Mercury Removal in a Wet FGD - mcilvainecompany.com · • the material balance around the FGD and...
Transcript of Mercury Removal in a Wet FGD - mcilvainecompany.com · • the material balance around the FGD and...
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Mercury Removal in a Wet FGD
Joe Stuart, TDC, A Genesis Energy Company
April 18, 2013
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Mercury Removal in a Wet FGD
Apply the Scientific Method
• Problem statement
• Two competing hypotheses
-- Suppress mercury re-emission.
-- Satisfy the material balance.
• Testing the hypotheses in a trial
• Explanation of observations
• Revisit the competing hypotheses
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Problem Statement
How to Achieve Reliable Removal of Mercury in a Wet FGD?
Finlet
Foutlet
Slurry Volume (V)
Fpurge
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Problem: How to Achieve Reliable Removal of Mercury in a Wet FGD?
Hypothesis A –
Suppress Mercury Re-emission.
Hypothesis B –
Satisfy the Material Balance.
Mercury is distributed into the streams leaving a wet FGD as determined by
• the material balance around the FGD and
• the physical properties of mercury.
Use existing, accepted engineering principals to adjust the distribution of mercury between the streams leaving the FGD.
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• It is known that oxidized mercury is soluble in water and therefore can be removed from the flue gas by the presence of a w-FGD.
• However, for certain w-FGDs, when oxidized mercury enters the scrubber it is reduced to elemental mercury which is not soluble, resulting in lower mercury capture efficiency and increased stack emissions.
• This phenomenon has been coined Mercury Re-emission and is defined as an increase in elemental mercury across the w-FGD scrubber.
• Suppression of mercury re-emission in the w-FGD offers a low cost solution to reducing mercury emissions.
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The Mercury Re-Emission Model
Gas Phase
Furnace / SCR
Liquid Phase
Hg0
Hg+2
Coal
Solid
Hg0
vapor
oxidation
Removed From System
Gas-Liquid Interface
liquid
Hg+2
aqueous
Hg+2
vapor
Hg+2
aqueous
purge
spontaneous
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reduction
suppression reagent
B&W / TDC
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Hg+2
aqueous
The Mercury Absorption Model
Hg0
vapor
Gas Phase
Furnace / SCR
Liquid Phase
Hg0
Hg+2
Coal
Solid
oxidation
reduction oxidation
Hg+2
vapor
Removed From System Hg
+2
aqueous
purge
Gas-Liquid Interface
Hg0
liquid
absorption
desorption
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Finlet
Foutlet
Slurry Volume (V)
Fpurge
Absorbing Mercury In a WFGD
An absorber is
• a special case of distillation technology
• where components in the gas phase are preferentially transferred to the liquid phase as allowed by
• thermodynamic driving forces (concentration gradients) and
• mass transfer capacity (contact area).
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A Trial to Test Our Hypotheses • Two weeks of testing was done in March 2013 at Michigan South Central Power
Agency’s Endicott Power Station in Litchfield , MI.
• MSCPA was seeking a plan to achieve reliable mercury MATS compliance – less than 1.2 lb Hg emissions / TBTU heat input on a 30-day average – at minimum cost.
• Endicott is a 55 MW unit burning eastern bituminous coal. The unit is equipped with low NOx burners, over-fire air for nitrogen oxide controls, an electrostatic precipitator for particulate control, and wet flue gas desulfurization for sulfur dioxide control.
• Coal is sourced from the East Fairfield Mine located in North Lima, Ohio.
• Continuous mercury monitors on both the scrubber inlet and outlet were provided by Ohio Lumex.
• Data was collected at one-minute intervals.
• NaHS solution was injected into the suction of the scrubber recycle pumps as indicated to precipitate mercury as a solid compound.
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Trial Results
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Trial Results
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Trial Results
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Trial Results
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Trial Results
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Trial Results
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Trial Results
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Trial Results
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Explanation of Observations • The mercury mass balance and physical properties appear to explain the
results well.
• The first-order problem appears to be accumulation of mercury in the slurry liquid.
– If saturated with both Hg0 and Hg+2 then Hg(out) = Hg(in).
– If saturated with Hg0 but not Hg+2 ,then Hg0 (out) > Hg0 (in).
– If sub-saturated in Hg0 , then Hg(out) < Hg(in).
• Occurrence of the first-order problem appears dependent on the recent history of the system.
• One could expect that satisfying the steady-state material balance for mercury would reliably avoid the first-order problem.
• Mass-transfer limitations in absorbing Hg0 can explain the observed second-order phenomena.
• Options exist for readily overcoming the mass-transfer limits by modest increases into the fraction of oxidized mercury in the inlet gas.
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Revisit the Hypotheses in Light of Our Scientific Testing. Apply Occam’s Razor to Make Your Own Decision
Hypothesis A –
Suppress Mercury Re-emission.
• It is known that oxidized mercury is soluble in water and therefore can be removed from the flue gas by the presence of a w-FGD.
• However, for certain w-FGDs, when oxidized mercury enters the scrubber it is reduced to elemental mercury which is not soluble, resulting in lower mercury capture efficiency and increased stack emissions.
• This phenomenon has been coined Mercury Re-emission and is defined as an increase in elemental mercury across the w-FGD scrubber.
• Suppression of mercury re-emission in the w-FGD offers a low cost solution to reducing mercury emissions.
Hypothesis B –
Satisfy the Material Balance.
Mercury is distributed into the streams leaving a wet FGD as determined by
• the material balance around the FGD and
• the physical properties of mercury.
Use existing, accepted engineering principals to adjust the distribution of mercury between the streams leaving the FGD.
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