Process for recovery of hydrogen from coal gasification products

Abstract

A process for recovering hydrogen from coal gasification products is provided. The process comprises contacting the coal gasification products with a metal-based Boudouard catalyst to produce a first product stream comprising hydrogen and carbon dioxide; and contacting the first product stream with a solid phase carbon dioxide adsorbent to separate hydrogen from the first product stream.

Description

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/793,028, filed Apr. 19, 2006, the entire disclosure of which is incorporated herein by reference.

FIELD

The present invention relates generally to a process for the production of pure hydrogen gas. More particularly, the invention relates to a process for the recovery of hydrogen from coal gasification products.

BACKGROUND

Energy consumption in the United States is growing twice as fast as energy production. While energy efficiency is increasing, there is still a need for additional energy sources and supplies to support economic growth.

Hydrogen can be used as a fuel in fuel cells, internal combustion engines, and turbines. It can also be used as a feedstock for liquid fuels and other chemicals. Hydrogen has the highest energy to weight ratio among fossil fuels. For example, one kilogram of hydrogen contains the same amount of energy as 2.1 kilograms of natural gas or 2.8 kilograms of gasoline.

Although hydrogen is the most abundant element on earth, it does not naturally exist in its elemental form. Currently, it is produced mainly by steam reformation, but can also be produced by partial oxidation, electrolysis, or gasification.

Steam reforming of natural gas or coal followed by water-gas shift reaction is commonly used for hydrogen production. However, such reactions for producing hydrogen require separation of the resulting hydrogen from the other gaseous reaction products for practical applications.

U.S. Pat. No. 6,509,000 discusses a process for the production of carbon monoxide-free hydrogen by catalytic decomposition of methane or natural gas at low temperatures. This approach avoids carbon build-up on the catalyst by its periodic removal. The exit gas, however, is a mixture of hydrogen and carbon dioxide and further separation is required.

U.S. Pat. No. 6,887,455 discusses a process for the catalytic generation of hydrogen. This approach uses the self-sustaining combination of partial oxidation and steam reforming of a hydrocarbon. The exit stream contains mostly hydrogen, carbon dioxide and nitrogen, with small amounts of carbon monoxide and methane.

U.S. Pat. No. 6,790,430 discusses a method for the production of hydrogen from coal or other carbonaceous substances. This approach attempts to alleviate the problem of ash and other impurities interfering with the gasification reaction and the calcium oxide carbonation reaction by using a gasification vessel and carbonation vessel. By using two vessels, there is no contamination with ash (from the coal) in the gasification vessel.

Thus, there is a need for a more effective process for producing hydrogen free of carbon dioxide and carbon monoxide from coal gasification products.

SUMMARY

The present invention is directed to a process for the production of pure hydrogen gas.

In one embodiment, the present invention provides a process for recovering hydrogen from coal gasification products. The process comprises contacting the coal gasification products with a metal-based Boudouard catalyst to produce a product stream comprising hydrogen and carbon dioxide; and contacting the product stream with a solid phase carbon dioxide adsorbent to separate hydrogen from the product stream.

In another embodiment, the present invention provides a process for recovering hydrogen from coal gasification products. The process comprises contacting the coal gasification products with a catalyst comprising metallic iron in a reactor at a temperature of from about 350° to about 900° C. to produce a first product stream comprising hydrogen and carbon dioxide; and contacting the first product stream with an adsorbent comprising calcium oxide to separate hydrogen from the product stream. The adsorbent is then regenerated by contacting the adsorbent with nitrogen and the metallic iron catalyst is regenerated by contacting the catalyst with oxygen.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation of separating hydrogen from syngas in accordance with the principles of the present invention;

FIG. 1B is a schematic representation of regenerating spent catalyst and metal-based adsorbent in accordance with the principles of the present invention;

FIG. 2A is a graphical representation showing the results of hydrogen separation at a catalyst to adsorbent ratio of 1:6 as described in Example 2;

FIG. 2B is a graphical representation showing the results of hydrogen separation at a catalyst to adsorbent ratio of 1:4.5 as described in Example 2;

FIG. 2C is a graphical representation showing the results of hydrogen separation at a catalyst to adsorbent ratio of 1:3 as described in Example 2;

FIG. 3A is a graphical representation showing the results of hydrogen separation at a temperature of 750° C. as described in Example 3;

FIG. 3B is a graphical representation showing the results of hydrogen separation at a temperature of 800° C. as described in Example 3;

FIG. 3C is a graphical representation showing the results of hydrogen separation at a temperature of 850° C. as described in Example 3;

FIG. 4A is a graphical representation showing the results of hydrogen separation at an inlet syngas partial pressure of 0.09 atm as described in Example 4;

FIG. 4B is a graphical representation showing the results of hydrogen separation at an inlet syngas partial pressure of 0.18 atm as described in Example 4;

FIG. 4C is a graphical representation showing the results of hydrogen separation at an inlet syngas partial pressure of 0.27 atm as described in Example 4;

FIG. 5 is a graphical representation showing results of outlet gas composition versus time for the reaction described in Example 5.

FIG. 6 is a graphical representation showing results during the regeneration of catalyst described in Example 5.

FIG. 7 is a graphical representation showing results during the regeneration of catalyst described in Example 5.

FIG. 8A is a graphical representation showing results during the regeneration of catalyst from steam experiments Example 6

FIG. 8B is a graphical representation showing results during the regeneration of catalyst from steam experiments Example 6

FIG. 8C is a graphical representation showing results during the regeneration of catalyst from steam experiments Example 6

FIG. 9A is a graphical representation showing results during the regeneration of catalyst from steam experiments Example 6

FIG. 9B is a graphical representation showing results during the regeneration of catalyst from steam experiments Example 6

FIG. 9C is a graphical representation showing results during the regeneration of catalyst from steam experiments Example 6

Corresponding reference numbers indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

The present invention provides an improved process for the separation and enrichment of hydrogen gas from coal gasification products. In particular, Applicants have found that hydrogen and carbon monoxide can be separated from the syngas produced from coal gasification using a single reactor system. Without being held to a particular theory, Applicants have discovered that hydrogen yield can be increased by contacting syngas with a metal-based Boudouard catalyst and a solid phase carbon dioxide adsorbent. The process further allows for the in-situ regeneration of catalyst and adsorbent such that hydrogen yield can be increased by the Boudouard reaction with little or no change to the catalyst structure and activity.

Referring now to FIG. 1A, an embodiment of the process of the present invention for hydrogen enrichment from coal gasification products is generally described. In this embodiment, coal gasification products 20 comprising hydrogen and carbon monoxide are introduced into a reactor 100 containing a metal-based Boudouard catalyst 101 and a solid phase carbon dioxide adsorbent 103. Contacting the coal gasification products 20 with a metal-based Boudouard catalyst 101 produces a product stream comprising hydrogen and carbon dioxide. Hydrogen is then recovered from the product stream by contacting the product stream comprising carbon dioxide and hydrogen with the solid phase carbon dioxide adsorbent 103.

The gas stream 105 exiting the reactor 100 consists mainly of hydrogen. The catalyst 101, which is now covered with carbon, and the carbon dioxide adsorbent 103 are regenerated as shown in FIG. 1B. The catalyst 101 is exothermically regenerated by carbon oxidation (to carbon dioxide), thereby providing the enthalpy needed to desorb the carbon dioxide from the adsorbent 103. The regenerated gas stream 109 leaving the reactor 100 contains carbon dioxide at relatively high temperature and pressure and can be used in a Rankine cycle to produce shaft work. The entire process can effectively produce pure hydrogen over multiple cycles. In addition, the process can also be utilized to modify the composition of synthetic gas (particularly the carbon monoxide to hydrogen ratio of synthetic gas) for efficient use in a Fisher-Tropsch reaction.

Generally, the coal gasification products fed to the reactor comprise syngas. In a first embodiment, the syngas comprises at least carbon monoxide, hydrogen and methane. In other embodiments, the syngas may comprise nitrogen, methane, carbon dioxide, hydrogen, and carbon monoxide.

Suitable metal catalysts for use as the metal-based Boudouard catalyst include, for example, iron or nickel catalysts. In a preferred embodiment, the Boudouard catalyst comprises metallic iron.

Solid phase carbon dioxide adsorbents for use in the process of the invention generally include any calcium-based adsorbent capable of adsorbing carbon dioxide, particularly calcium-based adsorbents of relatively high capacity. In a preferred embodiment, the adsorbent comprises calcium oxide.

Applicants have discovered that the process of the present invention can be advantageously conducted in a single reactor system. Suitable reactor systems for use in the present invention include reactors selected from the group consisting of a fixed bed reactor, a fluidized bed reactor and a packed bed reactor. When the process is conducted in a single reactor, the metal-based Boudouard catalyst and solid phase carbon dioxide adsorbent are present in amounts such that the ratio of metal-based Boudouard catalyst to solid phase carbon dioxide adsorbent is from about 1:3 to about 1:6.

The process of the present invention can generally be conducted at any temperature. However, in some embodiments, it has been found that the Boudouard reaction proceeds at an increased rate at temperatures above 350° C. Thus, in one embodiment, the coal gasification products and metal-based Boudouard catalyst are contacted at a temperature of from about 350° to about 900° C. Further, experience suggests that the Boudouard reaction may be increased in some embodiments at elevated pressure. Accordingly, in another embodiment, the coal gasification products and metal-based Boudouard catalyst are contacted at a pressure of from about 0 to about 250 psi.

In another embodiment, the coal gasification products are contacted with steam in the presence of the metal-based Boudouard catalyst to produce a product stream comprising hydrogen and carbon dioxide. Steam may be used to facilitate the water-gas shift reaction wherein carbon monoxide may be further converted to hydrogen.

In a particular embodiment, the metal-based Boudouard catalyst is generated in situ such that the process comprises contacting the syngas with a catalyst precursor to the metal-based Boudouard catalyst. In one embodiment, such a catalyst precursor comprises iron oxide. When the iron oxide catalyst precursor is contacted with the syngas, any methane in the syngas reacts with steam to form hydrogen and carbon monoxide in a three to one ratio as shown in reaction [1]. This reaction reduces the iron oxide to form metallic iron and a small fraction of ferric oxide, which both act as catalysts for the Boudouard reaction [2].
CH₄+H₂O→CO₂+3H₂  [1]
H₂+2CO→C+CO₂+H₂  [2]

A portion of the carbon monoxide produced from [1] and the carbon monoxide in the coal gasification products are water gas shifted [3].
CO+H₂OCO₂+H₂  [3]

Further, a portion of the carbon monoxide produced from [1] and the carbon monoxide in the coal gasification products are converted to carbon dioxide and carbon via the Boudouard reaction [2]. The Boudouard reaction [2] results in carbon deposition on the catalyst bed, leaving carbon dioxide and hydrogen in the first product stream.

The first product stream, comprised of carbon dioxide and hydrogen, is then contacted with a calcium oxide adsorbent to separate hydrogen as shown in reaction [4].
CO₂+H₂+CaO→CaCO₃+H₂  [4]

The calcium oxide acts as a carbon dioxide adsorbent and enhances both the water gas shift reaction [3] and the Boudouard reaction [2] by shifting them to the right. The net effect is the formation of calcium carbonate [5], thereby removing carbon dioxide and resulting in a pure hydrogen gas stream.
CaO+CO₂CaCO₃  [5]

In another embodiment, the catalyst and solid phase carbon dioxide adsorbent, which contains metallic iron, free carbon, and calcium carbonate after the reaction, can be regenerated. Oxygen is passed through the reactor to regenerate the iron catalyst and the calcium oxide adsorbent. The oxygen reacts with the carbon to form carbon dioxide [6] and with the iron to form iron oxide [7]. These reactions provide the enthalpy needed to desorb the carbon dioxide from the calcium oxide adsorbent [8].
2Fe+1.5O₂→Fe₂O₃  [6]
C+O₂→CO₂  [7]
CaCO₃CaO+CO₂  [8]

The exiting stream contains carbon dioxide at relatively high temperature and pressure and can be used in a Rankine cycle to produce shaft work. This entire process can effectively produce pure hydrogen over multiple cycles. In addition, this process can also be utilized to modify the composition of synthetic gas (especially the carbon monoxide to hydrogen ratio of the syngas) for efficient use in a Fisher-Tropsch reaction.

EXAMPLES

The following examples are merely illustrative, and do not limit this disclosure in any way.

Example 1

This example describes the theoretical yields achieved by the process of the present invention. Modeling was conducted to determine equilibrium concentrations of hydrogen produced from different feed gas compositions. The modeled the separation of hydrogen in a tubular plug flow reactor at a temperature of 400° C. using a metallic iron Boudouard catalyst and a calcium oxide adsorbent. Table 1 lists simulated data on equilibrium concentrations of the hydrogen stream produced. As shown, high purity hydrogen can be produced by the present invention. A mass and energy balance on the system assuming the absence of the Boudouard reaction achieved only 0.68 moles of hydrogen from each mole of carbon monoxide converted to carbon dioxide by water gas shift. However, when the Boudouard reaction occurs, the amount of hydrogen produced increased more than 33% to 0.91 moles per mole of carbon monoxide.

TABLE 1
Equilibrium Concentration of Hydrogen Product Stream at 400° C.
Initial Gas		After Ca-based
Composition	After catalyst bed	adsorbent bed
	Mole		Mole		Mole
Species	Fraction	Species	Fraction	Species	Fraction
Experiment 1
CO	0.125	CO	3.0 * 10−4	CO	3.93 * 10−5
CO2	0	CO2	5.5 * 10−3	CO2	7.2 * 10−4
H2	0.25	H2	0.402	H2	0.9992
H2O	0.625	H2O	0.589	H2O	0
Experiment 2
CO	0.063	CO	7.29 * 10−5	CO	8.6 * 10−6
CO2	0	CO2	1.72 * 10−3	CO2	2.0 * 10−4
H2	0.27	H2	0.344	H2	0.9997
H2O	0.667	H2O	0.653	H2O	0

Example 2

This example compares the effects of different catalyst loadings in the process of the present invention. The experiment comprised separating hydrogen from syngas in a ½″ diameter tubular plug flow reactor containing an iron oxide catalyst and a calcium carbonate adsorbent. Three experiments were conducted using different amounts of iron oxide catalyst. Experiment 1 used 0.28 g of iron oxide catalyst, Experiment 2 used 0.42 g of iron oxide catalyst and Experiment 3 used 0.56 g of iron oxide catalyst. In all three runs, the reactor contained 1.68 g of calcium carbonate adsorbent. The syngas, which comprised 52% carbon monoxide and 48% hydrogen, was fed into the reactor at 10 ml/min along with nitrogen. The temperature of the reactor was 750° C. and the syngas partial pressure was 0.18 to 0.2 atm. Results, which are shown in FIGS. 2A through 2C, generally indicated that increasing the catalyst loading increased the amount of pure hydrogen produced. Each of the experiments resulted in pure hydrogen being produced for at least 10 minutes.

Example 3

This example compares the effect of temperature on the separation of hydrogen in the process of the present invention.

The example comprised three experiments for the separation of hydrogen from syngas using the tubular plug flow reactor described in Example 1. In all three experiments, the reactor contained iron oxide catalyst (0.56 g) and calcium carbonate adsorbent (1.68 g). The syngas, which comprised carbon monoxide (52%) and hydrogen (48%), was fed into the reactor at 10 ml/min along with nitrogen. The three experiments were conducted at temperatures of 750° C., 800° C., and 850° C. respectively. Results, which are shown in FIGS. 3A through 3C, generally indicated that the cycle time for hydrogen production was faster at higher temperatures. Each of the experiments resulted in pure hydrogen being produced for at least 10 minutes.

Example 4

This example compares the effect of temperature on the separation of hydrogen in the process of the present invention.

The example comprised three experiments for the separation of hydrogen from syngas using the tubular plug flow reactor described in Example 1. In all three runs, the reactor contained iron oxide catalyst (0.56 g) and calcium carbonate adsorbent (1.68 g). The syngas, which comprised carbon monoxide (52%) and hydrogen (48%), was fed into the reactor at 10 ml/min along with nitrogen. The three experiments were conducted at syngas partial pressures of 0.09 atm, 0.18 atm, and 0.27 atm respectively. Results, which are shown in FIGS. 4A through 4C, generally indicated that high purity hydrogen was produced earlier at higher syngas concentrations with a constant total flow rate. It was also seen that for the given conditions, the maximum amount of pure hydrogen was produced at a syngas partial pressure of 0.18 atm. Each of the experiments resulted in pure hydrogen being produced for at least 10 minutes.

Example 5

This example demonstrates the regeneration of catalyst used in the process of the present invention.

The experiment comprised separating hydrogen from syngas using a tubular plug flow reactor as described in Example 1. The reactor contained metallic iron catalyst (0.5 g) and a calcium oxide adsorbent (0.56 g). Hydrogen was separated from syngas comprising carbon monoxide (52%) and hydrogen (48%). The temperature of the reactor was 850° C. When no change in concentration was obtained, syngas flow was stopped and flowing nitrogen was introduced followed by oxygen to regenerate the catalyst and adsorbent.

FIG. 5 shows the outlet gas composition versus time for the reaction and FIG. 6 shows the gas yield versus time for the reaction. FIG. 7 shows the profiles of CO₂ versus time during the two steps of the regeneration step. After the solids were regenerated, syngas was again introduced. Results for the post-regeneration separation of hydrogen are shown by the dotted lines in FIGS. 5 and 6. It is clearly seen from FIGS. 5 and 6 that no loss of activity occurred after one cycle and that the catalyst was regenerated to its original state. FIG. 7 shows that under nitrogen atmosphere, carbon dioxide is released from the calcium carbonate formed during the capture of carbon dioxide onto the calcium oxide adsorbent in the enrichment cycle. When all the carbon dioxide was released from the calcium carbonate, air was passed through the reactor and an increase in carbon dioxide was observed. This carbon dioxide was due to the oxidation of the carbon formed during the Boudouard process.

Example 6

This example describes the separation of hydrogen from syngas at three different temperatures followed by the regeneration of catalyst.

The experiments were conducted using a tubular plug flow reactor as described in Example 1. The reactor contained metallic iron catalyst (0.5 g) and a calcium oxide adsorbent (0.5 g). The three experiments comprised separating hydrogen was separated from syngas comprising hydrogen (48%) and carbon monoxide (52%) at temperatures of 750° C., 800° C., and 850° C., respectively. When no change in concentration was obtained, syngas flow was stopped and flowing nitrogen was introduced followed by oxygen to regenerate the catalyst and adsorbent. Results of the ex periments are shown in Table 2.

TABLE 2
Data on Experiments using catalyst (0.5 g) and CaO (0.5 g)
	H2 %	CO %
Inlet	48	52
	Enrichment	Regeneration
			H2	C		CO2 in
Temp	Gas Yield	H2	Recovery	removed	CO2 from C	CaCO3
° C.	%	%	%	%	%	%
750	78	60	80	82	—	—
800	48	98	95	92	35	65
850	68	72	100	54	20	80

Example 7

This example demonstrates the effect of steam on the process of the present invention.

The example comprised three experiments conducted with 3 different ratios of catalyst to calcium-based adsorbent. Each of the experiments was conducted in a tubular plug flow reactor as described in Example 1. Syngas (10 ml/min, 51% carbon monoxide, 49% hydrogen) was introduced to the reactor along with nitrogen (35 ml/min). The temperature was 750° C. In the first experiment, the reactor contained iron catalyst and calcium oxide adsorbent at a ratio of 1:2. In the second experiment, the ratio of iron catalyst to calcium oxide adsorbent was 1:6. In the third experiment, the reactor contained iron catalyst and calcium oxide adsorbent at a ratio of 1:12. Steam was added in the later stages of the experiment at a flow rate of 0.1 ml/min.

Results are shown in FIGS. 8A through 8C. The experiments demonstrate that the water gas shift reaction (when steam was added in the latter part of the experiment) increased the hydrogen yield. Thus, a high yield of pure hydrogen can be obtained when the separation is carried out in the presence of steam.

Example 8

This example demonstrates the effect of steam on the process in a reactor containing both catalyst and adsorbent.

The example comprised three experiments conducted in a tubular plug flow reactor as described in Example 1 using 80% steam in the syngas feed. In experiment 1, the reactor contained 1.68 g of calcium oxide adsorbent. In experiment 2, the reactor contained 0.56 g of metallic iron catalyst. In experiment 3, the reactor contained both 0.56 g metallic iron catalyst and 1.68 g calcium oxide adsorbent. Results are shown in FIGS. 9A through 9C. The experiments clearly show that the amount of pure hydrogen produced was highest when both the catalyst and carbon dioxide adsorbent were used. It can be also seen that the presence of steam was suspending methane formation.

Claims

1. A process for recovering hydrogen from coal gasification products, the process comprising:

contacting the coal gasification products with a metal-based Boudouard catalyst to produce a first product stream comprising hydrogen and carbon dioxide; and

contacting the first product stream with a solid-phase carbon dioxide adsorbent to separate hydrogen from the first product stream.

2. The process of claim 1, wherein the coal gasification products comprise syngas.

3. The process of claim 2, wherein the coal gasification products comprise nitrogen, methane, carbon dioxide, hydrogen and carbon monoxide.

4. The process of claim 3, wherein the coal gasification products and metal-based Boudouard catalyst are contacted at a temperature of from about 350° to about 900° C.

5. The process of claim 3, wherein the coal gasification products and metal-based Boudouard catalyst are contacted in the presence of steam.

6. The process of claim 5, wherein methane is converted to additional hydrogen and carbon monoxide.

7. The process of claim 2, wherein the coal gasification products comprise carbon monoxide, hydrogen and methane.

8. The process of claim 7, wherein the coal gasification products and metal-based Boudouard catalyst are contacted at a temperature of from about 350° to about 900° C.

9. The process of claim 7, wherein the coal gasification products and metal-based Boudouard catalyst are contacted in the presence of steam.

10. The process of claim 9, wherein methane is converted to additional hydrogen and carbon monoxide.

11. The process of claim 1, wherein the steps of contacting are conducted in a single reactor selected from the group consisting of a fixed bed reactor, a fluidized bed reactor and a packed bed reactor.

12. The process of claim 1, wherein the metal-based Boudouard catalyst comprises metallic iron.

13. The process of claim 12, wherein the process further comprises reducing carbon monoxide in the syngas to form carbon dioxide and hydrogen.

14. The process of claim 12, wherein the process further comprises converting carbon monoxide into carbon dioxide and free carbon.

15. The process of claim 1, wherein the solid phase carbon dioxide adsorbent comprises calcium oxide.

16. The process of claim 1, wherein the reaction is conducted at a pressure of from about 0 to about 250 psi.

17. The process of claim 1, wherein the ratio of metal-based Boudouard catalyst to solid phase carbon dioxide adsorbent is from about 1:3 to about 1:6.

18. The process of claim 1, wherein the process further comprises regenerating the solid phase carbon dioxide adsorbent by contacting the adsorbent with nitrogen; and

regenerating the metal-based Boudouard catalyst by contacting the catalyst with oxygen.

19. A process for recovering hydrogen from coal gasification products, the process comprising:

contacting the coal gasification products with a catalyst comprising metallic iron in a reactor at a temperature of from about 350° to about 900° C. to produce a first product stream comprising hydrogen and carbon dioxide;

contacting the first product stream with an adsorbent comprising calcium oxide to separate hydrogen from the product stream;

regenerating the adsorbent by contacting the adsorbent with nitrogen; and

regenerating the catalyst by contacting the catalyst with oxygen.

20. The process of claim 19, wherein the steps of contacting are conducted in a single reactor selected from the group consisting of a fixed bed reactor, a fluidized bed reactor and a packed bed reactor.