Biomass fast pyrolysis liquid (or bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better 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 source energy, air/fuel preheat, and equivalence ratio 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. Since the fuel is not fully distillable, it is important to have good atomization, thorough mixing, and increased recirculation to promote the burnout of nonvolatile material and decrease CO and hydrocarbon emissions. Air and fuel preheat are also important for reducing these emissions, although subsequent fuel boiling within the nozzle should be avoided in order to maintain flame stability. The amount of total primary air and atomizing air that can be used to improve turbulence, mixing, droplet burnout, and overall combustion quality is limited by the low volatility and tighter lean blow-out limit associated with bio-oil. The NO x produced in these flames is dominated by the conversion of fuel bound nitrogen. In order to reduce the NO x emissions without refining the fuel, the use of staged combustion is recommended.

Biomass fast pyrolysis liquid (or bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better 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 source energy, air/fuel preheat, and equivalence ratio on the particulate matter emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests. Increasing the residence time of spray droplets in the hot combustion zone by increasing the swirl number promotes the burnout of solid residues. Decreasing the mean diameter of fuel droplets by increasing the atomizing air flow rate has a similar effect. Ignition source energy, air/liquid fuel preheat, and equivalence ratio have either a weak or ambiguous effect on the measured particulate emissions. The residual material collected from the exhaust is composed of both carbonaceous matter and fly ash. However, the majority of particulate matter consists of ash, even at relatively poor combustion conditions. These results suggest that it is possible to provide enough oxygen availability and residence time for droplets to undergo nearly complete burnout during combustion. Under such conditions, the total particulate matter emissions could be further mitigated by reducing the inherent ash content in the fuel.

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 blowout and higher CO emissions. The NO x 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 NO x 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.