1-s2.0-S0896844614003271-main

J. of Supercritical Fluids 96 (2015) 3645

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

j o ur na l ho me page: www.elsev ier .com/ locate /supf lu

Water A magic solvent for biomass conversion

Andrea Ka Institute for C ermanb Institute of Ag

a r t i c l

Article history:Received 27 JuReceived in re24 SeptemberAccepted 25 SAvailable onlin

Keywords:BiomassFuelGasicationLiquefactionSupercritical wHydrothermalHydrogenCarbonizationHTC coal

ocessarrierlyst on an

reacth temages t com

sold .

1. Introdu

Hydrothtechniques fuels. The mis that wet converted contents besion proceshydrothermbase for biosary is that The water ivent. In ador catalyst water is conuid water a100 C; therthe vapor p

CorresponHohenheim, S

E-mail add(A. Kruse).

http://dx.doi.o0896-8446/ ction

ermal biomass conversion processes are interestingto produce renewable solid, liquid/tarry or gaseousost important advantage of hydrothermal processingbiomass, typically with 70 wt.% or more water can bewithout drying. Drying of biomass to optimal waterlow 10 wt.%, as necessary for dry biomass conver-ses, costs substantial amounts of energy [1]. Therefore,al conversions are attractive to extent the resourceenergy production. The reason why no drying is neces-hydrothermal processes are conducted in liquid water.nside the biomass is the same component as the sol-dition the reaction is supported by water as catalystprecursor as well as reactant. Every particular role ofnected with the special properties of superheated liq-

bove 100 C [26]. The reaction temperatures are aboveefore, hydrothermal processes require a pressure aboveressure of water at the corresponding temperature to

ding author at: Institute of Agricultural Engineering, University oftuttgart, Germany. Tel.: +49 459 24700; fax: +49 459 24702.resses: Andrea [email protected], [email protected]

keep water in its liquid phase. This means that hydrothermal pro-cesses are chemical reactions in a solvent; in this case a solventchanging its properties depending strongly on temperature. As aconsequence the chemical processes are inuenced by the differentsolvent properties depending on reaction conditions. By adjustingthe reaction parameters the reactions can be tuned selectively toobtain different products, namely solids (biochar), liquids, or gases(methane and hydrogen).

2. Overview of hydrothermal conversion processes

The hydrothermal processes can be assigned in terms of reactiontemperature and, accordingly, pressure (see Fig. 1). At relativelylow temperatures pretreatment methods are conducted. The mostimportant one is steam explosion [8,9]. Here, the biomass isheated up under pressure typically to 140240 C. Then, the pres-sure is reduced rapidly to ambient pressure e.g. by opening a valve.The water inside the biomass evaporates thereby disrupting itsstructure like in an explosion. This way the cellulose and the lignin,covering it, are separated. This is important because in non-treatedbiomass this lignin protects the cellulose bers against attack ofenzymes, solvents or other agents. Therefore, steam explosion is auseful pretreatment method e.g. for bio-ethanol production fromlignocelluloses by fermentation. By this pre-treatment, the yield isincreased because of the higher reactivity of cellulose [8,9]. This

rg/10.1016/j.supu.2014.09.0382014 Elsevier B.V. All rights reserved.rusea,b,, Nicolaus Dahmena

atalysis Research and Technology, Karlsruhe Institute of Technology (KIT), Karlsruhe, Gricultural Engineering, University of Hohenheim, Stuttgart, Germany

e i n f o

ne 2014vised form

2014eptember 2014e 2 October 2014

ater conversion

a b s t r a c t

Hydrothermal biomass conversion prcontent for the formation of energy cprocesses as solvent, reactant and cataprocesses of carbonization, gasicatiodiscussed for each of them. The highwater and its changing properties witproducts. Despite the obvious advanttions are rare. The main reason for nothat there are no products that can berisk, and with more simple processesy

es provide the opportunity to use feedstocks with high waters or platform chemicals. The water plays an active role in ther catalyst precursor. In this paper, the different hydrothermal

d liquefaction are introduced and the specic role of water isivity of the polar components of biomass in hot compressedperature are the key to obtain high selectivities of the desiredof hydrothermal conversion examples for industrial applica-mercial application of water in the high temperature state iswith prot and cannot be produced cheaper, with less capital

2014 Elsevier B.V. All rights reserved.

A. Kruse, N. Dahmen / J. of Supercritical Fluids 96 (2015) 3645 37

Fig. 1. Overvivapor pressure

points to anbe combineare carried conditions ais not only explosion [1

HydrothHere, the capolymerizecess usuallywith a lot ohas a heatinvolatile subimportant itoxicity of flikely a conare adsorbecoal overcofuel:

A) The heathigh oxysmaller avalue an

B) Mechaniof its higshow, thcarbonizapplied aof biomain the mto burn t

C) Biomassmelting Special tdle the a600 C. Tventiona130015sium is sash leadsis similar

In regarsuitable encoal can be m

or as an ion exchanger [25,26]. In the case that not biomass [27,28],but pure compounds like glucose are used in the process, advancedmaterials like micro- or nano-spheres as well as nano-tubes canbe produced [29]. At slightly higher temperature aqueous phase

ing (APR) can be performed by means of catalysts to pro-ydrogen or hydrocarbons [27,28]. This process for hydrogention is only possible at very low concentrations of the feedal because of thermodynamic limitation [30]. At higher con-tionss APRheset casss shos to ses [abilidrogo proocesroge

ormaion ossedhe tl liquthe ingil of omp

hydrodu

the s at l of an timst he

forml liqr gastemp

prod25 Mf po

. In aat is ctionand ls areeciahe goew of different hydrothermal biomass conversion processes and the curve of water (simplied) [7].

important aspect of hydrothermal processes: They cand with biological or biochemical processes because theyout in the same solvent [10]. Depending on the reactionnd especially in the case if acids are added the biomass

disrupted, but already partly hydrolyzed during steam1].ermal carbonization (HTC) occurs at around 200 C.rbohydrates are completely solved after hydrolysis and

subsequently to a product called HTC-coal. The pro- needs 24 h reaction time and has been demonstratedf different types of biomass [1217]. This HTC coalg value similar to lignite, but with a higher content ofstances and a higher biodegradability in soil, which isf it is used for soil improvement [18]. Here, the phyto-resh HTC coal is an important issue to be considered,sequence of water soluble products like phenols, whichd at the HTC coal surface [19]. The production of HTCmes three major disadvantages of untreated biomass as

ing value of untreated biomass is low because of thegen and water content. By the elimination of water (andmounts of CO2 [20]) the HTC coal has a higher heatingd lower oxygen content than biomass [14,16,17,21,22].cal dewatering of HTC products is very effective becausehly hydrophobic properties. Studies with sewage sludgeat the easier dewatering alone justies hydrothermalation as a rst drying step. Thermal drying, usuallys the second step, needs much more energy in the casess, where mechanical dewatering leaves a lot of water

reformduce hproducmatericentraFor thiSince tIn mosbiomaappearprocesterm stThe hystock tthis prfor hydAfter fformatis discu

In tthermaunder upgradtarry ooccur cysis. Insolid pally insecondysis oireactiovery favaporsthermasolid olower factionoil (20range osugarsysis thliquefawater pheno

A spHere taterial. On the other hand the water has to be removedhe biomass and to reduce transportation costs [12,23].

has a high potassium content. This leads to low ashtemperatures, which makes burning more complicate.echniques are required to prevent corrosion and to han-sh melt or the burning temperature is limited to belowhis is too low for efcient power generation, like in con-l power plants like pulverized coal ring systems with00 C. During hydrothermal carbonization the potas-olved in the water. The lower potassium content in the

to higher ash melting temperature of e.g. 1200 C. This to the ash melting temperatures of fossil lignite [13,24].

d of the aspects mentioned above, HTC coal is a moreergy carrier than untreated biomass. In addition, HTC

odied to be used as adsorbent for organic compounds

Lignin is lesfore a tempcatalyst.

Near thegasicationalysts are nsuitable fortwo approaThe subcritsolved and[45]. The su

1 The term of water as thsense that pre methane is the thermodynamically favored product. process catalyst like Ni, Pt, Pd and others are used [28].

are solid catalysts, they cannot work with solid biomass.es hydrogen-rich, soluble compounds produced fromw the highest hydrogen yields. This process is thereforebe useful for aqueous efuents of other hydrothermal31] or other aqueous efuents [32]. Poisoning and longty of the catalyst is often a challenge of this process [33].en produced can be used for hydrogenation of the feed-duce hydrocarbons [28] or to up-grade bio-oils [34]. Fors, in principle the same type of catalyst can be used asn production before, but the selectivity varies [27,28].tion of hydrogenated products, other reactions like thef aromatic rings can be carried out [35]. Therefore, APR

as process to produce a substitute for terephthalic acid.emperature range between 300 C and 350 C hydro-efaction takes place [3639]. The process is also knownoriginal SHELL trademark HTU(r) for Hydrothermal

[40]. Here, the biomass is converted to a highly viscousa high heating value [1]. Some signicant differencesared to the dry conversion technology, the ash pyrol-rothermal liquefaction and at optimized conditions noct is formed. In ash pyrolysis the solid yield is usu-range of 20 wt.%. Very short reaction times of a few500600 C are necessary to reach high yields of pyrol-round 60 wt.%. This oil is an intermediate and longere would convert it to gas and solid. As a consequenceating-up (ca. 1000 K/s) and rapid cooling down of theed during pyrolysis have to be realized. For hydro-

uefaction this is not necessary. No further reactions toeous products occur at reaction conditions, with mucherature. The heating value of the hydrothermal lique-uct (3036 MJ/kg) is much higher than that of pyrolysisJ/kg). The reason is that pyrolysis oil includes a wide

lar compounds like acids, alcohols, aldehydes and evenddition, ca. 1520 wt.% of water is formed during pyrol-in the pyrolysis oil after cooling down. In hydrothermal

polar, oxygen-containing compounds are solved in theonly the compounds of lower oxygen content, namely

found in the oil phase [1].l issue of liquefaction is the decomposition of lignin [41].al is to get phenols, e.g. for the production of resins.s reactive as lignocellulosic biomass and requires there-erature of around 400 C and maybe the support of a

critical point of water there is the range of catalyzed to produce methane. For this reaction noble metal cat-ecessary [4244]. The active metals are Ni, Rh, Pd, Pt,

hydrogenation of e.g. CO to methane [43]. There areches applying subcritical or supercritical1 conditions.ical approach has the advantage that salts are mostly

therefore plugging by salt precipitation is less likelypercritical reactions have the advantage that organic

sub- or supercritical here refers, as usually done, to the critical pointe solvent. This does not mean that the mixture is supercritical in thessure and temperature are above the critical point of the mixture.

38 A. Kruse, N. Dahmen / J. of Supercritical Fluids 96 (2015) 3645

Fig. 2. Properwater as funct

compoundscatalysts bysalt water sopportunititant aspectpotassium sthe supercrsalts are preof a hydro-c

In the rgasicationduce hydrothe other htor materiahydrogen acomplete. Oous efuenhurdles: prmass contephenols usuversion is nohigher convalyst includof the prod1 vol.%. Mocontents, bwater-gas scan be used

3. Propert

Water amolecules. the other hgases like hdensity decthermal modifferent wstable hydrexist at highlower densi[62]. Theredecreases. water behasolubility fosalts [63,64case that a pmolecules i

higher multi-valent cations the strong interaction to the oxygenatom leads to an elimination of H+ and a strong acidic behavior.Calculations for bi-valent cations (here Ca2+ and Sr2+, at water den-sities between 0.29 and 0.087 g cm3; [65]) lead to a density, which

easede ofion t

a ca neuuenssolvcome

is th7]. A

OH

o a e.g.erpen, eser [7one cper

iticalmicao, wassoci

at suandk-doally n rat

ionin of

a hy of anions ther

t in intn hat decte. Coestioielec,83]

c reamicahich

ensitrole o

the ater ties (density, ionic product and relative static dielectric constant) ofion of temperature at 25 MPa ([55,56] data from [57]).

are better solved [46]. In both cases poisoning of salts is observed [43]. Detailed investigations of theystems show very interesting properties and differentes to handle the salt deposition [46,47]. A very impor-

here is that some salts form solid and some, mainlyalts, form a liquid phase of higher density compared toitical water phase [47,48]. The possibilities to separatecipitation [46], solvation in salt brines [47] and the useyclone [49] (Fig. 2).ange of 600700 C the so-called supercritical water

(SCWG) is conducted [7,30,44]. Here the goal is to pro-gen, which is the reason for the high temperature. Onand this high temperature makes the choice of reac-l challenging [30]. In an ideal case, the conversion tond CO2 as major and methane as minor compound isnly the inorganic compounds should be left in the aque-t. Experimental studies show, on the other hand, someoteins limit the reaction rates [50,51] and higher drynts inhibit complete conversion [30]. Acetic acids andally are found in the efuent aqueous phase, if the con-t complete. To reach complete conversion, especially atersion and if possible, lower temperature different cat-ing activated carbon are tested [52,53]. The CO contentuct gas formed in biomass gasication is usually belowdel compound gasication usually leads to higher COecause the potassium salts in the biomass catalyze thehift reaction (see below) [7,30]. Then, the product gas

as synthesis gas to produce e.g. methanol [54].

ies of water

t ambient conditions is a polar solvent of polarIt is a good solvent for polar compounds and salts. Onand it is a weak solvent for non-polar compounds and

is incrthe casto the shell tousuallyThis inlike diHCl bereasonmic [6H+ andleads teratingmonotreactioof watinside ical temsub-cron che

Alsthe dihigherother hto breadrasticreactioby thefunctiosimplycentercondit

Ano[81]. Icase anreactiosolvention rathe qulocal ding [82organion cheguish wwith dof the

Forthat wydrogen or nitrogen [58]. If water is heated up, thereases. In consequence and because of the increasedvement of the water molecules, the interference of

ater molecules is weaker. This leads to fewer and lessogen bonds, although clusters by hydrogen bonds still

temperature [5961]. The second consequence of thety is that the relative static dielectric constant decreasesfore, with increasing temperature the solvent polarityAt supercritical conditions at e.g. 550 C and 20 MPa,ves as a nonpolar organic solvent like pentane with goodr organic components and gases and low solubility for]. But the single water molecules are still polar. In theolar compound or ion is present in the water, the watern its nearer environment are attracted. In the case of

solvent cha

4. Driving

In Fig. 3 tin case of thcose and phis that therany temperis the prefeuct at high (model comperatures a by a factor of 30100 compared to bulk density. In mono-valent anions the hydrogen atom comes closerhan the oxygen of the water molecule in the solventtion. This leads to the observation that salts, which aretral like NaCl become basic by formation of HCl [66].ces chemical reactions. As a rule of thumb, weak acidsed CO2 become stronger [67,68] and strong acids like

weaker [69]. From the thermodynamic perspective theat the dissociation of weak acids is usually endother-s water shows self-dissociation also the formation ofis inuenced by the strong clustering effect [70]. This

high local concentration of H+ and OH ions accel- the Beckmann and Pincol-Pinacolone rearrangement,ne alcohol synthesis as well as the Cannizzaro and Heckpecially slightly above or very close to the critical point178]. Here the H+ and OH ions can move very fastluster leading to a high reactivity slightly above the crit-ature, although the total concentration is lower than in

water [79]. (For a discussion of micro structuring effectl reactions see [56]).ter is a weak acid and base, becoming stronger becauseation is endothermic. Therefore, the ionic product isbcritical conditions than at ambient ones [62]. On the

the low density of water above its critical point leadswn of the solvent shell and the ionic product decreasesat low densities [80]. Looking at chemical reactions thees of acid-catalyzed reactions are higher than assumedc product. The reason might be that water fullls thethe H+ ion or micro-structuring effects [3]. In both casesdrogen atom of water comes very near to a reactive

organic molecule inducing a reaction, which at normalis caused by acids.

important aspect is the role of the dielectric constantuences solubility and also chemical reactions. In theermediate state of a chemical species during a certains a higher polarity than the educts or products a polarreases the activation energy and increases the reac-ncerning water at increased temperature and pressuren occurs, if the macroscopic dielectric constant or atic constant created by the solvent effects is determin-. Solvent shells around organic compounds inuencections in supercritical water. Concerning the inuencel reactions it is experimentally very difcult to distin-

property of water is relevant, because they all changey and temperatures [56]. For a more detailed discussionf water see [26].chemistry of biomass conversion it can be concludedis very reactive at every condition but the properties asnge. This inuences solubility and the stability of ions.

forces: thermodynamic and kinetics

he gas composition as function of temperature is shownermodynamic equilibrium assuming a mixture of glu-enol as a substitute for biomass. An important result

modynamics predict a nearly complete gasication atature. The second important nding is that methanerred product at low and hydrogen the preferred prod-temperatures at the given concentration like 10% (g/g)pound or dry matter content) [8486]. At low tem-

n increased hydrogen formation is possible also at low


Fig. 3. Calculafunction of temund 4.6 wt.% phmethod) Feed


other not. In addition a solvent with high heat transfer propertiesavoiding temperature differences inside biomass particles.

At conditions slightly above those of HTC, we nd the regionof aqueous phase reforming. Here, thermodynamics allow the for-mation of hsuccessfullywith biomacatalyst is dis that in thA direct rethe reactioof biomass.degradationbe present ring and asconversion.pounds wicatalysts usthe eliminahydrogen fodegradationconsecutiveThe thermobecause theincreased sthe increasheterogene[103,104].

At increAt this pointion with thof water [1ash pyrolyreaction timof several hup to 30 whigh heatinare intermenents furthare necessaperature mcompoundsthe reactionby hydrolyperatures apredict comformation rtion times.Although soprecipitate reports ndhydrothermare used asformation; pounds in sleads to a mpolymerizeIt is assumewater-gas shydrogenat

By fast psame produremarkableone at hydrdry conditioan assumpt

rrhenerim

pyroly

ays trolys-radsis throthe

of ws likend innols.

but t thehase undssionts foeacti

viscred tigh

], theexplained by the departing of small compounds formed fromydrates, catalyzed by potassium [111]. Here, water is impor-

solvent for the potassium salts and the compounds formingls to enable the reaction. However, in regard to process devel-t, the use or treatment of the aqueous phase formed duringhermal conversion has to be considered.mentioned above, the splitting of lignin, derived e.g. fromlp and paper production, is usually conducted at around. The reason for its lower reactivity is the less and, because ofmplex three-dimensional structure, hardly accessible ether

To understand the chemistry, studies of the degradationdel compounds like guaiacol are useful. Fig. 4 shows theius-plot of this reaction in water at different temperaturesThere is a kink between the sub-critical and near-critical dataing a change in the reaction mechanism. As the data in near-percritical water are very similar to the pyrolysis reactiont is likely that the high-temperature pathways occur via frees [113]. At lower temperatures, ionic hydrolysis pathways, which is not possible in dry pyrolysis. To conclude: In ligning the role of supercritical water is to act as a solvent. This isant because often hydrogen with a hydrogenation catalyst isydrogen at low educt concentrations. So far, this has been done with compounds derived from biomass notss itself. The fundamental question asks for what theoing. The challenge of applying heterogeneous catalystse beginning, there are two solids, catalyst and biomass.action is therefore very unlikely. More reasonable isn of solved molecules, produced by the degradation

In APR the temperature is high enough for complete. Anyway, higher molecular weight compounds would

in the mixture as well. This leads to a high risk of char- consequence, catalyst poising in the case of biomass

This is the main reason why usually monomeric com-th low molecular weights are used as educts. Theed in APR are hydrogenation catalysts. They catalyzetion/formation of hydrogen but also its reactions. Thermed from the biomass is able to react with the biomass

compounds to form hydrocarbons. Concerning this reactions the various catalysts behave differently [28].dynamic concentration limitation becomes irrelevant,

hydrogen leaves the equilibrium by this reaction. Theolubility for hydrogen, educts and products as well ased diffusion rates are important advantages in doingously catalyzed reactions in hot, compressed water

ased temperatures hydrothermal liquefaction occurs.t it makes sense to compare hydrothermal liquefac-e corresponding dry process to understand the role]. To produce a liquid fuel form dry biomass, fast orsis is applied usually around 500 C and at very shortes in the order of seconds. A complex product mix

undred components is formed (see above) containingt.% of water. The short reaction times combined withg and cooling rates are necessary, because pyrolysis oilsdiate products. At longer reaction time the oil compo-er reacts to coke and gases [2]. The high temperaturesry to split-up and volatize the biomass. At lower tem-ainly char is formed by elimination of smaller, volatile

from the solid particle. In the hydrothermal process with water enables complete conversion of biomass

sis instead of thermolysis. Consequently, lower tem-re necessary. At these temperatures, thermodynamicsplete conversion to gases (see above). In fact the gasate is low and there is no necessity to use short reac-

Obviously the gas formation is kinetically inhibited.me studies report the formation of solids, e.g. inorganicof incomplete conversion of too large particles, other

no or very low solid formation [36,39,105,106]. Foral liquefaction usually compounds like KOH or K2CO3

catalyst [107]. Basic catalysis may help to avoid cokethe main reason is the high solubility of different com-ubcritical water. The presence of this potassium saltseasurable reduction of HMF, which usually is able to

[108] or forms humines with carbohydrates [90,109].d that this is an indirect effect of the catalysis of thehift reaction (3), where the active hydrogen formedes the HMF [110].yrolysis and hydrothermal liquefaction in principle thects are formed but in different composition [1]. This is, because the chemical mechanism seems to be an ionicothermal conditions [2] but which cannot be ionic atns with missing stabilization by water as a solvent. Asion, water as solvent opens a low-temperature, ionic

Fig. 4. Awith expand via

pathwdry pybe freepyrolyat hydexcesspoundare fouof phestable,gases awith pcompoconverproducafter rhighercompatively h[2,111nols is carbohtant asphenoopmenhydrot

As the pu400 Cthe cobonds.of moArrhen[112]. indicatand surates, iradicaloccurssplittinimportius plot of guaiacol degradation in near critical water [112] comparedental values from experiments in sub- and supercritical water [115]sis [116] (Copyright 2012 Daniel Forchheim et al. [112]).

o products, which are formed via free radical reaction inis. Consecutive reactions of these products could onlyical ones with high activation energy. Therefore, in drye intermediates show fast consecutive reaction but notrmal conditions. The main difference is that due to theater and the phase separation after reaction, polar com-

organic acids and small polar components like alcohols the aqueous phase. The organic phase mainly consists

As seen in Fig. 3 the phenols are thermodynamic notobvious these compounds have no chance to react tose low temperature and in water. The reaction in waterseparation afterwards leads to an extraction of the polar. By support of water elimination in the hydrothermal, no sugars are found in any phase, which are typicalund in pyrolysis oil. This extraction of polar compoundson simply by phase separation leads to a product ofosity, lower oxygen content and higher heating valueo pyrolysis oils. An important observation is the rela-yield of phenols. Pure carbohydrates form also phenolsy are not only derived from lignin. The formation of phe-


Fig. 5. Typical products of glycerol conversion in water. Left: products from lowtemperature/high pressure pathway, Right: products from high temperature/lowpressure pathway [117].

used to support the lignin decomposition, and hydrogen solubilityis very high or even complete at these conditions.

Also, reactions studied in the context of up-grading bio-oils frompyrolysis are mainly dominated by free-radical reactions due to thelow number of reactive polar bonds [114].

A change in the reaction mechanism as shown in Fig. 4 has beenstudied before, e.g. for the reaction of glycerol in water [117]. Atsubcritical cpressure andiates are fo(Fig. 5). Largabove the cother produway [117,1a reaction nand H+aq focontrols thereactions frthe model, ture and hibecause of ture the ionionic produmeasured kglobal reacttions betwebecause theboth reactio

As a concation in the reactioprotein-con[119]. Anot

Fig. 6. Arrhenrectangles are

differences in the gas composition after gasication of alcoholsdepending of the number of carbon atoms. The dominance of -scission cracking leads to higher methane and lower hydrogenyield, if the number of carbons of the organic acid is even [120].

From ancritical watreactions. Tcal water oxbecause of early studiethe water-gshould enabmodynamicwater-gas scritical watThe effect otions of smaof higher co[122]. To coso-called hrate of the Early studie

arouminergyed cos selnt foirect

to focomp25,1uper

reareact

to atdynter-gike Nharams ifter areadiali sae of tw so

phaouggas s

the 650

veryen, fc com

normonditions and above the critical temperature at higherd similar density, products formed via ionic interme-und. These are different aldehyde, ketones and otherser amount of gases are only formed at lower densitiesritical temperatures of water (Fig. 5). These gases andcts like methanol, indicate a free radical reaction path-18]. For all products in the lower temperature rangeetwork was set-up basing on the catalysis by OHaqrmed from water. This way the ionic product of water

importance of this network. For the high-temperatureee-radical reaction pathways were created [117]. So inboth reaction networks compete: At lower tempera-gher density the ionic reaction pathways are superiorthe high ionic product of water. At higher tempera-ic reaction pathways are suppressed because of the lowct and the free-radical reaction pathway dominates. Theinetic rate coefcients and the calculated ones of theion rate are shown in Fig. 6. Near the kink, the deriva-en modeling and experimental results are larger, likely

model is not considering any interference between then networks.sequence of the free-radical nature of biomass gasi-supercritical water, free-radical scavengers reducen rate. These are Maillard products formed fromtaining biomass [50,51] and phenolic compoundsher consequence of the free radical mechanisms are

mationby assution enactivatchangediffereboth dforcedvated case [1

In sthe keyof this essarythermothe waalysts lbasic cproblecal waeffect lthe alkbecauseven lothe gas

Althwater-of salts(belowtion ishydrogorganicose inius-plot [117] of the over-all rate coefcient: Cycles are measured and calculated data.

via the degthe water-gdiscussed abehaves difreactor [87shift reactiopresses polreactive anhydrogen [1

In the gfor splitting historical point of view, free radical reactions in super-er have been intensively studied in view of oxidationhe goal was to destroy hazardous waste by supercriti-idation (SCWO) promising nearly complete conversion

the absence of mass and heat transfer limitations. Evens of SCWO show an unexpected high reaction rate ofas shift reaction (Eq. (3)) [121]. The presence of waterle the formation of hydrogen via this reaction by ther-

reasons. On the other hand the kinetics of the gas phasehift reaction was known and the reaction rate in super-er appeared to be higher than expected from this [121].f pressure can be considered by calculation: The reac-ll free radicals at increased pressures are faster becausellision numbers leading to a better energy nivellationnsider this, the reaction rate coefcient can be set to theigh-pressure limit [123,124]. By doing so, the reactionmeasured water-gas shift reaction was still very high.s assumed a lower activation energy by water shell for-nd the activated complex [121]. Later this was speciedg formic acid as intermediate and a lowering of activa-

for the formation of CO2 and H2 by solvation of themplex [125,126]. In the second case water as solventectivities, because the lowering of activation energy isr the two possible reaction pathways of formic acid inions of the water-gas shift reaction. Here formic acid isrm CO2 and H2 instead of CO and H2O, because the acti-lex formed with water has a lower energy in the rst26].critical water gasication the water-gas shift reaction isction as well [87,127]. In this case the kinetic inhibitionion is of importance. The presence of alkali salts is nec-tain the low CO and high hydrogen yields calculated byamic studies [30]. The reason is that alkali salts catalyzeas shift reaction [128], as also many heterogeneous cat-ickel [129]. The catalytic effect was connected with thecter of salts [128] although such argumentation causes

the changes in acidity/basicity in near- and supercriti-e considered [130]. So far the reaction pathway and theng to the catalysis are not clear today [130]. In additionlt concentration in supercritical water should be low,he decreased solubility. On the other hand, a certain andlubility is there and enables the effect of alkali salts. Inse only catalysis on salt particles is possible.h it is not fully clear how the alkali salts catalyze thehift reaction, the result is obvious. Only in the presence

thermodynamic predicted gas composition is reachedC, [131]). In addition the hydrogen formed via this reac-

reactive and inuences the reactions occurring. Thatormed via the water-gas shift reaction, hydrogenatespounds was shown be the reaction of deuterated glu-al water. Here H2 was added to double bonds formed

radation of glucose, and H2 could only be formed viaas shift reaction of normal water [111]. This effect wass reason, why a continues stirred tank reactor (CSTR)ferent concerning gas forming kinetics from a tubular,132]. Only in the CSTR hydrogen formed via water-gasn is able to react with early intermediates. This sup-

ymerization, because these early intermediates are veryd react with each other if they are not saturated by11].asication of biomass water is important as reactant

the macromolecules by hydrolysis and as hydrogen


source via the water-gas shift reaction. The change of the propertiesof water by passing its critical point makes the different betweenliquefaction and gasication in the case no heterogeneous metalcatalyst is present. As a solvent the high solubility of intermediatesand hydrog

6. Perspec

The diffebiomass depotential ofreaction pareaction temter of waterbonds in waof selectivitpared to drhigh. And tha rarely useapplicationof residues as from footant optionthe huge pHydrothermof a varietychemical pu

On the osub- and suapplicationto bridge frples are theis a multi-bsize (14 400twelve reactinuous hydBiotechnoloin Germanyof 100 kg/hcatalyzed vJapan. Herepanies are simportant ation procesSystem) prosolids) per hEven largerstrated in Jascale are th[139] with because it sThis seems ation. Althotypical sizeexperiences

On the research anindustry anmeans thatof biomass nal produtant, becauconnected tion, aspectmore intenessary to ge

is necessary to get reasonable heat efciency; this is a contra-diction. Good heat exchange leads to relative low heating rates.Pumping of concentrated biomass slurries is not possible; thereforemixing with hot process water instead of passing of the biomass

h a heat exchanger is suggested. In view of the energy bal-his isnd tore cduc

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tives and future directions

rent processes with their various product options fromgradation in hot compressed water show the large

hydrothermal conversions. Mainly, water opens newthways compared to dry processes, needing lowerperatures. This is a consequence of the polar charac-

and the high reactivity of biomass with its many polarter. The change of properties of water enables changesy therefore different products can be selected. Com-y processes the reached yields of the products are veryese are processes for wet types of biomass, which is stilld resource for bioenergy production apart from a fews such as biogas production by fermentation. The usefrom agriculture, forestry, and land cultivation as welld and feed production will be an increasingly impor-

in terms of energy supply. The complimentary use ofotential of wet and dry types biomass is a challenge.al processes are a promising basis for the production

of different intermediates to be used for energy andrposes.ne hand, we know a lot about chemical reactions inpercritical water. On the other hand, only few technicals exist. Very important are bench and pilot scale plantom lab-scale research to industrial application. Exam-

plant of AVA-CO2 for hydrothermal carbonization. Itatch process; the reactor of the pilot plant has the nal

L) in view of a production plant, the latter will be six ortors [133]. Examples for bench-scale plant with a con-rothermal carbonization are from the companies Artecgie GmbH [134], SunCoal [135] and terranova [24] also. The largest plants for gasication with a throughput

are at the KIT in Germany [49] and 410 kg/h by a coalersion in cooperation with the University of Hiroshima,, in contrast to other countries also some Japanese com-trongly involved to develop the technology [136]. Alsore plants in the USA in view of hydrothermal liquefac-ses, e.g. for the so-called STORS (Sludge to Oil Reactorcess with 30 kg of concentrated sewage sludge (20 wt.%our in the Battelle Pacic Northwest laboratories [137]., with 200 kg/h the liquefaction of sludge was demon-pan [138]. Related processes demonstrated in increasede hydrothermal sugar formation by Renmatrix, USA3 tons (dry mass) a day. This is of special importance,hows that not only fuels from biomass are interesting.to be also the impression of the BASF, starting cooper-ugh this summary is likely not complete, it shows thes of bench and pilot plants and that there are important

for scale-up.other hand, there seems to be a barrier betweend application. From the view of research, the needs ofd economy have to be addressed more strongly. This

whole process chains have to be developed. In the casethis means the chain from the plant on the eld to thect, which might be a fuel or a polymer. This is impor-se every process is part of a network, and the otherproduction lines have to be integrated, too. In addi-s important for scale-up [140] have to be investigatedsively. For example in SCWG high heating rate are nec-t the best gas yield. On the other hand, heat exchange

througance, teffect aeral, mare conthe cheffect can be

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[15] Mbp no solution [7]. Here more studies to understand theo develop an engineering concept are necessary. In gen-onversions of model compounds than of real biomassted. Model compounds give a better understanding ofal process, but studying them completely ignores thee biomass structure on the process. Some exceptionsd in HTC research [21,96,141].

role and needs of the market have to be considered. As, not only the quality of product but also the productionailable amounts have to be taken into account. Anyway,king in the eld of hydrothermal conversion research

more about products than about processes to bemunicate with industry. Such a better communication

goal in the future, e.g. by creation of special workshopss e.g. on Internet basis.lop process chains instead of single process steps, itelpful if there would be more and better funding pos-tween teams in different continents, e.g. Europe andould initiate new and powerful impacts on research

pment.

gement

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Water A magic solvent for biomass conversion1 Introduction2 Overview of hydrothermal conversion processes3 Properties of water4 Driving forces: thermodynamic and kinetics5 The role of water in the different processes6 Perspectives and future directionsAcknowledgementReferences

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