The invention relates to a method for starting up a process for production of methane rich gas.

The low availability of fossil liquid and gaseous fuels such as oil and natural gas has revived the interest in de ^¬ veloping technologies capable of producing combustible gas synthetically from widely available resources such as coal, petcoke, biomass as well as off-gasses from coke ovens. The produced gas goes under the name substitute natural gas or synthetic natural gas (SNG) having methane as its main con ^¬ stituent. The methane is formed by reaction between carbon oxides and hydrogen according to the following reactions:

CO+3H ₂ <=> CH ₄ +H ₂ 0 (1)

C0 ₂ +4H ₂ <=> CH4 + 2 H ₂ 0 (2)

CO+H2O <=> CO2 +H2 (3) The production of substitute natural gas from synthesis gas, is highly exothermal, and the reaction has a high ac ^¬ tivation energy, even when catalyzed. Under certain conditions there is a risk of carbon formation over the cata ^¬ lyst, and therefore the operation conditions are carefully monitored and compared to the so-called carbon limit, which is a function of gas composition, pressure and temperature for a specific catalyst, where the gas reacts to form car ^¬ bonaceous material on the catalyst surface. When consider ^¬ ing the gas composition for a methanation process, the mod- ule, M= ( H2 - CO2 ) / (CO+CO2 ) is often considered. A module equaling 3.0 reflects an optimal gas composition for methanation; when the module is above 3.0 excess ¾ is pre- sent and when the module is below 3.0 excess CO and/or CO ₂ is present.

During such start-up conditions and especially in connec- tion with load changes transient compositions are often ob ^¬ served. These transients may often involve significant var ^¬ iations of the module M, which could bring the catalyst within the so-called carbon formation regime, where carbon deposits are formed on the surface of the catalytically ac- tive material.

The present invention relates to a method for operating the SNG process, which involves an increase of the load of an SNG plant, while ensuring that the operating conditions have a safe margin to the carbon formation regime.

This is typically done by a single increase from 0% to 100% or by a sequential stepwise increase in load, with tempo ^¬ rary increases of the safety limit of a parameter reducing the propensity for carbon formation such as the module, ei ^¬ ther throughout the start-up period or at least during the period shortly before and after load change.

For the purpose of the present application the process load shall be understood as the amount of reactants supplied to the process. Where the load is presented in % this shall be construed as the % of design conditions.

For the purpose of the present application three modes of operation shall be considered, idle operation substantially without reaction, start-up operation with limited reaction and design or routine operation. A start-up procedure shall be understood as a procedure for increasing the process load from 0% to 100% of the design conditions.

For the purpose of the present application the module M shall be understood as the parameter defining the potential stoichiometric ratio between ¾ and CO, i.e. M= (¾ - CO2) / (CO + CO2) , If higher hydrocarbons are present the calculation of the module for the gas may be adjusted for this according to their reaction stoichiometry with respect to ¾ and CO, as would be clear to the person skilled in the art. The module could be determined based on gas compo ^¬ sition measurements made at the inlet of the plant or at convenient positions in the process, downstream or upstream the methanation reactors, possibly in combination with a compensation of the reaction estimated to occur between the point of measurement and the relevant reactor.

For the purpose of the present application carbon oxides shall be understood as CO and/or CO ₂ .

For the purpose of the present application a material cata- lytically active in methanation shall be understood as a material in the presence of which the activation energy of the reaction of carbon oxides with hydrogen to form methane is significantly reduced. Typically such materials comprise one or more of nickel and noble metals.

For the purpose of the present application the carbon for ^¬ mation regime shall be understood as the gas composition, pressure and temperature where undesired carbon species are formed on the surface of the catalyst. The gas composition, the pressure and the temperatures defining the carbon for- mation regime are specific for each type of catalytically active material.

For the purpose of the present application the methanation process shall be understood as a combination of the reac ^¬ tions 1-3.

For the purpose of the present application the method steps shall not be taken as strictly sequential or strictly syn- chronous, unless explicitly written as such.

For the purpose of the present application the method steps a feed gas shall be construed as the gas directed for methanation in a methanation reactor. Where recycle gas or steam is added to said feed gas this shall be considered equivalent to addition in any position upstream said reactor, unless otherwise stated.

For the purpose of the present application a dry or dried gas shall not be construed as an absolutely dry gas, but a gas comprising less water than a similar gas.

For the purpose of the present application carbon formation propensity shall be construed as the tendency to form car- bon deposits on the surface of a catalytically active mate ^¬ rial, which is dependent on a number of parameters. The combination of parameters at which there is a high risk for carbon deposits shall be called the carbon formation re ^¬ gime .

For a specific set of conditions certain parameters may re ^¬ duce the risk of carbon formation. For the purpose of the present application they shall be called parameters reduc ^¬ ing the propensity for carbon formation. The term removed from the carbon formation regime shall mean a low risk of carbon formation, and in this respect the term safety limit shall mean a further distance to the carbon formation regime - i.e. for parameters decreasing the risk of carbon formation with decreased value of the parameter the safety limit is a limit below the routine operation limit, and for parameters decreasing the risk of carbon formation with in- creased value of the parameter the safety limit is a limit above the routine operation limit.

In a broad form the present invention relates to a start-up method for a process for production of a methane rich gas, in which a feed gas mixture comprising one or more carbon oxides and hydrogen is directed to contact a material cata- lytically active in methanation, and a methane rich gas is withdrawn, said method being operable in a routine mode and a start-up mode comprising the steps during the start-up mode of

a) providing the material catalytically active in methana ^¬ tion at a temperature where exothermal reactions are ac ^¬ tive,

b) temporarily operating the process for production of me- thane rich gas, such that at least one parameter having po ^¬ tential for reducing the propensity for carbon formation has a set-point removed from the set-point of routine oper ^¬ ation such that the process conditions are removed from the carbon formation regime by a safety limit compared to the paramter value during routine mode, and c) inflicting a change in the process load, by means of a gradual or a stepwise change of one or more of the flow, the composition and the pressure of the feed gas mixture with the associated benefit of being able to change the process load e.g. by controlling the feed gas directed to contact the material catalytically active in methanation e.g. its flow rate, composition or pressure and thus limit ^¬ ing the strain on the equipment and the cost of startup, without risking carbon formation on the material catalyti- cally active in methanation, even under conditions with extraordinarily high fluctuations in composition.

In a further embodiment said safety limit is adjusted as a function of a parameter relating to variability of the feed gas stream such as time since the change in process load or measured variations of composition, pressure or temperature of feed gas or methane rich gas, such that a reduced varia ^¬ tion in the feed gas results in a reduced safety limit with the associated benefit of safely but without undue delay changing operation towards operation at 100% load.

In a further embodiment said at least one parameter having potential for reducing the propensity for carbon formation comprises the module of said feed gas and the set-point is higher than 3.0 by the positive safety limit being above 0.01, 0.2, 0.5 or 1.0 and below 2.0, 4.0 or 7.0 which has the effect that a module above but close to 3.0 will pro ^¬ vide a methane rich gas with further reaction potential at lower temperatures, which will maintain the temperature of downstream reactors, while a higher module will provide am ^¬ ple safety margin to carbon formation conditions. In a further embodiment said at least one parameter having potential for reducing the propensity for carbon formation comprises the amount of steam added to said feed gas mix ^¬ ture being above 5%, 10% or 25% and below 50%, 75% and 100% of the feed gas mixture excluding steam with the associated benefit of the presence of steam reducing the propensity for carbon formation, and with steam addition being rapid to control. In a further embodiment said at least one parameter having potential for reducing the propensity for carbon formation comprises the amount of methane rich gas being recycled and combined with said feed gas mixture, and the set-point of the amount of methane rich gas being recycled being in- creased over routine operation by a safety limit of from

5%, 10% or 25% to 125%, 150% or 200% of the routine opera ^¬ tion amount of methane rich gas being recycled with the as ^¬ sociated benefit of the recycled methane rich gas contrib ^¬ uting to limiting the reaction, as it thus comprises the products water and methane, and contributing to a reduced temperature increase by dilution.

In a further embodiment of the method, a heated substan ^¬ tially inert gas, such as nitrogen, is directed to contact the catalytically active material prior to steps b and c, with the associated benefit of providing a catalytically active material at a temperature required for activation of the methanation process. In a further embodiment the pressure of the feed gas mix ^¬ ture is increased from less than 10%, 20%, 50% or 75% of operation pressure to operation pressure in dependence of the variation of the feed gas mixture with the associated benefit of a low pressure matching the pressure of inert gas in the plant prior to introducing the feed gas mixture. In a further embodiment the initial molar flow of feed gas mixture relative to steady state operation is below 25%, 10% or 2% with the associated benefit of a reduced cost of start up by only providing a fraction of the feed gas, e.g. by leaving one or several gasifiers idle.

In a further embodiment the steps b and c are repeated in dependence of the variation in the load with the associated benefit of providing a stepwise increase in the load until a satisfactory operation occurs.

In a further embodiment the process for production of a me ^¬ thane rich gas involves one or more further steps compris ^¬ ing optionally condensing water in the methane rich gas providing a dried methane rich gas and a condensate, and directing the methane rich gas to a further material cata- lytically active in methanation for providing a further methane rich gas, and the start-up method of said process in ^¬ volves bypassing said second material catalytically active in methanation until the variation of the feed gas mixture is reduced, and then operating the process for production of methane rich gas at a set point of said parameter reduc ^¬ ing the propensity for carbon formation less removed from the routine operation until the composition of the methane rich gas is sufficient to maintain acceptable operating temperatures of the second material catalytically active in methanation, with the associated benefit of establishing stable operation of a first stage before effecting start-up of a second stage.

As mentioned the methanation process involves the following reactions between carbon oxides and hydrogen according to the reactions (1) , (2) and (3) :

CO+3H ₂ <=> CH ₄ +H ₂ 0 (1)

CO+H2O <=> CO2+H2 (3)

The mixture of carbon oxides and hydrogen is called synthe ^¬ sis gas, and may be obtained from gasification of carbona ^¬ ceous material such as coal or biomass in one or more gasi- fiers or a coke oven. Accurate and immediate control of a gasifier is difficult, and therefore fluctuations may occur upon changing the load of the gasifier. Furthermore the balance between CO, ¾0, CO ₂ and ¾ is often controlled by a shift section in which reaction (3) occurs, and a CO ₂ re- moval unit. The operation of these units can also show transients during changes in the process load.

The production of substitute natural gas from synthesis gas, is highly exothermal, and the reaction has a high ac- tivation energy, even when catalyzed, e.g. by the typical materials catalytically active in methanation including nickel and noble metals. Therefore an elevated temperature is required for activation of the processes. However, when the catalyst temperature becomes too high there is a ten- dency to carbon formation over the catalyst, which without being bound by theory is believed to occur via one or more of the following reactions: CH ₄ <=> C(s) + 2H ₂ (4)

2CO <=> C (s) + C0 ₂ (5)

CO + H ₂ <=> C(s) + H ₂ 0 (6)

Carbon formation is observed especially at higher tempera ^¬ tures and especially at conditions with a high amount of hydrocarbons including CH ₄ and relatively low H ₂ and H ₂ 0 concentrations, but carbon formation may also occur at low temperatures with excess of CO, i.e. so-called gum for ^¬ mation. Carbon formation when occurring is a rapid but not instantaneous process. Typically a significant amount of catalytically active material may be rendered useless in minutes or hours, i.e. a time comparable to the typical residence time in the methanation process which is in the range of minutes.

Other factors in carbon formation include increased pres ^¬ sure, which will shift reactions (5) and (6) to the right (and which with less significance will shift reaction (4) to the left) and the presence of water (in the form of steam) which will shift reaction (6) to the left.

By analysis of these elements it has been identified that a successful low-risk start-up procedure of an SNG plant re ^¬ quires that the initial temperature is sufficient for acti ^¬ vation of the methanation and water gas shift reactions (1), (2) and (3) (requiring the temperature at the inlet to the methanation reactor to be above 180°C to 300°C depend- ent on specific conditions and catalysts) , while the gas composition must be well controlled to avoid undesired re ^¬ actions. This may in practice be obtained by heating the process plant with an inert gas such as nitrogen at an ele ^¬ vated temperature, such as 300°C for the first methanation reactor, and possibly less for downstream methanation reactors, which is sufficient for activating the exothermal methanation process. Often the pressure in this idle mode is less than the pressure during operation, due to the practical availability of the inert gas. Then the inert gas is replaced with a feed gas, preferably having a module sufficiently above 3.0 to ensure that in the event of fluc- tuations the catalytically active material is not brought into the carbon formation regime. This may also be ensured by another parameter reducing the propensity for carbon formation, such as addition of steam or by recycle of cooled produced methane rich gas, or by a combination of these factors.

In a typical plant for production of SNG 2 to 5 methanation reactors may be configured in series, often with recycle around one or more reactors, typically with cooling between reactors and possibly also intermediary condensation of wa ^¬ ter, in one or two positions in the process.

To simplify the start-up procedure and to avoid the situa ^¬ tion where the methane rich gas comprises an insufficient amount of CO and ¾ for providing an exothermal reaction in downstream reactors the produced methane rich gas may be removed from the process during start-up, e.g. by flaring. This allows the plant to be divided into smaller sections, which can be started up one after the other. When one sec- tion is operating acceptably with little process variation, one or possibly more downstream sections may be sequential ^¬ ly included in the start-up until the all methanation sec- tions of the plant are running at the desired load. Subse ^¬ quently the module or another parameter reducing the propensity for carbon formation may temporarily be changed by a safety margin and the load may be increased, e.g. by put- ting an additional gasifier in operation. When stability is met, the module may be decreased.

For some cases it may also be chosen to perform the start ^¬ up operation in a single load step, in which the plant is changed from being idle, and filled with heated inert gas such as nitrogen to a temporary full load condition, operating with a parameter reducing the propensity for carbon formation with an elevated safety margin, e.g. a high value of the module. When the process variation is sufficiently low, the safety margin may then be reduced, to design oper ^¬ ation level in one or more steps.

Fig. 1 shows the configuration of a process plant for pro ^¬ duction of SNG,

Fig.2 shows load and module as a function of time during the start-up of an SNG process plant according to the prior art, Fig. 3 shows load and module as a function of time during the start-up of an SNG process plant according to the pre ^¬ sent disclosure, and

Fig. 4 shows load and module as a function of time during the start-up of a further SNG process plant according to the present disclosure. In Fig. 1 an example of a process layout for an SNG process plant is shown. At the time of start-up the plant is filled with 2 and at least the first methanation stage 32 is heated to a temperature such as 280°C sufficient for acti- vation of the desired reactions dependent on specific con ^¬ ditions and catalysts. A carbonaceous feed 10 is directed to a gasifier 12 and provides synthesis gas 14. Typically this synthesis gas is desulfurized in a sulfur guard reac ^¬ tor (not shown here) . The module of the synthesis gas is controlled by directing one portion to a shift reactor 16 converting CO and ¾0 to CO2 and ¾ . CO2 24 of the shifted gas 18 is removed e.g. by an acid gas removal wash in 22, using e.g. methanol or another solvent (or mixture of sol ^¬ vents) with high affinity for CO2. The module is controlled by the valve 20 controlling the amount of feed gas directed to water gas shift and the amount bypassing the shift reac ^¬ tor. The module adjusted gas is then combined with a recy ^¬ cled methane rich gas 40 and directed to a gas conditioning stage 28, in which CO reacts with ¾0 to form CO2 and ¾ providing a conditioned feed gas 30 under release of heat, followed by a high temperature methanation stage 32 having an inlet temperature of around 300 °C, in which CO and CO2 reacts with ¾ to form CH ₄ under release of further heat. The gas conditioning stage 28 may be omitted if an in- creased feed temperature to the methanation stage 32 is provided by other means. With time the methanation stage 32 will increase the temperature to around 550-675°C or even more, depending on the amount of recycle 36 and the design of the plant. With the increased temperature the first me- thane rich gas 34 will have a decreased content of methane, thus allowing for further reaction in the downstream stages . Where multiple methanation sections are used, each follow ^¬ ing section is often started up in a similar manner (heating in N ₂ , introducing feed gas, obtaining stable operating conditions) after awaiting stable operation of the previous section. The hot methane rich gas 46 may optionally be di ^¬ rected to one or more further methanation reactor (s), typi ^¬ cally before partially removing water 50 by condensation 48 before the dehydrated methane rich gas 52 is directed to one or more final methanation reactor (s) 54, providing a substitute natural gas 56.

Fig. 2 shows an example with arbitrary time scale on the x- axis of the startup conditions in a process for production of synthetic natural gas. The load is shown in solid line, increasing stepwise from 35% to 100%. Together with the load the set-point for the module (dashed line) , and the typical fluctuation of the module which increases at load changes (dotted line) is shown. In Fig. 2 it is seen that at each load change the module of the feed gas may drop be ^¬ low the critical carbon formation limit of 3.0 (shown as a heavy dashed line) , which must be avoided as even very short periods of time in the carbon formation regime may destroy the catalyst.

Fig. 3 shows an example with arbitrary time scale on the x- axis of the startup conditions in a process for production of synthetic natural gas according to the present disclo ^¬ sure. Here the module set-point is increased to a high val- ue of 5, at the time of load changes, which includes suffi ^¬ cient safety margin to the critical module below 3, where a high risk of carbon formation occurs. The set point of the module is adjusted by increasing the amount of hydrogen, and/or decreasing the amount of carbon monoxide and/or carbon dioxide, which typically is effectuated by directing more or less synthesis gas to the shift section and/or changing the amount of carbon dioxide removed in the carbon dioxide removal unit.

Fig. 4 shows a further example with arbitrary time scale on the x-axis of the startup conditions in a process for pro- duction of synthetic natural gas according to the present disclosure. Here the module set-point starts at a high val ^¬ ue of 10, and is gradually decreased as the load is in ^¬ creased - typically stepwise as shown. This provides suffi ^¬ cient safety limit to the critical module below 3 through- out the startup procedure.

The start-up of multiple methanation sections may be made at the same time, or one at a time, awaiting stable opera ^¬ tion of the previous section. The activation of further re- actors are typically made at the lowest load, before the load is increased for all reactors, but it may also be pos ^¬ sible to increase the load in one of more reactors before all reactors are started up. This may be done in considera ^¬ tion of the cost of operating the process during start-up without production of SNG of value, or in consideration of practical consideration specific to the process.

For feed gases rich in CO ₂ , such as coke oven gas where CO ₂ is often added to the raw feed gas in routine operation, and for other feed gases which are rich in CO ₂ the module may be controlled by addition of less or no CO ₂ or by in ^¬ creased carbon dioxide removal. An alternative or a supplement to the operation at an ele ^¬ vated set-point for the module is to reduce the carbon for ^¬ mation propensity by other means before or during the load change, such as increased recycle, increased addition of steam or decreased pressure.