The effect of plastic addition on the mechanical and thermal properties of fine coal extrudates

Abstract

Coal and plastics are significant sources of waste, not only in South Africa, but globally. During coal mining, coal fines are produced, and these fines are not widely used due to transportation and handling difficulties. The size of the coal fines needs to be increased to be integrated into most industrial processes. Multiple studies have investigated various potential agglomeration techniques and binders to improve the size of the coal fines. However, binders are often the most expensive part of agglomeration, therefore, waste streams as potential binders are being investigated. This study focusses on the extrusion, pyrolysis and gasification behaviour of discarded coal fines from the Highveld coalfield together with recycled waste low-density polyethylene (LDPE) and polypropylene (PP), respectively.

A twin-screw extruder with a 10 mm die and barrels heated to 220 °C was used to produce coal fine extrudates with varying plastic content (10, 25, 50, 75 and 100 wt.%) for both LDPE and PP. The 10% LDPE and 10% PP extrudates exhibited compressive strengths of 17.5 and 7.9 MPa before breaking, respectively. Extrudates containing more than 10% plastic did not break under load but merely deformed. All extrudates maintained similar compressive strengths after being submerged in water and absorbed less than 5% moisture after 24 hours in water.

The pyrolysis kinetics of the extrudates and raw materials were evaluated using Iso-conversional techniques (Kissinger–Akahira–Sunose, Starink, and Flynn–Wall–Ozawa) by means of Thermogravimetric analysis (TGA) under a nitrogen atmosphere, increasing temperature from ambient up to 900 °C. The extrudates exhibited lower activation energies and lower conversion temperatures, indicating synergy between plastic and coal fines during pyrolysis. Pyrolysis product yields and characteristics were further evaluated using Fischer Assay slow pyrolysis where the temperature was increased by 5 °C/min until the final pyrolysis temperature (520, 720, and 920 °C) was reached. The coal produced up to 83% char, whereas the main product derived from the plastics were overwhelmingly condensable liquids (< 90%). The small reactor volume and slow heating rate actively promoted condensable product formation. Both char and condensable product yields followed the additive model of the raw materials. Statistical models were developed which can estimate the pyrolysis product yield and characteristics using the composition of the original extrudate and the final pyrolysis temperature. The p-values indicate that temperature mostly interacts with the fine coal content, further supporting the observation that volatiles produced by the plastics leave the reactor before being exposed to higher temperatures. Furthermore, the condensable products derived from the PP containing extrudates have more components with higher boiling points than those produced from either coal or PP alone.

Chars derived from extrudates containing 0, 10, 25, and 50 wt.% plastic were analysed using petrographic carbon form, surface area and porosity, and XRD carbon crystallite analysis. Chars produced at 920 °C were pulverised (75 - 150 µm) before undergoing CO2 gasification at 800, 825 and 850 °C in the chemical-controlled regime. The chars’ crystalline structure became more ordered with an increase in plastic content. Even with the more ordered structure, the gasification reactivity increased similarly with an increase in both LDPE and PP content in the original extrudates. Petrographic carbon form analysis showed an increase in crack and pore formation with an increase in plastic addition during all stages of the chars’ evolution as the pyrolysis temperature increased. The low-pressure gas adsorption (LPGA) results report that maximum porosity occurred at 720 °C and decreased when the pyrolysis temperature further increased. This indicates that most of the pore growth occurred during pyrolysis and coalescence occurred during gasification. The volumetric reaction model (VRM) time factor showed a positive linear correlation with an increase in BET surface area, especially for the LDPE derived chars.Therefore, the observed reactivity increase with an increase in plastic content can be correlated to an increase in surface area.

The study suggests that the co-extrusion of recycled plastic with coal fines produces solid carbonaceous fuels with high hydrophobicity and mechanical strength compared to coal fines. The calorific value of the extrudates improves with an increase in plastic content since both LDPE and PP (46.5 MJ/kg) have calorific values more than double that of the coal fines. Additionally, the plastic content can be varied to vary the produced pyrolysis products. This alludes to the possibility that the fuel source can be adapted to suit a wide range of industries without causing large operational changes since the composition can be altered to be close to that of the fuel source currently in use. Furthermore, the gasification reactivity of the chars increased with an increase in plastic content. This allows processes to speed up the rate limiting char gasification step with the addition of plastic.