Modified water treatment sludge as high performing and high stability oxygen carriers for efficient chemical looping combustion and subsequent carbon capture

Abstract

Disclosed herein is an oxygen carrier for chemical looping combustion, comprising: iron; copper; calcium; aluminium; and silicon, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element. Also disclosed herein are a method of manufacturing the oxygen carrier, use of the oxygen carrier, and a method of chemical looping combustion.

Description

[0001] MODIFIED WATER TREATMENT SLUDGE AS HIGH PERFORMING AND HIGH STABILITY OXYGEN CARRIERS FOR EFFICIENT CHEMICAL LOOPING COMBUSTION AND SUBSEQUENT CARBON CAPTURE

[0002] Field of Invention

[0003] The present invention generally relates to water treatment sludges, and more particularly relates to modified water treatment sludges as oxygen carriers for chemical looping combustion.

[0004] Background

[0005] 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.

[0006] Chemical looping combustion (CLC) represents a promising and attractive technological approach with dual benefits of CO2 capture and energy production. The principle behind CLC involves the utilization of two interconnected fluidized bed reactors: the air reactor (AR) and the fuel reactor (FR). Within the system, the oxygen carrier (OC) undergoes reduction by the fuel in the FR and is transported to the AR, where it undergoes oxidation by air and then transported back into the FR, thereby completing a single cycle of the CLC process. One notable advantage of this configuration is the segregation of fuel and air, resulting in production of flue gas without the generation of NOX. The selection of the OC is critical to CLC. The chosen OC should be cost effective, sustainable, have high reactivity with fuel and air, be stable under multiple redox cycles and has minimal susceptibility to agglomeration. Transitional metal oxides are the active constituents in OCs, prompting extensive research into the utilization of low-cost natural ores and industrial solid wastes rich in these species.

[0007] CLC improves upon traditional combustion methods, primarily due to its capacity to produce an almost pure CO2 stream. This meant that it reduces the emissions of air pollutants, such as NOx, compared to conventional combustion processes. Consequently, this not only enhances energy efficiency but also translates into cost savings by removing the need for additional separation or purification steps to remove harmful air pollutants from the flue gas. Moreover, the high purity CO2 flue gas generated is promising as it facilitates carbon capture efforts, and therefore contributes to mitigating climate change. Without the requirement of separating pollutants from the flue gas, direct capture methods can be employed, either by utilizing carbon capture reactors to extract the CO2or through storage mechanisms to prevent carbon emissions into the environment. The combination of CLC with carbon capture exemplifies an innovative system that combines efficient waste management and carbon capture to achieve energy efficiency and cost savings while combating climate change and reducing the amount of waste heading into the landfill.

[0008] Existing literature encompasses a diverse array of natural ores and waste materials that can fulfil the criteria needed to be utilized as a low-cost OC. Among the natural ores, iron-based ores are popular with ilmenite ores demonstrating the ability to attain gas conversion efficiencies surpassing 90 % for syngas after undergoing 5 cycles of CLC. Similarly, copperbased natural ores exhibited syngas conversion efficiency exceeding 90 % but suffers from agglomeration after 5 cycles. In the context of waste materials, numerous options ranging from sludge to ash have been explored. For instance, Yin et al. (K. Yin et al., Chem. Eng. J. 2021 , 405, 127068) investigated the utilization of incineration bottom ash, resulting in a notable CO conversion efficiency of 80.2 % after 10 cycles. Shifting focus to sludge-based OCs, industrial, sewage, and water treatment sludge (WTS) have been studied due to their Fe2O3 content, which is suitable for CLC. A prior study explored the application of treated WTS as an OC, yielding more than 88 % combustion efficiency of synthetic syngas over 10 cycles (C. H. J. Koh Yang et al., Chem. Eng. J. 2023, 465, 142874). Thus, the aforementioned examples reveal the viability of both natural ores and waste materials as promising options for cost effective OCs for CLC.

[0009] However, there are critical issues that must be tackled when utilizing waste-derived OCs. Firstly, it is imperative to enhance and stabilize their reactivity while increasing the number of CLC cycles. Certain waste-derived OCs exhibits unstable reactivity and poor reactivity attributed to unfavorable properties, such as inadequate fluidization. In addition to reactivity concerns, several waste-derived OCs suffers from agglomeration. This phenomenon not only lowers the efficiency of the CLC process but also can damage the system and equipment in place.

[0010] Therefore, to overcome at least one of the aforementioned problems, there exists a need for new oxygen carriers for CLC processes.

[0011] Summary of Invention

[0012] Aspects and embodiments of the invention are provided in the following numbered clauses. 1 . An oxygen carrier for chemical looping combustion, comprising: iron; copper; calcium; aluminium; and silicon, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element.

[0013] 2. The oxygen carrier according to Clause 1 , wherein: iron is present in an amount of from 30 to 50 wt%; copper is present in an amount of from 5 to 20 wt%; calcium is present in an amount of from 15 to 30 wt%; aluminium is present in an amount of from 5 to 20 wt%; and silicon is present in an amount of from 1 to 15 wt%.

[0014] 3. The oxygen carrier according to Clause 2, wherein: iron is present in an amount of from 32.1 to 41 .2 wt% (e.g. from 35 to 40 wt%, such as about 36.65 wt%); copper is present in an amount of from 10 to 15 wt% (e.g. from 1 1 .32 to 15.24 wt%, such as about 13.28 wt%); calcium is present in an amount of from 20 to 25 wt% (e.g. from 21 .62 to 24.94 wt%. such as about 23.28 wt%); aluminium is present in an amount of from 10 to 15 wt% (e.g. from 12.78 to 13.2 wt%, such as about 12.99 wt%); and silicon is present in an amount of from 5 to 10 wt% (e.g. from 7.65 to 9.55 wt%, such as about 8.60 wt%).

[0015] 4. The oxygen carrier according to any one of the preceding clauses, wherein the oxygen carrier further comprises one or more trace elements selected from potassium, magnesium, manganese, sodium, sulfur, titanium and zinc, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element.

[0016] 5. The oxygen carrier according to Clause 4, wherein, when present: potassium is present in an amount of from 0.7 to 0.8 wt%, such as about 0.75 wt%; magnesium is present in an amount of from 1 .7 to 2.0 wt%, such as about 1 .77 wt%; manganese is present in an amount of from 0.18 to 0.25 wt%, such as about 0.21 wt%; sodium is present in an amount of from 0.40 to 0.45 wt%, such as about 0.43 wt%; sulfur is present in an amount of from 0.60 to 0.67 wt%, such as about 0.63 wt%; titanium is present in an amount of from 0.14 to 0.16 wt%, such as about 0.15 wt%; and zinc is present in an amount of from 0.63 to 0.67 wt%, such as about 0.65 wt%.

[0017] 6. The oxygen carrier according to any one of the preceding clauses, wherein the oxygen carrier displays characteristic peaks in an X-ray powder diffraction diffractogram of 20: 23.95°, 30.35°, 31.64°, 33.87°, 35.82°, 39.01°, 52.25° and 47.03°, such as 23.34°, 23.95°, 25.85°, 29.38°, 30.35°, 31.64°, 32.37°, 33.87°, 34.76°, 35.82°, 39.01 °, 43.59°, 47.03°, 48.89°, 52.25°, 52.94°, 58.53°, 63.1 1°, 66.67°, and 68.37°.

[0018] 7. The oxygen carrier according to any one of the preceding clauses, wherein the oxygen carrier, before use, comprises Fe2O3, Ca2Fe2O5, Ca2AI2SiO7, and CuO, optionally wherein Fe2Os and CuO are active materials, Ca2AI2SiO7 is a support material and Ca2Fe20s is a support material and / or an active material in chemical looping combustion.

[0019] 8. The oxygen carrier according to any one of the preceding clauses, wherein the oxygen carrier is capable of being used for 300 chemical looping combustion cycles, while retaining a high combustion efficiency, optionally wherein the combustion efficiency is greater than 90%, such as greater than 93%.

[0020] 9. The oxygen carrier according to any one of the preceding clauses, wherein the oxygen carrier is capable of being used for 700 chemical looping combustion cycles, while retaining a combustion efficiency greater than or equal to 89%.

[0021] 10. The oxygen carrier according to any one of the preceding clauses, wherein one or more of the following apply:

[0022] (a) the oxygen carrier has an experimental oxygen transport capacity of from 6 to 19, such as from 7 to 8.1 , such as about 7.9;

[0023] (b) a bulk density of from 0.6 to 0.8 g / cm3, such as about 0.75 g / cm3;

[0024] (c) a BET surface area of from 3 to 4 m3 / g such as about 3.44 m3g;

[0025] (d) a pore volume of from 0.01 to 0.03 mL / g, such as about 0.015 mL / g; and

[0026] (e) a particle size of from 0.15 to 1 mm, such as from 0.25 to 0.5 mm, .

[0027] 11. A method of manufacturing an oxygen carrier as described in any one of Clauses 1 to 10, the method comprising the steps of: (i) providing a cured mixture comprising water, a water treatment sludge, CuO, and high calcium aluminate cement; and

[0028] (ii) calcining the cured mixture for a first period of time at a temperature of from 900 to 1,500 °C, followed by grinding and sieving to provide the desired oxygen carrier material.

[0029] 12. The method according to Clause 11 , wherein the cured mixture composition is obtained by the steps of:

[0030] (ai) providing an uncured mixture comprising water, a water treatment sludge, CuO, and high calcium aluminate cement; and

[0031] (aii) placing the uncured mixture into a mould for a period of from 1 day to 10 days (e.g. 3 days) at a temperature of from 10 to 50 °C, such as about 25 °C to provide the cured mixture composition.

[0032] 13. The method according to Clause 12, wherein the uncured mixture is obtained by the steps of:

[0033] (bi) providing a mixture comprising from 40 to 70 wt% of a water treatment sludge, from 10 to 25 wt% of CuO, from 20 to 30 wt% of a high calcium aluminate cement; and (bii) adding water to the mixture in a solid to liquid ratio of about 1 .375:1 .

[0034] 14. The method according to any one of Clauses 11 to 13, wherein the water treatment sludge is once comprising iron and / or compounds thereof.

[0035] 15. Use of an oxygen carrier as described in any one of Clauses 1 to 10 in a chemical looping combustion process.

[0036] 16. A method of chemical looping combustion, the method comprising the steps of:

[0037] (ci) providing a chemical looping combustion reactor system comprising an oxygen carrier as described in any one of Clauses 1 to 10; and

[0038] (cii) subjecting a gas in need thereof to chemical looping combustion.

[0039] Drawings

[0040] Fig. 1 depicts a bench scale fluidized bed reactor set up for chemical looping combustion.

[0041] Fig. 2 depicts pictures of the oxygen carriers (OCs): (A) FSCuOO; (B) FSCuO2; (C) FSCuO5; and (D) FSCuOI O. Fig. 3 depicts characterization of the 4 OCs, FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0. (a) X-ray diffraction (XRD) patterns, (b) Oxygen transport capacity (OTC) profile obtained at 850 °C, (c) H2-TPR diagram, and (d) Field emission scanning electron microscopy (FESEM) imaging.

[0042] Fig. 4 depicts (a) FESEM Image of the OC, and (b) image of the OC.

[0043] Fig. 5 depicts (a) fluidization profile of FSCuOO and FSCuOl 0 under cold model, and (b) absorption and desorption curves of FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0 with their surface area and pore volume.

[0044] Fig. 6 depicts CO conversion when using FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0 as OC at 850 °C for chemical looping combustion.

[0045] Fig. 7 depicts FS CO combustion efficiency during chemical looping combustion at 850 °C.

[0046] Fig. 8 depicts XRD pattern of fresh, reduced, and spent (a) FSCuOO, (b) FSCuOl 0, (c) OTC of spent FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0 after 50 cycles of chemical looping combustion.

[0047] Fig. 9 depicts (a) elemental composition of various forms of FSCuO2, FSCuO5, and FSCuOl 0, and (b) FESEM-energy-dispersive X-ray spectroscopy (EDS) images of fresh and spent FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0 after 50 cycles of chemical looping combustion.

[0048] Fig. 10 depicts (a) FESEM images of spent and agglomerated FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0, (b) amount of attrition and agglomeration after 50 cycles of chemical looping combustion for FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0, and (c) surface area of fresh and spent FSCuOO, FSCuO2, FSCuO5, and FSCuOl 0.

[0049] Fig. 11 depicts CO conversion using FSCuOl 0 as OC at 850 °C for chemical looping combustion for 500 and 1000 cycles.

[0050] Description

[0051] It has been surprisingly found that the oxygen carriers disclosed herein have improved reactivity, stability, agglomeration and longevity, and they improve the performance of water treatment sludge in chemical looping combustion. In addition, the oxygen carriers disclosed herein may be prepared by economical and simple methods, allowing them to be easily mass produced.

[0052] Thus, in a first aspect of the invention, there is provided an oxygen carrier for chemical looping combustion, comprising: iron; copper; calcium; aluminium; and silicon, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element.

[0053] In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components / features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa.

[0054] The phrase, “consists essentially of’ and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

[0055] As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, reference to “an oxygen carrier” includes mixtures of two or more such oxygen carriers, reference to “the catalyst” includes mixtures of two or more such catalysts, and the like.

[0056] When used herein, the term “oxygen carrier” refers to a material that transfers oxygen from air to fuel. In conventional combustion, the chemical energy stored inside fuels such as coal has been utilized by combustion with oxygen with carbon dioxide and water as products. Similar reactions can be carried out if instead of oxygen, an oxygen carrier is used. Metal oxides such as an iron oxide can act as suitable oxygen carriers. However, unlike combustion of fuel with air, there is a relatively pure sequestration-ready carbon dioxide stream produced on combustion with the metal oxide carrier. The reduced form of the metal oxide (e.g. the metal) can then be reacted with air in a separate reaction chamber to liberate heat to produce electricity or reacted with water to form a relatively pure stream of hydrogen, which can then be used for a variety of purposes.

[0057] As mentioned above, the elements may be provided in one or both of their elemental form or ionic form as part of a compound comprising said element. For example, the elements may be provided in their ionic form as part of elemental oxides such as iron oxides, copper oxides, calcium oxides, aluminium oxides, and silicon oxides.

[0058] In some embodiments that may be mentioned herein, the oxygen carrier may be provided as a natural ore of the elements that are used. As such, the oxygen carrier may consist essentially of the elemental oxides (and elements per se that may be present in the ore naturally or following reduction in chemical looping combustion), but it may also contain other materials that are typically found as part of that ore.

[0059] For example, the oxygen carrier may further comprise one or more trace elements selected from potassium, magnesium, manganese, sodium, sulfur, titanium and zinc, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element. In certain embodiments that may be mentioned herein, potassium may be present in an amount of from 0.7 to 0.8 wt%, such as about 0.75 wt%; magnesium may be present in an amount of from 1 .7 to 2.0 wt%, such as about 1 .77 wt%; manganese may be present in an amount of from 0.18 to 0.25 wt%, such as about 0.21 wt%; sodium may be present in an amount of from 0.40 to 0.45 wt%, such as about 0.43 wt%; sulfur may be present in an amount of from 0.60 to 0.67 wt%, such as about 0.63 wt%; titanium may be present in an amount of from 0.14 to 0.16 wt%, such as about 0.15 wt%; and zinc may be present in an amount of from 0.63 to 0.67 wt%, such as about 0.65 wt%. For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges.

[0060] The oxygen carrier may be provided in any suitable form. A suitable form that may be mentioned herein is where the oxygen carrier may be provided in particulate form. For example, the oxygen carrier may be provided as particles (e.g. crushed particles) having a size of from 0.25 mm and 0.5 mm. The degree of uniformity of the size range of the particles may be achieved by sieving the particles through sieves that exclude particles that are larger and / or smaller than the desired particle size range. However, it will be appreciated that such sieves may still allow particles that are slightly larger (or smaller) to be retained in the final sieved product.

[0061] Any suitable amount of the elements may be present in the oxygen carrier of the present invention. In some embodiments that may be mentioned herein, iron may be present in an amount of from 30 to 50 wt%, copper may be present in an amount of from 5 to 20 wt%, calcium may be present in an amount of from 15 to 30 wt%, aluminium may be present in an amount of from 5 to 20 wt%, and silicon may be present in an amount of from 1 to 15 wt%.

[0062] In further embodiments that may be mentioned herein, iron may be present in an amount of from 32.1 to 41 .2 wt% (e.g. from 35 to 40 wt%, such as about 36.65 wt%), copper may be present in an amount of from 10 to 15 wt% (e.g. from 1 1 .32 to 15.24 wt%, such as about 13.28 wt%), calcium may be present in an amount of from 20 to 25 wt% (e.g. from 21.62 to 24.94 wt%. such as about 23.28 wt%), aluminium may be present in an amount of from 10 to 15 wt% (e.g. from 12.78 to 13.2 wt%, such as about 12.99 wt%), and silicon may be present in an amount of from 5 to 10 wt% (e.g. from 7.65 to 9.55 wt%, such as about 8.60 wt%).

[0063] In some embodiments that may be mentioned herein, the oxygen carrier may display characteristic peaks in an X-ray powder diffraction diffractogram of 20: 23.95°, 30.35°, 31.64°, 33.87°, 35.82°, 39.01°, 52.25°, and 47.03° such as 23.34°, 23.95°, 25.85°, 29.38°, 30.35°, 31.64°, 32.37°, 33.87°, 34.76°, 35.82°, 39.01°, 43.59°, 47.03°, 48.89°, 52.25°, 52.94°, 58.53°, 63.11 °, 66.67°, and 68.37°.

[0064] Details of the X-ray powder diffraction measurement technique are provided in Example 3 below.

[0065] In some embodiments that may be mentioned herein, the oxygen carrier, before use, may comprise Fe2O3, Ca2Fe2O5, Ca2Al2SiO7, and CuO, optionally wherein Fe2O3 and CuO are active materials, Ca2AlzSiO7 is a support material and Ca2Fe2O5 is a support material and / or an active material in chemical looping combustion. Without wishing to be bound by theory, the support material aids in agglomeration and attrition resistance, thus allowing the oxygen carrier to achieve good combustion efficiency.

[0066] In some embodiments that may be mentioned herein, the oxygen carrier may comprise a water treatment sludge (i.e. Fe2Os and Ca2Fe2Os), CuO, and high calcium aluminate cement (i.e. Ca2AlzSiO7).

[0067] In some embodiments that may be mentioned herein, the oxygen carrier may be capable of being used for 300 chemical looping combustion cycles, while retaining a high combustion efficiency. For example, the combustion efficiency may be greater than 90%, such as greater than 93%.

[0068] In some embodiments that may be mentioned herein, the oxygen carrier may be capable of being used for 700 chemical looping combustion cycles, while retaining a combustion efficiency greater than or equal to 89%.

[0069] Details of the combustion efficiency calculation are provided in Example 2 below.

[0070] In some embodiments that may be mentioned herein, one or more of the following apply:

[0071] (a) the oxygen carrier has an experimental oxygen transport capacity of from 6 to 19, such as from 7 to 8.1 , such as about 7.9;

[0072] (b) a bulk density of from 0.6 to 0.8 g / cm3, such as about 0.75 g / cm3;

[0073] (c) a BET surface area of from 3 to 4 m3 / g such as about 3.44 m3g;

[0074] (d) a pore volume of from 0.01 to 0.03 mL / g, such as about 0.015 mL / g; and

[0075] (e) a particle size of from 0.15 to 1 mm, such as from 0.25 to 0.5 mm. Details of the oxygen transport capacity, bulk density, morphology, BET surface area, pore volume, and particle size measurement techniques are provided in the examples section below.

[0076] An exemplary embodiment of the first aspect of the invention may be an oxygen carrier, comprising:

[0077] 45-70 wt% water treatment sludge with iron compounds in them;

[0078] 10-25 wt% pure CuO; and

[0079] 20-30 wt% high calcium aluminate cement.

[0080] An advantage of the oxygen carrier disclosed herein is that it has improved stability, longevity and reactivity over pure water treatment sludge. This means that the oxygen carrier disclosed herein can be used in the long term for great chemical looping combustion performance.

[0081] As will be appreciated, the present invention edges out other reutilized waste material with better performance, longer lifespan, and better stability compared to incineration bottom ash and sewage sludge. The present invention also shows promising performance compared to traditional oxygen carriers like natural iron ores. In addition to this, using waste materials and simple methods of preparation leads to lower cost of oxygen carrier production compared to synthetic oxygen carriers, made with pure metal oxides and produced by varying methods (e.g. impregnation, co-precipitation, solution combustion, etc.), thus making it suitable for mass production.

[0082] In a second aspect of the invention, there is provided a method of manufacturing an oxygen carrier as described in the first aspect of the invention, the method comprising the steps of:

[0083] (i) providing a cured mixture comprising water, a water treatment sludge, CuO, and high calcium aluminate cement; and

[0084] (ii) calcining the cured mixture for a first period of time at a temperature of from 900 to 1,500 °C, followed by grinding and sieving to provide the desired oxygen carrier material.

[0085] Any suitable period of time may be used in step (ii). For example, the first period of time may be 5 hours.

[0086] In some embodiments that may be mentioned herein, the cured mixture composition may be obtained by the steps of:

[0087] (ai) providing an uncured mixture comprising water, a water treatment sludge, CuO, and high calcium aluminate cement; and (aii) placing the uncured mixture into a mould for a period of from 1 day to 10 days (e.g. 3 days) at a temperature of from 10 to 50 °C, such as about 25 °C to provide the cured mixture composition.

[0088] In some embodiments that may be mentioned herein, the uncured mixture may be obtained by the steps of:

[0089] (bi) providing a mixture comprising from 40 to 70 wt% of a water treatment sludge, from 10 to 25 wt% of CuO, from 20 to 30 wt% of a high calcium aluminate cement; and

[0090] (bii) adding water to the mixture in a solid to liquid ratio of about 1 .375:1 .

[0091] In some embodiments that may be mentioned herein, the water treatment sludge may be once comprising iron and / or compounds thereof. In further embodiments that may be mentioned herein, the water treatment sludge may be once comprising iron, aluminium, calcium, copper and / or silicon, and / or compounds thereof.

[0092] An exemplary embodiment of the second aspect of the invention may be a method of manufacturing an oxygen carrier as described in the first aspect of the invention, the method comprising:

[0093] (c) mixing 45-70 wt% water treatment sludge, 10-25 wt% CuO, and 20-30 wt% high calcium aluminate cement;

[0094] (d) adding water at a 1 .375:1 solid to liquid ratio;

[0095] (e) pouring the mixture into cubic molds;

[0096] (f) curing the mixture at room temperature for 3 days;

[0097] (g) calcining the cubes at 950 °C for 5 hours; and

[0098] (h) grinding and sieving the material to obtain particle size between 0.25 mm and 0.5 mm.

[0099] As will be appreciated, the oxygen carrier disclosed herein can be prepared with low effort due to simple preparation methods including physical mixing, calcination, etc., and low cost as a waste material is used.

[0100] As discussed above, the present invention relates to an oxygen carrier made from water treatment sludge modified by cement and CuO for chemical looping combustions of syngas. Further aspects and embodiments of the invention may relate to the following numbered embodiments.

[0101] 1 . An oxygen carrier for chemical looping combustion, comprising: ferric sludge; calcium aluminate cement comprising 70-80% alumina; and copper oxide (CuO).

[0102] 2. The oxygen carrier of Statement 1 , wherein the ferric sludge is water treatment sludge.

[0103] 3. The oxygen carrier of Statements 1 and 2, wherein the ferric sludge is a byproduct of water treatment using ferric salts as coagulants.

[0104] 4. The oxygen carrier of any of the previous statements, wherein the oxygen carrier comprises 45-70 wt% ferric sludge of the total weight of the oxygen carrier.

[0105] 5. The oxygen carrier of any of the previous statements, wherein the oxygen carrier comprises 20-30 wt% high calcium aluminate cement of the total weight of the oxygen carrier.

[0106] 6. The oxygen carrier of any of the previous statements, wherein the oxygen carrier comprises 10-25 wt% copper oxide (CuO) of the total weight of the oxygen carrier.

[0107] 7. A method to produce an oxygen carrier of Statements 1 -6, comprising:

[0108] (a) calcinating the ferric sludge at 700-1000 °C for 5 hours;

[0109] (b) reducing the size of the calcined ferric sludge between 0.5 mm to 0.063 mm;

[0110] (c) mixing the ferric sludge from step 7.2, high calcium aluminate cement, copper oxide and water with a solid-to-liquid ratio of 1 .375:1 ;

[0111] (d) curing the mixture at room temperature for 3 days;

[0112] (e) calcinating the cured mixture at 950 °C for 5 hours; and

[0113] (f) crushing the calcined cured mixture to particle size between 0.25 mm and 0.5 mm.

[0114] 8. The method to produce an oxygen carrier of Statement 7, comprising mixing 45-70 wt% ferric sludge, 20-30 wt% high calcium aluminate cement, and 10-25 wt% copper oxide before adding water.

[0115] As noted hereinbefore, the oxygen carrier disclosed herein may be suitable for use in chemical looping combustion processes.

[0116] Thus, in a third aspect of the invention, there is provided a use of an oxygen carrier as described in the first aspect of the invention in a chemical looping combustion process. Examples of the use of the oxygen carrier in such processes are provided in the examples section below.

[0117] In a fourth aspect of the invention, there is provided a method of chemical looping combustion, the method comprising the steps of:

[0118] (ci) providing a chemical looping combustion reactor system comprising an oxygen carrier as described in the first aspect of the invention; and

[0119] (cii) subjecting a gas in need thereof to chemical looping combustion.

[0120] The chemical looping combustion may be run at any suitable temperature. For example, the chemical looping combustion may be run at a temperature of from 700 to 1 ,100 °C, such as 850 °C.

[0121] Any suitable gas may be subjected to the chemical looping combustion. For example, the gas may be a syngas. For example, the gas may comprise one or more of CO, CO2, H2, and N2. In some embodiments that may be mentioned herein, the gas may be composed of 50 ml / min CO, 50 ml / min CO2, 50 ml / min H2, and 213 ml / min N2.

[0122] Advantages of the present invention may include the following, which may or may not be described elsewhere herein.

[0123] • High reactivity, stability, and long lifespan allowing it to be used for chemical looping combustion of syngas, including municipal solid waste syngas, to achieve high combustion efficiencies.

[0124] • Use of the oxygen carrier disclosed herein in chemical looping combustion will aid in improving energy efficiency, cost savings, and being more sustainable compared to synthetic oxygen carriers.

[0125] • Usage of chemical looping combustion will improve energy efficiency and cost savings by removing the need to separate air pollutants from the flue gas while being sustainable as it reduces carbon emissions. Carbon emissions may be avoided by utilizing carbon capture technology together with the almost pure CO2flue gas stream. This may be achieved either by storing CO2to eliminate emissions, or pairing with a carbon capture reactor with high capture efficiency to reduce CO2released into the environment.

[0126] • Reuse of a modified waste product for a higher valued process compared to just landfilling the waste product. Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.

[0127] Examples

[0128] Materials

[0129] A composite mixture of high calcium aluminate cement was purchased from Jianai Special Aluminates Co. Ltd. CuO was purchased from WuXiShiYaTaiLianHeGongHua Pte. Ltd. The remaining materials and chemicals were purchased from commercial sources and used without purification unless otherwise stated.

[0130] Example 1. Preparation of oxygen carriers (OCs)

[0131] Dewatered ferric sludge (FS) was acquired from a water treatment facility in Singapore, where it was generated as a waste byproduct of employing ferric salts as coagulants in the water treatment process (T. Ahmad etal., J. Clean. Prod. 2016, 124, 1 -13). Subsequently, the ferric sludge (FS) was stored in a cold room at 7 °C. Pre-treatment by subjecting the ferric sludge to calcination at 950 °C for 5 h (heating rate of 15.5 °C / min) was completed before size reduction using a commercial grinder. A composite mixture of high calcium aluminate cement, calcined ferric sludge, and varying content of CuO was homogenously prepared in a mixer with a solid to liquid ratio of 1 .375:1 . After casting into cubic molds (5 cm x 5 cm) and allowed to cure for 3 days, the resulting cubes underwent calcination at 950 °C for 5 hours before being crushed and sieved to a size between 0.25 mm and 0.5 mm before being used for chemical looping combustion. The nomenclature for the various compositions is denoted as FSCuOO, FSCuO2, FSCuO5, and FSCuOlO, which reflects the varying addition rates of CuO. The content of calcium aluminate cement content was kept constant at 30% for the above oxygen carrier made. The mix of the oxygen carriers are described in Table 1.

[0132] Table 1 . Composition of the synthesized oxygen carriers.

[0133] Composition FSCuOO FSCuO2 FSCuO5 FSCuOl O

[0134] (%)

[0135] (%) (%) (%)

[0136] “FS 70 68 65 60

[0137] CuO 0 2 5 10 Cement 30 30 30 30

[0138] Example 2. Reactivity tests

[0139] To explore the stability and reactivity of the oxygen carriers, a bench-scale fluidized bed reactor 100 was employed as depicted in Fig. 1. The fluidized bed reactor 100 includes a programmable gas controller 110, a reactor 120 which includes modified oxygen carriers 121 , a furnace 130, a furnace control unit 140, a filter 150, and a gas analyzer 160. Constructed from quartz with an inner diameter of 11 mm, the reactor 120 was heated to 850 °C by an electrical furnace 130 during operation. Cyclic reduction and oxidation of the oxygen carriers 121 were conducted by switching the input gases according to the programmable controller 110. The gas switching program for a single cycle was: Purge 1 with N2 (3 min, 363 ml / min) -> Reaction with simulated syngas (5 min, 363 ml / min) -> Purge 2 with N2 (5 min, 363 ml / min) -> Regeneration with air (8 min, 363 ml / min). The simulated syngas was composed of 50 ml / min CO, 50 ml / min CO2, 50 ml / min H2, and 213 ml / min N2.

[0140] For each chemical looping combustion cycle, 10 g of oxygen carrier with particle size between 0.25 and 0.5 mm was introduced into the reactor, pictures of the oxygen carrier are provided in Fig. 2. The baseline chemical looping combustion experiment involved 50 cycles, while an extended chemical looping combustion experiment was conducted for 500 and 1000 cycles. Real-time monitoring of the gaseous products was carried out using an online gas analyzer (ABB Advance Optima Gas Analyser A02040). Efficiencies were reported with their average values and alongside error bars representing the standard deviations, ensuring comprehensive data representation. The calculation for the combustion efficiency of CO was performed as follows:

[0141] (1)

[0142] Where TJCO are the combustion efficiency for CO; y is the molar concentration of the reactant (in this case, CO) measured by the gas analyser; the in subscript is the results obtained from operating chemical looping combustion with an inert bed; the CLC subscript refers to the results obtained from operating chemical looping combustion with the modified oxygen carriers.

[0143] Before chemical looping combustion, the quantity of oxygen carrier introduced into the reactor will be weighed out. After chemical looping combustion, the oxygen carriers will be collected and sieved. Particles > 0.5 mm will be separated, weighed, and denoted as the agglomerated portion of the oxygen carrier. Subsequently, the remaining oxygen carriers will then be weighed. These 2 masses were then deducted from the mass of oxygen carrier introduced into the reactor to obtain the amount of attrition that occurred.

[0144] To understand the thermal properties of the oxygen carrier, thermogravimetric analysis (NETZSCH STA 449 F3 Jupiter) was performed. Reduction using H2(5 % H2 / N2, 210 ml / min) followed by oxidation using air (250 ml / min) at 850 °C allowed determination of the oxygen transport capacity (OTC). In addition, H2-temperature programmed reduction (H2-TPR) over a temperature range of 100 °C to 950 °C (heating rate of 5 °C / min) using H2(5 % H2 / N2, 210 ml / min) provided additional insights into the reducibility and lattice oxygen activity of the oxygen carrier.

[0145] Example 3. Characterizations of fresh oxygen carriers

[0146] Characterization techniques

[0147] The minimum fluidization rate (umf) was determined experimentally by conducting tests on a cold model. 10 g of oxygen carrier were placed within a quartz tube with an inner diameter of 22 mm. At room temperature, N2was introduced into the reactor with velocity ranging from 0 to 0.3 m / s, subsequently reducing it back to 0. The reactor was connected to a U-tube manometer to observe the pressure drop at different velocities. The umf was estimated from the distinctive plateau observed in the pressure drop curve as the velocity was increasing.

[0148] The elemental composition of the oxygen carriers was analyzed using inductively coupled plasma - optical emission spectrometry (ICP-OES, Perkin Elmer Optima 8300) following microwave acid digestion (MAD, Multiwave 5000, Anton Paar). During MAD, 0.1 g of oxygen carriers was digested using 4.4 ml HNOs, 2.2 ml HCI, and 1 ml HF for 9.5 min at 180 °C (heating rate of 18 °C / min) in adherence to the guidelines outlined in EPA 3052 (EPA, SW- 846 Test Method 3052: Microwave Assisted Acid Digestion of Siliceous and Organically Based Matrices, 1996). Subsequent de-complexation involved the use of 10 ml H3BO3 for 20 min at 150 °C (heating rate of 15 °C / min). Mineralogical composition, morphology, surface area, and porosity of the oxygen carriers were characterized through X-ray diffraction (XRD, Bruker D8 Advance), field emission scanning electron microscopy with energy-dispersive X-ray spectrometry (FESEM-EDS, JSM-7200F with Oxford Aztec Standard X-max80, JEOL), Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analysis (Quadrasorb EVO / SI, Quantachrome Instrument) respectively. Results and discussion

[0149] Fig. 3a presents the crystalline phases of the oxygen carriers, revealing the presence of brownmillerite (Ca2Fe2O5, COD 1008777), hematite (Fe2O3, COD 2300617), and calcium aluminosilicate (Ca2AI2SiO7, COD 1000048), in all of the oxygen carriers.

[0150] CuO (COD 1 100028): 39.01 , 48.89, 52.94, 58.53, 66.67.

[0151] Fe2O3(COD 2300617): 25.85, 30.35, 35.82, 43.59, 63.1 1 .

[0152] Ca2AI2SiO7(COD 1000048): 23.95, 29.38, 31.64, 52.25, 68.37.

[0153] Ca2Fe2O5(COD 1008777): 23.34, 32.37, 33.87, 34.76, 47.03.

[0154] The appearance of calcium aluminosilicate was attributed to the incorporation of high calcium aluminate cement. Notably, the main difference between the oxygen carriers lies in the presence of copper oxide (CuO, COD 1100028) in FSCuOl 0, FSCuO5, and FSCuO2 and not in FSCuOO which corresponds to the addition of CuO into the oxygen carrier. The XRD also revealed that the addition of CuO into FS did not form Cu mixed metal oxides (e.g. CuAI2O4, etc.). This is favorable as CuO has the best reactivity compared to other Cu mixed metal oxides and therefore will improve the reactivity of the oxygen carriers.

[0155] The appearance of the oxygen carrier will be brown, uneven and irregular particles (Fig. 4b). The usage of water treatment sludge will result in the shape, colour and size aforementioned while the use of synthetic materials will likely result in something more even. Similarly, the FESEM image will reveal porous and irregular structure due to the use of water treatment sludge while a synthetic material will be more even. The porous nature meant that the surface area of the oxygen carrier will be around 3.4 to 4.2 m2 / g though synthetic materials will be able to adjust their surface area based on different structures and arrangement of the compounds.

[0156] Table 2. Main elemental composition of FS and FSCuOl 0 (wt%).

[0157] Elements Ferric Sludge Cone. FSCuOl 0 Cone.

[0158] ± (S.D.) ± (S.D.)

[0159] Al 8.00+ (0.99) 12.99+ (0.21 )

[0160] Ca 35.44± (2.59) 23.28± (1 .66)

[0161] Cu 0.020± (0.00050) 13.28± (1.96)

[0162] Fe 42.72± (2.77) 36.65± (4.55) Si 0.68± (0.062) 8.60± (0.95)

[0163] For the oxygen carrier, the water treatment sludge provided the Fe, Ca and Al content. The Ca and Al content acts as a support for the active material Fe, allowing it to achieve good combustion efficiency. Water treatment sludge will have high Fe content of around 40 to 45% with a high Ca content of 32 to 38% due to the water treatment process. The addition of Cu was from CuO, improving the reactivity, reliability and stability of the oxygen carrier while the addition of high calcium aluminate cement provided additional Al, Ca and Si to improve the mechanical stability and property of the oxygen carrier making it more resistant to agglomeration and attrition. The composition of FSCuOlO will be Fe at around 35 to 40%, Ca at 20 to 25%, Al and Cu at 10 to 15% and Si at 5 to 10%. Another defining factor of this oxygen carrier would be the trace elements that comes with the water treatment sludge. A synthetic oxygen carrier will not have trace elements due to the pure nature of the synthesis process. Even the use of natural minerals and ores will not replicate the trace elements of water treatment sludge. So, certain trace elements (>0.1%) are provided below in Table 3.

[0164] Table 3. Minor elemental composition of FS and FSCuOl O (wt%).

[0165] Elements Ferric Sludge Cone. FSCuOlO Cone.

[0166] ± (S.D.) ± (S.D.)

[0167] K 0.53± (0.028) 0.75± (0.059)

[0168] Mg 3.44± (0.25) 1.77± (0.15)

[0169] Mn 0.25± (0.015) 0.21 ± (0.029)

[0170] Na 0.099± (0.031 ) 0.43± (0.026)

[0171] S 7.99+ (0.98) 0.63+ (0.26)

[0172] Ti 0.16± (0.013) 0.15± (0.019)

[0173] Zn 0.54± (0.033) 0.65± (0.071 )

[0174] Fresh oxygen carrier will contain Fe2O3and Ca2Fe2O5 from the water treatment sludge, Ca2AI2SIO7 from the addition of high calcium aluminate cement and CuO. Fe2Os and CuO are the active constituents during chemical looping combustion. Ca2AI2SiO7 will be the support material aiding in agglomeration and attrition resistance. While Ca2Fe2O5 can act as both a support material and active constituent during chemical looping combustion. Overall, these are the chemical and physical characteristics that may define this oxygen carrier.

[0175] The composition of the base water treatment sludge and the modified material is characterized by ICP-OES analysis as shown in Table 4. Due to the addition of cement and CuO, there is a reduction in Fe and Ca while Cu, Al, and Si saw an increase. Besides these main components, there are traces of other heavy metals in both the base material and the modified material.

[0176] Table 4. Composition of FS and FSCuOlO (wt%).

[0177] Elements Ferric Sludge Cone. FSCudio Cone.

[0178] 1 (S.D.) + (S.D.)

[0179] Al 8.00+ (0.99) 12.99+ (0.21)

[0180] As 0.00551 (0.0026) N.D.

[0181] Ba 0.0241(0.0026) 0.0311(0.0031)

[0182] Ca 35.441(2.59) 23.281(1.66)

[0183] Cd 0.00201(0.00050) 0.00411(0.0016)

[0184] Co 0.00461(0.00050) 0.00341(0.00031)

[0185] Cr 0.0551 (0.0032) 0.0561 (0.0074)

[0186] Cu 0.0201(0.00050) 13.281(1.96)

[0187] Fe 42.721 (2.77) 36.651 (4.55)

[0188] K 0.531(0.028) 0.751(0.059)

[0189] Mg 3.441(0.25) 1.771(0.15)

[0190] Mn 0.251 (0.015) 0.211 (0.029)

[0191] Mo 0.0151(0.00050) 0.0121(0.0016)

[0192] Na 0.0991(0.031) 0.431(0.026)

[0193] Ni 0.00231(0.0010) 0.00581(0.00092) Pb 0.014 (0.0016) N.D.

[0194] S 7.99+ (0.98) 0.63+ (0.26)

[0195] Si 0.68± (0.062) 8.60± (0.95)

[0196] Ti 0.16± (0.013) 0.15± (0.019)

[0197] V 0.016± (0.00050) 0.01 1 ± (0.0012)

[0198] Zn 0.54± (0.033) 0.65± (0.071 )

[0199] FSCuOl 0 has a bulk density of 0.75 ± (0.0037) g / cm3.

[0200] The OTC of the 4 oxygen carriers were experimentally determined and are presented as the isothermal reduction profiles illustrated in Fig. 3b. FSCuOl 0 exhibited the highest OTC among the other oxygen carriers, owing to the high reactivity and content of CuO. Comparing the theoretical OTC in Fig. 3b, the experimental OTCs were lower, which is expected as partial reduction is expected while full reduction was used to calculate the OTC. Morphological characteristics of the oxygen carriers also plays a role in their reactivity. The presence of lattice oxygen in both the surface and bulk phases of the oxygen carrier indicates that high surface area and porosity will facilitate penetration of reducing gas, promoting reaction with lattice oxygen in the bulk phase. The porous nature of the modified oxygen carriers, as evidenced by the FESEM in Fig. 3d, enhances the diffusion of the reduction gas to the bulk phase. This observation is substantiated by the high surface area and pore volume of the oxygen carriers presented in Fig. 5b. Fig. 3d also showcases FSCuOl 0 morphology that is porous looking, with surface area of 3.44 m3 / g and a pore volume of 0.015 ml / g. While FSCuOl 0 demonstrated the highest OTC, the similar surface area and porous nature of all the oxygen carriers yielded comparable results of <8%. Though the OTC may seem low, it is still suitable for chemical looping combustion. This adequacy is supported by the fact that many natural ores, such as ilmenite, demonstrated high performance with an OTC of 5% (B. FleilB et al., Fuel Process. Technol. 2022, 231, 107249), a value lower than that achieved by the modified oxygen carriers in the present disclosure.

[0201] The reducibility of the oxygen carriers was assessed through H2-TPR as depicted in Fig. 3c. FSCuOO exhibit reduction peaks for Fe2O3 and FesO4 at 500-600 °C and 600-700 °C, respectively. While for FSCuO2, FSCuO5, and FSCuOl 0, a broad reduction peak at 550- 650 °C can be attributed to the reduction of FesO4. In oxygen carriers with CuO substitution, a reduction peak for CuO can be observed between the temperature range of 200 °C to 300 °C. This reduction peak could also include the reduction of Fe2O3. The Fe3O4peak in oxygen carriers with CuO substitution was shifted to a similar lower temperature compared to FSCuOO as seen in Table 5, which indicates that the presence of CuO decreases the reduction temperatures and increases the reactivities of the oxygen carrier. All 4 oxygen carriers share a reduction peak at 850 °C to 900 °C, corresponding to the reduction of Ca2Fe2O5.

[0202] Table 5. Peak temperature of Fe3O4for FSCuOO, FSCuO2, FSCuO5, and FSCuOIO.

[0203] Temperature ( C) FSCuOO FSCuO2 FSCuO5 FSCuOI O

[0204] Fe3O4Reduction Peak 635 613 615 609

[0205] Additionally, fluidization tests were conducted to obtain the minimum fluidization velocities of the oxygen carriers. A cold model run with 10 g of oxygen carrier was performed and the measured profiles of pressure drop against the flow velocity of N2at room temperature can be seen in Fig. 5a while the umf is estimated based on the plateau in the pressure difference with increasing N2flow velocity. During actual fluidized bed operation, the gas velocity will be set at approximately 2 to 5 times higher than the umf to ensure effective fluidization. Calculation of the required gas flow at 850 °C from the umf obtained was carried out to account for the difference in temperatures, volume flow rates, flow velocities and dimensions of the reactor. This analysis indicates that a gas flow of 363 ml / min (0.064 m / s) at 850 °C is sufficient to ensure proper fluidization for both FSCuOO and FSCuOI O, requiring a gas flow range of 256 to 663 ml / min (0.045 to 0.12 m / s), and FSCuOI O, which requires 278 to 696 ml / min (0.049 to 0.12 m / s) of gas flow. The addition of CuO increases the density of the oxygen carrier as CuO is denser than FS, which in turn increases the pressure drop and therefore the umf required for the synthesized oxygen carrier as seen in Fig. 5a.

[0206] Example 4. Redox activity of modified FS with differing CuO content

[0207] Comparing the CO efficiency of the modified FS with differing CuO contents can be seen in Fig. 6. The efficiency increases with the increasing amount of CuO added into the oxygen carrier as expected. Conversely, the efficiency of FS-based oxygen carrier was expected to reduce with increasing number of cycles, as reported in C. H. J. Koh Yang etal., Chem. Eng. J. 2023, 465, 142874, but the addition of CuO stabilizes the performance even at a minimal loading of only 2%. Comparing the 5thand 50thcycle, FSCuO2 demonstrated average efficiencies of 62.75% and 61.30%, respectively. Only a minimal of 1.45% reduction in efficiency was observed after 45 cycles. While for FSCuO5, 68.94% was achieved in the 5th cycle and 70.53% was achieved in the 50thcycle, showcasing an increase of 1 .59% after 45 cycles, indicating the efficiencies are generally maintained throughout the cycles. Therefore, addition of CuO introduces a stabilizing effect on the oxygen carrier and increasing the CuO content from 2 to 5% led to increasing stability. Among the 4 modified oxygen carriers tested, FSCuOl O consistently demonstrated outstanding performance throughout 50 cycles, achieving a CO efficiency of 93.28% to 96.99%. Compared to the base material in Fig. 7, the reactivity, and stability of the invention was superior. This is expected due to the increased CuO content.

[0208] Besides this, 50 cycles could not be conducted on the base material (original water treatment sludge) due to severe agglomeration happening during chemical looping combustion, while for the present invention, 50 cycles were easily achieved with only 3.19% agglomeration. This proved that FSCuOl O has a longer lifespan as compared to the base material. This meant that the modification of adding CuO and cement improved the stability, reactivity, and longevity of the oxygen carrier, making this invention competitively viable.

[0209] From the results obtained, modification with CuO exhibited improved stability and reactivity of the oxygen carrier for FSCuOl O and FSCuO5. These results affirmed that the modification of FS with cement and CuO is competitively viable.

[0210] Example 5. Transformation of oxygen carriers during chemical looping combustion

[0211] From the XRD patterns in Figs. 8a and 8b, the reactions occurring during chemical looping combustion can be deduced. The addition of CuO in FSCuO2, FSCuO5, and FSCuOl O allowed for the reduction into Cu (COD 4105040). Given the selection of a lower operating temperature of 850 °C, this facilitates the reduction to Cu while preventing decomposition of CuO into CusO instead. Prior studies indicated an anticipated reduction into FesO4 (COD 9010941 ) and then into FeO (COD 9009770). However, in the present disclosure, FeO was further reduced into Fe (COD 9013485) for FSCuOl O and not FSCuOO. This phenomenon can be attributed to the presence of CuO. Addition of CuO accelerates the reduction of FeO into metallic Fe supporting the hypothesis that the modification of CuO improves the reactivity of the oxygen carrier as shown in Fig. 6. Due to low operating temperature chosen, there is a possibility of Ca2Fe20s being reduced in a one-step reaction into CaO and metallic Fe at a minimal rate, which could contribute to the chemical looping combustion process. Overall, the deep reduction suggests that excess syngas was involved in the reaction. Even though the loading of oxygen carrier in the reactor might not be sufficient for the complete reaction based on the experimental design of the present disclosure, this actually allowed for clearer identification of the impacts of CuO on the synthesized oxygen carrier for chemical looping combustion. The isothermal TGA profile, as presented in Fig. 8c, reveals a slight reduction in OTC for all 4 oxygen carriers after chemical looping combustion. FSCuOl 0 achieved the least reduction of 7.8% while FSCuO2, FSCuO5, and FSCuOO exhibited similar level of reduction of 10.0%, 10.0%, and 9.2% respectively. From these results, there is minimal increment of the OTC from the modification of CuO, but during chemical looping combustion, it showed drastic improvement in stability of the oxygen carrier even at 2% and improvement of reactivity which can especially be seen at 10%. The stability can also be noticed from the consistently high oxidation rate of the oxygen carriers in Fig. 8c.

[0212] Analysis of the ICP-OES results in Fig. 9a reveals marginal difference in elemental composition of major elements Fe, Ca, Al, and Cu. This observation demonstrates the stability of the oxygen carriers even after 50 cycles of chemical looping combustion. The remaining elements found in the oxygen carrier can be found in Table 6. Interestingly, examining the FESEM images in Fig. 9b exposes changes in the surface element composition, even though the overall concentration percentages of each element remain consistent before and after chemical looping combustion, as evidenced in Fig. 9a. Notably, active components, namely Cu and Fe, exhibit migration towards the oxygen carrier surface, whereby their surface composition is higher after chemical looping combustion compared to the fresh oxygen carrier. At this operational temperature, Cu migration can be expected and will enhance the reactivity of the oxygen carrier by mitigating the necessity for the reducing gas to penetrate deep into the oxygen carrier for reaction with CuO. Similar to Cu, Fe migration also contributes to improved reactivity. Overall, the chemical transformation results of the oxygen carrier during chemical looping combustion emphasized the competitive performance of the oxygen carrier and therefore, the usefulness of CuO modification in improving both reactivity and stability.

[0213] Table 6. Elemental composition of remaining elements of FSCuOO, FSCuO2, FSCuO5, and FSCuOl O before and after chemical looping combustion and FSCuOl O agglomerated particles.

[0214] Elements FSCuO2 FSCuO2 FSCuO5 FSCiiO5 FSCuOl O FSCuOl O FSCuOl O

[0215] Before (± After Before After Before After Agglom

[0216] S.D.)

[0217] (± S.D.) (± S.D.) (± S.D.) (± S.D.) (± S.D.) (± S.D.)

[0218] (%)

[0219] (%) (%) (%) (%) (%) (%) Ba 0.008± 0.020± 0.0067± 0.019± 0.031± 0.030± 0.022±

[0220] (0.0016) (0.0001 ) (0.0023) (0.001 ) (0.0031 ) (0.0039) (0.007)

[0221] Cd N.D. N D. N.D. N.D. 0.0041 ± N.D. N.D.

[0222] (0.0016)

[0223] Co N.D. N D. N.D. N.D. 0.0034± N.D. N.D.

[0224] (0.00031 )

[0225] Cr 0.05± (0) 0.06± 0.053± 0.061± 0.056± 0.054± 0.052±

[0226] (0.0022) (0.0023) (0.0025) (0.0074) (0.012) (0.0081 )

[0227] K 0.57± 0.83± 0.4± 0.69± 0.75± 0.59± O.63±

[0228] (0.026) (0.14) (0.023) (0.0097) (0.059) (0.070) (0.15)

[0229] Mg 2.82± 2.90± 2.65± 2.69± 1 .77± 1 .98± 1 .83±

[0230] (0.12) (0.11 ) (0.54) (0.29) (0.15) (0.27) (0.27)

[0231] Mn 0.18± 0.18± 0.19± 0.18± 0.21 ± 0.22± 0.16±

[0232] (0.0023) (0.0098) (0.012) (0.0066) (0.029) (0.052) (0.029)

[0233] Mo N.D. 0.006± N.D. 0.0055± 0.012± 0.013± 0.0037±

[0234] (0.00075) (0.00063) (0.0016) (0.0022) (0.0012)

[0235] Na 0.24± 0.62± 0.19± 0.57± 0.43± 0.36± 0.68±

[0236] (0.021 ) (0.071 ) (0.015) (0.012) (0.026) (0.089) (0.20)

[0237] Ni N.D. 0.0015± N.D. 0.0012± 0.0058± 0.0062± 0.0018±

[0238] (0.0008) (0.00054) (0.00092) (0.0014) (0.0029)

[0239] P 0.76± 0.51 ± 1.46± 0.98± 0.47± 0.63± 6.52±

[0240] (0.0046) (0.012) (0.12) (0.12) (0.089) (0.14) (4.55)

[0241] S 0.47± 0.56± 0.46± 0.65± 0.63± 0.48± 0.44±

[0242] (0.057) (0.065) (0.078) (0.054) (0.26) (0.15) (0.13)

[0243] Si 5.25± 5.03± 5.28± 4.49± 8.60± 8.40± 5.33±

[0244] (0.97) (0.15) (0.28) (0.72) (0.95) (1 -84) (1 .29)

[0245] Ti 0.12± 0.12± 0.12± 0.12± 0.15± 0.15± 0.12±

[0246] (0.0023) (0.0041 ) (0.0069) (0.0033) (0.019) (0.026) (0.029)

[0247] V 0.012± 0.023± 0.01 1 ± 0.022± 0.01 1 ± 0.01 1 ± 0.017±

[0248] (0) (0.0011 ) (0.0023) (0.0013) (0.0012) (0.0027) (0.001 ) Zn 0.63+ 0.66+ 0.54+ 0.58+ 0.65+ 0.45+ 0.51 +

[0249] (0.016) (0.051 ) (0.048) (0.0072) (0.071 ) (0.059) (0.073)

[0250] *N.D. refers to concentration lower than the detection limit (Cd < 0.38 mg / kg, Co < 0.76 mg / kg, Mo < 1 .89 mg / kg, and Ni < 1 .89 mg / kg) of the ICP-OES.

[0251] Comparative analysis of the FESEM surface images of the oxygen carriers before chemical looping combustion, after chemical looping combustion, and agglomerated particles, reveals evident surface agglomeration of the oxygen carrier particles. In Fig. 3d, the irregular and porous structure of the oxygen carriers surface before chemical looping combustion was apparent. Conversely, Fig. 10a illustrates noticeable surface agglomeration, with surface agglomeration being more pronounced for the agglomerated particles collected after chemical looping combustion. Surface agglomeration diminishes the available surface area, hindering the diffusion of reducing gas into the oxygen carrier and resulting in decreased reactivity, as evidenced in the OTC depicted in Fig. 8c. This reduction in surface area is further supported by Fig. 10c, where FSCuOlO exhibits the highest decline in surface area. Notably, this indicates that a higher CuO content correlates with increased surface agglomeration. This is due to CuO’s low Tamman temperatures at 526 °C, which coincides with the operating temperature condition at 850 °C during chemical looping combustion.

[0252] Despite the observed surface agglomeration, major inter-particulate agglomeration was not observed which is noteworthy. Excessive agglomeration is not favorable as it might result in defluidization and reduces the reactivity of the oxygen carrier. Prevention of defluidization is paramount, as it prevents channeling, thereby ensuring sufficient contact between the fuel and oxygen carrier. Agglomeration of FSCuOl O was the highest at 3.2% with marginal agglomeration observed for the other 3 oxygen carriers which can be seen in Fig. 10b. The minor agglomeration may be attributed to the following effects: (1 ) the agglomeration of CuO itself; (2) CuO modification improving the reduction of Fe leading to deep reduction of the oxygen carrier. Looking into the elemental composition of the agglomerated FSCuOlO particles in Fig. 9a, the Cu content was high with a high error bar. This meant that at different positions in the reactor, there are oxygen carriers with differing Cu content. Agglomeration worsen at positions with higher CuO content which is evidence that increased CuO concentration will cause more agglomeration. The uneven CuO contents in the oxygen carrier at different positions could be due to poor mixing or uneven introduction into the reactor. Nevertheless, the overall agglomeration and sintering phenomenon is reduced substantially for these newly synthesized oxygen carrier with the use of cement as compared to a previous study (C. H. J. Koh Yang et al., Chem. Eng. J. 2023, 465, 142874). This highlights the effectiveness of adding cement in mitigating drastic inter-particulate agglomeration after 50 cycles, crucial in preventing defluidization in the bed. The minimal agglomeration did not result in losses in reactivity as evidenced in the stable performance seen in Fig. 6.

[0253] Besides agglomeration, attrition of oxygen carriers was investigated too. From the results, attrition of the oxygen carrier during chemical looping combustion was low with the highest mass loss at 6.1 % after 50 cycles. Due to outward migration of Cu and Fe, high attrition can result in the loss of active material, thereby reducing reactivity. However, since attrition remained low after 50 cycles, the loss of active components was minimal which is evidenced by the ICP-OES data in Fig. 9a for all 3 oxygen carriers modified by CuO. The attrition for FSCuOl O was 3.44%, which was lower than the other three oxygen carriers. This suggests that some inter-particulate agglomeration during chemical looping combustion can enhance the mechanical strength of the oxygen carrier, thereby reducing attrition.

[0254] Overall, agglomeration and attrition of the oxygen carriers are significant parameters as they provide valuable insights into the effect of CuO and cement modifications and the lifespan of the oxygen carrier. The agglomeration and attrition data obtained provide more evidence that the modification of CuO is beneficial to the oxygen carrier. CuO improves both reactivity and stability of the oxygen carrier while also ensuring low agglomeration and attrition rates. This showcases the competitive advantage of CuO addition into FS modified by cement.

[0255] CuO modification resulted in an increase in OTC, although the improvement was minimal when considering the ratio between experimental and theoretical values of the oxygen carriers. The OTC measurement after 50 cycles yielded similar results, with minimal improvements for oxygen carriers with CuO modifications. Surprisingly, during chemical looping combustion experimental testing itself, the addition of CuO led to significant improvements in stability on top of the reactivity as evident from the chemical looping combustion performance shown in Fig. 6. The increased in addition of CuO at 10% also caused a slight increase in agglomeration but a minor decrease in attrition, while minor addition of CuO did not significantly improve agglomeration or attrition.

[0256] The improvements of oxygen carriers through CuO modifications can be explained as below: (I) the synergistic effect between CuO and Fe species enhanced the reducibility of Fe component, as evidenced by Fig. 6. It is interesting to note that the improvement observed in the present disclosure occurred without the formation of solid solutions such as CuFe2O4 which was identified to demonstrate superior oxygen carrier characteristics compared to Fe2Os and CuO single metal oxides. This would be due to the early reduction of CuO to Cu°, resulting in formation of metallic Cu nuclei. These nuclei can adsorb H2and CO, which can spillover to sites (i.e. Fe phases) close to the nuclei facilitating the improved reduction of the oxygen carrier, (ii) Outward migration of active components during the testing in multiple chemical looping combustion cycles. Oxygen carrier with CuO modification experiences major Fe migration as compared to FSCuOO, as seen in Fig. 9b. The addition of CuO promotes the outward migration of Fe and Cu during chemical looping combustion, reducing gas will not need to penetrate into the oxygen carrier to facilitate reactions, thus improving reactivity. Surface agglomeration observed in all oxygen carriers as seen in Fig. 10a hinders reactivity by preventing penetration into the oxygen carrier, therefore, the outward migration is favorable for the performance stability and reactivity of the oxygen carrier during the chemical looping combustion process. Further, despite the observed surface agglomeration, major interparticulate agglomeration was not observed as only 3.19% of the particles were agglomerated.

[0257] Since FSCuOl O performed the best, it underwent extended testing for 500 cycles to observe its performance. As seen in Fig. 11 , stable performance was observed for approximately 300 cycles, followed by a fluctuating decreasing trend. A follow-up experiment with 1000 cycles revealed that the fluctuating decreasing trend persisted for 400 cycles (from around 300thto around 700thcycle) before stabilizing for the last 300 cycles (from around 700thto 1000thcycle), achieving >89% combustion efficiency throughout each extended cycle run. This demonstrates that FSCuOlO has the potential to be used for 300 cycles before any reduction in efficiency and stability while it can be used for extended cycles if partial or full replacement of fresh oxygen carrier with spent oxygen carrier is performed.

[0258] Conclusion

[0259] The present disclosure aims to develop a high-performance oxygen carrier derived from ferric sludge with the modification of cement and CuO. Comprehensive characterization of the modified oxygen carriers has been conducted focusing on its heavy metal content and fluidization properties. CuO content was optimized considering the synthesis cost, reactivity and stability of the developed oxygen carriers. To ascertain the stability of the oxygen carriers, baseline runs of 50 cycles were carried out and extended runs of the most promising oxygen carriers were also completed. The agglomeration phenomena in the oxygen carriers are also investigated using the data obtained from XRD, FESEM, ICP-OES, and BET analysis. Lastly, an extended chemical looping combustion cycles was conducted to identify the duration of chemical looping combustion the developed oxygen carrier can undergo. The modifications applied to ferric sludge significantly enhanced its performance as an efficient oxygen carrier for chemical looping combustion with municipal solid waste (MSW) syngas. FSCuOl O, in particular, demonstrated high and stable combustion efficiencies, coupled with minimal agglomeration and attrition after 50 cycles. The success of these modifications, achieved through the addition of high aluminate cement to prevent interparticulate agglomeration and the incorporation of CuO to elevate oxygen carrier reactivity and stability, was evident. The present disclosure investigated the chemical and physical transformations of the oxygen carrier before and after chemical looping combustion, revealing outward migration of Fe and Cu, surface agglomeration, and the underlying reduction process associated with the modified oxygen carrier utilization. The findings unveiled that CuO addition brought about significant improvement of stability even at 2% inclusion, while also improving the reactivity of the oxygen carrier which is evident in the results obtained. Extended runs highlighted high stability and reactivity of FSCuOlO for 300 cycles before slight deterioration followed by stabilization from 700 to 1000 cycles. In essence, the present disclosure successfully transformed ferric sludge into an improved oxygen carrier for chemical looping combustion, achieving improved stability and reactivity with the help of both cement and CuO. The outcome is a novel and sustainable oxygen carrier for chemical looping combustion, aligning with the broader goal of circularity of waste products by reutilizing sludge as a value- added resource.

[0260] Therefore, the present disclosure focuses on enhancing the performance of water treatment sludge as an oxygen carrier for chemical looping combustion of MSW syngas. High aluminate cement was introduced to augment the oxygen carrier’s mechanical strength, concurrently preventing inter-particulate agglomeration, resulting in a commendable and low agglomeration rate of 3.44% after 50 cycles. The inclusion of CuO proves to be instrumental in boosting the oxygen carrier’s reactivity, mitigating the loss of active components due to the addition of the cement support. Notably, FSCuOl 0 (Ferric sludge with 10% CuO addition) demonstrates over 92% CO efficiency throughout 50 cycles, affirming the success of the dual modification of FS. The investigation also delves into comprehending the reduction process, agglomeration pathways, and the impact of varied CuO percentages on oxygen carrier performance. Extensive characterizations were conducted to elucidate oxygen carrier transformations and their consequential effects on reactivity, physical characteristics, and overall performance. A noteworthy observation includes the outward migration of iron and copper during chemical looping combustion, contributing to the stabilization of reactivity. In summary, the modifications implemented on FS yield an improved oxygen carrier for chemical looping combustion purposes, maintaining its novelty and cost-effectiveness. This research contributes to the circularity of waste-derived products, effectively repurposing water treatment sludge and contributing to improving sustainable waste management practices.

[0261] Hence, the present disclosure is positioned to achieve a high syngas combustion efficiency during chemical looping combustion, to maintain high syngas combustion efficiency throughout multiple cycles, and to reduce the impacts of severe agglomeration and attrition after multiple cycles.

Claims

Claims1 . An oxygen carrier for chemical looping combustion, comprising: iron; copper; calcium; aluminium; and silicon, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element.

2. The oxygen carrier according to Claim 1 , wherein: iron is present in an amount of from 30 to 50 wt%; copper is present in an amount of from 5 to 20 wt%; calcium is present in an amount of from 15 to 30 wt%; aluminium is present in an amount of from 5 to 20 wt%; and silicon is present in an amount of from 1 to 15 wt%.

3. The oxygen carrier according to Claim 2, wherein: iron is present in an amount of from 32.1 to 41 .2 wt% (e.g. from 35 to 40 wt%, such as about 36.65 wt%); copper is present in an amount of from 10 to 15 wt% (e.g. from 1 1 .32 to 15.24 wt%, such as about 13.28 wt%); calcium is present in an amount of from 20 to 25 wt% (e.g. from 21 .62 to 24.94 wt%. such as about 23.28 wt%); aluminium is present in an amount of from 10 to 15 wt% (e.g. from 12.78 to 13.2 wt%, such as about 12.99 wt%); and silicon is present in an amount of from 5 to 10 wt% (e.g. from 7.65 to 9.55 wt%, such as about 8.60 wt%).

4. The oxygen carrier according to any one of the preceding claims, wherein the oxygen carrier further comprises one or more trace elements selected from potassium, magnesium, manganese, sodium, sulfur, titanium and zinc, wherein the elements are provided in one or both of their elemental form or ionic form as part of a compound comprising said element.

5. The oxygen carrier according to Claim 4, wherein, when present: potassium is present in an amount of from 0.7 to 0.8 wt%, such as about 0.75 wt%; magnesium is present in an amount of from 1 .7 to 2.0 wt%, such as about 1 .77 wt%;manganese is present in an amount of from 0.18 to 0.25 wt%, such as about 0.21 wt%; sodium is present in an amount of from 0.40 to 0.45 wt%, such as about 0.43 wt%; sulfur is present in an amount of from 0.60 to 0.67 wt%, such as about 0.63 wt%; titanium is present in an amount of from 0.14 to 0.16 wt%, such as about 0.15 wt%; and zinc is present in an amount of from 0.63 to 0.67 wt%, such as about 0.65 wt%.

6. The oxygen carrier according to any one of the preceding claims, wherein the oxygen carrier displays characteristic peaks in an X-ray powder diffraction diffractogram of 20: 23.95°, 30.35°, 31.64°, 33.87°, 35.82°, 39.01°, 52.25° and 47.03°, such as 23.34°, 23.95°, 25.85°, 29.38°, 30.35°, 31.64°, 32.37°, 33.87°, 34.76°, 35.82°, 39.01 °, 43.59°, 47.03°, 48.89°, 52.25°, 52.94°, 58.53°, 63.1 1°, 66.67°, and 68.37°.

7. The oxygen carrier according to any one of the preceding claims, wherein the oxygen carrier, before use, comprises Fe2O3, Ca2Fe2Os, Ca2AI2SiO7, and CuO, optionally wherein Fe2Oa and CuO are active materials, Ca2AhSiO7 is a support material and Ca2Fe2Os is a support material and / or an active material in chemical looping combustion.

8. The oxygen carrier according to any one of the preceding claims, wherein the oxygen carrier is capable of being used for 300 chemical looping combustion cycles, while retaining a high combustion efficiency, optionally wherein the combustion efficiency is greater than 90%, such as greater than 93%.

9. The oxygen carrier according to any one of the preceding claims, wherein the oxygen carrier is capable of being used for 700 chemical looping combustion cycles, while retaining a combustion efficiency greater than or equal to 89%.

10. The oxygen carrier according to any one of the preceding claims, wherein one or more of the following apply:(a) the oxygen carrier has an experimental oxygen transport capacity of from 6 to 19, such as from 7 to 8.1 , such as about 7.9;(b) a bulk density of from 0.6 to 0.8 g / cm3, such as about 0.75 g / cm3;(c) a BET surface area of from 3 to 4 m3 / g such as about 3.44 m3g;(d) a pore volume of from 0.01 to 0.03 mL / g, such as about 0.015 mL / g; and(e) a particle size of from 0.15 to 1 mm, such as from 0.25 to 0.5 mm.

11. A method of manufacturing an oxygen carrier as described in any one of Claims 1 to 10, the method comprising the steps of:(I) providing a cured mixture comprising water, a water treatment sludge, CuO, and high calcium aluminate cement; and(ii) calcining the cured mixture for a first period of time at a temperature of from 900 to 1,500 °C, followed by grinding and sieving to provide the desired oxygen carrier material.

12. The method according to Claim 1 1 , wherein the cured mixture composition is obtained by the steps of:(ai) providing an uncured mixture comprising water, a water treatment sludge, CuO, and high calcium aluminate cement; and(aii) placing the uncured mixture into a mould for a period of from 1 day to 10 days (e.g. 3 days) at a temperature of from 10 to 50 °C, such as about 25 °C to provide the cured mixture composition.

13. The method according to Claim 12, wherein the uncured mixture is obtained by the steps of:(bi) providing a mixture comprising from 40 to 70 wt% of a water treatment sludge, from 10 to 25 wt% of CuO, from 20 to 30 wt% of a high calcium aluminate cement; and (bii) adding water to the mixture in a solid to liquid ratio of about 1 .375:1 .

14. The method according to any one of Claims 1 1 to 13, wherein the water treatment sludge is once comprising iron and / or compounds thereof.

15. Use of an oxygen carrier as described in any one of Claims 1 to 10 in a chemical looping combustion process.

16. A method of chemical looping combustion, the method comprising the steps of:(ci) providing a chemical looping combustion reactor system comprising an oxygen carrier as described in any one of Claims 1 to 10; and(cii) subjecting a gas in need thereof to chemical looping combustion.

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