A recent article published in Scientific Reports examined the co-pyrolysis of rice husk (RH) and low-density polyethylene (LDPE) using thermogravimetric analysis (TGA). Three blends with varying LDPE:RH ratios (50:50, 25:75, and 75:25) were analyzed.
Additionally, the thermal, physicochemical, and kinetic properties of these materials were investigated to understand the impact of LDPE on the co-pyrolysis process.
Study: Thermogravimetric analysis of rice husk and low-density polyethylene co-pyrolysis: kinetic and thermodynamic parameters. Image Credit: Tinasmos/Shutterstock.com
Background
The global energy market is heavily reliant on fossil fuels, contributing to environmental challenges and necessitating the development of sustainable alternatives. Biomass is emerging as a clean, biodegradable, and environmentally friendly energy source with significant potential.
This study focuses on utilizing rice husk (RH), an abundant agricultural byproduct in the Indian states of Punjab and Haryana. Despite its high annual production, much of the RH is either burned or landfilled, leading to environmental concerns. Similarly, plastic waste, particularly low-density polyethylene (LDPE), poses a severe ecological threat due to its non-biodegradability and harmful effects when openly burned.
The co-pyrolysis of RH and LDPE offers a sustainable pathway to address these issues by converting waste into valuable energy products. Thermogravimetric analysis (TGA) in an inert environment enables a deeper understanding of this process, aiding the design and optimization of industrial-scale pyrolysis systems. By blending RH and LDPE, this study aims to explore their synergistic thermal and kinetic behaviors to unlock their potential as renewable energy feedstocks.
Methods
Rice husk (RH) was sourced from Madhya Pradesh, India. It was thoroughly cleaned with tap water and sun-dried naturally for several days to remove impurities before sample preparation. Low-density polyethylene (LDPE), obtained from packaging plastic, was cut into uniform pieces, ensuring a particle size below 2 mm to match the RH particles. This step was critical to achieving a homogeneous mixture and optimizing heat transfer during pyrolysis.
Blended samples with LDPE:RH ratios of 50:50, 25:75, and 75:25 were prepared. These blends were dried at 101 °C until they reached a consistent weight, minimizing moisture content. The samples underwent proximate analysis using a muffle furnace to determine ash content, volatile matter, and fixed carbon. Elemental composition (carbon, hydrogen, nitrogen, sulfur) was analyzed using a CHNS analyzer, and calorific values were measured with a bomb calorimeter to evaluate their energy potential.
For thermogravimetric analysis (TGA)-based pyrolysis studies, the Kissinger Akahira Sunose (KAS) and Flynn Wall Ozawa (FWO) iso-conversion methods were employed to estimate kinetic and thermodynamic parameters, including changes in entropy (ΔS), enthalpy (ΔH), and Gibbs free energy (ΔG). These model-free methods were chosen for their robustness and ability to provide accurate activation energy (Ea) estimates without assuming specific reaction mechanisms.
The Coats–Redfern method was applied to calculate activation energy, pre-exponential factors, and reaction order. Additionally, the Comprehensive Pyrolysis Index (CPI) was used to evaluate the pyrolysis performance of the samples at different heating rates. TGA experiments were conducted in a nitrogen environment with heating rates ranging from 10 to 40 °C/min and a temperature range of 30 to 600 °C.
Results and Discussion
The synergistic interactions between rice husk (RH) and low-density polyethylene (LDPE) during co-pyrolysis were pivotal in enhancing both the yield and quality of the pyrolysis products. These interactions were influenced by the feedstock composition, operating parameters (e.g., heating rate and temperature), and the inherent properties of the materials.
Thermal degradation of RH began at a lower temperature compared to LDPE, initiating the co-pyrolysis process. At approximately 400 °C, solids produced from RH degradation acted as radical donors, facilitating the scission of LDPE polymer chains by supplying hydrogen radicals. Notably, the initial polymer chain scission was independent of the blending ratio of RH and LDPE, highlighting the robust nature of the interaction.
The thermodynamic analysis demonstrated consistency across the three iso-conversional model-free methods (KAS, FWO, and Coats–Redfern), confirming the reliability of the derived parameters. Variations in thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) corresponded to distinct stages of the pyrolysis process, reflecting the complexity of the reaction mechanisms.
The Comprehensive Pyrolysis Index (CPI) showed significant improvement with increasing heating rates, underscoring their role in promoting pyrolysis efficiency. At 600 °C under a nitrogen atmosphere, the CPI values varied with LDPE:RH ratios: 1.2–4.9 for 50:50, 2.1–4.1 for 25:75, and 1.9–6.5 for 75:25 blends. Both average and maximum decomposition rates increased with higher heating rates, further demonstrating the enhanced performance of the co-pyrolysis process under these conditions.
Conclusion
This study comprehensively investigated the co-pyrolysis kinetics and thermal behavior of rice husk (RH) and low-density polyethylene (LDPE) at blending ratios of 75:25, 50:50, and 25:75. The findings suggest that RH and LDPE can serve as promising feedstocks for thermochemical conversion processes, with the 50:50 ratio yielding the most favorable results.
Activation energy values, calculated using the Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) methods, were found to be 102 and 107 kJ/mol for the 50:50 blend, 100 and 101 kJ/mol for the 25:75 blend, and 110 and 117 kJ/mol for the 75:25 blend, respectively. The experimental higher heating value of the 75:25 blend (20.61 MJ/kg) closely matched the theoretical value (20.31 MJ/kg), highlighting the reliability of the study's findings.
This research significantly contributes to the understanding of co-pyrolysis and its potential application in the energy sector. It demonstrates how blending RH with LDPE enhances the pyrolysis process, offering a viable pathway for energy recovery and waste management. However, the study’s scope was limited to the co-pyrolysis of these two feedstocks.
Future research could explore the use of catalysts to improve reaction kinetics and product yields further, as well as investigate the practical applications of pyrolysis products such as bio-oil, char, and gases.
Journal Reference
Bisen, D. et al. (2024). Thermogravimetric analysis of rice husk and low-density polyethylene co-pyrolysis: kinetic and thermodynamic parameters. Scientific Reports, 14(1). doi: 10.1038/s41598-024-82830-9. https://www.nature.com/articles/s41598-024-82830-9
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