Cypress Wood and Bark Residues Chemical Characterization and Utilization as Fuel Pellets Feedstock

In order to meet the growing demand for raw material of solid biofuels, it is imperative to find alternative materials of low cost, underutilized so far. In this study, wood and bark material of two common cypress species (Mediterranean and Arizona cypress) were chemically characterized through gravimetric and spectroscopic (FTIR) analyses, to assess their potential to be used as raw materials in the production of fuel pellets. Low bark concentrations (0%, 2%, and 7%) were applied, and the mixtures were densified in a flat-die pellet press. The produced pellets were examined in terms of thermal, physical, hygroscopic, and mechanical properties, using standard ISO17225 thresholds as benchmark. The results revealed that the effect of bark presence in low content n pellet properties and quality was positive. The ash content of both wood species is adequately low for biofuels production, whereas their bark cannot be purely used as feedstock due to the high ash content. By using low bark contents (2% or 7%), the ash content of pellets was kept adequately low to be categorized in the highest quality classes (A1 and A2: for residential applications), while the produced pellets demonstrated improved dimensional stability, mechanical durability, and slightly improved calorific value. The moisture content, dimensions, and bulk density of all the produced pellet categories fulfilled the standard requirements. Even though the pellets of 2% bark share presented much lower ash contents, only the pellets of 7% bark share were proven to have considerably improved mechanical durability, suitable for residential use. The chemical composition of raw materials (especially the extractives and holocellulose) plays a major role in the mechanical durability of pellets.