Síntese e caracterização de hidrocarvão de casca de arroz (Oryza sativa): análise e aplicações

Abstract

The planet's climatic situation has been worsening at alarming rates. From 2011 to 2020, the global temperature rose by 1.1°C, a value that, in the past, was equivalent to the temperature increase observed between 1850 and 1900. The main contributor to this increase is CO2, which, despite being the least polluting greenhouse gas, is present in larger quantities due to the burning of fossil fuels. As a solution to reduce this problem, the search for energy alternatives that are part of the carbon cycle has sparked interest, such as hydrochar (HC). This material, produced from the hydrothermal carbonization (HTC) of biomass, is strongly influenced by process parameters such as temperature, residence time, biomass/water ratio, and heating rate. HC has distinct characteristics compared to both the source material and other carbonaceous materials, such as a large surface area with chemical functional groups, higher calorific value, and thermal stability, making it useful as a fuel, soil amendment, adsorbent, catalyst support, additive for cementitious composites, and supercapacitors. In this study, HC was synthesized using rice husk (RH) as a precursor at 180°C for 360 minutes, with a biomass/water ratio of 1g:5mL. Both the new material and the RH were analyzed, and it was observed through scanning electron microscopy that larger pores appeared in HC compared to RH, along with a more degraded surface. Energy dispersive spectroscopy revealed mass percentages of 61.52% C, 38.30% O, and 0.18% Si for RH, and 73.09% C, 24.26% O, and 2.65% Si for HC, confirming improved energy characteristics due to the decrease in the O/C mass ratio. Fourier-transform infrared spectroscopy showed the presence of several functional groups and aromatic rings originating from hemicellulose, cellulose, and lignin. HC exhibited an increase in calorific value to 19.14 Jg-1, compared to 17.56 Jg-1 for RH, and an energy densification of 1.090. X-ray diffraction demonstrated an increase in crystallinity for HC, with sharper peaks at 2θ = 16.55°, 22.60°, and 35.15°, attributed to cellulose and cellulose-based polysaccharides, while RH showed peaks at the same angles. Regarding surface area analysis, HC showed an increase in all parameters, with a surface area of 9.73 m²g-1, an average pore diameter of 11.51 nm, and a total pore volume of 0.028 cm³g-1, compared to RH with 0.54 m²g-1, 8.64 nm, and 0.0012 cm³g-1, respectively. Thermal analysis indicated improved thermal stability and performance in combustion processes for HC, as demonstrated by its ignition, peak, and burnout temperatures. Proximate analysis revealed a lower volatile matter content for HC (64.13%), indicating lower reactivity, a higher ash content (17.76%), making it less suitable as a fuel due to incrustation, and a higher fixed carbon content (18.11%), which improves its calorific properties, whereas RH had values of 73.06%, 12.35%, and 14.59%, respectively. Finally, HC's apparent density increased to 408.03 kgm-3, compared to RH's natural value of 150.30 kgm-3. These results position HC as a promising candidate for various applications such as solid fuel, catalyst support, adsorbent in aqueous streams, and soil amendment for immobilizing pollutants and providing phosphorus.