Question 6.4: A combined-cycle-based cogeneration plant consisting of a ga......
A combined-cycle-based cogeneration plant consisting of a gas turbine, an HRSG, and a condensing–extraction steam turbine is used for the production of power \mathrm{P_{el}} and useful heat \mathrm{Q_{u}}. The cogeneration plant (refer to Figure 6.8) is operating under the following conditions:
• Gas turbine capacity \mathrm{P_{GT}} is 360 MWe
• Gas turbine efficiency \mathrm{\bf{\eta}_{GT}} is 0.4
• Steam turbine capacity \mathrm{P_{ST}} is 176 MWe
• Fuel in gas turbine combustor is light fuel oil with LHV of 43 MJ/kg
• Plant useful heat production rate \mathrm{Q_{u}} is 254 MJ/s
Calculate
• Overall efficiency of the combined cycle
• Efficiency of the steam turbine
• Overall efficiency (EUF) of the cogeneration plant
• Fuel consumption rate of the cogeneration plant
1. Rate of fuel energy input to the gas turbine combustor
Q_{\mathrm{f}}\,=\,P_{\mathrm{GT}}/\mathbf{h}_{\mathrm{GT}}\,=\,360/0.4\,=\,900\,M\mathrm{J}/s2. Overall (electrical) efficiency of the combined-cycle power plant
{\bf h}_{\mathrm{CC}}\,=\,P_{\mathrm{el}}/Q_{\mathrm{f}}\,=\,(360\,+\,176)/900=\,0.5963. Efficiency of the steam turbine
{\bf h}_{ST}\,=\,({\bf h}_{CC}-{\bf h}_{GT})/(1-{\bf h}_{GT})=(0.596-0.4)/({1}-0.4)=0.3264. Energy utilization factor (overall efficiency) of the cogeneration plant
\mathrm{{E}}\mathrm{{UF}}=(P_{\mathrm{el}}+Q_{\mathrm{u}})/Q_{\mathrm{f}}\,=(360+176+254)/900=0.8785. Fuel consumption rate of the cogeneration plant
m_{\mathrm{f}}\,=\,Q_{\mathrm{f}}/\,\mathrm{LHV}\,=\,900/43\,=\,20.93\,\mathrm{kg/s}Performance parameters of a syngas-fired combined-cycle cogeneration plant are calculated in Example 6.5.