The Pyrolysis Characteristics of Bagasse Were Studied by TG-MS-FTIR

Abstract

Sugarcane bagasse is rich in cellulose and lignin, and the recycling of bagasse has become an important research field with the increasing global concern for sustainable development and environmental protection. In this paper, TG-MS-FTIR equipment was used to analyze the pyrolysis characteristics of bagasse from Guangxi under different heating rates and different atmospheres, which is conducive to the reuse of bagasse from the waste gas produced in the sugar plant. The results showed that the pyrolysis rate of sugarcane bagasse in the air atmosphere was faster than that in the nitrogen atmosphere and showed a double-peak trend, and the Coats–Redfern computational model could more accurately simulate the process of pyrolysis. The lower heating rate could overcome the heat transfer hysteresis phenomenon in the process of pyrolysis. In the air atmosphere, the contact time between oxygen and volatile products was shorter due to the high heating rate, and more and more complex species were precipitated at 10 °C/min than at 20 °C/min. In the nitrogen atmosphere, it was favorable to produce more kinds and quantities of gas products, because it did not react with oxygen. FTIR detected CH₄, CO, H₂O, CO₂, C-O-C, and C=O during pyrolysis in nitrogen, and some of C-O-C and C=O were cracked into small molecule compounds at high temperature.

1. Introduction

Traditional energy fuels, such as oil, natural gas, coal, etc., have limited reserves and low energy utilization, and the CO₂, SO₂, and NO_x produced by consumption are the main causes of the global greenhouse effect and atmospheric pollution [1]. Biomass energy has become an important energy source for replacing fossil fuels due to its environmentally friendly and clean abundant reserves and renewable characteristics [2]. At present, oil, coal, and natural gas account for 33%, 24%, and 19% of the global energy consumption, respectively, and the proportion of biomass energy is about 13% [3]. As a zero-carbon energy source, biomass is a good substitute for fossil fuels, and the application of efficient biomass energy conversion technology is one of the important ways to achieve carbon neutrality.

Among all conversion technologies, thermochemical technology is more convenient for converting biomass to fuel, because it is easy to operate and requires lower capital costs [4]. Pyrolysis in thermochemical technology is the only technology that can convert mixtures of biomass and different types of plastics into solid, liquid, and gaseous products [5].

As a kind of agricultural waste, bagasse is rich in cellulose, hemicellulose, lignin, and other organic substances, so the study of its pyrolysis characteristics has important value for energy and resource utilization [6]. Marchese L et al. [7] carried out thermograys analysis on malt bagasse under nitrogen atmosphere at different heating rates (10, 15, 25, and 40 °C/min) and found that the highest carbon mass percentage in the composition was 47.2 wt%. The second component had a high oxygen content of 39.8 wt%. It was found that, in the DTG curve, due to the insufficient heat transfer caused by the high heating rate, the peak value of the heating rate had a significant backward shift. Due to thermal lag, the higher heating rate caused the thermal decomposition to slow down in the direction of the high temperature. Zhang Songsong et al. [8] made a comparative analysis of the pyrolysis characteristics of four non-typical biomass, including bagasse and bark, through TG-GC-MS and found that woody bagasse produced the largest amount of combustible gas, and woody biomass was preferred for hydrogen production. Ma Nan et al. [9] analyzed the effects of different heating rates (10, 20, 30, and 50 °C/min) on the DTG curve of bagasse pyrolysis and found that, as the heating rate increased, the pyrolysis temperature range gradually shifted backward, and thermal hysteresis occurred as the temperature increased. Bustan M D et al. [10] pyrolyzed sugarcane Bagasse at a low temperature in an improved vacuum pyrolysis reactor and found that the generation of H₂, CO₂, CH₄, and CO could be effectively increased by applying an electromagnetic field.

China produces hundreds of millions of tons of sugarcane every year, and the output of bagasse is huge. In this paper, bagasse from Guangxi Province, a major sugarcane-producing province in China, was selected to study the bagasse degradation process. The combination of thermogravimetric analysis and Fourier-transform infrared (FTIR) spectroscopy was used to obtain the progress of thermogravimetric analysis of bagasse. The volatile thermoproducts were also analyzed, and the pyrolysis products at different temperatures were investigated by using TG-MS/FITR, which is of great significance for the effective utilization of the value of sugarcane bagasse biomass.

2. Materials and Methods

2.1. Materials

The sample was bagasse from Guangxi Province of China (abbreviated SB). Sample size ranges were from 0.5 mm to 1 mm. The proximate analysis, ultimate analysis, and heating value are shown in

Table 1. Proximate analysis, ultimate analysis, and heating value.

Sample	Proximate Analysis/Mass% (ad)				Ultimate Analysis/Mass%(ad)					Qnet, ar/ MJ·kg−1
	A	V	FC	M	C	H	O	N	S
SB	2.41	76.91	11.74	8.94	41.22	6.03	41.06	0.35	0.02	16.14

.

2.2. Instruments and Methods

The high-temperature thermal analyzer for SB was a SETARAM SETSYS EVOLUTION 16/18 from Calurie, France with a thermal scanning mode range of 0 to 900 °C at room temperature and heating rates of 10 °C/min and 20 °C/min in air atmosphere and 10 °C/min in a nitrogen atmosphere. The infrared spectrometer uses BRUKER Tensor 27 from Ettlingen, Germany to analyze the functional groups that generate the gas. The mass spectrometer experiments were carried out with the PFEIFFER OMNI star from Asslar, Germany for the analysis of the composition of the generated gas.

2.3. Kinetic Model

The Coats–Redfern method allows the calculation of kinetic parameters using different reaction models, such as the chemical reaction kinetic model, the kinetic model for the decomposition of reticulated coordination polymers, etc., which can be determined on the basis of experimental data. The Coats–Redfern method assumes that the rate of the reaction is a function of the temperature and that the reaction process is proportional to the temperature and the time. This method is an approximation that does not take into account the interactions between different reaction processes, whereas the decomposition of bagasse can be understood as biomass → volatiles + energy. Therefore, it is possible to determine the mechanism function at different heating rates and different pyrolysis stages.

3. Results and Analyses

3.1. TG and DTG Curve Analysis

3.1.1. TG and DTG Analysis of SB Under Different Gas Atmospheres

Figure 1a,b show the comparison of TG and DTG of SB biomass under nitrogen and air atmosphere conditions with a heating rate of 10 °C/min. As can be seen from the figure, the two TG curves for different atmosphere conditions are almost identical until 240 °C, and from 240 °C to 340 °C, the two curves appear to be separated, and the reaction rate of SB under air atmosphere is faster. From 340 °C to 465 °C, the TG curves of SB under air atmosphere showed a turn, and the reaction rate decreased, and this section was mainly the pyrolysis region of cellulose [11], with some crossover with lignin. SB under nitrogen atmosphere did not have a clear turn imagery of hemifibrillar and cellulosic pyrolysis. The pyrolysis was completed with almost the same slope from 240 °C to 370 °C, and this section was mainly the pyrolysis of hemicellulose, cellulose, and lignin pyrolysis. It proceeds from 370 °C onwards without a clear cut-off point. Under the air atmosphere, the reaction of biomass is almost completed at 465 °C, while, under the nitrogen atmosphere, the pyrolysis process is completed at nearly 900 °C. In other words, under an air atmosphere, the pyrolysis process is completed at nearly 900 °C. That is to say, under an atmosphere of air, the main weight loss region is clearly divided into three sections, while, under a nitrogen atmosphere, the main weight loss region is divided into two sections. Cellulose and lignin were converted faster and at a higher rate under the air atmosphere [12,13,14].

3.1.2. TG and DTG Analysis of SB at Different Heating Rates

Figure 2a,b show the TG and DTG curves of SB under air atmosphere with heating rates of 10 °C/min and 20 °C/min. The faster the heating rate, the smaller the slope of the TG curve and the slower the reaction. Under the conditions of different heating rates, the reaction process is almost the same, and the main difference is the speed of the reaction rate. The DTG curves with heating rates of 10 °C/min and 20 °C/min showed two sharp peaks, and the temperatures of the highest peaks were 329 °C and 340 °C, respectively. The peak value here is mainly related to the devolatilization of hemicellulose. The higher the heating rate, the higher the peak value. When the temperature reaches 350 °C, the cellulose is decomposed violently, and the decomposition temperature of lignin is wider, ranging from 160 °C to 900 °C. The passive pyrolysis stage occurs after 450 °C, and the pyrolysis temperature range of the high heating rate is slower than that of the low heating rate [7,9]. Under the given time conditions, a slower heating rate is favorable for the granular biomass to have the same temperature on the outer surface and inside, reducing the temperature gradient between them. On the contrary, a higher heating rate leads to a temperature difference between the outer surface and the inside of the biomass particles, slowing down the reaction rate [15]. Generally, at higher heating rates, hydrocarbons, proteins, and esters are pyrolyzed simultaneously, while, at lower heating rates, hydrocarbons, proteins, and esters are separated [16]; the biomass heating rates in this paper did not reveal a jagged DTG curve in the fast reaction zone, which also suggests that, at the 10 °C/min and 20 °C/min heating rates, the hydrocarbons, proteins, and esters were pyrolyzed simultaneously.

3.1.3. Chemical Reaction Kinetic Parameters

The heat weight loss curve of bagasse can be divided into four stages under air atmosphere and three stages under nitrogen atmosphere conditions (Figure 1). In the first stage, the weight loss curves of air atmosphere and nitrogen atmosphere almost overlap; in the second stage, the slope of the weight loss curve under air atmosphere is bigger, but the trend of both of them is the same; in the third stage, the slope of air atmosphere is smaller than that of the second stage, but it is obviously bigger than that of the weight loss curve under nitrogen atmosphere; in the fourth stage, the weight loss curve under air atmosphere almost becomes a straight line. From the analysis, it can be seen that, in fact, the weight loss curve under air atmosphere, in the third stage, is a significantly higher slope than the weight loss curve under nitrogen atmosphere. The activation energy of biomass, in general, varies with the conversion rate, indicating that the biomass weight loss reaction is not a single-step reaction but a multi-step reaction that varies with the conversion rate [17]. According to Formulas 1 to 10 in Reference [8], the reaction kinetic parameters of the main reaction zones of bagasse under air and nitrogen atmospheres were calculated, as shown in

Table 2. Reaction kinetic parameters of SB.

Biomass	Heating Rate β (°C·min−1)	Reaction Atmosphere	Temperature Rang T (°C)	Apparent Activation Energy E (KJ·mol−1)	Pre-Exponential Factor A (s−1)	Correlation Coefficient (R)
Bagasse	10	N2	240–380	117.70	5.28 × 108	−0.99772
			380–470	140.62	7.35 × 1010	−0.99801
		Air	240–340	164.50	7.76 × 1018	−0.99861
			340–400	83.93	1.64 × 107	−0.94465
			400–470	93.89	2.76 × 107	−0.93153
	20	Air	220–340	152.21	5.46 × 1017	−0.99458
			340–425	77.01	4.06 × 106	−0.97235
			425–490	125.25	3.28 × 108	−0.95268

.

From Table 2, it can be seen that the apparent activation energy is generally higher under a nitrogen atmosphere. The highest apparent activation energy is found under an air atmosphere at 10 °C/min and 20 °C/min between 240 and 340 °C, implying higher energy barriers to be overcome [18]; after which, the apparent activation energy decreases and then increases with the increasing temperature, and pyrolysis of cellulose is dominant between 300 and 400 °C. Cellulose pyrolysis can be seen to require fewer energy barriers to be overcome than hemicellulose and lignin pyrolysis [19,20]. During pyrolysis, cellulose first produces activated cellulose in the intermediate state, which has a smaller molecular weight and, therefore, a smaller energy barrier to overcome [21].

The biomass pyrolysis reaction can be calculated using different calculation models, such as Kissinger-Akahira-Sunose, Flynn-Wall-Ozawa, Distributed Activation Energy Model, and DAEM models [22], and the calculation results are also different. This study uses the Coats–Redfern method, which is calculated using 30 different mechanism functions [23]. When the segmentation is calculated, the smaller the segmentation, the closer the correlation coefficient is to 1. However, the segmentation is too small, and the significance of the subsequent reaction device design reference is obviously reduced. The data used in the design of the device should be within a temperature band, and this temperature band cannot be too small. Therefore, this paper selects the temperature band with almost the same slope as the straight line as the basis for partitioning, which is based on the findings in the literature [24]. At the same time, it combines with the actual situation of the research in this paper. For the same reaction zone, the mechanism function is not always the same. For example, under the air atmosphere, the mechanism functions of the 240–340 °C interval (heating rate of 10 °C/min) and 220–340 °C (heating rate of 20 °C/min) are (1 − 2/3α) − (1 − α)^2/3 and ${\alpha }^2$, respectively. The mechanism functions of the 340–400 °C interval (heating rate of 10 °C/min) and the 340–425 °C (heating rate 20 °C/min) are both [−ln(1 − α)]⁴, and those for 400–470 °C (heating rate 10 °C/min) and 425–490 °C (heating rate 20 °C/min) are [1 − (1 − α)^1/3]² and [−ln(1 − α)]², respectively.

3.1.4. TG-FTIR Analysis

Figure 3 shows the 3D IR spectra of SB in the nitrogen atmosphere at 10 °C/min and air atmosphere at 10 °C/min and 20 °C/min, and Figure 3a shows the FTIR of the nitrogen atmosphere at 10 °C/min. In a nitrogen atmosphere, due to the lack of oxygen, the pyrolysis of biomass mainly occurs as a series of dehydration and dehydrogenation reactions, and the products generated include volatile organic compounds (VOCs) and a number of unsaturated hydrocarbons. These products show specific absorption peaks in the FTIR diagram, which can be used for identification and quantitative analysis. Figure 3b shows the FTIR plot of the air atmosphere at 10 °C/min. Compared to the nitrogen atmosphere, the presence of oxygen in the air atmosphere promotes some oxidation reactions, which affects the composition of pyrolysis products. From the figure, it can be observed that the types and amounts of gaseous products in the air atmosphere are less than those in the nitrogen atmosphere, which is due to the fact that oxygen participates in some of the reactions, resulting in the reduction of volatile products. Figure 3c shows the FTIR plot for the air atmosphere at 20 °C/min. An increase in the rate of temperature rise leads to changes in the kinetics of the pyrolysis reaction, which affects the distribution of the products. It can be seen from the figure that the gas products produced at a heating rate of 10 °C/min have more species and are more complex than at 20 °C/min. This is due to the fact that, when the heating rate is increased, the time for oxygen to come into contact with the volatile products is reduced, which decreases the extent of the oxidation reaction and thus affects the composition of the final products.

3.1.5. Compositional Analysis of SB in Nitrogen

Figure 4 shows the changes in the gas composition of the SB samples during pyrolysis under a nitrogen atmosphere at a temperature increase rate of 10 °C/min. The specific temperature points include 100 °C, 250 °C (onset of fast pyrolysis), 357 °C, and 438 °C (two fast pyrolysis peaks). The peaks at 661 and 2351 cm⁻¹ correspond to CO₂ emission, 3739 cm⁻¹ corresponds to H₂O, and 2184 cm⁻¹ corresponds to CO. The other compositions are characterized by their specific wavelength: 1771 cm⁻¹ belongs to C=O bond stretching, 1537 cm⁻¹ belongs to the aromatic framework, and 1180 cm⁻¹ belongs to the C-O-C link stretching. SB releases CO₂ at different temperatures and shows C=O and C-O stretching vibrations at two fast pyrolysis peaks. The first peak has the most gas components, including H₂O, CH₄, CO₂, CO, etc. In Figure 4, the two peak periods of rapid pyrolysis have significantly more products than the beginning period of rapid pyrolysis of SB, which is because, at 100 °C and 250 °C, the water and other volatile components in SB begin to evaporate, and hemicellulose has begun to pyrolyze while releasing a small amount of CO₂. When the temperature increases to 357 °C, the pyrolysis of cellulose, hemicellulose, lignin, and other polymeric organic substances of SB is intense, resulting in the generation of volatile organic compounds (VOCs) such as CO, CH₄, CO₂, and other carbon oxides. The formation of CH₄ may come from the pyrolysis of methoxy (-OCH₃), methyl (-CH₃), methylene (-CH₂-), and other functional groups [25]. The formation of CO₂ may come from the dehydroxylation reaction and the fracture of the carbonyl group [26]. The formation of CO may come from the fracture of the C=O bond and ether bond [27].

Lignin, due to its more complex chemical structure, usually completes the final pyrolysis process at slightly higher temperatures. As the temperature rises to 438 °C, the production of volatiles decreases, and char formation starts to become more pronounced. The organic polymers decompose further, forming more solid residues (char) and small amounts of gaseous products, and functional groups such as C=O are significantly reduced at this stage. Of the four temperatures, only the temperature corresponding to the first pyrolysis peak has CO production. There is no CO production at the beginning and end of rapid pyrolysis, which should be controlled if the aim is to produce CO gas. C=O stretching will release a large amount of organic matter, including ketone, acid, ester, etc. [28], which, in the pyrolysis of biomass, are the necessary components for the formation of liquid products. Moreover, C-O stretching is the main body of tar formation during biomass pyrolysis. The biggest problem that hinders the application of biomass utilizing pyrolysis gasification on a large scale is tar. C-O stretching occurs both at the peak corresponding temperature and at the second peak temperature, so controlling the amount of tar should be done in the fast pyrolysis zone carried out.

3.1.6. Compositional Analysis of SB in Nitrogen and Air Atmospheres

Figure 5 shows the thermogravimetric weights of SB in the nitrogen atmosphere and air atmosphere at 10 °C/min selected for comparison. Under the nitrogen atmosphere, the first fast pyrolysis peak is 357 °C, and the second fast pyrolysis peak is 437 °C; under the air atmosphere condition, the first fast pyrolysis peak is 327 °C, and the second fast pyrolysis peak is 438 °C. Compared to the nitrogen environment, CH₄ is obviously absent in the air environment, where oxygen acts as a catalyst to promote the oxidative cleavage of hydrocarbons, -OCH₃, -OH, etc., in the biomass, thus inhibiting the generation of methane. Observing the peaks at 437 °C in nitrogen and 438 °C in air in Figure 5, a clear difference can be seen: the nitrogen atmosphere favors the production of more carbon dioxide. This is mainly due to the earlier onset of the fast reaction in air, leading to a significant decrease in carbon dioxide production as the process advances. These findings highlight the influence of the reaction atmosphere on the thermal decomposition and product distribution of SB, highlighting the importance of controlling pyrolysis conditions to optimize the yield and composition of pyrolysis products.

3.1.7. MS Analysis of SB Analyzed by Volatilization During Pyrolysis Under Nitrogen Atmosphere

Small molecule gases in pyrolytic volatile fractions are detected by mass spectrometry, and the absorbance intensity directly reflects the relative concentration of the gas (Lambert–Beer law). From the absorbance intensities, it can be seen that CO₂, acids, aldehydes, and ketones are the dominant gas species compared to other gases. Nevertheless, some other homogeneous diatomic gases, such as N₂ or H₂, are also part of the output gases but are not detectable by FTIR. This is consistent with the reaction in Figure 6. The main products of biomass pyrolysis in the figure are CO₂, C-O-C, C=O, CH₄, CO, and H₂O, where C-O-C and C=O are also cracked during pyrolysis to produce CO and CO₂, among others.

4. Conclusions

• The main heat loss regions of air and nitrogen do not coincide with faster and higher conversion of cellulose and lignin under air atmosphere conditions. Calculations of the activation energy verified that high cellulose pyrolysis requires fewer energy barriers to overcome than hemicellulose and lignin pyrolysis.
• The types and quantities of pyrolysis products in nitrogen atmosphere are higher than those in air atmosphere. Under air atmosphere, more types of pyrolysis were generated at a low heating rate than at a high heating rate.