"Apparatus for cleaning gasifier produced gas"

DESCRIPTION

[ 0001 ] The present invention relates to an apparatus for cleaning the gas produced by a gasifier. The gasifier is suitable for producing fuel gas from biomass of different origin or from mineral coal. The cleaning apparatus is designed to purify the fuel, gas produced by the gasifier.

[ 0002 ] For some time plants for the gasi fication of biomass, i.e. plants designed to produce fuel gas from biomass, have been known. The largest fraction of biomass (80-98%) consists of carbon (C), hydrogen (H) and oxygen (O) which are organized in the form of different types of molecules. The remaining fraction of biomass (2-20%) consists 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 occur during gasification are as follows:

[ 0004 ] C + 0 ₂ → C0 ₂ (combustion)

[ 0005 ] C + ½ 0 ₂ → CO (partial oxidation)

[ 0006 ] C + H_0(g)→ CO + H ₂ (carbon reforming)

[ 0007 ] C + C0 ₂ → 2CO (Boudouard reaction)

[ 0008 ] C + 2Ui→ CH ₄ (methanation)

[ 0009 ] CO + H ₂ 0 _(B) → C0 ₂ + H ₂ (Water/Gas Shift Reaction).

[ 0010 ] These reactions produce, in the presence of air, a gas composed of a mixture formed approximately by 50% N ₂ , 20% CO, 15% H ₂ , 10% C0 ₂ and 5% CI-I4. If the reactions lake place without the presence of air, the final mixture does not contain N ₂ and is referred to by the name "synthesis gas ^" , "producer gas" or syngas.

[ 0011 ] Generally modern internal combustion engines require a very high quality gas and are usually subject to severe limitations regarding the maximum temperature of the gas used io fuel them, its relative humidity and the dew poi nt of the tars present in it.

[ 0012 ] Recently two types of plant for cleaning gas produced by a biomass gasifier have been developed. With both these types of plant it is possible to obtain a gas of optimum quality suitable for reliable fuelling of internal combustion engines.

[ 0013 ] The plants o f the first type comprise, downstream of the reactor and a first dedusting unit, a scrubber inside which a stream of water is sprayed in atomized form. This water may be taken from the mains, in particular initially, or may be recirculation water which has been suitably treated and cooled. The scrubber is thus able to eliminate most of the suspended pollutants and, in particular, to lower the temperature of the gas from about the initial 600°C to the 50-70°C required to fuel the engine. In this type of scrubber, most of the heal of the gas is absorbed in the form of sensible heat by the water circulating i n the scrubber. The recirculating water, cooling the gas in the scrubber, increases its temperature. A suitable heat exchanger cools the recirculating water before it is used again in the scrubber. The gas output by the scrubber is therefore sufficiently cooled, but is still full of pollutants and generally saturated with water vapour. An electrostatic precipitator is therefore provided downstream of the scrubber so as to eliminate the suspended pollutants until, a gas of suitable quality is obtained.

[ 0014 ] As can be readily understood, such a plant provides, in addition to the gas, a large quantity of heat at a low temperature. Nearly all the heat removed from the gas is in fact removed during cooling of the scrubber recirculating water. Said cooling therefore provides a large quantity of fluid (typically water) at a temperature lower than or the same as the 50-60°C of the gas output by the scrubber. The heat available in this form is however, as known to the person skilled in the art, difficult to use.

[ 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 a first dedusting unit, an evaporative cooler inside which a stream of water is sprayed containing mixed therein, in a predetermined proportion, Lars from the gas itself. This aqueous mixture is continuously recirculated, with the sole provision of removing the excess tar. The evaporative cooler is thus able to eliminate some of the suspended pollutants and, in particular, to lower the temperature of the gas from about the initial 600°C 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 output by the evaporative cooler must therefore be cooled again, is stili full of pollutants and saturated with water vapour. An electrostatic condensing precipitator is therefore provided downstream of the scrubber so as to enable both cool i ng of the gas and removal of the suspended pollutants until a gas of suitable quality is obtai ned. In other words, suitable cooling means of the electrostatic precipitator cause condensation of the water present in the gas flow. The water, reconverted i nto the liquid state, is then recirculated back to the evaporative cooler.

[0017] As can be readily understood, this type of plant also provides, l ike the previous plant, a large quantity of low-temperature heat. Nearly all the heat removed from the gas is in fact removed during the cooling performed by the electrostatic condensing precipitator. Said cooling therefore provides a large quantity of fluid (typically water) at a temperature lower than or the same as the 50-60°C of the gas output by the precipitator. The heat available in this form is, however, as already mentioned above, difficult to use.

[0018] Moreover, in this second type of plant, the electrostatic condensing precipitator has dimensions greater than those which it couid have if it performed the sole function of removing the pollutants. The additional function of cooling the gas results in fact in the need to comply with more stringent design parameters, owing to the poor efficiency of the heat exchange between the precipitator wai ls and the gas flow. The design parameters imposed by cooling therefore mean thai, for the same amount of gas to be treated, substantially larger dimensions are required.

[ 0019] The object of the present invention is therefore to provide an apparatus for cleaning the gas produced by a gasifier, which is able to overcome at least partly the drawbacks mentioned with reference to the prior art.

[ 0020 ] In particular, a task of the present i nvention is to provide a gas cleaning apparatus which is able to produce gas with a degree of fil tration sufficient for il to be used also in modern and sophisticated internal-combustion engines.

[ 0021 ] Another task of the present invention is to provide a gas cleaning apparatus which is able to recover most of the thermal energy resulting from cooling of Lhe gas to temperatures higher than 70°C - 80°C, preferably higher than 100°C, thus increasing the overall energy efficiency.

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

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

[ 0024 ] Figure 1 is a schematic view of a plant for the production of energy from biomass, comprising the gas cleaning apparatus accordi ng Lo the invention;

[ 0025 ] Figure 2 shows a schematic view of another experimental plant similar to those according to the invention;

[ 0026 ] Figure 3 shows a schematic view of another plant according to the invention;

[ 0027 ] Figure 4 is a diagram which illustrates the behaviour of certain types of tars upon variation in the temperature.

[ 0028 ] In the remainder of the description reference will be made frequently to the concepts "top", "up", and the like, and to "bottom", "low", and the like. These concepts are to be understood as referring solely to She apparatus correctly assembled for operation and therefore subject to the force of gravity.

[ 0029 ] Reference will also be made, during the description of the path followed by the gas, to the concepts "upstream ^" and "downstream ^" . "Upstream ^" is understood as meaning a position along the path, relatively close to the gasifier reactor from which the gas is emitted at a high Lemperature, contaminated with dust and tar. On the other hand, "downstream ^" is understood as meaning a position along the path relatively far from the reactor.

[ 0030 ] In the accompanying figures, the reference number 100 denotes overall a planL for the production of energy from biomass. The plan! 100 comprises firstly 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 input line 16 suitable for conveying the flow of gas G to be cleaned, output by the gasifier 12;

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

• means 21 for spraying a mixture TAR based on tars without the presence of water, into the flow of gas G,

• a basin 22 suitable for storing in the first scrubber 20 a quantity of mixture TAR in the condensed state,

• a recirculation circuit 23 suitable for removing the mixture TAR from the basin 22 and supplying it to the spraying means 21 ,

^• a bleeder pipe 24 suitable for removing the condensed pollutants from the bottom of the basin 22 and conveying them to the gasifier 1 2, and

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

• a duct 27 for introducing 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, said precipitator 40 comprising:

• a tank 42 suitable for collecting the tars and the water condensed during cooling of the gas G,

• a bleeder pipe 44 suitable for removing the condensed tars from the bottom of the tank 42 and conveying them to the gasifier 1.2, and

• a discharge pipe 46 suitable for removing the excess aqueous mixture from the lank 42 and conveying it externally;

- an output line 18 suitable for conveying the flow of clean gas G to an external user unit.

[ 0032 ] Finally the apparatus 10 according to the invention comprises cooling means 50 able to remove a further quantity of heat from the gas G before the gas G is conveyed by the output line 18 to an external user unit. The cooling means 50 are posilioned between the first scrubber 20 and the wet electrostalic precipitator 40.

[ 0033 ] According to some embodiments of the apparatus 10, for example that shown in Figure 1, the cooling means 50 comprise a second scrubber 30 su itable for treating the flow of gas G leaving the first scrubber 20.

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

- means 31 for spraying a liquid LQ into the flow of gas G,

- a basi n 32 suitable for storing in the second scrubber 30 a quantity of liquid LQ in the condensed state,

- a recirculation circuit 33 suitable for removing the liquid LQ from the basin 32 and supplying it to the spraying means 31,

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

- a discharge pipe 36 suitable for removing the excess l iquid LQ from the basin 32 and conveying it externally;

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

[ 0035 ] In particular, the heat exchanger 35 is designed to cool the liquid LQ along its path to the spraying means 31 , so that the liquid LQ may then cool the gas G inside the second scrubber 30.

[ 0036 ] According to certain experimental embodiments of the apparatus 10, for example that shown in Figure 2, the cooling means 50 comprise a circuit 45 suitable for cool ing the walls of the electrostatic precipitator 40. in particular, the circuit 45 (described in detail further below) is suitable for cool ing the gas G directly inside the electrostatic precipitator 40,

[ 0037 ] According to certain embodiments of the apparatus 10, for example that shown in Figure 3. the cooling means 50 comprise a heat exchanger 43 situated along the line which conveys the gas G from the first scrubber 20 to the electrostatic precipitator 40. In particular, the heat exchanger 43 is suitable for cool ing directly the gas G before it accesses the electrostatic precipitator 40.

[ 0038 ] The gasifier 12, which is known per se, is preferably of the downdraft type. In this type of gasifier the biomass is introduced into the reactor from the top, the gasification reactions occur 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 - so-called updraft - gasifiers in which the gas produced is removed from the top of the reactor,

[ 0039 ] The downdraft gasifier offers a number of advantages. Firstly it produces a gas with a limited tar conLent and in particular it allows to use, as fuel, biomass with ash which has a low melting point. Moreover, the downdraft gasifier produces, as a by-product of the gas, a carbon residue, called charcoal, the demand for which is constantly increasing. Said charcoal is in fact used to improve the fertility of the ground (in this sense it may be called ^" biocharcoal-) and in particular to fix in an extremely stable manner the carbon present in the biomass. This carbon is derived from the carbon dioxide (C0 ₂ ) extracted from the atmosphere by the biomass. [ 0040 ] The gasifier 12 is preferably of the open-core or open-top type known per se. In this type of gasifier, the oxygen, which is needed for the carbon combustion and partial oxidation reactions, is generally provided by the air which is drawn in from the environment via the top mouth of Lhe reactor.

[ 0041 ] According to certain embodiments, the drawn-i n air may be enriched with oxygen which is injected in the vicinity of the top mouth of the gasifier 12, so as to increase its comburent capacity. This additional oxygen, for example equal to about 20-30% in volume, may be useful for improving the combustion of biomass with a particularly low calorific power, such as domestic sludges and fowl (or poultry) droppings.

[ 0042 ] The open-core gasifier has the advantage of being more simple and economical in terms of design and operation, since it operates at atmospheric pressure or under a slight vacuum. The open-core gasifier therefore does not require a costly pressurized reactor.

[ 0043 ] Moreover, the open-core gasifier does not have inlet points for comburent air inside the reactor. These inlet points for allowing air into the biomass, in the reacLors where they are present, give rise to very hot zones, so- called ^" hot spots ^" , which favour the melting of ash with a low melting point. Owing to the absence of hot spots, Lhe open-core 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 downdraft open-core or downdraft open-top type.

[ 0045 ] Patent application WO 2008/096387, in the name of the same Applicant, describes a gasificaLion planL which, in terms of the general configuration and the aspects known per, is similar to the plant 100 considered here. Reference may be made to said document for the aspects which are not described in detail here.

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

[ 0047 ] The main pollutants are charcoal dust, ash and tars in the vaporized or atomized state. The gas G output from the cleaning apparatus 10, in order to be effectively used downstream, must be cleaned as far as possible of the pollutants and must be cooled down to a temperature below S0C° and preferably below 60C°.

[ 0048 ] The scrubber 20 is designed to produce initial cooling of the gas G by means of close contact between a tar-based mixture TAR and the gas iLself. In other words, the spraying means 21 , situated inside the scrubber 20, spray the mixture TAR into the gas flow G. This mixture TAR absorbs heat coming into close contact with the gas and therefore lowering the temperature of the gas flow G. This mechanism functions by means of exchange of sensible heal of the gas G and sensible heat of the mixture TAR, while the tar evaporation and condensation phenomena are negligible.

[ 0049 ] In the simplest embodiment it is considered that the mixture TAR comprises mainly tars. In other possible embodiments of the apparatus 10 the mixture also comprises other additives which are suitable for solving specific contingent problems. These additives may be in the form of a solution, emulsion or suspension. One of these additives may be for example oil, preferably vegetable oil or products based on vegetable oil, such as biodiesel, which ensures better fluidi fication and pumpability of the tar mixture TAR in the scrubber 20.

[ 0050 ] The Applicant has conducted a series of experimental tests relating to the use of vegetable oil in the tar mixture TAR. These tests have shown that the use of vegetable oil in the tar mixture TAR prevents or solves problems of pumpability associated with long-chain tars (heavier tars). The oil must be preferably added to the tars in an amount such as to obtain a mixture TAR with a viscosity suitable for Lhe pump mounted on the recirculation circuit 23. The percentage of vegetable oil present in the mixture TAR varies greatly (from 0% to 95%) depending on the type of biomass used in the gasifier 12. [ 0051 ] The Applicant has noted that, in practice, the oil molecules surround Lhe tar molecules and the granules of coal dust which are emitted by the gasi fier 1.2, preventing their aggregation and forming in fact a colloidal solution. In this way, the tars remain dispersed in the oil and therefore the tar mixture TAR can be pumped more easily.

[ 0052 ] Moreover, vegetable oil in the mixture prevents or solves the problems of sedimentation which are also associated with heavy tars. In other words, with this oil it is possible to achieve subsequently easy separation of these heavy tars by means of gravity or by means of centrifuging.

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

- it is itsel f a bioniass and therefore, if suppl ied to the gasi fier. contributes to the production of gas;

- it has a specific weight of 0.85 - 0.9 kg/1, Lherefore less than that of tar which is equal to about 1. 1 kg/1 ;

- it is practically immiscible with most of the heavy tars which create problems of pumpability also at temperatures of about 100°C;

- it has a boiling point higher than 200°C, while it does not degrade over time in the case of temperatures below 200°C;

- it has a very low vapour pressure; therefore the loss of oil as a result of its conversion into the vapour state in the gas flow G during operation of the plant is very low;

- it is not toxic and may be handled by operators without the need for special precautions;

~ it is environmentally friendly;

it is not easily flammable and is not explosive.

[ 0054 ] The fact that vegetable ail is immiscible with the tars and has a lower specific weight means that it can be added to and removed easily from the tar mixture TAR. It in fact separates easily since the tars are deposited on the bottom of the basin 22 and then removed via the bleeder pipe 24. Alternatively, it is possible to use a special external settling tank or a centrifuge. In this way the oil may be conveniently separated from the tars and reused as an additive in the scrubber 20.

[ 0055 ] in general, the tar mixture TAR also comprises traces of the other elements present in the 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) and magnesium (Mg).

[ 0056 ] The temperature of the gas G leaving the scrubber 20 depends on the temperature of the tar mixture TAR injected into Lhe scrubber 20 by the spraying means 21. At the end of an initial, transient, the operating parameters of the scrubber 20 stabilize. If the scrubber 20 is correctly dimensioned, the gas G and Lhe tar mixture TAR have practically the same temperature. In this operati ng condition, cool ing of the gas occurs almost exclusively by means of absorption of sensible heat by Lhe tar mixture TAR.

[ 0057 ] At the temperatures of the tar mixture TAR which are considered to be advantageous for operation of the scrubber 20, the viscosity of said tars is sufficiently low, therefore ensuring optimum pumpability and spraying by the spraying means 21 . Said temperature l ies preferably between 70°C and 250°C. and even more preferably between 80°C and 140°C. These conditions are further improved by any addition of vegetable oil to the tar mixture TAR.

[ 0058 ] The tar mixture TAR, during the process for cooling of the gas G, gradually increases its temperature. It is therefore envisaged providi ng a suitable heat exchanger 25 which has the function of drawing heat from the tar mixture TAR and keeping its temperature at the desired val ue.

[ 0059 ] Owing to the relatively high temperature of the tar mixture TAR, it is possible to obtain, in an auxiliary circuit 26, a service fluid (for example glycolaLed water or diathermic oil) at a relatively high temperature or even obtain vapour. This improves substantially the energy efficiency of the plant 100 since it provides heat which can be used in normal heating, remote heal ing or drying systems which usually operate at temperatures higher than 80°C. Moreover, at temperatures above 100°C, the heat from the auxiliary circuit 26 can be used also in cool ing systems based on absorption refrigerating cycles.

[ 0060 ] Figure 4 is a diagram which illustrates the behaviour of different tar fractions depending on the temperature. As can be seen, the different tar fractions have behaviours which are markedly different at the same temperature.

[ 0061 ] In particular, at an equilibrium temperature plausible for operation of the scrubber 20, for example at the temperature of 125°C, different phases coexist. There are l ighter tars (aromatic tars) which are entirely in the vapour phase, other tars (light polyaromatic and heterocyclic tars) which are partly in the vapour phase and partly in the condensed phase and, finally, there are heavier tars (heavy polyaromatic tars) which are completely in the condensed phase.

[ 0062 ] The condensed tars accumulate in the basin 22 and increase in quantity over time.

[ 0063 ] The first bleeder pipe 24 is designed to remove from the bottom of the basin 22 part of the tars TAR which have collected there. They must be removed from the basin 22 in order to maintain the correct quanlity of liquid inside the scrubber 20. Moreover, the condensed tars present in the lar mixture TAR may be usefully conveyed back to the mouth of the gasifier 12 where they undergo again the oxidation and gasification processes. Tars are in fact colloidal systems comprising a large quantity of organic substance, in particular heterocyclic hydrocarbons and aromatic polycyclic hydrocarbons. From the Lars it is therefore possible to obtain a further quantity of gas G .

[ 0064 ] It should be noted here that the tar mixture TAR which has collected in the basin 22 during normal operation has a relatively high temperature. In this way the mixture itself has, despite the fact that the composition is almost exclusively based on tar and vegetable oil, a very low viscosity and is generally easily pumped.

[ 0065 ] In the accompanying figures, the scrubber 20 is shown as a conduct which is travelled along by the flow of gas G in a substantially horizontal direction. Inside the conduct the tar mixture TAR is sprayed in the same direction as that of the gas flow. The basin 22 is formed at a bottom point of the conduct and the condensed mixture TAR collects there by means of gravity.

[ 0066 ] In other embodiments, the scrubber 20 is formed as a conduct followed by the flow of gas G in a substantially vertical direction. Inside the conduct the tar mix Lure TAR is sprayed in the opposite direction to that of the flow of gas G. The basin 22 is formed at a bottom point of the conduct and the condensed mixture TAR collects there by means of gravity.

[ 0067 ] In some embodiments, for example that shown in Fig. 1 , the second scrubber 30 is situated downstream of the first scrubber 20 and has the function of cool ing the gas, output from the scrubber 20, to the desi red temperature (about 60°C).

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

[ 0069 ] Inside the second scrubber 30 a jet of liquid LQ is mixed thoroughly with the gas G, cooling it to the desired temperature. Part of the tars present in the gas in the vapour phase are condensed on the droplets of liquid which act as condensation nuclei, in this way the l iquid LQ which collects inside the basin 32 becomes charged with tars which are allowed to settle before being extracted via the bleeder line 34 and conveyed to the gasifier 12.

[ 0070 ] Different liquids may be advantageously used inside the second scrubber 30 in order lo meet specific requirements. Water is obviously the least expensive liquid most easily available for this use. A basic liqu id may be advantageously used in order to counterbalance the presence of acid components. A basic l iquid suitable for this purposes is for example lime milk, i.e. a suspension of calcium hydroxide particles in water. Finally, non-water based liqu ids may prevent the introduction of undesirable moisture into the flow of gas G. Non-water based l iquids suitable for this purposes are for example oil (typically, but not exclusively, vegetable oil), biodiesel and gas oil .

[ 0071 ] With reference again to the diagram shown in Figure 4 it can be seen how, at the temperature of about 60°C which is typically reached at the outlet of the scrubber 30, most of the tars which are still present in the gas G in the vapour phase and/or in aerosol form are condensed. The condensed liquids thus collect inside the basin 32.

[ 0072 ] The liquid LQ used to cool the gas G is gradually heated and a heat exchanger 35 is therefore required in order to extract heat from the liquid LQ and keep its temperature constant.

[ 0073 ] A wel electrostatic precipitator (WESP) 40 is si Luated downstream of the scrubber 20. The electrostatic precipitator 40, of the type known per se, therefore comprises ducts inside which an electrostatic field is maintained. In particular, the electrostatic precipitalor 40 comprises preferably tubular structures inside each of which a rod-shaped electrode is arranged. An electrostatic field may thus be formed between the tubular wall and the central electrode.

[ 0074 ] Moreover, in accordance with the experimental embodiment shown in Figure 2, the electrostatic precipitator 40 also comprises the cooling means 50 for final cooli ng of the gas G. In particular, the walls of the electrostatic precipitator 40 may be cooled by means of the action of the cooling circuit 45. With this structure it is possible to clean simultaneously the gas G of the suspended pollutants and cool it down to the desired temperature for final use (for example about 60°C). Cleaning of the gas is performed, in a manner known per se, by means of electrostatic attraction exerted on the pollutants by the walls of the precipitator 40.

[ 0075 ] The gas G enters into the precipitator 40 at a temperature substantially the same as the operating temperature of the scrubber 20, for example at about 125°C. The gas leaving the scrubber 20 carries in suspension form a fine spray of water droplets and in particular condensed Lars.

[ 0076 ] Inside the electrostatic precipitator 40 the water and tar droplets adhere to the inner walls of the precipitator 40, flow along them and collect inside the tank 42.

[ 0077 ] As mentioned above, the tank 42 is provided with a third bleeder pipe 44. In a similar manner to that already described above with reference Lo the first bleeder pipe 24 of the basin 22 of the scrubber 20, also the third bleeder pipe 44 allows removal from the bottom of the tank 42 of part of the tar mixture TAR. For this reason, the third bleeder pipe 44 draws advantageously from a bottom point of the tank 42 where the heavier tars spontaneously collect by means of gravity. The tars may be usefully conveyed back to the mouth of the gasifier 12 and undergo again the oxidation and gasification processes.

[ 0078 ] The lighter tar fraction remains volatile in the gas G also at the exit temperature from the precipitator 40, for example 60°C. This tar fraction is therefore conveyed together with the flow of the gas G to the subsequent user applications. Typically the use of the gas G envisages a combustion step, to which the light tars may also usefully contribute in view of their chemical nature.

[ 0079 ] In accordance with a number of experimental embodiments (see for example the diagram in Figure 2), the circuit 45 for cooling the walls of the precipitator 40 is used to circulate a cool ing flow which passes over the wal ls of the precipitator 40 on the side which is not exposed to Lhe How of gas G.

[ 0080 ] The cooling circuit 45 may be formed in a manner known per se, for example may be advantageously a closed circuit inside which a predetermined quantity of cool ing liquid circulates.

[ 0081 ] The cooling circuit 45 may comprise a radiator sui table for dispersing into the environment the heat absorbed by the cooling liquid, or may comprise a heat exchanger suitable for recovering the heat and conveying it to other user appl ications. Furthermore, this type of circuit may comprise, if needed, a cooling apparatus, should it be required to reach particularly low temperatures for cooling of the gas G.

[ 0082 ] These different types of circuit 45 for cooling the walls may advantageously exist alongside each other on different sections of the same precipitator 40. For example, it is possible for the first sections of the precipitator 40 to be 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 for sanitary water. It is also possible for the following sections of the same precipitator 40 to be equipped instead with a circuit comprising a radiator suitable for dispersing the low-temperature heat which otherwise cannot be used.

[ 0083 ] According to other embodiments, the circuit 45 for performing cool ing comprises fins on the outside of the walls of the precipitator 40. These fi ns are suitable in a manner known per se for increasing the dispersion surface area of the heat in the environment. Cool ing by means of fins or the like may be improved if necessary with the introduction of a forced ventilation system.

[ 0084 ] According to other embodiments of the apparatus 10, for example that shown in Figure 3, the cooling means 50 comprise a heat exchanger 43. This heat exchanger 43 is arranged along the l ine which conveys the gas G from the scrubber 20 to the electrostatic precipitator 40. The exchanger 43 may comprise different types of plant, substantially similar to those already described above for the cooling circuit 45. In particular, the exchanger 43 may comprise a radiator suitable for dispersing in the environment the heat removed from the gas G (as shown in Figure 3) or may also comprise a circuit suitable for recovering the heat and conveying it to other user applications. Moreover, the exchanger 43 may also comprise a cooling apparatus in the case where it is required to reach particularly low cooling temperatures of the gas G.

[ 0085 ] The Applicant has conducted a specific series of tests intended to verify the efficiency of the different embodiments of the apparatus 10 which were envisaged during the initial design stage and which have been described above. The outcome of this series of tests has shown that there was a marked preference for the embodiments of the apparatus 10 in which the cooling means 50 are positioned between the first scrubber 20 and the wet electrostatic precipitator 40, as shown in the diagrams of Figure 1 and Figure 3. These embodiments have proved to be distinctly preferable to the embodiment shown in Figure 2 in which the cooling means 50 coincide with the wet electrostatic precipitator 40.

[ 0086 ] One of the main disadvantages of the embodiment envisaged in Figure 2 is as follows: the flow of gas G is introduced into the electrostatic precipitator 40 at the temperature it is output from the first scrubber 20, at which relatively high temperature most of the tars are suspended in vapour form . In order for these tars to be removed from the gas G inside the electrostatic precipitator 40, the vapours must be cooled and condensed until they form smal l droplets so thai they are then subject to the force of electrostatic attraction. If Lhe incoming flow of gas G is very hot and removal of the heat is not efficient (as in fact occurs in the apparatus according to Figure 2), the residence time inside the electrostatic precipitator 40 will probably not be sufficient to remove all Lhe suspended tars. In particular, it is likely that a proportion of vapours will condense in the vicinity of the output line 18, i.e. too late for it to be captured by the force of electrostatic attraction of the electrostatic precipitator 40. In this case Lhe How of gas G leaving Lhe apparatus 10 could convey along with it a part of the tars present therei n from Lhe start.

[ 0087 ] For Lhe reasons explained above, the experimental embodiment according to Figure 2 may not be regarded as forming part of the invention.

[ 0088 ] In accordance with certain embodiments, Lhe output l i ne 1 8 also comprises heating means 70 suitable for heating the gas G again.

[ 0089 ] Cooling of the gas G, which is performed before or inside the electrostatic precipitator 40, in fact causes condensation of most of the water and Lhe tars present therein in the form of vapour. This condensed liqu id is removed from the gas G by the said electrostatic precipitator 40.

[ 0090 ] In general it is useful and particularly advantageous for the gas G to be cooled as far as possible in order to obtain better operation of Lhe user unit, in particular of the engines. Generally, therefore, the gas G is cooled to below the dew point which is typically in the region of 60°C for the gas produced by a wood-based biomass. In these cases, however, the gas G is emitted from the cool ing means 50 (whether they consist of a second scrubber 30, a condensi ng WESP 40, a heat exchanger 43, or the like) vapour-saturated, i.e. wi th a relative humidity of 100%.

[ 0091 ] In these conditions even a slight drop in temperature of the gas G therefore causes condensation of the vapours and the consequent formation of mists within the gas G. The occurrence of such a drop in temperature is highly likely along the output line 18 which conveys the gas G from the electrostatic precipitator 40 to the user unit. The consequent condensation of vapours and formation of mists would therefore risk soil ing the l ine I S and the user unit itself.

[ 0092 ] In order to overcome the abovementioned drawbacks, in accordance with certain embodiments of the invention, the temperature of the gas G may be raised again a few degrees (for example 10-20C ⁰ ). A reduction in the relative humidity is thus obtained which falls below 100%. In these changed conditions the gas G may be subject to slight temperature fluctuations, but without this giving rise to the formation of mists.

[ 0093 ] The heating means 70 may advantageously make use of the heat provided by other sections of the plant 100, such as the auxiliary service fluid 26.

[ 0094 ] According to one embodiment, the heating means 70 comprise a circuit which is suitable for the circulation of a heating fluid which supplies a sui table heat exchanger 71 situated along the output line I S which connects the electrostatic precipitator 40 and the external user unit.

[ 0095 ] The heating means 70 may be formed in a manner known per se, for example may comprise advantageously a closed circuit inside which a predetermined quantity of a heating liquid circulates.

[ 0096 ] The temperature of the gas G leaving the apparatus 10 is usually determined on the basis of the specific needs of the external user unit. In the description above a reference temperature of about 60°C has been frequently referred to. This temperature is in fact considered to be optimum for supplying most of the external user units. Obviously, in the case described above in which the output line I S also comprises the heating means 70, the temperature of the gas output by the precipitator 40 must be defined so as to take into account any action of said heating means 70.

[ 0097 ] Let us consider for example the case where the external user unit requires gas at 55°C. Let us also consider that the design ol ^' the plant 100 envisages heating the gas G by 10°C in order to reduce its relative humidity. In this case the cooling means 50 must have dimensions so as to obtain, at the outlet of the precipitator 40, a flow of gas G at the temperature of 45°C.

[ 0098 ] It should be noted how some embodiments of the plant 100 according to the invention are able to produce a flow of gas G containing an extremely low percentage amount of humidity depending on the cool ing temperature of the gas itself. The embodiments of the plant 100 shown in the accompanying Figures 2 and 3 are considered here firstly. Furthermore the embodiment shown in Figure 1 is considered, if neither water nor a water-based mixture is circulated in the second scrubber 30. In these embodiments of the plant 100, the gas G in fact does not encounter water either in the liquid state or in the vapour slate. Consequently, the only water present in the gas G is that which was already present in the biomass at the start of the gasification process. This quanLity of water may also be reduced when it passes through the electrostatic precipi tator 40, especial ly if the latter comprises the cooling means 50.

[ 0099 ] The fact that the humidity of the gas is lim ited to that of the biomass allows its value to be easily controlled. In particular, if a very dry biomass is used, a very dry gas will be obtained.

[ 00100 ] It should be considered that the biomass normally used for gasification may produce on average a gas G with a dew point of about 6()°C, which can be easily assimilated by any user unit. In more sophisticated internal- combustion engines a lower gas temperature, generally between 30°C and 40°C, i.e. lower than then the dew point of the gas itself, is sometimes required.

[ 00101 ] In the light of the above explanations, the person skil led in the art may easily understand how the plant 100 according to the invention is able to operate, during normal operating conditions, wiLhout having Lo use water from the outside or dispose of polluted water externally, if the temperature of the outgoing gas G is that of the dew point or higher. Obviously, if Lhe temperature of the outgoing gas G is lower than the temperature of the dew point, it is required lo dispose of a minimum amount of condensed contaminated water.

[ 00102 ] According to a number of embodiments of the plant 100 a unit .14 for dedusting the gas G is present between the gasifier 12 and the cleaning apparatus 10. This dedusting unit may for example comprise a cyclone {see for example the diagrams in Figures 1 to 3) or a high-temperature ceramic filler. Both these solutions are not described here in detail since they are known per se to the person skilled in the art.

[ 00103 ] Each of the bleeder pipes 24, 34 and 44 and the recirculation pipes 23 and 33 preferably comprises a pump suitable for moving the respective fl uid mixture MA even when it is rich in heavy tars such as those which must be conveyed back to the mouth of the gasifier 12. These pumps may be preferably gear pumps, screw pumps, lobe pumps, diaphragm pumps or peristaltic pumps which are suitable for moving fluids which may also be very viscous.

[ 00104 ] According to a number of embodiments, Lhe pump situated along the recirculation pipe 33 which suppl ies Lhe spraying means 31 of the second scrubber 30 may be advantageously a centrifugal pump. This type of pump is in fact suitable for providing a considerable throughput of l iquid LQ, provided thai il has a su fficiently low viscosity.

[ 00105 ] According to some possible embodiments, the discharge pipes 24, 34 and 44 may advantageously comprise a settling tank or a centrifuge suitable for separating further by means of gravity the tars from the oil or from the water, respectively. The tars recovered from the bottom of the settl ing tank or separated by the centrifuge may then be removed for storage or for conveying them back to the gasifier 12.

[ 00106 ] According to some possible embodiments, the plant 100 also comprises a blower mounted on the output line 18 and able to move the gas G along the entire plant 100, from the gasifier 12 via the dedusting unit 14 (if present), the first scrubber 20, the second scrubber 30 or the heat exchanger 43 (if present), and the electrostatic precipitator 40 as far as the output line 18 and beyond.

[ 00107 ] According to some possible embodiments, the plant 100 comprises, finally, an external gas user unit.

[ 00108 ] According to 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 electric energy may be typically connected.

[ 00109 ] In particular, it should be noted that the excellent qual ity of the gas G output by the cleaning plant 10 according to the invention allows the fuelling of modern reciprocating engines (both Otto cycle and Diesel cycle engines) and/or gas turbine engines.

[ 00110 ] According to other possible embodiments, the external gas user unit may comprise: burners and/or boilers for heating and/or for the production of hot sanitary water; headers for conveying the gas into a supply network; compressors for storing the gas in cylinders or tanks; units for filtering the gas by means of molecular filters or membranes for dividing up the producer gas into the individual constituent gases (H ₂ , CO, N ₂ , etc.); units for the production of liquid fuels by means of catalytic processes such as the Fischer-Tropsch process; and any other type of unit for using the gas known per se.

[00111] From thai stated above it will be clear to the person skil led in the art how the plant 100 in its entirety and in particular the apparatus 10 according to the invention overcome the disadvantages mentioned in connection with the prior art.

[00112] In particular, it will be clear to the person skilled in the art how the gas fil tration apparatus 10 is extremely compact and reliable and has a low manufacturing cost.

[00113] In particular, the gas cleaning apparatus 10 according to the present i nvention is able to produce gas with an optimum degree of filtration such that i t may be reliably used in modern and sophisticated internal-combustion engines.

[00114] Moreover, the gas cleaning apparatus according to the invention is able to recover about 90% of the thermal energy resulting from cooling of the gas and provide this energy in the form of a high-temperature service fluid, i.e. in a form which can therefore be easily used. As the person skilled in the art can easily understand, these characteristic features of the apparatus drastically increase the overall energy efficiency for the same amount of gas produced.

[00115] It is clear that the specific characteristics are described in connection with the various embodiments of the cleaning apparatus 30 with an exemplifying, non-limiting intent.

[00116] Finally the invention also relates to a method for cleaning the gas G produced by a gasifier 12. The different modes of implementation of the method are described briefly below since a detailed description of the operations involved has already been provided above during the description of the apparatus 10 and the plant 100.

[00117] The method for cleaning the 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 1.0 in accordance with the description given above; - conveying the flow of gas G from the input line 16 to the first scrubber 20;

- spraying a mixture TAR based on tars withou L the presence of water, into the flow of gas G;

- storing a quantity of mixture TAR in the condensed state inside a basin 22 of the first scrubber 20;

- recirculating the mixture TAR by removing it from the basin 22 and supplying it to the spraying means 21;

- removing the condensed pollutants from the bottom of the basin 22 and conveying them to the gasifier 12;

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

- conveying the flow of gas G from the first scrubber 20 to the cooling means 50;

- removing a further quantity of heat from the gas G;

- conveying the flow of gas G from the cooling means 50 to the wet electrostatic precipitator 40;

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

- collecting inside a tank 42 the tars and the water condensed duri ng cool ing of the gas G;

- removing the condensed tars from the bottom of the tank 42 and conveying them to the gasifier 1 2;

- removing the excess aqueous mixture from the tank 42 and conveying i t externally; and

- conveying the flow of clean gas G from the precipitator 40 via the output line 18 to an external user unit.

[ 00118 ] According to certain modes of implementation of the method, the sprayed mixture TAR comprises tars, vegetable oil and/or biodiesel.

[ 00119 ] in accordance with certain modes of implementation of the method, the step of removing the further quantity of heat from the gas G inside the cooling means 50 comprises the substeps of; - conveying the flow of gas G from the first scrubber 20 to a second scrubber 30;

- spraying a liquid LQ into the flow of gas G;

- storing in a second basin 32 of the second scrubber 30 a quantity of liquid LQ in the condensed state;

- recirculating Lhe liquid LQ by removing it from the second basin 32 and supplying it to the second spraying means 31 ;

- removing the condensed pollutants from the boUom of the second basin 32 and conveying them to the gasifier 12;

- removing the excess liquid LQ from the second basin 32 and conveying it externally;

- cooling the liquid LQ along its path from the second basin 32 to the spraying means 31 ; and

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

[ 00120 ] The liquid LQ sprayed in Lhe second scrubber 39 may be advantageously selected from the group comprising: water, lime milk, oil, vegetable oil and gas oil .

[ 00121 ] According to yet other modes of implementation of the method, the step of removing from the gas G the further quantity of heat inside the cooling means 50 comprises the substep of cooling Lhe gas G inside a heat exchanger 43 situated along the line which conveys the gas G from the first scrubber 20 Lo lhe electrostatic precipitator 40 (see Figure 3).

[ 00122 ] The step described above of exchanging high-temperature heat between the mixture TAR and a service fluid in an auxiliary circuit 26 may advantageously comprise the substep of obtaining vapour in the aux il iary circuit 26.

[ 00123 ] Finally, the step of conveying the flow of clean gas G lo an external user uni t may advantageously comprise the substep of heating the gas G again so as lo reduce its relative humidity. [ 00124 ] Obviously, a person skilled in the art. in order to satisfy specific contingent requirements, may make to the apparatus 10, plant 100 and method according to the present invention further modifications and variations, ail of which being moreover contained within the scope of protection of the invention, as defined by the following claims.