GB2456169A

GB2456169A - A method and associated apparatus for the production of hydrogen and/or electric energy

Info

Publication number: GB2456169A
Application number: GB0800165A
Authority: GB
Inventors: Pernille Merethe Sire Seljom
Original assignee: Nebb Tech AS
Current assignee: Nebb Tech AS
Priority date: 2008-01-04
Filing date: 2008-01-04
Publication date: 2009-07-08
Anticipated expiration: 2028-01-04
Also published as: GB0800165D0; GB2456169B

Abstract

The method and process comprise: feeding substantially pure oxygen and a carbonaceous fuel in a stochiometric ratio to a combustor 1, combusting the oxygen and carbonaceous fuel in the combustor 1 to form an energy rich exhaust gas comprising steam and CO2. At least a part of the energy of the exhaust gas exiting the combustor is converted into electric energy, and thereafter the exhaust gas is separated into a substantially pure water fraction 6 and a gaseous fraction containing CO2 5 by condensing the water fraction. The gaseous CO2 containing fraction is converted to means for storing or sequestering the CO2. The water 6 is then fed to an electrolysis reactor 2 by passing from 0 to 100 % of the condensed water, and the remaining water fraction from an external water source. From 0 to 100 % of the produced electric energy 11 is passed to the electrolysis reactor 2 for decomposing water to hydrogen gas and oxygen gas by electrolysis in the electrolysis reactor 2. The remaining fraction of the produced electric energy to is fed to a net grid, and the produced oxygen gas in the electrolysis reactor 2 to the combustor 1.

Description

Combined hydrogen and power production This invention relates to an energy efficient method and plant for production of hydrogen and optionally electric power from carbonaceous fuels without significant emissions of CO2.

Background

The transportation sector in all developed countries is practically totally dependent on fossil oil as energy carrier. Most vehicles, boats, airplanes, trains etc. use fuels made from fossil oil. But the world oil production is expected to reach a maximum and begin to decline sometimes in the near future. Estimates vary from having already reached the peak to up to 20 years from now. Thus it is necessary to begin developing and implementing alternative energy carriers for transportation vehicles.

One alternative is hydrogen. Hydrogen may for example react with oxygen in fuel cells to produce electric power with only water as the combustion product, and may thus be a environmentally sustainable alternative to fossil fuels.

Presently there are two industrial process routes for producing hydrogen. One is electrolytic decomposition of water to hydrogen and oxygen gas: 2 H20 + electric energy -2 H2 + 02. This process requires electric power, of which 2/3 in the world are produced in thermal power plants fed with fossil carbonaceous fuels. The other is producing the hydrogen directly from coal or hydrocarbons by reforming in excess of water steam. The reactions in the case of using coal are: C+H20-'H2+CO CO+H20 -iH2+C02 According to The World Coal Institute, the available coal reserves with present mining technology (in 2001) is in the order of one trillion metric tonnes, giving a global coal reserves-to-production ratio of more than 200 years. Thus it is expected that coal or hydrocarbon reforming will be a dominant energy source for production of hydrogen in the foreseeable future.

However, there is one serious drawback of using fossil carbonaceous fuels; they produce the greenhouse gas CO2. According to IPCC Fourth Assessment Report 2007, the world emissions of greenhouse gases are unsustainable, it is necessary to reduce the present world emissions of these gases by around 80 % in a couple of decades in order to halt the present global warming.

It is thus a need for production methods capable of large scale hydrogen production without significant emissions of greenhouse gases.

One suggested solution for avoiding significant C02-emissions by use of carbonaceous fuels is carbon sequestering from thermal power generation, capturing formed CO2 and then deposit it in earth formations.

Prior art

The presently known technological options for C02-capture from thermal power generation may be divided into the following categories: Pre-combustign Carbon in the fuel is extracted before (pre) the combustion take place. Pre combustion implies conversion of the fuel to CO2 and H2, followed by extraction of CO2 and combustion of H2 in air.

Post-combustion CO2 is extracted from the flue gas after (post) the combustion process. Post combustion implies direct combustion between fuel and air where CO2 is extracted from the combustion product gas.

Oxy-fuel The combustion takes place with pure oxygen, and the CO2 is extracted after the combustion by flue gas cooling. With stochiometric combustion, the flue gas will mainly contain CO2 and H20. The steam in the flue gas is condensed, and dry CO2 can be extracted. Examples of such technology are disclosed in i.e. US 5 724 805, US 5 956 937, US 6 389 814, US 6 598 398, or WO 2005/100754.

In these documents, there are shown different embodiments of power plants based on combustion of a pure carbonaceous fuel in the presence of pure oxygen and water, resulting in the production of a high-energy gas at high temperature and pressure consisting of only water and CO2 in a type of gas generator called an oxy-fuel generator. The thermal and mechanical energy in this gas is then utilised to produce electrical energy in conventional steam-driven multistage turbines. After the useful energy in the gas from the oxy-fliel generator is converted to electrical energy, the relatively cold gas mixture of steam and CO2 is separated by cooling until the steam is condensed into liquid water. The resulting gas phase consists of pure CO2 which is ready for pressurisation and depositing.

There are several advantages with the oxy-fuel process compared to the other capture categories. The unique feature of the oxy-fliel process is that the process captures --100 % of the produced CO2 and that -0 % of nitrogen oxides are produce since nitrogen is absent in the combustion process (provided that the fuel contains no nitrogen).

Objective of the invention The main objective of the invention is to provide an energy efficient method and plant for production of hydrogen and optionally electric power from carbonaceous fuels without significant emissions of CO2.

The objective of the invention may be obtained by the features as set forth in the following description of the invention and/or the appended claims.

Description of the invention

The invention is based on the realization that electrolytic production of hydrogen may be combined with an oxy-fuel based production of electric power to give an energy efficient production process of hydrogen, and optionally also electric power.

Thus in a first aspect, the present invention relates to a method for production of hydrogen and/or electric energy, comprising: -feeding substantially pure oxygen and a carbonaceous fuel in a stochiometric ratio to a combustor, -combusting the oxygen and carbonaceous fuel in the combustor for forming an energy rich exhaust gas comprising steam and CO2 at high temperature and pressure, -converting at least a part of the energy of the exhaust gas exiting the combustor into electric energy, and thereafter separating the exhaust gas into a substantially pure water fraction and a gaseous fraction containing CO2 by condensing the water fraction, -discharging the gaseous C02-containing fraction to means for storing or sequestering the C02, -feeding water to an electrolysis reactor by passing from 0 to 100 % of the condensed water and the remaining water fraction from an external water source, -passing from 0 to 100 % of the electric energy produced by the generator to the electrolysis reactor for decomposing water to hydrogen gas and oxygen gas by electrolysis in the electrolysis reactor, and passing the remaining fraction of the produced electric energy to a net grid, -feeding the produced oxygen gas in the electrolysis reactor to the combustor, and -collecting the produced hydrogen gas.

In a second aspect the invention relates to a plant for combined production of hydrogen and/or electric energy, comprising: -a combustor, -means for passing a substantially pure oxygen feed to the combustor, -means for passing a carbonaceous fuel in a stochiometric ratio of the oxygen feed to the combustor, -an electric energy producing unit which converts at least a part of the energy of the exhaust gas exiting the combustor into electric energy, -means for passing the exhaust gas from the combustor to the energy conversion unit, -an exhaust gas condenser separating the exhaust gas into a gaseous C02-fraction and a water fraction, S -means for passing the exhaust gas from the to electric energy conversion unit to the exhaust gas condenser and means for supplying a cooling medium to the exhaust gas condenser, -means for capturing and storing-or sequestering the gaseous C02-fraction exiting the condenser, -an electrolysis reactor, -means for feeding water to the electrolysis reactor by passing from 0 to 100 % of the condensed water in the condenser and the remaining fraction from an external water source, -means for passing from 0 to 100 % of the produced electric energy in the electric energy producing unit to the electrolysis reactor, and means for passing the remaining fraction of the produced electric energy in the electric energy producing unit to a net grid, -means for capturing and passing the formed oxygen in the electrolysis reactor to the combustor, and -means for capturing the produced hydrogen gas in the electrolysis reactor The combustion process may advantageously be controlled/cooled by insertion of water and/or steam in the combustor. The water insertion may also be advantageous for the downstream energy conversion process producing electric energy. Thus the invention may optionally include feeding water and/or steam into the combustor together with the carbonaceous fuel and the substantially pure oxygen fraction. The amount of water/steam needed to control/cool the combustion process and/or to form an energy rich exhaust gas suited for producing electric energy, depends on the type of carbonaceous fuel and energy conversion process which are being applied. The amount of injected water may be from 0.1 up to 80 mass% of the total mass-stream entering the combustor. It is of course also envisioned no injection of water and/or steam in the combustor. Another possible cooling/controlling aid is injection of recycled exhaust gas into the combustion chamber. In this case the invention includes extracting a part of the exhaust stream downstream of the energy conversion unit for production of electric energy. The exhaust gas may be more or less pure CO2 or a mixture of CO2 and steam. This feature may be used separately, or in combination with feature of insertion of water and/or steam into the combustor.

The term "combustor" as used herein means any type of chemical reactor able to sustain a continuous combustion of a carbonaceous fuel feed in a pure oxygen atmosphere, eventually with injection of water/steam. Examples of combustors includes, but are not limited to, steam boilers, gas turbines, combustion engines etc. The term "steam" as used herein means the gaseous phase that evolves when water boils. It may have a composition ranging from a substantially pure vapour phase (practically all H20-molecules in gaseous state) to a more foggy state comprising a mixture of gaseous H20 and liquid phase 1120-droplets.

The term "substantially pure oxygen" as used herein means any oxygen rich gaseous phase containing more than 80 molar% oxygen. The remainder should preferably be gaseous components that are inert during the combustion process in the combustor.

The oxygen supply, i.e. oxygen not produced in the electrolysis reactor, may advantageously be obtained by use of an air separation unit, which may be any unit or device able to separate atmospheric air into a substantially pure oxygen fraction and a residual fraction. Typically the oxygen fraction will contain from 90 to 95 molar% oxygen, the remainder is gases originating from the air supplied to the air separating unit. N2 and Ar are the most abundant residual gases. The air separation unit may be a cryogenic air separation unit or non-cryogenic air separation processes such as pressure swing adsorption, vacuum-pressure swing adsorption, or membrane separation. However, the invention may apply any present and future conceivable air separation unit able to provide a sufficient oxygen supply needed to run the combustion process a stochiometric conditions. The air separation unit may advantageously be able to separate the residual fraction into a substantially pure liquid nitrogen fraction, and possibly also substantially pure fractions of the noble gases present in atmospheric air. This will give the process according to the invention an improved economy by providing more products for sale.

The term "carbonaceous fuel" as used herein means any substance containing sufficient carbon to give an exothermic reaction with oxygen. Examples of suited fuels are, but not limited to coal, gasified coal, biomass, gasified biomass, natural gas, pet coke, etc. The fuel may advantageously consist of substantially pure carbon or hydrocarbon compounds in order to give an exhaust gas comprising mainly CO2 and H20. In this case the process need not involve removal of toxic and other unwanted impurity compounds/elements from the exhaust gas. However, it is envisioned use of any carbonaceous compound as long as the carbon content is sufficiently high to give an energy rich exhaust gas. This might involve impurity compounds/elements, such as heavy metal oxides, sulphur oxides, nitrogen oxides, dioxins, etc. that need to be extracted from the exhaust gas. The invention may employ any conventional and conceivable cleaning technology for purifying combustion gases. The specific impurity compounds/elements and the means for extracting them from flue gases from combustion processes of carbonaceous fuels are known to a skilled person, and need no further description. The same applies to where in the process line the cleaning means should be placed.

The means for converting energy in the exhaust gas to electric energy may advantageously be an expander that runs an electric generator. The term "expander" as used herein means any device which may extract energy from the high energy exhaust gas and convert it to mechanical energy. This may advantageously be multistage steam or gas turbines, etc. However, the invention is not tied to use of an expander, any conventional and conceivable method/means for extracting energy from the exhaust gas and convert it to electric energy may be employed.

The energy conversion process may be tuned to deliver heat energy in the form of hot water or other heated energy carrying fluids to external processes such as long distance heating facilities, industrial processes, etc. This may i.e. be obtained by regulating the condensation temperature when separating the exhaust gas into a substantially pure C02-fraction and water, and then either direct the hot condensation water to the external facilities or heat exchange the hot water with an external energy carrying fluid.

The term "electrolysis reactor" as used herein, means any device which may decompose water to hydrogen and oxygen gas by passing an electric current through a water phase. The invention may apply any known and conceivable electrolysis reactor including, but not limited to, high temperature electrolysis cells, high pressure cells, etc. The high temperature electrolysis process is more energy effective than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures. It is envisioned that the waste heat from the oxy-fliel process may be applied to raise the temperature of the electrolysis process to 100 °C or more. Conventional high temperature electrolysis processes operates in the area from about 100 °C to about 850 °C. A further advantage may be obtained by applying pressurized electrolysis, in this case the need for compressing the produced oxygen gas will be lowered, and thus reduce the energy need for providing compressed oxygen to the combustor. The pressurizing may be obtained by employing a high electrolysis temperature.

The water needed for the electrolysis process may, depending on type of fuel and amount of injected water/steam to the combustor, be fully supplied by the combustion process, i.e. by passing the condensed water from the exhaust gas.

Alternatively, there might of course be included an external water source for supplying the electrolysis reactor. It is also envisioned that all water needed for the electrolysis process is covered by an external source, that is, none of the condensed water in the condenser is used in the electrolysis cell.

The electric energy from the electric generator may in its entirety be used for producing hydrogen in the electrolysis reactor. It is also envisioned a combination of hydrogen and electricity production, this might be obtained by including an electric connection from the electric generator to the net grid. This option may be advantageous by allowing optimising the process to deliver electricity and/or hydrogen at various consumer demands. For example, the electricity demand is greater at day-time than at night, and many markets operates therefore with higher electricity prices at day-time and other peak demand periods. This might be exploited such that the process according to the invention delivers electricity at peak demand periods, and then switches to hydrogen production in-between the peak demand periods. The invention may be operated to give any ratio ranging from % hydrogen to 100 % electric energy delivered to the net grid.

As mentioned above, the oxy-fuel process has the great advantage that it allows combustion of carbonaceous fuels with practically 100 % capture of the formed C02, however at a penalty of using energy for forming a pure oxygen feed. The process according to the invention, eases this penalty by recycling oxygen from the electrolysis cell to the combustor. Thus the overall thermal efficiency of the process becomes higher compared to pure oxy-fuel processes. That is, the penalty for supplying pure oxygen to the combustion process is reduced due to formation of oxygen as a by-product in the electrolysis cell. It is estimated that up to 30 % of the oxygen need of the combustor may be supplied from the electrolysis cell. This will enhance the thermal efficiency by about 2 %.

The method according to the first aspect may be schematically illustrated as given in Figure 1. An oxy-fuel process I is fed with a stream of carbonaceous fuel 3 and oxygen 4 to produce a stream 5 of CO2 and a stream 6 of water, which may be fed to electrolysis process 2 by line 7 and/or discharged through line 8. The oxy-fuel process includes means for combusting the fuel, means for converting some of the energy in the exhaust gas to electric energy, and means for separating the exhaust gas into a C02-fraction and a water fraction. The produced electric energy may be fed to the electrolysis cell through line 11 and/or be sent to an electricity grid through line 12. The produced oxygen in the electrolysis cell is sent via line 10 to the oxygen supply 4 for the oxy-fuel process. There is an optional supply line 13 for feeding water to the oxy-fuel process and an optional line 9 for feeding water to the electrolysis process. The produced hydrogen is extracted through line 14.

Claims

1. Method for production of hydrogen and/or electric energy, comprising: -feeding substantially pure oxygen and a carbonaceous fuel in a stochiometric ratio to a combustor, -combusting the oxygen and carbonaceous fuel in the combustor for forming an energy rich exhaust gas comprising steam and CO2 at high temperature and pressure, -converting at least a part of the energy of the exhaust gas exiting the combustor into electric energy, and thereafter separating the exhaust gas into a substantially pure water fraction and a gaseous fraction containing CO2 by condensing the water fraction, -discharging the gaseous C02-containing fraction to means for storing or sequestering the CO2.

-feeding water to an electrolysis reactor by passing from 0 to 100 % of the condensed water, and the remaining water fraction from an external water source, -passing from 0 to 100 % of the produced electric energy to the electrolysis reactor for decomposing water to hydrogen gas and oxygen gas by electrolysis in the electrolysis reactor, and passing the remaining fraction of the produced electric energy to a net grid, -feeding the produced oxygen gas in the electrolysis reactor to the combustor, and -collecting the produced hydrogen gas.

2. Method according to claim 1, characterised in that the combustion process in the combustor is cooled by insertion from 0.1 to 80 mass% of the total mass stream entering the combustor.

3. Method according to claim I or 2, characterised in that the combustion process in the combustor is cooled by insertion of recycled exhaust gas extracted from the exhaust stream downstream of the conversion of the exhaust gas energy to electric energy.

4. Method according to claim 1, 2 or 3, characterised in that the electrolysis cell is a pressurised high temperature cell operating at temperatures from 100 °C up to about 850 °C.

5. Method according to any of the preceding claims, characterised in that the carbonaceous fuel is one or more of the following: coal, gasified coal, biomass, gasified biomass, natural gas, and pet coke.

6. Plant for combined production of hydrogen and/or electric energy, comprising: -a combustor, -means 4 for passing a substantially pure oxygen feed to the combustor, -means for passing a carbonaceous fuel 3 in a stochiometric ratio of the oxygen feed 4 to the combustor, -an electric energy producing unit which converts at least a part of the energy of the exhaust gas exiting the combustor into electric energy, -means for passing the exhaust gas from the combustor to the energy conversion unit, -an exhaust gas condenser separating the exhaust gas into a gaseous C02-fraction 5 and a water fraction 6, -means for passing the exhaust gas from the to electric energy conversion unit to the exhaust gas condenser and means for supplying a cooling medium to the exhaust gas condenser, -means for capturing and storing-or sequestering the gaseous C02-fraction exiting the condenser, -an electrolysis reactor 2, -means 7, 9 for feeding water to the electrolysis reactor by passing from 0 to % of the condensed water in the condenser and the remaining fraction from an external water source, -means 11 for passing from 0 to 100 % of the produced electric energy in the electric energy producing unit to the electrolysis reactor, and means 12 for passing the remaining fraction of the produced electric energy in the electric energy producing unit to a net grid, -means 10 for capturing and passing the formed oxygen in the electrolysis reactor to the combustor, and -means 14 for capturing the produced hydrogen gas in the electrolysis reactor.

7. Plant according to claim 6, characterised in that the combustor is one of the following: steam boiler, gas turbine or combustion engine.

8. Plant according to claim 6 or 7, characterised in that the combustor is fed with steam in an amount ranging from 0.1 to 80 mass% of the total mass stream entering the combustor.

9. Plant according to claim 6, 7 or 8, characterised in that the carbonaceous fuel is one or more of the following: coal, gasified coal, biomass, gasified biomass, natural gas, and pet coke.

10. Plant according to any of the preceding claims, characterised in that the electrolysis unit 2 is a pressurised high temperature electrolysis cell operating at temperatures from 100 °C up to about 850 °C.

11. Plant according to any of the preceding claims, characterised in that the energy conversion unit is an expander which runs an electric generator.

12. Plant according to claim 10, characterised in that the plant further comprises means for extracting heat energy from the condensed water exiting the condenser.