Introduction

Fossil fuels including coal, oil, and natural gas, are currently the world’s leading energy sources and account for more than 80% of the world’s energy consumption, resulting in rising prices and falling reserves of oil [1]. There is an urgent need to find raw materials other than fossil fuels because the oil reserves have been declining, and the energy demand is growing along with automotive fuels and petrochemicals. Therefore, lignocellulosic biomass is the only renewable source of fixed carbon that can be converted into liquid, solid, and gaseous fuels, in addition to heat and energy without contributing to emission of CO2 [2, 3].

In this context, it is apparent that two-thirds of the biomass of sugarcane is not used for the production of bio-fuels, such as ethanol. Only the cane juice, sucrose, is used for this purpose. Although bagasse is not yet used for producing bio-fuels, it is being harnessed to generate energy for the production of bio-electricity. The straw from sugarcane, which contains one-third of the total energy of the cane, is a residue of sugar and ethanol activity and is not used by this industry, thereby generating a negative environmental impact due to burning and high cost of handling.

Flash pyrolysis is one of the process, which has been widely studied [3, 4] since it involves a sustainable valorization of biomass to obtain transportation fuels and valuable products with different applications [5, 6]. As a thermochemical process, flash pyrolysis requires a greater knowledge of the heat balance for conducting the process economically and, consequently, the kinetic parameters involved in its reactions [4].

The thermal analysis techniques, in particular, differential scanning calorimetry (DSC) and thermogravimetric analysis (TG), allow obtaining such information easily and quickly. TG is the most common technique used to analyze the kinetics of devolatilization. There are several reports in the literature describing the TG analysis between various materials such as plastics, evaporation of water, natural fibers and different types of biomasses in the degradation process [7,8,9]. In recent years, an overall recovery of biomass has been suggested in the context of biorefinery to obtain transportation fuels and valuable chemicals, by various processes, such as pyrolysis, gasification and hydrolysis [3, 10,11,12]. Among these processes, pyrolysis is an endothermic process that occurs during the degradation of biomass at temperatures ranging from 673 to 1173 K, with production of various products such as coal, liquid pyrolytic bio-oil, and combustible gases.

In most of the pyrolysis processes, according to several authors, an inert atmosphere is used without oxygen. In the context of thermochemical processes, pyrolysis when compared with other processes such as combustion and gasification, has the advantage of a relatively low energy cost and can occur at a temperature range of 623–923 K, and whereas the gasification and combustion processes occur in at range of 1023–273 K [13,14,15]. However, a major problem for pyrolysis processes is the energy needed to heat and degrade the raw materials and the fluidizing gas according to the processing temperature. This energy is provided only partially by the fluidizing gas or sand, since the flexibility of the heating rate and the inlet gas temperature is low.

Thus, the oxidative pyrolysis with the addition of low concentrations of oxygen to prevent significant effects on combustion reactions, is a major breakthrough for the pyrolysis process, since the energy required for the process is supplied by combustion of the pyrolytic products, resulting in an autothermal operation, facilitating the integration of energy optimization and operational feasibility of the system [3]. Agricultural and agroindustrial residues are increasingly being investigated as biomass in the generation of energy and inputs, especially for pyrolysis processes, such as tobacco residue [16], sugarcane bagasse [17], and wood [18].

Recently, we performed an oxidative fast pyrolysis of sugarcane straw in a pilot plant, which has produced high liquid-fraction yield (50%) and molecules that can be upgraded further (Electronic Supplementary Material 1). Then, thermogravimetric and calorimetry investigation, which is very scarce regarding oxidative pyrolysis [19, 20], is quite important to optimize the energy consumption in future pyrolysis plants.

Thus, the main objective of this research was to evaluate the required heat and the kinetics of pyrolysis of sugarcane straw using different oxygen concentrations.

Materials and methods

Characterization of biomass

Proximate analysis of sugarcane straw

Sugarcane straw samples were obtained from the waste of sugar-ethanol industry in the state of Minas Gerais, Brazil. The dried sugar cane straw (biomass) was milled in a hammer mill, Nogueira Máquinas Agrícolas brand, with a sieve of 1 mm diameter. After milling, the biomass was separated by sieving, using finer sieves, with meshes of mean diameter of 0.45–0.22 mm. In the experiments used the samples obtained by sieving. For immediate analysis, the methods described by ASTM E871-82 E872-82, E1534-93, and D3172 were used. For the volatile content was used to ASTM E872-82 with adaptation to muffle, where the sample was subjected to a temperature of 1123 K for 7 min in a crucible with a lid. The ash content was determined based on the ASTM E1534-93. The content of fixed carbon was determined using the difference to that of previous analyses according to the method of ASTM Standard D3172.

Fourier transform infrared spectroscopy (FTIR)

FTIR measurements were performed in an Equinox 55 instrument by direct absorbance using KBr pellet technique. Each spectrum was recorded over 64 scans, in the range from 4000 to 400 cm−1 with a resolution of 4 cm−1. Background spectra were collected before every sampling. KBr was previously oven-dried to avoid interferences due to the presence of water.

Elemental analysis and calorific value

The contents of carbon, hydrogen and nitrogen were determined using the equipment CHNS/O Perkin Elmer 2400. The calorific value was determined in an IKA C2000 bomb calorimeter based on the method 8633/NBR NBR 11956. The oxygen content was determined by the difference, according to Eq. 1 as follows:

$$\% {\text{O}} = 100 - (\% {\text{C}} + \% {\text{H}} + \% {\text{N}} + {\text{Ash}})$$
(1)

Kinetic study by TGA

The kinetics of degradation involves determining the degree of conversion of the fraction decomposed (α), where, in non-isothermal analyses, (α) can be calculated using Eq. 2 [21] as follows:

$$\alpha = \frac{{(m_{0} - m)}}{{(m_{0} - m_{\infty } )}}$$
(2)

where m0 is the initial mass, m is the mass at each point in time, and \(m_{\infty }\) is the final mass.

The conversion rate of solid-state reaction kinetics (dα/dt) can be described as a function k (t) and f (α), according to Eq. 3 as follows:

$$\frac{{d_{\upalpha} }}{{d_{\text{t}} }} = k(T)f(\alpha )$$
(3)

where \(f\left( \alpha \right)\) is a function of the reaction mechanism, and k is the rate constant generally given by the Arrhenius equation (Eq. 4) as follows:

$$k(T) = A{\text{e}}^{{\frac{{ - \rm{E}_{\rm{a}} }}{\rm{RT}}}}$$
(4)

where \(E_{\text{a}}\) is the apparent activation energy (kJ mol−1), R is the ideal gas constant (8.314 J mol−1 K−1), A is the pre-exponential factor (1 min−1), and T is the absolute temperature (K). Combination of Eqs. 3 and 4 leads to Eq. 5 as follows:

$$\frac{{d_{\upalpha} }}{{d_{\text{t}} }} = Af(\alpha ){\text{e}}^{{\frac{{ - \rm{E}_{\rm{a}} }}{\rm{RT}}}}$$
(5)

In non-isothermal analyses, the rate of heating (\(\phi\)) can be varied, with temperature and time being dependent on the rate that can be described by Eq. 6 as follows:

$$(\phi ) = \frac{{d_{\text{T}} }}{{d_{\text{t}} }}$$
(6)

Thus, the reaction rate of a non-isothermal event is described as follows:

$$\frac{{d_{\upalpha} }}{{d_{\text{T}} }} = \left( {\frac{A}{\phi }} \right){\text{e}}^{{\frac{{ - \rm{E}_{\rm{a}} }}{\rm{RT}}}} f(\alpha )$$
(7)

The integral form of Eq. 7 results in:

$$g(\alpha ) = \int\limits_{0}^{\alpha } {\frac{{d_{\upalpha} }}{{d_{\text{T}} }}} = \left( {\frac{A}{\phi }} \right)\int\limits_{{T_{i} }}^{{T_{f} }} {{\text{e}}^{{\frac{{ - \rm{E}_{\rm{a}} }}{\rm{RT}}}} } {\text{d}}T = \frac{{AE_{\text{a}} }}{\phi R}p(x)$$
(8)

The FOW method is one of the isoconversional methods used to estimate the activation energy, and is based on Doyle integral using Eq. 9 [9, 22] as follows:

$$\ln (\phi ) = \ln \left( {\frac{{AE_{\text{a}} }}{Rg(\alpha )}} \right) - 5.331 - 1.052\frac{{E_{\text{a}} }}{RT}$$
(9)

Thus, a conversion is obtained from a series of tests performed at various rates of heating to form a linear regression. OriginLab 8.5 was used to plot graph \(\ln \left( \phi \right)\) versus \(\frac{1}{T}\). Equation 10 calculates the Ea of each degree of conversion as follows:

$${\text{Slope}} = - E_{\text{a}} \frac{1.052}{R}$$
(10)

The analyses were carried out in a thermogravimetric analyzer DTG Shimadzu-60 using an inert atmosphere and oxidizer (3 and 20% O2) of total flow rate of 50 mL min−1. Heating rates of 5, 10, and 20 K min−1 were used with a temperature of 303–873 K. The dough used in each experiment weighed about 5 mg.

Heat required for the pyrolysis process

The heat required for the pyrolysis of biomass has an important influence on the course of thermal conversion, being the sum of the following two components: the caloric requirement to heat the biomass, and the heat needed to complete the reactions of pyrolysis, since the process is endothermic and requires external energy sources. This parameter is very important in the design of a reactor, in the analysis of energy balance, and in the analysis of potential thermoconvertion processes [23].

Thus, the heat necessary for the process can be described as Eq. 11 as follows [24]:

$$C = m_{\text{b}} c_{{{\text{p}},{\text{b}}}} (T_{\text{r}} - T_{ 0} ) + m_{\text{b}} \Delta H_{\text{r}}$$
(11)

\(C\) is the heat required per kg (kJ kg−1), \(m_{\text{b}}\) the mass of biomass (kg), \(c_{{{\text{p}},{\text{b}}}}\) the specific heat of biomass, \(T_{\text{r}}\) the temperature of the reaction (pyrolysis), \(T_{0}\) the initial temperature and the process, \(\Delta H_{\text{r}}\) the heat of reaction (kJ kg−1).

DSC is a technique that measures the difference in energy supplied to the substance and a reference material inert in relation to temperature, while both are subjected to a controlled temperature program, so that the sample and the reference are maintained in isothermal conditions [25]. Using DSC, it is possible to visualize that the heating of biomass occurs in the heat required, presenting an endothermic reaction character. Integration of heat flows over time results in the total heat required for the pyrolysis process as a function of temperature [23], according to Eq. 12 as follows:

$$\frac{{d_{\text{c}} }}{{d_{\text{t}} }} = c_{\text{p}} \frac{{d_{\text{T}} }}{{d_{\text{t}} }} + \Delta H_{\text{r}} \to \int\limits_{0}^{t} {\left( {c_{\text{p}} \frac{{d_{\text{T}} }}{{d_{\text{t}} }} + \Delta H_{\text{r}} } \right)} d_{\text{t}}$$
(12)

Interference from biomass moisture was eliminated before integration of the DSC curves, since it influences the heat required for the process [24].

Results and discussion

Proximate and elemental analyses

The values obtained from the elemental analyses are shown in Table 1, which shows that the values obtained for sugarcane straw were quite similar to those reported in the literature. Moisture content of the straw was approximately 3.12%, which is considered as an adequate value (< 10%) for transformation in thermochemical processes such as pyrolysis.

Table 1 Proximate analysis of sugarcane straw
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The calorific value of a fuel is the amount of energy released during the complete combustion of a given mass of fuel. A higher calorific value takes into account the physical state of the products at the same initial test temperature—the water produced in the combustion is obtained in a liquid form. The higher calorific value measured for straw (16.42 MJ kg−1) is compatible with values found in the literature for lignocellulosic biomasses, with 17.70 and 17.74 MJ kg−1 for sugarcane straw [26], respectively, and 14.70 MJ kg−1 for elephant grass [27] and 16.35 MJ kg−1 for rice straw [28].

Fourier transform infrared spectrometry (FTIR) and chemical composition of biomass

The absorption spectrum in the Fourier transform infrared spectroscopy (FTIR) region for sugar cane straw can be observed in Fig. 1. The primary characteristics of this spectrum are attributed to the presence of lignin, hemicellulose and cellulose, characteristic of natural fibers. At higher frequencies, the band at 3422 cm−1 is associated with the axial deformation of the O–H group, which corresponds to the functional groups of phenols, alcohols, and carboxylic acids. The two subsequent bands at 2925 and 2846 cm−1 refer to the axial deformation vibrations of the CH group, which are confirmed by the deformation bands detected at 1515 and 1460 cm−1, which are strongly bound to the aromatic and phenolic rings present in lignin. At low frequencies, the band detected at 1734 cm−1 is associated with C–O–C stretching vibrations confirmed by its asymmetric strain band at 1266 cm−1. The presence of these two bands demonstrates the presence of cellulose chains and guaiacyl rings. The band at 1650 cm−1 is relative to the carbonyl group (C=O), associated with hemicellulose present in sugar cane straw. The band at 1425 cm−1 is associated with asymmetric deformation of the CH2 group present in cellulose. The band at 1375 cm−1 is associated with C–H deformation in cellulose and hemicellulose. The band corresponding to 1330 cm−1 refers to C–H vibration in cellulose and C–O vibration in syringyl derivatives. The bands at 1158 and 1044 cm−1 refer to C–O vibration in cellulose and hemicellulose. Finally, bands below 898 cm−1 are characteristic of the C–H deformations of the aromatic groups of lignin [29, 30].

Fig. 1
figure 1

The absorption spectrum in the Fourier transform infrared region (FTIR) for sugarcane straw

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Kinetic study

As show in Fig. 2, it can be noted that the curves show gradual displacement curves to higher temperatures, with increasing heating rates. This phenomenon is expected because the low heating rates generate large time intervals to produce the respective curves [31]. As reported by [32, 33], the displacement of the TG curves to higher temperatures for higher heating rates was observed as a consequence of the heat transfer limitations within the samples. The process of decomposition of biomass in different atmospheres at 20 K min−1 is shown in Fig. 3.

Fig. 2
figure 2

TG curves for the sugarcane straw in different atmospheres (inert and oxidizing) and heating rates

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Fig. 3
figure 3

TGA curves for sugar cane straw in inert and oxidizing (3 and 20% O2) atmospheres at 20 K min−1

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In this event, the mass loss in the three atmospheres was 64% (3 and 20% O2) and 67% (inert). The last event started at 633 and 673 K for oxidizing and inert atmospheres, respectively, is due to thermal degradation of lignin in the three atmospheres, since its structure is highly complex due to the presence of aromatic rings and multiple branches, reaching broad temperature ranges to be degraded, 373–1173 K [34]. In the atmosphere with 20% O2, the process terminates at a temperature of approximately 774 K (93% mass loss), with 3% O2, the process ends at 788 K (95% mass loss), and in the inert atmosphere, the process ends at a temperature of 873 K (92% mass loss), showing that the pyrolysis of biomass can be performed at temperatures below 973 K.

Figure 4 shows the DTG curves corresponding to pyrolysis in an inert and oxidizing atmosphere at a heating rate of 20 K min−1. In oxidative pyrolysis the first peak corresponds to the pyrolysis and heterogeneous oxidation, with temperatures of 603 K for the oxidizing atmosphere and 613 K for the inert atmosphere, while the second peak is assigned to the combustion of coal (704 and 733 K for 20 and 3% O2, respectively), since the second peak represents the degradation of lignin and occurs in a wider range of temperatures for the inert atmosphere [35]. According to [36], the degradation of sugar cane bagasse showed two peaks when they carried out the pyrolysis in an oxidizing atmosphere, starting from the exothermic reaction (combustion) at 573 K and ending at 773 K. It was observed that the inert atmosphere to the exothermic reaction started at 723 K and ended at 973 K.

Fig. 4
figure 4

DTG curves corresponding to pyrolysis in inert and oxidizing (3 and 20% O2) atmospheres at 20 K min−1

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Apparent activation energy (E a)

For the calculation of apparent Ea of sugarcane straw in the three atmospheres, a range of 20–80% conversion was analyzed. Through the thermal analysis method and the use of FOW method at different heating rates (5, 10, and 20 K min−1), the curves plotted in Fig. 5a–c (inert, 3% O2, and 20% O2 atmospheres, respectively) were obtained. The apparent Ea values are shown in Table 2.

Fig. 5
figure 5

Application of the FOW model to the sugarcane straw in different atmospheres

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Table 2 Values of apparent activation energy for the sugar cane straw in different atmospheres in rates of heating at 5, 10 and 20 K min−1 applying the method of FOW
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As show in Table 2, it can be observed that by varying the rate of conversion of 20–50%, no significant differences were observed in the Ea for the pyrolysis of straw in an inert atmosphere and 20% O2, with values ranging from 131 to 140 kJ mol−1. However, in 3% O2 atmosphere in the same ranges, the conversion values were lower, ranging from 101 to 130 kJ mol−1, possibly showing that small amounts of oxygen are responsible for reactions with combustion heat release and formation of few oxygenates resulting in lower energy ranges of activation. Considering the conversion rate of 60%, the activation energy in 20% O2 atmosphere significantly increased (from 137 to 198 kJ mol−1), whereas in the inert atmosphere (140–134 kJ mol−1) and 3% O2 (130–126 kJ mol−1), the activation energy had decreased. This highlights the participation of oxygen in the degradation reactions, which reacts with the substances present in the lignocellulosic material and can form various products during the stages of degradation, thus justifying the increase in activation energy in this conversion of straw in the atmosphere with 20% O2. These results show that sugarcane straw is more stable in an oxidizing atmosphere than in an inert atmosphere. This stability was also demonstrated for crushed sugar cane, noting that the Ea is larger and occurs at lower temperatures in the oxidizing atmosphere [36]. By increasing the conversion rate of 60–80%, it was found that the Ea decreased in the three atmospheres, track where almost the entire biomass had been degraded. In comparison with the other two atmospheres, the atmosphere with 3% O2 showed the lowest bands of apparent Ea for the degradation of sugarcane straw (101–130 kJ mol−1), presenting itself as the best atmosphere for autothermal pyrolysis process according to the working conditions studied, requiring low energy for the conversion of biomass.

Statistical analysis of activation energies by the FOW method (Table 3) showed that there was a significant difference between the atmospheres analyzed in relation to the activation energies, with p value equal to 0.0046, lower than the significance adopted (p < 0.05). In relation to the t test (p < 0.05), assuming different variances between two samples, a bi-caudal p value of 0.231 was observed between the inert and 3% O2 atmospheres, between inert and 20% O2 of 0.037 and between 3% O2 and 20% O2 a value of 0.007. According to the p value (two-tailed) found for the comparison between the atmospheres, there were significant differences in the energies of activation (p two-tailed < 0.05). The exception was between inert and 3% O2 atmospheres, which did not present significant differences between the energies of activation (p two-tailed > 0.05).

Table 3 One-way ANOVA table with significance of 5% for FOW method
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Tables 4 and 5 show the apparent Ea values and statistical analysis for Friedman method, respectively. As can be seen, the results for activation energy by the FOW and Friedman (Table 3) methods were close. Both methods of isoconversion were adequate to explain the decomposition process of sugarcane straw. [37] used non-isothermal TG experiments at different heating rates and then estimated the function of temperature using isoconversional technique. It is important to mention that, according to [38], the different methods of kinetic analysis are complementary and not competitive.

Table 4 Values of apparent activation energy for the sugar cane straw in different atmospheres in rates of heating at 5, 10 and 20 K min−1 applying the method of Friedman
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Table 5 One-way ANOVA table with significance of 5% for Friedman method
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Statistical analysis of activation energies by the Friedman method showed that there was a significant difference between the atmospheres analyzed in relation to the activation energies, with p value equal to 0.00361, lower than the significance adopted (p < 0.05). In relation to the t test (p < 0.05), assuming different variances between two samples, a bi-caudal p value of 0.247 was observed between the inert and 3% O2 atmospheres, between inert and 20% O2 of 0.030 and between 3% O2 and 20% O2 a value of 0.001. According to the p value (two-tailed) found for the comparison between the atmospheres, there were significant differences in the energies of activation (p two-tailed < 0.05). The exception was between inert and 3% O2 atmospheres, which did not present significant differences between the energies of activation (p two-tailed > 0.05). Statistically, the Friedman model presented similar comparisons regarding the atmospheres studied and their respective activation energy values, proving the similarity with the data presented by the FOW model.

Heat required for pyrolysis of sugar cane straw

Figure 6a–c and Table 6 show the values of the heat required for pyrolysis of sugar cane straw in different atmospheres and heating rates of 5, 10, and 20 K min−1, respectively (50 mL min−1), obtained by integrating the DSC curves.

Fig. 6
figure 6

Heat required for sugarcane straw pyrolysis in different atmospheres and heating rates of 5, 10 and 20 K min−1 (50 mL min−1), obtained by integrating the DSC curves

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Table 6 The heat required for pyrolysis of biomass in different atmospheres and rate of heating to a temperature of 773 K/50 mL min−1
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According to Fig. 6a–c and Table 6, it was observed that the heat required to convert the biomass into other products (bio-oil, coal, and gas) modified in accordance with the type of atmosphere and heating rate used in the process. Observing the changing rate of heating, it was noted that the higher the heating rate employed, the lower the heat required for the process of pyrolysis (773 K), which took place in all the three atmospheres studied, with values of 182, 210, and 161 kJ kg−1 (inert, 3 and 20% O2, respectively), at a rate of 20 K min−1, since the heat transfer is directly proportional to the rate generated of heat from the process, reducing the heat required for the degradation of biomass and conversion into by-products. Regarding the atmosphere used for the process, it was observed that the atmosphere with 20% O2 requires about 630 kJ to transform 1 kg of biomass at a heating rate of 5 K min−1, 134 and 125 kJ above the heat required in the inert atmosphere and 3% O2, respectively. At lower heating rates, the atmosphere with 20% O2 is not suitable for the pyrolysis of biomass since the energy expenditure was approximately 1.35 times greater than that in the other atmospheres studied. When comparing the heat required for the three atmospheres at a heating rate of 10 K min−1, there was a slight reversal in comparison with the rate of 5 K min−1 values in the inert atmosphere and 3% O2 (446 and 441 kJ kg−1, respectively). This shows that a small proportion of oxygen is capable of transforming biomass with less energy, according to a given heating rate. Regarding the heating rate of 20 K min−1, the 20% O2 atmosphere had the lowest amount of heat required compared to that in the other atmospheres with 161 kJ kg−1. This shows that the percentage of oxygen in the gas composition for the pyrolysis process at 773 K is important since it is sufficient to keep the process at high heating rates, generating an autothermic regime where coal combustion accompanies high warmups and keeps itself with the energy released by the burning of the sugarcane straw. The inert atmosphere in comparison with 20% O2 atmosphere showed a difference of 21 kJ kg−1 for the pyrolysis process, which shows an increased dependence on an external source to maintain the autothermic system, since no combustion occurs in the process, with no release of surplus energy, making it difficult to maintain the temperature required for the degradation of biomass. Analyzing the atmosphere with 3% O2, it was observed that the inverse phenomenon occurred, since the proportion of oxygen was not sufficient to maintain the process itself to a high rate of heating, with incomplete combustion of biomass, with possible formation of other products and increased heat required for cracking the compounds of biomass which can allegedly be evidenced by high heat value requirement (210 kJ kg−1) compared to that in the other atmospheres. [23] conducted a study on the heat required for pyrolysis in an inert atmosphere at 10 K min−1 at 773 K (50 mL min−1 gas flow) and found values in ranging from 267 to 434 kJ kg−1 for different biomasses (eucalyptus, poplar, sawdust, corn, sunflower, and straw). Regarding sugarcane straw, the value of heat required was 375 kJ kg−1. [23] found that the values of heat required were 600, 558, 465, and 389 kJ kg−1 at 773 K (10 K min−1) for pine, wheat straw, cotton stalk and peanut hulls, respectively.

Conclusions

The present study demonstrated that the use of synthetic air a diluted oxidizing agent with a constant flow of 3% of O2, increased the percentage of mass loss, as well as the decrease in the kinetic energy, for the pyrolysis of sugarcane straw, occurring through an autothermal process. This indicates that, under the studied operating conditions, the energy of the system was produced at low temperatures, with lower energy consumption and use of a reagent, which is less costly and aggressive to the environment.