Fire and explosion hazards of chemicals

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In recent years, among all the accidents that have occurred in China’s chemical industry, those caused by fires and explosions have resulted in the highest number of deaths, and they have also led to considerable direct economic losses. For example, in 1997, a severe fire and explosion occurred at the oil storage tank area of Beijing Dongfang Chemical Plant; within a short period of time, the entire tank area was engulfed in flames, resulting in 9 deaths, 37 injuries, and direct economic losses exceeding 100 million yuan. In 1993, a massive fire and explosion occurred at the Qingshuihe chemical hazardous materials warehouse in Shenzhen, resulting in 15 deaths and over 200 injuries, including 25 severe cases. The direct economic loss exceeded 250 million yuan. These accidents were all caused by the inherent fire and explosion hazards of the chemicals themselves. Therefore, understanding the fire and explosion hazards of chemicals, conducting proper hazard assessments, and taking timely preventive measures are of great significance for ensuring safe production and preventing accidents. 1. Combustion and explosion hazards of chemicals: The combustion and explosion hazards of chemicals are evaluated using different methods depending on their state. 1.1 Combustion and explosion hazards of combustible gases, vapors of combustible liquids, and combustible dusts (1) Explosive limit: Mixtures of combustible gases, vapors of combustible liquids, or combustible dusts with air do not explode at any mixture ratio; rather, an explosive reaction occurs only within a specific concentration range. Different combustible substances have their own specific concentration ranges. This fixed range is commonly referred to as the explosive range or explosion limit of that substance, and it is usually expressed as the volume percentage of combustible gases, vapors of flammable liquids, or combustible dusts in air. The lowest concentration at which an explosion can occur is called the lower explosive limit, while the highest concentration is known as the upper explosive limit. For example: the explosion range of ethanol is 4.3%–19.0%. 4.3% is referred to as the lower explosion limit, while 19.0% is referred to as the upper explosion limit. The explosive limit of gasoline is 1.0%–6.0% ； The explosive limit of natural gas is 4.8%–13.46% ； The explosive limit of hydrogen is 4.0%–75% ； The limit for carbon monoxide is 12.5%–74.2% ； The explosive limit of ammonia is 15.5%–27%, and so on. The wider the range of the explosion limit, the lower the lower explosion limit, and the greater the explosion hazard. 　　The explosion limit is determined under standard conditions such as normal temperature and pressure, and this range changes with variations in temperature and pressure. (2) Minimum ignition energy The minimum ignition energy refers to the smallest amount of energy required to cause a combustible mixture to burn and explode. For example, the minimum ignition energy for hydrogen is 0.019 mJ, for methane it is 0.25 mJ, for ethane it is also 0.25 mJ, for ethylene oxide it is 0.065 mJ, and for ethylene it is 0.096 mJ. The lower the minimum ignition energy value, the easier it is for that substance to be ignited. (3) Explosion pressure: The pressure generated when a mixture of combustible gases, vapors of combustible liquids, or combustible dusts with air, along with **materials, catches fire and explodes in a closed container is called explosion pressure. The maximum value of the explosion pressure is called the maximum explosion pressure. Explosion pressure is usually measured, but it can also be calculated based on the combustion reaction equation or the internal energy of the gas. Different substances have different explosion pressures; even for the same substance, its explosion pressure varies depending on factors such as the surrounding environment, initial pressure, and temperature. 　　The higher the maximum explosion pressure, the shorter the duration of that pressure, and the faster its rise rate, the greater the explosive power, indicating that the mixture or chemical is more dangerous. 1.2 The fire and explosion hazards of flammable or combustible liquids (1) Flash ignition and flash point: When a liquid burns, it first evaporates into vapor under the influence of an ignition source, and then the vapor undergoes oxidative decomposition and catches fire. On the surface of every liquid, there is a certain amount of vapor present. As the temperature of the liquid rises, the concentration of vapor increases as well. When this vapor concentration exceeds the lower limit of its explosive range, it can catch fire upon contact with a flame. At a certain temperature, a mixture of the saturated vapor of a flammable liquid and air can produce sparks when in contact with a flame, resulting in instantaneous combustion; this phenomenon is known as flash ignition. The temperature at which flash ignition occurs is called the flash point. When the temperature of a flammable liquid is above its flash point, it is at risk of being ignited by flames at any time. The lower the flash point, the more likely the chemical is to cause combustion and explosion. (2) Ignition point: The lowest temperature at which a combustible substance, when exposed to sufficient air and in contact with a flame, catches fire (producing a flame or glowing heat), and can continue to burn even after the flame is removed, is known as the ignition point or burning point of that substance. (3) Autoignition point: The lowest temperature at which a combustible material can catch fire when heated in air or oxygen, in the absence of a flame, electric spark, or other source of ignition, is known as the autoignition point (or ignition temperature). 　　There are two types of spontaneous combustion: Spontaneous combustion due to heat: Combustible materials experience an increase in temperature as a result of an external heat source, reaching their ignition point and thus burning on their own. 　　Spontaneous heating and combustion: A phenomenon in which a combustible material generates heat through physical, chemical, or biochemical processes occurring within it, without any external heat source; over time, this accumulated heat reaches the material’s autoignition point, causing it to burn on its own. 　　The reasons for the natural heating of substances include decomposition heat (such as celluloid), oxidation heat (such as unsaturated fats), adsorption heat (such as activated carbon), polymerization heat (such as liquid hydrogen cyanide), fermentation heat (such as hay), etc. Spontaneous heating and combustion are common phenomena in the storage and transportation of chemical products, and they pose a great threat.