WO2015091492A1

WO2015091492A1 - Method for roasting a carbonaceous feedstock comprising an optimised drying step

Info

Publication number: WO2015091492A1
Application number: PCT/EP2014/077975
Authority: WO
Inventors: Jeremy Gazarian; Laurent Bournay; Raphaël LEMAIRE; Norbert Ullrich
Original assignee: Axens; Bionext; Commissariat A L'energie Atomique Et Aux Energies Alternatives; IFP Energies Nouvelles; Sofiproteol; Thyssenkrupp Uhde Gmbh; Total Raffinage Chimie
Priority date: 2013-12-19
Filing date: 2014-12-16
Publication date: 2015-06-25
Also published as: FR3015513B1; FR3015513A1

Abstract

The present invention concerns a method for roasting a carbonaceous feedstock comprising at least one step of drying the carbonaceous feedstock and one step of roasting the dried carbonaceous feedstock, producing roasting gases and roasted biomass, in which at least a portion of the roasting gases originating in the roasting step is sent to a combustion step producing a combustion gas at a temperature greater than or equal to 700°C, at least a portion of which is recycled in the drying step, mixed with at least a gaseous effluent, the temperature of the gaseous mixture recycled in the drying step being between 200 and 900°C.

Description

METHOD FOR TORREFACTING A CARBON CHARGE COMPRISING A STEP

OPTIMIZED DRYING

FIELD OF THE INVENTION

The present invention relates to a method of roasting a biomass feedstock.

The potential value of roasted wood has long been known. It was used in the early twentieth century in the field of gas generators. These old processes did not make it possible to obtain torrefied wood under advantageous and reproducible economic conditions.

Recently, due to global warming and developments on non-polluting renewable energies, important research is taking place to valorize biomass. For example, thermochemical treatment processes have been developed to use biomass as a source of energy, such as for example biofuel production, power generation in coal or petroleum coke plants, heat generation. by means of steam boilers .... In the context of electricity production in coal-fired power plants, the roasting process makes it possible to substantially increase the heating value of the biomass and to modify some of its properties, in particular the ability of this material to be crushed into fines. particles easily and economically but also storage stability thanks to the hydrophobic character conferred by the roasting. In this sense, the roasted biomass has characteristics close to those of coal and petroleum coke, which allows the operator of this type of plant to mix this feedstock with the fossil fuel initially used without any modification of the process. investment in specific equipment such as a shredder or infrastructure such as the stockyard. As part of biofuel production, the biomass is roasted and crushed and then undergoes a gasification step to produce a synthesis gas. The preferred gasification technology is a high temperature and high pressure driven flow reactor, to ensure very high carbon conversion and avoid expensive recompression steps of the synthesis gas. The preferred system for injecting solid feed into the gasifier is by pneumatic transport of a finely divided feed by means of a carrier gas. The synthesis gas after thorough purification then allows to recompose a set of hydrocarbon cuts, including a gasoline cut, a diesel cut and a kerosene cut using Fischer-Tropsch synthesis. These cuts, and in particular the diesel cut, must undergo a hydrotreating step to meet the fuel specifications.

Roasting is a method of heat treating the biomass at temperatures between 200 ° C and 300 ° C in the absence of oxygen. In this temperature range, reactions take place and modify the chemical and physical properties of the biomass. Under these operating conditions, biomass begins to undergo extensive drying followed by partial destructuring of part of its lignocellulosic material. This transformation is accompanied by a loss of its mechanical strength and a decrease in its elasticity. It becomes easier to grind. Its calorific value is also increased due to an increase in the amount of carbon per unit mass. The products of the roasting operation are a solid, called roasted biomass and roasting gases. The roasted biomass is milled to obtain particles of desired particle size for subsequent gasification. The entire biofuel production chain (roasting, grinding, gasification, purification, Fischer-Trospch synthesis and hydrotreating) must have a high mass yield in order to maximize the production of biofuels.

If the biomass is heated below 150 ° C, the water in the biomass evaporates and the biomass dehydrates, but the degradation reactions do not take place. The biomass is then simply dried and not roasted. However, drying alone does not make it possible to obtain the properties under consideration (loss of mechanical strength, reduction in elasticity) leading to an easy grinding operation and producing a powder suitable for the gasification process. Similarly, if the dried biomass is heated to a temperature above 300 ° C, the reactions become exothermic. The temperature increases, leading to an acceleration of the kinetics of the chemical reactions. This phenomenon of runaway reactions leads to a very strong loss of mass of the solid fraction. However, this loss of mass reduces the material yield of the complete chain for the production of synthetic biofuels. Therefore, in a roasting process for the use of biomass to produce biofuels, it is particularly important to constantly monitor the temperature to avoid drying phenomena alone or thermal runaway phenomena. Moreover, before being roasted, biomass will be most often divided into solid particles whose characteristic dimension is of the order of one centimeter by a primary grinding operation. One of the key points of the roasting step will therefore be to ensure thermal transfer conditions that ensure homogeneous treatment of the charge not only between the particles but also within a particle.

The roasting of the biomass comprising a drying step and a roasting step requires a high heat input to first raise the temperature of the biomass to an appropriate level for the drying phase, then remove the water present. in the biomass, finally raise and maintain the temperature of the dry biomass at a level adequate for the roasting operation. This heat input is generally done by contact of the biomass with a hot gas flow.

Roasting gases, fuels, are usually burned to provide some of the heat needed for the roasting process. This thermal integration of the process is particularly important because, whatever the technology chosen to implement the roasting operation, the way in which the energy provided by the roasting gases is used directly influences the energy efficiency of the process as a whole. and therefore its economic and environmental performance.

PRIOR ART

The patent application US2010 / 0083530 (Wyssmont) describes a device and a method for roasting a lignocellulosic material containing water, said method using a single gas circulation loop for the drying and roasting operations. The raw biomass is introduced at the top of the oven and flows gravity towards the bottom of the oven. The drying operation is carried out in the first section of the oven. The temperature of this zone at the top of the oven is between 200 ° C. and 260 ° C. Once the biomass is dried, the roasting operation takes place on the lower trays and the roasting gases and the drying water are extracted at the top of the oven. The temperature of the zone of the furnace in which the roasting operation is carried out is typically between 200 ° C and 300 ° C and preferably between 260 ° C and 280 ° C. Part of the gases extracted at the top of the furnace is recycled to the bottom of the furnace after reheating of the latter by passing through a heat exchanger. The other part of said gases is first sent to an area where the drying water is condensed. The gases leaving this section are then sent to a zone of combustion for their recovery and generating combustion gases or hot fumes. The fumes then pass into the heat exchanger thus allowing the part of the roasting gases recycled to be recycled both at the top and at the bottom of the oven, in the drying and roasting zones. The energy supplied to the furnace therefore comes from the combustion of these roasting gases. The roasted biomass is extracted at the bottom of the oven.

In this process, the combustion gases or hot fumes generated by the combustion of a portion of the roasting gases are only used to heat the other part of the roasting gas recycled in the furnace, said combustion gases or hot flue gases. being neither recycled at the top of the oven in the drying zone, nor at the bottom of the oven, in the roasting zone.

The patent application US2013 / 228443 proposes to implement a thermochemical treatment of biomass in a vertical multi-stage furnace having several independent chambers with a controlled environment in terms of temperature and pressure. Each chamber can be connected to a control zone containing at least one combustion zone, a valve and a pump, the control zone being connected to an inlet and a gas outlet of each chamber. The control zone or zones therefore make it possible to supply the energy required within each chamber by supplying a hot gas generated in the control zone via the combustion zone, and reintroduced at the entrance of the chamber or chambers to a temperature ranging from 40 to 370 ° C. FIG. 4 more particularly describes a multi-stage oven having 5 independent chambers, each of the chambers being connected to a control unit of its own. Each control unit has the above characteristics. The biomass is introduced into the first chamber operating under aerobic atmosphere to be dried by a flow of heated gas in the control zone connected to said chamber and introduced into said chamber at a temperature between 94 and 204 ° C. The dried biomass is then transferred from one chamber to the other via a gastight valve with respect to the gas phase and is heated to higher and higher temperatures favoring the roasting operation as and when required. as it is transferred to the different chambers operating under anaerobic atmosphere. In this configuration each chamber within the furnace is independent on the gas phase. The patent application WO05 / 056723 (ECN) describes a process for the production of solids from raw material consisting of biomass and preferably lignocellulosic biomass in which the biomass whose moisture is generally between 30% and 60% by weight is sent in a drying step so as to reduce its moisture to contents of less than 15% by weight. The dry biomass is then sent to a roasting zone from which are extracted gases called roasting gas and a solid called roasted biomass which is then cooled and which can be packaged in a pelletizing step before transport and / or storage. Part of the roasting gases emitted during the roasting operation is sent to a heat exchanger after a step intended to increase their pressure. The other part of the roasting gases is recovered in a combustion chamber so as to generate combustion gases or hot fumes which are cooled by passing through a heat exchanger before being recycled in the drying stage so as to bring the required energy. The part of the roasting gases compressed at the end of the step intended to increase their pressure is then sent into the exchanger to be heated by heat exchange with the hot flue gases from the combustion chamber. The hot roasting gases leaving the exchanger are then recycled to the roasting zone to provide the thermal energy required for this operation.

In this case, the hot fumes generated by the combustion of a portion of the roasting gases are used to heat the other part of the roasting gases before their recycling in the roasting zone and the fumes, cooled, are then recycled, alone in the drying step.

The patent application US2012 / 137538 (Polysius) describes a device and a method for drying and roasting a material comprising carbon and preferably biomass in a multi-stage oven comprising a drying zone and a roasting zone. said zones having two separate gas circulation loops. The biomass is sent to a drying zone located in the upper section of the furnace in order to eliminate almost all the water it contains, the dried biomass is then sent to the roasting zone from which are extracted gases called roasting gas and roasted biomass. The drying gases containing mainly water vapor, withdrawn from the drying zone, circulate in an independent circulation loop and are sent to a heat exchanger in which they are heated to a temperature between 150 and 300 ° C before being recycled in said drying zone to provide the thermal energy necessary for this operation.

The roasting gases withdrawn from the roasting zone are sent into a combustion chamber so as to generate combustion gases or hot fumes which are cooled to a temperature greater than 300 ° C. by passing through said heat exchanger by heat exchange. with the drying gases circulating in the drying loop, before being partly recycled in the roasting zone so as to provide the required energy. The other part of the fumes cooled to a temperature above 300 ° C is recycled to the drying zone in admixture with the drying gases. The heat exchanger thus allows the heating of the recycle gas of the drying loop and the cooling of the gas of the roasting loop.

The energy required within the drying zone is therefore provided on the one hand by the recycle of the drying gases containing predominantly water vapor after reheating by passage through a heat exchanger, and on the other hand by a part of the combustion gases cooled after passage through said exchanger, originating from the roasting gas circulation loop, the mixture of the recycled gases having a temperature of between 150 ° C. and 300 ° C.

The combustion gases or fumes generated by the combustion of the roasting gases are thus cooled by passing through an exchanger before being recycled as a mixture with the drying gases heated in the drying zone.

All the schemes described in the prior art propose to send to the drying step:

or gases heated by exchange with the flue gases from a roasting gas combustion stage or any other heating means known to those skilled in the art

- Or to send fumes from the combustion of roasting gases and / or other fuels but having previously undergone at least one cooling step.

The temperature of the gases sent to the drying zone is never greater than a temperature of the order of 400 ° C. Processes according to the prior art are always forced to have a drying zone whose conditions are not optimal if we consider a sequence: drying zone and roasting zone. In the present invention, the Applicant has implemented a novel process for roasting a carbonaceous feed and preferably the biomass comprising a step of drying said feedstock and a step of roasting the dried carbonaceous feed producing a fuel gas and a feedstock. roasted carbonaceous feed and preferably roasted biomass, to increase the efficiency of the drying step by a significant increase in the temperature of the gases sent within said drying step, and in particular by a better valuation of the thermal level of the fumes generated by the combustion of at least a portion of the combustible gases resulting from roasting, possibly with a supplementary fuel gas stream. In particular, the method according to the invention proposes to send, within the drying stage, at least a portion of the combustion gases or hot fumes originating from the combustion of a portion of the roasting gases, without an intermediate cooling step. said combustion gases or fumes, mixed with at least one gaseous effluent, the temperature of the gaseous mixture being between 200 and 900 ° C. The contacting of the mixture comprising fumes, uncooled, at a very high temperature, in particular at least 700 ° C and at least one other gaseous effluent, said mixture having a temperature of between 200 and 900 ° C, with the charge carbonaceous and preferably biomass whose water content is generally between 5% and 70% by weight makes it possible to increase the efficiency of the drying step in a very significant manner.

An advantage of the present invention is therefore to reduce the contact area between the carbonaceous feedstock and preferably the biomass and the hot gas sent in the drying step, which thus makes it possible to reduce the flow of drying gas by a factor at least 1, 1.

Another advantage of the present invention is to increase the charge rate by a factor of at least 1.1.

Another advantage of the present invention is to allow the treatment of a filler having a moisture content greater than 0.5 wt%. In the rest of the text, it is important to note that the drying step in the sense of the present invention is defined as a step in which the carbonaceous feedstock and preferably the biomass is injected at a temperature close to room temperature and in which it is heated to its wet temperature. When this temperature is reached, the water contained in said solid charge evaporates and during this process, the solid temperature remains constant and equal to the wet temperature.

By wet temperature is meant the constant temperature reached by the liquid film present on the surface of the solid to be dried when, during drying, a stationary equilibrium is reached between the heat flux coming from the hotter gas phase and the heat flux. latent heat of evaporation of water. The concept of wet temperature is also described in the Handbook of industrial drying. OBJECT OF THE INVENTION

The present invention relates to a process for roasting a carbonaceous feedstock comprising at least one step of drying said carbonaceous feedstock and a step of roasting said dried carbonaceous feedstock producing roasting gases and roasted biomass, wherein at least one a portion of the roasting gases from the roasting stage is sent to a combustion stage producing combustion gases at a temperature of at least 700 ° C., at least a portion of which is recycled, without an intermediate cooling stage, in the drying step, mixed with at least one gaseous effluent, the temperature of the gaseous mixture recycled in the drying step being between 200 and 900 ° C.

An advantage of the present invention is therefore to provide a method for treating, in a given installation, a higher hourly rate of carbonaceous load in the drying zone. Another advantage of the present invention is to provide a method for treating, in a given installation, a load having an increased moisture content without changing the hourly flow rate of said load. Another advantage of the present invention is to provide a method having a minimized drying gas flow rate without changing the quality or hourly flow rate of the treated feedstock. The size of the drying loop can thus be reduced and thus be less bulky and less expensive.

DETAILED DESCRIPTION OF THE INVENTION

The carbonaceous feed used in the roasting process according to the present invention is preferably a biomass feedstock.

The biomass feedstock treated in the roasting process according to the invention may advantageously vary according to its origin. It can be wood or wood by-products, such as waste produced by logging (logging remnants), sawmills, wood-processing industries. It can also come from industry by-products such as sludge or agro-food waste. Biomass can also come from traditional agriculture and consist of residues such as straw, coppice, bagasse, as well as dedicated energy crops (miscanthus, short rotation coppice ...). Finally, organic waste, such as urban waste including sewage sludge, household waste can also be the burden.

Preferably, the biomass feedstock used in the present invention is lignocellulosic biomass or cellulose and preferably lignocellulosic biomass.

The lignocellulosic biomass essentially contains three natural constituents present in variable quantities according to its origin: cellulose, hemicellulose and lignin.

The lignocellulosic biomass feedstock is preferably used in its raw form, that is to say in its entirety of these three constituents cellulose, hemicellulose and lignin.

Preferably, the lignocellulosic biomass feedstock comprises a water content of between 5 and 70% by weight relative to the total mass of the biomass.

According to the invention, said carbonaceous feed is sent in a roasting process comprising at least one drying step within the meaning of the invention of said feedstock and a step of roasting the dried carbonaceous feed producing roasting gases and the feedstock. roasted carbon. Said drying step in the sense of the invention and said roasting step comprise respectively at least one drying zone and at least one roasting zone which may advantageously be a multi-stage oven in which the carbonaceous feedstock and preferably the biomass is introduced and flows gravitationally from an upper tray to the lower trays or other apparatus to ensure a sufficient residence time of the biomass load in this area.

The biomass feedstock comprising a water content advantageously between 5 and 70% by weight is thus introduced into said drying step so as to reduce its water content to contents generally less than 10% by weight and preferably less than 5%. weight. Said filler is introduced into the drying step at a temperature advantageously between 10 and 30 ° C.

During the drying step, gases called drying gases are generated. The drying gases mainly contain water vapor from the vaporization of the water contained in the biomass. In particular, said drying gases mainly comprise water, nitrogen, and CO ₂ and also contain minor compounds such as oxygen, and volatile compounds.

According to the invention, said drying step in the sense of the invention is carried out using a gaseous mixture entering said drying step at a temperature of between 200 and 900 ° C., preferably between 300 and 900 ° C. ° C, preferably between 350 ° C and 900 ° C, very preferably between 370 and 900 ° C, more preferably between 400 and 900 ° C and even more preferably between 450 and 900 ° C.

Preferably, the residence time of the carbonaceous feed and preferably of the solid biomass introduced into said drying step is between 5 minutes and 3 hours, preferably between 5 minutes and 2 hours, preferably between 5 minutes and 1 hour. hour and very preferably between 5 minutes and 30 minutes.

Preferably, said drying step is carried out at an absolute pressure of between 0.05 and 0.2 MPa and preferably between 0.08 and 0.15 MPa. The carbonaceous feed and preferably the biomass introduced into the drying step at a temperature advantageously between 10 and 30 ° C. is therefore brought into contact with a gaseous mixture having a temperature of between 200 ° and 900 ° C.

The injection of the gaseous mixture at high temperature into contact with the carbon charge whose temperature is close to ambient temperature makes it possible to increase the heat transfer which is proportional to the temperature gradient between the gas phase and the solid carbonaceous charge. This energy supply therefore increases in the first time the heating efficiency of the solid carbonaceous feedstock until it reaches its equilibrium temperature called the wet temperature. When this balance is reached, the water contained in the solid begins to evaporate and during this process, the solid temperature remains constant and equal to the wet temperature and the energy provided by the gas mixture allows evaporation of free water. The drying rate is therefore directly related to the contribution of this energy from the gas phase which is proportional to the temperature gradient between the gas phase and the solid phase. Injecting said gas mixture between 200 ° C. and 900 ° C. therefore makes it possible to increase the efficiency of this process all the more since the temperature of the solid remains constant. At the end of said drying step defined according to the invention and beyond this critical water content, the drying process slows down because the diffusion of water through the solid becomes limiting, the temperature begins to increase and the Drying speed slows down as residual water is removed.

Thus, the energy required at said drying step in the sense of the invention is provided by injecting a very hot gaseous mixture at a temperature between 200 and 900 ° C and consisting of a part of combustion gases or fumes generated by the combustion of a portion of the roasting gases alone or in mixture with other fuels and a gaseous effluent preferably heated by passage through at least one heat exchanger.

The contacting of the fumes that have not undergone an intermediate cooling step and therefore at a very high temperature and in particular at least 700 ° C and at least one other gaseous effluent, the mixture having a temperature between 200 and 900 ° C, with the carbonaceous charge makes it possible to increase the efficiency of the drying zone in a very significant manner and in particular to intensify the heat transfer and to increase the drying speed. The increase of the effectiveness of said drying step in the sense of the invention can be declined in different ways. Thus, for example all other things being equal, the introduction of a mixture of hot gases in the temperature range claimed in said drying step makes it possible, for example, to reduce the flow rates of the recycled dry gases by at least one factor. equal to 1, 1, which reduces the size of the pipes and compression steps related to the circulation of drying gas.

Likewise, other things being equal, the introduction of a mixture of hot gases in the temperature range claimed in the drying step also makes it possible to increase the amount of dried carbonaceous filler per unit of time by a factor. at least 1, 1. Thus the method according to the invention is capable of processing a larger charge rate.

In the case where a supplement of combustible gas is necessary for the combustion chamber to provide the energy required for the roasting and drying stages, all other things being equal, the introduction of a mixture of hot gases in the range of The temperature claimed in the drying step within the meaning of the invention also makes it possible to reduce the use of the addition of expensive combustible gases by a factor of at least 1, 1 by improving the use of thermal energy. The dried carbonaceous feed and preferably the dried biomass is then introduced into a roasting step advantageously comprising at least one roasting zone and producing roasting gases and roasted biomass.

Preferably, said roasting step is carried out at a temperature of the incoming gas phase of between 200 and 450 ° C, preferably between 250 ° C and 400 ° C and preferably between 270 and 350 ° C. The residence time of the dried biomass in said roasting step is advantageously between 5 and 600 minutes and preferably between 10 and 180 min and even more preferably between 10 and 90 minutes.

During the roasting process, combustible gases, called roasting gases, and a solid called roasted biomass in the case where the carbonaceous feedstock is a biomass feedstock are generated. At the end of the roasting step, the roasted carbonaceous feedstock and preferably the roasted biomass feedstock is advantageously sent to a cooling step before storage or transport to another operation. The roasting gases are rich in combustible material and preferably comprise carbon monoxide (CO), carbon dioxide (CO ₂ ), acetic acid, methanol, furfural, nitrogen, and also oxygen.

According to the invention, at least a part of said combustible roasting gases are recovered and sent to a combustion stage producing flue gases or fumes, having a temperature of at least 700 ° C., preferably greater than 750 ° C. preferably above 800 ° C, very preferably above 850 ° C and more preferably above 900 ° C. Preferably, said roasting gases are sent to a combustion step, alone or in admixture with at least one other fuel.

Thus, according to the amount of said roasting gas generated during the roasting operation and according to their heating value, and therefore according to the severity of the roasting operation, a fuel booster is introduced into said combustion step in order to cover the roasting operation. energy needs of the system and in particular to provide the required energy for the drying zone and the roasting zone.

Preferably, the fuel is natural gas.

According to the invention, at least a portion of said combustion gases or fumes is recycled, without an intermediate cooling step, in the drying step, mixed with at least one gaseous effluent, the temperature of the gaseous mixture recycled in the drying step being between 200 and 900 ° C, preferably between 300 and 900 ° C, preferably between 350 ° C and 900 ° C, very preferably between 370 and 900 ° C, more preferably between 400 and 900 ° C and 900 ° C and even more preferably between 450 and 900 ° C.

Preferably, the part of the combustion gases or fumes recycled in the drying step is sent in said drying step without prior passage in a heat exchanger. Similarly, the gaseous mixture sent in said drying step comprising at least a portion of said combustion gases or fumes and at least one gaseous effluent does not undergo any cooling step, preferably by passing through a heat exchanger before recycling in said drying step.

The gaseous effluent injected mixed with at least a part of said combustion gases or fumes in the drying step advantageously has a temperature between 120 and 550 ° C and preferably between 120 and 400 ° C so that the injected mixture in the drying step has a temperature between 200 and 900 ° C.

The gaseous effluent injected mixed with at least a part of said combustion gases or fumes in the drying step advantageously consists of:

or at least a portion of the drying gases from the drying step, said gases having previously been heated preferably by passing through a heat exchanger,

- Or part of the roasting gas not sent in a combustion step, said gas having previously been heated preferably by passing through a heat exchanger. In both cases, said gaseous effluent is preheated in a heat exchanger preferably by heat exchange with at least a portion of the combustion gases or fumes not recycled in said drying step.

Said drying step and said roasting step can advantageously be implemented in the same enclosure or in two different enclosures.

In the case where said drying step and said roasting step are carried out in the same chamber, said drying and roasting steps and preferably said drying and roasting zones may optionally be independent of one another or not.

In a first embodiment, said drying and roasting steps and preferably said drying and roasting zones are independent. Thus, operating conditions specific to each step can be applied independently.

In this case, a means for transferring the biomass feed such as for example a rotary valve is advantageously implemented between said drying and roasting steps and preferably between said drying and roasting zones.

According to this first embodiment, said drying and roasting steps comprise two separate gas circulation loops.

Drying gases containing mainly water vapor are advantageously extracted from the drying step and circulate in a gas circulation loop called drying gas loop.

The drying gases are sent to a heat exchanger in which they are heated by heat exchange with at least a portion of the combustion gases generated by the combustion of at least a part and possibly all of the roasting gases extracted from the roasting step.

The roasting gases are advantageously extracted from the roasting step and circulate in a gas circulation loop called roasting gas loop.

At least a portion and possibly all of the roasting gases are sent to a combustion chamber with possibly a supplement of fuel and the combustion gases or hot fumes generated are partly sent into a heat exchanger to be cooled before their recycling in the roasting step and the other part is sent directly without a previous cooling step by passing through a heat exchanger in the drying step, mixed with the heated drying gases circulating in the drying loop.

In the first embodiment, the gaseous effluent injected mixed with at least a portion of said combustion gases or fumes in the drying step consists of at least a portion of the drying gases, said gases having previously been heated. preferably by passing through a heat exchanger. In order to maintain a constant pressure within this drying gas loop, a part of the gases is advantageously sent to an outlet.

Likewise, in order to maintain a constant pressure within this roasting gas loop, part of the gases may possibly be sent to an outlet. Thus, in this case, the energy required at the roasting step is provided by the recycling of a part of the combustion gases or fumes generated by the combustion of a portion of the roasting gases after their cooling by exchange. of heat with the drying gases in a heat exchanger. In order to maintain a constant pressure within this roasting gas loop, part of the fumes generated is advantageously sent to an outlet.

In a second embodiment, said drying and roasting steps and preferably said drying and roasting zones are not independent. In this case, the biomass charge is introduced into a device and preferably a multistage furnace, comprising one or more trays through which the charge flows from an upper tray to the lower trays by gravity. The step of drying the biomass feed preferably takes place in the upper part of the device and the roasting step takes place in the lower part.

In this case, the drying and roasting zones are differentiated by the temperature of the biomass. In the drying zone, there is still water to evaporate, the heat supplied by the hot gases is used to vaporize this water and the temperature of the solid biomass particles remains at the wet temperature. In the roasting zone, the heat provided by the hot gases makes it possible to increase the temperature of the biomass to levels allowing the destructuring reactions that are suitable for roasting and to maintain this temperature for the necessary time.

According to this second embodiment, said drying and roasting steps comprise a single gas circulation loop called a roasting gas loop. The drying gases mainly comprising steam and the roasting gases generated are advantageously collected and extracted simultaneously at the top of the device.

In this case, the roasting gas is also called this mixture of gas extracted at the end of the drying and roasting steps operating within the same device. However, they have a different composition of the roasting gases extracted from the roasting step of the first embodiment according to the invention because they consist both of the gases generated. during roasting reactions and drying gases mainly comprising water vapor.

At least a part of said roasting gas may optionally be sent to a condenser making it possible to extract at least a portion and preferably all of the water vapor in the form of liquid water from the roasting gases. In order to maintain a constant pressure within the roasting gas loop, a part of the roasting gas optionally freed of at least a portion and preferably all of the water vapor is then advantageously sent to a combustion stage. for generating combustion gases or fumes having a temperature of at least 700 ° C, at least a portion of which is recycled in the drying step, without prior cooling step by passage through an exchanger, preferably to the upper trays of the device implementing the method according to the invention. The other part of the roasting gas that is not sent into the combustion stage is advantageously heated by heat exchange with the other part of the non-recycled fumes in the drying step, preferably by passing through a heat exchanger, and then recycled both in the drying step mixed with at least a portion of the fumes and in the roasting step.

In the second embodiment, the gaseous effluent injected mixed with at least a part of said combustion gases or fumes in the drying step consists of the part of the roasting gases not sent in a combustion step, said gases having previously been heated preferably by passing through a heat exchanger.

In order to maintain a constant pressure in the drying zone, a portion of the generated drying gases comprising the drying water and hot gases from the roasting gas loop constituting the recycled mixture is preferably sent to an outlet. DESCRIPTION OF THE FIGURES

FIG. 1 represents the first embodiment according to the invention in which the said drying steps within the meaning of the invention and of roasting are independent and whose separation is represented in the figure by line 20.

According to this first embodiment, said drying and roasting steps comprise two separate gas circulation loops. The drying gas loop is represented by the flows (4), (5) and (7) and the roasting gas loop is represented by the streams (8), (10), (1 1), (12) and ( 13).

The raw biomass load (1) containing a moisture generally of between 5 and 70% by weight enters a drying zone (A) in order to reduce its water content to levels generally less than 10% by weight and preferably less than 5% weight. The dried biomass (2) is sent to a roasting zone (B) independent of the drying zone.

The drying gases generated and represented by the flow (5) in the drying zone (A) are extracted and sent into a heat exchanger (C) to be heated by heat exchange and are extracted from the exchanger and represented by the flow (7). The heated gases are then mixed with a portion of the combustion fumes (13) and recycled to the drying zone (A). The mixture is represented by the flow (4).

The roasting gases extracted from the roasting zone (B) and represented by the flow (8) are sent to a combustion chamber (D) with a fuel booster (9) to generate combustion gases or fumes (10). having a temperature above 700 ° C. At least a portion of the combustion gases represented by the stream (13) is recycled without a pre-cooling step in the drying zone (A) mixed with the heated drying gases (7). The recycled gas mixture in the drying zone is represented by the flow (4). Thus, the temperature of the drying zone is controlled by the direct injection of the combustion gases or fumes (13) from the combustion chamber mixed with the heated drying gases (7). In order to maintain a constant pressure in the drying zone, part of the drying gases generated during the drying process and part of the combustion gases or fumes (13) from the roasting gas loop can be sent to an outlet ( 6). The roasted biomass feedstock (3) is removed from the roasting step (B). FIG. 2 represents the second embodiment according to the invention in which the said drying and roasting steps are not independent. Said drying and roasting steps comprise a single gas circulation loop called a roasting gas loop.

The raw biomass (1) enters a device in which it will be dried (zone (A)) and roasted (zone (B)). During the drying and roasting process, drying gases containing mainly water vapor and roasting gases are generated and collected simultaneously. They are represented by the flow (4). At least a part of these gases (5) is sent into a condenser (E) so as mainly to eliminate the water represented by the flow (7). In order to maintain a constant pressure within the roasting gas loop, part of the roasting gas (8) from the condenser is then sent to a combustion chamber (F) with a booster fuel (9) to generate combustion gases or fumes (10) having a temperature of at least 700 ° C. At least a portion of the flue gases or fumes represented by the flow (12) is mixed, without prior cooling step by passage in an exchanger, with a portion of the heated roasting gas (1 1) by passage through the exchange ( D) to the zone (A) corresponding to the drying zone, at the head of the device implementing the method according to the invention.

The other part of the roasting gas (6) not sent into the combustion stage is heated by heat exchange with the other part of the non-recycled fumes (14) in the drying step, by passing through a heat exchanger. heat (D), then recycled (1 1) both in the drying zone (A) (13) mixed with at least a portion of the uncooled fumes (12) and in the roasting zone (B).

The fumes cooled in the heat exchanger (D) are extracted from the exchanger is represented by the flow (15).

The examples illustrate the invention without limiting its scope. EXAMPLES

Example 1: comparative

An example not in accordance with the invention and in particular according to the method described in US patent 2012/0137538 is presented below: the example is not in accordance with the invention in that the combustion gases or fumes generated by burning the roasting gases are cooled by passing through an exchanger before being recycled in admixture with the drying gases heated in the drying zone.

A wood wafer flow rate comprising 30% moisture is sent to a drying step and then to a roasting step. Said steps comprising two separate gas circulation loops. The wet biomass is sent to a drying stage in order to eliminate almost all the water it contains, the dried biomass is then sent to the roasting zone from which gases called roasting gas are extracted and the roasted biomass.

Table 1 below gives an example of the flow rate and moisture content of the biomass through the different stages of drying and roasting.

Table 1

The gases withdrawn from the drying stage and containing mainly water vapor, circulate in an independent circulation loop and are sent to a heat exchanger in which they are heated and then mixed with a cooled flue gas flow rate. The mixture is then recycled in said drying step to provide the thermal energy necessary for this operation. In order to maintain a constant pressure in the drying gas loop, a portion of the gases leaving the drying step is sent to an outlet.

The gases withdrawn from the roasting zone are recovered in a combustion chamber so as to generate combustion gases or hot fumes. The fumes are cooled by passing through a heat exchanger by heat exchange with the drying gases circulating in the drying loop, before being partly recycled in the roasting zone so as to provide the energy required.

The following table 2 presents the properties of the gases of the drying loop:

Table 2

The energy required within the drying zone is therefore provided on the one hand by the recycling of drying gases containing mainly water vapor after reheating by passage through a heat exchanger, and on the other hand by a part of the combustion gases cooled after passage through said exchanger, originating from the circulation loop of the roasting gases.

The combustion gases or fumes generated by the combustion of the roasting gases are thus cooled by passing through an exchanger before being recycled as a mixture with the drying gases heated in the drying zone. The mixture of the flow of part of the cooled fumes and the recycling of the drying gases makes it possible to reach the temperature of 250 ° C. at the inlet of the drying step.

The energy required for the drying operation is about 2150 kW and is provided by the drying loop whose temperature goes from 250 ° C to 1 10 ° C.

In this example, the residence time required for this drying step is 30 minutes. The required drying gas flow rate is 43,000 Nm ³ / h.

Example 2 according to the invention

The method according to the invention is implemented with reference to FIG. The same flow rate of wood chips (1) is sent in a drying step (A) in order to eliminate almost all the water it contains, the dried biomass (2) is then sent to the zone of roasting not shown in Figure 3.

Table 3 below gives the properties of the biomass through the different stages of drying and roasting.

Table 3

The drying gases (5) containing mainly water vapor, withdrawn from the drying zone (A) circulate in an independent circulation loop and are sent into a heat exchanger (C). These gases are then heated by cooling a gas stream (1 1) and (12) before being recycled (7) in said drying step (A) to provide the thermal energy required for this operation. Fumes (13) at high temperature from the combustion of roasting gases from the roasting zone not shown in Figure 3 and a makeup of natural gas are mixed with the recycle gas (7) exiting the exchanger heat source (C) producing the gas mixture (4) which is recycled at high temperature to the drying zone (A). In order to maintain a constant pressure in the drying gas loop, a portion of the gases leaving the drying step is sent to an outlet (6).

Table 4 summarizes the characteristics of the different flows.

Table 4

The mixture of hot flue gas at a temperature of 900 ° C. (13) and heated drying gases (7) makes it possible to reach the temperature of 600 ° C. at the inlet of the drying zone (4). The energy required for the drying operation is about 2150 kW and is provided by the drying loop whose temperature goes from 600 ° C to 1 10 ° C.

In this example, the residence time required for this drying step is 15 min, hence a decrease of a factor 2 on the residence time compared to the previous example. The required drying gas flow rate is 13,000 Nm ³ / h resulting in a decrease of about 3 by factor.

The invention thus makes it possible to reduce the residence time required within the drying step within the meaning of the invention, allowing a reduction in the investment on the drying equipment as well as on all the gas lines and drying loop equipment due to the reduced flow required for the operation.

The invention also reduces by a factor 3 the electrical consumption of equipment for ensuring the flow of gas in the drying loop.

Claims

1. Process for roasting a carbonaceous feedstock comprising at least one step of drying the biomass and a step of roasting the dried carbonaceous feedstock producing roasting gases and a roasted carbonaceous feedstock, wherein at least a portion of the roasting gases from the roasting step is sent to a combustion step producing combustion gases at a temperature of at least 700 ° C, at least a portion of which is recycled without intermediate cooling step, in the drying step, mixing with at least one gaseous effluent, the temperature of the gaseous mixture recycled in the drying step being between 200 and 900 ° C.

2. Process according to claim 1, in which the carbonaceous feedstock is lignocellulosic biomass or cellulose.

3. Method according to one of claims 1 or 2 wherein the temperature of the gaseous mixture recycled in the drying step is between 370 and 900 ° C.

4. The method of claim 3 wherein the temperature of the gaseous mixture recycled in the drying step is between 400 and 900 ° C.

5. Method according to one of claims 1 to 4 wherein the temperature of the gaseous mixture recycled in the drying step is between 450 and 900 ° C.

6. Method according to one of claims 1 to 5 wherein the residence time of the carbonaceous feedstock introduced into said drying step is between 5 minutes and 3 hours.

7. Method according to one of claims 1 to 6 wherein, said roasting gas is sent to a combustion step, alone or in admixture with at least one other fuel.

8. Method according to one of claims 1 to 7 wherein, the portion of the combustion gases or fumes recycled in the drying step is sent in said drying step without prior passage in a heat exchanger.

9. Method according to one of claims 1 to 8 wherein, the gaseous mixture recycled in said drying step does not undergo any cooling step before recycling in said drying step.

10. Method according to one of claims 1 to 9 wherein said drying step and said roasting step are carried out in the same enclosure or in two different enclosures.

1 1. The method of claim 10 wherein, in the case where said drying step and said roasting step are carried out in the same chamber, said drying and roasting steps are independent of each other.

12. The method of claim 10 wherein, in the case where said drying step and said roasting step are carried out in the same chamber, said drying and roasting steps are not independent of one another .