Catalyst for supercritical water gasification

Abstract

CATALYST FOR SUPERCRITICAL WATER GASIFICATION The present invention relates to a catalyst for use in supercritical water gasification of biomass waste. Due to high cost and inaccessibility of conventional catalysts, a cost- effective catalyst produced from widely available materials through a simple preparation method is highly desired. The present invention provides a catalyst composition for supercritical water gasification comprising the combination of an intermetallic compound and activated carbon to achieve doubled catalyst support; and a method of preparing the catalyst in absence of prior treatment or intermediate treatment. Furthermore, the present invention provides a process for producing syngas by supercritical water gasification. The catalyst herein results in a significant increase in hydrogen and carbon dioxide yield from biomass, thus improving the hydrogen gasification efficiency and carbon dioxide gasification efficiency respectively.

Description

[0001] CATALYST FOR SUPERCRITICAL WATER GASIFICATION

[0002] FIELD OF INVENTION

[0003] The present invention relates to a catalyst for use in supercritical water gasification of organic biomass, comprising the combination of an intermetallic compound and activated carbon.

[0004] BACKGROUND OF INVENTION

[0005] Supercritical water gasification is a process for producing syngas from biomass feedstock at supercritical conditions with water as the reaction medium. The process is conducted at supercritical water point, in which the phase boundary between liquid and gas becomes weak. Water exhibits unique properties at this point, one of which is it becomes miscible with organic materials. Consequently, water becomes an ideal solvent for organic compounds, thus encouraging rapid decomposition of the biomass feedstock.

[0006] Concurrently, the high temperature of supercritical water gasification enables water-gas shift reaction. The reaction between the biomass and water therein produces hydrogen and carbon dioxide (CO + H2O CO2 + H2). Noble metal or alkaline salts are typically used as catalysts to improve product yields and reaction conditions. However, noble metal catalysts such as ruthenium and palladium are expensive and tend to become poisoned in the presence of sulphur, whereas alkaline salts may be insoluble in supercritical water as well as being corrosive to the reactor wall. As such, there is a need for a catalyst for supercritical water gasification that is cost-effective and accessible.

[0007] DI (Fidalgo, B., Arenillas, A. and Menendez, J. A. (2010) 'Synergetic effect of a mixture of activated carbon + Ni / A Os used as catalysts for the CO2 reforming of CHS, Applied Catalysis A: General, 390(1-2), pp. 78-83. doi: 10.1016 / j.apcata.2010.09.031) relates to a study on the catalytic activity of heterogeneous mixtures comprising activated carbon and an in-lab prepared Ni / Al2O3 in dry reforming process for the production of synthesis gas (H2 + CO). The Ni / AhOs catalyst is prepared by impregnating commercial pellets of alumina with an aqueous solution of Ni(NO3)2-6H2O, while the activated carbon FY5 is made from coconut shell. DI discloses a synergetic effect is observed between the carbonaceous fraction (FY5) and the metal-based fraction (Ni / Al2O3), in which CH4 decomposition occurs preferentially over the Ni-based fraction while CO2 tends to reactwith initial carbonaceous fraction. DI also teaches that the conversion of CO2 and CF over different mixtures of FY5 + Ni / AhOs are steady and higher than typically obtained from the mixtures law.

[0008] D2 (China Patent Publication No. CN115532266B published on 21 November 2023 entitled "Ni-Cu / AC catalyst for preparing gas fuel through hydrothermal conversion of indole and derivative thereof and preparation method of Ni-Cu / AC catalyst, Applicant: Xi'an Jiaotong University) relates to the preparation of a catalyst for the hydrothermal conversion of indole and its derivatives into gaseous fuels. D2 discloses the catalyst is prepared by dissolving nickel nitrate hexahydrate and copper nitrate trihydrate into a solution, adding activated carbon as a carrier to the Ni-Cu solution, and then drying the mixture to obtain a catalyst powder. D2 further teaches that the catalyst is used in supercritical water gasification reaction with a reaction temperature of 550-600°C and a reaction time of 30-40 min, while the catalyst loading amount is 10% of the mass of indole or its derivatives.

[0009] D3 (Japan Patent Publication No. JP4010369B2 published on 21 November 2007 entitled "Hydrothermal gasification catalyst, method for producing catalyst, and method for treating aqueous liquid using catalyst", Applicant: Kyoto University) relates to a novel hydrothermal gasification catalyst comprising at least cobalt catalyst supported on a carbon-based carrier, and the catalyst may be utilized in existing gasification systems as well as in supercritical systems. D3 discloses the catalyst may further carry another metal for catalytic support which is preferably nickel, and the combination form of the catalytic metals is not limited to an alloy as long as the catalyst can be obtained.

[0010] D4 (PCT Patent Publication No. WO2016095478A1 published on 23 June 2016 entitled "Composite catalyst for preparing hydrogen by supercritical water gasification of low-moisture-content dehydrated sludge and application of composite catalyst", Applicant: Zhejiang University of Technology) relates to a composite catalyst for hydrogen production through supercritical water gasification of low-moisture dehydrated sludge. D4 teaches the composite catalyst is prepared from a mixture of active nickel, a carbon fixation agent and an alkali metal salt, and the catalyst can be used for catalyzing low-moisture-content dehydrated sludge at a relatively low reaction temperature to generate hydrogen.

[0011] D5 (China Patent Publication No. CN103657649B published on 06 July 2016 entitled "A method for preparing carbon-supported nano-platinum-chromium intermetalliccompound as a cathode catalyst for proton exchange membrane fuel cells", Applicant: Shanghai Institute for Advanced Study, Chinese Academy of Sciences) relates to a method for preparing a carbon-supported nano-platinum-chromium intermetallic compound as cathode catalyst for a proton exchange membrane fuel cell. The catalyst utilises a mixture of platinum-containing compound and the chromium-containing compound to form the intermetallic catalyst while the carbon support is obtained from metal carbonyl cluster.

[0012] From the identified prior art, it is observed that an intermetallic catalyst with carbon support has limited application in supercritical water gasification with most of the relevant disclosures relating to the manufacture of fuel cells. Existing catalysts further require pre-treatment steps such as impregnation or deposition before being utilized in the relevant reaction. Therefore, a cost-effective catalyst produced is highly desired, especially a catalyst that is produced from widely available materials through a simple preparation method.

[0013] SUMMARY OF INVENTION

[0014] In a first aspect of the present invention, a catalyst composition for use in promoting supercritical water gasification reaction of biomass is provided. The catalyst composition comprises an intermetallic compound, further comprising a metallic element and a support element; and activated carbon as additional support for the intermetallic compound.

[0015] In another aspect of the invention, the intermetallic compound and the activated carbon has a ratio of 1:1.

[0016] In further aspect of the invention, the metallic element of the intermetallic compound comprises nickel and the support element comprises aluminium oxide. Preferably, the metallic element and the support element are provided in a ratio of 1:1.

[0017] In yet another aspect of the invention, the intermetallic compound is selected from commercial nickel-aluminium, Ni / AI catalyst.

[0018] In a second aspect of the invention, a method of producing a catalyst for supercritical water gasification of biomass is disclosed. The method comprises steps of preparing an intermetallic compound and activated carbon in a 1:1 ratio; and contacting theintermetallic compound and the activated carbon at ambient temperature. Furthermore, the step of preparing is conducted in absence of prior treatment or intermediate treatment; and the step of contacting the intermetallic compound and the activated carbon generates a doubled catalyst support effect.

[0019] In an aspect of the invention, the intermetallic compound and the activated carbon are contacted in solid phase.

[0020] In another aspect of the invention, the prior treatment comprises impregnation or precipitation; and the intermediate treatment comprises deposition, adsorption, drying, calcination or reduction.

[0021] In yet another aspect of the invention, the intermetallic compound comprises a metallic element and a support element; and further comprises nickel as the metallic element and aluminium oxide as the support element.

[0022] In further aspect of the invention, the intermetallic compound is selected from commercial nickel-aluminium, Ni / AI catalyst.

[0023] In a third aspect of the invention, a process for production of syngas by supercritical water gasification is disclosed. The process comprises providing a feedstock comprising biomass, preparing a catalyst according to the method of the second aspect, loading a reactor with water, feeding the feedstock and the catalyst simultaneously into the reactor, and performing gasification under a plurality of operating conditions.

[0024] In an aspect of the invention, the ratio of biomass to catalyst is 1:12.5.

[0025] In another aspect of the invention, the ratio of biomass to water is between 1:350 to 1:35.

[0026] In yet another aspect of the invention, the plurality of operating conditions comprises an operating temperature in the range of 257-391°C, an operating pressure in the range of 16-26 MPa and a runtime of up to 5 hours.

[0027] In further aspect of the invention, the operating temperature is supercritical temperature in the range of 373-380°C.In another aspect of the invention, the operating pressure is in the range of 21-23 MPa.

[0028] In yet another aspect of the invention, the biomass comprises algae biomass, agricultural waste, organic industrial waste or other organic waste.

[0029] DETAILED DESCRIPTION

[0030] Described below are preferred embodiments of the present invention. Each of the following preferred embodiments describes an example not limiting in any aspect.

[0031] Supercritical water gasification is a hydrothermal gasification process carried out to decompose biomass waste particularly under supercritical conditions with water as the solvent. Supercritical water gasification is notably beneficial for processing wet biomass as the high moisture content thereof does not heavily impact the efficiency of the gasification process. Prior drying of wet biomass is therefore unnecessary, reducing the overall energy consumption and effectively negate the carbon footprint from biomass waste. The biomass waste may derive from various abundant sources including algal biomass such as Spirulina, Chlorella vulgaris and Nannochloropsis; agricultural waste such as oil palm empty fruit bunches (EFB), palm kernel shell and palm oil mill wastewater; organic industrial waste as well as other organic waste not listed herein.

[0032] Catalysts are typically applied in the gasification reaction to optimise the overall process. Conventionally, catalysts are synthesized by conducting prior treatment methods such as precipitation and impregnation. For example, precipitation involves adding a precipitating agent to a dissolved catalyst precursor and filtering out the solid catalyst precipitated therein, whereas impregnation involves impregnating a dissolved catalyst precursor with a solid support material before calcining the impregnated mixture to obtain the final form of the catalyst. Furthermore, catalysts are typically introduced to the reactants through a medium for optimal contact with reactants. To obtain such medium, intermediate treatment such as deposition and adsorption are required to contact a catalyst precursor solution onto a support material through physical or chemical interactions. Additional steps and preparations are therefore necessary to produce a catalyst according to conventional methods, which may extend the overall period of the gasification process.

[0033] As previously noted, the high temperature applied during supercritical water gasification initiates a water-gas shift reaction to produce hydrogen and carbon dioxide from thebiomass waste. To further promote the production of syngas, especially hydrogen, the present invention provides a catalyst applicable under supercritical water conditions for gasification of biomass. A key objective of the invention is to produce a catalyst with doubled catalyst support achieved from the combination of two main components, namely an intermetallic compound and activated carbon. This is further described below.

[0034] The intermetallic compound is favoured for its robust physical properties and well-defined stoichiometry and typically formed by at least two metallic elements in solid phase. Unlike metal alloys, atoms in intermetallic compounds are highly ordered in unique crystalline structures as well as comprising numerous active sites. These characteristics form a stable compound with high catalytic activity which is desirable to achieve high conversion rates. In the present invention, the intermetallic compound comprises a metallic element and a metal-based support element.

[0035] In a preferred embodiment of the invention, nickel is selected as the metallic element of the intermetallic compound in view of its accessibility and cost effectiveness, in addition to high catalytic activity in hydrogen production. Aluminium oxide is selected as the metal-based support element for its thermal stability and its characteristic as a Lewis acid catalyst. Aluminium oxide is also porous, providing a large surface area for contact with the metal element. The combination of Ni / AI is especially preferred for its resistance to oxidation and high selectivity towards hydrogen. The ratio of the metallic and support elements in the intermetallic compound is preferably 1:1.

[0036] Another component in the catalyst of the present invention is activated carbon, which is favoured for its economic value and mass availability. The activated carbon may be obtained from industrial and / or commercial sources, such as commercially available charcoal. The porous structure of activated carbon provides a large surface area for contact, thus creating a structural support framework for the already-supported intermetallic compound. The structural framework from activated carbon generates a doubled catalyst support effect which improves the stability and durability of the catalyst of the present invention. The inertness of activated carbon further allows a stable gasification reaction under high temperature and pressure conditions. Another advantage of utilizing activated carbon is its capability for self-gasification, which enhances the production of gas and increases the carbon gasification efficiency.

[0037] An advantage of the present invention is that prior treatment of the catalyst through known methods such as impregnation or precipitation are not required. This is as theintermetallic compound and the activated carbon are contacted by directly mixing the components in solid phase, most preferably using the powders thereof. The mixing step is conducted at ambient temperature, which removes intermediate steps such as adsorption, drying and / or calcination that are typically required to obtain the final form of the catalyst. Preparing the catalyst using components in solid state further allows precise measurements of the reactants to accurately fulfill the stoichiometry in the gasification reaction. Intensive energy consumption from high temperature treatments is also avoided since ambient temperature is sufficient for obtaining the catalyst.

[0038] Furthermore, the mixture of the intermetallic compound and the activated carbon creates a synergistic effect, in which the activity of one catalyst enhances the other catalyst and vice versa. In particular, the synergistic effect is observed in application of the catalyst as tar production by the intermetallic compound is counteracted by the activated carbon as it concurrently catalyses the cracking and decomposition of the tar into hydrogen and carbon monoxide.

[0039] Another feature of the present invention is a method of preparing the catalyst for use in supercritical water gasification reaction of biomass. The catalyst comprises an intermetallic compound, further comprising a metallic element and a support element; and activated carbon as additional support for the intermetallic compound. Particularly, solid forms of the intermetallic compound and the activated carbon are utilized and prepared in a 1:1 ratio. As previously discussed, the intermetallic compound and the activated carbon are contacted at ambient temperature in absence of prior treatment or intermediate treatment, and the contact generates a doubled catalyst support effect in the catalyst.

[0040] The present invention further provides a process for producing syngas by supercritical water gasification. Feedstock comprising biomass and the catalyst prepared according to the above-described method are provided in a ratio of 1:12.5. A reactor is loaded with water, in which the ratio of biomass to water ratio is 1:350. Depending on the working capacity of the reactor, the ratio of biomass to water may be increased to 1:35. The catalyst and the biomass are fed into the reactor together by one-step addition. The gasification reaction is then conducted under a plurality of operating conditions. Particularly, the gasification is operable at, but not limited to, a temperature range of 257-391°C and a pressure range of 16-26 MPa over a runtime of up to 5 hours. The catalyst is activatable at this range of operating temperature, allowing for simultaneous reaction with the feedstock.With the one-step addition of the reactants, the present invention offers the advantage of eliminating the need for a catalyst medium. The simultaneous introduction into the reactor enhances contact between the catalyst and the biomass and subsequently promote the catalytic activity in the reaction. Moreover, the operating costs and requirements for the reactor may be reduced since a catalyst medium is no longer necessary, thus simplifying the gasification process.

[0041] In a preferred embodiment of the invention, the process is performed at supercritical water point with the optimum reaction temperature being in the range of 373-380°C. The process is also performed at the preferred operating pressure in the range of 21 to 23 MPa with over a runtime of up to 5 hours. For the intermetallic compound, the metallic element is preferably nickel, and the support element is preferably aluminium oxide, AI2O3, with the elements having a 1:1 ratio. Most preferably, the intermetallic compound is obtained from readily available commercial nickel-aluminium, Ni-AI catalyst. The activated carbon is acquired in powder form, preferably from commercially available charcoal. The ratio of the biomass to the catalyst is preferably 1:12.5. In the instance of conducting supercritical water gasification in a 1 -I iter continuous stirred tank reactor, the catalyst is prepared by mixing 6.25 g of Ni / AI intermetallic compound and 6.25 g activated carbon with respect to 1.0 g of biomass and 350 mL of water.

[0042] From the experimental results collected, it is noted that the use of the catalyst has produced steady levels of hydrogen yield over repeated runs of supercritical water gasification, indicating the stability of the catalyst under high operating conditions. Syngas production, especially hydrogen, is significantly enhanced from the use of the present catalyst due to high catalytic activity of the nickel-aluminium intermetallic compound and self-gasification quality of the activated carbon. Furthermore, the high ratio of biomass to catalyst applied in the reaction allows a generous distribution of the catalyst onto the biomass. This generates an overwhelming amount of catalytic activity to the biomass, thus promoting higher conversions of the biomass into the desired hydrogen and carbon dioxide product gases. For example, the use of the catalyst in gasification of Chlamydomonas algae biomass has resulted in over 200% increase of carbon dioxide yield while the yield of hydrogen increases over 500% compared to the use of carbon supported-ruthenium catalyst. This is further discussed in the examples.EXAMPLES

[0043] In the context of the present invention, the carbon gasification efficiency (CGE) and the hydrogen gasification efficiency (HGE) are used to determine the amount of carbon and hydrogen successfully converted into the respective product gas. The following formulae are used to calculate the values of CGE and HGE:

[0044] Total mass of carbon in gaseous product

[0045] Carbon gasification efficiency (CGE) x 100%

[0046] Mass of carbon in the feedstock

[0047] Total mass of hydrogen in gaseous product Hydrogen gasification efficiency (HGE) = x 100% Mass of hydrogen in the feedstock

[0048] The CGE and HGE values are further used to evaluate the performance of the catalyst of the present invention against conventional catalysts and consequently establishing the effectiveness of the catalyst.

[0049] Table 1 provides a comparison of the product gas yield obtained from the supercritical water gasification of Chlamydomonas algae sample using a conventional ruthenium-charcoal catalyst (Ru / C), conventional nickel-aluminium catalyst (Ni / AI), and the catalyst of the present invention (Ni / AI + AC).

[0050]

[0051] Table 1

[0052] Between the conventional Ru / C and Ni / AI catalysts, it is noted that the amount of hydrogen produced is comparable. However, the Ru / C catalyst is considerably more effective in producing higher carbon and hydrogen gasification efficiencies, which is consistent with prior literature.The results in Table 1 further indicate that the performance of the catalyst of the present invention is superior to the performance of conventional Ru / C and Ni / AI catalysts.

[0053] In comparison with the conventional Ni / AI catalyst, it is found that the catalyst of the present invention has increased the hydrogen yield significantly by 9 times while the carbon dioxide yield increased by almost 12 times when compared to the yields produced using the conventional Ni / AI catalyst. As a result, the present invention has drastically increased both the carbon and hydrogen gasification efficiencies by over 1000%. This substantial improvement demonstrates the significance of adding carbon to the Ni / AI catalyst to achieve additional catalyst support for the present invention.

[0054] In comparison with the conventional Ru / C catalyst, the catalyst of the present invention has produced a greater amount of product gas than the Ru / C catalyst with around 5 times increase in hydrogen yield and over 2 times increase in carbon dioxide yield. It is further demonstrated that the use of the catalyst of the present invention has increased the carbon gasification efficiency by almost 300% while the hydrogen gasification efficiency increased by over 500% compared to the Ru / C catalyst. It is therefore evident that the catalyst of the present invention outperforms the conventional Ru / C catalyst.

[0055] Table 2 discusses the effects of the operating temperature on the catalytic activity of the present invention towards Chlamydomonas algae sample during supercritical water gasification process.

[0056]

[0057] Table 2It is demonstrated therein that the product gas yield and the gasification efficiencies are highest when the supercritical water gasification reaction is conducted at supercritical water point at 373-380°C, whereas reactions conducted at temperatures lower and higher than the supercritical water point do not generate comparable product gas yields.

[0058] Furthermore, the above results indicate that the activity of the catalyst of the present invention peaks at supercritical water point, with around 22-40% increase in CGE and HGE values compared to the values achieved at 355°C and 391°C. While the catalyst of the present invention is applicable at a wider temperature range of 257-391°C, the results suggest that the catalyst is optimally utilised at supercritical water point in the range of 373-380°C to maximise its performance and achieve the highest yield of hydrogen and carbon dioxide gases.

[0059] Table 3 discusses the performance of the catalyst of the present invention on different types of biomass feedstock.

[0060]

[0061]

[0062] Table 3

[0063] The carbon-supported Ni / AI catalyst of the present invention is shown to improve the product gas yield and the gasification efficiencies of all five investigated feedstock. Notably, the use of the catalyst of the present invention with various biomass feedstock has at least tripled the hydrogen yield and at least doubled the carbon dioxide yield. As a result, the carbon gasification efficiency exhibited an increase between 200-400% for the above biomass samples while the hydrogen gasification efficiency increased between 300-1000%. The high conversions achieved through the use of the catalyst demonstrate the adaptability of the present invention with various organic compositions.

[0064] From the above examples, it is clear that the catalyst of the present invention demonstrates better performance than conventional catalysts in view of the substantial increase in product gas yield, and consequently, the improvement in the gasification efficiencies for carbon and hydrogen. It is further indicated that the optimum operating temperature for the catalyst of the present invention is at supercritical water point with the highest gas yield is produced at the temperature range of 373-380°C. Additionally, the catalyst of the present invention is found to significantly increase the product gas yields and gasification efficiencies of different types of biomass feedstock, establishing its effectiveness towards various biomass categories.

[0065] While the preferred embodiments of the present invention have been described herein, it should now be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope of the invention. Accordingly, the following claims are intended to embrace such changes, modifications, and areas of application that are within the scope of this invention.

Claims

CLAIMS1. A catalyst composition for promoting supercritical water gasification of biomass comprising:an intermetallic compound comprising a metallic element and a support element; andactivated carbon as additional support for the intermetallic compound.

2. The catalyst composition of claim 1, wherein the intermetallic compound and the activated carbon have a ratio of 1:1.

3. The catalyst composition of claim 1, wherein the metallic element comprises nickel and the support element comprises aluminium oxide, the metallic element and the support element having a ratio of 1:1.

4. The catalyst composition of claim 3, wherein the intermetallic compound is selected from commercial nickel-aluminium, Ni / AI catalyst.

5. A method of producing a catalyst for supercritical water gasification of biomass comprising steps of:preparing an intermetallic compound and activated carbon in a 1:1 ratio; andcontacting the intermetallic compound and the activated carbon at ambient temperature;wherein:the step of preparing is conducted in absence of prior treatment or intermediate treatment; andthe step of contacting the intermetallic compound and the activated carbon generates a doubled catalyst support effect.

6. The method of claim 5, wherein the intermetallic compound and the activated carbon are contacted in solid phase.

7. The method of claim 5, wherein the prior treatment comprises impregnation or precipitation; and the intermediate treatment comprises deposition, adsorption, drying, calcination or reduction.

8. The method of claim 5, wherein the intermetallic compound comprises a metallic element and a support element in a 1:1 ratio; and further comprises nickel as the metallic element and aluminium oxide as the support element.

9. The method of claim 8, wherein the intermetallic compound is selected from commercial nickel-aluminium, Ni / AI catalyst.

10. A process for producing syngas by supercritical water gasification comprising:providing a feedstock comprising biomass;preparing a catalyst according to the method of claim 5-9;loading a reactor with water;feeding the feedstock and the catalyst simultaneously into the loaded reactor; andperforming gasification under a plurality of operating conditions.

11. The process of claim 10, wherein a ratio of biomass to catalyst is 1: 12.5.

12. The process of claim 10, wherein a ratio of biomass to water is between 1:350 to 1:35.

13. The process of claim 10, wherein the plurality of operating conditions comprises an operating temperature in the range of 257-391°C, an operating pressure in the range of 16-26 MPa and a runtime of up to 5 hours.

14. The process of claim 13, wherein the operating temperature is supercritical temperature in the range of 373-380°C.

15. The process of claim 13, wherein the operating pressure is in the range of 21-23 MPa.

16. The process of claim 10, wherein the biomass comprises algae biomass, agricultural waste, organic industrial waste or other organic wastes.

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