Slow pyrolysis of coffee husk briquettes: characterization of the solid and liquid fractions

Abstract

The objective of this study was to investigate the kinetics of the biomass and to determine the influence of temperature on the distribution and quality of the products of slow pyrolysis. Coffee husk briquettes were produced by densification and subsequently subjected to slow pyrolysis at different final temperatures of 623 K, 673 K and 723 K at a heating rate of 0.5 °C/min. The results of the TGA showed that the pyrolysis of coffee husks occurred mainly at temperatures between 400 K and 700 K. The activation energy values ranged from 104.90 kJ/mol to 345.2 kJ/mol for the Friedman model and from 70.4 kJ/mol to 288.8 kJ/mol for the FWO model. In the composition of the bio-oil, at higher pyrolysis temperatures, there was a greater production of phenolic compounds and nitrogen compounds. Lower temperatures promoted the greater formation of hydrocarbons and acetic acid. The energy content of the biochar did not differ with the increasing temperature, but the fixed carbon was slightly higher at 723 K. It was concluded that the TGA provided information on the kinetics of the coffee husk pyrolysis, making it possible to improve the conversion process of this waste into products. Both isoconversional models were effective in estimating the Ea of the pyrolysis process with good accuracy. High final pyrolysis temperatures improve the quality of bio-oil, producing bio-oil with smaller amounts of acidic compounds, and this bio-oil is desirable for use as fuels. In addition, the bio-oil contains numerous chemical compounds that can be extracted and used in value-added chemical products.