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Status and outlook

Figure 3.18a Fuel cell system with pre-fuel cell CO2 capture. The carbon-containing fuel is first completely converted into a mixture of hydrogen and CO2. Hydrogen and CO2 are then separated and the H2-rich fuel is oxidized in the fuel cell to produce electricity. The CO2 stream is dried and compressed for transport and storage.

Figure 3.18a Fuel cell system with pre-fuel cell CO2 capture. The carbon-containing fuel is first completely converted into a mixture of hydrogen and CO2. Hydrogen and CO2 are then separated and the H2-rich fuel is oxidized in the fuel cell to produce electricity. The CO2 stream is dried and compressed for transport and storage.

This section reviewed a wide variety of processes and fuel conversion routes that share a common objective: to produce a cleaner fuel stream from the conversion of a raw carbonaceous fuel into one that contains little, or none, of the carbon contained in the original fuel. This approach necessarily involves the separation of CO2 at some point in the conversion process. The resulting H2-rich fuel can be fed to a hydrogen consuming process, oxidized in a fuel cell, or burned in the combustion chamber of a gas turbine to produce electricity. In systems that operate at high pressure, the energy conversion efficiencies tend to be higher when compared to equivalent systems operating at low pressures following the combustion route, but these efficiency improvements are often obtained at the expense of a higher complexity and capital investment in process plants (see Section 3.7).

In principle, all pre-combustion systems are substantially similar in their conversion routes, allowing for differences that arise from the initial method employed for syngas production from gaseous, liquid or solid fuels and from the subsequent need to remove impurities that originate from the fuel feed to the plant. Once produced, the syngas is first cleaned and then reacted with

Figure 3.18b Fuel cell system with post-fuel cell CO2 capture. The carbon-containing fuel is first converted into a syngas. The syngas is oxidized in the fuel cell to produce electricity. At the outlet of the fuel cell CO2 is separated from the flue gas, dried and compressed for transport and storage.

steam to produce more H2 and CO2. The separation of these two gases can be achieved with well-known, commercial absorption-desorption methods, producing a CO2 stream suitable for storage. Also, intense R&D efforts worldwide are being directed towards the development of new systems that combine CO2 separation with some of the reaction steps, such as the steam reforming of natural gas or water gas shift reaction stages, but it is not yet clear if these emerging concepts (see Section 3.5.3) will deliver a lower CO2 capture cost.

In power systems, pre-combustion CO2 capture in natural gas combined cycles has not been demonstrated. However, studies show that based on current state of the art gas turbine combined cycles, pre-combustion CO2 capture will reduce the efficiency from 56% LHV to 48% LHV (TEA, 2000b). In natural gas combined cycles, the most significant area for efficiency improvement is the gas turbine and it is expected that by 2020, the efficiency of a natural gas combined cycle could be as high as 65% LHV (IEA GHG, 2000d). For such systems the efficiency with CO2 capture would equal the current state-of-the-art efficiency for plants without CO2 capture, that is, 56%

Integrated Gasification Combined Cycles (IGCC) are large scale, near commercial examples of power systems that can be implemented with heavy oil residues and solid fuels like coal and petroleum coke. For the embryonic coal-fired IGCC technology with the largest unit rated at 331 MW , future improvements are expected. A recent study describes improvements potentially realisable for bituminous coals by 2020 that could reduce both energy and cost-of-electricity penalties for CO2 capture to 13% compared to a same base plant without capture. For such systems the generation efficiency with capture would equal the best efficiency realisable today without CO2 capture (i.e., 43% LHV; IEA GHG, 2003). Notably, all the innovations considered, with the exception of ion transport membrane technology for air separation (which is motivated by many market drivers other than IGCC needs) involve 'non- breakthrough' technologies, with modest continuing improvements in components that are already established commercially - improvements that might emerge as a natural result of growing commercial experience with IGCC technologies.

All fuel cell types are currently in the development phase. The first demonstration systems are now being tested, with the largest units being at the 1 MW scale. However, it will take at least another 5 to 10 years before these units become commercially available. In the longer term, these highly efficient fuel cell systems are expected to become competitive for power generation. Integrating CO2 capture in these systems is relatively simple and therefore fuel cell power generation systems offer the prospect of reducing the CO2 capture penalty in terms of efficiency and capture costs. For instance, for high temperature fuel cell systems without CO2 capture, efficiencies that exceed 67% are calculated with an anticipated 7% efficiency reduction when CO2 capture is integrated into the system (Jansen and Dijkstra, 2003). However, fuel cell systems are too small to reach a reasonable level of CO2 transport cost (IEA GHG, 2002a), but in groups of a total of capacity 100MWe, the cost of CO2 transport is reduced to a more acceptable level.

Most studies agree that pre-combustion systems may be better suited to implement CO2 capture at a lower incremental cost compared to the same type of base technology without capture (Section 3.7), but with a key driver affecting implementation being the absolute cost of the carbon emission-free product, or service provided. Pre-combustion systems also have a high strategic importance, because their capability to deliver, in a large scale and at high thermal efficiencies, a suitable mix of electricity, hydrogen and lower carbon-containing fuels or chemical feedstocks in an increasingly carbon-constrained world.

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