Oxy-fuel combustion characteristics of pulverized-coal in a drop tube furnace Gongliang Wang, René Zander, Mário Costa ⇑ Mechanical Engineering Department, Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal highlights  Oxy-coal combustion lowers the conversion of fuel-N to NO compared to air combustion.  The near burner region burnout levels increase as the O 2 concentration in the oxidizer increases.  C and N are released at about the same rate as total coal mass release, while H is released more rapidly.  The final char structure becomes more porous as the O 2 concentration in the oxidizer increases.  The final char elemental composition is essentially independent of the oxidizer composition. article info Article history: Received 19 April 2013 Received in revised form 17 July 2013 Accepted 18 July 2013 Available online 31 July 2013 Keywords: Drop tube furnace Oxy-fuel combustion Oxidizer composition Particle size abstract Experiments were conducted in a drop tube furnace (DTF) for a bituminous coal, two coal particle size distributions (unsieved coal with a mass mean diameter of 76 lm and sieved coal with a mass mean diameter of 29 lm) and three oxy-fuel atmospheres (21% O 2 /79% CO 2 , 26% O 2 /74% CO 2 and 31% O 2 / 69% CO 2 ) at a furnace temperature of 1100 °C. For comparison purposes, tests were also performed under air firing conditions. The data reported includes gas temperatures, major gas species concentration and particle burnout measured along the DTF. In addition, a number of selected final char samples were also morphologically and chemically characterized. Under oxy-fuel conditions: (i) the near burner region (NBR) temperature increases and the HC and CO concentrations decrease as the O 2 concentration in the oxidizer increases, regardless of the coal particle size; (ii) the NO x concentration levels along the DTF increase as the O 2 concentration in the oxidizer increases, regardless of the coal particle size, but, compared with air combustion, the results show that oxy-coal combustion lowers the conversion of fuel-N to NO; and (iii) the NBR burnout levels increase as the O 2 concentration in the oxidizer increases, regardless of the coal particle size. The combustion of the sieved coal yields higher NBR temperatures, lower NO x concentration and higher burnout levels along the DTF than the unsieved coal, regardless of the oxidizer composition. Carbon and nitrogen are released at about the same rate as total coal mass release, while hydrogen is released more rapidly regardless of the oxidizer composition and coal particle size. In the combustion of both unsieved and sieved coals, under oxy-fuel conditions, the final char struc- ture becomes more porous as the O 2 concentration in the oxidizer increases. The final char elemental composition is essentially independent of the oxidizer composition, but the combustion of the unsieved coal leads to chars with higher percentages of S and lower percentages of Cl than those from the sieved coal combustion. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Despite all efforts to do otherwise, coal combustion will con- tinue to account for the largest share of CO 2 emissions through the next decades. Under these circumstances, the oxy-fuel com- bustion process is very attractive for reducing emissions of CO 2 if it is sequestered (carbon capture and storage). Recent reviews on oxy-fuel combustion include those of Buhre et al. [1], Normann et al. [2], Toftegaard et al. [3] and Chen et al. [4], among others, and relevant previous related studies include those conducted in drop tube furnaces (DTF) (e.g., [5–9]), and in entrained flow reac- tors (EFR) (e.g., [10–15]). Bejarano and Levendis [5] investigated the combustion of single coal particles in O 2 /N 2 and O 2 /CO 2 environments in a DTF at 1127 °C and 1327 °C and observed that coal particles burned at 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.07.063 ⇑ Corresponding author. Address: Mechanical Engineering Department, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. Tel.: +351 218417186. E-mail address: mcosta@ist.utl.pt (M. Costa). Fuel 115 (2014) 452–460 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel