Manufacturing and Product Allocation

For a cradle-to-gate carbon footprint the final step is usually the production of the respective substance and depending on the chosen boundary limits potentially including filling and packaging.

The production is analyzed based on a balance of all material, utilities and energies flowing into and out of the unit. Usually the balance should be done on average numbers for a certain period of time, for example, one year of production. Non-continuous streams like for batch operations or for cleaning purposes should also be taken into account. The total balance should be representative for the goal and scope of the carbon footprint. Preparing a consistent, sound and representative balance is the most tedious part of the entire exercise for a cradle ^gate footprint.

We first look at the direct emissions of GHG from the unit. Typical sources for direct emissions in chemical production plants are

• Carbon dioxide mostly originates from incinerations. Some large petrochemical production units have significant emissions because of fuel [fired heating or yield loss in strong oxidation reactions. Examples are steam cracking or ethylene oxide production. Smaller amounts of CO 2 often originate from off gases or side reaction, gaseous purge streams or on - site incineration of organic waste streams. Large scale fermentation processes (e.g., ethanol production) also have larger volumes of CO2 emissions (note issues with bio-based raw materials).

• Nitrous oxides emissions occur on a larger scale mostly in processes with nitric acid (nitric acid and fertilizer production or nitration processes like adipic acid or other nitro-components). Smaller emissions can also originate from incineration processes.

• Methane can often be found in off gas streams, from incomplete conversion of chemical reactions or from leakage (e.g., natural gas leakage). Rotting of biodegradable matter also causes methane emissions.

• Most plants producing organic chemicals have some emissions of VOCs (volatile organic compounds). Their contribution to the carbon footprint depends on the individual composition. Most relevant are halogenated and fluorinated components. Their GWP is very high.

• Sulfur hexafluoride occurs predominantly in processes for its production. Use of SF6 with emissions is in magnesium production and mainly as inert gas in electrical high voltage applications.

All input material and utility streams into the production unit contribute to indirect emissions with their individual carbon footprint as described above.

Handling of output streams for the carbon footprint has to be distinguished according to the purpose of the respective streams. We can distinguish between main product stream(s), side product streams, and waste streams.

• Product streams are the desired output of the production unit. In the simple case of a continuously operated one-product plant, the total contribution of the carbon footprint is referred to this output stream and results in the total cradle-to-gate footprint of the respective product (e.g., in kg CO2e/kg product). Multipurpose plants produce a variety of different substances. If the majority of the consumed raw materials and utilities can be uniquely allocated to the individual product, separate carbon footprints for each product can be generated. This cannot be applied to typical coproduct plants (examples: crude oil refineries, steam crackers, NaCl electrolysis, air separation units, steam reformers, propylene oxide and styrene via the POSM process) where the majority of the consumed resources does not naturally split into fractions for each individual product. Here, allocation is always to a certain extent arbitrary. Typical allocation methods are by mass, mole or value of the products. See the example at the end of this section for more details.

• By-products are undesired outputs of the production unit. In contrast to waste streams these are used. We would differentiate them from coproducts mainly by their relevance. Typical examples are purge streams or residues used as fuel substitute. Off-spec product can sometimes also be handled like by- products. Handling of those streams is very case-dependent. A conservative approach is to set the carbon footprint of these streams to zero, so it does not reduce the total carbon footprint. Sometimes emissions are allocated to the side product based on a comparative use. An example is a process residue used as a fuel substitute in a power plant. If the incineration of the residue causes less GHG emission than the regular fuel (e.g., coal) the difference can be accounted to the side stream and hence reduces the total carbon footprint. However, this usually exceeds the cradle-to-gate scope as credit to the product is generated from an application downstream of the gate. Furthermore there is a risk of double counting if this effect is already included in the model for the supplied energy.

• Waste streams are of no use for the plant or the process and need to be disposed of. Handling of streams emitted into the environment is already explained above. If the stream does not carry any GHG relevant component, its carbon footprint would be set to zero. If the stream is cleaned or destroyed in a dedicated unit (e.g., off-gas incineration, waste water treatment), it needs to be decided if that unit is inside the system boundary limits. In that case the proportional fraction of emissions needs to be included.

The chlorine electrolysis example does not involve any direct emissions. Electrolysis is a typical coproduct or multi-output process, well suited to describe the options and issues from allocation. Allocation in NaCl electrolysis can be avoided if the impact is referred to an electrochemical unit (ECU) which is the combination of the products Cl2 and NaOH. Boustead [20] describes five methods for allocating raw materials and another 13 methods of allocating energies. This shows that there are manifold possibilities leading to very different results. It is not a question of correctness but of validity of the respective method. Frequently it is ambiguous which method is most appropriate and different opinions exist. This can only be decided case by case. For our example we will perform a mass and mole based allocation and show the impact on the results (Figure 1.6 and Table 1.4).

In an additional case we treat hydrogen as a by-product to substitute fossil fuel. The impact of the raw materials and energies are solely allocated to chlorine and NaOH and hydrogen is taken into account as a credit. This calculation is based on the assumption, that hydrogen will completely replace natural gas for thermal use (Figure 1.7).

First of all we can see from the graphs that power consumption is by far the major contributor to the carbon footprint. This is true even if we consider the energy carbon footprint from a local CHP plant (Section 1.2.2.1). In the base case raw materials have a share of 8.5% (based on total ECU carbon footprint) and transportation is negligible. The result can look completely different for other chemicals. Variance is too high to give typical or representative numbers for broader classes of products. From the scenarios we can also see the significant impact of different allocation procedures. The carbon footprint of hydrogen varies by a factor of 20 from mass to molar allocation. The credit of hydrogen if treated as a fuel substitute also has a relevant influence. Summarizing we can state that

lu 0.60

Figure 1.6 Cradle-to-gate carbon footprint for an ECU (electrolysis example).

Table 1.4 Mass and mole allocation of the electrolysis step.

Mass allocation

Mole allocation

Chlorine

46.4l%

25%

NaOH

52.29%

50%

Hydrogen

l.3l%

25%

the method applied for the total carbon footprint for an ECU would allow a sound comparison of different electrolysis processes at different sites including the complete cradle-to-gate value chain. Special regard must be given to the approach for modeling energy supply. However, for the single product chlorine, it is absolutely essential to analyze in closer detail the methods applied for the calculations before drawing conclusions from the resulting footprints. Electrolysis is a special example that we selected intentionally to discuss some of the issues. Chlorine is also a basic building component for the chemical industry and hence it has influence on many end products.

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