Water vapor emissivity

The effect of aircraft emissions on radiative forcing is summarized in Table 5. The global and annual radiative forcing due to subsonic aviation is calculated to be 0.036 Wm 2, which is larger than the value of 0.029 Wm 2 calculated in Section 2.1 (0.121-0.092 from Table 2), mainly due to the inclusion of water vapor emission here. The combined fleet provides a radiative forcing 0.055 Wm 2. Note that the water vapor emission from supersonic aviation plays an important role, providing 0.031 (=0.05-0.019) and 0.027 (=0.059-0.032) Wm-2 respectively in January and July. The results also show strong latitudinal and seasonal variations. 

Others expect water vapor emissions in a hydrogen energy economy (assumption is that half of the present energy supply is covered by hydrogen) to be reducible down to the respective level presently given by the fossil and nuclear economy which is a 0.005 % share of the total atmospheric water cycle. In order to reach the above reduction factor, hydrogen losses need to be decreased from currently estimated 10 % over the whole chain to 2 - 3 %. Today s world energy economy emits around 20 10 kg of water per year [134]. 

If car traffic in the city of Munich with 500,000 vehicles per day were converted to hydrogen-fueled traffic and assuming a 20 km cruising range per day and 1 kWh/km energy consumption, water vapor emissions would amount to about 1 million t/yr or 2 kg/(m yr) in the city area. This is about 0.1 % of the mean armual precipitation. A 1000 MW water-cooled power plant emits about 14 million t/yr of water vapor assuming 7000 operation hours [4]. 

Radiation differs from conduction and convection not only in mathematical structure but in its much higher sensitivity to temperature. It is of dominating importance in furnaces because of their temperature, and in ciyogenic insulation because of the vacuum existing between particles. The temperature at which it accounts for roughly half of the total heat loss from a surface in air depends on such factors as surface emissivity and the convection coefficient. For pipes in free convection, this is room temperature for fine wires of low emissivity it is above red heat. Gases at combustion-chamber temperatures lose more than 90 percent of their energy by radiation from the carbon dioxide, water vapor, and particulate matter. 

Water Vapor The contribution to the emissivity of a gas containing H9O depends on Tc andp L and on total pressure P and partial pressure p . Table 5-8 gives constants for use in evaluating . Allowance for departure from the special pressure conditions is made by multiplying by a correction factor C read from Fig. 5-21 as a function of (p + P) and p ,L. The absorptivity 0t of water vapor for blackbody radiation is evaluated from Table 5-8 but at T instead of Tc and at p LT /Tc instead of p, h. Multiply by (Tc/Ti)° . ... 

The correction factor C still applies. Spectral data for water vapor, tabulated for 371 wavelength intervals from 1 to 40 Im, are also available [Ferriso, Ludwig, and Thompson, J. Quant. Spectm.s. Radiat. Tran.sfer, 6, 241-273 (1966)]. The principal emission is in bands at about 2.55 to 2.84, 5.6 to 7.6, and 12 to 25 jlm. 

FIG. 5-21 Cor rection factor for converting emissivity of water vapor to values of P, and Fj other than 0 to 1 atm respectively. To convert atmosphere-feet to Idlopascal-meters, multiply by 30.89 to convert atmospheres to Idlo-pascals, multiply hy (1.0133)(10 ). 

Combined Soot W2O, and CO2 Radiation The spectral overlap of H9O and CO9 radiation has been taken into account by the constants for obtaining Ec- Additional overlap occurs when soot emissivity , is added. If the emission bands of water vapor and CO9 were randomly placed in the spectrum and soot radiation were gray, the combined emissivity would be Eg phis , minus an overlap correction g s- But monochromatic soot emissivity is higher the shorter the wavelength, and in a highly sooted flame at 1500 K half the soot emission hes below 2.5 [Lm where H9O and CO9 emission is negligible. Then the correction g s must be reduced, and the following is recommended ... 

Example 7 Radiation in Gases Flue gas containing 6 percent carbon dioxide and 11 percent water vapor by volume (wet basis) flows through the convection bank of an oil tube stiU consisting of rows of 0.102-m (4-in) tubes on 0.203-m (8-in) centers, nine 7.62-m (25-ft) tubes in a row, the rows staggered to put the tubes on equilateral triangular centers. The flue gas enters at 871°C (1144 K, 1600°F) and leaves at 538°C (811 K, 1000°F). The oil flows in a countercurrent direction to the gas and rises from 316 to 427°C (600 to 800°F). Tube surface emissivity is 0.8. What is the average heat-input rate, due to gas radiation alone, per square meter of external tube area ... 

Example 8 Ejfective Gas Emissivity Methane is hiimed to completion with 20 percent excess air (air half-saturated with water vapor at 298 K (60 F), 0.0088 mols H20/mol dry air) in a furnace chamber of floor dimensions. 3 X 10 m and heights m. The whole surface is a gray-energy sink of emissivity 0.8... 

Environmental Factors These inchrde (I) eqrripment location, (2) available space, (3) ambient conditions, (4) availabuity of adeqrrate rrtilities (i.e., power, water, etc.) and ancillary-system facilities (i.e., waste treatment and disposal, etc.), (5) maximrrm aUowable emission (air polhrtion codes), (6) aesthetic considerations (i.e., visible steam or water-vapor phrme, etc.), (7) contribrrtions of the air-poUrrtion-control system to wastewater and land poUrrtion, and (8) contribrrtion of the air-poUrrtion-control system to plant noise levels. 

Tonnage of air emissions, water emissions and liquid and solid effluent and tonnage of hazardous materials released into the environment. These two measures are related to one another. However, the first measure relates the total effluent, including nonpolluting materials. The second measure looks only at the tonnage of hazardous materials contained in the total effluent. Both measures can be important indicators. For example, for solid waste it is important to know the total volume of material for disposal and different upstream treatment techniques may affect the total volume. However, for ozone depleting chemicals, only the quantity of these gases is important and other components such as water vapor may be irrelevant. 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

Hydrogen can be used with vei y little or no pollution for energy applications. When hydrogen is burned in air, the main combustion product is water, with traces of nitrogen oxides. Wlien hydrogen is used to produce electricity in a fuel cell, the only emission is water vapor. 

Where human occupancy or wet process plant is present, the emission of water vapor will occur. Depending on external conditions and building fabric constmction, the attendant potential for excessive ambient humidity or surface condensation may exist. 

The environmental problem of sulfur dioxide emission, as has been pointed out, is very much associated with sulfidic sources of metals, among which a peer example is copper production. In this context, it would be beneficial to describe the past and present approaches to copper smelting. In the past, copper metallurgy was dominated by reverberatory furnaces for smelting sulfidic copper concentrate to matte, followed by the use of Pierce-Smith converters to convert the matte into blister copper. The sulfur dioxide stream from the reverberatory furnaces is continuous but not rich in sulfur dioxide (about 1%) because it contains carbon dioxide and water vapor (products of fuel combustion), nitrogen from the air (used in the combustion of that fuel), and excess air. The gas is quite dilute and unworthy of economical conversion of its sulfur content into sulfuric acid. In the past, the course chosen was to construct stacks to disperse the gas into the atmosphere in order to minimize its adverse effects on the immediate surroundings. However, this is not an en-... 

Water vapors are the most visible air emission from a pulp and paper mill, but are not usually regulated unless they form a significant obscurement or are a climate modifier. 

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