Environmental Engineering Reference
In-Depth Information
burdens allocated to the main product. Where system expansion cannot be applied and the
allocation cannot be avoided, ISO norms suggest that the inputs and outputs of the system
should be partitioned between its different products or functions in a way which reflects the
underlying physical relationships between them (physical allocation); i.e. they shall reflect the
way in which the inputs and outputs are changed by quantitative changes in the products or
functions delivered by the system (Curran 2007). Where physical relationship alone cannot be
established or used as the basis for allocation, the inputs should be allocated between the
products and functions in a way which reflects other relationships between them. If system
expansion cannot be applied, input and output data might be allocated between co-products in
proportion to thermodynamics parameters (such as energy or exergy content of outputs) or to
the economic value of products.
Allocation based on energy content of products can be easily carried out but its
application is inconsistent (i.e. lacking of a correct logical relation) and results in misleading
conclusions if there are some products which are not used as energy carriers (e.g. chemicals).
Allocation based on exergy overcomes this inconsistency but can be problematic to be
applied because of the difficulties for estimating the exergy content of substances (especially
new bio-based products). Allocation based on economic values focuses on external
characteristics of the products and has the disadvantages that do not take into account the
environmental perspective and the physical properties of the products, because is based on
their “value” in human societies; in addition, market values of products can fluctuate
consistently according to the reference year, production chain and geographical location
(Ekvall, 2001; Pierru 2007).
In order to do not disregard these issues and provide a sensitivity analysis on how the
different allocation procedures affect the final results, all the above mentioned allocation
methods are applied to the biorefinery system assessed in this chapter.
Concerning system expansion, the main product is assumed to be bioethanol and the
environmental benefits of co-products are assumed as credits, calculated thanks to the fossil
reference systems. These credits (i.e. the GHG and fossil energy saved by the co-products)
are then subtracted to the total GHG emissions and energy consumption of the whole system;
the resulting environmental burdens are completely assigned to the main product.
Allocation methods based on thermodynamics parameters (energy and exergy content)
and economic values of the products share the environmental burdens among the different
outputs, without identifying a main product.
Concerning allocation based on energy content, for biofuels the following heating values
are considered: bioethanol 27 MJ/kg, MTHF 32 MJ/kg, biomethane 34.75 MJ/kg, hydrogen
114 MJ/kg. The energy content of the material products has been estimated by means of the
Dulong's formula (furan resins 21.22 MJ/kg, FUMA 9.08 MJ/kg). Oxygen does not have a
heating value. Exergy content of products are collected from a specific database (Ayres et al.,
1996).
4.3. Life Cycle Impact Assessment
Life Cycle Impact Assessment (LCIA) stage deals with the evaluation of environmental
impacts of the biorefinery system and fossil reference system over their whole life cycle. The
results focus on two types of impact categories: greenhouse gas (GHG) emissions and
cumulative primary energy demand.
