Effects of potassium and calcium on the early stages of combustion of single biomass particles

Abstract

The main objective of this work is to evaluate the effects of potassium (K) and calcium (Ca) on the early stages of combustion of single biomass particles. The biomass used was grape pomace, sieved in the size range of 200–250µm. With the pre-treatments of demineralization and impregnation, a total of 12 different samples were obtained: raw grape pomace, demineralized grape pomace and impregnated grape pomace with 0.1, 0.5, 0.82, 3 and 6wt% of K, and 0.1, 0.5, 1.08, 3 and 6wt% of Ca. The tests were performed in an optical flat flame McKenna burner able to produce a confined laminar flow of combustion products, in which single particles were air-injected upward through a central hole. The equivalence ratio and the thermal input of the burner were adjusted to yield two operating conditions in the working zone: condition T1 with a mean temperature of 1575K and a mean dry O2 concentration of 5.4vol%, and condition T2 with a mean temperature of 1775K and a mean dry O2 concentration of 5.2vol%. A CMOS high-speed camera was used to record the early stages of the combustion process, particularly the ignition and the volatiles combustion events. The collected images were processed to calculate the ignition delay time and the volatiles combustion time. The results obtained showed that the demineralization pre-treatment used increased both the ignition delay time and the volatiles combustion time of the single biomass particles. The K impregnation pre-treatment led to a decrease in the ignition delay time as the concentration of K increased, while the Ca impregnation pre-treatment did not have a significant impact on the ignition delay time. Both impregnation pre-treatments decreased the volatiles combustion time as the concentration of K or Ca increased. Finally, the impregnation with K and Ca had a more significant impact on the volatiles combustion time than in the ignition delay time.