Methods and apparatuses for thermally converting biomass

Abstract

Methods and apparatuses for thermally converting or pyrolyzing biomass are provided. In one embodiment, a method of thermally converting biomass includes introducing the biomass to a reactor feed chamber. The method provides for flowing a low oxygen gas into the reactor feed chamber to purge the reactor feed chamber and biomass of oxygen. The method also includes delivering the purged biomass to a thermal conversion reactor and thermally converting the biomass in the thermal conversion reactor.

Description

TECHNICAL FIELD

The present invention generally relates to methods and apparatuses for thermally converting, or pyrolyzing, biomass and more particularly relates to methods and apparatuses for thermally converting biomass that operate at controlled oxygen levels.

BACKGROUND

Renewable energy sources are of increasing importance. They are a means of reducing dependence on oil and they provide a substitute for other fossil fuels. Also, renewable energy resources can provide for basic chemical constituents to be used in other industries, such as chemical monomers for the making of plastics. Biomass is a renewable resource that can supply some of the need for renewables-based chemicals and fuels.

Biomass includes, but is not limited to, lignin, plant parts, fruits, vegetables, plant processing waste, wood chips, chaff, grains, grasses, corn and corn husks, weeds, aquatic plants, hay, recycled and non-recycled paper and paper products, and any cellulose-containing biological material or material of biological origin. The economics of producing oil from biomass depend on the yield of oil produced from a quantity of biomass. When heated in an environment with low or no oxygen, biomass is thermally converted, or pyrolyzed, to generate a liquid known as pyrolysis oil. A modern form of pyrolysis, or rapid thermal conversion, is conducted under moderate temperatures, typically 400° C. to 600° C., and short residence times, such as less than 5 seconds. An example is flash pyrolysis that operates under such conditions and produces a pourable liquid product or pyrolysis oil from the thermal conversion of biomass feedstock or petroleum-based feedstock. Pyrolysis oil thermally converted from biomass feedstock has a higher energy density than the biomass feedstock. Further, the pyrolysis oil thermally converted from biomass feedstock is more easily stored and transported than the biomass feedstock. For economic reasons, it is typically desirable to maximize the yield of pyrolysis oil from the thermal conversion process.

In conventional flash pyrolysis processes, biomass is thermally converted in a reactor during a short contact duration, such as less than about 2 seconds, with a high temperature heat transfer medium, such as a solid heat carrier at about 500° C. This solid heat carrier can be silica sand, low activity catalyst, or other inert material. Typical thermal conversion processes allow oxygen to enter the thermal conversion reactor through the biomass inlet along with the biomass. Further, typical thermal conversion processes utilize equipment or instruments in the reactor system that must be protected from interference by the solid heat carrier or solid product from the thermal conversion of the feedstock. Generally, the instruments are purged with air to dislodge the solid matter or to prevent its intrusion into the instruments. However, the introduction of additional oxygen through the biomass inlet and instrument purge inlets reduces the pyrolysis oil yield proportionally to the amount of oxygen added. As a result, a typical thermal conversion unit exhibits up to about a 2% liquid yield loss due to the ingress of additional oxygen into the thermal conversion reactor.

Accordingly, it is desirable to provide methods and apparatuses for thermally converting biomass with improved pyrolysis oil yield. Further, it is desirable to provide methods and apparatuses for thermally converting biomass which inhibit ingress of oxygen. Also, it is desirable to provide methods and apparatuses for thermally converting biomass which control the oxygen level within a thermal conversion reactor. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

Methods and apparatuses for thermally converting biomass are provided. In accordance with an exemplary embodiment, a method of thermally converting biomass includes introducing the biomass to a reactor feed chamber. The method provides for flowing a low oxygen gas into the reactor feed chamber to purge the reactor feed chamber and biomass of oxygen. The method also includes delivering the purged biomass to a reactor and thermally converting the biomass in the reactor.

In accordance with another exemplary embodiment, a method for thermally converting biomass includes delivering the biomass to a thermal conversion reactor and introducing a carrier gas having a selected oxygen content to the thermal conversion reactor. The carrier gas carries the biomass through the thermal conversion reactor. The method includes thermally converting the biomass in the thermal conversion reactor and inhibiting the introduction of additional oxygen to the thermal conversion reactor.

In accordance with another exemplary embodiment, an apparatus for thermally converting biomass includes a reactor feed chamber for holding the biomass. The apparatus further includes a thermal conversion reactor configured to thermally convert the biomass and in communication with the reactor feed chamber for receiving the biomass. An instrument is provided in communication with the thermal conversion reactor and is adapted to monitor conditions in the thermal conversion reactor. Further, the apparatus includes a purge line in communication with the reactor feed chamber and the instrument and adapted to flow low oxygen gas into the reactor feed chamber and into the instrument to inhibit the introduction of oxygen into the thermal conversion reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the methods and apparatuses for thermally converting, or pyrolyzing, biomass will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:

FIG. 1 is a schematic illustrating an apparatus for thermally converting biomass in accordance with various embodiments herein;

FIG. 2 is a flow chart illustrating an exemplary method for thermally converting biomass which includes purging the biomass feed with an oxygen-free or low oxygen gas in accordance with an embodiment herein;

FIG. 3 is a flow chart illustrating an exemplary method for thermally converting biomass which includes purging process monitoring instruments with an oxygen-free or low oxygen gas in accordance with an embodiment herein;

FIG. 4 is a flow chart illustrating an exemplary method for thermally converting biomass which includes purging the heat transfer medium with an oxygen-free or low oxygen gas in accordance with an embodiment herein;

FIG. 5 is a flow chart illustrating an exemplary method for thermally converting biomass which includes forming an oxygen-free or low oxygen gas purge gas from a recycled flue gas in accordance with an embodiment herein; and

FIG. 6 is a flow chart illustrating an exemplary method for thermally converting biomass which includes generating an oxygen-free or low oxygen purge gas in accordance with an embodiment herein.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the methods and apparatuses for thermally converting biomass. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background or brief summary, or in the following detailed description.

It is contemplated herein that the thermal conversion of biomass can be improved under conditions in which oxygen levels are controlled at selected levels. Specifically, the methods and apparatuses for thermally converting biomass described herein can be used to limit the volume of oxygen introduced to a thermal conversion reactor. Conventional thermal conversion processes utilize a carrier gas having a desired oxygen level, such as no more than about 5 vol %, which enters the thermal conversion reactor and carries the biomass through the thermal conversion reactor during the thermal conversion reaction. However, in the conventional thermal conversion processes, additional oxygen enters the thermal conversion reactor, such as through the biomass inlet, through instrument purge inlets, and/or through the heat transfer medium inlet. The methods and apparatuses for thermally converted biomass described herein eliminate or inhibit the introduction of oxygen through the biomass inlet, instrument purge inlets, and/or heat transfer medium inlet through the use of oxygen-free or low oxygen purge gases. As used herein, “oxygen-free” refers to gases containing substantially 0 vol % oxygen, and “low oxygen” refers to gases having an oxygen content lower than that of air, i.e., less than about 20 vol % oxygen.

In accordance with the various embodiments herein, FIG. 1 illustrates an apparatus 10 for thermally converting biomass 12 to produce pyrolysis oil 14. As in a conventional system, the apparatus 10 includes a hopper or feed bin 18 for receiving the biomass 12. The hopper 18 is in communication with a reactor feed chamber 22 formed by, for example, an auger, a screw feed device, a conveyor, or other batch feed device. The reactor feed chamber 22 is further selectively connected to a thermal conversion or pyrolysis reactor 24 configured to thermally convert or pyrolyze the biomass 12. The thermal conversion reactor 24 includes a biomass inlet 26 for receiving the biomass 12 from the reactor feed chamber 22. Further, the thermal conversion reactor 24 includes a carrier gas inlet 28 for receiving a carrier gas 30. The thermal conversion reactor 24 may also include a solid heat transfer medium inlet 31 to receive hot heat transfer medium 32, such as sand, catalyst, or other inert particulate, via line 34. Alternatively, the heat transfer medium 32 may be mixed with and carried by the carrier gas 30 through the carrier gas inlet 28. As shown, the thermal conversion reactor 24 is outfitted with at least one instrument 36, such as temperature or pressure instruments for monitoring conditions within the thermal conversion reactor 24.

As the biomass 12 is heated by the heat transfer medium 32 to the thermal conversion or pyrolysis temperature, typically about 540° C., the thermal conversion or pyrolysis reaction occurs and pyrolysis vapor and char are formed in the thermal conversion reactor 24. The pyrolysis vapor and char, along with the heat transfer medium, are carried out of an outlet 38 in the thermal conversion reactor 24 and through a line 42 to a separator 46, such as, for example, a cyclone. The separator 46 separates the pyrolysis vapor 50 from the char and heat transfer medium 52. As shown, the pyrolysis vapor 50 is directed to a condenser 54 which condenses the pyrolysis vapor 50 to form the pyrolysis oil 14. Uncondensed gas 56 exits the condenser 54 and may be recycled as the carrier gas 30. Typically, the carrier gas 30 includes a low level of oxygen such as no more than about 5 percent by volume (vol %).

The char and heat transfer medium 52 are fed to a combustion unit 58, typically referred to as a reheater, for the purpose of reheating the heat transfer medium. As shown, a blower 60 feeds air 62 or another oxygen-containing gas into the combustion unit 58. Upon contact with the oxygen, the char combusts, heating the heat transfer medium and forming flue gas and ash. The hot heat transfer medium 32 exits the combustion unit 58 and is returned to the thermal conversion reactor 24 via line 34. The flue gas and ash exit the combustion unit 58 through line 64 and are directed to a separator 66, such as a cyclone. The separator 66 then removes the ash 68 which can be disposed of.

In an exemplary embodiment, the separated flue gas 69 exits the separator 66 and a portion 70 can be recycled for use as a low oxygen purge gas for inhibiting the entry of oxygen into the thermal conversion reactor 24. Typically, the recycled flue gas 70 will comprise carbon oxides, specifically carbon dioxide and carbon monoxide, nitrogen, water vapor, and a low level of oxygen, such as less than about 10 vol %, for example about 5 vol %.

Optionally, the recycled flue gas 70 may be fed to a cooler/separator 72 which condenses and removes the water vapor in stream 73. Further, to reduce the amount of oxygen in the recycled flue gas 70, it may be passed through an optional reduction unit 74 such as, for example, a membrane, a pressure swing adsorber or other adsorber, or a combustor. The unit 74 may be operated to remove substantially all oxygen from the recycled flue gas 70, or to reduce the oxygen level to a selected acceptable amount, such as no more than about 5 vol %. As shown, the recycled flue gas 70 is fed to a compressor 76 where it is compressed to an appropriate pressure for use in purging, such as about 20 psig to about 120 psig, for example to about 50 psig. The compressed recycled flue gas 70 is then delivered to a purge gas header 78 for use as the purge gas.

While the purge gas header 78 may be supplied with recycled flue gas 70 as described above, other exemplary embodiments may alternatively or additionally provide the purge gas header 78 with gas 80 supplied by an inert gas source 82. For example, the inert gas source 82 can be a generator, including a separator such as a pressure swing adsorber, a unit for removing reactive gases, or any other apparatus that generates a concentrated inert gas or combination of inert gases, such as nitrogen, argon, helium or others. The inert gas 80 may be delivered to the purge gas header 78 at a selected pressure, such as, for example, 100 psig.

As shown, the purge gas header 78 is connected to the reactor feed chamber 22 by a purge line 84. Therefore, when biomass 12 is received within the reactor feed chamber 22, the oxygen-free or low oxygen purge gas 86 in the purge gas header 78 may be flowed through the purge line 84 into the reactor feed chamber 22 and across the biomass 12 to purge any oxygen therefrom. Further, the instruments 36 within the thermal conversion reactor 24 can become jammed or otherwise impacted with particulate, such as heat transfer medium 32 or char. Apparatus 10 provides the thermal conversion reactor 24 with an instrument inlet 88 for each instrument 36. Each instrument inlet 88 is in communication with the purge gas header 78 via purge line 90. Therefore, the purge gas 86 can be flowed through the purge line 90 and instrument inlets 88 and into or over the instruments 36 to dislodge any heat transfer medium or char or prevent lodging of any heat transfer medium or char, and to maintain proper instrument operation. Also, apparatus 10 further provides a purge line 92 for connecting the purge gas header 78 to the line 34 carrying the reheated heat transfer medium 32 to the thermal conversion reactor 24. With this connection, the purge gas 86 can be flowed through purge line 92 and over the hot heat transfer medium 32 in line 34 to purge any oxygen from the medium's interstitial volume. Purge line 92 is of particular utility when the combustion unit 58 is run with excess air to provide temperature control. As a result of operating the combustion unit 58 with excess air, the oxygen content of recycled flue gas 70 may be as high as about 10 vol % and the heat transfer medium 32 may carry with it a non-insubstantial amount of oxygen. In such circumstances, performance of the thermal conversion reactor 24 is enhanced by purging the heat transfer medium 32.

In summary, the apparatus 10 provides for improved pyrolysis oil yield from biomass by purging the biomass 12, instruments 36, and heat transfer medium 32 with the oxygen-free or low oxygen purge gas 86 before introduction into the thermal conversion reactor 24. As a result, a controlled amount of oxygen enters the thermal conversion reactor 24 through biomass inlet 26, instrument inlet 88 and heat transfer medium inlet 31.

An exemplary method 200 for thermally converting biomass is illustrated in FIG. 2. As shown, method 200 introduces biomass to a feed chamber at step 202. Then, at step 204, the biomass is purged with low oxygen gas (including, in certain embodiments, oxygen-free gas). This purging step removes air from void space in and around the biomass. After the biomass is purged of air, it is delivered to the thermal conversion reactor at step 206. At step 208, the biomass is thermally converted into pyrolysis oil. As the thermal conversion reaction occurs, additional biomass is introduced to the feed chamber, and the process continues.

As discussed above, a carrier gas including a selected amount of oxygen also enters the thermal conversion reactor to carry the biomass through the thermal conversion reactor. As a result of the method 200 for thermally converting biomass, the introduction into the thermal conversion reactor of additional oxygen, i.e., oxygen not present in the carrier gas, is inhibited. Specifically, while a selected amount of oxygen may enter the thermal conversion reactor in the carrier gas through the carrier gas inlet, little or substantially no oxygen enters the thermal conversion reactor through the biomass inlet. In other words, the method 200 for thermally converting biomass purges the biomass of oxygen in the reactor feed chamber such that substantially no, or a limited amount of, oxygen enters the thermal conversion reactor through the biomass inlet.

FIG. 3 illustrates an exemplary method 300 for thermally converting biomass. The method 300 may be performed in addition to method 200 or alternatively to method 200, depending on the specific thermal conversion apparatus design and operation. As shown, method 300 thermally converts biomass in the thermal conversion reactor at step 302. At step 304, conditions within the thermal conversion reactor are monitored with instruments, such as pressure or temperature instruments. Because the instruments may be clogged or otherwise impaired by particulates inside the thermal conversion reactor, the method 300 provides for purging the instruments with low oxygen gas (including oxygen-free gas in certain embodiments) at step 306. By purging the instruments with low oxygen gas, particulates are dislodged from, or prevented from lodging in, the instruments. The thermal conversion, monitoring, and purging steps of the method 300 are continuous and concurrently performed during operation of the apparatus.

As a result of the method 300 for thermally converting biomass, the introduction into the thermal conversion reactor of additional oxygen, i.e., oxygen not present in the carrier gas, is inhibited. Specifically, while a selected amount of oxygen may enter the thermal conversion reactor through the carrier gas inlet, little or substantially no oxygen enters the thermal conversion reactor through the instrument inlets. In other words, the method 300 for thermally converting biomass purges the instrument with low oxygen gas such that substantially no, or a limited amount of, oxygen enters the thermal conversion reactor through the instrument inlets.

FIG. 4 illustrates an exemplary method 400 for thermally converting biomass. The method 400 may be performed in addition to methods 200 and/or 300 or alternatively to methods 200 and/or 300, depending on the specific thermal conversion apparatus design and operation. As shown, method 400 thermally converts biomass in the thermal conversion reactor at step 402. At step 404, char and the heat transfer medium are removed from the thermal conversion reactor. The char and heat transfer medium are fed to a combustion unit where the char is combusted to reheat the heat transfer medium at step 406. The reheated heat transfer medium then exits the combustion unit and is purged with low oxygen purge gas (including oxygen-free purge gas in certain embodiments) at step 408. Step 410 delivers the purged transfer medium to the thermal conversion reactor. As shown, the steps of method 400 are performed continuously and concurrently during a thermal conversion process.

As a result of the method 400 for thermally converting biomass, the introduction into the thermal conversion reactor of additional oxygen, i.e., oxygen not present in the carrier gas, is inhibited. Specifically, while a selected amount of oxygen may enter the thermal conversion reactor through the carrier inlet, little or substantially no oxygen enters the thermal conversion reactor through the heat transfer medium inlet. In other words, the method 400 for thermally converting biomass purges the heat transfer medium with low oxygen gas such that substantially no, or a limited amount of, oxygen enters the thermal conversion reactor through the heat transfer medium inlet.

FIG. 5 illustrates an exemplary method 500 for thermally converting biomass. The method 500 supplies the purge gas for the purging steps of methods 200, 300 and 400. In step 502, biomass is thermally converted in a thermal conversion reactor. Thermal conversion of biomass produces char. The char is removed from the thermal conversion reactor along with the heat transfer medium. Then the char is combusted at step 504 to reheat the heat transfer medium. Combustion of the char produces a flue gas and ash. In step 506, the flue gas is separated from the ash, such as with a cyclone. Optionally, the flue gas is delivered to a condenser where water vapor in the flue gas is condensed and separated from the remaining flue gas at step 508. In optional step 510, the oxygen level in the flue gas is reduced, such as by a membrane, a pressure swing adsorber or other adsorber, or a combustor. The flue gas is compressed at step 512 to a suitable pressure for purging processes, for example, to about 50 psig. Then, the compressed flue gas is delivered to a purge gas header at step 514. Step 516 performs a purge with the compressed flue gas, such as of the biomass, monitoring instruments, or the reheated heat transfer medium as discussed in relation to methods 200, 300 and 400.

FIG. 6 illustrates an exemplary method 600 for thermally converting biomass. The method 600 may be performed in addition to, or alternatively to, the method 500 and supplies the purge gas for the purging steps of methods 200, 300 and 400. In step 602, biomass is thermally converted in a thermal conversion reactor. In step 604, low oxygen gas (including oxygen-free gas) is generated, such as by separation of an external source of inert gas or by any process concentrating an inert gas (including an inert gas formed by multiple inert components). Nitrogen, helium, argon, other low oxygen gases, or a combination thereof may be generated according to known processes. For example, the inert gas may be generated by separating nitrogen from air, or by combusting oxygen out of air. In an exemplary embodiment, the generated gas has a pressure of about 100 psig. The generated gas is delivered to the purge gas header at step 606. Step 608 performs a purge with the generated gas, such as of the biomass, monitoring instruments, or the reheated heat transfer medium as discussed in relation to methods 200, 300 and 400. The method 600 may be repeated continuously during a typical thermal conversion process.

In an exemplary operation of the apparatus 10 of FIG. 1, any or all of methods 200, 300 and 400, and at least one of methods 500 and 600, is performed. For example, the operation begins with biomass 12 being introduced into the reactor feed chamber 22. Then, the purge gas 86 (sourced from recycled flue gas 70 or generated gas 80) is flowed into the reactor feed chamber 22 and purges oxygen from the reactor feed chamber 22 and from the biomass 12. Thereafter, the purged biomass 12 is delivered into the thermal conversion reactor 24 with little or substantially no oxygen. The carrier gas 30 having a selected oxygen content is introduced to the thermal conversion reactor 24 through the carrier gas inlet 28. In order to provide the necessary heat for thermal conversion, the heat transfer medium 32 is fed to the thermal conversion reactor 24 through the heat transfer medium inlet 31. First, however, the heat transfer medium 32 is purged of oxygen by flowing the purge gas 86 over or across the heat transfer medium 32 in line 34. The carrier gas 30 carries the purged biomass 12 and purged heat transfer medium 32 through the thermal conversion reactor 24, and the biomass 12 is thermally converted or pyrolyzed, producing pyrolysis vapor and char. Conditions within the thermal conversion reactor 24 are monitored by at least one instrument 36. In order to enable proper operation of the instrument 36, the purge gas 86 is flowed into or over the instrument 36 to dislodge, or to prevent the lodging of, any particulates, such as heat transfer medium or char.

During the exemplary operation of the apparatus 10 of FIG. 1 to thermally convert biomass, the introduction into the thermal conversion reactor 24 of additional oxygen, i.e., oxygen not present in the carrier gas 30, is inhibited. Specifically, while a selected amount of oxygen may enter the thermal conversion reactor 24 through the carrier gas inlet 28, little or substantially no oxygen enters the thermal conversion reactor 24 through other inlets, e.g., inlets 26, 31, 88. For example, the biomass 12 is purged of oxygen by the purge gas 86 in the reactor feed chamber 22 such that substantially no, or a limited amount of, oxygen enters the thermal conversion reactor 24 through the biomass inlet 26. Instruments 36 in the thermal conversion reactor 24 are purged with the purge gas 86 such that substantially no, or a limited amount of, oxygen enters the thermal conversion reactor 24 through the instrument inlets 88. Also, the heat transfer medium 32 is purged of oxygen by the purge gas 86 before introduction into the thermal conversion reactor 24 such that substantially no, or a limited amount of, oxygen enters the thermal conversion reactor 24 through the heat transfer medium inlet 31.

To briefly summarize, the methods and apparatuses described herein can be used to thermally convert biomass under conditions with a controlled level of oxygen. As a result, the methods and apparatuses herein can be used to efficiently convert biomass into pyrolysis oil with minimized loss in the yield of pyrolysis oil.

While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the processes without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application

Claims

1. A method, comprising: i) introducing char and a heat transfer medium into a reheater;ii) combusting at least a portion of the char with an excess amount of an oxygen-containing gas to form an oxygen-containing flue gas and a heated heat transfer medium;iii) adjusting said excess amount of the oxygen containing gas to control the reheater temperature;iv) forming a low-oxygen carrier gas, comprising: passing at least a portion of the oxygen-containing flue gas through an external reduction unit; andv) contacting a biomass with the heated heat transfer medium and a portion of the low-oxygen carrier gas in a fast pyrolysis reactor to form the char,

2. The method of claim 1, further comprising: dewatering the at least a portion of the oxygen-containing flue gas.

3. The method of claim 1, further comprising: compressing the low-oxygen carrier gas.

4. The method of claim 1, wherein the oxygen-containing gas is air.

5. The method of claim 1, wherein the heat transfer medium is sand.

6. The method of claim 1, wherein the reduction unit comprises a combustor.

7. The method of claim 1, wherein the reduction unit comprises a membrane.

8. The method of claim 1, wherein the reduction unit comprises an absorber.

9. The method of claim 1, wherein the oxygen-containing flue gas has an oxygen content of no more than 10 vol %.

10. The method of claim 1, wherein the low-oxygen carrier gas has an oxygen content of no more than 5 vol %.

11. The method of claim 1, wherein a further portion of the low-oxygen carrier gas is used to purge one or more instrument purge inlet components of the biomass flash pyrolysis reactor.

12. The method of claim 1, wherein a further portion of the low-oxygen carrier gas is used to purge the biomass prior to thermal conversion.

13. The method of claim 1, wherein a further portion of the low-oxygen carrier gas is used to purge the heated heat transfer medium prior to introduction of the heated heat transfer medium to the thermal conversion unit.

14. The method of claim 1, further comprising: vi) dewatering the at least a portion of the oxygen-containing flue gas; andvii) compressing the low-oxygen carrier gas to a pressure of between 20 psig and 120 psig,

15. The method of claim 14, wherein the heat transfer medium is sand.

16. The method of claim 1, further comprising: i) dewatering the at least a portion of the oxygen-containing flue gas; andii) compressing the low-oxygen carrier gas to a pressure of between 20 psig and 120 psig,

17. The method of claim 16, wherein the reduction unit is a combustor.

18. The method of claim 16, wherein the reduction unit is a pressure-swing absorber.