Experimental Investigation into Combustion Characteristics of Two Sub-bituminous Coals in O₂/N₂and O₂/CO₂Environments

Abstract

There are still arguments on the coal combustion reactivity under conventional and oxy-coal combustion atmospheres. The selection of the experimental device in such a study is crucial. A laminar entrained flow reactor system that is capable of producing a similar combustion environment of a utility furnace in terms of the high heating rate and temperature was adopted in this study. This study investigated the characteristics of sub-bituminous coal combustion in its processes of devolatilization and char oxidation by examining the structure and length of the flame. The effects of the particle size and oxygen molar fraction in N 2 and CO 2 diluent gases were studied. The O 2 mole fraction was varied from 0 to 50%. The flame became shorter as the O 2 concentration increased. It means that reactivity in both the devolatilization and char oxidation processes increased in a higher O 2 concentration environment. It was barely seen that the volatile burning realm shrunk, O 2 diffusion accelerated, and luminosity concentrated near the burning particle as the O 2 concentration increased. The larger size coals showed thicker and longer flames, and there is a small difference in the burning time between two different sub-bituminous coals. When the O 2 /CO 2 and O 2 /N 2 environment effects are compared, the flame was shorter in the oxy-fuel condition than in the O 2 /N 2 condition.

Volatile Release and Ignition Behaviors of Single Coal Particles at Different Oxygen Concentrations Under Microgravity

“…The results confirmed the previous findings obtained by various kinds of experimental methods (Chen et al 1994;Ponzio et al 2008;Shaddix and Molina 2009;Kim et al 2010;Qiao et al 2010). With a higher O 2 concentration, the mass diffusion of oxygen is stronger and less volatile matter concentration is needed for a mixture to be the flammable.…”

An overview of coal rank influence on ignition and combustion phenomena at the particle level

“…Lau and Niksa [65,66] for a 100 µm bituminous coal particle (they predicted a flame duration of 6.5 ms at 21% O 2 , T furnace =1250 K), again upon extrapolation to the higher temperature conditions that were used at the authors' laboratory. In simulated oxy-fuel combustion conditions (O 2 /CO 2 mixtures), the volatile flame durations are drastically shortened as oxygen mole fraction increases beyond the air level (21%) to 100% [15], which is also true in O 2 /N 2 mixtures (see also Murphy and Shaddix [40]; Molina and Shaddix [41], Kim et al [45], Khatami et al [16]). The effect of the background gas on the volatile flame duration was not as significant and was not always discernible [67]; lengthier flame durations were observed in the case of bituminous coals burning in CO 2 than in N 2 [14,67], but this was not observed in the case of sub-bituminous coals burning in CO 2 .…”

“…In parallel investigations, Shaddix and co-workers used the Sandia laminar entrained flow reactor to conduct comprehensive studies on the combustion of coals and chars of various ranks at oxygen partial pressures spanning 12-36%, in nitrogen and carbon dioxide [40][41][42]. Concurrently, Zhang et al [43,44] studied the combustion behavior of streams of a Victorian brown coal in a DTF, whereas Kim and coworkers [45] examined the combustion characteristics of sub-bituminous coals. Recently, Lee and Choi [46] recorded the volatile phase combustion characteristics of bituminous particles as a function of their size.…”

CFD modelling of oxy-coal combustion in an entrained flow reactor

“…Varhegyi et al [43] found that the reaction rate of char-CO 2 is slower than char-O 2 at temperatures up to 1223 K. Other researchers [37,44] have found that the reactivity of char in O 2 /CO 2 conditions may be increased due to the high CO 2 concentrations at high temperatures (up to 1600 K).…”