Method Of Regulating The Flow Of Combustible Gas During The Start-Up Phase Of A Reforming Furnace

Abstract

A method for regulating the flow of combustible gas conveyed to the burners of the reforming furnace of a steam reformer, enabling the gas flow feeding the ignited burners of said furnace to be regulated is provided.

Description

The present invention relates to a method for regulating the flow of combustible gas conveyed to the burners of the reforming furnace of a steam reformer, enabling the gas flow feeding the ignited burners of said furnace to be regulated.

Steam methane reforming or SMR makes it possible to produce synthesis gas, a mixture mainly composed of hydrogen and carbon monoxide from a gaseous charge of reactants substantially consisting of hydrocarbons and steam that react together in a tubular catalytic reactor. This technology, which is one of those most used for the production notably of hydrogen, is based on catalytic reactions at a high temperature (800-950° C.) between light hydrocarbons and steam. Strongly endothermic, these reactions require a heat input.

This heat is usually provided by the combustion of a fuel with air with the aid of burners situated in a radiant furnace in which the reforming tubes are disposed. Fumes from combustion circulate outside the tubes disposed in the furnace and provide the reactants with heat necessary for reforming by radiation and convection.

The reformers that we shall consider here are steam reformers with the normal geometry. The furnaces have a certain number of burners disposed in rows on the side walls of SMRs called “side-fired” and SMRs called “terrace wall”, or at roof level in the case of SMRs called “top-fired”. More rarely, burners are placed on the floor of “bottom-fired” SMRs. In all cases these burners are spaced relative to each other.

One of the difficulties in implementing this technology is regulation of the equilibrium between the heat provided and that required, namely an equilibrium between the amounts of heat generated by combustion and the heat required by the endothermic reactions.

Now, this equilibrium is essential for maintaining the temperatures of the tubes below the recommended maximum temperatures (commonly called design temperatures), failing which, the materials of which said tubes are made may become brittle or even crack, causing incidents and unexpected stoppages.

Control of the temperatures of the tubes is particularly difficult during the start-up phases (whether cold or hot). In point of fact, during start-up phases the charge gas is not introduced into the tubes and only an inert gas is made to circulate, and heat consumption is therefore low (by reason of the absence of endothermic reactions inside the tubes during these phases).

In addition, the burners are often able to operate with a large variety of fuels. In the case of units for producing H₂/CO, purge gases will notably be involved coming from the cold box (for the production of CO), and/or from the H₂ purification unit by pressure swing adsorption or PSA, as well as the hydrocarbon source (often natural gas) for the complement. Synthesis gas or even hydrogen may also be used when it is in excess relative to demand.

All the heating system, including among others the design of burners and instrumentation, is generally designed so as to be suited to the nominal working of the production unit. This includes operation with:

• • all (or in some cases the largest part) of the ignited burners, and
  • a combustible gas consisting mainly of recycled gas (notably offgas coming from the PSA, typically 90%) and hydrocarbons (natural gas for example).

However, during start-up of an SMR furnace, and therefore in the absence of endothermic reactions, only a portion of the burners is ignited for heating the reforming tubes and all the furnace, this in order to limit the heating power. A combustible gas flow and an air flow circulate in these burners.

The various burners are provided with individual ignition systems: those that participate in the start-up are distributed in the furnace so that the heat provided is regularly distributed over all the furnace, so as to limit hot spots to a maximum. For this same reason, ignition sequences are also defined for these burners.

The air flow feeding these burners should be such that it maintains sufficient excess air, “excess air” being defined here as being the percentage of air for combustion in excess of the air necessary to ensure the stoichiometry of the combustion reaction. A sufficient air excess guarantees lower flame temperatures and in this way reduces the risk of overheating the tubes. The air flow also acts to limit the risk of explosion by keeping the atmosphere of the furnace below 25% of the low explosivity limit or LEL, and this even though the flames of some burners are extinguished. This air flow is distributed over all the burners whether ignited or not.

The combustible gas flow is injected into the furnace through orifices in the ignited burners. Since the orifices are fixed, this flow is only dependant on the pressure of said gas in the feed pipes upstream of the burners.

Fuels generated during reforming (namely substantially purge gases) are not available on start-up. The only available fuels, generally the gas charge (natural gas, naphtha, light hydrocarbon mixture etc.), have a high calorific value. In order to provide 100% of the nominal heat (namely 100% of the heat necessary for the installation to operate when working nominally), a lower quantity of gas will be required. The gas flow should thus be lower the higher the heating value, and the combustible gas pressure in the feed gas pipes situated upstream of the burners should then also be lower.

This may be illustrated by the following non-limiting the following theoretical case:

Consider an installation of which the burners are normally fed with a mixture of combustible gas consisting of 90% residual PSA gas and 10% natural gas (NG) fed by a common manifold.

The nominal operating conditions are as follows:

• • the relative combustible gas pressure in the feed pipes upstream of the burners is 200 mbar,
  • the air flow conveyed corresponds to 100% of the nominal flow,
  • the heat release is 100%.

Note: relative pressure is understood to mean pressure measured relative to atmospheric pressure. All pressures expressed in the remainder of this text will be relative pressures unless explicitly mentioned.

If, all other conditions being moreover identical, the above fuel mixture (90/10) is replaced by a gas consisting of a 100% natural gas, the flow of fuel feeding the burners will have to be reduced considerably. The calorific value of natural gas being generally at least three times greater than that of PSA offgas, the fuel flow should be reduced by at least three and the pressure should be brought to 20 Mbar, (the flow circulating in the burner being proportional to the square root of the pressure difference).

In the start-up phase, the air flow to be conveyed is fixed (generally 50% of the nominal flow for reasons referred to above). A limited number of burners are ignited for heating the furnace and tubes. In order to determine the suitable pressure of the combustible gas during the start up phase, a person skilled in the art will make use of curves called “burner curves”. They make it possible to connect two quantities together; heat release and fuel gas pressure, and they are specific to each type of fuel.

In order to use these curves, a person skilled in the art will apply certain known accepted rules, including that consisting of limiting the start-up power to the value below the maximum start-up power so as not to damage the reforming tubes. It is at present accepted that this maximum power is 30% of the nominal power. In practice, however, a power of 25% will rather be set.

In order to make understanding easier, we will consider the following case in which the nominal power will correspond to a natural gas pressure of 22 mbar. A burner curve such as the curve reproduced in FIG. 1, shows that the natural gas pressure corresponding to the maximum start-up power (30% of the nominal power) is 2 mbar. Reading the curve then indicates that the natural gas pressure upstream of the burners during the start-up phase should not exceed 2 mbar.

This limiting upstream pressure is lower, the higher calorific value of the fuel.

However, it is in practice extremely difficult to regulate sufficiently low upstream combustible gas pressures. In point of fact, as described above, all the heating system and therefore the fuel feed is designed relative to nominal operation and the system is not able to deliver a combustible gas at a sufficiently low pressure for it to comply with the conditions required for a satisfactory start-up.

Also, lacking the power to reduce the combustible gas flow as much as is necessary, burners ignited during the start-up phase often operate under conditions that approach nominal conditions. The consequences of this are notably an increase in heat, less excess air and therefore less cooling. Flame temperatures are then extremely high and the risks of overheating the reforming tubes are thus considerably increased.

It is clear moreover that the use of combustible gases with an even higher calorific value than natural gas (butane, naphtha for example) tends to increase the risk of overheating still further.

In order to tackle this problem, installations for producing synthesis gas generally have several separate circuits in order to feed the burners with combustible gas. Each of the circuits is dimensioned according to the nature of the combustible gas or mixture of combustible gases that it conveys.

This solution leads to a multiplication of circuits and associated accessories, such as individual isolating valves for the burners. The object of the present invention is therefore to provide a simpler alternative solution that is more compact and more economical for the problem of overheating reforming tubes during the start-up of an SMR furnace, the solution consisting of ensuring, during this start-up, that the burners are provided in operation with a combustible gas with a sufficiently low calorific value adapted for feeding said burners during start-up of the SMR furnace.

The invention therefore relates to a method for regulating the flow of a combustible gas I designed to feed the ignited burners of a reforming furnace of a steam reformer during a start-up phase of said furnace, comprising steps of:

a) feeding a manifold with a gas flow of said combustible gas I,
b) feeding said manifold with a flow of inert gas II,
c) producing at the outlet of the manifold, a gas flow III consisting of a mixture of the combustible gas I and the inert gas II,
d) feeding the ignited burners of the reforming furnace with the gas mixture III coming from step c), via an assembly for distributing the gas mixture III to the ignited burners.

The combustible gas I may be of the natural gas, butane, propane, naphtha type, alone or mixed.

The inert gas II may be an inert gas of the nitrogen type, but a gas with a low calorific value may also be used. Low calorific value is understood to mean a calorific value substantially lower than that of the gas I.

Advantageously, the inert gas II is nitrogen.

According to a particular case, the combustible gas I is natural gas.

Advantageously, the flow of gas mixture III feeding the ignited burners of the furnace is at a pressure at least equal to 3-5 mbar. Preferably, said flow of gas mixture III feeding the ignited burners of the furnace has a partial pressure of the combustible gas I such that the heat released by combustion of the combustible mixture III with air feeding said burners is of the order of 25-30% of the nominal heat.

Advantageously, the ratio of the flow of inert gas injected II to the flow of combustible gas I is between 0.5 and 1.

The invention will be better understood in the light of the example below, accompanied by FIG. 2 that illustrates the principle of injecting nitrogen into the gas manifold situated on the circuit feeding the circuit with fuel.

Thus, according to FIG. 2, a flow of natural gas I and a flow of nitrogen II feed a common gas manifold 3 for the production of a combustible gas III via the pipes 1 and 2 respectively. The mixture III is led, via the feed pipes 4 to the burners 5 of the SMR furnace. Only a portion of the burners 5 is ignited. Only these ignited burners are fed with the combustible mixture III.

The SMR furnace is started up with a constant air flow equal to 50% of the nominal flow. This air flow is distributed over all the burners 5, whether ignited or not.

The flow of natural gas through the ignited burners should be minimized so as to ensure combustion with a large excess of air. This flow is proportional to the square root of the difference between the upstream pressure and the (relative) pressure in the combustion furnace chamber. Since this is considered to be constant (of the order of −1 mbar), it is therefore determined by the pressure measured upstream of the burners.

According to generally recognized practice, it is desired during start-up to apply a heating power of 25% of the nominal power.

Since the heating power is proportional to the combustible gas flow, this means therefore that the combustible gas flow should also be reduced by a factor of four.

Since the flow of gas is proportional to the square root of the pressure, in order to divide the flow by four, the pressure should be divided by sixteen.

Now, as has been recalled above, when, all other conditions being moreover identical, the conventional fuel mixture consisting of 90% PSA residual and 10% natural gas (NG) is replaced by a gas consisting of 100% natural gas, it is necessary to reduce the natural gas to 20 mbar in normal operation (namely for a heating power of 100%). During the start-up phase, the pressure of natural gas feeding the burners should therefore be brought to 20/16 mbar, namely 1.25 mbar.

It is difficult industrially to regulate sufficiently low flows of combustible gas in the feed pipes of burners, it being possible for the minimum pressure to be in practice 3-5 mbar when stabilized, which as the curve of FIG. 1 shows, corresponds for natural gas to a power of 40-50% of that of the nominal power at each burner and may therefore constitute an unacceptable value for preserving reforming tubes.

Nitrogen injection according to the invention enables these difficulties to be overcome. The following table shows the results obtained with various ratios of flows of natural gas and nitrogen [the pressure of the mixture III (natural gas+nitrogen) as well as the air flows are fixed constant].

	Molar ratio N2/NG
	0/100	25/75	50/50
	% nominal heat	50	35	22
	released

Thus, with an N₂/NG ratio of 50/50, the quantity of fuel is reduced by more than a half.

Since the nitrogen circuit is available in most reforming plants (essentially for ensuring start-ups and shut-downs), few modifications will be necessary for implementing the present invention and the quantities of nitrogen employed are moreover low, generally of the order of 500 to 2 500 Nm³/h for plants with a size of 20 000 to 100 000 Nm³/h of hydrogen. The excess operating cost therefore remains very marginal, especially if indirect gains are considered that are due to improved operational reliability.

By implementing the method as previously described, the flow of fuel gas injected I may be reduced, but nitrogen injection also offers advantages among which are:

• • provision of inert gas with regard to combustion makes it possible to increase the overall flow through the burners, thus contributing to their better regulation and therefore to obtaining more stable flames, which makes it possible to reduce substantially the risk of deterioration of tubes by unstable flames;
  • the pressure of the mixture (fuel+nitrogen) III in the feed pipe may be greater, which makes regulation easier and allows for better distribution of flows between burners, contributing in this way to improved thermal uniformity within the furnace;
  • since the portion of combustible gas in the overall flow is lower for each burner, it will be possible to ignite more burners in order to reach a given heating power, which also contributes to improved thermal uniformity within the furnace.

Claims

1-6. (canceled)

7. A method for regulating the flow of a combustible gas designed to feed the ignited burners of a reforming furnace of a steam reformer during a start-up phase of said furnace, comprising steps of:

a) feeding a manifold with a gas flow of said combustible gas,

b) feeding said manifold with a flow of an inert gas,

c) producing at the outlet of the manifold, a blended gas flow consisting of the mixture of combustible gas and inert gas,

d) feeding the ignited burners of the reforming furnace with the blended gas mixture coming from step c), via an assembly for distributing the blended gas mixture to the ignited burners.

8. The method of claim 7, wherein the combustible gas feeding step a) is selected from the group consisting of natural gas, butane, propane and naphtha, alone or mixed.

9. The method as of claim 7, wherein the inert gas feeding step a) is gaseous nitrogen.

10. The method of claim 7, wherein the flow of blended gas mixture feeding the ignited burners of the furnace has a total pressure of 3-5 mbar.

11. The method of claim 7, wherein the flow of blended gas mixture feeding the ignited burners of the furnace has a partial pressure of the combustible gas such that the heat released during combustion of the combustible mixture with air feeding said burners is of the order of 25% of the nominal heat.

12. The method of claim 7, wherein the ratio of the flow of inert gas injected to the flow of combustible gas is between 0.5 and 1.