Stereoscopic Camera-Based Investigation of Pulverized Solid Fuel Combustion - particle temperature, shape and burning rates

A newly developed stereoscopic camera system has been applied to measure the temperature, size and shape of burning coal and biomass particles in a lab-scale laminar flow reactor. The stereoscopic imaging method enables to measure the particle contour which can be used to determine the shape of the particle. As the camera system is constructed for two-color pyrometry, the simultaneous detection of the particle temperature is possible and provides data which can be used to calculate char burning rate parameters. Initial results are shown for biomass combustion, with measurements carried out in a O2/N2 atmosphere. Introduction The determination of char burning rate parameters is an important issue when the combustion properties of pulverized fuels (pf, dp~ 100 μm) are investigated. As char combustion is slow compared to the combustion of volatiles released during the pyrolysis phase, its profound knowledge is important to predict fuel conversion and thus heat release precisely when designing pf-boilers. Ratio pyrometry is a common technique for the investigation of char combustion. It is contactless and due to the high sensitivity of modern detectors, allows to measure particle temperature and size of pf-particles suspended in gas flows under realistic boundary conditions. It has to be distinguished between nonimaging or point detectors and imaging devices. Point detectors (e.g. PMTs) have been used in different setups. The determination of burning rate parameters is usually carried out in laminar drop tube (DTR) or entrained flow reactors (EFR). Typical experiments of this type have been conducted at Sandia National Laboratories 1,2 , by Levendis et al. 3,4 or Joutsenoja et al. 5 . Imaging two-color pyrometry has been introduced in the late 1990s 6 , when intensified (I)CCD-cameras with matching properties became available, which provide sufficient sensitivity for the detection of radiation on pfparticles in-flight, meaning short exposure times and thus little intensity. Both detector types have been used to investigate different phenomena, e.g. the influence of the fuel rank 2,7 , influence of enriched CO2 concentrations 2,8,9 (oxy-fuel combustion) and different questions on reaction models including effects like gasification reactions 10,11 . The performance of both detector types was compared in 12 . The results indicate that both approaches are on a comparable level and can be used for burning rate determination of pulverized coal. The spatial resolution of ICCD cameras was used to combine the measurement of particle temperature and particle shape of burning biomass particles in 13 , showing that the burning rate analysis of biomass particles can gain significant advantages when the shape of burning particles is considered in the calculations, although this work only used a camera system with one viewing direction, measuring only the projection of the particle shape in the focal plane. In order to measure the particle shape of burning biomass particles, a stereoscopic camera system has been developed, which is an upgrade of the system described in 8,12,13 . Its advantages are higher sensitivity, larger field of view, higher imaging frequency and, which is the major point, two viewing directions onto burning particles. The latter point not only creates the potential to detect the particle shape in terms of the particles outline, but also enables to measure the spatial position of the particle, which is necessary to prove the coincidence of particle position and focal plane. The pyrometer was tested with one coal and one biomass sample, both burned in an O2/N2 dominated atmosphere. The measurements illustrate the systems capabilities and its working principle, highlighting future possibilities and the necessity of this analysis method. Experimental Setup The experiments were carried out in a flat flame burner (FFB), which is a laminar reactor providing typical conditions for pf-combustion. The burner is realized by a ceramics honeycomb (guiding the oxidizer) with approx. 200 hypodermic tubes (guiding CH4 as fuel) equally distributed over the burner surface. Mass flow controllers are used to adjust the mixture of CH4/N2/O2/CO2. For the present experiments, an atmosphere containing 20% O2, 11.5% CO2 and 23% H2O (volumetric, N2 as balance, Tg= 1560 K) was chosen to avoid transport limitations of oxygen. The burner tube consists of quartz glass to provide optical access (inner diameter~ 65 mm). Particles are injected at the reactor center line with a small carrier gas flow (N2) at low particle feed rates to avoid particle-to-particle interactions. *Corresponding author: [email protected] Proceedings of the European Combustion Symposium 2015