COOPERATIVE RESEARCH CENTRE FOR BLACK COAL ... - CCSD

COOPERATIVE RESEARCH CENTRE FOR BLACK COAL UTILISATION
Established and supported under the Australian Government’s Cooperative Research Centres Program
ASH FORMATION DURING PF COMBUSTION AND
GASIFICATION IN A REDUCING ATMOSPHERE
FINAL REPORT – PROJECT 3.2
By
Terry Wall
Department of Chemical Engineering
University of Newcastle
October 2001
Advanced Technology Centre, The University of Newcastle
University Drive, Callaghan NSW 2308 AUSTRALIA
Telephone (02) 4921 7314 Facsimile (02) 4921 7168
Email: black-coal@newcastle.edu.au

Ash formation during pf combustion and gasification in a reducing atmosphere
Executive Summary
The characteristics of the products of combustion and gasification is of fundamental
importance to coal utilisation technologies. It was known that some Australian coals have
mineral occurrences and distributions which differ from Northern Hemisphere coals so that
empirical models which are used to predict operational issues in pf furnaces, which have
been developed for Northern Hemisphere coals, do not necessarily apply to Australian
coals. Thr research was undertaken to address this issue and is applicable to pulverised
fuel combustion in low NOx burners which have a region with a reducing atmosphere, ash
formation at pressure and dissolution in slagging gasifiers, formation of fume in reducing
conditions and its subsequent removal from the process as well as ash disposal issues.
Three PhD studies were completed with specific aims (i) identification of the differences in
the mechanisms by which ash is formed by Australian coals in reducing conditions
compared to the mechanisms in oxidising conditions and with overseas coals (ii)
characterisation of the associations of siderite related to deposition of ash in pulverised fuel
fired plants, along with a comparison of the previously established behaviour of pyrite, and
(iii) identification of the differences in ash formed at the high temperature and pressure of
IGCC gasifiers.
The main findings of the project were that:
• Siderite containing Australian coals will be associated with less slagging than other
coals containing pyrite with the same iron content, especially in reducing
conditions.
• Siderite containing coals are expected to slag primarily due to glass formation from
included siderite minerals.
• Ash formed at high pressure is finer than that formed at atmospheric pressure due
to the formation of the porous Group 1 chars, as they have been named by the CRC,
at pressure.
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Ash formation during pf combustion and gasification in a reducing atmosphere
The main project deliverables are slagging factors accounting for reducing atmospheres
and iron as siderite in coal to quantify the first two findings above, and new knowledge on
the mechanisms of ash formation during combustion and gasification in reducing
atmospheres.
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Ash formation during pf combustion and gasification in a reducing atmosphere
1. Introduction
2. Industry basis
3. Overview of the report
4. Research outcome
CONTENTS
4.1 Ash formation in reducing conditions
4.2 Siderite reactions and impact on slagging
4.3 Ash formation at pressure
5. Listing of publications arising
Attachments
Published papers
Page No.
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Ash formation during pf combustion and gasification in a reducing atmosphere
1. INTRODUCTION
The objective of Project 3.2 was to establish a mechanistic understanding of ash formation
processes relevant to Australian coals used in pf-fired furnaces and IGCC gasifiers.
Three PhD studies were completed with specific aims (i) identification of the differences in
the mechanisms by which ash is formed by Australian coals in reducing conditions
compared to the mechanisms in oxidising conditions and with overseas coals (ii)
characterisation of the associations of siderite related to deposition of ash in pulverised fuel
fired plants, along with a comparison of the previously established behaviour of pyrite, and
(iii) identification of the differences in ash formed at the high temperature and pressure of
IGCC gasifiers.
2. INDUSTRY BASIS
The characteristics of the products of combustion and gasification is of fundamental
importance to coal utilisation technologies. Previous work has established that ash
characteristics depends on coal mineralogy, the mineral distribution in the coal and the
presence of inorganic species. Australian coals have a mineral occurrence and distribution
which differs from Northern Hemisphere coals so that empirical models which are used to
predict fouling and slagging in pf furnaces, which have been developed for Northern
Hemisphere coals, do not necessarily apply to Australian coals. The work presented is
applicable to pulverised fuel combustion in low NOx burners, ash dissolution in slagging
gasifiers, formation of fume in reducing conditions and its subsequent removal from the
process as well as ash disposal issues.
3. OVERVIEW OF REPORT
The research findings from Project 3.2 were published as papers in international journals
and in three PhD theses. The report presents the papers and provides a summary of the
findings.
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Ash formation during pf combustion and gasification in a reducing atmosphere
4. RESEARCH OUTCOMES
This project has involved detailed laboratory scale experiments on a range of Australian
and overseas coals. Conditions have varied from oxidising to reducing reaction
atmospheres, pressures between 1 and 15 bar and temperatures between 1100 o C and
1600 o C. Synthetic ash mixtures and density separated minerals from coal have also been
studied.
4.1 Ash formation in reducing conditions
Experiments have shown that reducing combustion environments have had been found to
reduce the rate of oxidation of excluded pyrite and therefore prolong the existence of a
sticky FeO-FeS phase. This results in glassy ash particles particles that are sticky at
combustion temperatures.
Index established for Iron-Based Slagging for Pulverised coal Firing in Oxidising and
Reducing Conditions. A model for the prediction of iron based slagging precursors from
the combustion of iron containing coals is detailed. The model accounts for the form of
iron (pyrite or siderite), the distribution of iron within the pulverised coal, temperature and
oxidising or reducing conditions. The input required for the model is a CCSEM analysis of
the pulverised coal. For oxidising conditions, the index predicts similar behaviour for
pyrite and siderite-containing coals, with iron alumino-silicate ash particles becoming
sticky at temperatures greater than 1400°C. This suggests that for oxidising conditions, the
extent of included iron minerals is the most important factor. For reducing conditions, the
index predicts sticky ash particles are formed at lower temperatures, as low as 1000°C for
pyrite-containing coals as a result of the decomposition and partial oxidation of pyrite
forming sticky particles, and 1100°C for siderite-containing coals. For reducing
conditions, the level of excluded pyrite mineral for pyrite-containing coals and the level of
included iron containing minerals associated with clays for siderite and pyrite containing
coals are the most important factors determining slagging. An example of the model
predictions is presented in Figure 1 for CRC coal 299.
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Ash formation during pf combustion and gasification in a reducing atmosphere
kg sticky ash / tonne coal
70
60
50
40
30
20
10
reducing
0
1000 1100 1200 1300 1400 1500 1600 1700 1800
Temperature ( o C)
oxidising
Figure 1 – Model predictions for CRC coal 299.
4.2 Siderite reactions and impact on slagging
The high temperature behaviour of a range of siderite samples derived from mineral
samples and density separated coal samples, has been investigated at temperatures of
1100 o C and 1600 o C. Analysis of quenched combustion residues collected from the
oxidising combustion gases was conducted using scanning electron microscopy (SEM)
with energy dispersive spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF).
The FeOn-MgO-CaO ternary phase diagram was used to predict the behaviour of excluded
siderite grains under combustion conditions and indicated that siderite is likely to form
sticky or completely molten particles at 1600 o C when MgO < 5 wt.%, CaO < 37.5 wt.%
and (MnO + FeOn) > 60wt.%. The extent of melting observed for particles collected from
experiments agreed with these compositions. By comparison with published work for
excluded pyrite particles, residues from excluded siderite particles are found to be sticky at
higher temperatures than pyrite (1380 o C for siderite compared to ~ 910 - 1080 o C for
pyrite), and are expected to generate less fine ash via fume or fragmentation. Excluded
pyrite exists as a sticky FeO-FeS melt phase for a large portion of its decomposition time
whereas excluded siderite may only be molten at higher temperatures (>1369 o C). When
combustion and oxidation of the iron-containing minerals is complete, excluded siderite
residues would behave similarly to excluded pyrite residues. Slagging indices based on
iron in ash for coals containing pyrite will therefore not correctly predict the deposition
behaviour of coals containing a high proportion of excluded siderite.
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Ash formation during pf combustion and gasification in a reducing atmosphere
There have found to be similarities between the behaviour of included pyrite and siderite in
terms of formation of melt phases at lower temperatures when compared to their respective
excluded minerals. Included siderite and pyrite which react with included clay particles
form a glass with iron in the ferrous form which can be sticky in combustion chambers
based on the iron levels in the glass. Included siderite and pyrite grains, which do not
contact clays and are lost from the combusting char particle behave similarly to their
respective excluded mineral forms with oxidation being delayed due to char combustion.
CRC Case Study: Ash Deposits in a Power Station Related to Experimentally Measured
Ash Character. Boiler deposits collected from an Australian power station were
characterised and the observed deposit properties correlated with ash formation
mechanisms established from drop tube furnace experiments using two of the power
station coals noted for deposition problems. Mechanisms for the production of sticky ash
particles, based on the minerals present in the feed coals as determined by the CCSEM
technique developed in Project 1.1, were thereby related to the measured deposit
chemistries. At the high gas temperatures near the burners most ash particles are sticky so
that the chemistries of the total ash and deposits are similar. For the lower temperature
regions away from the burners only ash particles of high iron levels are sticky, resulting in
deposits being progressively higher in iron as temperatures reduce through the furnace as
illustrated in Figure 2. In Figure 2 the following areas are highlighted Point 1: Burner
deposits (fused) Gas Temperature ~ 1600°C; Point 2: line of constant SiO2/Al2O3 ratio;
Point 3: Bulk coal ash compositional range for the coalfields; Point 4: Wall deposits
(fused/semi-fused) Gas temperature ~1400°C; Point 5: Screen wall deposits (semifused/sintered)
Gas temperature

Ash formation during pf combustion and gasification in a reducing atmosphere
Figure 2 - Normalised XRF data for deposit samples presented on the FeOn-SiO2-Al2O3
ternary phase diagram with liquidus isotherms for the system in contact with metallic iron.
4.3 Ash formation at pressure
Char and ash samples generated in pilot scale reactors located in Victoria, Australia and
Stuttgart, Germany, have indicated that some differences in char and ash morphology can
be expected as a result of these intense conditions. It has been shown that the char
morphology (which is a result of the devolatilisation process and the subsequent char
combustion) has a dominant influence on the final ash particle size and composition
distribution. Detailed investigations into ash composition distributions and particle size
distributions using computer controlled scanning electron microscopy, scanning electron
microscopy, x-ray diffraction, Mossbauer specroscopy, x-ray fluorescence spectroscopy,
porosimetry and surface area measurements as well as physical characterisation of the char
and ash products has commenced.
A Field Emission Scanning Electron Microscope (FESEM) was used to investigate the
char samples collected after rapid heating devolatilisation of a coal sample at a pressure of
15 atm and a temperature of 1300°C. The images acquired at different accelerating
voltages show a significant difference in the exposed char structure and morphology as
shown in Figure 3. At low accelerating voltages, the SEM images tend to represent the
surface structure and morphology while images obtained at high accelerating voltages
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Ash formation during pf combustion and gasification in a reducing atmosphere
typically reflect the internal cellular structure. The images result in significant difference
in the "surface" pore profiles. The different structures observed for the same char particle
sample at various operating accelerating voltages are explained in the terms of the
mechanism for SEM image acquisition. Observations indicate the majority of the char
particles have a foam structure, which consists of a thin surface layer covering internal
pores. The thin layer is extremely non-uniform and the thickness is estimated to range
from several nanometers to several micrometers. This type of structure is believed to be
due to bubble creation and evolution as a result of volatile release in the highly fluid coal
particles during devolatilisation. Ash generated at high pressure was found to be much
finer than ash generated at low pressure due to the observed differences in char structure.
Char samples produced at high pressure have higher proportions of porous char particles
which fragment to form finer ash particles.
1 keV
5 keV
2 keV
10 keV
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Ash formation during pf combustion and gasification in a reducing atmosphere
15 keV 20 keV
Figure 3 – FEM images of the same particle at various accelerating voltages ranging from
1 to 20 keV
There were some unexpected results in this project, particularly the new finding that ash
formed at high pressure is different (ie finer) than that at atmospheric pressure and, through
recent collaboration with our Japanese colleagues, that this does have an impact on IGCC
gasifier performance. This is due to the formation of the porous Group 1 chars, as they
were then named by the CRC, at pressure. Current research is providing understanding of
the reasons why this char Group forms.
Fig 4 - Cartoon prepared by PhD student Hongwei Wu showing the association of the
CRC named char types with the size of ash formed, together with the differences in ash
size generated with burn-off, as observed from experiments. At high pressure Wu found
that coals form a higher proportion of group 1 chars.
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Ash formation during pf combustion and gasification in a reducing atmosphere
5. LISTING OF PUBLICATIONS ARISING
Publications arising from Project 3.2, which are attached to this report
Ash formation in reducing conditions
McLennan, A.R., Bryant, G.W., Stanmore, B.R. and Wall, T.F., Ash Formation
Mechanisms during pf Combustion in Reducing Conditions, Energy and Fuels, 14, 150-
159, 2000.
McLennan, A. R., Bryant, G. W., Stanmore, B. R. and Wall, T. F., An experimental
comparison of the ash formed from coals containing Pyrite and Siderite in oxidising and
reducing conditions, Energy and Fuels, 14, 308-315, 2000
McLennan, A. R., Bryant, G. W., Bailey, C.W, Stanmore, B. R. and Wall, T. F, Index for
iron based slagging for pulverised coal firing in oxidising and reducing conditions, Energy
and Fuels, 14, 349-354, 2000
Siderite reactions and impact on slagging
Bailey C.W., Bryant G.W., Matthews E. M., Wall T.F., Investigation of the High
Temperature Behaviour of Excluded Siderite Grains During Pulverised Fuel Combustion,
Energy and Fuels, 12 (3), 464-469, 1998.
Bailey C.W., Bryant G.W., Wall T.F., Ash Deposits in a Coal-Fired Power Station Related
to Experimentally Measured Ash Character, IFRF Combustion Journal,
http://www.journal.ifrf.net/library/october1999/199903bailey.pdf, 19pp, 1999
Ash formation at pressure
Wu H, Wall T, Liu G, Bryant G, Ash Liberation from Included minerals during
combustion of Pulverised Coal: The relationship with coal structure and burnout, Energy
and Fuels, 13, 1197-1202, 1999.
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Ash formation during pf combustion and gasification in a reducing atmosphere
Wu, H., Bryant, G. and Wall, T., The effect of pressure on ash formation during pulverised
coal combustion, Energy and Fuels, 14 (4), 745-750, 2000
Wall TF, Liu G, Wu H, Benfell K, The effect of pressure on char characteristics, burnout
and ash formation in entrained flow gasifiers, IFRF Combustion Journal,
http://www.journal.ifrf.net/library/may2001/20015guisu.pdf, 24pp, 2001
Project 3.2 theses, which can be obtained from the CRC or from the library of the
University of Newcastle
Angus McLennan, Ash formation in reducing conditions, PhD Thesis, Department of
Chemical Engineering, University of Newcastle, 1988
Chris Bailey, High temperature transformations of siderite and the performance of pf fired
plant, Department of Chemical Engineering, University of Newcastle, 1989
Hongwei Wu, Ash formation during pulverised coal combustion and gasification at
pressure, PhD thesis, Department of Chemical Engineering, University of Newcastle, 2000
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