TECHNICAL FIELD OF THE INVENTION
The present invention relates to a device and method for producing synthetic gas. It applies, in particular, to the field of producing synthetic natural gas by gasification of hydrocarbon compounds.
STATE OF THE ART
Biomethane is an "SNG" (Synthetic Natural Gas), and can be produced by thermochemical conversion of any hydrocarbon compound. This conversion is realized by a method consisting of the following four main steps:
- gasifying the hydrocarbon compound to produce syngas (synthetic gas), mainly composed of H2, CO, C02, CH4, H20 and Tars (C6+);
- purifying the syngas to remove the tars and the sulfated and/or chlorinated impurities;
catalytic methanation, consisting of converting the H2 and CO into CH4; and adjustment to specifications, aimed at eliminating the water, residual H2, and C02 to produce a synthetic natural gas as close as possible to the specifications of natural gas. Specification refers, in particular, to the following non-exhaustive list:
- high heating value, abbreviated to "HHV";
- Wobbe index; and maximum H2 and C02 content.
Gasification of the hydrocarbon compound is carried out within a reactor in which the biomass undergoes different reaction steps.
The first step is a thermal degradation of the hydrocarbon compound which is subjected, successively, to drying and then devolatilization of the organic matter to produce:
a carbonaceous residue (referred to as "char");
a synthetic gas, such as H2, CO, C02, H20 and CH4; and
condensable compounds contained within the syngas, such as tars.
The carbonaceous residue can then be oxidized by a gasification agent, such as water vapor, air or oxygen, to produce H2 and CO. Depending on its nature, this gasification agent may also react with the tars or the major constituent gases. Thus, if it is water vapor (H20) then a Dussan reaction, referred to as a "gas-to-water" reaction and

usually known under the acronym "WGS" (for Water-Gas Shift), occurs in the gasification reactor according to the following equilibrium:
CO + H20 ^^2 + C02
The reactor pressure has little effect on this reaction. In contrast, the equilibrium is closely linked to the reactor temperature and to the "initial" concentrations of reagents. For existing methods, the H2/CO ratio is generally less than 2 at the end of the gasification step. This ratio is an important factor for biomethane production in the subsequent purification and methanation steps that make the production of CH4 possible and on which the SNG production method is based.
The WGS reaction can be carried out in a specific reactor placed upstream from the methanation. However, in the case of certain fluidized-bed methods, the two reactions, methanation and WGS, can be carried out in parallel in the same reactor, the vapor necessary for the WGS reaction being injected into the reactor at the same time as the reactant mixture.
The production of biomethane from the syngas output from the gasification step is based on the catalytic methanation reaction of the CO or CO2, called the "Sabatier reaction". Methanation consists of converting the carbon monoxide or dioxide in the presence of hydrogen and a catalyst, generally based on nickel or any other transition metal of the periodic table, to produce methane. It is governed by the following competitively balanced hydrogenation reactions:
CO + 3H2 ^^CH4 + H20 AG298K = -206 kJ/g.mol C02 + 4H2 ^^ffl4 + 2H20 AG298K =-165 kJ/g.mol
Under the conditions generally used to produce SNG from the syngas obtained from gasification, the methanation reaction of the CO is largely favored because of the lack of H2 in the syngas produced.
The methanation reaction is an exothermic reaction with a reduction in the number of moles; according to Le Chatelier's principle, the reaction is encouraged by increasing the pressure and discouraged by increasing the temperature.
The production of methane from CO and H2 is optimum for a gas with a composition close to the stoichiometric composition, ie when the H2/CO ratio is close to 3. The syngas produced by gasification, especially of biomass, is characterized by a lower H2/CO ratio, of the order of 1 to 2. Also, to maximize the production of methane, this ratio must be adjusted by producing hydrogen using the reaction between carbon monoxide and water vapor through the WGS reaction.

At temperatures below 230°C, nickel, constituting the catalyst or present in the material making up the reactor walls, is likely to react with the carbon monoxide to form nickel tetracarbonyl (Ni(CO)4), a highly toxic compound. For this reason, it is essential that all portions of the reactor where CO and the metallic compound are present are always at a temperature above 150°C and preferably above 230°C to prevent this completely.
The heat generated during CO conversion is approximately 2.7 kWh during the production of 1 Nm of methane. Controlling the temperature inside the reactor, and therefore removing the heat produced by the reaction, is one of the keys to minimizing the deactivation of the catalyst, by sintering, and maximizing the methane conversion rate.
Among the methanation reactor technologies, some utilize a dense fluidized-bed reactor, the bed being formed by the methanation reaction catalyst. The heat produced by the reaction is therefore removed by exchangers immersed in the fluidized bed. However, because of the very high exothermicity of the reaction, the amount of heat to be removed, and therefore the exchange surface areas required, is very great. Therefore, the volume occupied by this exchanger leads to an overall over-dimensioning of the reactor size, and above all to making its design more complex.
The utilization of a fluidized bed is a simple solution for limiting the reaction temperature. Fluidization of the catalyst by the reactant mixture enables almost perfect homogenization of the temperatures at all points of the catalyst layer and the reactor can be assimilated to an isothermal reactor. The removal of the heat produced by the reaction is achieved by means of exchangers immersed within the fluidized bed. The thermal exchange coefficients between the fluidized layer and a wall immersed into the bed are very high (of the order of 400 to 600 W/K.m , comparable to those between a liquid and a wall) and make it possible to minimize the dimensions of the exchanger, and therefore the overall size of the reactor.
In the temperature range utilized in the fluidized-bed methanation reactors, the kinetics of the methanation reaction are very rapid and, as a result, the amount of catalyst required just for the chemical reaction is small. Consequently, the size of the reactor and the amount of catalyst used stem from the overall dimensions of the exchanger installed within the fluidized bed.
Because of the thermal exchange and the fluidization regime, a major drawback is attributable to this technology for high-pressure operation. The reduction in the gas volume due to increased pressure leads to a smaller cross-sectional area available for positioning the exchanger (for equivalent power). However, solutions of the person skilled in the art

exist for overcoming the adjustment of the effective surface area or the fluidization regime. For example, the non-exhaustive solutions are:
- reducing the number of tubes and, as a result, increasing the height of the catalyst layer, with a height limit linked to the slugging phenomenon with reduced thermal exchange properties, slugging being the movement of solids in packets with, between two packets, a gaseous pocket that fills the entire cross-sectional area of a reactor, which has the effect of producing an alternation between solid and gaseous packets instead of a mixture of gas and solids, as expected; and
- modifying the physical characteristics of the catalyst (particle size, density of the support) to preserve an equivalent fluidization for a lower volumetric flow rate.
In terms of flexibility, the fluidized bed naturally allows greater flexibility in terms of reactor flow rate and therefore power with regard to sizing conditions.
With regard to reactant, and unlike fixed-bed technologies or wall-cooled reactors, methanation of the syngas in a fluidized bed requires no pre-WGS. Co-injecting vapor with the syngas means the CO methanation and WGS reactions can be carried out in the same device.
The solutions currently proposed for this technological family are not really differentiated from each other by conversion efficiency, but mainly by the methodology utilized to cool the reactor.
Lastly, the function of the final step of adjustment to specifications is to separate the constituents of the gas produced by methanation in order to obtain a biomethane meeting the specifications for injection into the natural gas grid. This separation therefore generates the sub-products H2O, CO2 and H2. It is usually carried out in separate equipment with sometimes very different operating conditions.
On output from the methanation reactor, the water is first separated from the biomethane through cooling and condensation, by passing below the dew point temperature of the water under the conditions in question.
Next, the CO2 is extracted from the gas to meet the specifications of the grid. The technologies making this separation possible are relatively numerous and known. The main technological families are physical or chemical absorption, pressure-modulated adsorption, membrane permeation, and cryogenics.
Lastly, in the final step before the SNG produced is processed and injected, a significant fraction of the residual H2 from the methanation reaction has to be removed in order to meet the specifications concerning more particularly the HHV. The technique used

most frequently for this separation is membrane permeation, which can present a non-trivial level of complexity and costs, in terms of capital expenditure and operation, with a considerable impact on the value chain.
The composition of the raw SNG on output from the reactor is closely related to the operating conditions of the reactor, in terms of pressure, temperature, adiabatic or isothermal operating mode of the reactor, yet these conditions govern the chemical balances of the above reactions. These reactions generally form water, and consequently this species needs to be separated. With regard to the other species (CO, CO2 and H2), their respective concentrations can be modified by acting first on the operating mode of the reactor (adiabatic or isothermal) and second on the temperature or pressure. A high pressure and a low temperature will therefore allow the concentrations of these compounds to be reduced considerably. When the operation is carried out in an "adiabatic" reactor, a series of steps is also necessary to achieve an equivalent conversion quality to the isothermal reactor. In any event, the composition of the gas produced is generally incompatible with regard to the injection specifications, and consequently improvement steps are necessary to remove the CO2 and/or the residual H2. Therefore, the operating mode forms a block to simplifying the chain of methods.
Systems are known such as those described in document US 2013/0317126. In these systems, an adiabatic methanation reactor is utilized and a portion of the methanation products is recirculated in the input to said reactor.
This recirculation of methanation products is aimed, according to this document, at adjusting the temperature of the reagents input into the adiabatic methanation reactor so as to moderate the exothermicity of the adiabatic reaction occurring in the reactor.
In these solutions, the temperature reached on input to the methanation reactor is of the order of 310 to 330°C and the temperature on output from this reactor is of the order of 620°C, which results in a somewhat inefficient conversion limited by the thermodynamics of the reaction and therefore the presence of undesirable compounds, for example excess H2, CO, CO2 in the flow output from the reactor. This presence of undesirable compounds, especially hydrogen, makes a hydrogen separation step necessary downstream from the reactor to meet, for example, the specifications for injection into the natural gas distribution or transport grid.
The flow is said to meet injection specifications when the following characteristics of the flow:
- high heating value ("HHV");

- Wobbe index; and
- hydrogen content
are within predefined value ranges corresponding to the particular characteristics of the natural gas distribution or transport grid.
Systems such as those described in document US 3 967 936 are also known. In these systems, a series of adiabatic methanation reactors is utilized and a portion of the methanation products is recirculated in the input to each reactor of the series.
This recirculation of methanation products is aimed, according to this document, at adjusting the temperature of the reagents input into the adiabatic methanation reactor so as to moderate the exothermicity of the adiabatic reaction occurring in the reactor.
In the same way, these solutions require the separation, downstream from the reactor, of undesirable compounds such as hydrogen to meet, for example, the specifications for injection into the natural gas distribution or transport grid.
Systems are known such as those described in document US 2009/0247653. In these systems, a series of three adiabatic methanation reactors is utilized, the last reactor in the series being designed to produce additional synthetic methane. In these systems, a portion of the methanation products CO and H2 is recirculated after the second reactor towards the input flow of the first reactor in the series so as to adjust the CO and H2 ratio and adjust the temperature of the input flow of the first reactor in order to moderate the exothermicity of the adiabatic reaction occurring in the first reactor.
However, these solutions also require the separation, downstream from the reactor, of undesirable compounds such as hydrogen.
The adiabatic reactors, while they do not assume cooling within the methanation reactor, have several drawbacks:
a plurality of reactors in series is necessary to obtain a satisfactory methanation
yield; and
- in the case of CO methanation, a specific composition of the synthetic gas, by
adding a so-called gas-to-water catalyst step, is necessary to obtain a satisfactory
stoichiometry.
On the other hand, the design of adiabatic reactors is simpler, since they basically consist of a chamber having to resist generally high pressures (>30 bars) to achieve a satisfactory conversion.
In the case of methanation in an isothermal reactor, the operating pressure does not need to be as high (<20 bars), but requires the arrangement of surfaces immersed in the

catalyst layer, which generates a complex design and additional cost linked to the cooling system.
Therefore, the current systems do not make the adjustment to specifications for injection into the natural gas distribution or transport grid possible without a hydrogen separation step, downstream from the methanation step, taking place. SUBJECT OF THE INVENTION
The present invention aims to remedy all or part of these drawbacks.
To this end, according to a first aspect, the present invention envisages a synthetic gas production device, which comprises:
an isothermal methanation reactor comprising:
an inlet, for syngas produced by gasifying hydrocarbon material, connected to a syngas supply channel, and an outlet for synthetic natural gas;
a water separation means comprising:
an inlet for synthetic natural gas and
an outlet for dehydrated synthetic natural gas; and
a bypass for a portion of the dehydrated synthetic natural gas from the outlet of the
water separation means to the syngas supply channel in order to provide a mixture
of the syngas and the bypassed synthetic natural gas to the reactor.
As the water separation means cools the synthetic natural gas, the supply of this gas on input to the reactor allows the syngas to be cooled and means the reactor does not require a heat exchanger above a certain recirculation rate. Above this rate, the surface area required for an exchanger immersed in the reactor is reduced. The design of such a reactor, especially in terms of sizing, is made even simpler. In addition, the supply of dehydrated synthetic natural gas in the supply channel improves the Wobbe index and HHV of the methanation reaction products through the favorable modification of the reaction balances. Therefore, the device that is the subject of this invention allows a simplified sizing of the reactor and the simplification of the adjustment to specifications, no longer requiring separation of the hydrogen before processing for injecting into the gas grid.
In addition, the utilization of an isothermal reactor makes it possible to have a single methanation step to obtain an efficient synthetic natural gas conversion and obtain a gas of a quality close to the specifications for injection into the natural gas distribution or transport grid.

In some embodiments, the device that is the subject of the present invention comprises a means for separating carbon dioxide from the dehydrated synthetic natural gas, this separation means being positioned downstream from the bypass.
These embodiments improve the adjustment to specifications of the methanation reaction products.
In some embodiments, the device that is the subject of the present invention comprises a means for separating carbon dioxide from the dehydrated synthetic natural gas, this separation means being positioned upstream from the bypass.
These embodiments further improve the simplification of the adjustment to specifications of the methanation reaction products.
In some embodiments, the device that is the subject of the present invention comprises:
a sensor of a temperature inside or on output from the reactor; and
a recirculator of products input to the bypass, this recirculator being controlled as a
function of the temperature measured.
These embodiments make it possible to regulate the flow rate of reaction products recirculated as a function of the temperature measured. If the measured temperature is above a predefined temperature, corresponding to optimum methanation reaction conditions, the flow rate of recirculated products is increased to cool the reaction medium of the reactor. Conversely, if the measured temperature is below the predefined temperature, the flow rate of recirculated products is reduced.
In some embodiments, the device that is the subject of the present invention comprises, upstream from the input to the reactor, a means for preheating the mixture to a temperature higher than 150°C.
These embodiments make it possible to limit the formation of nickel tetracarbonyl in the methanation reactor, the nickel tetracarbonyl being formed at a temperature below 230°C, with a decrease between 150°C and 230°C.
In some embodiments, the preheating means heats the mixture to a temperature higher than 230°C, to avoid the production of toxic compounds, and less than the operating temperature of the reactor, to allow the reactor to be cooled.
In some embodiments, the device that is the subject of the present invention comprises a channel for injecting water vapor into the syngas supply channel upstream from the place for forming the mixture of syngas and the methanation products output from the bypass.

These embodiments make it possible to achieve a WGS reaction to adjust the H2/CO ratio in the reactor, so as to prevent the deactivation of the catalyst by coke deposit and improve the CH4 production yield.
In some embodiments, the device that is the subject of the present invention comprises a bypass channel, for a portion of the hot methanation reaction products, comprising:
an inlet positioned between the outlet from the reactor and the water separation
means; and
an outlet positioned upstream from the inlet to the reactor and downstream from the
preheating means.
These embodiments make it possible to keep the overall flow rate in the methanation reactor constant by bypassing a portion of the hot methanation products, on output from the reactor. Maintaining this flow rate gives the device increased flexibility in terms of syngas supply.
In some embodiments, the device that is the subject of the present invention comprises:
a means for measuring the syngas flow rate after injecting water vapor output from
the water vapor injection channel and after mixing with the gas output from the
bypass;
a recirculator of products input into the bypass channel, this recirculator being
controlled as a function of the flow rate measured.
These embodiments make it possible to maintain the overall rate of the flow input to the methanation reactor.
In some embodiments, the water separation means is configured to cool the synthetic natural gases to a temperature between -5°C and +60°C.
These embodiments enable increased water separation in the synthetic natural gases.
In some embodiments, the water separation means is configured to cool the synthetic natural gases to a temperature below the dew point temperature of the water at the operating pressure of the reactor in question.
In some embodiments, the reactor is configured to carry out a so-called "gas to water" Dussan reaction.
These embodiments enable a single reactor to be used to carry out the WGS reaction and the methanation reaction.

In some embodiments, the isothermal reactor is a fluidized-bed reactor.
In some embodiments, the device that is the subject of the present invention comprises at least one heat exchange surface positioned in the fluidized bed.
These embodiments make it possible to regulate the temperature inside the methanation reactor.
These embodiments allow the temperature in the catalyst layer of the isothermal reactor to be made uniform simply.
According to a second aspect, the present invention envisages a method for producing synthetic gas, which comprises:
a methanation reaction step, comprising:
a step of inputting syngas, produced by gasifying hydrocarbon material, into
an isothermal methanation reactor by means of a syngas supply channel,
and
a step of outputting synthetic natural gas;
a step of separating water, comprising:
a step of inputting synthetic natural gas and
a step of outputting dehydrated synthetic natural gas; and
a step of bypassing a portion of the dehydrated synthetic natural gas output from
the water separation step to the syngas supply channel in order to provide a mixture
of the syngas and the bypassed synthetic natural gas to the reactor.
The method that is the subject of the present invention corresponding to the device that is the subject of the present invention, the particular features, advantages and aims of this method are similar to those of the device that is the subject of the present invention. These features, advantages and aims are not repeated here.
In some embodiments, the method that is the subject of the present invention comprises:
a step of separating carbon dioxide from the dehydrated synthetic natural gas
output from the water separation step and/or
a step of bypassing a portion of the hot methanation reaction products, to upstream
of the methanation step.
BRIEF DESCRIPTION OF THE FIGURES
Other advantages, aims and particular features of the invention will become apparent from the non-limiting description that follows of at least one particular

embodiment of the device and method for producing synthetic gas that are the subjects of the present invention, with reference to drawings included in an appendix, wherein:
- figure 1 represents, schematically, a first particular embodiment of the device that is the subject of this invention;
- figure 2 represents, schematically, a second particular embodiment of the device that is the subject of this invention;
- figure 3 represents, schematically and in the form of a logical diagram, a particular series of steps of the method that is the subject of the present invention;
- figure 4 represents, in the form of a curve, the Wobbe index of synthetic gas obtained by the device and method that are the subjects of the present invention;
- figure 5 represents, in the form of a curve, the HHV of synthetic gas obtained by the device and method that are the subjects of the present invention;
- figure 6 represents, in the form of a curve, the relative reduction of the hydrogen molar flow in the synthetic gas as a function of the synthetic gas recirculation rate;
- figure 7 represents, in the form of a curve, the exothermicity of the reaction obtaining the synthetic gas during the utilization of the device and method that are the subjects of the present invention; and
- figure 8 represents, schematically, an example of system utilized in the state of the art.
DESCRIPTION OF EXAMPLES OF REALIZATION OF THE INVENTION
The present description is given as a non-limiting example, each characteristic of an embodiment being able to be combined with any other characteristic of any other embodiment in an advantageous way. In addition, each parameter of an example of realization can be utilized independently from the other parameters of said example of realization.
It is now noted that the figures are not to scale.
Figure 8 shows a schematic view of an example of system 80 utilized in the state of the art.
In these systems 80, methanation reagents enter into a methanation reactor 805, which can be part of a series (not shown) of such reactors.
On output from the methanation step, the water is separated from the methanation products by a means 825 for separating this water, such as a heat exchanger for example.

The dehydrated synthetic natural gas is then processed by a means 845 for separating carbon dioxide.
Lastly, the synthetic natural gas is processed by a means 855 for separating hydrogen so that this synthetic natural gas meets the specifications for injection into the natural gas distribution grid.
The steps of separating water, carbon dioxide and hydrogen can be carried out in any order.
Figure 1, which is not to scale, shows a schematic view of a first embodiment of the device 10 that is the subject of the present invention. This synthetic gas production device 10 comprises:
an isothermal methanation reactor 105 comprising:
an inlet 110, for syngas produced by gasifying hydrocarbon material, connected to a syngas supply channel 115, and an outlet 120 for synthetic natural gas;
a water separation means 125 comprising:
an inlet 130 for synthetic natural gas and
an outlet 135 for dehydrated synthetic natural gas; and
a bypass 140 for a portion of the dehydrated synthetic natural gas from the outlet of
the water separation means 125 to the syngas supply channel 115 in order to
provide a mixture of the syngas and the bypassed synthetic natural gas to the
reactor 105.
The reactor 105 is, preferably, an isothermal fluidized-bed methanation reactor operating at a predefined temperature. Fluidization of the catalyst by the reactant mixture enables almost perfect homogenization of the temperatures at all points of the catalyst layer and the reactor can be assimilated to an isothermal reactor. In some variants, this reactor 105 can be a boiling water reactor, known to the person skilled in the art under the abbreviation "BWR". In other variants, this reactor 105 can be a wall-cooled reactor or an exchanger reactor.
In some embodiments, the reactor 105 comprises at least one heat exchange surface 106 positioned in the fluidized bed of the isothermal reactor 105.
This surface 106 is, for example, a tube configured to form a loop for circulating a fluid from the outside the reactor 105 to the inside of this reactor 105, the fluid being cooled outside the reactor 105.
This fluid is, for example, superheated or saturated water vapor.

This reactor 105 is configured to carry out the methanation of the carbon monoxide and/or carbon dioxide.
This reactor 105 comprises the inlet 110 for syngas which is, for example, an aperture of the reactor 105 equipped with a connector (not shown) compatible with the syngas supply channel 115.
In some preferred embodiments, such as that shown in figure 1, the reactor 105 is configured to carry out a so-called "gas-to-water" Dussan reaction. In these embodiments, water vapor is injected into the syngas supplied to the reactor 105.
The syngas supply channel 115 is connected to a unit for gasifying hydrocarbon materials (not shown), such as biomass, carbon or waste. The syngas comprises elements of H2, CO, C02, H20, CH4, C2, C3+, etc. This supply channel 115 is sealed.
In some variants, the supply channel 115 receives H2 coming, for example, from a water electrolysis device.
In other variants, the supply channel 115 receives H2 output from a plurality of separate sources.
The synthetic natural gases leave the reactor 105 through the outlet 120 from the
reactor. This outlet 120 is, for example, an aperture connected to a connector (not
shown) making it possible to connect a sealed channel for transporting synthetic natural gas.
The water separation means 125 is, for example, a heat exchanger for cooling the synthetic natural gases to a temperature below the dew point temperature of the water. This temperature is preferably between -5°C and +60°C. Preferentially, this temperature is between -5°C and 40°C.
The water separated in this way is collected by an outlet 127 for water and can be used by an external device or heated to be transformed into water vapor that can, as indicated below, be injected into the syngas supply channel 115.
The water separation means 125 comprises the inlet (130) for synthetic natural gas. This inlet 130 is, for example, an aperture associated with a connector (not shown) to be connected to a sealed channel for transporting synthetic natural gas output from the reactor 105. This sealed transport channel is connected to the outlet 120 for synthetic natural gas from the methanation reactor 105.
The water separation means 125 comprises the outlet 135 for dehydrated synthetic natural gas. This outlet 135 is, for example, an aperture associated with a connector (not

shown) to be connected to a sealed channel (not shown) transporting dehydrated synthetic natural gas.
The bypass 140 is, for example, a sealed channel connected to the transport channel for dehydrated synthetic natural gas in order to capture a portion of the flow passing through this transport channel.
This bypass 140 injects the dehydrated synthetic natural gas into the syngas supply channel 115.
In this way, the syngas and the dehydrated synthetic natural gases, cooled by the water separation process, form a mixture which, in the reactor 105, reduces the exothermicity of the methanation reactor and also improves the characteristics of the synthetic natural gas output from the reactor 105 to meet the normal specifications of use.
In particular, the mixture produced makes it possible to avoid the downstream separation of H2. In some preferred embodiments, such as that shown in figure 1, the device 10 comprises:
a sensor 150 of a temperature inside or on output from the reactor 105 or a
temperature of the synthetic gas flow on output from this reactor 105; and
a recirculator 155 of products input to the bypass, this recirculator being controlled
as a function of the temperature measured.
The sensor 150 is positioned inside or on the outlet from the reactor 105. This sensor 150 captures the temperature of the catalyst forming the fluidized bed, the atmosphere of the reactor 105 and/or the wall of the reactor 105.
The recirculator 155 aims to offset the load, or pressure, losses successively in:
- the preheating means 160, the reactor 105, the water separation means 125 particularly for the device 10 described with reference to figure 1, and
- the preheating means 260, the reactor 205, the water separation means 225 and the carbon dioxide separation means 245 for the device 20 described with reference to figure 2
and all the connecting channels of these various items of equipment.
Such a load loss is estimated to be between 200 and 800 mbars, for example. The recirculator 155 is, for example, a fan, compressor or ejector. In the case of an ejector, the fluid used to realize the ejection mechanism is, for example, water vapor partially supplementing or replacing the vapor 165 of the WGS taking place in the reactor 105.
If the measured temperature is above a predefined temperature, corresponding to optimum methanation reaction conditions, the flow rate of recirculated products is

increased to cool the reaction medium of the reactor 105. Conversely, if the measured temperature is below the predefined temperature, the flow rate of recirculated products is reduced.
In some preferred embodiments, such as that shown in figure 1, the device 10 comprises, upstream from the inlet of the reactor 105, a means 160 for preheating the mixture to a temperature higher than 150°C. Preferably, this preheating means 160 heats the mixture to a temperature higher than or equal to 200°C. Preferentially, this preheating means 160 heats the mixture to a temperature higher than or equal to 230°C. Preferably, this preheating means 160 heats the mixture to a temperature below 280°C and preferentially to a temperature less than the operating temperature of the reactor 105.
The preheating means 160 is, for example, a fluid-gas or electrical heat exchanger configured to transmit a temperature higher than 150°C to the mixture. Preferably, the temperature of the mixture is brought to a temperature higher than or equal to 230°C.
In addition, the device 10 can comprise, in these embodiments, a temperature sensor 162 of the mixture downstream from the preheating means 160. The power supplied by the preheating means 160 varies as a function of the temperature captured on output from 160 and a predefined temperature setpoint. If the captured temperature is higher than the temperature setpoint, the power supplied by the preheating means 160 is reduced. Conversely, the power of the preheating means 160 is increased when the captured temperature is below the temperature setpoint.
In some preferred embodiments, such as that shown in figure 1, the device 10 comprises a channel 165 for injecting water vapor into the syngas supply channel 115 upstream from the place for forming the mixture of syngas and the methanation products output from the bypass 140.
The injection channel 165 is, for example, a sealed channel connected to a device (not shown) for producing water vapor. This water vapor production device heats, for example, the water separated from the synthetic natural gas to produce water vapor injected into the syngas. The water injected in this way makes it possible to produce a WGS reaction and to limit the formation of coke in the reactor 105. In addition, the vapor encourages, through the WGS reaction, the adjustment of the H2/CO ratio to close to optimum methanation reaction conditions. Analysis of the prior state of the art has shown that the WGS reaction could be carried out in a dedicated reactor located upstream from the reactor 105 or even within this reactor in parallel to the methanation reactions. To

benefit from the economic gains and simplification of the method, the methanation and WGS reactions are, preferably, carried out in a single device.
In some preferred embodiments, such as that shown in figure 1, the device 10 comprises a bypass channel 170, for a portion of the hot methanation reaction products, comprising:
an inlet 175 positioned between the outlet from the reactor 105 and the water
separation means 125; and
an outlet 180 positioned upstream from the inlet 110 to the reactor 105 and
downstream from the reheating means 160.
The bypass channel 170 is, for example, a sealed channel. The inlet 175 is, for example, an aperture emerging at the interior of the synthetic natural gas transport channel, upstream from the water separation means 125. The outlet 180 is, for example, an aperture for injecting synthetic natural gas into the mixture, downstream from the preheating means 160.
The synthetic natural gases, being hot, make it possible to keep the flow rate constant in the reactor 105.
The flow rate of the gasification syngas is entirely a function of the quantities of hydrocarbon compounds available. In order to maintain conversion stability during the methanation operation, the hydrodynamic conditions must be kept as constant as possible. However, if the available hydrocarbon material is insufficient and, as a result, the syngas flow rate is reduced, it is necessary to maintain a constant overall flow rate input to the reactor 105 or opt for a very flexible technology. Even in the case of the fluidized bed able to operate in a flow rate range of one to six, flow rates that are too low can cause degradation of the cooling, and therefore of the conversion. To overcome this difficulty, in the event of a significant drop in the syngas flow rate, the flow rate on output from the preheating means 160 is supplemented by a hot recirculation coming directly from the outlet 120 of the reactor 105 by means of the bypass channel 170. The fact of using a hot recirculation fluid does not cause a thermal imbalance of the reactor 105 but allows the device 10 to be made very flexible.
In some preferred embodiments, such as that shown in figure 1, the device 10 comprises:
a means 185 for measuring the syngas flow rate after injecting water vapor output
from the water vapor injection channel 165 and after mixing with the gas output
from the bypass 140;

a recirculator 190 of products input into the bypass channel 165, this recirculator
190 being controlled as a function of the flow rate measured.
The flow rate measurement means 185 can be any type known to the person skilled in the art that is suitable for measuring the flow rate of gases, such as an anemometer, Coriolis effect flowmeter, vortex effect flowmeter, or electromagnetic flowmeter, for example.
The recirculator 190 is similar to the recirculator 155 in structural terms. This recirculator 190 is controlled as a function of the flow rate measured by the measurement means 185 and a predefined flow rate setpoint value 187. If the measured flow rate is below a predefined flow rate setpoint 187, the recirculator 190 is actuated so as to make up the difference between the measured flow rate and the flow rate setpoint 187 by an equivalent flow rate of synthetic natural gas.
In some preferred embodiments, the device 10 comprises a means 145 for separating carbon dioxide from the dehydrated synthetic natural gas positioned downstream from the bypass 140.
The utilization of the device 10 that is the subject of the present invention makes it possible to obtain synthetic gas close to the specifications of the gas grid requiring few additional operations.
In addition, the device 10 can also be utilized for a pressure range of between one bar and one hundred bars, and a range of predefined temperatures of between 230°C and 700°C.
Figure 2, which is not to scale, shows a schematic view of a second embodiment of the device 20 that is the subject of the present invention. This synthetic gas production device 20 is similar to the device 10 described with reference to figure 1. Therefore, references 205, 210, 215, 220, 225, 227, 230, 235, 240, 250, 255, 260, 262, 265, 270, 275, 280, 285, 287 and 290 of device 20 correspond respectively to references 105, 110, 115, 120, 125, 127, 130, 135, 140, 150, 155, 160, 162, 165, 170, 175, 180, 185, 187 and 190 of device 10.
The device 20 also comprises a means 245 for separating carbon dioxide from the
dehydrated synthetic natural gas positioned upstream from the bypass 240. This
separation means 245 can be positioned upstream or downstream from the water separation means 225.

Figure 3 shows, in the form of a logical diagram of steps, a particular embodiment of the method 30 that is the subject of the present invention. This synthetic gas production method 30 comprises:
a methanation reaction step 305, comprising:
a step 310 of inputting syngas, produced by gasifying hydrocarbon material, into an isothermal methanation reactor by means of a syngas supply channel, and
a step 315 of outputting synthetic natural gas; a step 320 of separating water, comprising:
a step 325 of inputting synthetic natural gas and a step 330 of outputting dehydrated synthetic natural gas; a step 335 of bypassing a portion of the dehydrated synthetic natural gas output from the water separation step to the syngas supply channel in order to provide a mixture of the syngas and the bypassed synthetic natural gas to the reactor; and
- preferably, a step 340 of separating carbon dioxide from the dehydrated synthetic natural gas output from the separation step 320;
- preferably, a step (not shown) of bypassing a portion of the hot methanation reaction products, to upstream of the methanation step 305.
This method 30 is utilized, for example, by a device 10 or 20 that is the subject of the present invention and described with reference to figure 1 or 2.
In some variants, the method 30 that is the subject of the present invention comprises, upstream from the reaction step 305, a preheating step 340. This preheating step 340 is performed, for example, by a preheating means, 160 or 260, as described with reference to figure 1 or 2.
It is noted that figures 4 to 7 are the result of simulations carried out to determine the impact of the device and method that are the subject of the present invention. These results are compared to a simulation of a case of recirculation without dehydration as is, for example, the case in the first step of the boiling water reactor, for example. The aim of the device and method that are the subjects of the present invention is also to minimize the steps of adjustment to specifications while operating a single-stage methanation at moderate pressure that is acceptable in terms of costs. For the same reasons, the SNG is preferably compressed at the end of the production chain after the separations required for injection into the grid. The simulations carried out and presented below were carried out at 8 bars and a methanation temperature of 320°C.

Figures 4 and 5 respectively show the Wobbe index and HHV of the SNG before H2 separation for the reference configuration with recirculation of the humid SNG, recirculation of the dehydrated SNG and recirculation of the dehydrated, decarbonated SNG.
These results are presented as a function of the recirculation rate, which corresponds to the ratio of the volume flow rates under normal pressure and temperature conditions of the recirculated flow to the syngas flow. For the "reference configuration", the recycled flow is replaced by a humid SNG flow rate equivalent to the flow passing through the bypass channel. This humid SNG flow rate was achieved by bypassing a portion of the flow output from the reactor, upstream from the water separation.
Figure 4 shows, on the x-axis, the Wobbe index of the synthetic natural gas produced by the device, 10 or 20, as a function of the recirculation rate, on the y-axis, and of the nature of the recirculated synthetic natural gas on input to the methanation reactor, 105 or 205.
It is noted, in particular, that the recirculation of humid synthetic natural gas 405 has no effect on the Wobbe index of the synthetic natural gas produced by the device.
It is also noted that the recirculation of dehydrated synthetic natural gas 410 improves the Wobbe index of the synthetic natural gas produced by the device.
It is noted, lastly, that the recirculation of dehydrated, decarbonated synthetic natural gas 415 further improves the Wobbe index of the synthetic natural gas produced by the device, even with a recirculation rate of less than one.
Figure 5 shows, on the x-axis, the HHV of the synthetic natural gas produced by the device, 10 or 20, as a function of the recirculation rate, on the y-axis, and of the nature of the recirculated synthetic natural gas on input to the methanation reactor, 105 or 205.
It is noted, in particular, that the recirculation of humid synthetic natural gas 505 has no effect on the HHV of the synthetic natural gas produced by the device.
It is also noted that the recirculation of dehydrated synthetic natural gas 510 improves the HHV of the synthetic natural gas produced by the device.
It is noted, lastly, that the recirculation of dehydrated, decarbonated synthetic natural gas 515 further improves the HHV of the synthetic natural gas produced by the device, even with a recirculation rate of less than one.
According to the results obtained in terms of the Wobbe index and HHV, the recirculation rate of the humid gas, ie the reference configuration, has no impact on the quality of the gas and shows that the separation of the H2 is essential to achieve the

injection specifications. Recirculation after dehydration alone or with decarbonation leads to a larger or smaller increase in the Wobbe index and the HHV. These improvements may be interpreted as the result of a simple dilution, but figure 6 highlights a real improvement in the reaction balances, with a dramatic reduction in the H2 molar flow on output from device 10 or 20.
Figure 6 shows, on the x-axis, the relative reduction in the H2 molar flow on output from device, 10 or 20, as a function of the recirculation rate, on the y-axis, and of the nature of the recirculated synthetic natural gas on input to the methanation reactor, 105 or 205.
It is noted, in particular, that the recirculation of humid synthetic natural gas 605 has no effect on the H2 molar flow on output from the device.
It is also noted that the recirculation of dehydrated synthetic natural gas 610 causes a reduction in the H2 molar flow on output from the device.
It is noted, lastly, that the recirculation of dehydrated, decarbonated synthetic natural gas 615 also causes a reduction in the H2 molar flow on output from the device.
Despite the dilution by recirculation, the CO/H2O ratio is maintained on input to the reactor relative to the initial CO/H20 ratio when the reactor is started up. Therefore, the risk linked to deactivation of the methanation catalyst through coking remains relatively low. Between the two devices, 10 and 20, decarbonation upstream from the recirculation of the synthetic natural gas appears more effective in terms of molar reduction of the H2. Thus, to comply with the injectability criteria, the recirculation flow rate needs to be four to eight times greater for dehydration alone than in the solution with decarbonation. For the operating conditions used for the simulation, and when only dehydration is applied before recirculation, the minimum recirculation rate required to avoid H2 separation is estimated to be 1.6. When dehydration is supplemented by a decarbonation step, the required rate is 0.2. These respective rates effectively make it possible to meet the constraints related to injection, yet still require an internal system for cooling the reactor to maintain isothermality and, in that case, cannot be applied to non-cooled fixed bed technologies. With regard to exchanger reactors - boiling water or fluidized-bed reactor - this new feature, under these operating conditions, allows the exchanger surface area to be reduced by 10% and 25% respectively.
Figure 7 makes it possible to see the change in the reactor's normalized exothermicity, ie the heat to be removed compared to a case without recirculation, as a

function of the recirculation rate for the reference configuration and the two devices, 10 and 20, described above.
Figure 7 shows, on the x-axis, the exothermicity of the methanation reaction of the device, 10 or 20, as a function of the recirculation rate, on the y-axis, and of the nature of the recirculated synthetic natural gas on input to the methanation reactor, 105 or 205.
It is noted, in particular, that the recirculation of humid synthetic natural gas 705 reduces the exothermicity of the methanation reaction.
It is also noted that the recirculation of dehydrated synthetic natural gas 710 also reduces the exothermicity of the methanation reaction.
It is noted, lastly, that the recirculation of dehydrated, decarbonated synthetic natural gas 715 also reduces the exothermicity of the methanation reaction.
It appears that increasing the recirculation rate leads to a linear reduction in the exothermicity of the reactor. Here, the recirculated SNG plays the role of heat accumulator, which is more marked in the presence of H20 because of a higher heat capacity. Obtaining a recirculation level corresponding to an ideal operating temperature of the devices, 10 and 20, makes it possible to be liberated from the internal exchanger of the reactor and the H2 separation for adjustment to specifications. Further, the reactor is allothermal and therefore requires a heat supply to maintain the reactions.
The change in the molar fractions of the H2, CO2, CO and CH4 species as a function of the recirculation rate for the different configurations simulated is in the direction of improved SNG quality. However, the molar fraction of CO increases dramatically with the dehydrated SNG recirculation device 10.
In this case, dehydration alone leads to the CO2 content being over-concentrated on input to the reactor and shifts the balance of the WGS reaction towards the production of CO and consumption of H2. For the device 20, the CO2 extraction incorporated into the recirculation loop allows the WGS reaction to be encouraged towards the production of H2, which is then converted into CH4.

WE CLAIM
1. Synthetic gas production device (10, 20), characterized in that it comprises:
an isothermal methanation reactor (105, 205) comprising:
an inlet (110, 210), for syngas produced by gasifying hydrocarbon material,
connected to a syngas supply channel (115, 215), and
an outlet (120, 220) for synthetic natural gas; a water separation means (125, 225) comprising:
an inlet (130, 230) for synthetic natural gas and
an outlet (135, 235) for dehydrated synthetic natural gas; and a bypass (140, 240) for a portion of the dehydrated synthetic natural gas from the outlet of the water separation means to the syngas supply channel in order to provide a mixture of the bypassed syngas and synthetic natural gas to the reactor.
2. Device (10) according to claim 1, which comprises a means (145) for separating carbon dioxide from the dehydrated synthetic natural gas, this separation means being positioned downstream from the bypass (140).
3. Device (20) according to one of claims 1 or 2, which comprises a means (245) for separating carbon dioxide from the dehydrated synthetic natural gas, this separation means being positioned upstream from the bypass (240).
4. Device (10, 20) according to one of claims 1, 2 or 3, which comprises:
a sensor (150, 250) of a temperature inside or on output from the reactor (105, 205); and
a recirculator (155, 255) of products input to the bypass, this recirculator being controlled as a function of the temperature measured.
5. Device (10, 20) according to one of claims 1 to 4, which comprises, upstream from the inlet of the reactor (105, 205), a means (160, 260) for preheating the mixture to a temperature higher than 150°C.
6. Device (10, 20) according to claim 5, wherein the preheating means (160, 260) heats the mixture to a temperature higher than 230°C, to avoid the production of toxic compounds,

and less than the operating temperature of the reactor (105, 205), to allow the reactor (105, 205) to be cooled.
7. Device (10, 20) according to one of claims 5 or 6, which comprises a bypass channel
(170, 270), for a portion of the hot methanation reaction products, comprising:
an inlet (175, 275) positioned between the outlet from the reactor (105, 205) and the water separation means (125, 225); and
an outlet (180, 280) positioned upstream from the inlet (110, 210) to the reactor (105, 205) and downstream from the preheating means (160, 260).
8. Device (10, 20) according to claim 7, which comprises:
a means (185, 285) for measuring the syngas flow rate after injecting water vapor output from the water vapor injection channel (165, 265) and after mixing with the gas output from the bypass (140, 240); and
a recirculator (190, 290) of products input into the bypass channel, this recirculator being controlled as a function of the flow rate measured.
9. Device (10, 20) according to one of claims 1 to 8, which comprises a channel (165, 265)
for injecting water vapor into the syngas supply channel (115, 215) upstream from the
place for forming the mixture of syngas and the methanation products output from the
bypass (140, 240).
10. Device (10, 20) according to one of claims 1 to 9, wherein the water separation means (125, 225) is configured to cool the synthetic natural gases to a temperature between -5°C and +60°C.
11. Device (10, 20) according to claim 10, wherein the water separation means (125, 225) is configured to cool the synthetic natural gases to a temperature below the dew point temperature of the water at the operating pressure of the reactor (105, 205) in question.
12. Device (10, 20) according to one of claims 1 to 11, wherein the reactor (105, 205) is configured to carry out a so-called "gas-to-water" Dussan reaction.
13. Device (10, 20) according to one of claims 1 to 12, wherein the isothermal reactor (105) is a fluidized-bed reactor.

14. Device (10, 20) according to claim 13, which comprises at least one heat exchange surface (106) positioned in the fluidized bed.
15. Method (30) for producing synthetic gas, characterized in that it comprises:
a methanation reaction step (305), comprising:
a step (310) of inputting syngas, produced by gasifying hydrocarbon material, into an isothermal methanation reactor by means of a syngas supply channel, and
a step (315) of outputting synthetic natural gas; a step (320) of separating water, comprising:
a step (325) of inputting synthetic natural gas and a step (330) of outputting dehydrated synthetic natural gas; and a step (335) of bypassing a portion of the dehydrated synthetic natural gas output from the water separation step to the syngas supply channel in order to provide a mixture of the syngas and the bypassed synthetic natural gas to the reactor.
16. Method (30) according to claim 15, which comprises a step (340) of separating carbon dioxide from the dehydrated synthetic natural gas output from the separation step (320).
17. Method (30) according to one of claims 15 or 16, which comprises a step of bypassing a portion of the hot methanation reaction products, to upstream of the methanation step.