| Author | Suvit Tia |
| Call Number | AIT Diss. no. ET-90-1 |
| Subject(s) | Fluidized-bed combustion
|
| Note | A dissertation submitted in partial fulfillment of the requirement
for the degree of Doctor of Engineering, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Abstract | A study of pyrolysis and combustion kinetics of Thai lignite was carried out using the
Thermogravimetric (TO) technique. The experimental results on pyrolysis showed good agreement
with the prediction obtained from multireaction models proposed by Anthony et al. (1975) and
Suuberg et al. ( 1978). The latter model which enables to estimate the composition of volatile product
was selected for further studies. The activation energy for lignite char combustion obtained from
TO data by assuming first order reaction had a value of about 84 kJ.gmo1e·1
•
The pyrolysis kinetic parameters obtained from TO experiment were used as input in the
devolatilization and volatile combustion model of a single large lignite particle in a hot air stream.
In this model, both heat transfer to and through particle as well as chemical kinetics were considered.
The ignition of coal volatiles was assumed to occur inside the boundary layer and took place when
the gas phase reaction flux reached a critical value, while the extinction of volatiles flame was
assumed to occur when the flame radius was equal to the particle radius. The prediction results are
in good agreement with experimental results especially for the small particle size.
Visual observations of lignite and pyrolyzed electrode carbon combustion in a laboratory scale
spouted bed (SB) and spout-fluid bed (SFB) were carried out for both single particles and small
batches. In case of lignite, the volatiles burned as a diffusion flame surrounding the particle when
it was in the spout and fountain region; and it then fell back to the bed surface in annulus region and
went downward with the inert sand, in which the volatile flame was expected to extinguish due to
the quenching effect of the surrounding inert sand. The particle was then entrained again into the
spout to complete the cyclical movement. The fragmentation of lignite particle during devolatilization and char combustion periods was also observed. For the case of pyrolyzed electrode carbon,
glowing combustion with cyclic movement pattern was observed without any volatile flame and
fragmentation until burnout. The results from the case of SFB were similar.
The residence time of fuel particle in each region of SB was estimated based on the experimental particle cycle time, which was found to be decrease as particle diameter increased, and the
available hydrodynamic correlations. An adjustment of parameter was still needed due to the
uncertainties in these correlations when applied to high temperature beds such as combustors. The
particle cycle times in SFB was also experimentally studied for comparison purpose.
The single particle model for coal devolatilization and volatiles combustion in the SB was
developed based on the results of the devolatilization and volatile combustion model of a single
coal particle in a hot air stream, bed hydrodynamics, and residence time of coal particle in each
region of the bed. Model prediction showed good agreement with the experimental results obtained
from a laboratory scale SB. However, parameter adjustments were still needed. The extinction time
of volatile flame was found to be insensitive to the operation mode (SB or SFB) and air flowrate,
but sensitive to the bed temperature and increase with the particle diameter.
Finally, experimental and theoretical studies of batch combustion of carbon in a SB was carried
out. Both isothermal and non-isothermal particles were considered in the model development, using
the concept of particle residence time in combination with bed hydrodynamics and combustion
theory. The assumption of surface combustion was found reasonable for the case of high bed
temperature, and the prediction results agreed well with the experimental results. The non-isothermal
particle model also predicted well the particle combustion during heat-up period. No attempt have
been made to model carbon combustion in SFB due to the lack of hydrodynamic data. |
| Year | 1990 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Other Field of Studies (No Department) |
| Academic Program/FoS | Energy Technology (ET) |
| Chairperson(s) | Bhattacharya, Sribas C. ; P. Wibulwas
|
| Examination Committee(s) | Mora, Jean-Claude ;Tabucanon, Mario T. ;Basu, P.
|
| Scholarship Donor(s) | The Government of Japan |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1990 |