Processes for Managing Combustion of a Fuel Gas Supplied to a Combustion Zone

Abstract

Processes for managing combustion of a fuel gas in the presence of an oxidant supplied to a combustion zone, where the fuel gas undergoes an event that includes a pressure decrease and/or an average molecular weight increase, and where the fuel gas is supplied to the combustion zone via a valve. The process can include (I) detecting the event. Upon detecting the event in step (I), the process can also include (II-a) controlling the fuel gas supplied to the combustion zone by limiting an output of the valve to no more than a predetermined threshold output to prevent excessive fuel gas flooding the combustion zone and/or (II-b) controlling the fuel gas supplied to the combustion zone by adjusting the valve to maintain a pressure of the fuel gas after the valve to no higher than a predetermined threshold pressure to prevent excessive fuel gas flooding the combustion zone.

Claims

1. A process for managing combustion of a fuel gas in the presence of an oxidant supplied to a combustion zone, wherein the fuel gas may undergo an event including pressure decrease and/or average molecular weight increase, and wherein the fuel gas is supplied to the combustion zone via a fuel gas valve, the process comprising: (I) detecting the event; and subsequently carrying out (II-a) and/or (II-b) below: (II-a) controlling the fuel gas supplied to the combustion zone by limiting an output of the fuel gas valve to no more than a predetermined threshold output to prevent excessive fuel gas flooding the combustion zone; and (II-b) controlling the fuel gas supplied to the combustion zone by adjusting the fuel gas valve to maintain a pressure of the fuel gas after the fuel gas valve to no higher than a predetermined threshold pressure to prevent excessive fuel gas flooding the combustion zone.

2. The process of claim 1, wherein: before the event, the fuel gas is a H.sub.2-rich fuel gas at a first pressure, during the event, the fuel gas changes from the H.sub.2-rich fuel gas to a backup fuel gas, and the backup fuel gas has an average molecular weight greater than an average molecular weight of the H.sub.2-rich fuel.

3. The process of claim 2, wherein the backup fuel gas comprises one or more C.sub.1-C.sub.4 hydrocarbons and comprises less H.sub.2 than the H.sub.2-rich fuel gas.

4. The process of claim 2, wherein the backup fuel gas comprises natural gas, a methane-rich gas, an ethane-rich gas, a propane-rich gas, a butane-rich gas, or a mixture thereof.

5. The process of any one of claims 2 to 4, wherein: step (I) comprises monitoring a backup fuel valve in fluid communication with an inlet side of the fuel gas valve that supplies the fuel gas to the combustion zone, the backup fuel valve is configured to introduce the backup fuel gas into the fuel gas valve upon opening of the backup fuel valve, and detecting the event in step (I) comprises detecting the backup fuel valve opening.

6. The process of any one of claims 1 to 5, wherein step (II-a) is carried out, and wherein the predetermined threshold output is determined based on at least one of: a density of the fuel gas supplied to the combustion zone upon the detection of the event, an average molecular weight of the fuel gas supplied to the combustion zone upon the detection of the event, and the fuel gas valve output before the event is detected.

7. The process of any one of claims 1 to 5, wherein step (II-a) is carried out, and wherein the predetermined threshold output is a threshold output that was used after detecting one or more prior events.

8. The process of any one of claims 1 to 5, wherein step (II-a) is carried out, and wherein the predetermined threshold output is arrived at over a period of time using a predetermined algorithm.

9. The process of any one of claims 1 to 5, wherein step (II-b) is carried out, and wherein the predetermined threshold pressure is determined based on at least one of: a density of the fuel gas supplied to the combustion zone upon the detection of the event, an average molecular weight of the fuel gas supplied to the combustion zone upon the detection of the event, and the pressure of the fuel gas before the event is detected.

10. The process of any one of claims 1 to 5, wherein step (II-b) is carried out, and wherein the predetermined threshold pressure is a threshold pressure that was used after detecting one or more prior events.

11. The process of any one of claims 1 to 5, wherein step (II-b) is carried out, and wherein the predetermined threshold pressure is arrived at over a period of time using a predetermined algorithm.

12. The process of any one of claims 1 to 4, wherein step (I) comprises monitoring one or more process parameters and detecting a change in the one or more monitored process parameters.

13. The process of claim 12, wherein step (II-a) is carried out and detecting the change in the one or more monitored process parameters comprises detecting an increased opening of the fuel gas valve.

14. The process of claim 13, further comprising measuring a density of the fuel gas or an average molecular weight of the fuel gas supplied to the combustion zone upon detection of the event, wherein the predetermined threshold output is determined based on the measured density of the fuel gas or the measured average molecular weight of the fuel gas.

15. The process of claim 13, wherein the predetermined threshold output is a threshold output that was used after detecting one or more prior events.

16. The process of claim 12, wherein step (II-b) is carried out and detecting the change in the one or more monitored process parameters comprises detecting a reduction in a pressure of the fuel gas upstream of the fuel gas valve.

17. The process of claim 16, wherein the predetermined threshold pressure is a threshold pressure that was used after detecting one or more prior events.

18. The process of claim 16, further comprising measuring a density of the fuel gas or an average molecular weight of the fuel gas supplied to the combustion zone upon the detection of the event, wherein the predetermined threshold pressure is determined based on the measured density or the measured average molecular weight.

19. The process of claim 12, wherein: before the event, the fuel gas is supplied from a H.sub.2 supply source, a hydrogen recovery unit in a petrochemical plant, or a tail gas produced from a recovery section of an olefins production plant, and detecting the change in the one or more monitored process parameters comprises detecting one or more of the following: a trip signal from the H.sub.2 supply source, a trip signal from the hydrogen recovery unit in the petrochemical plant, a trip signal of a process gas compressor, or a trip signal of a turbo-expander supplying the tail gas.

20. The process of any one of claims 1 to 19, wherein carrying out step (II-a) or step (II-b) maintains an amount of O.sub.2 within the combustion zone that is at least 0.5 mol % greater than an amount of O.sub.2 required to combust the fuel gas introduced into the combustion zone after the event is detected.

21. The process of any one of claims 1 to 20, wherein the oxidant is supplied to the combustion zone via an induced draft fan or a forced draft fan, the process further comprising: (III) when the event is detected in step (I), controlling the oxidant supplied to the combustion zone by adjusting a parameter of the induced draft fan or the forced draft fan to maintain a flow rate of the oxidant into the combustion zone above a predetermined lower limit.

22. The process of any one of claims 1 to 21, wherein a rate of change of the pressure decrease is >10% per minute and/or a rate of change of the average molecular weight increase is >10% per minute.

23. The process of any one of claims 1 to 22, wherein: the fuel gas and the oxidant are supplied to two or more combustion zones, the fuel gas is supplied to each of the two or more combustion zones via a corresponding fuel gas valve, and upon detecting the event, step (II-a) is carried out such that the output of each corresponding fuel gas valve is limited to no more than the predetermined threshold output and/or step (II-b) is carried out such that each corresponding fuel gas valve is adjusted to maintain the pressure of the fuel gas after the fuel gas valve to no higher than the predetermined threshold pressure.

24. The process of any one of claims 1 to 23, wherein step (II-a) and/or step (II-b) is carried out automatically.

25. The process of any one of claims 1 to 24, wherein step (II-a) is carried out, the process further comprising using a predetermined period of time, a fuel gas header condition, or a combination thereof to return the threshold output of the fuel gas valve to an initial value that was used prior to detecting the event.

26. The process of any one of claims 1 to 24, wherein step (II-b) is carried out, the process further comprising using a predetermined period of time, a fuel gas header condition, or a combination thereof to return the threshold pressure to an initial value that was used prior to detecting the event.

27. A process, comprising: introducing a fuel gas via a fuel gas valve and an oxidant-containing feed via an induced draft fan or a forced draft fan into a combustion zone of a furnace; contacting at least a portion of the fuel gas with at least a portion of the oxidant-containing feed before or within the combustion zone to effect combustion within the combustion zone of at least a portion of the fuel gas to produce a combustion effluent; monitoring one or more process parameters during the introduction and combustion of the fuel gas; determining, based on the one or more monitored process parameters, the fuel gas has undergone an event comprising at least one of a pressure decrease and an average molecular weight increase; and controlling the fuel gas introduced into the combustion zone by (i) limiting an output of the fuel gas valve to no more than a predetermined threshold output to prevent excessive fuel gas flooding the combustion zone or (ii) adjusting the fuel gas valve to maintain the pressure of the fuel after the fuel gas valve to no higher than a predetermined threshold pressure to prevent excessive fuel gas flooding the combustion zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 depicts an illustrative system for managing combustion of a fuel gas in the presence of an oxidant supplied to a combustion zone of a furnace, according to one or more embodiments described.

[0009] FIG. 2 depicts another illustrative system for managing combustion of a fuel gas in the presence of an oxidant supplied to a combustion zone of a furnace, according to one or more embodiments described.

[0010] FIG. 3 depicts an illustrative set of curves for a comparative example that shows a fuel gas control response in a steam cracker furnace when a H.sub.2-rich fuel gas was unexpectedly replaced with natural gas (backup fuel gas).

[0011] FIG. 4 depicts an illustrative set of curves for an inventive example that shows a fuel gas control response in a steam cracker furnace when a H.sub.2-rich fuel gas was unexpectedly replaced with natural gas (backup fuel gas).

DETAILED DESCRIPTION

[0012] Various specific embodiments, versions and examples of the invention will now be described, including preferred embodiments and definitions that are adopted herein for purposes of understanding the claimed invention. While the following detailed description gives specific preferred embodiments, those skilled in the art will appreciate that these embodiments are exemplary only, and that the invention may be practiced in other ways. For purposes of determining infringement, the scope of the invention will refer to any one or more of the appended claims, including their equivalents, and elements or limitations that are equivalent to those that are recited. Any reference to the invention may refer to one or more, but not necessarily all, of the inventions defined by the claims.

[0013] In this disclosure, a process is described as comprising at least one step. It should be understood that each step is an action or operation that may be carried out once or multiple times in the process, in a continuous or discontinuous fashion. Unless specified to the contrary or the context clearly indicates otherwise, multiple steps in a process may be conducted sequentially in the order as they are listed, with or without overlapping with one or more other steps, or in any other order, as the case may be. In addition, one or more or even all steps may be conducted simultaneously with regard to the same or different batch of material. For example, in a continuous process, while a first step in a process is being conducted with respect to a raw material just fed into the beginning of the process, a second step may be carried out simultaneously with respect to an intermediate material resulting from treating the raw materials fed into the process at an earlier time in the first step. Preferably, the steps are conducted in the order described.

[0014] Unless otherwise indicated, all numbers indicating quantities in this disclosure are to be understood as being modified by the term about in all instances. It should also be understood that the precise numerical values used in the specification and claims constitute specific embodiments. Efforts have been made to ensure the accuracy of the data in the examples. However, it should be understood that any measured data inherently contains a certain level of error due to the limitation of the technique and/or equipment used for acquiring the measurement.

[0015] Certain embodiments and features are described herein using a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges including the combination of any two values, e.g., the combination of any lower value with any upper value, the combination of any two lower values, and/or the combination of any two upper values are contemplated unless otherwise indicated.

[0016] The indefinite article a or an, as used herein, means at least one unless specified to the contrary or the context clearly indicates otherwise. Thus, embodiments using a reactor or a conversion zone include embodiments where one or two or more reactors or conversion zones are used, unless specified to the contrary or the context clearly indicates that only one reactor or conversion zone is used.

[0017] In this disclosure, A, B, . . . or a combination thereof means A, B, . . . or any combination of any two or more of A, B, . . . and A, B, . . . , or a mixture thereof means A, B, . . . , or any mixture of any two or more of A, B, . . . .

[0018] The term hydrocarbon means (i) any compound consisting of hydrogen and carbon atoms or (ii) any mixture of two or more such compounds in (i). The term Cn hydrocarbon, where n is a positive integer, means (i) any hydrocarbon compound comprising carbon atom(s) in its molecule at the total number of n, or (ii) any mixture of two or more such hydrocarbon compounds in (i). Thus, a C2 hydrocarbon can be ethane, ethylene, acetylene, or mixtures of at least two of these compounds at any proportion. A Cm to Cn hydrocarbon or Cm-Cn hydrocarbon, where m and n are positive integers and m<n, means any of Cm, Cm+1, Cm+2, . . . , Cn1, Cn hydrocarbons, or any mixtures of two or more thereof. Thus, a C.sub.2 to C.sub.3 hydrocarbon or C2-C3 hydrocarbon can be any of ethane, ethylene, acetylene, propane, propene, propyne, propadiene, cyclopropane, and any mixtures of two or more thereof at any proportion between and among the components. A saturated C2-C3 hydrocarbon can be ethane, propane, cyclopropane, or any mixture thereof of two or more thereof at any proportion. A Cn+ hydrocarbon means (i) any hydrocarbon compound comprising carbon atom(s) in its molecule at the total number of at least n, or (ii) any mixture of two or more such hydrocarbon compounds in (i). A Cn-hydrocarbon means (i) any hydrocarbon compound comprising carbon atoms in its molecule at the total number of at most n, or (ii) any mixture of two or more such hydrocarbon compounds in (i). A Cm hydrocarbon stream means a hydrocarbon stream consisting essentially of Cm hydrocarbon(s). A Cm-Cn hydrocarbon stream means a hydrocarbon stream consisting essentially of Cm-Cn hydrocarbon(s).

[0019] Managing a rapid transition to a backup higher molecular weight fuel gas, e.g., natural gas, during a loss of a H.sub.2-rich fuel gas event is challenging due to current state of the art limitations. The initial decrease of the fuel gas header pressure associated with the loss of the H.sub.2-rich fuel gas causes the furnace fuel gas control valve to widely open to maintain a constant heat input to the process. When the backup higher molecular weight fuel gas is automatically introduced as a response to the loss of the H.sub.2-rich fuel gas, the initial large opening of the furnace fuel gas control valve can cause severe fuel rich or flooding conditions. This scenario based on state of the art control systems is described below with reference to FIG. 3.

[0020] FIG. 1 depicts an illustrative system 100 for managing combustion of a fuel gas in the presence of an oxidant supplied via lines 1040 and 1042, respectively, to a combustion zone 1082 of a furnace 1080, according to one or more embodiments. The system 100 can include a H.sub.2-rich fuel gas source 1002 and an oxidant source 1004. The fuel gas introduced via line 1040 into the combustion zone 1082 can be controlled via a fuel gas valve or first valve 1030 and the oxidant introduced via line 1042 into the combustion zone 1082 can be controlled via an oxidant valve or a second valve 1032. The fuel gas can be a H.sub.2-rich fuel gas that can be supplied via line 1008 from the H.sub.2-rich fuel gas source 1002 to an inlet of the first valve 1030 and the oxidant can be supplied via line 1010 from the oxidant source 1004 to an inlet of the second valve 1032. In some embodiments, the fuel gas via line 1040 and the oxidant via line 1042 can be introduced into the combustion zone 1082 via a burner 1083. The burner 1083 can mix the fuel gas and oxidant to produce a fuel and oxidant mixture prior to the introduction and combustion of the mixture within the combustion zone 1082. In other embodiments, the fuel gas via line 1040 and the oxidant via line 1042 can be introduced into and contacted, mixed, and combusted within the combustion zone 1082.

[0021] The system 100 can also include a backup fuel gas source 1006. The backup fuel gas source 1006 can be configured to supply a backup fuel gas to the first valve 1030. For example, as shown, line 1012 can be in fluid communication with an inlet of a backup fuel gas valve or third valve 1014, line 1015 can be in fluid communication with an outlet of the third valve 1014 and line 1008 at a location between the H.sub.2-rich fuel source 1002 and the first valve 1030. The backup fuel gas can be supplied via lines 1012, 1015, and 1008 to the inlet side of the first valve 1030 by opening the third valve 1014. In some embodiments, the furnace 1080 can be a steam cracking furnace or any other fired heater that can utilize a H.sub.2-rich fuel gas as a primary fuel that can, at times, utilize a backup fuel gas instead of the H.sub.2-rich fuel gas.

[0022] The H.sub.2-rich fuel gas supplied from the H.sub.2-rich fuel gas source 1002 can include molecular hydrogen (H.sub.2) in an amount in a range from 60 mol %, 65 mol %, 70 mol %, 75 mol %, or 80 mol % to 85 mol %, 90 mol %, 95 mol %, 97 mol %, 99 mol %, or even 100 mol %, based on a total weight of the H.sub.2-rich fuel gas. When the H.sub.2-rich fuel gas contains less than 100 mol % of molecular hydrogen, other components in the H.sub.2-rich fuel gas can be or can include, but are not limited to, one or more C.sub.1-C.sub.4 hydrocarbons, Ar, Ne, He, N.sub.2, CO.sub.2, or a mixture thereof. The oxidant supplied from the oxidant source 1004 can include, but is not limited to, air, enriched air, O.sub.2, O.sub.2 diluted by one or more inert gases, or any mixture thereof.

[0023] The backup fuel gas in line 1012 supplied from the backup fuel gas source 1006 can have an average molecular weight that can be greater than an average molecular weight of the H.sub.2-rich fuel gas. In some embodiments, the backup fuel gas can be or can include, but is not limited to, one or more C.sub.1-C.sub.4 hydrocarbons. In some embodiments, the backup fuel gas can include molecular hydrogen in addition to the one or more C.sub.1-C.sub.4 hydrocarbons. If the backup fuel gas includes any molecular hydrogen, the amount of molecular hydrogen in the backup fuel gas is less than the amount of molecular hydrogen in the H.sub.2-rich fuel gas. In some embodiments, the backup fuel gas can be or can include, but is not limited to, natural gas, a methane-rich gas, an ethane-rich gas, a propane-rich gas, a butane-rich gas, or a mixture thereof. The methane-rich gas, ethane-rich gas, propane-rich gas, and butane-rich gas can each include at least 40 mol %, at least 60 mol %, at least 80 mol %, at least 95 mol %, or more of methane, ethanc, propane, and butane, respectively, based on a total weight of the backup fuel gas. In some embodiments, the backup fuel gas can also include one or more inert gases, e.g., Ar, Nc, He, N.sub.2, CO.sub.2, or a mixture thereof.

[0024] In some embodiments, during operation of the furnace 1080, the oxidant can be supplied via line 1010 to the second valve 1032 and the fuel gas can be supplied via line 1008 to the first valve 1030. In other embodiments, during operation of the furnace 1080, the oxidant can be supplied via natural draft or indirectly via an induced draft fan (ID fan) and the fuel gas can be supplied via line 1008 to the first valve 1030. In still other embodiments, during operation of the furnace 1080, the oxidant can be supplied via a forced draft fan (FD fan), air ducts, and a flow control device and the fuel gas can be supplied via line 1008 to the first valve 1030.

[0025] The fuel gas can be contacted with the oxidant within the combustion zone 1082 to effect combustion of at least a portion of the fuel gas to produce a combustion effluent. The fuel gas supplied via line 1008 can be the H.sub.2-rich fuel gas supplied from the H.sub.2-rich fuel gas source 1002. During operation of the furnace 1080, the fuel gas supplied via line 1008 to the first valve 1030 may undergo an event. The event the fuel gas may undergo can be or can include, but is not limited to, a pressure decrease and/or an average molecular weight increase. More particularly, during operation of the furnace 1080, supply of the H.sub.2-rich fuel gas via line 1008 from the H.sub.2-rich fuel gas source 1002 can decrease or completely stop and introduction of the backup fuel gas via lines 1012, 1015, and 1008 to the first valve 1030 can automatically begin. In some embodiments, the pressure decrease and/or the average molecular weight increase can be sudden and large. As used herein, the phrase sudden and large pressure decrease means a rate of change in the pressure is >10%, >12%, >14%, >16%, >18%, or >20% per minute. Similarly, as used herein, the phrase sudden and large average molecular weight increase means a rate of change in the average molecular weight is >10%, >12%, >14%, >16%, >18%, or >20% per minute.

[0026] During operation of the furnace 1080 and before the event, the H.sub.2-rich fuel gas can be supplied via line 1008 from the H.sub.2-rich fuel gas source 1002 to the first valve 1030 at a first pressure. During the event, the fuel gas supplied via line 1008 to the first valve 1030 can change from the H.sub.2-rich fuel gas to the backup fuel gas supplied via lines 1012, 1015, and 1008. In some embodiments, a pressure of the backup fuel gas downstream of the control valve 1030 can be less than the first pressure of the H.sub.2-rich fuel gas and the average molecular weight of the backup fuel gas can be greater than the average molecular weight of the H.sub.2-rich fuel gas. In some embodiments, the gauge pressure of the backup fuel gas downstream of the control valve 1030 can be in a range of 30%, 40%, or 50% to 70%, 80%, or 90% of the pressure of the H.sub.2-rich fuel gas at equivalent firing rates. In some embodiments, when the event occurs, the fuel gas supplied via line 1008 to the first valve 1030 can undergo a pressure decrease in a range from 10%, 20%, or 30% to 50%, 60%, or 70%. In some embodiments, the average molecular weight of the backup fuel gas can be in a range from 2, 3, 4, 5, 6, 7, 8, 9, or 10 to 15, 20, 25, or 30 times greater than the average molecular weight of the H.sub.2-rich fuel gas. In some embodiments, when the event occurs, the fuel gas supplied via line 1008 to the first valve 1030 can undergo an average molecular weight increase in a range from 2, 3, 4, 5, 6, 7, 8, 9, or 10 to 15, 20, 25, or 30. In other embodiments, the pressure of the backup fuel gas can be greater than the pressure of the H.sub.2-rich fuel gas.

[0027] The event or transition from the H.sub.2-rich fuel gas to the backup fuel gas supplied to the first valve 1030 can take place over a relatively short period of time. In some embodiments, the transition from the H.sub.2-rich fuel gas to the backup fuel gas supplied to the first valve 1030 can take place in less than 15 minutes, less than 10 minutes, less than 5 minutes, less than 4 minutes, less than 3 minutes, less than 2 minutes, less than 1 minute, or less than 0.5 minutes. As such, in some embodiments, when the event occurs, the pressure of the fuel gas can decrease in a range from 10% to 70% and/or the average molecular weight can increase in a range from 2 to 30 times the original value over a time period of less than 15 minutes or less. Such a relatively sudden transition from the H.sub.2-rich fuel gas to the backup fuel gas that can have a large pressure decrease and/or a large average molecular weight relative to the H.sub.2-rich fuel gas can cause large furnace firing swings and fuel-rich or flooding conditions within the combustion zone 1082 of the furnace 1080. The particular magnitude and dynamics of the firing swings and flooding conditions can depend, at least in part, on the particular configuration of the furnace, the particular configuration of the fuel gas supply system, the specific composition of the H.sub.2-rich fuel gas, and/or the specific composition of the backup fuel gas.

[0028] The process for managing the combustion of the fuel gas introduced via line 1040 into the combustion zone 1082 of the furnace 1080 can include (I) detecting the event or transition from the H.sub.2-rich fuel gas supplied via line 1008 to the first valve 1030 to the backup fuel gas being supplied via lines 1012, 1015, and 1008 to the first valve 1030. In some embodiments, upon the detection of the event, the fuel gas supplied via line 1040 to the combustion zone 1082 can be controlled by (II-a) limiting an output of the first valve 1030 to no more than a predetermined threshold output to prevent excessive fuel gas flooding the combustion zone 1082. In some embodiments, this can be achieved by placing a high output limit on the valve 1030 but allowing the firing controls to remain online or by deactivating the normal firing controls and applying a set output to the valve.

[0029] The threshold output of the first valve 1030 can be immediately set or otherwise adjusted to a predetermined value or can be set or otherwise adjusted over a period of time using a constant rate or a predetermined algorithm. Said another way, in some embodiments, the predetermined threshold output can be arrived at over a period of time using a predetermined algorithm. In some embodiments, the predetermined algorithm can be FG CV OP (TFinal)=FG CV OP (T)*(1R), where FG CV is valve 1030, OP is valve position, (T) is the time at which the application is initiated and indicates the valve 1030 position at this time, (TFinal) is the time at which the valve 1030 is desired to be moved by and indicates the valve position at this time, and R is the desired fractional reduction in valve position (OP). Factor (R) may also be a function of a number of variables including but not limited to R=x[Density (T)Density (Tfinal)]/Density (T) where Density relates to the fuel gas density measured by sensor 1068 and x is a scaling factor. Factor R may also be a function of fuel gas supply pressure which may be measured by a sensor 1065 and a number of other related variables. Factor R may also vary with time either in response to live variable feedback through one or more instruments (1068/1065/etc.) or at a predetermined rate e.g. R (TLive)=R (TFinal)*Y where Y is a scaling factor and (TLive) is a time occurring within the transition from a high H.sub.2 fuel to a low H.sub.2 fuel and relates to the scaling factor R's value at this time. Factor Y may include a time delay from the initial activation at time (T).

[0030] In some embodiments, the output of the first valve 1030 can be temporarily limited. For example, in some embodiments, the output of the first valve 1030 can be limited for a predetermined period of time, e.g., 5 minutes, 10 minutes, 15 minutes, 20 minutes, or longer, and then the output of the first valve 1030 can be returned to the control of the normal firing controls that were being used prior to the event being detected. In other embodiments, the threshold output of the valve 1030 can be re-adjusted to the initial setting based on the fuel gas header process condition, e.g., using a maximum rate of change value of the fuel gas pressure (measured via sensor 1065) and fuel gas density (measured by sensor 1068). In a preferred embodiment, a combination of a predetermined period of time and fuel gas header condition can be used to return the threshold output of the valve 1030 to an initial value that was used prior to detecting the event. In some embodiments, the threshold output of the valve 1030 can be returned to its initial value via a steady ramp rate.

[0031] In other embodiments, upon the detection of the event, the fuel gas supplied via line 1040 to the combustion zone 1082 can be controlled by (II-b) adjusting the first valve 1030 to maintain a pressure of the fuel gas after the first valve 1030 to no higher than a predetermined threshold pressure to prevent excessive fuel gas flooding the combustion zone 1082. The threshold pressure can be immediately set or otherwise adjusted to a predetermined value or can be set or otherwise adjusted over a period of time using a constant rate or a predetermined algorithm. In other embodiments, when the event has been detected, the first valve 1030 can be controlled by (II-a) limiting the output of the first valve 1030 to no more than the predetermined threshold output and/or (II-b) adjusting the first valve 1030 to maintain the pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure to prevent excessive fuel gas from flooding the combustion zone 1082.

[0032] In some embodiments, the first valve 1030 can be temporarily adjusted to maintain the pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold. For example, in some embodiments, the first valve 1030 can be adjusted for a predetermined period of time, e.g., 5 minutes, 10 minutes, 15 minutes, 20 minutes, or longer, and then the predetermined pressure threshold can be returned to the initial pressure value that was being used prior to the event being detected. In other embodiments, the threshold pressure can be re-adjusted to the initial setting based on the fuel gas header process condition, e.g., using a maximum rate of change value of the fuel gas pressure 1065 and density 1068. In a preferred embodiment, a combination of the predetermined period of time and fuel gas header condition can be used to return the threshold pressure to an initial value that was used prior to detecting the event. In some embodiments, the threshold pressure can be returned to its initial value via a steady ramp rate.

[0033] In some embodiments, upon the detection of the event, step (II-a) and/or step (II-b) can be initiated immediately upon the detection of the event. In other embodiments, upon the detection of the event, step (II-a) and/or step (II-b) can be initiated within a time period ranging from 1 second, 5 seconds, 10 seconds, or 30 seconds to about 45 seconds, 1 minute, 1.5 minutes, 2 minutes, 3 minutes, or 5 minutes. In still other embodiments, upon the detection of the event, step (II-a) and/or step (II-b) can be initiated in less than 5 minutes, less than 3 minutes, less than 2 minutes, less than 1 minute, less than 45 seconds, less than 30 seconds, or less than 10 seconds.

[0034] In some embodiments, step (II-a) can be carried out. In other embodiments, step (II-b) can be carried out. In still other embodiments, step (II-a) and step (II-b) can be carried out simultaneously. In some embodiments, step (II-a) can be initiated and then step (II-b) can be initiated after step (II-a) has begun and before step (II-a) has been completed. In some embodiments, step (II-b) can be initiated and then step (II-a) can be initiated after step (II-b) has begun and before step (II-b) has been completed. In other embodiments, step (II-a) or step (II-b) can be carried out completely followed by the other of step (II-a) or step (II-b).

[0035] Detecting the event can be accomplished via one or more of a number of different ways. In some embodiments, the third valve 1014 can be monitored and detecting the event can include detecting the third valve 1014 opening. For example, in some embodiments, a process control unit 1050 can monitor the third valve 1014 via a communication link 1052. Upon detection that the third valve 1014 is moving from a closed position to an open position or from a first open position to a second open position that is greater than the first open position, the first valve 1030 can be controlled to limit the output of the first valve 1030 to no more than the predetermined threshold and/or to maintain the pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. For example, the process control unit 1050 can send a control signal via a communication link or physical connection 1054 to a pressure control unit 1056 that can via communication link or physical connection 1058 limit the output of the first valve 1030 to no more than the predetermined threshold output and/or to adjust the first valve 1030 to maintain the pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. In some embodiments, the process control unit 1050 can automatically send the control signal via the communication link or physical connection 1058 to limit the output of the first valve 1030 to no more than the predetermined threshold output and/or to adjust the first valve 1030 to maintain the pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. In some embodiments, the pressure of the fuel gas upstream of the first valve 1030 can be monitored via a pressure sensor 1065 and the pressure can be communicated via a communication link 1067 to the pressure control unit 1056 and/or the process control unit 1050.

[0036] In some embodiments, the predetermined threshold output can be based, at least in part, on the density of the fuel gas being supplied to the combustion zone 1082 after the event has occurred, an average molecular weight of the fuel gas being supplied to the combustion zone 1082 after the event has occurred, and/or an output from the first valve 1030 before the event occurs. The density or fuel gas molecular weight is measured by the analyzer 1068. In other embodiments, the predetermined threshold output can be based, at least in part, on a fixed threshold output determined after detecting one or more prior events. For example, a first event can be detected, and the threshold output utilized after detecting the first event can be used as the predetermined threshold output upon detection of a second and any subsequent event(s).

[0037] In some embodiments, the predetermined threshold pressure can be based, at least in part, on a density of the fuel gas being supplied to the combustion zone 1082 after the event has occurred, an average molecular weight of the fuel gas being supplied to the combustion zone 1082 after the event has occurred, a calculated composition of the fuel gas being supplied to the combustion zone 1082 after the event has occurred, and/or the pressure the fuel gas was supplied to the combustion zone 1082 before the event occurred. In some embodiments, the density and/or the average molecular weight of the fuel gas being supplied to the combustion zone 1082 can be continuously monitored and detecting an increase in the density and/or an increase in the average molecular weight of the fuel gas being supplied to the combustion zone 1082 can also be how the event can be detected. In other embodiments, the density and/or average molecular weight of the fuel gas being supplied to the combustion zone 1082 can be measured or otherwise determined in less than 5 minutes, less than 3 minutes, less than 2 minutes, less than 1 minute, less than 30 seconds, or less than 10 seconds upon the detection of the event, e.g., detecting the opening of valve 1014. In some embodiments, the calculated composition of the fuel gas can be based on a measured flow of the back-up fuel gas. In other embodiments, the predetermined threshold pressure can be based, at least in part, on a fixed threshold pressure determined after detecting one or more prior events. For example, a first event can be detected, and the threshold pressure utilized after detecting the first event can be used as the predetermined threshold pressure upon detection of a second and any subsequent event(s).

[0038] In some embodiments, upon detecting the event, limiting the output of the first valve 1030 to no more than a predetermined threshold output and/or adjusting the first valve 1030 to maintain a pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure can aim to maintain an amount of the oxidant, e.g., O.sub.2, within the combustion zone 1082 that can be at least 0.5 mol %, at least 0.7 mol %, at least 1 mol %, at least 1.3 mol %, at least 1.5 mol %, at least 1.7 mol %, or at least 2 mol % greater than an amount of oxidant required to combust the fuel gas introduced into the combustion zone 1082 upon detection of the event.

[0039] In some embodiments, the H.sub.2-rich fuel gas source 1002 can be a H.sub.2 supply source, e.g., a hydrogen production plant such as a blue hydrogen production plant, a green hydrogen production plant, etc., an industrial hydrogen supply pipeline, a hydrogen recovery unit in a chemical plant, and/or a tail gas produced from a recovery section of an olefins production plant, and combinations thereof. The H.sub.2-rich fuel gas source 1002 can supply the H.sub.2-rich fuel gas via line 1008 to the first valve 1030. Under certain circumstances, the H.sub.2-rich fuel gas source 1002 can experience a disruption that can reduce the flow of the H.sub.2-rich fuel gas to the first valve 1030 or can shut down causing supply of the H.sub.2-rich fuel gas into line 1008 to completely stop. When supply of the H.sub.2-rich fuel gas via line 1008 decreases or stops, the third valve 1014 can automatically open to supply the backup fuel gas via lines 1012 and 1008 to the first valve 1030. The backup fuel gas typically differs from the H.sub.2-rich fuel gas during normal operation in pressure and chemical composition. For example, natural gas at a differing pressure is routinely used as a backup fuel gas. The abrupt change of the fuel gas from H.sub.2-rich fuel gas to the backup gas can result in a sudden and large change in pressure and/or average molecular weight of the fuel gas passing through the first valve 1030. In such embodiments, the process control unit 1050 can monitor via a communication link 1064 the operation of the H.sub.2-rich fuel gas source 1002 and, upon detection of a trip signal or other signal indicating a decrease in or complete stoppage of the H.sub.2-rich fuel gas being supplied from the H.sub.2-rich fuel gas source 1002, can send a signal via the communication link 1054 to the pressure control unit 1056 that can via communication link or physical connection 1058 limit the output of the first valve 1030 to no more than the predetermined threshold output and/or to adjust the first valve to maintain a pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. In some embodiments, detecting a change in the one or more monitored process parameters can include detecting one or more of the following: a trip signal from the H.sub.2 supply source, a trip signal from the hydrogen recovery unit in the petrochemical plant, a trip signal of a process gas compressor, and/or a trip signal of a turbo-expander supplying a tail gas as the H.sub.2-rich fuel gas.

[0040] In some embodiments, the process control unit 1050 can monitor via a communication link 1066 one or more sensors 1068 and the event can be detected by detecting a change in one or more monitored process parameters. In some embodiments, the sensor 1068 can measure a density of the fuel gas and/or an average molecular weight of the fuel gas supplied to the combustion zone 1082. In some embodiments, the sensor 1068 can be a densitometer and/or a Wobbe analyzer. Upon detection of an increase in the density of the fuel gas and/or an increase in the average molecular weight of the fuel gas supplied to the combustion zone 1082, the process control unit 1050 can send a signal via the communication link 1054 to the pressure control unit 1056 that can via communication link or physical connection 1058 limit the output of the first valve 1030 to no more than the predetermined threshold output and/or to adjust the first valve to maintain a pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. In some embodiments, upon detection of an increase in the density of the fuel gas and/or an increase in the average molecular weight of the fuel gas supplied to the combustion zone 1082, the process control unit 1050 can automatically send a signal via the communication link 1054 to the pressure control unit 1056 that can automatically via communication link or physical connection 1058 limit the output of the first valve 1030 to no more than the predetermined threshold output and/or to adjust the first valve to maintain a pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. It should be understood that the sensor(s) 1068 can be located before the first valve 1030, as shown, and/or after the first valve 1030, not shown. In other words, one or more sensors 1068 can be located upstream of the valve 1030 and/or one or more sensors 1068 can be located downstream of the first valve 1030.

[0041] In some embodiments, the pressure control unit 1056 can monitor the pressure sensor 1065 via the communication link 1067 and, upon detecting a reduction in the pressure of the fuel gas in line 1008 upstream of the first valve 1030, the pressure control unit 1056 can via the communication link or physical connection 1058 limit the output of the first valve 1030 to no more than the predetermined threshold output and/or adjust the first valve 1030 to maintain a pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure. In some embodiments, the pressure control unit 1056 can monitor the pressure sensor 1065 via the communication link 1067 and, upon detecting a reduction in the pressure of the fuel gas in line 1040 upstream of the first valve 1030, the pressure control unit 1056 can automatically via the communication link or physical connection 1058 limit the output of the first valve 1030 to no more than the predetermined threshold output and/or adjust the first valve 1030 to maintain a pressure of the fuel gas after the first valve 1030 to no higher than the predetermined threshold pressure.

[0042] In some embodiments, the process for managing combustion of the fuel gas introduced via line 1040 into the combustion zone 1082 of the furnace 1080 can also include, upon detecting the event, (III) controlling the oxidant supplied via line 1042 to the combustion zone 1082 by adjusting the second valve 1032 to maintain a flow rate of the oxidant after the second valve 1032 above a predetermined lower limit. For example, upon detecting the event, the process control unit 1050 can send a signal via communication link 1070 to a pressure control unit 1072 that can via communication link or physical connection 1074 adjust the second valve 1032 to maintain a flow rate of the oxidant in line 1042 above a predetermined lower limit. In some embodiments, upon detecting the event, the process control unit 1050 can automatically send the signal via communication link 1070 to the pressure control unit 1072 that can automatically via communication link or physical connection 1074 adjust the second valve 1032 to maintain a flow rate of the oxidant in line 1042 above the predetermined lower limit.

[0043] The predetermined lower limit aims to be high enough to maintain an amount of the oxidant, e.g., O.sub.2, within the combustion zone 1082 that can be at least 0.5 mol %, at least 0.7 mol %, at least 1 mol %, at least 1.3 mol %, at least 1.5 mol %, at least 1.7 mol %, or at least 2 mol % greater than an amount of oxidant required to combust the fuel gas introduced into the combustion zone 1082 once the event occurs. In some embodiments, the predetermined lower limit can be based, at least in part, on a density of the fuel gas and/or an average molecular weight of the fuel gas supplied to the combustion zone 1082 once the event has been detected. In other embodiments, the predetermined lower limit can be based, at least in part, on a fixed lower limit determined after detecting one or more prior events. For example, a first event can be detected, and the predetermined lower limit utilized after detecting the first event can be used as the fixed lower limit upon detection of a second and any subsequent event(s).

[0044] In some embodiments, the predetermined lower limit can be based, at least in part, on a density of the fuel gas, a calculated fuel gas composition, and/or an average molecular weight of the fuel gas supplied to the combustion zone 1082 after the event has been detected and/or an output from the second valve 1032 before the event was detected. In other embodiments, the predetermined lower limit can be based, at least in part, on a fixed lower limit determined after detecting one or more prior events. For example, a first event can be detected, and the lower limit utilized after detecting the first event can be used as the fixed lower limit upon detection of a second and subsequent event. In other embodiments, the predetermined lower limit can be based on the valve 1032 position before the event is detected, i.e. preventing the valve to close when the H.sub.2-rich fuel gas flow decreases or stops.

[0045] The furnace 1080 can be any of a number of different types of furnaces. As shown in FIG. 1, the furnace 1080 can be a steam cracking furnace and the combustion zone 1082 can be located within a radiant section 1086 of the furnace 1080. A hydrocarbon feed via line 1092 supplied from a hydrocarbon feed source 1090 can be heated within a convection section 1084 of the furnace 1080 by flowing the hydrocarbon feed through one or more coils 1093 located within the convection section 1084 to produce a preheated hydrocarbon feed in line 1094. The preheated hydrocarbon feed via line 1094 can be introduced into one or more radiant coils 1096 disposed within the radiant section 1086 of the furnace 1086 and cracked therein to produce a steam cracker effluent via line 1098. In some embodiments, steam can be mixed with the hydrocarbon feed in line 1092 and/or with the preheated hydrocarbon feed in line 1094. Steam cracking furnaces and are well known to those skilled in the art. In some embodiments, the back-up fuel source 1006 and the hydrocarbon feed source 1090 can be the same source. For example, in some embodiments, the hydrocarbon feed source 1090 can supply ethane and/or propane via line 1092 and such ethane and/or propane feed can also be utilized as the backup fuel gas supplied via lines 1012, 1015, and 1008 to the first valve 1030. In some embodiments, the hydrocarbon feed in line 1092 can be steam cracked according to the processes and systems disclosed in U.S. Pat. Nos. 6,419,885; 7,993,435; 9,637,694; 9,777,227; 10,315,968; and 10,988,698.

[0046] In some embodiments, the H.sub.2-rich fuel gas source 1002 and back-up fuel 1006 can supply fuel gas to multiple furnaces. In such embodiments, when the H.sub.2-rich fuel gas source 1002 experiences a disruption, multiple furnaces can be affected. In such embodiments, the detection of the event can automatically trigger a response on all impacted furnaces. In some embodiments, the fuel gas and the oxidant can be supplied to two or more combustion zones, the fuel gas can be supplied to the each of the two or more combustion zones via a corresponding fuel gas valve, and upon detecting the event, (II-a) can be carried out such that the output of each corresponding fuel gas valve can be limited to no more than the predetermined threshold output and/or (II-b) can be carried out such that each corresponding fuel gas valve can be adjusted to maintain the pressure of the fuel gas after the fuel gas valve to no higher than the predetermined threshold pressure. In such embodiments, the predetermined threshold output of each corresponding fuel gas valve can be the same or different with respect to one another, and/or the predetermined threshold pressure of the fuel gas after each corresponding fuel gas valve can be the same or different with respect to one another.

[0047] In some embodiments, when the furnace 1080 is a steam cracking furnace, the hydrocarbon feed in line 1092 can be or can include any feed that includes hydrocarbon and is suitable for producing C.sub.2+ unsaturated hydrocarbons, e.g., ethylene and/or propylene, by pyrolysis, such as by steam cracking. Typical hydrocarbon feeds include 10% hydrocarbon (weight basis, based on the weight of the hydrocarbon feed), e.g., 50%, such as 90%, or 95%, or 99%. In some embodiments, the hydrocarbon feed in line 1092 can be primarily heavy hydrocarbons, e.g., C.sub.5+ hydrocarbons. In other embodiments, the hydrocarbon feed in line 1092 can be primarily light hydrocarbons, e.g., C.sub.4 hydrocarbons. Illustrative hydrocarbon feeds can be or can include, but are not limited to, crude, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids and/or gases, natural gasoline, distillate, virgin naphtha, atmospheric pipestill bottoms, vacuum pipestill streams such as vacuum pipestill bottoms and wide boiling range vacuum pipestill naphtha to gas oil condensates, non-virgin hydrocarbons from refineries, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, atmospheric residue, heavy residue, a C.sub.4/residue admixture, naphtha/residue admixture, hydrocarbon gases/residue admixture, hydrogen/residue admixtures, waxy residues, gas oil/residue admixture, relatively light alkanes, e.g., methane, ethane, propane, and/or butane, recycle streams that can include ethane, propane, ethylene, propylene, butadiene, or a mixture thereof, one or more condensates, fractions thereof, or any mixture thereof.

[0048] FIG. 2 depicts another illustrative system 200 for managing combustion of a fuel gas in the presence of an oxidant supplied via lines 1040 and 1042, respectively, to the combustion zone 1082 of the furnace 1080, according to one or more embodiments. The system 200 depicts a typical steam cracker furnace without air preheat. The system 200 is similar to system 100 with the main difference being that the system 200 introduces the oxidant via line 1042 into the combustion zone 1082 via an induced draft fan 2002. The induced draft fan 2002 can be driven via a motor 2004 or a steam turbine. In some embodiments, the furnace 1080 can include a damper 2008 that can be controlled or otherwise adjusted via an actuator 2006. It should be noted that the hydrocarbon feed source 1090, hydrocarbon feed line 1092, coil 1093, pre-heated hydrocarbon feed line 1094, radiant coil 1096, and steam cracker effluent line 1098 have been omitted in FIG. 2 for clarity. In other embodiments, the steam cracker furnace can include air pre-heat or can use a gas turbine exhaust as the oxidant and can include forced draft fans, air ducts, and a flow control device.

[0049] In some embodiments, an oxygen analyzer 2010 can monitor an oxygen content within the flue gas in the furnace 1080 and can send a control signal via a communication link 2012 to an oxygen control unit 2015 that can communicate via communication link 2016 with the pressure control unit 1072. The pressure control unit 1072 can also communicate via communication link 2013 with a pressure sensor 2014. In some embodiments, the pressure control unit 1072 can reset a bridge wall pressure set point measured by the pressure sensor 2014. In some embodiments, the pressure control unit 1072 can send a control signal via a communication link or physical connection 1074 to adjust, i.e., open or close, the damper 2008 to allow more or less oxidant into the combustion zone 1082. In other embodiments, the pressure control unit 1072 can send a control signal to the motor 2004 that can increase or decrease the speed of the induced draft fan 2002. In still other embodiments, the amount of oxidant introduced via line 1042 into the combustion zone 1082 can be controlled or otherwise adjusted via changing the speed at which the motor 2004 turns the induced draft fan 2002 and via changing the percentage the damper 2008 is open/closed via the actuator 2006. As noted above, in some embodiments, the system 200 can include a forced draft fan (FD fan), air ducts, and a flow control device instead of the induced draft fan 2002.

Examples

[0050] The foregoing discussion can be further described with reference to the following non-limiting examples.

[0051] FIG. 3 depicts an illustrative set of curves 300 for a comparative example that shows a fuel gas control response in a steam cracker furnace when a H.sub.2-rich fuel gas was unexpectedly replaced with natural gas (backup fuel gas) when a process gas compressor that supplied the H.sub.2-rich fuel gas to the furnace tripped and introduction of the natural gas to the furnace was automatically started. Curve 3002 indicates what the position the fuel gas valve was in before, during, and after the fuel gas composition was changed from the H.sub.2-rich fuel gas to natural gas. Curve 3006 indicates what the fuel gas pressure was before, during, and after the fuel gas composition was changed from the H.sub.2-rich fuel gas to natural gas. Curve 3010 indicates the amount of excess oxidant that was present within the combustion zone before, during, and after the fuel gas composition was changed from the H.sub.2-rich fuel gas to natural gas.

[0052] As shown in FIG. 3, the fuel gas valve was opened a significant amount (arrow 3004) to attempt to increase the pressure of the fuel gas when the fuel gas pressure suddenly dropped (arrow 3008). As also shown in FIG. 3, the amount of excess oxidant within the combustion zone dropped to 0% (arrow 3012) due to the combination of the large fuel gas valve opening in response to the fuel gas pressure drop and the large average molecular weight increase of the natural gas as compared to the H.sub.2-rich fuel gas.

[0053] FIG. 4 depicts an illustrative set of curves 400 for an inventive example that shows a fuel gas control response to a fuel system upset. Curve 4002 indicates what the position the fuel gas valve was in before, during, and after the fuel gas composition was changed from the H.sub.2-rich fuel gas to natural gas. Curve 4006 indicates what the fuel gas pressure was before, during, and after the fuel gas composition was changed from the H.sub.2-rich fuel gas to natural gas. Curve 4010 indicates the amount of excess oxidant that was present within the combustion zone before, during, and after the fuel gas composition was changed from the H.sub.2-rich fuel gas to natural gas.

[0054] As shown in FIG. 4, the fuel gas valve began to open but was almost immediately adjusted to limit the output of the first fuel gas valve to no more than a predetermined threshold output (arrow 4004) when the fuel gas pressure suddenly dropped (arrow 4008). As also shown in FIG. 4, the amount of excess oxidant within the combustion zone did not drop below the base line excess O.sub.2 (arrow 4012) due to following the process described herein. As also shown, the maximum threshold output is automatically returned to its initial value after over a period of time.

LISTING OF EMBODIMENTS

[0055] This disclosure may further include the following non-limiting embodiments.

[0056] A1. A process, comprising: introducing a fuel gas via a fuel gas valve and an oxidant-containing feed via an induced draft fan or a forced draft fan into a combustion zone of a furnace; contacting at least a portion of the fuel gas with the oxidant-containing feed before or within the combustion zone to effect combustion within the combustion zone of at least a portion of the fuel gas to produce a combustion effluent; monitoring one or more process parameters during the introduction and combustion of the fuel gas; determining, based on the one or more monitored process parameters, the fuel gas has undergone an event comprising at least one of a pressure decrease and a molecular weight increase; and controlling the fuel gas introduced into the combustion zone by (i) limiting an output of the fuel gas valve to no more than a predetermined threshold output to prevent excessive fuel gas flooding the combustion zone or (ii) adjusting the fuel gas valve to maintain the pressure of the fuel after the fuel gas valve to no higher than a predetermined threshold pressure to prevent excessive fuel gas flooding the combustion zone.

[0057] A2. The process of A1, wherein: before the event, the fuel gas is a H.sub.2-rich fuel gas at a first pressure, during the event, the fuel gas changes from the H.sub.2-rich fuel gas to a backup fuel gas, and a pressure of the backup fuel gas is less than the first pressure and an average molecular weight of the backup fuel gas is greater than an average molecular weight of the H.sub.2-rich fuel

[0058] A3. The process of A2, wherein the backup fuel gas comprises one or more C.sub.1-C.sub.4 hydrocarbons.

[0059] A4. The process of A2, wherein the backup fuel gas comprises natural gas, a methane-rich gas, an ethane-rich gas, a propane-rich gas, a butane-rich gas, or a mixture thereof.

[0060] A5. The process of any one of A2 to A4, wherein: monitoring the one or more process parameters comprises monitoring a backup fuel valve in fluid communication with an inlet side of the fuel gas valve that supplies the fuel gas to the combustion zone, the backup fuel valve is configured to introduce the backup fuel gas into the fuel gas valve upon opening of the backup fuel valve, and determining the fuel gas has undergone the event comprises detecting the backup fuel valve opening.

[0061] A6. The process of any one of A1 to A5, wherein step (i) is carried out, and wherein the predetermined threshold output is determined based on a density of the fuel gas, an average molecular weight of the fuel gas, or a combination thereof being supplied to the combustion zone after determining the fuel gas has undergone the event.

[0062] A7. The process of any one of A1 to A5, wherein step (i) is carried out, and wherein the predetermined threshold output is determined based on a fixed threshold output determined after determining the fuel gas has undergone one or more prior events.

[0063] A8. The process of any one of A1 to A5, wherein step (ii) is carried out, and wherein the predetermined threshold pressure is determined based on a density of the fuel gas, an average molecular weight of the fuel gas, or a combination thereof being supplied to the combustion zone after determining the fuel gas has undergone the event.

[0064] A9. The process of any one of A1 to A5, wherein step (ii) is carried out, and wherein the predetermined threshold pressure is determined based on a fixed threshold pressure determined after determining the fuel gas has undergone one or more prior events.

[0065] A10. The process of any one of A1 to A4, wherein determining the fuel gas has undergone the event comprises detecting a change in the one or more monitored process parameters.

[0066] A11. The process of A10, wherein step (i) is carried out and detecting the change in the one or more monitored process parameters comprises detecting an increased opening of the fuel gas valve.

[0067] A12. The process of A11, further comprising measuring a density of the fuel gas or an average molecular weight of the fuel gas supplied to the combustion zone after determining the fuel gas has undergone the event, wherein the predetermined threshold output is determined based on the measured density of the fuel gas or the measured average molecular weight of the fuel gas.

[0068] A13. The process of A11, wherein the predetermined threshold output is determined based on a fixed threshold output determined after determining the fuel gas has undergone one or more prior events.

[0069] A14. The process of A10, wherein step (ii) is carried out and detecting the change in the one or more monitored process parameters comprises detecting a reduction in a pressure of the fuel gas after the fuel gas valve.

[0070] A15. The process of A14, wherein the predetermined threshold pressure is determined based on a fixed threshold output determined after determining the fuel gas has undergone one or more prior events.

[0071] A16. The process of A14, further comprising measuring a density of the fuel gas or an average molecular weight of the fuel gas supplied to the combustion zone after determining the fuel gas has undergone the event, wherein the predetermined threshold pressure is determined based on the measured density or the measured average molecular weight.

[0072] A17. The process of A10, wherein: before the event is determined, the fuel gas is supplied from a process gas compressor or a turbo-expander, and detecting the change in the one or more monitored process parameters comprises detecting a process gas compressor trip signal or a turbo-expander trip signal.

[0073] A18. The process of any one of A1 to A17, wherein a rate of change of the pressure decrease is >10% per minute and/or a rate of change of the average molecular weight increase is >10% per minute.

[0074] A19. The process of any one of A1 to A18, wherein carrying out step (i) or step (ii) maintains an amount of O.sub.2 within the combustion zone that is at least 0.5 mol % greater than an amount of O.sub.2 required to combust the fuel gas introduced into the combustion zone after the event has been determined.

[0075] A20. The process of any one of A1 to A19, further comprising: after determining the fuel gas has undergone the event, controlling the oxidant supplied to the combustion zone by adjusting a parameter of the induced draft fan or the forced draft fan to maintain a flow rate of the oxidant introduced into the combustion zone above a predetermined lower limit.

[0076] A21. The process of any one of A1 to A20, wherein step (II-a) and/or step (II-b) is carried out automatically.

[0077] A22. The process of any one of A1 to A21, wherein the furnace is a steam cracking furnace.

[0078] Various terms have been defined above. To the extent a term used in a claim is not defined above, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Furthermore, all patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application and for all jurisdictions in which such incorporation is permitted.

[0079] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.