Oxy-fuel Combustion in Coal-fired Power Plants

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Oxy-fuel Combustion in Coal-fired Power Plants

• Sara Jones
• 24NOV08

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Background

• In most conventional combustion processes, air is
  used as the source of oxygen
• Nitrogen is not necessary for combustion and
  causes problems by reacting with oxygen at
  combustion temperature
• A high concentration of nitrogen in the flue gas
  can make CO2 capture unattractive
• With the current push for CO2 sequestration to
  ease global warming, it is imperative to develop
  cost-effective processes that enable CO2 capture
• The use of pure oxygen in the combustion process
  instead of air eliminates the presence of
  nitrogen in the flue gas, but combustion with
  pure oxygen results in very high temperatures

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Background

• In 1982 oxy-fuel combustion was proposed to
  produce CO2 for Enhanced Oil Recovery
• Recycling of hot flue gas has also been suggested
  to reduce furnace size and NOx emissions for
  metal heating furnaces
• Lately, interest has been paid to oxy-fuel
  combustion as a means to reduce pollutant
  emission control cost and create a CO2 gas stream
  that can easily be compressed and sequestered

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Process Variations

• Is the plant to be retrofitted or purpose-built?
• What is the optimum O2 proportion in the oxidant
  gas?
• What is the desired proportion of CO2 in the flue
  gas?
• To what extent will the flue gas be cleaned of
  NOx, SOx, and Hg?
• Will CO2 be fully/partially sequestered?

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Oxy-fuel combustion flow sheet

• Oxygen at greater than 95 purity and recycled
  flue gas are used for fuel combustion, producing
  a gas that is mainly CO2 and water
• Recycled flue gas is also used to control the
  flame temperature and replace the volume of the
  missing nitrogen needed to carry heat through the
  boiler

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Differences from Replacing N2

• To have a similar adiabatic flame temperature,
  oxygen must have a concentration of about 30
• For an oxygen concentration of 30, 60 of the
  flue gases are recycled
• Because of high concentrations of carbon dioxide
  and water, the furnace gas has a higher
  emissivity
• Flue gas volume after recycling is 80 smaller
  than conventional combustion, and its density is
  increased
• 3-5 excess of oxygen is required
• Species present in flue gases are in higher
  concentrations after oxy-fuel combustion
• Power must be provided for flue gas compression
  and air separation

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Differences

• Oxy-fuel combustion with CO2 sequestration
  involves oxygen separation, flue gas recycling,
  CO2 compression, and transport and storage
• A number of modifications to conventional pf coal
  technology must occur
• Running more processes leads to a reduction in
  availability
• The use of carbon sequestration increases capital
  and operating costs

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Oxy-fired PF Power Plant

• O2 is separated from air and mixed with the
  recycle stream from the boiler
• Fuel is fired into this mixed stream, and a
  portion of the flue gases is recycled
• Water vapor in the flue gas is condensed to form
  a stream of supercritical CO2 of high purity, 70
  by mass CO2 as compared to 17 with air-fired
  combustion
• CO2 can then be cooled and compressed for
  transportation and storage

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Design and Operational Issues

• Concentration of tri-atomic flue gas molecules is
  much higher in oxy-fuel combustion - changes the
  emissivity
• CO2 and H2O also have higher thermal capacities
  than nitrogen, which leads to a higher heat
  transfer in the convective section of the boiler
• Amount of gas passing through the boiler is
  lower, and heat transfer is increased in the
  radiative section of the boiler
• Result is lower heat transfer in the convective
  section of the boiler and a lower gas temperature
  at the furnace exit

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Design and Operational Issues

• Effects of flue gas recycling on trace elements
  emissions and fly ash size distribution have not
  been experimentally determined
• Several studies have shown problems with flame
  stability and ignition
• Currently, no studies have been conducted that
  assess the impact of oxy-fuel combustion on
  deposit formation and structure

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Pilot Scale Studies-EERC and ANL

• 10 Million Btu/hr boiler pilot facility
• With wet recycle, an oxygen concentration of
  23.8 in the burners was needed to match the heat
  transfer performance of air-fired combustion.
• For dry recycle, a 27 oxygen concentration was
  needed
• In-furnace gas temperature profile was similar
  with oxy-fuel combustion, but NOx and SOx
  emissions were lower (by 50 for NOx) and the
  carbon burnout was higher
• No operational difficulties were found

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Pilot Scale Studies - IFRF

• 2.5 MW furnace with air-staged swirl burner
• oxy-fuel combustion had radiative and convective
  heat transfer performance, in-flame gas
  composition trends, combustion performance, and
  flame length and stability comparable to
  air-fired combustion
• Optimum ratio for recycled flue gas was found to
  be 0.61
• Maximum flue gas CO2 concentration of 91.4 was
  achieved
• NOx emissions were reduced
• Low NOx burner technology was demonstrated to be
  practicable for oxy-fuel combustion

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Pilot Scale Studies IHI

• 1.2 MW combustion-test furnace with swirl burner
• Injection of pure O2 at the center of the burner
  improves flame stability and decreases the ashs
  unburnt carbon content
• NOx conversion was lower than with conventional
  combustion, but increased as oxygen concentration
  increased
• NOx reduction occurs because of rapid reduction
  of recycled NOx into HCN and NH3
• SOx emissions decreased from condensation of
  sulphates in ducts and adsorption of sulphur in
  ash

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Pilot Scale Studies Air Liquide

• 1.5 MW pilot-scale boiler with air staged
  combustion system
• Smooth transition from air to O2 with good flame
  stability and heat transfer achievable
• Less NOx than conventional below the 0.15
  lb/MMBtu standard for units installed after July
  1997
• Effective removal of SOx with wet FGD equipment
  and significant reduction of Hg emission (50)
• Large reduction in unburnt carbon in fly ash,
  leading to improved boiler efficiency

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Pilot Scale Studies CANMET

• 0.3 MW vertical combustor research facility
• Flue gas CO2 concentration close to theoretical
  (92)
• Equivalent flame temperature at 35 O2. O2
  purity (lt5 N2) had no significant effect on
  flame temperature
• Reduced NOx dependent on O2 concentration,
  flame temperature. Difference decreases
  significantly if 3 N2 present
• Increasing O2 concentration decreases CO
  emission. Decrease in CO concentration along
  flame is slower than conventional because of high
  CO2 concentration
• Experimental results compared to modeling efforts

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Advantages/Disadvantages

• Industry is familiar with this type of technology
• Viable for near-zero emissions
• Can be retrofitted to existing plant as oxy-fuel
  with CO2 liquefaction or direct flue gas
  liquefaction
• Can be implemented in new plant or put into new
  plant design for later retrofit
• Low NOx emissions

• Reduced efficiency
• Not demonstrated at commercial-scale might be
  unforeseen technical difficulties
• SOx removal might be required
• Oxygen separation plant needed

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Future RD

• Heat transfer performance of new and retrofitted
  plants
• Impact of O2 concentration and CO2 recycle ratio
• Assess retrofits for electricity cost and cost of
  CO2 avoided
• Combustion of coal in O2/CO2 ignition,
  burn-out, emissions
• Development of less costly O2 generation

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Conclusion

• Oxy-fuel combustion is technically feasible with
  current technologies
• CO2 in flue gas is relatively pure and is
  sufficient for sequestration
• Potential to reduce pollutant emissions,
  especially NOx