WO2002102943A1

WO2002102943A1 - Method for converting hydrocarbon-containing material to a methane-containing gas

Info

Publication number: WO2002102943A1
Application number: PCT/NL2002/000337
Authority: WO
Inventors: Renee Van Yperen; Anton Bastiaan Alderliesten; Mathieu Andre De Bas; Petrus Franciscus Maria Theresia Van Nisselrooy
Original assignee: Gastec N.V.
Priority date: 2001-05-28
Filing date: 2002-05-27
Publication date: 2002-12-27
Also published as: NL1018159C2; EP1390456A1

Abstract

The invention relates to a method for converting hydrocarbon-containing materials such as biomass to a methane-containing gas. More in particular, the invention relates to such a method, wherein the conversion takes place with high efficiency. It has been found that if at least a part of the hydrogen of the methane-containing gas flow, obtained by methanization of a fuel gas, is separated and is used in the methanization, reduced carbonization occurs and that, moreover, the methaniszation step can be carried out with simpler equipment.

Description

P54900PC00

Title: Method for converting hydrocarbon-containing material to a methane- containing gas.

The invention relates to a method for converting hydrocarbon-containing materials such as biomass to a methane-containing gas. More in particular, the invention relates to such a method wherein the conversion takes place with high efficiency. As a result of an increasing awareness of the necessity of fitting durable energy into the existing energy infrastructure, there has been a great increase in the interest in energy from hydrocarbon-containing materials such as biomass. The deployment of durable energy is becoming ever more important in tho at least partial replacement of the use of fossil fuels and the accompanying emission of CO2, which enhances the greenhouse effect. The contribution of energy from biomass or waste is to play an important part therein. As distinguished from fossil energy, durable energy is understood to mean energy obtained from renewable sources, for instance from biomass. In the state of the art, different methods have been described to use biomass as energy source.

Biomass, such as wood and other vegetable material and other hydrocarbon-containing materials, can be combusted directly, whether or not after admixing fossil fuels. The heat released can be used to generate heat and power, for instance in the form of electricity. Additionally, it is possible to biologically degrade certain sorts of biomass, waste or other organic flows, for instance by fermentation. The gasses which are released in such biological degradation can then be combusted, likewise yielding heat and/or power.

It is also possible to gasify hydrocarbon-containing materials such as biomass. Economic analyses show that from an economic point of view, gasifying is an interesting option, certainly for the future. In gasification processes, the hydrocarbon-containing material is converted to a gas rich in CO and H2 and often containing small amounts of hydrocarbon. To subsequently make the energy contained therein useful, these gasses need to be combusted, for instance in a (natural) gas engine. However, the engines used for converting chemical energy to work need to undergo adjustments in order to make them suitable for gasses which have been obtained in the conventional manner from biomass or other hydrocarbon- containing material. For instance, engines driven by fuel gas (a gas mixture containing substantially CO and H2) often have a considerably lower efficiency and specific load than natural gas engines. Additionally, the conversion of fuel gas to electricity has the great disadvantage that a part of the energy is lost in the form of heat. Although, by using a heat/onergy coupling (WKK), this disadvantage can be partly obviated, it often remains impossible to fully utilize the residual heat. Therefore, it is desired to first convert the fuel gas to a product which does not have these disadvantages. Particularly suitable for this purpose is the conversion to methane.

In the state of the art, different methods are known for converting fuel gas coming from hydrocarbons to methane.

GB-A-1 227 156 describes a method for converting hydrocarbons to a methane-containing gas. To this end, hydrocarbons such as, for instance, propane or naphtha are gasified by means of steam reforming, followed by methanization.

US-A-3 854895 describes the gasification of coal in the presence of steam and oxygen for obtaining fuel gas. This fuel gas is converted in three methanization steps to a methane-containing gas. An essential step in this process is converting a part of the CO to CO2 and hydrogen gas, followed by washing out of the CO2. for obtaining a proper ratio of hydrogen, carbon monoxide and carbon dioxide.

GB-A-1 391 034 describes a process corresponding to the one of US-A-3 854 895 for forming synthetic natural gas (SNG) from coal. In this process too, CO2 is washed out, however, this time, only at the end of the process, after the formation of methane.

US-A-3 642 460 describes a process for making methane from a paraffin flow. To this end, the hydrocarbons are subjected to a steam reforming step, followed by a methanization step. During both steps, water cooling takes place. The thus obtained steam is recirculated.

According to WO-A-00/21911, SNG is made from biomass and/or fossil fuel by first gasifying and then carrying out a methanization step. By feeding hydrogen gas to the gasification step, it is intended in this reactor to already produce a considerable amount of methane,

However, these known techniques have a number of disadvantages. One of these disadvantages is the result of carbon formation in the methanization step as a consequence of disproportioning of CO in carbon and CO₂. Coal or soot can be formed as undesired side-product in the methanization step, in particular on the surface of the methanization catalyst, resulting in its deactivation. Also, carbon deposition may lead to clogging and other problems when operating the installation. The problem of carbon formation occurs to an increased extent if, in addition to CO and H2, other hydrocarbons are present, in particular if these are unsaturated hydrocarbons. As indicated hereinabove, fuol gas often contains small amounts of hydrocarbons, in particular unsaturated hydrocarbons. A typical fuel gas obtained by gasification of biomass contains 1 - 5 % by volume of unsaturated hydrocarbons, among which approximately 0.5 vol.% of aromatic compounds, in particular so-called BTX'cs (i.e. benzene, toluene, xylene and naphthalene compounds). Another disadvantage is that for the usual implementations of the methanization, a specially cooled reactor needs to be used. This cooling is necefes^'ary because the methanization reaction is exothermic, while the temperature needs to be controlled carefully, because too high a temperature leads to formation of a product gas with relatively much hydrogen, which, according to the known processes, is indesired because this occurs at the expense of the methane yield.

The object of the present invention is to provide for a method for making a methane-rich product from hydrocarbon-containing material, such as biomass, which at least partly eliminates the above-mentioned disadvantages. It has been found that if at least a part of the hydrogen of the methane- containing gas flow, obtained by methanization of a fuel gas, is separated and is used in the methanization, this object can be achieved. Therefore, the present invention relates to a method for producing a methane-rich gas, comprising the following steps:

- conversion of hydrocarbon-containing material to fuel gas;

- methanization of the fuel gas, wherein a methane-containing gas flow is obtained, which also contains hydrogen and possibly COs;

- separation of at least a part of the hydrogen from the methane- containing gas flow, such, that a methane-rich product and a hydrogen-rich gas flow are obtained;

-re-circulation of the hydrogen-rich gas flow to the methanization step or a point upstream of the methanization stop.

What is meant by a methane-rich product is a product which contains at least 50 vol.%, preferably more than 75 vol.% and most preferably more than 90 vol.% CH₄. This methane-rich product can, inter alia, be used in SNG. For instance, in the Netherlands, for SNG, inter alia the following requirements are set:

Calorific value [MJ/NmS] 31.6-38.7

Wobbe-index [MJ/Nm^S] 43.41^4.46

CO [vol.%] < 0.8

CO₂ [vol.%] < 8

Aromatics [vol.%] < 1

O2 [vol.%] < 0.5

Total sulfur [mg/Nm-η < 45

HC1 + HF [mg Nm³] < 25

Fuel gas is understood to mean a gas which consists for an important part of CO and Ho, for instance of more than 30 vol.% CO and more than 10 vol.% H2. A typical fuel gas composition comprises 40-55 vol. % CO and 20-40 vol.% H2.

In principle, most hydrocarbon-containing materials are suitable to be used as feed for the method according to the invention, alone or in combination with biomass. Therefore, the method according to the invention can be very well used with both economic and environmental advantage, starting from a feed flow which comprises hydrocarbon-containing waste such as plastic.

A practical embodiment of the invention preferably comprises the following steps: a) converting hydrocarbon-containing material to a fuel gas in a gasifier; bl) cleaning the fuel gas in a cleaner for removal of contaminants; b2) passing the cleaned fuel gas through a guard bed; c) methanizing the product of the preceding step in a methanization reactor; whereby heat is released; d) removing water and drying the methanized gas; e) separating hydrogen from the methane-rich product of the preceding step, wherein this hydrogen is used in the methanization reaction or the preceding process; £) reprocessing the product of the preceding step in a reprocessing unit into a methane-rich product.

Due to the conversion of fuel gas to a methane-rich product according to the invention, a gas can be obtained which can be converted very effectively to useful energy via existing infrastructures, such as the gas network, in existing and new high efficiency apparatuses, such as central heating installations, combined heat and energy plants, gas engines, etc. Moreover, by preparing the fuel gas from renewable sources (such as biomass), the gas obtained is durable. Methane has a higher energy density or calorific value (indicated in J/m³) than fuel gas. Moreover, the methane-rich product produced according to the method of the invention can be reprocessed to a product with a Wobbe-index and calorific value comparable to that of natural gas of a particular origin (the composition of natural gas varies per country and/or per location). The Wobbe- index is a measure for the amount of enthalpy which can be added with a particular gas composition per unit of time to a system, for instance an engine or a stove. By providing a product with a Wobbe-index comparable to that of natural gas, the methane-rich product obtained according to the invention can be used very well as a replacement of or in addition to natural gas. Moreover, the efficiency obtained per unit of hydrocarbon-containing material is much higher than when the fuel gas is used directly for the production of electricity, or higher than when the hydrocarbon-containing material is directly converted to electricity, not via gasification but via conventional routes. The methane- rich product can be used directly for, for instance, heating, or as a fuel for WK -units with, for instance, gas engines.

The separate steps of the method according to the invention will presently be further elucidated.

a) Conversion of hydrocarbon-containing material into fuel gas

According to the invention, hydrocarbon-containing material such as biomass (vegetable or animal) is gasified, i.e. converted to a gas mixture containing substantially CO and H2. To this end, use is made of techniques known per se. In the art, three different types of gasifiers are known which can be distinguished according to the gasification medium used: air, oxygen or steam. Combinations thereof are also possible. The use of an air gasifier has as a drawback that, with it, nitrogen from the air is introduced into the process. This nitrogen needs to be removed so that, generally, the costs of operation of the installation according to the invention will prove to be higher than when one of the gasifiers of the other type is used. Moreover, the costs of investments will be higher as a result of the larger volume which needs to be treated because of the presence of inert nitrogen. For this reason, oxygen gasifiers, steam gasifiers and combinations thereof are preferred. Another great advantage of oxygen gasifiers, steam gasifiers and combinations thereof is that the temperature in the gasifier and at the end of the gasifier can be considerably higher than in an air gasifier. As a result, the amount of tar present in the fuel gas decreases strongly. The gasification process with oxygen is exothermic, hence, heat is released.

The gasification process with steam is endothermic, hence, it requires heat. By combining the gasification process with steam and the gasification process with oxygen, the gasification process with oxygen can produce the heat for the gasification process with steam. Also, the heat can, at least partly, be added to the feed flow (biomass, waste, etc. and/or the oxygen and steam to be used) by heating this with the heat released elsewhere in the process or by using it to cool the fuel gas coming from the gasifier. Through the combination of oxygen and steam gasification and the use of residual heat for the steam gasification process, more CO and H can be produced from the same amount of biomass, waste or other organic flow, because less of the feed flow (or fossil fuel) needs to be combusted to CO2 and H2O and the steam is also an extra source of hydrogen. When a sufficient amount of energy from residual sources is fed into the gasification process, even with use of biomass, a part of the CO₂ formation as a result of the presence of oxygen in the biomass can be prevented.

As in gasification processes both CO and H2O are present, also the so-called shift-reaction can occur;

CO + H2O <=> CO2 + H2; ΔJΪo = -41.1 k /mol (I) wherein ΔHo is the formation enthalpy with standard conditions.

Because of the establishment of the shift equilibrium, the ratio of CO and H2 in the fuel gas after the gasifier can to some extent be controlled by setting the pressure and the temperature at the end of the gasifier.

After the gasifier, the fuel gas can be cooled with the oxygen and/or steam feed flows by heat exchange before the gasifier. As a result, a large part of the heat of the fuel gas after the gasifier can effectively be returned to the gasification process to improve the total efficiency of the process. Also, the fuel gas can be cooled down while forming high pressure steam, which can be converted with a steam turbine to useful work, for instance into electricity or compression work.

bl) Cleaning

After gasification, in a cleaning step, the contaminating components in the fuel gas are reduced to an acceptable level for the methanization step. In particular the concentrations of sulfur compounds, halogen compounds and nitrogen compounds need to havo an acceptable level. If necessary, other contaminating components such as tars, ammonia, (heavy) metals and dust also have to be reduced to an acceptable level. For cleaning the fuel gas, use is made of conventional methods for cleaning gasses, such as processes based on washing, adsorption and/or particle separation.

To be distinguished are the removal of dust, tar, alkali metals, heavy metals, NH3/HCN, HaS/COS and halogen compounds (HC1 + HF).

For the removal of dust and tar, known commercial techniques are available/, cyclones, rotating particle separators, cloth filters, ceramic filters, sand beds, active carbon filters, quenching and washing towers and Venturi scrubbers. For gasifiers reaching temperatures of 800^βC to 1000°C, catalytic and thermic tar cracking is also a possibility. Alkali metals precipitate at temperatures below 600°C and, together with dust and tar, remain behind in filters, adsorption beds and quenching and washing towers.

Most heavy metals are separated from the gasifier with bottom and fly ash. Exceptions are the volatile metals: mercury (Hg), cadmium (Cd) and possibly Se. These metals can be captured with wet gas cleaning (washing towers, condensation). With the aid of adsorption to active carbon, mercury can be captured in an effective manner.

NH3, HiS and halogens are preferably removed by washing processes (scrubbing). By correctly setting the acidity degree of the washing liquids, a very effective cleaning is achieved. When relatively small amounts of contaminations are removed, the saturated washin liquid can be purified in a waste water treatment plant.

HCN and COS are converted via hydrolysis in the washing liquid to, inter alia, NHa and HgS. An alternative is simultaneous catalytic hydrolysis of HCN and COS at temperatures above 200*C.

Depending on the scale, it can be desirable, in particular for H₂S, to use regenerative washing processes and to reprocess the released H₂S in a Claus- unit (suitable for > 20 tons S/day) to sulfur. For 1 to 20 tons S/day, washing processes exist which oxidize H2S in the washing liquid to sulfur.

With relatively low productions of NH3 and H2S, it may be economically advantageous to reprocess the washed out NHs and H2S in situ to N₂ and sulfur by means of biologic oxidation processes.

b2) Guard bed

With regard to the removal of dust, tar, alkali metals and heavy metals, the above-mentioned cleaning techniques meet the requirements for properly carrying out the method according to the invention.

However, by using washing processes typically suitable for fuel gas cleaning, in general, 5 to 10 ppm (mol mol) HaS and halogen compounds remain in the cleaned gas. For properly carrying out the method according to the invention, it is desirable to reduce the concentration of H2S and halogen compounds to less than 200 ppb (mol/mol), preferably less than 100 ppb (mol/mol). The fact is that certain catalysts which are used for the methanization reaction are highly susceptible to contamination by sulfur compounds and halogen compounds. Too high a concentration of, in particular, the sulfur compounds in the feed of the methanization step would considerably shorten the life span of these methanization catalysts and hence render the process economically less attractive. Additionally, the requirements with regard to the quality of the produced methane-containing gas (for instance if this is used as SNG) are often such that also a guard bed is required for reducing the contaminations to acceptable values.

For reducing the contents of sulfur compounds and halogen compounds, physical cleaning methods can be used, such as adsorption to active carbon, but chemically binding the contaminants to, for instance, metal oxides is preferred. Adsorption by way of chemical binding, compared to physical binding, generally results in a higher degree of loading of the adsorbent and, with a comparable price of the adsorbent, in an economically attractive ratio of standing time and adsorbent volume. Sufficiently low concentrations of H2S and halogen compounds can be obtained by placing a guard bed before the methanization reactor. This bed contains a solid adsorbent which adsorbs these residual contaminations. As stated, for reducing the content of the sulfur and halogen compounds, a dry adsorption on the basis of chemical binding to metal oxides or a physical binding to, for instance, activated carbon are especially suitable.

For a proper adsorption of the contaminants, the through-put speed of the gas through the guard bed is bound to a maximum. This results in a minimal size of the adsorbent volume in the guard bed. The amount of adsorbent and the through-put speed through the guard bed depend, inter alia, on the desired degree of purity, the frequency with which the beds are replaced and the degree of contamination in the preceding cleaning step.

One of the current materials for adsorbing H2S is, inter alia, activated alumina. A very suitable chemical adsorbent for H₂S is zinc oxide (ZnO). This adsorbent acts optimally at approximately 200-350°C. This is advantageous as such temperatures are well in line with the required temperatures for the shift reaction and the methanization reaction. As a result, it is possible to omit a cooling or heating step between the guard bed and the methanization step. The guard bed based on ZnO makes it possible to reduce the content of sulfur compounds to less than 100 ppb (mol/mol).

Halogen compounds in the gas can react with ZnO to volatile and corrosive zinc halogenides, which are transported with the process gas to the methanization reactor, where they precipitate on the catalyst and deactivate it. To prevent deactivation by zinc halogenides, an adsorption bed on the basis of ZnO can be combined with a layer of an adsorbent which adsorbs the halogen compounds before the process gas reaches the ZnO adsorbent. The layer can consist of, for instance, activated alumina or sodium aluminate on an alumina carrier material.

As, according to the invention, the guard bed has been placed before the methanization reactor, also possible precipitation of dust and metals on the methanization catalyst can be strongly reduced, since the guard bed captures these substances in an efficient manner in case they had not been removed to a sufficient extent yet.

The guard bed can be designed in a so-called two-bed system. Then, two beds with adsorbent are arranged next to each other and the feed can be circuited thus, that it is guided over one of the two beds. The bed which is not in operation can then be replaced or regenerated, without the processing needing to be interrupted. After replacement or regeneration, the flow can be diverted for flowing through the regenerated bed so that the other bed can be replaced or regenerated. Particularly effective is the so-called "lead/lag" configuration, which comprises at least two beds, and wherein a ("lag") bed contains the regenerated or fresh adsorption material. The lag-bed is serially connected with a partly loaded ("lead") bed, which is the first to be flowed through. When the lead-bed has been loaded to a particular value, it is regenerated or renewed, while, temporarily, only the lag-bed provides for the adsorption. After regeneration, the regenerated bed is deployed as lag-bed and the cycle can be repeated. This configuration can easily be obtained by switching the flows. In this manner, a maximum loading of the adsorbent is obtained. A typical fuel gas composition such as it can be obtained by gasification of hydrocarbon-containing materials and after cleaning and passage through the guard bed, is represented in Table 1.

Table 1

Typical fuel gas composition after gasifier, cleaning step and guard bed.

Typical composition

CH₄ 0 - 10 vol.%

Ha 20 - 40 voL%

CO 40 - 55 vol.%

CO2 5 - 20 vol.%

Na 0 - 1 vol.%

H₂S -f- COS <150 ppb (mol/mol)

HC1 + HF < 30 ppb (mol mol)

Air or oxygen gasifiers and gasification feeds with a relatively low moisture content give a fuel gas with a H2 CO ratio of typically 0.5 - 1. Otherwise comparable feeds with a high moisture content (30 - 40%) and/or steam gasification yield a H2/CO ratio of >1. The use of O2 and steam in the gasifier under the conditions wherein the water-gas-shift reaction occurs can increase the Hs CO ratio. In this manner, a H2/CO ratio of 2 to 3 can be achieved.

c) Methanization reaction Methanization of the cleaned fuel gas takes place in a methanization reactor. The methanization of fuel gas can proceed according to reaction (ID. and/or (III).

CO + 3H_z → CH₄ + H₂0 ΔHo = -206.2 kJ/mol (11) CO₂ + 4H₂ -> CH₄ -t- 2H₂O Δ#₀ - -165.0 kJ/mol (III)

In fuel gas obtained according to known techniques insufficient hydrogen is present for converting all CO and CO2 to methane. Generally, the amount of hydrogen in conventionally produced fuel gasses is even too low for complete conversion of CO alone to methane. Without additional measures, the use of such a fuel gas will therefore lead to a low CH4 yield in the methanization step. A further consequence of the presence of relatively much CO is the above- mentioned disproportioning of CO to carbon and CO2, which may lead to carbonization and deactivation of the methanization catalyst. For fuel gasses which, in addition to CO andCO2, also contain carbon in the form of unsaturated hydrocarbons (olefms, aro atics, and the like), the H₂ need is even greater. Without sufficient H for converting these hydrocarbons to saturated hydrocarbons, preferably to CH*, the methanization catalyst deactivates.rapidly through crack reactions and carbonization. Although it is possible to add externally acquired hydrogen - durably or not durably obtained (for instance obtained from green current or other CO₂- neutral methods of preparation) - this is often economically not interesting. Another possiDinτy would be to produce hydrogen via the water-gas-shift reaction (I) by means of adding steam, in situ, i.e. in the methanization reactor. The hydrogen formed with the aid of the shift reaction could then be used to convert the remaining CO to methane. However, it has appeared that the kinetics of this reaction are relatively slow, so that the hydrogen production starts insufficiently rapidly and the above-mentioned carbon formation processes are not sufficiently prevented, even less so when the fuel gas contains (unsaturated) hydrocarbons. By presently, according to the invention, adding H2 in excess and operating the methanization reactor such that after the methanization step the residual H2 is sufficient to be subsequently separated and recirculated, these drawbacks do not occur, while still a favorable conversion to CH₄ can be obtained. The recirculation of the hydrogen-rich gas flow is preferably carried out by guiding this flow to the entrance of the methanization step. Naturally, this recirculated gas flow can also be passed to a point situated before, i.e. upstream of the methanization step, for instance in one of the preceding purification or separation steps. It is also possible to pass the hydrogen- rich gas flow to the fuel gas production step. Also if the fuel gas production step is carried out in the presence of oxygen (for instance from air), the recycling of the hydrogen-rich gas flow can be advantageously used. In the oxygen gasifier two zones can be distinguished. In the first zone, combustion of the hydrocarbons takes place, whereby water, CO2 and heat are produced. In the second zone, all oxygen has been used up and the hydrogen can be safely, i.e. without an explosive mixture being formed, be recycled.

Preferably, according to the invention, so much H2 is added that the methanization step can be carried out with an amount of H2 which is sufficient for converting both the unsaturated hydrocarbons (if present) and the CO. If the amount of H2 (mol) with which the methanization is carried out is given by:

H2 (mol) = a x [mol C-atoms in unsaturated hydrocarbons]

+ b x [mol CO] then α is preferably equal to 1 to 2 (mol mol), more preferably 1.5 to 2 and b is preferably at least 0.4, more preferably greater than 0.7 and most preferably greater than 1 (mol/mol).

In this manner, a favorable CH₄ production is obtained, while the formation of carbon is minimized, or can even be completely pre ented. Also, H2 in these amounts can easily bo separated from the methane-rich gas flow after the methanization reaction, for instance and preferably by using a membrane.

Overall, a part of the CO will be converted with the aid of water vapor to hydrogen and CO2 and the remaining part of the CO will be converted with the aid of hydrogen to methane and water. Eventually, therefore, a gas mixture is obtained in which, besides the excess amount of H₂, also CH₄, CO₂ and water are present. The following overall reactions occur:

4y CO + 2y H₂O → y CH₄ + 3y CO₂

(x + 4y)CO + Sx H2 + (2 -x)H₂O → (*+y)CH₄ + 3y CO₂ wherein x and y are stoichiometric factors. Therefore, the required excess amount of H2 depends on the amount of water vapor in the fuel gas.

The requirements regarding the Wobbe-index and the calorific value of the produced gas can differ, for instance depending on the country where the gas will be distributed and, generally, need to fall within a certain range. To obtain a methane-rich gas with a Wobbe-index and calorific value corresponding to the existing requirements, often a part of the CO₂ and the water needs to be removed and, optionally, other elements need to be added. If desired, the obtained methane-rich gas can be mixed with methane-rich gas, such as natural gas, so as to meet the respective requirements. Although the equilibrium for forming CH₄ from CO and H2 shifts towards the side of methane with decreasing temperature, the temperature should not be chosen to be too low, because the reaction will then proceed too slowly. In practice, it appears that the reaction proceeds with sufficient velocity from 250°C.

As, according to the invention, the available amount of H2 for the methanization step increases with the methanization temperature, it is possible to work at a higher temperature than in conventional processes without the thermodynamic CH yield decreasing distinctly. This is favorable in particular because the methanization reaction proceeds more rapidly at higher temperatures than at lower temperatures. As a result, the CH₄ yield is not limited by the catalyst kinetics. At higher temperatures, the available amount of H2 reaches a maximum and will start to decrease at still higher temperatures, so that the thermodynamic CH4 equilibrium concentration also starts to decrease. For these reasons, the initial temperature of the methanization catalyst is preferably not higher than 600^βC. More preferably, the methanization reaction is carried out at a catalyst initial temperature of 350-500°C, most preferably of 375-425°C.

An important advantage of the method according to the present invention is that the temperature at which the methanization is carried out is far less critical than in conventional processes. As a result, the methanization can be carried out in a simpler reactor, without there being the necessity of complex cooling systems. The reason for this is that due to the excess of H₂ which is present, up to the temperature mentioned of 600°C, there will always be sufficient CH4 production. The unreacted H2 is separated from the CH₄ and reused for the methanization.

Operating at higher pressure yields a still greater flexibility with regard to the temperature range within which ther odynamically, still a good conversion can be achieved. As a result, fluctuations in the fuel gas can be compensated more easily. The pressure will therefore be generally chosen on economic grounds, and will preferably be between 1-100 bara, preferably 1- 50 bara, most preferably 1-15 bara. When the methane-rich product is added to a gas network, the pressure of the product will have to be adjusted to the pressure in the gas network. When, for instance, in the Netherlands, the product in the form of SNG is to be added to the 9 bara natural gas network, the methane-rich product will have to be brought to approximately 10 bara. In this case, it is economically advantageous to operate the methanization reactor at approximately 12 bara. Consequently, this gives a margin of 2 bar for loss of pressure over the reprocessing steps (steps c to f). For the methanization and shift reaction, different types of catalysts can be used. Suitable catalysts are catalysts based on nickel, on platinum and/or on ruthenium.

Catalysts based on nickel are economically advantageous. However, nickel catalysts are more susceptible to contaminations, in particular to contamination by sulfur and halogen compounds, than the platinum and ruthenium -based catalysts. However, as the requirements with regard to contaminations in, for instance, SNG are very strict, these components will still have to be removed for a large part, so that, technically and economically, the uso of nickel catalysts is of very great interest. Typical nickel catalysts for methanization comprise 20 - 80 % by weight, preferably 35 - 70% by weight, for instance 60% by weight of nickel on an alumina carrier, while the weight percentages are defined as grams of metallic nickel per gram of catalyst.

Due to the exothermic reactions, in the reactor a large amount of heat is released which increases the temperature strongly. In conventional methanization processes, through the shift of the thermodynamic equilibrium position, this has a negative effect on the CH₄ yield and the heat needs to be almost entirely dissipated. However, the methanization according to the invention can be carried out without, or with limited heat dissipation. Optionally, after the methanization, the heat can be recovered from the product gas and be usefully employed elsewhere in the process. Through this process integration, the efficiency of the entire installation increases even more.

With particular advantage, in the cooling of the product gas, use is made of cooling by evaporation of water. This has as an advantage that a high- pressure steam can be produced. Moreover, a part of the produced steam can be added to the feed of the methanizing reactor, to thus have sufficient water in the reactor for the shift reaction.

With the recovered heat, in a steam turbine, electric power can be generated. This power can for instance be utilized for driving a compressor which compresses the fuel gas or the final methane-rich product to the desired pressure. This compressor can be placed after the gasifier, preferably after step bl). As the gas is compressed, the temperature rises, thereby bringing it closer to the temperature required for step b2). Generally, placing the compressor has as an advantage that consequently, higher pressures can be realized, as a result of which the equipment can bo of more compact design. Moreover, a higher pressure is favorable to the position of the equilibrium of the methanization reaction. Therefore, the compressor can also be advantageously placed after step b2). By placing the compressor, an important amount of electricity for heating the gas flow is saved. The steam, which partially condenses in the turbine, is condensed after the steam turbine in an air cooler and recycled with a pump to the storage tank for the cooling water of the reactor.

Also, the heat which is recovered from the product gas after the methanization reactor can be used for operating a heat pump. With such a heat pump known in the art, elsewhere in the process, for instance in the reprocessing, cooling can take place. d) Removal of the water fraction

For removing water, a cooling system can be used, with the gas being cooled to approximately 40°C, if the methanization is carried out at increased pressure. A part of the cooling path can be done by bringing at least a part of the product gas in heat exchange with (a part of) the gas to be methanized. For further cooling to approximately 40°C, use can be made of cooling water, which can be recooled in air of in a cooling tower with, for instance, river water.

Finally, the methanized gas needs to be dried, so as to free it from the remaining residues of water, so that the required dew point (in the Netherlands, for instance, -20°C) is reached. This can be accomplished in different manners. In addition to deep cooling, where the gas is cooled electrically to below the required dew point, it is possible to carry out the dehydration step on the basis of adsorption or absorption. In case of adsorption, the gas can be dried by passing it through a fixed bed of silica gel or molecular sieves. Another suitable manner for carrying out the drying step is based on the adsorption with a suitable absorbent, such as glycol (for instance triethylene glycol).

e) Separation and reuse of hydrogen The product gas of the methanization reactor is subjected to a step wherein hydrogen is separated. In this manner, a hydroge -low methane- containing gas flow and a hydrogen-rich gas flow are obtained.

The hydrogen-rich gas flow is recirculated and can, for instance, be fed to the fuel gas at a suitable location. In principle, the hydrogen-rich gas flow can be supplied at any suitable point before the methanization step. Naturally, if applicable, the above-mentioned considerations should be taken into account if the hydrogen-rich gas flow is guided to a fuel gas production step operated with oxygen.

The separation of hydrogen can be carried out with conventional means, but is preferably carried out with a membrane filter. In comparison to the conventional separation techniques which are principally based on adsorption, a membrane filter entails lower investment and operating costs. Further, the desired purity can be controlled in a relatively simple manner by setting the permeate pressure. Also, membrane units are relatively compact and the required ground surface is small in comparison to conventional units.

The membrane separation of H2 can also be combined with partial H2O- removal, whereby the permeate is formed by a mixture of H2 and H2O. An advantage of this combined H2 H2O separation is that, in principle, the abovementioned cooling to 40°C in step d) can be omitted. It suffices to collect water behind the membrane, for instance by passing the mixture into a drum. After removal of a part of the water, thus, a water-saturated flow of H2 is obtained. This water-saturated flow of H2 can then, for instance, be guided to the methanization step.

f) Reprocessing

Reprocessing the methane-rich product comprises techniques common in the gas industry for removing CO2, any residual water, nitrogen and/or other undesired components so that the desired specifications can be met. Also, in this step, an odorant (for instance totrahydrothiophene, THT) can be added. The reprocessing of the methane-rich product to a product suitable for supply to the natural gas network can for instance be carried out as follows.

To obtain an SNG having the desired composition, for instance expressed in the Wobbe-index and or calorific value, generally, the CO2 concentration needs to be set. For removing CO2, use can be made of membrane technologies, pressure swing absorption (PSA) technologies, membrane gas absorption (MGA) technologies, scrubber technologies, etc.

If use is made of a PSA system, preferably a system is used with two or more absorbers filled with CMS (Carbon Molecular Sieves), so that the separation process can be carried out semi-continuously. ϊn addition to the removal of CO2, also (a part of) nitrogen and the water, if present, can be removed.

Apart from the above-mentioned techniques for separating CO₂ (PSA, scrubber-technologies), this can be carried out very effectively with the aid of membranes. Preferably, polyimide-membrane systems are utilized. These have a relatively high chemical resistance and, in comparison to other polymer membranes, can be used to a relatively high temperature (up to 150°C).

It is also possible to include a step for separation of the CO2 before the methanization reactor, so that the CO2 content in the fuel gas flow is reduced. This pre-separation has as an advantage, that the volume flows can be smaller and therefore the equipment too. Another advantage is that less steam needs to be added to the reactor, since the risk of carbonization has considerably decreased due to the removal of CO2, while the final SNG production remains guaranteed. The separation of CO2 before the methanization step therefore not only leads to an extra reduction of the total volume flows (and a corresponding reduction of the equipment), but also increases the efficiency of the total plant.

During the production cycle, the Wobbe-index of the produced methane- rich product is measured. The Wobbe-index is determined to a considerable extent by the CO2 content of the SNG and is therefore strongly dependent on the efficiency of the CO2 removal. With PSA systems, for instance, the Wobbe- index decreases in time because of saturation of the CMS in the absorber. The production phase is stopped at that moment when the time average Wobbe- index meets the quality requirements for natural gas. The Wobbe-index is for instance determined as described in EP-A-0 665 953. To obtain a synthetic natural gas with a constant Wobbe-index, after the CO2 separation, a mixing of the product gas in time takes place by means of a buffer vessel. This happens in two series-connected mixing vessels in which a perforated tube provides for proper mixing.

Eventually, to the methane-rich product, the odorant THT can be added with the aid of a dosing pump. Finally, with the aid of an officially approved gas chromatograph or Wobbe-meter, the methane-rich product can be analyzed before it will be added as SNG to the gas network.

The methane content of the product gas can be set on the basis of the desired use and contains an amount of methane such that when it is added to a gas network, the respective requirements with regard to the quality of the gas are met. For other uses, technical and economic factors play a role in determining the ideal amount of methane and CO2 in the product gas.

The invention is presently elucidated with reference to Figs. 1 and 2 in which process diagrams are represented of possible embodiments of the method according to the invention.

According to the embodiment in Fig. 1, a fuel gas which has been obtained by gasifying hydrocarbons in a gasification step (not represented) and largely cleaning in the cleaning step bl (not represented) is raised in pressure with the aid of a compressor to 13 bara. As a result of the compression of the fuel gas, the temperature of the gas increases considerably. This is favorable since the successive guard beds operate at a temperature of 200 - 350"C. In step (b2) the sulfur compounds and halogen compounds are virtually completely removed (sulfur compounds <150 ppb (mol/mol) and halogen compounds < 30 ppb (mol/mol). Then, the cleaned gas is passed to the methanization reactor (c), together with an amount of steam. In the methanization reactor, the conversion of CO and hydrogen to methane takes place.

The methanized gas from the reactor is cooled for condensing the added steam. This can be combined with the preheating of the feed of the methanization reactor. Optionally, last residues of water can be removed in a supplemental drying step (not represented) so that the desired requirements (for instance to a dew point of -20°C) are met.

The thus obtained product gas consists mainly of CO2, CH₄, and H2. Then, the hydrogen is separated from the product gas. In Fig. 1, this i& done by means of membrane separation. The hydrogen-rich gas is returned to the feed of the reactor by adding this to the fuel gas before the compression step.

After the hydrogen has been removed, the greater part of the COa is removed from the methane-rich product gas. According to Fig. 1, this also takes place in a membrane separation step, but it can also be carried out in, for instance, a PSA-installation. The separation is carried out such, that a methane-rich product gas having the desired Wobbe-index is obtained. Reprocessing the gas comprises, for instance, odorizing with the aid of THT. After this, the gas is suitable for distribution. In Fig. 2, a preferred embodiment of the invention is represented, identical to the one of Fig. 1, but supplemented with a step wherein the greater part of the CO2 in the feed is separated in a pre-separation step. The separated CO2 is transported to the last CO2 separation step. Optionally, and advantageously, it is possible, in addition to CO2, to also separate CR4 in this pre-separation step if this is present in the fuel gas to a sufficient extent.

Consequently, the methanizing reactor and the supplementary equipment can be of smaller dimensions, without this having to lead to a decrease of the SNG- production.

Claims

1. A method for producing a methane-rich gas, which metho comprises the following steps:

- conversion of hydrocarbon-containing material to fuel gas,

- methanization of the fuel gas, wherein a methane -containing gas flow is obtained which also contains hydrogen and possibly CO^,

- separation of at least a part of the hydrogen from the methane-containing gas flow, while obtaining a methane-rich product and a hydrogen-rich gas flow,

- recirculation of the hydrogen-rich gas flow to the methanization step or a point before the methanization step.

2. A method according to claim 1, wherein the hydrogen-rich gas flow is recirculated to the fuel gas production step.

3. A method according to any one of the preceding claims, further comprising a CO2 separation step, wherein at least a part of the CO2 present n the methane-containing gas flow is separated.

4. A method according to claim 3, wherein the CO2 separation step is carried out such that the methane-rich gas flow undergoes no or only a slight pressure decrease.

5. A method according to claim 3 or 4, wherein the CO2 separation step is carried out after the hydrogen separation step.

6. A method according to claim 3 or 4, wherein the CO2 separation step is carried out before the hydrogen separation step.

7. A method according to any one of claims 3 - 6, wherein the CO₂ separation step is carried out by using a membrane and/or by performing a washing step.

8. A method according to any one of the preceding claims, wherein the hydrogen separation step is carried out by using a membrane.

9. A method according to any one of the preceding claims, wherein in the hydrogen separation step also water is separated from the methane-rich gas flow by using a membrane.

10. A method according to any one of the preceding claims, further comprising a CO2 pre-separation step, wherein before the methanization step at least a part of the CO2 present in the fuel gas is separated while a CO2-rich gas is obtained.

11. A method according to claim 10, wherein the CO2-rich gas is guided to the CO2 separation step mentioned in claims 3 - 4.

12. A method according to any one of the preceding claims, wherein the methanization step is carried out at 250 - 600^βC.

13. A method according to claim 12, wherein the methanization step is carried out at 350 - 500°C, preferably at 375 - 425°C.

14. A method according to any one of the preceding claims, wherein the methanization step is carried out in an uncooled reactor.

15. A method according to any one of the preceding claims, wherein a nickel-based catalyst is used in the methanization step.

16. A method according to any one of the preceding claims, wherein the methani ation step is carried out at a pressure of 1 - 100 bara, preferably 1-50 bara, and most preferably 1 - 15 bara.

17. A method according to any one of the preceding claims, wherein said fuel gas flow, the methane -containing gas flow, the methane-rich product and/or the hydrogen-rich gas flow ate cooled, while the released heat and/or the heat which ie released in the methanization process is usefully employed, preferably for the production of high-pressure steam.

18. A method according to claim 17, wherein said heat which is recovered upon cooling is used for driving a heat pump.