How to Measure Flare Vent Gas Composition and Net Heating Value

Paper from the AFRC 2015 conference titled How to Measure Flare Vent Gas Composition and Net Heating Value

Developing regulations and recent consent decrees aim to ensure that assisted flares operate with high combustion efficiency (CE) or destruction efficiency (DE), consequently minimizing emissions of uncombusted materials (particularly volatile organic compounds (VOCs)). Currently, there is no proven technology that can be permanently installed at a plant to continuously and reliably measure flare CE and DE. Due to this limitation, regulations and recent consent decrees rely on the measurement of flare operating parameters such as vent gas composition, vent gas net heating value, vent gas flow rate, and assist media (i.e., steam or air) flow rate. These measurements are used to calculate other flare operating parameters and these calculated parameters are correlated to CE and DE. Several of these parameters have been identified from recent flare tests. The most effective of these parameters are determined "in the combustion zone"-that is, the calculated parameters take into account the addition of steam or air assist. The two parameters that correlate best with CE and DE are:...

How  to  Measure  Flare  Vent  Gas  Composition  and   Net  Heating  Value:     A  Review  of  Existing  Instrumentation                   Prepared  for  the  American  Flame  Research  Council   8/25/15       Prepared  by     Dan  Pearson     How  to  Measure  Flare  Vent  Gas  Composition  and  Net  Heating  Value                                          Page  2         Overview   Developing  regulations  and  recent  consent  decrees  aim  to  ensure  that  assisted   flares  operate  with  high  combustion  efficiency  (CE)  or  destruction  efficiency  (DE),   consequently  minimizing  emissions  of  uncombusted  materials  (particularly  volatile   organic  compounds  (VOCs)).  Currently,  there  is  no  proven  technology  that  can  be   permanently  installed  at  a  plant  to  continuously  and  reliably  measure  flare  CE  and   DE.  Due  to  this  limitation,  regulations  and  recent  consent  decrees  rely  on  the   measurement  of  flare  operating  parameters  such  as  vent  gas  composition,  vent  gas   net  heating  value,  vent  gas  flow  rate,  and  assist  media  (i.e.,  steam  or  air)  flow  rate.   These  measurements  are  used  to  calculate  other  flare  operating  parameters  and   these  calculated  parameters  are  correlated  to  CE  and  DE.     Several  of  these  parameters  have  been  identified  from  recent  flare  tests.  The  most   effective  of  these  parameters  are  determined  "in  the  combustion  zone"  -  that  is,  the   calculated  parameters  take  into  account  the  addition  of  steam  or  air  assist.  The  two   parameters  that  correlate  best  with  CE  and  DE  are:     1.) Net  Heating  Value  of  the  Combustion  Zone  (NHVcz)   NHVcz  is  a  calculated  term  representing  the  net  heating  value  of  all  components   in  the  combustion  zone.  This  includes  total  assist  media  to  the  flare  tip.  The   combustion  zone  is  directly  above  the  flare  tip  and  is  the  point  at  which  all   materials  mix  and  combust.  The  NHVcz  is  therefore  the  resultant  heat  content   from  the  mixture  of  the  vent  gas  from  the  flare  header  and  the  total  assist  media.   Air  inspirated  into  the  combustion  zone  with  steam  assist  is  not  included.  NHVcz   is  calculated  as  follows.       𝑄!" ∙ 𝑁𝐻𝑉!" 𝑁𝐻𝑉!" = Where:   𝑁𝐻𝑉!"   𝑄!"   𝑄!"   𝑁𝐻𝑉!"     =   =   =   =   𝑄!" + 𝑄!"   Equation  1   combustion  zone  gas  net  heating  value  (Btu/scf)   volumetric  vent  gas  flow  rate  (scf/hr)   volumetric  assist  media  flow  rate  (scf/hr)   net  heating  value  of  vent  gas  (Btu/scf)   ! 𝑁𝐻𝑉!" = 𝑥! ∗ 𝑁𝐻𝑉!   Equation  2   !!! Where:   𝑁𝐻𝑉!"   𝑥!   𝑁𝐻𝑉!   𝑛       =   =   =   =   net  heating  value  of  vent  gas  (BTU/scf)   concentration  of  individual  component  in  vent  gas  (vol  fraction)   net  heating  value  of  pure  individual  component  i  (Btu/scf)   number  of  individual  pure  components  in  vent  gas       How  to  Measure  Flare  Vent  Gas  Composition  and  Net  Heating  Value                                          Page  3         2.) Lower  Flammability  Limit  of  the  Combustion  Zone  (LFLcz)   This  is  the  calculated  lower  flammability  limit  after  assist  media  has  been  added.   Air  inspirated  into  the  combustion  zone  with  steam  assist  is  not  included.  LFLcz   is  a  calculated  parameter  and  is  not  the  "true"  LFL  in  the  combustion  zone  since   gas  heating,  mixing,  and  air  inspiration  are  not  taken  into  account.  There  are   many  documented  instances  in  flare  tests  of  combustion  occurring  well  below   the  LFLcz  level.  There  is  a  direct  mathematical  relationship  between  NHVcz  and   LFLcz  -  i.e.,  one  may  be  mathematically  transformed  into  the  other  when   hydrogen  is  taken  into  account.  LFLcz  is  calculated  as  follows.         𝐿𝐹𝐿!" = Where:   𝐿𝐹𝐿!"   𝑄!"   𝑄!!   𝐿𝐹𝐿!"     =   =   =   =   =   =   =   =   𝑄!" ∙ 𝐿𝐹𝐿!"   Equation  3   lower  flammability  limit  of  combustion  zone  gas  (volume  fraction)   volumetric  vent  gas  flow  rate  (scf/hr)   volumetric  assist  media  flow  rate  (scf/hr)   lower  flammability  limit  of  vent  gas  (volume  fraction)   𝐿𝐹𝐿!" = Where:   𝐿𝐹𝐿!"   x!   𝐿𝐹𝐿!   𝑛     𝑄!" + 𝑄!" 1   ! 𝑥! ! 𝐿𝐹𝐿 ! Equation  4   lower  flammability  limit  of  vent  gas  (volume  fraction)   concentration  of  individual  component  in  vent  gas  (volume  fraction)   lower  flammability  limit  of  individual  component  i  (volume  fraction)   number  of  individual  pure  components  in  vent  gas     Once  NHVcz  or  LFLcz  is  known,  CE  and  DE  can  be  estimated.  The  calculations  show   that  vent  gas  volumetric  flow  rate,  assist  media  volumetric  flow  rate,  and   composition  are  required  for  these  calculations.  Another  approach  is  the  direct   measurement  of  NHVvg,  which  can  be  used  to  determine  NHVcz.  This  paper   evaluates  different  technologies  that  can  be  used  to  measure  vent  gas  composition   or  net  heating  value.             How  to  Measure  Flare  Vent  Gas  Composition  and  Net  Heating  Value                                          Page  4         Technologies   Complex  gas  mixtures  comprise  flare  vent  gas  streams.  These  gas  mixtures  typically   include  hydrocarbons,  nitrogen,  hydrogen,  and  sulfur  containing  compounds.  As  the   speed  and  accuracy  of  vent  gas  composition  or  net  heating  value  measurements   improve,  the  ability  of  a  facility  to  optimize  its  flare  performance  improves  as  well.   Instruments  based  on  the  following  analytical  methods  are  currently  available  to   characterize  vent  gas  composition  or  net  heating  value:     1.) Gas  Chromatography   2.) Calorimetery   3.) Mass  Spectrometry   4.) Raman  and  FTIR  Spectrometry     Gas  Chromatograph  (GC)   At  this  time,  a  GC  is  the  instrument  most  commonly  used  to  measure  flare  vent  gas   composition.  A  GC  operates  by  separating  the  individual  compounds  of  a  complex   gas  matrix  by  injection  of  the  sample  into  a  narrow  column  and  subsequently   detecting  them  using  a  variety  of  available  detectors.  The  variation  in  chemical  and   physical  properties  of  different  compounds  in  a  gas  mixture  cause  the  individual   compounds  to  elute  at  different  intervals.  Several  factors  can  be  adjusted  to   optimize  the  ability  of  a  GC  to  separate  compounds  in  a  gas  mixture.  These  factors   include  carrier  gas  flow  rate,  the  number  of  columns,  column  lengths,  and   temperature.     The  primary  advantages  of  a  GC  are  that  it  can  measure  all  vent  gas  compounds  and   that  many  instrument  technicians  are  already  familiar  with  the  technology.  The   primary  disadvantage  of  using  a  GC  is  that  the  analytical  cycle  is  much  longer  (5-­‐20   minutes)  than  any  of  the  other  technologies  discussed  in  this  report.  This  long   analytical  cycle  limits  the  effectiveness  of  using  NHVcz  or  LFLcz  for  controlling  flare   performance.  In  addition  to  the  long  cycle  time,  the  practicality  of  a  GC  is  also   limited  by  the  extensive  the  calibration  requirements  necessary  to  generate  quality   data.  The  monthly  calibration  of  a  GC  requires  a  wide  range  of  calibration  gases.   Calibration  gases  can  either  be  pure  component  or  blends,  but  all  measured   components  of  the  vent  gas  must  be  contained  in  the  calibration  gases.     Calorimeter   A  calorimeter  is  an  instrument  that  can  be  used  to  measure  the  net  heating  value  of   flare  vent  gas  directly.  There  are  different  types  of  calorimeters  based  on  different   operating  principles  for  determining  net  heating  value  of  the  sample  gas  mixture.   The  two  primary  types  of  calorimeters  are  metered  flame  calorimeters  and  residual   oxygen  calorimeters.         How  to  Measure  Flare  Vent  Gas  Composition  and  Net  Heating  Value                                          Page  5         The  primary  advantage  of  a  calorimeter  is  that  it  is  relatively  inexpensive   instrument,  especially  when  compared  to  the  other  technologies  evaluated  in  this   report.  Additionally,  a  calorimeter  requires  only  a  single  reference  gas  for   calibration.  A  disadvantage  of  using  a  calorimeter  for  vent  gas  net  heating  value   measurement  is  that  the  vent  gas  cannot  be  speciated.     The  U.S.  EPA  has  allowed  the  use  of  a  hydrogen  adjusted  net  heating  value  of  1212   Btu/scf  rather  than  the  actual  value  of  274  Btu/scf  when  calculating  vent  gas  net   heating  values  from  vent  gas  composition  data.  Because  the  vent  gas  hydrogen   concentration  is  not  measured  with  a  calorimeter,  flare  operators  cannot  take   advantage  of  this  adjustment  factor  without  the  use  of  additional  instrumentation.     Mass  Spectrometer  (MS)   A  mass  spectrometer  (MS)  can  be  used  to  measure  flare  vent  gas  composition  and  is   currently  being  used  on  flare  systems  at  select  refineries.  This  instrument  ionizes   the  sample  gas  mixture  and  detects  the  charged  fragments  by  measuring  the  mass-­‐ to-­‐charge  ratio  (m/z).     The  primary  advantage  of  using  of  using  a  MS  for  flare  vent  gas  measurement  is  the   rapid  analytical  cycle.  The  short  analytical  cycle  is  ideal  for  controlling  flare   performance.  The  primary  disadvantages  of  using  a  MS  is  that  some  instruments   have  difficulty  distinguishing  compounds  with  similar  mass-­‐to-­‐charge  ratios  and   sulfur  compounds,  and  some  instruments  may  be  limited  by  the  number  of   compounds  that  can  be  measured  simultaneously.  However,  these  claims  vary   between  vendors.  Additionally,  a  MS  requires  the  same  effort  and  range  of   calibration  gases  as  a  GC.     Raman  /  FTIR  Dual  Spectrometer   A  Raman  /  FTIR  dual  spectrometer  (Raman/FTIR)  uses  both  Raman  and  FTIR   spectroscopy  to  measure  flare  vent  gas  composition.    The  Raman  Spectrometer  uses   a  laser  to  excite  the  molecules  of  a  gas  mixture  and  measures  the  inelastic  scattering   of  the  light  to  determine  composition  of  the  gas  mixture.  The  FTIR  spectrometer   measures  gas  composition  by  collecting  high  resolution  infrared  absorbance  spectra   and  comparing  them  to  known  references.     The  primary  advantages  of  using  an  instrument  that  combines  Raman  and  FTIR   analytical  methods  are  that  every  vent  gas  compound  can  be  measured  and  the  fast   analytical  cycle  is  ideal  for  controlling  flare  performance.  A  Raman  /  FTIR  uses  a   Raman  spectrometer  for  measuring  homonuclear  diatomics  (nitrogen,  oxygen,   hydrogen,  etc.)  and  other  hydrocarbons  found  in  percent  level  concentrations.  The   FTIR  spectrometer  can  be  used  to  measure  compounds  found  in  parts  per  million   level  concentrations.  An  FTIR  spectrometer  obtains  a  data  point  roughly  every   second  while  the  Raman  obtains  a  data  point  roughly  every  minute.         How  to  Measure  Flare  Vent  Gas  Composition  and  Net  Heating  Value                                          Page  6         In  contrast  to  other  analytical  methods,  a  Raman/FTIR  requires  only  two  calibration   gases  (one  for  each  spectrometer)  in  order  to  obtain  quality  data.  The  Raman/FTIR   is  the  only  instrument  capable  of  measuring  the  full  spectrum  of  components  found   in  flare  vent  gas  with  at  a  speed  sufficient  to  effectively  control  flare  performance.     Instrument  Summary  Table   The  table  below  summarizes  the  key  aspects  of  the  four  types  of  instruments   discussed  above.     Instrument   Measures   Calibration  Gases   Response  Time   Gas  Chromatograph   (GC)   All  vent  gas   compounds   Either  pure   component  or  gas   blends;  must  contain   300-­‐1,200  seconds   all  vent  gas   compounds   Calorimeter   Vent  gas  net  heating   value   Single  reference  gas   Mass  Spectrometer   (MS)   Either  pure   component  or  gas   Hydrocarbon  groups,   <  1  second  per   blends;  must  contain   diatomics   compound   all  vent  gas   compounds   Raman  /  FTIR  Dual   Spectrometer   All  vent  gas   compounds   Two  calibration   gases   3-­‐30  seconds   FTIR  :  1  second   Raman:  60  seconds   Table  1:  Instrumentation  Summary     Summary   There  is  currently  no  instrument  that  can  be  permanently  installed  at  a  facility  and   used  to  measure  flare  CE  and  DE  on  an  ongoing  basis.  The  most  effective  way  to   control  a  flare  is  to  have  instrumentation  and  control  logic  necessary  to  calculate   either  NHVcz  or  LFLcz.  There  are  several  instruments  available  that  are  capable  of   measuring  either  flare  vent  gas  composition  or  flare  vent  gas  net  heating  value.  The   calibration  requirements,  components  measured,  analytical  cycle,  and  cost  all  vary   so  the  optimum  instrument  may  vary  from  flare  to  flare.