Study on Gas-side Fouling of Oil-fired Boilers by Means of Controlled Temperature Tubes in a Test Furnace

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Chemical analysis indicates that Na, V, and S are principal elements of gas-side deposits of oil-fired boilers. There has, however, been no generally accepted view as to the molecular composition of substances which initially precipitate on the outermost surface of the deposit on the tube. After reaching the outermost surface, the substance or the substances react chemically with already existing deposits and/or with the tube metal.<BR>This experimental study was carried out by forming fouling compounds on test tubes with predetermined surface temperatures ranging from 75 to 680°C. The test tubes were arranged in a duct which accept combustion gas from a test furnace burning city gas. Na, V and S were selected as the fouling elements. The gas temperature at the test tube was from 750 to 960°C. Each fouling material containing Na, V or S separately was supplied to the. furnace and the formed deposits were investigated. Then fouling materials containing any two of Na, V and S were supplied to the furnace, and finally all three fouling elements, i.e., Na, V and S were supplied to the furnace.<BR>It was found that the gas-side fouling can be classified into low, intermediate and high temperature fouling. In the low temperature fouling, only sulphur trioxide precipitates at the outermost surface. In the intermediate temperature fouling, sodium sulphate and vanadium pentoxide precipitate from the gas, and these two compounds absorb sulphur trioxide from the gas after deposition to form sodium pyrosulphate on the tube surface. High temperature fouling is characterized by the fact that the precipitating materials are sodium sulphate and vanadium pentoxide and that sodium vanadyl vanadates are formed on the tube surface.<BR>The quantity of fouling deposits is dependent also on the gas temperature because the gas-film directly in contact with the outermost deposit surface is affected by the bulk gas temperature. When the tube metal temperature is raised gradually, the deposits quantity sharply decreases when the low temperature fouling is over. When the metal temperature goes above 250°C, the quantity gradually increases, reaches a maximum and decreases because the gas-film temperature goes beyond the dew points of vanadium pentoxide and sodium sulphate vapours.
公益社団法人日本マリンエンジニアリング学会の論文

Study on Gas-side Fouling of Oil-fired Boilers by Means of Controlled Temperature Tubes in a Test Furnace

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