Coal Composition and Properties

Overview

Coal composition determines combustion efficiency, emissions characteristics, and heating value in boiler applications. Understanding both proximate and ultimate analysis methods enables proper fuel selection and system design for coal-fired HVAC equipment.

Proximate Analysis

Proximate analysis provides practical fuel characteristics through standardized laboratory tests per ASTM D3172. This method quantifies four fundamental parameters:

Moisture Content (M): Water present in coal, reported on as-received or dry basis. Excessive moisture reduces heating value and increases handling costs.

$$Q_{\text{net,ar}} = Q_{\text{net,dry}} \times \left(1 - \frac{M}{100}\right)$$

Volatile Matter (VM): Components released as gas when coal is heated without air. Higher volatiles indicate easier ignition but potential for incomplete combustion.

Fixed Carbon (FC): Solid combustible residue remaining after volatile matter release. Calculated by difference:

$$\text{FC} = 100 - (M + VM + A)$$

Ash Content (A): Noncombustible mineral residue remaining after complete combustion. High ash reduces heating value and increases disposal requirements.

The mass balance relationship for proximate analysis:

$$M + VM + FC + A = 100%$$

Ultimate Analysis

Ultimate analysis determines elemental composition per ASTM D3176, critical for combustion calculations and emissions prediction:

Element	Typical Range (wt%, dry basis)	Combustion Significance
Carbon (C)	65-95%	Primary heat source
Hydrogen (H)	2-6%	Secondary heat, water formation
Oxygen (O)	2-25%	Reduces heating value
Nitrogen (N)	0.5-2%	Forms NOx emissions
Sulfur (S)	0.3-8%	Forms SO₂, corrosion risk

The stoichiometric air requirement calculation uses ultimate analysis data:

$$A_0 = 11.5C + 34.5\left(H - \frac{O}{8}\right) + 4.3S \quad \text{(kg air/kg fuel)}$$

Composition by Coal Rank

Coal rank significantly affects composition and combustion properties:

Coal Rank	Fixed Carbon (%)	Volatile Matter (%)	Moisture (%)	Heating Value (MJ/kg)
Anthracite	86-92	3-8	2-4	30-34
Bituminous	45-86	20-40	2-10	24-32
Sub-bituminous	35-50	40-50	15-30	18-26
Lignite	25-35	40-50	30-45	10-20

Sulfur Content and Classification

Sulfur exists in three forms in coal:

Pyritic Sulfur: Iron sulfide minerals (FeS₂), partially removable by physical cleaning
Organic Sulfur: Chemically bonded to coal structure, not removable by cleaning
Sulfate Sulfur: Negligible in fresh coal, forms during weathering

Classification by total sulfur content:

Low Sulfur: < 1.0%
Medium Sulfur: 1.0-3.0%
High Sulfur: > 3.0%

Sulfur dioxide emissions calculation:

$$\dot{m}{\text{SO}2} = \dot{m}{\text{coal}} \times \frac{S}{100} \times \frac{64}{32} \times (1 - \eta{\text{FGD}})$$

where η_FGD represents flue gas desulfurization efficiency.

Analysis Methods Overview

graph TD
    A[Coal Sample] --> B[Sample Preparation]
    B --> C{Analysis Type}
    C -->|Proximate| D[Moisture Test<br/>ASTM D3173]
    C -->|Proximate| E[Volatile Matter<br/>ASTM D3175]
    C -->|Proximate| F[Ash Content<br/>ASTM D3174]
    C -->|Proximate| G[Fixed Carbon<br/>By Difference]
    C -->|Ultimate| H[CHNS Analyzer<br/>ASTM D5373]
    C -->|Ultimate| I[Oxygen<br/>By Difference]
    C -->|Sulfur Forms| J[Sulfur Analysis<br/>ASTM D2492]
    D --> K[Composition Report]
    E --> K
    F --> K
    G --> K
    H --> K
    I --> K
    J --> K
    K --> L[Heating Value<br/>ASTM D5865]
    L --> M[Combustion Design<br/>Parameters]

Ash Properties

Ash composition affects fouling, slagging, and corrosion in boiler systems:

Ash Fusion Temperature: Temperature at which ash begins to soften and flow, critical for preventing clinker formation. ASTM D1857 defines four characteristic temperatures:

Initial deformation temperature (IDT)
Softening temperature (ST)
Hemispherical temperature (HT)
Fluid temperature (FT)

Silica Ratio: Predicts slagging tendency:

$$R_s = \frac{\text{SiO}_2}{\text{SiO}_2 + \text{Fe}_2\text{O}_3 + \text{CaO} + \text{MgO}}$$

Values Rs < 0.6 indicate high slagging potential.

Trace Elements

Coal contains trace elements with environmental and operational significance:

Mercury (Hg): 0.01-1.5 ppm, requires emission controls per MACT standards
Arsenic (As): 0.5-80 ppm, toxic emissions concern
Chlorine (Cl): 50-5000 ppm, causes high-temperature corrosion
Selenium (Se): 0.5-10 ppm, environmental regulation target

HVAC Application Considerations

Coal composition directly impacts boiler design and operation:

High Fixed Carbon Coals (Anthracite): Require higher combustion air temperatures and longer residence times. Lower fouling rates but slower ignition.

High Volatile Coals (Sub-bituminous, Lignite): Easier ignition, shorter flame length, higher reactivity. Increased moisture necessitates larger pulverizers and higher auxiliary power.

High Sulfur Coals: Mandate acid dewpoint consideration for air preheater design and flue gas desulfurization systems.

High Ash Coals: Require larger ash handling systems, increased tube spacing, and more frequent cleaning cycles.

The heating value on an as-received basis incorporates all composition effects:

$$\text{HHV}{\text{ar}} = \text{HHV}{\text{daf}} \times \left(1 - \frac{M + A}{100}\right)$$

where HHV_daf represents higher heating value on a dry, ash-free basis.

Standards Reference

ASTM D3172: Standard Practice for Proximate Analysis of Coal and Coke
ASTM D3176: Standard Practice for Ultimate Analysis of Coal and Coke
ASTM D3174: Standard Test Method for Ash in the Analysis Sample of Coal
ASTM D3175: Standard Test Method for Volatile Matter in the Analysis Sample of Coal
ASTM D2492: Standard Test Method for Forms of Sulfur in Coal
ASTM D5865: Standard Test Method for Gross Calorific Value of Coal and Coke
ASTM D1857: Standard Test Method for Fusibility of Coal and Coke Ash

Understanding coal composition through standardized analysis methods enables accurate boiler sizing, fuel procurement decisions, and emissions compliance for coal-fired HVAC systems.