Evaluation of Pyrolysis Reactivity, Kinetics, and Gasification Reactivity of Corn Cobs after Torrefaction Pretreatment

Abstract

To reveal the effect of torrefaction pretreatment on pyrolysis and gasification reactivity of biomass, corn cob was first subjected to torrefaction pretreatment in a fixed-bed reactor at various reaction temperatures. The pyrolysis reactivity, kinetics, and gasification reactivity of torrefied corn cob were systematically assessed by various methods, proving that torrefaction pretreatment has a substantial influence on the physicochemical properties of corn cobs. The O/C and H/C molar ratios of corn cobs considerably drop with the increasing torrefaction temperature, and their higher heat-ing value (HHV) and energy density rise as well. It is found that torrefaction improves the pyrolysis reactivity of corn cobs because hemicellulose degradation is more severe than cellulose degradation during torrefaction, resulting in an increase in the percentage of cellulose in torrefied corn cobs. However, the severe depolymerization, polycondensation, and carbonization reaction during torre-faction of corn cobs at 280–300 °C can lead to a significant decline in the pyrolysis reactivity of corn cobs. Torrefaction pretreatment increases the pyrolysis activation energy of corn cobs, in addition to decreasing the char gasification reactivity of corn cob. The average char gasification reactivity of corn cobs drops when torrefaction severity increases. The passivation of active sites on the char surface may cause condensation and carbonation reactions of corn cobs during torrefaction. These findings provide new sights into the reasonable design of efficient torrefaction methods for appli-cation prior to pyrolysis and gasification of biomass.

1. Introduction

The combination of limited fossil reserves and serious environmental issues such as CO₂ emissions has driven the search for alternative and environmentally friendly renew-able energy sources [1,2,3,4]. In particular, in response to key issues such as global warming and ocean acidification caused by CO₂ emissions, more than 100 countries and regions formally signed and entered into force the Paris Agreement in 2016. Moreover, the Chi-nese government has made a solemn pledge to reach peak carbon dioxide emissions be-fore 2030 and achieve carbon neutrality before 2060. Currently, 124 countries around the world have reached an agreement to achieve carbon neutrality by 2050 or 2060 [5]. Bio-mass can be exploited to replace fossil fuels and produce liquid fuels and chemicals be-cause it is the only form of organic carbon that can regenerate. Biomass can be efficiently converted into liquid fuels and chemicals via pyrolysis and gasification [6,7]. However, the inherently poor quality of biomass, as reflected by low structural homogeneity, low bulk/energy density, high moisture content, high hygroscopicity, poor grindability, and low resistance to biodegradation, is associated with significant difficulties with respect to its handling, storage, and transportation, impeding its widespread utilization and neces-sitating effective pretreatment [8,9,10]. Common pretreatment techniques for the upgrad-ing of biomass include biological, physical, and chemical pretreatment. Biomass can undergo various pretreatment methods to alter its physicochemical characteristics and chemical structure, improving the economic benefits of both biomass logistics and subse-quent pyrolysis and gasification conversion [11].

Among pretreatment methods that can overcome the poor quality of biomass, torre-faction is the most promising [12,13]. Torrefaction is a pyrolysis process that energetically densifies and homogenizes biomass without oxygen at low temperatures [14]. The major reactions that occur during the torrefaction of biomass involve depolymerization (the cleavage of glycosidic bonds), side-chain cleavage, ring opening, polycondensation, and carbonization reactions mainly involving hemicellulose and lignin [15,16]. These reactions can destroy the structure of biomass, making it more finely ground and effectively in-creasing its energy density. Given the abovementioned advantages, torrefaction pretreat-ment has received increasing attention. Chen and colleagues reported that torrefaction pretreatment can effectively improve the heating value of biomass by up to 40%. When the torrefaction temperature is maintained at 250 °C for more than 1 h, the grindability of the torrefaction wood can be considerably enhanced [17]. Park and colleagues demon-strated that severe torrefaction of biomass results in the appearance of condensed structures coupled with lignin degradation through demethoxylation. Torrefaction was re-ported to improve the aromaticity of biomass from 36 to 60% [18]. Chen and colleagues proposed that torrefaction severity is related to the operating conditions of torrefaction and biomass species and can be used as a predictor of torrefaction performance, including energy densification and energy yield [19]. Zheng and colleagues reported that the quality of bio-oil derived from pine can be effectively improved by torrefaction pretreatment. With the application of torrefaction pretreatment, the moisture content of bio-oil was re-duced from 28% to 20%, and the high-heating value increased from 15.74 MJ/Kg to 17.23 MJ/Kg [20,21]. Chen and colleagues showed that the energy density of biomass can be increased by torrefaction pretreatment, the O/C ratio can be decreased, and the syngas quality and cold gas efficiency can be improved during subsequent gasification [22].

Despite these advances, the effect of torrefaction on the pyrolysis reactivity, kinetics, and gasification reactivity of corn cobs has not been explored in detail in previous litera-ture. Corn cob is a kind of agricultural waste residue with enormous potential to produce liquid fuels and chemicals. Herein, we evaluate the effect of torrefaction pretreatment on the pyrolysis behaviors and reaction kinetics of corn cobs, as well as their char gasification reactivity.

2. Experimental Section

2.1. Torrefaction Experiment

Before the torrefaction experiments, raw corn cobs were first dried for 12 h to remove moisture at a temperature of 105 °C. A tubular quartz reactor with a 5.37 cm inner diam-eter and a length of 12.5 cm was used to conduct the torrefaction experiments. For each experiment, approximately 3.0 g of raw corn cob was placed in a quartz vessel located at the head of the quartz tubular reactor. Then, 300 mL/min nitrogen (99.999%, Shengying Chemical Co., Ltd. Guangdong, China) was used to sweep the reactor for 1 h, followed by heating of the reactor. When the temperature reached the set value, the quartz vessel was moved to the reaction zone in the middle of the reactor and maintained for 40 min. Sub-sequently, the quartz cup was moved to the ambient zone at the top of the quartz tube and cooled by purging it with a stream of nitrogen (99.999%) for 10 min. The torrefied samples were then collected and weighed for further experiments. An elemental analyzer (Vario EL, Elementar Analysensysteme, Langenselbold, Germany) was applied to analyze the elemental composition (C, H, N, and S) of raw and torrefied corn cobs.

2.2. Pyrolysis and Gasification Experiment

A thermogravimetric analyzer (STA449 F3, NETZSCH, Germany) was applied to in-vestigate the pyrolysis behaviors of raw and torrefied corn cobs. In each experiment, 10 approximately mg samples were used and heated at 10 °C/min from 30 °C to 900 °C. Here high-purity argon carrier gas (99.999%, Shengying Chemical Co., Ltd. Guangzhou, China) was used at a flow rate of 20 mL/min. A rapid heating thermogravimetric apparatus (SDT650, Waters, Milford, WI, USA) was applied to investigate the pyrolysis kinetics of raw and torrefied corn cobs. Using argon (99.999%) at a flow rate of 40 mL/min, the samples, each weighing about 5 mg, were heated from 30 °C to 900 °C. All data ob-tained from the thermogravimetric analyzers were normalized for further analysis. Heat-ing rates of 5 °C/min, 10 °C/min, and 20 °C/min were applied to collect kinetic information. To obtain the kinetic parameters of samples, we applied a distributed activation energy model (DAEM) that assumes that the pyrolysis or combustion reaction of a sample in-volves an infinite number of independent parallel first-order reactions, each with its own independent activation energy, and that the activation energy is continuously distributed. The detailed DAEM is shown in our previous study [23]. In a fixed-bed reactor with same parameters (700 °C with a residence time of 15 min), pyrolysis char was prepared from raw and torrefied corn cobs. A thermogravimetric analyzer (STA449 F3, NETZSCH, Selb, Germany) was applied to quantitatively evaluate the CO₂ gasification reactivity of pyrol-ysis char. Under 60 mL/min nitrogen (99.999%), 10 mg of pyrolysis char was heated at 30 °C/min from 30 °C to 700 °C and maintained for 10 min. Then, the carrier gas was replaced with CO₂ at a rate of 60 mL/min; the temperature was then increased to 1100 °C at 10 °C/min and maintained for 40 min. Carbon conversion (x_t), instantaneous gasifica-tion reactivity (r_t, min⁻¹), and average gasification reactivity (r_a, min⁻¹) were determined according to the following formulae, respectively:

x_t = (w₀ − w_t)/(w₀ − w_ash)

(1)

r_t = −1/(w₀ − w_ash)·(dw_t)/d_t = (dx_t)/d_t

(2)

r_a = (∫₀^tr_tdt)/t

(3)

where w₀ represents the initial weight, w_t represents the weight at time t, and wash rep-resents the weight of the ash left after the gasification reaction.

3. Results and Discussion

3.1. The Effect of Torrefaction Temperature on the Mass Yields and Elemental Composition of Torrefied Corn Cobs

Figure 1 shows the mass yields of torrefied corn cobs. It is obvious that torrefaction has a significant impact on the mass yield of torrefied corn cobs. The yield of torrefied corn cobs steadily decreases from 95.1% to 45.80% when the torrefaction temperature in-creases from 220 °C to 300 °C. The weight loss of corn cobs is attributed to the devolati-lization of hemicellulose and lignin within their structure during torrefaction. Side-chain cleavage reactions of hemicellulose and lignin, such as deacetylation and demethoxyla-tion, first occur at the beginning. When the torrefaction temperature increases, the depol-ymerization of hemicellulose and lignin begins. The presence of inherent alkali and alka-line earth metals (AAEM) promotes the ring-opening reaction of hemicellulose, which in turn promotes the formation of light oxygenates. When the torrefaction temperature is exceeds 270 °C, significant cellulose depolymerization begins, and hemicellulose and lig-nin undergo polycondensation and carbonization reactions. As a result, the color of the corn cobs gradually changes from light yellow to dark yellow and, finally, to dark brown. A previous study showed that pinewood torrefied at 300 °C for 40 min still produced a solid yield of nearly 70%, which is considerably higher than the yield of torrefied corn cobs obtained under the same reaction conditions [24], possibly owing to the higher hem-icellulose content of corn cobs compared to woody biomass. Hemicellulose has been iden-tified as the most reactive component during torrefaction [25], and the thermal stability of three dominant components follows the order of hemicellulose < lignin < cellulose [16]. Thus, the hemicellulose content of biomass determines its mass loss during torrefaction.

Elemental analysis results of raw and torrefied corn cobs are summarized in

Table 1. The elemental analysis, heating value, and energy yield of raw and torrefied corn cobs.

Feedstock		Raw	Torrefaction Temperature/°C
			220	240	260	280	300
Elemental analysis/wt.%	C	44.70	46.5	49.45	54.02	58.86	64.21
	H	6.12	6.181	5.468	5.76	5.222	4.807
	N	0.23	0.25	0.37	0.36	0.49	0.56
	O	48.95	47.07	44.71	39.86	35.43	30.42
H/C molar ratio		1.64	1.60	1.33	1.28	1.06	0.90
O/C molar ratio		0.82	0.76	0.68	0.55	0.45	0.36
HHV (MJ/kg)		18.50	19.05	19.40	20.88	21.88	23.11
Energy densification ratio		1.00	1.03	1.05	1.13	1.18	1.25
Energy yield (%)		100	96.59	80.71	75.43	62.71	57.22

. Torrefaction has a considerable influence on the elemental composition of corn cobs. The carbon content of torrefied corn cobs gradually increases from 44.70 to 64.21% with in-creasing torrefaction temperature, and the oxygen content continuously decreases from 48.95% to 30.42%. During torrefaction, the hydrogen content of corn cobs first increases at 220 °C and then decreases with increased torrefaction temperature. Both the H/C and O/C molar ratios of torrefied corn cobs drop drastically increased torrefaction temperature. Therefore, torrefaction is general deoxygenation and dehydrogenation reaction that can be used to enrich carbon. During torrefaction, oxygen in the corn cob is removed, primari-ly in the form of H₂O, with minor portions of CO, CO₂, and other oxygenated compounds, such as acetic acid [26]. Hence, the O/C molar ratios of raw and torrefied corn cobs are linearly related to their H/C molar ratios (Figure 2). The higher heating value (HHV) of corn cobs can be enhanced by torrefaction pretreatment. The HHV of corn cobs increases from 18.50 to 23.11 MJ/kg with increased torrefaction temperature, and the energy densification ratio improves from 1 to 1.25 (Table 1). During torrefaction, the energy densificat-ion of corn cobs can be attributed to the deoxygenation and carbonization reactions. The energy yields of torrefied corn cobs decline from 96.59 to 57.22% when the torrefaction temperature increases from 220 °C to 300 °C, which are higher than the corresponding mass yields. Therefore, torrefaction is an effective approach to achieving energy densific-ation of biomass with high energy efficiency.

3.2. The Effect of Torrefaction Temperature on Pyrolysis Reactivity and Kinetics of Corn cobs

Figure 3 shows the TG/DTG curves of both raw and torrefied corn cobs. It is obvious that torrefaction pretreatment can significantly affect the pyrolysis behaviors of corn cobs. An obvious feature of the DTG curve of raw corn cobs is that there are two shoulder peaks with comparable intensity around 300 °C. The former refers to the pyrolysis of hemicellul-ose, and the latter refers to the pyrolysis of cellulose. The peak strength of hemicellulose gradually decreases as the torrefaction temperature increases and then disappears at approximately 240 °C, suggesting that hemicellulose can be decomposed effectively at this torrefaction temperature. The maximum weight loss rate (D_max) and its corresponding temperature (T_max) can be obtained from the DTG curves of raw and torrefied corn cobs. When the torrefaction temperature is less than 260 °C, the D_max of torrefied corn cobs changes slightly. However, as the torrefaction temperature subsequently increases to 280 °C, the D_max of torrefied corn cobs decreases drastically from 28.5 to 12.2%, indicating that the significant depolymerization of cellulose when the torrefaction temperature is 280 °C. The T_max of torrefied corn cobs varies slightly in the range of 319.3 to 325.2 °C when the torrefaction temperature is less than 280 °C. With a further increase in torrefaction temperature up to 300 °C, the T_max of torrefied corn cobs improves considerably from 323.5 to 379.8 °C, implying that the severe polycondensation and carbonization of corn cobs take place at this torrefaction temperature. Torrefaction also has a substantial influence on the initial pyrolysis temperature and the yield of char. With increasing torrefaction temp-erature, the initial pyrolysis temperature of corn cobs increases, and the yield of char (pyrolysis residue) increases. These results can be attributed to the devolatilization, polyc-ondensation, and carbonization reactions of corn cobs caused by torrefaction.

As shown in

Table 2. The pyrolysis characteristic parameters obtained from the TG/DTG curves for pyrolysis of raw and torrefied corn cobs.

Feedstock	Raw	Torrefaction Temperature/°C
		220	240	260	280	300
The yield of char at 700 °C	24.6	28.8	40.0	38.3	53.2	65.1
Dmax (%/min)	26.2	26.6	24.5	28.5	12.2	6.1
Tmax (°C)	324.4	325.2	319.3	325.2	323.5	379.8
Ti (°C)	199.2	214.1	227.5	243.1	250.6	271.3
CPI a (10−4%/(min·°C2))	3.23	3.68	4.18	5.34	2.59	0.74

^a CPI = D_max/[T_max(T_f − T_i)], T_f = 2T_max − T_i, where T_i is the initial pyrolysis temperature, and T_f is the final pyrolysis temperature.

, the pyrolysis reactivity of torrefied corncob is quantitatively described by the comprehensive pyrolysis index (CPI) [27]. When the torrefaction temper-ature increases, the CPI of torrefied corn cobs first increases and reaches the maximum value at 260 °C. The enhancement in the pyrolysis reactivity of corn cobs is attributed to the increase in the percentage of cellulose in torrefied corn cobs as a result of the more severe decomposition of hemicellulose due to torrefaction. As the torrefaction temperat-ure further increases from 260 to 300 °C, the CPI of torrefied corn cobs decreases conside-rably, owing to the severe depolymerization of cellulose and the polycondensation/carb-onization of hemicellulose/lignin during the torrefaction of corn cobs at this temperature. Therefore, for effective pretreatment prior to biomass pyrolysis, the torrefaction temperat-ure should be not higher than 260 °C. A higher torrefaction temperature could cause sev-ere devolatilization, polycondensation, and carbonization of biomass, resulting in a cons-iderable reduction in the pyrolysis reactivity and resulting liquid yield.

Based on the experimental results reported above, three torrefaction (220, 240, and 300 °C) were selected to represent light torrefaction (LT), medium torrefaction (MT), and severe torrefaction (ST), respectively. The distributed activation energy model (DAEM) was applied to calculate the kinetic parameters for the pyrolysis of raw and torrefied corn cobs with varying severity. Figure 4 shows the resulting Arrhenius plots at various conv-ersions (α) for the pyrolysis of raw and torrefied corn cobs with varying severity, and Figure 5 shows the activation energies that were determined based on the slopes at vario-us conversion rates. As shown in Figure 5, the pyrolysis activation energies of the raw corn cob ranged from 115 kJ/mol to 163 kJ/mol, and the fitted correlation coefficient was better more than 0.99, showing that the DAEM is applicable for the calculation of the act-ivation energy for the pyrolysis of corn cob. The pyrolysis behaviors of raw corn cobs at the conversion rates of 0.1 to 0.3 are mainly attributed to the decomposition of hemicellulose in corn cobs, and its activation energy gradually increases. We speculate that the hemicellulose in the corn cob proceeds first by side-chain cleavage, which requires relatively low activation energy. Hemicellulose then undergoes depolymerization and ring-opening reactions, which require increased activation energies. The pyrolysis pro-cesses of raw corn cobs at the conversion rates of 0.3 to 0.7 are primarily related to the pyrolysis of cellulose. When the conversion is greater than 0.7, the pyrolysis processes of raw corn cobs may be associated with the depolymerization and condensation reactions of lignin. The average pyrolysis activation energy of raw and torrefied corn cob follow the order of ST (243.0 kJ/mol) > MT (215.9 kJ/mol) > LT (199.7 kJ/mol) > Raw (138.9 kJ/mol), indicating that torrefaction pretreatment improves the pyrolysis activation energy of corn cobs, and its average pyrolysis activation energy increases with increased torrefaction se-verity. The devolatilization, polycondensation, and carbonization reactions of corn cob may be responsible for this phenomenon. Furthermore, the activation energy required for pyrolysis of ST is as high as 399.9 kJ/mol when the conversion is 0.1, possibly due to the severe carbonization reaction of hemicellulose in corn cob at this torrefaction severity.

3.3. The Effect of Torrefaction Severity on Char Gasification Reactivity of Corn cobs

The effect of torrefaction severity on conversion during CO₂ gasification of pyrolysis char derived from raw and torrefied corn cobs is shown in Figure 6A. The graph shows that torrefaction reduces the carbon conversion of corncob char. When the gasification temperature is 800 °C, the carbon conversion of char derived from raw corncob is 38.8%. The carbon conversion of char obtained by torrefaction of corn cob at 220 °C is reduced to 33.3%, and the lowest carbon conversion (24.4%) is obtained by char derived from corn cob torrefied at 300 °C. When the char gasification temperature is 900 °C, the carbon con-version of raw corn cob is 72.8%, and when the torrefaction temperature increases from 220 °C to 300 °C, the carbon conversion of char derived from torrefied corn cobs decreases from 60.3% to 43.9%. Figure 6B shows the instantaneous char gasification reactivity of corn cobs. Torrefaction is effective in reducing the instantaneous char gasification reactiv-ity of corn cobs. The char gasification of raw corn cob under a CO₂ atmosphere comprises two stages. The char gasification reaction rate increases gradually at first and then de-creases rapidly, an reaches the maximum value at approximately 878 °C. As the severity of torrefaction increases, the instantaneous char gasification reactivity of torrefied corn cobs decreases gradually. Their maximum char gasification reaction rate shifts toward higher temperatures, and the time required for termination of the char gasification reac-tion also increases. The instantaneous char gasification reactivity of corn cobs as a function of carbon conversion is illustrated in Figure 6C. The resulting average char gasification reactivity is shown in Figure 6D. Torrefaction pretreatment can reduce the average char gasification reactivity of corn cobs. With increased torrefaction severity, the average char gasification reactivity of corn cobs drops gradually, possibly as a result of the passivation of the active sites on the surface of corncob char by the torrefaction pretreatment.

4. Conclusions

In this study, we demonstrated that torrefaction pretreatment has a considerably influence on the physicochemical properties of corn cobs. With increasing torrefaction temperature, the O/C and H/C molar ratios of corn cobs decrease significantly, and their HHV and energy density increase. Severe decomposition of hemicellulose can lead to an in-crease in the percentage of cellulose in torrefied corn cobs, and the pyrolysis reactivity of biomass can be improved by torrefaction. However, when the torrefaction temperature is less than 280 °C, the pyrolysis reactivity of corn cobs decreases, owing to the severe de-polymerization of cellulose in their structure during torrefaction at this temperature. Tor-refaction pretreatment increases the pyrolysis activation energy of corn cobs and reduces the CO₂ gasification reactivity of corncob char. The average char gasification reactivity of corn cobs decreases with increased torrefaction severity, possibly due to the passivation of active sites on the char surface caused by the polycondensation and carbonization re-actions of corn cobs during the process of torrefaction. The study results indicate a quan-titative relationship between the torrefaction severity of corn cobs and their pyrolysis and gasification reactivity, which providing a theoretical and methodological basis for optimi-zation of the torrefaction severity of biomass prior to pyrolysis of gasification.