Question 12.7: A cogeneration plant using a CFB combustor is designed for t......
A cogeneration plant using a CFB combustor is designed for the following operat-ing conditions:
• Electric capacity of the cogeneration plant P_{el} is 19 MWe
• Thermal capacity of the cogeneration plant Q_{u} is 26 MWt
• Fuel: bituminous coal with LHV of 30 MJ/kg
• Efficiency of power generation in a separate power plant \eta_{el} is 0.43
• Efficiency of useful heat production in a separate heating plant \eta_{h} is 0.85
• Overall efficiency (energy utilization factor) of the cogeneration plant \eta_{CG} is 0.9
Calculate (i) the fuel requirements of the cogeneration plant and the separate plants, and (ii) the fuel savings of the cogeneration plant per day.
1. Fuel requirements of the CFB cogeneration plant
m_\mathrm{f,CG}=(P_{\mathrm{el}}+Q_{\mathrm{u}})/(\mathrm{LHV~\eta}_{\mathrm{CG}})=(19+26)/(30\times0.9)=1.67\mathrm{~kg/s}2. Total fuel requirements of two separate plants together
m_{ \mathrm{f,sep}}=(P_{\mathrm{el}}/\eta_{\mathrm{el}}+Q_{\mathrm{u}}/\eta_{\mathrm{h}})/ \mathrm{LHV}=(19/0.43+26/0.85)/30=2.49~{\mathrm{kg/s}}3. Fuel savings rate of the CFB cogeneration plant
\Delta m_\mathrm{f}=m_{ \mathrm{f,sep}}-m_{ \mathrm{f,CG}}=2.49-1.67=0.82\ {\mathrm{kg/s\,or}} \,0.82/2.49=33\%4. Daily fuel savings rate of the CFB cogeneration plant
\Delta m_\mathrm{f,d}=\Delta m_\mathrm{f}\times24\times3600=0.82\mathrm{~kg/s}\times24\mathrm{~h/day}\times3600\mathrm{~s/h}=70,848\mathrm{~kg/day}