Question 6.7: Compare a gas/diesel engine-based cogeneration plant (refer ......
Compare a gas/diesel engine-based cogeneration plant (refer to Figure 6.10) with two plants for separate power and useful heat production in a conventional steam power plant and a boiler, respectively.
Both the cogeneration plant and two conventional plants produce 500 MWh of electrical energy and 650 MWh of useful heat over the same period of time.
The efficiencies of steam power plant and heating boiler are \eta_{el} = 0.42 and \eta_{b} = 0.83, respectively, and the energy utilization factor of the cogeneration plant is EUF = 0.89. The fuel used in both cogeneration and separate plants is fuel oil with a heating value LHV of 43 MJ/kg.
Calculate the fuel energy and fuel savings achievable in the cogeneration plant.
1. Fuel energy requirements of the cogeneration plant
E_{\mathrm{CG}}=(E_{\mathrm{el}}+Q_{u})/\mathrm{EUF}=(500+650)/0.89=1292.1\,\mathrm{MW}\mathrm{h}2. Fuel energy requirements for separate production of power and useful heat
E_{\mathrm{sep}}\,=\,E_{\mathrm{el}}/\mathbf{h }_{\mathrm{el}}+Q_{\mathrm{u}}/\mathbf{h}_{\mathrm{b}}\,=\,500/0.42+650/0.83\,=\,1973.6~\mathrm{MWh}3. Fuel consumption in the cogeneration plant over the specified period of time
m_{C G}=E_{C G}/\mathrm{LHV}=1292.1\,\mathrm{MWh}\times3600\,\mathrm{s/h}/43\,\mathrm{MJ}/\mathrm{kg}=108,179\,\mathrm{kg}4. Fuel consumption in two separate power and heat production plants over the same period of time
m_{\mathrm{sep}}\,=\,E_{\mathrm{sep}}/\mathrm{LHV}\,=\,1973.6\,\mathrm{MWh}\times3600\,\,\mathrm{s/h}/43\,\mathrm{MJ}/\mathrm{kg}\,=\,165,232\,\mathrm{kg}5. Fuel savings of the cogeneration plant in comparison with separate power and heat production
\Delta m_\mathrm{f}\,=\,m_{\mathrm{sep}}\,-\,m_{\mathrm{CG}}\,=\,165,232-108,179\,=\,57,053\,\mathrm{kg}or
\Delta m_\mathrm{f}/m_{\mathrm{sep}}\;=\;57,053/165,232=0.345\;=\;34.5 {\mathrm{\%}}Thus, the fuel savings in the cogeneration plant is nearly 35% in comparison with the power and heat production in two separate plants.
The main purpose of the cogeneration is the simultaneous production of process steam and/or useful heat for hot water and space heating together with electrical energy in one installation based on a heat engine, for example, steam or gas turbine or gas/diesel engine.