Comprehensive kinetic study of Imperata Cylindrica pyrolysis via Asym2sig deconvolution and combined kinetics

Highlights

• Pyrolysis kinetics via iso-conversional and master plot are examined for analysis of kinetic parameters of pseudo components.

• A combination of Asym2sig deconvolution and combined kinetics is applied.

• Activation energy of pseudo cellulose was lower than the hemicellulose and lignin.

• The kinetic parameters from combined kinetic agreed well with the experimental data.

Abstract

This study examined the non-isothermal kinetics of the slow pyrolysis of Imperata Cylindrica (IC). Pyrolysis conditions were developed under the pure N₂ flow and non-isothermal conditions at the heating rates of 2.5, 5, 10, and 17.5 K/min and over the temperature range of 303–1173 K. The IC pyrolysis profiles could be identified into three parallel reactions, each of which corresponded to pseudo-hemicelluloses (P-Hem), pseudo-cellulose (P-Cell), and pseudo-lignin (P-Lig) decomposition. A systematic kinetic study of the pyrolysis of IC via thermogravimetric analysis (TGA) deconvolution using Asymmetric Double Sigmoidal (Asym2sig), Friedman differential iso-conversional and combined kinetics of biomass pseudo-components was carried out. The kinetics parameters of pseudo components fitted well with the pyrolysis experimental data for all the heating rates. Differential master-plots showed that the reaction mechanisms for pseudo hemicellulose (P-Hem) and pseudo cellulose (P-Cell) were diffusional and order based, and high order based (3rd order) for the pseudo lignin (P-Lig). Mechanism of P-Hem, P-Cell and P-Lig could be further reconstructed to Sestak and Berggren model of $f\left(\alpha \right)={\alpha }^{-0.9875}{\left(1-\alpha \right)}^{1.325}{\left[-\text{l}\text{n}(1-\alpha )\right]}^{0.0209},$ $f\left(\alpha \right)={\alpha }^{0.3313}{\left(1-\alpha \right)}^{1.4731}{\left[-\text{l}\text{n}(1-\alpha )\right]}^{0.0215}$ and $f\left(\alpha \right)={\alpha }^{-2.9551}{\left(1-\alpha \right)}^{2.7642}{\left[-\text{l}\text{n}(1-\alpha )\right]}^{0.0074}$, respectively. The combined kinetic reported the activation energies of pseudo-components were as 194.709 kJ/mol, 179.968 kJ/mol and 219.226 kJ/mol for P-Hem, P-Cell and P-Lig, respectively.

Introduction

The consumption of non-renewable energy to come upon the growing energy demands has deleterious effects on the environment, such as acidic rain, greenhouse gas emissions, and climate change [[1], [2], [3], [4], [5]]. Furthermore, the reserves of fossil fuel are anticipated to be depleted in the near future. Hence, increasing attention has been paid towards the development and application of cleaner and renewable sources of energy to achieve sustainable future frameworks. Biomass has advantages over other renewable energy resources because it is widely available and carbon-neutral resource. In addition, it can be transformed into biofuel in solid, liquid, and gaseous forms by applying various conversion processes [[6], [7], [8], [9]]. Imperata cylindrica (IC) is one of prospective biomass, also known as cogongrass, alang-alang in Southeast Asia. IC can grow well without any specific nutritional treatments over a wide variety of soil types and is found in various environments, such as grasslands, abandoned fields, highway and railway edges, pine and hardwood jungles, and recreational areas [10].

Among thermochemical technologies, pyrolysis process decompose biomass directly into value-added products in the form of solid (bio-char), liquid (bio-oil), and gaseous biofuels [[11], [12], [13], [14]]. The analysis of the kinetics of the biomass pyrolysis reactions provides fundamental information on how it is decomposed, allowing optimization of the process, and also provides useful data for the reactor design to convert biomass effectively. Kinetics analysis also provides important information on reaction mechanism models to develop mathematical models that describe the pyrolysis process [15,16].

Kinetic studies on biomass pyrolysis can be estimated in two ways: iso-conversional methods and model-fitting methods [17]. Iso-conversional methods have been developed to examine the kinetic parameters of solid biomass reactions with simple thermal characteristics. In recent studies, the kinetic parameters of biomass included IC were evaluated using these methods [[18], [19], [20], [21]]. On the other hand, the reaction model analysis familiar with a single step method via master-plots owing to homogeneity in the biomass pyrolysis process were evaluated [21].

The data from the single components biomass pyrolysis mechanism cannot describe the thermal behaviour of biomass. This is considered contradictory rather than complementary in the complexity of the reaction kinetics [17,22]. The multi components analysis of biomass pyrolysis provided more detailed knowledge about the simulation and modelling of thermochemical reactions [23]. International Confederation for Thermal Analysis and Calorimetry (ICTAC) [24] recommended comprehensive kinetic analysis to represent the distributed reactivity analysis because biomass components are complex materials, in which the decomposition occurs through multiple, independent and parallel reactions. Oladokun et al. [20] performed model fitting for a kinetic study by the multi-step de-volatilization of IC by assuming the reaction model of pseudo-components as parallel first-order reactions. Furthermore, numerous kinetic studies of biomass decomposition through Distributed Activation Energy Model (DAEM) and nth order-based kinetics have been reported [25,26]. Poletto et al. [27] reported that the first-order or n-order models to analyse the degradation process have an erroneous physical meaning and order because of the combined effects of some mechanisms. In this context, an investigation of comprehensive pyrolysis kinetic analysis via combination deconvolution and combined kinetic would be important. The systematic approaches in multi-step kinetics are thermogravimetric analysis (TGA) deconvolution, iso-conversional method, master plot analysis and combined kinetics. Systematic multi-step kinetics can offer more precise info on the modelling and simulation of pyrolysis reactions [23]. Based on the deconvolution method, the reaction rates are separated into three main components in biomass: pseudo hemicellulose, cellulose, and lignin. The obtained deconvolution profiles of the pseudo-components of biomass can be analysed by the kinetic parameters and reaction model by iso-conversional method, master plot and combined kinetics [24].

In various studies regarding the deconvolution of biomass pyrolysis, thermogram profile was deconvoluted by symmetric functions (Logistic, Gaussian, and Lorentz functions) [[28], [29], [30]] or asymmetric functions (Asymmetric Double Sigmoidal, Weibull, Fraser–Suzuki functions and Bi-Gaussian) [21,[30], [31], [32]]. Among these functions, recent studies [[32], [33], [34]] have shown that Asymmetric Double Sigmoidal (Asym2sig) functions gave an appropriate deconvolution for the kinetic model. To the best of the authors knowledge there are no studies regarding the comprehensive deconvolution and combined kinetic pyrolysis of IC reported yet in literature. Hence, this study to proposed the systematic kinetic pyrolysis of IC via thermogravimetric analysis (TGA) deconvolution using Asym2sig, iso-conversional method, master plot and combined kinetic is the first report of this kind. The systematic multi-step kinetic offer reliable calculation of kinetic parameters and good model description of pyrolysis. Thermogravimetric analysis and differential thermal gravimetric analysis (TGA/DTG) analysis at four different heating rates were carried out. In addition to this, IC biomass was also characterized by the proximate and ultimate analysis, compositional analysis, and Fourier transform infrared (FTIR) to report its bioenergetic value. This study is expected to put forward the beneficial management solution of Imperata Cylindrica biomass to produce bio-oil for the energy requirements, which at the present are heavily dependent on the fossil fuels.