Characterization of Boiler deposits and possible preventive measures

• By: Mitra S.K ADMIN
• December 31 2024
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Introduction:

A boiler is an enclosed vessel, which is used to transfer  the heat generated from combustion  of a fuel into water to convert it into steam. The steam so formed is used to drive certain industrial processes such  as  electricity  generation. Boilers utilize coal,  oil, natural  gas  or  biomass  as  fuels  for  heat generation and  they  are  designed  in such  a way to  utilize maximum  heat  from  the  combustion reaction. Performance of a boiler mainly depends on heat transfer efficiency. Metals being good heat conductors, all the contact  surfaces  between the fire side and water  side are either  metal or metal alloys. During continuous  operation of a boiler, scale formation  on the  exposed  metal  surfaces  are unavoidable.  The scaling on boiler tubes  is predominantly governed  by the  feed  water  chemistry, which in turn affects the heat transfer efficiency of boilers. Deposition of scales and sludges on boiler tube walls leads to the following detrimental effects:

1. Corrosion of the Copper alloy containing boiler tubes
2. Excess fuel consumption for steam formation as efficiency decreases.
3. Decreased heat transfer  between the surfaces.

Continuously evaporating  circulating water of the boiler leaves behind progressively increased concentrations of dissolved salts. Concentration of dissolved salts after  reaching a saturation point tend  to precipitate on the inner walls of the boiler. Loose and slimy precipitate, generally found on the colder parts of the boiler is called Sludge (e.g. CaCl2, MgCO3) while hard and adhering precipitates are called Scales (e.g. CaSiO3, Mg(OH)2).

Scales can  be  formed  by various  mechanisms  from  the  components present therein.  Ca(HCO3)2 decomposition can lead to the formation of a sticky CaCO3 scale in the following way:

 Ca(HCO3)2 ⟶ CaCO3 ↓ + H2O  +  CO2 ↑

Similarly, scale formation  can also occur by hydrolysis of Magnesium salts: 
MgCl2 + 2 H2O ⟶ Mg(OH)2 + 2HCl

There are  many  reasons  for  scale  formation  like poor  pre-treatment of the  incoming  water  or contamination of the  condensate which gets collected  in the  cooler parts  of the  boiler, where  the circulating steam condenses.

Boiler Corrosions:

Boiler corrosion  which is detrimental for boiler health  and subsequent performance can occur via various mechanisms. Pre-boiler corrosions occur when metals (like Iron or Copper) transport to boiler from external equipment. The corrosion can be due to various reasons. It can be either oxygen related corrosion,  where  metal surfaces start  acting like galvanic cells, resulting in gradual corrosion of the surface or pH related  corrosion, where the changing pH of the feed water can lead to degradation of the metal surface.  The force of the water  hitting against the different  bends of the boiler tubes can lead to erosion of boiler surfaces, leading to corrosion. Excluding the external factors, internal boiler corrosion is a general concept  that  defines all corrosion mechanisms  that  originate inside the boiler tubes. Caustic corrosion, acid corrosion and Hydrogen embrittlement are the three  main contributing factors of internal corrosion. 

In a nutshell,  different  types  of scales or deposits  found  at the  various parts  of boilers essentially demand  identification/understanding of the  chemistry  comprising  of a host  of metals  and  metal bearing mineralogical phases. Consequently, a proper understanding of the scale/deposit composition is the  key to the  root  cause  analysis of the  corrosion  or performance issues that  can considerably improve the health and safety aspects of the high value machinery and impact the economical side of the affairs in a positive note.

Experimental Section:

Materials and Methods:

Experiments were carried out on 6 boiler sludge samples, which were received from a coal fired boiler (having a fire-side model). The composition  of the sludges was of particular interest to us. This could be a helpful tool in analysing the working condition of the boilers and their efficiency. The techniques used for studying the sludges were:

1. Powdered  XRD to understand the various phases present in the sample
2. Elemental composition of the deposits was studied using X-Ray Fluorescence spectroscopy
3. FTIR studies were also done to support  the data produced  from PXRD and XRF

FTIR analysis:

The samples were sized down to 200 mesh and dried at 250C. They were then diluted with KBr and pelletized  prior  to  subjecting  them   to  IR  spectroscopy  on  a  Thermo  Scientific Nicolet  iS10 IR
spectrometer. The recorded IR spectra of the samples have been shown below (Fig.1)

Fig. 1: IR Spectra of the boiler sludge samples

The IR spectra of the samples clearly indicate the presence of different phases of silica and Iron oxides in them. Other than these two major phases, aluminium oxides are also present. The IR peaks of O-H indicate the presence of moisture,  which is rather  unavoidable  in spite of drying the samples at high temperature prior to measurement. 

EDXRF studies:

The  samples  were  analysed  in an  ARLTM   QUANT’X  EDXRF  Spectrometer.  Table  1 represents the summarized elemental composition.

Table 1: Elemental composition of Sample 1-6

The XRF results corroborates with the FTIR results and indicate presence of major phases of Silica and
Iron oxide, along with some other minor phases.
 

PXRD studies:
The samples (-200 mesh size) were subjected to X ray diffraction in a Bruker D2 Phaser powder X ray diffractometer. The PXRD patterns have been provided in Figure 2 and the composition of the samples have been provided in Table 2. 

Fig. 2 PXRD patterns of the samples  1-6

Table 2 : Phase composition of samples  1-6

The XRD data of the sludge samples clearly reveals the presence of various phases, mainly comprising Iron oxides (Magnetite  and Hematite),  Silicon Oxides (SiO2). These data  go hand in hand with the IR and XRF data, proving that the sludge and scales are mainly composed  of Iron and Silicon Oxides. The Iron oxides might result  from the  corrosion  of the  steel  tubes,  while the  feed  water  might be the source of the Silicon oxides. 

Conclusion:
Here  6 sludge  samples  collected  from  a coal fired boiler  were  subjected to  chemical  analysis by employing FTIR, EDXRF and XRD technique. This revealed  that  the sludge samples  mainly consist of Silicon and  Iron  oxides,  resulting  from  contaminated  feed  water   and  corroded   boiler  tubes, respectively.  As suggested  by the current data set, the deposits mainly consist of Iron oxides and can be  eliminated  by proper  deaeration of  the  feed  water  using  suitable  oxygen  scavengers.  Such corrosion  of boiler  tubes  can  also  be  prevented by pH monitoring  of the  incoming feed  water. Although presence of considerable amount of Calcium and Magnesium salts are not observed in these samples, passing the feed water  through  a simple cation exchanging water softener is an answer  to such issues. High Silica based  sludges can be dealt with by proper  filtration of the feed water.  Thus, proper identification of the deposit composition  is can provide the remedy and it remains the aim of our future work which will include thorough  collection of sludge and scale samples from a variety of boiler models and predict their origin thereby  finding the means to extend  lifetime and enhance the performance of high value installations.

Contributed by Aradhita Bhattacharjee and Arijit Goswami