Pyrolysis Characteristics of Forest Residues Obtained from Different Harvesting Methods

Abstract

Although forests promise an abundant supply for residue biomass for biorefining and biopower applications, heterogeneity generated in residue due to collection methods or selection process (limb, top, and understory etc.) may influence biofuel production in both the thermochemical and fermentation pathways. This article focuses on effects of collection methods on the fuel properties, thermochemical decomposition behavior, and properties of pyrolysis bio-oil and char. Forest residues studied in this research include: clean wood chips (CW), horizontal grinder material (GM), pre-commercially thinned biomass chips (PC-chips), biomass chips produced from pine limbs and tops (T-chips), and biomass chips produced from hardwood understory stems and pine limbs and tops (TU-chips). All biomass samples were characterized by measuring bulk density, moisture, chemical composition (cellulose, hemicellulose, lignin, ash, and extractives), energy density, proximate analysis (moisture, fixed carbon, volatile matter, and ash), and ultimate (C, H, N, S, O) analysis. Pyrolysis was performed on partially dried biomass samples to produce char and bio-oil. The yields of pyrolysis products were calculated. Char was characterized by proximate and ultimate analysis, energy density, and iodine number; and, bio-oil was characterized by pH, viscosity, ultimate analysis, energy density, and water content. In addition, fixed carbon yields, char carbon efficiency, and energy conversion efficiency of char were also calculated. Results showed that the GM had the highest ash content (5.9%) and the lowest energy density (17.03 MJ/kg) among all forest residues evaluated. TU-chips had higher concentration of extractives which resulted in an additional decomposition peak at 300°C beyond the main peak which appeared at 370°C responsible for decomposition of cellulose and hemicelluloses in thermogravimetric analysis. Results from this study showed no significant difference in char and bio-oil yields in the various forest residues evaluated; however, GM char which was large in ash content, because of contamination by soil inclusion had lower heating value and adsorption capacity. All residues evaluated produced bio-oil with similar fuel properties.