METHOD FOR CONVERTING ALGAL BIOMASS INTO A GAS OR INTO BIOCRUDE BY HYDROTHERMAL GASIFICATION OR HYDROTHERMAL LIQUEFACTION, RESPECTIVELY

Abstract

The invention relates to methods for converting algal biomass into a gas or into biocrude comprising (a) gasification or hydrothermal liquefaction of an algal biomass in at least one first reactor, (b) separation of the gas or biocrude produced thereby from the aqueous effluents and the CO.sub.2 produced, at the outlet of the first reactor, (c) recovery of the aqueous effluents, and (d) oxidation of the aqueous effluents in at least one second reactor. Continuous processes for culture of algal biomass and conversion of the algal biomass cultivated into a gas or into a biocrude are also disclosed.

Claims

1-21. (canceled)

22. A method for converting algal biomass into a gas or biocrude with a view to producing a fuel or a motor fuel, notably a liquid fuel, or some other synthetic product, comprising: (a) gasification or hydrothermal liquefaction of an algal biomass in at least one first reactor; (b) separation between respectively the gas or biocrude produced and the aqueous effluents and the CO.sub.2 produced at the outlet of the first reactor; (c) recovery of the aqueous effluents; and (d) oxidation of the aqueous effluents in at least one second reactor; wherein step (d) is oxidation by a wet process or a hydrothermal oxidation (HTO) in supercritical conditions.

23. The method of conversion according to claim 22, wherein step (a) is a liquefaction process carried out at a temperature ranging from 150° C. to 374° C.

24. The method of conversion according to claim 22, wherein step (a) is a liquefaction process carried out at a pressure ranging from 0.5 MPa to 35 MPa.

25. The method of conversion according to claim 22, wherein step (a) is a gasification process carried out at a temperature ranging from 374° C. to 800° C.

26. The method of conversion according to claim 22, wherein step (a) is a gasification process carried out at a pressure ranging from 22.1 MPa to 35 MPa.

27. The method of conversion according to claim 22, wherein step (a) is carried out for a residence time ranging from 15 seconds to 1 hour.

28. The method of conversion according to claim 22, wherein step (d) is a wet-process oxidation carried out at a temperature ranging from 150° C. to 374° C.

29. The method of conversion according to claim 22, wherein step (d) is a wet-process oxidation carried out at a pressure ranging from 0.5 MPa to 35 MPa.

30. The method of conversion according to claim 22, wherein step (d) is a hydrothermal oxidation carried out at a temperature ranging from 374° C. to 800° C.

31. The method of conversion according to claim 22, wherein step (d) is a hydrothermal oxidation carried out at a pressure ranging from 22.1 MPa to 35 MPa.

32. The method of conversion according to claim 22, wherein step (d) is carried out for a residence time in the second reactor ranging from 15 seconds to 1 hour.

33. The method of conversion according to claim 22, wherein step (d) is a wet-process oxidation carried out with part of the biocrude produced injected into the second reactor.

34. The method of conversion according to claim 22, wherein the CO.sub.2 produced and separated is injected, as solvent, in the biocrude product.

35. The method of conversion according to claim 22, wherein the heat produced during step (d) of oxidation in the second reactor is supplied to the first reactor for carrying out step (a) of hydrothermal liquefaction or gasification.

36. The method of conversion according to claim 22, wherein the heat produced during step (d) of oxidation in the second reactor is supplied for preheating the oxidizing gas.

37. The method of conversion according to claim 22, wherein step (a) of hydrothermal liquefaction and step (d) of oxidation are carried out in a double-jacket reactor, said double-jacket reactor comprising an inner jacket delimiting the chamber of the first reactor internally, and an outer jacket surrounding the inner jacket, with the space between the inner jacket and the outer jacket defining the chamber of the second reactor.

38. The method of conversion according to claim 22, wherein step (a) of hydrothermal liquefaction or gasification is carried out in several first reactors fluidically in parallel.

39. The method of conversion according to claim 22, wherein step (d) of oxidation is carried out in several second reactors fluidically in parallel.

40. A continuous process for culture of algal biomass and conversion of the algal biomass cultivated into a gas or into a biocrude, comprising: (a′) culturing the algal biomass in a zone containing a culture medium; (a″) harvesting the cultivated algal biomass; (a) gasification or hydrothermal liquefaction of an algal biomass in at least one first reactor; (b) separation between respectively the gas or biocrude produced and the aqueous effluents and the CO.sub.2 produced at the outlet of the first reactor; (c) recovery of the aqueous effluents; (d) oxidation of the aqueous effluents in at least one second reactor; and (d′) injecting the water, nutrients, and any CO.sub.2 obtained at the outlet of the second reactor, into the culture zone.

41. The continuous process according to claim 40, further comprising (d″) recovery of any oxygen (O.sub.2) produced by the algal biomass to inject it, as oxidant, upstream of the second reactor before the oxidation step (d).

Description

DETAILED DESCRIPTION

[0069] Other advantages and features of the invention will become clearer on reading the detailed description of the invention given for purposes of illustration and nonlimiting, referring to the following figures, where:

[0070] FIG. 1 is a schematic view of an example of a system employing the continuous process for culture of algal biomass and conversion of the algal biomass cultivated by hydrothermal liquefaction into biocrude according to the invention;

[0071] FIG. 2 is a perspective schematic view of an advantageous embodiment of a part of the system according to the invention with a double-jacketed reactor;

[0072] FIG. 3 is a schematic view of an example of a system using a continuous process for culture of algal biomass and conversion of the algal biomass cultivated by hydrothermal liquefaction into biocrude according to the state of the art;

[0073] FIG. 4 is a schematic view of an example of a system using a continuous process according to the invention and applied directly to the system according to FIG. 3;

[0074] FIG. 5 is a graph indicating the preferred ranges of temperature and pressure for carrying out step a/ of hydrothermal liquefaction in the method according to the invention.

[0075] In the description given hereunder, the terms “inlet”, “outlet” “upstream” and “downstream” are used by reference to the direction of circulation of the products obtained within the system employing the continuous process according to the invention.

[0076] The notation X wt % signifies a percentage X by weight of a compound.

[0077] FIG. 1 shows schematically a first example of a system employing the continuous process for culture of photosynthetic microorganisms and conversion thereof by hydrothermal liquefaction according to the present invention.

[0078] The system comprises firstly a zone 1 for culture of the algal biomass. This culture zone is either in the open air, with one or more basins, or of the closed type, with one or more photo-bioreactors (PBR).

[0079] The open-air basin may be of the type with a loop or loops, usually called “raceway”.

[0080] For culture of the following species of microalgae and cyanobacteria, the following preferred temperature ranges may be envisaged: [0081] Arthrospira platensis: 25-35° C. (optimal temperature=30° C.), [0082] Chlorella pyrenoidosa: 35-45° C. (optimal temperature=38.7° C.), [0083] Chlorella vulgaris: 25-35° C. (optimal temperature=30° C.), [0084] Chlamydomonas reinhardtii: 15-30° C. (optimal temperature=25° C.), [0085] Phaeodactylum tricornutum: 20-25° C. (optimal temperature=22.5° C.), [0086] Porphyridium cruentum: 15-30° C. (optimal temperature=19.1° C.), [0087] Scenedesmus sp.: 20-33° C. (optimal temperature=26.3° C.), [0088] Nannochloropsis oceanica: 20-33° C. (optimal temperature=26.7° C.), [0089] Dunaliella tertiolecta: 30-39° C. (optimal temperature=32.6° C.).

[0090] The algal biomass obtained by culture is then harvested.

[0091] Various techniques may be used for harvesting, such as flocculation, filtration, centrifugation.

[0092] The subsequent step of hydrothermal liquefaction offers the advantage of operation with a high moisture level, up to 80 wt % of water, or even more. In contrast to other conversion technologies, this makes it possible to simplify the steps of harvesting and drying.

[0093] The harvested biomass may be sent, notably by means of a suction pump 10, to a reactor 2 for transforming the algal biomass into biocrude (oily phase) by hydrothermal liquefaction. Besides the biocrude at reactor outlet 2, there is formation of an aqueous phase, containing organic residues and nutrients of the culture medium, and a gas phase, containing primarily CO.sub.2. In other words, the reaction of hydrothermal liquefaction, which is endothermic, allows the transformation of an algal solution, partially concentrated in its culture medium, into biocrude of interest, an aqueous effluent, and CO.sub.2. The biocrude obtained may then undergo a post-treatment, of hydro-liquefaction for example, to be transformed into biofuel of the biodiesel type.

[0094] According to the invention, at the outlet of reactor 2, separation is performed between the aqueous effluents, CO.sub.2, and the biocrude by means of a suitable device 3, which may be a usual device.

[0095] Then the aqueous effluents, preferably with the CO.sub.2, are sent via a return line to the inlet of a reactor 5, in which a wet-process oxidation reaction will take place.

[0096] The oxidation reaction, which is an exothermic reaction, allows the aqueous effluents obtained by the liquefaction reaction in reactor 1 to be transformed into water containing the nutrients present in the initial algal solution, and into CO.sub.2. For it to take place, this reaction requires an oxidant, which may be air, oxygen, or some other. For the wet-process oxidation reaction to be viable energetically, it is preferable that the separation of the phases at the outlet of liquefaction take place without cooling or depressurizing the mixture.

[0097] Then, at reactor outlet, a mixture of nutrients with water and optionally CO.sub.2 is obtained, which may be injected via line 6 into the culture zone 1. This mixture is therefore recyclable to the algal culture.

[0098] Thus, according to the invention, the aqueous effluents are treated by oxidation at the outlet of hydrothermal liquefaction, for fresh algae to be cultured.

[0099] Moreover, the invention described allows the best possible energy utilization.

[0100] In fact, the reaction of liquefaction requires heating of the algal solution and is endothermic, whereas the oxidation reaction is exothermic.

[0101] Thus, as shown schematically in FIG. 1, the heat derived from the exothermic wet-process oxidation reaction that takes place in reactor 5 is supplied to reactor 2 for carrying out or participating in the hydrothermal liquefaction.

[0102] Thus, to combine these two reactions, a so-called double-jacketed reactor 7 as illustrated in FIG. 2 may be used advantageously. In this reactor 7, the inner jacket delimits the chamber of reactor 2 in which the reaction of hydrothermal liquefaction takes place and the space between the inner jacket and the outer jacket delimits the chamber of reactor 5 in which the wet-process oxidation reaction takes place (or vice versa).

[0103] The structure and operation of said double-jacketed reactor 7 will now be explained.

[0104] Reactor 7 is tubular and its length gives a sufficient contact surface between the two reaction zones, and thus improves the heat exchange.

[0105] Moreover, a small diameter makes it possible to minimize the thickness required for containing the internal pressure. The outer jacket must withstand the mechanical stresses: pressure and temperature, and the chemical stresses connected with oxidation of organic matter and the ions derived from the mineral salts. Typically, the outer jacket of reactor 7 may be made of steel 316L or Inconel 625®. If necessary, to improve the chemical durability of the reactor, titanium lining may be envisaged.

[0106] The inner jacket is used with equal pressure between the two reaction zones, therefore it does not have to withstand pressure. It must, however, withstand the thermal stresses, provide good thermal conductivity, and withstand the chemical stresses of the two reactions, i.e. both of liquefaction in chamber 2 and of oxidation in chamber 5. Typically, an inner jacket of titanium may be envisaged, which can withstand chemical and thermal stresses, but it has a lower thermal conductivity. Steel 316L or Inconel 625® may also be used.

[0107] Instead of using a double-jacketed reactor 7, it is also possible to the use other heat exchanger systems (tubular exchangers, tube-bundle exchangers, spiral-tube exchangers, etc.), or else utilize the energy produced by the oxidation reaction in some other way.

[0108] FIG. 3 shows a system using a method with continuous conversion of algal biomass culture and conversion of the cultivated algal biomass by hydrothermal liquefaction into biocrude according to the state of the art.

[0109] In a system of this kind, the aqueous effluents and the CO.sub.2 produced are troublesome wastes.

[0110] To overcome this drawback, the inventors thus proposed the method described above with optimized recycling: [0111] by oxidation of the aqueous effluents recovered at the outlet of the separating device 3, [0112] and then by injecting a stream consisting of water, CO.sub.2 and nutrients produced by oxidation in culture zone 1.

[0113] For performing the wet-process oxidation reaction it is necessary to add an oxidant to the mixture. This may be air, oxygen, hydrogen peroxide, or some other. Advantageously, it is possible to recover the O.sub.2 produced during growth of the algae in the culture zone and inject it before the step of wet-process oxidation, i.e. at reactor inlet 5 as indicated in FIG. 4.

[0114] Thus, according to the invention, the coproducts of the reaction of hydrothermal liquefaction, which are troublesome in the system according to the state of the art, are actually utilized in the method of culture of the algal biomass.

[0115] With the additional oxidation step according to the invention, the energy efficiency of the hydrothermal liquefaction process is increased, and reprocessing and utilization of the troublesome effluents at process outlet become possible.

[0116] The preferred parameters and conditions for carrying out the various steps of the method according to the invention are presented below.

[0117] Hydrothermal Liquefaction Step

[0118] i. Products at Inlet

[0119] At the inlet of reactor 2, various types of algae may be injected (Nannochloropsis, Chlorella, Neochloris, Spirulina, Chlamydomonas, Dunaliella, etc.), at varying concentration in their culture medium.

[0120] For performing a liquefaction test, the algal solution advantageously has a concentration between 10 wt % and 40 wt % of dry matter (organic and inorganic matter), knowing that the elemental chemical composition of the dry matter is within the following ranges of values: [0121] 20 to 70 wt % of carbon [0122] 5 to 40 wt % of oxygen [0123] 5 to 10 wt % of hydrogen [0124] 5 to 10 wt % of nitrogen [0125] 3 to 50 wt % of inorganic salts (compounds of P, K, Cl, Na, S, Mg, Ca, Fe, Al, F, etc.).

[0126] Hereafter, an intermediate concentration of 20 wt % of dry matter is considered, made up as follows: 55 wt % of carbon, 25 wt % of oxygen, 7 wt % of hydrogen, 8 wt % of nitrogen and 5 wt % of salts. This composition (C.sub.6.0H.sub.9.1O.sub.2.0N.sub.0.7) corresponds to a gross calorific value (GCV) of the dry matter of about 24.5 MJ/kg.

[0127] ii. Operating Conditions

[0128] The reaction temperature is in the range 200 to 350° C. for pressures varying between 5 and 25 MPa.

[0129] Typically, for the liquefaction reaction to take place, the temperature and pressure conditions must be in the operating domain in FIG. 5.

[0130] The pressure/temperature pair must be maintained in such a way that the medium is not in the phase gas.

[0131] The residence time in these conditions may be within a range from 1 to 60 minutes.

[0132] Hereafter, a test at 290° C., and 10 MPa, with a residence time of 5 minutes, is considered. It goes without saying that this is only one example among the multitudes of possible operating conditions.

[0133] Treatment of one kilogram of algal solution, with density equal to 1 to a first approximation, requires about 1000 kJ to heat the water present in the algal solution from 20 to 290° C., and about 125 kJ to heat the organic and inorganic matter.

[0134] This constitutes a first approximation of the energy to be supplied ideally for heating the reaction mixture. The enthalpy of the liquefaction reaction is to be added to this value. According to the estimates in publication [4], this can be estimated at 27 kJ in these conditions.

[0135] The operating conditions may have a considerable effect on the results.

[0136] Thus, the temperature, pressure, residence time, type and strain of algae, their concentrations, and their methods of culture, will cause the level of biocrude obtained and its quality to vary, as well as the nature of the aqueous effluent and its concentration of carbon compounds.

[0137] iii. Products at Outlet

[0138] In the conditions stated above, we expect approximately 7 wt % of biocrude (with a gross calorific value close to 32 to 38 MJ/kg), 1 wt % of CO.sub.2, supercritical in these conditions of temperature and pressure, and 92 wt % of aqueous effluent.

[0139] The latter contains 6 wt % of organic carbon, or 54.8 g of carbon per liter of algal solution treated.

[0140] Wet-Process Oxidation Step

[0141] When a double-jacketed reactor 7 is used, this wet-process oxidation reaction takes place in the space between the inner and outer jackets (or conversely in the internal zone).

[0142] iv. Products at Inlet

[0143] The aqueous effluents resulting from the liquefaction reaction, as well as an oxidant such as air or oxygen, are introduced at the inlet of reactor 5.

[0144] The aqueous phase, containing carbon compounds and CO.sub.2, thus undergoes wet-process oxidation in these conditions of temperature and pressure, through addition of an oxidant.

[0145] To a first approximation, the 54.8 g of carbon present in the aqueous effluents can release 1800 kJ during oxidation (according to the elemental composition of the carbon compounds, the value will in reality be between 1000 and 1800 kJ).

[0146] To a first approximation, this value is therefore sufficient to supply the necessary energy, as heat for the reaction of hydrothermal liquefaction and for heating of the oxidant.

[0147] v. Operating Conditions

[0148] The wet-process oxidation reaction takes place in the same conditions as hydrothermal liquefaction, i.e. in a temperature range from 200 to 350° C., and a range of pressure between 5 and 25 MPa. The oxidant must be added in an amount above stoichiometric (1.5 for example).

[0149] The residence time required is thus of the order of 1 to 60 minutes.

[0150] For 54.8 g of carbon we require a theoretical minimum of 146.1 g of oxygen to ensure complete theoretical oxidation.

[0151] In reality, above-stoichiometric is favorable. For example, for 1.5 above stoichiometric, 219.2 g of oxygen is required.

[0152] Since the oxygen arrives at room temperature, 54.5 kJ is required to heat it to the reaction temperature. If the oxidant is air, 767.6 g of nitrogen would be added, requiring 213.3 kJ for heating from 20 to 290° C.

[0153] vi. Products at Outlet

[0154] At the outlet of the oxidation reactor 5, the stream is composed of a favorable medium for culture of algae, containing water, CO.sub.2 and the nutrients initially present in the algal solution.

[0155] These elements may be injected into the culture zone directly.

[0156] Other variants and improvements may be envisaged while remaining within the scope of the invention.

[0157] The invention is not limited to the examples that have just been described; it is notably possible for characteristics of the examples illustrated in variants that are not illustrated to be combined together.

[0158] The expression “comprising a” (or “comprising one”) must be understood as being a synonym of “comprising at least one”, unless stated otherwise.

REFERENCES CITED

[0159] [1]: J. Pruvost et al. “Industrial production of microalgae and cyanobacteria”, Techniques de l'ingénieur, Ind, 200, November/2011. [0160] [2]: Julia L. Faeth, et al.: “Fast Hydrothermal Liquefaction of Nannochloropsis sp. to Produce Biocrude”, Energy Fuels, 2013, 27 (3), pp 1391-1398. [0161] [3]: Yan Zhou, et al.: “A synergistic combination of algal wastewater treatment and hydrothermal biofuel production maximized by nutrient and carbon recycling”, Energy & Environmental Sciences, 2013, 6, 3765-3779. [0162] [4]: Mariane Audo: “Evaluation of the rheological potential of oils derived from microalgae for applications as bitumen substitutes” Doctorate Thesis, 2013, Nantes University, doctoral school 3MPL.