Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review

Increasing energy demand, especially in the transportation sector, and soaring CO2 emissions necessitate the exploitation of renewable sources of energy. Despite the large variety of new energy carriers, liquid hydrocarbon still appears to be the most attractive and feasible form of transportation fuel taking into account the energy density, stability and existing infrastructure. Biomass is an abundant, renewable source of energy; however, utilizing it in a cost-effective way is still a substantial challenge. Lignocellulose is composed of three major biopolymers, namely cellulose, hemicellulose and lignin. Fast pyrolysis of biomass is recognized as an efficient and feasible process to selectively convert lignocellulose into a liquid fuel—bio-oil. However bio-oil from fast pyrolysis contains a large amount of oxygen, distributed in hundreds of oxygenates. These oxygenates are the cause of many negative properties, such as low heating value, high corrosiveness, high viscosity, and instability; they also greatly limit the application of bio-oil particularly as transportation fuel. Hydrocarbons derived from biomass are most attractive because of their high energy density and compatibility with the existing infrastructure. Thus, converting lignocellulose into transportation fuels via catalytic fast pyrolysis has attracted much attention. Many studies related to catalytic fast pyrolysis of biomass have been published. The main challenge of this process is the development of active and stable catalysts that can deal with a large variety of decomposition intermediates from lignocellulose. This review starts with the current understanding of the chemistry in fast pyrolysis of lignocellulose and focuses on the development of catalysts in catalytic fast pyrolysis. Recent progress in the experimental studies on catalytic fast pyrolysis of biomass is also summarized with the emphasis on bio-oil yields and quality.

Catalytic fast pyrolysis of lignocellulosic biomass

Pyrolytic biofuels have technical advantages over conventional biological conversion processes since the entire plant can be used as the feedstock (rather than only simple sugars) and the conversion process occurs in only a few seconds (rather than hours or days). Despite decades of study, the fundamental science of biomass pyrolysis is still lacking and detailed models capable of describing the chemistry and transport in real-world reactors is unavailable. Developing these descriptions is a challenge because of the complexity of feedstocks and the multiphase nature of the conversion process. Here, we identify ten fundamental research challenges that, if overcome, would facilitate commercialization of pyrolytic biofuels. In particular, developing fundamental descriptions for condensed-phase pyrolysis chemistry (i.e., elementary reaction mechanisms) are needed since they would allow for accurate process optimization as well as feedstock flexibility, both of which are critical to any modern high-throughput process. Despite the benefits to pyrolysis commercialization, detailed chemical mechanisms are not available today, even for major products such as levoglucosan and hydroxymethylfurfural (HMF). Additionally, accurate estimates for heat and mass transfer parameters (e.g., thermal conductivity, diffusivity) are lacking despite the fact that biomass conversion in commercial pyrolysis reactors is controlled by transport. Finally, we examine methods for improving pyrolysis particle models, which connect fundamental chemical and transport descriptions to real-world pyrolysis reactors. Each of the ten challenges is presented with a brief review of relevant literature followed by future directions which can ultimately lead to technological breakthroughs that would facilitate commercialization of pyrolytic biofuels.

Top ten fundamental challenges of biomass pyrolysis for biofuels.

Reaction mechanisms and multi-scale modelling of lignocellulosic biomass pyrolysis

Electrochemical cells and systems play a key role in a wide range of industry sectors. These devices are critical enabling technologies for renewable energy; energy management, conservation, and storage; pollution control/monitoring; and greenhouse gas reduction. A large number of electrochemical energy technologies have been developed in the past. These systems continue to be optimized in terms of cost, life time, and performance, leading to their continued expansion into existing and emerging market sectors. The more established technologies such as deep-cycle batteries and sensors are being joined by emerging technologies such as fuel cells, large format lithium-ion batteries, electrochemical reactors; ion transport membranes and supercapacitors. This growing demand (multi billion dollars) for electrochemical energy systems along with the increasing maturity of a number of technologies is having a significant effect on the global research and development effort which is increasing in both in size and depth. A number of new technologies, which will have substantial impact on the environment and the way we produce and utilize energy, are under development. This paper presents an overview of several emerging electrochemical energy technologies along with a discussion some of the key technical challenges.

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Emerging electrochemical energy conversion and storage technologies.

Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify α-cyclodextrin as an appropriate small-molecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.

/pdf/revealing-pyrolysis-chemistry-for-biofuels-production-4l4k9mxnvv.pdf

Revealing pyrolysis chemistry for biofuels production: Conversion of cellulose to furans and small oxygenates

The direct utilization of carbonaceous fuels is examined in a solid oxide fuel cell (SOFC) with a molten Sb anode at 973 K. It is demonstrated that the anode operates by oxidation of metallic Sb at the electrolyte interface, with the resulting Sb2O3 being reduced by the fuel in a separate step. Although the Nernst Potential for the Sb-Sb2O3 mixture is only 0.75 V, the electrode resistance associated with molten Sb is very low, approximately 0.06 Ωcm2, so that power densities greater than 350 mW cm−2 were achieved with an electrolyte-supported cell made from Sc-stabilized zirconia (ScSZ). Temperature programmed reaction measurements of Sb2O3 with sugar char, rice starch, carbon black, and graphite showed that the Sb2O3 is readily reduced by a range of carbonaceous solids at typical SOFC operating conditions. Finally, stable operation with a power density of 300 mW cm−2 at a potential of 0.5 V is demonstrated for operation on sugar char.

A direct carbon fuel cell with a molten antimony anode

An energy-storage concept is proposed using molten Sb as the fuel in a reversible solid-oxide electrochemical cell (SOEC). Because both Sb and Sb2O3 are liquids at typical SOEC operating temperatures, it is possible to flow Sb from an external tank and use it as the fuel under fuel-cell conditions and then electrolyze Sb2O3 during recharging. This concept was tested using a button cell with a Sc-stabilized zirconia electrolyte at 973 K by measuring the impedances under fuel-cell and electrolyzer conditions for a range of stirred Sb-Sb2O3 compositions. The Sb-Sb2O3 electrode impedances were found to be on the order of 0.15 Ωcm2 for both fuelcell and electrolyzer conditions, for compositions up to 30% Sb and 70% Sb2O3. The open circuit voltages (OCV) were 0.75 V, independent of oxygen composition. Some features of using molten Sb as an energystorage medium are discussed. Disciplines Biochemical and Biomolecular Engineering | Chemical Engineering Comments Suggested Citation: Javadekar, A., Jayakumar, A., Gorte, R. J., Vohs, J. M., & Buttrey, D. J. (2012). Energy Storage in Electrochemical Cells with Molten Sb Electrodes. Journal of the Electrochemical Society. 159, A386-A389. DOI: http://dx.doi.org/10.1149/2.050204jes © The Electrochemical Society, Inc. 2012. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in the Journal of the Electrochemical Society, Volume 159, Issue 4, pp. A386-A389 (2012). Publisher URL: http://scitation.aip.org/ JES/ This journal article is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/155 A386 Journal of The Electrochemical Society, 159 (4) A386-A389 (2012) 0013-4651/2012/159(4)/A386/4/$28.00 © The Electrochemical Society Energy Storage in Electrochemical Cells with Molten Sb Electrodes Ashay Javadekar,a Abhimanyu Jayakumar,b,∗ R. J. Gorte,b,∗∗,z J. M. Vohs,b,∗∗ and D. J. Buttreya aDepartment of Chemical Engineering, University of Delaware, Newark, Delaware 19716, USA bDepartment of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA An energy-storage concept is proposed using molten Sb as the fuel in a reversible solid-oxide electrochemical cell (SOEC). Because both Sb and Sb2O3 are liquids at typical SOEC operating temperatures, it is possible to flow Sb from an external tank and use it as the fuel under fuel-cell conditions and then electrolyze Sb2O3 during recharging. This concept was tested using a button cell with a Sc-stabilized zirconia electrolyte at 973 K by measuring the impedances under fuel-cell and electrolyzer conditions for a range of stirred Sb-Sb2O3 compositions. The Sb-Sb2O3 electrode impedances were found to be on the order of 0.15 cm2 for both fuel-cell and electrolyzer conditions, for compositions up to 30% Sb and 70% Sb2O3. The open circuit voltages (OCV) were 0.75 V, independent of oxygen composition. Some features of using molten Sb as an energy-storage medium are discussed. © 2012 The Electrochemical Society. [DOI: 10.1149/2.050204jes] All rights reserved. Manuscript submitted December 8, 2011; revised manuscript received January 6, 2012. Published January 24, 2012. Renewable energy sources, such as wind or solar energy, could help decrease our dependence on fossil fuels; but they produce intermittent power that may not be available when energy demand is highest. To take full advantage of these energy sources, as well as provide buffering for the electrical grid as a whole, the ability to store electrical energy at times when demand is low for later use when demand is high would be very desirable. Conventional ways to store energy are based primarily on compressed air or elevated water.1 With compressed air, off-peak power is used to run a compressor. At times of high demand, the compressed air can then be used to drive a turbine. In hydroelectric power plants, energy is stored by pumping water back to higher elevations. Unfortunately, both of these approaches require special geographic conditions, making them infeasible in many cases. One alternative for energy storage involves electrochemical devices, such as static Li-ion batteries,2 metal-air batteries,3 and flow batteries.4–6 With these devices, energy is stored in the form of chemical energy and electricity is generated later by reversing the chemical reaction. An intrinsic advantage of electrochemical energy storage is the direct conversion of chemical energy into electricity with few moving parts. Among the various types of flow batteries, those based on zinc or vanadium redox cycles have received the most attention.4, 5 Reversible fuel cells are also a type of flow battery. In the conventional use of reversible fuel cells, surplus electricity would be sent to the cell to electrolyze water to produce H2; electricity would be generated later by flowing the H2 back to the cell, which would then be operated in the fuel cell mode.7 The low density of gaseous H2 makes its storage challenging and typically requires that a significant amount of energy be used in compressing the H2. Several variations on the idea of reversible fuels cells have been proposed to reduce this storage problem. Because H2O and CO2 can be electrolyzed with equal efficiency in solid oxide fuel cells (SOFC),8–14 Zhan, et al.14 has suggested electrolyzing mixtures of H2O and CO2 to produce syngas, which could in turn be converted to methane, greatly reducing the volume of gas that needs to be stored. In another variation, H2 produced in the electrolyzer is used to reduce FeO to Fe.13 By passing steam over the Fe, H2 can be generated again for use in the fuel cell. In this paper, we propose a new type of flow battery based on solidoxide, O2−-conducting electrolytes and molten Sb as one electrode. Previous work from our laboratories has shown that it is possible operate a fuel cell with molten metals as the fuel, according to the following anode reaction.15–17 M + nO2− → MOn + 2ne− [1] ∗ Electrochemical Society Student Member. ∗∗ Electrochemical Society Active Member. z E-mail: gorte@seas.upenn.edu Cells using metals like Sn, In, or Pb exhibit a very high AreaSpecific Resistance (ASR) because the respective metal oxides that are formed in Reaction 1 are solids at a typical operating temperature of 973 K and form insulating layers over the electrolyte.15, 16 Further oxidation is then limited by oxygen transport through this layer and the low solubility of oxygen in the metals (e.g. 0.0233% in liquid Sn at 973 K18). However, the electrode impedance of an SOFC with molten Sb as the anode can be near zero, in part because Sb2O3 is also a liquid at this temperature.17 Because both Sb and Sb2O3 are liquids at typical SOFC operating conditions, it should be possible, in principle, to flow pure Sb from an external tank to the cell during fuel cell operation and flow mixtures of Sb and Sb2O3 to the cell during electrolyzer operation. In the present paper, we set out to determine whether electrolysis of Sb2O3 is possible and to measure the electrode impedances for Sb-Sb2O3 mixtures, information required for the design of a flow battery based on Sb and Sb2O3. What we will demonstrate is that the impedances for this system are indeed very low over a wide range of compositions, so that the overpotential losses in a reversible fuel cell based on molten Sb could be low. The high densities of Sb and Sb2O3 allow the storage volumes for a reversible fuel cell to also be low, making this a potentially attractive technology.

/pdf/energy-storage-in-electrochemical-cells-with-molten-sb-2ldtnf1djz.pdf

Energy Storage in Electrochemical Cells with Molten Sb Electrodes

The stability of direct carbon fuel cells with molten Sb and Sb–Bi alloy anodes

Molten silver as a direct carbon fuel cell anode

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