AP Environmental Science

## Learning Outcomes

- Understand which major air pollutants come from coal-fired power plants
- Identify the correct pollution-control device for **SO₂**, **NOₓ**, particulate matter, and mercury
- Explain how scrubbers reduce acid rain by lowering sulfur dioxide emissions
- Compare electrostatic precipitators and baghouses for particulate removal
- Distinguish between **SCR** and **SNCR** as nitrogen oxide controls
- Understand how the **1990 Clean Air Act Amendments** and the **Acid Rain Program** reduced emissions
- Evaluate trade-offs of coal plant pollution controls, including waste, cost, and fuel switching

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand coal plant air-pollution control methods, with a focus on the following syllabus points:

- Know that coal-fired power plants release **sulfur dioxide (SO₂)**, **nitrogen oxides (NOₓ)**, **particulate matter**, **mercury**, and **carbon dioxide**
- Identify **scrubbers** or **flue-gas desulfurization** as the main control for **SO₂**
- Identify **electrostatic precipitators** and **baghouse filters** as controls for particulate matter such as fly ash
- Understand that **SCR** and **SNCR** are used to reduce **NOₓ**
- Know that **activated carbon injection** is used mainly for mercury control
- Explain how reducing **SO₂** emissions lowers acid deposition
- Understand that the **1990 Clean Air Act Amendments** created the **Acid Rain Program**, a cap-and-trade system for **SO₂**
- Distinguish between primary pollutants from coal plants and the secondary pollutants they create in the atmosphere

## 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. Which coal plant pollution-control device is mainly used to remove sulfur dioxide?
2. How does an electrostatic precipitator differ from a baghouse filter?
3. What is the main difference between **SCR** and **SNCR**?
4. How did the **Acid Rain Program** allow power plants to comply with sulfur dioxide limits?
5. Why does removing **SO₂** from flue gas help reduce acid rain?

## Introduction

Coal-fired power plants are a major source of air pollution because coal combustion releases several harmful substances at the same time. For AP Environmental Science, you need to think about this topic as a matching exercise: each pollutant has a main control method, and exam questions often test whether you can link the right device to the right pollutant.

This topic also connects Unit 6, Unit 7, and Unit 9. Coal is an energy source, but burning it creates atmospheric pollution and contributes to global change. That means you need to know not only what devices are used, but also why they matter for problems such as **acid deposition**, health effects from particulates, and regulation under the **Clean Air Act**.

> **Key Term: coal-fired power plant**
> A power station that burns coal to heat water, make steam, spin a turbine, and generate electricity.

### Pollutants from coal plants and why they matter

Coal plants do not release just one pollutant. They release a mixture, and each pollutant creates a different problem.

The main pollutants are:

- **Sulfur dioxide (SO₂)**
- **Nitrogen oxides (NOₓ)**
- **Particulate matter**, especially fly ash
- **Mercury**
- **Carbon dioxide (CO₂)**

For APES, the most tested idea is that **SO₂** and **NOₓ** are **primary pollutants** released directly from the plant, while sulfuric and nitric acids formed later in the atmosphere are part of **acid deposition**, which is a secondary effect.

> **Key Term: primary pollutant**
> A pollutant released directly into the atmosphere from a source, such as SO₂ or NOₓ from a power plant.

Why these matter:

- **SO₂** contributes strongly to acid rain.
- **NOₓ** contributes to acid deposition and photochemical smog.
- **Particulate matter** harms lungs and can travel long distances.
- **Mercury** can enter food chainss and later bioaccumulate.
- **CO₂** contributes to climate change.

A common student mistake is to assume one device “cleans” everything. Coal plants usually need a series of controls because different pollutants behave differently. Gases such as **SO₂** must often be removed chemically. Solids such as fly ash are usually removed physically.

> **Key Term: flue gas**
> The hot mixture of gases and particles that leaves a combustion system and travels toward the smokestack.
>
> **Test Tip:** If a question asks you to match a pollutant to a device, start with the easiest pairings: **SO₂ -> scrubber**, **PM -> ESP or baghouse**, **NOₓ -> SCR or SNCR**, **mercury -> activated carbon injection**.

### Matching pollutants to control devices

The fastest way to revise this topic is to organize it by pollutant.

#### Sulfur dioxide: scrubbers and flue-gas desulfurization

The main control for **SO₂** is the **scrubber**, also called **flue-gas desulfurization (FGD)**.

A scrubber works by bringing flue gas into contact with an alkaline substance. Because **SO₂** is acidic when dissolved in water, it reacts with alkaline materials such as limestone or lime. This removes it from the gas stream before the gas exits the smokestack.

> **Key Term: flue-gas desulfurization (FGD)**
> A pollution-control process that removes sulfur dioxide from flue gas by reacting it with an alkaline material such as limestone or lime.

The APES-level idea is simple: if you remove **SO₂** before it enters the atmosphere, less sulfuric acid can form later, so acid rain decreases.

A representative reaction in a limestone-based system is:

CaCO₃ + SO₂ + 1/2 O₂ + 2H₂O -> CaSO₄·2H₂O + CO₂

You do not need advanced chemistry to use this. The key idea is that sulfur dioxide is converted into a removable solid product, often **calcium sulfate**. In many wet systems, that product can be processed into **gypsum**.

Wet scrubbers can be very effective. Depending on design and operation, FGD systems can reduce **SO₂** by about **50% to 98%**.

There are several forms:

- **Wet scrubbers**: use a slurry, often limestone slurry
- **Dry or semi-dry scrubbers**: spray a lime-based material into hot gas, then capture the dry products

Wet systems are common because they are effective, but they can create sludge or wastewater that must be managed. This is a classic APES trade-off: reducing one kind of pollution can create another management issue.

> **Key Term: scrubber**
> A device that removes acidic gases, especially sulfur dioxide, from smokestack emissions by reacting them with an alkaline substance.

#### Particulate matter: electrostatic precipitators and baghouses

Coal combustion produces solid particles called **fly ash**, along with other particulates. These are not mainly removed by scrubbers. Instead, coal plants use **electrostatic precipitators** or **fabric filters**.

> **Key Term: electrostatic precipitator (ESP)**
> A device that removes particulate matter from flue gas by electrically charging particles and collecting them on plates.

An **ESP** works by charging particles in the flue gas. Once charged, the particles are attracted to collector plates, where they build up and are later removed. This is a physical separation process, not a chemical one.

A **baghouse** works differently. Flue gas passes through fabric filters, and the particles are trapped in the fabric.

> **Key Term: baghouse filter**
> A particulate control device that removes fly ash and other particles by passing flue gas through fabric filters.

Both devices target particulate matter, but students often confuse them.

Compare them clearly:

- **ESP**: uses electric charge and plates
- **Baghouse**: uses physical filtration through fabric

Both are very effective. Baghouses often perform especially well for very fine particles, while ESPs are also highly efficient and very common in coal plants.

A frequent exam trap is to say an ESP removes sulfur dioxide. It does not mainly do that. It removes **solid particles**.

#### Nitrogen oxides: SCR and SNCR

**NOₓ** forms during high-temperature combustion. Coal plants reduce it in two broad ways:

- reducing how much forms during combustion
- treating the flue gas after combustion

One combustion-based method is the use of **low-NOₓ burners**, which reduce peak combustion temperature and control fuel-air mixing. But the main post-combustion controls you should know are **SCR** and **SNCR**.

> **Key Term: selective catalytic reduction (SCR)**
> A method of reducing nitrogen oxides by reacting them with ammonia or urea over a reactive material to form mainly nitrogen gas and water.
>
> **Key Term: selective non-catalytic reduction (SNCR)**
> A method of reducing nitrogen oxides by reacting them with ammonia or urea without a chemical additive.

Both methods convert **NOₓ** into less harmful products, mainly **N₂** and **H₂O**. The main difference is the key difference.

Compare them side by side:

- **SCR** uses a reagent and is generally more effective
- **SNCR** does not use a special component and is generally less effective

This is highly testable because the names look similar. If you remember that the **C** in **SCR** stands for catalytic, you can separate them quickly.

Some systems use ammonia injection into flue gas, which the exam may describe in plain language rather than naming SCR or SNCR directly. If ammonia or urea is used to remove **NOₓ**, think nitrogen oxide control.

> **Test Tip:** If an FRQ says a plant “sprays ammonia into flue gas to reduce emissions,” do not say scrubber unless the question clearly targets **SO₂**. Ammonia or urea points toward **NOₓ** control, especially **SCR** or **SNCR**.

### Mercury control, acid rain policy, and trade-offs

#### Mercury control

Coal can contain trace amounts of mercury. When burned, that mercury can enter the flue gas. The main dedicated control technology is **activated carbon injection (ACI)**.

> **Key Term: activated carbon injection (ACI)**
> A mercury-control method in which activated carbon is injected into flue gas so mercury sticks to it and can then be captured.

The logic is straightforward:

1. Activated carbon is injected into the flue gas
2. Mercury adsorbs onto the carbon
3. The carbon-mercury particles are then captured by a baghouse or ESP

This means **ACI** often works together with particulate controls. It is another example of a pollution-control train, not a single-device solution.

#### Why sulfur dioxide control matters for acid rain

Acid deposition forms when sulfur and nitrogen compounds react in the atmosphere and later return to Earth in wet or dry form. Coal plants are especially important here because they have historically been major **SO₂** sources.

Removing **SO₂** matters because it breaks the chain before sulfuric acid forms. Less atmospheric **SO₂** means:

- less acid rain
- less acidification of lakes and streams
- less nutrient leaching from soils
- less damage to forests and buildings

This causal chain is a favorite APES prompt.

Worked reasoning:

- coal combustion releases **SO₂**
- atmospheric reactions convert that sulfur to acidic compounds
- those compounds enter rain, snow, fog, or dry deposition
- oceanss acidify
- so, reducing **SO₂** at the smokestack lowers downstream environmental damage

#### The Acid Rain Program

The main policy example for this topic is the **Acid Rain Program**, created by the **1990 Clean Air Act Amendments**.

> **Key Term: Acid Rain Program**
> A U.S. cap-and-trade program created in 1990 to reduce sulfur dioxide emissions, mainly from fossil-fuel power plants.

This program used **cap-and-trade**. Instead of ordering every plant to cut by the exact same amount, it set an overall cap and gave plants flexibility.

One allowance generally allowed emission of **one ton of SO₂**.

Important dates:

- **1995**: Phase I began, targeting the largest and highest-emitting coal units
- **2000**: Phase II expanded the program to many more units

Plants could comply by:

- installing scrubbers
- switching to lower-sulfur coal
- reducing operation of dirtier units
- buying or selling allowances
- retiring units

This is an excellent APES example of market-based regulation. It is not the same as a simple ban or a universal emission cut for every plant.

A strong evidence point is that U.S. power-sector **SO₂** emissions fell much faster than coal generation. That shows regulation plus technology can cut emissions even before coal use disappears entirely.

#### Trade-offs and evaluative thinking

Pollution controls reduce harmful emissions, but they are not cost-free.

Key trade-offs include:

- **Scrubbers** reduce air pollution but can create wastewater or sludge
- **Baghouses** and **ESPs** capture particulates, but the captured ash still needs disposal
- **SCR** reduces **NOₓ**, but adds complexity and cost
- **ACI** helps with mercury, but may affect fly ash reuse
- None of these devices mainly solve the **CO₂** problem

That last point matters. Students often overstate what coal plant controls do. These technologies mainly reduce conventional air pollutants, not climate-changing carbon dioxide.

A broader comparison is fuel switching. Replacing coal with natural gas generally lowers:

- **SO₂**
- particulate matter
- usually **NOₓ**
- and also lowers **CO₂** at the power plant

But even then, natural gas is still a fossil fuel.

If you are writing an FRQ, a balanced answer is strongest: coal controls can sharply reduce some pollutants, especially **SO₂** and particulate matter, but they do not eliminate all environmental costs, and replacing coal entirely may provide bigger long-run gains.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Coal-fired power plants release **SO₂**, **NOₓ**, particulate matter, mercury, and **CO₂**
- **SO₂** is mainly controlled by **scrubbers** or **flue-gas desulfurization**
- Scrubbers reduce acid rain by lowering the amount of sulfur dioxide entering the atmosphere
- Wet scrubbers often use limestone and can produce calcium sulfate or gypsum
- **Electrostatic precipitators** remove particulate matter by charging particles and collecting them on plates
- **Baghouse filters** remove particulate matter by trapping particles in fabric filters
- **SCR** and **SNCR** both reduce **NOₓ**, but **SCR** uses a reactant and is usually more effective
- **Activated carbon injection** is mainly used for mercury control
- The **1990 Clean Air Act Amendments** created the **Acid Rain Program**
- The **Acid Rain Program** used cap-and-trade, with one allowance generally equal to one ton of **SO₂**
- Phase I of the Acid Rain Program began in **1995** and Phase II began in **2000**
- Coal pollution controls reduce major air pollutants but do not mainly remove **CO₂**

## Key Terms and Concepts

- coal-fired power plant
- primary pollutant
- flue gas
- flue-gas desulfurization (FGD)
- scrubber
- electrostatic precipitator (ESP)
- baghouse filter
- selective catalytic reduction (SCR)
- selective non-catalytic reduction (SNCR)
- activated carbon injection (ACI)
- Acid Rain Program


Reduction of Air Pollutants - Coal plant pollution controls