WO1995035148A1

WO1995035148A1 - Regenerable manganese-based sorbent pellets

Info

Publication number: WO1995035148A1
Application number: PCT/US1995/007549
Authority: WO
Inventors: Rachid Ben-Slimane; Malcolm T. Hepworth
Original assignee: Regents Of The University Of Minnesota
Priority date: 1994-06-17
Filing date: 1995-06-14
Publication date: 1995-12-28
Also published as: AU2773895A

Abstract

The present invention provides regenerable manganese-based sorbent pellets for removing hydrogen sulfide from a hot gaseous mixture, methods of manufacturing the pellets and methods of using the pellets for desulfurization.

Description

REGENERABLE MANGANESI--BASED SORBENT PFIIKIS

FIELD OF THE INVENTION

The present invention relates to the field of sorbents for the desulfurization of coal gases. More particularly, the present invention relates to the field of regenerable sorbent pellets containing manganese and alumina for removing hydrogen sulfide from a hot gaseous mixture, methods of maniifacturing the same and methods of using the pellets for desulfurization.

BACKGROUND OF THE INVENTION Current methods for the conversion of chemical energy from coal to electrical energy tend to be cumbersome, wasteful of the energy within the fuel, and environmentally unsound. In a typical system, coal is reacted in a burner system which exchanges heat to a boiler to generate steam. This steam then passes through a turbine to drive a generator and produce electricity. This process, based upon the Rankine cycle, wastes more than 60% of the energy originally present in the coal. Energy is also lost in transmitting the generated electricity through power lines to users causing the process to be, overall, only 30 to 35% efficient. In addition, solid waste streams are generated and must be disposed of in increasingly expensive landfill space.

Stricter government regulations on pollutant emissions and the need to improve process efficiency led to the development of advanced power generation systems. Emerging technologies which are reaching the commercial stage include the integrated gasification combined cycle (IGCC), the molten-carbonate fuel cell (MCFC), and the pressurized fluidized-bed combustor (PFBC). The IGCC is one of the most promising proposed processes for advanced electric power generation that is likely to replace conventional coal combustion. In this emerging technology, the mixture of carbon monoxide and hydrogen produced through gasification is burned; the very hot exhaust is routed through a gas turbine to generate electricity; and the residual heat in the exhaust is used to boil water for a conventional steam- turbine generator thus producing more electricity. These technologies are principally based on coal gasification, i.e. the production of gaseous fuels from coal. In a coal gasifier, unlike coal combustion processes, the sulfur in coal is released in the form of hydrogen sulfide, H₂S, rather than sulfur dioxide, S0₂. The conventional technology of scrubbing the gas for H₂S is not practical in these situations because the scrubbing processes operate at low temperature and, therefore impose a severe thermal penalty, because thermal efficiencies are directly dependent upon the difference in temperature between the process heat and the heat sink. Thus the IGCC process must employ hot- gas cleanup techniques to remove sulfur and other impurities in the fuel gas stream, principally to protect turbine components from the corrosive action of H₂S. This approach completely eliminates the more costly, less efficient method in which a liquid-based scrubbing system is employed at low- temperatures for scrubbing the fuel-gas. Desulfurization with sorbents is essentially a process of removal of the predominant sulfur-bearing species, H₂S, from the gas phase. As H₂S is removed, the other minority sulfur-bearing species, such as COS, equilibrate in the gas phase and also are proportionately reduced. The high-temperature desulfurization can be successfully accomplished by using solid sorbents such as oxides of those metals that form stable sulfides.

The effectiveness of a desulfurizing agent in treating coal gases is related to the predicted equilibrium partial pressure of sulfur which is present in a phase combination of the reduced form of sulfide and oxide phases. A sulfur concentration limitation of approximately 150 ppmv (parts per million by volume) for IGCC systems has been established; therefore, a sorbent system capable of reducing H₂S concentration from about 5000 to 150 ppmv is sought.

The focus of much current work on hot coal-derived fuel gas desulfurization is primarily in the use of zinc ferrite and zinc titanate sorbents. The choice of zinc oxide is typically based on the thermodynamic considerations that indicated very low concentration levels of H₂S in equilibrium with ZnO, ZnS, and H₂0 vapor. To improve the process economics further, it would be desirable to have an easily regenerable sorbent which would not only reduce the cost of sorbent but also the costs associated with frequent loading and unloading of the reactors with sorbent and the costs associated with disposal of the spent sorbent. For these reasons, research has focused almost entirely on making the zinc-based sorbents durable. However, research casts doubt that zinc ferrite or zinc titanate can be utilized even for the mild conditions associated with fixed-bed operation.

One major problem associated with zinc-based sorbents is sulfate formation which is found to occur whenever S0₂ is present in the regeneration gas. Sulfation leads to a volume expansion and swelling of sorbent pellets eventually causing them to spall and crack. For example: a 5 to 6-mm diameter loaded pellet expands to 9 mm after regeneration when sulfate is formed. Certainly, in a fixed-bed operation some of the pellets will be saturated with sulfur and the regeneration gas will contain some S0₂.

Therefore, sulfate formation is unavoidable along with accompanying spalling, cracking, and an undesired major volume change within the fixed bed reactor which, in turn, can generate high radial stresses within the vessels containing the sorbent pellets. The role of chlorides in the gas system should also be considered because of the potential volatility of metal chlorides. The presence of hydrochloric acid (HC1) in the coal gas causes extensive volatilization of zinc in the form of ZnCl₂. Losses of zinc as high as 5-10% by weight from the exit of a fixed-bed reactor have been reported, even after one cycle of loading and regeneration.

In addition, zinc oxide can be reduced under coal gasification conditions and tends to volatilize, limiting the temperatures for which desulfurization can be effectively accomplished for recycling of sorbent to values less than 750 °C. Given the limited success of the zinc-based sorbents, interest has been shown in formulating and testing manganese-based sorbent pellets. There are a number of studies that led to the consideration of Mn-based sorbents. Research based upon thermodynamic considerations alone has predicted manganese oxide stability and a high degree of desulfurization to temperatures in excess of 1000 °C. In addition, manganese showed desulfurization potential in the range of 600 to 700 °C where traditional metal oxides known to be reactive with H₂S proved unsatisfactory.

These kinetic studies determined the initial rates for the reaction between H₂S and MnO, CaO, ZnO, and V₂0₃ over a temperature range of 300 to 800 °C. The relative magnitude of reaction rates decreased in the order: MnO > CaO « ZnO > V₂0₃. They concluded that MnO possessed favorable properties for a high temperature desulfurization process and highly recommended that further studies be carried out.

The feasibility of using manganese oxide pellets to desulfurize hot reducing gases has been investigated. It was found that a mixture of manganese ore and alumina formed into pellets in a 3:1 weight ratio readily accepted sulfur from hot (800 °C) H₂-H₂S gas mixtures and that the pellets could be regenerated with air, or other oxidizing gases.

The research also explored the cyclic loading and regeneration of these pellets through 18 consecutive cycles. The tested pellets exhibited high strength and rapid loading kinetics which did not show a decline in reactivity or capacity through the regimes which they explored. Repeated cycling of the pellets resulted in improved sulfidation and regeneration kinetics, unlike the situation with Zn-based sorbents, as discussed above. This desirable phenomenon with manganese pellets was attributed to transport between pellet pores, possibly by development of cracks. Other research based on zinc ferrite included substituting MnO for some of the zinc oxide as a means of enhancing the durability and reactivity of the zinc ferrite and reported favorable results. In a recent study, a systematic approach for the evaluation of the behavior of single and mixed- metal sorbents for removing sulfur from hot coal-derived fuel gases, based on thermodynamic considerations was developed. A range of metal systems including iron, nickel, magnesium, calcium, manganese, copper, sodium, and zinc were examined. This study singled out manganese oxide as a prime candidate sorbent capable of being utilized under a wide temperature range, irrespective of the reducing power (deteirnined by C0₂/CO ratio) of the fuel gas.

Another problem faced by many of the pellet-based sorbent systems is that the pellets are typically formed through extrusion processes which provide essentially cylindrical-shaped pellets. Although cylindrical pellets are expedient from a manufacturing standpoint, they do not provide optimum packing efficiency. Furthermore, the cylindrical pellets have edges at either end which are the center of stress and the site of degradation of the pellets, resulting in the loss of sorbent capacity.

Some of the research discussed above has indicated that manganese based pellets may provide a useful sorbent for coal gas desulfurization. None of the research, however, has provided a formulation of pellets and a process of manufacturing the same which is economically feasible and provides pellets which have the characteristics necessary for successful commercial application.

SUMMARY OF THE INVENTION The present invention provides regenerable manganese-based sorbent pellets for removing hydrogen sulfide from a hot gaseous mixture, methods of manufacturing the pellets and methods of using the pellets for desulfurization. The durable and highly-reactive manganese-based sorbent pellets according to the present invention are capable of effectively removing hydrogen sulfide from coal-derived fuel gas ranging in temperature from about 700 to 1200°C. The pellets are readily regenerated by air or oxygen- deficient air in the range 800 to 1200°C with possible recovery of sulfur in a useful form. The sorbent pellets are also able to maintain their physical integrity and chemical reactivity after repeated use in a large number of sulfidation and regeneration cycles.

The durability provided both by the materials used to manufacture the pellets as well as their spherical shape allows the pellets to maintain their desired physical and chemical characteristics in long-term cyclic sulfidation and regeneration in a high-temperature desulfurization operation. The desired physical characteristics of pellets according to the present invention include high crush strength, resistance to attritioning or physical disintegration, and high porosity. The desired chemical characteristics are: a high sulfur sorption capacity (gS/100 g pellets) during the sulfidation step; and a high reaction rate and extent for both sulfidation and regeneration.

The sorbent pellets according to the present invention are also produced by a relatively inexpensive process comprising mixing combinations of manganese-containing compounds and an alumina-based matrix with organic or inorganic binders. The mixed powder is then fed to a pelletizer, along with a fine spray of water to form spherical pellets. These pellets are dried to impart green strength for further handling. The dried pellets are then indurated at high temperature to obtain the desired combination of physical properties such as crush strength and porosity.

To minimize costs, the preferred processes use commercially available reagents such as 95% pure regular manganese carbonate, an African rich pyrolusite (Mn0₂) ore, alundum, bentonite, and dextrin. The pelletizing operation requires the following commercially available equipment: mix- muller, pelletizer, drying oven, and induration furnace.

One advantage of the manganese-oxygen-sulfiir system used for desulfurization with pellets according to the present invention is its resistance to reduction to elemental manganese under the range of most of the fuel gas compositions for which it would be utilized as a sorbent.

Manganese also has a significantly lower vapor pressure in the elemental state than zinc hence it is not as likely to undergo depletion from the sorbent surface upon loading and regeneration cycles. Analysis of the manganese-sulfur-oxygen (Mn-S-O) system shows it to be much less amenable to sulfation than zinc-based sorbents. Potential also exists for utilization of manganese at higher temperatures than zinc ferrite or zinc titanate which would boost the efficiency of power generation from coal. Furthermore the sulfidation and regeneration can be carried out at the same temperature, for example (800 °C) without the formation of sulfates. Another advantage of the manganese-oxygen-sulfur system is its resistance to fusion as a result of temperature rises which may occur in the exothermic step of regeneration since the system is relatively refractory and non-volatile (i.e., lacks low melting-point phases) even in the presence of trace chloride impurities.

Fuel gas typically contains some HC1 and exit gas temperatures from commercial gasifiers are likely to be in excess of 650 °C. Zinc chloride (ZnCl₂) has a boiling point of 1005 °K (732 °C) and, as a result, when zinc- based compositions are used as the sorbent, it was found that temperature excursions as small as 50 °C could significantly reduce the sorbent reactivity through loss of the reactive component. In contrast, manganese chloride (MnCl₂) has a boiling point of 1504 °K (1231 °C) which makes manganese- based sorbents much less susceptible to volatilization and losses from the sorbent than zinc. Manganese-based pellets are also capable of much higher temperatures of loading and regeneration than zinc without degradation. To prevent the loss of zinc by vaporization, gas desulfurization processes employing zinc must be performed at lower temperatures with higher H₂0/H₂ ratios in the gas than is desirable for optimum gasification conditions (usually ratios less than 0.1) at desulfurization temperatures of greater than about 800 °C. Under these conditions manganese pellets containing alumina according to the present invention maintain open porosity. As a result, a manganese oxide/alumina sorbent pellet according to the present invention can be used over a wider range of operating temperatures than a zinc-based sorbent. The spherical shape of the pellets manufactured by the process according to the present invention and the high crush strength imparted to them by the presence of the inorganic binder, bentonite, result in desirable handling and packing characteristics. The increased packing density of spherical pellets as compared to typical cylindrical pellets allows more sorbent material to be contained within a reactor. That results in increased capacity as well as reaction rate. The spherical shape also eliminates the edges which are the site of degradation due to stresses during use as well as handling of typical cylindrical sorbent pellets.

These and other features and advantages of the present invention will be readily apparent upon reading the detailed description of the invention and the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic diagram of a system for simulating the desulfurization of gases using regenerable manganese-based sorbent pellets according to the present invention.

FIGURE 2 is a cross-sectional schematic diagram of a reactor used in the system depicted in FIGURE 1.

FIGURE 3 is a graph depicting the reproducibility of loading tests at 950°C with 3% H₂S-H₂ gas mixtures showing average value and error bars. FIGURE 4 is a graph depicting the effect of temperature on regeneration kinetics of one embodiment of Mn-based sorbent pellets according to the present invention.

FIGURE 5 is a graph depicting the effect of 0₂ concentration in N₂/air mixtures on regeneration kinetics. FIGURE 6 is a graph depicting the effects of repeated cyclical loading and regeneration on reaction kinetics and capacity of one embodiment of Mn- based sorbent pellets according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION The processes used to manufacture manganese-based pellets according to the present invention essentially comprise mixing combinations of manganese-containing compounds and an aliimina-based matrix material with organic or inorganic binders. The mixed powder is then fed to a pelletizer, along with a fine spray of water to form spherical pellets. These pellets are dried to impart green strength for further handling. The dried pellets are then indurated at high temperature to obtain the desired combination of physical properties such as crush strength and porosity. The raw materials used to form the pellets comprise a source of manganese as either a carbonate or an oxide and an aliimina-based matrix to provide the strength needed to maintain porosity and provide the mechanical structure in the pellets. In the compositions employing manganese carbonate, it is preferred to also include an inorganic binder to provide further cementing actions to increase strength in the pellets as well as to facilitate formation into spherical pellets. In compositions employing manganese oxides, the raw materials can include an organic binder to provide additional porosity to the pellets as the organic binder volatilizes during induration as well as to provide a mechanism for formation of the raw materials into pellets. Organic binders can also be included in formulations using manganese carbonates, but are typically not needed because of the porosity-enhancing effects of MnC0₃ dissociation.

One preferred formulation, a combination of technical grade manganese carbonate (approximately 93-95% pure), alundum (96.6% A1₂0₃, 2.6% Ti0₂), and bentonite as an inorganic binder, provides excellent sulfidation and regeneration characteristics. No further modifiers or surface area or porosity promoters are needed since the carbonate, upon dissociation and volatilization of the contained carbon dioxide during induration of the pellets, produces the required porosity.

The spherical shape of the pellets and the high crush strength imparted to them by the presence of the inorganic binder result in desirable handling and packing characteristics. The inorganic binder also preferably provides a strong, permanent bridging action between manganese oxide and matrix materials. It will be understood that the amount of inorganic binder is preferably limited, not only to reduce the cost of the finished pellets, but also to provide a greater amount of chemically active sorbent to maximize both sorption capacity and reaction rates.

The preferred inorganic binder comprises bentonite, although other inorganic binders could be substituted for bentonite, including, but not limited to kaolin or Portland cement. The presence of an organic binder, such as, for example, dextrin, also enhances the porosity as this material burns away during the induration process. Other organic binders which could be used in the process according to the present invention include, but are not limited to starches, methyl cellulose, and molasses.

The preferred finished manganese-based pellets have a composition of 30-50% by weight of manganese, 20-30% alumina, and up to about 2% by weight of inorganic binder. The alumina may desirably contain about 1-5% impurities such as Si0₂ and Ti0₂. The preferred size of the finished pellets is about 5 mm in diameter. It is preferred that the pellets have a crush strength of about 22 N/mm of diameter to maintain their integrity during use.

Examples The following examples are provided to illustrate the process of manufacturing regenerable manganese-based sorbent pellets according to the present invention. The examples are not to be construed as limiting the scope of the invention which is defined by the claims.

Four pellet formulations were prepared using combinations of a manganese-containing compound, an alumina-based matrix, and an organic or inorganic binder, based on desired compositions determined on a dry weight- percent basis. The composition of the initial make up of the formulations prepared using this procedure is reported in Table I.

Table I. Initial Make up of Formulations

Formulation Raw Material Content in initial make-up, t% No.

Moanda Ore Alundum MnC0₃ Bentonite Dextrin

1 68.18 22.73 - - 9.09

2 73.53 24.51 - 1.96 -

3 - 15.51 75.4 - 9.09

4 - 16.73 81.31 1.96 -

The preferred raw materials used to form the pellets included Moanda manganese ore (aka Comilog ore) (77% Mn0₂, 3.36% MnO, 6% A1₂0₃, 2.81% Fe, 2.56% Si0₂, 5.16% H₂0 (bound)) obtained from the Prince Manufacturing Company in Quincy, IL; regular manganese carbonate (approximately 93-95% pure) obtained from Chemetals, Inc., Baltimore, MD; Alundum (96.6% A1₂0₃, 2.6% Ti0₂) obtained from Industrial Ceramics Corporation (Norton Materials), Worcester, MA; bentonite, an inorganic binder, was obtained from the Aldrich Chemical Company, Inc., Milwaukee, WI; and dextrin (type IV), an organic binder, was obtained from the Sigma Chemical Company, St. Louis, MO.

In the formulations comprising manganese carbonate, precalculation mass of the MnC0₃ was chosen to produce 75% by wt. MnO in the finished pellets because calcination of manganese carbonate effectively reduces its mass upon loss of carbon dioxide.

The preferred particle sizes of both the manganese compounds and matrix compounds is less than about 100 μm, and more preferably less than about 50 μm to facilitate pellet formation.

Feed is prepared by adding a binder to the dry mix of manganese- containing compound and alumina-based matrix and mulling the mixture while slowly adding water to produce a cohesive, but not sticky, consistency. The amount of water needed is typically in the range of about 5 to 10% by weight. The mix is then rubbed through a 10-mesh sieve to form fines. Pelletization is accomplished using a balling tire at a 40 to 60° inclination to the horizontal and a rotational speed of 30 to 50 rpm. Initially, the tire interior is moistened and then wiped clean. About 7% by weight of the prepared feed in the form of fines is slowly sprinkled into the rotating tire. When seeds begin to form (as evinced by micro-pellets), moisture is added in discrete increments (of about 0.1% by weight of the original solids) via a finely atomized hand-held sprayer (spray droplets 5 to 15 μm in diameter) along with additional fines to allow the seeds to grow. After growth, the seeds are removed and screened to pass a 6-mesh screen, and the fines are returned to the tire.

This procedure is continued until approximately one-third of the original feed (which is set aside) is converted to agglomerates which would ideally range in size from 2 to 5 mm in diameter. At this point, the feed is divided into three portions. In the subsequent pellet making operation, a portion of the seeds are returned to the tire and sprayed with the above fine spray of water until the glistening point is noted. This glistening point is a visual observation of a shiny reflective surface appearance on pellets which originally appeared as "flat", i. e. non-glossy. Fines are then added in small increments to cause the seeds to grow.

The rate of addition of fines is controlled so as to avoid sticking and also to promote the free-rolling of the pellets. Too slow a rate of addition causes the seeds to stick together; whereas, too rapid an addition causes the formation of unnecessary new seeds in competition with the desired growth of the original seeds. Immediately after this procedure of addition of the fines, water is sprayed once again to bring the pellets to the glistening point. This procedure of spraying pellets to the glistening point and the subsequent addition of fines is repeated until the pellets grow to the desired final size. At this point, the pellets are removed from the pelletizer and screened; the 7- to 10-mm fraction is stored in desiccators pending evaluation of their strength and appearance, and the undersize pellets are returned to the tire for further growth. The required moisture content for the feed depends on its average density. Balling is optimal at about 30-35% moisture on a volume basis, i.e., the approximate void fraction in dried pellets.

The pellets were then dried to a constant weight at 110°C and then indurated. In duration was conducted for a 12 hour cycle in air and involved heating from 25°C to about 300°C over one hour; heating from about 300°C to about 400°C over a period of about four hours; heating from about 400°C to about 1200°C over a period of about 5 hours and then holding the Moanda ore based samples at 1200°C for one to two hours while the manganese carbonate based samples were heated to about 1250°C and held there for one to two hours.

The pellets produced using the above procedures had the desired strength of greater than 22 N/mm of pellet diameter (5 lbs/mm).

The strongest formulations for the pellets are bentonite-bonded. At the conclusion of these tests, four formulations were selected for thermogravimetric analysis (TGA). They were given the following designations: FORMl-A, FORM2-A, FORM2-B, and FORM4-A. Letter "A" corresponds to induration for 2 hours at 1200 °C for the ore-based pellets, and 1250 °C for carbonate-based pellets; whereas, letter "B" corresponds to an induration time of 1 hour.

Table II summarizes the manganese assays and strengths of the formulations which were subjected to TGA analysis. In addition, BET surface area tests on indurated pellets have shown them to have a surface area less than 0.2 m²/g. Typical specific surface area values are 0.055 and 0.127 m²/g for FORM2-A and FORMl-A, respectively. These low specific surface area values are caused by rapid sintering which occurs during induration at high temperature. Porosity tests made by combination of aqueous and mercury pycnometry on crushed pellets from formulation 2 showed the pellets to have a porosity of about 36%. Table II. Manganese Assays and Strengths of Formulations Studied.

Sample Designation Weight, %V-n Strength, N/ m

FORMl-A 41.14 25.5

FORM2-A 37.48 64.7

FORM2-B 39.83 52.4

FORM4-A 44.22 23.8

The formulation FORM4-A exhibits the most favorable loading characteristics of all the formulations tested. It is made up of 81.31% by weight manganese carbonate, 16.73% by weight Alundum (containing 96.6% A1₂0₃), and 1.96% bentonite as an inorganic binder.

Upon forming pellets, they were first dried at 110°C then heated to at least 350°C to allow for weight losses as a result of dissociation of the carbonate (the manganese carbonate decomposes at about 343°C). They were then heated to 1250°C and indurated for 2 hours in an air atmosphere before removal from the furnace.

The average size of the pellets was 4.8 mm and they had an average strength of 5.4 lbs/mm of diameter. The final manganese assay of the indurated pellets was 44.22% by weight, which corresponds to a theoretical sulfur capacity of 34.4% sulfur, (i.e., 34.4 gS/lOOg sorbent).

Methods of Use To test the efficiency and operation of the sorbent pellets according to the present invention, an experimental fixed-bed reactor was constructed and simulated coal gasifier fuel gas was introduced to test the desulfurization and regeneration of the sorbent pellets.

The overall schematic diagram of the experimental arrangement for fixed-bed tests is shown in Figure 1. The split or two-zone furnace 10 is positioned with respect to the sorbent pellet bed 21 to accomplish gas preheating and careful control of the bed temperature. Gas combinations for flows can be produced via control of bottled gases through the valves and flowmeters shown in the diagram. The sulfidation gases include hydrogen sulfide 14, carbon monoxide 16, carbon dioxide 17, hydrogen 18 and nitrogen 19. The relative amounts of the carbon monoxide 16, carbon dioxide 17, hydrogen 18 and nitrogen 19 are controlled with a bank of flowmeters 20. After mixing, the combination of carbon monoxide 16, carbon dioxide 17, hydrogen 18 and nitrogen 19 is preheated to simulate the exit temperature of fuel gas from a gasifier.

The water content of the system is controlled by a metering pump 12 acting on liquid water which is discharged into heated tube 11 along with the simulate fuel gas to vaporize the water. Hydrogen sulfide 14 is mixed with the other gases and water vapor using flowmeter 13 and the complete composition is introduced into reactor 22. The gas composition required to simulate a modified form of Tampella-U gas (from a Tampella U-gas air-blown gasifier) is reported in Table III. This composition was chosen, in part, because of its relatively low water vapor content (5.35% by volume). The extent of desulfurization attainable with a Mn-based sorbent is dependent on the water vapor content of the exit gas, i.e., lower water vapor content results in higher levels of desulfurization.

The primary modification made in the gas composition is an increase in the amount of H₂S, which is typically contained at about 1% by volume in Tampella-U gas. The modified composition included 3% by volume of H₂S to decrease the amount of time required to achieve "breakthrough" as discussed below.

The choice of low water vapor gas should, however, be indicative of the performance of the sorbent pellets according to the present invention because gasifier systems typically add water vapor to the exit gas stream to enhance desulfurization with zinc-based sorbents. If a Mn-based sorbent is used, the water vapor addition can be discontinued. That would provide additional advantages as addition of water vapor poses two major disadvantages. First, the heating value of the exit gas is significantly reduced with the introduction of water vapor and, second, the volume of the gas is increased which requires corresponding increases in the size and cost of the equipment in the desulfurization system and other equipment downstream. Table III. Gas Composition for the Simulation of the Tampella-U Fuel Gas.

Gas Volume Percent

Hydrogen 13

Nitrogen 50

Carbon Monoxide 24

Carbon Dioxide 5

Water Vapor 5

Hydrogen Sulfide 3

The details of the reactor system are depicted in Figure 2. This figure shows the two-inch (I.D.) closed-end alumina reactor tube 22 equipped with an alumina perforated-disc plate 24 suspended over alumina chips 26.

The coal-derived fuel gas simulating the Tampella-U Gas is passed through a small central tube 28 (without contacting the reaction zone of manganese pellets) through the perforated disc 24 and into a preheat zone of alumina chips 26 which supports the disc.

The gas is then discharged from the small central tube 28 into the larger tube 22 and moves upward, after reaching temperature, into gas preheat zone 30 of the two-zone furnace 10. The gas, now at temperature ranging from 800 to 1100 °C, contacts the sorbent bed 21 consisting of pellets ranging in size from 4 to 5 mm in diameter. The off-gas is removed from the reactor 22 through tube 32 and, in the experimental arrangement, a portion is transported to a gas chromatograph for testing. Gas temperatures of 900 or 1000°C show further increases in the reaction rate.

Loading of the pellet bed 21 will continue at a rate of 3.5 liters of gas per minute until breakthrough is observed. Breakthrough is defined as a sudden rise in the sulfur content of the gas as determined by gas chromatography. After breakthrough, the bed 21 is flushed with nitrogen 40 to expel remaining hydrogen and a regeneration gas consisting of air 42 plus nitrogen 40 is introduced into reactor 22 in the same way as the simulated Tampella-U Gas. E)uring regeneration, the sulfur content of the gas exiting the reactor 22 through line 32 is continuously analyzed for sulfur dioxide. The regeneration process is discontinued when the concentration of sulfur dioxide in the exit gas is measured below a desired value, typically about 500 ppmv. Other methods of determining when to discontinue regeneration include deteirnining when the bed is 95% regenerated as determined by an approximate mass balance of the system based upon the calculated sulfur content of the off-gas. The loading and regeneration cycles can then be repeated.

The sulfur loading capacity (defined as grams of sulfur per 100 grams of original pellet mass) shows improvement with each cycle. The results from twenty consecutive cycles indicate that loading capacity ranges from 21.1 to 23.1% which corresponds to over 90% bed utilization. The sulfur capacity is calculated using mass balances, based on gas analyses for H₂S entering and leaving the reactor. Before breakthrough the H₂S concentrations in the desulfurized gas more closely approaches the equilibrium value, which is an indication of an improvement in pellet reaction kinetics with repeated cyclic loading. Thermogravimetric Analysis (TGA) tests were also performed to study the effects of regeneration parameters (temperature and 0₂ content) and on repeated loading and regeneration of the selected FORM4-A pellets. The reduction and sulfidation were carried out under standard conditions of 950°C and 3% H₂S-H₂ gas mixtures at 500 cc/min delivery for a total of approximately 2 grams of pellets. Reduction was carried out with pure hydrogen for the first 45 minutes and then hydrogen sulfide was added.

Figure 3 shows the reproducibjlity of loading tests resulting from five separate tests on fresh pellets (i.e., first loading cycle) at 950°C with 3% by volume hydrogen sulfide in hydrogen. This figure gives the error bars on the data to indicate the range of the results. Although not shown for the reduction portion of the curve the reproducibility is particularly good, with reduction essentially completed in fifteen minutes. Figure 3 shows that deviations in extent of loading occur even though during each test the macroscopic properties (mass and size) of the pellets were kept as constant as possible. These deviations may be due to the difficulty associated with controlling temperature, flow rates, etc. However, the reproducibility of sulfidation is excellent beyond the first 30 minutes after the initial reduction period (total lapse time of 75 minutes). The solid line represents the average values and therefore serves as an indication for future test results.

A total of five regeneration tests were then carried out using five pellets during each test. The regeneration gas was air flowing at 500 cc per minute and regeneration temperature varied between 800 and 1000°C in increments of 50°C. The purpose was to determine the temperature at which loaded pellets could be fully regenerated in reasonable time. The results from these tests are reported in Figure 4. The loading curve corresponds to the solid line shown in Figure 3; however, the right-hand portion of this of this figure shows the regeneration portion after a total time lapse of 165 minutes.

These results appear to be in strict accordance with thermodynamic predictions, viz.: at 800°C the loaded pellets first oxidize (and hence lose weight) for about 3 minutes at which time sulfation with an concurrent weight gain becomes predominant. When approximately 60% of the MnS is converted to MnSO sulfate decomposition starts and the pellets begin to show a weight loss. Continuing at higher temperatures, at 850°C, the same trend is observed; however, the oxidation of the pellets appears to be faster than sulfation; this is reasonable since the driving force for the decomposition of sulfate is greater. Regeneration kinetics appear to be a weak function of temperature over the range 900 to 1000°C. In this range regeneration is essentially complete in 10 to 15 minutes and sulfation does not occur, as concluded from the prior thermodynamic analysis of the Mn-S-0 system. It is preferred that regeneration be conducted at 900°C or above to avoid sulfate formation. Furthermore, it is more preferred that regeneration temperatures reach 1000°C or higher to provide for full regeneration (i.e., W/W₀=l). Figure 5 shows the effect of 0₂ concentration in N₂/air mixtures on regeneration kinetics at four oxygen levels (5, 10, 15 and 21%(air) by volume). Even for the lowest oxygen level, the flow rates and mass of samples were chosen so that the rate of gas delivery was not rate-limiting. As shown, the regeneration rate increases with increasing oxygen content. These results indicate the need to control the amount and rate of oxygen delivery to the reactor bed due to the exothermic nature of the regeneration process. If regeneration is accomplished too quickly, the temperature of the reactor bed can reach critical levels, leading to sintering and deterioration of the pellet reactivity. The FORM4-A pellets exhibited a limit of about 1 liter per minute of air during regeneration to avoid critical levels when using a reactor bed of 2 inch diameter by 6 inch depth (approx. 19 in.³) containing pellets with diameters of 4-5 mm.

Over a five-day period, one loading and regeneration cycle was completed each day at 950°C for reduction and sulfidation and 1000°C for regeneration. The TGA data for these five tests are shown in Figure 6. The arrows show the direction of increasing cycle sequence numbers, i.e. for loading, the capacity and reaction kinetics improve for each of the five cycles with the change between the first and second cycles being the most pronounced. For the tail of the curve teπninating the regeneration step, the weight loss drop is steepest for the fifth cycle: again showing that the rate of regeneration increases with pellet re-use.

An optical examination at 30 magnifications with a binocular microscope as well as petrographic thin sections examined at lOOx showed the presence of radial cracks. The penetration distance of the sulfur as exhibited by the higher reflectivity of the manganese sulfide phase was densest adjacent to these cracks, indicating that the diffusion distance for permeation of sulfur (and countertransport of oxygen) was decreased by the change in diffusion geometry occasioned by the cracking. Whereas the original pellet experiences diffusion for a gross pellet size of ^***2.5 mm in radius; upon cracking of the pellet, the solid-state diffusion distance is somewhat reduced. This permits gaseous species to be transported by the relatively faster mechanism of gaseous diffusion into the cracks compared with the slower process of the solid state counter diffusion of oxygen out of the pellet to exchange for sulfur into the pellet.

This is a unique feature of manganese-based pellets, i.e. their retained strength upon micro-cracking and improved kinetics compared with zinc-based sorbents, in which the reaction kinetics and sorbent loading capacity and pellet strength decrease upon cyclic loading.

Although specific methods and examples have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific methods and examples described. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims

We claim:

1. A pelletized sorbent for removing sulfur from hot coal gases comprising manganese and alumina.

2. A pelletized sorbent according to claim 1, further comprising an inorganic binder.

3. A pelletized sorbent according to claim 2, wherein the inorganic binder comprises about 2% by weight of the pelletized sorbent.

4. A pelletized sorbent according to any one of claims 1-3, wherein the manganese comprises about 30-50% by weight of the pelletized sorbent.

5. A pelletized sorbent according to any one of claims 1-3, wherein the manganese comprises about 35-45% by weight of the pelletized sorbent.

6. A pelletized sorbent according to any one of claims 1-5, wherein the alumina comprises about 20-30% by weight of the pelletized sorbent.

7. A pelletized sorbent according to claim 6, wherein the alumina further comprises additional materials chosen from the group consisting of Si0₂ and TiO_j.

8. A pelletized sorbent according to claim 2, wherein the inorganic binder comprises bentonite.

9. A pelletized sorbent according to any one of claims 1-8, wherein pellets formed of the pelletized sorbent have a crush strength of about 22 N/mm (dead weight load) of pellet diameter or greater.

10. A pelletized sorbent for removing sulfur from hot coal gases comprising about 35-45% by weight manganese, about 20-30% by weight alumina and about 2% by weight of inorganic binder, wherein the alumina further comprises additional materials chosen from the group consisting of Si0 and Ti0₂, and further wherein the inorganic binder comprises bentonite.

11. A method of using sorbent pellets to desulfurize gas and regenerate the sorbent pellets after desulfurization comprising the steps of: a) providing sorbent pellets comprising manganese and alumina; b) desulfurizing a gas comprising H₂S by directing the gas past the pellets, the gas having a temperature of about 800°C or above; c) regenerating the pellets by directing regeneration gas comprising oxygen past the pellets, the regeneration gas having a temperature of 800°C or above.

12. A method according to claim 11, wherein the regeneration gas has a temperature of about 900°C or greater.

13. A method according to claim 11, wherein the regeneration gas reaches a temperature of about 1000°C or greater.

14. A method according to any one of claims 11-13, wherein the step of providing sorbent pellets further comprises providing pellets which include an inorganic binder.

15. A method according to claim 14, wherein the inorganic binder comprises about 2% by weight of the pelletized sorbent.

16. A method according to any one of claims 14 or 15, wherein the inorganic binder comprises bentonite.

17. A method according to any one of claims 11-16, wherein the step of providing sorbent pellets further comprises providing pellets in which the manganese comprises about 30-50% by weight of the pelletized sorbent.

18. A method according to any one of claims 11-16, wherein the step of providing sorbent pellets further comprises providing pellets in which the manganese comprises about 35-45% by weight of the pelletized sorbent.

19. A method according to any one of claims 11-18, wherein the step of providing sorbent pellets further comprises providing pellets in which the alumina comprises about 20-30% by weight of the pelletized sorbent.

20. A method according to claim 19, wherein the alumina further comprises additional materials chosen from the group consisting of Si0₂ and Ti0₂.

21. A method according to any one of claims 11-20, wherein the step of providing sorbent pellets further comprises providing pellets having a crush strength of about 22 N/rnm (dead weight load) of pellet diameter or greater.

22. A method of using sorbent pellets to desulfiirize gas and regenerate the sorbent pellets after desulfurization comprising the steps of: a) providing sorbent pellets comprising about 30-50% by weight manganese, about 20-30% by weight alumina and about 2% by weight of inorganic binder, wherein the alumina further comprises additional materials chosen from the group consisting of Si0₂ and Ti0₂, and further wherein the inorganic binder comprises bentonite; b) desulfurizing a gas comprising H₂S by directing the gas past the pellets, the gas having a temperature of about 800°C or above; c) regenerating the pellets by directing regeneration gas comprising oxygen past the pellets, the regeneration gas having a temperature of 800°C or above.

23. A method according to claim 22, wherein the step of providing sorbent pellets further comprises providing pellets in which the manganese comprises about 35-45%o by weight of the pelletized sorbent.

24. A method according to any one of claims 22 or 23, wherein the regeneration gas has a temperature of about 900°C or greater.

25. A method according to any one of claims 22 or 23, wherein the regeneration gas reaches a temperature of about 1000°C or greater.

26. A method of using sorbent pellets to desulfurize gas and regenerate the sorbent pellets after desulfurization comprising the steps of: a) providing sorbent pellets comprising about 35-45% by weight manganese, about 20-30% by weight alumina and about 2% by weight of inorganic binder, wherein the alumina further comprises additional materials chosen from the group consisting of Si0₂ and Ti0₂, and further wherein the inorganic binder comprises bentonite; b) desulfurizing a gas comprising H₂S by directing the gas past the pellets, the gas having a temperature of about 800°C or above; c) regenerating the pellets by directing regeneration gas comprising oxygen past the pellets, the regeneration gas having a temperature of 900°C or above.

27. A method for rnanufacturing regenerable manganese-based sorbent pellets for removing sulfur from a gas, the method comprising the steps of: a) providing a rnixture comprising a manganese containing compound, an alumina-based matrix material and a binder; b) pelletizing the mixture with water to form substantially spherical pellets; and c) indurating the pellets to form strong, porous sorbent pellets.

28 A method according to claim 27, wherein the manganese containing compound comprises manganese carbonate.

29. A method according to claims 27 or 28, wherein the manganese containing compound comprises manganese oxide.

30. A method according to any one of claims 27-29, wherein the binder comprises an inorganic material.

31. A method according to claim 30, wherein the inorganic binder is selected from the group consisting of bentonite, kaolin and Portland cement.

32. A method according to any one of claims 27-31, wherein the binder comprises an organic material.

33. A method according to claim 32, wherein the organic binder is selected from the group consisting of dextrin, starches, methyl cellulose, and molasses.

34. A method according to any one of claims 27-33, wherein the step of pelletizing further comprises:

1) adding water to the manganese containing compound, matrix material and binder to form feed; and

2) forming substantially spherical pellets from the feed.

35. A method according to any one of claims 27-34, wherein the step of pelletizing further comprises:

1) preparing feed comprising the manganese containing compound, matrix material, binder and about 5-10% by weight water; 2) passing the feed through a sieve to form fines;

3) placing the fines in a balling tire; 4) adding water to the fines in the balling tire, thereby causing the fines to form seeds; and

5) adding fines to the seeds to cause the seeds to grow.

36. A method according to any one of claims 27-35, wherein the step of indurating further comprises holding the pellets at temperatures ranging from about 1150 to 1275°C for about 1 to 2 hours.

37. A method according to any one of claims 27-36, wherein the manganese containing compound in the mixture comprises manganese carbonate and is about 81% by weight of the mixture, wherein the matrix material comprises about 16% by weight of the mixture, and further wherein the binder is inorganic and comprises about 2% by weight of the mixture and is selected from the group consisting of bentonite, kaolin and Portland cement.

38. A method according to any one of claims 27-37, wherein the matrix material further comprise one or more additional materials comprising about 1-5%) by weight of the matrix material, wherein the additional materials are chosen from the group consisting of Si0₂ and Ti0₂.

39. A method according to any one of claims 27-36 or 38, wherein the manganese compound in the mixture comprises manganese carbonate and is about 75%o by weight of the mixture, wherein the matrix material comprises alumina and is about 15% by weight of the mixture, and further wherein the binder is organic and comprises about 9% by weight of the mixture and is selected from the group consisting of dextrin, starches, methyl cellulose, and molasses.

40. A method of manufacturing regenerable manganese-based sorbent pellets comprising about 30-50% by weight manganese, about 20-30% by weight alumina and about 2% by weight of inorganic binder, the sorbent pellets for removing sulfur from a gas, the method comprising the steps of: a) providing a mixture comprising manganese carbonate, an alumina-based matrix material and an inorganic binder; b) pelletizing the mixture with water to form substantially spherical pellets; and c) indurating the pellets at temperatures ranging from about 1150 to 1275°C for about 1 to 2 hours.