Thermal Degradation and Conversion of Plant Biomass into High

Chapter 8

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Thermal Degradation and Conversion of Plant Biomass into High Value Carbon Products Xinfeng Xie*,1 and Barry Goodell2 1Division

of Forestry & Natural Resources, West Virginia University, Morgantown, West Virginia 26506 2Department of Sustainable Biomaterials, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 *E-mail: [email protected].

Plant biomass materials are thermally degradable due to their polymeric nature. Treatment of plant biomass at high temperatures removes all carbohydrates with the resulting carbon-rich material highly durable and is not subject to biological degradation due to the lack of nutrient sources for microorganism and insects. Considerable interest has developed in recent years in the use of plant biomass as an inexpensive and renewable feedstock to produce advanced carbon materials for engineering and energy applications. Plant biomass derived carbon has shown great potential for production of carbon-polymer composites, carbon-carbon composites, carbide ceramics, and carbon fiber. Both non-graphitic and graphitic nanostructures have been produced from plant biomass, and they are promising alternatives to petroleum-based carbon nanomaterials. New studies have improved our understanding on the evolution of the carbon structure in carbonized plant materials. Recent studies demonstrated the possibility to produce carbon nanotubes (CNTs) and mesoporous carbon with regularly arranged channels directly from plant biomass materials.

© 2014 American Chemical Society In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Thermal Decomposition of Plant Biomass in Inert Atmosphere


Plant biomass materials, such as wood, decompose on heating due to their polymeric nature. There are four basic types of products generated during a thermal degradation process under ambient pressure (1): 1) Non-condensable gases, including carbon monoxide, carbon dioxide, hydrogen and methane. They are produced at temperatures between 200°C and 450°C with a maximum production at about 350°C to 400°C. 2) Condensable pyroligneous material, containing about 50% moisture. The production rate of pyroligneous material reaches a maximum from 250°C to 300°C and ceases at about 350°C. 3) Tar with no moisture; formed from 300°C to 450°C. 4) Solid carbon-rich residue.

One previous study (2) indicated that only gases were produced as wood was pyrolyzed up to 170°C. At these temperatures, except during long-term heat treatment, and other than the products of wood decomposition, water vapor predominates among the gases. Baileys et al. (3) found that extreme mass loss during wood thermal degradation occurs between 300°C and 350°C due to the rapid decomposition of cellulose. When higher temperatures are used, the carbon content of the solid residue increases further while the hydrogen and oxygen content decreases (4). The process of increasing the carbon content of an organic polymer material by pyrolysis is also called carbonization. The thermal decomposition of wood is a superpositioning of the thermal degradation of its three major polymer components, i.e. cellulose, lignin and hemicelluloses (1, 5). Zeriouh and Belkbir (6) investigated the thermal decomposition of a Moroccan wood under a nitrogen atmosphere and found that decomposition of hemicellulose, cellulose and lignin occurred discretely during wood thermal degradation. A study by Beall (7) indicated that hemicelluloses were thermally the least stable wood component. Their decomposition is almost completed before cellulose starts to decompose. The thermal stability of lignin is greater than hemicellulose and less than that of cellulose. The decomposition of lignin starts at about 200°C and does not complete until about 650°C (8). Studies by Shafzadeh (9) indicated that cellulose decomposes upon heating via two pathways. The first pathway dominates at temperatures below 300°C and it involves reduction in the degree of polymerization (DP). The major decomposition products of this pathway are CO, CO2, H2O, and solid carbon-rich residue. The second pathway, which dominates at temperatures greater than 300°C, involves cleavage of molecules and disproportionation reactions to produce a mixture of anhydro tar sugars and low molecular weight volatiles. Intensive oxidation of the solid carbonaceous residue gives glowing combustion, while violent oxidation of the combustible volatiles gives flaming combustion. The decomposition of the polymers in plant biomass is generally complete at about 800°C, because there is no significant mass loss after the material is carbonized at a temperature greater than 800°C (10). 148 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


The ability to obtain higher carbon yields by chemically modifying lignocellulosic precursors has been studied, initially with the intention of suppressing the flammability of the materials. In the 1960’s, cellulose fiber was carbonized in an hydrogen chloride atmosphere, resulting in a carbon yield 14% greater than that would obtained when nitrogen was used (11). It is well known that alkali and alkaline earth metals, including potassium, sodium, calcium, and magnesium, are strong catalysts for decomposition of lignocellulosic polymers. It has been reported that these metals can increase the carbon yield in pyrolysis of biomass materials at temperatures lower than 500°C (12, 13). In addition, phosphorus and boron containing compounds also can catalyze the formation of solid carbon during pyrolysis of lignocellulosic materials by promoting the dehydration reactions in the materials at temperatures lower than 300°C (14, 15). Because of their ability to increase solid carbon yield and reduce the production of flammable volatiles at relatively low temperatures, many phosphorus and boron compounds have been extensively used as fire retardant chemicals for lignocelluloses-based materials and composites.

Evolution of the Carbon Structure during Carbonization of Plant Biomass Materials The properties of a material are generally determined by its structure. In the development of advanced carbon materials from plant biomass, it is of great importance to understanding the evolution of the carbon structure during the carbonization process. There are two critical factors governing the carbonization process and the material properties: carbonization temperature and heating speed.

Table 1. Mass Yield at Different Carbonization Temperatures Carbonization temperature (°C)

Slow heating rate 3°C / hour

Fast heating rate 60°C / hour

600

31.26% (0.0039)

29.71% (0.0023)

800

30.61% (0.0029)

28.18% (0.0028)

1000

30.44% (0.0033)

27.93% (0.0017)

Data are from reference (10). Values in the brackets are standard deviation.

A previous study (10) on carbonization of solid wood indicated that slower heating rates lead to significantly higher mass yield when the material is carbonized to the same temperature (Table 1). Carbonized wood exhibited an anisotropic shrinkage behavior when the processing temperature was higher than 600°C, with the least shrinkage in the longitudinal direction and the greatest the tangential direction. The difference was more prominent when a slower heating rate was used (Table 2). 149 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Carbonization temperature (°C)

Slow heating rate 3°C / hour

Fast heating rate 60°C / hour

Longitudinal

Tangential

Longitudinal

Tangential

600

17.7% (0.0009)

36.1% (0.0067)

17.5% (0.0027)

33.9% (0.0041)

800

20.4% (0.0021)

37.8% (0.0029)

21.4% (0.0039)

37.3% (0.0116)

1000

20.9% (0.0017)

38.4% (0.0037)

21.7% (0.0006)

37.9% (0.0024)


150


Table 2. Shrinkage at Different Carbonization Temperatures

In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


At atomic level, carbonized plant biomass has a turbostratic crystallite carbon structure, which is non-graphitic and exhibits 2-dimensional peaks in x-ray diffraction profiles. Plant biomass materials undergoing carbonization between 300°C and 1000°C are characterized by a continuous increase in the amount of turbostratic crystallites and a continuous growth in the dimension of graphene sheets, while the number of graphene layers in the turbostratic crystallites does not significantly change (10, 16, 17). The graphene sheets in the turbostratic crystallites are preferentially oriented parallel to the longitudinal direction of plant biomass cells (10, 16, 18, 19) with a certain angle to the circumference of the cell walls in the cross-sectional plane of the plant cell (10). The orientation of the graphene sheets was believed to be attributed to the original orientation of cellulose microfibrils in the plant cell wall structure. However, recent studies have shown that the phenyl-propane units of lignin follow the cellulose microfibril arrangement, and also have a preferred orientation along the fiber axis (20–22). Given that lignin generates a higher carbon yield compared to cellulose, the preferred orientation of lignin may also contribute to the orientation of the graphene sheets. The preferred orientation of the turbostratic crystallites primarily contributes to the anisotropic properties of carbonized plant biomass materials.

Figure 1. ER of samples prepared at different carbonization temperatures. SHR: slow heating rate at 3°C per hour; FHR: fast heating rate at 60°C per hour. Each point is an average of 24 measurements of 3 samples, error bar: standard deviation. Data are from reference (10). 151 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Carbonized plant biomass can be considered to be a two-phase system including disordered carbon and turbostratic crystallites with large graphene layers. Turbostratic crystallites possess higher electric conductivity and mechanic strength compared to the disordered carbon. The electric resistivity (ER) of carbonized wood along the fiber longitudinal direction decreases 3-4 levels of magnitude when carbonization temperatures increase from 600°C to 800°C (Figure 1). This dramatic decrease in electric resistivity is closely related to a significant increase in the size of graphene layers in the same temperature range (10, 23). Both high carbonization temperature and slow heating rate can promote the formation and growth of graphene sheets in the turbostratic crystallites (10). Therefore, important properties, such as ER and Young’s modulus, which are developed at high carbonization temperatures, can also be obtained at lower temperatures if a slow heating rate is used (Figure 1 and Table 3).

Table 3. Young’s Modulus of Samples Prepared at Different Temperatures Carbonization temperature (°C)

Slow Heating Rate 3°C / Hour (Gpa)

Fast Heating Rate 60°C / Hour (Gpa)

600

8.74 (0.4022)

6.67 (0.3385)

800

14.45 (1.0448)

12.49 (1.1699)

1000

19.66 (0.5630)

15.34 (0.2072)


Large crack-free monolithic carbon blocks have been produced from thick solid wood and medium density fiberboard (MDF) using slow heating rates, which reduces shrinkage stresses associated with the decomposition differential between the exterior and interior parts of the material. Thermally conductive materials, such as graphite powder, sand or granular silica, and graphite plates, usually were used to surround the wood during the heating process to reduce uneven heating of the samples. The carbon obtained has excellent machinability, high reactivity, and outstanding dimensional stability, making the material an excellent net-shape preform for producing carbide ceramics, carbon-polymer composites, and carbon-carbon composites. Figure 2 shows a sample of 1 inch thick crack-free, carbonized oak (the original thickness was 1.5 inches), two pieces of carbonized MDF-polymer composites, a cylinder-shaped wood carbon, and three pieces of carbon/carbon composite made from phenolic resin infused carbonized MDF. Because MDF is more homogeneous compared to solid wood at the millimeter scale, large deformation-free monolithic carbon panels were produced from MDF (Figure 3) using heating rates slightly faster than those for carbonization of solid wood. 152 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 2. From back to front: 1) a large monolithic carbon block (1 inch think) produced from oak using slow heating rates; 2) carbonized MDF-polymer composite; 3) carbonized solid wood machined into a cylinder; 4) carbonized MDF based carbon/carbon composite.

Figure 3. Large monolithic carbon panels produced from MDF using slow heating rates. 153 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Thermal Conversion of Plant Biomass into Carbon Nanostructures Carbon nanostructures including graphene, nanotubes, nanofibers, nanohorns, nanocapsulates, nanocages, and ordered meso- and microporous carbon have drawn much attention recently because of their great potential for applications in electrochemical energy storage systems, such as rechargeable batteries and supercapacitors, and in energy conversion systems, such as solar cells and artificial leaf devices (24). Plant biomass provides a renewable, abundant and inexpensive carbon source for the production of carbon nanostructures. Certain types of biomass also provide a promising alternative to petroleum-based carbon precursors. Although previous studies have shown some promising results in transferring plant biomass carbon into high performance carbon nanostructures, much fundamental research and technical development is needed before large scale production and applications of biomass-derived carbon nanostructures. The carbon from plant biomass is non-graphitizable, which means that it cannot be transformed from non-graphitic into graphitic carbon solely by heating the material to 3000°C at atmospheric or lower pressure (25). However, most of the carbon nanostructures aforementioned are in graphitic carbon forms. In order to promote the transformation of plant biomass carbon from non-graphitic to graphitic at relatively low carbonization temperatures ranging up to 1300°C, catalyzed heat treatments using transition metals, such as nickel and iron, have been developed (26–28). Some aluminum compounds were also found to be effective in the catalytic graphitization of wood charcoal as well, but at a much higher temperature of 2200°C (29). The use of other catalysts, including cobalt and copper in the catalytic graphitization of plant biomass has not been reported, although they were found effective in graphitizing non-graphitic carbons from synthetic polymers. There are two widely accepted mechanisms for catalyzing the graphitization of non-graphitic carbon (30, 31): 1) dissolving of non-graphitic carbon into metals or metallic compounds, and subsequent precipitation of graphitic carbon; and 2) formation and subsequent decomposition of intermediate carbide into metal and graphite. The difference in free energy between non-graphitic carbon and graphitic carbon is the driving force behind the mechanisms (32). During catalytic graphitization only the non-graphitic carbon in contact with the metal catalyst will be converted into graphitic carbon. Therefore the graphitization is rather localized. Catalytic graphitization of solid carbons derived from lignocellulosic biomass produces an inhomogeneous material with localized regions of graphitic carbon surrounded by non-graphitic carbon. However, the range of order of catalytically graphitized lignocellulosic materials at the nanometer level can be as good as traditional petroleum-based graphite. One study (28) reported that the ordering of the graphitic carbon was comparable to that of the pitch-derived graphite when samples of hardwood species were soaked in nickel nitrate solution under vacuum for 120 hours and then treated at 1600°C for 6 hours. 154 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Using catalytic techniques, graphitic carbon nanostructures have been fabricated from lignocellulosic biomass. The process generally includes five important steps (26, 27): 1) oven drying of lignocellulosic materials; 2) impregnation of the dried materials with a solution of metallic salt catalyst; 3) heat treatment of the catalyst-loaded material to enable catalytic graphitization; 4) removal of the metal catalyst using acids; and 5) removal of the non-graphitic carbon by oxidation.

Figure 4. Homogeneous morphology of carbonized plant material without pretreating at 250°C.

In addition to catalytic graphitization, non-catalytic technologies have been developed as well to produce carbon nanostructures from lignocellulosic materials. Traditional production of charcoal involves heating woody biomass continuously to high temperatures with limited or no oxygen supply. The carbon produced retains the structures of the original biomass material at cellular or fiber level, but within the carbonized cell wall, at nanometer level, the material is homogeneous (Figure 4). New studies have demonstrated that the original arrangement of the cellulose microfibril and lignin-containing matrix can be retained when a step-wise oxidative carbonization process at defined temperatures is employed (33, 34). A typical step-wise oxidation process includes treating plant materials in air at about 250°C followed by oxidation at temperatures higher than 400°C. Carbon nanotubes (CNTs) (33) and mesoporous carbon with nanochannels (Figure 5) have been observed in carbonized plant materials produced using step-wise oxidation processes. 155 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 5. Nanochannels in carbonized plant fibers prepared using step-wise oxidative carbonization method. It is believed that the production of CNTs and nanochannels within the plant cell wall is attributed to preferential ablation of cellulose microfibrils within the lignin-containing matrix of the intact secondary plant cell walls (33). It is hypothesized that the ablation of cellulose microfibrils results in the formation of nanochannels in the carbonized plant cell wall, while the nanochannels, formed from carbonized lignin residues, may act as a template that facilitates the formation of CNTs from the volatilized cellulose carbon gases.

Table 4. Apparent Kinetic Parameters of Cellulose Carbon and Lignin Carbon Carbonization temperature

Cellulose Carbon

Lignin Carbon

n

E (kJ mol-1)

n

E (kJ mol-1)

400°C

1.05

89.8

0.65

98.4

500°C

0. 75

101.2

0.50

109.7

700°C

0. 65

143.4

0.55

141.1

1000°C

0.55

167.3

0.50

165.8

Data are from reference (35).

A study (35) focused on a comparison of the oxidation behavior of cellulose and lignin carbons prepared at different temperatures reported the discovery that cellulose carbon had a higher reaction order (n) and lower activation energy (E) 156 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

than lignin carbon when oxidized in air if they were prepared under identical conditions, and at temperatures lower than 500°C. The difference in oxidation decreased dramatically when the initial carbonization temperature was at 700°C or greater (Table 4).


Table 5. Pore Volume and Surface Area of Cellulose Carbon and Lignin Carbon Carbonization temperature

Total pore volume (cm3 g-1)

BET surface area (m2 g-1)

Cellulose carbon

Lignin carbon

Cellulose carbon

Lignin carbon

400°C

0.0092

0.0011

2.602

1.321

500°C

0.0108

0.0087

5.244

0.846

700°C

0.2238

0.2221

437.63

448.10

1000°C

0.2256

0.2130

449.06

432.33

Data are from reference (35).

Experimental data from the Fourier transform infrared (FTIR) absorption studies verified that the oxidative differences observed in the cellulose carbon and lignin carbon were influenced primarily by the chemical structure of the carbonized materials (35). Cellulose carbon contained more paraffinic carbon structures than lignin carbon when these carbons were formed at lower temperatures. However, the chemical structures were similar from the perspective of carbonization, when higher temperature carbons were compared. The results from nitrogen adsorption at 77 K comparing the pore volume and Brunauer-Emmett-Teller (BET) surface area of both materials indicated that the surface and porosity properties played only a minor role in the oxidation of cellulose carbon and lignin carbon (Table 5).

References 1.

2. 3. 4. 5. 6. 7. 8.

Beall, F. C.; Eickner, H. W.; Forest Products Lab. Thermal degradation of wood components: a review of the literature; U.S.D.A. Forest Service: Madison, WI, 1970; pp 2–3. Beall, F. C. Wood Sci. 1972, 5, 102–108. Baileys, R. T.; Blankenhorn, P. R. Wood Sci. 1982, 15, 19–28. Setton, R. In Carbon Molecules and Materials, 1st ed.; Setton, R, Bernier, P., Lefrant, S., Ed.; Taylor & Francis Inc.: New York, 2002; pp 1–50. Beall, F. C. Wood Sci. Technol. 1971, 5, 159–175. Zeriouh, A.; Belkbir, L. Thermochim. Acta 1995, 258, 243–248. Beall, F. C. Wood Fiber 1969, 1, 215–226. Fierro, V.; Torné-Fernández, V.; Montané, D.; Celzard, A. Themochim. Acta 2005, 433, 142–148. 157 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

9. 10. 11.


12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

Shafizadeh, F. In The Chemistry of Solid Wood; Rowell, R., Ed.; American Chemical Society: Washington, DC, 1984; pp 489–529. Xie, X.; Goodell, B.; Qian, Y.; Peterson, M.; Jellison, J. Holzforschung 2008, 62, 591–596. Shindo, A.; Nkanishi, Y.; Soma, I. In High Temperature Resistant Fibers from Organic Polymers: American Chemical Society Symposium; Preston, J., Ed.; Applied Polymer Symposia; Interscience Publishers: New York, 1969; Vol. 9, pp 271–284. Raveendran, K.; Ganesh, A.; Khilar, K. C. Fuel 1995, 74, 1812–1822. Wang, Z.; Wang, F.; Cao, J.; Wang, J. Fuel Process. Technol. 2010, 91, 942–950. Grexa, O.; Horvathova, E.; Besinova, O.; Lehocky, P. Polym. Degrad. Stab. 1999, 64, 529–533. Wang, Q.; Li, J.; Winandy, J. E. Wood Sci. Technol. 2004, 38, 375–389. Paris, O.; Zollfrank, C. Carbon 2005, 43, 53–66. Kercher, A. K.; Nagle, D. C. Carbon 2003, 41, 15–27. Byrne, C. E.; Nagle, D. C. Carbon 1997, 35, 267–273. Greil, P.; Lifka, T.; Kaindl, A. J. Eur. Ceram. Soc. 1998, 18, 1961–1873. Fromm, J.; Rockel, B.; Lautner, S.; Windeisen, E.; Wanner, G. J. Struct. Biol. 2003, 143, 77–84. Åkerholm, M.; Salmén, L. Holzforschung 2003, 57, 459–465. Deng, Y.; Feng, X.; Yang, D.; Yi, C.; Qiu, X. BioResources 2012, 7, 1145–1156. Nishimiya, K.; Hata, T.; Imamura, Y.; Ishihara, S. J. Wood. Sci. 1998, 44, 56–61. Su, D. S.; Centi, G. J. Energy Chem. 2013, 22, 151–173. Edwards, I. A. S. In Introduction to Carbon Science; Marsh, H., Ed.; Butterworth & Co. (publishers) Ltd: London, 1989; pp 1–36. Kodama, Y.; Sato, K.; Suzuki, K.; Saito, Y.; Suzuki, T.; Konno, T. Carbon 2012, 50, 3486–3496. Sevilla, M.; Sanchís, C.; Valdés-Solís, T.; Morallón, E.; Fuertes, A. B. J. Phys. Chem. C 2007, 111, 9749–9756. Johnson, M. T.; Faber, K. T. J. Mater. Res. 2011, 26, 18–25. Bronsveld, P.; Hata, T.; Vystavel, T.; DeHosson, J.; Kikuchi, H.; Nishimiya, K.; Imamura, Y. J. Eur. Ceram. Soc. 2006, 26, 719–723. Marsh, H.; Warburton, A. P. J. Appl. Chem. 1970, 20, 133–142. Ōya, A.; Marsh, H. J. Mater. Sci. 1982, 17, 309–322. Fitzer, E.; Kegel, B. Carbon 1968, 6, 433–436. Goodell, B.; Xie, X.; Qian, Y.; Daniel, G.; Peterson, M.; Jellison, J. J. Nanosci. Nanotechnol. 2008, 8, 2472–2474. Xie, X.; Goodell, B.; Qian, Y.; Daniel, G.; Zhang, D.; Nagle, D.; Peterson, M.; Jellison, J. For. Prod. J. 2009, 59, 26–28. Xie, X.; Goodell, B.; Zhang, D.; Nagle, D. C.; Qian, Y.; Peterson, M.; Jellison, J. Bioresour. Technol. 2009, 100, 1797–1802.

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Thermal Degradation and Conversion of Plant Biomass into High

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