A Hydrogen Economy’s Potential Environmental Impacts

1. A Hydrogen Economy’s Potential Environmental Impacts Chun Zhao Evan Cobb Ga Tech - EAS 6410 - Air Chemistry Group Presentation

2. A Hydrogen Economy www.gii.com.hk Ga Tech - EAS 6410 - Air Chemistry Group Presentation

3. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

4. Hydrogen in troposphere The sources and sinks of hydrogen in troposphere Ga Tech - EAS 6410 - Air Chemistry Group Presentation

5. 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? Note for table of sources and sinks Ga Tech - EAS 6410 - Air Chemistry Group Presentation

6. Hydrogen production from oxidation of organic compounds Ga Tech - EAS 6410 - Air Chemistry Group Presentation

7. Reduction in OH • H2 behaves like CO (takes up one OH and releases one HO2 radical + H20) Michael Prather, 2003. Ga Tech - EAS 6410 - Air Chemistry Group Presentation

8. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

9. Hydrogen in the Stratosphere The main hydrogen reservoirs in Stratosphere: Molecular hydrogen (H2) Water vapor (H2O) Methane ( CH4) Ga Tech - EAS 6410 - Air Chemistry Group Presentation

10. H2 Sources in the Stratosphere CH4 OH, O(1D), Cl CH3 O2HHO,OH,HCl CH3O2 NO NO2 CH3O O2 HO2 CH2O hv OH CO+H2HCO+HHCO+H2O Ga Tech - EAS 6410 - Air Chemistry Group Presentation

11. H2 sinks the in stratosphere Reaction with OH, O(1D), Cl: • H2 + OH H2O + H • H2 + O(1D) OH + H • H2 + Cl HCl + H Ga Tech - EAS 6410 - Air Chemistry Group Presentation

12. 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. Ga Tech - EAS 6410 - Air Chemistry Group Presentation

13. Main reactions of H2 in the stratosphere Ga Tech - EAS 6410 - Air Chemistry Group Presentation

14. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

15. 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 Environmental Impact Overview Ga Tech - EAS 6410 - Air Chemistry Group Presentation

16. 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. Ga Tech - EAS 6410 - Air Chemistry Group Presentation

17. 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. Ga Tech - EAS 6410 - Air Chemistry Group Presentation

18. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

19. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

20. 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 GWP Increase for CH4 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

21. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

22. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

23. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

24. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

25. 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 Ga Tech - EAS 6410 - Air Chemistry Group Presentation

26. Questions? Ga Tech - EAS 6410 - Air Chemistry Group Presentation