4.4. Carbon Emissions

Bioenergy cycles can affect carbon emissions in two main ways: (1) they can provide energy that displaces fossil fuel energy, and (2) they can change the amount of carbon sequestered on land. The net carbon benefit depends on what would have happened otherwise; the amount and type of fossil fuel that would have otherwise been consumed; and the land-use that would have prevailed if biomass were not grown and harvested for energy. Since this counterfactual situation is speculative, it is impossible to calculate the carbon benefit of a given bioenergy cycle with certainty. However, carbon benefits can be estimated using some illustrative assumptions.

Fossil Fuel Displacement

The carbon benefit of displacing fossil-fuel consumption depends on the bioenergy cycle considered. For electricity generation, the carbon emissions depend on how efficient the generation technology is and how much fossil fuel was used to produce the biomass. Table 4.2 gives some approximate values for the carbon emissions of selected technologies. It assumes that the biomass feedstock is carbon-neutral - that is, the carbon released during combustion balances the carbon extracted from the atmosphere during growth, and there is no net change in carbon on the land. Production and transport of either biomass or fossil fuel would give rise to some additional carbon emissions, but these are minor and are considered comparable (Borjesson, 1996b).

Table 4.2. Approximate Carbon Emissions from Sample Biomass and Conventional Technologies

(a) The energy of the biomass produced divided by the energy of the fossil fuel consumed to produce the biomass.

Land Changes

In fact, there will be some change in the amount of carbon sequestered on land. This change is highly dependent on the particular details of the land in question and the proposed biomass feedstock system. Three cases are considered here.

In the first case, natural forest is cleared in unsustainable ways to provide fuel for a bioenergy facility, leaving a denuded site that does not regenerate. In this case, the carbon emissions from the bioenergy cycle are comparable to or greater than carbon emissions from a fossil-fuel cycle generating an equivalent amount of energy. There is no justification for this fuel cycle from a greenhouse gas perspective, nor from any other environmental perspective. Unfortunately, this is a frequently used model for production of non-energy biomass, and could be the most cost-effective strategy for a bioenergy project from the standpoint of an investor guided solely by short-term profits.⁴ Measures might be required to prevent this from happening.

⁴ A100 MW power plant with a thirty-year lifetime would require about 50 thousand hectares of land (assuming a 35 percent efficiency and a one-time yield of 250 dry tonnes of harvested fuel per hectare), roughly the amount of land within a 13-kilometer radius of the plant. Alternatively, producing biomass on an energy plantation with a yield of 15 dt/ha/yr would call for roughly 30 thousand hectares of land, roughly the amount of land within a 10-kilometer radius of the plant. From a strictly financial perspective, the choice between these options would depend on a number of factors, including relative cost of land versus the cost of managing a plantation, and the expected future plans for the land.

In the second case, natural forest is cleared and replanted with an energy plantation harvested sustainably to supply a bioenergy facility with biomass continuously. The carbon formerly sequestered in the natural forest will be released. The amount of carbon released depends on the type of forest, but a rough figure is 300 tonnes of carbon per hectare (tC/ha) (Brown, Cabarle, Livernash, 1997). As biomass feedstock is grown and harvested in cycles, carbon will be sequestered on the land, partly compensating for the carbon released when the natural forest was cut down. Averaged over a growth cycle, a typical amount of carbon sequestered on the plantation land might be 30 tC/ha.⁵ The natural forest therefore sequesters 270 tC/ha more than the energy crop. If the biomass is used to displace fossil fuels, thereby reducing carbon emissions, this 270 tC/ha difference will eventually be compensated over a period of roughly 45 years.⁶ Thus, depending on the precise stocks of carbon involved, there might be a case based on carbon benefits for clearing natural forest to plant energy plantations. However, it is not a very compelling case, even without taking into account the loss of ecosystem services that would accompany clearing of natural forest. Environmental and social considerations such as preserving habitat, protecting watersheds, etc., might more than outweigh the carbon benefits.

⁵ This assumes a 7.5 tC/ha/yr growth rate and an eight-year harvest cycle. (In this rough approximation, carbon in soils and litter is assumed unchanged relative to the natural forest.)

⁶ This assumes that a tonne of fossil fuel carbon is displaced by 1.25 tonnes of biofuel carbon. (The factor 1.25 accounts for differences in power conversion efficiency and fuel carbon content, and also the fossil fuels inputs consumed for biomass production.) Then it will take 45 years [(270 tC/ha * 1.25) / (7.5 tC/ha/yr) = 45] to make up for the initial release of carbon from the natural forest. This is a worst case scenario, in that it assumes that the original natural forest biomass is not used, like the purpose-grown biomass, to displace fossil fuels or other non-renewable resources. If some fraction of the biomass is suitable for use as fuel, the carbon benefits of this case will improve, and the breakeven time will be less than the 45-year worst-case situation.

In the third case, unproductive land, such as degraded land that would benefit from revegetation, is converted to bioenergy crop plantation. The degraded land probably sequestered considerably less carbon than the plantation, including the increase in carbon sequestered in the soil and other below-ground biomass. In this case, the change in land use will have carbon benefits over and above the benefits resulting from displacing fossil fuels, as well as other ecosystem benefits.

Bioenergy cycles can affect carbon emissions in two main ways: [by] displac[ing] fossil fuel energy, and chang[ing] the amount of carbon sequestered on land.