Patent Application
20130247846
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+O2→CO2 (combustion)
C+½O2→CO (partial oxidation)
C+H2O(g)→CO+H2 (coal reforming)
C+CO2→2CO (Boudouard reaction)
C+2H2→CH4 (methanation)
These reactions produce, in the presence of air, a gas (called “producer gas”) composed of a mixture consisting in dry form of about 50% N2, 20% CO, 15% H2, 10% CO2 and 5% CH4. If the reactions take place without the presence of air, the final mixture does not contain N2 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:
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
The macroapparatus 10 according to the invention comprises:
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:
The macroapparatus 10 described above may preferably comprise one or more of the following auxiliary components:
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
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.
According to this embodiment, the gasifier 12 comprises means 120 for raising the outer jacket 121.
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
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
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).
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
In accordance with that shown in the accompanying
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
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
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 H2O(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 H2O(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 O2 in the oxygenated gas A, a system for supplying oxygen O2 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 O2 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% O2), 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. H2, H2O, N2, CO, CO2) 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 H2O(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
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 H2O(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 H2, 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 H2O(g). It is thus possible to convey a significant amount of water vapour H2O(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 H2 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 H2O(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.
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:
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
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
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 (H2, CO, N2, 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:
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2010A 002158 | Nov 2010 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2011/055065 | 11/14/2011 | WO | 00 | 5/23/2013 |
