Simple CO2 Balance

Based on the numbers in Table 1.1 we can make very simple calculations of the impact of certain emissions on climate change. Assuming we drive a distance of 100 km with a passenger car that has emissions resulting from fuel combustion of 150 g CO2km-1, yields a GWP of 15 kg CO2e compared with simply staying where you are. This is a very simple form of a carbon balance, which is used to compare different activities with one another. These numbers are only relevant information in the simple cases where we compare certain activities with respect to their direct emissions of GHGs with one another or with no activity (which means staying where you are in this example). A few examples where such calculations can lead to meaningful results:

• Comparison of different power generation technologies, for example, a conventional fossil-fired power plant and a power plant that uses carbon capture and storage (CCS, see Chapter 11) technologies with respect to direct emissions.

• Reducing the impact on global warming of a chemical plant by implementing a N2O reduction technology (e.g., nitric acid, adipic acid).

• Emissions of methane from cattle: calculation of the amount of CO2 that needs to be extracted from the atmosphere and fixed to compensate for the emissions of methane from a single beast in one year (approx. 100 kg methane = 2100 kg CO2e).

Let's return to the passenger car example. Staying where you are is often not an alternative to driving somewhere. We want to compare alternatives to solve the task of transportation. For example, we want to understand what difference it would make, if we were to use an electrically powered vehicle. This vehicle has no direct emissions at all but it is obvious that the simple comparison does not tell the entire story. We would want to know where the power for charging the battery came from and assign the respective emissions to the trip. And if it is a solar powered vehicle, we maybe want to include the GHGs that have been emitted for manufacturing the photovoltaic panels as we know that this is an energy- and hence emission-intensive process. We now also consider a train as a third alternative for moving forward. If it is electrically powered there are again no direct emissions. The heavy-weight train will certainly have the highest indirect emissions caused by power consumption but shouldn' t we compare it on a per passenger basis rather than a per vehicle basis? How many people do we assume then traveling in one train and in one car? Different approaches and assumptions will lead to significantly different results. However, this does not mean that only one result is correct and all others are wrong. Assuming we are doing the math, physics and chemistry right, the results will all be correct. Which one is most reasonable depends on what exactly we want to know, that is, the scope of the problem and how we want to allocate the emissions. Even this apparently trivial problem illustrates that a calculated number standalone is not an adequate answer to the question 'What is the CO2 balance of this?'

The basic calculations for CO - balances rely mostly on mass and energy balances. We will discuss this together with the aspect of data acquisition later in this chapter (Section 1.2.2). The challenges with carbon balances are predominantly in the areas of defining the scope and the base case, choosing system boundaries, making appropriate assumptions, reasonable allocations, and approximations. In the remainder of this section we will highlight a few examples.

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