Process and system for hydrothermal carbonization

Abstract

Biomass hydrothermal carbonization process and plant for implementation thereof, wherein said process comprises: - a thermal recovery operation A which involves heating a biomass to be treated, as a result of cooling of a treatment sludge, with heat exchange by means of indirect contact between sludge and biomass; wherein said biomass to be treated is intended for a hydrothermal carbonization treatment and wherein said treatment sludge results from hydrothermal carbonization treatment; - an operation R for regulating the volume of the treatment sludge and / or of the biomass to be treated which exchange heat by means of indirect contact during operation A, so as to obtain a predetermined operating temperature of the treatment sludge resulting from step A.

Description

[0001] PROCESS AND SYSTEM FOR HYDROTHERMAL CARBONIZATION

[0002] TECHNICAL SECTOR

[0003] The present disclosure relates to a process and a plant, in particular a compact plant, for the hydrothermal carbonization of a biomass.

[0004] PRIOR ART

[0005] Nowadays it is known to use hydrothermal carbonization, which consists in the transformation of biomass by means of a thermochemical process carried out in the presence of pressurized hot water in such a way that the biomass is completely immersed and therefore has moisture values higher than about 80% and not less, in any case, than 60% of the total volume.

[0006] The reactors known nowadays may have volumes for containment of the material to be treated depending on the overall treatment capacity of the plant in which they are intended to be installed and, in them, it Is necessary to maintain a high pressure designed to the prevent the evaporation of the water which instead must remain in the liquid state for correct implementation of the hydrothermal carbonization.

[0007] Compared to the usual operating temperatures generally of between 160°C and 300°C, in particular between 180°C and 250°C and, especially, at 210°C-220°C, the internal pressure of the reactors must be kept in general between 5 and 90 bar and especially at 20-24 bar.

[0008] In these working conditions the usual time for carrying out the hydrothermal carbonization may vary from a few tens of minutes to several hours.

[0009] In these traditional plants it is known to use pressurized air, injected into the plant, as driving means for moving the material being treated, for example, in order to feed the biomass to be treated into each reactor and then discharge from it the hydro-carbonized material (henceforth called “treatment sludge”, better known as “HTC slurry”).

[0010] In order to obtain this functionality it is usual to provide a suitable pressure difference, for example of about 2 bar, between the upstream section of the plant, namely that from where the material is transferred, and the downstream section of the plant, namely that where the material is transferred to. Typically, in the upstream plant section, namely in the vicinity of the reactors, the pressure is greater than in the downstream section of the said plant.

[0011] The use of the transfer means, consisting of pressurized air, in the plant sections however obviously proves to be costly, both in view of the high pressures which must be reached and maintained and in view of the volumes of the reactors and the other chambers in the plant and therefore the volume of the air which must be pressurized and kept under pressure.

[0012] Moreover, these means have proved to be ineffective since the actual displacement of the material on which the pressurized air acts depends not only on the controllable variable, consisting in the difference in pressure between the upstream and downstream sections of the plant, but also on the variable which cannot be directly or easily controlled, consisting of the resistance which the material exerts and opposes to its movement.

[0013] Therefore, the inertia of the displacement means consisting of the compressed air is significant and poses a major problem in the sector. In other words, the pressurization of the air for moving the material, since it cannot be instantaneously varied, results in unavoidable inertial displacement of material which is difficult to quantify a priori.

[0014] In fact, unless pressurized air buffer tanks are available for ensuring a substantially instantaneous supply of the volumes of compressed air required to push the material, said pushing force depends on the gradual increase in pressure induced by pressurization if this is performed as needed, namely only when the displacement of material is required.

[0015] A plant known nowadays is described in the patent document EP3898905 in the name of the same Applicant.

[0016] SUMMARY

[0017] The problem underlying the present invention is therefore to optimize a process and plant for the hydrothermal carbonization of biomass.

[0018] The task of a biomass hydrothermal carbonization process and plant according to the present invention is therefore to solve this problem.

[0019] In connection with this task an object of the invention is to propose a biomass hydrothermal carbonization process and plant which are able to reduce drastically the amount of energy used for the same quantity of treated product.

[0020] Another object of the invention is to propose a biomass hydrothermal carbonization process and plant which allow simple and reliable control of the flow of treated biomass, as well as greater reliability with regard to the continuity of treatment of the biomass compared to the solutions known hitherto.

[0021] A further object of the invention is to propose a biomass hydrothermal carbonization process and plant which are able to significantly reduce the dimensions of the plant for the same quantity of treated product.

[0022] Other objects of the present invention will become clear below.

[0023] This task is achieved by a biomass hydrothermal carbonization process and plant according to the attached independent claims.

[0024] Detailed characteristics of a biomass hydrothermal carbonization process and plant according to the invention are described in the attached dependent claims. In particular, with a biomass hydrothermal carbonization process and plant according to the present invention it is possible to obtain simpler and much more efficient control, resulting in major energy savings compared to the conventional solutions which use pressurized air in order to promote a fluid movement of biomass to be treated and treatment sludge in the plant.

[0025] In particular, the same Applicant in the patent text IT102018000020320 has disclosed a hydrothermal carbonization plant which, by means of a heat exchange device, allows preheating of the biomass to be treated, which is supplied to a reactor, using the heat of the treatment sludge discharged from a reactor.

[0026] This known plant and process are able to achieve a high degree of heat recovery and therefore an excellent energy optimization.

[0027] A process and plant according to the present invention are able to increase further the energy optimization; in fact, they allow regulation of the flow of biomass to be treated, supplied for example by a rehydrator, and fed to the reactors, by means of a volumetric pumping apparatus for example, which determines in a efficient and precise manner the volume of biomass supplied to the reactors, avoiding the use of compressed air for the displacement of the biomass and the treatment sludges inside the plant, which has a high inertia (unless pressurized air buffer tanks are provided).

[0028] In other words, a process and a plant according to the present invention allow regulation of the volume of biomass to be treated and treatment sludge which exchange heat. In fact, by regulating the volume, it is possible to regulate the mass of the biomass flow to be treated and the treatment sludge which exchange heat.

[0029] In this way, it is easy to maintain a volumetrically constant flow of treatment sludge to be subjected to a procedure for extracting therefrom the carbonaceous solid referred to in technical jargon as “hydrochar”

[0030] In fact, by regulating the volume of biomass to be treated at the inlet of the heat exchange unit it is possible to determine the temperature of the treatment sludge at the outlet of the heat exchange unit which, to ensure optimum operation of the procedure for extraction of a carbonaceous solid from the treatment sludge, may range between 50°C and 100°C, in particular between 70°C and 80°C. In this way, the temperature of the treatment sludge at the outlet of the heat exchange unit preferably corresponds to the optimum temperature for supplying the treatment sludge to a centrifuge for separation of the liquid fraction therefrom and, subsequently, for dehydration until hydrochar is obtained.

[0031] Together with the volumetric control of the biomass to be treated, for the supply of the heat exchange unit at the discharge outlet thereof, on the treatment sludge side, control means may be used to control the flow volume of the treatment sludge output from the heat exchange unit, such as a valve with an adjustable cross-section or a rotary valve.

[0032] The use of these means for controlling the flow volume of the treatment sludge helps improve the control of the heat exchange between the treatment sludge and the biomass to be treated, for example depending on the temperature of the latter, which generally is the ambient temperature (namely typically between 5°C and 25°C), so as to obtain a desired volume and temperature of the treatment sludge at the discharge outlet of the heat exchange unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further characteristic features and advantages of the invention will emerge more clearly from the description of a preferred, but non-exclusive embodiment of a biomass hydrothermal carbonization process and plant according to the invention, illustrated in an embodiment provided solely by way of a non-limiting example in the attached sets of drawings listed below where:

[0034] - Figure 1 shows a simplified diagram of an example of embodiment of a biomass hydrothermal carbonization plant according to the present invention.

[0035] DETAILED DESCRIPTION

[0036] With particular reference to the said figures, 100 denotes overall a biomass hydrothermal carbonization plant according to the present invention.

[0037] The plant 100 has a particular feature in that it is configured to perform a biomass hydrothermal carbonization process which, according to the present invention, comprises:

[0038] - a thermal recovery operation A which involves heating a biomass to be treated, intended for a hydrothermal carbonization treatment, as a result of cooling of a treatment sludge resulting from a hydrothermal carbonization treatment; said heating and cooling are obtained from a heat exchange by means of indirect contact between sludge and biomass, in particular while they are flowing;

[0039] - an operation R for regulating the volume (i.e. volumetric regulation) of the treatment sludge and / or the biomass to be treated which exchange heat during operation A, so as to obtain a predetermined operating temperature of the treatment sludge resulting from step A; said desired temperature is determined in accordance with calculations and evaluations as explained below. The process according to the present invention may involve an operation B for determining a value of at least a first operating parameter, which is thermodynamically correlated to the operating temperature; there is therefore a predetermined (more or less simple) function which links the fist operating parameter to the operating temperature. “Operating temperature” is understood as meaning, in the context of the present invention, a temperature assumed by the treatment sludge following the execution of said operation A.

[0040] In the context of the present invention, the expression “treatment sludge” is understood as meaning essentially the hydro-carbonized material, known in the sector as “HTC slurry”, resulting from step A.

[0041] The operation R may be provided such that the first operating parameter satisfies an optimization criterion chosen in such a way that the treatment sludge may be continuously fed, or supplied as an input, to a procedure for extraction of a carbonaceous solid from the treatment sludge. In other words, the operation R may be provided in such a way that the first operating parameter assumes a value such that the treatment sludge obtained at the end of the operation A may be directly subjected to an operation or procedure for extraction of a carbonaceous solid.

[0042] Clearly, the first operating parameter may consist directly of the said operating temperature, or may be a different thermodynamic parameter which is directly or indirectly correlated thereto; so that the control of this operating parameter results in a direct or indirect control of the operating temperature.

[0043] The operation R may envisage modulation of the volume of biomass to be treated intended for operation A. In particular, operation R may be implemented by means of a volumetric pumping unit 105 which has an adjustable flowrate so as to control precisely the volume of the treatment sludge and the biomass to be treated which exchange heat during operation A.

[0044] Control of the operating temperature, according to the present invention, is able to guarantee a continuous process for the formation of hydrochar. In fact, the procedure for extraction of the latter from the treatment sludge requires a continuous supply of material which is not at a temperature which is too high for treatment by a centrifugal separation unit.

[0045] The control of the operating temperature, as a result of the present invention, is able to provide at the outlet of the heat exchange unit a treatment sludge at a temperature of between 50°C and 100°C, preferably between 70°C and 80°C and, more preferably around 70°C. This therefore allows said treatment sludge to be fed to, i.e. supplied to the input of, the extraction procedure directly without requiring:

[0046] - time pauses, due to cooling, which would slow down and / or result in discontinuity in the feeding, or input supply, to the extraction procedure, or

[0047] - cooling devices which could ensure the desired continuity of the process at the cost of a greater complexity of the plant 100 and therefore greater economic outlay.

[0048] The operation A may envisage implementing a heat exchange between a hot (typically flowing) treatment sludge and a cold (typically flowing) biomass to be treated, wherein:

[0049] - the treatment sludge is derived from previous hydrothermal carbonization reaction and is intended for directly subsequent dehydration and drying the biomass to be treated may result from rehydration and is intended for a directly subsequent hydrothermal carbonization reaction.

[0050] Thus, the cooling of the treatment sludge facilitates the execution of a subsequent dehydration and drying operation for which it is destined and, at the same time, the heating of the biomass to be treated results in a lower amount of energy required for heating thereof during the subsequent hydrothermal carbonization.

[0051] Therefore, in connection with operation A and the further operations of the process according to the present invention, the expression “biomass to be treated” is understood as meaning typically a flow of biomass, intended to undergo a hydrothermal carbonization reaction, while the expression “treatment sludge” is understood as meaning typically a flow of material resulting from a hydrothermal carbonization reaction of a biomass to be treated.

[0052] As already mentioned, the process according to the present invention may comprise an operation B which involves determining a value of at least a first operating parameter which is thermodynamically correlated to the operating temperature of the treatment sludge.

[0053] The operation R may therefore involve:

[0054] - regulation of the volume of the treatment sludge and / or of the biomass to be treated so that the first operating parameter satisfies an optimization criterion chosen in such a way that the treatment sludge may be continuously fed to a procedure for extraction of a carbonaceous solid from the treatment sludge;

[0055] - (preferably) modulation of the volume of biomass to be treated by means of a volumetric pumping unit 105 which has an adjustable flowrate so as to control precisely the volumes of the treatment sludge and the biomass to be treated which exchange heat during operation A.

[0056] Said optimization criterion may consist of a criterion according to which the operating temperature of the treatment sludge is optimum or in a range around an optimum temperature so that the treatment sludge is continuously fed, or supplied to the input of, a hydrochar extraction procedure which may be performed by a hydrothermal carbonization plant 10 according to the present invention.

[0057] Operation R may involve regulation of the volume of the treatment sludge which exchanges heat in operation A, preferably by means of a rotary valve 105a or an adjustable opening valve.

[0058] Essentially, according to the present invention, one or each of the temperatures at the outlet of the (at least one) heat exchanger used to heat the flowing biomass and to cool the flowing sludge may be controlled by regulating one or each of the flowrates - the flowrate corresponding to a volume per unit of time.

[0059] For simple control, the process according to the present invention may also comprise a definition or determination operation C which involves defining or determining a value of a second operating parameter, which is a function of the biomass volume to be treated which exchanges heat with the sludge to be treated in said operation A; there is therefore a predetermined (more or less simple) function which links the second operating parameter to the volume of biomass to be treated. In other words, the second operating parameter is thermodynamically correlated to the volume of the biomass to be treated which exchanges heat with the flow of sludge to be treated in operation A.

[0060] According to this preferred aspect, operation C also involves setting a target value for the value of the first operating parameter. According to said optimization criterion, in this case the value of the second operating parameter may be determined so that, following execution of operation A, the value of the first operating value tends towards the target value or falls within a range around the target value; for example, according to a feedback control of the volume of the flow of biomass to be processed with respect to the first operating parameter.

[0061] Preferably the process according to the present invention is implemented so that, during execution of operation A, operations B, C and R are performed cyclically and in sequence continuously or at predefined time intervals, so as to obtain effective and efficient control of the first and / or the second operating parameter.

[0062] The aforementioned first operating parameter may correspond to the temperature of the treatment sludge detected following the execution of operation A.

[0063] The target value of the first operating parameter may be fixed as being not lower than 50°C and not higher than 100°C, being preferably between 60°C and 90°C, and more preferably between 70°C and 80°C, so as to allow direct introduction of the treatment sludge, for example into a centrifuge of the separation procedure suitable for obtaining the hydrochar. The target value of the first operating parameter is even more preferably about 70°C.

[0064] The aforementioned second operating parameter may correspond to the flowrate of the biomass to be treated which is supplied to the reactor or reactors.

[0065] Preferably, the process according to the present invention comprises said hydrothermal carbonization treatment which may be carried out at a temperature of between 160°C and 300°C, preferably 180°C-250°C and more preferably 210°C-220°C.

[0066] Moreover, the process according to the present invention may comprise a pressurization operation E, which involves keeping the biomass to be treated at a pressure of between 6 bar and 86 bar and, preferably, between 10 bar and 50 bar, and more preferably at a pressure of not less than 23 bar and not more than 28 bar or, in any case such that, depending on the temperature adopted in the hydrothermal carbonization treatment, it is suitable for preventing boiling of the water present in the biomass to be treated.

[0067] For completeness, the process according to the present invention may comprise the said extraction procedure, which involves:

[0068] - an operation F of - preferably mechanical - separation of water from the treatment sludge, for example by means of a centrifuge;

[0069] - an operation G of drying the treatment sludge, carried out subsequent to the operation F, so as to obtain from the treatment sludge a carbonaceous solid having a residual moisture preferably of less than 15% by mass.

[0070] The process according to the present disclosure may also comprise an operation H of rehydration of a biomass, which precedes operation A and which, following the execution thereof, is intended to ensure that said biomass to be treated has a moisture content preferably of between 85% by mass and 95% by mass.

[0071] It can therefore be seen how a hydrothermal carbonization process according to the present disclosure is able to obtain a substantial energy optimization and at the same time a high working continuity, making it possible to obtain with continuity a hydrochar from the biomass which forms the raw material of said process.

[0072] The present invention also relates to the hydrothermal carbonization plant 100 configured to implement the hydrothermal carbonization process disclosed here.

[0073] Structurally speaking, the plant 100 comprises:

[0074] - a feeding unit 101 for feeding biomass to be treated, which may comprise a rehydrator into which a biomass 10 and water 11 are introduced;

[0075] - a first reaction group 102 and a second reaction group 102b and, preferably, a third reaction group 102c, each comprising at least one reactor 103 configured to receive a biomass to be treated, in order to carry out hydrothermal carbonization of the biomass to be treated so as to obtain a treatment sludge, to be discharged from the reactor 103;

[0076] - a heat exchange unit 104, which is connected to the feeding unit 101 and to the first and second reaction groups 102a and 102b as well as, preferably also, to a possible third reaction group 102c and which is configured to perform operation A so as to carry out a heat exchange between:

[0077] I. a hot treatment sludge, discharged from a reactor 103 of the first reaction group 102a or, alternatively, from a reactor 103 of the second reaction group 102b (or, alternatively, from the third reaction group 102c);

[0078] II. and a cold biomass to be treated, directed so as to feed a reactor 103 of the first reaction group 102a or, alternatively, a reactor 103 of the second reaction group 102b (or, alternatively, a reactor 103 of the third reaction group 102c, if present);

[0079] - a volumetric pumping unit 105, connected to the heat exchange unit 104 and to the feeding unit 101 and configured to move a volume of the biomass to be treated towards the reaction groups 102a, 102b, 102c, so as to control the volume of the biomass to be treated which exchanges heat with the sludge to be treated in the heat exchange unit 104 and before reaching one of the reactors 103;

[0080] - a control device (not shown) (which may be suitable for realizing operation B) connected to the volumetric pumping unit 105 so as to activate it and configured to operate the volumetric pumping unit 105 so as to perform operation R.

[0081] The volumetric pumping unit 105 advantageously comprises a volumetric pump (which in the attached figures is indicated, for the sake of simplicity, by the reference number 105), but preferably may comprise also means for controlling the volume of the treatment sludge from the heat exchange unit 104, such as a valve with adjustable cross-section or a rotary valve 105a.

[0082] In this way, the control device may, by operating in a coordinated manner the volumetric pump 105 and the rotary valve 105a (or adjustable valve), regulate the speed and the quantity of biomass to be treated and treatment sludge which cross the heat exchange unit 104 so as to keep the operating temperature of the treatment sludge at the outlet of the heat exchange unit (for example the rotary valve 105a) at the desired value.

[0083] In order to maintain an ambient pressure inside the reactors 103 such as to prevent the boiling of the water contained in them, in a manner conventional per se the plant 100 may comprise a compression unit (not shown).

[0084] The latter is connected, by means of valves 107, to the reactors 103 of the first and second reaction groups 102a and 102b, and of the third reaction group 102c, if present, and configured to supply them with compressed air so as to maintain in them an operating pressure such as to counteract boiling of the water present in them during a hydrothermal carbonization operation.

[0085] Preferably, the control device is connected to the compression unit and to the valves 107 in order to operate them.

[0086] The plant 100 may comprise a valve system comprising:

[0087] - first valves 108a placed in fluid connection between the heat exchange unit 104 and an inlet of each of the reactors 103;

[0088] - second valves 108b placed in fluid connection between a discharge outlet of each of the reactors 103 and the heat exchange unit 104.

[0089] The control device may be connected to the first valves 108a and to the second valves 108b so as to operate them selectively according to:

[0090] - a first connection arrangement, in which the first valves 108a connected to the reactors 103 of the first reaction group 102a and the second valves 108b connected to the reactors 103 of the second reaction group are opened at the same time, while the first valves 108a connected to the reactors 103 of the second reaction group 102b and the second valves 108b connected to the reactors 103 of the first reaction group 102a are closed at the same time, so that in the heat exchange unit 104 said biomass to be treated directed to the first reaction group 102a receives thermal energy from the treatment sludge supplied from the second reaction group 102b;

[0091] - a second connection arrangement, in which the first valves 108a connected to the reactors 103 of the first reaction group 102a and the second valves 108b connected to the reactors 103 of the second reaction group are closed at the same time, while the first valves 108a connected to the reactors 103 of the second reaction group 102b and the second valves 108b connected to the reactors 103 of the first reaction group 102a are open at the same time, so that in the heat exchange unit 104 said biomass to be treated directed to the second reaction group 102b receives thermal energy from a treatment sludge supplied from the first reaction group 102a.

[0092] The presence of only a first reaction group 102a and a second reaction group 102b results typically in a discontinuous production of treatment sludge.

[0093] In fact, each of the reaction groups 102a and 102b typically operates in accordance with a procedure which comprises three steps:

[0094] - a loading step, which involves loading the reactor 103 with a biomass to be treated;

[0095] - an active step, which involves hydrothermal carbonization of the biomass to be treated and the production of said treatment sludge;

[0096] - an unloading step where the treatment sludge is unloaded from the reactor 103.

[0097] The unloading of treatment sludge from the reaction groups 102a and 102b may tend to be discontinuous or intermittent because, while the reactor 103 of one of the reaction groups 102a or 102b unloads treatment sludge, the other one is performing the loading step, therefore the steps subsequently performed by the reactors 103 will be the loading step and the hydrothermal carbonization step, respectively, resulting in discontinuous unloading of treatment sludge.

[0098] For this reason, in the case where the plant 100 is provided with only two reaction groups 102s and 102b, the control device may be advantageously preset to operate in such a way as to control the flow of treatment sludge and biomass to be treated so that the unloading of cooled treatment sludge from the heat exchange unit 104 is as continuous as possible.

[0099] The provision of a third reaction group 102c allows an increase in the continuity of production of the treatment sludge.

[0100] Advantageously, the provision of a third reaction group 102c, for example as shown in the attached figure, is able to guarantee a more continuous production of treatment sludge suitable for supplying the heat exchange unit 104.

[0101] In this case the valve system will comprise corresponding first valves 108a and second valves 108b arranged upstream and downstream of the reactor 103 of the third reaction group 102c.

[0102] In fact, in this case, the control device may cyclically activate the first valves 108a and the second valves 108b and the reaction groups 102a, 102b, 102c so that, at each cycle, while the reactor 103 of one of the three reaction groups 102a, 102b and 102c performs the loading step, the reactor 103 of another reaction group 102a, 102b and 102c may perform the hydrothermal carbonization step while the last reaction group 102a, 102b and 102c may have the reactor 103 which may perform the unloading step.

[0103] Thus, for each cycle there is always a reactor 103 which unloads treatment sludge and a reactor 103 which loads biomass to be treated.

[0104] In order to carry out the aforementioned extraction procedure, the plant 100 may comprise a tank 109 with centrifugal separator and, in sequence, a centrifuge 110 and a dryer unit 111.

[0105] For heating of the reactors 103, the plant 100 may comprise a supply of heated diathermic oil, the flow of which for heating the reactors 103 is regulated by valves 112 which may be connected to the control device so as to be activated by it.

[0106] The problem of optimizing a process and plant for hydrothermal carbonization of biomass is therefore solved by the present invention. In particular, according to the present invention, a biomass hydrothermal carbonization process and plant which are able to reduce significantly the dimensions of the plant for the same amount of treated product, without the need for a series of twin reaction groups as described in the patent document EP3898905 in the name of the same Applicant, are proposed.

[0107] A biomass hydrothermal carbonization process and plant according to the present invention therefore allow the energy consumption levels to be drastically reduced for the same amount of treated product.

[0108] Moreover, the plant according to the present invention allows simple and reliable control of the flow of biomass by means of regulation of the volume of biomass to be treated, directed towards the reaction groups, and / or the volume of treatment sludge discharged from the heat exchange unit.

[0109] This therefore results in a greater reliability in the continuity of treatment of the biomass compared to the solutions known today.

[0110] Where the operational characteristics and the techniques mentioned in the following claims are followed by reference numbers or symbols, these reference numbers or symbols have been assigned with the sole purpose of facilitating understanding of the said claims and consequently they do not limit in any way the interpretation of each element which is identified, purely by way of example, by said reference numbers or symbols.

Claims

CLAIMS1. Biomass hydrothermal carbonization process which comprises:- a thermal recovery operation A which involves heating a biomass to be treated, as a result of cooling of a treatment sludge with heat exchange by means of indirect contact between sludge and biomass; wherein said biomass to be treated is intended for a hydrothermal carbonization treatment and wherein said treatment sludge results from a hydrothermal carbonization treatment;- an operation R for regulating the volume of the treatment sludge and / or of the biomass to be treated which exchange heat by means of indirect contact during operation A, so as to obtain a desired operating temperature of the treatment sludge resulting from step A.

2. Process according to claim 1 , wherein the operation R involves regulating the volume of the treatment sludge which exchanges heat in operation A via a rotary valve or an adjustable opening valve.

3. Process according to claim 1 or 2, wherein the operation R involves regulating the volume of the biomass to be treated by means of a volumetric pumping unit which has an adjustable flowrate.

4. Process according to claim 1 , 2 or 3, wherein said heat exchange between sludge and biomass takes place while sludge and biomass are flowing.

5. Process according to claim 1 or 2 or 3 or 4 which comprises an operation B, for determining a value of at least a first operating parameter, said first operating parameter being correlated by means of a first predetermined function to an operating temperature, said operating temperature being a temperature assumed by the treatment sludge following the execution of saidoperation A; wherein the operation R involves:- said regulation of the volume of the treatment sludge and / or of the biomass to be treated being performed such that the first operating parameter satisfies an optimization criterion chosen in such a way that the treatment sludge can be continuously fed to a procedure for extraction of a carbonaceous solid from the treatment sludge.

6. Process according to claim 5, which comprises a definition or determination operation C which involves defining or determining a value of a second operating parameter, said second operating parameter being correlated by means of a second predetermined function to a volume of biomass to be treated which exchanges heat with a volume of treatment sludge in said operation A, and which also involves setting a target value for the value of said first operating parameter; wherein said optimization criterion results in the value of the second operating parameter being determined in such a way that, following execution of operation A, the value of the first operating parameter tends towards said target value or falls within a range around said target value.

7. Process according to claim 6 wherein, during the execution of operation A, operations B, C and R are performed cyclically and in sequence.

8. Process, according to one of the preceding claims 5 or 6 or 7, wherein said first operating parameter consists of the temperature of said treatment sludge detected following the execution of operation A; said target value being between 50°C and 100°C, preferably between 60°C and 90°C and more preferably between 70°C and 80°C.

9. Process, according to one of the preceding claims, which comprises said hydrothermal carbonization treatment at a temperature of 160°C - 300°C, preferably 180°C - 250°C and more preferably 210°C - 220°C, and a pressurization operation E, which involves maintaining said biomass to be treated at a pressure of between 8 bar and 86 bar and, preferably, between 10 bar and 50 bar, and more preferably at a pressure of not less than 23 bar and not greater than 28 bar, depending on said temperature for avoiding boiling of the water present in said biomass to be treated.

10. Process according to one of the preceding claims which comprises said extraction procedure, which involves:- an operation F of - preferably mechanical - separation, by means of a centrifugal action, of the water from said treatment sludge;- an operation G of drying said treatment sludge, carried out subsequent to said operation F, so as to obtain from said treatment sludge a carbonaceous solid having a residual moisture preferably of less than 15% by mass.11 . Process according to one of the preceding claims which comprises an operation H involving rehydration of a biomass in order to obtain said biomass to be treated, where, following the execution of said operation H, said biomass to be treated has a moisture content preferably of between 85% by mass / volume and 95% by mass / volume.

12. Hydrothermal carbonization plant (100) configured to implement the process according to any one of the preceding claims, which comprises:- a feeding unit (101 ) for feeding biomass to be treated;- a first reaction group (102a) and a second reaction group (102b) each comprising at least one reactor (103) configured to receive a biomass to betreated, in order to carry out a hydrothermal carbonization of said biomass to be treated so as to obtain a treatment sludge, to be discharged from said reactor (103);- a heat exchange unit (104) which is connected to said feeding unit (101 ) and to the first and second reaction groups (102a, 102b) and which is configured to perform operation A so as to carry out a heat exchange between a hot treatment sludge, discharged from a reactor (103) of the first reaction group (102a) or, alternatively, from a reactor (103) of the second reaction group (102b), and a cold biomass to be treated, directed so as to feed a reactor (103) of the second reaction group (102b) or, alternatively, a reactor (103) of the second reaction group (102b), respectively;- a volumetric pumping unit (105), connected to the heat exchange unit (104) and to the feeding unit (101 ) and configured to move a volume of the biomass to be treated towards the reaction groups (102a, 102b), so as to control the volume of the biomass to be treated which exchanges heat with the sludge to be treated in the heat exchange unit (104) and before reaching one of said reactors (103);- a control device connected to the volumetric pumping unit (105) so as to activate it and configured to operate the volumetric pumping unit (105) so as to perform operation R.

13. Plant (100) according to claim 12, which comprises a compression unit (107) connected to the reactors (103) of the first and second reaction groups (102b) and configured to supply them with compressed air so as to maintain an operating pressure in them such as to counteract boiling of the water present in them during a hydrothermal carbonization operation.

14. Plant (100) according to either one of claims 12 or 13 comprising a valve system comprising:- first valves (108a) placed in fluid connection between the heat exchange unit (104) and an inlet of each of said reactors (103);- second valves (108b) placed in fluid connection between a discharge outlet of each of said reactors (103) and the heat exchange unit (104); wherein said control device is connected to said first valves (108a) and to said second valves (108b) so as to operate them selectively according to:- a first connection arrangement, in which the first valves (108a) connected to the reactors (103) of the first reaction group (102a) and the second valves (108b) connected to the reactors (103) of the second reaction group are opened at the same time, while the first valves (108a) connected to the reactors (103) of the second reaction group (102b) and the second valves (108b) connected to the reactors (103) of the first reaction group (102a) are closed at the same time, so that in the heat exchange unit (104) said biomass to be treated directed to the first reaction group (102a) receives thermal energy from the treatment sludge supplied from the second reaction group (102b);- a second connection arrangement, in which the first valves (108a) connected to the reactors (103) of the first reaction group (102a) and the second valves (108b) connected to the reactors (103) of the second reaction group are closed at the same time, while the first valves (108a) connected to the reactors (103) of the second reaction group (102b) and the second valves (108b) connected to the reactors (103) of the first reaction group (102a) are open at the same time, so that in the heat exchange unit (104) said biomass to be treated directed to the second reaction group (102b) receives thermal energyfrom a treatment sludge supplied from the first reaction group (102a).

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