Example 1.1 Carbon Emissions and Water Needs of a Coal-Fired Power Plant. Consider an average PC plant with a heat rate of 10,340 Btu/kWh burning a typical U.S. coal with a carbon content of 24.5 kgC/GJ (1 GJ = 10 J). About 15% of thermal losses are up the stack and the remaining 85% are taken away by cooling water. a. Find the efficiency of the plant. b. Find the rate of carbon and CO2 emissions from the plant in kg/kWh. c. If CO2 emissions eventually are taxed at $10 per metric ton (1 metric ton = 1000 kg), what would be the additional cost of electricity from this coal plant (c/kWh)? d. Find the minimum flow rate of once-through cooling water (gal/kWh) if the temperature increase in the coolant returned to the local river cannot be more than 20F. e. If a cooling tower is used instead of once-through cooling, what flow rate of water (gal/kWh) taken from the local river is evaporated and lost. Assume 144 Btu are removed from the coolant for every pound of water evaporated. Solution a. From Equation 1.1, the efficiency of the plant is 3412 Btu/kWh 10,340 Btu/kWh 0.33 = 33% b. The carbon emission rate would be 24.5 kgC 10,340 Btu C emission rate = 10J kWh 1055 J Btu = 0.2673 kgC/kWh Recall, that CO2 has a molecular weight of 12 + 2 16 = 44; so 0.2673 kgC 44 gCO CO2 emission rate = = 0.98 kg CO2/kWh kWh 12 gC This is a handy rule of thumb, that is, 1 kWh from a coal plant releases close to 1 kg of CO2.