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Chemical production plants are often located in larger manufacturing sites with on-site energy generation. The heat demand is large and independent of the time of year and hence ideally suited for combined heat and power (CHP) generation. In this situation it makes sense to perform a detailed analysis of the power plant to calculate the carbon footprint of the energies provided. A CHP power plant is a typical example where allocation cannot be avoided. That means we have to decide how to distribute the emissions to heat and to power. There are various options available, for example:

1) Based on enthalpy or energy content. Electricity and heat are regarded as of equal value. This approach is used very often.

2) Based on the exergy. This is a thermodynamic concept to assess different values of form of energy. Electricity has 100% of exergy as it is theoretically fully convertible into any other form of energy, whereas the exergy of heat depends on its temperature. This approach makes most sense from a scientific point of view but is not consistent with some publicly available datasets.

3) The electricity gets its carbon footprint from a publicly available dataset, for example, the country footprint. All remaining inputs and outputs are allocated to steam. A rationale for this could be that electrical power supply is tightly linked with the country grid.

4) Inputs and outputs caused by producing steam in a boiler house are allocated to the steam and the remaining inputs and outputs are allocated to the electricity. This would make sense if the power plant is mainly used for producing steam and electrical power is only generated part-time.

Commercial energy generation datasets are available for many countries for a variety of power plant types (brown and anthracite coal, natural gas, hydroelectric, etc.). The specific energy mix results in a considerably lower carbon footprint for countries with a large fraction of nuclear energy use compared with countries dominated by coal-fired power plants. Thus, when using commercial datasets, it should be verified that the datasets for the respective country fit to the example.

Similar methods have to be applied for other forms of energies, like refrigeration, chilling, cooling water. Compressed air and technical gases like nitrogen or oxygen are usually also treated as energies in chemical plants. Their carbon footprint is normally made up only of the energy contributions required for the generation of these utilities. Like CHP power plants, air separation units require allocations (see also Section 1.2.2.4).

Example: The electrolysis plant is located on production site with a natural gas-fired CHP plant providing electricity and steam. We want to assess the contributions from energy consumption for chlorine production using different methods. Therefore we need to calculate the carbon footprint of power and steam for the power plant. The following parameters of the CHP plant are assumed:

• overall energetic plant efficiency 65%;

- Direct emissions from combustion: 204kg CO2eMWh-1 natural gas

- Emissions caused by upstream chain: 21.4 kg CO2eMWh-1 natural gas;

• Assumed exergy factors: 1 for power, 0.33 for steam.

Applying the described methodologies leads to the following results (Table 1.3): As an alternative to this concrete analysis we can use country specific carbon footprints from public sources for power (Database: GaBi):

Germany (2002): 706 kg CO2e MWh-1

France (2002): 150 kg CO2e MWh-1

Great Britain (2002): 664kg CO2eMWh-1

Norway (2002): 31 kg CO2e MWh-1

We can see that the numbers from databases and the numbers from a detailed analysis vary significantly. All numbers are equally correct. Which one should be applied depends on the detailed target of the analysis. For the case of the

Table 1.3 Carbon footprints of power and steam from different methods, kg CO2eMWh '.

Exergy

Enthalpy

Power from

Steam from

natural gas fix

natural gas fix

Power

473

347

535a)

419

Steam

157

347

65

239b)

a) Database: GaBi Power from natural gas (2002).

b) Database: GaBi Steam from natural gas (94%) (2002).

a) Database: GaBi Power from natural gas (2002).

b) Database: GaBi Steam from natural gas (94%) (2002).

electrolysis we arbitrarily use the country factor for Germany as the base case and use the enthalpy allocation for a case scenario.

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