Dry anaerobic digester for lignocellulosic biomass with unique hydrodynamics (dadfor lcb)

Abstract

The present invention provides a reactor for the anaerobic digestion of lignocellulosic biomass for biogas and biomanure (digestate) generation. The innovation is designed primarily for the treatment of lignocellulosic biomass such as rice straw / rice husk with a total solids consistency in the slurry between 20 - 40 %. One of the major obstacles in the AD of lignocellulosic biomass is the formation of scum layer on the top surface of the slurry with the fibrous material leading to choking of pipelines. Scum layer formation occurs due to the phase separation of the material into solid and liquid, poor contact between the organic matter and the microorganisms, low heat and mass transfer. Phase separation of solid and liquid occurs particularly in low density materials such as lignocellulosic biomass. Therefore, the reactor is designed in a way that the maximum solids including the fibrous material is retained inside the reactor for efficient digestion. The reactor comprises of a feed hydrolysis duct situated inside the reactor. The hydraulic residence time of the feed hydrolysis duct is delinked with that of the main reactor. Further the reactor is provided with open and closed, perforated cones to ensure efficient movement of the slurry from the top to bottom. Pressurized biogas purging in the bottom elongated cone area of the reactor ensures no settling of solids in the cone, while the slurry recirculation and the pressurized biogas purging inside the digester helps promote efficient mixing of the reactor contents in the digester preventing the formation of phase separation, scum layer formation, choking etc. The feed slurry is initially allowed to flow through the central duct towards the bottom of the reactor which then expands creating an upward flow regime from the bottom due to the presence of a truncated cone. The perforated truncated cones ensure retainment of maximum solids in the reactor arresting any biomass washout. The reactor is also facilitated with a feed preparation system, digestate collection tank, biogas balloon, feed inlet and digestate outlet valve mechanism for controlled feed in and digestate out.

Description

[0001] DRY ANAEROBIC DIGESTER FOR LIGNOCELLULOSIC BIOMASS WITH UNIQUE HYDRODYNAMICS (D I); / / for LCB)

[0002] FIELD OF INVENTION

[0003] The present invention relates to a reactor for the dry anaerobic digestion of lignocellulosic biomass for biogas and biomanure (digestate) generation. In particular, the present invention provides a reactor that can be operated at high solids in the range of 20 - 40 % and which can handle low density feedstocks such as lignocelluloses. More particularly, the reactor of the instant invention in its configuration consists of truncated cones and a central duct that creates unique hydrodynamic flow of the material from top to bottom of the reactor. The arrangement of the cones is provided in such a way that the formation of scum layer on the surface of the material is completely prevented. Another highlight of the reactor is the de-linking of hydraulic residence time of the central duct meant for hydrolysis and the main reactor. The developed reactor is immensely useful in the sectors of solid waste management, especially renewable energy generation and lignocellulosic waste management. The invention shall help attain the 7thsustainable development goal of affordable and clean energy.

[0004] BACKGROUND OF THE INVENTION AND DESCRIPTION OF PRIOR ART

[0005] The objectives of the solid-state anaerobic digestion process are to operate the digester with slurry consisting of more than 20 % total solids (TS) for the generation of biogas and biomanure. Anaerobic digestion of high solids biomass helps in the usage of lower volume of water required for dilution of feedstock to obtain desired consistency. The quantity of digestate or stabilized solids generation in solid-state anaerobic digestion is low.

[0006] The history of anaerobic digestion (AD) dates back to long years, beginning as early as the 10thCentury. The first application of anaerobic biological process to the treatment of organic wastes was the septic tank, invented in 1895. A variety of applications of the anaerobic concept have evolved over the past nearly 100 years. Early applications, beginning in 1918, were for the treatment of sludge from domestic sewage. Much later, beginning in the 1950's, a process that was called "anaerobic contact" and "anaerobic activated sludge" was applied to livestock slaughtering wastewaters. This process made use of a clarifier unit to separate solids from the liquid. Still later, beginning in the 1970's, anaerobic treatment applications included various attached growth (up flow, down flow, and expanded bed) treatment approaches.

[0007] AD is basically classified into two types namely, slurry AD (< 15 % TS) and dry AD (> 20 % TS) based on solids consistency of the feed material. The performance of an anaerobic digester depends on the number of internal and external factors such as substrate type, organic loading rate temperature, pH, HRT, mixing of slurry and C / N ratio. Majority of the indigenous biogas technologies in India are suitable slurry digestion of organic wastes. One of the major disadvantages in slurry type digestion is the generation of huge volumes of digestate, that makes its handling difficult on a daily basis. The separation of solids and liquids from the digestate using mechanical equipment adds on to the operational and capital costs. Low density substrates like lignocellulosic biomass requires huge volumes of water if they have to be treated in slurry digester. In addition to this, bio methanation of low density materials in slurry digesters results in the formation of a scum layer on the top of the surface due to solid liquid phase separation leading to chocking of the entire system.

[0008] The main disadvantage of the present anaerobic digesters is the long residence time typically required to digest the organic wastes. Mostly, the anaerobic digesters are suitable for the treatment of organic solid waste are "batch" or one-stage digesters. The batch digester is a closed or domed vessel within which very large quantities of organic waste is fermented. Anaerobic batch digesters take 40 to 50 days to adequately digest the organic solids (U.S. Pat. No. 5,637,219). Studies were reported on batch type digesters of KVIC and DENAB ANDHU models for production of biogas from solid organic waste, which are otherwise used for dung digestion (Rajashekhar Reddy et al., 1996). These reactors have drawbacks of high residence time (40 - 50days) and formation of scum. No mixing mechanism is available in this digester. Moreover, these batch type plants are not suitable for the treatment of large quantities of solid organic waste. As a result, many municipal and industrial wastes are processed using aerobic digestion systems or a combination of aerobic with anaerobic systems (U.S. Pat. No. 4,885,094).

[0009] Higher solid content feed to digester has a potential to increase the efficiency of conversion of organic matter in the biomass to biogas, but the higher solid content in the range of 20 - 40 % in the digester can cause reduction in mass and heat transfer between bacteria, enzymes, and substrate in the digester due to poor hydrodynamics (mixing of reactor contents). Hydrodynamics is a major factor that contributes in the mass transfer, structure and metabolism of microbial community in the AD process. It was mentioned that the close contact between acetogens and methanogens can lead to effective methanogenesis which can achieved by smooth and adequate mixing. Negative impacts of inadequate mixing are observed as lower methane yield, loss of digester volume (dead zones), increase in operational expenses, clogging and chocking of lines of the entire system. Therefore, the tank geometry, placement of inlets and outlets provides a means of efficient mixing in the digester. Therefore, it is of utmost importance that the anaerobic digesters require adequate mixing that enhances the homogeneous distribution of nutrients and micro-organisms and can avoid formation of surface crust and sedimentation. Mixing in the anaerobic digester can be attained by various methods such as slurry recirculation, biogas recirculation and provision of impellers.

[0010] Mixing in the anaerobic digesters meant for the treatment of lignocellulosic biomass is very important because the lignocellulosic biomass is a low density material and density is a characteristic property of the material and does not depend on the amount of the substance. Poor mixing in the digesters treating low density materials can result in the phase separation (solid / liquid) and poor degradation due to which the surface active substances can accumulate at air / liquid interface and enhances the surface activity and potentially foaming. Although the importance of mixing in anaerobic digestion is noted by many researchers to enhance the performance but the optimum mixing method is still a debatable subject. Higher mixing intensities and continuous mixing disrupt the syntrophic relationships and the microbial flocs among the bacteria and methanogens. Mixing is mainly associated with various costs like equipment, maintenance cost and operation cost.

[0011] DI (WO2021112440) document discloses anaerobic digestion apparatus which comprises: an anaerobic digestion reactor comprising an outer vessel and an inner vessel, wherein the outer vessel is a container sealed from the outside, and the inner vessel is vertically installed inside the outer vessel to thereby partition the outer vessel into an inner anaerobic reaction unit and an outer anaerobic reaction unit; a liquid reverse-flow pipe and a liquid reverse-flow valve, wherein the liquid reverse-flow pipe is vertically installed below the height of the inner vessel inside the inner anaerobic reaction unit and guides liquid in the inner anaerobic reaction unit to the outer anaerobic reaction unit, and the liquid reverse-flow valve controls opening and closing of the liquid reverse-flow pipe; and an impulse mixer for transferring liquid to the inner anaerobic reaction unit from the outer anaerobic reaction unit. The anaerobic digestion apparatus thusly configured prolongs the retention time of the organic solid matter and the anaerobes and enables uniform mixing thereof, and thus can improve digestion efficiency of the organic solid matter and increase the production amount of biogas standards over a period of several months.

[0012]

[0013] Batch Process / Systems

[0014] Batch processes are easy to handle as it requires low operational and capital cost with less control systems (Liu et al., 2018). The biogas generation in batch process is very less in the initial days and it continuously increases reaching a peak point followed by gradual decrease and seize of biogas due to the depletion of the organic material. A solid state anaerobic batch system can handle the organic waste with a total solid concentration in the range of 20 to 40%. In a batch system, the reactor is loaded with fresh substrate and it is discharged then loaded with fresh substrate. Batch systems have higher reaction rates with higher methane yield (Li et al., 2011). By reducing the inoculum percentage in a batch process the efficiency of solid state anaerobic digester (SSAD) increases along with retention time (Han et al., 2017). In order to reduce the water intake in the batch digester, leachate can be collected and recirculated in to the system (Xing et al., 2020). The recirculate can directly be added in to the substrate, this may reduce the volume of the fresh culture. Other advantages of batch systems include, it is a simple ad robust process and it is economically cheap, reliable process (Li et al., 2011). Disadvantages of batch system include the uncontrolled biological process, intermediate products such as VFA can generated it reduced the pH and decreased the activity of methanogens (Kothari et al., 2014). This may be rectified by using existing reactor inoculum or leachate.

[0015] Bekon Batch digester system

[0016] A company called Bekon in Germany has a largest batch anaerobic system called “BEKON DIGESTER”. It is also called as garage type percolating reactor which is mainly used for anaerobic digestion of agricultural waste, source separated waste, OFMSW and yard waste for the generation of biogas. In this digester system, a recirculation system is arranged for the recirculation of the leachate generated from the feedstock. The leachate is pumped to the surface of the substrate to maintain suitable the moisture in the system. In this system, recirculation of the leachate under optimized conditions at regular interval of time with a quantified amount of leachate is followed to keep the microorganisms in contact with the substrate that improves the mass transfer resulting in enhanced biogas generation (Qian et al., 2017). BEKON digesters can be operated at mesophilic and thermophilic temperature.

[0017] BIOFerm Batch systems

[0018] BIOFerm digesters are batch systems suitable for solid-state anaerobic digestion of high solids containing wastes at mesophilic temperature. BIOFerm digesters were used for feed stocks with total solid concentrations in the range of 25-30%. Feedstock is loaded in to the chambers and remained for 28 days (Andre, et al., 2018). Heat is supplied in to the system by in-wall radiant process at the same time, the percolate is also sparged on the surface of the feedstock through the sprinklers. In this process, the percolate which is sprayed on the entire biomass acts as the inoculum for the AD process. BIOFerm solid state digesters were suitable for substrates such as OFMSW and yard waste.

[0019] Continuous Process / system

[0020] In the continuous reactor systems, the fresh feedstock is continuously fed in to the digester and the same amount of the digested material is removed from the reactor on a daily basis. The main difference between batch and continuous system is the stages involving in batch system are well defined whereas in continuous process there is consistency in feed stock and the reactors resulting in a constant generation of biogas. In a continuous process, for the continuous movement the inlet and outlet valve mechanism is used to feed and withdraw the slurry. The design of the continuous reactor decides the operation of the entire process (Kothari et al., 2014). The main advantages of continuous process compared to batch process are inoculum requirement is less in continuous process compared to batch and the continuous reactors can handle high OER with small land usage and the biogas yield is very high. Most of the SSAD continuous reactors are designed as plug flow reactors (Mahnert et al., 2005). The disadvantage of the continuous process is continuous removal of the effluent which is a combination of completely digested material (digestate) and partially digested feed. In order to avoid the biomass washout, design of the reactor should be taken in to consideration. Technologies available for continuous SSAD

[0021] DRANCO Process

[0022] A Belgian company named as “organic waste systems” developed the DRANCO process which is known as solid state anaerobic digester. First DRANCO plant was started in 1992 in the city of Brecht, Belgium (12,000 t / y). There are several plants in operation currently working based on Dranco process, such as Bassum, Germany with the capacity of 13,500 t / y one more in Kaiserslautern, Germany with the capacity of 20,000 t / y and in the city of Salzburg, Austria with the capacity of 20,000 t / y (Rapport et al., 2008). Dranco process is a complete SSAD process of organic fraction of MSW in a continuous plug flow reactor system. Hydrolysis, acidogenesis, acetogenesis and methanogenesis occurs in a single reactor with the total solids concentration of substrate ranging between 15 to 40% (Van et al., 2019). The entire process is operated at thermophilic temperature with a retention time of 20 days. The feedstock enters the reactor from top and the digestate is collected from the bottom of the reactor. No internal mixing is provided in the process. Internal mixing in the system through agitators is avoided by providing recirculation of the leachate.

[0023] KOMPOGAS Technology

[0024] The KOMPOGAS technology was established by W. Schmid of Glattbrugg, Switzerland in 1980s. The Kompogas technology is based on the use of thermophilic fermentation process where the operational temperature of the process is between 50-60°C and the hydraulic residence time for KOMPOGAS process is 15 to 20 days. The kompogas reactor is a horizontal tank which is made up of steel. Axial mixers were arranged in the reactors with slow rotating speed which is meant for the transfer the feedstock from inlet to outlet of the reactor. Kompogas process is able to treat the organic waste with TS concentration of 23 to 28% and it is a single stage system (Hartmann et al., 2006). Recyclable digestate and process water may be added to the feed stream in order to reduce the solid content.

[0025] VALORGA Process

[0026] The VALORGA Technology was developed in France and was acquired by a Germany company called Steinmuller Valorga Sari. Valorga technology operates at mesophilic temperature under semi solid state conditions suitable for the treatment of organic fraction of MSW with total solids concentration of 25 to 35% (Fernandez et al., 2008). Valorga reactor is a vertical cylinder type reactor with horizontal plug-flow behaviour. Two openings are introduced on either side of the wall for inlet and outlet of the organic matter. The walls of the reactor intensifies the organic matter and moves in a circular way in order to transport round and it continuously covers the entire surface of the reactor. There is a pneumatic system for mixing the high solid organic matter. The biogas generated from the reactor is injected with high pressure from the bottom of the reactor in a periodic manner and it allows the efficient vertical mixing (Fu et al., 2018). Due to this process mechanical processes can be avoided for transfer or circulation of the organic matter and also suitable interactions happen between fresh feed stock and digestate. The advantages of the Valorga process include optimal degradation of organic matter due to the recirculation of pressurized biogas and it is a reliable process, it eliminates the pathogens due to the perfect degradation of organic matter (Li, et al., 2011).

[0027] Despite the availability of the hitherto known anaerobic reactors or processes for lignocellulosic biomass degradation, an unfilled need exists to provide an improved reactor for lignocellulosic biomass treatment at high solids (20 - 40 % solids) with unique hydrodynamic behaviour of material for the anaerobic digestion of lignocellulosic biomass with total solids ranging between 20 - 40% in the slurry and wherein it comprises the features such as dealienation of scum formation issue, elimination of choking and clogging, short-circuit of feed in the digester which are common issues in conventional digesters.

[0028] OBJECTIVES OF THE INVENTION

[0029] The main objective of the present invention is therefore to provide a dry anaerobic digester for lignocellulosic biomass with unique hydrodynamics, which obviates the drawbacks of the hitherto reported prior art.

[0030] Another objective of the present invention is to provide a dry anaerobic digestor with unique hydrodynamics for the generation of biogas and biomanure from lignocellulosic biomass with a solids consistency in the range of 20 - 40 % in the slurry.

[0031] Still another objective of the present invention is to provide a reactor that delinks the residence time of the feed hydrolysis duct from the residence time of the main reactor for efficient digestion of the low-density material without any phase separation.

[0032] Yet another objective of the present invention is to provide a reactor that comprises middle perforated truncated cones fixed to the wall of the cylindrical reactor and the top and bottom truncated cones meant to create turbulence in the reactor disturbing the flow behaviour of the slurry, ensuring proper mixing of the material inside the digester without any mechanical equipment. Still another objective of the present invention is to provide a reactor wherein the arrangement of truncated cones creates unique hydrodynamics inside the reactor even with materials such as lignocellulosic biomass by preventing phase separation and biomass washout.

[0033] Yet another objective of the present invention is to provide a reactor wherein the perforated truncated cone above the bottom truncated cone ensures the movement of slurry towards upward flow regime through the perforations (20 mm) while the solid material is retained in the bottom part of the reactor.

[0034] Still another objective of the present invention is to provide a reactor wherein the bottom truncated cone elongated through the central duct is provided to ensure the retainment of solids and divert the flow from bottom to towards the middle perforated truncated cone.

[0035] Yet another objective of the present invention is to provide a reactor wherein the top truncated cone is provided for the easy movement of the thinner slurry towards the perforated plate.

[0036] Still another objective of the present invention is to provide a reactor wherein the conical bottom of the reactor is a place where maximum solids are retained and the mixing of solids in the cone is ensured through pressurized biogas purging.

[0037] Yet another objective of the present invention is to provide a reactor wherein the perforated plate ensures further retainment of solids and allows the liquid to flow upwards which is then withdrawn as the final digestate. Still another objective of the present invention is to provide a reactor which has the capacity of being scaled up to handle a wide range of waste quantities from smaller capacity to larger capacity.

[0038] Yet another objective of the present invention is to provide a reactor having the capability of intermittent mixing of digester slurry by using the autogenerated biogas pressure and slurry recirculation from bottom to top.

[0039] SUMMARY OF THE INVENTION

[0040] Accordingly, the present invention provides a reactor for dry anaerobic digestion of lignocellulosic biomass (20 - 40 % solids) comprising a vertical cylindrical digestion tank with elongated conical bottom and a domed top, a central duct, a perforated plate and three truncated cones (two closed truncated cones fixed to the central duct and one open truncated perforated cone fixed to the wall of the cylinder). A perforated plate is placed on top of the truncated cones. A truncated cone is a cone with the tip straight cut off with a larger base circle and the top surface with the smaller circle. Two sample ports are provided to the digester just below the perforated truncated cone and bottom truncated cone to withdraw samples for characterization and also for mixing the feed to ensure appropriate buffering. The feed inlet mechanism and the gas collection point are both located at the top of the reactor. The feed prepared from lignocellulosic biomass consisting of 20 - 40 % solids is transported to the bottom of the reactor through the central hydrolysis duct using pumping mechanism. Further, the reactor is provided with gas purging line for pressurized biogas purging in the main reactor as well as the elongated cone area where maximum solids are retained to ensure effective mixing of material inside the digester. Further, the reactor is provided with digestate recirculation mechanism so that the digested liquid can be withdraw from the top and recirculated back to the reactor.

[0041] In an embodiment, the present invention provides a dry anaerobic digester for lignocellulosic biomass with unique hydrodynamics comprising a reactor (1) for anaerobic digestion of a lignocellulosic biomass slurry, wherein the said reactor comprises of:

[0042] (i) a feeding line with inverted cone fixed in the centre of the perforated plate (2 and 3);

[0043] (ii) a central duct (4) known as feed hydrolysis duct;

[0044] (iii) a closed truncated cone (5) fixed to the central duct;

[0045] (iv) an open truncated perforated cone fixed to the wall of the cylinder (6);

[0046] (v) a closed truncated cone (7) fixed to the central duct;

[0047] (vi) a sample point (8);

[0048] (vii) an elongated conical bottom (9);

[0049] (viii) water sprinklers (10);

[0050] (ix) a drain valve at the bottom (11);

[0051] (x) means for pre and post processing mechanisms / elements such as the feed preparation tank (FPT); digestate collection tank (DCT);

[0052] (xi) the biogas collection lines connected to the biogas balloon;

[0053] (xii) digestate recirculation ports at the bottom of the reactor and on top of the elongated cone;

[0054] (xiii) a compressor, feeding pumps (slurry pumping and recirculation); and a biogas purging line.

[0055] In another embodiment, the present invention provides a dry anaerobic digester, wherein the lignocellulosic biomass (rice husk / rice straw / wheat straw etc) slurry consisting of 20 to 40 % solids is pumped into the reactor from top through the central duct (4) that flows towards bottom of the reactor (1) in a distributed manner.

[0056] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the slurry inlet mechanism (2) is provided with valve mechanism for feeding the slurry to the tank under atmospheric conditions and the gas inlet is provided with a gas distribution system.

[0057] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein the working of the reactor starts with the preparation of feed slurry of required solids consistency in the FPT, followed by its pumping to the main reactor through the feeding line with an inverted cone that is mounted to the feed hydrolysis duct (4), wherein the feed slurry is initially allowed to flow through the central duct towards the bottom of the reactor which then expands creating an upward flow regime from the bottom due to the presence of a truncated cone.

[0058] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the central duct in the reactor acts as a feed hydrolysis duct where the feed slurry gets hydrolysed, and the residence time of the feed hydrolysis duct is delinked with the residence time of the main reactor.

[0059] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein the perforated truncated cone above the bottom truncated cone ensures the movement of slurry towards upward flow regime through the perforations, while the solid material is retained in the bottom part of the reactor, wherein the bottom truncated cone (7) elongated through the central duct (4) is provided to ensure the retainment of solids and divert the flow from bottom towards the middle perforated truncated cone, wherein the perforated cone (6) consists of perforations with a diameter of 20 mm to ensure the flow of high solids material upwards.

[0060] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the pressure build-up inside the feed hydrolysis duct stimulates the downward movement of the acidified slurry towards the bottom and the presence of truncated cone directs the flow behaviour regime upwards for further digestion of the organic material.

[0061] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein the top truncated cone (5) is provided for the easy movement of the thinner slurry towards the perforated plate and the perforated plate (3) with 10 mm perforations ensures further retainment of solids and allows the liquid to flow towards up which is then withdrawn as the final digestate. In still another embodiment, the present invention provides a dry anaerobic digester, wherein the middle perforated truncated cone (6) fixed to the wall of the cylindrical reactor and the top and bottom truncated cones are meant to create turbulence in the reactor disturbing the flow behaviour of the slurry.

[0062] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein formation of scum layer on the top surface of the slurry with the fibrous material leading to choking of pipelines is avoided with the presence of truncated cones.

[0063] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the arrangement of truncated cones (5, 6, and 7) creates unique hydrodynamics inside the reactor with materials such as lignocellulosic biomass.

[0064] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein in addition to the truncated cone (5, 6, and 7), the recirculation of the slurry from the bottom to top ensures the creation of flow regime upwards.

[0065] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the pressurized biogas purging further improves the hydrodynamic behaviour of the thick slurry inside the digester and ensures appropriate mixing without any mechanical equipment for efficient heat transfer, nutrients exchange between the microorganisms and organic material.

[0066] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein the water sprinklers (10) mounted on the wall of the vertical cylinder ensure the periodic sprinkling of water to arrest any foaming that forms on the top of the digested slurry.

[0067] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the biogas generated from the reactor is stored in biogas balloon which is then compressed and sent back to the reactor partially through the gas purging line.

[0068] In yet another embodiment, the present invention provides a dry anaerobic digester, wherein the digestated liquid accumulated above the perforated plate is collected in the digestate collection tank which is partially utilized as a buffering agent and a diluent in the feed slurry, wherein samples from the sample points (8) can be collected for analysis periodically to check the health of the reactor.

[0069] In still another embodiment, the present invention provides a dry anaerobic digester, wherein the elongated conical bottom (9) of the reactor is a place where the maximum solids are retained and the mixing of solids in the cone is ensured through biogas purging, wherein the solids can be drained out using the drain valve (11) as per the need.

[0070] BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0071] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawing, wherein:

[0072] FIGURE 1 illustrates a sectional view of the reactor in accordance with an embodiment of the present invention.

[0073] Skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily been drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help to improve understanding of aspects of the present invention. Furthermore, the one or more elements may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

[0074] DETAILED DESCRIPTION OF THE INVENTION

[0075] Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.

[0076] While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention as defined by the appended claims. The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that one or more devices or sub-systems or elements or structures proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or additional devices or additional sub-systems or additional elements or additional structures. Similarly, a method step proceeded by “comprising” or any variation thereof, does not, without more constraints preclude the existence of additional steps or repetitive steps.

[0077] Referring to figure 1, it can be noticed that the reactor in accordance with one embodiment of the present invention comprises:

[0078] • a vertical cylindrical digestion tank (1) known as the main reactor;

[0079] • a feeding line with inverted cone fixed in the centre of the perforated plate (2 & 3);

[0080] • a central duct (4) known as feed hydrolysis duct;

[0081] • a closed truncated cone (5) fixed to the central duct;

[0082] • an open truncated perforated cone fixed to the wall of the cylinder (6);

[0083] • a closed truncated cone (7) fixed to the central duct;

[0084] • sample point (8);

[0085] • an elongated conical bottom (9);

[0086] • water sprinklers (10) and

[0087] • a drain valve at the bottom (11).

[0088] The innovation also contains pre and post processing mechanisms / elements such as the feed preparation tank (FPT); Digestate collection tank (DCT); the biogas collection lines connected to the biogas balloon; digestate recirculation ports at the bottom of the reaction and on top of the elongated cone; a compressor, feeding pumps (slurry pumping and recirculation); and a biogas purging line.

[0089] The innovation is designed primarily for the treatment of lignocellulosic biomass such as rice straw / rice husk with a total solids consistency in the slurry between 20 - 40 %. One of the major obstacles in the AD of lignocellulosic biomass is the formation of scum layer on the top surface of the slurry with the fibrous material leading to choking of pipelines. Scum layer formation occurs due to the phase separation of the material into solid and liquid. Phase separation of solid and liquid occurs particularly in low density materials such as lignocellulosic biomass. Therefore, the reactor is designed in a way that the maximum solids including the fibrous material is retained inside the reactor for efficient digestion. The working of the reactor starts with the preparation of feed slurry of required solids consistency in the FPT, followed by its pumping to the main reactor (1) through the feeding line with an inverted cone that is mounted to the feed hydrolysis duct (2). The feed slurry is initially allowed to flow through the central duct (4) towards the bottom of the reactor which then expands creating an upward flow regime from the bottom due to the presence of a truncated cone (5). The central duct in the reactor acts as a feed hydrolysis duct where the feed slurry gets hydrolysed. The residence time of the feed hydrolysis duct is delinked with the residence time of the main reactor.

[0090] The perforated truncated cone (6) above the bottom truncated cone ensures the movement of slurry towards up through the perforations while the solid material is retained in the bottom part of the reactor. The bottom truncated cone (7) elongated through the central duct is provided to ensure the retainment of solids and divert the flow from bottom to towards the middle perforated truncated cone (6). The perforated cone consists of perforations with a diameter of 20 mm to ensure the flow of even denser material (15 - 30 % solids) towards up. The pressure build up inside the feed hydrolysis duct stimulates the downward movement of the acidified slurry towards the bottom and the presence of truncated cone directs the flow behaviour regime towards up for further digestion of the organic material.

[0091] The top truncated cone is provided for the easy movement of the thinner slurry towards the perforated plate. The perforated plate (3) ensures further retainment of solids and allows the liquid to flow towards up which is then withdrawn as the final digestate. A middle perforated truncated cone fixed to the wall of the cylindrical reactor and the top and bottom truncated cones are meant to create turbulence in the reactor disturbing the flow behaviour of the slurry.

[0092] The presence of truncated cones alleviates the issue of scum formation or floating of the fibrous material on the top surface of the slurry. This arrangement of truncated cones creates unique hydrodynamics (flow behaviour from top to bottom and bottom to top) without any mechanical equipment inside the reactor even with materials such as lignocellulosic biomass. In addition to the truncated cone, the recirculation of the slurry from the bottom ensures the creation of flow regime towards upwards. The pressurized biogas purging further improves the mixing for efficient heat and mass transfer to take place between the substrate and the microorganisms ensuring suitable hydrodynamic behaviour of the thick slurry inside the digester and ensures appropriate mixing.

[0093] The water sprinklers mounted on the wall of the vertical cylinder is to ensure the periodic sprinkling of water to arrest any foaming that forms on the top of the digested slurry. The presence of truncated cones in the reactor is provided to ensure proper mixing without the provision of any mechanical equipment and to retain maximum solids in the digester to prevent biomass washout. The biogas that is generated from the reactor is stored in biogas balloon which is then compressed and sent back to the reactor partially through the gas purging line.

[0094] The digestate that gets accumulated on the top of the perforated plate is collected in the digestate collection tank which is partially utilized as a buffering agent and a diluent in the feed slurry. The conical bottom of the innovation is a place where maximum solids are retained and the mixing of solids in the cone is ensured through pressurized biogas purging. Therefore, the innovation is provided with suitable arrangements for the expansion of slurry at the bottom and its flow towards up creating upward flow regime, retention of solids through perforated cone and plate, delinking of residence time of the reactor as well as the feed hydrolysis duct, provision of slurry recirculation and biogas purging mechanism.

[0095] Although not illustrated in figure 1, the reactor constructed in accordance with the teachings of the present invention also includes a feed preparation system with provision to add lignocellulosic biomass to the reactor vessel and arrangement for feeding a calculated quantity of homogenized slurry to the feed tank of dry anaerobic digester (DADUH for LCB). The feed preparation system may be integrated with the reactor or the lignocellulosic biomass with 20 - 40 % solids may be transported from the feed preparation system to the reactor using pumps. The feed preparation system may be of a rectangular / circular PPFRP / HDPE / RCC construction with suitable capacity depending on feed rate of lignocellulosic biomass and fitted with an air compressor to homogenize the slurry. The feed preparation system in a preferred embodiment has a slanting bottom with bottom discharge to remove the grits. In the normal course of action, the digested slurry from digester (tank) is collected in gravity settler tank (digestate collection tank). This unit is suitably designed to receive calculated quantity of digested slurry from the digester (tank). The gravity settler can be in the form of a rectangular / circular tank having suitable volume depending upon feeding rate to the reactor. The gravity settler has supporting structure with top mounted with mesh so that solids are retained on the top whereas liquid is settled to the bottom. The liquid is pumped back (recycled) to feeding system for the preparation of feed slurry along with fresh water.

[0096] The reactor of the present invention functions as a continuous reactor and everyday known amount of slurry can be fed to the reactor from top controlling the valves of the tank and equal quantity of the digested slurry can be discharged from the top zone of the tank (above perforated plate) to a digestate collection tank. The production of methane can be estimated by methods known in the art. See Ch. 8, Metcalf & Eddy, Inc. (1991).

[0097] The reactor was operated with lignocellulosic biomass namely rice husk and rice straw for a period of 100 days. The rice husk and rice straw were initially pre-treated for size reduction in a pulveriser machine with a particle size of < 1mm. The pulverized powder of rice husk and rice straw was used as substrate in the digester for the generation of biogas under dry anaerobic digestion conditions. The total solids in the slurry was varied from 12 % to 20 % followed by 30 and 40 %. The reactor (present innovation) of 70 L volume with an active volume of 50 L was used in the experimental studies. Comparative experimental studies with a conventional slurry digester was also performed. The experiments in the present innovation were performed in batch as well as continuous mode. The examples for continuous mode of operation is given below as reference.

[0098] EXAMPLES

[0099] The functioning of the reactor constructed in accordance with the teachings of the present invention is described with reference to the following examples, which are explained by way of illustration only and should not therefore be construed to limit the scope of the present invention.

[0100] Example 1(a): Anaerobic digestion of rice husk in a conventional slurry digester

[0101] Experiments were conducted in the conventional anaerobic slurry digester having capacity of 70L. The effective volume of the reactor was 50 L of which the digester was inoculated with anaerobic sludge (40 % of reactors effective volume i.e., 20 L) obtained from a digester treating market vegetable waste. The digester was operated in continuous mode (equal amount of feed and digested slurry are fed and withdraw to and from the reactor respectively) and fed with lignocellulosic biomass (rice husk) at (12 % TS concentration) initially at a feed rate of 0.35 L / day at an HRT of 90 days which was gradually reduced to 30 days reaching a feed rate of feed rate of 1 L / day, 2.4 kg TS / m3 / day, 1.68 kg VS / m3 / day, and HRT of 30 days. During the stabilization of the reactor, steady VS destruction rate and biogas production was measured. The biogas generation was measured and yields were calculated resulting in 0.3 m3 / kg VS destroyed at a corresponding CH4 content of 55 % in biogas. Phase separation, choking of pipelines, reactor clogging were the noticeable issues observed when the slurry digester was operated with slurry TS consistency > 12 %.

[0102] Example 1(b): Anaerobic digestion of rice straw in a conventional digester

[0103] Example 2(a): Anaerobic digestion of rice husk in a DADUH for LCB

[0104] Experiments were conducted in the conventional anaerobic slurry digester having capacity of

[0105] 70 L. The effective volume of the reactor was 50 L of which the digester was inoculated with anaerobic sludge (40 % of reactors effective volume i.e., 20 L) obtained from a digester treating market vegetable waste. The digester was operated in continuous mode (equal amount of feed and digested slurry are fed and withdraw to and from the reactor respectively) and fed with lignocellulosic biomass (rice husk) at (12 %, 20 %, 30 % and 40 % TS concentration) initially at a feed rate of 0.35 L / day at an HRT of 90 days which was gradually reduced to 30 days reaching a feed rate of feed rate of 1 L / day, 2.4 kg TS / m3 / day, 1.68 kg VS / m3 / day, and HRT of 30 days with a slurry of 12 % TS, while the values for 20 %, 30 % and 40 % TS slurry is given in the table below. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. During the stabilization of the reactor, steady VS destruction rate and biogas production was measured. The biogas generation was measured and yields were calculated resulting in biogas of 0.4, 0.5, 0.45 and 0.4 m3 / kg VS destroyed at a corresponding TS of 12 %, 20 %, 30 % and 40 % respectively with CH4 content in the range of 62 - 65 % in biogas.

[0106] Example 2(b): Anaerobic digestion of rice straw in a DADUH for LCB Experiments were conducted in the conventional anaerobic slurry digester having capacity of

[0107] 70 L. The effective volume of the reactor was 50 L of which the digester was inoculated with anaerobic sludge (40 % of reactors effective volume i.e., 20 L) obtained from a digester treating market vegetable waste. The digester was operated in continuous mode (equal amount of feed and digested slurry are fed and withdraw to and from the reactor respectively) and fed with lignocellulosic biomass (rice straw) at (12 %, 20 %, 30 % and 40 % TS concentration) initially at a feed rate of 0.35 L / day at an HRT of 90 days which was gradually reduced to 30 days reaching a feed rate of feed rate of 1 L / day, 2.4 kg TS / m3 / day, 1.68 kg VS / m3 / day, and HRT of 30 days with a slurry of 12 % TS, while the values for 20 %, 30 % and 40 % TS slurry is given in the table below. At each loading rate, during the stabilization, steady VS destruction rate and gas production was ensured. During the stabilization of the reactor, steady VS destruction rate and biogas production was measured. The biogas generation was measured and yields were calculated resulting in biogas of 0.43, 0.6, 0.6 and 0.45 m3 / kg VS destroyed at a corresponding TS of 12 %, 20 %, 30 % and 40 % respectively with CH4 content in the range of 62 - 65 % in biogas. ADVANTAGES OF THE INVENTION

[0108] • The design of anaerobic digester is with unique hydrodynamics for superior biogas generation suitable for lignocellulosic biomass treatment with total solids concentration in the range of 20 - 40 % in the slurry.

[0109] • Delinking of the residence time of the feed hydrolysis duct from the residence time of the main reactor for efficient digestion of the low-density material without any phase separation.

[0110] • Middle perforated truncated cones fixed to the wall of the cylindrical reactor and the top and bottom truncated cones are meant to create turbulence in the reactor disturbing the flow behaviour of the slurry, ensure proper mixing of the material inside the digester without any mechanical equipment.

[0111] • The arrangement of truncated cones create unique hydrodynamics inside the reactor even with materials such as lignocellulosic biomass by preventing phase separation and biomass washout.

[0112] • The perforated truncated cone above the bottom truncated cone ensures the movement of slurry towards upward flow regime through the perforations (20 mm) while the solid material is retained in the bottom part of the reactor.

[0113] • The bottom truncated cone elongated through the central duct is provided to ensure the retainment of solids and divert the flow from bottom to towards the middle perforated truncated cone.

[0114] • The top truncated cone is provided for the easy movement of the thinner slurry towards the perforated plate.

[0115] • The conical bottom of the reactor is a place where maximum solids are retained and the mixing of solids in the cone is ensured through pressurized biogas purging.

[0116] • The perforated plate ensures further retainment of solids and allows the liquid to flow towards up which is then withdrawn as the final digestate.

[0117] • Scaling up of digester to handle a wide range of waste quantities from smaller capacity to larger capacity.

[0118] • Intermittent mixing of digester slurry by using the autogenerated biogas pressure and slurry recirculation from bottom to top.

[0119] • De-alienation of scum formation issue, elimination of choking and clogging, short- circuit of feed in the digester which are common issues in conventional digesters.

Claims

We claim:

1. A dry anaerobic digester for lignocellulosic biomass with unique hydrodynamics comprising a reactor (1) for anaerobic digestion of a lignocellulosic biomass slurry, wherein the said reactor comprises of:(i) a feeding line with inverted cone fixed in the centre of the perforated plate (2 and 3);(ii) a central duct (4) known as feed hydrolysis duct;(iii) a closed truncated cone (5) fixed to the central duct;(iv) an open truncated perforated cone fixed to the wall of the cylinder (6);(v) a closed truncated cone (7) fixed to the central duct;(vi) a sample point (8);(vii) an elongated conical bottom (9);(viii) water sprinklers (10);(ix) a drain valve at the bottom (11);(x) means for pre and post processing mechanisms / elements such as the feed preparation tank (FPT); digestate collection tank (DCT);(xi) the biogas collection lines connected to the biogas balloon;(xii) digestate recirculation ports at the bottom of the reactor and on top of the elongated cone;(xiii) a compressor, feeding pumps (slurry pumping and recirculation); and a biogas purging line.

2. The dry anaerobic digester as claimed in claim 1, wherein the lignocellulosic biomass (rice husk / rice straw / wheat straw etc) slurry consisting of 20 to 40 % solids is pumped into the reactor from top through the central duct (4) that flows towards bottom of the reactor (1) in a distributed manner.

3. The dry anaerobic digester as claimed in claim 1, wherein the slurry inlet mechanism (2) is provided with valve mechanism for feeding the slurry to the tank under atmospheric conditions and the gas inlet is provided with a gas distribution system.

4. The dry anaerobic digester as claimed in claim 1, wherein the working of the reactor starts with the preparation of feed slurry of required solids consistency in the FPT, followed by its pumping to the main reactor through the feeding line with an inverted cone that is mounted to the feed hydrolysis duct (4), wherein the feed slurry is initiallyallowed to flow through the central duct towards the bottom of the reactor which then expands creating an upward flow regime from the bottom due to the presence of a truncated cone.

5. The dry anaerobic digester as claimed in claim 1, wherein the perforated truncated cone above the bottom truncated cone ensures the movement of slurry towards upward flow regime through the perforations, while the solid material is retained in the bottom part of the reactor, wherein the bottom truncated cone (7) elongated through the central duct (4) is provided to ensure the retainment of solids and divert the flow from bottom towards the middle perforated truncated cone, wherein the perforated cone (6) consists of perforations with a diameter of 20 mm to ensure the flow of high solids material towards up.

6. The dry anaerobic digester as claimed in claim 1, wherein the top truncated cone (5) is provided for the easy movement of the thinner slurry towards the perforated plate and the perforated plate (3) with 10 mm perforations ensures further retainment of solids and allows the liquid to flow towards up which is then withdrawn as the final digestate.

7. The dry anaerobic digester as claimed in claim 1 , wherein the arrangement of truncated cones (5, 6, and 7) creates unique hydrodynamics inside the reactor with materials such as lignocellulosic biomass.

8. The dry anaerobic digester as claimed in claim 1, wherein the water sprinklers (10) mounted on the wall of the vertical cylinder ensure the periodic sprinkling of water to arrest any foaming that forms on the top of the digested slurry.

9. The dry anaerobic digester as claimed in claim 1, wherein the digestated liquid accumulated above the perforated plate is collected in the digestate collection tank which is partially utilized as a buffering agent and a diluent in the feed slurry, wherein samples from the sample points (8) can be collected for analysis periodically to check the health of the reactor.

10. The dry anaerobic digester as claimed in claim 1, wherein the elongated conical bottom (9) of the reactor is a place where the maximum solids are retained and the mixing of solids in the cone is ensured through biogas purging, wherein the solids can be drained out using the drain valve (11) as per the need.

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