WO2008099252A1

WO2008099252A1 - Conversion of co2 captured from combustion systems or other industrial processes into methane through anaerobic digestion combined with biomasses

Info

Publication number: WO2008099252A1
Application number: PCT/IB2008/000288
Authority: WO
Inventors: Cesarino Salomoni; Enrico Petazzoni
Original assignee: Busi Impianti S.P.A.
Priority date: 2007-02-14
Filing date: 2008-02-09
Publication date: 2008-08-21
Also published as: ITMI20070267A1

Abstract

A quick method for converting CO2 into methane is provided. According to this method, CO2 is first combined with suitably formulated and treated biomasses according to predefined ratios and then subjected to an optimized anaerobic digestion process based on bacterial populations of specially selected high-density acetogenic and methanogenic microorganisms. The method described herein integrates four modules: a first module adapted to prearrange a concentrated CO2 flow; a second module for preparing, by using industrial methods, nutritional substrates obtained from cocktails of highly degradable, low-cost vegetable and animal matrices; a third module for preparing, by using agro-industrial methods, alcoholic substrates from low-cost vegetable matrices having a high carbohydrate content; a fourth module for carrying out the industrial conversion of the CO2 obtained from the first module into acetates and subsequently into methane, by combining it according to accurately defined ratios with optimal nutritional and alcoholic substrates respectively obtained from the second and third modules and then by subjecting said combination to a totally engineered anaerobic digestion process. The proposed method is very useful for industries emitting greenhouse gases as well as for any plants involved in the production of energy from biomasses or in the disposal of organic waste.

Description

TITLE

CONVERSION OF CO₂ CAPTURED FROM COMBUSTION SYSTEMS OR OTHER INDUSTRIAL PROCESSES INTO METHANE THROUGH ANAEROBIC DIGESTION

COMBINED WITH BIOMASSES DESCRIPTION

FIELD OF APPLICATION

The present invention concerns four application fields:

- industries having to comply with CO₂ emission regulations; production of energy from renewable sources; waste disposal industry; agricultural production.

Accordingly, it pursues four objects:

- reducing the emission of greenhouse gases into the atmosphere;

- producing biogas/energy from CO₂ and biomasses;

- recycling organic waste, as opposed to landfill disposal; using agro-energetic cultures and agricultural residues.

BACKGROUND ART

The increasing demand for energy throughout the world has highlighted several problems: in the foreseeable future, the technologies and raw materials required for large-scale energy production will still be the current ones based on the combustion of fossil fuels.

Unfortunately, the certainty of an adequate availability of such sources for every country has become a delicate and even strategic matter; the same can be said for the problem of the cost thereof on account of their ever increasing prices.

On the other hand, general climatic considerations as well as international agreements have set strict limitations to the emission of greenhouse gases thus generated.

Moreover, generalized social behaviours characterized by the cultural syndrome known as NIMBY (Not In My Back Yard) make it difficult to implement most of the actions so far commonly recommended for facing the energetic and environmental issues taken into consideration herein.

Also, for the present time the only renewable source which could potentially replace the traditional ones in large amounts, as is absolutely necessary for modern production plants, e.g. power plants, is liquid phase biomass, such as palm oil, colza oil, sunflower oil, etc., due to its continuous availability and concentrated energy content. However, economical and cultivable surface constraints make such a prospect unfeasible. Theoretically, biomass transformed into biogas could also provide the same result, but in such a case the above constraints would be complemented by another one related to the limits imposed by an anaerobic digestion process still lacking those technologic innovations which would make it really effective. The same can be said for the present state of development of pyrolisis and syngas. Wholly different considerations relate to the nuclear perspective, since opinions are quite different in this regard: on the one hand, there are people who think that nuclear plants are only one of the bad things necessary for achieving a definitive solution to the problem of large-scale energy production, while on the other hand there are also people who think that safety aspects take absolute priority and hinder any such perspective. Both parties however agree that the times and costs involved in such a choice will make it feasible only in the future. Therefore, as long as the available possibilities remain limited to the current ones, there is still an irresolvable contraposition between indispensable fossil sources and inadequate non-polluting or renewable sources. Among other things, such a contraposition is bound to limit the development of the latter.

Finally, underground CO₂ capture and sequestration carried out in fixed combustion systems is certainly a first and so far perhaps the only interesting perspective for solving only the environmental issues among all of the above-mentioned problems. Notwithstanding this limitation, this solution still remains insufficient and vulnerable from an economical and regulatory viewpoint as concerns the risks caused by the "NIMBY syndrome".

For these reasons, it is provided herein a wholly alternative industrial technology which is capable of facing all economic, environmental, energetic and cultural aspects at once. The most important feature of this technology is its ability of potentially recycling all the CO₂ generated by combustion and then captured into a quantity of methane equal to that burned in fixed plants, even large ones. The conversion of unlimited masses of said CO₂ into methane with procedures, times and plant dimensions which are typical of an industrial application is described for the first time in the present patent. This methane is produced during an anaerobic digestion process fed with humid biomass. As mentioned, said process is very fast and compact because it incorporates advanced biotechnological innovations. It follows that this renewable energy source can be alternative to fossil sources when used within the same plant where the capture/transformation processes are carried out, or it can be complementary to fossil sources when, after conversion, it is sent to domestic users or small to medium businesses, or for automotive use, and in general to users who do not carry out the gas capture process.

Thus, unlike underground sequestration, this approach offers a sharp reduction in the CO₂ being present in the atmosphere, while at the same time overcoming any technical or regulatory obstacles related to long-term sequestration and implying minimized costs as well as maximized profits. Furthermore, its possible use along with fossil sources, especially coal, promotes its generalized development and application. More particularly, when viewing this approach as a basis for producing energy in general without being limited to large plants, and placing it into the most appropriate context, i.e. biomass energy production, it can be noted that, in a first comparison with the case of solid biomass combustion, the lower investment costs it requires as well as the easier-to-find, simpler and cheaper humid biomass compared to dry biomass lead to results being much more interesting for energy producers and agricultural/agro- industrial operators, for whom the disposal of surplus, waste, machining residue, waste waters, etc. will no longer be a cost, but an additional profit to be combined with the profit opportunity offered by energetic cultures currently boosted by the drastic restructuring of the European Union's agricultural policy. Similarly, the value of the process residue thus produced will also be increased considerably. In addition, if we consider that in this case it is also possible to use a part of the urban solid waste in order to cut down supply costs and to solve a serious environmental problem, as currently attempted by using waste-to-energy plants, it must be pointed out that anaerobic digestion creates no social alarm, whereas waste combustion stimulates it more than any other activity in the energy industry.

In conclusion on this matter, it is worth noting that, although a decentralized distribution of variously sized digesters over the territory allows to minimize transportation and logistic costs, the methane thus produced can be used at the single digesters themselves or, when all digesters are connected to an integrated gas distribution network, it may also be used for supplying large or very large plants, as would never be possible by using solid combustible biomass due to plain physical reasons. In a second comparison with the case of second-generation ethanol production, which represents the most important revolution in biomass utilization which can be expected to take place in the not-too-far future, it must be pointed out that, according to such a prospect, only high carbohydrate content materials can be used for this purpose, unlike those lipid and/or protein rich materials as can be found, for example, in food industry residues or in the organic fraction of solid urban waste, and that neither can any CO₂ produced be recycled; for the most part, the final product must be distilled to a very high purity degree and then esterified for use in existing plants; in any case, the process will involve a very high energy consumption. None of these limitations applies to the technology disclosed by the present patent.

A third comparison between this approach and the most innovative methods among those currently available, though seldom used, for anaerobic biomass digestion highlights the quality jump represented by the decision to: (a) prepare biomass combinations appropriately selected from very assorted biomass types as opposed to randomly collected biomass; (b) pre-treat said biomass specifically in distinct manners, all of which incorporate or re-invent some biotechnologically advanced discoveries among the wide range of recent findings in various fields, instead of using generic matrices; (c) organize a raw material and semi-finished product supply and storage system, even located in the countryside, to solve the serious season problems that can normally jeopardize such operations; (d) use specialized microorganisms for the different process phases in order to create optimal environmental, nutritional and, most of all, stimulating conditions, instead of those conditions simply provided by the presence of solid organic material to be digested; (e) and use CO₂ as a main raw material, producing methane quickly therefrom with the aid of bacteria feeding on biomass, as opposed to producing methane slowly by starting from biomass, with the contribution of CO₂ in order to set up a suitable environment.

For all these reasons, it is apparent that the present approach allows to overcome the basic limitations of traditional anaerobic digestion, i.e.: those related to the availability of biomass and raw materials in general, to process times, to plant sizes and, therefore, to the quantity of methane which can potentially be produced. In short, the economical advantage of this approach is incomparably superior.

US patents no. 4,022,665 and 4,696,746 and patent application no. WO2006108532, describe some of the best systems available in this field: in the former two patents, which are strongly interconnected, it can be noticed that, besides using selected bacteria populations, dividing the process into specialized stages and providing optimal operational/environmental conditions for each stage, as in the most advanced solutions, it finally accomplishes the most important innovative step of recirculating the CO₂ obtained as a by-product of anaerobic digestion. However, this is also the very limit of this technology: it produces methane from biomass and uses process-endogenous CO₂ in order to make the process more effective: the total quantity of methane produced is therefore limited by the quantity of degraded organic material as well as by the quantity of CO₂ naturally produced during the digestion process.

A partially similar critical comment can be made with respect to patent application no. WO2006108532: as in the above case, an effort is made to specify the optimal general conditions, which are by now widely known, for setting up an anaerobic digestion process, with particular reference to depuration sludge and to the organic fraction of solid urban waste. Moreover, also in this case CO₂ is used in an attempt to push the process to its limits; however, unlike the above case, the CO₂ used is not limited to the quantity obtained as a digestion by-product; on the contrary, it is introduced exogenously after having been captured from combustion systems, and the quantity thereof is solely limited by the requirements of the existing bacterial populations, which in this case as well are used for degrading organic materials in order to produce methane. Even after having overcome the limit of CO₂ availability, there is still a problem to be solved pertaining to solid matrices and the digestion speed thereof. Patent application no. WO2006108532 also proposes, though in a laborious or hardly feasible manner, to adjust the pH value in the anaerobic system by using bicarbonate obtained during the capture operations in addition to CO₂. On the contrary, in the present invention the pH value adjustment is carried out in a much simpler way by using CO₂ only and/or by regulating the substrate flows. Thus, because of the stability of the new and much more deeply structured process, bicarbonate is no longer required, resulting in economical and operational advantages.

The general picture of the systems found in the literature which at least partly deal with the same technology as the present patent is completed by mentioning US patent no. 6,664,101 and DE patent no. 4230644: along with the long-term sequestration approach, the former patent proposes to introduce the large quantities of CO₂ captured from big combustion plants into underground geologic formations; unlike simple sequestration, it also proposes to convert said CO₂ into methane during an anaerobic digestion process carried out by introducing into such spaces industrial substrates such as hydrogen, ammonia, acetates, methanol and formato, combined with methanogenic bacterial masses.

The limitations of this approach are apparent: two borderline cases can be distinguished. The first case refers to a deep geologic formation with no further specifications except those related to CO₂ containment, conditions which are not themselves frequently fulfilled: in this case, in addition to the morphology of these articulated, extended and almost inaccessible cavities being substantially unknown, one must also take into consideration the presence of any pits, vaults, gradients creating humid and dry areas, rocks rich in favourable or unfavourable bacteria, etc.; such conditions, which include a great variety of microenvironments, will imply a huge waste of nutritional substances and bacteria, as well as a large quantity of unconverted CO₂ and very long process times. The opposite case, which is so special as to be virtually inexistent, specifies hydraulic, biological and other conditions which are suitable for hosting the process, which anyway utilizes the above-mentioned expensive substrates together with unselected bacterial masses. Any real site lies necessarily between these two extremes, most likely much closer to the former than to the latter; it is therefore impossible to establish an actual control over the hosted process, e.g. as regards the operating conditions or the simple distribution of bacteria and substrates. This leads to the obvious consequence that the input quantities used, the quantity of methane produced, and the conversion process times are those of a semi-natural system as opposed to an industrial one, thus also implying that costs cannot be controlled by any means.

Referring now to DE patent no. 4230644, it should be stated beforehand that it relates to the particular case of methane conversion of CO₂ produced by a gasified coal burning plant.

In fact, should it be applied to cases of combustion in the presence of air, the described gas flow, wherein CO₂ is the predominant component, would be replaced by much greater flows, wherein the dominant gas, in a quantity five times greater for coal and twenty- five times greater for methane, would be nitrogen; this would involve unfeasible huge systems and very high energy costs, apart from a gaseous output which, unlike the one described in said patent, would not consist of CH₄ and CO₂ only, since these substances would be dispersed into great quantities of nitrogen.

Notwithstanding this limitation, it should be noted that this patent is the one which is closer than any other patent to the technology proposed herein; the most important differentiating features will be compared below. Said features are all related to three aspects of the optimization work: in the first place, the nutritional substances used in the cited patent for feeding the bacteria are of the expensive industrial type, as opposed to those used in the present proposal, which are derived from biomass, i.e. cheap; in the second place, the single-stage configuration of DE patent no. 4230644, which is similar to the one commonly used in traditional biomass digestion, is much less effective than the multi-stage configuration which is typical of the modern technology illustrated herein, thus implying that a much larger quantity of substrates is required; finally, this very lack of separation between the acetogenic fermentation stage and the methane production stage involves a slower overall speed of the conversion process, thus requiring bigger plants and higher energy consumption. In conclusion, it is a much more expensive way to obtain the same result. SUMMARY OF THE INVENTION

The main and general object of the present invention is to provide a new method for converting into methane the CO₂ being present in a high CO₂ content flow generally derived from a low CO₂ concentration emission, said CO₂ being typically previously captured from combustion systems or other plants used for industrial processes. Said object is achieved through the process incorporating the features set out in the appended claims, which are intended as an integral part of the present description. According to further aspects, the present invention also relates to plants adapted to carry out said process.

In particular, according to the present invention, CO₂ is combined according to predefined ratios with suitably formulated and pre-treated biomass in order to subject the resulting combination to an optimized anaerobic fermentation process based on specially selected bacterial populations.

The reason for such an approach lies in the necessity of creating industrial methods which, instead of separating and sequestrating underground CO₂ in expensive and environmental-unfriendly manners, can economically recycle it into a valuable energetic product having a large market, while at the same time having favourable consequences on the environment. According to a typical implementation of the present invention, CO₂ is captured and separated from a flue gas flow coming in particular from combustion chambers of fixed plants such as, for example, incinerators, electric or thermal power plants, steel mills, cement works, glassworks, ceramics works, oil refineries, etc. (reference is made herein to the most common type of combustion, i.e. the one which uses air as an oxidant), or from other industrial processes. Said CO₂ is then converted into methane during an industrial multi-stage anaerobic digestion process which uses specialized bacterial populations of acetogenic and methanogenic microorganisms kept at high density and fed with nutritional and alcoholic substrates as well as stimulators. Said substrates are obtained from cocktails of matrices selected among a range which comprises, preferably in multiple combinations, ad hoc agricultural and aquatic cultures, garden and forest prunings, slaughter house wastes, agricultural and agro-industrial wastes, animal sewage, depuration sludge, organic fraction of solid urban refuse, and agro-industrial semi-finished products, all of which are specifically pre-treated according to industrial and/or agro-industrial methods.

The proposed process turns out to be more effective and require lower investment and running costs compared with prior-art processes; the term "prior art" is used herein to designate any of the best existing processes for anaerobic CO₂ conversion into methane among the following: the process is started from flows of flue gases produced by the combustion of gasified coal (i.e. combustion in the absence of N₂), by using expensive industrial substrates in a low-efficiency single-stage system in the presence of unselected bacterial populations (DE patent no. 4230644); the conversion is carried out in large natural environments wherein CO₂ is stored and very expensive industrial substrates (such as hydrogen, ammonia, acetates, methanol and formato) and large, expensive bacterial masses are artificially introduced, without any possibility of controlling the process because of the widely unknown morphology of these environments and of the inaccessibility thereof, i.e. in the absence of the minimum conditions required for obtaining an effective and low-cost process (US patent no. 6,664,101); only the small quantity of endogenous CO₂ formed as a by-product of biomass anaerobic digestion is recycled, thus limiting methane production without even tackling the problem of recycling the CO₂ captured from combustion effluents (US patents no. 4,022,665 and 4,696,746); a large quantity of CO₂ captured from flue gases is used for widely improving the degradation of organic waste material, resulting in the methane production being strictly limited by biomass quantity and digestion time, i.e. not related to the very large mass of available CO₂ and to its very short conversion time (patent application no. WO2006108532).

According to the present invention, the above-described drawbacks are overcome through a process which essentially comprises four phases (see claim 1) and through a suitable system (see claim 30) which combines four modules in an integrated manner. In particular, the process and the system comprise the following modules/phases:

A) a first module/phase adapted to prepare a concentrated CO₂ flow, which typically captures and separates CO₂ from a diluted CO₂ flow, thus producing a concentrated CO₂ flow;

B) a second module/phase for preparing, by using industrial methods, nutritional substrates obtained from cocktails of highly degradable vegetable and animal matrices;

C) a third module/phase for preparing, by using agro-industrial methods, alcohol-rich substrates from high carbohydrate content vegetable matrices;

D) a fourth module/phase for the industrial conversion, first into acetates and then into methane, of the CO₂ captured in the first module/phase, by combining it according to predefined ratios with optimal nutritional and alcoholic substrates obtained from biomass suitably formulated and pre-treated in the second and third modules/phases, respectively, and then subjecting said combination to an optimized anaerobic fermentation process based on specially selected bacterial populations of high-density acetogenic and niethanogenic microorganisms.

In particular, in the embodiment example described below the first module/phase comprises the following stages:

- CO₂ removal from flue gas in an extraction zone, by means of an alkaline solution of sodium minerals and CO₂ dissolution and hydration catalyzed by immobilized carbonic anhydrase;

- precipitation by sodium carbonate of the species resulting from CO₂ hydration and obtainment of an oversaturated sodium bicarbonate solution;

- removal of the sodium bicarbonate precipitate from the solution, regeneration thereof through heat treatment and obtainment of a concentrated flow of gaseous CO₂. In particular, the second module/phase comprises different combinations of the following stages depending on the nature of the available matrices:

- supply of organic material cocktails having the desired nutritional characteristics by starting from fresh, ensiled or semi-finished vegetable and animal matrices selected among a range which comprises, necessarily in multiple combinations, ad hoc agricultural and aquatic cultures, agricultural and agro-industrial wastes, slaughter house wastes, animal sewage, depuration sludge, organic fraction of solid urban refuse, agro-industrial semi-finished products, etc., thus obtaining large quantities of easily available raw materials for a low-cost industrial process;

- detailed preparation in accordance with chemical-physical methods of cocktails of vegetable and animal nutritional matrices having the desired characteristics in terms of homogeneity, size, solid content and carbohydrate, protein, lipid, macroelement and microelement composition, for the purpose of making the substrates extremely effective in feeding microorganisms;

- moderate heat and pressure pre-treatment of the biomass cocktails through injection of gaseous CO₂ coming from the first module/phase, the pressure being quickly reduced after a short pre-programmed time, thus sanifying said biomass cocktails and making them more susceptible to the subsequent acid enzymatic hydrolysis;

- acid enzymatic hydrolysis of the nutritional cocktails in a CO₂-saturated environment through multi-enzymatic complexes (cellulase, amylase, protease and lipase) and/or mixed hydrolytic and cellulolitic bacterial populations kept at high density, which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Clostridium sp., Pseudomonas sp. and Bacillus sp.; as a result, the conversion of polymers (polysaccharides, proteins, nucleic acids and lipids) into oligomers and monomers (sugars, amino-acids, purines, pyrimidines, fatty acids and glycerol, etc.) is sped up significantly;

- separation of solids, substrate particles and microorganisms being present in the acid hydrolyzed effluent by using any of the various techniques known in the art, followed by recirculation for an increased retention thereof, and output of the liquid component to the fourth module/phase.

In particular, the third module/phase comprises different combinations of the following stages depending on the nature of the available matrices;

- supply of fresh, ensiled or semi-finished vegetable material having the desired energetic characteristics, represented by matrices rich in carbohydrates (sugars, starch, hemicelhαlose and cellulose), selected among a range which comprises ad hoc agricultural and aquatic cultures, agricultural, garden and forest prunings, agro- industrial and industrial wastes, semi-finished agro-industrial products, etc., resulting also in this case in the supply of large quantities of easily available raw materials for the implementation of a low-cost industrial process;

- detailed preparation in accordance with biological and chemical- physical methods of vegetable matrices providing the desired characteristics of homogeneity, size, solid content, high carbohydrate content and low protein content, thus making the use of low- cost substrates for the production of alcoholic hydrolyzed products extremely effective;

- treatment of high carbohydrate content matrices mainly consisting of substances having a heavy molecular weight, such as hemicellulose, cellulose and lignin, by using multi-enzymatic ligninolitic complexes (especially phenoloxidasi) and/or through mixed inoculation of populations of microorganisms which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Basidiomycetes, Pseudomonas sp. and Actinomycetes sp.; as a result, any pollutants and lignin are removed, thus producing a cellulosic material which is more susceptible to the subsequent saccharification step;

- saccharification of high carbohydrate content matrices mainly consisting of substances having a heavy molecular weight, such as cellulose, hemicellulose, and rich in other polysaccharides, through multi-enzymatic complexes (cellulase) and/or a plurality of populations of microorganisms which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Clostridium sp., Trichoderma sp. and Pseudomonas sp., in order to obtain a more effective production of oligosaccharides and free sugars;

- alcoholic fermentation of high carbohydrate content matrices mainly consisting of oligosaccharides and free sugars through populations of microorganisms which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Saccaromyces sp., Zymomonas sp. and Chalara sp., resulting in the obtainment of a high alcohol content hydrolyzed product;

- separation of solids, substrate particles and microorganisms being present in the alcoholic hydrolyzed effluent by using any of the various techniques known in the art, followed by recirculation for an increased retention thereof, and delivery of the liquid component to the fourth module.

In particular, the fourth module/phase comprises the following stages:

- distribution of CO₂, nutritional hydrolyzed product and alcoholic hydrolyzed product obtained from the first, second and third modules/phases, respectively, to an acetogenic digester, thereby attaining the result of providing the microorganism cultures with the necessary environment and substrates, which are readily available for implementing an extremely fast fermentation process;

- fermentation of CO₂ and nutritional and alcoholic hydrolyzed products, integrated with stimulators, into a digester containing mixed acetogenic bacterial populations kept at high density in an acid environment, thereby obtaining the result of making the production of large quantities of acetates extremely economical and fast;

- separation of substrate particles and microorganisms from the effluent of the acetogenic fermenter by using any of the various techniques known in the art, followed by recirculation for an increased retention thereof; delivery of the liquid component to the next methane production digester for the purpose of providing a rich feeding material mainly consisting of acetates;

- delivery of gas obtained through acetogenic fermentation to the methane production digester in order to integrate the acetate supply with hydrogen and CO₂ supply;

- methane production in a dedicated digester from the liquid and gaseous effluents, integrated with stimulators, coming from the aceto genesis reactor, conducted by using selected mixed methanogenic bacterial populations kept at high density in order to obtain a more effective production of a large quantity of methane from acetates, CO₂ and hydrogen;

- removal of biogas from the methane production reactor for its collection and utilization;

- separation of substrate particles and microorganisms being present in the effluent of the methane production reactor by using any of the various techniques known in the art, followed by recirculation for an increased retention thereof, and delivery of the liquid component for depuration/use.

The adjustment of the pH value in the fermentation reactor and in the methane production reactor is carried out by using CO₂ captured in the first module/phase as an exclusive chemical agent, and/or by regulating the substrate flows to and/or among the reactors; due to the stability of this process, no bicarbonate needs to be used, whether produced in the first module/phase or supplied by an external source. By mixed bacterial populations it is meant a culture of two or more aceto genie and methanogenic anaerobic microbial populations which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, as far as acetogenic populations are concerned, by Butyribacterium sp., Eubacterium sp, Clostridium sp., Ruminococcus sp. and Morella sp., or, as far as methanogenic populations are concerned, by Methanosarcina sp., Methanosaeta sp., Methano coccus sp. and Metanobacterium sp.; these populations are isolated by ecosystems comprising: waste waters, anaerobic digesters, natural sediments, termite intestine, rumen, non- ruminant intestinal tract; or else coming from extreme environments in terms of pH value, salinity and temperature; or, finally, found in collections of microbial cultures, kept by scientific institutions or biotechnological firms.

The most important new and unique element in the present invention, identified by the above-described four modules, rests with the integration of said processes, which has never before been suggested. The possibility of capturing and separating large quantities of CO₂ and of converting it into methane by combining it according to predefined ratios during an efficient anaerobic process with optimal nutritional and alcoholic substrates prevalently in solution, obtained from selected cocktails of low- cost organic matrices and from low-cost treatment processes, is very useful for industries emitting greenhouse gases as well as for any plants involved in the production of energy from biomasses or in the disposal of organic waste. First module/phase

In general, according to the present invention the first module/phase can be provided in many different ways, some of which are extremely simple and/or already known in the art.

However, as aforementioned, in the embodiment example described herein it is also an object of the present invention to provide and use a process which can capture the CO₂ being present in a flue gas and generate a concentrated flow of gaseous CO₂. In the embodiment example described herein, a characteristic feature of the invention is the elimination, in the first module/phase, of the need to operate in very caustic conditions while maintaining a quick and efficient CO₂ capture by means of a combination of an absorption process and a chemical reaction in the presence of a solution of alkaline metals supplemented by an immobilized biocatalyst, i.e. carbonic anhydrase, which promotes CO₂ hydration. The advantages of this new process are low investment and running costs and better environmental safety.

In the embodiment example described herein, it is a further object of the present invention to provide a CO₂ capture and separation process which can significantly contain the circulating solution volume and the energy consumption associated with alkaline agent regeneration. Said object is attained in the first module/phase by maintaining an efficient CO₂ capture in liquid phase while running the alkaline agent regeneration in solid phase in small plants and at relatively low temperatures. In the embodiment example described herein, it is another object of the present invention to provide a CO₂ capture and separation process which can significantly contain biocatalyst and alkaline agent degradation. This problem can be solved, for example, by retrofitting the CO₂ capture module/phase to plants used for removing traditional pollutants (NOx, particulate, SOx) and by using temperatures which may be compatible with the management of the chemical and biotechnological processes contemplated in the present invention. The advantage so achieved is that, in such a condition, the biocatalyst and the alkaline agent last for a long time and thus their cost per ton of captured CO₂ is correspondingly reduced. Second module/phase

It is a further object of the present invention to provide integrated processes for the production of hydrolyzed nutritional substrates specially formulated for properly maintaining and growing acetogenic and methanogenic bacterial populations specifically selected for reductive environments rich in dissolved and gaseous CO₂. Said processes comprise different combinations of biological and/or chemical-physical stages among those listed above in the description of the second module, as are needed from time to time depending on the nature of the available vegetable and animal matrices (fresh, ensiled or semi-manufactured). This approach, based on a wide range of raw matrices and on various specific storage and treatment methods, allows to overcome any cost and seasonal problems which are typical of these tasks. Third module/phase

It is a further object of the present invention to provide integrated processes for the production of hydrolyzed alcoholic substrates capable of maximizing, when supplied to those populations specified for the fourth module/phase, the negative oxido-reductive potential needed by the anaerobic CO₂ conversion process; this is an extremely critical factor for speeding up said process. In this case as well, the wide range of raw matrices and the various specific treatments to which said matrices are subjected allow to minimize the costs of these tasks. In addition, these features allow to solve the above- mentioned seasonal problems which, as far as the alcoholic hydrolyzed product is concerned, are already limited by the possibility of storing said product as a final product of this module/phase. It should also be added that some of the processes mentioned herein take place at farms' level, where costs are lower. Fourth module/phase

In the fourth module/phase, which is the one intended for actual methane production, this object is achieved by the present invention through a new process which uses large quantities of CO₂ from the first module/phase and nutritional and alcoholic substrates obtained in the second and third modules/phases, thus specializing the mixed populations of microorganisms being present in or added to the anaerobic digestion system and providing an environmental conditioning/selection thereof. These bacterial populations are capable of providing a high rate of conversion of CO₂ into acetates during the acetogenesis stage and a high rate of conversion of acetates, CO₂ and hydrogen into methane during the methano genesis stage. The key elements, which are combined together to create the unique, innovative features of this process, allow to obtain a fast and effective CO₂ conversion thanks to the creation of the most favourable conditions for acetogenesis and methano genesis, i.e.: a large quantity of exogenous CO₂ introduced and dissolved in the fermenter and made readily available to acetogenic and methanogenic populations; acid pH value created within the fermenter, which can inhibit the methanogenic microorganisms and block the pathway for methane conversion of CO₂ and hydrogen in this reactor. optimal quantity and composition of the nutritional substrates, integrated with stimulators, which are supplied to the mixed bacterial populations, combined with the negative oxido-reductive potential created in the CO₂ fermentation reactor when introducing the alcoholic hydrolyzed product used as a reductive agent; greater availability, in the methane production reactor, of acetates and hydrogen coming from the previous acetogenic stage, integrated with nutritional and alcoholic substrates coming from the other two modules. The industrial application of the present invention is essentially feasible in two different basic versions.

In the first version, which can be defined as "closed loop", substantially all of the CO₂ produced and captured in a fixed methane combustion plant is transformed into methane through processes fed with biomasses (which supplies the required energy), thus generating a quantity of methane which is substantially equal to the quantity of methane which has to be burnt in that very same plant.

In the second version, which can be defined as "open loop", the CO₂ produced in a fixed plant using any kind of fossil fuel is transformed into methane through a biochemical biomass fermentation process, and said methane is then sent to decentralized users not involved in CO₂ capture.

In both cases, reference is typically made to the most common type of combustion, i.e. the one which uses air as an oxidant. The second version also applies to the case of CO₂ released by typical industrial processes, e.g. cement works or oil refineries.

Finally, the integrated technologic components proposed herein represent an engineered system wherein dimensions, speeds, costs and process control are optimized both separately and as a whole.

The way in which the present invention can be implemented by those skilled in the art will be made clear by the following description and drawings. Said description and drawings refer to a particularly advantageous, but non-limiting, embodiment example.

BRIEF DESCRIPTION OF THE 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/phase of the process of Fig. 1 : the drawing shows the functional structure and the equipment used.

Fig. 3 represents a schematic basic configuration of the second module/phase of the process of Fig. 1 : the drawing shows the functional structure and the equipment used.

Fig. 4 represents a schematic basic configuration of the third module/phase of the process of Fig. 1 : the drawing shows the functional structure and the equipment used.

Fig. 5 represents a schematic basic configuration of the fourth module/phase of the process of Fig. 1 : the drawing shows the functional structure and the equipment used.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a diagram of a method for converting into methane CO₂ previously captured from combustion systems or other industrial processes. In particular, in accordance with said method, the CO₂ is combined according to predefined ratios with suitably formulated and pre-treated biomasses in order to subject the resulting combination to an optimized anaerobic fermentation process based on specially selected bacterial populations. As shown, the process consists of four integrated modules/phases: the first one is called "gaseous emission treatment", the second one is called "nutritional substrate preparation", the third one is called "alcoholic substrate preparation", and the fourth one is called "optimized anaerobic fermentation and methane production".

According to the present invention, the object of capturing the CO₂ contained, in most cases, in a flue gas and of generating a concentrated CO₂ flow at low investment and running costs is attained, for example, in the first module/phase, which represents an optimal way to implement this part of the invention, through a process illustrated schematically in Fig. 2 and described below.

The embodiment example described herein relates to a flue gas produced by sludge or urban waste incinerators, gas or coal thermal and/or electric power plants, etc. and subjected to removal of traditional pollutants (NOx, particulate, SOx); if the gas has a low CO₂ concentration and a typical volumetric concentration between 3% and 20% of the total flue gas volume, the optional first module/phase can be useful; if the flue gas is already a concentrated CO₂ flow, said module/phase has no purpose and can be omitted. The removal treatments bring the inflowing gas temperature to values ranging between 30 and 8O⁰C, which are compatible with the management of the chemical and biotechnological processes contemplated in the present invention.

As shown in Fig. 2, the CO₂ rich flue gas enters into the extraction zone, which consists of a contact and dissolution reactor (IA), through a line (1) directly connected to the emission flow. The contact between the gas and the alkaline solution of sodium minerals and between the latter, enriched with CO₂, and carbonic anhydrase may take place in whatever reactor designed for gas/liquid reactions and capable of ensuring that the biocatalyst be kept at all times in liquid phase or hydrated. The reactor has inlet and outlet ports for the gaseous emission flow; it also has an inlet port for the capture solution coming through line (2) and an outlet port, located at the bottom of the reactor, where the resulting solution is collected to be discharged through an exit line (3). The capture reactor is so equipped as to make it possible to control the two separate flows, i.e. the gaseous emission flow and the capture solution flow. The carbonic anhydrase enzyme is immobilized, according to methods and onto supports known in the art. The alkaline solution of sodium minerals flowing into the reactor has a pH value between 8.3 and 9.6.

The contact process between the CO₂ contained in the flue gas and the alkaline solution of sodium minerals, aided by carbonic anhydrase, determines a significant increase in the concentration of carbonic acid, protons and bicarbonate ions. Sodium carbonate in the alkaline solution reacts with the stoichiometric quantities of the species resulting from CO₂ dissolution, further augmenting in this way the concentration of bicarbonate ions, and consequently their own input into the subsequent precipitation reactor. The obtained solution has a 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 at low temperatures. On the other hand, the sodium carbonate reaction rate with the species resulting from CO₂ dissolution to form bicarbonate is lower at low temperatures. Therefore the alkaline solution temperature must be maintained at such a level as to obtain CO₂ dissolution and hydration in line with the desired rate of reaction between sodium carbonate and the species formed as a result of CO₂ dissolution. Temperature is maintained below 75°C and preferably in the range between 35°C and 6O⁰C.

The gas outflowing from the capture reactor is sent to a demister (IB) and then released into the environment.

The solution containing bicarbonate and other species resulting from CO₂ dissolution and from their reactions with sodium carbonate is first collected in the first reactor and then transferred to a second precipitation reactor (1 C), where solid sodium carbonate is added to obtain an oversaturated sodium bicarbonate solution. This second reactor may consist of any type of container known in the art which, in terms of dimensions and equipment, can contain and maintain the solution in suspension as long as it takes to fully convert all the added sodium carbonate into sodium bicarbonate. In the precipitation reactor, the pH value is controlled by increasing or decreasing the pH value of the collected solution, i.e. by dissolving and hydrating a smaller or larger quantity of CO₂. Alternatively, the pH value is controlled by increasing or decreasing the quantity of solid sodium carbonate introduced into the solution. As a further alternative, the pH value can be controlled by introducing into the solution protons or substances which may affect it. In the precipitation reactor, the best pressure and temperature conditions are maintained in order to obtain sodium, bicarbonate precipitate. Furthermore, the solution is agitated until almost all of the added sodium carbonate is converted into precipitated sodium bicarbonate. "Almost all" is to be understood as any value between 90 and 100% of sodium carbonate added to the solution. Following the addition of sodium carbonate, the pH value of the solution never falls below 9, being preferably between 9 and 9.6. The precipitation reactor temperature may vary between 35 and 60°C. To minimize the consumption of alkaline agents, the solution collected in the precipitation reactor, which contains suspended solid sodium bicarbonate, is transferred to an apparatus (ID) known in the art designed for solid/liquid separation. The solution thus obtained is then transferred through a line (4) to a storage unit (IE) to be recycled in the first capture reactor through line (2).

On the other hand, the solid bicarbonate obtained by separation is transferred through line (5) to a subsequent regeneration unit (IF), where CO₂ and steam are released by calcination at a constant temperature ranging between 120 and 140⁰C. For sodium bicarbonate, the following endothermic reaction takes place inside the regeneration reactor:

2NaHCO₃ = CO₂ + H₂O + Na₂CO₃ + 32.4 Kcal/mol

The carbonate (Na₂CO₃) produced in the regeneration unit is recycled as a reagent into the precipitation reactor through line (6), while the gas (CO₂ + H₂O) is sent through line (7) to an apparatus (IG) for separation and concentration of gaseous CO₂. The steam is condensed and the released and separated CO₂ is compressed and stored in a container (IH) in view of its further uses in the second "nutritional substrate preparation" module/phase and in the fourth "optimized anaerobic fermentation" module/phase for methane conversion.

According to the present invention, the object of the industrial production of optimal nutritional substrates for anaerobic microorganisms from selected cocktails of vegetable and animal matrices and specific treatment processes, both of which are essential components of a low-cost solution, is achieved in the second module/phase through the process diagrammatically illustrated in Fig. 3 and described below. In particular, the second module/phase comprises different combinations of the pre- treatment stages included in said module/phase as needed from time to time depending on the nature of the available matrices capable of producing optimal substrates which can readily be used by anaerobic bacteria.

As shown in Fig. 3, there is a first stage called "supply and storage", wherein fresh, ensiled or semi-finished vegetable and animal matrices selected among a range including ad hoc agricultural and aquatic cultures (e.g. cereals, forages and macroalgae), agricultural and agro -industrial wastes, slaughter house wastes, animal sewage, depuration sludge, organic fraction of solid urban refuse, agro-industrial semifinished products, etc. enter through line (8).

It should be pointed out that the raw matrices selected for feeding the second stage have specific distinctive features; in particular, they may vary considerably as to qualitative and quantitative composition, homogeneity, fluid dynamics and biodegradability; some matrices may contain 1% of total solids, while other matrices may contain over 40%; the organic material content may vary between 70% and 95% of total solids; the nutritional ratio (C:N) may vary between 6 and 500; the distribution of organic macromolecules such as carbohydrates, proteins and lipids may also change substantially among the different matrices; all of these features are extremely important, since the composition and high degradability of said matrices, obtained during the various pre-treatments provided, will lead to the formation of all the fundamental components making up the main substrate readily available to bacteria in the fourth module/phase.

It should be noted that a limited series of organic matrices cannot be used in the second module/phase because of high pre-treatment costs, presence of contaminants and/or inhibitors, low degradability within the context of the pre-treatments to be carried out, or hygienic risks (e.g. high lignin content matrices, unselected urban waste, materials belonging to class 1 according to the hygienization guidelines of EC Regulation No. 1774/2002).

Moreover, since optimal organic material cocktails must be available throughout the year, it is also necessary to provide adequate storage of the most important constituent matrices. Among the latter, the most important one is vegetable material containing high levels of non- structural carbohydrates, which might get degraded in improper storage conditions; therefore, the storage method contemplated by this invention is ensilage (at farms), a process traditionally used for preserving forage for animal feeding. During this type of process, soluble carbohydrates contained in vegetable organic materials undergo lactic acid fermentation, which causes the pH value to drop and inhibits the growth of undesired microorganisms; in addition, lactic acid fermentation can be controlled through acidification or else by inoculating bacterial populations or enzymes in order to degrade the cellular wall of vegetable cells as well and to release soluble intracellular carbohydrates to be used for lactic acid fermentation. Ensilage therefore allows to obtain intermediate products for the formulation of optimal cocktails as well as to partially degrade any structural polysaccharides contained in the vegetable material. Ensilage storage, which may last two to six months, can thus be considered as a pre-treatment also ensuring a more appropriate utilization of the stored matrix within the overall pre-treatment process of the second module/phase.

All of the matrices selected from time to time in order to formulate optimal cocktails are thus stored ready for use, according to the type thereof, in containers designated (2A), (2B), (2C) and (2D).

From the "supply and storage" stage, an adequate portion of each selected matrix is taken and sent through line (9) to the second stage, defined as "detailed preparation" (2E), wherein the various matrices are mixed, most preferably in multiple combinations, and homogenized in order to obtain the optimal formulations.

The formulations of the optimal cocktails are obtained on the basis of the parameters and the respective ranges listed below: a) the particle size of the solids being present in the cocktails is preferably in the range between 0.5 and 3 cm, more preferably between 0.5 cm and 1.5 cm; b) the total solid content is preferably in the range between 10 and 35%, more preferably between 10 and 20%; c) the volatile solid content is preferably between 70 and 95% of total solids, more preferably between 85 and 95 % of total solids; d) as to the distribution of organic macromolecules, the carbohydrate content is preferably between 40 and 60% of total solids, raw protein content is preferably between 20 and 40% of total solids, raw lipid content is preferably between 10 and 30% of total solids; e) as to main minerals (e.g. K, Na, Mg, Mn, etc.), a mixture thereof is constantly present, at least in traces; f) as to main metals (e.g. Co, Ni, Fe, etc.), a mixture thereof is constantly present, at least in traces. Depending on the nature of the organic matrices and on the ratios of their constituents, different actions performed by using different equipment and settings may become necessary; however, since the latter are all well known in the art, they will not be detailed herein.

The optimal biomasses cocktails formulated according to the desired characteristics of homogeneity, size, solid content, volatile solid content, general nutritional ratio, and carbohydrate, lipid, macroelement and microelement composition are wholly transferred through line (10) into the reaction container (2F) of the thermo-cheniical and pressure treatment stage; alternatively, in the event that distinct treatments must be carried out for the different cocktail components, only a portion of said cocktails will be sent through line (10) to the reaction container (2F), while the remaining portion will be sent directly to the enzymatic hydrolysis stage (2H) through line (11) without going through the thermo-chemical and pressure treatment stage.

In the reaction container (2F), the cocktails undergo a thermal pre-treatment by direct or indirect heating or a combination thereof, according to methods known in the art, in the range between 36 and 160°C; when the desired temperature has been reached, the reaction container (2F) is pressurized by injecting gaseous CO₂ supplied by the first module/phase through line (12) up to a level preferably comprised between 3 and 50 bar, more preferably between 3 and 12 bar, for a variable time preferably between 5' and 1 hour, more preferably between 5' and 30'; when the programmed time has elapsed, the reaction container (2F) is slowly depressurized down to a level preferably comprised between 3 and 5 bar, more preferably between 3 and 4 bar; the treated cocktails in the reactor (2F) are thus wholly transferred by sending the gas outputted from the head of the reaction container (2F) to the fourth module/phase through line (13), by means of a quick and complete depressurization through line (14), which connects the bottom of the reaction container (2F) to the expansion container (2G), which in turn is also connected to the fourth module/phase through line (15), which allows to recover the gas used for pressurization. The described treatment allows to obtain a partial break down of the cellular wall of the cocktail constituents, as well as to obtain a safe hygienization thereof (in accordance with the health provisions contained in EC Animal By-Product Regulation No. 1774 /2002) and an increase in the biodegradability of the matrices, resulting in the latter being more susceptible to the subsequent acid enzymatic hydrolysis. Those cocktails which have been subjected to the thermo-chemical and pressure treatment are transferred through line (16) to the container (2H) in the enzymatic hydrolysis stage.

The enzymatic hydrolysis of the nutritional cocktails is conducted in the container (2H) within an environment saturated with CO₂ coming from the first module/phase through line (17); the CO₂ partial pressure is so adjusted at this stage as to keep the pH value of the cocktails preferably in the range between 3 and 6, more preferably between 4 and 5.5, while temperature is kept preferably in the range between 15 and 60°C, more preferably between 45 and 55 ⁰C.

Multi-enzymatic complexes (cellulase, amylase, protease and lipase) are introduced into the container through line (18), while mixed hydrolytic and cellulolitic bacterial populations, which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Clostridium sp., Pseudomonas sp. and Bacillus sp., are introduced through line (19) together with any necessary integrating stimulators, catalysts and nutritional factors.

Through line (20), the acid hydrolyzed effluent obtained through enzymatic hydrolysis in sent to the apparatus (21), dedicated to the separation of solids, substrate particles and microorganism particles, by means of any of the various techniques known in the art. In order to keep the bacterial populations at high density in the hydrolysis container (2H) and to improve the degradation of solid particles in the acid hydrolyzed product, the solid component separated from the liquid component is partly recirculated through line (21), whereas the other portion that cannot be used for this purpose is sent through line (22) to an industrial application or to the third module/phase for further treatment; on the other hand, the liquid component is sent to the fourth module/phase through line (23). The hydrolysis stage attains the result of significantly speeding up the conversion of polymers (polysaccharides, proteins, nucleic acids and lipids) into oligomers and monomers (sugars, amino-acids, purines, pyrimidines, fatty acids, glycerol, etc.) and of making the nutritional substances being present in the liquid hydrolyzed product readily available to and usable by the bacterial populations of the fourth module/phase. According to the present invention, the object of an agro -industrial production of optimal alcoholic substrates for anaerobic microorganisms from high carbohydrate content vegetable matrices and specific treatment processes, both of which are essential components of a low-cost solution, is achieved in the third module/phase through the process diagrammatically illustrated in Fig. 4 and described below. In particular, the third module/phase comprises different combinations of the pre- treatment stages included in said module/phase as needed from time to time depending on the nature of the available matrices capable of producing alcoholic substrates containing different types and/or combinations of alcohol, as properly exemplified, among others, by ethanol, methanol and butanol, all of which can readily be used by anaerobic bacteria.

As shown in Fig. 4, a first stage called "supply and storage" is supplied with matrices consisting of fresh, ensiled or semi-finished vegetable material having the desired energetic characteristics of high carbohydrate content (sugars, starch, cellulose, hemicellulose), selected among a range preferably comprising, but not limited to, ad hoc agricultural and aquatic cultures, agricultural, garden, forest, agro-industrial and industrial wastes, agro-industrial semi-finished products, etc. It should be pointed out that the raw matrices used for feeding this module/phase may have distinct features while still maintaining the basic characteristic of a high carbohydrate content. By high carbohydrate content matrices it is meant a total value of the different components (sugars, starch, cellulose, hemicellulose) preferably comprised between 50 and 85% of the matrix dry weight, more preferably between 70 and 85% of the matrix dry weight.

Consequently, the series of treatments to which they are subjected varies depending on their characteristics; matrices rich in carbohydrates mainly consisting of structural substances having a high molecular weight, such as hemicellulose, cellulose and lignin, which enter the container (3A) through line (24), will undergo the whole series of treatments executable in the module/phase, whereas matrices rich in carbohydrates consisting of substances having a high molecular weight, such as cellulose, hemicellulose and other polysaccharides, but a limited quantity of lignin, which enter the container (3B) through line (25), will only go through the saccharification and alcoholic fermentation processes, and matrices rich in carbohydrates consisting almost only of simple sugars, which enter the container (3C) through line (26), will only go through the final alcoholic fermentation stage.

As previously described for matrices intended for the production of nutritional substrates in the second module/phase, for the matrices used for producing alcoholic substrates it is also necessary that the alcoholic hydrolyzed product be available throughout the year; this need can be fulfilled in the first place by storing the raw matrices appropriately through ensilage, and in the second place more advantageously by storing the alcoholic hydrolyzed product itself. Such storage operations, which are carried out in the countryside, imply very low costs.

The different matrices selected from time to time are thus stored according to the type thereof in containers designated (3 A, 3B, 3C), and are then taken and sent through lines (27, 28 and 29), respectively, to the second stage called "detailed preparation" in containers designated (3D, 3E, 3F), respectively, wherein they are made homogeneous in size and brought to the desired solid content.

Depending on the nature of the organic matrices and on the ratios of their constituents, different actions performed by using different equipment and settings may become necessary; however, since the latter are all well known in the art, they will not be detailed herein.

Matrices rich in carbohydrates prevalently consisting of structural substances having a high molecular weight, such as hemicellulose, cellulose and lignin, are transferred through line (30) from the container (3D) to stage (3G), wherein a hydrolysis process is carried out by means of ligninolitic multi-enzymatic complexes (mainly phenoloxidasi) introduced through line (31) and/or by inoculating mixed populations of microorganisms, which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Basidiomycetes, Pseudomonas sp. and Actinomycetes sp., which are introduced through line (32) together with any necessary integrating stimulators, catalysts and nutritional factors, thereby obtaining as a result the removal of any pollutants and lignin, as well as the production of a cellulosic material which is more susceptible to the subsequent saccharification stage. The hydrolyzed product thus obtained is then sent through line (33) to the next saccharification stage (3H), together with the other matrices rich in carbohydrates consisting of substances having a high molecular weight, such as cellulose, hemicellulose and other polysaccharides, but a limited quantity of lignin, coming from stage (3E) through line (34). Saccharification is carried out through multi- enzymatic^{^} complexes (cellulase) introduced through line (35) and/or by means of a plurality of populations of microorganisms, which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Clostridium sp., Trichoderma sp. and Pseudomonas sp., which are introduced through line (36) together with any necessary integrating stimulators, catalysts and nutritional factors for the purpose of improving the efficiency of the production of oligosaccharides and free sugars.

The sacchariferous hydrolyzed product outputted from stage (3H) is then transferred to the alcoholic fermentation stage (31) through line (37), together with the matrices rich in carbohydrates prevalently consisting of oligosaccharides and free sugars coming from stage (3F) through line (38). Alcoholic fermentation is carried out by means of populations of microorganisms which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Saccaromyces sp., Zymomonas sp. and Chalara sp., which are introduced through line (39) together with any necessary additional stimulators, catalysts and nutritional factors, resulting in the obtainment of a high alcohol content hydrolyzed product containing different alcohol types and combinations, as properly exemplified, among others, by ethanol, methanol and butanol, all of which can readily be used by anaerobic bacteria.

The effluent from stage (31) is sent through line (40) to the solid separation apparatus (3L), mainly consisting of ligno-cellulosic materials and microorganisms; from this apparatus, the latter are recirculated and/or sent to an industrial application through line (41), while the alcohol-rich liquid component is sent to the fourth module/phase through line (42).

According to the present invention, the object of converting CO₂ into methane is attained in the fourth module/phase through the process diagrammatically illustrated in Fig. 5 and described below.

In particular, the first stage of the fourth module/phase comprises the distribution of CO₂ supplied by the storage container (IH) of the first module/phase through line (43), of nutritional hydrolyzed product supplied by the apparatus (21) of the second module/phase through line (23), of alcoholic hydrolyzed product supplied by the apparatus (3L) of the third module/phase through line (42), as well as of any integrating stimulators, catalysts and nutritional factors introduced through line (44), to the fermentation reactor (4A), thereby providing the microorganism cultures being present in or supplied to the reactor (4A) through line (45) with all necessary substrates and components readily available for the implementation of an extremely fast fermentation process, thus allowing for a low-cost production of large quantities of acetates from CO₂. The optimized nutritional substrate is supplied into the reactor (4A) on a daily basis, according to a direct relationship, expressed in terms of weight, with the bacterial population biomass being present at that time, said ratio being preferably in the range between 1 : 8 and 1 :25, more preferably between 1 :12 and 1 :25; CO₂ is supplied daily up to the maximum total load that can be used by the acetogenic populations being present therein; the alcoholic substrate is supplied daily up to the maximum total load that can stimulate the desired oxido-reductive reactions in the fermentation reactor. The fermentation reactor (4A) can be any reactor known in the art designed to sustain the growth of suspended bacteria or bacteria fixed to inert supports and capable, in terms of dimensions and equipment, of containing and maintaining the exogenously introduced CO₂ in solution or mixed with the bacterial culture medium for a time long enough to allow substantially all of the exogenously introduced CO₂ to be converted into acetates.

The selected mixed acetogenic bacterial populations being present in and/or supplied to the reactor (4A) may belong to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Butyribacterium sp., Eubacterium sp, Clostridium sp., Ruminococcus sp. and Morella sp.. Acetogenic fermentation is obtained at a density of said mixed bacterial populations, expressed in terms of dry weight, preferably in the range between 6 and 12%, more preferably between 9 and 12%; environmental conditions are kept constant and are defined by a pH value preferably in the range between 4.5 and 6.3, more preferably between 5 and 6, and by a temperature preferably between 30 and 80 °C, more preferably between 30 and 60 °C. On the one hand, the pH value of the reactor must not rise over 6.3, so that hydrogenotrophic methano genesis stays blocked, thus preventing hydrogen from being consumed by methanogenic microorganisms, while on the other hand it must not drop below 4.5 in order to sustain a competitive growth of acetogenic bacteria. Furthermore, since acetogenic populations are more resistant to ammonia than methanogenic ones, it is also possible to use higher nitrogen content substrates, in particular with ammonia concentrations over 1.2 grams per litre.

The effluent from the acetogenic fermenter (4A) is sent through line (46) to the apparatus (4B) for the separation of substrate particles and microorganisms being present therein, which may use any of the various techniques known in the art; after having been collected, these solid materials are recirculated through line (47) into the reactor itself or else they are recirculated for degradation to the head of the module/phase 2, whereas the liquid component is conveyed through line (48) to the next methane production digester (4C); the gas in the upper part of the fermentation reactor (4A), which contains unused undissolved CO₂, is also collected in the upper part of the same reactor as a mixed gas (CO₂, H₂, other) and sent to a storage container (4D) through line (49), after which it is recirculated into the same fermentation reactor still by using line (49) or else conveyed to the lower part of the methane production digester (4C) through line (50).

The output of the methane production reactor (4C) therefore comprises both the distribution of a high acetate content liquid substrate through line (48), integrated through line (51) with any necessary stimulators, catalysts and nutritional factors, and the distribution of a gaseous substrate which combines the acetate supply with the hydrogen and CO₂ supply being present in the gas itself, thereby ensuring an efficient production of a large quantity of methane from said substrates.

After optimization, all the liquid substrate exiting the apparatus (4B) is supplied into the reactor (4C) on a daily basis, according to a direct relationship, expressed in terms of weight, with the bacterial population biomass being present at that time, said ratio being preferably in the range between 1 : 8 and 1 :25, more preferably between 1 : 12 and 1 :25, while the gas coming from the reactor (4A) is supplied daily up to the maximum total load that can be used by the methanogenic populations being present therein. The methane production reactor (4C) may be any reactor known in the art designed to sustain the growth of suspended bacteria or bacteria fixed to inert supports and capable, in terms of dimensions and equipment, of containing and maintaining the CO₂ and hydrogen introduced from the container (4D) in solution or mixed with the bacterial culture medium for a time long enough to allow substantially all of the introduced acetates, CO₂ and hydrogen to be converted into methane.

The selected mixed methanogenic bacterial populations being present and/or introduced through line (52) in the reactor (4C) may belong either to the same or different genera as well as to the same or different species, as properly exemplified by Methanosarcina sp., Methanosaeta sp., Methanococcus sp. and Metanobacterium sp.. Methane production is obtained at a density of said mixed bacterial populations, expressed in terms of dry weight, preferably in the range between 6 and 12%, more preferably between 9 and 12%; environmental conditions are kept constant and are defined by a pH value preferably in the range between 7 and 9, more preferably between 7.5 and 8.5, and by a temperature preferably between 30 and 80 ⁰C, more preferably between 30 and 60 °C.

The effluent from the methane production reactor (4C) is sent through line (53) to the apparatus (4E) for the separation of particles and microorganisms being present therein, which may use any of the various techniques known in the art; after having been collected, these solid materials are recirculated through line (54) into the reactor itself for maintaining/increasing the density of the active bacterial populations, or else they are recirculated for degradation to the head of the module/phase 2, whereas a portion of the liquid component is conveyed to the base of the fermentation reactor (4A) through line (55) for contributing to the regulation of the pH value and of the retention time in this latter reactor; the remaining portion is sent to subsequent agricultural or agro- industrial applications or to depuration through line (56).

The biogas thus produced is collected in the upper part of the reactor (4C) as a mixed gas (CH₄, CO₂, other) and is then recirculated in the lower part of the same reactor or else conveyed through line (57) to a downstream storage unit (4F) and processing unit (not shown) before the biogas/methane can be used for feeding fixed power plants or delivered to a network of final users who are not involved in CO₂ capture. The fermentation reactor (4A) and the methane production reactor (4C) are equipped with a suitable monitoring system capable of detecting the most important process parameters (temperature, pH value, output gas composition, etc.), which are then used for controlling and optimizing the process for converting CO₂ into methane. The main controlled functions comprise: temperature and pH value; supply flow rate and composition of nutritional and alcoholic substrates, integrators and stimulators, and CO₂; circulation rate of liquids and gases between the fermentation reactor and the methanogenic reactor, and vice versa. REFERENCES Patents and patent applications

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- Bond, G. M., Stringer J., Branvold, D. K., Simsek, F. A., Medina, M. G. and Egeland, G., (2001 a) Development of integrated system for biomimetic CO₂ sequestration using the enzyme carbonic anhydrase, Energy & Fuels, 15: 309-316

- 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 2003Muller, V. (2003), Energy Conservation in Aceto genie Bacteria, App. Environ. Microbiol. Vol. 69 No 11 : 6345-6353

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

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Claims

1. Anaerobic process for converting CO₂ into methane, said. CO₂ being captured from a combustion system or another industrial process, which uses suitably formulated and pre-treated biomasses and which is based on specially selected mixed bacterial populations, said process comprising the following phases:

A) prearrangement of a concentrated CO₂ flow,

B) preparation of nutritional substrates containing monomeric and/or oligomeric organic substances for said selected mixed bacterial populations by starting from a first portion of said biomasses,

C) preparation of high alcohol content substrates for said selected mixed bacterial populations by starting from a second portion of said biomasses, and

D) feeding said selected mixed bacterial populations with said substrates and supplying the CO₂ of said flow to said selected mixed bacterial populations, so that it is converted into methane during a multi-stage anaerobic process.

2. Process according to claim 1, wherein during said phase A CO₂ is captured from a flue gas flow produced by combustion or other industrial processes to generate said concentrated CO₂ flow, said combustion being typically of the type which uses air as an oxidant.

3. Process according to claim 1, wherein said first biomasses portion is made up of fresh, ensiled or semi-finished vegetable and animal matrices, all of which are highly degradable.

4. Process according to claim 1, wherein said second biomasses portion is made up of fresh, ensiled or semi-finished vegetable matrices, all of which are rich in carbohydrates, in particular sugars, starch, hemicellulose and cellulose.

5. Process according to claim 3, wherein during said phase B optimal biomasses cocktails are formulated which contain desired characteristics in terms of homogeneity, size, total solid content, volatile solid content, general nutritional ratio, and carbohydrate, protein, lipid, macroelement and microelement composition.

6. Process according to claim 5, wherein during said phase B said biomasses and/or cocktails thereof are subjected to a thermal pre-treatment through direct or indirect heating or a combination thereof, to CO₂ pressurization, and to a subsequent enzymatic and/or bacterial hydrolysis.

7. Process according to claim 6, wherein during said phase B said biomasses is subjected to a thermal treatment at a temperature preferably in the range between 36 and 160 °C and at a CO₂ pressure preferably in the range between 3 and 50 bar, more preferably between 3 and 12 bar, for a variable time preferably between 5' and 1 hour, more preferably between 5' and 30'.

8. Process according to claim 6, wherein during said phase B said biomasses and cocktails thereof are subjected to enzymatic hydrolysis in a CO₂ saturated environment, which includes contact with multi-enzymatic complexes consisting of cellulase, amylase, protease, lipase and/or with mixed high-density hydrolytic and cellulolitic bacterial populations; the latter may belong either to the same or different genera as well as to the same or different species, as properly exemplified by Clostridium sp., Pseudomonas sp. and Bacillus sp.; CO₂ pressure is so adjusted as to keep the pH value of said cocktails preferably in the range between 3 and 6, more preferably between 4 and 5.5; temperature is kept preferably in the range between 15 and 60 °C, more preferably between 45 and 55°C.

9. Process according to claim 4, wherein during said phase C combinations of pre- treatment stages included in said phase are carried out as needed from time to time depending on the nature of the available matrices capable of producing alcoholic substrates containing different types and/or combinations of alcohol, which are properly exemplified, among others, by ethanol, methanol and butanol.

10. Process according to claim 9, wherein during said phase C, when the selected matrices are rich in carbohydrates mainly consisting of substances having a high molecular weight such as hemicellulose, cellulose and lignin, a first pollutant and/or lignin reduction and/or removal stage, a second saccharification stage and a third alcoholic fermentation stage are carried out.

11. Process according to claim 9, wherein during said phase C, when the selected matrices are rich in carbohydrates mainly consisting of substances having a high molecular weight such as cellulose, hemicellulose and other polysaccharides, a saccharification stage and a subsequent alcoholic fermentation stage are carried out.

12. Process according to claim 9, wherein during said phase C, when the selected matrices are rich in carbohydrates mainly consisting of oligosaccharides and simple sugars, only an alcoholic fermentation stage is carried out.

13. Process according to claim 10, wherein the treatment of matrices rich in carbohydrates mainly consisting of substances having a high molecular weight such as hemicellulose, cellulose and lignin is carried out, for the purpose of producing high cellulose content material which can readily be saccharified, through contact with ligninolitic multi-enzymatic complexes (mainly phenoloxidasi) and/or by inoculating mixed populations of microorganisms, which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Basidiomycetes, Pseudomonas sp. and Actinomycetes sp..

14. Process according to claim 11, wherein the saccharification treatment of matrices rich in carbohydrates mainly consisting of substances having a high molecular weight such as cellulose, hemicellulose and other polysaccharides is carried out, for the purpose of producing material rich in oligosaccharides and simple sugars, through contact with multi- enzymatic complexes (cellulase) and/or with a plurality of populations of microorganisms, which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Clostridium sp., Trichoderma sp. and Pseudomonas sp..

15. Process according to claim 12, wherein the alcoholic fermentation treatment of matrices rich in carbohydrates mainly consisting of oligosaccharides and free sugars is carried out, for the purpose of producing a high alcohol content substrate, through populations of microorganisms which may belong either to the same or different genera as well as to the same or different species, as properly exemplified, among others, by Saccaromyces sp., Zymomonas sp. and Chalara sp..

16. Process according to claim 1, wherein said phase D comprises all or several of the following stages: i) distribution of said CO₂, said nutritional substrate and said alcoholic substrate coming from phases A, B and C, respectively, as well as of further integrating stimulators, catalysts and nutritional factors, to an acetogenic digester; ii) conversion of said CO₂ and said nutritional and alcoholic substrates into acetates, acetic acid and other organic acids through fermentation with specially selected mixed acetogenic bacterial populations kept at high density in an acid environment; iii) separation of substrate particles and microorganisms from the effluent of stage ii), followed by recirculation thereof within the same stage or at the head of phase B, and output of the liquid component to the next stage v); iv) output of the gas collected from stage ii) to the subsequent stage v) or recirculation thereof within the same stage; v) methane conversion of acetates, CO₂ and hydrogen being present in the liquid effluent and/or in the gaseous effluent coming from stage ii) by means of specially selected mixed methanogenic bacterial populations kept at high density; vi) removal of biogas from stage v) and recirculation thereof within the same stage or output thereof to downstream storage and processing units; vii) separation of substrate particles and microorganisms being present in the effluent from stage v), followed by recirculation thereof within the same stage or at the head of phase B, and output of the liquid component to the previous stage ii) or to depuration.

17. Process according to claim 16, wherein stage i) comprises a daily supply of said nutritional substrate according to a direct relationship, expressed in terms of weight, with the bacterial population biomass being present at that time in stage ii), said ratio being preferably in the range between 1 : 8 and 1 :25, more preferably between 1 : 12 and 1 :25.

18. Process according to claim 17, wherein stage i) comprises a daily supply of said CO₂ up to the maximum total load that can be used by said acetogenic bacterial populations.

19. Process according to claim 18, wherein stage i) comprises a daily supply of said alcoholic substrate up to the maximum total load that can stimulate the desired oxido- reductive reactions in the fermentation reactor.

20. Process according to claim 16, wherein stage ii) comprises at least one acetogenic fermentation step.

21. Process according to claim 20, wherein acetogenic fermentation is obtained by means of specially selected mixed high-density bacterial populations which may belong either to the same or different genera as well as to the same or different species, as properly exemplified by Butyribacterium sp., Eubacterium sp., Clostridium sp., Ruminococcus sp., Morella sp., etc.

22. Process according to claim 21, wherein acetogenic fermentation is obtained at a density of said specially selected mixed bacterial populations, expressed in terms of dry weight, preferably in the range between 6 and 12%, more preferably between 9 and 12%.

23. Process according to claim 20, wherein acetogenic fermentation is obtained in environmental conditions defined by a pH value preferably in the range between 4.5 and 6.3, more preferably between 5 and 6, and by a temperature preferably in the range between 30 and 80 °C, more preferably between 30 and 60 ⁰C.

24. Process according to claim 16, wherein stage v) comprises the total daily supply of said liquid effluent coming from stage ii), integrated with any necessary stimulators, catalysts and nutritional factors, according to a direct relationship, expressed in terms of weight, with the bacterial population biomass being present at that time, said ratio being preferably in the range between 1 : 8 and 1 :25, more preferably between 1 : 12 and 1 :25.

25. Process according to claim 16, wherein stage v) comprises a daily supply of said gas collected from stage ii) up to the maximum total load that can be used by said specially selected methanogenic bacterial populations.

26. Process according to claim 16, wherein stage v) comprises at least one acetoclastic methanogenesis and one hydrogenotrophic methanogenesis.

27. Process according to claim 26, wherein methanogenesis is obtained by means of specially selected mixed high-density bacterial populations which may belong either to the same or different genera as well as to the same or different species, as properly exemplified by Methanosarcina sp., Methanosaeta sp., Methanococcus sp., Metanobacterium sp., etc.

28. Process according to claim 27, wherein said acetoclastic and hydrogenotrophic methanogeneses are obtained at a density of said specially selected mixed bacterial populations, expressed in terms of dry weight, preferably in the range between 6 and 12%, more preferably between 9 and 12%.

29. Process according to claim 26, wherein said acetoclastic and hydrogenotrophic methanogeneses are obtained in environmental conditions defined by a pH value preferably in the range between 7 and 9, more preferably between 7.5 and 8.5, and by a temperature preferably in the range between 30 and 80 ⁰C, more preferably between 30 and 60 °C.

30. Plant for producing CH₄ from CO₂ and biomasses, said CO₂ being derived from a first module adapted to provide a concentrated CO₂ flow, comprising: a second module adapted to prepare nutritional substrates containing monomeric and/or oligomeric organic substances for specially selected mixed bacterial populations by starting from a first portion of said biomasses,

- a third module adapted to prepare high alcohol content substrates for specially selected mixed bacterial populations by starting from a second portion of said biomasses,

- a fourth multi-stage module, adapted to use said nutritional and alcoholic substrates and said concentrated CO₂ flow by means of specially selected mixed anaerobic bacterial populations in order to produce CH₄.

31. Plant according to claim 30, characterized by comprising said first module.

32. Plant according to claim 30 or 31, wherein said first module is adapted to capture CO₂ from a flue gas flow produced by combustion or another industrial process and then to generate said concentrated CO₂ flow.

33. Plant according to claim 30 or 31, wherein said second module is adapted to prepare nutritional substrates having an optimal content of monomeric and/or oligomeric substances for mixed bacterial populations by starting from a first portion of said biomasses.

34. Plant according to claim 30 or 31, wherein said third module is adapted to prepare optimal high alcohol content substrates for mixed bacterial populations by starting from a second portion of said biomasses,

35. Plant according to claim 30 or 31, wherein said fourth module is adapted to use said nutritional and alcoholic substrates, integrated with stimulators, catalysts and nutritional factors, and said concentrated CO₂ flow by means of said specially selected mixed anaerobic bacterial populations in order to produce CH₄.

36. Plant according to claim 30 or 31, characterized by being adapted to carry out the process according to any of claims 1 to 29.

37. Plant for producing energy from biomasses and CO₂, comprising:

- a first plant for producing CH₄ from a gaseous flow containing CO₂ and from biomasses, according to any of claims 30 to 36, a second combustion plant adapted to burn CH₄ produced by said first plant, thus emitting a gaseous flow containing CO₂; wherein said gaseous flow emitted by said second plant is supplied to said first plant.

38. Plant according to claim 37, wherein said second plant is adapted to use air as an oxidant.

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