A Novel Integrated Hydrothermal Liquefaction and Solar Catalytic Reforming Method for Enhanced Hydro...

Anuradha Shende, Richa Tungal, Rajneesh Jaswal, Rajesh Shende

  Open Access OPEN ACCESS  Peer Reviewed PEER-REVIEWED

A Novel Integrated Hydrothermal Liquefaction and Solar Catalytic Reforming Method for Enhanced Hydrogen Generation from Biomass

Anuradha Shende1, Richa Tungal1, Rajneesh Jaswal1, Rajesh Shende1,

1Department of Chemical and Biological Engineering, South Dakota School of Mines & Technology, Rapid City, South Dakota, USA


Short energy intensive hydrothermal liquefaction (HTL) of biomass in the presence of Ni salt catalyst selectively generates H2 in the product gas and biocrude mainly containing C1-C3 acids (formic, lactic, propionic, acetic), HMF and furfural. The H2 mass balance indicated that only 3.12 vol% H2 in biomass (cotton) was released as product gas; 48.7 vol% was captured in the C1-C3 acids while the remainder H2 was trapped in oxygenated compounds and char. Continuing HTL after 120 minutes caused no further increase in gas phase H2 yields. To enhance the H2 yields with minimal energy input, solar photocatalytic reforming (PR) of the biocrude with Pt/TiO2 catalyst was investigated. Photocatalysis of activated carbon (AC) treated biocrude generated an additional H2, 17.82 wt%. H2 yields from photoreforming of simulated biocrude acid mixture and actual biocrude were compared. Enhanced H2 generation was observed with integrated HTL-PR of biomass.

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Cite this article:

  • Shende, Anuradha, et al. "A Novel Integrated Hydrothermal Liquefaction and Solar Catalytic Reforming Method for Enhanced Hydrogen Generation from Biomass." American Journal of Energy Research 3.1 (2015): 1-7.
  • Shende, A. , Tungal, R. , Jaswal, R. , & Shende, R. (2015). A Novel Integrated Hydrothermal Liquefaction and Solar Catalytic Reforming Method for Enhanced Hydrogen Generation from Biomass. American Journal of Energy Research, 3(1), 1-7.
  • Shende, Anuradha, Richa Tungal, Rajneesh Jaswal, and Rajesh Shende. "A Novel Integrated Hydrothermal Liquefaction and Solar Catalytic Reforming Method for Enhanced Hydrogen Generation from Biomass." American Journal of Energy Research 3, no. 1 (2015): 1-7.

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1. Introduction

World’s current energy demand is primarily fulfilled by the use of fossil fuels, however, very little fraction of energy is derived from renewable resources [1]. Biomass is a renewable source of environmentally friendly high energy density (142 MJ/kg) hydrogen (H2) fuel. In industry, majority of H2 is produced by steam reforming of non-renewable resources such as the natural gas and petroleum. With rapid depletion of these resources, many researchers are exploring biological [2], pyrolysis [3], gasification [4], hydrothermal liquefaction (HTL) [5, 6] and photoreforming [7, 8] processes for H2 generation from renewable biomass, biomass waste and biomass components. However, each of these technologies suffer from drawbacks ranging from high energy input to low conversion efficiencies pointing to a need to develop energy efficient viable routes for H2 generation from renewable sources.

Hydrothermal liquefaction (HTL) under subcritical conditions transforms a biomass into gaseous products, liquid biocrude and solid biochar. The composition of the biocrude and product gas is largely influenced by catalyst. In our previous work [9] on HTL of cellulose, xylan, lignin, wastepaper and pinewood, homogeneous Ni salt catalysts were found effective in generating H2. HTL of pinewood at 275°C with Ni(NO3)2 yielded 12.26 mol% H2 [6]. Under similar experimental conditions, the authors reported HTL of wastepaper resulted in 51% liquefaction with 10.2 mol % H2 generation [5]. These HTL studies revealed longer processing time that resulted only in slight increase in H2 yield with bicrude mainly forming CO2 in the product gas. Biocrude typically contains intermediates such as saccharides, anhydrosugars, HMF, furfural, ketones, alcohols, carboxylic acids, lignin derived compounds [5]. Our studies with pinewood and waste paper have shown that the Ni salt catalyst promotes higher quantities of C1-C3 carboxylic acids in the biocrude. Several C1-C3 acids have been shown to be excellent sacrificial agents in photocatalyis for H2 generation. There are many reports on photocatalysis of single acids for H2 production. For e.g. photoreforming of lactic, acetic, benzoic, formic and oxalic acids has been investigated with the aim of simultaneous H2 production and remediation of acidic wastewater [10, 11, 12]. However, H2 yields from few real multi-component aqueous systems have been studied [13, 14]. Photoreforming of wastewater systems containing multiple electron donors showed that the H2 yields depend on several factors such as TiO2 surface adsorption, chemical structure, concentration and the ratio of the electron donors [10]. To the best of our knowledge, photocatalysis of the complex multi-component biocrude for H2 production is not yet reported.

In addition to acids and sugars, HTL derived biocrude contains several other compounds that may either contribute to the H2 yield or may negatively affect photoreforming process. Lignin, tannin, and sugars interfering in downstream processes such as the microbial fermentation have been removed using activated charcoal (AC) [15, 16]. AC has also been used to remove toxic components from biocrude obtained from HTL of starch for downstream microbial processing [17]. Therefore, in this investigation, cotton biocrude was treated with AC to remove potential interfering compounds. Both, untreated and AC-treated biocrude were tested with PR process for the H2 generation.

Chemically inert and inexpensive TiO2 catalyst [18] was chosen for PR of biocrude. The band gap energy (Eg) of TiO2 permits excitation of TiO2 only by the UV-portion of the solar spectrum. Efforts can be found in the literature on modifying TiO2 with suitable dopants to achieve visible light activation. Very recently, we reported TiO2 modified ZnO/TiO2 photocatalysts [19] with lower Eg that yielded higher H2 volume from aqueous methanol.

Figure 1. Schematic of integrated hydrothermal liquefaction (HTL) and photoreforming (PR) process for H2 generation (GC: gas chromatography, GC-MS: gas chromatography-mass spectrometry, HPLC: high pressure liquid chromatography, TOC: total organic carbon, MD: magnetic drive, RD: rupture disk, SS: stainless steel, C: cooling water, C1: condenser, PG: pressure regulator, V1/V2/V3/V4: control needle valves, T: thermocouple, W: vessel SS304 mounted for gas sampling, AI: impeller)

Additionally, the rapid recombination of the photogenerated electrons with the holes limits H2 production. Using co-catalyst such as platinum on TiO2 surface traps photogenerated electrons and partially prevents electron-hole recombination [20]. The PR of biomass model compounds, such as the primary, secondary and tertiary alcohols, glycerol, sugars and a few carboxylic acids have been studied for H2 generation [21]. It is expected that H2 and CO2 production from biocrude components will follow the general reforming scheme:


Green solar photoreforming of the biocrude to H2 will not only remediate the biocrude but also permit significantly higher H2 abstraction from the biomass. In this study, HTL of cellulosic biomass was carried out at 250°C in the presence of NiSO4 catalyst for the production of H2 and biocrude containing C1-C3 carboxylic acids. The biocrude and its carboxylic acids were individually photoreformed to demonstrate additional yields of H2. A schematic describing the combined HTL and PR process is shown in Figure 1. The results show that integrated HTL and PR process has a potential to develop into technologically feasible route for enhanced H2 production.

2. Materials and Methods

2.1. Materials

Commercially available cotton balls (98% cellulose) were selected as biomass feedstock. Nickel sulphate hexahydrate and dihydrogen hexachloroplatinate (IV) hydrate, Premion (99.999% metal basis) were purchased from Alfa Aesar, MA. Titanium (IV) oxide (P25 nanoparticles), 99.7% anatase (particle size ~ 25 nm, surface area 45-55 m2/gm) was obtained from Sigma Aldrich. Filtrasorb 600 activated charcoal (Iodine number 850 mg/g) was purchased from Calgon Carbon.

2.1.1. Elemental Composition of Biomass

Biomass, cotton (1 gram) was dried at 45°C overnight in the oven and weighed the next day to determine moisture content. The moisture free cotton was supplied to the Midcontinent Testing Labs, Inc., Rapid City, SD, USA for the elemental analysis.

2.1.2. Preparation of Platinum Coated TiO2 Nanoparticles

Platinum coating on TiO2 nanoparticles was achieved by the wet incipient method. TiO2 nanoparticles (0.5 g) were placed in water (12 ml) and sonicated for 30 min. To this dispersion, hexachloroplatinic acid (1 wt% Pt equivalent) was added and sonication was continued for additional 3 hrs. The dispersion was dried at 100°C and calcined at 600°C for 1 hrs. The platinum loading was determined by EDX analysis using Zeiss Supra 40VP scanning electron microscope/energy dispersive X-ray spectrometer.

2.1.3. Hydrothermal Liquefaction (HTL) of Biomass

A 300 ml high-temperature, high-pressure (HTHP) stainless steel (SS316) reactor (PARR Instrument Co, Illinois) fitted with gas inlet, outlet valves, a liquid sampling valve, pressure gauge, rupture disc, magnetic drive and a thermocouple was used for HTL of biomass. Slurry of 1.0 gram cotton balls cut into 0.5 cm2 pieces was made in 150 ml of distilled water containing 5 wt% NiSO4 catalyst. The slurry was loaded in the reactor and stirred by an impeller at a constant speed of 1300 rpm. The reactor was pressurized to 40 psi with N2 at room temperature and heated to 250°C to perform HTL for 120 min. During the HTL processing, gas and liquid samples were withdrawn periodically and analyzed by GC, HPLC and GC-MS. The processed slurry was centrifuged at 13000 rpm for 20 min to recover supernatant biocrude.

2.1.4. Characterization of Product Gas from HTHP Reactor Headspace

While HTL of biomass was in progress, product gas samples were withdrawn from the HTHP reactor headspace and analyzed on the Agilent7890A gas chromatograph (GC) equipped with thermal conductivity detector (TCD) and Porapak-Q stainless steel packed column (6 ft x 0.25 in x 2.1 mm) from Supelco. The GC oven was operated at 40°C and N2 at 1 ml/min was used as a carrier gas. He (99.99% purity) was used to characterize gaseous carbon species. Calibration curves were prepared using specific gas standards purchased from the Matheson Gases, SD, USA and the product gas volumes were determined at the NTP (20°C and 1 atm) conditions.

2.1.5. Activated Carbon (AC) Treatment of Biocrude and Characterization of AC-treated Biocrude

To the biocrude obtained from HTL, activated carbon was added at two different concentrations of 10 mg/ml and 50 mg/ml. The mixture was left on a stirrer for 12 hours after which it was centrifuged. The clear supernatant was kept at 4°C until ready to use. The biocrude was analyzed on a HPLC (Shimadzu) equipped with Rezex ROA 300 x 7.8 mm column and UV-vis detector. A mobile phase of 0.005M H2SO4 was used. Ethyl acetate extract of the biocrude was analyzed on an Agilent GC-MS (7890 GC/5975C MSD) equipped with Agilent HP- 5ms capillary column. Helium (99.999%, UHP grade) with column flow rate of 1.197 ml/min was used as carrier gas. The oven was programmed as follows: 40°C (8 min hold), ramped up (2°C/min) to 120°C (8 min hold), ramped up (5°C/min) to 220°C (no hold) and (20°C/min) to 250°C (no hold). The temperature of injector and detector was maintained at 300°C. The compounds were identified using National Institute of Standards and Technology (NIST) mass spectral library (2008, version 2.0f). The total organic carbon (TOC) content of the AC-treated HTL derived cotton biocrude was determined using a Sievers InnovOx Laboratory TOC analyzer (GE Instruments). Similar analyses were carried out with the samples of untreated.

2.1.6. Photocatalytic Reforming (PR) of Biocrude and Carboxylic Acids to H2

Pt/TiO2 photocatalyst (4.0 mg) was dispersed in 10 ml de-ionized water for 30 minute using ultrasonic bath and the dispersion was purged with N2 for 30 min to displace oxygen. Lactic (LA), formic (FA), acetic (AA) and propionic (PA) acids were individually and in a mixed form dissolved in water to a final concentration of 50 mM. In another experiment LA, AA, FA PA were mixed in ratio of 3.65:1.80:1.27:1 to form biocrude acids stimulant. The reaction mixture (7 ml) was added to a quartz reactor and sealed with a butyl septum. The reactor headspace (4 ml) was purged with N2. The headspace gas was sampled to confirm absence of O2.The mixture was stirred by a magnetic needle and exposed to simulated solar irradiation obtained with 240 W Xe lamp and AM1.5G filter. The power density measured at the center of the reactor window was 100 mW/cm2. During PR, headspace gas was withdrawn periodically and analyzed for H2 on the Agilent 7890GC using the same column and method used for the analysis of product gas from HTL. Photreforming of AC-treated and untreated biocrude for H2 generation was carried out using the same procedure as described earlier for individual acid and acid mixture. To achieve maximal reforming of the biocrude to H2, the headspace was evacuated after 3 hour and purged with N2 prior performing the next photoreforming cycle. The photoreforming cycles were continued until H2 levels were below detection limit in GC analysis.

3. Results and Discussion

3.1. HTL of Cotton in the Presence of NiSO4 Catalyst

Elemental analysis of cotton revealed that C, H, N and O were 45 wt%, 7.11 wt%, 0.2 wt% and 47.69 wt%, respectively. Figure 2a shows the product gas analysis from HTL of 1 gram cotton in the presence of 5 wt% NiSO4 at 250°C and 650 psi. At “zero” reaction time, practically no H2 was observed, however, after 60 min of reaction 4.3 ml H2 volume was detected. A total H2 volume of 11.54 ml was observed after 120 min reaction time suggesting an almost linear increase in H2 volume generation. The concentration of CH4 in product gas was almost negligible at zero reaction time and did not change over the period of 2 hrs of reaction time. However, the volume of CO and CO2 observed at zero reaction was about 25 ml. As the reaction progressed further, the volume of CO increased to 28 ml and remained constant.

Starting from zero reaction time to 2 hrs, the volumetric ratio of H2: CO gaseous products increased from 0.0438 to 0.43 indicating 10 fold increase in H2 over CO. Thus it appears that NiSO4 has increased H2 selectivity in the product gas phase. It is to be noted that in the absence of catalyst negligible quantities of H2 were observed (data not shown). The CO2 volume observed at zero reaction time (25 ml) was increased to 33 ml after 120 min reaction time. These results indicated 9.2%, 23% and 90% (volume basis) increase in CO, CO2 and H2, respectively over the period of 2 hrs reaction time at 250°C. The total product gas was estimated to be 10 wt% of the 1 gram cotton that was processed at 250°C for 2 hrs. After the HTL, the reactor contents were cooled to room temperature and centrifuged to recover the biocrude and residue. Based on the TOC analysis of the biocrude (1720 mg/L) and carbon content of cotton (estimated TOC 2811 mg/L), 61.2 wt% liquefaction of cotton was estimated (Figure 2b). The observed char residue was 28.8 wt%.

Figure 2. a) Product gas components observed during HTL of cotton at 250°C in the presence of NiSO4 catalyst and b) mass balance showing liquefied carbon, total product gas, and char
3.2. Composition of the Cotton Biocrude

The liquefied cotton biocrude was analyzed by HPLC and GC-MS and the results obtained are presented in Table 1 and Table 2, respectively. HPLC analysis revealed presence of acetic, formic, lactic and propionic acids. Among these acids, lactic acid is the main component with the concentration of 1675 mg/L, whereas propionic acid was observed in minimum concentration of 459 mg/L. The total acids were estimated to be 79.88 wt% of cotton biocrude as per the following equation-

Table 1. HPLC Analysis of C1-C3 acids in the Cotton Biocrude Obtained after HTL at 250°C in presence of 5 wt% NiSO4

HMF and furfural were observed to be 1.69 wt% of liquefied biocrude. The remaining 18.4 wt% of biocrude was assigned to C6 sugars (mainly glucose) and oxygenated hydrocarbons, which were thoroughly analyzed by GC-MS using HP-5ms column and inbuilt NIST library. The complete mass balance of different compounds observed in liquefied cotton biocrude is shown in Figure 2b. The GC-MS analysis of the biocrude showing only major compounds (peak area >1% and confidence level >95%) is presented in Table 2. It indicates cyclic ketones and substituted cyclic ketones, quinone derivatives, phenols and substituted phenols, alcohols and acids. However, based on the mass balance analysis, it appears that the pathways leading to the formation of C1-C3 carboxylic acids dominated with the use of NiSO4 catalyst and under the experimental conditions employed.

Table 2. Major compounds observed in the biocrude and AC-treated biocrude as analyzed by GCMS; >95% confidence level

3.3. Activated Charcoal (AC) Treatment of Biocrude

The cotton biocrude was treated with 10 mg/ml and 50 mg/ml activated charcoal and the visible difference in biocrude coloration is shown in inset of Figure 3. Activated charcoal at 10 mg/ml did not completely remove biocude color while colorless biocrude was obtained with 50 mg/ml activated charcoal treatment for 12 hours. Equal volumes (1 ml) of untreated biocrude and AC-treated samples were tested for photocatalytic H2 generation using 0.4 mg/ml Pt/TiO2 catalyst concentration. The gas phase H2 volume (ml at NTP) was estimated for 150 ml biocrude. As seen from Figure 3, PR of untreated biocrude did not generate any H2 whereas 10 mg/ml and 50 mg/ml AC-treated biocrude generated 0.6 ml and 2.14 ml of H2, respectively after 180 min.

Figure 3. H2 volume generated during PR of untreated cotton biocrude and biocrude treated with activated carbon at different concentrations

Although the colorless biocrude provided a six fold increase in H2 volume generation, increasing AC concentration to 50 mg/ml contributed to loss of the C and H content of the biocrude. HPLC analysis of AC-treated cotton biocrude (Table 1) indicates significant removal of C1-C3 carboxylic acids as compared to the untreated biocrude. Alternate methods to selectively remove only the interfering compounds without affecting the carboxylic acids and sugars are currently under investigation. It can also be noticed from Table 2 that several oxygenated hydrocarbon compounds including the major ones such as 1H-pyrazole, 1,5-dimethyl-, 1,23-benzenetriol, 1,2,4 benzenetriol and 1,2 cyclopentanedione, 3-methyl have been removed by the AC treatment. It is to be noted that the AC has significant Ni ions adsorption capacity (97.8%) [18]; therefore, we inferred that negligible Ni ions were present during PR of AC-treated biocrude.

3.4. Photocatalysis of Biocrude Acid Component and AC-treated Biocrude

To further understand the role of the four major acid components of the biocrude towards the H2 production, photoreforming of 50 mM LA, AA, FA and PA was carried out using procedure outlined in the Experimental section. Clearly, as seen from Figure 4 the acids differ in relative proficiencies as electron donors. Formic acid which is structurally simplest C1 acid with one α hydrogen appears to be best electron donor producing maximum H2 volume during 180 min reaction time. Under identical PR conditions, formic acid yielded 9.7 ml H2 at NTP, which was 133 fold, 12.77 fold, and 2.95 fold higher than AA, PA and LA, respectively. LA, which is major acid in the biocrude, is also a better electron donor compared with the other two C1-C3 acids. On the other hand, acetic acid is relatively recalcitrant to photoreforming to H2. LA, a C3 acid differs from PA by the presence of a single hydroxyl group. The hydroxyl group in addition to the carboxyl group probably increases adsorption affinity of LA on the TiO2 surface. This could facilitate better transfer of the photogenerated electrons to LA generating higher H2 yields. A mixture containing 50 mM of FA, AA, PA and LA was also investigated for H2 generation under identical PR conditions. Although the total H2 content of this mixture was higher, the H2 volume generated (6.65 mL) was about 1.5 fold lower than produced by the FA during PR, however, it was higher than LA, PA, and AA. This suggests that recalcitrant acids such as AA and PA in the acid biocrude probably reduce H2 generation. This warrants further investigation of Pt/TiO2 surface reactions of acid mixture during PR. In a separate experiment LA, PA, AA and FA were mixed in the concentration ratio found in the cotton biocrude. This acids mixture is referred to as ‘biocrude acid simulant’. During 180 min of PR reaction, 1.16 mL H2 was generated from the biocrude simulant. The biocrude stimulants were prepared with 0.1 and 10 fold acids concentrations to study the effect of concentration on H2 evolution. The 0.1 fold biocrude stimulant generated significantly higher H2 volume (4.13 mL), whereas 10-fold biocrude stimulant generated only 0.8 ml at NTP during PR. As the catalyst loading was held constant (0.4 mg/ml), higher number of active sites were available for the acid moieties present in 0.1 fold biocrude stimulant, which reflected into higher H2 volume generation.

Figure 4. Comparative H2 volume generated during PR of carboxylic acids and their mixture

It can be concluded that under the experimental conditions employed in this study, NiSO4 catalyzed HTL process generates sufficient quantities of acids (~79 wt%) in the downstream biocrude. The biocrude can be used without dewatering or concentrating the acids for PR to enhance H2 generation.

3.5. Combined Yields of H2 from Integrated HTL and PR Processing of Cotton

To investigate total H2 volume generation capacity of the biocrude simulant and actual biocrude during PR, multiple PR cycles were performed in the time interval of 180 min. After 180 min of PR reaction, the reactor headspace was evacuated by purging with N2 until H2 was undetectable in the GC analysis. The reactor was again exposed to simulated solar irradiation and next PR cycle was performed for 180 min. The H2 volume generated from the biocrude acid simulant during four PR cycles is shown in Figure 5a. During PR cycle-1, cycle-2, cycle-3 and cycle-4, the maximum H2 volume of 5.35 ml, 3.75ml, 1.98 ml, and 0.75 ml, respectively was observed after 180 min reaction time. The H2 volume generated from the actual biocrude during four PR cycles is shown in Figure 5b. The maximum H2 volume observed during PR cycle-1, cycle-2, cycle-3 and cycle-4 was 2.17 ml, 1.6 ml, 0.87 ml and 0.56 ml, respectively. These results indicate lower H2 volume generation from the actual biocrude as compared with the biocrude acid simulant, which infers that the additional oxygenated compounds (Table 2) present in actual biocrude probably inhibited some of the H2 generating reactions. The volumes of H2 generated from each PR cycle were combined to obtain the total H2 volume.

Figure 5. a: H2 volume generated during PR of C1-C3 acids mixture, and b) AC-treated cotton biocrude

The H2 yield (vol%) was determined as per the following equation-

The total H2 volume of acids mixture was estimated based on the theoretical H2 content of all acids and their concentrations used to prepare the acids mixture. For biocrude, the H2 yield was determined as the ratio of total H2 volume generated in all PR cycles and H2 volume content of 150 ml AC-treated biocrude. The total H2 volume for the AC-treated biocrude was estimated based on the acids concentration reported in Table 1. These acids were present in the biocrude after AC treatment. The H2 yields in terms of vol% are presented in Figure 6. It shows a total H2 vol% produced from actual biocrude during four PR cycles was 17.82%. When H2 vol% from PR and HTL is combined, a total of 20.93 vol% H2 was extracted from cotton biocrude. This shows that by integrating HTL and PR processes, higher H2 volume generation could be achieved. A maximum of 57.5 vol% H2 was observed from biocrude acid simulant that contained only carboxylic acids during four PR cycles. This leads to the notion that the acids can be separated from the biocrude and treated with PR for higher H2 volume generation.

Figure 6. Total H2 (vol%) extracted by integrating HTL of biomass and PR of HTL derived biocrude

4. Conclusions

HTL of cotton at 250°C in presence of NiSO4 yielded 10 wt% product gas, 61.2 wt% liquefied biocrude and 28.8 wt% char. The product gas phase during HTL was found to contain 3.12 vol% H2 whereas the PR of biocrude performed during cycles 1 to 4 amounts to a total of 5.22 ml. When the H2 volume generated from HTL was combined with the total H2 volume produced during four PR cycles, a total of 20.94 vol% was extracted from the biocrude. Thus by integrating HTL and PR, higher H2 volume generation could be achieved.


Authors acknowledge the financial and research assistantship support from NSF EPS-0903804 and Chemical and Biological Engineering at South Dakota School of Mines & Technology.


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