1. Bed agglomeration

[GLOSS]Bed agglomeration[/GLOSS] problems in fluidised-bed conversion ([GLOSS]fluidised bed combustion[/GLOSS] or gasification) are related to a high content of alkali metals in the fuel.

Combined with high contents of sulphur (in combustion), chlorine, silica (from the fuel or the bed material) and phosphorus, low-melting compounds or low-melting mixtures of several compounds, so called [GLOSS]eutectic[/GLOSS]s, are formed, which become deposited on the bed particles, coating them with a sticky [GLOSS]ash[/GLOSS] layer acting as to glue particles together.

The particles may form large [GLOSS]agglomerate[/GLOSS]s, which decrease the mixing of the bed and may result in collapse of the fluidised bed, i.e. defluidisation.

Bed agglomeration in fluidised-bed conversion of [GLOSS]biomass[/GLOSS] is related to a high content of potassium. Growing plants selectively concentrate potassium, which along with nitrogen and phosphorous are the key nutrients for plant growth. Therefore potassium is rather concentrated in fast growing (annual) plants. Likely problematic fuels are: residues of agricultural crops, young [GLOSS]energy crops[/GLOSS] or other biomass containing young organic material.

2. Mechanism of bed agglomeration

There are three [GLOSS]sintering[/GLOSS] mechanisms relevant in bed agglomeration:

1)        Partial melting: sintering in the presence of a reactive non-viscous liquid phase consisting of molten alkali salts, where the solid phase is partly soluble in the liquid at the sintering temperature. The high alkali content of fuel combined with sulphur and/or chlorine form low-melting eutectics even below 700^oC.

2)       Viscous flow sintering or [GLOSS]vitrification[/GLOSS]: sintering due to viscous flow of a vitreous silicate phase. When silica is heated to the melting temperature range, a highly viscous liquid phase forms. Due to its high viscosity, the liquid remains viscous on rapid cooling below the melting temperature range, forming a glassy phase; this glassy phase has a viscosity low enough to cause sintering of particles at temperatures as low as 700 to 800^oC.

3)       Chemical reaction: sintering due to reaction between the particles or the particles and the gas, to form a new compound binding the particles together. This mechanism is reported to be dominant in sintering of ashes rich in calcium. CaO in a gas with high CO₂ partial pressure gives particle-to-particle bonding via CaCO₃ formation at temperatures between 600^oC and 800^oC. Above 800^oC these decompose to CaO and CO₂. CaO in high SO₂ concentration environment form CaSO₄ crystals, resulting in similar sintering; however at temperature over 500^oC.

The main influencing factors are: concentration of potassium, chlorine, sulphur, silica, type of bed material (silica (SiO₂), alumina (Al₂O₃), mullite (Al₂O₃.SiO₂)), fluidisation conditions, bed and fuel particle size, temperature and ash recirculation from cyclones.

3. Result of bed-agglomeration: Defluidisation

Defluidisation is indicated by a sudden decrease of the pressure drop over the bed to a low level. The pressure drop declines slowly before defluidisation, suggesting a segregation of large agglomerates in the bottom of the bed.

The temperature profile inside the bed can be an indicator of defluidisation. When the bed is in its normal fluidization state, the bed temperature is very uniform.  A difference in temperature between the bottom and the center of the bed is indicative of poor mixing caused by large agglomerates.

In bubbling beds (relatively low fluidisation velocity), defluidisation may be due to:

a)        Agglomerates disturbing the good mixing and causing hot spots, aggravating agglomeration and;

b)        Increased particle size and inter-particle force due to the sticky coating, increasing the minimum fluidisation velocity.

In circulating fluidised bed, where the velocity is much higher, agglomerates are expected at the relatively relaxed areas as in the stand pipe, non-mechanical valve, blocking the recirculating system.

4. How to predict agglomeration?

Laboratory methods for assessing the slagging, fouling and agglomeration are difficult to apply since the ash produced in a laboratory environment is significantly different from the ash formed in an industrial environment. However, together with large-scale tests and mathematical modelling, the following methods and measures may give better understanding of the mechanisms:

1)        Fuel analysis:

a.        [GLOSS]Slagging index[/GLOSS],[GLOSS]Fouling index[/GLOSS]

b.        Reactive alkali and chlorine content

2)       Ash melting, sintering and agglomeration temperatures analysis:

a.        Standard ASTM ash fusion test

b.        Empirical correlations between standard ash fusion temperatures and the ash chemical composition

c.       [GLOSS]Differential thermal analysis[/GLOSS],[GLOSS]Thermogravimetric analysis[/GLOSS]

d.        Electrical resistance and shrinkage method

e.        Laboratory sintering method (pressure strength measurement of heat-treated ash pellets)

f.        Heat treatment mixtures of laboratory-ash and bed material and other similar agglomeration tests

g.        Flow properties of heat treated ash

The question of how to prevent agglomeration in fluidised beds is treated in CF191.

Acknowledgements

Acknowledgement is due to Mr. Bram van der Drift, ECN Biomass, for his help in literature research.

Sources

[1] A. van der Drift and A. Olsen: Conversion of biomass, Prediction and Solution Methods for Ash Agglomeration and Related Problems. Final Report, Non-Nuclear Energy Programme Joule 3 by European Commission, contract JOR3-95-0079.

[2] W. Lin and K. Dam-Johansen: Agglomeration in Fluidized Bed Combustion of Biomass – Mechanisms and Co-Firing with Coal, Proceedings of the 15th International Conference on Fluidized Bed Combustion May 16 – 19, 1999 Savannah, Georgia