Thermal conversion behaviors and products of spent mushroom substrate in CO₂ and N₂ atmospheres: Kinetic, thermodynamic, TG and Py-GC/MS analyses

Highlights

• SMS pyrolysis in the CO₂ and N₂ atmospheres were quantified.

• The char-gasification occurred at 750℃ in the CO₂ atmosphere.

• The R2 model was determined to best describe the volatile decomposition.

• The pyrolysis performance was better in the N₂ than CO₂ atmosphere.

• Phthalan, toluene, and benzene were the major pyrolysis products of SMS.

Abstract

This study aims at characterizing pyrolysis/gasification behaviors and products of spent mushroom substrate (SMS) in the CO₂ and N₂ atmospheres. The major decomposition stages occurred between 200 and 600 °C with the mass losses of 60.4 and 61.5% at 20 °C/min in the CO₂ and N₂ atmospheres, respectively. The maximum mass loss rate grew with the increased heating rate, while DTG curves shifted toward a higher temperature. Volatiles were released easier in the N₂ than CO₂ atmosphere with a higher comprehensive devolatilization index and decomposition rate. At above 750 °C, the char gasification in the CO₂ atmosphere resulted in a significant mass loss as well as a less char yield. Average activation energies by the Flynn-Wall-Ozawa method were estimated at 212 and 214 kJ/mol in the CO₂ and N₂ atmospheres, respectively. The higher thermodynamic parameters in the N₂ than CO₂ atmosphere indicated the higher reactivity of the pyrolysis in the N₂ atmosphere. The reaction mechanisms of the volatiles decomposition were best described by g(α) = (1-α)⁻¹-1 (R2 model) in the range of 200–370 °C in both atmospheres. The major pyrolysis products at 800 °C were identified using Py-GC/MS and composed mostly of aromatic compounds such as toluene and x-methyl-naphthalenes.

Introduction

The increased consumption rate of the limited fossil fuel reserves, and its association with global climate change render it necessary to seek alternative renewable energy sources and technologies [1,2]. Recently, this has brought the thermochemical conversions of agricultural and forestry wastes to the forefront owing to their CO₂ neutrality, renewability, rich labile substances, high calorific value, low ash content and waste reduction [3]. Globally, renewable resources and biomass wastes were reported to meet about 19% of the total annual energy consumption, with its share increasing at an annual rate of 2.5% [4]. Biomass wastes explored to generate energy using (co-)combustion and (co-)pyrolysis included pomelo peels [5], buckwheat and wheat straws [6], bagasse [7], and hard/softwoods [8].

The total amount of mushrooms produced in China accounts for 75% of the annual world production with five kilograms of spent mushroom substrate (SMS) per each kilogram of mushroom produced [9]. The growing amount of SMS waste by the mushroom industry has generated a severe public health concern in China due to its volatiles content and traditional disposals such as landfills, and composts [10]. SMS also contains mycelia, wood chips, hydrocarbons, and residual nutrients [11], thus presenting a great opportunity as a solid biofuel to bridge the gap between the environmental and economic goals towards the sustainable development of the mushroom industries.

Among the thermochemical conversions, pyrolysis can be considered to be one of the most environmentally and commonly used technique in the absence of oxygen owing to its reasonable cost, simple operation and value-added by-products such as bio-oils, and bio-chars [12,13]. The gasification of biomass using CO₂ is regarded as an effective way to substantially reduce CO₂ emission when compared to fossil fuels [14]. Pyrolysis/gasification serves to use biomass waste as a renewable energy source, reduce their volume and remove pathogens. The devolatilized gases of pyrolysis can be condensed as bio-oils which can be used as a feedstock or a value-added product [12]. In addition, bio-chars generated by the pyrolysis were found to have excellent adsorption properties to reduce total nitrogen (TN) and COD_Cr leaching [9]. The temperature and atmosphere type are the major drivers of the thermochemical processes as well as the difference in reactivity [15]. For example, the pyrolysis of biomass wastes was reported to lead to a significantly less char yield in the pure CO₂ than N₂ atmosphere [16]. Pyrolysis in the CO₂ atmosphere was found to enhance C4 hydrocarbon-cracking, impede the formation of benzene derivatives and generate less condensable hydrocarbons (tar) and chars with a higher specific surface area [17,18]. Li et al. [19] found that the char gasification occurred at a high temperature zone, while the mass loss rate was higher in the N₂ than CO₂ atmosphere in a low temperature zone. Mafu et al. [20] reported that the gasification of bio-chars resulted in higher reactivity than did that of coal chars.

Thermogravimetric (TG) analysis is essential to a better understanding of pyrolysis behaviors and products as a function of time and temperature in a controlled atmosphere [4,21]. The TG data provide the kinetic parameters to design and optimize the thermal conversion systems. Cumming and McLaughlin [22] emphasized the role of the TG data in the industrial-scale generation of bioenergy. Li et al. [23] optimized the operational conditions for the co-combustion of tobacco residue and high-ash anthracite coal by using TG analysis. Based on TG/DSC-FTIR analyses, Yang et al. [24] pointed out that hemicellulose, cellulose and lignin degraded at different temperatures and emitted different gas products during pyrolysis due to their different functional groups and chemical structures. Zhao et al. [25] showed that CO₂, CO, and CH₄ were most evolved from the pyrolysis of hemicellulose, cellulose, and lignin, respectively, using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in the TG experiments. Hemicellulose, cellulose, and lignin also play an important role in the pyrolysis/gasification behaviors and gas products of SMS as a lignocellulosic by-product of the mushroom industry.

Model-free and model-fitting methods based on TG data have been successfully used to understand the kinetic and thermodynamic performance parameters of solid wastes during the pyrolysis [25,26]. The model-free methods such as Flynn-Wall-Ozawa (FWO), Starink, and distributed activation energy model (DAEM) have the following two major advantages: (1) no assumption about the kinetic models, and (2) conversion degree-dependent activation energy estimates [11,27]. The model-fitting approach such as Coats and Redfern method (CR) has been adopted to explain the mechanisms of the different reaction stages [2,28]. However, activation energy estimates by the model-fitting method are an average value for the overall degradation process and do not capture the interaction among kinetic parameters, temperature, and conversion degree [29].

In light of the above related literature, there still exists a significant knowledge gap about the pyrolytic products, and kinetic and thermodynamic behaviors of SMS in a changing atmosphere, theoretically and operationally essential to the development of industry-scale pyrolysis technologies to boost the bioenergy generation from solid wastes. The objectives of this study were to quantify (1) the thermal degradation behaviors of SMS as a function of four heating rates in the CO₂ and N₂ atmospheres using TG analysis, (2) their pyrolysis performances and reaction mechanisms using the kinetic and thermodynamic parameters, and (3) gas products using Py-GC/MS analysis.