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Module 6: Air Pollutants and Control Techniques - Sulfur Oxides - Control Techniques
Introduction
Absorption
Adsorption
Alternative Fuels
Practice
Problems
Objective
- Describe the basic techniques and systems used to control sulfur dioxide and sulfuric acid emissions.
Air pollution control systems for sulfur dioxide removal are large and sophisticated. Sulfur dioxide is controlled by three different techniques: absorption, adsorption, and the use of low-sulfur fuels.
The control systems used for sulfur dioxide are usually not designed to remove sulfuric acid. The sulfuric acid concentrations are usually below the levels where it is economically feasible or environmentally necessary to install control systems.
Absorption processes use the solubility of sulfur dioxide in aqueous
solutions to remove it from the gas stream. Once sulfur dioxide has dissolved
in solution to form sulfurous acid (H2SO3), it reacts
with oxidizers to form inorganic sulfites (
)
and sulfates (
).
This process prevents the dissolved sulfur dioxide from diffusing out
of solution and being re-emitted. For more information about the process
of absorption, see the lesson on Gas Absorption in Module 4.
The most common type of sulfur dioxide absorber is the limestone wet scrubber. An example flowchart is shown in Figure 1.
Limestone is the alkali most often used to react with the dissolved sulfur dioxide. Limestone slurry is sprayed into the sulfur dioxide-containing gas stream. The chemical reactions in the recirculating limestone slurry and reaction products must be carefully controlled in order to maintain the desired sulfur dioxide removal efficiency and to prevent operating problems. Wet scrubbers used for sulfur dioxide control usually operate at liquid pH levels between 5 to 9 to maintain high efficiency removal. Typical removal efficiencies for sulfur dioxide in wet scrubbers range from 80 to 95%.
-
#1
- Which of the following types of wet scrubbers has the best design
for sulfur dioxide removal?
- Venturi scrubber
- Impingement plate scrubber
- Spray tower
- Venturi scrubber
The wet scrubber (absorber) vessels do not efficiently remove particulate matter smaller than approximately 5 micrometers. However, as in the case with low-efficiency particulate wet scrubbers, the particulate removal efficiency increases rapidly with particle size above 5 micrometers. Usually, a moderate-to-high efficiency particulate control system is used upstream from the sulfur dioxide absorber to reduce the particulate matter emissions in the less than 3 micrometer size range. These upstream collectors also reduce the quantity of particulate matter that is captured in the absorber.
The evaporation of water that occurs in wet scrubber vessels can keep gas temperatures relatively cold, in the range of 110 to 140°F. These gas temperatures are well below the typical operating temperatures of other air pollution control systems used on sources that generate sulfur dioxide emissions.
Another type of absorption system is called
a spray atomizer dry scrubber (see Figure 2),
which belongs to a group of scrubbers called spray-dryer-type dry scrubbers.
In this case, an alkaline slurry is sprayed into the hot gas stream at
a point upstream from the particulate control device. As the slurry droplets
are evaporating, sulfur dioxide absorbs into the droplet and reacts with
the dissolved and suspended alkaline material.
Large spray dryer chambers are used to ensure that all of the slurry droplets evaporate to dryness prior to going to a high efficiency particulate control system. The term "dry scrubber" refers to the condition of the dried particles approaching the particulate control system. Fabric filters or electrostatic precipitators are often used for high efficiency particulate control. The system shown in Figure 3 has a fabric filter.
Spray-dryer-type absorption systems have efficiencies that are similar to those for wet-scrubber-type absorption systems. These generate a waste stream that is dry and, therefore, easier to handle than the sludge generated in a wet scrubber. However, the equipment used to atomize the alkaline slurry is complicated and can require considerably more maintenance than the wet scrubber systems. Spray-dryer-type absorption systems operate at higher gas temperatures than wet scrubbers do and are less effective for the removal of other pollutants in the gas stream such as condensable particulate matter.
The choice between a wet-scrubber absorption system and a spray-dryer absorption system depends primarily on site-specific costs. The options available for environmentally sound disposal of the waste products are also an important consideration in selecting the type of system for a specific application. Both types of systems are capable of providing high efficiency sulfur dioxide removal.
Sulfur dioxide can be collected by adsorption systems. In this type of control system, a dry alkaline powder is injected into the gas stream. Sulfur dioxide adsorbs to the surface of the alkaline particles and reacts to form compounds that cannot be re-emitted to the gas stream. Hydrated lime (calcium hydroxide) is the most commonly used alkali. However, a variety of alkalis can be used effectively. A flowchart for a dry-injection-type dry scrubber (adsorber) is shown in Figure 4.
A dry-injection-type dry scrubber can be used on smaller systems as opposed to using the larger, more complicated spray-dryer-type dry scrubber. However, the dry injection system is slightly less efficient, and requires more alkali per unit of sulfur dioxide (or other acid gas) collected. Accordingly, the waste disposal requirements and costs are higher for adsorption systems than absorption systems.
Other techniques used for limiting the emissions of sulfur dioxide are simply to switch to fuels that have less sulfur or to convert to synthetic (processed) fuels that have low sulfur levels. The sulfur dioxide emission rate is directly related to the sulfur levels in coal, oil, and synthetic fuels. However, not all boilers can use these types of fuels. Each type of boiler has a number of very specific and important fuel characteristic requirements and not all low sulfur fuels meet these fuel-burning characteristics.
Practice Problems
Sulfur Oxides - Control Techniques
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.