Is Graphite Powder Flammable?

Einführung

Graphite is a layered carbon-based material, which is widely used in various fields due to its excellent properties. The flammability of its powder has been the subject of much discussion. Although it is traditionally considered non-flammable, research and cases have shown that it can catch fire under certain conditions.

Structural Characteristics of Graphite powder and the Foundation of Combustion Reactions

The crystal structure of graphite

The crystal structure of graphite is composed of a layered structure of hexagonal units, and the Kohlenstoffatome within each layer form covalent bonds through sp² hybridization. Its bond lengths are 0.142 nm and bond energies are 413 kJ/mol, so it can form stable six-membered ring planes. The layers are connected by van der Waals forces, with a layer spacing of 0.335 nm. And the force of action is relatively weak, approximately 20 kJ/mol, which endows graphite with unique physical and chemical properties. The high stability within the layers gives it extremely high heat resistance. The weak interlayer forces make it easily separable into thin sheets or powders.

The chemical stability of graphite

From the perspective of chemical thermodynamics, the reaction between graphite and oxygen produces carbon dioxide. (The chemical equation for this reaction is C + O2= CO2.) This has a Gibbs free energy change of -394.36 kJ/mol under standard conditions, which indicates thermodynamic spontaneity. However, the actual occurrence of this reaction requires overcoming the kinetic energy barrier. With the atomic ordered arrangement on the surface of graphite, the effective collision probability with oxygen molecules is low. At room temperature, the annual oxidation rate < 0.01%, so the oxidation rate can be negligible. So it has kinetic inertness, which is considered non-flammable in conventional environments.

Basic conditions of combustion reactions and the influence of powder characteristics

 Basic conditions of combustion reactions

Combustion will occur only when three conditions are met simultaneously: combustible material, oxidant (usually oxygen), and a temperature reaching the ignition point.

The influence of powder characteristics

Specific surface area

The specific surface area of graphite powders of different particle sizes varies, which significantly increases as the particle size decreases. The specific surface area of 100μm graphite powder is approximately 0.5 m²/g. The specific surface area of 1μm powder can reach 50 m²/g, and that of nanometer-sized powder even exceeds 100 m²/g. The large specific surface area increases the contact area with oxygen, reducing the activation energy of the reaction. Thus making the combustion easier to occur.

Dispersion state

When the powder forms a suspension in the air, it can fully mix with oxygen. When the concentration of graphite powder is 15-45 g/m³, it is within the “explosion limit” range. At this point, the heat generated by the local combustion will trigger a chain reaction through heat radiation, thereby causing an explosion.

Ignition point

The ignition point of block graphite is approximately 800°C. But when in powder form, as the specific surface area increases, the ignition point of the material significantly decreases. It is  shown that the ignition point of 50μm graphite powder is 750°C. While 20μm powder drops to 680°C, and 5μm powder can be as low as 600°C.

Analysis of Factors Affecting the Combustion of Graphite Powder

The critical role of particle size

The particle size is the key factor determining the flammability of graphite powder, which it can be discussed in three levels:

Micron level (1 – 100μm)

This is the grade of conventional industrial Graphitpulver. When dispersed, if the concentration reaches above 15g/m³, it can explode upon encountering an ignition source with energy ≥ 0.2mJ. But it is difficult to sustain a continuous combustion.

Sub-micron level (0.1 – 1μm)

The specific surface area of this level of powder significantly increases, and the oxidation reaction rate accelerates. For example, 0.5 μm graphite powder can sustain combustion at 700℃ in an oxygen-rich environment. Its combustion rate reach 0.8g/(cm²·s) and it releases approximately 32MJ/kg of heat.

Nanometer level (< 100 nm)

Nanometer graphite powder exhibits special combustion characteristics due to its extremely high surface energy. In dry air, the oxygen molecules adsorbed on the surface accumulate heat through slow oxidation. And when the temperature rises to 60℃, it can trigger self-ignition. Additionally, the explosion limit concentration of nanometer powder is lower, which is approximately 5 – 30g/m³, increasing the safety risk.

Synergistic effect of oxygen concentration and environmental temperature

The influence of oxygen concentration

In an air environment, which oxygen concentration is 21%, the combustion of graphite powder requires a higher temperature. But in an oxygen-rich environment, which oxygen concentration is greater than 30%, the activation energy of the reaction decreases. And the ignition point can drop by 100-200°C, thereby the combustion rate is three times higher than in the air environment.

The influence of environmental temperature

If the environmental temperature increases, the oxidation process will accelerate. For example, when the environmental temperature rises from 25℃ to 300℃, the oxidation rate of graphite powder increases by 10 times. This positive correlation significantly increases the combustion risk of graphite powder in high-temperature environments.

Energy threshold of the ignition source

The different ignition sources have different energies, which will affect the possibility of the graphite powder burning.

Static sparks

Its energy ranges from 0.2 to 1 mJ, which can ignite micron-sized graphite powder within the explosion limit.

Cigarette ash

Its surface temperature is 300-400℃, so it has insufficient energy and is unable to ignite conventional graphite powder.

Welding sparks

Its temperature > 1000℃, energy > 10 mJ, so it can directly ignite graphite powder below 50μm and trigger a chain reaction.

In addition, the duration of the ignition source is also crucial: short pulse ignition requires higher energy to initiate combustion. And continuous heat sources are more likely to cause the graphite powder to reach its ignition point.

Safety Management of Graphite Powder in Industrial Practice

Risk prevention and control in the production environment

Concentration Control

You need to install a central dust removal system and local ventilation devices to keep the concentration of graphite powder in the workshop below 10g/m³.

Ignition Source Elimination

This requires that open flames be strictly prohibited in the production area, and the equipment be grounded to eliminate static electricity. While you should select explosion-proof motors and lamps, and clear the surrounding dust before welding operations.

Monitoring and Early Warning

You need to install a dust concentration sensor and a temperature alarm device. So that they can automatically trigger an alarm while activating the ventilation and fire suppression systems.

Specifications for Storage and Transportation

You need to store the graphite powder in a well-ventilated and dry warehouse to prevent it from spontaneously igniting. When packaging, you need to use a sealed container to prevent the powder from suspending. Besides, keep at least 3 meters away from oxidants and heat sources. During transportation, you should avoid the extreme vibrations and equip a dry powder fire extinguisher to prevent dust from flying and causing a secondary explosion.

Schlussfolgerung

In conclusion, the combustion of graphite powder requires certain conditions, which requires sufficient activation energy and an appropriate supply of oxygen. So it is non-flammable under normal circumstances, but can be flammable or even self-igniting under extreme or specific conditions.