International Journal of Greenhouse Gas Control 5S (2011) S36–S46 Contents lists available at ScienceDirect International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc Pulverized coal stream ignition delay under conventional and oxy-fuel combustion conditions Yinhe Liu 1 , Manfred Geier, Alejandro Molina 2 , Christopher R. Shaddix ∗ Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA article info Article history: Received 1 February 2011 Received in revised form 13 May 2011 Accepted 14 May 2011 Available online 12 June 2011 Keywords: Ignition Pulverized coal Coal combustion Group combustion Oxy-fuel combustion abstract The coal stream ignition process is critical to the performance of modern pulverized coal burners, par- ticularly when operating under novel conditions such as experienced in oxy-fuel combustion. However, experimental studies of coal stream ignition are lacking, and recent modeling efforts have had to rely on comparisons with a single set of experiments in vitiated air. To begin to address this shortfall, we have conducted experiments on the ignition properties of two U.S. and two Chinese coals in a laminar entrained flow reactor. Most of the measurements focused on varying the coal feed rate for furnace tem- peratures of 1230–1320 K and for 12–20 vol.% O 2 in nitrogen. The influence of coal feed rate on ignition with a carbon dioxide diluent was also measured for 20 vol.% O 2 at 1280 K. A second set of measurements was performed for ignition of a fixed coal feed rate in N 2 and CO 2 environments at identical furnace temperatures of 1200 K, 1340 K, and 1670 K. A scientific CCD camera equipped with a 431 nm imaging filter was used to interrogate the ignition process. Under most conditions, the ignition delay decreased with increasing coal feed rate until a minimum was reached at a feed rate corresponding to a particle number density of approximately 4 × 10 9 m -3 in the coal feed pipe. This ignition minimum corresponds to a cold flow group number, G, of ∼0.3. At higher coal feed rates the ignition delay increased. The igni- tion delay time was shown to be very sensitive to (a) the temperature of the hot coflow into which the coal stream is introduced, and (b) the coal particle size. The three high volatile bituminous coals showed nearly identical ignition delay as a function of coal feed rate, whereas the subbituminous coal showed slightly greater apparent ignition delay. Bath gas CO 2 content was found to have a minor impact on ignition delay. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction The topic of coal particle ignition, including considerations of homogeneous (gas-phase, volatile) versus heterogeneous (particle- surface) ignition, has been reviewed by Essenhigh et al. (1989), Wall et al. (1991), and Annamalai et al. (1994). These reviews show that most studies of coal particle ignition have focused on the minimum temperature for ignition of a cloud of particles, irrespective of res- idence time. These studies have generated relatively low ignition temperatures (associated with a long thermal ‘soak’ time) that are directly relevant to evaluations of fire safety and coal mine explo- ∗ Corresponding author at: Sandia National Labs, MS 9052, 7011 East Avenue, Livermore, CA 94550, USA. Fax: +1 925 294 2276. E-mail address: crshadd@sandia.gov (C.R. Shaddix). 1 Permanent Address: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, China. 2 Permanent Address: Bioprocesses and Reactive Flows, Faculty of Mines, National University of Colombia, Medellín, Car. 80 No. 65-223, Medellín, Colombia. sions, but have little relevance to the problem of flame holding in pulverized coal (pc) burners, wherein high velocity streams of cool coal particles turbulently mix with hot flame products and must ignite in tens of milliseconds. For application to pc burners then, the key issue is the characteristic ignition delay of coal particles introduced into hot surroundings. To-date, most of the information on coal ignition delay has been gleaned from experiments and modeling of individual reacting coal particles, in most cases using particles substantially larger than those characteristic of pulverized coal. Although single-particle studies are undoubtedly useful for the study of ignition and combustion of dilute particle streams, particle group effects are likely important to the flame-holding process for practical burn- ers. Therefore, an understanding of the ignition characteristics of a continuous flow of pulverized coal particles at different parti- cle mass loadings is needed to address actual industrial practice. An improved understanding of particle stream ignition is impor- tant because it influences many aspects of pc burner performance, including NOx production, char burnout, flame stability, flame shape, and flame length. 1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2011.05.028