Combining Solar Steam Processing and Solar Distillation for Fully Off

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Combining Solar Steam Processing and Solar Distillation for Fully Off-grid Production of Cellulosic Bioethanol Oara Neumann, Albert D. Neumann, Shu Tian, Christyn Thibodeaux, Shobhit Shubhankar, Julius Mueller, Edgar Silva, Alessandro Alabastri, Sandra Whaley Bishnoi, Peter Nordlander, and Naomi J. Halas ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00520 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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Combining Solar Steam Processing and Solar Distillation for Fully Off-Grid Production of Cellulosic Bioethanol Oara Neumann1,6, #, Albert D. Neumann3, #, Shu Tian3,6, #, Christyn Thibodeaux1,6, Shobhit Shubhankar1,6, Julius Müller4,6, Edgar Silva5, Alessandro Alabastri1,6, Sandra W. Bishnoi1,6, Peter Nordlander2,6, and Naomi J. Halas1,2,4,6,* 1

Department of Electrical and Computer Engineering, 2Department of Physics and Astronomy, 3

Department of Civil Engineering, 4Department of Chemistry, 5Department of Mechanical

Engineering, 6Laboratory for Nanophotonics and the Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005 Corresponding Author * Naomi J. Halas Phone: (+) 1-(713)348-5612 Fax: (+) 1-(713)348-5686 E-mail: [email protected] #

Equal contribution

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ABSTRACT

Conventional bioethanol for transportation fuel typically consumes agricultural feedstocks also suitable for human consumption and requires large amounts of energy for conversion of feedstock to fuel. Alternative feedstocks, optimally those not also in demand for human consumption, and off-grid energy sources for processing, would both contribute to making bioethanol far more sustainable than current practices. Cellulosic bioethanol production involves three steps: the extraction of sugars from cellulosic feedstock, the fermentation of sugars to produce ethanol, and the purification of ethanol through distillation. Traditional production methods for extraction and distillation are energy intensive and therefore costly, limiting the advancement of this approach. Here we report an initial demonstration of the conversion of cellulosic feedstock into ethanol by completely off-grid solar processing steps. Our approach is based on nanoparticle-enabled solar steam generation, where high-efficiency steam can be produced by illuminating light-absorbing nanoparticles dispersed in H2O with sunlight. We used solar-generated steam to successfully hydrolyze feedstock into sugars, then used solar steam distillation to purify ethanol in the final processing step. Coastal hay, a grass grown for livestock feed across the southern U. S., and sugar cane as a control, are successfully converted to ethanol in this proof-of-concept study. This entirely off-grid solar production method has the potential to realize the long-dreamed-of goal of sustainable cellulosic bioethanol production.

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Bioethanol is an alternative fuel candidate that has long been advocated for its potential to considerably reduce our need for fossil fuels. Bioethanol is currently produced using agricultural products such as corn and sugar cane, utilizing valuable agricultural land that could be utilized for human food production. In contrast, the long-term goal of achieving bioethanol using cellulosic feedstock, such as agricultural residue, native grasses such as switchgrass, livestock feed such as coastal hay, or woody biomass, has yet to be realized in a cost-effective manner.

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cellulosic feedstock into bioethanol requires pretreatment to extract the polysaccharides from plant matter and convert them into monosaccharides for bacterial conversion into ethanol. Current methods used for pretreatment typically require the input of energy in the form of heat and the use of acids, bases, or enzymes to degrade plant cell walls.1-6 This pretreatment adds a substantial energy cost to bioethanol production. This is in addition to the already energy-intensive and costly distillation of the final ethanol product, which has been estimated to constitute 70-85% of the total energy costs of bioethanol production. Sustainable, low-cost methods for conversion of cellulosic feedstock into bioethanol are critically needed to make this approach a practical and sustainable reality. Degradation of the hemicellulose and lignin constituents of plant walls is essential to reach the cellulose core and release the available stored sugars. Cellulose is a polymer composed of Dglucose monomers linked by a β-1,4 glycoside bond. Cellulose polymers have an amorphouscrystalline structure due to hydrogen bonds between the hydroxyl groups of glucose; these bonds pack the glucose polymeric chains tightly, making hydrolysis difficult. Since glucose converts most efficiently relative to other constituent monosaccharides or disaccharides in the production of bioethanol, a primary challenge is the dissociation of cellulose into glucose monomers. Several


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hydrolysis methods have been developed for this process, including enzymatic methods, acidcatalysis methods, ultra-high temperature and pressure steam explosion methods, microwavebased alkali pretreatment, ammonia water-pretreatment, and hydrothermal processes.

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these methods, however, chemical or biological additives are typically needed; requiring waste neutralization cycles or corrosion resistant equipment, increasing overall costs. 17, 22-24 Recently we showed that illuminating a solution of light-absorbing and light-scattering nanoparticles dispersed in water can result in steam generation without heating the fluid volume. 25, 26

This general effect has also been observed using porous, buoyant graphenic materials. 27-31 For

particle concentrations in the 109 to 1010 NP/mL range, light trapping caused by multiple scattering prior to absorption concentrates the light absorption into a small volume of the liquid for efficient photothermal conversion.25 For top-down illumination, the energy is focused into a thin region just below the liquid-air interface enabling steam generation at high efficiencies. 25 This effect has been demonstrated to be useful for powering a portable, standalone solar autoclave, and may have a range of promising applications,27 including the harnessing of solar power. 19, 32-35 We have also initially demonstrated that ethanol can be distilled from an H 2O-ethanol mixture using nanoparticles to absorb the focused sunlight, yielding fractions significantly richer in ethanol content than simple thermal distillation. 26, 36 Here we show how combining the two processes of solar steam generation and nanoparticle-enabled solar distillation can convert a cellulosic feedstock to ethanol using direct solar irradiation as the only energy source. We demonstrate a nanoparticle-based solar steam generation system that is capable of delivering steam at 150°C180°C into a 7.5 L vessel to degrade the cellulose chains of the feedstock into monomeric sugars. Glucose obtained in this initial processing step is fermented, and then the resulting ethanol is


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distilled by solar distillation. This proof-of-concept demonstrates a small scale, entirely off-grid method for producing ethanol from cellulosic feedstock start to finish. RESULTS AND DISCUSSION Conversion of biomass into alcohol involves three basic steps: (I) hydrolysis, (II) fermentation, and (III) distillation. In these three steps, the feedstock/biomass is degraded into sugars, which are then converted to ethanol using bacteria, such as yeast, which is then extracted/purified by distillation. This all-solar process is depicted in Figure 1. Here we use solar steam generation, first as a solar hydrolysis system (Figure 1A) followed by with a standard fermentation system (Figure 1B), then as a solar distillation system (Figure 1C).


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Solar Hydrolysis Solar steam generator

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Feedstock Treatment Module

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Copyright Oara Neumann Rice University

Figure 1: Schematic representation and photographs of solar biofuel three-step process. A. Solar hydrolysis: light from a solar collector unit is absorbed by nanoparticle and generates steam. The steam is directed into the feedstock treatment module where the cellulose is degraded into small glucose units. B. Fermentation: the extracted sugar is converted to ethanol by yeast. C. Solar distillation: the bioethanol is extracted from the fermented solution–Au nanoshell mixture using sunlight. An image of each of the solar biofuel processes are presented below each respective schematic.


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There are several ways to hydrolyze biomass, chemical, biochemical, combustion, thermochemical processes, etc.2, 7-18, 37-40 In our experiment, solar steam generation results in steam generated at temperatures and pressures sufficient to dissociate cellulose into sugar monomers. The high temperature and pressure has two main roles: to soak and soften the dry feedstock material, and to self-ionize the water, lowering its pH, which facilitates hydrolysis. Solar steam hydrolysis was accomplished with a custom solar parabolic trough system. The solar parabolic trough system (Figure 1A) has two main parts: the concentrated solar power system and the high pressure reactor vessel. The concentrated solar steam generator consists of a parabolic trough and a collector element. The concentrated light from the solar collector unit is focused onto a transparent glass tube containing an aqueous solution of Au nanoshells (Au-NS) to generate high temperature/pressure steam.26 The parabolic trough focuses the sun’s rays with a 3 x 6 feet curved mirror created with three sheets of Anolux mirror sun KKS (from Anomet Inc., Canada) onto the collector element located in the optical focal line of the parabola. The solar parabolic mirror tracks the sun continuously by a dual axis tracker with a 50 second time delay remote sun sensor V1.0 (from Heliotrack.com). The collector element consists of an 80 mm OD X 122 cm length transparent glass tube with a 9 mm thick wall, isolated with a 110 mm OD vacuum jacket with a 3 mm wall to prevent heat loss (Scientific Glass & Instruments, Inc., Houston TX) and a 28 mm tubing connection to ½ OD water hose. However, the fragile glass tub-collector may be replaced by a corrosion resistant metal vessel containing a quartz window that allows the sunlight to enter the vessel. The heat collector tube was half-filled with a ~109 particles/mL Au-NS solution (approximately 2 L). (The optical properties of the Au-NS are shown in Figure S1, Supporting Information). The steam generated in the glass tube flows into the reactor vessel into which the feedstock was placed (7.5 L), where the pressure and temperature required for feedstock hydrolysis


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was achieved. For comparison, we performed the thermal hydrolysis process using electrical heating (a Parr Instruments Model 4621 pressure vessel equipped with an electrical heating unit; Supporting Information Figure S2). In this control experiment, the biomass was placed in a 1 L autoclave, heated to ~135°C for a defined time period, then left to cool to room temperature prior to pressure release. Two types of feedstock were investigated for steam driven hydrolysis: dry coastal hay (from OXBOW Animal Health, NE) and non-fresh sugar cane as a comparative control (from Fiesta Supermarket, Houston, TX). Coastal hay was chopped into small fragments (1-4 cm in length) to assist the penetration of steam into the biomass, and sugar cane was sliced into approx. 1 cm lengths. Two solutions were prepared: 50 g coastal hay in 300 mL water and 50 g sugar cane in 300 mL water. The biomass samples were placed one at a time into the solar autoclave and then heated to 135°C for approximately 15 minutes. After heating, the solutions were cooled to room temperature prior to pressure release. The time-dependent temperature and pressure of the solar and the nonsolar hydrolysis step is shown in Figure 2. Under solar illumination, the steam heats the biomass to a maximum temperature of 136°C (Figure 2A) with a corresponding steam pressure of 70 psig. The variation in the pressure vs. time data is due to the interrupted sunlight from surrounding clouds. Under electrical heating (Figure 2B), the temperature of the mixed solution, controlled by a thermocouple, was set to 136°C to match solar reactor conditions. The vapor temperature reached 120°C in the electrical heating unit with a maximum pressure of 20 psig. The average heating time was approximately 10 minutes and the reactions were stopped by reducing the reactor temperature. The time interval required to reach 136°C is longer in the case of solar steam heating compared to electrical heating source due to the larger autoclave size 7.5 L (solar) versus 1 L (electrical). Note that despite the same liquid temperature in both systems, there is a


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large difference in the vapor pressure between the solar and electrical heating processes. The system evolves in different ways based on the heating method. For the electrical heating system the phase-change process takes place in a quasi-equilibrium state, while in the nanoparticleassisted solar steam generation system, the evaporation process happens locally, at the surface of the nanoparticles, which is a nonequilibrium process. Solutions were also left to cool down naturally to room temperature without any release of pressure. The approximate time for cooling was 3 hrs. After each experiment, solid matter was separated from liquids by filtering. The pH was measured to be in the neutral range of 6.0-7.0 to prevent damage to the HPLC column; otherwise, pH adjustments were performed by addition of Ca(OH) 2 when needed. Conditions were optimized by analyzing multiple batches of feedstock with varying temperatures and incubation times (Supporting Information Figure S3 and Table T1). The optimal reactor temperature was found to be 150°C for cellulose hydrolysis, with minimal precipitation of the extracted sugars onto the pretreatment solids.41 However, for safety concerns the maximum temperature achieved using our solar collector was 136°C. When incubation times were varied from 5 min to 120 min, no significant difference in glucose content was observed as a function of time, though some differences could be seen among the other hydrolysed sugars (Table T1, Supporting Information).

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Figure 2: The thermal and pressure time profile of the steam hydrolysis process under (A) Solar Steam and (B) Electrical Heating. The autoclave size was 7.5 L for solar steam and 1L for electrical heating.

The sugar samples isolated after steam hydrolysis were neutralized with Ca(OH) 2 to pH 6, filtered to avoid column damage, and characterized by high-pressure liquid chromatography (HPLC) (Shimadzu, USA) using a refractive index detector and a Rezex RPM-Monosaccharide Pb+2 (8 %) LC column of dimensions: 300 mm x 7.8 mm (from Phenomenex, US). HPLC grade water was used as the mobile phase in all experiments. The column was operated at a flow rate of 0.4 mL/min and a temperature of 60°C. The sample run time was 50 min with a post-run time of 30 min between injections. The HPLC chromatogram of both the glucose concentration of sugar cane and coastal hay as a result of solar steam-based hydrolysis is presented in Figure 3.


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Figure 3: Separation of sugars from sugar cane and coastal hay feedstocks: HPLC chromatograms of sugars present in (A) sugar cane feedstock: D-glucose and D-mannose/L-arabinose at 22 and 28 min retention times and (B) coastal hay feedstock: D-cellobiose, D-glucose, D-xylose, Dgalactose, and D-mannose/L-arabinose at 16, 22, 24, 26, and 28 min retention times (* indicates the glucose retention time). Glucose concentration extracted from the: (C) sugar cane feedstock and (D) coastal hay feedstock using electrical and solar heating sources. The hydrolysis temperature was ~132°C.


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The HPLC chromatogram of sugars extracted from sugar cane showed peaks at 22 and 28 min retention time, corresponding to D-glucose, and D-mannose/L-arabinose (Figure 3A), while the sugars extracted from coastal hay showed peaks at 16, 22, 24, 26, and 28 min retention time, corresponding to D-cellobiose, D-glucose, D-xylose, D-galactose, and D-mannose/L-arabinose (Figure 3B). We concern ourselves only with glucose content here, since glucose is the primary sugar converted by the Saccharomyces cerevisiae yeast used. Details of the HPLC chromatogram of all sugar units extracted from cellulosic biomass analysis are presented in Supporting Information Figure S4 and Table T2. A standard calibration curve (range 0.25-10 mg/mL) was generated for glucose with pure samples dissolved in water to determine sample concentrations. The glucose concentration was observed to be slightly higher when solar steam was used for hydrolysis, compared with electrically heated hydrolysis (Fig. 3C and D). For example, in the case of sugar cane, the glucose concentration obtained with solar steam hydrolysis was 7.90 mg/mL, while with electrical heating was 6.23 mg/mL (Fig. 3C). The same trend was observed in the hydrolysis of coastal hay, where the glucose concentration obtained with solar heating was 1.38 mg/mL, while with electrical heating was 1.17 mg/mL (Fig. 3D). We compared the energy-toglucose conversion of both solar and electric hydrolysis processes with regards to energy usage. For sugar cane, the energy rate was 2.55 GJ/kg and 3.42 GJ/kg for solar and electric heating respectively, while for coastal hay, the energy rate was 14.63 GJ/kg and 18.18 GJ/kg. These results suggest solar irradiation to be a valid alternative to electrical heating. Electric energy has been assumed to be produced from natural gas with 43% efficiency. Details of the calculation steps are presented in the Supporting Information. Both systems used the same cellulosic solution temperature during hydrolysis; however, a higher pressure was obtained in the solar steam system. With increasing pressure, the cellulose in the feedstock would become better hydrated, leading to


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increased hydrolysis.10,

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To compute the effectiveness of the solar hydrolysis process, an

analytical procedure for standard biomass analysis (National Renewable Energy Laboratory, CO) has been used to quantify the entire cellulose content in coastal hay and sugar cane extracted from biomass43 (results are summarized in the Supporting Information, Table T3). Using the solar steam process, we extracted 39.7 % of the total glucose available in sugar cane and 45.5 % of the total glucose available in coastal hay. This is slightly greater than the yield obtained by thermal hydrolysis (30.7 % glucose for sugar cane and 38.5 % glucose for coastal hay). Many other factors can impact the overall amount of extractable sugar from a biomass sample, including harvest date, geographic location, and grass maturity of feedstock.38 In addition, increasing the number of hydrolysis cycles can also dramatically increase the extracted sugar content. One of the most sensitive variables for glucose yield can also be preparation of the initial material prior to hydrolysis. A smaller feedstock size yields a higher available surface area, which improves cellulose hydrolysis and allows for a higher glucose content extraction. Each of these parameters could provide a path toward increasing glucose yield in future solar hydrolysis experiments. Following sugar extraction, a standard fermentation process was used to convert sugar to ethanol. Yeast was selected as the fermentation organism due to its tolerance of high alcohol content and acids, and reasonable incubation temperatures. Yeast cells under anaerobic conditions consume sugar and produce ethanol and carbon dioxide. The fermentation step was conducted with the filtered solutions obtained from hydrolysis, concentrated from 1.38 mg/mL to 6.5 mg/mL in the case of coastal hay and from 7.90 mg/mL to 162.10 mg/mL in the case of sugar cane. A vacuum evaporator at 45°C was used for concentration to avoid glucose degradation. A solution containing 0.2 g yeast, 5 mg nutrient, and 5 mL of H2O (pre-boiled for sterilization) was prepared and added to the 5 mL solution of the concentrated extracted sugar solutions. Solution pH was then verified


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to be within the range required by the yeast (pH 3.4-5.5). The solutions were sealed with a Ushaped tube filled with water to avoid the entrance of air into the system (Fig. 1B). Optimization of the glucose-bioethanol conversion was important in identifying the ideal yeast. Saccharomyces cerevisiae yeast provided the best glucose-ethanol conversion compared to the other yeast species investigated (Supporting Information Figure S5). The fermentation took place for 2-3 days in a water bath at 34°C. Once the sugar-to-ethanol conversion was complete, the bioethanol was extracted from the mixture by solar distillation (Figure 1C). Recently, we have shown that a solar illuminated aqueous nanoparticle solution can drive water-ethanol distillation, yielding fractions significantly richer in ethanol content than simple thermal distillation. 26, 36 Briefly, 20 mL of filtered, fermented biomass solution was dispersed with Au NS (~109 particles/mL) contained in a 40 mL glass vessel coated with a vacuum jacket to prevent losses. The vessel was then illuminated by focusing the light through a 26 x 26 cm Fresnel lens with a focal length of 44.5 cm. The resulting distillates were collected using a water condenser.26 The extracted samples were diluted (1/1000 in water) and analysed by gas chromatography (GC) on a Hewlett-Packard 5890 GC equipped with a glass column containing 80/120 Carbopack B-DA*/4% Carbowax 20 M (Supelco, Bellefonte, PA, USA) and a flame-ionization detector (Agilent Technologies). A 5 μL sample was injected into the GC unit and 250, 110, and 250°C temperatures were used for the injector, oven, and detector, respectively. Identification and quantification of distillates were performed using a calibration curve with ethanol standards prepared by diluting 200-proof ethanol with MQ water.


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Vapor mole fraction of bioethanol

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Figure 4: Mole fraction of bioethanol in vapor phase yielded from the sugar fermented mixture under solar/laser exposure. The Au NS concentration was 2.5x109particles/mL.

The mole fraction of bioethanol obtained by solar simple distillation is presented in Figure 4. We extracted 0.474 ethanol mole fraction for 162 mg/mL fermented glucose from sugar cane and 0.05 ethanol mole fraction from 6.5 mg/mL fermented glucose from coastal hay. As expected, the ethanol extracted from sugar cane is higher than the ethanol extracted from coastal hay, because the overall sugar concentration is higher in the case of sugar cane (contains free glucose, fructose, and sucrose also fermented by yeast). In summary, we have demonstrated an entirely solar-driven process of cellulosic feedstock into ethanol by combining solar steam processing for cellulose degradation with standard fermentation, followed by solar steam-based distillation of the fermentation product. This proof-of-concept


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demonstration clearly shows that off-grid, start-to-finish bioethanol production is possible. While there are many strategies that can be explored for optimization of the individual processing steps, this work shows that bioethanol production can in principle be pursued using nonhumanconsumable feedstock, and could be practiced in remote locations, eliminating the need for costly transport of feedstock to processing plants. This approach also ultimately opens the door to portable, modular bioethanol processing facilities.

ASSOCIATED CONTENT Supporting Information Extinction spectra of Au nanoshells; Schematic representation and photographs of solar biomass thermo-hydrolysis process: A Parr Instruments Model 4621 pressure vessel equipped with an electrical heating unit has been used. The loaded vessel was heated to a certain temperature which was kept constant for a defined time period; The D-Glucose concentration versus temperature as a produce of thermal sugar cane hydrolysis used to establish the thermal extraction condition; Sugar concentrations versus time as a result of thermo-hydrolysis process using an electrical source. The feedstock was heated and kept at 150° C for different amount of time, without releasing the pressure; The HPLC chromatogram of the sugar units extract from cellulosic biomass; The sugar yield from different feedstock as a result of thermal-hydrolysis process using electrical heating source; Details of the calculation steps of the energy comparison of the solar and electrical based pre-treatment steps; The carbohydrates, lignin, and free sugars extracted accordingly with the NREL Laboratory Analytical Procedures; Mole % of bioethanol extracted from fermented sugar cane versus different type of yeast (a) Saccharomyces bayanus-Champagne yeast, (b)


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Saccharomyces cerevisiae, (c) Saccharomyces bayanus-Premier Cuvee. The Au nanoshell concentration was 0.25 x 1010 part/mL.

AUTHOR INFORMATION Include email address, URL of the group website, or Twitter handle, if any. Notes The authors declare no competing financial interest. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. #These authors contributed equally. ACKNOWLEDGMENT We gratefully acknowledge the Robert A. Welch Foundation (C-1220 and C-1222); the J. Evans Attwell-Welch Fellowship (L-C-0004), for financial support, Microbac Laboratories for biomass analysis, Jared Day, Luke Henderson, Alex Urban, Fangfang Wen, and Mark Knight for helpful discussions.

REFERENCES 1. He, C.; You, F., Toward more cost-effective and greener chemicals production from shale gas by integrating with bioethanol dehydration: Novel process design and simulation-based optimization. AIChE J. 2005, 61, 1209–1232. 2. Cheng, J. J.; Timilsina, G. R., Advanced Biofuel Technologies: Status and Barriers. Renewable Energy 2011, 36, 3541-3549. 3. Jacques, K. A.; Lyons, T. P.; Kelsall, D. R., The Alcohol Textbook. Areference for the beverage, fuel and industrial alcohol industries. Nottingham University Press, Manor Farm, Main Street, Thrumpton, Nottingham, NG11 0AX, United Kingdom 2003, 4th Edition. 4. Kohlmann, K.; Westgate, P. J.; Weil, J.; Ladisch, M. R., Biological-Based Systems for Waste Processing. S. A. E. Tranzaction 1993, 102-1, 1476-1483.


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Solar steam-based bioethanol generation process 254x190mm (96 x 96 DPI)


Combining Solar Steam Processing and Solar Distillation for Fully Off

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