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Mineral Matter Transformations in Combustion Systems: Reactions and Deposition, Slides of Geochemistry

Dhirubhai Ambani Institute of Information and Communication TechnologyGeochemistry

The physical and chemical mechanisms of mineral matter transformations and deposition in combustion systems. It discusses the influence of particle size, coal rank, and ash loading on the formation of fly ash and fouling. The document also covers the importance of alkali and alkaline earth reactions in convective pass fouling and the implications for heat transfer and harmful pollutant species formation.

Typology: Slides

2012/2013

Uploaded on 08/30/2013

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Class 13
Mineral Matter Transformations
and Deposition
Question 1
• Discuss some of the different physical mechanisms that
result in deposition of mineral matter on furnace walls
and boiler tubes. Also, please discuss the influence of
particle size (in relative terms) on each mechanism.
Transport Mechanisms Low Temperature Fouling
Deposits on Tube Diameter Dependence
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Download Mineral Matter Transformations in Combustion Systems: Reactions and Deposition and more Slides Geochemistry in PDF only on Docsity!

Class 13

Mineral Matter Transformations

and Deposition

Question 1

  • Discuss some of the different physical mechanisms that result in deposition of mineral matter on furnace walls and boiler tubes. Also, please discuss the influence of particle size (in relative terms) on each mechanism.

Transport Mechanisms Low Temperature Fouling

Deposits on Tube Diameter Dependence

Summary of Convective Pass Deposits

2. Slagging vs. Fouling

(Slagging is in radiant section, Fouling is not)

From STEAM by Babcock & Wilcox

Another View of Pathways

Physical Transformations of Inorganics

1. Coalescence of individual mineral grains within

char particle

2. Shedding of ash particles

3. Incomplete coalescence due to disintegration of

the char

4. Convective transport of ash from the char

surface during devolatilization

5. Fragmentation of inorganic mineral particles

6. Formation of cenospheres

7. Vaporization and subsequent condensation of

inorganic components upon gas cooling

Assignment: Please draw one of these on the board

Size Transformations

(Baxter’s Results)

Alkali and Alkaline Earth Reactions

  • Extremely important with respect to convective pass fouling
  • Behavior of alkalies during combustion depends strongly on their form in the coal
  • Carboxyl-bound alkalies or those in solution will volatilize
  • Na is bound to carboxyl group in lignite and volatilizes to form NaOH
  • Na also reacts with clay and quartz in vapor phase to form sodium-alumino-silicate slag droplets (lower melting point) or flyash
  • NaOH also reacts with SO 2 to form NaSO (^4)
    • Low melting point (884C)
    • Becomes part of initial sticky slag layer (fouling)

Na, K Mg, Ca

Transformation Implications to Deposition

Conclusions

  1. The study of mineral matter transformations in combustion systems leads to better understanding of fouling/slagging, heat transfer, and harmful pollutant species formation.
  2. Physical transformation of ash during char oxidation usually behaves between the limit of 1 ash particle per coal particle or 1 ash particle per mineral grain. - Swelled bituminous coal fragments readily, giving a larger particle size distribution than lignite coals
  3. Different reaction mechanisms for iron, alkali and alkaline earths, and acid-base chemical groups have been suggested for several combustion regimes
  4. The predictions of fouling tendencies of a certain coal can be done based on experience and based on the general trends discussed previously
  5. A priori predictions are not feasible at this time due to the complex processes involved.

Wall et al. (1979) Prog. Energy Comb. Sci.

Deposition Mechanisms

3. Sootblowing

  • Blow in air or steam to remove ash deposits (not soot)
  • Performed routinely in cyclic manner
  • Frequency of sootblowing must be short enough to prevent fusing of deposit - Deposit surface temperature increases with increasing deposit thickness

Figures from STEAM by Babcock & Wilcox

Deposit Characteristics

Overview of Fouling & Slagging

Model

Ash Disposal

  1. Landfill ---> Getting harder to do
  2. Cement ---> Needs low C in ash
  3. Roadfill ---> Low level radiation? High C in ash (> 10%)
    • Gray ash
    • Decreased ESP performance
    • Loss of efficiency Example: 10% ash in parent coal

a. 20% C in ash ---> 97.2% daf burnout b. 10% C in ash ---> 98.8% daf burnout c. 5% C in ash ---> 99.4% daf burnout

, 0

, 0

1

a

a

a

x

x

x Burnout daf

Coal Cleaning (Why?)

A. Reduce Sulfur!!!!!

(Pyrite may occur in excluded mineral grains)

– Clean air act says SOx must be decreased

– Eastern coals have a lot of sulfur

– Power generation needed in East

– Transportation of western coals is $$, even

though they are low in sulfur

B. Reduce mineral matter

– less ash ---> less deposits)

Coal Cleaning (Methods)

A. Wash with H2O B. Microbubble Flotation Process (MFP)

  • finely ground coal (< 44 mm)
  • mix with water in a column
  • froth with air bubbles (db ~ 100 mm) C. Spherical Oil-Agglomeration Process (SOAP)
  • finely ground coal (< 44 mm)
  • mix with water
  • mix with heptane (binds coal together into 1 mm particles)
  • steam treat to remove heptane (50 ppm to 0.2 wt% heptane in final SOAP product) D. Molten Caustic Leach
  • molten NaOH bath ($$, reduced volatiles)