Method For Lighting The Burners In A Reforming Furnace

Abstract

A method for igniting the burners of a hydrocarbon steam reforming furnace is provided.

Description

The present invention relates to a method for igniting the burners of a hydrocarbon steam reforming furnace.

Hydrocarbon steam reforming processes, widely called SMR (steam methane reforming) processes produce a mixture predominantly composed of H₂ and CO, which is called syngas or synthesis gas. These processes are based on what are called reforming reactions, which are highly endothermic catalytic reactions at high temperatures (800-950° C.). For this purpose, the hydrocarbons and steam are introduced in catalyst-filled reforming tubes, said tubes being placed in a reforming furnace. The syngas is obtained at the outlet of the tubes.

The furnace is composed of one or more combustion chambers made of refractory walls and burners placed on these walls. The burners are designed so as to transfer the heat of combustions thereof to hydrocarbon/steam mixture through the wall of the tubes, generally by the heat of the flame radiating on the refractory walls of the combustion chamber.

The reforming tubes used in these processes are subjected to very severe conditions and have limited lifetimes. Exposure of these tubes to temperatures above the design temperature, even for a limited time, even locally, may be extremely prejudicial to their lifetime.

The risks of overheating are particularly high during the transient phases, and especially the startup phase because of the absence of endothermic reactions inside the tubes. This is because, during the startup phase, there is only a small amount of inert gas (nitrogen, steam, etc.) flowing in the tubes and the heat generated by the burners is essentially absorbed by the sensitive heat of the tubes, the catalysts and the walls of the furnace.

Furthermore, it is also necessary to take into account the fact that measurements of the temperatures inside a furnace are not necessarily reliable during this startup phase. This is because in general there is no direct measurement of the temperature of the tubes, the sole means available being the temperature measurements carried out at the point where the flue gases are discharged and at the outlet of the reforming tubes, in a manifold. These measurements provide no indication as to the possible local overheating during this transient phase.

Thus, there is a further risk of the tubes overheating due to the problem of measuring the temperature inside the tubes during the startup phase.

The risks are even higher in the case of side-fired SMR furnaces, that is to say those with lateral heating, for which there may be local overheating inside the tubes if the burners ignited are not sufficiently and judiciously distributed.

For these side-fired SMR furnaces, it is therefore important to control the total heating power, which must be kept low enough, and the heat distribution, which must be distributed over the various rows of burners so as to ensure good heating uniformity.

Now, the compromise between these two requirements is becoming at the present time even more difficult because as the performance of burners increases, an additional aim is to minimize the number of burners and/or rows of burners.

Currently at furnace startup, the burner ignition procedures carried out often result from standard practices, often differing according to the plant. If these are not optimized, they may lead to highly nonuniform heating of the reforming tubes, thus running the risk of overheating some of them, at least locally.

Thus, the present invention relates most particularly to side-fired furnaces for which there may be serious local overheating of certain tubes if the burners are not correctly distributed upon ignition.

During the startup phase, thermal equilibrium is not established, the heat produced by the ignited burners not being consumed by the reactions since the reactants are not flowing in the tubes. There is therefore both a risk of excess heat supplied in certain zones of the furnace and a risk of heat not being consumed. To this is also added the difficulty of taking the temperature measurements necessary for protecting the tubes.

There is therefore a need for a reliable operating method for heating the reforming tubes, upon igniting the burners of the furnace, which minimizes the risk of overheating the tubes and improves the uniformity of heating over the length thereof.

One objective of the present invention is to provide a method for igniting the burners of a side-fired reforming furnace enabling the risk of all or some of these tubes locally overheating to be limited.

Another objective of the present invention is to provide a method for igniting the burners of a side-fired reforming furnace enabling all of the tubes to be uniformly heated.

These various objectives are achieved by the use of predetermined burner startup sequences that define the order in which the various horizontal rows of burners will be ignited upon starting up an SMR furnace.

For this purpose, the invention relates to a method for igniting the burners of a hydrocarbon steam reforming furnace at the startup thereof, said furnace comprising at least:

• • a combustion chamber having at least two vertical walls made of refractory material facing each other;
  • at least vertically aligned reforming tubes at mid-distance between the two facing vertical refractory walls; and
  • burners placed on each of the two facing vertical refractory walls, said burners being aligned vertically in the form of columns and horizontally in the form of n rows Ri (where i varies from 1 to n and i increases from the bottom of the furnace upward),in which the burners are ignited according to an ignition sequence consisting of successive steps so that, at the end of ignition, all of the burners necessary for the reforming are ignited,characterized in that the burners ignited during a given step all belong to one particular row and in that the order of the first steps of the ignition sequence is the following:
  • 1—ignition of the burners of a row chosen from the highest rows; then
  • 2—ignition of the burners of a row chosen from the lowest rows; then
  • 3—ignition of the burners of a row chosen from the rows of the central zone, preferably in the upper half; and then
  • 4—ignition of the burners of a row chosen from the rows of the lower half, the order of ignition of the rows being chosen so as to improve the uniformity of the heating throughout the furnace.

The ignition performance of the burners of the furnace according to the invention is further improved when, on completing the sequence up to ignition of all of the rows of burners, the heating over the entire height of the furnace during the following ignition steps is balanced by providing a low/middle/high/middle alternation, etc.

The term “ignition step” is understood to mean the successive ignition of burners in a predetermined order.

The term “ignition sequence” is understood to mean a series of ignition steps carried out in a defined order.

When the invention states that “all of the burners ignited during a step lie in the same row Ri”, this means that the step consists in igniting a set of burners of a given row Ri but this does not mean, however, that all of the burners of said row have to be ignited.

Thus, for example, it will be possible in a given row to ignite only one burner in two, or even fewer.

The expression “all the burners necessary for reforming” is understood to mean all the burners that will operate during the reforming that follows the startup. This is because there may be certain reforming operations that do not require the use of all of the heating power of the furnace, and in this case certain burners are not ignited. Thus, it is common practice in particular not to use the upper row of burners; in this case, the row Ri of highest index “i” that has to be ignited is Rn-1.

According to a preferred way of implementing the invention, at least the row Ri corresponding to the highest “i” is ignited during the first ignition step.

Preferably, when two rows are ignited in succession, they are chosen nonadjacent among the not-yet ignited rows.

FIGS. 1 to 3 illustrate the invention in the particular case of a side-fired reforming furnace with five rows of burners placed symmetrically on both sides of the furnace. FIG. 1 shows a reforming furnace while FIGS. 2 and 3 show the temperature profiles of the reforming tubes (and of the flue gas) after a certain heating time with burners in only one row being ignited (FIG. 2a according to the invention and 2b according to the usual practices of a person skilled in the art) and then both rows (FIG. 3a according to the invention and 3b according to the usual practices).

FIG. 1 shows a side-fired reforming furnace comprising reforming tubes 1 placed at mid-distance between two refractory walls 2 and 3. The refractory walls 2 and 3 are fitted with burners 4 arranged in the form of columns and rows. The rows of burners are referenced R1 to R5 from the bottom upward.

The startup practices of the prior art recommend igniting one burner in two in each row, starting with the lowermost rows of the furnace (R2 or even R1) and then continuing with other rows. This choice is dictated in particular by the ease of implementation—it is easier to ignite the rows in order starting from the bottom of the furnace.

Certain startup practices also recommend igniting the burners in two adjacent rows at the top of the furnace (R4 and R5) during hot startup.

Several types of problem are potentially caused by igniting the burners in the lower rows (R1 or R2), i.e. especially:

• • local overheating;
  • nonuniform heating inside the furnace;
  • excessively long duration of the startup phase, owing to overly slow heating of the convection zone.

FIGS. 2 a and 2b show the temperature profiles of the reforming tubes (and of the flue gas) measured after a given heating time (which is identical in both cases) with burners on only one row being ignited (FIG. 2a according to the invention and 2b according to the usual practice).

Comparing FIGS. 2a with FIG. 2b shows that igniting the row of burners located at the top of the furnace (R5) results in more uniform tube temperatures (FIG. 2a) than that of the burners in row R2 (second from the bottom of the furnace). Moreover, igniting the burners in the highest row also results in a higher flue gas temperature at the outlet of the furnace, which makes it possible to heat the convection zone more rapidly and to generate necessary steam at SMR startup more rapidly.

When a second row of burners is then ignited, the ignition sequence consisting in igniting the burners in row R5 and then in row R2 results, as shown in FIG. 3a, in good temperature uniformity over the entire length of the tubes and limits the highest temperature levels reached. However, igniting burners in two adjacent rows, in particular when the rows are located in the lower part of the furnace, results in an excessive maximum tube temperature localized in the rows in question, as FIG. 3b shows.

The following table compares the maximum temperatures of the reforming tubes reached after a certain heating time with the sequence R5-R2 of FIG. 3a and the various sequences for igniting the rows of burners, including the sequence R2-R3 of FIG. 3b.

TABLE 1
Rows of ignited burners Maximum tube temperatures
R5-R2                   Base
R4-R5                   +53° C.
R2-R4                   +66° C.
R3-R4                   +84° C.
R2-R3                   +154° C.

It may be seen that the advantage of the novel sequence according to the invention is clearly demonstrated, the risk of overheating therein being minimized.

On the other hand, ignition of a bottom row, followed by that of the adjacent row, according to the standard practice of a person skilled in the art, proves to be the most unfavorable both in terms of localized overheating of the tubes, and therefore the risk of accelerated deterioration thereof, and in terms of heating uniformity over the entire furnace.

The ignition performance of the burners of the furnace according to the invention is further improved when, on completing the sequence up to the ignition of all of the rows of burners, the heating during the subsequent ignition steps is balanced over the entire height of the furnace.

Thus, for a side-fired SMR furnace, in accordance with FIG. 1, that is to say one having five rows of burners, the following novel ignition sequences are recommended:

• • ignition of burners in various rows according to the following sequences:
    • R5→R1→R3→R2→R4 or R5→R2→R3→R1→R4;
  • it may also be useful to limit the heating power with limited burner ignition per row, possibly up to one in three burners or even one in four burners, if this is necessary.

It is therefore preferential to start the ignition via the top of the furnace, followed by a low/middle/high alternation. It will also be preferable to avoid igniting adjacent rows (however, admittedly when the number of rows is small this constraint cannot be respected).

The invention thus makes it possible:

• • to distribute the heating power uniformly within a furnace;
  • to limit local overheating;
  • to shorten the startup time by producing steam more rapidly in the convection chamber, and heating the tubes more rapidly because of the good uniformity of the tube temperatures obtained.

The invention is applicable to improving the operation of existing reforming furnaces.

Claims

1-3. (canceled)

4. A method for igniting the burners of a hydrocarbon steam reforming furnace at the startup thereof, said furnace comprising: a combustion chamber having at least two vertical walls made of refractory material facing each other;at least vertically aligned reforming tubes at mid-distance between the two facing vertical refractory walls; andburners placed on each of the two facing vertical refractory walls, said burners being aligned vertically in the form of columns and horizontally in the form of n rows Ri (where i varies from 1 to n and i increases from the bottom of the furnace upward),

5. The method of claim 4, wherein at least the row Ri corresponding to the highest “i” is ignited during the first ignition step.

6. The method of claim 4, wherein, when two rows are ignited in succession, they are chosen nonadjacent among the not-yet ignited rows.

7. The method of claim 4, wherein the igniting of the burners of a row chosen from the rows of the central zone, comprise the upper half.