Quick Way To Estimate NOx Emissions

NOx Level. NOx emissions for gas-turbine based plant are usually specified in terms of volumetric concentration in the exhaust gas on a dry basis as “ppmvd” normalized to a certain percent of oxygen in the exhaust, such as 25 ppmvd at 15% O2.


Flow rate. Instead of ppmvd, however, project develop-ers and plant operators must file data for a plant’s NOx emissions on an hourly (or annual) mass flow rate basis in pounds per hour or tons per year.


To come up with a precise figure requires a considerable amount of exact data on the chemical composition and den-sity of the exhaust gas (as an hourly or annual average) and on the exhaust mass flow which is difficult to measure.


For a quick way to estimate the flow rate in lb/hr the En-vironmental Protection Agency allows the use of estimat-ing factors to be found in its so-called Method 19 specs.


You must know only two numbers to apply Method 19: 1) the NOx emission rate in ppmvd at 15% oxygen and 2) the fuel consumption, as MMBtu/hr.


Natural gas fuel. Method 19 rule-of-thumb for estimating NOx emissions (at 15% O2) for gas turbines running on natural gas fuel is as follows:


1 ppmv NOx = 0.0036 lb/MMBtu


For pounds per hour, simply multiply the actual exhaust NOx concentration (as ppmvd) and the fuel consumption (as MMBtu/hr) by the .0036 factor.


Assume for instance a typical 60Hz F-class 300MW combined cycle plant (1x1 configuration) operating with a fuel consumption of approximately 2,000 MMBtu/hr with an exhaust NOx concentration of 25 ppmvd.


Multiply both those values by the Method 19 factor as follows to estimate the NOx flow rate:


 25 (ppmv) x .0036 (lb/MMBtu) x 2,000 (MMBtu/hr) = 180 lb/hr


Then multiply the flow rate (lb/hr) by the number of hours of plant operation per year divided by 2,000 (lb/ton) to estimate the NOx emissions in ton/year.


Assuming that the plant operates base load for 7500 hours per year, the annual NOx flow would be 675 ton/year.


Distillate fuel. For operation on distillate fuel oil, the same equation holds except that the multiplier factor becomes 0.004 rather than 0.0036.


Accordingly, in the example above, for the same 25 ppmvd emission level, the estimated NOx flow rate would be 11% higher (in lb/MMBtu) than on natural gas fuel, i.e. 750 lb/hr.


To obtain plant fuel consumption in MMBtu/hr units, multiply the plant heat rate (Btu/kWh) by the plant’s power output in kW and then divide the result by 1,000,000.

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