FR2918656A1 - METHOD OF CONTROLLING THE FLOW OF COMBUSTIBLE GAS DURING THE STARTING PHASE OF A REFORMING OVEN. - Google Patents

Abstract

The invention relates to a method for regulating the flow rate of a fuel gas I intended to supply the burners of a furnace for reforming a steam reformer during a start-up phase of said furnace, a process in which one said burners ignite in a mixture III formed from the fuel gas I and an inert gas II present on the site.

Description

The present invention relates to a method for regulating the flow of gas

  fuel fed to the burners of the reformer furnace of a steam reformer for controlling the flow of fuel gas supplying the burners on said furnace.

  Steam reforming (SMR) is used to produce synthesis gas, a mixture composed mainly of hydrogen and carbon monoxide from a gaseous charge of reactants consisting essentially of hydrocarbons and carbon monoxide. water vapor which react together in a catalytic tubular reactor. This technology, one of the most used for hydrogen production in particular, is based on the catalytic reactions at high temperature (800-950 ° C) of light hydrocarbons with water vapor. Highly endothermic, these reactions require heat input. This heat is usually provided by the combustion of a fuel with air using burners located in a radiant furnace in which the reforming tubes are arranged. The fumes from the combustion flow outside the tubes in the furnace and provide the reactants, by radiation and convection, the heat required for reforming. .

  One of the difficulties of the implementation of this technology is the control of the equilibrium between the supply and the heat demand, ie the balance between the quantities of heat generated by the combustion and the quantities of heat. heat demanded by endothermic reactions. However, this balance is essential to maintain tube temperatures below the recommended maximum temperatures (commonly known as design temperatures), otherwise the materials constituting said tubes can become fragile or even crack, causing incidents and unexpected stops. The temperature control of the tubes is particularly delicate during the start-up phases (whether cold or hot). In fact, during these start-up phases, the feed gas is not introduced into the tubes, only an inert gas is circulated therein, the heat consumption is therefore low (due to the absence of endothermic reactions in the atmosphere). inside the tubes during these phases).

  In addition, the burners are often able to operate with a wide variety of fuels. In the case of H2 / CO production units, this will include purge gases from the cold box (for the production of CO) and / or offgas of the H2 purification unit by adsorption by modulation of pressure swing (adsorption pressure in English, or PSA), as well as the source of hydrocarbons (often natural gas) for the complement. It is also possible to use synthesis gas or even hydrogen when it is surplus to the demand. The entire heating system, including, among other things, the burner design, the instrumentation, is generally designed to be adapted to the nominal running of the production unit. This includes working with: - all (or in most cases most) burners, and - a combustible gas consisting mainly of recycled gas (including offgaz from PSA, typically 90%) and hydrocarbons (natural gas for example). However, when starting an SMR furnace, and therefore in the absence of endothermic reactions, only a portion of the burners are ignited to heat the reforming tubes and the entire oven, in order to limit the heating power. . In these burners flow a fuel gas flow and air flow. The air flow must be such that it maintains sufficient air excess, excess air is here defined as the percentage of combustion air in excess of the air required to ensure stoichiometry of the combustion reaction. Sufficient excess air guarantees lower flame temperatures and thus reduces the risk of overheating of the tubes. The airflow also has the role of limiting the risk of explosion by keeping the atmosphere of the furnace below 25% of the low explosive limit (Low explosivity limit in English or LEL), and this even if the flames of some burners are blown. This air flow is distributed over all the burners, whether they are lit or not. The flow of the combustible gas is injected into the furnace through the burnt burner ports. The orifices being fixed, this flow rate is solely dependent on the pressure of said gas in the supply pipe upstream of the burners.

  Fuels generated during reforming (ie essentially purge gases) are not available at startup. The only fuels available, usually the gaseous filler (natural gas, naphtha, light hydrocarbon mixture, etc.), are of high calorific value. To provide 100% of the nominal heat (ie 100% of the heat required for the operation of the installation in nominal operation), it will require a smaller amount of gas. The gas flow must therefore be all the lower as its heating value is important; the fuel gas pressure in the supply pipe located upstream of the burners must therefore be lower too. This can be illustrated by the following theoretical case, which is not limiting: Either an installation whose burners are normally supplied by a mixture of combustible gas consisting of 90% of PSA waste gas and 10% of natural gas (NG) fed by a common collector.

  The nominal operating conditions are as follows: - the relative pressure of combustible gas in the feed pipe upstream of the burners is 200mbar, - the air flow sent corresponds to 100% of the nominal flow, - the heat released (heat release) is 100%.

  Note: relative pressure means the pressure measured with respect to the atmospheric pressure ... All the pressures expressed in the remainder of this text will be relative pressures, except explicit mention. If we replace, all the other conditions being identical, the fuel mixture (90/10) above with a gas consisting of 100% natural gas, it will significantly reduce the fuel flow to the burners. As the heating value of natural gas is generally at least 3 times higher than that of the PSA offgaz, the fuel flow must be reduced by at least 3 and the pressure should be reduced to 20mbar (the flow circulating in the burners being proportional at the root of the pressure difference).

  In the start-up phase, the air flow rate to be sent is fixed (generally 50% of the nominal flow rate for the reasons mentioned above). A limited number of burners are turned on to heat the entire oven and tubes. In order to determine the fuel gas pressure adapted during the start-up phase, those skilled in the art have curves called burner curves in the English language. They allow to link together the two quantities released heat (heat release) and fuel gas pressure; they are specific to each type of fuel. To use these curves, those skilled in the art apply certain known rules, including that of limiting the starting power to a value less than a maximum starting power, so as not to damage the reforming tubes. It is generally accepted that this maximum power is 30% of the nominal power. In practice, we will rather set a power of 25%. For ease of understanding, consider the following case in which the rated power would correspond to a natural gas pressure of 22 mbar. A curve of the burners such as the curve reproduced in FIG. 1 shows that the natural gas pressure corresponding to the maximum starting power (30% of the nominal power) is 2 mbar. The reading of the curve indicates that the natural gas pressure upstream of the burners during the start-up phase must not exceed 2 mbar. This upstream limit pressure is lower as the calorific value of the fuel is high. However, it is in practice extremely difficult to regulate upstream fuel gas pressures sufficiently low. Indeed, as described above, the entire heating system, and therefore fuel supply is designed relative to the nominal running; the system is not able to deliver a fuel gas at a sufficiently low pressure to comply with the conditions required for a satisfactory start. Also, failing to reduce as much as necessary the fuel gas flow, the burners on during the startup phase often operate under conditions that are close to the nominal conditions. The consequences include an increase in heat, less air in excess and therefore less cooling. The flame temperatures are then extremely high, the risks of overheating of the reforming tubes are thus considerably increased. It is also clear that the use of even higher calorific fuel gases than natural gas (butane, naphtha, for example) tends to increase the risk of overheating. In order to overcome this problem, the synthesis gas production facilities generally have several separate circuits for supplying the burners with combustible gas. Each of the circuits is dimensioned according to the nature of the fuel gas or fuel gas mixture it conveys. This solution leads to a multiplication of circuits and associated accessories, such as individual burner isolation valves. The object of the present invention is therefore to provide a simpler, more compact and more economical alternative solution to the problem of overheating of the tubes. reforming when starting the furnace of the SMR, the solution consisting in ensuring, during this start-up, the supply to the burners in operation, of a fuel gas of sufficiently low calorific value, suitable for supplying said burners at the start of operation. SMR oven. The invention thus relates to a method for regulating the flow rate of a fuel gas I intended to supply the burners of a steam reformer reforming furnace during a start-up phase of said furnace, comprising the steps of a) supplying a collector with a gaseous flow of said fuel gas I, b) supplying said collector with a flow of inert gas II, c) producing at the outlet of the collector a flow of gas III consisting of the fuel gas mixture I and inert gas II, d) supplying the burners ignited from the reforming furnace to the gaseous mixture III resulting from stage c) via a distribution assembly of the gaseous mixture III towards the burners ignited. The fuel gas I may be type natural gas, butane, propane, naphtha alone or mixed.

  The inert gas It can be an inert gas, of the nitrogen type, but it is also possible to use a gas with a low calorific value. By low calorific value is meant a heating value substantially lower than that of the gas I. Advantageously, the inert gas It is nitrogen. In a particular case, the fuel gas I is natural gas.

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

  Advantageously, the flow ratio of the inert gas It injected / fuel gas flow I is between 0.5 and 1. The invention will be better understood from the example below, accompanied by Figure 2 which illustrates the principle the injection of nitrogen into a gas manifold located on the fuel supply circuit of the burners. Thus, according to FIG. 2, a stream of natural gas I and a stream of nitrogen 11 supply via a line 1 and 2 respectively, a common gas manifold 3 for the production of a combustible gas mixture III; The mixture III is fed via the feed pipe 4 to the burners 5 of the SMR furnace.

  Only a part of the burners 5 is lit. Only these ignited burners are fed with combustible mixture III. The start of the SMR furnace is carried out with a constant air flow and equal to 50% of the nominal flow rate. This air flow is distributed in all burners 5, whether on or off.

  Natural gas flow through burning burners should be minimized to ensure a high excess of air combustion. This flow rate is proportional to the square root of the difference between the upstream pressure and the (relative) pressure in the furnace combustion chamber. Since this is considered constant (of the order of -1 mbar), it is therefore determined by the pressure measured upstream of the burners. In accordance with generally accepted practice, it is desirable to have a heating power of 25% of the nominal power during startup. Since the heating power is proportional to the flow rate of the fuel gas, this means that the flow rate of the fuel gas must be reduced by a factor of four as well. The flow of the gas being proportional to the square root of the pressure, to divide the flow by four, it is necessary to divide the pressure by sixteen. However, as has been recalled above, since all the other conditions are identical, the conventional fuel mixture consisting of 90% of PSA waste and 10% of natural gas is replaced ( GN) by a gas consisting of 100% natural gas, it is necessary in normal operation (ie for a heating power of 100%), reduce the pressure of natural gas to 20mbar. During the start-up phase, the natural gas pressure supplied to the burners must therefore be reduced to 20/16 mbar, or 1.25 mbar.

  It is difficult industrially to regulate sufficiently small fuel gas flows in the burner feed pipe; the minimum value of the pressure which can in practice be stabilized is 3-5mbar, which, as shown in the curve of figure 1, corresponds for natural gas to a power of 40-50% of the nominal power at each burner and can therefore be an unacceptable value for the preservation of the reforming tubes. The nitrogen injection according to the invention makes it possible to overcome these difficulties. The following table shows the results obtained with different ratios of natural gas and nitrogen rates [the pressure of the mixture III (natural gas + nitrogen) as well as the air flow are fixed constant]. Ratio molar N2 / GN 0/100 25/75 50/50% of the heat 50 35 22 released nominal Thus, with a ratio of N2 / GN of 50/50, the amount of fuel is reduced by more than half.

  Since the nitrogen circuit is available in most reforming plants (essentially for starting and stopping), little modification will be required for the practice of the present invention; the quantities of nitrogen involved are also low, generally of the order of 500 to 2500 Nm3 / h for plants whose size ranges from 20 000 to 100 000 Nm3 / h of hydrogen. The additional operating cost therefore remains very marginal, especially considering the indirect gains obtained thanks to improved operational reliability. By carrying out the process as previously described, it is thus possible to reduce the flow rate of fuel gas I injected, but the nitrogen injection also offers other advantages, among which: the supply of inert gas under combustion increases the overall flow through the burners, thus helping to better regulate their operation and thus to obtain more stable flames, this significantly reduces the risk of damage to the tubes by unstable flames; the pressure of the mixture (fuel + nitrogen) III in the feed pipe can be greater, which facilitates its regulation and allows a better distribution of the flows between the burners, thus contributing to improving the thermal uniformity in the furnace; As the proportion of the combustible gas in the overall flow is lower for each burner, more burners can be ignited to reach a given heating power, which also contributes to improving the thermal uniformity in the furnace.

Claims (6)

  A method for regulating the flow rate of a fuel gas I for supplying the burners of a steam reformer reforming furnace during a start-up phase of said furnace, comprising the steps of: a) feeding a collector with a gaseous flow of said fuel gas I, b) supplying said collector with a flow of inert gas II, c) producing at the outlet of the collector a flow of gas III consisting of the mixture of fuel gas I and gas II, d) feeding the burners of the reforming furnace into the mixture III from step c) via a distribution assembly of the gaseous mixture III to the burners ignited.   2. Method according to claim 1 characterized in that the fuel gas I feeding step a) is selected from natural gas, butane, propane, naphtha alone or in admixture.   3. Method according to one of claims 1 or 2 characterized in that the inert gas It feeding step a) is nitrogen gas.   4. Method according to one of claims 1 to 3 characterized in that the gas flow III feeding the burners of the furnace has a total pressure of at least 3-5 mbar.   5. Method according to one of the preceding claims, characterized in that the gas flow III feeding the burners of the furnace has a partial pressure of fuel gas I such as the heat released during the combustion of said fuel mixture III with the Air supplying the so-called burners is of the order of 25% of the nominal heat.   6. Method according to one of the preceding claims characterized in that the ratio of inert gas flow It injected / fuel gas flow I is between 0.5 and 1.