of biofuels has seen an unprecedented increase in the last decade,

however, it is warned that too-rapid development will lead to catastrophic

mistakes being made, namely to local environments, habitat destruction and

concerns surrounding the fuel for food debate.4 Most of these concerns spread

from the over eager use of first generation biofuels derived from food crops,

namely ethanol. Time to reflect on current technologies and their impacts will

give chance for the more advanced second and third generation biofuels to

reveal their potential and break into the biofuels market. This report looks at

some of the future potential contributions and research challenges for the use of

biofuels in gasoline, spark ignition engines.

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2 FUELS

2.1 Gasoline

Gasoline is a mixture of four major hydrocarbons with 5-12 carbon atoms each;

olefins, aromatics, parafins and napthenes with some contaminants such as

sulphur, nitrogen and oxygen.5 The properties of gasoline such as octane

number, density, reactivity and composition, vary with the source of the crude oil

which results in blending of different gasoline to meet specifications. Key

parameters are volatility and octane number. Gasoline spark ignition engines are

designed to be most efficient at specific volatilities and burning fuels outside the

specified range will result in poor performance, especially during cold start,

engine warming and acceleration. The octane rating is an indicator of a fuels

resistance to detonation under compression. A higher octane rating fuel can

withstand higher compression ratios without detonating, a positive feature of a

spark ignition fuel. Typical gasoline ratings are 91-99.

Is important in this current youthful state of biofuel utilisation in internal

combustion engines that biofuels match gasoline properties to ensure their

uptake in the market and compatibility with existing combustion techniques,

distribution and storage methods. The European Normal for gasoline can be

found in appendix I.

2.2 Ethano l

Ethanol is the same alcohol found in drinks, has the chemical formula C2H5OH

and is the most produced biofuel worldwide.6 Bioethanol can be produced from

sugars or starch by fermentation and has the potential to be used in unmodified

gasoline engines with ethanol blends up to 10% (E10) but specific engines can

run higher blends up to E85.7,8

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Ethanol is perfectly miscible and hydroscopic with water and creates an

azeotrope due to hydrogen bridges formed with the OH branch. This can lead to

the absorption of water vapour into fuel blends from atmospheric humidity

creating storage issues. Petrol-ethanol blends have a higher vaporisation

pressure and low temperatures can cause water and fuel separation. In these

conditions the ethanol will be removed from the gasoline as it will remain in

solution with the water, essentially loosing the ethanol to the atmosphere and

changing the fuel characteristics.9

The use of ethanol fuels in internal combustion engines reduces overall

emissions of particulate matter and CO, however ethanol and acetaldehyde,

which can form dangerous secondary particulate matter, are both released and

resistant to existing catalytic converters. It is rather undetermined if alcohols

increase or decrease NOx emissions, with reports on both sides.10, It is suggested

that NOx formation is dependant on air-fuel ratios. Ethanol poses many threats to

surface soils and ground water, figure 2.2. The corrosive nature of ethanol and

its miscibility with water enable rapid soil and ground water infiltration, risking

damage to underground steel storage and pipe work and requiring new

distribution infrastructure. Clays are dehydrated by ethanol posing building

foundation threats.

Bioethanol has a higher octane rating, flame speed, heat of vaporization and

wider flammability range than gasoline.11 Ethanol does however have a lower

energy density than gasoline, containing about 70% of that in gasoline, due to

constituent water. This lower energy density results in a higher fuel demand for

the same work rate, increasing total CO2 emissions from exhaust gasses. Taking

into account the carbon life cycle of bioethanol results in a marginal CO 2 saving

of 1-5% against gasoline, making it very expensive per tonne of CO2 saved

(103/tonne).10 The net energy value (NEV) of ethanol is heavily debated and

there are voices arguing that ethanol has a NEV deficit whilst others argue

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otherwise.12 Positive ratios appear marginal however, with ratios such as 1.24,13

appendix II. Energies into the production of ethanol include the use of farm

machinery, the cultivation of yeasts, distillation and transport between different

stages. The NEV needs to be optimized for ethanol to be economically and

environmentally viable and this will stem from research into production

processes.

Figure 2.2 - Schemat ic representation of the environmental impacts of ethanol in

gasoline (2005-Niven)

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2.3 Butanol

Butanol is an alcohol of four carbon atoms, C4H9OH and has the potential to be

used in conventional gasoline engines in blends up to 85% butanol.14 Of the

higher order alcohols, 1-butanol has proven to be the easiest to produce using

microbial technology. Butanol shows a number of advantages over its shorter

chained counterpart ethanol. It easily mixes with gasoline, has a similar calorific

value, is less susceptible to water contamination and has a lower vapour

pressure than ethanol.15,16 These properties allow the high potential blends and

eliminate any alterations to conventional engines and fuel distribution and

storage infrastructure. Higher alcohols over ethanol have the ability to be

branched forming isomers, which have higher octane numbers to their straight

chain counterparts.17

Sources of biobutanol from fermentation are the same feedstock as used for

ethanol and bioethanol plants can be retro fitted simply and cost effectively.

Butanol production from sugar is however, three times less efficient than the

production of ethanol.18 Engine combustion characteristics of butanol are not

well known and more research is needed to determine and control the multitude

of potential reaction pathways. Recent studies suggest relatively high emission

levels of carbon monoxide, methane and propene.19

British Petroleum (BP) and DuPont have recently joined their market forces and

biotechnology capabilities to develop the biobutanol process methods.15 This will

be produced at an existing British Sugar mill that currently produces bioethanol

with aims to have a commercially suitable method during 2010.

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2.4 Biogasoline

It has been claimed recently that gasoline can be made directly from poplar and

switch grass biomass, although results are yet to be published. The chemical

changes of cellulosic biomass during a rapid heating and cooling process over

catalysts has been investigated at the University of Massachusetts, USA.20

Cellulose is heated rapidly and briefly and immediately cooled in one step into

gasoline components. At a heating rate of 1000oK/s, half the cellulose energy is

transformed into naphthalene (two fused benzene rings, C10H8) and toluene (a

benzene ring with a single methyl group, C6H5CH3), liquid state high octane

aromatic hydrocarbons which are found in petrol. The catalyst is readily available

and used in the petroleum industry,21 although specifics have been withheld. The

rates of heating and cooling determine the products; slow heating produces coke

and carbon whilst too rapid heating produces vapours. The chemistry of this

process are not yet fully understood, but current yields lie at 50% and estimates

cost 100% yields at $1 a gallon. It may be possible to synthesize further

products from the naphthalene and toluene that are closer to the gasoline blend,

or alternatively add these to existing blends to increase the octane rating.

Alternative methods include the generation of specific fatty acids with carbon

chains of length and composition to those found in gasoline, a similar concept to

biodiesel from waste cooking oils.22 Generic engineering ofE. colihas lead to the

creation of the fatty acids and the enzymes to refine them into equivalent

mineral fuels.23

The creation of a biofuel with the same chemical structures as those of gasoline

would allow seamless integration of the new fuel, using existing infrastructure

and internal combustion engines. If this can be done cost effectively, the future

use of biofuels in the transport sector will be secured for energy security and

independence from fossil fuel reasons alone.

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3.M ICROORGANISMS

There are many microorganisms from a vast range of environments that have

the ability to produce a variety of chemicals that are of use to industry, including

natural fuels. The most common of these natural processes is fermentation bythe yeast Saccharomyces cerevisiaeproducing ethanol. Propanediol and butanol

have also been found to be generated by fermentation of glucose by

microorganisms.24 Species of bacteria, fungi, algae and yeasts have been found

to produce oils, table 3.1. An algal species, Botryococcus brauniihas regularly

been cultivated to produce 50wt% hydrocarbons between C27H52 and C34H58

which individually have high heating values of around 33.8 MJ.kg-1.25,26 The

simplicity of

algae enable very high photosynthetic efficiencies, resulting inincredibly rapid growth rates, far exceeding those of food crops. Biomass can be

doubled in under a day, and can be grown on any land condition in closed cycle

systems.27 Further products can be synthesised from those produced by

microbial activity with additional synthetic processes, such as the cracking of

natural oils into conventional diesel or gasoline.

Table 3.1 Oil content of some m icro-organisms (Meng, 2009)

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Key fields of development in microbial fuel production surround understanding of

the complicated product synthesis pathways, figure 3.1, with the aim to increase

target product yield, predictability and reliability whilst reducing environmental

stresses to the microorganism, production of less desirable products and

economic cost.28 Natural yields of fuel products are often low as increasing yields

tend to create environments in which the organism can no longer function. For

example, butanol is toxic to the bacteria Clostridium acetobutylicum that can

produce it through acetone-butanol-ethanol (ABE) fermentation.29 This makes it

necessary for continuous removal of the product to keep the microbial process

functional, but results in a dilute harvest requiring further processing and

increasing production costs.

Figure 3.1 Schematic representation of 1-butanol production in engineered E. coli.

(2008, Atsumi)

There exists a vast array of sequenced genomes which aids the genetic

engineering of microorganisms to increase the variety of chemicals that can be

produced. Genetic engineering of a single trait often requires manipulation of

multiple genes, which has knock on effects to other processes of the cell.

Inserting genes to yeast for ethanol fermentation of xylose weakens the microbe

and reduces ethanol tolerance.12 Genetic engineering of Escherichia coli is

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showing great potential for the biofuels industry and has proved to be successful

in producing butanol.30 Currently, potential microorganisms producing molecules

of interest are metabolically engineered to enhance the creation of these

products which are then assessed alongside growth rates. Bottlenecks to the

process are determined and attempts to remove them are performed by cellular

profiling, adjustments and repeating of the cycle, figure 3.2. The creation of

systematic models describing microbial pathways are needed to aid the

understanding of cellular physiology enabling rapid prototyping, testing,

optimisation, diagnosis of problems and suitable solutions to further the potential

use of microorganisms to generate fuel products.31

The formation of higher alcohols has required glucose as feedstock. This will

need to be expanded to lower cost feedstock to be economically viable. Future

research will include changing the fermentation process from anaerobic to

aerobic, interspecies transfer of genes for further products, to ensure continued

growth of the organism and formation of the product in higher alcohol

concentrations.24,30

Figure 3.2 Flowchart of systems biology applied to achieving a production target

(2008, Mukhopadhyay)

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fermentation.35 Ethanol derived from sugars or starch crops behave with greater

similarity to gasoline than those from oils or lignocellulosic fibres.36

Removing ethanol from water is performed by distillation which proves to be

energy intensive, most notably when achieving purities over 95%. It has been

estimated that 60% of the energy provided by the combustion of produced

ethanol is needed in its distillation.12 The future of fermentation may be limited

economically by the relatively long, unpredictable nature and low yields of the

process.37 Improvements in separation techniques are evidently required to

improve the process of fermentation and provide a continuous system.

Possibilities aside from distillation include pervaporation (the partial vaporisation

of the batch through a selective membrane), online solvent extraction and

selective adsorption of ethanol from a small stream of fermentation broth. The

latter requires highly selective ethanol adsorbents, alternating adsorption -

desorption columns but has the potential of producing pure ethanol. Powdered

adsorbents are most rapid due to increased surface area.

4.2 Acetone-butanol-ethanol (ABE) fermentation

Solventogenic closteria bacteria can ferment sugars into acetone, ethanol and

most uniquely, butanol.38 The aim of ABE fermentation is to increase the yield of

butanol production and reduce the yield of acetone and ethanol. Yields of

15.8g/L.h have been achieved, however butanol is toxic to the bacteria and

hence needs to be continuously removed.29 This can be done by gas stripping,

using the by bubbling the by-products, hydrogen and carbon dioxide, through

the medium to capture the butanol which is collected after condensing and the

gasses recycled. Alternatively, liquid-liquid extraction is possible by adding oleyl

alcohol (C18H35OH) which is insoluble in the aqueous fermentation broth but

miscible with butanol and floats atop the broth.

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ABE fermentation is not yet commercially viable however. Higher productivity

rates are required via microbial development. Cheaper collection techniques and

developing the ability to use lignocellulosic biomass will reduce the cost of

butanol production by this method. Clostridium acetobutylicumhas a particularly

complicated genetic make-up and E. coli is receiving interest to develop this

field.39

4.3 Saccharification

Saccharification is the degeneration of cell walls into sugars and is required as an

intermediate step to ferment lignocellulosic feedstocks which have carbohydrate

structures such as lignin, cellulose, xylan and starch.40 Cellulose is a linear

crystalline chain of glucose (C6H12O6) containing up to 10,000 units per chain.

Hemi cellulose is a branched polymer with a mix of glucose and xylose (C5H10O5),

with their respective isomers, and is found predominantly in woody biomass.

The cells cellulose crystalline structure wall first needs to be physically broken.

Mechanical methods include chipping, milling and grinding resulting in

approximate sizes of 10-30mm, 2mm ad 0.2mm respectively. Low temperature

pyrolysis (


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than is needed in mechanical processing to the same size, and is most effective

in hardwoods and agricultural residue. Steam explosion may generate inhibitory

compounds to later processes requiring a wash which will also remove any

soluble sugars, however the addition of small amounts of H2SO4, SO2 or CO2 has

been found to decrease the production of such compounds.

Ammonia fibre explosions (AFEX) do not produce inhibitory components, works

at lower temperatures and id good for herbatious crops and grasses. The

ammonia must be recycled however to protect environment, is not very good for

the break down of hemicellulose and longer residence times (30min) are

required. Carbon dioxide explosions are more cost effective than AFEX and dont

form inhibitory compounds but result in lower yields. It is envisaged that the CO2

will form carbonic acid which will increase the rate of hydrolysis.

4.5 Hydrolysis

Hydrolysis is the chemical break down of carbohydrates and starch into simple

sugars.41The equation for starch into glucose is given as equation (4.2).

-[C6H10O6]-n + n(H2O) => n(C6H12O6) + nO2 (4.2)

Hydrolysis can be carried out by use of an acid or by enzymes and is most

effective at the breaking down of cellulose.42 Acid hydrolysis typically uses

Sulphuric acid which is also successful at breaking down xylon, which can

account for 30% of lignocellulosic carbohydrates. Continuous processes require

high temperatures (>160oC) and low loading whilst batch processes can use

lower temperatures and allow higher loading (


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Enzymatic hydrolysis makes use of microorganisms that produce a collection of

cellulose enzymes. The process can be performed in aerobic or anaerobic

conditions by bacteria or fungi although the aerobic conditions of fungi are

preferred. Low substrate concentrations are needed as high concentrations

inhibit enzymatic hydrolysis. Lignin must be removed before enzymatic hydrolysis

as it forms a barrier against access to cellulose. Mild conditions are required and

the process is not corrosive, so is cheaper than acid hydrolysis, but the results

are less predictable and longer times are required.

The development of pre-treatment to fermentation in the production of ethanol

continues to search for a solution that not only produces the highest yield of

simple sugars, namely glucose, but also has a low energy demand and doesnt

produce fermentation or hydrolysis inhibitors. These include eliminating the need

of acids with a movement to enzymatic methods, novel grinding techniques,43

production of many biofuels from one feedstock and process44 and specific yeast

cell surface engineering which may provide direct ethanol production from

lignocellulosic feedstock, bypassing saccharification and hydrolysis all together.45

4.6 Fischer-Tropsch Synthesis

Carbon monoxide and hydrogen (syngas) from gasification (the partial burning of

solid biomass or wastes in reduced oxygen levels) can be converted into liquid

hydrocarbons, or directly to methanol, by use of a catalyst in the Fischer-Tropsch

process. Different temperatures, catalysts and syngas H/C/O ratios will yield

products in varying ratios.46 The two most common reactions are given as

equation (4.3) and (4.4). The most suitable catalysts are iron, ruthenium or

cobalt based. Cobalt is almost 1000 times more expensive than iron, but

produces a higher activity and a longer life. Products contain no nitrogen or

sulphur compounds because syngas is used, creating fuels that are more

environmentally friendly than those derived from fossil oil.47

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5.CONCLUSION

Gasoline fuel replacements to date are limited to alcohols. Ethanol is the most

widely used biofuel for gasoline replacement. The benefits of ethanol have been

debated in recent years and a call to slow down the utilisation of biofuels is

primarily down to the destructive nature in which ethanol has been produced

from food crops. Ethanol is miscible with water which causes storage, transport

and combustion problems when mixed with gasoline. New fuels are emerging to

the market with properties closer to those of gasoline, most notable butanol

which is hydrophilic and highly miscible with gasoline. Fuel research focuses on

creating products from biomass with chemical compositions and properties

replicating those of gasoline. Progress has been made in the lab, creating some

part of the chemicals found in gasoline, but results are yet to be published.

Research in biofuel synthesis aims to improve the cost effectiveness of products

by reducing the creation of un desirable products whilst increasing the yields of

fuels. As alcohols dominate the gasoline biofuel market, most of this research

surrounds fermentation, and the microorganisms that carry out this process.

Genetic engineering of bacteria and fungi to increase yields, ferment morecomplicated sugars and produce higher alcohols to ethanol proves to be the

cutting edge, but modelling of changing microbe genetics would prove to speed

up this field dramatically. Aside from genetic engineering, improvements of

feedstock preparation by breaking down complex carbohydrates by non-

biological means continues to develop whilst catalyst research to convert syngas

into suitable hydrocarbons unearths new methods, but is yet to be economically

viable.

It must be the aim of future research to peruse the creation of a biofuel

replicating gasoline to ensure its success by enabling seamless integration with

existing infrastructure and spark ignition engines.

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APPENDIX I

EN228 European Normal for Petrol 98 (biofuels platform)


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APPENDIX I I

Energy inputs and outputs for the life cycle analysis of mature ethanol

manufacture. (2007 Grands)


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Documents

Biofuels in Gasoline Engines - 2009