ITMI20100763A1 - GASIFICATION PROCEDURE WITH THREE STAGES WITH FIXED BED FOR SOLID FUELS - Google Patents

Description

â € œThree-stage fixed bed gasification process for solid fuelsâ €

TEXT OF THE DESCRIPTION

The patent relates to an innovative process concerning the gasification of solid fuels on a fixed bed.

PREMISE. The appliances that carry out gasification are called gas generators. Fixed bed gas generators are closed appliances, in parallelepiped or cylindrical form with metal walls internally lined with refractory material indicated with the number 1 in the various figures of the patent; the gas generator is equipped at the top with a device 2 for feeding the solid fuel to be gasified; the latter passes through a supply duct 3 and falls on the bed of material 4 supported below by a mobile or fixed metal grid 5. If the velocity of the gases through the bed is lower than certain values, the particles that make up the bed are lean one on top of the other; if vice versa the speed exceeds certain limits, the particles of the bed detach from each other and are kept in suspension by the gas, moving continuously: in the first case we speak of a fixed bed and in the second of a fluid bed; as already said, the patent concerns fixed bed gas generators.

Our patent relates to a gasification process to be carried out on a fixed bed.

If, as generally happens, the combustible material is accompanied not only by humidity, but also by non-combustible material, the latter cannot be gasified and together with a little residual carbon constitutes a waste product called slag; these slags are extracted from the gas generator, by the slag extractor 6.

The LFU, therefore, works very well with low humidity fuels without hydrogen; the ideal material for it is coke.

The LFU would give a bad syngas in the gasification of fuels with plastic, rubbers, waste or derivatives and a syngas with too much tar and humidity even in the case of wood, especially if wet.

LFU, on the other hand, behaves well with fuels with a high presence of inerts, because the latter reach the lower part of the gas generator where there is the highest temperature and where the carbon meets the gasification air in the best conditions for oxidation; the result is a good quality of the waste discharged by the gas generator itself in the sense that they contain a low quantity of unburned carbon.

If an LFD gas generator is used, represented schematically in Figure 2 of Table 1, the presence of tars and sludge is drastically reduced because the gases, crossing the bed from top to bottom, encounter increasingly hot layers which perform an action of craking on complex substances and favor the oxidation of the fuel by breaking, for example, the macromolecules of polymers.

The lack of LFD gas generators is that, if the fuel has a high content of inert materials, especially low melting ones, when they reach the carbon oxidation zones where the temperatures are at their maximum, they melt and the molten mass determines heavy disruptions to the flow of the air and fuel through the bed, forcing the gas generator to stop.

The extraction of the gas from the bottom of the LFD gas generators is more complex than that of the LFU gas generators, because it interferes with the mechanisms of the grid, which must not be damaged by the temperature and by contact with the syngas.

INNOVATIVE AND ADVANTAGEOUS ASPECTS OF THE NEW 3-STAGE GASIFICATION PROCESS. With the new gasification process, both the defect of the LFU of producing syngas with tars and condensable products such as water, and the defect of the LFD of not functioning correctly when the fuel has high quantities of inert materials, will be eliminated. especially if they are low melting.

The three-stage fixed bed gasification process (LF3S), object of this patent, is represented in the block diagram of Figure 3 of Table 2.

in this process, the material is sent to the first stage consisting of an LFU, where the evaporation of the humidity of the fuel and the gasification of the volatile components and part of the carbon takes place; in the ideal functioning, in the lower part of the bed, the material that passes to the second stage is made entirely of fixed carbon or char.

In the first stage the walls 1 are conceptually and practically the same as those of conventional gas generators for which the same code is used; the same thing happens for the feeding device 2 and for the feeding channel 3.

The fuel falls on the bed 11 whose height, with the same capacity and fuel, is lower than that of the conventional gas generator LFU, for which a different code has been used; grid 12 is also different from the conventional one, in the sense that not all of the gasification takes place in it but only a part for which, in principle, it will have a lower surface.

The quantity of air 13 and steam 14 fed under the grate are lower than those of the conventional gas generator, because the gasification that takes place in the 1st stage is partial; in freeboard 15 the gases have a different composition from the final one, so the volume of the freeboard can be reduced. In the 1st stage freeboard the gases are at a relatively low temperature (indicatively but not limited to 400 - 500 ° C) and contain water vapor and volatile products of all kinds, both low and high molecular weight and has, especially with fuels with low humidity and high hydrogen content, a good calorific value.

From the bed of the 1st stage the material passes to the bed of the 2nd stage through the drop channel or the mechanical conveyor 16 while the syngas passes from the freeboard 15 of the 1st stage to the freeboard 20 of the 2nd stage through the duct 17. In the ideal operation, the material solid that reaches the second stage consists of fixed carbon (in Italian) or char (in English), which is composed only of carbon and solid non-combustible materials (inert); char can no longer gasify by simple heating, as it does not contain volatile materials or oxygen through which carbon gasification could be achieved by partial oxidation of the same to carbon monoxide (CO). Given that the functioning of the 2nd stage will be treated later, for now we limit ourselves to saying that the bed of material 18 that is formed in the 2nd stage is moved by a mobile grating 19 from which the material, through the the fall channel or the mechanical conveyor 22, passes over the fixed bed 24 of the 3rd stage moved by the grid 25. When the solid material on the grid 25 has reached the end of its path it is made up only of inert materials with a little of carbon that can no longer gasify, that is, it has become slag and as such is discharged from the gas generator by means of the extractor 6, conceptually and practically the same as that of conventional gas generators.

Apart from a small quantity that ends up in the syngas in the form of powder, the aggregates pass through the 1st and 2nd stages and reach the 3rd stage mixed with carbon and without volatile materials that have already developed in the previous stages; the bed of the 3rd stage, therefore, is made up of char with the highest concentration of aggregates.

The 3rd stage is of the LFU type, but since only the char is present in it, it is in ideal operating conditions and can withstand the high content of inert materials present in it; the air 26 and the steam 27 are blown from the grate; the air in the lower area of the bed oxidizes the carbon to C02; this reaction is strongly exothermic, so it heats the char that goes downwards, including the inerts present in it, and heats the gases that rise upwards; in this way the ideal conditions are created to gasify the carbon at the highest level, reducing its presence in the slag, with a similar reduction of energy losses due to unburned carbon in the slag.

The melting of the aggregates in the bed is avoided by mixing the gasification air with a suitable quantity of steam so that, when the reaction of the water gas, which is endothermic, does not reach the melting point of the ashes.

The gas that comes out of the 3rd stage bed is the classic mixed gas with discrete calorific value and passes from the 3rd stage freeboard 28 to the 2nd stage freeboard 20 through conduit 23.

OPERATION OF THE 2ND STAGE. The 2nd stage freeboard performs the fundamental function of cracking complex molecules coming especially from the 1st stage, possibly dividing them at the level of single molecules. In order for the cracking to take place, the gases must be brought to a high temperature (indicatively but not limited to 1000 -1500 ° C) and to reach this temperature they must be partially oxidized through the air 21, with the formation of CO and C02.

In the freeboard 20 of the 2nd stage there is the presence of water vapor coming especially from the freeboard 15 of the 1st stage and any further addition of water vapor 22 to optimize the process; the high gas temperature of the 2nd stage, as well as being necessary for cracking, also heats, by radiation and convection, the char present in bed 18 of the 2nd stage, so that the endothermic reactions of the gas can take place of water and reduction of C02. It should be noted that, unlike what happens in conventional LFDs, no oxygen is blown into the bed, so there is no danger of overheating which causes the aggregates to melt.

The endothermic reactions, in addition to producing an excellent syngas, are also positive in the sense that, as you go down towards the grid, the temperature of the syngas is lowered, so that the grid, possibly cooled in the more delicate metal parts, will be able to withstand the thermal stresses due to contact with the syngas that passes through it.

Another favorable peculiarity of the system is constituted by the fact that the bed 18 of the 2nd stage also carries out a physical filter action in respect of the possible presence of tars and sludge, for which the syngas is extracted from the bottom without such substances.

GLOBAL OPERATION OF THE LF3S. Summarizing the above on the operation of the LF3S, the advantage of the system consists in the fact that the gasification is divided into 3 phases, for each of which it is possible to operate in such a way that the bed of the material to be gasified and the reactions that must take place inside are optimized, being able to dose the gasification air and steam in the best way.

By breaking the bed into 3 parts it is possible to have, in each bed, a height of material lower than that which would be obtained by operating on a single bed, so that the fluid dynamics of the solid material that constitutes the fixed bed is better controlled. .

For example, in the 1st stage, it is easier to cope with the possible formation of blocks in the bed 11 which could occur due to the presence of low melting materials such as, in particular, plastics; similarly, in the 3rd stage, it is easier to keep under control the melting of inert materials that could take place inside the bed 25.

The syngas 17 coming from the 1st stage, which has an excessive quantity of volatile materials with high molecular weight and water vapor and the syngas 24 coming from the 3rd stage, which tends to have excessive quantities of C02, are further processed in the 2nd stage, so that in the final syngas 15 there is no presence of the negative substances contained in 17 and 24.

The cracking action that takes place in the 2nd stage freeboard is very important, to ensure that the final syngas is made up only of low molecular weight substances, also due to its reflections in the syngas purification phase.

For example, in LFU reactors, if chlorine is present in the fuel, dioxin is easily formed, the molecule of which contains chlorine strongly bound to aromatic rings. With normal wet or dry purification systems, it is not possible to obtain a selective chemical reaction between the chlorine and the purification reagent (a basic substance such as, for example, NaOH or NA2C03 or CA (OH) 2). If the same dioxin molecule is instead brought above 1000 ° C, even in the absence of oxygen, it is divided into smaller parts and there is a considerable probability that the chlorine remains bound only to hydrogen, forming hydrochloric acid, which it reacts easily with the basic substances mentioned above, forming solid salts in the form of powder, which can be easily separated from the syngas in a bag filter.

The same thing happens for other pollutants such as sulfur, which binds in the form of H2F and fluorine, which binds in the form of HF.

With LFUs, the alternative to the intermediate cracking solution is to bring the syngas to a high temperature at the outlet of the gasifier by suitably oxidizing it and in this case there is a loss of calorific value of the syngas, which strongly penalizes gasification.

When the syngas is made up of elementary substances with a low molecular weight, it is possible to cool the gas by means of exchange surfaces, with which the sensible heat can be recovered and reused to preheat the air and to obtain steam. water to be used in gasification, with a considerable advantage on the final yield.

When in the syngas there is the presence of sludge and tars, the exchange surfaces cannot be used and the classic system is that of carrying out a sudden cooling of the gas with water; after this process, the syngas is made up of H, CO, CH4, N2, while the sludge, tars and condensed water vapor are found in the washing water, all with the presence of acid pollutants.

This type of syngas purification involves a considerable loss of heat which lowers the efficiency of the gasification and poses problems of disposal of the washing water which are not easy to solve.

DELULF3S GEOMETRY AND ITS USE IN THE INDUSTRIAL FIELD.

The process described above, in the industrial field, can be carried out in several ways, using both rectangular-shaped grids and circular-shaped gasification grids; in a non-limiting manner, some examples of gas generators that carry out the process object of the patent are reported.

GASOGEN WITH VERTICAL RECTANGULAR GRIDS. The 3-stage fixed bed gasification process represented in Figure 3 of Table 2 can be carried out with a gas generator having a vertical parallelepiped shape such as that represented schematically in Figures 4 - 5 - 6 of Table 3. Figures 4 and 5 are a section longitudinal on the shorter side of the section in plan, in which you can see 3 rooms, placed one above the other vertically, each of them composed of the grid that supports the fixed bed and the freeboard, in which the process described in 1 takes place °, in the 2nd and in the 3rd stage. The gas generator is delimited by the internally refractory metal walls 1; the solid fuel to be gasified enters by means of the device 2 and through the supply channel 3 passes by gravity into the fixed bed 11 supported by the grate 12. In figure 4 of Table 3 the arrows indicate the movement of the solid material and it can be seen that it passes from the bed 11 to the first zone of the bed 18 through the drop channel 16. The movable grid 19 moves the material through the whole bed 18 and makes it fall at the beginning of the bed 25 through the channel 23; the grate 26 moves the material along the bed 25 and makes it fall into the slag discharge channel 30 from which the gas generator is extracted outside by the slag extractor 6. In Figure 5 of Table 3 the arrows refer to the movement of the fluids in the gas generator and it is noted that under the grate 12 of the 1st stage the air 13 and the steam 14 are fed and through the grate 12 they are uniformly distributed at the base of the bed 11 where the first gasifications take place and the syngas leaves the top of bed 11 and is collected in the plenum 15. From this plenum the syngas of the 1st stage passes to the plenum 20 of the 2nd stage through the ducts 17. In a similar way to what happens in the 1st stage, the air 27 and the steam 28 are fed under the grate 26 which distributes them uniformly in the bed 25; the syngas coming out of this bed collects in the freeboard 29 of the 3rd stage and from there it passes into the freeboard 20 of the 2nd stage, in which the air 21 and the steam 22 are injected. The air oxidizes part of the substances gaseous fuels present in the freeboard 20 and the syngas present therein increases in temperature, with the result that the craking of high molecular weight substances occurs. The gas, possibly added by the water vapor 22, for the reactions of the water gas, passes from the freeboard 20 through the bed 18 and is collected in the plenum of the subgrid from where it is sucked outwards by a fan.

GASOGEN WITH HORIZONTAL RECTANGULAR GRIDS. The 3 grids for gasification LFU of the 1st stage, LFD of the 2nd stage and LFU of the 3rd stage can be placed on the same plane, one after the other, as shown in Figure 7 of Table 4. Each grid is characterized by the fact of having a plenum in the lower part which in the 1st and 3rd stage LFU collects the air and steam to be fed into the bed while in the 2nd stage LFD collects the final syngas coming from the bed 18.

The conveyor 16 is installed between the grate 12 and the grate 19, which is identical to the grate, except that it has no holes for the air-steam mixture, which are not blown in this area because the gases they produce could bypass the freeboard 15 going immediately into the plenum of the 2nd stage grid 19. Similarly, the conveyor 23 is installed between the grids 19 and 25; the length of the conveyors 16 and 23 will be suitably greater than the height of the bed to avoid the above bypass. Also to force the 1st and 3rd stage syngas not to bypass its own freeboard, partitions 31 and 32 can be installed. Figure 7 in Table 4 is accompanied by all the codes that identify the various parts of the system, which are identical to those reported on Tables 2 and 3 for which we believe that the 3-stage operation of the gas generator is clear even with this geometry.

GASOGEN WITH RECTANGULAR GRIDS AT DIFFERENT HEIGHTS ON THE PLANT. The solution is represented in Figure 8 of Table 5, in which, compared to the vertical solution of Figures 4, 5 and 6, there is a reduction in height of the gas generator, against a greater extension in plan. GASOGEN WITH OVERLAPPING CIRCULAR GRIDS. The solution is represented in Figure 9 of Table 6. The equipment is similar to the vertical rectangular grid one in Figures 4, 5 and 6, with the only difference that the gas generator section is circular instead of rectangular. CONVENTIONAL GASOGENS, DISTINCT AND SEPARATE, CONNECTED CHAR SIDE AND SYNGAS SIDE, in order to realize the three-stage process: the solution is represented in Figure 10 of Table 7. This solution appears geometrically very dispersed.

Claims (13)

CLAIMS 1) Process for the gasification of solid fuels on a fixed bed, carried out in three stages (numbered in the direction of the path of the solid material), each of which is characterized by the fact that it consists of a mobile metal grid that supports and moves the material to be gasified , from a freeboard inside a refractory chamber that collects the syngas, from the pipes for the injection of air and steam necessary for gasification. The first stage works with gas that crosses the bed from the bottom up (updraft), the second with the gas that crosses the bed from top to bottom (downdraft), the third with the gas that passes from the bottom upward updraft. In the first stage, a mixture of air and water vapor is fed into the plenum under the grate, which is uniformly distributed by the grate in the lower part of the bed and through which part of the solid material is gasified. The same thing is done in the third stage; the gases emerging from the first and third stages flow into the freeboard of the second stage, above the bed of solid material supported and moved by the grid. In the freeboard of the second stage, the gases are oxidized in order to raise their temperature up to the values necessary to effect the cracking of the high molecular weight components (coming in particular from the first stage) and to be able to carry out the endothermic reactions for the production of gas. € ™ water and C02 reaction when the gases pass through the 2nd stage bed in the downdraft direction. In the 2nd stage freeboard, if necessary, water vapor is also added to make the above mentioned water gas reactions take place even if the fuel is not very humid. The gases pass through the second stage bed from top to bottom so that this stage functions as a fixed bed downdraft gas generator and is then extracted from the bottom of the grid. The 3-stage process allows to have a synthesis gas characterized by the absence of tars and sludge and consisting only of low molecular weight substances and with a reduced amount of water vapor, in order to allow the syngas to be cooled with surface exchangers and be purified only with dry systems. This procedure is schematically represented in Figure 3 of Table 2. At the conclusion of claim 1 it should be noted that there are gas generators with multiple gas streams that have only one fixed bed but there are no gas generators in which the fixed bed is broken into 3 parts, the 1st and 3rd of which are updraft functioning and the 2nd downdraft. 2) Gasification process, as in claim 1, characterized by the fact that the fuel to be gasified is loaded into the first stage where air and steam are fed into the lower part of the grate, so that the updraft movement of the gases through the fixed bed, causes the gasification of part of the material through mechanisms of pyrolysis, partial oxidation of carbon and reaction of the water gas, together with the evaporation of the humidity contained in the fuel so that the material passing from the 1st at the 2nd stage is made up only of char. 3) Gasification process, as per claim 1, characterized by the fact that, from the freeboard of the first stage, the gases, which are of poor quality due to an excessive presence of high molecular weight substances and excessive presence of vapor ™ water, and those coming from the 3rd stage freeboard that may have an excessive presence of C02 pass into the second stage freeboard to be further processed thus not lowering the quality of the final svnaas. 4) Gasification process, as per claim 1, characterized by the fact that in the freeboard of the 2nd stage, the gases coming from the freeboards of the 1st <0> and of the 3rd stage mix with each other, are added, if necessary, with a suitable quantity of water vapor and are brought to high temperature by partial oxidation of the syngas obtained by injecting air into the 2nd stage freeboard; in this way the conditions are prepared to be able to carry out the gasification of the char of the 2nd stage by means of the reactions of water gas production (C H20 = CO H2) and reduction of C02 (C02 C = 2 CO) which are both endothermic. 5) Process of raising the temperature, as in claim 4, characterized by the fact that the temperature is increased up to values such that the cracking of high molecular weight substances occurs, which are harmful as they produce sludge and tars and prevent a efficient gas purification. 6) Gasification process, as in claim 1 characterized by the fact that, with the 3-stage process, in the bed of the 2nd stage, the char is gasified only by means of endothermic reactions, thus avoiding the melting of low-melting inert materials which is the defect of conventional LFD gas generators in which exothermic reactions that can melt the inerts also take place. The decrease in temperature caused by the endothermic reactions will also allow easier extraction of the syngas from the 2nd stage grid. 7) Gasification process, as in claim 1, characterized by the fact that the process takes place in such a way that in the 2nd stage the flow of gases that crosses the bed is of the downdraft type, and in this way the char bed works as a physical filter against sludge and tars. 8) Gasification process, as per claim 1, characterized by the fact that in the 3rd stage the char coming from the 2nd stage is gasified, by means of water and steam, with an updraft type process which is the most suitable, as the char of the third stage contains a very high quantity of aggregates, because they have not been eliminated in the least by the gasification which took place in the first and second stage. 9) Gasification process, as in claim 1, characterized by the fact that the 3-stage process is carried out in a gas generator in which the three stages consist of parallelepiped chambers, arranged one above the other, with rectangular-shaped grids, with fuel supply from the top and passage of the char from one grid to another by gravity, with suitable pipes for the connection between the freeboard of the first stage with the gases that go down from the top to the bottom and the freeboard of the third stage with the gases passing from the bottom to the top, all as shown in Figures 4, 5 and 6 of Table 3. Figure 4 shows the flow of fuel through the gas generator and Figure 5 shows that of the syngas develops in the 3 stages. 10) Gasification process, as per claim 1, characterized by the fact that the 3-stage process is carried out in a gas generator in which the three stages are made in a parallelepiped chamber equipped with two intermediate septa to divide the three freeboards; the grids of the three stages are arranged one after the other, with the interposition of a char conveyor between the grids of the 1st stage and that of the 2nd stage and similarly with the interposition of a char conveyor between the grids of the 2nd stage and that of the 3rd stage, the conveyors are identical to the grids (except for the length) but without the holes for the injection of air and steam into the bed, all as shown in Figure 7 of Table 4. 11) Gasification process, as in claim 1, characterized by the fact that the 3-stage process is carried out in a gas generator in which the three stages consist of a paraffefepiped chamber, with two intermediate septa to divide the three freeboards, with grids to rectangular shape positioned at different heights, all as shown in Figure 8 of Table 5. 12) Gasification process, as in claim 1, characterized by the fact that the 3-stage process is carried out in a gas generator in which the three stages consist of cylindrical chambers, arranged one above the other, with circular grids, with fuel supply from the top and passage of the char from one grid to another by gravity, with suitable pipes for the connection between the freeboard of the first stage with the gases that go down from the top to the bottom and the freeboard of the third stage with the gases passing from bottom to top, all as shown in Figure 9 of Table 6. 13) Gasification process, as per claim 1, characterized by the fact that the 3-stage process is carried out by means of three conventional, distinct and separate gas generators, each of which represents a stage, connected with suitable mechanical conveyors on the char side and with suitable ducts on the side syngas, all as shown in Figure 10 of Table 7.