Differences between coal combustion in an oxygen bomb and combustion in the atmosphere

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When coal burns in an oxygen bomb, the atmosphere inside the oxygen bomb is high-pressure pure oxygen. Under these special conditions, the combustion reaction of coal differs significantly from that under normal atmospheric conditions. (1) Under high-pressure oxygen conditions, nitrogen (including the nitrogen present in the small amount of air in the oxygen bomb) is partially converted into higher-valent nitrogen oxides; these higher-valent nitrogen oxides react with water to form nitric acid. This process is exothermic. When coal burns in the atmosphere, it does not produce high-valent nitrogen oxides, nor does it generate nitric acid in boilers that would release heat. Obviously, coal releases more heat when burning in an oxygen bomb. (2) When the combustible sulfur in coal (organic sulfur and pyrite sulfur) burns in an oxygen bomb, due to the presence of high-pressure oxygen, the generated SO2 is converted into SO3, which then reacts with water to form H2SO4. H2SO4 dissolves in water to create dilute sulfuric acid. This process is also exothermic. When coal burns in the atmosphere, the vast majority of its combustible sulfur is released in the form of SO2. Obviously, due to the presence of sulfur in coal, the heat released when coal burns in an oxygen bomb is greater than the heat released when it burns in the atmosphere. (3) The adsorbed water in coal, as well as the water generated by the combustion of hydrogen in coal, exist as liquid water in an oxygen bomb, whereas when coal burns in the atmosphere, water is released into the atmosphere in the form of vapor. Water that changes from steam to a liquid state releases a large amount of heat. It can be seen that, due to the different forms in which water exists, the heat released when coal burns in an oxygen bomb is greater than that released when it burns in the atmosphere. (4) The combustion of coal in an oxygen bomb is a constant-volume combustion, while its combustion in the atmosphere is a constant-pressure combustion. When burning under constant pressure, the increase in gas volume requires work to be done on the surroundings, thereby reducing the amount of heat released. When burning in an oxygen bomb, there is no issue of doing work on the environment, so more heat is released. The heat released when coal burns under constant pressure and constant volume conditions differs little, so no correction is generally applied, and the results measured under constant volume conditions are used directly.