The Environmental Impact of Electricity Production from Biomass: A Comparison of Poplar and Ethiopian Mustard

Spain has set the goal of producing 12% of its primary energy demand through renewable sources in its Renewable Energies Plan 2000–2010, and power generation from biomass represents an important contribution to meet this goal. Butner et al. (2010) evaluated the environmental performance of two biocrops used for electricity production, poplar and Ethiopian mustard.  Using a Life-Cycle Assessment (LCA) approach, the authors calculated environmental impacts for electricity generation from the two crops and compared values to natural gas generation and the Spanish electricity mix in order to assess whether or not these crops are environmentally competitive with conventional sources of power.  They found poplar to be less environmentally impactful than Ethiopian mustard, mostly due to higher production yields. Electricity from biomass had more impact in three of six environmental categories than natural gas power and less impact in all categories compared to the mix of electricity supplied to the Spanish grid.—Lucy Block
Butnar, I., Rodrigo, J., Gasol, C. M., and Castells, F., 2010. Life-cycle assessment of electricity from biomass: Case studies of two biocrops in Spain. Biomass and Bioenergy 34, 1780–1788.

          Isabela Butnar from the Universitat Rouvira I Virgili along with Julio Rodrigo, Carles M. Gasol, and Francesc Castells selected poplar and Ethiopian mustard for the study due to their high yields in Spain. They chose poplar in particular because of its strong environmental performance and high yields in Mediterranean areas compared with annual herbaceous crops, though its cultivation also consumes large amounts of water.  The two crops also differ significantly from one another: poplar is a perennial crop with a sixteen-year cultivation period, and Ethiopian mustard an annual herbaceous species.

The authors considered cradle-to-grave impacts of the different processes required to produce electricity from poplar and Ethiopian mustard, including field work, the use of farm machines, and the transport of materials associated with production such as fertilizer, herbicides, and packaging, as well as the transport of produced biomass to the power plant.  To calculate this last impact, Butnar et al. considered two different distances, 25 km and 50 km.  The authors based these distances on the percentage of available land for biocrop production at different plant capacities: when the required cultivated area of biomass for a given plant capacity exceeded 15% of the regional irrigated arable land, they calculated a distance of 50 km from the field; otherwise, they calculated a distance of 25 km. 
Along with distance values, Butnar et al. varied the power plant capacities and productivity yield values to calculate the environmental impacts of twelve different scenarios.  The three power plant capacities considered were 10, 25, and 50 MW.  The authors used two different productivity values for each crop, the lower limit of their productivity yield range and the average value.  For poplar these values were 9 and 13.5 t/ha, respectively, and for Ethiopian mustard they were 4.72 and 8.07 t/ha.
          Using SiAGROSOST, a software tool created by their research group, the authors found optimum values for minimizing environmental impact in the variety of scenarios mentioned above for ten different environmental indicators, though only six indicators were included in the report: acidification, global warming, human toxicity, ozone layer depletion, abiotic depletion, and photochemical oxidation.
          As expected, for all indicators, environmental impact decreased when biomass productivity increased.  Because of its higher productivity per hectare, poplar outperformed Ethiopian mustard, having less environmental impact across all indicators.  In general, a greater distance between fields and power plants (50 km as opposed to 25 km) implied greater environmental impact.  In turn, this had implications for the optimal power plant size—the larger capacity of 50 MW plants required more biomass cultivated area than 15% of the regional irrigated arable land, and thus the quantity of biomass required to run the plant at full capacity was not available at a 25 km distance.  This increased the environmental impact of 50 MW plants, despite their generally higher efficiency in energy production. 
          The two crops performed differently in their relationship with productivity, transport, and environmental impacts.  Poplar’s impacts were more closely associated with transport, and Ethiopian mustard’s with productivity.  When transport distances increased, poplar’s environmental impact increased more than Ethiopian mustard’s environmental impact did.  When productivity decreased, Ethiopian mustard’s environmental impact increased more than poplar’s did.  This information has implications for planning the use of different types of crops for biomass electricity generation: while both productivity and transport distance are important, either factor may be more important for different crops.
          By calculating the contribution of individual fieldwork activities—between fertilizers, harvesting, pesticides, and others—to the impact of crop cultivation, the authors found that fertilization is by far the most impactful step of cultivation (accounting for up to 78% of acidification and up to 82% of global warming impact).  However, replacing mineral fertilizers with natural fertilizers such as livestock manure could significantly reduce the environmental impact of fertilization, which, along with soil characteristics and weather, greatly contributes to crop productivity. 
          Butnar et al. compared their calculated impact values for biomass with electricity production from natural gas and the electricity put into the Spanish grid, which is largely dependent on fossil fuels.  They found that biomass electricity had a worse environmental profile than natural gas in the areas of acidification, human toxicity, and photochemical oxidation, and a better environmental profile in the areas of global warming, abiotic depletion, and ozone depletion.  Biomass electricity had a better environmental profile than the Spanish electricity mix in all areas.  However, it is important to note that Butnar et al. did not calculate the impact of transport and distribution of electricity in their biomass LCA, while values for natural gas and the Spanish mix include transport and distribution.  Additionally, Butnar et al. failed to account for the disposal of ashes from the combustion of biomass in their LCA.

          Butnar et al. found that electricity generation from poplar is more environmentally competitive than from Ethiopian mustard, and that power plants of 10 and 25 MW are more environmentally competitive for biomass electricity generation than 50 MW plants in the region of Tarragonès.  The authors recommend thoughtful biomass management plans that include recycled residues, e.g. from the cleaning of public parks.  They also state that in order to keep environmental impact of electricity generation from biomass low, biomass productivity must be optimized, and distances between field and power plant must be minimized. 

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