Problems in the use of heavy oil
Fuel for boilers contains sulfur content more or less. When the fuel is burned, the sulfur oxidizes in the furnace, generating mainly sulfur dioxide gas (SO2), part of which is further oxidized into sulfur trioxide gas (SO3). Recently, high-sulfur fuel such as residual oil is often used, increasing SO3 generation, which causes the following problems.
Corrosion of equipment such as precipitators and ducts installed as the flue gas treatment system
- SO3 gas reacts with moisture in the flue gas at a temperature range around 200 degrees, generating sulfuric acid (H2SO4), which produces sulfuric acid mist (SO3 mist) once cooled in the flue gas treatment system.
- SO3 mist is condenced sulfuric acid, which is "dusted" with a great deal of fly ash when coal is used as fuel. Thus, corrosion is rarely a problem, except with some kinds of coal like US coal, which has very high sulfur content. However, fuels such as oil containing little ash often cause corrosion because there is only a small amount of "ash" to be dusted.
- Fuels containing extremely high sulfur content, such as heavy oil (including residual oil, asphalt, and petroleum coke) can generate a great deal of SO3, possibly causing severe problems of corrosion by SO3 in particular.
Visible plume problems caused by effluent of "bluish plume"
- Since SO3 mist is made up of extremely fine particles condensing in the gaseous phase, even if a flue gas desulfurization system is installed, most of particles are not collected, as they slip through and are discharged as "bluish plume" (Note) from the stack.
- This "bluish plume" has a small particle size and hardly disperses after discharge from the stack into the air, thus lingering on conspicuously. Recently, it has often recognized as visual pollution and measures to reduce visual plume have been required.
* What is bluish plume?
Smoke from a lit cigarette left on an ash tray looks bluish depending on the lighting condition. The particle size of such tobacco smoke is on the submicron (0.1 micrometer - 1 micrometer) order and looks bluish under the influence of scattering light. Because of this, tobacco smoke is often called bluish smoke. SO3 mist, which is contained in the flue gas discharged from boiler stacks, has also submicron size particle similar to tobacco smoke from an ash tray, looks also bluish because of scattering light, and is often called "blue plume" just like tobacco smoke.