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    • 专利摘要:
    A method and an apparatus for protection of a combustion unit of a chemical process against over firing, the burner(s) of the combustion unit are limited by a fuel and duty limiter which limits the duty based on process feeds, combustion gas and fuel flows.
    公开号:DK201900332A1
    申请号:DKP201900332
    申请日:2019-03-19
    公开日:2019-03-27
    发明作者:Krog Jensen Anne；Henrik Speth Christian；Bruun Jensen Peter；De Sarkar Sudip；Andreas Tjärnehov Emil
    申请人:Haldor Topsøe A/S；
    IPC主号:
    专利说明:

    Over firing protection of combustion unit
    FIELD OF THE INVENTION
    This invention relates to a method and an apparatus to protect a furnace against over firing. The furnace may comprise one or a plurality of burners. More particular, the invention relates to the protection against overfiring of a chemical reactor or a fired heater, which otherwise may entail in-efficient operation, damage to equipment and production loss.
    BACKGROUND OF THE INVENTION
    In a number of industrial processes, combustion units are used as a necessary part of a chemical process. Typically, the combustion units have at least one burner and often a plurality of burners, producing a flame from combustion of one or more fuels in a combustion gas.
    When operating the one or more burners in the combustion unit, there is a risk of over firing of the combustion unit. The requirement for fuel to the combustion unit is to provide the necessary duty to keep the chemical process running, to heat up the combustion gas and fuel mixture and also during start-up, transient and shut down operation to heat up the furnace. If a to small amount of fuel is supplied to the burner(s), the chemical process is not running optimal, but if too much fuel is supplied and if too many or too few burners are in operation, there is a risk of over firing, which may lead to damage to the process equipment and production loss. Burners are commonly controlled by systems comprising temperature measurements, but in general and especially during start-up, transient and shut down operation, a change in firing of the burners will be observed with a delay on flue gas and process fluid temperature measurements. Thus, especially during start-up, transient and shut down operation of a combustion unit, the time delay is long and the risk of over firing
    DK 2019 00332 A1 is high.
    Therefore, a need exists for a method and an apparatus for protecting a combustion unit with one or more burners against over firing, which does not rely only on temperature measurements of the flue gas and process fluids.
    In known art, EP0614047 discloses an electronic control device for gas burners of heating installations. To simplify the electronic control device for gas burners of heating installations, the microcomputer of the automatic firing unit is extended to take over tasks from the heating regulator. The microcomputer or the device equipped therewith is provided with a signal generator, a comparator, a controller and a temperature watchdog. The signal generator generates, in particular, pulse-width-modulated control signals, which are used for controlling a D.C. motor, which is used as drive element for an air blower. The comparator compares rotational speed current values of the blower generated by a rotational speed sensor with rotational speed desired values or limiting values stored in the memory and, as a function of the type and/or the magnitude of the difference values, triggers control signals or influences the latter. Furthermore, the microcomputer outputs control signals to the D.C. motor of the blower during the operational time of the burner as a function of parameters controlling the boiler temperature, and takes over temperature watchdog tasks.
    US4915613 discloses a method and apparatus to monitor fuel pressure in a heating system where a controller controls actuation of fuel valves. A fuel pressure limit signal is provided to the controller for determining if the fuel pressure crosses predetermined thresholds. In order to avoid nuisance shutdowns, the fuel pressure limit signal is ignored by the controller for a predetermined time interval after the controller has actuated a fuel valve.
    In WO9906768 a fault detection apparatus for a boiler system is disclosed which comprises a first pressure sensor in air supply line, downstream of fan
    DK 2019 00332 A1 and damper; and a second pressure sensor in the fuel supply line, downstream of the valve. Pressures P1 and P2 sensed respectively by sensors are fed to microprocessor, together with an indication from sensor of the temperature of the air supply. The microprocessor stores a range of pressure valves across a range of temperatures which are indicative of optimum combustion conditions. Having selected the reference valves appropriate for the temperature sensed, microprocessor compares them with P1 and P2 and produces a measured response in dependence upon the results of the comparison, and ranging from further monitoring (slight deviation between stored and sensed valves) to emergency shutdown (major deviation between stored and sensed valves).
    In US2008233523 a video analytics system for characterization of a flare is disclosed. A video of a flare may be taken for obtaining information so as to appropriately control the flare in an interest of reducing emissions not necessarily favourable to the environment. The system may incorporate a control scenario involving one or more parameters of a flare which are to be controlled in view of a flare characterization from an algorithmic analysis of the video.
    US3217782 describes forced draft air-gas burners utilizing a main gas supply line including a pressure responsive manual reset main gas valve, a primary air source, and a pilot gas supply line including a manual reset pressure response pilot valve, a safety system comprising: means connecting said pressure response pilot valve to be responsive to air pressure of said primary air source to permit said pilot valve to be manually opened and permit gas flow there through into said pilot gas supply line; means connecting said pressure responsive main gas valve to be responsive to pressure in said pilot gas supply line and also responsive to air pressure from said primary air source to permit opening of said main gas valve and establish gas flow in said main gas supply line, a high gas pressure sensing valve, means connecting said high gas pressure sensing valve to be responsive to unsafe high pressure in said main gas supply line to shut-off air pressure from said primary air source to said main
    DK 2019 00332 A1 gas valve; said main gas valve through its connecting means arranged to be responsive to shut-off of air pressure from said primary air source to close said main gas valve and shut-off supply of gas through said main gas supply line; a low gas pressure sensing valve means operably responsive to unsafe low pressure in said main gas supply line to shut off air pressure from said primary air source to said pilot valve to cause the latter to close and shut off supply of pilot gas line pressures to said main gas valve and thereby cause said latter valve to close and shut off supply of gas to said main gas supply line.
    Despite this mentioned prior art, there is still a need for solving the problem of protecting against over firing in a combustion unit in a chemical process, given the challenge of delay of feed-back of flue gas and process fluid temperature measurements especially during changes in firing, especially during start-up, transient and shut down operation of the combustion unit.
    SUMMARY OF THE INVENTION
    To lower the risk of over firing of a combustion unit, the present invention comprises a fuel and duty limiter that limits the duty to the furnace based on process feeds, combustion gas and fuel flows.
    The requirement for fuel is to heat up the steam/process gas mixture, combustion air/fuel mixture and also, during start-up, transient and shut down operation, to heat up the furnace. A change in firing, especially during start-up, will be observed with a delay on the flue gas and process gas temperature measurement. Thus, at low loads/initial phase of the restart the time-delay is long and the risk for over-firing is high. To lower risk of over-firing it is suggested to have a fuel/duty limiter that limits the duty to the reformer based on the steam, process gas, combustion gas and fuel flows. It is to be understood that there may be a number of feed and fuel flows, and there can also be a plurality of fuel headers. The combustion gas may be air, Oxygen or any range of gases comprising Oxygen.
    DK 2019 00332 A1
    Apart from fuel/duty limiter it is also possible to utilize existing temperature measurement for feedback to protect against excess firing in top/bottom of furnace or failure of the duty control system. Flue gas and process gas outlet temperature measurements are used (flue gas temperature has fastest response) where the flue gas temperature is limited based on the capacity of the plant process gas outlet temperature has a fixed limit.
    To prevent over-firing/flame impingement during a start-up/hot restart, the firing should be increased by igniting more burners and not by increased fuel pressure. Therefore, it is suggested to limit the fuel pressure close to minimum heat release for the burners. When sufficient burners have been lit, fuel pressure may be increased accordingly as a function of number of lit burners.
    The above does not protect against an un-symmetrical firing pattern. Therefore, the firing pattern should be linked to the process control system with an alarm when unsymmetrical pattern is used.
    In an embodiment of the invention no burners are lit at start up. When a burner is ignited the panel operator will tick the burner on in the process control system after confirmation from field operator. The process control system will register that the burner is lit and will keep track of how many burners are operating.
    The set point for the fuel pressure controller is forced (locked) to a value close to minimum heat release for the burners and the duty controller is forced (locked) to manual with 0% output. When sufficient number of burners has been ignited the system shall release duty controller, allowing the duty controller to be taken inline. Also the pressure controller may be released allowing the set point of the pressure controller to be changed if required.
    In an embodiment of the invention the maximum duty requirement is calculated during the entire operation, used as limitation for firing. The duty limitation shall override the duty control. Thus, the duty cannot be increased above the limiting
    DK 2019 00332 A1 value without permission by key or similar from the supervisor. The pressure controller shall still overrule and maintain pressure above minimum pressure for the burner via high selector to avoid unstable flames.
    In a further embodiment of the invention, the flue gas temperature can be used to limit firing as an extended over firing protection. In an embodiment of the invention, a burner matrix is added where the operator must click ignited burners on. When a burner is ignited the panel operator will tick the burner on in the process control system after confirmation from field operator. The process control system will register that the burner is lit and will keep track of how many burners that are lit. The system can also suggest which burner to ignite next. Further, a function that checks the symmetry of the lit burners can warn the operator if there is non-symmetry.
    FEATURES OF THE INVENTION
    1. A method for protecting a combustion unit having at least one burner, the method comprising the steps of
    a) acquiring a value for the flow of process feeds,
    b) acquiring a value for the flow of fuel,
    c) acquiring a value for the flow of combustion gas,
    d) calculating a value for the provided duty to process, provided by the combustion unit based on inputs comprising the value of step b),
    e) calculating a value for the maximum allowable duty to process based on input comprising the value of step a), and c),
    f) comparing the value of step d) with the value of step e)
    g) generating an alarm state output if the value of step d) exceeds the value of step e).
    2. A method for protecting a combustion unit according to feature 1, the method comprising the steps of
    a) acquiring a value for the flow of process feeds,
    DK 2019 00332 A1
    b) acquiring a value for the flow of fuel,
    c) acquiring a value for the flow of combustion gas,
    d) calculating a value for the provided duty to process, provided by the combustion unit based on inputs comprising the value of step b),
    e) calculating a value for the maximum allowable duty to process based on input comprising the value of step a), b) and c),
    f) comparing the value of step d) with the value of step e)
    g) generating an alarm state output if the value of step d) exceeds the value of step e).
    3. A method for protecting a combustion unit according to feature 1 or 2, wherein the fuel addition is limited if the value of step d) exceeds the value of step e).
    4. A method for protecting a combustion unit according to any of the preceding features, wherein the combustion unit has a plurality of burners and the method further comprises a step of controlling the pattern of the burners which are ignited, prescribing which burner or burners can be ignited next, and generating an alarm state output if the ignited burners are not in accordance with a range of an acceptable pattern.
    5. A method for protecting a combustion unit according to feature 4, wherein the operational state of the burners is detected by means of a flame detection device.
    6. A method for protecting a combustion unit according to any of the features 4 5, wherein said flame detection device comprises a human operator.
    7. A method for protecting a combustion unit according to any of the features 4 6, wherein said flame detection device comprises at least one camera with a view of the plurality of burners.
    DK 2019 00332 A1
    8. A method for protecting a combustion unit according to any of the features 4 7, wherein the number of burners which shall be in operation is calculated on the basis of the value of the flow of fuel in step b), the number of burners which are in operation is detected by means of the position of shut-off valves on the fuel lines feeding each of the burners, and the number of burners which shall be in operation is compared to the number of burners which are in operation.
    9. A method for protecting a combustion unit according to any of the preceding features, wherein the method further comprises the step of limiting the pressure of the fuel in accordance with the number of burners which are in operation.
    10. A method for protecting a combustion unit according to any of the preceding features, wherein the method further comprises the steps of acquiring a value for the flue gas temperature down-stream of the burners, acquiring a value for the temperature of the process gas outlet temperature or outlet gas temperatures and generating an alarm state output if said values are not within a pre-set range.
    11. A method for protecting a combustion unit according to feature 10, wherein the pre-set range of the values varies with the capacity of the combustion unit.
    12. A method for protecting a combustion unit according to any of the preceding features, wherein the alarm state output comprises visual and/or acoustic alarms.
    13. A method for protecting a combustion unit according to any of the preceding features, wherein the alarm state output comprises reducing the fired duty, or shutting down one or more of said burners.
    14. A method for protecting a combustion unit according to any of the preceding features, wherein said values are acquired and said calculation are executed at periodic time intervals following the time intervals of the flow measures provided
    DK 2019 00332 A1 to the process control system.
    15. A method for protecting a combustion unit according to feature 14, wherein the length of said periodic time intervals are dependent of whether the combustion unit is in a start-up phase, a steady operation phase or a shut-down phase.
    16. Apparatus for protecting a combustion unit having at least one burner, said apparatus comprising a computer operatively coupled with means to receive
    a) a value for the flow of process feeds,
    b) a value for the flow of fuel, and adapted to calculate a value for the provided duty to process provided by the combustion unit based on the inputs comprising the values of b), the computer is further adapted to calculate a value for the maximum allowable duty to process based on inputs comprising the value of a) and b), and adapted to comparing the value for the provided duty to process with the value for the maximum allowable duty to process and generating an alarm state output if the value for the provided duty to process exceeds the value of the maximum allowable duty to process.
    17. Use of an apparatus according feature 16 for a chemical reactor or a fired heater.
    EXAMPLES
    Fuel pressure limitation
    In a case, the limit for number of burners ignited is estimated as 60%, to be verified during commissioning of a system. A check of fuel pressure and duty has been made. The number of burners lit is estimated as:
    _Q_
    No of burners = (Q = fired duty, HRb = heat release per burner)
    DK 2019 00332 A1
    In the case, already at approximately 15-25 % lit burners starts there is an increase in fuel pressure, which is not according to operating manual. The new pressure limitation according to the invention will force up to 60% burners lit before fuel pressure can be increased.
    Duty/fuel limitation
    The duty limitation is given by:
    QFurnace = QAir + QFuel = (mCA + mFG) X CpFL X ATfuRN (alternatively QFurnace = mFL x CpFL x ATfurn )
    QReformer = Qn2 + QpS + QpG + QrEAC = (mN2 x CpN2 + mPS x CpPS + mPG x CpPG) x ATref + AHr x mPG
    Qloss = k1 x (QFurnace + QReformer)
    Qmax = QFurnace + QReformer + Qloss+ Qmargin = k2 x (QFurnace + QReformer) + Qmargin
    For self inspirating burners, flow measurement of combustion air is not possible. Instead a new flue gas measurement must be installed with two independent measurements in the flue gas stack. The limit switch of the false air dampers may be used to confirm closed dampers. Alternatively, the oxygen analyzer may be used to estimate the combustion air flow from fuel gas flow and oxygen content.
    Parameters are shown in table below
    VariableDescriptionFixed /VariableValueUnitmiMassflow, measured or calculatedfrom volume flow and molecularweightVariableCpFLHeat capacity flue gasFixed0.30kcal/kg/°C
    DK 2019 00332 A1
    CpN2Heat capacity nitrogenFixed0.27kcal/kg/°CCppsHeat capacity process steamFixed0.52kcal/kg/°CCppGHeat capacity process gasFixed0.95kcal/kg/°CATfurnTemperature increase in furnace(air to flue gas)Fixed925°CATrefTemperature increase in reformer(TpG,out - TpG,in)Fixed300°cAHrAverage enthalpy of reactionFixed2500kcal/kgQMarginMargin on fired dutyFixed0.0Gcal/h
    The duty is measured by fuel flow why the range requirement for the measurement is large and therefore a separate fuel valve and fuel flow measurement is required.
    The margin for maximum duty (Qmargin) and the constants above are fixed values but shall be adjustable and checked/adjusted during commissioning of the plant.
    Flue gas temperature limitation
    As mentioned, the flue gas temperature can be used to limit firing as an extended overfiring protection. Maximum flue gas temperature as per calculated in a system for capacity 50 - 110%:
    TICflue max = A [°C] + B [°C/%] x Cap [%] where A and B are estimated based on 50% and 100% operating case.
    0 In a case, at 100% load the flue gas temperature is approximately 10001030°C. At 50% load flue gas temperature is 800-850°C Design tube wall temperature is 916°C which is used as maximum up to 50% capacity. The
    DK 2019 00332 A1 function is therefore 0 - 50% capacity, maximum flue gas temperature 916°C.
    100% capacity, maximum flue gas temperature 1050°C
    50-100% capacity, linear function, 916°C to 1050°C.
    To protect the top coil in the waste heat section the temperature indication on the process gas side is used to verify that temperature is below mechanical design temperature. If there is no flow on process gas side flue gas temperature shall be restricted to a fixed temperature. The steam flow + nitrogen flow measurement is used as an indicator that flow is present, i.e at no steam flow flue gas is restricted to a fixed temperature (step function at 0% capacity).
    CASE EXAMPLE
    In a case, when performing a hot restart of an ammonia plant, an uncontrolled and rapid increase in flue gas temperature was noticed in the primary reformer. After natural gas feed-in to the primary reformer, it was observed that a rupture of several catalyst tubes had occurred and the plant was tripped immediately. Haldor Topsoe A/S was requested to take the lead technical advisor role in a thorough and rigorous root-cause analysis, performed to identify the reason(s) that led to the tube rupture incident.
    The results of this comprehensive root-cause investigation will be described in detail, and the means, to assure such incident will not happen again, will be revealed. The incident with ruptured tubes not only impaired plant capacity utilization until the re-tubing of the furnace, but more importantly, the leaked gas from the ruptured tube could have resulted in unsafe conditions. Therefore, the possibility of avoiding this type of an incident is of paramount significance to all plants operating with any type of tubular reformers. Following this incident, an automated over-firing protection management system that provides some elements of protection against over-firing of primary reformer tubes was developed.
    DK 2019 00332 A1
    Over the years, it has been observed that over-firing and consequently rupture of primary reformer catalyst tubes during operation, especially during start-up, has been one of the common problems in the reforming section in ammonia plants. Apart from its economic impact, such incident might also create potentially unsafe condition. Therefore, a thorough root-cause analysis was performed. A few action points were suggested to eschew similar incident in future. Also an automated system to provide better protection against over-firing of primary reformer catalyst tubes especially during start-up has been made.
    About the reformer
    The primary reformer in the ammonia plant is a side-fired Haldor Topsoe design with 264 catalyst tubes installed in a duplex furnace box. The necessary heat for the endothermic reforming reaction is supplied uniformly to the catalyst tubes by burning fuel through radiant wall burners, arranged on both sides of catalyst tubes in seven rows on the furnace walls. The process feed gas to the primary reformer is evenly distributed to the catalyst tubes through inlet hairpins at the top of furnace. The reformed process gas, coming out of the catalyst tubes at a normal operating temperature of 800oC (1472°F), passes through the outlet hairpins, below the bottom of the furnace box to six hot collectors. The reformed process gas further flows down to the refractory lined cold collector and transfer line, going to the secondary reformer. The hot flue gas with a normal operating bridge wall temperature (BWT) of 1045oC (1913°F) leaves the top of the furnace box and flows in the downward direction through vertical part of the convection section.
    Incident
    The tube rupture incident occurred in the primary reformer. The sequence of events, that finally culminated in several tube ruptures is described in following section. This information is based on retrieved data from the distributed control system (DCS).
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    Sequence of events / observations
    11:00 hrs., 09 February
    The ammonia plant was running stable at 112% of nameplate capacity when the plant tripped due to power failure.
    04:00 hrs., 11 February
    The plant was restarted with nitrogen circulation through the reformer, prior to ignition of burners in accordance with normal start-up procedure.
    21:00 hrs., 11 February
    The plant tripped again when operating at 24% load due to trip of auxiliary boiler.
    00:48 hrs., 12 February
    A hot restart (i.e. reintroduction of process steam while reigniting burners) was commenced but with a low process steam flow of approximately 3 t/h (6600 lb/h). Due to declining temperatures in the cold and hot collectors the natural gas fuel flow was stepwise increased to approximately 7500 Nm3/h (280 MSCFH).
    01:14 hrs., 12 February
    The increasing fuel gas flow and low process steam flow resulted in steep increase in flue gas temperature, but only modest increase in hot collector temperatures.
    01:57 hrs., 12 February
    When the BWT exceeded 1000oC (1832°F), one of the six hot collector temperatures suddenly increased by 115°C (207°F) above the other hot collector temperatures and the process vent valve to the flare system closed completely.
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    02:10 hrs., 12 February
    Gradually, about 40t/h (88,200 Ib/h) of process steam was introduced to primary reformer in 30 minutes, which initiated a sharp temperature increase in the hot collectors followed by a significant temperature increase in the BWT. The fuel flow to the burner was then reduced in steps to ~6,500 Nm3/h (243 MSCFH) and natural gas feed was introduced. The feed flow was further increased to 4,000 Nm3/h (149 MSCFH) in 15 minutes to absorb the excessive firing in the furnace box by the endothermic steam methane reforming reaction inside the catalyst tubes.
    02:46 hrs., 12 February
    Instead of cooling down the furnace the introduction of natural gas feed to the primary reformer resulted in a dramatic temperature increase of BWT and subsequently in hot and cold collectors. The fuel gas flow was interjected when the BWT exceeded 1200°C (2192°F) and the plant was tripped when rupture of several tubes was observed in the furnace.
    February
    The plant was restarted after inspection of the furnace box, transfer duct to the convection section, convection section, hairpins and hot collectors. All of the ruptured tubes, except one, were replaced due to shortage of spare tubes. All damaged burners were replaced as well. The damage of the remaining tubes was assessed by metallographic replica tests and ultrasonic and eddy current inspection.
    All significant process parameters were carefully monitored during re-start of the plant. Heating was done slowly and a burner light up sequence was followed to minimize firing in the expected damaged areas where potential over-firing has taken place.
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    Visual observation from the end peepholes into the furnace box confirmed a tube rupture in the area next to the replaced catalyst tubes and the plant was tripped immediately.
    Furnace inspection
    Rupture of 19 catalyst tubes in the first section of chamber AB was observed during inspection. 10 of them were completely ruptured with circumferential cracks and the rest had longitudinal cracks, facing the reformer furnace wall.
    In chamber CD, 18 and 3 catalyst tubes were ruptured in first and second section respectively. All of these tubes had longitudinal cracks on the side facing the outer furnace wall. Many of the burners were exposed to extremely high temperature causing melting of the burner tip.
    Root-cause analysis
    The incident of tube rupture was followed by an in-depth root-cause analysis (RCA). The RCA consisted of metallurgical examination of both ruptured and remaining catalyst tubes in the furnaces; and analyses of the collected operating data retrieved from the DCS.
    Metallurgical analysis
    Over-heating of the reformer tubes lasted for approximately 1.5 hours and resulted in rupture of 40 catalyst tubes. For the remaining tubes in the furnaces, the significant over-heating was expected to have resulted in thermal damage of the tube material by changing the microstructure, i.e. coarsening of the primary carbides in the grain boundaries and dissolution the secondary carbides in the grains. Dissolution of secondary carbides and coarsening of primary carbides decrease the creep strength significantly and therefore accelerate the creep damage when the tubes are put back in operation.
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    Metallographic replica of the tube surface was made on the remaining tubes to assess the surface microstructure. It was found that the secondary carbides were absent in most of the exposed tubes, which confirmed that the tubes had been exposed to a metal temperature above 1150°C (2102°F) and had experienced severe thermal damage. It was likely that the thermal damage had propagated all the way through the tube wall due to the relative long exposure time, low flow through the tubes and high heat conductivity of the tube material. Based on this information, it was decided to replace the remaining 224 tubes.
    Formation of creep defects, like cavities, and alignment of cavities and formation of micro-cracks are normally not expected to occur in connection with short duration of overheating. Normally, it takes longer time for cavities to form, align and develop into cracks. Therefore, the remaining tubes in the furnace were subjected to ultrasonic and eddy current inspection to ensure that the tubes were not at the brink of rupture due to severe creep defects when the plant would be restarted.
    DCS data analysis
    The operating data from DCS were analyzed to get a better overview of what went wrong during the hot re-start. The actual operating data was compared with the maximum allowable parameters derived from computer simulation of the reformer operation. The firing intensity during the start-up exceeded recommended practice by Haldor Topsoe A/S. The actual heat input ( ) was
    ζ] Γ7Γ] Τ' more than required maximum duty ( ) in reformer for approximately 1.5 hours. The heat input corresponded to ~20% plant load, but with only 3 t/h (6600 Ib/h) process steam flow in the catalyst tubes. Moreover, actual BWT was way above maximum allowable BWT for extended period of time. These factors together led to the over-firing incident which resulted in rupture of 40 catalyst tubes and severe damage of the remaining tubes in the primary reformer.
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    The tube rupture started on February 12th at 01:57 hrs, when one of the six hot collector temperatures suddenly increased above the other hot collector temperatures and the process vent valve to the flare system closed completely. The reason for the sudden temperature increase in one of the collector was due to the back flow of steam from the other hot collectors to the ruptured tubes. When this steam flow passed the refractory lined cold collector, having a temperature of approximately 450°C (842°F), it was heated up from 325°C (617°F) to 440°C (824°F). The vent valve gradually closed, as the process steam escaped through the ruptured tubes. In addition to that, some data from previous start-ups were also analyzed. During an earlier start-up in, both BWT and actual duty in primary reformer went above the maximum limit. Although the maximum allowable BWT and maximum allowable duty ( v ) were marginally exceeded, lifetime of some of the catalyst tubes might have been reduced as a consequence of this upset.
    Protection elements
    The analysis of DCS data revealed more than required heat input to the primary reformer during start-ups. Dissolution of secondary carbides in metallurgical examinations further corroborated the fact that the primary reformer catalyst tubes were over-heated for an extended period of time during start-up.
    As a follow-up action, a number of suggestions were provided in order to avoid over-heating of catalyst tubes. These recommendations have further been developed into the current over-firing protection management system (OFP). This system enforces a safe way of starting up of primary reformer by preventing the possibility of both ‘local’ and ‘global’ over-firing. The ‘local’ overfiring may happen due to higher fuel pressure, especially during start-up, with the possibility of flame impingement to catalyst tubes. Asymmetrical heating of catalyst tubes during start-up can also result in ‘local’ over-firing. ‘Global’ overfiring considers the possibility of higher heat input that is actually required.
    Thus, different stages of protections are:
    DK 2019 00332 A1 • Fuel header pressure limitation • Symmetric burner ignition sequence • Duty limitation • Bridge wall temperature (BWT) limitation • Minimum excess air limitation • Heating rate limitation
    Fuel header pressure limitation
    To ensure efficient heat distribution without over-firing during start-up, it is essential to keep fuel pressure as low as possible by igniting as many burners as possible. This is achieved by forcing fuel header pressure to a value that corresponds to stable flame with minimum heat release of the burner, until sufficient number of burners are ignited. The number of burners used for this limitation should be defined during commissioning.
    OFP forces the actual fuel header pressure to a pre-set value until a certain number of burners are ignited. This pre-set value is marginally higher that lower fuel pressure trip value to ensure stable firing without trip due to low fuel pressure.
    Symmetric burner ignition sequence
    During start-up, it is important to ignite burners in a particular pattern to ensure that the heat distribution is as even as possible and there is no concentrated local heating. This can be achieved by following a particular burner ignition sequence. OFP does it proactively by enforcing the operation to follow symmetric burner ignition sequence. Moreover, it supervises the compliance with the suggested burner ignition sequence and reactively, limits the fuel flow to the burners, if the sequence is not followed.
    OFP system ensures the following:
    - similar burner ignition pattern on opposite walls in the same furnace chamber
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    - similar burner ignition pattern on walls inside different furnace chambers
    - minimum difference between number of ignited burners in each column
    - avoidance of adjacent burner ignition unless decent heat distribution is obtained
    - no ignition of top row burners during initial start-up
    If the abovementioned suggestions are not followed, then an ‘asymmetric firing' signal is sent to DCS. The asymmetric signal doesn't allow any increase in heat input to the reformer. However, reduction in heat input to primary reformer, if needed, is possible in this situation. Increase of heat input is possible only when the problem(s) for asymmetric signal is resolved.
    Furthermore, OFP also estimates number of ignited burners based on the required heat input to the reformer and burner heat release curve. Actual numbers of ignited burners in operation must be close to estimated numbers of ignited burners, otherwise heat input to the primary reformer is restricted by sending ‘asymmetric firing' signal to DCS.
    If the reformer is provided with hot collectors, the deviation in temperatures of the collectors will be monitored and in case the deviation exceeds a pre-defined value the above mentioned actions will take place. Alternatively, the fuel valves could be provided with limit switches or the limited edition of Topsoe Furnace Management system could be installed to monitor burner ignition sequence.
    Duty limitation
    Mismatch between heat input to primary reformer and heat uptake as reaction heat and sensible heat can result in over-heating of the primary reformer catalyst tubes. Therefore, it is important not to provide more heat than what is required. A simplified heat balance is shown below.
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    where,
    Qfilfl.V
    QflHF typVffli
    Qpg
    Q.fg
    Qfurx
    Ofviin maximum required duty to reformer required transferred duty to process gas required transferred duty to flue gas side reformer in process gas energy flow reformer out process gas energy flow reformer in flue gas energy flow reformer out flue gas energy flow
    The duty input to the reformer is partially taken by the process gas via catalyst tubes and the rest goes to flue gas side. Thus, maximum required duty ( ' ') can be expressed in terms of the duty transferred to the process gas (and to flue gas in the furnace ( as follows:
    Qmax = (Qi-ιληλ Ή Qhff)
    Qpff = (Qfg - Qfg) = tø™ 20
    DK 2019 00332 A1
    Qfuiw — where, fyreac
    Qtisat
    MPl, t-'Pn,
    ΔΤ,
    CC/fWI/V ^Fl/fl/v) — ^FI ' ^‘PrL ' ΔΪ'γ required duty for endothermic steam reforming reaction required duty to heat up reformed gas at reformer outlet temperature flue gas mass flow specific heat capacity of flue gas temperature difference between BWT and ambient
    The reformer duty ( ) consists of required duty for endothermic steam reforming reaction ( ) and sensible heat transferred to process gas ( ).
    _ ,, O/Pijr: , Q.tffifjt. , ,.,,, , ,-,-,
    Both and can be estimated, based on measured feed flow, process steam flow, reformer inlet and outlet temperature, set point of steam to carbon ratio, and manual input for carbon number in feed. Qref can be estimated to:
    + <2 + Qmn.r ~ Qhsat Qrtac
    Whereas Qheat can be expressed as
    Qfie-o-i QfieafcrfHC + Qheat,PS
    Impact of and thus, on is negligible and therefore, it can be ignored.
    With an approximation that specific heat capacity varies linearly with temperature, the terms in above formula can be written as
    Qheat.UC
    Qheat.PS (TO - CpfrOPC
    -(TO-T0-Cp^(T9 +
    Mp, (T - TÜ) Cp^ (Γ0
    DK 2019 00332 A1
    where,
    In a case, during normal operation ¢70- 7') = - 484 K and (Tl! - TO) = 775 K.
    These values are used as constants in order to avoid fluctuation due to change in primary reformer inlet and outlet temperatures. It results in slightly more heat input during start-up which partially takes care of heating refractories and steel structure.
    can be expressed as follows f^CNO)
    TC'·')
    M'PS = MPS + MHC-{1-/WO)}
    Af = MP5 + AfHC-{l-r(cJVO^,7C'r)}
    From analyses it is found that Cp value for PS remains the same, irrespective of i ir different T and I . However, Cp value for HC stream is project specific and corresponds to the value in normal operation
    The duty transferred to flue gas side is estimated based on flue gas flow ( ') and measured increase in flue gas temperature ( in the reformer. Flow of flue gas ( ), used for the duty estimation, is the lowest value among three different measurements/calculations. One of them is the direct flow measurement at the stack. Apart from this, flue gas flows are also estimated by measuring excess oxygen at the bridge wall and pressure at the suction of flue gas fan. The impact of false air damper opening is considered while using the
    Cpm.
    last method. Specific heat of flue gas ( ) remains constant during both startup and normal operation.
    DK 2019 00332 A1
    Bridge wall temperature limitation
    The impact of the extent of firing is reflected in two temperature measurements - bridge wall temperature (BWT) and reformer outlet process gas temperature. Between these two, the second has a lag and therefore, a fastest response, in case of over-firing, can be obtained by monitoring BWT. This concept is used to provide more protection against over-firing due to mismatch in heat input and utilized in reformer.
    The BWT is limited by a function that increases proportionally with the plant capacity. This function, to limit BWT during operation, was obtained by performing simulations of at various capacities. The constants, used in the function, must be defined after commissioning. BWT can be limited to a particular temperature during initial start-up. Protection against over-firing by BWT limitation remains active at all plant capacities.
    Minimum excess air limitation
    Insufficient combustion air flow corresponding to fuel flow at a particular operating condition might lead to sub stoichiometric combustion. If sufficient combustion air is not provided post combustion inside the furnace box can occur resulting in very high local temperatures near the reformer tubes. This overheating of the reformer tubes may ultimately lead to tube ruptures. The over-firing protection system ensures a minimum excess air flow all the time to avoid any such uncontrolled combustion. A simple correlation can be used to predict minimum requirement of combustion air flow based on fuel flow and capacity.
    Predicted combustion air flow = f(CAP)*k1*stoichiometric air flow Where f(CAP) is minimum excess air flow as a function of capacity k1 is a constant dependent on whether the plant is heated up or cooled down
    DK 2019 00332 A1
    If combustion air flow goes below corresponding fuel flow, OFP reduces heat input by decreasing fuel flow to the reformer burners.
    Heating rate limitation
    High heating rate may cause high thermal stresses in the reformer and reformer tubes and which reduces the lifetime of the reformer tubes. The OFP system monitors the bridge wall temperature, hot collector temperatures and reformer outlet temperature. If the heating rate is exceeding maximum allowed heating rate any further increase in the fuel flow is stopped until the heating rate has reduced.
    权利要求:
    Claims (5)
    [1] 1. A method for protecting a combustion unit having at least one burner, the method comprising the steps of
    a) acquiring a value for the flow of process feeds,
    b) acquiring a value for the flow of fuel,
    c) acquiring a value for the flow of combustion gas,
    d) calculating a value for the provided duty to process, provided by the combustion unit based on inputs comprising the value of step b),
    e) calculating a value for the maximum allowable duty to process based on input comprising the value of step a), and c),
    f) comparing the value of step d) with the value of step e)
    g) generating an alarm state output if the value of step d) exceeds the value of step e).
    [2] 2. A method for protecting a combustion unit according to claim 1, the method comprising the steps of
    a) acquiring a value for the flow of process feeds,
    b) acquiring a value for the flow of fuel,
    c) acquiring a value for the flow of combustion gas,
    d) calculating a value for the provided duty to process, provided by the combustion unit based on inputs comprising the value of step b),
    e) calculating a value for the maximum allowable duty to process based on input comprising the value of step a), b) and c),
    f) comparing the value of step d) with the value of step e)
    g) generating an alarm state output if the value of step d) exceeds the value of step e).
    [3] 3. A method for protecting a combustion unit according to claim 1 or 2, wherein the fuel addition is limited if the value of step d) exceeds the value of step e).
    [4] 4. A method for protecting a combustion unit according to any of the preceding
    DK 2019 00332 A1 claims, wherein the combustion unit has a plurality of burners and the method further comprises a step of controlling the pattern of the burners which are ignited, prescribing which burner or burners can be ignited next, and generating an alarm state output if the ignited burners are not in accordance with a range of an acceptable pattern.
    5. A method for protecting a combustion unit according to claim 4, wherein the operational state of the burners is detected by means of a flame detection device.
    6. A method for protecting a combustion unit according to any of the claims 4 5, wherein said flame detection device comprises a human operator.
    7. A method for protecting a combustion unit according to any of the claims 4 6, wherein said flame detection device comprises at least one camera with a view of the plurality of burners.
    8. A method for protecting a combustion unit according to any of the claims 4 7, wherein the number of burners which shall be in operation is calculated on the basis of the value of the flow of fuel in step b), the number of burners which are in operation is detected by means of the position of shut-off valves on the fuel lines feeding each of the burners, and the number of burners which shall be in operation is compared to the number of burners which are in operation.
    9. A method for protecting a combustion unit according to any of the preceding claims, wherein the method further comprises the step of limiting the pressure of the fuel in accordance with the number of burners which are in operation.
    10. A method for protecting a combustion unit according to any of the preceding claims, wherein the method further comprises the steps of acquiring a value for the flue gas temperature down-stream of the burners, acquiring a value for the temperature of the process gas outlet temperature or outlet gas temperatures
    DK 2019 00332 A1 and generating an alarm state output if said values are not within a pre-set range.
    11. A method for protecting a combustion unit according to claim 10, wherein the pre-set range of the values varies with the capacity of the combustion unit.
    12. A method for protecting a combustion unit according to any of the preceding claims, wherein the alarm state output comprises visual and/or acoustic alarms.
    13. A method for protecting a combustion unit according to any of the preceding claims, wherein the alarm state output comprises reducing the fired duty, or shutting down one or more of said burners.
    14. A method for protecting a combustion unit according to any of the preceding claims, wherein said values are acquired and said calculation are executed at periodic time intervals following the time intervals of the flow measures provided to the process control system.
    15. A method for protecting a combustion unit according to claim 14, wherein the length of said periodic time intervals are dependent of whether the combustion unit is in a start-up phase, a steady operation phase or a shut-down phase.
    16. Apparatus for protecting a combustion unit having at least one burner, said apparatus comprising a computer operatively coupled with means to receive
    a) a value for the flow of process feeds,
    b) a value for the flow of fuel, and adapted to calculate a value for the provided duty to process provided by the combustion unit based on the inputs comprising the values of b), the computer is further adapted to calculate a value for the maximum allowable duty to process based on inputs comprising the value of a) and b), and adapted to comparing the value for the provided duty to process with the
    DK 2019 00332 A1 value for the maximum allowable duty to process and generating an alarm state output if the value for the provided duty to process exceeds the value of the maximum allowable duty to process.
    [5] 5 17. Use of an apparatus according claim 16 for a chemical reactor or a fired heater.
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    同族专利:
    公开号 | 公开日
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    法律状态:
    2019-03-27| PAB| Application published on request|Effective date: 20190326 |
    2020-08-04| PHB| Application deemed withdrawn due to non-payment or other reasons|Effective date: 20200721 |
    优先权:
    申请号 | 申请日 | 专利标题
    DKPA201800138|2018-03-28|
    DKPA201800138|2018-03-28|
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