NTPC LECTURE-2010Sl.No.

Group of Minerals

Representative Minerals

Formula Frequency of Occurrence

%(approx)ofTotal M.M

1. Shale a. Illite

b.Montmorillonite

c. Muscovite

Na2O.3Al2O3.6SiO2.2H2OMgO.CaO.Al2O3.5SiO2.nH2OK2O.3Al2O3.6SiO2.2H2O

Frequent & Common 5-10

2. Kaoline a. Kaoliniteb. Metahalloysite

Al2O3 2SiO2.2H2OAl2O3.2SiO2.4H2O

Very frequent 60-85

3. Sulphide a. Iron Pyrites (cubic)b. Mercasite (Rhombic)

FeS2 Less common About0.5

4. Carbonate a. Calciteb. Dolomitec. Siderited. Ankerite

CaCO3

CaCO3.MgCO3

FeCO3

2CaCO3.MgCO3.FeCO3

Frequent & Common 2-3

5. Oxide a. Quartzb. Rutilec. Haematite

SiO2

TiO2

Fe2O3

Very frequent & Very common Frequent & CommonFrequent & Common

2-200.7-0.81-3

6. Other accessory Minerals group

a. Felspar- Soda/ / Potash Felspar

b. Apatite- Chloro/Fluro Apatitec. Magnetited. Zircone. Vivinite

Na2O.Al2O3.6SiO2/K2OAl2O3.SiO2

3Ca3(PO4)2.CaCl2/3Ca3(PO4)2.CaF2

Fe3O4

ZrO2

Fe3(PO4)2.8H2O

Common

0.5-1.0

Sl.No. Decomposition temperature. 0C

Mineral Matter

1. 370-570 Water of hydration Starts at 370& complete at 5702. 300-800 FeS2 starts at 300Max. at400 &complete at 8003. 500-575 FeCO3 starts at 500 Max at 5754. 540-760 MgCO3 starts at 5405. 750-800 CaCO3,MgCo3 at 7606. 850-950 CaCO3,

The principal changes are:

1. Dehydration of hydrated minerals: Kaoline (Shale)-Al2O3.xSiO2.yH2O → Al2O3.xSiO2+yH2O CaSO4.2H2O → CaSO4+2H2O

2. Decomposition of carbonates of lime, magnesia and iron bearing oxides of residues CaCO3 → CaO+CO2, MgCO3 → MgO+CO2, FeCO3 → Fe2O3+2CO2

3. Decomposition of sulphides:- Pyrites, 2 FeS2+3 ½O2 → Fe2O3

4. Volatilisation of chlorides of the alkali metals, this is usually small and may be neglected.

In a combustor, at a relatively high temperature (1400c and above) the mineral matter may undergo following reaction in the thermal environment.

1. Kaolinite-mullite formation to glass formation

(Kaolinite) Al2O3.2SiO2.2H2O (Metakaolinite) Al2O3.2SiO2+2H2O

(Silicon spinel) 2Al2O3.3SiO2 1:1 Mullite type Al2O3.SiO2 3:2 Mullite

type phase 3Al2O3.2SiO2 C Liquid

2. Alpha quartz (SiO2) changes to tridymite to crystabalite

α –quartz β quartz Tridymite Crystabalite Liquid

3. Iron carbonate changes to Fe2O3

4. CaO, MgO from carbonate may react with acidic component and form binary phase with siliceous component of kaolinite and shale.

5. In milding reducing conditions, Fe2O3 may be reduced to FeO and may react to form low melting FeSiO3 & Ferrous alumina silicates. Thus, there may be incipient fusion to clinker formation in the boiler but due to silicious nature of Indian coal, chances of general slag formation is less.

NTPC Vindhyachal

Sl.no. Oxide Ash Analysis Result1 SiO2 66.262 Al2O3 24.213 Fe2O3 5.194 TiO2 1.125 P2O5 0.186 SO3 0.36 7 CaO 0.798 MgO 0.349 Na2O 0.4010 K2O 1.14