COMBUSTION OF SOLID FUELS: PF,FBC AND WASTE
POSSIBLE CAUSES OF THE VARIABILITY OF FBC ASH BEHAVIOUR
E. M. Bulewicz, K. Dudek, D. Gora and J. Taraszka
Institute of Inorganic Chemistry and Technology Cracow University of Technology,
Three lots of ash from a small bubbling fluidized bed combustor, each made up of
three fractions and two lots of ash from a large industrial scale circula6ng
fluidized bed installation were examined. Limestone sorbents were used in both
cases. Ash samples were subjected to phase analysis "as received" and after
prehydration at ambient conditions and under pressure. Determinations of the
proportion of free CaO were also made for selected samples, including some to
which a controlled amount of CaO or silica had been added before hydration. When
possible, the thermal effect associated with mixing the samples with an excess
of water was measured. It was concluded that during the prehydration process CaO
can be liberated from Ca-containing coal ash components but it can also be
consumed in reactions with silica or silicates. These processes can affect the
ASSESSMENT OF CATALYST DEACTIVATION MODEL FOR SULFUR RETENTION IN FLUIDIZED BED
Engin Degirmenci, Yusuf Gögebakan and Nevin Selçuk
Department of Chemical Engineering, Middle East Technical University,
06531 Ankara, TURKEY
Tel:+90 (312) 210 2603, Fax:+90 (312) 2I0 1264, E-mail:email@example.com
An improved version of a combustion-desulfurization model for fluidized bed
combustors burning high ash and sulfur content low quality lignite in its own
ash with the addition of limestone is presented. The desulfurization model
considers sulfur retention by both limestone and ash and is based on first order
catalyst deactivation where the kinetics of desulfurization is described by two
rate constants; one for the initial surface reaction, the other for the rate of
pore plugging or deactivation. The accuracy of the complete model with
experimentally determined deactivation rate constants was tested by applying it
to a 0.3 MWt AFBC test rig and comparing its predictions with measurements.
Comparisons show that the model produces reasonably accurate predictions of
sulfur retention provided that the rate constants are determined experimentally
under the fluidized bed combustor operating conditions.
Department of Mechanical and Mechatronic Engineering, University of Sydney, NSW,
Sugar Research Institute, 239 - 255 Nebo Rd Mackay, Australia 4740
Bagasse fired furnaces have long been used in the sugar industry but suffer from
stability problems due to the high moisture content of the fuel (45-50% as
fired). For a wide range of furnace conditions a cyclic pattern of material
accumulating, drying and then burning can be observed at various places on the
grate. For some furnace conditions the pattern occurs simultaneously over the
entire grate leading to large fluctuations in furnace pressure, reductions in
steam output and generally unstable furnace behaviour. In extreme cases
explosions have occurred, leading to severe damage to the furnace itself. A
time-dependant computational fluid dynamics model of the combustion processes is
developed. In particular, the accumulation and burnout of material deposited on
the grate has an important influence on the stability of the furnace. The model
has an ignition criterion and together with the drying and burnout rates this
controls the frequency of the combustion fluctuations. FURNACE predictions are
compared with gas temperature measurements at three separate points 0.72 meters
above the grate. The qualitative behaviour of the gas temperature is predicted
well at all three positions.
THERMAL ANNEALING OF CHARS FROM DIVERSE ORGANIC PRECURSORS UNDER COMBUSTION-LIKE
Hong-Shig Shim, James Andriotis, Robert H. Hurt
Brown University, Providence, RI 02912
Extensive studies have documented the strong dependence of char reactivity on
heat treatment history, but there have been relatively few studies focusing on
the short time scales relevant to combustion processes. This paper presents
measurements of reactivity loss in coal and alternate fuel chars as a function
of heat treatment under combustion-like conditions. An all-graphite transient
heat treatment device has been developed capable of temperatures from 700 - 3000
K, maximum heating rates of 2*103 K/s and hold times at peak temperature from
2 - 120 seconds. The experiments were augmented by TGA analysis of very young
chars generated by dilute-phase oxidative pyrolysis in a flame-supported
entrained flow reactor to explore the role of annealing at very short residence
times (< 50 msec).
The low temperature (500 - 800°C) oxidation reactivity of chars was observed to
vary by almost six orders of magnitude as a function of organic precursor and
heat treatment conditions. Rapid heat treatment (~2 seconds) between 700°C and
2400°C has a very large effect on reactivity, causing a variation in reactivity
by a factor that ranges from 30-30,000 depending on precursor. The propensity
to deactivate shows a strong rank dependence (the low-rank, reactive chars
deactivate more), but there are significant fuel-to-fuel differences in addition
to the overall rank trend. An example is the high annealing propensity of the
anthracite (despite its low initial reactivity) which can be rationalized by
structural changes observed by high resolution TEM fringe imaging.
UTILIZATION OF DIFFERENT BIOMASS TYPES THROUGH CIRCULATING FLUIDIZED BED
Liviu Dragos - S.C. ICPET Cercetare S.A.
Mihai Macavescu - S.C. ICPET Cercetare S.A.
Alexandru Savu - S.C. ICPET Cercetare S.A.
Mihu Gîrjoabâ - S.C. ICPET Cercetare S.A.
Ioana Ionel - "Politehnica" University of Timisoara
The background and current state of biomass resources and utilization in Romania
is presented. Important quantities result annually especially from agriculture
and wood processing factories, but most of the biomass is not used at all or is
burned inefficiently in old boilers.
Researches begun at ICPET Cercetare to develop a more efficient and environment
friendly technology for biomass combustion, suitable for local conditions. The
experiments were performed on two laboratory-scale facilities - one fluidized
bed and one circulating fluidized bed (CFB) combustor - and on a 1 MWt CFB pilot
plant. All three facilities are described, with a special attention conferred to
the fuel feeding systems.
Experimental results are discussed for several biomass types tested - wood
chips, sawdust and sorghum waste. Figures, tables and diagrams are included.
EVOLUTION OF REACTIVITY OF HIGHLY POROUS CHARS
FROM RAMAN MICROPROBE SPECTROMETRY
Tsachi Livneh2, Ezra Bar-Ziv1,
Osvalda Senneca3 and Piero Salatino3
1Department of Mechanical Engineering, Ben-Gurion University of the Negev,
P.O.Box 653, Beer-Sheva, ISRAEL
2Nuclear Research Center - Negev, P.O.BoX 9001 Beer-Sheva, ISRAEL
3Dipartimento di Ingegneria Chimica Universita' degli Studi di Napoli Federico
II, P.le Tecchio, 80125 Napoli, ITALY
The objective of this study is to investigate the effect of heat treatment on
the evolution of reactivity of highly porous chars, without the presence of
mineral matter, from Raman scattering measurements. Pure synthetic char
particles (Spherocarb) were used as a model material for a class of highly
porous coal-chars. Two sets of experiments were carried out: A) Samples were
oxidized in air at 600°C without heat treatment and Raman spectra were measured
as a function of conversion. B) Samples were heat-treated in the range 900-1400°C, in nitrogen, then characterized by micro-Raman spectroscopy, in the range
800-1800 cm-1, and then oxidized by air at 500 °C and by CO2 at 900 °C in a
thermogravimetric balance. In all Raman scattering measurements, three-band
spectra were obtained. The G and D bands were dominant and a weak "1180 cm-1"
band was apparent. The spectra were fitted to a 3-Lorentzian function from which
width, peak position and intensity of each band were determined and used for
analysis of the evolution of reactivity: The following conclusions can be drawn
from results of experiments A and B respectively:
A) Throughout reaction with oxygen the most significant change in system characteristics (reactivity, Raman
features) occurs in the conversion range 0-30%. This is consistent with
previous measurements of thermal resistivity and intrinsic reaction rate.
B) Heat treatment up to 900 °C at time duration up to 300 minutes did not show any
effect on reactivity. Decrease in reactivity was apparent at 1200 °C for 2-min
treatment, above which the effect was more pronounced.
The characteristics (width, peak location, and intensity) of each of the three
Raman bands showed clear correlation with reactivity changes both in air and
CO2. These were attributed to morphological (dimensions and ordering) changes in
the rearrangement of the carbon network structure, as previously suggested. We
propose in the present study an interpretation of these changes and their effect
Key Words: Char, Reactivity, Porous Structure, Oxidation, Annealing, Raman
D. Boavida, P. Abelha*, I. Gulyurtlu, and I. Cabrita
Departamento de Tecnologias de Combustâo, INETI Azinhaga dos Lameiros, 1699
Lisboa Codex, Portugal
*Coaltec e Ambiente, Estrada do Paço do Lumiar, 1699 Lisboa Codex, Portugal
This paper discusses the results obtained from an experimental combustion work
undertaken to investigate the behaviour of multicomponent briquettes, prepared
by mixing two different particle sizes of coal and two different types of binder
Single briquettes were burned over a wide range of temperatures in a laboratory
scale fluidised bed combustor facility.
Nitrogen (NOx and N20) and Sulphur (SO2) oxides emissions resulting from the
combustion of these briquettes were constantly monitored during the time of
burning. The levels of O2, CO2 and CO were also recorded during the same period.
Experimental results showed that coal particle size influenced burn-out times
and emission levels of some of gaseous species. The binder type was also found
to have a major influence on the emissions of different pollutants.
The temperature was observed to significantly influence the extent of the
effects of the other operating parameters studied.
A. Bouhafid*, J.P Vantelon**
*Laboratoire de chimie physique, Faculte des Sciences Semlalia, BP 515,
**Laboratoire de Combustion et Détonique, UPR 9028 CNRS, ENSMA, Universite de
Poitiers, Site futuroscope, Futuroscope Cedex, France
Surface area of Tarfaya shale particles was measured for large and small
particles formed by attrition and fragmentation during combustion in fluidized
reactor. Bed temperature was held constant between 350°C and 750°C. During
fluidization, each particle size changes continuously and results in a particle
size distribution. Parent particle fragments to complete destruction after a
period of 4 hours for bed temperature equal to 350°C. This time was reduced to
few second (20 - 40 s) if bed temperature exceeded 750°C. For each experiment,
particles were collected out of the reactor, screened and weighed at different
times of combustion. The specific area of each class of particle was determined
by nitrogen adsorption desorption. Results obtained show that surface area of
bed material depends on particles size distribution which range from the parent
to the fine particles. Bed temperature and initial size particle act together to
change the shale porosity at any stage of the combustion process. Particles
processed at 400°C present a surface area 1.5 order of magnitude greater than
the original ones. It was found that fine particles, generally formed by
abrasion, present a higher surface area for bed temperature greater than 450°C.
This is due to the fact that oxidation occurs mainly on external part of the
particle and they are free of organic matter once detached from the parent
particle. Surface area of parent, particle, not fragmented during fluidization
varies slightly from surface area of the original particle. For bed temperature
exceeding 600°C, all the class of particles present in the bed have surface area
greater than 4 m2/g and tend to rapidly devolatilise and change their surface
area to reach 7 m2/g.
SOUND-ASSISTED FLUIDIZED BED COMBUSTION OF FINE PARTICLES
Riccardo Chirone, Sabato Russo and Marco Serpi
Istituto di Ricerche sulla Combustione - CNR, P.le V. Tecchio, 80 - 80125 Napoli, Italy, e-mail:firstname.lastname@example.org
Piero Salatino and Fabrizio Scala
Dipartimento di Ingegneria Chimica - Universitâ Federico II di Napoli P.le V., Tecchio, 80 - 80125 Napoli, Italy
The sound-assisted fluidized bed combustion of powders made of tyre derived fuel
(TDF) and of a biomass (Robinia Pseudoacacia) has been studied to assess the
benefits deriving from the application of acoustic fields of different frequency
on the performance of the combustor. The parameters monitored were: the carbon
elutriation rate, the carbon loading in the bed and the degree of fixed carbon
conversion. Both fuels are characterized by strong propensity to generate fines
during pyrolysis and/or combustion. Elutriable particles are mainly formed
during devolatilization for TDF, during char burn-off for Robinia. Experiments
consisted of steady combustion, either ordinary or sound-assisted, of TDF and
Robinia fine particles. Acoustic fields of 150dB and frequency of 80, 120 and
240Hz were used in sound assisted experiments. High intensity acoustic fields of
120Hz reduce carbon elutriation rates by a factor of about 1.5, regardless of
the fuel used. Correspondingly, fixed carbon conversion efficiency increases by
5-8% and 2-3% for TDF and Robinia respectively. Sound frequencies either lower
or higher than 120Hz reduce the effectiveness of sound, approaching the
performances of ordinary fluidized bed combustors.
Results have been analyzed considering that fine particles are present in the
bed either as fines freely moving in the interstices of the bed (free fines) or
as fines attached onto coarse inert bed particles (attached fines). Accordingly
the inventory of carbon present under steady state conditions in the bed depends
on the competition between the following processes: elutriation of free fines,
adhesion and/or cohesion of elutriable fines onto bed material, generation of
elutriable fines by attrition of fine-coarse aggregates and combustion of fine
particles. In this framework, acoustic fields have the property of enhancing the
formation of fine-coarse aggregates.
ON THE APPLICATION OF DIFFERENT TURBULENCE MODELS FOR THE COMPUTATION OF FLUID
FLOW AND COMBUSTION PROCESSES IN SMALL SCALE WOOD HEATERS
H. Knaus, S. Richter, S. Unterberger, U. Schnell, H. Maier, K.R.G. Hein
Institute for Process Engineering and Power Plant Technology, University of
Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germany,
Phone:+49 - 711 - 6853487, Fax:+49 - 711 - 6853491, e-mail:email@example.com
In order to optimize the combustion process in small-scale wood heaters with a
thermal capacity of up to 15 kW using natural wood logs an important task is to
investigate the mixing and combustion processes within the reaction zones of
such firing systems. For a basic understanding of the formation and
decomposition of different gas components and the dependency on mixing intensity
and therefore on turbulent behaviour of the reacting flow numerical studies were
carried out using the simulation program AIOLOS. A comparison of results
obtained during the numerical modeling studies applying the k,e -, low Reynolds-
number k,e - and the Reynolds-stress turbulence model on different grids with
detailed experimental data shows a good correspondence both for isothermal
conditions as well as during combustion. The main aim is to validate the
numerical model enabling its use as an engineering tool to investigate the
influence of parameters such as mixing conditions, combustion air distribution
and furnace geometry on the combustion process within the wood heater on a
complete combustion for reduced emission of unburned components, such as CO,
hydrocarbons and soot.
D. Boavida, P. Abelha*, I. Gulyurtlu, and I. Cabrita
Departamento de Tecnologias de Combustâo, INETI
Azinhaga dos Lameiros, 1699 Lisboa Codex, Portugal
*Coaltec e Ambiente, Estrada do Paço do Lumiar, 1699 Lisboa Codex, Portugal
Combustion studies of five coals of different origin were carried out in a
laboratory scale fluidized bed combustor. Five blends prepared by mixing two
coals based on their petrological characterisation, in varying amounts, were
selected to study the possibility of reduction NOx, N2O and SO2 emissions.
The results showed that some blends had the opposite behaviour concerning the
release of NOx and SO2 in relation to parent coals, and the emissions were
higher than expected. The N2O amounts observed were, however, in almost all
blends tested, lower than predicted values.
With some blends, the mixing levels intended to reduce SO2 were not always found
to correspond to those for simultaneous decrease of NOx.
Most of the blends studied showed some evidence of interaction between them.
Varying the proportion of the blend components was observed to alter the
temperatures at which interactions were stronger.