We define CNHCs as those whose oxidation does not result in a net increase in atmospheric CO2 concentrations. Hydrocarbon fuels can be made carbon neutral either directly by manufacturing them using carbon captured from the atmosphere, or indirectly by tying the production of fossil fuels to a physical transfer of atmospheric carbon to permanent storage. The indirect route allows for a gradual transition from the current infrastructure, based on petroleum, to a sustainable system based on atmospheric sources of carbon.
It is vital to distinguish negative emissions achieved by permanent physical storage from economic offsets (carbon credits) or the sequestration of carbon in the active biosphere. While the use of carbon offsets such as those allowed under the clean development mechanism may have some benefits, they are not equivalent to non-emission (Wara 2007). There are also tangible benefits to increasing stocks
fuel use C02 to storage
Figure 7.2 Schematic of routes to CNHCs.
fuel use C02 to storage
of carbon in soils or standing biomass, but such organic stores are highly labile and may be quickly released back to the atmosphere by changes in management practices or climate. Geological storage reservoirs for CO2 may also leak. However, the retention time for CO2 in geological reservoirs is at least 103 times longer than that for carbon stored in the biosphere. In most cases, a very large fraction of CO2 placed in geological storage is expected to be retained for time scales exceeding 108 years (IPCC 2005).
Direct and indirect routes to CNHCs both begin by capturing CO2 from the atmosphere. Carbon can be captured from the atmosphere by either harvesting biomass from sustainable plantations or direct industrial processes referred to as air capture (Keith et al. 2006). Once captured, the CO2 can be transferred to storage either in geological formations or other means such as mineral sequestration (IPCC 2005).
Alternatively, it may be returned to the fuel cycle through incorporation into a synthetic fuel or conventional biofuels. The synthetic fuel pathway depends on a source of primary energy to drive the required chemical reactions including the supply of hydrogen. As with hydrogen and electricity, these synthetic hydrocarbons are an energy carrier produced from a primary energy source such as wind, nuclear power or fossil fuels with CCS. Unlike hydrogen and electricity, they are carbonaceous fuels that are nevertheless carbon neutral as they were derived from the atmosphere. The relationship between all of the options is presented in Figure 7.2.
We first review the technologies for capturing carbon from the air, using either biomass growth or air capture. The review is followed by discussions on transforming the carbon, in the form of high-purity CO2, into hydrocarbon fuels. The objective is to outline the important process steps so that they can be quantified in the economic comparison that follows. The comparison does not include fugitive emissions from individual process steps. These include emissions associated with harvesting and processing biomass, potential leakage from industrial air capture and CO2 emissions associated with hydrogen production from fossil fuels (estimated at 7-28 kg CO2 GJ-1: IPCC 2005). As such, the processes considered here will not be completely 'carbon neutral' unless accompanied by the removal of additional CO2 from the atmosphere.
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