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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35

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45

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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

FFF 2014Source: P Vainikka, VTT

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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

FFF 2014Source: P Vainikka, VTT

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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