lecture 3: the science of oxy-fuel - university of newcastle · lecture 3: the science of oxy-fuel...

Lecture 3:

The Science of Oxy-fuel

System differences in flames, heat transfer, coal combustion and emissions

APP OFWG Capacity Building Course, Sunday/Monday 11/12 September, 2011 Capricorn Resort, Yeppoon AUSTRLIA

Rohan Stanger University of Newcastle, Australia

Related Literature

Woodhead Publishing

February 2011

Volume 5, Supplement 1, Pages S1- S238 (July 2011)

Oxyfuel Combustion Technology -

Working Toward Demonstration

and Commercialisation

TEXTBOOK JOURNAL Special Edition WEBSITE

http://nl.sitestat.com/elsevier/elsevier-com/s?ScienceDirect&ns_type=clickout&ns_url=http://www.sciencedirect.com/science/journal/17505836

Lecture Outline

• Flames & heat transfer

• Coal Behaviour

• Emissions

• Impurity Impacts during CO2 compression

Lecture context and content

Oxyfuel science used here to compare air and oxy-fuel furnace performance, for retrofit of an existing air-fired boiler while maintaining heat transfer, considering

– Conditions for matched heat transfer– Changed burner flows, with flame and heat transfer impacts– Coal reactivity and burnout impacts

Developments and gaps in knowledge

will be suggested

Basic Oxy-fuel Circuit

ESP Sequestration Site

Transport (pipeline, truck, etc)

Air Separation Unit

Coal Handling

O2

N2

Recycled Flue Gas ~70%

CO2compression

Heat Transfer

Flame & Coal Behaviour

Emissions & Impurities Impacts

~30%

Presenter

Presentation Notes

This diagram shows the main components of a pulverised coal oxy-fuel plant. First generation oxy-fuel plants are expected to have an air separation unit or ASU to supply O2 to the boiler. Recycled flue gas is used to temper the flame temperature and control the heat transfer. For retrofit applications the goal of recycling the gas is to match the heat transfer that the steam side is designed for, while new oxy-fuel plants can have a number of variations in terms of recycle rate, O2 content, premixing or direct O2 injection. A major benefit of recycling the flue gas is that the amount of gas to be treated and compressed is only around 30% of the total flue gas exiting the boiler. The downside to recycling the flue gas is that SOx concentration is higher, which can cause corrosion issues. The exiting flue gas must then be cooled, compressed and treated for impurities before being sequestered at a suitable site.

Oxy-fuel: differences of combustion in O2 /CO2 compared to air firing

• To attain a similar AFT the O2 proportion of the gases through the burner is ~ 30%

• The high proportions of CO2 and H2 O in the furnace gases result in higher gas emissivity's

• The volume of gases flowing through the furnace is reduced (longer residence time)

• The volume of flue gas (after recycling) is reduced by about 80%.

Recycle gases have higher concentrations in the furnace

Presenter

Presentation Notes

The characteristics of oxy-fuel combustion with recycled flue gas differ with air combustion in several aspects primarily related to the higher CO2 levels and system effects due to the recirculated flow, including the following: To attain a similar adiabatic flame temperature (AFT) the O2 proportion of the gases passing through the burner is higher, typically 30%, than that for air (of 21%), necessitating that about 60% of the flue gas is recycled. The high proportions of CO2 and H2O in the furnace gases result in higher gas emissivities, so that similar radiative heat transfer for a boiler retrofitted to oxy-fuel will be attained when the O2 proportion of the gases passing through the burner is less than the 30% required for the same AFT. The volume of gases flowing through the furnace is reduced somewhat, and the volume of flue gas (after recycling) is reduced by about 80%. The density of the flue gas is increased, as the molecular weight of CO2 is 44, compared to 28 for N2 Without removal in the recycle stream, species (including corrosive sulphur gases) have higher concentrations than in air firing.

Flames and heat transfer

Fixed velocity

27 O2

% v/v fixed for same HT

Therefore secondary RFG reduced

~3% v/v O2

Burner

Burner flow comparisons for a retrofit

Presenter

Presentation Notes

Figure shows air combustion (solid lines) and retrofit oxy combustion (dashed lines). Higher Cp values in oxy combustion, requires higher O2 levels to establish similar adiabatic flame temperature for air firing. For a fixed velocity for primary stream, higher mass flow is required and the balance (which is lower) is sent through secondary stream.

Gas property ratios for CO2

and N2

at 1200 KProperties from Shaddix, 2006

Impact for air to oxyfuel retrofitHigher O2 thru burner

Lower burner velocity, higher coal residence time in furnace

Slower flame propagation velocity

Property/ratio Gas property differences

AFT of air and oxy cases

1000

1400

1800

2200

2600

0.18 0.22 0.26 0.30 0.34 0.38

O2 fraction at burner inlet

Adia

batic

flam

e Te

mpe

ratu

re (K

)

airoxy-wetoxy-dry

Presenter

Presentation Notes

Oxy-fuel recycled flue gas will be a mixture of CO2 and H2O and a number of impurities. The higher Cp of the recycled gases lowers the adiabatic flame temperature. Therefore, the O2% must be raised to compensate for the loss in heat transfer. However, ADF is only one half of the heat transfer problem. The second factor is gas emissivity.

Gas property differences : Emissivity Triatomic gas (H2 O+CO2 ) emissivity ~ beam length

comparisons

Gupta et al (2006)

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80 90

Beam Length (L) (m)

Gas

Em

issi

vity

(-)

Oxy-fuel fired furnace

Air fired furnace

30 500 1050 MWe

Presenter

Presentation Notes

Spectral band models must be used for oxyfuel emissivity estimations for large furnaces, rather than Hottel charts, We see that gas emissivituies are substantially greater in oxyfuel, with the furnace beam length indicated her for different MWe outputs.

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60Beam length (L) (m)

Emis

sivi

ty (-

)

4 grey gas model,

4-GGM

Gupta et al, (2006)

3 grey gas model,

3-GGM

Smith et al, (1982)

Oxy-fuel fired furnace

CFD radiative transfer inputs

]1[)( )(

0,

22 Lppk

ii

OHCOieTa

30 MWe

500 MWe

Presenter

Presentation Notes

For cfd models for prediction of radiative transfer in furnaces the Sum Of Weighted Gas Model is used when the equation given here is matched to the emissivity ~ beam length function with a number of grey gases. For air firing three grey gases are used, with the coefficients (a in the equation here) from Smith for three grey gases used. For oxyfuel we have established that four gases are needed for large furnaces greater than about 50 MWe , with a typical fit as given here. The coefficients have not yet been published. A critical point to make is that this gas emissivity model has yet to be verified because oxy-fuel is still only operating at the 10MWe scale. This represents possibly the largest unknown in building an oxyfuel plant.

Preheated air/RFG: primary 350 - 400K and secondary 450 - 550 K, Wall 1200 K

Parameter Full load Partial load

Air case Oxy case Air case Oxy case

Coal flow rate kg/hr 120 120 72 72

Primary velocity m/s 20 23 17 21

Secondary velocity m/s 35 21 18 12

Secondary swirl number - 0.2 0.2 0.2 0.2

Primary momentum flux kg/s.m2/s2 35.7 54.1 20.9 36.8

Secondary momentum flux kg/s.m2/s2 270.2 74.1 38.2 16.4

Momentum flux ratio (Pri/Sec) - 0.13 0.73 0.55 2.25

1 MWt

test conditions

Presenter

Presentation Notes

From the calculations, the following input conditions were derived as used during experiments. Notable here is the momentum flux ration term. Higher for oxy compared to air case. Also for partial load conditions. Also note that the primary gas flow remains relatively fixed, while the secondary flow has a large variation. This has practical implications in running a burner because the O2 is typically added only in the secondary flow, which means that if a lower flow is used then this will directly impact the O2% in the windbox, since the actual amass flow of O2 is fixed.

Type-0

Type-1

Type-2

Low S

Hi S (S>0.6) , Low v2

Hi S, Hi v2

IFRF Flame types from swirl burners

Presenter

Presentation Notes

Flame Types were categorized from the IFRF study. Current experimental study found Type-0 flames which is characteristic of low swirl flows and no internal recirculation. How is this relevant specifically to oxy: reduced secondary flow

Air-case

X 1.5m

Type-0 flame

Oxy-case

1 MWt

–

Temperature contours at full load

Presenter

Presentation Notes

Ignition was located at off axis. The predicted axial ignition location (referred by high temperature) is different for air and oxy combustion, with oxy combustion delaying ignition. It can also been seen that the oxy-fuel flame is longer, which can impact on the heat transfer.

30 MWe

–

heat transfer results

150

170

190

210

230

250

45 51 56 56Furnace wall area (m2)

Tota

l sur

face

hea

t flu

x (k

w/m

2)

airoxy-fuel

Front wall Rear wall Side wall1 Side wall2

1 MWt

Temperature comparisons for matching furnace heat transfer

Flame

FEGT

Sensitivity analysis –

full & partial load

Presenter

Presentation Notes

Figure represents predictions for full and partial load operation. In both cases, the partial load operation shows ignition delay. The effect is due to the effect of primary momentum flux being higher for partial load operation

Confirms the significance of momentum flux

and Gas properties

on flame ignition

Effect of momentum flux

21% O2 /CO2

Presenter

Presentation Notes

An theoretical analysis was conducted in which the ratios of momentum flux were matched to that of air case, to understand the influence of gas composition on ignition location. Thus the ignition delay can be explained by two factors: different gas properties and higher momentum flux. Thus jet aerodynamics also influences ignition location and flame types.

Particle imaging of ignition and devolatilisation of pulverized coal during oxy-fuel combustion.

Shaddix, C. R. and A. Molina (2009)

CO2 atmosphere delays ignition and

devolatilisation

• higher Cp of gas

• Reduced radical pool

• Lower diffusivity of O2, CxHy

(Murphy and Shaddix 2006)

Coal combustion in Sandia’s entrained flow reactor under the intermediate gas temperature conditions.

O2% affects flame

length and type

Controlled through

recycle rate

Important when matching heat transfer in retrofit applications

Recycle Rate

Normal Air Fired

OXY recycle ratio = 0.76

(low O2%)

OXY recycle ratio=0.58

(high O2%)

O2 Flow is set by stoichiometry

Recycle Rate changes O2%, AFT, radiative/convective HT

Tan, Corragio, Santos, IFRF 2005 Review

Flame & Heat Transfer Summary

• Properties of oxy-fuel recycle gas

• Lowers adiabatic flame temperature (higher Cp)• Delays flame ignition (higher Cp, lower O2 diffusion)• Affects radiative heat transfer (higher emissivity)

• Matching heat transfer in a retrofit is a balance of

• adiabatic flame temperature, • recycle rate of flue gas (for O2%)• gas emissivity (model validation needed)

• Managing oxy-fuel flame has more parameters to consider (eg flame type, length) but offers more control (eg O2 Injection)

Coal Behaviour

25

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

500 600 700 800 900 1000 1100 1200 1300

Temperature, T p (K)

Rea

ctiv

ity, R

m,p

(s-1

)

Coal 100% N2Coal 100% CO2

Pyrolysis of Coal D in N2 & CO2TGA Experiments

Char gasification begins

CO2

N2

Devolatilisation reactivities are similar in both N2 and CO2 atmospheres.

Char-CO2 gasification reaction is clearly evident in a CO2 atmosphere.

CO2

reactivity with char

devolatilisation reactivities

are similar

Rathnam et al, OCC1, 2009

26Comparison of Apparent Volatile Yields in N2 & CO2

DTF Experiments at 1400 oC

Higher apparent volatile yield at higher temperatures and heating rates in DTF at 1400 oC

compared to the proximate analysis volatile matter.

Higher apparent volatile yield in CO2

atmosphere -

attributed to the char-CO2

gasification reaction.

76.3

6

46.7 60

.0

50.9

66.6

36.1

8 48.5

7

87.4

5

48.7

74.3

59.4 70

.2

35.8

0 49.1

9

57.1

9

37.0

1

37.9

3

35.5

1 51.4

6

32.6

1

36.5

1

0

10

20

30

40

50

60

70

80

90

100

67.51 75.00 78.40 79.10 81.60 81.86 83.91

App

aren

t vol

atile

yie

ld (w

t. %

daf

bas

is)

N2CO2Proximate Analysis

C in Coal (wt. %, daf basis)


27Char Swelling in N2 & CO2

0102030405060708090

100

1 10 100 1000Particle diameter (μm)

Cum

ulat

ive

volu

me

perc

enta

ge CoalN2 CharCO2 Char

Coal A

0102030405060708090

100


Cum

ulat

ive

volu

me

perc

enta


Coal B

0102030405060708090

100


Cum

ulat

ive

volu

me

perc

enta


Coal C

Chars were formed in the DTF at 1400 oC

Coal A Coal B

Coal C

Swollen chars

Similar N2

and CO2

chars

Larger CO2

chars


28

100

150

200

250

300

350

400

450

500

60 65 70 75 80 85 90

C content in coal (wt. % daf basis)

Mic

ropo

re s

urfa

ce a

rea

(m2 /g

) N2 CharCO2 Char

Char Micropore

Surface Areas of Chars

Chars were formed in the DTF at 1400 oC


Presenter

Presentation Notes

Coal A has the largest internal micropore surface area. Some CO2 chars have larger micropore surface areas in comparison to the N2 chars. Coal A’s CO2 char has a lower micropore surface area probably because of the C-CO2 reaction in the pores which results in the collapse of the pore surface area.

29Coal A Reactivity in O2/N2 & O2/CO2 TGA Experiments

REACTIVITY

Endothermic gasification

Constant Heating Rate – 25C/min

- Lower reactivities & heat flows at low temperatures in O2 /CO2 combustion

- Endothermic gasification reduces heat flows and increases reactivities at high temperatures

0

0.01

0.02

0.03

0.04

0.05

0.06

0 200 400 600 800 1000

Temperature (oC)

Rea

ctiv

ity (m

in-1

)

0

0.04

0.08

0.12

0.16

0.2Coal A

21% O2

CO2

3% O2

10% O2

N2

-100

-50

0

50

100

150

200

0 200 400 600 800 1000

Temperature (oC)

Hea

t Flo

w (m

W)

Coal A

3% O2

10% O2

21% O2

N2

CO2

HEAT FLOW


Presenter

Presentation Notes

The effect of CO2 at high temperatures and high O2 levels are not clear from heating experiments. Hence, isothermal experiments at 1000 oC were performed at various O2 levels on char formed in DTF in a N2 atmosphere.

30

405060708090

100

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Oxygen concentration (atm)

Coa

l bur

nout

(wt.

%, d

af b

asis

)

O2/N2

O2/CO2

Coal A

Coal A Burnout & Reactivity

60

70

80

90

100

800 900 1000 1100 1200 1300 1400 1500

Furnace temperature (oC)

Coa

l bur

nout

(wt.

%, d

af b

asis

)

O2/N2

O2/CO2

Coal A

DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS

0

0.2

0.4

0.6

0.8

1

0 0.05 0.1 0.15 0.2 0.25Oxygen Concentration (atm)

Rea

ctiv

ity (m

in-1

)

Air - O2/N2Oxy - O2/CO2

COAL A 1400N2 CHAR1000 OC, 50% CONVERSION

0

0.2

0.4

0.6

0.8

1

600 700 800 900 1000 1100 1200

Temperature (oC)

Rea

ctiv

ity (m

in-1

)

Air - O2/N2Oxy - O2/CO2

COAL A 1400N2 CHAR10% O2, 50% CONVERSION


Char reactivity comparison for air and oxyfuel conditions at the same O2

level

1/Tp (K-1)

Low temperatures Reaction kinetics controlled (Regime I)

Moderately high temperatures Reaction kinetics & internal diffusion limited (Regime II)

Very high temperatures Bulk diffusion controlled (Regime III)

Oxy-fuel (O2/CO2) combustion

Air (O2/N2) combustion

Rea

ctio

n ra

te

Presenter

Presentation Notes

Transition temperature (I to II and II to III). Coal reactivity towards oxygen

32

Variable Technique Difference in O2

/CO2

or CO2atmosphere

Apparent Volatile Yield DTF Higher in CO2Pyrolysis Rate TGA Similar peak devolatilisation

Enhanced at higher temperature

(due to gasification)

Char Swelling DTF/PSD VariableMicropore Surface Area

DTF/BET Different results in CO2 to N2

Coal Burnout DTF Higher in O2 /CO2 conditions in some coals

Char Reactivity TGA Higher in O2 /CO2 conditions

SUMMARY OF COAL RESULTS

33

There are significant differences in pulverised coal reactivity

in air and oxy‐fuel

conditions. The extent of the differences depend on the coal type (rank).

The devolatilisation reactivity of coal in N2

and CO2

is similar

as shown by TGA

results. The apparent volatile yield appears to be higher in CO2

due to mass loss

during the char‐CO2


Depending on the coal type, there are also significant differences in the

characteristics of char formed in N2

and CO2

atmospheres. Some coals exhibit

greater swelling in a CO2

atmosphere.

Higher char reactivity in O2

/CO2

conditions, especially at high temperatures and

low O2

levels, is attributed to the char‐CO2


Reactivity parameters for char combustion need to be estimated separately for oxy‐

fuel conditions including the char‐CO2

gasification reaction and other relevant

differences need to be accounted for when modelling pulverised coal combustion

in O2

/CO2

conditions.

Coal Behaviour Conclusions

Emissions

PM, SOx, NOx, Hg Concentration in the flue gas and in furnace

Particulates

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Dust Air, g/Nm3

Dus

t O

xy, g

/Nm

3

Coal ACoal BCoal C

0

200

400

600

800

1000

0 200 400 600 800 1000

Gas Flow Air, Nm3/h

Gas

Flo

w O

xy, N

m3/

h

Coal ACoal BCoal C

Lower gas flow due out of Furnace (before recycle) due to higher O2% at burner

Dust concentration higher due to lower gas flow

Higher concentration of dust & longer residence time for flue gas/particulates

NOx Concentration & Emission – 1MWt

Coal ACoal BCoal C

Coal ACoal BCoal C

Wall, T., et al., An overview on oxyfuel coal combustion--State of the art research and technology development. Chemical Engineering Research and

Design, 2009. 87(8): p. 1003-1016.

NOx emission much lower due to “reburning effect” in furnace and lack of N2

NOx concentration higher due to lower gas flow & recycled flue gas

SO2 Concentration & Emission – 1MWt

Coal A

Coal BCoal C

Coal A

Coal BCoal C


Design, 2009. 87(8): p. 1003-1016.

SO2 concentration higher due to lower gas flow & recycled flue gas

SO2 emission lower due to enhanced sulphur retention in fly ash & possible SO3 deposition

SO3 Concentration & Emission- 1MWt


Design, 2009. 87(8): p. 1003-1016.

Coal ACoal BCoal C

Coal A

Coal BCoal C

SO3 concentration higher due to lower gas flow & recycled flue gas

SO3 emission lower due to enhanced sulphur retention in fly ash & possible SO3 deposition

SO3 Pilot Comparison

0

10

20

30

40

50

60

70

0 500 1000 1500 2000

SO2, ppm

SO3,

ppm

IHI/Callide Coal A-Air

IHI/Callide Coal A-Oxy

IHI/Callide Coal B-Air

IHI/Callide Coal B-Oxy

IHI/Callide Coal C-Air

IHI/Callide Coal C-Oxy

ANL-Air [17]

ANL-Oxy [17]

CANMET-Air [13,30]

CANMET-Oxy [13,30]

IVD Stuttgart-Air [31]

IVD Stuttgart-Oxy [31]

Utah-Air

Utah-Oxy

E.on-Air

E.on-Oxy

Alstom-Air

Alstom-Oxy

0

1

2

3

4

5

0 1 2 3 4 5

SO3/SO2 Conversion -Air Firing, %

SO3/

SO2

Con

vers

ion-

Oxy

fuel

Firi

ng, %

Stuttgart

ANL

CANMET

IHI/Callide

Utah

E.on

Alstom

0

20

40

60

80

100

120

140

160

180

200

0 5000 10000 15000 20000

SO2, ppm

SO3,

ppm

Low range SO3 High range SO3

• Uncertainty in SO3 due to difficulty of measurement

• Controlled condensation method with inertial separation of fly ash prevents gas-solid interactions

• Results are approaching consensus, but understanding?

SO3 Behaviour

Eddings, Ahn, Okerlund, Fry,

IEA SO2/SO3/Hg Workshop 2011

1.5MW PC Firing

Marier and Dibbs, Thermochimica Acta, 1974.

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500

Temperature,oC

Equi

libriu

m

SO2H2SO4

SO3

TEMPERATURE• Cooling rate

FLY ASH CATALYSIS & CAPTURE

• Fly Ash species + dust load

• Residence time

EQUILIBRIUM• Gas Species

Hg in oxyfuel

• Becoming more important due to downstream corrosion of aluminium heat exchangers in inerts (O2, N2, Ar) separation in cold box

• Difficult to measure due to low concentration and manual methods (similar to SO3)

• Removal depends on species of Hg

– Hg0 (elemental Hg) not water soluble, but absorbed on unburned carbon in fly ash (HgP), potentially captured as Hg(NO3 )2 (aq) in compression

– Hg2+ (ionic Hg, mainly HgCl2 ) is water soluble

• Hg tends to coat walls and piping, taking a long time to reach equilibrium due to low concentrations

• Measurements in large pilot systems may never reach this point

Hg0 (g)

HgCl2(g)Halogenation

Cl/HCl/Cl2NO/NO2

SO2/SO3 Hg0 Sorption

Postcombustion

CatalyticOxidation Hg2+ X(g) Species

Hg(p) Species

Hg(NO3)2HgO

HgCl2HgOHgSO4HgSHgSe

Combustion

Vaporization

Ash Formation andParticle Growth

AmalgamAuHgAgHg

FuelHgSpecies

HgBr2(g)HgF2(g)

Hg0 (g)

HgCl2(g)Halogenation

Cl/HCl/Cl2NO/NO2

SO2/SO3 Hg0 Sorption

Postcombustion

CatalyticOxidation Hg2+ X(g) Species

Hg(p) Species

Hg(NO3)2HgO

HgCl2HgOHgSO4HgSHgSe

Combustion

Vaporization

Ash Formation andParticle Growth

AmalgamAuHgAgHg

FuelHgSpecies

HgBr2(g)HgF2(g)

Pavlish, J. Understanding the fate of Hg during oxyfuel combustion. IEA Workshop on

SO2/SO3/Hg/Corrosion in Oxyfuel. 2011.

Popular Science website

Hg in oxyfuel - results

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

CFB Air

CFB Oxy

L1500 Air

L1500 Oxy

Carbon

in Ash

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

CFB Air

CFB Oxy

L1500 Air

L1500 Oxy

HgP

Hg2+

Hg0

HgTG

Fry, A., et al. Mercury Speciation & Emission from Pilot Scale PC Furnaces under Air & Oxyfired Conditions. IEA Workshop on

SO2/SO3/Hg/Corrosion in Oxyfuel. 2011

• Little differences observed in Hg speciation between Air and Oxy firing

• Mainly attributed to differences in firing systems

• Further trials necessary before conclusions made

Impact of Emissions/Impurities on

CO2 processing

PM, SOx, NOx, Inerts, Hg

Li, H., J. Yan, et al. (2009)

Impurity impacts on the oxyfuel process(A generic diagram)

CRR=92.15%

Presenter

Presentation Notes

Sweden, Royal Institute of Technology, Malardalen Uni, Vattenfall R&D

Li, H., J. Yan, et al. (2009)

Impurity impacts on the oxyfuel process-Particulates

Natural SO2 capture by

CaO & MgO fly ash species

Natural Hg capture by

unburned carbon in fly ash

Fly Ash removed prior to recycle to avoid excessive fouling

Particulates can damage compressors

Fabric Filter/ESP

Absorbed SO3 can improve ESP performance

Presenter

Presentation Notes


Li, H., J. Yan, et al. (2009)

Impurity impacts on the oxyfuel process-SOx

SO3 absorbs on active Hg sites in activated carbon

reducing efficacy

SO3 forms H2SO4 which

condenses <160 C in colder

sections (eg recycle, mill)

If not removed SO2 would report with CO2 product,

corroding transport materials and acidifying storage media

Furnace corrosion enhanced

by high SO2/H2S

FGD placement before/after recycle depends on furnace corrosion (fuel-S<1%)

FGD & FGC

Remaining SOx removed as H2SO4 in corrosive

condensate

Presenter

Presentation Notes


Li, H., J. Yan, et al. (2009)

Impurity impacts on the oxyfuel process-NOx

SCR catalyst may enhance SO3 formation

If not removed NO2 would report with CO2 product,

corroding transport materials and acidifying storage media

Direct O2 injection can lower NOx

NOx recycled to furnace is “re-burned” and reduced to N2 in furnace

SCR

NO2 forms at higher pressure and oxidises

SO2 to H2SO4

Remaining NOx removed as HNO3 in corrosive

condensateRemaining NO would be vented as a stack

gas

Presenter

Presentation Notes


Li, H., J. Yan, et al. (2009)

Impurity impacts on the oxyfuel process-Inerts

N2 & Ar IN

• O2 purity

• Air ingress

N2 & Ar OUT

Dilutes CO2 + affects CO2 VLE & capture rate

Reduces storage media capacity

O2 in CO2 product can corrode pipelines &

support biological growth, lowering injection rates

Presenter

Presentation Notes


Li, H., J. Yan, et al. (2009)

Impurity impacts on the oxyfuel process-Hg

Hg can corrode brazed aluminium HEX used in the

“cold box”

during CO2 distillation

Activated carbon “guard beds”

are affected by SO3 absorption at low pressure

and may be explosive under high pressure with O2

Experience with Hg comes from natural gas industry where high pressure activated carbon beds are commonly used

without O2

Hg is expected to be removed as Hg(NO3)2 with

NOx

condensates

Presenter

Presentation Notes


Impacts Summary

• Particulates

– Can capture extra SOx and Hg in fly ash – Can increase the load on ESP x– Can be abrasive to compressor if not removed x

• SOx

– Can be corrosive in coal mill, boiler and convective pass– Can be corrosive if condensed as H2SO4 (in recycle lines & compression)– Higher acid dew point in oxy-fuel (

SO2,

SO3,

H2O)– Higher SO3 on fly ash can enhance ESP

• H2O

– Involved in corrosion (H2SO4, HNO3)– Can form solid “ice-like” hydrates with compressed CO2, causing blockages

Impacts Summary

• NOx

– Less of a problem as lower emission than with air firing (re-burning effect)– Environmental limit of NO in stack (as ppm and/or mg/MJ)– Could form HNO3 in compression (corrosive to plant, pipeline)

• Hg

– Hazardous to health, emissions becoming limited by regulation– Can be extremely corrosive to cryogenic aluminium heat exchangers (for

separation of inerts O2/N2)

• Inerts

– Lowers CO2 purity & reduces pipeline/storage capacity (affects economics)– O2 corrosive to pipeline, and some storage media– O2 supports biological growth, – Affects compression energy

Additional research required

• Heat transfer and combustion

• Hot spot evaluation, and burner impacts (cfd based)• Validate emissivity modelling at large scale

• Gas quality and furnace

• Sulphur gases and corrosion • Fly ash-SOx interactions (coal specific?)• NOx/SOx interaction at higher pressure• Mercury levels and form, and impact/control in CO2 handling

Gas quality for transport, compression and storage

• Plant impacts, regulation and safety issues• Pipeline corrosion and effect in storage media

Thank you for your attention!

lecture 3: the science of oxy-fuel - university of newcastle · lecture 3: the science of oxy-fuel...

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