Chemical Engineering and Processing 36 (1997) 423–432
A study on the importance of dependent radiative effects in
determining the spectral and total emittance of particulate ash
deposits in pulverised fuel fired furnaces
S.P. Bhattacharya
a,b,
*, T.F. Wall
b
, M. Arduini-Schuster
c
a
Cooperatie Research Centre for New Technologies for Power Generation from Low-rank Coal, 8 /677 S pringale Road, M ulgrae,
Victoria 3170, A ustralia
b
Department of Chemical Engineering, The Uniersity of Newcastle, Newcastle 2308, A ustralia
c
Baarian Center for Applied Energy Research, Am Hubland, D -97074 Wu ¨rzburg, Germany
Received 10 June 1996; received in revised form 5 May 1997
Abstract
This paper presents the results of an experimental and theoretical investigation on the importance of dependent effects in
determining the emittance of ash deposits. A model has been developed to predict the spectral emittance (directional, normal,
hemispherical) of semi-transparent and opaque particulate deposits without considering dependent effects. Predictions from this
model have been presented to illustrate the effects of particle size and composition. Hemispherical transmission and reflection
measurements have been performed on semi-transparent and opaque particular deposits over a wavelength range typical of pf
fired furnances and spectral emittance has been deduced therefrom. Well characterised slag particles, with composition and size
similar to ash deposits formed in pf fired furnances, have been used in the measurements. For the opaque deposit, difference
between the model predictions and measurements, in terms of emittance, have been found to be significant at wavelengths up to
7 m. This difference is believed to be largely due to dependent effects. Such measurements and comparisons appear to be the first
of its kind and more experiments are recommended to quantify the effects. © 1997 Elsevier Science S.A.
Keywords: Fuel fired furnaces; Particulate ash deposits; Radiative effects
1. Introduction
Knowledge of the spectral emittance of the particu-
late deposits is necessary for monitoring of temperature
and condition, and prediction of heat transfer through
ash deposits formed in a pulverised fuel (pf) fired
furnace [1]. As the need increases for higher efficiency
and better pollution control in pf fired furnances, there
is a need for increased understanding of the factors that
affect the emittance of deposits. Emittance ( ) of a
surface is commonly defined as the ratio of its emission
rate to that from a blackbody at the same temperature
( =E /E
B
). Emittance can be defined as hemispherical
(
h
) or normal (
n
) depending on the direction of mea-
surement and as spectral (
) or total (
t
) in which case
it is referred to part or all of the wavelength spectrum
with reference to the wavelength range and direction,
and can be calculated from spectral emittance [1]. It can
be defined as a combination of any two of the above.
Since all radiation incident on a surface must be either
reflected, absorbed or transmitted, one can write:
+ + =1 (1)
where , and are reflectance, absorbance and trans-
mittance, respectively, of the surface. On a spectral
basis, absorbance is equal to emittance so that one can
deduce emittance of a surface or deposit from the
reflection and transmission measurements from it as
follows:
=
=1 -
-
(2)
for a semi-transparent layer and
=
=1 -
(3)
for an opaque layer.
*Corresponding author. Tel.: +11 61 3 92390813; fax: +11 61 3
95610710.
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