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Making Hydrogen a Competitive Alternative to Fossil Fuels

through Anaerobic Digestion of Waste

By Neil Scheibelhut

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Table of Contents

1. Introduction

1.1 Signifcance

1.2 Motivation

1.3 Current Technologies

1.4 b!ective

2. Current Technologies

2.1 Natural "as #e$or%ing

2.2 Bio%ass "asifcation

2.3 Coal "asifcation

2.4 &lectrolysis

2.' Microbial Bio%ass Conversion

3. (ro)osed Bioreactor

3.1 *)+o, -naerobic Sludge Blan et #eactor

3.2 /ood 0aste as /eedstoc

3.3 (retreat%ent o$ Munici)al Sludge

3.3.1 eat Shoc Treat%ent

3.3.2 -cid -l ali Treat%ent

3.3.3 -eration

3.3.4 *ltraviolet Irradiation

3.4 I%)ortance o$ "enetic Se uencing

4. /uture #esearch

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'. Conclusion

1 !ntroduction

1.1 Signifcance

-ccording to the Intergovern%ental (anel on Cli%ate Change greenhouse

gases $ro% hu%an activities are the %ost signifcant driver o$ observed

cli%ate change since the %id526 th century. Carbon dio7ide $ro% burning

$ossil $uel $or energy and heat is the leading greenhouse gas e%ission

8/igure 19. ver :'; o$ the ,orld. nly China

82?;9 )roduces %ore o$ the ,orld.

0ith no %itigation by the year 2166 the &arth. International concerns over global ,ar%ing

are at an all ti%e high there$ore the need $or alternatives to $ossil $uels as

energy sources has never been greater.

Figure 1 Figure "

Figure #

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1.2 Motivation

ydrogen is considered a )ro%ising $uel source as it )roduces only ,ater

u)on co%bustion and is getting increasing attention as a clean energy

alternative to $ossil $uels. Co%)ared to conventional $ossil $uels hydrogen

has the )otential to )roduce t,o and a hal$ ti%es %ore energy than natural

gas or gasoline ='>. In 266' the *.S. &nergy (olicy -ct ,as )assed and

directed the energy secretary to conduct a research and develo)%ent

)rogra% in consultation ,ith other $ederal agencies and the )rivate sector

on technologies related to the )roduction )urifcation distribution

storage and use o$ hydrogen energy $uel cells and related in$rastructureD

=A>.

ydrogen $uel cell technology has been around since the N-S- "e%ini

days and they continue to use hydrogen as an energy source to this day. It

is )ollution $ree ,ith only ,ater )roduced as a by)roduct and scalable.

Because a single $uel cell only )roduces about one volt o$ )o,er they are

co%bined in series to $or% a stac ,hen %ore )o,er is needed =:>. So $uel

cells can be custo%iEed $or any a))lication. -dditionally hydrogen $uel

cells are very eFicient co%)ared ,ith $ossil $uel energy generators. -

gasoline co%bustion engine is only about 26; eFicient ,ith %ost o$ the

energy lost as heat. Co%bustion based )o,er )lants are little %ore

eFective at about 3'; eFiciency. o,ever hydrogen $uel cells can be u) to

A6; eFicient =?>. Because o$ this versatility and eFiciency the *.S.

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Ge)art%ent o$ &nergy is currently )er$or%ing research to %a e the% %ore

eFicient and less e7)ensive to %anu$acture o)erate and %aintain.

o,ever their research also includes hydrogen )roducing technologies as

the cost o$ hydrogen is currently too e7)ensive to be co%)etitive ,ith $ossil

$uels. The Ge)art%ent o$ &nergy esti%ates that a ilogra% o$ hydrogen

,hich is a))ro7i%ately e uivalent to one gallon o$ gasoline needs to cost

bet,een H2.66 and H4.66 by the year 2626 =@>.

1.3 Current Technologies

Currently the only viable hydrogen )roduction technology is natural gas

re$or%ing ,hich uses a t,o ste) a))roach to convert %ethane 8natural

gas9 into hydrogen. In the frst ste) called stea%5%ethane re$or%ing

natural gas is %i7ed ,ith ,ater va)or to create carbon %ono7ide and

hydrogen gas. The carbon %ono7ide is then %i7ed ,ith ,ater va)or in

,hat is re$erred to as the ,ater5gas shi$t reaction ,hich converts the

carbon %ono7ide and ,ater va)or into carbon dio7ide and hydrogen. This

)rocess )roduces hydrogen at a cost o$ H3.?6 )er gallon o$ gasoline

e uivalent 8gge9 dis)ensed. o,ever there are %any dra,bac s to this

technology ,hich ,ill %a e costs )rohibitively e7)ensive to e7)and into the

%ass %ar et =16>. Because o$ this the Ge)art%ent o$ &nergy is

researching other technologies to create hydrogen through gasifcation o$

bio%ass and coal electrolysis and %icrobial bio%ass conversion. &ach o$

these technologies in their current states ho,ever are )rohibitively

e7)ensive and %ore research %ust be )er$or%ed to %a e the% viable.

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1.4 Objective

In order to %a e hydrogen a co%)etitive $uel source to $ossil $uels research

%ust be )er$or%ed to lo,er the cost o$ )roduction.

" Current Technologies

2.1 Natural Gas Re or!ing

Currently @'; o$ all hydrogen is being )roduced in large central

%anu$acturing )lants ,hich utiliEe natural gas re$or%ing technology. It

,or s by co%bining e7tre%ely hot ,ater va)or 8:66 ° C 5 1666 ° C9 ,ith the

%ethane contained in natural gas at high )ressure. In the )resence o$ a

)ro)er catalyst carbon %ono7ide and hydrogen is $or%ed in a )rocess

called the stea%5%ethane re$or%ing reaction 8& uation 19.

819 C 4 2 eat → C 3 2

Ne7t the carbon %ono7ide is co%bined ,ith stea% in the )resence o$ a

catalyst ,hich converts the carbon %ono7ide into carbon dio7ide and

hydrogen 8& uation 29.

829 C 2 → C 2 2 eat

/inally i%)urities are re%oved $ro% the gas strea% leaving )ure hydrogen

in a )rocess called )ressure5s,ing adsor)tion.

This technology is used currently because natural gas is relatively

ine7)ensive and even though carbon dio7ide is )roduced in the )rocess it

is a relatively s%all a%ount ,hen co%)ared ,ith the co%bustion o$ $ossil

$uels =11>. *n$ortunately there are dra,bac s to this technology. Ca)ital

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costs to create satellite re$or%ers a,ay $ro% the large central )lants is too

high to %eet )rice )er gge targets. #e$or%ing units are currently designed

and built one at a ti%e leading to )rohibitively large and e7)ensive units.

-dditionally the re$or%ing stations are too large $or urban settings. nce

built the re$or%ing stations need e7cessive %aintenance and in )erson

%onitoring leading to high o)eration costs. /inally control sa$ety and

environ%ental )rotocols are needed to address concerns ,ithout escalating

costs ,hile %eeting de%ands avoiding lea age and %ini%iEing e%issions

=16>.

2.2 "io!ass Gasifcation

Bio%ass is considered to be any rene,able organic resource that can be

used to )roduce hydrogen. &7a%)les o$ bio%ass include %unici)al solid

,aste ani%al ,aste agricultural ,aste and even cro)s gro,n $or the

s)ecifc )ur)ose o$ being used $or energy use. Bio%ass gasifcation

converts organic carbon based %atter into hydrogen carbon dio7ide and

other gases by heating the bio%ass to over :66 ° C but not burning it. -

controlled a%ount o$ o7ygen and stea% are used in this )rocess ,hich

yields carbon %ono7ide carbon dio7ide hydrogen and other gases

8& uation 39.

839 CA 12 A 2 2 → C C 2 2 other gases

Just li e natural gas re$or%ation the ne7t ste) is a ,ater5gas shi$t reaction

to re%ove the )oisonous carbon %ono7ide $ro% the gas strea% 8& uation

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29. -dsorbers or %e%branes fnally se)arate the hydrogen $ro% the gas

strea% =12>.

This technology is )ro%ising because o$ the abundance o$ bio%ass as a

resource. -ccording to the *.S. Ge)art%ent o$ &nergy by the year 2636

the *nited States ,ill )roduce over one billion dry tons o$ bio%ass that

could )otentially be utiliEed $or gasifcation. Sources o$ this bio%ass

include $orestland agricultural ,aste and energy cro)s =13>. -dditionally

because )lants re%ove carbon dio7ide $ro% the at%os)here the carbon

dio7ide )roduced during gasifcation is oFset ,hich results in a recycling o$

carbon dio7ide ,hich lo,ers greenhouse gas e%issions.

The challenges to bio%ass gasifcation center around high ca)ital costs $or

)urchasing e ui)%ent and high costs associated ,ith bio%ass $eedstoc

=12>.

2.3 Coal Gasifcation

Ki e bio%ass coal can be gasifed through the introduction o$ o7ygen and

stea% under high te%)eratures. This results in synthesis gas ,hich

consists o$ %ostly carbon %ono7ide and hydrogen 8& uation 49. nce

again the ,ater5gas shi$t reaction 8& uation 29 converts the carbon

%ono7ide to carbon dio7ide once the i%)urities o$ the synthesis gas are

re%oved.

849 C 6.? 2 2 → C C 2 2 other gases

The abundance o$ coal resources in the *nited States %a es this technology

attractive due to the lo, cost o$ the coal. o,ever coal gasifcation

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)roduces a large a%ount o$ carbon dio7ide. Gevelo)ing carbon ca)ture

technologies to ensure the dangerous greenhouse gas is not released into

the at%os)here during the )roduction )rocess is needed =14>.

2.4 #lectrol$sis

0ater can be bro en do,n into hydrogen and o7ygen by introducing it to

electricity. This )rocess is called electrolysis. &lectrolysis is )ro%ising as

it generates hydrogen ,ith no greenhouse gas e%issions. o,ever the

electricity used to s)lit ,ater into its ele%ental co%)onents is usually

generated using $ossil $uels. In addition to fnding green electricity sources

ca)ital costs and hydrogen )ac aging costs are challenges that need to be

overco%e $or electrolysis to beco%e a viable source o$ hydrogen =1'>.

2.% Microbial "io!ass Conversion

*nder anaerobic conditions so%e %icroorganis%s $er%ent organic %atter

and create hydrogen. Because no light is needed by these %icroorganis%s

this )rocess is called dar anaerobic $er%entation. This technology is

attractive $or so%e o$ the sa%e reasons as bio%ass gasifcation bio%ass is

an abundant resource and in %any cases it consists o$ ,aste )roducts

,hich can be obtained at little to no cost. -dditionally the %icroorganis%s

use the bio%ass as an energy source so the bioreactors ,hich are used to

generate hydrogen use little )o,er another cost saving attribute. o,ever

hydrogen )roduction is slo, ,ith this technology. #esearch %ust be

)er$or%ed to i%)rove %icrobial co%%unities reactor syste%s and

$eedstoc sources to create higher yields =1A>.

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# $roposed %ioreactor

3.1 &'(o) *naerobic Slu+ge "lan,et Reactor

Gar anaerobic $er%entation has great )otential to create lo, cost

hydrogen. o,ever to ensure hydrogen costs stay lo, the )roduction

)rocess %ust concentrate on using lo, cost $eedstoc and a bioreactor

design ,hich uses lo, energy. The %ost co%%only utiliEed bioreactor

design $or dar anaerobic $er%entation is the u)+o, anaerobic sludge

blan et reactor 8/igure 49. This reactor is good $or lo, cost hydrogen

)roduction because there is no %echanical stirring that %ust be )o,ered.

Mi7ing o$ the substrate ,ith the %icroorganis%s is obtained through gas

generation. -s the bacterial sludge $er%ents the organic substrate ,hich is

)u%)ed into the reactor $ro% the botto% bubbles o$ biogas are $or%ed

,hich churn the li uid causing the desired %i7ing eFect and increases the

rate o$ $er%entation =1:>. o,ever there are still challenges to )roducing

lo, cost hydrogen using this technology. /eedstoc costs %ust be e)t lo,

the bacterial co%%unity ,hich $er%ents the $eedstoc %ust be o)ti%iEed to

increase )roduction rates and lo, energy reactor syste%s %ust be

i%)le%ented to ee) o)erating costs lo,.

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Figure &

3.2 -oo+ aste as -ee+stoc,

-naerobic $er%entative bacteria turn a carbon source usually a fve5carbon

sugar li e glucose into hydrogen carbon dio7ide and organic acids acetic

acid butyric acid and )ro)ionic acid. 0hen considering a $eedstoc $or

biohydrogen 8hydrogen )roduced by living organis%s such as bacteria9

)roduction it should be a %aterial that contains a high )ercentage o$ fve5

carbon sugars. In addition because the $eedstoc is consu%ed there is a

uni ue o))ortunity to use ,aste %aterial to )rovide a solution $or ,aste

re%oval and storage. In a,aii )articularly on ahu $ood ,aste %a es u)

the largest )ortion 812.:;9 o$ all ,aste that %a es its ,ay to ,aste

)rocessing $acilities. ver 126 666 tons o$ $ood ,aste are )roduced each

year on ahu %a ing it a )lenti$ul rene,able resource $or biohydrogen

)roduction not to %ention ee)ing 126 666 tons o$ %atter out o$ the

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island. -dditionally the local diet is rich in carbohydrates

li e rice and tro)ical $ruits so the )otential $or biohydrogen )roduction

$ro% $ood ,aste on ahu %ay be )otentially greater than that o$ %ainland

*S-.

3.3 /retreat!ent o Munici'al Slu+ge

Many %unici)al ,aste ,ater treat%ent $acilities utiliEe bacterial sludge in

their treat%ent )rocess but hydrogen is not a %a!or co%)onent o$ the

biogas )roduced by this )rocess. Because o$ the )resence o$ %any %ethane

)roducing %icroorganis%s in %unici)al sludge %ost o$ the hydrogen

)roduced in the $er%entation )rocess gets converted into %ethane ,hich is

a greenhouse gas that gives oF carbon dio7ide ,hen co%busted.

/ortunately there are %any technologies that can be used to inhibit or

eli%inate %ethanogenic %icroorganis%s. In %ost cases the technologies

)rovide harsh conditions $or the %icroorganis%s in the sludge to induce

s)ore $or%ation. Certain %icroorganis%s create s)ores ,hen )laced in an

environ%ent in ,hich they cannot survive. The s)ores they create

ho,ever can survive very harsh conditions and i$ conditions return to a

state in ,hich the %icroorganis%s can thrive the s)ores hatchD into a ne,

version o$ the original. Methanogenic bacteria have sho,n they are non5

s)ore $or%ing and there$ore are illed in such harsh conditions ,hile %any

hydrogenic bacteria are s)ore $or%ing. By creating such harsh conditions

the %ethanogenic bacteria )erish leaving the desirable hydrogenic bacteria

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to survive and thrive. Theoretically a sludge that has been )retreated in

this %anner should )roduce biogas rich in hydrogen.

3.3.1 0eat Shoc, Treat!ent

eat Shoc Treat%ent 8 ST9 is the %ost co%%only used )retreat%ent

%ethod. It uses heat to create a harsh environ%ent ,hich induces s)ore

creation by hydrogenic bacteria "acillus and Clostri+iu! ,hile illing oF

%ethanogenic bacteria. o,ever as ,ith %any )retreat%ent %ethods that

rely on inducing s)orulation other s)ore )roducing bacteria survive the

)retreat%ent )rocess ,hich )roduces a %i7ed culture that includes

acetogenic bacteria such as "acterioi+es and #ubacteriu! ,hich utiliEe

hydrogen =1@>. Most o$ the ti%e an increase o$ hydrogen yield is seen a$ter

ST co%)ared to an untreated control ,ith a %a7i%u% re)orted yield o$

2.3 %ol hydrogen )er %ol glucose. =26>. Ty)ically sludge is )retreated at a

te%)erature bet,een @65166 ° C $ro% any,here bet,een 16%in to 1h =21>.

3.3.2 *ci+ *l,ali Treat!ent

Because %ethanogenic bacteria survive at neutral ) the addition o$ acid

or al ali to )retreat sludge has been investigated. Ty)ically hydrogen

)roducing bacteria can survive at a ,ider ) range than %ethanogenic

bacteria. Because s)ore $or%ers are not the only bacteria to survive a

%ore diverse %icrobial co%%unity e7ists a$ter acid al ali )retreat%ent =26>.

The %ost co%%on acidic )retreat%ent uses Cl 2S 4 N 3 and Cl 4

=22 23> ,ith Na being the %ost )o)ular al aline treat%ent %ethod =24>.

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Ty)ically the )retreat%ent lasts $or 24h =2'> ,ith a %a7i%u% observed

yield o$ 1.' %ol hydrogen )er %ol glucose =2A>.

3.3.3 *eration

The )ur)ose o$ aeration is to a))ly o7idative stress to obligate anaerobes

,hich causes the% to die. Most %ethanogenic bacteria are obligate

anaerobes ho,ever %any hydrogen )roducing bacteria are also obligate

anaerobes ,hich %a es this %ethod rather ineFicient. -$ter aeration $or

24 hours %a7i%u% hydrogen yield ,as very lo, at 6.: %ol hydrogen )er

%ol glucose =2A>. o,ever aeration has sho,n a very large acetate to

butyrate ratio co%)ared ,ith other %ethods 826.'? L 6.2'9 =24>. This is

signifcant because the acetate )ath,ay )roduces $our %oles o$ hydrogen to

every %ole o$ glucose ,hile the butyrate )ath,ay only )roduces t,o.

-dditionally because o$ the diFerent ind o$ stress this treat%ent )laces on

the sludge co%)ared to other %ethods it results in a diFerent %icrobial

co%%unity. Most )hysical )retreat%ents sho, Clostri+iu! s)). as the

do%inant s)ecies a$ter )retreat%ent but aeration sees #nterobacter s)).

and lebsiella s)) =2A>. This )rovides a uni ue o))ortunity to co%bine

sludge that has been )retreated ,ith o7idative stress ,ith heat shoc ed

sludge $or e7a%)le %a ing $or a %ore robust %icrobial co%%unity.

3.3.4 &ltraviolet rra+iation

*ltraviolet irradiation bo%bards the sludge ,ith ,avelengths $ro% 16n% to

3?6n% creating an environ%ent ,hich sti%ulates s)orulation o$ hydrogenic

bacteria and inhibition or eli%ination o$ %ethanogenic bacteria. This

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%ethod is )roven to increase hydrogen yield by 6.3@ $old co%)ared to

untreated sludge ,hen irradiated $or 1' %in =2:>. The co%)lication o$ this

%ethod is that sludge is usually dar and o)a ue ,hich bloc s the * light

$ro% )enetrating $arther than !ust the sur$ace %a ing this an ineFicient

%ethod. n the other hand this %ethod )resents a uni ue )ossibility to

ee) costs do,n. Solar irradiation could )otentially be used as a substitute

$or * irradiation through electric )o,ered la%)s. #esearch %ust be

)er$or%ed to deter%ine the )ro)er )ara%eters $or this %ethod such as

translucence o$ sludge de)th o$ sludge and ti%e o$ e7)osure to solar

irradiation.

3.4 !'ortance o Genetic Se uencing

"enetic se uencing can be an i%)ortant tool in %a7i%iEing hydrogen

)roduction ,hen utiliEing )retreat%ent technologies. ne se uencing

techni ue called bar coding can be a valuable tool in identi$ying the

%icrobial co%%unity o$ )retreated %unici)al sludge. Bar coding utiliEes

s)ecifc areas o$ the geno%e ,hich are co%)letely uni ue to each s)ecies o$

bacteria. nce such area o$ the geno%e is the 1AS region o$ the riboso%al

#N- =2?>. Isolating this region is uite %anageable and it can be a%)lifed

through (C# easily so the se uence can be obtained %ulti)le ti%es over to

ensure accuracy. -nother se uencing techni ue that can be use$ul is ,hole

geno%e se uencing. -lthough it is a longer %ore e7)ensive and diFicult

)rocess it yields %ore accurate results because entire geno%es are being

co%)ared to database in$o not !ust a short se uence li e those utiliEed in

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bar coding. By using genetic se uencing techni ues to identi$y the

%icrobial co%%unity o$ )retreated sludge a )ossible correlation can be

%ade by co%)aring the hydrogen )roduction o$ each ty)e o$ )retreated

sludge ,ith the %icrobial co%%unity )resent in the sludge. This ,ay

individual s)ecies o$ bacteria can be identifed as vital to the hydrogen

)roducing )rocess. nce the i%)ortant bacteria are identifed an o)ti%al

%icrobial co%%unity %ay be designed to %a7i%iEe hydrogen )roduction.

& Future 'esearch

Ma7i%iEing hydrogen yield through dar anaerobic $er%entation ,hile

%ini%iEing cost is a $uture reality. Many )ara%eters %ust be researched

be$ore this technology is ready $or )roduction on an industrial scale. The

)otential %icrobial co%%unities that could be utiliEed are al%ost infnite

and research %ust be done to identi$y not only the %ost eFicient bacterial

co%%unity $or hydrogen )roduction but also the right co%%unity $or the

s)ecifc $eedstoc being considered. #esearch %ust also be )er$or%ed to

identi$y lo, cost $eedstoc o)tions and ,hether they are better $or dar

anaerobic $er%entation as a )ure $eedstoc or one that has been %i7ed ,ith

another $or%. #eactor designs %ust be researched to o)ti%iEe hydrogen

)roduction ,hile ee)ing o)erating and ca)ital costs lo,. )ti%al

$eedstoc concentration #T te%)erature and ) %ust be identifed.

-dditionally lo, energy reactor syste%s %ust be integrated to re%ove and

)uri$y the hydrogen and drive the syste%.

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( Conclusion

0ith research being )er$or%ed to decrease the cost o$ hydrogen

)roduction biohydrogen is a )ro%ising technology. Biohydrogen $ro%

,aste is even %ore enticing because it solves t,o )otential )roble%s

,aste %anage%ent and $eedstoc cost. (retreat%ent o$ %unici)al sludge

to %a7i%iEe biohydrogen )roduction can be utiliEed to this end. o,ever

%ore research %ust be )er$or%ed to o)ti%iEe hydrogen yields and drive

hydrogen costs do,n. By using genetic se uencing techni ues %ore

insight can be gained into the co%%unity o$ %icroorganis%s that %ay be

best $or hydrogen )roduction. This techni ue can )otentially be used to

o)ti%iEe %icrobial co%%unities $or each ty)e o$ $eedstoc in order to use

ine7)ensive %aterials or even ,aste to %a e hydrogen even %ore

aFordable. Ko, energy bioreactors such as an u)+o, anaerobic sludge

blan et reactor can be used to ee) o)erating costs do,n. o,ever

research %ust be )er$or%ed to identi$y the o)ti%al o)erating s)ecifcations

and reactor syste%s ,hile ee)ing energy invest%ent lo,. I$ hydrogen can

beco%e a lo,er cost alternative to burning $ossil $uels $or energy it %ay be

utiliEed %ore ta ing a,ay the need $or $ossil $uels and turning to a %ore

sustainable and clean source o$ energy. Microbial bio%ass conversion could

be the %eans to this end.

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