Improved Acid Gas Burner

1 © Foster Wheeler 2013. All rights reserved.

Improved Acid Gas Burner Design Debuts in Major European Refinery Upgrade

Scott Kafesjian, PE Foster Wheeler

10876 S River Front Parkway, Suite 250 South Jordan, UT USA 84095

SUMMARY

Increasingly strict environmental regulations, including reduced SO2, CO, and NOx emissions from sulfur recovery units, present ongoing challenges to refiners world-wide. Meeting these regulations is not a matter of choice, it is imperative as a condition to operating the refinery. One of the key refinery units subject to stringent emission regulations is the Sulfur Recovery Unit (SRU), which converts hydrogen sulfide produced in processing units into high purity elemental sulfur. An SRU must perform its process intent as designed, exhibit the utmost in process and personnel safety, and include individual equipment items that have been demonstrated to function with a high degree of availability.

Foster Wheeler sulfur recovery technology was recently installed in a European refinery as part of a major facility upgrade that greatly increased the capacity and ability to process sour crude. The sulfur recovery area included four parallel Claus trains of 535 mt/day combined capacity. To meet required overall recovery and SO2 emission limits, further tail gas processing was done in two parallel sub-dewpoint units, which extend the Claus reaction at reduced temperature. Two tail gas incinerators with waste heat recovery boilers handled the tail gas from the sub-dewpoint units. Low-level oxygen enrichment capability was also a design requirement to allow full design capacity to be met in the event of an outage of one Claus train.

These four Claus units featured the first installation of Foster Wheeler’s recently improved proprietary acid gas burner. The burner has been demonstrated to be successful in processing high-NH3 feed streams in the Claus units, as was the case in the design of this unit. The background of the burner, design studies, CFD modeling, and design improvements and benefits will be reviewed and discussed in this paper. The improvements address the discovery of a potential burner operational issue and design modification to eliminate the potential problem.

INTRODUCTION

More stringent regulatory standards for air emissions, coupled with the long-term trend towards the processing of more heavy sour crude feed as a margin-improver, have created economic challenges for refiners. Substitution of heavy sour crude feed improves operating margins, but significant additional processing steps are needed to produce on-spec products, and sulfur removal and recovery becomes a key operation. In particular, given that SRU capacity is adequate, the efficiency and reliability of the SRU become very high priority design requirements. High sulfur recovery efficiency is required to meet allowed SO2 emission limits, and reliability is critical since, in many cases, operating permits do not allow


flaring except in emergencies. As a result, an SRU outage due to an unreliable design or equipment mis-operation can cause refinery throughput, hence profitability, to be curtailed. Equipment in a SRU is specialized and unique. Proprietary technology that is not commonly available is applied to much of the design of SRU equipment.

Foster Wheeler has recently acquired proprietary technology for all aspects of sulfur recovery. The acquired technology includes proprietary processes, equipment design methods and fabrication details, and in-depth know-how in amine treating of sour gases and liquids, amine regeneration, sour water stripping, Claus sulfur recovery, hydrogenation/amine based tail gas treating, tail gas incineration, alternate tail gas treating processes, sulfur degassing, and sulfur handling. By adding sulfur recovery to its technology portfolio, Foster Wheeler offers complete heavy oil upgrading facilities, including ancillary units, to clients in the refining industries.

The Claus sulfur recovery unit (SRU), shown schematically in Figure 1, is the “heart” of the refinery sulfur block. Acid gas streams containing H2S and lesser amounts of ammonia and CO2 are fed to the acid gas burner at the front end of the process. The reaction furnace, or thermal reactor (depending on terminology used), operation is critical to the overall operation and reliability of the entire SRU. Functions of the reaction furnace include:

• Partial oxidation of H2S to SO2 to create the correct ratio of reactants to accomplish the sulfur forming reaction 2 H2S + SO2> 2 H2O + 1/x Sx,

• Destruction/conversion of contaminants in feed (NH3, hydrocarbons, BTX, etc.) to innocuous compounds,

• Convert H2S and SO2 to elemental sulfur vapor, • Provide a means for achieving thorough mixing of all reactants and products in a

complex, kinetically-limited reactor volume, • Operate safely and contain an aggressive, high-temperature process stream containing

numerous highly toxic, highly flammable chemical constituents for up to 5 years between shutdown/turnaround.

If the SRU is the heart of the sulfur block, the acid gas burner is the heart of the SRU. Without a reliable, predictable burner that performs its intended function as designed, most aspects of process performance that the SRU is intended to accomplish will not be achieved. A poorly performing SRU that does not meet its design parameters can be a serious impediment to the operation and profitability of the entire refinery.


Figure 1: Schematic representation of a three-reactor Claus Sulfur Recovery Unit Claus technology is generally well-known and mature, but incremental improvements continue to be made in the areas of equipment design, controls and instrumentation, catalysts, reliability, and safety. As an example, this paper discusses one such incremental improvement that has recently been implemented in Foster Wheeler’s proprietary Acid Gas Burner. Burner history and key design features are presented, and the motivation and methods used to evaluate design improvements are described. Finally, the first installation of the improved burner is discussed, including process requirements and performance testing results.

FOSTER WHEELER ACID GAS BURNER HISTORY

The Foster Wheeler acid gas burner design was conceived and developed over 40 years ago because existing burners were found to be unsuitable for operation in the environment of the SRU thermal reactor. Typically, off-the-shelf burner designs were used in early SRUs.The SRU thermal reactor presents unique operating conditions that compromised the performance of these burners. The need to operate with fuel or natural gas during startup and shutdown, widely varying stoichiometry (air/fuel ratio), elevated operating pressure, very high operating temperatures, and the presence of contaminants such as ammonia, aromatic hydrocarbons, and other difficult to combust species resulted in unreliable operation, limited operating flexibility, and unacceptably short burner life.


A burner design was conceptualized, developed, tested and validated that eliminated the problems noted above and allowed for significant design flexibility. The new acid gas burner was first installed in an SRU in 1973. The resulting operational success demonstrated that a new standard for acid gas burner reliability and performance was set. In 1976, a patent on the acid gas burner was granted to its designers. Over 150 burners have been put in service since the initial commercial installation.

BURNER DESIGN HIGHLIGHTS

The conceptual design of the Foster Wheeler acid gas burner is shown in Figure 2 below. Advantages and benefits of the design are numerous, including

• Elimination of nearly all radiative heating of burner metal parts • No refractory required due to self-cooling design • Operates on acid gas or natural/fuel gas alone or co-fired • Sub- and super-stoichiometric operation • Moderate pressure drop on air and acid gas • Unobstructed view of thermal reactor interior and waste heat boiler inlet • Significant mixing to enhance contaminant destruction • High energy pilot remains in place during operation • Wide turndown range in excess of 5:1 has been demonstrated • Meets pressure vessel design codes • Flame scanner and sight ports remain clear and cool without purges • Easy operator access to flame scanner, sight port, pilot • Custom design tailored to each specific unit and operating regimes

Figure 2: Conceptual cutaway sketch of Foster Wheeler acid gas burner


A typical installation in a refinery SRU is shown in Figure 3. This figure illustrates the simple arrangement and good accessibility of the sight glass, pilot, and flame scanners. Figure 4 is an actual photograph taken by aiming a camera through the sight glass and illustrates what an operator would see in observing the operation: a clear view of the burner interior, injection holes, pilot, burner tile, thermal reactor checkerwall, and waste heat boiler inlet. It is highly beneficial to the start-up, operation, and shutdown of the SRU to have a clear view of the thermal reactor interior. A clear view of the waste heat boiler inlet allows identification of tube leakage caused by refractory or ceramic ferrule damage and subsequent overheating of tube-to-tubesheet joints.

Figure 3: Typical Foster Wheeler acid gas burner installation with high energy pilot

Figure 4: View of operating Foster Wheeler acid gas burner through sight glass with boiler inlet tube sheet visible beyond checker wall

With minor design alterations, the Foster Wheeler burner can be applied in tail gas unit feed heater and reducing gas generating service. In these services, the burner is designed for the preferred fuel, usually natural gas, due to its stable composition and minimal content of heavier hydrocarbons. Oxygen breakthrough and soot formation are potential problems with burners operating in the highly sub-stoichiometric mode that is required by these applications as well as the SRU thermal reactor. The Foster Wheeler burner has been demonstrated to provide very long life, no soot formation when operated according to guidelines, and no catalyst degradation due to these phenomena.

BURNER STUDIES – CFD MODELING

Over the course of time since the initial commercial installation, minor improvements have been made to the burner, primarily focused on improving the mechanical integrity and ease of fabrication. The basic design and configuration remain essentially unchanged, given the long and successful operating history, with over 150 burners installed. Many burners have been in service over 10 years, with no reported problems.


Long-term trends and future projections for the refining industry, as well as the challenges refiners are facing due to environmental regulations, economic pressures, and reliability issues, motivated Foster Wheeler to undertake a review and reevaluation ofthe burner design. In general, much of the world’s crude oil supply has been and will continue trending toward being heavier and higher in sulfur and nitrogen. Regulations limiting the sulfur content of transportation fuels have become more prevalent world-wide, following the lead of Europe and the US in reducing emissions from mobile sources. The future will bring additional sulfur limitations on heavier fuels such as heavy fuel oil, bunker fuels and perhaps jet fuel. More countries will undoubtedly move toward low-sulfur or ultra-low sulfur fuels for an ever-increasing number of vehicles. To meet these regulations, more and deeper hydroprocessing capacity is required, resulting in increased H2S production. Nitrogen, when present in hydroprocessed streams, is converted to NH3, which ultimately is routed to the SRU as a component of the acid gas produced when stripping sour water effluents. NH3 presents additional challenges to the acid gas burner. If not destroyed to a sufficiently low level in the thermal stage of the SRU, NH3 can cause significant operating problems in downstream equipment where temperatures are lower.

In many regions, environmental regulations restrict or prohibit flaring. In the past, an SRU outage may have been tolerable because the acid gas feed could be flared while the unit was repaired and brought back online. However, with the increased regulatory pressure, flaring may no longer be an option, and production curtailment is mandated if the SRU is unavailable. The economics of curtailment have put pressure on SRU designs for increased reliability and availability to enable uninterrupted operation throughout the duration of hydroprocessing unit runs, which are typically 3-4 years.

Acknowledging these long term industry trends and recognizingthe importance of the acid gas burner to the operation of the SRU, Foster Wheeler has committed to studying, understanding, and improvingthe performance of the burner. An effective tool to study burners and combustion phenomena is Computational Fluid Dynamic analysis (CFD). CFD has been increasingly applied to study flow patterns in burners and other process equipment in recent years as computing power has increased. Chemical reaction modeling, including so-called finite rate chemistry, is often incorporated into CFD, although the resulting models become very computationally complex due to the large numbers of chemical species and reaction pathways that are possible.Due to the complex and specialized nature of the analysis, Foster Wheeler decided to engage an experienced combustion modeling firm in the study of the acid gas burner.

Initial CFD modeling efforts focused on understanding the fuel (acid gas feeds) and air flow patterns that develop within the burner shell and the interaction of the burner geometry with the fuel/air mixture. In these studies, simplified equilibrium chemistry models were used to reduce computational time. An existing burner design, installed and operating in a relatively small SRUprocessing both sour water stripper gas and ammonia acid gas, was used to define the model geometry and feed streams. Model refinement was then done by applying finite rate kinetic models to the major combustion reactions, thereby allowing a better prediction of heat release patterns and flame location.


MODEL PREDICTIONS AND EVALUATION

The amount of information produced by CFD models is extensive. Much has been learned about burner operation, gas compositions, temperatures, mixing, and gas velocity through the results of the study. This paper will focus on information related to gas velocities and predicted velocity field.

Figure 5 shows the computational model used, including the inlet air nozzle, air barrel, acid gas injection holes, and burner tile (the model continues into the thermal reactor, to the right, but for clarity the thermal reactor is not shown). Figure 6 shows a detail of the model in the region of the acid gas injection holes showing the refinement made to accurately capture the geometry and predict flow phenomena occurring near the holes.

Figure 5: Computational model of Acid Gas Burner air inlet, air plenum, air barrel, acid gas injection holes, and burner tile.

Figure 6: Refinement of computational grid in vicinity of acid gas injection holes.

Results of the modeling are presented in Figure 7, which shows the predicted velocity magnitude at all locations within the model. Increasing velocity is represented by color change from blue (lowest) to green to yellow to red (highest). Highly symmetrical air velocity distribution within the air barrel is noted, as the air flow proceeds from the inlet nozzle to the plenum outside the air barrel, and then turns into the air barrel. The function of the plenum is to uniformly distribute the air flow around the air barrel.

At the location of the acid gas injection holes, increased velocity is apparent, starting at the air barrel wall and progressing into the bulk air stream just downstream of the holes. Penetration of the acid gas jet into the air stream is uniform around the circumference of the air barrel. As combustion reactions progress and the temperature of the gas mixture rises, significant velocity increase occurs as the flame attaches at the burner tile and expands into the thermal reactor.


Inlet nozzle

Acid gas injection holes

Figure 7: Velocity plot of air, acid gas, and combustion gas streams

A similar CFD analysis was performed for a burner designed to be installed in a medium-sized SRU. The diameter of the air barrel in this burner is about 2.83 times larger than that of the first, smaller burner and the external shell diameter is about 1.72 times bigger. The resulting velocity field is shown in Figure 8, which shows velocity vectors and a velocity field. It is immediately apparent that there isan unexpected asymmetrical velocity distribution within the air barrel.

The asymmetry was attributed to flow phenomena occurring in the inlet plenum and transition to the air barrel, and the larger diameter of the air barrel itself, relative to the position of the air inlet nozzle. The asymmetric flow begins at the leading edge of the air barrel as the air flow turns to enter the air barrel.

Further downstream in the air barrel, ahead of the injection holes, the asymmetric velocity distribution develops into a recirculation that causes the acid gas jet penetration into the air stream to be non-uniform. When the combustion reactions initiate and progress, the flame attaches properly at the end of the burner tile, but expands asymmetrically into the thermal reactor volume.

Air barrel


Figure 8: Velocity vectors (left) and velocity field (right) showing asymmetric air flow and recirculation inside the air barrel

A potential undesirableoutcome of the predicted asymmetric flow and recirculation is the possibility of movement of hot combustion gas from the flame area, upstream into the air barrel. The standard material of construction of the air barrel upstream of the acid gas injection holes is carbon steel, which could be rapidly compromised by the sulfidation that occurs on exposure to hot, H2S-laden gas at a temperature in excess of 343°C (650°F). Elimination of the possibility of hot gas recirculation was made a study priority because of the potential for equipment damage during operation.

Effort to eliminate recirculationfocusedon burner geometry alterations. Several geometry modifications were considered and evaluated, including:

• Reduce width of gap between air barrel inlet and rear head • Vary the length to diameter ratio of the air barrel • Increase the distance from the air inlet nozzle to the end of the air barrel • Insert distribution baffle(s) in the air inlet plenum • Introduce air through two nozzles

Several combinations of the above changes were also evaluated. Geometry changes were “built” in a model grid and analyzed, and the resulting flow and velocity fields evaluated. A total of 12 different geometry modifications were analyzed and compared. Each of the above listed variations made a discernible improvement when compared to the asymmetric flow observed in the base design. However, it was decided that any geometry change that violated long-standing proven burner design parameters that set the internal dimensions should be considered with reservations, especially since there would be no opportunity to validate the modeling results with physical experiments. This philosophy reduced the likelihood of adopting any of the first three approaches above.


DESIGN REVISION

After evaluating and comparing numerous 2D and 3D graphs, videos, and other representations and visualizations of the CFD model results, it was decided that introducing combustion air through two nozzles, located 180° apart on the burner shell, would be the best fix to be implemented. Advantages of selecting this option were that it resulted in the highest degree of flow and velocity symmetry in the air barrel, it would be the simplest to implement, and it did not violate any established burner design parameters. Figure 9 shows the predicted velocity field within the burner with the air introduced through diametrically opposed nozzles. Elimination of the potential for recirculation due to asymmetric flow is expected to improve the burner reliability and service life in larger size burners.

Figure 9:Velocity vectors (left) and velocity field (right) showing symmetric air flow and no recirculation inside the air barrel, resulting from revised air inlet nozzle configuration.

IMPROVED BURNER – OPERATION AND TESTING

The initial installation of the improved acid gas burner was in a European refinery that was undergoing a major upgrade, which included addition of a Foster Wheeler Delayed Coking Unit (DCU). As part of the sulfur block design, four parallel SRU trains were installed with a combined capacity of 535 metric tons per day of sulfur. To meet the overall sulfur recovery requirements, further processing of the SRU tail gas was accomplished in two sub-dewpoint technology units, each processing the tail gas from two Claus sections.


Figure 10: Installed redesigned acid gas burner The design feed to the SRUs was relatively high in ammonia, at 22% by volume of the total feed. Short-term low-level oxygen enrichment was desired to allow operation at full capacity when one of the four trains was unavailable. Startup occurred in September 2011, with performance test runs completed in February 2012. Required performance in the SRU (Claus section) included 97.9% sulfur recovery when feed gas containing ammonia was processed, and 98.0% when no ammonia was fed to the unit.

Initial light-off of the burner on natural gas was witnessed and directed by Foster Wheeler personnel. The light-off proceeded as planned and all indications were that burner performance was as predicted. Introduction of acid gas proceeded smoothly, and ammonia feed was introduced later. Observations of burner and SRU operation continued to validate the predicted performance, with no unexpected behavior. Burner and SRU performance was confirmed in a three-day testing period, and the unit was accepted by the owner’s team.

Continuous operation without evidence of ammonia salt formation was also required. Although ammonia was not directly measured in the thermal reactor effluent, the unit has met this requirement, with no observed pressure drop build-up. Operation with oxygen enrichment (up to 28 volume percent oxygen in air) was confirmed over a three day period. To demonstrate capacity, feed to each train was temporarily increased to meet the maximum design capacity and operation sustained for a pre-determined period. The owner’s acceptance certificate was issued after meeting all performance objectives.

CONCLUSIONS

The Foster Wheeler acid gas burner was one of the earliest burner designs created with the unique operation and environment of Claus sulfur recovery units in mind. Over 150 acid gas burners have been placed in operation since the first commercial installation in 1973. Recent investigations using CFD analysis revealed insight into the behavior of the burner and the influence of the internal geometry on flow patterns and flame location. A potentially problematic flow phenomenon caused by asymmetric velocity distribution was revealed by CFD analysis of a burner designed for a recent SRU project. The asymmetry was corrected by a relatively simple modification to the burner design, introduction of the


air flow through diametrically opposed nozzles. The revised burner design was demonstrated in a refinery installation, with startup and performance testing completed in February 2012. Performance requirements were met.

Foster Wheeler plans to utilize CFD modeling to evaluate additional design enhancements to further increase the performance, reliability, and flexibility of the acid gas burner, thereby providing refiners with even greater confidence that the Foster Wheeler design will have a positive impact on their operations.

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Improved Acid Gas Burner