121

Fuel oil sulphur: 2.25%

Table 17-2 Different Ways of Stating the Same Emissions from a Low-Speed Diesel Engine. Source: MAN B&W

Table 17-2 Different Ways of Stating the Same Emissions from a Low-Speed Diesel Engine. Source: MAN B&W

oils and natural gas.

• Equipment-specific factors. These include factors such as age, design, and logistics.

• Target level of emissions. Required percent reductions or allowable levels for equipment vary depending on regulatory distinctions, as discussed in the previous two chapters. Required control technology will depend on the project and attainment area in which it is located.

• Existing control measures. Many of the control methods described in this chapter can be used in conjunction with others already in place. It should be noted, however, that the effect of controls is not necessarily additive.

Pollutant Units Of Measurement

Emissions measurements and standards may be expressed in a variety of ways. They are generally listed as dry values with a correction factor for oxygen content because added air dilutes the emissions without actually reducing the amount emitted. The concentration of emissions in stack gases are usually corrected to a standard amount of dilution, expressed as excess oxygen content on a dry basis, as if the moisture in the flue gas were removed. Boilers typically use a correction factor to 3% O2. Combustion gas turbines and reciprocating engines, which have significantly higher dilution levels than boilers, typically use a correction factor to 15% O2, though sometimes other correction factors, such as 5% O2, are used.

Table 17-2 shows different ways of stating emissions for an uncontrolled low-speed Diesel engine. The data represents the calculated exhaust gas composition at an ambient temperature of 77°F (25°C) and relative humidity of 50%. Unit values are broken into two categories — concentrations and emissions factors. Common methods of calculating emissions rates are indicated in Table 17-3.

The advantage of relating pollutants to output (i.e., shaft power) rather than input is that output takes into account efficiency. If, for example, two units consume the same amount of fuel, but one produces more useful output, the more efficient unit would be allowed a higher level of pollutant per energy unit consumed relative to the less efficient unit. The same applies to methods used to express process emissions factors. Nevertheless, this is not a common practice for boilers or process heaters.

Parts per million (ppm) is a measure of concentration, usually by volume. To express emissions in units of output and time, ppm must be converted to mass (weight). The conversion depends on units of measurement, molecular weight of the pollutant, variation of flow with time, and the basis on which the numbers are expressed (dry or wet, corrected for oxygen or uncorrected). The configuration of pollutant monitoring systems will dictate the conversions required to generate the corrected values from as measured raw data.

The formula used to correct to a fixed reference oxygen content when expressing NOX concentrations is:

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