WO2011055296A2

WO2011055296A2 - Plant and method for the production of gas from biomass

Info

Publication number: WO2011055296A2
Application number: PCT/IB2010/054954
Authority: WO
Inventors: Giovanni Cappello
Original assignee: A.G.T. Srl
Priority date: 2009-11-05
Filing date: 2010-11-02
Publication date: 2011-05-12
Also published as: ITMI20091934A1; WO2011055296A3; IT1396596B1

Abstract

The invention relates to a plant 100 for the production of gas from biomass, comprising a gasifier 8 of the open- core downdraft type and an apparatus 10 for cleaning the gas G. The apparatus comprises: - an inlet line 12 for the gas to be cleaned; - an evaporative cooler 16 having: means 18 for spraying an aqueous mixture MA into the gas; a basin 20 for containing the condensed mixture; and a first bleed pipe 22 for removing the pollutants; - a condensing wet electrostatic precipitator 26 having: means 28 for cooling the walls 29 in contact with the gas; a tank 30 for collecting the mixture condensed by cooling; a recirculation pipe 31 for removing the mixture and conveying it to the evaporative cooler; a second bleed pipe 32 for removing the pollutants from the tank; and a discharge pipe 34 for removing the excess aqueous mixture; - an outlet line 36 for conveying the flow of cleaned gas G to an external utilization unit 38. The invention also relates to a method for treating the gas.

Description

DESCRIPTION

"Plant and method for the production of gas from biomass"

[0001] The present invention relates to a plant for the production of gas from biomass. The plant comprises a gasifier and an apparatus for cleaning the gas produced by the gasifier. The gasifier is designed to produce fuel gas from biomass of various sources or from mineral coal. The cleaning apparatus is designed to purify the fuel gas produced by the gasifier.

[0002] Plants for the gasification of biomass, i.e. plants designed to produce fuel gas from biomass, have been known for a long time. The most significant fraction of the biomass (80-98%) is made up of carbon (C) , hydrogen (H) and oxygen (0) organized in different types of molecules. The remaining fraction of biomass (2-20%) is made up of other molecules and other inorganic elements including, in particular, silicon (Si) , potassium (K) , calcium (Ca) and magnesium (Mg) .

[0003] In a manner known _. per se, the main reactions which take place during gasification are:

[0004] C + 0₂ → C0₂ (Combustion)

[0005] C + ½ 0₂ → CO (Partial oxidation)

[0006] C + H₂0(g) → CO + H₂ (Coal reforming)

[0007] C + C0₂ → 2C0 (Boudouard reaction) [0008] C + 2H₂ → CH₄ ( ethanation)

[0009] CO + H₂0₍g) → C0₂ + H₂ (Water/gas shift reaction) .

[0010] From these reactions, in the presence of air, a gas which is made up of a mixture composed of about 50% N₂, 20% CO, 15% H₂, 10% C0₂ and 5% CH₄ is obtained. If the reactions take place in the absence of air, the final mixture does not contain N₂ and is called "synthesis gas", "producer gas" or syngas.

[0011] Several types of gasification plants are known and they are differentiated on the basis of the reactor structure, the path that the gas follows inside the reactor, the type of filtration apparatus employed, etc.

[0012] Gasification plants of the known type are not however without drawbacks.

[0013] Current gasification plants can be grouped into two major categories. Plants of the first category are produced mainly for experimental purposes, are characterized by large dimensions (power levels typically greater than 1 megawatt) and employ sophisticated technologies. Such dimensions and the fact that they are built generally as unique examples effectively make their large-scale commercialization impossible.

[0014] Plants of the second category are characterized by small dimensions, employ rudimentary technologies and are designed above all for the rural situations of developing countries. The technological backwardness of such plants effectively makes their large-scale use in the Western energy market impossible.

[0015] In the 1940s, extremely compact gasification plants were produced. They were generally fitted to motor vehicles to compensate for the lack of petroleum-derived products. Indeed, such plants enabled the associated internal-combustion engines to be powered with wood or charcoal. They were characterized by small dimensions, but had a poor efficiency and produced a gas of unacceptable quality by current standards.

[0016] Modern internal-combustion engines require a very high quality of gas, in general with stringent limits on the maximum temperature of the gas which powers them, on its relative humidity and on the dew point of the tars present in the same.

[0017] Achieving such a quality of gas, and above all lowering of the temperature of the gas powering the engine, involves the formation of condensate which is particularly contaminated with organic substances (phenols, ammonia, benzenes, etc.) which are difficult to treat and eliminate. [0018] The aim of the present invention is therefore to provide an apparatus for cleaning the gas produced by a gasifier, suitable for overcoming, at least partly, the drawbacks indicated with reference to the prior art.

[0019] In particular, a task of the present invention is to provide a gas cleaning apparatus which has small overall dimensions, which is of economic construction and which is capable of producing a gas with a sufficient degree of filtration so that it can also be used in modern sophisticated internal-combustion engines.

[0020] This aim and these tasks are achieved by means of a gas cleaning apparatus according to Claim 1.

[0021] To better understand the invention and appreciate its advantages, a number of non-limiting examples of embodiments thereof are described hereinafter with reference to the accompanying drawings, in which:

[0022] Figure 1 is a schematic view of a plant for the production of energy from biomass, comprising the gas cleaning apparatus according to the invention;

[0023] Figure 2 is a schematic view of a plant similar to that of Figure 1;

[0024] Figure 3 is a diagram representing the behaviour of some types of tars with a variation in temperature; [0025] Figure 4 is a side view of a plant similar to that of Figure 2;

[0026] Figure 5 is a plan view of the plant of Figure 4;

[0027] Figure 6 is a detail of the cross-section along the line VI-VI of Figure 5.

[0028] Hereafter in the description, reference will often be made to the concepts of "top", "upper" and the like, and to the concepts of "bottom", "lower" and the like, respectively. These concepts are to be understood as referring specifically to the apparatus correctly fitted and in working order, and therefore subject to the force of gravity.

[0029] Reference will also be made, in the description of the path followed by the gas, to the concepts of "upstream" and "downstream". "Upstream" is understood to mean a position along the path relatively close to the reactor of the gasifier from which the gas exits at high temperature contaminated with dust and tars. On the other hand, "downstream" is understood to mean a position along the path relatively far from the reactor.

[0030] In the accompanying drawings, the reference 100 indicates overall a plant for the production of energy from biomass. The plant 100 according to the invention firstly comprises a gasifier 8 of the open-core downdraft type and an apparatus 10 for cleaning the gas G.

[0031] The apparatus 10 in turn comprises:

[0032] - an inlet line 12 suitable for conveying the flow of gas G to be cleaned exiting the gasifier 8 and/or a dedusting unit 14;

[0033] - an evaporative cooler 16 suitable for treating the incoming flow of gas G from the line 12, the evaporative cooler 16 comprising:

means 18 for spraying an aqueous mixture MA into the flow of gas G,

a basin 20 suitable for keeping in the evaporative cooler 16 a reserve of mixture MA in the condensed state, and

a first bleed pipe 22 suitable for removing from the bottom of the basin 20 the condensed pollutants and conveying them to the gasifier 8;

[0034] - a condensing wet electrostatic precipitator 26 suitable for treating the outgoing flow of gas G from the evaporative cooler 16, said . precipitator 26 comprising :

means 28 for cooling the walls 29 in contact with the gas G,

a tank 30 suitable for collecting the aqueous mixture MA condensed during cooling of the gas G, a recirculation pipe 31 suitable for removing from the tank 30 excess aqueous mixture MA and conveying it to the evaporative cooler 16,

a second bleed pipe 32 suitable for removing the aqueous mixture MA containing condensed tars from the bottom of the tank 30 and conveying it to the gasifier 8, and

a discharge pipe 34 suitable for removing from the tank 30 excess aqueous mixture MA and conveying it externally;

[0035] - an outlet line 36 suitable for conveying the flow of cleaned gas G to an external utilization unit 38.

[0036] The gasifier 8, known per se, is of the downdraft type. In this type of gasifier the biomass is introduced into the reactor from the top, the gasification reactions take place in the bottom part of the reactor and the gas produced is removed from the bottom of the reactor. This type of gasifier therefore differs from other gasifiers, referred to as "updraft" gasifiers, in which the gas produced is removed from the top of the reactor.

[0037] The downdraft gasifier offers some notable advantages. First of all, it produces a gas with a limited tar content and above all enables biomass with ashes having a low melting point to be used as fuel. Furthermore, the downdraft gasifier produces, as a byproduct of the gas, a carbonaceous residue called charcoal, the demand for which is always increasing. Indeed, this charcoal is used for improving soil fertility (in this context, it is called "biochar") and above all for fixing in an extremely stable form the carbon present in the biomass. This carbon is derived from carbon dioxide (C0₂) taken from the atmosphere by the biomass.

[0038] The gasifier 8 is of the open-core type, known per se. In this type of gasifier, the oxygen, required for the combustion and partial oxidation reactions of the carbon, is supplied by the air drawn in from the environment at atmospheric pressure through the upper inlet of the reactor. This type of gasifier therefore differs from other gasifiers in which the oxygen is supplied by a suitable plant. The open-core gasifier has the advantage of being simpler and more economical in production and operation.

[0039] Patent application WO 2008/096387, in the name of the same Applicant, describes a gasification plant which, in its general configuration and in aspects known per se, is similar to the plant 100 considered here. Reference is made to this document for aspects not described here in depth.

[0040] The output of gas G from the gasifier 8 has a temperature of about 400-800°C and carries a considerable quantity of pollutants. The main pollutants are charcoal dust, ashes and tars in the vaporized or atomized states. The gas output from the cleaning apparatus 10, in order to be able to be used effectively downstream, must be cleaned as far as possible of the pollutants and must have a temperature of less than 80°C, preferably less than 60°C.

[0041] The evaporative cooler 16 is suitable for achieving a first cooling step of the gas G by means of the evaporation of an aqueous mixture MA until the gas is saturated. In other words, the spraying means 18, situated inside the evaporative cooler 16, spray the aqueous mixture MA into the flow of gas G. This mixture MA evaporates absorbing heat and therefore lowering the temperature of the flow of gas G. This mechanism operates in an efficient manner until such time as the vapours produced by the evaporation of the aqueous mixture MA saturate the gas G.

[0042] In the embodiments described hereinafter, it is considered that the aqueous mixture MA comprises mainly water, polar tars (therefore soluble in water) and non- polar tars (not soluble) . In other possible embodiments of the apparatus 10, the aqueous mixture MA could also comprise other additives suitable for solving specific contingent problems. Such additives can be in solution, emulsion or suspension form and in any case mixed in the water. One of these additives can for example comprise particles of calcium hydroxide Ca(OH)₂ which, when suspended in water, form so-called "lime milk". Using lime milk provides for neutralization of any acidic components present in the gas.

[0043] In general, the aqueous mixture MA also comprises traces of other elements present in the gas G, such as carbon (C) in the form of charcoal and other inorganic elements in the form of ashes such as silicon (Si) , potassium (K) , calcium (Ca) and magnesium (Mg) .

[0044] The spraying means 18 may advantageously comprise a connection 17 to the water supply network or to another water source. It is thus possible to compensate for possible losses of water which occur in the evaporative cooler 16 or downstream of the latter.

[0045] The evaporative cooler 16 also comprises a basin 20 in which a reserve of aqueous mixture MA in the condensed state collects. The presence of the mixture MA in the liquid phase ensures the effective saturation of the gas G in transit in the evaporative cooler 16. Furthermore, any condensation due to an excess of mixture MA in the vapour phase can in turn be collected in the basin 20.

[0046] The temperature of the gas at the outlet of the evaporative cooler 16 depends on the concentration of tars mixed in the water and on the types of said tars. At the end of an initial transient, the operating parameters of the evaporative cooler 16 become stabilized. If the evaporative cooler is correctly dimensioned, the saturated gas G and the condensed aqueous mixture MA have practically the same temperature. In such an operational state, cooling of the gas takes place almost exclusively through absorption of latent heat of evaporation by the aqueous mixture MA.

[0047] If pure water is employed in place of the aqueous mixture MA, the equilibrium temperature would settle at about 7 5°C. Upon increasing the concentration of tars mixed in the water, the ebullioscopic constant of the said mixture also increases and the equilibrium temperature therefore rises.

[0048] At concentrations of tar in the mixture which are considered advantageous for operation of the evaporative cooler 16, the equilibrium temperature is preferably between 75°C and 100°C. If the concentration of water in the mixture MA reduces, the evaporative cooling effect also gradually reduces causing a rise in the temperature of the mixture itself.

[0049] Above 100 °C, the quantity of water in the mixture is sharply reduced and therefore almost exclusively evaporation of the lighter tars occurs. However this phenomenon, due to the small quantity of such tars and their reduced latent heat of vaporization, is not sufficient to cool the gas efficiently. Therefore in this case the mixture MA, formed by tars only, would warm up appreciably reaching equilibrium at a temperature only slightly less than that of the gas entering the evaporative cooler 16.

[0050] Figure 3 shows a diagram representing the behaviour of various tar fractions as a function of temperature. As can be seen, the various tar fractions have markedly different behaviours at the same temperature .

[0051] In particular, at an equilibrium temperature plausible for operation of the evaporative cooler 16, for example at a temperature of 95°C, various phases coexist. There are the lighter tars (aromatic tars) completely in the vapour phase, other tars (light polyaromatic and heterocyclic tars) partly in the vapour phase and partly in the condensed phase, and lastly there are heavier tars (heavy polyaromatic tars) completely in the condensed phase .

[0052] The condensed tars collect in the basin 20 together with the water forming the aqueous mixture MA. The aqueous mixture MA comprises both polar tars which pass into solution and non-polar tars which remain simply mixed with the water, but which would tend to separate if the aqueous mixture MA were left to rest.

[0053] The first bleed pipe 22 is suitable for removing from the bottom of the basin 20 some of the mixture MA which collects there. The mixture must be removed from the basin 20 in order to keep the correct quantity of liquid inside the evaporative cooler 16. Furthermore the condensed tars present in the mixture MA can usefully be brought back to the inlet of the gasifier 8 so as to undergo again the oxidation and gasification processes. Tars are in fact colloidal systems comprising a high quantity of organic substance, above all heterocyclic and polycyclic aromatic hydrocarbons. It is therefore possible to extract a further quantity of gas G from the tars.

[0054] It is worthwhile noting here that the aqueous mixture MA accumulated in the basin 20 has during operation a relatively high temperature. Thus, the same mixture has, notwithstanding the presence of tar, a very low viscosity and can be easily pumped.

[0055] In the accompanying Figure 1, the evaporative cooler 16 is shown as a channel passed through by the flow of gas G in a substantially horizontal direction. Inside the channel, the aqueous mixture MA is sprayed in the same direction as that of the gas flow. At a low point of the channel, the basin 20 is provided and collects the condensed mixture MA by the action of gravity .

[0056] In the accompanying Figure 2, the evaporative cooler 16 is represented as a channel passed through by the flow of gas G in a substantially vertical direction. Inside the channel, the aqueous mixture MA is sprayed in a direction opposite to that of the flow of gas G. At the bottom of the channel, the basin 20 is provided and collects the condensed mixture MA by the action of gravity .

[0057] The condensing Wet Electrostatic Precipitator 26, also referred to as a condensing WESP, is situated downstream of the evaporative cooler 16. It is a wet electrostatic precipitator, of a type known per se, which also comprises means 28 for cooling the walls 29 in contact with the gas G. The electrostatic precipitator 26 therefore comprises channels inside which an electrostatic field is maintained. In particular, the electrostatic precipitator 26 preferably comprises tubular structures inside each of which an electrode 27 is situated. An electrostatic field can thus be formed between the walls 29 and the electrode. Furthermore, the walls 29 can be cooled by means of the action of the means 28.

[0058] With this structure it is possible both to clean the gas G of suspended pollutants and cool it to the desired temperature for final use. Cleaning of the gas takes place, in a manner known per se, by the electrostatic attraction operated on the pollutants by the walls 29 of the precipitator 26. Cooling of the gas G takes place by heat removal via the walls 29 of the precipitator 26.

[0059] The means 28 for cooling the walls 29 of the precipitator 26 enable the temperature of the gas G to be lowered, removing heat from it.

[0060] Cooling of the gas G also causes the condensation of most of the vapours present within it. The gas G enters the precipitator 26 at a temperature, defined by the passage in the evaporative cooler 16, of between about 75 °C and about 100 °C. At the outlet of the precipitator 26, the gas must assume a temperature of preferably less than 60°C.

[0061] Cooling of the gas G causes the condensation of the aqueous mixture MA. The formation of water droplets provides condensation nuclei for the tar molecules, and conversely, the formation of tar droplets provides condensation nuclei for the water molecules. Thus, the aqueous mixture MA passes from the vapour phase to the liquid phase, adheres to the walls 29 of the precipitator 26, runs down along the walls 29 and collects in the tank 30.

[0062] With reference again to the diagram of Figure 3, it is possible to note how, at temperatures that are typically reached at the outlet of the precipitator 26, most of the tars still present in the gas G in the vapour phase and/or in aerosol form condense. Naturally at such a temperature most of the water also condenses. The condensed liquids hence collect in the tank 30.

[0063] As mentioned above, the tank 30 is equipped with a second bleed pipe 32. In a manner similar to that already described above with reference to the first bleed pipe 22 of the basin 20 of the evaporative cooler 16, the second bleed pipe 32 also provides for removal of some of the aqueous mixture MA from the bottom of the tank 30. The removed mixture contains water, polar tars in solution form and above all non-polar (water-insoluble) tars which collect at the bottom of the tank 30. For this reason, the second bleed pipe 32 draws advantageously from a low point of the tank 30, where the heavier non- polar tars collect naturally by the action of gravity. The tars, both polar and non-polar, can be usefully conveyed back to the inlet of the gasifier 8 so as to undergo again the oxidation and gasification processes.

[0064] The lighter fraction of tars remains volatile in the gas G also at the outlet temperature of the precipitator 26. This tar fraction is therefore conveyed with the flow of gas G for subsequent applications. Typically the use of the gas G requires a combustion step, to which step the light tars can also contribute usefully in view of their chemical nature.

[0065] The tank 30 of the precipitator 26 also comprises a recirculation pipe 31. This pipe advantageously draws at an intermediate height of the tank 30, so as to remove from the tank 30 an aqueous mixture MA composed mainly of water and tars of similar density (and therefore miscible) and/or with polar behaviour (and therefore soluble) . This aqueous mixture MA is in fact intended to be conveyed again upstream to the evaporative cooler 16.

[0066] In accordance with some embodiments (see for example the diagrams of Figure 1 and Figure 4), the recirculation pipe 31 draws off the aqueous mixture MA from the tank 30 and introduces it into the basin 20. A second recirculation pipe 24 draws off the aqueous mixture MA from the basin 20 and feeds it to the spraying means 18.

[0067] In accordance with other embodiments (see for example the diagram of Figure 2), the recirculation pipe 31 draws off the aqueous mixture MA from the tank 30 and feeds it directly to the spraying means 18.

[0068] The tank 30 of the precipitator 26 comprises, lastly, a discharge pipe 34. This pipe is suitable for removing from the tank 30 excess aqueous mixture MA and for conveying it to the outside. An apparatus (not described) for removal of the pollutants present in the aqueous mixture may be preferably situated downstream of the pipe 34.

[0069] In accordance with some embodiments (see for example the diagram of Figure 1) , the means 28 for cooling the walls 29 of the precipitator 26 comprise a circuit 40 suitable for allowing the circulation of a cooling fluid which passes over the walls 29 of the precipitator 26 on the side not exposed to the flow of gas G.

[0070] The circuit 40 can be produced in a manner known per se, for example it can advantageously be a closed circuit inside which a pre-established quantity of cooling liquid circulates.

[0071] The circuit 40 may comprise a radiator suitable for dispersing the heat absorbed by the cooling liquid into the environment or it may comprise a heat exchanger suitable for retrieving the heat and conveying it away for other uses. Furthermore, this type of circuit may, if necessary, comprise a refrigeration apparatus in the event that particularly low cooling temperatures for the gas G are required.

[0072] Such different types of cooling plant for the gas G may advantageously be arranged next to each other on various sections of the same precipitator 26. It is for example possible for the first sections of the precipitator 26 to be equipped with a circuit comprising a heat exchanger, suitable for exploiting for other uses the relatively high temperature of the gas G, for example for heating rooms and/or for sanitary hot water. It is then possible for successive sections of the same precipitator 26 to be equipped, instead, with a circuit comprising a radiator suitable for dispersing the low- temperature heat that can no longer be used elsewhere.

[0073] In accordance with other embodiments (see for example the diagram of Figure 2), the means 28 used for cooling comprise external fins on the walls 29 of the precipitator 26. These fins are designed in a manner known per se to increase the surface area for dispersion of the heat in the environment. Cooling by means of fins or the like can if necessary be improved with the introduction of forced ventilation.

[0074] In accordance with some embodiments, the electrostatic precipitator 26 further comprises means 42 for heating the gas G again after cooling it. These means 42, where present, are associated with the last section passed through by the gas G inside the electrostatic precipitator 26, immediately upstream of the outlet line 36.

[0075] Cooling of the gas G inside the electrostatic precipitator 26 in fact causes condensation of most of the water and tars present in it in the form of vapour. The gas G however remains saturated with vapours, i.e. it remains with a relative humidity of 100%. Under these conditions lowering, even slightly, of the temperature of the gas G causes condensation of the vapours and consequent formation of mists inside the gas G. The occurrence of such a drop in temperature is extremely probable along the line 36 which carries the gas G from the electrostatic precipitator 26 to the utilization unit 38. The consequent condensation and formation of mists would therefore risk soiling the line 36 and the said utilization unit 38.

[0076] To avoid the abovementioned drawbacks, in accordance with some embodiments of the invention, the temperature of the gas G can be raised again a few degrees (for example, 10-20°C). Hence, a reduction in the relative humidity, which drops below 100%, is achieved. Under these changed conditions, the gas G can endure slight changes in temperature without thereby giving rise to the formation of mists.

[0077] Furthermore, since heating of the gas takes place inside the electrostatic precipitator 26, the effect described above of the increase in temperature is combined with the effect of the electrostatic field, thereby succeeding in removing from the gas G even the last remaining fraction of water and tars in liquid form. Hence, a very dry gas G is obtained.

[0078] The heating means 42 can advantageously exploit the heat made available by other sections of the plant 100, such as for example the gas utilization unit 38 (as schematically shown in Figure 1) or the means 28 for cooling the walls 29 of the said precipitator 26.

[0079] In accordance with one embodiment, the heating means 42 comprise a circuit suitable for allowing the circulation of a heating fluid which passes over the walls 29 of the precipitator 26 on the side not exposed to the flow of gas G.

[0080] The circuit can be formed in a manner known per se, for example it may be advantageously a closed circuit inside which a pre-established quantity of heating liquid circulates .

[0081] In accordance with some embodiments of the plant 100, a unit for dedusting the gas G is present between the gasifier 8 and the cleaning apparatus 10. Such a dedusting unit may comprise, for example, a cyclone (see, for example, the diagram of Figure 1) or a high-temperature ceramic filter (see for example the diagram of Figure 2) . Both these solutions are not described here in detail since they are well-known per se to the person skilled in the art.

[0082] Each of the bleed pipes 22 and 32 and the recirculation pipes 31 and 24 (where present) comprises preferably a pump suitable for moving the aqueous mixture MA even when it is charged with heavy tars such as those which must be conveyed back to the inlet of the gasifier 8. Such pumps may be preferably gear pumps or peristaltic pumps, suitable for moving fluids, including those with a high viscosity. In accordance with some embodiments, the pump situated on the recirculation pipe which feeds the spraying means 18 may be advantageously a centrifugal pump. This type of pump is in fact suitable for providing a notable flow rate of aqueous mixture MA, provided the latter has a sufficiently low viscosity.

[0083] In accordance with some possible embodiments, the bleed pipes 22 and 32 may advantageously comprise a settling tank suitable for further separating the tars from the water by the action of gravity. The tars retrieved from the bottom of the settling tank can then be drawn off for storage or for conveying them back to the gasifier 8.

[0084] In accordance with some possible embodiments, the plant 100 further comprises a blower 37 mounted on the line 36 and suitable for moving the gas G along the whole of the plant 100, from the gasifier 8, through the dedusting unit 14 (where present) , the evaporative cooler 16, the condensing electrostatic precipitator 26, as far as the line 36 and beyond.

[0085] In accordance with some possible embodiments, the plant 100 comprises, lastly, a unit 38 for using the gas .

[0086] In accordance with the embodiments of the plant 100 shown in the accompanying Figures 1 and 2, the gas utilization unit 38 comprises an internal-combustion engine to which a generator for the production of electrical energy may be typically connected.

[0087] In particular, it should be noted that the optimum quality gas at the outlet of the cleaning apparatus 10 according to the invention may be used for powering present-day reciprocating engines (both Otto cycle and Diesel cycle) and/or gas turbine engines.

[0088] In accordance with other possible embodiments, the gas utilization unit 38 may comprise: burners and/or boilers for heating and/or for the production of sanitary hot water; manifolds for conveying the gas into a distribution network; compressors for storing the gas in cylinders or tanks; units for filtering the gas via molecular filters or membranes for fractioning the producer gas into its individual component gases (H₂, CO, N₂, etc.); units for the production of liquid fuels via catalytic processes such as the Fischer-Tropsch process; and any other type of gas utilization unit 38 known per se .

[0089] The present invention also relates to a method for treating the gas G produced by a gasifier 8. The method comprises, in steady state, the steps of:

- providing an apparatus 10 comprising an inlet line 12, an evaporative cooler 16, a condensing wet electrostatic precipitator 26 and an outlet line 36;

- conveying the flow of gas G to be cleaned coming from the gasifier 8 into the evaporative cooler 16;

- spraying an aqueous mixture MA into the flow of gas so as to achieve elimination of some of the dust suspended in the gas G and evaporation of at least some of the sprayed aqueous mixture MA and hence provide a first cooling step for the gas G by absorption of latent heat of the evaporation in the aqueous mixture MA;

- keeping in the evaporative cooler 16 a reserve of mixture MA in the condensed state;

- removing from the bottom of the reserve of condensed aqueous mixture MA the condensed pollutants and conveying them to the gasifier 8;

- conveying the flow of gas from the evaporative cooler 16 into the condensing wet electrostatic precipitator 26; cooling the walls 29 of the condensing wet electrostatic precipitator 26 in contact with the gas so as to obtain a second cooling step for the gas G by heat removal ;

- applying an electrostatic field to the flow of gas G so as to remove from the flow of gas G at least some of the suspended pollutants;

- collecting the aqueous mixture MA which has condensed during the second cooling step of the gas in the condensing wet electrostatic precipitator 26 and which contains the pollutants removed from the flow of gas G;

- removing the aqueous mixture MA condensed in the condensing wet electrostatic precipitator 26 and conveying it to the evaporative cooler 16;

- removing from the aqueous mixture MA the pollutants condensed in the condensing wet electrostatic precipitator 26 and conveying them to the gasifier 8;

removing any excess aqueous mixture MA in the condensing wet electrostatic precipitator 26 and conveying it to the outside; and

- conveying the flow of cleaned gas G to an external utilization unit 38.

[0090] In accordance with some embodiments of the method according to the invention, in the spraying step, a quantity of aqueous mixture MA is sprayed such that the first cooling step cools the gas G from a temperature of between about 400°C and about 800°C, at which it is conveyed into the evaporative cooler 16, to a temperature of between about 75 °C and about 100 °C, at which it is conveyed into the condensing wet electrostatic precipitator 26.

[0091] In accordance with some embodiments of the method according to the invention, in the step of cooling the walls 29 of the condensing wet electrostatic precipitator 26 in contact with the gas, the second cooling step cools the gas G from a temperature of between about 75°C and about 100°C, at which it is conveyed into the condensing wet electrostatic precipitator 26, to a temperature of less than or equal to about 60 °C, at which it is conveyed to an external utilization unit 38.

[0092] In accordance with some embodiments of the method according to the invention, in the step of applying an electrostatic field to the flow of gas G, all the pollutants that are not in the vapour phase, i.e. all the pollutants present in the solid state (typically in the form of dust) , in the liquid state (typically in aerosol form) and/or in the colloidal state, are removed.

[0093] In accordance with some embodiments of the method according to the invention, the step of removing excess aqueous mixture MA has the purpose of keeping the entire plant 100 in water equilibrium. Specifically, during operation, the quantity of water which is introduced into the plant 100 together with the biomass is generally equal to the quantity of water in the vapour phase removed by the flow of gas G and expelled outside at a temperature of about 60 °C. Naturally, the quantity of water removed by the flow of gas G is reduced upon a reduction in the temperature of the gas G. For this reason, if the external utilization unit requires colder gas, for example at 40 °C, the water equilibrium of the plant 100 is upset and there is an excess of water in the condensed state inside the tank 30 of the condensing electrostatic precipitator 26. In this case, it is required to perform the step of removing the aqueous mixture MA in excess of the water equilibrium.

[0094] From the above description, it will be clear to the person skilled in the art how the plant 100 in its entirety and in particular the apparatus 10 and the method according to the invention overcome the drawbacks indicated in relation to the prior art.

[0095] In particular, it will be clear to the person skilled in the art how the apparatus 10 for the filtration of the gas is extremely compact, efficient and economical to be produced.

[0096] It is clear that the specific characteristics are described in relation to the various embodiments of the cleaning apparatus 10 by way of example and in a non- limiting manner.

[0097] Clearly, a person skilled in the art, with the aim of satisfying the contingent and specific requirements, may make further modifications and variations to the apparatus 10 according to the present invention, all of which, however, fall within the scope of protection of the invention, as defined by the following claims.

Claims

1. Plant (100) for the production of gas from biomass, comprising a gasifier (8), and an apparatus (10) for cleaning the gas (G) produced by the gasifier (8), wherein the gasifier is of the open-core downdraft type and wherein the apparatus (10) for cleaning the gas comprises :

- an inlet line (12) suitable for conveying the flow of gas to be cleaned exiting the gasifier (8) and/or a dedusting unit (14);

- an evaporative cooler (16) suitable for treating the incoming flow of gas from the line (12), the evaporative cooler (16) comprising:

means (18) for spraying an aqueous mixture (MA) into the flow of gas,

a basin (20) suitable for keeping in the evaporative cooler (16) a reserve of mixture (MA) in the condensed state, and

a first bleed pipe (22) suitable for removing from the bottom of the basin (20) the condensed pollutants and conveying them to the gasifier (8); a condensing wet electrostatic precipitator (26) suitable for treating the flow of gas exiting the evaporative cooler (16) , said precipitator (26) comprising :

means (28) for cooling the walls (29) in contact with the gas,

a tank (30) suitable for collecting the contaminated aqueous mixture (MA) condensed during cooling of the gas,

a recirculation pipe (31) suitable for removing from the tank (30) excess aqueous mixture (MA) and conveying it to the evaporative cooler (16) ,

a second bleed pipe (32) suitable for removing the condensed pollutants from the bottom of the tank (30) and conveying them to the gasifier (8), and a discharge pipe (34) suitable for removing the excess aqueous mixture (MA) from the tank (34) and conveying it to the outside; and

- an outlet line (36) suitable for conveying the flow of cleaned gas to an external utilization unit (38).

2. Plant (100) according to Claim 1, wherein the evaporative cooler (16) comprises a channel passed through by the flow of gas (G) in a substantially horizontal direction, and wherein the aqueous mixture (MA) is sprayed inside the channel in a direction the same as that of the flow of gas (G) .

3. Plant (100) according to Claim 1, wherein the evaporative cooler (16) comprises a channel passed through by the flow of gas (G) in a substantially vertical direction, and wherein the aqueous mixture (MA) is sprayed inside the channel in the opposite direction to that of the flow of gas (G) .

4. Plant (100) according to any one of the preceding claims, wherein the recirculation pipe (31) draws the aqueous mixture (MA) from the tank (30) and introduces it into the basin (20); and a second recirculation pipe (24) draws the aqueous mixture (MA) from the basin (20) and feeds it to the spraying means (18).

5. Plant (100) according to any one of Claims 1 to 3, wherein the recirculation pipe (31) draws the aqueous mixture (MA) from the tank (30) and feeds it to the spraying means (18).

6. Plant (100) according to any one of the preceding claims, wherein the means (28) for cooling the walls (29) of the precipitator (26) comprise a circuit (40) suitable for allowing the circulation of a cooling fluid which passes over the walls (29) of the precipitator (26) on the side not exposed to the flow of gas (G) .

7. Plant (100) according to the preceding claim, wherein the circuit (40) is a closed circuit inside which a pre- established quantity of cooling liquid circulates.

8. Plant (100) according to the preceding claim, wherein the circuit (40) comprises a radiator suitable for dispersing into the environment the heat absorbed by the cooling liquid.

9. Plant (100) according to Claim 7 or Claim 8, wherein the circuit (28) comprises a heat exchanger suitable for retrieving the heat absorbed by the cooling liquid and conveying it away for other uses.

10. Plant (100) according to any one of Claims 7 to 9, wherein the circuit (28) comprises a refrigeration apparatus.

11. Plant (100) according to any one of the preceding claims, wherein the means (28) for cooling comprise external fins on the walls (29) of the precipitator (26) .

12. Plant (100) according to the preceding claim, further comprising means for forced ventilation of the external fins .

13. Plant (100) according to any one of the preceding claims, wherein the electrostatic precipitator (26) further comprises means (42) for heating the gas (G) , said means (42) being associated with the last section passed through by the gas (G) inside the electrostatic precipitator (26) , immediately upstream of the outlet line (36) .

14. Plant (100) according to any one of the preceding claims, comprising a unit (14) for dedusting the gas (G) , which unit (14) is situated downstream of the gasifier (8) and upstream of the apparatus (10) for cleaning the gas (G) .

15. Plant (100) according to any one of the preceding claims, further comprising a unit (38) for using the gas (G) comprising one or more of the devices selected from the group comprising: reciprocating engines, gas turbine engines, burners and/or boilers for heating and/or for the production of sanitary hot water; manifolds for conveying the gas into a distribution network; compressors for storing the gas in cylinders or tanks; units for filtering the gas via molecular filters or membranes for fractioning the gas (G) ; units for the production of liquid fuels via catalytic processes.

16. Method for treating the gas (G) produced by a gasifier (8) comprising, in steady state, the steps of:

- preparing an apparatus (10) comprising an inlet line (12), an evaporative cooler (16), a condensing wet electrostatic precipitator (26) and an outlet line (36) ;

- conveying the flow of gas to be cleaned coming from the gasifier (8) into the evaporative cooler (16);

- spraying an aqueous mixture (MA) into the flow of gas so as to achieve elimination of some of the dust suspended in the gas (G) and evaporation of at least some of the sprayed aqueous mixture (MA) and provide a first cooling step for the gas (G) by the absorption of latent heat of evaporation in the aqueous mixture (MA) ;

- keeping in the evaporative cooler (16) a reserve of mixture (MA) in the condensed state;

- removing from the bottom of the reserve of condensed aqueous mixture (MA) the condensed pollutants and conveying them to the gasifier (8); - conveying the flow of gas from the evaporative cooler (16) into the condensing wet electrostatic precipitator (26) ;

cooling the walls (29) of the condensing wet electrostatic precipitator (26) in contact with the gas so as to achieve a second cooling step for the gas (G) by heat removal;

- applying an electrostatic field to the flow of gas (G) so as to remove from the flow of gas (G) at least some of the suspended pollutants;

- collecting the aqueous mixture (MA) which has condensed during the second cooling step of the gas in the condensing wet electrostatic precipitator (26) and which contains the pollutants removed from the flow of gas (G) ; removing the aqueous mixture (MA) condensed in the condensing wet electrostatic precipitator (26) and conveying it to the evaporative cooler (16) ;

- removing from the aqueous mixture (MA) the pollutants condensed in the condensing wet electrostatic precipitator (26) and conveying them to the gasifier (8); removing any excess aqueous mixture (MA) in the condensing wet electrostatic precipitator (26) and conveying it to the outside; and

- conveying the flow of cleaned gas (G) to an external utilization unit (38) .

17. Method according to the preceding claim, wherein, in the spraying step, a quantity of aqueous mixture (MA) is sprayed such that the first cooling step cools the gas (G) from a temperature of between about 400 °C and about 800 °C, at which it is conveyed into the evaporative cooler (16), to a temperature of between about 75°C and about 100 °C, at which it is conveyed into the condensing wet electrostatic precipitator (26) .

18. Method according to Claim 16 or Claim 17, wherein, in the step of cooling the walls (29) of the condensing wet electrostatic precipitator (26) in contact with the gas, the second cooling step ^~ cools the gas (G) from a temperature of between about 75 °C and about 100 °C, at which it is conveyed into the condensing wet electrostatic precipitator (26) , to a temperature of less than or equal to about 60 °C, at which it is conveyed to an external utilization unit (38).