Growing New Forests for Biofuels

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Analysis by the author suggests that unharvested plantations are much more effective in saving GHG emissions over a 34-year period than if they are harvested for ethanol production. Using the carbon sequestration model of the Australian Government (2007) the comparison was made between the amount of CO2 removed from the atmosphere by a hectare of hoop pine (Araucaria cunninghamii) grown in north Queensland and the carbon dioxide savings of a plantation that was clear felled, with the resulting biomass being used for ethanol production. Forest thinnings prior to harvest were also used for ethanol production.

Ethanol is derived from wood at a rate of 313 to 355 liters of ethanol per tonne of biomass (Perlack et al, 2005; Malmsheimer et al., 2008). Ethanol derived from wood is assumed to emit 90.9 percent less CO2e

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than the gasoline that it replaces, which produces 2.3kg of CO2e per liter burned. Both types of forest, the unharvested and the harvested, remove CO2e from the atmosphere and sequester it as carbon in biomass. When the forest is thinned and harvested it gives up a portion of its sequestered carbon for conversion to ethanol.

Discounting the future savings in CO2 emissions at 2 percent gave a result more than 2 to 1 in favor of leaving the plantation unharvested rather than harvesting it for ethanol. It should be emphasized that this simple analysis ignores the life-cycle emissions involved in the growing and harvesting of the trees, and in the transport of ethanol and gasoline and the production of gasoline. However, it is unlikely that the inclusion of these extra emissions, on both sides of the ledger, would alter the conclusion.

A similar result was obtained by Johnson and Heinen (2007) in comparing the GHG implications of growing trees or growing rapeseed for biodiesel. Replacing biodiesel with petroleum diesel and devoting the land to forest was twice as effective, in terms of reducing GHG emissions, as producing biodiesel to replace petroleum diesel.

Despite the likelihood that the GHG benefits of carbon sequestration exceed those of bioethanol production, both the US and Europe are bent on policies that will require substantial sources of cellulosic biomass in order to meet their targets for biofuel replacement of petroleum-based fuels. In the US much of this is expected to come from unexploited available sources, better use of residues and perennial and fast-growing trees in short rotation such as hybrid poplar and willow (Geyer and Melichar, 1986; De La Torre Ugarte et al., 2003). Residues require no land-use change and come at a low financial cost, while fast-growing tree plantations deliver cellulose with far less fossil fuel use than annual crops (Wang et al., 2007).

If the price is high enough for biomass, land will be switched out of crop, pasture and the CRP to the growing of herbaceous species such as switchgrass and short rotation forests for cellulosic ethanol production (De La Torre Ugarte et al., 2003). At a price around $30 per dry ton, bioenergy crops offer greater profits than existing land uses, and produce 8.51 billion gallons of ethanol, 8.2 million hectares of CRP lands being converted where sensitive lands are excluded. If the price for dry biomass rises to around $40 per dry ton, 16.7 billion gallons of ethanol would be forthcoming from land switched to cellulosic production, almost a third (12.9 million acres) being CRP land. Conventional crop prices rise under both scenarios.

In contrast to the US, the contribution of forests in the EU is constrained and cellulosic biomass requirements are more likely to be imported, as concluded above.

It should be emphasized that while marginal or degraded land in both the US and EU have been considered to be available for cellulosic biomass production, it is likely that yields will be low from such lands and production may well be uneconomic. Moreover, marginal lands may harbor considerable biodiversity and this should be considered when contemplating the benefits and costs of their conversion to short-rotation forest monocultures.

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Guide to Alternative Fuels

Guide to Alternative Fuels

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