A Detailed Kinetic Study of Pyrolysis and Oxidation of Glycerol (Propane-1,2,3-triol)

GCST #664006, VOL 184, ISS 7–8

A DETAILED KINETIC STUDY OF PYROLYSIS ANDOXIDATION OF GLYCEROL (PROPANE-1,2,3-TRIOL)

Emma Barker Hemings, Carlo Cavallotti, Alberto Cuoci,Tiziano Faravelli, and Eliseo Ranzi

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A Detailed Kinetic Study of Pyrolysis and Oxidation of Glycerol(Propane-1,2,3-triol) Emma Barker Hemings, Carlo Cavallotti,Alberto Cuoci, Tiziano Faravelli, and Eliseo Ranzi

A DETAILED KINETIC STUDY OF PYROLYSIS ANDOXIDATION OF GLYCEROL (PROPANE-1,2,3-TRIOL)

Emma Barker Hemings, Carlo Cavallotti, Alberto Cuoci,Tiziano Faravelli, and Eliseo Ranzi

5Department of Chemistry, Materials and Chemical Engineering ‘‘G.Natta’’,Politecnico di Milano, Milan, Italy

Glycerol (propane-1,2,3-triol) has been an object of interest in recent years as a byproduct

in the production of biodiesel, a parental molecule for syngas production, and a representa-

tive compound of the tar components derived from the carbohydrate fraction of biomass.10Kinetic models describing the thermal degradation of glycerol toward lighter hydrocarbons

and noncondensable gases are considered beneficial to an effective implementation of pro-

cesses for its valorization. The aim of this work is to analyze and discuss the primary propa-

gation reactions of glycerol, together with the ones of acetol (1-hydroxypropan-2-one) and

3-hydroxypropanal (the primary products of glycerol). This kinetic model is then evaluated15in comparison with three independent sets of experimental measurements. The first one

refers to pyrolysis experiments at intermediate temperatures, while syngas is obtained from

glycerol pyrolysis at higher temperatures in the second one. The model is also tested against

experiments on combustion of droplets of propanol–glycerol mixtures.

Keywords: Biofuels; Glycerol; Kinetics; Pyrolysis

20INTRODUCTION

Glycerol (propane-1,2,3-triol) is an oxygenated organic compound that hasbeen successfully and widely used in the chemical industry in the last decades. Majorapplications of glycerol can be found in the detergency industry, as well as in drugsand pharmaceuticals productions (Pagliaro et al., 2007). Other relevant uses of gly-

25cerol are summarized in Figure 1, which represents the market for glycerol in termsof % volumes in industrial use.

Glycerol has been traditionally synthesized from chlorine and propylene, viaan epichlorohydrin intermediate (Kneupper and Saathoff, 2000). However, recently,the increase in the production of biodiesel has resulted in a relatively large surplus of

30glycerol (Adhikari et al., 2007), which is formed as a byproduct. The production of

Received 7 May 2011; revised 28 November 2011; accepted 1 February 2012.

Published as part of the Seventh Mediterranean Combustion Symposium Special Issue with Guest

Editors Federico Beretta, NevinSelcuk, Mohy S. Mansour, and Andrea d’Anna.

Address correspondence to Eliseo Ranzi, Department of Chemistry, Materials and Chemical

Engineering ‘‘G.Natta,’’ Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milan, Italy. E-mail:

[email protected]

Combust. Sci. Technol., 184: 1–15, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0010-2202 print=1563-521X online

DOI: 10.1080/00102202.2012.664006

3b2 Version Number : 7.51c/W (Jun 11 2001)File path : P:/Santype/Journals/TandF_Production/GCST/v184n7-8/gcst664006/gcst664006.3dDate and Time : 21/06/12 and 19:13

1

biodiesel, indeed, goes through the transesterification of renewable biological sourcessuch as vegetable oils and animal fat oils with an alcohol using alkaline or acidcatalysts (Ma and Hanna, 1999; Tapanes et al., 2008Q1 ). The process yields 1mol ofglycerol for every three of fatty acid methyl esters (FAME) (Dou et al., 2009), with

35the crude glycerol attaining 10% wt of the overall product fraction (Fernandez et al.,2009). The production of biodiesel is expected to grow in the near future: The figureindicates a probable production of 3.23 million tons worldwide, which would put anextra 323,000 tons of glycerol on the market (Pagliaro et al., 2007). This would rep-resent an increase of roughly 40% in the global glycerol market, which was estimated

40to be around 800,000 tons in 2005. A profitable exploitation of glycerol is there-fore crucial: This would promote the biodiesel industry, and decrease in price [from850 USD=ton to 770 USD=ton according to Pagliaro et al., (2007)] could potentiallytransform glycerol in a platform chemical for various applications. The opportunityfor glycerol as a reactant in several catalytic processes for the production of

45commodities has been reviewed in the recent literature (Pagliaro et al., 2007). Gly-cerol is also widely used as a feedstock for the production of acrolein (Chaiet al., 2007), hydrogen (Cortright et al., 2002), or synthesis gas (Simonetti et al.,2007), and eventually additional transportation fuels via Fisher–Tropsch synthesis(Valliyappan et al., 2008a). Crude glycerol from the biodiesel process often contains

50many impurities and is a very poor fuel, which cannot be directly used in eitherpetrol or diesel engines (Slinn et al., 2008). Glycerol shows a strong tendencyfor decomposition and polymerization, which could result in engine problems athigh temperature. Nevertheless, glycerol can be processed through several chemistryroutes (including acetylation, sterification, etherification, or acetalation with

55adequate chemicals) and transformed into fuel additives. As most of the oxygenatedadditives, glycerol derivatives contribute to increment octane number and combus-tion quality, therefore reducing particulate emissions and CO production (Rahmatet al., 2010). Furthermore, 1mol of glycerol can theoretically produce up to 4molof hydrogen gas and 3mol of CO as gaseous products. Pyrolysis, gasification, or

60steam reforming could lead to successful performances. Hydrogen production fromthe steam reforming reaction of glycerol over ceria-supported Ir, Co, and Ni cata-lysts was studied (Zhang et al., 2007). Similarly, the reforming of glycerol and its

Figure 1 The market for glycerol. Adapted from Pagliaro et al. (2007).

2 E. BARKER HEMINGS ET AL.

aqueous solution over a nickel-based catalyst was investigated (Buhler et al., 2002).The gas phase pyrolysis of glycerol in steam in a laminar flow reactor has been

65successfully reported by Stein et al. (1983). Syngas production from the pyrolysisof glycerol in N2 in a packed bed tubular reactor was also performed (Valliyappanet al., 2008b). The thermal decomposition of glycerol in near-critical and super-critical water was also carried out by Buhler et al. (2002). The pyrolysis of the crudeglycerol from a biodiesel production plant was investigated by thermogravi-

70metry coupled with Fourier transform infrared spectroscopy in a recent work(Dou et al., 2009).

The study of glycerol also goes in the direction of the demand for new energysources and processes. It was successfully demonstrated that hydrogen can beproduced from sugars and alcohols at temperatures near 500K in a single-reactor

75aqueous-phase reforming process using a platinum-based catalyst (Valliyappanet al., 2008b). Similarly, the production of renewable hydrogen by autothermalsteam reforming of volatile carbohydrates is accounted for in Dauenhauer et al.(2006). Methanol, ethylene glycol, glycerol, and their solutions in water were studiedusing platinum- and rhodium-based catalysts supported on alumina foams. These

80findings are of outstanding importance, when considering that the above-mentionedcarbohydrates and products of further decomposition (glycerol, for instance) can bederived from the thermal processing of biomass material, representing indeed theproducts of the so-called secondary phase of biomass thermal decomposition (Evansand Milne, 1987). Data showing that the yield of H2 obtained from both pyrolysis

85liquid and crude glycerol are close to 80% of the stoichiometric value are reported(Czernik et al., 2002), which represents an interesting opportunity. A detailedaccount of the products of biomass decomposition appears to be necessary. Usuallyvery simplified representations of tar formation and conversion are adopted. Mostwork in the area of combustors and gasifiers simply refers to global reactions

90(Gomez-Barea and Leckner, 2010). However, efforts in the direction of a kineticapproach that chooses a limited set of primary reference species to represent anddescribe the primary tar, released during devolatilization (e.g., levoglucosane, xylan,hydroxyl-acetaldehyde, acetol, anisole and substituted phenols, benzene, toluene,and naphthalene), and proposes a multistep kinetic model of biomass pyrolysis

95found in literature (Ranzi et al., 2008). In addition to the primary reactions of ref-erence tar lumps, the kinetic scheme accounts for the interactions between the lumpsand the surrounding gases. A general and detailed kinetic scheme of pyrolysis andoxidation of hydrocarbon fuels is applied to this purpose (Ranzi et al., 2001). Testingof the proposed kinetic scheme, with specific reference to the production of bio-oils,

100was performed and offered good results, particularly in terms of product yields(Calonaci et al., 2010). Within this perspective, this work aims to broaden the hor-izon of prediction of the referenced model, by specifically detailing the secondaryreactivity of glycerol, a model compound for the tar fraction originated by carbohy-drates present in biomass feedstocks. The major reactive steps in the pyrolysis of gly-

105cerol are described in the following. Particular consideration is given to theH-abstraction reactions, with reference to the different types of hydrogen atomsavailable in the glycerol molecule. Molecular dehydrations appear to be interestingreaction pathways as well. The model is tested against several sets of experimentaldata, offering good agreement between predicted and measured values.

PYROLYSIS, OXIDATION OF GLYCEROL (PROPANE-1,2,3-TRIOL) 3

110PRIMARY PROPAGATION REACTIONS OF GLYCEROL, ACETOL, AND3-HYDROXYPROPANAL

Table 1 summarizes the initiation and primary propagation reactions of glyce-rol. The initiation paths involve either (1) the cleavage of a C-C bond or (2) thecleavage of a C-O terminal bond, with the consequent formation of, respectively,

115hydroxyacetaldehyde and acetaldehyde, together with smaller radicals. As in mostof the radicalic reaction schemes, the propagation reactions are of major interest.Particularly, the abstraction of hydrogen on the original glycerol molecule can leadto the formation of either acetol or 3-hydroxypropanal, which represent importantprimary intermediates. These compounds can undergo further reactions, which are

120reported in Table 1. The further decomposition and=or oxidation reactions of smal-ler radicals and molecules are already described elsewhere (Grana et al., 2010; Ranzi,2006). The overall kinetic scheme is based on hierarchical modularity and is consti-tuted by more than 300 species involved in more than 7000 reactions. The complete

Table 1 Primary propagation reactions of glycerol, acetol, and 3-hydroxypropanal

GLYCEROL: C3H8O3 (CH2OH-CHOH-CH2OH) 1,2,3-propanetriol

Reactions A Eatt

GLYCEROL>CH2OHþHþHAA 1.00Eþ 16 80000

GLYCEROL>CH2OHþOHþMECHO 1.00Eþ 16 80000

RþGLYCEROL>RHþOHþC3H6O2 4 H primary alpha

RþGLYCEROL>RHþOHþACETOL 1 H secondary alpha

RþGLYCEROL>RHþCH2OHþHAA 1 H oxydril

RþGLYCEROL>RHþCH2OþOHþMECHO 2 H oxydril

GLYCEROL=ACETOLþH2O 4.00Eþ 13 65000

GLYCEROL=C3H6O2þH2O 2.00Eþ 13 65000

ACETOL: C3H6O2 (CH3-CO-CH2OH) hydroxyacetone

Reactions A Eatt

ACETOL=CH2OHþCH3CO 1.00Eþ 16 80000

ACETOL>CH2OHþCH3þCO 1.00Eþ 16 80000

RþACETOL>RHþCH2COþCH2OH 3 H primary

RþACETOL>RHþCH3þGLYOX 2 H primary alpha

RþACETOL>RHþCH2OþCH3CO 1 H oxydril

ACETOL=MECHOþCH2O 2.00Eþ 13 53000

C3H6O2: (CHO-CH2-CH2OH) 3-hydroxypropanal

Reactions A Eatt

C3H6O2¼R1ETOþHCO 1.00Eþ 16 82000

C3H6O2¼CH2OHþR2CHO 1.00Eþ 16 81000

RþC3H6O2>RHþR1ETOþCO 1 H acylic

RþC3H6O2>RHþACROLþOH 2 H secondary

RþC3H6O2>RHþMECHOþHCO 2 H primary alpha

RþC3H6O2>RHþCH2OþR2CHO 1 H oxydrilic

C3H6O2¼ACROLþH2O 2.00Eþ13 51000

C3H6O2¼MECHOþCH2O 2.00Eþ13 54000


mechanism in CHEMKIN format is available online (http://www.chem.polimi.it/125CRECKModeling/).

The H-abstraction or metathesis reactions are systematically described in theirgeneral form in Equation (1):

Rþ GLYCEROL > RH þ RC3H7O3 ð1Þ

where R and RH stand, respectively, for all the possible H abstracting radicals and130for the corresponding saturated species. The molecule of glycerol contains eight

hydrogen atoms, that can be divided and classified into four primary H-atoms ina position, one secondary H-atoms in a position, and three hydroxyl H-atoms.The metathesis reactions, therefore, can possibly lead to the formation of four differ-ent radicals, corresponding to the general formula of RC3H7O3. These radicals, how-

135ever, are not explicitly mentioned in the kinetic scheme, as a rapid evolution is seen,leading to an enolic organic and a smaller radical. The enolic compounds, moreover,are easily converted in their tautomericaldehydic forms, which are listed as thefinal products of the metathesis reactions in Table 1. The extended mechanism ofthe H-abstraction reactions (3–6) is represented in Figure 2. The formation of the

140RC3H7O3 radical is expected to be the rate determining step, which determines thekinetic constant for the process.

RATE CONSTANTS FOR THE H-ABSTRACTION REACTIONS

As suggested above, the H-abstraction reactions plays an important role in thereactive system, therefore a careful account of their kinetic constants should be

145given. In a recent work (Carstensen and Dean, 2010), electronic structure methodshave been used to calculate the kinetic constants for H-abstraction reactions from

Figure 2 Primary radicals from H-abstraction of glycerol, and their further rearrangements.


alcohols. As stated by the authors, this approach enables to estimate with goodaccuracy the rate constants for small reactants, and to extend the results to the cor-responding reactions of larger compounds containing the same functional groups,

150through the application of proper estimation rules. The reactivity of butanol andits isomers has been studied as well in a previous work of our research group (Granaet al., 2010), and the two results were found to be in good agreement. Table 2summarizes the above mentioned results.

The tabled values refer to a single abstraction; the overall kinetic constant155should therefore be multiplied by the number of H atoms of the selected type that

are present in the original molecule. A comparison of the values in the temperaturerange of interest for the pyrolysis of glycerol (i.e., 900–1100K) shows that they are invery good agreement. The rate constants that our research group proposed in Granaet al. (2010), and were confirmed with comparison to Carstensen and Dean (2010),

160were used to perform simulations in the present work. It can be interestingly noticedthat the rate of reaction for the abstraction of an hydroxylhydrogen is approximatelyclose to 70% of the rate of removal of a primary H atom from a methyl group. Onthe other hand, the removal of hydrogen from the a carbon atoms (both primary andsecondary) is almost 50% higher than the rate values of the corresponding removal

165of H atoms from alkanes. The enhanced reactivity related to the effect of the OHgroup on the reactivity of neighboring C-H bonds is negligible when consideringcarbon atoms in the b-position. The extension of this rule to the generalized caseof the presence of more than one OH group (as, for instance, in glycerol) is suggestedfrom a series of data on ethylene glycol (Carstensen and Dean, 2010).

170MOLECULAR DEHYDRATION REACTIONS

As already observed in previous works on alcohol fuels (Grana et al., 2010;Frassoldati et al., 2010), molecular dehydration is an important reaction class inbiomass pyrolysis. Several dehydration reactions are reported in Table 1, for bothglycerol and its primary intermediate products. The different nature of simple alco-

175hols seems to have little impact on the reference rate value for this reaction class(Carstensen and Dean, 2010). On the contrary, significantly larger deviations areobserved for substituted aldehydes, when water elimination yields products withconjugated double bonds. This is typical, for instance, of 3-hydroxypropanal andacetol. As clearly shown in Table 1, the rate constants for these reactants are consis-

180tently larger than the ones estimated for saturated alcohols, according to the abovementioned approach of generalization through estimation rules. In other words, the

Table 2 Kinetic constants for H-abstraction reactions according to Carstensen and Dean (2010) and

Grana et al. (2010)

Type of H atom

Kinetic constant from Carstensen and

Dean (2010), [cm3mol�1s�1], T in [K]

Kinetic constant from Grana et al. (2010),

[cm3mol�1s�1], T in [K]

Hydroxyl 3.6� 106�T2.11� exp(�9830=R=T) 1.7� 106�T2� exp(�6525=R=T)

Primary a-C-H 2.7� 106�T2.16� exp(�4140=R=T) 3.6� 106�T2� exp(�3950=R=T)

Secondary a-C-H 2.4� 107�T1.88� exp(�2460=R=T) 3.6� 106�T2� exp(�2660=R=T)


aldehyde moiety influences the reactivity by stabilizing the transition state and theproducts (Carstensen and Dean, 2010). This stabilizing effect is transduced in anactivation energy of the process, which is roughly 10 kcal=mol lower than the

185corresponding alcohol. Figure 3 represents the major dehydration pathways forglycerol, 3-hydroxypropanal, and acetol (1-hydroxypropan-2-one).

MODEL PREDICTIONS IN COMPARISON WITH EXPERIMENTALMEASUREMENTS

The kinetic model of glycerol pyrolysis described in the previous section is190tested against three independent sets of experimental measurements. The first one

refers pyrolysis experiments performed at intermediate temperatures (Stein et al.,1983), while in the second one glycerol is pyrolyzed at high temperatures for the pro-duction of syn gas (Valliyappan et al., 2008b). Finally the model is further tested incomparisons with experiments on combustion of droplets of propanol–glycerol

195mixtures (Dee and Shaw, 2004).

PYROLYSIS OF GLYCEROL AT INTERMEDIATE TEMPERATURES

Pyrolysis of the glycerol in the steam mixture was analyzed in a tubular, laminarflow quartz reactor (Stein et al., 1983). After a rapid preheat (residence time�0.015 s),a steady flow of a glycerol in the steam mixture enters the isothermal zone of the

200reactor, which is characterized by a relatively small surface-to-volume ratio (ca.5 cm�1) and residence times of �0.1 s. Due to the short residence time, only 15% ofthe initial glycerol decomposes at 650�C, while at 750�C glycerol is almost completelyconverted. At high temperatures, there are significant successive decomposition reac-tions of the primary products. This indicates that, if the major interest of the process

205is in maximizing bio-oil production, the optimal temperature range is 650–700�C.

Figure 3 Dehydration reactions of glycerol, 3-hydroxypropanal, and acetol.


A comparison between the experimental results of Stein et al. (1983) and the predictedvalues obtained with the proposed kinetic mechanism is reported in Table 3. Theresults show a satisfactory agreement between the experimental data and model pre-dictions. Particularly, the major products of the pyrolysis reaction are acrolein and

210acetaldehyde. The simulations seem to shift the peak of production of both at highertemperatures with respect to the experimental results. This goes with the fact that

Table 3 Products of the pyrolysis of glycerol in steam at 650=675=700�C and 1 atm

[Experimental data of Stein et al. (1983)]

Temperature

650�C 675�C 700�C

Exp. Sim. Exp. Sim. Exp. Sim.

Glycerol (mole %) 1.69 1.69 0.9 0.9 1.04 1.04

Conversion 17.6 17.66 34 27.4 25 24.2

Residence time [s] 0.13 0.13 0.13 0.13 0.048 0.05

Products yield (%)

Acrolein 52 38.6 39 38.2 34 38.7

Acetol – 13 – 6 – 7.7

3-Hydroxypropanal – 9 – 3.3 – 4.4

Acetaldehyde 48 28.2 38 35.6 42 34.3

CH2O – 28.8 34.9 – 34.2

CO – 10.8 35 19.1 58 16

CO2 – 0.96 0.3 1.7 1 1.4

Hydrogen – 13 29 14.5 44 13

Methane – 2 5 3.8 11 3

Ethylene – 5.6 10 8.4 17 7

Figure 4 Major products from glycerol (1.69%) pyrolysis in steam at 700�C and 1 atm (model predictions).


there is no experimental evidence of the reaction intermediates of acetol and 3-hydro-xypropanal. A possible explanation of these facts is the following: On one hand,recent studies (Carstensen and Dean, 2010) suggest that the second dehydration of

215glycerol (yielding acrolein and acetaldehyde) is strongly favored with respect to thefirst one, which leads to acetol and 3-hydroxypropanal. Figure 4 proposes the pre-dicted concentration profiles of the major species involved in the reaction system,and a clear intermediate behavior is visible for both acetol and 3-hydroxypropanal.On the other hand, the detail of the experimental results is limited, and the proximity

220(both in terms of structure, and particularly of C-groups) of the investigatedcompounds is likely to make a precise distinction complex.

A similar consideration can be put forward with respect to formaldehyde:Asstated in the original work (Stein et al., 1983), a quantitative detection was difficultdue to the modest concentration (original glycerol is highly diluted in steam as well).

225The decomposition of formaldehyde to CO and H2 in the product stream can also beseen, and would explain the low yields of both in the simulations, together with thesimulated data on formaldehyde, which appear to have no correspondence in theexperimentally detected products.

SYNGAS PRODUCTION AT 900–1100K

230The pyrolysis of glycerol for the production of syngas, useful for instance as afeed stock for the Fisher–Tropsch synthesis to produce transportation fuel, isreported in recent literature (Valliyappan et al., 2008b). The pyrolysis of glycerolwas carried out at various temperatures (650–800�C, and varying flow rates in atubular reactor at atmospheric pressure, using different packing materials. The

235major product was syngas, in which traces of CO2, CH4, and C2H4 were also mea-sured. Composition of the product gas ranges were between 70–93mol% for syngas,3–15mol% for CH4, and 2–12mol% for C2H4. Correspondingly, the heating value ofthe gaseous product ranged from 13 to 22MJ=m3. Figure 5 shows a comparison ofthe experimental measurements and the model simulation results in terms of product

240yields. The liquid and gas yields show a good agreement, whereas the major devi-ation in the model description comes from the fact that no char prediction is given.The carbonaceous residue found experimentally can probably be due to the succes-sive polymerization of tar products that stick to the particle material within the rea-ctor at low temperatures. Acrolein, for instance, is well known for its ability to

245polymerize at room temperature. These phenomena are not included in the gas phasereactive kinetic scheme that is used for simulation purposes. Table 4 shows a detailedcomparison between experimental measurements and model predictions for the gascomposition. The results obtained from the simulation are generally in good agree-ment with the reported experimental values. Particularly, syngas yields are correctly

250predicted. The only major deviation that demands some explanation is the discre-pancies between the results for CH4 and C2H4 at high temperature (800�C). Experi-mental results show a strong decrease in light hydrocarbons at high temperature,suggesting that their decomposition should take place. Moreover, the increase inthe syngas yield seems to suggest that the steam reforming of methane is occurring

255within the reactor. However, there is no evidence of matching in the simulation; onthe contrary, both cracking of light HC and steam reforming of methane are


expected to become relevant in the homogeneous phase only at higher temperatures.A possible explanation of this discrepancy could come from an equilibrium calcu-lation. Thermodynamic equilibrium indicates that at 800�C, the most stable products

260in the system are CO and H2, whereas methane and other hydrocarbons shoulddecompose. If we admit that some catalytic effect is present in the reactor, then thiscould promote the steam reforming reactions of methane and ethylene:

CH4 þH2O ! COþ 3H2 ð2Þ

C2H4 þ 2H2O ! 2COþ 4H2O ð3Þ

265and shift the final composition towards the equilibrium value, which sees the pre-dominance of syngas. Evidence of analogous concerted heterogeneous–homogeneous

Table 4 Effect of temperature on gas product composition during pyrolysis of glycerol at carrier gas flow

rate 50mL=min [Experimental data of Valliyappan et al. (2008b)]

Temperature [�C]650 700 750 800

Components (mol %) Exp. Mod. Exp. Mod. Exp. Mod. Exp. Mod.

H2 17 23.9 22.1 22.3 27.7 22.8 48.6 23.6

CO 54 41.4 50 45.2 45.7 46.4 44.9 47

CO2 0.2 4.8 0 3.9 1.1 3 1 2.4

CH4 14.2 10 14.5 13.3 14.1 14.7 3.3 15.7

C2H4 10.1 19.3 9.6 13.8 9.1 11.8 2 10.5

C2H6 2.2 0.7 2 1.6 1.5 1.2 0.1 0.8

C3H6 2.4 0 1.7 0 0.8 0 0.1 0

H2þCO 71 65.3 72.1 67.5 73.4 69.2 93.5 70.6

Figure 5 Effect of temperature on product yields during pyrolysis of glycerol. Experimental data from

Valliyappan et al. (2008b) (dashed lines and empty symbols) and model predictions (solid lines and

symbols).


processes are reported in literature (Donazzi et al., 2011). Table 5 presents the finalproduct distribution obtained by coupling the results of the simulation performedusing the proposed kinetic mechanism for glycerol with the mentioned steam refor-

270ming reactions of methane and ethylene, for which a degree of conversion of 65%is assumed. Results show a very satisfactory agreement with the experimental data.It is also interesting to observe that the experimental data report the formation ofa solid carbonaceous residue, which is not present in the simulation results. This devi-ation can be explained by taking into consideration the eventual polymerization of

275some intermediate pyrolysis products, for instance acrolein. The high surface-to-volume ratio in the reactor, together with the possible catalytic effect of the nickel-chrome alloy that constitutes the reactor tube, could enhance the formation of depos-its on the walls.

COMBUSTION OF PROPANOL–GLYCEROL MIXTURE DROPLETS

280To our knowledge, data on the combustion of glycerol in relatively simple sys-tems are not available in the literature to date, and therefore the evaluation of theproposed kinetic scheme for glycerol in oxidative conditions has limited opportu-nities. However, recent and useful data can be found on reduced-gravity experimentson combustion of propanol–glycerol mixture droplets, performed by Dee and Shaw

285(2004) in air at standard temperature and pressure. Due to the high viscosity of gly-cerol, propanol=glycerol mixtures were investigated. This choice is justified with thescientific interest in propanol and alcohols as alternative fuels. It might be worthnoticing that glycerol can influence convective mixing rates inside droplets by reduc-ing liquid convective flow velocities. Droplets were initially �1mm in diameter with

290initial glycerol mass fractions of 0.05 and 0.2. Experiments were conducted at the2.2 s drop tower at the NASA Glenn Research Center. Droplets are dispensed ontoa fiber and are ignited by a hot wire igniter with a controlled current.

Figure 6 shows that the square of the droplet diameter decreases linearly withtime, i.e., the d2 law is basically followed by both the droplets. Experimental flame

295standoff ratios (i.e., instantaneous ratios of flame and droplet diameters) are shownas well for times prior to flame contraction and after early ignition transients havedecayed. Flame standoff ratios are nearly constant with values of �5, indicating aquasi steadiness of the region between the droplet and the flame. Computations

Table 5 Results corrected with the introduction of the steam reforming (SR)

reactions. [Experimental data of Valliyappan et al. (2008b)]

Mole fractions EXP MOD MODþ SR

H2 48.6 23.6 49

CO 44.9 47 43.1

CO2 1 2.4 1.47

CH4 3.3 15.7 3.6

C2H4 2 10.5 2.4

C3H6 0.2 0.8 0.5

H2þCO 93.5 70.6 92.1


have been performed following the numerical approach already discussed elsewhere300(Cuoci et al., 2005), in which the conservation equations of mass, momentum,

energy, and chemical species are solved in the gas phase under the hypothesis ofmicrogravity conditions. Numerical simulations are performed considering a dif-fusion coefficient equal to 10�8m2=s (Dee and Shaw, 2004) for the liquid phase.Following the suggestions reported in Dee and Shaw (2004), the liquid diffusivity

305was increased by a correction coefficient in order to account for the internalrecirculations.

According to the performed calculations, the value of the correction coefficientthat best fits the experimental data is �4. The agreement between numerical resultsand experimental data is satisfactory: The flame stand-off ratio is well captured at

310every time, and the vaporization rate is correctly predicted. The model is not ableto describe the initial droplet growth (associated to the droplet heating), but thisphase is strongly affected by the characteristics of the hot wire igniter and the heattransfer through the suspending fiber, for which additional data are necessary. Themodel can effectively predict the flame extinction observed in the experiments corre-

315sponding to an initial glycerol mass fraction of 0.20. The diameter of the dropletafter extinction is accurately captured, whereas a delay in the estimated extinctiontime with respect to the experimental data is noticeable.

Figure 6 Histories for droplet and flames sizes with glycerol mass fraction 0.05 and 0.2. Symbols:

Experiments (Dee and Shaw, 2004); lines: model.


CONCLUSION

This work presents a model of the primary reactions of glycerol and of its320primary products (acetol and 3-hydroxypropanal). Particular attention is paid to the

H- abstraction reactions on the different hydrogens of the molecules. The study ofthe reactivity of the hydrogen atoms on the carbon in the a position is emphasized,as it shows the enhancing effect of the neighboring hydroxyl group. Furthermore, therole of molecular dehydrations is discussed, and the effect of the aldehyde moiety

325that is produced within the molecule from the first dehydration is underlined. Thesecond dehydration of3-hydroxypropanal to acrolein is indeed favored, with adecrease in the activation energy of about 10 kcal=mol.

The proposed model is tested against experimental data found in the literature.Pyrolysis of glycerol in steam and pure glycerol for syngas production are con-

330sidered. The model generally captures the system reactivity and the main productdistribution. Major deviations are taken into consideration, and catalytic effects ofthe reactor walls, together with limitations in the analytical details, are invoked ascontributions in the explanation. The capability of the mechanism to reproducethe combustion behavior of glycerol is tested against experimental data taken from

335tests on droplets composed of a mixture of propanol and glycerol mixture, whichwere combusted in reduced gravity. Despite the complexity of the system, the modelshows a good agreement with the measurements in term of the square diameter ofthe droplet, and flame diameter for different compositions of the mixture.

The importance of the proposed results is two fold. On one hand, there is a340growing interest in a profitable exploitation of glycerol, due to its importance as

the major byproduct of biodiesel production. On the other hand, glycerol is an effec-tive reference component for the intermediate components, which can be formedfrom biomass pyrolysis and gasification, and consequently its investigation contri-butes to the identification of the main reaction classes involved in these processes

345and their kinetic parameters.

ACKNOWLEDGMENTS

The authors wish to acknowledge the project ‘‘CNR-Biofuels’’ for the financialsupport.

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Documents

A Detailed Kinetic Study of Pyrolysis and Oxidation of Glycerol (Propane-1,2,3-triol)