Chemistry of Fire
In this essay I will be talking about different areas and types of a fire, including how they are started, how they spread, how they can be prevented and the chemistry behind a fire. Pyrolysis is decomposition brought about through high temperatures, thermally decomposed to a point where gas is released which causes a fire to be produced.
Fires can spread in three different ways, the heat energy is transferred by the use of conduction, radiation and convection. Radiation is the emission of energy in rays or waves. When heat is traveling through the atmosphere it moves as energy waves. Radiation is a common type of heat that is easily recognisable, it’s how we feel heat when you run your hand over a candle. Radiation waves always travel in straight lines, because of this and in this example it cannot conduct through human flesh without causing damage. If you place your hand over a lit candle you’d only be able to feel the heat on your palms and not the back of your hands. Also the closer you get to a fire the more radiation waves you’ll receive will increase. Secondly, conduction is defined as the transmission of heat through solids. In solids the particles are much closer together and in sync compared to the particle arrangement of liquids and gases, the result is that solids will conduct heat better than liquids and gases. When temperature increases it causes the particles to start vibrating. Because the solid particles are packed tight to each other the greater the amount of collisions that occur between the particles the more amount of thermal energy that is being transmitted increases. The solids have the greatest collision rate out of liquids, gases and solids. Heat is transmitted through solids which mean that when there is enough heat present it will cause damage if there is fuel near by. Conduction allows for fuels to be ignited through other materials and objects of the heat is great enough. It’s obvious that materials that are more combustable than others will fall victim to conduction. Conduction will occur when two different objects at different temperatures are in contact with each other, heat will flow from the hotter object to the cooler object until they are both the same temperature. When a fire produces enough heat energy it means that objects which are in a close enough radius can potentially catch alight. An example of conduction is the Grenfell tower tragedy. As the fire in the flat where it first started caught a light it grew started to spread. After destroying the flat it then ignited the cladding. It had to use conduction to actually get to the cladding and set it alight. Once the heat had been successfully transferred to the cladding via conduction the fire then spread to the other flats. The heat was able to conduct itself through doors and spread to curtains, beds and many more flammable materials. Finally the last process is convection, which is the transmission of heat within a gas or liquid. Convection heats up molecules which cause them to move around erratically. The process of convection is a cycle which occurs when a hotter or warmer area of gas or liquid starts to move and rise above the much cooler gas or liquid. The convection cycle is never ending as along as there’s a constant source of heat. The hotter area of liquid and gas will rise and the cooler temperature will fill in the area where the hotter matter used to be, because of this one temperature will always be replacing one another so one will always be heated. When one the hotter liquid or gas is being heated the oxygen is being used which allows for the chemical chain reaction to take place which is the rising gas from the heat transfer. This cycle is a circular motion. In the cycle as the hotter part rises its trying to get to the cooler area to reduce its temperature.
A fire will always have different phases. All fires change between the different phases during their life cycle. The life cycle of a fire and the events go in order of first ignition. Ignition occurs when oxygen a fuel source and heat all react together to create the fire ignition can only happen when all elements in the fire tetrahedron are involved. Then the fire grows until it reaches the fully developed phase. By the fully developed point the fire has reached its plateau point and it will continue to burn at a steady state for a very short time. After it has been burning for enough time it will burn itself out because there will then be a massive shortage of oxygen. All elements of the fire tetrahedron must be sustained for the fire to keep growing. By this point the fire has hit the decay phase. The first type of fire I will be talking about is the glowing and smouldering part. The glowing a smouldering part of the fire is normally characterised by a lower temperature and no naked flame will be both visible and present. This type of the fire mostly happens in all flaming fires at one point as the life cycle has reaches the pinnacle point and is entering the decay phase, but glowing fires are defiantly the most common fire. An example of where they can be found is in cigarettes. Cigarettes are an example of being started through primary ignition but the rest of the life cycle is glowing because for them to be lit to smoke they require exposure to a naked flame, but once they have been lit there isn’t a naked flame and the temperature of the fire is lower than a flaming fire would be. Secondly another type of fire is the self heating part. Self heating can occur when a material of any kind which is flammable increases in temperate without being exposed to a naked flame otherwise known as secondary ignition. The material will self ignite when it has been heated to the point of ignition. But it can only combust into flames when every element of the fire tetrahedron is present. This means that there will need to be more than 16% oxygen present at all times minimum for the fire to ignite and be sustained. Its not this simple though, because of many different factors for self heating to be successful and reach the ignition point it is dependant of the environments and the surroundings to ignite. Because it’s not primary ignition where a naked flame will be exposed it means that the rate in which heating takes place must be more sufficient than how fast heat is being disposed of. When referring to fuel controlled combustion and ventilation controlled combustion it’s the same idea as using the fire tetrahedron. With fuel controlled ventilation when you add more fuel to the fire it has the ability to grow. As it’s controlled it suggests that depending on what type of fuel wether it be a liquid eg. Methanol or a gas eg. Ethane then depending on how much you feed the fire it will react with a growth if all of the required elements of the fire tetrahedron are present or the fire will start to extinguish itself if you start to starve it by decreasing how much fuel you expose it to. The same principle applies with ventilation controlled combustion however instead of being focused on fuel oxygen is the focus. The fire will become extinct if starved of oxygen or oxygen levels drop below 16%. But the fire will rapidly grow if an excess amount of oxygen is available to the fire.
The two contrasting ignition sources I will be comparing are a lighter which is a primary ignition source and an electric heater which is a secondary ignition source. Firstly, the lighter is a clear example of a pure naked flame fire, it’s easily controllable and won’t set anything a light without someone using it in that way, whereas the electric heater could very easily set something a light without the use and intent by humans. An electric heater fire could be ignited by a fuse blowing or some liquid getting into dodgy wiring. From both fires you could suffer burns, the three types of burns from an electrical fire you could encounter are electrical, ARC and thermal contact burns. From a normal fire depending upon how long you’re in contact with it the most likely burns would be first, second and third degree burn. The maximum flame temperature that is burning wood can reach 1029 degrees celsius. I decided to use wood as my example because in most house/ flat fires wood is a predominant material found which is flammable, it is found in doors, sofas etc, and because of this the fire is more like going to spread. Unless the lighter used was a jet then the flame would be a calm yellow flame. If it was a jet then it would be a hotter blue flame. From an electrical fire the flame would be a yellow orange flame indicating that the temperature would be around 1300-1400 degrees celsius at the peak of the flames life cycle. The damaging effects of these two ignition sources can be terrible. A lighter has the ability to completely destroy everything around it if used to ignite something which is flammable. As it’s a naked flame it has the ability to completely disintegrate household items like sofas or curtains. Arguably the lighter has one of the most damaging effects for various reasons. Another reason for this is because it can be used to set something alight and then you can very easily stop using the lighter and put it back in your pocket whereas the electric heater will be much more difficult to put out. The electric fire has the ability to fuse sockets which could give people electric shocks post fire. It could also damage other electrical equipment that’s plugged into the same circuit.
The fires above are two different types and must be extinguished in different ways. First off the fire which was produced using a lighter as the primary ignition source can be extinguished using either a water, carbon dioxide, foam or dry powder extinguisher. In this case all four different types of extinguisher that I named wouldn’t have to be used but could all extinguish a different type of fire that they’re individually designed for. The reason that I have selected all four of these is because my primary ignition source is a lighter which is a pure naked flame meaning it has the ability to set anything alight which is flammable. For example, if the fire is a simple house fire which is being sustained by being transferred to the sofas and carpet then an appropriate extinguishment method would be either a water, dry foam or carbon dioxide extinguisher. On the other hand if the fire was a flammable liquid fire started with the lighter then the appropriate response would be to use dry powder extinguisher or a carbon dioxide extinguisher. The different types of extinguishment methods will all take out the fires differently. The water extinguisher will cool the fire where as the foam will smother and cool and the dry powder will smother and starve the fire. The carbon dioxide will only starve the fire. The next type of fire which is different from the above because the secondary ignition source is an electrical heater immediately indicating that the fire is an electrical. Because of this it narrows down which extinguishment types are available. For this type of fire the two extinguishers that can be used are a dry powder and a carbon dioxide extinguisher. The dry powder extinguisher can extinguish fires which are combustible materials, flammable liquid fires, flammable gas fires, flammable metal fires, and electrical fires so this extinguisher would be the perfect method to use because it can put an end to the the surrounding environment fire. If it’s in an apartment fire and the wallpaper was alight then it could be put out with the dry powder but most importantly the dry powder can directly extinguish the source of the fire which is electrical by staving the fire. However the carbon dioxide method can only extinguish the electrical source and flammable liquids if there are any. So it’s effective but wouldn’t be the best type for the specific type of fire.
The fire tetrahedron is the fire triangle that includes all the elements needed to combust a fire and maintain it. The fire tetrahedron is made up of oxygen, heat, fuel and in the middle is the chemical chain reaction which in other words is combustion. If theres excess oxygen then it would be complete combustion if not then it would be incomplete combustion. A fire can only be sustained if all oxygen, heat and fuel remain. If one of these three becomes extinct then it’s impossible for the fire to be sustained as a combustion process. Incomplete combustion is a chemical chain reaction with only small amounts of burning fuel, this could possibly be caused from a lack of oxygen, in incomplete combustion carbon monoxide is formed. Complete combustion is the same reaction but with lots of oxygen and theres no carbon monoxide produced.
To conclude this essay I will sum up some of the best ways to prevent fires. The first way to prevent fires is to make sure you never store or briefly place any liquids near to electrical wires in the walls or around the house, this is because you may not be aware of any damages to these goods but it doesn’t mean they are not there, by not keeping liquids there it will decrease the chances of any spillages causing an electrical fire. Secondly is to make sure that after any fire has already occurred to properly extinguish the fire. Make sure that no smouldering bits are still burning because this could lead to a fire reigniting anywhere from minutes later to days. My final point is to make sure that every room around the house and all rooms in any building is safely and correctly fitted with working fire alarms. These alarms will be able to detect smoke. A good idea to keep the alarms accurate is to regularly check them, this includes changing the batteries and doing sound tests.
( I personally sourced my images used above from google images.)