A Hydrogen Economy’s Potential Environmental Impacts

4/20/2006 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

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A Hydrogen Economy’s 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 troposphereThe 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

• 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

Shultz et al. 2003

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3

HC

N

S

SKOH


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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

CO+H2 HCO+H HCO+H2O


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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

• “[H2O]…would 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, ΤCH4

would increase by 26%

• Radiative forcing of 0.5 W*m-2

• Prather

• Doesn’t take NOx reductions into account

• Increases lifetime of CH4

– 0.60 ppm H2 increase

– GWP of ~0.026 W*m-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/noctilucent_clouds/)

•“An increase in the mesosphere of H2O derived from H2 could lead to an increase in noctilucent clouds, with potential impact on Earth’s albedo and mesopheric chemistry.” –Tromp et al. 2003

http://lasp.colorado.edu/noctilucent_clouds/


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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?

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A Hydrogen Economy’s Potential Environmental Impacts