Question 6.6: A gas/diesel engine cogeneration plant (refer to Figure 6.10......
A gas/diesel engine cogeneration plant (refer to Figure 6.10) is operating under the following conditions:
• Useful heat output Q_{u} = 23 MW
• Heat-to-power ratio HPR = 1.3
• Fuel mixture: natural gas (97%) and light fuel oil (3%)
• LHV of natural gas is 49 MJ/kg; LHV of light fuel oil is 45 MJ/kg
• Fuel mixture consumption m_\mathrm{f} = 0.95 kg/s
Calculate (i) the plant electrical efficiency, and (ii) the cogeneration plant over-all efficiency, that is, EUF
1. Heating value of the fuel mixture
\mathrm{LHV}=(m_{\mathrm{gas}}\mathrm{LHV}_{\mathrm{gas}}+m_{\mathrm{oil}}\mathrm{LHV}_{\mathrm{oil}})=0.97\times49+0.03\times43=48.82\mathrm{~MJ} /\mathrm{kg}2. Rate of heat input to the engine from the fuel combustion
Q_{\mathrm{in}}=m_\mathrm{f} \mathrm{LHV}=0.95\times48.82\ =46.38\,\mathrm{MJ}/s3. Electric power output of the cogeneration plant
P_{\mathrm{el}}=Q_{\mathrm{u}}/{\mathrm{HPR}}=23/1.3=17.69\,M\mathrm{W}4. Electrical efficiency of the cogeneration plant
{\bf h}_{\mathrm{el}}\,=\,P_{\mathrm{el}}/Q_{\mathrm{in}}\,=\,17.69/46.38\,=\,0.381\,=\,38.1\%5. Overall efficiency of the cogeneration plant, that is, energy utilization factor
\mathrm{E U F}=(P_{\mathrm{el}}+Q_{u})/Q_{\mathrm{in}}=(17.69+23)/46.38=0.877=87.7\%Example 6.7 presents the calculation of fuel savings achieved in a gas/diesel engine cogeneration plant in comparison with two separate facili-ties—one for power generation and another for heat production.