Method for quantifying the pyrogenic carbon present in a sample of an organo-mineral or mineral matrix comprising charcoal and / or biochar

Abstract

The present invention relates to a method for quantifying the pyrogenic carbon content of a sample comprising an organo-mineral or mineral matrix as well as charcoal and / or biochar. The method comprises heating the sample in an inert atmosphere and then heating the residue from the heating in an inert atmosphere in an oxidising atmosphere. A Gaussian deconvolution is then applied to the measurement curve of the amount of CO2 released as a function of the temperature during the heating in an oxidising atmosphere, and first and second Gaussian deconvolutions are determined which are centred respectively on a first temperature between 380°C and 540°C, preferably between 415°C and 425°C, and very preferably equal to 420°C, and a second temperature between 500°C and 600°C, preferably between 570°C and 580°C, and preferably equal to 576°C. The pyrogenic carbon content is determined from the surfaces of the first and second Gaussian deconvolutions.

Description

[0001] METHOD FOR QUANTIFYING THE PYROGENIC CARBON PRESENT IN A SAMPLE OF AN ORGANOMINERAL OR MINERAL MATRIX COMPRISING

[0002] CHARCOAL AND / OR BIOCHAR

[0003] Technical field

[0004] The present invention may relate in particular to the field of soil sciences, agronomy and the environment, and more generally to any field concerned with the quantification of biochar and / or carbon present in a mineral or organo-mineral matrix.

[0005] The present invention may, for example, relate to the field of the production and marketing of biochar, in particular when it is marketed in the form of a mixture with an organo-mineral or mineral matrix, for example for use as an organic amendment (biochar-sediment) or as a construction material for buildings, like hybrid green concrete (biochar-calcium carbonate) or simply by mixing the biochar with concrete (mineral matrix).

[0006] In the environmental field, in order to address environmental challenges and in particular to limit global warming to 1.5°C compared to the pre-industrial era, it is necessary to achieve the objective of "Zero Net Emissions" by 2050. To achieve this, it is necessary not only to drastically reduce our CO2 emissions, but also to develop negative emission technologies. Biochar, a product resulting from the pyrolysis of biomass, constitutes a negative emission technology since it is a carbon-rich and biologically stable material. Furthermore, in recent decades, pyrogenic organic matter has been recognized as an important component of the highly fertile Amazonian soils. Indeed, this material can have beneficial effects on soil fertility, particularly for water and nutrient retention and the liming effect. The name given to pyrolyzed organic matter differs depending on its end use.Biochar is used when organic matter is pyrolyzed to improve soil properties. In the literature, black carbon describes the volatile components resulting from the incomplete combustion of biomass. In some studies, the term black carbon is also used to describe a broader range of materials, forming a continuum from charcoal to soot. Charcoal, or sometimes charcoal, is used when organic matter is deliberately pyrolyzed for use as fuel or filter media, for example. It should be noted that coal differs from coal in its production methods and in its very different chemical and physical properties.Unlike coal from biomass pyrolysis, geological coal (e.g., hard coal) is produced by geological processes under high pressure and high temperature, over a longer time scale (several thousand years). Notably, geological coal does not contain pyrogenic carbon. Thus, forms of coal other than geological coal (i.e., coals such as charcoal, char, or biochar) may be present in a soil due to several contexts, such as vegetation fires and intentional amendment to improve soil properties. Since coal is rich in biologically stable carbon (i.e., aromatic compounds), its quantification can be done through the pyrogenic carbon that constitutes the majority of it. The benefits of being able to quantify pyrogenic carbon in the soil are multiple and depend on the context in which it was introduced.In the case of wildfires, pyrogenic carbon content is a marker of fire frequency. This marker is particularly useful for archaeological studies, since high fire frequencies on the same site may indicate previous human occupation, while lower intensities are more likely to result from natural fires. In the case of biochar amendments, quantifying pyrogenic carbon in an amended plot allows monitoring of the remaining biochar content in the soil. Indeed, biochar can easily be subject to physical degradation. Furthermore, quantifying pyrogenic carbon outside the amended plot allows monitoring of the fate of biochar in the environment and particularly of its deposition areas.

[0007] Prior art

[0008] The following documents will be cited during the description:

[0009] Arroyo-Kalin, M. A. (2008). Steps towards an ecology of landscape: A geoarchaeological approach to the study of anthropogenic dark earths in the central Amazon region, Brazil (Doctoral dissertation, University of Cambridge).

[0010] Aubertin, M. L. (2022). Biochar-compost mixtures: interactions and impact on carbon sequestration and soil fertility (Doctoral dissertation, Sorbonne université).

[0011] Behar, F., Beaumont, V., & Penteado, H. D. B. (2001). Rock-Eval 6 technology: performances and developments. Oil & Gas Science and Technology, 56(2), 111-134.

[0012] Chalk, P., & Smith, C. J. (2022). 13C methodologies for quantifying biochar stability in soil: A critique. European Journal of Soil Science, 73(3), e13245. Cuypers, C., Grotenhuis, T., Nierop, K. G., Franco, E. M., de Jager, A., & Rulkens, W. (2002). Amorphous and condensed organic matter domains: the effect of persulfate oxidation on the composition of soil / sediment organic matter. Chemosphere, 48(9), 919-931.

[0013] Glaser, B., Haumaier, L, Guggenberger, G., & Zech, W. (1998). Black carbon in soils: the use of benzenecarboxylic acids as specific markers. Organic geochemistry, 29(4), 811-819.

[0014] Llorente, M., Turriôn, M. B., & Glaser, B. (2018). Rapid and economical quantification of black carbon in soils using a modified benzene polycarboxylic acids (BPCA) method. Organic Geochemistry, 115, 197-204.

[0015] Paterson, G. A., & Heslop, D. (2015). New methods for unmixing sediment grain size data. Geochemistry, Geophysics, Geosystems, 16(12), 4494-4506.

[0016] Poot, A., Quik, J. T., Veld, H., & Koelmans, A. A. (2009). Quantification methods of Black Carbon: Comparison of Rock-Eval analysis with traditional methods. Journal of Chromatography A, 1216(3), 613-622.

[0017] Sebag, D., Disnar, J. R., Guillet, B., Di Giovanni, C., Verrecchia, E. P., & Durand, A. (2006). Monitoring organic matter dynamics in soil profiles by ‘Rock-Eval pyrolysis’: bulk characterization and quantification of degradation. European journal of soil science, 57(3), 344-355.

[0018] Sebag, D., Garcin, Y., Adatte, T., Deschamps, P., Ménot, G., & Verrecchia, E. P. (2018). Correction for the siderite effect on Rock-Eval parameters: application to the sediments of Lake Barombi (southwest Cameroon). Organic Geochemistry, 123, 126-135.

[0019] Simpson, M.J., & Hatcher, P.G. (2004). Overestimates of black carbon in soils and sediments. Naturwissenschaften, 91(9), 436-440.

[0020] There are various known techniques for quantifying pyrogenic carbon in soil or sedimentary material, based on chemical, magnetic, optical or thermal differences between the soil and the coal or by the presence of molecular markers.

[0021] A commonly used technique is the quantification by extraction of benzene polycarboxylic acids (BPCAs) following chemical oxidation of aromatic structures, by gas chromatographic analysis, as described for example in (Glaser et al., 1998), or by elemental analysis, as described for example in (Llorente et al. 2018). However, extraction is time-consuming and can add bias to the quantification of BPCAs.

[0022] The use of natural carbon isotopy is a direct, precise and reproducible quantification method, making it possible to distinguish the source of a carbon from a mixture of two carbon components with an isotopic signature (5 13 C) significantly different, as described for example in the paper (Aubertin et al., 2022). However, isotopic analysis can only be applied in the case of a coal-soil mixture, where the two components have significantly different isotopic signatures. Isotopic enrichment methods can also be used to quantify pyrogenic carbon, but this involves (time-consuming) incubation and the results may be biased by the non-uniform distribution of the added labeled carbon in the plant, as described for example in the paper (Chalk and Smith, 2022).

[0023] Other methods for quantifying coal are based on observing differences in color or density of coal particles from a microscope photo, as described for example in the document (Arroyo-Kalin, 2008). However, these methods are time-consuming (time for preparing thin sections and for handling the sample), only take into account particles larger than a certain diameter, are not very reproducible because they are manipulator-dependent, and only approximate the mass of the coal, based on a surface count.

[0024] Among the thermal methods, the thermochemical oxidation method involves a chemical oxidation pretreatment with an acid to remove inorganic carbon and then the separation of pyrogenic and non-pyrogenic carbon with combustion at temperatures around 350 °C for at least 2 hours, followed by analysis of the residual carbon by NMR. 13C or elemental analysis, as described for example in (Poot et al. 2009). Besides the time-consuming aspect of this method, it can also induce overestimations of pyrogenic carbon due to the formation of pyrogenic carbon during the combustion phase, as described for example in (Simpson and Hatcher, 2004). Thermogravimetric analysis measures several emission peaks during heating, but it is difficult to differentiate emission peaks related to pyrogenic carbon and soil components with this method, as described for example in (Cuypers et al., 2002). One method for quantifying pyrogenic carbon in soil is the technique of differential scanning calorimetry, known by the acronym DSC (Differential Scanning Calorimetry), in which the sample causes changes in heat flux depending on the temperature rise gradient.A close correlation can be made between variations in heat fluxes and the amount of carbon. To quantify pyrogenic carbon, it is sufficient to differentiate between the amount of carbon above a threshold temperature, around 400°C, of ​​a sample of the same soil / sediment with and without carbon. However, DSC is an indirect measure of stable carbon in a sample, which can introduce imprecision in carbon quantification. Thus, despite its obvious interest, quantifying pyrogenic carbon in soil often remains difficult to achieve and the various existing techniques have disadvantages due, for example, to their price, analysis time, precision, or the fact that they are not always well reproducible.

[0025] Thermal analysis methods for soil organic matter are also known, based on measurements of the quantities of hydrocarbon compounds (HC), carbon monoxide (CO) and / or carbon dioxide (CO2) released over time by a sample subjected to a sequence of temperatures in an inert atmosphere (pyrolysis phase) and / or to a sequence of temperatures in an oxidizing atmosphere (oxidation phase). These methods were initially developed in the petroleum industry for the purpose of characterizing the organic fraction of sedimentary rocks. For example, the "ROCK-EVAL® BULK ROCK" method is known, initially developed for conventional source rock samples, which makes it possible to distinguish pyrolyzed organic carbon from refractory organic carbon (Behar et al., 2001). The document (Poot et al., 2009) describes that the amount of refractory carbon measured during such a thermal analysis can be used to approximately quantify the pyrogenic carbon in a soil or sediment sample. More specifically, this document describes that the "ROCK-EVAL® BULK ROCK" method allows for the easy and rapid separation of pyrolyzable carbon (PC) from residual carbon (RC). RC corresponds to refractory organic carbon, derived from organic matter thermally resistant to the pyrolysis phase and which is oxidized during the oxidation phase. This document thus proposes the idea of ​​approximating RC as a measure of pyrogenic carbon, which it calls "black carbon" and which it defines as a continuum ranging from pyrolyzed biomass char to soot. However, this latter variant remains imprecise, since RC can also be partly produced during the pyrolysis phase.Thus, the pyrogenic carbon analyzed with this method may be slightly overestimated.

[0026] The present invention makes it possible to overcome these drawbacks. In particular, the present invention makes it possible to quickly and precisely quantify pyrogenic carbon in a sample of an organo-mineral or mineral matrix such as a soil, by means of a thermal analysis, including in particular the analysis of carbon emissions during an oxidation phase of the sample.

[0027] Summary of the invention

[0028] The present invention relates to a method for quantifying the pyrogenic carbon content present in a sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar. The method according to the invention comprises at least the following steps: A) said sample is heated in an inert atmosphere according to a first temperature sequence of which an initial temperature (T0) is between 100 and 300°C, and is preferably 200°C, and a final temperature (TF) is between 500 and 800°C, and is preferably 650°C;

[0029] B) a residue of said sample from said heating in an inert atmosphere is heated in an oxidizing atmosphere according to a second temperature sequence of which an initial temperature (TO') is between 100 and 300°C, and is preferably 200°C, and a final temperature (TF') is between 700 and 1000°C, and is preferably 850°C, said second temperature sequence comprising at least one thermal gradient of between 1°C and 50°C / min, preferably between 15°C / min and 35°C / min, very preferably 25°C / min, and at least one quantity of CO2 (QCO2) released during said second temperature sequence is measured;

[0030] C) from a curve representative of the evolution as a function of the temperature of said quantity of CO2 released during said heating in an oxidizing atmosphere, a Gaussian deconvolution is applied to said curve, so as to determine a first and a second Gaussian centered respectively on a first and a second temperature, said first temperature being between 380°C and 540°C, preferably between 415°C and 425°C, and very preferably being 420°C, and said second temperature being between 500 and 600°C, preferably between 570 and 580°C, and preferably being 576°C;

[0031] D) From the surfaces of said first and second Gaussians, said pyrogenic carbon content X3 present in said sample is determined according to a formula of the type: where X ±and X2 are carbon contents determined respectively from the surfaces of said first and second Gaussians, A is a coefficient representing the proportion of said matrix in said second Gaussian relative to said first Gaussian, and B is a coefficient representing the proportion of said coal and / or said biochar in said first Gaussian relative to said second Gaussian.

[0032] According to an implementation of the invention, said first temperature sequence may comprise an isothermal plateau of a predetermined duration at said initial temperature (T0) of said first temperature sequence, followed by a thermal gradient to reach said final temperature (TF) of said first temperature sequence, said predetermined duration of said isothermal plateau of said first temperature sequence being able to be between 1 and 5 minutes, and being preferably 3 minutes, and said thermal gradient of said first temperature sequence being able to be between 1 and 50°C / min, preferably between 15°C / min and 35°C / min, and being very preferably 25°C / min.

[0033] According to an implementation of the invention, said second temperature sequence may further comprise an isothermal stage of a predetermined duration at a temperature between 500 and 600°C, preferably between 570 and 580°C, and preferably 576°C, said predetermined duration of said isothermal stage of said second temperature sequence being able to be between 1 and 5 minutes, and preferably 3 minutes.

[0034] According to one implementation of the invention, it is also possible to have a sample of said pure organo-mineral or mineral matrix, and it is possible to determine said coefficient A of step D) in advance in the following manner: steps A) to C) are applied to said sample of said pure organo-mineral or mineral matrix, and coefficient A is determined by calculating the ratio between the surface area of ​​a second Gaussian and the surface area of ​​a first Gaussian determined from said sample of said pure organo-mineral or mineral matrix.

[0035] According to one implementation of the invention, a sample of said biochar and / or said pure coal may also be available, and said coefficient B of step D) may be determined beforehand in the following manner: steps A) to C) are applied to said sample of said biochar and / or said coal, and coefficient B is determined by calculating the ratio between the surface area of ​​said first Gaussian and the surface area of ​​said second Gaussian determined from said sample of biochar and / or pure coal.

[0036] According to one implementation of the invention, step D) can be applied by means of a coefficient A between a value of 0.17 and a value of 0.73 and preferably 0.19, and / or by means of a coefficient B between a value of 0.10 and a value of 4.98 and preferably 0.32.

[0037] According to one implementation of the invention, a total mass of pyrogenic carbon Qc, bcjnix present in said sample can be determined according to a formula of the type:

[0038] Qc, bcjnix = X3 / C * K, where C is a ratio between a carbon content determined from a surface of a second Gaussian determined for a sample of biochar and / or pure charcoal, and a total mass of carbon in said sample of biochar and / or pure charcoal, and where K is a multiplicative coefficient between 12.0 and 12.5, and preferably 12.2.

[0039] Alternatively, a total mass of pyrogenic carbon Qc, bcjnix present in said sample can be determined according to a formula of the type: Qc, bc nix = X3 / C * K, where C is a ratio between a value of 0.51 and a value of 1.76, and preferably 1.06, and where K is a multiplicative coefficient between 12.0 and 12.5, and preferably 12.2. Other characteristics and advantages of the method according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below.

[0040] List of figures

[0041] Figure 1A schematically illustrates the evolution of the temperature as a function of time of the temperature sequence of the first step of the method according to the invention.

[0042] Figure 1B schematically illustrates the evolution of the temperature as a function of time of a variant of the temperature sequence of the first step of the method according to the invention.

[0043] Figure 2 shows, in an example of application, the CO2 curve measured during step 2) of the method according to the invention, as well as the first and second Gaussians resulting from the Gaussian deconvolution according to step 3) of the method according to the invention.

[0044] Figure 3 schematically presents the share of the organo-mineral or mineral matrix and the share of biochar and / or coal in the first Gaussian resulting from the Gaussian deconvolution according to step 3) of the method according to the invention, as well as the share of biochar and / or coal and the share of the organo-mineral or mineral matrix in the second Gaussian resulting from the Gaussian deconvolution according to step 3) of the method according to the invention.

[0045] Description of the embodiments

[0046] The invention relates to a method for quantifying pyrogenic carbon present in a sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar.

[0047] Pyrogenic carbon means the organic fraction that has undergone pyrolysis, i.e. that has undergone the effect of fire and / or heat (temperature >200°C) in the absence or low concentration of oxygen.

[0048] An organo-mineral or mineral matrix is ​​an unconsolidated, porous material consisting of a mixture of organic and / or mineral particles of varying size and chemical and / or mineralogical composition. Charcoal is the solid residue of a chemical transformation under the effect of a rise in temperature, resulting from pyrolysis or incomplete combustion of plant or animal biomass.

[0049] Biochar means charcoal produced with the intention of using it as an organic amendment, particularly to improve the physicochemical properties of a soil or its carbon storage.

[0050] The method according to the invention requires having at least one sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar.

[0051] According to one implementation of the invention, the sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar may be a sample of a soil comprising coal and / or biochar. By soil is meant all the outer layers of the Earth's surface formations. A soil sample may be taken manually from a pit or by coring using an auger.

[0052] According to one implementation of the invention, the sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar may be a sample of an amendment comprising an organo-mineral or mineral matrix as well as coal and / or biochar.

[0053] According to one implementation of the invention, the sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar may be a soil sample, a sample of natural sediment or sediment polluted by fire residues, or a sample of mineral materials (concrete, excavated earth, sediments) mixed with coals / biochars. The soil may be agricultural soil or an anthrosol deliberately enriched with biochar.

[0054] Advantageously, the sample can be sieved using a sieve with orifices having a diameter of 2 mm, dried at a temperature below 40°C, then ground until fragments having dimensions of less than 200 μm are obtained.

[0055] Preferably, it is also possible to have a sample of said pure organo-mineral or mineral matrix, i.e. one comprising neither biochar nor coal.

[0056] Advantageously, a sample of the biochar and / or charcoal present in the sample in question can also be available.

[0057] The method according to the invention can be advantageously but not limitatively implemented using the ROCK-EVAL® device (IFP Energies nouvelles, France), as described in patents FR 2227797 (US 3953171) and FR 2472754 (US 4352673). Indeed, the ROCK-EVAL® device comprises at least:

[0058] - a pyrolysis oven in a non-oxidizing atmosphere,

[0059] - means of transferring pyrolysis residues into an oxidation furnace,

[0060] - an oxidation furnace in an oxidizing atmosphere,

[0061] - means of measuring the quantity of hydrocarbon compounds (HC) released during pyrolysis,

[0062] - means of measuring carbon monoxide (CO) and carbon dioxide (CO2).

[0063] The method can also be implemented using a single pyrolysis furnace, which can operate in both a non-oxidizing atmosphere and an oxidizing atmosphere, cooperating with a device for measuring the quantity of hydrocarbon compounds released during pyrolysis, and a device for measuring carbon monoxide and carbon dioxide.

[0064] The method according to the invention comprises at least the following steps:

[0065] 1) Heating sequence under inert atmosphere (pyrolysis)

[0066] 2) Heating sequence under oxidizing atmosphere (oxidation)

[0067] 3) Gaussian deconvolution

[0068] 4) Determination of pyrogenic carbon content

[0069] The steps of the method according to the invention are detailed below.

[0070] 1. Heating sequence under inert atmosphere (pyrolysis)

[0071] During this step, the sample comprising an organo-mineral or mineral matrix as well as biochar and / or coal is heated under an inert atmosphere (such as for example under a flow of nitrogen or helium) according to a sequence of temperatures of which the initial temperature (denoted T0 hereinafter) is between 100 and 300°C and is preferably 200°C, and the final temperature (denoted TF hereinafter) is between 500 and 800°C, and is preferably 650°C.

[0072] Preferably, the sequence of temperatures under an inert atmosphere may comprise at least one isothermal plateau at the initial temperature T0, followed by a predetermined thermal gradient so as to raise the temperature of the sample to the final temperature TF. Figure 1A schematically illustrates the evolution of the temperature T as a function of time t of such a sequence of temperatures, presenting an isothermal plateau at the temperature TO, followed by a thermal gradient until reaching the temperature TF.

[0073] Advantageously, the temperature sequence under an inert atmosphere of this embodiment may further comprise a second isothermal stage, at the final temperature TF. In other words, a second isothermal stage at the final temperature TF follows the phase of the temperature sequence in the form of a thermal gradient. This makes it possible to continue, if necessary, the cracking of the compounds having a cracking temperature close to the final temperature TF of the temperature sequence under an inert atmosphere according to the invention. Figure 1B schematically illustrates the evolution of the temperature T as a function of time t of such a temperature sequence, having two isothermal stages, at temperatures TO and TF as defined above, and linked together by a thermal gradient.

[0074] According to one implementation of the invention, the initial temperature TO is preferably 200°C. This temperature is in fact sufficient to release the most labile organic compounds present in most samples of soil, organic amendment or sediment.

[0075] According to one implementation of the invention, the final temperature TF is preferably 650°C, so as to avoid obtaining CO and CO2 curves presenting incomplete peaks at the end of pyrolysis measured in particular on natural samples (fresh and dried plant tissues, litter, peat, and plant composts, organo-mineral and mineral soils, surface formations).

[0076] According to one implementation of the invention, the isothermal stage(s) of the temperature sequence under an inert atmosphere may have a predetermined non-zero duration (for example, greater than half a minute), preferably between 1 and 5 minutes, and very preferably 3 minutes. Such durations make it possible to consider that the cracking of the compounds having a cracking temperature close to the temperature of the isothermal stage is complete. According to the implementation of the invention in which the temperature sequence under an inert atmosphere according to the invention comprises several isothermal stages and in particular two isothermal stages at temperatures TO and TF, the duration of an isothermal stage may be different from the duration of the other isothermal stages.

[0077] According to one implementation of the invention, the thermal gradient(s) of the temperature sequence under an inert atmosphere may be between 1 and 50°C / min, preferably between 15° and 35°C / min, and are preferably 25°C / min. Such values ​​constitute compromises allowing the thermal cracking of the compounds, while limiting the duration of implementation of the method. According to one implementation of the invention, it is possible to measure, continuously (i.e. continuously over time), a quantity of hydrocarbon compounds released during heating under an inert atmosphere, and / or a quantity of CO2 and / or a quantity of CO contained in an effluent resulting from said heating. In other words, during this sequence, it is possible to continuously measure the quantity of HC, CO and CO2 released by the sample by thermal cracking of the organic matter and by the thermal decomposition of the carbonate minerals.The measurement of the quantity of hydrocarbon compounds can be carried out using a flame ionization detector (FID). The measurement of the quantity of CO and CO2 released can be carried out using an infrared (IR) detector. Alternatively, other means for measuring the quantity of HC, CO and / or CO2 can be used. According to this implementation, it is possible to obtain, at the end of this step applied to a given sample, a first curve representative of the quantity of hydrocarbon compounds released over time during the pyrolysis phase, as well as two other curves representative of the quantity of CO and CO2 released over time, during the pyrolysis phase.Such measurements can help determine standard parameters for such thermal analysis, in particular the parameter denoted TOC (for "Total Organic Carbon") which corresponds to the carbon content of the sample, determined from the quantity of hydrocarbons released by the sample and the quantities of CO and CO2 released below threshold temperatures during the pyrolysis phase and the oxidation phase; and the parameter denoted MinC (for "Ore Carbon") which corresponds to the mineral carbon content of the sample, determined from the quantities of CO and CO2 released by the sample above threshold temperatures during the pyrolysis phase and the oxidation phase. A description of these general parameters can be found in the document (Behar et al., 2001).

[0078] Generally speaking, this particular heating sequence under an inert atmosphere is sufficient to allow the thermal cracking of classes of compounds comprising mineral carbon and organic carbon, in particular: highly thermally labile compounds, which are particularly abundant in fresh biological tissues, and which are generally released at temperatures between approximately 80 and 360°C; thermally labile compounds, which are predominant in organic samples such as litter or peat, and which are generally released at temperatures between approximately 360 and 420°C; thermally resistant compounds, which are predominant in organo-mineral (soils) or mineral (alluvium, colluvium) samples, and which are generally released at temperatures between approximately 420°C and 470°C;thermally refractory compounds, which are generally released at temperatures between about 470 and 520°C; and very thermally refractory compounds, which are present in larger proportions in decomposition residues or exogenous fractions, such as pyrogenic or petrogenic organic materials, and which are generally released at temperatures between about 520 and 650°C.;

[0079] According to one implementation of the invention, the sequence of temperatures under an inert atmosphere according to the invention may be preceded by a phase of temperature rise of the pyrolysis furnace, which may be in the form of a thermal gradient, for example between 1 and 50°C / min, preferably between 20 and 25°C / min, or any other form of temperature rise curve of the pyrolysis furnace. This preliminary phase of temperature rise of the pyrolysis furnace makes it possible to bring the pyrolysis furnace to the initial temperature of the sequence of temperatures under an inert atmosphere according to the invention. This preliminary phase may contribute to starting the thermal cracking of compounds whose cracking temperature is lower than the initial temperature of the sequence of temperatures under an inert atmosphere according to the invention, in particular in the case of fresh biological tissues.

[0080] According to one implementation of the invention, the sequence of temperatures under an inert atmosphere according to the invention may be followed by a phase of lowering the temperature of the pyrolysis furnace, which may be in the form of a thermal gradient, for example between -1 and -50°C / min, preferably between -20 and -25°C / min, or any other form of temperature reduction curve of the pyrolysis furnace. This final phase of lowering the temperature of the pyrolysis furnace makes it possible, if necessary, to complete the thermal cracking of the associated compounds at the final temperature of the sequence of temperatures under an inert atmosphere according to the invention.

[0081] 2) Heating sequence under oxidizing atmosphere (oxidation)

[0082] During this second step, the solid residue of the sample obtained at the end of the heating sequence under an inert atmosphere as described in step 1 above is subjected to oxidation according to a predefined temperature sequence, the initial temperature (hereinafter referred to as T0') of which is between 100 and 300°C and is preferably 200°C, and the final temperature (hereinafter referred to as TF') is between 700 and 1000°C, and is preferably 850°C (so as to exhaust the stock of mineral carbon). In addition, according to the invention, the temperature sequence of this heating under an oxidizing atmosphere comprises at least one thermal gradient of between 1 and 50°C / min, preferably between 15° and 35°C / min, and is preferably 25°C / min.

[0083] Generally speaking, the preferred temperature range for the initial temperature TO' of the temperature sequence under an oxidizing atmosphere makes it possible to avoid episodes of instantaneous combustion of the sample residue at the start of the oxidation cycle.

[0084] According to one implementation of the invention, the sequence of temperatures under an oxidizing atmosphere may further comprise an isothermal plateau at the initial temperature TO' of a predetermined non-zero duration (for example greater than half a minute), and may preferably be between 1 and 5 minutes, very preferably 3 minutes.

[0085] According to the invention, at least a quantity of CC>2 (and optionally a quantity of CO) released during this second temperature sequence is continuously measured. According to one implementation of the invention, this measurement can be carried out using an infrared (IR) type detector. Note that such a sensor provides values ​​measured in millivolts (mV). Conventionally, a quantity of CO2 released during this second temperature sequence, denoted Xtot thereafter, is determined by determining an area under the curve measured (possibly between predefined temperatures) by this sensor, according to a formula of the type: where SurfC corresponds to the area under the curve (also called thermogram) representing the quantity of CO2 released during this second temperature sequence, mass corresponds to the mass of the sample, and where Xtot is expressed in mg / g of sample. Alternatively, other means of measuring the quantity of CO2 can be used.

[0086] Advantageously, the sequence of temperatures under an oxidizing atmosphere may further comprise an isothermal stage at a temperature between 500 and 600°C, preferably between 570 and 580°C, and preferably 576°C. This isothermal stage makes it possible to better separate, in a curve representing the evolution as a function of temperature of the quantity of CO2 released during heating under an oxidizing atmosphere, a component attributed to the biochar or to the carbon from a component attributed to the mineral or organo-mineral matrix present in the sample considered. This makes it possible in particular to contribute to improving the result of step 3) of the method according to the invention described below. This isothermal stage may be of a predetermined non-zero duration (for example greater than half a minute), preferably between 1 and 5 minutes, very preferably 3 minutes.According to this embodiment, the temperature sequence under an oxidizing atmosphere may further comprise an additional thermal gradient (i.e. in addition to the at least one thermal gradient of the temperature sequence of step 2) of the method according to the invention), comprised between 1°C and 50°C / min, preferably between 15 and 35°C / min, and very preferably being 25°C / min. Thus, according to this implementation, the isothermal plateau at a temperature comprised between 500 and 600°C, preferably between 570 and 580°C, and preferably being 576°C may be both preceded and followed by the at least two thermal gradients of this embodiment.

[0087] 3) Gaussian deconvolution

[0088] During this step, from a curve representative of the evolution as a function of the temperature of said quantity of CO2 released during heating under an oxidizing atmosphere, a Gaussian deconvolution is applied to said curve, so as to determine (at least) a first and a second Gaussian centered respectively on a first and a second temperature, said first temperature being between 380°C and 540°C, preferably between 415°C and 425°C, and very preferably being 420°C, and said second temperature being between 500 and 600°C, preferably between 570 and 580°C, and preferably being 576°C.

[0089] Gaussian deconvolution means a decomposition of a curve (in this case the curve representing the evolution as a function of temperature of the quantity of CO2 released during the second temperature sequence) into elementary components each corresponding to a Gaussian distribution.

[0090] Thus, this step aims to approximate the curve representing the evolution as a function of temperature of the quantity of CO2 released during the second temperature sequence by two Gaussians, more precisely, a first Gaussian centered on a temperature between 380°C and 540°C, preferably between 415°C and 425°C, and very preferably worth 420°C, and a second Gaussian centered on a temperature between 500 and 600°C, preferably between 570 and 580°C, and preferably worth 576°C. Indeed, the temperature range of the first Gaussian, between 380°C and 540°C, is characteristic of the temperature range of the main peak of a curve representing the quantity of CO2 released by a sample of pure mineral or organo-mineral matrix, regardless of the type of mineral or organo-mineral matrix, as for example described in the document (Sebag et al. 2018).Indeed, it has been shown that for this type of samples (mineral or organo-mineral matrix), the largest CO2 peak is emitted at such temperatures during the oxidation phase. These temperatures are lower than those where the maximum CO2 is emitted for biochars or coals, due to the lower thermal stability of the mineral or organo-mineral matrix. Furthermore, the temperature range of the second Gaussian, between 500 and 600°C, is characteristic of the temperature range of the peak of a curve representative of the quantity of CO2 released by a sample of pure coal and / or biochar, regardless of the type of coal and / or biochar, as for example described in the document (Aubertin et al., 2022). Indeed, it has been shown that biochar or coal samples emit a majority of CO2 over this temperature range.In other words, the peak temperatures of the two Gaussians according to the invention are in some way “signatures” of the mineral or organo-mineral matrices and of the coal and / or biochar, whatever their origin and composition.

[0091] According to a non-limiting implementation of the invention, the residual Gaussian deconvolution method described in the document (Sebag et al., 2006) can be used. More precisely, this method consists of progressively subtracting the Gaussians centered on the main peaks of the signal.

[0092] According to another implementation, one can use the Gaussian deconvolution method by means of an analysis of the mixing of the end members ("End-Member Mixing Analysis" in English) described in particular in the document (Paterson and Heslop, 2015), and which consists of determining, by means of an algorithm, the Gaussian components allowing to best describe the signal, with a given number of Gaussians.

[0093] Figure 2 illustrates the result of the decomposition of a curve C measuring the quantity of CO2QCO2 released during step 2) of the method according to the invention into two Gaussians G1, G2.

[0094] 4) Determination of pyrogenic carbon content

[0095] During this step, from the surfaces of the first and second Gaussians determined in step 3), the content of pyrogenic carbon present in the sample considered is determined. More precisely, according to the invention, the content of pyrogenic carbon present in the sample considered, denoted X3 hereinafter, is determined according to a formula of the type: where X and X2 are carbon contents determined respectively from the surfaces of said first and second Gaussians, A is a coefficient representing the proportion of said matrix in said second Gaussian relative to said first Gaussian, and B is a coefficient representing the proportion of said coal and / or said biochar in said first Gaussian relative to said second Gaussian.

[0096] According to an implementation of the invention, ^ can be determined according to the formula:

[0097] X1= SurfGl (3) and X2 according to the formula:

[0098] X2= SurfG2 (4) where SurfGl and SurfG2 are respectively the surfaces of the first and second Gaussians determined at the end of step 3 of the method according to the invention.

[0099] Equation (2) arises from the fact that deconvolution is imperfect for separating the contribution of the organo-mineral or mineral matrix from the contribution of biochar and / or charcoal in a curve measuring the quantity of CO2 released during an oxidation phase. In other words, the first Gaussian resulting from the deconvolution according to the invention certainly mainly includes a contribution linked to the organo-mineral or mineral matrix (denoted X l mat P ar the following), but also includes a contribution of biochar and / or charcoal (denoted X l bc subsequently). Similarly, the second Gaussian resulting from the deconvolution according to the invention certainly mainly includes a contribution from biochar and / or coal (noted X2, bcsubsequently), but also includes a contribution linked to the organo-mineral or mineral matrix (denoted l, bc hereafter). This is notably illustrated in Figure 3, which schematically represents the share of the organo-mineral or mineral matrix X1,mat and the share of biochar and / or coal X1,bc in the first Gaussian G1, as well as the share of biochar and / or coal X2,bc and the share of the organo-mineral or mineral matrix X2,mat in the second Gaussian G2. With these notations, the coefficients A and B according to the invention can be described by the following formulas: And

[0100] According to a first variant of the invention, the coefficient A and / or the coefficient B can be determined from respectively a sample of the pure organo-mineral or mineral matrix and a sample of pure biochar and / or coal, representative of the organo-mineral or mineral matrix and of the biochar and / or coal present in the sample considered, to which steps 1), 2) and 3) described above are applied. The coefficient A can then be determined by the ratio between the surface area of ​​the second Gaussian and the surface area of ​​the first Gaussian determined from the sample of pure organo-mineral or mineral matrix. The coefficient B can be determined by the ratio between the surface area of ​​the first Gaussian and the surface area of ​​the second Gaussian determined from the sample of pure biochar and / or coal.According to a second variant of the invention, and in particular if samples of the pure organo-mineral or mineral matrix and / or of pure biochar and / or coal, representative of the organo-mineral or mineral matrix and of the biochar and / or coal present in the sample considered, are not available, equation (2) above can be implemented by means of a coefficient A between a value of 0.17 and a value of 0.73 and preferably 0.19, and / or a coefficient B between a value of 0.10 and a value of 4.98 and preferably 0.32. These ranges and preferred values ​​of the coefficients A and B were determined from a plurality of samples of pure organo-mineral or mineral matrix and a plurality of samples of pure biochar and / or coal, of different types, to which the method described above was applied.In particular, samples of pure organo-mineral or mineral matrix of soil and sediment type from varied climatic conditions and with varied total organic carbon contents were used, and samples of pure biochar and / or charcoal from varied plant biomass and varied pyrolysis temperatures, between 450 °C and 650 °C. The preferred value of coefficients A and B corresponds to the median of the values ​​thus determined for the plurality of samples.

[0101] Thus, at the end of this step, we obtain the pyrogenic carbon content present in the sample considered, comprising both a mineral or organo-mineral matrix and coal and / or biochar.

[0102] According to one implementation of the invention, the total mass of pyrogenic carbon present in the sample considered, subsequently denoted Qc, bc nix, can be determined according to a formula of the type:

[0103] Qc, bcjnix = X3 / C * K (7)

[0104] Or

[0105] - C is the ratio between the carbon content determined from the surface of the second Gaussian determined in the case of a sample of biochar and / or pure charcoal, noted X2, bc, and the total mass of carbon in the sample of biochar and / or pure charcoal; in other words, the ratio C can be written

[0106] C = X2, bc / (TOC, bc * Qty, bc) (8) where TOC, bc and Qty, bc correspond respectively to the total organic carbon and the total mass of a sample of pure biochar / charcoal. According to one implementation of the invention, and in particular if a sample of pure biochar and / or charcoal, representative of the biochar and / or charcoal present in the sample considered, is not available, equation (2) above can be implemented by means of a ratio C between a value of 0.51 and a value of 1.76 and preferably 1.06. This range and this preferred value were determined from a plurality of samples of pure biochar and / or charcoal, of different types. The preferred value corresponds to the median of the values ​​thus determined for the plurality of samples.

[0107] - K is a multiplicative coefficient. According to one implementation of the invention, the coefficient K can be between 12.0 and 12.5, and is preferably 12.2. Such values ​​were determined from a plurality of samples of pure biochar and / or coal, of different types.

[0108] Examples

[0109] The characteristics and advantages of the method according to the invention will appear more clearly on reading the application example below.

[0110] The present invention is applied for the purpose of determining the mass of pyrogenic carbon present in a sample corresponding to a soil-biochar mixture,

[0111] A plurality of samples are generated, for different mass ratios between biochar and soil, by a homogeneous mixture between a soil of agricultural origin, and an industrial biochar of herbaceous plants. The total organic carbon (TOC) of the biochar and the soil amounts to 82.60% and 5.55% respectively. Each sample thus formed is dried at a temperature lower than or equal to 40 °C until its weight stabilizes, then ground below 200 pm.

[0112] Each sample is subjected to heating under an inert atmosphere according to the invention, then its residue is subjected to heating under an oxidizing atmosphere according to the invention. Figure 2 shows curve C of the quantity of CO2 QCO2 measured during step 2) of the method according to the invention, as well as the result of step 3) of the method according to the invention, in the form of two Gaussians G1, G2, in the case of a sample containing 1% by mass of biochar.

[0113] Table 1 shows the mass of pyrogenic carbon (given in mg of carbon, mgC) present in the samples considered, determined at the end of step 4 of the method according to the invention applied according to the first variant described above (determination of coefficients A and B of equation (2) from pure soil and biochar samples; column Qc,bc_mix_V1) and according to the second variant described above (determination of coefficients A and B of equation (2) from their preferred values ​​defined above; column Qc,bc_mix_V2), as well as the actual mass of pyrogenic carbon present in the samples considered (column Qc,bc_mix_REAL), as a function of their mass ratio between biochar and soil (column Ratio).It can be observed that the mass of pyrogenic carbon determined by the present invention, applied according to its first variant or its second variant are very close to the real values ​​(average error of 2.31% and maximum of 76.45% (sample with lower biochar / coal content) for the first variant; average error of -3.54% and maximum of 29.63% for the second variant).

[0114] These results were obtained in less than 90 minutes for each sample, a duration which essentially corresponds to the heating time in an inert atmosphere and the heating time in an oxidizing atmosphere for each sample. Thus, the present invention makes it possible to quickly and precisely quantify pyrogenic carbon in a sample of an organo-mineral or mineral matrix further comprising biochar or coal, by means of a simple thermal analysis.

Claims

Claims 1. Method for quantifying the content of pyrogenic carbon present in a sample comprising an organo-mineral or mineral matrix as well as coal and / or biochar, characterized in that: A) said sample is heated in an inert atmosphere according to a first sequence of temperatures of which an initial temperature (T0) is between 100 and 300°C, and is preferably 200°C, and a final temperature (TF) is between 500 and 800°C, and is preferably 650°C; B) a residue of said sample from said heating in an inert atmosphere is heated in an oxidizing atmosphere according to a second temperature sequence of which an initial temperature (T0') is between 100 and 300°C, and is preferably 200°C, and a final temperature (TF') is between 700 and 1000°C, and is preferably 850°C, said second temperature sequence comprising at least one thermal gradient of between 1°C / min and 50°C / min, preferably between 15°C / min and 35°C / min, very preferably 25°C / min, and at least one quantity of CO2 (QCO2) released during said second temperature sequence is measured; C) from a curve representative of the evolution as a function of the temperature of said quantity of CO2 released during said heating in an oxidizing atmosphere, a Gaussian deconvolution is applied to said curve, so as to determine a first and a second Gaussian centered respectively on a first and a second temperature, said first temperature being between 380°C and 540°C, preferably between 415°C and 425°C, and very preferably being 420°C, and said second temperature being between 500 and 600°C, preferably between 570 and 580°C, and preferably being 576°C; D) From the surfaces of said first and second Gaussians, said pyrogenic carbon content X3 present in said sample is determined according to a formula of the type: where X ±and X2 are carbon contents determined respectively from the surfaces of said first and second Gaussians, A is a coefficient representing the proportion of said matrix in said second Gaussian relative to said first Gaussian, and B is a coefficient representing the proportion of said coal and / or said biochar in said first Gaussian relative to said second Gaussian.

2. Method according to claim 1, wherein said first temperature sequence comprises an isothermal plateau of a predetermined duration at said initial temperature (T0) of said first temperature sequence, followed by a thermal gradient to reach said final temperature (TF) of said first temperature sequence, said predetermined duration of said isothermal plateau of said first temperature sequence being between 1 and 5 minutes, and preferably being 3 minutes, and said thermal gradient of said first temperature sequence being between 1 and 50°C / min, preferably between 15°C / min and 35°C / min, and very preferably being 25°C / min.

3. Method according to one of the preceding claims, in which said second temperature sequence further comprises an isothermal stage of a predetermined duration at a temperature between 500 and 600°C, preferably between 570 and 580°C, and preferably 576°C, said predetermined duration of said isothermal stage of said second temperature sequence being between 1 and 5 minutes, and preferably 3 minutes.

4. Method according to one of the preceding claims, in which a sample of said pure organo-mineral or mineral matrix is ​​also available, and in which said coefficient A of step D) is previously determined in the following manner: steps A) to C) are applied to said sample of said pure organo-mineral or mineral matrix, and said coefficient A is determined by calculating the ratio between the surface area of ​​a second Gaussian and the surface area of ​​a first Gaussian determined from said sample of said pure organo-mineral or mineral matrix.

5. Method according to one of the preceding claims, in which a sample of said biochar and / or said pure coal is also available, and in which said coefficient B of step D) is previously determined in the following manner: steps A) to C) are applied to said sample of said biochar and / or said coal, and coefficient B is determined by calculating the ratio between the surface of said first Gaussian and the surface of said second Gaussian determined from said sample of biochar and / or pure coal.

6. Method according to one of the preceding claims, in which step D) is applied by means of a coefficient A between a value of 0.17 and a value of 0.73, and preferably having a value of 0.19, and / or by means of a coefficient B between a value of 0.10 and a value of 4.98, and preferably having a value of 0.

32.

7. Method according to one of the preceding claims, in which a total mass of pyrogenic carbon Qc, bcjnix present in said sample is determined according to a formula of the type: Qc, bcjnix = X3 / C * K, where C is a ratio between a carbon content determined from a surface of a second Gaussian determined for a sample of biochar and / or pure coal, and a total mass of carbon in said sample of biochar and / or pure coal, and where K is a multiplicative coefficient between 12.0 and 12.5, and preferably 12.

2.

8. Method according to one of claims 1 to 6, in which a total mass of pyrogenic carbon Qc, bc nix present in said sample is determined according to a formula of the type: Qc, bcjnix = X3 / C * K, where C is a ratio between a value of 0.51 and a value of 1.76, and preferably being 1.06, and where K is a multiplicative coefficient between 12.0 and 12.5, and preferably being 12.2.

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