2. Materials and methods
2.1. Goal and scope definition
The LCA used is of the attributional comparative type, it analyses the environmental performance of three systems producing thermal energy for domestic use with forestry logging residues as biomass fuel. The term logging residues refers, in this context, to the crown mass (tops and branches with leaves, also called slash) and stumps (Helmisaari et al., 2014), produced as a result of commercial logging operations for the production of industrial wood (sawlogs and pulpwood). We did not include logs from any thinning operation.
We study three pathways: loose residues burned in a log-stove; a district heating plant utilizing forest chips and a domestic stove fuelled with wood pellets (see Fig. 1). The analysis is divided into two stages. In a first stage we focus on the supply chain impacts of the three bioenergy systems and we compare them to a fossil reference supply chain system using natural gas (NG) (Fig. 2). This approach is the one applied in European legislation for GHG emissions (e.g. typical and default GHG emissions values in EU (2009a)).
In the second stage we go beyond the EU methodology limitations and we expand the system boundaries to include the forest system. This approach reveals additional information on the land-use impacts of bioenergy as compared to the non-bioenergy system. We quantify the implications on the forest carbon balance and we review other possible risks and benefits posed to the ecosystem by an increased removal of logging residues.
The functional unit considered is 1 MJ of useful thermal energy; this includes losses due to start-up and shutdown, partial loads, thermal inertia and losses in the heat distribution system (Obernberger and Thek, 2010).
The environmental impact categories evaluated are: global warming, acidification, particulate matter and photochemical ozone formation. The physico-chemical properties of the wood are summarized in Table S1. We use the characterization models at midpoint recommended by the ILCD (2012) (Table S2). The characterization factors used are detailed in Tables S3–S4. The model used to calculate the response of global surface temperature to the emission profiles from the systems is detailed in the Supplementary Material (SM). Infrastructures are not included. The geographical scope of the paper is the EU-28 countries. The software used is Gabi 6.3 from PE International. No allocation of emissions from timber logging operations is considered.
2.2. Life Cycle Inventory (LCI)
All the datasets related to collection and processing of the logging residues are the same as the ones presented in JRC (2014) (see SM).
We modify a few assumptions compared to the JRC report. Firstly, the conversion efficiency and pollutants' emissions associated to the final conversion of the biomass fuel to thermal energy are now based on published data (Table 1) as opposed to the standard conversion efficiency applied in the JRC report. Secondly, transport distances for the biomass fuels have been reduced to reflect more realistic conditions (100 km) compared to the fixed distance of 500 km considered in the JRC report. Thirdly, updated GWP(100) factors are used in this work for CO2, CH4 and N2O (see Table S3), as compared to the GWP(100) values from the 4th IPCC Assessment Report used in the JRC report. Finally, the data for fossil fuel supply and combustion emissions represent average European conditions (PE, 2014) as compared to the values in the JRC report.