Unusual combustion behaviour when burning SA low grade coals and their Impact on the efficiency of one spreader stoker boiler

RL Taole1

1School of Chemical and Metallurgical Engineering, University of the Witwatersrand

Co-authors: RMS Falcon1 & M Andrews2

2OEN Enterprises

PROBLEM STATEMENT

ó SOUTH AFRICA HAS APPROXIMATELY 6 000 INDUSTRIAL COAL-FIRED BOILERS in operation throughout the country. These were bought and installed between 1950 and 1970.

ó THE DESIGNS OF THE BOILERS were based upon high grade products typical of European or American Carboniferous coals.

ó THE QUALITY OF COAL now available in South African is low to medium grade thermal products, including discards and duff.

ó THE RESULT OF BURNING SUCH LOW GRADE COAL is high inefficiencies and excessive emissions; this is due to the incompatibility between coal performance, design of boiler and operating conditions.

ó THE CONSEQUENCE is frequent maintenance stoppages, long downtime, short life spans of equipment, high gaseous and particulate emissions, high cost of steam and energy output per unit coal .

CAN SOUTH AFRICA USE THE REMAINING COAL RESERVES, GIVEN THEPROPORTION OF COALS IN THE HIGH RANGES OF ASH CONTENT?

(after Petrick ; Horsfall, 1977)

0

5

10

15

20

25

30

35

40

45

50

% 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

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 xxx x

x

x xxx x

x

4

Local Coal Products: Quality Issues

Coa

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duct

s fr

om m

ine

Mar

ketin

g

EXPORT

POWER GENERATION

SYNFUELS

SMALL SCALE INDUSTRY

METALLURGICALHEAT AND POWER GENERATION -pulp and paper, textiles, agriculture, sugar, tobacco, mining, brick and tile, cement, lime, manufacturing factories, hospitals, town gas

MINNG INDUSTRY – Gold, uranium and other mining activities

DOMESTIC HOUSEHOLDS – coal, anthracite, low smoke fuels, etc.

SOUTH AFRICAN INDUSTRIES USING COAL

The Challenge

§ to establish how the current coals burn

§ to predict the combustion properties

§ to render the combustion process more efficient, and thereby reduce costs, maintenance time and damage, and

§ to reduce environmental emissions in order to comply with the latest NEMA Air Quality Acts (PM, GHGs, NOx, SOx)

CASE STUDY – Boiler Design Data

ó BOILER TYPE

§ Spreader stoker - by Babcock & Wilcox, installed 1973

§ Coal-fired water-tube boilers

§ Evaporation at MCR - 45 t/hr

§ Effective grate area ~ 19m2

§ Stoker type - Rotograte

§ Saturated steam @ 31Barg; ~ 35MWt

CASE STUDY – Coal Spec Data

ó COAL SPECIFICATIONS EX BOILER MANUFACTURER

§ CV ~ 28 MJ/kg – GRADE A

§ VM = 25% ad;

§ FC = 57% ad;

§ Ash =14%;

§ Inherent Moisture = 2.4%

Source: Operating Instruction Manual ( B&W Contract No. 81269, 1973)

CASE STUDY – Implats Spreader Stoker

[Courtesy of Impala boiler photo archives]

Coal Feeders

STRUCTURE OFTHE SPREADER STOKER

Coal distributor

Over fire air system above grate

Direction of coal fling from the coal distributor /feeder

Coarse ash dropping off

Water tubes

Coarse burning coal travelling backwards on metal grate surface

Travelling chain grate

Undergrate air is fed at various points along the grate

FOUR COALS SELECTED FOR TESTING Qualities compared to Manufacturer’s Specifications

Coal IDProximate Analysis (% ad) Commercial

Grades of coal sold on

CV valuesVM Ash H2O Tot S CV

(MJ/kg)

MANUFACTURER SPECIFICATIONS

25.0 14.0 2.4 - 28.0 A

A 29.2 15.6 4.8(9.55)

1.46 26.13 C

B 24.6 17.3 3.4(7.32)

0.46 26.42 C

C 23.5 14.7 3.7(9.1)

0.51 26.99 B

D 30.2 16.7 4.5(5.9)

1.23 25.54 D

*E*No further tests due to flame-out

24.9 23.1 5.1(6.2)

0.49 22.19 DIII

COAL

SPETROGRAPHIC ANALYSES (% mmf)

TOTAL VITRINITE

(%)

TOTAL REACTIVES

TOTALINERTINITE

(%)

Micro-Lithotype

Inertite(%)

ABNORMAL CONDITIONWeathered

(%)

RANK RoVr%

A 47 63 49 24 14 0.61

B 28 56 68 33 19 0.70

C 24 54 70 40 22 0.76

D 44 64 47 20 25 0.63

FOUR COALS SELECTED FOR TESTING Selected results of petrographic analyses

ANALYTICAL RESULTS - 1

1. CHEMICO-PHYSICAL ANALYSES Calorific values range from Grades B to C and D

I. - Coal C is top grade (Grade B, 26,9 MJ/kg)II. - Coals A and B are moderate grade (26,1 and 26,4 MJ/k respectively)III. - Coal D is lowest grade (25,5 MJ/kg)

Volatile matter contents range from 23,5 to 30,2%ad, I. - Coal C and A are top (30,2 and 29,2% ad respectively); II. - Coals B and C are considerably lower (24,6 and 23,5%ad respectively)

Ash contents range from 14,7 to 17,3% ad.I. - Coal C has the lowest ash content (14,7% ad)II. - Coals A and D have 15,6 and 16,7% ad respectivelyIII. - Coal B has the highest ash content (17,3% ad)

Hardgrove analyses rage from 52 to 70I. - Coal D has the highest value (70) meaning softest and most friable)

ANALYTICAL RESULTS - 2

2. PETROGRAPHIC ANALYSES - Rank (maturity) of the coals ranges from 0,61 to 0,76 RoVr% in rank (i.e.

low rank Bituminous C in range)- Maceral composition (vitrinite) ranges from 24 to 47% mmf.- Abnormal (weathered) material ranges from 14 to 25% mmf

3. CARBON IN ASH- Carbon-in- ash in fly ash samples ranged from 30,1 to 42,8% (65%)- Carbon-in- ash in the bottom ash ranged from 16,2 to 22,7%

4. THERMAL EFFICIENCY- Thermal efficiency ranges from 71,0 to 79,7%

3.

METHODOLOGY Tr

ials

-Stockpile coal

-Burn a specific coal over 5 days (4 types)

-Composite samples across feeders @ 30mins intervals over a 4 hour period

-Boiler @ 70-90%MCR. All other operating parameters constant

Mea

sure

men

ts&

Ana

lysi

s -Feed coal & ash analyses

-Thermography using thermal imaging camera

-Petrography

-Combustion gases in flue

Resu

lts in

terp

reta

tion Coal Quality

vs Combustion efficiency

vs Steam output

vsTemperature profiles

vsUnburnt carbon

THERMOGRAPHIC VISUAL TESTING SYSTEM

DURAG, D-VTA 100-10 Video and Thermographic Camera System

STRUCTURE OF THE SPREADER STOKER

Camera’s field of view

Coal distributor

Overfire air system above grate

Direction of coal fling from the coal distributor

Coarse ash dropping off

Water tubes

Travelling chain grate

Coarse burning coal travelling backwards on metal grate surface

TEST RESULTS 1 - Thermography

Photographs, graphs and temperatures captured by the thermographic camera

COAL A:CV*26.13; Ash*15.6%; VM*29.2%.

Key: ROI1 = Top FreeboardROI3 = In FireballROI5 = Just above Grate

ToC Range:

ROI1 = 1509

ROI3 = 1540

ROI5 = 1549

Thermography: Coal A

COAL B: CV*26.42; Ash*17.3%; VM*24.6%.

Key: ROI1= Top FreeboardROI3= In FireballROI5= Just above Grate

ToC Range:

ROI1 = 1709

ROI3 = 1789

ROI5 = 1779

Thermography: Coal B

COAL C: CV*26.99; Ash*14.7%; VM*23.5%.

Key: ROI1= Top FreeboardROI3= In FireballROI5= Just above Grate

ToC Range:

ROI1 = 1616

ROI3 = 1678

ROI5 = 1722

Thermography: Coal C

COAL D: CV*25.54; Ash*16.7%; VM*30.2%.

Key: ROI1= Top FreeboardROI3= In FireballROI5= Just above Grate

ToC Range:

ROI1 = 1771

ROI3 = 1788

ROI5 = 1793

Thermography: Coal D

A: Excellent, on grate and in fireball –1549oC

B: Irregular, on grate and small fireball –1741 oC 1789oC

C: Poor ignition on grate –1721oC

D: Massive, throughout combustion chamber –1793oC

SUMMARY AND COMPARISON OF COMBUSTION CHARACTERISTICS

Calorific value and proximate analyses compared to Boiler Combustion Efficiency and UBC

ó Table1: Empirical

Coal ID Proximate Analysis (% ad) Overall Efficiency

VM Ash H2OInherent(Total)

Tot S CV -Grade(MJ/kg)

ε(%) Comb efficiency

(%) UBC

Fly-ash (Bottom)

D (15/06) 30.2 16.7 4.5(5.9)

1.23 25.5 D 79.13 31.90(16.26)

A (26/05) 29.2 15.6 4.8(9.6)

1.46 26.1 C 77.98 30.13(16.97)

B (02/06) 24.6 17.3 3.4(7.3)

0.46 26.4 C 75.54 42.76(22.70)

C (09/06) 23.5 14.7 3.7(9.1)

0.51 26.9 B 71.05 42.81(21.62)

*E (14/06)*No further tests due to flame-out

24.9 23.1 5.1(6.2)

0.49 22.1 DII 65.00 27.56(23.53)

• Primary ranking in order of combustion efficiency; • Increasing combustion efficiencies correlate with increasing volatile matter, and inversely with UBC; • There is no correlation between combustion efficiency, CV and ash

Petrographic composition compared to Boiler Combustion Efficiency and UBC

ó Table2: Fundamental

COAL

Petrographic Analyses (% mmf) Combustion

Efficiency and% Unburnt Carbon

(UBC)TOTAL

VITRINITE (%)

TOTAL REACTIVES

(%)

TOTALINERTINITE

(%)

Micro-LithotypeInertite

(%)

ABNORMAL CONDITIONWeathered

(%)

RANK RoVr% ε

(%) Comb efficiency

(%) UBC

Fly-ash (Bottom)

D 44 64 47 20 25 0.63 79.13 31.90(16.26)

A 47 63 49 24 14 0.61 77.98 30.13(16.97)

B 28+ 56 68 33 19 0.70 75.54 42.76(22.70)

C 24 54 70 40 22 0.76 71.05 42.81(21.60)

• Highest combustion efficiency and lowest UBC correlates with high reactive organic macerals• Low combustion efficiencies and high UBC correlate with high inertinite coals

Calorific value, proximate analyses and boiler efficiencycompared to Highest Thermographic flame temperatures)

ó Table1: Empirical

CoalID

Proximate Analysis (% ad) Combustion Efficiency , % Unburnt Carbon and

Thermographic flame temperaturesVM Ash H2O

Inherent(Total)

Tot S CV (MJ/kg)

ε(%) Comb efficiency

(%) UBC Fly-ash

(Bottom)

Flame ToClowest

Flame ToCHighest

D 30.2 16.7 4.5(5.9)

1.23 25.5D

79.13 31.90(16.26) 1771 1793

A 29.2 15.6 4.8(9.6)

1.46 26.1C

77.98 30.13(16.97) 1509 1549

B 24.6 17.3 3.4(7.3)

0.46 26.4C

75.54 42.76(22.70) 1709 1779

C 23.5 14.7 3.7(9.1)

0.51 26.9B

71.05 42.81 (21.62) 1616 1722

There is no correlation between flame ToC and CV, combustion efficiency, UBC, ash or volatiles

Petrographic composition compared to Thermographic Flame Temperatures

ó Table2: Fundamental

COAL

S Petrographic Analyses (% mmf) Combustion Efficiency,% Unburnt Carbon and

Thermographic flame temperatures

TOTAL VITRINITE

(%)

TOTAL REACTIVES

TOTALINERTINITE

(%)

Micro-LithotypeInertite

(%)

ABNORMAL CONDITIONWeathered

(%)

RANK RoVr% Ε (%)

Comb efficiency

(%)

UBC

Fly-ash(Bottom)

Flame ToC

lowest

Flame ToC

Highest

D 44 64 47 20 25 0.63 79.13 31.90(16.26)

1771 1793

A 47 63 49 24 14 0.61 77.98 30.13(16.97)

1509 1549

B 28 56 68 33 19 0.70 75.54 42.76(22.70)

1709 1779

C 24 54 70 40 22 0.76 71.05 42.81 (21.62)

1616 1722

High flame ToC correlates with high inertinite/inertite and rank (B and C) and potentially to weathered and oxidised components (Coal D).

Petrographic composition compared to Actual Boiler readings - Steam Output and O2

ó Table2: FundamentalCOAL

S

Petrographic Analyses (% mmf) Overall

EfficiencyThermograph

Flame ToCBOILER

OUTPUT

TOTAL VITRINITE

(%)

TOTAL REACTIVES

(%)(Liptinites)

TOTALINERTINITE

(%)

Micro-LithotypeInertite

(%)

ABNORMAL CONDITIONWeathered

(%)

RANK RoVr% ε

(%) Comb efficiency

(%)UBCFly-ash

(Bottom)

Flame ToC

LowestRO1

Flame ToC

HighestRO5

Average Steam Output(O2 in flue gas %)

D 44 64(9)

47 20 25 0.63 79.13 31.90(16.26)

1771 1793 41,76(10,27)

A 47 63(4)

49 24 14 0.61 77.98 30.13(16.97)

1509 1549 38,59(11,5)

B 28 56(4)

68 33 19 0.70 75.54 42.76(22.70)

1709 1779 37,79(11,4)

C 24 54(5)

70 40 22 0.76 71.05 42.81 (21.62)

1616 1722 34,56(11,7)

HIGHEST STEAM OUTPUT correlates with high vitrinite contents, highest efficiency and lowest unburnt carbon BUT DOES NOT CORRELATE WITH FLAME TEMPERATURE

.

Calorific value, proximate analyses , boiler efficiency, UBC and flame ToCcompared to Actual Boiler performance - Steam Output and O2 in flue gas

ó Table2: FundamentalCOAL Proximate Analyses

Fuel RatioBoiler

EfficiencyThermography

Flame ToCBOILER

OUTPUT

CV MJ/kg

Volatile matter ad %

Fixed CarbonAd% FUEL

RATIO

ε(%) Comb efficiency

(%)UBC

Fly-ash (Bottom)

Flame ToCLowest

RO1

Flame ToC

HighestRO5

Average Steam Output(O2 in

flue gas %)

D 25,5 30.2 48,6 1,61 79.13 31.90(16.26)

1771 1793 41,76(10,27)

A 26,1 29.2 50,4 1,73 77.98 30.13(16.97)

1509 1549 38,59(11,5)

B 26,4 24.6 54,7 2,22 75.54 42.76(22.70)

1709 1779 37,79(11,4)

C 26,9 23.5 58,1 2,47 71.05 42.81 (21.62)

1616 1722 34,56(11,7)

HIGHEST STEAM OUTPUT correlates with high volatile matter and high combustionefficiency, but correlates INVERSELY with Fuel ratio and unburnt carbon.

OBSERVATIONS - 1

• Combustion behaviour is clearly illustrated by thermography. The coals under review all showed different combustion characteristics despite similar ash contents and CV.

• There is a strong correlation between thermographic results, combustion behaviour and petrographic analyses. This is not reflected in calorific values ash or fixed carbon contents.

• Flame temperature measurements for all coals tested displayed higher than expected temperatures in the range of 1600 to ~1800oC. i.e. Such hightemperatures lead to excessively high back end temperatures, slagging in apparently non-slagging coals and high temperatures on the grate which, in turn, leads to melting and fusing of chain grate links, water tubes and refractory linings. Burning high temperature coals will require high temperature materials of construction (MOC) in future.

•

OBSERVATIONS – 2

v Combustion efficiency correlates with high volatile matter, high vitrinite contents and high Steam Output. It bears no relationship to flame ToC readings, CV or ash content

v Highest unburnt carbons in coarse ash and fly ash occur when burning coals with low volatile matter and high inertinite macerals contents. They bear no relationship with flame ToC readings, CV or ash content

v Vitrinite and, to some extent volatile matter, appear to play the major role in the combustion behaviour of coal, with an increased content guaranteeing better combustion, while lower contents (i.e. higher inertinite contents) lead to poorer ignition, delayed combustion and higher unburnt carbon.

OBSERVATIONS – 3

ó The combined results show that coals A and D exhibit the highest combustion efficiencies and steam outputs compared to coals B and C but they differ in combustion behaviour and flame temperature.

ó Coal A burns on the grate and in a fireball, and it has a lower flame temperature, whereas

ó Coal D burns in a massive all-encompassing flame at extremely high temperatures – these could damage boiler equipment

ó Coal A would therefore be the coal of choice.

The impact of the more advanced approach to combustion assessment and boiler operation would lead to:

v- Improved selection of suitable feed coals using key parameters for better selection

v- Improved design or adaptation of main boiler and auxiliary equipment (precips, bag filter - new, or retrofitting old) in order to adapt to the low grade coals now being used

v- Extended boiler plant availability and reduced maintenance by minimising hot spots, boiler tube rupture, slagging

v- Improved combustion efficiency leading to greater cost effectiveness

v- Reduced emissions by using more suitable coal feedstocks, less coal leads to lower CO2 emission.

CONCLUSIONS

“The illiterate of the 21st century will not be those who can not read and write, but those

who cannot learn, unlearn, and relearn”

Alvin Toffler (The Future shock)

ó THANK YOUó NDO LIVHUWAó ENKOSI

ó REA LEBOHAó BAIE DANKIEó INKOMU

ó SIYATHOKOZAó SIYABONGAó REA LEBOGA