Method and plant for the pyrocarbonization of organic matter and in particular of organic waste

Abstract

The invention relates to a method for the pyrocarbonization of organic matter, the method comprising the following steps: - supplying a first, indirect dryer (3) with organic matter; - supplying a second, direct dryer (4) with the organic matter from the first dryer (3); - pyrolyzing the organic matter from the second dryer (4); - burning the pyrolysis gases (32) and extracting post-combustion fumes (34); - transferring heat from the post-combustion fumes (34) to a heat-transfer fluid (38) heating a heating surface (7) of the first dryer (3); and - transferring the latent heat of condensation from the vapor (36) extracted from the first dryer (3) to a drying gas (37) heating the second dryer (4); the invention also relates to a plant for implementing this method.

Description

DESCRIPTION Title of the invention: Process and installation for the pyrocarbonization of organic matter, and in particular organic waste |Field of invention

[0001] The invention relates to the field of pyrolysis of organic matter.

[0002] It relates more specifically to a process and an installation for pyrocarbonizing wet organic matter, and in particular organic waste, such as sewage sludge for example. Technological background

[0003] The pyrolysis of materials containing an organic fraction in general, and of waste in particular, is known.

[0004] Pyrolysis is the thermal decomposition of organic matter at temperatures exceeding 350°C, generating three main products, namely: - a synthesis gas, also called syngas, which is a mixture of gases mainly composed of carbon monoxide (CO) and hydrogen (H2), with varying amounts of carbon dioxide (CO2), methane (CH4) and some other light hydrocarbons such as ethanes (C2H6); - a pyrolytic oil or crude oil, also called tars, composed of condensable hydrocarbons; and - a solid residue rich in carbon called char, and more specifically biochar if the organic raw material is biogenic.

[0005] The mass distribution and quality of pyrolysis products depend on various factors: the quality of the raw material, the final pyrolysis temperature, the heating rate, the residence time in the reactor, the residence time of the pyrolysis gases in the reactor, the pressure, the use or not of a catalyst, and the use or not of an oxidizing gas (steam, air, O2, CO2).

[0006] Depending on the desired end product, the pyrolytic process has a different name: - pyrogasification aims to produce pyrolysis gas or syngas; - Pyroliquefaction aims to produce pyrolytic oil; and - Pyrocarbonization aims to produce char.

[0007] Historically, several factors have favored pyrolytic processes for treating sewage sludge. Firstly, they offer an alternative to incineration, which often faces social acceptance issues and requires investments considered too large for small and medium-sized wastewater treatment plants. Secondly, they allow for the concentration of energy in easily transferable products (pyrolytic gas, pyrolytic oil, or char) and the transformation of sludge into higher value-added products.

[0008] However, among these processes, pyrocarbonization has long been the least popular pyrolytic process, mainly due to a lack of interest in sustainable carbon sequestration solutions and an insufficient understanding of the benefits of biochar for soil fertilization.

[0009] However, it has recently been identified as a process of technical and economic interest. On the one hand, char sequesters a significant amount of carbon, which has increased its appeal due to the climate crisis. For example, one ton of char obtained by pyrocarbonizing sewage sludge can sequester 800 to 1600 kg of CO2. On the other hand, biochar obtained by pyrocarbonizing sewage sludge contains a significant fraction of phosphorus that is readily accessible and usable by plants (up to 80% of available P2O5, measured as ammonium citrate). Pyrocarbonization thus offers an effective technology for recycling the phosphorus contained in sewage sludge to fertilize soils.

[0010] The material intended for pyrolysis must have a very low moisture content, i.e., a dryness level of at least 60% and preferably above 80%. A preliminary drying operation of the organic matter is therefore necessary, prior to pyrolysis, to achieve such a dryness level.

[0011] However, this preliminary drying operation is costly and energy-intensive. This preliminary drying operation is all the more critical for the energy and thermal efficiency of the pyrolysis process when the organic matter to be treated is very moist, as is the case dewatered sewage sludge which on average has a very low dryness rate, typically between 15 and 35%.

[0012] It is certainly known to use pyrolysis gas as a fuel for drying and pyrolysis. However, when the organic matter to be pyrolyzed is wet, as is the case with the aforementioned sewage sludge, the amount of water to be removed during drying is so significant that the combustion of pyrolysis gases proves insufficient to meet all the overall energy needs of the pyrocarbonization process, including both drying and pyrolysis.

[0013] Thus, it has been observed that, in a conventional process of pyrocarbonization of sewage sludge, the energy needs of the dryer are only met to about 50% by the combustion of pyrolysis gases, which makes the energy balance of pyrocarbonization much less interesting than that of incineration which is generally autothermal or almost.

[0014] Therefore, with current pyrocarbonization processes, it is necessary to supply primary heat to the dryer, i.e. to use an external heat source to maintain the drying process.

[0015] Several options have been explored to address this need, including: - the use of heat generated by the combustion of natural gas; - the use of heat produced in a combined heat and power unit, using biogas from a methanization process of sewage sludge; - solar drying; and - the use of bio-drying, i.e. a drying process which exploits the heat generated by microbial activity degrading organic matter in sludge.

[0016] However, none of these options is entirely satisfactory.

[0017] Burning natural gas to provide the heat needed for pyrolysis results in the replacement of biogenic CO2 sequestered in the fuel with an emission of fossil CO2. This reduces the environmental benefits of the process, making it similar to conventional incineration in terms of gas emissions. greenhouse gas emissions. In addition, this increases operating costs due to the higher energy bill.

[0018] Using the heat produced by a cogeneration unit fueled by biogas from sludge methanation requires an on-site digestion facility. Therefore, this solution is not viable for wastewater treatment plants without a digestion unit.

[0019] Solar drying, with or without heat input, is a viable option for wastewater treatment plants with sufficient land and located far from inhabited areas due to odor concerns. While the installation of odor control units can mitigate this problem, the high total cost of these units, due to the large volumes of air to be treated, compromises the economic viability of this solution.

[0020] Bio-drying reduces the amount of carbon sequestered by pyrolysis and can emit other potent greenhouse gases. While this method replaces the primary energy required for conventional drying with biological oxidation, it does not necessarily improve the environmental balance due to potential emissions of nitrous oxide (N2O) and methane (CH4). Furthermore, these units require a large area of ​​land.

[0021] Furthermore, designers of pyrolytic carbonization plants often neglect drying issues. Most of them are still using their first generation of technology, and energy optimization is not yet a priority at this stage. Summary

[0022] Also, one problem that the present invention aims to solve is to develop a process and installation for the pyrocarbonization of organic matter, capable of limiting, or even eliminating in a permanent system, external heat inputs, while maintaining a favorable environmental and economic balance.

[0023] In order to solve this problem, and according to a first objective, a pyrocarbonization process for organic matter is proposed, comprising the following operations: - to feed a first indirect type dryer with organic matter in a wet state and to extract the organic matter from said first dryer in a pre-dried state; the first dryer comprising a heating surface heated by a heat transfer fluid, said heat transfer fluid being heated in a heat transfer fluid heating circuit; - to feed a second direct type dryer with the organic matter in a pre-dried state and to extract the organic matter from the second dryer, in a dried state; the second dryer being heated by a drying gas circulating in the second dryer, said drying gas being heated in a drying gas heating circuit; - to feed a pyrolysis reactor with organic matter in a dried state, to pyrolyze said organic matter and to extract pyrolysis gases and char from said pyrolysis reactor; - burn pyrolysis gases to produce after-combustion fumes; - heat the pyrolysis reactor with the post-combustion fumes; - to direct the post-combustion fumes to a heat exchanger in the heat transfer fluid circuit and transfer heat from said post-combustion fumes to the heat transfer fluid in said heat exchanger; and - extract vapors from the first dryer, conduct said vapors into a first heat exchanger of the drying gas heating circuit and transfer a latent heat of condensation from the vapors to the drying gas in said first heat exchanger.

[0024] Thus, the aforementioned arrangement of the first indirect-type dryer and the second direct-type dryer, in combination with the use of pyrolysis gases to heat the first dryer and the latent heat of vapors from the first dryer to heat the drying gas of the second dryer, optimizes the thermal efficiency of the drying operations and, more generally, of the pyrocarbonization process as a whole.

[0025] From then on after the start of the process, the process requires an external heat input to maintain it which is low, and advantageously zero when the thermal energy generated by the combustion of the pyrolysis gases is sufficient to power the two dryers as well as the pyrolysis reactor.

[0026] For the purposes of this application, the "wet", "pre-dried" and "dried" states of organic matter describe the evolution of the dryness rate of the matter during the process, the dryness rate of the matter in the wet state being lower than that of the matter in the pre-dried state, the latter being itself lower than that of the matter in the dried state.

[0027] According to some embodiments, such a process may include one or more of the following characteristics.

[0028] According to one embodiment, the pyrolysis gas is burned in a combustion chamber.

[0029] According to one embodiment, the combustion chamber is separate from the pyrolysis reactor and the post-combustion fumes are extracted from the combustion chamber and conveyed to the pyrolysis reactor.

[0030] According to another embodiment, the combustion chamber is constituted by a zone of the pyrolysis reactor.

[0031] In one embodiment, the drying gas heating circuit includes a second heat exchanger, and heat is transferred from the post-combustion flue gases to the drying gas in said second heat exchanger. This further improves the thermal efficiency of the process.

[0032] According to one embodiment, heat is transferred directly between the post-combustion fumes and the drying gas in the second heat exchanger.

[0033] In another embodiment, heat is transferred from the heat transfer fluid to the drying gas in the second heat exchanger. In this case, the heat transfer from the post-combustion flue gases to the drying gas is indirect, meaning it occurs via the heat transfer fluid.

[0034] In one embodiment, the drying gas is extracted from the second dryer to the drying gas heating circuit and condensed in a condenser of said drying gas heating circuit upstream of the first heat exchanger. This allows for the recycling of the drying gas, which further improves the thermal efficiency of drying.

[0035] In one embodiment, after transferring the latent heat of condensation from the vapors to the drying gas in the first heat exchanger, the vapors are condensed in a condenser, and then the resulting non-condensable gases are burned. This eliminates the need for a dedicated deodorization unit to deodorize the non-condensable gases.

[0036] In one embodiment, a portion of the drying gas is extracted from the drying gas heating circuit, preferably downstream of the first heat exchanger and upstream of the second heat exchanger, and burned. This prevents—or at least limits—the accumulation of micropollutants in the second dryer.

[0037] According to one embodiment, at least part of the post-combustion fumes are conducted to the heat exchanger of the heat transfer fluid heating circuit from the pyrolysis reactor.

[0038] In one embodiment, the pre-dried organic materials are shaped, for example in a perforated grid extruder, to form cords before being fed into the second dryer. This optimizes the heat exchange between the organic material and the drying gas in the second dryer.

[0039] Furthermore, carrying out this shaping between the two drying stages, and not upstream of the first, presents a notable technical advantage: it allows obtaining, at the outlet of the second dryer, an organic material in a dried state exhibiting an increased apparent density and a reduced geometric dispersion.

[0040] These improvements are particularly crucial in the context of a pyrocarbonization process. Indeed, optimizing char production during pyrocarbonization requires precise adjustment of operating conditions, especially the residence time in the pyrolysis reactor, depending on the physical characteristics (and particularly the particle size) of the introduced organic matter. Furthermore, although particle size has an impact, for other pyrolysis processes, such as pyrogasification and In contrast, pyroliquefaction processes require the pyrolysis reactor to be fed with the finest particles in order to accelerate the heating rate and promote the formation of gas or tar rather than char.

[0041] Thus, by limiting the geometric dispersion of the strands through intermediate extrusion, it becomes possible to fine-tune the process parameters, thereby improving char yield. This also helps reduce heat production fluctuations that could disrupt the overall heat balance of the process and consequently require additional external energy inputs to stabilize its operation.

[0042] Furthermore, since the primary objective of a pyrocarbonization process is precisely the production of char, intended for subsequent use, for example as a soil amendment or solid fuel, increasing the density of the dried product upstream of the reactor directly leads to a higher density of the resulting char. This characteristic is beneficial not only in terms of ease of transport, but also for the use and commercial value of the final product.

[0043] According to an advantageous embodiment, the pre-dried organic material cords are formed using calibrated dies with dimensions specifically chosen to obtain cords with a diameter greater than 5 mm, for example, between 6 mm and 8 mm. In other words, the diameter of the extruder die openings is greater than 5 mm, and advantageously between 6 mm and 8 mm.

[0044] It has been found that choosing these dimensions is particularly advantageous. On the one hand, the diameter of the beads is large enough to limit the formation of fine char dust downstream of the process, thus reducing the risk of explosive atmospheres. On the other hand, it remains sufficiently small to prevent the formation of excessively large beads, which allows for control of the residence time in the pyrolysis reactor and avoids the development of excessive thermal gradients within the beads. These gradients could indeed lead to incomplete or uneven pyrocarbonization, compromising the quality and uniformity of the resulting char. The sizing The precision of the processes thus constitutes a parameter allowing for the optimization of both the safety, yield and quality of the char produced.

[0045] According to one embodiment, pyrocarbonization is specifically configured to produce a solid char fraction in which the organic fraction represents at least 20% by mass, and for example between 20 and 30%, of the initial volatile matter in the organic matter.

[0046] According to one embodiment, the organic matter in its wet state has a dryness content of less than 50%.

[0047] According to one embodiment, the organic matter in its wet state has a dryness rate of between 15 and 35%.

[0048] According to one embodiment, the organic materials in the pre-dried state have an intermediate dryness level Ti which is between 35 and 55%, and preferably between 40 and 50%.

[0049] According to one embodiment, the organic materials in the dried state have a dryness rate of between 60 and 100%, advantageously greater than 80% and preferably greater than 90%.

[0050] According to one embodiment, the organic matter in a wet state comprises or is made up of dehydrated sewage sludge.

[0051] According to one embodiment, the heating surface is heated to a temperature between 0.8 Tb and 1.2 Tb, and preferably between 0.9 Tb and 1 Tb, with Tb: the boiling point of water under pressure conditions inside the first dryer.

[0052] According to one embodiment, the heat transfer fluid is chosen from thermal oils and water vapor.

[0053] According to one embodiment, the heat transfer fluid is heated in the heat exchanger of the heat transfer fluid heating circuit to a temperature between 150 and 250 °C and preferably between 180 and 200 °C.

[0054] According to one embodiment, the post-combustion fumes have a temperature between 600 and 1000°C and preferably between 700 and 800 °C, at the inlet of the heat exchanger of the heating circuit of the heat transfer fluid.

[0055] According to one embodiment, at the inlet of the second dryer, the drying gas has a temperature between 40 and 120°C, and preferably between 90 and 100°C.

[0056] According to one embodiment, at the outlet of the second dryer, the drying gas has a temperature between 30 and 60 °C, and preferably between 50 and 60 °C.

[0057] According to one embodiment, the vapors are extracted from the first dryer at a temperature between 90 and 100°C and the drying gas, at the outlet of the first heat exchanger, has a temperature between 60 and 90°C.

[0058] According to one embodiment, the pyrolysis temperature in the pyrolysis reactor is between 300 and 800°C and preferably between 500 and 600°C.

[0059] According to one embodiment, the pyrolysis gases are conducted into the combustion chamber at a temperature above 400°C.

[0060] According to one embodiment, the pyrolysis gases are burned at temperatures between 850 and 1600 °C, and preferably between 900 and 1000 °C.

[0061] According to a second item, a pyrocarbonization installation is proposed comprising: - a first dryer, of indirect type, configured to be fed with organic matter in a wet state and to deliver the organic matter in a pre-dried state; the first dryer comprising a heating surface configured to be heated by a heat transfer fluid; - a second dryer, of the direct type, configured to be fed with organic matter in a pre-dried state and to deliver organic matter in a dried state; the second dryer being configured to be heated by a drying gas; - a pyrolysis reactor intended to be heated with post-flue gases combustion and configured to pyrolyze organic matter in a dried state and deliver pyrolysis gases and char; - a combustion chamber configured to burn pyrolysis gases and deliver post-combustion fumes; - a heat transfer fluid heating circuit comprising a heat exchanger configured to transfer heat from the post-combustion flue gases to the heat transfer fluid; - an extraction device configured to extract fumes from the first dryer; and - a drying gas heating circuit comprising a first heat exchanger configured to transfer latent heat of condensation from the vapors to the drying gas.

[0062] Depending on the embodiment, such an installation may include one or more of the following characteristics.

[0063] According to one embodiment, the first dryer is chosen from thin film dryers, disc dryers, and paddle dryers.

[0064] According to one embodiment, the pyrolysis reactor is chosen from among screw (auger), rotary drum, stepped hearth and fluidized bed pyrolysis reactors.

[0065] According to one embodiment, the second dryer is a belt dryer.

[0066] According to one embodiment, the installation includes a condenser arranged to receive the vapors at the outlet of the first heat exchanger.

[0067] In one embodiment, the drying gas heating circuit includes a second heat exchanger configured to transfer heat from the post-combustion flue gases to the drying gas. This heat exchange can be direct or indirect, i.e., using an intermediate fluid, such as a heat transfer fluid.

[0068] According to one embodiment, the drying gas heating circuit includes an extraction duct for extracting the drying gas from the second dryer and a condenser upstream of the first heat exchanger of said drying gas heating circuit.

[0069] According to one embodiment, the installation includes a condenser to condense the vapors at the outlet of the first heat exchanger and a conduit to carry the non-condensable gases resulting from the condensation of the condenser stops to the combustion chamber in order to burn them.

[0070] According to one embodiment, the installation includes a conduit to carry at least part of the post-combustion fumes from the pyrolysis reactor to the heat exchanger of the heat transfer fluid heating circuit.

[0071] According to one embodiment, the installation includes a forming device arranged to form cords with the organic material in a pre-dried state and convey them to the second dryer.

[0072] According to one embodiment, the forming device is an extruder, comprising calibrated dies and a screw arranged to transport the organic materials in a pre-dried state through the calibrated dies in order to form the cords. Brief description of the figures

[0073] Other features and advantages of the invention will become apparent from the following description of particular embodiments of the invention, given by way of example but not limitation, with reference to the attached drawings in which:

[0074] [Fig. 1] is a diagram of a pyrocarbonization installation according to one embodiment. Description of the implementation methods

[0075] Referring to Figure 1, a process and installation for the pyrocarbonization of organic waste are described below. This process and installation are specifically designed to treat wet organic waste, i.e., waste with a dry matter content of less than 50%, for example, between 15% and 35%.

[0076] The dryness percentage of a material is defined as the ratio of its dry mass to its initial mass before drying, that is: Dryness percentage = dry mass / initial mass * 100; where: - dry mass: the mass of the sample after it has been dried until all the water has been removed; and - initial mass: the mass of the sample before drying, including water.

[0077] In the following description, the process is mainly illustrated by the pyrocarbonization treatment of dewatered sewage sludge.

[0078] The term "dewatered sewage sludge" refers to the solid residues that remain after the treatment of water in a wastewater treatment plant, this sludge having undergone a mechanical dewatering operation to reduce its water content, thus increasing its proportion of dry matter.

[0079] The dryness rate of sewage sludge after dewatering generally varies between 15% and 35%, depending on the dewatering technologies used (such as filter presses, centrifuges, or drying beds).

[0080] It should be noted, however, that the applications of the pyrocarbonization process and installation according to the invention are not limited to the treatment of this type of organic matter. They can also be used for the pyrocarbonization of other wet organic materials, such as methanation digestate or wet biomass.

[0081] As illustrated in Figure 1, the installation 1 includes a pyrolysis reactor 2 and two separate drying units: a first indirect dryer 3 and a second direct dryer 4. As will be detailed later, the combustion of the pyrolysis gases 32 is used to heat the heat transfer fluid 38 that heats the first dryer 3, while the latent heat of vapors 36 extracted from the first dryer 3 is used to heat the drying gas 37 of the second dryer 4, thus optimizing the thermal efficiency of the drying operations.

[0082] The first dryer 3 is fed with dewatered sewage sludge 5 by a pump 6. The first dryer 3 is of the indirect type, meaning that heat is transferred to the material to be dried via a heating surface 7 rather than by direct contact with hot gases. The surface of heating 7 is heated via a heat transfer fluid 38 which is itself heated in a heat transfer fluid heating circuit 8.

[0083] The heating surface 7 is maintained at a temperature between 0.8 Tb and 1.2 Tb, and preferably between 0.9 Tb and 1 Tb, with Tb: the boiling point of water under the pressure conditions inside the first dryer 3. Thus, the heating surface 7 has a temperature between 80 and 120°C and preferably between 90 and 100°C, when the pressure in the first dryer 3 is equal to or substantially equal to atmospheric pressure.

[0084] The heat transfer fluid circuit 8 is equipped with a pump 21 which circulates the heat transfer fluid 38 through a heat exchanger 22 where it will be heated by post-combustion fumes 34, i.e. fumes generated by the combustion of pyrolysis gases 32.

[0085] The heat transfer fluid 38 of the heat transfer fluid circuit 8 is preferably a thermal oil or water vapor.

[0086] At the outlet of pyrolysis reactor 2, the post-combustion flue gases 34 have a temperature between 600 and 1000°C and preferably between 700 and 800°C. The heat transfer fluid 38 is thus heated in the heat exchanger 22 to a temperature between 150 and 250°C and preferably between 180 and 200°C.

[0087] The first dryer 3 is, for example, chosen from: - thin film dryers which are arranged to bring a thin layer of the material to be dried into contact with the heating surface; - disc dryers, which consist of a series of heated rotating discs; and - paddle dryers which have rotating paddles inside a heated drum.

[0088] The first dryer 7 is equipped with an extraction device 16 fitted with a fan 17, which extracts the vapors 36 from the first dryer 7. These vapors contain air, water vapor, and non-condensable materials, rich in odorous micropollutants such as VOCs and NH3. The vapors 36 are conveyed to a heat exchanger 18 where their latent heat of condensation is transferred to the drying gas 37 of the second dryer 4.

[0089] The vapors 36 are then conveyed to a condenser 19. A first portion of the vapor is condensed in the heat exchanger 18, and the remaining portion is condensed in the condenser 19, using a heat exchanger with water, for example. The condensate 20 is collected and then discharged.

[0090] At the outlet of the condenser 19, the non-condensable gases resulting from the condensation of the vapors 36 are advantageously directed to the combustion chamber 23 in which the pyrolysis gases 32 are burned. This allows the non-condensable species contained to be oxidized and thus avoids the use of a dedicated deodorization unit.

[0091] The pre-dried sludge 39 is extracted from the first dryer 3. At the outlet of the first dryer 3, it has an intermediate dryness level Ti, which is typically between 35 and 55%, for example between 40 and 50%.

[0092] At the outlet of the first dryer 3, the pre-dried sludge 39 is advantageously conveyed into a forming device 9 before being conveyed into the second dryer 4. The forming device 9 is configured to give the pre-dried sludge 39 a shape optimizing its contact surface with the heating fluid circulating in the second dryer 4.

[0093] According to one embodiment, the forming device 9 is an extruder, also called a spaghetti maker, which includes a screw 10 transporting the pre-dried sludge 39 through calibrated dies 11 in order to form pre-dried sludge cords 12. Advantageously, the calibrated dies 11 are dimensioned to obtain pre-dried sludge cords having a diameter greater than 5 mm, for example between 6 and 8 mm.

[0094] Subsequently, the pre-dried sludge cords 12 are brought into the second dryer 4, which is a direct dryer, i.e. equipment where the heat required for drying is directly transferred to the material by contact with a drying gas 37, which is for example air.

[0095] The second dryer 4 is, for example, a belt dryer, that is to say, equipment in which the pre-dried sludge cords 12 are deposited on one or more perforated conveyor belts 14 which move through a drying chamber 15.

[0096] The second dryer 4 is equipped with a drying gas heating circuit 13. This heating circuit comprises, on the one hand, an injection duct 24 for injecting the heated drying gas 37, and, on the other hand, an extraction duct 25 for extracting the cooled drying gas 37, which is now enriched with water vapor and non-condensable gases. The heating circuit 13 includes one or more fans 26 for circulating the drying gas 37 from the extraction duct 25 to the injection duct 24.

[0097] At the inlet of the second dryer 4, i.e. in the injection duct 24, the drying gas 37 has a temperature between 40 and 120°C. At the outlet of the second dryer 4, i.e. in the extraction duct 25, the drying gas, laden with moisture, has a temperature between 30 and 60°C, and preferably between 50 and 60°C.

[0098] The drying gas heating circuit 13 includes a condenser 27 designed to condense the water vapor contained in the drying gas 37 in order, on the one hand, to dehumidify the drying gas 37 and, on the other hand, to recover the condensate 20.

[0099] At the outlet of the condenser 27, the drying gas 37 is led to the heat exchanger 18 where it will be heated by the vapors 36, extracted from the first dryer 3. [000100] In the embodiment shown, the heat exchanger 18 is a gas / gas exchanger. [000101] In another embodiment, the heat transfer between the vapors 36 extracted from the first dryer 3 and the drying gas 37 of the second dryer 4 is carried out by means of a heat transfer fluid, such as water. In other words, said heat transfer fluid is used to absorb the latent heat of the vapors 36 extracted from the first dryer 3 and transfer it to the drying gas 37 of the second dryer 4. [000102] The vapors 36 extracted from the first dryer 3 preferably have a temperature between 90 and 100°C and thus allow the drying gas 37, at the outlet of the heat exchanger 18, to be heated to a temperature between 60 and 90 °C. [000103] In the embodiment shown, the drying gas heating circuit 13 includes a second heat exchanger 28 which allows the drying gas 37 to be heated even further. In this embodiment, the temperature of the drying gas 37 in the injection duct 24 is therefore higher, preferably between 90 and 120 °C, for example around 100 °C, whereas it is between 70 and 90 °C and, for example, around 80 °C, in the absence of such a second heat exchanger 28. In the second heat exchanger 28, the drying gas 37 is heated by a heat transfer from the post-combustion flue gases 34. [000104] In the embodiment shown, this transfer from the post-combustion fumes 34 is done via the heat transfer fluid circuit 8, that is to say that heat is transferred from the post-combustion fumes 34 to the heat transfer fluid 38 by the heat exchanger 22 and then from the heat transfer fluid 38 to the drying gas 37 by the heat exchanger 28. [000105] In an alternative embodiment, not shown, the heat transfer takes place directly from the post-combustion fumes 34 to the drying gas 37 of the second dryer 4 via the second heat exchanger 28. [000106] Furthermore, in order to limit the concentration of micropollutants in the second dryer 4, a portion of the drying gas 37 is extracted from the drying gas heating circuit 13 and then directed to the combustion chamber 23 for combustion. This extraction is advantageously carried out between the first and second heat exchangers 18, 28. This extraction of a portion of the drying gas 37 is compensated by the injection of an equivalent quantity of fresh drying gas 37. [000107] The dimensions of the second dryer 4, and in particular its drying surface, are chosen according to the temperature of the drying gas 37 and the intermediate dryness rate Ti of the pre-dried sludge 39 at the inlet of the second dryer 4 in order to obtain, at the outlet of the second dryer 4, dried sludge 29 having a dryness rate between 60 and 100%, advantageously greater than 80% and, preferably, greater than 90%. [000108] At the outlet of the second dryer 4, the dried sludge 29 is conveyed to the pyrolysis reactor 2 via a hopper 30 allowing The dried sludge 29 is temporarily stored and supplied to the pyrolysis reactor 2 with said dried sludge 29. The reaction in the pyrolysis reactor 2 is described as slow or carbonization, meaning that it proceeds at a slower rate than other pyrolysis reactions and promotes char formation. The pyrolysis temperature is between 300 and 800°C and preferably between 500 and 600°C. [000109] The pyrolysis reactor 2 can be of many different types, including: - screw (auger) which uses a worm screw to convey the material through a heating zone; - a rotary drum where the material is continuously mixed and heated in a rotating cylinder, - a multi-tiered hearth oven that heats the material on several levels of fixed or moving trays, - a fluidized bed where the material is suspended and mixed in a bed of particles fluidized by a flow of air or gas, - or any other specific configuration adapted to the needs of the pyrolysis process. [000110] The pyrolysis reaction generates, on the one hand, biochar 31, and on the other hand, pyrolysis gases 32. [000111] Biochar 31 is extracted from pyrolysis reactor 2, cooled and then stored for later use as a soil amendment, fuel or raw material for various industrial applications. [000112] According to an advantageous embodiment, the pyrolysis reactor 2 is supplied with a low flow of air, which allows partial oxidation of the pyrolysis gas 32 so as to reduce pollutant emissions and increase the quality of said pyrolysis gas 32. [000113] In the embodiment shown, the pyrolysis gases 32 extracted from the pyrolysis reactor 2 are conducted to a particle filter 33 in which the solid particles and soot residues present in the pyrolysis gases 32 are captured and removed. [000114] The pyrolysis gases 32 are then introduced into a combustion chamber 23 in which they are burned. The pyrolysis gases 32 are preferably conducted into said combustion chamber 23 at a temperature above 400 °C, which prevents heavy hydrocarbons contained in the pyrolysis gas 32 from condensing in the conduits connecting the pyrolysis reactor 2 to the combustion chamber 23. [000115] The combustion of the pyrolysis gases 32 takes place at temperatures between 850 and 1600 °C, preferably between 900 and 1000 °C. The residual fumes from this combustion, also referred to as post-combustion fumes 34, are then directed to the pyrolysis reactor 2, where they are used to heat it and thus provide the heat required for the pyrolysis process. [000116] The heat exchange between the post-combustion fumes 34 and the material to be pyrolyzed can be direct, i.e. the heat is transferred directly from the post-combustion fumes 34 to the dried sludge 29, or indirect, i.e. the combustion fumes heat a heating surface, such as an external wall of the pyrolysis reactor 2. [000117] The post-combustion fumes 34, which have been used to provide the heat necessary for the pyrolysis process, are then conveyed to the heat exchanger 22 in which they heat the heat transfer fluid 38, which provides the heating for the first dryer 3. At the outlet of said heat exchanger 22, the post-combustion fumes 34 advantageously have a temperature between 160 and 180 °C and are conveyed to a fume treatment device 35 before being released into the atmosphere with a quality that complies with the regulations in force. [000118] According to an advantageous embodiment, the flue gas treatment device is equipped with a plume-reduction device for reducing, or even eliminating, the visible appearance of a smoke plume at the chimney outlet. Such a plume-reduction device comprises a cooling device disposed upstream of a heating device, the cooling device being configured to cool the post-combustion flue gases 34, so as to cause the condensation of the water vapor contained in these flue gases, and then the heating device, positioned downstream, for reheating the post-combustion flue gases. combustion at the outlet beyond their dew point. This double thermal treatment makes it possible to obtain dehumidified and heated flue gases at the outlet, the emission of which does not generate any visible plume, even in cold conditions. [000119] The cooling device can be achieved by means of any suitable system, and in particular by means of a water spray device, so as to lower the temperature of the fumes, for example down to a value of around 15 °C. [000120] According to an advantageous embodiment, the heating device includes a heat exchanger arranged to ensure heat transfer between the post-combustion fumes located upstream of the cooling device and the post-combustion fumes located downstream of said cooling device. [000121] According to an advantageous, complementary or alternative variant, the heating device includes one or more heat exchangers arranged to ensure heat transfer of the condensates 20 from the vapors 36 and / or the drying gas 37 to the post-combustion fumes, downstream of the heating device. [000122] In the embodiment shown and described above, the combustion of the pyrolysis gases 32 is carried out in a dedicated combustion chamber 23. However, in other embodiments not shown, the air flow rate injected into the pyrolysis reactor 32 can be adapted so that all or part of the pyrolysis gases 32 are burned in a zone of the pyrolysis reactor 2. In other words, a combustion chamber is constituted by a zone of the pyrolysis reactor 2 in addition to or instead of the combustion chamber 23. [000123] According to a preferred embodiment, the intermediate dryness rate Ti of the material at the outlet of the first dryer 3 is optimized so that the combined thermal consumption of the first and second dryers 3, 4 is as low as possible. [000124] The aforementioned arrangement of the first dryer 3 and the second dryer 4, in combination with the use of the latent heat of the vapors 36 from the first dryer 3 to heat the drying gas 37 of the second dryer 4, makes it possible to obtain a consumption, for the drying operations, between 500 and 700 kWh / tonne of H2O evaporated, that is to say significantly less than the 1000 kWh / tonne of H2O evaporated of a conventional dryer. [000125] The table below presents implementation conditions for the process according to an example of an embodiment with a combined consumption of the first dryer 3 and the second dryer 4 of 500 kWh / tonne of water evaporated. [000126] The table below presents other conditions for implementing the process with a combined consumption of the first dryer 3 and the second dryer 4 of 600 kWh / tonne of water evaporated. [000127] In this embodiment, the process is therefore autothermal, that is to say that the heat required for drying the dewatered sewage sludge and for maintaining the pyrolysis reaction is entirely supplied by the combustion of the pyrolysis gases 32. In other words, after the process has started, the process no longer requires an external heat input to maintain it, because the thermal energy generated by the combustion of the pyrolysis gases 32 is sufficient to supply the two dryers 3, 4 as well as the pyrolysis reactor 2. [000128] Although the invention has been described in connection with several particular embodiments, it is clearly evident that it is by no means limited to them and that it includes all technical equivalents of the means described as well as their combinations if these fall within the scope of the invention, as defined by the claims. [000129] The use of the verb "comprise", "comprendre" or "include" and its conjugated forms does not exclude the presence of other elements or other steps than those stated in a claim. [000130] In the claims, any reference sign in parentheses shall not be interpreted as a limitation of the claim.

Claims

1. DEMANDS

1. [Process for the pyrocarbonization of organic matter comprising the following operations: - to feed a first indirect type dryer (3) with organic matter in a wet state and to extract from said first dryer (3) the organic matter, in a pre-dried state; the first dryer (3) comprising a heating surface (7) heated by a heat transfer fluid (38), said heat transfer fluid (38) being heated in a heat transfer fluid heating circuit (8); - to feed a second direct-type dryer (4) with the organic matter in a pre-dried state and to extract the organic matter from the second dryer (4) in a dried state; the second dryer (4) being heated by a drying gas (37) circulating in the second dryer (4), said drying gas (37) being heated in a drying gas heating circuit (13); - to feed a pyrolysis reactor (2) with the organic matter in the dried state, to pyrolyze said organic matter and to extract from said pyrolysis reactor (2) pyrolysis gas (32) and char (31); - burn pyrolysis gases (32) to produce afterburning fumes (34); - heat the pyrolysis reactor (2) with the post-combustion fumes (34); - to conduct the post-combustion fumes (34) to a heat exchanger (22) of the heating circuit of the heat transfer fluid (8) and to transfer heat from said post-combustion fumes (34) to the heat transfer fluid (38) in said heat exchanger (18); and - extract vapors (36) from the first dryer (3), conduct said vapors (36) into a first heat exchanger (18) of the heating circuit of the drying gas (13) and transfer a latent heat of condensation of the vapors (36) to the drying gas (37) in said first heat exchanger (18).

2. A pyrocarbonization process according to claim 1, wherein the heating circuit for the drying gas (13) comprises a second heat exchanger (28) and in which heat is transferred from the post-combustion fumes (34) to the drying gas (37) in said second heat exchanger (28).

3. Pyrocarbonization process according to claim 1 or 2, wherein the drying gas (37) is extracted from the second dryer (4) to the drying gas heating circuit (13) and wherein the drying gas is condensed in a condenser (27) of said drying gas heating circuit (13) upstream of the first heat exchanger (18).

4. Pyrocarbonization process according to any one of claims 1 to 3, wherein, after transferring the latent heat of condensation of the vapors (36) to the drying gas (37) in the first heat exchanger (18), said vapors (36) are condensed in a condenser (19) and then non-condensable gases resulting from the condensation of the vapors (36) are burned.

5. Pyrocarbonization process according to any one of claims 1 to 4, wherein at least a portion of the post-combustion fumes (34) are conducted to the heat exchanger (22) of the heat transfer fluid heating circuit (8) from the pyrolysis reactor (2).

6. Pyrocarbonization process according to any one of claims 1 to 5, wherein the organic materials in the pre-dried state are shaped to form cords before feeding the second dryer (4).

7. Pyrocarbonization process according to claim 6, wherein the cords of organic matter in the pre-dried state are formed by means of calibrated dies having dimensions specifically chosen to obtain cords having a diameter greater than 5 mm, for example between 6 mm and 8 mm.

8. Pyrocarbonization process according to any one of claims 1 to 7, wherein the organic matter feeding the first dryer (3) comprises dehydrated sewage sludge.

9. Pyrocarbonization installation comprising: - a first dryer (3), of indirect type, configured to be fed with organic matter in a wet state and deliver organic matter in a pre-dried state; the first dryer (3) comprising a heating surface (7) configured to be heated by a heat transfer fluid (38); - a second dryer (4), of the direct type, configured to be fed with organic matter in a pre-dried state and to deliver organic matter in a dried state; the second dryer (4) being configured to be heated by a drying gas (37); - a pyrolysis reactor (2) intended to be heated with afterburner fumes (34) and configured to pyrolyze organic matter in a dried state and deliver pyrolysis gases (32) and char (31); - a combustion chamber (23) configured to burn pyrolysis gases (32) and deliver post-combustion fumes (34); - a heat transfer fluid heating circuit (8) comprising a heat exchanger configured to transfer heat from the post-combustion flue gases (34) to the heat transfer fluid (38); - an extraction device (16) configured to extract vapors (36) from the first dryer (3); and - a drying gas heating circuit (13) comprising a first heat exchanger (18) configured to transfer latent heat of condensation of the vapors (36) to the drying gas (37).

10. Installation according to claim 9, wherein the drying gas heating circuit (13) includes a second heat exchanger (28) configured to transfer heat from the post-combustion flue gas (34) to the drying gas (37).

11. Installation according to claim 9 or 10, wherein the drying gas heating circuit (13) includes an extraction duct (25) for extracting the drying gas (37) from the second dryer (4) and a condenser (27) upstream of the first heat exchanger (18) of said drying gas heating circuit (13).

12. An installation according to any one of claims 9 to 11, comprising a condenser (19) for condensing the vapors at the outlet of the first heat exchanger (18) and a conduit for conducting the gases non-condensable gases resulting from the condensation of vapors (36) from the condenser (19) towards the combustion chamber (23) in order to burn said non-condensable gases.

13. Installation according to any one of claims 9 to 12, comprising a conduit for conducting at least part of the post-combustion fumes from the pyrolysis reactor (2) to the heat exchanger (22) of the heating circuit of the heat transfer fluid (8).

14. Installation according to any one of claims 9 to 13, comprising a forming device (9) arranged to form cords with the organic matter in the pre-dried state and convey them to the second dryer (4).

15. Installation according to claim 1, wherein the forming device (9) is an extruder having calibrated dies having dimensions which are specifically chosen to obtain cords having a diameter greater than 5 mm, for example between 6 mm and 8 mm.

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