The ignition point of coal is a key indicator that helps to measure the propensity of coal to catch alight easily and soon. Specifically, coal ignition point is the lowest temperature at which a coal sample will sustain ignition when exposed to the surrounding environment. It is generally measured in degrees Celsius (°C). The measure of a coals ignition point enables one to identify the danger a fire carries in the coal mining industry, coal fuelled power plants and various other coal related industries.
Two measures of a coals ignition point exist; the one is the Initial Ignition Temperature (IIT), and the other is the Self-ignition Temperature (SIT). The IIT is the minimum temperature at which a coal sample can be held for a certain period of time and still ignite spontaneously when exposed to a flame or spark. The SIT is the minimum temperature at which a coal sample will self-ignite (ignite without any external spark) when heated in oxygen-free conditions.
The values of coal ignition points can be determined through experimentation with samples of coal which are tested in a laboratory. The coal samples are placed into a furnace and heated gradually to progressively higher temperatures. At intervals, the operator introduces a flame or spark into the furnace in order to observe whether the coal ignites. If the coal does ignite, the temperature of the furnace is recorded as the coals ignition point.
The specific coal definition for IIT and SIT changed in 2001 when the World of Coal Ash Conference (WOCA) published a revised ignition point for coal. This revised value is considered to be the most accurate figure available. WOCA defines the IIT as being the lowest temperature at which a given coal sample combusts within 10 seconds after exposure to a hot air-flame source. This temperature is measured according to ASTM procedure D project 4795. The SIT is defined as being the temperature at which the coal sample self-ignites and combusts in less than 10 minutes but greater than 200 seconds. It is measured using a specially constructed combustion vessel.
The two measures of coal ignition point can also be determined using a ‘cone calorimeter.’ This device uses a cone-shaped test specimen to measure the heat released and/or consumed by combustible material when exposed to a flame or spark. The cone calorimeter also measures the combustion rate of the test specimen at various temperatures, allowing for an accurate assessment of the ignition point of the material.
The ignition points of different coals vary and are dependent on the type of coal sample and its composition. Generally, the ignition point of coal with a high moisture content is very low, often below 200°C. For example, lignite and peat may be expected to ignite easily when heated to temperatures as low as 50°C. On the other hand, high-temperature coals (anthracite and bituminous coals) have relatively high ignition points ranging from 500 - 700°C.
In order to determine the exact ignition point of a given coal sample, it is important to carry out the tests in a laboratory using all of the recommended safety precautions. Without the correct safety measures in place, carrying out tests on a coal sample may be a dangerous activity which could lead to injury or death. Therefore, it is necessary that the tests be conducted in compliance with all safety regulations in order to protect all persons conducting the tests and those who may be exposed to the risks associated with testing coal.
