Optimization of bio-oil yields by demineralization of low quality biomass

Lignocellulosic biomass contains inorganic elements (ash) that are known to act catalytically during fast pyrolysis and result in a decrease of the process' selectivity towards the desired liquid product. In this work, the removal of inorganic elements from two forestry residues, two agricultural residues and two energy crops was studied with the aim to maximize the liquid product yield from the fast pyrolysis process. The biomass samples were washed with water and solutions of nitric or acetic acid and the effects of time, temperature, acid type and acid concentration in the washing solution were studied. The original and the pretreated biomass samples were then pyrolyzed in a bench-scale fixed bed reactor in order to investigate the effects of the demineralization on the yields and composition of the pyrolysis products. Washing with water was effective for the removal of up to 42% of the inorganics in the biomass samples, whereas washing with acidic solutions achieved inorganics removal higher than 90%. The most important factor during the biomass pretreatment proved to be the washing temperature; higher temperatures (50 °C) were more effective for the removal of the inorganics than room temperature. The type of acid used in the washing solution was also an important factor, as nitric acid proved to be more effective than acetic acid. In the pyrolysis experiments, the demineralized biomass samples yielded less gas products and solid residue, while the selectivity towards the liquid product was substantially increased in all cases.