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At standard temperature and pressure, the specific volume of natural gas is:

1 �÷ (0.5934 x 0.075 lb per cu ft) = 22.469 cu ft per lb

Thus, 1 lb of natural gas has a heat content of:

1,032.2 Btu per cu ft x 22.469 cu ft per lb = 23,192.5 Btu per lb

One MMBtu of natural gas, then, would equal:

(1,000,000 Btu) �÷ (23,192.5 Btu per lb) = 43.117 lb natural gas

Because 1 lb of natural gas produces 2.723 lb of carbon dioxide, 43.117 lb of natural gas would produce 116.9 lb of carbon dioxide (43.117 times 2.712), or burning 1 MMBtu of natural gas would produce 116.9 lb of carbon dioxide.

For other fuel constituents and the typical chemical equations that apply, see Chapter 28 of the 2009 edition of ASHRAE Handbook ¡ª Fundamentals.

GENERATION OF ELECTRICITY

A large amount of the electricity produced today is generated in an enhanced Rankine steam cycle. Typically, coal or natural gas is fired in a boiler, producing high-pressure steam that is delivered to a steam turbine. In the turbine, energy is released, turning an electrical generator. The expanded steam condenses at a low temperature and is pumped back to the boiler, where the cycle is completed.

The performance of such plants often is measured by gross station heat rate:

Gross station heat rate (Btu per kwh) = (W_f x HR) �÷ (kwh generated)

where:

W_f = amount of fuel burned per hour in pounds or cubic feet

HR = heating value of fuel in British thermal units per pound or British thermal units per cubic foot

kwh = electric energy produced by generator

Such a calculation is a good measure of boiler-turbine efficiency, but does not address parasitic loads, such as feedwater and cooling-water pumping. To better account for the overall efficiency of a plant in delivering electrical energy to end users, net station heat rate often is utilized. Net station heat rate is based on station output minus the power consumed by plant electrical auxiliaries:

Net station heat rate (Btu per kwh delivered) = (W_f x HR) �÷ (kwh generated - kwh used in plant)

Note that net station heat rate always will be higher numerically than gross station heat rate.

For steam Rankine-cycle plants operated by the local utility in St. Louis, the average gross station heat rate is approximately 9,800 Btu per kilowatt-hour, while the average net station heat rate is in the range of 10,300 Btu per kilowatt-hour (or an equivalent overall thermal efficiency of 3,413 divided by heat rate, or 33 percent).

For a coal-burning Rankine-cycle plant, carbon-dioxide emissions per kilowatt-hour of electricity delivered to end users would be:

(212.6 lb CO₂ per 1 MMBtu coal) x (10,300 Btu per kwh) x (1 MMBtu coal �÷ 1,000,000 Btu) = 2.19 lb CO₂ per kwh

For a natural-gas-burning Rankine-cycle plant, carbon-dioxide emissions per kilowatt-hour of electricity delivered to end users would be:

(116.9 lb CO₂ per 1 MMBtu natural gas) x (10,300 Btu per kwh) x (1 MMBtu natural gas �÷ 1,000,000 Btu) = 1.20 lb CO₂ per kwh

In St. Louis, where the average regional electric emissions rate is 1.844 lb of carbon dioxide per kilowatt-hour, the majority of electricity is generated through the use of coal. (Electricity generated with wind energy, solar energy, hydroelectric energy, and nuclear energy has equivalent net emissions rates approaching 0.0 lb of CO₂ per kilowatt-hour of electricity generated.)

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