AP Environmental Science

## Learning Outcomes

- Understand how **coal rank** is used to classify different types of coal
- Identify the order of the four main coal ranks from highest to lowest energy content
- Explain how carbon content, moisture content, and heat content change across coal types
- Distinguish between **peat** as a precursor and the four main coal ranks used in AP Environmental Science
- Describe the common uses of anthracite, bituminous coal, subbituminous coal, and lignite
- Explain how sulfur in coal links to **sulfur dioxide (SO₂)** emissions and **acid deposition**
- Apply coal rank trends to multiple-choice questions, graphs, and free-response responses

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand coal as a major nonrenewable fuel source, with a focus on the following syllabus points:

- Know that the four main coal ranks are **lignite, subbituminous, bituminous, and anthracite**
- Identify the order of coal rank from lowest to highest, and from highest to lowest energy yield
- Explain that as coal rank increases, **carbon content** and **heat content** increase while **moisture content** decreases
- Understand that **peat** forms earlier in the sequence but is not usually counted as one of the four main coal ranks
- Recognise that **anthracite** is the highest-rank, highest-energy coal and **lignite** is the lowest-rank, lowest-energy coal
- Understand that **bituminous coal** is commonly linked with higher sulfur pollution in AP-level treatments
- Explain how burning sulfur-containing coal releases **SO₂**, contributing to air pollution and acid deposition
- Understand that all coal combustion releases **CO₂**, even if some coal types burn more cleanly than others

## 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 are the four main coal ranks in order from lowest to highest?
2. How do carbon content and moisture content change as coal rank increases?
3. Why does anthracite generally produce more useful energy per unit mass than lignite?
4. What is the difference between peat and lignite?
5. How can sulfur in coal lead to acid deposition?

## Introduction

Coal is one of the most tested fossil fuels in AP Environmental Science because it connects energy production, mining, air pollution, and climate change. Students are often asked not just to name coal types, but to compare them. That means you need more than a list. You need to understand the pattern behind the list.

Coal types are best understood through the idea of **rank**. Rank tells you how far plant material has progressed from partly decayed organic matter toward a harder, drier, more carbon-rich fuel. Once you understand that progression, many facts become easier to remember: higher-rank coal usually has more carbon, less moisture, and greater heat value. Lower-rank coal tends to be wetter and less energy-dense.

> **Key Term: coal rank**
> The classification of coal based on how far it has progressed through coalification, especially its carbon content, moisture content, and heat value.

## Introduction to coal formation and rank

Coal did not begin as coal. It began as dead plant matter, especially from ancient swampy environments. Over very long periods of time, that plant material was buried, compressed, and heated. As burial continued, the material lost water and became more concentrated in carbon.

This progression matters because AP questions often test the difference between **formation sequence** and **rank classification**. In the formation sequence, plant material first becomes peat, then lignite, then subbituminous coal, then bituminous coal, and finally anthracite. But in the usual four-rank coal classification, peat is left out.

> **Key Term: coalification**
> The natural process by which buried plant material becomes denser, drier, and more carbon-rich over geologic time, forming coal.
>
> **Key Term: peat**
> Partially decayed organic material that forms in wet, low-oxygen conditions and is a precursor to coal, not usually one of the four main coal ranks.

Why is peat usually excluded? Because it is not fully coal in the standard APES sense. It is an early stage, still rich in moisture and only partly transformed. Students often lose marks by listing peat as one of the four main coal ranks when the question asks specifically for the ranks.

A useful way to keep the sequence clear is:

- Formation sequence: plant material → peat → lignite → subbituminous → bituminous → anthracite
- Four main coal ranks: lignite → subbituminous → bituminous → anthracite
  > **Test Tip:** If a question asks for the "four main coal ranks," do not include peat unless the wording specifically asks about the formation sequence.

## The four main coal ranks and their properties

### Lignite: the lowest rank

**Lignite** is the lowest-rank coal. It is often called **brown coal**. It has the lowest carbon content of the four main ranks and the highest moisture content. Because so much of its mass includes water and less carbon-rich material, it provides the least heat energy per unit mass.

That matters in practice. When a fuel contains more moisture, some energy released during combustion is effectively spent heating that water rather than producing as much useful heat for electricity generation. This is one reason lignite is considered lower-quality coal.

Lignite is commonly used for electricity generation, often near the place where it is mined. Transporting it long distances is less attractive because its low energy density makes it a less efficient fuel to move.

> **Key Term: lignite**
> The lowest-rank coal, with low carbon content, high moisture, and low heat value; often used for electricity generation.

A common mistake is to assume lower-rank means automatically lower pollution. That is not always true. Because lignite gives less energy per unit mass, more of it may need to be burned to generate the same electricity. That can mean more emissions per unit of useful energy produced.

### Subbituminous coal: intermediate but lower sulfur in many cases

**Subbituminous coal** sits above lignite and below bituminous coal. It has more carbon and less moisture than lignite, so it gives a better heat yield. However, it is still lower in energy than bituminous and anthracite.

Subbituminous coal is also widely used for electricity generation. In AP Environmental Science, it is often treated as lower in sulfur than bituminous coal, though sulfur content is not perfectly fixed by rank everywhere. That is an important distinction. On the exam, you should treat subbituminous as commonly linked to lower sulfur emissions than bituminous unless the question gives different data.

> **Key Term: subbituminous coal**
> A middle-rank coal above lignite and below bituminous, with moderate carbon content and heat value, often used in power generation.

Why does this distinction matter? Because sulfur affects air pollution. If a question compares two coals for likely **SO₂** emissions, bituminous is often the one associated with greater sulfur concerns.

### Bituminous coal: common, energy-rich, and often sulfur-linked

**Bituminous coal** is one of the most important coal types for APES. It has high carbon content, relatively high heat value, and is widely used in electricity generation. In simple exam treatments, it is also commonly associated with higher sulfur content than subbituminous coal.

That combination makes bituminous coal both useful and environmentally significant. It burns well and produces substantial heat, which makes it attractive for steam-electric generation. But if sulfur is present, burning it releases sulfur dioxide, contributing to industrial air pollution and acid deposition.

> **Key Term: bituminous coal**
> A high-rank coal below anthracite, with high carbon content and heat value; widely used in electricity generation and commonly linked with sulfur pollution.

Bituminous coal may also be used in making coke for iron and steel production, though APES most often emphasizes its electricity role.

### Anthracite: the highest rank

**Anthracite** is the highest-rank coal. It has the highest carbon content, the lowest moisture content, and the greatest heat value of the four main coal ranks. It is hard, dense, and almost pure carbon compared with lower-rank coal.

Because it contains more carbon-rich material and less water, anthracite generally burns hotter and more efficiently by mass than other coal ranks. That is why it is often described as the cleanest-burning coal in relative terms.

> **Key Term: anthracite**
> The highest-rank coal, with very high carbon content, low moisture, and the greatest heat value.

But "cleanest-burning" is only a relative description. Anthracite still releases carbon dioxide when burned. It is not a clean fuel in the wider climate sense. Students sometimes confuse "cleaner than other coal" with "clean." Those are very different claims.

> **Test Tip:** If an exam question asks which coal has the highest energy content, the answer is **anthracite**. If it asks which has the lowest, the answer is **lignite**.

## The trend across ranks: what changes and why

### The core pattern you must know

The single most important exam pattern is this:

From **lignite → subbituminous → bituminous → anthracite**:

- carbon content increases
- heat content increases
- moisture content decreases

This trend is the logic behind coal rank. Higher-rank coal has experienced more heat and pressure during formation, which drives off water and concentrates carbon. More carbon-rich fuel means more chemical energy available during combustion.

Students often memorize the order but not the trend. That is risky, because AP questions may avoid naming the ranks directly and instead describe their properties in a table. For example, a question may ask which sample is highest-rank based on lowest moisture and highest heating value. If you only memorized names, you may miss it.

### Worked comparison example

Imagine four coal samples:

- Sample A: lowest moisture, highest heat value
- Sample B: high heat value, but lower than A
- Sample C: moderate moisture and moderate heat value
- Sample D: highest moisture, lowest heat value

Step 1: Match the trend.

- Highest rank = lowest moisture + highest heat value = Sample A = anthracite
- Next = Sample B = bituminous
- Next = Sample C = subbituminous
- Lowest rank = Sample D = lignite

Step 2: Explain the reasoning.
As coal rank rises, burial and coalification remove water and increase carbon concentration. That raises the fuel's heating value.

This kind of reasoning is exactly what multiple-choice items often test.

### Cleaner-burning versus lower total pollution

Students also need one subtle distinction. Higher-rank coal may burn more cleanly in relative terms because it contains fewer impurities and less moisture. But all coal combustion remains a major pollution source.

Coal burning releases:

- carbon dioxide
- particulates
- nitrogen oxides
- sulfur oxides, if sulfur is present
- heavy metals such as mercury

So even anthracite should not be described as environmentally safe. It is only less dirty than lower-rank coal in some combustion respects.

## Coal type, sulfur, and environmental impact

### From sulfur in coal to sulfur dioxide in air

One of the most testable cause-and-effect chains is the sulfur pollution chain.

When sulfur-containing coal is burned:

coal sulfur → **SO₂** released → reactions in atmosphere → acid deposition

> **Key Term: sulfur dioxide (SO₂)**
> A gas released when sulfur-containing fuels such as coal are burned; it contributes to air pollution and acid deposition.
>
> **Key Term: acid deposition**
> The transfer of acidic substances from the atmosphere to Earth’s surface as wet deposition or dry deposition, often linked to SO₂ and nitrogen oxides.

In the atmosphere, sulfur dioxide can react further and help form acids that lower the pH of rain, soils, lakes, and streams. This can damage aquatic life, weaken forests, and corrode buildings.

### Why bituminous coal is often emphasized

At AP level, **bituminous coal** is commonly the coal type most strongly linked to sulfur concerns. That does not mean every bituminous deposit has the same sulfur level, but it is the exam-friendly association you should know.

By contrast, **subbituminous coal** is often treated as lower sulfur. This is why some questions may imply that subbituminous has lower SO₂ emissions than bituminous when burned.

### Coal pollution links across units

Coal belongs to Unit 6, but it connects directly to Unit 7 and Unit 9.

- Unit 6: coal as a nonrenewable energy resource
- Unit 7: coal combustion as a source of **SO₂**, particulates, and industrial smog
- Unit 9: coal combustion as a major source of anthropogenic **CO₂**

That means when you revise coal ranks, you are also building links to air pollution and climate change questions.

## How AP questions test coal types

### Common multiple-choice patterns

AP Environmental Science commonly tests coal through comparisons rather than obscure detail. Expect questions like:

- Which coal has the highest energy content?
- Which coal has the highest moisture?
- Which coal is commonly called brown coal?
- Which coal type is most associated with sulfur pollution?
- What is the correct order of coal ranks?
- How does coal rank relate to carbon content?

If a table gives you carbon, moisture, or heating value, use the main trend rather than guessing.

### Common mistakes students make

Students often make three errors:

1. **Including peat as one of the four ranks**
   Peat is part of the formation sequence, but not usually one of the four main coal ranks.

2. **Saying bituminous is the highest rank**
   It is not. Anthracite is higher.

3. **Calling anthracite clean**
   Anthracite is cleaner relative to other coal types, but it still emits **CO₂** and other pollutants during combustion.

### FRQ-style explanation model

If you had to explain why one coal is better for energy production than another, a strong answer would say:

Higher-rank coal such as anthracite has greater carbon content and lower moisture content than lower-rank coal such as lignite. Therefore, it produces more heat per unit mass when burned. Lower-rank coal contains more water and less carbon-rich material, so it provides less useful energy and may require more fuel to generate the same electricity.

That answer shows the causal link, not just the fact.

> **Test Tip:** In FRQs, use a full chain: **higher rank → more carbon and less moisture → higher heat content → more useful energy per unit mass**. That earns more credit than naming the coal type alone.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Coal rank classifies coal by degree of coalification
- The four main coal ranks are lignite, subbituminous, bituminous, and anthracite
- Peat comes before lignite in coal formation but is not usually counted as one of the four main coal ranks
- As coal rank increases, carbon content increases
- As coal rank increases, heat content increases
- As coal rank increases, moisture content decreases
- Anthracite is the highest-rank coal and has the highest energy content
- Lignite is the lowest-rank coal and has the highest moisture content
- Bituminous coal is widely used for electricity generation and is commonly linked with sulfur pollution
- Burning sulfur-containing coal releases sulfur dioxide, which can lead to acid deposition
- All coal types release carbon dioxide when burned
- AP questions often test rank through trends in moisture, carbon content, and heating value rather than names alone

## Key Terms and Concepts

- coal rank
- coalification
- peat
- lignite
- subbituminous coal
- bituminous coal
- anthracite
- sulfur dioxide (SO₂)
- acid deposition


Fuel Types and Uses - Types of coal