Hydrogen Sulfide in VRLA Cells

© Philadelphia Scientific 2001 Philadelphia Scientific

Hydrogen Sulfide in VRLA Cells

Harold A. Vanasse

Frank J. Vaccaro

Volen R. Nikolov

INTELEC 2001


Presentation Outline

• H2S is produced in VRLA Cells.

• H2S is absorbed on the lead dioxide of the positive plate.

• Resultant H2S equilibrium concentration is less than 1 ppm.


Background

• H2S is known in:– Flooded cells.– VRLA cells in thermal runaway.

• Measured in our lab in Negative Active Material testing (Intelec 2000).

• Supported in literature:– Reduction reaction.– MeS + 2H+ = H2S + Me2+


Finding H2S

• Goals: – Prove that H2S could be produced at

normal float voltages and temperatures.– Identify sources.

• Early testing eliminated many candidates as main factors.

• Reaction between pure lead and acid became our focus.


Test Rig

• Test run at 40ºC.• 1.300 specific gravity

acid.• Test run at a variety of

voltages.• Three identical test rigs

used.


Results

• H2S Concentration independent of voltage.

• Results repeated over multiple tests.

0

100

200

300

400

500

600

2.25 2.35 2.45 2.55 2.65 2.75

Cell voltage (V)

H2S

con

cent

ratio

n (p

pm)


Another Surprise

• H2S concentration declines over time or repeated rounds of testing.

• Results repeated over multiple tests.

0

100

200

300

400

500

600

1 2 3 4

Test round #

H2S

me

asu

red

in t

he

co

llect

ed

gas

(p

pm

)


Interim Findings

• Sulfuric acid + charged negative plate = H2S.

• Liberation of H2S is not voltage dependent.

• H2S concentration high at first, but decreases over time.

• If this were the case, we would smell rotten eggs around new VRLA cells.


H2S is removed by Positive Plate

• Lead dioxide reactions predict absorption:– PbO + H2S = PbS + H2O

– 4PbO2 + H2S = PbSO4 + 3PbO + H2O

– 4PbO2 + 3H2SO4 + H2S = 4PbSO4 + 4H2O

• Two experiments lead to proof.


Experiment 1: Reactor Test

Material to be tested

Reactor

H2 +

H2S

10

0 p

pm

GC

H2 Gas with 100 ppm of

H2S


Experiment 1: Reactor TestResults

• Input: 108 ppm H2S in H2 @ 50 ml/min.

• Output: Connected to GC.

• Measurements taken every 15 minutes.

Test Material

Amount (grams)

Breakthrough Time

(minutes)

Empty 0.0 0.01

PbO 2.2 120

PbO2 2.0 360


Experiment 2: H2S Through a VRLA

Cell

H

2 +

H2S

10

0 p

pm

H2 Gas with 100 ppm of

H2S

VRLA cell

GC

2.27V


Experiment 2: Results

• H2S clearly being removed in the cell.

• Output H2S significantly lower than input concentration.

0

20

40

60

80

100

120

0 5 10 15 20 25 30

Elapsed Time (hours)

H2S

co

nce

ntr

atio

n (

pp

m) Inlet Concentration

Outlet Concentration

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25 30

Elapsed Time (hours)

Gas

flo

wra

te (

ml/

min

)

Gas Flowrate through the VRLA Cell


H2S Interactions

• H2S Generated at the Negative Plate.

• H2S Absorbed or Oxidized at the Positive Plate.

• Follow on Questions:– Which process is dominant?

– What H2S equilibrium concentration level is established?


GC Analysis of VRLA Cells

• Multiple cells from multiple manufacturers sampled weekly for H2S.

• All cells on float service at 2.27 VPC at either 25°C or 32° C.

• Age of cells: New to 6 years old.


Results of GC Sampling

• H2S concentration: 0 ppm < 1 ppm, but always less than 1 ppm.

• Found across all cells tested.

• Analytical proof of the presence of H2S in VRLA cells.

• Maximum equilibrium threshold established for float conditions.


Conclusions

• H2S can be produced by VRLA cells through the reduction of sulfur-containing compounds.

• H2S can be absorbed within a VRLA cell by the positive plate active material.

• In cells on float, H2S concentration levels are less than 1 ppm.


Impact

• H2S is a poisonous gas that corrodes metal.

• H2S can poison precious metal catalysts.

• We have built a filter into our latest catalyst design to protect against H2S.

Hydrogen Sulfide in VRLA Cells

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