Oxy-fuel Combustion in the Claus Process and Using CFD Modeling in Burner Design Optimization

Paper presented at AFRC 2014 Industrial Combustion Symposium

Oxy-fuel combustion in the Claus process and using CFD modelling in burner design optimization Xiaoping Tian CEng MIMechE, Technical Specialist, Linde/BOC UK Dr. Andrew Richardson, Linde Technologist, Linde North America Dr. Bernhard Schreiner, Senior Expert Chemical Process, Linde AG The definition of combustion is a sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by production of heat and conversion of chemical species. Using oxygen instead of air as the oxidant offers several advantages including higher temperatures, reduced exhaust volume and heat losses, and a reduction in emissions. Historically, most oxy-fuel applications have evolved from high temperature heating or melting processes typically found in the metals and glass production industries. This paper will look at oxy-fuel combustion from a different aspect - the conversion of chemical species; and its application in the sulphur recovery-Claus process found in oil refineries and gasification plants. This paper will also look at the special characteristics of Claus furnace combustion and show how Linde/BOC tackles various challenges using CFD modelling as a powerful tool for Claus furnace acid gas burner design optimisation. Introduction of sulphur recovery - the Claus process The Claus process is by far the most significant gas desulphurizing process. Its first iteration was patented by Carl Friedrich Claus in 1883. Today's Claus process was developed and introduced by IG Farben in Germany around 80 years ago. It converts highly toxic hydrogen sulphide gas into harmless elemental sulphur. This partial oxidation is referred to as "Sulphur Recovery". The feed stream to the Claus process is typically the acid gas stream stripped from a loaded liquid, such as found in a chemical or physical gas treatment unit, e.g. amine scrubbers at oil refineries or methanol scrubbers of gasification plants. These streams often contain high concentrations of hydrogen sulphide, a precondition for applicability of the Claus process. The Claus process consists of a thermal section followed by a cascade of catalytic steps. Around 60-70% of the sulphur is converted in the thermal stage at high temperatures above 925°C. Down-stream at much lower temperatures, i.e. between 340°C and 180°C, sulphur production is realized catalytically by H2S reacting with SO2. The catalytic stages include condensing, reheating and catalytic conversion. The catalytic procedure is repeated between two and three times at a decreasing temperature level in order to increase sulphur recovery efficiency. Then the tail gas, still containing H2S, SO2 and sulphur vapour, is normally sent to a Tail Gas Treatment Unit, before it is oxidized in the incinerator. The overall chemical reactions occurring in the Claus process can be represented by the following reaction: H2S + 1/2 O2  S + H2O In the catalytic converters, the chemical reaction can be represented by the so-called Claus reaction as follows: 2H2S + SO2  3S + 2H2O The air demand analyser placed in the tail gas is used to measure the ratio of H2S/SO2 in the tail gas stream, thus controlling the air inlet flow to achieve a typical H2S/SO2 ratio of 2:1. In the thermal stage, this drives the partial oxidation of a third of the H2S to SO2 to allow close to stoichiometric Claus reaction in the subsequent catalytic stage: 3H2S + 3/2 O2  2H2S + SO2 + H2O The main equipment involved in the thermal stage is the Burner, the Reaction Furnace and the subsequent Waste Heat Boiler. Physically they are generally contained in one integrated vessel. Oxygen enrichment in Claus process Many refiners nowadays are facing more and more stringent legislation aimed at reducing sulphur content in automotive fuel products, even as sulphur levels in crude oil increase. This current trend has led to an increasing demand for sulphur processing capacity. Traditionally, new air-based plants are built at a considerable cost; especially if there is an additional requirement for Tail Gas Treatment. Oxygen enrichment at different levels, up to 100% oxygen, can be used to intensify existing Claus units. This can add significant capacity increase with relatively few Figure 1: Claus Process 2H2S + SO2  3S +2H2O modifications, minimum capital expenditure and it requires little additional plot area. Therefore, oxygen enrichment in the sulphur recovery process has been widely used since the mid-1980. To date, there are over 200 Claus units operating around the world that have been retrofitted with various oxygen enrichment technologies. The principle of employing oxygen in the Claus combustion is to eliminate part or all of the nitrogen in air by reduction of the air flow, thus allowing more acid gas to be processed while maintaining the same total volumetric flow through the plant. It also offers other benefits to the process such as energy savings and increased contaminant destruction. Due to different limitation thresholds there are three levels of oxygen enrichment in the Claus process: Low level enrichment - up to 28% oxygen enrichment Low level enrichment is accomplished by injecting oxygen via an oxygen diffuser into the existing air main pipe to the Claus process. Generally, no equipment modifications on the sulphur plant are required, other than providing the respective tie-in point for the oxygen injection in the combustion airline. A maximum oxygen enrichment level of 28% provides up to around 30% increase in capacity. The scheme is shown in Figure 2. Medium level enrichment - 28% to 40% oxygen enrichment - SURE™ single combustion At levels above 28%, the air-only designed burners are not suitable for handling oxygen-rich air. Therefore, oxygen should be introduced into the reaction furnace separately from the air supply. Linde/BOC supplies the proprietary SURE™ burner Figure 2: Low level oxygen enrichment specially designed for this purpose. The exact level of enrichment, and associated capacity increase will vary with the feed composition. However, an oxygen enrichment level of 40% can generally be achieved, resulting in about 75% of sulphur recovery capacity increase. The scheme is shown in Figure 3. High level enrichment- up to 100% oxygen - SURE™ double combustion Operations with high levels of oxygen (> 40% oxygen enrichment) can achieve up to a 150% capacity increase. Acid gas is combusted in two stages to mitigate high flame temperatures in the reaction furnace. Some of the oxygen is diverted to the second stage combustion. The process gas is cooled down in the first Waste Heat Boiler before reacting with the remaining oxygen injected in the second Reaction Furnace. Figure 3: Medium level oxygen enrichment Figure 4: Double combustion The process can be configured with an additional Reaction Furnace and Waste Heat Boiler installed upstream of the existing equipment to accommodate the first stage combustion; or with a new furnace consisting of a two-pass Waste Heat Boiler arrangement. Figure 4 and Figure 5 show the two different set-ups separately. Challenges of oxy-fuel combustion in Claus process Although the concept of using oxygen enrichment in the Claus process to increase capacity seems fairly straightforward, the combustion phenomenon is particularly complex. The variety of components and impurities in the acid feed gas results in a very complicated reaction chemistry. The high heat release associated with oxy-fuel combustion can be utilized in some respect; however it has to be mitigated in other respects. Feed gas complexity There is an increasing trend to treat high levels of sour water stripping gas (so-called SWS gas, typically: H2S/NH3/H2O) and other waste streams in the reaction furnace. In fact, sulphur recovery units are often referred to as the "dust-bins" of the refinery. In dependence of the respective source there can be significant amounts of contaminants like ammonia and/or hydrocarbons in the acid feed gas, in addition to hydrogen sulphide. It is well known that the presence of ammonia and hydrocarbons downstream of the thermal section can cause serious problems, such as equipment plugging with ammonia salt, soot formation, and acid corrosion, to name just a few. As contaminant destruction is kinetically limited, its efficiency is dependent on 3 Ts: high Temperature; Turbulence and sufficient residence Time. It is therefore crucial to have a well-designed burner to ensure effective mixing and achieve the desired fluid dynamic interaction of the gases with the configuration of the reaction furnace. Figure 5: Double combustion- Two-pass Waste Heat Boiler High temperature With high levels of oxygen during combustion, the temperature is increased significantly when compared with air only combustion. High temperature is favoured for proper contaminant destruction. However, this has to be managed carefully to ensure it is within the refractory design temperature limit. From a process point of view, double combustion is one solution that can be used to mitigate the high temperature resulting from very high levels of oxygen enrichment. From a burner design point of view, flame shaping is also important to generate a desirable temperature profile in the reaction furnace. Methodology of Claus furnace combustion CFD modelling Linde/BOC has been using Computational Fluid Dynamics (CFD) modelling to assess the interaction between the oxy-acid gas burner design and the geometry of the reaction furnace since the early 1990s when the initial package was developed. Unlike thermodynamic models that predict bulk adiabatic temperatures in the furnace, the CFD model uses kinetic relationships to enable a detailed furnace temperature profile to be generated. In addition, profiles of individual species can be plotted throughout the furnace to predict the complete destruction of contaminants. The CFD model used to simulate the Claus furnace environment is particularly complex. It requires not only fluid flow simulation, but also various chemical reactions. It is extremely challenging to condense around 300 reactions derived from Claus chemistry into a number of manageable global reaction scheme inputs for a CFD model. It is also critical to incorporate reaction rates into the turbulence modelling in order to give an accurate prediction of flame length and location. Another specific feature of Claus furnace combustion is the radiation properties of flame and associated gases. In the combustion situations we are often familiar with, CO2 and H2O are the only radiative gases. However, in the Claus furnace, not only are CO2 and H2O acting as radiative gases, but there is also a significant amount of H2S and SO2 contributing to radiative heat transfer. Correct estimation of radiation is vital to determine the heat flux to the furnace wall, as well as to predict the gas temperature which in turn controls the reaction rates. In particular, for the oxygen enrichment operation, the partial pressures of these radiative gases are higher than in air operation. Therefore, the CFD model has to be adapted to include all the radiative gases over the entire combustion temperature range. These special characteristics of Claus furnace simulation coupled with the basic mass and energy equations created quite a demanding computing activity at the time. As with all simulation techniques, it is essential to validate the model to give confidence in the predicted results. That is the reason in the early 1990s, alongside the development work in the CFD package, Linde/BOC also invested in a highly instrumented pilot plant facility with sophisticated gas sampling points and state-of-art temperature measurement to understand Claus reaction fundamentals. This research and development programme was run intensively for four years. During this period, we gained first-hand operational experience and amassed valuable experimental data that has been used in the development of today's oxygen burners and to validate the CFD package. Application of CFD modelling in Claus furnace and burner design Nowadays, CFD modelling software has evolved rapidly and computing power is easily accessible. However, to our knowledge, no commercial codes have been specifically generated for Claus furnace CFD modelling, possibly due to its complexity and a lack of understanding of the chemistry taking place in the Claus furnace. Linde/BOC has always been at the forefront of oxygen enrichment technology in the Claus process. We have utilized CFD modelling techniques for all our commercial projects to verify burner design and its performance in the reaction furnace environment. The burner, the reaction furnace, as well as the critical parameters of the Waste Heat Boiler, are included to form a three dimensional model. Although the methodology adopted is universal, every CFD model varies. Each burner is individually designed according to its process specifications. Most Claus reaction furnace designs have varying geometries, for example different aspect ratios, cylindrical head shapes, and some have additional features like a choke ring and checker walls. Waste Heat Boilers have different capacities and various ferrule installations. Together with the different feed streams, these features can all impact the flow fields, the thermodynamics and the kinetics inside the furnace. Furthermore, two different firing configurations - end-fired and tangentially fired - can result in very different design criteria for the burner itself. Figure 6 and Figure 7 below show an example of a tangentially fired burner simulation results. Figure 6 shows the temperature profile of the bulk gas at a cross-section of the furnace, which also shows the prediction of the flame length and shape on this plane. Figure 7 shows the reaction furnace inner wall temperature. For tangential fired burners, it becomes necessary to design the burner to avoid overheating of the refractory wall opposite the burner. Figure 6: Bulk gas temperature profile- tangential fired Figure 8 below shows the bulk gas temperature profile at a cross-section area for an end-fired burner. This burner is operating in a reaction furnace design with multi-injection nozzles around the choke-ring. Due to the symmetry of the burner design and reaction furnace shape, a quadrant of the reaction furnace and burner is generated as the geometry for the model. This is to best utilize the computing power to achieve a more refined calculation around the close-burner area. For end-fired burners, the reaction furnace shape becomes more important to generate the desired recirculation flows to enhance mixing. Figure 7: Reaction furnace wall temperature profile - tangential fired Figure 8: Bulk gas temperature profile - end fired Some reaction furnaces have checker walls installed to give additional protection of the downstream Waste Heat Boiler tubes. These installations are normally found in an end-firing burner arrangement. In addition, checker walls can bring about very positive benefits to the recirculation flow generation, hence enhancing mixing in the furnace. Figure 9 shows an example of the velocity vectors display at a cross-section area along the reaction furnace with a checker wall arrangement. Individual species distribution inside the furnace can also be generated in the CFD results. For example, Figure 10 shows the oxygen mole fraction profile, which demonstrates all the oxygen consumed at the near burner region. It is important to ensure there is no likelihood of oxygen breakthrough from the reaction furnace, as this may causes fire in the downstream catalyst beds. Figure 9: Velocity profile -Checker wall Figure 10: Oxygen mole fraction Figure 11 shows the ammonia mole fraction profile at the same plane. This also proves total ammonia destruction in the flame region. The destruction of contaminants such as ammonia is essential to achieve optimum performance of the Claus unit. To ensure satisfactory ammonia destruction the temperature within the furnace needs to be above 1250°C. Achieving the high temperatures necessary when firing with oxygen or at the maximum air firing is typically not a problem, but it is also necessary to assess the ammonia destruction at the minimum turndown situation. ° Conclusions All the Claus furnaces are continuously operating over long period of time with highly toxic gas components in a very harsh environment. Such natures of these furnaces have prevented any detailed examination of the performance and behaviour of the furnaces and the burners. The pilot plant that Linde/BOC purposely built for research and development with high level of instrumentation has enabled us to collect valuable empirical data to gain understanding of the furnace and burner performance under various feed compositions and different oxygen enrichment levels. These data have been utilized to validate the CFD package developed for Claus furnace simulation. This has provided us with a very powerful tool in burner design, scaling and optimisation in all the commercial projects. References S.R. Graville; R Hull; J.S. Norman; R.W. Watson; "Burner development technology for oxygen use in Claus plants" J.S. Norman; R.W. Watson; "CFD facilitates the design process" B.Schreiner "Oxygen enriched vs air-only operation at Claus units field tests are teaching lessons- especially in view of ammonia destruction" Figure 11: Ammonia mole fraction