The role of atomization in the spray combustion of a fast pyrolysis bio-oil

The influence of atomization on spray, combustion, and gas- and solid-phase emissions was investigated experimentally for a fast pyrolysis bio-oil (FPBO), also known as bio-oil or pyrolysis oil, in a 10 kW swirl burner. Two different single-hole twin-fluid nozzles, namely externally-mixed and internally-mixed, with the same equivalent exit diameter were studied for applications in micro gas turbine engines. Over the range of air-to-liquid mass flow ratio, 0.25 < ALR < 0.65, the spray generated by the internally-mixed nozzle comprised finer droplets with approximately 50% smaller diameter than the externally-mixed one. Two empirical correlations were modified for the prediction of Sauter mean diameter (SMD) of the droplets generated by each nozzle. The combustion tests showed that while reaching a 100% FPBO stable flame was possible using the internally-mixed nozzle, 10% by volume fraction ethanol (EtOH) was needed when using the externally-mixed one to improve the volatility and subsequent ignition characteristics of the larger size fuel droplets generated by this nozzle. The quantitative pollutant emissions measurements also demonstrated that, using 90/10 FPBO/EtOH as a fuel and at the same ALR, the spray flame generated by the internally-mixed nozzle had less pollutant emissions than the externally-mixed one due to the atomization effects. Conducting combustion tests in conjunction with spray testing showed that, while the spray SMD plays a critical role in the ignition and combustion of FPBO, other flow-field characteristics such as near nozzle shear-forces and hot-zone residence times of droplets are also important in generating a stable 100% FPBO flame with minimum emissions.