Fluidized Bed Coal Combustion

In this chapter, existing literature has been surveyed based on the study of the different efforts made by various researchers to simulate, optimise and model the combustion mathematically in CFBC boilers, Ash agglomeration and related issues. Finally it is tried to identify the gaps so that problem can be formulated for this research attempt.

2.1 EXISTING RESEARCH EFFORTS

Various related literature such as transactions, proceeding of various national and international conferences and journals which available on google scholar, Elsevier, IEEE, Science direct etc. are reviewed and the contribution of researchers in the area of CFBC boilers, Ash agglomeration and related issues are categorised in following three groups:

• Literature related to ash experimental investigations related to ash agglomeration.

• Literature related tomodelling of ash agglomeration

• Literature related toremoval of ash agglomeration

2.1 Literature review on ash agglomeration causes and detection

Marinov et al. 1992 [1] studied and revealed the chemical processes causing ash agglomeration in fluidized bed gasifiers that are fed with lignites of high sulphur content. Two kinds of ash agglomeration centres can be distinguished: centres of melting and centres of sintering. The first should be held responsible for the ash agglomeration considered in this study because of the coincidence of the melting temperatures of FeS• FeO eutectic and the temperature (930OC ), at which the process of gasification of Elhovo lignites is broken off in the pilot installation. This conclusion is supported by the identification of β-pyrrhotite and wustite in agglomerate samples taken from the pilot plant.

Borio et al. 1995 [3] stated that the presence of ash deposits and fly ash can create the following problems in a boiler:

  1. Reduced heat transfer

  2. Impedance of gas flow

  3. Physical damage to pressure parts

  4. Corrosion of pressure parts

  5. Erosion of pressure parts

These problems will lead to reduced generating capacity, unscheduled outages, reduced availability, and costly modifications. It is also concluded that the ash formation and deposition process is not fully understood and traditional ASTM analyses do not always provide information that can be used to make predictive judgments at the confidence levels desired.

Brown et al. 1996 [4] determined physical and chemical reactions which lead to the undesired agglomeration of bed material during fluidized bed combustion of coal and to relate these reactions to specific causes. It is concluded that results obtained using the laboratory-scale combustor may be applied to industrial-scale combustors. The method of feeding coal into industrial-scale combustors is of utmost importance and if coal feed is limited to one location localized reducing zones may occur. These localized reducing zones may lead to agglomeration as shown by this research. It is also important to maintain a well-mixed bed. Poor lateral bed mixing may lead to fuel rich zones or local hot spots and subsequent agglomeration. Experiments performed in a laboratory-scale FBC support the hypothesis that a reducing atmosphere during fluidized bed coal combustion contributes to the formation of agglomerates.