WO2006108532A1

WO2006108532A1 - Co2 capture and use in organic matter digestion for methane production

Info

Publication number: WO2006108532A1
Application number: PCT/EP2006/003041
Authority: WO
Inventors: Cesarino Salomoni; Enrico Petazzoni
Original assignee: Cesarino Salomoni; Enrico Petazzoni
Priority date: 2005-04-08
Filing date: 2006-04-04
Publication date: 2006-10-19
Also published as: EP1984098A1; ITBO20050217A1

Abstract

A process to provide an alternative to CO2 sequestration in depleted gas or oil fields is presented. An integrated process combining a first module, for capturing and separating CO2 from exhaust gases, with a second module, for exploiting the captured CO2 in the anaerobic digestion of organic matter to produce methane, is provided. In the first module CO2 is removed from the exhaust gas by contact and mass transfer to an alkaline metal solution and more particularly of sodium. The CO2 dissolved in the solution is rapidly hydrated into carbonic acid by means of carbonic anhydrase which is immobilized onto an inert support. Finally, the hydrated CO2 is then reacted with alkaline metal and upon adding alkaline metal carbonate to the solution, a precipitate of alkaline bicarbonate is obtained, which is then calcinated to produce a concentrated flow of CO2. In the second module such concentrated flow of CO2 is used to provide a perfectly suitable environment for the hydrolysis of organic matter and for hydrogen production in distinct and specialized sections aimed at acidification and fermentation respectively; at the same time mixed and high density microbial populations, already there or selectively added to the fermentation process, are being conditioned to utilize CO2 and hydrogen to increase the production of acetates and consequently of methane. The outlined process may be used e.g. by greenhouse gas emitting firms as well as by those involved in organic waste management.

Description

Title

CO₂ CAPTURE AND USE IN ORGANIC MATTER DIGESTION FOR METHANE PRODUCTION

Description 1. Technical field

The present invention relates to an integrated process for capturing CO₂ particularly from exhaust gases and using the same CO₂ to increase the efficiency of the anaerobic digestion of organic matter to produce methane. The process according to the present invention typically comprises two parts. The first part of the process relates to the use, in a solution, of alkaline reagents together with a biological catalyst, carbonic anhydrase, immobilized onto an inert support, for absorbing, through a chemical reaction, emitted CO₂ to obtain a bicarbonate precipitate which is later regenerated by calcination leading to a concentrated gaseous CO₂ flow. The second part of the process relates to the use of CO₂ microbiologically stepwise controlled anaerobic digestion of organic matter, organized in subsequent steps, specialised in acidification and methane production respectively, with intermediate steps devoted to the concentration and recycling of biomass. 2. Background art The growing CO₂ concentration in the atmosphere, due to the increasing use of fossil fuels derived energy, has a major impact on climate change. Numerous governments had to adopt necessary measures to reduce CO₂ emissions into the atmosphere (The Kyoto Protocol, EU Emission trading directive). Unfortunately one of the most desirable approaches to the solution of such problem, as viewed for the standpoint of the existing technologies for energy production and other important industries, the one known as CO₂ capture and sequestration in depleted gas or oil fields, is encountering great difficulties in gaining legal recognition, due to the large technical uncertainties that become apparent as soon as they are considered in the context of the very long run. With similar preoccupations, with respect to organic wastes of different types (organic fraction of urban refuse, depuration sludge, wastes from parks and gardens, from agro- industrial productions, etc.) waste management firms are looking for solutions to limit the recourse to landfills and consequently to limit occupied volumes, produced odours and greenhouse gases, risks of polluting waters, and, most of all, to limit costs by recycling, in efficient ways, matter and energy from organic waste. 2.1. CO₂ capture

By examining the numerous existing technologies for the capture and separation of CO₂ from exhaust gases, it becomes obvious that they all suffer high capital and running costs per unit of captured CO₂; this situation is particularly serious when such technologies are considered in the context of exhaust gases with low CO₂ concentrations. Further, with the respect to long term sequestration of CO₂, in the international arena various solutions are being discussed, envisaging, in the main, the burying of CO₂ in depleted gas and oil fields or, alternatively, the ocean storage of it. As opposed to such a perspective, the systematic use of CO₂, aimed at obtaining compounds which can be consumed with no serious limitations on the demand site, represents an highly desirable and sought after alternative to the sequestration because it avoids the problems arising for the lack of availability and the costs of the underground sites or the uncertainties associated with the ocean ones.

In this context, however, it is necessary to take into account that an efficient industrial utilization of CO₂, through chemical or biochemical process, having gaseous emissions as the starting point, demands a preceding capture and concentration step. The relatively more widely used technique, to remove and concentrate CO₂ from exhaust gases containing it at low partial pressure, is that of the extraction by means of alkaline agents. In this relatively most widespread technique, represented by the "scrubbing", the exhaust gas is put into contact with an alkaline solution where, by keeping an high pH value, an equally high power to remove CO₂ from the exhaust gas is maintained. Unfortunately, in order to keep the pH value sufficiently high, a large quantity of alkaline agents is required because of the fact that CO₂ reacts with them. In aqueous solution the most common alkaline agents for CO₂ extraction are ammines, which react with CO₂, in an acid-base reaction, to produce a soluble salt carbamate. The reaction is reversible at high temperature, making it possible to regenerate the alkaline agent. Even when absorption, through chemical interaction, is very efficient this process has a few disadvantages, amongst them the most relevant ones are the large energy consumption for regenerating the separating agent and the potential degradation of the agent itself in the face of pollutants: ammines, in fact, have a limited life span of utilization, due to the degradation resulting from their oxidation; they also lead to serious corrosion of plants.

In the art, other types of agents are used, among them, as an example, there are aqueous solutions of alkaline metals. Processes utilizing such solutions are characterized by running costs which are relatively lower than the ones based on ammines; this is true mainly for systems leading to the formation of bicarbonate, which enable the utilization of a less endothermic regeneration reaction and therefore a less expensive one, from an energy viewpoint, as compared to those based on carbamate.

Nevertheless, to reach a significant rate of capture and separation of CO₂, even the alkaline metals based agents need to be employed in high concentrations and at high pH values, implying thus high capital costs due to the need to build reactors out of special materials which have to be adequately resistant to corrosive action.

During the above mentioned "scrubbing" operations what takes place is the mass transfer of CO₂ from the gaseous phase to the liquid one according to the chemical reactions shown in the following equations from (1) to (3).

Gaseous CO₂ upon contacting the solution is rapidly dissolved to produce an aqueous form which is strictly hydrated.

CO₂ (gas) = CO₂ (acq) (1)

The CO₂ aqueous form can then react both with water or, at high pH value, with hydroxide ions.

CO₂ (acq) + H₂O = H₂CO₃ (2a)

H₂CO₃ - H⁺ + HCO₃ ^" (2b)

CO₂ (acq) + OH^" = HCO₃ ^" (3)

At pH < 8 reaction (3) is irrelevant due to the lack of OH^" ions; at pH between 8 and 10 both reactions (2) and (3) occur, while at pH >10 reaction (3) is predominant.

The step limiting the velocity of the indicated sequence of reactions, with the exception of the high pH conditions (3), is the CO₂ hydration reaction (2a) which makes the industrial application of it very difficult.

It is also known that when the agent is an alkaline metal carbonate, in order to bring the absorption rate of CO₂ up to the needs of industrial applications, activating substances are added to the solution, like, as an example, piperazine and others, which proved able to increase the initial rate of absorption of CO₂; subsequently CO₂ is chemically bound to the carbonate, thus increasing the global absorbing power of the solution. Unfortunately, even upon adding activating substances, there remains the problem of the low alkaline agent concentration in the absorbing solution, which involves well known economic disadvantages in terms of investment (the reference is to the plant dimensions) and of large energy requirements (the reference is to the vast quantity of dilution water per unit of reagent to be regenerated).

In order to limit the energy penalty and therefore the significant costs associated with alkaline agents regeneration and CO₂ stripping, which are both typical features of solution based processes, solid state alkaline metal carbonates have recently been utilized as regenerable agents. Such systems, nonetheless, though advantageous in the regeneration stage, keep suffering the important disadvantage coming from the sluggishness of the capture process consequent upon the interaction between CO₂ and solid phase alkaline metals. (Green et al., 2003).

The CO₂ quick capture has also been tackled by using a biocatalyst, the carbonic anhydrase enzyme, one of the fastest known enzymes, capable of catalyzing the rapid interconversion of CO₂ and water into carbonic acid, protons and bicarbonate ions. This enzyme can increase the reaction efficiency one million times, in particular the human carbonic anhydrase isoenzyme (CA II) can hydrate some 1.4 million CO₂ molecules per second. Carbonic anhydrase is an ubiquitous enzyme, it can be found in different forms or isoenzymes in animals, plants and microorganisms (Budger and Price, 1994; Seltemeyer et al., 1993; Brown, 1990; Maren and Sanyal, 1983).

Carbonic anhydrase facilitates the CO₂ transfer from gaseous to liquid phase following well known laws governing gas mass transfer.

The technical feasibility of carbonic anhydrase catalyzed systems, designed both for CO₂ capture with bicarbonate and hydrogen ions production and for CO₂ capture and sequestration in the form of magnesium and calcium carbonates, has been proved (Bond et al., 2001a; Bond et al., 2001b, Bond et al., 1999).

Specifically in the US patent application n. 20040219090 a process to remove CO₂ from a gaseous stream and turn it into stable solid from is disclosed. In the sequestration process the CO₂ enriched gas is passed through a gas diffusion membrane to transfer the CO₂ to a fluid medium. The CO₂ rich fluid is then passed through a matrix containing carbonic anhydrase to accelerate the conversion of CO₂ to carbonic acid. In the final step, a mineral ion is added to the reaction so that a precipitate of carbonate salt is formed. The difficulty in the implementation of solutions such as this rests with finding low cost sources of minerals, with permanently disposing of the vast quantities of carbonate salts obtained with precipitation, and with disposing of the capture liquid medium after the removal of carbonate solids.

Alternatively, in international patent application n. 9855210, a process and an apparatus to capture CO₂ by means of carbonic anhydrase have been presented, based on a liquid solvent spray which flows countercurrent to a CO₂ enriched gaseous stream in a column reactor. The reactor contains carbonic anhydrase immobilized onto an inert inorganic support; such anhydrase catalyses the reversible hydration of CO₂ to form bicarbonate and hydrogen ions. In the suggested process the obtained solution can be subjected to an ion exchange resin to immobilize the bicarbonate ions. As an alternative the hydrogen and bicarbonate ions containing solution can be fed into a second reactor, identical to the one used for capturing CO₂, and there the ions are converted into water and CO₂. Because of the costs associated with the ion exchange technology or with the pressure input which is needed to have the exhaust gas overcome the pressure drops and obtain a CO₂ concentrated flow into the two column reactors, the two above mentioned solutions seem to be applicable only for limited exhaust gas streams.

In a different vein, with US patent application n. 20040259231, a process is proposed whereby CO₂ is removed from the exhaust gas by means of a reactor containing carbonic anhydrase immobilized on a porous substrate, A type of reactor is described such that immobilized carbonic anhydrase is employed through a catalytic contact with water spray to concentrate and dissolve the CO₂ contained in an exhaust gas. This system can limit pressure drops in the exhaust gas all along the process and does not require therefore expensive pressure inputs to the incoming gas as in the case of water column reactors, providing thus a better economy for the process. In such system, nevertheless, the availability of CO₂, once more, only in a solution sets this as a limit to its utilization or else it forces one to turn to further and still expensive chemical or biological conversions apt to different uses. Furthermore methods to immobilize carbonic anhydrase with acrylamide, alginate and chitosan-alginate matrices are well known to expert in the art, and yet other systems as well may be employed like those generally applied in enzyme immobilization (Bhattacharya et al., 2003; Simsek-Ege et al., 2002; Liu et al., 2001). As for the economic use of carbonic anhydrase as a catalyst in industrial processes to capture CO₂ following the described method, important progress has been made in as much as a robust and economic way to get the enzyme, by producing it via biotechnological process, has been identified. Furthermore carbonic anhydrase and its analogues proved able to function efficiently in dealing with the composition of the capturing solution and with pollutants (like SOx, NOx, led, mercury or arsenic ions) that can be found in exhaust gases (Bond et al., 2001a; Bond et al., 2001b). The most important problems still left wide open with the systems such as these are mainly to do with the optimization of their biological and chemical-physical conditions as well as with the engineering of a not yet adequate process. 2.2. CO₂ use and treatment of organic matter

At laboratory stage experiments have been performed for the anaerobic fermentation of carbon monoxide and/or carbon dioxide and of hydrogen in order to produce acetic acid, acetate salts and other commercial products like ethanol (Vega et al., 1989a; Vega et al., 1989b; Klasson et al., 1990; Klasson et al., 1992).

More recently, to obtain the very same products, methods and apparatuses have been proposed (US patent n. 6753170; US patent n. 6340581 ; international patent application n. 0208438; European patent application n. 0909328; US patent n. 5173429) for the conversion of exhaust gases coming from refuse of industrial processes like those of oil and cool refining, and from ammonia and methanol production. Depending on the product that one wants to get specific anaerobic bacterial strains or mixed bacterial populations (US patent n. 5593886) are used, on top of appropriate nutritional, pH, temperature, pressure, gas flow and other conditions. The many advancements in the knowledge of these systems notwithstanding, the commercial production via fermentation of acetic acid is not carried out because of the economic impediments related to the recovery of the acetic acid which is diluted in the fermentation medium. Acetic acid can be produced, according to these systems, at relatively low concentrations (around 5%) and its recovery by means of distillation or of other more complex processes makes still too expensive the global process. It is therefore economically more profitable to use directly the anaerobic fermentations products, coming from the refuse gas, in the colture medium and do this in less expensive ways, like that of bioconversion to another product easier to separate. Coming now to examine the state of the art concerning anaerobic digestion processes, one has to note that in the industry they are employed mainly for the treatment of organic refuse and wastewaters of diverse origins, to obtain methane and other products. The most widely used technology is the one characterized by the fact that the anaerobic digestion is carried out in a single reactor.

The organic matter digestion process consists of four main phases which are supported by different microbial populations: the hydrolytic phase involves the conversion of complex organic substances into soluble organic substances; the acidogenic phase concerns the conversion of soluble organic substances into volatile fatty acids, alcohols and hydrogen; the acetogenic phase is the one where volatile fatty acids and alcohols are turned into acetic acid, carbon dioxide and hydrogen; the methanogenic phase contemplates both acetic acid and hydrogen as well as carbon dioxide being transformed into methane. There are, moreover, other minor phases or biochemical reactions that take place in between the four major phases. The anaerobic digestion of organic matter had in the past and still has at present some problems which put and are still putting limitations to its generalized industrial use: the volatile organic solids reduction is far from completed; the organic matter needs a long retention time to complete the transformation of its slowest degrading fraction and this leads to large plant dimensions; finally the risk of instability of reactors and the consequent efficiency drops need to be mentioned, they are often caused by excessive organic or hydraulic load or by toxic phenomena.

In the last few years the anaerobic digestion process schemes and the digester functional structure have enormously evolved to increase the organic matrices degradation rate and to decrease plant dimensions. Depending on the nature of organic matrices the hydrolytic phase, as far as the organic solids component is concerned, and the methanogenic one, regarding the organic soluble component, have been identified as "limiting"; the acidogenic and the acetogenic phases only rarely are considered capable of limiting the degradation rate and the process velocity. The suggested solutions contemplate the introduction of chemical-physical pre-treatment methods, the process phases separation, say the acid phase from the methanogenic one to give an example, the introduction of biomass retention methods, say anaerobic filters or high growth rate of bacteria, attached to inert supports, to give another (international patent application n. 03035558). It is widely known that the incomplete degradation of organic matrices is due to the fact that volatile solids generally comprise constituents showing very different hydrolysis rates; and some constituents are in fact much slower to degrade than others. As an example the most common soluble chemical intermediates like glucose or acetic acid degrade rapidly, while particulate or colloidal material like proteins, fats, vegetable oils, bacterial cell walls, cellulose and lignin degrade slowly or do not degrade al all. Further, the incomplete hydrolysis, by provoking the accumulation of not degraded substances, reduces the productively usable space in the digesters. This situation often limited the industrial application of anaerobic digestion to specific substrates or to small organic loads in mixed substrates to guarantee satisfactory operational management.

Ways to improve the hydrolysis have been proposed in European patent application n. 0119430, which suggests the introduction of a second reactor, separate from the first, where the slower hydrolysis material is treated, and the solids recycling from one reactor to the other in order to keep a high concentration of the substrate and the bacterial enzymes. In other studies it is proposed to keep an higher temperature in the second reactor. Further solutions that are suggested in order to obtain a supplementary hydrolysis, as thought in US patent n. 5670047 and US patent n. 5015384, include the thickening of the partially digested solid constituents and their separation from the effluent stream, to recycle them in the same reactor in order not to lose substrate and bacterial enzymes. In US patent n. 5785852 a mechanical and pressurized thermal pre- treatment process is suggested in order to liquefy solid sludge before digestion and get rid of inorganic solids from the system effluent. The proposed solutions do reach the goals of increasing the hydrolyses and partially reducing the volumes in the digesters, nevertheless they still have high investment costs, for the equipment which is needed for running the operational management, and high energy consumption. The methanogenesis limitations, due to the low growth rate of the methanogenic bacterial populations and to their high sensitivity to environmental factors, have been tackled by proposing the separation of the conventional anaerobic process into stages which operate at different pH values: the first one being acid while the second is neutral or moderately alkaline, and with retention in the latter of the microbial biomass. International patent application n. 03016220, US patent n. 5630942, US patent n. 5525228 contain examples of separation into stages. US patent n. 4022665 further proposed for the methanogenic stage the introduction of a series of functional separation and specific operational conditions referred to organic load and retention time. Another proposed solution is that of functional separate units in parallel operation to independently manage the gaseous and the liquid effluent coming from the acid stage, as in US patent n. 4696746: in the liquid phase biomethanation of fatty acids, while in the gaseous one biomethanation of carbon dioxide and hydrogen take place respectively, maintaining different microbial populations in the two functional units. Multistage systems proved to be good at treating large quantities of organic matrices coming from diverse origins, at operating with higher concentrations of microbial populations and specialized enzymes in smaller digester, at operating with shorter hydraulic retention times while obtaining treatment levels similar to the ones of conventional systems; unfortunately they display high investment costs either in the building of new ones or in the restructuring of existing conventional ones. There is a still ample room for improvement in such systems with respect to the efficiency of the solid organic matrices treatment as well as to the possibility of treatment integration between that of matrices and the fermentation one of exhaust gases; in such a way no significant further investment is required on top of that needed to increase the efficiency of the organic matrices treatment by itself. Many scientific publications and patents concentrate, beyond the aspects related to the stage separation, on the need to identify suitable equilibrium between production and consumption of the different intermediate compounds in each of the different stages resulting from the separation of the process.

The degree of mutual dependence among the different microbial populations can vary considerably: on the one hand the organisms at the end of anaerobic process always depend upon the preceding ones for their substrates, on the other, in each stage, the substrates using organisms have a significant influence over the preceding ones, by removing their products.

During primary fermentations polymers (polysaccharides, proteins, nucleic acids and lipids) are first turned, typically by extracellular hydrolytic enzymes, into oligomers and monomers (sugars, amino acids, purines and pyrimidines, fatty acids and glycerol); the produced monomers are further fermented into fatty acids, succinates, lactates, alcohols, etc. Some of these fermentation products, mainly acetates, hydrogen, carbon dioxide and other one carbon atom substances, can be turned directly into methane and carbon dioxide by methanogenic microorganisms. For the degradation of other fermentation products (fatty acids with the chain longer than two carbon atoms, alcohols with chain longer than one carbon atom, aromatic acids, etc.) a second group of fermentative bacterial populations (secondary fermentative or protons reducing) convert their substrates into acetates, hydrogen and carbon dioxide which are used by methanogenic organisms in the traditional anaerobic digestion process. Hydrogen has been identified as an anaerobic process component capable of exercising an influence on the structure of it and on the ways of its development, by regulating the proportion of the different intermediate products both in fermentation and methanation. During primary and secondary fermentations hydrogen is produced by the electrons which are generated in the oxidation of NADH (nicotinamide-adenin-dinucleotide) and its formation is facilitated only when its partial pressure is maintained at low levels, as in the case where hydrogen is rapidly metabolized by methanogenic organisms responsible for the final step of methanogenesis. At hydrogen low partial pressures, the NADH electrons flow, generated during glycolysis, is oriented towards protons reduction, leading to hydrogen formation; hydrogen allows pyruvates to be degraded into acetates, hydrogen and carbon dioxide. When hydrogen partial pressure increases, the NADH electrons flow switches from hydrogen production to the formation of other reduced products like propionates or long chain fatty acids and to the formation of lactates or ethanol from pyruvates. Therefore in anaerobic processes where hydrogen is efficiently used fermentative bacteria produce large quantities of acetates and hydrogen while produce in small quantities, or do not produce at all, ethanol or lactates and produce less propionates or butyrates. Hydrogen exercises also a control over the process during which acetogenic bacteria act on fatty acids, organic acids and alcohols to produce acetic acid and hydrogen.

Competition among microorganisms is also influenced, beyond their intrinsic characteristics, by environmental conditions like pH value, redox potential, temperature, nutritional supplies, hydraulic and solids retention time. Methanogenesis inhibiting conditions (e.g. significantly lower than neutral pH, nitrates presence in the organic material, high variability organic loads) may allow the acetogenic organisms to become dominant populations in hydrogen utilization. Also sulfate-reducing bacteria may compete for hydrogen, but only when sulfates are present in high concentrations, which is rare in organic refuse or wastewaters. That notwithstanding, if hydrogen concentration increases for whatever reason, (e.g. excessive fermentative substrate, hydrogen consumption inhibition due to a drop in pH value or to the presence of toxic substances) the fatty acids pool in the digesters significantly increases and may determine a further drop in the pH value leading to further inhibition of hydrogen utilization. The consequence may be the fermentation block and the accumulation of large quantities of fatty acids: this situation can be observed reasonably frequently in industrial digesters of organic refuse. Obviously in the case where the hydrogen utilizing organisms are the methanogenic ones they act as primary regulators in the total niethanogenic conversion process and the fatty acids oxidizing bacteria are very severely conditioned by an hydrogen removal failure (Schink, 1997).

Carbon dioxide and bicarbonate (inorganic dissolved carbon) are themselves inherent products to the anaerobic process and they represent furthermore bacterial growth indispensable factors as well as important electrons acceptors. In anaerobic processes CO₂ utilization needs an adequate hydrogen supply. Clostridium spp. and Eubacterium spp. type organisms produce hydrogen from carbohydrates, Syntrophobacter wolinii and Syntrophomonas wolfei type organisms produce hydrogen from propionates or butyrates oxidation. Anyway, as it was previously noted, an efficient hydrogen production is possible, for thermodynamic reasons, only when hydrogen does not accumulate. In anaerobic reactors there are microorganisms of different types: in part they compete for the same substrates, as in the case of hydrogen, in part they use for growing partially different substrates; only methanogens and acetogens organisms (e.g. Acetobacterium woodii and Clostridium thermoaceticum) use mainly CO₂ and hydrogen to produce methane and acetic acid/acetates respectively. Competition for hydrogen takes place in between reductive-acetogen and methanogen organisms on the basis of their affinity with it, this is higher in the methanogens than in the acetogens ones: therefore when there is methanogenesis there cannot be reductive-acetogenesis. Nevertheless reductive-acetogen organisms are always present, even when methanogenesis determines an hydrogen concentration which is lower than their affinity for it: they survive in the digesters thanks to the mixotrophic characteristic. In standard conditions the hydrogen methanogenic oxidation produces more energy than the acetogenic one and one should think therefore that acetogenic organisms may have little chances of success in the competition with the methanogenic ones at low hydrogen concentrations. That notwithstanding, the fact that acetogenic organisms may take advantage from their own metabolic versatility put them in the position to compete in various metabolic processes thanks to their ability to utilize two or more substrates simultaneously (Mϋller, 2003; Diekert et al., 1994). Such simultaneous utilization of more substrates can increase the affinity for each of them, as it was proved in the case of Escherichia coli (Lendenmann, 1996; EgIi, 1995).

Anaerobic microorganisms, known for their ability to convert hydrogen and carbon dioxide or carbon monoxide and water into acids, acid salts and alcohols, comprise Aceto bacterium kiwui, A. woodii, Clostridium aceticum, Butyribacterium methylotrophicum, C. acetobutylicum, C. formoaceticum, C. kluyveri, C. thermo aceticum, C. thermocellum, C. thermohydrosulfuricum, C. thermosaccarolyticum, Eubacterium limosum, C. Ijungdahlii, Peptostreptococcus productus; they have a metabolic capability that differentiate them from other acetogenic organisms synthesizing acetates from other metabolic processes. As it is clear from the reported list, acetogenic organisms are, from a phylogenetic point of view, a rather diversified group and they have been isolated in equally differentiated ecosystems: wastewaters, anaerobic digesters, natural sediments, termite gut, rumen, not ruminants intestinal tract, extreme environments with respect to pH, salinity and temperature. It has been estimated that acetogenesis globally produces billions of tons of acetates every year and that acetogenic organisms perform a relevant role in the carbon cycle (Drake et al., 2004).

It is well known that anaerobic digestion produces a biogas which can be very variable in its concentration of CO₂ and methane. Depending on methane concentration the biogas may require "scrubbing" treatment to convert the produced biogas into a usable form with higher energy value. To avoid treating the produced biogas numerous solutions have been suggested concerning the digestion process: the introduction of gaseous hydrogen both in the acid and in the methanogenic stages (e.g. US patent n. 3383309), the use of caustic substances in the methanogenic stage (E. Colleran, 1982), the keeping of specified volume/interface ratios to facilitate a preferential methane diffusion while CO₂ is maintained in liquid phase (US patent n. 4040953); these are all excellent solutions from a technical viewpoint but still very expensive from an economic viewpoint. In the last few years control instruments and procedures have been developed to obtain a better stability in the anaerobic digestion process with respect to total production and composition of the biogas and supplementary checks have been introduced concerning pH value, alkalinity, volatile fatty acids concentrations, total solids, etc. The management problems correction in multistage systems, so as to guarantee optimal specific conditions in each process stage, is often obtained by regulating the pH value, in case by adding alkaline material, the organic load flow and the one recycling liquid and biomass in between reactors (European patent application n. 1181252; Water Research 1994). The main problem comes from the difficulty in regulating hydrogen and volatile fatty acids which, at high concentrations, may inhibit the treatment of organic matrices to reach high methane concentration and which create some time impediments to a stable and economic continuous management of the process. The alkaline substances, used to obtain simultaneously an high concentration of methane in the produced biogas and an adequate pH control in the methanation reactor, come in two main groups: those that capture gaseous CO₂ and turn it into bicarbonate (strong bases and carbonate salts) and those that add directly to the liquid medium bicarbonate alkalinity (bicarb onates).

If a strong base (e.g. calcium or sodium hydroxide) or a carbonate salt (e.g. sodium carbonate) is added, the ionic equilibrium is reached very rapidly and CO₂ is removed from the gaseous phase to form the needed alkalinity by bicarbonate. The process control, by means of the cited chemical substances which trap the gaseous CO₂, demands, nevertheless, that their adding be done gradually, in subsequent steps, to make possible the reaching of the ionic equilibrium after each addition of chemical substance without having large pH jumps which could be dangerous for the methanogenic microorganisms. In particular, if the chosen chemical product is calcium hydroxide, as it is relatively common because it is cheap, then it traps CO₂ and turns it into bicarbonate; yet, when bicarbonate reaches the 0,5-1 g per liter concentration, whatever new adding is done it leads to the formation of insoluble carbonate. In such a situation CO₂ is removed from the gas but the system alkalinity is not augmented. The CO₂ partial pressure drop in the gas determines a rapid increase in the pH value, but, in view of the fact that alkalinity was not augmented, the pH is instable so that when the biological activity resumes in an efficient way, the pH value rapidly drops once more. As a consequence calcium hydroxide can be proposed only in operational conditions where the pH value is less than 6.5 and can be added in limited amounts to allow an increase in pH value to no more than 6.8. If, on the contrary, it is bicarbonate to be added, the buffering capacity is exercised at a moderately alkaline pH value; these are particularly suitable conditions to the methanogenic microbial populations growth, and do not imply large jumps even in the case of excessive doses of the chemical substance. Furthermore bicarbonate is easier to handle when compared to hydroxides and carbonates, it is not corrosive and toxic if used in appropriate ways, it is soluble in water with no difficulties, it does not form hard deposits and does not demand frequent cleaning and maintenance. In view of what above mentioned, a need is felt in the art for practical and economic methods to capture CO₂ from exhaust gases; furthermore, a need is felt for new processes and organisms capable of utilizing CO₂ to increase organic matter degradation and methane gas production. Brief disclosure of invention

The main task of the present invention is to provide a process for capturing and separating the CO₂ contained in a gaseous flow and for using the CO₂ so captured. The above problem is solved, according to the present invention, by a process having the features set out in the annexed claims that are to be considered an integral part of the present description.

According to the present invention, the captured CO₂ is used in the anaerobic treatment of organic matter to produce methane; such process is more efficient, requiring smaller investment and lower running costs than the ones known to the prior art.

According to the present invention, there are combined, in an integrated way, a first module for CO₂ capture and separation with a second module for use of the captured CO₂ in the treatment of organic matter. In particular, the first module comprises the following stages: - CO₂ removal from the exhaust gas in an extraction zone, by means of an alkaline metal solution and more particularly of sodium and CO₂ dissolution and hydration catalysed by immobilized carbonic anhydrase;

- precipitation by alkaline metal carbonate of the species resulting from CO₂ hydration and obtainment of an over saturated alkaline metal bicarbonate solution

- removal of the alkaline metal bicarbonate precipitate from the solution, transfer to the second module of part of it and regeneration of the remainder by calcination while obtaining at the same time a concentrated CO₂ gaseous flow. In particular, the second module comprises the following stages:

- organic matter preparation

- CO₂ dissolution and hydration in a liquid medium containing organic matter

- CO₂ assisted organic matter hydrolysis and acidification, in an acid zone - separation and recyrculation of solids contained in the effluent out flowing from the acid zone

- methanogenesis, in a specialised zone, of the liquid effluent coming from the acid zone, after solids removal, with pH buffed by bicarbonate addition separation and recyrculation or disposal of solids contained in the effluent outflowing from the methanogenic zone.

The possibility of CO₂ capturing and separating and then using it to treat more efficiently organic matter producing more methane is a very useful tool in the hands of e.g. greenhouse gas emitting firms as well as of those treating organic refuse. First module Another task of the present invention is that of providing a new and better process to capture CO₂ contained in an exhaust gas and generate a concentrate gaseous CO₂ and/or solid bicarbonate flow.

A characteristic feature of the invention is to remove, in the first module, the need to operate in very caustic conditions while maintaining quick and efficient the CO₂ capture by means of a combination of an absorption process and a chemical reaction in an alkaline metals solution supplemented by an immobilized biocatalyst, carbonic anhydrase, favouring CO₂ hydration. The advantages of this new process are low investment and running costs and better environmental safety. A further task of the present invention is to provide, with the bicarbonate produced in the first module, one of the essential elements to make more efficient and stable the process taking place in the second module. This task is performed by adding bicarbonate in the second module methanation zone in order to increase methane concentration in biogas and to maintain a slightly alkaline pH, this condition is particularly suitable to methanogenic bacterial populations growth. A further task of the present invention is to provide a CO₂ capture and separation process which may significantly contain the circulating solution volume and the energy consumption associated with the alkaline agent regeneration. This task is performed in the first module by coming out an efficient CO₂ capture in liquid phase while running, on the other hand, the alkaline agent regeneration in solid phase, in reasonably small plants and at equally low temperature.

A further task of the present invention is to provide a CO₂ capture and separation process which may significantly contain biocatalyst and alkaline agent degradation. This problem can be solved by retrofitting the CO₂ capture module to traditional pollutants (NOx, particulate, SOx) removal plants and by using temperatures which may be compatible with the management of chemical and biotechnological processes contemplated in the present invention. The advantage so achieved is that the biocatalyst and the alkaline agent, in such a condition, last for a long time and thus their cost per tonne of removed CO₂ is correspondently reduced. Second module

In the second module the present invention provides a new method for using the CO₂ in treating organic matter in order to reach, on the one hand, a fuller degradation of volatile solids which are contained in the organic matter and, on the other, an increased methane production. Specific and unique characteristics of the invention are, firstly, the specialization of mixed anaerobic micro organisms populations which are present or can be added to the anaerobic digestion process and, secondly, their environmental conditioning determined mainly by the CO₂ and bicarbonate introduction into the process. In the liquid medium of the reaction zone, named pre-acidification zone, there are or can be introduced high density mixed hydrolytic bacterial populations; in the liquid medium of the fermentation reactor there are or can be introduced mixed high density acidogenic and acetogenic anaerobic bacterial populations; in the liquid medium of the methanation reactor there are or can be introduced mixed high density methanogenic microbial populations. What is meant by mixed populations is a colture of two or more microbial populations which have been isolated from ecosystems comprising: wastewaters, anaerobic digesters, natural sediments, termites intestines, rumen, non-ruminants intestinal tracts; or else coming from extreme environments with respect to pH value, salinity and temperature; or, finally, found in collections of microbial cultures, kept by scientific institutions or biotechnological firms.

A further task of the present invention is to provide a new organization of the process in separate stages, with biomass retention, so as to gain an higher efficiency in every stage and a better overall stability. These objects and some more are reached, in the second module, by introducing two anaerobic subsystems, acid the first and methanogenic the second, and by using specialised functional stages, with intermediate concentration and biomass recyrculation steps. The key element, which is the innovative and unique feature characterising the process, is in the fact that the increased degradation rate of the polymeric materials, especially the volatile solids contained in organic matter, is obtained by introducing gaseous CO₂ into two sections: pre-acidification, the first one, fermentation, the second, where suitable conditions are created to hydrolysis and hydrogen production respectively and able also to inhibit methanogenic microorganisms, and to block CO₂ and hydrogen conversion pathway to methane. In the fermentation section, the introduced CO₂ is immediately used, together with hydrogen, by bacterial populations so as to increase the overall acetates production. In the more suitable conditions to the methanogenic microbial populations growth, which have been brought about by the introduction of bicarbonate, the increased methane production, in the methanogenic stage, is made possible by the augmented acetates availability coming from the preceding acid stage and by CO₂ reduction to methane.

A further task is to obtain a process characterised by stability and robustness in handling variations in composition and quantity of organic matter and optimised with respect to the maximization of exogenous CO₂ that can be introduced into the system. These object are reached by controlling the volatile fat acids content and the pH value in the acid and methanogenic stages. The fatty acids concentration and the pH value in each reactor are free to fluctuate in between stage specific predetermined levels. The volatile fatty acids content is checked discontinuously, while the pH value, more conveniently, is checked and controlled continuously to guarantee process stability. The pH values are regulated, on the one hand, by using as the only chemical agents bicarbonate and CO₂ coming from the CO₂ capture and separation module, on the other by regulating the substrate flows amongst reactors.

The process proposed in the second module is economically efficient because, on top of consuming exogenous CO₂, it requires smaller digesters per unit mass of treated organic matter, and one should note that the larger share in investment costs for anaerobic digestion plants is due precisely to digesters volume. The process can be run in existing anaerobic digestion plants after limited changes in structure and equipments, thus overcoming possible difficulties in siting new plants. The process shows important advantages with respect to operational costs as well, in as much as it reduces the solid fraction remaining after digestion and increases the possibility of dehydrating it: the consequence is a reduction in landfilling, in incineration or in possible further uses costs; finally, the large increase in net methane production brings in equally larger revenues.

The present invention, together with its advantages, will become more clear from the following description of an embodiment thereof to be considered in conjunction with the annexed drawings; this embodiment is provided by way of example and therefore shall not be interpreted limitatively. Brief description of drawings

Fig. 1 represents a schematic basic configuration of an integrated process according to the present invention.

Fig. 2 represents a schematic basic configuration of the first module of the process of Fig. 1 : in it, the functional structure and the equipments composition are illustrated. Fig. 3 represents a schematic basic configuration of the second module of the process of Fig. 1 : in it, the functional structure and the equipments composition are illustrated. Detailed disclosure of invention

Fig. 1 shows the scheme of a new integrated process, suggested by the present invention, for capturing CO₂ and using it in order to increase the efficiency in the anaerobic digestion of organic matter so as to increase methane production. As it is shown the process consists in two integrated modules: the first one is named "gaseous emissions treatment" (GET), the second is named "treatment of organic matter" (TOM). Following the present invention the goal of capturing the CO₂ contained in an exhaust gas and of generating a concentrated Cθ₂/bicarbonate flow, with low investment and running costs, is reached in the GET module with the help of a process schematically presented in Fig. 2 and described in what follows.

In the present invention we refer to an exhaust gas typically as the one outflowing from combustion chambers in incinerators or electric power, thermal power, steel, cement, paper, glass, ceramic plants or oil refineries, after removal of traditional pollutants (NOx, particulate, SOx); this exhaust gas has low CO₂ concentrations, typically something between 3 and 20% of total volume. The removal treatments bring the inflowing gas temperature to values ranging between 30 and 80⁰C which are compatible with the management of the chemical and biotechnological processes contemplated in the present invention.

As it is shown in Fig. 2, the CO₂ containing exhaust gas enters into the extraction zone, which consists of a contact and dissolution reactor (1), through a line (a) directly connected to the emissions flow. The contact between the exhaust gas and the alkaline metal solution and more particularly of sodium and again between this one, being CO₂ enriched, and the carbonic anhydrase, may take place in whatever reactor initially designed for gas/liquid reactions and subsequently perfected to guarantee that the biocatalyst be maintained, at all times, in liquid phase or hydrated. The reactor has a flow in and a flow out point for the gaseous emissions; further it has a flow in point for the capture solution coming through line (b) and a flow out point, located at the bottom of the reactor where the solution is collected to discharge it through exit line (c). The capture reactor is equipped so as to make possible controlling the two separate flows, that of gaseous emissions and the one of the capture solution. The carbonic anhydrase enzyme is immobilized in ways and onto supports known to the art. The alkaline metal solution and more particularly of sodium minerals flowing into the reactor has a pH value set between 8.3 and 9.6.

The contact process, between the CO₂ rich exhaust gas and the alkaline metal solution and more particularly of sodium, aided by carbonic anhydrase, determines a significant increase in the concentration of carbonic acid, protons and bicarbonate ions. Alkaline metal carbonate and more particularly of sodium in the solution react with the stoichiometric quantities of the species resulting from CO₂ dissolution and hydration, further augmenting in this way the bicarbonate ions concentration, and consequently their own input into the subsequent precipitation reactor. The obtained solution has pH value between 7.5 and 8.3.

The temperature in the capture reactor may vary between 35 and 75⁰C. One thing to remember is that the CO₂ dissolution rate into water is higher the lower is the temperature. On the other hand the alkaline metal carbonate reaction rate, with the species resulting from CO₂ dissolution, to form bicarbonate is lower at lower temperature. Therefore the alkaline solution temperature must be maintained at a level such as to obtain CO₂ dissolution and hydration in line with the desired rate of reaction between alkaline metal carbonate and the species resulted from CO₂ dissolution and hydration. Temperature is maintained below 75 °C and preferably in the interval between 35°C and 60°C.

The gas outflowing from the capture reactor is sent to a demister (2) and then released into the environment. The solution, containing bicarbonate and other species resulting from CO₂ dissolution and hydration and from their reactions with alkaline metal carbonate, is first harvested in the first reactor and then transferred to a second precipitation reactor (3), where solid alkaline metal carbonate is added to obtain an over saturated alkaline metal bicarbonate solution. This second reactor consists of whatever container be known to the art and which, by dimensions and equipments, may contain and maintain, in suspension the solution, for as long as it takes to fully convert all the added alkaline metal carbonate into alkaline metal bicarbonate. In the precipitation reactor the pH value is controlled by increasing or decreasing the pH value in the harvested solution, that is by dissolving and hydrating a smaller or larger CO₂ quantity. Alternatively, the pH value is controlled by increasing or decreasing the quantity of solid alkaline metal carbonate which is introduced into the solution. As a further alternative the pH value can be controlled by introducing into the solution protons or substances which may effect it.

In the precipitation reactor the best pressure and temperature conditions are maintained in order to obtain alkaline metal bicarbonate. Further the solution is agitated up to the point when almost all added alkaline metal carbonate is converted into precipitated alkaline metal bicarbonate. "Almost all" is to be understood as whatever fraction, ranging between 90 and 100%, of the alkaline metal carbonate which was added to the solution. The solution pH value, after adding alkaline metal carbonate, is never below 9, and better still is between 9 and 9.6. The precipitation reactor temperature may vary between 35 and 6O⁰C.

To minimize alkaline agents consumption, the solution harvested in the precipitation reactor and containing suspended solid alkaline metal bicarbonate, is transferred to an apparatus (4) known in the art and designed for solid/liquid separation. The obtained solution is transferred, through line (d), to a storage unit (5) to be recycled in the first capture reactor, through line (b).

On the other hand the solid bicarbonate obtained by separation is transferred, through line (e), into a subsequent regeneration unit (6) where CO₂ and steam are released by calcination, at a constant temperature ranging between 120 and 140°C. Inside the regeneration reactor, as for the alkaline metal bicarbonate and more particularly of sodium, the following endothermic reaction takes place:

2NaHCO₃ = CO₂ + H₂O + Na₂CO₃ + 32,4 Kcal/mol

The carbonate, produced in the regeneration unit, is recycled as a reagent into the precipitation reactor through line (f), while the gas (CO₂ + H₂O) is sent, through line (g), to an apparatus (7) for the separation and concentration of gaseous CO₂. The steam is condensed and the released and separated CO₂ is compressed and stored (8) in view of its further uses in the anaerobic digestion second module or for even different uses. As an alternative to regeneration, part of the solid alkaline metal bicarbonate is transferred, through line (h) to the methanation zone, in the TOM module, in order to keep the most suitable conditions for growing methanogenic microbial populations. Following the present invention, the goal of using CO₂ to produce methane by means of anaerobic microorganisms which may be present, or else added, in a microbiological system for the anaerobic digestion of organic matter, is obtained in the TOM module according to a process which is schematically presented in Fig. 3 and described in what follows.

As it is shown in Fig. 3 the organic matter enters, through line (i), into the functional storage and pre-treatment section (9) where it is adequately prepared for the anaerobic digestion process, made homogeneous, of appropriate dimensions, with the desired solids concentration and pollutions free. Depending on the nature of organic matter and on the ratios of its constituents, different actions with the different equipments (pumps, filters, mixers, hydropulper, etc.) and in different settings may become necessary, but being all well known to the art they are not presently specified. The organic matter, made available in the desired conditions, is transferred to the pre-acidification section (10) through line (i).

Organic matter enters into the pre-acidification section with a suspended solids content ranging between O and 35% and a chemical oxygen demand (COD) ranging between O and 200 grams per litre. Into the same section, also gaseous CO₂, coming from the storage container, is introduced through line (1). Such introduction may be performed by injection into the organic material carrying line or else by diffusion though a porous membrane, or by whatever method known to the art. The CO₂ which is not dissolved, is harvested in the upper part of the present section and then recycled into the very same section. The acid condition, derived from CO₂ dissolution and hydration, starts a controlled degradation of solid and soluble complex organic substances (carbohydrates, proteins, lipids) into monomers in soluble form (sugars, amino acids, long chain fatty acids). The CO₂ partial pressure in this section is regulated so that the organic matter pH value preferably contained in the interval 3 to 6, and better still 4 to 5.5. Depending on the conversion speed of complex organic substances and of the solid ones into monomers in soluble form, the hydraulic retention time (HRT) may range approximately between 6 hours and 2 days, while the solids retention time (SRT) between 1 and 3 days. Temperature is approximately maintained between 20 and 8O⁰C. The effluent from the pre-acidification section, through process line (m) feeds the section, in the same acid zone, represented by the fermentation reactor (11) which is structured to sustain the growth of suspended or fixed to inert supports bacteria. Here more gaseous CO₂ is introduced by means of an hydraulic or, if present, a gas based moving system; alternatively by means of whatever other method known to the art. The CO₂ which is not dissolved and hydrated nor utilized is harvested in the upper part of the same reactor as a mixed gas (CO₂, H₂, other) and recycled into the same fermentation reactor. In the fermentation reactor a trophic network is maintained, made up of different groups of microorganisms, each characterised by the substrates as well as by the products of its metabolism. In such network mixed hydrolytic bacterial populations carry out the last reduction of complex organic substances into monomers; mixed fermentative acidogenic bacterial populations oxidise simple organic substrates into volatile fatty acids and alcohols, which represent, in the next step, the main substrates for the mixed acetogenic bacterial populations producing acetic acid, formic acid, CO₂ and H₂. The process parameters, suitable to the competitive growth of mixed acetogenic bacterial populations, capable of reducing CO₂ to acetates, include a total organic load ranging between 3 and 150 kg per cubic meter of reactor per day and the introduction of a CO₂ quantity defined with respect both to the organic load and to the culture volume which is present in the reactor. Further, the pH value in the reactor is allowed to vary between an upper bound, equal to 6.3, below which the hydro geno trophic methanogenesis totally blocked or significantly reduced and hydrogen consumption by methanogenic microorganisms is avoided, and a lower bound, equal to 4.3, to sustain acetogenic bacteria's competitive growth. Because of the fact that these populations are more resistant to ammonia than the methanogenic ones, they are able to operate even with high nitrogen content organic matter, which can give higher than 1.2 gr. per litre ammonia concentrations. The pH value is maintained within desired limits by regulating the organic and the CO₂ load, and by recycling the alkaline effluent from the successive methanogenesis reactor, through line (t). To fully convert the slowest degrading volatile solids and to remove the volatile fatty acids from the culture with no process inhibiting phenomena, the hydraulic retention time (HRT) is approximately regulated between 12 hours and 3 days, while the solid retention time (SRT) between 1 and 7 days and temperature between 30 and 60 °C.

The effluent from the fermentation reactor, through a process line (n) feeds a concentrating and conditioning apparatus (12) where a solid/liquid separation takes place: part of the concentrated solid is recycled, through line (o) into the pre- acidification tank (10); the excess solids (non degradable organic substances and non volatile solids) may be disposed of through line (s). The excess solids disposal depends upon inorganic substances or inert solids concentrations as well as upon the efficiency of the liquid/solid separating apparatus which is employed.

The liquid part, containing dissolved organic components (mainly acetates), on top of CO₂, hydrogen and other substances, is, on the contrary, sent, through line (p) to a methanation reactor (13) in the successive methanogenic zone where bicarbonate, coming from the GET module, through line (h), may be added in order to control alkalinity and pH value. Microbial growth fixed onto inert supports, separation and subsequent recycling of the solid component make possible, in the fermentation reactor, the separate regulating of the hydraulic and the solids retention times, thus prolonging the time available to treat the harder to digest matter and increasing the degradation efficiency due to the higher concentration of the active biomass in the acid digester. In the methanation zone, in a specialised reactor (13) which is structured to support microbial growth, either suspended or fixed onto inert supports, there are different mixed populations formed by acetoclastic methanogenic organisms, which transform acetic acid into methane and CO₂, and hydro genophilic methanogenic organisms, which carry out the hydrogen anaerobic oxidation and the CO₂ reduction to methane; it is through these populations that a high methane content gas and a digested effluent are produced.

The methane containing gaseous product is collected through line (q) and sent to a gasometer (14). The digested effluent is sent, though line (r) to a separator/concentrator (15). In view of the low rate of growth of methanogenic populations and in order to maintain a high concentration of them in the reactor, the solid fraction (microorganisms and non degraded solids), separate from the liquid effluent, is recycled into the methanogenic reactor. The excess solids are disposed of through line (s). The liquid effluent, on the other hand, is recycled through line (t) into the fermentation reactor or else finally discharged.

The operations conditions in the methanation zone include a total organic load ranging between 1 and 70 kg per cubic meter of reactor per day, on top of dissolved CO₂ and hydrogen. The pH value in the reactor is allowed to vary between 9.0 and 7.2: such values are optimal for methanogenesis. The efficient volatile fatty acids and CO₂ conversion is obtained by regulating the hydraulic retention time (HRT) between 4 and 12 days and the solids' one (SRT) between 8 and 20 days, approximately, with temperature ranging between 30 and 60 ⁰C.

A crucial innovation in the present proposal is represented by the introduction of a large amount of CO₂ into organic matter anaerobic digestion process. Such element causes an important discontinuity in the environmental conditions where bacterial populations are commonly found.

The discontinuity, on top of breaking the existing equilibrium in the composition of the populations, generates, through selection and adaptation processes, a new long run dynamic equilibrium in the populations composition. Therefore, in the new situation, the management parameters (organic load, HRT and SRT) of the system improve dramatically with respect to those in the initial state.

Every reactor is equipped with an adequate monitoring system. The most important process parameters (temperature, pH value, produced gas composition, etc.) are measured to be used for controlling and optimising the digestion process. The main controlled functions comprise: temperature and pH value in every reactor; the incoming CO₂ and organic matrix flows to the fermentation reactor; the recycling rate between the fermentation and the methanation reactors and vice versa; the bicarbonate dosing into the methanation reactor. Operationally, when the pH value in the acid reactor reaches the predetermined lower bound the recycling of the liquid effluent from the methanation reactor is started, while, when the pH value reaches the upper bound, the recycling is ended. In the methanation reactor, on the other hand, when the pH value falls below the lower level, the flow into the reactor is buffered by adding bicarbonate, which is coming from the first module, up to the point where the predetermined pH minimum value is passed. To guarantee optimal process conditions solid and hydraulic retention times in each reactor are also controlled.

According to a further aspect, the present invention relates to a system for capturing CO₂ in a low CO₂ concentration gaseous flow and for using it to digest organic matter in order to produce methane; this system is adapted to realize the process according to the present invention.

The low CO₂ concentration gaseous flow is typically the exhaust gas outflowing from combustion chambers in incinerators or electric power, thermal power, steel, cement, paper, glass, ceramic plants or oil refineries.

As already said above, the process according to the present invention comprise a first module and a second module.

These two modules may take place in two plants, in particular two distinct and separated plants; additionally, these two plants may be located even far apart. In this last case, the second plant may receive the high CO₂ concentration gaseous flow from the first plant through a pipeline; in principle, other means might be used for transporting such CO₂ from the first plant to the second plant.

References

Patents and Patent applications : US20040259231

US20040219090

US6753170 WO03035558

WO03016220

EP1181252 WO0208438

US6340581

EP0909328

WO9855210 US5785852

US5670047

US5630942

US5593886 US5525228

US5173429US5015384

US4696746EP0119430

US4040953

US4022665 US3383309

Other references :

Bhattacharya, S., Schiavone, M., Chakrabarti, S. and Bhattacharya, S.K. (2003) CO₂ hydration by immobilised Carbonic Anhydrase, Biotechnol. Appl. Biochem. 38: 11-

117. Bond, G.M., Stringer J., Branvold, D.K., Simsek, F.A., Medina, M. G. and Egeland, G.,

(2001a) Development of integrated system for biomimetic CO₂ sequestration using the enzyme carbonic anhydrase, Energy & Fuels, 15: 309-316.

Bond, G. M., Medina, M. G. and Stringer J. (2001b) "A biomimetic CO₂ scrubber for safe and permanent CO₂ sequestration", Eighteenth Annual International Pittsburgh Coal Conference Proceeding (December 3-7, 2001, Newcastle, NSW, Australia).

Bond, G.M., Egeland, G., Branvold, D.K., Medina, M.G. and Stringer J. (1999) in

Mishra, B., editor, EPD Congress, TMS, Warrendale, 763-781.

Brown, R.S. in Aresta, M. and J. V. Schloss, editor; Enzymatic and Model

Carboxylation and Reduction for Carbon Dioxide Utilisation, NATO ASI Series, Series C: Mathematical and Physical Sciences, Vol. 314, Kluver Academic Publishers,

Dordrecht (1990), 145-180.

Budger, M.R. and Price, G.D. (1994) The role of carbonic anhydrase in photosynthesis,

Ann. Rev. Plant Physiol. Plant MoI. Biol. 45: 369-392.

Colleran, E. in "The Application of the Anaerobic Filter Design to Biogas Production from Solid and Liquid Agricultural Wastes", Proceedings of the Symposium on Energy

Biomass and Waste, Institute of Gas Technology, Chicago, 111. (1982).

Diekert, G., and G. Wohlfarth.(1994), Metabolism of homoacetogens. Antonie

Leeuwenhoek Int. J. Gen. Microbil. 66: 206-221. Drake, H. L., K. Kusel, and C. Matthies, Acetogenic prokaryotes. In M. Dworkin et al.

(eds). The prokaryotes, 3rd ed., Springer-Verlag, New York, N.Y. (2004).

EgIi, T. (1995), The ecological and physiological significance of the growth of heterotrophic microorganisms with mixtures of substrates, Adv. Micro. Ecol. 14: 305- 386.

EU Emission Trading Directive 2003/87/EC of the European Parliament and of the

Council of 13 October 2003 establishing a scheme for greenhouse gas emission allowance trading within the Community and amending Council Directive 96/6 I/EC.

Green, D.A., Turk B. S., Portzer, J. W., Gupta, R., McMichael, WJ., Liang, Y., Moore, T. and Harrison D.P., Carbon Dioxide Capture From Flue Gas Using Dry Regenerable

Sorbents, Quarterly Technical Progress Report, Research Triangle Institute, August

2003.

Klasson, K. et al, "Bioconversion of Synthesis Gas into Liquid or Gaseous Fuels",

Enzyme Microb. Technol., 14:602-608 (1992). Klasson, K. et al, "Biological Production of Liquid and Gaseous Fuels from Synthesis

Gas", Applied Biochemistry and Biotechnology, 24/25:857-873 (1990).

Lendenmann, U., M. Snozzi, and T. EgIi (1996), Kinetics of the simultaneous utilization of sugar mixtures by Escherichia coli in continuous culture, Appl. Environ.

Microbiol. 62: 1493-1499. Liu, D. M., Chen, I- Wei (2001), Encapsulation of biomaterials in porous glass-like matrices prepared via an aqueous colloidal sol-gel process, U.S. Pat. No. 6 303 290.

Maren, T. H. and Sanyal, G. (1983), The Activity of sulfonamides and anions against the carbonic anhydrase of animal, plants and bacteria, Ann.Rev. Pharmacol. Toxicol.

232: 439-459. Muller, V. (2003), Energy Conservation in Acetogenic Bacteria, App. Environ.

Microbiol. Vol. 69 No 11 : 6345-6353.

Seltemeyer D., Schmidt, C, Fock, H. P. (1993), Carbonic anhydrase in higher plants and aquatic microorganisms, Physiol. Plant 88: 179-190.

Schink, B. (1997), Energetics of the Syntrophic Cooperation in Methanogenic Degradation, Microbiology and Molecular Biology Review, Vol. 61 No 2: 262-280.

Simsek-Ege F.A., Bond; G.A., Stringer, J. (2002), Matrix molecular weight cut-off for encapsulation of carbonic anhydrase in polyelectrolyte beads, J. Biomater. Sci. Polym.

Ed 13: 1175-87. The Kyoto Protocol to the United Nations Framework Convention on Climate Change strengthens the international response to climate change. Adopted by consensus at the third session of the Conference of the Parties (COP3) in December 1997, it contains legally binding emissions targets for Annex I (developed) countries for the post-2000 period.

Vega, J. L. et al, "The Biological Production of Ethanol from Synthesis Gas", Applied Biochemistry and Biotechnology, 20/21 :781-797 (1989a).

Vega, J. L. et al, "Study of Gaseous Substrate Fermentations: Carbon Monoxide Conversion to Acetate. 2. Continuous Culture", Biotechnology and Bioengineering, 34:785-793 (1989b).

Water Research (1994),Vol.28, No.2, pp.475-478.

* rk * * A "k *

Claims

1. A process to capture CO₂ in a low CO₂ concentration gaseous flow, particularly in an exhaust gas outflowing from combustion chambers in incinerators or electric power, thermal power, steel, cement, paper, glass, ceramic plants or oil refineries, and use it to digest organic matter in order to produce methane, characterized by the fact of comprising the modules of:

A) generating an high CO₂ concentration gaseous flow from the above mentioned low CO₂ concentration gaseous flow, and

B) treating, in anaerobic environment, organic matter, using the above mentioned high CO₂ concentration gaseous flow, to produce methane.

2. The process, as claimed in claim 1, wherein module A contemplates that the CO₂ contained in said low CO₂ concentration gaseous flow be put into contact with an alkaline solution in the presence of a biocatalyst, in particular such alkaline solution is made out of minerals of an alkaline metal and more particularly of sodium minerals.

3. The process, as claimed in claim 1 or 2, wherein module A contemplates the precipitation of a salt of the carbonic acid, which is generated by using said low CO₂ concentration gaseous flow, such salt of carbonic acid being in particular an alkaline metal bicarbonate, more particularly sodium bicarbonate.

4. The process, as claimed in claim 1 or 2 or 3, wherein said module B is carried out through a microbial anaerobic digestion process.

5. The process, as claimed in claim 4, when it depends on claim 3, wherein said salt of the carbonic acid generated in said module A is also used in said module (B), in particular to control alkalinity or acidity in a methano genie zone.

6. The process, as claimed in anyone of claims from 1 to 5, comprising two modules wherein: a) the first module comprises all or some of the following stages: a.i) capture and separation of CO₂ contained in exhaust gas flow by means of an alkaline metal solution and more particularly of sodium, in the presence of a biocatalyst, leading to a conversion into carbonic acid; a.ii) precipitation of the carbonic acid obtained in stage a.i into a salt by adding alkaline metal carbonate and more particularly of sodium; a.iii) regeneration of the salt obtained in stage a.ii leading to the generation of a concentrated CO₂ gaseous flow; b) the second module comprises the use of CO₂ and of the carbonic acid salt obtained in the first module in an organic matter degrading system based on a microbiological anaerobic digestion process to produce methane.

7. The process, as claimed in claim 6, wherein: - in stage a.i the CO₂ removal from the exhaust gas takes place in an extraction zone and the biocatalyst is immobilized carbonic anhydrase; in stage a.ii the precipitation of CO₂ hydration resulting species is created by adding alkaline metal carbonate and more particularly of sodium in order to obtain an over saturated solution; - in stage a.iii the alkaline metal bicarbonate and more particularly of sodium precipitate is partially transferred into the second module and the production of a concentrated CO₂ gaseous flow, in the process of regenerating the salt, is realised by means of thermal treatment.

8. The process, as claimed in claim 6 or 7, wherein the second module is characterized by separate stages and includes all or some of the following sections: b.i) a preparation section where organic matter is made homogeneous, of appropriate size, suspended or dissolved in the liquid medium at the desired concentration; b.ii) a pre-acidification section, where the organic matter prepared in section b.i is combined with a liquid medium containing CO₂ coming from the first module and where mixed anaerobic bacterial populations selected to produce acid effluent are maintained; b.iii) a fermentation section, where high density mixed anaerobic bacterial populations, selected to produce acetates, are maintained and where the acid effluent coming from section b.ii is transferred to continue its hydrolyses accompanied by acidification and aceto genesis, in section b.iii the organic matter being suspended or solubilised and into the liquid medium more CO₂ coming from the first module is introduced; b.iv) a separation section where the solids, contained in the effluent coming from the fermentation reactor, are separated and the liquid component, containing the produced fatty acids is sent to the following section b.v; b.v) a methanation section, where the liquid component resulting from section b.iv is sent to a methanation reactor, where high density mixed bacterial populations, selected to produce methane, are maintained and where a high methane content gaseous effluent is obtained on top of the digested part out of which, after separation, the solid component is recycled into the same reactor, while liquid component is recycled into the fermentation reactor and/or disposed of.

9. The process, as claimed in anyone of claims from 6 to 8, wherein in the CO₂ extraction zone the exhaust gas is put into contact with an alkaline metal solution and more particularly of sodium which, at the point of entry, has pH value between 8.3 and 9.6; temperature in the contact and biocatalyzed CO₂ solubilisation reactor is maintained between 35 and 60°C and the solution at the point of exit from the capture reactor has a pH value between 7.5 and 8.3.

10. The process, as claimed in anyone of claims from 6 to 9, wherein in the precipitation zone of alkaline metal bicarbonate and more particularly of sodium and the other species resulting from the CO₂ biocatalyzed hydration, alkaline metal carbonate and more particularly of sodium is added to the solution, coming from the extraction zone, until an over saturated alkaline metal bicarbonate solution and more particularly of sodium is obtained and, specifically, is set to have a bicarbonate percentage content between 90 and 100% and a pH value between 9 and 9.6.

11. The process, as claimed in anyone of claims from 6 to 10, wherein the alkaline metal bicarbonate and more particularly of sodium, harvested in the precipitation zone, having a bicarbonate percentage content between 90 and 100%, is introduced into the second module methanation zone in order to maintain a pH value between 7.2 and 9 and to increase methane concentration in the biogas.

12. The process, as claimed in anyone of claims from 6 to 11, wherein CO₂ capture takes place in liquid phase, the harvested alkaline metal bicarbonate and more particularly of sodium is regenerated to carbonate in solid phase and a concentrated CO₂ gaseous flow, together with steam, is liberated by means of thermal treatment at a temperature ranging between 120 and 14O⁰C.

13. The process, as claimed in anyone of claims from 6 to 12, wherein in the liquid medium of the pre-acidification zone, are found and/or introduced mixed high density hydrolytic bacterial populations, ordinarily present in hydrolytic biological process, capable of carrying out acid hydrolysis in environmental conditions, defined by a pH value between 3 and 7, preferably between 4 and 6, by temperature ranging between 20 and 80 °C, by an hydraulic retention time ranging between 6 hours and 2 days and a solids retention time between 1 and 3 days.

14. The process, as claimed in anyone of claims from 6 to 13, wherein in the liquid medium of the pre-acidifϊcation zone, are found and/or introduced mixed high density hydrolytic bacterial populations, present in the long run equilibrium composition generated by selection/ adaptation, capable of carrying out acid hydrolysis in environmental conditions, defined by a pH value between 3 and 7, preferably between 3 and 5, by temperature ranging between 20 and 80°C, by an hydraulic retention time ranging between 1 hours and 2 days and a solids retention time between 6 hours and 3 days.

15. The process, as claimed in claim 13 or 14, wherein the bacterial populations are selected from the group consisting of or comprising Anaerovibrio, Bacillus, Batteroides, Butyrivibrio , Clostridium and Rumino coccus.

16. The process, as claimed in anyone of claims from 6 to 15, wherein in the liquid medium of the fermentation reactor are found and/or introduced high density mixed acidogenic and acetogenic bacterial populations, ordinarily present in anaerobic process, performing in environmental conditions defined by a total organic load between 3 and 150 kg per cubic meter of reactor per day, a pH value between 4.3 and 6.3, a temperature between 30 and 60°C, an hydraulic retention time between 12 hours and 3 days and a solids retention time between 1 and 7 days.

17. The process, as claimed in anyone of claims from 6 to 16, wherein in the liquid medium of the fermentation reactor are found and/or introduced high density mixed acidogenic and acetogenic bacterial populations, present in the long run equilibrium composition generated by selection/adaptation, performing in anaerobic environmental conditions defined by a total organic load between 3 and 250 kg per cubic meter of reactor per day, a pH value between 4 and 6, a temperature between 30 and 6O⁰C, an hydraulic retention time between 8 hours and 3 days and a solids retention time between 1 and 7 days.

18. The process, as claimed in claim 16 or 17, wherein the bacterial populations are selected from the group consisting of or comprising Clostridium, Desulfovibrio,

Ruminococcus, Selonomonas, Syntrophobacter, Syntrophomonas, Acetobacterium, Acetogenium and Eubacterium.

19. The process, as claimed in anyone of claims from 6 to 18, wherein in the liquid medium of the methanation reactor are found and/or introduced high density mixed methanogenic microbial populations, ordinarily present in anaerobic process, performing in environmental conditions defined by a total organic load between 1 and 70 kg per cubic meter of reactor per day, a pH value between 7.2 and 9.0, a temperature between 30 and 60°C, an hydraulic retention time between 4 and 12 days and a solids retention time between 8 and 20 days.

20. The process, as claimed in anyone of claims from 6 to 19, wherein in the liquid medium of the methanation reactor are found and/or introduced high density mixed methanogenic microbial populations, present in the long run equilibrium composition generated by selection/adaptation, performing in environmental conditions defined by a total organic load between 1 and 100 kg per cubic meter of reactor per day, a pH value between 7.2 and 9.0, a temperature between 30 and 6O⁰C, an hydraulic retention time between 2 and 12 days and a solids retention time between 5 and 20 days.

21. The process, as claimed in claim 19 or 20, wherein the bacterial populations are selected from the group consisting of or comprising Metanosarcina, Metanotrix, Metanobacterium and Metano coccus.

22. The process, as claimed in anyone of claims from 1 to 21, wherein all or some of the following parameters are monitored and/or regulated: - gas production measured in cubic meters per cubic meter of reactor per day;

- methane percentage content in produced gas;

- volatile fatty acids content;

- pre-acidifϊcation reactor pH value;

- fermentation reactor pH value; - methanation reactor pH value.

23. The process, as claimed in anyone of claims from 1 to 22, wherein the fatty acid concentration and/or the pH value in a or each reactor used for the process are allowed to vary between reactor specific predetermined limits and preferably the pH regulation is carried out by using as only chemical agents CO₂ and sodium bicarbonate coming from the first module and/or regulating the substrate flows to and between reactors.

24. A system to capture CO₂ in a low CO₂ concentration gaseous flow, particularly in an exhaust gas outflowing from combustion chambers in incinerators or electric power, thermal power, steel, cement, paper, glass, ceramic plants or oil refineries, and use it to digest organic matter in order to produce methane, characterized by being capable of realizing the process described in one or more of the preceding claims.

25. The system, as claimed in claim 24, wherein said module A takes place in a first plant and said module B takes place in a second plant.

26. The system, as claimed in claim 25, wherein said first and second plants may be located far apart.

27. The system, as claimed in claim 26, wherein said second plant receives the low CO₂ concentration gaseous flow from said first plant particularly through a pipeline.

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