Hydrocarbon Processing - October 2021 - 40

Plant Safety and Environment
TABLE 1. Baseline data of a refinery furnace
Parameters
Absorbed heat duty in furnace
Total fuel flowrate
Oxygen level measured at arch
Fuel quality
Flue gas temperature leaving
the heater
Total flue gas load
CO2 emitted
Secondly, higher excess air levels increase the mass of flue
Values
23 MMKcal/hr
3,128 kg/hr
6 vol% (dry)
Specific gravity of 1.01 (Type 6 fuel oil)
C/H ratio of 8.196
Sulfur content of 0.5 wt% max
400°C
1,512 metric tpd
242 metric tpd
it is fervently hoped that the technology is soon demonstratively
proven on commercial scale refinery furnaces, alleviating the inherent
apprehensions.
The impact of the first two strategies has been examined for
this case study on a moderate duty furnace commonly found
in oil refineries. As it is proven that a reduction in emissions is
intricately linked to fuel consumption rate-or, in other words,
an increase in operational efficiency-effort has been made to
enhance the furnace efficiency to its best achievable figures.
The most common strategies include:
* Optimizing the current operation and plugging areas
of inefficiency
* Retrofitting the furnace with an air preheat system to
effectively utilize residual heat from outgoing flue gas
* Switching to fuel gas firing from existing fuel oil firing.
The first two steps exemplify efficiency improvement. Fuel
substitution is demonstrated by the third step of fuel switchover,
as well as an additional check case where fuel gas containing
60% hydrogen by volume has been investigated.
OPERATION ANALYSIS
AND BASELINE DEFINITION
A Southeast Asian refiner was operating a furnace with the
conditions detailed in TABLE 1. It was evident from the operating
parameters that ample scope existed for improving performance.
The refinery was facing an uphill task of curbing carbon emissions.
Among many other furnaces in the refinery, the subject
furnace was further investigated with respect to CO2
emissions.
A deeper analysis indicated that the furnace was running at
far from ideal operating conditions. For example, the furnace's
fuel efficiency was a mere 75%, primarily due to the absence of
air preheating or other heat recovery. In fact, the arch oxygen
level of 6 vol% was quite high, which was a consequence of excess
air being maintained at 38%. Therefore, it was evident the
furnace presented ample opportunity to improve on operating
parameters, which will also help in lowering the carbon emissions
from its current level of 242 metric tpd.
The following steps emerged in pursuit of these strategies.
Step 1: Working on current operational lacuna-Excess
air levels. Excess air levels impact the process from multiple directions.
The higher excess air dilutes the flame zone with products
that do not contribute to heat release, resulting in a lower
heat source temperature and reducing heat transfer by radiation.
40 OCTOBER 2021 | HydrocarbonProcessing.com
Step 4: Tightening the noose-Fine-tuning fuel gas firing
parameters. Having established a substantial reduction
in CO2
levels, it was time for the final fine-tuning. The inherently
clean character of refinery fuel gas presented an opportunity
to further enhance heat recovery by cooling the outgoing
flue gas within 25°C of the acidic dew point. For refinery fuel
gas with amine-treating facilities installed, the acid dew point
is generally within 110°C-115°C with a level of 100 ppm hydrogen
sulfide (H2
gas, increasing the sensible heat loss through flue gases. With
more excess air, more heat is wasted in heating it from ambient
temperature to the combustion temperature. Excess air leads
to the burning of additional fuel, which is not desirable from
an operational cost perspective nor from an emissions viewpoint.
Accordingly, the excess air level was adjusted to 25%
from the existing level of 38%; this 25% figure was in accordance
with standard API guidelines, as well as common operating
procedures for natural draft systems.
The results of this optimization exercise are shown in
TABLE 2. It can be seen that excess air optimization-an essentially
zero-investment solution-can lead to a carbon emissions
reduction of *8 metric tpd.
Step 2: Opting for an air preheat system. It was evident
that continuing with the current natural draft system was
insufficient for substantial cuts in emissions. Reducing fuel
consumption was expected to impact emissions, so the installation
of an air preheat system was evaluated in detail. Sometimes,
fuel may be so inexpensive that the installation of an air
preheat system may not be economically justified; however,
emissions reduction will always justify higher heat recovery
investments. Accordingly, an air preheat system was envisaged
for the case studied here and appreciable carbon reductions
were achieved. Note: The excess air level could be trimmed
further to 20% with the use of air preheat systems in lieu of
better control over the combustion medium.
Step 3: Exploring avenues for further reduction-Exercising
the fuel gas firing option. After the evaluation of
an air preheat system, it was decided to utilize a lower-carbon
fuel source. Accordingly, refinery fuel gas was evaluated for
furnace firing. It is vital to judge the overall refinery fuel balance
before proceeding with this fuel shift. A sudden shift to
fuel gas would have created a dearth in the fuel gas network,
leaving the refinery with excess fuel oil. It is important to consider
the use of this left-over fuel oil on a pan-refinery level for
effective utilization. Common residue processing units, such
as a delayed coker unit (DCU), can process this leftover fuel
oil and generate valuable products. A study on fuel gas consumption
was conducted and results are tabulated in TABLE 2.
S). Further, better burning characteristics
and ease of combustion also presented an opportunity to further
optimize the excess air level to within 15%. The results of
this exercise are shown in TABLE 2.
Check case: Hydrogen-rich fuel gas firing. Hydrogen
(H2
) has been touted as the " energy of the future " and a promising
solution to multiple persisting environmental and energy
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