Coal analysis & it's importance in coal based power plants

Rajesh Patil

Proximate & Ultimate Analysis of Coal

A.0. Introduction

Coal is an extremely complex material but it is the main fuel for power plants.

Hence it becomes necessary to devise acceptable methods for coal analysis in the hope that it would be possible to correlate coal composition and coal properties with coal behavior.

B.0. Proximate analysis

Proximate analysis serves a quick characterization of coal in respect of its quality, rank and type and thus broadly indicate it’s suitability for a particular mode of use. 

The analysis is done on air-dry sample of coal passed through 212 micron and it involves the determinations of moisture, ash, volatile matter and fixed carbon percentages.

 The test procedure of proximate analysis is stipulated by IS 1350 (Part-I) – 1969.

B.1. Moisture

Coal is hygroscopic means its moisture content depends on the atmospheric conditions of humidity and temperature. 

The forms of moisture which may be present in coal under various conditions are as follows:

 (1) Free moisture

 (2) Bed moisture

(3) Air-dried moisture

(4) Total moisture. 

Free moisture means water excess to the capacity/Bed moisture. 

Bed moisture means, seam moisture/capacity moisture and it is referred to the equilibrated moisture of coal at the condition of 96% RH & at 40oC. 

Moisture which is present in coal under normal laboratory condition is known as air-dried moisture. 

Free water plus the moisture within the coal is called total moisture. 

Proximate analysis is reported on the equilibrated moisture basis (60% RH & at 40oC).

However, for low moisture coal (moisture less than 2%), the analysis is recorded on air-dried basis.

B.2.  Ash

          Ash is the inorganic residue left when coal sample is incinerated in air to a constant weight under specified conditions. 

Coal ash thus obtained has a quantitative relation with the mineral matter present in coal substance. 

Pure coal substance approximately equates to: 100-(1.1 A+M) where A and M are ash and moisture percentage respectively on the same basis. 

The quality of coal substance is compared on dry mineral matter free basis.

B.3. Volatile matter (VM)

Volatile matter is the total loss of weight minus the moisture when coal is heated out of contact with air under specified conditions. 

It contains mainly carbon dioxide, carbon monoxide, methane, hydrogen, unsaturated hydrocarbons, water, tar-vapours, hydrogen sulphide, and ammonia etc. 

These are the pyrolyzed products of coal substance and evolve as volatiles. 

Volatile matter (volatiles) present is expressed on dry mineral matter free basis to know whether a coal is low or high volatile. 

It is to be noted that high volatile coal contains high moisture.

B.4. Fixed Carbon (FC)

Fixed carbon is obtained by subtracting the sum of moisture, ash and volatiles percentages from hundred. 

On dry ash free basis fixed carbon contains: (Carbon 97.0%, hydrogen 0.6%, Nitrogen 1.4%, Sulphur 0.5% and Oxygen 0.5% approximately irrespective of the rank of coal), but the F.C. % depends on the rank of coal.

 Precision of testsRepeatabilityReproducibility
    
MoistureUp to 3%+ 0.1 units+ 0.2 units
Over 3%+ 3% of result+ 6% of result
 
AshUp to 10%+ 0.2 units+ 0.4 units
Over 10%+ 2% of result+ 3% of result
 
Volatile Matter-----+ 1% of result+ 1.5% of result

C. Calorific Value (CV)

Calorific value of fuel is determined in bomb calorimeter at constant volume and expressed as gross C.V. Kcals/kg. 

Net C.V. is calculated from gross C.V.

there are two types of calorimeters, one is “isothermal” and the other is “adiabatic”. 

Radiation correction of heat is required for isothermal bomb calorimeter. 

The standard method of test procedure regarding determination of calorific value is guided by  IS: 1350 (part II) – 1970. 

There is safety precaution in connection with the experiment in bomb under high pressure of oxygen (25-30 atmosphere) and pressure in bomb shoots up due to sudden release of heat.

 For precaution and also for accuracy, heat release in the bomb should not be allowed to become more than ten thousand calories.

D. Ultimate Analysis

Analysis of coal for its elementary constituents, carbon, hydrogen, nitrogen, sulphur and oxygen is called ultimate analysis. 

The importance of ultimate composition of coal in deciding the extent of coalification (rank) has been recognized. 

From lignite to anthracite, the carbon in the pure coal substance increases progressively.  The hydrogen showing a small change up to 90% carbon after which it starts to fall off abruptly. 

Carbon and hydrogen values are required for combustion calculations.

D.1 Carbon and hydrogen

Standard method of carbon and hydrogen determination is stipulated by IS:1350 (Pt.IV/Sec.I) 1974 or IS:1351 – 1959.

The main principle which is being adopted for the determination of carbon & hydrogen is as follows:

  1. Coal of 212-micron size is burned in a silica combustion tube in a current of purified oxygen.
  2. The product gases so produced contain mainly water vapor from burning of hydrogen, carbon dioxide from burning of carbon and some interfering gases such as oxides of sulphur & nitrogen, chlorine etc, which can be selectively removed by using suitable packing reagents inside the tube. 
  3. Finally, the outcoming water and carbon dioxide gases are to be absorbed in suitable absorbents and determined gravimetrically. 

Percent carbon and % hydrogen figures can be computed from the weight of carbon dioxide and water respectively. 

Suitable corrections for carbonate carbon, water of hydration and moisture are needed to get correct figures of carbon and hydrogen.

D.2 Sulphur

Sulphur in coal generally occurs in three forms.

  (a) Sulphate

 (b) Pyritic

 (c) Organic Sulphur.

Total Sulphur can be determined as per IS:1350 –Pt.III – 1969, by Eschka method. 

Main principle is as follows:

  1. The crushed 212-micron coal is intimately mixed with Eschka mixture and heated in an oxidising atmosphere until complete combustion takes place. 
  2. The incinerated mass brought into solution and barium sulphate is precipitated by using barium chloride in a specified condition.
  3. Sulphate in Sulphur is determined by extracting coal with dilute hydrochloric acid and after removing iron, sulphur is to be determined as above.  Non pyritic iron is to be done simultaneously from the iron hydroxide precipitate.
  4. Dilute nitric acid can dissolve pyritic sulphur which is insoluble in dilute HCl.

Total iron (Pyritic and non-pyritic) is to be determined by extracting coal with dil. HNO3.  Pyritic iron can be obtained by deducting non pyritic iron from total iron.

Pyritic Sulphur can be calculated from the pyritic iron.  Organic sulphur is calculated by subtracting the percentage of sulphate sulphur and pyritic Sulphur from total Sulphur of coal.

D.3 Nitrogen

Following the IS:1351 – 1959 procedure, the nitrogen % of coal can be determined by Kjeldahl method or by modified Badami & Whitaker method. 

Nitrogen is converted into ammonium sulphate which release ammonia during distillation with sodium hydroxide. 

The released ammonia is to be absorbed in boric acid and by subsequent titration with standard sulphuric acid nitrogen percent can be calculated.

D.4 Oxygen

Oxygen is generally determined by subtracting from 100 the sum of the percentage of moisture, mineral matter, carbon, hydrogen, sulphur and nitrogen. (100 – (% moisture + % Mineral matter + % Carbon + % Hydrogen + % Sulphur + % Nitrogen)).

Precision:

 RepeatabilityReproducibility
Carbon+ 0.25% absolute+ 0.6% absolute
   
Hydrogen+ 0.12% absolute+ 0.25% absolute
   
Sulphur upto 2%+ 0.02% absolute+ 0.05% absolute
   
2 to 10%+ 0.04% absolute+ 0.05% absolute
   
Nitrogen+ 0.02% absolute+ 0.04% absolute

Modern Instrumental Methods in Coal Analysis:

Proximate Analysis

Proximate Analyzer determines weight changes of charge under appropriate heating conditions.

 The same sample aliquot is used for determining moisture, volatile matter and ash in the same order.

In one system, moisture and VM tests are done under nitrogen.  After this, the heating continues under oxygen to form the ash. 

In this system, the balance is built-in.  The specimens are loaded in crucibles set in a turn-table which places each crucible periodically on the balance. 

The entire process is automatic.  Calculated percentage results are issued as a print out. 

Provision exists for 2 furnace control.  One furnace, one run – 19 determinations (single).  Run-time, about 2 ½ hours.

Calorific Value

The determination is carried out by using bomb calorimeter.

 All operations including corrections are automatic.  Only sample weighing and initial loading of the sample into the system are done by human effort.

In one system, for increasing the turn over 3 bomb units (and container vessels) can be handled simultaneously, resulting in 14 determinations per hour.

Result print out is obtainable as B T U/lb, M J/kg or Cal/gm.

Ultimate Analysis (C, H, N, S & O)

There are two different varieties of systems are available. 

  1. In one variety, C-H-N determination is on one sample aliquot while for S and O separate attachments are used with the same main unit.  Here the sample is 1 to 2 mg.  So proper representation is difficult with Indian coals (of high ash content).
  • In another system C-H-N determination is carried out using 0.2 mg of coal. 

Combustion is carried out under oxygen.  Helium is used as carrier gas. 

Oxides of Sulphur are removed by suitable absorption.  Oxides of nitrogen are reduced to N2

Final products measured are CO2, H2O and N2 – former using infrared detector and latter by thermal conductivity detector. 

Through appropriate calibration and weight fractionation, final results are issued as percent C, H and N in the form of a print out.  Run-time, 5 mins.

Rajesh Patil
Mr. Rajesh Patil is having 25 years of experience in power plant chemistry as well as water treatment plant. Commissioning of water treatment plant , boiler water ,cooling water treatment in the power plant. Also experience in establishing of water treatment plant & coal laboratory with handling various types of analytical equipment.

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