GB2454226A

GB2454226A - Biomethane purifying process and cell wall disruption process

Info

Publication number: GB2454226A
Application number: GB0721467A
Authority: GB
Inventors: Christopher Maltin
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-11-01
Filing date: 2007-11-01
Publication date: 2009-05-06
Also published as: GB0721467D0; WO2009056856A2; WO2009056856A3

Abstract

There is provided a method for producing biomethane comprising the steps of: providing a source of biogas, compressing the biogas using a water piston (20a-d) to cause contaminants to dissolve in the water; and extracting purified compressed biomethane for use as a transport fuel. Also provided is a method a treating biomass comprising the steps of: providing biomass; pre-treating the biomass by exposing it to a fluid under pressure in a cell wall disruption unit (50); and reducing the pressure of the biomass/fluid using an expansion vessel. Apparatus for conducting the methods is also provided.

Description

BIOGAS PURIFYING PROCESS & CELL WALL DISRUPTION

PROCESS

The present invention relates generally to the processing of various materials including biogas and biomass and particularly to apparatus and methods for processing such materials which can optionally be combined.

Biogas typically refers to a (biofliel) gas produced by the anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or biodegradable feedstock under anaerobic conditions.

It usually consists of between 60-80% methane and 20-40% carbon dioxide, with other trace gases such as hydrogen suiphide, ammonia and hydrogen. If biogas is generated from landfill sites it may also contain further contaminants such as siloxanes.

Biogas can be used unmodified as a fuel, for example for stoves and lamps, to run small machines and to generate electricity. However, biogas is not currently suitable for use as a transport fuel. The carbon dioxide and other contaminants must be removed so that the majority of the gas remaining is methane. The methane is typically then compressed (usually to around 250 bar) into storage cylinders using a multi-stage gas compressor and then is transferred to storage cylinders (usually at around 200 bar) on a vehicle where it can be used as a fuel.

A first aspect of the present invention seeks to provide an improved process for "upgrading" raw biogas to provide a usable transport fuel.

According to a first aspect of the present invention, therefore, there is provided a method for producing biomethane comprising the steps of: providing a source of biogas; compressing the biogas using a water piston to cause contaminants to dissolve in the water; and extracting purified, compressed biomethane.

The use of water pistons to compress biogas combines the purification of methane with its compression ready for storage and use. This process therefore provides a significant cost reduction and increase in efficiency compared to separate purification and compressing steps.

Compressing the biogas with water serves to remove contaminants because the surface of the water exposed to and compressing the biogas acts as a molecular sieve. The methane molecules are very much larger of those of the other components and so dissolve less easily into the water (although it is likely that a small quantity will also dissolve in the water).

Typically the fluid used to compress and filter the biogas will be water, although other fluids, including mixtures of water with other fluids, could conceivably be used.

The products of the biogas upgrading process are purified, compressed biomethane and pressurised water containing carbon dioxide and other contaminants.

The biomethane released from the process may then be passed through a gas dryer prior to delivery to a storage cylinder ready for use.

The contaminated water could simply be discharged. However, it has been appreciated by the applicant that this "waste product" could have further uses.

The contaminated water still contains useful components, including carbon dioxide. The water may be further processed to removed useful components prior to discharge and/or recycling back into the purification cycle.

Although the contaminated water could simply be discharged or further purified, it has been appreciated by the applicant that because it is under very high pressure it could have further uses in the processing of organic matter. Furthermore, in some cases the contents of the water may make it impossible to discharge safely without further processing. The method and apparatus of the first aspect may therefore be used in combination with the second aspect defined herein.

When organic matter is digested by bacteria different components are processed at different rates. In particular, the bacteria are not able to break down the lignocellulose and fibrous parts of the organic matter in the same time-scale which they break down higher fractions. The result of this is that when bacteria are used in industrial digesters, where the organic matter is only processed for a certain period of time, a majority of the cells including cellulose protected by lignin are not processed. These cells remain in the digestate after the processing and are usually spread onto land where they improve the soil by replacing organic matter removed during crop growth. However, because the cells are not broken down by the bacteria they are not converted into biogas.

According to a second aspect of the present invention there is provided a method of treating biomass comprising the steps of: providing biomass; exposing the biomass to a fluid under pressure; and reducing the pressure of the biomass/fluid.

By subjecting the biomass to a fluid under pressure and then releasing the biomass/fluid down to a lower pressure the mechanical destruction of cell walls results. In particular, the lignin material which protects the cellulose cell walls is ruptured which dramatically increases the proportion of the biomass available for conversion by the bacteria within existing industrial digestion processes.

In other words an explosive release takes place which breaks up fibres and ligno cellulose contained in the biomass solution, making these parts of the biomass rapidly available to bacteria for digestion. Typically methanogenic bacteria can be used in such digestion processes to provide biogas.

This process therefore increases biogas yield, speeds up biogas production and reduces biomass particle sizes for increased efficiency in bacterial digestion.

Once the biomass has been treated it may be fed into a conventional digestion process. The treatment method may in fact be thought of as a pre-treatment step for the digestion process.

It can be seen that the treatment system of the second aspect of the present invention could be provided as a stand-alone system in which pressurised fluid is provided to effect the cell wall disruption process. However, it will also be seen that the source of the pressurised fluid could be the bi-product of the biogas upgrading process of the first aspect of the present invention. Accordingly, pressurised contaminated water produced by the method and apparatus of the first aspect of the present invention could be fed into the method and apparatus of the second aspect of the present invention to effect the cell wall disruption process.

If contaminated water from the first aspect of the present invention is used in the cell wall disruption process, when the process fluid is reduced in pressure the carbon dioxide will come out of solution and can be tapped off for use in its own right.

Biogas produced in the subsequent digestion process may then be fed into the biogas upgrading system of the first aspect of the present invention.

The present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

S

Figure 1 is a schematic representation of a method and apparatus according to the first aspect of the present invention; Figure 2 is a schematic representation of a method and apparatus according to the second aspect of the present invention; Figure 3 is a schematic representation of a method and apparatus formed in accordance with a combination of the first and second aspects of the present invention; and Figure 4 is a schematic representation of the combined method and apparatus shown in Figure 3 together with a bacterial digestion system.

Referring first to Figure 1 there is shown a biomethane extraction process and apparatus generally indicated 10.

Raw biogas is introduced to the system through inlet pipe 15. The biogas is fed into a network of water pistons. In this embodiment four water pistons 20a, 20b, 20c, 20d are provided in series. The pistons are supplied with water from a water inlet 25 which is pressurised by a water pump 30.

The water pistons compress the biogas so as to separate biomethane from the other components of the biogas. Purified biomethane is removed from the water pistons and vented into an outlet pipe 35 which feeds the biomethane into a gas dryer station 40. The now purified and dried compressed biomethane is then fed through an outlet pipe 45 to a compressed gas storage facility ready for use as a vehicle bio fuel.

The contaminated water from the pistons can be released through outlet pipe 47 for further processing or discharge.

Referring now to Figure 2 there is shown a biomass treatment system generally indicated 50.

A pressure vessel 55 is provided and includes a fluid inlet 60 and an organic matter feed inlet 65.

The pressure vessel 55 further comprises an outlet leading to a connecting pipe 70 which interconnects the pressure vessel 55 with an expansion vessel 75. The expansion vessel comprises an inlet for receiving flow from the pressure vessel 55 and an outlet 80 for releasing its contents, which in this embodiment are fed to a bacterial digestion system.

In use, organic matter is introduced into the pressure vessel 55 through the inlet 65.

Simultaneous with and/or after introduction of a batch of biomass fluid under pressure is introduced into the vessel 55 through the inlet 60. Thereafter, the mixture of fluid and biomass which is under pressure is released through the pipe into the expansion vessel which significantly reduces the pressure of the process fluid. The now reduced pressure fluid is then onwardly fed from the expansion vessel 75 through the outlet 80 to a bacterial digestion system (not shown).

S

Referring now to Figure 3 there is shown a combined biomethane production facility and biomass pre-treatment facility generally indicated 85.

The facility 85 comprises the system 10 shown in Figure 1 combined with the system 50 shown in Figure 2.

More specifically, the pressurised waste water resulting from the system 10 is fed through the outlet pipe 47 directly into the pressure vessel 55 for use in the cell wall disruption process. An intermediate holding tank (not shown) for receiving waste water and storing it under pressure until it is required by the system 50 may also be provided.

Referring now to Figure 4 there is shown a biogas/biomass processing facility generally indicated 90.

The facility 90 comprises a facility of the type shown in Figure 3. Additionally, the facility 90 comprises a bacterial digestion system generally indicated 95. The expansion vessel outlet 80 feeds pre-treated biomass into the digestion system 95.

The digestion system 95 comprises a digestion vessel 100 which is elongate and has a uniform section defined by a peripheral wall 105. The wall 105 has two lower lobed portions interconnected about a vertical median plane of symmetry of the container. The container 100 also has two flat end walls 110, 115 with the same peripheral shape as the section of the peripheral wall 105 and sealed to the ends to close the container 100.

The opposite end walls 110, 115 each have closable apertures 111, 116 respectively for the introduction of process fluid into the container and its removal therefrom in a potentially continuous process.

The container 100 further includes a plurality of gas inlet apertures 120 which are formed in a spacer bar 125 that extends longitudinally of the container 100. The spacer bar 125 is positioned in the central median plane of the container, between the two edge portions of the peripheral wall 105 which are bent inwardly of the container and sealed to the bar 16.

The container 100 also has a plurality of gas outlets 130 in the uppermost part of the peripheral wall which open into a common manifold 135 fixed to the top of the container.

In use, process fluid is received from the expansion vessel through the inlet aperture 116 whereupon it is subjected to a digestion process which is well known to those skilled in the art and will not be described in more detail herein.

As part of the digestion process biogas may be released from within the container 100. It is possible for biogas to be removed from the container and fed either directly or indirectly into the system 85 described in relation to Figure 3.

As will be appreciated, each of the components of the complex system described in relation to Figure 4 could be used together or separately.

Claims

1. A method for producing biomethane comprising the steps of: providing a source of biogas; compressing the biogas using a water piston to cause contaminants to dissolve in the water; and extracting purified compressed biomethane.

2. Apparatus for producing biomethane comprising -a water piston arrangement for receiving raw biogas and compressing it to cause contaminants to dissolve in the water, and -means for separating purified, compressed biomethane from contaminated water.

3. A method of treating biomass comprising the steps of: providing biomass; pre-treating the biomass by exposing to a fluid under pressure; and reducing the pressure of the biomass/fluid.

4. A method of treating biomass as claimed in Claim 3, further comprising the step of: subjecting the pre-treated biomass to bacterial digestion.

5. Apparatus for the treatment of biomass comprising: -a pressure vessel for receiving biomass; -means for introducing a fluid under pressure into the pressure vessel; -an expansion vessel for receiving the contents of the pressure vessel and reducing the pressure thereof.

6. Apparatus for the treatment of biomass as claimed in Claim 5, further comprising a digestion vessel for receiving the contents of the expansion vessel and conducting bacterial digestion.

7. A method for producing biomethane substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

8. Apparatus for producing biomethane substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

9. A method of treating biomass substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

10. Apparatus for the treatment of biomass substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.