Hydrocarbon Processing - February 2022 - 40

Bio-Based Processing
straight-through (one burner, one chamber)
configuration, the reaction furnace
temperature is strictly a function of the
process chemistry. The extra CO2
from
the biofeedstock only cools the furnace.
When processing SWAG, if the resulting
furnace temperature is expected to be
less than the 1,250°C target value, then
more ammonia will leave the furnace
and increase the risk of ammonia salt
formation and plugging (FIG. 9) in the
downstream waste heat boiler and subsequent
equipment.
Although increasing the furnace temperature
is possible by warming the feed
gas(es) and combustion air with preheaters,
there is a considerable capital investment
in addition to the extra operating
cost and maintenance complications.
Alternatively, the temperature can be increased
by co-firing with some type of
fuel gas or by oxygen enrichment of the
combustion air stream.
Fuel gas is also used in the SRU for
startup, hot standby and shutdown operations,
as a reliable source of energy
to manage the furnace temperature. The
fuel may be natural gas, refinery fuel gas,
hydrogen, etc. Whichever gas is used, it is
critical that it is of constant composition
to maintain good process control. Large
fluctuations in the gas composition, and,
thus, combustion air demand, will interfere
with control of the H2
S:SO2
ratio
target, meaning lost recovery efficiency
and other possible dangers. If continuous
co-firing is selected as the heating source,
the design of the burner must first be reviewed.
Next, ensure the SRU's maximum
throughput or hydraulic load can handle
the extra load from co-firings, as co-firing
will add a large amount of inert gas to the
SRU. The added dilution also reduces the
sulfur recovery efficiency, which may be
unacceptable. Finally, the CO2
emissions
of the facility will increase, which may be
a concern in some locations.
A cleaner and better way to improve
the furnace temperature is to increase the
oxygen concentration of the combustion
air. Oxygen enrichment has been used
successfully in SRUs that are processing
lean (i.e., low concentration H2
S and more
CO2) acid gas to achieve the desired furFIG.
10. Views of a straight-through furnace
configuration (A) and a split-flow furnace
configuration (B).
nace temperature. However, this requires
some modification to the system. The target
oxygen concentration will determine
the extent of the changes required-lowlevel
oxygen is quite a simple addition,
while greater concentrations (up to 100%
oxygen) require a special burner and materials
with a dedicated oxygen lance.20
The source and cost of oxygen must also
be considered. It is important to check the
turndown capability of the burner, since
the addition of oxygen to the process increases
not only the flame temperature,
but also the speed of combustion. Usually,
systems equipped with oxygen enrichment
require more throughput to ensure that
the burner operates in a safe regime to protect
the metal and refractory components.
Split-flow furnace. In the split-flow
process, the SWAG containing the ammonia
is processed directly through the
main burner with a portion of the AAG.
The balance of the AAG is sent to the rear
zone of the furnace and is used to control
the temperature in the front zone (FIG. 11).
The CO2
with biofeedstock processing, may reduce
the effectiveness of the bypass design
since this increases the gas volume without
adding heat of combustion. If the target
temperature cannot be achieved with
the split-flow design alone, then co-firing
or oxygen enrichment can be used.
The rear zone of the furnace must
also be hot enough to destroy the BTEX
in the bypassed acid gas. This requires a
minimum burn temperature of 1,050°C,21
but this threshold was determined using
straight-through reaction furnaces only
with sufficient residence time, which is
another serious process consideration.
Conservatively, hotter operation of the
second zone is recommended, but BTEX
destruction can only be safely confirmed
by field analytical testing.
The impact of biofeed co-processing
on SRU reaction furnace temperature,
and the effect of mitigation methods
(such as natural gas co-firing, oxygen enrichment
and operation with a split-flow
furnace) are discussed further in this article's
case study.
Although more CO2
in the feed(s)
will form more carbonyl sulfide (COS)
in the reaction furnace, losses associated
with this species can be managed in the
first Claus converter with temperature
control, catalyst choice and catalyst activity.
Similarly, more CS2
is formed when
FIG. 11. Acid gas bypass rate vs. front-zone reaction furnace temperature in a
SWAG-processing SRU.
40 FEBRUARY 2022 | HydrocarbonProcessing.com
co-firing, which is also managed in the
first converter. For the many facilities
that have an amine-based TGTU after
the Claus section, virtually all losses from
content of the SWS, especially
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - February 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - February 2022
