University  of  South  Florida        

         

   

Investigation of Methods and Processes to Increase Efficiency for Carbon Activation Processes

     

 

 

 

 

 

 

 

 

 

 

 

 

 

Joshua  Dang  

ENC  3246  

Dr.  Dianne  Donnelly  

June  20,  2014  

  2  

Abstract:     The   demand   for   activated   carbon   keeps   growing   while  supply   will   dwindle.     As   a   result   economic   feasibility   of   such   an  important   material   will   become   nonexistent.     Alternative   sources   of  carbon   rich   raw   material   need   to   be   used,   along   with   innovative  methods  of  activating  carbon,  which  currently  is  a  high  energy  and  high  cost   endeavor.   As   a   result,   possible   improvements   could   be   made   to  increase   the   overall   efficiency   of   the   activation   process,   increase  effectiveness  of   activated   carbon  desired   characteristics,   and  decrease  the   detrimental   impacts   to   the   environment.     These   current  improvements   when   applied   together   within   the   process   chain   will  allow   for   greater   stability   of   raw   resources,   make   activated   carbon  pound  for  pound  more  effective,  and  preserve  the  environment.    

   INTRODUCTION  The  material  known  as  activated  carbon  affects  countless   lives.    The  application  of  this  material   can  be   found   in  products   throughout  household   items,   including  soft  drinks  and   shampoos   to   large-­‐scale   industries   including   removal  of  mercury   from  natural  gas,  and  carbon  dioxide  from  fermentation  processes  [8].      The  reason  to  the  wide  application  of  activated  carbon   is  due   to  molecular  carbons  special  structure  characteristics,   which   has   a   great   capacity   and   affinity   for   impurities.   Activated  carbon   is   not   found   naturally   with   these   characteristics   but   needs   to   be   created  through  an   extended  process.     It   is   said   that   any   carbon   rich   raw  material   can  be  used   as   a   precursor   to   activated   carbon   [1];   however   current   raw   materials   are  mainly  sourced  from  coal  and  wood  [9],  and  are  activated  through  physical  means,  which   require   a   high-­‐energy   input.     These   current   sources   and   methods   present  several   issues   both   economically   and   environmentally.     Activation   of   carbon   is   a  high  energy  and  cost  endeavor;  however  as  research  continues  for  more  sustainable  sources   such   as   agricultural   byproducts,   viable   processing   methods   such   as  chemical  activation,  and  recycling  techniques  such  as  regeneration  of  used  activated  carbon,   the   cost   will   decrease   as   well   as   an   increased   impact   to   preserve   the  environment.     An   investigation   of   the   activated   carbon   process   will   result   in  measures   to   increase   the   overall   efficiency   of   the   activation   process,   increase  effectiveness  of  activated  carbon  desired  characteristics,  and  decrease  detrimental  impacts  to  the  environment.      BACKGROUND  History    The  earliest  use  of  activated  carbon  has  been  lost  in  history  [10].  It  is  believed  that  the  earliest  application  dates  back  to  3750  B.C.  where  activated  carbon  was  used  by  ancient  Hindus  in  India  as  a  process  for  water  filtration  [11].  The  first  documented  

  3  

use  of  activated  carbon  was  found  on  Egyptian  papyrus  dating  back  to  1500  B.C.  as  a  method  to  absorb  unpleasant  odors  [10].        The  desirable  characteristic  of  activated  carbon  have  been  known  for  more  than  one  and   a   half   millennia,   however   its   main   application   today   are   still   targeted   at  fundamentally  similar  organic  impurities.  As  late  as  the  18th  century  sources  of  raw  carbon   were   derived   from   blood   and   animals,   which   were   then   used   to   purify  liquids  [11].    Documented  uses  of  activated  carbon,  which  became  noted  in  medical  journals,   were   a   treatment   for   ingested   poisons   [10].     Early   uses   were   also   for  medicinal  purposes,  and  most  widely  accepted  in  the  19th  century  were  uses  found  in   the   treatment   of   poultices,   sloughing   ulcers,   and   gangrenous   sores   [10].   Some  noticeable   improvement   pertains   to   the   manufacturing   process   the   produces   a  different   shape   and   size   of   activated   carbon.     These   different   shapes   allow   for  longevity  of  the  carbon  purification  performance  as  well  as  improved  shipping  and  handling  durability.    At  the  beginning  of  the  twentieth  century  activated  carbon  was  only   available   in   a   powder   form.     During   the   First  World  War   granular   activated  carbon  was   used   in   gas  mask   to   capture   deadly   organic   gases   [11],   this   granular  processing   eventually   lead   to   the  widespread  manufacturing  of   granular   activated  for  other  applications  such  as  water  treatment,  and  gas  purification.      The  wide  application  and  available  sources  of  activated  carbon  throughout  history  and   until   this   day   is   a   testament   to   the   imperative   usefulness   of   this   material.  Through  activated  carbons  intrinsic  physical  and  chemical  properties  the  usefulness  has  been  experienced  and  applied  to  a  vast  array  of  situations.    Very  prominent  to  this  day   is   the  application  of  activated  carbon  to  purify  both  gaseous  and  aqueous  phases  of  substances  to  prevent  environmental  harm.      Current  Application    Widespread   uses   of   activated   carbon   can   be   found   in   industrial,   pharmaceutical,  water  treatment  processes.      Great  focus  is  put  on  the  protection  of  the  environment  and  the  health  effects  from  emission  of  gases  and  waste  products  in  industrial  and  manufacturing   processes.     These   emissions   include   volatile   organic   compounds  (VOCs)  and  are  known  to  cause  cancer   in  tested  animals  [12].    Activated  carbon  is  vital   in   the   many   processes   that   involve   VOC’s   to   stay   within   EPA   regulation   of  emissions   for   health   concerns.     Impurities   in   an   aqueous   phase   are   also   an  important  consideration  in  many  products.    These  organic  impurities  are  in  the  form  of  chemical  solvents  used   in  production  processes   for  products  such  as  paints  and  household   cleaners   [12].    Activated   carbon  due   to   its   ability   to   conduct   electricity  can   also   be   found   as   a   catalyst   in   many   vital   electronic   components   including  batteries,  supercapacitors,  and  fuel  cells   [7].    A  major  use  of  activated  carbon   is   in  the  purification  of  water  for  human  consumption.    When  using  this  absorbent  in  the  water  purification  process   it   is   layer  after  sand  and  before  chlorination  [11].      The  use   of   activated   carbon   not   only   decreases   bad   odors   and   taste   but   also   removes  harmful   contaminants   found   in   the   water   sources,   such   as   synthetic   organic  

  4  

compounds   (SOC).     These   current   applications   are   a   constant   issue,   as   nations  economical   and   industrial   development   demands   more   activated   carbon   to  continually  produce  quality  products.        Molecular  Characteristics    Activated  carbon  has  a  desired  physical  and  chemical  structure  due  to  the  porosity  and   surface   area   different   to   non-­‐activated   carbon.   The   porosity   describes   the  amount   of   microscopic   cavity   between   carbon   molecules   and   affects   the   total  surface  area  per  unit  mass.    Another  important  aspect  is  the  pore  size,  which  is  due  to   the   process   of   activation   as  well   as   the   raw   source   for   carbon.     Impurities   can  range   from  one  one  thousand  of  a  micron  to  ten  thousand  microns.    To  effectively  capture   these   contaminants   a   correct   pore   size  must   be   used   to   allow   for   proper  mechanical   fit,   which   is   then   preceded  my   chemical   interactions.     Surface   area   is  directly  related  to  the  capacity  to  hold   impurities.      To  achieve  a  high  surface  area  the  pore  structure  must  be  extensive,   in   that  many  channels  are  present   [2].     It   is  evident   that   the   level   of   desired   molecular   characteristics   can   be   altered.     This  provides  a  variable  in  the  effort  to  increase  the  overall  efficiency  of  the  production  of  activated  carbon.    PROCCESSING  RAW  CARBON  Current  and  Alternative  Sources    Current  raw  carbon  sources  as  a  precursor  are  from  coal  and  wood.     In  the  recent  pass,  the  selected  source  must  meet  several  of  the  following  requirements.    It  must  have  the  potential   to  produce  high  quality  activated  carbon,  which   is  a   function  of  the  porosity  and  resulting  surface  area.     It  must  have   large  available  supplies;   this  will   in   effect   lower   the   cost.     And   finally   it  must   have   the   ability   to   be   stored   for  extended  periods  of  time  [3].    Both  coal  and  wood  have  passed  these  conditions  as  leading   sources   for   the   production   of   activated   carbon,   however   an   important  requirement   have   been   dismissed   in   past   decisions.     This   important   neglected  requirement   is   how   does   sourcing   of   this  material   effect   the   environment?    With  130,000   tons   per   year   of  wood   and   100,000   tons   per   year   of   raw  material   being  harvested,  the  impact  to  the  environment  is  one  of  great  magnitude.      Wood  and  coal  are  currently  the  leading  source  for  the  production  of  carbon.    Coal   comes   from   surface   and   underground  mines,  with   the  majority   from   surface  mines  at  sixty  percent  [13].    Coal  is  considered  a  non-­‐renewable  source  due  to  the  time   it   takes   to   create   it.     Estimates   have   said   supplies   of   coal  will   last   only   until  2035  [14].  This  estimates  does  not   take   into  account   the  coal   that   is   too  deep  and  costly   to  mine.    This  presents  an   issue   to   the  economics  of  using  coal  as  a   source.    Alternative  sources  must  be  researched  and  tested  to  keep  supplies  of  raw  carbon  stable.     Without   proper   preparation   a   sudden   decrease   in   supply   will   trigger  

  5  

staggering  price  hikes,  which  will  effect  how  companies  operate,  prices  of  products,  and  could  even  shut  down  industrial  processes  and  slow  the  economy.    Alternative   sources   need   to   meet   all   constraints   previously   set   as   well   as   an  additional   constraint   of   sustainability.     This   will   be   the   basis   to   analyzing   the  potential  of  new  sources.        A  study  on  the  use  of  corncobs  proves  the  economic  feasibility  and  sustainability  of  this  agricultural  waste  byproduct  as  a  potential  source.    In  a  published  article  from  American  Chemical  Science  Journal  the  use  of   corncobs  have   two  main  advantages,  the   first   is   the   wide   availability,   and   second   is   the   intrinsic   thermodynamics  properties  of  corncobs  [15].    There  is  a  vast  amount  of  corncobs  that  are  wasted  in  the   production   of   food   and   ethanol.     A   51%   portion   of   total   U.S.   grown   corn   is  dedicated  to  the  production  of  food  and  ethanol;  within  these  productions  only  the  kernels  are  used  [16].    These  waste  products  can  be  recycled  and  processed  to  into  a  high   value   material   of   activated   carbon.     The   thermodynamic   characteristics   of  corncobs  allows   for   “a   low  carbonization   temperature  compared   to  other  biomass  residues”   [15].     This   allows   for   a   lower   temperature   during   the   activation   stage  where  all  the  undesired  components  existing  within  the  raw  material  are  vaporized.    Vaporization  of  any  material  takes  a  great  amount  of  energy;  this  is  due  to  how  heat  is   distributed   within   a   substance.     The   energy   input   is   converted   into   thermal  energy,  which   then   flows  down  a  gradient  of   temperature  differences.    Only  when  the  gradient  is  at  equilibrium  at  the  boiling  point  of  the  substance  does  vaporization  initiate.   Therefore,   corncobs   with   low   carbonization   temperature   will   allow   for   a  lower  input  of  thermal  energy.    This  material  has  potential  as  an  alternative  source.    The  source  of  municipal   refuse   is  numerous   in  supply.      This  refers   to  solid  waste  consisting   of   everyday   trash   and   garbage.     The   process   to   which   the   raw   refuse  originates   is   through   the   sorting   out   of   glass   and   metal   leaving   a   source   full   of  carbonaceous  material   ready   to   be   activated   [5].     The   desired   characteristic   from  the  municipal  refuse  was  on  the  same  standard  as  those  that  are  from  coal  and  wood  [5].    Pass   considerations   for  using   refuse  have  been  disregarded  due   to   the   cheap  and  highly  available  supplies  of  wood.    The  economics  and  profit  margins  were  the  key  driving  force  to  choosing  less  sustainable  sources.  However  as  supplies  of  coal  and   the   ever-­‐increasing   cost   to   produce   lumber   these   readily   available   waste  sources  will  become  comparatively  economical.      Activation  Methods    Without   an   improvement   to   chemical   and   physical   characteristics   of   the   carbon  precursor,  the  effectiveness  of  carbon  as  an  absorbent  could  be  17  to  25  times  less  absorbent.    The  possible  range  is  due  to  the  source  of  raw  materials  used,  which  is  a  factor   in   the  pore  structure  and  surface  area.    Activation   is  also  crucial   in  creating  certain  structure,  which  have  a  greater  affinity  for  targeted  impurities.    Activation  is  done   either   physically   or   chemically;   however,   both   techniques   have   been   used  

  6  

simultaneously  to  yield  even  higher  absorption  and  adsorption  capacities  [5]  at  the  expense  of  higher  cost.  

   

Figure 1: Processes for chemical and physical activation [5].    In  figure  1  both  physical  and  chemical  process  are  outlined.  A  physical  activation  is  also   referred   to   as   a   thermal   activation,   due   to   required   high   temperature  conditions.   Physical   means   of   activation   generally   required   two   steps,   as   seen   in  above  figure.    The  first  step  is  carbonization.    This  involves  pyrolysis  in  the  absence  of  oxygen,  which  is  the  breakdown  of  the  raw  carbon  rich  organic  matter  [6].    This  is  done  with  a  high-­‐energy  input  to  raise  temperatures  to  a  level  in  which  a  precession  of  vaporization  of  volatile  components  are  possible.    To  achieve  a  condition  without  the   presence   of   oxygen,   inert   gases   are   pumped   into   the   system.     Inert   gases   are  non-­‐reactive   agents;   this   prevents   side   reactions,   which   is   desired   for   the  conservation  of  pure  carbon.      The  result  of  pyrolysis  is  a  reduction  of  raw  material,  but  also  an  increase  in  the  quality  and  purity  of  carbon  atoms  [3].    The  second  stage  is   activation;   this   process   is   carried   out   with   oxygen   or   steam.     The   purpose   of  activation  is  to  increase  the  porosity  of  the  structure  as  well  as  increase  the  surface  area.     High-­‐energy   cost   due   to   high   temperature   processes   is   associated  predominately  with  physical  means  of  activation.        In  contrast  to  a  high  temperature  process,  chemical  means  of  activation  is  a  one  step  process  and  allows  for  carbonization  at  a  significantly  lower  temperature,  as  a  result  

  7  

there  is  a  greater  porous  structure  [7].    However,  chemical  precursors  are  needed,  which  are  typically  an  acid,  or  strong  base  [4].    Raw  materials  are  impregnated  with  the   chemicals,   which   begins   the   process   of   removing   the   impurities   through  dehydration   which   effects   pyrolytic   decomposition   of   impurities.     Chemical  activation   allow   for   less   thermal   energy   to   be   expended,   however   washing   to  remove  the  impregnated  chemicthals  and  drying  are  required  [5].    These  chemicals  are  a  hazard  to  the  environment  if  not  recycled  and  reused.    The  potential  to  recycle  pyrolytic   chemical   present   an   advantage   over   physical   means   of   activation.     The  economics  associated  with  recycle  stream  within  industrial  application  saves  fresh  material,  which  is  directly  associated  with  lower  cost  of  purchasing  these  chemicals.    For   every   unit   of   mass   activated   carbon   is   created   there   is   small   portion   of  chemicals  that  are  required.    The  ability  to  use  the  fraction  of  the  activated  carbon  product  to  remove  the  impurities  of  the  chemical  for  reuse  of  the  chemical  presents  a   possible   solution   to   lower   energy   consumption   within   the   activated   carbon  process.    Taking  into  account  the  large  surface  area  that  is  created  due  to  activation,  only  a  small  portion  of  the  total  product  needs  to  be  used  in  the  recycling  process  of  the  pyrolytic   chemicals;  Overall,   chemical   activation   if   a  more  viable  method   than  physical  activation.    ENERGY  &  ENVIROMENT    Energy  Consumption    Energy  considerations  for  processing  will  be  discussed  starting  from  the  sourcing  of  raw   materials.     The   mining   of   coal   in   itself   is   a   high-­‐energy   process.     The   large  machinery   and   transportation   of   raw   carbon   accounts   for   most   of   the   energy  expenses  in  the  extraction  of  coal.    Its  estimated  that  15%  of  the  production  cost  is  due  to  transportation  and  mining  of  coal  alone  [14].        Energy  consumption  of  wood  as  precursors  also  account  for  a  significant  portion  of  the   production.     Shipping   of   wood   from   less   rural   area   and   overseas   present  significant   fuel   usage.   Temporary   bridges   must   be   built   over   small   rivers   and  streams   to   gain   access   to   depleting   supplies   of   hard   woods.     This   takes   large  equipment  to  get  to  these  areas.    After  the  wood  is  harvested  it  has  to  be  dried,  this  requires  a  kiln,  which  is  a  thermally  insulated  chamber  where  heat  can  be  added  to  evaporate  water  [18].    High-­‐energy  inputs  are  required  with  vaporization  processes.    A  physical  activation  technique  requires  great  amount  of  expended  energy  to  raise  temperature  to  pyrolyze  all  the  undesired  substances.    The  needed  temperature  of  physical   activation   is   on   a   scale   of   magnitude   twice   that   required   of   a   chemical  activation.  High  temperatures  are  needed  in  the  two-­‐step  process  of  carbonization  and   activation,   which   is   approximately   1000˚C   and   700˚   C   for   carbonization   and  activation   processes,   respectively.   It’s   intuitively   known   and   explained   by   the   2nd  law  of  thermodynamics  that  heat  flows  from  high  temperature  to  low  temperatures.    If  a  substance  at  a  lower  temperature  needs  to  be  at  a  state  of  higher  temperature,  energy  from  the  surroundings  must  be  inputted  in  to  the  system.    This  is  the  main  

  8  

reason   to   experienced   high   operational   cost   associated  with   a   physical   activation  process.        Chemical   activation   is   a  more   economical   and  energy   efficient  method   to  physical  activation.    This  method  also  requires  energy  inputs  to  raise  temperature  in  order  to  start  reactions,  however  these  temperature  are  well  below  at  approximately  500  ˚C  depending  on  certain  raw  materials.    Another  advantage  of  a  chemical  means  is  the  one  step  process,  which   lowers  cost  of   special  equipment  and   larger   facilities.  For  every   action   there   is   a   reaction,   and   with   energy   consumption   there   is   a  environmental  impact.    Environmental  Effects  of  Activation  Methods    Through   a   physical   means   of   activation   high   temperatures   are   required   for  extended  periods  to  vaporize  impurities  within  the  raw  material.    The  implication  of  this   method   is   a   high-­‐energy   consumption,   and   therefore   high   environmental  effects.    The  method   to  heating  of   the  carbon  precursor   is  done   though  resistance  heat  coil  or  through  the  burning  of  natural  gas  [5].    Both  of  these  heating  techniques  have  detrimental   impacts  on   the  environment.    With  heat   coils,   electricity   is  used.    Electricity   is   produce  mainly   from   nonrenewable   resources,   with   39%   coal,   27%  natural   gas,   and   19%   nuclear.     These   sources   of   energy   pollute   the   environment  with  the  harmful  emissions  such  as  nitrogen  oxide  (NOx),  which  is  known  to  cause  cancer   in   animals,   destroy   natural   environments,   and   affect   the   health   of   the  ecosystem  [17].          A   chemical   means   of   activation   requires   far   less   energy   than   with   thermal  activation.    With  300,000  tons  a  year  of  activated  carbon  produce  solely   for  water  treatment  processes,  a  huge  impact  can  be  clearly  seen  in  small  energy  saving  [5].    A  potential  detrimental  effect  to  the  environment  can  be  seen  in  the  use  of  chemicals  to  pyrolyze  raw  carbon.    Possible  contamination  can  happen  due  to  waste  chemicals  not   being   handled   properly,   however  with   EPA   regulations   it  would   be   rare.   This  case   would   also   be   unlikely   because   it   is   uneconomically   to   waste   high   value  solvents;  a  common  practice  would  be  to  recycle  and  reuse.    Lower  energy  and  use  of  recyclable  solvents,  chemical  activation  is  a  better  for  the  environment.    DISSCUSSION  &  SUMMARY  A  Cost  Effective  and  Sustainable  Model    As   a   result   of   the   investigation  possible   improvements   could  be  made   to   increase  the  overall  efficiency  of  the  activation  process.    A  sustainable  model  dictates  the  use  of  a  recycling  method.     In   figure  2  below,  a  comparison  of   the  main  aspects  of   the  process   is   depicted.     It   all   starts   with   sustainable   sources   those   that   would   have  otherwise   been  wasted   such   as   corncob   and  municipal   refuse   to   list   just   the   few  possibilities.    With  supplies  of  current  carbon  precursors  becoming  scarce,  this  will  provide   the   shift   in   economical   feasibility  of  waste   sources.    The  benefits  of  using  

  9  

waste   byproducts   will   positively   impact   emission   levels,   natural   habitat  preservation,  and  save  natural  resources.    The  energy  saved  from  initial  production  cost  of  equipment,  labor,  and  transportation  will  then  be  decreased  due  to  recycle  of  the  material  from  waste  byproducts.          Next   the  activation  of   carbon  will  be  processed  with  a  chemical  pyrolysis  method.    This   will   provide   significant   lower   thermal   energy   cost   than   current   physical  methods  due  to  chemical  reactions   instead  of  vaporizations.    The  ability  to  recycle  chemical   activating   solvent   will   provide   yet   another   advantage   in   economically  feasibility  and  sustainability.    With  chemical  reactions  the  ability  to  control  precise  characteristics  in  pore  structures  will  help  to  increase  the  effectiveness  of  activated  carbon  for  specific  materials  and  situations.                                                          

Figure 2: A comparison of the process chain in the production of activated carbon. On the left is the how the majority of activated carbon is currently produced, and on the right is a more efficient process with sustainable sources and efficient activation methods.

  10  

Conclusion  

The   gap   is   widening   and   the   rising   cost   of   current   sources   is   soaring.     With  resources  becoming  scarce  and  difficult   to  access   the  economics  of   the  production  will  not  allow  current  prices  and  profits.    Following   this   standard  sustainable  and  efficient   model,   supply   shortages   and   environmental   harm   will   be   avoided.     The  ecosystem   as   a   whole   is   being   affected,   but   with   new   highly   efficient   activation  techniques   and   sustainable   sources,   the   detrimental   effect   of   the   destruction   to  forest,   and  natural   reserves  will  be   limited.    With   the  described  cost   effective  and  sustainable  model,  this  guideline  provides  a  path  to  increase  the  overall  efficiency  of  the   activation   process,   increase   effectiveness   of   activated   carbon,   and   decrease  detrimental  impacts  to  the  environment.                                                                  

  11  

   References:    [1] J. T. Nwabanne and P. K. Igbokwe, STATISTICAL OPTIMIZATION OF

PRODUCTION OF ACTIVATED CARBON DERIVED FROM OIL PALM EMPTY FRUIT BUNCH [Online], 04 ed., Nigeria: Nnamdi Azikiwe University: Department of Chemical Engineering, 2013. Available: http://www.journalcra.com/sites/default/files/Download%202171.pdf

[2] Y. Chiang, P. Chiang, and C. Huang, Effects of pore structure and

temperature on VOC adsorption on activated carbon [Online], Singapore: Nanyang Technological University, 2000. Available: http://ntur.lib.ntu.edu.tw/bitstream/246246/96682/1/16.pdf

[3] Michael David Sufnarhki, Regeneration of Granular Activated Carbon

[Online], Texas: The University of Texas at Austin, 1999. Available: http://www.dtic.mil/dtic/tr/fulltext/u2/a362534.pdf

[4] M. K. Stevenson, J. O. Leckie, Preparation and Evaluation of Activated

Carbon Produced from the Municipal Refuse, California: Standard University Department of Civil Engineering, 1972. Available: http://books.google.com/books?id=N9EDAAAAIAAJ&printsec=frontcover&dq=production+of+activated+carbon&hl=en&sa=X&ei=pziKU6-iHsORqgar9oGQDQ&ved=0CEQQ6AEwAQ#v=onepage&q=production%20of%20activated%20carbon&f=false

[5] A. H. Abdullah, A. Kassim, Z. Zainal, D. Kuang, F. Ahmad, and O. S.

Wooi, Preparation and Characterization of Activated Carbon from Gelam Wood Bark (Melaleuca cajuputi), Malaysia: University of Malaysia, 2000. Available: http://www.researchgate.net/publication/237464297_Preparation_and_Characterization_of_Activated_Carbon_from_Gelam_Wood_Bark_(Melaleuca_cajuputi)/file/72e7e52d69b5bbc478.pdf.

[6] Burning of wood, Finland: VTT Technical Research Centre of Finland,

2000. Available: http://virtual.vtt.fi/virtual/innofirewood/stateoftheart/database/burning/burning.html

  12  

[7] B. Viswanathan, P. Indra Neel and T. K. Varadarajan, Methods of Activation and Specific Applications of Carbon Materials, Chennai: NATIONAL CENTRE FOR CATALYSIS RESEARCH DEPARTMENT OF CHEMISTRY INDIAN INSTITUTE OF TECHNOLOGY, 2009. Available: http://nccr.iitm.ac.in/e%20book-Carbon%20Materials%20final.pdf

[8] Gas Processing with Activated Carbon, Chemviron Carbon, 2014.

Available: http://www.chemvironcarbon.com/en/applications/process/gas-

processing [9] Raw Materials Used in the Production of Activated Carbon, Virginia

Polytechnic Institute and State University, 2014. Available: http://www.webapps.cee.vt.edu/ewr/environmental/teach/gwprimer/group23/acraw_materials.html

[10] THE HISTORY OF ACTIVATED CARBON, Cabot Norit Activated

Carbon, 2014. Available: http://www.norit.com/carbon-academy/the-history-of-activated-carbon/

[11] Ferhan Cecen, Water and Wastewater Treatment: Historical Perspective of

Activated Carbon Adsorption and its Integration with Biological Processes, Wiley, 2011. Available:

http://www.wiley-vch.de/books/sample/3527324712_c01.pdf [12] Volatile Organic Compounds (VOCs), United States Environmental

Protection Agency. Available: http://www.epa.gov/iaq/voc.html [13] Environmental impacts of coal power: fuel supply, Massachusetts: Union

of Concerned Scientists. Available: http://www.ucsusa.org/clean_energy/coalvswind/c02a.html

[14] ZANE SELVANS, WARNING: FAULTY REPORTING ON US COAL

SUPPLIES, Colorado: Clean Energy Action, 2013. Available: http://cleanenergyaction.org/2013/10/30/warning-faulty-reporting-on-us-coal-supplies/

  13  

[15] J. M. Ketcha, D. J. D. Dina1, H. M. Ngomo1 and N. J. Ndi, Preparation and Characterization of Activated Carbons Obtained from Maize Cobs by Zinc Chloride Activation, Douala Cameroon: American Chemical Science Journal, 2012. Available: http://www.sciencedomain.org/download.php?f=1353500396-Ketcha%20et%20al_242012ACSJ1806.pdf&aid=689.

[16] Gary W. Brester, Corn, Montana: Agricultural Marketing and Resource

Center, 2012. Available: http://www.agmrc.org/commodities__products/grains __oilseeds/corn_grain/

[17] Natural Gas, Environmental Protection Agency: Agricultural Marketing and Resource Center, 2013. Available: http://www.epa.gov/cleanenergy/energy-and-you/affect/natural-gas.html

[18] Phil Mitchell, LUMBER PROCESSING EFFICIENCY, YIELD AND

VALUE, North Carolina: North Carolina State University. Available: http://www.ces.ncsu.edu/nreos/wood/RMOG/Yield.htm