Rheology of Coal Ash in Fluidised Bed Combustion of Low Rank Coal
N Tonmukayakul and Q Nguyen CRC for Clean Power from
Lignite University of Adelaide, Adelaide
Development of more effective technologies for utilizing low rank coals
for power generation has been driven by a demand for higher efficiency,
low capital costs and minimal environment impacts. Fluidised bed systems
are regarded as one of the most promising alternative technologies for
power generation to overcome the disadvantages of the existing pulverized
coal burning power generation plants for low rank coals. However, ash deposition
and bed agglomeration are potential problems in fluidised bed processing
of high alkali coals. Thus, in order to gain a better understanding the
mechanism of agglomeration in fluidised bed technologies, a good knowledge
of the rheological behavior of coal ash deposits at high temperatures and
under processing conditions is necessary. This paper presents a detailed
investigation of rheological characterisation of high alkali low rank coal
ashes under different operating temperatures. The rheological properties
of the coal ashes were measured using the unique high temperature ash rheometer
recently developed in our laboratory. The experimental results obtained
will be presented the effect of the alkali content on the rheological characteristics
discussed in terms of the implication of coal ash rheology on agglomeration.
Evaluation of the Slag Flow Characteristics of Australian Bituminous
Coals in Slagging Gasifiers
J H Patterson, H J Hurst, A Quintanar CSIRO Energy Technology, Brisbane;
R Boyd and H Tran Pacific Power International, Sydney
The slag viscosity and crystallisation characteristics of coal ash
slags have been further examined, confirming the suitability of Australian
bituminous coals for use in slagging gasifiers. Data has been obtained
for the reference coals of the CRC for Black Coal Utilisation. For a slag
tapping temperature of 1450oC, about half the coals examined are shown
to require a limestone flux addition of <3% CaCO3 by weight of coal.
However, high temperatures of critical viscosity proved more of a problem
than expected, particularly for Queensland coals, increasing flux requirements
at 1400oC. Limitations arising from slag crystallisation appear to be associated
with SiO2/Al2O3 ratios of < 2 and iron contents > 10% FeO. Empirical
viscosity models have been developed for Australian bituminous coals ash
slags in the range 10-15% FeO. Relative coal value modelling has been used
for economic optimisation of limestone flux addition and gasification temperature.
The efficacy of coal blending has been demonstrated for reducing flux requirements
and lowering the temperature of critical viscosity.
Fates and Roles of Alkali and Alkaline Earth Metal Species during
Gasification of Victorian Lignite
D Mody, H Wu and C Z Li CRC for Clean Power from Lignite,
Melbourne
The transformation of alkali and alkaline earth metal (AAEM) species
in a Victorian lignite during the pyrolysis and subsequent gasification
in CO2 was studied in a novel quartz fluidised-bed reactor. Lignite samples
prepared by physically adding NaCl and ion-exchanging Na+ and Ca++ into
the lignite were used to investigate the effects of chemical forms and
valency of the AAEM species in the substrate lignite on their transformation
during pyrolysis and gasification. Carboxyl-bound Na was found
to be less volatile than Na present as NaCl, but more volatile than carboxyl-bound
Ca, during pyrolysis at temperatures between 400°C and 900°C.
However, the carboxyl-bound Na was volatilised to a much greater extent
than the carboxyl-bound Ca in the same lignite during pyrolysis.
It was seen that the loading of NaCl into the lignite did not significantly
affect the char reactivity in the fluidised-bed reactor at 900°C.
Predicting Coal Ash Slag Flow Characteristics (Viscosity Model in
Al2O3-CaO “FeO”-SiO2 System)
A Kondratiev and E Jak University of Queensland, Brisbane
A model has been developed which enables the viscosities of coal ash
slags to be predicted as a function of composition and temperature under
reducing conditions. The model describes both completely liquid and heterogeneous,
partly crystallised slags in the Al2O3-CaO-“FeO”-SiO2 system in equilibrium
with metallic iron. The Urbain formalism has been modified to describe
the viscosities of the liquid slag phase over the complete range of compositions
and a wide range of temperatures. The computer package F*A*C*T was used
to predict the proportion of solids and the composition of the remaining
liquid phase. The Roscoe equation has been used to describe the effect
of presence of solid suspension (slurry effect) on the viscosity. The model
provides a good description of the experimental data in liquid phase, and
liquid + solid mixtures, over the complete range of compositions and a
wide range of temperatures. This model can now be used for viscosity predictions
in industrial slag systems. Examples of the application of the new model
to coal ash fluxing and blending are given in the paper.
Application of new FACT Database for the Prediction of melting Behavior
of Coal Mineral Matter
E Jak and P Hayes University of Queensland, Brisbane
A range of technological coal utilisation issues, including prediction
of the Ash Fusion Temperatures (AFT), fluxing in IGCC, coal blending, slagging
and fouling in PF combustion, are related to the melting behaviour of the
coal mineral matter. A new thermodynamic database for the system
Al-Ca-Fe-O-Si has recently been developed. This new database, in
conjunction with the F*A*C*T computer package, can now be used to calculate
the proportions and compositions of liquid and solid phases, liquidus temperatures
and other phase equilibrium information over a wide range of compositions,
temperatures and atmospheric conditions. These predictions, examples
of which are given in the paper, are important for the characterisation
of melting behaviour of the coal ash slags and are applied to assist in
solving a range of the related coal utilisation technological issues.
AFTs are widely used as a measure of coal ash fusibility and melting
behaviour, and hence is a widely used parameter in coal marketing and utilisation.
The research described in the present paper examines the relationship between
measured AFTs and F*A*C*T liquidus calculations for a selected set of coals.
The formation of the solid phases in these coal ashes is also examined
and compared with the F*A*C*T equilibrium predictions. A model for
the AFTs predictions based on the F*A*C*T calculations is developed and
applied to coal blending.
Slagging in a Pulverised-Coal-Fired Boiler
G Devir, J Pohl University of Queensland, Brisbane Australia; and
R Creelman RA Creelman and Associates, Sydney
This paper describes a technique to evaluate the severity of slagging
of a coal in a pulverised-coal-fired boiler. From an operator’s perspective,
the performance of a coal can be gauged by the time lapsed between sootblowing
cycles that allows control of furnace deposits. This translates to the
time taken for the superheater metal tube surface temperature to exceed
a nominal value. However, to ultimately describe the slagging performance
of pulverised coals in a boiler, several descriptors of deposit behaviour
need to be nominated and quantified. There is precious little data in the
literature on the nature of in-situ boiler slags, their rate of growth
and/or their strength properties relevant to sootblowing. The latter is
thought to be of more concern to boiler operators and gives rise to the
significance of selecting suitable strength tests. As well as standardised
methods for characterising pulverised coal performance in a boiler, several
novel and less popular techniques will be discussed in detail. A suite
of three sub-bituminous coals from the Callide Coalfields, Biloela (600
kilometres north of Brisbane), has been selected for slagging tests in
the 350 MWe units of Callide ‘B’ Power Station. Disposable air-cooled mild
steel slagging probes have been constructed to simulate the conditions
for deposit formation in the boiler region. To date, tests for one of these
coals has been completed and preliminary results are presented below. Once
testing for the remaining coals has been completed, it is anticipated that
the differences exhibited in deposit growth and strength may be correlated
with typical variations in physical and chemical properties of the pulverised
coal.
The Heterogeneous Nature of Mineral Matter, Fly-Ash and Deposits
R Creelman RA Creelman and Associates; J Pohl, G Devir and
S Shi University of Queensland, Brisbane
Deposits that cannot be easily removed cost the Utility Industry several
billion dollars per year in reduced capacity and maintenance. These deposits
are cause by complex mineral matter transformations to fly ash and the
interaction of fly ash during combustion and in the deposits. This paper
reports on a series of studies looking at the heterogeneous nature of mineral
matter, fly ash, and deposits and how this heterogeneity affects deposition.
The data comes from Low Temperature Ashing (LTA) of coal, fly ash from
boilers, and deposits from pilot scale furnaces and boilers. The paper
presents optical and Scanning Electron (SEM) micrographs, Electron Micro
Probe Analysis (EMPA) and Energy Dispersive X-Rays Analysis (EDXA) analysis
of mineral matter, individual fly ash particles, and isolated regions of
the deposits. During combustion, included mineral matter is transformed
into fly ash, melts and partially adheres to the char surface, and may
form agglomerated masses. Excluded mineral matter has little chance of
encountering another ash particle and agglomerating in the gas phase, but
can react with other particles in the wall deposits. Certain fly ash particles
adhere to the wall where they can combine with other fly ash particles.
Analyses of molten regions of deposits have shown, so far, three types
of minerals, made of mixed minerals, to be responsible for forming difficult
deposits with melting points below deposit temperatures which range from
1200-1350. These compounds are iron cordierite (2FeO-2Al2O3-5SiO2) with
melting points around 1100-1300 C, Juniper, et al (1993); albite (Na2O-Al2O3-6SiO2)
and its silica undersaturated equivalent nepheline (Na2O-Al2O3-2SiO2) with
melting points around 850 - 1100 C Su (1999), anorthite (CaO-Al2O3-2SiO2)
with melting points around 1300-1400 C Pollman (1967), and compounds with
ratios of CaO/P2O5 of 2.3-2.5 and melting points around 1300 C, Su (1999).
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