Bed Agglomerate and Ash Deposit Formation.

Bed agglomeration and ash deposit formation in combustion and gasification systems can lead to efficiency and reliability problems. Through our advanced analysis methods, MTI assists our clients in determining the cause of bed agglomeration and ash deposition and preventing these problems in the future.

Our analysis has shown that the accumulation of bed agglomerates and ash deposits is:

Typically due to sulfate/sulfide and/or silicate-based bonding phases that are temperature dependent
Impacted by alkali and alkaline earth species in the coal or biomass
Impacted by reduced iron species in the form of metals, oxides, sulfides, and silicates
Impacted by vanadium and nickel species in petroleum coke-fired systems

Based on this information, we can advise our clients in the proper selection of fuel, operating condition, bed materials, and cleaning methods—important factors in minimizing ash deposition and bed agglomeration.

Viscosity impact on bed agglomerate and ash deposit formation
The ability of particles to stick together, become bonded, and fuse is dependent upon the temperature and viscosity of the liquid phases present. The viscosity of a liquid phase is calculated based on its chemical composition as a function of temperature. In the case illustrated below, the liquid phase components are concentrated on the surfaces of the bed particles and caused the initial bonding that led to the formation of the agglomerate.

Viscosity and temperature criteria are defined as follows:

Initial sticking temperature where particles stick is where the viscosity of the liquid phase is 5.5 log¹⁰ poise
Particle-to-particle bonding initiation temperature is where the viscosity of the liquid phase is at 4.5 log¹⁰ poise
Particle and the formation of continuous fused materials is at a temperature where the viscosity of the materials is below 3.5 log¹⁰ poise
Molten material where the slag materials are flowing are at a temperature where the viscosity is 2.4 log¹⁰ poise and is also call the T²⁵⁰ temperature.

Viscosity-temperature relationship for bed agglomerate material, showing viscosity differences by phase.

Backscattered electron image of cross-sectioned deposit showing agglomerated particles and fusing.

Bonding phase identification using SEMPC analysis
SEMPC provides information on the degree of melting and interaction of ash particles and bed material, and provides quantitative information on the phases present. This information is used to identify the agglomerating material. SEMPC can be used to identify and quantify the amount of melted phases and their viscosities. Based on SEMPC data, the following temperatures can be determined:

Initial particle sticking

Particle-to-particle bonding initiation

Ash deposit and agglomerate fusing

Bonding material identification using morphological analysis
Scanning electron microscopy morphology analysis provides images and chemical compositions of selected features in deposits and agglomerates. By utilizing the information gained from the morphology analysis, such as structure and compositional changes, mechanisms of formation may be determined.

Point and area analysis of selected points in agglomerate cross-section (wt%).

Backscattered electron images of agglomerated material in corss-section. Brighter areas are richer in higher-atomic-number elements. Numbers represent the point or area analysis provided in the table above.