Page 332 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.9 Proper Layout for Post-combustion Air Pollution Control Devices 309

Since more than one piece of equipment is installed to control different air
pollutants, they have to be installed in a proper sequence in order for them to
function properly. As a general rule, particulate matter has to be removed as much
as possible before dealing with gaseous pollutants. However, when limestone is
injected in the duct to remove SO x or catalyst is used to remove NO x , particulate
control devices should be arranged even further upstream in order to capture the
excessive particles.
A typical air pollution control system for stationary sources like power plants is
composed of a dry sorbent injection followed by filtration devices. Selection of the
exact equipment depends on the fuel quality, government regulations of concerns
and process type. The gas temperature plays an important role in the positioning of
the equipment; so do the cross-effects between air pollution control processes. An
attempt to list the most important of interactions is given in Table 10.4. Considering
the interactions between different air emissions, a good planning is needed for
optimized air emission control.

10.10 Practice Problems

1. A power plant burns coal at a rate of 5000 tons/day. The coal has 5 % of ash
content. Assume 25 % of ash falls to the bottom of the furnace and 75 % becomes
fly ash. In order to meet a particulate matter emissions limit of 2 tons/day find out
the efficiency required of a final control device.
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2. Calculate the number of bags required in a bag-house to filter 17,000 m /min of
air at a temperature of 200 °C and pressure of 1 atm. Each bag is 3.5 m long and
0.3 m in diameter and air to cloth ratio is 0.9 m/min.
3. A power plant is using a large bag-house to control air pollution. The air is
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flowing through at a rate of 60 m /s with a dust loading of 230 mg/m . Cal-
culate the collection efficiency for the bag-house if emission regulation for the
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plant is 100 µg/m .
4. A power plant produces a flue gas with a mass flow rate of 1.75 million lb per
hour. Assume that the mass flow rate of sulfur dioxide in flue gas is 2000 lb/hr.
This flue gas is passed through a wet scrubber to meet emission standards. The
efficiency of scrubber is 90 %. Determine the rate of sulfur dioxide emitted to
the atmosphere.
5. The pressure drop through a freshly cleaned bag-house is 125 Pa and the bag is
expected to be cleaned to remove dust cake when the pressure drop reaches
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500 Pa in 1 h. The gas being cleaned has a flow rate of 300 m /min, and the a
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particle loading is 10 g/m . If the average mass filtration efficiency is 99 %, and
the dust cake has a thickness of 5 mm, a solidity of 0.5 and permeability of
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1 10 12 m . Assume the real dust material is 2,000 kg/m . Estimate the total
filtration area.