ABSTRACT
Boilers fired with high-calcium coals from the western United States often experience fouling of steam tubes at temperatures below those at which the ash is sticky. To determine the causes of low-temperature fouling, a multisponsor program entitled "Project Calcium: Calcium-Based Ash Deposition in Utility Boilers" was initiated at the University of North Dakota Energy & Environmental Research Center (EERC). During the course of the project, detailed sampling of five utility boilers and laboratory-scale studies coupled with advanced inorganic analysis methods were conducted. The testing determined that five types of ash fouling deposits occur in the convective passes of boilers burning high-calcium, low-sulfur coals. They include conventional high-temperature fouling deposits and four types of low-temperature deposits known as upstream massive, upstream enamel, double-crested upstream, and downstream powder deposits. The low-temperature deposits contain large amounts of sulfur fixed from the gas stream by alkaline (basic) elements. The sulfation occurs after deposition at temperatures below 1050°C through chemical vapor deposition. Above that temperature, calcium is more stable as a silicate or oxide, and sulfur is more stable as SO2. Sodium, potassium, and phosphorus are also enriched in deposits formed below 1050°C, whereas concentrations of other elements show little temperature dependence. Each of the deposit types formed under different temperature and gas flow conditions. Consequently, deposition mechanisms and size distributions of depositing particles also varied with deposit type.
