Hydrocarbon Processing - October 2021 - 41

Plant Safety and Environment
TABLE 2. Comparative parameters after stepwise optimization
Values
Parameters
Absorbed heat duty in furnace
Total fuel flowrate
Oxygen level measured
at arch
Fuel quality
Flue gas temperature leaving
the convection section
Flue gas acid dew point
Flue gas temperature leaving
the air preheater
Total flue gas load
CO2 emitted
Step 1: Excess air
optimization
23 MMKcal/hr
3,028 kg/hr
4.3 vol% (dry)
Step 2: Air preheat
system retrofit
23 MMKcal/hr
2,636 kg/hr
3.6 vol% (dry): 20%
excess air
Same as defined in TABLE 1
390°C
131°C
390°C (no APH)
1,333 metric tpd
234 metric tpd
Parameters
TABLE 3. Check case: Hydrogen-rich fuel gas firing
Values
Absorbed heat duty in furnace
Total fuel flowrate
Oxygen level measured at arch
Fuel quality
Flue gas temperature leaving
the heater convection section
Flue gas temperature leaving
the air preheater
Total flue gas load
CO2 emitted
23 MMKcal/hr
1,705 kg/hr
2.4 vol% (dry)
H2: 60 vol%
CH4: 40 vol%
~350°C
135°C (assuming same
exit temperature)
987 metric tpd
95 metric tpd
security issues. It was determined that the study would be incomplete
without evaluating the H2
For the sake of this study, fuel gas with a H2
can be accommodated in an existing furnace
with minor changes in hardware, although it is always recommended
to thermally evaluate the furnace to match this situation.
Results are shown in TABLE 3.
As evident, H2
-rich fuel gas will reduce CO2
-rich fuel gas firing case.
content of 60 vol%
(and residual as methane content) was evaluated, considering
that 60 vol% H2
-rich fuel gas firing shows remarkable reemissions
to < 40%
370°C
131°C
155°C
1,116 metric tpd
204 metric tpd
Step 3: Switchover
to fuel gas firing
23 MMKcal/hr
2,366 kg/hr
3 vol% (dry)
Step 4: Optimizing fuel
gas firing conditions
23 MMKcal/hr
2,304 kg/hr
2.9 vol% (dry)
Fuel gas with following major component composition in mol%:
H2: 25%, N2: 6.5%, CH4: 36%, C2H6: 16%, C2H4: 11.8%; Others: balance
363°C
111°C
155°C
1,040 metric tpd
144 metric tpd
360°C
111°C
135°C
1,012 metric tpd
140 metric tpd
TABLE 4. Critical performance parameters pre- and
post-optimization
Parameters
Radiant duty, MMKcal/hr
Convective heat duty,
MMKCal/hr
Radiant heat flux, Kcal/hr.m2
Maximum calculated tube
metal temperature, °C
Maximum calculated film
metal temperature, °C
Bridgewall temperature, °C
Ex-convection temperature, °C
Baseline condition After Step 4
17.01
5.94
15.42
7.52
33,478
584
514
820
400
36,938
592
515
838
360
sults when evaluated for carbon emissions reductions. Firing
60 vol% H2
of the baseline figure. However, practical limitations of refinery
and commercial fuel pricing must be evaluated before this
strategy can be adopted. Enriching the fuel gas composition by
H2
produced from fossil sources will not help the cause, as the
overall CO2 generated in a hydrogen generation unit (HGU)
may be higher than the reduction achieved in this step.
Critical checkpoint: Evaluation of furnace performance
parameters. As shown in TABLES 1, 2 and 3, the
conversion of a natural draft furnace to a balanced draft furnace-and
further shifting to fuel gas firing-appreciably reduces
flue gas quantity and causes a major shift in the heat
profile within the furnace. A lower flue gas quantity results in
a lower convective heat transfer and, therefore, a higher radiant
heat flux. A higher radiant heat flux leads to a higher metal
temperature and film temperature in the radiant section. This
must be evaluated well before such a conversion is attempted.
A comparison of critical parameters is provided in TABLE 4. An
analysis of these tabulated parameters was critical to ascertain
the following points:
* Although the radiant flux increases as one moves
to Step 4, the impact of these critical parameters
can be absorbed within the case study furnace as
sufficient design margins were already available.
* Tube metal temperature and film temperature are
critical parameters that showed an increase. Although
these can be managed within the existing design (of the
studied furnace), in many cases, the increase in these
parameters may force internal hardware modifications
to operate the furnace within a safe envelope.
* An appreciable increase exists in the bridgewall or
arch temperature, which can be managed within
the existing design in lieu of inherent design
margins and superior tube support metallurgy.
Further, the shift to fuel gas firing augmented with an air
Hydrocarbon Processing | OCTOBER 2021 41
Hydrocarbon Processing - October 2021

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