Proceedings of the Australian Combustion Symposium December 2-4, 2009, The University of Queensland __________________________ * Corresponding author: Phone: (+61) 8 8303 3117 Email: woei.saw@adelaide.edu.au - 1 - Surface temperature measurement of a burning black liquor droplet using two-colour optical pyrometry W.L. Saw 1,* , G.J. Nathan 1 , P.J. Ashman 2 , Z.T. Alwahabi 2 , M. Hupa 3 , Centre of Energy Technology, Schools of 1 Mechanical and 2 Chemical Engineering The University of Adelaide, SA 5005, Australia 3 Process Chemistry Centre,Åbo Akademi Biskopsgatan 8, FI-20500 Åbo, Finland Abstract This paper reports on the implementation of two-colour optical pyrometry to measure the distribution of surface temperature of a burning black liquor droplet in a flat flame environment. The black liquor was burned in the flame provided by the flat flame burner at two flame conditions, fuel lean (Ø bg = 0.8), and fuel rich (Ø bg = 1.25). Two identical digital single- lens reflex (SLR) cameras were used to allow the simultaneous measurement of the surface temperature and particle size. This provides an opportunity for more reliable measurements of surface temperature for black liquor than has previously been available. Keywords: Surface temperature, Two-colour optical pyrometry, Black liquor. 1. Introduction Black liquor is a complex and important source of renewable fuel in the pulp and paper industry. It consists of both organic components of wood, mainly lignin, and inorganic components derived from sodium based pulping chemicals. The sodium can be bound to both organic and inorganic compounds [1]. During black liquor combustion the organically bound sodium, which is associated with the phenolic hydroxyl and carboxyl compounds, decomposes and converts to Na 2 CO 3 [1]. The purpose of burning black liquor in a kraft recovery boiler is to recover both the pulping chemicals and the heat generated from the black liquor combustion, which is converted to electricity to power the pulp mill through a steam turbine. The black liquor is sprayed into a recovery boiler, typically as droplets from 0.5–5 mm in diameter [2]. Combustion of a single droplet of black liquor combustion can be divided into the four stages of drying, devolatilisation, char combustion and smelt oxidation [3]. Black liquor swells significantly during the devolatilisation stage, by 10–60 times in volume [4], due to gas evolution within the highly viscous droplet [2]. The drying and devolatilisation stages of combustion may overlap due to large temperature gradient within the swollen droplet (Biot number > 1) [2, 5]. During the char combustion stage, char is oxidised by reaction with H 2 O, CO 2 and O 2 , and by the reduction of Na 2 CO 3 and Na 2 SO 4 [2]. A molten bead of smelt, consisting of inorganic components (mainly Na 2 CO 3 and Na 2 S), is formed at the end of the char combustion stage and the oxidation of Na 2 S can occur if O 2 is available [2]. Details of the black liquor combustion by measuring swelling and surface temperature simultaneously at each stage of the combustion are required to provide an insight to support the development of reliable models. Two-colour optical pyrometry has been well- established and widely used to measure the surface temperature at a single point on burning coal particles [6–10]. The technique was further improved by Stenberg et al. [11] to measure the surface temperature of a burning black liquor droplet in a hot, radiating environment. It was then adapted by Frederick et al. [12] to study the influence of oxygen on the surface temperatures of a burning black liquor particle, again at a single point. Increasing the oxygen concentration in a N 2 /O 2 gaseous environment increases the surface temperature of the droplet during the char combustion stage. At lower oxygen concentrations, the surface temperature of the droplet during the char combustion stage was found to be constant [12]. However, the average surface temperature was found to reach as high as 300ºC above the furnace temperature for droplets burned in air [12]. Later, the swelling, mass loss and the surface and internal temperatures of a burning black liquor droplet were measured simultaneously by Ip et al. [13]. A digital video camera fitted with two infra-red filters was used to measure the surface temperature by detecting the radiation emitted from the burning black liquor droplet at two wavelengths. This technique requires the grey emissivity (ε) of the droplet to be assumed. Nevertheless, it provided two-dimensional imaging of the surface temperature of a burning black liquor droplet. A non-uniform distribution of the surface temperature of the burning droplet was observed, along with a large temperature difference between the surface and internal temperatures throughout the combustion stages. However, no statistical