GB2571413A

GB2571413A - Methods of manufacturing hydrogen

Info

Publication number: GB2571413A
Application number: GB1820678.9A
Authority: GB
Inventors: Edward Graves John; Bains Narinder; Anthony Beddow James; Mendel Williams Sharon
Original assignee: Coventry University
Current assignee: Coventry University
Priority date: 2017-12-20
Filing date: 2018-12-19
Publication date: 2019-08-28
Anticipated expiration: 2038-12-19
Also published as: GB201721388D0; GB2571413B; GB201820678D0

Abstract

A method of manufacturing hydrogen comprises the steps of: (a) converting urea from the excreta of livestock into ammonia; and/or providing human sewage and/or food waste containing ammonia; and/or converting urea from human sewage and/or food waste into ammonia; (b) stripping ammonia from the livestock excreta, human sewage and/or food waste; and (c) converting the ammonia to hydrogen via electrolysis. Preferably, the livestock is pigs. A ureolytic enzyme, such as urease 12, may be used to convert urea to ammonia. The human sewage and/or food waste can comprise ammonia in the form of ammonium ions in solution. The ammonia may be stripped in step (b) using a desorption process. The electrolysis may comprise electrooxidation of ammonia coupled with electrolysis of water. The electrolysis may be performed in an electrolyser 24 comprising a metal oxide film anode and a steel-based or nickel-based cathode. The electrolyser can be connected to a micro fuel cell 30, wherein the fuel cell may be capable of converting hydrogen produced in the electrolyser into electricity 32 and heat 34.

Description

Methods of Manufacturing Hydrogen

Technical Field of the Invention

This invention relates to methods of manufacturing hydrogen, and in particular, but not exclusively, to methods of generating hydrogen for use in generating power and heat and/or in natural gas grids.

Background to the Invention

The generation of hydrogen as a source of energy, and as a fuel per se, is currently a priority for researchers around the world. Hydrogen is seen as a relatively clean energy source compared to fossil fuels, as it can react with oxygen to produce water as the sole reaction product. The use of hydrogen as a fuel therefore does not produce greenhouse gases such as carbon dioxide and methane.

Hydrogen can be used as fuel per se, for instance in hydrogen-powered vehicles, or can be used in hydrogen-powered fuel cells for electricity generation.

Around 95% of the world’s hydrogen is still derived from other fossil fuels (such as natural gas, oil, coal and coke) or from non-fossil fuels or reactants which are expensive, or are difficult to produce in large enough quantities to be a commercially viable source of hydrogen (for example from biomass).

The most effective way of utilising hydrogen as a renewable energy source is in socalled “power to gas” processes. Power-to-gas (often abbreviated P2G) is a technology that converts electrical power to a gas fuel. Standard P2G methods use electricity to split water into hydrogen and oxygen by means of electrolysis, and the hydrogen is then incorporated into downstream power generation.

The hydrogen produced can then be used in three common methods to produce energy. In the first method, hydrogen is injected into the natural gas grid or is used in transport or industry, such as in hydrogen fuel tanks for vehicles. The second method combines hydrogen with carbon dioxide and reacts the two gases to form methane. The methane then be fed into the natural gas grid. Finally, the third method uses the output gas of a wood gas generator or a biogas plant, after the biogas upgrader is combined with hydrogen formed by electrolysis. In each case, the initial electricity used in the electrolysis of hydrogen may be generated using renewable energy sources, such as wind, wave or solar power, for example. In each method hydrogen is produced electrolytically by the electrolysis of water, which is a relatively expensive process, often requiring high temperatures and large energy input.

It would therefore be advantageous to implement a hydrogen-generating process in which the source of hydrogen is renewable, and in which the source of hydrogen itself is a waste product normally disposed of in an expensive and relatively environmentally unfriendly way; and to feed the hydrogen pre-cursor or source into a relatively environmentally-friendly P2G process, especially one in which electricity used in the electrolysis of the hydrogen-source (to produce the hydrogen), is a non-fossil fuelderived electricity source (such as wind, solar or wave power).

In this regard, two potential streams of renewable hydrogen generation are livestock waste and municipal waste.

Agriculture experts warn that pig farmers are braced for a difficult time, as a glut of pork on the global markets is leading to low profits on the sale of traditional pig products and is prompting an increasing number of pig farmers out of business. On the other hand, pig urine contains high levels of urea and there is an opportunity to use this cheap, abundant waste stream to generate a source of truly renewable hydrogen, a highly valued product. Generating renewable hydrogen from pig urine would add value to the waste stream generated from pig farms, bring in extra revenue, increase competitiveness and allow farmers to compete with other sources of hydrogen generation.

The removal of urea and ammonia from human waste is also currently relatively expensive and resource intensive, requiring specific anaerobic or aerobic bacterial strains to convert the ammonia into nitrates, followed by further processing of the nitrates. Aerobic bacteria used in such nitrification processes also require specific conditions and large quantities of oxygen to work effectively.

It would therefore be advantageous to provide a method for the production of hydrogen from renewable energy sources, in an amount that is commercially viable for use in heat and power generation, and/or in a natural gas grid.

It would also be advantageous to provide a method for the production of hydrogen which utilises waste products as the hydrogen precursor, which waste products are normally expensive and/or difficult to process using current technologies.

It is therefore an aim of embodiments of the present invention to overcome or mitigate at least one problem of the prior art.

Summary of the Invention

According to a first aspect of the invention there is provided a method of manufacturing hydrogen, the method comprising the steps of

a) converting urea from the excreta of livestock into ammonia; and/or providing human sewage and/or food waste containing ammonia; and/or converting urea from human sewage and/or food waste into ammonia;

b) stripping ammonia from the livestock excreta, human sewage and/or food waste; and

c) converting the ammonia to hydrogen via electrolysis.

By “excreta” we mean urine, faeces and/or any other biological fluid, but especially urine. The livestock may be any suitable livestock such as pigs, cows, chickens, ducks or sheep, for example, but in some embodiments the livestock is pigs.

The term “sewage” may be used interchangeably with “domestic waste” or “municipal waste”, and also includes any form of human waste which includes excreta, as defined hereinabove. Domestic waste may include the wastewater from residences and institutions, carrying bodily wastes (primarily faeces and urine), washing water, food waste, laundry waste, and/or other waste products of normal human living; and is sometimes called domestic sewage or sanitary sewage.

By “food waste”, as well as including food waste which is part of sewage (or domestic or municipal waste), we also include food waste from farms, agriculture and other industrial sources of food waste, and any by-product of farming or food production which is discarded in the preparation of foodstuffs (including, but not limited to chaff, spoiled agricultural products, rinds, peelings, plant parts, straw etc.) as well as food waste from household and retail establishments such as homes, supermarkets, shops and restaurants.

By “ammonia”, as well as ammonia gas we also include ammonium ions in solution. Furthermore, in many of the processes and raw materials described herein, ammonia is in solution as an equilibrium of ammonia and ammonium ions, and this is intended to be covered under the definition of “ammonia”, as well as ammonia gas per se and ammonium ions in solution. For example liquid human sewage and food waste comprising ammonia generally has ammonia present as an equilibrium of ammonia and ammonium ions in solution.

In some embodiments the livestock excreta may comprise the excreta filtrate left after digesting excreta and removing any solids (such as sludge). The excreta may comprise the liquor produced by dewatering excreta sludge after excreta digestion. Alternatively, or additionally, the livestock excreta may comprise the excreta sludge per se or a combination of sludge and filtrate/digestate. The livestock excreta may be untreated or raw excreta.

In some embodiments the human sewage may comprise at least partially treated domestic and/or municipal waste, which may be sewage digestate or filtrate (e.g. the liquor produced after digesting sewage and removing the solids, such as sludge), or sewage sludge, or a combination thereof. The human sewage may be raw sewage (i.e. untreated).

The method, in step a) may comprise one or both of converting urea in livestock excreta into ammonia and providing human sewage and/or food waste containing ammonia.

In embodiments wherein the method comprises providing human sewage and/or food waste containing ammonia, the method, in step a) may further comprise converting at least a portion of any urea in the human waste and/or food waste into ammonia.

The method, in step a) may comprise converting urea from the following sources into ammonia: human sewage and livestock excreta; human sewage and food waste;

livestock excreta and food waste; or a combination of human sewage, food waste and livestock excreta.

The method of the first aspect of the invention provides a highly effective process to produce hydrogen which may be utilised to produce energy or in further chemical reactions. The conversion of urea to ammonia ensures high volumes of ammonia are present in or derived from the livestock excreta for subsequent stripping and conversion. Whilst livestock excreta naturally contain quantities of ammonia, the methods of the invention ensure that a commercially viable source of ammonia from livestock may be utilised.

The method also enables a high volume of hydrogen to be produced, compared to prior art methods of hydrogen production from waste products, and enables two hitherto waste products, which are difficult and expensive to dispose of, to be converted into useful and potentially revenue-generating product.

Step a) may comprise utilising a ureolytic enzyme, such as a urease enzyme, to convert urea to ammonia.

The urease may be a synthetic urease enzyme. In preferred embodiments the urease is isolated from the excreta or the livestock itself, and may comprise bacteria containing urease, the bacteria being isolated from the excreta or livestock and/or isolating urease expressed from bacteria and adding the urease to the excreta and/or sewage/ food waste.

In some embodiments the urease may comprise plant urease, and may comprise a bean urease. Suitable beans which include a significant quantity of urease include, for example, soy bean, urad bean, hyacinth bean, white lupin bean, sword bean, jack bean and horse gram bean. Therefore, suitable plant-derived ureases include soy bean urease, urad bean urease, hyacinth urease, lupin urease, sword bean urease, jack bean urease and horse gram urease, for example.

The plant-derived urease may be isolated from the plant or bean and the urease may be added to the excreta in isolated form. In other embodiments the excreta may be seeded with plant pieces or whole beans comprising the urease.

The urease may be immobilised, and in some embodiments may be immobilised on a porous substrate, such as a membrane or sheet.

Step a) may comprise using more than one different urease to convert urea to ammonia. For example, step a) may comprise using a synthetic urease and at least one urease isolated from livestock excreta and/or livestock.

The ammonia produced in step a) may comprise ammonium ions in solution, especially ammonium hydroxide ions. For the avoidance of doubt the term ‘ammonia’ when used to describe ammonia in excreta or sewage, also includes ammonium ions in solution.

The livestock excreta may comprise one or both of urine and faeces, but preferably includes urine and may consist of urine in some embodiments.

In normal human sewage or wastewater treatment, ammonia is converted, during treatment, into nitrates, by aerobic or anaerobic bacteria. This process can be resourceintensive as large amounts of oxygen are required for the aerobic nitrification processes, expensive anaerobic or aerobic bacteria must be provided, and resource and timeintensive bacterial sludge digestion must be performed. The process is usually carried out on sewage sludge in a digester, to produce a digestate.

In some embodiments, the ammonia is stripped from the human sewage and/or food waste separately to ammonia being stripped from the livestock excreta. Step c) may comprise isolating and stripping ammonia directly from human sewage and/or food waste, which may be undertaken at any suitable point in human sewage treatment and/or food waste disposal. The ammonia in the sewage or food waste may be in the form of ammonium ions in solution or an equilibrium of ammonia and ammonium ions in solution. By removing ammonia from the sewage, food waste or wastewater therefrom, expensive nitrification processes can be avoided or reduced.

Ammonia stripping may be undertaken using a desorption process in which ammonium (hydroxide) ions present in the livestock excreta (after the urea is converted to ammonia), human sewage, food waste or wastewater are reacted with a strong alkali such as lime or caustic soda until free ammonia gas is generated, which can then be stripped from the excreta, sewage, food waste or wastewater using counter-current or cross-flow stripping, as is well known in the art. If the excreta, sewage or wastewater has a particularly high concentration of ammonium hydroxide ions, in excess of 100 mg/1, then other techniques, such as steam stripping or biological methods, may be employed.

Additionally, or alternatively, step c) may comprise utilising a urease enzyme to convert any urea in the sewage and/or food waste to ammonia. Thus, more ammonia may be generated in the sewage, food waste and/or wastewater, which may then be stripped as described above, for conversion into hydrogen. The urease may be a synthetic urease enzyme. In preferred embodiments the urease may comprise urease in micro-organisms present in the human sewage, or may comprise urease added to the human sewage and/or food waste. Urease-containing microorganisms, such as bacteria may be added to the human sewage and/or food waste. More than one different urease may be used to convert urea in the human sewage and/or food waste to ammonia. For example, at least one synthetic urease and at least one urease isolated from human sewage may be used.

Step c) may comprise combining the ammonia from ammonia-enriched excreta from step a) with the ammonia from human sewage and/or food waste of step a) and stripping the ammonia from the combined product. In such embodiments, both the livestock excreta and human sewage and/or food waste may be treated with urease after they are combined, but in other embodiments the livestock excreta is treated with urease before combining it with the human sewage and/or food waste.

The method may therefore comprise the steps of:

a) converting urea in the excreta of livestock into ammonia;

b) combining the ammonia produced in step a) with ammonia from human sewage and/or food waste to produce combined ammonia product;

c) stripping the ammonia from the combined ammonia product produced in step

b); and

d) converting the ammonia to hydrogen.

The ammonia produced in step a) and/or the ammonia in the human sewage and/or food waste used in step b) may be in the form of ammonium ions in solution.

Step b) may further comprise optionally converting any urea in the human sewage and/or food waste or urea in the combined ammonia produced in step b) to ammonia. Urea may be present in the human sewage and/or food waste, and thus the extra step of converting any urea into ammonia enhances the total concentration of ammonia in the combined waste, for stripping. Step b) may comprise feeding the ammonia produced in step a) in solution, and feeding the ammonia in solution from human sewage and/or food waste, into a single reaction vessel, such as a digester, for example, and stripping the ammonia from the combined streams. Step b) may alternatively comprise feeding partially treated or untreated human sewage and/or food waste, containing ammonia, into the reaction vessel. The human sewage and/or food waste may comprise urea, which may be converted to ammonia in the reaction vessel, preferably using a urease as described hereinabove.

Steps b) and c) may be performed in a reactor vessel, and thus step b) may comprise feeding the ammonia from excreta, human sewage and/or food waste into the reactor.

In a preferred embodiment the method may comprise:

a) using a bacterial urease or plant urease to convert urea in the excreta of livestock into ammonia;

b) optionally, but preferably, combining the ammonia produced in step a) with ammonia from human sewage and/or food waste to produce a combined ammonia product;

c) stripping the ammonia from the combined ammonia product produced in step b); and

d) converting the ammonia to hydrogen.

Step a) may comprise incubating batches of livestock excreta with bacteria containing urease, which bacteria have been isolated from the excreta from which urea will be converted, or from livestock providing the excreta from which urea will be converted.

Ammonia stripping may be carried out in a reactor vessel as described in GB2504505, for example. Ammonia stripping may be performed at a temperature of between 5 °C and 80°C, at a pH of between 7 and 13 and under an airflow at between 1 bar and 10 bar, for example.

Converting of the ammonia to hydrogen may be performed using electrolysis and may comprise the coupling of electro-oxidation of ammonia with hydrogen generation, such as by using the electrons generated in the electro-oxidation of ammonia in the electrolysis of water to hydrogen.

The ammonia stripped from the human sewage, food waste and/or livestock excreta may be dissolved in an electrolyte. In this way the ammonia can be delivered to an electrochemical cell or electrolyser to undergo electrolysis. Suitable electrolytes include sodium and potassium hydroxide, sodium sulphate, potassium sulphate, potassium nitrate, sodium nitrate, sodium chloride, potassium chloride or any combination thereof. The electrolyte may have a concentration of between 0.1 and 1 mol dm-3, for example.

Stripping ammonia from the excreta and human sewage and/or food waste (or combined waste in some embodiments) mitigates or avoids impurities entering the electrolyte and hence mitigates or prevents fouling of the electrolyser unit.

In some embodiments the electrolysis is performed in an electrolyser unit in which the anode comprises an iridium oxide film anode, such as a thermally decomposed iridium oxide film (TDIROF) anode. Other anode materials include nickel, Raney nickel, platinum, rhodium, ruthenium and ruthenium dioxide or combinations or alloys thereof. Precious metal electrodes may be coated onto a titanium mesh support and can be combined with tantalum.

In some embodiments the electrolysis is performed in an electrolyser unit in which the cathode comprises stainless steel, steel, nickel, nickel plated steel, high surface area electrodes such as nickel foams and stainless-steel foam, or any combination thereof.

During electrolysis the ammonia is converted to nitrogen and water through oxidation, and coupled to hydrogen generation at the cathode (through reduction).

An example of the production of hydrogen from ammonia, using electrolysis, and using a hydroxide electrolyte, may be as shown in the reaction below:

NH3 is oxidized in the presence of OH according to the following reaction:

2NH₃ + 6OH--------> N₂ + 6H2O + 6e

At the cathode a solution of KOH or NaOH is supplied and water is reduced in alkaline medium according to the following reaction:

6H2O + 6e -------> 3H₂ + 6OHThe overall reaction is given by:

2NH3(aq.)-------> N₂(g) + 3H₂(g) mole of urea can therefore generate 3 moles of hydrogen using this process.

According to a second aspect of the invention there is provided a method of manufacturing ammonia, the method comprising the steps of:

a) converting urea in the excreta of livestock into ammonia;

b) optionally, but preferably combining the ammonia produced in step a) with ammonia from human sewage and/or food waste to produce a combined ammonia product;

c) stripping the ammonia from the combined product produced in step b)

The urea, ammonia, livestock excreta, food waste and human sewage may be as defined hereinabove for the first aspect of the invention, and all combinations described above are equally applicable for the method of the second aspect of the invention.

The ammonia produced in the second aspect of the invention may be used for purposes other than hydrogen production.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments will now be described, by way of example only, with reference to the accompanying drawing, of which:

Figure 1 is a flow chart illustrating a hydrogen manufacturing system 2 for use with an embodiment of the method of the first and second aspects of the invention.

Referring to Figure 1, a system 2 for use in a method of manufacturing hydrogen from ammonia from pig excreta and human sewage is illustrated.

The system 2 comprises a pig farm 4 connected to an ammonia generation apparatus

8. Additionally, the system 2 comprises a sewage treatment plant 16. The ammonia generation apparatus 8 and sewage treatment plant 16 are connected to a reactor 20.

The reactor 20 is connected to an electrolyser 24, which comprises an electrochemical cell able to convert ammonia to hydrogen. The electrolyser 24 is connected to a micro fuel cell 30, downstream of the electrolyser 24. The fuel cell 30 is capable of converting hydrogen produced in the electrolyser 24 into electricity 32 and heat 34.

In use, the system 2 is operated as follows. Excreta, in the form of pig urine 6, is produced at the pig farm 4. The ammonia generation apparatus 8 is seeded with urease 12 produced in a urease production facility 10. The urease 12 is fed into the ammonia generation apparatus 8. Urease 12 may be produced via any one of a number of methods, but is preferably manufactured either from plant material such as soy bean urease, or comprises urease within bacteria isolated from the excreta produced by the pigs in the pig farm 4. The urine 6 is fed into the ammonia generation apparatus 8, which comprises the urease 12. Digestion of the excreta and urine 6 takes place in the ammonia generation apparatus 8, and the urease 12 converts urea in the pig’s urine 6 into ammonia (as a solution of ammonium ions in waste liquor). The ammonia 14, produced in the ammonia generation apparatus 8 is then fed into a reactor 20. At the same time, ammonia 18 produced in the sewage treatment plant 16 (via anaerobic or aerobic digestion) is also fed into the reactor 20. The reactor 20 may comprise a desorption chamber in which ammonium hydroxide ions present in the ammonia solutions 14 and 18 produced in the ammonia generation apparatus 8 and sewage treatment plant 16, are reacted with a strong alkali such as lime or caustic soda to produce free ammonia gas 22 which is stripped and transported using counter current or cross-flow stripping. The ammonia gas 22 is fed into an electrolyser 24 which includes an anode comprising thermally decomposed iridium oxide film (TIDROF), and a nickel cathode. The electrolyser converts the ammonia into hydrogen 28 and nitrogen 26. The nitrogen 26 is transported for further processing and use, whilst the hydrogen gas 28 is fed into a micro combined heating and power (CHP) fuel cell 30. The fuel cell 30 converts the hydrogen into electricity 32 and heat 34. The heat 34 generated by the fuel cell 30 may be fed back into the pig farm 4 in order to provide heating for the pigs in the pig farm 4. The electricity 32 may also be fed back into the pig farm 4 to provide electricity, but is also preferably fed into the national electricity grid 36 of the country in which the system 2 is situated.

In alternative embodiments of the method described above for the system 2, the hydrogen gas 28 may be fed into the national gas grid of the country in which the system 2 is situated.

In an alternative embodiment of the system 2 shown in Figure 1, the reactor 20 may be fed with human sewage from the sewage treatment plant 16 directly; and thus the reactor 20 will strip ammonia directly from human sewage, rather than an ammonia stream 18 produced via anaerobic or aerobic digestion of human sewage in the sewage treatment plant 16. Thus, in this embodiment, ammonia generated in the ammonia generation apparatus 8 will be combined directly with human sewage which contains ammonia 18, in the reactor 20.

In other embodiments, ammonia entering the reactor 20 may be from a single source selected from livestock excreta, human sewage and food waste. The sewage treatment plant 16 may provide the ammonia from human sewage, directly or via conversion of urea to ammonia within the plant 16; while food waste may be provided in an appropriate format, containing ammonia and/or urea which is then converted to ammonia using, for example a urease as described herein. The livestock excreta may be provided from the pig farm 4, as described above. Preferred embodiments comprise stripping the ammonia from both livestock excreta in which urea has been converted to ammonia and also from human sewage and/or food waste and using electrolysis to convert the ammonia.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method of manufacturing hydrogen, the method comprising the steps of:

c) converting the ammonia to hydrogen via electrolysis.

2. A method of manufacturing hydrogen, as claimed in claim 1, the method comprising the steps of:

a) converting urea from the excreta of livestock into ammonia and providing human sewage and/or food waste containing ammonia;

b) stripping the ammonia from the livestock excreta and human sewage and/or food waste; and

c) converting the ammonia to hydrogen.

3. A method as claimed in claim 1 or 2 wherein the livestock is pigs.

4. A method as claimed in any one of claims 1 to 3 wherein step a) comprises using a ureolytic enzyme to convert urea to ammonia.

5. A method as claimed in claim 4 wherein the ureolytic enzyme is urease.

6. A method as claimed in any preceding claim wherein the ammonia produced in step a) comprises ammonia and/or ammonium ions in solution.

7. A method as claimed in any preceding claim wherein the human sewage and/or food waste comprises ammonia in the form of ammonium ions in solution.

8. A method as claimed in any preceding claim wherein the ammonia from step a) is stripped in step b) using a desorption process.

9. A method as claimed in any preceding claim wherein step a) comprises converting urea from the excreta of livestock into ammonia and providing human sewage and/or food waste containing ammonia and step b) further comprises using a urease enzyme to convert any urea in the sewage and/or food waste to ammonia before or during stripping the ammonia produced in step a).

10. A method as claimed in any preceding claim comprising the steps of:

a) converting urea in the excreta of livestock into ammonia;

b) combining the ammonia produced in step a) with ammonia from human sewage and/or food waste to produce a combined ammonia product;

d) converting the ammonia to hydrogen;

11. A method as claimed in claim 10 wherein step b) further comprising converting any urea in the human sewage and/or food waste or combined ammonia product to ammonia.

12. A method as claimed in claim 10 or 11 wherein step b) comprises feeding partially treated or untreated human sewage and/or food waste, containing ammonia, into a reaction vessel and feeding the ammonia produced in step a) into the reaction vessel.

13. A method as claimed in claim 12 wherein steps a) and b) are performed in the reaction vessel.

14. A method as claimed in any preceding claim comprising the steps of:

b) optionally combining the ammonia produced in step a) with ammonia from human sewage to produce combined ammonia product;

c) stripping the ammonia produced from step a) or from the combined ammonia product produced in step b); and

d) converting the ammonia to hydrogen.

15. A method as claimed in any preceding claim, wherein the electrolysis comprises electrooxidation of ammonia coupled with electrolysis of water.

10

16. A method as claimed in claim 15 wherein the electrolysis is performed in an electrolyser comprising a metal oxide film anode and a steel-based or nickel based cathode.