WO2014207703A1

WO2014207703A1 - Assembly for the production of methane from soil gas emitted by degassing zones

Info

Publication number: WO2014207703A1
Application number: PCT/IB2014/062635
Authority: WO
Inventors: Paolo DEIANA; Claudia BASSANO; Vincenzo Barbarossa; Giuseppina VANGA
Original assignee: Agenzia Nazionale Per Le Nuove Tecnologie, L'energia E Lo Sviluppo Economico Sostenibile (Enea)
Priority date: 2013-06-26
Filing date: 2014-06-26
Publication date: 2014-12-31
Also published as: ITRM20130367A1

Abstract

An assembly for the production of methane comprising at least one methanation reactor (6) suited to convert carbon dioxide into methane by means of catalytic hydrogenation, an electrolyzer device (5) suited to produce the necessary hydrogen to be introduced into the methanation reactor (6), collection means (2) for collecting from degassing zones a soil gas comprising carbon dioxide, and a treatment device (3) for desulphurization of the soil gas from the soil collection means (2) and suited to purifying the carbon dioxide to be introduced into the methanation reactor. The electrolyzer device (5) is powered with electric energy from renewable sources.

Description

"ASSEMBLY FOR THE PRODUCTION OF ME THANE FROM SOIL GAS EMI TTED BY DEGASSING ZONES"

TECHNICAL FIELD

The present invention concerns an assembly for the production of methane from soil gas emitted by terrestrial or marine degassing zones.

Here and below, by soil gas we mean a gas produced by natural degassing zones.

BACKGROUND ART

As is known, some dormant volcanic areas are characterised by a more or less continuous emission of soil gas from the ground, consisting mainly (90 to 99% vol.) of carbon dioxide (CO₂) · In areas where the phenomenon of natural degassing from the ground occurs, the soil gas emitted comprises, in addition to CO₂, also nitrogen, water vapour, hydrogen sulphide, methane and, to a lesser extent, other components.

The gaseous emissions are associated with faults that run through structural highs of buried carbonate rocks with aquifers below. The CO₂ content in these areas can reach values in the order of 50,000 g/m²/day.

The need is therefore felt to use the large quantity of CO₂ emitted by the ground in a productive manner, at the same time limiting its environmental hazardousness .

For a correct understanding of the present invention, a few words need to be said about the problem of the storage of energy from renewable sources.

The incidence of production from renewable sources in the global electrical market is increasing yearly, in both Italy and Europe. A typical characteristic of these renewable sources (wind, sun, biomass, etc.) is that they cannot be planned over time. Due to the consequent uncertainty of the feed-in of energy to the grid, solutions need to be identified that allow stabilisation of the national electricity system.

This is the background to the accumulation systems that store the energy surplus when the transmission network is not able to safely dispose of all the power generated by the non- programmable renewable sources. Generally, the electric accumulation systems can be defined as systems that store the electric energy converting it into another form of energy (chemical, mechanical, electrostatic, electromagnetic) . The most widespread energy storage systems are electrochemical accumulators, also known as batteries; said systems allow medium-term storage (<1 day) with use limited by their low energy, power density and duration. Other systems include pumping stations and hydroelectric production.

For long-term conservation and seasonal balancing of renewable energy sources, the chemical storage systems that convert electric energy into energy vectors such as hydrogen or methane can be considered the most suitable.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a solution with technical characteristics such as to meet the needs relative both to the emissions of CO₂ from the ground and storage of the energy from renewable sources.

The subject of the present invention is an assembly for the production of methane from soil gas emitted by the ground, the essential characteristics of which are described in claim 1, and the preferred and/or auxiliary characteristics of which are described in claims 2-6. A further subject of the present invention is a method for the production of methane, the essential characteristics of which are described in claim 7, and the preferred and/or auxiliary characteristics of which are described in claims 8 and 9.

BRIEF DESCRIPTION OF THE DRAWING

Below an embodiment example is given purely for illustrative non-limiting purposes with the help of the figure of the accompanying drawing, which schematically illustrates the assembly subject of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the figure the number 1 indicates as a whole an embodiment of the assembly subject of the present invention.

The assembly 1 comprises collection means for collecting the soil gas from the ground 2, a treatment device for treating the soil gas 3, generation means for generating electric energy from renewable sources 4, an electrolyzer device 5, a methanation section 6 and a treatment device for treating the methane produced 7.

The collection means for collecting the soil gas from the ground 2 are produced with systems that provide for containment of the gaseous flow from natural degassing phenomena from the ground in a confined environment.

Said collection means preferably comprise a layer of material permeable to the soil gas, a layer of material impermeable to the soil gas arranged above the permeable layer and suited to prevent outflow of the soil gas, and a suction system arranged to act directly on said permeable layer and suited to guarantee both the progress of the soil gas through the permeable layer and draw-off of said soil gas. As may seem obvious to a person skilled in the art, the suction system is arranged either across the layer of material impermeable to the soil gas or where said impermeable layer is not present in order to act directly on the layer of material permeable to the soil gas without the interposition of the layer of impermeable material.

Preferably, the material permeable to the soil gas comes from the group consisting of stones, gravel, sand, natural fibre fabrics such as cotton, jute and wood fibres whereas the impermeable material consists of clay, synthetic materials such as rubber sheaths, plastic materials and cementitious material . The soil gas thus drawn off is conveyed into the soil gas treatment device 3, the main job of which is to remove from the soil gas the sulphuric compounds in order not to jeopardise the subsequent methanation step by deactivation of the catalyst.

The sulphuric compounds can be removed via technologies which entail wet absorption or adsorption on sorbents at high temperature. The wet desulphurization entails washing with water or with ¾S-selective solvents based on soda or amines, which remove the sulphuric compounds by absorption in the liquid medium. Generally, the absorption is performed in packed towers or columns and is favoured by low temperatures in the order of the ambient temperature and pressures which according to the type of absorption - chemical or physical - can range from the ambient pressure to higher pressures. Wet absorption comprises a solvent regeneration section with consequent energy expenditure and recirculation of the regenerated solvent to the absorber. Said technology is commercially widespread and in industrial applications is suitable for large plants in view of the complexity and costs of the system. Preferably, the soil gas treatment device 3 performs desulphurization at high temperature, offering the advantages connected with the possibility of treating solids and non- liquids with obvious simplification in running and costs.

The desulphurization at high temperature is based on the adsorption of H₂S on alkaline and transition metal oxides, capable of removing the sulphides up to parts per million and which can be regenerated via oxidisation with air.

The main distinction between the two types of oxides is the possibility or otherwise of regenerating the sulphide that forms. The non-regenerable adsorbents contain alkaline metals (Ca, Ba, Sr) including limestone and dolomite. Vice versa the regenerable adsorbents contain transition metals (Fe, Zn, Mn, Cu, Ni, etc.) and can be based on single oxides, combination of different oxides and combinations of oxide and aggregates. Generally, the pure oxide phase is confined on supports that increase the surface area and reduce the tendency to sinter.

With Me indicating the generic metal used, the rations involved can be summarised as follows:

MeO+H₂S → MeS+H₂0 adsorption ΔΗ<0

The reaction occurs above 250°C while the regeneration occurs in nitrogen dilution air atmosphere or in a vapour current and develops in the temperature range of 500°C-900°C with the following reaction:

MeS+3/20₂ → MeO+S0₂ regeneration ΔΗ>0

The presence of the means for generation of electric energy from renewable sources 4 is an option that may not be provided if the assembly 1 is directly interfaced with the electric grid. The production of electric energy from renewable sources has the function of covering the electrical requirement of the assembly 1 as a whole. In particular, this refers mainly to the electrical requirement of the electrolyzer and, secondarily, the electrical consumption of auxiliaries, e.g. water recirculation pumps, compressors, suction units and controls. The generation means for generating electric energy from renewable sources 4 can preferably comprise a photovoltaic plant sized in order to cover internal uses or larger in the case of feeding of the surplus electric energy into the grid. This type of plant is immediately available on the market. A small to medium-sized wind generation plant could also be used in said system.

Alternatively and as previously mentioned, the assembly 1 can be connected directly to the electric grid in order to absorb the production surplus at reasonable costs (application as energy storage) .

The electrolyzer device 5 can be of alkaline type which uses an aqueous solution of a hydroxide-based alkaline electrolyte with concentration between 25% and 35%. The conventional alkaline electrolyzers operate at a pressure near to ambient pressure and with operating temperatures varying between 70°C and 90°C and a cell voltage ranging from 1.8 to 2.25 V. The consumption relative to the production of 1 Nm³ of hydrogen is around 4-6 kwh/Nm³H₂ with efficiency between 60 and 70%. Generally this type of electrolyzer can produce hydrogen at pressures in the order of 1 to 30 bar.

Alternatively, the electrolyzer device 5 can be of the type with polymer electrolyte membrane (PEM) cells. This type of electrolyzer, with the same efficiency, allows much higher energy density and power to be achieved but requires catalysts in the platinum group and therefore has higher costs than the alkaline electrolyzers. The values of the energy required in kwh by the process for producing one Nm³ of ¾ in the case of electrolyzers with PEM technology are 4-5 kwh/Nm³H₂. The presence of the electrolyzer device 5 in the assembly 1 allows the system to receive the variable supply of electric power. The hydrogen produced (with a 99% degree of purity) is sent to a storage tank (known and therefore neither described nor illustrated for the sake of simplicity) which allows decoupling of the discontinuous operation of the electrolyzer from the continuous operation of the plant. For a correct analysis of the economic advantages of the assembly 1 subject of the present invention, it should be noted that the oxygen produced by the electrolyzer device, having a high degree of purity (99.5%vol), can be advantageously utilised by the market.

The methanation section 6 consists of one or more reactors in series/parallel and provides the catalytic conversion of CO₂ into CH . The main reactions involved in the process are listed below.

C0₂ (g) + 4 H₂ (g) → CH₄ (g) +2 ¾0 (g) [1]

AG°298 °K = - 27150.4 cal; ΔΗ°298 °K = - 37531 cal .

CO (g)+ 3 H₂ (g) → CH₄ (g) +H₂0 (g) [2]

AG°= -53806 + 60.34 T cal/mole for T between 600 and 1500 °K AG°298 °K = -33967 cal; ΔΗ°298 °K = - 49271 cal.

If small quantities of 0₂ are also present, the following reaction takes place:

0₂ (g) +2 H₂ (g) → 2 H₂0 (g) [3]

AG°298 °K = - 115596 cal; ΔΗ°298 °K = - 109270 cal.

Given the exothermicity of the reaction, to obtain high conversions it is necessary to operate at moderate temperatures . To obtain acceptable reaction speeds, catalysts are used. Many catalysts have been tested (Ni, Cu, Ir, Co, Fe, Pt, Pd, Mo, W , Ru and Rh, all with different supports like AI₂O₃, Ti0₂ , Ce0₂ , -MgO-, -MgAl₂0₄-, -K₂0-MgAl₂0₄- , Si0₂, -Cr₂0₃, ksr, -MgO- ksr, Zr0₂ , Al₂0₃-CaO , La₂0₃) but the most active are those based on nickel and nickel oxides (70 ± 80 %) and A1₂0₃ oxides (30 ± 20%) .

The nickel and nickel oxide catalysts are poisoned by the sulphuric compounds and by the arsenic and therefore the soil gases must be pre-treated to remove the sulphuric compounds.

The reaction temperature is maintained around 300°C and, given the exothermicity of the reactions, the temperature in the reactor must not rise above 430°C otherwise the catalytic activity will be reduced.

Since the reaction occurs with reduction in the number of moles, it is possible to operate at a pressure ranging from a few bars up to 60 bars. In particular, in the methanation section the fixed bed technology is used (catalyst bed shaped into cylinders or pellets) equipped with pressure, temperature and composition controls. The soil gas supplied is heated to approximately 250°C and sent to the reactor, adjusting the flow rate so that the temperature does not rise beyond the desired values.

The methane produced is cooled and sent to the treatment device 7 provided to eliminate its water content. The methane is then compressed to the pressure necessary for the specifications of the network into which it will be fed or to the storage specifications in the case of local consumption.

The condensed water downstream of the methanation is subsequently conveyed to the electrolyzer 5 by means of a dedicated transport line 8. As will appear evident from the above description, the assembly and the method subject of the present invention offer the significant advantage of converting the CO₂ emitted from the ground into methane via a process that uses the hydrogen produced by the electrolysis which in energy terms is supplied by renewable sources. In this way a gas is produced, with zero emissions, which substitutes the natural gas. The particular advantage of the solution of the present invention is the generation of a standardised product, i.e. the methane, which can be fed into the network or stored for local use.

It should be taken into account, in fact, that the assembly and the method proposed could respond, integrating other storage systems, to the need for disposal of the energy production surplus not otherwise absorbable by the transmission network.

Claims

1. An assembly for the production of methane comprising a methanation reactor (6), which is suited to convert carbon dioxide into methane by means of catalytic hydrogenation, and an electrolyzer device (5), which is suited to produce the necessary hydrogen to be introduced into the methanation reactor (6) ; said assembly being characterised in that it comprises collection means (2) to collect from degassing zones soil gas comprising carbon dioxide and a treatment device (3) for the desulphurization of the soil gas coming from said collection means (2) and suited to purify the carbon dioxide to be introduced into the methanation reactor (6) , and in that at least said electrolyzer device (5) is supplied with electric energy coming from renewable sources.

2. An assembly for the production of methane according to claim 1, characterised in that it comprises electric energy generation means (4) for the generation of electric energy from renewable sources.

3. An assembly for the production of methane according to claim 1 or 2, characterised in that it comprises a further treatment device (7), which is suited to dehydrate the methane produced .

4. An assembly for the production of methane according to claim 3, characterised in that it comprises a water transportation line (8), which is suited to transport the water produced by the methanation reactor (6) to the electrolyzer device (5) .

5. An assembly for the production of methane according to one of the preceding claims, characterised in that said soil gas collection means (2) comprise a layer of material permeable to the soil gas, a layer of material impermeable to the soil gas arranged above the permeable layer and suited to prevent outflow of the soil gas, and a suction system suited to guarantee both the progress of the soil gas through the permeable layer and draw-off of said soil gas.

6. An assembly for the production of methane according to one of the preceding claims, characterised in that the treatment device (3) for desulphurization of the soil gas uses a technique of adsorption on sorbents at a temperature higher than or equal to 250°C.

7. Method for the production of methane comprising a methanation step in which carbon dioxide is converted to methane by means of catalytic hydrogenation and an electrolysis step in which the water is decomposed into oxygen and hydrogen which is fed to said methanation step; said method being characterised in that it comprises a collection step for collecting from degassing zones a soil gas comprising carbon dioxide, a desulphurization step for desulphurizing the soil gas from the preceding collection step and an energy production step for producing energy from renewable sources suited to supply power at least to said electrolysis step; the soil gas from said desulphurization step being fed to said methanation step.

8. A method for the production of methane according to claim 7, characterised in that it comprises a dehydration step to dehydrate the methane produced.

9. A method for the production of methane according to claim 7 or 8, characterised in that it integrates with the energy storage systems suited for the disposal of the surplus electricity .