Analysis of thermal degradation of banana (Musa balbisiana) trunk biomass waste using iso-conversional models

Abstract

The thermo-chemical characterization (proximate and ultimate analyses and higher heating value) of banana trunk biomass waste has been carried out. The thermo-gravimetric and differential scanning calorimetric (DSC) investigations have been made at heating rates of 10, 15, 20 and 25 °C/min. The TGA data have been used to carry out kinetic analysis and evaluate the kinetic and thermodynamic parameters using iso-conversional models. The values of activation energy increase with conversion (α) irrespective of the iso-conversional model used. The average values of activation energies (E_α) are found to be 386.21, 355.43, 385.77, 355.01, 379.67, and 292.78 kJ/mol for Flynn-Wall-Ozawa (FWO), Starink, Kissinger-Akahira-Sunose (KAS), Tang, Vyzovkin and Vyzovkin AIC model, respectively. The average values of change in enthalpy, Gibbs free energy, and entropy have been calculated. The reaction mechanisms of pyrolysis have been predicted using Criado’s method.

Introduction

The rising environmental pollution level due to over exploitation of finite fossil fuels (i. e. coal, gas and oil) has aroused global interest in renewable energy resources as possible options. Out of the various renewable energy resources, the virgin and waste lingo-cellulosic biomasses from various sectors are being considered as sustainable and viable options due to their abundant availability throughout the year in various forms. The ligno-cellulosic biomass can be converted to gaseous, liquid and solid products suitable for use either as fuel or as feed-stock for producing other value-aided products through several routes (Mishra and Mohanty, 2018a). The bio-chemical, physical and thermo-chemical conversion processes are commonly used for this purpose (Kumar et al., 2019a). Bio-gas, bio-hydrogen, ethanol, and butanol using enzymes and/or microorganisms are the main products of biochemical conversion processes. The physical conversion processes produce solid fuels such as briquettes and pellets (Bartocci et al., 2017). The thermo-chemical conversion processes (combustion, gasification, liquefaction, and pyrolysis) are used to convert biomasses into bio-oil, gaseous fuels and char. Out of this pyrolysis is the simplest thermal conversion technology and is useful for producing gaseous, liquid, and solid products.

Over the past two decades several researchers across the world have tried to evaluate the suitability of different types of agro-wastes for their use as fuel. Banana is the most widely grown fruity plant in the world. It is produced in Asia, Latin America, and Africa, each contributing around 50.82, 32.97 and 14.09%, respectively to the total world production. Europe (0.57%) and Oceania (1.55%) also contribute insignificant amounts. Some African countries, Brazil, China, Ecuador, India, and Philippines are the major banana producing countries. During 2010 to 2017, India produced on an average 29 million tonnes of banana, China around 11 million tonnes, Philippines around 7.5 million tonnes and Ecuador and Brazil each around 7 million tonnes. Colombia, Costa Rica, and Guatemala are other Latin American countries producing sizeable amount of banana. The banana production area in the world has increased steadily from 3.6 million hectares in 1993 to to 5.6 million hectares in 2017 (FAOSTAT, 2014). Between the years 2000 to 2017, the global production of banana increased at the compound annual rate of 3.2% reaching 114 million tonnes in 2017 from around 67 million tonnes in 2007.

Banana is a tropical, herbaceous and perennial plant belonging to the Musaceae family, which produces a large flower cluster, bears fruits and then dies. The plant is cut to bring the crop down and harvest the fruits. The main stem (or pseudo-stem or trunk) of banana is a juicy cylindrical cluster of leaf stalk bases. The subterranean stem is known as corm and the part that supports fruits is known as peduncle, stalk or rachis. After harvesting the fruit all these portions become waste biomass. At the packaging and processing plant, the banana bunch rachis and damaged/rotten fruits also become additional residual biomass. The banana biomass waste production is to the tune of 220 tonnes per hectare (Padam et al., 2012). In addition to the plant biomass wastes, the rejected fruits that fail to meet the required quality standards may contribute between 8 and 20 percent of additional biomass waste. For every tone of banana produced, about 100 kg of fruit rejects and 4 tons of biomass waste comprising leaves, pseudo-stem, rotten fruits, fruit-bunch stem and rhizomes are produced, totalling to approximately 4-times the banana fruit produce (Subayo and Chafidz, 2018).

The banana biomass waste thus generated is either left in the field or taken to open dumps. In both cases, the ligno-cellulosic biomass wastes produce greenhouse gases as they decompose. Thus it is seen that banana farming and processing generates thousands of tons of waste biomass, which is currently being improperly managed causing serious environmental problems (Padam et al., 2014). There is a need to evaluate the potential of this huge biomass waste for utilization as a fuel or feed-stock for biochemical and thermo-chemical conversion processes.

Banana biomass waste has a long history of direct use. In several African and Latin America countries and elsewhere banana leaves are used as cover and wrappings, matting, and partition walls. Fibres from banana pseudo-stem (or banana trunk) are used for making various types of cloth and rope (Subayo and Chafidz, 2018). A small portion of banana biomass is also used as animal fodder and feed for making compost and producing bio-gas.

Biochemical conversion of banana biomass waste to fuel was attempted as early as in 1986. Tewari et al. (1986) produced ethanol from banana peel waste and Bardiya et al. (1996) converted it to methane. Jena et al. (2017) produced methane from dry banana leaves. The conversion of banana biomass waste to energy and value added products through thermal route received attention only in the beginning of the 21st Century (Tock et al., 2010). Thermochemical characterization of banana bunch and stem was reported by Gańán et al. (2004). Bilba et al. (2007) investigated thermal degradation of banana fiber. Fernandes et al. (2012) evaluated the suitability of banana leaves and pseudo-stem wastes for producing bio-oil and bio-char. Fernandes et al. (2012) concluded that due to high fixed carbon banana biomass wastes are suitable for thermo-chemical conversion. Fernandes et al. (2013) reported thermo-chemical characteristics and thermal degradation behaviour of semi-dry and dry banana leaves. The two types of biomasses showed different mass loss and energy release intensities.

Branca and Blasi (2015) reported combustion characteristics of banana peel waste. Memon et al., 2008, Selvarajoo and Hanson, 2015, Zhou et al., 2017a, Zhou et al., 2017b, and Selvarajoo et al. (2019) carbonized banana peels to obtain bio-char and used as adsorbent for the removal of heavy metals like Cd, Cr and Pb. Sellin et al. (2016) investigated oxidative fast pyrolysis of banana leaves biomass for assessing its suitability as feedstock for gasification and pyrolysis processes.

Abdullah et al. (Abdullah et al., 2014) characterized and compared banana pseudo-stem and fruit-bunch-stem and concluded that both biomasses are suitable as feed-stock for energy generation. Cheng et al. (2016) reported that the thermal degradation banana stem took place in three conversion ranges- 0 to 0.2, 0.2 to 0.9 and 0.9 to 1.0 corresponding to the pyrolysis of hemi-cellulose, cellulose and lignin, respectively and it followed the 3D diffusion model. Kabenge et al. (2018) reported that the thermo-chemical characteristics of banana peels biomass were similar to those of leaves and pseudo-stem and hence both are suitable for pyrolysis. Luna et al. (2019) carried out thermo-gravimetric analysis of banana pseudo-stem biomass and recommended its suitability as feedstock for pyrolysis and gasification. Selvarajoo et al. (2019) considered the effect of time and temperature on the weight loss and used feed-forward neural network to model the pyrolysis of banana peel to produce bio-char. They reported a char yield of 47%. Kumar et al. (2019d) investigated the effect of Ru and Fe impregnation on the kinetics of pyrolysis and gasification of banana pseudo-stem and obtained lower activation energy and reduced weight loss compared to un-impregnated biomass. These results indicated that metals catalyze the pyrolysis process. Kumar et al. (2019d) investigated pyrolysis of banana peel at 600 °C in a fixed bed reactor at three heating rates (5, 10, 15 °C). The bio-oil yield depended on the heating rate, but varied only marginally (11.47–12.88 mL/100 g of biomass). The calculated average activation energy showed a two-fold variation and varied from 108.42 kJ/mol for Kissinger model to 201 kJ/mol for Coats-Redfern model.

India generates huge amount of banana biomass waste, but most of it is improperly managed and poorly utilized. It may be useful to assess its suitability for use as a renewable feed-stock for energy generation and production of value added products. To the best of our knowledge there are no reported results on the thermal degradation behaviour of Indian banana biomass waste. The results available for non-Indian banana biomass wastes may not be directly applicable to Indian banana biomass wastes thus necessitating the need for their characterization and evaluation. In this work the thermo-chemical characteristics and thermal degradation behaviour of Indian banana trunk (or pseudo-stem) biomass waste have been investigated