Anthracite oxy-fuel combustion

in fluidized bed

Isabel Guedea, Irene Bolea, Carlos Lupiáñez, Luis I. Díez, Luis M. Romeo

CIRCE, University of Zaragoza, Spain

Pedro Otero, Jesús Ramos

CIUDEN Ciudad de la Energía, Spain

Ponferrada, September 12th 2013

Anthracite oxy-fuel combustion in fluidized bed

1. Introduction

2. Oxy-fuel facility

3. Experimental activities

4. Modelling

5. Conclusions

2

3

Anthracite oxy-fuel combustion in fluidized bed

1. Introduction

2. Oxy-fuel facility

3. Experimental activities

4. Modelling

5. Conclusions

3

4

1. Introduction

Introduction

• Objective: Characterization of oxy-fuel combustion for a wide range of O2

in the fluidizing stream, and a combination of other operating variables

• Combustion efficiency

• CO2 in flue gases

• Control of emissions: SO2 and NOx

• Selection of fuel: Anthracite

• Design fuel for oxy-firing at CIUDEN facilities

• Methodology: Experimentation in small-scale BFB and modeling to support experiments and simulate different conditions

5

Anthracite oxy-fuel combustion in fluidized bed

1. Introduction

2. Oxy-fuel facility

3. Experimental activities

4. Modelling

5. Conclusions

5

2. Oxy-fuel facility

6

2. Oxy-fuel facility

7

Main design features

• Dual air- and oxy-fired bubbling fluidized bed reactor (850ºC, 1.0-1.2 m/s)

• 0.1 MWth thermal input

• Two independent hoppers to feed coal, biomass, inert and/or sorbent

• O2/CO2 cylinders + mixer and wet flue gas recirculation

• Flue gas circuit: cyclone, heat recovery, fabric filter

• Water-cooling to control bed temperature

• Modifications after initial design: secondary oxidant supply, fall chamber, several on-load solid sampling, fouling probes

Operation flexibility

• O2 in the mixture from 20% to 50%

• Recycling ratios from 0% to 60%

• Variety of fuels: anthracite, bituminous, lignite, culm waste, forest biomass

Anthracite oxy-fuel combustion in fluidized bed

8

1. Introduction

2. Oxy-fuel facility

3. Experimental activities

4. Modelling

5. Conclusions

3. Experimental activities

9

Research objectives

• Effect of O2/CO2 atmospheres and bed temperatures in:

• Fluid-dynamics

• Combustion efficiency and CO2 production

• SO2 capture (limestone addition and sulphation mechanism)

• NOx control: oxygen staging and effect of limestone

Test campaigns

• Low volatile fuels: anthracite

• Spanish high-sulphur lignite

• Blends and co-firing

3. Experimental activities

10

Anthracite Bituminous Lignite

Proximate analysis (% d.a.f.)

Volatile 15.07 19.89 45.81

Fixed carbon 84.93 80.11 54.19

Ultimate analysis (% d.a.f.)

C 89.58 88.29 72.19

H 3.22 4.00 7.25

N 1.67 2.27 0.50

S 1.44 0.44 11.85

Mean Particle size (mm) 0.8 0.7 1.0

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3. Experimental activities

11

3.1. Effect of

• Bed temperature

• Excess oxygen

• Selection of limestone

… on NOx emissions for anthracite

oxy-combustion

3. Experimental activities

12

Fluidizing gas

Limestone Ca:S ratio

Bed temperature, Tbed (ºC)

Oxygen excess,

Air None 0 800–875 1.6–1.7

#1 4 850 1.6

#2 4 850 1.6

40/60 O2/CO2 None 0 850, 875 1.6

#2 2.5 850 1.6

50/50 O2/CO2 #2 2.5 850–950 1.6

25/75 O2/CO2 #1 4 850, 900 1.1–1.7

#2 4 850 1.7

50/50 O2/CO2 #1 4 900 1.3–1.7

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3. Experimental activities

13

• Use of limestone has been reported as a relevant factor affecting

NOx emissions in fluidized bed combustion (Miccio et al., de

Diego et al., Hayhurst & Lawrence)

• Addition of limestone enhances NO formation, mainly due to the

catalytic effect of CaO, but also CaCO3 and CaSO4 can influence

some formation/depletion reactions

• CaO increases NO formation rates, enhancing the presence of free

radicals (–O, –H, –OH) and favoring the NH3 conversion, rather

than HCN

3. Experimental activities

14

No limestone

3. Experimental activities

15

Limestone #1, Tbed = 900ºC

25/75 O2/CO2

50/50 O2/CO2

3. Experimental activities

16

Tbed = 850ºC

3. Experimental activities

17

3. Experimental activities

18

Tbed = 850ºC

3. Experimental activities

19

3.2. Effect of

• Oxygen-staging

… on NOx emissions for anthracite

and lignite oxy-combustion

Two tangential ports for secondary supply: 40 cm and 80 cm over the

perforated plate

Runs: 10%-20% secondary supply, 30/70 and 50/50 atmospheres

20

3. Experimental activities

Effect of O2 stagging (lignite, oxy-firing)

50/50 O2/CO2

30/70 O2/CO2

21

Effect of O2 stagging (anthracite, oxy-firing)

3. Experimental activities

50/50 O2/CO2

30/70 O2/CO2

Anthracite oxy-fuel combustion in fluidized bed

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1. Introduction

2. Oxy-fuel rig

3. Experimental activities

4. Modelling

5. Conclusions

4. Modelling

24

Small-scale OF bubbling fluidized bed reactors: fluid-dynamics

• 1D, suitable for small-scale reactors

• Combination of empirical correlations and own experimentation

• On-line calculation of voidage, pressure distribution, gas and solid transfers in the bed and the free-board

4. Modelling

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Small-scale OF bubbling fluidized bed reactors: solid conversion

• Specific fittings:

• Devolatilization

• Primary fragmentation

• Char conversion

• On-line calculation of conversion rates and species released

• Separated model for SO2 capture is also available, based on limestone reactivity

4. Modelling

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Small-scale OF bubbling fluidized bed reactors: global model

• Coupling of all considered phenomena in a global model to predict the performance of small-scale OF bubbling bed reactors

• Iterative process for a spatial discretization (grid independence)

• Local evolution of all relevant variables (pressure, temperature, solids concentration, chemical species, heat transfer rates)

• Validated with data gathered in experimentation at CIRCE OF-BFB

4. Modelling

27

Tests to show model validation

ANTHRACITE LIGNITE BITUMINOUS

Test # 1 2 3 4 5 6 7 8 9 10

Fluidizing gas Air 30/70 30/70 30/70 Air 25/75 40/60 Air 35/65 40/60

uf (m/s) 0.87 0.97 0.86 0.81 1.26 0.90 0.84 1.30 0.90 0.83

Ca:S ratio 4 4 4 4 2.5 4 2.5 2.5 2.5 2.5

Secondary gas (%) 0 0 10 20 10 10 10 0 0 0

Tbed (ºC) 840 875 880 875 805 830 890 870 885 865

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Temperature: model vs. experiment

AIR OXY OXY

0

10

20

30

40

50

60

70

80

1070 1120 1170 1220H (cm)

Tb(K)

LigniteExp #5 Model #5

Exp #6 Model #6

Exp #7 Model #7

AIR OXY

0

10

20

30

40

50

60

70

80

1070 1120 1170 1220

H (cm)

Tb(K)

Anthracite

Exp #1 Model #1

Exp #2 Model #2

Exp #3 Model #3

Exp #4 Model #4

4. Modelling

29

CO2 in flue gases: model vs. experiments

OXYOXY

OXY OXYOXY OXY

OXY

AIR

AIRAIR

8

13

18

23

28

84

89

94

99

1 2 3 4 5 6 7 8 9 10

CO2(%

) ‐AF

CO2(%

) ‐OF

Test

ANTHRACITE BITUMINOUSLIGNITE

ModelExp.

4. Modelling

30

Small-scale OF bubbling fluidized bed reactors: global model validation

4. Modelling

ModelExp.

AIR OXY OXY OXY

AIR

OXYOXY

AIR

OXY

OXY

0

500

1000

1500

1 2 3 4 5 6 7 8 9 10

CO (ppm)

TestANTHRACITE LIGNITE BITUMINOUS

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Detailed distribution in the reactor

Evolution of particle temperature

Air 40/60 O2/CO2

4. Modelling

32

Anthracite oxy-fuel combustion in fluidized bed

1. Introduction

2. Oxy-fuel facility

3. Experimental activities

4. Modelling

5. Conclusions

5. Conclusions

33

• Selection of limestone is a relevant issue affecting NOx emissions

in fluidized bed oxy-combustion

• Despite the increase of bed temperature can be suitable for a

higher SO2 capture efficiency (calcining conditions), NO

formation ratios are also enhanced due to CaO availability

• Effect of oxygen staging has a different extent depending on the

coal rank, not always leading to an effective reduction

• Oxy-combustion behaviour of anthracite is shown to be good,

with low CO records, and CO2 in flue gases over 93%

Thanks for your attention

www.fcirce.es

Luis I. Díez, luisig@unizar.es

Acknowledgments:

Spanish Ministry of Science and Technology, R&D National Program

Fundación CIUDEN, Ciudad de la Energía