Energy quality of pellets produced from coffee residue: Characterization of the products obtained via slow pyrolysis

The growing market for pellets used to produce energy requires research on new feedstocks and technologies (pyrolysis) that increase the energy potential of these products. Thus, the objective of this study was to produce pellets from coffee residues, evaluate their energy potential, pyrolyze the pellets via slow pyrolysis and characterize the final products of the process (bio-oil and biochar). The pellets were produced from coffee husks in a pelletizer; some of the pellets were used for the combustion test, and others were used in the slow pyrolysis process. The efficiency in compacting the bulk biomass to obtain solid fuels was demonstrated by calculating the energy density. The pellet formation efficiency was also evaluated using stereoscopic images. Slow pyrolysis was performed at three final temperatures: 400 °C, 450 °C and 500 °C; 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 proximate analysis, higher heating values and Fourier transform infrared spectroscopy (FTIR) of the biochar were determined. The results showed that the prior addition of water to the coffee husk particles resulted in unstable pellets. The energy density of the pellets was statistically higher than that of the coffee husk. The combustion test allowed the combustion profile of the pellets to be determined, in which high temperatures and uniform burning were obtained. The pyrolysis product yields were influenced by the process final temperature. The biochar yield decreased with increasing temperatures; however, the fixed carbon and calorific values of the biochar collected at 450 °C were higher than the values of the biochar collected at other temperatures. The maximum yield of bio-oil was obtained at 450 °C, while the gas yields were the highest at 500 °C. The bio-oils obtained by higher pyrolysis temperatures comprised greater amounts of phenolic (phenol, phenol, 2-methyl- and phenol, 3-methyl-) and nitrogenous (caffeine) compounds. Lower temperatures yielded more esters (hexanedioic acid and bis(2-ethylhexyl) ester). The low atomic ratios H/C and O/C proved that temperatures ≥ 400 °C made the biochar more aromatic and carbonaceous, this fact was evidenced by the FTIR analysis. It is concluded that the coffee husk pelletizing process is a convenient alternative to obtain higher energy efficiency. Furthermore, from the energetic properties of the biochar the coffee husk has the potential to be used as fuel.