Gt

HRSG —►Steam

Fig. 26-41 Conventional Simple-Cycle Cogeneration System.

Source: Solar Turbines

Fig. 26-43 Simple-Cycle Cogeneration System with Supplementary

Firing. Source: Solar Turbines

Fig. 26-41 Conventional Simple-Cycle Cogeneration System.

Source: Solar Turbines featuring a 10 MW gas turbine with HRSG and water injection for NOx emissions control in a simple cogeneration cycle. The baseline arrangement produces about 48,000 lbm/h (22,000 kg/h) of saturated steam at 150 psig (11.4 bar). The range of electric and steam output is shown in the lower left corner of the energy map in Figure 26-46. As Table 26-9 indicates, the overall thermal fuel efficiency of this system is 76%. The net electric generation efficiency is 60%, which corresponds to an FCP of 5,688 Btu (6,011 kJ) per kWh, and the ETR is 0.68.

2. Figure 26-42 illustrates a conventional combined-cycle cogeneration system featuring the addition of an extraction steam turbine. With this arrangement, the HRSG is designed to produce superheated steam at a condition of 600 psig/800°F (42.4 bar/427°C). The operating range for this arrangement, with varying amounts of extraction steam being passed on to process, is shown near the bottom center of the energy map in the region labeled unfired combined-cycle. When operating in full condensing mode, the combined-cycle system can produce more than 14 MW of power. At the selected comparative design point for Table 26-9, 50% of the steam flow, or 21,000 lbm/h (9,500 kg/h), is extracted and the total system

Fig. 26-43 Simple-Cycle Cogeneration System with Supplementary

Firing. Source: Solar Turbines electric output is 12.6 MW At this design point, both the overall thermal fuel efficiency and the net electrical efficiency are the lowest of all the arrangements considered, while at 1.88, the ETR is the highest.

3. Figure 26-43 illustrates the addition of a duct burner to the simple-cycle cogeneration system for the purpose of increasing steam production. With an upgraded HRSG, the steam production can be increased to 126,000 lbm/h (57,000 kg/h), with supplementary firing increasing the exhaust temperature from 920 to 1,800°F (493 to 982°C). Compared with the baseline system, this arrangement provides higher overall thermal fuel efficiency (85%) and net electrical efficiency (79%), with an FCP of 4,321 Btu (4,558 kJ) per kWh. This is because all fuel energy added to the duct burner produces useful steam, with the stack temperature from the HRSG remaining constant. However, at 0.26, the ETR is the lowest of all arrangements considered, due to the increase in steam production with no corresponding increase in electric output. Similar to the use of only a BPST electric generation system, despite the low ETR, if there is sufficient process steam requirement, this operating arrangement is highly economical as indicated by the high net electrical efficiency. However,

Fig. 26-42 Conventional Combined-Cycle Cogeneration System. Source: Solar Turbines
Fig. 26-44 Advanced Combined-Cycle Cogeneration System with Back-Pressure Steam Turbine and Supplementary Firing. Source: Solar Turbines

capital costs are increased due to the addition of the duct burner and the need for an upgraded HRSG.

4. Figure 26-44 illustrates a combined-cycle system arrangement featuring supplementary firing and the use of a BPST. Under the steam conditions used in the conventional combined-cycle arrangement example, the steam turbine adds about 3 MW of power output to the system. However, in this example, steam conditions are increased significantly, compared with current practice for smaller systems, to 1,250 psig/1,050°F (87.2 bar/566°C), and a maximum system electrical output of 14.5 MW is achieved by this advanced cycle. This is shown on the upper-middle portion of the energy map. As shown in Table 26-9, due to the elevated steam conditions and the inherent high net electrical efficiency of back-pressure steam turbine (BPST) operation, this arrangement produces similar overall thermal fuel efficiency and net electrical efficiency to the conventional cogeneration system with supplementary firing. However, the ETR is increased due to the additional power provided by the BPST. While operation is very economical, capital costs are increased further due to the addition of the steam turbine and additional upgrades required for operation at elevated steam temperature and pressure conditions.

5. Figure 26-45 illustrates the addition of a low-pressure condensing turbine to the arrangement shown in Figure 26-44. With the combination of back-pressure and condensing turbines, the steam flow to process can be modulated between 0 and 110,000 lbm/h (50,000 kg/h). With no extraction steam, the electrical output of the full condensing advanced combined-cycle, at the elevated pressure and temperature conditions shown in Figure 26-45, is 22.7 MW. At

Fig. 26-45 Advanced Combined-Cycle Cogeneration System with Back-Pressure and Condensing Steam Turbines and Supplementary Firing. Source: Solar Turbines o o o

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