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 and charcoal and / or biochar. The method comprises a step of heating the sample in an inert atmosphere and then a step of heating the residue from the step of heating the sample in an inert atmosphere in an oxidising atmosphere, and a step of measuring the amount of CO2 released during the step of heating the residue in an oxidising atmosphere. The measured curve is decomposed into a first component corresponding to the portion of the curve associated with temperatures below a limit temperature and a second component corresponding to a portion of the curve associated with temperatures higher than or equal to the limit temperature, the limit temperature being between 500°C and 550°C, and preferably equal to 530°C. The pyrogenic carbon content is determined from the surfaces of the first and second components.

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.

[0013] 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. 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 as a function of 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.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.

[0025] 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.

[0026] Summary of the invention

[0027] 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;

[0028] B) a residue of said sample from said heating in an inert atmosphere is heated in an oxidizing atmosphere according to a second sequence of temperatures 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, and at least a quantity of CO2 (QCO2) released during said second sequence of temperatures is measured;

[0029] 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, said curve is broken down into at least a first and a second component, said first component corresponding to a part of said curve associated with temperatures lower than a limit temperature, and said second component corresponding to a part of said curve associated with temperatures greater than or equal to said limit temperature, said limit temperature being between 500°C and 550°C, and preferably being 530°C;

[0030] D) From the surfaces of said first and second components, 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 components, A is a coefficient representing the proportion of said matrix in said second component relative to said first component, and B is a coefficient representing the proportion of said coal and / or said biochar in said first component relative to said second component.

[0031] 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 preferably being 3 minutes, and said thermal gradient of said first temperature sequence being able to be between rC / min and 50°C / min, preferably between 15°C / min and 35°C / min, and very preferably being 25°C / min.

[0032] According to one implementation of the invention, said second temperature sequence may comprise at least one thermal gradient between 1°C / min and 50°C / min, preferably between 15°C / min and 35°C / min, and 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 490 and 600°C, preferably between 500 and 550°C, and preferably 520°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 component and the surface area of ​​a first component 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 component and the surface area of ​​said second component 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 0.08 and a value of 0.76 and preferably 0.09, and / or a coefficient B between a value of 0.90 and a value of 46.60 and preferably 20.73.

[0037] According to one implementation of the invention, a total mass of pyrogenic carbon Qc, bc_mix 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 area of ​​a second component 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 3.5 and 4.0, and preferably 3.7. Alternatively, a total mass of pyrogenic carbon Qc, bc nix present in said sample can be determined according to a formula of the type: Qc, bcjnix = X3 / C * K, where C is a ratio between a value of 0.04 and a value of 0.74 and preferably 0.07, and where K is a multiplicative coefficient between 3.5 and 4.0, and preferably 3.7.

[0039] 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 components resulting from the decomposition according to step 3) of the method according to the invention.

[0044] Figure 3 schematically shows the share of the organo-mineral or mineral matrix and the share of biochar and / or coal in the first component resulting from the decomposition according to step 3) of the process 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 component resulting from the decomposition according to step 3) of the process 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 been subjected to the effect of fire and / or heat (temperature >200°C) in the absence or low concentration of oxygen. Organo-mineral or mineral matrix means an unconsolidated, porous material consisting of a mixture of organic and / or mineral particles of varying size and chemical and / or mineralogical composition.

[0048] Coal is understood to mean the solid residues 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, it is also possible to have a sample of the biochar and / or coal present in the sample in question. The method according to the invention can be advantageously but not limited to 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:

[0057] - a pyrolysis oven in a non-oxidizing atmosphere,

[0058] - means of transferring pyrolysis residues into an oxidation furnace,

[0059] - an oxidation furnace in an oxidizing atmosphere,

[0060] - means of measuring the quantity of hydrocarbon compounds (HC) released during pyrolysis,

[0061] - means of measuring carbon monoxide (CO) and carbon dioxide (CO2).

[0062] 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.

[0063] The method according to the invention comprises at least the following steps:

[0064] 1) Heating sequence under inert atmosphere (pyrolysis)

[0065] 2) Heating sequence under oxidizing atmosphere (oxidation)

[0066] 3) Decomposition into two components

[0067] 4) Determination of pyrogenic carbon content

[0068] The steps of the method according to the invention are detailed below.

[0069] 1. Heating sequence under inert atmosphere (pyrolysis)

[0070] 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.

[0071] 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 T0, followed by a thermal gradient until reaching the temperature TF.

[0072] 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 1 B 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.

[0073] 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.

[0074] 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).

[0075] 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.

[0076] 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 process.

[0077] 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 by means of a flame ionization (FID) type detector. The measurement of the quantity of CO and CO2 released can be carried out by means of an infrared (IR) type 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).

[0083] According to one implementation of 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. In particular, this second temperature sequence may comprise such a thermal gradient to connect the initial temperatures T0' and final temperatures TF' of the temperature sequence under an oxidizing atmosphere.

[0084] Generally speaking, the preferred temperature range for the initial temperature T0' 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.

[0085] 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 T0' 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.

[0086] 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.

[0087] Advantageously, the sequence of temperatures under an oxidizing atmosphere may further comprise an isothermal stage of a predetermined duration at a temperature between 490 and 600°C, preferably between 500 and 550°C, and preferably 520°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 non-zero predetermined duration (for example greater than half a minute), preferably between 1 and 5 minutes, very preferably 3 minutes.According to this embodiment, the sequence of temperatures under an oxidizing atmosphere may comprise two thermal gradients, preferably between 1 and 50°C / min, preferably between 15 and 35°C / min, and very preferably 25°C / min. According to this implementation, the isothermal plateau at a temperature between 490 and 600°C, preferably between 500 and 550°C, and preferably 520°C may be both preceded and followed by the at least two thermal gradients of this embodiment.

[0088] 3) Decomposition into two components

[0089] During this step, a curve representative of the evolution as a function of the temperature of said quantity of CO2 released during the second heating sequence is broken down into at least a first and a second component, the first component corresponding to the part of said curve associated with temperatures lower than a limit temperature, and said second component corresponding to the part of said curve associated with temperatures greater than or equal to said limit temperature, said limit temperature being between 500°C and 550°C, and preferably being 530°C.

[0090] In other words, during this step, two components are delimited on the CO2 curve measured in step 2), as a function of a vertical line passing through the limit temperature according to the invention. This is illustrated in Figure 2, which shows 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 components C1, C2 as a function of a vertical line D passing through the limit temperature TL, 520°C for this illustration.

[0091] The limit temperature according to the invention corresponds to the temperature below which the majority of the CO2 from the mineral or organo-mineral matrix is ​​emitted and above which the majority of the CO2 from the biochar or coal is emitted. This limit temperature could be observed on several samples of soil or sediments comprising biochar or coal. We thus obtain a first component which represents the portion of CO2 released mainly by the mineral or organo-mineral matrix composing the sample considered, and a second component which represents the portion of CO2 released mainly by the biochar and / or the coal composing the sample considered.According to one implementation of the invention, the limit temperature according to the invention can be determined by determining the temperature of an inflection point of the curve representing the evolution as a function of the temperature of the quantity of CO2 released during the second heating sequence in a part of this curve between 500°C and 550°C.

[0092] 4) Determination of pyrogenic carbon content

[0093] During this step, from the surfaces of the first and second components 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 components, A is a coefficient representing the proportion of said matrix in said second component relative to said first component, and B is a coefficient representing the proportion of said coal and / or said biochar in said first component relative to said second component.

[0094] According to an implementation of the invention, ^ can be determined according to the formula:

[0095] X ± = SurfGl (3) and X2 according to the formula:

[0096] X2= SurfG2 (4) where SurfGl and SurfG2 are respectively the surfaces of the first and second components determined at the end of step 3 of the method according to the invention.

[0097] Equation (2) arises from the fact that the decomposition according to step 3) is imperfect for separating the contribution of the organo-mineral or mineral matrix from the contribution of the biochar and / or the charcoal in a curve measuring the quantity of CO2 released during an oxidation phase. In other words, the first component resulting from the decomposition according to the invention certainly mainly comprises a contribution linked to the organo-mineral or mineral matrix (denoted X l mat subsequently), but also includes a contribution of biochar and / or charcoal (denoted X l bc subsequently). Similarly, the second component resulting from the decomposition according to the invention certainly mainly comprises a contribution of biochar and / or coal (noted X 2 bcsubsequently), but also includes a contribution linked to the organo-mineral or mineral matrix (denoted XI, 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 component C1, 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 component C2, the components C1 and C2 being delimited by the straight line D. With these notations, the coefficients A and B according to the invention can be described by the following formulas: And

[0098] 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 component and the surface area of ​​the first component 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 component and the surface area of ​​the second component determined from the sample of pure biochar and / or coal.

[0099] 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 0.08 and a value of 0.76 and preferably being 0.09, and / or a coefficient B between a value of 0.90 and a value of 46.60 and preferably being 20.73. 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. Thus, at the end of this step, the pyrogenic carbon content present in the sample considered, comprising both a mineral or organo-mineral matrix and charcoal and / or biochar, is obtained.

[0100] 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:

[0101] Qc, bcjnix = X3 / C * K (7)

[0102] Or

[0103] - C is the ratio between the carbon content determined from the surface area of ​​the second component 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

[0104] 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 0.04 and a value of 0.74 and preferably 0.07. 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.

[0105] - K is a multiplicative coefficient. According to one implementation of the invention, the coefficient K can be between 3.5 and 4.0, and is preferably 3.7. Such values ​​were determined from a plurality of samples of pure biochar and / or coal, of different types.

[0106] Examples

[0107] The characteristics and advantages of the method according to the invention will appear more clearly on reading the application example below.

[0108] 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. 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 less than or equal to 40°C until its weight stabilizes, then ground below 200 μm.

[0109] 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, in the case of a sample containing 1% by mass of biochar, the curve C of the quantity of CO2 QCO2 measured during step 2) of the process according to the invention, as well as the result of step 3) of the process according to the invention, in the form of two components C1, C2 delimited by the vertical line D passing through the limit temperature TL equal to 520°C.

[0110] 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 1.36% and maximum of 26.37% for the first variant; average error of -18.43% and maximum of -46.80% for the second variant).

[0111] 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.

[0112] Thus, the present invention makes it possible to rapidly 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 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 700 and 1000°C, and is preferably 850°C, and at least a quantity of CO2 (QCO2) released during said second sequence of temperatures 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, said curve is broken down into at least a first and a second component, said first component corresponding to a part of said curve associated with temperatures lower than a limit temperature, and said second component corresponding to a part of said curve associated with temperatures greater than or equal to said limit temperature, said limit temperature being between 500°C and 550°C, and preferably being 530°C; D) From the surfaces of said first and second components, 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 components, A is a coefficient representing the proportion of said matrix in said second component relative to said first component, and B is a coefficient representing the proportion of said coal and / or said biochar in said first component relative to said second component.

2. Method according to claim 1, wherein said first temperature sequence comprises an isothermal plateau of a predetermined duration at said initial temperature (TO) 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°C / min 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 comprises at least one thermal gradient between rC / min and 50°C / min, preferably between 15°C / min and 35°C / min, and very preferably 25°C / min.

4. 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 490 and 600°C, preferably between 500 and 550°C, and preferably 520°C, said predetermined duration of said isothermal stage of said second temperature sequence being between 1 and 5 minutes, and preferably 3 minutes.

5. 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 coefficient A is determined by calculating the ratio between the surface area of ​​a second component and the surface area of ​​a first component determined from said sample of said pure organo-mineral or mineral matrix.

6. 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 area of ​​said first component and the surface area of ​​said second component determined from said sample of biochar and / or pure coal.

7. Method according to one of the preceding claims, in which step D) is applied by means of a coefficient A between a value of 0.08 and a value of 0.76 and preferably worth 0.09, and / or a coefficient B between a value of 0.90 and a value of 46.60 and preferably worth 20.

73.

8. 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 area of ​​a second component 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 3.5 and 4.0, and preferably 3.

7.

9. Method according to one of claims 1 to 7, 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 0.04 and a value of 0.74 and preferably worth 0.07, and where K is a multiplicative coefficient between 3.5 and 4.0, and preferably worth 3.7.

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