Power generation technologies

The substantial thermal efficiency reductions resulting from incorporation of carbon capture technology to conventional pulverised coal combustion power plants have led to increased attention being given to investigations examining alternative fossil fuel-based host power generation technologies. These technologies need to provide high thermal efficiencies when coupled with carbon capture.

Two notable examples of alternative host power generation technologies which provide improved fundamentals for carbon capture are described.

CHEMICAL LOOPING COMBUSTION

Chemical Looping Combustion (CLC) technology is a form of oxyfuel combustion without the use of an air separation plant.

It can be used for combustion of coal for power generation. It relies on the use of paired fluidised beds (an oxidiser and a reducer) and the use of a solid oxygen carrier. The reducer exit gas contains almost all of the CO2 generated by the system and CLC therefore can be said to exhibit ‘inherent carbon capture’, as water vapour can easily be removed from the reducer exit gas via condensation, leading to a stream of almost pure CO2The production of a sequestration-ready CO2 stream therefore does not require any additional separation units and there is no energy penalty or reduction in power plant efficiency (NETL 2008a).

Theoretically, the efficiency penalty for CO2 capture only comes from the compressors that give the CO2 stream the right pressure for subsequent transport and geologic storage.

Analysis of CLC system performance has indicated that the thermal efficiency of a CLC system can be expected to be over 41 per cent with carbon capture (Global CCS Institute 2012b). The units are expected to be able to start up and then adjust their power production rate in a similar manner to a conventional pulverised coal combustion power plant.

A key current requirement for CLC is to scale-up the technology. Currently, the largest CLC system being demonstrated is the 1 MWth unit at University of Darmstadt in Germany. Once successful, this will see the technology achieve TRL-5. The test work in 2012 on this CLC system is seen as being critical to the development of CLC.

DIRECT INJECTION CARBON ENGINE

A Direct Injection Carbon Engine (DICE) power generation unit is based around a large low-speed diesel engine that is fuelled with micronised refined carbon fuel, which is made from coal and water instead of diesel oil.

Analysis of DICE system performance has indicated that the thermal efficiency of a DICE system can be expected to be up to 50 per cent with carbon capture (Wibberley 2012). One reason for this is the inherently high efficiency of the diesel thermodynamic engine cycle. Another key reason is that a DICE, unlike other power cycles, has large quantities of highly usable low-grade heat present in cooling streams. This heat can be used to regenerate post-combustion capture solvents without reducing net power output. For improved post-combustion capture solvent, the quantity of usable low-grade waste heat is a close match with solvent regeneration requirements. This is a significant advantage compared to applying post-combustion capture to conventional pulverised coal-fired plants and to natural gas combined cycle power plants, which both are likely to experience significant reductions in net power output as a result of the regeneration energy requirements of post-combustion solvent capture.

This process is currently the subject of RD&D work being undertaken by CSIRO in Australia building on the earlier work done by the US DOE in 1980–90s. The work is currently at TRL-4.