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FIELD OF THE INVENTION

The present invention relates to a biogas upgradation system and more particularly it relates to a biogas upgradation system with reduced methane slippage, which can extract maximum amount of methane from the raw biogas and offer it as fuel grade methane.

BACKGROUND OF THE INVENTION

The rising price of fossil fuels and the need to fight against global warming promotes the development of alternative energy production techniques that are environmentally friendly. This has led to an observed rise in the use of organic waste, from agriculture, food industries, landfills, waste water treatment plants, etc. which can be used as an input source for the generation of biogas. The biogas with the help of advanced technology can then be upgraded into biomethane also known as renewable natural gas, having similar properties to natural gas.

Biogas refers to a mixture of different gases produced by the breakdown of organic matter through anaerobic digestion or fermentation of waste waters and solid residues. The material digested can include any type of organic matter such as manure, sewage sludge, agricultural waste, municipal waste, plant material, green waste or food waste.

To compresses, transport or make use of biogas as a replacement for CNG or LPG, it must be "enriched" or "upgraded" to remove its impurities. The typical steps for upgrading biogas include cleaning, drying & refining the biogas. The biogas needs to be dried and devoid of hydrogen sulphide. Furthermore, removal of most of the carbon dioxide raises the methane content sufficiently.

The biogas upgradation depends on the type of feed material used, climatic condition, production site and type of the technology adapted for the production. The main constituents of biogas are methane (CFh) and carbon dioxide (CO2) along with some traces of gases such as water vapor, hydrogen sulfide (FhS), nitrogen, hydrogen and oxygen. Carbon dioxide and traces of gases must be removed because the presence of hydrogen and water vapor may cause corrosion on metal surfaces, in addition to reducing the heating value of the upgraded (product) gas.

While most countries have their own defined parameters for biomethane, typically the upgraded biogas has to have a methane content of more than 90%. There are various techniques for biogas upgradation such as water scrubbing or pressurized water scrubbing, membrane permeation, amine gas treating or amine absorption, cryogenic upgradation and pressure swing adsorption.

One such technique is "Water scrubbing" or "Pressurized water scrubbing" which involves passing the biogas stream through an absorber or scrubber packed with media to provide a high contact surface area at a gas/water interface. The biogas is brought into the contact with water at elevated pressure and low pH of absorbent water; components of the biogas are absorbed into the water until saturation equilibrium is reached. The gas which is not absorbed is discharged from the scrubber. This discharged gas is rich in methane but also contains residual traces of hydrogen sulphide and carbon dioxide. Further, this discharged gas is saturated with water.

Various prior arts have been disclosed describing the biogas upgradation system by water scrubbing method. The prior art document US 20100107872A1 discloses a water scrubbing method for biogas upgradation. Described therein is biogas purification/upgradation of biogas generated from anaerobic digestion of waste material. The limitations of this method are clogging of packing due to the growth of bacteria and formation of elemental sulphur, especially when incoming biogas has a high level of hydrogen sulphide in it.

Another technique is "Membrane permeation" which works on the principle of pressure difference between the gases. The membranes which are available for the separation can be grouped into two main types i.e. high and low pressure membranes. The low pressure membrane system works at pressure close to the atmospheric pressure whereas the high pressure membrane system works at the pressures of up to 20 Bar g. The carbon molecular sieve (CMS) hollow fiber membranes are used under high pressure for carbon dioxide removal from the raw biogas. In multistage membrane separation, the waste gases from the first stage are recycled within the process to enrich the methane content of the final gas output.

US Patent No. 6,565,626 B1 discloses membrane separation process with three membrane separation stages, wherein a retentate stream and a permeate stream from stage 2 and 3 respectively are recycled to the crude gas stream. Limitations of the above said method are high initial investment due to membrane cost, sensitivity of the membranes to trace impurities leading to fouling of membranes and thus, high replacement costs. Another technique "Pressure swing adsorption" (PSA) is a process by which multiple gas species from the gases mixture can be separated. The adsorbents such as zeolite, activated carbon and molecular sieves play a crucial role in adsorption. The separation method relies on high pressure, the higher the pressure, the more gas is adsorbed. The raw biogas is compressed to a high pressure using compressors and then fed into an adsorption column which retains only carbon dioxide but not methane. When column material gets completely saturated with carbon dioxide, the pressure is released and carbon dioxide gets desorbed. To operate the PSA process continuously, several columns are required which operate sequentially.

US Patent No. 7,025,803 B2 discloses a methane recovery process by PSA method. It utilizes a plurality of pressure swing adsorption units to remove some of the volatile organic compounds from the feed mixture. Various disadvantages related of the aforesaid method are poisoning of porous media, disposal of the media after poisoning or at the end of the useful life, replacement of the media and methane loss to atmosphere. Further, from this conventional PSA technique, methane loss to the atmosphere is high, which is undesired & harmful as methane is a greenhouse with a higher global warming potential compared to carbon dioxide. Apart from the aforesaid drawbacks, the methane recovery from a typical PSA based upgradation system is less than 90%. During the adsorption process, the molecular sieve unintentionally adsorbs some of the methane along with the carbon dioxide which is an undesired outcome. If methane escapes along with the carbon dioxide in the exhaust, it is a significant financial loss to the producer. Also, methane is a greenhouse gas and allowing it to escape into atmosphere causes severe environmental impacts in the long run.

Hence, it is a desperate need to develop a system and process which reduces the methane slippage as compared to existing processes and offers the highest possible methane capture rate. The present invention mainly focuses on offering methane recovery greater than 99%. The process is exhaustively described in the detailed description.

OBTECT OF THE INVENTION The main object of the present invention is overcoming the drawbacks of the existing PSA method by providing an improved biogas upgradation system and method with reduced methane slippage.

Another object of the present invention is to provide a biogas upgradation system which enables capture of methane contained in the raw biogas stream with the least methane slippage compared to the existing PSA methods at the same throughput, and hence reduced environmental pollution by this strong greenhouse gas.

Yet another object of the present invention is to provide a biogas upgradation system which mainly focuses on methane recovery by directing the exhaust gas stream from the dryer and active MPSA tower into another similar cycle for extracting maximum amount of methane (more than 99%) from input gas and providing a useful choice to the industry.

Still another object of the present invention is to provide a biogas upgradation system having buffer balloon, reduced energy consumption, high production efficiency and low equipment cost.

Further object of the present invention is to provide a biogas upgradation system which reduces methane slippage to less than 1% and thus, makes the process more financially viable for the producer.

SUMMARY OF THE INVENTION

The present invention relates to a biogas upgradation system with reduced methane slippage by extracting maximum amount of methane and offering it in the product gas. The aforesaid system mainly comprises a pre-treatment unit (1), a drying unit (2), a methane enrichment unit (3), and a methane recovery unit (4). The pre-treatment unit (1) removes the hydrogen sulphide from the raw biogas feed and sends it to the drying unit (2) for moisture removal. Then, the dried biogas is directed to the methane enrichment unit (3) for extracting the methane by adsorbing the carbon dioxide into the molecular sieve. An exhaust stream from the drying unit (2) and the methane enrichment unit (3) is directed to the methane recovery unit (4) for extracting the maximum amount of methane. By re-circulating the exhaust gas into the methane recovery unit (4), the present invention significantly reduces methane loss to less than 1%. BRIEF DESCRIPTION OF THE DRAWING

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the present embodiment when taken in conjunction with the accompanying drawings. Figure 1 depicts a schematic view of a biogas upgradation plant with reduced methane slippage according to the present invention.

DETAILED DESCRIPTION

Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the construction and arrangement of parts illustrated in the accompanied drawing(s). The invention is capable of other embodiments, as depicted in different figure(s) as described above and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

The present invention discloses a biogas upgradation system with reduced methane slippage by extracting maximum amount of methane, from the biogas feed and offering it as fuel grade methane. The upgradation system according to present invention significantly reduces the methane loss to less than 1%.

Further, it is to be also understood that the term "ACF" refers to activated carbon filter, "MS" refers to moisture separator and "MPSA" refers to medium/minor pressure swing adsorption. Further the term "VSP" refers to vacuum suction port and "VDP" refers to vacuum discharge port.

Now referring to Fig. 1, an exemplary embodiment of the present invention, the biogas upgradation system (A) mainly comprises a pre treatment unit (1), a drying unit (2), a methane enrichment unit (3), and a methane recovery unit (4).

It is to be understood that the biogas feed stream refers to the mixture of methane, carbon dioxide, water, hydrogen sulphide and other trace elements.

Further, referring to Fig. 1, said pre-treatment unit (1) comprises a biogas inlet (11) for receiving the raw biogas, a blower (12), an activated carbon filter (ACF) (13), and a pre-cooler (14). Said raw biogas is sucked by the blower (12) and discharged ahead into the ACF (13). In this unit, the ACF (13) removes hydrogen sulphide content from the incoming biogas and reduces it to less than 10 ppm. Initially, the raw biogas contains 100-1000ppm hydrogen sulphide which is beyond the standards prescribed by the bureau of Indian Standards. Hydrogen sulphide and water vapor are corrosive in nature and lead to corrosion when they come in contact with metal surfaces and also reduce the heating value of the product/upgraded gas. Hence, in order to reduce hydrogen sulphide content, the raw biogas is passed through the ACF (13). After removing hydrogen sulphide content through the ACF (13), the treated biogas enters into the pre-cooler (14). The pre-cooler (14) is basically a heat exchanger, which reduces the temperature of the gas coming from the ACF (13). The cooled biogas is directed to a Moisture separator for removing majority of the moisture after which it is introduced in the drying unit (2) for further drying.

Due to cooling of the biogas, it carries water vapor. Hence, said water vapor needs to be removed from the pre-treated biogas to increase the efficiency or energy input; prevent corrosion of engine, pipe lines and other components; partial removal of hydrogen sulphide, ammonia, siloxane and other water soluble gases, and reduce the contamination. In order to attain said objectives, the cooled biogas is fed into the drying unit (2)· The drying unit (2) comprises of a moisture separator (21), and a pair of drying towers (22, 23). Said MS (21) removes excess water vapor from the biogas coming from the pre-cooler (14). However, after passing from the MS (21), said biogas still carries some moisture. Hence, in order to remove remaining moisture content from the treated biogas, it is further fed into the desiccant based drying towers (22, 23). Said desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its vicinity. Said pair of the drying tower (22, 23) is operated sequentially according to the pre-defined cycle time (5-10 minutes). So, at one moment of time only one dryer (either 22 or 23) is active. At the same time, the other drying tower (either 22 or 23) is regenerating under the vacuum. So, after removing the water vapor from the pretreated biogas into the moisture separator (21), the treated biogas enters into the active drying tower (22, 23) which dries the gas and reduces its due point to less than -60°C. It is to be understood that the dew point is the temperature to which air must be cooled to become saturated with water vapor. When said treated biogas is cooled further, the airborne water vapor will condense to form liquid water (dew). The dried biogas from the said active dryer (22, 23) is then fed into the methane enrichment unit (3).

As described in the Fig. 1, the methane enrichment unit (3) comprises of a pair of medium/minor pressure swing adsorption (MPSA) towers (31, 32) and a biogas surge tank (33). Said dried biogas is fed into the active MPSA tower (31, 32) from the active dryer (either 22 or 23) for the removal of carbon dioxide using various grades of zeolite based molecular sieves. Similar to the dryers (22, 23), the pair of MPSA tower (31, 32) also works sequentially based on the saturation of the molecular sieve by adsorbing carbon dioxide. The dried biogas is pressurized up to 0.7 bar g (1.7 bar absolute) in the MPSA tower (31, 32). Due to pressurization, the molecules are usually loosely adsorbed in the cavities of the molecular sieve but not irreversibly bound. The molecular sieve specifically separates a number of different gaseous compounds from the dried biogas. Said molecular sieve possess specific affinity for the carbon dioxide. Before the molecular sieve of the MPSA tower (31, 32) is completely saturated with the carbon dioxide, the stream of dried biogas is directed into another MPSA tower (31, 32) that has been regenerated and switched into adsorption mode to achieve continuous operation. Regeneration of the saturated molecular sieve is performed by depressurization using a vacuum pump. The selectivity of adsorption is achieved by the different mesh sizes and/or application of pressure. When the pressure is released, the compounds extracted from the biogas are desorbed. The process is therefore often called "pressure swing adsorption" (PSA). Said PSA methods takes place over four phases namely pressure build-up, adsorption, depressurization and regeneration as described above.

Thus, after the application of 0.7 bar g (1.7 bar absolute) pressure, only carbon dioxide gets adsorbed in the molecular sieve which ultimately leaves the product gas which is rich in methane. Then, said product gas is allowed to flow into the biogas surge tank (33). The gas in the surge tank is tested for the desired quality, after which it is pressurized based on its end application like PNG or CNG.

After carrying out this method for capturing methane from the raw biogas feed as discussed above, the exhaust gas in the regenerating dryers (22, 23) and the regenerating MPSA tower (31, 32) still contain traces of methane which is lost when the gas is exhausted without being further processed. Hence, in order to prevent such methane slippage, the exhaust gas from the regenerating dryer (22, 23) and the regenerating MPSA tower (31, 32) is directed to the methane recovery unit (4).

The methane recovery unit (4) comprises of a vacuum suction port (VSP) (41), a plurality of vacuum pumps (42, 43), a vacuum discharge port (VDP) (44), a biogas buffer balloon (45), a blower (46), a pre-cooler (47), a moisture separator (MS) (48), a pair of MPSA towers (49, 50), a surge tank (51) and an exhaust (52). The said unit is a miniature version of the methane enrichment unit (3) as described above. The exhaust gas from the regenerating dryer (22, 23) and the regenerating MPSA tower (31, 32) is directed to the VSP (41), with the help of vacuum pump (42). Said vacuum pump (42) further conveys the exhaust gas to the VDP (44). The VDP (44) serves a similar function to the MS (21, 48). Then after the exhaust gas is introduced into the biogas buffer balloon (45) through the VDP (44). Said buffer balloon (45) stores the exhaust gas collected from the dryer (22, 23) and the regenerating MPSA tower (31, 32). The blower (46) sucks and pressurizes the exhaust gas being stored into the biogas buffer balloon (45) at 0.7 bar g (1.7 bar absolute) and sends it to the pre-cooler (47) for reducing the temperature of the exhaust gas. Said cooled exhaust gas is then fed into MS (48) from the pre-cooler (47) for residual moisture removal. After that the dried exhaust gas is introduced into the active

MPSA tower (49, 50) for removal of residual carbon dioxide in the manner as described above in detail. Once the said dried exhaust gas enters into the active MPSA tower (49, 50), the remaining methane is extracted from it and then directed into the surge tank (51) while the carbon dioxide adsorbed by the molecular sieve is allowed to vent out into the atmosphere or collected if required through the VDP (53) from said exhaust (52) via vacuum pump (43). The extracted methane gas from the surge tank (51) is allowed to flow back into the pre-treatment unit (1), drying unit (2) and methane enrichment unit (3) for recycling as discussed above. The methane rich stream is then collected from the biogas surge tank (33) and pressurized depending on the end application.

The present invention is experimented and illustrated more in details in the following example. It describes and demonstrates embodiments within the scope of the present invention. It is given solely for the purpose of illustration and is not to be construed as limitations of the present invention, as many variations thereof are possible without departing from spirit and scope.

Example 1:

Various parameters regarding input and output gas for the pre-treatment unit (1), the drying unit (2) and the methane enrichment unit (3) are as mentioned below:

Various parameters regarding input and output gas for the methane recovery unit (4) are as mentioned below:

Composition of the exhaust stream from the methane recovery unit (4) is as mentioned below:

From the above described invention, it is observed that when the exhaust gas from the regenerating dryer (22, 23) and the regenerating MPSA tower (31, 32) is directed to the buffer balloon (45) for another cycle, passing through vacuum discharge port (44) with the help of vacuum pump (42), more than 99% of the methane was recovered. Further, methane loss to the atmosphere was reduced to less than 1%. Hence, negative environmental effects of methane as a global warming agent can be prevented.

The invention has been explained in relation to specific embodiment. It is inferred that the foregoing description is only illustrative of the present invention and it is not intended that the invention be limited or restrictive thereto. Many other specific embodiments of the present invention will be apparent to one skilled in the art from the foregoing disclosure. All substitution, alterations and modification of the present invention which come within the scope of the following claims are to which the present invention is readily susceptible without departing from the spirit of the invention. The scope of the invention should therefore be determined not with reference to the above description but should be determined with reference to appended claims along with full scope of equivalents to which such claims are entitled.

List of reference numerals:

Pre-treatment unit (1)

Drying unit (2)

Methane enrichment unit (3)

Methane recovery unit (4)

Biogas inlet (11)

Blower (12, 46)

Activated carbon filter (ACF) (13)

Pre-cooler (14, 47)

Moisture separator (MS) (21, 48)

Pair of drying tower (22, 23)

Pairs of medium pressure swing adsorption (MPSA) towers (31, 32 and 49, 50)

Biogas surge tank (33, 51)

Vacuum suction port (VSP) (41)

Vacuum pump (42, 43)

Vacuum discharge port (VDP) (44, 53)

Biogas buffer balloon (45)

Exhaust (52)