Life Cycle Inventory
 The basis of any LCA is the creation of a model, that contains the
amounts of all inputs and outputs of processes that occur during the life
cycle of a product.
This includes the production phase, distribution, use and final disposal
of the product. To avoid you need to collect
too much data, you need to set boundaries.
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Introduction to LCA
(PDF, 1MB) for an extensive introduction in LCA, and the
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Introduction A product's life cycle can be presented as a
process tree. 
Example of a process tree. Each box represents a process which forms part of
the life cycle. Every process has defined inputs and outputs.
Process inputs can be divided into two kinds: - Inputs of
raw materials and energy resources (environmental input).
-
Inputs of products, semi-finished products or energy, which are outputs
from other processes (economic input).
Similarly, there are two kinds of outputs: - Outputs
of emissions (environmental output).
- Outputs of a
product, a semi-finished product or energy (economic output).
With information about each process and a process tree of the life cycle, it
is possible to draw up a life cycle inventory of all the environmental
inputs and outputs associated with the product. The result is called the
table of impacts . Each impact is expressed as a particular quantity of
a substance.
The table below displays an example of a small part of the table of impacts for
the production of two materials. A complete table can have hundreds of rows! | |
Polyethylene | |
Glass | |
Unit | |
emission | | | | |
CO2 | |
1.792 | |
0.4904 | |
kg | |
NOx | |
1.091 | |
1.586 | |
g | |
SO2 | |
0,987 | |
2.652 | |
g | |
CO | |
670.0 | |
57.00 | |
mg |
An example of
impacts from the production of 1 kg of polyethylene and 1 kg of glass.
It will be clear that such a table does not provide an immediate answer to a
question such as whether 1 kg of polyethylene is more or less
environmentally friendly than 1 kg of glass.
Impact assessment methods have been developed which simplify this task
of interpretation. Before going into these, there are some problems to be
considered regarding the calculation of the table of impacts.
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Inventory process in greater detail The
inventory process seems simple enough in principle. In practice, it is
subject to a number of practical and methodological problems. They are as
follows: System boundaries
In breaking the life cycle down into processes, it is not always clear how
far one should go in including processes belonging to the product concerned.
In the production of polyethylene, for example, oil has to be extracted; this
oil is transported in a tanker; steel is needed to construct the tanker, and the
raw materials needed to produce this steel also have to be extracted. For
practical reasons a line must be drawn. For example, the production of capital
goods is usually excluded.
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Processes that generate more than one product For example
the electrolysis of salt to produce chlorine. The environmental effects of
the electrolysis process cannot be ascribed entirely to chlorine alone, as
caustic soda and hydrogen are also produced. A suitable allocation
rule is needed here, for instance allocation on mass basis or economic
value of the products.
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Avoided impacts When a disposal process generates a
profitable output, such as energy generation at a municipal waste
incineration plant, it not only causes impacts. It also saves impacts as it
is no longer necessary to produce the energy or the material in a normal
way. To allow for this, avoided impacts are introduced.
These are equivalent to the impacts that would have occurred in actual
production of the material or energy. The avoided impacts of a process are
deducted from the impacts caused by other processes. In SimaPro both the
attribution of impacts concept and the avoided emissions concept can be
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Geographical variations An electrolysis plant in Sweden
uses much less environmentally detrimental electricity than an identical
plant in Holland, as hydroelectric power is abundantly used in Sweden.
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Data quality
Publications on environmental process data are often incomplete or inaccurate.
Moreover, the data are subject to obsolescence; there are many cases where
processing industries have cut emissions by 90% during the last ten years. The
use of obsolete data can therefore cause distortions.
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Choice of technology A distinction can be made between
worst, average, and best (or modern) technology.
Before starting to collect data it is important to be aware of which type of
technology you are interested in. In
SimaPro we have collected average technology as far as possible.
Despite these problems, it is often quite feasible to carry out an impact
inventory. It is unreasonable, however, to treat the results as an absolute
truth. Factors such as the choice of technology and system boundaries, data
quality etc. have to be taken into account when interpreting them. This is
why there always seems to be disagreement among experts about the
environmental soundness of a product.
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