The increasing energy demand and the growing concern over the environmental impact caused by the combustion of fossil fuels require the development of effective alternative fuel sources. Biodiesels and bio-alcohols are attractive alternatives to liquid petroleum-based fuels. Biodiesels, such as soy methyl ester can be produced from renewable resources by transesterification of vegetable oils. Methyl esters have properties similar to those of petroleum-based fuels which allow them to be blended with petroleum fuels and to be used in existing infrastructure with minimal or no modification. Alcohols can be produced from renewable resources through the distillation of sugar, and starch crops. Adding alcohols such as butanol to a biofuel-petroleum blend extends the miscibility limits of the blend, increases the content of renewable components, and the concentration of fuel-bound oxygen in the fuel. The characteristics of the blends of alternative fuels and petroleum-based fuels (up to 20% content of alternative fuels) did not vary significantly with respect to a neat petroleum fuel in terms of performance indicators such as heating value or adiabatic flame temperature.

Porous media combustors offer several unique characteristics and advantages when compared to conventional burners, such as an enhanced heat transfer between the combustion products and the reactants, improved mixing of the unburned mixture, and improved evaporation of liquid fuels. These unique characteristics allows for flame stabilization at lean and ultra-lean combustion conditions which suppress the formation of pollutants like NOx and CO. Porous media burners have been developed for application such as furnaces, gas turbines, steam generators, and heating systems.

The porous media burner used in this study consisted two different chambers; a flame chamber fixed on top of a spray chamber with the porous media encased between them. The flame chamber (4.3 cm each side, and 27 cm tall) was manufactured out of stainless steel, it was fitted with tempered glass windows in the front and rear sides to allow flame visualization. In addition, it had two lateral 1 cm wide slots to perform probe or thermocouple measurements. The spray chamber had slightly larger dimensions (5 cm square, and 30 cm high) and was located upstream the flame chamber and the porous media casing. It was fitted with four tempered glass windows in order to observe the quality of the spray. Two silicon carbide porous media were used in the burner; an evaporation porous medium (EPM) was the upstream segment of the porous burner. The EPM had pores of diameter 0.75 mm and a pore density of 31 pores per centimeter. The EPM served to enhance the spray evaporation by transferring trapped combustion heat to the fuel/air unburned mixture. It also functioned as a flashback barrier since the pore diameter was small enough to quench the reactions. The second porous medium was a combustion porous medium (CPM). The CPM was characterized by its low pore density (8 pores per cm) and relatively larger pores (diameter >1 mm). The CPM helped to enhance the mixing and stabilize the reactions. In this study, the flame was located downstream of the CPM. The porous media were held together in a two-part stainless steel casing with an inlet and outlet of square dimensions of 3.75 cm. Both halves of the casing had inner dimensions corresponding to those of the porous media, and outer dimensions of 10 cm on each side.

Fuel was injected into a preheated (463 K) co-flowing environment using an air-blast atomizer upstream of the porous media. The experiments were carried using three different fuels Jet-A, a blend of 10% SME with 90% Jet-A (SA 10), and a 10% butanol-10%SME-80% Jet-A blend (BSA 10-10), and their combustion characteristics were studied at three different equivalence ratios, ϕ=0.5, ϕ=0.6, and ϕ=0.7 which were selected based on the stability of the flames in the fuel lean operation regime. Equivalence ratio was varied by changing the fuel flow rate and keeping the total air flow rate constant. Flame appearance was measured by taking photographs of the flame; exhaust global emissions and in-flame species concentration at 25%, 50% and 75% flame height were performed using a NOVA gas analyzer and a quartz sampling probe of 1mm aperture and a body of 6mm ID and 7.2mm OD; temperature measurements were carried using an in-house built R-type thermocouple at 25%, 50%, and 75% flame height; soot volume fraction was measured using a laser and a power meter, and the integrity of the burner after operation was addressed by measuring the pressure drop through each PM after operation.

The flames generated in the porous media burner were non-luminous, and blue due to the lean combustion conditions, enhanced evaporation of the reactants, and mixing mechanisms. Additionally, the presence of fuel-bound oxygen in the blend affected the luminosity and visible height of the flames as oxidation of the fuel occurred faster when the concentration of fuel-bound oxygen was higher. The appearance of the flames and the soot volume fraction measurements showed that no measurable amount of soot was being generated by the flames. The emission indices of NOx were comparable for the all the fuel blends at a given equivalence ratio. Peak emission index for NOx was found to occur at ϕ= 0.6. It was concluded that the thermal NOx formation mechanism was the dominant mechanism in all cases studied since the highest peak in-flame temperatures for all fuels were recorded at ϕ=0.6. Radial in flame temperature profiles were uniform through the span of the PM as a consequence of the enhanced mixing, and homogeneous reaction produced by the presence of the porous media. Peak temperatures were similar for all flames, at all conditions, on every flame height recorded. The emission indices of CO were comparable for the all the fuel blends at any given equivalence ratio. Peak emission index for CO was found to occur at ϕ= 0.6. CO emission indices were significantly lower than those of open spray flames, and open flames which demonstrate the effects of the porous media burner in suppressing CO emissions. After 100 hours of operation while using blends the pressure drop across the EPM increased by 59 Pa, while pressure drop increased 2 Pa for the CPM.