Spray atomization of bio-oil/ethanol blends with externally mixed nozzles

Authors: LUJAJI, FRANK - Nelson Mandela African Institute Of Science And Technology; Boateng, Akwasi; Schaffer, Mark; MTUI, PETER - Nelson Mandela African Institute Of Science And Technology; MKILAHA, IDDI - Nelson Mandela African Institute Of Science And Technology

Submitted to: Experimental Thermal and Fluid Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2015
Publication Date: 1/1/2016
Publication URL: http://handle.nal.usda.gov/10113/61662
Citation: Lujaji, F.C., Boateng, A.A., Schaffer, M.A., Mtui, P.L., Mkilaha, I.S. 2016. Spray atomization of bio-oil/ethanol blends with externally mixed nozzles. Experimental Thermal and Fluid Science. 71:146-153.

Interpretive Summary: The fuel properties of pyrolysis oil (bio-oil) produced from the fast pyrolysis of biomass are not easily compatible with burner systems. This is caused by its poor fuel properties such as high acidity, high water content, high oxygen content, wide volatility, and the presence of char, all of which cause problems in combustion systems. We sought to explore increased pyrolysis oil utilization in combustion systems through blending with ethanol and the use of twin fluid atomization nozzles that have previously worked successfully for high viscous fuels. The spray characteristics observed without combustion, including the variation of droplet sizes and discharge velocity of bio-oil blends were measured and compared with those exhibited by diesel fuel (No 2 oil) using different sizes of air-assisted atomization nozzles. We found that ethanol in bio-oil improved the bio-oil flow properties with the 20:80 bio-oil:ethanol blend showing good fuel characteristics comparable to diesel fuel. While the higher bio-oil concentration in the blend e.g.,40:60 bio-oil:ethanol blend showed good spray droplet sizes, their discharge velocities were slightly slower, suggesting potential impact on flame length and an attribute that must be taken into consideration when switching from diesel fuel to up to 40% bio-oil content. The results of this work will be helpful in providing information of bio-oil blends spray characteristics. The spray behavior is an important information needed when deciding on nozzle selection for the combustion of bio-oil in burners for home heating and industrial furnaces.

Technical Abstract: Experiments were conducted to investigate the properties of sprays of pyrolysis oil from biomass (bio-oil) using an air assisted atomization nozzle operated without combustion to explore the potential of pyrolysis oil combustion in industrial and home furnaces. Bio-oil was blended with ethanol to improve its flow characteristics. The fuel samples investigated were bio-oil blends with ethanol (EtOH), 100% ethanol and diesel. The bio-oil:EtOH blends were prepared in concentrations of 20:80 and 40:60 vol%. Twin-fluid nozzles SU2, SU4, and SU5 representing 0.711, 1.52, 2.54 mm, orifice diameters respectively were used in the spray experiments with the liquid and atomizing air pressure as well as temperature controlled to maintain a constant fuel flow rate equivalent to 30 and 50 kW energy input. Images of the atomized spray droplets for the fuel mixtures were analyzed to determine droplet size and velocity. Results show that it is possible to spray fuel mixtures with up to 40% of bio-oil blended with 60% ethanol. High viscosity and gelling were the main drawback, however, the 20:80 bio-oil:EtOH blend and 100% ethanol exhibited spray characteristics closer to that of diesel fuel.