Retrofitting Existing Plants

Addressing the future of coal-based power generation raises many key issues and challenges. A number of these are analyzed next. To achieve really significant reductions, CO2 emissions from the existing coal fleet will have to be reduced. This will require retrofitting CO2 capture on existing units, or repowering them with high-efficiency technology with CO2 capture such as IGCC-CCS or replacing them with other technology.

Retrofitting existing units involves several factors that significantly affect the economics and viability of the unit. These include unit age, size, and operating efficiency, as well as land availability or other space constraints at the plant site. Existing units are frequently smaller, have low generating efficiency, and may not have highly efficient emissions control systems relative to large, new builds. The energy requirement for CO2 capture is usually higher for retrofits because of less efficient heat integration for sorbent regeneration in an existing plant. For power generation, plant output reduction approaches 40% vs. the 30% reduction for purpose-built plants [39, 43-45]. Existing plants that are not equipped for adequate NOx control or with a flue gas desulfurization (FGD) system for SO2 control must be retrofitted or upgraded for high-efficiency sulfur capture in addition to the CO2 capture and recovery system. All these factors lead to higher overall costs for retrofits. Figure 2.18 illustrates the retrofit of a subcritical PC unit with MEA (monoethanolamine) flue gas scrubbing. The original unit had a generating efficiency of 35% (HHV) without CO2 capture; after retrofit with CO2 capture the original 500 MWe unit produces only 294 MWe and has a generating efficiency of 20.5% (HHV) or a 41.5% derating. The efficiency reduction for a

Fig. 2.18 Retrofit of a subcritical PC unit with amine CO. capture

CO2 capture purpose-built unit would be about 28% (HHV) or a 28% derating. For the purpose-built unit, everything is optimally sized; for the retrofit unit, steam is diverted from the turbine for sorbent regeneration, and the turbine is operating at about 58% of design loading, far from its conditions for optimum performance.

If the original unit is fully paid off, the cost of electricity after retrofit could be slightly less to somewhat more than that for a new purpose-built PC plant with CO2 capture based on the new capital required [43, 44]. However, an operating plant will usually have some residual value, particularly if flue gas clean-up technology has recently been added; and the reduction in plant efficiency and output, increased on-site space requirements, and unit downtime are all complex factors not fully accounted for in this analysis. For smaller, older units, rebuilding the entire boiler and power generation sections or replacing them with IGCC (repowering) may be the best alternatives [44, 45]. Generally, the cost of CO2 avoided is expected to be 30-40% higher than for a purpose-built capture-plant. For example, an MEA retrofit of a supercritical PC is projected to cost almost as much as a new unit on a $/kWe basis from an Alstom retrofit design study [43]. Retrofit capture costs have been projected to range from 2 to 70/kWe-h from best to worst case scenarios considered with 90% CO2 capture in a feasibility study by Alstom [46]. Further, retrofits require case-by-case detailed design-based examination. Although there is no one answer for existing subcritical PC units, CO2 capture will likely be achieved through repowering with a supercritical PC unit with CO2 capture or with oxyfuel or with IGCC-CCS or other technology, rather than retrofitting. A recent MIT symposium on retrofitting PC plants for CO2 control addressed all of these issues but found no easy, cheap solutions [45]. The option of producing fuels and power from biomass and coal is a new option that is discussed in Chap. 3.

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