How to: Model chemicals in LCA

Learn how to model chemicals while performing a Life Cycle Assessment (LCA).

Emily Lalonde avatar
Written by Emily Lalonde
Updated this week

Some chemicals can be modeled directly using an existing dataset in the database. For example, the processes of producing urea in any part of China can be accounted for using a dataset “Urea Production|Urea|cut off, U-CN.” This dataset captures the foreground and background data associated with urea production.

Sometimes, chemical compounds cannot be found in databases, especially when they are more complex or less commonly used chemicals. How can you choose a fitting dataset in this case?

This article outlines the steps to model chemical compounds that aren't found in databases.


Dataset workarounds

Step 1: Estimate the contribution of the impact of the chemicals in the process.

A less accurate approximation is practical if the chemical at hand contributes a tiny amount to the product's total impact. It will take less time to model a less accurate approximation without significantly affecting the result. By contrast, if the chemical is one of the key components of the product, then a more precise approach is necessary.

Step 2: Check for the chemical

Sometimes chemicals are present in ecoinvent with a different name, for example:

  • 2-Aminoethanol = monoethanolamine

  • Phenylethane = ethylbenzene

  • Esterquats = Quaternary ammonium compounds

A helpful website we often use at Ecochain to find chemical synonyms is PubChem.

Step 3. Check whether the chemical belongs to a more generic category

Check whether the chemical belongs to a more generic category and can be modeled as such:

  • Non-ionic surfactant

  • Ethoxylated alcohol

  • Organic solvent

  • Foaming agent

For example, Tween 80, broadly categorized as a Polysorbate, can be modeled as a non-ionic surfactant.

Step 4. Use a generic dataset (chemical organic/chemical inorganic)

Use generic datasets for organic (carbon-containing) or inorganic chemicals such as the “market for chemical, organic | chemical, organic | Global” or “market for chemicals, inorganic | chemical, inorganic | Global” dataset from ecoinvent.


Stoichiometry workarounds

If adopting the abovementioned options is infeasible, the chemicals can be modeled using stoichiometry. For better understanding, an example of pyrogenic silica (SiO2) produced from Silicium tetrachloride (SiCl4) is described in detail.

Step 1: Atom weights of common elements.

Chemical element

Atom weight

Unit

C

12,011

g·mol−1

H

1,008

g·mol−1

O

15,999

g·mol−1

Cl

35,453

g·mol−1

Si

28,086

g·mol−1

Step 2: Determine atom weights (molar masses) of the chemical reaction.

Chemical element

Atom weight

Unit

Input

-

-

SiCl4

169,898

g·mol−1

2H2

4,032

g·mol−1

O2

31,999

g·mol−1

Sub-total

205,928

g·mol−1

Output

-

-

SiO2

60,084

g·mol−1

4HCl

145,844

g·mol−1

Sub-total

205,928

g·mol−1

Step 3: Relate atom weights (molar masses) of the chemical reaction to the reference chemical.

Chemical element

Atom weight

Unit

Calculation

Kg chemical /kg pyrogenic silica (SiO2)

Input

-

-

-

-

SiCl4

169,898

g·mol−1

169,898/60,084

2,828

2H2

4,032

g·mol−1

4,032/60,084

0,067

O2

31,999

g·mol−1

31,999/60,084

0,533

Output

-

-

-

-

SiO2

60,084

g·mol−1

60,084/60,084

1,000

4HCl

145,844

g·mol−1

145,844/60,084

2,427

Step 4: Correction for yield Assumed yield = 90%.

Chemical element

Atom weight

Unit

Calculation

Kg chemical /kg pyrogenic silica (SiO2)

Input

-

-

-

-

SiCl4

169,898

g·mol−1

2,828/0,9

3,142

2H2

4,032

g·mol−1

0,067/0,9

0,075

O2

31,999

g·mol−1

0,533/0,9

0,592

Output

-

-

-

-

SiO2

60,084

g·mol−1

-

1,000

4HCl

145,844

g·mol−1

-

2,427

Step 5: Find correct ecoinvent datasets.

· SiCl4 = silicon tetrachloride production | silicon tetrachloride | Global

· 2H2 = chichibabin amination | hydrogen, liquid | Europe

· O2 = -

· 4HCl = market for hydrochloric acid, without water, in 30% solution state | hydrochloric acid, without water, in 30% solution state | Europe

Step 6: Add chemical reaction to Mobius and put ' minus (-) in front of the property for 4HCl.

This should be done because the system is multifunctional. No allocation should be applied. Instead, the impact of 4HCl should be corrected. This can quickly be done in Mobius by simply putting a minus (-) in front of the property. This gives -1 kg mass.

A point to note when applying this approach is that the process energy requirement needs to be considered while considering the stoichiometric calculations.

Note: When applying this approach, the process energy requirement must be considered while considering the stoichiometric calculations.

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