Combustion of single biomass particles in air and in oxy-fuel conditions

Abstract

The combustion behaviors of four different pulverized biomasses were evaluated in the laboratory. Single particles of sugar cane bagasse, pine sawdust, torrefied pine sawdust and olive residue were burned in a drop-tube furnace, set at 1400 K, in both air and O 2 /CO 2 atmospheres containing 21, 30, 35, and 50% oxygen mole fractions. High-speed and high-resolution images of single particles were recorded cinematographically and temperature-time histories were obtained pyrometrically. Combustion of these particles took place in two phases. Initially, volatiles evolved and burned in spherical envelope flames of low luminosity; then, upon extinction of these flames, char residues ignited and burned in brief periods of time. This behavior was shared by all four biomasses of this study, and only small differences among them were evident based on their origin, type and pre-treatment. Volatile flames of biomass particles were much less sooty than those of previously burned coal particles of analogous size and char combustion durations were briefer. Replacing the background N 2 gas with CO 2 , i.e., changing from air to an oxy-fuel atmosphere, at 21% O 2 impaired the intensity of combustion; reduced the combustion temperatures and lengthened the burnout times of the biomass particles. Increasing the oxygen mole fraction in CO 2 to 28-35% restored the combustion intensity of the single biomass particles to that in air.

Single particle flame-combustion studies on solid biomass fuels

“…The experiment examined ignition, devolatilisation and char burn-out for cylindrical particles with similar mass (12.5mg) with aspect ratios ranging from 1 to 6 and for gas temperatures ranging from 1200¡C to 1600¡C (with oxygen concentrations ranging from 5 to 20% for the burn-out tests). Similar experiments performed by Riaza et al [17] using a drop tube furnace and high speed camera on moving particles milled to <150µm, report burnout times for various biomass and oxygen concentrations.…”

Biomass fuel flexibility in future conventional power generation

“…One method for examining this variability is to combust single particles of fuel in a controlled environment and measure the burn-out time. This has been the basis of experimental investigations undertaken by Riaza et al [11] and Mason et al[12]. In the latter, an empirical relationship between particle size and the duration of devolatilisation and char-burn out was obtained from single particles burned in a methane flame.…”

Observations on the combustion of torrefied biomass

“…Renewable biomass has advantages of near-zero net CO2 emission, as it absorbs carbon from atmospheric carbon dioxide while it grows and then it returns carbon dioxide to the atmosphere when it is burned; and this creates a closed-loop carbon cycle. However, biomass has some disadvantages as fuel (low calorific value, high moisture content, low grindability, high biodegradability, high emissions of acid gases, and smoking during combustion) [11][12][13].…”

“…Torrefied biomass is more stable, has higher energy density, lower atomic O/C and H/C ratios and moisture content, higher friability and grindability, less biodegradability, less smoking during combustion, and higher hydrophobicity [11,12,[18][19][20][21]. Figure 1 shows changes in biomass properties before and after torrefaction, taken from Chen at al.…”

“…Also, due to the fibrous nature of raw biomass particles cannot not be pulverized to the same size small particle sizes as coal [77,[92][93][94][95][96][97]. On the other hand, the larger particle sizes of biomass can induce lengthier ignition delays and can generate larger quantities of unburned residues than coal [12,77,[98][99][100][101]. Hence finding the right biomass particle size to co-fire with coal is important to achieve good combustion efficiency.…”