Hydrothermal gasification of a biodiesel by-product crude glycerol in the presence of phosphate based catalysts

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 4 0 ( 2 0 1 5 ) 1 4 8 0 6 e1 4 8 1 5

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Hydrothermal gasification of a biodiesel by-product crude glycerol in the presence of phosphate based catalysts Nihal U¨remek Cengiz a,*, Gu¨ray Yıldız b, Murat Sert a, lam a, Mithat Yu¨ksel a, Levent Ballice a €kkaya a, Mehmet Sag Dilek Selvi Go a b

_ Ege University, Engineering Faculty, Department of Chemical Engineering, 35100 Bornova, Izmir, Turkey Ghent University, Department of Biosystems Engineering, Ghent, Belgium

article info

abstract

Article history:

Energy from biomass can be provided in various ways, such as burning the solid wastes,

Received 7 May 2015

production of biogas (by anaerobic digesters), biofuels (i.e. methanol, ethanol, biodiesel,

Received in revised form

and derivatives), and methane via the utilization of landfills. Biodiesel is a widely used

13 August 2015

biofuel produced by the conversion of first-generation biomass feedstock via bio-chemical

Accepted 22 August 2015

conversion platforms.

Available online 26 September 2015

Crude glycerol is the by-product of biodiesel production being 10 wt.% of the produced biodiesel. The objective of this study is to utilize this glycerol fraction by converting it to

Keywords: By-product glycerol

fuel gas or to chemical feedstock. In this study, the concentration of glycerol feedstock solution and the catalyst con-

Hydrogen

centration were 50 g/L and 5 g/L, respectively. Crude glycerol was gasified in a sub and

Methane

supercritical water medium by using a batch autoclave with an inner volume of 100 ml. A

Supercritical water gasification

temperature range from 300 to 600
C was studied. Experiments were performed with pure and crude glycerol samples in the absence and the presence of homogeneous acidic and alkali catalysts, namely H3PO4, KH2PO4, K2HPO4, and K3PO4. These were used to obtain higher gasification efficiencies and hydrogen and/or methane yields. Subsequent to each experiment liquid, solid, and gaseous products were collected and analyzed by GC, TCA (total carbon analyzer), HPLC, and GC/MS. The order of the effectiveness of the catalysts on gasification was found as: K3PO4 > K2HPO4 > H3PO4 > KH2PO4 for crude glycerol and K3PO4 > K2HPO4 > KH2PO4 > H3PO4 for pure glycerol. K2HPO4 and K3PO4 were found to be more effective in terms of hydrogen production while H3PO4 and KH2PO4 showed the best performance for the maximized methane production. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ90 232 3114125; fax: þ90 232 3887776. ¨ . Cengiz). E-mail address: [email protected] (N.U http://dx.doi.org/10.1016/j.ijhydene.2015.08.097 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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 4 0 ( 2 0 1 5 ) 1 4 8 0 6 e1 4 8 1 5

Introduction The processing and use of fossil fuels cause significant environmental problems, such as increasing the levels of greenhouse emissions and toxic gases (i.e. CO2, SO2 and NOx) resulting in global warming and climate change. In the production process of biodiesel, vegetable oils and animal fats are converted by transesterification using alcohol. During this process, 10 wt.% of glycerol is formed as a by-product of the main reactant. Since biodiesel-derived glycerol consist of unreacted triglycerides, methanol, free fatty acids and inorganic salts it requires costly refining operation to have a higher purity. The research on the utilization of excess amount of glycerol, a variety of techniques could be applied to convert it to value added products [1e7]. Around the supercritical point of water (Tc: 374
C and Pc: 22.1 MPa), it becomes a unique reaction medium with the aid of the changes in its physical and chemical properties. High degree of solubility avoids the tar formation; it promotes the decomposition of feedstock rapidly, and the yields of reactions are high due to strong diffusion characteristics comparing the classical gasification [8e10]. SCW becomes strongly reactive at supercritical conditions and behaves as acidic/basic catalyst providing to perform process in the absence of catalyst [1]. Selective production of more valuable organic substances such as, hydrogen, acrolein, acetic acid and acetaldehyde, 1-propanol, i-propanol, 1, 2-propanediol, 1, 3-propanediol is possible by SCWG [11e16]. Researches on SCWG of glycerol were very few until the last three-four years. The process parameters [1e3], catalyst types [2,17e22], thermodynamic analysis [23e25], reaction pathways and kinetics [5,26] of hydrothermal gasification (HTG) of glycerol were gained more importance in recent years. Ortiz et al. have been focused on the reforming of glycerol using SCW in a tubular reactor operated at 750e850
C and 240 bar in the absence of catalyst by using initial glycerol concentrations of up to 30 wt.%. They achieved a high-yield of hydrogen production and high conversion of glycerol without added catalyst at elevated temperatures and long residence times [1]. Catalytic SCWG of glycerol was overviewed by Markocic et al. shortly [27]. Sulfiric acid [6], sulfates [17,21,28], alkali catalysts [2,29], ruthenium based catalysts [16,30,31], metallic catalysts and alloy [18], carbon and activated-C catalysts [32] were used in the SCWG of glycerol in various researches. In the last couple of years, Nickel-based catalysts [19,33,34] were suggested as the catalyst by the researchers commonly. Onwudili and Williams [29] studied with crude glycerol under sub- and supercritical conditions adding NaOH as catalyst. The results were verified that more H2 was formed at higher concentration of NaOH and less CO, CO2 and hydrocarbons produced. Reaction of glycerol with water in sub-critical and supercritical conditions was investigated by Antal [28] and Ramayya [6] as first to develop a new method for the production of acrolein. At 500
C and 34.5 MPa, acrolein, acetaldehyde and a gas mixture consisting of H2, CO, CO2, CH4, C2H4, and C2H6, were obtained. In the presence of acid catalyst (NaHSO4), the formation of acrolein increased from 24% to 70%. In recent

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years, efforts on the production of acrolein from glycerol have been addressed again in some studies [11,35]. Guo et al. [26] studied on reaction pathway and the kinetics model for the main gaseous products (H2, CO, CH4, CO2) in SCWG of glycerol. They proposed a model consisting four reaction types: pyrolysis, steam reforming, wateregas shift and methanation reaction. It is concluded that most of the hydrogen was produced in pyrolysis and steam reforming of intermediates. To specify the dominant reaction pathways of glycerol in supercritical water, extensive studies have been conducted in Karlsruhe-Germany (FZK-ITC-CPV) [5,36] within the temperature and pressure ranges of 622e748 K and 25e45 MPa in various retention times (from 32 to 165 s). The glycerol decomposition conversions (mol %) varying from 0.4 to 31 were obtained in tubular reactor with molar ratios of water/glycerol mixture ranging from 131 to 503. Comprehensive studies have been conducted with glycerol and various biomass feedstocks on the production of H2 and CH4 by SCWG in the University of Twente (The Netherlands) [31,37]. The conversion was reached to 100% at an initial glycerol concentration of 1 wt.% and it drops to 90e75% for the feed concentrations higher than 5 wt.% [31]. Chakinala et al. investigated the gasification behavior of carboxylic acids and alcohols in SCWG by considering the molecular structures and chain lengths [37]. They have studied at 600
C and 250 bar, with feedstock concentrations of 10 and 20 wt.%. The results indicated that alcohols gasified easier than the corresponding acids. As the number of OH groups increased the gasification efficiency and yields of CO2 and H2. H3PO4, KH2PO4, K2HPO4 and K3PO4 were suggested as catalyst in by-product glycerol utilization for the first time in literature evaluating the effects of them on the CGE (Carbon Gasification Efficiency) and CLE (Carbon Liquefaction Efficiency). Experimental runs were conducted at a wide range of reaction temperatures and aqueous and gaseous products were analyzed comprehensively.

Experimental Feedstock and catalysts In this study, the aqueous phase of crude glycerol solution prepared as a model of biodiesel by-product was used as feedstock as well. Besides, pure glycerol was used for comparison of the obtained experimental results. Deionized water was used in the preparation of the solutions. The composition of the feedstock solutions of pure and crude glycerol are given in Table 2. Experiments were performed with pure and crude glycerol in the absence and presence of acidic and alkali catalysts, namely H3PO4, KH2PO4, K2HPO4, and K3PO4.

Experimental setup and procedure The sub and supercritical water gasification experiments of aqueous glycerol solutions were performed in a stainless steel (SS 316) batch reactor with an electrical heater and autoclave system with an inner volume of 100 mL. The maximum allowable temperature and pressure are 650
C and 500 bar,

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Table 1 e Effects of the various temperatures and catalysts on the TOC of aqueous products (ppm) belong to pure glycerol (PG) and crude glycerol (CG) hydrothermal gasification.

PG

CG

T,
C

No catalyst

H3PO4

KH2PO4

K2HPO4

K3PO4

300 400 500 600 300 400 500 600

5417 4828 1023 443 8496 6991 3275 1470

4668 2134 726 459 7232 4990 1964 1664

5163 5407 750 234 7765 6319 2871 1667

5364 3667 546 209 7359 4953 1769 1568

5065 2996 539 179 8217 4966 1676 1278

Table 2 e Composition in 15 mL of aqueous feedstock solutions of pure (PG) and crude (CG) glycerol in each experimental run (NC: Non-catalytic). Weight, g

Glycerol Methanol Sodium hydroxide Fatty acids Catalyst (if used) Water in NC runs (approximate) Water in catalytic runs (approximate)

PG

CG

0.75 e e e e ~14.4 ~14.3

0.75 0.375 0.015 0.0018 0.075 ~13.9 ~13.8

respectively. During an experiment, the temperature is kept constant by means of a PID controller. The schematic of the experimental setup is represented in Fig. 1. A feedstock solution of 15 mL was placed into the reactor. In the case of the catalytic experiments, the catalyst was added in the solution. The cover of the reactor was closed and sealed tightly to avoid gas leakages. The purging of the air inside the reactor was done with an inert (nitrogen) gas. The system was heated to the desired reaction temperature and the operation continued for a run time of 1 h. The experiments

were carried out at 300
C, 400
C, 500
C, and 600
C with a corresponding pressure range of 8.5e33.0 MPa. Subsequent to each experiment, the reactor was cooled to the room temperature. The volume of the gaseous products was measured by a gas flow meter with an accuracy of ±10%. Gas tight syringes were used to collect gas samples for the analyses in a gas chromatography device (HP 6890). The liquid product was collected from the reactor by washing it and then filtering the collected liquid. Product efficiencies were calculated by closing the carbon balance between the products (gaseous and aqueous) and the feedstocks. The total efficiencies ranged between 95 and 99 wt.%. The detailed analytical procedures and the analytical devices used are given in Analysis of the gaseous and aqueous products.

Analysis of the gaseous and aqueous products In this study, the quantification of gaseous products was accomplished using an HP 6890 gas chromatography device. The commonly available GC detectors, the thermal conductivity detector (TCD), and the flame ionization detector (FID) are well suited for the application. The HP Plot Q (30 m long, and 0.53 mm i.d.) and HP Molesieve (30 m long, and 0.53 mm i.d.) columns are serially connected, and argon is used as carrier gas. The Plot Q column is used to retain the CO2 gas for a while, to prevent the contamination caused by the gas fed. The molesieve column was used to analyze hydrogen, C1eC4 hydrocarbons, CO, N2, and O2. The oven temperature program is as followed: 45
C isothermal for 2 min, 40
C/min to 60
C, 60
C isothermal for 14 min, 10
C/min to 250
C, 250
C isothermal for 10.5 min. The aqueous products were analyzed with a Total Organic Carbon analyzer (Shimadzu TOC-VCPH, Japan) to determine the total organic carbon (TOC) content of the aqueous phase. Qualitative analyses of the liquid phase was carried out with a gas chromatographyemass spectroscopy (GC/MS) device. All MS analyses were carried out using an Agilent Technologies 7890A GC System with 5975C VL MSD type Triple-Axis

Fig. 1 e Schematic of experimental setup.

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CLE

CGE

100

80 Efficiency, %

Detector. The compounds with considerable amounts in the aqueous phase due to GC/MS results were analyzed qualitatively with a High Performance Liquid Chromatography (HPLC) device. The amounts of aldehydes and ketones present in the aqueous solutions were analyzed with a Shimadzu LC20A series liquid chromatography device. The HPLC system consisted of a DGU-20AS degassing module, LC-20AT gradient pump, CTO-10ASVP chromatography oven, and a SPD-20 multi-wavelength ultraviolet detector.

60

40

20

Results and discussion

P Carbon gasification efficiency ðCGE; %Þ ¼

Carbon liquefaction efficiency ðCLE; %Þ ¼

i

ni Ci PV M RT

vTOCgly

 100

TOCaq V  100 vTOCgly

The results of the GC/MS analyses indicate that small amounts of chemicals were formed in the SCWG of pure glycerol at 300 and 600
C. It may be concluded that the

300

400

500

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

In the gasification of biomass, the yield and composition of the formed gases and the aqueous products depend on the composition of the feedstock catalyst type, operating conditions and the reactor system [38e42]. In this experimental study, the parameters investigated were the temperature and the catalyst type. Effects of these parameters on the product efficiencies and on the compositions were determined for pure and crude glycerol solutions as feedstock. The experimental results are shown in Figs. 2e5. The reaction temperatures may be categorized as subcritical (300
C) and supercritical (400, 500, and 600
C). Carbon gasification efficiency (g C in gaseous/g C in biomass) and carbon liquefaction efficiency (g C in aqueous/g C in biomass) were expressed by using the following formulas;

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

0

600

Temperature, °C

Fig. 3 e The effects of various temperatures and catalysts on the product yields of crude glycerol in hydrothermal gasification. amount of reacted glycerol with water is low at 300
C and the formed aqueous product is low. Conversely at 600
C, glycerol and the formed aqueous products reacted with water at a significant ratio to produce gaseous products and the yield of the chemicals in the aqueous form decreased. The aqueous products obtained from the experiments performed at 400 and 500
C include more aldehydes and ketones than 300 and 600
C. A similar finding was obtained by were reached with this condition May et al. [16]. They investigated glycerol gasification at 510e550
C at 350 bar and the glycerol was decomposed completely at 550
C with 8 s of residence time. The highest yields of acetic acid, acetaldehyde, and

C2-C4

CO

CO2

H2

CH4

100 CLE

CGE

100

80

300

400

500

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

0

600

Temperature, °C

Fig. 2 e The effects of various temperatures and catalysts on the product yields of pure glycerol in hydrothermal gasification.

20 0

300

400

500

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

20

40

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

40

60

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

60

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

Molar percentage, %

Efficeincy, %

80

600

Temperature, °C

Fig. 4 e The effects of various temperatures and catalysts on gaseous product compositions of pure glycerol in hydrothermal gasification.

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C2-C4

CO

CO2

H2

CH4

Isobutryaldehyde Butryaldehyde Acetone Propionaldehyde Acetaldehyde Formaldehyde Hydroxyacetone

600

100

500

Concentration, ppm

60

40

300

400

200

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4 500

Catalyst type

600

Tempreature, °C Fig. 5 e Temperature and catalyst effects on gaseous product compositions of crude glycerol in hydrothermal gasification.

hydroxyacetone then the degradation of acetic acid and acetaldehyde by decarboxylation and decarbonylation in SCW conditions caused a decrease in their yields. Based on these facts the amounts of the identified aqueous products were given for only 400
C and 500
C in Figs. 6e9. Table 1 represents the total organic carbon (TOC) content of the aqueous product obtained in the non-catalytic and catalytic cases.

Fig. 7 e Variation of the yields of aqueous product (ppm) for hydrothermal gasification at 500
C of pure glycerol with temperature and catalyst.

Variation in the yields of the gaseous products and the compositions under the effect of various temperatures and catalysts The total degradation of the glycerol was evaluated by two different reaction mechanisms and was indicated as dominant [5]. The first path is the ionic reaction stages, while the second path consists of free-radical degradation. In general, at high pressure and/or low temperature ionic reactions dominate while at low pressure and/or high temperature radical reactions become more effective. The formation of the gaseous products takes place by means of free radicals at high temperatures (>500
C) and the yields of them increase with

Isobutryaldehyde Butryaldehyde Acetone Propionaldehyde Acetaldehyde Formaldehyde Hydroxyacetone

600 500 400 300 200

500 400 300 200

100

100

0

0

Catalyst type Fig. 6 e Variation of the yields of aqueous product (ppm) for hydrothermal gasification at 400
C of pure glycerol with temperature and catalyst.

Isobutryaldehyde Butryaldehyde Acetone Propionaldehyde Acetaldehyde Formaldehyde Hydroxyacetone

600

Concentration, ppm

Concentration, ppm

300

0 No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

0

400

100

20

No Catalyst H3PO4 KH2PO4 K2HPO4 K3PO4

Molar percentage, %

80

Catalyst type Fig. 8 e Variation of the yields of aqueous product (ppm) for hydrothermal gasification at 400
C of crude glycerol with temperature and catalyst.

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 4 0 ( 2 0 1 5 ) 1 4 8 0 6 e1 4 8 1 5

Isobutryaldehyde Butryaldehyde Acetone Propionaldehyde Acetaldehyde Formaldehyde Hydroxyacetone

600

Concentration, ppm

500 400 300 200 100 0

Catalyst type

Fig. 9 e Variation of the yields of aqueous product (ppm) for hydrothermal gasification at 500
C of crude glycerol with temperature and catalyst.

increasing temperatures. Fig. 10 represents simplified reaction pathways for glycerol hydrothermolysis in SCW [16]. The pure and crude glycerol were gasified successfully in all runs and no solid residue was observed. Since the catalyst used were dissolved in the reaction medium homogenously, there is no tar or coke formation due to the catalyst addition. The highest CGE and the lowest CLE were obtained at supercritical conditions as expected. The effect of the catalyst was also prominent on the product yields. In terms of gasification, the most effective parameter was determined as the temperature for both feedstocks. The gaseous product was composed mainly of H2, CH4, and CO2. There is a little amount of CO and C2eC4 in the gaseous product mixture in both the catalytic and non-catalytic cases. Bennekom et al. [43] investigated the HTG of pure glycerol, crude glycerol, and methanol at a range of 723e923 K and 25.5e27.0 MPa. The conversion is higher at elevated temperatures and longer residence times. They

14811

observed the effect of the alkali catalyst at low temperatures were more vigorous than the effect at elevated temperatures. The CGE were promoted in the presence of a catalyst at significant ratios. At 600
C, the CGE obtained from the experiments with pure glycerol and crude glycerol increased by 8.2 wt.% and 6.6 wt.% with K3PO4, respectively. The K3PO4 improved the CGE by 8.8% and 29.3% of pure and crude glycerol respectively, at 500
C (Figs. 2 and 3). It shows a compatible effect with the mentioned work of Bennekom et al. previously [43]. The order of effectiveness of the catalysts on gasification was found as: K3PO4>K2HPO4>H3PO4>KH2PO4 for crude glycerol and K3PO4 > K2HPO4 > KH2PO4 > H3PO4 for pure glycerol. Since the effect of H3PO4 is more in the case of the crude glycerol gasification, the interaction of the compounds in crude glycerol with the catalyst may be higher than to that of glycerol and the last term depends on the feedstock type. The reason for this difference can be explained as the effect of organic fatty acid and methanol degradation in the crude glycerol solution. L. Dianningrum et al. found that the crude glycerol mixture content had a great effect on the gas yields and CGE [44]. Yu-Wu et al. found that light hydrocarbons were formed in the HTG of crude glycerol while H2 was favored in the HTG of pure glycerol proposing this difference by the presence of sodium salts in the crude glycerol [45]. H3PO4, as an acidic catalyst, converts organic fatty acids and methanol into CO, H2, and also CH2O and increases the yield of gaseous products. There is no organic impurity effect on the gasification efficiency in the SCWG experiments of pure glycerol. Carbon monoxide is formed from formaldehyde that was an intermediate product produced during the gasification reactions (Fig. 10). It is converted to carbon dioxide by the wateregas shift reaction (1). Methane is produced from CO and CO2 by methanation reactions (2e3) in biomass gasification.

CO þ H2O ⇔ CO2 þ H2

Fig. 10 e Simplified reaction pathways for glycerol hydrothermolysis in SCW [17].

(1)

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CO þ 3H2 ⇔ CH4 þ H2O

(2)

CO2 þ 4H2 ⇔ CH4 þ 2H2O

(3)

The CO yields were lower than 10 wt.% in all cases. In the presence of H3PO4 and at 500
C, CO formation accelerated while H2 and CO2 yields decreased. In this case, the wateregas shift reaction may be shifted to the reversed direction. The yields of C2eC4 hydrocarbons were lower than 15 wt.% in all cases and the acidic catalysts improved their formation. The compositions of the gaseous products were found to be significantly affected by the temperature and the catalyst [27,39,42,46]. At 500
C, hydrogen yields reached their maximum with 42% in SCWG of pure glycerol and with 53% in SCWG of the crude glycerol. K2HPO4 and K3PO4 showed a catalytic effect on hydrogen formation due to their alkaline nature. Guo et al. studied with a continuous flow tubular reactor at 718e873 K and 25 MPa in the presence of NaOH, Na2CO3, KOH, and K2CO3. The results indicated that no char formation was observed, higher temperatures increased CGE, and the alkali catalysts enhanced the wateregas shift reaction [2]. The pH of the catalyst medium was approximately 9.5 (K2HPO4) and 12 (K3PO4) respectively. In the presence of K2HPO4 and K3PO4, at 500
C and with pure glycerol, the yield of hydrogen was increased by 20.4% and 20.6%, respectively. Acidic characteristics of H3PO4 and KH2PO4 promoted the formation of methane and its yield was above 15% with the effect of the catalyst at 500
C and at 600
C. To the best of our knowledge there is no data submitted, to include gaseous product yields individually and CGE at around 400
C in the presence of acidic catalyst in literature. The fact that the metallic catalyst, the long residence time, and decreasing water/glycerol ration in the feed enhances the methane yields dominantly were proposed in various works [16,18,27,30]. In general, hydrogen yields were higher in the SCWG of crude glycerol than pure glycerol because NaOH in the crude glycerol solution produces an alkali catalyst effect.

Variation in the aqueous product yields and the product compositions based on the effects of various temperatures and catalysts The yields of the aqueous products decreased with respect to the increased CGE resulting from the increasing reaction temperature. Since the radical reaction mechanism dominates at elevated temperatures, gaseous product formation is promoted and less aqueous products are formed. The CLE decreased within the range of 88 and 95% from 300
C to 600
C for all catalyst types and non-catalytic experiments in the SCWG of pure glycerol. The decrement ratio varied between 76 and 82% in the SCWG of crude glycerol. Similarly, the TOC content decreased with the increasing temperature and in the presence of catalyst as shown in Table 1. In the presence of various catalysts for all temperatures, the TOC contents generally decreased while the gasification rate of the glycerol increased compared to that of the non-catalytic cases. The results are in accordance with what was were found in the work of Q. Yu-Wu et al. [45]. They observed the TOC value of

the aqueous products decreases as the temperature increases and in the absence of a catalyst caused a slower decrease in the glycerol concentration and TOC. The organic carbon contents of the crude glycerol was significantly higher than that of the pure glycerol because of the methanol and fatty acid content of the crude glycerol. In the aqueous product, some aldehydes and ketones were quantified by the derivation with 2e4 dinitrophenylhydrazine and analyzed by the technique given in literature [47]. The compounds detected were hydroxyacetone, formaldehyde, acetaldehyde, propionaldehyde, acetone, butryaldehyde, and isobutryaldehyde. The distributions of these compounds are given in Figs. 6e9. As can be seen from the TOC values, there are high amounts of non-analyzed compounds (carboxylic acids, phenols, etc.) in the aqueous product. It was shown by Anna May et al. that at first, three unstable radicals are formed as CH2eCHOHeCH2OH (Int1), COHeCHOHeCH OH (Int2), and COHe(CH OH) by hydrogen 2 2 2 transfer (Int1&2), and radical isomerization (Int3 from Int2) in the glycerol hydrothermolysis as given in Fig. 10. Int1 was converted to propionaldehyde and allyl alcohol, whereas, the Int2 produced acrolein, acrylic acid, and formaldehyde. Additionally, Int3 was converted to acetaldehyde and acetic acid [16]. In this study, the compounds produced in the highest amounts in the aqueous product of SCWG were found as acetaldehyde, propionaldehyde, and acetone. The presence of these compounds verifies the proposed route from the specified radicals in the previous work [16]. In the work of Buhler et al. the pathways of glycerol decomposition in subcritical and supercritical water were investigated [5]. They obtained a liquid product containing methanol, allyl alcohol, acetaldehyde, acrolein, and formaldehyde that is a compatible composition with our work considering the identified aqueous products. H3PO4 shows a strong catalytic effect on the aqueous product formation in pure glycerol at 400
C. Due to the low pH of it, cracking reactions are promoted and the highest yields were obtained in the presence of H3PO4. The acidic catalyst are very effective in the aqueous product formation of HTG of glycerol [6,35,48]. The catalytic effect of H2SO4 was expressed as the dissociation of it in water above 673 K and at high pressure and results in the heterolytic bond cleavages [6]. The main objective of using acidic catalysts and metal salts is to increase the acrolein conversion basically at a temperature range of 190e400
C [11,6,17,12,28]. Concentrations of the major aqueous products were obtained as: Formaldehyde 54.6 ppm, acetaldehyde 295.9 ppm, propionaldehyde 79.0 ppm, and acetone 57.1 ppm. There are some supporting references that the main product of glycerol degradation is acetaldehyde with SCW conditions [5,28,49]. Glycerol was directly degraded to acetaldehyde (and also formaldehyde) and propionaldehyde with an intermediate radical. In the case of crude glycerol being used as feedstock at 400
C in the presence of H3PO4, methane and gaseous product formation increased and aqueous product yield decreased. Some points may be emphasized in the SCWG studies of crude glycerol in the presence of a catalyst. KH2PO4 are found as the most effective catalyst in the aqueous product formation at 400
C and 500
C. This may be explained as the lower

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gasification efficiencies obtained with this catalyst because of its weak acidic property. Glycerol and the organics in the crude glycerol feedstock solution were converted to acetone and propionaldehyde mainly in the presence of KH2PO4. K3PO4 is more effective at high temperatures (Effectiveness 500
C > Effectiveness 400
C) due to the dominance of aldehyde and acetone formation reactions in the supercritical water medium. As seen from the TOC values (at 400
C: 4966 ppm and at 500
C: 1676 ppm), the amounts of nonanalyzed compounds are higher than the detected compounds. The SCWG studies of pure glycerol showed that the compound produced in the highest quantity within the detectable compounds is acetaldehyde at 400
C. Propionaldehyde formation is dominant in the non-catalytic and in the presence of acidic catalyst cases at 500
C.

Conclusions  Supercritical water gasification of pure and crude glycerol in a batch autoclave system were carried out successfully with the valuable chemicals.  Non catalytic and catalytic experiments were carried out in this research. H3PO4, KH2PO4, K2HPO4, and K3PO4 were used as catalysts which improved the CGE and decreased the CLE. The catalyst effectiveness may be ordered as: K3PO4 > K2HPO4 > H3PO4 > KH2PO4 for the crude glycerol and K3PO4 > K2HPO4 > KH2PO4 > H3PO4 for the pure glycerol. The degradation reactions of organic fatty acid and methanol were catalyzed in the presence of H3PO4 to form CO, H2, and also CH2O.  At 500
C, hydrogen yields reached their maximum for pure and crude glycerol within the mentioned experimental conditions. K2HPO4 and K3PO4 showed a catalytic effect on hydrogen formation due to the alkaline nature. The acidic characteristics of H3PO4 and KH2PO4 promoted methane formation and the yields of methane increased to above 15% at supercritical temperatures.  It may be concluded that the catalysts used in this study are suitable for the HTG of pure and crude glycerol. In the case of the production of hydrogen K2HPO4 and K3PO4 are more effective while H3PO4 and KH2PO4 are preferable in methane production.  Operating temperature of 300
C may be considered as sufficient by the evaluation of the proposed formation mechanisms of aldehydes, ketones, carboxylic acids, and phenols, however, at 400
C a higher conversion to these products takes place. Due to the decrease in thermodynamically stable structure of them with increasing temperature we can make an assessment as they start to decompose to the gaseous products at 500
C and higher temperatures.  The most produced compounds in the aqueous product of SCWG were identified as acetaldehyde, propionaldehyde, and acetone. Acidic catalysts improved the formaldehyde yields and the production of acetone was favored by the catalysts.

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Acknowledgments The financial support of The Scientific and Technological _ ¨ BITAK) Research Council of Turkey (TU (Project No: 107M480) is gratefully acknowledged. We also thank Mr. G. Serin for his support during the experimental studies.

Nomenclature Ci HTG ni M P R T V V TOCaq TOCgly

concentration of component ‘i’ in the gas product, vol.% hydrothermal gasification number of carbon atoms of component ‘i’ in the gas product molar mass of carbon, g mol1 pressure, Pa universal gas constant, 8.3143 J mol1 K1 temperature, K volume of gas product under ambient conditions, L volume of aqueous product under ambient conditions, L total organic carbon content of the aqueous product, g L1 total organic carbon content of the feedstock glycerol solution, wt.%

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