A study on initiation of ash agglomeration in fluidized bed gasification systems

• Gravity separated fractions analyses reveal local initiation of agglomeration.
• FactSage calculations help to predict slag formation in fluidized bed gasification.
• Slag formation occurs even at fluid bed temperatures in high rank coal gasification.
• Hematite increases alumino-silicate slag formation temperature, until it reduces.
• Integrated ash agglomeration model was developed to predict agglomeration kinetics.

Agglomeration in fluidized beds begins locally by the sticking of slag–liquid-covered particles. Analysis of the composite fuel is not adequate in predicting agglomeration problems. Separation of the high rank Pittsburgh seam coal into particle classes based on specific gravity (SG1: <1.3 g/cm3; SG2: 1.3–1.6 g/cm3; SG3: 1.6–2.6 g/cm3 and SG4: >2.6 g/cm3) and particle size (PS1 through PS7) helped to identify important particle-level slag–liquid formation tendencies. Slag–liquid formation tendencies under fluidized bed operating temperatures were determined both computationally and experimentally. Particles rich in certain iron and calcium phases melt at very low temperatures that are well within fluidized bed operating conditions. The iron rich particle classes (SG3 and SG4) showed the presence of several phases containing iron in different oxidation states. The presence of these iron phases was not detected in the composite bulk fuel. The possibilities of equilibrium liquid phase formation in the presence of different ratios of these iron and calcium oxides to alumino-silicates were determined. Presence of hematite was found to delay slag–liquid formation. Each of the particle classes showed distinct slag–liquid formation tendencies that indicate initiation of agglomeration around SG3 and SG4 particles. The study revealed the importance of particle class-level differences in mineral matter composition for the prediction of agglomeration during fluidized bed gasification.A novel integrated ash agglomeration model that accounts for particle hydrodynamics as well as particle class level ash chemistry has been outlined to predict agglomeration kinetics.