Methods, Systems, and Devices for Deep Desulfurization of Fuel

Gases

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

[0001] This invention was made with Government support under

Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0002] Syngas generated by the gasification of coal or biomass has many potential applications, including the production of hydrocarbon transportation fuels, chemicals (including hydrogen), and electric power. Most syngases contain impurities that may make it unsuitable for various end uses. Sulfur- containing molecules, primarily H2S and COS, are especially troublesome catalyst poisons that must be removed to the parts per billion levels for the production of fuels and chemicals. Although technical approaches exist for removal of these sulfur species, these approaches typically tend to be rather costly, require temperature fluctuations, and in many cases backup sacrificial adsorbents. Since catalytic processes for the production of fuels and chemicals typically operate in the range of 225-300 ⁰ C, a process that requires the cooling of syngas followed by a re-heating is energy inefficient. A process that is capable of removing sulfur gases to the 50 ppb level at or somewhat above the temperature of the synthesis step is much preferred.

[0003] Syngas composition is a function of several parameters, including

gasifier type, operating conditions and fuel source. In the case of coal, a

combination of zinc oxide with a regenerable downstream polishing bed is a

promising approach. For biomass, which typically generates less than 100 ppm

of sulfur gases, a stand-alone regenerable sulfur sorbent may provide an

attractive approach. In the past, the development of regenerable metal sorbents

has typically been stymied by the strong tendency of the metals to sinter or

aggregate during the regeneration process. This results in a loss of surface area

and therefore sulfur adsorbent capacity. What is needed, therefore, is a method,

system and device that allows for deep (ppb) desulfurization of fuel gasses.

What is also needed is a regenerable desulfurization system. What is also

needed is a desulfurization system that performs effective desulfurization at

warm temperatures. The present invention provides a solution to these needs.

[0004] Additional advantages and novel features of the present invention

will be set forth as follows and will be readily apparent from the descriptions

and demonstrations set forth herein. Accordingly, the following descriptions of

the present invention should be seen as illustrative of the invention and not as

limiting in any way.

SUMMARY OF THE INVENTION

[0005] The present invention is a highly effective and regenerable

method, system and device that enables the deep desulfurization of warm

fuel gases by passing these warm gasses over metal-based sorbents arranged

in a mesoporous substrate. This technology protects Fischer-Tropsch

synthesis catalysts and other sulfur sensitive catalysts, without drastic

cooling of the fuel gases, and without sintering or agglomeration of the metal

sorbents during the regeneration process. This allows for much more energy

efficient processes for deep desulfurization to be designed. The system is

characterized by active metal-based sorbents such as transient metals like Cr,

Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Pt, Au and mixtures and alloys thereof that

are attached to a porous substrate, particularly a mesoporous silicate such as

SBA-16.

[0006] These devices enable a process whereby warm fuel gasses

such as natural gas, syngas, Ih, CO, and hydrocarbon gases, mixtures of

hydrocarbon gasses, and mixtures of hydrocarbon gases and inert gases,

having a temperature between 20 and 900 degrees C are passed over these

sorbents to effect desulfurization of these gases down to a ppb level. These

devices also allow for the regeneration of these materials after use by a

cycling "oxidation-reduction" process that utilizes H2, CO, O2, N2, air, inert

gases and steam in various combinations, at temperatures between 100 ⁰ C and

900 ⁰ C. These processes can be utilized in a process either alone or alongside

other separation processes. For high sulfur coal gas cleanup, the

nanostructured metal-based sorbent bed may be attached to a separate

process that can remove sulfur to ppm level (such as zinc oxide and related

oxide absorbents). The present invention will then polish this gas stream by

removing sulfur to ppb level. The present invention allows the total sulfur

in such a gas to be reduced to less than 100 ppb and in some instances as low

as 10 ppb.

[0007] The present invention utilizes stabilized active metal sorbent

particles dispersed on a controlled nanoporous substrate that enables for

desulfurization of warm fuel gases. In some embodiments of the invention the

metal-based sorbents comprise 0.1 to 100 weight percent metals relative to the

substrates. Examples of various types of metals that can act as sorbents include:

Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Pt, Au and their alloys. Of this

group particular success has been shown in systems that utilize transient metals

selected from the group consisting of Ni, Cu, Fe, Ag, Co and their alloys.

[0008] Examples of various types of substrates include silica-based

materials (including mesoporous silica in any form, fumed silica in any form,

and zeolites in any form). These types of materials may be combined in a variety

of forms and may be utilized to treat a variety of gasses including but not limited

to natural gas, syngas (including both coal and biomass syngas), H2, CO, and

other hydrocarbon gases, their mixtures, and their mixtures with inert gases.

Examples of the types of materials that may be captured include but are not

limited to H2S, COS, mercaptans, sulfides, disulfides, and thiophenes.

[0009] These metal-mesoporous materials may be incorporated into a

system wherein warm fuel gases having a temperature in the range between

ambient temperature (20°C) to 900 ⁰ C can be effectively treated. In one

embodiment of the invention this temperature is preferably between 100 ⁰ C to

700 ⁰ C and more preferably from 15O ⁰ C to 500 ⁰ C. After these sorbents have been

filled in this treatment process, these sorbents can be regenerated through a

combination of "oxidation-reduction" treatments, over a series of cycles.

Typically, this regeneration step is performed between one and ten cycles and

preferably between 2 to 5 cycles. This "oxidation-reduction" regeneration

process can be carried out using any gas stream that can oxidize metal sulfides

(such as O2 and its mixtures with inert gases and steam) and any gas stream that

can reduce metal oxides (such as H2, CO, syngas, and their mixtures with inert

gases and steam). Examples of such a configuration include those wherein the

oxidative gas is 2% O2 in Ni.

[0010] The present invention enables deep desulfurization of warm

gasses. The mesoporous structures described prevent metal particle

agglomeration and pore blocking, and maintaining access to the active metal

sites. These nanostructured sorbents can reduce the sulfur concentration from

warm fuel gases to well below 50 ppb, and can be repeatedly regenerated using

a combined oxidation and reduction process.

[0011] The purpose of the foregoing abstract is to enable the United States

Patent and Trademark Office and the public generally, especially the scientists,

engineers, and practitioners in the art who are not familiar with patent or legal

terms or phraseology, to determine quickly from a cursory inspection the nature

and essence of the technical disclosure of the application. The abstract is neither

intended to define the invention of the application, which is measured by the

claims, nor is it intended to be limiting as to the scope of the invention in any

way.

[0012] Various advantages and novel features of the present

invention are described herein and will become further readily apparent to those

skilled in this art from the following detailed description. In the preceding and

following descriptions I have shown and described only the preferred

embodiment of the invention, by way of illustration of the best mode

contemplated for carrying out the invention. As will be realized, the invention is

capable of modification in various respects without departing from the

invention. Accordingly, the drawings and description of the preferred

embodiment set forth hereafter are to be regarded as illustrative in nature, and

not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figures IA and IB are schematic views of a preferred embodiment of the present invention.

[0014] Figure 1C is TEM view of the preferred embodiment of the invention shown in Fig. IA and IB.

[0015] Figure 2A is a schematic view of a prior art desulfurization method

[0016] Figure 2B is a schematic view of one embodiment of the present invention.

[0017] Figure 3 is a graph showing the sulfur removing capacity of one embodiment of the present invention

[0018] Figure 4 is a graph showing a summary of test results on one embodiment of the invention.

[0019] Figure 5 is a chart showing one regeneration cycle.

[0020] Figure 6 is a table showing the testing results of various embodiments and materials.

[0021] Figure 7 is a chart showing the results of testing on an embodiment of the present invention .

[0022] Figure 8 is a chart showing the results of testing in a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The following description includes the preferred best mode of one

embodiment of the present invention. It will be clear from this description of the

invention that the invention is not limited to these illustrated embodiments but

that the invention also includes a variety of modifications and embodiments

thereto. Therefore the present description should be seen as illustrative and not

limiting. While the invention is susceptible of various modifications and

alternative constructions, It should be understood, that there is no intention to

limit the invention to the specific form disclosed, but, on the contrary, the

invention is to cover all modifications, alternative constructions, and equivalents

falling within the spirit and scope of the invention as defined in the claims.

[0024] Figures 1-8 show a variety of embodiments of the present

invention. Referring first to Fig. IA and IB, schematic views of one embodiment

of the present invention is shown. In the first preferred embodiment a

regenerable sulfur gas sorbent that may find general use for cleanup of gasifier-

derived syngas for synthesis applications is described. In the preferred

embodiments of the invention a metal such as nickel (Ni) or copper (Cu) is

utilized within a nanoporous material. This nanoporous material allows for

surface chemisorption rather than bulk sulfide formation and enables a 50 ppb

maximum residual sulfur level to be achieved. Prior attempts to utilize metal-

based sulfur getters have been limited to sacrificial adsorbents principally

because of the strong tendency of the metals to sinter or aggregate during the

regeneration process. This sintering or agglomeration process results in the loss

of surface area and sulfur adsorbent capacity.

[0025] This embodiment shows a the structure and application of unique

metal-based adsorbents in which small metal particles (comprising Ni-Cu alloys)

are contained and stabilized within nano-porous silica structure. The isolation

and stabilization of the small metal particles allows regeneration of the fully

loaded sorbents through multiple cycles with minimal sintering and loss of

capacity. In the embodiment shown in Fig. 1, nickel metal was selected as the

active adsorbent material loaded within a three-dimensional cubic pore structure

mesoporous silica (SBA-16) . In addition to providing a high surface area and

excellent chemical inertness, SBA 16 also included a unique three-dimensional

interconnected channel structure (~3 nm diameter) with relatively larger pore

diameter (5 nm) at the channel intersections. The nickel particles, which fill the

channel intersections of SBA-16, are constrained due to the small connecting

pore diameter of 3 nm.

[0026] The structure of SBA-16 prevents Ni particles from sintering and

allow easy access of sulfur-containing molecules to the Ni particles by diffusion

through a 3-dimensional interconnected pore structure. While SBA -16 was

described here in it is to be distinctly understood that the invention is not limited

thereto but that a variety of other types of mesoporous materials may also be

utilized within the spirit and scope of the present invention. In this

embodiment the nickel was introduced by impregnation of the silica support (at

16.6 wt.%) using a nickel salt solution, following by drying, air oxidation

(calcination), and in situ reduction. While this method of preparation is

described it is to be distinctly understood that the invention is not limited

thereto but may be variously embodied according the respective needs and

necessities of a particular user. Figure 1C provides a transmission electron

microscopy (TEM) image of fresh sulfur adsorbent comprising 16.6wt% Ni in

SBA-16.

[0027] In use a warm stream of a fuel gas such as syngas is passed over

these sorbents. Referring now to Figures 2A and 2B schematic views of a prior

art method 2A and one embodiment of the inventive method 2B are shown.

Referring first to Figure 2A a high level flow sheet for sulfur removal from

syngas using Rectisol process is shown. Such a process requires that the

temperature of the gas stream fluctuate so as to allow for sulfur removal at -4O ⁰ C

using cold methanol. Such a process is energetically inefficient because of the

temperature swings that must be accomplished in order to raise and lower the

temperature of the gas for desulfurization to take place.

[0028] Figure 2B shows a high level flow sheet of one embodiment of the

method of the present invention wherein high level flow sheet of deep sulfur

removal from warm syngas using ZnO and NiCu-loaded SBA-16 composite

sorbent. This process shows that sulfur removal at 350 ⁰ C (ZnO-based bed) and

300 ⁰ C (NiCu-loaded SBA-16 bed). Regeneration of the sorbents at 650 ⁰ C (ZnO-

based bed, with air/Nz) and 500 ⁰ C (NiCu-loaded SBA-16 bed, with alternative

treatment of air/N2 and clean syngas/N2). During regeneration, off-gas from ZnO

bed and from oxidation treatment of NiCu-SBA-16 bed will be combined for

sulfur production. Off-gas from NiCu-SBA-16 bed during reduction treatment

will go through an off-gas treatment system to oxidize the reductants and absorb

sulfur. This off-gas stream can also be combined with the main off-gas stream for

S production.

[0029] In one example, coal gas deep desulfurization was performed

using syngas containing 10 ppm H2S (representative of a post-ZnO bed), a sulfur

capacity of 0.75 wt% was achieved before 100 ppb H2S was observed in the

treated gas. A rough calculation of the sulfur-to-Ni atomic ratio confirms that

the H2S removed can be accounted for by chemisorption on the Ni surface

(capacity of -1.0 wt.% assuming Ni2S surface stoichiometry). Thus, bulk nickel

sulfide formation is unlikely to contribute significantly to the overall sulfur

capacity. The result of this reaction is shown in Fig. 3.

[0030] Figure 4 shows a summary of the test results for the Ni-SBA-16

sorbent through five cycles. The 2 ^nd cycle gave the highest sulfur capacity,

possibly due to metal redistribution. The third cycle, which employed clean

syngas (14% CO2, 38% IHh, 48% CO) gave lower subsequent capacity than when

pure hydrogen was employed. A steady state performance level of 0.68 wt%

sulfur capacity was reached in the 4 ^th and 5 ^th cycles. Although this capacity value

appears somewhat low, as a regenerable guard bed that would be required to

reduce sulfur gases from 3 ppm to 50 ppb, and for the flow rates used in our

tests, the time between regeneration cycles would be approximately 100 hours.

For ZnO-based sorbent to reduce high sulfur coal gases from 1000 ppm to 3

ppm, the time between regeneration cycles would be approximately 20 hours.

Thus, in an integrated regenerable bed, the relative weight of this Ni-SBA-16

sorbent to ZnO would be about 1 to 5.

[0031] After use, regeneration of the sulfided Ni-SBA-16 was carried out

by a sequence of oxidation-reduction cycles. In these cycles the oxidizing gas

was 10% air in Ar, and the reducing gas was pure H2. Monitoring the off-gas by

mass spectrometry during the regeneration process indicated that under both the

oxidative and reductive sequences SO2 is the primary sulfur species. This

"oxidation-reduction" process was typically repeated 5 times. In this process

various reactions took place: These reactions include:

Oxidation: Ni ₂ S + 2O ₂ = 2NiO + SO2 (2)

2Ni ₂ S+ 5O ₂ = 2NiSO ₄ + 2NiO (3)

2NiO + 2SO ₂ + O2 = 2NiSO ₄ (4)

2Ni(buik) + O2 = 2NiO(buik) (5)

Reduction: NiSO ₄ + H ₂ = NiO + H2O + SO2 (6)

2NiSO ₄ + 6H2 = Ni ₂ S + SO ₂ + 6H2O (7)

NiO(bulk and surface) + H ₂ = Nϊ(bulk and surface) + H2O (8)

3H ₂ + SO ₂ = H ₂ S + 2H ₂ O (9)

H ₂ S + 2Ni = Ni ₂ S + H ₂ (10)

During the oxidation step, both adsorbed sulfur and any non-sulfided

nickel are oxidized. The oxidized sulfur is partially released as SO ₂ , but some

nickel sulfate remains, either by direct oxidation of surface Ni ₂ S or by

subsequent combination reaction of SO ₂ with NiO in the presence of oxygen.

During the subsequent reduction step, the nickel sulfate is converted to Ni, SO ₂

and water, along with nickel sulfide, which can be produced either by direct

reduction or through intermediate H ₂ S re-adsorbing on the reduced Ni sites. No

elemental sulfur was observed downstream of the adsorbent bed. The above

reaction schemes require several redox cycles to fully regenerate the used

adsorbents. Overall, this oxidation-reduction regeneration is more effective than

the simple reduction regeneration because all the reactions are

thermodynamically favored. A graph showing these reactions is set forth in Fig.

5.

[0032] Fig. 6 shows examples and comparisons of various substances. A

blank test was run using SBA-16 without any nickel. No sulfur removal was

observed. Ni supported on commercial fumed SiO∑ showed some regenerable

sulfur capacity. However this capacity is low because large Ni particles are

present and it is easy for the Ni particles to agglomerate and grow during

regeneration. Nickel supported within a different mesoporous S1O2 (two-

dimensional hexagonal SBA-15) gave very high first cycle sulfur removal

capacity (3 wt%). However, the adsorbent could not be regenerated using the

"oxidation-reduction" process. TEM analysis confirmed that nickel loaded into

the hexagonal channels as very fine particles. However, after adsorption and

regeneration, the nickel particles were found to have migrated out of the

mesopore structure and sintered. A Ni adsorbent using γ-Abθ3 as support was

also synthesized and tested. This adsorbent gave a non-regenerable 1.5 wt%

capacity. However, the cubic mesostructure of SBA-16 appears to provide a 3-

dimensional framework for retention of small Ni particles which is considered

most effective in this application. However this result is not intended to be

exhaustive of all of the various embodiments of the present invention. In

addition to Ni other types of materials may also be utilized for particular

advantage.

[0033] The performance of Ni in SBA-16 can be further improved by

adding a small amount of copper to the nickel. Nickel and copper form an alloy,

although there is evidence that the surface of the alloy tends to be enriched in

copper under reducing conditions. Addition of copper improves the reducibility

of the Ni-based sorbent. As a result, diluted H2 or syngas may be used for the

reductive regeneration step with Ni-Cu, whereas pure hydrogen was previously

found to be required with Ni which significantly increases the operation cost.

Also, pure hydrogen will not typically be available at a gasifier-synthesis facility

unless hydrogen is intentionally produced. The surface enrichment by copper in

the Ni-Cu alloy provides an additional benefit, as it is known to significantly

reduce the Ni methanation activity.

[0034] The sulfur removal-regeneration performance of an adsorbent

containing 1.6wt% Cu and 15.0wt% Ni in SBA-16 under the following test

conditions is shown in Fig. 7. Sulfur removal from warm coal syngas using 1.6

wt% Cu and 15.0 wt% Ni-doped SBA-16 adsorbent. Test conditions: T=300°C;

coal gas composition: 23% H ₂ , 29% CO, 8% CO ₂ , 30% H ₂ 0, 10% He, 10 ppm H ₂ S;

flow rate: 12,000 hr ¹ GHSV. Regeneration conditions: four "oxidation-reduction"

treatments at 500 ⁰ C. Oxidation in 10% air in Ar at 24,000 hr ¹ GHSV. Reduction in

2% H ₂ in Ar at 24,000 hr ¹ GHSV (for desulfurization cycle 1 to 5) and 5% H ₂ O-

and S-free syngas in Ar at 24,000 hr ¹ GHSV (for desulfurization cycle 6 to 8).

Three minutes purge with Ar between oxidation and reduction treatment.

[0035] These results demonstrate a stable breakthrough capacity of

approximately 0.75 wt% sulfur was maintained through 8 desulfurization cycles,

very similar to the capacity demonstrated with the pure nickel sorbent. The

regenerations were performed at 500 ⁰ C using 2% H2 (for cycles 1 to 5) and 5%

clean syngas (for cycles 6 to 8) as the reducing gas, and 10% air as the oxidation

gas. Four redox treatments were carried out for each regeneration. Methane

production with fresh Ni-Cu-SBA-16 was 0.16 mol%, vs. 0.7 mol% for the

regenerated copper-free nickel adsorbent. No changes in concentration of CO,

CO2, and H2 in the treated syngas were observed during sulfur removal. Pure

Cu-SBA-16 was also evaluated for sulfur removal effectiveness. A high initial

sulfur capacity was achieved (0.8 wt%), very similar to the Ni and Ni-Cu

samples, but despite the confining pore structure of the SBA-16 the capacity

reduced to less than 0.2% following regeneration, indicative of metal Cu

sintering.

[0036] The Ni-Cu-SBA-16 sorbent (15 wt.% Ni, 1.6% Cu) was then tested

for desulfurization of syngas simulated from a biomass gasifier. Test conditions:

T=300°C; biomass gas composition: 18% H ₂ , 12% CO, 10% CO ₂ , 50% H ₂ O, 4% He, 36 ppm H ₂ S; flow rate: 12,000 hr ^"1 GHSV. Regeneration conditions: four "oxidation-

reduction" treatments at 50O ⁰ C. Oxidation in air at 14,000 hr ^"1 GHSV. Reduction in clean dry syngas at 14,000 hr ^"1 GHSV. Three minutes purge with Ar between oxidation and reduction treatment. The sulfur capacity with the biomass-based syngas is

significantly higher than with the coal-based syngas, at 2.3 wt.% approximately a

factor of 3 increase. A similar uptake capacity was also obtained with carbonyl

sulfide was used as the sulfur gas. This is a very positive result, as adsorbents

such as zinc oxide are less effective in removing COS than H2S. The higher sulfur

concentration in the feed likely contributes to these higher capacities (uptake

capacity increases with H2S partial pressure), but in addition the higher

concentration of steam and lower concentration of CO may also contribute to the

better performance. At this uptake capacity, sulfur removal cannot be explained

simply by a surface adsorption mechanism, and bulk formation of nickel sulfide

must be invoked which was clearly observed via XRD analysis of sulfur-loaded

sorbent. The performance over multiple regeneration cycles is given in Fig. 8,

showing that the oxidation-reduction regeneration procedure is equally effective

with a bulk metal sulfide.

[0037] A simplified "oxidation-reduction" procedure was found to be as

effective as the multi-cycle procedure is. This procedure requires only two steps:

oxidation at 700 ⁰ C in air for 20 hours and reduction at 500 ⁰ C in a reducing gas for

4 hours, such as in diluted or no-diluted clean syngas stream. At 700 ⁰ C, NiSO4

and CuSO4 are not stable; they decompose to metal oxides and SO2. As a result,

almost all the sulfur on the sorbent can be removed during this step. This new

procedure can be easily integrated with the regeneration procedure of used

ZnO-based sorbents, which also requires high temperature (~700°C) oxidation of

ZnS. Major reactions occur during this regeneration procedure include:

Oxidation (700 ⁰ C): NixS + (l+x/2)O ₂ = xNiO + SO ₂ (11)

CuxS + (l+x/2)O ₂ = xCuO + SO2 (12)

Reduction (500 ⁰ C): NiO + H ₂ = Ni + H ₂ O (13)

In the nickel and copper loading configurations described above a significant

fraction of voids within the mesoporous structure of SBA-16 remain. This

suggests that higher loadings of metal within the material are possible,

providing a means to increase sulfur sorption capacity. At higher capacity, it is

possible to operate the Ni-Cu-SBA-16 sorbent as a stand-alone device, without

the need for an upstream zinc oxide bed.

[0038] In summary, by trapping Ni and Ni-Cu alloy nanoparticles in three

dimensional mesostructured silica SBA-16, we have developed a class of metal-

based adsorbents that can remove sulfur from gasifier-produced syngas from

either coal or biomass to less than 50 ppb levels. A combination of sulfur

chemisorption and (at higher uptakes) bulk sulfide formation appears to occur.

A sequential oxidation-reduction treatment can effectively regenerate the sulfur-

loaded adsorbents. This solid adsorbent-based approach can provide economic

advantages compared with existing technologies based on ambient or lower

temperature solvent-based cleanup systems. With coal-based syngas that may

contain several thousand ppm of sulfur, these adsorbents could be used in

combination with a higher capacity zinc oxide absorbent, providing the

necessary sub-ppm polishing capability that cannot be provided by zinc oxide

alone. With biomass-based syngas, which typically may contain 30-80 ppm

sulfur gases, these sorbents could form the basis for a sub-ppm, stand-alone

desulfurization system.

[0039] While various preferred embodiments of the invention are shown

and described, it is to be distinctly understood that this invention is not limited

thereto but may be variously embodied to practice within the scope of the

following claims. From the foregoing description, it will be apparent that various

changes may be made without departing from the spirit and scope of the

invention as defined by the following claims.