GB2466554A

GB2466554A - Process for the manufacture of town gas from landfill gas

Info

Publication number: GB2466554A
Application number: GB0921466A
Authority: GB
Inventors: David Gibson
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2008-12-24
Filing date: 2009-12-08
Publication date: 2010-06-30
Anticipated expiration: 2029-12-08
Also published as: GB2466554B; GB0823449D0; WO2010073026A1; GB0921466D0; HK1142358A1

Abstract

A process for the manufacture of town gas from landfill gas comprises the steps of;(a) recovering a landfill gas from a landfill site, (b) removing contaminants and water from the landfill gas and adjusting the carbon dioxide level in the resulting landfill gas to give a purified mixed gas stream comprising carbon dioxide and methane,(c) forming a hydrogen-containing gas stream by reforming a first portion of the mixed gas stream, and(d) mixing a second portion of the mixed gas stream with the hydrogen-containing gas. Volatile organic contaminants may be removed by a cryogenic capture step. An apparatus for carrying out the process is also described.

Description

Process This invention relates to a process for the manufacture of town gas from landfill gas.

Town gas is a mixture of hydrogen, methane and carbon oxides and is used as an industrial and domestic fuel. It is often manufactured by the thermal cracking of naphtha fuel. Such processes can be uneconomic where no local source of naphtha is available.

Landfill gas, which is generated by the action of bacteria on cellulose material in landfill waste sites, contains methane and carbon dioxide and therefore offers the potential for use as a fuel.

However, landfill gasses generally contain high levels of water and a range of contaminants that have to be removed before the gas may be used. Furthermore, the high concentration of carbon dioxide in landfill gas is not well suited to its direct use as a fuel for many applications.

GB2347432 describes a process for the generation of naphtha gas, or town gas, from a landfill gas. The landfill gas is collected; water, volatile organic contaminants and hydrogen sulphide are removed. The resulting mixed gas stream comprising 50:50 carbon dioxide and methane is split and a first portion is subjected to methane recovery. A portion of the recovered methane is steam reformed and the reformed gas subjected to the water-gas shift reaction to produce a hydrogen-containing gas comprising hydrogen and carbon dioxide at a ratio of about 4:1. The hydrogen-containing gas is then blended with a second portion of the mixed gas stream and the remaining recovered methane to give a naphtha gas. Carbon dioxide separated in the methane recovery step may also be blended-in.

The process described in GB2347432 requires complex methane separation and careful blending of the various gas streams in order to achieve the desired naphtha gas composition.

Methane purification by e.g. vacuum pressure-swing adsorption, C02-liquid adsorption, or using selective membranes, was performed to increase the hydrogen content of the resulting gas mixture to the required levels. Whereas the mixed gas stream containing carbon dioxide could be used as a fuel used to heat the hydrogen reformer system, it was not used directly to generate the hydrogen-containing gas for fear that the carbon dioxide levels would be too high.

We have developed a process including a step of reforming a gas mixture comprising carbon dioxide and methane that overcomes the drawbacks of the known processes.

Accordingly the invention provides a process for the manufacture of town gas from landfill gas comprising the steps of; (a) recovering a landfill gas from a landfill site, (b) removing contaminants and water from the landfill gas and adjusting the carbon dioxide level in the landfill gas to give a purified mixed gas stream comprising carbon dioxide and methane, (c) forming a hydrogen containing gas stream by reforming a first portion of the mixed gas stream, and (d) mixing a second portion of the mixed gas stream with the hydrogen-containing gas.

The invention further provides apparatus for producing a town gas from a landfill gas comprising: (a) landfill gas recovery apparatus operatively coupled to (b) a purification unit that removes water and volatile organic contaminants and adjusts the content of carbon dioxide in said landfill gas to produce a mixed gas stream comprising methane and carbon dioxide, (c) a hydrogen generation unit comprising a reforming unit operatively coupled to said purification unit so that a first portion of the mixed gas stream is passed to the reforming unit to form a hydrogen-containing gas and (d) a mixing unit, also operatively coupled to said purification unit so that a second portion of said mixed gas stream is mixed with said hydrogen-containing gas stream.

Herein, the term "landfill gas" is taken to mean any gas derived from a landfill site that includes a mixture of methane, carbon dioxide, nitrogen and oxygen gases. Typically, landfill gas may comprise about 40-70% by volume methane, 30-60% by volume carbon dioxide, 6-13% nitrogen (on a dry gas basis) and trace amounts of contaminants such as volatile organic compounds, hydrogen sulphide, heavy metals and oxygen. The landfill gas is also often saturated with water.

The landfill gas is recovered from a landfill site. Any known landfill gas recovery method and equipment may be used in the process. The landfill gas is preferably recovered from the landfill site through a series of wells and pipe laterals which extend into the landfill area.

The collected landfill gas is preferably subjected to a pre-treatment step in which the water content of the landfill gas is reduced by cooling the landfill gas to condense water and removing the condensate in one or more separators. Removing water in this way to form a de-watered landfill gas also has the benefit of potentially removing some of the more soluble contaminants.

The landfill gas may be suitably compressed at this stage. The compression for water removal need not be high and pressures in the range 0.5-5 barg (1.5-6 bara or 1.5x105 -6x105 Pa) may be used. At these pressures the water may be effectively removed in two stages by cooling with cooled water below 40°C and chilled water below 10°C, preferably about 5°C.

Alternatively, regenerable absorbents may be used such as zeolites. Unlike the aforesaid GB2347432, which requires a substantially dry gas for effective methane purification, in the process of the present invention is not necessary to remove all the water from the landfill gas prior to its use in hydrogen generation.

The contaminants in the landfill gas stream present an obstacle to efficient hydrogen generation and use of the landfill gas as a fuel. Typical contaminants include alkanes, aromatics, halo-hydrocarbons, oxygenated compounds, other hydrocarbons, silicon compounds, heavy metals and sulphur compounds. The contaminants, particularly the sulphur compounds such as hydrogen sulphide and thiols, heavy metals such as mercury, lead and arsenic and silicon compounds such as organo-siloxanes are often poisons or the source of poisons for catalysts used in reforming processes. Therefore, these contaminants as well as other volatile organic contaminants that may be hazardous in the resulting town gas such as halogen-compounds should be removed from the landfill gas or reduced to acceptably low levels.

Contaminant removal may be accomplished using known methods for purifying landfill gas such as absorbent columns containing non-reactive absorbents such as activated carbon, or reactive absorbents such as zinc oxide. If hydrogen is available a hydrodesulphurisation catalyst comprising Co, Ni, W or Mo on a support, e.g. alumina, may also be used to convert sulphur compounds to hydrogen sulphide that may then be absorbed using zinc oxide compositions. Heavy metals may also be trapped on the hydrodesulphurisation catalyst.

In a particularly preferred embodiment, volatile organic contaminants are removed by cryogenic capture using one or more refrigeration units operating at very low temperatures. Compression to pressures �= lObarg (�= libara), preferably �= lsbarg (�= l6bara), e.g. 21 barg (22 bar abs) is desirable for effective VOC removal and this may be performed on the de-watered landfill gas, followed by stages of cooling to temperatures in the range -20 to -50°C. Preferably the de-watered landfill gas is cooled first to a temperature in the range -20 to -30°C to condense the impurities, and then the gas is passed through a filter material.

A further advantage of using cryogenic capture units is that they may be operated at temperatures and pressures that cause some of the carbon dioxide present in the landfill gas to be effectively separated, thereby reducing the burden on the hydrogen generation equipment.

Hence, preferably the process includes the step of cryogenically adjusting the carbon dioxide level by condensing and removing a portion of the carbon dioxide from the purified landfill gas.

In a particularly preferred embodiment, the de-watered landfill gas stream at a pressure �= lObarg, preferably �= lsbarg, more preferably �= 2Obarg, is cooled to a temperature in the range -20 to -30°C to condense impurities, the gas is separated from the condensate and filtered, and then further cooled to a temperature < -50°C, preferably < -60°C, more preferably < -80°C to condense carbon dioxide. This removal of contaminants and reduction in the carbon dioxide content of the landfill gas offers a particular efficiency improvement over the known process.

Cryogenic capture units suitable for removing contaminants and carbon dioxide from landfill gas are described in WO 2007/021183. However, there is no suggestion whatsoever in this disclosure that the resulting purified landfill gas may be used as the feed for a reforming process to produce a hydrogen-containing gas stream that may be combined with a portion of the purified landfill gas to produce a town-gas.

Alternatively, or in addition to a cryogenic capture unit, known carbon dioxide removal technology such as pressure swing absorption, liquid absorption or selective membrane separation may be used although this is less preferred. In the present invention, the carbon dioxide in the landfill gas is not completely removed, i.e. there is no methane separation step and the feed to the hydrogen generation system is not substantially pure methane. We have found that, unlike GB2347432, pure methane reforming is not required in order to generate an

acceptable town gas.

The resulting purified mixed gas stream comprising carbon dioxide and methane typically has a composition of 70-85 mole % (dry) methane, 1-lSmole % (dry) carbon dioxide, 10-20 mole % (dry) nitrogen and < 1.5 mole% oxygen, and may still contain very small amounts of sulphur contaminants. Preferably, the purified mixed gas stream has a composition comprising 75-85 mole % (dry) methane, 5-10 mole % (dry) carbon dioxide, 5-10 mole % (dry) nitrogen and 1-5 mole% oxygen.

Heavy metals and oxygen may not be effectively removed using refrigeration and therefore, where a cryogenic capture unit is employed, suitable catalysts and absorbent columns may, if desired, be used downstream to remove contaminant metals and free oxygen from the landfill gas. In particular we have found that hydrogen or carbon monoxide conversion catalyst may be effective in removing the oxygen present in the partially purified, C02-depleted landfill gas.

The gas mixture is preferably passed over the conversion catalyst at a suitable temperature and pressure in order to react the hydrogen with the free oxygen to produce steam.

Alternatively or additionally, the conversion catalyst may convert the free oxygen into carbon dioxide by reaction with any carbon monoxide present in the mixed gas stream. The conversion catalyst may be any shown to display activity for the oxidation of hydrogen and/or carbon monoxide at low temperatures, and preferably is a supported Group 8 transition metal catalyst. For example the catalyst may comprise one or more of Co, Ni, Pt, Pd, Rh, Ir or Ru on an oxidic support such as ceria, magnesia, alumina, titania, zirconia or silica. Au may also be present. Metal sulphide supports may also be used. Preferably the catalyst comprises Au, PtSn, PtFe, PtCo, Pt, Pd, Co or Ni on alumina, e.g. �=5% wt Pt on alumina. The conversion catalyst may be in the form of a woven, nonwoven or knitted mesh, particulates such as pellets or extrudates, a foam, monolith or coating on an inert support. The conversion of the free oxygen is preferably performed at < 300°C.

As stated above, remaining metal compounds, e.g. of Hg, Pb, V, As, may be absorbed on suitable catalyst/absorbents such as Go-or Ni-containing hydrodesulphurisation catalysts, which may be followed by absorbent materials such as zinc oxide compositions. This may also remove any remaining H2S that has not been captured upstream.

The aforesaid GB2347432 and WO 2007/021183 are silent upon heavy metal and oxygen contamination of the landfill gas. We have found that their removal in a conversion/purification unit can be particularly important to the effective operation of the process.

The resulting purified mixed gas stream comprising carbon dioxide and methane typically will have a composition of 70-85%, preferably 75-85% by volume of methane, 1-15%, preferably 6- 12% by volume of carbon dioxide with a balance of nitrogen and contaminants of <1 ppb total sulphur.

A first portion of this mixed gas stream is sent forward to a hydrogen generation stage and a second portion of this gas is sent to a blending stage. The portion of the mixed gas that is subjected to reforming in the hydrogen generation step is dependent upon the desired town gas composition, but is preferably between 40 and 55 %, more preferably between 45 and 55% of the purified landfill gas.

Hydrogen generation in the process may be achieved by reforming or a by a combination of reforming and water-gas shift steps. The reforming step may comprise catalytic steam reforming, a combination of catalytic steam reforming and partial oxidation or just partial oxidation. Steam reforming in a heat exchange reformer is preferred, particularly in combination with one or more water-gas shift stages.

In steam reforming, the separated portion of the mixed gas stream comprising carbon dioxide and methane is mixed with steam and the mixture passed at elevated temperature and pressure over a suitable catalyst, generally a supported transition or precious metal on a shaped alumina or calcium-aluminate support, disposed in externally heated tubes in a heat exchange reformer. Methane reacts with steam to produce hydrogen and carbon oxides. Any hydrocarbons containing two or more carbon atoms that are present are converted to methane, carbon monoxide and hydrogen. In addition, the reversible water-gas shift reaction occurs.

The mixed gas stream comprising carbon dioxide and methane may be mixed with steam by means of a saturator or direct injection of steam. The amount of steam required is such as to give a steam ratio of 0.5 to 3.5, i.e. 0.5 to 3.5 moles of steam per gram atom of hydrocarbon carbon in the purified, C02-depleted landfill gas. Water recovered from the landfill gas and suitably purified may, if desired, be used to generate at least part of the steam required.

If desired, the gas mixture may be subjected to an initial step of adiabatic low temperature steam reforming. In such a process, the mixed gas stream /steam mixture is heated, typically to a temperature in the range 350-650°C, and then passed adiabatically through a bed of a suitable catalyst, usually a particulate supported nickel catalyst having a high nickel content, for example above 40% by weight. During such an adiabatic low-temperature reforming step any hydrocarbons higher than methane react with steam to give a mixture of methane, carbon oxides and hydrogen. The use of such an adiabatic reforming step, commonly termed pre-reforming, may be desirable to ensure that the feed to the heat exchange reformer contains no hydrocarbons higher than methane and also contains some hydrogen. This is desirable in order to minimise the risk of carbon formation on catalyst in the subsequent reformer apparatus.

The pre-reformed gas mixture or the mixed gas stream /steam mixture may be heated if necessary to the heat exchange reformer inlet temperature, which is typically in the range 300- 500°C and then passed through the catalyst-filled tubes. During passage through the tubes containing the reforming catalyst, the endothermic reforming reaction takes place with the heat required for the reaction being supplied by a hot gas flowing past the exterior surface of the tubes. In one embodiment the hot gas is combusting landfill gas, preferably purified landfill gas, or town gas, or a mixture thereof.

The temperature of the reformed gas leaving the reforming stage is preferably in the range 650-850°C.

The reforming catalyst may comprise nickel supported on a refractory support such as rings or pellets of calcium aluminate cement, alumina, titania, zirconia and mixtures thereof.

Alternatively, particularly when a steam ratio less than 1.0 is employed, a precious metal catalyst may be used as the reforming catalyst. Suitable precious metal catalysts include rhodium, ruthenium and platinum between 0.01 and 2% by weight on a suitable refractory support such as those used for nickel catalysts. Alternatively a combination of a nickel and precious metal catalyst may be used. For example, a portion of the nickel catalyst may be replaced with a precious metal catalyst, such as a ruthenium-based catalyst.

The reformed gas mixture comprising hydrogen, carbon dioxide, carbon monoxide and unreacted methane and steam is preferably subjected to the water-gas shift reaction in one or more stages with intermediate cooling, if desired. The water-gas shift reaction increases the hydrogen content of the gas mixture by converting carbon monoxide and steam into hydrogen and carbon dioxide. The reaction may be performed by passing the reformed gas through one or more beds of a particulate shift catalyst disposed in one or more shift reactors, preferably with inter-reactor cooling. The shift reaction is promoted by many different catalysts but preferably the gas mixture is passed over one or more beds of reduced iron-and/or copper-based catalysts. The iron-based catalysts are typically based on haematite, often with small amounts of chromia and may be used at temperatures in the range 350-450°C. Copper-based catalysts such as the known copper-zinc oxide/alumina catalysts are more active but require operation at lower temperatures (e.g. 200-250°C) to prevent deactivation.

The resulting hydrogen-containing gas stream comprising hydrogen, steam and carbon dioxide and small amounts of methane and carbon monoxide typically will have a composition of 40- 50% volume of hydrogen, 10-15% by volume of carbon dioxide, a steam content of < 45%, a methane level of 5-7% and a carbon monoxide level of < 2%. Small amounts of nitrogen and ammonia may also be present.

Howsoever the reforming and shift steps are performed, cooling of the hydrogen-containing gas mixture is desirable to recover useful heat energy, e.g. for raising steam, and in order to remove steam as water. Accordingly the hydrogen-containing gas mixture is subsequently cooled to below the dew point of steam at which water condenses. Such cooling may be effected using a stream of cold water and/or by indirect heat exchange. The water condensate is separated from the resulting hydrogen-containing gas using for example, a separator.

The portion of purified landfill gas not sent for reforming may be combined with the reformed gas mixture before or after any cooling thereof, i.e. the blending or mixing step may be upstream or downstream of one or more of the heat recovery stages. In a preferred embodiment, the purified landfill gas is blended with the reformed gas mixture before any cooling thereof. In the blending step, the hydrogen containing reformed gas mixture and the second portion of gas mixture containing carbon dioxide and methane are combined to form the town gas composition.

Downstream of the heat recovery process, a portion of the product gas, which contains hydrogen, may be recycled to the conversion/purification unit, if present. The remaining product gas may be further dried and mixed with a stenching agent to provide the final town gas for distribution. The composition of the town gas is preferably 46.3-51.8 mole % (dry) hydrogen, 1.0-3.1 mole % (dry) carbon monoxide, 16.3-25.9% mole % (dry) combined carbon dioxide and nitrogen, and a methane level of 28.2-30.7 mole % (dry).

The invention is further illustrated by reference to the following drawings in which; Figure 1 is a flowsheet depicting one embodiment of a process according to the present invention; Figure 2 is a flowsheet depicting a pre-treatment stage, which may be used to remove water from a landfill gas; Figure 3 is a flowsheet depicting a cryogenic capture unit (CCU), which may be used to remove volatile organic contaminants and carbon dioxide from the resulting de-watered landfill gas; Figure 4 is a flowsheet depicting a catalyst and absorbent system, which may be used to remove oxygen, heavy metals and any remaining sulphur compounds from the resulting partially purified, C02-depleted landfill gas, Figure 5 is a flowsheet depicting hydrogen generation by steam reforming and water-gas-shifting of a first portion of the purified, C02-depleted landfill gas and also the blending of the hydrogen-containing gas and a second portion of the purified, C02-depleted landfill gas to give a town gas.

The process as depicted in Figures 1 to 5 was modelled based on a landfill gas having the following composition; (on a dry gas basis); Methane 52 mole % Carbon Dioxide 41.5 mole % Nitrogen 6 mole % Oxygen 0.5 mole % Hydrogen Sulphide 100 ppmv + SOppmv organic sulphur species The landfill gas was saturated with water and the pH was 5-8.

The range of contaminants present in the landfill gas and their upper limit levels were; Organosulphur (as dimethyl sulphide) ppmv 20 Organochlorine (as Freon 113) ppmv 25 Organofluorine (as Freon 113) ppmv 10 Freonl2 ppmv 5 Methanol ppmv 3,000 Ammonia mg/m3 1,000 Hydrocarbons: Butane 20 ppmv, Hexane 5 ppmv, Heptane 10 ppmv, Octane 25 ppmv, Decane 25 ppmv, Undecane 15 ppmv, CS to C6 95 mglm3, C7 to C8 75 mg/m3, C9 to ClO 180 mg/m3, Cli toCl2Smg/m3, Heavy Metals: Mercury 130 tg/m3, Lead 160 tg/m3, Arsenic 540 tg/m3, Vanadium S tg/m3, Oil Mist: C13 to CiS 1 mg/m3, C17 to C20 1 mglm3, C21 to C30 1 mg/m3, VOC Compounds: cis-1,2-Dichloroethylene 5 ppmv, Benzene 5 ppmv, Trichloroethane 5 ppmv, Toluene 30 ppmv, Ethyl Benzene 15 ppmv, m-& p-Xylene 15 ppmv, o-Xylene 10 ppmv, Styrene 5 ppmv, 1,3,5-Trimethylbenzene 5 ppmv, 1,2,4-Trimethylbenzene 10 ppmv, 1,4-Dichloro-benzene 5 ppmv, Acetone 5 ppmv, Methyl Ethyl Ketone 5 ppmv, Cyclohexane 5 ppmv, 4-Ethyltoluene 5 ppmv, 1H-Pyrazole -4,5-dihydro-4,5-dimethyl-l5ppmv, 2 Methyl Heptane 15 ppmv, Alpha Pinene 50 ppmv, Bicyclo[3.1.1]heptane,6,6-dimethyl-2-me 15 ppmv, Limonene 15 ppmv, Benzene, 1-methyl-2-(1-methylethyl)-65 ppmv, Dichlorodifluoromethane 3,800 ppbv, Chloromethane 1 ppbv, 1,2-dichloro-1,1,2,2-tetra-fluoroethane 1 ppbv, Vinyl chloride 3,000 ppbv, Chloroethane 300 ppbv, Trichlorofluoromethane 1,100 ppbv, 1,1-dichloroethene 1 ppbv, 1,1,2-trichloro-1,2,2-trifluoroethane (Freon 113) 65 ppbv, Methylene chloride 610 ppbv, 1,1-dichloroethane 240 ppbv, cis-1,2-dichloroethene 2,600 ppbv, Chloroform 10 ppbv, 1,1,1-trichloroethane 15 ppbv, 1,2-dichloroethane 25 ppbv, Carbon Tetrachloride 1 ppbv, 1,2-dichloropropane 1 ppbv, Trichloroethylene 450 ppbv, cis-1 3-dichloropropene 1 ppbv, trans-i,3-dichloropropene 1 ppbv, 1,1,2-trichloroethane 1 ppbv, Tetrachloroethene 730 ppbv, Chlorobenzene 5 ppbv, 1,1,2,2-tetrachloroethane 1 ppbv, 1,3,5- trimethylbenzene 3,500 ppbv, 1,2,4-trimethylbenzene 5 ppbv, m-Dichlorobenzene 1 ppbv, p-Dichlorobenzene 3,800 ppbv, o-Dichlorobenzene 55 ppbv, Organosilicon: Hexamthylcyclotrisiloxane 5 mg/ms, Hexamethyldisiloxane 5 mg/ms, Octamethylcyclotetrasiloxane (as hexamethyldisiloxane) 30 mg/m3, Octamethyltrisiloxane 1 mg/m3, Decamethylcyclopentrisiloxane 5 mg/ms, Decamethyltetrasiloxane 1 mg/ms, Decamethylcyclopentasiloxane (as hexamethyldisiloxane) 5 mg/ms, and Dodecamethylcyclohexasiloxane (as hexamethyldisiloxane) 5 mg/ms.

In Figure 1, a raw landfill having the above composition is collected from a landfill site and delivered via line 110 to a pretreatment stage 112 in which it is pressurised and cooled to condense water and other soluble/liquid impurities that are removed via line 114. The resulting de-watered landfill gas is further compressed then fed via line 116 to a cryogenic capture unit (CCU) 118 that cools the gas stream to condense volatile organic contaminants, which are removed via line 120. The gas is further cooled in the CCU to condense a portion of the carbon dioxide, which is removed via line 122. A portion of the resulting partially purified, C02-depleted gas stream is taken for use as fuel in the downstream reforming and the remainder mixed with a portion of a hydrogen-containing gas fed via line 124, preheated and passed via line 126 to a conversion/purification unit 128. Alternatively, all the partially purified, C02-depleted gas stream may be combined with the recycled hydrogen-containing stream and passed through the preheater before diverting a portion off as fuel to the reformer (not shown).

The unit 128 comprises a reactor containing a conversion catalyst that converts any oxygen present into steam or carbon dioxide and a vessel containing hydrodesulphurisation catalyst and a reactive absorbent that remove heavy metals and any remaining H2S. The resulting purified, C02-depleted landfill gas leaving the unit 128 is then split with a first portion fed via line 130 to hydrogen generation and a second portion via line 132 to the product gas. The first portion in line 130 is combined with steam from line 134 and fed to steam reforming apparatus 136 where it is passed through externally-heated catalyst filled tubes. The tubes are externally heated by combusting the separated portion 138 of the partially purified, C02-depleted gas stream obtained from unit 118 as fuel. The reformed gas stream from reformer 136 is cooled and passed via line 140 to a shift reactor 142 containing a water gas shift catalyst that increases the hydrogen content of the reformed gas stream. The resulting hydrogen containing stream is then mixed with the purified landfill gas in line 132 and cooled by one or more steps of heat exchange in one or more heat exchangers 144 to below the dew point to condense water. The condensate is removed via line 146 and the resulting gas and fed via line 148 to drying and odorising units 150 to create the final town gas 152. A portion of the cooled, dried gas from line 148 is sent back via line 124 to the conversion/purification unit 128 as a source of hydrogen.

Figures 2 to 6 depict in detail the features of the embodiment described in Figure 1.

In Figure 2, the landfill gas 110 is delivered from a metering station (not shown) to the pretreatment unit 112 at a maximum temperature of 60 deg C and pressure of 0.069 barg (6.9 kPag). The pretreatment unit 112 is based on three identical compression trains (2A-C) plus an optional additional unit (2D). In each of the trains, a first stage of compression raises the pressure of the landfill gas to 131 kPag using a Rootes Blower 210 to keep the main compressor size and costs to a minimum. The gas temperature after this compression stage rises to approximately 144 deg C. After compression, the hot landfill gas enters a Landfill Gas Cooler 212 where it is cooled with cooling water 214 to approximately 50 deg C, in a shell and tube heat exchanger, with the landfill gas on the tube side of the unit. A knock out drum 216 is located downstream of the cooler 214 to separate the gas from the condensed liquid effluent 218. The liquid effluent 218 is routed to an oil water separator before being discharged to a landfill leachate treatment facility (not shown). The knock out drum 216 is fitted with a level control system to maintain a liquid seal. The landfill gas leaving the knock out drum 216 is then compressed in compression unit 220, which is cooled with cooling water 222. The compression unit 220 increases the gas pressure to 21 barg (2lOOkPag). The gas is cooled in the unit to 45°C by cooling water fed via line 222, and the condensed liquids removed in a knock-out drum. The condensed liquids are recovered via line 224 and sent for appropriate effluent treatment. The compressed gas from the compression units 220 of each train 2A-2C are combined and directed via line 116 to the cryogenic capture unit 118.

In one embodiment, an additional train 2D is provided to assist with startup of the facility and provide electrical generation. Unit 2D comprises a Rootes blower 230 that raises the pressure of the gas to 1.2barg (l2OkPag), a forced draught aftercooler 232 to maintain the gas temperature at 50°C, a knock-out drum 234 to separate liquid waste 236 for effluent treatment and a siloxane removal unit 240. In the unit 240, the siloxane compounds are removed from the landfill gas to prevent serious abrasive damage to gas engines during fuel combustion.

The cleaned gas may then be fed directly via line 242 to the gas engines driving the feed gas compressor units 220 in trains 2A, 2B & 2C and via line 244 to electrical power generators 246.

It will be understood that, if used, only a small proportion of the feed gas is required to operate unit 2D and that the majority of the feed gas is treated in trains 2A-2C and passed to the Cryogenic Capture Unit 118.

The Cryogenic Capture Unit 118, is specifically designed to remove landfill gas contaminants that will poison the steam reforming catalyst and also to control the amount of carbon dioxide passing into the reforming section. This latter feature is a requirement to allow composition control of the final product gas. Particularly suitable refrigeration apparatus is described in the aforementioned WO 2007/02 1 183.

In Figure 3, one train of the cryogenic capture unit (CCU) is depicted. In operation there may be one or more trains, e.g. four or five trains, operating in parallel, capable of treating 3300 Nm3/h of landfill gas at an inlet pressure of 20.Sbarg (2O5OkPag). The incoming landfill gas, fed via line 116, is divided equally amongst the trains. In this embodiment, in each train, incoming landfill gas at 45 deg C is cooled in a gas/gas interchanger 300 against the outgoing treated gas. The pre-cooled gas is filtered in a knock out drum 304 to remove a portion of the contaminants 306, before entering the condensation unit 308 comprising a number of heat exchangers. In the condensation unit 308, the gas is cooled to -26 degC to condense the contaminants therein. Gas leaving the condensing unit 308 is filtered in a knock out drum 310 to remove the condensed contaminants 312 then polished in vessel 314 over an activated carbon bed or proprietary sorbent such as SOXSIATM available from Gas Treatment Services by and described in the aforementioned WO 2007/021183. The polished gas is then subjected to further cooling in heat exchanger 316 to -40 to -50 deg C to condense carbon dioxide, thus enriching the methane content of the purified landfill gas. A CO2 separator 320 is located downstream of heat exchanger 316 and the recovered liquefied CO2 is sent via line 322 to storage tank 324. The partially purified, C02-depleted landfill gas 326 is sent for further cooling and separation of carbon dioxide in a CO2 condensation unit 328 comprising a number of heat exchangers. The crude product stream 330 is fed to a further CO2 separator 332 from which liquid CO2 is recovered and sent to storage in tank 324. The resulting product gas stream emerging from CO2 separator 332 is fed via line 318 to be used as coolant in heat exchanger 316 and then via line 334 to be used as coolant in heat exchanger 300 before being combined with the product gas from the other streams and fed to the next stage via line 126. By using the product gas in this way it is usefully reheated. The condensed contaminants recovered from knock-out drums 304 and 310 may be combined and sent via line 120 for disposal.

The CCU unit is constructed to control the amount of carbon dioxide removed. This is achieved by using Wobbe Meters on the exit of each train. The amount of carbon dioxide removed in the CCU will be governed by the required product gas composition.

The gas composition leaving the CCU is typically (on a dry gas basis) Methane 80.7 mole % Carbon dioxide 11.5 mole % Nitrogen 7.2 mole % Oxygen 0.6 mole % The gas leaving the CCU still contains heavy metals and oxygen that are desirably removed prior to the gas being used as reformer feed. This may be achieved in a conversion/purification unit by a two-stage process using a catalytic reactor and an absorption column.

In Figure 4, the Landfill gas from the CCU fed via line 126 is divided into streams 400 and 402.

Stream 402 is heated in the shell side of fuel gas heat exchanger 404 to form reformer fuel stream 138. The remaining portion 400, is mixed with product town gas containing hydrogen from line 124 and preheated in a Landfill Gas Preheater 406. The hydrogen recycle rate is based on a minimum of 3 mole% hydrogen (dry basis). The exchanger 406 heats the landfill gas from approximately 44 deg C to 250 deg C using hot product gas to achieve the desired reaction temperature (> 200°C) for the catalytic oxidation and metal contaminant removal. The heated gas stream is then passed from exchanger 406 via a feed line to a reaction vessel 408 containing a fixed bed of a particulate supported precious metal catalyst. The oxygen present in the preheated landfill gas is reacted with hydrogen from the recycled converted gas stream 124 to give water vapour and/or with carbon monoxide to give CO2. The inlet temperature to the reactor is controlled to nominal 250 deg C. The outlet temperature is desirably controlled to 370 deg C or less to prevent thermal damage to the catalyst. The product gas mixture from reactor 408 is split and a recycle portion 412, cooled in recycle exchanger 414, compressed in recycle compressor 416 and added to the reaction vessel feed line. The landfill gas portion 418 that is not recycled passes to an absorption unit 420 containing separate beds of a hydrodesulphurisation catalyst and zinc oxide. The gas leaving the absorption unit is then split, with one stream 130 feeding forward as reformer feed gas and the balance of the gas 132 being passed forward for combination with the reformed gas mixture.

In Figure 5, purified landfill gas at 305 deg C from line 130 is mixed with superheated steam 134 at approximately 360 deg C at a molar steam to carbon ratio of about 2.8:1. The mixed feed is then fed to steam reforming apparatus 136 comprising a reformer vessel 500 containing a plurality of catalyst-filled tubes 506, a fluegas duct 502 containing heat exchangers 504, 512, 514 and 516 and a stack 520. The mixed feed is heated to 420 deg C in a Mixed Feed Preheater 504, located in the flue gas duct 502. The mixed feed flowrate to the steam reformer apparatus 136 has been calculated as approximately 10,908 Nm3/h (wet) with the following composition; Methane 24.2 mole % Hydrogen 1.5 mole % Carbon Monoxide 0.1 mole % Carbon Dioxide 3.9 mole % Nitrogen 2.2 mole % Water vapour 68.1 mole % The mixed feed is passed though the catalyst-filled tubes 506 in a fired steam reformer 500.

The tubes are externally heated by combusting purified landfill gas from line 138 with air fed by line 510. Whereas only three tubes are depicted it will be understood that in a steam reformer many tens or hundreds of tubes may be present. The mixed feed is reformed over the steam reforming catalyst as it passes through the tubes 506 into a gas mixture comprising hydrogen and carbon oxides. The reformed gas is collected and removed from the steam reformer. The com busted gases are sent to the stack 520 via flue gas duct 502.

The flue gas duct 502 in this embodiment comprises four heaters; a) Mixed Feed Preheater 504, b) Flue Gas Boiler 512, c) Steam Superheater 514, and d) Combustion Air Preheater 516.

The Flue Gas Boiler 512 is used to generate steam at4O barg. Boiler water is recirculated through the boiler via a reformer steam drum (not shown). The Mixed Feed Heater 504 is used to raise the temperature of the mixed feed to 480 deg C for reformer feed. The Steam Superheater 514 is used to superheat saturated steam for use in the reformer. The Combustion Air Preheater 516 is used to raise the temperature of the combustion air from 20 deg C to approximately 165 deg C. A motor driven Combustion Air Fan 518 is used to control the flow of combustion air 510 to the reformer burners and is controlled using a variable speed drive. The flue gas leaving the duct 502 enters an ID Fan (not shown) at approximately 135 deg C before being discharged to atmosphere via the Flue Gas Stack 520. The ID Fan is used to control the furnace pressure and is motor-driven with speed control by a variable speed drive.

The reformed gas 140 leaving the Steam Reformer 500 enters a Mixed Gas Boiler 522 at 740 deg C. The boiler is a shell and tube type heat exchanger with the process gas on the tube side and the boiler water on the shell side and will be designed to cool the reformed gas from 740 deg C to 340 deg C for the shift conversion reaction, and is supplied with an internal bypass to control the temperature entering the shift converter. Water is re-circulated from the steam drum (not shown) through the boiler raising steam at 40 barg. After being cooled in the mixed gas boiler 522 the gas mixture is sent to a shift converter 142.

The CO Shift Converter 142 is a stainless steel reactor vessel containing a fixed bed of a commercially available particulate iron-based high temperature water-gas shift catalyst 524.

Reformed gas from the mixed gas boiler 522 enters the Shift Converter 142 at about 340 deg C. The carbon monoxide is reacted with steam over the catalyst bed and converted to carbon dioxide and hydrogen by the water gas shift reaction. There is a slight exotherm across the Shift Converter, thus increasing the temperature of the converted gas to approximately 383 deg C. The converted gas composition exiting the shift converter 142 has been calculated as; Methane 6.4 Mole % Hydrogen 47.0 mole % Carbon Monoxide 1.5 mole % Carbon Dioxide 13.6 mole % Nitrogen 1.3 mole % Water vapour 29.4 mole % The converted hydrogen-containing gas mixture leaving the Shift Converter 142 is then blended with the purified landfill gas 132 that has bypassed the reformer to give the desired town gas composition. An inline static mixer may be installed before the heat exchange steps to ensure good mixing.

The hot town gas mixture is then passed to the tube sides of a number of heat exchangers in series. Firstly it is fed to the landfill gas preheat exchanger 406 at approximately 383 deg C. The exchanger is a shell and tube type heat exchanger with the mixed town gas on the tube side, landfill gas on the shell side and is designed to cool the mixed gas from approximately 383 deg C to 264 deg C. Then the mixed town gas leaving the landfill gas preheat exchanger enters a boiler feedwater preheater 526 at approximately 264 deg C. The preheater is a shell and tube type heat exchanger with the mixed town gas on the tube side, boiler feed water on the shell side and is designed to heat the boiler feed water from approximately 104 deg C to 181 deg C. The hot town gas mixture is cooled from approximately 264 deg C to 170 deg C. The hot town gas mixture gas leaving the preheater 526 enters the Fuel Gas preheater 404 at approximately 170 deg C. The fuel gas preheater 404 is a shell and tube type heat exchanger with the hot town gas mixture on the tube side, fuel gas 402 on the shell side and is designed to heat the fuel gas from approximately 35 deg C to 140 deg C. The hot town gas mixture gas is cooled from approximately 170 deg C to 162 deg C. After the fuel gas preheater, the hot town gas mixture is cooled in another tube and shell heat exchanger 528, this time using water as coolant. The water may be treated water from the process. Exchanger 528 is designed to cool the hot town gas mixture from approximately 162 deg C to 126 deg C. A knock out drum 530 is located after the heat exchanger 528 to separate the condensed fluids. The condensed liquid is water and this is returned to a Condensate Collection Tank or deaerator (not shown) via line 146.

The gas leaving the knock out Drum 530 then enters a Product Gas Cooler 532 at approximately 126 deg C. The cooler is a shell and tube type heat exchanger with the converted gas on the tube side, cooling water on the shell side and is designed to cool the converted gas from approximately 126 deg C to 45 deg C. The cooling water flowrate is based on a 9 deg C rise. A further knock out drum 534 is located after the Product Gas Cooler 532 to separate remaining condensed liquid. The condensed liquid is water and this is returned to the Condensate Collection Tank or deaerator via line 146.

The town gas emerging from the top of knock out drum 534 is split with a small portion fed via line 124 to the conversion/purification unit 128. The remaining town gas may be used directly, however it is preferred to further dry it and add an odorizing agent before passing to the distribution mains. Therefore the town gas saturated at 45°C from knock out drum 534 is passed via line 148 to a Gas Drying Plant 538. The drying plant dries the gas to a dew point of 0°C at 700 kPag. The drying plant preferably comprises a suitable absorption system. The product gas flowrate, where stream 2D is included, has been calculated as approximately 13,614 Nm3/h (drybasis) at the maximum capacity. Before discharge into the 700 kPag distribution main, the town gas is injected with tetrahydrothiophene as a stenching agent in an odoriser unit 540 using known techniques. The product town gas is obtained from unit 540 via line 152.

The town gas production and composition arising from this embodiment has been calculated as; Flowrate 13,607 Nm3/h (dry) Wobbe No. 24.13 MJ/Sm3 Calorific Value 17.20 MJ/Sm3 Flame Speed 34.1 m/s SG 0.51 Pressure 700kPag Temperature <40°C Dew Point 0°C at 700 kPag Methane 29.9 mole % Hydrogen 47.6 mole % Carbon Monoxide 1.5 mole % Carbon Dioxide 17.2 mole % Nitrogen 3.4 mole % The Overall Efficiency has been calculated as 75.0 % The Thermal Efficiency has been calculated as 90.4 % Selected calculated gas composition, flowrates, temperatures and pressures for the various streams depicted in Figures 1 -5 are given in the following Tables.

Table 1.

Stream 110 116 126 Component Flowrate CH4 371.2 306.2 272 (kmol/hr) CO2 296.2 244.4 36.8 N2 42.8 35.3 32.0 02 3.6 2.9 2.5 H20 127.4 3.04 0.32 H25 0.07 0.06 0.001 Methanol 2.14 1.78 0.00017 Ammonia 1.09 0.9 0.12 C5-C12 Hydrocarbons 0.07 0.06 - Organosulphur/chlorine/fluorine 0.04 0.03 - VOC's 0.24 0.2 -Heavy metals (Pb, Hg, V, As) 0.00014 0.00012 0.00010 Siloxanes (mglm3) 57 57 2 Volume flowrate (dry basis) Nm3/hr 16000 13200 8183 Molar Flowrate (dry basis) kmol/hr 713.8 589 365.1 Mass flowrate (wet basis) kg/hr 22771 16927 7024 Operating pressure Barg 0.07 20.5 18.5 Operating temperature degC 60 45 250

Table 2

Stream 124 130 132 134 138 140 152 Component Flowrate CH4 13.0 118.3 153.7 -46.0 40.9 181.5 (kmol/hr) CO2 7.0 16.0 20.8 -5.3 48.6 98.1 CO 0.7 0.28 0.36 --41.1 9.0 N2 2.0 13.9 18.0 -5.3 11.9 28.0 02 ----0.4 --H2 20.8 6.9 9.5 --257.8 289.8 H20 0.3 2.3 3.0 331 1.5 223.6 0.25 H25 ------- Methanol -----4.0 - Ammonia -----4.1 -

COS -------

Volume flowrate 979.5 3485.4 4525.4 -1279.8 9153.9 13607 (dry basis) Nm3/hr Molar Flowrate 43.7 155.5 201.9 -57.1 408.4 607.1 (dry basis) kmol/hr Mass flowrate 643 3055.7 3969 5969 907.5 9024.5 8879.5 (wet basis) kg/hr ________ ________ _______ _______ _______ ________ _______ Operating pressure 18.7 16.9 16.9 15.0 0.2 11.6 7.0 Barg _______ _______ _______ _______ _______ _______ _______ Operating temperature 116 318 318 358 138 340 45 degC _______ _______ _______ _______ _______ _______ _______ In a second embodiment, the process was modelled on a process similar to that depicted in Figures 1-5 based on a feed gas composition as follows (on a dry gas basis); Methane 52 mole % Carbon Dioxide 35 mole % Nitrogen 12 mole % Oxygen 1 mole % Hydrogen Sulphide 250 ppmv + SOppmv organic sulphur species The landfill gas was saturated with water and the pH was 5-8.

The process was also amended by; i) mixing all the partially purified, C02-depleted gas stream with the recycled hydrogen-containing stream 124 and passing it through the preheater before diverting a portion off as fuel stream 138 to the reformer. In this way the fuel stream comprises some hydrogen and separate heat exchange of the fuel stream is avoided. The heat exchanger 404 may then be used to heat the treated water instead of fuel gas and accordingly exchanger 528 may be omitted.

ii) Omitting train 2D. This increases the gas flow through the purification and increases the productivity of the process.

iii) Installing a bulk desulphurisation vessel containing a desuiphurisation material upstream of the cryogenic capture unit 118 to capture a portion of the sulphur compounds to reduce the loading on the unit. The bulk desulphurisation unit may contain conventional zinc oxide desulphurisation materials or other known desulphurisation catalysts/sorbents.

iv) Using the compression unit 220 to raise the gas pressure to 30 barg (3000kPag).

The calculations show the process will provide the following gas compositions; a) the gas composition leaving the CCU (on a dry gas basis); Methane 79.9 mole % Carbon dioxide 0.2 mole % Nitrogen 18.4 mole % Oxygen 1.5 mole % b) The mixed feed to the steam reformer (12,000Nm3h); Methane 24.2 mole % Hydrogen 0.9 mole % Carbon Monoxide 0.1 mole % Carbon Dioxide 0.5 mole % Nitrogen 5.7 mole % Water vapour 68.6 mole % c) The converted gas composition exiting the shift converter; Methane 9.4 Mole % Hydrogen 40.2 mole % Carbon Monoxide 0.4 mole % Carbon Dioxide 10.0 mole % Nitrogen 4.5 mole % Water vapour 35.5 mole % d) The town gas production and composition; Flowrate 16,000 Nm3/h (dry) Wobbe No. 24.13 MJ/Sm3 Calorific Value 17.20 MJ/Sm3 Flame Speed 34.1 mIs SG 0.51 Pressure 700kPag Temperature <40°C Dew Point -4°C at 700 kPag Methane 30.6 mole % Hydrogen 47.1 mole % Carbon Monoxide 0.05 mole % Carbon Dioxide 11.8 mole % Nitrogen 10.0 mole % The Overall Efficiency has been calculated as >70.0 % The Thermal Efficiency has been calculated as >90.0 %

Claims

Claims.A process for the manufacture of town gas from landfill gas comprising the steps of; (a) recovering a landfill gas from a landfill site, (b) removing contaminants and water from the landfill gas and adjusting the carbon dioxide level in the landfill gas to give a purified mixed gas stream comprising carbon dioxide and methane, (c) forming a hydrogen containing gas stream by reforming a first portion of the mixed gas stream, and (d) mixing a second portion of the mixed gas stream with the hydrogen-containing gas gas.
2. A process according to claim 1 wherein the landfill gas is subjected to a pre-treatment step in which the water content of the landfill gas is reduced by cooling the landfill gas to condense water and removing the condensate in one or more separators, thereby forming a de-watered landfill gas.
3. A process according to claim 2 wherein the pre-treatment comprises increasing the pressure of the landfill gas to a pressure in the range 0.5 -5 barg and cooling with cooling with cooled water below 40°C and optionally, chilled water below 10°C.
4. A process according to any one of claims 1 to 3 wherein volatile organic contaminants are removed by cryogenic capture using one or more refrigeration units operating under conditions that cause volatile organic contaminants to condense, followed by removal of the condensed contaminants.
5. A process according to claim 4 wherein the cryogenic capture of volatile organic contaminants is performed on the landfill gas compressed to a pressure �= 10 barg and cooled to a temperature in the range -20 to -50°C, preferably -20 to -30°C.
6. A process according to any one of claims 1 to 5 wherein the carbon dioxide content of the landfill gas is reduced by cryogenic capture using one or more refrigeration units operating under conditions that cause carbon dioxide to condense, followed by removal of the condensed carbon dioxide.
7. A process according to claim 6 wherein the cryogenic capture of carbon dioxide is performed on the landfill gas compressed to a pressure �= lobarg, preferably �= lsbarg, and cooled to a temperature <-50, preferably < -60°C, preferably < -80°C.
8. A process according to any one of claims 1 to 7 wherein both volatile organic contaminants are removed and the content of carbon dioxide in the landfill gas is reduced by cryogenic capture using one or more refrigeration units operating under conditions that cause volatile organic contaminants and carbon dioxide to condense, followed by removal of the condensed contaminants and condensed carbon dioxide.
9. A process according to claim 8 wherein the condensed carbon dioxide is used to cool the landfill gas following removal of the volatile organic contaminants and wherein the purified mixed gas stream obtained after separation of the condensed carbon dioxide is used to cool the landfill gas prior to the step of condensing volatile organic contaminants.
10. A process according to any one of claims 1 to 9 wherein oxygen is removed from the landfill gas using a conversion catalyst to react free oxygen present in the landfill gas with hydrogen and/or carbon monoxide also present in said landfill gas.
11. A process according to claim 10 wherein the conversion catalyst is a supported Group 8 transition metal catalyst.
12. A process according to claim 10 or claim 11 operated following a step of cryogenic capture of volatile organic contaminants and carbon dioxide according to claim 8 or claim 9.
13. A process according to any one of claims 1 to 11 wherein heavy metal compounds are removed from the landfill gas by reaction with a Go-or Ni-containing catalyst, which may be followed by an absorbent material, such as a zinc oxide composition.
14. A process according to claim 13 operated following a step of oxygen removal according to anyone of claims lOto 12.
15. A process according to any one of claims ito 14 wherein the first portion of landfill gas that is subjected to reforming in order to generate hydrogen is between 45 and 55% of purified mixed gas stream.
16. A process according to any one of claims ito 15 wherein the hydrogen formation step is achieved by reforming or a by a combination of reforming and water-gas shift steps, in which the reforming step comprises one or more stages of catalytic steam reforming, a combination of catalytic steam reforming and partial oxidation or just partial oxidation.
17. A process according to claim 16 wherein the hydrogen generation step comprises steam reforming in a heat exchange reformer.
18. A process according to claim 17 wherein the steam reforming is achieved by passing a mixture of steam and the purified mixed gas stream comprising methane and carbon dioxide through a plurality of externally-heated catalyst-filled tubes.
19. A process according to claim 18 wherein the catalyst comprises nickel, or a combination of nickel and a precious metal catalyst, supported on a refractory support.
20. A process according to claim 18 or claim 19 wherein the tubes of the reformer are externally heated by combusting a portion of the purified mixed gas stream
21. A process according to any one of claims 17 to 20 wherein the reformed gas stream is subjected to one or more water-gas shift stages over an iron-or copper-containing catalyst.
22. A process according to any one of claims 1 to 21 wherein any steam or water in the hydrogen-containing gas is removed before or after mixing the hydrogen-containing gas with the second portion of the mixed gas stream that has by-passed the reforming step.
23. A process according to claim 22 wherein the hydrogen-containing gas is cooled in one or more stages to below the dew point and condensed water removed using one or more separators.
24. A process according to anyone of claims 10 to 23 wherein a portion of the hydrogen-containing reformed gas mixture is used to provide hydrogen for the oxygen removal stage
25. A process according to any one of claims 1 to 24 wherein the combined hydrogen-containing gas and mixed gas stream are subjected to one or more subsequent drying steps, and an odorising step.
26. Apparatus for producing a town gas from a landfill gas comprising: (a) landfill gas recovery apparatus operatively coupled to (b) a purification unit that removes water and volatile organic contaminants and adjusts the content of carbon dioxide in said landfill gas to produce a mixed gas stream comprising methane and carbon dioxide, (c) a hydrogen generation unit comprising a reforming unit operatively coupled to said purification unit so that a first portion of the mixed gas stream is passed to the reforming unit to form a hydrogen-containing gas, and (d) a mixing unit, also operatively coupled to said purification unit so that a second portion of said mixed gas stream is mixed with said hydrogen-containing gas stream.
27. Apparatus according to claim 26 comprising a pre-treatment unit having compression means, heat exchange means and separation means that pressurise the landfill gas and cool it so that water present in said landfill gas is condensed and separated upstream of the purification unit.
28. Apparatus according to claim 26 or 27 wherein the purification unit comprises a cryogenic capture unit having compression means, heat exchange means and separation means that pressurise the landfill gas and cool it so that volatile organic contaminants and carbon dioxide present in said landfill gas are condensed and separated.
29. Apparatus according to any one of claims 26 to 28 comprising an oxygen removal unit, downstream of said purification unit, said oxygen removal unit comprising a reactor containing a supported group 8 conversion catalyst able to react hydrogen and/or carbon monoxide present in the landfill gas with oxygen.
30. Apparatus according to any one of claims 26 to 29 comprising a heavy metals removal unit, downstream of said purification unit, said heavy metals removal unit comprising a reactor containing a Go-or Ni-containing catalyst, which may be followed by an absorbent material, such as a zinc oxide composition.
31. Apparatus according to any one of claims 26 to 30 wherein the hydrogen generation unit comprises a steam reformer having a plurality of externally heated catalyst-filled tubes through which a mixture of steam and the mixed gas stream may be passed.
32. Apparatus according to claim 31 wherein the one or more water-gas-shift converters containing a water-gas shift catalyst are operatively coupled downstream of said steam reformer unit.
33. Apparatus according to any one of claims 26 to 32 wherein condensing means comprising one or more heat exchangers and separators are coupled to the hydrogen generation unit to cool the hydrogen-containing gas and separate the condensate.
34. Apparatus according to claim 33 wherein the mixing unit is installed upstream or downstream of said condensing means.