Advancing the Application of Pyrolysis Liquid (Bio-oil) by the Improvement of Its Fuel Properties by Thermo-Catalytic Reforming

Biomass-derived fast pyrolysis liquid, also called bio-oil (FPBO), can replace petroleum fuels in thermal devices, but several undesirable properties hinder its widespread application. This paper aims to improve these properties and quantify their effect on its combustion performance. In this study, higher quality bio-oil having similar viscosity and surface tension as FPBO was obtained through the thermo-catalytic reforming bio-oil (TCRBO) process. The effect of fuel chemical composition, thermogravimetric analysis (TGA) residue, fuel volatility, heating value, water content, solid content, and ash content was examined in a 10 kW spray burner. Gas phase emissions and particulate matter (PM) were also measured and compared to its operation on fuel oil #2 for which diesel was used. TCRBO ignited irrespective of the primary air preheat temperature, and its evaporation characteristics (i.e., TGA residue of less than 3%) were more like diesel. TCRBO does not display any flash atomization. Both FPBO and TCRBO displayed coking potential; however, for TCRBO, no coking was observed if the primary air was heated above 110 °C. FPBO droplets were larger in size (∼100 μm) and could not achieve complete burnout due to increasing emissions. TCRBO exhibits lower emissions than FPBO at any given operating point, primarily due to better atomization achieved due to its higher fuel heating value, which reduced the fuel flow rate by almost 40%. FPBO resulted in the highest PM emissions, which can be attributed to the formation of secondary char particles. Higher fuel ash content also contributed to the PM emissions. The reduction in fuel water content (i.e., ∼2 wt %) made the TCRBO flame less susceptible to blow out, and a stable flame was observed without the pilot flame. Swirl and pilot were necessary to stabilize the FPBO flame. The TCRBO’s higher fuel nitrogen content significantly increased its NOₓ emissions. Overall, thermo-catalytic reforming significantly improved the combustion characteristics of pyrolysis bio-oil. Although TCRBO was obtained from lower quality feedstock (e.g., sewage sludge), its superior combustion performance is attributed to the ability of the TCR process to produce higher quality liquid fuels from various biomass sources.