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.

A thorough knowledge of liquid fuel properties is required for optimizing the design of spray combustion systems. To this effect, the density, rheological characteristics, and surface tension of a hardwood-derived bio-oil fraction were measured between 25 and 80 °C. Particular attention was paid to how these properties would govern the potential spray behavior of the fuel. Whenever possible, results were compared to No. 6 residual or heavy fuel oil, which is the closest petroleum counterpart. Steady and dynamic rheological tests were performed that indicate that the bio-oil is mildly shear thinning, weakly elastic, and predominantly Newtonian in nature. Direct visualization of hanging pendant drops allowed for the measurement of non-equilibrium and equilibrium surface tension. Small differences were found between these values, and the dependence of surface tension on the temperature was weak. Details about the microstructure of the fuel were examined using optical microscopy. Although the bio-oil demonstrates macroscopic homogeneity and phase stability, it is microscopically composed of a tar-like, char-laden phase immersed within a bulk aqueous phase. The atomization quality of the liquid fuel was evaluated using the Ohnesorge number. On the basis of its liquid properties, the bio-oil is expected to produce a spray that is comparable to heavy fuel oil. Heating value, water content, pH, and elemental composition were also measured and fell within the expected range for bio-oils. Special considerations and the adaptation of standard experimental techniques to accommodate working with the fuel have been emphasized to facilitate future studies.

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.