REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of the Greek Patent Application No. 20210100526, filed on August 02, 2021.

BACKGROUND

[0002] The process of gasification has been used for energy production for more than 180 years. Initially, the supply consisted of coal and peat which were gasified in gasification plants to produce gas for lighting and cooking purposes. Subsequently, the method of gasification was used both to produce gas, which in turn was used in the production of electricity, and in blast furnaces. Currently, the most common use of gasification is for the production of synthetic chemicals. Through gasification and proper treatment of the process’ derivatives, high-quality gases and/or liquid fuels are produced. This process was adopted on a large scale during both World Wars to counter-measure oil shortages during the Wars.

[0003] The gas produced by the gasification of biomass can be standardized in terms of its quality and used as a pure combustible gas for heating, power generation or as feedstock for chemical synthesis. The advantages of gasification over combustion are the same as those which characterize a gaseous fuel, compared to a solid fuel, i.e. higher heat release rates, higher combustion efficiency, reduced environmental impact, fewer ash-related problems, direct combustion of the gas in internal combustion engines and application in combined cycles, as well as easy distribution of the gas over short distances.

[0004] Due to the flexibility of the producer gas, for small-scale power plants (solid fuel consumption less than 150 kg/h on a dry basis) gas engine coupling is the optimal solution in terms of efficiency/cost ratio. Such small-scale plants are particularly suitable for areas and installations which produce solid residues (i.e., solid fuels of low energy quality and high ash content) from which quantities of combustible gas can potentially be produced for energy production. Such areas are agricultural areas where organic residues remain after harvest completion (i.e. harvesting of sugar cane, corn, cereals, etc.), areas with developed logging, plants where agricultural products are processed (i.e. olive mills, stone fruit jam production plants, etc.), livestock units (where feces of high straw content, such as poultry, sheep, etc. are produced), wood processing plants, organic waste management facilities (biogas plants, waste- water treatment facilities) and more. Gasification of these solid residues aims at converting a difficult to manage and low-quality solid fuel into a fluid fuel that has a higher energy density, does not degrade over time and is easier to manage (transport, storage).

[0005] As the by-products of the agri-food sector are produced in local scales and in a distributed manner, it is economically disadvantageous to transfer them to central exploitation units (collection radius of more than 30 km). For this reason, it is particularly useful to exploit them on site. This is possible by installing easily transportable, small-scale gasification units directly at the locations where the solid residues are produced.

[0006] Basic gasification technologies and their disadvantages

[0007] Gasification technologies are distinguished according to the direction of the supplied air and fuel flow in the reactor (gasifier). The main gasifier concepts are: the fixed bed of updraft or downdraft current, the fluidized bed and the entrained flow reactors. The fixed bed reactor is the most common type in small-scale applications. Depending on the flow direction of the produced gas, reactors are classified as updraft, downdraft, or cross-flow reactors.

[0008] In updraft reactors the fuel is supplied from the top part and the air from the bottom part of the reactor, through a support grate. The gasification solid residue is concentrated on the grate where it is burned at a temperature of 1000°C, the ash is concentrated at the lowest point while the hot gases move upwards, undergoing a reduction process. The main advantages of this type of reactor are its simplicity, the high conversion of carbon residues and the internal heat exchange rate, leading to low gas outlet temperature. Due to reactor design, the incoming fuel is dried at the top of the bed and it is therefore possible to use fuel with high humidity (up to 60%) without requiring pre-treatment. The main disadvantage is the particularly increased production of tars (50-100 g/Nm ³ ).

[0009] In downdraft reactors the fuel and air flow in the same direction. The gaseous products exit the reactor after passing through the hot fuel particles zone, thus facilitating the partial decomposition of the tars generated during the pyrolysis step. Since gases exit the reactor at a high temperature (900-1000°C) the energy efficiency is low, as heat is contained in the hot gases. The gas tar content is low (1-2 g/Nm ³ ). The design and operation of this type of reactor is relatively simple. The producer gas tar content is low, but it is practically impossible to get rid of it completely. The main disadvantage is considered to be the high ash content in the outgoing gas and the strict fuel particle size requirements, which must be evenly shredded from 40-10 mm so as not to block the cross-section of the reactor and allow pyrolysis gases to be heated by the oxidation zone. The maximum humidity limit of the fuel is set at 25%. [0010] The fluidized bed reactor has been used extensively for the gasification of fuels such as lignite, coke, woody biomass and sludge. Its advantage over fixed bed reactors is the uniform temperature distribution in the gasification zone. This is achieved through the use of a pneumatically agitated, through the vertical upward flow of the process air, fine-sand bed which enhances mixing between the hot sand bed, the inlet fuel particles and the produced gases.

[0011] The bed temperature is set to 700-900 °C and is maintained by controlling the air/fuel/sand mass ratios. In contrast to fixed bed reactors, in this reactor type, there is no separation into process zones due to the intense mixing of the fuel with the oxidative fluid and the fine-grained fluidization material. Drying, pyrolysis and gasification, all take place simultaneously throughout the volume of the reactor, where uniform mixing and a constant temperature prevail, achieving almost complete conversion of the fuel. For these reasons fluidized bed gasifiers can convert, without melting and agglomeration problems, fuels with high ash content and low ash melting points such as agricultural solid fuels. The disadvantages are summarized in the complex operation and control of the process as well as the increased pollutant load of the producer gas in suspended particulates and tar (10 g/Nm ³ ). Due to their complexity, fluidized bed reactors are usually applied on a large scale where there is no space limitation for the height requirement of the gasifier to avoid pneumatic transport of the fuel particles and for the inclusion of the bed removal and additional auxiliary systems.

[0012] Main filtration technologies and their disadvantages

[0013] A filtration system for the purification of the producer gas is necessarily placed downstream the gasifier and upstream the producer gas utilization system such as a power production device. The main pollutants treated in the filtration system are the particulate load and the tar content as their presence in the gas stream is substantial and can create critical operational problems. Based on the operating temperature, cleaning technologies are distinguished in cold and hot methods. Cold methods, in turn, are divided into 'dry' and 'wet' methods. Wet cleaning methods operate at temperatures around 150-250°C. They have an efficiency around 99%, in terms of particle separation, and around 20-80% (depending on temperature and active filter surface) in terms of tar retention. In wet scrubber type methods, the gas comes into contact with a jet of water or other liquid chemical (e.g. diesel oil), and is cooled at temperatures of 25-55°C. Thus, the scrubber cleanses the gas of particles, tar, and various nitrogen compounds (ammonia). Disadvantages of this technology include the significant cooling of the gas as well as the need to install an additional system for the recovery of the washing liquid.

[0014] Regardless of gasification technology, usually the first stage of gas cleaning is categorized as hot and consists of a gravitational or centrifugal separator (cyclone type). This hot process removes much of the particulate load. Generally, the cyclone filter can remove up to 90% of particles with a diameter of more than 5 pm, is partially efficient for particles between 1-5 pm diameters, while such systems are generally unable to filter particles less than 1 pm in diameter due to their operating principle.

[0015] Additional components for the complete removal of particles and tar are placed downstream the cyclone filter. While in cold methods the tar is removed from the gas through condensation followed by separation or adsorption, in hot methods tar is removed through breaking it down into chemical compounds with lower molecular weight which do not cause clogging problems in producer gas utilization equipment. Tar breakdown is done either thermally (at temperatures above 1000°C i.e. by oxidation) or by means of a catalyst (600 - 900°C). Due to the presence of carbon particles inhibiting the proper function of the catalyst, usually the particulate load of the gas is removed by a hot filtration process, upstream of the tar decomposition device. Due to the high temperatures, conventional filters used in cold methods (bag filters, sand/sawdust/straw bed filters) are not suitable. High temperature filters consist of ceramic or metal materials. They separate, through absorption, sulfides and chlorides, retaining even the smallest particles.

[0016] The retention of these particles gradually creates a solid filter cake which partially clogs the filter and causes an increase in pressure drop. For this reason, it is necessary to periodically clean the filters. Cleaning of the filters is usually done through channeling a pulse of compressed air or inert gas. This causes the detachment of the filter cake which, due to gravity, settles and is collected with appropriate mechanisms.

[0017] The most widely used high temperature filter technology concerns ceramic and metal "candle" type filters. According to publications, these types of filters usually fail after 3,000 hours at temperatures above 400°C making their frequent change economically unviable. The main causes of failure are attributed to filter design, the manufacturing material, temperature variations and ash deposition. Notably, in the case of ash, since the deposition of particles takes place on the external side of the candle, the accumulation of material in neighboring candles leads to the linkage of these particle deposits, creating a "bridge" of particles which due to different thermal properties from the candle material quickly lead to candle structural failures. Various solutions have been proposed in the literature on the construction of filters resistant to mechanical and thermal stress, with the adoption of ceramic filters seeming more promising but without providing a satisfactory solution to the related problems while attention should also be focused on filter support and fastening. More specifically, it is necessary to ensure that their, usually metallic, support base, does not create operational problems. These problems, in turn, arise from the mechanical stress that is applied on the ceramic filter when the latter changes dimensions during the contraction-expansion cycle attributed to the temperature variations during system operation.

[0018] In addition, the cleaning of filters, which is usually done through channeling air under pressure in the opposite direction to that of their operation, strains the filters both mechanically and thermally as a thermal shock is created due to the temperature difference of the air that is used for cleaning and the temperature of the filter itself.

[0019] Both these stresses are causes of filter failure, which in turn loses part of its properties, creating the need for its replacement with the respective cost for the supply and installation of the new filter but also for the interruption of the system's operation. Due to their geometry, candle-type filters have a restriction on the porosity of the material, and respectively on the filtering surface, in order to maintain structural strength, deeming them unsuitable for limited-space applications.

SUMMARY

[0020] The present disclosure provides methods and systems that address the deficiencies and technical problems associated with existing gasification systems and processes.

[0021] The existing technologies which utilize solid ligno-cellulosic biomass on a small scale are limited to the utilization of fuel low in ash content (1-2% w/w) and strict particle size and moisture standards, which is practically applied only in the cases of standardized chips and pellets with the corresponding fuel cost. It is therefore necessary to design an innovative small- scale gas production system through gasification, utilizing solid fuels of high heterogeneity and ash content, offering the possibility of using low- or even negative-cost biomass.

[0022] In certain implementations, an innovative small-scale system for the production of producer gas through gasification, utilizing solid fuels with high ash content, is presented.

[0023] According to the present invention, the innovative system consists of a fluidized bed gasifier, a stage for removal of large suspended particles (> 5 pm), a monolithic honeycomb filters with plugged alternate channels unit suitable for cleaning of the filter during the operation of the system, a tar condensation and removal system, and a solid fuel pre-drying system. The innovative system can be configured and dimensioned to be contained in a standard freight container properly configured for easy access to the equipment, i.e. the subunits of the system. It has the capability to couple with a gas generator-set utilizing the heat produced either for the pre-drying needs of the solid fuel or to cover external thermal needs.

[0024] The proposed solution introduces the following innovations: (a) the operation of a small-scale gasifier, designed for limited-space applications (for example a standard freight container), for the production of gas, from solid fuel high in ash content, at operating temperatures lower than the melting point of the solid fuel ash, (b) the design of a filter unit consisting of independent filters, connected in parallel, capable of selectively isolating one or more filters from the system, for the purpose of their cleaning through providing a gaseous pressurized medium at an appropriate temperature for filter or filters cleaning (c) achieving a desired filtration surface through the use of monolithic ceramic or metal honeycomb filters, of plugged alternate channels, at temperatures at which melting or softening of the ash and its consequent retention by the filter or filters is avoided, as well as the condensation of the tar so that the tar remains in gaseous form as it penetrates the filter, (d) tar condensation, after removal of the gas particulate load, and its easy extraction during operation, (e) mechanical or pneumatic ash removal units at the gasifier, at the first stage of particle collection and at the filter unit, and (f) a feeding and system operation control unit, for the gasifier parameters adjustment, the detection of the need to clean the filters and the implementation of the cleaning operation aiming at the uninterrupted clean gas production at the system outlet, the protection of the filters from failure during their cleaning and the effective tar and other substances removal from the producer gas. Various other sub-units may be added to or removed from the system. The indispensable sub-units for system operation include the gasifier, the filters, and a unit of controlled producer gas cooling which is located between the gasifier and the filters.

DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 illustrates a diagram with an example of a clean gas production system, by solid fuel gasification, according to the state of the art.

[0026] FIG. 2 illustrates a simplified diagram of a clean gas production system, by solid fuel gasification, according to this invention.

[0027] FIG. 3 illustrates a simplified diagram of the system of this invention and the operating temperatures of its individual units. [0028] FIG. 4 illustrates examples of filters of the system of this invention and their principle of operation.

[0029] FIG. 5 presents simplified examples of filter mounting in FIG. 4 into the system of the present invention.

[0030] FIG. 6 presents a simplified system operation control flowchart.

DETAILED DESCRIPTION

[0031] System for the production of clean gas through solid fuel gasification according to the state of the art

[0032] FIG. 1 illustrates a diagram with an example of a small-scale clean gas production system (10-500 kWe) through solid fuel gasification according to the state of the art. The system (100) typically consists of a fixed-bed gasifier (110), a centrifugal cyclone (120), a candle filter system followed by a gas cooler, in case the cleaning is performed at high temperatures, or a heat exchanger system (130) for cooling the gas followed by a fixed bed or fabric filter (140) if the cleaning is performed at low temperatures.

[0033] The most common type of fixed bed gasifier at the above-mentioned scales is that of the downdraft due to the minimal tar amounts in the producer gas. In these gasifiers the fuel material (112) is typically imported from the upper part and fills the entire volume of the reactor. The oxidizing agent, typically atmospheric air (114) is also injected at the upper part at a slightly lower point than the supply height of the fuel material. At the air supply point, temperatures may rise to 1250°C as the oxidation stage of the contained volatile matter, including tar, is realized. The synthesized producer gas (111) exits from the bottom after passing through the reduction stage, at temperatures up to 1050°C. Due to the consumption of the solid fuel by the gas medium, the former shrinks and flows evenly to the bottom of the reactor. Consequently, the produced gas contains minimal tar amounts. However, this type shows increased sensitivity to changes in raw material humidity and particle distribution. This is due to the fact that the fuel does not rest on the grate, on account of the high temperatures at the bottom, but maintains the structure of the bed through the geometry of the raw material. For this reason, these gasifiers have a limitation to the minimum particle size of the fuel material. Also, due to the high temperatures, ash content melts and creates agglomerates that prevent the downward flow of the material. Therefore, there is a restriction both in the amount of ash in the incoming fuel and at the melting point of it, which should be above 1100°C to avoid problems. [0034] The gas exits at the bottom of the gasifier, dragging ash and fuel particles that have not reacted. It is then supplied either to a heat exchanger to cool the gas to a temperature suitable for conventional bag filters or fixed bed filters (e.g. WO2018/037152 Al) or cleaned at high temperature in a candle type filter system (e.g. CA 2937445 Al).

[0035] Disadvantages of state-of-the-art systems

[0036] Cooling the gas before removal of the particles facilitates the condensation of tar on the flying particles that act as condensation nuclei. The produced waste has the flow characteristics of sludge and causes operational problems as it deposits on downstream equipment while its removal from the heat exchanger is a challenge and typically requires system shut-down for system cleaning and reassembly. After cooling, the gas typically passes through a fixed bed filter which is filled with low-cost filtration mediums, such as sawdust. To achieve successful retention of particles, low gas velocities are required within the filter to prevent particle drag by the gas leading to large filter cross-sections and space requirements. Additionally, in order to clean the filters it is necessary to interrupt the process of cleaning/purifying the gas, and thus utilization of the gas, with the corresponding economic effects. More specifically, the filter container is opened, and its contents (sawdust and particles) are removed and replenished.

[0037] In the case which the gas is rid of the particulate load at high temperatures using candle type filters, cleaning is more direct and faster with the reverse flow method mentioned. However, apart from the structural failures of these filters as mentioned, such systems are practically only applied to large-scale installations (with capacities greater than 1 MWe) in which there are no special spatial constraints. Due to their geometry, candle-type filters have a limited filtration area thus a significant number of filters are required to adequately refine the gas.

[0038] Another reason why candle-type filters are not applied on small scale systems concerns the cleaning of the filter itself. As mentioned, the cleaning is done by reverse flow of compressed gas at the filter outlet. Typically, a nozzle is placed at the edge of each candle and the cleaning is performed on each candle sequentially so as not to interrupt the operation of the rest of the filter array. In large-scale units where the candles which constitute an array are many, the addition of inert (e.g. of nitrogen) or not (e.g., air) gas due to the pulse does not significantly affect the quality of the produced gas. In the case of a small-scale system however, where an array of filters may contain from 4 to 8 candles, a pulse can significantly affect the instantaneous quality of the gas, obstructing the operation of the power generation system. This problem may be solved through isolating and operating the filters in parallel as in the present invention but in the case of candle-type filters this would require an extremely large space and high cost of mechanical equipment such as isolation valves for each candle.

[0039] System for the production of clean producer gas through solid fuel with high ash content gasification according to the presented invention

[0040] Gasifier

[0041] FIG. 2 illustrates a simplified diagram of a clean gas production system through solid fuels gasification according to this invention. The system depicted in FIG. 2 may optionally include additional elements which are not depicted and part of which is described below. The system (200) consists of three subsystems, the solid fuel supply subsystem (dryer), the fuel conversion to gas subsystem (fluidized bed gasifier) and the producer gas treatment subsystem (first combustible gas cooling device, filter unit, second combustible gas cooling device) [0042] The solid fuel supply subsystem consists of a storage tank for the untreated solid fuel at the base of which a screw conveyor or similar device which serves to transport the solid combustible material is located. The tank has a geometry suitable for maximizing useful volume and ensuring the vertical untreated fuel flow due to gravity without forming agglomerates and cavities. At the lower end of the tank in the flow direction of the material there is an opening through which the combustible material passes and is supplied to an inclined screw conveyor. The inclined screw conveyor rotates in a metal shell compelling the fuel to the highest point of the device where there is a discharge orifice at the bottom of the shell. The base of the middle part of the shell consists of a perforated metal sheet and is encased in an airtight, metallic duct. Drying air, heated by the heat from the gas cooling system, passes through a suitable orifice into the air duct and through the perforated plate within the screw conveyor which it scavenges heating the material and removing the moisture content through an orifice in the upper part of the shell through which it is dispersed to the ambient. The two screw conveyors have a fixed volumetric capacity ratio and rotate simultaneously to avoid supercharging or under-feeding of the inclined screw conveyor. The simultaneous rotation is ensured by a system of gears and chains. Alternatively, each conveyor is coupled with its own electric motor and rotates independently.

[0043] In alternative realization examples of the solid fuel supply system, the storage tank is replaced by a tank with a hydraulic floor which moves the material towards the screw conveyor. The hydraulic floor may be perforated allowing the drying air pass through. In any case, because the level of the material in the tank is not constant, part of the drying air passes through the inclined screw conveyor in order to maximize the time of contact between the bulk material and the hot air stream.

[0044] The conversion subsystem consists of a gasifier device (210) in which solid fuel is inserted from solid fuel inlet (212) into the side of the gasifier (210) while air (214) enters from the bottom part of the gasifier (210) reacting with the fuel at high temperature (650-950°C) and converting the solid fuel into combustible gas (209) which exits the gasifier at its upper part (210). Fluidizing material (210) of specific particle size, is also injected into the gasifier (216) which is heated at the base of the gasifier (210) and the air (214) is injected through the sand bed (216) inside the gasifier (210). The high temperatures (about 650-950°C) prevailing at the base of the gasifier (210) and the air supply (214) make the sand (216) behave like a fluid, exchanging heat with solid combustible fuel material and air acting as a heat storage medium. The gasifier consists of a cylindrical metal tube internally lined with high temperature resistance refractory. The gasifier is divided into three inter-communicating flow parts. The upper part (201) is the main body or reactor, the middle part (202) is the air distributor, and the lower part (203) is the ash removal device. The ash removal device consists of a cylindrical cross-sectional duct with a blind metal plate at its base. A screw-conveyor, or a similar bulk material conveying system, is located at the center of the device, which removes ash and other inert and non-inert-material from the bottom of the gasifier by means of an outflow duct and transfers it out of the system into an airtight container. In order to facilitate the flow of ash to the screw conveyor, appropriate inclination is formed at the bottom of the lower part (203). The bottom (203) is connected to the air distributor (202) through a flange or other appropriate connection. The distributor (202) consists of two concentric cylindrical metal ducts between of which a heat-insulating material is placed or a cooling medium flows. The outer cylinder is welded to flanges or other equivalent connecting devices on each end. Before it enters the gasifier, the supply air is led into two identical collectors which are oppositely positioned with respect to the gasifier. Within each collector an electrical resistance is coaxially mounted for the rapid preheating of the air during the preheating phase of the gasifier. The air enters the collector where it is heated to temperatures higher than 200°C and then distributed to three or more air distribution pipes which are plugged at their end. Each pipe has several orifices of a maximum diameter of 3 mm through which air enters the gasifier in the form of a jet. The orientation of the orifices is preferably horizontal but may have any inclination to the crosssection of the gasifier. The orientation of the orifices of each distribution pipe is preferably vertical but may be in any other direction. The upper part (201) consists of two cylindrical ducts which are axially connected through an expansion. Considering an upward flow, the diameter of the reactor upstream the expansion is always smaller than the corresponding diameter downstream the expansion. In the lower cross-section, a flange or any other suitable connection device is placed to connect to the air distributor. In the axial distance between the connection flange with the distributor and the expansion, there are two openings which communicate with the external environment through metal ducts. The first opening is the solid fuel inlet (212) from which the solid combustible material is inserted while the second opening (213) is used to regulate the active height of the fluidizing sand bed either through mechanically removing the content of the gasifier, or through mechanical addition of ash that has already been removed either from the bottom of the gasifier or from the collector of the centrifugal cyclone. Downstream the expansion there are also two or more openings while at its upper point the gasifier is connected to a blind flange or other isolation device. The opening (215) serves to supply fine-grained solids or liquids to the gasifier, such as bed material (sand) or liquid fuels respectively. Opening (211) is as close as possible to the upper point of the gasifier and through a metal duct the produced gas is discharged from the reactor.

[0045] The solid fuel material enters the gasifier from an opening (212) through mechanical bulk material transportation equipment such as screw feeder. The solid fuel material, that may optionally be dried, flows into a temporary storage tank which is connected to the orifice at the inlet of the solid fuel (212). The storage tank has an integrated moisture meter suitable for use in bulk raw materials which analyses in real time the moisture content of the solid fuel after the drying stage. The tank is isolated flow-wise from the screw conveyor through a standard hydraulic isolation device with or without pressure adjustment. The hydraulic isolation device consists of two or more valves of the knife gate type or other technology (e.g. rotary valve) suitable for use with powders and particles, between of each a vessel of given volume is placed equipped with solid fuel material detection sensors. This device enables a pseudo-continuous supply of fuel in small batches. The batch is transported to the screw conveyor by the following procedure. Initially, the first valve is connected to the screw conveyor at the inlet of the aforementioned vessel and remains open while a second valve at the outlet of the vessel is closed. The process starts with the closure of the first valve and the subsequent opening of the second valve, followed by the gravity flow of solid fuel accumulated in the tank. When the level sensor detects material, the first valve closes and then the second valve opens releasing the combustible material into the socket of the screw conveyor. In this way, a controlled fuel supply is achieved, preventing the return of the producer gas to the tank as well as the adverse effects that this backflow phenomenon entails (premature pyrolysis of fuel and formation of agglomerates). To help reduce the returning flow, it is possible to supply a relatively small air supply or some inert gas downstream of the second valve to push the heated gases, which have the potential to return, back inside the gasifier. In the case of air use, the flow rate shall be known and subtracted from the total air supply to the process in order to keep the air-fuel flow ratio constant. The same principle is followed in case more than two isolating valves are included in the system as long as solid fuel material is present in each independent volume.

[0046] The appropriate selection of the gasifier’s operating parameters (210), i.e. pressure, temperature, solid fuel supply (212), air (214) and additives (216) can be used to produce gas with specific characteristics (e.g. content of particulate matter, tar, etc.) and at a desired temperature (e.g. in the range of 650-950 °C ). The choice of the operating parameters of the gasifier (210) is not important in the case of the present invention, except for the control of the temperature of the producer gas (209), so that it does not approach or exceed the threshold of 900°C, i.e. the fuel ash melting point.

[0047] Innovative Features and Operating Parameters of the proposed System

[0048] The operation of the gasifier in the selected temperature range is of significance as, in contrast to other known systems, the present invention achieves an easier, more effective and more efficient ash removal from the produced gas (209), by not melting the ash contained in the solid fuel. Thus, most of the ash settles due to gravity in the lower part of the gasifier (210) where it is mixed with the fluidized sand (216). Ash may be removed through a special removal unit, which has the form of a screw conveyor (217) and is mounted on the lower part (218) of the gasifier (210).

[0049] The layout of the gasifier allows for efficient processing in applications of limited space and especially limited height. It is an optimal compromise between the minimum hydraulic height to avoid pneumatic transport of fine particles and the maximum reactor diameter to ensure uniform distribution of solid fuel within the gasifier bed. The reactor height minimization is achieved through the combination of the horizontal mechanical ash removal and the specifically designed air distributor. In large-scale applications, such as industrial fluidized bed applications, removal of the bed material is achieved at the bottom of the bed with a vertical hydraulic isolation valve (lock hopper) system. The main advantage of the ash removal subsystem is the minimization of its height over other devices, through its horizontal orientation, ensuring reliable removal with mechanical means without requiring vital space under the bed. At the same time, by placing the air distributor collector outside the reactor, the amount of effective volume of gasification that is preserved, allows for the total height of the bed to be reduced. The basic principle of distributor design is the uniform air supply throughout the reactor cross-section according to the criteria of V. E. Senecal (V. E. Senecal, “Fluid distribution in Process Equipment”, Ind. Eng. Chem. 1957, 49, 6, 993-997).

[0050] In existing gasification systems (see CN 104093481 A) the gas plenum or wind box is located under the bed and is distinguished from it by perforated or porous plates or rings which constitute the air distributor. The placement of the plenum under the bed balances the pressure and allows the accumulation of air throughout the volume of the plenum and therefore uniform flow through the distributor orifices. In these cases, ash removal is a challenge and is usually achieved through a declined duct at a chosen height above the dispenser, so that there is always an inaccessible volume between the distributor and the entrance of the declined duct, in which ash accumulates.

[0051] In other applications (see CN 106336905 A, WO 2020071908 Al), for the complete removal of ash there is a vertical opening at the bottom of the gasifier. In the opening perimeter, and in order not to prevent the downward flow of the bed, the air distributor consists of a perforated conical sheet within the bed which internally separates the plenum from the bed. In these cases, ash is completely removed however achieving the uniform flow of air from the distributor is problematic.

[0052] The present invention places the collector out of the gasifier without affecting the internal diameter and the height of the bed. Through the symmetrical air distribution, uniform flow is achieved inside the gasifier while avoiding ash and sand deposition on the heating elements inside the plenum avoiding reduction in thermal efficiency and service life reduction of those heating elements.

[0053] By use of a moisture sensor in the temporary fuel supply tank, the control system calculates the actual energy content of the solid fuel material that enters the gasifier and adjusts the air supply accordingly in order to maintain the desired air equivalence ratio (0.2 to 0.5) enabling the utilization of non-standardized solid fuels with minimal effect on the stability of the gasifier's operation. The control of the air and solid fuel supply is implemented through appropriate variable speed drives on each feeder.

[0054] Alternative Realizations of the Gasifier

[0055] In alternative examples of gasifier realization (210), the cross-section of the reactor may be square, rectangular or of any other shape while piping for process flow air preheating before entering the distributor may run through the refractory lining. In an alternative example the gasifier is not internally lined and metal tube protection is achieved through the preheating of the process air. Additionally, internal baffles may be placed along the gasifier height (i.e. means to increase the gas residence time in the gasifier) at an appropriate distance and inclination to create internal recirculation or to increase the actual distance traveled by the producer gas at high temperature which increasing residence time. In alternative examples the opening (213) may be inclined with an angle of < 90° to the ground and act as hydraulic overflow protection to control the bed level in the gasifier.

[0056] In alternative examples of gasifier realization, the air distributor may consist of vertical or inclined pipes. Instead of perforated pipes, perforated sheets or meshes capable of holding the weight of the bed may be used. In each realization example, there is a sufficient gap typically greater than 2 cm in the gasifier cross-section so as not to hinder the free downward flow of the material to the ash removal device located at the bottom of the gasifier.

[0057] In alternative examples of the solid fuel supply subsystem, the screw conveyor is replaced by an inclined duct with an angle of > 90° to the ground through which the fuel is gravitationally transported into the gasifier bed. In other examples, the screw conveyor may be replaced by two or more screw conveyors with intermediate stages of isolation which serve to minimize returning hot gas flow from the inside of the gasifier to the supply subsystem.

[0058] In alternative examples for the ash removal device, the ash extraction unit may be moved elsewhere in the lower part of the gasifier (210), while the screw conveyor may be replaced by another mechanism. An example of such a mechanism may be a gate or other valve mechanism which opens and closes accordingly, to allow ash and sand to be extracted (216) by gravity.

[0059] Optionally the gasifier (210) is connected via the ash and valve extraction mechanism (219) to a sand recovery unit which separates the sand (216) from the ash and feeds the recovered sand (216) back to the gasifier (210). The sand cleaning unit (205) may be in the form of a sieve with an appropriate mesh size to retain the sand or alternatively may be in the form of a vortex or other known technology suitable for the gravitational separation of sand (216) from ash.

[0060] Innovative Features and Parameters of Operation of the proposed Cyclone

[0061] The gas treatment subsystem consists of three purification stages. In the first stage (first producer gas cooling device (220)) the combustible gas is cooled at a temperature suitable for supply to the next stage and large diameter particles are optionally removed through a cyclone, in the second stage (filter unit (240)) the entire particulate load of the gas is removed using barrier filters, while in the optional third stage (second fuel gas cooling device (230)) condensate is removed by gas cooling and droplet separation. [0062] The produced combustible gas, which contains particles of different diameters, is supplied from the outlet (211) of the gasifier (210) through a pipe into a centrifugal cyclone unit (220) which retains at its lower part particles of larger selected diameters (e.g. 5 pm) suspended in the combustible gas (209). The cyclone unit may be of wet or dry type, while in alternative realization examples it may be replaced by other types of large particle retention unit based on a different technology (e.g. electrostatic precipitator) or even eliminate this separation stage entirely.

[0063] The separated particles are extracted from the lower part of the cyclone unit (220) using a suitable removal device, which has the form of a screw conveyor (222) and is based at the lower part of the cyclone unit (220).

[0064] Alternative Realizations of the Cyclone (220)

[0065] In alternative realization examples, the cyclone may either be completely eliminated or operated in series with a second more efficient cyclone or with a filter of other technology (e.g. electrostatic precipitator). In alternative realization examples of the cyclone unit (220), the large particle extraction unit may be moved elsewhere in the lower part of the cyclone unit (220), while the screw conveyor may be replaced by another transport mechanism. An example of such a mechanism is a gate-type mechanism which allows the collection of retained particles through gravity flow. The valve (221) is connected to the output of the large particle extraction unit and specifically after the conveyor screw (222) when the latter is connected to the cyclone unit (220).

[0066] Innovative Characteristics and Operating Parameters of the proposed Filter Unit [0067] The combustible gas outlet, optionally free of large particles, is performed through a pipe (225) which feeds the gas into a filter unit (240) to remove the remaining particulate load consisting mainly of particles in the diameter range of 0.05-5 pm. The filter unit (240) consists of at least two identical (or not) filter arrays in a parallel connection layout. Through the use of at least two filters, in parallel connected to each other, which may operate simultaneously or alternately, the creation of appropriate conditions for the uninterruptible operation of the system is achieved (200) and more specifically the possibility of controlled cleaning of the filters without affecting the unit productivity, that is, without interrupting the operation of the unit during the cleaning of one or more filters.

[0068] For the sake of a clearer illustration, the example in FIG. 2 presents two filters, the first filter (241) and the second filter (242) in parallel connection. Individuals with technical domain knowledge of the invention may easily recognize that more than the two filters depicted may be connected in parallel without deviating from the intended purpose of the invention. For this purpose, i.e. to achieve the possibility of cleaning one (or more filters) without interrupting the operation of the system (200), each filter (241, 242) is placed in a container independent of the containers of the other filters. The container is connected at its entrance with an independent pipe (226, 227), respectively for the first (241) and second (242) filter, and at its exit with an independent pipe (236, 237), respectively for the first (241) and second (242) filter. The pipes (226, 227) comprise a branching of the pipe (225) connecting the outlet of the cyclone unit (220) with the entrance of the filter unit (240), and the tubes (236, 237) converge in the pipe (239) connecting the outlet of the filter unit (240) to the tar condenser (230). The pipes (226, 227) include valves (246, 247) before filter inlets (241, 242), respectively, and valves (234, 235) after filter outlets (241, 242), respectively.

[0069] By use of the filters of the filter unit (240), the filters (241, 242) retain almost the entire particulate load of the gas that is fed to the filter unit (240). Gradually, the accumulation of particles on the surface of the filters creates a "filter cake" mass, which causes an increase in the pressure drop at the filters and reduces the energy efficiency of the system (200). For this reason, filters (241, 242) must be cleaned periodically and preferably when the pressure drop exceeds a predetermined upper limit.

[0070] Innovative Features and Operating Parameters of the proposed Condenser

[0071] Particle-free gas exiting the filter unit (240) is channeled, through pipe (239) in which pipes (236, 237) converge, into the condenser (230) to remove tar and other condensed compounds. Tar, water vapors and other compounds condense as a consequence of gas cooling and deposit on the walls of the device and, due to gravity, flow towards the lower part of the condenser (230) where a collection tank is located. The condensate from the collection tank is evacuated using a simple liquids pump (232), which is located inside or outside the collection tank.

[0072] The condenser operating principle is the cooling of the gas from around 400°C to the temperature of around 50°C in order to condense the moisture and tar content and remove them from the producer gas. Indicatively, a concentric tube-in-tube heat exchanger is applied. The cooled gas flows in the internal tube of the exchanger while in the external tube, which encloses the inner one, the coolant flows. In this application, gas cooling is achieved in two stages. In the first stage, the process air flows into the external tube and is preheated before entering the gasifier by recovering part of the heat. In the second stage, a coolant flows in the external tube which delivers the recovered heat load to the heat exchanger responsible for heating the drying air. The double tube is preferably vertically orientated, and a certain number of tube passes is performed to achieve the required heat exchange surface. At the lower point of each pass there is a condensate collection duct connected to the condensate collection tank. In the desired implementation, the inner tube consists of straight tube sections without internal configurations to enable easy tube wall cleaning during maintenance. The external tube contains fins which amplify the heat transfer coefficient. In the desired realization, the fins have the form of an endless screw and force the coolant into a helical path around the internal tube, increasing the coolant speed and thus the convection coefficient. The fins may have any other form that amplifies the heat transfer coefficient between the fluid streams thin the two tubes, for example transverse rings or longitudinal bars.

[0073] By removing almost the entire particulate load of the produced gas, the condensates created in the condenser do not contain solid impurities and therefore have more favorable flow characteristics, compared to condensates that create muddy effluents as they condense around particles. The favorable characteristics allow easier flow into the condensate collector tank by minimizing agglomeration in the walls and valves of the condenser resulting in lower need for maintenance and thus, more economical operation. Also, due to the fluid form of the condensate, its removal is easily performed through a non-specialized pump without need to shut down the gas production plant.

[0074] Alternative Realizations of the Condenser

[0075] In alternative realization applications of the condenser, the heat exchanger may be of another type such as a tube-and-shell of one or more routes or a helical heat exchanger. In alternative condenser (230) realization examples, the condensate extraction unit may be placed elsewhere in the lower part of the condenser (230), while the pump may be replaced by another mechanism or device. The condenser (230) usually has the form of a tube-in-tube heat exchanger, but it may also take the form of an electrical cooler or gas refrigerator or a venturi type scrubber or a liquid precipitation device.

[0076] The gas is then discharged from the condenser exit (230) clean and is channeled through a pipe (260) to a utilization system such as an internal combustion engine for electricity generation or stored in a gas tank under pressure for future use. The engine and the tank are not shown in FIG. 2.

[0077] The system also includes an operating control unit which is not shown in FIG. 2. [0078] Cleaning of filters without interruption of the operation of the clean gas production system

[0079] In contrast to the state of the art, this invention allows filters (241, 242) to be cleaned by use of compressed air without interrupting the proper operation of the system (200) while a compressed air temperature control system ensures the minimization of thermal stress on the filtration elements during cleaning. The valves (246, 234) as well as the valves (247, 235) are open when the system operates so that the gas is led into the filters (241, 242).

[0080] Choosing as an example the cleaning of the first filter (241), while the second filter (242) is in operation, the control unit of the system (200) (not depicted in FIG. 2) proceeds to isolate the first filter (241) from the rest of the system (200) by closing the valves (246) at the entrance and (234) at the exit of the first filter (241) and its container. After closing valves (246, 234) the import and export of gas to the first filter (241) and its container stops.

[0081] The filters (241, 242) are cleaned through compressed air injection from the filter exit towards the filter inlet, i.e. in the direction opposite to that of the gas-flow during filter operation. Compressed air is stored in a pressure vessel (251) with air supplied by a compressor or fan (250) through tubes (254, 255) which are connected to the filter container outlets (241, 242), respectively. The pressure vessel is equipped with a stored gas temperature control system consisting of one or more electric heaters and thermostat. The tubes (254, 255) are connected to valves (252, 253) respectively, which valves (252, 253) are closed during filter operation (241, 242) so that the particle-free gas at the filter exit (241, 242) is not injected into the compressor (250) but into the tubes (236, 237).

[0082] When cleaning the first filter (241), in addition to the closed valves (246, 234), valve (252) opens to supply compressed air to the filter outlet (241) in order to clean it. By channeling compressed air into the filter (241), the layer cake of material is detached from the filter input channels and hits the bottom surface of the filter container (241), while closing the valve (252), isolates the filter (241) and its container from the compressor (250). The particulate mass resulting from filter (241) cleaning is deposited in the lower part of the filter container (241) from where it is extracted using a suitable extraction unit, which is in the form of a screw conveyor (244). The air that has been released into the container then re-penetrates the filter and is extracted clean to the ambient through valves (272, 273) which open while the exit valves (234, 235) to the condenser remain closed. The valves (272, 273) in the depicted realization are connected to the extraction units (244, 245), while more commonly, in an alternative realization (which is not depicted), they are connected to branches of the pipelines (236, 237), which are located before the valves (234, 235), respectively. [0083] Alternative filter unit and filter cleaning process realizations

[0084] In alternative realization examples of the filter unit (240), the small particles extraction unit may be placed elsewhere in the lower part of the filter unit (240), while the screw conveyor may be replaced by another transport mechanism. An example of such a mechanism may be a gate-type mechanism which opens and closes to allow the removal of fine particles by pneumatic transfer or gravity.

[0085] The injection of compressed air for the back flushing of the first filter (241) may be repeated before the new charging cycle, if deemed necessary by the control system and specifically by the pressure drop’s rate of increase in the filter during its charging cycle.

[0086] Advantages of different realizations of the filter unit and their cleaning process

[0087] The reason the filter (241) cleaning process with a single pressurized air pulse is effective is related to the way the system (200) and the filter (241) operate. More specifically, the system operates at temperatures lower than the ash melting or softening point, while tars and other volatile compounds are still in gaseous form as the temperature of the gas while entering the filter unit is between 350 and 550°C. The result of the operating and design conditions of the system (200) allow even a gas with a high ash content, such as that produced from solid agricultural etc. residues, to be filtered by the filter unit (240) without tar condensation and deposition on the pipes, the filter, and the particulate layer on the filter surface. Thus, the retained material does not contain viscous substances such as tar or softened ashes and may be easily detached by injecting compressed air (up to 6 bar) of a single pulse. More pulses may be used in cases where an anomaly is observed and more specifically a sharp increase in the pressure drop between two sequential charge cycles of the same filter.

[0088] With the above-mentioned cleaning method, the pressurized air injected into the filter does not lead to mechanical stress on the filter elements capable of causing damage either during the cleaning of the filter or over time after a plethora of cleaning cycles.

[0089] Moreover, the air injection for the filter (241) cleaning does not lead to a thermal shock capable of causing damage to the filter (241) during cleaning, nor over time after a plethora of cleaning cycles as the temperature of the compressed fluid entering the filter (241) and its container is controlled so that there is no critical temperature difference between the filter to be cleaned (241) and the air channeled through it. Depending on the material and geometry of the filtering element, the tolerable temperature difference between the filter to be cleaned (241) and the air injected into the filter (241) may be calculated experimentally so as not to create a thermal shock that could cause a filter (241) material failure. Indicatively, the temperature difference between the filter to be cleaned (241) and the air injected into the filter (241) is ±50°C. Such a temperature difference is acceptable in order to avoid the development of axial stresses due to thermal strain greater than the strength limit of ceramic filtering elements from cordierite material, (M^AUSisOis) and wall thickness more than 1 mm.

[0090] In the present invention the problem of thermal shock is solved by utilizing the above experimental data for the selected filter type (241) and regulating the injecting of compressed air into the filter to be cleaned when said filter (241) is isolated from the rest of the system (200), when the filter (241) temperature is lower than a threshold which in combination with the temperature of compressed air do not make a temperature difference between the filter (241) and the compressed air capable of creating a thermal shock harmful to the filter (241) as this temperature difference has been experimentally calculated for the selected filter (241).

[0091] The above procedure requires a specific amount of time for the filter (241) to be cooled to the desired temperature, the exact duration depends on the operating temperature of the filter (241), the temperature of the compressed air, the ambient temperature around the filter unit (240) and the rest of the filters and their containers contained in the filter unit (240).

[0092] In an alternative system (200) realization, instead of compressed air, the filter is purified with the reverse flow expansion of inert gas such as nitrogen or argon at a controlled temperature. In another example, the filter may be cleaned with the use of steam or purified producer gas at the filter operating temperatures, during the cleaning cycle. In another example, an auxiliary supply of steam or air is injected into the filter during loading in mixture with the gas to be cleaned. Through steam or air supply, the oxygen necessary for the oxidation of the carbon particles is supplied to the filter, lengthening the loading cycle due to the consumption of part of the particulate load during filter loading. In an alternative realization example, cleaning is done in two stages. During the first stage, the cleaning described in the desired realization is carried out and during the second stage a high flow rate of air at atmospheric pressure is supplied to the filter for the oxidation of the particulate load that has remained on the filter surface.

[0093] In an alternative realization example of the system (200), filter elements of the filter system (240) are coated or impregnated with a catalytic material suitable for breaking down tar and/or other volatile compounds so that they, in the absence of condensation, as the filter unit is connected to the system (200) upstream the condenser (230), are broken down into gaseous components in order to reduce the amount of tar and the need to condense and remove it in the liquid phase. In this realization example, the filter operating temperature depends on the catalytic material used and may be increased up to 800°C.

[0094] In a different realization example of the system (200), an optional heating unit (251) is placed per pipeline (254, 255) so that more than one filter may be cleaned simultaneously, in cases where the filter unit (240) contains more than two filters. This allows the accurate control of the compressed air desired temperature for the simultaneous cleaning of each and every filter within the filter unit (240).

[0095] At the end of the filter cleaning process (241) through compressed air injection, collection of the detached particles at the lower part of the filter container (241), and their extraction from the lower part of the filter container (241) using the extraction unit (e.g. the screw conveyor (244)) valve (272) closes, while valve (252) and valves (234, 246) are already closed, so that the filter (241) and its container are isolated from any other unit of the system (200), from external systems and the external environment, while the filter (242) operates normally, having the valves (247, 235) open and the valves (273, 253) closed and remain in this state until the new loading cycle begins.

[0096] Process of restoring a cleaned filter to the operation of the system

[0097] To return the filter (241) to working condition after cleaning it, valves (246, 234) open and gas is channeled into the filter (241), in the same way already being channeled into the filter (242), through pipe (225) connecting the cyclone unit exit (220) to the filter unit (240) entrance.

[0098] The same cleaning procedure described for the first filter (241) is followed, respectively, for the second filter (242), and for each additional filter in case several filters are included in the filter unit (240). In the case of the second filter (242) the valves (247, 235, 253, 273), the pipes (227, 237, 255) and the screw conveyor (245) are used.

[0099] Parameters of operation of a clean gas production system

[00100] FIG.3 illustrates a simplified diagram of the system in this invention and the operating temperatures of its individual units. The clean gas production system (300) consists of, inter alia, a gasifier (310), a first stage fuel gas cooler (320), a filter unit (340), a condenser (330), and an air compressor (350).

[00101] The gasifier (310) is of the fluidized bed type and may be adjusted so that in its lower part it operates at a temperature range around 650-950°C. This temperature decreases axially towards the upper part of the gasifier (310) from which it exits in the temperature range of 500- 800°C. The temperature drop is due to the endothermic chemical reactions that take place inside the gasifier (310) between the solid fuel and the air introduced into the gasifier (310). The temperatures within the gasifier (310) may be adjusted by regulating the flow rate, the flow rate ratios of solid fuel, air and sand supply and the ash outlet supply, so that the producer gas temperature in the upper part of the gasifier is approximately 500-800°C. These adjustments allow (with an accuracy of some °C, e.g. 10-50°C) to regulate the desired gas temperature so that, on the one hand, no ash melting occurs in the bed and on the other hand the bed temperature is maintained at the desired levels to maximize the rate of solid fuel conversion into gaseous fuel. To maximize the conversion rate, the gasifier (210) is adjusted to deliver fuel gas to the lower part (203) at a superficial speed less than 3m/sec.

[00102] With the flow of the gas from the gasifier (310) to the first gas cooling device (320) large flying particles (> 5pm) are removed from the gas and at the same time the gas temperature is reduced so that the gas is channeled to the filter unit at temperatures around 400°C. In alternative realization examples of the system (200, 300) the temperature of the gas channeled to the filter unit may be adjusted using an optional heat exchanger (not depicted) to cool the gas to the desired temperature.

[00103] The gas introduced into the filter unit is at temperatures much lower (about 350-550°C or even lower in alternative realization examples) than the melting or softening point of the ash and at the same time higher than the condensation temperature range of the tar and other gasified compounds so that they do not deposit on the filters, causing clogging and making the cleaning operation of the filters difficult, costly and damaging to the filters.

[00104] After passing through the filter unit and subsequently cleansed from suspended particles, the gas passes through the condenser (330), which cools it down to a temperature of around 50°C in order for the tars to be condensed also dissolving other gasified compounds contained in the gas such as hydrogen sulfide. At the condenser exit (330), the gas is now clean and suitable for use in an engine for electricity generation, or for storage and future use. In different realization examples, the gas is not cooled but is utilized at high temperature with tars remaining in gaseous form (e.g. gas turbine or gas burner).

[00105] To clean the filters, the compressor (350) channels compressed air into one or more filters of the filter unit (340) and approximately at the temperature of the filter to be cleaned. To avoid thermal shock on the filters during their cleaning, the system (200, 300) allows compressed air supply to the filter (or filters) for cleaning, after isolating a priori the specific filter (or filters) from the gas flow, at a controlled temperature. [00106] Alternative examples of system operating conditions

[00107] In alternative examples of system operating conditions, the air supplied to the process is replaced by steam or oxygen or carbon dioxide or a mixture of air with steam and/or oxygen/carbon dioxide in order to produce a gas with a lower nitrogen content and therefore a higher calorific value.

[00108] In an alternative example of operating conditions, the oxidizing agent is replaced by an inert gas such as nitrogen or argon. Under these conditions there is no oxidation, and the gasifier operates under pyrolysis conditions to produce bio-oil and char. The bio-oil is condensed and collected in the condenser while the char is collected from the ash removal screw conveyor at the bottom of the gasifier. During pyrolysis, a small amount of gas is still produced which follows the cleaning stages of the invention and is used for energy production. The corresponding operating temperatures of the gasifier under pyrolysis conditions are in the range of 500 - 700°C.

[00109] Selection and use of filters in the clean gas production system

[00110] FIG. 4 illustrates examples of filters of the system in the present invention and their principle of operation. Filters (400) are examples of filters configurations. Different configurations are possible without deviating from the scope and intended protection of the present invention. The different configurations are obvious to persons of ordinary skill in the art related to the present invention and are therefore not specifically mentioned.

[00111] The preferred filter option for the system (200, 300) is monolithic ceramic honeycomb filters with plugged alternate channels. Alternatively, other filter types can be used depending on the temperature of the gas channeled to the filter inlet. The following paragraphs give examples of the preferred filters, but similar conditions apply to the alternative types of filters that could be used in alternative realization examples of the system (200, 300).

[00112] The filter (410) is illustrated in a side-frontal view having a square cross-section and, in this example, consisting of longitudinal channels, in an AxB matrix arrangement, where A>2 and B>2, and where one channel in relation to the other channels is positioned in parallel and with the input and output of each channel aligned with the filter input and output or vice versa. The channel cross-section is depicted square, but it may also be circular, rectangular or any other shape or size and is formed into monolithic ceramic material. At the input (401) of the filter (410) the channels (442) are sealed, while the channels (443) are open, and at the filter exit the channels (442) are open and the channels (443) are sealed. The sealing or plugging of the channels (442, 443) may be done with any suitable material, resistant to high temperature and with a low expansion coefficient, such as inert cement, etc. Each channel (442, 443) may be open only at one end and is alternately placed inside the filter (410) so that when the gas enters the open channels (443) it penetrates the length of the channels (443) up to their closed end at the exit (402) of the filter (410). The gas, unable to exit the closed end of the channels (443) (nor to enter through the clogged entrance of the channels (442)) and due to the gas pressure, is forced through the porous walls of the channels (443) and enters the neighboring channels (442) from which it cannot exit through their plugged end at the entrance (401) of the filter and thus finds an outlet from the open end of the channels (442) at the exit (402) of the filter (410). The arrows (445) indicate the gas flow path when filtered by the filter walls (410), where the particles contained in the gas are deposited since the ceramic material wall pores of the filter (410) have a smaller cross-section than the cross-section of the smallest particle they may retain.

[00113] The outer surface of the filter (410) has no pores and may comprise of the same or different material from the material of the channels (442, 443). The choice of material of the outer filter surface (441) can be made to offer protection to the filter (410) during use, its connection to the filter unit, etc.

[00114] The filter (420) is depicted in cross-section view. It is the same as the filter (410) both in terms of structural elements and function. The only difference lies in the fact that the channels (448, 449) are in a circular matrix layout, as opposed to the filter (410). The cross section of the channels is depicted square but may be of any shape and size, as analyzed for the filter (410) and its method of manufacturing is the same as that of the filter (410).

[00115] How to secure filters in the filter unit + advantages

[00116] FIG. 5 presents simplified examples of securing the filters in FIG. 4 into the system in the present invention. The filter (510) depicted in a side-frontal view, is the same as the filter (410) and contains plugged alternate channels (542, 544) in a square matrix layout. When placed in the filter unit (240, 340), the filter (510) is placed in a (usually metallic) quadrilateral frame (545) of dimensions larger than the filter (510), and between the filter (510) and the frame (545) a sealing material is inserted (543) suitable for securing the filter (510) in the frame (545), where the material (543) is wrapped around the periphery of the filter (510) and is made of a material resistant to the filter operation (510) high temperatures, capable of absorbing vibrations and suitable for expanding to allow the space between the filter (510) and the frame to be sealed (545). In this way the material (543) absorbs any forces that could be applied on the filter (510) when moving the system (200, 300), securing and releasing the filter (510) in the frame (545), during filter operation (510), and during contraction and expansion cycles of the frame (545). An example material for the construction of the sealant (543) is a layer of needle-shaped polycrystalline fiber.

[00117] Alternative filter realizations

[00118] The filter (520) depicted in a cross-section, is the same as the filter (510) and contains plugged alternate channels (547, 548) in a circular matrix arrangement. When placed in the filter unit (240, 340), the filter (520) is placed in a (usually metallic) cylindrical frame (550) of dimensions larger than the filter (520), and between the filter (520) and the frame (550) a material (549) suitable for securing the filter is inserted (520) in the frame (550), said material (549) is wrapped around the outer surface of the filter (520) and is made of a material resistant to high temperatures and capable of absorbing vibrations and deformations, thus allowing the filter to be secured (520) in the frame (550). In this way the material (549) absorbs any forces that could be applied on the filter (520) when moving the system (200, 300), securing and releasing the filter (520) in the frame (550), during filter operation (520), and during the contraction and expansion cycles of the frame (550). An example of a material for the construction of the sealant (549) is a layer of needle-shaped polycrystalline fiber.

[00119] Filters (410, 420) may consist of arrays of identical filters grouped into square, circular or other shapes and size matrices, and contain a different number of channels than those shown in FIG. 5. It is also possible that the matrices and/or arrays of filters are encased within frames of a similar shape for ease of transport, securing and protecting from external forces. The monolith or array of monoliths that make up the filter (510, 520) is placed vertically within the container in which it is anchored at one end, while the other end is free to move to prevent the formation of compressive or tensile stresses on the ceramic material due to contraction and expansion of the metallic housing.

[00120] Due to its compact form, the filter (410, 420) possesses high mechanical strength in combination with a high filtration area thanks to the dense channel structure. Such ceramic filters are widely used in the automotive industry for the filtration of soot particles from diesel engine exhaust gases called Diesel Particulate Filters or DPFs. Space savings in the automotive industry are of critical importance, so DPF ceramic filters, through their high filtration surface and small volume, are ideal for small spaces and therefore small-scale gasification units in limited space applications. More specifically, they occupy at least two times less space than ceramic and metallic candle type filters used in similar larger-scale applications. In DPF filters the lack of empty space between adjacent channels increases the static strength of the ceramic filter while the empty space between adjacent candles in other applications is a source of problems regarding the structural strength of the filter and the lack of protection against the accumulation of ash on the outer surfaces of candles.

[00121] Control of operation of a clean gas production system

[00122] FIG. 6 shows a simplified system operation control flowchart. The operation control (600) of the system (200, 300) begins with the reception of data (610) from temperature and pressure sensors at various points of the system (200, 300), position sensors on the valves, and motion sensors in other moving parts of the system (200, 300) as well as level, weight and humidity sensors mounted on the solid raw material feed system or at the dryer’s outlet. The sensors may be selected among all known temperature, pressure, position and motion sensor technologies as well as other sensor technologies which may be used in ways that provide data equivalent to temperature, pressure, distance and humidity sensors.

[00123] Step measurements (610) are received from a control unit (e.g. microprocessor, computer, programmable logical controller (PLC), etc.) which executes software stored in a memory drive or hard disk or other physical storage medium (e.g. magnetic or optical medium) or in a the cloud or on another computer to which it is connected. Alternatively, the computational unit is of the type of an Application-Specific Integrated Circuit (ASIC) or equivalent and/or executes firmware.

[00124] The control unit, hereinafter the computer, proceeds to adjust and control the operation of one or all of the following: the gasifier (620), the cyclone unit (630), the filter unit (640) and the condenser (650). Steps (610)-(650) are performed until the system is shut down (660) (200, 300) and undertake to implement all the necessary functions for the gasification of the solid fuel, the cleaning of the combustible gas and the cleaning of the filters. More specifically, the control unit may be designed or programmed to implement the adjustment of at least one of the following: the fluidized bed gasifier (210) supply with solid fuel and air, the operation of the first combustible gas cooling device (220), the operation of the filter unit (240), and the operation of the second combustible gas cooling device (230). The computer may also record the data of the sensors and the actions performed in the steps (610)-(650) in order to process them (or assign the data processing to an external computer) for the purpose of their statistical, or other, analysis and the use of the results of the analysis to optimize the actions of the steps (610)-(650). The data analysis and the optimization of the steps (610)-(650) may be done using appropriate software, firmware, hardware, or a combination of all and may include amongst others, artificial intelligence techniques and machine learning. [00125] The examples used above to describe the present innovative solution should not be considered as restrictive to the scope of this innovative solution. This innovative solution may be applied to scenarios and arrangements other than those described in the examples presented above. This innovative solution should be considered as applicable to any system of gasification of solid fuels of all types and cleaning of the gas produced.

[00126] The average person with relative knowledge of the state of the art understands that the shape and dimensions of the parts of the present invention, as presented in the exemplary embodiments, may be modified without deviating from the scope and intended protection of the present invention.

[00127] The above exemplary embodiment descriptions are simplified and do not include parts that are used in the embodiments but are not part of the current invention, are not needed for the understanding of the embodiments, and are obvious to any user of ordinary skill in related art. Furthermore, variations of the described exemplary embodiments are possible, where, for instance, some parts of the exemplary embodiments may be rearranged, omitted and replaced with equivalent, or new parts may be added, as well as, the existing parts may be interconnected differently than the described manner, under the provision that the different interconnection is compatible with the technical effect of the parts of the invention, being characteristics of the invention. Similarly, the modification of the shape and dimensions of the presented parts is assumed to fall within the scope of protection of the present innovative solution to the degree that these modifications are equivalent to the described exemplary embodiments, or do not add tangible and unanticipated or non-obvious improvements to the technical effect they offer. Thus, the present text is not intended to be limited only to the presented exemplary embodiments but it should be given the broadest possible scope according to the principles and the new characteristics it discloses.

[00128] Unless specifically otherwise noted, it is the intention of the inventor that the words and phrases in the specification and claims be given the ordinary and accustomed meanings to those of ordinary skill in the applicable art(s).

[00129] The foregoing description of a preferred embodiment and best mode of the invention known to the applicant at this time of filing the application has been presented and is intended for the purposes of illustration and description. It is not intended to be exhaustive or limit the invention to the precise form disclosed and many modifications and variations are possible in the light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application and to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.