ITMI20100467A1 - GAS CLEANING SYSTEM PRODUCED BY GASIFICATORS. - Google Patents

Description

Description

â € œEquipment for cleaning the gas produced by gasifiers with heat recoveryâ €

[0001] The object of the present invention is an apparatus for cleaning the gas produced by a gasifier. The gasifier is suitable for producing combustible gas from biomass of different origins or from mineral coal. The cleaning apparatus is suitable for purifying the combustible gas produced by the gasifier.

[0002] Plants for the gasification of biomasses have been known for some time, ie plants suitable for producing combustible gas starting from biomass. The most relevant fraction of biomass (80-98%) is made up of carbon (C), hydrogen (H) and oxygen (O) organized in different types of molecules. The remaining fraction of biomass (2-20%) is made up of other molecules and other inorganic elements including above all silicon (Si), potassium (K), calcium (Ca), magnesium (Mg).

[0003] In a known way, the main reactions that occur during gasification are:

[0004] C O2â † ’CO2 (Combustion)

[0005] C 1⁄2 O2â † ’CO (Partial oxidation)

[0006] C H2O (g) â † ’CO H2 (Reforming of coal)

[0007] C CO2â † ’2CO (Boudouard reaction)

[0008] C 2H2â † ’CH4 (Methanation)

[0009] CO H2O (g) â † ’CO2 + H2 (Water / Gas Shift Reaction).

[0010] From these reactions, in the presence of air, a gas is obtained composed of a mixture composed of approximately 50% N2, 20% CO, 15% H2, 10% CO2 and 5% CH4. If the reactions take place in the absence of air, the final mixture does not contain N2 and takes the name of â € œsynthesis gasâ € â € œproducer gasâ € or Syngas.

[0011] Modern internal combustion engines require very high gas qualities, generally with stringent limitations on the maximum temperature of the gas that feeds them, on its relative humidity and on the dew point of the tars present therein.

[0012] Two types of plants have recently been developed for cleaning the gas produced by a biomass gasifier. Both these types of systems allow to obtain an excellent quality gas suitable for reliably feeding internal combustion engines.

[0013] The plants of the first comprise, downstream of the reactor and of a first dedusting unit, a scrubber in which a stream of water is nebulized. This water can be withdrawn from the network, especially during the initial phases, or it can be recirculated water suitably treated and cooled. The scrubber thus allows to reduce part of the suspended pollutants and, above all, to lower the gas temperature from the initial 600 ° C up to the 50 ° 70 ° C required to power the engine. In this type of scrubber, most of the heat of the gas is absorbed in the form of sensible heat by the water circulating in the scrubber. The recirculating water, by cooling the gas in the scrubber, increases its temperature. A suitable heat exchanger cools the recirculated water before it is used again in the scrubber. The gas leaving the scrubber is therefore sufficiently cooled, but is still full of pollutants and is generally saturated with water vapor. Downstream of the scrubber there is therefore an electrostatic precipitator which allows the removal of suspended pollutants until a gas of adequate quality is obtained.

[0014] As can be immediately understood, such a plant makes available, in addition to gas, a large quantity of low temperature heat. Almost all of the heat removed from the gas is in fact removed in the cooling of the recirculation water of the scrubber. This cooling therefore makes available a large quantity of fluid (typically water) at a temperature lower than or equal to the 50 ° · 60 ° C of the gas leaving the scrubber. The heat available in this form is however, as known to the expert, scarcely usable.

[0015] A plant of this first type is described for example in the international patent application WO 2008/096387 filed in the name of the same applicant.

[0016] The gas treatment plants of the second type comprise, downstream of the reactor and of a first dedusting unit, an evaporative cooler inside which a flow of water is nebulized in which they are mixed, in a predetermined proportion , of the tars coming from the gas itself. This aqueous mixture is continuously recirculated, with the sole care of removing the excess tars. The evaporative cooler thus allows to reduce part of the suspended pollutants and, above all, to lower the gas temperature from the initial 600 ° C up to 80 ° 90 ° C. In this type of evaporative cooler, most of the sensible heat of the gas is absorbed by the water in the form of latent heat of evaporation. The gas leaving the evaporative cooler must therefore still be cooled, it is still full of pollutants and is saturated with water vapor. Downstream of the scrubber there is therefore an electrostatic condensation precipitator which at the same time allows the gas to be cooled and the suspended pollutants to be removed until a gas of adequate quality is obtained. In other words, suitable cooling means of the electrostatic precipitator cause the condensation of the water present in the gas flow. The water, returned to the liquid state, is then recirculated to the evaporative cooler.

[0017] As can be immediately understood, this type of plant also makes available, like the previous one, a large quantity of low temperature heat. Almost all of the heat removed from the gas is in fact removed in the cooling of the condensing electrostatic precipitator. This cooling therefore makes available a large quantity of fluid (typically water) at a temperature lower than or equal to the 50 ° · 60 ° C of the gas leaving the precipitator. The heat available in this form is however, as already noted above, scarcely usable.

[0018] Furthermore, in this second type of plant, the condensing electrostatic precipitator is oversized compared to what it could be if it had the sole function of removing pollutants. The additional function of cooling the gas imposes in fact more stringent construction parameters, due to the poor efficiency of the heat exchange between the walls of the precipitator and the gas flow. The constructive parameters dictated by the cooling therefore translate into a considerably increased dimensioning for the same flow rate of gas to be treated.

[0019] The object of the present invention is therefore to make available an apparatus for cleaning the gas produced by a gasifier, suitable for at least partially overcoming the drawbacks indicated with reference to the known art.

[0020] In particular, the aim of the present invention is to make available a gas cleaning apparatus which is capable of producing a gas with a degree of filtration sufficient to be used also in today's sophisticated internal combustion engines.

[0021] Another task of the present invention is to make available a gas cleaning apparatus that is capable of recovering most of the thermal energy coming from the cooling of the gas at temperatures above 70 ° C Ã · 80 ° C, preferably above 100 ° C, thus increasing overall energy efficiency.

[0022] This object and these tasks are achieved by means of a gas cleaning apparatus according to claim 1.

[0023] In order to better understand the invention and appreciate its advantages, some exemplary and non-limiting embodiments thereof are described below, with reference to the attached drawings, in which:

[0024] Figure 1 is a schematic view of a plant for the production of energy from biomass including the gas cleaning apparatus according to the invention;

[0025] figure 2 is a schematic view of another plant according to the invention;

[0026] figure 3 is a schematic view of another plant according to the invention;

[0027] Figure 4 is a diagram which represents the behavior of some types of tars as the temperature varies.

[0028] In the remainder of the description, reference will often be made to the concepts of "high", "higher" and the like and, respectively, to the concepts of "low", "lower" and the like. These concepts are to be understood unambiguously with reference to the apparatus correctly assembled in order of operation, therefore subject to the force of gravity.

[0029] Reference will also be made, in the description of the gas route, to the concepts of â € œupstreamâ € and â € œdownstreamâ €. With â € œupstreamâ € we mean a position along the path relatively close to the gasifier reactor from which the gas comes out at a high temperature and dirty with dusts and tars. Conversely, 'downstream' means a location along the path that is relatively far from the reactor.

[0030] In the attached figures, the reference 100 generally indicates a plant for the production of energy from biomass. The plant 100 first of all comprises a gasifier 12 and an apparatus 10 for cleaning the gas G, according to the invention.

[0031] The apparatus 10 according to the invention in turn comprises: - an inlet duct 16 suitable for conveying the flow of gas G to be cleaned out of the gasifier 12;

- a first scrubber 20 suitable for treating the gas flow G entering the duct 16, the scrubber 20 comprising:

nebulization means 21 of a TAR mixture, based on tars without the presence of water, in the gas flow G,

a basin 22 suitable for maintaining a reserve of TAR mixture in the condensed state in the first scrubber 20,

a recirculation circuit 23 suitable for removing the TAR mixture from the basin 22 and feeding it to the nebulization means 21

a purge pipe 24 suitable for removing the condensed pollutants from the bottom of the basin 22 and conveying them to the gasifier 12, and

a heat exchanger 25 suitable for exchanging heat between the TAR mixture in the recirculation circuit 23 and a service fluid in an auxiliary circuit 26;

a channel 27 for inserting additives into the basin 22;

- a wet electrostatic precipitator 40 suitable for treating the gas flow G already treated by the first scrubber 20, called precipitator 40 comprising: a tank 42 suitable for collecting the tars and water condensed during the cooling of the gas G,

a purge pipe 44 suitable for removing the condensed tars from the bottom of the tank 42 and conveying them to the gasifier 12, and

a drain pipe 46 suitable for removing the excess aqueous mixture from the tank 42 and conveying it towards the outside;

- an outlet duct 18 suitable for conveying the flow of clean gas G to an external unit of use.

[0032] Finally, the apparatus 10 according to the invention comprises refrigerating means 50 suitable for removing a further amount of heat from the gas G before the gas G is conveyed from the outlet duct 18 to an external utilization unit.

[0033] In accordance with some embodiments of the apparatus 10, for example that shown in Figure 1, the refrigerating means 50 comprise a second scrubber 30 suitable for treating the flow of gas G leaving the first scrubber 20.

[0034] In particular, the second scrubber 30 advantageously comprises:

nebulization means 31 of a liquid LQ in the gas flow G, a basin 32 suitable for maintaining in the second scrubber 30 a reserve of liquid LQ in the condensed state,

a recirculation circuit 33 suitable for removing the liquid LQ from the basin 32 and feeding it to the nebulization means 31,

a purge pipe 34 suitable for removing the condensed pollutants from the bottom of the basin 32 and conveying them to the gasifier 12,

a drain pipe 36 suitable for removing the excess LQ liquid from the basin 32 and conveying it towards the outside;

a heat exchanger 35 located along the recirculation circuit 33 of the second scrubber 30.

[0035] In particular, the heat exchanger 35 is suitable for cooling the liquid LQ on its way to the nebulization means 31, so that the liquid LQ then cools the gas G in the second scrubber 30.

[0036] In accordance with some embodiments of the apparatus 10, for example that shown in Figure 2, the cooling means 50 comprise a circuit 45 suitable for cooling the walls of the electrostatic precipitator 40. In particular, the circuit 45 (described in detail below) is suitable for cooling gas G directly in the electrostatic precipitator 40.

[0037] In accordance with some embodiments of the apparatus 10, for example the one represented in Figure 3, the refrigerating means 50 comprise a heat exchanger 43 located along the duct which from the first scrubber 20 conducts the gas G to the electrostatic precipitator 40. In particular, the heat exchanger 43 is suitable for directly cooling the gas G before it reaches the electrostatic precipitator 40.

[0038] The gasifier 12, known per se, is preferably of the downdraft type. In this type of gasifier the biomass is introduced into the reactor from above, the gasification reactions take place in the lower part of the reactor and the gas produced is removed from the bottom of the reactor. This type of gasifier therefore differs from others, called updraft, in which the gas produced is removed from the top of the reactor.

The downdraft gasifier offers some notable advantages. First of all it produces a gas with a limited content of tars and above all it allows to use biomass with low melting point ashes as fuel. Furthermore, the downdraft gasifier produces, as a by-product of the gas, a carbonaceous residue called charcoal (or charcoal) whose demand is increasing more and more. In fact, this charcoal is used to improve the fertility of the land (in this sense it is called biochar) and above all to fix the carbon present in the biomass in an extremely stable form. This carbon derives from carbon dioxide (CO2) removed from the atmosphere by biomass.

[0040] The gasifier 12 is preferably of the opencore or opentop type, known per se. In this type of gasifier, the oxygen, necessary for the combustion and partial oxidation reactions of carbon, is supplied by the air sucked in from the environment through the upper mouth of the reactor.

[0041] In accordance with some embodiments, the sucked air can be enriched with oxygen injected near the upper mouth of the gasifier 12, so as to increase its combustive capacity. This addition of oxygen, for example to the extent of 20-30% by volume, can be useful for improving the combustion of biomass with particularly low calorific value, such as for example civil sludge and poultry manure (or manure).

[0042] The opencore gasifier has the advantage of being simpler and cheaper in construction and management, as it works at atmospheric pressure or in slight depression. The opencore gasifier therefore does not require an expensive pressurized reactor.

[0043] Furthermore, the opencore gasifier has no punctual inlets of combustion air inside the reactor. These punctual inputs of air into the biomass, in the reactors in which they are present, give rise to extremely hot points (called hot spots) which favor the melting of low melting ashes. Thanks to the absence of hot spots, the opencore gasifier has the further advantage of being able to use a wider variety of biomass.

[0044] Even more preferably, the gasifier 12 of the plant 100 is of the opencore downdraft (or opentop downdraft) type.

[0045] The patent application WO 2008/096387, in the name of the same applicant, describes a gasification plant which, in the general configuration and in the aspects known per se, is similar to the plant 100 considered here. Reference should be made to this document for the aspects not described here in detail.

[0046] The gas G leaving the gasifier 12 carries a significant quantity of pollutants and has a temperature usually between 400 ° C and 800 ° C, more often between 500 ° C and 700 ° C.

[0047] The main pollutants are charcoal dusts, ash and tars in the vapor phase or nebulized. In order to be effectively used downstream, the gas leaving the cleaning apparatus 10 must be cleaned as much as possible of pollutants and must be brought to a temperature below 80C °, preferably below 60 ° C.

[0048] The scrubber 20 is suitable for obtaining a first cooling of the gas G by means of the intimate contact of a mixture based on TAR tars with the gas itself. In other words, the nebulization means 21, placed inside the scrubber 20, nebulize the mixture TAR in the gas flow G. This TAR mixture absorbs heat entering into intimate contact with the gas and therefore lowering the temperature of the gas flow G. This mechanism works by exchanging the sensitive heat of the gas G and the TAR mixture while the evaporative and condensation phenomena of the tars are negligible.

In the simplest embodiment, the TAR mixture is considered to comprise mainly tars. In other possible embodiments of the apparatus 10 it also comprises other additives suitable for solving specific contingent problems. These additives can be in solution, emulsion or suspension. For example, one of these additives can be oil, preferably vegetable oil or vegetable oil based products such as biodiesel, which allows a better fluidization and pumpability of the TAR tar mixture in the scrubber 20.

[0050] The applicant has carried out a campaign of experimental tests relating to the use of vegetable oil in the mixture of TAR tars. These tests have shown that the use of vegetable oil in the TAR mix prevents or solves the pumpability problems associated with longer chain (heavier) tars. The oil must preferably be added to the tars in a quantity such as to obtain a TAR mixture with a viscosity suitable for the pump mounted on the recirculation circuit 23. The percentage of vegetable oil in the TAR mixture varies greatly (from 0% to 95%) depending on the type of biomass used in the gasifier 12.

[0051] The applicant has found that in practice the oil molecules surround the tar molecules and the coal dust grains coming from the gasifier 12, preventing their aggregation and effectively forming a colloidal solution. In this way the tars remain dispersed in the oil and therefore the mixture of TAR tars remains easily pumpable.

[0052] Furthermore, the vegetable oil in the mixture prevents or solves the problems of sedimentation, also connected to heavy tars. The oil makes it possible to subsequently obtain an easy separation of these heavy tars by gravity or by centrifugation.

[0053] Vegetable oil also has various properties which make it suitable for being advantageously used as an additive in the mixture of TAR tars. The most relevant of these properties of vegetable oil are listed below.

∠’It is itself a biomass and therefore, if sent to the gasifier, it participates in the production of gas.

∠’It has a specific weight of 0.85à · 0.9 kg / l therefore lower than that of the tar which is about 1.1 kg / l.

â ’It is practically immiscible with most heavy tars which cause pumpability problems even at temperatures around 100 ° C.

∠’It has a boiling point above 200 ° C while it does not degrade over time for temperatures below 200 ° C.

∠’It has a very low vapor pressure; therefore the loss of oil due to its passage in the vapor phase in the gas flow G during the operation of the plant is very low.

∠’It is not toxic and can be handled by operators without special precautions.

∠’It does not represent a danger to the environment.

∠’It is not easily flammable and it is not explosive.

[0054] The fact that the vegetable oil is immiscible with the tars and that it has a lower specific weight allows it to be added and removed easily from the TAR mixture. In fact, it separates easily since the tars are deposited on the bottom of the basin 22 to be subsequently removed by means of the purge tube 24. Alternatively, you can choose to use a suitable external decanting tank or a centrifuge. In this way the oil can be conveniently separated from the tars and reused as an additive in the scrubber 20.

In general, the TAR tar mixture also includes traces of the other elements present in gas G, such as carbon (C) in the form of charcoal and other inorganic elements in the form of ash such as silicon (Si), potassium (K), calcium (Ca), magnesium (Mg).

[0056] The temperature of the gas G leaving the scrubber 20 depends on the temperature of the mixture of tars TAR injected into the scrubber 20 by means of the nebulization means 21. At the end of an initial transient, the operating parameters of the scrubber 20 settle. If the scrubber 20 is well sized, the gas G and the tar mix TAR are practically the same temperature. In this regime, the cooling of the gas occurs almost exclusively by absorption of sensible heat by the mixture of TAR tars.

[0057] At the temperatures of the TAR tars mixture which are considered advantageous for the operation of the scrubber 20, the viscosity of said tars is sufficiently low thus allowing excellent pumpability and nebulization by means of nebulization means 21. Said temperature is preferably included between 70 ° C and 250 ° C, even more preferably between 80 ° C and 140 ° C. These conditions are further improved by the possible addition of vegetable oil in the mixture of TAR tars.

[0058] The mixture of tars TAR, in the process of cooling the gas G, progressively increases its temperature. Therefore a suitable heat exchanger 25 is provided with the function of subtracting heat from the mixture of TAR tars and maintaining its temperature at the desired value.

[0059] The relatively high temperature level of the TAR tar mixture makes it possible to obtain, in an auxiliary circuit 26, a service fluid (for example glycol water or diathermic oil) at a relatively high temperature or even to obtain steam. This fact significantly improves the energy efficiency of the plant 100 as it makes available heat that can be used in normal heating, district heating or drying systems which generally work at temperatures above 80 ° C. Furthermore, with temperatures above 100 ° C, the heat of the auxiliary circuit 26 can also be used in cooling systems based on absorption refrigeration cycles.

[0060] Figure 4 shows a diagram representing the behavior of the different tar fractions as a function of temperature. As can be seen, the different tar fractions have distinctly different behaviors at the same temperature.

[0061] In particular, at a plausible equilibrium temperature for the operation of the scrubber 20, for example at a temperature of 125 ° C, different phases coexist. There are the lighter (aromatic) tars completely in the vapor phase, other tars (light poly-aromatic and heterocyclic) partially in the vapor phase and partially in the condensed phase, and finally there are the heavier tars (heavy poly-aromatic) completely in condensed phase.

[0062] The condensed tars collect in the basin 22, increasing their quantity over time.

[0063] The first purge pipe 24 is suitable for removing part of the TAR tars which collect there from the bottom of the basin 22. They must be removed from the basin 22 to maintain the correct quantity of liquid inside the scrubber 20. Furthermore, the condensed tars present in the TAR mixture can be usefully returned to the mouth of the gasifier 12 and subjected again to the oxidation and gasification processes. Tars are in fact colloidal systems comprising a high quantity of organic substance, especially heterocyclic and polycyclic aromatic hydrocarbons. It is therefore possible to extract a further quantity of G gas from the tars.

[0064] It should be noted here that the mixture of TAR tars collected in the basin 22 has a relatively high temperature at steady state. In this way the mixture itself has, despite the composition being almost exclusively attributable to tar and vegetable oil, a very low viscosity and is generally easily pumpable.

[0065] In the attached figures, the scrubber 20 is represented as a conduit crossed by the gas flow G in a substantially horizontal direction. Inside the duct the mixture of TAR tars is nebulized in a direction concordant with that of the gas flow. In a low point of the duct is the basin 22, in which the condensed TAR mixture is collected by gravity.

[0066] In other embodiments, the scrubber 20 is made as a conduit through which the gas flow G flows in a substantially vertical direction. Inside the vertical duct the TAR tars mixture is nebulized in countercurrent with respect to the gas flow G. At the bottom of the duct there is the basin 22, in which the condensed TAR mixture is collected by gravity.

[0067] In some embodiments, for example the one shown in fig. 1, downstream of the first scrubber 20 there is the second scrubber 30 which serves to cool the gas leaving the scrubber 20 to the desired temperature (about 60 ° C).

[0068] The gas G leaving the scrubber 20 has a temperature between 75 ° C and 250 ° C, typically of about 125 ° C. This gas G is not saturated with water vapor and contains tars in the vapor phase and condensed tars in the form of aerosols.

[0069] In the second scrubber 30 a jet of liquid LQ intimately invests the gas G cooling it to the desired temperature. Part of the tars present in the gas in the vapor phase condense on the droplets of liquid which act as a condensation core. In this way, the liquid LQ which collects in the basin 32 is loaded with tars which are left to settle and are then extracted from the purge 34 and sent to the gasifier 12.

[0070] Different liquids can advantageously be used in the second scrubber 30, in order to satisfy specific requirements. Water is naturally the cheapest and most readily available liquid for such use. A basic liquid can be advantageously used to balance the presence of acidic components. A basic liquid suitable for this purpose is for example milk of lime, a suspension of calcium hydroxide particles in water. Finally, non-aqueous-based liquids can avoid the introduction of unwanted humidity into the G gas flow. Non-aqueous liquids suitable for this purpose are, for example, oil (typically vegetable oil but not only), biodiesel and diesel.

[0071] Referring again to the diagram of figure 4, it is possible to note how, at the temperature of about 60 ° C that is typically reached at the exit from the scrubber 30, most of the tars still present in the gas G in the vapor phase condense and / or in the form of an aerosol. The condensed liquids are thus collected in the basin 32.

[0072] The liquid LQ used for cooling the gas G heats up progressively and a heat exchanger 35 is therefore necessary to remove heat from the liquid LQ and keep its temperature constant.

[0073] Downstream of the scrubber 20 there is the wet electrostatic precipitator 40, also called Wet ElectroStatic Precipitator or WESP. The electrostatic precipitator 40, of a per se known type, therefore comprises ducts inside which an electrostatic field is maintained. In particular, the electrostatic precipitator 40 preferably comprises tubular structures inside each of which a rod-shaped electrode is placed. An electrostatic field can thus form between the tubular wall and the central electrode.

[0074] Furthermore, in accordance with the embodiment of figure 2, the electrostatic precipitator 40 also comprises the refrigerating means 50 for the final cooling of the gas G. In particular, the walls of the electrostatic precipitator 40 can be cooled by means of the action of the cooling circuit 45. This structure allows at the same time to clean the gas G of suspended pollutants and to cool it to the desired temperature for the final use (for example about 60 ° C). The cleaning of the gas takes place, in a per se known manner, due to the electrostatic attraction exerted on the pollutants by the walls of the precipitator 40.

[0075] The gas G enters the precipitator 40 at a temperature substantially equal to the operating temperature of the scrubber 20, for example at about 125 ° C. The gas that comes out of the scrubber 20 carries in suspension a mist of water droplets and above all of condensed tars.

[0076] Inside the electrostatic precipitator 40, the water and tar droplets adhere to the internal walls of the precipitator 40, run along them and collect in the tank 42.

[0077] As mentioned above, the tank 42 is equipped with a third bleed pipe 44. Similarly to what has already been described above with reference to the first bleed pipe 24 of the bowl 22 of the scrubber 20, the third bleed pipe 44 also allows to remove part of the TAR tar mixture from the bottom of the tank 42. For this reason, the third purge pipe 44 advantageously draws in a low point of the tank 42, where the heavier tars are collected spontaneously by gravity. The tars can be usefully returned to the mouth of the gasifier 12 and subjected again to the oxidation and gasification processes.

[0078] The lighter fraction of tars remains volatile in the gas G even at the outlet temperature from the precipitator 40, for example 60 ° C. This fraction of tars is then conveyed with the flow of gas G to subsequent uses. Typically, the use of gas G involves a combustion phase, a phase to which even light tars can usefully contribute in view of their chemical nature.

[0079] In accordance with some embodiments (see for example the diagram in Figure 2), the circuit 45 for cooling the walls of the precipitator 40 hosts the circulation of a cooling fluid which laps the walls of the precipitator 40 from the side not exposed to the gas flow G.

[0080] The cooling circuit 45 can be made in a manner known per se, for example it can advantageously be a closed circuit inside which a predetermined quantity of cooling liquid circulates.

[0081] The cooling circuit 45 can comprise a radiator suitable for dispersing the heat absorbed by the cooling liquid into the environment, or it can comprise a heat exchanger suitable for recovering the heat and conveying it to other uses. Furthermore, this type of circuit can, if necessary, include a refrigerating apparatus in the case in which it is desired to reach particularly low cooling temperatures of the G gas.

[0082] These different types of circuit 45 for cooling the walls can advantageously exist side by side on different sections of the same precipitator 40. For example, it is possible that the first sections of the precipitator 40 are equipped with a circuit comprising a heat exchanger, suitable for exploiting the relatively high temperature of the gas G for other uses, for example for heating rooms and / or sanitary water. It is also possible that the subsequent sections of the same precipitator 40 are instead equipped with a circuit comprising a radiator, suitable for dispersing the heat at low temperatures which can no longer be used otherwise.

[0083] In accordance with other embodiments, the cooling circuit 45 comprises fins external to the walls of the precipitator 40. These fins are suitable in a known way to increase the heat dispersion surface in the environment. Cooling by means of fins or the like can possibly be improved by introducing forced ventilation.

[0084] In accordance with other embodiments of the apparatus 10, for example that of Figure 3, the cooling means 50 comprise a heat exchanger 43. This heat exchanger 43 is located along the duct that leads from the scrubber 20 gas G to the electrostatic precipitator 40. Also the exchanger 43 can include different types of plant, substantially similar to those already described above for the cooling circuit 45. In particular, the exchanger 43 can comprise a radiator suitable for dispersing in the environment the heat removed from gas G (as shown in figure 3), or it can include a circuit suitable for recovering the heat and conveying it to other uses. Furthermore, also the exchanger 43 can, if necessary, include a refrigerating apparatus in the case in which it is desired to reach particularly low cooling temperatures of the gas G.

[0085] In accordance with some embodiments, the outlet duct 18 further comprises heating means 70 suitable for reheating the gas G.

[0086] The cooling of the gas G, obtained before or inside the electrostatic precipitator 40, in fact causes the condensation of most of the water and tars present in it in the form of vapor. Such condensed liquids are removed from the gas G by the electrostatic precipitator 40 itself.

[0087] It is generally useful and particularly appreciated that the gas G is cooled as much as possible to obtain a better functioning of the utilization unit, in particular of the engines. In general, therefore, gas G is cooled down to below the dewpoint, which is typically around 60 ° C for gas G produced from a wood-based biomass. In these cases, however, the gas G leaves the refrigerating means 50 (whether represented by a second scrubber 30, a condensing WESP 40, a heat exchanger 43, or other) saturated with vapors, i.e. with a relative humidity equal to 100%.

[0088] In these conditions, even a slight lowering of the temperature of the gas G therefore determines the condensation of the vapors and the consequent formation of mists inside the gas G. The occurrence of a similar lowering of temperature is extremely probable along the duct outlet 18 which carries the gas G from the electrostatic precipitator 40 towards the utilization unit. The consequent condensation of vapors and the formation of mists would therefore risk dirtying the duct 18 and the unit of use itself.

[0089] To avoid the aforementioned drawbacks, in accordance with some embodiments of the invention, the temperature of the gas G can be raised again by a few degrees (for example 10 ° 20C °). This results in a reduction in relative humidity which falls below 100%. Under these changed conditions the gas G can undergo slight changes in temperature without giving rise to the formation of mists.

[0090] The heating means 70 can advantageously exploit the heat made available by other sections of the plant 100, such as for example the auxiliary service fluid 26.

[0091] According to an embodiment, the heating means 70 comprise a circuit suitable for hosting the circulation of a heating fluid which feeds a suitable heat exchanger 71 located on the outlet duct 18 which connects the electrostatic precipitator 40 and the € ™ external use unit.

[0092] The heating means 70 can be made in a known way, for example they can advantageously comprise a closed circuit inside which a predetermined quantity of heating liquid circulates.

[0093] The temperature of the gas G leaving the apparatus 10 is usually determined on the basis of the specific needs of the external use unit. In the above description, reference has often been made to a reference temperature of about 60 ° C. This temperature is in fact considered optimal for the power supply of most of the external utilization units. Naturally, in the case just described in which the outlet duct 18 also includes the heating means 70, the temperature of the gas at the outlet from the precipitator 40 must be defined in such a way as to take into account the possible action of these heating means 70 .

[0094] Consider for example the case in which the external use unit requires gas at 55 ° C. It should also be considered that the plant 100 project envisages heating gas G by 10 ° C to reduce its relative humidity. In this case the refrigerating means 50 must be sized in such a way as to obtain, at the outlet of the precipitator 40, a flow of gas G at a temperature of 45 ° C.

[0095] It should be noted that some embodiments of the plant 100 according to the invention are capable of producing a flow of gas G containing an extremely low percentage of humidity as a function of the cooling temperature of the gas itself. First of all, the embodiments of the plant 100 shown in the attached figures 2 and 3 are considered. Furthermore, the embodiment shown in figure 1 is considered, if neither water nor a mixture is circulated in the second scrubber 30. aqueous base. In these embodiments of the plant 100, the gas G in fact does not meet water, either liquid or in the vapor state. Consequently, the only water present in the gas G is that which was already present in the biomass at the beginning of the gasification process. This quantity of water can also be reduced in passing through the electrostatic precipitator 40, especially if it includes the cooling means 50.

[0096] The fact that the humidity of the gas is limited to that of the biomass allows it to be easily controlled. In particular, using a very dry biomass will result in a very dry gas.

[0097] It should be considered that the biomasses normally used for gasification can on average originate a gas G, with dewpoint (dew point) at about 60 ° C, easily assimilated by any unit of use. In more sophisticated internal combustion engines, a lower gas temperature is sometimes required, generally between 30 ° C and 40 ° C, which is therefore lower than the dewpoint of the gas itself.

[0098] In the light of the above, the expert will be able to easily understand how the plant 100 according to the invention can work, when fully operational, without the need to use water coming from outside or to dispose of it towards the € ™ external polluted water, if the outlet gas temperature G is that of dewpoint or higher. Naturally, if the outlet gas temperature G is lower than the dewpoint temperature, it is necessary to dispose of a minimum quantity of condensed polluted water.

[0099] In accordance with some embodiments of the plant 100, between the gasifier 12 and the cleaning apparatus 10 there is a unit 14 for the dedusting of gas G. Such a dedusting unit can for example comprise a cyclone (see for example the diagrams of Figures 1 to 3) or a high temperature ceramic filter. Both of these solutions are not described in detail here because they are well known to the skilled person.

[00100] Each of the purge pipes 24, 34 and 44 and of the recirculation pipes 23 and 33 preferably comprises a pump suitable for handling the respective fluid mixture even when it is rich in heavy tars such as those which must be returned to the mouth of the gasifier 12. These pumps can preferably be gear pumps, screw pumps, lobe pumps, diaphragm pumps or peristaltic pumps, suitable for handling even very viscous fluids.

[00101] According to some embodiments, the pump placed on the recirculation tube 33, which feeds the nebulization means 31 of the second scrubber 30, can advantageously be a centrifugal pump.

This type of pump is in fact suitable for supplying a considerable flow rate of LQ liquid, as long as it has a sufficiently low viscosity.

[00102] In accordance with some possible embodiments, the purge pipes 24, 34 and 44 can advantageously comprise a decanter or a centrifuge suitable for further separating the tars from the oil or, respectively, from the water by gravity. The tars recovered from the bottom of the decanter or separated from the centrifuge can then be taken for their storage or to return them to the gasifier 12.

[00103] In accordance with some possible embodiments, the plant 100 further comprises a blower mounted on the outlet duct 18 and suitable for moving the gas G along the entire plant 100, from the gasifier 12, through the dedusting unit 14 (if present), the first scrubber 20, the second scrubber 30 or the heat exchanger 43 (if present), the electrostatic precipitator 40 up to the outlet duct 18 and beyond.

[00104] In accordance with some possible embodiments, the plant 100 finally comprises an external unit for using the gas.

[00105] In accordance with some embodiments of the plant 100, the external unit for using the gas G comprises an internal combustion engine to which a generator for the production of electrical energy can typically be connected.

[00106] In particular, it should be noted that the excellent quality of the gas leaving the cleaning apparatus 10 according to the invention allows the powering of today's alternative engines (both Otto cycle and Diesel cycle ) and / or gas turbine engines.

[00107] In accordance with other possible embodiments, the external unit for using the gas can comprise: burners and / or boilers for heating and / or for the production of domestic hot water; manifolds for conveying gas in a distribution network; compressors for storing gas in cylinders or tanks; gas filtering units using membranes or molecular filters for the fractionation of the producer gas into the individual gases that constitute it (H2, CO, N2, etc.); units for the production of liquid fuels by catalytic processes such as the Fischer-Tropsch process; and any other type of gas utilization unit known per se.

[00108] From what has been said above, it will be clear to the skilled person how the plant 100 as a whole and in particular the apparatus 10 according to the invention and overcome the disadvantages noted in relation to the known art.

[00109] In particular, it will be clear to the skilled person that the gas filtration apparatus 10 is extremely compact, effective and economical to make.

[00110] In particular, the gas cleaning apparatus 10 according to the present invention is capable of producing gas with an excellent degree of filtration, so that it can be reliably used in today's sophisticated internal combustion engines.

[00111] Furthermore, the gas cleaning apparatus according to the invention is capable of recovering about 90% of the thermal energy coming from the cooling of the gas and making this energy available in the form of a service fluid at high temperature, therefore easily usable form. As the expert can easily understand, these characteristics of the apparatus drastically increase the overall energy efficiency for the same amount of gas produced.

[00112] It is clear that the specific characteristics are described in relation to the different embodiments of the cleaning apparatus 10 with an illustrative and non-limiting intent.

[00113] Finally, the invention relates to a method for cleaning the gas G produced by a gasifier 12. The method in its different embodiments is described briefly below, since a detailed description of the phenomena has already been reported above in the course of the description of the apparatus 10 and of the plant 100.

[00114] The method for cleaning gas G according to the invention comprises the steps of:

- conveying the flow of gas G to be cleaned from the gasifier 12 to an apparatus 10 in accordance with what is described above;

- conveying the gas flow G from the inlet duct 16 to the first scrubber 20;

- nebulize a TAR mixture, based on tars without the presence of water, in the gas flow G;

- maintaining a reserve of TAR mixture in the condensed state in a basin 22 of the first scrubber 20;

- recirculate the TAR mixture by removing it from the basin 22 and feeding it to the nebulization means 21;

- remove the condensed pollutants from the bottom of the basin 22 by conveying them towards the gasifier 12;

- exchanging heat at high temperature between the TAR mixture and a service fluid in an auxiliary circuit 26;

- conveying the flow of gas G from the first scrubber 20 to the wet electrostatic precipitator 40;

- treating the flow of gas G in the precipitator 40;

- collect in a tank 42 the tars and water condensed during the cooling of gas G;

- remove the condensed tars from the bottom of the tank 42 by conveying them towards the gasifier 12;

- remove the excess aqueous mixture from the tank 42 by conveying it towards the outside;

- conveying the flow of clean gas G from the precipitator 40 to the outlet duct 18 towards an external use unit; and then

- remove a further amount of heat from the gas G before the gas G is conveyed from the outlet duct 18 to an external utilization unit.

In accordance with some embodiments of the method, the nebulized TAR blend comprises tars, vegetable oil and / or biodiesel.

[00116] In accordance with some embodiments of the method, the step of removing the further amount of heat from the gas G before conveying it to the external unit of use includes the sub-phases of:

- conveying the flow of gas G from the first scrubber 20 to a second scrubber 30;

- nebulize a liquid LQ in the gas flow G;

- maintaining a reserve of liquid LQ in the condensed state in a second basin 32 of the second scrubber 30;

- recirculating the liquid LQ by removing it from the second basin 32 feeding it to the second nebulization means 31;

- removing the condensed pollutants from the bottom of the second basin 32 by conveying them towards the gasifier 12;

- remove the excess LQ liquid from the second basin 32 and convey it towards the outside;

- cooling the liquid LQ on its way from the second basin 32 towards the nebulization means 31; And

- convey the flow of gas G from the second scrubber 30 to the wet electrostatic precipitator 40.

[00117] The liquid LQ atomized in the second scrubber 30 can be advantageously selected from the group comprising: water, milk of lime, oil, vegetable oil, and diesel oil.

[00118] In accordance with other embodiments of the method, the step of removing the additional amount of heat from the gas G before conveying it to the external utilization unit comprises the sub-step of cooling the walls of the electrostatic precipitator 40 ( see figure 2).

[00119] In accordance with still other embodiments of the method, the step of removing the additional amount of heat from the gas G before conveying it to the external utilization unit comprises the sub-step of cooling the gas G in an exchanger of heat 43 located along the duct which from the first scrubber 20 leads the gas G to the electrostatic precipitator 40 (see Figure 3).

[00120] The step described above of exchanging heat at high temperature between the TAR mixture and a service fluid in an auxiliary circuit 26 can advantageously comprise the subphase of obtaining steam in the auxiliary circuit 26.

[00121] Finally, the phase of conveying the flow of clean gas G to an external use unit can advantageously comprise the sub-phase of reheating the gas G so as to reduce its relative humidity.

[00122] Obviously, a person skilled in the art, in order to meet contingent and specific needs, can make further modifications and variations to the apparatus 10, the plant 100 and the method according to the present invention, all of which are contained in the Scope of the invention, as defined by the following claims.

Claims (20)

CLAIMS 1. Apparatus (10) for cleaning the gas produced by a gasifier (12), comprising: - an inlet duct (16) suitable for conveying the flow of gas G to be cleaned out of the gasifier (12); - a first scrubber (20) suitable for treating the gas flow G entering the duct (16), the scrubber (20) comprising: nebulization means (21) of a TAR mixture, based on tars without the presence of water, in the gas flow G, a basin (22) suitable for maintaining a reserve of TAR mixture in the condensed state in the first scrubber (20), a recirculation circuit (23) suitable for removing the TAR mixture from the basin (22) and feeding it to the nebulization means (21), a purge pipe (24) suitable for removing the condensed pollutants from the bottom of the basin (22) and conveying them to the gasifier (12), and a high temperature heat exchanger (25) suitable for exchanging heat between the TAR mixture in the recirculation circuit (23) and a service fluid in an auxiliary circuit (26); a channel (27) for inserting additives into the basin (22); - a wet electrostatic precipitator (40) suitable for treating the flow of gas G already treated by the first scrubber (20), called precipitator (40) comprising: a tank (42) suitable for collecting the tars and water condensed during the cooling of gas G, a third purge pipe (44) suitable for removing the condensed tars from the bottom of the tank (42) and conveying them towards the gasifier (12), and a drain pipe (46) suitable for removing the aqueous mixture from the tank (42) excess and to convey it towards the outside; And - an outlet conduit (18) suitable for conveying the flow of clean gas G to an external use unit; and in which said apparatus (10) further comprises refrigerating means (50) suitable for removing a further amount of heat from the gas G before the gas G is conveyed from the outlet duct (18) to an external utilization unit. Apparatus (10) according to claim 1, wherein the TAR blend comprises tars, vegetable oil and / or biodiesel. Apparatus (10) according to claim 1, wherein the refrigerating means (50) comprise, downstream of the first scrubber (20), a second scrubber (30) suitable for treating the flow of gas G leaving the first scrubber ( 20), called the second scrubber (30) comprising: second nebulization means (31) of a liquid LQ in the gas stream G; a second basin (32) suitable for maintaining in the second scrubber (30) a reserve of liquid LQ in the condensed state; a second recirculation circuit (33) suitable for removing the liquid LQ from the second basin (32) and feeding it to the second nebulization means (31); a second purge pipe (34) suitable for removing the condensed pollutants from the bottom of the second basin (32) and conveying them towards the gasifier (12); a drain pipe (36) suitable for removing the excess LQ liquid from the second basin (32) and conveying it towards the outside; And a heat exchanger (35) located along the recirculation circuit (33) of the second scrubber (30) and suitable for cooling the liquid LQ on its way to the nebulization means (31). Apparatus (10) according to claim 3, wherein said liquid LQ is selected from the group comprising: water, milk of lime, oil, vegetable oil, and gas oil. Apparatus (10) according to claim 1, wherein the cooling means (50) comprise a circuit (45) suitable for cooling the walls of the electrostatic precipitator (40). Apparatus (10) according to claim 1, wherein the refrigerating means (50) comprise a heat exchanger (43) located along the duct which from the first scrubber (20) conducts the gas G to the electrostatic precipitator (40). Apparatus (10) according to any preceding claim, wherein the heat exchanger (25) is suitable for obtaining steam in the auxiliary circuit (26). Apparatus (10) according to any preceding claim, wherein the outlet conduit (18) comprises heating means (70) suitable for reheating the gas G so as to reduce its relative humidity. 9. Plant (100) for the production of energy from biomass, comprising a gasifier (12), an apparatus (10) according to any preceding claim for cleaning the gas (G) produced by the gasifier (12), and a units for the use of gas G. Plant (100) according to the preceding claim, in which the gasifier (12) is of the downdraft type. Plant (100) according to claim 9 or 10, wherein the gasifier (12) is of the opencore type. Plant (100) according to any one of claims from 9 to 11, wherein the gasifier (12) comprises means for injecting oxygen near the upper mouth to enrich the sucked air. 13. Method for cleaning gas G produced by a gasifier (12), comprising the steps of: - conveying the flow of gas G to be cleaned from the gasifier (12) to an apparatus (10) in accordance with any one of claims from 1 to 8; - conveying the gas flow G from the inlet duct (16) to the first scrubber (20); - nebulize a TAR mixture, based on tars without the presence of water, in the gas flow G; - maintaining a reserve of TAR mixture in the condensed state in a basin (22) of the first scrubber (20); - recirculate the TAR mixture by removing it from the basin (22) and feeding it to the nebulization means (21); - remove the condensed pollutants from the bottom of the basin (22) by conveying them towards the gasifier (12); - exchanging heat at high temperature between the TAR mixture and a service fluid in an auxiliary circuit (26); - conveying the flow of gas G from the first scrubber (20) to the wet electrostatic precipitator (40); - treating the flow of gas G in the precipitator (40); - collect the tars and water condensed during the cooling of gas G in a tank (42); - remove the condensed tars from the bottom of the tank (42) by conveying them towards the gasifier (12); - remove the excess aqueous mixture from the tank (42) by conveying it towards the outside; - convey the flow of clean G gas from the precipitator (40) to the outlet duct (18) towards an external use unit; and then - remove a further amount of heat from gas G before gas G is conveyed from the outlet duct (18) to an external use unit. The method according to claim 13, wherein the nebulized TAR blend comprises tars, vegetable oil and / or biodiesel. 15. Method according to claim 13 or 14, in which the step of removing a further amount of heat from the gas G before the gas G is conveyed from the outlet duct (18) to an external utilization unit comprises the sub-phases of : - conveying the flow of gas G from the first scrubber (20) to a second scrubber (30); - nebulize a liquid LQ in the gas flow G; - maintaining a reserve of liquid LQ in the condensed state in a second basin (32) of the second scrubber (30); - recirculate the liquid LQ by removing it from the second basin (32) feeding it to the second nebulization means (31); - remove the condensed pollutants from the bottom of the second basin (32) by conveying them towards the gasifier (12); - remove the excess LQ liquid from the second basin (32) and convey it towards the outside; - cooling the liquid LQ on its way from the second basin (32) towards the nebulization means (31); And - convey the flow of gas G from the second scrubber (30) to the wet electrostatic precipitator (40). A method according to claim 15, wherein said liquid LQ is selected from the group comprising: water, milk of lime, oil, vegetable oil, and gas oil. 17. Method according to claim 13 or 14, in which the step of removing a further amount of heat from the gas G before the gas G is conveyed from the outlet duct (18) to an external utilization unit comprises the sub-phase of cool the walls of the electrostatic precipitator (40). 18. Method according to claim 13 or 14, in which the step of removing a further amount of heat from the gas G before the gas G is conveyed from the outlet duct (18) to an external utilization unit comprises the sub-phase of cooling the gas G in a heat exchanger (43) located along the duct which from the first scrubber (20) leads the gas G to the electrostatic precipitator (40). Method according to any one of claims 13 to 18, wherein the step of exchanging high temperature heat between the TAR mixture and a service fluid in an auxiliary circuit (26) comprises the sub-phase of obtaining steam in the auxiliary circuit (26) . 20. Method according to any one of claims 13 to 19, in which the step of conveying the flow of clean gas G to an external utilization unit comprises the step of reheating the gas G so as to reduce its relative humidity .