Beneficiation of sugarcane bagasse for production of GreenCoal

Abstract

The sugarcane mills in South Africa are to some extent self-sustaining by using sugarcane bagasse and coal to generate heat and power; however, due to the inferior physicochemical properties of sugarcane bagasse, there is potential to add maximum value to the entire crop by investing in a bio-refinery concept. Hydrothermal liquefaction has the potential to add value to the sugarcane crop by converting sugarcane bagasse to hydrochar and using it to produce heat and power. This study aimed to evaluate GreenCoal pellets, prepared from coal and hydrochar, for combustion and gasification applications. The objectives of this study were to characterise the sugarcane bagasse, hydrochar, GreenCoals and coal, and to evaluate the effect the hydrochar fraction had on the pellet properties and behaviour during thermochemical processing. In this study, a continuous HTL pilot plant upgraded sugarcane bagasse with the main reactor operating at 270°C under a system pressure of 70 bar and a flow rate of 120 L/hr. The feed was a slurry containing 4% biomass. The solid yield was 20.47%. The samples were characterised according to the chemical and structural characteristics. Chemical and structural analyses included proximate analysis, elemental analysis, higher heating value (HHV), Fourier-transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), X-ray powder diffraction (XRD) and X-ray fluorescence. Additional analysis conducted on the sugarcane bagasse and hydrochar included fibre analysis (NDF, ADF, and ADL). Following HTL of sugarcane bagasse, all samples were pelletised and evaluated for the strength, as well as the combustion -, and CO₂-gasification behaviour. The results showed that GreenCoal pellets had superior properties. The energy density of hydrochar and GreenCoal pellets (31.61 GJ/m3 ─ 33.28 GJ/m3) was significantly higher than the sugarcane bagasse pellets (21.24 GJ/m3), whereas both the mass and energy density of hydrochar and GreenCoal pellets were higher than that of coal pellets, following a linear relationship with the hydrochar/coal ratio. The mechanical strength and durability of GreenCoal pellets improved with higher hydrochar/coal ratios, showcasing the natural binding effect of hydrochar. Furthermore, all GreenCoal pellets and hydrochar pellets showcased excellent hydrophobic properties, with the pellets remaining intact after submerging the pellets for two hours in water and showing negligible amounts of water absorbed. In contrast, sugarcane bagasse and coal pellets dissolved within five minutes. FTIR analysis and SEM images suggested that the hydrochar consisted out an aromatic hydrochar with a high degree of aromatisation. The thermal degradation of hydrochar under inert conditions revealed two phases: the secondary- and primary hydrochar. The hydrochar demonstrated improved combustion behaviour opposing the sugarcane bagasse pellets with prolonged combustion at higher temperatures and lower reactivity. The ignition and peak temperatures increased with 78°C ─ 117°C and 137°C ─ 168°C, respectively. The results showed that hydrochar had higher ignition and peak temperatures than that of coal; however, the GreenCoal pellets demonstrated synergetic behaviour with lower ignition and peak temperatures. Both the ignition index and comprehensive combustion index of GreenCoal pellets were favourable against the pure hydrochar and pure coal pellets and showed that GreenCoal 1:1 demonstrated the best combustion behaviour. The hydrochar pellets showed favourable, lower reactivity of the chars in opposition to the sugarcane bagasse pellets. At a conversion of 50%, the reactivity (R50%) of sugarcane bagasse and hydrochar pellets was 107.9 min⁻¹ and 28.2 min⁻¹, respectively. Coal demonstrated the lowest reactivity, with R50% of 17.7 min⁻¹. The benefit of producing GreenCoal blends was observed, with GreenCoal pellets showing synergistic behaviour. Overall, the effect of the hydrochar/coal blend was less apparent at higher gasification temperatures.