Air Emissions Treatment

Because air pollutants vary in size many orders of magnitude, many different types of treatment devices are required for emissions treatment.

Air pollutant devices can be treated as black box separators, like we studied in Chapter 3. Recovery efficiency can be described by the equation: R 1 = (x 1 /x 0 ) x 100 R 1 = recovery of component 1, % x 1 = amount of pollutant collected by the device per unit time, kg/sec x 0 = amount of pollutant entering the device per unit time, kg/sec

Example An air pollution control device removes a particulate that is being emitted at a concentration of 125,000 : g/m 3 at an air flow rate of 180 m 3 /sec. The device removes 0.48 metric toms per day. What are the concentration of the emission and the recovery of collection? Pollution control deviceFeed x 0 x 0Escaped x 2 x 2 Removed x 1 x 1 At steady state the mass balance is: Rate of particulates in = rate of particulates out Rate of particles in = rate of particles removed + rate of particles escaping

Particulates in: 180 m 3 /sec x 125,000 : g/m 3 x : g/g = 22.5 g/sec Particles collected: 0.48 tons/day x 10 6 g/ton x 1 hr/3600 sec x 1 day/24 hr = 5.5 g/sec Mass balance: 22.5 = particles that escape Particles that escape = 17 g/sec Emission concentration: (17 g/sec x 10 6 : g/g)/ 180 m 3 /sec = 94,000 : g/m 3 Recovery: R = (x 1 x 100)/x 0 = (5.5 x 100)/22.5 = 24%

Treatment Devices Settling Chambers The simplest of treatment devices. Wide spot in the exhaust flue that allows particles to settle out. Only very large particles (> 100 : m) will settle effectively.

Cyclones

Bag Filter or Bag House

Spray Tower or Scrubber

Electrostatic Precipitators

Process Selection A)Settling Chamber B)Simple Cyclone C)High-efficiency Cyclone D)Electrostatic Precipitator E)Spray Tower wet scrubber F)Venturi Scrubber G)Bag Filter

Control of Gaseous Pollutants Wet Scrubbers Adsorption Incineration or flaring Catalytic Combustion

Wet Scrubber Venturi Scrubber

Wet Scrubbers

Adsorption

Carbon Adsorption System

Incineration and Flaring

Control of Sulfur Oxides (SO x ) Emission of SO x one of the major causes of acid precipitation Major source of SO x is coal-fired power plants Several options for control: Change to Low-sulfur fuel Desulfurize the coal Sulfur in coal is either organic (usually about 60%) or inorganic (about 40%). The inorganic form is iron pyrite (FeS 2 ) that can be removed from the coal by washing. The removal of organic sulfur requires a chemical reaction that is accomplished best if the coal is gassified first. Gasified coal is like natural gas.

SO x Control Options (continued) Tall Stacks Flue-Gas Desulfurization Contact with lime SO 2 + CaO  CaSO 3 Contact with limestone SO 2 + CaCO 3  CaSO 4 + CO 2 Calcium sulfite (CaSO 3 ) and calcium sulfate (CaSO 4 ) are solids that have low solubilities so they can be removed from the water by settling, but then there is a disposal problem