Solid-state anaerobic digestion method

Abstract

The invention relates to a novel dry-state anaerobic digestion method comprising pretreatment of the input with an additive selected from among zeolite, clay, pozzolan and biochar.

Claims

1. A dry methanization process comprising the following steps: a) pretreatment of an input, the pretreatment comprising the addition of an additive selected from zeolite, clay, pozzolana and biochar whose particle size is less than 700 m followed or preceded by a hydrolysis, wherein the input is an organic substance with methanogenic capacity, and the additive is added in proportions ranging from 5% to 15% by weight of dry matter of the inputs; b) anaerobic digestion; c) separation of the solid phase and the liquid phase of the digestate; wherein production of biogas via the dry methanization process is carried out for a period of more than 10 days.

2. The process according to claim 1, wherein the additive is zeolite.

3. The process according to claim 2, wherein the zeolite is selected from clinoptilolite, chabazite, phillipsite, ferrierite, mordenite and erionite.

4. The process according to claim 3, wherein the zeolite is clinoptilolite.

5. The process according to claim 1, wherein the additive is added in proportions ranging from 7% to 12% by weight of dry matter of the inputs.

6. The process according to claim 1, wherein the additive is added in proportions ranging from 6% to 14% by weight of dry matter of the inputs.

7. The process according to claim 1, wherein in step a), the pretreatment is performed prior to the hydrolysis.

8. The process according to claim 1, wherein step b) is conducted under mesophilic or thermophilic conditions.

Description

(1) FIG. 1 shows the reaction scheme of the methanization process.

(2) FIG. 2 shows a conventional plant for the implementation of the methanization process.

(3) FIG. 3 shows the experimental results obtained during the measurement of cumulated biogas production as a function of the particle size of the used zeolite.

(4) FIG. 4 shows the experimental results obtained during the measurement of cumulated biogas production as a function of the particle size and the zeolite concentration used in the substrate.

(5) Step a) of the process according to the present invention therefore corresponds to a pretreatment step of the input comprising in particular the addition of an additive selected from zeolite, clay, pozzolana and biochar whose particle size ranges from 100 m to 1000 m followed or preceded by a hydrolysis.

(6) Preferably, step a) of the process according to the present invention is conducted under the following conditions, taken alone or in combinations: the additive is selected as the zeolite; the particle size of the additive ranges from 400 m to 900 m; still preferably from 500 m to 800 m, the additive is added in proportions ranging from 5% to 15% by weight of dry material of the inputs. More preferably, the additive is added in proportions ranging from 6% to 14% by weight of dry material of the inputs. In a totally preferred manner, the additive is added in proportions ranging from 7% to 12% by weight of dry material of the inputs; the inputs are selected from straw manure (bovine, equine or any similar straw-based material), the seeds, the straw menu or the raw vegetables; the dry material content of the input ranges from 20% to 90%, preferably from 28% to 55%; the additive is added to the raw material (input) and mixed manually or with any appropriate means, allowing a homogeneous distribution of the additive in the substrate. the mixture of the additive and the inputs is produced before the beginning of the hydrolysis phase, the hydrolysis is carried out by adding water and/or recycled liquid digestate at the end of the separation step of the solid phase and the liquid phase of the digestate (step c) of the process according to the present inventionFIG. 2); the temperature at which the addition of the additive and the inputs is carried out ranges from 20 C. to 37 C.; the hydrolysis time varies from 24 h to 90 h; and/or the temperature at which the hydrolysis is carried out varies from 20 C. to 55 C.

(7) At the end of step a) of the process according to the present invention, the pretreated inputs (or substrates) are injected into a digester in order to undergo an anaerobic digestion (step b)).

(8) The anaerobic digestion step may be conducted by any means known to those skilled in the art, in particular by the use of a vertical or horizontal digester.

(9) Two temperature regimes are mainly used: mesophilic (from 30 C. to 40 C.) or thermophilic (from 45 C. to 60 C.).

(10) The anaerobic digestion step b) is carried out in the presence of an inoculum added to the substrate, said inoculum capable of being any inoculum known to those skilled in the art adapted to the nature of the used input and to the methanization conditions. Preferably, step b) of the process according to the present invention is conducted under the following conditions, taken alone or in combinations: the digester is of the horizontal type with a sealed piston type or box type feed, provided with systems of drainage, recirculation of juices by pumping and gas management; the temperature regime is of the mesophilic or thermophilic type; the stirring is ensured by recirculation of the juices or by a horizontal blade shaft whose rotation is sequenced to degas the substrate and facilitate the progress of the load; and/or the temperature holding is ensured by a plate heat exchanger

(11) At the end of step b) of the process according to the present invention, the solid phase and the liquid phase are separated of the obtained digestate (step c) of the process according to the present invention).

(12) The main function of this step is to separate a liquid fraction slightly loaded with dry material (DM) to be capable of bringing the inputs to the good dryness and to obtain solid digestates which can be handled without free juice or whose structure facilitates the air circulation in order to ensure the composting or possibly the drying with heated air thereof by heat recovery of cogeneration plant for example.

(13) The solid phase and the liquid phase of the mixture obtained in step b) may be separated by any means known to those skilled in the art. In particular, it will be possible to use a filter press or proceed by gravity separation and extraction of the liquid phase.

(14) Preferably, the digestates output from the anaerobic phase are directly introduced into the separator without pretreatment other than that specific to the particularities of the recipe or the installation.

(15) At the end of the process according to the present invention, three distinct products are obtained; biogas mainly consisted of carbon dioxide (between 30 and 50%) and methane (between 50 and 70%); water saturated and containing traces of gaseous impurities (NH.sub.3, H.sub.2S, N.sub.2, CO); a liquid phase containing water and solubilized salts and fine particles of dry material (DM). Generally the dry material content (DM) is lower than 7%; preferably comprised between 3 and 5% and are called juices; and a solid phase constituted of coarse dry materials, with a high content of stable organic matter and still containing a quantitative majority of juices absorbed or adsorbed (generally from 65 to 72%). Generally, the solid phase is exempt of free juices.

(16) The non-recycled solid or liquid fractions containing organic matter and nutrients for the plants (mainly nitrogen, phosphorus and potassium) have a waste status and are usable in spreading plans for agriculture.

(17) Nevertheless, the obtained digestates have some agronomic properties and are likely to support transformations to enrich them, and facilitate the storage, the handling and the dosage thereof at the application. The first transformation is generally a drying to concentrate the organic matter and the nutrients by using heat produced by the power generation. The product accordingly dried may be mixed with other fertilizing materials and transformed, for example, into granules.

(18) The present invention is illustrated in a non-limiting manner by the following examples.

Example: Methanization Process According to the Invention

AExperimental Protocol

1.1Pretreatment (Step a)

1.1.1Used Input

(19) The used input is constituted of 79% of manure (straw horse manure) and 21% fresh vegetables (carrots, courgettes, green beans in substantially equal parts).

(20) The manures were ground beforehand to obtain busted fibers of maximum length of 2 cm.

(21) Vegetables, for their part, are cut so that their size varies from 1 to 2 cm.

1.1.2Used Additive

(22) The used additive is a natural zeolite of the clinoptilolite class (Aquavista Limited company), brand name TerraSorb.

(23) The grade <700 m was tested as supplied.

(24) The grade 1 to 3 mm was ground using a conventional hammer mill, then the ground sample was dry screened using a laboratory screening apparatus: RUSSEL Finex Screen vibrating screenVSIAA14550, with different stacked screen stages of the smallest mesh (lower row) to the largest mesh (top row), so as to have different, separate, identified particle sizes.

(25) The used screens are the following:

(26) TABLE-US-00001 Mark Mesh Brand Reference 11 3.15 mm SAULAS NFX 11 10 2.5 mm Prfsieb AFNOR 547037 9 1.25 mm Prfsieb DIN 4188 8 1 mm Prfsieb AFNOR NFX 11.501 7 800 m Prfsieb AFNOR NFX 11.501 6 500 m Prfsieb AFNOR NFX 11.504 5 400 m Prfsieb AFNOR NFX 11.501 4 125 m SAULAS NFX 11.501 3 100 m Prfsieb AFNOR 11955 2 50 m SAULAS AFNOR 18 1 40 m SAULAS AFNOR 17

(27) Within the scope of the assays which have been carried out, different zeolite particle sizes have been tested:

(28) TABLE-US-00002 Assay Used zeolite reference TerraSorb grade 1 to 3 mm: ground natural zeolite, Z 40-50 screened between screen marks 1 & 2 - Size 40 to 50 TerraSorb grade 1 to 3 mm: ground natural zeolite, Z 100-125 screened between screen marks 3 & 4 - Size 100 to 125 TerraSorb grade 1 to 3 mm: ground natural zeolite, Z 400-500 screened between screen marks 5 & 6 - Size 400 to 500 TerraSorb grade 1 to 3 mm: ground natural zeolite, Z 500-800 screened between screen marks 6 & 7 - Size 500 to 800 TerraSorb grade<700: natural zeolite, Z <700 commercial product TerraSorb grade 1 to 3 mm: ground natural zeolite, Z 1-1.25 mm screened between screen marks 8 & 9 - Size 1 mm to 1.25 mm TerraSorb grade 1 to 3 mm: ground natural zeolite, Z 2.5-3.15 mm screened between screen marks 10 & 11 - Size 2.5 mm to 3.15 mm

1.1.3Pretreatment

(29) The zeolite is manually and gradually mixed with the substrate to ensure a homogeneous distribution. Water is gradually added when mixing the additive and the substrate in order to obtain a substrate adjusted up to about 40% of dry material.

(30) The zeolite is added in proportions of 5%, 10% or 15% of dry material according to the assay which is carried out.

(31) The obtained substrate is subsequently stored at ambient temperature for about 32 hours prior to methanization. The hydrolysis phase takes place during this storage time.

1.2Anaerobic Digestion (Step b)

1.2.1Preparation of the Inoculum

(32) The used inoculum comes from a pilot dedicated to the production of inoculum. This pilot is maintained in a continuous methanization in an infinitely mixed system (DM about 7%) and held at 54 C., with a regular and daily stirring. It is fed twice a week with a mixture of cereals, green waste and sludges from sewage treatment plants. The proportions of this mixture are 8 parts for the sludges, 1 part for the green waste, 1 part for the cereals, so as to obtain a substrate of about 7% of DM.

(33) At each feed, the stirring is stopped and a known volume (from 5 to 15%) of the digestate is removed and replaced by an equivalent volume of the substrate. The output digestate constitutes the inoculum for the methanization assays.

(34) Prior to the use of the inoculum, the inoculum is tested to determine its activity level and its capability to degrade the organic matter. For this assay, a known volume of inoculum is brought together with a known amount of acetate and put in thermophilic incubation for a duration of about 48 hours. The inoculum is considered active when at least 80% of the introduced acetate is degraded.

1.2.2Anaerobic Digestion

(35) The used digesters are PVC enclosures of about 1 liter or 6 liters manufactured specifically for the methanization. Each enclosure is provided with a feed cap and a biogas outlet orifice.

(36) The substrate obtained in the previous step 1.1.3 is added into the digester, then inoculated with the inoculum obtained in the previous step 1.2.1. The added inoculum amount is about 39% of the total substrate amount present in the digester. Water is added in order to adjust the dry material content up to about 24%.

(37) The temperature in the digester is held at 54 C. using a water bath. The mixture is manually and daily stirred (working days) for 2 to 3 minutes.

(38) The produced biogas amount is measured once a day throughout the duration of the assay.

BObtained Experimental Results and Conclusions

2.1Test 1Particle Size of the Zeolite

(39) The assay was carried out according to steps a) and b) described in the previous examples 1.1 and 1.2 under the following operating conditions:

(40) TABLE-US-00003 Volume of the used reactor (in l) 1 Amount of inputs (in g) 83.3 (horse manure: 61 g/vegetables: 16.3 g/water: 6 g) Amount of zeolite (% by weight of dry 10% (that being 2.7 g) material of the inputs) Amount of inoculum (in g) 56

(41) The Zeolites Z 40-50, Z<700, Z 1-1.25 mm and Z 2.5-3.15 mm were used.

(42) Cumulative biogas production for each of these substrates was measured for 30 days and compared to that of a blank sample a substrate to which no zeolite was added.

(43) The obtained results are reported in FIG. 3.

2.2Assay 2Particle, Size, and Concentration of the Zeolite

(44) The assay was carried out according to steps a) and b) described in the previous examples 1.1 and 1.2 under the following operating conditions:

(45) TABLE-US-00004 Volume of the used reactor (in l) 6.5 Amount of digestate (in g) 520 (Horse manure: 380 g/vegetables: 100 g/water: 40 g) Amount of zeolite (% by weight of 10% (that being 21 g) dry material of the inputs) or 15% (that being 31.5 g) Amount of inoculum (in g) 350

(46) The zeolites Z 100-125, Z 400-500, Z 500-800 and Z<700 were used at different concentrations (10% or 15% by weight of dry mass of the inputs).

(47) The cumulative biogas production for each of these substrates was measured for 30 days and compared to that of a blank sample substrate to which no zeolite was added.

(48) The obtained results are reported in FIG. 4.

2.3Conclusions

(49) The provision of a zeolite of particle size greater than or equal to 100 m and lower than or equal to 1000 m allows accelerating the biogas production of an existing plant and thus increasing the daily charge in fresh waste; which results in an increase of produced energy(energies), recoverable in the same proportion to installation and constant fixed costs.

(50) Furthermore, the addition of zeolite in a proportion ranging from 5% to 15% appears optimum in terms of efficiency relative to the cost.