Patent Publication Number: US-2013247846-A1

Title: Macroapparatus for the production and treatment of gas obtained from mineral coal

Description:
The present invention relates to a macroapparatus for the production, cooling and cleaning of gas produced by a gasifier. The gasifier is of the updraft type and is suitable for producing fuel gas from mineral or fossil coal. The cleaning apparatus is designed to purify the fuel gas produced by the gasifier. 
     For some time plants for the gasification of mineral coal, i.e. plants designed to produce fuel gas from coal, have been known. The largest fraction of mineral coal (80-98%) consists of carbon (C), hydrogen (H) and oxygen (O) which are organized as different types of molecules. The remaining fraction of the coal (2-20%) consists of other molecules and other inorganic elements including silicon (Si), potassium (K), calcium (Ca) and magnesium (Mg). 
     In a manner known per se, the main reactions which occur during gasification are as follows: 
     C+O 2 →CO 2  (combustion)
 
C+½O 2 →CO (partial oxidation)
 
C+H 2 O (g) →CO+H 2  (coal reforming)
 
C+CO 2 →2CO (Boudouard reaction)
 
C+2H 2 →CH 4  (methanation)
 
     CO+H 2 O (g) →CO 2 +H 2  (Water/Gas Shift Reaction). 
     These reactions produce, in the presence of air, a gas (called “producer gas”) composed of a mixture consisting in dry form of about 50% N 2 , 20% CO, 15% H 2 , 10% CO 2  and 5% CH 4 . If the reactions take place without the presence of air, the final mixture does not contain N 2  and is referred to by the name of “synthesis gas” or syngas. 
     Various types of gasification plants are known, these differing on the basis of the reactor structure, the path followed by the gas within the reactor, the type of filtration apparatus used, etc. 
     Gasification plants of the known type are, however, not without defects. 
     The present-day gasification plants may be classified as two main types. The plants of the first type are constructed mainly for experimental purposes, are characterized by large dimensions (power output typically greater than 1 Megawatt) and use sophisticated technology. These dimensions and the fact that they are built generally on a “one-off basis” mean that these plants are not suitable for large-scale commercialization. 
     The plants of the second type are characterized by small dimensions, use rudimentary technology and are suitable in particular for rural environments in developing countries. The technological backwardness of these plants is such that they cannot be used on a large scale in the Western energy market. 
     In the 1940s extremely compact gasification plants were built. These plants were generally mounted on motor vehicles in order to compensate for the absence of petroleum-derived products. These plants in fact enabled internal combustion engines to be run using wood or mineral coal. They were characterized by small dimensions, but were inefficient, produced a gas of unacceptable quality by modern standards and generally created serious problems in terms of environmental pollution. 
     Present-day internal combustion engines require gas of a very high quality. In general, further—relatively severe—restrictions are also applicable with regard to the maximum temperature of the supply gas, its relative humidity and the dewpoint of the tars present therein. 
     Achieving such a quality of gas, and in particular lowering the temperature of the gas used to fuel an engine, results in the formation of condensation water. This water is heavily contaminated with organic substances (phenols, ammonia, benzenes, etc.) and therefore gives rise to major problems with regard to the treatment and elimination of said substances. 
     The object of the present invention is therefore to provide an apparatus for cooling and cleaning the gas produced by an updraft gasifier for mineral coal, which is able to overcome at least partly the drawbacks mentioned with reference to the prior art. 
     In particular, a task of the present invention is to provide an apparatus for cooling and cleaning the gas which has generally compact dimensions and which can be built at a low cost, easily implemented on an industrial scale and optimized for efficient gasification of mineral coal. 
     Moreover, a task of the invention is to provide an apparatus and a method which are able to produce high-quality gas at a temperature sufficiently low for use also in present-day internal combustion engines. 
     This object and these tasks are achieved by means of a macroapparatus according to Claim  1 , by means of plant according to Claim  6  and by means of a method according to Claim  9 . 
    
    
     
       In order to understand the invention and to appreciate its advantages, a number of exemplary and non-limiting embodiments thereof are described below with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic view of a plant for the production of energy from mineral coal, comprising the macroapparatus for the production and treatment of the gas and generation of wet air according to the invention; 
         FIG. 2  is a diagram which illustrates the behaviour of certain types of tars upon variation in the temperature; 
         FIG. 3  shows a perspective view of the assembly consisting of the evaporative cooler and the humidifier forming part of an embodiment of the macroapparatus according to the invention; 
         FIG. 4  shows a plan view of the assembly according to  FIG. 3 ; 
         FIG. 5  shows a side view of the assembly according to  FIG. 3 ; 
         FIG. 6  shows a cross-sectional view along the line VI-VI of  FIG. 4 ; 
         FIG. 7  shows a cross-sectional view of an embodiment of the gasifier forming part of the macroapparatus according to the invention, in the closed configuration; 
         FIG. 8  shows a cross-sectional view of the gasifier of  FIG. 7  in the open configuration; 
         FIG. 9  shows an enlarged view of the detail indicated with IX in  FIG. 7 ; and 
         FIG. 10  shows an enlarged view of the detail indicated with X in  FIG. 8 . 
     
    
    
     In the remainder of the description reference will often be made to the concepts “up”, “top” or the like, and respectively to “low”, “bottom” or the like. These concepts are to be understood as referring solely to the apparatus correctly assembled for operation and therefore subject to the forces of gravity. 
     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 reactor inlet through which the mineral coal is fed. On the other hand, “downstream” is understood as meaning a position along the path relatively far from the reactor inlet. 
     In the accompanying figures, the reference number  100  denotes overall a plant for the production of energy from fuel, in particular from mineral coal CM. The plant  100  comprises firstly a macroapparatus  10  for the production and treatment of the gas G. In accordance with the embodiment shown in  FIG. 1 , the plant  100  also comprises, downstream of the macroapparatus  10 , other components which will be described below. These further components preferably include an electrostatic filter  40  and a unit  80  for using the gas G. 
     The macroapparatus  10  according to the invention comprises:
         a gasifier  12  suitable for receiving a flow of wet oxygenated gas AU and fuel CM and emitting a flow of gas G;   a line suitable for conveying the flow of gas G from the gasifier  12  to a dedusting unit  14 ;   a dedusting unit  14 ;   a line suitable for conveying the flow of gas G from the dedusting unit  14  to an evaporative cooler  20 ;   an evaporative cooler  20  suitable for treating the flow of gas G, comprising:   means  21  for spraying an aqueous mixture MA into the flow of gas G,   a basin  22  suitable for allowing settling (decanting) and for storage in the evaporative cooler  20  of a quantity of aqueous mixture MA in the condensed state,   a recirculation circuit  23  suitable for removing the aqueous mixture MA from the basin  22  and supplying it to the spraying means  21  of the evaporative cooler  20 ; and   a bleeder pipe  24  suitable for removing the condensed pollutants from the bottom of the basin  22  and conveying them externally;   a line suitable for conveying the flow of gas G from the evaporative cooler  20  to a scrubber  30 ;   a scrubber  30  suitable for treating the flow of gas G, comprising:   means  31  for spraying an aqueous mixture MA into the flow of gas G,   a basin  32  suitable for allowing settling (decanting) and for storage in the scrubber  30  of a quantity of aqueous mixture MA in the condensed state,   a recirculation circuit  33  suitable for removing part of the aqueous mixture MA 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 externally; and   a heat exchanger  35  located along the recirculation circuit  33  of the scrubber  30 , and   a compensation pipe  37  suitable for removing part of the aqueous mixture MA from the basin  32  and supplying it to the basin  22  of the evaporative cooler  20 ;   a line suitable for conveying the flow of gas G from the scrubber  30  outside of the macroapparatus  10 .       

     Finally, the macroapparatus  10  according to the invention comprises a humidifier  60  suitable for receiving a flow of oxygenated gas A and of aqueous mixture MA and for emitting a flow of wet oxygenated gas AU, the humidifier  60  comprising in turn:
         a line  61  for supplying a flow of oxygenated gas A;   means  63  for spraying an aqueous mixture MA into the flow of oxygenated gas A;   a pipe  65  suitable for removing a quantity of aqueous mixture MA from the basin  22  of the evaporative cooler  20  and for supplying the aqueous mixture MA to the spraying means  63 ;   a line  62  suitable for conveying the wet oxygenated gas AU from the humidifier  60  to the bottom part of the gasifier  12 .       

     The macroapparatus  10  described above may preferably comprise one or more of the following auxiliary components:
         a pipe  27  suitable for introducing from the outside water into the macroapparatus  10 , for example supplying it to the spraying means  21  or to the basin  22  of the evaporative cooler  20 ;   a bleeder pipe  36  suitable for removing the excess aqueous mixture MA from the basin  32  of the scrubber  30  and conveying it externally; and/or   a recirculation circuit  64  suitable for removing any condensed aqueous mixture MA from the bottom of the humidifier  60  and conveying it to the basin  22  of the evaporative cooler  20 .       

     The gasifier  12  is suitable, in a manner known per se, for treating different fuels such as coal of various kinds, charcoal or mineral coal. 
     The gasifier  12  is preferably of the updraft type known per se. In this type of gasifier the fuel CM is introduced into the reactor from above, the gasification reactions occur in the bottom part of the reactor and the gas produced is removed from the top of the reactor, i.e. in the vicinity of the gasifier inlet. This type of gasifier therefore differs from other—so-called downdraft—gasifiers in which the gas produced is removed from the bottom of the reactor. 
     The mineral coal CM is introduced into the top of the gasifier  12  by means of a valve system  124  (see in particular  FIGS. 7 and 8 ). These valves, in a manner known per se, prevent the entry of external air into the gasifier  12  or the escaping of gas G from the gasifier  12  into the external environment. The valves  124  may comprise, for example, a pair of shutter elements arranged in series (as shown in the accompanying figures) or other devices known per, such as rotary valves. 
     The updraft gasifier offers a number of notable advantages during the gasification of mineral coal since it is able to exploit entirely the carbon C contained therein, therefore achieving a very high energetic efficiency. Moreover, the updraft gasifier, as a by-product of gasification of mineral coal, produces a solid inert residue commonly called ash. 
     The gasifier  12  is preferably of the updraft open-top or open-core type known per se. In this type of gasifier, the oxygen which is needed for the reactions of combustion and partial oxidation of the carbon is generally provided by the air which is drawn in from the environment at atmospheric pressure via the proper opening present in the bottom of the reactor. This type of gasifier therefore differs from other types in which the oxygen which is necessary for the combustion/gasification reactions is supplied at a pressure greater than atmospheric pressure by an apposite plant or is injected at given points directly inside the reactor. 
     The open-core gasifier has the advantage of being very simple and economical in terms of construction and management thereof. 
       FIGS. 7 to 10  show a particular embodiment of the gasifier  12  included in the macroapparatus  10  according to the invention. 
     According to this embodiment, the gasifier  12  comprises means  120  for raising the outer jacket  121 . 
       FIG. 7  shows the gasifier  12  in the closed configuration. This configuration is the configuration which is maintained during operation of the gasifier  12 , but also during the normal non-operative periods. In this configuration, the outer jacket  121  therefore separates the inside of the gasifier  12  from the external environment. 
       FIG. 8  shows the same gasifier according to  FIG. 7 , but in its open configuration. This configuration is the configuration which is maintained during maintenance of the gasifier, typically during the removal of foreign bodies  123 . In this configuration, the outer jacket  121  is raised and therefore defines an opening which establishes communication between the inside of the gasifier  12  and the external environment. 
     A fairly common occurrence is the involuntary introduction into the gasifier  12 , during introduction of the coal CM, of foreign bodies  123 . These foreign bodies  123  are typically bodies which, owing to their physical and chemical nature, are unable to take part in the gasification reactions, i.e. typically stones or metal waste. For this reason, the foreign bodies  123  pass through all the zones of the gasifier  12 , their structure and mass remaining more or less unchanged, and are deposited finally on the bottom grille  122  supporting the coal CM. 
     The accumulation of foreign bodies  123  on the bottom grille  122  may in the long run result in an unacceptable alteration in the process conditions, for example owing to the anomalous mechanical stresses acting on the grille cleaning members. In the light of the above, it is therefore required to remove periodically the foreign bodies  123  from the bottom grille  122 . 
     In this connection, operation of the gasifiers of the known type must be periodically interrupted in order to allow disassembly of the outer casing of the reactor so that the grille can be accessed. This solution, although widely used, results in long downtimes of the plant, said downtimes being necessary in order to allow cooling of the reactor and enable the operating staff to disassemble the reactor, remove the foreign bodies  132  and restore the working configuration of the reactor. As the person skilled in the art can easily understand, these long downtimes have a negative impact on the productivity of the plant. 
     With the embodiment of the gasifier  12  shown in  FIGS. 7 to 10  it is instead possible to complete the operation of removal of the foreign bodies  123  in an extremely simple manner. The raising means  120  in fact allow the outer jacket  121  of the reactor to be raised so that there is direct access to the bottom grille  122 . Removal of the foreign bodies  123  may thus be performed in a simple and rapid manner, without the need for lengthy stoppage of the plant. 
       FIGS. 9 and 10  show a detail of the raising means  120 , in the closed operating configuration and in the open maintenance condition, respectively. In the specific embodiment shown in the figures, the raising means  120  comprise a plurality of jacks which are arranged on the outer perimeter of the jacket  121 . 
     According to certain embodiments, not shown in the accompanying figures, the macroapparatus  10  also comprises means for evacuation of the dust and/or ash which collects, respectively, inside the gasifier  12  and the dedusting unit  14 . 
     These means comprise advantageously one or more screw conveyors which are designed to remove the dust and/or ash and to convey them to special collection or storage containers. Moreover, the dust and/or ash evacuation means preferably comprise valves which are similar to the valves  124  described above. These valves are in fact designed to allow the dust and/or ash to be removed from the dedusting unit  14 , without at the same time allowing either the entry of air from the outside towards the inside or, vice versa, the escaping of gas G from the inside towards the outside of the macroapparatus  10 . These valves are particularly advantageous in the case where the inside of the plant  100  is kept under a vacuum in order to obtain the movement of the gas G along the plant itself. In this case, the absence of the valve on the dust outlet from the dedusting unit  14  would result in the entry of air and the consequent explosion of the gasifier  12  itself. 
     The gas G leaving the gasifier  12  has a temperature of about 400° C.-800° C. and conveys a considerable quantity of pollutants. The main pollutants are dust from mineral coal CM, ash and tars in the vaporized or atomized state. In fact, the normal mineral coal CM found in nature (anthracite, lignite, etc.) always contains a fraction of volatile hydrocarbons which, during the process of heating and gasification inside the gasifier  12 , evaporate and are converted into the vapour state in the gas flow G. 
     The gas G, in order to be effectively used in a user unit  80 , must be as far as possible free of pollutants and must be cooled down to a temperature below 70° C. and preferably below 60° C. 
     The unit  14  for dedusting the gas G may for example comprise a cyclone (see for example the diagram in  FIG. 1 ) or a high-temperature ceramic filter (not shown). Both these solutions are not described here in detail since they are well known per se to the person skilled in the art. 
     The evaporative cooler  20  is designed to obtain initial cooling of the gas G by means of evaporation of a water-based mixture MA until the gas G is saturated. In other words, the atomization means  21  situated inside the evaporative cooler  20  spray the aqueous mixture MA into the flow of gas G. Part of this mixture MA evaporates, absorbing the heat and therefore lowering the temperature of the flow of gas G. This mechanism functions efficiently until the vapours produced by the evaporation of the aqueous mixture MA saturate the gas G. 
     In the embodiments described below it is considered that the aqueous mixture MA comprises mainly water, polar (hence water-soluble) tars and non-polar (non-soluble) tars. 
     In other possible embodiments of the apparatus  10  the aqueous mixture MA could also comprise other additives which are suitable for solving specific contingent problems. These additives may be in the form of a solution, emulsion, suspension or in any case mixed with water. One of these additives may, for example, comprise particles of calcium hydroxide Ca(OH) 2  which, in an aqueous suspension, form so-called lime milk. The use of lime milk allows neutralization of any acid compounds present in the gas G. Another useful additive could in some cases be an anti-foaming agent. 
     In general, the aqueous mixture MA also comprises traces of the other elements present in the gas G, such as carbon (C) in the form of mineral coal CM and other inorganic elements in the form of ash such as silicon (Si), potassium (K), calcium (Ca) or magnesium (Mg). 
     As already described above, the spraying means  21  may advantageously comprise a connection  27  to the water mains or to some other water supply outside the plant  100 . It is thus possible to compensate for any water losses occurring in the evaporative cooler  20  or downstream thereof. 
     The evaporative cooler  20  also comprises a basin  22  which stores a quantity of aqueous mixture MA in the condensed state. The presence of the mixture MA in the liquid phase ensures effective saturation of the gas G passing through the evaporative cooler  20 , but in particular is necessary in order to be able to achieve effective washing of the gas G. 
     A quantity of aqueous mixture MA much greater than that required to saturate the gas G is injected into the evaporative cooler  20  by the spraying means  21 . The excess quantity of aqueous mixture MA which therefore passes through the gas G in the liquid state results in removal of the suspended condensed tars and the fine dust which cannot be retained by the dedusting means  14 . Obviously the aqueous mixture MA which is sprayed in excess of that producing saturation of the gas G, and which therefore remains in the liquid state, is collected inside the basin  22 . 
     The temperature of the gas G leaving the evaporative cooler  20  depends on the concentration of tars in the aqueous mixture MA and the type of said tars. At the end of an initial transient, the operating parameters of the evaporative cooler  20  stabilize. If the evaporative cooler  20  is correctly dimensioned, the saturated gas G and the condensed aqueous mixture MA have practically the same temperature. In this operating condition, cooling of the gas G occurs almost exclusively by means of absorption of latent evaporation heat by the aqueous mixture MA. 
     If pure water were used in place of the aqueous mixture MA, the equilibrium temperature inside the evaporative cooler  20  would stabilize at about 80° C. With an increase in the concentration of tars in the aqueous mixture MA the ebullioscopic constant of the aqueous mixture MA also increases and therefore the equilibrium temperature rises. 
     The basin  22  of the evaporative cooler  20  is preferably formed so that it is possible to achieve efficient sedimentation of the non-soluble tars in the aqueous mixture MA, the dust from mineral coal CM and the ash produced by the gasifier and not eliminated by the dedusting unit  14  so that the concentration thereof suspended in the aqueous mixture MA is as low as possible. These separated tars and dusts are then discharged via the pipe  24 . 
     At the tar concentration levels in the mixture considered advantageous for operation of the evaporative cooler  20 , the equilibrium temperature is preferably between 75° C. and 90° C. This temperature is achieved by favouring the sedimentation of a given quantity of non-soluble tars in the aqueous mixture present in the basin  22 . In this case the aqueous mixture MA has a particularly low viscosity which favours the use of low-cost centrifugal pumps. If the concentration of water in the mixture MA should decrease, the evaporative cooling effect would gradually decrease causing a rise in temperature of the mixture. 
     The use of evaporative cooling of the gas G, which occurs generally at temperatures higher than 80° C., eliminates the need to cool the aqueous mixture MA by means of a heat exchanger located along the recirculation circuit  23  supplying the spraying means  21 . 
     A heat exchanger  23  located along this recirculation circuit  23  would in fact be easily soiled and would therefore require frequent maintenance. The aqueous mixture MA contains in fact a large quantity of tars which complicate and increase the cost of any heat exchange in a conventional heat exchanger, for example of the tube bundle type. Moreover, in order to obtain a suitable heat exchange action, the surfaces of the heat exchanger should be substantially colder than the aqueous mixture MA flowing over them. This would thus cause a localized increase in the viscosity of the aqueous mixture MA, with consequent problems, both of a thermal nature (poorer heat transmission) and of a hydraulic nature (clogging of the exchanger). 
       FIG. 2  is a diagram which illustrates the behaviour of different tar fractions upon variation in the temperature. As can be seen, the different tar fractions have behaviours which are markedly different at the same temperature. 
     In particular, at an equilibrium temperature plausible for operation of the evaporative cooler  20 , for example at the temperature of 85° C., different phases coexist. There are lighter 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. 
     The condensed tars are collected in the basin  22  together with water, forming the aqueous mixture MA. The aqueous mixture MA comprises both polar tars which are converted into solution and non-polar tars which generally have a density greater than water and which, if left to settle suitably, form layers on the bottom of the basin  22 . 
     The bleeder pipe  24  is designed to remove from the bottom of the basin  22  most of the sedimented tars which collect there. They must be removed from the basin  22  in order to maintain the correct quantity of mixture MA inside the evaporative cooler  20 . 
     In the accompanying  FIG. 1 , the evaporative cooler  20  is shown in the form of a duct along which the flow of gas G goes in a substantially horizontal direction. Inside the duct the aqueous mixture MA is sprayed in the same direction as that of the gas flow. The basin  22  is formed at a bottom point of the duct and the condensed mixture MA collects there by means of gravity. 
     In accordance with that shown in the accompanying  FIGS. 3 to 6 , the evaporative cooler  20  may have the structure of a duct along which the flow of gas G goes in a substantially vertical direction. Inside this duct the aqueous mixture MA is sprayed for example in the same direction as that of the flow of gas G. The basin  22  is formed at the bottom of the duct and the condensed mixture MA collects there by means of gravity, in a similar manner to that described above. 
     The evaporative cooler  20  preferably comprises means  25  for breaking up the encrustations which form inside it during treatment of the flow of gas G. 
     The crushing means  25  are preferably situated between the upper zone where spraying of the aqueous mixture MA take place and the basin  22  inside which the aqueous mixture MA collects in the condensed state. 
     The crushing means  25  preferably comprise a series of blades  250  movable with respect to a grille  251 . In accordance with the embodiment shown in  FIG. 6 , the blades  250  are mounted radially on a rotating shaft arranged in the vicinity of the surface of the grille  251 . In accordance with this embodiment, following rotation of the shaft, the blades  250  periodically pass through the surface of the grille  251 . 
     Below it is described how a number of undeniable advantages may be achieved with this solution. Firstly it must be mentioned here that, in this zone of the plant  100 , the flow of gas G conveys along with it a large quantity of pollutants since it has been treated only by the dedusting unit  14 . Most of the pollutants (whether they be in the form of vapours, aerosols or fine dust) are preferably removed further downstream, typically inside the electrostatic precipitator  40  (described in detail below). 
     Inside the evaporative cooler  20  the flow of gas G undergoes a drastic reduction in temperature: from about 400-800° C. to about 75-100° C., preferably about 80° C. Owing to said cooling, some of the tars condense to form droplets which, in turn, act as aggregation nuclei for the dust present in the gas G. 
     The phenomenon described above therefore produces, in the zone where the flow of hot gas G encounters the flow of sprayed aqueous mixture MA, the formation of solid or semi-solid encrustations which may also be of considerable size. These encrustations, separating from the walls of the evaporative cooler  20 , may also be conveyed into the basin  22  and prevent correct operation thereof. The encrustations are in fact of a size such as to cause blockage in a very short amount of time of the bleeder pipe  24  and/or the recirculation circuit  23 . 
     The crushing means  25  reduce instead the size of these encrustations so as to allow them to flow out correctly through the bleeder pipe  24 . In particular, during operation of the evaporative cooler  20 , the encrustations are deposited by means of gravity onto the grille  251  where the regular passing movement of the blades  250  breaks them down into parts sufficiently small for them to be able to fall through the grille  250  and then be conveyed together with the liquid tars through the bleeder pipe  24 . 
     As may be noted in  FIG. 6 , in this particular embodiment of the evaporative cooler  20 , a screw conveyor  220  is provided on the bottom of the basin  22  so as to be able to remove all the pollutants which collect there (whether they be more or less fluid tars or pieces of the encrustations previously broken up by the crushing means  25 ). 
     The basin  22  also comprises an outlet suitable for removing the aqueous mixture MA. This outlet is situated in a position within the stored quantity of aqueous mixture MA so as not to draw off either the heavier pollutants which are deposited on the bottom of the basin  22  or the foam and the lighter pollutants which float on the surface of the aqueous mixture MA. The outlet supplies the recirculation circuit  23  which delivers the aqueous mixture MA to the spraying means  21  of the evaporative cooler  20 . Moreover the outlet also supplies the pipe  65  which delivers the aqueous mixture MA to the spraying means  63  of the humidifier  60 . 
     As already mentioned above, the scrubber  30  comprises a heat exchanger  35  situated along the recirculation circuit  33 . The heat exchanger  35  is designed to cool the aqueous mixture MA along the path which it follows towards the spraying means  31 , so that the aqueous mixture MA may in turn cool the gas G inside the scrubber  30 . 
     Moreover, the scrubber  30  may advantageously comprise a discharge pipe  36  suitable for removing the excess aqueous mixture MA from the basin  32  and conveying it externally. 
     Inside the scrubber  30  the gas G is cooled down to the temperature required for correct operation of the gas user unit  80 , which temperature is typically between 40° C. and 50° C. During the process of cooling of the gas G, some of the water and some of the tars which are present in the gas G in the vapour phase upon leaving the evaporative cooler  20  are condensed. These condensed products accumulate in the basin  32  forming the aqueous mixture MA. This aqueous mixture MA is continuously removed and pumped to the evaporative cooler  20  via the special compensation pipe  37 . 
     In this connection it should be noted that in the evaporative cooler  20  the quantity of mixture MA present in the condensed state in the basin  22  gradually diminishes owing to its continuous evaporation within the flow of gas G. On the other hand, inside the scrubber  30  the quantity of mixture MA present in the condensed state in the basin  32  increases gradually owing to the continuous condensation of the vapours present in the gas G during cooling. In the macroapparatus  10  according to the invention the excess amount of aqueous mixture MA present in the basin  32  is used in order to supplement in a continuous manner the losses of mixture MA inside the basin  22 . 
     If the temperature of the gas G leaving the scrubber  30  is higher than that of the dewpoint of the gas G, the condensed water inside the basin  32  is less than the water evaporated inside the evaporative cooler  20 . In these operating conditions, even displacing continuously the aqueous mixture MA from the basin  32  to the basin  22 , it is required to introduce water into the macroapparatus  10  drawing upon an external supply via the line  27 . 
     If the temperature of the gas G leaving the scrubber  30  is the same as the temperature of the dewpoint of the gas G, the condensed water in the basin  32  is equal to the evaporated water in the evaporative cooler  20 . In these operating conditions, by displacing continuously the aqueous mixture MA from the basin  32  to the basin  22 , the macroapparatus  10  is in hydric equilibrium. 
     If the temperature of the gas G leaving the scrubber  30  is lower than that of the dewpoint of the gas G, the condensed water in the basin  32  is greater than the evaporated water in the evaporative cooler  20 . In these operating conditions, even displacing continuously the aqueous mixture MA from the basin  32  to the basin  22 , it is required to dispose of water from the macroapparatus  10  externally. 
     According to certain embodiments of the invention, the macroapparatus  10  comprises a heat exchanger instead of the scrubber  30 . This heat exchanger is preferably of the tube-bundle type in which the hot gas G exchanges heat with a service liquid. 
     The water vapour H 2 O (g)  necessary for the gasification reactions of the mineral coal CM may be usefully produced by the humidifier  60 . The humidifier therefore allows the oxygenated gas to be humidified before it is introduced into the gasifier  12 . 
     Since the temperature of the aqueous mixture MA sprayed into the humidifier  60  is about 80° C., the mass of water which may be evaporated may be as high as about 0.5 kg for each kilogram of oxygenated gas A, for example air. 
     This evaporation inside the humidifier  60  enables water vapour H 2 O (g)  to be produced, using a quantity of heat made available at low temperature and therefore generally not utilizable for other purposes. 
     The flow of wet oxygenated gas AU may be generated in different ways. One extremely simple way is that of using a fan  66  mounted so as to draw air from the environment and blow it into the line  61  leading to the humidifier  60 . As an alternative or in addition to the fan  66 , another fan (not shown) may be mounted so as to draw the oxygenated gas AU already treated by the humidifier  60  and/or the gasifier  12 . These solutions envisage therefore the use of ambient air as oxygenated gas A. 
     In accordance with other possible embodiments, intended to obtain a greater percentage of oxygen O 2  in the oxygenated gas A, a system for supplying oxygen O 2  under pressure, for example a gas cylinder or other pressurized oxygen tank, is provided alongside or instead of the fans. These embodiments potentially enable the percentage of oxygen O 2  in the oxygenated gas flow to be increased as required. The composition of the oxygenated gas A may therefore vary between that of air (i.e. a mixture containing about 20% oxygen) and that of pure oxygen (100% O 2 ), the intermediate compositions being generally definable as oxygenated air. The increase in the percentage of oxygen may be useful in certain particular cases of operation of the gasifier  12 , for example in order to obtain gas G with a higher calorific power. 
     The gases produced by the gasification of the mineral coal CM inside the gasifier  12  (i.e. H 2 , H 2 O, N 2 , CO, CO 2 ) are extracted via a proper pipe from the top of the reactor of the gasifier  12  and are conveyed to the dedusting unit  14 . These products have a temperature of about 400-800° C. 
     The coal reforming reaction, which is favoured by the presence of water vapour H 2 O (g)  in the oxidation zone of the reactor (which in the updraft gasifier is located in the bottom of the reactor, just above the bottom grille  122  visible in  FIGS. 7 and 8 ) as well as the Boudouard reaction (described above) are highly endothermic reactions. These reactions therefore absorb the heat released by the other exothermic reactions which occur inside the gasifier  12  (typically the reactions produced by combustion of the carbon and the pyrolysis gases) and thus limit the temperatures in the oxidation zones to values in the region of 800° C.-900° C. 
     These considerations are extremely important for the gasifier  12  of the updraft type, owing to the fact that it operates substantially in the presence of carbon C alone. The generalized combustion reaction of the carbon C could result in extremely high temperatures, in excess of 1500° C., being reached inside the gasifier  12 . The introduction of water vapour H 2 O (g)  into this type of reactor and the consequent generation of endothermic reactions is therefore even more important in order to limit the temperature inside the reactor and generate hydrogen H 2 , improving at the same time the quality of the gas G. 
     As already explained above, the aqueous mixture MA, removed from the basin  22  and sprayed into the humidifier  60  at a temperature equal to or greater than 80° C., enables a significant flow of oxygenated gas A to be easily saturated with water vapour H 2 O (g) . It is thus possible to convey a significant amount of water vapour H 2 O (g)  inside the gasifier  12 . Moreover, as already explained above, the heat required to obtain evaporation of the water which saturates the flow of oxygenated gas A is provided by cooling the aqueous mixture MA. 
     This heat, owing to the relatively low temperature at which it is available, may not generally find a useful application. On the other hand, in the case of the macroapparatus  10  according to the invention, this heat may be used to improve gasification of the mineral coal CM inside the gasifier  12 . All this enables the energetic efficiency and the overall quality of the entire gasification process to be increased. The increase in the energetic efficiency may be as high as about 10% depending on the type of mineral coal CM and its ash content, this meaning a reduction in the specific consumption of coal CM of up to about 10% and an increase in the amount of hydrogen H 2  contained in the gas G produced by the plant. 
     This increase in the energetic efficiency is due to the use of part of the heat for cooling the gas G to generate water vapour H 2 O (g)  to be used in the gasification processes. The use of this heat, which is generally not used, avoids having to draw heat from the actual gasification reactions. 
     It should be noted here that another solution is known for generating steam, said solution being historically used in mineral coal gasifiers. This known solution makes use of the high-temperature heat which is available on the outer walls of the reactor. A solution of this type, however, does not produce energetic advantages since the heat required for evaporation of the water is taken from the gasification reactions. 
       FIGS. 3 to 6  show an embodiment of the evaporative cooler  20  and the evaporator  60  which are integrated in a single assembly  50 . This embodiment of the invention is particularly advantageous. It in fact enables the heat dispersion to be minimized along the line supplying the aqueous mixture MA from the basin  22  to the spraying means  63  of the humidifier  60 . To ensure optimum operation of the invention, the aqueous mixture MA must in fact be sprayed at the highest possible temperature from among those already available inside the plant  100 . The high temperature of the aqueous mixture MA (equal usually to about 80° C.) ensures efficient saturation of the flow of oxygenated gas A. In view of the above, the use of heat made available by the plant  100  avoids having to resort to external heat sources which would result in a drastic reduction in the overall energetic efficiency of the process. 
     In accordance with the embodiments shown in the accompanying figures, the plant  100  also comprises an electrostatic precipitator  40  suitable for treating the flow of gas G already treated by the macroapparatus  10 . The gas leaving the macroapparatus  10  has in fact undergone the desired cooling and initial dedusting, but still contains numerous pollutants in suspended form. These pollutants (fine dusts, tars in aerosol and vapour form) may be advantageously removed from the gas G by means of an electrostatic precipitator  40 . 
     In particular, the plant  100  preferably comprises a wet electrostatic precipitator  40  of the type known per se. Said precipitator comprises ducts inside which an electrostatic field is maintained. In particular, the electrostatic precipitator  40  comprises preferably tubular structures inside each of which an electrode is arranged. An electrostatic field may thus be formed inside each tubular structure, between the walls and the central electrode. 
     Moreover, the precipitator  40  may comprise preferably:
         a tank  42  suitable for collecting the tars and the water removed from the gas G;   a bleeder pipe  44  suitable for removing the condensed tars from the bottom of the tank  42  and conveying them externally;   a discharge pipe  46  suitable for removing the excess aqueous mixture from the tank  42  and conveying it towards the basin  32 ; and   an output line  18  suitable for conveying the flow of clean gas G to a user unit  80 .       

     The electrostatic precipitator  40  allows the gas G to be cleaned of the suspended pollutants so as to obtain the desired quality for the user unit  80 . 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 . 
     Immediately upstream of the electrostatic precipitator  40 , cooling of the gas G inside the scrubber  30  causes condensation of the vapours still present inside it. The formation of water droplets provides condensation nuclei for the tar molecules and, vice versa, the formation of tar droplets provides condensation nuclei for the water molecules. In this way the aqueous mixture MA is converted from the vapour phase to the liquid phase, thus forming a tar and water aerosol. 
     Once it has reached the inside of the electrostatic precipitator  40 , the aerosol suspended in the gas G is attracted towards the walls of the precipitator itself by the effect of the electrostatic field maintained within it. The aerosol therefore adheres to the walls of the precipitator  40  and, flowing along the walls, is collected inside the tank  42 . 
     With reference again to the diagram shown in  FIG. 1 , it is possible to note how the condensed liquids, which are removed from the gas G in the electrostatic precipitator  40 , are collected inside the tank  42  also provided with a bleeder pipe  44 . 
     In similar manner to that already described above with reference to the bleeder pipes of the basins  22  and  32  of the evaporative cooler  20  and the scrubber  30 , respectively, the bleeder pipe  44  also allows removal from the bottom of the tank  42  of at least part of the aqueous mixture MA. The mixture removed contains water, polar tars in solution form and in particular non-polar (water-insoluble) tars which collect on the bottom of the tank  42 . For this reason, the bleeder pipe  44  draws advantageously from a bottom point of the tank  42  where the heavier non-polar tars spontaneously collect by means of gravity. The tars, both polar and non-polar, may be usefully conveyed outside of the plant  100 . 
     The lighter tar fraction remains volatile in the gas G also at the exit temperature from the electrostatic precipitator  40 . 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. 
     The tank  42  of the precipitator  40  comprises finally a discharge pipe  46 . This pipe is suitable for removing any excess aqueous mixture MA from the tank  42  and conveying it towards the basin  32 . 
     According to certain embodiments, the electrostatic precipitator  40  comprises a thermal insulation able to prevent as far as possible heat exchange with the external environment. The gas G flows inside the electrostatic precipitator  40  at a temperature generally greater than the ambient temperature, usually at a temperature of between 40° C. and 60° C. In these conditions the gas G would tend to release heat spontaneously to the external environment, cooling and triggering further condensation phenomena. These phenomena are therefore limited in nature owing to the thermal insulation of the electrostatic precipitator  40 . 
     According to certain embodiments the plant  100  also comprises means  70  for heating the gas G gain after having cooled it. These means  70 , if present, may for example be associated with the output line  18 . 
     Cooling of the gas G, in particular inside the evaporative cooler and the scrubber  30 , causes condensation of most of the water and the tars present therein in the form of vapour. The gas G, however, remains saturated with vapours, i.e. it remains with a relative humidity of 100%. In these conditions even a slight drop in temperature of the gas G causes condensation of the vapours and the consequent formation of mist within the gas G. The occurrence of such a drop in temperature is highly likely along the line  18  which conveys the gas G from the electrostatic precipitator  40  to the external user unit  80 . The consequent condensation and formation of mist would therefore risk soiling the line  18  and the user unit  80  itself. 
     In order to overcome the abovementioned drawbacks, according to certain embodiments of the invention, the temperature of the gas G may be raised again a few degrees (for example 10-20° C.). A reduction in the relative humidity which falls below 100% is thus obtained. In these changed conditions the gas G may be subject to slight temperature fluctuations, but without this giving rise to the formation of mist. 
     The heating means  70  may advantageously make use of the heat provided by other sections of the plant  100 , such as the evaporative cooler  20  or the gas user unit  80  (which preferably comprises an internal combustion engine or other form of burner). 
     The circuit may be formed in a manner known per se, for example may be advantageously a closed circuit inside which a predetermined quantity of heating liquid circulates. 
     Each of the bleeder pipes  24 ,  34  and  44  (if present) and each of the recirculating circuits  23 ,  33  and  64 , as well as the pipe  65 , preferably comprises a pump suitable for moving the aqueous mixture MA even when it is rich in heavy tars such as those which must be conveyed back to the inlet of the gasifier  12 . These pumps may be preferably centrifugal pumps, gear pumps or peristaltic pumps suitable for moving fluids which may also be very viscous. According to certain embodiments, the pump situated in the recirculation circuits  23  and  33  which respectively supply the spraying means  21  and  31  of the evaporative cooler  20  and the scrubber  30  may be advantageously centrifugal pumps. This type of pump is in fact suitable for providing a considerable throughput of aqueous mixture MA provided that it has a sufficiently low viscosity. 
     According to certain possible embodiments, the discharge pipes  36  and  46  may advantageously comprise a settling tank suitable for separating further by means of gravity the tars from the water. The tars recovered from the bottom of the settling tank may then be removed for storage or disposal thereof. 
     According to certain possible embodiments, the plant  100  also comprises a blower mounted on the line  18  and able to move the gas G through the entire plant  100 , from the humidifier  60  via the gasifier  12 , the dedusting unit  14 , the evaporative cooler  20 , the scrubber  30  and the electrostatic precipitator  40  (if present) as far as the line  18  and beyond. 
     According to certain possible embodiments, the plant  100  comprises, finally, a gas user unit  80 . 
     According to the embodiment of the plant  100  shown in the accompanying  FIG. 1 , the gas user unit  80  comprises an internal combustion engine to which a generator for the production of electric energy may be typically connected. 
     In particular, it should be noted that the excellent quality of the gas G output from the plant  100  according to the invention can be used to fuel modern reciprocating engines (both Otto cycle and Diesel cycle engines) and/or gas turbine engines. 
     According to other possible embodiments, the gas user unit  80  may comprise: burners and/or boilers for heating and/or for the production of sanitary hot water; headers for conveying the gas in 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 its 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  80  known per se for using the gas. 
     The invention also relates to a method for the production and treatment of the gas G. 
     The method comprises, during normal operation, the steps of:
         providing a macroapparatus  10  in accordance with that described above;   providing a flow of wet oxygenated gas AU and fuel CM to the gasifier  12 ;   conveying the flow of gas G from the gasifier  12  to the dedusting unit  14 ;   conveying the flow of gas G from the dedusting unit  14  to the evaporative cooler  20 ;   spraying inside the evaporative cooler  20  the aqueous mixture MA into the flow of gas G so as to perform washing of the gas G and a first cooling step for absorption of latent evaporation heat in the aqueous mixture MA;   storing and allowing to settle inside the basin  22  of the evaporative cooler  20  a quantity of aqueous mixture MA in the condensed state;   removing the aqueous mixture MA from the basin  22  and supplying it to the spraying means  21  of the evaporative cooler  20 ;   removing the condensed pollutants from the bottom of the basin  22  and conveying them externally;   conveying the flow of gas G from the evaporative cooler  20  to the scrubber  30 ;   spraying inside the scrubber  30  the aqueous mixture MA into the flow of gas G so as to perform a second cooling step for removal of heat by the aqueous mixture MA;   storing and allowing to settle inside the basin  32  of the scrubber  30  a quantity of aqueous mixture MA in the condensed state;   removing from the basin  22  part of the aqueous mixture MA and supplying it to the spraying means  31 ;   removing from the basin  32  part of the aqueous mixture MA and supplying it to the basin  22  of the evaporative cooler  20 ;   removing from the bottom of the basin  32  the condensed pollutants and conveying them externally;   cooling by means of the heat exchanger  35  the aqueous mixture MA along the path from the basin  32  to the spraying means  31 ;   introducing a flow of oxygenated gas A into the humidifier  60 ;   removing a quantity of aqueous mixture MA from the basin  22  of the evaporative cooler  20 ;   spraying inside the humidifier  60  the aqueous mixture MA removed from the basin  22  into the flow of oxygenated gas A so as to obtain a flow of wet oxygenated gas AU;   conveying the wet oxygenated gas AU to the gasifier  12 ; and   conveying the flow of gas G from the scrubber  30  outside of the macroapparatus  10 .       

     According to a mode of implementation of the invention, the first cooling step lowers the temperature of the gas G from the 400-800° C. at which it leaves the gasifier  12  to 75-90° C. This first cooling step therefore involves absorption of a large quantity of heat, present in the gas G in the form of sensible heat. In the method according to the invention, the aqueous mixture MA removes preferably from the gas G a large quantity of heat, absorbing it in the form of latent evaporation heat, while a minimum part is absorbed in the form of sensible heat. During this first (evaporative) step there is therefore no exchange of heat towards the outside of the system and the heat therefore remains inside the flow of vapour-saturated gas G. 
     According to a mode of implementation of the method, the second cooling step lowers the temperature of the gas G from the 75-90° C. at which it leaves the evaporative cooler  20  to the 40-60° C. which are optimum for operation of the user unit  80 . This second step of cooling of the gas G involves removal of a large quantity of heat present in the form of latent heat in the vapours generated by the aqueous mixture MA (water vapour and light tars in the vapour phase) and mixed with the gas G. In the method according to the invention, the aqueous mixture MA removes preferably the heat, absorbing it in the form of sensible heat and therefore increasing its own temperature. During this second (condensation) step there is therefore an exchange of heat with the outside of the system via the exchanger  35 . 
     The present invention also relates to a gasifier  12  of the updraft open-core type comprising means  120  for raising the outer jacket  121 . These raising means, as already described above in connection with the gasifier  12  forming part of the macroapparatus  10 , are suitable for converting the outer jacket  121  from a closed operating configuration into an open maintenance configuration. 
     From that stated above it will be clear to the person skilled in the art how the plant  100  in its entirety, the macroapparatus  10  in particular and the method according to the invention overcome the disadvantages highlighted in relation to the prior art. 
     It is clear that the specific characteristics are described in relation to the various embodiments of the plant  100  by way of a non-limiting example. 
     Obviously, a person skilled in the art, in order to satisfy any specific requirements which might arise, may make to the macroapparatus  10  and/or the plant  100  according to the present invention further modifications and variations, all of which moreover contained within the scope of protection of the invention, as defined by the following claims.