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METHODOLOGICAL GUIDANCE Sample calculation for Option 2: CO2 captured from flue gas A range of different technologies are available for captu- ring CO2 from power plant exhaust gas streams. One of the best known technologies is capture by chemical absorpti- on, which, as an end-of-pipe technology, can therefore be retrofitted to existing power plants. Carbon capture occurs by absorbing the CO2 from the flue gas stream by means of a solvent. In this example we consider an advanced gas-fi- red power plant that requires about 45 kg of natural gas to generate 1 GJ of electrical energy. The combustion process generates around 120 kg of CO2. Emissions of approxima- tely 0.5 kg CO2-eq are associated with the fuel supply chain (gas extraction and transport), but these will be ignored in the current calculation. The amine scrubbing method of CO2 capture is carried out in two stages: 1. Absorption of the CO2 by an aqueous amine solution and 2. Regeneration of the solvent by heating the saturated solvent to release (desorb) the CO2. The large vo- lume streams involved, the relatively low concentrations of CO2 in the flue gas (approx. 12% in the crude gas due to pre- sence of air nitrogen) and the limited absorption capacity of the amine solvent mean that a comparatively large amount of energy is needed to drive the capture process. In this ex- ample, solvent regeneration requires 2.5 GJ of steam per me- tric ton of captured CO2. This reduces the overall efficiency of the power plant by eight percentage points and results in greater fuel consumption in the power plant for no change in the amount of electricity generated. We assume here that the carbon capture process has an efficiency of 90%. Other common methods of allocation lead to highly ske- wed results: At the process level, one could argue that in the case ‘without carbon capture’, the production of 1 GJ of electricity from natural gas generates average GHG emissi- ons of 120 kg CO2-eq. If, however, the CO2 is captured (with an efficiency of 90%), the remaining GHG emissions are on- ly 14 kg CO2-eq., which is then allocated between the two products.8 As electrical energy and CO2 do not have any common physical properties, environmental burdens can only be allocated on the basis of market value. If we assume prices of € 0.10/kWh and €8/t CO2, electricity is assigned 96% of the burden, while 4% is allocated to the CO2. A power plant operator would no doubt be very pleased with such an approach, as the electricity generated is now essen- tially ‘emissions free’ (13 kg CO2 instead of 120 kg CO2 per 1 GJ of electricity). The recipient of the CO2 (‘the downstre- am processor’), who utilises the CO2 and is responsible for the carbon inventory for the remaining life of any products produced, would clearly be less happy with this scenario, as the burden is in this case 1 kg CO2-eq. per 126 kg of CCU product. In addition to the cost of processing and conver- ting the CO2 into a marketable product, the downstream processor must bear the full emissions burden from the end-of-life combustion of the product. This burden will be approximately equal to the 126 kg of CO2 captured from the fuel combustion process in the power generating plant, but no negative emissions credits would be issued in this case as the CO2 has not been removed from the atmosphere but generated from a technical process. Without CO2 capture Input Natural gas 45 kg Output Output Electrical energy CO2 emission 1.000 MJ 120 kg With CO2 capture Input Natural gas 52 kg Output Output Electrical energy CO2 production CO2 emission 1.000 MJ 126 kg 14 kg System expansion is not possible without additional in- formation on how the captured CO2 will be utilised. The problem of how to allocate the environmental burdens from the production process has to be addressed: the CO2 is no longer a climatically harmful emission, but a co-pro- duct; and what was a single-function process (electricity generation) has now become a multifunctional process (producing electricity and CO2). The various approaches to allocation discussed in the main text yield the following results when applied to the present example: a) The product from the CO2 utilisation stage (the ‘CCU product’) is a ‘zero-emission’ product at end of life (iLUC Directive (EU) 2015/1513): The main burden of 126 kg CO2, which would be released when the product from the CO2 utilisation stage undergoes end-of-life combus- tion, is in this case assigned in full to the electricity gene- ration process, plus the residual emissions of 14 kg CO2. As this is greater than the emissions burden from gene- rating 1 GJ of electrical energy without carbon capture, there is clearly no incentive to the power plant operator to capture CO2. b) 50:50 split between both processes: Each product (elec- tricity and CO2) is assigned a burden of 70 kg CO2-eq. The emissions burden associated with electricity pro- duction is therefore lower (70 kg instead of 120 kg CO2- eq./GJ) and the CCU product also generates a smaller en- vironmental burden at its end-of-life combustion than if it had been produced from fossil carbon (70 kg instead of 126 kg CO2-eq.). 8 As the calculation is normalised to the production of 1 GJ of electricity, reduced efficiency requires the consumption of more natural gas. The total quantity of CO2 (from the gas combustion process) therefore increases to 140 kg. 303PDF Image | Chemical Processes and Use of CO2
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