Evaluation of the physical-mechanical and energy properties of coffee husk briquettes with kraft lignin during slow pyrolysis

Abstract

The objective of the study was to evaluate the properties of briquettes produced with coffee husks with the addition of kraft lignin for the production of solid biofuels of high physical-mechanical and energetic quality and to analyze the feasibility of converting these briquettes into co-products with higher energy value at from the pyrolysis process. The ratios of kraft lignin to the coffee husk mass were: 10%, 20% and 50%. The briquetting was performed at temperature 120 °C and at a pressure of 15 MPa for 15 min. The physical mechanical characteristics such as apparent density and compressive strength were determined. Slow pyrolysis was performed in 350 °C, 400 °C and 450 °C at a heating rate of 5 °C/min; the final residence time was 1 h. The product yields were calculated at the end of the pyrolysis process. The chemical compounds present in the bio-oil were identified by gas chromatography mass spectrometry (GC-MS) and the biochar was evaluated for proximate analysis, higher heating value (HHV) and Fourier transform infrared spectroscopy with total reflectance attenuated (FTIR-ATR). The briquettes produced with 50% of lignin kraft showed higher apparent density and compressive strength. The pyrolysis performed at 350 °C of the briquettes with 50% lignin favored the production of biochar, on the other hand the pyrolysis performed at 350 °C of the briquettes with 10% lignin provided a higher bio-oil yield. In the bio-oil obtained in the pyrolysis at 400 °C of briquettes with 50% lignin kraft, the greatest production of phenolic compounds occurred. On the other hand, the greatest yield of nitrogen compounds (caffeine) occurred in pyrolysis at 350 °C of briquettes with 20% kraft lignin. The chemical and energetic properties of biochars did not vary considerably in the studied pyrolysis temperature ranges. However, the use of the thermochemical process enhanced the energetic properties and the FTIR analysis showed that biochars have become more aromatic and carbonaceous, in relation to the raw agro-industrial residues. It is concluded that the combination of the different agro-industrial residues (coffee husk and kraft lignin) can offer a sustainable solution for part of the energy scenario, considering that both the properties of the briquettes and the characteristics of the pyrolysis products have been improved.