METHODS FOR FUEL CONVERSION

Abstract

In one embodiment described herein, fuel may be converted into syngas by a method comprising feeding the fuel and composite metal oxides into a reduction reactor in a co-current flow pattern relative to one another, reducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides, transporting the reduced composite metal oxides to an oxidation reactor, regenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant in the oxidation reactor, and recycling the regenerated composite metal oxides to the reduction reactor for sub-sequent reduction reactions to produce syngas. The composite metal oxides may be solid particles comprising a primary metal oxide and a secondary metal oxide.

Description

BACKGROUND

1. Field

The present disclosure relates to chemical looping systems and methods, and specifically to systems and methods for producing syngas from feedstock fuels.

2. Technical Background

There is a constant need for clean and efficient energy generation systems. Many of the commercial processes that generate energy carriers such as steam, hydrogen, synthesis gas (syngas), liquid fuels, and/or electricity are based on fossil fuels. Furthermore, the dependence on fossil fuels is expected to continue in the foreseeable future due to the lower costs compared to some renewable sources. Current conversion methods of carbonaceous fuels may emit large quantities of carbon dioxide to the environment and may require significant capital and operational costs. Sulfur and nitrogen compounds may also be generated in these processes due to the complex contents of coal.

A need is present for improved systems and methods for converting fuel, and system components therein, which can convert fuel effectively while reducing pollutants.

SUMMARY

In one embodiment, fuel may be converted into syngas by a method comprising feeding the fuel and composite metal oxides into a reduction reactor in a co-current flow pattern relative to one another, reducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides, transporting the reduced composite metal oxides to an oxidation reactor, regenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant in the oxidation reactor, and recycling the regenerated composite metal oxides to the reduction reactor for subsequent reduction reactions to produce syngas. The composite metal oxides may be solid particles comprising a primary metal oxide and a secondary metal oxide.

In another embodiment, natural gas may be used as a fuel and may be converted into syngas by a method comprising feeding the fuel and composite metal oxides into a reduction reactor in a co-current flow pattern relative to one another, reducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides, transporting the reduced composite metal oxides to an oxidation reactor, regenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant in the oxidation reactor, and recycling the regenerated composite metal oxides to the reduction reactor for subsequent reduction reactions to produce syngas. The composite metal oxides may be solid particles comprising a primary metal oxide and a secondary metal oxide. The composite metal oxide may comprise iron oxide and titanium oxide, or the composite metal oxide comprises iron oxide and aluminum oxide. The iron oxide may be reduced in the reduction reaction from FeO_x to FeO_y and 1.5>x>1>y>0.3.

According to yet another embodiment, fuel may be converted by a method comprising reducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides, and regenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant. The composite metal oxides are solid particles may comrprise primary metal oxide and a secondary metal oxide. The composite metal oxide may comprise iron oxide and titanium oxide, or the composite metal oxide may comprise iron oxide and aluminum oxide. The reducing of the composite metal oxides and the oxidizing of the composite metal oxides may be carried out in a fixed bed with a gas switching system. Alternatively, the reducing of the composite metal oxides may occur in a reduction reactor, wherein the reduction reactor is a moving bed reactor comprising two gas outlets and the reducing of the composite metal oxides produces syngas and CO₂. Alternatively, the reducing and oxidizing of composite metal oxide may occur in a membrane based reactor, wherein the composite metal oxides are integrated to the fuel side of the membrane based reactor.

Additional features and advantages of the devices and methods for chemical conversion systems and methods and processes for manufacturing the same will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

FIG. 1 is a graph comparing the equilibrium carbon distribution difference with various ratios between methane and single/composite metal oxides, according to one or more embodiments shown and described herein;

FIG. 2 is a graph comparing the equilibrium syngas composition difference between pure FeO and composite metal oxides, according to one or more embodiments shown and described herein;

FIG. 3( a) is a schematic flow diagram of a fuel conversion system using a co-current reactor for the reduction reaction of composite metal oxide using gaseous fuel, according to one or more embodiments shown and described herein;

FIG. 3( b) is a schematic flow diagram of a fuel conversion system using a co-current reactor for the reduction reaction of composite metal oxide using gaseous fuel, according to one or more embodiments shown and described herein;

FIG. 4( a) is a schematic flow diagram of a fuel conversion system using a co-current reactor for the reduction reaction of composite metal oxide using solid fuel, according to one or more embodiments shown and described herein;

FIG. 4( b) is a schematic flow diagram of a fuel conversion system using a co-current reactor for the reduction reaction of composite metal oxide using solid fuel, according to one or more embodiments shown and described herein;

FIG. 5( a) is a schematic flow diagram of a fuel conversion system using a co-current reactor for the reduction reaction of composite metal oxide using liquid fuel, according to one or more embodiments shown and described herein;

FIG. 5( b) is a schematic flow diagram of a fuel conversion system using a co-current reactor for the reduction reaction of composite metal oxide using liquid fuel, according to one or more embodiments shown and described herein;

FIG. 6 is a schematic flow diagram of a fuel conversion system using a fixed bed reactor, according to one or more embodiments shown and described herein;

FIG. 7 is a schematic flow diagram of a fuel conversion system using a counter-current moving bed reactor with syngas produced from the middle and concentrated CO₂ stream from the top in the reduction reaction of composite metal oxide, according to one or more embodiments shown and described herein;

FIG. 8 is a schematic flow diagram of a fuel conversion system using a moving bed reactor with fuel introduced in the middle and syngas produced from the bottom and concentrated CO₂ stream from the top in the reduction (endothermic) reaction of composite metal oxide, according to one or more embodiments shown and described herein;

FIG. 9 is a schematic flow diagram of a fuel conversion system with additional hydrogen production from steam in a separate counter-current moving bed reactor, according to one or more embodiments shown and described herein;

FIG. 10 is a schematic flow diagram of a fuel conversion system using solar energy to provide the heat of reaction in the two-step conversion scheme, according to one or more embodiments shown and described herein;

FIG. 11 is a schematic flow diagram of a fuel conversion system using a OTM/ITM based membrane reactor with the composite metal oxide material at fuel side for syngas production, according to one or more embodiments shown and described herein;

FIG. 12 is a schematic diagram of the co-current moving bed experimental apparatus;

FIG. 13 is a graph illustrating CO/CO₂ and H₂/CO ratios as a function of time for co-current moving bed experimental setup using coal as the feedstock fuel, according to one or more embodiments shown and described herein;

FIG. 14 is a graph illustrating CO/CO₂ and H₂/CO ratios as a function of time for a co-current moving bed experimental setup using both coal and methane as feedstock, according to one or more embodiments shown and described herein; and

FIG. 15 is a graph illustrating co-current moving bed experimental data using methane as feedstock, according to one or more embodiments shown and described herein.

DETAILED DESCRIPTION

Described herein are systems and method for converting fuel sources, sometimes referred to as feedstock fuels, into syngas. Generally, syngas comprises carbon monoxide and hydrogen, and may comprise some other chemicals, such as, but not limited to, carbon dioxide and steam (H₂O). In one embodiment, a reduction reaction of composite metal oxides may produce syngas from a fuel and an oxidation reaction may regenerate the reduced composite metal oxides. As used herein, the reduction reaction may be referred to as a “first step” or “step one”, and the oxidation reaction may be referred to as “second step” or “step two”. The reduction reaction may take place in a reduction reactor and the oxidation reaction may take place in a separate oxidation reactor. In some embodiments, syngas may comprise at least about at least about 50 mol %, at least about 60 mol %, at least about 70 mol %, at least about 80 mol %, or even at least about 85% of the combination of carbon monoxide and hydrogen, such that the sum of the mol % of carbon monoxide and the mol % of hydrogen is at least about 50 mol %, at least about 60 mol %, at least about 70 mol %, at least about 80 mol %, or even at least about 85%. The stoichiometric ratio of carbon monoxide to hydrogen in the syngas produced may be about 1:2, such as between about 1:3 and about 1:1. However, the ratio may be controlled by the process parameters such as reaction conditions and reactants. The syngas may have little carbon dioxide and steam present, such as, for example, less than about 10 mol %, less than about 5%, or even less than about 2% of carbon dioxide and less than about 10 mol %%, less than about 5%, or even less than about 2% steam, respectively. The syngas may have little carbon formation, such as less than about 10 mol %. The syngas may be ready for use in downstream synthesis reactions to produce various hydrocarbons (C≧1) such as, but not limited to, methanol, dimethyl ether, gasoline, and diesel. The reduced composite metal oxide from the reduction reaction (after the syngas production) may be regenerated by oxidation with air, or another oxidant such as oxygen, steam, carbon dioxide, or combinations thereof, and then may be recycled back to the initial reduction reactor, such that the composite metal oxides may be recycled and may be continually used in the oxidation and reduction reactions. The cyclic reduction (endothermic) and oxidation (exothermic) reactions of composite metal oxides may form a reaction and heat integrated process loop that may perpetuate.

The fuel and composite metal oxides may flow in a co-current pattern relative to one another in the reduction reactor. For example, in one embodiment the co-current gas-solid flow may be either upward or downward. The reduction reactor may be, for example, a moving bed reactor, rotary kiln, riser, downer, or gas switching fixed bed. In one embodiment, the co-current reactor design may allow the natural gas, or other fuel, conversion to achieve completion wherein the composite metal oxides may be reduced to an oxidation state that provides a high quality of product syngas (i.e., at least about 90 mol % carbon monoxide and hydrogen). The composite metal oxides may act as a heat and oxygen transfer media to balance the energy and mass between the two steps (the oxidation and reduction reactions).

The composite metal oxides may act as oxygen carrying materials in the processes described herein. The composite metal oxides may comprise primary metal oxides and secondary metal oxides, including, but not limited to, FeO_x (primary)—Al₂O₃ (secondary) and FeO_x (primary)—TiO₂ (secondary), where 0.3<x<1. The primary to secondary metal oxide weight ratios may be about 15:85 to about 85:15, which may promote high CO/CO₂ and H₂/H₂O ratios (i.e., at least about 6) in the product syngas and to avoid carbon formation. Primary metal oxides may comprise, for example, oxides of Fe, V, Cr, Mn, Co, Ni, Cu, Zn, W, Pb, Ce, Sn, Mo, or combinations thereof. Secondary metal oxides may comprise, for example, oxides of Ti, Al, Si, Ca, Y and Zr, or combinations thereof. The composite metal oxide may further comprise dopants/promoters, such as, but not limited to, Ca, Ce, Pt, Ru, Rh, La, Fe, Cu, oxides thereof, and combinations thereof. The dopants/promoters may assist in the oxidation and/or reduction reactions and to enhance the rate of reactions, and may serve as an inert support or binder that may enhance the mechanical properties of the composite metal oxides. Through the reduction reaction, the composite metal oxide may provide a high quality of product syngas at the outlet of the reactor. Following the reduction reaction, the reduced state composite metal oxide (any weight percentage) may comprise primary and secondary metal oxides, such as FeO_x—Al₂O₃ or FeO_x—TiO₂, at oxidation states of the +0 state, i.e., Fe, and the +2 state, i.e., FeO.

In addition to natural gas, other carbonaceous fuels in the form of gas, liquid and solid may also be used, such as feedstock fuels including, but not limited to, coal, biomass, petroleum coke, naphtha, residual oil, shale gas, C₂-C₄ light hydrocarbons, and combinations thereof. The systems and methods described herein may convert these feedstock fuels using the same type of reactors and same types of composite metal oxides as for natural gas, to carbon monoxide and hydrogen at near the stoichiometric ratio, e.g., 1:0.6 to 1:0.8 for biomass, with little carbon dioxide presence (less than about 10 mol %). In one embodiment, feedstock fuel may be co-injected with a carbon-rich or hydrogen-rich reactant to change the carbon monoxide to hydrogen ratio of the syngas. For these feedstocks, the CO/H₂ ratio of the product syngas may be adjusted to any desired ratio for downstream product synthesis, such as 1:2, by means of co-injection of these feedstock fuels with “carbon-rich” (such as CO₂ and coal) or “hydrogen-rich” (such as H₂O and CH₄) reactants. For example, with biomass as the feedstock fuel, the CO/H₂ ratio may be adjusted to 1:1 by co-injection of the biomass with methane, a hydrogen-rich reactant. Such syngas composition adjustment in the system and method described herein may require only a minimal amount of co-injecting reactants and may be significantly lower compared to the amount of the co-injecting reactants used in the conventional methods.

In another embodiment, the CO/H₂ ratio may be adjusted to produce a high hydrogen content syngas by reaction of the reduced composite metal oxide with steam to produce hydrogen. The hydrogen may then be used to adjust the syngas CO/H₂ ratio to a higher level in H₂ contents. The steam oxidized composite metal oxide may be oxidized by air to its original oxidation state of the composite metal oxide for reuse in an oxidation reaction in an oxidation reactor.

Conventional syngas production methods from natural gas or other carbonaceous feedstocks may require cost intensive heat-exchanger type reactor, air separation unit, and/or a large amount of CO₂/H₂O reactants for controlling the feedstock conversion and product quality. The system and method described herein that carries out the selective oxidation reaction by the redox cycle of composite metal oxides may eliminate the need for the use of complicated reactors, air separation unit and excessive CO₂/H₂O reactants. The specially tailored composite metal oxides described herein coupled with the gas-solid co-current flow reactor design may directly convert the feedstock fuel to a high quality syngas yielding a high feedstock fuel conversion efficiency, flexible syngas product CO to H₂ ratio, low CO₂ and H₂O concentrations in the product gas, and little carbon deposition. The overall process may be auto-thermal with composite metal oxides transferring heat between the reactions. The operation temperature may range from about 500° C. to about 1200° C. for the reduction (endothermic) reaction of composite metal oxide, and from about 600° C. to about 1250° C. for the oxidation (exothermic) reaction of composite metal oxide. The operation pressure may range from about 1 to about 50 bars, which may depend on the pressure of feedstocks as well as the requirement of downstream syngas conversion process. The feedstocks such as air and fuel may be preheated up to 1000° C. to increase the fuel to syngas conversion efficiency.

Syngas may be a chemical precursor for synthesis of liquid fuels and chemicals. It may be a mixture of predominantly carbon monoxide (CO) and hydrogen (H₂) produced from the partial oxidation of a variety of feedstocks including, but not limited to, natural gas, shale gas, coal, biomass, naphtha, residual oil, petroleum coke, etc. Depending on the feedstocks and processing methods, the syngas composition and quality may vary significantly. Table 1 compares some conventional syngas production approaches, including steam methane reforming (SMR), dry methane reforming (DMR), partial oxidation (POX), autothermal reforming (ATR)/two-step reforming, dry coal gasification, and coal slurry gasification.

TABLE 1
Conventional Syngas Production Processes
		CO:H2
		molar	CO2	Temperature,
Process	Feedstocks	ratio	level	° C.	Reactor(s)
SMR	CH4, H2O	1:2.8-4.8	High	800-900	externally heated tubular
					catalytic reactor
DMR	CH4, CO2	1:1-3	high	800-900	externally heated tubular
	H2O				catalytic reactor
PDX	CH4, O2	1:1.7-1.8	low	>1300	high temperature non-
					catalytic reactor
ATR/two step	CH4, O2	1:1.8-4	high	900-1100	combination of SMR and
	H2O				POX
Dry coal	Coal, O2	1:1-1.2	low	1500	Entrained bed with heat
gasification					recovery
Coal slurry	Coal, O2,	1:1.2-1.5	high	1400	Entrained bed
gasification	H2O

The CO to H₂ ratio may directly affect the downstream application of the produced syngas. For example, a molar ratio of CO to H₂ of 1:2 may be commonly used for the synthesis of liquid fuels such as, but not limited to, gasoline, diesel, and methanol, while a ratio of about 1:1 may be used for production of acetic acid, acetic anhydride, or formaldehyde. Ratios less than 1:3 may be used in combination with a water gas shift unit for hydrogen production and ammonia synthesis.

As shown in Table 1, a conventional SMR process may produce a hydrogen rich syngas due to the excess amount of steam introduction, which may limit its application to hydrogen and ammonia synthesis. Due to the reduced steam flow and slow CO₂ reaction kinetics of DMR, carbon formation from methane decomposition may result. The reactions occurring in the SMR and DMR processes may be endothermic, favoring higher operating temperature for greater fuel conversion. Both processes may be commonly performed using costly heat exchanger type reactors, where fuel combustion externally provides the heat necessary to drive the catalytic reactions for syngas production. Considering the reactor materials, the SMR and DMR processes may operate below 900° C., thermodynamically restricting the methane conversion.

POX and dry coal gasification processes may be operated at a much higher temperature as the partial oxidation of the fuel with oxygen may be exothermic and may provide the heat for the process. The CO to H₂ ratio from these processes may depend upon the atomic carbon and hydrogen content of the fuel feedstock, which may pose limitations on downstream chemical and fuel synthesis. Their associated high process temperatures also may require capital-intensive heat recovery systems. In ATR, two-step methane reforming, and coal slurry gasification processes, steam and/or water may be introduced to promote a water gas shift reaction and boost hydrogen content. H₂O reactions with C and CH₄ may be endothermic, which lowers the operation temperature, allowing for high fuel conversion with cost-effective reactor design and construction. However, these syngas generation technologies all require air separation units (ASU) to supply concentrated oxygen and account for 40% to 50% of the overall capital and operating costs of a chemical/liquid fuel production plant.

The content of CO₂ and H₂O in the syngas may be an important aspect for the syngas quality control. In many existing processes, excess amounts of O₂ and H₂O may be usually introduced, which may result in promoting combustion and water gas shift reactions, respectively. A significant amount of CO₂ and/or H₂O may exist in the syngas stream, lowering the syngas production selectivity and efficiency.

In processes such as DMR, carbon deposition and formation may occur, when the fuel may be exposed to metallic substances such as Ni and Fe in a low CO₂ and low H₂O content environment. The metallic substance may catalyze the methane decomposition to carbon and hydrogen, where the absence of oxidizing gas, such as CO₂, H₂O and O₂, prevents the deposited carbon from being gasified. The negative effects of carbon deposition are two-fold: it may reduce the fuel conversion efficiency and selectivity, and may cause catalyst deactivation.

Thus, controlling the CO₂/H₂O ratio and concentration, and preventing carbon deposition may be two opposing challenges in many conventional syngas production processes. Many of these processes use excess CO₂/H₂O to suppress carbon formation. Such tradeoff may result in a syngas product stream with more than 15% CO₂ and/or H₂O, requiring downstream syngas purification steps and may result in a decreased fuel to syngas production efficiency. The process described herein may minimize excess CO₂ and/or H₂O use and production while preventing carbon formation and deposition, greatly improving the syngas production efficiency.

Described herein are chemical looping processes that have been developed for natural gas conversion to syngas. Oxides such as, but not limited to, Fe₂O₃, NiO, ZnO, PbO, CoO, Cr₂O₃, BaSO₄ have been considered as oxygen carriers to partially oxidize methane to CO and H₂. The reduced metal oxides and sulfates may be regenerated with air in a separate reactor. The chemical looping approach avoids the direct mixing between methane and air, and thus eliminates the need for an ASU and has the potential to significantly reduce syngas production costs.

However, in order to produce a low H₂O and CO₂ content syngas (less than 10 mol % respectively) from the chemical looping system, the thermodynamic phase equilibrium of single metal oxides and/or sulfates dictates that complete reduction to metallic phase or metal sulfide may be required in the fuel reactor (reduction reactor). The complete reduction may irreversibly change the oxygen carrier structure, causing its deactivation during the redox cycles. In addition, the extensive formation of the reduced metallic phase may accelerate methane decomposition resulting in carbon formation/deposition. For example, when 70% of Fe₂O₃ is reduced to Fe, carbon deposition may become a dominant process. Also, the reduction extent of metal oxides and sulfates affects the CO to H₂ ratio, which may require a careful control of the solid circulation rate and operation condition.

The composite metal oxide may be in the form of particles, pellets, or monolith, depending on the reactor design. The pellet size may range from 300 microns to 4000 microns, which may be suitable for moving bed operation. In one embodiment, the density of the composite metal oxide may be from about 1000 to about 5000 kg/m³. The relatively large pellet size and relatively high density may also assist the separation of process fines when solid fuels are used as the feedstock fuel. The composite metal oxide materials may be chemically and physically stable during the redox cycle. The synthesis procedure includes dry/wet mixing, particle/pellet formation, and sintering. Alternative techniques include sol-gel, wet impregnation, and dry-freezing.

In addition to natural gas, the feedstock fuels may be any gas, solid, and liquid fuel, or combinations thereof. Gaseous fuels include shale gas (including dry gas and wet gas), tail gas containing light hydrocarbons from downstream syngas conversion and hydroprocessing units such as Fischer-Tropsch synthesis. The composite material may be effective in converting C1-C4 hydrocarbons as well as CO₂ and H₂O to high quality syngas from the feedstock fuels sources. In certain embodiments, the composite material may handle sulfur compound in the fuel gas without deactivation. Therefore, no fuel pretreatments, such as CO₂ removal and sulfur removal, may be needed. The methods described herein may also convert solid fuels such as coal, petroleum coke, and biomass, as well as liquid fuels, such as naphtha and residual oil. In one embodiment, when solid fuels are introduced with the metal oxide composite in a moving bed process, the solid fuel may be in pellet form with a size ranging from about 300 to about 4000 microns. Such operation may ease the solid fuel injection and enhance the solid-solid distribution inside the reactor bed.

In some embodiments, solar energy may be used as a heat source for the endothermic reactions of the fuel conversion in the reduction reaction. Such arrangement may directly convert CO₂ and H₂O into syngas and other fuel forms for solar energy storage and utilization. In another embodiment, oxygen or oxygen releasing material may be introduced in the reduction reaction to increase reaction kinetics for syngas production. In yet another embodiment, the composite metal oxide materials may release oxygen for fuel conversion.

In one embodiment, a co-current flow reactor may be used for syngas production in the first step (reduction reaction). In one embodiment, a moving packed bed reactor may be used with both fuel and the composite metal oxides fed from the top and discharged from the bottom. The co-current downward flow moving bed reactor may ensure a full conversion of fuel into syngas as well as a desired composite metal oxide conversion which, may control the syngas product composition and may avoid carbon formation. The solid flow in the reduction reactor may be controlled by a non-mechanical valve system positioned at the bottom of the reactor. The reduced composite metal oxide may be then transported to the second step, an air fluidized bed reactor that oxidizes the composite metal oxide to a higher oxidation state. Alternative reactor designs for the first step include rotary kiln, riser, and downer, which may provide similar gas solid co-current flow patterns. The overall process may be auto-thermal with composite metal oxides transferring heat between the reactors.

In one embodiment, the two-step conversion (reduction and oxidation) may be conducted in a fixed bed reactor and a gas switching system for syngas production from gaseous fuel. The fixed bed may be filled with composite metal oxide pellets, particles or a monolithic bed structure. The fuel gas may be introduced to the fixed bed, where the composite metal oxide may be reduced within a certain extent and high quality syngas may be produced. When the composite metal oxide conversion reaches a determined state, the fuel gas may be switched to a preheated air stream for the composite metal oxide regeneration. In one exemplary embodiment, the fuel and air streams may be preheated and the gas switching system may be operated at high temperatures (at least about 1000° C.).

In yet another embodiment, a counter-current moving bed reactor may be used for the first step with fuel introduced from the bottom and composite metal oxide introduced from the top. A conversion profile of the composite metal oxide may be formed inside the moving bed reactor. There may be two gas outlets in the moving bed reactor, one at the top for concentrated CO₂ and H₂O production, another in the middle of the bed where the composite metal oxide conversion may be suitable for high quality syngas production. For the second step, the reduced composite metal oxide may be reoxidized with air. In this embodiment, high quality syngas may be obtained with sequestration ready CO₂ stream. The yield between syngas and CO₂ stream may be adjusted by the process heat balance and flowrates from the two gas outlets of the moving bed reactor.

In another embodiment, a moving bed reactor may be used for simultaneous high quality syngas and high purity CO₂ productions. For the first step, the composite metal oxide may be introduced from the top, and the fuel may be introduced at the middle. Two gas outlets may be positioned at the top and bottom of the moving bed reactor. The gas coming out from the top may encounter higher oxidation state composite metal oxides resulting in high purity CO₂ and H₂O production, while the gas from the bottom may be high quality syngas controlled by the reduced composite metal oxide. The reduced composite metal oxide may be reoxidized by air in another reactor for the second step. For this two-step process, high quality syngas may be obtained with sequestration ready CO₂ stream. The yield between syngas and CO₂ stream may be adjusted by the process heat balance and the fuel split ratio in the moving bed reactor.

In another embodiment, the reduced composite metal oxide from the first step may be oxidized by H₂O, CO₂ or a mixture thereof, for production of H₂, CO or syngas in an additional reactor. In one embodiment, the additional reactor may be a moving bed reactor with counter-current gas (H₂O, CO₂)—solid (reduced composite metal oxide) contacting pattern. The operation temperature may be from about 500° C. to about 1100° C.

The oxidation state of the single metal oxide may significantly affect the syngas composition and possibility of carbon formation. As shown in FIG. 1 and FIG. 2, for example, when FeO may be used to partially oxidize methane to syngas, a low selectivity to CO₂ and H₂O may be obtained when the FeO to CH₄ ratio may be below 2. In such condition, however, metallic Fe may be formed and may catalytically decompose methane causing carbon deposition. Carbon deposition/formation may be avoided by reducing the extent of the metal oxide reduction by increasing the FeO to CH₄ ratio. Conversely, increasing this ratio results in an increase in CO₂ and H₂O levels to more than 30 mol %, lowering the syngas selectivity and yield. Thus, the use of a single metal oxide material pose challenges in syngas quality control.

The subject matter of the present disclosure may be integrated with various fuel and chemical synthesis processes in light of its ability to produce flexible CO to H₂ ratio. Depending on the downstream application, the CO to H₂ ratio may be adjusted to the desired value in the first step by introducing CO₂ or H₂O together with the fuel. The CO₂ and H₂O may also enhance the initial conversion of methane and carbon via reforming and gasification reactions, respectively. Thus, the need for cost and energy intensive downstream processes to adjust the syngas composition may be removed or reduced to a minimal. In some embodiments, the reduced composite metal oxide may be also used for hydrogen production by steam oxidation. The hydrogen produced may be used for hydroprocessing or product upgrading. In certain embodiments, less than 20 wt % of dopants may be added to the composite to catalyze certain reactions such as methane decomposition, carbon gasification, and tar cracking. The dopants may be selected from the group consisting of at least one of Ca, Ce, Pt, Ru, Rh, La, Fe, Cu, and oxides thereof. In yet another embodiment, the composite metal oxide material may also release oxygen for fuel conversion. Binders such as bentonite and PVC material may be also used for binding purpose in the synthesis process.

The composite metal oxide material may be chemically and physically stable during multiple redox cycles. Single metal oxide materials may not be able to sustain multiple redox cycles due to changes in mechanical and crystal structure. The composite metal oxide materials may be synthesized by dispersing active metal oxide compounds in a physically stable structure, and thus may be repeatedly used in the process with little change to its reactivity and oxygen carrying capacity.

In other embodiments, as shown in FIG. 3(a) and FIG. 3(b), a co-current flow reactor may be used for syngas production in the first step from a natural gas or shale gas feedstock fuel. The CO to H₂ ratio in the syngas may be about 1:2, which may be suitable for F-T and methanol synthesis. The tail-gas of the syngas conversion step containing C₁-C₄ hydrocarbons may be recycled to the first step reactor to maximize the fuel to product yield and efficiency. The CO:H₂ ratio may be adjusted to other ratios by introducing minimal amount of the CO₂ and H₂O in step 1. For example, when CO₂ or other carbon rich feedstocks may be introduced together with methane, the CO to H₂ ratio may be adjusted to 1:1, suitable for acetic acid, acetic anhydride, and/or formaldehyde synthesis. In one embodiment, a moving packed bed reactor may be used with both fuel and the said composite metal oxides fed from the top and discharged from the bottom. The co-current downward flow moving bed reactor may promote a full conversion of fuel into syngas as well as a desired composite metal oxide conversion which may control the syngas product composition and avoid carbon formation. For example, the composite metal oxide may be introduced as FeO_x—TiO₂ at the top, and may be converted to FeO_y—TiO₂ at the bottom of the reactor. In one embodiment, the operation range of 1.5>x>1>y>0.3 may be used to increase the particle oxygen carrying capacity and control the syngas quality. Alternatives to the first step reactor designs include rotary kiln, riser, and downer, which may provide similar gas solid co-current flow pattern as the moving packed bed reactor. The reduced composite metal oxide may be then transported to the second step, an air fluidized bed reactor that oxidizes the composite metal oxide to a higher oxidation state. For example, the main reactions in the first and second steps may be:

2FeOTiO₂+CH₄=2FeO_0.5TiO₂+CO+2H₂ ΔH=235 kJ@1000° C.

2FeO_0.5TiO₂+1/2O₂=2FeOTiO₂ ΔH=−359 kJ@1000° C.

Still referring to FIGS. 3(a) and 3(b), the overall heat of reaction may be thus exothermic, rendering a flexible process heat balance. In one embodiment, the operation temperature may be from 500° C. to 1200° C. for the first step, and from 600° C. to 1250° C. for the second step. The fuel and oxidizing gas feedstocks for the first and second step, respectively, may be preheated up to 1000° C. to increase the fuel to syngas efficiency. The operation pressure may range from 1 to 50 bars, depending on the pressure of feedstocks as well as the requirement of the downstream syngas conversion process.

In other embodiments, as shown in FIG. 4(a) and FIG. 4(b), a co-current flow reactor may be used for syngas production from solid fuels such as biomass and/or coal. The CO to H₂ ratio in the syngas may be about 1:0.6 to about 1:0.8, and may depend on the composition of the feedstock fuels. Co-injection of hydrogen rich feedstocks such as CH₄ and H₂O may adjust the CO to H₂ ratio to about 1:1 or about 1:2, which may be suitable for downstream chemical and fuel synthesis. In one embodiment, a moving packed bed reactor may be used with similar sized solid fuel and the said composite metal oxides fed from the top and discharged from the bottom. The co-current downward flow moving bed reactor may promote full fuel conversion into syngas as well as a desired composite metal oxide conversion which may control the syngas product composition and avoid carbon formation. For example, the composite metal oxide may be introduced at the top of the first step reactor as FeO_xTiO₂ and may be converted to FeO_yTiO₂ when it reaches the bottom. In one embodiment, the operation range of 1.5>x>1>y>0.3 may be used to maintain a high particle oxygen carrying capacity and to control the syngas quality. The solid flow may be controlled by a non-mechanical valve system positioned at the bottom of the reactor. The reduced composite metal oxide may be then transported to the second step, an air fluidized bed reactor that oxidizes the composite metal oxide to a higher oxidation state. In one embodiment, the operating temperature may range from about 500° C. to about 1200° C. for the first step, and from about 600° C. to about 1250° C. for the second step. The fuel and oxidizing gas feedstocks for the first and second step, respectively, may be preheated up to at least about 1000° C. to increase the fuel to syngas efficiency. The operating pressure may range from about 1 bar to about 50 bars, depending on the pressure of the feedstocks as well as the requirement of the downstream syngas conversion process. Alternatives to the first step reactor designs include rotary kiln, riser, and downer, which may provide similar gas solid co-current flow pattern as the moving packed bed reactor.

In other embodiments, as shown in FIG. 5(a) and FIG. 5(b), a co-current flow reactor may be used for syngas production from liquid fuels such as, but not limited to, naphtha. In one embodiment, a moving packed bed reactor may be used with preheated liquid fuel and the said composite metal oxides fed from the top and discharged from the bottom. The co-current downward flow moving bed reactor may promote full fuel conversion into syngas as well as a desired composite metal oxide conversion which may control the syngas product composition and avoid carbon formation. For example, the composite metal oxide may be introduced in the first step reactor as FeO_xTiO₂ at the top, and may be converted to FeO_yTiO₂ when it reaches the bottom. In one embodiment, the operation range of 1.5>x>1>y>0.3 may be used to maintain a high particle oxygen carrying capacity and control the syngas quality. The solid flow may be controlled by a non-mechanical valve system positioned at the bottom of the reactor. The reduced composite metal oxide may then be transported to the second step, an air fluidized bed reactor that oxidizes the composite metal oxide to a higher oxidation state. In one embodiment, the operating temperature ranges from 500° C. to 1200° C. for the first step, and from 600° C. to 1250° C. for the second step. The fuel and oxidizing gas feedstocks for the first and second step, respectively, may be preheated up to 1000° C. to increase the fuel to syngas efficiency. The operating pressure may range from 1 to 50 bars, depending on the pressure of feedstocks as well as the requirement of the downstream syngas conversion process.

In another embodiment, as shown in FIG. 6, the two step conversion may be conducted in a fixed bed reactor with a gas switching system for syngas production from gaseous fuel. The fixed bed may be filled with composite pellets, particles or a monolithic bed structure. The fuel gas may be introduced to the fixed bed, where the composite metal oxide may be reduced within a certain range and high quality syngas may be produced. When the composite metal oxide conversion reaches a certain extent of reduction, the fuel gas may be switched to preheated air for the composite metal oxide regeneration. For example, the composite metal oxide may be reduced in the first step reactor from FeO_xTiO₂ to Feo_yTiO₂ before switching the gas feed to the second step. In one embodiment, the operating range of 1>x>y>0.3 may be used to control the syngas quality. In one embodiment, the fuel and air streams may be preheated and the gas switching system may be operated at high temperatures. In one embodiment, the operating temperature may range from about 500° C. to about 1200° C. for the first step, and from about 600° C. to about 1250° C. for the second step. The fuel and oxidizing gas feedstocks for the first and second step, respectively, may be preheated up to at least about 1000° C. to increase the fuel to syngas efficiency. The operating pressure may range from about 1 bar to about 50 bars, depending on the pressure of the feedstocks as well as the requirement of the downstream syngas conversion process.

In another embodiment, as shown in FIG. 7, a counter-current moving bed reactor may be used for the first step with fuel introduced from the bottom and composite metal oxide introduced from the top. This gas-solid contact design may form a conversion profile of the composite metal oxide along the height of the moving bed reactor. Two gas outlets may be placed on the moving bed reactor, one at top for concentrated CO₂ and H₂O production, and the other at or near the middle where the composite metal oxide conversion may be suitable for high quality syngas production. In the second step, the reduced composite metal oxide may be reoxidized with air. For this two-step process, high quality syngas may be obtained in conjuction with a high purity/sequestration ready CO₂ stream. For example, the composite metal oxide may be introduced in the first step reactor as FeO_xTiO₂ at the top, and may be converted to FeO_yTiO₂ when it reaches the middle, and may be converted to FeO_zTiO₂ when it reaches the bottom. In one embodiment, the operation range of 1.5>x>1>y>z>0.3 may be used to maintain a high particle oxygen carrying capacity and control the syngas quality. The yield between syngas and CO₂ stream may be adjusted by the process heat balance and flowrates from the two gas outlets of the moving bed reactor. The operating temperature may range from about 500° C. to about 1200° C. for the first step, and from about 600° C. to about 1250° C. for the second step. The fuel and oxidizing gas feedstocks for the first and second step, respectively, may be preheated up to at least about 1000° C. to increase the fuel to syngas efficiency. The operating pressure may range from about 1 bar to about 50 bars, depending on the pressure of the feedstocks as well as the requirement of the downstream syngas conversion process.

In another embodiment, as shown in FIG. 8, a moving bed reactor may be used for full fuel conversion to high quality syngas and high purity CO₂ co-production. The composite metal oxide may be introduced from the top, and the fuel may be introduced at or near the middle of the reactor. Two gas outlets may be positioned at the top and bottom of the moving bed reactor. The gas coming out from the top may encounter higher oxidation state composite metal oxides resulting in high purity CO₂ and H₂O production, while the gas from the bottom may be high quality syngas controlled by the reduced composite metal oxide. For the second step, the reduced composite metal oxide may be reoxidized with air. For this two-step process, high quality syngas may be obtained in conjuction with a high purity/sequestration ready CO₂ stream. In one embodiment, the operation range of 1.5>x>1>y>z>0.3 may be used to maintain a high particle oxygen carrying capacity and control the syngas quality. The yield between syngas and CO₂ stream may be adjusted by the process heat balance and the fuel split ratio in the moving bed reactor. The operating temperature may range from about 500° C. to about 1200° C. for the first step, and from about 600° C. to about 1250° C. for the second step. The fuel and oxidizing gas feedstocks for the first and second step, respectively, may be preheated up to at least about 1000° C. to increase the fuel to syngas efficiency. The operating pressure may range from about 1 bar to about 50 bars, depending on the pressure of the feedstocks as well as the requirement of the downstream syngas conversion process. In certain embodiments, as shown in FIG. 9, the reduced composite metal oxide may be also used for hydrogen production by steam oxidation or syngas production using a mixture of CO₂ and H₂O. The hydrogen produced may be used for hydroprocessing or product upgrading.

In another embodiment, as shown in FIG. 10, the reduced composite metal oxide from the first step may be used for hydrogen production by steam oxidation or for syngas production using a mixture of H₂O, CO₂. In one embodiment, the additional reactor may be a moving bed reactor with countercurrent gas solid contacting pattern. The operation temperature may be from about 500° C. to about 1100° C. and the heat may be supplied directly from an external heat source such as a solar energy collecting system.

In yet another embodiment, as shown in FIG. 11, the composite metal oxide material may be coated on the fuel side of a membrane system, such as an OTM/ITM based membrane system for syngas production. Here, the composite material may be stabilized with an x value between 0.3 and 1 to control the syngas quality.

It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.

In another embodiment, the fuel conversion system may be designed in compact, modular, mobile mode and used for offshore and remote well applications.

EXAMPLES

The various embodiments of systems and methods for converting fuel will be further clarified by the following examples. The examples are illustrative in nature, and should not be understood to limit the subject matter of the present disclosure.

Example 1

A fixed bed experiment was conducted to study the methane to syngas reaction. The lower section of the reactor was filled with 23.1 g FeO—TiO₂ particles, and the upper section was filled with 8.3 g Fe₂O₃—TiO₂ particles. When the temperature of the reactor reached 990° C., 50 mL/min CH₄ and 50 mL/min N₂ was injected into the reactor by digital mass controllers. The outlet gas composition was analyzed using a CAI gas analyzer as well as a gas chromatography. The gas concentration at the outlet reached a quasi-steady state in half an hour with methane conversion >95%, CO:CO₂ ratio around 10, CO:H₂ ratio around 1:2.

Example 2

A moving bed reactor test apparatus was constructed, as shown in FIG. 12, and experiments were conducted to study the solid feedstock to syngas reaction using Powder River Basin (PRB) coal as prototype coal. The PRB coal tested has about 25% moisture and the corresponding molecular formula may be CH_0.8O_0.2 on a dry basis. The coal powder was mixed with particles at the mass ratio of 1:5 and then fed into the moving bed system from the top in a batch mode. The solid flowrate was controlled to be 20 g/min by the screw feeder at the bottom. The temperature was controlled by the external heating to be 1030° C. At the steady state, the coal conversion is over 90%, H₂/CO ratio fluctuated between 0.5-0.7 and CO/CO₂ ratio fluctuated between 5-12, as shown in FIG. 13, respectively. The fluctuation is due to the batch feeding mode of the solid material. Furthermore, CH₄ was also co-injected to adjust the H₂/CO ratio. The co-injection of CH₄ (870 ml/min, coal 2.7 g/min, particles 20 g/min) could give adjusted the CO/H₂ ratio effectively around 1.1 with CO/CO₂ ratio maintained high, as shown in FIG. 14.

Example 3

A moving bed reactor test was conducted to study the solid feedstock conversion to syngas using biomass. The corresponding molecular formula was CH_1.4O_0.6. The biomass material was mixed with particles at a mass ratio of 1:3 and then injected into the moving bed system from the top. The solid fuel and composite particles followed a co-current contacting mode. The composite particle flowrate was controlled to 20 g/min using the screw feeder at the bottom. The temperature was controlled to 1040° C. using the external heaters. At steady state operation, the biomass conversion was over 95% with CO:CO₂ ratio around 10 and CO:H₂ ratio around 1:0.8. In this case, the carbon monoxide and hydrogen concentration in the high quality syngas from the syngas production reactor was higher than 91 mol %. Additionally, CH₄ was co-injected to adjust the CO:H₂ ratio. The co-injection of CH₄ (540 ml/min, biomass 5 g/min, particles 26 g/min) gave an almost complete fuel conversion (95%) with CO:H₂ ratio about 1:1. Given longer fuel residence time, a 100% fuel conversion may be achieved.

Example 4

A moving bed reactor test was conducted to study the gas feedstock to syngas reaction using methane as a feedstock. FIG. 15 shows a graph illustrating the co-current moving bed experimental data using methane as feedstock.

Claims

1. A method for converting fuel into syngas, the method comprising: feeding the fuel and composite metal oxides into a reduction reactor in a co-current flow pattern relative to one another, wherein the composite metal oxides are solid particles comprising a primary metal oxide and a secondary metal oxide;reducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides;transporting the reduced composite metal oxides to an oxidation reactor;regenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant in the oxidation reactor; andrecycling the regenerated composite metal oxides to the reduction reactor for subsequent reduction reactions to produce syngas.

2. The method of claim 1, wherein the fuel is a gas.

3. The method of claim 1, wherein the fuel is natural gas.

4. The method of claim 1, wherein the fuel is natural gas, coal, biomass, petroleum coke, naphtha, residual oil, shale gas, C2-C4 light hydrocarbons, and combinations thereof.

5. The method of claim 1, wherein the oxidizing reactant comprises air, oxygen, steam, carbon dioxide, or combinations thereof.

6. The method of claim 1, wherein the ratio of hydrogen to carbon monoxide can be controlled.

7. The method of claim 1, wherein the syngas comprises a stoichiometric ratio of carbon monoxide to hydrogen of about 1:2 and the fuel is natural gas.

8. The method of claim 1, wherein the fuel is co-injected with a carbon-rich or hydrogen-rich reactant to change the carbon monoxide to hydrogen ratio of the syngas.

9. The method of claim 1, wherein oxygen or oxygen releasing material is introduced in the reduction reaction to increase reaction kinetics for syngas production.

10. The method of claim 1, wherein the reducing of the composite metal oxides occurs at a temperature in the range of between about 500° C. and about 1200° C.

11. The method of claim 1, further comprising reacting the reduced composite metal oxides with steam to produce hydrogen used to adjust the carbon monoxide to hydrogen ratio of the syngas.

12. The method of claim 1, wherein the syngas comprises at least about 85 mol % carbon monoxide and hydrogen.

13. The method of claim 1, wherein the syngas comprises less than about 10 mol of carbon dioxide and less than about 10 mol steam.

14. The method of claim 1, wherein the reduction reactor is a moving bed reactor, rotary kiln, riser, gas switching fixed bed reactor, downer, or combinations thereof in series.

15. The method of claim 1, wherein: the primary metal oxides comprises oxides of Fe, V, Cr, Mn, Co, Ni, Cu, Zn, W, Pb, Ce, Sn, Mo, or combinations thereof; andthe secondary metal oxides comprises oxides of Ti, Al, Si, Ca, Y and Zr, or combinations thereof.

16. The method of claim 1, wherein the composite metal oxides further comprises a dopant/promoter comprising Ca, Ce, Pt, Ru, Rh, La, Fe, Cu, oxides thereof, or combinations thereof.

17. The method of claim 1, wherein: the composite metal oxides comprises iron oxide and titanium oxide; or the composite metal oxides comprises iron oxide and aluminum oxide.

18. The method of claim 17, wherein the iron oxide is reduced in the reduction reaction from FeOx to FeOy and 1.5>x>I>y>0.3.

19. The method of claim 1, wherein the CO to H2 ratio may be adjusted by introducing CO2, H2O, or both, into the reduction reaction.

20. The method of claim 1, wherein the process is auto-thermal, the fuel is preheated, and solar energy is introduced to the reduction reactor.

21. A method for converting fuel into syngas, the method comprising: feeding the fuel and composite metal oxides into a reduction reactor in a co-current flow pattern relative to one another, wherein:the composite metal oxides are solid particles comprising a primary metal oxide and a secondary metal oxide;the fuel is natural gas; andthe composite metal oxide comprises iron oxide and titanium oxide, or the composite metal oxide comprises iron oxide and aluminum oxide; andreducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides, wherein the iron oxide is reduced in the reduction reaction from FeOx to FeOy and 1.5>x>I>y>0.3;transporting the reduced composite metal oxides to an oxidation reactor;regenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant in the oxidation reactor; andrecycling the regenerated composite metal oxides to the reduction reactor for subsequent reduction reactions to produce syngas.

22. A method for converting fuel into syngas, the method comprising: reducing the composite metal oxides with the fuel to form syngas and reduced composite metal oxides; andregenerating the composite metal oxides by oxidizing the reduced composite metal oxides with an oxidizing reactant, wherein:the composite metal oxides are solid particles comprising a primary metal oxide and a secondary metal oxide; andthe composite metal oxide comprises iron oxide and titanium oxide, or the composite metal oxide comprises iron oxide and aluminum oxide.

23. The method of claim 22, wherein the reduction of the composite metal oxides and the oxidizing of the composite metal oxides is carried out in a fixed bed reactor with a gas switching system.

24. The method of claim 22, wherein the reduction of the composite metal oxides occurs in a reduction reactor, wherein the reduction reactor is a moving bed reactor comprising two gas outlets and the reduction of the composite metal oxides produces syngas and CO2.

25. The method of claim 22, wherein the reduction and oxidation of the composite metal oxides occurs in a membrane based reactor, wherein the composite metal oxides are integrated to the fuel side of the membrane based reactor.