Chemistry: Fire
Saturday, May 13, 2006
Fuel and Oxygen
Most combustible fuels begin as solids, such as wood, wax, and plastic. Many fuels that people burn for energy, including gasoline and methane (natural gas), begin as either a liquid or a gas. Any fuel must be in a gaseous state (so that it can react with oxygen) before a fire can occur. Heat from the fire’s ignition source, and later from the fire itself, decomposes solid and liquid fuels, releasing flammable gases called volatiles. Some solids, such as the wax in a candle, melt into a liquid first. The liquid then evaporates, giving off volatiles that may then burn. Other solids, such as wood and cotton, decompose and evaporate directly. In a wood fire, gases given off by the decomposing wood enter the flame, combine with oxygen from the surrounding air, and ignite. The heat from the flame decomposes more wood, thus adding more flammable gases to the flame and creating a self-supporting process.
Most common fuels consist of compounds containing the elements carbon and hydrogen. Fuels often also contain oxygen, nitrogen, chlorine, and sulfur. Cellulose is the principle combustible compound in wood, paper, and cotton. It contains carbon, hydrogen, and oxygen. Plastics that burn, such as polyvinylchloride (PVC), polystyrene, polymethyl methacrylate (PMMA), nylon, and polyurethane, are composed mostly of carbon and hydrogen. Liquid fuels include oil and gasoline, while gaseous fuels include methane, propane, and hydrogen. All of these fuels (except pure hydrogen) contain both carbon and hydrogen.
The final requirement for a fire is a chemical chain reaction. The heat of the ignition source starts the reaction, and heat from the fire’s flame continues the reaction. The flame needs to heat the fuel and make it release enough flammable gases to continuously support the chemical reaction. A common example of combustion is the burning of wood. When an ignition source heats wood to a sufficient temperature, about 260°C (500°F), the cellulose in the wood decomposes, producing volatile gases and char. The average composition of the gases can be represented by the compound CH2O, where C stands for carbon, H stands for hydrogen, and O stands for oxygen. Under ideal conditions, CH2O reacts with oxygen in the air and produces carbon dioxide (CO2) and water vapor (H2O). In the real world conditions are not ideal, so fires often produce other products as well, such as carbon monoxide (CO) and soot.
Products of fire
1. Light and Heat
Once a material ignites, a flame forms. The flame consists of volatile gases moving upward, and it is the region in which the combustion reaction occurs. The gases in the flame move upward because they are hotter—and therefore lighter—than the surrounding air. The colors in the flame come from unburned carbon particles that glow and travel upward as the flame heats them.
The flame continues to burn as the volatile gases streaming from the fuel combine with oxygen from the surrounding air. Different parts of the flame have different temperatures. Most common fuels are compounds called hydrocarbons, and they produce about the same flame temperature, roughly 1200°C (2200°F). The maximum theoretical flame temperature for most hydrocarbons is about 1300°C (2400°F).
Different fuels produce varying amounts of heat. The rate at which a fire generates heat is equal to the burning rate of the fuel (measured in grams per second, or g/s) multiplied by the amount of heat produced by the combustion reaction. This second factor is called the effective heat of combustion, and scientists measure it in units of kilojoules per gram (kJ/g). When a gram of wood burns, for example, it produces 8 kJ of heat energy. Wood’s effective heat of combustion is therefore 8 kJ/g. Polyurethane’s effective heat of combustion is about 18 kJ/g. Polyurethane’s burning rate is also about twice that of wood under similar conditions. Multiplying the burning rates for these two substances by their effective heats of combustion, one finds that polyurethane fires produce heat at about 4.5 times the rate of wood fires under similar conditions.
Gases
Fires can produce a number of different gases, including some that are harmless and some that are toxic. Carbon dioxide (CO2) and water vapor (H2O) are two relatively harmless gases produced by fires. Toxic gases from fires include carbon monoxide (CO), hydrogen cyanide (HCN), sulfur dioxide (SO2), and hydrogen chloride (HCl).
from: http://encarta.msn.com/encyclopedia_761563809_3/Fire.html
posted by Sin Hong @ 6:46 PM,
