The efficiency ofCCS as a measure for lowering CO2 emissions is described using various parameters:
• energy requirements for CO2 capture, transport and storage;
First of all, in the process itself, the CO2 capture rate is an important characteristic value and design parameter. It describes how much of the CO 2 that would otherwise be emitted into the atmosphere is captured from the process and stored. In the three methods developed for CO2 capture from power plants, the capture rate is typically some 90%, possibly even 92%. A much higher capture rate can only be achieved with disproportionately large outlays. The CO2 capture rate takes no account of the energy consumption for capture, transport and storage, and, hence, ignores the CO2 emissions associated with such energy consumption.
In assessing the overall CCS process chain, it is of course important to quantify the energy requirements. Depending on the capture process, the chief consumers are: air separation, desorption of the CO - from the scrubbing solution, and any conversion steps. Irrespective of the capture method, the CO2 must be compressed to a supercritical pressure (only transportation by pipeline as the standard case is considered here, ignoring the ship, railway and truck options as special cases with any given amount of complexity.) In this respect, energy is used both in the form of heat and in the form of electricity. In the power plant sector, the energy requirements can be covered by the power station process, so that optimal integration is possible. Any heating steam, for instance, is made available as turbine extraction steam, which makes this the most efficient concept. For power stations, therefore, the entire energy requirements for CCS can be combined and stated as a reduction in net electric output or efficiency. For the three capture processes, including transport and storage, the equivalent energy requirements per ton of captured CO2 are in a range of roughly 0.25-0.3 MWh electricity, so that efficiency is lowered by approx. 10 percentage points. CO2 compression, which is necessary for transport and storage irrespective of carbon capture, is responsible for a pro-rated efficiency fall of 3-4 percentage points.
To determine the effective energy requirements for CO2 capture in other industrial plants, a precise examination of the provenance of heating steam and electricity is necessary.
For climate protection, finally, it is the amount of CO2 avoided, that is, the effective reduction in CO- emissions, that is relevant. This is the result of the two parameters described above, the CO2 capture rate and energy consumption with the associated additional CO2 emissions. The basis for calculating the avoided CO2 should be the target quantity of the product to be made. The CO2 amount avoided is then obtained from the difference of the CO2 emissions with and without CCS. In this respect, account must be taken of all components in the process chain, including the CO2 emissions occurring in the provision of auxiliary energies. In plants where, as in power stations, all CCS components constitute an integrated, self- contained system for the energy supply, the specific CO2 emissions of the product made can be established relatively simply (in power generation: ton of CO2 per MWh of electricity). The specific CO2 emissions for plants with and without CCS then very graphically express the efficiency of the CCS measure.
In summary, the following rough values may be estimated for a rough balance and assessment of CCS:
• fall in efficiency: 10 percentage points.
The effectively avoided CO2 quantity depends on the efficiency of the basic power plant without CCS:
where aeff: effectively avoided CO2 amount compared with the basic power plant n»sic: efficiency of the basic power plant without CCS nCCS* efficiency ofthe power plant with CCS cr: C02 capture rate
If the efficiency of the basic power plant is 50%, we obtain-with the rough values for the CO2 capture rate and the fall in efficiency-effective CO2 avoidance of 87.5% compared with the basic power plant. (NB: Of course, the CO2 quantity avoided, relative to an existing power plant to be replaced, is obtained using a completely different calculation, and is often above the capture rate, since in many cases the new CCS power station will have much higher efficiency than the existing decades-old power plant.)
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