Over firing protection of combustion unit

Abstract

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.

Claims

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), b) and c), f) comparing the value of step d) with the value of step e), and g) generating an alarm state output if the value of step d) exceeds the value of step e); 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.

2. The method for protecting a combustion unit according to claim 1, wherein fuel addition is limited if the value of step d) exceeds the value of step e).

3. The method for protecting a combustion unit according to claim 1, wherein the operational state of the burners is detected by a flame detection device.

4. The method for protecting a combustion unit according to claim 3, wherein said flame detection device comprises a human operator.

5. The method for protecting a combustion unit according to claim 3, wherein said flame detection device comprises at least one camera with a view of the plurality of burners.

6. The method for protecting a combustion unit according to claim 1, wherein the number of burners which should 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 the position of shut-off valves on the fuel lines feeding each of the burners, and the number of burners which should be in operation is compared to the number of burners which are in operation.

7. The method for protecting a combustion unit according to claim 1, 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.

8. The method for protecting a combustion unit according to claim 1, wherein the method further comprises the steps of acquiring a value for a flue gas temperature down-stream of the burners, acquiring a value for a process gas outlet temperature or outlet gas temperatures and generating an alarm state output if said values are not within a pre-set range.

9. The method for protecting a combustion unit according to claim 8, wherein the pre-set range of the values varies with the capacity of the combustion unit.

10. The method for protecting a combustion unit according to claim 1, wherein the alarm state output comprises visual and/or acoustic alarms.

11. The method for protecting a combustion unit according to claim 1, wherein the alarm state output comprises reducing the fired duty, or shutting down one or more of said burners.

12. The method for protecting a combustion unit according to claim 1, 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.

13. The method for protecting a combustion unit according to claim 12, 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.

14. An apparatus for protecting a combustion unit having at least one burner, said apparatus comprising a computer configured 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; 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.

15. A method comprising using the apparatus according claim 14 for a chemical reactor or a fired heater.

Description

EXAMPLES

(1) Fuel Pressure Limitation

(2) 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:

(3) $\mathrm{No}\mathrm{of}\mathrm{burners}=\frac{Q}{{\mathrm{HR}}_B}$$\left(Q=\mathrm{fired}\mathrm{duty},{\mathrm{HR}}_B=\mathrm{heat}\mathrm{release}\mathrm{per}\mathrm{burner}\right)$${\mathrm{HR}}_B=\sqrt{\frac{P_{\mathrm{fuel}}}{P_{\mathrm{norm}}}}$$\left(P_{\mathrm{norm}}=2.5\mathrm{kg}/{\mathrm{cm}}^{2}g\mathrm{normal}\mathrm{pressure},P_{\mathrm{fuel}}=\mathrm{measured}\mathrm{fuel}\mathrm{pressure}\right)$

(4) 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. In the case it was reported that the burners are unstable at pressure below 1.0 kg/cm.sup.2 g which explains the large difference between P.sub.actual, data and P.sub.actual, desired at the ignition phase.

(5) Duty/Fuel Limitation

(6) The duty limitation is given by:
Q.sub.Furnace=Q.sub.Air+Q.sub.Fuel=(m.sub.CA+m.sub.FG)×Cp.sub.FL×ΔT.sub.FURN (alternatively Q.sub.Furnace=m.sub.FL×Cp.sub.FL×ΔT.sub.FURN)
Q.sub.Reformer=Q.sub.N2+Q.sub.PS+Q.sub.PG+Q.sub.REAC=(m.sub.N2×CP.sub.N2+m.sub.PS×Cp.sub.PS+m.sub.PG×Cp.sub.PG)×ΔT.sub.REF+ΔH.sub.R×m.sub.PG
Q.sub.loss=0.015×(Q.sub.Furnace+Q.sub.Reformer)
Q.sub.max=Q.sub.Furnace+Q.sub.Reformer+Q.sub.loss+Q.sub.margin=1.015×(Q.sub.Furnace+Q.sub.Reformer)+Q.sub.margin

(7) No heating term has been included for heating of refractory, tubes and catalyst. 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.

(8) Parameters are shown in table below

(9) TABLE-US-00001 Fixed/ Variable Description Variable Value Unit m.sub.i Massflow, measured or calculated Variable from volume flow and molecular weight Cp.sub.FL Heat capacity flue gas Fixed 0.30 kcal/kg/° C. Cp.sub.N2 Heat capacity nitrogen Fixed 0.27 kcal/kg/° C. Cp.sub.PS Heat capacity process steam Fixed 0.52 kcal/kg/° C. Cp.sub.PG Heat capacity process gas Fixed 0.95 kcal/kg/° C. ΔT.sub.FURN Temperature increase in furnace (air to Fixed 925 ° C. flue gas) ΔT.sub.REF Temperature increase in reformer Fixed 300 ° C. (T.sub.PG, out − T.sub.PG, in) ΔH.sub.R Average enthalpy of reaction Fixed 2500 kcal/kg Q.sub.Margin Margin on fired duty Fixed 0.0 Gcal/h

(10) 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.

(11) The margin for maximum duty (Q.sub.margin) and the constants above are fixed values but shall be adjustable and checked/adjusted during commissioning of the plant.

(12) Flue Gas Temperature Limitation

(13) As mentioned, the flue gas temperature can be used to limit firing as an extended over firing protection. Maximum flue gas temperature as per calculated in a system for capacity 50-110%:
TIC.sub.FLUE(SP)=A[° C.]+B[° C./%]×Cap[%]
where A and B are estimated based on 50% and 100% operating case.

(14) This function is extended to lower capacities by:
TIC.sub.FLUE(SP)=MAX(TD.sub.TWT,A[° C.]+B[° C./%]×Cap)
where TD.sub.TWT is design max tube wall temperature.

(15) In a case, at 100% load the flue gas temperature is approximately 1000-1030° 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 function is therefore

(16) 0-50% capacity, maximum flue gas temperature 916° C.

(17) 100% capacity, maximum flue gas temperature 1050° C.

(18) 50-100% capacity, linear function, 916° C. to 1050° C.

(19) 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 500° C. 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 500° C. (step function at 0% capacity).