GB2470452A

GB2470452A - Electrical Energy Generating System

Info

Publication number: GB2470452A
Application number: GB1004605A
Authority: GB
Inventors: Alan Rogers; John Rutt
Original assignee: CLEAN ENERGY Ltd; INTARK Ltd
Current assignee: CLEAN ENERGY Ltd; INTARK Ltd
Priority date: 2009-03-19
Filing date: 2010-03-19
Publication date: 2010-11-24
Anticipated expiration: 2030-03-19
Also published as: GB201004605D0; GB2470452B; GB0904661D0

Abstract

An electrical energy generating system comprises an electricity generator 4, a biowaste processor 2 and a farm housing 6, wherein the biowaste processor breaks down a biowaste product to a methane-rich biogas and is in fluid communication with both the generator and the farm housing, the system further including gas transport elements which are capable of transporting the biogas to the generator 32 and carbon dioxide 72 to the farm housing; and wherein the system further includes a biogas treatment station 12 located between the biowaste processor and the generator, the treatment station including a scrubber including water adapted to remove CO2from the methane-rich biogas. A method of production of biogas with the apparatus is also disclosed.

Description

Electrical Energy Generating System The present invention relates to an electrical energy generating system and in particular a system which minimises so-called "greenhouse gases" being released into the atmosphere.

Current fossil fuel power stations generate electrical energy through the burning of fossil fuels, such as oil and gas. The combustion process generates as a by-product significant amounts of greenhouse gases, such as carbon dioxide, and other atmospheric pollutants, which are released into the atmosphere.

Thus, conventional power stations have a double negative impact on the environment: they consume valuable natural resources and they release pollutants into the atmosphere.

The present invention seeks to address the problems of commercial power stations by using a renewable fuel source to power the generator and to minimise pollutants that are released into the atmosphere.

The present invention provides an electrical energy generating system including an electricity generator, a biowaste processor and a farm housing, wherein the biowaste processor breaks down a biowaste product to a methane-rich biogas and is in fluid communication with both the generator and the farm housing, the system further including gas transport elements which are capable of transporting the biogas to the generator and carbon dioxide to the farm housing; and wherein the system further includes a biogas treatment station located between the biowaste processor and the generator, the treatment station including a scrubber including water adapted to remove carbon dioxide from the methane-rich biogas.

The biowaste processor generates a methane-rich biogas from a biowaste source and is suitably a biodigester or a pyrolysis apparatus for biowaste. In an embodiment of the invention, the biowaste processor is a biodigester.

Biodigesters are known in the art and convert appropriate biowaste to a methane-rich gas via microbial action. Thus the generator is effectively powered by biowaste which is renewable, and not by natural resources such as fossil fuels.

The pyrolysis apparatus pyrolyses a biowaste product to generate the methane-rich biogas.

In addition, the system utilises the carbon dioxide by-product of the biodigester and/or the generator to produce a carbon dioxide-rich environment in the farm housing. This minimises the amount of this greenhouse gas that is released into the atmosphere. In addition it enhances the growth of the crops, which thrive in a carbon dioxide-rich environment.

The system should operate in a way which is carbon neutral or in some cases, carbon negative.

Definitions: Biodigester: a reactor which is capable of generating a methane-rich gas from organic waste products via the action of micro-organisms, e.g. bacteria.

Biogas: the gas derived from the biodigester which is used to power the generator.

Biowaste: a carbon-rich organic waste, typically derived from animal or plant waste.

Digestate: the non-gaseous output from the biodigester.

The biowaste processor may include a plurality of individual units which may be linked in series or in parallel to provide sufficient biogas to power the generator.

In an embodiment of the invention, the biowaste processor is a biodigester.

In a further embodiment of the invention, the biodigester includes a pre-treatment station which is adapted to break down lignin in the biowaste prior to the biodigester being fuelled with the biowaste.

Lignin in the biowaste is difficult for the biodigester to degrade. As such, too much lignin may cause the biodigester to stall, which in turn stops or severely limits the output of the generator. In order for the biodigester to operate at optimum efficiency, it is therefore useful to break down the lignin content of the biowaste.

Lignin is indigestible by animal enzymes, but some fungi and bacteria are able to secrete ligninases which can bidegrade the polymer. Thus, the pre-treatment station may comprise fungi and/or bacteria which secrete one or more ligninases.

Lignin is suitably broken down by an aerobic process typically found in mushroom production. As such, the pre-treatment station may include a mushroom farm. In such an embodiment, the lignin-rich biowaste is used as part of a growing medium for the mushrooms where the ligninases secreted by the mushrooms break down the lignin content in the biowaste which is then used as a fuel for the biodigester after the mushrooms have been harvested.

The embodiment described immediately above provides an additional feature to the system in that the mushrooms generated as part of the process to break down lignin in the biowaste can then be sold.

The invention includes a biogas treatment station located between the generator and the biowaste processor, wherein the biogas treatment station suitably includes a gas input in fluid communication with the processor and a gas output in fluid communication with the generator and the treatment station outputs a desired ratio of gases for powering the generator from the gas mixture received from the processor.

Certain generators operate more efficiently with a fuel which contains more methane as a volume percentage than is generated by the biowaste processor. In such cases, it is desired to increase the volume percentage of methane in the biogas fuel for the generator. Such an increase in the amount of methane is achieved by the biogas treatment station.

The volume percentage of methane in the biogas fuel for the generator is increased by removing carbon dioxide from the gas output of the biowaste processor. This is achieved by washing the gas output from the processor with water, which is capable of dissolving carbon dioxide in preference to methane. Thus, the biogas treatment station may include a scrubber containing water.

The skilled person will appreciate that the water scrubber will also remove an amount of certain other non-methane gases from the biogas, such as hydrogen sulphide and sulphur dioxide, for

example.

The water may be under pressure and/or be within a controlled temperature range as desired to dissolve the carbon dioxide. Suitably, the gas output from the processor is bubbled through the water. The water within the scrubber may be a static pool or it may be a flow of water. Accordingly, the scrubber may include a water inlet and a water outlet such that the water flows through the scrubber from the inlet to the outlet.

The system may be used to provide environmentally-friendly power for a specific site, or it may be used to provide power to a local or national power grid, or any combination of these. Accordingly, the generator may be connected to a local or national power grid.

In addition to fuelling the generator, the system may provide surplus methane or methane-rich biogas which may be utilised as a fuel for, say, a boiler/heater, stored for later use or sold as a fuel source.

The farm housing is typically an enclosed structure which houses crops being grown. The structure is suitably enclosed as this minimises the amount of carbon dioxide which is allowed to enter the atmosphere. As crops tend to require light for growth, the structure may be formed at least partly from glass. For example, the farm housing may be a greenhouse.

The crops grown within the farm housing need not be limited to traditional plant crops such as flowers, fruits and vegetables, but may include algae, plankton, etc. Any surplus carbon dioxide from the system may be purified and/or stored for later use or sold as a by-product.

In a still further embodiment of the invention, the processor is a biodigester which includes a digestate conditioning station in which the spent biowaste is conditioned for use as a fertiliser. The digestate conditioning station typically includes a dryer which is adapted to dry the digestate. The dryer may include for example a source of heated air and a blower to direct the hot air flow over the digestate. The conditioning station may also include a grinder or pelletiser to powder or pelletise the dried digestate. A powdered or pelletised form of the dried digestate is a useful form for packaging, storage and transportation.

The system may include a catalytic converter downstream of the generator to minimise further any environmental impact of the system. In such an embodiment, the exhaust gases of the generator pass through the catalytic converter where the nitrogen oxides are broken down and carbon monoxide is oxidised to carbon dioxide, which may then be captured by a scrubbing process similar to that described above. The carbon dioxide removed from the generator exhaust may be combined with the carbon dioxide removed from the biogas and fed into the farm housing.

The generator typically produces heat as a by-product of burning the biogas and this heat is usually dissipated using a coolant system, suitably containing water as a coolant. However, this heat may be used to generate additional power by the use of a waste heat generator system. Such systems typically operate on the principals of the Rankine cycle and include a refrigerant which is heated by the waste heat from the generator to form a heated pressurised vapour, which is then used to drive an electric generator. The vapour is then cooled and condensed back to a liquid to re-start the cycle.

Thus, the generator may include a cooling system, such as a water coolant system, coupled to a waste heat generator which is operable to convert the heat to electrical energy.

To increase the efficiency of the cooling system for the generator, the cooling fluid may be further cooled between the waste heat generator and being returned to the biogas-powered generator. This may be achieved by using geothermal cooling, such as, for example, passing the coolant fluid through a heat exchanger located underground in an aquifer.

Additionally or alternatively, the heat produced by the biogas-powered generator may be used to dry the digestate. Furthermore, the waste heat may be used in a local heating system, for example to heat the farm housing.

According to a second aspect of the invention, there is provided a method of powering a generator, the method including charging a biowaste processor with biowaste, isolating from the processor a methane-rich biogas, removing a percentage of the carbon dioxide from the biogas via a scrubber containing water, fuelling the generator with the isolated biogas and supplying excess carbon dioxide to a farm housing.

As mentioned above, where the processor is a biodigester, the presence of lignin in the biowaste may have a detrimental effect on the efficiency of the biodigester. As such, the method may include a step in which the biowaste is pre-treated to reduce the lignin content therein to an acceptable level.

The pre-treatment step may include subjecting the lignin-rich biowaste to a source of bacteria and/or fungi which are capable of secreting one or more ligninases for a period of time sufficient to reduce the lignin content to a desired level.

In an embodiment of the invention, the lignin content is reduced by an aerobic process using fungi-produced ligninases, such as is found in mushroom farms. The mushrooms may be sold to increase operating revenues of the system.

In order to increase the volume percentage of methane in the biogas used to power the generator, the method includes a biogas conditioning step in which carbon dioxide is removed from the output gas of the biowaste processor. Thus, the conditioning step includes passing the biogas through a scrubber containing water. The removal of carbon dioxide is achieved by the use of water as a solvent, which preferentially dissolves gases such as carbon dioxide compared with methane. The water-based scrubber may also dissolve other non-methane gases, such as hydrogen sulfide and sulfur dioxide, which further increases the volume percentage of methane in the biogas.

Where the solvent is water, this is suitably used at a temperature between 2 and 15°C to wash the gas. The gas may be washed by bubbling it through the water.

Carbon dioxide has a greater solubility in water in the temperature range 2 to 15°C, suitably 2 to 6°C, more suitably about 4°C. Accordingly, water in this temperature range is able to dissolve a greater amount of carbon dioxide. Additionally or alternatively, the water may be pressurised to increase the carbon dioxide capable of being dissolved therein. The water pressure may be 1 to 200 kPa (0.01 to 2 bar).

The water may be static within the scrubber, i.e. the scrubber may contain a pool of water through which the biogas is bubbled, or a flow of water may pass through the scrubber or be cycled through the scrubber. Thus, the scrubber may include a water inlet and a water inlet and the water may flow through the scrubber from the inlet to the outlet.

The system may also include a carbon dioxide release step in which the water is warmed to release the dissolved carbon dioxide from the water. The non-dissolved carbon dioxide can then be collected and pumped into the farm housing or may be compressed for storage.

Alternatively, the carbon dioxide-rich water may be used directly on the crops being grown in the farm housing.

In addition to removing excess carbon dioxide from the biogas, the system may also include an exhaust gas conversion step, in which the exhaust gases from the generator are passed through a catalytic converter. The conversion step may further include a scrubbing step to remove carbon dioxide from the exhaust gases downstream of the catalytic converter.

For embodiments where the biowaste processor is a biodigester, the digestate from the biodigester may be used as a fertiliser. To provide the digestate in a more suitable form for use as a fertiliser, it may be conditioned. Thus, the method may include a step of conditioning the digestate. The conditioning step may include drying the digestate, for example by the use of a heated airflow.

Additionally or alternatively, it may include a step of grinding or pelletising the digestate.

The present invention will now be described, by way of example only, with reference to the accompanying drawing, which shows a schematic representation of an electrical energy generating system according to the invention.

Figure 1 shows a schematic representation of an electrical energy generating system according to the invention. The system includes a biodigester 2 which is a conventional "plug-type" biodigester which contains appropriate bacteria to generate a methane-rich biogas from a biowaste source 8.

The biogas source 8 is either fed directly into the biodigester 2 via paths 20 and 22a if it contains a relatively low lignin content, or it is fed into a mushroom farm 10 via paths 20 and 22b if the lignin content is relatively high. The mushrooms in the mushroom farm 10 secrete one or more ligninases which decomposes at least some of the lignin to a form which can be converted to biogas within the biodigester 2. The mushroom farm includes two outputs: a more acceptable biowaste which is fed into the biodigester as shown by step 24, and a crop of mushrooms which are harvested according to step 26.

The biogas produced within the biodigester 2 is carried by a conduit 30 to a biogas treatment station 12.

In addition to the bio gas, the biodigester 2 produces a digestate, which is the spent biowaste after it has been subjected to bacterial decomposition in the biodigester 2. The digestate is conveyed from the biodigester 2 to a digestate conditioning station 18 via step 40. The digestate is dried and pelletised within the conditioning station 18 and the digestate pellets are output via step 42, where they can be packaged and sold as fertiliser.

The biogas treatment station 12 scrubs the biogas with chilled water at 4°C which is fed into the biogas treatment station 12 under pressure from a water source 16 via a conduit 34. The scrubbing of the biogas with the chilled pressurised water removes a percentage of carbon dioxide from the biogas which effectively increases the percentage of methane in the biogas. The methane-rich biogas is carried from the biogas treatment station 12 to an electricity generator 4 via a conduit 32.

The water from the biogas treatment station 12 is carried to a degassing station 70 via a conduit 36 where the water is allowed to warm and is de-pressurised to release at least some of the carbon dioxide which was dissolved therein. The carbon dioxide is then carried to a greenhouse 6 via a conduit 72 and the water is carried to the greenhouse 6 via a conduit 74.

The greenhouse maintains an atmosphere which is rich in carbon dioxide and moisture and outputs crops which thrive in such environments via step 62.

The generator 4 receives the methane-enriched biogas which is used as fuel to generate electricity, which is output from the generator via path 50. The generator may be electrically connected to one or more electrical apparatus located nearby or it may be electrically connected to a local or national power grid.

The exhaust gases are carried by an exhaust conduit 54 to an exhaust gas conversion station 14 which includes a catalytic converter (not shown) which converts carbon monoxide from the generator to carbon dioxide and breaks down nitrogen oxide compounds. The gases downstream of the catalytic converter are scrubbed with chilled pressurised water in a similar process to the biogas treatment station 12 and the carbon dioxide-rich water is carried to the greenhouse 6 via a conduit 58, where the carbon dioxide is released from the water into the greenhouse atmosphere and the water is used for irrigation. The non-dissolved gases downstream of the catalytic converter are vented to the atmosphere via an exhaust port 56.

The generator 4 is cooled by a water cooling system. Relatively cool water is carried to the generator 4 from the greenhouse 6 via a conduit 60 and passed around the generator 4 where heat from the generator 4 is absorbed by the water. The heated water is then output from the generator 4 via a conduit 52, where is may be recycled through the system via a radiator (not shown).

The skilled person will appreciate that various features of the above-described schematic representation of the invention can be varied within the scope of the invention as claimed. In particular, various components of the system, for example pumps, valves, control systems, etc. are not described in detail as these are well known in the field of fluid transfer. In addition, details of the biodigester, the generator, the greenhouse, etc. are not provided, as these individually are known components.

Claims

Claims 1. An electrical energy generating system including an electricity generator, a biowaste processor and a farm housing, wherein the biowaste processor breaks down a biowaste product to a methane-rich biogas and is in fluid communication with both the generator and the farm housing, the system further including gas transport elements which are capable of transporting the biogas to the generator and carbon dioxide to the farm housing; and wherein the system further includes a biogas treatment station located between the biowaste processor and the generator, the treatment station including a scrubber including water adapted to remove carbon dioxide from the methane-rich biogas.
2. An electrical energy generating system according to Claim 1, wherein the biowaste processor is a biodigester and the system includes a pre-treatment station which breaks down the lignin in the biowaste prior to the biodigester being fuelled with the biowaste.
3. An electrical energy generating system according to Claim 2, wherein the pre-treatment station includes a mushroom farm.
4. An electrical energy generating system according to any of Claims 1 to 3, wherein the biogas treatment station removes a pre-determined percentage of carbon dioxide from the biogas.
5. An electrical energy generating system according to any of Claims 1 to 4, wherein the scrubber includes a flow of water.
6. An electrical energy generating system according to any preceding claim, wherein the generator is electrically connected to a local or national power grid.
7. An electrical energy generating system according to any preceding claim, wherein the farm housing is a greenhouse or glass enclosure within which crops are grown.
8. An electrical energy generating system according to Claim 7, wherein the crops are selected from plants, plankton and algae.
9. An electrical energy generating system according to any preceding claim, wherein the biowaste processor is a biodigester and the system includes a digestate conditioning station in which the spent biowaste is conditioned for use as a fertiliser.
10. An electrical energy generator system according to any preceding claim, wherein the generator includes a coolant system coupled to a waste heat generator.
11. A method of powering a generator, the method including charging a biowaste processor with biowaste, isolating from the processor a methane-rich biogas, removing a percentage of the carbon dioxide from the biogas via a scrubber including water, fuelling the generator with the isolated biogas and supplying excess carbon dioxide to a farm housing.
12. A method according to Claim 11, wherein the biowaste processor is a biodigester and the biowaste is pre-treated to reduce the lignin therein to an acceptable limit prior to being charged into the biodigester.
13. A method according to Claim 12, wherein the lignin is broken down by an aerobic process.
14. A method according to any of Claims 11 to 13, wherein the water is between 2 and 15°C.
15. A method according to any of Claims 11 to 14, wherein the scrubber includes a water inlet and a water outlet and the water flows from the inlet to the outlet.
16. A method according to any of claims 11 to 14, wherein the biowaste processor is a biodigester and the method further includes the step of conditioning the digestate for use as a fertiliser.
17. An electrical energy generating system substantially as described herein with reference to the accompanying drawings.
18. A method of powering a generator substantially as described herein with reference to the accompanying drawings.