EMISSION MEASUREMENT TECHNOLOGY
Emissions monitoring is essential in controlling industrial environments and processes to ensure good air quality standards are maintained. It is also required in order that the various regulations and guidelines related to air quality are met. In addition to gaseous emissions, such as sulfur dioxide, carbon monoxide, nitrogen oxides, hydrocarbons, and many others, the emissions of particulate material and heavy metals must also be controlled.
Guiding values for particulate material PMJ0 (aerodynamic diameter smaller than 10 |j. m) are given by the United States and the European Union. Major debates are in progress regarding the importance of introducing values for the size PM2 5.
Air pollution control legislation is aimed at protecting human health and the overall environment. It also provides a means of reducing economic losses and a decline in environmental amenity as a consequence of air pollution.1
In industry, many process streams are involved in the gas phase.
The reasons for measuring these streams are process optimization, mandated regulatory information, analyzer application development, and identification of process or effluent abnormalities.2
13.5.1.1 Air Quality and Emissions
Basic Properties of Air
In practical calculations relating to air quality analysis, ideal gas laws can be applied with negligible error. The water vapor in air often varies from ideal conditions somewhat more than gases; however, the errors involved in using the
Ideal gas laws for water vapor can be ignored. ’ In calculations, reference is made to standard conditions with the temperature of 273.15 K and the pressure of 101.325 kPa. The volume of one mole of gas at these conditions is 22.41 L.
In conversion calculations between the state functions temperature (Ti, pressure (p) and volume (V), the ideal gas law states that
Where
P = absolute pressure
V = volume
N = number of moles
R = universal gas constant, and
T = absolute temperature.
Thus, the volume of one mole of gas at a temperature Tj is approximately (Tj/273) x 22.4 L, if the pressure remains the same. The value of R is 8.314 J/(K mol). The units must be consistent.
Example
Particulate emission is sampled at a temperature of 70 °C by an isokinetic sampling nozzle, i. e., the sampling velocity is the same as the gas velocity around the nozzle in the duct.
Gas velocity is measured over an aperture in the heated zone of the sampling train, at a temperature of 110 °C, to remove the moisture by heating. Determine the gas velocity at the sampling nozzle if the measured velocity is 28 m s_1 for the sampling diameter used. The water concentration determined from the condensate is 75 g m-?(n).
Solution The gas volume under the same pressure is greater by 383/343 at the higher temperature. When the sampling diameter remains the same, the gas velocity at the higher temperature is greater by the same factor. Water is assumed to be in vapor form, behaving as a gas, as the concentration is less than that in saturated air.
The gas velocity at the nozzle is thus (343/383) x 28 m s-1 = 25.1 m s~!.
Water in droplets: At 100 °C the droplets evaporate producing an additional volume of M/18 x 22.4 x 383/273 L at 110°C, where M is the mass (in grams) of the water droplets.
Air Composition
The main components of standard air are listed in Table 13.19.
Measurement of Atmospheric Emissions
The behavior of gases in air, as shown by studies in atmospheric physics and chemistry, depends on the physical and chemical properties of these gases, such as density and reactivity. Examples of gas and air densities are given in Table 13.20.
Emission measurements are required for many purposes. They can be used as the basis for emission and air quality studies, as well as for process control and specific technologies to reduce emissions. The reliability of the measured values is constantly improving with developments in monitoring techniques.
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TABLE 13.19 Chemical Composition Of Standard Air3
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In many industrial gas measurements, water vapor is present in high concentrations. The sample cell of the measurement instrument and sample line can be heated up to 200 °C to remove water vapor. Sometimes, the sample gas is dried by condensation or by using Peltier gas dryers.
Particulate material consists of solid or liquid substances that may be visible or invisible.3 The particles affect visibility and can be transported over long distances by wind. The small particles, less than PM10, are particularly dangerous to human health as they can pass through nostril hairs (cilia) and enter the lungs.
Aerosols are small particles dispersed in gas. In aerosols, the particles are relatively large compared with the gaseous particles.
Aerosol dynamics are based on spherical particles, a premise which almost never exists in practice.3 However, if there is consistency in handling the aerosol dynamics calculations, the aerodynamic diameter (see Section 13.5.2.2) that is measured gives fairly accurate predictions of aerodynamic behavior. As a result, the difference between the real shape and size of the particles and the aerodynamic shape and size is unimportant for most practical purposes.
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TABLE 13.20 Densities of Air and Some Gases at 273 К, 101.3 kPa (kg nr3)4
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