Download Report

What is fluidised bed agglomeration?
A Combustion File downloaded from the IFRF Online Combustion Handbook
ISSN 1607-9116
Combustion File No:
50
Version No:
1
Date:
23-06-2003
Author(s):
Jenő Kovács
Source(s):
See CF
Sub-editor:
Neil Fricker
Referee(s):
Mikko Hupa
Status:
Published
Sponsor:
EuroFlam
1. Bed agglomeration
Bed agglomeration problems in fluidised-bed conversion (fluidised bed combustion 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 eutectics, are formed, which become deposited on the bed
particles, coating them with a sticky ash layer acting as to glue particles together.
The particles may form large agglomerates, 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 biomass 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 energy crops or other biomass containing young organic
material.
2. Mechanism of bed agglomeration
There are three sintering 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 700oC.
2) Viscous flow sintering or vitrification: 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 800oC.
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 600oC and 800oC. Above 800oC these
decompose to CaO and CO2. CaO in high SO2 concentration environment form
CaSO4 crystals, resulting in similar sintering; however at temperature over
500oC.
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.
Slagging index, fouling index
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.
Differential thermal analysis, Thermogravimetric analysis
d. Electrical resistance and shrinkage method
e.
Laboratory sintering method (pressure strength measurement of heattreated 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.
Glossary terms
Agglomerate - A cluster of individual particles in which the particles are held together
by surface forces resulting from a change of environment. Opposite of agglomeration is
dispersion.
Ash - Ash is the non-combustible material that is contained by a liquid or solid fuel,
which is left as a residue after the completion of the combustion process. (See also Flyash)
Bed agglomeration - The process when separate bed particles adhere to each other to
form larger particles.
Biomass - Biomass comprises all growing organic matter, such as plants, trees, grasses,
and algae. Biomass is renewable fuel from organic origin, residues from forestry,
agriculture and energy crops.
Differential thermal analysis - A method used in the analysis of phase changes and the
heat involved in the change, in which the temperature difference between a test sample
and an inert reference sample is recorded in a uniform heating process.
Energy crops - Crops grown specifically for their fuel value. These include food crops
such as corn and sugarcane, and non-food crops such as poplar trees and switchgrass.
Eutectic – A mixture of substances having the lowest freezing point of all possible
mixtures of the substances.
Fluidised bed combustion - Combustion based on the fluidisation technology.
Fouling index - An index, which can be calculated from the ash analysis and which, gives
an indication of the propensity for that coal to cause fouling problems during
combustion.
Sintering - The process in which fine particles initially touching each other become
attached to each other due to a temperature that is sufficient for atomic diffusion.
Slagging index - An index which can be calculated from the ash analysis and which gives
an indication of the propensity for that coal to cause slagging problems during
combustion.
Thermogravimetric analysis - Instrumental technique that measures the weight of a
sample and how this weight diminishes as the sample reacts. The sample temperature is
controlled.
Vitrification – Formation of a glass (a viscous silicate melt that does not crystallize
when cooled to lower temperatures).
Keywords
Agglomerates; agglomeration; alkali; ash; bed material; biomass; defluidisation; energy
crops; fluidised bed combustors; melting; potassium; silica; sintering
Related Combustion Files
CF 85 – What are the main fluidised bed combustion phenomena?
CF 191 – How do I prevent agglomeration in fluidised beds?
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
Acknowledgements
Acknowledgement is due to Mr. Bram van der Drift, ECN Biomass, for his help in
literature research.
File Placing
[Burners]; [Fluidised Beds]; [General]
Access Domain
[Members’ Domain]
Parity between this pdf and the present html version of this Combustion File
The information contained in this pdf Combustion File edition is derived from html
edition of the same number and version, as published in the IFRF Combustion Handbook
(http://www.handbook.ifrf.net).
The information published in this pdf edition, is that which was included in the original
html edition and has not been updated since. For example there may have been minor
corrections in the html version, of errors, which have been drawn to our attention by our
readers. What is more important is that with the passage of time and the continuous
growth of the handbook, a number of other changes may have been made to the
published html version, such as:
•
The related combustion files may have been augmented;
•
The filing system may have been further developed;
•
The Access Domain may have changed.
These changes can be made without substantial changes being made to the main text and
graphics. If there have been substantial changes made, then a new version of the
Combustion File will have been published.
Thus to be sure of up-to-date information, go to the Handbook and download the latest
html version of the Combustion File.
Limits of Liability
A full Limits of Liability declaration is shown at the entry of the IFRF ONLINE
Combustion Handbook at www.handbook.ifrf.net. Through possession of this
document, it is assumed that the holder has read and accepted the limits. The essential
limitation is that:
The International Flame Research Foundation, its Officers, its Member
Organisations its Individual Members and its staff accept no legal liability or
responsibility whatsoever for the consequences of unqualified use or misuse of the
information presented in the IFRF Combustion Handbook or any results derived from
the Combustion Files which comprise this Handbook.
 IFRF 1999 - 2003