Wet oxidation of digestate from anaerobic digestion

Abstract

A process for producing biogas that includes (i) anaerobically digesting lignocellulosic feedstock in an anaerobic digestion to produce a digestate; (ii) treating at least some of the solids from the digestate in a wet oxidation with lime; (iii) precipitating and recovering calcium from liquid from the wet oxidation as calcium carbonate; (iv) sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and (v) recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.

Description

Wet Oxidation of Digestate from Anaerobic DigestionTechnical Field

[0001] The present disclosure relates to a process for producing biogas in which digestate is subjected to wet oxidation.Background

[0002] Biogas is a renewable source of energy, and its use could aid in meeting net zero carbon emission targets set by regulators. The gas is produced by the anaerobic digestion of organic matter and contains methane, the primary compound in natural gas. Biogas is usually upgraded to remove impurities, such as carbon dioxide, and the resultant renewable natural gas (RNG) can be distributed using natural gas pipelines and / or can be used in any application in which conventional natural gas is used (e.g., transportation, household heating, or industrial processes).

[0003] Despite the potential of RNG to reduce greenhouse gas (GHG) emissions, biogas production produces digestate as a byproduct. The digestate includes liquid and / or solid residue remaining after anaerobic digestion. Large-scale biogas production can produce large quantities of digestate as a byproduct and managing and / or processing such large quantities of digestate can be challenging. For example, the amount of digestate that can be land- applied is often limited due to concerns of over fertilizing, the build-up of nutrients and / or potentially toxic compounds in the soil, and / or the contamination of ground and / or surface water. Accordingly, if a large amount of digestate is produced, the process can require trucking at least some of the digestate long distances and / or landfilling at least some of the digestate. This increases the cost and operating expenses, has environmental consequences, and / or complicates the process (e.g., requires infrastructure to accommodate an increasing number of vehicles and / or causes logistical problems).

[0004] Therefore, there is a need in the art to address the problems of handling and / or disposing of digestate from anaerobic digestion. There is also an ongoing need to find processes that will facilitate the reduction of GHG emissions to ultimately achieve net zero emissions targets.Summary

[0005] The present disclosure seeks to address one or more of the foregoing problems and / or provide useful alternatives thereof.

[0006] The present disclosure provides a process in which digestate from biogas production is subjected to wet oxidation using lime. Feeding at least some of the digestate to wet oxidation can help degrade undigested material from the anaerobic digestion (e.g., lignin) so that at least some of the undigested material can be converted to additional biogas by anaerobic digestion (e.g., to increase biogas yield). The use of lime as alkali in wet oxidation allows for the recovery of calcium carbonate (CaCCh), which is sent to a calcination to produce calcium oxide (CaO) and carbon dioxide (CO2). At least some of the CaO or Ca(OH)2derived therefrom is recycled to wet oxidation, thereby reducing chemical usage of digestate processing. Yet further, in some embodiments, a liquid separated from the process can be recycled to incoming lignocellulosic feedstock and / or anaerobic digestion and contains reduced levels of sodium than would otherwise arise from the use of sodium hydroxide in wet oxidation. This is advantageous since high sodium concentrations can negatively impact the performance of anaerobic digestion. In some further or alternative embodiments, CO2produced during the process (e.g., originating from biogas and / or from calcination) is used within the process (e.g., to lower the pH of a liquid and precipitate out calcium carbonate (CaCOs)) and / or is used to reduce life cycle GHG emissions of the biogas, or product produced using the biogas (e.g., is sequestered).

[0007] According to an aspect of the disclosure, there is provided a process for producing biogas, the process comprising: (i) anaerobically digesting lignocellulosic feedstock in an anaerobic digestion to produce a digestate; (ii) treating at least some of the solids from the digestate in a wet oxidation with lime; (iii) precipitating and recovering calcium from liquid from the wet oxidation as calcium carbonate; (iv) sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and (v) recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.

[0008] According to an aspect of the disclosure, there is provided a process of producing biogas, the process comprising: (i) anaerobically digesting lignocellulosic feedstock in an anaerobic digestion to produce biogas and digestate; (ii) treating at least some of the solidsfrom the digestate in a wet oxidation with lime at a pH of at least 10; (iii) precipitating and recovering calcium from liquid from the wet oxidation as calcium carbonate; (iv) sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and (v) recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.

[0009] According to an aspect of the disclosure, there is provided a process for producing biogas, the process comprising: anaerobically digesting lignocellulosic feedstock in a first anaerobic digestion to produce biogas and digestate; treating at least some of the solids from the digestate in a wet oxidation with lime; conducing a solids-liquid separation on effluent from the wet oxidation to produce liquid and solids; feeding at least some of the liquid to a second anaerobic digestion, wherein calcium carbonate is precipitated out; sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.

[0010] In some embodiments, the process further comprises capturing the carbon dioxide and providing the captured carbon dioxide for sequestration.

[0011] In some embodiments, the wet oxidation is carried out at a pH of at least 9 or at least 10.

[0012] In some embodiments, the process further comprises carrying out a solids-liquid separation on the digestate, thereby obtaining a separated digestate liquid and the solids that are treated in (ii), and wherein at least a portion of the calcium carbonate is precipitated by combining digestate liquid with the liquid from the wet oxidation.

[0013] In some embodiment, the process further comprises a further anaerobic digestion after the wet oxidation.

[0014] In another embodiment, the liquid from the wet oxidation is from the solids-liquid separation carried out on material treated by the wet oxidation and / or from liquid obtained from the further anaerobic digestion carried out after the wet oxidation.

[0015] In a further embodiment, the liquid from the wet oxidation is the liquid obtained from the further anaerobic digestion, which liquid is from solids-liquid separation of digestate from the further anaerobic digestion.

[0016] In some embodiments, the precipitation of calcium carbonate further comprises addition of carbon dioxide.

[0017] In further embodiments, the carbon dioxide is introduced to the separated liquid from the digestate of the further anaerobic digestion to precipitate the calcium carbonate.

[0018] In an embodiment, the carbon dioxide from the calcination is sequestered (e.g., underground).

[0019] In a further embodiment, a stream comprising precipitated calcium carbonate is sent to a solids-liquid separation to produce a separated liquid stream and a solids stream comprising the recovered calcium carbonate, which separated liquid is recycled to the lignocellulosic feedstock.

[0020] In another embodiment, the wet oxidation comprises addition of oxygen.

[0021] A further embodiment comprises capturing the carbon dioxide and using the captured carbon dioxide for the precipitation of the calcium carbonate.Brief description of figures

[0022] Figure 1 shows an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium. The calcium is recovered as calcium carbonate that is subjected to calcination to produce CO2 and CaO.

[0023] Figure 2 shows an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime followed by a solids-liquid separation (SLS) with recovery of calcium from the liquids and solids. The calcium is recovered as calcium carbonate that is subjected to calcination to produce CO2 and CaO.

[0024] Figure 3 shows an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium from a slurry containing liquids and solids. The calcium is recovered as calcium carbonate that is subjected to calcination to produce CO2 and CaO.

[0025] Figure 4 shows an embodiment of a process in which digestate solids from anaerobic digestion are treated by wet oxidation using lime with recovery of calcium. The calcium is recovered as calcium carbonate precipitated out by adding liquid from the process. The calcium carbonate is subjected to calcination to produce CO2 and CaO.

[0026] Figure 5 shows an embodiment of a process in which digestate solids from anaerobic digestion are treated by wet oxidation using lime with recovery of calcium. The calcium is recovered as calcium carbonate precipitated out by adding carbon dioxide. The calcium carbonate is subjected to calcination to produce CO2 and CaO.

[0027] Figure 6 shows an embodiment of a process in which digestate solids from anaerobic digestion are treated by wet oxidation using lime with recovery of calcium. The calcium is precipitated out upstream, during, and / or downstream of a second anaerobic digestion. The recovered calcium carbonate is subjected to calcination to produce CO2 and CaO.

[0028] Figure 7 shows an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium. The calcium is precipitated out upstream, during, and / or downstream of a second anaerobic digestion. The recovered calcium carbonate is subjected to calcination to produce CO2 and CaO.

[0029] Figure 8 shows an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium. The calcium is precipitated out upstream, during, and / or downstream of a second anaerobic digestion. The recovered calcium carbonate is subjected to calcination to produce CO2 and CaO.

[0030] Figure 9 shows an embodiment of a process in which digestate (e.g., at least the solids) from anaerobic digestion are treated by wet oxidation using lime with recovery of calcium. The calcium is precipitated out by the carbon dioxide formed during the wet oxidation. The calcium carbonate is subjected to calcination to produce CO2 and CaO.Detailed description

[0031] Certain exemplary embodiments of the invention now will be described in more detail, with reference to the drawings, in which like features are identified by like reference numerals. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0032] The terminology used herein is for the purpose of describing certain embodiments only and is not intended to be limiting of the invention. For example, as used herein, the singular forms "a," "an," and "the" may include plural references unless the context clearly dictates otherwise. The terms “comprises”, "comprising", “including”, and / or “includes”, as used herein, are intended to mean "including but not limited to." The term “and / or”, as used herein, is intended to refer to either or both of the elements so conjoined. The phrase “at least one” in reference to a list of one or more elements, is intended to refer to at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements. Thus, as a non-limiting example, the phrase “at least one of A and B” may refer to at least one A with no B present, at least one B with no A present, or at least one A and at least one B in combination. In the context of describing the combining of components by the “addition” or “adding” of one component to another, or the separating of components by the “removal” or “removing” of one component from another, those skilled in the art will understand that the order of addition / removal is not critical (unless stated otherwise). The term “stream” includes a liquid, solid or slurry arising and / or that is a part of a batch, continuous or fed- batch processing step or steps. The terms “upstream” and “downstream”, as used herein, refer to the disposition of a step / stage in the process with respect to the disposition of other steps / stages of the process. For example, the term upstream can be used to describe a step / stage that occurs at an earlier point of the process, whereas the term downstream can be used to describe a step / stage that occurs later in the process. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art.Lignocellulosic feedstock

[0033] Lignocellulosic feedstock refers to plant biomass that includes cellulose, hemicellulose, and lignin. The cellulose and hemicellulose fractions may be considered carbohydrate polymers, whereas lignin may be considered an aromatic polymer.

[0034] In one embodiment, the lignocellulosic biomass has a combined content of cellulose, hemicellulose and lignin that is greater than about 25 wt%, that is greater than about 50 wt%, or is greater than about 75 wt%. In one embodiment, sucrose, fructose, and / or starch are also present, but in lesser amounts than cellulose and hemicellulose.

[0035] In one embodiment, the lignocellulosic feedstock is selected from: (i) energy crops; (ii) residues, byproducts, or waste from the processing of plant biomass in a facility or feedstock derived therefrom; (iii) agricultural residues; (iv) forestry biomass; (v) waste material derived from pulp and paper products; (vi) pulp and paper waste; and / or (vii) municipal waste including components removed from municipal waste.

[0036] Energy crops include biomass crops such as grasses, including C4 grasses, such as switch grass, energy cane, sorghum, cord grass, rye grass, miscanthus, reed canary grass, C3 grasses such as Arundo donax, or a combination thereof.

[0037] Residues, byproducts, or waste from the processing of plant biomass include residues remaining after obtaining sugar from plant biomass (e.g., sugar cane bagasse, sugar cane tops and leaves, beet pulp, Jerusalem artichoke residue), and residues remaining after grain processing (e.g., com fiber, com stover, and bran from grains). Agricultural residues include, but are not limited to soybean stover, com stover, sorghum stover, rice straw, sugar cane tops and / or leaves, rice hulls, barley straw, wheat straw, canola straw, oat straw, oat hulls, com fiber, and com cobs.

[0038] Forestry biomass includes hardwood, softwood, recycled wood pulp fiber, sawdust, trimmings, and / or slash from logging operations. Pulp and paper waste includes waste from chemical pulping such as black liquor, spent sulfite liquor, sludge, and / or fines.

[0039] Municipal waste includes post-consumer material or waste from a variety of sources such as domestic, commercial, institutional and / or industrial sources.

[0040] In one embodiment, the lignocellulosic feedstock is an energy crop or biomass crop. In one embodiment, the lignocellulosic feedstock comprises an agricultural residue. In one embodiment, the lignocellulosic feedstock comprises a non-woody lignocellulosic feedstock.

[0041] In one embodiment, the lignocellulosic feedstock comprises hardwood. In one embodiment, the lignocellulosic feedstock comprises softwood.

[0042] In one embodiment, the lignocellulosic feedstock comprises wheat straw, or another straw. In one embodiment, the lignocellulosic feedstock comprises stover. The term “straw” may refer to the stem, stalk and / or foliage portion of crops remaining after the removal of starch and / or sugar containing components for consumption. Examples of straw include, butare not limited to, sugar cane tops and / or leaves, bagasse, oat straw, wheat straw, rye straw, rice straw and barley straw.

[0043] The term “stover” may include the stalk and foliage portion of crops after the removal of starch and / or sugar containing components of plant material for consumption. Examples of stover include, but are not limited to, soybean stover, sorghum stover, and com stover. In one embodiment, the lignocellulosic feedstock is a mixture of fibers that originate from different kinds of plant materials, including mixtures of cellulosic and non-cellulosic feedstock. In one embodiment, the lignocellulosic feedstock is a second-generation feedstock.

[0044] In one embodiment, the lignocellulosic feedstock is mixed with another type of feedstock (e.g., manure, algae, sewage sludge, etc.). In one embodiment, the lignocellulosic feedstock is mixed with manure (e.g., swine manure, cow manure, chicken manure, etc.). Mixing the lignocellulosic feedstock with manure (e.g., upstream and / or within the anaerobic digestion) can be advantageous in terms of decreasing the carbon to nitrogen ratio of the feedstock (e.g., lignocellulosic feedstock can have carbon to nitrogen ratio that is relatively high relative to manure). In one embodiment, the lignocellulosic feedstock is mixed with cow manure. Mixing the lignocellulosic feedstock with cow manure can be advantageous in terms of decreasing the carbon to nitrogen ratio, introducing inoculum, and / or reducing fresh water usage in the anaerobic digestion. While cow manure often contains cellulose, hemicellulose and lignin, the term “lignocellulosic feedstock,” as used herein, does not include material that has been consumed by an animal (i.e., for purposes herein, manure is not a lignocellulosic feedstock). In one embodiment, the lignocellulosic feedstock is mixed with cow manure and the lignocellulosic feedstock makes up at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the total feedstock fed to the anaerobic digestion, where the percentage is based on dry weight.

[0045] Although size reduction may not be required for the anaerobic digestion, it may be advantageous (e.g., with regards to increasing biogas yield and / or conveying the lignocellulosic feedstock to the anaerobic digestion, particularly when the lignocellulosic feedstock includes agricultural residues or forestry biomass). In general, the lignocellulosic feedstock may have been subjected to a size reduction if the biomass is obtained from a stream of a first or second generation ethanol production process or from another process such as a pulp and paper process (i.e., processed lignocellulosic feedstock). Optionally,before, during or subsequent to size reduction, the lignocellulosic feedstock may be slurried in liquid (e.g., water), which allows the feedstock to be pumped.

[0046] The desired weight ratio of water to dry lignocellulosic feedstock solids in the slurry is determined by factors such as pumpability, pipe-line requirements, and other practical considerations. For example, in one embodiment, the consistency of the lignocellulosic feedstock is between about 1% and about 20% or between about 1% and about 10%. However, it should be understood that the feedstock need not be slurried, but rather could be fed to the anaerobic digestion without prior addition of liquid. This would generally occur when the required water is added to the anaerobic digestion separately.Anaerobic digestion

[0047] The lignocellulosic feedstock is subjected to anaerobic digestion to produce biogas. Anaerobic digestion refers to the biological breakdown of organic matter by anaerobic microorganisms and is typically conducted in anaerobic or low oxygen conditions and may involve a series of microorganism types and processes (e.g., hydrolysis, acidogenesis, acetogenesis, and methanogenesis). In general, the anaerobic digestion of lignocellulosic feedstock can be conducted in any suitable environment (e.g., one or more anaerobic digesters arranged in series and / or in parallel). Each anaerobic digester can be a holding tank, or another contained volume, such as a covered lagoon or sealed structure, configured to facilitate the anaerobic digestion and collection of biogas.

[0048] The biogas typically is a gas mixture that contains methane (CH4) and carbon dioxide (CO2), and may further comprise water (H2O), nitrogen (N2), hydrogen sulfide (H2S), and / or ammonia (NH3), depending on the lignocellulosic feedstock from which it is produced. The gas mixture produced from anaerobic digestion often has a methane content between about 35% and about 75% (e.g., about 60%) and a carbon dioxide content between about 15% and about 65% (e.g., about 35%). The percentages used to quantify biogas composition and / or a specific gas content, as used herein, are expressed as mol%, unless otherwise specified. More specifically, they are expressed by mole fraction at standard temperature and pressure (STP), which is equivalent to volume fraction.

[0049] In one embodiment, the biogas produced from anaerobic digestion is subjected to biogas upgrading. Such upgrading refers to a process where the gas mixture (e.g., also referred to in the art as a “raw” or “crude” biogas, or simply “biogas”) is treated to removeone or more components (e.g., CO2, N2, H2O, H2S, NH3, and / or particulates), wherein the treatment increases the calorific value of the gas mixture and produces upgraded biogas (e.g., partially purified biogas or RNG). For example, such upgrading typically includes removing carbon dioxide and / or nitrogen from the mixture. In general, such upgrading can be conducted using any suitable technology or combination of technologies known in the art. Such upgrading, which is well-known to those of skill in the art, often includes one or any combination of absorption, adsorption, membrane separation, and cryogenic upgrading. As will be understood by those skilled in the art, the technology or combination of technologies utilized may be dependent on the composition of the gas mixture and / or how such mixture is produced. Since the gas mixture often has a significant carbon dioxide content, biogas upgrading often includes at least one system for separating methane from carbon dioxide. Without being limiting, the carbon dioxide can be removed from the gas mixture by one or any combination of absorption (e.g., water scrubbing, organic physical scrubbing, chemical scrubbing (e.g., amine)), adsorption (e.g., pressure swing adsorption (PSA), which includes vacuum PSA, or temperature swing adsorption), membrane separation (e.g., CO2 selective membranes based on polyimide, polysulfone, cellulose acetate, poly dimethylsiloxane), and cryogenic separation.

[0050] Advantageously, the biogas produced can be used as fuel (e.g., to produce electricity) and / or as feedstock for another process. For example, in embodiments where the biogas is upgraded (e.g., to produce partially purified biogas or RNG), the upgraded biogas can be used as fuel (e.g., as transportation fuel and / or to generate electricity) and / or can be an intermediate for producing a fuel or other product (e.g., can be feedstock for an industrial process). Depending on the purity of the upgraded biogas, it can be provided as a compressed gas (e.g., at 3600 psig), as a liquid, and / or injected into a natural gas distribution system. Further advantageously, any carbon dioxide removed during biogas upgrading can be used within the process (e.g., to precipitate calcium carbonate as described below) and / or can be sequestered (e.g., injected into a carbon dioxide distribution system).

[0051] In one embodiment, at least some of the biogas or upgraded biogas (e.g., RNG) is used to generate electricity (e.g., to power plant operations). In such embodiments, the biogas is optionally not upgraded or only partially upgraded to reduce the impurities content. In such embodiments, the biogas may simply be burned to generate energy.Digestate

[0052] The slurry removed from an anaerobic digestion (e.g., removed at the end of a batch process or removed periodically or continuously during a continuous process) is often referred to as whole digestate. Whole digestate contains liquid and solids (e.g., suspended solids such as undigested cellulose, undigested hemicellulose, lignin, nutrients, by-products of the anaerobic digestion, and / or microbial biomass). The composition of whole digestate (e.g., including the amount of nutrients, cellulose, hemicellulose, lignin, and microbial biomass) can be dependent on the feedstock for anaerobic digestion and / or various conditions of the anaerobic digestion (e.g., retention time).

[0053] In one embodiment, the digestate (e.g., whole digestate) undergoes at least one solids- liquid separation. The term “solids-liquid separation,” as used herein, refers to methods wherein one or more devices separate at least some of the solids in a material (e.g., a slurry such as whole digestate) from at least some of the liquid in the material (e.g., based on centrifugation, filtration, sedimentation, pressing and / or other dewatering technologies). Such devices include, but are not limited to, centrifuges (e.g., decanter centrifuge or discontinuous centrifuge), screw presses, filter presses (e.g., belt filter press, plate and frame filter press), screens, settling tanks, cyclone cleaners, and / or the like. Optionally, a flocculant, coagulant, and / or surfactant is added to enhance the solids-liquid separation. In one embodiment, the digestate (e.g., whole digestate) is fed to a screw press, screening drum press (vibrating screen), and / or centrifuge. The use of a screw press can be advantageous for digestate produced from lignocellulosic feedstocks as such digestate can have a relatively high fiber content.

[0054] A solids-liquid separation carried out on digestate typically produces a liquid fraction (e.g., also termed “liquid digestate” or “liquid”) and a solids fraction (e.g., also termed “solid digestate” or “solids”). In general, the solids fraction has a higher undissolved solids (UDS) than the feed to the solids-liquid separation, and typically contains the larger suspended solid particles in the digestate (e.g., undigested cellulose, undigested hemicellulose, lignin, and microbial biomass). The liquid fraction, which has a lower UDS than the feed to the solids- liquid separation, is typically aqueous and may contain smaller suspended solids (e.g., fines).

[0055] In one embodiment, the anaerobic digestion is conducted with a UDS between about 1% and about 20%, between about 2% and about 15%, between about 4% and about 20%, between about 5% and about 15%, or between about 6% and about 12%. In one embodiment,a solids-liquid separation conducted on the digestate (e.g., having a UDS between about 8% and about 12%) produces solids having a UDS greater than about 18%. In one embodiment, a solids-liquid separation conducted on the digestate (e.g., having a UDS between about 4% and about 12%) produces solid digestate having a relatively high moisture content (e.g., having a UDS between about 10% and about 18%).

[0056] UDS is a measurement of the amount of solids in a material that is not in solution (e.g., cannot pass through a given filter, or can be settled by a centrifuge). For purposes herein, the UDS of a sample is measured by weighing the sample, separating solid particles in the sample from at least some of the liquid (i.e., using a 1.6 pm glass filter and optionally using a centrifuge if it does not filter readily), washing the solid particles, and drying the solid particles at 37°C to constant weight; the UDS is the weight of the dried solid particles to the weight of the sample, and can be expressed as a percentage.Wet oxidation

[0057] In general, the term "wet oxidation" refers to an aqueous phase oxidation (e.g., containing free water), which may comprise oxidation and hydrolysis reactions.

[0058] In general, the term “lime” refers to CaO, hydrated lime, Ca(OH)2, or any combination thereof.

[0059] Wet oxidation of digestate may be accomplished by heating at least some of the digestate to a predetermined temperature in a closed and / or pressure-controlled vessel into which an oxidant is introduced. Some examples of suitable oxidants include air, oxygen, ozone, and hydrogen peroxide. It is particularly advantageous to use oxygen and / or air as the oxidant due to cost and simplicity (e.g., use of an air compressor). For example, the wet oxidation may be a wet air oxidation (WAO). In some cases, it may be advantageous to use an increased concentration of oxygen. In one embodiment, the oxidant is an oxygen stream purified from air.

[0060] The wet oxidation is operated at alkaline pH, such as a pH of at least about 9, or about 10. Increasing the pH to values of at least about 9, or about 10 can improve lignin solubility and / or generally promote wet oxidation reactions. In one embodiment, the wet oxidation is conducted at a pH that is equal to or greater than about 9, about 10, or about 11. In general, the pH in the wet oxidation can change throughout the wet oxidation (e.g., for a batch processthe pH may decrease with time). For purposes herein, the pH that the wet oxidation is conducted at is measured at the start of wet oxidation at ambient temperature and pressure, unless otherwise specified. In practice, this can require taking a sample of the incoming feed (e.g., if lime is added upstream of the reactor) and / or of the contents of the reactor near the inlet of the reactor (e.g., for a batch or continuous process). In one embodiment, the wet oxidation is conducted such that the pH of the material treated by the wet oxidation (i.e., the material that leaves the reactor) is equal to or greater than about 6, about 7, about 8, about 9, or about 10, as measured at ambient temperature and pressure.

[0061] To achieve a suitable alkaline pH, the wet oxidation of the present disclosure comprises the addition of lime, which includes CaO and / or hydrated lime (Ca(OH)2). The use of lime rather than other alkali, such as caustic soda, which has the chemical formula NaOH, can be advantageous. For example, the use of lime instead of caustic soda avoids the build-up of and / or reduces sodium concentrations in anaerobic digestion (e.g., when liquid separated from the material treated by wet oxidation is carried through to anaerobic digestion). Such increased concentration of sodium salts may compromise the performance of the anaerobic digestion due to its inhibitory effects on microorganisms. It should be appreciated, however, that the addition of small amounts of NaOH may not significantly affect the performance of anaerobic digestion.

[0062] In some cases, it may be advantageous to add CaO to wet oxidation, but Ca(OH)2(hydrated lime) may be added to the wet oxidation or a combination of both CaO and Ca(OH)2. The hydrated lime can be produced when CaO is added to water. For example, in one embodiment, Ca(OH)2is added to the wet oxidation via an upstream mixing and / or buffer vessel. The use of such vessel(s) can be advantageous in that it can provide the time needed for the pH of the digestate to increase to the desired level before it enters the wet oxidation reactor. In addition, such vessel(s) can be adapted to and / or be integrated with a system to pre-heat the digestate.

[0063] The combination of using lime and conducting the wet oxidation at or above a pH of about 10 is believed to be particularly advantageous. For example at pH values above about 10 more lignin may be solubilized, more lime may be solubilized, more carbon dioxide will remain in solution, and / or the carbon dioxide that is in solution is predominately in the carbonate form (e.g., present as CO32', as opposed to as carbonic acid, carbon dioxide molecules, and / or bicarbonate (HCO3')). Keeping more carbon dioxide in solution (e.g., asopposed to releasing carbon dioxide gas in off gas from the wet oxidation) and / or providing more carbon dioxide in the carbonate form, can increase the yield of CaCCf produced (e.g., in downstream steps or stages). As discussed below, such CaC'Ch is sent to calcination to produce CaO and CO2 and thus it is generally desirable to improve its yield. In a preferred embodiment, the lime (CaO and / or Ca(OH)2) is recycled to the wet oxidation to reduce chemical usage and / or the CO2is captured and sequestered to reduce the carbon that would otherwise be emitted to the atmosphere.

[0064] In general, a wet oxidation may be run at elevated temperatures and pressures. For example, when the oxidant is air, the temperature within the reaction vessel may be greater than about 100°C, whereas the pressure within the closed vessel may be greater than ambient pressure (e.g., the initial partial pressure of the oxygen within the closed vessel may be equal to or may exceed the ambient partial pressure of oxygen). In one embodiment, the pressure within the closed vessel is selected and maintained at a value sufficiently high to prevent excessive evaporation.

[0065] Digestate is fed to wet oxidation either as a solution, slurry (e.g., high or low consistency), or as separated solids. In embodiments wherein the digestate has a high consistency and / or includes dry fiber, an appropriate amount of water and / or steam may be added to the reaction vessel to allow wet oxidation to occur. Feeding a low consistency slurry to the wet oxidation is advantageous for ease of pumpability. For example, in one nonlimiting example, the stream subjected to wet oxidation may contain at least about 80% by weight water (w / w) or at least about 90% by weight water (w / w).

[0066] In general, the wet oxidation conditions will be selected to achieve a suitable level of oxidation and may depend on the selected oxidant, and / or reaction temperature that is utilized. As will be appreciated by those of skill in the art, higher concentrations of oxidant, higher reaction temperatures, and longer residence times within the reactor typically correspond to a more complete oxidation. Other factors that may affect the level of oxidation include pH and / or the presence of a catalyst.

[0067] Wet oxidations typically occur at temperatures above the normal boiling point of water (100°C). For example, in one embodiment, when the oxidant is air, the treatment temperature is greater than about 120°C. In one embodiment, wherein the oxidant is air or oxygen, the treatment temperature is between about 140°C and about 330°C. In anotherembodiment wherein the oxidant is air or oxygen, the treatment temperature is between about 225°C and about 275°C. In one embodiment, wherein the wet oxidation is a WAO, the treatment temperature of is between about 150°C and about 330°C, whereas the total pressure is between about 1 MPa (-150 psi) and about 22 MPa (-3200 psi). In general, the pressure of the system may be selected and / or maintained to provide a specific concentration of oxidant and / or to prevent excessive evaporation. In one embodiment, the partial pressure of oxygen measured at ambient temperature is between about 0.15 MPa (-22 psi) and about 11 MPa (-1600 psi). In another embodiment, the partial pressure of oxygen is between about 0.3 MPa (-50 psi) and about 1.4 MPa (-200 psi). In one embodiment, wherein an air stream is used, the total pressure is between about 2 MPa (-290 psi) and about 22 MPa (-3200 psi).

[0068] In one embodiment, the oxidant may be added at a concentration corresponding to about 30% to about 250% of the chemical oxygen demand (COD) of the digestate. In one embodiment, the oxidant may be added at a concentration corresponding to about 150% of the COD. In general, the duration of the wet oxidation may be carried out for any suitable time period. In one embodiment, wherein the oxidant is air or oxygen and the treatment temperature is between about 140°C about 330°C, the reaction time is between about 10 minutes and about 2 hours. In one embodiment, wherein ozone is the oxidant and the treatment temperature is between about 0°C to about 60°C, the treatment duration is between about 5 and about 30 minutes. In general, the pH may vary depending upon the biomass and / or whether the biomass has been pretreated.

[0069] In one embodiment, the wet oxidation is conducted at a temperature within the range of about 175°C and about 225°C, at a pressure within the range of about 250 psig and about 750 psig, at a pH greater than about 10, and for a duration between about 0.5 hour and about 2 hours. Such conditions can be advantageous in terms of treating the digestate so as to increase the yield of biogas and / or facilitate the precipitation of calcium carbonate. In one embodiment, the wet oxidation is conducted at a temperature within the range of about 175°C and about 275°C (e.g., about 250°C), at a pressure within the range of about 500 psig and about 800 psig, at a pH greater than about 11, and for a duration between about 0.5 hour and about 1.5 hours.

[0070] In one embodiment a catalyst is added to the wet oxidation. Catalytic wet air oxidation (CWAO), wherein the stream to be oxidized is passed over a catalyst at elevated temperatures and pressures, is believed to be particularly cost-effective for streams having aCOD greater than about 10,000 mg / L. Although the process and / or equipment may be simpler if the temperature within the reactor is between about 100°C and about 374°C, while the pressure is below about 22 MPa (e.g., such that there is a liquid component), in other embodiments, the temperature and / or pressure is higher. For example, in one embodiment, the temperature and / or pressure is above the mixture’s thermodynamic critical point such that the wet oxidation is a supercritical water oxidation (SCWO).

[0071] The wet oxidation may be conducted in batch or continuous mode. Conducting the wet oxidation in continuous mode is generally advantageous in large scale applications. One example of a known commercially available unit for conducting a continuous mode wet oxidation 10 is aZimpro® wet oxidation unit available from Siemens. In Zimpro® wet oxidation systems, the stream fed to wet oxidation is typically pressurized and then fed to a heat exchanger wherein the input stream is preheated by indirect heat exchange before entering the wet oxidation reactor, wherein it is mixed and reacted with the oxidant. The organic material in the stream is oxidized at the predetermined temperature and a pressure that is controlled to maintain a liquid phase. The oxidized stream and off-gases pass from the reactor to the heat exchanger, where it is cooled. In one embodiment, the wet oxidation is conducted in a pipe reactor (e.g., with or without baffles provided for mixing). In one embodiment, oxygen is injected via injectors located on the sides of the pipe (e.g., held a pressure between about 200 psig and about 400 psig).

[0072] In one embodiment, the wet oxidation is substantially complete (e.g., most of the organic compounds are converted to carbon dioxide and / or a solid residue (e.g., a char like substance, referred to herein as “char”)).

[0073] In one embodiment, the wet oxidation is conducted such that the material fed to the wet oxidation is converted to saturated carboxylic acids and / or the corresponding salts (e.g., acetate salts), low molecular weight alcohols such as methanol, and / or furans such as furfural. Low molecular weight carboxylic acids (e.g., R-CO2H), and acetic acid (CH3COOH) in particular, may be resistant to oxidation, and thus can accumulate at the latter stages of wet oxidation. Advantageously, since calcium oxide and / or calcium hydroxide is added to the wet oxidation, the liquid from the wet oxidation can contain a significant amount of soluble calcium acetate, which can form a significant amount of insoluble calcium carbonate (e.g., when reacted with bicarbonate and / or carbon dioxide).

[0074] In one embodiment, the material treated by the wet oxidation contains solids (e.g., which can be separated from the liquid via a solids-liquid separation), which can be land applied as a fertilizer and / or soil amendment, and / or from which some nutrients can be recovered for fertilizer production. In some embodiments, the solids can include calcium carbonate.Precipitation and recovery of calcium carbonate

[0075] Calcium is precipitated and recovered from the liquid from the wet oxidation as calcium carbonate.

[0076] The phrase “liquid from the wet oxidation” when used with reference to liquid from which calcium carbonate is precipitated, includes liquid arising directly or indirectly from the wet oxidation and / or arising from a subsequent step, such as anaerobic digestion or any other stage, process and the like, including any solids-liquid separation and / or washing. The liquid may be part of a stream having some amount of solids or a separated liquid from a solids- liquid separation or washing. For example, calcium carbonate may be precipitated during the wet oxidation and / or during one or more processes conducted on material treated by the wet oxidation.

[0077] In one embodiment, calcium carbonate is recovered from a liquid from a solids-liquid separation conducted right after wet oxidation (e.g., as in the embodiment of Figure 1 or 2). In one embodiment, calcium carbonate is alternatively, or additionally, recovered from solids produced from a solids-liquid separation conducted right after wet oxidation (e.g., as in the embodiment of Figure 2 or Figure 9). In one embodiment, calcium carbonate is recovered from a slurry containing liquid and solids (e.g., as in the embodiment of Figure 3). In one embodiment, calcium carbonate is recovered from subsequent step (e.g., from a subsequent anaerobic digestion as in the embodiments of Figures 6, 7, or 8).

[0078] The calcium carbonate may be precipitated from such liquid using any suitable conditions and / or reaction(s). For example, calcium carbonate can be precipitated from the liquid by adding carbon dioxide (e.g., bubbling in carbon dioxide in an amount sufficient to reduce the pH of the liquid). Advantageously, such carbon dioxide addition can reduce the pH of the liquid to a level that is compatible with microorganisms (e.g., in the first or a second anaerobic digestions). As described herein, such carbon dioxide can be obtained from biogas upgrading and / or from the calcination (e.g., captured from the biogas and / or limekiln). Alternatively, or additionally, the carbon dioxide can be generated in situ (e.g., during a second anaerobic digestion as described herein and / or from a substantially complete wet oxidation). Further alternatively, or additionally, calcium carbonate can be precipitated out by introducing liquid (and optionally some solids) containing certain ions (e.g., sodium and / or potassium, with carbonates and / or hydroxides). For example, as the liquid from wet oxidation can contain a significant amount of calcium acetate, such calcium can be precipitated out as calcium carbonate by adding a liquid containing sodium bicarbonate, sodium carbonate, potassium bicarbonate, and / or potassium carbonate. Further alternatively, or additionally, calcium carbonate can be precipitated out by removing certain ions.

[0079] In one embodiment, the calcium precipitation is aided and / or conducted by the addition of carbon dioxide (e.g., produced from the anaerobic digestion and / or calcination). In one embodiment, the carbon dioxide is in relatively pure form (e.g., captured from biogas upgrading and / or indirect calcination). In one embodiment, the carbon dioxide is introduced as a gas mixture (e.g., as raw biogas, partially purified biogas, tail gas from biogas upgrading, and / or flue gas from calcination). Introducing the carbon dioxide as a gas mixture can be advantageous in that it may reduce costs (e.g., reduce costs associated with gas purification). For example, feeding flue gas from calcination that contains carbon dioxide and nitrogen, can eliminate carbon dioxide / nitrogen separations. In some embodiments, feeding raw biogas into calcium recovery provides at least a first step of carbon dioxide removal, and thus can reduce biogas upgrading costs.

[0080] In one embodiment, the calcium precipitation is conducted by the addition of carbon dioxide together or separately with alkali, such as a carbonate, bicarbonate salt or a combination thereof. In one embodiment, the addition is conducted at a pH greater than about 5.0 to ensure sufficient solubility of carbon dioxide and low solubility of the calcium carbonate. The alkali and carbon dioxide may be added separately, or they may be combined to make a carbonate salt that is then added to the material resulting from wet oxidation or a separated liquid thereof. Furthermore, optionally, any magnesium originating from the feedstock can be removed by this precipitation step as well.

[0081] In one embodiment, the alkali used to precipitate calcium carbonate is a bicarbonate salt, such as potassium bicarbonate (KHCO3). The potassium bicarbonate (KHCO3) may arise from liquid separated from digestate. A non-limiting example of such a stream is a separated liquid 14 of digestate 8 in Figs. 4 and 5, which is described in more detail below.

[0082] The precipitation may be conducted at a pH of between about 3 and about 11. For example, the pH may be about 3, about 3.5, about 4.0, about 4.5, about 5.0, about 5.5, about 6.0, about 6.5, about 7.0, about 7.5, about 8.0, about 8.5, about 9.0, about 9.5, about 10, about 10.5 or about 11. In some cases, it can be advantageous for the precipitation to be conducted at, or to end at, a pH that is compatible with conditions for anaerobic digestion. In one embodiment, the final pH of the liquid following calcium recovery is between about 6 and about 8.5. In one embodiment, the final pH of the liquid following calcium recovery is between about 7 and about 8.

[0083] The calcium carbonate, and optionally magnesium carbonate, is recovered from the aqueous solution of soluble salts by allowing the salt to precipitate and then separating the precipitate using known solids-liquid separation methods (e.g., gravity separation, floatation, centrifugation, microfiltration, plate and frame filtration, crossflow filtration, pressure filtration, vacuum filtration, and the like). The resulting calcium carbonate solids are optionally dried and then sent to calcination described below. Alternatively, the calcium carbonate may be provided in the form of an aqueous slurry. Thus, the calcium carbonate sent to calcination in some embodiments is primarily composed of solids, is a moist cake, or is an aqueous slurry of calcium carbonate.

[0084] The precipitation of calcium carbonate, and optionally additionally with coprecipitation of magnesium carbonate, may be carried out at any suitable temperature, for example, between about 20°C and about 95°C, or any temperature range therebetween. In one embodiment, the temperature range is between about 40°C and about 80°C, or any temperature range therebetween. For example, the temperature may be about 20°C, about 25°C, about 30°C, about 35°C, about 40°C, about 45°C, about 50°C, about 55°C, about 60°C, about 65°C, about 70°C, about 75°C, about 80°C, about 85°C, about 90°C or about 95°C. These conditions are maintained for any suitable amount of time to allow the insoluble calcium precipitates to form, or longer as desired. In embodiments where carbon dioxide is added it may be advantageous to first cool the material (e.g., liquid) from which the calcium carbonate (and / or other carbonates) is precipitated (e.g., to ambient temperature). In some cases, it can be advantageous for the precipitation to be conducted at, or to end at, temperature that is compatible with conditions for anaerobic digestion (or at least does not require heating). In one embodiment, the precipitation is conducted at, and / or ends at, a temperature within the range between about 35°C and about 60°C.

[0085] The precipitation can be a batch or a continuous process. The calcium precipitates may form, for example, after about 5 to about 60 minutes, or any time range therebetween, more typically between about 10 and about 30 minutes, or any time range therebetween, although the total holding time in the vessel wherein the precipitation is carried out may be greater than this. For instance, the stream containing the insoluble calcium precipitates may be stored for a certain amount of time in the precipitation vessel prior to its use (e.g., before being fed to calcination). Moreover, separation of the calcium carbonate precipitate from the stream may be conducted in a precipitation vessel. Thus, the total holding time in the precipitation vessel may be greater than the amount of time required for precipitation to occur. Consequently, the total duration of the precipitation step may be from about 5 minutes to about 48 hours, or any time therebetween, or between about 15 minutes and about 24 hours, or any time therebetween.Further anaerobic digestion

[0086] In one embodiment, at least some of the material from wet oxidation is subjected to further anaerobic digestion (e.g., is recycled to one or more anaerobic digesters into which the incoming lignocellulosic feedstock is fed, or is fed to another digester). Since the wet oxidation can help degrade the digestate and / or make the digestate less resistant to digestion, feeding the material from the wet oxidation to further anaerobic digestion can produce additional biogas from the feedstock, and thus increase biogas yield.

[0087] It can be advantageous to feed the material from the wet oxidation to a second anaerobic digestion (i.e., conducted in one or more digesters separate from those used for the incoming feedstock). For example, as described herein, since the material from the wet oxidation can contain a significant amount of calcium acetate, calcium carbonate can precipitate out upstream of the second anaerobic digestion, during the second anaerobic digestion, and / or downstream of the second anaerobic digestion (e.g., with the addition of carbon dioxide, carbonate salts, and / or bicarbonate salts).

[0088] In one embodiment, carbon dioxide is added to the liquid from the wet oxidation upstream of the second anaerobic digestion to reduce the pH for the second anaerobic digestion (e.g., reduce the pH from about 7, about 8, about 9, about 10, or higher, to within the pH range from about 6 to about 8.5). Advantageously, using carbon dioxide (carbonic acid) to reduce the pH for the second anaerobic digestion advantageously reduces and / orobviates the need for stronger acids, such as hydrochloric acid and / or sulfuric acid, which may have a negative effect on the further anaerobic digestion, and / or can result in at least some of the calcium precipitating out as calcium carbonate upstream of the second anaerobic digestion.

[0089] In one embodiment, at least some of the calcium is precipitated out as calcium carbonate during and / or downstream of the second anaerobic digestion. Advantageously, the second anaerobic digestion can facilitate such precipitation of calcium carbonate by: 1) digesting acetic acid and / or acetate carried through from the wet oxidation, thereby reducing the amount of calcium acetate, and / or 2) generating carbon dioxide used for the precipitation of the calcium carbonate. Accordingly, providing a second anaerobic digestion can be beneficial in terms of recovering calcium, increasing biogas yield, and / or optimizing the microorganisms (e.g., there can be different populations in the two anaerobic digestions). Further advantageously, there is decreased risk of the microorganisms in the second anaerobic digestion being negatively affected by the alkali used in wet oxidation because the alkali does not contain high sodium concentrations (e.g., as would occur if NaOH was used in the wet oxidation) and / or because the calcium precipitates out as calcium carbonate.

[0090] In general, the second anaerobic digestion of wet oxidized material can be conducted in any suitable environment as described previously in connection with the anaerobic digestion of lignocellulosic feedstock. Each anaerobic digester can be a holding tank, or another contained volume, such as a covered lagoon or sealed structure, configured to facilitate the anaerobic digestion and collection of biogas.

[0091] As with the anaerobic digestion of the incoming lignocellulosic feedstock, the biogas from the second anaerobic digestion typically is a gas mixture that contains methane (CH4) and that can contain some carbon dioxide (CO2), water (H2O), nitrogen (N2), hydrogen sulfide (H2S), ammonia (NH3), oxygen (O2), volatile organic compounds (VOCs), depending on the lignocellulosic feedstock from which it is produced. However, since the liquid from wet oxidation can contain a significant amount of soluble calcium (e.g., present as calcium acetate), as the carbon dioxide is generated from the anaerobic digestion, calcium carbonate may precipitate out as the anaerobic digestion progresses. Alternatively, or additionally, the calcium carbonate can be precipitated out from the material treated by the second anaerobic digestion (e.g., by adding additional carbon dioxide and / or part of a process stream containing a bicarbonate). When the feed to the second anaerobic digestion is liquid obtainedfrom a solids-liquid separation, relatively pure precipitated calcium carbonate can be obtained from a subsequent solids-liquid separation. In one embodiment, all or some of the calcium carbonate fed to the calcination system is obtained from the second anaerobic digestion and / or is obtained from an effluent of the second anaerobic digestion.

[0092] In one embodiment, the biogas produced from the second anaerobic digestion is subjected to at least partial upgrading (e.g., separate or together with biogas from the first anaerobic digestion). In one embodiment, the relatively pure biogas (e.g., which has a lower carbon dioxide content due to the precipitation of calcium carbonate) is otherwise used (e.g., to generate onsite electricity).Calcination of calcium carbonate with recycle of lime

[0093] In some embodiments, the calcium carbonate that is recovered during or subsequent to precipitation is sent to calcination. Calcination is a thermal treatment of CaCCfi in which the temperature is elevated sufficiently to cause at least partial thermal decomposition.

[0094] Calcination of calcium carbonate produces carbon dioxide and calcium oxide according to the following reaction: heat CaCO3- ► CaO + CO2.

[0095] In general, the calcination can be conducted at any suitable temperature (e.g., at and or above about 400°C or about 500°C). Some non-limiting examples of a suitable temperature range is about 400°C to about 1500°C or about 550°C to about 1150°C.

[0096] In general, the calcination can be conducted in any suitable reactor or reactors (e.g., a calcination system that includes one or more direct-or indirect-fired lime kilns). In one embodiment, the calcination is conducted in a direct-fired lime kiln. In one embodiment, the calcination is conducted in an indirect-fired lime kiln. The term “lime kiln”, as used herein, refers to any system (e.g., a calcination system) capable of calcination (e.g., to produce products such as calcium oxide, magnesium oxide, and / or other oxides).

[0097] The fuel for the calcination can be any suitable fuel. For example, calcination systems can be fueled by coal, fuel oil, natural gas, biomass, and / or biogas (raw, partially purified, or fully purified). In one embodiment, the fuel for the calcination includes product and / or by-product from the process (e.g., part of the digestate from the first and / or a subsequent anaerobic digestion, organic residue from the wet oxidation, tail gas from biogas upgrading, RNG, etc ).

[0098] The feed to calcination can be primarily calcium carbonate (e.g., dry or wet), or can be a mixture of calcium carbonate and other material (e.g., ash, char, lignin, and / or other compounds present in the wet oxidized material).

[0099] Depending on the calcination system (e.g., whether it is direct-fired or indirect-fired), the feed to calcination, and / or the fuel provided for the calcination, the exhaust gas from a calcination system can include nitrogen, carbon dioxide from the combustion, carbon dioxide from the calcination, oxygen, steam, and / or dust. In some cases, it can be advantageous to select the calcination system to either make the carbon capture more economical (e.g., by producing substantially pure carbon dioxide and / or an exhaust gas for carbon capture that does not contain a significant amount of nitrogen) or to maximize the amount of carbon dioxide that can be captured (e.g., from the calcination and from the combustion).

[0100] In one embodiment, the calcination is conducted in a calcination system configured to keep the carbon dioxide from calcium carbonate decomposition separated from the flue gas from the combustion. In one embodiment, the calcination is conducted with small amounts of oxygen or with no oxygen.

[0101] In one embodiment, the calcination is conducted in a calcination system wherein the combustion is based on an oxy-fuel process. Using a calcination system where the combustion is based on an oxy-fuel process (e.g., wherein the combustion is conducted with pure oxygen or a mixture of oxygen, water, and / or carbon dioxide) can be beneficial in producing relatively pure carbon dioxide and / or carbon dioxide that is suitable for sequestration (e.g., does not contain significant amounts of nitrogen, which can be costly to separate out). In one embodiment, the calcination is conducted in a calcination system wherein an oxygen enriched stream is fed to combustion.

[0102] It will be appreciated by those of ordinary skill in the art that calcium oxide produced from the calcination can be added directly to the wet oxidation, in which case it may be converted to calcium hydroxide upon addition, or the calcium oxide may be first hydrated and then added to the wet oxidation.

[0103] In one embodiment, at least some of the carbon dioxide from calcination is captured and sequestered. This capture of carbon dioxide may reduce the amount of carbon that is otherwise emitted to tatmosphere by the process or may delay such emissions. The term “captured” is used herein in a non-limiting sense and includes any method, process and / or technique for obtaining carbon dioxide from the calcination. Depending on whether impurities are present, in some embodiments, the carbon dioxide can be captured using one or more methods, processes and / or techniques described above in connection with biogas upgrading (e.g., to remove nitrogen if present).

[0104] In one embodiment, at least some of the carbon dioxide thus captured from calcination is sequestered. This includes, without limitation, any methodology for sequestration of carbon dioxide, including any storage (e.g., underground) and / or use that is carried out so that most or all of the carbon dioxide is prevented from entering the atmosphere or in which entry to atmosphere thereof is delayed. The carbon dioxide may be introduced underground in a geological formation, such as an underground reservoir that sequesters the carbon dioxide. In some non-limiting embodiments, measures may be taken to reduce leakage of carbon dioxide from the geological formation. In another non-limiting example, the carbon dioxide is sequestered in concrete by its introduction to a concrete manufacturing process using known methods. In another non-limiting example, the carbon dioxide is sequestered as a result of its use in enhanced oil recovery (EOR) or for the production of one or more products (e.g., plastics). Sequestration of carbon dioxide often includes compressing the captured carbon dioxide (e.g., to produce liquid carbon dioxide or for injection into a carbon dioxide distribution system) and transporting the captured carbon dioxide for sequestration (e.g., by vehicle and / or a carbon dioxide distribution system). As will be understood by those of skill in the art, it can be advantageous to sequester the captured carbon dioxide using a method recognized by the applicable regulatory authority for reducing GHG emissions and / or mitigating climate change. In one embodiment, the carbon dioxide is provided for sequestration by transporting it (e.g., by pipeline or vehicle) to a carbon capture and sequestration hub or site.Recycle of separated liquid with optional purge of inorganic and / or organic components

[0105] A further advantage of the process of the disclosure is that separated liquid from one or more solids-liquid separations can be recycled (e.g., thereby reducing fresh water usage). In one embodiment, separated liquid is sent to incoming lignocellulosic feedstock, to theanaerobic digestion, the wet oxidation and / or any other step or stage that has liquid addition, such as a wash step. Since the relatively high pH used in the wet oxidation is achieved with lime (e.g., rather than sodium hydroxide), there can be a relatively low concentrations of Na+. Accordingly, sodium concentrations that inhibit the microorganisms in anaerobic digestion can be avoided or the concentrations can be low enough so as to not significantly impact the performance of anaerobic digestion (e.g., reduces or avoids inhibition of microorganisms).

[0106] In one example, separated liquid from the wet oxidation is subjected to calcium carbonate precipitation and recovery and liquid from a solids-liquid separation of calcium carbonate is recycled to the anaerobic digestion, the wet oxidation and / or any other step or stage that includes liquid addition, such as a wash step.

[0107] In one embodiment, before or during such recycle, part of the recycle liquid may be purged and / or treated (e.g., to remove and / or recover organics and / or inorganics, such as nitrogen and / or phosphorus).

[0108] In one embodiment, at least part of one or more of the recycling liquids and / or materials derived therefrom are purged using known techniques to reduce or prevent any build-up of potentially inhibiting material.

[0109] Certain exemplary embodiments of the invention now will be described in more detail, with reference to the drawings, in which like features are identified by like reference numerals. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.Description of Figure 1

[0110] As shown in Figure 1, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of calcium carbonate to produce CaO that is recycled in the process.

[0111] Lignocellulosic feedstock 2 is slurried with recycle liquid 3 and fed to anaerobic digestion 4. The anaerobic digestion 4 produces biogas 6 that contains methane and other compounds such as carbon dioxide (CO2), water (H2O), nitrogen (N2), hydrogen sulfide (H2S), and / or ammonia (NH3), depending on the lignocellulosic feedstock and / orconfiguration of the anaerobic digestion. The biogas 6 produced from anaerobic digestion 4 can be subjected to an optional biogas upgrading 5 to remove one or more components (e.g., CO2, N2, H2O, H2S, NH3, and / or particulates), wherein the treatment increases the calorific value of the gas mixture and produces upgraded biogas 44 (e.g., illustrated as a CH4rich gas). The separated CO246 can be used within the process (e.g., to precipitate CaCO3as described hereinafter) and / or sequestered (not shown).

[0112] The anaerobic digestion 4 produces digestate 8, which contains liquid and solids (e.g., digestate can contain suspended solids such as undigested cellulose, undigested hemicellulose, lignin, nutrients, by-products of the anaerobic digestion, and / or microbial biomass). The digestate 8 and CaO 17 are fed to wet oxidation 18 to which an oxidant 20 is added (e.g., illustrated as air and / or oxygen). The CaO and digestate can be added to the wet oxidation together and / or separately (e.g., the amount of CaO can be added to provide a certain initial pH and / or to keep the pH above a certain level). An initial amount of CaO 17 may be added to the digestate during start-up. In one embodiment, during wet oxidation 18, the pH is above about 10 initially and / or is maintained above 10.

[0113] The wet oxidized material 24 is sent to a solids-liquid separation 26 to produce separated wet oxidized solids 28 and liquid 30. Depending on the feedstock and / or its handling (e.g., whether there is substantial sand removal), the operating conditions of the anaerobic digestion and / or wet oxidation, and / or any processing downstream of wet oxidation and upstream of the solids-liquid separation, the wet oxidized solids 28 from solids-liquid separation 26 may include sand, ash, precipitated calcium carbonate, char, lignin, etc., Optionally, at least some of these solids are fed to the calcination 68 (e.g., as combustion fuel and / or source of calcium carbonate). Alternatively, or additionally, at least some of these solids are optionally processed to recover nutrients (e.g., phosphorus) and / or are provided for land application.

[0114] The wet oxidized liquid 30 is subjected calcium recovery 40, which is one or more processes wherein CaCO3is precipitated and solids containing the precipitated CaCO3are collected (e.g., via a solids-liquid separation, not shown). For example, such precipitation can be achieved by adding at least some of the CO2 generated from the anaerobic digestion 4 (e.g., CO246 separated during the optional biogas upgrading 5), adding at least some of the CO272 generated from the calcination 68, and / or adding another precipitating agent (e.g., some of the liquid from the digestate from the anaerobic digestion 4). The resultant streamcontaining precipitated CaCCh 49 is sent to a calcination 68 (e.g., illustrated as being carried out in a lime kiln). The thermal decomposition of CaCCh in the calcination 68 produces CaO 70 and CO272. At least some of the CaO 70 is recycled to wet oxidation 18. The CO272 can be captured and reused (e.g., within the process) and / or sequestered underground (not shown).

[0115] Accordingly, the process allows for recycle of CaO to wet oxidation. Advantageously, the liquid 48, which can contain some Na+ from the feedstock, will contain less Na+than if NaOH was used instead of lime in the wet oxidation 18, and thus this liquid can be recycled and / or fed to anaerobic digestion with less risk that it will negatively affect the performance of the anaerobic digestion.

[0116] Optionally, the liquid 48 is subjected to one or more processes 50 that removes and / or recovers inorganics (e.g., sodium plus nutrients such as nitrogen (N), phosphorus (P), and / or potassium (K)). Such processes can include fdtration (e.g., using vibrating sieves, reverse osmosis, ultrafiltration, nanofiltration, and / or microfdtration), evaporation (e.g., falling fdm evaporator), stripping (e.g., ammonia stripping), ion exchange, struvite precipitation, and / or biological treatment (e.g., aerobic biological treatment). In one embodiment, such processes can precipitate out more calcium carbonate. Optionally, such processes can produce byproducts and / or purge streams 51 (e.g., for disposal). For example, recovered nitrogen and / or phosphorus can be provided for use as a fertilizer and / or for producing fertilizer.Description of Figure 2

[0117] As shown in Figure 2, similar to Figure 1, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is optionally used in the process and / or sequestered).

[0118] Figure 2 differs from Figure 1 in that the solids 28 from the solids-liquid separation 26 are treated and / or washed to recover additional calcium (e.g., solids containing precipitated calcium carbonate can be washed with an acid solution (e.g., containing acetic acid) to produce soluble calcium acetate that is sent for calcium recovery 40). Depending on the composition of these relatively clean solids, they can be disposed of (e.g., if largely sand),provided for soil amendment (e.g., if largely containing ash, char, and / or lignin), and / or can be combusted (e.g., if largely residual organics, such as lignin).Description of Figure 3

[0119] As shown in Figure 3, similar to Figure 1, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is optionally used in the process and / or sequestered).

[0120] Figure 3 differs from Figure 1 in that the wet oxidized material 24 is fed to calcium recovery (e.g., the precipitation of CaCC 41 and solids-liquid separation 36) without an intermediate solids-liquid separation. The subsequent solids-liquid separation 36 produces separated liquid 48, which is fed to optional inorganic removal 50, and solids that contain precipitated calcium carbonate 49. Depending on the feedstock and / or its handling (e.g., whether there is substantial sand removal), the operating conditions of the anaerobic digestion, and / or the operating conditions of the wet oxidation, the solids 49 can include residual organics (e.g., lignin and / or char) that can be combusted as fuel for the lime kiln 69 (e.g., together with another fuel). Such embodiments may be more attractive when there is not a significant amount of material in the solids that may negatively affect the combustion and / or lime kiln (e.g., sand, salts, etc.).Description of Figure 4

[0121] As shown in Figure 4, similar to Figure 1, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is optionally used in the process and / or sequestered).

[0122] Figure 4 differs from Figure 1 in that digestate is subjected to a solids-liquid separation 10 upstream of the wet oxidation 18. This solids-liquid separation 10 produces solids 12 that are fed to the wet oxidation (e.g., after being mixed with lime 17) and liquid 14. The separated digestate solids 12 can, for example, contain undigested feedstock (e.g., a significant portion of which may be lignin), microbial biomass, and / or sand. The separateddigestate liquid 14 can, for example, contain organic matter (e.g., organic acids, humic substances, etc.), nutrients (e.g., nitrogen, phosphorus, potassium), and / or salts (e.g., potassium bicarbonate (KHCO3)). For example, the separated digestate liquid can contain organic acids and / or salts, such as acetate salts, including calcium acetate.

[0123] In some cases, separating the digestate into solids 12 and liquid 14 upstream of wet oxidation 18 can be advantageous. As lignin is not normally anaerobically digestible, the digestate 8 can contain a significant amount of lignin containing solids. Feeding the lignin containing solids to wet oxidation 18 can oxidize such solids such that the resulting material can be digested anaerobically, and thus can increase the biogas yield. Feeding the solids 12 to the wet oxidation, and not the liquid 14, reduces the amount of material that needs to be treated by wet oxidation and / or allows the liquid 14 to be otherwise treated and / or used in the process.

[0124] Figure 4 also differs from Figure 1 in that calcium recovery 40 includes providing at least some of the liquid 14 from the solids-liquid separation 10 for the calcium recovery 40. Since the liquid 14 can contain a significant amount of carbonate salt (e.g., potassium bicarbonate), contacting this liquid 14 with the liquid from the liquid 30 from the solids- liquid separation 26, can result in calcium carbonate precipitating out (e.g., at least part of this liquid 14 is used to help recover calcium used in the wet oxidation). Optionally, some of the liquid 14 is recycled back to the anaerobic digestion 4 and / or is further treated (e.g., via inorganic removal 50).Description of Figure 5

[0125] As shown in Figure 5, similar to Figure 4, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is used in the process and / or sequestered).

[0126] Figure 5 differs from Figure 4 in that the calcium recovery 40 uses carbon dioxide (e.g., and that the addition of at least some of the liquid 14 to calcium recovery is optional). The carbon dioxide provided for calcium recovery 40 can be produced from the anaerobic digestion 4 (e.g., provided in biogas 5), a second other anaerobic digestion (not specifically shown in Fig. 5), and / or the calcination 68. In general, the carbon dioxide provided can berelatively pure (e.g., may have already been subjected to one or more purification processes) or can be provided as a gas mixture (e.g., provided as raw biogas or flue gas).Description of Figure 6

[0127] As shown in Figure 6, similar to Figure 5, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is used in the process and / or sequestered).

[0128] As shown in Figure 6, lignocellulosic feedstock 2 is slurried with recycle liquid 3 and fed to anaerobic digestion 4. The anaerobic digestion 4 produces biogas 6 that contains methane and other compounds such as carbon dioxide (CO2), water (H2O), nitrogen (N2), hydrogen sulfide (H2S), and / or ammonia (NH3), depending on the lignocellulosic feedstock and / or configuration of the anaerobic digestion. In some examples, the biogas 6 produced from anaerobic digestion 4 is subjected to an optional biogas upgrading 5 to remove one or more components (e.g., CO2, N2, H2O, H2S, NH3, and / or particulates), wherein the treatment increases the calorific value of the gas mixture and produces upgraded biogas 44 (e.g., partially purified biogas or RNG). In some examples, separated CO246 is used within the process (e.g., to precipitate CaCO3as described hereinafter) and / or is sequestered (not shown).

[0129] The anaerobic digestion 4 produces digestate 8, which contains liquid and solids (e.g., the digestate can contain suspended solids such as undigested cellulose, undigested hemicellulose, lignin, nutrients, by-products of the anaerobic digestion, and / or microbial biomass). The digestate 8 is fed to a solids-liquid separation 10 upstream of the wet oxidation 18. This solids-liquid separation 10 produces solids 12 that are fed to the wet oxidation (e.g., after being mixed with lime 17) and liquid 14. The separated digestate solids 12 can, for example, contain undigested feedstock (e.g., a significant portion of which may be lignin), microbial biomass, and / or sand. The separated digestate liquid 14 can, for example, contain organic matter (e.g., organic acids, humic substances, etc.), nutrients (e.g., nitrogen, phosphorus, potassium), and / or salts (e.g., potassium bicarbonate (KHCO3)).

[0130] The digestate solids 12 and CaO 17 are fed to wet oxidation 18 to which an oxidant 20 is added (e.g., illustrated as air and / or oxygen). The CaO 17 and digestate solids 12 can be added together and / or separately (e.g., the amount of CaO can be added to provide a certaininitial pH and / or to keep the pH above a certain level). An initial amount of CaO 17 may be added to the digestate solids 12 during start-up. It can be advantageous to conduct the wet oxidation 18 at a pH that is equal to or greater than about 10.

[0131] The wet oxidized material 24 is sent to a solids-liquid separation 26 to produce separated wet oxidized solids 28 and liquid 30. Depending on the feedstock and / or its handling (e.g., whether there is substantial sand removal), the operating conditions of the anaerobic digestion and / or wet oxidation, and / or any processing downstream of wet oxidation and upstream of the solids-liquid separation, the wet oxidized solids 28 from solids-liquid separation 26 may include sand, ash, precipitated calcium carbonate, char, lignin, etc.). Optionally, at least some of these solids are fed to the calcination 68 (e.g., as combustion fuel and / or source of calcium carbonate).

[0132] The wet oxidized liquid 30 is fed to a second anaerobic digestion 41 A (e.g., after an optional pH adjustment with added carbon dioxide). The second anaerobic digestion 41 A converts organic material in the liquid 30 (e.g., dissolved and / or fine solids) to biogas, thereby increasing the biogas yield. As organic material (e.g., as acetate) is digested and / or as carbon dioxide is produced, some or all of the calcium in solution can precipitate out as calcium carbonate. Optionally, additional carbon dioxide (e.g., from the biogas and / or calcination) and / or a portion of the liquid 14 is added to the anaerobic digestion 41 A and / or downstream of the anaerobic digestion 41A (not shown). Advantageously, this second anaerobic digestion 41 A has a relatively short residence time (e.g., the acetate and / or other organics can digest in under a week, whereas the first anaerobic digestion can take more than 2, 3, 4, 5, or 6 weeks). Further advantageously, since the wet oxidation uses CaO to increase the pH rather than NaOH, and since CaCO;, can precipitate out, there is less risk that the alkali provided to increase the pH in wet oxidation will negatively affect the performance of the second anaerobic digestion 41 A. Yet further advantageously, since the second anaerobic digestion 41A is generally fed only liquid, the subsequent solids-liquid separation 41B can provide relatively pure carbonate salt (e.g., calcium carbonate). The resultant stream containing precipitated CaCO;, is sent to a calcination 68 (e.g., illustrated as being carried out in a lime kiln). The thermal decomposition of CaCCti in the calcination 68 produces CaO 70 and CO2 72. At least some of the CaO 70 is recycled to wet oxidation 18. The CO2 72 can be captured and reused (e.g., within the process) and / or sequestered (e.g., underground).

[0133] In this embodiment, the liquid produced from the solids-liquid separation 41B is disposed of (e.g., after further purification and / or concentration). In other embodiments, this liquid is recycled within the process and / or further treated to remove inorganics.Description of Figure 7

[0134] As shown in Figure 7, similar to Figure 6, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is used in the process and / or sequestered).

[0135] Figure 7 differs from Figure 6 in that there is no solids-liquid separation between the anaerobic digestion 4 and the wet oxidation 18. Accordingly, the organic material in liquid and solids components of the digestate can be converted to carbon dioxide and / or to carboxylic acids and / or salts thereof via the wet oxidation.

[0136] Figure 7 also differs from Figure 6 in that an optional separate precipitation 42A is shown downstream of the anaerobic digestion (e.g., via the addition of carbon dioxide) and that the liquid from the solids-liquid separation 41B is optionally subjected to inorganic removal and recycling to the first anaerobic digestion 4.Description of Figure 8

[0137] As shown in Figure 8, similar to Figure 7, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is used in the process and / or sequestered).

[0138] Figure 8 differs from Figure 7 in that there is no solids-liquid separation between the wet oxidation 18 and the second anaerobic digestion 41 A. Accordingly, the organic material in liquid and solids components of the wet oxidized material 24 can be converted to additional biogas and / or all of the solids can be fed to the calcination 68. Figure 8 also differs from Figure 7 in that the optional precipitation 42A is not shown.Description of Figure 9

[0139] As shown in Figure 9, there is provided an embodiment of a process in which digestate from anaerobic digestion is treated by wet oxidation using lime with recovery of calcium carbonate and calcination of the calcium carbonate to produce CaO that is recycled in the process (and CO2 that is used in the process and / or sequestered).

[0140] Lignocellulosic feedstock 2 is slurried with recycle liquid 3 and fed to anaerobic digestion 4. The anaerobic digestion 4 produces biogas 6 and digestate 8. The whole digestate 8, or the solids 12 produced from the optional solids-liquid separation 10a, are fed to wet oxidation 18.

[0141] In this embodiment, the wet oxidation 18 is conducted close to completion (e.g., most of the organic compounds are converted to carbon dioxide and possibly also to a solid residue). As carbon dioxide is formed (i.e., as a product of the wet oxidation), CaCCL can precipitate out. The resulting wet oxidized material 24 (i.e., containing the precipitated CaCCL) is sent to a solids-liquid separation 26 to produce separated wet oxidized solids 28 and liquid 30. Depending on the conditions of the wet oxidation, the separated wet oxidized solids 28 can be predominately CaCCL, or can be a mixture of CaCCL and other solids. In this embodiment, the separated wet oxidized solids 28 are fed directly to the calcination 68. Alternatively, in another embodiment, the separated wet oxidized solids 28 can be treated and / or washed to recover the calcium (not shown), before being fed to calcination 68.

[0142] Advantageously, since all or most of the calcium is precipitated during the wet oxidation, the recovery of CaCCL is at least partially conducted via the solids-liquid separation 26, and there is no need for a separate precipitation (e.g., all or most of the carbon dioxide captured from biogas upgrading 5 and / or calcination 68 can be reused (e.g., outside of the process) and / or sequestered (e.g., underground).

[0143] Of course, the above example and / or embodiments have been provided as examples only. It will be appreciated by those of ordinary skill in the art that various modifications, alternate configurations, and / or equivalents will be employed without departing from the scope of the invention. Accordingly, the scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims

CLAIMS1. A process for producing biogas, the process comprising:(i) anaerobically digesting lignocellulosic feedstock in an anaerobic digestion to produce biogas and digestate;(ii) treating at least some of the solids from the digestate in a wet oxidation with lime;(iii) precipitating and recovering calcium from liquid from the wet oxidation as calcium carbonate;(iv) sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and(v) recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.

2. The process of claim 1, further comprising capturing the carbon dioxide and providing the captured carbon dioxide for the process, for sequestration, or for a combination thereof.

3. The process of claim 1 or 2, wherein the wet oxidation is carried out at a pH of at least 9.

4. The process of claim 1 or 2, wherein the wet oxidation is carried out at a pH of at least 10.

5. The process of any one of claims 1 to 4, further comprising carrying out a solids- liquid separation on material treated by the wet oxidation, thereby producing separated solids and the liquid from the wet oxidation from which at least some of calcium is precipitated and recovered.

6. The process of any one of claims 1 to 5, further comprising carrying out a solids- liquid separation on the digestate, thereby obtaining a separated digestate liquid and the solids that are treated in (ii).

7. The process of claim 6, wherein (iii) comprises combining at least some of the separated digestate liquid with the liquid from the wet oxidation.

8. The process of any one of claims 1 to 7, where (iii) comprises introducing carbon dioxide to the liquid from the wet oxidation.

9. The process of claim 8, wherein the introduced carbon dioxide comprises carbon dioxide from the biogas, carbon dioxide produced in (iv), or a combination thereof.

10. The process of any one of claims 1 to 9, further comprising feeding at least some of the liquid from the wet oxidation to further anaerobic digestion.

11. The process of claim 10, wherein the further anaerobic digestion comprises a second other anaerobic digestion.

12. The process of claim 11, further comprising reducing a pH of the liquid from the wet oxidation for the second anaerobic digestion, the reducing comprising adding carbon dioxide to the liquid from the wet oxidation.

13. The process of claim 11 or 12, wherein (iii) comprises collecting calcium carbonate from the liquid from the second anaerobic digestion.

14. The process of any one of claims 1 to 13, further comprising upgrading the biogas.

15. The process of claim 14, wherein upgrading the biogas comprises capturing carbon dioxide from the biogas, and wherein at least some of the captured carbon dioxide from the biogas is sequestered.

16. The process of any one of claims 1 to 15, wherein a stream comprising precipitated calcium carbonate is sent to a solids-liquid separation to produce a separated liquid stream and a solids stream comprising the recovered calcium carbonate, which separated liquid is recycled to the lignocellulosic feedstock.

17. The process of any one of claims 1 to 16, wherein the wet oxidation comprises addition of oxygen.

18. The process of any one of claims 1 to 17, wherein the calcination comprises addition of an oxygen enriched gas.

19. The process of claim 5, further comprising providing the separated solids for land application, or providing nutrients recovered from the separated solids for use as fertilizer.

20. A process of producing biogas, the process comprising:(i) anaerobically digesting lignocellulosic feedstock in an anaerobic digestion to produce biogas and digestate;(ii) treating at least some of the solids from the digestate in a wet oxidation with lime at a pH of at least 10;(iii) precipitating and recovering calcium from liquid from the wet oxidation as calcium carbonate;(iv) sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and(v) recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.

21. A process for producing biogas, the process comprising: anaerobically digesting lignocellulosic feedstock in a first anaerobic digestion to produce biogas and digestate; treating at least some of the solids from the digestate in a wet oxidation with lime; conducing a solids-liquid separation on effluent from the wet oxidation to produce liquid and solids; feeding at least some of the liquid to a second anaerobic digestion, wherein calcium carbonate is precipitated out; sending the calcium carbonate to calcination to produce carbon dioxide and calcium oxide; and recycling at least some of the calcium oxide produced during the calcination or calcium hydroxide derived therefrom to the wet oxidation.