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Climate change responses: carbon offsets, biofuels and the life cycle assessment contribution

10.3.1 Using biomass in energy generation 131
As with any LCA, the first consideration in determining potential savings from a biomass
energy technology is the analysis boundary. A simple boundary condition would be green-
house gas emissions solely at point-of-use, counting net emissions from the biomass upon
combustion and subtracting emissions due to avoided fossil fuel combustion, to provide an
overall indication of the greenhouse benefit of the technology. However, this does not neces-
sarily provide a realistic approximation of net greenhouse gas emissions from the combustion
of biomass. The system boundary should be extended to include inputs from growing, harvest-
ing and transporting the biomass, and the contribution of soil carbon sequestration. This
extension should also include a consideration of the ‘upstream’ fossil fuels system, including
mining, processing and transporting fossil fuels. The greater the level of extension, the more
comprehensive the consideration of greenhouse gas emissions from the biomass technology,
although as the boundary expands so does the level of data and uncertainty in assumptions
about these expanded elements.
For example, fertiliser, herbicide and pesticide production all contribute to greenhouse gas
emissions associated with (non-organic) biomass crops. There are variations in both the type
and quantities of inputs applied to a given quantity of biomass. Practices vary, local conditions
and soil optimisation requirements vary, and emissions factors vary between electricity systems
within which these products are manufactured. Further uncertainties occur for short rotation
crops, since a percentage of fertiliser applied at a given time may be for the current crop to use
although a further percentage may be for use by a subsequent crop, and therefore it needs to be
allocated.
There is also uncertainty associated with changes in carbon storage in soils and soil-related
greenhouse gas sources, as discussed in Section 10.2. This is especially important, as these
effects may be significant in the total greenhouse gas balances of biofuels. For example, where
switchgrass was substituted for coal in an existing coal-fired electricity generation facility, soil
carbon sequestration accounted for 29% of the net benefit from the fuel switch, but it contrib-
uted 62.5% of the total uncertainty (Ney and Schnoor 2002).
Another important consideration in calculating greenhouse gas balances for biomass
energy technologies relates to the specific substitution situation. In the case of local off-grid
co-firing, the calculation will be based on the primary fuel or mix of fuels being replaced. In
the case of a stand-alone biomass plant that contributes to a region or country’s electricity grid,
the calculation of greenhouse benefits is more complex. Arguably, it is inappropriate to select
an expensive, unused, inefficient or obsolete fossil fuel technology for comparison in calculat-
ing net benefits. Logically, the appropriate comparison is with a fossil fuel technology that
could reasonably be adopted given the market conditions and current technologies in the
selected setting. One study (Schlamadinger et al. 1997) suggests that a reference energy system
should be ‘the least-cost fossil energy system with the lowest greenhouse gas emissions and
minimised environmental impacts, fulfilling the same goals as the bioenergy system’.
Another approach is to compare the marginal electricity generator, provided the biomass
energy alternative can meet marginal demand requirements (i.e. it can be easily and quickly
brought on-stream at energy demand peaks and shut down following them). For example, in a
coal-dominated electricity grid, the marginal electricity generator might be a gas-fired plant.
The addition of new capacity from a biomass project would displace the gas-fired plant, and
therefore gas rather than coal, or a mix of gas and coal, would be the fossil fuel being replaced.
Where electricity demand exceeds supply, the addition of the biomass project may increase
overall energy consumption rather than displace an existing fuel. In this case, greenhouse gas
emissions would not necessarily be reduced, and the only benefit of using biomass technology

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