Comparison of the Spray Combustion Characteristics and Emissions of a Wood-Derived Fast Pyrolysis Liquid-Ethanol Blend with Number 2 and Number 4 Fuel Oils in a Pilot-Stabilized Swirl Burner

Abstract

Biomass fast pyrolysis liquid (bio-oil) is a cellulose-based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition (pilot) energy, and total primary combustion air on the stability and emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests, and the results were compared to the burner operation with number 2 and number 4 fuel oils. Bio-oil exhibits higher emissions than number 2 fuel oil at any given operating point. This is primarily due to better atomization quality with number 2 fuel oil, although not as a consequence of viscosity differences, which are minor at the measured fuel temperature in the nozzle (>80 °C). The disparity in atomization quality is caused by differences in the relative amount of atomizing air to liquid fuel flow rate provided to the nozzle at a fixed energy throughput. Another factor that contributes to higher bio-oil blend emissions is worse overall distillation characteristics compared to number 2 fuel oil. As a fully distillable fuel that evaporates high-energy compounds, number 2 fuel oil is far less sensitive to changes in flame stability or blow-off brought upon by varying the swirl number, atomizing air, pilot energy, or primary combustion air flow rate. Because of a combination of these atomization quality and fuel volatility effects, the bio-oil blend cannot operate over as wide of a range of primary air or atomizing air flow rates as number 2 fuel oil. The bio-oil blend has a lower boiling point than diesel and is much more susceptible to flashing-induced combustion instabilities, which lead to intermittent blow-out and higher CO emissions. The NOx and particulate emissions of number 2 fuel oil are lower than bio-oil because of the negligible fuel nitrogen and ash contents in the fuel, respectively. Number 4 fuel oil is more comparable to bio-oil because of its nondistillable fraction, fuel nitrogen, and ash contents. CO and hydrocarbon emissions are lower than the bio-oil blend, but total particulates and carbonaceous residues are higher for number 4 fuel oil. This is despite better atomization and a lower nondistillable fraction, suggesting that the fuel-oil residues are more difficult to burn out completely. Fuel NOx conversion efficency of number 4 fuel oil is similar to the bio-oil blend. There are differences in fly ash mineral composition between the two fuels, as well as a much higher sulfur content for number 4 fuel oil. Carbon burnout analysis indicates that all fuels can achieve high carbon conversion efficiency (>99%) at the best operating conditions. The bio-oil blend has the highest amount of unburned carbon, of which the majority is in the form of gaseous CO.