IMPACT OF COAL QUALITIES WHEN BURNING LOW GRADE … · 2015-02-01 · Rosemary Falcon 2012 IMPACT...
Transcript of IMPACT OF COAL QUALITIES WHEN BURNING LOW GRADE … · 2015-02-01 · Rosemary Falcon 2012 IMPACT...
Rosemary Falcon 2012
IMPACT OF COAL QUALITIES WHEN BURNING LOW GRADE COALS IN BUBBLING FLUIDISED BED
COMBUSTION BOILERS IN SOUTH AFRICA
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R M S Falcon, R Taole, D Brook and S Bada
November 2014IFSA 2014
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CONTENTS
1. Relevance of Coal in South Africa2. Qualities of coal in South Africa3. Low Carbon Economy planned for South Africa4. Experiences in BFBC in South Africa and Botswana5. CFBC test work conducted on South African coals in Finland6. Conclusions for the future
RELEVANCE OF COAL IN SOUTH AFRICA
South Africa is the ó 7th largest producer of coal in the worldó 6th largest exporter of coaló Largest supplier to the European Unionó Possesses 3,5% of world coal reserves (IEA)
Coal in SA accounts for ó Highest foreign exchange earnings in the country (2011, 2012, 2013)ó Highest mining income earning commodity, beating gold and platinumó 95% of SA energy production, 81% of the regions’ energyó >90% of carbon reductants in the metallurgical industry ó >40% of petrol, diesel and other liquid fuel requirementsó >200 major chemicals for 1000s of carbon-based products (Sasol)
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Countries heavily dependent on coal for electricity
4Source: XM Prevost 2013
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CONTENTS
1. Relevance of Coal in South Africa2. Qualities of coal in South Africa3. Low Carbon Economy planned for South Africa4. Experiences in BFBC in South Africa and Botswana5. CFBC test work conducted on South African coals in Finland6. Conclusions for the future
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% Proportion
0-10 10 to 15 15-20 20-25 25-30 30-35%% Ash in in-situ coal
Proportion of SA CoalReserves in ash ranges (%)ex Petrick Horsfall
PROPORTION OF COALS IN THE HIGH RANGES OF ASH CONTENT?(after Petrick ; Horsfall, 1977)
CAN SOUTH AFRICA USE THE REMAINING COAL RESERVES, GIVEN THE
Very little left
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LOW ASH COAL –HIGH VOLATILES - all volatiles combustible
HIGH ASH COAL (CLAYS) –HIGH VOLATILES - most incombustible
Clays
CLEAN COAL Vitrinite - no visible
minerals
DIRTY COAL Vitrinite + abundant layered clays (black)
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EFFECT OF HIGH ASH COALS: LIMITED LIBERATION OF MINERALS DUE TO NATURE OF THEIR DISTRIBUTION IN COAL
Scale: 200 microns from side to side
In each photograph
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QUALITY OF COAL Should definition change for future high ash reserves?
Boundaries based upon the International Classification of Seam Coal : UN-ECE 1993
COAL:Definition to increase
to 65 or 70% ash?
ROCK?
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CLASSIFICATION AND CATEGORISATION OF COALS
USING PETROGRAPHIC PARAMETERS
DEGREES OF INCREASING DIFFICULTY IN IGNITION AND COMBUSTION
AND HIGHER COMBUSTION TEMPERATURES
USA and European Carboniferous coals
Witbank-Highveld coals
KWZ Natal coals
Reactive –Vitrinite)
Inert (Inertinite)
Variations in Organic Composition of
Coals from different Regions x x
xx x
x
x xxx x
x
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Local Coal Products: Quality Issues
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CONTENTS
1. Relevance of Coal in South Africa2. Qualities of coal in South Africa3. Low Carbon Economy planned for South Africa4. Experiences in BFBC in South Africa and Botswana5. CFBC test work conducted on South African coals in Finland6. Conclusions for the future
0 1 000 2 000 3 000 4 000 5 000 6 000 7 000
ChinaUSA
RussiaIndia
JapanGermany
CanadaUK
IranKorea
ItalyMexico
AustraliaSaudi Arabia
IndonesiaFrance
BrazilSouth Africa
SpainUkraine
Million tons CO2
BUT THE RELATIVE COST OF ABATEMENT
IS VERY VERY HIGH
SA is responsible for 1.1% of total global CO2 emissions, but South Africa is committed to reducing its emissions by
35 and 45 % within the next 15 years.
CO2 EMISSIONS FROM SOUTH AFRICA IN ABSOLUTE TERMS IS VERY VERY SMALL
Source IEA - Top 20 CO2 Country emitters, 2008FFF 2014 11
EFFECT OF USING INCREASING PROPORTIONS OF POORER GRADES OF COAL (HIGH ASH AND INERT ORGANIC MATTER)
ON CO2 EMISSIONS
Rise in CO2 in Eskom over 10 years
Eskom 2012
Eskom grid emission factor
Discards51.8 Mt
Electricity120.8 Mt
Exports64.0 Mt
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LOW CARBON ECONOMY IN SA - 1IRP2010 - SA’S Integrated Resource Plan 2010
The Role of Coal going forward....
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LOW CARBON ECONOMY IN SA - 2IRP2010 - SA’S Integrated Resource Plan 2010
The Role of Coal going forward....
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CONTENTS
1. Relevance of Coal in South Africa2. Qualities of coal in South Africa3. Low Carbon Economy planned for South Africa4. Experiences in BFBC in South Africa and Botswana5. CFBC test work conducted on South African coals in Finland6. Conclusions for the future
water
steam
coal
dust collectors
stack
FLUIDISEDBED COMBUSTION
FLUIDISEDBED
COMBUSTION
combustion air
limestone
8500C
EFFECT OF INERT ORGANIC MATTER IN BUBBLING FLUIDISED BED BOILERS (BFBC)
• Difficult temperature control• Agglomeration of particles within the moving bed • Ash deposits dropping down to the base • Difficulties in coarse ash removal• Corrosion of the sparge pipes below the rims
16- POOR EFFICIENCY -
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The Mondi Multi-Fuel Boiler: Fuels
• Limestone (Typical)– CaCO3 94%– MgCO3 2,2%– Size < 200µm– Injection Overbed
• Operational Conditions– Steam Output 90tph– Pressure/Temp 63bar/485oC– Environmental SOx 400mg/Nm3 typical 100-350mg/Nm3
NOx <690mg/Nm3 typical 200-350mg/Nm3CO <250Particulates <20mg/Nm3
Fuel CV kJ/kg Ash % H2O % Size (mm)
Bark 7 182 0,8 55 < 100
Sawdust 7 394 0,8 54 < 5
Sludge 3 124 15 64 Fibre
Fine Ash (CFB) 11 814 64 0,4 < 200 µm
Coal 26 120 14 4 6-9 mm
Gas – Sasol 34 000 - - -
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EFFECT OF HIGH ASH COALS: DIFFICULT ASH HANDLING, HIGH PARTICULATE EMISSIONS,
SLAGGING AND DAMAGE TO WATER TUBES AND BOILER WALLS- POOR EFFICIENCY -
EXAMPLE OF A SINTERED BED IN A BUBBLING FBC BOILER IN SOUTH AFRICA
Bed Sinter – Active Formation
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So, what went wrong?• Coal characteristics
– Analysis revealed 3 different rank coals:• 50% Low rank bituminous (Witbank)• 40% Mid bituminous – coking coal (KZN)• 10% Highly heat affected in situ – semi anthracitic
– Mixed coal had:• A low proportion of reactive material 37%• A high proportion of inert forms of inertinite• Weathered and heat affected chars Dwhich emonstrated limited porosity
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So, what went wrong?• Coal characteristics
– Analysis revealed 3 different rank coals:• 50% Low rank bituminous (Witbank)• 40% Mid bituminous – coking coal (KZN)• 10% Highly heat affected in situ – semi anthracitic
– Mixed coal had:• A low proportion of reactive material 37%• A high proportion of inert forms of inertinite• Weathered and heat affected chars demonstrated limited porosity
– These factors produced:• Unreactive coal chars at bed temperatures in desirable range for co-
combustion of biomass based materials (sub 900C)• Slow bed reaction to increases in fuel inputs (+ 20 minutes)• High inventory of coal char• Higher level of unconsumed char in the bed waste, than expected• Higher levels of combustion temperature in association with the inert
and semi-reactive coal-char particles
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Coal Micrographs – Mixed Qualities
Normal Coal
Cracked Heated – Likely DepositFFF 2014 22
Micrographs – illustrating Dense Inert forms of Char
Dense, compact inert forms of
“inertinite-derived” chars
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Micrographs – illustrating Porous Reactive forms of Char
Mosaic-textured semi-porous
partially “reactive” forms of char derived from
higher rank vitrinite(typical form of “coke”)
Smooth-texturedhighly porous semi-molten
highly reactive forms of lower rank
“vitrinite-derived” char
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Micrographs – illustrating Semi-Porous Partially-Reactive forms of Char
Mixed Chars composed of Reactive and Inert Macerals in intimate association –i.e. porous bands (white, mosaic-textured) associated with dense inert carbon material (grey)
inert
inertreactive
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Microscopic View of Clinker -Backscatter Electron Image
Angular Quartz with interstitial smooth glass
Detail of Quartzite Grains (Grey) and Sintering
Ca-Silicate
Scale bar 1mm
Scale bar 200µm
Photographs of clinker illustrating cause of agglomerationbetween minerals
i.e. Ca-Si-Al glass “gluing” particles together
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Clinker Analysis Revealed
Bulk Analysis Sintering phaseMiddle Left Front Middle Left Front Clinker
Na2O 0.77 0.53 0.69 0.47 0.33 0.78
MgO 0.35 0.77 0.59 2.27 1.56 0.99 1.05
Al2O3 1.61 1.85 1.6 5.63 3.22 1.56 30.57SiO2 91.64 90.31 90.86 71.14 78.42 67.33 61.06P2O5 0.53 P2O5
SO3 0.74 0.85 0.26 SO3
K2O 0.370 1.3
CaO 4.78 4.69 4.5 17.47 14.38 28.65 2.71
TiO 0.37 1
Fe2O3 0.25 0.78 1.27 0.72 0.96
Semi-quantitative elemental proportions
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Sinter Conclusions• Sinter is Angular Quartz (Si) and Calcium Silicate• Features implied quartz reacting with Ca-enriched sinter phase• Ca and Al concentrations suggested that these were mobile and
thus a solid state reaction likely• Liquidus temperature of sinter phase had to exceed 1400oC• Note: Bed temperature recorded as max 960oC• Probable sinter phase reaction CaO and quartz, with pressure,
temperature and time – or • Sinter arose in the presence of high-temperature burning semi-
inert, inert macerals
Recommendation: Avoid static conditions and ensure best control over fluidization
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Operational Changes• Have to recognize you can’t change the fuel• Complex physical and chemical reaction – research
underway • Focus on factors within control
• Solution Pt 1 - Reduce energy inventory -Crush coal smaller 6 – 9mm
• Solution Pt 2 - Reduce time for reaction- Reduce purge cycle for over-bed
CO to minimum
• Result – SuccessFFF 2014 29
CONTENTS
1. Relevance of Coal in South Africa2. Qualities of coal in South Africa3. Low Carbon Economy planned for South Africa4. Experiences in BFBC in South Africa and Botswana5. CFBC test work conducted on South African coals in Finland6. Conclusions for the future
Secondarycyclone
Fuel container 1 and 2 Zone 1
Zone 2
Gas coolig
Primarycyclone
Observation port Sampling port/Deposit probe
Zone 3
Zone 4To stack
Sampling port
Sampling port
Observation and Sampling port/Deposit probe
Additivecontainer
Air
Secondary air(preheated)
Primary gas heating
Nitrogen
PC control and data logging system
Sampling port
Sampling port
Bag filter
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CIRCULATING FLUIDISED BED BOILER
–VTT Pilot Scale Test facility
Tests:1. Test high-low ash, high-low reactives 2. Temperature profiles in the boiler 3. Combustion efficiency of coals4. Proportions of coarse to fly ash
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700 800 900T [°C]
Dis
tranc
e fro
m g
rate
[m]
SA ISA IIDCRD-SA IIRUS
Freeboard top
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TEMPERATURE PROFILES AND COMBUSTION EFFICIENCY
RESULTS
1. Temperature profiles varied between 820oC and 870oC
2. Despite significant differences in coal quality and composition, all four coals achieved 98-100% combustion efficiency
4 Coals were tested: 1 Russian and 3 South African
• CV - 5 to 26 MJ/kg• Ash - 15 to 73 %• Volatiles 12 to 34 %
Source: VTT, Metso Finland: Vainikka 2011FFF 2014
Highly reactive low ash coal – high volatiles
all volatiles combustible
CLEAN COAL Vitrinite - no visible
minerals33
Microphotos of Coal – Russian coal-char
Thin walled, highly porous fluid char
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CHAR - highly porous - no visible
minerals
Micrographs – illustrating South African Dense Inert forms of Char
Dense, compact inert forms of
“inertinite-derived” chars
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Micrographs – illustrating South African Semi-Porous Partially-Reactive forms of Char
Mixed Chars composed of Reactive and Inert Macerals in intimate association –i.e. porous bands (white, mosaic-textured) associated with dense inert carbon material (grey)
inert
inertreactive
Inert organic forms
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0 %
20 %
40 %
60 %
80 %
100 %
RUS SA I SA II DCRD
Inertinite
Liptinite
Vitrinite
Total reactivemacerals
Combustionefficiency
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COMBUSTION EFFICIENCY
Despite significant differences in coal quality and composition, all four coals achieved 98-100% combustion efficiency
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31 %
55 %
29 %
69 %
69 %
45 %
71 %
31 %
0 %
20 %
40 %
60 %
80 %
100 %
RUS SA I SA II DCRD-SA II
Fly ash
Bottom ash
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PROPORTIONS OF COARSE ASH TO FLY ASH
The distribution of ash in bottom and fly ash is shown here. The primary cyclone separates particulates larger than 100µm from the flue gases exiting the circulation loop and returns them to the boiler.
In RUS and SA II coals (with lower ash and highest reactive coals), 70% of the ash ends up in fly ash exiting the circulation. SA I and DSCRD samples have highest coarse ash which is associated with highest mineral and inert organic matter. FFF 2014Source: P Vainikka, VTT
CONTENTS
1. Relevance of Coal in South Africa2. Qualities of coal in South Africa3. Low Carbon Economy planned for South Africa4. Experiences in BFBC in South Africa and Botswana5. CFBC test work conducted on South African coals in Finland6. Conclusions for the future
CONCLUSIONS• Fluidised bed technology is likely to be the technology of choice
for IPPs due to the high ash coals of the Southern African sub-continent
• South African coals have been shown to burn at high temperatures
• Coal particles in FBC boilers will burn at their natural inherent high temperatures despite the FB bed being kept at <900oC
• Great care needs to be taken to prevent slagging in FB boilers as a result of particles temperatures
• CFBC boilers are less likely to encounter slagging and ash agglomeration than BFBC boilers
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Coal is, and will be, the primary source of Industrialisation
in Southern Africa for decades to come, but this must be
addressed appropriately in order to meet the needs
of the Low Carbon Economy
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About 600 million people, and more than 10 million micro-enterprises, across Africa have
no access to electricity! There is an urgent need to switch the light on in Africa.
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Acknowledgments
DST/NRF SARChI funding under Clean Coal Technology umbrella
VTT, Finland for collaboration, training and test work
OEN Engineering for the thermal camera usage
Prof N Wagner for petrographic contributions
Dr C van Alphen for CCSEM analysis
THANK YOU FOR YOUR ATTENTION
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