POWER PLANT

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of priority of Singapore patent application No. 10201510688V filed on 28 December 2015, the content of which is incorporated herein by reference in its entirety for all purposes.

Field of Invention This invention relates to electrolyzing/co-electrolyzing water and carbon dioxide in the flue gas emitted by fossil fuel power plants to produce synthetic gas as a means of storing excess power generated by renewable energy resources and/or fossil fuel power plants. At the same time, a by-product of this process, high purity oxygen, can be fed back into the fossil fuel power plant to increase the combustion efficiency.

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

In recent years, fossil fuels are being replaced by renewable resources at an accelerated rate to reduce carbon emissions and at the same time to minimize the depletion of said fossil fuels. A balance between the supply and demand of electrical energy is a fundamental issue faced by making more use of renewable resources to provide domestic and commercial power. For example, solar power and wind power are intermittent because of fluctuations in the amount of sunlight and wind received in a particular region - and the peak production periods for these resources may differ significantly from the peak usage periods. As for power plants (e.g. fossil fuel power plants), the power produced is always higher than the power consumption at any given time in order to cater for any changes in demand for power (such as a surge in demand caused by advert breaks in a live television event). It would be useful to efficiently store the excess energy so that it can be used when there is greater demand. A possible environmentally-friendly method to store energy in the form of fuel may be through the electrolysis/co-electrolysis of water and carbon dioxide. In such a system, excess power from renewable energy sources and power plants can be channelled to electroiyzers coupled to power plants emitting flue gas that contains steam and carbon dioxide. However, flue gas from fossil fuel power plants consists of not only steam and carbon dioxide, but also contains other ingredients such as excess oxygen, sulfur dioxide, nitrogen, nitrous oxide and traces of other gases, all of which may act as potential contaminants to an electrolyzer. Thus, it is essential to keep the fuel electrode stable in the presence of these additional gases - especially excess oxygen and sulfur dioxide.

The development of low-temperature water electrolysis technologies such as alkaline and proton exchange membrane based water electroiyzers have been ongoing for many years (Electrochimica Acta, 2013, 100(0), 249-256, Chemical Engineering Science, 2013, 98(0), 282-290). High-temperature steam electrolysis using solid oxide electrolyzer cells (SOECs), which is the reverse of solid oxide fuel cells (SOFCs), has gained much attention lately due to it providing advantages over low-temperature electrolysis for its excellent efficiency in hydrogen production (28-39 kWh/kg H2) (IEA, April 2007, Energy Technology Essentials (ETE 05)). One of the potential applications of SOECs is to co-electrolyze steam and carbon dioxide in the flue gas emitted by power plants running on natural gas. However, like many other hydrocarbon fuels, natural gas contains excess oxygen and sulfur (in the form of 1- 3ppm H ₂ S), which causes oxidation and poisoning of the state-of-the-art nickel-yttria- stabilized zirconia (Ni-YSZ) fuel electrode used in SOECs. Using high resolution X-ray computed tomography, Shearing et al. have observed volume changes associated with the redox-cycling of nickel-based fuel electrodes in solid oxide cells, which can cause significant microstructure degradation (Solid State Ionics, 2012, 216, 69-72). This will cause severe degradation in performance of such SOECs coupled with power plants. The performance loss of solid oxide fuel cells in sulfur-containing fuels depends on two factors (Journal of Power Sources, 2007, 168(2), 289-298):

1 ) physical adsorption/chemisorption of H ₂ S/S0 ₂ at surface-active sites (triple- phase boundaries), which causes the reduction of the active surface area available for catalytic activity; and

2) sulfidation of the fuel electrode material caused by the reaction between sulfur and the fuel electrode material (e.g. Ni + S— NiS), resulting in the loss of catalytic activity and degradation.

The form and reactivity of sulfur species can have a significant effect on the sulfur poisoning of SOFC fuel electrodes. At elevated temperatures (i.e. -1000K), the reactions of H ₂ S found in hydrocarbon fuels and their reversible potentials are given as: H ₂ S + 30 ^2" <→ H ₂ 0 + S0 ₂ + 6e (E° = +0.785V) (1)

H ₂ S + O ^2" *→ H ₂ 0 + S + 2e (E° = +0.761V) (2)

H ₂ S H H ₂ + S (3)

S + 20 ^2" <→ S0 ₂ + 4e ^" (E° = +0.748V) (4)

H ₂ + 0 ^2~ → H ₂ 0 + 2e (E° = +0.998V) (5)

(Journal of The Electrochemical Society, 1998, 145(5), 449-1454) In the flue gas coming from a power plant running on natural gas, most of the H ₂ S will be converted into S0 ₂ . Although S0 ₂ is considered to be less adverse for SOFC fuel electrodes, as compared to H ₂ S, due to its thermodynamic stability, it still has a significant effect on SOFC operation and degradation. A study by atsuzaki et al. indicated that, as operating temperature decrease, the minimum amount of H ₂ S needed to cause a significant degradation of a Ni-YSZ electrode also decreases, while an increase in performance loss was observed at an intermediate temperature range below 1123 K for H ₂ S concentrations of from 0.2 to 20 ppm (Journal of The Electrochemical Society, 2000153(11), A2023-A2029). Bartholomew et al. further noted that adsorbed sulfur is more stable than bulk sulfide (sulfidation) at intermediate temperatures and for low H ₂ S concentrations (~10 ppm), where as much as 90% surface coverage may occur (Advances in Catalysis, 1982, 31 , 135-242, Applied Catalysis A: General, 2001 , 212, 17-60). Pujare et al. were the first to use thiospinel sulfides as electrocatalytic fuel electrodes for H ₂ S oxidation on a H ₂ S/air fuel cell (Journal of The Electrochemical Society, 1987, 134(10), 2639-2640). Using CuFe ₂ S ₄ as the fuel electrode material, an initial open circuit voltage (OCV) of 1.04 V at 1173K was achieved, which is much higher than the theoretical OCV for H ₂ S oxidation. The same group later discovered that NiFe ₂ S ₄ , WS ₂ and CuCo ₂ S ₄ are the most active fuel electrode electrocatalysts (Journal of The Electrochemical Society, 1989, 136(12), 3662-3678). These results show the potential of using thiospinel materials as the fuel electrode in SOCs with sulfur-containing fuels. Liu et al. used metal sulfides (WS ₂ , CoS ₂ ) as fuel electrode materials to improve the performance of H ₂ S oxidation in SOCs (Journal of The Electrochemical Society, 2003, 150(8), A1025-A 029). They examined several composite sulfides and found that when Fe, Co, or Ni was doped into MoS ₂ , the sulfide volatility was reduced at high temperatures and their catalytic activity was comparable to those achieved by Pt electrodes. Scandia-doped zirconia (SSZ) was investigated as a possible zirconia-based electrolyte in SOFCs due to its excellent ionic conductivity {Chemical Reviews, 1970, 70(3), 339-376). A further study led to the finding that a rise in the output voltage could be achieved by an SOFC using a Ni-YSZ fuel electrode in the presence of H ₂ S at 5 ppm concentration and at 1073K, with the substitution of YSZ with SSZ as electrolyte {Journal of The Electrochemical Society, 2006, 153(11 ), A2023-A2029). These findings show that an increase in ionic conductivity may improve sulfur tolerance of a Ni-cermet electrode. Due to the good performance and low cost of doped and undoped ceria oxides, they are commonly used as sulfur-tolerant materials in metal cermet electrodes. High sulfur tolerance and good electro-catalytic performance can be achieved by applying a combination of Cu, Ce0 ₂ and YSZ in SOC fuel electrodes {Electrochemical and Solid-State Letters, 2005, 8(6), A279-A280). When Ce0 ₂ is used together with a Ni-cermet electrode, sulfur poisoning of the Ni is suppressed, indicating that Ce0 ₂ is effective in absorbing H ₂ S in SOFCs {Science, 2006, 312, 3). However, there is a challenge in the application of Cu-ceria electrodes to SOFC systems operating at high temperatures with hydrocarbon fuels due to the agglomeration of Cu particles. In another study, composite Ni electrodes with gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC) both exhibited higher performance and a remarkable sulfur tolerance in comparison to pure Ni-YSZ electrodes (Aravind, "Proceedings of the 9th International Symposium on Solid Oxide Fuel Cells." 6, 2005).

In recent years, mixed ionic and electronic conductor (MIEC) oxides with perovskite structures have received increasing interest as sulfur tolerant fuel electrode materials due to their good ionic and electronic conductivity at high temperatures and in reducing environments. Munkundan et al. has found that the AB0 ₃ structure of perovskite is less reactive with H ₂ S than Ni-based materials {Electrochemical and Solid-State Letters, 2004, 7(1 ), A5-A7). However, few reported perovskites exhibit both good sulfur tolerance and catalytic activity for H ₂ oxidation equivalent to that provided by Ni-doped ceria or Ni-YSZ electrodes. Thus, these materials have found limited use to date.

Lanthanum-doped strontium titanate (LST) has been investigated as a potential material for SOC fuel electrodes due to its high electronic conductivity and stability in reducing environments amongst different perovskite materials. LST exhibits good conductivity at high operating temperatures, but it does not exhibit comparable electro-catalytic performance to Ni-cermet electrodes for H ₂ oxidation. It was found that a maximum conductivity of 360 Scm ^"1 was achieved at 1273K when the occupancy of lanthanum approaches 0.4 in the A- site (Solid State Ionics, 2002,149(1-2), 21-28). In order to promote the catalytic performance of LST, recent studies have found that low-level doping of Mn and Ga and over-percolation doping of Mn on the B-site can be done to provide a useful enhancement in electrocatalytic performance of LST (Journal of The Electrochemical Society, 2006, 153(4), D74-D83; and Nature, 2006, 439(7076), 568-57 ).

In a study using strontium doped lanthanum vanadate (La^Sr _x VOs) as a sulfur tolerant fuel electrode in SOCs, good stability in a 48hr testing period and superior performance with fuels containing high H ₂ S concentrations (5-10%) were discovered (Journal of Power Sources, 2004, 135(1-2), 17-24). However, its performance with pure H ₂ or hydrocarbon fuels is relatively low because SrV0 ₃ in LSV tends to favour H ₂ S oxidation instead of H ₂ or other hydrocarbons. Trembly et al. developed a solution for optimizing the usage of LSV due to its insufficient activity by applying LSV as a current collector outside the Ni-YSZ layer, which is exposed to the fuel environment (Journal of Power Sources, 2006, 158(1 ), 263-273). In this case, the high conductivity and high H ₂ S oxidation rate of LSV yields the advantages of forming a sulfur-tolerant layer, by removing excess H ₂ S, and conducting electrons to the Ni-YSZ electrode at the same time.

Strontium doped lanthanum chromate (LSC) has been modified to be used as a fuel electrode in SOCs for methane oxidation due to its better stability and sulfur tolerance than a Ni electrode. However, LSC has a rather low conductivity in reducing environments, so transition metal cations with lower coordination numbers (such as Mn, Co, Fe or Ni) have been doped onto the B-site to improve its catalytic activity and ionic conductivity (Science, 2006, 312(5771 ), 254-257, Journal of Catalysis, 2001 , 202, 229-244). (Lao.75Sro. ₂ 5)o. ₉ Cro. ₅ Mno.5 (LSCM) with its Mn dopant exceeding percolation level (33%) has been found to exhibit comparable performance as a Ni-YSZ electrode in fuels containing H ₂ and methane (Science, 2006, 312(5771 ), 254-257). Testing for sulfur-tolerant applications has been conducted for both LSC and LSCM. It has been found that the sulfur tolerance decreases as Mn content increases, while the opposite trend is found for fuel electrode performance (Electrochemical and Solid-State Letters, 2004, 7(1 ), A5-A7).

Tao et al. (Journal of The Electrochemical Society, 2004, 151 (2), A252-A259) discovered that LSCM is compatible with YSZ to at least 1573 K and good performance has been achieved using LSCM as a fuel electrode in wet H ₂ . The electrode polarization may be further reduced by coating a thin layer of GDC between the LSCM electrode and YSZ electrolyte or when graded electrode is applied. Good performance is achieved for methane oxidation without using excess steam and stable performance is recorded for at least 4 h operation in wet methane.

Zha et al. (Journal of Solid State Chemistry, 2005, 178(6), 1844- 850) noted that all different LSCM compositions have excellent tolerance to redox conditions but its crystal structure transforms from hexagonal in air to orthorhombic in H ₂ . The sulfur resistance of LSCM increases with the increase of Cr content whereas the conductivity increases with the increase of Mn content. Sulfur tolerance decreases as Mn content increases due to the poisoning effects by La ₂ 0 ₂ S and MnS impurity phases. Fuel cells using LSCM as fuel electrode exhibit very good performance when using pure H ₂ as the fuel, yielding a peak power density of 375 mWcm ^"2 at 1223 K.

Chen et al. discovered that the Lao.75Sro.25Cro.5Mno.5O3/Gdo. ₂ iCeo.8OL9 (LSCM-GDC, 50:50 wt.%) composite electrode exhibits much better hydrocarbon stability and sulfur tolerance under anodic conditions than Ni-based electrodes (Journal of The Electrochemical Society, 2007, 154(1 1 ), B1206-B1210). After cell stabilization for 10 h, the cell only suffers a small degradation of 0.017%/h for the subsequent 110 h period at a constant potential of -0.5 V when subjected to sulfur-containing methane fuel. Yang et al. (Journal of Materials Chemistry, 2008, 18(20), 2349-2354) found that LSCM may be used for the fuel electrode in high temperature SOECs when cells operate in atmospheres with a low content of H ₂ . Considerably improved performance was observed compared to Ni-based electrodes in low H ₂ conditions in electrolyzer mode. In other work, Yue et al. (Solid State Ion., 225, 131-135; Electrochemical and Solid State Letters, 15(3), B31-B34) suggested that LSCM/GDC is a promising fuel electrode for carbon dioxide electrolysis.

None of the above literature discusses the use of a SOEC to directly electrolyze/co- electrolyze the flue gas emitted by a fossil-fuel power plant. Thus, there remains a need for materials and systems that can be used to efficiently capture the feedstocks contained within the flue gas of a fossil fuel power plant, which contains contaminants such as sulfur oxides and oxygen, which may affect the performance of materials used in an SOEC.

Summary of Invention

It has been surprisingly found that electrolyzer cells, particularly solid oxide electrolyzer cells, can be used to produce synthetic gas and high purity oxygen from flue gas containing sulfur oxides and nitrogen oxides emitted continuously from a fossil fuel power plant/furnace. This discovery enables excess power generated from renewable energy resources and power plants (e.g. fossil fuel power plants) to be stored during off-peak hours in the form of syngas, which can then be converted into energy-dense feedstocks, such as methane (or hydrogen gas separated out from the syngas), or directly used to provide energy itself during peak periods of power consumption.

In a first aspect of the invention, there is provided a fossil fuel flue gas energy recovery system, comprising:

an electrolyzer cell;

an energy source to provide power to the electrolyzer cell; and

a storage means or apparatus for storing syngas, wherein

the electrolyzer cell is operable in an unscrubbed fossil fuel flue gas where sulfur oxides and nitrogen oxides are present.

In embodiments of the first aspect of the invention:

(a) the electrolyzer cell may be operable for at least 75 (e.g. at least 100, at least

5,000 or, more particularly, at least 10,000) hours in an unscrubbed fossil fuel flue gas where sulfur oxides and nitrogen oxides are present (e.g. from 10,000 hours to 50,000 hours);

(b) the electrolyser cell may be fitted to an industrial plant where it is in fluid communication with unscrubbed fossil fuel flue gas emissions;

(c) the electrolyzer cell may be selected from the group consisting of solid oxide electrolyzer cells, proton exchange membrane electrolyzers, and alkaline electrolyzers (e.g. the electrolyzer cell may be a solid oxide electrolyzer cell which comprises a fuel electrode arranged to be operable in a fossil fuel flue gas, an electrolyte and an air electrode, optionally:

(i) the fuel electrode may be selected from the group consisting of strontium and manganese doped lanthanum chromate (LSC ), lanthanum-doped strontium titanate (LST), and strontium doped lanthanum vanadate (LSV) mixed with doped ceria, optionally wherein the fuel electrode further comprises platinum as a dopant in an amount of from 0.1 wt.% to 10.0 wt.%, such as 3.0 wt.% (e.g. the doped ceria may be one or more of the group consisting of gadolinium doped ceria (GDC), samarium doped ceria (SDC), and yttria doped ceria (YDC), optionally the fuel electrode may be a composite material comprising LSCM and one or both of GDC and YDC);

(ii) the fuel electrode may be a composite material comprising LSCM and GDC in a weight percentage ratio of LSCM.GDC of from 90:10 to 50:50 (e.g. the electrolyter, may be selected from one or more of the group selected from yttria- stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), gadolinium doped ceria (GDC), samarium doped ceria (SDC), and yttria doped ceria (YDC));

(iii) the air electrode is selected from the group consisting of strontium doped lanthanum manganite (LSM), strontium doped lanthanum cobaltite and ferrite (LSCF), where the LSCF is mixed with one or more of the group consisting of yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), gadolinium doped ceria (GDC), samarium doped ceria (SDC), yttria doped ceria (YDC);

(d) the energy source may be one or more of a power plant and a renewable energy source, optionally wherein the energy source is provided by excess power generated from the power plant and the renewable energy source (e.g. the power plant may be one or more of a fossil fuel power plant, a nuclear power plant and a geothermal power plant. The renewable energy sources may be any one or more of hydroelectric, solar, wind, marine, osmosis and biomass energy);

(e) the system further comprises a means or apparatus to convert the syngas obtained from the electrolyzer cell into one or more of hydrogen gas, ammonia, methanol and synthetic hydrocarbons

(f) when the electrolyzer cell is a solid oxide electrolyzer cell which comprises a fuel electrode arranged to be operable in a fossil fuel flue gas, an electrolyte and an air electrode and the system is attached to an industrial plant with fossil fuel flue gas emissions, the system may further comprise a means or apparatus to channel back the oxygen generated by the solid oxide electrolyzer cell to the plant to enhance the combustion efficiency;

(g) the electrolyzer cell may have a tubular or planar design.

In a second aspect of the invention, there is provided a process for producing syngas from an unscrubbed fossil fuel flue gas where sulfur oxides and nitrogen oxides are present, comprising the use of the system according to the first aspect of the invention and any technically sensible combination of its embodiments.

In a third aspect of the invention, there is provided a process for manufacturing a system according to the first aspect of the invention and any technically sensible combination of its embodiments, wherein, when the electrolyzer cell is a solid oxide electrolyzer cell, the process comprises:

(a) synthesizing the fuel electrode material by sol-gel or combustion synthesis; and/or (b) fabricating the electrolyzer cell by dip-coating, dry-pressing, tape casting, extrusion forming or screen-printing.

Further aspects and embodiments of the invention are provided below.

1. The concept of energy storage disclosed herein relates to storing excess off-peak power generated from renewable resources and power plants (not limited to fossil fuel power plants) by converting this excess power to fuel utilizing the flue gas emitted from fossil fuel power plants. By coupling electrolyzers, in particular SOECs, with an energy source (e.g. renewable energy resources and/or a power plant's excess off-peak power) with the flue gas of a fossil fuel power plant, several beneficial outcomes can be achieved simultaneously as described herein. For example, a system as described herein may use an electrolyzer cell, for example but not limited to solid oxide electrolyzer cells (SOECs), to electrolyze/co-electrolyze the water and carbon dioxide in the flue gas to produce synthetic gas (H ₂ and CO).

2. In one example, the solid oxide electrolyzer cell (SOEC) comprises of a fuel electrode, an electrolyte, and an air electrode. The SOEC is connected at the fuel and air electrodes to complete the circuit. Flue gas from fossil fuel power plants can be fed into the fuel electrode side. After electrolyzing/co-electrolyzing water and carbon dioxide in the flue gas from the fossil fuel power plant (or other combustion means or apparatus, such as a furnace), a fuel mixture comprising hydrogen and carbon monoxide is released from the fuel electrode, while pure oxygen is released from the air electrode. 3. For this application, the fuel electrode should be able to operate under an oxidizing atmosphere with the presence of sulfur contaminants.

4. A preferred fuel electrode may be strontium and manganese doped lanthanum chromate (LSCM) or lanthanum-doped strontium titanate (LST) or strontium doped lanthanum vanadate (LSV) mixed with doped ceria (Gd doped ceria, Sm doped ceria, Y doped ceria) but not limited to these materials.

5. The fuel electrode material may be synthesized by chemical synthesis, such as sol-gel, or combustion synthesis but not limited to these processes. 6. A preferred electrolyte may be yttria-stabilized zirconia (YSZ) or scandia-stabilized zirconia (SSZ) or gadolinium doped ceria (GDC) or samarium doped ceria (SDC) or yttria doped ceria (YDC) or a mixture of these materials. 7. A preferred air electrode may be strontium doped lanthanum manganite (LSM) or strontium doped lanthanum cobaltite and ferrite (LSC, LSCF) mixed with yttria-stabilized zirconia (YSZ) or scandia-stabilized zirconia (SSZ) or gadolinium doped ceria (GDC) or samarium doped ceria (SDC) or yttria doped ceria (YDC) but not limited to these materials.

8. The structure of the electrolyzer may be tubular or planar but not limited to these designs.

9. The solid oxide electrolyzer cell may be fabricated by dip-coating, dry-pressing, tape casting, extrusion forming or screen printing but not limited to these processes.

10. The gas fed into the fuel electrode chamber for co-electrolysis is flue gas from power plants.

Drawings

In order to demonstrate the feasibility of the invention for real life practical applications, we provide non-limiting example embodiments of the present invention, where the descriptions are accompanied by illustrative drawings. Fig. 1 illustrates the overall process of storing excess power from renewables and power plants by co-electrolyzing water and carbon dioxide in the flue gas using electrolyzers, such as SOECs.

Fig. 2 illustrates the (a) current-voltage curves and (b) impedance spectra of half cells with different fuel electrode materials for H ₂ 0 electrolysis.

Fig. 3 illustrates the (a) current-voltage curves and (b) impedance spectra of half cells with different LSC to GDC ratios for H ₂ 0 electrolysis. Fig. 4 illustrates the comparison of (a) current-voltage curves, (b) impedance spectra and (c) cell durability (tolerance) with different sulfur compositions between the optimized LSC - GDC (50:50 wt.%) and Ni-YSZ (60:40 wt.%) fuel electrode for H ₂ 0 electrolysis. Fig. 5 illustrates the comparison of (a) current voltage curves and (b) cell durability (tolerance) with different sulfur compositions based on the optimized LSCM-GDC (50:50 wt.%) fuel electrode for co-electrolysis of H ₂ 0 and C0 ₂ .

Fig. 6 illustrates the (a) current-voltage curve with 0 ₂ ; (b) impedance spectra without 0 ₂ ; and (c) impedance spectra with 0 ₂ for the optimized LSCM-GDC (50:50 wt.%) fuel electrode for co-electrolysis of H ₂ 0 and C0 ₂ . Fig. 7 illustrates the comparison of (a) current-voltage curves with 6.5% 0 ₂ ; (b) current- voltage curves without 0 ₂ ; and (c) impedance spectra without 0 ₂ between the impregnated and non-impregnated fuel electrode for co-electrolysis of H ₂ 0 and C0 ₂ .

Fig. 8 illustrates the SEM images for the microstructure of (a) LSCM, (b) LSCM-GDC, (c) LSCM-YSZ and (d) LSCM-(GDC-YSZ) at their electrode/electrolyte interface.

Description

This invention seeks to address at least one of the problems noted hereinbefore for existing technologies. In general terms, we disclose a system to store excess off-peak power generated from renewable resources and power plants (not limited to fossil fuel power plants) by converting this excess power to fuel using flue gas emitted from fossil fuel power plants. One of the possible ways to achieve this is by using electrolyzers, in particular but not limited to solid oxide electrolyzer cells (SOECs).

The current invention enables energy storage by utilizing the flue gas emitted by fossil fuel power plants. Flue gas from power plants contains moisture and carbon dioxide, which are useful for the purpose of energy storage, along with several other gases which include but are not limited to nitrogen, excess oxygen, sulfur dioxide and nitrous oxide. By coupling an electrolyzer, in particular an SOEC, with renewable energy resources and/or fossil fuel power plants in order to utilize their flue gas emitted as well as excess off-peak power, many beneficial outcomes can be achieved simultaneously.

1) Energy storage

Main components of the flue gas from fossil fuel power plants include C0 ₂ and H ₂ 0, which can be electrolyzed/co-electrolyzed in a SOEC to produce synthetic gas (H ₂ and CO). H ₂ 0 + C0 ₂ + excess power→ (H ₂ + CO) + 0 ₂

The produced syngas can be further converted into synthetic hydrocarbon fuels, for example, with the Fischer-Tropsch method.

2) Reduction of carbon footprint

In 2012 alone, 13.37 billion tons of C0 ₂ was emitted globally for the purpose of electricity and heat production (IEA statistics, 20 4). The integration of electrolyzers, such as SOECs, with fossil fuel power plants would be one of the most practical solutions to reduce C0 ₂ emissions in the atmosphere and at the same time producing high quality synthetic gas.

3) Removal of SO _x and sulfur tolerance capability

Electrolyzers, such as SOECs, are similar to an oxygen pump which transfer oxygen ions from the fuel electrode to the air electrode through the electrolyte. Sulfur oxide may be split electrochemically into sulfur and oxygen, and sulfur may react with carbon to form C ₂ S or with hydrogen to form H ₂ S. This option can be achieved by using a sulfur tolerant electrode where the co-electrolysis of water and carbon dioxide as well as the desulfurization of flue gas will happen simultaneously. 4) Removal of NO _x (deNO _x )

In relation to this case, nitrous oxide compounds (NO _x ) can be removed by splitting them into nitrogen and oxygen at the fuel electrode. Most nitrous oxide compounds from fossil fuel power plants exist as NO and N0 ₂ . The possible routes of splitting NO and N0 ₂ are as follows:

I. 2N0 ₂ → 2NO + 0 ₂

II. 2NO→ N ₂ + 0 ₂

III. 2N0 ₂ -→N ₂ + 20 ₂ 5) Oxygen recycling

During the co-electrolysis reaction, syngas is produced at the fuel electrode side while oxygen is derived at the air electrode side. Oxygen, which is a precious commodity, has many potential uses. One of its uses is that the oxygen produced can be reused by mixing with air to give 0 ₂ -rich air or to directly use the pure oxygen, which in both cases, can be channeled back to the thermal power plants to enhance their combustion efficiency. In the case of oxy-combustion, the flue gas contains no nitrogen which would increase the concentrations of C0 ₂ and H ₂ 0 for higher co-electrolysis efficiency, without further formation of the acid gas NO _x during the combustion process.

6) Reusing waste heat

Typically all types of power plants produce a significant amount of heat as a byproduct. Usually, this heat possesses no value and will be discharged to the environment. In this novel concept, this waste heat can be utilized to heat up the electrolyzers, in particular but not limited to SOECs, to their optimum operating temperature. Compared with the existing technologies, the current invention has the following advantages:

1. Achievement of several beneficial outcomes simultaneously, i.e. energy storage, reduction of carbon footprint, removal of NO _x and SO _x compounds, recycling of oxygen and reusing waste heat as described earlier in this section.

2. Direct use of flue gas from fossil fuel power plants for the electrolysis/co-electrolysis of water and carbon dioxide, which is highly efficient.

3. Low operating cost due to the direct use of flue gas from fossil fuel power plants.

4. The energy required for electrolysis/co-electrolysis comes from the excess power of renewable energy resources and power plants, which energy can be stored efficiently. 5. The electrolyzer in the invention is a solid structure, so it has minimum danger of leakage, corrosion and explosion.

6. The LSCM-based fuel electrode material for SOECs used in this invention is stable under an oxidizing environment and has much higher sulfur tolerance than the state-of- the-art Ni-YSZ fuel electrode material.

Thus, the invention relates to a fossil fuel flue gas energy recovery system, comprising an electrolyzer cell, an energy source to provide power to the electrolyzer cell and a storage means or apparatus for storing syngas, where the electrolyzer cell is operable in an unscrubbed fossil fuel flue gas where sulfur oxides and nitrogen oxides are present.

It will be appreciated that the electrolyzer cell is intended to be operable in an unscrubbed fossil fuel flue gas to generate syngas.

When used herein "comprising" is intended to be an open-ended term that covers the closed terms "consists of and "consists essentially of. It is specifically contemplated that the open- ended term "comprising" may be replaced by the closed terms "consists of or "consists essentially of at each occurrence thereof - and vice versa. As discussed hereinbefore, an advantage of the system described above is that it can be used (i.e. is "operable") for extended periods of time while in contact with an unscrubbed fossil fuel flue gas. The term "extended periods of time", relates to a time where the electrolyzer cell is operated in a fossil fuel flue gas for at least 75 hours or at least 00 hours, such as 1 ,000 hours, such as at least 5,000 hours or, more particularly 10,000 hours. For example, the system may be capable of operating in a fossil fuel flue gas for from 10,000 hours to 100,000 hours (e.g. from 15,000 to 50,000 hours or, alternatively from 75 to 1 ,000 hours, such as from 80 to 250 hours, such as from 90 to 100 hours.). It is expected that a SOFC system according to the current invention would be able to operate for as long as 50,000 hours so that it can be integrated into and/or compete with traditional solutions for power generation. However, the solution provided by the current invention is not only for power generation, but is mainly for enabling the full use of excess power and carbon footprint reduction. As such, it is expected that the system would be able to operate for at least 10,000 hours.

As will be apparent, the system is intended to be attached to an industrial plant that produces unscrubbed fossil fuel flue gas emissions. Such plants may include power stations, or the generation of heat or steam for use in a plant associated with the manufacture of paper, food, petroleum, chemicals, and metal/mineral products. For example, industrial plants in the cement industry have to heat up limestone to 1450°C as part of the process of making cement, which is done by burning fossil fuels to create the required heat. As such, the system may be integrated into an industrial plant in such a way that the electrolyser cell is in fluid communication with unscrubbed fossil fuel flue gas emissions.

As will be appreciated, any suitable electrolyzer cell may be used herein, provided that can operate in an unscrubbed fossil fuel flue gas for an extended period of time (i.e. at least 1 ,000 hours, such as at least 5,000 hours, such as at least 10,000 hours). Suitable electrolyzer cells may be selected from, but are not limited to, the group of solid oxide electrolyzer cells, proton exchange membrane electrolyzers, and alkaline electrolyzers. The structure of the electrolyzer may be tubular or planar in design, though any suitable shape and construction may be used.

A particular electrolyzer cell that may be mentioned in embodiments of the invention is a solid oxide electrolyzer cell which may comprise a fuel electrode arranged to be operable in a fossil fuel flue gas, an electrolyte and an air electrode. In use, the flue gas from a fossil fuel power plant is injected into the fuel electrode side. After co-electrolyzing water and carbon dioxide present in the flue gas, the fuel mixture which then contains hydrogen and carbon monoxide will be released from the fuel electrode and pure oxygen will be released from the air electrode. For this application, the fuel electrode of the solid oxide electrolyzer cell should be able to operate under an oxidizing atmosphere with the presence of sulfur contaminants. Examples of materials to make such fuel electrodes include, but are not limited to, strontium and manganese doped lanthanum chromate (LSCM), lanthanum-doped strontium titanate (LST), and strontium doped lanthanum vanadate (LSV) mixed with doped ceria. Suitable doped ceria for use herein includes, but is not limited to gadolinium doped ceria (GDC), samarium doped ceria (SDC), yttria doped ceria (YDC), and mixtures thereof. In certain embodiments, the fuel electrode may further comprise platinum as a dopant in an amount of from 0.1 wt.% to 10.0 wt.%, such as 3.0 wt.%. In particular embodiments of the invention, the fuel electrode of the solid oxide electrolyzer cell may be a composite material comprising LSCM and one or both of GDC and YDC. For example, the fuel electrode may be a composite material comprising LSCM and GDC in a weight percentage ratio of LSCM:GDC of from 90:10 to 50:50, optionally further comprise platinum as a dopant in an amount of from 0.1 wt.% to 10.0 wt.%, such as 3.0 wt.%. A particular fuel electrode that may be mentioned herein may essentially consist of LSCM and GDC in a weight percentage ratio of LSCM:GDC of 50:50, optionally the composition may further contain platinum as a dopant in an amount of from 1.0 wt.% to 5.0 wt.%, such as 3.0 wt.%. The electrolyte of the solid oxide electrolyzer cell may be yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), gadolinium doped ceria (GDC), samarium doped ceria (SDC) or yttria doped ceria (YDC) or a mixture of these materials.

The air electrode of the solid oxide electrolyzer cell may include, but is not limited to, strontium doped lanthanum manganite (LSM), strontium doped lanthanum cobaltite and ferrite (LSCF), where the LSCF is mixed with one or more of yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), gadolinium doped ceria (GDC), samarium doped ceria (SDC) or yttria doped ceria (YDC). It will be appreciated that mixtures of these materials may be used as the air electrode.

A SOFC device that may be mentioned in embodiments of the current invention includes one a fuel electrode that may be a combination of La ₀ .75Sro.25Cro. ₅ Mno.5 0 ₃ . ₅ with Gdo.2Ce ₀ .80 ₂ -5 (LSC -GDC), an electrolyte that may be Y ₂ 0 ₃ -Zr0 ₂ (YSZ) and an air electrode that may be La0.6Sr0.4Co0.2Fe0.8 0 _3-δ mixed with Gdo.2Ceo.eO2.* (LSCF-GDC).

As is noted herein, the system includes an energy source, which may be any suitable source that can provide power to the electroiyzer cell. A suitable energy source may include a power plant, a battery or a renewable energy source. Additionally or alternatively, the energy source may be provided by excess power generated from the power plant and the renewable energy source. The power plant can be any of fossil fuel power plant, nuclear power plant and geothermal power plant. The renewable energy sources can be any of hydroelectric, solar, wind, marine, osmosis and biomass energy.

It will be appreciated that as the system described herein provides syngas, that this product may be utilised downstream. For example, the system may include a means or apparatus to convert the syngas obtained from the electroiyzer cell into one or more of hydrogen gas, ammonia, methanol and synthetic hydrocarbons. This may be achieved in any suitable manner according to known techniques, processes and apparatus, which may be integrated into the system described herein.

In addition to the generation of syngas, the system also provides pure oxygen as a by- product. The oxygen may be vented to the atmosphere or may be captured by a suitable means or apparatus connected to the system - whereupon it may be stored or sold. Alternatively or additionally, when the system is attached to an industrial plant with fossil fuel flue gas emissions, the system may further include a means or apparatus to channel back at least part of the oxygen generated by the solid oxide electroiyzer cell to the plant to enhance combustion efficiency of the source of the fossil fuel flue gas emissions.

It will be appreciated that the system described hereinbefore is intended to be used to produce syngas from an unscrubbed fossil fuel flue gas where sulfur oxides and nitrogen oxides are present.

As mentioned herein, the system includes an electroiyzer cell which may be a solid oxide electroiyzer cell. There is also disclosed herein a method for manufacturing said solid oxide electroiyzer cell, which process comprises:

(a) synthesizing the fuel electrode material by sol-gel or combustion synthesis; and/or

(b) fabricating the electroiyzer cell by dip-coating, dry-pressing, tape casting, extrusion forming or screen-printing. It will be appreciated that the materials used in the processes above will be suitable precursor materials for the solid oxide electrolyzer cells described hereinbefore. The current invention will now be described in further detail by way of non-limiting examples.

Examples

The supported electrolyte is fabricated by dry pressing, and the electrodes are printed (i.e. screen-printed) onto the electrolyte. The electrolyte is fabricated from YSZ, the fuel electrode is fabricated from the composite of LSCM with GDC, YSZ or both and the counter and reference electrodes are fabricated from platinum. The fabrication processes are as follows:

1. LSCM perovskite powder was synthesized by a combined citrate and EDTA complexing (Sol-gel) method. EDTA, which is a complexing agent, was added into NH ₃ .H ₂ 0 while maintaining the solution at 353 K with constant stirring. Necessary amounts of the different nitrate salts, Cr(N0 ₃ ) ₃ -9H ₂ 0, Sr(N0 ₃ ) ₂ , Μη(Ν0 ₃ ) ₂ ·4Η ₂ 0, La(N0 ₃ ) ₃ -6H ₂ 0, all obtained from Sigma-Aldrich, were then dissolved in the EDTA-NH ₃ .H ₂ 0 solution under constant stirring. When all the nitrates had dissolved, an appropriate amount of citric acid was introduced into the solution at a molar ratio of EDTA: citric acid: total metal ions of 1 :1.5:1. The solution was allowed to condense until a dark purple gel was formed. The gel was then heated at 473 K for 3 h to form a powder precursor before calcination at 1073 K for 5 h to obtain the crystalline LSCM powder. From the XRD results of this LSCM powder, it is confirmed that the synthesized powder has a perovskite structure with a similar phase composition obtained by Tao et al. (Tao and Irvine, 2004). . Similar to the LSCM powder, the GDC powder was synthesized by the sol-gel method. A proper amount of citric acid was dissolved in a small amount of deionized water. Necessary amounts of nitrate salts Ce(N0 ₃ ) ₄ and Gd(N0 ₃ ) ₃ , all obtained from Sigma- Aldrich, were then dissolved in the citric acid solution under constant stirring at 353 K.

EDTA-NH ₃ .H ₂ 0 solution was then added into the mixture to obtain a molar ratio of EDTA: citric acid: total metal ions of 1 :0.5:1 followed by the addition of NH ₃ .H ₂ 0 into the mixture to obtain a pH of 4. The solution was allowed to condense until a transparent gel was formed. The gel was then heated at 473 K for 3 h to form a powder precursor before calcination at 1073 K for 5 h to obtain the crystalline GDC powder. 3. YSZ electrolyte disks were fabricated with 2 g of 8 mol% YSZ (Tosoh, Japan) powder by uniaxial die pressing and sintering at 1723 K for 4 h. The electrolyte disks had a thickness and diameter of about 1 mm and 20 mm, respectively. In order to improve the quality of contact between the electrode and the electrolyte by increasing the surface roughness, the surface of the electrolyte disks was ground using sandpapers.

4. For the LSC -based electrode, powders of LSCM, GDC and/or YSZ were mixed with polyethylene glycol 400 at a 1 :1 ratio (for LSCM:GDC or LSCM:YSZ) or a 1 :0.5:0.5 ratio (for LSCM:GDC:YSZ) followed by screen-printing onto the YSZ substrate and sintering at 1373 K for 2 h. The thickness of each fuel electrode after sintering was about 40 μηη with a surface area of 0.5 cm ² .

5. For the Ni-YSZ electrode, the powders of Ni and YSZ were mixed with polyethylene glycol 400 at a 1 :1 ratio followed by screen-printing onto the YSZ substrate and sintering at 1573 K for 2 h. The thickness of each fuel electrode after sintering was about 40 μιη with a surface area of 0.5 cm ² .

6. For the counter and reference electrodes, platinum paste was painted on the opposite side of the fuel electrode on the YSZ substrate followed by sintering at 1223 K for 30 min. The surface area of each counter electrode was 0.5 cm ² and the distance between the counter and reference electrode was 4 mm.

7. The half-cells were sealed in the test furnace by ceramic paste. 8. The furnace was heated to 1073 K, and the different gas mixtures (70%/30% H ₂ 0/H ₂ , 70%/30% H ₂ 0/H ₂ with S0 ₂ and 10%/20%/70% C0 ₂ /H ₂ 0/N ₂ with S0 ₂ ) were fed into the fuel electrode.

9. The gas was fed through a heated water bath to obtain the desired water content.

However, S0 ₂ was only added to the gas at the outlet of the heated water bath and not with the carrier gas through the heated water bath to avoid solubility in water.

When used in the process described hereinbelow, a current of 00 mA cm ^"2 is equivalent to 0.2 V.

Fig. 1 shows the overall concept of the invention where an electrolyzer cell, for example a SOEC, is used to electrolyze/co-electrolyze water and carbon dioxide in the flue gas emitted by a fossil fuel power plant. This process produces hydrogen and carbon monoxide at the fuel electrode which can be further converted into liquid hydrocarbon fuel, for example, via the Fischer-Tropsch method. Electrical power from any renewable energy resources as well as excess power from power plants can be used to power the electrolyzers. Waste heat generated by power plants can be used to heat up the electrolyzers to their operating temperature and the byproduct produced at the air electrode which is oxygen can be recycled in the power plants to increase their combustion efficiency.

Fig. 2 shows the current-voltage curves and normalized impedance spectra at an electrolysis bias of 100 mA cm ^"2 of the half cells with different fuel electrode materials for H ₂ 0 electrolysis. Polarization curves show that LSCM-GDC (50:50 wt.%) exhibits the highest current density over the range of operating voltages, with an area specific resistance (ASR) of only 1.68 Ω cm ² as compared to 4.28 Ω cm ² , 3.03 Ω cm ² , and 26.9 Ω cm ² for pure LSCM, LSCM-(GDC-YSZ) (50:(25:25) wt.%) and LSCM-YSZ (50:50 wt.%) respectively. Impedance spectra measured at 100 mA cm ^"2 bias show the lowest polarization resistance for LSCM- GDC (50:50 wt.%) and they were in good agreement with the polarization curves obtained.

Fig. 3 shows the current-voltage curves and impedance spectra at an electrolysis bias of 100 mA cm ^"2 of the half cells with different ratios of LSCM to GDC composite for H ₂ 0 electrolysis. According to the current-voltage curves, LSCM-GDC (50:50 wt.%) exhibits the highest electrolysis performance with an ASR of 1.68 Ω cm ² as compared to 1.79 Ω cm ² and 1.82 Ω cm ² for LSCM-GDC (70:30 wt.%) and LSCM-GDC (90:10 wt.%), respectively. Impedance spectra measured at 100 mA cm ^"2 bias show the lowest polarization resistance for LSCM- GDC (50:50 wt.%) and were in good agreement with the polarization curves obtained.

Fig. 4 shows the comparison of electrolysis performance and sulfur tolerance between the optimized LSCM-GDC (50:50 wt.%) and Ni-YSZ (60:40 wt.%) for H ₂ 0 electrolysis. The results from the current-voltage curves and impedance spectra show that the LSCM-GDC electrode may exhibit better electrolysis performance than the Ni-YSZ electrode. According to the polarization curves, at higher current densities, the cell with a LSCM-GDC fuel electrode has an ASR of 1.68 Ω cm ² whereas Ni-YSZ has an ASR of 1.75 Ω cm ² . From the durability test, LSCM-GDC exhibits much higher tolerance towards S0 ₂ , which is present in flue gas, as compared to the Ni-YSZ electrode for at least up to 48 hours at an operating current density of 100 mA cm ^"2 .

Fig. 5 shows the co-electrolysis performance and sulfur tolerance of the optimized LSCM- GDC (50:50 wt.%) for H ₂ 0 and C0 ₂ co-electrolysis. The current-voltage curve shows that the LSCM-GDC fuel electrode exhibits relatively good co-electrolysis performance with an ASR of 2.52 Ω cm ² at higher current densities without reducing agent (H ₂ ) in the feedstock gas. From the durability test, the LSCM-GDC fuel electrode proved to be stable for co- electrolysis of H ₂ 0 and C0 ₂ without reducing agent and with the presence of sulfur dioxide at an operating current density of 600 mA cm ^"2 (current of 300 mA normalised with the surface area)for at least up to 48 hours.

Fig. 6 shows the current-voltage curves and impedance spectra of the optimized LSCM- GDC (50:50 wt.%) fuel electrode for the co-electrolysis of H ₂ 0 and C0 ₂ , with additional 0 ₂ in the feedstock gas. By comparing with the polarization curve in Fig. 5(a), the polarization curve in Fig. 6(a) shows that the cell performance increases significantly at the lower operating current density region with the presence of an additional 6.5% 0 ₂ . The impedance spectra plots also show a lower polarization resistance with the addition of 0 ₂ in the feedstock gas, especially when the cell is operating under OCV conditions.

Fig. 7 shows the current-voltage curves and impedance spectra of the optimized LSCM- GDC (50:50 wt.%) fuel electrode impregnated with 3 wt.% platinum (Pt) for the co- electrolysis of H ₂ 0 and C0 ₂ . It can be observed from the polarization curves that the optimized fuel electrode with Pt impregnation has a slight improvement in co-electrolysis performance, which is more significant in the case where no 0 ₂ is present in the feedstock gas. In the case where no 0 ₂ is present, the ASR measured from the linear portion of the polarization curves at higher operating current densities dropped by about 20% with Pt impregnation as compared to only a slight drop in ASR in the case where 0 ₂ is present. The impedance spectra plots show that both the polarization and ohmic resistances of the cell dropped slightly with Pt impregnation, which signifies an increase in electronic conductivity throughout the fuel electrode as well as an increase in overall electrocatalytic activity of the fuel electrode.

Fig. 8 shows the microstructure of LSCM, LSCM-GDC, LSCM-YSZ and LSCM-(GDC-YSZ). It can be seen that all the cells are intact with their electrode properly attached to their electrolyte. The addition of only YSZ seems to cause the electrode to become denser as shown in (c). In (d), the LSCM particles seem to be more agglomerated as compared to pure LSCM with the addition of both GDC and YSZ. Nevertheless, a higher porosity can be observed as compared to the LSCM-YSZ electrode in (c). From (b), addition of GDC did not have much effect on the electrode microstructure as compared to the pure LSCM electrode shown in (a).