Steam cracking process comprising a separation step and differential treatment of the obtained particles according to a threshold value

Abstract

The invention relates to the field of solid biofuels obtained by steam cracking. More particularly, the invention relates to a process for the treatment of lignocellulosic biomass by steam cracking in which the obtained powder is treated in order to separate the particles into two categories according to a threshold value, and each category is treated differently.

Description

The present invention relates to the field of solid biofuels obtained by steam cracking. More particularly, the present invention relates to a process for the treatment of lignocellulosic biomass by steam cracking, in which the obtained powder is treated in order to separate the particles into two categories according to a threshold value, each category being treated differently. Field of the invention The production of energy (electrical and thermal) of renewable origin can be achieved from the environment (sun, wind, tide, swell, geothermal, hydraulic) or from biomass. With the exception of biomass, river or barrage hydraulics, and geothermal, these renewable energies are intermittent, unless the energy storage means develop. Only biomass is a primary energy which can be transported to the site of its transformation into heat and / or electricity. However, the biomass is in fact an energy which is not very dense, is variable, and is perishable. The transformation of lignocellulosic biomass (wood, agricultural waste, co-products of agriculture and the agro-industry) into an energy-dense, transportable, and easily storable compound, makes it possible to develop and consolidate a stationary energy industrial sector (biofuel used at a fixed point, at home, in contrast with biofuel oils), and to reduce the environmental impacts (CO2 fossil emission, with a biomass without fertilizers or phytosanitaries). The heat treatment of the biomass by steam cracking allows for this densification of energy, by homogenizing the biomass into a steam-cracked powder according to precise parametric conditions, in particular size grading, temperature, and residence time. The powder is then pelletized in order to facilitate the transport, storage and use thereof. It is thus a question of ensuring that the steam cracking method is capable of ensuring a constant product, i.e. an output powder that is stable in terms of quality, and is capable of being transformed or used downstream of the method, for the desired application. This powder has a calorific value and a composition suitable for being used in combustion, and is able to be pelletized. Today, the best solution for guaranteeing the best result of the steam cracking is to make a regular and controlled supply of biomass of the same quality. However, with a view to increasing the volumes of treatment of biomass, for the purpose of production of solid biofuels, it appears to be of interest to be able to also treat a heterogeneous biomass. The parameters of the steam cracking process are critical, and, in order to facilitate the comparison of different options, a model has been developed that is based on the hypotheses that the kinetics of the process is of the first order and obeys the Arrhenius law, making it possible to develop the ordinate of the reaction (R0): RO =Jt exp [(Tr - Tb) / 14.75] dt Where Tr is the reaction temperature (°C), Tb is the baseline temperature (boiling point of water at atmospheric pressure: 100°C), t is the residence time (min), and 14.75 is the conventional activation energy, supposing that the general method is hydrolytic, and the general conversion is of the first order. The log10 value of the ordinate of the reaction gives the severity factor (or severity) which is used to represent the effects of the steam explosion on the biomass: Severity = log10 (R0) The problem encountered when treating a heterogeneous biomass is that the powder obtained is itself heterogeneous, causing problems of pelletization. Indeed, depending on the nature of the initial biomass, and the conditions applies, the steam cracking will not have the same effect in terms of destructuration of the material. Thus, when the input material is heterogeneous, the severity conditions must be adjusted so as to not totally destructure the least resistant materials, which would adversely affect the pelletization. When the severity conditions are adjusted depending on the less resistant biomass, the steam cracking generates residual particles originating from the more resistant biomasses, which also disrupt the pelletization process. Failure to respect the final quality and the pelletization specifications may be fatal, whether due to a treatment that leaves large particles, or a treatment that is too drastic and degrades the product and its behavior upon pelletization. There is therefore a need to have access to a steam cracking process which is suitable for treatment of a heterogeneous biomass, for the manufacture of black pellets of a controlled quality. The steam-cracking differs from hydrothermal pre-treatment, also referred to as aqueous fractionation, solvolysis, hydrothermolysis, or hydrothermal treatment, in that the latter consists in using water at a high temperature and high pressure in order to promote the disintegration and the separation of the lignocellulosic matrix. This technique is not suitable for the production of black pellets, since the products obtained are largely liquid. The pyrolysis is the chemical decomposition of an organic compound, by intense heating in the absence of oxygen. The compounds obtained following pyrolysis differ, in terms of their characteristics, from those obtained by steam cracking. The steam cracking cannot be likened to a pyrolysis technique, in that it uses steam explosion and is carried out in the presence of oxygen. It is also necessary to distinguish torrefaction processes, which are characterized by a thermochemical treatment of between 100 and 300°C, which makes it possible to modify some of the organic material in order to break the fibers, while eliminating the water. Prior art The Chinese patent CN102 949 969A discloses a system for pyrolysis of solid carbon material or material having an increased viscosity, using a duvet mixer and a catalytic heat transfer fluid heater, and a process for using the reactor system. This process further comprises a solid / solid or solid / liquid separator, for allowing the separation of the solid or liquid pyrolyzed products from the used catalytic heat transfer fluid. The American patent US2012 / 260563A1 describes a process for thermochemical transformation of biomass by hydropyrolysis (conventional pyrolysis liquid reaction, with hydrogen and in the presence of solid catalysts) in order to obtain liquid products. This process comprises in particular inertial separation devices. The patent US2014 / 298716A1 relates to a process for drying and torrefaction of lignocellulosic biomass. The drying and torrefaction facility comprises two circuits. The second circuit comprises a cyclone separator which makes it possible to eliminate all the large volatile particles. Finally, the patent US2016 / 251611A1 discloses a process for growing a microbial organism, comprising the cultivation of the microbial organism in the presence of a hydrolyzed composition obtained from a lignocellulosic feedstock that has undergone a step of steam explosion. The treated lignocellulosic biomass furthermore comprises a step of separation of the fibers depending on a threshold value, such as the size of the fibers. Disadvantages of the prior art The solutions of the prior art are not entirely satisfactory, since they use techniques or use a non-pulverulent product. Indeed, these are techniques of: - pyrolysis leading to obtaining compounds different from those obtained by steam cracking - torrefaction, which is a technology having an acceptable yield (10% to 20% loss), but the cost remains prohibitive, and the technology is not mature. Disclosure of the invention In order to meet this need, the inventors have developed a process for steam cracking, with a given severity factor, characterized in that it comprises a step of treatment of the pulverulent products in order to separate the particles above a threshold value and the particles below said threshold value, and of differentiated treatment of particles of the first category and particles of the second category. Said process makes it possible, on the one hand, to pelletize a homogeneous powder from which resistant particles have been removed, and, on the other hand, to treat the overflow of poorly deteriorated material by a return to the steam cracking tank or another deferred treatment. The invention also relates to a steam cracking facility for implementing this process, comprising a particle separator. Advantages of the invention The invention thus consists in implementing a screen for separating the poorly degraded resistant particles from the powder which can be pelletized, or the powder having too fine a size grading from the powder which can be pelletized. The main advantage of this process is that it makes it possible to produce black pellets of quality, from any type of biomass, and in particular from heterogeneous biomass. Indeed, eliminating the poorly degraded particles from the powder to be pelletized improves the quality of the pellets, which are thus more cohesive, more hydrophobic, and higher-energy. This process thus makes it possible to comply both with the specifications of the overall pelletization process, and of the specification of the final product, in terms of quality and constancy of quality. This process makes it possible to recover the poorly degraded particles and to re-use them. It may be a question of subjecting them a second time to a steam cracking treatment, or of using them for other purposes. It is also possible to eliminate, by the screen, the finest powders which pelletize poorly, and to keep only the powders which are capable of clogging, with the aim of producing pellets. It is possible, for example, to envisage using the finest powders in biotransformation processes. It will be noted that, if the increase in the severity of the treatment could manage to overcome the issue of particles that remain coarse, this would result in degradation of the more labile wood particles, and would also increase the material loss on account of too advanced degradation of molecules in polymer form into volatile compounds (for example the hemicelluloses that are most sensitive to the temperature and to the residence time). There is therefore an advantage in recycling, into the head of the process, the post-steam cracking screen overflow of the powder, rather than increasing the severity, or indeed subjecting them, in isolation or in a manner grouped together, to more suitable severity conditions. DETAILED DESCRIPTION OF THE INVENTION The invention firstly relates to a process for steam cracking of a lignocellulosic biomass, characterized in that it comprises a step of treatment of the pulverulent products in order to separate the particles above a threshold value and the particles below said threshold value, and of differentiated treatment of particles of the first category and particles of the second category. A process of this kind comprises the following steps: - steam cracking of a lignocellulosic biomass - screening the pulverulent products obtained by steam cracking, in order to separate the particles above a threshold value (referred to as the “first category”) and the particles below said threshold value (referred to as the “second category”) - differentiated treatment of the particles of the first category and of the particles of the second category. The severity factor applied during the steam cracking step will be determined depending on the biomass to be treated. It is determined from the value of the Log10 of the ordinate of the reaction. Severity = log10 (R0) This is a characteristic that is inherent to the steam cracking process, which a person skilled in the art knows how to measure and adapt. Typically, the severity factor will be between 4.0 and 4.02, and more precisely between 4.05 and 4.15. This process implements screening which aims to separate the steam cracking particles into two categories, with the aim of differentiated treatment. The threshold value can for example be the mass, the dimension, the cross-sectional area, or the density of the particles, but also their aeraulic behavior, or any other criterion which makes it possible to separate the particles that are too fine, too large, too dense, etc. compared with a threshold value. In a preferred embodiment, one of the two categories of particles is pelletized for the preparation of black pellets. In a particular embodiment, the particles to be pelletized correspond to the category of particles that are above the threshold value. It is a question of eliminating the largest and / or the most dense and / or the most resistant particles. In an alternative embodiment, the particles to be pelletized correspond to the category of particles that are above the threshold value. It is a question of eliminating the particles that are too fine and are not able to be pelletized. Among the other expected qualities such as the water resistance, the mechanical strength, and the high calorific value, in a general manner the black pellets may be qualified by a main constitution (> 80%) of particles of a size smaller than 500 pm, and a few percent of particles larger than a millimeter, or indeed several millimeters. The resistance of the pellets to water can be evaluated by soaking, i.e. the product must not take up much water during soaking, and the qualities of the product, such as its mechanical strength, must not be altered following soaking. The mechanical strength of the pellets is associated with an impact resistance and a mechanical durability. The calorific value of the pellets must be increased with respect to that of the initial biomass. The severity factor of the steam cracking step is fixed according to the nature of the biomass and the type of powder which it is desired to collect. The particles which are eliminated undergo a treatment different from pelletization, and are collected and recycled for example by reintroduction into the steam cracking tank, immediately after separation or in a deferred manner, or by re-use in other processes. There are as many different possibilities for treatment of the eliminated particles as there are particular embodiments of the invention. In a particular embodiment, the particles above the threshold value (resistant, too large and / or too dense, for example) are reintroduced directly into the steam cracking tank, in order to be treated again within the continuous process. In another particular embodiment, the particles above or below the threshold value are collected in order to be recycled in a deferred manner. They can be subsequently reintroduced into the steam cracking tank under different severity conditions, or be reused in other processes. The particles that are too fine to be pelletized can for example be used as a substrate for biotransformation reactions, such as biotechnological processes and green chemistry, etc. The particles that are too large can be re-used by means of local combustion in a boiler, or can be reintegrated into biomaterials. This process thus makes it possible to treat a heterogeneous biomass and to re-use all of this biomass. Said heterogeneous biomass may consist in a mixture of young woods and old woods, a mixture of woods originating from different species, waste wood, etc. The invention secondly relates to a steam cracking facility comprising at least one steam cracking unit, and a separator for separating particles into two categories depending on a threshold value, and a means for the transport of the particles thus separated. The steam cracking facility is a conventional facility with regard to the elements up to the powder collector located at the outlet of the steam cracking tank. The particle separator may be of any type, and depends on the criterion selected for the screen. The screening means may thus be a separation by size selection, such as a rotary screen, a perforated or inclined vibrating table, dynamic densimetric separation such as a ballistic screen, or a centrifugal effect. The means for differential transport of the particles makes it possible to transport the overflow powder to a site of immediate treatment or to a storage site for deferred treatment. A transport means of this kind may be a conveyor, a chute, a screw, a chain conveyor, a pneumatic system, etc. The facility may furthermore comprise items of equipment associated with the treatment of said overflow downstream.

Claims

1. Process for steam cracking of a lignocellulosic biomass, characterized in that it comprises:- a step of steam cracking of a lignocellulosic biomass- a step of screening the pulverulent products obtained at the end of the steam cracking step, in order to separate the particles above a threshold value in a first category and the particles below said threshold value in a second category, in which said first category of particles is removed while said second category is pelletized, and- a step of collection and recycling of the eliminated particles from the first category, in which the said particles are reintroduced into the steam cracking chamber, immediately after separation or in a deferred manner.

2. Process according to claim 1, wherein said threshold value is selected from the mass, the dimension, the cross-sectional area, the density of the particles, the aeraulic behavior of said particles.

3. Process according to claim 1, wherein said eliminated particles are above the threshold value and are re-used in local combustion or integrated into biomaterials.

4. Process according to claim 1, wherein said eliminated particles are below the threshold value and are re-used in biotechnology or green chemistry processes.

5. Process according to any of the preceding claims, wherein said biomass is heterogeneous.