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Assessing the Effects of Different Process Parameters on the Pyrolysis Behaviors and Thermal Dynamics of Corncob Fractions
Xiwen Yao,a Kaili Xu,a,* and Yu Liang b
The influence of heating rate, gas flow, and biomass particle size on the pyrolysis and thermal dynamics of corncobs (CC) was investigated experimentally using the quantitative method of thermogravimetric analysis (TGA) coupled with mass spectrometry (MS), and the obtained results were compared in depth. For the examined heating rates of 5, 10, and 20 °C/min, the CC pyrolysis at higher heating rates resulted in a more complete decomposition. The initial pyrolysis temperature decreased when gas flow was increased from 30 to 90 mL/min, whereas the weight loss increased. Particle sizes (d ≤ 74 μm, 74 μm < d ≤ 154 μm, 154 μm < d ≤ 280 μm, and 280 μm < d ≤ 450 μm) had pronounced effects on the thermal decomposition and bio-syngas compounds (CO, CO2, CH4, and H2) distribution. The emission intensities of most the gaseous products increased at the elevated heating rate, while they decreased with increasing gas flow. In sum, the pyrolysis of CC particles of 154 μm < d ≤ 280 μm under 20 °C/min and in a gas flow of 30 to 60 mL/min was the most appropriate for bio-syngas production in industrial applications.
Keywords: Corncob; Pyrolysis; Heating rate; Gas Flow; Particle Size; Thermal Dynamics; Bio-syngas
Contact information: a: School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, PR China; b: College of Information Science and Engineering, Northeastern University, Shenyang 110819, PR China; *Corresponding author: neu_kailixu@126.com
INTRODUCTION
Considering the depletion of fossil fuel sources and the high sensitivity of the public toward environment protection from the conventional energetic systems, the exploration of alternative and renewable energy sources has attracted increasing attention (Shen et al. 2013). As an alternative to fossil fuels, biomass, such as agricultural and wood wastes, has become one of the most significant elements of the sustainable energy system due to its abundance, renewability, and environmental benefits (Czernik and Bridgwater 2004). Biomass pyrolysis, as the key sub-category process of biomass thermo-chemical conversion, converts biomass waste in the absence of oxygen to obtain an array of gaseous, liquid, and solid products (Mohan et al. 2006; Lv et al. 2012), which can be further used to produce fuels, chemicals, heat, and electricity (Meng et al. 2013).
The technique of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) coupled with mass spectrometry (MS) has become more important for studying the weight loss characteristics, thermal stability, gas products distribution, and the correlation between pyrolysis reactions and chemical structure in biomass (Singh et al. 2012; Alshehri et al. 2013; Wang et al. 2014). The characteristic parameters of the thermal pyrolysis can be obtained from the thermogravimetric (TG) and differential thermogravimetric (DTG) data during the TGA experiments to further investigate the thermal degradation mechanism (Gai et al. 2013).
The pyrolysis behaviors of a variety of agricultural wastes, such as apple pomace (Baray et al. 2014), hardwood residues (Mazlan et al. 2015), corn stalk (Pittman et al. 2012), peanut shells (Yao et al. 2016a), grape residues (Xu et al. 2009), rice husk and straw (Worasuwannarak et al. 2007), wheat and corn straw (Lanzetta and Di Blasi 1998), and cherry seeds (Duman et al. 2011), have been investigated for different purposes. Miura and Maki (1998) and Arora et al. (2009) have focused on developing mathematical dynamic models for predicting the pyrolysis characteristics of various biomass wastes. In these studies, the chemical composition and pyrolysis behavior of different biomass fractions were greatly different from each other. In the present research, corncobs (CC) waste was chosen as the source of renewable energy due to its abundant availability in China and good potential to produce high value-added products (Yao et al. 2016b).
Biomass pyrolysis is an extremely complex process that involves complicated chemical processes and complex physical processes such as mass transfer, heat transfer, and their interactions (Lv et al. 2012; Gai et al. 2013). There have been many studies on the pyrolysis of CC (Cao et al. 2004; Feng et al. 2006; Ates and Isikdag 2009; Ioannidou et al. 2009; Zhang et al. 2009; Trninic et al. 2012; Liu et al. 2014). Most of these studies focused on developing kinetics models to predict the weight loss characteristics of CC and the reaction dynamics, and only a few were concentrated on the emission regularities of bio-syngas products (consisting primarily of CO, CO2, CH4, and H2) during the pyrolysis as well as on the key factors affecting the thermal degradation.
Furthermore, the pyrolysis of CC remains uncertain in many details, especially in terms of heat and mass transfer, thermal dynamics, and gas products distribution affected by different thermolysis conditions, biomass particle size, and indirectly, the catalytic effects. In general, biomass composition is one of the most significant factors affecting the pyrolysis of biomass, which is composed of cellulose, hemicellulose, and lignin (Ates and Isikdag 2009). According to Meng et al. (2013), the pyrolysis of biomass involves the superposition of the pyrolysis of these three biomass components, and it can be also strongly influenced by the experimental conditions.
Therefore, elucidating the influence of different reaction parameters (heating rate, the pyrolysis temperature, the carrier gas, and the pyrolysis medium, etc.) together with many other factors (biomass feed particle size and the catalyst, etc.) on the pyrolysis of CC fractions is essential for a better understanding of their thermochemical conversion. In this study, the influence of heating rate, gas flow, and biomass particle size on the thermal degradation, heat transfer, kinetics information and organic reaction mechanisms in chemistry, and releasing characteristics of bio-syngas products during the pyrolysis of CC was investigated using the quantitative method of TGA-DSC-MS.
In general, TGA analysis is very useful in determining the preliminary kinetics of biomass pyrolysis. It has been used extensively for the characterization of various feedstock. But, there is a lack of kinetics information on the thermal pyrolysis of CC. Besides, the organic reaction mechanisms in chemistry during the pyrolysis process is also unclear. Therefore, conducting a comprehensive investigation on the pyrolysis of CC is very essential, and the objectives of this study are to obtain the characterization of CC related to thermo-chemical conversions, and to analyze the effects of different process parameters on the kinetics information of CC as well as the organic reaction mechanisms in chemistry. This study can provide a more integrated understanding of the complete pyrolytic process of CC fractions.
EXPERIMENTAL
Preparation of Materials and Characterization
The CC biomass samples were collected from the countryside of Shenyang, northeast China. The raw biomass samples were oven-dried at 105 °C ± 0.5 °C for 24 h, ground and pulverized using a high-speed rotary cutting mill, and sieved with a 100-mesh sieve (0.154 mm in size). The materials that passed through the sieve and those not passed through the sieve were gathered in two different closed containers and kept for analysis. The proximate, ultimate, and component analyses of those CC materials are listed in Table 1, with each value being the mean value of three tests.
The proximate analysis was conducted on an as-received basis, while ultimate analysis and low heating value (LHV) was performed on a dry basis. The volatile matter, fixed carbon, and ash content were measured by 5E-MACIII Infrared Speediness Coal Analyzer (Kaiyuan Co., Changsha, China). The moisture was determined by a Sartorius Moisture Analyzer IMA 30 (Hamburg, Germany). Ultimate analysis was performed on a Vario MACRO Elemental Analyzer (Elementar, Hanau, Germany). The LHV of CC fractions was measured by IKA Calorimeter System C2000 (IKA Co., Staufen, Germany). The proximate and ultimate analyses, and the chemical components of the CC fractions were analyzed according to GB/T 28731-2012 (2012) and ASTM E1758-01 (2015), respectively. All the values in Table 1 presented a good reproducibility with a relative standard deviation less than 2.0%, and the precision of these measurements was 0.5%.
Table 1. Proximate, Ultimate, and Component Analyses of CC Samples
TGA-MS Experiments and Quantitative Method
The TGA tests of those prepared CC samples (less than 0.154 mm in size) were performed in a sensitive thermal balance (NETZCH-STA449 F3, Selb, Germany) at the heating rates of 5, 10, and 20 °C/min with a gas flow of 10 mL/min from 25 °C up to a final temperature of 1200 °C. For determining the influence of gas flow on the pyrolysis prepared CC samples, high purity helium (99.99%) in the gas flow of 30, 60, and 90 mL/min at the heating rate of 30 °C/min was used as the carrier gas. To study the effects of biomass particle size on the thermal degradation and gas products distribution in the pyrolysis, the pyrolysis behaviors of CC fractions of varying diameter (d ≤ 74 μm, 74 μm < d ≤ 154 μm, 154 μm < d ≤ 280 μm, and 280 μm < d ≤ 450 μm) at the heating rate of 20 °C/min with a gas flow of 30 mL/min were compared.
The sensitivity of this thermal balance was 1 μg and 0.01 °C, and the CC samples needed for each test were about 5 mg. To guarantee the accuracy of pyrolysis experiments and minimize the errors, the best particles sizes for TGA experiments using this thermal balance should be not more than 0.50 mm. Here, the pretreated CC fractions that passed through a 100-mesh (0.154 mm) sieve were further sieved using a 200-mesh (0.074 mm) screen, and thus both the particles of d ≤ 74 μm and those of 74 μm < d ≤ 154 μm were obtained. Similarly, the CC particles that had not passed through the 100-mesh screen, namely particles of d > 154 μm, were further sieved by a 60-mesh (0.28 mm) sieve, and then sieved by a 40-mesh (0.45 mm) sieve, and thus the CC particles of 154 μm < d ≤ 280 μm and 280 μm < d ≤ 450 μm were obtained sequentially.
The MS measurements were carried out by a quadrupole MS spectrometer (QMS 403D, Pfeiffer Vacuum Technology, Selb, Germany) coupled to the thermal balance to measure typical gas products. The MS spectrometer was performed in the EI mode with 70 eV of electron energy. To avoid secondary reactions, the gases released were purged to the MS spectrometer immediately to obtain gas evolution curves. The transfer line and gas cell were preheated to 200 °C to avoid cold spots and to prevent the condensation of semi-volatile products. A quantitative method on the MS was implemented by calibration for the following species: CO, CO2, CH4, and H2, based on the quantitative analysis on signals for their mass numbers. Details of this method can be found in a previous paper (Yao et al. 2016a). The calibration is taken on the MS spectrometer directly to obtain the method file before the measurement.
Thermal Dynamic Analysis
Thermal dynamic parameters were determined by the integral Coats-Redfern method, which has been successfully employed to investigate biomass pyrolysis kinetics (Vamvuka et al. 2003; Sun et al. 2010). The global kinetics of the biomass pyrolysis based on the first-order-reaction can be expressed as follows (Rath and Staudinger 2001),
(1)
where k represents the reaction rate constant, T stands for thermodynamic temperature (K), R is the universal gas constant (8.314 J·K-1·mol-1), E represents the activation energy (kJ·mol-1), and A is the pre-exponential factor (s-1). The degradation rate of biomass can be expressed by Eq. 2,
(2)
where dα/dt denotes the process rate, α is the fractional conversion and defined as (w0–w)/(w0–wf), and wo, wf, and w are the mass at the starting, end, and at a specific time t, respectively. Because temperature serves as the function of time, and it increases with an invariable heating rate β, the following equation can be derived.
(3)
Differentiation of the above correlativity results in Eq. 4.
(4)
Eq. 2 can be expressed as follows in Eq. 5.
(5)
The integration of Eq. 5 results in Eq. 6,
(6)
where.
Eq. 6 is integrated by the internal Coats-Redfern method to produce Eq. 7,
(7)
where g(α) represents the function of kinetic mechanism in an integral type.
The 2RT/E term is negligible because it is far less than 1. Thus, Eq. 7 is simplified to Eq. 8.
(8)
For a given heating rate, the ln(g(α)/T2) term varies as 1/T with a slope of –E/R linearly, and the intercept of the line corresponds to ln (AR/βE). Thus, pyrolysis kinetic parameters such as activation energy (E) and pre-exponential factor (A) in the main pyrolysis stage were calculated by the above equations.
Fig. 1. DTG nomenclature for the studied temperature range
Here, it is important to note that the temperature points at which the temperatures were appointed are defined in Fig. 1, and the investigated temperature range is precisely between the named T1 and T2 (Yao et al. 2016c).
RESULTS AND DISCUSSION
Influence of Heating Rate on the Thermal Decomposition of CC Fractions
Figures 2(a)-(c) show the TG, DTG, and DSC curves, respectively, obtained for CC pyrolysis under different heating rates within the temperature range of 25 to 1200 °C. As shown by the TG curves in Fig. 2(a), CC pyrolysis followed stepwise mechanisms and mainly occurred in four steps that can be explained from its chemical composition. The first weight loss (below 140 °C) started at around 70 °C; it reflected the evaporation of unbound moisture in biomass. As observed from the DTG curves in Fig. 2(b), the initial weight loss rates for all heating rates were quite slow, accompanied by a small shoulder peak within 70 to 140 °C. Munir et al. (2009) suggested that the weight loss occurring near 100 °C represents the initial degradation of lignin and hemicellulose components in biomass. Simultaneously, the DSC curves in Fig. 2(c) show an endothermic peak around 100 °C, indicating the energy adsorbed during the volatilization of moisture from CC.
Fig. 2. Thermogram curves of CC under different heating rates (a) TG, (b) DTG, and (c) DSC
The second decomposition stage mainly took place from 140 °C to 420 °C, accompanied by two sharp peaks in the DTG curves. The weight loss in this pyrolysis zone made great contributions to the total weight loss (about 80 to 90 wt.%) during pyrolysis, and it was caused by the emission of volatiles from thermal degradation of the three fundamental components in biomass, namely cellulose, hemicellulose, and lignin. Cellulose, hemicellulose, and lignin decompose within 277 to 427 °C, 197 to 327 °C, and 277 °C to 527 °C, respectively (Du et al. 1990). The DTG curves show that the temperature relevant to the maximum weight loss rate tended to shift to a higher temperature zone as the heating rate was increased.
The third decomposition stage mainly occurred above 420 °C, and in this stage, the pyrolysis of CC proceeded at a relatively slower weight loss rate, which is mainly attributable to the decomposition of lignins at higher temperatures. After 600 °C, there was no significant weight loss, and the pyrolysis of CC was fundamentally complete.
The obtained TG and DTG data derived from TGA experiments are often used to establish thermal degradation profiles that clarify vital parameters. To evaluate the thermal performance of biomass more correctly, a comprehensive devolatilization parameter was proposed by Zeng et al. (2013) and is defined as follows,
(9)
where (dw/dt)max is the maximum weight loss rate, (dw/dt)mean is the mean weight loss rate, Ts is the starting temperature for volatile release and weight loss, Tmax is the temperature of maximum weight loss rate, △T1/2 reflects the temperature of full width at half maximum for the main peak of DTG curves, and D reflects the volatile release index.
Table 2. Pyrolysis Feature Parameters of CC under Different Heating Rates
Table 2 contains the characteristic parameters of starting temperature (Ts), peak temperature (Tmax) of the main weight loss, along with the maximum degradation rate (dw/dt)max and the mean weight loss rate (dw/dt)mean. All of these values increased remarkably with increased heating rate. The maximum degradation rate tended to rise at relatively faster heating rates. This effect was attributed to the fact that more thermal energy is released at faster heating rates, which promotes thermal transmission nearby and within CC (Baray et al. 2014). When the heating rate was 5, 10, and 20 °C/min, the total weight loss (Wt) was 47.79%, 69.65%, and 77.23%, respectively. Thus, CC pyrolysis at a higher heating rate resulted in a more complete decomposition.
Influence of Gas Flow on the Thermal Decomposition of CC Fractions
Figures 3(a)-(c) show the TG, DTG, and DSC curves, respectively, obtained for the thermal pyrolysis of CC fractions in different gas flow within the temperature range of 25 to 1200 °C. The main characteristic parameters determined from TG and DTG profiles for various gas flow cases are listed in Table 3.
Taken together, the TG curves in Fig. 3(a) and the results in Table 3 show that the total weight loss of CC increased slightly with increased gas flow, whereas the starting temperature of the pyrolysis decreased as the gas flow increased. Ferdous et al. (2001) demonstrated that carrier gas flow is closely related to the residence time of gaseous products released in the pyrolysis reaction zone. Hence, the residence time of the gaseous products decreased with elevated gas flow. These gaseous products released from the pyrolytic process could be pumped away the pyrolysis region in time by the elevated gas flow, and this could further accelerate the thermal cracking of biomass.
However, the influence of gas flow on the decomposition rate was not obvious, as observed from the DTG curves in Fig. 3(b). The DTG curves in different gas flow were almost completely overlapping. This phenomenon was consistent with the results in Table 3; the peak temperature (Tmax) corresponding to the maximum weight loss rate ((dw/dt)max), the temperature of full width at half maximum (△T1/2), the mean weight loss rate ((dw/dt)mean), and the volatile release index (D) showed little variation and association with the increase of gas flow.
Fig. 3. Thermogram curves of CC in different gas flow (a) TG, (b) DTG, and (c) DSC curves
Table 3. Pyrolysis Feature Parameters of CC under Different Gas Flow
As shown in the DSC curves in Fig. 3(c), the strength of the endothermic peak caused by the volatilization of moisture within 100 to 200 °C was evidently enhanced by the relatively higher gas flow. With the increase of pyrolysis temperature, the heat of exothermic reactions during the pyrolysis increased gradually. Notably, the DSC curves in both cases of 60 and 90 mL/min presented evident endothermic peaks within the high temperature zones (between 500 and 800 °C), while the DSC curve for case of 30 mL/min exhibited no obvious endothermic peak in this temperature zone, as it was primarily exothermic (Fig. 3(c)). This result suggested that when the gas flow increases, the carrier gas needs to absorb more heat and so further heats up the reaction temperature.
Influence of Particle Size on the Thermal Decomposition of CC Fractions
The CC particles of varying diameter (d ≤ 74 μm, 74 μm < d ≤ 154 μm, 154 μm < d ≤ 280 μm, and 280 μm < d ≤ 450 μm) were used to investigate the effects of particle size on thermal decomposition characteristics. The TG, DTG, and DSC results within 25 to 1200 °C for different particle sizes are shown in Figs. 4(a)-(c), respectively.
Fig. 4. Thermogram curves of CC particles of varying size (a) TG, (b) DTG, and (c) DSC
Generally speaking, the smaller particles have a relatively higher surface/volume ratio, which allows the primary pyrolysis fragments to escape more rapidly into the vapor. Thus, the centers of these smaller particles can reach the reactor temperature more rapidly than those of large particles (Zhang et al. 2007). In contrast, Figs. 4(a) and (b) clearly show that within the examined particle size range (d ≤ 450 μm), the total weight loss and the peak weight loss rate for particles of d ≤ 74 μm were the minimum. The particles between 280 and 450 μm in diameter were most easily pyrolyzed, followed by those with diameter between 154 and 280 μm, 74 < d ≤ 154 μm, and d ≤ 74 μm. These findings were confirmed by the characteristic parameters summarized in Table 4.
Given the bulk density of biomass powders and the heat-transfer limitations, these results and phenomena from the above analyses can be well explained. The bulk density of finer particles is larger than that of coarse particles. In general, larger particles have a smaller bulk density, and thus, the gaps in these particles are wider. Besides, as for the particles with large size, the gas diffusion resistance is much lower than that of particles with relatively smaller size, which is quite conductive to the diffusion of those gaseous products produced from the pyrolysis process. Moreover, during the thermal cracking process, increased biomass particle size favors the production of small-molecule gases, which further increased the total weight loss of biomass pyrolysis. Altogether, for the studied particles of d ≤ 450 μm, the pyrolysis performance of larger particles was much better than that of small particles, and biomass feed particle size can significantly influence the thermal degradation of CC fractions.
Furthermore, Fig. 4(c) shows that the DSC for all particles in the tested size range could be divided into two categories based on the trend in DSC after 500 °C, which was similar for (1) the particles with a diameter smaller than 154 μm or (2) larger than 154 μm. For the particles of d > 154 μm, there was an evident and sharp endothermic peak within 500 to 800 °C, while there was no strong endothermic reaction for particles of d ≤ 154 μm in this temperature stage. In contrast, their DSC curves reflected the general feature of exothermic reactions. According to Yang et al. (2007), the thermal degradation of hemicelluloses, celluloses, and lignin in biomass primarily occurs at 220 to 315 °C, 315 to 400 °C, and 160 to 900 °C, respectively. Thus, this variation tendency of heat flow within this higher temperature zone (500 to 800 °C) can be primarily ascribed to the thermal degradation of lignin components in CC.
Table 4. Pyrolysis Feature Parameters of CC Particles with Different Size
For smaller particles of d ≤ 154 μm, the bulk density and specific surface area were relatively much larger than in large particles, which is much more advantageous to the heat transfer in pyrolysis. As a result, the heat released from the thermal degradation of celluloses and hemicelluloses in the early pyrolysis stage can basically provide the heat required for the degradation of the lignin components during the later pyrolysis stage. However, with respect to the larger particles of 154 μm < d ≤ 450 μm, their bulk density and specific surface area were lower than those of small particles, and their heat conductive performance was not as good as that of finer particles. Thus, the pyrolytic process needs to absorb more heat to guarantee the continued pyrolysis of lignin components in the CC samples at higher temperatures.
Influence of Process Parameters on the Release of Bio-syngas Products from CC Pyrolysis
Pyrolysis of biomass can produce raw synthesis gas, which mainly contains CO, CO2, CH4, and H2. The experimental gasification tests of CC waste conducted by Biagini et al. (2014) in a downdraft reactor at a demonstrative scale (nominal thermal throughput of 350 kW) suggested that the synthesis gas by CC pyrolysis was composed of 22.4-22.6% vol CO, 15.8-17.3% vol H2, 11.3-12.3% vol CO2, and 1.9-2.3% vol CH4. Besides, in their study, they also found that the performance parameters (specific gas production 2 m3/kg, syngas heating value 5.6 to 5.8 MJ/m3) were comparable with those obtained with woody feedstock, which further indicates that the performance of CC in synthesis gas by pyrolysis mass can make great contribution to the syngas production in practice.
The evolution profiles of typical bio-syngas products (i.e., CO, CO2, CH4, and H2) during the pyrolytic process of CC samples under different process parameters were studied using the quantitative MS technique. The spectra for those gaseous products varied with the increase of temperature and are shown in Figs. 5 through 7. The evolution of bio-syngas compounds was in accordance with the DTG curves under different reaction parameters (Figs. 2(b), 3(b), and 4(b)).
Fig. 5. Bio-syngas products varied with temperature under different heating rates
As shown in Figs. 5(a), 6(a), and 7(a), there were no obvious peaks of CO during pyrolysis. Meng et al. (2013) suggested that the CO was mainly was released from the cracking of carboxyl (C=O) and carbonyl (C–O–C) groups during the pyrolysis of hemicelluloses and of lignins above 600 °C. Notably, the emission intensity of CO increased with increased heating rate and decreased with increased gas flow. There was no consistent decreasing or increasing trend for the CO evolution with increased biomass feed particle size. Thus, the thermal pyrolysis of CC fractions in a relatively lower gas flow and at a higher heating rate may be more suitable for the production of CO.
As can be observed in Figs. 5(b), 6(b), and 7(b), the shape of CO2 emission fits well with the DTG curves below 400 °C, while the rest does not fit well with the DTG curves after 400 °C. CO2 emission was mainly from the degradation of hemicelluloses, while CO was most likely generated from the pyrolysis of celluloses (Lv et al. 2012). The distinct emission of CO2 appeared from 200 °C, and it reached a high peak intensity within 300 to 400 °C, which is the major pyrolysis zone of celluloses. The peak around 350 °C was ascribed to the decomposition of aromatic and aliphatic carboxyl groups in celluloses. Yang et al. (2007) suggested that below 500 °C, the abundant presence of C=O groups in hemicelluloses was favorable for CO2 formation. As temperature increased, more stable ether structures and oxygen-bearing heterocycles in lignin are also decomposed into CO2 (Wang et al. 2014). The variation in the evolution profiles of CO2 (Figs. 5(b), 6(b), and 7(b)) as well as CH4 (Figs. 5(c), 6(c), and 7(c)) with the change of heating rate and gas flow are similar to those of CO, implying that CO, CO2, and CH4 were the three main small-molecule gaseous products of pyrolysis.
Fig. 6. Bio-syngas products varied with temperature under different gas flow
As observed from Figs. 5(c), 6(c), and 7(c), the shape of CH4 emission curves under different parameters indicated that the release of CH4 during the pyrolysis was more complicated than the CO and CO2 emissions. The highest concentration emission of CH4 occurred at about 380 °C. As previously noted (Hodek et al. 1991; Arenillas et al. 2003), the CH4 emission at relatively lower temperatures was most likely released from the C-C bond cleavage in aliphatic chains, whereas the CH4 emission at higher temperature zones was mainly produced from the cracking of a weakly bonded methoxyl-O-CH3 group as well as the break of having higher bond energy of methylene group -CH2– (Ferdous et al. 2001). From the perspective of biomass components pyrolysis, the CH4 was mainly released from the pyrolysis of lignin because it contains more methoxyl-O-CH3 chemical groups than hemicelluloses and celluloses (Liu et al. 2008).
Fig. 7. Influence of particle size of CC on releasing characteristics of bio-syngas products
The emission of H2 (Figs. 5(d), 6(d), and 7(d)) took place in a much broader temperature range compared with the other gases, which primarily occurred in two temperature stages. The first H2 emission stage was from 200 to 500 °C, and in this period, the H2 emission was mainly generated from the thermal degradation of celluloses and hemicelluloses (Wu et al. 2013). At the second stage beyond 500 °C, the curves of the evolved H2 exhibited a decreasing trend under different heating rates (Fig. 5(d)), further revealing that the pyrolysis of lignin at relatively higher temperatures could also generate H2. When the pyrolysis was carried out in various gas flows, especially for the cases of 60 and 90 mL/min (Fig. 6(d)), the emission of H2 arrived at the second highest concentration peak at temperatures up to about 800 °C; this peak represented the thermal degradation of heterocyclic compounds or the condensation of aromatic and hydro-aromatic structures (Arenillas et al. 2003).
Furthermore, as shown in Fig. 5, the emission intensities of CO, CO2, and CH4 during CC pyrolysis at relatively faster heating rates were much higher than those values at low heating rates. H2 emission showed the opposite effect, further indicating that the pyrolysis of CC at faster heating rates is more suitable for the production of bio-syngas products, especially for CO, CO2, and CH4. Figure 6 clearly shows the influence of gas flow on the release of these gaseous products, suggesting that the residence time of gas-phase products in the pyrolysis region was closely related to the gas flow. With increased gas flow, the emission intensities of CO, CO2, and CH4 decreased noticeably. Less residence time for volatiles released from the primary pyrolysis was needed to undergo secondary pyrolysis reactions, which further promoted the formation of small-molecule gases, especially CO and CH4 (Jegers and Klein 1987). However, among these three cases (30, 60, and 90 mL/min), the H2 emission intensity was the highest in the 60 mL/min gas flow. This result was consistent with the findings of Ferdous et al. (2001), who reported that the H2 yield could be improved by reduced residence time when the yields of CO and CH4 were decreased.
Figure 7 shows that the biomass feed particle size also had remarkable effects on the distribution of bio-syngas products, but the effects were not consistently changing. The maximum gas concentrations resulted from the pyrolysis of feed particles of 154 μm < d ≤ 280 μm, and the minimum concentrations resulted from the pyrolysis of particles of 74 μm < d ≤ 154 μm. Thus, in the examined ranges of heating rate, gas flow, and particle size, the pyrolysis of CC fractions with 154 μm < d ≤ 280 μm under the heating rate of 20 °C/min and in a gas flow of 30 to 60 mL/min was the most appropriate for the production of bio-syngas in industrial applications. In particular, as for the production of combustible gases (mainly including CO, CH4, and H2) in the synthesis gas product of the pyrolysis, the pyrolysis under the heating rate 20 °C/min and in a gas flow of 30 mL/min are the optimal for CO production and CH4 production, while the pyrolysis under 5 °C/min and in a gas flow of 60 mL/min can be regarded as the optimal for H2 production.
Influence of Different Process Parameters on Thermal Dynamics of CC
The TGA data obtained from experiments under different process parameters can be used for determining important kinetics and for investigating the effects of heating rate, gas flow, and biomass particle size on the thermal dynamics variation of CC. Based on the integral Coats-Redfern method, the linear variation between the ln(g(α)/T2) term and the slope of –E/R (namely, 1/T) under various thermolysis conditions was obtained. Figures 8(a), (b), and (c) show the variation between ln(g(α)/T2) and 1/T at different heating rates, gas flows, and particle sizes, respectively. As can be seen from Fig. 8, there was a good linear relationship between ln(g(α)/T2) and 1/T.
The pyrolysis kinetic parameters in the main pyrolysis stage, such as the activation energy (E) and pre-exponential factor (A), were determined by these equations. The apparent activation energy, fitting equation, pre-exponential factor, and correlation between ln(g(α)/T2) and 1/T with different heating rates, gas flow, and particle sizes are summarized in Tables 5, 6, and 7, respectively. All these calculated values presented a good reproducibility with the standard deviation of less than 0.5%.
Tables 5 through 7 show the E values determined from the Coats-Redfern method under different process parameters varied from 64 to 80 kJ·mol-1, which was much lower than that of coal (generally 120 to 230 kJ·mol-1) recorded in the literature (Liang and Kozinski 2000). Besides, Wang et al. (2008) reported that the E values of sawdust calculated by distributed activation energy model (DAEM) ranged from 161.9 to 202.3 kJ/mol, and Yang et al. (2010) demonstrated that the E values of wheat straw calculated by the methods of Kissinger and Ozawa were respectively 103.92 and 107.69 kJ/mol, all of which are much higher than that of the studied CC samples. Thus, these results further indicate that CC can decompose more easily. Moreover, with increased heating rate and gas flow, the E values decreased gradually, revealing that the energy needed for the pyrolysis at a relatively faster heating rate or in a higher gas flow is certainly more than that of pyrolysis at a slower heating rate or in a lower gas flow.
Fig. 8. The variation of correlation between the ln[g(α)/T2] term and 1/T from Coats-Redfern method (a) at different heating rates, (b) in various gas flow, (c) for particles with different sizes
Table 5. Thermal Dynamics of CC Fractions under Different Heating Rates
Table 6. Thermal Dynamics of CC Fractions in Different Gas Flow
Table 7. Thermal Dynamics of CC Fractions with Varying Particle Size
Table 7 shows that the biomass particle size had an evident effect on the thermal dynamics of CC, but the effect regularity was not obvious for different particle sizes. In addition, the linear correlations (R) relevant to the linear fittings under different thermolysis conditions were all greater than 0.98, suggesting a good linear dependence between ln(g(α)/T2) and 1/T. This result further indicated that the pyrolytic process of CC fractions can be explained in terms of the first-order-reaction with the Arrhenius theory (Masgrau et al. 2003; Mangaraj et al. 2015).
CONCLUSIONS
- TGA results showed evident differences in corncob (CC) pyrolysis under different conditions. Increasing the heating rate and gas flow promoted the thermal cracking of CC and further increased its weight loss. The activation energies determined from the Coats-Redfern method varied from 64 to 80 kJ·mol-1. Particle size also had obvious effects on thermal dynamics, but the effect regularity was not clear on various levels of particle size.
- In the examined particle size range (d ≤ 450 μm), the pyrolysis behavior of larger CC particles was better than that of smaller particles. The pyrolysis of particles of d > 154 μm presented an endothermic peak within 500 to 800 °C, while the particles of d ≤ 154 μm reflected the general feature of exothermic reactions in this temperature zone.
- The evolution profiles of bio-syngas products corresponded well with thermogram curves. The emission intensities of CO, CO2, and CH4 increased with the increased heating rate, while they decreased with elevated gas flow. Using CC particles of 154 μm < d ≤ 280 μm under 20 °C/min and in the gas flow of 30 to 60 mL/min resulted in the optimal bio-syngas production.
ACKNOWLEDGMENTS
The authors are very grateful for the support of Rural Energy Comprehensive Construction Fund of the Ministry of Agriculture of China (Grant. No. 2015-36).
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Article submitted: December 16, 2016; Peer review completed: February 3, 2017; Revised version received and accepted: February 17, 2017; Published: February 22, 2017.
DOI: 10.15376/biores.12.2.2748-2767