Comparison of biomass and coal char reactivities Page: 3 of 6
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expect both fuels to exhibit comparable reactivities and to produce similarly low levels of unburned
carbon. Past investigations yielded preliminary evidence to support this hypothesis [Wornat, 19931, but
the work is incomplete. This paper presents an investigation of char reactivity of biomass chars and
compares them to our observations for coal chars of various ranks.
We believe that a relationship exists between a char's reactivity and its temperature-time profile [Hurt,
1993]. Normalized temperature-time profiles of chars are best suited to illustrate this relationship and
to compare the reactivities of different chars. Because absolute peak temperature and exact extinction
time are not necessary in determining a char's reactivity, these two parameters are normalized to allow
for a comparison of reactivities for various types of chars. In fact, peak temperatures and extinction
times vary greatly for different types of chars.
The high-temperature reactor in Sandia's Coal Combustion Laboratory (CCL) was used to measure the
temperature-time profiles during combustion of two high-rank coal chars (Illinois #6 and Pocahontas
#3), two low-rank coal chars (Beulah and Dietz), and two biomass chars (pine and switchgrass). Data
were obtained with the Captive Particle Imaging (CPI) diagnostic system that has been discussed in
previous reports [Hurt and Davis, 1994]. Figure 1 presents a schematic of the CPI. The system
provides optical access for high-resolution video imaging of complete particle combustion for
individual 100 m to 200 gm char particles in a-well-controlled combustion environment. Particles are
placed on an alumina fiber bed supported by fine platinum wires. The particles are oxidized in a
laminar flow of vitiated air with 6 mole-% excess oxygen at a temperature of approximately 1600 K.
The particle is positioned along the reactor centerline at the focal point of a long-focal-length
microscope. During the positioning process a water-cooled coil surrounding the particle holder
provides local cooling and therefore prevents premature reaction of the particle. This microscope is
connected to a video system capable of simultaneously imaging both reflected light and near infrared
(IR) emission from the reacting particles. The IR images can be used for determination of radiance
temperatures. Radiance temperature is defined as the temperature of a hypothetical black body emitting
the same radiative power as the real object (particle) in the wavelength range of interest (here 700 to
1000 nm). For particles that have undergone low to intermediate extents of burnout, emissivities are
approximately 0.8 [Baxter et al., 1988] and true particle temperatures will be approximately 20 K
greater than the reported radiance temperatures.
Long Focal Length
-- CCL Flow Reactor
Figure 1. Schematic of the Captive Particle Imaging System
For the temperature-time profiles, the radiance temperatures are normalized to their corresponding peak
temperatures (i.e., normalized temperature equals the actual temperature divided by the maximum
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Huey, S.P.; Davis, K.A. & Hurt, R.H. Comparison of biomass and coal char reactivities, article, August 1, 1995; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc627637/m1/3/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.