On the oxy-combustion of blends of coal and agro-waste biomass under dry and wet conditions

Integrating bioenergy into carbon capture and storage systems (Bio-CCS) is a novel concept aiming at reducing CO2 emissions, pointing to a short-term need to increase the use of non-conventional biomasses. The main objective of this experimental research is to characterize the behavior of two agro-waste biomasses under oxy-co-firing conditions, as concerns fuel conversion and NO formation, compared to the use of typical raw pine wood. The effect of replacing CO2 with H2O in the firing atmosphere is also sought. Two different biomass shares in the blends, 20 % and 50 %, are selected. The experiments are conducted in a lab-scale entrained flow reactor for two O-2 concentrations (21 % and 35 %) and four H2O concentrations (0 %, 10 %, 25 %, 40 %). Some operating conditions are kept the same to enable the comparisons: mean residence time (3 s), initial reactor temperature (1000 degrees C) and oxygen excess (1.25). New results have been obtained from the experiments, optimizing burnout degrees and reducing NO levels. Minimum differences in conversions are detected for the 35 % O-2 cases when the agro-biomasses replace the pine wood: less than 0.7 and 1.1 percentage points. Burnout degrees are maximized when 25 % CO2 is replaced with H2O in most cases, with maximum values in the range 97.3-97.7 %. The higher the agro-biomasses share in the blend, the higher the N-fuel to NO conversion, consistent with their larger nitrogen contents. Significant decreases of NO are detected when CO2 is replaced with H2O, with maximum reductions of 17.6 %. The extent of these NO reductions shows a clear dependence on the volatiles-to-char ratios for the fired blends: the higher the ratio, the lower the decrease. For the largest steam additions (40 %), the NO depleting effect caused by H2O is partially compensated with the enhancement of the N-volatiles oxidation, limiting the NO reductions to 1.7-7.3 % compared to the dry atmospheres.