Insight into soot formed in coal combustion flame: Evolution of physiochemical structure, oxidation reactivity

Probing into Volatile Combustion Flame and Particulate Formation Behavior During the Coal and Ammonia Co-firing Process

“…Coal combustion experiments were carried out on a flat-flame combustion system, in which coal samples were heated and ignited by the premixed combustion of designed gaseous fuels (i.e., CO, CH 4 , O 2 , and N 2 ). 27 As sketched in Figure 1, the flat-flame combustion system was made up of a McKenna type flat-flame burner (burner outlet diameter: 60 mm, Holthuis & Associates Co.) with a special central fuel tube (diameter: 4 mm), a quartz tube reactor with a vertical slit (width: 10 mm) shielding the burner outlet, two fine coal supporting rods, a gas supply module, and a water-cooling module. During the experiments, premixed CO, CH 4 , O 2 , and N 2 were supplied into the burner inner chamber through the premixed fuel tube, mixed in the chamber, flew out of the burner top through the sintered porous plate, and finally ignited.…”

“…The selected pulverized coal (200 ± 1 mg) was compressed into a pellet (diameter: 8 mm and height: 3 mm) with identical pressure (2 MPa). 27 A single coal sample was settled above the burner at the height above burner (HAB) = 5 mm, supported by two parallel fine refractory ceramic rods (diameter: 0.4 mm). The placement height of the coal sample was specially selected to facilitate investigating the thermal and chemical effects of ammonia co-firing, according to the gas temperature profile and gas distribution profile of the hot flue gas generated by premixed fuel combustion without a coal pellet (see Supporting Information).…”

“…A thermophoresis sampling system was used in soot sampling, which consisted of a self-closing tweezer, a high-speed cylinder, an electromagnetic valve with controller, and a compressed air supply module. 27 During the thermophoresis sampling, a transmission electron microscope grid (diameter: 3 mm) held on the tip of a tweezer was rapidly inserted into flame and stayed at the sampling site for a total dwell time of 70 ms. Uniformly, the samplings were performed at 35 s after igniting the premixed fuel stream in all experimental conditions; yet, the sampling sites were set at the flame centerline at HAB = 14, 17, 20, 24, 28, and 32 mm in fuel-lean conditions and at HAB = 20, 24, 28, 32, 36, and 40 mm in fuel-rich conditions according to the flame morphology.…”

“…As shown in Figure 4, samples collected at the bottom of flame (HAB = 14 mm) were mainly liquid droplets, which were formed by the condensation of heavy volatile matter (i.e., tar) released from coal devolatilization, consistent with the observations in our previous studies. 27 Briefly, large molecule species such as polycyclic aromatic hydrocarbons (PAHs) were intensively released from the large organic components in coal via pyrolysis. These large molecule species usually had low saturated vapor pressures and therefore condensed into liquid tar in the relative low temperature region surrounding the coal due to the endothermic pyrolysis.…”

“…Knowledge on this aspect is urgently required as it is essential not only to control pollutant emission but also to achieve stable flame during ammonia–coal co-firing. Moreover, detailed information on soot properties such as particle loadings, size, and fractal structure is essential for combustion design because of its special role in radiation heat transfer in furnaces. − …”