The large carbon fluxes between the atmosphere and the terrestrial biosphere (approx. 60 Gtonne carbon per year), in combination with our substantial control over terrestrial biotic productivity (Vitousek et al. 1986), grant us a powerful lever for manipulating atmospheric CO2. If biomass is cyclically harvested so that stocks of standing biomass are not decreased, it provides a means of simultaneously capturing both carbon and solar energy. As discussed below, once harvested biomass can be used to produce conventional biofuels or CNHCs; alternatively, the carbon can be permanently sequestered allowing for the indirect production of CNHCs.
Large-scale use of biomass presents enormous challenges and poses risks of substantial environmental, social and economic side effects. As noted by the IPCC (2007), 'biomass production and use imply the resolution of issues relating to competition for land and food, water resources, biodiversity and socio-economic impact'. These competing issues make it very unlikely that conventional biomass fuels can be used as the dominant solution to emissions from the transportation sector. There is also a degree of risk associated with solving the climate change problem using a technology dependent on the climate (Fargione et al. 2008; Searchinger etal. 2008).
Estimates of biomass cost and availability vary widely. For example, the cost of switchgrass ranges from $33 tonne-1 at a yield of 11 tonnes biomass ha-1 to $44 tonne-1 at 7 tonne ha-1 in Oklahoma (Epplin 1996). Walsh estimated switchgrass costs at $20-25 tonne-1 depending on the location in the USA with woody crops (poplar) in the range $22-35 tonne-1 (Walsh et al. 2003). Other researchers estimate the cost of short rotation crops in Sweden at $89 tonne-1 with forestry residues slightly more expensive at $110 tonne-1 (Gustavsson et al. 2007), using a conversion value of $1.00 = €0.72 and an energy content for dry woody biomass of 20 GJ tonne-1 (Khesghi et al. 2000). Alternative studies for combination biomass with CCS have assumed costs of $50-54 tonne-1 (Audus & Freund 2004; Rhodes & Keith 2005).
The dedication of large amounts of land to energy crops may also raise the price of agricultural products. Estimates vary from 10 per cent (Walsh et al. 2003) to 40 per cent (Searchinger et al. 2008). The current biofuel boom in North America appears to have increased agricultural prices significantly, even though its contribution to fuel supplies is minimal. This illustrates a negative impact of biofuel production, although it does not prove that larger-scale biomass production could not succeed using better choices of crops and incentive mechanisms.
Ignoring the negative side effects of biomass harvesting discussed above, we assume that the cost of large-scale biomass delivered to centralized facilities ranges from $40 to $80 per dry tonne or $2-4 GJ-1. It seems plausible that negative non- and macro-economic impacts of biomass production might limit biomass availability. Since we have no basis to estimate the effects of such impacts, we treat them parametrically, calculating the biomass price that would make biomass more expensive than air-capture routes to CNHCs.
Biomass contains both carbon and energy. Production of ethanol from biomass uses the energy content of the biomass to drive the conversion process. In order to provide process energy, most of the carbon in the input biomass stream is oxidized and released to the atmosphere as CO2. Even in advanced cellulosic ethanol production, which has not yet been applied at a commercial scale, only about one-third of the carbon content in the input biomass ends up in the fuel. As a means of recycling atmospheric carbon to liquid fuels, these processes make inefficient use of biomass carbon.
Here, we consider only the production of CNHCs from biomass using external energy inputs to make more efficient use of the carbon captured in the biomass. This choice is based on the assumption that land-use constraints will be the most important barrier to biomass-based fuels and the observation that the cost of large-scale carbon-free energy at a biomass processing plant is substantially less than that of delivered fuel energy. For example, hydrogen and heat might be supplied from coal with CCS at costs substantially less than those of delivered CNHCs or conventional gasoline. The use of external energy/hydrogen can convert a larger fraction of the input carbon to hydrocarbon fuel and reduce land use by a factor of 2-3 when compared with systems based on biomass alone (Agrawal et al. 2007).
There is a large suite of methods that might be employed to produce CNHCs from biomass. Examples include the following:
• electricity production with CO2 capture followed by CO2 hydrogenation using externally supplied H2; the CO2 capture step might use oxy-fuel, post- or pre-combustion capture;
• gasification to produce synthesis gas followed by production of CNHCs using the Fischer-Tropsch (F-T) process and CO2 hydrogenation using externally supplied H2; and
• biological processing to produce hydrocarbons or alcohols combined with carbon capture followed by CO2 hydrogenation using externally supplied H2.
For simplicity, we examine only the first route since there have been several assessments of biomass electricity with CO2 capture. Moreover, it allows a direct comparison with air capture allowing us to consider both direct and indirect routes from biomass to CNHCs. In reality, biomass co-firing or co-feeding with fossil fuels seems a more likely near-term prospect. Such methods would produce hydrocarbon fuels with reduced life-cycle CO2 emissions, but will not produce CNHCs. One might consider these options as a blend of CNHCs with conventional fossil fuel use.
The process of air capture comprises two components: absorption and regeneration. The absorption phase refers to dissolving the CO2 contained in the atmosphere into solution or onto a solid sorbent, while the regeneration phase refers to producing a concentrated stream of CO2 from the medium used for absorption. Most recently, published work has addressed systems that use a strong base, typically NaOH, as the sorbent and chemical caustic recovery as the means of regeneration. Earlier works assumed an electrochemical system based on carbon-free electricity. The challenge with electrochemical methods is the electricity consumption during regeneration, 308 kJe mol-1 CO2 (Bandi et al. 1995), which can be converted to a cost of $100200 tCO-1 for carbon neutral electricity costing $0.05-0.10 (kW h)-1.
The thermal process, as outlined in the literature, consists of four reactions: absorption; causticization; regeneration; and hydration (Baciocchi etal. 2006; Keith et al. 2006; Zeman 2007). The CO2 is absorbed into sodium hydroxide to form sodium carbonate. The carbonate ion is transferred from sodium to calcium ions in the causticization process, which results in the precipitation of calcium carbonate. The CO2 is regenerated by thermal decomposition of the calcium carbonate in a kiln while the lime produced is hydrated to complete the cycle. The absorption reaction is an established engineering technology dating back several decades (Spector & Dodge 1946). The other reactions are at the heart of the pulp and paper industry and can be directly applied to air capture with the addition of conventional CCS technologies (Keith et al. 2006), although conversion to an oxygen kiln significantly reduces energy demand (Baciocchi et al. 2006; Zeman 2007). Experimental work has shown conventional vacuum filtration technology sufficient for dewatering the precipitate and causticization at ambient temperatures to be feasible (Zeman 2008).
While technically feasible, the amount of energy consumed and its form are critical components in terms of neutralizing emissions from the transportation sector. The thermal requirements are dominated by the decomposition of calcium carbonate, which requires a minimum of 4.1 GJtCO-1 of high-temperature heat (Oates 1998) with the potential to recover 2.4 GJtCO-1 at lower temperatures via steam hydration (Zeman 2007). Using existing technologies, the actual thermal load is 5.1GJtCO-1 for an 80 per cent efficient kiln (Oates 1998). Some electrical energy is required to power blowers for air movement, pumps for sorbent circulation as well as oxygen production with load estimates varying from 126 to 656 kWhtCO-1 (0.45-2.36 GJe tCO-1: Zeman 2007). The cost of air capture was estimated, using technologies from other industries, for the system above at approximately $150 tCO-1 (Keith etal. 2006). We note that this cost estimate was intended to be a minimum estimate for long-run costs and that it was based on a simplistic combination of existing technologies rather than on an integrated plant design. Other capture processes have been considered (Steinberg & Dang 1977) as well as advanced regeneration cycles based on titanates that reduce high-temperature heat requirements by approximately 55 per cent (Nohlgren 2004). In this work, we consider air capture costs ranging from $100 to 200 tCO-1 recognizing that there are no a priori reasons why costs could not eventually be lower.
Air capture, as opposed to biomass growth, is not limited by land area but rather by the rate of CO2 diffusion to the boundary layer, analogous to the physics that limits wind turbine spacing (Keith et al. 2006). Previous work has shown that air-capture rates are at least one order of magnitude larger than biomass growth (Johnston et al. 2003). This value reflects the large-scale limitations of CO2 transport in the atmospheric boundary layer. Note that air-capture systems need only occupy a small portion of the land area in order to capture the maximum large-scale CO2 flux, just as wind turbines have a small footprint yet capture much of the large-scale kinetic energy flux. The effective flux, based on the air-capture plant boundary, can be expected to be at least two and quite likely three orders of magnitude larger than biomass growth with the remainder of the land area available for other uses, such as agriculture.
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