Conversion of Woody Biomass Materials by Chemical Looping

Article pubs.acs.org/IECR

Conversion of Woody Biomass Materials by Chemical Looping ProcessKinetics, Light Tar Cracking, and Moving Bed Reactor Behavior Siwei Luo, Ankita Majumder, Elena Chung, Dikai Xu, Samuel Bayham, Zhenchao Sun, Liang Zeng, and Liang-Shih Fan* William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University , Columbus, Ohio 43210, United States ABSTRACT: In recent years, chemical looping has evolved into a promising technique for carbonaceous fuel conversion. The chemical looping technology using biomass material as feedstock could provide a process that is environmentally attractive and sustainable. The focus of this study is to examine the kinetics of biomass decomposition and to evaluate the feasibility of using biomass as a renewable source for chemical looping processes. A series of thermogravimetric experiments were carried out under inert and reactive gas environments to investigate the kinetics of the devolatilization and gasification stages of biomass decomposition. Since tar derived from biomass pyrolysis is a major concern for many biomass conversion techniques, the cracking of biomass derived light tar using iron oxide-based composites was also investigated in a fixed bed reactor. Furthermore, a bench scale moving bed reactor was used to study the feasibility of biomass chemical looping in both cocurrent and countercurrent gas−solid contact modes.

1. INTRODUCTION Biomass conversion technologies have garnered more attention in recent years. The rationales of biomass utilization include long-term energy security, environmental benefits by recycling wastes and residues, reduction of greenhouse emissions, and economic development of rural areas.1 Wood and wood wastes are the most abundant biomass resource, comprising almost 64% of the energy that derives from biomass.2 A large number of thermochemical treatments, particularly pyrolysis and gasification, have been demonstrated to produce fuels and chemicals from biomass.3 However, conventional technologies such as direct combustion are inefficient due to the low heating value of the biomass feedstock. Even relatively new technologies such as gasification result in inconsistent fuel quality. Tar formed from biomass gasification can be a particularly problematic issue. Tars are generally considered to be the condensable organic compounds that are largely aromatic molecules, including benzene.4 Presence of tar in the system is highly undesirable because it will hinder the continuous running of the gasification system and will also hamper all of the downstream processes. As condensable tar is one of the main byproducts of biomass gasification, gas conditioning to remove tar is often a necessary step prior to any post-gasification processing units, such as the water-gas shift reactor. In traditional biomass conversion processes, tar can be removed either thermally or via steam and catalytic oxidative conversions. Among these approaches, catalytic cracking has been found to be the most effective method for tar removal.5 Numerous studies on in situ tar cracking utilize materials such as limestone, dolomites, zeolites, and metal oxides as bed additives in the gasifier.6 However, there are fundamental challenges associated with biomass pyrolysis for biofuels. For example, lignocellulosic biomass consists of structures varying over multiple orders of © 2013 American Chemical Society

magnitude on the length scale. Additionally, the structural complexities are compounded by the fact that biomass pyrolysis is multiphase in nature.7 As shown in recent studies, solid woody biomass pyrolysis is a process that involves the solid phase, liquid phase, and gaseous phase.8 These phase changes, along with the thermal reactions, make pyrolysis a very difficult process to model. Therefore, efficient biomass conversion strategies are highly desirable. In recent years, chemical looping has evolved into a promising technique for carbonaceous fuel conversion. It is regarded as one of the ultimate technologies for carbon sequestration in the U.S. Department of Energy’s innovative CO2 control technology roadmap. It is environmentally benign, economically favorable, and technically feasible.9 Chemical looping processes are projected to have higher energy conversion efficiency due to their minimized exergy.10−12 One of the main reasons chemical looping is feasible for biomass conversion is its flexibility with respect to fuel types. Extensive lab scale and subpilot scale studies have been conducted using coal, syngas, and natural gas as feedstocks.13 The limited work on biomass fueled chemical looping systems include studies done by Shen et al., Cao et al. and Acharya et al.14−16 Figure 1 illustrates a novel biomass chemical looping (BCL) system consisting of a moving bed reactor.17 In this chemical looping system of three sections, oxygen is transferred to the fuel by means of a metal oxide oxygen carrier, thus avoiding direct contact between the air and the fuel during combustion. The oxygen carriers are reduced by the biomass fuel in the reducer section. The reduced oxygen carriers are then partially oxidized by steam in the oxidizer Received: Revised: Accepted: Published: 14116

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process convert volatiles, tar and char to CO2 either directly or indirectly. In the presence of enhancing gases, like steam or CO2, char reacts with the enhancing gas to form CO and/or H2. The resulting gas would be further oxidized to CO2 and H2O by oxygen carriers. For the BCL system, the properties of the metal oxide particles used in the redox cycle play a key role in developing a successful process.23,24 The oxygen carrier designed for the BCL system is expected to undergo multiple redox reactions without loss of physical and chemical integrity. Early research in this field revealed pure metal oxides failed to function as optimum oxygen carriers due to degradation in their performance over multiple cycles. Primary metal oxides combined with supports are generally used as oxygen carriers to enhance their physical and chemical recyclability. The main objective of this work is to study several key aspects of biomass decomposition to be able to successfully integrate it into the chemical looping scheme. The decomposition kinetics of soft woody biomass is studied through thermogravimetric experiments. The destiny of light tar, with molecular weight between 78 and 200 and boiling point below 200 °C, is studied with multiple fixed bed experiments with and without oxygen carriers. Furthermore, since biomass conversion primarily occurs in the reducer, this study also includes the bench scale reducer tests using biomass. The feasibility of an efficient biomass chemical looping system was demonstrated by studying the biomass conversion and CO2 selectivity in both cocurrent and countercurrent bench scale moving bed experiments.

Figure 1. Biomass chemical looping gasification system with high purity hydrogen generation and in situ CO2 capture.

section to produce hydrogen and subsequently fully oxidized by air in the combustor section. In the BCL process, ideally all of the carbonaceous species, including volatile gas, tar, and char will be converted to CO2 and H2O in the reducer by the oxygen carriers. As illustrated in Table 1, the base biomass gasification plant is 30% efficient for

2. MATERIALS AND METHODS 2.1. Materials. 2.1.1. Biomass. The biomass used in this study is from BMQ Inc. It is a blend of softwood pines from the Midwest and Southern United States. Dried wood pieces were fed through a pellet mill to form uniform, pressed pellets. In this study, the biomass pellets were further milled for 10 min in a ball mill to obtain a powdered form with a particle size of less than 800 μm. The ultimate analysis for the biomass sample is shown in Table 2. 2.1.2. Oxygen Carrier. The oxygen carrier used in this study is iron-based composite metal oxide that has been extensively tested in the previous research of this group.24,25 These ironbased metal oxides have crucial properties required for successful large scale chemical looping operations. These factors are high oxygen-carrying capacity, good fuel and oxidant conversions, high reaction rates, long-term recyclability and durability, good mechanical strength, suitable heat capacity, high melting point, high resistance to carbon deposition and contaminants, minimal health and environmental impacts, low cost, and easy scalability of particle synthesis. 2.2. Experiment Procedure. 2.2.1. Biomass Decomposition Kinetics. A Setaram SETSYS Evolution Thermogravimetric Analyzer (TGA) was used to perform the experiments to study the decomposition kinetics. It has a maximum temperature limit of 1000 °C and a maximum heating rate of 100 K/min. The kinetics of the two stages of biomass decomposition, namely devolatilization and char gasification,

Table 1. Comparisons among Various Biomass Conversion Systems (Based on 50-MWth System)18,19 technology biomass conversion power efficiency (%HHV) CO2 capture rate (%) LCA CO2 emission (ton/ton biomass) cost of power (cent/kWh)

BCL with 100% CO2 capture

biomass IGCC

100% 38% 100% −1.36

100% 30% 0% 0.42

9.5

18

electricity generation without CO2 capture, whereas BCL is 40% efficient with 100% CO2 capture. For the case of hydrogen cogeneration, BCL is 69% efficient.18 The self-sustainability and cost effectiveness of a BCL plant make this technology very attractive for rural applications.19 Also biomass is considered as a carbon neutral renewable energy resource. By sequestrating CO2, BCL would also be carbon negative from the Life Cycle Analysis (LCA) standpoint. Therefore, BCL provides a promising strategy to convert biomass to electricity and hydrogen.20−22 Biomass conversion occurs in several sequential steps. First, devolatilization of the biomass releases the volatiles and tar, leaving behind just char and ash. Oxygen carriers in the BCL

Table 2. Ultimate Analysis of the Biomass Sample Used for This Study wt % (as received)

wt %, dry basis

residual moisture

ash

carbon

hydrogen

nitrogen

sulfur

chlorine

oxygen by difference

5.47

1.07

51.33

6.27

120 amu) were formed. These experimental results were consistent with the tar maturation scheme proposed by Evans et al.33 A generally accepted mechanism for aromatics formation is the free radical mechanism among olefins. Dehydration and decarbonylation reactions are also possible mechanisms resulting in the transformations observed.34 The mass spectra of the gaseous product from the fixed-bed reactor experiments at 900 °C for the two cases, with and without oxygen carriers, are shown in Figure 5(b). Although the number of light tar components was less than that in the slow pyrolysis experiment due to the thermal cracking, the conversion of light tar was not very high. Significant amounts of light tar still remained unconverted. Comparison of the two mass spectra showed a dramatic difference in the variety and amounts of chemical species present during the two experiments. For the experiment involving oxygen carriers, the effluent from the fixed bed consisted mainly of CO2. The effluent from the other experiment, without oxygen carriers,

Figure 4. Plots for calculating the kinetic parameters of char gasification. (a) DTG curves for isothermal gasification of char under CO2 atmosphere. (b) Arrhenius plot for isothermal gasification of char under CO2 atmosphere. (c) Arrhenius plots for rate constants of devolatilization and gasification reactions under CO2 atmosphere. 14121

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Figure 5. Mass-spectrometer data from the fixed bed experiments (a) At various temperatures to study slow pyrolysis of biomass derived light tar. (b) With and without iron oxide based oxygen carriers to study light tar cracking by oxygen carriers at 900 °C.

Table 4. Outlet Gas Composition from the Moving Bed Reactor as Measured by The GC test

H2

N2

CH4

CO

CO2

biomass conversion

CO2 selectivity

cocurrent countercurrent

0.14 0.09

40.29 14.71

0 2.11

0.25 14.94

59.31 67.15

∼100% ∼100%

∼100% ∼74%

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consisted mainly of CO accompanied by a variety of other compounds as seen from the corresponding spectrum. From the MS data, it can be concluded that the oxygen carriers were effective in cracking and oxidizing the biomass derived light tar. The oxygen carriers converted the light tar to lighter hydrocarbons, where CO2 was the major component of the product gases. 3.3. Biomass Conversion in Moving Bed Reactor. Table 4 shows the gas composition as measured by the Micro-GC in the moving bed reactor gas outlet. As a result of the short contact time between the oxygen carriers and biomass, the countercurrent moving bed mode produced a gas mixture consisting of CO, CH4, H2, and CO2. In addition, a little amount of condensed tar was also observed in the ventilation line. It is reasonable to assume that biomass decomposed into volatiles, tar, and char at the upper section. Subsequently, most of the volatiles and tar were converted to CH4 and CO by the oxygen carrier. However, they cannot be further oxidized to CO2 due to gas channeling or the short residence time. Char, left after biomass devolatilization, moved downward in the moving bed reactor. An external CO2 stream was introduced from the bottom of the reactor as an enhancing gas to accelerate the slow solid−solid reaction. The solid biomass char reacted with CO2 to produce CO according to the reverse Boudouard reaction. This CO was subsequently oxidized by oxygen carrier to CO2. Almost all of the carbon was converted to gaseous specious of CH4, CO, and CO2, based on the mass balance. In the cocurrent mode, since the volatiles and tar have a sufficient residence time, minimal concentrations of CO, CH4, H2, and condensed tar were observed at the gas outlet. Also, in this mode, there was no need for an external source of CO2 as an enhancing gas since the CO2 produced by volatile oxidation was sufficient to gasify char. The residence time was much longer for tar to achieve full conversion, and the oxygen carriers were able to successfully crack the biomass derived tar into CO2. However, it should be noted that the oxygen carrier to biomass mass ratio (36) must be much higher than that used in the countercurrent mode (21) to guarantee the high CO2 selectivity due to thermodynamic constraints. In both countercurrent and concurrent moving bed tests, solids were sampled from the bottom of the reactor for a carbon analyzer (CO2 Coulometer, UIC, Inc.) test. It was confirmed that there was no carbon in the solid samples, which was consistent with the conclusion that all the carbon in biomass exited from the reactor in gaseous form. In a large scale demonstration unit, a countercurrent moving bed with biomass injection in the middle section is a more favorable design. In the countercurrent moving bed configuration, CO2 selectivity is high and the oxygen carrier to biomass ratio is low. Oxygen carrier particles are fed from the top of the reducer, while biomass is pneumatically conveyed to the middle section of the reactor using CO2. A small amount of CO2, recycled from the reducer gas outlet, can also be introduced at the bottom of the reducer to enhance char conversion. The biomass injection port divides the reducer into two sections, as shown in Figure 6. Biomass will be devolatilized in the pneumatic injection zone, resulting in volatiles, tar, and char. The function of the upper section (Stage I) is to ensure full conversion of gaseous species, including volatiles and tar, to CO2 and H2O, whereas the lower section (Stage II) is used to maximize the char conversions. Volatiles and tar will move upward and be oxidized by oxygen carriers that enter from the top of the reducer. The countercurrent

Figure 6. Moving bed reducer schematic for large scale demonstration unit.

interaction between the biogas and iron oxide particles ensures the complete conversion of gaseous fuels. The mixing of the devolatilized biomass char and partially reduced iron oxide particles continues to occur as particles descend from Stage I to Stage II. In Stage II, the biomass char is progressively gasified by the CO2 formed at the lower portion of the reducer. Provided that an adequate residence time is given, biomass char can be fully converted. Further, the Fe2O3 particles can be reduced to a mixture of metallic Fe and FeO. Biomass ash will exit from the bottom of the reducer along with the reduced particles. Thus, reaction product streams from the reducer include a solid particles stream that exits from the bottom of the reducer containing Fe, FeO, and ash.

4. CONCLUSIONS This study demonstrated that biomass chemical looping technology can be an effective renewable energy conversion strategy. Iron oxides based oxygen carriers can successfully crack light tar from woody biomass. The Ea and ko values are ∼155 kJ/mol and ∼1 × 1011 s−1, respectively, for woody biomass devolatilization. The Ea and ko values are 113 kJ/mol and 1 × 104 s−1, respectively, for woody biomass char gasification. Almost full conversions of biomass to gaseous species were achieved in a bench scale moving bed reactor. A desirable countercurrent moving bed reactor with biomass injection in the middle section is proposed for larger scale chemical looping demonstrations.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 614-688-3262. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 14123

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial assistance provided by the United States National Science Foundation (Project No. CBET-1236467). The authors would also like to acknowledge Andrew Tong and Omar McGiveron for helpful discussions.

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