Thermochemical Conversion of Sugarcane Bagasse: A Comprehensive Analysis of Ignition and Burnout Temperatures

Abstract

The Brazilian sugarcane industry generates a significant amount of waste each year, which should be properly analyzed and studied to allow an adequate recovery and application supported by the best understanding of its properties. The present work reports the ignition and burnout temperatures of sugarcane bagasse (SCB) obtained after performing a thermal analysis using four different heating rates. The intersection method (IM) and deviation method (DM) were employed to approach the ignition and burnout temperatures of the sugarcane bagasse. The ignition temperatures of the SCB measured from IM are between 250 and 263 °C, and their burnout temperatures are between 357 and 377 °C. The ignition temperature was in the range of 205 °C to 236 °C for the DM. IM is recommended for determining the ignition and burnout temperatures. In TGA, the heating rates in the range of 10 °C·min⁻¹ and 15 °C·min⁻¹ are suggested due to their accuracy and the contribution to timesaving in the analysis.

1. Introduction

Sugarcane is one of the most important crops in the world, having an almost strategic status in countries such as Brazil, India, and the Philippines [1,2,3]. Currently, the interest that sugarcane presents is not only linked to the food industry, for the production of sugar, which is one of the most consumed foods globally, but also to the possibility (already a reality) of sugarcane also contributing to the production of biofuels, such as bioethanol [4,5]. This possibility gives sugarcane a very important role in the decarbonization of the energy matrix, as bioethanol is a direct substitute for gasoline, and the replacement of this helps mitigate carbon emissions from fossil origins [6,7,8,9].

Sugarcane, a plant originally from Asia, was taken by Europeans to all parts of the world due to its ancestral importance for the supply of food both to humans and animals, and because it has always been associated with the production of alcohol [10]. Examples are Cachaça from Brazil, Poncha from Madeira (Portugal), Grogue from Cape Verde, and Rhum from the Caribbean [11]. However, sugar production and its distillation are not impact-free since the sugar extraction processes result in significant amounts of waste, namely sugarcane bagasse (SCB) [12,13,14,15].

Despite being a waste of organic origin, its management is not problem-free, namely of an environmental nature, since the vast volumes generated are a large-scale challenge. A part of the waste generated in sugarcane juice extraction is used as an energy source in sugar production plants and alcohol distilleries. However, as SCB volumes are very high, the surplus that is not valued is often left in piles. This material abandonment makes these piles, still with high sugar levels, ferment and emit greenhouse gases into the atmosphere. It is also common for these batteries to ignite (both naturally and caused by man), contributing to even more greenhouse gas emissions.

The relationship between sugarcane and Brazil dates back to the time of its discovery by the Portuguese, who began to cultivate sugarcane there about 500 years ago. Currently, Brazil is responsible for the yearly production of approximately 600 million tons of sugar, which corresponds to about one-third of the world’s production [16,17,18,19]. As mentioned, SCB is the waste obtained after sugarcane juice extraction. This residue, which is a herbaceous-type material, can also be used to recover more sugar through enhanced recovery processes, and also for the production of first-generation ethanol [20,21,22]. The second option is considered a positive alternative for the sugarcane industry, as it plays a critical role in climate change mitigation by supplying an alternative to fossil fuel replacement [23,24]. However, even after this recovery of the still very high sugar content present in the SCB, which remained from the first extraction, the predominantly woody material continues to be a problem for which a solution needs to be found. In this way, recovery through combustion, or using thermochemical conversion processes, such as torrefaction or pyrolysis, should be an option to be taken into consideration.

The efficiency of a biofuel depends on its chemical composition and production conditions. For this reason, it is fundamental to understand all the characteristics and properties, such as the chemical composition of this residual biomass, and, this way, predict the most efficient thermochemical process to convert the residual materials into biomass-derived fuels [25,26]. The thermal analysis provides the assessment of biofuel properties, such as the initial and final temperatures in their combustion processes. In addition, it allows knowing other properties such as the maximum reactivity temperature and the total combustion time, which are both widely used to characterize the thermal process or the kinetics of biomass pyrolysis and combustion [20,27,28]. Biochar particle ignition and burnout are subjects that have raised great interest in several studies, such as those presented by Essenhigh et al. [29], and are essential steps used to describe the biochar combustion characteristic of biomass energy conversion [30,31]. Studies of the ignition and burnout mechanisms of biochar particles are essential for understanding biochar conversion and controlling its combustion process efficiently [30].

The ignition temperature of a material can be defined as being the minimum temperature that allows the self-ignition of that material without the presence of an external source of energy [20,32]. The quantification of the ignition temperature is very important, e.g., for biochar, because it can be directly associated with safety issues during storage and transportation [33]. The occurrences of self-ignition of different biomass-derived fuels or materials are widely documented in the literature, both for non-heat treated biomass, as presented in the works by Schwarzer et al. [34], by Castells et al. [35], and by Restuccia et al. [36], but also for materials resulting from thermochemical conversion processes, such as the approaches presented by Liu et al. [37], Zhou et al. [38], Restuccia et al. [39], Liu et al. [40], or Nunes [41]. The latter author even describes a series of incidents that occurred in industrial units to produce torrefied biomass and associated the risk of self-heating and self-ignition with the presence of black particles and the high content of volatiles. On the other hand, the burnout temperature of a material classifies its reactivity [42,43,44,45]. However, despite the importance of the subject, studies approaching ignition and burnout temperatures of SCB are scarce. Thermogravimetry analysis (TG) and derivative thermogravimetry (DTG) measurements can be used to determine the kinetics of the thermal decomposition of biomass materials since the use of these analytical methods allows the comprehension of the thermal behavior of the samples [46,47]. Thus, the analysis of these parameters can provide more data for optimized biofuel production. In this way, the main objective of the present study is to investigate the ignition and burnout temperatures of SCB. The laboratory tests were conducted under different TG approaches and different heating rates. It is expected that with the results obtained in the present study, which are understood to be innovative mainly for the type of residual biomass in question, practical solutions can be found for the recovery of these residual materials, namely through energy recovery. This energy recovery can follow two paths, the most direct being the combustion path, or else, in order to obtain products with greater added value, the thermochemical conversion path. In both situations, processes can be optimized by knowing the kinetics of reactions and their parameters and thermal behavior.

2. Materials and Methods

2.1. Sample Preparation

The SCB samples were previously washed to remove mineral impurities and the excess sugar remaining. After that, the samples were dried at room temperature for 24 h naturally and, in sequence, were submitted to forced drying at 105 °C using a Solab model SL-100/42 oven (Piracicaba, São Paulo, Brazil). After that, the dried samples were ground using a Wiley mill, MA048—Marconi model (Swedesboro, NJ, USA), and then sieved using Solotest equipment (Bela Vista, São Paulo, Brazil), using an NBR #100 (0.149 mm) sieve.

2.2. Thermogravimetric Analysis

For the TG analysis, a simultaneous Differential Scanning Calorimetry—Thermogravimetric Analysis (DSC-TGA) equipment, from TA Instruments, model SDT Q600 (New Castle, DE, USA) was used. The analysis was carried out at four different heating rates, namely, at 5 °C·min⁻¹, 7.5 °C·min⁻¹, 10 °C·min⁻¹,and 15 °C·min⁻¹. During the tests, nitrogen and air were used as purge gases with a flow rate of 120 mL·min⁻¹. To eliminate the effects of mass and heat transfer limitations, small quantities of biomass, about 1.5 mg, were inserted into alumina crucibles for each test round. The equipment software provided the weight loss curve (TG) and the derivative weight loss curve (DTG), recorded as a function of temperature and time.

2.3. Ignition and Burnout Temperatures

2.3.1. Intersection Method

The ignition temperature (T_i) can be defined as the temperature at which there is a visible decrease in the DTG curve [48]. The ignition temperature was determined from the TG, and two different methods were used in the present study. Using the Intersection Method (IM), two points on a TG curve were identified. As presented in Figure 1a, A is the point where a vertical line from the first DTG peak (the highest value in the DTG curve) crosses the TG curve. B is the point where devolatilization begins. A tangent at A on the TG curve and a horizontal line through B are drawn. The corresponding temperature at the intersection of the two lines is identified as the T_i. The ignition index (D_i) is calculated using Equation (1):

$${\mathrm{D}}_{\mathrm{i}}=\frac{{\left(\raisebox{1ex}{$\mathrm{dw}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{dt}$}\right.\right)}_{\max }}{{\mathrm{t}}_{\mathrm{p}}{\mathrm{t}}_{\mathrm{i}}}$$

(1)

where (dw/dt)_max is the maximum combustion rate, t_p is the corresponding time of the maximum combustion rate, and t_i is the ignition time.

The burnout temperature (T_f) is the temperature on the DTG curve where the oxidation is complete [48,49]. The burnout temperature of biomass was determined from the single peak of a DTG curve [27,28]. In Figure 1a, C is the position on the TG curve where a vertical line from the second peak of the DTG curve crosses the TG, and D is the position where the TG curve becomes steady. A temperature corresponding to the intersection of the tangent on the TG curve at C and the horizontal line through D is defined as the burnout temperature (T_f). In this study, the burnout index was used to analyze the thermal behavior of the samples, as described by Equation (2):

$${\mathrm{D}}_{\mathrm{f}}=\frac{{\left(\mathrm{dw}/\mathrm{dt}\right)}_{\max }}{\Delta {\mathrm{t}}_{1/2}{\mathrm{t}}_{\mathrm{p}}{\mathrm{t}}_{\mathrm{f}}}$$

(2)

where (dw/dt)_max is the maximum combustion rate, ∆t_1/2 the time zone of (dw/dt)/(dw/dt)_max = 1/2, t_p is the corresponding time of (dw/dt)_max, and t_f is the burnout time.

2.3.2. Deviation Methods

The Deviation Method (DM) presents the ignition temperature determined from the deviation of the two DTG curves, using an air and nitrogen atmosphere to study biomass combustion and pyrolysis. The ignition temperature was defined on the DTG curves for the combustion and pyrolysis processes at the exact point where they deviate. Figure 1b presents the determination of the ignition temperature based on DM. The ignition temperature was identified where the relative difference between the values of the two DTG curves reaches 3%.

Table 1. Temperature and intensity of DTG peaks at different heating rates.

Material	Heating Rate (°C·min−1)
	5	7.5	10	15
Bagasse	Temperature (°C)
DTG peak	346.82	347.40	353.49	358.79
	Intensity (%·min−1—%·°C−1)
DTG peak	5.13–0.99	7.15–0.94	9.95–0.98	12.41–0.82

presents the temperature and intensity of the DTG peaks at each of the heating rates.

3. Results and Discussion

The mass loss during thermal conversion at different heating rates of SCB samples was depicted using differential curves of the biomass samples under different atmosphere (air and nitrogen) concentrations. Different stages of mass loss were observed for the SCB. The first stage was due to the moisture loss and the release of some light volatiles at temperatures below 180 °C. Thermal decomposition of lignin can occur in a temperature range from 180 to 500 °C, and the mass loss rate is not as straightforward as in the case of hemicellulose and cellulose, which decomposition has been estimated to be in the range from 210 to 330 °C with a maximum mass loss rate at about 300 °C. The maximum weight loss was observed in the range from 300 to 400 °C, reaching 36.81% in an inert atmosphere and 39.73% in the temperature range from 290 to 357 °C using air, suggesting that the biomasses were more stable using an inert atmosphere. This behavior can be explained by the presence of other particles that influence single-particle thermal exchange. The degradation pattern was very similar to all the heating rates used, varying according to the atmosphere applied. The TG curves for SCB in a nitrogen atmosphere at the four heating rates are shown in Figure 2a, while the corresponding DTG curves expressed in %·min⁻¹ and %·°C⁻¹ are shown in Figure 2b,c, respectively.

Table 2. Ignition and burnout temperatures of sugarcane bagasse at four heating rates.

Material	Heating Rate (°C·min−1)
	5	7.5	10	15
Bagasse	Ignition temperature—Ti (°C)
IMa	250.50	256.45	259.54	263.73
DMb	205.52	227.64	234.03	236.16
	Burnout temperature—Tf (°C)
IM	357.69	366.29	373.57	377.89

Intersection method (using nitrogen as the carrier gas). Deviation method (using air and nitrogen as the carrier gases).

shows the different temperatures of the DTG peaks according to the various heating rates studied.

The determination of the fuel ignition and burnout temperatures can be potentially influenced by the heating rate, biochar quality, particle size, and volatile matter content. All the samples presented a small biochar particle size (approximately 100 µm), and it is estimated that its ignition type did not change when the particle size increased. However, it did change when the heating rate increased. The ignition of SCB started with the degradation of hemicellulose and volatile matter. According to the IM results, the ignition temperature of SCB was in the range of 250 to 263 °C and tended to rise when the heating rate increased. This is possibly due to the cellulose content present in the biomass samples. The ignition temperature of SCB based on DM is in the range of 205 to 236 °C, which is lower than those based on IM. The lower ignition temperature shows the high volatile contents of biomass and allows easier ignition. The results suggest that cellulose content in the biomass changed the ignition characteristics and decomposition of lignin since cellulose can be readily volatilized due to its composition and structural arrangement.

The burnout temperature indicates that oxidation of SCB occurred between 357 °C and 377 °C, according to the IM results. These lower burnout temperatures reveal that SCB does not require a longer residence time or higher temperature to achieve complete combustion due to the lower burnout temperature. Higher burnout temperatures, 448 °C from coconut fiber and 417 °C from eucalyptus leaves, were previously observed by Liu et al. [28]. SCB using a heating rate of 5 °C·min⁻¹ showed the lowest burnout temperature because, using a low heating rate, the volatile matter of the biomass was harder to decompose and the ash content was lower than that found when using other heating rates. The results also showed that the lower the carbon content of biochar, the shorter the burnout time. Figure 3 and Figure 4 show that the ignition and burnout temperatures vary according to the different heating rates. The heating rate affected the ignition and burnout temperatures obtained from IM. A higher heating rate shows higher ignition and burnout temperatures. The ignition and burnout index can be observed in

Table 3. Characteristic combustion parameters of SCB.

Sugarcane Bagasse	Combustion Parameters			Ignition Characteristics		Burnout Characteristics
Heating Rate (°C·min−1)	DTGmax (%·min−1)	tp (min)	ti (min)	tf (min)	Ti (°C)	Di	Tf (°C)	Df
5	5.10	65	45	68	250.50	1.74 × 10−3	357.69	1.56 × 10−5
7.5	7.05	44	32	47	256.45	5.01 × 10−3	366.29	7.03 × 10−5
10	9.93	33	23	35	259.54	1.31 × 10−2	373.57	2.15 × 10−4
15	12.41	23	17	25	263.73	3.17 × 10−2	377.89	1.19 × 10−3

. The ignition index agrees with the results of ignition temperature, and its higher value is indicative of better ignition performance. The influence of heating rates on burnout time must be considered, so burnout time represents the burnout performance of biomass. Higher index values represent better combustibility.

As can be seen from the results presented in Table 3, the heating rates used, respectively, of 5 °C·min⁻¹, 7.5 °C·min⁻¹, 10 °C·min⁻¹,and °C·min⁻¹, seem to influence both ignition temperatures and burnout temperatures significantly. For a heating rate of 5 °C·min⁻¹, the ignition temperature was 250.50 °C, while the burnout temperature was 357.69 °C. For the heating rate of 7.5 °C·min⁻¹, the burnout temperature was 366.29 °C. For the heating rate of 10 °C·min⁻¹, the ignition temperature was 259.54 °C, while the burnout temperature was 373.57 °C. Finally, for a heating rate of 15 °C·min⁻¹, the ignition temperature was 263.73 °C, and the burnout temperature was 377.89 °C. Another critical factor that must be considered is that the ignition time decreases with increasing heating rate. A similar situation exists during burnout time. This apparent relationship between the increase in the heating rate and the subsequent decrease in ignition and burnout times (with the increase in ignition and burnout temperatures) could be related to the faster release of volatiles that occurs with the increase in the heating rate, which leads to an acceleration of the combustion process, as stated by Nunes [41]. However, this situation still needs to be further studied.

4. Conclusions

Sugarcane presents an excellent opportunity for decarbonizing the global energy matrix, especially if it is framed in a circular bioeconomy perspective, where even the waste generated during the sugarcane juice extraction process and the production of first-rate bioethanol generation, as is the case with sugarcane bagasse, are utilized. The valorization of sugarcane bagasse through combustion is an additional possibility that allows for adding another way for the complete valorization of sugarcane. The results obtained allowed us to determine the most suitable parameters for adopting the most appropriate methods and heating rates to obtain the most accurate ignition and burnout temperatures for carrying out combustion tests with SBC in the most accurate, optimized, and efficient method. It was observed that, in the IM and DM tests, there was an increase in ignition and burnout temperatures with increasing heating rates, which resulted from thermal lag in biomass particles. It was also found that the ignition temperatures in the MI are higher than those in the DM, as the combustion of cellulose has a dominant role in the MI, while the combustion of hemicellulose has a dominant role in the DM. Ignition temperatures at IM range between 250 °C and 263 °C. On the other hand, the burnout temperatures are in the range of 357 °C to 377 °C, and the ignition temperature in the DM is in the range of 205 °C to 236 °C. The results obtained also suggest that heating rates of 10 °C·min⁻¹ and 15 °C·min⁻¹ are the best options for processing SCB.