Emissions Comparison of Cogeneration vs no Cogeneration

Paramount to the concept of controlling pollution, and air emissions in particular, is the understanding that, in addition to the use of less polluting fuels and more effective pollution control technologies, the best solution is often simply to use less fuel by improving efficiency. The least environmentally harmful fuel unit is the one that is not consumed.

Figure 3-13 compares the NOx and CO2 (greenhouse gas) emissions of a representative combined-cycle cogen-eration system applied on site against the comparable emissions of a utility-built combined cycle plus an industrial (gas-fired) boiler producing steam for industry. The point of comparison in the figure (100% Emissions Ratio) is based on a total emissions by the central utility and the local industrial boiler. Relative emissions at varying steam sales are indicated for a combined-cycle cogeneration plant equivalent to Alternative 3 in the previous FCP comparison.

The simple cycle (Alternative 1) has its break point at an even higher steam/power ratio, because this HRSG is producing steam at the process application pressure (in this example 150 psig or 11.4 bar) rather than at the turbine pressure (900 psig or 63 bar). With firing, this cycle continues to improve until it reaches its maximum cycle efficiency at an FCP of just below 6,000 Btu/kWh (6,330 kJ/kWh).

Fig. 3-13 Comparison of Cogeneration Cycle Emissions with No Cogeneration. Source: Cogen Designs, Inc.

The assumptions are that the combined cycle is equipped with low-NOx combustors (25 ppm) and supplemental burners (0.15 lbm NOX/MMBtu or 0.068 gram/MJ), and that the industrial boiler was also equipped with low-NOX burners (0.15 lbm NOX/ MMBtu or 0.068 gram/MJ). In this case, the lowest emissions (both NOX and CO2) fall at the point where supplemental firing would just start to be needed to produce more steam. Figure 3-14 shows specific CO2 emissions from different applied power plant technologies as a function of net station efficiency.

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