WO2012153003A1

WO2012153003A1 - Method for intensifying the combustion process

Info

Publication number: WO2012153003A1
Application number: PCT/FI2012/050453
Authority: WO
Inventors: Markku Lampinen
Original assignee: Aalto University Foundation
Priority date: 2011-05-10
Filing date: 2012-05-10
Publication date: 2012-11-15
Also published as: FI20115447A0; EP2712414A4; FI126249B; FI20115447A; EP2712414A1

Abstract

The present invention relates to a combustion method, wherein a burner is used, which contains one or more tubular, semi-permeable burner membranes laid parallel, whereby the fuel is introduced into the combustion space formed of the membranes, and both the oxygen and the nitrogen of the combustion air, in ionized form, are transported through the membrane into the combustion space without the assistance of a compressor, whereby the fuel is reacted with the oxygen of the combustion air. The present invention also relates to such a burner containing one or more tubular semi-permeable membranes.

Description

METHOD FOR INTENSIFYING A COMBUSTION PROCESS

Background to the invention Scope of the invention

The present invention relates to a method of boosting the combustion process by creating a combustion which generates less entropy than a conventional combustion. The invention also relates to a burner which is formed of tubular membranes and which is suitable for use in this method.

Description of the prior art

Research into exergy losses in combustion is relatively limited. Conventional thermal combustion processes are constant pressure combustion (such as takes place in gas turbine plants) and constant volume combustion. The latter has a better coefficient of efficiency, whereas a disadvantage of constant pressure combustion is the considerable generation of entropy and a corresponding loss of energy, which, depending on the fuel and the coefficient of air, amounts to approximately half of the calorific value of the fuel used. The ideal combustion process is the "isentropic combustion process", in which no entropy is generated at all. The closest to the principle of the present invention is electrochemical combustion, i.e. use of a fuel cell in combination with constant volume combustion with higher efficiency. Oxygen separated from air by using cryotechnology is used in conventional oxygen combustion, and the cooling is carried out by means of recirculation of carbon dioxide through heat exchangers. An apparatus according to the present invention creates a combustion which generates less entropy, i .e. it is closer to a state of equilibrium than conventional combustion is.

Also, the use of so-called oxygen membranes in combustion has been studied (for example, US 2008/0141672) and it has been discovered that such membranes reduce the amount of energy required by the combustion. The property of semi-permeable membranes to transport oxygen ions is exploited in these oxygen membranes. Equilibrium of the combustion reaction is achieved by means of a difference in the pressure of the oxygen on each side of the membrane. The reaction is carried out under conditions which separate the oxygen from the other components of the combustion air. Publications EP 1172135 and 2026004 describe the use of a similar oxygen membrane in combustions in which transportation of nitrogen through the membrane is avoided by the presence of nitrogen, which prevents the generation of a product stream which is as carbon dioxide-rich as possible. These publications also state that the transporting of the nitrogen reduces the efficiency of the combustion process because energy is required to heat the nitrogen.

An article by Hashim S. M. et al (Current status of ceramic-based membranes for oxygen separation from air, Advances in Colloid and Interface Science 160 (2010) 88-100), in turn, focuses on materials and development of such oxygen membranes.

In fact, use of the well-known oxygen membrane already makes it possible to carry out pressurised oxygen combustion. Problems occurring include the very high combustion temperatures, which make the method difficult to apply as such in thermal

power generation processes.

The present invention presents a solution to this problem, specifically a membrane which is permeable to oxygen gas and also to nitrogen gas, in which case the nitrogen acts as a cooler and, at the same time, is itself pressurized when passing through the membrane. Accordingly, in the present invention, both the oxygen gas and the nitrogen gas in the air are transported from the combustion air to the pressurized space, an action which is carried out without work required by a compressor, thus improving considerably the efficiency of the heat engine which is connected to the combustion process. Brief description of the invention

One of the objects of the present invention is to provide a new combustion process which is more efficient than the known solutions. In particular, it is an object of the present invention is to provide a new combustion process in which the generation of entropy is minimized compared to the use of oxygen

membranes of the prior art. Thus, the present invention relates to a method which allows transportation of oxygen and nitrogen in the combustion air through a membrane to a space which is pressurized by means of fuel gas, by means of a combustion reaction, without using a compressor. The method can be carried out by using a burner made of tubular membranes. The tubular membrane is a ceramic coating which conducts oxygen and nitrogen ions and which coating is placed onto the inner surface of a porous ceramic capillary tube which acts as a support structure.

In this way, both the combustion air and the nitrogen gas required for cooling can be imported into the pressurized space without the use of a compressor.

More specifically, the method of transporting nitrogen ions, according to the present invention, is characterized by what is stated in the characterizing part of Claim 1. The combustion method according to the present invention is, in turn, characterized by what is stated in the characterizing part of Claim 3.

The burner according to the present invention is characterized by what is stated in the characterizing part of Claim 10.

Numerous advantages are achieved with the present invention compared with prior art solutions. The energy content of the fuel provides more energy which is suitable for power generation (exergy), and, when used in a combustion engine, more mechanical energy than is generated in the known combustion solutions. The present invention differs from some of the oxygen combustions used in that here the oxygen is taken from the air through a membrane, whereas in conventional oxygen combustion, oxygen taken from the air by means of cryotechnology is used. In addition, nitrogen gas is now used to reduce the high temperature of the combustion space. Brief description of the drawings

Figure 1 is a schematic illustration of the principle of the operation of a known so-called oxygen membrane (oxygen-transporting membrane), which oxygen membrane is used to determine the oxygen content by measuring voltage, which makes it possible to calculate the ratio of partial pressures of oxygen across the membrane by means of the Nernst equation.

Figure 2 is a schematic illustration of the principle of the operation of the so-called self- pressurized membrane combustion process according to the present invention, in which case Figure 2A shows a principle which makes it possible, by using oxidation of the fuel gas, to generate an electric field inside the membrane, and Figure 2B illustrates how the oxygen and nitrogen ions move in the membrane. Figure 3a is a graphical presentation of a traditional turbo-charged diesel engine combustion process, and Figure 3b a corresponding combustion process having an external, self-pressurized membrane combustion process, according to the present invention. Figure 4 A is a graphical presentation of ideal combustion in a gas turbine combustion process, where combustion of coal takes place isentropically and the heat released in the combustion process is converted to work in the gas turbine, and Figure 4B shows a preferred embodiment of the combustion process shown in Figure 4A. Detailed description of preferred embodiments of the invention

The present invention relates to a method of combustion designed to produce an exhaust gas stream at a pressure as high as possible. In this combustion method, a burner is used which comprises one or more tubular, semi-permeable burner membranes laid parallel in order to form, in turn, one or more tubular, pressurized combustion spaces. The fuel, preferably at the pressure of the combustion space, is fed into the combustion space and, at the same time, the oxygen and the nitrogen, both in ionized form, of the combustion air are carried through the burner membrane into the combustion space without assistance of a compressor. The force propelling the oxygen and the nitrogen ions is the internal electric field in the membrane, which field is generated as a result of the combustion process.

The present invention also relates to a method of transporting nitrogen ions through a burner membrane which is permeable to oxygen ions, by which method oxygen ions are transported through the burner membrane by means of the difference in the partial pressures of oxygen, propelled by the combustion reaction, in which case an electric field is formed in the combustion membrane, which makes it possible to transport through the same membrane not only oxygen, but also a substantial number of nitrogen ions. Also, the difference in the partial pressures of oxygen influences the transportation of the nitrogen ions.

Furthermore, the present invention relates to a burner to be used in the combustion process, which burner comprises one or more tubular combustion spaces laid parallel, each of which comprises a semi-permeable combustion membrane, and a porous ceramic support layer surrounding it. This burner can be utilized in the above-mentioned method.

The present invention aims at developing the energy efficiency of the combustion process by reducing the generation of entropy and, accordingly, the loss of exergy. Here, "exergy" means the share of the energy that is available for work, whereas "entropy" is used to describe the distribution of energy and matter in the system, in this case the combustion space and the environment surrounding it. The reduction of the generation of entropy (here the reduction of the disorder and the guidance of the matter to a desired state) is known to require outer work/force, and this cannot be achieved with a complete efficiency.

However, the invention aims at optimizing this efficiency.

The combustion air of the burning comprises oxygen and nitrogen, and optionally also other components. Preferably, the combustion air comprises essentially the same components, and which are essentially of the same concentrations, as the atmospheric air, i.e. about 78 % by volume nitrogen and approximately 21 % by volume oxygen.

The material of the tubular burner membrane is permeable to oxygen and nitrogen ions, for instance zirconium oxide stabilized with yttrium (YSZ). More preferably, the membrane is doped with nitrogen. By having a perovskite phase, which is more preferably positioned in the membrane material, it is possible to transport the electrons within the material, in which case the voltage difference between the anode and the cathode can be transferred into the electrolyte (which here is the membrane material, such as YSZ). This membrane can be placed onto the inner surface of the porous tube, such as a ceramic capillary tube, which acts as a support structure. Preferably, the material of the capillary tube acting as a support structure is zirconium oxide.

Thin, hollow tubes are formed of these materials. Preferably, a number of these tubes, for example 3-1000, in particular 5-100 tubes, are bundled together, and a packet is formed of them (hereinafter referred to as membrane tube system).

The thickness of the wall of the capillary tube that functions as a support structure is preferably 0.1-0.5 mm and the diameter is 2-4 mm. The thickness of the burner membrane is preferably 1-100 μπι.

However, the size of the membrane burner can be varied to suit a given application. The area of the reaction surface is relatively easy to compress into a very small volume, for example by using tubes having a diameter of approximately one millimetre. The operation of an oxygen membrane according to the prior art is shown in Figure 1. Here, a mixture of zirconium oxide and yttrium oxide (Zr0₂:Y₂03 92:8), i.e. zirconium oxide stabilized with yttrium (YSZ, 8 %) is used. The operation of the semi-permeable membrane, according to the present invention, in turn, is illustrated in Figures 2A and 2B. Here, the membrane material used is zirconium oxide stabilized with yttrium (YSZ, 8 %), which membrane comprises a perovskite phase and is doped with nitrogen (for example 7.5 %).

The membrane material used in the invention is similar to the materials used as electrolytes in solid oxide fuel cells (SOFC), and in which the purpose is to generate a large potential difference between the conductive (electron-conducting) electrodes of the membrane

(because the point at which the work is taken out is here) and a small potential difference across the electrolyte (because it is all loss). However, in the present invention, the situation is the opposite to the conventional use of SOFC materials. The aim is to generate a large internal potential difference across the electrolyte of the membrane material (YSZ), and to use it for pumping oxygen and nitrogen ions. Preferably, perovskite, which conducts electrons, is positioned into the membrane structure, which perovskite, by short-circuiting the anode and the cathode, transfers the potential difference to the electrolyte (YSZ), as shown in Figure 2A.

Compression of the combustion air into the combustion space takes place in the form of ions and by using the electric field, in accordance with the

following basic thermodynamic principles:

1 ° . In one embodiment of the present invention, oxygen gas is fed, in the form of O^2" oxygen ions, at low pressure into a pressurised space, which makes it possible to feed oxygen without back leakage (Figure 2A). At a sufficiently high temperature (for instance approximately 900 °C), the ceramic membrane material (such as zirconia oxide stabilised with yttrium (YSZ)) starts to conduct oxygen ions (and nitrogen ions) reasonably well. At the combustion point on the surface of the membrane, part of the oxygen reacts directly with the fuel and part of the oxygen is gasified and, in turn, reacts after that with the fuel. A potential difference (U) is generated inside the membrane material (thickness As), the size of which potential difference can be estimated, according to a simplified study model, with the help of electrochemical oxidation reactions (1) and (2), (Figure 2A):

20²- + ½ CH4 (g)→ ½ C0₂(g) + H₂0 (g) + 4e ( 1 )

The generated electric field E = U/As, having a magnitude of 10 -10⁶ V/m, creates a force in the oxygen ions, which force pulls oxygen towards the reaction surface, despite the oxygen being in a pressurized state.

2°. Electrons released from the oxygen ions 20²-→ 0₂(g) + 4e (2) are carried to the oxygen intake point, i.e. to the other surface of the membrane, along a separate phase (perovskite), the electrical conductivity of which is sufficient and which prevents recombination of the electrons. In this way, the electrons are settled across the membrane at approximately the same potential, and a potential difference vis-a-vis the membrane material (YSZ) is generated.

3 °. By using a ceramic membrane doped with nitrogen (for example YSZ: N 7.5 %), it is possible to make also the nitrogen ions (N^3") move in the crystal structure of the membrane material. An electric field (E), which is generated as a result of the reaction of oxygen, causes the nitrogen to be led to the combustion space, despite the counter-pressure of nitrogen in the combustion space being substantially higher than its partial pressure in the combustion air. Gasification of nitrogen can take place through ionization of oxygen, in accordance with the following balance equation (Figure 2A):

2N³- + 3/2 0₂(g) = 30^2' + N₂(g) (3)

The nitrogen gas brought to the combustion space cools the exhaust gases, the temperature of which remains very high in a pure oxygen combustion.

In the method according to the present invention, control of the temperatures is based on an adequate two-way flow between the combustion process inside the fibres and the combustion air. With these, it is possible to generate an even distribution of temperature across the membrane, thus ensuring that the operation temperature prevails throughout the tube, which temperature is high enough to bring about a good combustion. Preferably, the combustion space temperature is maintained within a range of 500-1000 °C, most suitably close to the temperature of 900 °C. In this way, it is possible to further minimize the generation of entropy.

In the burner according to the present invention, the porous ceramic support structure which surrounds the combustion membrane is preferably formed of nanoparticles. Very high strengths are achieved by means of support structure tubes which are made of nanoparticles, even though the structure is porous. Such strength allows relatively high pressures inside the tube. The tube-membrane system should tolerate pressure differences as high as 15 bar, or higher, such as up to 20 bar.

The operation of the membrane according to the present invention is controlled in such a way that, in the combustion surface, the oxygen ions react with the fuel (Formula 2), creating a potential difference between the combustion surface and the oxygen-receiving surface. The perovskite phase enables the potential difference to be transferred to the electrolyte, i.e. the YSZ structure (Figure 2A). This results, inside the membrane, in a force which propels the oxygen onto the combustion surface and, as a result, that oxygen will start flowing into the combustion space.

Also, nitrogen gas starts to flow into the combustion space, because the nitrogen gas, too, is ionized at the high temperature of the membrane, and the electric field active in the electrolyte (YSZ) pulls nitrogen ions towards the combustion surface.

Accordingly, in a preferred embodiment of the present invention, the burner according to the invention operates in such a way that the fuel is introduced into one end of the tubular ceramic combustion space, whereas the combustion air is brought into the space surrounding the tubular ceramic material (or in the middle of the above-mentioned membrane tube system or tube package), where it is distributed longitudinally evenly, in which case the oxygen and the nitrogen of the combustion air penetrate the support structure and the membrane material, and thus come into contact with the fuel, whereby a reaction is achieved. As the temperature rises, the tube material becomes increasingly oxygen- and nitrogen-conductive.

In a preferred embodiment of the present invention, the combustion is brought, at atmospheric pressure, outside the tubular structure. Gaseous fuel (such as CH}(g)), in turn, is introduced, at an elevated pressure (in the gas network or the fuel carburetor), into the inner side of the tubular membrane, where an elevated temperature prevails (higher than the gasification temperature of the fuel). Inside the membrane, the combustion takes place, too, on its surface, in which case the fuel reacts and generates carbon dioxide and water. At a high temperature, the oxygen and nitrogen molecules penetrate first the porous support material and then, in ionized form, the semi-permeable membrane. The fuel, however, is unable to significantly penetrate the membrane.

The exhaust gases generated during the oxidation of the fuel, i.e. the combustion reaction, are removed from one end of the tubular membrane (exit end) approximately at the same pressure as that at which fuel gas is fed into to the tube at the other end (input end). Since the volume of the exiting exhaust gases is larger than the volume of fuel gases, the gas flow rate increases inside the membrane-like tube, in the flow direction of the tube.

The burner according to the present invention can be utilized in a combustion system which comprises, besides a burner, also a fuel feeding system and an exhaust

gas processing system. The following non-limiting examples illustrate the present invention and its advantages.

Examples

Example 1 - Losses of energy during combustion in a combustion engine process

This example examines a conventional combustion engine (turbo-charged diesel engine). The following formula can be derived for the theoretical efficiency of a combustion engine:

η = 1 ^

- [H(B_S) - H(A)] where Om corresponds to the generation of entropy throughout the combustion engine process, and ~[H(B_S) - H(A)] is the calorific value equivalent to the isentropic combustion.

In a combustion engine according to the example in Figure 3a, the process of combustion accounts for approximately 79 % of the total exergy loss. The corresponding overall efficiency of the engine in driving conditions is approximately 48 %. Assuming that the exergy loss during combustion is eliminated, the efficiency would be approximately 89 % (Figure 3b).

The thermodynamic efficiency of this engine is subject to the same restrictions as the efficiency of a fuel cell (see Figure 3). Comparative example 1 - Combustion losses in a conventional gas turbine process

The role of exergy losses during combustion is also highly crucial for the gas turbine process. Generally, a compressor is used to introduce air for combustion into the combustion space. In the example case, the compressor takes 54 MW of shaft power from the gas turbine. The temperature of the exhaust gases coming to the gas turbine is 1100 °C and the pressure 11 bar. The total power of the gas turbine is 94 MW, of which the power transferred to the generator is 94 - 54 = 40 MW. Example 2 - Exergy losses during combustion in an ideal gas turbine process

By sufficiently reducing the loss of entropy during the combustion in the comparative example 1, an adequate self-pressurization of 11 bar is generated in the combustion space, in which case no compressor is needed and all of the 94 MW can be transferred to the generator. Figure 4A shows the principle of an ideal gas turbine process in which the combustion process is isentropic, the corresponding theoretical self-pressurization of which is very high (746 bar), and Figure 4B shows a similar process, according to a preferable embodiment.

Claims

1. A method of transporting nitrogen ions through an oxygen-ion permeable burner membrane of a burner, characterized in that an electric field is formed by means of a combustion reaction, by means of which field both oxygen and nitrogen ions are transported through the burner membrane.

2. The method according to Claim 1, characterized in that an electric field is formed in the electrolyte structure of the membrane by means of a phase positioned in it, such as perovskite, which conducts electrons.

3. A method of burning gaseous fuel, wherein the fuel is brought to the combustion space of the burner, into which space also combustion air is brought, whereby the combustion space is pressurized, after which the fuel is reacted with the oxygen of the combustion air in said pressurized combustion space, characterized in that a burner containing one or more semi-permeable tubular burner membranes laid parallel, which thus form one or more tubular combustion spaces, is used as the burner, whereby both the oxygen and the nitrogen of the combustion air, in unreacted form, are transported through the burner membrane into the combustion space, without the assistance of a compressor.

4. The method according to Claim 3, wherein the tubular membrane is formed of a ceramic material, which conducts oxygen and nitrogen ions, and which also contains a perovskite phase, and on the inner surface of which the fuel is reacted with the oxygen and the nitrogen of the combustion air.

5. The method according to Claim 3 or 4, wherein the conditions in the burner are adjusted to allow the oxygen and nitrogen of the combustion air to be transported in ionized form through the semi-permeable membrane, by utilizing the combustion reaction.

6. The method according to any of Claims 3-5, wherein the tubular membrane is placed inside a porous support material, such as a ceramic capillary tube, which acts as a support material, whereby the membrane and the support material form a tubular combustion space.

7. The method according to any of Claims 3-6, wherein unreacted combustion air is brought to the outer side of the tubular membrane, through the pores of a support structure and at atmospheric pressure.

8. The method according to any of Claims 3-7, wherein fuel is introduced, in a gaseous form, into the inner side of the tubular membrane, and at a pressure which is higher than atmospheric pressure.

9. The method according to any of Claims 3-8, wherein the temperature is kept constant throughout the area of the combustion space, preferably at 500-1000 °C, most suitably at approximately 900 °C.

10. A burner to be utilized in a combustion process, characterized in that it contains one or more tubular combustion spaces laid parallel, each of which contain a semi-permeable combustion membrane, which is doped with nitrogen, and a surrounding porous ceramic support layer.

1 1. The burner according to Claim 10, the combustion membrane of which is formed of a ceramic material permeable to oxygen and nitrogen ions.

12. The burner according to Claim 10 or 1 1, the membrane material of which is zirconium oxide stabilized with yttrium (YSZ), which material is doped with nitrogen.

13. The burner according to any of Claims 10-12, the combustion membrane of which is placed inside a porous support material, which acts as a support structure, such as a ceramic capillary tube.

14. Use of the burner according to any of Claims 10-13 in the method according to any of Claims 3-9.