Generally, coal burns slowly but the fine coal dust in mines can burn so rapidly it will cause an explosion.?

Question

Explain why this could be.

Answer 1

It's all about the fuel-to-oxygen mixture. Fire needs 3 things: fuel, oxygen and spark. By compacting coal into briquettes, you allow fewer air particles in between the particles of raw fuel, so the fire is kind of "smothered" by its own fuel and burns slower, longer. But if you have the exact same fuel separated by generous amounts of air, that makes optimum conditions for a fire, and in that case a chain-reaction explosion.
Alot of people don't realize that by the time you smell LP gas, for example, there's usually too much of it in the room to create such an explosion. Same principle.

Answer 2

How rapidly something burns is proportional (not necessarily linearly) to the ratio of surface area to mass, provided you can get the right mixture of air to support the reaction. More surface area means the combustible mass has easier access to oxygen molecules, a required component of combustion. So the higher the ratio of surface area to mass, the faster the substance will burn.

For example, if you have a block of wood that's an inch on a side with a mass of 1 oz, your ratio of surface area to mass is 6*(1x1) in^^2/oz. If you cut the block in half in each of the three dimensions, the ratio for all those smaller blocks added together becomes 8*6*(1/2*1/2)in^^2/oz., or 12in^^2/oz - the ratio doubled. Keep doing this until you're down to dust size and you can see how the potential burn rate increases. And as Mark said, looking at the ratio from a mass standpoint, a lower mass for the same surface area makes the particle easier to heat to the point of ignition.

Why do we use small sticks for kindling to start a fire? Same reason - higher surface area per unit mass than big branches or logs.

Ever hear of a grain elevator (where they store harvested grain) blowing up? Sometimes the grain dust inside those things gets to the right proportion with air, then a motor somewhere will spark, and BOOM! Dust and spark mitigation are serious considerations for any similar type of work environment.

I used to work for a chemical company that produced 1/3 of the world's supply of aspirin. We had a safety demo once where they injected air into a small bin of aspirin dust, then generated a spark - it made a fireball about 6 feet in diameter, and you could feel the intense heat about 60 feet away. And aspirin probably isn't the first thing I'd pick to build a fire from...

Answer 3

The same process of burning ocurrs with a lump of coal vs a dust speck of coal.

The small particle size of coal dust greatly increases the surface area of the coal. This makes it easier to mix the fuel(coal) with oxygen.
At the same time a small dust speck has a low mass which makes it easier to rasie it temperature with a given amount of thermal energy. Therfore a small amount of thermal energy would be needed to burn a small dust speck. As that dust speck burns its close proximity to adjacent dust specks allows the thermal energy to raise the temperature of the adjacent small mass specks initiating there combustion. If the correct balance of air to fuel is obtained then this is a self sustaining reaction and can resut in a rapid combustion wave (explosion) propigating from the ignition source through the suspended dust.

To explode there has to be a correct mixt of fuel to air. Too much fuel (rich mixture - Upper explosive limit) or not enough fuel(lean - Lower Explosive Limit) and an explosion is not possible. Between the UEL and LEL is the explosive range.

As a safe guard there are meters which measure the explosive nature of an atmosphere and provide a read out in percent of LEL. This meas that as fuel is added to the air the LEL rises from 0% until the mixture become explosive at 100% LEL and enters the explosive range. OSHA regulations consider anything above 10% LEL as a potentially hazardous atmosphere which requires venting.

Answer 4

Probably has something to do with the size of the coal dust as well as the oxygen mixture in the mine. There are gasses inside a mine that cannot dissipate as rapidly as on the surface.

Answer 5

Because coal dust is a mixture of coal particles and air (which contains lots of oxygen). Fine particles of just about anything, mixed with lots of oxygen, will burn very rapidly. The classic experiment/demonstration involves custard powder!

Answer 6

high-quality airborne dirt and dust is plenty greater probable to explode because of the fact it has greater floor area advert reactions take place on the exterior. present day coal fired flowers grind there coal and upload extra oxygen to burn the coal greater thoroughly, get greater warmth and reason much less pollutants.

Answer 7

iif you put the dust on a fire it will burn real slow,but floating around in the mine mixed wth a lot oxygen and gas it has a high flash point

Answer 8

no its usually pockets of gas that cause the explosion when mining thats why they have to have gas levels checked all the time

Answer 9

smaller particles, less dense, but high concentration.
fueled with oxygen, and you got yourself a nice explosion.