Question 6.5: A combined-cycle-based cogeneration plant supplies power and......
A combined-cycle-based cogeneration plant supplies power and useful heat for industrial processes. The plant consists of two syngas-fired gas turbines, HRSG, steam turbine, and electric generator (refer to Figure 6.8). The plant is operating under the following conditions:
• Gas turbine number n = 2
• Power output of the gas turbine \mathrm{P_{GT}} = 175 MWe
• Efficiency of the gas turbine plant \mathrm{\eta_{GT}} = 40%
• Power output of the steam turbine \mathrm{P_{ST}} = 200 MWe
• Heat rate of the cogeneration plant HR = 6000 kJ/kWh
• Overall efficiency of the cogeneration plant EUF = 0.9
• Fuel in gas turbine combustor is syngas with LHV of 25 MJ/kg
• Process steam is raised in HRSG using the gas turbine exhaust heat
Calculate
• Overall (electrical) efficiency of the cogeneration plant
• Heat-to-power ratio of the cogeneration plant
• Electric power output of the cogeneration plant
• Rate of useful heat production of the cogeneration plant
• Fuel rate of the cogeneration plant
• Specific fuel consumption per MWh of energy output of the cogeneration plant
1. Electrical efficiency of the combined-cycle-based cogeneration plant
\mathbf{h}_{\mathrm{CC}}\,=\,3600/\mathrm{HR}\,=\,3600/6000=0.62. Efficiency of the steam turbine plant
\mathbf{h}_{\mathrm{ST}}\,=\,(\mathbf{h}_{\mathrm{CC}}-\mathbf{h}_{\mathrm{GT}})/(1-\mathbf{h}_{\mathrm{GT}})=(0.6-0.4)/(1-0.4)=0.3333. Fuel energy input required for both gas turbine combustors
Q_\mathrm{f}=nP_{G T}/\mathbf{h}_{GT}\,=\,2\times175/0.4=875\,\mathrm{MW}4. Heat-to-power ratio of the cogeneration plant
{\mathrm{H P R}}={\mathrm{E U F }}/\mathbf{h}_{CC}\ -1=0.9/0.6\ -1=0.55. Electric power output of the cogeneration plant
P_{\mathrm{el}}=n~P_{GT}+P_{ST}=2\times175+200=550~\mathrm{M W}6. Rate of useful heat production
Q_{u}={\mathrm{HPR}}\times P_{\mathrm{el}}=0.5\times550=275\,\mathrm{MJ}/s7. Fuel rate of the cogeneration plant
m_\mathrm{f}\,=\,Q_\mathrm{f}/\mathrm{LHV}\,=\,875/25\,=\,35\,\mathrm{kg/s}8. Specific fuel consumption per MWh of the cogeneration plant useful energy output
\mathrm{SCF}=3600\;m_\mathrm{f}/(P_{\mathrm{el}}+Q_\mathrm{u})=3600\;\mathrm{s/h}\;35~\mathrm{kg/}\!s/(550+275)\;MW=152.73~\mathrm{kg/}\mathrm{MWh}