AU2015100328A4 - Power production from ucg product gas with carbon capture - Google Patents

Abstract

The invention provides methods of generating electric power from underground coal seams with capture of carbon dioxide. co (c0 N-N LC)O L,, 'IC"D No Li Co N N0-

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: Linc Energy Ltd Invention Title: POWER PRODUCTION FROM UCG PRODUCT GAS WITH CARBON CAPTURE The invention is described in the following statement: 1 POWER PRODUCTION FROM UCG PRODUCT GAS WITH CARBON CAPTURE TECHNICAL FIELD [0001] The present invention pertains to methods of electric power generation that utilise underground coal gasification (UCG) product gas as a fuel source for producing combustion reactions, with low carbon dioxide emissions. BACKGROUND ART [0002] Underground coal gasification is a process by which product gas is produced from a coal seam by combusting and gasifying the coal in situ in the presence of an oxidant. The product gas is typically referred to as synthesis gas or syngas and can be used as a feedstock for various applications, including clean fuels production, chemical production, and electricity generation. [0003] Wells are drilled into the coal seam to allow for oxidant injection and product gas extraction. The wells are linked or extended to form an in-seam well channel to facilitate oxidant injection, cavity development, and product gas flow. [0004] The well allowing the injection of oxidant is called an injection well. The well from which product gas emerges is called a production well. Both horizontal and vertical well regions can be used for injection and production. Underground coal gasification may also utilise one or more vertical wells (service wells) located between the injection and production wells. [0005] A coal seam having an in-seam well channel is typically referred to as a coal gasifier. The gasifier will have a combustion zone within which coal is combusted in the presence of an oxidant, a gasification zone located downstream of the combustion zone in which coal is gasified and partially oxidized to produce product gas, and a downstream pyrolysis zone in which pyrolysis of coal occurs. Hot product gas flows downstream from the gasification zone and exits the ground from a well head of the production well. As coal is consumed or gasified, a gasifier (gasification) cavity within the coal seam develops and grows in size.

2 [0006] Typically, UCG product gas will contain: (1) main syngas components (e.g., CO, H 2 , C02, N 2 , and CH 4 ); (2) solid particles/particulates (e.g., soot, ash and coal particles); (3) water; (4) minor components such as C2-C6 hydrocarbons, oxygen, argon, sulphur containing components (e.g., H 2 S, COS, CS 2 , mercaptans, and thiophenes), nitrogen based components (e.g., NH 3 and HCN), hydrocarbon components (e.g., coal condensate, BTEX (benzene, toluene, ethylbenzene and xylenes) and PAHs (polycyclic aromatic hydrocarbons)); and (5) trace components such as heavy metals (arsenic and mercury) and chlorides. [0007] Rankine cycle power generation systems (e.g., steam power plants) have long been in use as a source of electric power. A typical steam power plant includes a boiler, which heats a pressurised working fluid (typically water) to form high temperature/pressure steam. The high temperature/pressure steam is then fed into a turbine where it is expanded to a lower pressure and reduced to a lower temperature. The turbine outputs power from the power plant. Thereafter, the steam is typically condensed back to a liquid for recirculation in the system. [0008] Coal is a preferred fuel source for Rankine cycle power generation systems due its low cost. However, coal combustion based Rankine cycle power generation systems generate significant amounts of carbon dioxide. [0009] While Rankine cycle power generation systems fuelled by coal are highly effective in producing electric energy, there is a need for systems that capture carbon dioxide from coal combustion, thus enabling coal-based Rankine cycle power generation systems with low carbon dioxide emissions. SUMMARY OF INVENTION [0010] An object of the present invention is to provide a coal-based Rankine cycle power generation system and method for power generation with low carbon dioxide emissions (i.e., capture of carbon dioxide produced). [0011] In one aspect, the invention provides a method of generating electric power from an underground coal seam with carbon dioxide capture, including the 3 steps of: (i) gasifying the underground coal seam in situ, (ii) collecting UCG product gas from the underground coal seam, (iii) passing the product gas through a water wash, (iv) removing sulphur from the product gas, (v) combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture, (vi) expanding the high temperature steam/carbon dioxide mixture through a plurality of turbines to generate electric power, (vii) condensing the steam to recover the carbon dioxide, (viii) dehydrating the recovered carbon dioxide, and (ix) compressing the recovered carbon dioxide, wherein gasifying the underground coal seam in situ includes injecting substantially pure oxygen and carbon dioxide into the underground coal seam. [0012] In another aspect, the invention provides a method of generating electric power from an underground coal seam with carbon dioxide capture, including the steps of: (i) gasifying the underground coal seam in situ, (ii) collecting UCG product gas from the underground coal seam, (iii) removing particulates from the product gas, (iv) combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture, (v) expanding the high temperature steam/carbon dioxide mixture through a plurality of turbines to generate electric power, (vi) condensing the steam to recover the carbon dioxide, (vii) dehydrating the recovered carbon dioxide, (viii) compressing the recovered carbon dioxide, and (ix) removing SOx and/or NOx from the recovered carbon dioxide, wherein gasifying the underground coal seam in situ includes injecting substantially pure oxygen and carbon dioxide into the underground coal seam. [0013] In one embodiment, the carbon dioxide injected into the underground coal seam is recovered carbon dioxide (i.e., recovered from combustion of the UCG product gas and substantially pure oxygen in the gas generator). [0014] In another embodiment, the plurality of turbines include a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine.

4 [0015] In a further embodiment, the high temperature steam/carbon dioxide mixture is passed through a reheater after leaving the high-pressure turbine and prior to entering the intermediate-pressure turbine to increase the temperature of the mixture. [0016] Suitably, the reheater combusts UCG product gas with substantially pure oxygen. [0017] In yet another aspect, the invention provides a method of generating electric power from an underground coal seam with carbon dioxide capture, including the steps of: (i) gasifying the underground coal seam in situ, (ii) collecting UCG product gas from the underground coal seam, (iii) passing the product gas through a water wash, (iv) removing sulphur from the product gas, (v) combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture, (vi) using the high temperature steam/carbon dioxide mixture to generate steam in a pressurised boiler, (vii) expanding the generated steam through a turbine to generate electric power, (viii) condensing the steam in the steam/carbon dioxide mixture to recover the carbon dioxide, (ix) dehydrating the recovered carbon dioxide, and (x) compressing the recovered carbon dioxide, wherein gasifying the underground coal seam in situ includes injecting substantially pure oxygen and carbon dioxide into the underground coal seam. [0018] In a further aspect, the invention provides a method of generating electric power from an underground coal seam with carbon dioxide capture, including the steps of: (i) gasifying the underground coal seam in situ, (ii) collecting UCG product gas from the underground coal seam, (iii) removing particulates from the product gas, (iv) combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture, (v) using the high temperature steam/carbon dioxide mixture to generate steam in a pressurised boiler, (vi) expanding the generated steam through a turbine to generate electric power, (vii) condensing the steam in the steam/carbon dioxide mixture to recover the carbon dioxide, (viii) dehydrating the recovered carbon 5 dioxide, (ix) compressing the recovered carbon dioxide, and (x) removing SOx and/or NOx from the recovered carbon dioxide, wherein gasifying the underground coal seam in situ includes injecting substantially pure oxygen and carbon dioxide into the underground coal seam. [0019] In one embodiment, the carbon dioxide injected into the underground coal seam is recovered carbon dioxide. BRIEF DESCRIPTION OF DRAWINGS [0020] Figure 1 is a flow diagram of a method for generating electric power from an underground coal seam with carbon dioxide capture, according to an embodiment of the present invention. [0021] Figure 2 is a flow diagram of another method for generating electric power from an underground coal seam with carbon dioxide capture, according to an embodiment of the present invention. [0022] Figure 3 is a flow diagram of yet another method for generating electric power from an underground coal seam with carbon dioxide capture, according to an embodiment of the present invention. [0023] Figure 4 is a flow diagram of a further method for generating electric power from an underground coal seam with carbon dioxide capture, according to an embodiment of the present invention. DESCRIPTION OF EMBODIMENTS [0024] Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to mean the inclusion of a stated integer, group of integers, step, or steps, but not the exclusion of any other integer, group of integers, step, or steps. [0025] In the figures, like reference numerals refer to like features.

6 [0026] Referring to the drawings, there is shown in Figure 1 a flow diagram of a direct method for generating electric power from an underground coal seam with carbon dioxide capture. This direct method utilises a "cold clean-up" (i.e., water wash) process to remove entrained particulates, water soluble contaminants, and coal condensate from the raw UCG product gas. [0027] Raw UCG product gas 10 produced by an underground coal gasifier 12 is passed through a water wash process 15 for removal of entrained particulates, water soluble contaminants, and coal condensate 17, and a sulphur removal process 20 for removal of sulphur 22, to provide clean syngas 25. [0028] Clean syngas 25 is fed into a gas generator 27 for combustion with substantially pure oxygen 30 in the presence of water 32, which is injected into the gas generator 27, to produce a high temperature steam/CO 2 mixture 35. [0029] The high temperature steam/CO 2 mixture 35 is expanded through a high pressure turbine 37 to generate electric power 40. Optionally, the high temperature steam/CO 2 mixture 35 is further expanded through an intermediate-pressure turbine 42 and a low-pressure turbine 45 to generate electric power 40. [0030] Optionally, the high temperature steam/CO 2 mixture 35 is passed through an optional reheater 47 after leaving the high-pressure turbine 37 and prior to entering the intermediate-pressure turbine 42. The reheater 47 improves overall cycle efficiency by increasing the temperature of the steam/CO 2 mixture 35 before it enters the intermediate-pressure turbine 42. [0031] The reheater 47 combusts clean syngas 25 with substantially pure oxygen 30. [0032] A lower temperature steam/CO 2 mixture 50 exits the one or more turbines and is fed into a de-saturator or condenser 52 where the steam is condensed to water 55 to recover carbon dioxide 57.

7 [0033] By "de-saturator" is meant a water wash that recirculates and cools the condensed fluid (i.e., water). The cooled, condensed fluid is directly mixed with the incoming stream to cool it. [0034] As will be understood by one of ordinary skill in the art, typical condensers include shell and tube heat exchangers, with the steam on the shell side and cooling water on the tube side. Small amounts of non-condensable gases can be removed by air extraction pumps. [0035] Suitably, shell and tube condensers are utilised for low pressure/temperature designs, and de-saturators for high pressure/temperature designs. [0036] The recovered carbon dioxide 57 is dried, purified, and compressed 60 to provide liquefied carbon dioxide 62 (i.e., captured carbon dioxide). [0037] Captured carbon dioxide 62 is suitable for use in chemical production (e.g., urea and methanol), enhanced oil recovery, enhanced coal bed methane production, and geo-sequestration. [0038] According to an important aspect of the present invention, a portion of the recovered carbon dioxide 57 is recirculated to the underground coal gasifier 12 for use as a diluent of the substantially pure oxygen 30, both of which are injected into the underground coal gasifier 12 to support combustion and gasification of the underground coal seam. Dilution of the injected oxygen with carbon dioxide provides multiple benefits, including allowing the use of low cost materials for injection piping, avoiding nitrogen entering the system (which will reduce the purity of the recovered C02), and increasing overall thermal efficiency compared to injection of steam, or water (which first needs to be evaporated before reacting in the gasifier, contributing unwanted hydrogen to the UCG product gas). [0039] Water 55 from the de-saturator or condenser 52 is passed through a water treatment process 65 for clean-up, to produce clean water 67. Contaminated water 8 70 from the water wash process 15 is also passed through the water treatment process 65 for clean-up. [0040] According to an important aspect of the present invention, water 55 from the de-saturator or condenser 52 is recirculated to the gas generator 27 via the water treatment process 65, as injected water 32. [0041] Substantially pure oxygen 30 is provided by an air separation unit 72. [0042] Referring to Figure 2, a flow diagram of another direct method for generating electric power from an underground coal seam with carbon dioxide capture is shown. This direct method utilises a "hot clean-up" (i.e., filtration) process to remove entrained particulates from the raw UCG product gas. [0043] Raw UCG product gas 10 produced by an underground coal gasifier 12 is passed through a filtration process 16 for removal of entrained particulates, to provide hot syngas 26. [0044] Any suitable type of filtration process for removal of entrained particulates can be employed. For example, particulate removal can be done in two stages; the first stage for removal of larger particles, and the second stage for removal of very fine particulates that remain in the hot syngas 26 after the first stage. [0045] As will be known by one of ordinary skill in the art, suitable particulate removal systems include hot candle filters (for removal of particulates as a dry solid), water scrubbers (for removal of particulates as a slurry), electrostatic precipitators, and separators, such as cyclone separators and cyclone separators employing water scrubbers. Preferably, the filtration process will reduce the particulate content of the hot syngas 26 to a level below about 1 mg/Nm 3 , but the actual required level will depend on downstream equipment requirements, and can be ascertained by one of ordinary skill in the art.

9 [0046] Hot syngas 26 is fed into a gas generator 27 for combustion with substantially pure oxygen 30 in the presence of water 32, which is injected into the gas generator 27, to produce a high temperature steam/CO 2 mixture 35. [0047] The high temperature steam/CO 2 mixture 35 is expanded through a high pressure turbine 37 to generate electric power 40. Optionally, the high temperature steam/CO 2 mixture 35 is further expanded through an intermediate-pressure turbine 42 and a low-pressure turbine 45 to generate electric power 40. [0048] Optionally, the high temperature steam/CO 2 mixture 35 is passed through an optional reheater 47 after leaving the high-pressure turbine 37 and prior to entering the intermediate-pressure turbine 42. The reheater 47 improves overall cycle efficiency by increasing the temperature of the steam/CO 2 mixture 35 before it enters the intermediate-pressure turbine 42. [0049] The reheater 47 combusts hot syngas 26 with substantially pure oxygen 30. [0050] A lower temperature steam/CO 2 mixture 50 exits the one or more turbines and is fed into a de-saturator or condenser 52 where the steam is condensed to water 55 to recover carbon dioxide 57. [0051] The recovered carbon dioxide 57 is passed through a drying, compressing, and contaminant removal process 61 to provide purified and liquefied carbon dioxide 62. The process of contaminant removal removes SOx/NOx contaminants 64. [0052] As will be known by one of ordinary skill in the art, suitable SOx/NOx clean-up processes include low temperature, high pressure water scrubbing plus a lead chamber process, and separation by liquefaction/distillation. Both processes may be required depending on required C02 purity. [0053] According to an important aspect of the present invention, a portion of the recovered carbon dioxide 57 is recirculated to the underground coal gasifier 12 for 10 use as a diluent of the substantially pure oxygen 30, both of which are injected into the underground coal gasifier 12 to support combustion and gasification of the underground coal seam. [0054] Water 55 from the de-saturator or condenser 52 is passed through a water treatment process 65 for clean-up, to produce clean water 67. [0055] According to an important aspect of the present invention, water 55 from the de-saturator or condenser 52 is recirculated to the gas generator 27 via the water treatment process 65, as injected water 32. [0056] Substantially pure oxygen 30 is provided by an air separation unit 72. [0057] Referring to Figure 3, a flow diagram of an indirect method for generating electric power from an underground coal seam with carbon dioxide capture is shown. This indirect method utilises a "cold clean-up" (i.e., water wash) process to remove entrained particulates, water soluble contaminants, and coal condensate from the raw UCG product gas. [0058] Raw UCG product gas 10 produced by an underground coal gasifier 12 is passed through a water wash process 15 for removal of entrained particulates, water soluble contaminants, and coal condensate 17, and a sulphur removal process 20 for removal of sulphur 22, to provide clean syngas 25. [0059] Clean syngas 25 is fed into a gas generator 27 for combustion with substantially pure oxygen 30 in the presence of water 32, which is injected into the gas generator 27, to produce a high temperature steam/CO 2 mixture 35. [0060] The high temperature steam/CO 2 mixture 35 is used to generate steam 75 in a pressurised boiler 77, and the steam 75 is expanded through a turbine 80 to generate electric power 40.

11 [0061] A lower temperature steam/CO 2 mixture 50 exits the pressurised boiler 77 and is fed into a de-saturator or condenser 52 where the steam is condensed to water 55 to recover carbon dioxide 57. [0062] The recovered carbon dioxide 57 is dried, purified, and compressed 60 to provide liquefied carbon dioxide 62. [0063] According to an important aspect of the present invention, a portion of the recovered carbon dioxide 57 is recirculated to the underground coal gasifier 12 for use as a diluent of the substantially pure oxygen 30, both of which are injected into the underground coal gasifier 12 to support combustion and gasification of the underground coal seam. [0064] Water 55 from the de-saturator or condenser 52 is passed through a water treatment process 65 for clean-up, to produce clean water 67. Contaminated water 70 from the water wash process 15 is also passed through the water treatment process 65 for clean-up. [0065] According to an important aspect of the present invention, water 55 from the de-saturator or condenser 52 is recirculated to the gas generator 27 via the water treatment process 65, as injected water 32. [0066] Substantially pure oxygen 30 is provided by an air separation unit 72. [0067] Referring to Figure 4, a flow diagram of another indirect method for generating electric power from an underground coal seam with carbon dioxide capture is shown. This indirect method utilises a "hot clean-up" (i.e., filtration) process to remove entrained particulates from the raw UCG product gas. [0068] Raw UCG product gas 10 produced by an underground coal gasifier 12 is passed through a filtration process 16 for removal of entrained particulates, to provide hot syngas 26.

12 [0069] Preferably, the filtration process will reduce the particulate content of the hot syngas 26 to a level below about 1 mg/Nm 3 , but the actual required level will depend on downstream equipment requirements, and can be ascertained by one of ordinary skill in the art. [0070] Hot syngas 26 is fed into a gas generator 27 for combustion with substantially pure oxygen 30 in the presence of water 32, which is injected into the gas generator 27, to produce a high temperature steam/CO 2 mixture 35. [0071] The high temperature steam/CO 2 mixture 35 is used to generate steam 75 in a pressurised boiler 77, and the steam 75 is expanded through a turbine 80 to generate electric power 40. [0072] A lower temperature steam/CO 2 mixture 50 exits the pressurised boiler 77 and is fed into a de-saturator or condenser 52 where the steam is condensed to water 55 to recover carbon dioxide 57. [0073] The recovered carbon dioxide 57 is passed through a drying, compressing, and contaminant removal process 61 to provide purified and liquefied carbon dioxide 62. The process of contaminant removal removes SOx/NOx contaminants 64. [0074] According to an important aspect of the present invention, a portion of the recovered carbon dioxide 57 is recirculated to the underground coal gasifier 12 for use as a diluent of the substantially pure oxygen 30, both of which are injected into the underground coal gasifier 12 to support combustion and gasification of the underground coal seam. [0075] Water 55 from the de-saturator or condenser 52 is passed through a water treatment process 65 for clean-up, to produce clean water 67. [0076] According to an important aspect of the present invention, water 55 from the de-saturator or condenser 52 is recirculated to the gas generator 27 via the water treatment process 65, as injected water 32.

13 [0077] Substantially pure oxygen 30 is provided by an air separation unit 72. [0078] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics can be combined in any suitable manner in one or more combinations. [0079] Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. It will therefore be appreciated by those of skill in the art that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention.

Claims (14)

1. A method of generating electric power from an underground coal seam with carbon dioxide capture, comprising the steps of: a. gasifying the underground coal seam in situ; b. collecting UCG product gas from the underground coal seam; c. passing the product gas through a water wash; d. removing sulphur from the product gas; e. combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture; f. expanding the high temperature steam/carbon dioxide mixture through a plurality of turbines to generate electric power; g. condensing the steam to recover the carbon dioxide; h. dehydrating the recovered carbon dioxide; and i. compressing the recovered carbon dioxide, wherein gasifying the underground coal seam in situ comprises injecting substantially pure oxygen and carbon dioxide into the underground coal seam.

2. The method of claim 1, wherein the carbon dioxide injected into the underground coal seam is recovered carbon dioxide.

3. The method of claim 1 or claim 2, wherein the plurality of turbines comprise a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine.

4. The method of claim 3, wherein the high temperature steam/carbon dioxide mixture is passed through a reheater after leaving the high-pressure turbine and prior to entering the intermediate-pressure turbine to increase the temperature of the mixture.

5. The method of claim 4, wherein the reheater combusts UCG product gas with substantially pure oxygen. 15

6. A method of generating electric power from an underground coal seam with carbon dioxide capture, comprising the steps of: a. gasifying the underground coal seam in situ; b. collecting UCG product gas from the underground coal seam; c. removing particulates from the product gas; d. combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture; e. expanding the high temperature steam/carbon dioxide mixture through a plurality of turbines to generate electric power; f. condensing the steam to recover the carbon dioxide; g. dehydrating the recovered carbon dioxide; h. compressing the recovered carbon dioxide; and i. removing SOx and/or NOx from the recovered carbon dioxide, wherein gasifying the underground coal seam in situ comprises injecting substantially pure oxygen and carbon dioxide into the underground coal seam.

7. The method of claim 6, wherein the carbon dioxide injected into the underground coal seam is recovered carbon dioxide.

8. The method of claim 6 or claim 7, wherein the plurality of turbines comprise a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine.

9. The method of claim 8, wherein the high temperature steam/carbon dioxide mixture is passed through a reheater after leaving the high-pressure turbine and prior to entering the intermediate-pressure turbine to increase the temperature of the mixture.

10. The method of claim 9, wherein the reheater combusts UCG product gas with substantially pure oxygen.

11. A method of generating electric power from an underground coal seam with carbon dioxide capture, comprising the steps of: 16 a. gasifying the underground coal seam in situ; b. collecting UCG product gas from the underground coal seam; c. passing the product gas through a water wash; d. removing sulphur from the product gas; e. combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture; f. using the high temperature steam/carbon dioxide mixture to generate steam in a pressurised boiler; g. expanding the generated steam through a turbine to generate electric power; h. condensing the steam in the steam/carbon dioxide mixture to recover the carbon dioxide; i. dehydrating the recovered carbon dioxide; and j. compressing the recovered carbon dioxide, wherein gasifying the underground coal seam in situ comprises injecting substantially pure oxygen and carbon dioxide into the underground coal seam.

12. The method of claim 11, wherein the carbon dioxide injected into the underground coal seam is recovered carbon dioxide.

13. A method of generating electric power from an underground coal seam with carbon dioxide capture, comprising the steps of: a. gasifying the underground coal seam in situ; b. collecting UCG product gas from the underground coal seam; c. removing particulates from the product gas; d. combusting the product gas with substantially pure oxygen in the presence of water in a gas generator to produce a high temperature steam/carbon dioxide mixture; e. using the high temperature steam/carbon dioxide mixture to generate steam in a pressurised boiler; f. expanding the generated steam through a turbine to generate electric power; 17 g. condensing the steam in the steam/carbon dioxide mixture to recover the carbon dioxide; h. dehydrating the recovered carbon dioxide; i. compressing the recovered carbon dioxide; and j. removing SOx and/or NOx from the recovered carbon dioxide, wherein gasifying the underground coal seam in situ comprises injecting substantially pure oxygen and carbon dioxide into the underground coal seam.

14. The method of claim 13, wherein the carbon dioxide injected into the underground coal seam is recovered carbon dioxide.