GB2514809A - Chemical looping combustion process - Google Patents

Abstract

A chemical looping combustion process comprising feeding a fuel gas 10 comprising hydrogen and/or combustible hydrocarbons over a solid state metal oxide, the fuel gas containing at least one sulphur compound which is oxidised by the metal oxide in step 1, re-oxidisation of the metal is carried out in step 2 with an oxidising gas at high temperature. The fuel gas remains substantially un-oxidised by the metal oxide. Preferably the process occurs at a temperature where the hydrocarbons will not be oxidised. The metal oxide is preferably in particulate form and the metal is at least one transition metal of groups VIIA, VIIIA, IB or IIB, in particular Iron, Copper, Nickel, Manganese or Cobalt. Preferably the metal oxide is on a refractory inert support. The metal oxide is preferably contained in a fixed bed 14 and steps 1 and 2 above are alternated in sequence. Sulphur oxide is preferably extracted from the exit gas by Calcium Oxide.

Description

Chemical Looping Combustion Process The present invention concerns a chemical looping combustion (CLC) process, and in particular, a CLC process aimed at removing impurities from a gas stream, such as a stream of fuel gas.

The CLC process is an emerging technology aimed at cleaner combustion. In broad terms, the CLC process involves the coupling of two or more chemical reaction steps in order to effect oxidation of a "fuel" molecule, instead of performing combustion in a single reaction step. An additional reactive species is required to act as an oxygen carrier, that oxygen carrier being repeatedly reduced and re-oxidised in the coupled reaction steps. As an example, a simple hydrocarbon may be oxidised in a CLC process, using a metal oxide as the oxygen carrier.

A typical such CLC process, in this case using CuO as the oxygen carrier and a hydrocarbon fuel gas, is illustrated in the following reaction scheme:- (2x+y)CuO + CH2 -(2x+y)Cu + xCO2 + yH2O 1 (2x±y)Cu + ((2x+y)/2)02 (2x+y)CuO 2 This is on the simplified assumption that the oxide is completely reduced to the base metal; of course, such complete reduction is not essential, and the metal may simply be reduced to a lower net oxidation state. In any case, the net reaction is equivalent to gas phase oxidation of the hydrocarbon fuel gas, typically as follows:-CH2 + ((2x-'-y)/2)02 xCO2 + yH2O 3 Alternative metal oxides, such as those of transition metals of Group VIlIb of the Periodic Table (for example, Fe, Co or Ni) or of Group VII of the Periodic Table (for example, Mn) may also be used as the oxygen carriers in such a reaction scheme. In such oxides, when the metals are in divalent form, then the appropriate metal may be substituted for Cu in the above equations 1 and 2. Where the metal in the oxide is other than divalent, the equations are adjusted accordingly, as will be apparent to the person skilled in the art. The metal oxide is typically on a support comprising, for example, silica and/or alumina, or a silicate and/or aluminate.

There is considerable interest in using a CLC process in combustion for power generation and carbon capture, where the reaction generally takes place at high temperature, such as greater than 800°C, and in coupled fluidised beds where the oxygen carrier physically moves between a combustion and re-oxidation reactors; CO2 capture takes place in step 1 via simple moisture condensation step. Such systems have undergone pilot studies at up to 120kW generation capability utilising oxides of Cu, Mn and Fe as the oxygen carrier.

In contrast, very little work has been carried out on the CLC process using a fuel gas stream at lower temperatures, because the typical aim of the prior art CLC process is to make the combustion of the gas stream as complete as possible. We have, however, found that, for certain gas streams, there can be advantages in carrying out a CLC process without complete combustion.

According to the invention, therefore, there is provided a chemical looping combustion process, which comprises: (a) feeding a combustible fuel gas over a solid state oxygen carrier comprising a metal oxide, in which at least some of the metal is in a first, positive, valence state, so as to reduce at least some of the metal in the oxide to a lower valence state (such as to the elemental metal) and oxidise at least part of the fuel gas; and (b) re-oxidising the lower valence state of the metal back to the first valence state using an oxidising gas at elevated temperatures; wherein the fuel gas contains at least one combustible sulphur compound and the fuel gas is fed over the oxygen carrier in step (a) under conditions such that the at least one sulphur compound reduces the metal oxide and is oxidised thereby, and such that hydrogen or hydrocarbons present in the fuel gas are substantially unoxidised by the oxygen carrier.

Within a given flow regime, combustible molecules present in any given fuel gas will initiate their respective oxidation reactions with the oxygen carrier at different temperatures, dependant on a combination of thermodynamic and kinetic restrictions. One of the most important such restrictions is the residence time of the respective combustible molecule on the surface of the solid state oxygen carrier (the respective metal oxide). This temperature at which the oxidation reaction initiates, sometimes known as the light off temperature', for a given combustible molecule can be as low as about 270°C for CC, or as high as 600°C for CH4. For hydrogen, the "light off temperature" is 350-370°C.

The trend is shown in Figure 1 of the accompanying drawings, which are graphs showing results from a series of experiments on a small scale reactor operating at a pressure of 5 bar absolute, a flow of 5 slm and a bed retention time of 0.5 seconds. The graphs show normalised conversion versus reactor temperature in degrees Celsius, for, from left to right, CC, H2S, H2 and CH4.

The graphs indicate that the trend in "light off temperature" is as follows: CO and oxygenatesc C34 hydrocarbons c H2 c C2 hydrocarbons c CH.

H2S has been reported to have a "light off temperature" of about 300-350°C, while organo-sulphur compounds are expected to behave as oxygenates, with "light off temperatures" below 350°C. Hence, in the process according to the invention, if the reactor temperature is controlled at below the "light off temperature" of hydrogen or combustible hydrocarbons known to constitute the fuel gas, then sulphur compounds can be oxidised without substantial oxidation of the hydrogen or hydrocarbons. The particular combustible hydrocarbons present in a specific gas can be found by routine analysis of the fuel gas (ignoring minor ingredients only present in impurity amounts).

The temperature in step (a) can be controlled at up to about 550°C in the case where the combustible hydrocarbon fuel gas consists predominantly of methane. In the case where the gas also contains hydrogen, C2 hydrocarbons and C3 to C4 hydrocarbons, the temperature in step (a) may be controlled at up to about 350°C.

Thus, it is preferred that step (a) of the process according to the invention is carried out with reactor temperatures of below 550°C (depending on the nature of the combustible fuel gas, as indicated above),whereby sulphur compounds can be oxidised by means of the oxygen carrier, without oxidation of hydrogen or lower hydrocarbons in the fuel gas. The resultant desulphurised combustible fuel gas thus obtained may be stored for subsequent use as a combustible fuel, or passed directly to a combustion process. In either case, the ultimate combustion proceeds with advantageously lowered amounts of corrosive (acidic) sulphur oxides being produced in the resultant exhaust gases or the poisoning of catalysts used to enhance the conversion of the fuel gas into hydrogen (such as for electrical power in a fuel cell).

In a CLC process according to the invention, the possible oxidation steps (again using Cu as the metal in the metal oxide constituting the oxygen carrier) are illustrated below, using H2S as the combustible sulphur compound:- 3CuO+H2S->3Cu+H20+S02 4 CuO+H2S>CuS+H2O 5 For low levels (less than 100 ppm) of such combustible sulphur compounds in the relevant stream of the fuel gas, reaction 4 overwhelmingly dominates, while at higher concentrations reaction 5 occurs, along with the slow oxidation of the sulphur to SO2 when the challenge is removed.

Depending on the intended application, the resulting SO2 can be trapped using a simple dry reactor filled with a basic material such as sodium bicarbonate or calcium oxide (CaO); in the latter case, the product of trapping the SO2 is solid calcium sulphite, CaSO3 -which has the significant advantage of being a benign by-product of the process according to the invention.

Thus it is generally possible according to the invention to selectively substantially remove combustible sulphur-containing ingredients or impurities from hydrogen-or hydrocarbon-containing fuel gas streams, simply by selecting the appropriate temperature and retention time (flow) regime for a packed bed reactor filled with appropriate oxygen carrier.

In one embodiment, the process according to the invention may be advantageously used for removal of H2S or other-sulphur-containing compounds present as entrained impurities in a combustible fuel gas stream. A known fuel gas, namely biogas, may also contain siloxanes, which advantageously can be removed along with the sulphur-containing compounds in the process according to the invention.

In another embodiment, the process according to the invention may be used for the removal of intentionally included (entrained) ingredients, such removal being prior to combustion of the combustible gas stream. For example, tertiary butyl mercaptan and ethyl mercaptan are conventionally added to domestic natural gas, along with sulphides in the UK, to odorise the gas, and to thereby allow leaks to be detected. In continental Europe, the odorant tetrahydothiophene is also frequently used for this purpose. The process according to the invention may permit removal of such ingredients before combustion, thus resulting in cleaner exhaust gases with little or no content of corrosive oxides of sulphur in the exhaust gases.

In the step of the CLO process according to the invention in which the oxygen carrier is reoxidised to the first valence state, the oxidising gas is typically a gas stream containing oxygen (such as air) at elevated temperature. The reoxidation then takes place according to equation 2 as illustrated above (when the metal is copper).

S

Alternatively, the oxidising gas may comprise steam (especially superheated steam) instead of oxygen. In this case, the reoxidation -namely step (b) -proceeds according to the following equation: M+H2OMO+H2 6 By this means, it can be seen that the CLC process can generate hydrogen in the resultant combustible fuel gas, by using steam rather than oxygen as the oxidising gas. This method of hydrogen generation can be used with a mixed combustible fuel gas containing various combustible molecules including sulphur compounds; an example of such a mixed combustible fuel gas is a low grade kerosene.

It is possible in some embodiments of the invention to physically move the oxygen carrier from a first reactor (to which the hydrocarbon fuel gas has been supplied) to a second reactor for reaction (regeneration or re-oxidising) with the oxidising gas. However, in preferred embodiments of the invention, the oxygen carrier is employed in a fixed bed, and there is cyclical switching of the gas supply (alternating between supply of the combustible fuel gas and then of the oxidising gas) to the bed of oxygen carrier.

This latter approach readily lends itself to gas clean-up applications, because the technology of the process according to the invention can be completely scalable, with gas flow from as little as 10 sccm (standard cubic centimetres per minute) up to 1,000 slm. Because the reactions are "driven", with the equilibrium being overwhelmingly to the right, small reactors can remove impurities to the tens of ppb level even at high flows and high impurity concentrations up to the % level; this is not the case with more conventional gas purifiers which often rely on equilibrium adsorption during the removal process; such conventional gas purifiers are severely concentration and flow limited.

In the process according to the invention, the solid state oxygen carrier, which is generally in particulate form, typically comprises transition metal oxide particles. Preferred such oxides are those of one or more transition metals of Group VIIA, VIllA, lB or IIB of the Periodic Table of Elements, of which preferred oxides are those of the transition metals Fe (iron), Cu (copper), Ni (nickel), Mn (manganese) and/or Co (cobalt).

Such a transition metal oxide is optionally on a refractory inert support of, for example, at least one oxide of Group IIA, IVA, IIIB or IVB of the Periodic Table of Elements, or an oxide of a Lanthanide metal. Especially preferred such supports are one or more of alumina (A1203), titanium dioxide (hO2), silicon dioxide (SiC2), magnesium oxide (MgO), zirconium dioxide (Zr02), and a mixed oxide such as a silicate or aluminate.

The process according to the invention has the advantages that entrained H2S and organo-sulphur compounds may be removed from the gas stream, without the need for the oxygen carrier to be compositionally optimised for the particular mix of organo-suiphur compounds present. Operating temperatures of typically about 350°C (or in some cases not more than about 360°C) can ensure that harmful levels of residual H2S are minimised in the resultant stream of combustible gas. As indicated previously, temperatures of up to about 550°C (such as about 450°C) can be used when the combustible hydrocarbon fuel gas consists predominantly of methane.

When the process according to the invention is carried out on biogas, which in addition to having H2S at levels up to 10-100 ppm, can also contain siloxanes which can be substantially removed according to the invention, thereby avoiding forming silica in a subsequent oxidation step, which could otherwise coat catalyst surfaces and create wear in combustion based generators.

Another potential application of the process according to the invention is to use the ability to generate hydrogen directly from a CLC reactor without steam reforming and water gas shift reactions. This has the advantage of having an "in-built" desulphurisation step. However, whist all the individual steps of a CLC based hydrogen generator have been proven, to date, there are no examples of complete systems operating. Such a hydrogen generator could utilise low grade, high sulphur, feedstock not suitable for more conventional systems The present invention can result in improvements in overall efficiency and lowered emissions of CO2 and harmful sulphur oxides.

The present invention will now be illustrated in more detail with reference to Figures 2 and 3 of the accompanying drawings, which illustrate preferred embodiments of the invention.

Figure 2 shows a typical configuration for the removal of the added sulphur odorising compounds from domestic natural gas thereby allowing the gas to be used in a fuel cell to generate electricity. Inlet gas stream 10 passes through a heat exchanger 13 and enters a packed bed reactor 14 which is maintained at a temperature of about 380°C. The packed bed reactor 14 comprises two zones, a first zone 15 containing a chemical looping combustion material in cylindrical pellet form (typically of diameter about 3mm, length 5mm) and a second zone 17 containing calcium oxide particles of typical dimension 3-5mm.

The sulphur odorising compounds, which are at low concentrations <1 ppm, are oxidised stoichiometrically to sulphur dioxide, carbon dioxide and moisture in the CLC zone 15 and pass into the second zone 17 where they are captured by the calcium oxide at high capture efficiency. The gas 18 exiting from the packed bed reactor 14 exits via the heat exchanger 13 and is substantially tree of any sulphur compounds i.e. <0.01 ppm.

Figure 3 shows a typical arrangement for the removal of hydrogen sulphide from a gas stream 20 (which may be biogas from an anaerobic digester, typically comprising methane -60%, carbon dioxide -37%, moisture -2.5% and hydrogen sulphide -0.5%). Gas stream 20 enters compressor 21 and is compressed to a pressure of between 3 and 8 bar. The compressed gas passes through heat exchanger 23 and into packed bed reactor 24 held at a temperature of 40000 containing CLC material 25. During the transit of the bed 24 the hydrogen sulphide reacts stoichiometrically with the CLC material, thereby generating sulphur dioxide and moisture. This gas mixture passes through the heat exchanger 23 and into a second packed bed reactor 26, held at approximately ambient temperature, the packed bed reactor containing sodium bicarbonate 27. The sulphur dioxide is stoichiometrically captured by the sodium bicarbonate releasing carbon dioxide and moisture. The exit gas 28 is substantially free of hydrogen sulphide i.e. < 0.01 ppm. The invention is also suitable for regeneration.

Claims (4)

1. CLAIMS1. A chemical looping combustion process, which comprises: (a) feeding a fuel gas comprising hydrogen and/or at least one combustible hydrocarbon over a solid state oxygen carrier comprising a metal oxide, in which at least some of the metal is in a first, positive, valence state, so as to reduce at least some of the metal in the oxide to a lower valence state; and (b) re-oxidising the lower valence state of the metal back to the first valence state using an oxidising gas at elevated temperatures; wherein the fuel gas contains at least one combustible sulphur compound and the fuel gas is fed over the oxygen carrier in step (a) under conditions such that the at least one sulphur compound reduces the metal oxide and is oxidised thereby, while the fuel gas is substantially unoxidised.
2. 2 A process according to claim 1, wherein step (a) is carried out a temperature not exceeding the light off temperature (as defined herein) of hydrocarbons known to constitute the combustible fuel gas.
3. 3 A process according to claim 1 or 2, wherein the oxygen carrier is in particulate form, and comprises particles of at least one transition metal oxide.
4. 4. A process according to claim 3, wherein the transition metal is of Group VIIA, VIllA, lB or IIB of the Periodic Table of Elements.

   A process according to claim 4, in which the transition metal is Fe (iron), Cu (copper), Ni (nickel), Mn (manganese) and/or Co (cobalt).

   6 A process according to any of claims 1 to 5, in which the oxygen carrier is on a refractory inert support 7. A process according to claim 6, wherein the support comprises an oxide of at least one element of Group IIA, IVA, IIIB or IVB of the Periodic Table of Elements, or an oxide of a Lanthanide metal.8. A process according to claim 7, wherein the support comprises one or more of alumina (A1203), titanium dioxide (TiC2), silicon dioxide (SiC2), magnesium oxide (MgO), zirconium dioxide (Zr02), and a mixed oxide.9. A process according to any of claims 1 to 8, wherein the fuel gas comprises biogas, natural gas containing methane, or liquid petroleum gas containing propane and/or butane.10. A process according to any of claims 1 to 9, wherein the oxygen carrier is employed in a fixed bed, and steps (a) and (b) aie performed in alternating sequence to the fixed bed of the oxygen carrier.11. A process according to any of claims 1 to 10, wherein the oxidising gas comprises oxygen oi superheated steam.12. A process according to any of claims ito 11, the exit gas from step (a) is passed through a basic material in order to trap oxides of sulphur from the exit gas.