Energy & Fuels 2009, 23, 1067–1075
1067
Numerical Modeling of a Biomass Pellet Domestic Boiler Jacobo Porteiro,* Joaquin Collazo, David Patin˜o, Enrique Granada, Jorge Carlos Moran Gonzalez, and Jose´ Luı´s Mı´guez UniVersity of Vigo, Thermal Engines, E.T.S.I.I.-Lagoar Marcosende s/n, Vigo, Spain 36200 ReceiVed October 6, 2008. ReVised Manuscript ReceiVed December 18, 2008
This paper presents a computational fluid dynamic analysis of a small-scale commercial biomass pellet boiler. Combustion of the particles in the bed plays a key role in the analysis of this kind of systems, and a stand-alone code was used in this work to provide the inlet conditions for the CFD analysis. Model predictions were compared with the experimental gas temperature and species concentration measurements, which were in good agreement. On the basis of the CFD analysis it can be concluded that the interaction of the particles in the bed and the poor mixing of the gases in the furnace are the key factors leading to the high emission levels that are typical of small-scale systems.
1. Introduction The increasing consumption of fossil fuels in recent decades has led to a greater interest in improving energy availability and reducing pollutant emissions. Biomass has long served as the primary energy form used for everyday activities such as heating and cooking. Before industrialization, the world’s biomass was the main energy source, but the use of fossil fuels has now replaced biomass in the developed world. However, growing demand and the well-known problems caused by the use of conventional fuels1 have revived interest in biomass. Within this framework, the biomass boiler industry has recently expanded. However, the combustion behavior of biomass in these systems has lately received increasing attention, although it still requires more thorough research into the characteristic processes involved. Computer simulations of these combustion systems can give better insights into the flow and combustion phenomena occurring inside each individual particle of biomass2-5 and within the entire boiler.6-8 These simulations may also be used as a design and analysis tool to increase efficiency, reduce pollutant emissions, and improve the system’s overall performance.9 However, available computer models still need to be previously validated by experimental data. Most of the codes available serve as a framework where more specific models, specially developed for each application, can be employed. Therefore, any numerical simulation of a biomass boiler necessarily includes several * To whom correspondence should be addressed. Telephone: +34 986812604. E-mail:
[email protected]. (1) Mı´guez Tabare´s, J. L.; Granada, E.; Moran, J.; Porteiro, J.; Murillo, S.; Lo´pez Gonza´lez, L. M. Energy Source, Part A 2006, 28, 501–515. (2) Saastamoinen, J. ; Richard, J. Drying, Pyrolysis and Combustion of Biomass Particles Research. In Thermochemical Biomass ConVersion; Bridgwater, A. V., Kluester, J. L., Eds.; Elsevier Applied Science: London, 1988; Vol. 1, pp 221-225. (3) Peters, B.; Bruch, C. Chemosphere 2001, 42, 481–490. (4) Galgano, A.; Di Blasi, C.; Horvat, A.; Sinai, Y. Energy Fuels 2006, 20, 2223–2232. (5) Ouedraogo, A.; Mulligan, J. C.; Cleland, J. G. Combust. Flame 1998, 114, 1–12. (6) Griselin, N.; Bai, X. S. IFRF-Combust. J. 2000, Article no. 200009. (7) Klasen, T.; Goerner, K. IFRF-Combust. J. 2002, Article no. 200202. (8) Cooper, J.; Hallet, W. L. H. Chem. Eng. Sci. 2000, 55, 4451–4460. (9) Saastamoinen, J. Modelling of Wood Combustion in Small StoVes; Nordic Workshop on Combustion of Biomass: Norway, 1991.
submodels to account for the different processes taking place, including fluid dynamics, homogeneous chemical reactions, radiative heat transfer, particle transport by the gases, and biomass drying, devolatilization, and heterogeneous combustion. Each submodel may have been validated individually or in conjunction with one or more of the others.10,11 Some three-dimensional simulations of large combustion systems have already been presented and validated by other authors.12 Nevertheless, most of these studies have undergone difficulties in obtaining precise data on a combustion system whose size and power do not allow for modification of several of the most important operating conditions. This impediment has, to some extent, limited the scope of the conclusions derived from this research. Furthermore, large-scale combustion systems also complicate the process of experimentation and limit the amount of data available for validation of the simulation. On the other hand, test plants are easier to modify and measure, although they do have certain drawbacks. The main aspects that are known to be problematic in small-scale systems are the relatively large size of the fuel particles (especially problematic when dealing with pellets), the lower system inertia,13 and the intrinsic influence of the feeding dynamics on the system’s behavior.14 Simulation of practical combustion systems usually requires large computational efforts involving high mesh densities, especially in the regions where higher gradients are expected, and a high number of equations regarding combustion chemistry have to be solved in order to capture the combustion and heat transfer details taking place inside the system. At present, there are still only a limited number of numerical simulations of biomass fixed-bed combustion systems employing detailed combustion models for both the bed and the gas phases.15-17 (10) Grønli, M.; Melaaen, M. Energy Fuels 2000, 14, 791–800. (11) Yang, Y. B.; Sharifi, V. N.; Swithenbank, J.; Ma, L.; Darvell, L. I.; Jones, J. M.; Pourkashanian, M.; Williams, A. Energy Fuels 2008, 22, 306– 316. (12) Zarnescu, V.; Pisupati, S. V. Energy Fuels 2002, 16, 622–633. (13) Moran, J. C.; Granada, E.; Porteiro, J.; Mı´guez, J. L. Biomass Bioenergy 2004, 27, 577–583. (14) Nevalainen, H.; Jegoroff, M.; Saastamoinen, J.; Tourunen, A.; Ja¨ntti, T.; Kettunen, A.; Johnsson, F.; Niklasson, F. Fuel 2007, 86, 2043–2051. (15) Shin, D.; Choi, S. Combust. Flame 2000, 121, 167–180.
10.1021/ef8008458 CCC: $40.75 2009 American Chemical Society Published on Web 01/27/2009
1068 Energy & Fuels, Vol. 23, 2009 Table 1. Ultimate and Proximate Analyses of the Fuel Used in This Work (wt % as received) ultimate analysis
wt %
proximate analysis
wt %
carbon hydrogen oxygen nitrogen sulfur
47.0 5.03 37.9 0.117
