The presence of suspended char particulate and ash in fast pyrolysis bio-oil produced from the fast pyrolysis of high ash forestry materials poses a significant technical challenge for the direct utilization and/or catalytic upgrading of these low-carbon renewable fuels. Cross-flow microfiltration is a physical upgrading process strategy that can remove suspended solids and ash from the fast pyrolysis bio-oil. To develop data sets on operational characteristics of fast pyrolysis bio-oil cross-flow microfiltration, experimental research was undertaken. Using a variety of filtration media with nominal pore sizes between 1 and 40 μm, typical solids and ash rejection ranged from 80 to 95% and 4–45%, respectively. An empirical modeling procedure was developed to predict the throughput and resistance associated with cross-flow microfiltration of fast pyrolysis bio-oil, which demonstrated good agreement with generated experimental data. Key operating parameters were also studied, and it was found that transmembrane pressures less than 1 bar and fluid preheat temperatures up to 60 °C were ideal for maximizing the pseudo steady-state flux of the process. Furthermore, the use of low viscosity, miscible solvents, and/or initial solids reduction pathways prior to microfiltration offered additional routes to potentially improve the throughput of such a process.

Mineral matter can negatively influence liquid yield and product properties from the fast pyrolysis of woody biomass residuals. Biomass pretreatment approaches to reduce the ash content represent a potential pathway to expand the feedstock flexibility of fast pyrolysis. In this work, ash reduction in biomass pyrolysis oil via sieving the fines portion of a hog fuel as well as washing the fractionated hog fuel with nitric acid was investigated prior to the conversion in CanmetENERGY-Ottawa’s 5–10 kg/h fast pyrolysis system. It was found that sieving the material was much less influential on the liquid yield and product properties relative to nitric acid washing. Product analysis showed up to 40% increase in organic liquid yield and up to 30% decrease in biochar yield on a dry, ash-free basis by nitric acid washing the fractionated hog fuel. Pyrolysis reaction water was minimized when nitric acid washing the feedstock, which has important implications for the phase separation of the pyrolysis liquids. Through nitric acid washing, the ash content in the liquid was reduced up to 87% relative to the liquid produced from the pyrolysis of the untreated material. The chemical quantification of the produced pyrolysis liquids demonstrated that the chemical composition was significantly altered after nitric acid washing the hog fuel, indicating that the removal of the ash species impacted the pyrolysis reaction chemistry.

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Cross-flow microfiltration of fast pyrolysis bio-oil is a physical treatment pathway capable of reducing the filtered product's solids content to acceptable levels for fuel utilization and/or upgrading. However, the overall throughput of the process is hindered by the fouling that occurs on and/or within the filtration media over short operating periods. To improve the throughput from fast pyrolysis bio-oil cross-flow microfiltration, the use of offline and on-line cleaning techniques were experimentally evaluated. On-line cleaning strategies using permeate, solvent, and compressed air confirmed the reversibility of the accumulated fouling layer over a small (<10) number of cleaning cycles. The use of compressed air as a simple on-line cleaning strategy was further examined over extended operating times, including a total of 31 consecutive backflushing cycles. Compared to the reference case (no cleaning), on-line compressed air backflushing increased the overall throughput of low solids permeates by more than 100%. Ultimately, the demonstration of an on-line fouling remediation strategy for fast pyrolysis bio-oil cross-flow microfiltration increases the likelihood that it could be a viable treatment pathway for suspended solids and/or ash removal in pyrolysis liquids before end-use and/or upgrading of the biofuel.

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