A Hydrogen Economy

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A Hydrogen Economys Potential Environmental
Impacts

• Chun Zhao
• Evan Cobb

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A Hydrogen Economy
www.gii.com.hk
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Hydrogen characteristics in the atm.

• Observed global hydrogen burden 182 Tg
• Global sink 74.4 Tg/yr
• Lifetime 2.5 years Rahn etc. 2003
• Current Mixing Ratio of H2 510ppbv
• Tropospheric hydrogen
• Stratospheric hydrogen

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Hydrogen in troposphere

• The sources and sinks of hydrogen in troposphere

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Note for table of sources and sinks

• Main source of Hydrogen oxidation of organic
  compounds
• Main sink of Hydrogen soil uptake
• Man-made sources fossil fuel combustion
• CO H2O H2 CO2
• Main chemical sink
• OH H2 H H2O
• Debate How is H2 lost?
• What portion of H2 is consumed by soil/microbial
  activity?

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Hydrogen production from oxidation of organic
compounds
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Reduction in OH

• H2 behaves like CO (takes up one OH and releases
  one HO2 radical H20)
• Michael Prather, 2003.

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Reduction in OH by ?NOx
Shultz et al. 2003

• Decrease in OH is largely driven by the reduction
  in NOx emissions
• Importance of HOx/NOx coupling leads to
  non-linear dependence of OH on NOx levels

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Hydrogen in the Stratosphere

• The main hydrogen reservoirs in Stratosphere
• Molecular hydrogen (H2)
• Water vapor (H2O)
• Methane ( CH4)

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H2 Sources in the Stratosphere

• CH4
• OH, O(1D), Cl
• CH3
• O2
  HHO,OH,HCl
• CH3O2
• NO NO2
• CH3O
• O2 HO2
• CH2O
• hv
  OH
• COH2 HCOH
  HCOH2O

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H2 sinks the in stratosphere

• Reaction with OH, O(1D), Cl
• H2 OH H2O H
• H2 O(1D) OH H
• H2 Cl HCl H

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The fate of Hydrogen in stratosphere

• The H2 mixing ratio in the lower and middle
  stratosphere is nearly constant, the net hydrogen
  cycling in the stratosphere can be regarded as a
  loss in methane and a production of water.

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Main reactions of H2 in the stratosphere
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Potential chemical changes in the stratosphere

• H2 OH ? H2O H
• H2Owould result in cooling of the lower
  stratosphere, and the disturbance of ozone
  chemistry, which depends on heterogeneous
  reactions involving hydrochloric acid and
  chlorine nitrate on ices of H2O. Tromp et al.
  2003 pg. 1740

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Environmental Impact Overview

• Tropospheric Effects
• Reduced oxidative capacity of atm. (OH)
• Reductions in NOx, soot, sulfates, CO2, O3
• Increased surface H2 concentrations
• Change in atmosphere-biosphere reactions
• Global warming

• Stratospheric Effects
• Increase of water vapor
• Cooling in lower layers
• Enhanced ozone destruction chemistry
• Increase in noctilucent clouds
• Global warming

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Impacts ALL hinge upon

• Production methods of H2
• Clean or dirty
• Leak rates from system
• Current networks in Germany achieve 0.1
• Natural gas pipelines 0.5-1.5
• 10-20 losses possible from uncontrolled
  evaporation from liquid storage tanks
• Complete fossil fuel replacement and 3 leak rate
  would ? H2 emissions 1.35-2 times.

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Increased Surface H2

• H2 source is from system leaks
• H2 burden could increase by 30-120
• Increased partial pressures of H2 could affect
  microbial colonies
• More pronounced changes in N.H. than S.H.

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Environmental ImpactsSchultz et al. 2003

• Increased H2 concentrations lead to a reduction
  in OH and an increased lifetime of CH4 and
  without reductions in NOx, increases in
  tropospheric O3

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Global Warming Impacts

• Increased lifetime of CH4
• Changes of tropospheric and stratospheric ozone
  levels
• Noctilucent cloud formation (albedo change)
• Dependent on generation processes
• Dependent on level of fuel cell replacement
• Massive reductions in CO, CO2, NOx, and other
  combustion emission if made cleanly

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GWP Increase for CH4

• Schultz et al.
• In some models, ?CH4 would increase by 26
• Radiative forcing of 0.5 Wm-2

• Prather
• Doesnt take NOx reductions into account
• Increases lifetime of CH4
• 0.60 ppm H2 increase
• GWP of 0.026 Wm-2

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GWP of H2 Production

• Reductions of greenhouse gas emissions
• How much?
• Increase of greenhouse emissions
• Emissions of CO2 ?34, CH4 ?19
• H2 made by
• Hydrolysis after electricity from coal power
• Gasification of coal
• Natural gas reforming

Shultz et al. 2003
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Reduction of Tropospheric O3

• Up to 50 reductions of NOx and CO by 100 fuel
  cell replacement of surface fleet reduces
  tropospheric ozone
• Assumes all H2 is produced using emission-free
  processes
• Reduction of 1-8 ppbv in surface ozone throughout
  N.H.
• Shultz et al. 2003

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Reduction of Stratospheric Ozone

• Increases of H2 to stratosphere result in
• Increase of H20
• Decrease of columnar O3
• Tromp et al. 2003

Increase of H20
Decrease of O3
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Noctilucent Clouds

• Clouds at extremely high altitude, about 85 km,
  that literally shine at night. They form in the
  cold, summer polar mesopause and are believed to
  be ice crystals. (http//lasp.colorado.edu/noctilu
  cent_clouds/)
• An increase in the mesosphere of H2O derived
  from H2 could lead to an increase in noctilucent
  clouds, with potential impact on Earths albedo
  and mesopheric chemistry. Tromp et al. 2003

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Summary

• An H2 economy could provide substantial
  improvements in local, regional, global air
  quality and lower greenhouse gas emissions
  depending upon production processes.
• Large uncertainties remain
• NOx, CH4, CO2 emission changes
• H2 lifetime

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Questions?