|Eco-restructuring: Implications for sustainable development (UNU, 1998, 417 pages)|
|Part I: Restructuring resource use|
|6. Fuel decarbonization for fuel cell applications and sequestration of the separated CO2|
Consider a world situation in 2100 where the hydrogen fuel cell vehicle is a well established technology and where natural gas resources are in limited supply, so that the primary options for producing hydrogen at low cost are from coal and biomass. Suppose further that some regions with a capacity to produce biomass on large scales agree to seek to help bring about deep reductions in global CO2 emissions, but that other regions must depend on coal and either have no ready access to secure sequestering sites or are unwilling to incur the cost penalties for storage, however modest. In these circumstances it may still be feasible to achieve low global emissions levels because of the large "emissions offset potential" offered by biomass-derived hydrogen (see tables 6.1 and 6.2).
To illustrate the possibilities, suppose that: (i) the world population in 2100 is 10.5 billion; (ii) there are then 0.4 cars per capita in the world (the average for the industrialized market countries in 1985) - some 4.2 billion cars altogether (10 times the present number); (iii) the average car is driven 13,400 km/year (the average for industrialized market countries in 1985); (iv) these cars are all equipped with hydrogen fuel cells; (v) the average automotive fuel economy is 42.5 km/litre (100 mpg) of gasoline equivalent.
With so many cars, would it be possible to have zero net CO2 emissions worldwide from the automotive sector, if some cars were run on hydrogen derived from coal without sequestration of the separated CO2 (so that net lifecycle emissions would be +41.2 kg C/GJ of hydrogen see table 6.1) and if the rest were run on hydrogen derived from biomass with sequestration of the separated CO2 in saline aquifers and/ or depleted natural fields (so that the net lifecycle emissions would be-18.4 kg C/GJ of hydrogen - see table 6.1)?
Under the above conditions, there would be zero net emissions from the automotive sector if a fraction "a" of all cars is operated on coal-derived hydrogen, where "a" is determined from the following equation based on the net lifecycle emission rates for the two hydrogen-producing systems: 41.2*a - (1 - a)*18.4 = 0, or a = 0.31. Thus net global emissions from cars would be zero if 31 per cent of the hydrogen were derived from coal and 69 per cent from biomass. The amount of hydrogen consumed worldwide for cars would be 46 EJ per year. Assuming an efficiency of 64 per cent for making hydrogen from coal or biomass (see Appendix A), some 22 EJ and 51 EJ per year of coal and biomass, respectively, would be needed for fuelling automobiles in 2100. For comparison, total global coal consumption in 1985 was 90 EJ per year; and SO EJ per year is approximately the average rate at which noncommercial biomass is consumed in the world today (Hall et al. 1993). Assume that in 2100 all this biomass would be grown on plantations. It is reasonable to expect an average yield of, say, 20 dry tonnes per hectare per year at that time (Williams 1995). With an energy content of 20 GJ/dry tonne, some 128 million hectares of plantation area would be needed worldwide. There will very likely be far more land available for biomass energy plantations than this (Larson et al. 1995). The required sequestering rate of 0.75 GtC per year is modest in relation to the estimated capacities of aquifers and depleted natural gas and oil fields.