Spray combustion of fast pyrolysis bio-oils: Applications,challenges, and potential solutions

This article provides a comprehensive review of the spray combustion of fast pyrolysis bio-oil (FPBO, also called bio-oil, pyrolysis oil or pyrolysis liquid biofuel), which is widely regarded as one of the most economically feasible renewable resources to facilitate the replacement of fossil oils. The utilization of FPBO as a fuel is challenging due to its unique atomization and combustion characteristics but it is important given the need to develop a more sustainable energy infrastructure. Significant efforts have been made in utilizing FPBO as a practical alternative fuel and the first FPBO facilities for heat and/or power generation have been brought online in recent years. FPBO-fueled burners, boilers, and furnaces are ready from a technical perspective for large-scale industrial use, and even small-scale systems show excellent flame stability, low emissions, and minimal requirements for secondary fuel usage. FPBO applications in gas turbine and compression-ignition engines are technically more challenging, currently having had only limited successes in larger-scale units and for short time intervals in smaller ones. With recent research and technological advances, however, FPBO use in small-scale combustion engines appears to be technically feasible. In the literature, extensive research efforts have been dedicated to this topic either as a fuel itself or its utilization for practical applications. Nonetheless, inadequate considerations have been given to the critical role of FPBO atomization and its subsequent fuel/air mixing, which in turn controls the combustion efficiency and emission characteristics of a system. Understanding the spray combustion properties of FPBO is especially important because of the fuel's unfavorable properties compared to fossil oils including low energy density, high viscosity, high water content, containing suspended solid particulates and non-volatile residue, chemical instability, and an incompatibility with conventional fossil oils. The information presented herein, therefore, focuses on understanding the challenges and constraints that are unique to FPBO applications, along with proposing several strategies to properly atomize and combust this fuel in order to enhance combustion efficiency and reduce pollutant emissions in practical systems. Although substantial progress has been made in understanding the FPBO spray combustion as revealed by this review, better standardization of FPBO properties, more efficient techniques for optimizing atomization and combustion for different applications, and more studies to understand the long-term reliability of devices running on FPBO are needed.