A New Method to Inhibit Bed Agglomeration Problems in Fluidized Bed Boilers

2003 ◽  
Author(s):  
Jaani Silvennoinen
Keyword(s):  
Fluidized Bed ◽  
Industrial Scale ◽  
Fluidized Bed Combustion ◽  
Natural Sand ◽  
Worst Case ◽  
Free Quartz ◽  
Bed Materials ◽  
Bed Agglomeration ◽  
High Alkali ◽  
Bed Material

Fluidized bed combustion (FBC) technology was commercialized in the 70s. Both bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) technology are capable of handling a wide variety of solid fuels. Natural sand is typically used as the fluidizing material. However, the properties and behavior of some solid fuel ash may limit the use of these fuels due to bed agglomeration problems. Natural sand contains several minerals, typically mainly consisting of 20–50 wt.-% of plagioclase (NaAlSi3O8 + CaAlSi3O8), 10–30 wt.-% of potash feldspar (KAlSi3O8), and 25–100 wt.-% of quartz (SiO2). Biomass based fuels contain high amounts of alkali. Ash high in alkali may react with the free quartz of the natural sand, producing an alkali silicate mixture with low melting point. This mixture may act as an adhesive between fluidized bed particles and may, in the worst-case, result in serious fluidization problems. This problem can be avoided by using AGGLOSTOP™ quartz-free bed material. Four different bed materials were tested in a 15 kW laboratory-scale FBC test rig with plywood residue, which is known to cause severe fluidization problems in FB boilers. Two of the tested bed materials were quartz-free. When quartz-free bed materials were used, no signs of bed agglomeration were observed. The other two bed materials containing free quartz caused total defluidization at a temperature of around 750°C after about half an hour of operation. The concept of using AGGLOSTOP™ quartz-free bed material with high alkali fuels has been successfully applied in two industrial scale BFB boilers (15 and 74 MWth). The use of AGGLOSTOP™ fluidized bed material enables energy production in FB boilers based on high alkali fuels, which were earlier impossible to utilize due severe bed agglomeration problems. This paper focuses on the bed agglomeration phenomenon by discussing the results from laboratory and industrial-scale boilers and presents a new solution to extend the use of high alkali fuels in FB boilers.

Alternative Bed Materials for High Alkali Fuels

2005 ◽  
Author(s):  
Matts Almark ◽  
Matti Hiltunen
Keyword(s):  
Fluidized Bed ◽  
Particle Sizes ◽  
Fluidized Bed Combustion ◽  
Minimum Level ◽  
Natural Sand ◽  
Suitable Form ◽  
Bed Materials ◽  
High Alkali ◽  
Silicate Layers ◽  
Sodium And Potassium

Fluidized bed combustion of high alkali fuels may cause agglomeration and defluidization of the bed as sodium and potassium react with quartz particles in the bed, forming alkali-silicate layers causing the particles to stick together. In certain cases the use of quartz free bed materials has been shown successful, like “Ofita” in the combustion of the residues from olive oil production in Spain. There are alternatives available to be used as quartz-free bed materials but some of them may be more expensive compared to natural sand. They are not always found in suitable form or particle sizes. Furthermore, the knowledge of the alternatives, and when it would beneficial to use them, is limited. The economics are not always favorable to using the alternative, effective but expensive bed materials since the build-up of harmful elements in bed usually can be avoided with sufficient make-up sand addition. A minimum level of make-up material addition is in any case necessary when combusting fuels with ashes that do not maintain the bed inventory. A study of minerals and materials available in Finland and elsewhere as possible bed make-up materials was carried out. The differences between these materials compared to quartz as a reference are investigated. The economics of the use of the alternative bed materials are discussed.

Bed Agglomeration During the Fluidized Bed Combustion of Olive Husk

2005 ◽  
Author(s):  
Antonio Cammarota ◽  
Riccardo Chirone ◽  
Fabrizio Scala
Keyword(s):  
Fluidized Bed ◽  
Dynamic Pressure ◽  
Mediterranean Area ◽  
Operating Conditions ◽  
Pressure Gradients ◽  
Fluidized Bed Combustion ◽  
High Propensity ◽  
Olive Husk ◽  
Bed Agglomeration ◽  
High Alkali

The fluidized bed combustion of a biomass residue (olive husk) common in the Mediterranean area has been investigated in a bench scale reactor. The focus of the study was the high propensity of this fuel to give rise to bed agglomeration problems during combustion, as a consequence of the high alkali content of the ash. Bed agglomeration characteristic times as well as temperature and pressure gradients were measured at different operating conditions. In addition, a diagnostic tool based on the measurement of the dynamic pressure signal inside the bed was tested for its capability to predict the bed agglomeration onset.

Experimental Research on Impacts of Operation Parameters on Agglomeration Characteristics during CFB Biomass Gasification

2014 ◽  
Vol 556-562 ◽  
pp. 375-379
Author(s):  
Xiao Xu Fan ◽  
Li Guo Yang ◽  
Hui Liang Zhang ◽  
Hong Jian Chen
Keyword(s):  
Fluidized Bed ◽  
Biomass Gasification ◽  
High Alumina ◽  
Cotton Stalk ◽  
Air Velocity ◽  
Bed Materials ◽  
Operation Parameters ◽  
Bed Agglomeration ◽  
Bed Material ◽  
Return Valve

The impacts of operation parameters on agglomeration characteristics during biomass gasification in fluidized bed were studied experimentally in a 0.02MWt CFB gasifier using cotton stalk pellet as fuel. The experimental results indicated that among the temperature range (600 °C-800 °C), bed agglomeration would occur after a period running with sand, high alumina bauxite or periclase as the bed material, and potassium gathered on the surface of bed materials. In the process of the fluidized-bed gasification of biomass, air velocity affected the degree of bed agglomeration, and the agglomeration problem in return valve was more serious than that in main bed.

Bed Agglomeration Characteristics in Fluidized-Bed Combustion of Biomass Fuels Using Olivine as Bed Material

2012 ◽  
Vol 26 (7) ◽  
pp. 4550-4559 ◽  
Author(s):  
Alejandro Grimm ◽  
Marcus Öhman ◽  
Therése Lindberg ◽  
Andreas Fredriksson ◽  
Dan Boström
Keyword(s):  
Fluidized Bed ◽  
Fluidized Bed Combustion ◽  
Biomass Fuels ◽  
Bed Agglomeration ◽  
Bed Material

Biomass Ash – Bed Material Interactions Leading to Agglomeration in FBC

2003 ◽  
Author(s):  
H. J. M. Visser ◽  
S. C. van Lith ◽  
J. H. A. Kiel
Keyword(s):  
Coating Thickness ◽  
Problem Area ◽  
Critical Conditions ◽  
Fluidized Bed Combustion ◽  
Biomass Ash ◽  
Fundamental Study ◽  
Fluidization Velocity ◽  
Bubbling Fluidized Bed ◽  
Bed Agglomeration ◽  
Bed Material

In (bubbling) fluidized-bed combustion and gasification of biomass, several potential problems are associated with the inorganic components of the fuel. A major problem area is de-fluidization due to bed agglomeration. The most common found process leading to de-fluidization in commercial-scale installations is “coating-induced” agglomeration. During reactor operation, a coating is formed on the surface of bed material grains and at certain critical conditions (e.g., coating thickness or temperature) sintering of the coatings initiates the agglomeration. In an experimental approach, this work describes a fundamental study on the mechanisms of de-fluidization. For the studied process of bed de-fluidization due to sintering of grain-coating layers, it was found that the onset of the process depends on: a) a critical coating thickness, b) on the fluidization velocity when it is below approx. four times the minimum fluidization velocity and c) on the viscosity (stickiness) of the outside of the grains (coating).

Bed Agglomeration Characteristics during Fluidized Bed Combustion of Biomass Fuels

2000 ◽  
Vol 14 (1) ◽  
pp. 169-178 ◽  
Author(s):  
Marcus Öhman ◽  
Anders Nordin ◽  
Bengt-Johan Skrifvars ◽  
Rainer Backman ◽  
Mikko Hupa
Keyword(s):  
Fluidized Bed ◽  
Fluidized Bed Combustion ◽  
Biomass Fuels ◽  
Bed Agglomeration

The Phase Transformations during Fluidized-Bed Combustion of Biomass

2013 ◽  
Vol 419 ◽  
pp. 366-369 ◽  
Author(s):  
Hai Peng Teng ◽  
Bin Yang ◽  
Bin Liang
Keyword(s):  
Gas Phase ◽  
Fluidized Bed ◽  
Solid Phase ◽  
Fluidized Bed Reactor ◽  
Biomass Combustion ◽  
Material Surface ◽  
Fluidized Bed Combustion ◽  
Biomass Ash ◽  
Equilibrium Modeling ◽  
Bed Material

FactSage6.1 was used to study the phase transformation at high temperature when biomass combustion in a fluidized bed reactor. The results show that eutectic was formed during the reaction process, the eutectics are formed mainly by the reaction between the silica in bed particles and the alkali species in biomass ash. The solid phase transformed to melt layer on the surface of sands particle mainly contains potassium, some calcium and magnesium, and also a few phosphorus and chlorine are found in the melt layer. The result utilizing FactSage equilibrium modeling shown that the distribution ratio of potassium in the gas phase increased with the increase of temperature, moreover, the melt of bed material surface increased when defluidized occurred.

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