Effective heat of combustion and HRR

[mcgratta]

Discussed in #16334

^{Originally posted by AmirrezaHBSH June 11, 2026}
Hello,

I would greatly appreciate your help with a question regarding the calculation of the effective heat of combustion and the area under the HRR curve in FDS. According to the FDS User Guide, the total heat released, represented by the area under the HRR curve, should be equivalent to: m(1-nu_char nu_ash)\Delta h_eff

As discussed in the sample case WUI/char_oxidation_1.fds, this equivalence appears to be satisfied when a single-step combustion reaction is used. However, in my case, the vegetative fuel produces multiple gaseous fuels during pyrolysis, including CO, CO2, CH4, and H2. For this case, the integrated HRR is approximately 6300 kJ, whereas the effective heat of combustion calculated from the species yields is approximately 17,000 kJ. This discrepancy is significantly larger than expected.

For the calculation of (\Delta h_c) in Eq. 17.28 of the User Guide, I used the following formulation:

\Deltah_c (total) = (NU_SPEC_i + NU_SPEC_i,HM)* \Deltah_i, which accounts for the contributions of both the light gas species produced directly during pyrolysis and the gaseous species generated from heavy molecular cracking. Could you please advise what might cause the discrepancy between the integrated HRR and the effective heat of combustion in this case? Is there an additional consideration when multiple gaseous fuels are included?

&REAC FUEL = 'HEAVY MOLECULAR'
SPEC_ID_NU = 'HEAVY MOLECULAR','CARBON MONOXIDE','CARBON DIOXIDE','METHANE','HYDROGEN','SOOT',
NU = -1, 0.402, 0.002, 0.153, 4.742, 12.227
HEAT_OF_COMBUSTION = 42 /

&REAC FUEL='CARBON MONOXIDE',
SPEC_ID_NU='CARBON MONOXIDE','AIR_COMB','CARBON DIOXIDE','NITROGEN',
NU=-1, -2.380952, 1.0, 1.880952 /

&REAC FUEL='HYDROGEN', SPEC_ID_NU='HYDROGEN','AIR_COMB','WATER VAPOR','NITROGEN',
NU=-1, -2.380952381 , 1 , 1.880952381/

&REAC FUEL='CARBON DIOXIDE', SPEC_ID_NU='CARBON DIOXIDE','CARBON MONOXIDE','OXYGEN',
NU=-1, 1 , 0.5/

&REAC FUEL='METHANE',
SPEC_ID_NU='METHANE','AIR_COMB','CARBON DIOXIDE','WATER VAPOR',
'CARBON MONOXIDE','HYDROGEN','NITROGEN',
NU=-1, -8.619047619, 0.872341, 1.747659, 0.127659, 0.252341, 6.809047619 /

&MATL ID = 'MOISTURE'
DENSITY = 1000
CONDUCTIVITY = 0.6
SPECIFIC_HEAT = 4.190
A = 600000
E = 48200
N_T = -0.5
NU_SPEC = 1
SPEC_ID = 'WATER VAPOR GAS'
HEAT_OF_REACTION = 2259 /

&MATL ID = 'DRY VEGETATION'
DENSITY = 544
ALLOW_SHRINKING = F
N_S = 3.342
CONDUCTIVITY = 0.46
SPECIFIC_HEAT = 2.19
A = 0.0346
E = 7.9E4
NU_MATL = 0.21
MATL_ID = 'CHAR'
NU_SPEC(1,1) = 0.0636
SPEC_ID(1,1) = 'CARBON DIOXIDE'
NU_SPEC(2,1) = 0.1332
SPEC_ID(2,1) = 'CARBON MONOXIDE'
NU_SPEC(3,1) = 0.0202
SPEC_ID(3,1) = 'METHANE'
NU_SPEC(4,1) = 0.003
SPEC_ID(4,1) = 'HYDROGEN'
NU_SPEC(5,1) = 0.57
SPEC_ID(5,1) = 'HEAVY MOLECULAR'
HEAT_OF_REACTION = 336/

&MATL ID = 'CHAR'
DENSITY = 300
CONDUCTIVITY = 0.05
SPECIFIC_HEAT = 1.6
SURFACE_OXIDATION_MODEL = T
A = 465
E = 68000
NU_MATL = 0.12
MATL_ID = 'ASH'
SPEC_ID = 'AIR_COMB','PROCHAR'
NU_SPEC = -7.17,8.05
HEAT_OF_REACTION = -25000 /

&MATL ID = 'ASH'
DENSITY = 67
CONDUCTIVITY = 50.1
SPECIFIC_HEAT = 1/

[mcgratta]

Jason -- for the case posted in the Discussion post, there is 0.05312 kg of dry vegetation to start with, which generates 824 kJ of heat from both the gas phase and char oxidation reactions. This corresponds to an effective HoC of about 15512 kJ/kg. However, when you look at the input file, there are actually 5 gas phase reactions involving various amounts of CO, H2, CO2, CH4, and C12H10. The HoC for the CO2 reaction is given as -6430 kJ/kg in the .out file. I cannot figure out how the effective HoC is being calculated in this case. I'm not sure what FDS even does with a negative HoC.

[drjfloyd]

On leave this week will look at it when I return. Looking at the first post if you are referring to the reaction of CO2->CO+1/2O2, that is endothermic and should have a negative HoC.

…

On Mon, Jun 22, 2026, 13:36 Kevin McGrattan ***@***.***> wrote: *mcgratta* left a comment (firemodels/fds#16351) <#16351 (comment)> Jason -- for the case posted in the Discussion post, there is 0.05312 kg of dry vegetation to start with, which generates 824 kJ of heat from both the gas phase and char oxidation reactions. This corresponds to an effective HoC of about 15512 kJ/kg. However, when you look at the input file, there are actually 5 gas phase reactions involving various amounts of CO, H2, CO2, CH4, and C12H10. The HoC for the CO2 reaction is given as -6430 kJ/kg in the .out file. I cannot figure out how the effective HoC is being calculated in this case. I'm not sure what FDS even does with a negative HoC. — Reply to this email directly, view it on GitHub <#16351?email_source=notifications&email_token=ADBUZ4NZPO2IW2INNTVVF3L5BFVBVA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINZXGEYTAMZVGE3KM4TFMFZW63VGMFZXG2LHN2SWK5TFNZ2KYZTPN52GK4S7MNWGSY3L#issuecomment-4771103516>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ADBUZ4MIIM4FZMMCKSL724D5BFVBVAVCNFSNUABEKJSXA33TNF2G64TZHMZTOMJUGY3DEMR3JFZXG5LFHM2DOMJZGA3DCNZYG2QXMAQ> . Triage notifications, keep track of coding agent tasks and review pull requests on the go with GitHub Mobile for iOS <https://github.com/notifications/mobile/ios/ADBUZ4P5E3NAK4KLXVIEURD5BFVBVA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINZXGEYTAMZVGE3KM4TFMFZW63VGMFZXG2LHN2SWK5TFNZ2KUZTPN52GK4S7NFXXG> and Android <https://github.com/notifications/mobile/android/ADBUZ4JEBBGGPBOGSNGEAYL5BFVBVA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINZXGEYTAMZVGE3KM4TFMFZW63VGMFZXG2LHN2SWK5TFNZ2K4ZTPN52GK4S7MFXGI4TPNFSA>. Download it today! You are receiving this because you were assigned.Message ID: ***@***.***>

[mcgratta]

The HoC is -6430 kJ/kg. But the bigger issue is that I cannot determine how FDS is calculating the HRR in the _hrr.csv file.

[drjfloyd]

That value looks like it is correct. It is approximately 10000 kJ/kg for CO->CO2. In the other direction the kJ/mol value is the same but 1 kg of CO2 is less moles by 28/44.

Is it that the units do not look correct for the HoC in the HEAVY MOLECULAR?

&REAC FUEL = 'HEAVY MOLECULAR'
SPEC_ID_NU = 'HEAVY MOLECULAR','CARBON MONOXIDE','CARBON DIOXIDE','METHANE','HYDROGEN','SOOT',
NU = -1, 0.402, 0.002, 0.153, 4.742, 12.227
HEAT_OF_COMBUSTION = 42 /

Also some reactions are using AIR and some O2 (NU_SPEC_ID are tracked and not primitive) but there is no O2 unless the CO2 decomposes.

[drjfloyd]

Oh never mind that first reaction is a decomposition.