Burning characteristics of grape marc under mild combustion conditions

In this experimental study, the burning characteristics of grape marc, as a biomass fuel, under MILD (Moderate or Intense Low-oxygen Dilution) combustion conditions are explored. Specifically, the impact of grape marc particle size on flame structure, stability and pollutant emissions, under MILD combustion, is investigated. Three different particle sizes of the grape marc are considered, in the range of 150–250 μm, 250–355 μm and 355–500 μm. The particles are injected through a central insulated jet into a hot and vitiated coflow inside a vertical MILD combustion furnace. All other operating conditions remain the same. CO2 is employed as a carrier gas and the central jet bulk Reynolds number is set to the value of Rejet = 13,040. The vitiated coflow had ~6% oxygen molar fraction and a flow bulk temperature of about Tco-flow = 1330 K. The heat input from the grape marc fuel is kept constant at 10 kW, corresponding to a mass flow rate of 1.63 kg/h. Stable MILD combustion conditions have been successfully achieved for all cases. The temperature gradient along the vertical and the radial direction of the furnace is limited to below 100 K for all the cases. The variation of particle size does not significantly impact on the furnace temperature and exhaust emissions. Albeit, the case of largest particle size exhibits a higher temperature by ~40 K as compared to the other two cases, and the highest NO production is identified for the middle particle size's case showing peak value of approximately 478 ppm at the exhaust at 3% excess O2. No remarkable NO-reburning process is observed for any cases, which points towards the intense volatile combustion of grape marc under MILD combustion conditions.