Do All Carbonized Charcoals Have the Same Chemical Structure? 1

Jun 26, 2007 - A half century ago, Rosalind Franklin identified two distinct families of organic ... even after heating to 3000 °C.” Franklin concl...
1 downloads 0 Views 557KB Size
Download PDF
Ind. Eng. Chem. Res. 2007, 46, 5943-5953

5943

Do All Carbonized Charcoals Have the Same Chemical Structure? 1. Implications of Thermogravimetry-Mass Spectrometry Measurements Erika Me´ sza´ ros,† Emma Jakab,† Ga´ bor Va´ rhegyi,† Jared Bourke,‡ Merilyn Manley-Harris,‡ Teppei Nunoura,§ and Michael Jerry Antal, Jr.*,§ Chemical Research Center, Institute of Materials and EnVironmental Chemistry, Hungarian Academy of Sciences, P.O. Box 17, Budapest, Hungary, The Department of Chemistry, UniVersity of Waikato, PriVate Bag 3105, Hamilton, New Zealand, and The Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822

A half century ago, Rosalind Franklin identified two distinct families of organic materials: those that become graphitic during carbonization at high temperatures and those that do not. According to Franklin, sucrosederived biocarbons showed “no trace of homogeneous development of the true graphitic structure, even after heating to 3000 °C” [Proc. R. Soc. A 1951, 209, 196-218]. Franklin concluded that “non-graphitizing” carbons (e.g., sucrose biocarbons) are typically formed from oxygen-rich or hydrogen-poor substances that develop a “strong system of cross-linking, which immobilizes the structure and unites the crystallites in a rigid mass”. In this work, we show that there is a spectrum of non-graphitizing biocarbons ranging from those that release little CO during carbonization at temperatures approaching 1000 °C to those that strongly and persistently emit CO during carbonization at temperatures approaching 1000 °C. Typically, very low-ash biocarbons are not persistent CO emitters, but biocarbons with moderate ash contents can also be a member of this class if their ash lacks the catalytic species K, P, Mg, and/or Na that appear to be responsible for persistent CO evolution at 1000 °C. Introduction A half century ago, Rosalind Franklin1 identified two distinct families of organic materials: those that become graphitic during carbonization at high temperatures and those that do not. Her key finding resulted from X-ray diffraction (XRD) studies of biocarbons prepared from “sugar” (presumably sucrose) at heat treatment temperatures (HTT) of 1000, 2160, 2700, and 3000 °C. These sucrose-derived biocarbons showed “no trace of homogeneous development of the true graphitic structure, even after heating to 3000 °C.” Franklin concluded that “nongraphitizing” carbons (e.g., sucrose biocarbons) are typically formed from oxygen-rich or hydrogen-poor substances that develop a “strong system of cross-linking, which immobilizes the structure and unites the crystallites in a rigid mass. The resulting carbons are hard, and their fine-structure porosity is large and is preserved at high temperatures.”1 Subsequent research confirmed Franklin’s finding that organic materials having high net hydrogen content are prone to graphitization and vice versa.2-4 Also, subsequent research revealed the catalytic influence of mineral matter (e.g., clay minerals such as Al, Si, K, and Fe found in ash) on the graphitization behavior of various coals.2,4-6 This paper concerns the properties of charcoals and carbonized charcoals derived from biomass. Biomass is primarily composed of oxygen-rich carbohydrates (as well as lignin and small amounts of extractives). As a result of Franklin’s seminal work, we know that the carbohydrate sucrose is non-graphitizing. In what follows, we refer to a “family” of biocarbons as those charcoals and carbonized charcoals that are derived from a single biomass substance (e.g., sucrose, abbreviated as Suc) and subjected to various HTT. Thus, Suc1000 represents a * To whom correspondence should be addressed. E-mail: [email protected]. † Hungarian Academy of Sciences. ‡ University of Waikato. § University of Hawaii at Manoa.

member of the sucrose family of biocarbons that was carbonized at an HTT of 1000 °C. Consider now the family of biocarbons prepared by pyrolyzing glucose at increasing HTT. Recall that the non-graphitizing sugar sucrose is a dimer of the hexoses glucose and fructose; consequently, we presume that charcoals derived from glucose and fructose are non-graphitizing. In light of Franklin’s conclusions (above), it seems reasonable to hypothesize that other hexoses and pentoses (e.g., xylose, the principal component of biomass xylan) are also non-graphitizing. Because crystalline glucose is composed of pyranose rings, we might expect its charcoals to contain condensed 6-member rings (e.g., pyranones7) and their fragments.8 Alternatively, consider the family of biocarbons derived from the pyrolysis of Kraft lignin (KL) at increasing HTT. Kraft lignin is largely composed of phenyl-propane units; consequently, the family of Kraft lignin biocarbons should be composed primarily of aromatic ring structures and their fragments. Are these biocarbons non-graphitizing? Does the chemical structure of the biocarbons formed from Kraft lignin at increasing HTT (i.e., KL600, KL1000, ..., KL2160, ...) approach that of sucrose biocarbons formed at increasing HTT? Finally, consider the family of biocarbons derived from corncob (CC). CC500 has a volatile matter (VM) content of about 20%; whereas, CC600 has a VM content of about 13%, and CC900 has a VM content of about 2%. CC2700 and CC3000 are virtually pure carbons (except for some remaining ash). Recall that corncob is 26.3% cellulose (a polymer of glucose), 25.2% hemicellulose (largely polymers of pentoses), 16.3% lignin, and 3.48% ash.9 Because corncob is primarily composed of carbohydrate polymers, we presume that the CC biocarbons are non-graphitizing. Nevertheless, prior work suggests that the ash content of the corncob might catalyze the graphitization of its charcoal.2,4-6 Obviously the chemical structure of all these biocarbons evolves as they are heated to higher temperatures. Our principal

10.1021/ie0615842 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007

5944

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007

interest is the structure of biocarbons with low VM content (i.e., less than 10%). To emphasize the low VM content of these materials, we often refer to them as “carbons”. Usually these carbons are produced by a high-temperature heat treatment (i.e., a “carbonization”) of a parent charcoal and may be called a “carbonized charcoal”. Occasionally, the charcoal is produced at such a high temperature that it possesses little VM. Because this charcoal has a low VM content, we also refer to it as a “carbon”. We pose the following question: “Do biocarbonss derived from different biomass precursors (e.g., sucrose, glucose, lignin, corncob)smerge in structure as they are carbonized at higher and higher temperatures? Or do carbonized charcoals retain some memory of the structure of their biomass parent?” In other words, “Do carbonized charcoals inherit any properties from their parent substrate, or are they all the same?” Although thermogravimetry-mass spectrometry (TG-MS) does not provide direct measurements of the chemical structure of the solid residue, TG-MS measurements can provide insights into its structure. In this paper, we use TG-MS data obtained from a wide variety of biomass charcoals (typically formed at temperatures below 800 °C) to gain insights into the evolution of their chemical structures as they are carbonized at increasing temperatures. First, we examine the behavior of these charcoals over a wide range of HTT. Then, we focus on their behavior at high temperatures (typically above 800 °C) wherein the solid residue is a nearly pure carbon. Apparatus and Experimental Procedures Charcoal Preparation. Charcoals were prepared from the following biomass feedstocks: the simple monomeric and dimeric sugars glucose, fructose, and sucrose, the small polysaccharide inulin, the lignocellulose corncob and demineralized corncob, oak wood, and Kraft lignin. According to the supplier (Alfa Aesar), the glucose, fructose, and inulin are 98-99 wt % pure. We estimate the sucrose to be 99.95 wt % pure. Note that crystalline fructose is composed of pyranose rings, distinct from fructose in the dimer sucrose that is in its furanose form. Glucose is also a crystalline solid at room temperature that is composed of pyranose rings. As mentioned earlier, the hexoses glucose and fructose melt before they carbonize. The melt phase permits the isomerization of gluco- and fructopyranose to gluco- and fructofuranose (respectively) via oxonium ion intermediates.10-12 In water, the furanose structure is thermodynamically favored at higher temperatures;13,14 consequently, we might anticipate that biocarbons derived from glucose and fructose at elevated temperatures could contain condensed 5-member rings (e.g., furanones) and their fragments. Following this logic, inulin, a small linear polysaccharide composed of ∼35 (2 f 1) linked β-D-fructofuranose monomers for every 1 glucopyranose monomer,15 is an ideal candidate feedstock.16 It differs from cellulose and hemicellulose because it is a polyketose rather than a polyaldose at room temperature.17 Of course, biomass is not only composed of oxygen-rich carbohydrates; it also contains lignin, a complex three-dimensional aromatic structure composed of phenyl propane units. Taking this into account, a charcoal derived from Kraft lignin was prepared and presumed to contain condensed 6-membered rings analogous to graphite. The pyrolysis chemistry associated with individual biomass components (e.g., cellulose, hemicellulose, lignin) may differ from that of the combined substrate. Consequently, charcoals derived from waste oak (Quercus sp.) wood floorboards and corncob were prepared. The oak was supplied by the Cowboy Charcoal Co., and the cob was obtained from the Waimanalo Research Farm on the island of Oahu, Hawaii. The cob was

Table 1. Proximate Analyses of 20/40 (425 µm) Mesh Charcoals Proximate Analysis (%)a oak wood charcoal 090704c demineralized corncob charcoal 060804 corncob charcoal 311203 fructose charcoal 180205 Kraft lignin charcoal 220305 glucose charcoal 250405 inulin charcoal 120405 sucrose charcoal 131204

sectionb

VM

FC

ash

middle middled middle bottom top top top top top top

22.3 19.8 18.8 13.7 6.9 6.8 5.7 5.1 2.5 1.8

77.1 79.4 79.9 84.6 90.7 93.1 90.4 94.7 96.7 98.2

0.6 0.9 1.3 1.7 2.4 0.1 3.9 0.2 0.8 0.0

a Dry basis, ASTM D1762-84 (reapproved 1990). b Refers to the position of the charcoal within the lab-scale flash carbonization canister. c Number corresponds to date produced ddmmyy. d Demineralized corncob charcoal. The top, middle, and bottom sections refer to three split portions in the top half of the canister only.

broken into smaller pieces typically 2-3 cm in length by 1 cm in diameter prior to carbonization. Finally, we also prepared a charcoal from demineralized corncob in order to investigate the potential catalytic effect of mineral matter on biomass carbonization. All the charcoals were produced using flash carbonization (FC) technology, a process that quickly and efficiently produces charcoal from biomass.18,19 Charcoals derived from feedstocks, which proceeded through a distinct liquid phase during pyrolysis, were prepared within a small stainless steel beaker positioned at the top of the packed bed of biomass within the FC canister. Table 1 displays proximate analyses of the charcoals. Note that the volatile matter content of these charcoals ranges from 22.3% (oak wood) to 1.8% (sucrose). As expected, the sugar charcoals (sucrose, fructose, and glucose) contain very little ash. Prior to analysis by TG-MS, the corncob charcoal samples were ground as described in a previous work.20 The other charcoal samples were studied as received. Demineralization Procedure. Chopped corncob was partially demineralized by use of a 10 L insulated silica glass wash cylinder and 7 L of hot (∼90 °C) 0.1 mol L-1 citric acid solution. A continuous flow of hot citric acid was percolated through the 0.75 kg cob bed for 120 min. The buoyancy of the chopped corncob was counteracted by a weighted 1 mm Ø stainless steel cage. The acid wash was followed by a neutralization step that involved the percolation of boiled and ambient temperature deionized water through the cob bed. Subsequent to drying at 105 °C, the partially demineralized corncob was converted into charcoal. Elemental Analysis. Charcoal substrates destined for elemental analyses were carbonized by use of Coors porcelain crucibles and a Barnstead Thermolyne FB1215M muffle furnace. Typically, 5-10 g of 20/40 mesh charcoal was placed inside a crucible. The charcoal sample with the lid in place was then subjected to a volatile matter analysis as per ASTM method D1762-84 with one exception. Instead of the crucible being moved to the back of the furnace with the furnace door closed for 6 min, the charcoal sample was left to soak in the furnace for 30 min at 950 °C. Ash Analysis. Charcoal ash samples were prepared at the University of Hawaii according to ASTM D1762-84 and analyzed according to ASTM D6349-01 by the Huffman Laboratory with some minor modifications. The Huffman Laboratory pre-ashed the ash samples at 750 °C in small platinum dishes, weighed the residue to calculate loss on ignition, and then fused the entire residue in the same dish with

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007 5945

Figure 1. Digital photos of selected flash carbonized feedstocks: (a) corncob charcoal; (b) sucrose charcoal; (c) R-D-glucose charcoal; (d) β-D-fructose charcoal covering a piece of corncob charcoal; (e) inulin charcoal; (f) partially carbonized Kraft lignin charcoal.

lithium metaborate at 950 °C. The cooled fusion melt was dissolved in 5% v/v nitric acid, and the resulting solution was measured on a Perkin-Elmer Optima 3000 ICP-AES spectrophotometer for total Al, Ca, Fe, Mg, Mn, P, K, Si, Na, and Ti. Total S was determined on a separate portion of the as submitted sample using a Leco SC-144DR dedicated sulfur analyzer. All elements were calculated as oxides and reported on an as received sample weight basis, along with the measured loss on ignition. Thermogravimetry-Mass Spectrometry. The TG-MS system consists of a Perkin-Elmer TGS-2 thermobalance coupled to a Hiden HAL 301 F/PIC quadrupole mass spectrometer through a heated capillary transfer line. The measurements were carried out using argon purge gas with a flow rate of 140 mL min-1. Prior to the experiments, the apparatus was purged with the carrier gas for 45 min. Depending upon the volatile matter content of the substrate, 7-17 mg of the sample was heated from 20 to 1000 °C at a heating rate of 20 °C min-1 in a platinum sample pan. A portion of the volatiles formed as a result of thermal decomposition was introduced to the ion source of the mass spectrometer. The mass spectrometer was operated in electron impact ionization mode at 70 eV. The ion intensities were normalized to the sample mass and to the intensity of the

38Ar

isotope of the carrier gas to eliminate errors resulting from a shift in MS intensities. As some of the main decomposition products of charcoal (water, carbon monoxide, and carbon dioxide) give rise to fragment ions of m/z 16 and m/z 28, corrections are necessary. The contributions of water, CO, and CO2 were subtracted from m/z 16, and the contribution of CO2 (only) was subtracted from m/z 28. Thus, the m/z 16 and m/z 28 ions represent mainly the molecular ions of methane and carbon monoxide, respectively. The amount of H2O, CO, CO2, and H2 evolved from the sample was determined based on integration of the mass spectrometric curves. The calibration for these compounds was carried out by measuring the thermal decomposition of calcium oxalate monohydrate (for H2O, CO, and CO2) and titanium hydride (for H2). Principal Component Analysis. Principal component analysis (PCA) is a useful tool for distinguishing similarities and differences in the thermal behaviors of the charcoal samples. Samples with similar thermal properties lie close to one another in the principal components space, while the distance between samples with considerably different properties is large. As a result, samples with similar properties fall into groups. Thus, PCA is useful for identifying samples that possess similar properties.

5946

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007

Figure 2. TG and DTG curves of roughly ground (in a mortar by hand) and finely ground (particle size e 11 µm, milled as described in ref 20) middle section corncob (a) and demineralized corncob charcoal (b) and the MS curves of carbon dioxide (m/z 44) of the finely and roughly ground middle section corncob (c) and demineralized corncob charcoal (d).

The following data were used to perform the principal component analysis: • the amounts of hydrogen (m/z 2), water (m/z 18), carbon monoxide (m/z 28), and carbon dioxide (m/z 44) evolved from each sample • values of DTGmax (% s-1), Tpeak (°C), and mass loss in the 50-1000 °C temperature range for each sample. Results Charcoal produced from most lignocellulosic biomass retains the form and structure of its biomass precursor to such an extent that the appearance of the charcoal can be used to identify its origin.21 This behavior is evidenced in Figure 1a that depicts corncob charcoal. Note the retention of the corncob’s form and structure. On the other hand, charcoals derived from glucose, fructose, sucrose, inulin, and Kraft lignin retained none of their original solid structure because they proceeded through a distinct liquid phase during pyrolysis. The visual appearance of each charcoal formed from a melt phase (i.e., a “melt charcoal”) was unique (see Figure 1b-f). The charcoal resulting from sucrose pyrolysis was hard, sparkly, and porous and when mechanically broken apart formed hard nuggets. Glucose charcoal was fragile and shiny and appeared less porous, resembling a solidified, viscous, tarry liquid. Fructose charcoal was extremely lustrous, fragile, metallic looking, and glassy and fractured easily into

thin films. The charcoal resulting from inulin pyrolysis was again quite fragile and had a fine porosity and a sheen similar to the charcoal derived from glucose. Visual inspection of the Kraft lignin charcoal led to the assessment that the feedstock may have gone through a more viscous melt phase than the sugars prior to carbonization. The Kraft lignin charcoal was quite similar in appearance to glucose charcoal with a possible exception in the manner by which it fractured upon mechanical grinding, forming what appeared to be small discrete particles glued together. Figure 2a and b display the TG and differential thermogravimetric (DTG) curves of finely and roughly ground corncob (a) and demineralized corncob charcoal (b). These figures show that particle size (below a certain minimum) has no significant effect on the TG and DTG data. The MS curves of the above samples were also compared. We found that the shapes of the curves and the peak heights are quite similar, implying that the particle size (below a certain minimum value) has no influence on the evolution of volatile products. As an example, the MS curves of carbon dioxide are shown in Figure 2c and d. These findings are in good agreement with our earlier work.22-26 In order to set the stage for further discussion, we first present the results of a PCA applied to all samples contained in this paper. The characteristics of the TG and DTG curves and the amounts of the most important volatile products (Table 2) have been used as input data for these chemometric calculations.

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007 5947

Figure 3. Principal component loading plot (a) and score plot (b) prepared with the evaluation of the thermogravimetric data and the amounts of the most important volatile products shown in Table 2. Table 2. Amounts of the Most Important Volatile Products Formed from the Charcoal Samples during Thermal Decomposition and Characteristics of the DTG Curves and Mass Loss

oak wood demineralized CC middle CC bottom CC top CC fructose Kraft lignin glucose inulin sucrose a

H2 (%)a

H2O (%)

CO (%)

CO2 (%)

DTGmax (% s-1)

Tpeak (°C)

width (°C)

mass loss (%)

1.4 1.8 1.6 2.1 1.0 1.6 1.4 1.5 0.8 0.7

9.6 3.8 9.1 4.2 2.9 1.7 2.2 1.7 0.7 0.8

9.7 7.6 7.9 4.8 3.8 4.9 4.4 4.4 2.4 1.5

5.2 2.9 4.3 3.5 3.0 1.8 2.2 1.8 1.4 1.2

0.0254 0.0157 0.0234 0.0134 0.0074 0.0104 0.0108 0.0093 0.0046 0.0041

554 593 541 601 739 667 786 777 795 740

295 399 250 579 512 267 208 228 210 226

27.4 22.1 24.1 19.4 12.4 10.5 9.2 9.0 5.0 4.3

Percentage of initial sample mass.

According to the loading plot (Figure 3a), the second component is determined mainly by the DTG peak width and the amount of hydrogen evolved from the sample. The amount of water, carbon dioxide, and carbon monoxide, the DTGmax value, and the peak temperature are represented mainly by the first component. It can also be seen that samples with higher VM contents (i.e., mass losses as measured by TG and indicated in Table 2) have higher DTG maxima and lower the peak temperatures (as expected). Also, the figure shows that the amount of H2 and the DTG peak width do not correlate; however, these parameters have the most important role in determining the second principal component. The score plot (Figure 3b) shows that the oak wood and the middle corncob charcoal, which exhibit relatively high mass loss in the TG-MS experiments between 50 and 1000 °C (27% and 24%, respectively; see Table 2), have similar thermal behavior. Likewise, charcoals with low mass loss constitute a separate group. Inulin and sucrose lie very close to one another, reflecting the fact that their mass loss is very low (with mass losses of 4.3-5 wt %). Charcoals with intermediate VM contents (with mass losses of 9-12%) are positioned nearby but in the general direction of the high VM charcoals. The thermal behavior of the demineralized corncob charcoal with a mass loss of 22.1% is in between that of the middle and the top section charcoals. The corncob charcoal obtained from the bottom part of the reactor with a mass loss of 19.4% clearly behaves differently from the other charcoals as a result of its large (negative) second component. In summary, the PCA shows that the amount of volatile compounds (described by the mass loss in the given temperature interval) of a charcoal is the key determinant of its thermal properties as quantified by TG-MS. In addition, charcoals taken

from the bottom of the reactor, which experienced a large flux of volatiles derived from the carbonization of feedstock in the upper part of the reactor, appear to have unusual properties as measured by their hydrogen evolution and DTG peak width. With this overview in mind, we now examine the TG/DTG curves and gas evolution behavior of the individual charcoal samples. As expected, a comparison of the TG and DTG curves of the high VM charcoals (Figure 4a) shows that the DTG curves of the oak wood and corncob charcoals from the middle section are rather similar. But unexpectedly, the behavior of the demineralized corncob charcoal resembles that of the bottom section corncob charcoal in the 400-1000 °C temperature region. In the case of the bottom section corncob charcoal, a small peak can be observed between 100 and 300 °C; whereas, there is a sharp peak on the DTG curve of the demineralized corncob sample at about 360 °C which may result from the evaporation of volatile matter formed during the charcoal production process that condensed back into the pores of the charcoal. In Figure 4b, the TG and DTG curves of the low VM charcoals are depicted. As the PCA results imply (Figure 3b), the decomposition behaviors of the inulin and sucrose charcoals are similar. The main decomposition peak occurs at about 750800 °C. The DTG peaks of glucose and Kraft lignin charcoals are roughly in the same temperature range as those of sucrose and inulin charcoals; however, the DTGmax values of glucose and Kraft lignin charcoals are considerably higher. The fructose charcoal decomposes over a very broad temperature range (500-950 °C), and its DTG profile differs from the other curves. The behavior of the corncob charcoal obtained from the top

5948

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007

Figure 4. TG and DTG curves of high VM (a) and low VM (b) charcoals.

Figure 5. TG, DTG curves, and evolution profiles of the main volatile products formed from the middle section (a), bottom section (b), and demineralized (c) corn cob charcoals and the oak wood charcoal (d).

section (the highest temperature region) of the reactor is significantly different from the other low VM charcoals. The evolution of the volatile products of thermal decomposition was monitored with the mass spectrometer as described in the experimental section. The products that evolved from all charcoal samples were hydrogen, methane, water, carbon monoxide, and carbon dioxide. Besides these products, some other compounds were also detected from the high VM charcoals. First of all, we compare the evolution profiles of the main products for the various high VM charcoals (see Figure

5). The scales of the MS curves of different products are different, and they are scaled only to show the relative ordering of the measured intensities. However, the scales of given products are the same for the different samples, so the MS curves of the volatile products of each charcoal are comparable. We observe that during the first stages of decomposition mainly water and carbon dioxide evolve from these charcoal samples. The H2O and CO2 may be simply physisorbed or chemisorbed species, either as remnants of the gas in the FC reactor or chemisorbed from the air. At higher temperatures

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007 5949

Figure 6. TG, DTG curves, and evolution profiles of the main volatile products formed from the top section corn cob (a), fructose (b), R-D-glucose (c), Kraft lignin (d), inulin (e), and sucrose charcoal (f).

(above 300 °C), carbon monoxide and methane are also produced. The CO may be derived from chemisorbed O2. Finally, above 500 °C, the evolution of hydrogen begins. As displayed in Figure 5, the evolution profiles of the products from all the high VM charcoals are similar. In particular, the TG and DTG curves and the evolution profiles and peak heights of carbon dioxide and methane from the middle section corncob and the oak wood charcoals are nearly identical, whereas a larger amount of carbon monoxide was formed from the oak wood charcoal than from the middle section corncob. Concerning the low VM charcoals (see Figure 6), in the initial stages of decompositionsas was the case with the high VM

charcoalsssmall amounts of water and carbon dioxide are formed. At higher temperatures, more water and carbon dioxide are formed, accompanied by the evolution of carbon monoxide and methane. In the last stages of decomposition, hydrogen evolution begins. The temperature ranges in which given products can be detected are similar for most volatiles. However, the lower the volatile matter content, the higher the initiation of hydrogen evolution. It is also interesting to note that as the mass loss of the samples decreases, the relative amounts of methane, water, and carbon dioxide decrease significantly (see also Table 2). The main products of carbonization are carbon monoxide and hydrogen from the lowest VM charcoals (i.e.,

5950

Ind. Eng. Chem. Res., Vol. 46, No. 18, 2007

Figure 7. TG, DTG curves, and evolution profiles of some organic volatile products formed from the middle section corn cob (a and c) and oak wood charcoal (b and d).

the inulin and sucrose charcoals). Also, the top section corncob sample continues to evolve CO at the highest temperatures employed in this work. The high VM charcoals evolve other products besides hydrogen, methane, water, carbon monoxide, and carbon dioxide upon heating. Among these, aliphatic and aromatic fragments and carbohydrate decomposition compounds can be identified. In Figure 7, the MS curves of some products formed from middle section corncob and oak wood charcoal are shown. The product evolution profiles and temperatures are the same for these two charcoals. Nevertheless, some differences can be observed in the heights of the peaks (i.e., in the amounts of the products). For example, the yields of aromatic products are higher from the corncob charcoal. Features in the MS curves imply that aliphatic compounds are present in both charcoals (e.g., the presence of m/z 27 for C2H3+). Besides these, the appearance of the CH3CO fragments and HCHO suggests the presence of aldehydes and other oxygen containing compounds (see Figure 7a and b). Some of the aromatic compounds (toluene and phenol) are released in the same temperature range as well, but benzene is evolved at higher temperatures (Figure 7c and d). The compounds shown in Figure 7 also were released from the bottom section corncob charcoal and the demineralized corncob charcoal. In the case of the demineralized charcoal, other products were also identified (Figure 8) at low temperatures (below 400 °C). The peak at 150 °C is derived from methyl acetate, a pyrolytic remnant of the citric acid wash that was employed to demineralize the corncob. The sharp peak at about

Figure 8. TG, DTG, and some MS curves of the demineralized corn cob charcoal sample.

360 °C represents oxygen containing compounds that can be attributed to various furan compounds and their derivatives. Discussion From the preceding discussion, we can conclude that biomass charcoals with differing VM contents certainly have differing chemical structures. This is not a surprise. But what about charcoals that possess a low VM content (e.g., 800 °C) behavior of the lowest VM content charcoals. Both the sucrose charcoal and the inulin charcoal are melt charcoals. Figure 6 shows the gas evolution profiles for these two charcoal samples: H2, CO2, and H2O exhibit similar behavior; however, the rate of release of CO from the inulin carbon is higher than that of the sucrose carbon at 1000 °C. In a very recent paper, Giroux et al.27 report persistent CO evolution from coal at 1000 °C, resembling the behavior of the inulin carbon. Table 3 compares values of the rate of CO evolution with the rate of mass loss for these biocarbons at 1000 °C, as well as summarizing our most important observations. The rates of mass loss (DTG) of the two samples are similar, but the rate of release of CO (I28) by inulin is 32% higher than that of sucrose. Now consider the high-temperature spectra (>800 °C) of the glucose, Kraft lignin, and fructose charcoals. These samples are all melt charcoals. As was observed with the sucrose and inulin charcoals, the behaviors of H2, CO2, and H2O are similar above 800 °C (see Figure 6), but the rate of emission of CO from the Kraft lignin carbon is much higher than that of the glucose and fructose carbons at 1000 °C. Likewise, the rate of mass loss of the lignin biocarbon is higher than that of the fructose and glucose carbons, and the ratio of the CO emission rate to the rate of mass loss (I28/(1000‚DTG)) for the lignin carbon (3.4 in Table 3) is much larger than that of the fructose carbon (1.6 in Table 3) or the glucose carbon (1.9 in Table 3). This means that CO evolution accounts for more of the ongoing weight loss of the Kraft lignin biocarbon at 1000 °C than of the other two biocarbons. Finally, consider the TG-MS data from four corncob and the oak wood charcoal samples. These charcoals did not experience melting before carbonization. Above 800 °C, the evolution of H2, CO2, and H2O from the low VM, top section corncob carbon decreases quickly (see Figure 6), just like the other low VM charcoal samples. At 1000 °C, the rate of mass loss of the top corncob carbon (0.0025% s-1 in Table 3) is similar to that of the melt biocarbons, but its rate of CO release (15.1 in Table 3) is much higher. Likewise, the ratio of the CO release rate to the rate of mass loss for the top corncob carbon (6.0 in Table 3) is much higher than that of the melt biocarbons. This means that the rate of CO emissions accounts for much more of the rate of mass loss for the top corncob carbon than for the other biocarbons. The top corncob carbon is an unusually strong and persistent emitter of CO at 1000 °C. Likewisesexcept for CO emissionssabove 800 °C, the middle, bottom, and demineralized corncob charcoals and the oak charcoal (see Figure 5) display nearly identical behavior. This behavior resembles that of the other biocarbons at high

Table 4. Elemental Analyses of Charcoal Samples Carbonized at 950 °C (dry basis, Measured by Huffman Laboratories, Inc., USA)

oak wood middle CC bottom CC top CC fructose Kraft lignin glucose inulin sucrose

C (wt %)

H (wt %)

O (wt %)

N (wt %)

S (wt %)

ash (wt %)

96.19 94.01 92.92 92.99 96.62 91.31 96.33 96.10 96.70

0.65 0.71 0.64 0.72 0.75 0.68 0.74 0.71 0.76

1.54 1.92 2.19 2.25 2.24 4.62 2.53 2.61 2.17

0.20 0.56 0.64 0.54 0.13 0.51 0.11 0.13 0.12