AP Environmental Science

## Learning Outcomes

- Understand how wet and dry scrubbers reduce sulfur dioxide emissions from smokestacks
- Explain why scrubbers are linked to lower acid deposition
- Distinguish wet scrubbers from dry scrubbers, baghouses, and electrostatic precipitators
- Calculate scrubber removal efficiency using inlet and outlet pollutant data
- Compare the advantages and drawbacks of wet and dry scrubber systems
- Apply scrubber knowledge to AP Environmental Science free-response and multiple-choice questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand methods of reducing air pollutants using scrubber technology, with a focus on the following syllabus points:

- Know that scrubbers are used mainly to reduce **sulfur dioxide (SO₂)** from coal-burning power plants and other stationary sources
- Identify **flue-gas desulfurization (FGD)** as the general process of removing SO₂ from exhaust gases
- Explain the difference between **wet scrubbers** and **dry scrubbers** in terms of water use, sorbents, byproducts, and removal efficiency
- Understand how removing SO₂ before release reduces **acid deposition**
- Distinguish scrubbers from **electrostatic precipitators (ESPs)** and **baghouses**, which mainly remove particulate matter
- Recognise that wet scrubbers often achieve higher SO₂ removal than dry systems, but use more water and create slurry or wastewater handling issues
- Apply the scrubber removal efficiency equation using inlet and outlet concentrations or mass rates
- Be able to state one advantage and one disadvantage of each scrubber type in exam responses

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What pollutant are scrubbers mainly designed to remove from coal power plant emissions?
2. How does removing sulfur dioxide from smokestack gases reduce acid deposition?
3. What is one major difference between a wet scrubber and a dry scrubber?
4. Why are electrostatic precipitators not the same thing as scrubbers?
5. If flue gas enters a scrubber with 500 ppm SO₂ and leaves with 50 ppm SO₂, what is the removal efficiency?

## Introduction

Scrubbers are one of the most testable air-pollution control technologies in AP Environmental Science because they connect several topics at once: fossil fuel combustion, sulfur dioxide emissions, acid deposition, and pollution control. If you see a question about coal-fired power plants, smokestacks, or reducing acid rain, scrubbers are one of the first technologies you should think about.

In simple terms, a scrubber removes harmful pollutants from exhaust gases before those gases leave a smokestack. On the AP exam, the most important pollutant linked to scrubbers is **sulfur dioxide (SO₂)**. Because SO₂ can react in the atmosphere to form sulfuric acid, removing it at the power plant helps stop pollution before it becomes a wider regional problem.

> **Key Term: flue-gas desulfurization (FGD)**
> The process of removing sulfur dioxide from exhaust gases, usually from coal-burning power plants, before the gases are released into the atmosphere.

### Why scrubbers matter for acid deposition

Coal often contains sulfur. When coal is burned, that sulfur is released mainly as sulfur dioxide. Once in the atmosphere, sulfur dioxide can react with water vapor and oxygen to form acidic compounds. These acids can return to Earth as **wet deposition** such as acid rain, fog, or snow, or as **dry deposition** such as acidic particles and gases settling onto surfaces.

That cause-and-effect chain is crucial:

1. Coal is burned
2. **SO₂** is released
3. SO₂ reacts in the atmosphere
4. Sulfuric acid forms
5. Acid deposition damages soil, lakes, streams, forests, and buildings

Scrubbers interrupt this chain at step 2. They remove SO₂ before it escapes into the air. That is why they are a direct method of reducing acid deposition.

> **Key Term: sulfur dioxide (SO₂)**
> A primary air pollutant released mainly by burning sulfur-containing fossil fuels, especially coal and oil.
>
> **Key Term: acid deposition**
> The return of acidic pollutants from the atmosphere to Earth as wet forms, such as rain and snow, or dry forms, such as particles and gases.

Students sometimes confuse scrubbers with solutions that repair damage after acid rain has already happened, such as adding limestone to lakes. That is remediation. Scrubbers are different because they are a **prevention** strategy. They reduce the pollutant before it enters the atmosphere.

A common AP application is a question asking how a coal plant could reduce the falling pH of soils or lakes downwind. A strong answer is: install scrubbers to reduce sulfur dioxide emissions from smokestacks.

> **Test Tip:** If a question asks for a way to reduce acid rain from a coal plant, the safest first answer is usually "use scrubbers to remove SO₂ from flue gas."

### How wet scrubbers work

A **wet scrubber** removes pollutants by bringing dirty exhaust gas into contact with a liquid, usually water mixed with an alkaline substance such as limestone or lime. Because sulfur dioxide is acidic in behavior, it reacts with the alkaline material and is captured instead of being released.

In coal power plants, the common alkaline sorbents are:

- **limestone**: calcium carbonate, CaCO₃
- **lime**: calcium hydroxide, Ca(OH)₂

The basic idea is chemical neutralization. The sulfur dioxide in the flue gas reacts with calcium compounds and becomes part of a solid or slurry that can be removed.

One representative overall wet-scrubber reaction is:

CaCO₃ + SO₂ + 1/2 O₂ + 2H₂O → CaSO₄·2H₂O + CO₂

The product **CaSO₄·2H₂O** is gypsum.

This matters because the scrubber is not just "washing" the gas in a physical sense. It is also driving a chemical reaction. That is why alkaline materials are required. Plain water alone would not give the same level of sulfur dioxide control.

> **Key Term: wet scrubber**
> A pollution control device in which exhaust gas contacts a liquid, usually water mixed with limestone or lime, to remove sulfur dioxide and other acid gases.

Wet scrubbers are often preferred when plants need very high sulfur dioxide removal, especially if the coal being burned has a high sulfur content. Typical wet systems can remove more than 90% of SO₂, and modern systems may reach roughly 90-98% depending on design and operation.

A common AP mistake is to say that wet scrubbers only move air pollution into water pollution. That is partly true in the sense that there is wastewater or slurry to manage, but it is incomplete. In many systems, the sulfur ends up as a removable solid byproduct such as gypsum, not just contaminated water.

#### Wet scrubber advantages and drawbacks

Advantages of wet scrubbers:

- high SO₂ removal efficiency
- strong reduction of acid deposition risk
- can also reduce some other acid gases such as HCl and HF
- may capture some oxidized mercury

Drawbacks of wet scrubbers:

- high water use
- sludge, slurry, or wastewater must be handled
- systems are more complex and expensive than simpler dry systems
- corrosion and maintenance can be significant issues

The key trade-off is simple: **higher removal, but more water and more complicated waste handling**.

### How dry scrubbers work

A **dry scrubber** captures sulfur dioxide using an alkaline material too, but it uses far less water. On AP Environmental Science, it is best to think of dry scrubbers as lower-water alternatives to wet scrubbers.

Two common dry or semi-dry approaches are:

- **spray dry absorber (SDA)**, also called a lime spray dryer
- **dry sorbent injection (DSI)**

In a spray dry absorber, a lime slurry is sprayed into hot flue gas. The water quickly evaporates, leaving dry reaction products behind. In dry sorbent injection, a dry alkaline powder is injected into the flue gas, where it reacts with SO₂.

A typical dry-system reaction is:

Ca(OH)₂ + SO₂ → CaSO₃ + H₂O

With further oxidation, sulfate can also form.

> **Key Term: dry scrubber**
> A pollution control device that uses a dry or semi-dry alkaline sorbent, such as lime, to react with sulfur dioxide and form removable dry solids.

The solids produced in dry systems usually include:

- calcium sulfite
- calcium sulfate
- unreacted sorbent
- fly ash

These solids are then collected by downstream devices, often baghouses or electrostatic precipitators.

This is one place where students get mixed up. The scrubber captures the gas pollutant chemically. The baghouse or ESP then helps collect the solid products. These are linked systems, but they are not the same device.

Dry scrubbers generally use less water, are often easier to retrofit, and create dry waste that can be simpler to handle than wet slurry. However, they usually have lower maximum SO₂ removal than wet scrubbers. Conventional dry systems are often below 80%, though some newer designs can approach about 90%.

#### Wet vs dry scrubbers compared

| Feature                 | Wet scrubber                               | Dry scrubber                                  |
| ----------------------- | ------------------------------------------ | --------------------------------------------- |
| Main target pollutant   | SO₂                                        | SO₂                                           |
| Water use               | High                                       | Low                                           |
| Sorbent                 | Limestone or lime slurry                   | Lime or other dry/semi-dry sorbent            |
| Typical waste/byproduct | Slurry, wastewater, gypsum in some systems | Dry solids                                    |
| Typical SO₂ removal     | Usually >90%                               | Often lower, though some designs approach 90% |
| Best use                | High removal needs, higher-sulfur fuel     | Lower water use, simpler waste handling       |

> **Test Tip:** For compare/contrast questions, use this structure: wet scrubber = higher SO₂ removal but higher water use; dry scrubber = lower water use but usually lower removal efficiency.

### Scrubbers vs particulate-control devices and worked efficiency example

Students often confuse scrubbers with **electrostatic precipitators** and **baghouses** because all three may appear in questions about coal plants. The clearest distinction is this:

- **scrubbers** mainly target gaseous pollutants, especially SO₂
- **ESPs** and **baghouses** mainly target particulate matter
  > **Key Term: electrostatic precipitator (ESP)**
  > A device that uses electric charges to remove particulate matter from exhaust gases.
  >
  > **Key Term: baghouse**
  > A pollution control system that passes dirty gas through fabric filters to trap particulate matter.

Why the distinction matters:

- If the question asks about reducing **acid deposition**, think scrubbers because SO₂ is the key pollutant
- If the question asks about reducing **fly ash**, soot, or particulate matter, think ESPs or baghouses
- If the question involves a dry scrubber setup, remember that a baghouse or ESP may be used after the scrubber to collect reaction products

Scrubbers may remove some particulates depending on design, but they are not mainly installed as particulate-control devices on the AP exam. The exam emphasis is much more straightforward: **scrubber = sulfur dioxide control**.

#### Worked example: removal efficiency

You may be asked to calculate how effective a scrubber is. The removal efficiency formula is:

η = ((C_in - C_out) / C_in) × 100%

Where:

- C_in = pollutant concentration entering the scrubber
- C_out = pollutant concentration leaving the scrubber

Example:

A coal plant’s flue gas enters a scrubber with **500 ppm SO₂** and exits with **50 ppm SO₂**.

Step 1: Write the formula

η = ((C_in - C_out) / C_in) × 100%

Step 2: Substitute the values

η = ((500 - 50) / 500) × 100%

Step 3: Simplify

η = (450 / 500) × 100%

η = 0.9 × 100%

η = **90%**

So the scrubber removes **90% of the sulfur dioxide**.

> **Test Tip:** Always subtract outlet from inlet. If you reverse them, you may get a negative answer, which is a common exam error.

#### Common student errors

Here are the mistakes students make most often:

1. **Confusing SO₂ with NOₓ**
   Scrubbers are mainly associated with sulfur dioxide, not nitrogen oxides.

2. **Mixing up scrubbers and ESPs**
   Scrubbers mainly remove acid gases like SO₂; ESPs mainly remove particulates.

3. **Assuming all scrubbers are wet**
   Dry and semi-dry scrubbers also reduce SO₂ and also help reduce acid deposition.

4. **Forgetting the environmental trade-off**
   Wet scrubbers reduce SO₂ very well, but use more water and create slurry or wastewater issues.

5. **Overstating what scrubbers do**
   Scrubbers are not complete pollution-control systems by themselves. A coal plant may also need particulate controls and other devices.

#### How to use this in FRQs

If an FRQ asks you to propose a way to reduce emissions from a coal-burning power plant, a strong answer can be built in three steps:

1. **Identify the technology**
   Install a wet or dry scrubber.

2. **Name the pollutant**
   The scrubber removes sulfur dioxide from flue gas.

3. **Explain the environmental result**
   Less sulfur dioxide enters the atmosphere, so less sulfuric acid forms and acid deposition decreases.

That gives you a clear cause-and-effect chain, which is exactly what AP Environmental Science rewards.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Scrubbers are mainly used to reduce **sulfur dioxide (SO₂)** from smokestack emissions
- **Flue-gas desulfurization (FGD)** is the general term for scrubber-based SO₂ removal
- Removing SO₂ lowers **acid deposition** because less sulfuric acid can form in the atmosphere
- Wet scrubbers use a liquid alkaline slurry, usually limestone or lime
- Dry scrubbers use dry or semi-dry alkaline sorbents and much less water
- Wet scrubbers usually achieve higher SO₂ removal than dry scrubbers
- Wet scrubbers may produce gypsum or slurry waste, while dry scrubbers usually produce dry solids
- Scrubbers mainly target gaseous pollutants such as SO₂, not primarily particulate matter
- **Electrostatic precipitators** and **baghouses** are mainly particulate-control devices
- Scrubber efficiency can be calculated with the inlet-outlet removal formula
- A 500 ppm to 50 ppm SO₂ reduction gives a **90% removal efficiency**
- A strong FRQ answer links scrubbers to SO₂ reduction and then to lower acid rain risk

## Key Terms and Concepts

- flue-gas desulfurization (FGD)
- sulfur dioxide (SO₂)
- acid deposition
- wet scrubber
- dry scrubber
- electrostatic precipitator (ESP)
- baghouse


Reduction of Air Pollutants - Wet and dry scrubbers