Hydrogen and its competitors, 2004

62Risø Energy Report 3Hydrogen and the environment6.14. Carbon dioxide emissionsIn a recent study, today's surface traffic fuels wereassumed to be replaced by hydrogen generated fromrenewable sources leading to a 20% CO 2 emission reduction[14]. However, as long as hydrogen is generatedfrom fossil fuels, CO 2 emissions from reforming caneasily rival today's emissions from power plants andtraffic. From the standpoint of avoiding CO 2 emissionsin the short to medium term, centralized facilities appearpreferable, because this might allow efficient capturingand storing of the CO 2 produced. 9 In the long term, it isobvious that hydrogen generation has to be based onrenewable sources to avoid the environmentally adverseeffects of carbon dioxide.5. Air quality effectsProbably the most immediate implication of a large-scaleshift to hydrogen, especially in road transport, would bea significant drop in emissions of air pollutants (NO x ,benzene and other VOCs) and an associated decrease inground-level ozone concentrations.In most regions of the world, ozone formation is limitedby the amount of NO x in the atmosphere. However,many big cities emit such large amounts of NO x thatozone formation is limited instead by the availability ofVOCs and carbon monoxide, which are produced byplants and soil bacteria as well as vehicles and industrialprocesses.As a result, it is well known that lowering emission levelscan actually increase ozone concentrations at first; onlyif the reduction is large enough will ozone concentrationsdecrease. This effect probably lies behind observationsthat though emissions of ozone precursors havedecreased significantly in Europe over the past decade orso [1], summertime surface ozone concentrations haveremained stable. All this means that the reduction ofNO x and VOC emissions following the introduction ofhydrogen vehicles could initially increase ozone levels insome areas. However, the reduced NO x and VOCconcentrations will bring their own health benefits, andozone levels downwind of the city will decrease. As aresult, the damaging effects of ozone on crops andnatural ecosystems will be reduced.Simulations also show the positive trend for the airquality that drastic cuts in NO x emissions are especiallyeffective in reducing peak ozone concentrations and thenumber of days on which air quality standards areexceeded [13].Thus there is little reason to doubt that the widespreaduse of hydrogen should bring significant improvementsof air quality. This would only be untrue if coal-firedpower stations with little emission control were used toproduce the hydrogen (in particular in developing countries,e.g. China). In this case NO x emissions might actuallyincrease compared to today, with a further rise intropospheric ozone concentrations.ConclusionsWhile there are still large uncertainties about the currentbudget of atmospheric hydrogen and the consequencesof a large-scale shift towards a hydrogen economy,present knowledge indicates that there are no majorenvironmental risks associated with this energy carrier,and that it bears great potential for reducing air pollutionworld wide, provided that the following rules arefollowed:• Hydrogen should not be produced using electricitygenerated by burning fossil fuels. Instead, natural gasor coal reformers should be used at first, and replacedby renewable energy sources as soon as possible. CO 2capture from reformers should be seriously considered.• Hydrogen should be used predominantly on theground rather than in aircrafts, and to achieve fullbenefits, fuel cells would be preferable against internalcombustion engines.• Leakage in the hydrogen energy chain should belimited to 1% wherever feasible, and global averageleakage should not exceed 3%.Atmospheric hydrogen concentrations should be carefullymonitored. Enough research should be carried outto obtain a better understanding of hydrogen sourcesand sinks, and to provide an early warning system in casewe have overlooked something.9. Using electricity from coal fired power plants, for example, could increase CO 2 emissions by a factor of 2-4. But, as long as efficient technology isemployed we would not expect significant change in CO 2 emissions in the coming decades.