Patent Publication Number: US-2011067611-A1

Title: Combustion material process and related apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a process for the combustion of materials and a related apparatus, suitable in particular for waste-to-energy plants. 
     BACKGROUND ART 
     In many sectors the destruction of materials by combustion has for a very long time been considered only as a method for getting rid of unwanted and bulky amounts. In recent decades models have spread which also propose to make use of the energy generated during the combustion of such materials. The considerations which follow, which can be extended to any type of materials, above all relate to waste, which because of its dimensions, quantities and environment risk, provides a striking example of what is indicated above. 
     Today the disposal of municipal solid waste is a big problem, especially in Italy, where approximately 75% of waste produced still ends up in landfill sites: this causes widespread pollution of the territory and high disposal and clean-up costs. 
     The development of alternative waste management models is therefore made necessary by the unsustainable nature of the current situation. Without doubt, a step in that direction is represented by waste-to-energy (incineration with energy recovery), by means of which the waste, understood to be a renewable energy source, can contribute to the generation of energy. 
     Waste-to-energy is a modern, efficient system which spread from Europe to the rest of the world. Thanks to it, the heating value of waste can be used and the heat released by waste combustion can be converted into electricity (or heat energy that can be used for district heating), reducing the overall impact on the environment. 
     A waste-to-energy plant is a waste incinerator able to use the heat content of waste to generate heat, to heat water (or other fluids) and finally to produce electricity or convey heated water to environments and areas to be heated. Therefore, it differs from old incinerators which only carried out the thermal destruction of waste, without producing energy. The use of waste-to-energy plants seems to represent a way out of the problem of overfull landfill sites. 
     Incinerators are plants mainly used for waste disposal by a high temperature combustion process (incineration) whose end products are a gaseous effluent, ashes and dusts. 
     The main categories and predominant quantities of wastes which can be incinerated are municipal solid waste (MSW) and special wastes. 
     Special categories can be added to these, such as sewage sludges, medical waste or chemical industry waste. 
     Before incineration, the waste may be treated using processes designed to eliminate non-combustible materials (glass, metals, inert items) and the wet fraction (organic material such as food waste, agricultural waste, etc.). Waste treated in this way is defined refuse derived fuel or more commonly eco-bales. 
     Operation of an incinerator may be divided into a sequence of steps. First, the waste arrives from selection plants located throughout the territory (but also directly from waste collection), the combustible fraction is produced (RDF—refuse derived fuel) and is incinerated after biological dehydration of the waste followed by separation of inert items (metals, minerals, etc.) from the combustible fraction. 
     Combustion takes place, during which a forced air flow is conveyed in the furnace to introduce the necessary amount of oxygen, which allows the best combustion, keeping the temperature high (usually close to 1000° C.). 
     Prior art incinerators have quite a large amount of residues (normally within a range of from 25% to 35% relative to the initial total mass). This large amount of residues is a serious problem, since they must be suitably stored. Said operation involves high costs which reduce the productivity of the entire plant (from an economic viewpoint). 
     It should also be noticed that the maximum specific energy production of prior art plants is approximately 200/300 kWh/t. Such values are low and not enough to guarantee economically successful management of the plants. 
     DISCLOSURE OF THE INVENTION 
     The main purpose of the present invention is to provide a process for the combustion of materials suitable for minimising the residues. 
     Within the scope of the technical purpose, the present invention also has for an aim to provide a process for the combustion of materials which has high efficiency and low operating costs. 
     The present invention has for another aim to provide a process for the combustion of materials which is suitable for operating with very high temperatures. 
     Another aim of the present invention is to provide an apparatus suitable for the combustion of materials according to the process, having a simple structure and substantially compact dimensions. 
     Another aim of the present invention is to provide a process for the combustion of materials, having a maximum specific energy production value which is very high compared with prior art plants. 
     Another aim of the present invention is to provide a process for the combustion of materials and related apparatus, particularly for waste-to-energy plants, which are inexpensive, simple to implement/produce and safe to apply. 
     The present invention achieves this purpose and aim with the present process for the combustion of materials which consists of inserting the suitably compacted materials in a reaction chamber and closing the chamber; injecting a flow of combustible gas and a corresponding flow of a comburent gas, which are in the correct stoichiometric ratio to each other, in the reaction chamber, so as to activate combustion of the materials; continuing the thermochemical reaction of the oxidizable elements with the oxygen present in the materials without introducing any more gases; again injecting comburent gas to feed the thermochemical reactions of the remaining oxidizable elements, at least until the increase in temperature stops; and opening a throttle valve to expel the gases, while continuing to introduce comburent gas at a substantially constant pressure until completion of the thermochemical reaction of the oxidizable elements, subjecting to pyrolysis the strongly bound oxides and oxidizing the metals present. 
     This purpose and this aim are also achieved by the present apparatus, suitable for implementing the process described above, in particular for waste-to-energy plants, of the type comprising a reaction chamber, having an inlet for the insertion of materials, an outlet through which the gases can flow out, and suitable circuits for introducing reagent gases, characterised in that the comburent gas oxygen enrichment fraction (F), expressed as a percentage, the apparatus maximum operating pressure (P), expressed in bars, the reaction chamber free inner volume (V), expressed in cubic metres, the mass (M) of materials inserted in the reactor, expressed in tons, and the maximum temperature (T) reached in at least one portion of the reaction chamber, expressed in Kelvins, are linked according to the formula 
         FPV/M≧ 5.24×10 −2 ×( T   2 −314.73 ×T )
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details are more apparent from the detailed description which follows of a preferred, non-limiting embodiment of a process for the combustion of materials and related apparatus, particularly for waste-to-energy plants, illustrated by way of example, but without limiting the scope of the invention, in the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a process for the combustion of materials according to the invention; 
         FIG. 2  is a side view in cross-section according to a longitudinal axial plane, of an apparatus for the combustion of materials, particularly for waste-to-energy plants according to the invention; 
         FIG. 3  is an enlarged view of a detail of  FIG. 2 ; 
         FIG. 4  is a side view in cross-section according to a transversal plane, of an apparatus, particularly for waste-to-energy plants according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     In particular with reference to the accompanying drawings, the numeral  1  denotes as a whole a process for the combustion of materials X and the numeral  2  denotes the related apparatus, particularly for waste-to-energy plants. 
     The process  1  for the combustion of materials X consists of a sequence of five steps, the first three being essential. 
     During a first step (a), similar to that also carried out in normal prior art incinerators, the materials X, preferably compacted, without particular pre-treatments, must be inserted in a reaction chamber  3  and the chamber  3  must be closed. 
     In prior art incinerators, the materials X inserted in the reaction chamber  3  must usually be suitably pre-treated, eliminating wetness and all non-combustible materials or materials which may produce harmful emissions during combustion (such as chlorinated polymers). The process  1  according to the invention and the related apparatus  2  allow work on materials X which have not been pre-treated, with the certain economic advantage of reducing or limiting a complex and expensive operation. As described below, the process  1  according to the invention can also be applied to materials X which comprise fractions of non-combustible materials and other materials, since the operating cycle is particularly effective. 
     During a second step (b), a flow of combustible gas and a corresponding flow of comburent gas, the two gases being in the correct stoichiometric ratio to each other, must be injected into the reaction chamber  3 . 
     The flow of combustible gas, for example methane, together with its particular stoichiometric oxygen, triggers spontaneous combustion of the materials X (for example the waste) contained in the reaction chamber  3 , producing, during the combustion reaction, carbon dioxide and water vapour. When the combustion is started, at temperatures which may vary from approximately 100-150 to around 600-650° C. depending on the type of materials X being processed, the introduction of comburent gas is stopped. 
     During a third step (c), it is necessary to continue the thermochemical reaction of the oxidizable elements (for example, the oxidation of carbon) by the oxygen present in the materials X without introducing any more gases. This means that the oxidation will continue, being sustained exclusively by using carbon, and the other oxidizable elements, and the loosely bound oxygen contained in the material in the reaction chamber  3 . Said elements will produce carbon dioxide, brining the temperature of the material to 800-900° C., and higher. 
     A fourth step (d), to be started when the temperature increase stops, involves injecting more comburent gas to feed the oxidation reactions of at least part of the remaining carbon and in general of the oxidizable elements, at least until the temperature again stops increasing, usually at temperatures of between 1600 and 2200° C., again depending on the materials X processed. 
     The oxygen injected oxidizes part of the remaining carbon (producing carbon dioxide) and other oxidizable elements, until a maximum temperature (T) and a maximum internal pressure (P) are reached. In practice, the temperature may reach 2000-2200° C. and the pressure 35-50 bar. Embodiments are not excluded which may operate with pressure and temperature values outside said ranges, still covered by this patent. 
     In a fifth and final step (e), at an outlet  4  for the gases, a throttle valve  5  has to be opened to allow the gases to be expelled, at the same time continuing to introduce comburent gas at a substantially constant pressure to complete the thermochemical reaction of the oxidizable elements. For example, until all of the remaining carbon has been oxidized, the strongly bound oxides have been subjected to pyrolysis and the metals present have been oxidized (it should be noticed that iron and aluminium are often found in municipal waste). 
     Completion of the fifth and final step (e) and therefore of the process  1  can be verified when a negative gradient is seen in signals sent by temperature-pressure probes which can be installed in the reaction chamber  3 . 
     The valve  5  is kept open until the end of the process  1 , that is to say, until the internal pressure has reached the same value as the atmospheric pressure. 
     It must be emphasised that during the final step (e), controlled opening of the throttle valve  5  is needed (using a suitable control and management apparatus for maintaining a predetermined pressure value in the reaction chamber  3 ) until the pressure in the reaction chamber  3  is equal to the atmospheric pressure. 
     The gases which come out of the reaction chamber  3  through the throttle valve go to a suitable container  6  for mixing with cooling air if necessary and the fractional deposition of oxidized metals contained in them, substantially in the form of powders. 
     For that purpose the container  6  comprises at least one suitable compartment  7  in which the oxidized metals can be deposited and which can be inspected for their removal. 
     Before the gases come out of the apparatus  2 , they pass through a suitable device  22  for scrubbing fumes (located downstream of the container  6  along a path suitably identified for the purification of fumes), comprising a copper-based catalyst  23 . 
     The copper-based catalyst  23  comprises a continuous band made of refractory steel links, coated on both sides with a copper deposit, which passes, by sliding on rollers, from one to the other, and vice versa, of the two compartments of a box divided into two by a partition made of refractory steel. 
     Oxygen is introduced into one of the two compartments, and the combustion fumes into the other, the fumes containing molecules of hydrogen and carbon monoxide, harmful to the environment. At the temperatures present, equal to several hundred ° C., the copper in the first compartment oxidizes, so that the copper oxide which reaches the second compartment, at said temperatures, reacts with the hydrogen and carbon monoxide, forming water vapour and carbon dioxide, which do not harm the environment. 
     The fume scrubbing device  22  also comprises a “scrubber”, in which a process of halogen gas acidification and elimination takes place. 
     In this way, downstream of the fume scrubbing device  22 , only carbon dioxide comes out, at a temperature substantially not higher than 80° C. 
     The apparatus  2 , suitable for implementing the procedure  1 , particularly suitable for installation in waste-to-energy plants, comprises a reaction chamber  3  having an inlet  8  for the insertion of materials X, an outlet  4  through which the gases flow out and suitable circuits  9  for introducing reagent gases. 
     In an apparatus  2  according to the invention, the comburent gas oxygen enrichment fraction (F), expressed as a percentage, the apparatus  2  maximum operating pressure (P), expressed in bars, the reaction chamber  3  free inner volume (V), expressed in cubic metres, the mass (M) of materials X inserted in the combustion chamber  3 , expressed in tons, and the maximum temperature (T) reached in at least one portion of the reaction chamber  3 , expressed in Kelvins, are linked according to the formula 
         FPV/M≧ 5.24×10 −2 ×( T   2 −314.73 ×T )
 
     In particular, the apparatus  2  comprises a sealed, hollow outer shell  10  and an inner casing  11  which matches the shell  10  cavity. 
     The casing  11  is suitably made of refractory material with thicknesses suitable for withstanding the mechanical load supplied by the pressure inside the reaction chamber  3  and the very high temperature to which it will be subjected. 
     According to an embodiment of particular interest for practical purposes and implementation, the casing  11  is made in a hollow cylindrical shape (as shown in  FIG. 4 ) with suitable thickness, for example around 200 mm. 
     The cylinder has stiffening ribs, for example six ribs distributed radially, so that the diameter of the circle circumscribing the ribs is the same as the internal diameter of the shell  10 . 
     Interposed between the shell  10  and the casing  11  there is a space  12  having the shape of a cylindrical ring and being narrow, for example 100 mm thick, so that they are separate. 
     The space  12  comprises an inlet channel  13  and an outlet channel  14  for the passage of coolant fluid. 
     According to a possible embodiment of interest for application of the invention, the coolant fluid is a flow of atmospheric air pumped into the space  12  to cool the walls of the shell  10  and the casing  11 . A suitable computerised control and management station (not illustrated in the accompanying drawings) adjusts a valve present in the outlet channel  14  to guarantee that the pressures inside the reaction chamber  3  and in the space  12  are constantly equal. 
     The apparatus  2  comprises suitable sensors  15  for checking the pressure and the temperature in the reaction chamber  3 . 
     The apparatus  2  comprises suitable nozzles for the introduction of combustible fluid (nozzle  16 ) and comburent fluid (nozzle  17 ) into the reaction chamber  3 . Appropriately, said nozzles  16  and  17  introduce those fluids according to respective stoichiometric ratios for adjusting and controlling combustion in the reaction chamber  3 . 
     The outlet  4  through which the gases flow out is intercepted by a throttle valve  5  substantially consisting of a plug  18  shaped to match a respective hole  19  in the inner casing  11  made of refractory material. 
     The hole  19  is in communication with the reaction chamber  3 . 
     The plug  18  is forced into the hole  19  and blocks it by means of a pusher  20  with controlled, adjustable action. In this way, the intensity of the pusher  20  action is determined by the pressure to be maintained (or reached) in the reaction chamber  3  (and therefore by the current step of the process  1 ). 
     According to a possible embodiment, the shell  10  consists of a plurality of shell portions which can be joined together. In this way, disassembly of the shell portions allows extraction of the inner casing  11  made of refractory material for its substitution and maintenance. After many successive cycles, the refractory material may show signs of deterioration which will prevent perfect operation of the apparatus  2 . The possibility of substitution simplifies management of the apparatus  2  according to the invention compared with prior art incinerators. 
     The inlet  8  for insertion of the materials X houses a lid  21  made of refractory material, its shape and dimensions matching those of the inlet  8 . 
     It must be emphasised that one of the possible combustible fluids which can be used in this apparatus  2  is methane (the possibility of using other hydrocarbons in a gaseous or liquid state is not ruled out or even other combustible substances in a solid state, which may be powdered). 
     In such a case the comburent fluid must comprise gaseous oxygen. For that purpose, it is possible to introduce atmospheric air, mixtures of air enriched with oxygen or even pure oxygen, depending on the intensity of the reaction to be obtained in the reaction chamber  3 . Having already labelled the oxygen fraction F, when F increases the dimensions of the chamber  3 , and therefore of the apparatus  2 , will change in inverse proportion. 
     The new process  1  is a discontinuous chemical-physical process which consists of a prior step (a) followed by 4 subsequent steps (b), (c), (d) and (e) and allows a predetermined mass of materials X, such as municipal solid waste, to be rapidly brought to temperatures of between 1800° C. and 2200° C., causing their sublimation, that is to say, vaporisation without passing through the liquefaction stage. The process  1  takes place inside a reaction chamber  3 , for example having a tubular shape, with a temperature/pressure gradient of up to 2200° C./50 bar. 
     The suitable throttle valve  5  calibrated to the maximum pressure tolerable, causes throttling of the vapours as they come out, subjecting them to adiabatic expansion and cooling and introducing them into the container  6 , from which they will then be sent to the heat exchangers for the production of superheated steam for obtaining energy. 
     In the material X to be processed, the oxygen is normally present as a component with large molecules (loosely bound oxygen) and as an oxide of elements, for example calcium and silicon (strongly bound oxygen). Carbon is present as a loosely bound element. 
     As already indicated, during step (b) a flow of combustible gas, for example methane, together with its particular stoichiometric oxygen, brings the material to a temperature of up to 600-650° C., producing carbon dioxide and water vapour. 
     During the next step (c), in which gases are not injected from the outside, the carbon, the other oxidizable elements and the loosely bound oxygen contained in the material produce carbon dioxide and other oxides, bringing the temperature of the material up to temperatures which may reach 2200° C. 
     At this point the step (d) involves injecting comburent gas, usually containing oxygen, which oxidizes part of the remaining carbon and other oxidizable elements, producing carbon dioxide and oxides, until the achievement of the maximum temperature T (expressed in Kelvins) and the maximum internal pressure P, expressed in bars. In practice, the temperature may reach 2000-2200° C. and the pressure 35-50 bar. 
     Then a step (e) is needed in which the throttle valve  5  is opened and comburent gas continues to be introduced until all remaining carbon has oxidized, the strongly bound oxides have been subjected to pyrolysis and the metals present, for example iron and aluminium, have oxidized. 
     The start of the end of step (e) and of the process  1  is indicated by the negative gradient of the signals sent by the sensors  15 , for example comprising temperature-pressure probes. The valve  5  is kept open until the end of the process  1 , that is to say, until the internal pressure has reached the same value as the atmospheric pressure. 
     Starting from step (e), the vapours introduced into the container  6  may be mixed with external air to reach maximum temperatures compatible with the exchangers. 
     After introduction in the container  6  and during passage through the exchangers, the gradual cooling causes fractional deposition of oxidized metals, in powdery form, which may be collected in suitable compartments  7 . 
     Downstream of the exchangers, a device for scrubbing the fumes acidifies and eliminates halogen gases. Therefore, carbon dioxide at a temperature lower than 80° C. comes out of the chimney. 
     The oxygen may be cryogenic or obtained by means of “molecular sieve” available on the market. Consumption of methane and oxygen per ton of material processed are for example approximately 30 Nm 3  (normal cubic metres: the unit of measurement for the volume of gases used, in “normal” conditions, that is to say, at atmospheric pressure and at a temperature of 0° C.) for methane and 400 Nm 3  for oxygen. 
     For a material such as Municipal Solid Waste the process causes the development of heat which is much greater than its NHV (the Net Heating Value is the amount of heat released during complete combustion of a fuel, without considering the evaporation heat of the water vapour) and energy production equal to around 2.5 to 3 times that of a normal waste-to-energy plant, as well as drastically reducing the mass of residues (5-10 kg/ton rather than 300-320) and therefore the need to use special landfill sites. 
     The process can also be applied to inorganic materials defined as incombustible, with an increase in the methane/oxygen ratio and a reduction in the combined heat and power generation. 
     The formula 
         FPV/M≧ 5.24×10 −2 ×( T   2 −314.73 ×T )
 
     created based on studies and experiments can allow the sizing of apparatuses  2  for every possible pair of values representing process temperature/mass of material inserted depending on the comburent gas oxygen enrichment fraction. 
     The valve  5  allows the take up of the play caused by wear on the refractory material. A suitable vent tube may prevent (if present) the build-up of pressure in the area inside the reaction chamber  3 . 
     Therefore, as indicated the invention achieves the preset aims. 
     The invention described above may be modified and adapted in several ways without thereby departing from the scope of the inventive concept. 
     For example, it is possible to make an opening in the casing  11  (at the areas close to the valve  5 ) which allows the gases to flow in conventional heat exchangers for energy recovery then in a conventional “scrubber” (fume scrubbing tower) for dehalogenation then to the chimney. 
     The reaction chamber  3  is delimited by the casing  11  which has suitable thickness, for example 200 mm, equipped with ribs, for example six ribs (as shown in  FIG. 4 ), so that the diameter of the circle circumscribing the ribs is the same as the internal diameter of the shell  10  and so that between the shell  10  and the outer cylindrical part of the casing  11  there remains a space having the shape of a cylindrical ring which is narrow (the space  12 ), for example 100 mm thick, so that it is separate, even if not in a completely sealed fashion, from the chamber  3 . 
     The lid  21  is equipped with a refractory plate anchored to it by metal clamps and rests on the flange with a gas-seal elastic toroidal metal ring interposed between them. The lid  21  may be guided at the back by horizontal bars, allowing a loading hopper (in one possible embodiment) to unload the ready-made bale of material X to be processed. The movement of the lid  21  may be obtained by means of a pneumatic or hydraulic pusher which inserts the bale of material X in the chamber  3  and guarantees a pressurised gas-seal. 
     The start of step (b) of the process  1  is guaranteed by the introduction of the stoichiometric combustible gas and comburent gas by the nozzles  16 ,  17  and its ignition by a suitable igniter. 
     Moreover, all details of the invention may be substituted by other technically equivalent elements. 
     In the example embodiments illustrated, individual features, shown relative to specific examples, may be interchanged with other different features, existing in other example embodiments. 
     Moreover, it should be noticed that everything which during the procedure for obtaining the patent was revealed to be prior art, is not claimed and shall be considered removed from the claims (disclaimer). 
     The present invention shall be implemented in the strictest compliance with laws and rules relating to the products which form the subject matter of the invention or related to them and, if necessary, subject to authorisation from the relevant authorities, particularly with reference to regulations regarding safety, environmental pollution and health. 
     In practice the materials used, as well as the shapes and dimensions, may vary depending on requirements, without therefore departing from the protective scope of the appended claims.