Coal Analysis

Chemical analyses and testing of a coal sample are generally done off site in a laboratory. The main purpose of coal sample analysis is to determine the quality or rank of the coal along with its intrinsic characteristics. Furthermore this data will be used as the fundamental consideration for coal trading and its utilization. General coal analysis and testing include the following.

1. Proximate analysis: Total moisture, Inherent moisture, ash content, volatile matter, fixed carbon; Gross Calorific Value

2. Ultimate analysis: carbon, hydrogen, oxygen, nitrogen, sulphur;

In addition, special coal analyses may also be performed on trace elements, coal hardness, ash fusion temperature, chlorine, fluorine, boron, arsenic, mercury, selenium, phosphorus, size analysis, and so on. Standardization of the procedures and conditions is essential for obtaining results that are comparable within any one laboratory and between different laboratories. The standard test methods used in routine laboratory analyses of coal will be presented in the following sections. The variables are expressed as a percentage fraction (weight percent wt. %) and are calculated in several different bases. As received (ARB) basis puts all variables into consideration and uses the total weight as the basis of measurement. Air dried (ADB) basis neglects the presence of moisture other than inherent moisture while dry basis (DB) leaves out all moisture, including surface moisture, inherent moisture, and other moisture. Dry ash free (DAFB) basis neglects all moisture and ash constituents in coal while dry mineral-matter-free (DMMFB) basis leaves out the presence of moisture and mineral matters in coal, for example: quartz, pyrite, and calcite.

Proximate Analysis:

The   proximate   analysis   of   a   coal   determines   the   moisture   content,   volatile   matter   (VM),   ash   and,   by   difference,   the   fixed   carbon   within   the   coal   sample.   It   is   the   simplest   and   most   common   form   of   coal   evaluation,  and  constitutes  the  basis  of  many coal  purchasing  and  performance  prediction  indices  used  by   utility  operators.

Moisture:

Moisture may occur in various forms within a coal:

• Surface or free moisture: water held on the surface of coal particles or macerals;
• Inherent (residual) moisture: water held by capillary action within the pores/capillaries of coal;
• Decomposition moisture: water produced from the thermal decomposition of organic constituents of coal
• Mineral moisture (water of hydration of mineral matter): water which comprises part of the crystal structure of hydrous silicates such as clays and inorganic minerals in coal.

There are various standardized tests to determine the moisture content of coal and they are classified depending upon content of coal and they are classified depending upon the type of moisture content tested:

Total Moisture:

Take 1kg of coal sample of 12.5 mm size dry it at lab atmospheric environment for 24 hr up to constant weight. Take final weight after constant weight this is called as a surface moisture (X). After that sieve it by using 2.8mm sieve and take a 10 gm. of sample and kept it in the oven for drying for 1.5 hr to 3 hr at 108 deg c.. this drying is called as oven drying method (Y)

Calculation:

TM % = X+Y (1-X/100)

Inherent Moisture:

Take 1.5 gm. of 212 mic coal sample in petri dish dry it in oven for 1.5 hr to 3 hrs. At 108 deg c.

Calculation:

IM% = (SW-FW/SW-IW)x100

Equilibrium Moisture:

Take a 5 gm. of 212 mic sample in beaker and boil it for 10 min and filter it with normal filter paper kept it in humidity chamber for 72 hrs at 40 deg c. temp. And 60 % humidity. After that analyzed same as like inherent moisture.

Ash:

Ash is the residue remaining after the combustion of coal in an air and is derived from inorganic complexes present in an original coal substance and from associated mineral matter. Therefore the result of the determination is ash and not ash content as coal does not contain any ash.

Ash   is   determined   by   weighing   the   residue   remaining   after   burning   approximately   1gm   of   coal   under   rigidly   controlled   conditions   of   sample   weight,   temperature,   time, atmosphere, and equipment specifications.  In general  analysis   test   sample   of  coal  (ground to  pass  a  sieve  of   212  μm aperture), accurately weighed,  is heated in  air in a furnace from room temperature  to  500°C over  a  period  of 60  minutes  at a  uniform heating  rate and  is  held at  this temperature  for 30  minutes The sample is  then heated  to  815°C  ±  10°C and  maintained at  this  temperature  for  a minimum of 60 minutes  until  the  sample is constant  in  mass. When the  incineration period  is  complete,  the  sample  is  allowed  to cool  and  then weighed.  The results are reported as a percentage mass fraction on an air dried basis.

Calculation:

Ash% = (FW-IW/SW-IW) x100

Volatile matter:

Volatile  matter  (VM)  in  coal  refers  to  the  thermal  decomposition  products liberated  when coal  is  heated   at  high  temperature  in  the  absence  of  air. The  VM  obtained  during  the analysis  of  coal  consists  mainly  of   combustible   gases   such   as   hydrogen,   carbon   monoxide,   hydrocarbons,   tar,  ammonia   as   well   as   incombustible  gases  like  carbon dioxide  and steam.  VM  content  may  be  used  to  establish  the  rank  of  coals,   to   provide the   basis   for   purchasing   and   selling,   or   to   establish   burning   characteristics   such   as combustibility  (reactivity)  of  a  coal,  and  ease  of ignition  and  hence   flame  stability.  VM  is determined  by   establishing   the loss  in  weight  resulting   from  heating  a  coal  sample  under rigidly controlled  conditions.   The measured weight loss, corrected  for moisture  as determined using  standard  methods,  establishes  the   volatile  matter content. Take a 1 gm. of coal sample in VM crucible and ignite in muffle furnace for 7 min at 900 deg c in absence of oxygen.

Calculations:

VM% = (SW-FW/SW-IW) x100 – IM%

Fixed Carbon:

Fixed  carbon  in  coal  is  the  carbon  that  remains  in  the  coal  sample  after volatile  matter  is driven  off.  This   differs   from   the   ultimate   carbon   content   of   the   coal   because   some   carbon is lost in   hydrocarbons with volatiles.  The ratio   of   fixed carbon to proximate VM, the   fuel   ratio, is used as a measure   of combustion   reactivity and burnout.  Fixed  carbon is  not determined directly,   it  is calculated  by subtracting   from  100   the resultant  summation  of moisture,  VM  and ash,  all as  a percentage  mass  fraction  on  the  same  moisture   reference basis.

Calculation

FC% = 100 – (IM%+VM%+ASH %)

Gross Caloric Value:

Calorific   value  (CV)  is  a   direct  indication   of   the   heat   content   (energy   value)   of   the   coal.   It  is   one   of   the   most   important   parameters   by   which   the   coal   is   mined   for   combustion   applications,   and   is   the   most commonly  used  benchmark  of  coal  quality and hence  its  economic  value. The  CV  is  usually  expressed  as   the  gross  calorific  value  (the higher heating  value)  or  the  net  calorific  value  (the  lower  heating  value).  The   difference between  the  gross  calorific  value  (GCV)  and  the  net  calorific value  (NCV)  is  the latent  heat of   condensation  of  the  water  vapors produced  during  the  combustion  process.  The  GCV assumes  that  all  of   the vapors   produced   from   coal   moisture  during   the   combustion   process   is   fully   condensed.   The   NCV   assumes  that  the  water  is  removed  with  the combustion  products  without  being  fully  condensed. Standard  methods  for determining  the CV  of  coal  employ  calorimeters  and  burning  coal in oxygen under   pressure  in  a  closed system.  The  bomb  calorimeter  provides  the most  suitable  and  accurate  apparatus  for   determination   of   the   CVs   of   solid   and   liquid  fuels,   and   is   adopted   in   ASTM   D5865 (Standard   test   method  for  gross  calorific  value  of  coal  and  coke)  and  ISO  1928:2009  (Solid mineral fuels – Determination of   gross   calorific   value   by   the bomb   calorimetric method and calculation of net  calorific   value).

The   procedures  specified  in  the  standards  involve  burning  a  weighed portion  of  the analysis  sample  of  coal  in   high pressure  oxygen  in  a  bomb calorimeter  under  specified conditions.  The  effective  heat  capacity  of  the   calorimeter   is   determined   in   calibration   tests   by   combustion   of   certified   benzoic   acid   under   similar   conditions,  accounted  for  in the certificate.  The temperature  measurements  required  for  the  evaluation  of   the   corrected   temperature   rise   are  made   during   a   fore period,   a  main   period   (reaction   period)   and   an   after period.   For   the   adiabatic type   calorimeter   (ASTM,   ISO),   the   CV   is   calculated   from   the   corrected temperature  rises  during  the   fore and  after periods.  For  the  isoperibol (isothermal  jacket)  (ASTM,  ISO)   and   the   static jacket type   calorimeters   (ISO),   the   CV   is   computed   from   the   corrected   temperature   rise   observed  during   the   fore,  main and  after period.  Allowances  are  made   for contributions   from  ignition   energy,   combustion   of   the   fuse(s),   for   thermochemical corrections   and,   if   appropriate,   for   heat   losses   from  the calorimeter  vessel  to  the  water  jacket.  Water  is  added  to  the  bomb  initially to  give  a saturated   vapour   phase   prior   to   combustion,   thereby   allowing   all   the   water   formed   from   the   hydrogen   and   moisture   in   the   sample   to   be   regarded   as   liquid   water.   The   result   obtained   is   the   GCV   of   the analysis   sample  at  constant  volume  with  all  the  water of  the  combustion products  as  liquid  water,  and  is  usually   expressed  in  J/g  (or  kJ/g, MJ/kg), cal/g  (or  kcal/g,  kcal/kg).  Btu/lb  is  also  used  in  some  parts  of  the  world   such   as   the   USA.   In   practice,   fuel   is   burned   at   constant   (atmospheric) pressure   and   the   water   is   not   condensed   but  is   removed  as   vapour with   the   flue  gases.  Under   these   conditions, the   operative   heat   of combustion is   the NCV of   the   fuel at constant pressure.  The standards provide  equations   for  converting   GCV  to  NCV  of  the  fuel  at  constant pressure  and  at consent  volume.

Ultimate analysis:

Ultimate   analysis   determines   the   percentage   mass   fraction   of   the   major   constituents   of   coal   such   as   carbon, hydrogen, sulphur, nitrogen, and, usually by difference, oxygen. The ultimate  analysis  is  used  with   the  calorific  value  of  the  coal  to  perform  combustion calculations  such  as  the  determination  of coal  feed rates,  boiler performance and  sulphur emissions. All ultimate analysis done by CHNS analyzer.

 ADB to ARB Conversion of GCV

ARB GCV % = (100-TM%/100-IM %) X ADB GCV

ADB to Dry Basis conversion of GCV

DB GCV % = (100/100-IM%)x ADB GCV

Conversions – Units

From kcal/kg to MJ/kg multiply kcal/kg by 0.004187

From kcal/kg to Btu/lb multiply kcal/kg by 1.8

From MJ/kg to kcal/kg multiply MJ/kg by 238.8

From MJ/kg to Btu/lb multiply MJ/kg by 429.9

From Btu/lb to kcal/kg multiply Btu/lb by 0.5556

From Btu/lb to MJ/kg multiply Btu/lb by 0.002326