Duplex-A Creative Innovation in Industrial Combustion Technology

Conference Paper

Duplex™  -  A  Creative  Innovation  in  Industrial  Combustion  Technology         Roberto  Ruiz  and  Donald  Kendrick   ClearSign  Combustion  Corporation   Paper  presented  at  the  American  Flame  Research  Committee  Industrial  Combustion  Symposium   Kauai,  HI  -­‐  September  2016     Abstract   Duplex  Technology  has  been  installed  in  a  Vertical  Cylindrical  (VC)  reformer-­‐splitter  reboiler   process  heater  with  a  maximum  firing  capacity  of  11.25  MMBtu/hr  at  a  California  refinery.    The   technology  has  successfully  demonstrated  the  capability  to  operate  with  unmatched  NOx   emissions  of  sub-­‐6  ppm  (corrected  to  3%  O2),  without  external  flue  gas  recirculation  (EFGR)  or   selective  catalytic  reduction  (SCR),  over  a  wide  range  of  refinery  process  conditions.     Background   Duplex  is  a  revolutionary  gaseous  fuel  combustion  technology  that  has  proven  its  capability  to   reduce  environmental  emissions  of  nitrogen  oxides  (NOx),  a  highly  regulated  pollutant  in   industrial  applications,  to  the  very  low  levels  required  by  the  most  stringent  regulations  in  the   country,  while  enhancing  heat  transfer  characteristics.    The  technology  has  been  successfully   implemented  in  Once-­‐Through  Steam  Generators  (OTSGs)  in  Enhanced  Oil  Recovery  (EOR)   applications1,  and  more  recently  in  refinery  process  heaters.     Duplex  incorporates  a  high-­‐temperature  porous  ceramic  matrix  (i.e.,  Duplex  surface)  positioned   at  a  distance  a  few  feet  away  from  where  the  fuel  and  air  are  introduced  in  the  furnace  (Figure   1).    Fuel  and  air  are  thoroughly  mixed  before  reaching  the  Duplex  surface.    The  fuel/air  jet   entrains  internal  flue  gases  diluting  the  mixture.    The  diluted  mixture  is  then  ignited  by  the  hot,   glowing  Duplex  surface  where  the  bulk  of  the  combustion  process  takes  place  within  the  matrix   porous  passages.    Duplex  provides  an  effective  combustion  stabilization,  flame  confinement  and   radiation  source.                     Figure  1.    Schematic  of  Duplex  in  a  VC  heater                                                                                                                     1  "Duplex™  Technology  Demonstrates  Sub-­‐5  PPM  NOx  and  CO  Simultaneously  Without  SCR,  FGR,  or  High  Excess   Air"  -  D.  Karkow,  et.al.,  AFRC  2015  Symposium         1     #12;The  technology  integrates  several  NOx  reduction  techniques  that  combine  to  deliver  extremely   low  NOx  emission  levels:   • Pre-­‐mixed  combustion  behavior.    Fuel  and  air  are  introduced  in  the  furnace  in  a  manner   consistent  with  a  diffusion  flame  burner;  however,  in  contrast  with  conventional  raw   gas  burners  in  which  ignition  takes  place  at  the  burner  throat  immediately  after  the   fuel  and  air  start  mixing,  Duplex  ignition  does  not  take  place  until  fuel  and  air  reach  the   Duplex  surface.    Although  fuel/air  injection  is  configured  as  a  diffusion  system,  fuel  and   air  have  the  opportunity  to  thoroughly  mix  as  they  travel  the  distance  before  ignition   occurs.    The  system  behaves  as  a  premixed  system  with  its  inherent  NOx  reduction   benefits  but  without  the  disadvantages  of  pre-­‐mixed  combustion  (i.e.,  flashback).       • Fuel/air  mixture  dilution.    Internal  flue  gas  is  entrained  along  the  length  the  fuel/air  jet   travels  providing  enhanced  dilution  of  the  jet.    This  dilution  contributes  to  reduced   peak  flame  temperatures  for  better  control  of  thermal  NOx.       • Radiation  cooling.    Because  combustion  primarily  occurs  within  the  Duplex  surface,  the   majority  of  the  energy  transfer  takes  place  as  solid  body  radiation  (gray  body).    Solid   body  radiation  is  a  considerable  more  effective  energy  transfer  mechanism  than  flame   radiation,  the  primary  energy  transfer  in  conventional  burners.    This  is  because  of  the   highly  spectrally  dependent  manner  in  which  gaseous  fuel  flames  radiate  energy.    The   enhanced  radiation  heat  transfer  by  Duplex  contributes  to  thermal  NOx  reduction  by   radiative  cooling  of  the  flame2.     In  addition  to  being  able  to  operate  with  very  low  emissions  of  NOx,  Duplex  has  also  shown  to   provide  other  benefits1:   • Elimination  of  flame  impingement.    A  NOx  reduction  strategy  in  the  design  of  ultra-­‐low   NOx  (ULN)  burners  incorporates  delayed  fuel/air  mixing  (i.e.,  flame  stretching).     Unfortunately,  stretching  flames  creates  buoyant  flames  that  are  often  impacted  by   furnace  currents  or  coalesce  with  adjacent  flames  leading  to  flame  impingement  on   process  tubes  or  shock  tubes  in  the  convection  section.    Duplex  combustion  is  confined   within  the  Duplex  surface  eliminating  any  possibility  for  flame  impingement.   • Enhanced  CO  oxidation.    With  Duplex,  fuel  and  air  are  thoroughly  mixed  prior  to  ignition,   eliminating  fuel  rich  and  fuel  lean  zones  that  may  contribute  to  CO  production  in   conventional  burners.    Furthermore,  Duplex  provides  very  uniform  temperature  on  the   surface  further  enhancing  CO  oxidation.   • Enhanced  radiation  heat  transfer.    As  previously  described,  Duplex  provides  a   considerable  enhancement  in  radiation  heat  transfer  over  conventional  burner  flames   as  it  radiates  energy  as  a  gray  body.    In  addition,  radiation  is  further  improved  by  the   porous  nature  of  the  Duplex  surface.    The  cavities  in  the  porous  surface  enhance   emissivity  properties  of  the  high-­‐temperature  ceramic  material  as  a  result  of  the   apparent  emissivity  phenomena3.     • Noise  reduction.    Laboratory  experiments  have  demonstrated  that  sound  pressures   levels  can  be  reduced  from  an  average  of  85  dB  in  burner  mode  to  an  average  of  72  dB   in  Duplex  mode.    This  is  attributed  to  the  fact  that  the  turbulent  nature  of  combustion  is   essentially  eliminated  in  Duplex  mode  as  laminar  combustion  takes  place  mostly  within   thousands  of  small  cavities  in  the  Duplex  surface.                                                                                                                   2  "Radiative  cooling  in  a  flame  holder  for  NOx  reduction"  -  Breidenthal,  R.;  et.al.,  American  Physical  Society   Division  of  Fluid  Dynamics  Fall  (2014).    Bibliographic  Code:  2014APS..DFDA34005B.   3  "Thermal  Radiation  Heat  Transfer,"  R.  Siegel  &  J.  Howell,  Published  by  Taylor  &  Francis,  4th  Edition,  2002         2     #12;   Duplex  Modes  of  Operation   • Cold  furnace  start  up  and  warm  up  (burner-­‐mode  operation).    Conventional  process   heater  start-­‐up  procedures  are  followed.    That  is,  burner  pilots  are  ignited  followed  by   proof  of  ignition.    Burner  ignition  follows  according  to  standard  refinery  process  heater   and  burner  procedures.      The  heater  operates  in  burner  mode  (i.e.,  flames  stabilized  at   the  burner  throat/tile)  during  the  furnace  and  Duplex  surface  warm  up  period.    Standard   burner  ramp-­‐up  and  furnace  soaking  periods  should  be  followed.   • Transition.    Once  the  standard  heater  warm  up  procedures  have  been  met,  Duplex   surface  temperature  is  verified  to  ensure  it  is  above  the  ignition  temperature  of  the  gas.     There  is  no  direct  ignition  temperature  measurement  but  the  temperature  is  typically   correlated  to  the  heater  bridge  wall  temperature  and/or  the  Duplex  surface  glow.    Once   ignition  temperatures  are  verified,  fuel  supply  to  the  burner  nozzles  is  interrupted  and   fuel  supply  to  the  Duplex  fuel  nozzles  is  started  concurrently  effecting  transition  to   Duplex  operation.    A  few  seconds  later  and  upon  confirmation  of  Duplex  operation,   burner  pilots  are  shut  off.     • Duplex-­‐mode  operation.    Once  transition  has  been  accomplished,  the  heater  can  be   ramped-­‐up  to  its  design  capacity.                    Burner  mode              Transition                            Duplex  mode   Figure  2.    Modes  of  operation       Duplex  Retrofit  in  a  Refinery  Process  Heater   Duplex  Technology  was  retrofitted  in  a  VC  reformer  splitter  reboiler  process  heater  at  a  refinery   in  California.  Project  objective  was  to  meet  NOx  emissions  ≤  6  ppm,  corrected  at  3%  O2                                               (≤  0.007  lbs  of  NOx/MMBtu  )  over  a  wide  range  of  refinery  process  conditions.     The  reboiler  VC  furnace  heats  fluid  fed  from  a  distillation  tower  that  fractionates  process  fluid   fed  from  a  reformer  unit  ("the  unit").    The  heater's  radiant  section  dimensions  are:  outside  shell   diameter  of  9'  6  ½"  and  height  of  17'  8  ½".    Three  natural  draft  ULN  burners  with  a  maximum   capacity  of  3.75  MMBtu/hr  each  are  installed  on  the  furnace  floor  firing  refinery  fuel  gas   vertically  up.    Historical  refinery  fuel  compositions  obtained  over  a  24-­‐week  period  immediately   prior  to  the  initiation  of  the  testing  activities  was  obtained.    A  summary  of  selected  data  is   presented  in  the  table  below:       Maximum   Minimum   Average   H2     (vol.  %  @  STP)   68.7   22.8   43.8   CH4     (vol.  %  @  STP)   55.6   12.3   31.7   LHV   (BTU/scf)   1462   636   892   Table  1.    Fuel  Composition         3     #12;The  retrofit  into  the  VC  process  heater  took  less  than  2  days  of  furnace  downtime  and  it   encompassed  the  following:   • Modifications  to  the  existing  burners  were  made  to  make  the  existing  UNL  burners   compatible  with  Duplex  operation.    The  existing  fuel  manifolds,  risers  and  tips  were  kept   intact  and  separate  Duplex  fuel  manifolds,  risers  and  tips  were  installed.    Figure  3  is  a   schematic  representation  of  the  burner  modifications  that  were  made  to  the  existing   ULN  burners  installed  in  the  VC  heater.    A  doughnut-­‐shaped  fuel  manifold  was  added  at   the  bottom  of  each  burner.    Each  Duplex  manifold  delivers  fuel  to  four  risers  with   Duplex  tips  positioned  in  the  burner  throat  within  the  core  of  the  air  supply.    Valves  in   each  burner  fuel  supply  lines  were  installed  to  provide  independent  fuel  supply  to   accommodate  burner-­‐mode  operation  and  Duplex-­‐mode  operation.     • Duplex  surface.    A  Duplex  surface  ceramic  structure  resting  on  supports  welded  to  the   furnace  shell  was  installed.    The  high-­‐temperature  porous  ceramic  matrix  rested  on  this   structure  (Figure  4).   • Instrumentation.    Additional  valving  and  instrumentation,  as  required  for  Duplex   operation,  was  installed  for  proper  control  and  safety  monitoring.    This  included  a  UV   scanner  to  detect  flame  on  the  Duplex  surface,  and  oxygen  control.         Figure  3.    Duplex  Burner  Modifications       Duplex  Surface Ceramic structure Support (welded  to furnace  shell) Figure  4.    Duplex  surface             4     #12;Results   After  Duplex  was  installed  and  commissioned,  data  was  collected  to  verify  NOx  performance   under  typical  process  conditions  experienced  at  the  refinery.    NOx  data  was  obtained  using  a   Testo  350  analyzer  with  a  low-­‐NOx  cell.    Flue  gas  samples  were  drawn  from  the  furnace  stack   and  were  conditioned  using  a  sample  dryer  with  a  fast  loop.    Wet  oxygen  data  was  obtained   using  an  in-­‐situ  zirconium  oxide  oxygen  probe  installed  downstream  of  the  convection  section.     More  than  100  data  points  were  obtained  during  a  six-­‐week  period.    Data  points  presented   below  represent  averaged  daily  data.         The  thermal  load  in  the  reboiler  process  heater  is  dependent  on  the  reformer  unit  process   conditions.    Because  of  this,  refinery  operations  requested  to  monitor  Duplex  operation  as  a   function  of  the  unit's  charge  rate  (barrels  per  day  -  bpd).     Daily  unit  and  reboiler  charge  rates  were  kept  at  constant  values  and  experienced  only  minor   daily  deviations.     A  summary  of  results  is  presented  in  the  following  graphs:     NOx  (corr.  @  3%  O2)   7   6   5   4   3.7  ppm  average   3   2   1   0   1100   1200   1300   1400   Bridge  Wall  Temperature  (°F)   1500       Figure  5.    NOx  vs.  Bridge  Wall  Temperature         5     #12;NOx  (corr.  @  3%  O2)   7   6   5   4   3   2   1   0   2.5   3.0   3.5   4.0   4.5   O2  (%)     Figure  6.    NOx  vs.  Zirconium  Oxide  Oxygen  Probe  Measurement     NOx  (corr.  @  3%  O2)   7   6   5   4   3   2   1   0   1300   1600   1900   2200   2500   Unit  Charge  Rate  (bpd)     Figure  7.    NOx  vs.  Reformer  Unit  Charge  Rate           6     #12;Heater  Charge  Rate  (bpd)   4450   4400   4350   4300   4250   4200   4150   1300   1600   1900   2200   Unit  Charge  Rate  (bpd)   2500   Figure  8.    Reboiler  Heater  Charge  Rate  vs.  Reformer  Unit  Charge  Rate     NOx  results  met  and  exceeded  project  objectives.    In  addition,  average  CO  emissions  were  25   ppm  (corrected  @  3%  O2)  and  were  consistently  lower  than  50  ppm  (below  regulatory   requirements)  in  spite  of  the  relatively  low  bridge  wall  temperature  conditions.  Furthermore,   Duplex  performance  was  very  stable  over  all  process  conditions  encountered  during  the   evaluation  period.         The  NOx  data  is  scattered  and  does  not  appear  to  reveal  any  trends.    However,  the  dynamic   conditions  typically  encountered  in  refinery  operations  in  which  thermal  loads,  fuel  heating   values  and  other  process  variables  that  may  change  rapidly  and  may  not  be  monitored   continuously,  make  it  difficult  to  establish  any  trends.     The  pictures  below  document  Duplex  surface  glow  that  can  also  be  used  as  a  qualitative   measure  of  optimum  Duplex  performance.         7     #12;   Figure  9.    Duplex  Surface  Glow.   Viewing  port  located  on  furnace  shell  (left)  and  viewing  port  located  on  furnace  floor  (right)     Conclusions   Duplex  is  a  revolutionary  and  highly  innovative  technology  that  has  been  successfully   implemented  in  a  refinery  VC  process  heater.    Duplex  demonstrated  its  capability  to  consistently   operate  with  NOx  emissions    ≤  6  ppm,  corrected  at  3%  O2  (≤  0.007  lbs  of  NOx/MMBtu)  over  a   wide  range  of  process  conditions.    This  emissions  performance  satisfies  the  most  stringent  NOx   regulations  and  is  un-­‐matched  by  any  other  retrofit  combustion  technology.    It  also  provides  a   very  attractive  and  competitive  option  against  SCR  systems.     Duplex  is  also  being  considered  by  other  refineries  as  a  solution  to  eliminate  flame  impingement   on  furnace  process  tubes  brought  about  by  long  and  buoyant  flames  characteristic  of  ULN   burners.               8