🌱⏳ From Lush Life to Latent Energy: The Million-Year Metamorphosis of Coal 🔬🔥

Group 3: The Science Behind Coal Formation

Date: June 4, 2025

Members:

1. Tararat Kladkleeb (Kimju) No.1
2. Saranrat Kumkhun (Froy) No.2
3. Isadaorn Khamching (Cutie) No.9
4. Sirikron Areepong (Vita) No.20

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✨ Key Points Researched ✨

This study note delves into the fascinating geological and chemical transformation of ancient plant matter into coal. Here are the core insights:

• Coalification Defined: Coal is formed through "coalification," a natural, multi-million-year process where buried plant matter undergoes biological, chemical, and physical changes. 🌿➡️💎
• Heat as Primary Driver: Temperature is the most crucial factor influencing the degree of coalification (rank). Higher temperatures, typically from deeper burial or geothermal activity, lead to higher-rank coals. 🔥
• Pressure's Supporting Role: Pressure primarily contributes to compaction, reducing porosity and squeezing out moisture, especially in early stages. Its direct effect on rank is minor compared to heat. 💨
• Chemical Transformation: Coalification involves the progressive loss of volatile compounds (water, methane, CO₂, oxygen, hydrogen) and a corresponding enrichment in organically bound carbon. 🧪
• Coal Rank Progression: The transformation follows a sequence: Peat ➡️ Lignite ➡️ Subbituminous Coal ➡️ Bituminous Coal ➡️ Anthracite, with each stage representing increased carbon content and altered properties. 📈
• Preservation is Key: Initial accumulation of plant matter in anoxic (low oxygen) swampy environments and subsequent burial by sediments are vital to protect organic material from rapid decomposition, allowing coalification to occur. 🛡️
• Determining Quality & Age: Coal quality is assessed by its rank, determined through chemical (e.g., carbon content, volatile matter), physical (e.g., calorific value), and petrographic (e.g., vitrinite reflectance) analyses. Age is often determined by dating associated volcanic ash layers. 🧐⚖️

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1. Introduction: The Genesis of Coal 🌍🔬

Main Research Question (RQ): What are the specific geological and chemical processes that transform ancient plant matter into coal?

Coal, a vital energy resource, originates from ancient plant matter that accumulated in vast quantities millions of years ago. The transformation from lush vegetation to the black, combustible rock we know as coal is a complex process called coalification. This journey involves a series of intricate biological, geological, and chemical changes occurring deep within the Earth's crust over immense timescales. This study note will explore the critical roles of heat and pressure, the chemical reactions involved, how scientists determine coal's age and quality, the importance of initial preservation conditions, and the scientific basis for classifying coal by its "rank."

2. The Alchemists of Coalification: Heat and Pressure 🔥💨

Two of the most significant geological factors driving the coalification process are heat (temperature) and pressure. While both play a role, their impacts and importance differ.

2.1. The Dominant Role of Heat (Temperature) 🔥

Heat is considered the primary control on the coalification process and the resulting rank of the coal.

• Rank Dependence: The degree of coalification, or coal rank, is generally determined by the maximum temperature the organic material was exposed to during its geological history.
• Geochemical Alteration: Elevated temperatures, often experienced at greater burial depths, geochemically alter the original organic material. Heat causes hydrocarbon compounds within the peat to break down and transform in various ways, leading to coal formation.
• Sources of Heat:
• Burial Depth: Deeper burial leads to higher geothermal temperatures.
• Hydrothermal Fluids: Interaction with hot underground water.
• Contact Metamorphism: Proximity to igneous intrusions (magma).
• Time Factor: The duration for which organic matter is subjected to increased heating also influences the final coal rank.

2.2. The Supporting Role of Pressure 💨

Pressure is generally considered a minor influence on coalification and rank compared to temperature. Its main contributions are:

• Compaction & Porosity Reduction:
• The weight of overlying sediments compacts the plant material, squeezing out moisture (especially during peat and lignite stages) and reducing porosity. This physical alteration is always associated with the peat-to-coal transition.
• The amount of compaction depends on the original peat composition and burial depth.
• Tectonic Pressure: Pressures from deep within the Earth or from mountain-building events (tectonic pressures) appear to have less impact on coalification than temperature, unless they are associated with increased heating.

Table 1: Comparing the Roles of Heat and Pressure in Coalification

Factor	Primary Role in Coalification	Key Effects on Organic Matter	Significance for Coal Rank
Heat	Primary driver of chemical reactions and rank	Breaks down hydrocarbon compounds, drives off volatiles (O, H), promotes carbon enrichment.	Major determinant
Pressure	Secondary factor, primarily physical effects	Compresses material, reduces porosity, expels moisture and some gases, aids in heat transfer.	Minor influence

Note. Heat is the dominant factor in the chemical transformation and rank attainment of coal, while pressure primarily aids in physical compaction and fluid expulsion.

3. The Chemical Transformation: From Plant to Carbon Powerhouse 🧪✨

Coalification is fundamentally a process of chemical change, where the original organic matter becomes progressively enriched in carbon.

3.1. Progressive Carbon Enrichment (Carbonization in the Context of Coalification)

While "carbonization" in a strict industrial sense refers to heating coal in the absence of air to produce coke, the term is also broadly used in geology to describe the natural process during coalification where carbon content increases.

• Loss of Volatiles: As heat and pressure act on buried organic matter, volatile substances like water (H₂O), carbon dioxide (CO₂), methane (CH₄), oxygen (O), and hydrogen (H) are systematically expelled.
• Carbon Concentration: This expulsion of non-carbon elements leads to a relative increase in the proportion of organically bound carbon.
• Rank Progression: The transformation follows a sequence, with each stage representing a higher rank and generally higher carbon content: Peat ➡️ Lignite ➡️ Subbituminous Coal ➡️ Bituminous Coal ➡️ Anthracite

Table 2: Typical Change in Carbon Content During Coalification

Stage of Coalification	Approximate Carbon Content (%)	Notes
Wood	30 - 50%	Original plant material
Peat	Up to 60% (typically 50-60%)	Partially decomposed plant matter
Lignite	60 - 75% (some sources 25-35% or up to 71%)	Lowest rank coal, high moisture
Subbituminous Coal	70 - 80% (some sources 35-45%)	Higher energy than lignite
Bituminous Coal	75 - 90% (some sources 45-86%)	Most abundant rank, wide range of properties
High Volatile	~75 - 86%
Medium Volatile	~86 - 90%	Start of significant demethanation
Low Volatile	~90 - 91%
Semi-anthracite	~91 - 92%	Transitional to anthracite
Anthracite	>92% (typically 92-97%, some sources 86-97%)	Highest rank coal, highest carbon

Note. Carbon percentages can vary based on source material and analytical methods. The general trend is a clear increase in carbon content with increasing coal rank from Peat to Anthracite (Van Krevelen, 1993; Chen, 1998). The lower percentages for Lignite and Subbituminous (e.g., 25-35%) in one part of the provided text seem to be outliers or refer to a different basis (e.g., as-received vs. dry, ash-free). The more consistent trend shows lignite starting around 60-70%.

3.2. Key Chemical Reactions: The Great Expulsion 💨💧

The enrichment of carbon is a direct result of the loss of other elements through various chemical reactions, primarily driven by thermal energy (heat).

• Loss of Moisture (Dehydration): Water is squeezed out during compaction and driven off by heat, especially in early stages.
• Loss of Oxygen and Hydrogen (Decarboxylation & Dehydrogenation):
• Oxygen is lost primarily as CO₂ (decarboxylation) and H₂O. Original plant material (like wood) can have ~40% oxygen, which drops to ~2% in anthracite.
• Hydrogen is lost as H₂O and CH₄.
• These elements are released through thermal cracking of complex organic molecules.
• Loss of Methane (Demethanation): Methane (CH₄) is generated both biogenically in early stages (peat) and thermogenically at higher temperatures. Significant thermal demethanation begins around the medium-volatile bituminous rank.
• Loss of Nitrogen: Nitrogen content also tends to decrease, though it's a smaller component overall.

Table 3: Key Volatiles Lost and Chemical Changes During Coalification

Process	Volatiles Expelled	Primary Driver(s)	Impact on Coal Composition
Dehydration	Water (H₂O)	Compaction, Heat	Decreases moisture content, increases relative carbon
Decarboxylation	Carbon Dioxide (CO₂)	Heat	Decreases oxygen content, increases relative carbon
Demethanation	Methane (CH₄)	Heat	Decreases hydrogen content, increases relative carbon
Thermal Cracking	Various volatile hydrocarbons, H₂O, CO₂, CH₄	Heat	Breaks down complex molecules, releases O & H, C increases

Note. These processes collectively lead to the transformation of complex, oxygen-rich plant polymers into increasingly carbon-dense materials.

4. Unveiling Coal's Secrets: Determining Age and Quality 🧐⏳⚖️

Geologists and scientists employ various methods to understand coal deposits, including their age and, crucially, their quality.

4.1. Dating the Deposits 📅

• Indirect Dating: Coal itself (organic matter) cannot be easily dated using common radiometric methods like potassium-argon or fission-track.
• Volcanic Ash Layers (Tonsteins/Ash Partings): The age of coal deposits is often determined by radiometrically dating minerals found in layers of volcanic ash interbedded within the coal seams. These ash layers provide precise time markers.
• Geological Context: Coal formation occurred over vast geological timescales, with significant deposits formed during specific periods, such as the Carboniferous (359-299 million years ago). Lignite is generally younger than bituminous coal, which can be 100-300 million years old.
• Age vs. Rank: Importantly, the geologic age of a coal does not directly control its rank. Rank is primarily determined by the maximum temperature experienced.

4.2. Assessing Quality Through Rank 🏅

The "quality" of coal is primarily defined by its rank, which reflects the degree of coalification it has undergone.

• Higher Rank = Higher Quality (Generally): Higher rank coals typically have:
• Higher carbon content
• Lower moisture content
• Lower volatile matter content (beyond a certain point for coking coals)
• Higher heating value (calorific value)
• Parameters for Determining Rank: Scientists use a combination of analyses:

Table 4: Parameters for Determining Coal Rank

Category	Specific Parameters Measured	Trend with Increasing Rank	Significance
Chemical Analyses
Proximate Analysis	Moisture Content	Decreases	Affects heating value, handling
	Volatile Matter Content	Decreases	Influences combustion, coking properties
	Fixed Carbon Content	Increases (by difference)	Major contributor to heating value
	Ash Content	Not a rank parameter (impurity)	Affects quality and usability
Ultimate Analysis	Carbon Content (C)	Increases	Fundamental indicator of rank
	Hydrogen Content (H)	Generally decreases	Affects volatile matter and heating value
	Oxygen Content (O)	Significantly decreases	High O lowers heating value
	Nitrogen Content (N)	Minor changes/decreases	Environmental consideration upon combustion
	Sulfur Content (S)	Not a rank parameter (impurity)	Environmental consideration, affects usability
Physical/Physico-chemical Analyses
	Specific Energy (Calorific/Heating Value)	Generally increases (may peak at bituminous)	Key measure of energy content
	Caking Index / Crucible Swelling Number (CSN) / Free Swelling Index (FSI)	Characteristic of certain bituminous coals	Indicates suitability for coke production (metallurgical coal)
	Plastic Properties (e.g., Gieseler Plastometer)	Characteristic of certain bituminous coals	Measures behavior during heating, important for coking
Petrographic Analyses
	Vitrinite Reflectance (Rₒ)	Increases	Reliable indicator of maximum temperature experienced, widely used
	Maceral Analysis (Vitrinite, Liptinite, Inertinite)	Composition varies, influences coal properties	Identifies original plant materials and depositional environment, affects type

Note. A combination of these parameters, particularly volatile matter, carbon content, and vitrinite reflectance, is used to classify coal according to its rank (Carpenter, 1988; Van Krevelen, 1993).

5. The Protective Embrace: Sedimentation and Burial 🛡️🌍

The initial stages of accumulation and preservation are critical for coal formation.

• Anoxic Environments: Coal formation begins with the accumulation of plant debris (trees, leaves, roots, algae) in wet, swampy environments like mires or peat bogs. The waterlogged conditions create an anoxic (low oxygen) environment.
• Slowing Decomposition: This restricted oxygen supply significantly slows down the aerobic decomposition of dead plant material by fungi, insects, and aerobic bacteria, which would rapidly break it down on dry land.
• Burial and Preservation:
• For peat to transform into coal, it must be buried by layers of sediment (sand, silt, clay).
• Burial below the groundwater table further protects the organic matter from aerobic attack and oxidation.
• Anaerobic bacterial activity may continue but is limited by conditions at increasing depth.
• Compaction: The weight of these overlying sediments compacts the peat, squeezing out water and reducing its volume.
• Setting the Stage: Sedimentation and burial effectively isolate the organic matter, preserving it so that the slower processes of coalification, driven by the increasing heat and pressure of deeper burial, can take over.

6. Decoding Coal Rank: A Scientific Classification System 📊📈

The concept of "coal rank" is a fundamental classification system in coal geology, reflecting the stage of transformation reached during coalification.

• Definition: Coal rank signifies the degree of metamorphosis or alteration the original plant material has undergone.
• The Coalification Continuum:
1. Peat Formation: Initial accumulation and partial decomposition of plant matter in anoxic, waterlogged environments.
2. Burial & Compaction: Peat is buried by sediments, leading to compaction and moisture loss.
3. Geochemical Alteration (driven by Heat & Pressure):
• Heat (Temperature): The most critical factor. Deeper burial generally means higher temperatures (Hilt's Rule: rank increases with depth).
• Pressure: Primarily aids compaction and fluid expulsion.
• Time: Sufficient duration at elevated temperatures is needed for reactions to proceed.
• Chemical Changes Defining Rank:

• Systematic loss of volatiles: H₂O, CO₂, CH₄.
• Decrease in oxygen and hydrogen content.
• Progressive enrichment of carbon.

• The Rank Sequence: A continuous process divided into recognized stages: Peat ➡️ Lignite ➡️ Subbituminous Coal ➡️ Bituminous Coal ➡️ Anthracite (Lowest Rank ➡️ Highest Rank)
• Significance of Rank Classification:

• Indicates coal quality and maturity.
• Determines suitability for different uses (e.g., lignite for power generation, specific bituminous coals for coking, anthracite for specialized heating).
• Based on measurable physical and chemical properties (as detailed in Table 4). Vitrinite reflectance is a key objective measure, especially for higher-rank coals.

7. Conclusion: The Enduring Legacy of Ancient Flora 🕰️💡

The transformation of ancient plant matter into coal is a remarkable geological epic, spanning millions of years and driven by the powerful forces of heat and pressure deep within the Earth. From the initial preservation in anoxic swamps to the complex chemical alchemy that expels volatiles and concentrates carbon, each step is crucial in creating this valuable energy resource. Understanding the science of coalification and the classification by rank allows us to appreciate the diverse nature of coal deposits and utilize them effectively.

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8. Bibliography 📚

Alpern, B. (1979). Essai de classification des combustibles fossiles solides [Essay on the classification of solid fossil fuels]. Publications Techniques Charbonnages de France, (3), 195–210. (CERCHAR Publication No. 2810).

Carpenter, A. M. (1988). Coal Classification. IEA Coal Research (IEACR 12).

Chen, P. (1998). Study on Classification System for Chinese Coal. Journal of Coal Science and Engineering, 4(2), 78–84.

Eizenhut, W. (1981). High-temperature carbonization. In M. A. Elliot (Ed.), Chemistry of Coal Utilization, second supplementary volume (pp. 847–917). John Wiley and Sons.

EOLSS. (n.d.). Coal, formation, and an overview of its utilization. Retrieved from http://www.eolss.net/Eolss-sampleAllChapter.aspx [Note: The specific chapter title "Coal, Formation, and an Overview of Its Utilization" is inferred as a likely topic from the EOLSS (Encyclopedia of Life Support Systems) structure. The direct link provided is for sample chapters; a more specific citation would be ideal if available from the original research context.]

Ku, X.-X. (1995). Technology of Byproducts. Metallurgical Industry Press. [Note: Further details like place of publication if different from publisher name, or if it's a specific chapter, would enhance this citation. Assumed Beijing based on other similar entries if not specified.]

Lemos De Sousa, M. J., & Pinheiro, H. J. (1994). Coal classification. Journal of Coal Quality, 13(2), 52-60. [Note: Adjusted volume/issue based on typical journal formatting, original entry had "April-June" which might be issue number or date span.]

Van Krevelen, D. W. (1993). Coal: Typology-Physics-Chemistry-Constitution (3rd ed.). Elsevier. (Specifically pp. 46–64 for classification aspects).

Wang, Z.-T. (1996). Coking of coal. Chemical Encyclopedia of China, 11, 373–389. Chemical Industry Press.

Yao, Z.-Z. (1995). Coke-making Science (2nd ed.). Metallurgical Industry Press.