Burners and Firing Systems for Oxyfuel Combustion

Oxyfuel Combustion Capacity Building Course

Wuhan, China, 27th October 2015

Introduction

Burners and Firing Systems for Oxyfuel Combustion

Presentation covers (briefly)….

• Underlying principles that underpin low NOx burner design

• Differences between air and oxyfuel firing, and how they affect burner design

• Key points about milling plant

• Burner testing, and how operating parameters affect performance

• Transition between air and oxyfuel firing

• Thermal performance, and how the burner affects this in a test facility

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Principles of Burner Operation

Modern coal burners are

aerodynamically stabilised and

minimise NOx by air staging

• Swirling flow creates an internal recirculation zone (IRZ), drawing hot flue gas back to the burner inlet.

• PF is injected into the IRZ where it is rapidly heated and devolatilises.

• Interaction of PF injection and IRZ is important to achieve stable flame.

• Volatiles burn, consume PA, and release heat in the near-burner region.

• Char particles spread out to periphery of the IRZ.

• SA, then TA gradually mixes in with IRZ, and supports combustion of char particles.

• Radiant heat from adjacent flames further supports combustion.

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Burners typically operate under air staged conditions with λ between 0.8 and

1.0; the remaining combustion air is supplied via overfire air (OFA) ports

Principles of Burner Operation

Doosan Babcock Source: Pictures from Hupa et al. IFRF Today, FFRC

Liekkipäivä, Åbo Akademi, Turku, 24th January 2006

Swirl has a major impact

on flame shape

• Type 0: Long jet flame, minimal secondary swirl

• Type 1: Combination of type 0 & 2 flame, low swirl where fuel jet penetrates through IRZ

• Type 2: Short intense flame with closed IRZ, moderate to high swirl, used for wall-fired boilers

• Type 3: longer but still intense flame with a second IRZ downstream, very high swirl

Gas

Gas

Gas

Oil

Propane

Coal

Coal

Differences Between Air and Oxyfuel Firing

In oxyfuel firing N2 is removed from the combustion process and replaced by recycled flue gas

• Flue gas typically contains >75% CO2 (dry basis)

• CO2 properties differ from N2 (see below)

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• Increased density leads to

lower velocities in the burner

• Increased heat capacity

means that temperature is

reduced for a given energy

input

• Reduced thermal and mass

diffusivity has a negative

impact on flame propagation Source: Wall & Yu, APP OFWG capacity building course,

Daejeon, Korea, 5th~6th February 2009

Oxyfuel Burner Design - Aerodynamics

Gas properties impact on burner aerodynamics

• Velocities in the coal pulveriser and PF transport lines must be kept similar to air firing to keep the coal particles in suspension

• Mass flow of primary stream is increased due to higher density of CO2

compared to N2

• Mass flow of secondary stream (windbox) is reduced; combined with the higher density of CO2 the velocity of the secondary stream is considerably lower

• Momentum ratio of Primary to Secondary stream increased

• More forward momentum detrimental to flame stability • Flow through swirl generators reduced

• Less swirling energy reduces strength of IRZ; detrimental to stability • It is therefore important to supply sufficient flow to the windbox to ensure

that the burner aerodynamics are able to deliver a stable flame structure (strong recirculation zone, primary jet momentum low enough to prevent it “punching through” the IRZ) • e.g. reducing recycle leads to increased flame length as relative

primary momentum increases and swirl is reduced, eventually leading to the flame being blown off

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Source: Black, CFD Modelling and Scaling of a 40MW Burner for Oxyfuel Combustion,

18th IFRF Members Conference, Freising, Germany 1st~3rd June 2015

Oxyfuel Burner Design - Ignition

Gas properties affect on coal ignition

• Higher specific heat and lower thermal diffusivity has a negative impact on flame propagation

• Higher specific heat leads to lower temperatures

• Replacing N2 with CO2 makes it more difficult to ignite coal (MCIT is higher for 21% O2 in CO2 than 21% O2 in N2) • Safe operation of milling plant is assured

by limiting O2 to 21% in oxyfuel firing, but the penalty is more difficult early ignition at the burner

• Increased O2 in the secondary stream compensates to support later ignition when secondary stream mixes with primary

• Delayed ignition and potential flame stand-off from the burner is possible for oxyfuel firing • Attention must be paid to flame stability

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Source: Verplaetsen, Assessing Explosion Risk in Oxy-Coal Combustion Systems,

18th IFRF Members Conference, Freising, Germany 1st~3rd June 2015

Minimum Cloud Ignition Temperature (MCIT)

Oxyfuel Burner Design – Emissions (NOx)

Gas properties affect devolatilisation

• Higher specific heat leads to lower temperatures and longer time to heat particles • This tends to reduce the volatiles released from the

coal • Presence of high concentrations of CO2 and H2O leads to

greater significance of “gasification” reactions during devolatilisation at higher temperatures • This has the potential to increase the volatile yield

from the coal at a given temperature

• Low NOx burners work by rapidly releasing volatiles in a

region of reduced stoichiometry in the flame

• Reduced volatile yield and delayed volatile release lead to increased NOx emission

• However….. FGR recycles NOx to the burner where it can react with CHi from the volatiles (reburn reactions) to reduce NOx Lower temperature and N2 concentration leads to lower thermal NOx

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Source: Al-Makhadmeh, Maier, Scheffknecht, Coal pyrolysis

and char combustion under oxy-fuel conditions, 34th

International Technical Conference on Coal Utilization & Fuel

Systems, Clearwater, 2009

Oxyfuel Boiler Design - The Role of Flue Gas Recycle (FGR)

Recycle flue gas flow rate can be used to vary radiant and convective heat transfer

• Increased recycle flow leads to: • Greater mass per unit heat input → lower adiabatic flame temperature; less radiant heat transfer • Greater mass flow through boiler → higher gas velocity and more convective heat transfer

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Source: Woycenko et al, Combustion of Pulverised Coal in a Mixture of Oxygen and

Recycled Flue Gas, IFRF Report F98/y/1, 1994

Oxyfuel Burner Design – The Role of Flue Gas Recycle (FGR)

FGR is a key operational parameter

• Flue gas recycle rate specified such that the required boiler thermal performance is achieved, but has many impacts on burner performance…..

• Increased FGR leads to: • Reduced oxygen partial pressure • Reduced adiabatic flame temperature • Reduced residence time

• Increased swirl • Increased secondary stream momentum

• Even “simple” operating parameters have both

positive and negative impacts on burner

performance

• Interactions are complex

• Modelling and large scale testing are required

to establish oxyfuel burner performance

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Reduced combustion efficiency

Delayed ignition

Reduced NOx or Increased NOx?

Stronger IRZ

Doosan’s Clean Combustion Test Facility (CCTF), Renfrew, Scotland

Doosan tested their oxyfuel burner in

2009 ~ 2010

• 1 x 40MWt burner • Horizontally fired • Pre-dried bituminous coal • Refractory lined furnace with water jacket • Heat recovery boiler

• Demonstration of full-scale burner

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Vattenfall’s Oxyfuel Pilot Plant (OxPP), Schwarze Pumpe, Germany

Doosan tested their oxyfuel burner at

Schwarze Pumpe over the period 2011~2012

• 1 x 30MWt burner • Down fired • Pre-dried lignite • Radiant natural circulation furnace • Convective superheater • Convective economiser • Spray attemporation

• Demonstration of near full-scale burner

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CIUDEN’s es.CO2 Pilot Plant, Ponferrada, Spain

CIUDEN undertook testing in 2014 & 2015

• 4 x 5MWt burners (similar design to Doosan Mk3) • Opposed wall fired • Pre-dried bituminous coal • Radiant natural circulation furnace • Radiant + Convective superheaters • Convective economiser • Spray attemporation

• Investigation of burner interaction • Investigation of furnace heat transfer in a realistic

arrangement

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Effect of FGR

FGR affects flame shape and stand-off….

……though the range of FGR tested in CCTF was quite narrow

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Wider flame

Flame root less well attached

Effect of FGR

Increased FGR reduces NOx

• Baseline (air firing) NOx at the es.CO2 facility is higher than at the OxPP plant • Higher burner zone stoichiometry • Coal has lower volatile content

• Comparable NOx emissions from each facility

for oxyfuel, despite differences in the coal

• Greater impact of FGR at the es.CO2 facility • Operational mode is slightly different • At the OxPP facility the fuel is supplied via

a dense phase system with no added O2; this mixes with primary comburant; the primary comburant and windbox comburant have the same O2 concentration

• At the es.CO2 facility the primary comburant has fixed O2 concentration, and is independently controlled from the windbox comburant

• Demonstrates that recycling NOx into the fuel rich part of the flame is effective in minimising NOx formation (via “reburn” reactions)

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Transition from Air to Oxyfuel Firing

Burner operating conditions are especially important during transient operation

• First transition from air to oxyfuel firing under automatic control when testing Doosan’s burner at Vattenfall’s OxPP test facility • Control logic as-found by Doosan on arrival

• Exceptionally bright flame and excessively high flame temperatures observed during transition

• Post-mortem established that • Air flow is reduced, oxygen flow is increased • However FGR flow is not increased during oxygen flow increase, but later in the

transition – low FGR flow leads to increased temperature • There is a period when the burner is operated with close to pure oxygen; the

flame was observed to be “white hot” • We do not understand why this control logic was applied for the transition • The transition, in our opinion, was not safe

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Transition from Air to Oxyfuel Firing

Successful implementation of automated control logic by Doosan

• Doosan developed a revised control logic based on our previous testing experience of a 40MWt oxyfuel burner in Renfrew and applied it to the OxPP

• Control logic modified to increase FGR flow ahead of increase in oxygen flow • Outcome was a stable automated transition over a 20 minute period, with a stable rooted

flame at all times, and acceptable flame temperature

• Safe and repeatable

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Heat Transfer - Furnace

Furnace effectiveness appears to be higher for air firing

• Furnace effectiveness falls with time due to deposition • FEGT and spray flow increase

• Furnace effectiveness increases following sootblowing • FEGT reduces • Spray flow reduces, then increases when superheater is sootblown

• Furnace effectiveness is higher for air firing than oxyfuel firing at a comparable time following sootblowing • Rate of change is the same for air and oxyfuel firing, implying similar deposition rates

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Heat Transfer - Furnace

The higher furnace effectiveness for the air firing

test is probably due to the flame, and the FGR

rate used in the oxyfuel test

Causes of increased furnace performance include….. • Cleaner furnace

• Flame temperature

• Air firing has appreciably higher flame temperature (~200°C), leading to increased radiation to furnace walls

• Flame luminosity

• Air firing has a brighter flame (note reduced exposure time for air firing photo!), also leading to increased radiation to furnace walls

• The lower flame temperature and luminosity for oxyfuel firing was due to operation at high FGR rate, which dilutes the heat input • Also note slightly wider flame at flame root

due to increased swirling energy at high FGR

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Oxyfuel Firing

Air Firing

Conclusions

Understanding the principles that underpin the design of pulverised coal burners and milling

plant allows us to extend the design to oxyfuel firing

• Increased density of CO2 vs. N2 leads to lower velocities • Primary stream flow has to be increased to maintain velocities in milling plant • Secondary stream flow reduces as a consequence • Burner process design has to be modified for oxyfuel firing

• It is more difficult to ignite a coal particle in CO2 than N2

• Operation of milling plant is inherently safer for oxyfuel firing if O2 is limited to 21% • There may be an increased tendency for the flame to stand-off the burner due to delayed

ignition

• FGR is a key operational parameter – it affects both the burner (flame shape, flame stand-off, NOx emission, CIA & combustion efficiency) and the boiler (heat transfer performance)

• All the major OEM hardware suppliers have successfully developed oxyfuel burners and oxyfuel

firing systems • We are ready to build oxyfuel fired plant

Lessons Learned on Oxyfuel Technology Page 20

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