**1. Introduction**

The excessive utilization of fossil fuel sources for diverse energy purposes engenders a grave global concern. The combustion of these fuel sources results in the emission of greenhouse gases (GHGs), which have been implicated in global warming and climate change phenomena [1,2]. Again, fossil fuels, which are non-renewables, are being heavily depleted due to an increasing rate of exploitation. Consequently, attention is shifting more

**Citation:** Balogun, A.O.; Adeleke, A.A.; Ikubanni, P.P.; Adegoke, S.O.; Alayat, A.M.; McDonald, A.G. Study on Combustion Characteristics and Thermodynamic Parameters of Thermal Degradation of Guinea Grass (*Megathyrsus maximus*) in N2-Pyrolytic and Oxidative Atmospheres. *Sustainability* **2022**, *14*, 112. https://doi.org/10.3390/ su14010112

Academic Editors: Farhad Taghizadeh-Hesary and Han Phoumin

Received: 2 November 2021 Accepted: 1 December 2021 Published: 23 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

toward biomass resources because they are viewed as carbon neutral and abundantly available as inexpensive residues, and they possess widespread availability and huge sustainability potential. It has been reported that energy from biomass has replaced about 14% of the global energy consumption [1]. Diverse biomass materials have been investigated through thermochemical means for probable bioenergy applications and biofuel production. These include rice husk, corn cobs [3], sugarcane straw [4], peanut shells [1], sorghum bicolor glume [5], coffee residues [6], and different woody samples [7–9]. Recently, interest has been turning toward the exploration of different energy crops, which include diverse varieties of grass species, for thermal analysis [10–12].

There are a number of reasons for the recent keen interest in the use of grasses as feedstock materials for biofuel production. The interest may have been prompted by the availability of large expanses of degraded lands, upon which grasses can be cultivated in some reclamation efforts. For instance, Cai et al. [13] stated that about 1107–1411 Mha of degraded land is available globally for the cultivation of energy crops. Grasses have also been identified as short-rotation, non-food crops, as well as low-input, high-yielding biomasses [11,14]. For example, relative to corn feedstock, which has a yield of about 7 Mg/(ha\*year), the yield of grasses could reach as high as 40 Mg/(ha\*year) [15,16]. This makes them viable substitutes for alleviating the unwholesome competition that arises from the use of food crops, such as corn and sugarcane, for biofuel production. It was noted that, in 2014, 91 billion liters of bioethanol was produced worldwide, mainly through the physicochemical processing of grains and sugarcane—stirring ethical issues from the public [11]. Another attraction to grasses is that though woody biomass residues have been widely investigated, relative to grass their use is characterized by some challenges that include limited land availability, a low annual biomass yield, a slow growth rate, and difficulty in harvesting due to higher energy requirements [14]. Guinea grass (*Megathyrsus maximus*), in particular, has a reputation for being a prolific energy crop in the sub-Saharan region of Africa. It exhibits rapid growth, tolerance to low soil fertility, and resistance to adverse weather conditions [17]. Aside from these characteristics, it is also a lignocellulosic biomass whose polymeric structure is characterized by an intricate matrix of hemicellulose, cellulose, and lignin constituents.

It is important to note that the polymeric structure of grasses differs significantly as a function of certain factors, such as grass variety, maturity stage, and environmental conditions [15]. In terms of climatic conditions, grasses may be broadly classified as tropical (C4) or temperate (C3) region grasses. The major constituents of the former are sucrose and starch, while the latter is predominantly rich in fructose and sucrose [15]. Specifically, it has been noted that *Miscanthus* x *giganteus*, a native Asian grass, has the distinctive feature of a high lignocellulose yield—hemicellulose (20–40%), cellulose (40–60%), and lignin (10–30%) [18]. In contrast, the polymeric constituents of Napier grass (native to Africa) and Bermuda grass (mainly grown in the United States) are comparable to the one mentioned earlier. Switch grass exhibits a slightly different structural composition—the hemicellulose, cellulose, and lignin, respectively, are 25–29%, 37–40%, and 18–25% [15]. Relative to the previous grasses, tall fescue, timothy, yellow flag, and meadow foxtail show markedly different compositions (Table 1). From the foregoing discussion, grasses represent a suitable lignocellulosic feedstock for biofuel production. However, given the wide variation in their structural makeup, it is imperative to undertake detailed characterization analyses prior to deployment for bioenergy purposes. Not only does the information from such analyses provide valuable insight into the chemical character of the feedstock, but it is also profoundly useful in the design of reactors and the modeling and optimization of the associated thermal processes.


**Table 1.** Polymeric composition of selected grasses [15].

The basic characterization efforts, including proximate and ultimate analyses and higher heating value (HHV) determination, have been utilized extensively in evaluating the composition of diverse biomass feedstock [6,19,20]. The wet chemistry method, typically based on a two-step acidic hydrolysis, is an analysis that has proven reliable in providing insight into biomass composition. However, it is time-consuming and labor-intensive and requires pre-conditioning [21]. Another notable method is infra-red spectroscopic analysis. It is a powerful technique that can be utilized for gathering both quantitative and qualitative data. It is a non-destructive test that is fast and precise. In addition, it is devoid of elaborate sample preparation and the use of expensive and dangerous chemicals. Balogun et al. [22] subjected brewer's spent grain (BSG) to pyrolysis and then undertook the physico-chemical, thermal, and spectroscopic analyses of the BSG and its biochar. They reported a significant variation between the structural configuration of the original BSG and its biochar based on the condensation index and the cellulose crystallinity content. Research has been conducted on the comprehensive characterization of five different biomass samples and revealed that there were distinct differences in the chemical and structural constituents of the samples [23].

Notably, the thermogravimetric analysis (TGA) represents another critical characterization technique that provides a rich source of information regarding the thermal behavior of lignocellulosic biomass. The thermal decomposition of a solid by TGA can be performed isothermally, or otherwise, in an inert or oxidative atmosphere. From the TGA data, deductions can be made on the decomposition mechanism, the kinetic and thermodynamic parameters, and the combustion characteristics of a sample. Though kinetic investigation is suitable for small-sized particles and low-heating-rate processes, it has been widely used because of its high predictability and simplicity. Furthermore, the kinetic data produce sub-models that can be incorporated into complex transport phenomena models to yield practical descriptions of either pyrolysis or combustion processes. The kinetic models, including the one-step global kinetic model, the independent parallel and competitive reaction models, the detailed lumped kinetic model, and the distributed activation energy model, have been formulated and extensively applied [24,25]. Typically, in a kinetic study, the reaction rate is given as a function of temperature and the conversion ratio, and the temperature dependence is expressed as an Arrhenius' equation. The best-fit model, applied to the TGA data, is utilized for the determination of the kinetic parameters and subsequently for simulation. The mathematical approach deployed for solving the rate equation results in two notable techniques, namely iso-conversional and model fitting.

In the model-fitting technique, prior knowledge of the reaction mechanism is required for the selection of an appropriate reaction model. This is achieved by successively fitting different reaction models to the TGA data to select the one with the highest correlation. A popularly utilized model-fitting method is the Coats–Redfern (CR) integral technique. The kinetics of solid-fuel pyrolysis was analyzed using the CR technique and the model identified the probable reaction mechanisms at different stages of pyrolysis [26]. The direct differential and CR methods were deployed to deduce the non-isothermal kinetic parameters of the pyrolysis of pure and crude glycerol, and the distinctive activation energy values were observed [27]. There have been more recent comparative kinetics studies that involved the CR model-fitting method [8,28–31]. The iso-conversional technique, on the

other hand, forestalls the need for any foreknowledge of the reaction mechanism. Rather, it relies on the use of several TGA measurements at varied heating rates for the evaluation of the kinetic parameters, and it is based on an approximation technique of the temperature integral. Some kinetic modeling studies have been undertaken through iso-conversional methods, including the Kissinger, Starink, Kissinger–Akahira–Sunose (KAS), and Flynn– Wall–Ozawa (FWO) models [7,25]. It has been demonstrated that thermal degradation of biomass follows a multi-step reaction mechanism because the kinetic parameters vary with the conversion degree [22,30,32].

Globally, energy recovery from biomass is predominantly from combustion processes (about 90%) [33]. Biomass combustion can yield low GHG emissions with efficient monitoring and control. Therefore, the combustion characteristics of specific biomass feedstock need to be quantified for the optimum design and modeling of the combustors and scrubbers. Furthermore, it is also critical to gather information on the feasibility of thermal-conversion processes as well as the energy measurements. This can be achieved by calculating the changes in enthalpy, Gibbs free energy, and entropy from the kinetic parameters [34]. There is limited information on the thermal decomposition of grasses of tropical origin. The objective of this study was to thermally decompose guinea grass in inert and oxidative environments, with the focus on evaluating the kinetic and thermodynamic parameters and the combustion characteristics. The kinetic study will entail the use of model-fitting and iso-conversional techniques, while the feedstock characterization will involve proximate, elemental, compositional, and spectroscopic analyses.
