Integration of LCA in a thermo-economic model for synthetic natural production from woody biomass

Environmental impacts of emerging technologies such as the production of biofuels have become an important concern. To assess these impacts, life cycle assessment (LCA) is a well-established and widely used method. However, conventional LCAs for these technologies are generally based on an average technology at lab- or pilot-scale. Therefore, the changes in process design, future installation size and technology evolution are not considered. Moreover, it is not possible for engineers to use the LCA as a design tool at the conception stage to target not only economic competitiveness and energy efficiency, but also minimal environmental impacts. The objective of the present survey was to develop a methodology allowing to link process design and scaling with the LCA. It is illustrated by an application to an existing thermo-economic model used for the multi-objective optimization of the thermo-chemical production of synthetic natural gas (SNG) from woody biomass. First, the functional unit (FU) and the system limits are defined for the LCA model, according to the established LCA methodology. In the present case, a cradle-to-gate LCA approach was applied, and the chosen FU was 1 MJout of SNG, ready to be injected in the gas grid. All the energy or materials flows of environmental concern falling within these boundaries as input or output are identified and form the life cycle inventory (LCI). Secondly, the driving parameters which allow for quantifying the LCI flows as a function of the process design and scale are identified. These driving parameters are then directly taken from the thermo-economic model to perform the scaling of the LCI within the LCA model. If some of these parameters relevant for the LCA are not available from the thermo-economic model, the latter is extended. Each LCI flow is thus adapted to the process design and scale. Necessary process equipment for plant infrastructure is also included in the LCI, and its impacts are scaled. The life cycle impact assessment (LCIA) is then performed with one or more established impact assessment methods in order to interpret a particular process design in terms of environmental consequences. The obtained model including both the LCA and thermo-economic parts can be termed as a thermo-environomic model. The environmental indicators given by the LCIA of such a model were used in the frame of an economic optimization at multiple scale for six different scenarios for SNG production, representing different technological choices and stages of technology evolution. The effects of economic optimization on the environmental impacts were then studied. It appeared that for five of the six scenarios studied, there was a trade-off between SNG production costs and environmental impacts, since costs were decreasing while impacts were increasing with respect to installation size.

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