Detailed measurements in a laboratory furnace with reburning C. Casaca, M. Costa ⇑ Mechanical Engineering Department, Instituto Superior Técnico, Technical University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal article info Article history: Received 22 February 2010 Received in revised form 3 December 2010 Accepted 19 December 2010 Available online 1 January 2011 Keywords: Reburning Detailed measurements NO x emissions Particle burnout abstract This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O 2 , CO, CO 2 , HC and NO x concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NO x reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NO x reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NO x emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, a number of investigations, stimulated by the need to control the emissions of nitrogen oxides (NO x ) from com- bustion systems, have been devoted to the study of the reburning [1]. Reburning is an in-furnace NO x control technology that involves the establishment of three distinct reaction zones along the height of a furnace: (1) a primary reaction zone where most of the fuel is burned under fuel-lean conditions; (2) a secondary reaction zone, the so-called reburn zone, where additional fuel (the reburn fuel) is added to create reducing conditions to convert part of the NO x formed in the primary zone to molecular nitrogen; and (3) a tertiary reaction zone, the so-called burnout zone, where overfire air (tertiary air) is added to complete the combustion. Many fuels have been investigated as reburning fuels, particu- larly gaseous fuels such as natural gas and other hydrocarbons [2–4], liquid fuels such as residual fuel oil [5] and solid fuels such as coal [6–13] and biomass [14–21]. These studies indicate that this technology is capable of reducing NO x emissions by more than 50%, regardless of the reburning fuel used, with gaseous fuels being identified as the most effective reburn fuels due to the null or minimal content of nitrogen in their composition and due to their ability to produce easily active hydrocarbon radicals which react with NO [2]. Recently, there has been an increasing interest in using biomass fuels in the reburning process, partly because of the reasonable cost level of biomass, in comparison to other renewable energy sources, and partly because of its advantage of reducing the carbon dioxide and sulfur dioxide emissions, com- pared to fossil fuels [16]. A number of studies have evaluated the potential of alternative fuels to be used as reburn fuels. Maly et al. [14] examined the reburning performance of a variety of fuels, including biomass and refuse derived fuel, in a pilot scale combustor, and found that their performance was similar to or slightly better than that of nat- ural gas reburning, with over 70% NO x reduction achievable at reburn heat inputs above 20%. Harding and Adams [15] evaluated the behavior of two different biomasses, a hardwood and a soft- wood, as reburning fuels in a laboratory combustor (U-furnace) and concluded that wood is as effective as natural gas or coal as a reburning fuel, with NO x reductions of as high as 70% attainable at reburn heat inputs between 10% and 15%. More recently, Casaca and Costa [17,19] studied the effectiveness of the reburning pro- cess using rice husk as reburn fuel in the present laboratory furnace and concluded that the reburning performance of the rice husk is comparable to that of the natural gas reburning, with al- most 60% NO x reduction achievable at reburn heat inputs of 25% and 30%. In a similar furnace, Ballester et al. [18] performed a de- tailed experimental characterization of biomass reburning and concluded that the use of oak sawdust as a reburn fuel originates 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.12.020 ⇑ Corresponding author. Tel.: +351 21 841 7186; fax: +351 21 847 5545. E-mail address: mcosta@ist.utl.pt (M. Costa). Fuel 90 (2011) 1090–1100 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel