A method and a compact equipment to increase the calorific value of biogas by microreaction

Abstract

The present invention discloses a method and a compact equipment to increase the calorific value of biogas by methanation of CO2 with H2 (Sabatier reaction), relayed on a microreaction system that use geometric periodic open cellular structures (POCS), in which the catalytic system is embedded, to intensify the gas reaction. The compact equipment enables the delocalized biogas producers to benefit the CO2 fraction by increasing the calorific value of gas, avoiding the emission of this gas to atmosphere.

Description

[0001] A METHOD AND A COMPACT EQUIPMENT TO INCREASE THE CALORIFIC VALUE OF BIOGAS BY MICROREACTION

[0002] TECHNICAL FIELD

[0003] This invention concerns an upgrading process of biogas to increase its calorific value using a compact equipment for the methanation of carbon dioxide fraction. Moreover, the equipment is based on a microreaction system that use geometric periodic open cellular structures (POCS), in which the catalytic system is embedded, to intensify the gas reaction.

[0004] BACKGROUND ART

[0005] Biogas is obtained from the breakdown of biomass by microorganisms and bacteria in the absence of oxygen. Biogas is considered a renewable source of energy, like solar energy and wind energy, and it is environmentally friendly. It is obtained in a suspended monoxide decomposition process by anaerobic bacteria or in a fermentation process of decomposable materials such as agricultural manure, sewage, municipal waste, green waste (gardens and parks), plant material and agricultural products, among others. Biogas is a mixture of that includes 50-65% of methane (CH4) by volume, 35-50% of carbon dioxide (CO2), and traces of other gases, like hydrogen sulphide (H2S, 0.005-2%), nitrogen (0-2%), ammonia (NH3, <1 %), siloxanes (0-0,02%), carbon monoxide and water vapor (H2O, up to 5%).

[0006] Biogas is typically exploited in production sites as fuel for feeding internal combustion engines capable of producing heat and electric energy (cogeneration engines). However, the high concentration of carbon dioxide means that the calorific value of biogas is limited, it has a low combustion efficiency, and its applications are restricted. To overcome these drawbacks, biogas upgrading is a purification process that removes impurities and carbon dioxide, obtaining a high purity biomethane. The type and the number of pollutants depend upon the biogas source and determine which cleaning and upgrading techniques are the most suitable for gas purification. “Cleaning” is referred to as the pretreatment that allows the removal of all pollutants but carbon dioxide, while “upgrading” is the treatment for CO2 removal. The benefits of removing carbon dioxide and minor contaminants are numerous: longer process equipment lifespans, higher calorific value, decrease in undesired emissions of greenhouse gas and an overall boost of the process profitability.

[0007] Thus, the upgrading process therefore allows a more valuable fuel to be obtained, with characteristics suitable for use as automotive fuel and, more generally, as biofuel. The commercial exploitation of biomethane, however, is strictly connected to the possibility of cost effectively obtaining biomethane with characteristics that are such as to be able to be introduced into natural gas transport and distribution networks.

[0008] Various upgrading processes of biogas to biomethane are known and used in the state of the art. These processes, however, are generally characterized by a certain plant complexity, significant energy consumptions and high investment and management costs of the plants. Many technologies for biogas upgrading have been developed to date, and some of them are commercially available.

[0009] The main upgrading technologies of biogas applied on an industrial scale, to split the CO2 from CH4, are the physical absorption in water (so-called water scrubbing), organic solvents and chemical absorption in reactive solutions, adsorption on solid sorbents (e.g. activated carbons) and membrane separation.

[0010] However, all these technologies require high-cost and complex facilities, Moreover, as it can be inferred, to producing high purity biomethane, a significant amount of residual CO2 is obtained as a consequence of the separation and must be somehow used or disposed of with additional costs. Such CO2 is generally sold to other plants / companies and / or stored, for example underground, with the technique known as Carbon Capture and Storage (CCS). The recent increase in biomethane production plants entails the risk of leading, in the short term, to the saturation of the CO2 market, making it necessary to find alternative solutions to its reuse and / or disposal.

[0011] On the other hand, the CO2 can be converted into methane (methanation) by a reaction with hydrogen called the Sabatier reaction, as it shows following:

[0012] CO2+ 4H2CH4+ 2H2O

[0013] The common molar ratio of H2to CO2should ideally be 4:1 to maximize methane production, the optimal temperature range is considered between 300°C to 400°C (572°F to 752°F). Temperatures above 400°C can lead to undesirable side reactions or catalyst degradation. Moderate pressure is usually applied, around 1 to 30 atmospheres (atm). Higher pressure can enhance reaction rates and methane yield but requires more energy.

[0014] Document EP 4001381 A1 relates to a biogas upgrading process characterized in that said biogas is subjected to at least two steps of biogas upgrading, with intermediate removal of H2O, using the direct methanation reaction of the CO2 that is present in the biogas without separation from methane.

[0015] Document WO 2022 / 201061 A1 relates a method of direct upgrading of CO2 using two cyclic adsorptive reactors, in parallel, following by a hydrogen gas stream for reactive regeneration. Document EP 2963107 B1 relates a method that supply a part of said biogas stream to a cogeneration system to produce a heating fluid and electric energy. Then, the said electric energy produces a cooling fluid. The remaining part of said biogas stream is suppling to a treatment system comprising a couple of treatment unit, each comprising at least one solid sorbent.

[0016] Furthermore, microreaction technology is currently one of the most innovative techniques in the field of chemical synthesis and similar fields. Compared to conventional chemical reactors, the microreactor has certain unique advantages as reduced diffusion distance between the reactants, excellent mass transfer and excellent heat transfer due to its high surface area-to-volume ratio. The intimate contact between molecules allows optimization of both the product-catalyst interface and energy transfer, leading to higher efficiency and yield in endothermic reactions. In addition, microreactors can be directly scaled up for rapid production, which is difficult for conventional reactor.

[0017] The amounts of reactants in the microreactor are relatively small, which can lead to better process safety, especially for the hazardous substances production and exothermic reactions, as methanation of CO2. As the methanation reaction is highly exothermic, heat management is crucial to avoid thermal runaway in the reactor ensuring compliance with the design specifications of the reactor and catalyst. Operating the methanation reactor at a lower temperature is thermodynamically favourable, shifting the reaction toward the product side. However, a lower operating temperature affects the kinetics negatively, leading to a reduced reaction rate.

[0018] Either the CO2 from biogas can be used in a methanation process as pure CO2 after biogas upgrading (mostly described as indirect biogas methanation), or the biogas can be directly used as feed in the methanation process (known as direct biogas methanation). The presence of CH4in biogas methanation feeding mixture aids to avoid extreme thermal runaway; however, it is not thermodynamically favourable to have one of the reaction products at the inlet of the methanation reactor, so it displays lower exergy efficiency.

[0019] Structured catalytic reactors became popular in different industrial processes as an alternative for conventional packed fixed-bed reactors. Structuring of process units and / or their internals is one of the main strategies for process intensification. The aim is to realize a well-defined, uniform, and easily controllable and adjustable processing experience for all molecules, thereby approximating the optimal process route. This ultimately leads to highly efficient, flexible, resilient, and safe processes. Periodic open cellular structures (POCS) represent a promising new class of structured internals as next-generation catalyst supports in reactors. POCS feature a well-defined morphology and can be fabricated with high reproducibility even for complex geometries by means of additive manufacturing. In recent years, additive manufacturing techniques offer the possibility to manufacture highly reproducible POCS with a well-defined and ordered geometry and with the desired dimensions directly from CAD model. Additive manufacturing saves time and energy, as well as reducing costs, because it is capable of directly producing technical components. Additive manufacturing enables the use of metals as stainless steel, titanium, cobalt chromium, alloys or aluminium alloys.

[0020] The use of POCS results in a uniform and easily controllable flow field, which allows for adjusting the heat and mass transport processes to realize optimal process conditions. The development of the microreaction technology is facilitated by the increasing demand of the distribution and portability. It is likely to have small or temporary chemical equipment in the chemical process, so the microreactors are easily placed in the desired location or carried.

[0021] Document WO 2012032325 A1 disclosed a method to the manufacture catalysts in POCS by additive layer manufacturing.

[0022] An increasing number of biogas plants with biogas upgrading units are emerging in Europe the recent years (Angelidaki, I., Treu, L., Tsapekos, P., Luo, G., Campanaro, S., Wenzel, H., & Kougias, P. (2018). Biogas upgrading and utilization: Current status and perspectives. Biotechnology Advances, 36(2), 452-466), but all technologies require the installation of a factory comprising specific facilities for the different steps of the upgrading process (cleaning, CO2 absorption, methanation...). This also requires the transportation of biogas from the production plants to the upgrading facility, which implies high transportation costs and limitations in shipping and processing distances. As available organic matter for anaerobic digestion is mostly located in rural areas, with small-scale anaerobic digestion plants, economic viability of upgrading is nowadays very difficult due to the associated cost of biogas logistic is still too high to reach viability (O'Connor, S., Ehimen, E., Pillai, S. C., Black, A., Tormey, D., & Bartlett, J. (2021). Biogas production from small-scale anaerobic digestion plants on European farms. Renewable and Sustainable Energy Reviews, 139, Article 110580). On the other hand, upgrading plants are high investment facilities with high processing costs, therefore attention should be addressed to the development of costefficient solutions for biogas upgrading at small-scale. The presence of a high volume of carbon dioxide in the biogas not only reduces the calorific value but also makes the biogas not economically viable for transportation and compression during offsite utilization. Therefore, a compact equipment that allows the methanation of CO2 from biogas, with low cost, easy installation and processing and high efficiency, would significantly increase the benefit of biogas and reduce the emission of greenhouse gases. The previous art displays several methanation microreactors like document US 2020 / 0324265 A1 , that disclosed a micro-reactor, based in microchannels, for methanation that comprises a reactor shell, a reaction space, a cooling space, and fluid - tight separate inlets for at least one fluid reactant and for a cooling fluid, there being at least two inlets for the at least one cooling fluid each with at least one convoluted channel and column structure with reversal of flow. Other POCS microreactors have been described in literature (Stiegler, T., Meltzer, K., Tremel, A., Baldauf, M., Wasserscheid, P., & Albert, Jakob. (2019). Development of a Structured Reactor System for CO2 Methanation under Dynamic Operating Conditions. Energy Technology. 7; Gonzalez-Castano, M., Baena-Moreno, F., Navarro de Miguel, J.C., Miah, K.U.M., Arroyo-Torralvo, F., Ossenbrink, R., Odriozola, J. A., Benzinger, W., Hensel, A., Wenka, A., & Arellano-Garcia, H. (2022) 3D-printed structured catalysts for CO2 methanation reaction: Advancing of gyroid-based geometries. Energy Conversion and Management, Volume 258, 115464), nevertheless the behaviour of these reactors is usually simulated by computer or tested with model gases in case of real construction, thus their implementation in industry depends on overcoming several challenges derived from real process such as pollutants that deactivate the catalytic system, pressure gradients, thermodynamic equilibrium of side-reactions, yields, selectivity, flow integration and others.

[0023] The present invention discloses a method and a compact equipment to increase the calorific value of biogas by methanation of CO2 with H2 (Sabatier reaction), relayed on a microreaction system that use POCS, in which the catalytic system is embedded, to intensify the gas reaction. This method is able of avoiding the poisoning of catalytic system and working with different configurations (e.g. direct methanation, CO2 fraction separation) to find the appropriate thermodynamic conditions for each raw material. Moreover, this compact equipment enables the delocalized biogas producers to benefit from the CO2 fraction by increasing the calorific value of gas, avoiding the emission of this gas to the atmosphere. Thus, the method and the compact equipment overcome the exposed upgrading process shortcomings: the valorisation of CO2 from biogas and the development of a compact installation suitable to be implemented in delocalized biogas facilities.

[0024] SUMMARY OF INVENTION

[0025] Throughout the description and claims the word "comprise" and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word "comprise" encompasses the case of "consisting of". Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

[0026] For the purposes of the present invention, the ranges indicated include the lower and upper ends of the range. The ranges given, such as temperatures, times, ratios and the like, must be considered approximate, unless specifically stated. For the purposes of the present invention, the terms “methanation reaction” and “Sabatier reaction” are used interchangeably and they refer to a chemical reaction of CO2 with H2 in presence of a catalytic system to obtain methane and water.

[0027] For the purposes of the present invention, the term “compact” is used according to the Oxford dictionary's interpretation: a) smaller than is usual for things of the same kind, b) closely packed together. Thus, the “compact equipment” of this invention refers to a biogas upgrading equipment able to increase the calorific value of biogas, smaller than usual upgrading facilities and with the capability of being packed and hauled together.

[0028] For the purposes of the present invention, the term “catalytic system” refers to the assembly composed of the catalytic active metal, in any form (salts, oxides, mixtures thereof), and the porous material on which it is supported.

[0029] As previously mentioned, the removal of carbon dioxide from biogas during the upgrading process creates the possibility of carrying out a methanation process to obtain biomethane, rather than storing it or releasing it into the atmosphere. Also, the direct methanation of biogas is also possible although with unfavourable thermodynamics. Furthermore, the environmental benefit of biogas methanation is not only reduced to the utilization of the CO2 fraction of the biogas, but also avoids greenhouse gas emissions and other impacts derived from the civil works required for biogas logistics. From this perspective, the implementation of microreactors capable of carrying out this reaction at anaerobic medium-small biogas production facilities is a more cost-effective and environmentally friendly alternative to transporting it to upgrading plants located at great distances. Therefore, the biogas producers will increase the energetic efficiency by means of the increasing of gas calorific value with no environmental impact.

[0030] Accordingly, this invention seeks to provide a method and a compact equipment to increase the calorific value of biogas, with harnessing of CO2, relayed on a microreaction system that use geometric periodic open cellular structures (POCS), produced by additive manufacturing, in which the catalytic system is embedded, to intensify the gas reaction. The method and the compact equipment comprise the following operations and parts:

[0031] A) biogas cleaning,

[0032] B) optionally, separation of CO2 from the CH4 in the biogas stream, and

[0033] D) CO2 methanation in a microreactor relying on periodic open cellular structures with the catalytic system embedded.

[0034] Biogas cleaning (A) consists in the removal from biogas stream of several secondary impurities such as moisture, hydrogen sulphide, siloxanes, nitrogen oxides (NOx), volatile organic compounds (VOCs) and halogens. Biogas impurities can cause different public health concerns (like pulmonary and respiratory diseases) and environmental impacts (such as global warming and climate change). Regarding to effects into the biogas upgrading process, these impurities can lead to the failure of process functions of pipelines, power equipment, connections, and nozzles, triggering the process equipment to corrode, as they tend to build-up or accumulate. In the compact equipment, the impurities removal is carried out by at least one solid sorbent selected from the group of active carbon, zeolites, silica gel, polymers, ion exchange resins, single metal oxides, composite metal oxides, ordered mesoporous silica, metal-organic frameworks, membranes and mixtures thereof. The solid sorbent is placed in a thermostatic container in powder, and / or as a cartridge, called cleaning reactor. The biogas stream is passed at least through one of these cleaning reactors, thus the secondary impurities remain absorbed in the solid sorbent, while the biogas stream reaches the module of compression. Once the solid sorbent saturates its removal capacity, it will be replaced. Optionally, in prevention of stopping the process, preferably two (or more) parallel cleaning reactors can be installed, so that, when the solid sorbent of one reactor is saturated, the second line come into operation while the first one is replaced. The cleaning process conditions (pressure, temperature and flow, among others) will be those appropriate for the optimum absorption of secondary contaminants in the sorbents and they can readily be defined by those skilled in the art according to knowledge field and / or recommendations of the suppliers. The water content can be removed with a condenser with a cooling fluid placed before the cleaning reactor and / or after them.

[0035] Compression of biogas consists in the increase of biogas stream pressure, reducing the volume of gas. Biogas small-medium producers are normally run nearly under ambient pressure, but Sabatier reaction is performed under pressure, normally between 1 and 30 bar. The common operating pressure of biogas stations is between 8-60 mbar above the local atmospheric pressure. Thus, the compact equipment requires a pressure increasing before methanation step. This gas compression is carried out but at least one compressor located in the compact equipment and / or outside it. This compressor optionally can be followed by a pressurized gas collection tank and / or cylinder. In one embodiment the compression is done by piston compressors. In one embodiment the compression is done by rotary screw compressors. In one embodiment the compression is done by rotary vane compressors. In one embodiment, the methanation reaction is performed directly from biogas and it is compressed between 1 bar and 30 bar. In one embodiment, the methanation reaction is performed directly from biogas and it is compressed between 3 Bar and 20 Bar. In one embodiment, the methanation reaction is performed directly from biogas and it is compressed between 5 bar and 15 bar. In one embodiment, the CO2 fraction is split from biogas, and it is compressed between 1 bar and 30 bar. In one embodiment, the CO2 fraction is split from biogas, and it is compressed between 3 bar and 20 bar. In one embodiment, the CO2 fraction is split from biogas, and it is compressed between 5 bar and 15 bar.

[0036] Preferably, the biogas streaming (CO2+CH4), after compression step, can be used in a direct biogas methanation, going directly to methanation, but optionally, a separation of CO2 from the CH4 in the biogas stream (C) may be implemented. This step consists in the split of CO2 from biogas stream. As methane is the desired product of the reaction, its presence in the methanation reactor modifies the thermodynamic equilibrium of the Sabatier reaction, which can lead to a decrease in the final methane yield and selectivity. This can be especially relevant in case of biogas streams with high methane content. In these cases, it may be desirable to split the CO2 stream first and move it to the methanation phase afterwards.

[0037] The separation can be conducted by the retention of CO2 in a solid reversible sorbent (after that, CO2 will be desorbed by changing in physical conditions) and / or by a membrane system capable of splitting CO2 / CH4 stream.

[0038] Adsorption processes of CO2 on solid sorbents typically use activated carbons or molecular sieves (e.g. zeolites) as sorbent materials. Solid sorbents are capable of selectively adsorbing CO2 and possibly other small sized gaseous molecules (e.g. O2 and N2), allowing a high purity biomethane to be produced. The adsorption process is favourable at high pressures (e.g. 7 bar) whereas the desorption process, necessary for regenerating the sorbent, is carried out at atmospheric pressure or under a slight vacuum (adsorption effected in so-called Pressure Swing Adsorption-PSA) mode.

[0039] Membrane technology is an alternative to the conventional absorption-based biogas upgrading system. The main principle of the technology relies on the selective permeability properties of membranes allowing the separation of the biogas components. For example, based on the relative permeation rates, the different molecules contained in biogas permeation can be ordered hierarchically from the slowest to the faster permeation (Angelidaki, I., Treu, L., Tsapekos, P., Luo, G., Campanaro, S., Wenzel, H., & Kougias, P. (2018). Biogas upgrading and utilization: Current status and perspectives. Biotechnology Advances, 36(2), 452-466). Depending on the separation media, the process can either be performed with dry (gas / gas separation) or wet (gas / liquid separation) techniques. Those membranes can be inorganic membranes, polymeric membranes and / or composite membranes.

[0040] In one embodiment, a solid reversible sorbent is placed in a thermostatic container in powder, and / or as a cartridge, called absorption reactor. The solid reversible sorbent captures the CO2 from CH4 stream at certain temperature. Once the solid is saturated with CO2, the gas will be released through temperature modification. Optionally, a flow of an inert carrier gas (e.g. nitrogen) can be used. In prevention of stopping the process, preferably two parallel lines can be installed, so that, when one absorption reactor is saturated and it releases the gas, the second line come into operation to capture CO2. Optionally, the outlet stream of CO2 may be compressed in order to adapt the pressure to that required in the methanation module.

[0041] The separation process conditions (pressure, temperature and flow, among others) will be those appropriate for the optimum absorption / release of CO2 in the sorbents / membranes and they can readily be defined by those skilled in the art according to knowledge field and / or recommendations of the suppliers.

[0042] In one embodiment, the solid reversible sorbent is selected from the group of zeolites, silica, active carbon and / or other carbon molecular sieves. In one embodiment, separation is carried out by at least one solid reversible sorbent selected from the group of active carbon, zeolites, silica gel, polymers, potassium carbonate, single metal oxides, composite metal oxides, ordered mesoporous silica, metal-organic frameworks, zeolite Z4A, zeolite HZ4A- 1-3, zeolite 5A, zeolite 13X and combinations thereof.

[0043] In one embodiment, the release the CO2 from solid reversible sorbent uses a carrier gas

[0044] In one embodiment, the CO2 is split by membrane system placed in a container as a cartridge. Gas separation membranes work on the principle of selective permeation through a membrane surface. The driving force for permeation of the gas through the membrane is the difference between the partial pressures of the gas on the retentate side (the interior of the hollow fibre) and the permeate side (the exterior of the hollow fibre). In one embodiment, the membrane system is a single-stage process with at least one absorption reactor. In one embodiment, the membrane system is a single-stage process, with at least one absorption reactor, where the gas permeated (a mixture of CO2+CH4) is total o partially recycled.

[0045] CO2 methanation (D) consists in the reaction of CO2 with H2 in a microreactor relayed on periodic open cellular structures, produced by additive manufacturing, with the catalytic system embedded in the mentioned POCS cells. The microreactor consists of at least one metallic hollow tube contained at least one POCS placed into it (figure 4), with at least one inlet connection in one extreme and an at least one outlet connection in the opposite extreme, placed in a thermostatic device. One inlet stream is biogas, and / or CO2, coming from the compression step and / or from separation step, and the second inlet stream is compressed hydrogen. Both gases and catalytic system embedded in POCS cells interact inside the metallic hollow tube, inducing the chemical reaction between CO2 and H2 molecules to obtain methane, water and other byproducts. As the reaction produces water, the gas stream arising from methanation reactor goes through a condenser to remove this water. Finally, both streams (CH4 from original biogas and CH4 produced by Sabatier reaction) are ready to be used either together or detached. This reactor is an easily scalable configuration: simply varying the number of tubes, the hourly capacity of the plant could be changed.

[0046] The catalytic system for Sabatier reaction is usually based on active metal as ruthenium (Ru), iron (Fe), nickel (Ni), rhodium (Rh), palladium (Pd), platinum (Pt), cerium (Ce) cobalt (Co), tin (Sn), copper (Cu) and combinations thereof. Those metals are supported on a porous ceramic material, which is mechanically and thermally stable, such as alumina (AI2O3), silica (SiCh), titanium dioxide (TiCh), Mg / AhCh, AIMgCU, ZrC>2, CeCh / ZrCh, silicon carbide, ceramic materials with a high specific surface and mixtures thereof.

[0047] In general, periodic open cellular structures (POCS) are three-dimensional lattice structures with unit cell dimensions from 0.1 to 50 mm. Unlike solid foams (open- and closed-cell), POCS are categorized as non-stochastic cellular solids with a regular strut and node arrangement and controlled organization of their unit cells. Of the many existing three- dimensional geometric figures, only some can create a complex three-dimensional structure that completely fills the space by folding and adding individual figures. A broad range of unit cell types can be used. The most common are cubic cells, diamond cells, Kelvin cells and gyroid cells, but several more exits. The choice of POCS design plays a big role in determining its physical, mechanical and thermal properties. The shape and structure of the unit cell influence on many desired features, such as a high strength-to-weight ratio, large surface area, excellent energy absorption, low heat conductivity, substantial acoustic and thermal insulation properties and minimized material requirements.

[0048] There are several options for impregnation of the catalytic system in the POCS. Commonly, the impregnation of catalytic system in POCS surface request the previous dispersion of the catalytic system in a solvent and / or mixture of solvents to obtain a slurry. Then, the printed POCS is wash coated by immersing and withdrawing in the slurry followed by the elimination of the excess. In wash coating procedures, POCS cells are dipped into slurries, kept in the particle dispersion for a certain period of time and finally withdrawn. Once the metallic POCS is withdrawn it must be drained and the excess slurry eliminated. Usually, excess slurry is removed either by air blowing and / or centrifuging. In general, by gravitational draining or by applying some form of pressure or vacuum to clear the channels of the excess but the adhered catalyst layer.

[0049] In some cases, catalysts are introduced into the POCS in vapor form (technique called chemical Vapor Deposition). The catalyst vapor reacts or condenses on the internal surfaces of the POCS, forming a thin layer of active catalyst within the pores.

[0050] In some cases, the catalysts are deposited onto the internal surfaces of the porous POCS by electrochemical deposition, using an electric current to drive a chemical reaction. Afterwards, to form a thin oxide layer on the metal surface the POCS cell is dried and calcined to suitable temperatures. Finally, at least one POCS is placed into at least one metallic hollow tube to create the microreactor. The catalytic system charge density ranged from 10 to 250 grams of catalytic system over square meter of POCS, optionally from 20 to 200 g / m2, further optionally from 30 to 150 g / m2.

[0051] In one embodiment, the metallic hollow tube is between 10 and 1000 centimetres long. In one embodiment, the metallic hollow tube is between 50 and 750 centimetres long. In one embodiment, the metallic hollow tube is between 100 and 500 centimetres long. In one embodiment, the metallic hollow tube has between 0.01 and 20 centimetres of diameter. In one embodiment, the hollow metallic tube has between 1 and 15 centimetres of diameter. In one embodiment, the hollow metallic tube has between 2 and 10 centimetres of diameter. In one embodiment, the thermostatic device is an oven. In one embodiment, the oven is an electric oven. In one embodiment, the oven is a gas oven. In one embodiment, the microreactor involves one tube. In one embodiment, the microreactor involves between 2 and 10000 tubes. In one embodiment, the microreactor involves between 10 and 1000 tubes. In one embodiment, the microreactor involves between 50 and 500 tubes.

[0052] In one embodiment, the POCS are based in a cell unit selected from the group of cubic cell unit, diamond cell unit, Kelvin cell unit, gyroid cell unit and a combination thereof. In one embodiment, the POCS is impregnated with a catalyst system based on a metal selected from the group of as ruthenium (Ru), iron (Fe), manganese (Mn), zirconium (Zr), yttrium (Y), nickel (Ni), rhodium (Rh), palladium (Pd), platinum (Pt), cerium (Ce), lanthanum (La), europium (Eu), cobalt (Co), tin (Sn), copper (Cu) and combinations thereof. In one embodiment, the catalyst metals are supported on a porous ceramic material selected from the group of alumina (AI2O3), silica (SiO2), titanium dioxide (TiO2), Mg / AI2O3, AIMgO4, ZrO2, CeO2 / ZrO2, silicon carbide, ceramic materials with a high specific surface and combinations thereof.

[0053] All these operations are integrated in a compact equipment such as it is described in figure 1. The biogas (A) stream (1) from biogas facility comes into cleaning step and, first, it passes through a water condenser to remove the moisture content (B) and then, it passes through at least one cleaning reactor, where secondary contaminants are removed. These cleaning reactors, as well the rest of the reactors, can be thermotasted and equipped with control probes for temperature, pressure and flow, according to process control requirements. Then, the cleaning biogas stream is compressed and, optionally, stored in a gas collection tank or cylinder.

[0054] Preferably, the compressed clean biogas goes directly into methanation reactor, stream bypassing the separation step, where it is mixed with the hydrogen (D) stream (4). Optionally, the compressed clean biogas stream comes into separation step where the CO2 is captured in separation reactors, while the purified CH4goes out the equipment (2). The CO2 can be desorbed with the help of a nitrogen (C) stream. The split CO2 / N2 stream (3) then comes into methanation step. If it is necessary, this stream can be compressed to increase the pressure. Then, hydrogen stream was added, along with stream 3, into methanation reactors (4). Inside these reactors, the POCS embedded with catalyst system intensifies and allows Sabatier reaction to obtain CH4as a main product. As a result of Sabatier reaction, in addition to methane, water is also produced, so this water (E) is removed by a condenser (5). Finally, both streams (CH4from original biogas and CH4produced by Sabatier reaction) are ready to be used and they can be joined together (6).

[0055] BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1. Scheme of a possible embodiment of the compact equipment according to the present invention.

[0057] Figure 2. Details of gyroid cell POCS used in direct methanation embodiment.

[0058] Figure 3. Details of cubic cell POCS used in indirect methanation embodiment.

[0059] Figure 4. Example of possible POCS packing inside the hollow metal tubes.

[0060] DESCRIPTION OF EMBODIMENTS

[0061] The following examples and embodiments are provided by way of illustration, and they are not intended to be limiting for this presentation.

[0062] The effectiveness of a compact equipment, object of the present invention, was evaluated on a demonstrative device designed to process 12.000 Nm3 / year of biogas. This amount of biogas is a representative value of a small facility (hotel, airport, farm, resort, resident community) that generates between 70 and 90 tons annually of organic residues. This type of facility generates enough organic waste to install a biogas digester together with an electric cogenerator, while its production would not be sufficient to evaluate an upgrading process with CO2 harnessing. This demonstrative equipment is connected to a biogas plant outlet stream and processes a flow of 25 litre per minute of biogas. The four parts of the equipment are installed in a container of 3.5 (length) x 2,0 (width) x 2,7 (height) meters. In this example, the term “percentage (%)” refers always to volume / volume (v / v) percentage unless otherwise specifically stated. Example 1 : direct methanation.

[0063] The cleaning of the biogas comprising two series cleaning reactors of 2,8 Liter (25 cm height x 11 .2 cm internal diameter), filled with 600 grams of active carbon and a gas contact time of 8 seconds. The first one removes H2S with Filtracarb® SA62 CX. The removal of NH3is carried out with Filtracarb® SA66 activated carbon. Both active carbons were supplied by CPL Activated Carbons Iberia. The temperature inside of cleaning reactors was 30C and the pressure 0.011 Bar.

[0064] The compression of biogas stream is performed by a heavy-duty rotary screw compressor, in this embodiment there is no gas collection tank after compressor. The compressor increases the pressure of biogas from 0.011 Bar up to 8 Bar and its dimensions are 1.2 (length) x 1 .6 (width) x 2,2 (height) meters.

[0065] In the present example, the compressed cleaning biogas is directly feeding to methanation reactor.

[0066] The catalyst system consisting of ruthenium and nickel as active metals supported over spinel (MgAhCh), thus the catalyst system is named as 0,5Ru / 5Ni_MgSph. The catalyst system is embedded in gyroid cells (figure 2) of 10 cm of length and 4,5 cm of external diameter. The catalytic system charge density is 98,3 grams of catalytic system over square meter of POCS.

[0067] The biogas stream (1) is cleaned and compressed after the methanation process. Then, this stream (25 l / min) is mixed with a with another stream of H2(4 l / min) to feed the methanation reactor (4). The methanation reactor has one bed with a length of 20 cm and internal diameter of 4,5 cm, so two POCS cells are placed into. The reactor provides 74,2% of CO2conversion, 72,2% of yield to CH4 and 13,04% of yield to CO. The methanation reaction carries out at 8 Bar and 450°C. After methanation, the product gas pass through one condenser to remove the water content (5). As there is not CH4stream (2), stream 5 correspond to stream 6. As it is showed in table 1 , the calorific value of final stream is three times the calorific value of inlet biogas.

[0068] Table 1

[0069]

[0070] Example 2: indirect methanation with CO2 splitting.

[0071] Cleaning and compression operations are the same as example 1.

[0072] In this example, the CO2 is split from biogas by a separation module. The separation is performed on a biogas composition (v / v) of 54.43% CH4 and 45.57% CO2, maintaining the feed rate of 25 l / min. The biogas stream is passed through the Zeolite 13X. Then, a stream of 11 l / min of CH4 is generated, while practically all the CO2 remains trapped in the zeolite.

[0073] Two reactors are arranged to alternate the CO2 capture process with the CO2 release process. Each reactor uses 1500 grams of 13X zeolite, arranged in cylindrical containers of 20 cm diameter and 15 cm high.

[0074] The capture passes off at 30 °C and the release is performed by heating the loaded reactor up to 350 °C, with a flow of 0.250 l / min of nitrogen, which generates a flow of 1 l / min of CO2 together with 0.250 l / min of inert N2 corresponding to a mixture of 80%of CO2 and 20% of N2. This stream is mixed with another stream of 4 l / min of H2. Thus, the feed methanation stream consists of (v / v) 19,04% of CO2, 76,19% of H2 and 4,76% of N2. This stream is compressed up to 8 Bar.

[0075] The catalyst system consisting of nickel as active metal supported over spinel (MgAhOa), thus the catalyst system is named as 5Ni_MgSph. The catalyst system is embedded in cubic cells (figure 3) of 10 cm of length and 4,5 cm of external diameter. The catalytic system charge density is 85,7 grams of catalytic system over square meter of POCS. The methanation reactor has one bed with a length of 30 cm and internal diameter of 4,5 cm, so three POCS cells are placed into. The reactor provides 70,1 % of CO2 conversion, 68,3% of yield to CH4 and 12,36% of yield to CO. The methanation reaction carries out at 8 Bar and 450°C.

[0076] After methanation, the product gas pass through one condenser to remove the water content (5). Then, gas coming from methanation (5) and split CH4 (2) reassemble in stream 6. As it is showed in table 2, the calorific value of final stream is three times the calorific value of inlet biogas. Table 2

Claims

CLAIMSClaim 1. A method for increasing the calorific value of raw biogas through direct or indirect methanation of the CO2fraction, performed in a compact, modular and transportable equipment operable in off-grid conditions, characterized in that it comprises:(a) feeding raw biogas, containing methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor and siloxanes,(b) subjecting the mixture to a cleaning process comprising at least one solid-phase module for selective removal of H2S, siloxanes and water,(c) introducing hydrogen (H2) into the gas stream,(d) compressing the cleaned gas to a pressure between 1 and 30 bar, and(e) passing the compressed mixture through a methanation module consisting of at least one microreactor incorporating periodic open cellular structures (POCS) fabricated by additive manufacturing and embedded with a catalyst selected from Ni, Ru, or Rh.Claim 2. The method according to claim 1 wherein the CO2methanation reaction is carried out in at least one tubular reactor filled with at least one periodic open cellular structure, with the catalytic system embedded, based on at least one three-dimensional geometric figures selected from the group of cubic cells, diamond cells, Kelvin cells and gyroid cells.Claim 3. The method according to claims 1-2 wherein the CO2methanation operation is carried out with a catalytic system that uses as active catalyst at least one metal, and / or its oxides and / or its salts, selected from the group as ruthenium (Ru), iron (Fe), manganese (Mn), zirconium (Zr), yttrium (Y), nickel (Ni), rhodium (Rh), palladium (Pd), platinum (Pt), cerium (Ce), lanthanum (La), europium (Eu), cobalt (Co), tin (Sn), copper (Cu) and combinations thereof.Claim 4. The method according to claims 1-3 wherein the CO2methanation operation is carried out with a catalytic system that uses as supporter at least one material selected from the group of alumina (AI2O3), silica (SiO2), titanium dioxide (TiO2), Mg / AI2O3, AIMgO4, ZrO2, CeO2 / ZrO2, silicon carbide, ceramic materials with a high specific surface and combinations thereof.Claim 5. A compact and modular equipment, transportable and operable in off-grid conditions, for increasing the calorific value of raw biogas through direct or indirect methanation of the CO2fraction, characterized in that it comprises:(a) an inlet module for raw biogas,(b) a cleaning stage comprising at least one solid-phase unit selected from activated carbon, zeolites, iron oxides or alumina, configured for selective removal of H2S, siloxanes and water vapor,(c) an inlet module for hydrogen (H2),(d) a compression unit configured to pressurize the cleaned mixture of gases to between 1 and 30 bar,(e) a methanation module comprising at least one microreactor cartridge containing a 3D-printed periodic open cellular structure (POCS), embedded with a methanation catalyst, and(f) a support structure configured to allow mechanical insertion and replacement of said microreactor cartridges.Claim 6. The equipment according to claim 5 wherein at least one separation reactor of CO2is implemented.Claim 7. The equipment according to claims 5-6 wherein the biogas cleaning stage comprises at least one solid sorbent selected from the group of active carbon, zeolites, silica gel, polymers, ion exchange resins, single metal oxides, composite metal oxides, ordered mesoporous silica, metal-organic frameworks, membranes and combinations thereof.Claim 8. The equipment according to claims 6-7 wherein the CO2 separation comprises at least one solid reversible sorbent and / or a membrane system.Claim 9. The equipment according to claims 5-8 wherein the CO2methanation module comprises a microreactor between 10 and 1000 centimetres of length, optionally between 50 and 75 centimetres of length, further optionally between 100 and 500 centimetres of length.Claim 10. The equipment according to claims 5-9 wherein the CO2methanation module comprises a reactor with an internal diameter between 0.01 and 20 centimetres, optionally between 1 and 15 centimetres, further optionally between 2 and 10 centimetres.Claim 11. The equipment according to claims 5-10 wherein the CO2methanation module comprises a reactor with at least one tubular reactor filled with at least one periodic open cellular structure, with the catalytic system embedded, based on at least one three- dimensional geometric figures selected from the group of cubic cells, diamond cells, Kelvin cells and gyroid cells.Claim 12. The equipment according to claims 5-11 wherein the CO2methanation module comprises a catalytic system that uses as active catalyst at least one metal, and / or its oxides and / or its salts, selected from the group as ruthenium (Ru), iron (Fe), manganese (Mn), zirconium (Zr), yttrium (Y), nickel (Ni), rhodium (Rh), palladium (Pd), platinum (Pt), cerium18(Ce), lanthanum (La), europium (Eu), cobalt (Co), tin (Sn), copper (Cu) and combinations thereof.Claim 13. The equipment according to claims 5-12 wherein the CO2 methanation module comprises a catalytic system that uses as supporter at least one material selected from the group of alumina (AI2O3), silica (SiO2), titanium dioxide (TiO2), Mg / AI2O3, AIMgO4, ZrO2,CeO2 / ZrO2, silicon carbide, ceramic materials with a high specific surface and combinations thereof.

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