Kinetic studies of co-pyrolysis of rubber seed shell with high density polyethylene

Highlights

• Co-pyrolysis of biomass and plastic waste in thermogravimetric analyzer.

• Investigation of thermal degradation behavior in different feedstocks.

• Synergistic effect of the biomass and plastic waste mixture is investigated.

• Kinetic parameters using one step integral method are determined.

Abstract

This paper investigates the thermal degradation behavior of rubber seed shell (RSS), high density polyethylene (HDPE), and the HDPE/RSS mixtures (0.2:0.8 weight ratio) using thermogravimetric analyzer under non-isothermal condition in argon atmosphere at flowrate of 100 ml min⁻¹. Cellulose, hemicellulose, and lignin are also analyzed in this study for comparison of pyrolysis behavior with RSS. The experiments were conducted at different heating rates of 10, 20, 30, and 50 K min⁻¹ in the temperature range of 323–1173 K. The kinetic data is generated based on first order rate of reaction. It is observed that the thermal degradation behavior of the main components in biomass such as hemicellulose, cellulose, and lignin differs during pyrolysis process due to the structural differences that leads to distinctive pathways of degradation of feedstock. It is found that there are one, two, and three stages of decomposition occurring in HDPE, RSS, and HDPE/RSS mixtures respectively during the pyrolysis process. The remaining solid residue increases with an increase in heating rate regardless of the type of samples used. The activation energies (E_A) for RSS, HDPE, HDPE/RSS mixtures are 46.94–63.21, 242.13–278.14, and 49.14–83.11 kJ mol⁻¹ respectively for the range of heating rate studied.

Introduction

Co-pyrolysis of biomass and plastic wastes is an attractive and promising alternative in reducing the dramatically increasing amount of municipal solid waste (MSW) generated globally each year and also a substitute for conventional methods such as incineration and landfill that are commonly practiced in most countries when dealing with MSW [1], [2]. This method can potentially (i) reduce the consumption of fossil fuels, (ii) reduce the volume of waste generated, (iii) reduce the secondary pollution problems, and (iv) produce high value fuel [3], [4], [5]. Grieco and Baldi [6] had mentioned several advantages of using pyrolysis over incineration. The crucial reasons that pyrolysis is a more technological and environmental viable option compared to incineration method are the absence of oxygen supplied to the system. Thus, has a low tendency of producing dioxins and furans, and electrical energy can be attained from more options of higher efficiency technologies such as internal combustion engines, and gas turbines instead of limiting to Rankine cycle [6].

The understanding of thermal decomposition or devolatilization occurring during the pyrolysis process of the biomass–plastic mixtures is very important as kinetics is intrinsically related with the decomposition mechanisms [7]. Hence, thermogravimetric analysis (TGA) is used in this study as it is said to be the commonly used technique to investigate the solid-phase thermal degradation and also the determination of kinetic triplets namely pre-exponential factor (A), activation energy (E_A), and order of reaction model (n) from the kinetic analysis of solid state decomposition [4], [5], [8], [9]. The advantages of using TGA is the ability to monitor the mass of a substrate during heating or cooling process at a specific heating rate with respect to time or temperature and also to measure the decrease in substrate mass from the effect of devolatilization during thermal decomposition [10]. In addition, the maximum reaction rate can be determined by taking the first derivative thermogravimetric curves (−dw/dt), known as derivative thermogravimetry (DTG) [11]. First order reaction is the most commonly used approach in most kinetic studies for solid fuel pyrolysis [12], [13], [14]. In this approach, the overall rate constant is calculated from the weight loss curve of polymer and biomass waste data provided from the TGA. The determination of E_A and A are obtained from the logarithmic form of the Arrhenius equation [15], [16].

Specific studies related to the thermal degradation and determination of kinetic parameters on the mixtures of polymer and lignocellulosic biomass during the pyrolysis process had been carried out by other researchers. Pinthong [14] investigated the co-pyrolysis of rice husk, high density polyethylene (HDPE), low density polyethylene (LDPE), and polypropylene (PP) mixtures using TGA. The E_A and A of waste plastics (HDPE, LDPE, and PP) decomposition are in ranges of 279.0–455.1 kJ mol⁻¹ and 4.95 × 10¹⁹–1.48 × 10³¹ min⁻¹ respectively. It is found that the E_A of plastic decomposition reaction reduced when plastics are mixed with rice husk. The E_A of the mixture are in range of 221.2–317.3 kJ mol⁻¹ while the A are in range of 2.11 × 10¹⁵–7.18 × 10²¹ min⁻¹. Rotliwala and Parikh [13] studied the thermal degradation behavior of mixtures of rice bran and HDPE using TGA in nitrogen atmosphere and compared with that of individual materials. It is reported that there are one, two, and three stages of decomposition occurring in HDPE, rice bran, and mixtures of HDPE and rice bran (1:1) respectively. The E_A for HDPE in the first stage is in the range of 234.99–257.80 kJ mol⁻¹. The E_A for rice bran in the first and second stage respectively are in the range of 13.08–15.49 kJ mol⁻¹ and 44.78–46.33 kJ mol⁻¹. Meanwhile, the E_A for the mixtures of HDPE and rice bran in first, second, third stage respectively are in the range of 11.62–14.54 kJ mol⁻¹, 33.51–33.57 kJ mol⁻¹, and 165.76–174.96 kJ mol⁻¹. A similar observation by Pinthong [14] reported that the E_A for the mixture is less compared to individual components.

The aim of this present work is to study the co-pyrolysis of HDPE with rubber seed shell (RSS) in the ratio of 20/80 to investigate the thermal decomposition behavior and predicting the kinetic parameters such as E_A and A using TGA equipment under non-isothermal condition in argon atmosphere at four different heating rate of 10, 20, 30 and 50 K min⁻¹. In addition, the thermal degradation behavior of HDPE/RSS mixture is compared with the individual components of HDPE and RSS. Furthermore, the synergistic effect of the HDPE/RSS mixture in the pyrolysis process is investigated. The understanding of chemical kinetics is vital as it depends on the rate of reaction which provides essential substantiation on the mechanisms of the chemical processes involved. Hence, the accuracy of experimental data collection is important to produce reliable and suitable kinetic models for scaling up to a full scale industrial reactor suitable for this process when using this feedstock. The HDPE/RSS weight ratio of 0.2/0.8 selected in this study is based on the optimum condition obtained for co-pyrolysis of RSS and HDPE mixtures for syngas production from previous study [3]. RSS is selected as lignocellulosic biomass in view of the fact that Malaysia is one of the major rubber growing countries with an acreage of 1,229,940 hectares of rubber plantation in the year of 2007 [17] and it is expected that the annual production of RSS is about of 1.2 million metric tons from an average value of 1000 kg RSS produced per hectares per year [17].

Hence, there is an immediate need to study the potential of rubber wastes utilization, not merely to reduce the excessive waste volume, but also taking advantage to convert them in a manner that is more energy efficient, climatically sound, and environment friendly to human. In addition, rubber waste can contribute a positive and promising prospect as a source of renewable energy with regards to the current state of energy crisis with high price of crude petroleum.