Integrated slow oxidative pyrolysis process and anaerobic digestion for the production of biomethane

Abstract

The invention concerns an integrated process of slow oxidative pyrolysis (SOP) of an organic material, e.g. biomass, and an anaerobic co-digestion (AD) process of the aqueous phase of APL pyrolysis in order to enhance the production of biomethane.

Description

[0001] "INTEGRATED SLOW OXIDATIVE PYROLYSIS PROCESS AND ANAEROBIC DIGESTION FOR THE PRODUCTION OF BIOMETHANE”

[0002] *****

[0003] DESCRIPTION

[0004] Field of the Invention

[0005] The invention relates to an integrated plant and process for the production of biomethane by means of anaerobic digestion of the aqueous pyrolysis liquids (Aqueous Pyrolysis Liquids, “APL”), resulting from thermochemical treatment of slow oxidative pyrolysis (SOP) of biomass, e.g. lignocellulosic biomass, manure or other suitable material, e.g. municipal waste.

[0006] Background art

[0007] It is known that the pyrolysis process basically consists of the thermal decomposition of organic material in the absence of oxygen.

[0008] The main products of the process are a carbonaceous solid with a porous structure (biochar), and a pyrogas (pyrolysis vapours), which in turn is composed of noncondensable gases and a condensable fraction. The latter is composed of two distinct phases, both produced by the thermal decomposition of the organic material, which tend to separate spontaneously: a phase rich in heavy organic substances, poor in water, (so-called "oily" fraction), and a liquid organic fraction (APL), mainly consisting of water but also containing light organic substances soluble in it, such as acetic acid, methanol, and others. The two phases of the condensable fraction can be separated both in the condensation phase itself, for example by adopting a fractional condensation system operating at different temperature levels, and subsequently, for example, through systems such as sedimentation, centrifugation, or other apparatus designed to separate the aqueous fraction from the oily one.

[0009] A significant advantage of the pyrolysis process is the ability to produce products of potential industrial interest. In particular, the vapors can be condensed to obtain a liquid fraction normally called bio-oil (or pyrolysis oil) from which to obtain a liquid fuel or high value-added chemicals while only a fraction of the vapors produced inside the reactor is composed of non-condensable gases, mainly carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and hydrogen (H2).

[0010] The carbonaceous solid, thanks to its porous structure, can find numerous applications for example as a filter material, soil improver, insulating material, molecule carrier, in human and animal nutrition, in paints and varnishes, in sensors, cement and steel, and numerous other fields.

[0011] Finally, the APL liquid fraction, being rich in organic acids and alcohols, in particular acetic acid, can be used in anaerobic digestion processes to be converted into biomethane.

[0012] The yield of pyrolysis products varies depending on the material being treated and the reaction conditions, especially temperature and residence time in the reactor.

[0013] Pyrolysis is in fact generally classified according to the operating conditions of the process into fast, intermediate and slow, as shown indicatively in table 1 .

[0014] Table 1 also compares the main operating parameters and typical product yields of the three types of pyrolysis.

[0015] Tabella 1:

[0016] Typical reaction conditions and product yields for slow, intermediate and fast pyrolysis in the case of lignocellulosic biomass.

[0017] Fast pyrolysis is therefore characterized by high heating rates and low residence times of hot vapors, in order to maximize the liquid yield and reduce the cracking of the organic molecules thus formed. In contrast, slow pyrolysis is characterized by low heating rates and high residence times of hot vapors, so as to promote char production. Finally, intermediate pyrolysis combines long residence times of hot vapors in the reactor and moderate heating rates to achieve combined production of char and bio-oil.

[0018] The pyrolysis process can also be carried out in a slightly oxidizing atmosphere, if a moderate amount of air is injected into the reactor, to promote exothermic reactions. In this case, the process is called oxidative pyrolysis, or autothermal, and the heat necessary for the process is provided internally to the reactor by the partial oxidation of the biomass itself and the pyrolysis products (the pyrogas previously defined). Since the partial oxidation reactions take place inside the reactor, there is no longer any need for heat carriers or auxiliary indirect heat exchange equipment (allothermic pyrolysis), greatly simplifying the process and plant layout.

[0019] In general, for the reasons stated above, the pyrolysis process is considered interesting in the recovery processes of materials such as biomass, waste, sludge and more, because as the treated material varies, the composition and percentage of the resulting products varies, which can be used in different sectors of industry.

[0020] In particular, within the scope of the present invention, examples of combining pyrolysis processes with anaerobic digestion processes of the aqueous phase of APL pyrolysis are known.

[0021] As an example, from "Integration of pyrolysis and anaerobic digestion - Use of aqueous liquor from digestate pyrolysis for biometano production Hubner T. et al., 2015" https: / / d .Org / 1 Q.1016 / i bsorteoh 2015.02.037 the APL phase is obtained by pyrolysis of ground solid digestate, treated at three different temperatures (330, 430, 530 °C). In the process described here, pyrolysis was conducted in a rotary furnace with a solids retention time of 45±15 minutes and continuous washing with nitrogen. From the above study, it is clear that the main focus is only the optimization of the anaerobic digestion process of APL and, secondly, that the only variable that can be optimized in the slow inert pyrolysis process is explicitly mentioned to be the maximum process temperature. The APL investigated thus presents itself with a high organic load of inhibitors (phenols, furans, PAHs) compared to VFAs, partially inhibiting the anaerobic digestion process.

[0022] A method for the treatment of biological waste has been known for WO2022043478 in which the liquid fraction resulting from the pyrolysis of a biomass is treated through subsequent anaerobic digestion for the production of biomethane. In the previous document mentioned, a slow inert pyrolysis process is used, and the main focus of the patent is to propose integrated and compatible solutions aimed at minimizing waste and enhancing the residues recovered in anaerobic digestion. However, this document does not go into detail on the quantitative and qualitative aspects of the APL obtainable. Therefore, at present, in the production of biomethane the combination of pyrolysis processes with anaerobic digestion processes of the liquid pyrolysis phase there is room for improvement through the advantages offered by the use of SOP for the production of APL. The proposed scheme, in fact, focuses on the integration of the two processes (SOP and AD), using the slow oxidative pyrolysis process in order to obtain an APL with better characteristics for the purposes of anaerobic digestion, as mass yield and product quality are better than what can be obtained from classic slow pyrolysis, i.e. inert. In addition, this scheme is accompanied by advantages that can also be found in terms of optimization of the industrial process (energy saving, process intensification) and on the quality of the solid residual product obtainable (increasing the porous structure of the biochar and increasing its stability).

[0023] Aim of the invention

[0024] The present invention therefore intends to enhance the integration of pyrolysis and anaerobic digestion processes through the oxidative pyrolysis process, which guarantees lower energy consumption than inert pyrolysis, a better quality of the solid residue (biochar) and the production of an APL particularly suitable for interaction with DA as it can be modulated in the content and quality of the organics present.

[0025] Summary of the invention

[0026] These results were achieved by implementing an integrated process according to at least the main claim, in which a process of slow oxidative pyrolysis (SOP) of an organic material, preferably a biomass, is integrated with a process of anaerobic digestion of the aqueous pyrolysis phase for the production of biomethane with an additional digestion substrate.

[0027] A first advantage consists in the fact that the addition of an oxidizing agent (e.g. air, oxygen) during oxidative pyrolysis, in addition to constituting an advantage in terms of energy saving of the process, plant configuration and process intensification (higher input flow rate processed by the oxidative plant compared to a similar non- oxidative one), promotes oxidation reactions that significantly affect the yields and composition of pyrolysis products; in particular, with substantially the same yield of char produced, as the equivalence ratio ("ER", quantity of oxidant introduced compared to the stoichiometric quantity required for complete combustion) increases, an increase in the percentage of aqueous phase (APL) on the total recoverable condensable products is observed, and therefore a higher overall efficiency of the combined pyrolysis process and anaerobic digestion of the aqueous pyrolysis phase for the production of biomethane.

[0028] A further advantage is that in the oxidative pyrolysis process it is possible to adjust the equivalence ratio in order to reduce the concentration of unwanted compounds (i.e. phenols, furans, nitrogenous aromatic compounds and heavy compounds precursors of bioil) to the advantage of the compounds desired for degradation into DA (e.g. volatile fatty acids "VFA" and alcohols).

[0029] Although a slight reduction in char yield was found as the ER increased, the oxidizing atmosphere resulted in a reduction in the H:C and O:C molar ratios of the biochar produced with a consequent increase in the degree of aromaticity and greater stability often accompanied by an increase in surface area.

[0030] List of drawings

[0031] These and other advantages will be better understood by a skilled person from the description below and from the attached drawings, given as a non-limiting example, in which:

[0032] - Fig.1 schematically shows an integrated process according to the invention. Detailed Description

[0033] With reference to the attached drawings, an integrated process is described, according to the invention, between a slow oxidative pyrolysis process of an organic material, hereinafter called "SOP", and an anaerobic digestion process "AD" in the presence or absence of an organic substrate (SD) for the production of biomethane. The SOP process receives a controlled amount of organic material or FS feedstock, preferably composed of biomass or waste, which is subjected to a pyrolysis process in a heated process chamber of a reactor RP.

[0034] In different process implementation examples, the RP pyrolysis reactor can be screw, rotary furnace, fixed bed, fluidized bed type.

[0035] The processed material FS is heated with a heating rate preferably less than 50°C min-1 and kept preferably in a range between 350°C and 650°C and is fed, preferably with adjustable speed, in one direction of feed inside from the feeding section to a pyrolysis solids extraction section of said process chamber so as to maintain a residence time of the FS material in the reactor preferably greater than 20 min.

[0036] The process also involves the insufflation of a controlled quantity of AIR air inside said process chamber into one or more insufflation points in order to promote a controlled oxidation process between the oxygen of the introduced air and the vapors and gases developed by the pyrolysis process.

[0037] In particular, through the controlled insufflation of air, the effect of maintaining the equivalence ratio ER between 0 and 0.3 is obtained.

[0038] The pyrolysis process produces "BIOCHAR" coal which is extracted at an extraction section of the reactor RP, non-condensable gases which are extracted from the reactor, and condensable vapors VC which are extracted and fed into a separator CD to be converted into pyrolysis liquids, separated in turn into a predominantly organic fraction "OIL" of pyrolysis oils and an aqueous fraction "APL", composed mainly of water and water-soluble organic compounds, especially acetic acid.

[0039] According to the invention, the APL aqueous fraction obtained with the SOP process of oxidative slow pyrolysis described above is introduced into an AD anaerobic digestion reactor for the production of biomethane in the presence or absence of an organic substrate SD.

[0040] Advantageously, the main organic constituents of APL (organic acids and alcohols) are easily degraded by the microbial consortium of anaerobic digestion and in particular acetic acid, which constitutes its main organic constituent, is directly converted into biomethane.

[0041] It has also been observed that advantageously, thanks to the SOP process, any inhibitors of the anaerobic digestion process typically present in APL (i.e. phenols, furfurals, nitrogen-based aromatic compounds, etc.) are less concentrated than those degradable in anaerobic digestion.

[0042] In particular, it was found that it is possible to control the SOP process to lower the concentration of inhibitory components by modulating the ER. In addition, it was found that by the continuous addition of APL in the AD reactor, the microorganisms responsible for the anaerobic digestion process are able to adapt (acclimatization process), increasing their ability to degrade even potentially inhibitory compounds; in fact, a particularly effective bacterial community is created and suitable for the digestion of APL, which thus increases the biomethane yields from the AD of the same. Although the composition of APL and the concentration of compounds still depend on the FS raw materials used in the SOP pyrolysis process and the pyrolysis conditions themselves, in order to optimize the production and composition of the APL aqueous phase to feed the AD process, it was found that in an integrated process of the type described, the control of the pyrolytic process parameters, in particular, temperature, residence time of solids, residence time of vapours, oxygen / air content in the reactor, allows the optimization of the composition of the APL product required for the subsequent AD treatment (and in particular the concentration of acetic acid) and to control and modify the APL parameters such as pH and organic content by "customizing" the aqueous condensate for the AD process required in a given application.

[0043] Once the APL aqueous phase has been produced with the SOP process, the APL recovered from pyrolysis can be mixed into water or other liquid substrates and integrated into an AD anaerobic digester, e.g. a Stirred-Tank Reactor (CSTR) or a dedicated "Up-flow Anaerobic Sludge Blanket" (UASB) reactor, possibly also fed with other co-digestion substrates SD.

[0044] In different implementation examples, APL can be added not only in wet AD processes, but also in semi-dry and dry systems such as Plug Flow Reactors (PFRs), within the limits of the moisture content required for the process.

[0045] In a further implementation example, it was also found that the solid pyrolysis product biochar obtained downstream of a described SOP oxidative slow pyrolysis process can usefully be used as an additive together with APL for the anaerobic digestion process in the AD reactor where it improves and accelerates the process by interacting with both the microorganisms and the medium (interaction with the DIET-Direct Electron Transfer, but also buffering capacity and pollutant sequestration for some substrates, etc.) and contributes to improving the conversion of APL to biomethane, for example by sequestering AD inhibitors contained in APL and creating a favourable environment for AD microorganisms. EXAMPLE 1

[0046] In this example, anaerobic co-digestion tests were carried out in the presence of different concentrations of APL where a consistent increase in biomethane production was observed compared to samples without APL.

[0047] SOP Process

[0048] APL was produced from a SOP process of slow oxidative pyrolysis of a lignocellulosic biomass (Ulmus Pumila) with the following parameters Equivalence ratio ER 0.04

[0049] Maximum process temperature 550°C

[0050] Average heating rate 17.6°C min-1 residence time of the material 30 minutes.

[0051] The fraction of APL obtained from condensation was subsequently separated by exploiting the different density of the two components.

[0052] AD Process

[0053] The APL produced by the SOP process was fed into anaerobic digestion bioreactors (laboratory-scale batch process) in co-digestion with organic substrate (com silage). The recipe provided for a fixed volume of the reactors filled only with mesophilic bacterial inoculum suitable for anaerobic digestion, an addition of substrate (com silage) fixed on the organic load of the inoculum and the same in all reactors, and the addition of a volume of water equal to the volume of the inoculum until a fixed final volume value was reached and the same in all reactors. The control samples received only ultra-pure water as a complement while the other samples received an increasing amount of APL in place of a portion of the complement water, according to different percentages in volume (compared to the total complement) equal to 2.5% I 7.5% I 15% I 25% while keeping the final volume of the recipe unchanged. The APL was neutralized by NaOH pellets upstream of the AD process, and the organic load was analyzed by analysis of the Chemical Oxygen Demand (COD equal to 115 g / l) and the main organic constituents by HPLC as shown in the table below.

[0054] Table 2. Composition of the main organic constituents of APL produced by lignocellulosic biomass and 550° C and ER 0.04, residence time of the solid 30 min.

[0055] Compound g / L

[0056] Glycerol 0,75

[0057] The measurement of biomethane produced was carried out until the plateau phase was reached (no production for at least 5 days) by all the AD reactors used to define the methane potential (BMP-BioMethane Potential), and tripled for each case study. Results All samples containing APL, in triplicate, produced higher quantities of biomethane than the control samples without APL with production yields 18% higher up to more than 100% more biomethane than the control, these production increases are therefore attributable to the conversion of the compounds contained in the APL added in the reactors, especially acetic acid. Table 3. Biomethane yield increase for samples containing

[0058] APL in increasing amount, compared to control (without APL).

[0059] EXAMPLE 2

[0060] A flow rate of 1.5 kg / h of lignocellulosic biomass, in this example poplar chips, was subjected to the SOP process of the invention at constant temperature (500°C) but at an increasing ER from inert, ER equal to 0, up to ER equal to 0.3.

[0061] The resulting vapours were condensed as pyrolysis liquids and separated by density into an oily organic phase and an APL aqueous phase.

[0062] The APL phase was then characterized in terms of water content, by Karl Fisher titration, and the complementary percentage was assumed to be constituted by the organic fraction of APL.

[0063] By HPLC analysis it was possible to classify the main compounds constituting APL, in particular carboxylic acids (i.e. acetic acid) and alcohol (i.e. methanol, glycerol) considered compounds easily degradable by DA; and aromatics (phenolic compounds) and ketones (i.e. furfural, HMF) considered more difficult to degrade and above certain inhibitory concentrations of the DA process.

[0064] The remaining organic component of water is considered to be made up of heavier organic compounds as oligomers precursors of the oil phase and generally considered difficult to degrade into biogas by microorganisms and potentially inhibiting.

[0065] As the ER increased, a growing increase in compounds related to anaerobic digestion was observed compared to other organics undesirable to the DA process. As the ER increased it was also observed an increase in the ratio of the APL phase to the total pyrolysis liquids, and consequently a lower production of the oily organic phase.

[0066] Results

[0067] The organic oil phase was easy to separate from APL with consequent less contamination of oily organic compounds (undesirable in DA, as inhibitors for the microorganisms involved in the process) in the same.

[0068] It was then found that as the ER increases, more APL is produced (ranging from yields of 27% m / m on the input biomass, in the inert case ER 0, up to 33.2% in the case of ER 0.3) with a decrease in the organic load; there is a decrease in the workforce in APL of about 40% at ER 0.04 compared to the inert case up to values above 60% with ER greater than 0.1 .

[0069] The ratio of desired organics (i.e. carboxylic acids and alcohols) and undesirable organics (i.e. aromatics, furans and heavy compounds) already increases by 30% at ER 0.04, up to more than 90% in the ER 0.3 case, with central values at ER 0.07 and 0.12 of about 70 and 90% respectively. In conclusion, as the ER increases, the amount of water in the APL increases but the organic share produced is more similar to anaerobic digestion as it is enriched with organic acids and alcohols and lightened of undesirable compounds and therefore more favorable to the production of biomethane through an anaerobic digestion process. The invention has been described with reference to a preferred form of implementation, but it is understood that equivalent modifications may be made without however leaving the scope of protection granted to the present industrial right.

Claims

CLAIMS1. Integrated pyrolysis process of organic material (FS) and anaerobic digestion of the aqueous pyrolysis phase for the production of biomethane, comprising the following steps feeding a feed section of a process chamber of a pyrolysis reactor (RP) with a controlled flow of organic material, heating said organic material (FS) with a heating rate up to 50 °C / min, advancing said organic material in an advancement direction from the feed section to an extraction section of pyrolysis solids of said process chamber, insufflating a controlled amount of air (AIR) inside said process chamber in one or more insufflation points in order to maintain an equivalence ratio (ER) between the oxygen contained in the introduced air and that necessary for the complete combustion in stoichiometric conditions comprised between 0 and 0.3, and promoting a controlled oxidation process between the oxygen of the introduced air and the vapours and gases developed by the pyrolysis process, extracting said pyrolysis gases and vapours (GP) from one or more extraction points along said process chamber, separating from said pyrolysis vapours, by means of a condenser (CD), an organic fraction comprising pyrolysis oils (OIL) and an aqueous phase fraction (APL) comprising acetic acid and other organic compounds miscible with water, maintaining the maximum process temperature inside the reactor between 350°C and 700°C, extracting the solid pyrolysis products (BIOCHAR) at said second section of said process chamber after a residence time in the process chamber greater than 20 minutes, introducing said separated fraction in aqueous pyrolysis phase (APL) into a reactor (AD) for the production of biomethane by means of anaerobic digestion of an organic digestion substrate (SD).

2. Integrated process according to claim 1 , wherein said aqueous phase (APL) is continuously fed into said anaerobic digestion reactor.

3. Integrated process according to claim 1 , wherein said aqueous phase (APL) ismixed with water downstream of said step of introduction into the anaerobic digestion reactor.

4. Integrated process according to one of the preceding claims, wherein said step of extracting the pyrolysis vapours is carried out after an adjustable residence time of the vapours in the reactor comprised between 1 and 30 seconds, preferably between 7 and 15.

5. Integrated process according to one of the preceding claims, wherein a fraction of the pyrolysis solid (biochar) produced by the pyrolysis reactor (RP) is fed into said anaerobic digestion reactor (AD).

6. Integrated process according to one of the preceding claims, wherein said anaerobic digestion reactor (AD) can be fed with further co-digestion substrates (SD) together with the APL, for example dedicated crops (i.e., silage), livestock waste, residues of the agri-food industry, OFMSW (Organic Fraction of Municipal Solid Waste), sludge etc.

7. Integrated process according to one of the preceding claims, wherein said organic material (FS) consists of lignocellulosic type biomass.

8. Integrated process according to one of the preceding claims, wherein said pyrolysis reactor is a screw-type, rotary kiln, fixed bed, fluid bed reactor.

9. Integrated process according to one of the preceding claims, wherein said anaerobic digestion reactor is of the continuous-flow Stirred-Tank Reactor (CSTR) type or of the “Up-flow Anaerobic Sludge Blanket” (UASB) type or of the semi-dry and dry type such as Plug Flow Reactor (PFR) type.

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