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
Doosan Babcock
Principles of Burner Operation
Modern coal burners are
aerodynamically stabilised and
minimise NOx by air staging
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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.
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
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Principles of Burner Operation
Swirl has a major impact
on flame shape
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Type 0: Long jet flame,
minimal secondary swirl
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Type 1: Combination of
type 0 & 2 flame, low swirl
where fuel jet penetrates
through IRZ
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Type 2: Short intense flame
with closed IRZ, moderate
to high swirl, used for wallfired boilers
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Type 3: longer but still
intense flame with a second
IRZ downstream, very high
swirl
Gas
Propane
Gas
Gas
Oil
Coal
Coal
Source: Pictures from Hupa et al. IFRF Today, FFRC
Liekkipäivä, Åbo Akademi, Turku, 24th January 2006
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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)
• Increased density leads to
lower velocities in the burner
• Increased heat capacity
means that temperature is
reduced for a given energy
input
Source: Wall & Yu, APP OFWG capacity building course,
Daejeon, Korea, 5th~6th February 2009
• Reduced thermal and mass
diffusivity has a negative
impact on flame propagation
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Oxyfuel Burner Design - Aerodynamics
Gas properties impact on burner aerodynamics
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Velocities in the coal pulveriser and PF transport lines must be kept similar
to air firing to keep the coal particles in suspension
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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
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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
Source: Black, CFD Modelling and Scaling of a 40MW Burner for Oxyfuel Combustion,
18th IFRF Members Conference, Freising, Germany 1st~3rd June 2015
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Oxyfuel Burner Design - Ignition
Gas properties affect on coal ignition
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Higher specific heat and lower thermal
diffusivity has a negative impact on flame
propagation
Higher specific heat leads to lower
temperatures
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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
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Delayed ignition and potential flame stand-off
from the burner is possible for oxyfuel firing
• Attention must be paid to flame stability
Minimum Cloud Ignition Temperature (MCIT)
Source: Verplaetsen, Assessing Explosion Risk in Oxy-Coal Combustion Systems,
18th IFRF Members Conference, Freising, Germany 1st~3rd June 2015
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Oxyfuel Burner Design – Emissions (NOx)
Gas properties affect devolatilisation
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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
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Low NOx burners work by rapidly releasing volatiles in a
region of reduced stoichiometry in the flame
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Reduced volatile yield and delayed volatile release lead to
increased NOx emission
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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
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
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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
Source: Woycenko et al, Combustion of Pulverised Coal in a Mixture of Oxygen and
Recycled Flue Gas, IFRF Report F98/y/1, 1994
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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:
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Reduced oxygen partial pressure
Reduced adiabatic flame temperature
Reduced residence time
Reduced combustion efficiency
Delayed ignition
Reduced NOx or Increased NOx?
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Increased swirl
Increased secondary stream momentum
Stronger IRZ
• 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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Doosan’s Clean Combustion Test Facility (CCTF), Renfrew, Scotland
Doosan tested their oxyfuel burner in
2009 ~ 2010
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1 x 40MWt burner
Horizontally fired
Pre-dried bituminous coal
Refractory lined furnace with water jacket
Heat recovery boiler
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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
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1 x 30MWt burner
Down fired
Pre-dried lignite
Radiant natural circulation furnace
Convective superheater
Convective economiser
Spray attemporation
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Demonstration of near full-scale burner
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CIUDEN’s es.CO2 Pilot Plant, Ponferrada, Spain
CIUDEN undertook testing in 2014 & 2015
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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
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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
Flame root less well attached
Wider flame
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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
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Comparable NOx emissions from each facility
for oxyfuel, despite differences in the coal
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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
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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
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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
Air Firing
• Flame luminosity
• Air firing has a brighter flame (note reduced
exposure time for air firing photo!), also
leading to increased radiation to furnace walls
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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
Oxyfuel Firing
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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
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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)
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All the major OEM hardware suppliers have successfully developed oxyfuel burners and oxyfuel
firing systems
• We are ready to build oxyfuel fired plant
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Lessons Learned on Oxyfuel Technology
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