ES2715527A1 - Renewable carbonaceous material and process for its production from biomass (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Renewable carbonaceous material and procedure for its production from biomass. The present invention refers to a renewable carbonaceous material with added value, a process to obtain it by hydrothermal carbonization from lignocellulosic biomass with a quantity of water comprised between 50% and 90% by weight, and its use as a substrate or Additive without phytotoxic effect for the soil in agricultural plantations or as a solid fuel.

Description

DESCRIPTION

Renewable carbonaceous material and process for its production from biomass

The present invention relates to a value-added renewable carbonaceous material, a process for obtaining it by hydrothermal carbonization from lignocellulosic biomass with an amount of water comprised between 50% and 90% by weight, and its use as a substrate or additive without phytotoxic effect for the soil in agricultural plantations or as solid fuel.

BACKGROUND OF THE INVENTION

In 2007 Antonietti and collaborators of the Max-Planck Institute in Potsdam, Germany, published a paper on the conversion of biomass in the presence of water, hydrothermal carbonization (Chem. Mater. 2007, 19, 4205-4212). This article describes the conversion of lignocellulosic biomass with a high content of water, such as oak leaves or orange peel, into a renewable carbonaceous material (with a slight characteristic smoke odor) by an autoclave treatment at a temperature between 180 and 250 ° C.

This process arouses great interest because low value resources of great accessibility can be used, which currently have no application due to their high water content. After the hydrothermal carbonization treatment, the uses of the resulting products as solid fuels have been described (Catal. Today, 2015, 257, 154-159.). It has been demonstrated that during the process of hydrothermal carbonization (HTC) a carrier of energy is produced and its production consumes only 25-35% of the energy contained in the product, to provide the heat and electricity required to carry out the production process (Green Chem., 2016, 18, 1051-1060).

Apart from the use of resources without other alternative use, hydrothermal carbonization can also be considered as a means to mitigate climate change, since carbonizing the humid resources avoids the formation of methane during a possible composting, controlled or uncontrolled. In this context, for To increase the effect of mitigation, it is also desirable to use the carbonaceous product obtained by hydrothermal carbonization or hydrochar for other purposes, which is considered as an alternative to the use as solid fuel. In the latter case, carbon dioxide is rapidly released into the atmosphere.

Another alternative application of the hydrochar is that of additive for the soil for applications in agriculture. It is known that the "Terra Preta de Índio" is an excellent substrate for the growth of plants and contains a high carbon content (Geoderma 2017, 288, 154-165.).

However, the direct application of hydrochar does not have positive effects on the growth of plants. In many cases, inhibitory effects on growth were observed when hydrochar was applied, since it has been found to have a phytotoxic effect when used as a substrate without soil (J. Environ, Manage , 2017, 191, 237-243). This phytotoxic effect can be eliminated or reduced by subsequent treatments, such as a treatment with diluted nitric acid, degradation of harmful substances over time, or that are soluble in water and washed out.

The phytotoxic effect of hydrochar is due to its content in a number of chemical compounds with a toxic effect, and that are almost impossible to identify separately by chemical analysis. In addition, the hydrochar has a low concentration in the usual pollutants such as polyaromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), dioxins or heavy metals and therefore, the phytotoxic effects can not be explained with standard analyzes, which makes this work. The substances that cause this toxic or phytotoxic effect are not yet known, nor is it known if it is one or several (Ecotoxicol, Environ, Saf , 2013, 97, 59-66.).

Apart from a partially harmful properties of hydrochar with respect to its application to the field, hydrochar is considered a valuable product. At the end of a European project the participants concluded that the price of hydrochar may be too high for its application to improve the properties of the field. In addition, they recommend treatments for the elimination of phytotoxic effects. On the one hand, they propose chemical processes such as oxidations and on the other, biological processes such as hydrochar composting. According to the researchers, these treatments are effective to improve the characteristics of the hydrochar for its application in agriculture, but of course, these additional processes would also increase the cost of production of the material, in the same way as washing processes mentioned above.

On the other hand, in patent ES2339320, whose content is incorporated in the present application by reference, the hydrothermal carbonization process is described from an aqueous mixture of biomass, which is carried out during a period of 2 to 12 hours at a temperature between 180 and 225 ° C and a pressure between 10 to 25 bar. As a result of the process, an aqueous mixture of carbonized biomass with porous particles of different sizes is obtained, which is collected from the lower part of the inverted vertical flow reactor in which the hydrothermal carbonization process takes place. In particular, the water content can vary between 80% and 90%. This process produces a renewable carbonaceous material that has phytotoxic properties and a characteristic odor, with a low average fixed carbon content in comparison with other renewable carbonaceous materials (for example, biochar obtained by pyrolysis).

European patent application EP3053929 describes how the lignin-based material is treated by subjecting the lignin separated from the lignocellulosic raw material to the hydrothermal carbonization process at an elevated temperature, where carbonized lignin having a higher carbon content is obtained, and stabilizing the carbonized lignin obtained in an inert atmosphere at a stabilization temperature higher than the temperature of the hydrothermal carbonization process.

In summary, it can be verified that hydrochar obtained by a hydrothermal carbonization process (HTC) at temperatures between 180 and 250 ° C, could be a medium-term fertilizer material for agricultural use and at the same time contribute to the mitigation of climate change by carbon sequestration and by prevention of methane formation. However, hydrochar has phytotoxic properties in the short term and a high price that hinder its application. Washing or composting relieves these effects, but as additional processes, which increases the cost of production.

Therefore, it is necessary to find a process to obtain a carbonaceous product with improved phytotoxic properties and through an energy more sustainable procedure.

DESCRIPTION OF THE INVENTION

The present invention provides a process for obtaining a product carbonaceous material obtained by hydrothermal carbonization (or hydrochar) with improved properties. The process produces a solid with an increased amount of fixed carbon and absence of phytotoxic, which improves its properties as a solid fuel and increases its stability in the soil.

The solid product resulting from the process of the present invention is a hydrochar:

● without phytotoxic effects or with reduced phytotoxic effects,

● less odorous or odorless, and

● Useful as a raw material for value-added materials.

Furthermore, the process of the present invention does not produce additional residues with respect to the traditional hydrothermal carbonization process and has a good economic balance, more favorable because it lowers the energy costs. Surprisingly, a part of the hydrochar obtained by the process of the invention can be used to supply the hydrothermal carbonization plant with energy and convert it into autarkic in terms of energy, and at the same time improve the properties of the hydrochar.

On the other hand, the present invention is clearly a technological advance for the elimination of phytotoxic compounds with respect to the methodologies known up to now, since in none of the known processes it is possible to valorise the compounds energetically because they are obtained in diluted form in a aqueous solution, or freely released to the environment as probably occurs in composting.

Another advantage of the method of the present invention is that the stages of "finishing" and "energy recovery" can be easily integrated into a hydrothermal carbonization plant without having to increase the space, since the "finishing" stage "Would imply a simple modification in the drying process and present in the plant, and the stage of "energy recovery" would only need to add a conduit to the power generation unit; while an additional composting, which is an established procedure for the elimination of phytotoxic substances, requires much more space because the residence time is much higher, this is about 28 days, compared to a few hours (eg 8 h, in any case less one day) that are used in the hydrothermal carbonization process. The residence time for the finishing process is also considered a quick process of one hour or less. In addition, this process is carried out without water, which reduces the volume by 60-80% with respect to the HTC reactor.

Taking into account these considerations, a first aspect of the present invention relates to a method for obtaining a renewable carbonaceous material characterized in that it comprises the following steps:

(a) hydrothermal carbonization of an aqueous mixture of lignocellulosic biomass with an amount of water of between 50% and 90% by weight with respect to the total weight of the mixture, at a temperature between 180 ° C and 235 ° C, a pressure of between 10 and 25 atm, and a pH of less than 7; and further reducing the water content in the resulting wet solid mass to a range between 40% and 60% by weight with respect to the total weight of the mixture;

(b) heating the wet solid material resulting from step (a), in the absence of oxygen, at a temperature of between 200 ° C and 400 ° C, and a pressure of between 1 and 5 atm; Y

(c) generation of thermal energy from the vapors and liquids obtained in step (b) for use in steps (a) and (b).

With this stage of valorisation of the gaseous flow coming from stage (b) the formation of products is avoided, beyond the CO2. That is to say, the phytotoxic compounds are recovered as energy carriers, avoiding the release of polluting products into the environment and, therefore, the additional costs for the management of these compounds, apart from alleviating the energy costs of the process. Hydrothermal carbonization.

On the other hand, the hydrothermal carbonization process carried out in step (a) of the process of the invention allows a high moisture content in the raw material (50% -90% by weight with respect to the total weight of the mixture) without this adversely affecting the economic balance of the process. For this reason, residual resources such as the organic fraction of urban waste (FORSU), pruning remains, grass, skins and fruit and vegetable shells, and even mixtures of all of them can be used as raw material.

In a preferred embodiment, the amount of water present in the aqueous starting mixture in step (a) is between 60% and 85% by weight relative to the total weight of the mixture; and water can come from biomass or other sources.

On the other hand, the heat treatment carried out in steps (a) and (b) of the process of the invention, does not exceed 400 ° C, since it is not intended to modify the structure of the solid product but to eliminate compounds already existing in the solid, while, in classical procedures are used as optimal temperatures in the range of 500 and 600 ° C, since it is intended to transform carbonaceous compounds in a more stable.

In addition, in these conventional processes relatively high pressures are used, between 20 and 40 atm, while in the present invention lower pressures are used, preferably less than 20 atm.

In a preferred embodiment, step (a) is carried out at a temperature of between 200 ° C and 212 ° C, and a pressure of between 15 and 20 atm.

Third, in the present invention this first step is carried out at a maximum pH of seven, preferably more acid, with a pH of around 5. This pH is necessary for the degradation of hemicellulose and cellulose which, in a later process, they polymerize. On the contrary, in procedures described in the literature, part of the lignin is separated or concentrated from the biomass, to the detriment of cellulose and hemicellulose. This means that it does not use the entire biomass but only a part, the one enriched in lignin; and for this separation a basic pH is used.

In a preferred embodiment, the pH of the mixture in the reactor in step (a) is between 4 and 6.

In another preferred embodiment, the water content in the wet solid mass resulting from the hydrothermal carbonization of step (a) is adjusted to a range of between 40% and 50% by weight with respect to the total of the mixture before passing to heating stage (b).

In another preferred embodiment, this reduction in water content is carried out by mechanical processes, avoiding the evaporation of water. Such mechanical processes include, but are not limited to filtration or centrifugation.

In another preferred embodiment the heating temperature in step (b) is between 250 ° C and 300 ° C.

In another preferred embodiment, the vapors and liquids produced in step (b) and recovered in step (c) are conducted to a unit of thermal energy generation of the HTC plant by a conduit heated to the same temperature of the stage ( b) or higher, that is to say, at a temperature comprised between 200 ° C and 400 ° C, and more preferably between 250 ° C and 350 ° C.

In a particular embodiment, the method of the invention may comprise a further step, after adjusting the moisture of the solid in step (a) and prior to heating the wet solid material in step (b), grinding the solid and / or of a pelletization of the solid for obtaining particles with an average particle diameter of between 1 and 10 mm, preferably between 1 and 5 mm.

A second aspect of the invention is a renewable carbonaceous product obtained from the process of the invention, which has an added value with respect to classical hydrochar, that is, with respect to the product obtained from step (a) of the process of the invention , and characterized in that the amount by weight of fixed coal increases by at least 33% with respect to the value in the classic hydrochar, that is, with respect to the product obtained from step (a) of the process of the invention, and more preferably 40% by weight of the value in the classic hydrochar, ie, with respect to the product obtained from step (a) of the process of the invention; and decreases the amount by weight of volatiles by at least 15% of the value in the classic hydrochar, ie, with respect to the product obtained from step (a) of the process of the invention, and more preferably by 20% by weight of the value classic hydrochar, that is, with respect to the product obtained from step (a) of the process of the invention.

Another aspect of the invention is the use of the renewable carbonaceous product of the invention as a substrate and / or additive without phytotoxic effect for the soil in agricultural plantations.

Another aspect of the invention is the use of the renewable carbonaceous product of the invention as a solid, odorless fuel.

In the context of the present invention, "lignocellulosic biomass" is understood as a composition comprising cellulose, hemicellulose, and lignin The biomass may also comprise additional components, such as starch, oligosaccharides and / or monosaccharides, proteins and / or lipids.

In the context of the present invention, "hydrochar" is understood to mean the solid carbonaceous product obtained from a hydrothermal carbonization process, in particular "classic hydrochar" is understood as the product resulting from the hydrothermal carbonization process, that is, from the stage ( a) of the method of the invention, without further processing; and is characterized by having a high volatile content, a low fixed carbon content, and the amount of ash depends solely on the concentration of non-organic materials, such as metals or metal oxides, in the biomass that is fed to the HTC reactor.

In the context of the present invention it is understood that the "fixed carbon" is the material that is not included in the ashes or in the volatiles, starting from a dry carbonaceous material.

In the context of the present invention, "ash" is understood to be the remains of the combustion of the material at 815 ° C for one hour.

In the context of the present invention, "volatile" is understood as the part eliminated by a treatment of the material at 900 ° C for 7 min in a closed container.

Throughout the description and the claims the word "comprises" and its Variants do not intend to exclude other technical characteristics, additives, components or steps. For those skilled in the art, other objects, advantages and characteristics of the invention will emerge partly from the description and partly from the practice of the invention. The following examples and figures are provided by way of illustration, and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Multidimensional Gas Chromatogram (GCxGC) for (A) untreated hydrochar; (B) hydrochar after an extraction with a buffer solution of pH 3.5.

FIG. 2: Representation of the results of the decrease of the phytotoxic compounds by washes at pH 3.5; 6.0; 9.5; and deionized water.

FIG. 3: Representation of the results of the decrease of phytotoxic compounds by treatment at different temperatures.

FIG. 4: Comparison of the finishing treatment at 200 ° C on a hydrochar obtained from pruning biomass and a hydrochar obtained from the organic fraction (left column) of the urban solid waste (right column).

EXAMPLES

Next, the invention will be illustrated by means of tests carried out by the inventors, which show the effectiveness of the product of the invention.

Example 1: Preferred embodiment of the method of the invention

The method of the invention comprises at least the following steps:

● Stage of "reaction" (a), which consists of a hydrothermal carbonization (HTC) that is carried out in an HTC reactor at a temperature of between 180 and 235 ° C, to obtain a carbonaceous material with a content in water of 40-60% by weight with respect to the total weight of the mixture.This solid, without the application of the next "finishing" step (b), is termed "classic hydrochar" in the present invention, as has been described above.

The starting product of this stage is lignocellulosic biomass without any previous chemical pretreatment, independently of the contents of cellulose, hemicellulose and lignin and the other organic components (for example, proteins, oils, natural compounds such as terpenes, etc.) and inorganic ( salts, metal cations, metal oxides, glass, metals, etc.).

● Stage of "finishing" (b), which consists of drying in the absence of oxygen, at a higher temperature than the "reaction" stage, between 200 and 400 ° C, which eliminates water and substances that are They can evaporate at these temperatures. Temperatures are relatively mild so as not to induce pyrolysis reactions that can occur from 400 ° C and that would degrade the carbonaceous material. This degradation would convert desired material as a product, such as fixed carbon or other macromolecules, into more volatile, unwanted parts. Apart from a possible influence on the properties of the material, the yield to the solid product is also reduced and this induces an increase in the cost per weight of the product obtained.

The product of this stage is on the one hand a carbonaceous material (> 60% by weight) characterized by a low volatile content and a flow of gaseous and / or boiling liquid substances at the temperature of the "finishing" stage.

● Stage of "energy recovery" (c), in which the flow of boiling gases and liquids from the "finishing" stage is conducted directly to the thermal energy generation unit to feed the processes that take place in the stages (a) and (b).

Example 2: Hydrothermal carbonization of biomass

Following the procedure described in the literature (Green Chem. 2016, 18, 1051 1060.), 9240 kg of organic fraction of urban solid waste is deposited in a reactor, obtained by selective collection, with a humidity of 75% with respect to the total mass, and it is processed to obtain 1047 kg of dry hydrochar. The treatment was carried out at a temperature between 200 and 212 ° C and a pressure between 15 and 20 atm, and a pH below 7. As a reactor effluent, a mixture (suspension type) of a carbonaceous material and water is obtained , and with a filter-press the water amount up to about 50% by weight. Part of the hydrochar is thermally dried. This hydrochar has an ash content of 13.2% by weight, 68.2% by weight of volatiles and 18.8% by weight of fixed carbon, all on a dry basis.

Example 3: Elimination of phytotoxic compounds by washing hydrochar (Comparative example)

Three buffer solutions with pH 3.5 were prepared; 6.0 and 9.5. For pH 3.5, a mixture of acetic acid / acetate was used, for pH 6.0 a mixture of KH2PO4 / K2HPO4, and for pH 9.5 a mixture of NH4Cl / NH4.

In a closed glass bottle 0.50 g of hydrochar, produced in the reactor according to Example 2, was suspended in 45 mL of buffer solution for each of the prepared buffers, or deionized water. The suspension was stirred overnight at room temperature. The hydrochar was separated by filtration and dried overnight at 100 ° C in an oven.

The change in less phytotoxic materials was documented by an advanced analysis of multidimensional gas chromatography (GCxGC). To do this, the signals were integrated for all the substances observed in the initial sample produced and used in the washings. The value obtained with the initial sample was stipulated as 100%. Phytotoxic substances should be included in them, according to the literature, without being identified in more detail. Figure 1 shows an example of these analyzes in an untreated hydrochar and a hydrochar after an extraction with a buffer solution of pH 3.5. The different compounds (phytotoxic substances) are represented by circles whose size is in proportion to the amount detected.

The results of the decrease of the phytotoxic compounds by different washes are summarized in Figure 2 in a comparative manner, where the (area) values were obtained by multidimensional gas chromatography analysis (GCxGC) and are shown in reference to the value obtained with the initial sample (classic hydrochar).

Example 4: "Finishing" step for the renewable carbonaceous material obtained in Example 2 at different temperatures

The carbonaceous material obtained in Example 2 is placed on a porous plate in a vertical quartz tube. A flow of nitrogen is passed, the tube is heated to the desired temperature with a ramp of 3 ° C / min and the temperature is maintained for 1 h.

This procedure is carried out at the following temperatures, specified in the following Table (example (a) represents the classic hydrochar without treatment):

The samples obtained in the described treatment are analyzed by an advanced analysis of GCxGC as described in Example 3 to evaluate the phytotoxicity of the samples.

It is observed in Figure 3 that the treatment at 200 ° C is more effective than the wash treatment carried out in Example 2. The amount of possibly phytotoxic compounds is reduced to 12% of the original value at that temperature. After treatment at 250 ° C, only 2% of these compounds are detected and after treatment at 275 ° C these types of compounds are no longer detected.

It is also observed that after the thermal treatment the solid has lost its characteristic odor and is odorless. At least part of the substances removed in the heat treatment can be condensed at room temperature. This mixture has a strong odor, similar to the carbonaceous material before treatment.

On the other hand, if the mixture is analyzed by gas chromatography coupled to a mass detector, guaiacol (known for its smoked and availlinated odor) can be identified in the mixture, and other derivatives with additional methyl or methoxy groups.

Example 5: Energy difference before and after the step of "finishing"

The calorific value of the samples prepared in Example 4 was measured and the values are shown in the following Table, together with the loss of mass in each treatment.

From these two data can be calculated the amount of energy recovered in the solid, with respect to the initial energy of the solid before treatment.

It is observed that after the treatments at 250 and 275 ° C about two thirds of this energy is recovered. As the treatment is carried out in the absence of oxygen, it can be assumed that the rest of the energy, a third of the initial amount, is contained in the liquids and vapors that are produced in this treatment.

Example 6: "Finishing" stage for the renewable carbonaceous material obtained from pruning biomass,

Hydrochar is prepared from pruning biomass in the manner described in Example 2. The "finishing" step is carried out as described in Example 4 at a temperature of 200 ° C at t = 0, or by maintaining the temperature for one hour. "Finished" are analyzed as described in Example 3 to evaluate the phytotoxicity of the samples.

It can be observed in Figure 4 that the treatment is just as effective in the elimination of potentially phytotoxic substances for this hydrochar as for the hydrochar obtained in Example 2, obtained from the organic fraction of the urban solid waste.

Claims (15)

1. Process for obtaining a renewable carbonaceous material characterized in that it comprises the following stages: (a) hydrothermal carbonization of an aqueous mixture of lignocellulosic biomass with an amount of water of between 50% and 90% by weight with respect to the total weight of the mixture, at a temperature between 180 ° C and 235 ° C, a pressure of between 10 and 25 atm, and a pH of less than 7; and further reducing the water content in the resulting wet solid mass to a range between 40% and 60% by weight with respect to the total weight of the mixture; (b) heating the wet solid material resulting from step (a), in the absence of oxygen, at a temperature of between 200 ° C and 400 ° C, and a pressure of between 1 and 5 atm; Y (c) generation of thermal energy from the vapors and liquids obtained in step (b) for use in steps (a) and (b). The process according to claim 1, wherein the amount of water present in the starting mixture in step (a) is between 60% and 85% by weight with respect to the total weight of the mixture. Method according to any one of the preceding claims, wherein stage (a) is carried out at a temperature of between 200 ° C and 212 ° C, and a pressure of between 15 and 20 atm. 4. Process according to any of the preceding claims, wherein the pH of the mixture in step (a) is between 4 and 6. Method according to any one of the preceding claims, wherein the water content in the wet solid mass resulting from the hydrothermal carbonization of step (a) is adjusted to a range between 40% and 50% by weight with respect to the total of the mixture before going to stage (b). Method according to the preceding claim, wherein the reduction of the water content is carried out by mechanical processes avoiding the evaporation of the water. Method according to the preceding claim, wherein the mechanical processes are selected from filtration and centrifugation. 8. Method according to any of the preceding claims, wherein the heating temperature in step (b) is between 250 ° C and 300 ° C. The method according to any of the preceding claims, wherein the vapors and liquids produced in step (b) and recovered in step (c) are conducted to the thermal power generation unit of the HTC plant through a heated duct the same temperature of step (b) or higher, preferably between 250 ° C and 350 ° C. Method according to any of the preceding claims, comprising a further step, after adjusting the moisture of the solid in stage (a) and prior to heating the wet solid material in step (b), grinding the solid and / or of a pelletization of the solid for obtaining particles with an average diameter of between 1 and 10 mm, preferably between 1 and 5 mm. 11. Renewable carbonaceous product obtained from the process according to any of the preceding claims, characterized in that the amount by weight of fixed carbon is increased by at least 33% with respect to the value in the product obtained from step (a), and the amount by weight of volatiles is decreased by at least 15% of the value in the product obtained from step (a). Product according to claim 11, wherein the amount by weight of fixed carbon is increased in a range between 33% and 40% of the weight value in the product obtained from step (a). 13. Product according to any of claims 11 to 12, wherein the amount of volatiles is decreased in a range between 15% and 20% of the weight value in the product obtained from step (a). 14. Use of a product as defined in any of claims 11 to 13 as a substrate and / or additive for the soil. 15. Use of a product as defined in any of claims 11 to 13 as a solid fuel.