hydrogen synthesis from biomass pyrolysis with in situ

8/13/2019 Hydrogen Synthesis From Biomass Pyrolysis With in Situ

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Hydrogen synthesis from biomass pyrolysis with in situ

carbon dioxide capture using calcium oxide

Meilina Widyawati a, Tamara L. Church a, Nicholas H. Florin a,b, Andrew T. Harris a,*a Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, University of Sydney, NSW, 2006, Australiab Grantham Institute for Climate Change, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom

a r t i c l e i n f o

Article history:

Received 23 August 2010

Received in revised form

14 November 2010

Accepted 26 November 2010

Available online 2 March 2011

Keywords:

Hydrogen

Carbon dioxide capture

Pyrolysis

Pine

CaOThermogravimetric-mass spectro-

metric analysis

a b s t r a c t

Hydrogen (H2) and other gases (CO2, CO, CH4, H2O) produced during the pyrolysis of

cellulose, xylan, lignin and pine (Pinus radiata), with and without added calcium oxide

(CaO), were studied using thermogravimetry-mass spectrometry (TG-MS) and thermody-

namic modeling. CaO improved the H2 yield from all feedstocks, and had the most

significant effect on xylan. The weight loss of and gas evolution from the feedstocks were

measured over the temperature range 150e950 C in order to investigate the principle

mechanism(s) of H2formation. Without added CaO, little H2was produced during primary

pyrolysis; rather, most H2 was generated from tar-cracking, reforming, and char-decom-

position reactions at higher temperatures. When CaO was added, significant H2 was

produced during primary pyrolysis, as the water-gas shift reaction was driven toward H2formation. CaO also increased the formation of H2 from reforming and char gasification

reactions. Finally, CaO increased the extent of tar cracking and char decomposition, and

lowered their onset temperatures. The production of H2 from pine over the course ofpyrolysis could be modeled by summing the H2 evolutions from the separate biomass

components in relevant proportions.

Copyright 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

The concept of a hydrogen energy economy has attracted

attention from the scientific, environmental, and political

communities. The properties of H2 make it a promisingchemical feedstock, fuel, energy carrier and energy-storage

medium[1]. Currently, about 96% of global H2is derived from

fossil fuels [2], either through catalytic steam reforming,

partial oxidation, or a combination of the two [1]. Producing H2from these finite fuel resources has raised concerns about

energy security and about environmental sustainability, as

the processes collectively liberate 0.4 Gt(CO2)/a [3]. Vastly

preferable would be to produce H2from a renewable resource,

such as water or biomass. H2can be obtained from water by

electrolysis, thermolysis, or biologically assisted photolysis, or

from biomass via pyrolysis or steam gasification (pyrolysis in

the presence of added H2O)[4e7].

Pyrolysisuses heat to degradea feedstock, in the absenceofan external oxidizing agent, to a combustible mixture of gases

[1,8] that contains mainlyH2,CO,CO2, CH4 andH2O, along with

tarand charfractions (Table 1,eq.(1)) [6]. The processoccursin

three stages: (i) pyrolysis to produce the main gas products,

volatiletar, andsolid char;(ii) crackingand reformingof tarand

gases, and (iii) char gasification [7]. During each stage, the

primary products participate in multiple, interdependent

secondary reactions that can alter the nature and distribution

* Corresponding author. Tel.:61 2 9351 2926; fax: 61 2 9351 2854.E-mail address:[email protected](A.T. Harris).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / h e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 8 0 0 e4 8 1 3

0360-3199/$ e see front matter Copyright 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2010.11.103
mailto:[email protected]://www.sciencedirect.com/http://www.elsevier.com/locate/hehttp://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://www.elsevier.com/locate/hehttp://www.sciencedirect.com/mailto:[email protected]


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sawdust[32]. Engineering aspects of steam gasification in the

presence of CaO have also been considered [7,33]. The pyrolysis

reactionsofhemicelluloseandlignininthepresenceofCaOhave

not been studied, nor have the effects of various biomass

components on one another during CaO-assisted pyrolysis. To

address some of these gaps in understanding, we performed

multi-technique analyses of the pyrolysis behaviours, with and

without CaO present, of a biomass source (P. radiata, hereafterreferred to as pine) and three biomass constituents: cellulose,

xylan (a hemicellulose), and lignin. This allowed us to study and

compare the behaviours of each feedstock during pyrolysis, and

to evaluate the effect of CO2 sorption by CaO on each, while

focusing specifically on the generation of H2gas. We were also

abletoconsidertheformationofH2 frompineinthecontextofH2production from its components.

2. Materials and methods

2.1. Feedstocks

Avicel cellulose (Fluka), xylan from beechwood (Sigma) and

lignin (organosolv, mixture of lignins from maple (50%), birch

(35%), and poplar (15%), Aldrich) were selected as model

biomass components. Pine wood (P. radiata, from Pollards

Sawdust Supplies, Australia) was selected as a representative

biomass sample, as pine products are commonly studied in

biomass-gasification studies [15,17,21,31,32,34,35]. Pine does,

however, pyrolyze differently from some hardwoods, such as

holm oak and eucalyptus [34]. The as-received feedstocks

were ground using a mortar and pestle, and sieved to 53/

212 mm and CaCO3 (Aldrich) was sieved to


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the range 1e150. The lower heating rate was chosen for these

experiments to facilitate the evolution of low-molecular-

weight compounds [36]. The ions identified in this process

were targeted for MS scanning during the pyrolysis experi-

ments in order to obtain better resolution. Focusing on the

detection of lighter species was a trade-off. Though it limited

our ability to directly observe the effects of CaO on tar

cracking, it allowed the evolution of light gases to be studied

more accurately. By considering the equations in Table 1,

insights on tar cracking can still be gleaned from the forma-

tion of light gases, primarily H2and CH4.

We examined several methods of reporting ion intensities

[27,36,37]. Following three identical experiments on the

pyrolysis of each biomass component, we found the best

reproducibility using the relative integrated peak linear

intensity to total integrated peak linear intensities normalized

to sample weight, eq.(13).

RInti Inti=X

Inti m

(13)

where R(Int)i is the relative integrated peak linear intensity

(hereafter referred to as relative intensity), Inti is the inte-grated intensity of a gas species, and m is themass of biomass.

Though generally reproducible, this method of processing

MS data returned highstandard deviations forsome speciesin

some reactions, and this limited our ability to make definitive

statements in some cases. However, TG-MS remains the only

technique available to simultaneously measure the thermal

decomposition and H2 output of a very small sample, and

a small sample is necessary to minimize the interference of

heat and mass transfer effects. Nevertheless, we cannot rule

out heat and mass transfer as sources of error in these

experiments.

The prominent species for study were selected based on

their relative intensities across the temperature range150e950 C and on their relevancy. Ion fragments with R

(Int) < 0.5 nA min/mg were not considered prominent. Five ion

species stood out as the most intense and relevant. The first of

these, atm/e 2, is assigned to H2. The peak atm/e 15 shows

the release of methyl groups, eCH3, which are formed

primarily from methane. The parent H2O peak atm/e 18 was

studied rather than the fragment peak atm/e 17. CO is the

major contributor to the peak at m/e 28, though contribu-

tions from other compounds, especially CO2and ethylene, are

also likely [38]. Finally, the peak at m/e 44 indicates the

release of CO2; acetaldehyde is a possible minor contributor to

this peak. The most prominent minor ion species are given in

the supporting data (Table S2).

3. Results and discussion

3.1. Predicted impact of CaO

We have considered the importance of the equilibria inTables

1 and 2 on biomass pyrolysis, and used thermodymanic

modeling to study the effect of CaO on the steam gasification

of cellulose [27]. For the present work, we used similar

calculations to predict the impact of CaO on the products ofthe pyrolysis reactions of cellulose, xylan, lignin, and pine.

These studies are for comparison purposes only; in a real

system, biomass pyrolysis does not reach equilibrium for

several reasons. At low temperatures, equilibrium is not

approached because most of the material remains trapped in

the biomass sample, and because the kinetic energy of the

system is insufficient. The greater kinetic energy present at

higher temperatures allows the reaction to come closer to

equilibrium; however, it remains under kinetic control, as

evidenced by the effect of catalysts on the reaction [39].

Moreover, at hightemperatures, the reaction will divergefrom

the particular equilibrium calculations described in this

article because some components have been selectivelyremoved from the system (i.e. the ones that were volatilized

and swept away at low temperature). Nevertheless, the ther-

modynamic calculations offer insight into the effect of CaO on

biomass pyrolysis.

Our calculations, which are discussed in more detail in

the supporting data, predict that CaO will increase the

molar fraction (dry basis) of H2 in the pyrolysis products of

all four biomass sources when T < 720 C (see Figs. S1eS4).

At higher temperatures, the release of CO2from CaCO3 (the

reverse of eq. (10) is favoured and CO2is no longer absorbed,

thus negating the thermodynamic effects of CaO. The H2concentrations were maximized at 640 C. CaO (nCa/nC 0.5)

increased the theoretical maximum H2concentration in theproduct gas produced from the pyrolysis of cellulose, xylan,

lignin, and pine by 52, 49, 27, and 44%, respectively. Thus

the effect of CaO on H2 concentration was inversely

proportional to the C/O and H/O ratios of the feedstocks (see

Fig. S5), and was therefore predicted to be greater in the

pyrolysis of the polysaccharide biomass components than

of lignin.

Aside from altering the thermodynamic balance of gaseous

products during pyrolysis, CaO catalyzes tar cracking[21e23].

Further, even a small amount of carbonate salt affects the

initial products of rapid cellulose pyrolysis quite significantly

[17]. The CaO added to our samples, and the Ca(OH) 2 and

CaCO3 that are formed in situ, may catalyze one or more

Table 4 e Composition of ash formed from the combustion of xylan and pine.a

Ash source Content in ash sample [%]

SiO2 Al2O3 Fe2O3 TiO2 K2O MgO Na2O CaO SO3 CO2 P2O5

Xylan 0.5 0.5 0.2 0.03 0.8 0.6 51.8 6.5 0.5 34.5 7.30

Pine 4.0 1.3 24.0 0.09 12.6 15.7 2.0 37.8 2.1 0.2b 0.17

a Ash was analyzed using ICP-OES after combusting to 815C, fusing with borate, and dissolving in acid.b Calculated by difference.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 8 0 0 e4 8 1 3 4803
http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103http://dx.doi.org/10.1016/j.ijhydene.2010.11.103


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reactions during pyrolysis; however, we are unable to distin-

guish kinetic and thermodynamic effects in our experiments.

Thus, in the discussions below,we will consider the formation

of various products within the context of the reactions shown

inTables 1 and 2. The catalytic effects of calcium compounds

will not be discussed, but they are possible whenever these

are present.

3.2. Pyrolysis of biomass and components

The main thermogravimetric features and the total

ion intensities for major ions in the pyrolyses of cellulose,

xylan, lignin and pine are summarized in Table 5, and the

temperatures at which various ion productions are maxi-

mized are listed inTable S3.

3.2.1. Cellulose

Even beforethe gasification of biomasswas commonly studied

asasourceofH2, cellulosepyrolysis wasstudied with thegoals

of understanding fires [40] and deriving chemicals from

biomass [41]. Cellulose samples are more uniform than

hemicellulose, lignin, or biomass samples [42,43], making

them easier to study. Thus the pyrolysis of cellulose is betterunderstood than those of other biomass components, and the

kinetics of the process have been reviewed [44]. The ther-

mogravimetric behaviour of cellulose is also well-known

[14e16,45e48]. It generally begins with a large, sharp weight

loss withTmaxw355 C; the preciseTmaxdepends on the heat-

ing rate [11e14,40]. This is followed by a smaller, more gradual

weight loss at Tw650e700 C. Little char remains after pyrol-

ysis. We observed a large, sharp weight loss with Tmax 362 C

and a much smaller feature that peaked at 705 C(Fig. 1a). H2production was slow and relatively constant for T


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a feature at 415 C in the thermal decomposition of cellulosewith added Ca(OH)2 [15]; however, much less calcium was

used in that case (nCa/nC 0.02e0.05). In any case, H2production declined during this shoulder, reaching

a minimum near 435 C, then slowly increased to a broad

peak at 580 C. The rate of mass loss was quite low in the

first half of this broad peak, suggesting that H 2 was mainly

produced by hydrocarbon reformation and tar cracking (eqs.

3e5). This is supported by an increase in intensity of the ion

at m/e 15 (Tpeak 500 C), which could correspond to CH4formation during tar cracking (eq. (5)). Some of the CH4produced during tar cracking could then be consumed to

produce H2 (eq. (3)). These reactions occurred at higher

temperatures when no CaO was added (Fig. 1a).

Above 500 C, as the detection of methyl groups decreased

but H2production continued to increase, the rates of weight

loss and CO formation also began to increase (Tpeak 680 C

for weight loss). Thus char decomposition (eqs. 6e8) likely

began around this temperature. No char was detected at the

end of cellulose pyrolysis in the presence of CaO. CO2production also increased during this period, likely from both

char (eq. (7)) and carbonate (reverse of eq. (10)) decomposition.Both processes contributed to the weight loss. The decompo-

sitions of carbonate and char are not independent; the

increased pressure of CO2formed in the former may favour

char decomposition via the Boudouard reaction (eq. (8)),

accounting for some of the CO formed (though its contribution

is expected to be limited below 800 C); whereas the increase

in CO2 pressure from carbonate decomposition disfavours

char decomposition. The wateregas reactions (eqs. 6 and 7)

may also have contributed to weight loss, as they had in the

absence of CaO (vide supra); this is also consistent with the

coincident peak in CO formation. CO may also have been

formed by tar cracking, which is catalyzed by CaO[21e23].

Overall, CaO appeared to increase the relative intensity ofH2 formation over the course of cellulose pyrolysis (Table 5,

entries 1 and 2), consistent with the results of our experiments

under steam gasification conditions [14]. In this case,

however, the high uncertainty in H2 production from cellulose

alone (entry 1) precludes a statistically significant comparison.

3.2.2. Xylan

There are fewer pyrolysis studies of hemicellulose than

cellulose[50], though thermogravimetric analyses have been

reported for the pyrolysis reactions of hemicelluloses under

inert atmosphere[45,46,48,51]and under syn gas and H2[11].

Consistent with previous studies, the thermal decomposition

we observed for xylan (Fig. 2a) occurred chiefly between 200and 330 C and left 24 wt% char at 850 C. In addition to the

largest weight loss peak (Tmax 304 C), we observed

a shoulder atT 250 C. This is likely due to the specific xylan

used, and may indicate that the sample was inhomogeneous.

Though several studies have examined the thermal decom-

position of xylan, only one has measured the H2 released

during pyrolysis under inert atmosphere. Yang et al. used

GC-TCD to follow gas production from xylan pyrolysis in

a fixed bed system, but were not able to measure the weight

loss simultaneously [48]. Bymeasuring H2 formation and mass

loss concurrently, we found that xylan produced little H2during its primary thermal decomposition (200e330 C);

rather, H2O was the main gas detected during this period. Asecond (but unresolved) weight-loss peak occurred with

a maximum at 430 C. In addition to continued H2O release,

this weight loss was associated with minor peaks in the

production of CO (Tpeak 470 C) and CO2(Tpeak 430 C) that

were likely due to the continued breakdown of the biomass

sample. The wateregas oxidations of carbon (eqs. 6 and 7) are

also possible sources of carbon oxides, but are likely minor

contributors in this temperature range.

Following the production of CO and H2O during the weight

loss with maximum at 430C, H2and methyl-group formation

increased, signaling the onset of the second stage of the

reaction. At first, the H2and methyl peaks grew concurrently,

suggesting that they originated from the same source; this

Fig. 1 eThe rate of weight loss and the release of major

gases during the pyrolysis of (a) cellulose and (b) cellulose

in the presence of CaO, as measured by TG-MS.

vT/vt[ 40 C/min. MS data is presented in arbitrary units,

and the curve for H2was multiplied by 20 to improve

visibility. In (b), nCa/nC [ 0.5.



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was most likely tar cracking (eq. (5)). The evolution of CH4peaked at 495 C and then declined, whereas H2 production

continued to increase (Tpeak 535 C) even as the rate of mass

loss continued to decrease. Thus, atT > 495 C, CH4was likely

consumed by steam reformation to produce H2 and CO (eq.

(3)). Accordingly, CO production began to increase around

550 C, and continued to rise as the char began to decompose,

producing a final peak in the rate of mass loss at 755 C.

AddingCaO hada significant effecton thepyrolysis ofxylan

(Fig. 2b). Thermal decomposition began much as the reaction

without CaO had, with mass loss beginning at the same

temperature (Tinit 195 C), and H2O being evolved. Again,

some H2O was likelytaken upby CaO (eq.(11)).The weight loss

peakedat294 C,nearpeaksinCOandCO2 evolution.However,

in contrast to the reaction without added CaO, H 2production

also peaked during this time. H2 formation increased as CO

production decreased, suggesting that the wateregas shift

reaction (eq. (2)) was important during this stage of the reac-

tion. Immediately following the first large weight loss, anotherwas observed. This weight loss had a maximum at 435 C, and

was primarily associated with H2O evolution. The H2O

production in this case likely derived from Ca(OH)2via dehy-

dration (the reverse ofeq. (11)) orcarbonation(eq.(12)). Therate

of weight loss in this case (0.15 %wt/C) was consistent with

that seen for cellulose (0.14 %wt/C; Fig. 1b) at the same

temperature. Towardthe end of the second large mass loss, H2production declined, then increased sharply (Tpeak 525 C).

The increasedH 2 production wasaccompanied by increases in

methyl-group and CO production, consistent with tar cracking

(eq. (5)). Some CH4and CO may also have been reacted to form

H2via the steam-gasification (eq. (3)) and wateregas shift (eq.

(2)) reactions, respectively. H2production decreased slowly asthetemperatureincreasedfurther,and a thirdweightlosspeak

occurred, reaching a maximum at 690 C. This peak was asso-

ciated with CO and CO2 release, and is assigned to the

decompositions of CaCO3 (the reverse of eq.(10)) andchar (eqs.

(6)e(8)); tar-cracking reactions likely also contribute to the CO

and CO2release. Evincing char decomposition, only 3% char

remained after xylan pyrolysis in the presence of CaO (cf. 24%

whenCaOwasnotadded).Overall,CaOincreasedH2 formation

from xylan by 165%.

3.2.3. Lignin

The pyrolysis behaviours of a wide range of lignins have been

studied [11,12,17,45,46,48,52e56]. Contrary to the morechemically uniform polysaccharide components of biomass,

lignins are generally pyrolyzed over a broad temperature

range under inert atmosphere [12,45,46,48,52e56]and under

H2and syngas[11]. Both the different chemical compositions

of lignins from different sources [42,43,50,52,55,56] and the

different methods of lignin extraction[17,50,54,56]contribute

to the variation among the published data. The thermogravi-

metric data we measured for lignin (Fig. 3a) was generally

consistent with that in previous reports; decomposition

occurred over wide temperature range (215e825 C;

Tmax 408 C), and produced 29 wt% char at 850 C.

Compared to cellulose and xylan, lignin produced a higher

relative intensity of methyl groups, but a lower relativeintensity of H2O (see Table 5, entries 1, 3, and 5). It also

produced a higher relative intensity of minor fragments (see

Table S2). Several studies have examined the products of

lignin pyrolysis [12,17,46,48,52,53,55], and a few have moni-

tored H2 evolution in the reaction. Unlike Yang et al. (who

studied commercial alkali lignin) [48] and Fushimi et al.

(commercial lignin) [12], we observed a small peak in H2production at Tmax (408 C). Faix et al. observed maxima at

440 C (i.e. slightly aboveTmax) in the pyrolysis of lignins from

beech and bamboo woods [55]. In addition to a peak in H2formation, we also observed peaks in the formation of CO,

methyl groups, H2O, and CO2at this temperature, indicating

that primary pyrolysis was occurring. Several studies have


gases during the pyrolysis of (a) xylan and (b) xylan in the

presence of CaO, as measured by TG-MS. vT/vt[ 40 C/

min. MS data is presented in arbitrary units, and the curve

for H2was multiplied by 10 to improve visibility. In (b),

nCa/nC [ 0.5.



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observed maxima in these gases atTmax[12,48,52], though the

relative sizes of these peaks varied among the reports. Thus

most lignins appear to undergo primary pyrolysis at similar

temperatures, but with varying product distributions. MS

studies by Faix et al. found peaks in the release of several

oxygenated species, including HCHO, CHO, H3COH, and

H3CO inthe 420e500 C range, but little ofthese species above

540 C, and therefore concluded that most of the oxygen was

removed during this phase[55].

The greatest H2 production from lignin occurred at high

temperatures. Previous studies observed the greatest rate of

H2 release between 640 and 800 C, depending on the lignin

source [12,48,55]. We found a broad peak in H2 production

between 500 and 900 C (Tpeak 750 C), and peaks in CO and

CO2formation at 700 C. The latter result was consistent with

the observations of Yang et al. and Fushimi et al. [12,48],

though others have reported declining CO and CO2 levels atthis temperature [52,55]. In the present system, the coinci-

dent occurrence of the peaks in CO, CO2, and H2suggest that

char decomposition via the wateregas reactions (eqs. 6 and 7)

is the dominant process at 700 C, though the dehydrogena-

tion of aromatic rings is also important [55]. Finally, we

observed a decrease in H2formation at T > 750 C. This was

followed by an increase in H2O detection, suggesting that

some H2 may have been consumed to form water via the

reverse of the wateregas shift reaction (i.e. the reverse of

eq. (2)). However, the decreasing rate of CO detection at these

temperatures eliminates the wateregas shift reaction as

a major contributor to H2O production. Rather, the increase

in H2O formation was likely caused by the primary pyrolysisof recalcitrant compounds, consistent with the coincident

small peak in weight loss that ends as H2O generation

decreases (T > 800 C).

CaO did not affect the primary weight loss of lignin, which

still peaked just above 400 C (Tmax 407 C;Fig. 3b). A new

weight loss peak at 700 C, associated with carbonate and char

decompositions, was also observed, just as it was in the

pyrolysis reactions of cellulose and xylan in the presence of

CaO. The release of H2O from Ca(OH)2may have occurred at

w425 C, as it had in the cases of cellulose and xylan pyrolysis

in the presence of CaO; however, no new peak was visible due

to the peak in lignin decomposition in this temperature range.

The pyrolysis of lignin in the presence of CaO produced 70%more H2 than that without CaO (Table 5, compare entries 5

and 6), and yielded only 5% char (cf. 29% without CaO).

In the absence of CaO, H2had been a minor product of the

primary pyrolysis (i.e. near Tmax). With CaO, however, it had

a significant peak in this temperature range. Thus CaO likely

promoted the wateregas shift reaction (eq. (2)) during the

primary pyrolysis of lignin, as it had during the primary

pyrolysis reactions of cellulose and xylan. H2 production

slowed to a minimum at Tw500 C, then began to increase

(Tpeak635 C,withashoulderat710 C)evenastheweightloss

rate of the sample stayed was relatively stable. Considered

together with the concurrent decrease in the relative intensity

ofCH4, this suggeststhatthe steam reforming of methane andhydrocarbons (eqs 3 and 4), promoted by the removal of CO2(and consequently CO) by CaO, were likely important at this

temperature. The rate of H2 formation continued to increase as

the weight loss rate increased, suggesting that these processes

continued while the more stable lignin compounds, as well as

char and carbonate, decomposed.

3.2.4. Pine

The gasifications of various pine species have been examined

[15,17,21,31,32,34,35] and the thermal decompositions of two

pine specieshave been considered in terms of cellulose, xylan,

and lignin pyrolysis [35]. In the present study, the thermal

decomposition of pine (Fig. 4a) occurred in the range


gases during the pyrolysis of (a) lignin and (b) lignin in the



for H2was multiplied by 10 to improve visibility. In (b),nCa/nC [ 0.5.



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200e530 C, with the greatest weight loss being completed

whileT < 425 C. The major pyrolysis peak (Tmax 388 C) was

unsymmetrical, with a large shoulder at lower temperatures,

as previously discussed in detail by Grnli et al.[35]. H2O, CO2,

and CO were the major gaseous products of primary pyrolysis,

though methyl groups were also detected in this temperature

range (Tpeak 385 and 440 C, overlapping). Contrary to the

pyrolysis reactions of its components, the thermal decompo-

sition of pine did not show a high-temperature peak. Never-

theless,the pyrolysis of pine produced lesschar (at 850 C, 15%

of the original weight of pine remained) than the thermal

decompositions of xylan and lignin, but more than that of

cellulose.

Except for a very small, sharp peak at 385 C, the rate of H2formation from pine increased steadily over the course of the

pyrolysis, reaching a maximum at 750 C. Similarly to the

reactions with biomass components, the pyrolysis of pine

formed the greatest relative intensity of H2when the rate of

weight loss was low. Thus most of the H 2 formed from pinecame from secondary reactions. These were likely the

wateregas shift reaction (eqs. (2)) at lower temperatures, and

hydrocarbon reforming (eqs 3 and 4) and wateregas reactions

(eqs 6 and 7) at higher temperatures.

Having examined the formation of H2from pine, we turned

our attention toward the reaction in the presence of added

CaO. Though the pyrolysis of pine [15,34] and the steam

gasifications of pine sawdust[32,34]and bark[31]have been

studied in the presence of CaO, H2 production from pine

pyrolysis in the presence of CaO has not. We found that added

CaO had little effect on Tinit and Tmax of thermal pine degra-

dation (seeFig. 4b andTable 5, entries 7 and 8), but added the

same two features that had appeared when the componentswere pyrolyzed in the presence of added CaO: a low shoulder

(T 412 C) associated with the evolution of H2O (at approxi-

mately the same weight loss rate that was observed for

cellulose and xylan) and an additional weight loss (maximum

at 680 C) associated with CO and CO2 release (char and

carbonate decomposition, tar cracking).

H2 became an important product of primary pyrolysis

when CaO was present. Toward the end of the rapid weight

loss, H2formation declined as the rate of weight loss declined,

but it increased again after the temperature passed 450 C. The

detection of methyl groups peaked at 510 C and declined as

that of H2increased, implying that steam reformation (eqs. 3

and 4) was occurring. H2formation continued to increase asCO and CO2production increased again, suggesting that char

decomposition by the wateregas reactions was producing H2.

The maximum relative intensity of H2came at 660 C. Overall,

the relative intensity of H2increased by 82% in the presence of

added CaO.

The pine and component studies indicate that adding CaO

can significantly increase the H2yield from biomass pyrolysis,

decrease char yields, and lower the temperature at which the

relative intensity of H2 is maximized. A comparison of the

thermogravimetric and gas-evolution data allows some

general deductions to be made about which reactions are

most affected by CaO. At lower temperatures (T < 450 C), the

increased relative intensity of H2and complete suppression ofCO2reveal the importance of the exothermic wateregas shift

reaction (eq. (2)). This was especially visible in carbohydrate

(cellulose or xylan) pyrolysis. For all feedstocks, added CaO

also lowered the Tpeak for methyl-group detection. This indi-

cates enhanced tar cracking (eq. (5)) at lower temperatures,

though we cannot determine whether this is due to kinetic

factors (catalysis by CaO) or thermodynamic changes(the heat

released from carbonation and the shifts in the equilibria that

govern the system). Regardless, increased tar cracking

releases more methane and small hydrocarbons that can

undergo steam reformation (eqs 3 and 4). At moderate

temperatures (500 < T < 700 C), pyrolysis in the presence of

CaO produces more H2 at the expense of methyl-group


gases during the pyrolysis of (a) pine and (b) pine in the



for H2was multiplied by 20 to improve visibility. In (b), nCa/

nC [ 0.5.



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formation, supporting the involvement of steam-reforming

reactions (eqs 3 and 4). This is most noticeable in the pyrolysis

of lignin, which gives high relative intensity of methyl groups.

CaO also increased the extent, and decreased the onset

temperature, of char decomposition. This resulted in higher

H2 production at higher temperatures (T > 600 C), and

decreased the amount of char left after the reaction. In future

work, it would be interesting to consider the influence ofheating rate and residence time on these effects, as the

reactions involved in pyrolysis may be quite different under

very rapid heating or in a bed reactor.

Though added CaO increased H2 production from all

feedstocks (the difference was not statistically significant in

the case of cellulose), the effect is by no means optimized. We

were not able to judge the conversion of CaO to CaCO3, so it is

possible that not all of the added CaO was available to absorb

CO2. We were also unable to evaluate the degree to which char

deposition on the reactive surface of CaO hindered CO2sorp-

tion; this could seriously impact CaO activity in a large reactor

system, and requires study before these results can be applied

to large-scale biomass-gasification systems. Further, forpractical reasons (see Materials and Methods), we used

nCa/nC 0.5 in these experiments, but more CaO may benefit

the pyrolysis of biomass even more; for example, we have

previously shown that nCa/nC 1 is ideal for H2 production

from cellulose[27].

3.3. Component additivity

There is ongoing discussion in the literature regarding

whether biomass pyrolysis can be understood as the sum of

the pyrolysis behaviours of its components, i.e., whether

a simple additivity relationship applies between the pyrol-

ysis of cellulose, hemicellulose, and lignin [57]. To evaluatethis idea, several groups have pyrolyzed mixtures of biomass

components, and compared the results to those of the pyrol-

ysis reactions of individual components or of natural biomass

samples. Caballero et al. reported that the thermal decom-

position of a mixture of lignin and holocellulose (a mixture of

cellulose and hemicellulose in relevant proportions) did not

replicate that of almond shells [58]. Yang et al. studied the

thermal decomposition of cellulose, xylan, and lignin, as well

as of mixtures of two or three components, and reported

negligible interaction among them [45]. For samples with

known cellulose, hemicellulose, and lignin components, they

were able to predict the fraction of sample weight that would

be lost in three different temperature ranges, though theconverse prediction was less reliable. Couhert and co-workers

determined that even the method of mixing impacted the CO2yields obtained in the flash pyrolysis of mixtures of cellulose,

hemicellulose, and lignin [42,43]. Wang et al. [11]pyrolyzed

sawdust, cellulose, hemicellulose and lignin under syn gas

and under H2, and found that the presence of lignin and

hemicellulose affected the thermogravimetric behaviour of

cellulose. Evans and Milne concluded that the organic

components of biomass did not impact one another during

flash pyrolysis, but that the inorganic species present in real

biomass samples impacted their pyrolysis [17]. Hosoya and

co-workers compared the flash pyrolysis of untreated and

demineralized Japanese Oak samples, and concluded that the

mineral species in wood could not fully account for the

differences between product distributions from biomass and

component mixtures [59]. Nowakowski and Jones, on the

other hand, found that HCl treatment had little effect on the

thermogravimetric behaviour of cellulose and lignin [46].

However, chemical treatments are known to impact the

pyrolysis behaviours of biomass components [17], making

definitive conclusions from these studies difficult.Aside from the interactions between biomass components

and the presence of inorganic salts, several other factors

complicate the modeling of biomass pyrolysis using compo-

nent additivity. For example, lignins vary among species

[42,43,50,52,55,60], as do hemicelluloses [42,43,50,60]. In fact, it

is not trivial to find a component that is representative of

hemicellulose [61]. Moreover, real biomass samples contain

low-molecular-weight organic extractives [18] that are also

pyrolyzed. Finally, the problem is compounded by the varia-

tion in methods for determining the composition of a biomass

sample[62]. Nevertheless, many authors have attempted to

use mathematical models to relate the pyrolysis behaviour of

biomass to the separate pyrolysis behaviours of biomasscomponents. In general, models that describe thermogravi-

metric behaviour[18,35,61e64] are more common and, thus

far, more successful than models that aim to predict product

compositions [42,43,58,62]. However, models that accurately

predict thermogravimetric behaviour can require the use of

6e10 parameters. Better results are obtained when these

parameters relate to the weight loss of a sample in different

temperature ranges rather than its actual cellulose, hemi-

cellulose, and lignin contents[18,45,63]. Among models that

attempt to predict the formation of gases from biomass,

Caballero et al. found good agreement between the predicted

and experimental yields of CO, CO2, and H2O from almond

shells, although those of CH4, methanol, formaldehyde, andhigher hydrocarbons were not accurately predicted. Couhert

et al. showed that physically meaningful results were not

obtained when building models based on the H2, CO, and CH4yields from the rapid pyrolysis of hardwood, softwood, wood

bark, rice husk, and grass [42]. To our knowledge, no study has

predicted the H2formation profile of a biomass sample, and

we therefore examined this possibility using the pyrolysis of

pine, with and without added CaO.

As thermogravimetric models of biomass pyrolysis are

more common than gas-formation models, we first attempted

a quantitative comparison of the thermal decomposition of

pine with those of its components by combining the cellulose,

xylan, and lignin traces in the ratio 4:3:3 [65]. The results,shown in the supporting data (Figs. S6 and S7for pine and

pine/CaO, respectively), were unimpressive. The predicted

and experimental traces showed some similarities, but the

simple weighted sum of the component results was not an

accurate predictor of the thermal decomposition of pine. This

is likely due to the simplicity of the model, as Grnli et al.

produced much better models for the thermogravimetric

behaviour (for T 150e450 C) of two pine species upon

optimizing ten parameters[35].

In the component studies, H2 formation was not neces-

sarily proportional to the rate of thermal decomposition;

rather, it was often at a maximum when the rate of thermal

decomposition was low. Thus, despite the failure of



11/14

component additivity to predict the thermal decomposition of

pine, we examined the relationships between H2 formation

during pine pyrolysis and during the pyrolysis of the biomass

components. The results are shown in Figs. 5 and 6 for

pyrolysis in the absence and presence of CaO, respectively.

The H2data were collected as relative intensity, and the data

from the three different components were normalized (to the

same maximum R(Int)) before comparison, so quantitative

conclusions are inappropriate. Qualitatively, though, the

weighted sum of H2 formation from each of the three

component traces was a surprisingly accurate predictor of the

experimentally observed trace (Figs. 5a and 6a). Most notably,

the separate component traces predicted the temperature at

Fig. 5 e H2production during the pyrolysis of pine. MS data

is presented with arbitrary units. vT/vt[ 40 C/min. (a)

Predicted and experimental H2production. The predicted

profile was calculated as the weighted sum of the

normalized component decompositions according to: R

(Int)pine [ 0.4 R(Int)cellulose D0.3 R(Int)xylan D0.3 R(Int)lignin.

(b) H2production during the pyrolysis of pine compared to

H2production from the biomass components.

Fig. 6 eH2production during the pyrolysis of pine in the

presence of CaO. MS data is presented with arbitrary units.

nCa/nC [ 0.5; vT/vt[ 40 C/min. (a) Predicted and

experimental H2production. The predicted profile was

calculated as the weighted sum of the normalizedcomponent decompositions according to: R(Int)pine [ 0.4 R

(Int)cellulose D0.3 R(Int)xylan D0.3 R(Int)lignin. (b) H2production during the pyrolysis of pine in the presence of

CaO, compared to H2production from the biomass

components in the presence of CaO.



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which the relative intensity of H2was maximized; this is an

important parameter if biomass pyrolysis is to be used for H2synthesis.

2The agreement between the predicted and experimental

H2-evolution curves provides some license to consider H2formation from the components independently. A compar-

ison of the H2 formation from pine with those from the

biomass components (Fig. 5b), suggests that, in the earlystages of the reaction, H2was likely produced by the pyrolysis

of cellulose, as the cellulose and pine traces were very similar

forT < 300 C. Lignin began to contribute to H2formation just

beforeT 400 C. Though xylan undergoes thermal decom-

position at a lower temperature than cellulose and lignin do,

it releases little H2at low temperatures. It was thus unlikely

to be an important contributor to H2 formation until just

below 500 C. The contribution from lignin pyrolysis

increased significantly shortly thereafter, and continued to

increase even as the overall H2 production declined. This is

consistent with the results of Demirbas, who showed that the

H2 yields from pyrolysis were higher for cellulose and

hemicellulose at lower temperatures, but higher for ligninabove 675 C[66]. The relative intensity of H2was maximized

at 730 (experimental) and 720 C (predicted). When CaO was

added, cellulose remained the main source of H2 during

primary pyrolysis, although xylan and lignin also contributed

in this case (Fig. 6b). All three components contributed to H2formation at higher temperatures, and the maximum relative

intensity of H2 came at 660 (experimental) and 620 C

(predicted).

H2 formation during pine pyrolysis can be qualitatively

modeled using the H2formation traces from the pyrolyses of

cellulose, xylan, and lignin, both in the absence and pres-

ence of CaO. This suggests that the H2production from pine

can be understood as the simple weighted sum of H2productions from these components, despite that the ther-

mogravimetric behaviour of pine is modeled less accurately

using this strategy. This result, taken together with the

observation that H2formation is often high at temperatures

when the rate of weight loss is low, reaffirms the impor-

tance of gas-phase reactions in determining the product

distribution obtained from biomass pyrolysis. It also

suggests that component additivity may be useful for pre-

dicting the formation of H2 from other sources of biomass.

The results obtained by Couhert et al. [42,43], however,

illustrate an important caveat to these predictions: compo-

nent additivity may not be useful when the biomass and

components are from very different sources. These authorsobtained very similar H2 yields in the flash pyrolysis of

beechwood and of a mixture of spruce and fir, but found

that mathematical models based on these two samples plus

the pyrolysis data from grass, wood bark, and rice husk gave

unreasonable results. Thus component additivity models

such as the ones shown in Figs. 5a and 6a, which were

developed using hemicellulose and lignin from wood sour-

ces, may only be useful for predicting the behaviour of

woods. More investigations will be necessary before we

can properly evaluate the scope of this model; however,

we hope that the results of this will be useful for the

further development of advanced biomass-gasification

technologies.

4. Conclusion

The combination of thermogravimetric and mass-spectro-

metric analyses offered insight into H2 formation from

biomass pyrolysis. Although significant relative errors were

obtained forthe total relative intensities of the species studied

in repeated experiments, the changes in gas formation withtemperature were quite reproducible. Thus comparing the

evolution profiles of the various gases, along with the rate of

mass loss, allowed conclusions to be drawn about the domi-

nant processes operating during the reaction. Most of the H2generated from biomass pyrolysis is actually produced from

secondary reactions among CO, CO2, H2O and CH4, and

between these gases and heavier products.

CaO is known to improve the H2 yield from biomass

pyrolysis by acting as a CO2 sorbent, andwe have examined its

specific actions during the reaction. Adding CaO increased H 2production from several of the gas-phase and gasesolid

reactions that influence H2formation. At lower temperatures,

the H2 yield from the wateregas shift reaction greatlyincreased when CO2was being absorbed. As the temperature

was raised, CaO encouraged the cracking of tars, and drove

the steam reformations of methane and hydrocarbons to

produce more H2. Finally, CaO lowered the onset temperature

of char decomposition, and decreased the char yield. The

result was more H2produced from the wateregas reactions.

We also found that the H2 evolution from pine could be

qualitatively predicted from the weighted sum of the

component H2-evolution traces, despite that the thermogra-

vimetric behaviour was not predicted as accurately. This was

true both with and without added CaO. These results allow the

synthesis of H2from pine to be considered in the context of its

components, and may be applicable to other biomass sources.

Acknowledgements

The authors thank EON and the Australian Research Council

for their support of this research through an EON Research

Initiative Grant and DP666488, respectively. M.W. is grateful to

the Asian Development Bank and the University of Sydney

World Scholars Programme for scholarships.

Appendix

Supplementary data related to this article can be found online

atdoi:10.1016/j.ijhydene.2010.11.103.

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hydrogen synthesis from biomass pyrolysis with in situ

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