April 10-14, 2016
Implementation of currently considered and available CCS technologies into fossil power plants brings inevitable technical, energy and economic penalty. This is getting even larger when fossil fuels such as low rank coal are being utilized. All three generally considered CCS technologies were modelled – oxyfuel combustion and ammonia based post-combustion (subcritical power plant with fuel drying) and pre-combustion (IGCC with Rectisol method for CO2 separation).
After traditional methods of system optimization there was considered another way for increasing system efficiency. CCS technologies produce waste heat streams, which can be converted to electricity by small modular units with unit cost comparable to the whole plant, some of which are already commercially available. Here we consider technologies based on steam microturbine, Organic Rankine Cycle (ORC) and absorption power cycle.
CCS technologies generally produce significant amounts of waste heat from CO2 compressor intercooling which pressurize the CO2 for state for the transport and storage. Post-combustion method provides possibility for waste heat recovery partly on cooling down of flue gas before entering absorber and from cooling down of desorbed CO2 stream. Oxyfuel combustion and IGCC with oxygen gasification provide also large amounts of waste heat from intercooling of air, eventually oxygen and nitrogen compressors of air separation unit. Fluidized bed fuel dryer exhaust also contains some potential for work. Pre-combustion IGCC plant provides other possibilities for waste heat recovery from low temperature syngas cooling and from very clean flue gas at low temperatures, which are already impossible to be utilized by regular steam part of combined cycles.
In order to utilize the waste heat streams and increase plant efficiency, there are often designed sophisticated but complicated systems, especially for feed water preheating. Although they slightly increase the plant efficiency, the resulting system has low flexibility. It is presented here that by decoupling waste heat streams from main steam cycle and by low cost in modular waste heat recovery units there can be at the same time increased both plant efficiency and flexibility, while the negative effects associated with these measures are minimal.
Detailed results (technical and economic) are presented for a case scenarios of 250 MWe coal fired power plants, applied to specific conditions of central Europe. The considered fuel for subcritical oxyfuel plant is a low rank coal, lignite, with heating value (LHV) down to 8.5 MJ/kg, water content up to 35% and ash content up to 40% and for the IGCC plant is used coal of LHV about 16.5 MJ/kg, water content over 30% and ash content around 9%.
Vaclav Novotny, Monika Vitvarova, Michal Kolovratnik, and Zdenek Hrdina, "Minimizing the energy and economic penalty of CCS power plants through waste heat recovery systems" in "CO2 Summit II: Technologies and Opportunities", Holly Krutka, Tri-State Generation & Transmission Association Inc. Frank Zhu, UOP/Honeywell Eds, ECI Symposium Series, (2016). https://dc.engconfintl.org/co2_summit2/49