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CALCIUM CYCLE FOR EFFICIENT AND LOW COST CO2 CAPTURE IN FLUIDIZED BED SYSTEMS

Final Report Summary - C3-CAPTURE (Calcium cycle for efficient and low cost CO2 capture in fluidized bed systems)

The C3-CAPTURE project aimed to develop an advanced dry CO2 capture system applicable for both pulverised firing (PF) and circulating fluidised bed (CFB) boiler systems. The project's full title was: 'Calcium cycle for efficient and low cost CO2 capture in fluidized bed systems'. Two options for CO2 capture from boiler systems were investigated: an integrated atmospheric fluidised bed system for post-combustion capture from PF or CFB boilers and an in-situ capture system for PFBC boilers.

The quantifiable objectives of the development were:
- low CO2 capture costs (<2 0 euro/ton for atmospheric, < 12 euro/ton for pressurised systems);
- low efficiency penalty for CO2 capture (= 6 % ?el including CO2 compression to 100 bar). The efficiency penalty is even lower (= 4 %) when considering the integration with a cement plant where pre-calcined feed reduces energy consumption and CO2 release;
- > 90 % carbon capture for new power plants and > 60 % for retrofitted existing plants;
- A calciner gas stream containing > 95 % CO2 (dry base);
- A solid product usable for cement production;
- Simultaneous sulphur and CO2 removal with sulphur recovery option.

The project was structured into the following work packages (WPs):
- WP 1: Process definition and boundary conditions
- WP 2: Selection and improvement of sorbent materials
- WP 3: Experimental testing of the process
- WP 4: Technical and economic process evaluation
- WP 5: Pilot plant design.

By integrating a closed carbonation-calcination loop in the flue gas of a conventional CFB boiler, the CO2 in the flue gas can be removed. The heat required for calcination is released during carbonation and can be utilised efficiently (i.e. at high temperature) in the steam cycle of the boiler. Highly concentrated CO2 can be generated when using oxygen blown calcination. Because the fuel required for supplying sufficient heat for calcination is only a fraction of the total fuel requirements, the required oxygen is only about 1/3 to 1/2 of the oxygen required for oxyfuel processes. This CO2 capture technology can be compared to wet scrubbers using amines for CO2 absorption. These scrubbers need as well a regenerator operating at higher temperatures than the absorber, but at a considerably lower temperature level compared to the lime loop. The lime based 'dry scrubber' operates at typical steam cycle temperatures, so the energy can be directly reused. Therefore, the efficiency penalty of such a capture system is typically very low (< 6 %) compared to other capture technologies. Since limestone is a low cost material with good geographical distribution, it allows the use of local limestone resources from power plants for CO2 capture with minimal limestone-related infrastructure investment.

The following findings could be concluded during the early phases of the project:
- Taking into account the costs and the scale of the processes of carbon dioxide recovery from flue gas and recognising experimentally the highest attrition resistance of the sorbents obtained by calcination of CaCO3 of mineral origin, it is recommended to use this type of sorbent in the post combustion carbon capture process.
- There is the common property of sorbents produced from the fines- they must be reprocessed back to the required powder form before reuse. It was found that the sophisticated but expensive process of physico-chemical agglomeration of fines combined with thermal reactivation is needed so as to produce the necessary powders.
- The low cost process of physical agglomeration of fines, which uses only water and inexpensive reagents (surfactants), produces porous sorbent of high reactivity, which could be utilised in desulfurisation of flue gases without the necessity of activation of the granules by calcination. This product could be used for preliminary removal of SO2 in the combustors.
- The fines collected from the 'dusty' outlet gases, after calcination produce the fine porous particles, which could be used as the effective support in preparation the dry sorbents of high activity. Those are of increasing interest in CO2 capture from humid flue gases at low temperatures.

Additional findings included the following:
- It is possible to establish a commercial system with both fluidised beds working under parameters close to standard fluidised circulating beds.
- It is possible to recover most of the heat available in this capture system in a new supercritical water-steam power cycle with very high efficiency. This extra power generated reduces considerably the costs of the CO2 captured. An efficiency of more than 36 % is possible for a configuration with steam-driven compressors and steam feeding water pump.
- The integrated system, consisting of the existing power plant and the C3 Capture technology plant, could reduce the costs of the CO2 avoided in the range of 20-25 euro/t CO2 which is very low when comparing with other systems like MEA or oxy-fuel combustion which exhibits values over 30 euro/t CO2.
- One of the parameters with most influence in the final cost of the CO2 captured and in the cost of electricity is the annual equivalent operation ratio. The goodness of this system came from the extra electricity produced in the plant. The income of this electricity covers the high investment cost of the system. So, it is necessary to guarantee a minimum value of operation hours. On the other hand, the possibility of selling the deactivated material purged from the system to a cement industry could reduce the final economical balance notably.
- The three main inconveniences of this capture system are the high area needed for the erection, (normally this is a critical issue in existing sites), the necessity of a limestone provider and an own infrastructure for limestone storage and pre-treatment, and the high investment cost needed for carrying out the project. Nevertheless, new extra power is installed with integrated CO2 capture systems. That new power compensates the extra space and investment cost needed, and reduces considerably the cost of the electricity of the integrated system.

VTI analysed the performance of the design for two types of coals: bituminous coal and brown coal from a heat balance analyse for the cycle, and for two different pressures in the carbonator: atmospheric pressure and 1.6 MPa.

Next results were achieved:
- In case of bituminous coal when reactor-carbonator works at the atmospheric pressure for own needs 72 % of heat are spent.
- In case of bituminous coal when reactor-carbonator works at pressure 1.6 ?P? for own needs 37 % of heat are spent.
- In case of a brown coal when reactor-carbonator works at the atmospheric pressure for own needs 83 % of heat are spent.
- In case of a brown coal when reactor-carbonator works at pressure 1.6 ?P? for own needs 37 % of heat are spent.

The experimental tests carried out in the C3-CAPTURE project have demonstrated the viability of the process in a basic research size. Nevertheless, it is necessary that a new experimental facility of bigger scale is built, where hydrodynamics of the process could be analysed in detail. Large pilot testing will provide an intermediate validation step between the lab and the demonstration 20 / 50 MWe and will allow for preparation of back up plans to produce CO2 according to specification. A 1 MWth pilot will allow testing of the process in conditions closed to an industrial unit.

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