Article pubs.acs.org/EF
Impact of Inorganic Matter on the Low-Temperature Oxidation of Cornstalk and Cellulose Chars Shuang Fan and Changdong Sheng* School of Energy and Environment, Southeast University, 2 Sipailou, Nanjing 210096, People’s Republic of China ABSTRACT: This paper reports an investigation on low-temperature oxidation prior to the ignition of biomass char from slow pyrolysis with a focus on the impact of inorganic matter. A set of chars was produced from the slow pyrolysis of cornstalk, cellulose, and their pretreated samples (water-leached cornstalk and KCl-doped cellulose) at 500 °C. The data for simultaneous thermogravimetric analysis and differential scanning calorimetry and isothermal calorimetry showed that the presence of active inorganic matter, particularly K, in the raw materials enhances the reactivity and favors the heat generation for the lowtemperature oxidation of the chars. These observations further suggest that water leaching of biomass to remove inorganic species may be an effective strategy to reduce the self-heating tendency of the char. The in situ diffuse reflectance infrared Fourier transform spectroscopy results indicated that active inorganic matter, particularly K, may provide active sites overtaking aliphatic C−H and O−H sites for O2−char reactions at low temperatures. The presence of active inorganic species, such as K, affects the formation behavior of total CO groups but appears to have an insignificant influence on the formation of individual CO groups, which were observed to be more dependent upon the char material. chars,16−19 including their self-heating-related behavior, are known to vary widely depending upon not only the raw material but also the production and processing conditions specified for the products. For example, slow heating pyrolysis is typically employed to produce the char as the main product (fuel or biochar), whereas fast pyrolysis aiming at liquid biofuel production generates the char as a byproduct, which can be used as fuel or for other purposes.1,3 The resulting chars from slow and fast pyrolysis present significantly different properties related to their applications.1 Thus, considering the various modes of production and the range of applications, understanding low-temperature oxidation behavior is essential for evaluating the proneness of a biomass char to spontaneous combustion and describing the process of self-heating. Any biomass material contains a certain amount of inorganic matter, and the content and composition are highly variable.20 Inorganic matter can cause severe problems, such as fouling, for operating biomass thermochemical conversion systems. To address the related issues, water leaching has been suggested as a pretreatment to remove inorganic matter from biomass.21−24 Leaching is known to lower the high-temperature oxidation reactivity25,26 and increase the ignition temperature of biomass char.15 However, there is still a lack of knowledge of how inorganic matter influences the low-temperature oxidation and related heat evolution behavior of biomass char. This study investigated the low-temperature oxidation of biomass char prior to ignition from slow pyrolysis, with a focus on the impact of inorganic matter. Cornstalk and cellulose were used as raw materials to generate chars. The cornstalk and
1. INTRODUCTION Biomass char is a solid product from the conversion of biomass material in oxygen-limited environments, such as pyrolysis and gasification.1,2 Biomass char has broad applications3 as renewable energy and carbon sources, including solid fuel,4,5 precursor for activated carbon,6 and biochar for soil amendment and carbon sequestration.7,8 The significantly increased production of biomass char and its widespread use inevitably necessitate bulk transport and storage. As notoriously reactive materials, biomass chars are typically highly susceptible to selfheating and spontaneous combustion4 in the chain from production to end use. For example, the risk of freshly produced wood charcoal is well-known. Therefore, the susceptibility to self-heating must be evaluated for the safe handling of bulk biomass char to prevent it from spontaneous combustion. The self-heating of carbonaceous materials, including coal and char, involves various physical and chemical processes. Among these processes, the interaction between the material and atmospheric oxygen, i.e., low-temperature oxidation, is recognized as one of the major mechanisms.4,9 Even during the storage of biomass material, chemical oxidation of the char formed by biomass charring at lower temperatures is regarded as one of the important forces driving self-heating toward spontaneous combustion.10−12 Indeed, low-temperature oxidation itself is also a complex process.13 During the self-heating prior to ignition, oxygen adsorption together with surface reactions is a key process4 at relatively lower temperatures, whereas both oxygen chemisorption and direct carbon oxidation contribute to heat generation at temperatures higher than 125−140 °C.14,15 In contrast to extensive investigations on the low-temperature oxidation of coal13 and biomass,10 few efforts have focused on the low-temperature oxidation of biomass char.15−19 Moreover, recent studies have focused more on assessments of the flammability of biomass chars18,19 rather than the self-heating process. The properties of biomass © XXXX American Chemical Society
Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 30, 2015 Revised: January 20, 2016
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DOI: 10.1021/acs.energyfuels.5b02287 Energy Fuels XXXX, XXX, XXX−XXX
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The sample was first heated in a purge of N2 with a flow rate of 50 mL/min to 105 °C and dried for 30 min, followed by cooling to 25 °C. The purge gas was then switched to air (20% O2 in N2), and the sample was heated at a rate of 5 °C/min to a final temperature of 600 °C. During the test, the sample mass and heat evolution were recorded, from which characteristic parameters and kinetics related to low-temperature oxidation were derived. Each test was replicated to ensure reproducibility. 2.4. Test of the Isothermal Heating of Char with Isothermal Calorimetry. Isothermal calorimetric tests were conducted on the chars using an eight-channel TAM Air microcalorimeter (TA Instrumentals). Each channel is constructed in a twin configuration with two 20 mL ampules for holding the sample and its reference. Approximately 1 g of the sample and the equivalent mass of reference material (quartz sand) were weighed and filled into the sample and reference ampules, respectively. The ampules were then sealed and loaded into the measuring positions. The heat power of the tested sample was recorded as the heat flux difference between the two ampules. The test was run at 50 °C for 48 h to yield the time-resolved heat evolution. Duplicate samples of each char were measured, and the average was reported. 2.5. Analysis of in Situ DRIFTS on the Char. In situ DRIFTS was used to investigate the surface behavior of the chars exposed to air under both temperature-programmed and isothermal heating. A Nicolet 6700 spectrometer (Thermo Scientific) equipped with a high-temperature reaction chamber (Harrick Scientific) was employed to monitor the evolution of the functional groups on the char surface during oxidation. The spectrum was collected by co-adding 32 scans at a resolution of 4 cm−1 in the scanning range of 4000−1000 cm−1 for each record. All recorded spectra were converted into Kubelka−Munk units. The in situ DRIFTS analysis procedure for temperatureprogrammed oxidation was set to be the same as the procedure for TGA−DSC. The char was placed in the sample cup in the reaction chamber and first underwent drying at 105 °C and cooling to 25 °C in N2. After collection of the base spectrum, the purge gas was switched to air. The sample was then heated at 5 °C/min to a final temperature of 250 °C, and difference spectra with respect to the base spectrum were sampled at intervals of 5 min (i.e., 25 °C). Measurements at temperatures higher than 250 °C were not attempted to avoid char ignition and intensive oxidation. The in situ DRIFTS analysis for isothermal oxidation was performed under a procedure similar to the procedure for isothermal calorimetry in TAM Air. After the char sample was heated in N2 to 50 °C and the base spectrum was recorded, the sample was exposed to air at 50 °C for 5 h and analyzed at every 5 min to collect difference spectra. To examine the spectral features of the functional groups most affected by the oxidation, the spectra in the corresponding wavenumber ranges were deconvoluted by curve fitting with Gaussian bands.
cellulose were also pretreated to change the inorganic matter content by water leaching and KCl doping, respectively. Two thermal analysis methods, simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA−DSC) and isothermal calorimetry, were applied to examine the behavior and characteristics of the low-temperature oxidation. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to monitor the behavior of the char surface during the low-temperature oxidation. The impact of inorganic matter on the low-temperature oxidation behavior was explored through systematic comparisons between the chars.
2. EXPERIMENTAL SECTION 2.1. Materials and Their Treatment. Cornstalk was milled to less than 500 μm as a raw material. A portion was leached with deionized water to partially remove the inorganic matter. The cornstalk was soaked in water with a liquid/solid ratio of 30 mL/g for 6 h at room temperature. The mixture was then filtered, washed, and oven-dried at 50 °C. The compositions of the raw and water-leached (WL) cornstalk are presented in Table 1. The contents of active basic elements indicate that 96% K, 30% Na, 32% Ca, and 64% Mg were leached.
Table 1. Composition of the Materials for Char Preparation cornstalk
WL cornstalk
K-cellulose
cellulose
Elements of Organic Matter (wt % on a Dry and Ash-Free Basis) C 42.34 42.38 44.29 44.73 H 5.41 5.72 6.30 6.32 O 51.55 51.46 49.30 48.87 N 0.48 0.25 0.03 0.01 S 0.22 0.19 0.08 0.07 Elements of Inorganic Matter (mg/kg of Dry Matter) K 1552 77 172 18 Na 31 26 73 39 Ca 776 639 11 19 Mg 1575 684 2 6
Considering that K is the major active element removed from cornstalk by water leaching and KCl is the major K-bearing species in herbaceous biomass,20,26 pure microcrystalline cellulose doped with KCl was prepared as the model biomass. The doping was performed by soaking the cellulose in 0.05 mol/L KCl aqueous solution at a liquid/solid ratio of 10 mL/g. After stirring at room temperature for 3 h, the mixture was filtered and oven-dried at 50 °C to obtain KCldoped cellulose, denoted as K-cellulose. For comparison purposes, the cellulose was also treated with water soaking. The procedure was the same as KCl doping, except that deionized water was used instead of the KCl solution. The treated cellulose is still called cellulose hereafter. The compositions of the cellulose and K-cellulose are also listed in Table 1, indicating that a small amount of K was doped. 2.2. Char Preparation and Characterization. Raw and WL cornstalk, cellulose, and K-cellulose were pyrolyzed to generate chars, denoted as CK, WL-CK, CE, and K-CE char, respectively. The pyrolysis was conducted in a N2 environment in a horizontal tube furnace by heating a sample at 10 °C/min to 500 °C and then holding for 30 min. The conditions, including the treatment temperature, heating rate, and holding time, were set to be typical for slow pyrolysis.1 The resulting chars were stored under airtight conditions before the characterization and testing. The chars were characterized by elemental analysis and DRIFTS at room temperature to determine the influence of pretreating raw materials on the char structure. 2.3. Test of the Temperature-Programmed Oxidation of Char with TGA−DSC. TGA−DSC was performed on a Netzsch STA 449F3 Jupiter apparatus (Netzsch-Gerätebau GmbH, Selb, Germany) with alumina crucibles as the sample and reference holders. Approximately 5 mg of char sample was loaded in the sample crucible.
3. RESULTS AND DISCUSSION 3.1. Char Characteristics. Table 2 presents the elemental composition of the four chars. As shown, WL-CK char has a considerably higher carbon content and lower oxygen content than CK char, reflecting the influence of water leaching on char formation. The two cellulose chars are similar in composition, indicating the slight effect of KCl doping, likely because of the small amount of K doped on the cellulose (Table 1). Table 2. Elemental Composition of the Chars (wt % on a Dry and Ash-Free Basis)
C H O N S B
CK char
WL-CK char
K-CE char
CE char
72.22 4.23 22.63 0.70 0.22
76.71 4.21 18.19 0.69 0.20
84.39 3.98 11.26 0.09 0.28
83.02 4.00 12.38 0.18 0.42
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the extractives, mainly sugar and organic acids,23,24 are the most labile constituents of the biomass material. They decompose or vaporize earlier at lower temperatures and are expected to contribute to char formation only slightly. When the spectra are compared between the chars from two raw materials (Figure 1), the main observable differences are the higher intensities of OH modes associated with the cellulose chars and the higher intensities of CO groups (∼1700−1600 cm−1) with the cornstalk chars. These differences reflect the raw material dependence of char functionality development. 3.2. Characteristics of the Low-Temperature Oxidation Derived from TGA−DSC. Variations in the weight, weight change rate, and heat flux with the temperature during the heating of the chars in air at 5 °C/min are compiled in Figure 2 and shown as thermogravimetry (TG), derivative thermogravimetry (DTG), and DSC curves. For all of the chars, the heat evolution presents two exothermic stages. One stage occurs at higher temperatures as one broad peak or two overlapping strong peaks; another stage occurs at lower temperatures as a broad shoulder. The two stages have often been observed in the temperature-programmed oxidation of cellulose- and biomass-originated chars generated at similar temperatures.31,32 The exothermic event at the higher temperature stage is associated with a large weight loss, as indicated by the TG and DTG curves, clearly as a result of intensive burning of the char. The lower temperature stage is accompanied by a slight weight gain but generates a considerable amount of heat (panels a and b of Figure 2), which is attributed to lowtemperature oxidation. Figure 2a shows that, at the higher temperature stage, WLCK char burns at higher temperatures but with slower rates of weight loss and heat release than CK char. This behavior is expected because water-leaching removed the majority of the active inorganic species, particularly K (Table 1), to significantly reduce their catalytic role in char carbon burning.26 During the low-temperature oxidation stage, WL-CK char presents a considerably lower heat generation rate than CK char, implying that removing water-washable inorganic species also reduces the heat evolution rate of low-temperature oxidation. When comparing the two cellulose chars, for the higher temperature stage, K-CE char burns and generates heat faster at relatively lower temperatures (Figure 3b). Considering the small amount of K doped on cellulose (Table 1), the effect of K on char burning is significant. In contrast, the heat evolution of the two chars at the lower temperature stage is similar but with only slightly more heat generated from K-CE char in the
Figure 1 shows the infrared spectra of the four chars, sampled by DRIFTS using KBr as the background. The observable
Figure 1. Spectra of the chars analyzed with DRIFTS. The upper three spectra were shifted up by adding 0.5, 1.0, and 1.5 to their original Kubelka−Munk values, and the data denoting the peaks are rough wavenumbers for the bands.
bands, assigned according to refs 27−29, are summarized in Table 3. Common bands detected on the four chars include C− H stretching modes (aliphatic at 3000−2800 cm−1 and aromatic at 3100−3000 cm−1), CC stretching vibrations of aromatic rings or highly conjugated hydrogen-bonded CO at ∼1600 cm−1, CH2 and CH3 bending at ∼1435 cm−1, and CH3 bending groups at ∼1375 cm−1 and the modes in the 1300− 1110 cm−1 region assigned to ether groups, as well as C−O stretching and O−H bending vibrations in phenoxy structures. A comparison of the spectra illustrated that there is only a slight difference between the two cellulose chars but a marginally higher intensity of CO groups (∼1700−1600 cm−1) in the K-CE char, implying that a small amount of KCl doping on the cellulose has a marginal influence on the char surface structure. When comparing the two cornstalk chars, the distinct differences are that the CK char presents remarkably higher intensities in the C−H stretching modes at 3000−2800 cm−1, C−H bending modes at ∼1435 and ∼1375 cm−1, and band at ∼1600 cm−1, indicating that more C−H functional groups formed than in the WL-CK char. Considering that the CK char has less carbon and a higher oxygen content than the WL-CK char (Table 2), the higher intensity at ∼1600 cm−1 is likely to be from the CO modes rather than the CC stretching vibrations of aromatic rings. The differences showed the watersoluble inorganic matter of the cornstalk having an effect on surface functionality development during char formation.30 Aside from inorganic matter, water leaching the cornstalk also removed approximately 15% of the organic material. However,
Table 3. Main Bands Observable on Char Spectra of DRIFTS and Their Functional Group Assignments27−29 bands (cm−1) 3700−3100 3030 2950 2920, 2850 1835 1775−1765 1735 1720−1630 1600−1500
bands (cm−1)
assignment O−H stretching, free O−H, and associated OH aromatic C−H stretching CH3 aliphatic −CH, CH2, and CH3 CO, anhydride CO in ester with electron-withdrawing group attached to single-bonded oxygen CO in ester CO in aldehyde, ketone, or −COOH CO in −COOH and in salt from −COO− C
assignment
1600 1450
aromatic ring CC stretching CH2 and CH3 bending
1375 1300−1110
CH3 groups C−O stretching and O−H bending in phenoxy structures, ethers
1100−1000
aliphatic ethers, alcohols
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Figure 3. Heat powers of the chars measured with a TAM Air microcalorimeter at 50 °C. The 15 h results are shown, whereas tests lasting for 48 h showed continuous decreases in heat power.
Table 4. Characteristic Data for Low-Temperature Oxidation of the Chars Derived from TGA−DSC MWG (%) TMWG (°C) Ti (°C) Qi (kJ/g) Ea (kJ/mol) QA (W/g)
CK char
WL-CK char
K-CE char
CE char
0.19 240 282 1.90 43.12 3.56 × 106
1.21 296 337 3.02 (1.26)a 42.33 1.72 × 104
2.91 312 368 4.35 29.73 1.05 × 103
3.90 327 378 4.50 (4.07)a 32.38 1.67 × 103
a
The values in parentheses present the heat release at the ignition temperature of the counterpart char.
energy Ea and pre-exponential factor QA) of low-temperature oxidation. Ti was determined by defining the ignition at the temperature of 1% weight loss of the char on a dry and ash-free basis. Qi was calculated by integrating the DSC data before ignition. The kinetic parameters were derived following the approach of kinetic analysis based on non-isothermal TGA− DSC at a single heat rate.34 Considering the heat release power (W) q of the char sample at time t ⎛ dm ⎞ ⎟ = QmA exp( − E / RT ) q = Q ⎜− a ⎝ dt ⎠
(1)
Rearranging eq 1 gives Figure 2. Results of simultaneous TGA−DSC analysis on the chars: (a) DTG and DSC profiles of two cornstalk chars, (b) DTG and DSC profiles of two cellulose chars, and (c) TG curves.
ln(q/m) = ln(QA) − Ea /RT
(2)
where m is the sample mass (g) on a dry and ash-free basis at time t, Q is the heat of the reaction (J/g), A is the preexponential factor (s−1) of the reaction rate constant, T is the temperature (K), and R is the universal gas constant (=8.314 J K−1 mol−1). With q and m derived from TGA−DSC, kinetic parameters QA and Ea were determined by fitting the Arrhenius plot of eq 2 over the temperature range from the heat release sensibly measurable to ignition. Table 4 indicates that WL-CK and CE chars achieve more weight gains at higher temperatures than CK and K-CE chars, respectively. The lower Ti values in Table 4 reflect that CK and K-CE chars are more susceptible to spontaneous combustion than their counterparts. WL-CK and CE chars generate more heat before their ignition. However, at the Ti values of their counterparts, i.e., CK and K-CE chars, they release less heat (values in parentheses in Table 4) than CK and K-CE chars, respectively. These data imply that CK and K-CE chars generate heat at higher rates and have more potential for self-
temperature range of 150−300 °C, implying a slight effect of K on low-temperature oxidation and heat evolution.15,33 Comparing the DSC curves in panels a and b of Figure 2 illustrates that CK char is more reactive at both higher and lower temperatures than the two cellulose chars. However, pretreating the cornstalk brings the heat evolution of the char at the lower temperatures down to almost the same as those of the cellulose chars and confirms the effectiveness of water washing in reducing the self-heating tendency of cornstalk char. To further characterize the low-temperature oxidation behavior of the chars, characteristic parameters and kinetics were derived from the data of TGA−DSC at the lower temperature stage prior to ignition. The results are summarized in Table 4, including the maximum weight gain (MWG), temperature at MWG (TMWG), ignition temperature (Ti), heat released before ignition (Qi), and kinetic parameters (activation D
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oxidation and heat evolution of CK char. The similarity between the heat evolutions of WL-CK and CE chars (Figures 2 and 3) indicates that the inorganic matter left in WL-CK char has only a slight impact on the heat evolution in lowtemperature oxidation, in contrast to the strong catalytic role of the inorganic matter in high-temperature char burning (Figure 2). The higher heat power of K-CE char in isothermal calorimetry (Figure 3) and its slightly greater heat evolution in TGA−DSC (Figure 2b and Table 4) compared to CE char suggest that K enhances the low-temperature oxidation, despite the small quantity of doping on the cellulose. Considering the large amount of water-soluble K in the cornstalk, it is not surprising that K makes a greater contribution to the selfheating of the CK char. Nevertheless, in addition to K, considerable proportions of other basic elements (Na, Ca, and Mg) in the cornstalk were also water-leachable (Table 2). Water-soluble Na is likely to have the same effect as K, but its content in the cornstalk is considerably lower than the K content. Ca and Mg may play their roles in the low-temperature oxidation of the char, which is worthy of future investigation. 3.4. Evolution of the Surface Functional Groups When Char Is Exposed to Air under Temperature-Programmed Heating. Figure 4 illustrates the evolution of the in situ DRIFTS difference spectra with the temperature for the chars heated in air at 5 °C/min. At 125 °C and above, CK and K-CE chars show remarkable increases with the temperature in the intensities of the CO modes at 1840−1500 cm−1, with the most apparent feature widely observed on low-temperature oxidation of carbonaceous materials.40−43 In addition to the
heating. With regard to low-temperature oxidation kinetics, CK char has a similar Ea but a higher QA, indicating a higher reactivity than WL-CK char. Despite the slight difference in Ea, the kinetic parameters reflect that K-CE char is slightly more reactive than CE char, particularly at relatively lower temperatures. The activation energies of the four chars are well within the range of values reported in the literature based on TGA and DSC measuring the oxygen chemisorption and related heat evolution of various carbonaceous materials.14,35 In summary, the ignition temperature, heat generation, and kinetics derived from TGA−DSC all suggest that inorganic matter in the biomass enhances the low-temperature oxidation reactivity of the resulting char. Weight gain is typically linked to oxygen adsorption of carbonaceous materials,14,30,31,36 which actually corresponds to the difference between oxygen adsorbed and product gas desorbed during low-temperature oxidation. Figure 2c and Table 4 show that the more reactive char releases more heat but has a smaller increase in weight during low-temperature oxidation. Using TGA examining a wide rank range of coals, Avila et al.36 found that coals with high spontaneous combustion potential generally showed lower weight gains, if any, consistent with the observation here for the chars. They suggested that apart from the oxygen adsorption, less stable surface oxides are formed but decomposed rapidly at low temperatures.36 More intensive reactions of oxide complexes occur on more reactive char, whereas the active inorganic species may catalyze the reactions, leading to releasing more heat. 3.3. Characteristics of Low-Temperature Oxidation Based on Isothermal Calorimetry. Figure 3 presents the results of isothermal calorimetric tests on the four chars at 50 °C. For all of the chars, heat powers follow the same trend, increasing rapidly to a maximum in the initial period and then decreasing gradually with time (only the measured data at 15 h are shown in Figure 3). The trend is consistent with the trends observed using microcalorimeters to study the self-heating of coal37 and biomass materials.38,39 The initial increase to the maximum is mainly due to the sharp exothermic adsorption of oxygen to active sites and associated reactions on the char surface. The gradual decrease after the peak may be attributed to the decrease in the O2 concentration in the sealed ampule38,39 and the deactivation caused by various reasons, likely including the decrease in the number of active sites and the inhibition effect of product gases.13 Figure 3 shows that pretreating cornstalk results in the char generating heat at a considerably lower rate over the entire measurement process than the char from raw material, implying that water leaching is able to greatly reduce the propensity of cornstalk char to self-heating at the low temperature. When the two cellulose chars are compared, K-CE char releases heat at a higher rate, except for the initial period. Pre-pyrolysis doping KCl on cellulose also favors heat generation in the resulting char. The comparisons of both pairs of the chars indicate that the presence of active inorganic matter is favorable to char selfheating. Figure 3 shows that the heat power of WL-CK char is close to the heat power of CE char and further suggests that partially removing inorganic matter from the cornstalk with water leaching is an effective approach to reduce the selfheating propensity of the char. Analyses of both TGA−DSC and isothermal calorimetry demonstrate that water-washable inorganic species in the cornstalk play a significant role in the low-temperature
Figure 4. Evolution of the in situ DRIFTS difference spectra of the chars exposed to air at a heating rate of 5 °C/min: (a) cornstalk and WL-cornstalk chars and (b) cellulose and K-cellulose chars. A twodirection axis is used to compare the evolution of the spectra to the temperature between char pairs. E
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on the two cornstalk chars. The higher content of CO functional groups in the initial cornstalk chars (Figure 1) may have limited their further formation during oxidation. CE char forms more CO functional groups than K-CE char at the same temperatures. WL-CK char forms slightly more CO functional groups than CK char, with the exception of temperatures above 225 °C. Forming more CO functional groups may be caused by intensified oxidation at higher temperatures close to TMWG of the CK char, which changes the behavior of the CO groups. Nevertheless, considering that the formation of the CO functional groups is the dominant reaction associated with the weight gain,40,43 the trends of the intensity variations of the four chars shown in Figure 5a generally agree with the weight gains measured by TGA−DSC (Figure 3). Figure 5b presents the evolution of the integral intensities of the total C−H functional groups. The fitting revealed that aliphatic C−H groups comprise more than 90% of the total intensities, dominating aromatic C−H groups,40,42 and their contribution proportion changes only slightly with the temperature and char materials (not shown here). Figure 5b shows that the integral intensities of the C−H groups of WLCK and CE chars decrease significantly with increasing temperatures, whereas the integral intensities of the CK and K-CE chars change only slightly. As shown in Figure 4, O−H groups generally follow the same trend as C−H groups, even though the O−H bands were not fitted. The increases in intensity indicate that the formation of C O functional groups on the char surface increases with the reaction temperature, whereas the decreases in intensity suggest losses of O−H and C−H functional groups from the surface. These changes can be explained by the fact that, at low temperatures, char oxidation occurs mainly on aliphatic sites, leading to the formation of CO surface groups.40,43 The behavior of the WL-CK and CE chars follows this tendency. However, for CK and K-CE chars, only slight changes in the intensities of O−H and C−H groups were observed, despite the slightly higher amount of C−H groups in the CK char than in the WL-CK char, as shown in Figure 1, implying that active inorganic matter, mainly K, may provide active sites overtaking the aliphatic sites for O2−char reactions at low temperatures. Even a small amount of K doped on cellulose has a significant effect on the behavior of the char surface functional groups and clearly demonstrates the significant impact of inorganic matter on the low-temperature oxidation behavior of biomass char. Although the TGA−DSC analysis shows small differences in the heat evolution between K-CE and CE chars (Figure 2b), the in situ DRIFTS analysis indicates that the differences in the evolution of CO and C−H functional groups are notable (Figure 5). For the two cornstalk chars, the extent of these differences appears to be opposite the extent of these differences between the two cellulose chars. These observations can be explained as follows. TGA−DSC results are the global effects of low-temperature oxidation involving complex reactions, including oxygen adsorption, reactions of surface oxide complexes, and product gas desorption. The weight gain is the net weight change resulting from oxygen adsorption and product desorption; the heat evolution is the total heat effect of these reactions. In situ DRIFTS also reflects the resulting effects of these reactions on surface functionality but goes into more detail regarding the various functional groups. For the two cellulose chars, the major formation of CO groups, which dominates the consumption of C−H and OH groups (Figure
profound increases in the intensities of CO modes, WL-CK and CE chars present significant decreases in O−H modes at 3700−3100 cm−1 (negative bands) and evidence decreases in C−H groups at 3100−2840 cm−1 (negative bands) with increasing temperature. These decreases suggest that lowtemperature oxidation is associated with the aliphatic structure of the chars.40−42 The differences in the behavior of O−H and C−H groups in Figure 4 demonstrate the significant impact of the inorganic matter in the raw materials on the surface behavior of the resulting chars during low-temperature oxidation. The differences also show the significant effect of raw materials. To investigate the details of the CO groups evolving with the temperature, curve fitting was performed to deconvolute the spectra in the range of 1900−1500 cm−1 into five bands. These bands were set at 1830−1800, 1775−1865, 1740−1730, 1720−1630, and 1600−1500 cm−1, corresponding to the CO modes assigned in Table 3. The spectra related to the C−H groups were also deconvoluted into the bands, including aromatic C−H at 3100−3000 cm−1 and aliphatic C−H at 3000−2800 cm−1. The results are compiled in Figure 5. Figure
Figure 5. Evolution of integrated intensities of total (a) CO and (b) C−H functional groups during the chars heated in air at 5 °C/min to 250 °C. A negative intensity denotes negative bands of the difference spectra. Error bars present the standard deviation of curve fitting. Curves show the trends of the evolution.
5a shows the integrated area intensities of the total CO functional groups varying with the temperature for the four chars. The changes in the intensities are detectable at temperatures above 125−150 °C, after which the intensities increase rapidly with increasing temperature. A comparison of the intensities indicates that many more CO functional groups formed at higher rates on the two cellulose chars than F
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Figure 6. Ratios of the area intensity of individual bands to the total intensity of CO groups evolved with the temperature during the heating of the chars in air at 5 °C/min to 250 °C. Curves show the trends of the evolution.
The details of the fitted CO functional groups with varying temperature are shown in Figure 6, with the ratios of the area intensity of the individual band to the total intensity of the C O groups. For the two cornstalk chars, the 1600−1500 cm−1 band is the major band at lower temperatures, indicating the formation of mainly carboxylic acids or salts, which were often observed to form in the early stage of coal oxidation.40 The proportion decreases rapidly with gradual increases in the other bands. After 150 °C, the CO stretching suggests that aldehydes and ketones are the main functional groups. Except for more CO stretching for aldehydes and ketones at 1720− 1630 cm−1 for CK char at lower temperatures, there is no clear difference between the two cornstalk chars, implying that the inorganic matter of the cornstalk has no significant effect on the formation behavior of individual CO groups. For the cellulose chars, CO groups behave differently from the CO groups of the cornstalk chars. The carboxylic band at 1600−1500 cm−1 and the ether band at 1775−1765 cm−1 present a drastic evolution with the temperature (panels a and b of Figure 6), as is typically observed in the low-temperature oxidation of coal.10,40−42 Despite the lower intensity ratios compared to the intensity ratios of the cornstalk chars, the carboxylic CO is also the major group formed at lower temperatures, and its ratio decreases rapidly with the temperature. Meanwhile, the intensity ratio of the ether band at 1775−1765 cm−1 increases with the temperature and overtakes the carboxylic CO to become the dominant component at higher temperatures. When the two cellulose chars are compared, the K-CE char has lower intensity ratios for the 1775−1765 cm−1 band at temperatures below 175 °C, possibly as a result of the presence of K, but is unable to be
5a), indicates extensive oxygen adsorption and explains their weight gain more than the cornstalk chars. In comparison to the CE char, K in the K-CE char replaces aliphatic sites for O2− char reactions, causing the lower consumption of C−H and O− H groups and likely also promoting surface reactions, leading to the greater decomposition of CO groups to release gases. These two aspects result in considerable differences in surface behavior between K-CE and CE chars (Figure 5). However, the two aspects have opposite effects on the weight, particularly for the CE char, explaining the slightly greater weight gain of the CE char than the K-CE char. Although adsorption, surface reactions, and desorption all generate heat, considerable fewer CO groups formed on the K-CE char imply more extensive surface reactions and desorption. The heat effects are believed to lead to even slightly more heat evolution than in the CE char. With regard to the two cornstalk chars, the high content of active inorganic matter, particularly K, in CK char strongly enhances the low-temperature oxidation, which clearly results in less weight gain and more heat evolution for the WL-CK char (Figure 2a). However, the strong effects on surface reactions and desorption cannot be fully reflected by the functional group behavior observed by the in situ DRIFTS. Figure 4a shows that the most profound difference between CK and WL-CK chars is in O−H groups rather than in CO and C−H groups. Additionally, likely as a result of the material features, the lower weight gains of the two cornstalk chars compared to the weight gains of the cellulose chars imply more extensive oxidation at relatively lower temperatures (Figure 2a and Table 4), where the effect on the behavior of the CO and C−H groups may also narrow the differences between the CK and WL-CK chars. G
DOI: 10.1021/acs.energyfuels.5b02287 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
continuous loss of C−H groups over time. In comparison to the two cornstalk chars, both the K-CE and CE char lose C−H groups more rapidly. This difference may be related to the different features of the chars from different raw materials. In comparison, the intensities of the WL-CK and CE chars decrease more rapidly than the intensities of their counterparts, indicating the influence of active inorganic matter, particularly K, on char oxidation behavior by providing active sites to replace the aliphatic C−H sites for the reaction to some extent, similar to the temperature-programmed heating. Although some K as well Ca and Mg is left in the cornstalk after water leaching (Table 1), the inorganic matter is believed to have only a slight impact on the low-temperature oxidation, as shown in Figure 3. Therefore, the differences in the intensities and behavior of the C−H groups as well as the O−H groups between WL-CK and CE chars is more likely to be related to the char material, which may be the main reason why the difference in the intensity of the C−H groups between K-CE and CE chars is generally larger than the difference in the intensity of the C−H groups between CK and WL-CK chars in both isothermal heating (Figure 7b) and temperatureprogrammed heating (Figure 5b).
confirmed for CK char. The behavior of the individual CO functional groups indicates that, although their formation is dependent upon the char materials, the impact of inorganic matter is inconclusive and worthy of further investigation. 3.5. Evolution of Surface Functional Groups with Time during Char Exposed to Air at 50 °C. The recorded spectra for the chars at 50 °C (isothermal) were also curvefitted in the wavenumber ranges of the CO and C−H functional groups. The total integral intensities obtained varying with time are presented in Figure 7. The general
4. CONCLUSION Cornstalk, cellulose, and their samples pretreated respectively by water leaching and KCl doping were pyrolyzed at 500 °C under slow heating conditions. The resulting chars were systematically characterized with TGA−DSC, isothermal calorimetry, and in situ DRIFTS to investigate the characteristics and behavior of low-temperature oxidation prior to ignition of the biomass char, focusing on the impact of inorganic matter. Analysis of the chars exposed to air under temperature-programmed heating in TGA−DSC showed that the presence of active inorganic matter, particularly K, in the parent materials increases the reactivity and heat generation of the chars during low-temperature oxidation prior to ignition. Isothermal calorimetry also revealed that active inorganic matter enhances heat generation at 50 °C. Both thermal analyses showed that water leaching of the cornstalk significantly reduces the heat generation of the resulting char close to the level of the cellulose char at low-temperature oxidation, suggesting the effectiveness of biomass water leaching in reducing the self-heating tendency of the char. The results of in situ DRIFTS indicated that active inorganic matter may provide active sites overtaking aliphatic C−H and O−H sites for O2−char reactions at low temperatures in both temperature-programmed and isothermal heating conditions. The presence of inorganic matter affects the formation behavior of the total CO groups but appears to have an insignificant influence on the formation behavior of individual CO groups, which was observed to be more dependent upon char material.
Figure 7. Evolution of integrated intensities of total (a) CO and (b) C−H functional groups during the exposure of chars to air at 50 °C. Curves just show the trends. Error bars present the standard deviation of curve fitting, which is not presented for CO groups to show the trends more clearly.
trend observed in Figure 7a is the intensities of the CO groups increasing with time. For the CK and K-CE chars, the intensities increase nearly linearly after 50−100 min. In contrast, the intensities of WL-CK and CE chars rise more rapidly in the initial 50−100 min and then increase gradually. The initial faster increases in CO intensities may explain why the heat power of the CE char is stronger than the heat power of the K-CE char at the initial heating in TAM Air (Figure 3). The fitting showed that the carboxylic mode at 1600−1500 cm−1 is also dominant in the intensities of the CO groups of the chars (not shown here), similar to the observation with the programmed heating (Figure 6). Integral intensities of the total C−H bands are shown in Figure 7b. For the CK char, there is only a slight change in the intensity with time, implying that its C−H functional groups are only slightly affected by oxidation. In contrast, the intensity decreases gradually for the WL-CK char, indicating the
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DOI: 10.1021/acs.energyfuels.5b02287 Energy Fuels XXXX, XXX, XXX−XXX
