The US consumes 1.3 × 1010 GJ of electrical energy each year. Of this about 45%
ID: 233092 • Letter: T
Question
The US consumes 1.3 × 1010 GJ of electrical energy each year. Of this about 45% is from coal burning power plants. About 600 lbs, 0.3 tons, of CO2 are released for each GJ of electricity produced by burning coal while about half of this, 300 lbs, 0.15 tons, of CO2 are released for each GJ of electricity produced by burning natural gas. The total amount of CO2 released by the US is 6 Gtons per year.
a) If all of the coal burning power plants in the US were replaced by natural gas burning ones operating at the same efficiency as the coal plants, by what percentage would the CO2 emission of the US be reduced?
b) The efficiency of a combined-cycle natural gas plant is 60% while the efficiency of most existing coal power plants is 33%. If all of the coal burning power plants in the US were replaced by combined-cycle natural gas ones, by what percentage would the CO2 emission of the US be reduced?
c) What is a combined-cycle natural gas plant?
Explanation / Answer
total energy consumed = 1.3*10^10 GJ/year
coal plants produced energy = 45% = 1.3*10^10 x 0.45 = 0.585 x 10^10 GJ/year
total CO2 released per year = 6 Gtons
CO2 produced by burning natural gas/GJ = 0.15 tons
a) If all of the coal burning power plants in the US were replaced by natural gas burning ones operating at the same efficiency as the coal plants then all the produced energy per year would be from Natural Gas.
hence total CO2 produced = 1.3 x 10^10 x 0.15 = 1.95 Gtons
%reduction in CO2 emission = {(initial CO2-current CO2)/ initial CO2 } x 100
= (6-1.95)X100/6 = 67.5% per year
b) new CO2 emission = 1.3 x 10^10 x 0.15/0.60 = 3.25 Gtons
%reduction in CO2 emission = {(initial CO2-current CO2)/ initial CO2 } x 100 = (6-3.25) x100 / 6 = 45.8% per year
c) a combined cycle power plant uses both gas turbine and steam turbine to produce electricity from the same fuel.
This is how a combined-cycle plant works to produce electricity and captures waste heat from the gas turbine to increase efficiency and electrical output.
Gas turbine burns fuel.
Heat recovery system captures exhaust.
Steam turbine delivers additional electricity.
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