Wet Flue Gas Desulphurisation FGD at Oxyfuel conditions

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Wet Flue Gas Desulphurisation (FGD) at Oxyfuel
conditions

• Brian B. Hansen Søren Kiil Jan E. Johnsson
• CHEC Annual Day, DTU, October 1st, 2009

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Wet Flue Gas Desulphurisation

• The wet FGD process
• CaCO3(s) SO2(g) 2H2O ½O2 ? CaSO4?2H2O(s)
  CO2
• Major reactions (Kiil, 1998)
• ? Absorption of SO2
• ? Oxidation of HSO3- to SO42-
• ? Limestone dissolution
• ? Gypsum crystallisation
• Important parameters
• ? Desulphurisation degree
• ? Gypsum moisture content
• ? Gypsum impurity content (limestone etc.)

Courtesy of DONG Energy
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The oxy-fuel concept
Oxy-fuel combustion ? Substitution of fuel air
with oxygen ? Flue gas recycle in order to
control temperature/convective heat transfer ?
Gas phase enriched in CO2 H2O ? Capture and
compression of CO2 Flue gas desulphurization ?
Ensuring a clean product i.e. transport and
geological storage considerations ? Wet FGD
before compression (traditional) ? As part of
the compression (research stage)
Courtesy of Vattenfall
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Project objective

• ? To experimentally investigate and simulate the
  changes in
• wet FGD plant operation facilitated by oxyfuel
  conditions
• - CO2 atmosphere
• - Higher saturation temperature
• - Potentially higher SO2 concentration
• - Potentially reduced flue gas flow rate

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Process changes

• ? CO2 atmosphere
• - CO2 absorption (absorber) and release (holding
  tank)
• - Increased HCO3- concentration (wt CaCO3)
• CaCO3 2H ? Ca2 HCO3- H ? Ca2
  H2O CO2
• - Increased limestone availability in absorber
  (?)
• ? Higher saturation temperature
• - Decreased gas solubility (?)
• - Increased diffusion coefficients (wt CaCO3)
• ? SO2 concentration/flue gas flow rate
• - Prolonged gas phase residence time (?)
• - Increased importance of liquid phase
  resistance (?)

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Pilot plant outline

• Overview
• ? Flue gas Gas burner/cylinders
• ? SO2 added to flue gas stream
• ? SO2 removed in falling film column
• ? Slurry withdrawn continuously
• (no hydrocyclones)
• Model describing pilot plant
• ? Steady state model
• - Plug flow (absorber)
• - Well-mixed tank (holding tank)
• - 4 rate determining steps
• - Verified against experimental data

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Experimental methodology

• Experimental procedure
• ? Steady state
• - 5 days desulphurisation
• - 1000 ppm SO2 flue gas (natural gas burner)
• ? Oxy-fuel experiments
• - Until a steady limestone consumption rate
• - 1000 ppm SO2 flue gas (gas cylinders)
• Comments
• ? Water evaporation in the top of column
• - Contributes to flue gas flow rate
• ? Flue gas flow rate measurement
• - Gas property dependent

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Experimental Overview

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Experimental Results (1/2)
CaCO3 2H ? Ca2
HCO3- H ? Ca2 H2O CO2 ? An increased
content of residual limestone - Increased
bicarbonate concentration (lower dissolution
rate) ? An increased desulphurization degree
- Higher absorber pH (residual limestone and
carbonate buffer) ? A minor decrease of
operating pH compensate the changes facilitated
by oxy-fuel conditions
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Experimental Results (2/2)
? Oxy-fuel An increased
absorber pH desulphurization degree
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Conclusions and future work

• Desulphurisation of an Oxyfuel flue gas stream
• ? An increased degree of desulphurisation (?)
• - Higher absorber pH HCO3- buffer and
  increased limestone dissolution
• ? An increased content of residual limestone
• - Higher HCO3- concentration in holding tank
• ? A slightly lower slurry pH counters the
  effects
• Future work
• ? Limestone PSD
• ? Evaluation of experimental results
• ? Evaluation of model assumptions concerning CO2
  content of gas phase
• ? Simulations

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Tak for opmærksomheden/ Thanks for the Attention !