Patent Publication Number: US-2010107494-A1

Title: Method and installation for variable power gasification of combustible materials

Description:
The present invention relates to a method for variable power gasification of products such as biomass and organic byproducts (plants, animals, domestic refuse, sewage sludges), it being understood that:
         By gasification, is meant a thermochemical method for converting a solid fuel into a gas fuel. This is an incomplete combustion since it should result in combustible chemical products.   By combustion, is meant an exothermic chemical reaction with rapid oxidation of a fuel.   By gasifier, is meant a reactor allowing the transformation of a solid fuel into a gas fuel.   By reactor, is meant an enclosure which allows thermochemical transformations.   By sole, is meant a surface on which rests the organic material to be treated, consisting of openworked elements such as grids.   By airspace, is meant a single empty zone above the bed where the oxidation reactions take place.   By biomass, is meant all carbonaceous products directly or indirectly stemming from photosynthesis and notably but not in a limiting way, plants, animals, diverse organic waste, comprising domestic refuse, water sewage sludges, etc.       

     Generally, it is known that many solutions have already been proposed for the purpose of beneficiating biomass and organic byproducts. 
     Thus, notably, French patent FR No. 78 31356 describes a gas producer with a fixed bed comprising a horizontal treatment chamber in which the materials to be treated are introduced through one of the ends, and then are driven inside the chamber by a driving device up to a discharge aperture formed in the lower portion of the wall of the chamber at its second end. Between both ends of the chamber, the wall comprises two outlets spaced apart from each other, i.e.:
         a first outlet located on the side of the first end of the chamber, and   a second outlet which forms the gas outlet of the gas producer.       

     The first outlet is connected through a recycling circuit to an injection nozzle, located below a pre-heated air injector, so that the hot gases produced by the reaction of the recycled gases and of the pre-heated air are injected towards the first end of the chamber, at a level corresponding to that of the base of the slope formed in front of the material contained in the chamber. Consequently, the material particles which are the closest to the aperture are attacked by the hottest gases (about 1,200° C.) so that the ashes are rejected after the carbon has been totally gasified. 
     This solution notably has the advantage of reducing the drying and pyrolysis times by the forced hot gas circulation generated by the recycling. Further, the use of hot gases is optimized in order to obtain complete and rapid gasification. 
     Nevertheless, the drawback of the solution as described in this document consists in that it does not comprise a self-adaptive structure capable of optimizing the gasification process according to the flow rate of the treated material or, conversely, according to the required power output, and this for variable flow rates or powers in relatively wide ranges. 
     Patent FR No. 80 16854 proposes improving the treatment method as described earlier by having a heated-up gas flow resulting from recycling pass through the material to be treated, no longer axially as earlier, but transversely relatively to the longitudinal direction of progression of the materials during the treatment in the chamber. More specifically, according to this method, the treatment chamber comprises a succession of treatment modules, each comprising its own recycling, air admission and combustible gas extraction means. This is therefore a relatively expensive solution. Moreover, the goal at which this solution is aimed, is obtaining an optimization of the flow characteristics of the gases through the crossed material layer and not adapting the operation of the treatment chamber according to the material flow rate and/or the required power output, because of the presence of several oxidation zones and of a single gasification zone. 
     The object of patent application US 2007.0006528 is a method and a corresponding device allowing transformation of a solid carbonaceous material into a combustible gas with low tar content, this transformation being carried out in a vertical gasification chamber-reactor. This method notably comprises the following main steps:
         introduction of the carbonaceous material into the chamber;   transformation of a first portion of the carbonaceous material into coal in a flame pyrolysis zone;   control of a plurality of temperatures along the chamber by injecting an oxidizing gas at several levels in the gasification chamber;   controlling an amount of oxidizing gas injected from one of said levels;   modification of the location of the flame pyrolysis zone by increasing or by reducing an amount of oxidizing gas injected upstream or downstream from the pyrolysis zone;   controlling the porosity of the coal and of a second portion of the carbonaceous material in the gasification chamber-reactor by applying at least one force to the chamber;   transformation of the coal and of the second portion of the carbonaceous material into a low tar content combustible gas, inside the gasification chamber-reactor.       

     The object of patent application FR 2 263 290, as for it, is a method and installation for treating bituminous shales and asphaltic limestones by pyrogenation. This method mainly consists in a treatment in a vertical gas producing oven of rocks containing exploitable organic material and notably bituminous shales, in which these rocks are first submitted to a pyrogenation reaction and then to a gasification reaction. This method is characterized in that:
         the diverse required gases are injected into the zone where the gasification reaction is triggered at different levels, the operation for dosing and mixing these gases being automatically adjusted in devices located outside the oven, and in that   after its extraction from the reaction zone and before exiting the oven, the mineral residue is cooled in the lower portion of the oven by means of a gas in a closed circuit with heat recovery.       

     It is found that the object of both patent applications US 2007/0006528 and FR 2 263 290 is a reactor and an oven, the bed of which is vertical, with which it is not possible to obtain perfect homogenization of the temperature inside the bed and control of the flow rate. Further, the flow of material to be treated in the device, object of patent application US 2007/0006528, may be hindered by the injection ramps. 
     Therefore more particularly, the object of the invention is a gasification method with which the operation of the treatment chamber may notably be adapted according to the nature of the material and/or to the required power output, by means of functional adaptation of the treatment chamber and this without any significant physical modifications, and without increasing significantly the cost of the installation. 
     This method involves an installation which comprises a reactor comprising a treatment chamber in which the materials to be treated successively pass through a drying/pyrolysis zone of variable dimensions in which pyrolysis gas extraction is carried out, and then through a gasification zone of variable dimensions in which synthesis gas extraction is carried out, the pyrolysis gas extracted in the drying/pyrolysis zone being injected into the airspace of the reactor with an oxidizing gas, so as to generate an exothermic oxidation reaction providing the energy required for pyrolysis and gasification reactions. 
     According to the invention, the dimensions and/or the position of the drying/pyrolysis and gasification zones are adjusted according to the amounts of material to be treated introduced into the treatment chamber, to their nature, notably on the their grain size and to their hygrometry level and/or to the power output needs, and in that the combustible material substantially circulates horizontally by means of a pusher or the like allowing the combustible material to advance from upstream to downstream from the reactor. 
     Moreover, the treatment chamber may comprise between the drying/pyrolysis zone and the gasification zone, a mixed zone in which either extraction of pyrolysis gas or extraction of synthesis gas may be carried out, the type of extraction carried out in this zone being determined according to the amounts of materials introduced into the treatment chamber, to the nature of this material and/or to the power output needs. 
     Further, at least one of the aforesaid zones may comprise several successive controllable gas extraction areas, the variation of the dimensions and/or of the position of said zones being obtained by partial or total deactivation of said areas. 
     The invention also relates to a gasification installation allowing the application of the method defined earlier, this installation comprising a fixed a bed reactor which comprises a treatment chamber comprising a sole on which the bed of combustible material substantially circulates horizontally, said bed being divided into at least three zones, i.e.:
         a first upstream zone where only one drying/pyrolysis process is carried out, this zone being connected to pyrolysis gas extraction means with variable flow rate, these extraction means being connected to a common circuit for extracting pyrolysis gas connected to a burner supplied with oxidizing gas such as air or oxygen and located so as to generate an exothermic oxidation reaction in the airspace of the chamber of the reactor, the latter providing the energy required for the pyrolysis, gasification and degradation reactions of tars or other organic molecules contained in the pyrolysis gases,   a last zone where only one gasification process is carried out resulting from a reduction phase produced upon passage of the gas generated by the oxidation reaction through the carbonized bed during the drying/pyrolysis process, this downstream zone being equipped with means for extracting at a variable flow rate the synthesis gas obtained by this gasification process, connected to a common circuit for extracting synthesis gases,   a multifunctional zone located between the first and second zone, this multifunctional zone may totally or partly be a drying/pyrolysis zone, and/or totally or partly be a gasification zone and/or totally or partly be deactivated, and is connected to extraction means connected to the common circuit for extracting pyrolysis gases via a circuit with adjustable flow rate on the one hand, and to the common circuit for extracting synthesis gases via a circuit with adjustable flow rate on the other hand.       

     Thus, when an extraction means or a circuit with variable flow rate is closed, the zone of the treatment chamber corresponding to this extraction means or to this circuit is made at least partly inactive. It therefore becomes possible to distribute the active and inactive zones of the treatment chamber according to the nature of the material to be treated, to the amounts of material to be treated and/or to the desired power output. The presence of the multifunctional intermediate zone in which the extraction means may be connected to the pyrolysis gas extraction circuit or to the synthesis gas extraction circuit notably allows axial displacement of the location where the separation is carried out, between the pyrolysis gas extraction zone and the synthesis gas extraction zone. 
     Advantageously:
         the circulation rate of the combustible material to be treated inside the treatment chamber may be variable and may be adjusted according to the amounts of material to be treated and/or to the power output needs,   the relative flow rates of the oxidizer injected into the reactor and of extraction of the gasification combustible gas may be adjusted so as to maintain the reactor depressurized,   the produced power output may be controlled by the combustible material supply, the displacement rate of the combustible material by means of a piston or the like, by the flow rate and the quality of the injected oxidizer, the volume of the pyrolysis zone, the recirculation flow rate, the volume of the gasification zone, the extraction flow rate of gasification gas.       

     Moreover, with the purpose of improving the energy yield of the gasification installation described earlier, it is desirable to provide at the outlet of the gasifier, a system for treating high temperature gas loaded with many residual bothersome elements, notably tars or other organic molecules, this system comprises at least one piece of equipment notably having the purpose:
         of cooling the synthesis gas which is extracted at a temperature which may range from 400° C. to 650° C. in order to bring it to a temperature below 150° C. allowing application of a scrubbing process (which is generally performed at a moderate temperature),   of reducing or even removing the tar content,   of recovering the substantial heat of the synthesis gas thereby increasing the thermal yield of the gasification system.       

     This treatment equipment (tar condenser) is preferably designed so as to carry out wet treatment of the gas under ambient conditions, with a cooling step which is performed on a gas-water tube exchanger with which the substantial heat of the synthesis gas may be recovered and the tars may be separated from the gas. This three-fluid exchanger may comprise a plurality of vertical tubes in which the synthesis gas circulates as well as means with which a falling film formed by circulation of oil may be generated in the tubes. This falling film has the effect of trapping the dusts and tars thereby preventing fouling of the tubes. By cyclically drawing off oil, it is possible to maintain its quality by deconcentrating it by adding new fluid. 
    
    
     
       An embodiment of such an installation will be described hereafter, as a non-limiting example, with reference to the appended drawings wherein: 
         FIG. 1  is a schematic illustration of a fixed bed gasification installation; 
         FIG. 2  is an elevational view with partial cut-aways of a three-fluid exchanger-condenser device which may be used in the installation illustrated in  FIG. 1 . 
     
    
    
     In this example, the gasification installation involves a fixed bed reactor  1  of tubular shape, for example with a circular or polygonal section, comprising a treatment chamber  8 . This reactor  1  and this chamber  8  are connected at one of their ends (upstream) to a combustible material supply system  2  and comprise at the other downstream end a system  3  for extracting ashes. 
     Here, the supply system  2  comprises a worm-screw conveyor  4  or any equivalent device located in the storage area  5  of the combustible material. This conveyor  4  delivers onto a belt conveyor  6  which feeds a vertical supply air-lock  7  which opens out inside the treatment chamber  8  of the reactor  1  at right angles to a discharge area  9  of the bed poured into said chamber  8 . The material delivered by the air-lock  7  onto this discharge area  9  is pushed back towards the inside of the chamber  8  by a pusher  10  with an alternating movement. 
     Beyond the discharge area  9 , the sole of the treatment chamber  8  is formed by a succession of gas extraction areas on which the bed of combustible material may circulate under the driving effect of the pusher  10 . This sole comprises one or more grids  11 - 15  covering portions under each of which a hopper T 1 -T 5  is positioned, the lower portion of which is provided with an obturator or register  16 - 20  intended for discharging fine material particles passing through the grid(s)  11 - 15 . 
     According to this embodiment, the sole successively comprises two pyrolysis gas extraction areas (grids  11  and  12 ), two mixed or polyvalent extraction areas (grids  13  and  14 ) and an area for extracting synthesis gases (grid  15 ). 
     The hoppers T 1 -T 4  are each connected to the suction inlet of a pyrolysis gas extraction circuit  21  and of a turbine  23 , via suction conduits  24 ,  25 ,  26 ,  27 , equipped with valves  28 ,  29 ,  30 ,  31 . 
     Also, the hoppers T 3 , T 4 , T 5  are connected to the suction inlet of a synthesis gas extraction circuit  37  via suction conduits  31 ′,  32 ,  33  equipped with valves  34 ,  35 ,  36 . The extraction circuit  37  successively comprises the primary of a gas/air heat exchanger  38  and, optionally, a scrubber system for the gases  39 . It is connected to the suction inlet of a turbine  40 , the outlet of which is for example connected to a synthesis gas distribution network. 
     The downstream end of the treatment chamber  8  is provided with a well  41  for extracting ashes, the lower end of which is immersed in water  42  contained in a tank  43  for recovering ashes, which extends under the treatment chamber  8 . 
     Also, the obturators (or registers)  16 - 20  are connected to sleeves M which are immersed in the water of the tank  43 . 
     The particles of ashes or of combustible materials collected by the tank are carried away by a conveyor  44  and poured at a height above the water level of the tank  43  into an area  45  for storing ashes and residues. 
     The turbine  23  of the pyrolysis gas extraction circuit is connected through its outlet to a burner  50  which injects into the airspace of the treatment chamber  8  a gas mixture comprising the pyrolysis gas as well as an oxidizer which may consist in preheated air from a circuit  46  passing through the secondary of the heat exchanger  38  and from a turbine  47  or in oxygen from a distribution circuit  48  controlled by a valve  49 . 
     The starting of the installation is moreover ensured by means of a burner B using natural gas from a circuit C controlled by a solenoid valve E. This burner B is maintained in operation until the reaction temperature is reached. 
     Inside the treatment chamber  8 , above the extraction areas  11 ,  12  (and  13 ,  14  insofar that the valves  30  and  31  are open and the valves  34  and  35  are closed), there exists a drying/pyrolysis zone in which the oxidized gas flow from the burner  50  and circulating in the airspace of the chamber  8  passes through the bed of material lying on the sole (grids  11 - 14 ) while causing by their supply of energy, drying of the material and the pyrolysis reaction, the tars contained in the pyrolysis gases being degraded during the oxidation reaction which is carried out in the airspace of the chamber  8  at a high temperature. 
     In the first portion of the treatment chamber (drying and pyrolysis zone), the gases are extracted at a temperature of the order of 500° C.-700° C. 
     In the second portion of the reactor, when the gas from the oxidation passes through the carbonized bed during the pyrolysis phase, a reduction phase occurs and the gas is extracted through the suction conduit  33  and the suction conduits  31 ′ and  32  insofar that the valves  34  and  35  are open. 
     It therefore appears that the dimensions and the position of the gas extraction zones (pyrolysis and gasification) may be modified depending on the (open or closed) condition of the valves  28 - 31  and  34 - 36 . 
     Also, it is possible to adjust the extraction flow rates in these zones. It is therefore possible to generate active or inactive zones of the bed of material according to the power output need. 
     More practically, variation of the power output of such an installation is achievable by a combination of the following actions:
         varying the supply flow rate of material to be treated by acting on the actuation rate of the pusher and on the cycle for supplying material to be treated,   varying the dwelling time in the drying and pyrolysis zone by adjusting the dimension of this zone by closing or opening the valves  28 ,  29 ,  30  and  31 ,   varying the recycled pyrolysis gas flow rate, by adjustment of the turbine of the pyrolysis gas extraction circuit,   varying the dwelling time of the material to be treated in the gasification zone by adjusting the dimension of this zone by closing or opening the valves  34 ,  35 ,  36 ,   varying the synthesis gas flow rate by adjustment of the turbine rate of the synthesis gas extraction circuit.       

     These actions are here controlled by a processor P which controls the rate of supply of the airlock with combustible material, the condition of the registers  16 - 19  and of the valves  28 - 31  and  34 - 36 , the speed of rotation of the turbines  23 ,  40 ,  47 , the speed of rotation of the motor driving the conveyor  44  which ensures extraction of the ashes and of the residues. 
     This processor is moreover connected to detectors (notably a temperature detector DT and a pressure detector DP) with which the different parameters of the installation may be measured in order to provide the controls and safety measures. 
     Moreover, for a given power, the operation of the installation is ensured by three control loops:
         a control loop for the depressurization in the reaction chamber, by action on the synthesis gas extraction flow rate. This action may be carried out by adjusting the speed of the turbine or by acting on the valves; this control loop is inevitable for safety issues.   a control loop for the temperature of the oven by acting on the flow rate of injected oxidizer (air or oxygen) into the burner. This action may be carried out by adjusting the speed of the turbine for the oxidizing gas.   a control loop for the temperature of the pyrolysis gases by acting on the recirculation flow rate of the pyrolysis gases. This action may be carried by adjusting the speed of the turbine; the temperature of the pyrolysis gas reflects the temperature in the reactor and the pyrolysis quality.       

     Further, many adjustable parameters may directly contribute to the performance of the installation, i.e., notably:
         the tilt angle of the sole of the reaction chamber; although the sole is substantially horizontal in the example described earlier, it is of course possible to provide a sole having a predetermined tilt,   the progression rate of the material to be treated on the sole,   the recirculation gas rate in the pyrolysis bed,   the gas rate in the gasification bed,   the flow rate and the nature of the oxidizing gas,   the temperature in the oxidation zone of the reactor,   the surface area of the effective zones of the sole,   the positioning and the size of the pyrolysis zone,   the positioning and the size of the gasification zone;       

     By means of the arrangements described earlier, the gasification installation has the following advantages:
         The possibility of varying (or regulating) the capacity or the power of the installation without having to change the structure and/or the physical dimensions of the installation; this power variation is achievable over a significant range. A gas producer designed for an average power of 1MW may have its power vary from several hundred kW to several MW.   The power changes may be carried out instantaneously and very easily (total automation) without any consequence on the quality of the synthesis gas. With the high recycling rate, it is possible to momentarily operate the system in a standby mode without any negative effect upon changing over to the rated output mode. It is sufficient to slow down the inflows of oxidizer, which allows high reactivity and if the situation persists, to slow down the inflow of solid. This control capability is essential because for certain applications (cogeneration or pure thermics), the variations of the instantaneous heating value have to be compensated by an inversely proportional variation of flow rate in order to maintain constant power. For other uses, the power demand has to be able to be followed instantaneously.   The polyvalence of combustible materials: progression of the solid in the reactor is no longer by gravity, the density of the material is no longer a limiting selection criterion, which opens new possibilities for using varied products (in nature and in packaging) and so that an operation with increased reliability may be foreseen.   The absence of tar in the gas: the recirculation of the gas described earlier has another very favorable consequence in addition to the thermal homogenization effect and to the mechanical diffusion effect. The multiple successive crossings of the bed allow total conversion of carbon into CO and CO 2  with an excellent CO/CO 2  ratio and of hydrogen into H 2  and H 2 O with there also an excellent H 2 /H 2 O ratio. The pyroligneous juices commonly called tars which are more or less long hydrocarbon chains which subsist because of incomplete reactions, are gradually destroyed as they form in this type of gasifier. This is a crucial point for using synthesis gas in cogeneration groups but even more for use in hydrogen chemistry (fuel cell or conversion into biofuel).   The possibility of using an oxygen oxidizer: if the intention is to produce a gas with a better instantaneous heating value per unit mass, its inert nitrogen fraction (50% by volume in air gas) has to be removed. The latter is mainly brought by the oxidizing air. By gasifying with oxygen, it is possible to reduce by half the gas flow rate while keeping endogenous energy. In other words, the instantaneous heating value of the gas is doubled by reducing by half the substantial thermal heating loss.   A risk of limited blocking of the mechanism for moving the combustible material to be treated forwards. Anyhow, the optional curative action is clearly simplified. Internal interventions are accomplished by opening a front door upon standstill of course, but without complete emptying of the reactor as in the prior solutions.       

     In the example illustrated in  FIG. 2 , the gas scrubbing system  39  may consist of a tar condenser comprising a vertical tubular column CT closed at both of its ends and the internal volume of which comprises from top to bottom:
         a gas admission chamber  51  into which a gas admission conduit  52  opens out radially.   an exchanger  53  delimited by two radial axially spaced partitions  54 ,  55 , crossed by a plurality of vertical tubes  56  which extend axially between said partitions  54 ,  55 . The volume  57  delimited by the tubes  56 , both partitions  54 ,  55  and the wall of the CT column is filled with water which circulates countercurrently between a water inflow conduit  58  located in a lower portion and a water outflow conduit  58 ′ located in an upper portion.   a gas outflow chamber  59  into which a gas outflow conduit  60  opens out radially.   an oil reserve  61  in which is positioned a double wall in which runs tubing or any other coil-shaped device  62 , the purpose of which is to cool the oil and to preheat the water, and one of the ends  63  of which is connected to the water inlet  58  while the other end is connected to a “cold” water supply circuit  64 . The bottom of the column CT which forms the bottom of the oil reserve  61  has a conical shape at the centre of which is positioned an orifice connected to an oil purge pipe  65 .       

     In this device, the synthesis gas forms the fluid to be treated. Water is used as a main heat transfer fluid which absorbs part of the heat released by the gas. The oil which is used for capturing the tars also plays the role of a secondary transfer fluid ensuring preheating of the water. 
     The gas which enters the admission chamber  51  at a relatively high temperature, flows through the exchanger  53  from top to bottom inside the tube  56  while cooling upon contacting the latter. 
     After having been preheated in the coil  62 , the water passes through the exchanger  53  from bottom to top while heating up upon contacting the tubes  56 . The oil which is supplied in the admission chamber  51  (by a circuit not shown) enters the tubes  56  by overflow and forms films falling along the inner walls of the tubes  56  before finally reaching the reserve  61 . The substantial heat of the gas is transferred to the water by passing through oil films and the walls of the tubes  56 . The tars present in the gas are captured by the oil films during the direct tar/oil contact. The oil present in the reserve  61  may be regularly drawn off by means of a purge pipe  65 .