2006 biljana potic

8/13/2019 2006 Biljana Potic

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GASIFICATION OF BIOMASS INSUPERCRITICAL WATER

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Promotion committee:

Prof.dr. W.H.M. Zijm Chairman University of TwenteProf.dr.ir. A. Bliek Secretary University of TwenteProf.dr.ir. W.P.M. van Swaaij Promoter University of Twente

Dr.ir. W. Prins Assistant-promoter University of Twente/BTG BVDr. S.R.A.Kersten Assistant-promoter University of TwenteDr.ir. L. van de Beld BTG B.V.Prof.dr.ir. J.A.M. Kuipers University of TwenteProf.dr.ir. M. Wessling University of TwenteProf.dr. E. Dinjus Forschungszentrum Karlsruhe

The research reported in this thesis was executed under:

1. A grant of the Netherlands Organization for Scientific Research – Chemical Sciences

(NWO-CW) in the framework of the research program “Towards Sustainable

Technologies”, subproject BIOCON with the financial contributions from Shell Global

Solutions International B.V. and the Dutch Ministries of Economic Affairs

(EZ/SenterNovem) and Environmental Affairs (VROM).

2. A grant of the European Commission in the framework of the “Super Hydrogen”

program EC-contract: ENK5-CT2001-00555.

3. A grant of the New Energy and Industrial Technology Development Organization

(NEDO) from Japan in the framework of the research program “Technical Feasibility

of Biomass Gasification in a Fluidized Bed with Supercritical Water”.

Cover designer: Vesna Smiljanić

Publisher: Wöhrmann Print Service, Zutphen, The Netherlands

© Biljana Potic, Enschede, The Netherlands, 2006

No part of this work may be reproduced by print, photocopy or any other means withoutthe permission in writing from the author.

ISBN 90-365-2367-2

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GASIFICATION OF BIOMASS IN SUPERCRITICAL WATER

Proefschrift

ter verkrijging vande graad van doctor aan de Universiteit Twente,

op gezag van de rector magnificus,prof. dr. W.H.M. Zijm,

volgens besluit van het College voor Promotiesin het openbaar te verdedigen

op vrijdag 12 mei 2006 om 13.15 uur

door

Biljana Potic

geboren op 20 september 1970te Novi Sad, Servië en Montenegro

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Dit proefschrift is goedgekeurd door de promotor:

Prof.dr.ir. W.P.M. van Swaaij

en de assistent-promotoren:

Dr.ir. W. PrinsDr. S.R.A. Kersten

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Luna, Mila and Goran, you are my life and my orientation.I love you!

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Contents

Summary 1

Samenvatting 5

Chapter 1 Introduction 11

Chapter 2 A High-throughput Screening Technique for Conversion in

Hot Compressed Water

37

Chapter 3 Gasification of Model Compounds and Wood in Hot

Compressed Water

53

Chapter 4 Pilot Plant 79

Chapter 5 SCWG Experiments in a Micro Continuous Flow Reactor 97

Chapter 6 Fluidization with Supercritical Water in Microreactors 111

Chapter 7 Reactor Design Considerations for Biomass Gasification in

Hot Compressed Water

131

Acknowledgements 141

List of Publications 145

Curriculum Vitae 147

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SSSuuummmmmmaaarrryyy

Application of biomass and waste, as a renewable and possibly sustainable energy source, has gained

an important role in the world’s future energy policy. The Dutch government, for instance, has set a

target for 2020, which states that 5% of the total primary energy production must be based on biomass

and waste input. For the year 2040 this could rise to 30%. The European Committee has put similar

directives forward.

An important fraction of the available biomass and waste streams has a high moisture content (say

more than 70 wt % water). Wet streams cannot be converted economically by thermal conversion

techniques like combustion, pyrolysis and gasification because of the large amount of energy required

for evaporation of water. Partial conversion of wet biomass by anaerobic biological processes,

producing convenient energy carriers like methane or ethanol, is possible and practiced for suitable

feedstock materials. Over the last decades research activities worldwide have been devoted towards

the development of new thermochemical processes, which can convert wet biomass efficiently and

economically. One of the novel technologies for conversion of wet biomass and waste streams is

gasification in supercritical water (SCWG: T > 3750C, P > 221 bar). Hydrogen and/or methane can be

produced by SCWG with selectivities that can be controlled by the process conditions and catalysis.

The research described in this thesis is dealing with the SCWG process. Due to the severe process

conditions (typically: T = 600°C, P = 300 bar and a corrosive environment), experimental

investigation on SCWG is expensive and time consuming. Despite these challenges, experiments were

conducted by different groups in the world revealing the influence of process conditions (temperature,

pressure, residence time, concentration of the organics, catalysis) on the yields and the selectivity of

the desirable gas products. All laboratory scale experiments reported in the past were conducted in

metal reactors. This was demonstrated to cause a difficult to quantify catalytic effect of the reactor

wall that makes interpretation and comparison of the available data uncertain. Therefore, part of the present work is focusing on gasification experiments not influenced by catalytic effects.

For this non-catalytic gasification, a high throughput technique for mapping of the reaction space has

been developed. In this technique, quartz capillaries of 1 mm inside diameter are used as batch

reactors. SCWG experiments could then be performed in a safe, cheap, and quick manner; one

capillary measurement takes about 5 min. compared to several hours for earlier batch and continuous

methods. Via validation measurements with formic acid and glucose solutions it has been


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demonstrated that the technique is sufficiently reliable for screening purposes including trend

detection.

The quartz capillary batch reactor technique has been subsequently used to study the non-catalytic

gasification of glycerol, glucose and pinewood in supercritical water. Mapping of the reaction space

has been done by performing over 700 experiments in which T, P, the heating time, the reaction time

and the concentration of the feedstock were varied. The most important observations were that the

pressure turned out to have no effect on the conversion rate to gas-phase products and product yields,

and that, without catalytic effects, complete conversion to the gas-phase is only possible for very

diluted feedstock solutions (


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process conditions. This burn-of or regeneration may be an important aspect of an industrial SCWG

reactor system, where the catalyst will most likely be in a fluidized state. As ultimately this convenient

continuous reactor set-up would be used for catalytic SCWG with a highly active catalyst compared to

the moderate catalytic influence of the reactor wall, the catalytic activity of the wall was accepted.

To pave the way for introducing a fluidized catalyst bed in a SCWG reactor and especially in a

continuous micro plant, a new micro-fluid bed technique for investigation of supercritical water

fluidization was designed and operated. A cylindrical quartz reactor of only 1 mm internal diameter

with a quartz ball as distributor was used for process conditions up to 5000C and 244 bar. Properties of

the fluid bed like: minimum fluidization velocity (Umf ), minimum bubbling velocity (Umb), bed

expansion, and the identification of the fluidization regime were investigated by visual observation.

Dedicated 2D and 3D Discrete Particle Models (DPM) models were used to simulate the micro-fluid

beds for gas-solid fluidization (fluid density range from 16 to 230 kg/m3). It has been found that the

results for Umb from the simulations are in accordance with experimental data and somewhat higher

than predictions from existing empirical relations. Experiments in three different scale cylindrical

reactors namely: 26 mm, 12 mm and 1 mm inside diameter, showed that to mimic large-scale

homogeneous fluidized bed systems in the 1 mm reactor only small particles (d < 100 micron) can be

used.

Finally, based on the present work and literature results, the different requirements for a SCWG

system are listed and discussed. A preliminary conceptual design is proposed including

fluidized/circulating bed systems, heat-exchangers and a regeneration section. Such a system could

possibly cope with the operational problems expected for a pilot plant or commercial SCWG system

processing real biomass, but much more research and development work is required before it can be

realized.


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SSSaaammmeeennnvvvaaattttttiiinnnggg

Het gebruik van biomassa en afval als een hernieuwbare, en mogelijk duurzame energiebron, wordt

wereldwijd een steeds belangrijker onderdeel van het energie beleid. De Nederlandse regering

bijvoorbeeld, heeft in haar doelstelling voor 2020 bepaald dat 5% van de totale primaire

energieproductie gebaseerd moet zijn op biomassa en afval als grondstof. De Europese Commissie

heeft vergelijkbare richtlijnen voorgesteld.

Een belangrijk deel van de beschikbare biomassa- en afvalstromen heeft echter een hoog vochtgehalte

(meer dan 70 gewichtsprocent water). En zulke natte stromen kunnen niet op economisch

verantwoorde wijze worden omgezet met de traditionele thermische conversietechnieken

(verbranding, pyrolyse en vergassing) vanwege de grote hoeveelheid energie die er nodig is voor de

verdamping van water. Gedeeltelijke omzetting van natte biomassa met behulp van biologische

processen waarbij nuttige energiedragers zoals methaan of ethanol worden gevormd, is mogelijk en

wordt in de praktijk veelvuldig toegepast voor bepaalde typen biomassa.

De afgelopen twintig jaar is er op een aantal plaatsen in de wereld ook onderzoek verricht m.b.t. de

ontwikkeling van nieuwe thermochemische processen om natte biomassa efficiënt en economisch

verantwoord om te zetten in nuttige producten. Eén van de nieuwe technologieën voor de verwerking

van natte biomassa en afval is de vergassing in superkritiek water, in het Engels aangeduid met de

afkorting SCWG (Super Critical Water Gasification). Bij temperaturen boven 375oC en drukken boven

221 bar (het superkritieke punt van water) kan er waterstof en/of methaan gemaakt worden. De

productselectiviteit wordt geregeld met de keuze van procescondities en de toepassing van

katalysatoren.

Het onderzoek dat in dit proefschrift is beschreven gaat over het SCWG-proces. Gewoonlijk zijn de

experimenten tijdrovend en kostbaar vanwege de strenge procescondities (reactie in een corrosief

medium, typisch bij T = 600oC en P = 300 bar). Ondanks deze bezwaren zijn er door verschillende

onderzoeksgroepen in de wereld experimenten uitgevoerd waarbij duidelijk is geworden wat de

invloed is van de procescondities (temperatuur, druk, verblijftijd, biomassaconcentratie en katalyse) op

de opbrengst en de verdeling van de gewenste producten. Alle laboratoriumexperimenten waarover in

het verleden is gerapporteerd zijn uitgevoerd in reactoren van metaal. Inmiddels is ook aangetoond dat

dit de interpretatie en vergelijking van resultaten bemoeilijkt omdat de metaalwand een onbekend


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katalytisch effect veroorzaakt. Daarom was een deel van het onderzoek voor dit proefschrift gericht op

vergassingsproeven waarbij zulke ongewenste katalytische effecten zijn uitgesloten. Er is een techniek

ontwikkeld om snel, en veel experimenten te doen (high-throughput screening) ten einde de mogelijke

reactiecondities in kaart te brengen. Hierbij zijn kwartsglazen capillairen van 1 mm inwendige

diameter gebruikt als “batch” reactor. De SCWG experimenten konden op deze wijze veilig, goedkoop

en snel worden uitgevoerd; een experiment met zo’n capillair duurt maar vijf minuten terwijl in het

verleden een experiment vele uren in beslag nam.

De techniek is eerst gevalideerd, door de omzetting van mierenzuur en glucose te bestuderen, en bleek

voldoende betrouwbaar voor snelle verkennende proeven en het vastleggen van trends. Vervolgens is

de capillaire techniek gebruikt om de niet-katalytische vergassing van glycerol, glucose en dennenhout

in superkritiek water te onderzoeken. Het operatiegebied voor de vergassingsreactie is in kaart

gebracht door 700 experimenten uit te voeren waarbij de temperatuur, de druk, de opwarmsnelheid, en

de concentratie van de organische stof in het water zijn gevarieerd. De belangrijkste waarnemingen

waren: a) dat de druk geen invloed bleek te hebben op de snelheid van de omzetting naar gasfase

producten en de productopbrengsten, en b) dat een volledige omzetting naar gasfase producten zonder

toepassing van katalyse alleen maar mogelijk is voor verdunde oplossingen (minder dan 2

gewichtsprocent organische stof in water). In vergelijking met het eerdere werk uitgevoerd in metalen

reactoren, bleek de omzettingssnelheid en de uiteindelijke omzetting naar gasfase producten lager te

zijn terwijl ook de productsamenstelling afweek van eerder gepubliceerde resultaten. Door ruthenium

op een TiO2-drager als katalysator toe te voegen aan de capillairen, kunnen glucoseoplossingen met

een concentratie variërend van 1 tot 17 gew. % al bij 600oC volledig vergast worden, waarmee de

potentie van katalyse voor SCWG is aangetoond.

In dit proefschrift wordt ook het ontwerp, de bouw en het testen van een volledige pilot-plant

installatie beschreven. Deze SCWG pilot-plant heeft een capaciteit van 30 liter vloeistof per uur, en is

bedoeld om procesgerelateerde problemen van allerlei aard te identificeren en op te lossen, en ervaring

te krijgen met continue operatie. Voedingen die geen coke vormen, zoals glucose en methanol, kondenin deze pilot-plant goed worden vergast. Maar de vergassing van zetmeel, leidde tot vervuiling en

tenslotte tot verstopping van de warmtewisselaar en de reactor, waaruit valt af te leiden dat er al

problemen ontstaan tijdens de opwarming van de voeding naar de gewenste reactietemperatuur. De

resultaten van succesvolle pilot-plant zijn vergeleken met die van de kleinschalige

laboratoriumproeven en de waargenomen verschillen zijn verklaard met de katalytische activiteit van

de metalen wand van de pilot-plant reactor.


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Het bedrijven van de pilot-plant is duur en lastig gebleken. Daarom is er een veel kleinere opstelling

gebouwd voor continue SCWG-vergassingsproeven. Helaas bleek het niet goed mogelijk om in deze

micro-opstelling een kwartsglazen reactor te gebruiken. Met name de verbinding met staal leidde vaak

tot breuk tijdens het op druk en temperatuur brengen van de reactor. Er is tenslotte besloten om met

een metalen buisreactor te werken van 1 mm inwendige diameter, gemaakt van Inconel 625 of

roestvrij staal (SS-316). De opstelling is gebruikt om de invloed van de procescondities, en het type

metaal van de reactorwand, op de conversie van de voeding naar de verschillend productgassen te

onderzoeken. En in een tweede serie experimenten is er een gepakt bed van katalysatorpoeder in serie

geplaatst met een lege buisreactor om de invloed van de katalysator op de conversie en selectiviteit te

bestuderen. De beproefde katalysatormaterialen zijn 3 gew.% ruthenium op TiO2 en kool van

beukenhout. Na de vergassing werd de geaccumuleerde kool in de reactoren afgebrand met lucht, en

de hoeveelheid ervan bepaald door de CO2 productie te meten. Er bleek dat de ruthenium katalysator

de koolstofconversie naar gasfaseproducten sterk verbetert, waarschijnlijk doordat intermediare

producten, die zonder de aanwezigheid van een katalysator door de reactor slippen en als opgeloste

componenten in de waterfase terechtkomen, in dit geval wel worden omgezet. Het gebruik van glucose

als voeding veroorzaakte koolachtige bijproducten in de vorm van waarneembare afzettingen in de

reactor. In enkele proeven zijn deze koolafzettingen verbrand met lucht onder gecontroleerde

omstandigheden. Het afbranden, c.q. uitbranden van koolafzettingen zal voor een grootschalig SCWG-

proces erg belangrijk zijn om het systeem te reinigen en de katalysator (mogelijk in een fluid bed

reactor) te regenereren.

De kleine continue opstelling is gemakkelijk te bedrijven en zal in de toekomst verder worden

gebruikt voor het testen van katalysatoren voor SCWG. In dat verband is een beperkte katalytische

activiteit van de wand acceptabel omdat deze wegvalt tegen de veel grotere activiteit van de

toegepaste katalysatordeeltjes.

Om de introductie van een fluid bed als katalytische reactor in SCWG voor te bereiden, en met name

ook voor toepassing van fluïdisatie in kleine laboratoriumopstellingen, (microreactor), is er een

nieuwe microtechniek ontwikkeld en toegepast om de fluïdisatie van katalysatordeeltjes insuperkritiek water te onderzoeken. Een cilindrische kwartsbuis van slechts 1 mm inwendige diameter,

met een kwartskogeltje onderin om het fluïdisatie medium te verdelen, is gebruikt voor proeven bij

temperaturen tot 500oC en drukken tot 244 bar. Eigenschappen van het gefluïdiseerde bed, zoals de

minimum fluïdisatiesnelheid (Umf ), de minimale belsnelheid (Umb), de bedexpansie, en het

fluïdisatieregiem, zijn onderzocht aan de hand van visuele waarnemingen. Bovendien zijn er

geavanceerde twee- en driedimensionale modellen toegepast (DPM: discrete particle models) om gas-

vast fluïdisatie te simuleren voor zulke micro fluid bedden met dichtheden variërend van 16 tot 230

kg/m3

. De uitkomsten van de simulaties zijn vergelijkbaar met de experimentele resultaten, maar zijn


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iets hoger dan wat de gangbare empirische correlaties voorspellen. Experimenten waarbij de

reactordiameter is gevarieerd (26, 12, en 1 mm inwendige diameter) hebben laten zien dat het gedrag

van grootschalige homogeen gefluïdiseerde bedden goed kan worden nagebootst, mits er in de 1mm

reactordeeltjes worden gebruikt die kleiner zijn dan 100 µm.

Tenslotte is er, op basis van dit onderzoek en de literatuur, een overzicht gemaakt van de verschillende

eisen die aan een SCWG-systeem gesteld moeten worden. Er is een voorlopig ontwerp gepresenteerd

waarin gefluïdiseerde bedden en vaste stof circulatie zijn opgenomen, om warmtewisseling tussen

voeding en reactor effluent alsmede katalysatorregeneratie, mogelijk te maken. Zo’n systeem zou

mogelijk de operationele problemen die te verwachten zijn van een pilot-plant of een commerciële

SCWG installatie voor echte biomassa, kunnen ondervangen. Maar er is meer onderzoek en

ontwikkelingswerk nodig om dit allemaal te realiseren.


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C C C h h h a a a p p p t t teee r r r 111 IIInnntttrrroooddduuuccctttiiiooonnn

Biomass as sustainable energy source

During the last three decades, concerns on the use of fossil sources have increased enormously. Care

about the environment and uncertainties about the security of energy supply are the main reasons for

these concerns.

It is a well-established fact that the concentration of carbon dioxide in the atmosphere has increased

notably over the past 150 years (United Nations Environment Programme, 2005). There is also

considerable evidence that the increase of the carbon dioxide concentration in the atmosphere since the

industrial revolution is primarily caused by human activity (United Nations Environment Programme,

2005). How the combustion of fossil fuels and the resulting unbalanced global CO 2 flows will affect

life on earth on the long term is strictly speaking not known. The fact that the consequences are

unknown should however be enough to start considering sustainable energy systems. Our measures to

counteract the increasing CO2 level should be guided by the precaution principle. It is by far the safest

strategy to assume that mankind’s increasing fossil CO2 emissions turns out to do irreversible harm to

the well-being and prosperity of generations to come. At present, we are able to develop the required

technologies for the conversion of sustainable sources into heat, power, fuels, and chemicals. The most

important hurdles for rapid development of such technologies are economic and mainly related to the

still low price of crude oil, natural gas, and coal.

Besides these concerns on global warming, the world’s current political situation also gives rise for

reconsideration of energy strategies. The majority of the crude oil reservoirs and reserves (90%,

(World Energy Council, 2004)) are located in potentially political unstable countries in the Middle

East, which makes availability principally unsure. This motivates several countries to invest in the

development of alternatives from locally available fossil resources such coal, shale and natural gas, but

also increasingly from renewable sources like hydro, solar, wind, and biomass (e.g. State of the Union


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Speech of US president Bush, January 2006), (Bush, 2006, The National Renewable Energy

Laboratory's, 2006).

Moreover, it is projected by most scenarios that the world’s oil and gas reserves are limited to ca. 60

years (e.g. BP statistical review (BP, 2005)). Obviously, this will have far reaching consequences for

the security of supply in a fossil dominated energy system. For the distant future it is evident that we

have to rely on sustainable sources, no matter if the fossil reserves turn out to sufficient for 200, 500 or

1000 years.

Because biomass is built-up from carbon extracted from the atmosphere in the form of CO 2, which, in

a short cycle, is returned again to the atmosphere after decay or combustion, the utilization of biomass

does not overall influence the atmospheric CO2 concentration. While considering alternatives for fossil

feedstocks, it becomes clear that biomass is the only sustainable source containing carbon and thus

allows direct matching with existing fossil derivatives like transportation fuels and chemicals.

Integrating and partnering with the existing fossil-based industries and logistic facilities is of utmost

importance in the transition towards a completely sustainable society, because it lowers the capital

investments and offers guaranteed markets for the products (Van Swaaij et al., 2004). Without this

initial partnering with the fossil industry it is to be doubted if the very large capital investments

required for achieving the directives on sustainable energy of the Dutch government and the European

Union will ever be brought up. Moreover, the total amount of biomass in the form of forestry,

agricultural and plantation residues is huge (say the equivalent of half the world’s crude oil

production, (Groeneveld, 2000). Moreover, even possible to grow biomass especially for energy

production (e.g. seeds for bio-diesel, and corn or sugar cane for bio-ethanol), also as part of

agricultural policies. When combining biomass-based processes with CO2 sequestration, the CO2

concentration in the atmosphere can even be reduced. Biomass as a renewable source should of course

be used in a sustainable fashion. Several factors should be considered like competition with the food

chain, energy for production and transportation of biomass, ecological aspects, bio-diversity soil

exhaustion, etc. As an example it should be realized that biomass always contains minerals, which

should be carefully considered in relation to a sustainable and ecological responsible use of biomass.


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Biomass conversion

For the conversion of biomass to energy or energy carriers, the moisture content of the biomass is of

crucial importance in view of the net lower heating value and the flame temperature (see Figure 1.1).

The net lower heating value is defined here as:

LHVnet = (1 – Xmoisture) x LHV – Xmoistue x ∆Hvap

in which Xmoisture is the mass fraction of moisture in the biomass, LHV is the lower heating value of the

dry fraction and ∆Hvap the enthalpy of vaporization of water. This definition of the net lower heating

value thus includes the energy required for vaporization of the moist.

Biomass with a moisture content of more than 60% on dry weight basis cannot be combusted

efficiently. Drying of the biomass is expensive and will consume rapidly a large fraction of the energy

available in the wet biomass unless special measures are taken. Water can be removed by methods like

sun drying, mechanical pressing, utilization of hot flue gas, multiple effect evaporation, vapor

recompression, and superheated steam drying. Sun drying in the open air is the cheapest but not

always possible. Other drying techniques are costly and generally not yet optimized for bulky biomass.

That is why conversion technologies for wet biomass streams, which do not request the initial

evaporation of the water associated with the biomass, can be very attractive. Examples of wet biomass

streams are by-products from food processing and certain streams in future biorefineries.

0.0 0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

moisture content [kg/lg]

Flame temperature [oC/100]

LHVnet

[MJ/kg]

Figure 1.1 Influence of the moisture content on the net heating value of the feedstock and the

adiabatic flame temperature in air.

In this introduction we will consider briefly the technologies that utilize dry or dried biomass, after

which the conversion technologies for wet biomass will be discussed.

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Dry biomass conversion

Examples of dry biomass are: rice husk, wood chips, straw, saw dust, cotton stalk, nut shell, etc. Dry

biomass can be used as a solid fuel in direct combustion or be converted thermochemically for the

production of various secondary energy carriers. Figure 1.2 gives an overview.Apart from the classical combustion in household stoves, grate boilers etc., also more advanced boilers

like fluidized bed and circulating fluidized bed boilers are used now to generate energy from e.g. wood

and straw (Quaak et al., 1999). A recent development is large-scale combustion of biomass in coal-

based power stations, by co-firing up to a level of 10 wt % (IEA, 1998). This is indeed a quick and

efficient way to reduce fossil carbon-based CO2 emissions and produce “green electricity”, for which

reason it is stimulated in some countries by fiscal measures and subsidies. The combustion with

energy generation of organic fractions in waste incineration is also counted sometimes as a

contribution to the reduction of CO2 emissions. Here, of course, only organic fractions from non-fossil

sources should be counted, e.g. thus excluding most plastics.

Dry Biomass

directcombustion

flashpyrolysis

gasificationoil

pressing

ash ash ash

organic liquid(+ water)

fuel gas orsynthesis gas

oil

liquid fuelshydrocarbons

methanol etc.

Dry Biomass

directcombustion

flashpyrolysis

gasificationoil

pressing

ash ash ash

organic liquid(+ water)

fuel gas orsynthesis gas

oil

liquid fuelshydrocarbons

methanol etc.

Figure 1.2 A simplified overview of the conversions of dry biomass into secondary energy carriers.

Flash pyrolysis (Bridgwater et al., 1999; Wang et al., 2005) is a process in which dry biomass is

heated rapidly to produce vapors that can be condensed to a liquid product that is usually indicated as

bio-oil. If carried out properly, about 70 - 80% of the product can be converted into bio-oil. Some char

and incondensable gas is produced, which can be used to generate heat and power for the process

itself, or for the local market. Conversion to bio-oil increases the bulk density of the material from

applied directly in engines, furnaces, turbines etc. After gasification in existing oil gasifiers (from 200-

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300 kg/m3 to 1300 kg/m

3 allowing for long distance transport and easy storage. The oil can be SHELL,

LURGI or TEXACO), the produced bio-oil syngas can be used for the production of chemicals,

Fischer Tropsch diesel, or as a fuel-cell feedstock. Biomass can also be pyrolyzed slowly, that is in a

more traditional way, to produce e.g. charcoal from wood as a secondary solid fuel. But that will not

be discussed here.

Dry biomass can be gasified (Beenackers and Maniatis, 1997; Kersten, 2002; Kersten et al., 2003) by

partial oxidation with air or oxygen/steam mixtures, to transfer the combustion value of the biomass to

fuel gas or syngas respectively. A wide range of processes to achieve this gasification has been

developed. Over the last decades, the research group in Twente has analyzed the performance and

made experimentally verified reactor models of various gasifier types, such as the co-current moving

bed (Groeneveld, 1980), fluidized bed (Van den Aarsen, 1985), and circulating fluid bed (Kersten,

2002). For large-scale operation entrained flow gasifiers are attractive (Higman and Van der Burgt,

2003). Product gas can be fired in boilers, turbines and engines or shifted to hydrogen for application

in fuel cells. It can be converted to liquid fuels like methanol or hydrocarbons via the Fischer-Tropsch

synthesis.

If biomass consists of oilseeds, oil can also be extracted and converted to “green diesel” but this will

not be discussed here (Boerrigter et al., 2003).

Wet biomass conversion

Examples of wet biomass streams are vegetable, fruit and garden waste, waste streams from

agricultural, food and beverage industries, manure, sewage sludge, and some household wastes.

Seaweed and micro-algae are examples of cultivated wet biomass crops. According to estimates of

ECN (Hemmes, 2004), between 5.3 and 12 million ton (dry matter) of wet streams are annually

available in The Netherlands. Besides, these streams that are already available it is expected that

within a future biorefinery also diluted side streams (by-products) are present. A bio-refinery is an

integrated concept aiming at full utilization of biomass, in which different fractions of biomass are

converted in large-scale plants (economy of scale) or in standardized small-scale units (economy of

numbers) in an economically optimal product slate.

This thesis deals with the conversion of wet biomass streams. An overview of these processes is given

in Figure 1.3. By biological processes methane can be produced which will easily separate from the

bioreactor fluid, leaving a non-fermentable organic fraction (Hill and Bolte, 2000).


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“ Wet” Biomass

Pressure cooker

P=300 bar, T=600oC

Pressure cooker

P=150 bar, T=300oC

Phase

separation

Water + Ash Methane,

alcohols etc.

Phase

separation

Water + Ash Hydrophobic

oi l

Phase

separation

Water + Ash Gaseous fuel

(H2)

HTU SCWG

Figure 1.3: Simplified overview of conversion processes for wet biomass.

Bio-ethanol can be produced also from various feedstock materials, especially from starch and sugars.

It can also be recovered from the bioreactor by refined distillation or extraction processes (Reith et al.,

2002). Biological processes with their special feedstock and operation conditions differ considerably

from the thermochemical processes and will not be further discussed here. Biological and

thermochemical conversion process will both be needed to develop a biorefinery that utilizes the

whole lignocellulosic feedstock. In this refinery a multitude of conversion steps will be present; by-

products from biological conversions will be used as feedstock for thermochemical conversions and

vise versa.

Two other conversion processes indicated in Figure 1.3 are thermochemical, viz. HTU® and

supercritical water gasification (SCWG). These processes are operated at elevated temperatures, and

under high pressure to avoid the evaporation step of water. They both require extensive heat exchange

between the feed inlet and product outlet streams in counter current operation to become energy

efficient.

HTU® or HydroThermal Upgrading is a thermochemical process in subcritical water in which

biomass is converted to liquid bio-crude with a reduced oxygen content (Goudriaan and Peferoen,

1990). In some hydrothermal processes by application of a catalyst the production of methane is aimed

for (Elliott et al., 2004).

Gasification of biomass in supercritical water for the production of hydrogen or methane rich gas is

the topic of this thesis.

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Gasification of wet biomass feedstocks in supercritical water

As stated above, the high moisture content of so-called wet biomass streams makes conventional

thermochemical technologies inefficient due to the high-energy requirement for water evaporation (2.4

MJ/kg at atmospheric conditions). Although wet biomass (moisture > 60 wt %) has a very low overallheating value, products with a high heating value can still be extracted from it by applying advanced

conversion processes. Biomass gasification in hot compressed water (SCWG, 600°C, 300 bar) is

considered as a promising technique to convert such wet streams into a gas that is rich in either

hydrogen or methane depending on the operating conditions and applied catalysis. In hot compressed

water (P > 200 bar), the heat effect associated with water evaporation is marginal compared to that at

ambient conditions (∆Hvap becomes zero at Pc). Therefore, by practicing counter-current heat

exchange between the feed stream and the reactor effluent, high thermal efficiencies can be reached

despite the low dry matter content of the feedstock. In Figure 1.4 a conceptual flow sheet of the

SCWG process is given.

Figure 1.4 Conceptual flow sheet of the SCWG process.

It is therefore crucial for the process that the sensible heat content of the reactor effluent is utilized as

far as possible to pre-heat the feedstock stream (mainly water) to reaction conditions (see Figure 1.4).

The efficiency of the heat exchange in relation to the applied pressure can be calculated from the heat

balance for a counter-current shell and tube heat-exchanger. The result is presented in Figure 1.5, in

which the heat-exchanger efficiency is plotted as a function of the operating pressure and the available

17


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area per unit throughput in kilogram per second. In case of an infinitely long heat-exchanger, the steep

asymptotic approach to 100% efficiency above 200 bar is a result of the sharp decrease of ∆Hvap to

zero beyond that pressure. For heat-exchangers with finite surface area, the effect of the operating

pressure is less pronounced. In practice, a hundred percent transfer of the available heat in the reactor

effluent to the feedstock stream is impossible. In fact, efficiencies of ca. 75% are typical for liquid-

liquid shell and tube heat-exchangers (Kersten et al., 2004). For such efficiency the operating pressure

should be ca. 200 bar in case of 50 m2 per kg/s throughput or 300 bar in case of 25 m

2 per kg/s

throughput (see Figure 1.5).

0 50 100 150 200 250 300 3500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

cold,out cold,in

HE

hot,in cold,in

H -Hη =

H -H

e f f i c i e n c y (

H E

) [ - ]

pressure [bar]

AHE

= 10

2550

100

Supercritical

Pressure

Figure 1.5 Calculated efficiencies of a water-water counter-current heat-exchanger plotted versus the

operating pressure for different surface areas, AHE = area (m2) per unit throughput (kg/s). The flow

rates (kg/s) on both sides were assumed to be equal. U = 1000 W/(m2.K), Inlet conditions: Thot,in =

600oC, Tcold,in = 25

oC.

Promises of biomass gasification in supercritical water are that:

● The technology is suitable for efficient processing of wet feedstock (> 70 wt % moisture);

evaporation of water is avoided and feedstock/product heat exchange is quite well possible.

● Contrary to anaerobic digestion and fermentation processes, the technology allows in principle

for complete feedstock conversion.

● The product gas is made available at high pressure (> 250 bar) and, for its application,

expensive gas compression can be avoided.

18


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19

● The product gas is clean; minerals, metals, and the undesired gases like CO2, H2S and NH3

(which have a high solubility in compressed water) remain in the water phase and can thus be

recovered.

● The product gas is not diluted with inert gas.

● The selectivity towards either methane or hydrogen can be controlled with temperature,

pressure and the application of catalysts.

● Sequestration of (pure) CO2 seems convenient.

These promises however go together with a series of problems that need to be solved in the process

development. Pumping of biomass slurries to pressures of up to 300 bar is a challenge. The high

temperatures and pressures involved put serious demands on the construction materials to be used,

especially because corrosion problems are expected. Heat exchange between the reactor feed and

effluent is required to make the process efficient, but heating of a biomass slurry is likely to cause

fouling and plugging in the heat-exchanger as the biomass starts to decompose already around 250oC.

Ash deposits may cause similar problems, and an effective ash removal system must therefore be part

of the reactor/process. Although carbon formation can be suppressed by applying high temperatures or

using a catalyst, the process should include an option to burn this carbon off the catalyst (when

applied) and the reactor walls.

When the future process development is successful in solving the problems of heat exchange,

corrosion, and fouling, interesting applications are foreseen.

Compressed methane can be used for blending with synthetic natural gas and compressed natural gas

for motorcars. In The Netherlands CH4/H2 mixtures could be fed to the natural gas pipelines.

Compressed hydrogen is an attractive feedstock for upgrading processes within the bio refinery

concept and for storage of hydrogen for mobile and stationary fuel cells. CO2 sequestration is

especially interesting because it would, together with the use of biomass as a feedstock material, create

the opportunity to reduce the amount of CO2 in the atmosphere (carbon sink).


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Relevant properties of supercritical water

The vapor pressure curve in the P-T diagram of water, which indicates the co-existence of liquid and

vapor, ends in the critical point at Tc = 373.95 and Pc = 220.64 bar.

Figure 1.6 Phase diagram of water.

At the critical point, the distinction between vapor and liquid disappears. The right upper quadrantenclosed by the critical isotherm Tc and the critical isobar P = Pc is the area where the fluid is

supercritical. The properties of supercritical water are quite different from those of the normal liquid

or steam at atmospheric pressure. For instance, at the critical point the density ρ is around 0.3 g/cm3

(versus 1 g/cm3 for liquid water at ambient conditions), the dielectric constant e is 5 (versus 80 for

liquid water at ambient conditions) and the ion product Kw = [H+][OH-] is 10-11

(versus 10-14

for

liquid water at ambient conditions). The most striking feature of supercritical water is the possibility to

manipulate and control its properties around the critical point by tuning the temperature and pressure.

For instance, by raising the pressure from 10 to 50 MPa at a temperature of 400 oC, the dielectric

constant is increased from 2 to 14, the ion product from 10-28

about to 10-12

, and the density from less

than 0.1 to about 0.5 g/cm3. As an important consequence of the change in the dielectric constant,

supercritical water behaves like a non-polar solvent and exhibits a high solubility towards non-polar

organic compounds like benzene. Also gases like oxygen, nitrogen, carbon dioxide and methane are

completely miscible in supercritical water. Contrary, the solubility of inorganic salts like NaCl is

decreased to very low values. All relevant properties of (supercritical) water are known accurately (see

e.g. NIST).

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21

Chemistry and thermodynamics

Biomass gasification in supercritical water is the result of thermal decomposition reactions (catalyzed

or not), followed by a multitude of reactions between intermediates and end-products (Kruse et al.,2002; Kruse and Gawlik, 2003). In the vicinity of the critical point where the ion product is high (10-

11), the H+ concentration is about 30 times higher than at ambient conditions offering increased

opportunities for acid catalyzed reactions (Buhler et al., 2002; Kabyemela et al., 1997; Kabyemela et

al., 1997; Kabyemela et al., 1998; Kabyemela et al., 1999). Presumably, at the higher temperatures

where hydrolysis as a result of the very low ion product Kw is impossible, radical reactions (pyrolysis

and cracking) will control the chemistry (Antal et al., 1993; Antal et al., 1999; Antal et al., 2000; Hao

et al. 2003; Lee et al., 2002; Potic et al., 2002; Potic et al., 2004; Potic et al., 2005; Schmieder, 1999).

Steam reforming and methanization of the biomass cover the extremes of possible stoichiometric

equations:

C6H10O5 + 7H2O → 6CO2 + 12H2 (1)

C6H10O5 + 1H2O → 3CO2 + 3CH4 (2)

Thermal decomposition of biomass and the subsequent reforming of intermediate fragment molecules

at higher temperatures could profit from the supercritical conditions, because of the good miscibility

of organic compounds and gases in supercritical water. It offers an opportunity to conduct chemistry in

a single phase that otherwise would have to occur in a multiphase system. As a consequence, there are

no interphase mass transfer limitations reducing the reaction rates, and higher concentrations of

reactants and intermediate products can be maintained. There is an extensive literature (Kabyemela et

al., 1997; Kabyemela et al., 1997; Kabyemela et al., 1997; Kabyemela et al., 1998; Kabyemela et al.,

1999; Kruse et al., 2002; Kruse and Gawlik, 2003; Kruse et al., 2003; Minowa et al., 1998; Mok and

Antal, 1992; Saka and Ueno, 1999; Sinag et al., 2003; Sinag et al., 2004; Stein et al., 1983) available

on the elucidation of the chemical pathways of biomass conversion in hot compressed water. Although

there seems to agreement on certain elements of the whole degradation chain, an overall model

suitable for reactor design is not present yet. As there is no mechanistic reaction path model available,

equilibrium calculations could be used to produce indicative results for comparison with experimental

results.

The thermodynamic calculations for supercritical gasification of C6H10O5 (cellulose), of which the

results are presented in Figure 1.7, are obtained with Gibbs free energy minimization model (Kyle,

1999) using the predictive Soave Redlich Kwong equation of state to calculate the required fugacity


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22

coefficients (Bertucco et al., 1995; Soave, 1972; Soave et al., 1993). Such calculations have a limited

quantitative value in case the reactions involved are too slow to reach equilibrium, but they may be

useful in predicting trends and the results desired upon application of an appropriate catalyst. In Figure

1.7 yields are plotted that are defined as the moles produced of a certain component out of one mole of

C6H10O5. If water is consumed in the reaction the corresponding water yield is negative.

Complete gasification of wet feedstock is thermodynamically possible. Actually, at 600°C and 250 bar

dry matter concentrations of up to 50 wt % do not have thermodynamic or stoichiometric limitations

(see Figure 1.7a) regarding the full conversion the gas-phase. At a lower temperature of 350°C,

complete conversion is only possible for feedstocks with less than 30 wt % organics (see Figure 1.7a).

In the thermodynamic calculations the non gasified part feedstock remains as solid carbon.

Thermodynamics predict that high temperature gasification would produce a hydrogen-rich gas (at

least for dry matter contents of less than 10 wt %) while at low temperature a methane-rich gas is

produced (see Figures 1.7b, 1.7c and 1.7d). Complete reforming of the feed (Eq. 1) to hydrogen is

thermodynamically possible at temperatures above 600°C and concentrations of organics lower than 1

wt %. Below 400°C while using feedstocks with more than 15 wt % organics, complete conversion of

the feed to methane (Eq 2) is favored by the Second Law. Between 350°C and 600°C there is a large

difference in composition of the produced gas (compare Figures 1.7b and 1.7c), while increasing the

temperature from 600°C to 700°C results in only minor changes (compare Figures 1.7c and 1.7d). It is

also interesting to note that according to thermodynamics water is net consumed in the decomposition

reactions under all relevant conditions (see Figures 1.7b, 1.7c, 1.7d, 1.7e and 1.7f).

For low temperature gasification, the content of dry matter in the feed does not influence the product

distribution to a large extent; the yields are almost unaffected (see Figure 1.7b). On the contrary, at

higher temperature there is a continuous varying product distribution ranging from nearly pure

hydrogen for very low weight percentages of dry matter, to a mixture hydrogen methane for high

organic fractions in the feed (see Figures 1.7c and 1.7d). Below 200 bar, the pressure has a profound

influence on the product distribution (see Figure 1.7e). Once above 200 bar, the operating pressure

does not influence the product distribution to a large extent (see Figure 1.7e). It is worthwhile to note

that, from a thermodynamic point of view, high temperature gasification should be carried out at thelowest possible pressure to achieve maximal hydrogen yields (see Figure 1.7e). Figure 1.7f shows

predictions for typical SCWG conditions, viz. 250 bar and a feed stream that contains 10 wt %

organics. For these conditions, according to thermodynamics, there is strong shift from methane

towards hydrogen and carbon monoxide upon increasing the temperature. Methane-rich gas can be

produced up to temperatures of approx. 500oC, higher temperatures favor the production of hydrogen.


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23

100 20 40 60 80 00.0

0.2

0.4

0.6

0.8

1.0600

oC & 250 bar

350

o

C & 250 bar

f r a c t i o n c a r b o n g a s i f i e d

wt.% organics in feed

0 5 10 15 20 25 30

-6

-3

0

3

6

9

12

CO

CO2

CH4

H2

H2O

350oC & 250 bar

Y i e l d s , m o l e p r o d u c e d / m o l e C

6 H

1 0

O 5


(a) (b)

0 5 10 15 20 25 30

-6

-3

0

3

6

9

12 CO

CO2

CH4

H2

H2O

600oC & 250 bar


6 H

1 0

O 5


0 5 10 15 20 25 30

-6

-3

0

3

6

9

12

CO

CO2

CH4 H

2

H2O

700oC & 250 bar

Y i e l d s , m o l e p r o d u c e d / m o l e

C 6

H 1 0

O 5


(c) (d)

0 100 200 300 400

-6

-3

0

3

6

9

12

CO

CO2

CH4

H2

H2O


6 H

1 0

O 5

Pressure, bar

10 wt% organics in feed, 600 o

C

300 400 500 600 700 800

-6

-3

0

3

6

9

12

CO

CO2

CH4

H2

H2O

10 wt% organics in feed, 250 bar


6 H

1 0

O 5

T,oC

(e) (f)

Figure 1.7 Results of equilibrium calculations of cellulose (C6H10O5) gasification in hot compressed

water.


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24

Literature review

Here a short literature review is presented dealing with those reported results that are relevant for

process development. For a more complete and detailed review the reader is referred to a joint paper of

the SCWG community (Matsumura et al., 2005), which was co-authored by the author of this thesis.The reported experimental results concerning the effects of the operating conditions and applied

catalysis are analyzed from a reactor engineering point of view. There is also significant information

available concerning the chemical pathways of model compounds near the critical point (see e.g.

Buhler et al., 2002; Kabyemela et al., 1998; Kruse and Gawlik, 2003; Savage, 1999). These results are

however less relevant for the severe conditions of SCWG. Listing and discussing this information is

therefore beyond the scope of this work.

Historical background

In the mid seventies of the last century, researchers at MIT (Amin, 1975; Woerner, 1976) discovered

that biomass could be liquefied without producing char by processing it in supercritical water. In 1985,

Modell (Modell, 1985) reported these experiments. It concerned experiments in which glucose,

cellulose, hexanoic acid, polyethylene and maple wood sawdust samples were quickly immersed in

sub- en supercritical water. Most experiments were performed with glucose or maple wood sawdust,

of which 10 and 2 g were injected respectively in ca. 300 g of water. Char was collected as reaction

product when operating below the critical point of water, while just above the critical point char was

not observed. Other interesting observations were that polyethylene produced char under sub- and

supercritical conditions and that using 30 g of glucose instead of 10 g resulted in significant char

formation. Also in 1985, Elliot and Sealock (Elliott and Sealock, 1985) presented work that showed

that the combined advantage of a high-pressure water environment and a metal catalyst (nickel) could

compensate in the slow reaction kinetics typical for gasification at lower temperature. In supercritical

water of 450°C, 80% of the carbon present in the feedstock (10 wt % wood flour in water) could be

converted to gases. In hindsight, the results presented in these papers should be regarded as the

pioneering work that set off the research on gasification of biomass in hot compressed water. It is

interesting to note that the motivation for this early research was not to develop a process for the

effective conversion of wet biomass streams, but to minimize char production and to optimize steam

reforming of biomass. At that time, the carbonaceous by-product of gasification and liquefaction

processes was considered a serious problem. It was reasoned that char decreased the yield of the fluid

products and could cause technical processing difficulties. Already in 1978 biomass was proposed as a

potential feedstock for hydrogen production via steam reforming (Antal, 1978) and studies were

reported of the reaction kinetics of cellulose pyrolysis in steam. It was found that biomass did not react

directly with steam at atmospheric pressure to produce the desired products. Significant amounts of tar


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and char were formed, and the gas contained higher hydrocarbons in addition to the desired light gases

(Antal, 1985; Antal et al., 1987; Antal and Mok, 1988; Mozaffarian et al., 2004; Stein et al., 1983).

Since the mid eighties, roughly speaking, two approaches to gasification in hot compressed water have

been investigated in terms of reaction temperature ranges. In low-temperature catalytic gasification at

350 to 500oC, the feedstock is gasified with the help of a catalyst into a methane-rich gas. High

temperature supercritical gasification is carried out in the range of 500 to 750oC, with or without

catalysis, and aims at producing primarily hydrogen.

The carbon efficiency is often used in literature to characterize the gasification process; it is defined as

fraction of carbon in the feedstock that has been converted to permanent gases.

Low temperature gasification

Looking at the reported results, the catalytic low-temperature process (350 - 500°C) must be regarded

as very promising. It provides the opportunity to produce a pressurized methane-rich gas at relatively

low temperatures. Even for feedstocks with a dry matter concentration of up to 10 wt %, carbon

efficiencies of 90% and up were achieved in batch and continuous bench-scale facilities, while without

catalyst the carbon efficiency is limited to ca. 20% (Elliott et al., 1993; Elliott et al., 1994; Elliott et al.,

2004). However, near 100 percent carbon efficiency was never reached yet; typically the carbon

efficiency was in the range of 94 to 98% when using the optimal catalyst at 400°C. The comparison of

several feedstocks using Ru and Ni catalyst showed the highest reactivity with manure, lignocellulosic

feedstocks showed lower activity as a group. The observed effect of temperature was obvious (higher

carbon efficiency at higher temperature), but there was no dramatic effect noticeable passing the

supercritical point of water (374oC). Only a limited range of catalytic metals and supports can be used

in the process because of the oxidation and degradation problems in the hot-water environment

(Matsumura et al., 2005). New catalyst formulations for low-temperature gasification include

combinations of metals stable under the applied conditions, such as ruthenium or nickel bimetallics

and stable supports, such as certain titania, zirconia, or carbon. For example, the ruthenium on rutile

titania extrudate is particularly effective in this process (Matsumura et al., 2005).

High temperature gasification

All reported laboratory results on high temperature gasification (500 - 750 °C) were obtained in metal

reactors. Stainless steel, Inconel, Hastelloy and corroded Hastelloy were used as construction

materials. Both empty tubular reactors and stirred cells were used in the laboratory work that was

carried out so far. Results obtained by processing model compounds and various wet biomass species

in tubular reactors were reported mainly by Antal and co-workers (Antal et al., 1993; Antal et al.,

2000). Hao et al. (Hao et al., 2003) and Lee et al. (Lee et al., 2002) reported experiments in tubular


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reactors with glucose solutions as feedstock while varying the operating conditions. The diameter of

the tubular reactors was in the range of 1.44 mm to 6.22 mm with a corresponding specific possible

catalytic wall area of 2778 m2 to 643 m

2 per unit volume reactor. Sinag et al. (Sinag et al., 2004)

conducted SCWG experiments in a 190 mL metal autoclave equipped with a stirrer. In their

experiments a cold feed stream was continuously injected into the hot autoclave. Literature data

indicate that the reactor material has a significant effect on the gasification process. Yu et al. (Yu et

al., 1993), for instance, reported results of gasification of glucose in the same experimental set-up

under identical conditions, but using other metals for the reactor. They found large differences

between experiments carried out in Inconel, Hastelloy and corroded Hastelloy reactors. For a test

series using 0.6 M acetic acid they observed 14% carbon efficiency in the Inconel reactor whereas in

the corroded Hastelloy reactor the carbon efficiency was as high as 53%.

In a nutshell the results reported for non-catalytic (not taking into account the catalytic effect of the

reactor wall) SCWG in the range of 500 - 750°C can be summarized as follows:

● The metal reactor wall itself and deposits on it are catalytically active influencing both the

carbon efficiency and the product distribution. Due to this catalytic effect of the wall, which

may also be influenced by corrosion, published data are difficult to compare and interpret.

Besides, the catalytic activity of small laboratory equipment cannot be translated to large-scale

reactors, which have a much smaller wall area over volume ratio.

● Temperature is the most important process parameter; at 500°C without catalyst hardly any gas

is produced, while at 600°C using a low concentrated feedstock complete gasification can be

achieved. It is important to note here that results reported by Antal and co-workers before

report the temperature in the last section of the tubular reactor while it turned out later that in

the entrance (mixing) region peak temperatures were present that were more than 100°C higher.

● Complete conversion of the feedstock to the gas-phase is achieved only for very diluted

solutions of glucose and glycerol (< 5 wt %). Higher concentrations (> 5 wt %) of model

compounds and more complicated feedstocks (e.g. wood, lignin, starch) cannot be gasified

completely. The carbon conversion is typically between 75 and 90% for solutions with morethan 5 wt % organics in it.

● The overall reaction is reasonably fast reaching the maximum conversion at 600°C within one

minute.

● Processing complicated feedstocks such as wood and starches that contain more than 10 wt %

organics cause severe plugging problems in the reactor due to deposits of carbonaceous and

mineral material.


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● The catalytic activity of the reactor wall suggested that heterogeneous catalysis might be

employed to increase the extent of gasification for concentrated feeds. Antal and co-workers

employed carbonaceous catalysts to investigate this possibility (Antal et al., 2000; Xu et al.,

1996; Xu and Antal, 1998) and concluded that the available surface area of the carbon does not

significantly influence the results. Of the tested carbons, coconut shell activated carbon was the

most active one. They obtained complete gasification of a 22 wt % glucose solution at 600°C

(peak temperature not given). Besides glucose also other model compounds (e.g. ethylene

glycol, phenol) and various realistic biomass feedstocks such as sewage sludge, wood sawdust,

and starches were tested. Although complete gasification was not always possible,

carbonaceous catalysts were found effective for all the compounds tested. To achieve

reasonable carbon efficiencies (> 85%) for feedstocks such as potato starch catalyst bed

temperatures of up to 750°C had to be applied.

High temperature gasification pi lot plants

SCWG is in an early stage of development. Due to its promises with respect to possible conversion of

waste materials to a valuable gas, the laboratory research is developing rapidly. However, large-scale

commercial installations do not yet exist. The gap between small-scale testing in laboratories to

practical demonstration of a new process is bridged by experimentation with pilot plants. At present

there are two pilot plants being operated in the world. The largest plant, in operation since the

beginning of 2003, is the one of Forschungszentrum Karlsruhe (FzK) in Germany (Boukis et al.,

2002). It has a design capacity of 100 l/h, and was built to demonstrate supercritical gasification of wet

residues from wine production. EU subsidies plus a grant awarded by the Japanese NEDO enabled the

construction of a well-equipped process development unit (PDU) in Enschede, The Netherlands, with

a maximum throughput capacity of 30 l/h (Potic et al., 2002, Potic et al., 2004, Van de Beld et al.,

2003). BTG Biomass Technology Group B.V. has been responsible for the technical realization and

start-up of this small pilot plant.


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Outline of this thesis

The further development of the SCWG process requires input from various disciplines. On the

feedstock side, pre-treatment and pumping problems have to be tackled. Due to the severe operating

conditions, studying of the long-term performance of selected construction materials is of utmostimportance. Detailed chemical studies that provide more insight in the reaction pathways and kinetics

at the severe SCWG conditions would help to select the optimal process conditions.

This thesis focuses on the reactor engineering aspects of SCWG. Four main research lines (goals) will

be addressed:

● Development of high-throughput experimental techniques.

● Mapping of the operating window under catalytically inert conditions.

● Investigating the potential of heterogeneous catalysis.

● Development of elements of a SCWG that can deal with heterogeneous catalysis under foulingconditions.

Below the relevance of these research lines will be elucidated.

1. Development of high-throughput experimental techniques. Due to the severe operating

conditions of SCWG, conventional laboratory equipment (e.g. autoclaves with significant

volumes) requires extensive safety measures, and is costly and time intensive. In such

systems, typically, 1 to 2 experiments can be performed per day, while 10 to 20 would be

desirable to speedup the pace of the research. In this thesis, a high-throughput batch screening

technique using quartz capillaries of only 1 mm internal diameter (Chapter 2) has been

developed. With this method 20 cheap and safe tests per day have been performed. For

catalyst screening and to study possible ways to recover or to avoid carbonaceous deposits

when processing realistic feedstock materials, a continuous flow micro system (1 mm i.d.) is

designed and applied (Chapter 5). In the same system, fluidization with sub- and supercritical

water has been studied (Chapter 6).

2. Mapping of the operating window under catalytically inert conditions. It is know that metal

reactor walls are catalytically active and influence the gasification process. Due to this

catalytic effect of the wall published data are difficult to compare and interpret. Besides, the

catalytic activity of small laboratory equipment cannot be translated to large-scale reactors,

which have a much smaller wall area over volume ratio. In the present work, catalytically inert

quartz capillaries have been used for the investigation of gasification in hot compressed water

(Chapters 2 and 3). These results are supposed to represent the plain performance of the

process and can provide reliable information on the chemistry and kinetics of biomass and

waste gasification in high temperature and high-pressure water.


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3. Investigating the potential of heterogeneous catalysis. Results of previous research already

made it clear that with catalysis only very diluted feed streams (say < 5 wt % organics) can be

gasified completely. When processing feedstock with higher concentrations, carbonaceous

deposits (not always e.g. not for glycerol) and water soluble organics are produced next to

gases. Carbonaceous deposits cause fouling and blocking problems within the process. The

organics containing effluent water requires an additional waste water treatment system, which

obviously affects the economics of the process negatively. In this thesis, a catalyst developed

for subcritical conversion has been tested under SCWG conditions in the microreactors

(Chapters 3 and 5). Both the gasification performance and the burn-off characteristics of this

catalyst have been investigated. The latter aspect is of importance for the development of the

SCWG process.

4. Development of elements of a SCWG process that can deal with heterogeneous catalysis

under fouling conditions. Like mentioned above, catalysis is needed when dealing with

concentrated feedstocks. Besides, when processing charring feedstock fouling will occur.

These two items define the severe boundary conditions for process development; viz. the

SCWG should be able to deal with catalysis under fouling conditions. In Chapter 7, based on

results gathered in Chapters 2 to 6, an integrated heat-exchanger/reactor system based on fluid

beds and circulating solids is proposed.

Notation

AHE Area per unit throughput, m2/(kg/s)

H Enthalpy, J/kg

∆Hvap Enthalpy of vaporization of water, MJ/kg

Kw Ion product

LHV Lower heating value of the dry fraction, MJ/kg

LHVnet Net lower heating value, MJ/kg

Pc Critical pressure, bar

Tc Critical temperature,oC

Thot,in Inlet temperature of the hot stream,oC

Tcold,in Inlet temperature of the cold stream,oC

U Overall heat transfer coefficient, W/(m2.K)

Xmoisture Mass fraction of moisture in the biomass

ηHE Heat-exchanger efficiency


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u

C C C h h h a a a p p p t t teee r r r 2 2 2 AAA HHHiiiggghhh---TTThhhrrrooouuuggghhhpppuuttt SSScccrrreeeeeennniiinnnggg TTTeeeccchhhnnniiiqqquuueee

f f f ooorrr CCCooonnnvvveeerrrsssiiiooonnn iiinnn HHHooottt CCCooommmppprrreeesssssseeeddd WWWaaattteeerrr

Abstract

Conversion in hot compressed water (e.g. 600oC and 300 bar) is considered to be a promising

technique to treat very wet biomass or waste streams. In this chapter, a new experimental method is

described that can be used to screen the operating window in a safe, cheap, and quick manner (one

measurement takes about 5 min). Small sealed quartz capillaries (i.d. = 1 mm) filled with biomass or

model compounds in water are heated rapidly in a fluidized bed to the desired reaction temperature.

The reaction pressure can be controlled accurately by the initial amount of solution in the capillary.

After a certain contact time, the capillaries are lifted out of the fluidized bed, rapidly quenched, and

destroyed to collect the produced gases for GC analysis.

Results of measurements for formic acid and glucose solutions have shown that the technique is

reliable enough for screening purposes including trend detection. For conversions above 30%, three

identical measurements are sufficient to produce reasonably accurate average values with a

confidence level of 95%.

This chapter has been published in Ind. Eng. Chem. Res, 43 (2004), p. 4580


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Introduction

Process intensification is a novel design approach in which reduction of equipment size leads to less

energy consumption, improved safety, lower capital costs, and less pollution (Burns and Ramshaw,

1999; Dummann et al., 2003; Rebrov et al., 2003). In this chapter, the intensification principle has been applied to a screening technique for the conversion of biomass in hot compressed water, in

particular gasification of biomass/waste in supercritical water (SCWG). Very wet biomass (moisture

content > 70 wt %) cannot be converted economically by traditional techniques such as combustion

and gasification due to the energy required for water evaporation (2.4 MJ/kg). In SCWG, water

evaporation is avoided and intensive countercurrent heat exchange is practiced. Therefore, SCWG is

considered as a promising technique to convert wet streams into a hydrogen-rich gas (Antal et al.,

1993; Holgate et al., 1995; Modell, 1985). Biomass is converted in the presence of water, e.g. via:

C6H12O6 + 6H2O 6CO2 + 12H2

The above stoichiometric equation is highly idealized; in practice, also CO, CH4, C2-3-components,

liquids (including H2O), and polymers are formed next to CO2 and H2 (Schmieder et al., 2000). The

product distribution appears to be a strong function of the reactor temperature and the weight

percentage of organic material in the feedstock (Kruse et al., 1999). Antal and co-workers (Yu et al.,

1993) found that the wall material (Hastelloy, Inconel) of their bench-scale continuous flow reactor

had a large effect on the obtained results. This finding points to an important role of catalysis in

SCWG. Consequently, data obtained in small-scale metal reactors are obscured by undefined catalytic

effects, and therefore difficult to interpret and extrapolate. The novel method uses quartz capillaries of

1 mm i.d., 2 mm o.d., and 150 mm length as batch reactors, which have no catalytic activity.

Unfortunately, the number of experimental SCWG data published is small, while at the same time the

range of conditions explored is narrow. This is partly due to the severe operating conditions (300 bar

and 600oC), which make laboratory testing at bench-scale (1 - 100 g/h, 10 mL < V reactor < 1 L)

problematic,

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