3462 r2010 American Chemical Society pubs.acs.org/EF Energy Fuels 2010, 24, 3462–3470 : DOI:10.1021/ef901583k Published on Web 05/20/2010 On the Burning of Sawdust in a MILD Combustion Furnace Bassam. B. Dally,* ,† Sung Hoon Shim, ‡ Richard. A. Craig, † Peter J. Ashman, § and George G. Szeg€ o † † School of Mechanical Engineering, The University of Adelaide, SA 5005, Australia, ‡ Eco Machinery Division, Korea Institute of Machinery & Materials, 171, Jang-dong, Yuseong-gu, Daejeon 305-343, Korea, and § School of Chemical Engineering, The University of Adelaide, SA 5005, Australia Received January 13, 2010. Revised Manuscript Received May 2, 2010 The purpose of this work is to extend the applicability of moderate or intense low oxygen dilution (MILD) combustion to solid biomass fuels. A laboratory-scale furnace fitted with a parallel jet burner was operated in conventional nonpremixed flame mode, and in MILD combustion mode, using either natural gas or pine sawdust particles. Sawdust with particle sizes in the range of 212-355 μm were injected into the furnace using either air, CO 2 , or N 2 as a carrier gas. Measurements of in-furnace wall temperatures and exhaust gas emissions of O 2 , CO, NO x , and ash are presented, together with visual observations at the burner exit region. It was found, through detailed comparisons, that MILD combustion was established without air preheat for both gaseous and solid fuels, suggesting that the parallel jet burner system is suitable for MILD combustion. A 3-fold reduction in NO x emissions and an increase in CO were recorded during the transition from conventional to MILD combustion using natural gas. The optimal equivalence ratio (φ) to reduce both CO and NO x emissions, when burning sawdust, was determined to be in the range of φ = 0.71-0.75, with CO 2 as the carrier gas, and at φ ≈ 0.75, with N 2 as the carrier gas. Ash content analysis showed that the extent of carbon burnout was low, which is thought to be due to the relatively short furnace residence times. Introduction Heat and exhaust gas recirculation is an innovative approach to create a distributed reaction zone, slow the chemical reaction, and increase the net radiant heat flux (and, thus, thermal efficiency). It is now well-established that a mixture of reactants diluted with combustion products, at a temperature above that of autoignition, can achieve the desired outcome of reduced pollutant emissions and enhanced thermal efficiency. The appli- cation of these principles to practical systems has taken different routes, and different names were used to describe the process. Some relied on a descriptive form of the resulting combustion process, i.e. flameless oxidation, whereas others described the features of the reactants streams, i.e., high temperature air combustion. The term used in this paper is moderate or intense low oxygen dilution (MILD) combustion. 1 A good volume of work has been published on MILD combustion, which was reviewed in a recent book. 2 Gaseous, liquid, and solid fuels were investigated and some commercial products are now available. 3,4 Further application of this combustion mode to waste-to-energy technologies is also being investigated. 5 Despite the above, this relatively new field has many unresolved issues that require further attention and considera- tion. Some of those issues relate to air preheating and its impact on the system thermal efficiency. Dally’s group has proven unequivocally through a series of papers, 6-14 as also reported below, that air preheat is not needed and strong internal recirculation provide a simpler and effective alter- native. Tied to this is the location and approach to injecting the reactants into the furnace to achieve MILD combustion. This issue is particularly crucial for solid fuels and, to the authors’ knowledge, has not been resolved thus far. What follows is a brief summary of the current knowledge of MILD combustion under gaseous conditions, as well as sample *Author to whom correspondence should be addressed. Tel.: þ61- 8-8303 5397. Fax: þ61-8-8303 4367. E-mail: bassam.dally@adelaide. edu.au. (1) Cavaliere, A.; de Joannon, M. MILD Combustion. Prog. Energy Combust. Sci. 2004, 30, 329–366. (2) Tsuji, H.; Gupta, A. K.; Haskgawa, T.; Katsuki, M.; Kishimoto, K.; Morita, M. High Temperature Air Combustion-From Energy Con- servation to Pollution Reduction; CRC Press: Boca Raton, FL, 2003. (3) Blasiak, W.; Yang, W. Volumetric Combustion of Coal and Biomass in Boilers. In Proceedings of the HITAC Conference, Phuket, Thailand, 2007. (4) Blarino, L., Fantuzzi, M., Malfa, E. Zanusso, U. Tenova Flex- ytech burners: Flamesless Combustion for very los NO x Reheating Furnaces. In Proceedings of the HITAC Conference, Phuket, Thailand, 2007. (5) Yoshikawa, Kunio R&D commercialization of Innovative Waste- to-Energy technologies. In Proceedings of the HITAC Conference, Phuket, Thailand, 2007. (6) Dally, B.; Karpetis, A.N.; Barlow, R. S. Proc. Combust. Inst. 2002, 29, 1147-1154. (7) Dally, B. B.; Karpetis, A. N.; Barlow, R. S. 2002 Australian Symposium on Combustion and The Seventh Australian Flame Days, January 2002, Adelaide, Australia (8) Christo, F. C.; Dally, B. B. Modelling Turbulent Reacting Jets Issuing into a Hot and Diluted Coflow. Combust. Flame 2005, 142, 117– 129. (9) Medwell, P. R.; Kalt, P. A. M.; Dally, B. B. Combust. Flame 2007, 148, 48–61. (10) Medwell, P. R.; Kalt, P. A. M.; Dally, B. B. Imaging of Diluted Turbulent Ethylene Flames Stabilised on a Jet in Hot Coflow (JHC) Burner.Combust. Flame 2008, 152 (1-2), 100-113 (11) Dally, B. B.; Riesmeier, E.; Peters, N. Effect of Fuel Mixture on MILD Combustion. Combust. Flame 2004, 137, 418–431. (12) Szeg€ o, G., Dally B. B., Nathan, G. J. Scaling of NO x Emissions from a Laboratory-scale MILD Combustion Furnace. Combust. Flame 2008, 154, (1-2), 281-295 (13) Szeg€ o, G., Dally, B., Nathan, G., Christo, F. The Sixth Asia-Pacific Conference on Combustion, Nagoya, Japan, May 20-23, 2007; pp 231-234. (14) Mi, J.; Li, P.; Dally, B. B.; Craig, R. A. Energy Fuels 2009, 23, 5349–5356.