**3. Pyrolysis behaviors of plant fibers**

#### **3.1 Pyrolysis characteristic of plant fibers**

In general, the process of biomass pyrolysis includes four stages [5]. The first stage (<150° C) is dehydration, where the biomass releases external water by absorbing heat. This registers as a slight weight loss on the TG curve. The second stage (150–250°C) is evaporation and distillation of the volatile and semi-volatile components. At the same time, lignin begins to lose weight and releases some small molecules, and the crystalline regions of cellulose transform to amorphous partly. The major pyrolysis stage (250– 500°C) shows the maximum weight loss peak on the DTG curve of the biomass. The last stage (>500°C) corresponds to the further slow decomposition of residues that generated from incomplete pyrolysis. **Figure 1** shows the TGA curve of coniferous fiber at a heating rate of 10°C/min and it presents the typical pyrolysis characteristics of biomass under inert atmosphere. The initial pyrolysis temperature *T*s and terminal temperature *T*h are obtained by the tangent method [28, 29], that are 339°C and 362.4°C, respectively.

**Figure 1.** *TG/DTG curves of coniferous fiber sheet under N2 atmosphere (heating rate β = 10°C/min) [27].*

The pyrolysis behaviors of various plant fibers are quite distinct because of their different morphologies, chemical components, and pyrolysis conditions. **Table 2** demonstrates the pyrolysis characteristic parameters of various plant fibers. With an increase in heating rates, the values of these parameters *T*s, *T*h, and the peak temperature *T*max show an increasing trend [27]. Zhao [30] investigated the pyrolysis rate elevated linearly with the increase in heating rates. The heating rate has an appreciable impact on the temperature difference for heat transfer and the temperature gradient between the measuring point and the sample, besides, that causes an additional endothermic amount to balance the thermal hysteresis. The distribution of cellulose, hemicellulose, and lignin demonstrates differences across species of plant fibers [31**–**33]. The pyrolysis temperature of the hemicellulose is the lowest, while that of lignin behaves the highest. As shown in **Table 2**, the maximum mass loss rate − (dm/dt)max of the fibers rises with the increase of the heating rate, which −(dm/dt)max of cotton fiber is the highest, and −(dm/dt)max of grass fiber is the lowest. When the heating rate is 15°C/min, the values of −(dm/dt)max are 35.2%/min and 24.1%/min, respectively. In addition, the pyrolysis index P [34, 35], which reflects the pyrolysis degree of fibers, enhances with the increase of heating rates.

#### **3.2 Pyrolysis and combustion kinetics of plant fibers**

As a typical biomass, the pyrolysis and combustion kinetics of various plant fibers are essential for the multipurpose utilization of biomass materials [6**–**9]. The TG/ DTG data obtained at different heating rates (5, 10, and 15° C/min) were applied to get the relationship between α β ln *<sup>d</sup> dT* and 1 *T* for each plant fiber, at selected <sup>α</sup>

values based on the Friedman method, as shown in **Figure 2**. The activation energy (Ea) corresponding to the α values could be obtained from the slope of the fitting curve, as shown in **Table 3** [27]. The value of α ranged from 0.05 to 0.95, and the step size was 0.05.

α

α

Due to the complicated pyrolysis processes of biomass, the apparent kinetics could be described by the appropriate models based on the global weight loss, while


*Study on Pyrolysis Behaviors of Various Plant Fibers DOI: http://dx.doi.org/10.5772/intechopen.109294*

*β: heating rate; Ts: initial decomposition temperature; Th: terminal decomposition temperature; Tmax: peak temperature; −(dm/dt)max: maximum mass loss rate;* **△***T1/2: peak width at half-height; and P: pyrolysis index.*

#### **Table 2.**

*Pyrolysis characteristic parameters of fibers [27].*

#### **Figure 2.**

*Friedman results of pyrolysis of different fiber sheets under N2 atmosphere (a) coniferous fiber, (b) broadleaf fiber, (c) bamboo fiber, (d) flax fiber, (e) grass fiber, and (f) cotton fiber [27].*


*Study on Pyrolysis Behaviors of Various Plant Fibers DOI: http://dx.doi.org/10.5772/intechopen.109294*

the pyrolysis mechanism of a specific component is still an intractable problem. Plant fiber is composed of complex multi-components and its pyrolysis is a multistep reaction process. As indicated in **Table 3**, the resulting apparent activation energies of plant fibers are various with the conversion rate. The average apparent activation energies of coniferous, broadleaf, bamboo, flax, grass, and cotton fibers are 193.79, 173.30, 201.10, 184.77, 176.78, and 186.28 kJ/mol, respectively, among the conversion range of 0.05**–**0.85. The average apparent activation energy of broadleaf fiber is the lowest and that of bamboo fiber is the highest.

Although biomass pyrolysis is generally carried out under an inert atmosphere, some research has been performed under an oxygen atmosphere [27]. The characteristic parameters, both of pyrolysis process of fibers and combustion of residual fixed carbon, as *Ts*, Th, and Tmax, demonstrate a high-temperature-shifting with the increase of the heating rate. The Tmax of all fibers behaves are lower than those under nitrogen condition, and the apparent activation energies show a similar variation trend, especially among the conversion between 0.05 and 0.65. Cotton fiber shows the highest Tmax, compared with grass fiber, which is the lowest. Improving the heating rate benefits the combustion performance of plant fibers and the combustion characteristic index (S) [27] increases accordingly. The result shows that the broadleaf fiber has the largest combustion characteristic index, while grass fiber has the smallest. The apparent activation energy of plant fiber pyrolysis in an oxygen atmosphere is lower than that in nitrogen, indicating that oxygen atmosphere can promote the pyrolysis reactions of plant fibers.
