PCF from Cradle to Gate

In the following we will discuss the contributions for a cradle -to-gate footprint. There will be a general part on relevance and options for retrieving the data including a discussion on consequences. Calculations are illustrated using an example from the bulk chemical industry, chlorine production via electrolysis. The data is

7.2 Product Carbon Footprints (PCF) \ 43 Coding Sulfuric steam water acid

7.2 Product Carbon Footprints (PCF) \ 43 Coding Sulfuric steam water acid

Hydrogen

Downstream processing

Hydrogen

Figure 1.5 Block diagram for the example chlor-alkali electrolysis.

taken from public literature [16] for a diaphragm process with some adjustments. Please note that the example is simplified for the purpose of making it understandable. It is not a comprehensive accurate calculation of a carbon footprint for an electrolysis.

Figure 1.5 shows a block diagram of the production process and Table 1.2 the major consumption numbers for this process. The generation of such a balance is usually the first step and required for the decision on scope and on the contributions that need to be taken into account or neglected. The major raw materials are salt and water. Electrolysis processes have large energy consumptions determined mainly by electrical power and some steam for product processing. The target product is Cl2, and, per t Cl2, 1.0971 of NaOH (in 50% water) and 2.817 kg of H2 (stoichiometric) are produced as coproducts (Table 1.2).

The carbon footprint to be calculated will be used to assess the overall impact of chlorine production on climate change. We apply the cradle-to-gate scope which in this case covers

• raw materials used for the production with their upstream contribution;

• transportation of raw materials to the production unit;

• energy consumption;

• considerations on coproducts.

The example will be used in the following sections to demonstrate the calculation of the individual contributions. For this example we have used the tool GaBi [17] to perform the calculation. GaBi's data sources and methods are used unless otherwise quoted.

Table 1.2 Utility demand for the process steps of the example chlor-alkali electroylsis.

Electrolysis Chlorine NaOH Hydrogen Total

Raw materials

Electrolysis Chlorine NaOH Hydrogen Total

Raw materials

Rock salt

t/t Cl2

1.67

1.67

Sodium carbonate

t/t CI2

0.024

0.024

Hydrochloric acid

t/t CI2

0.016

0.016

Calcium chloride

t/t CI2

0.013

0.013

Sulfuric acid

t/t CI2

0.009

0.009

Utilities

Steam

t/t CI2

0.22

0.02 2.86

3.10

Electricity

MWh/t Cl2

2.95

2.95

Process water

m3/t Cl2

4.20

4.20

Cooling water

m3/t Cl2

69.00

69.00

Inert gas

Nm3/t Cl2

24.00

24.00

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