Hot Coal Gas Desulfurization With Manganese-Based Sorbents

The objective of this project is to develop a pellet formulation which is capable of achieving low sulfur partial pressures and a high capacity for sulfur, loaded from a hot fuel gas and which is readily regenerable. Furthermore the pellet must be strong for potential use in a fluidized bed and regenerable over many cycles of loading and regeneration. Regeneration should be in air or oxygen-depleted air to produce a high-concentration sulfur dioxide. Fixed-bed tests were conducted with several formulations of manganese sesquioxide and titania, and alumina. They were subject to a simplified fuel gas of the oxygen-blown Shell type spiked with a 30,000ppmv concentration of H2S. Pellet crush strengths for 4 and 2mm diameter pellets was typically 12 lbs per pellet and 4 lbs per pellet, respectively. For the most favorable of the formulations tested and under the criteria of break-through at less than 100ppmv H2S and loading temperatures of 500oC and an empty-bed space velocity of 4,000 per hour, breakthrough occurred an effective loading of sulfur of 27 to 29% over 5 loading and regeneration cycles. At 90% of this saturation condition, the observed level of H2S was below 10ppmv. For regeneration, a temperature of 900oC is required to dissociate the sulfide into sulfur dioxide using air at atmospheric pressure. The mean sulfur dioxide concentration which is achieved during regeneration is 8% with empty-bed space velocities of 700/hr. TGA tests on individual pellets indicate that bentonite is not desirable as a bonding material and that Mn/Ti ratios higher than 7:1 produce relatively non-porous pellets. Whereas the reactivity is rapid below 12% conversion, the kinetics of conversion decreases significantly above this level. This observation may be the result of plugging of the pellet pores with sulfided product creating inaccessible pore volumes or alternately an increase in diffusional resistance by formation of MnS. 1 Research sponsored by the U. S. Department of Energy's: University Coal Research Program, under contract DE-FG-2294PC94212. Introduction Department of Energy is actively investigating hot fuel gas desulfurization sorbents for application to the Integrated Gasification Combined Cycle Power Generation (IGCC). A sorbent must be highly active towards sulfur at high temperatures and pressures, under varying degrees of reducing atmospheres. High conversion of the metal oxide and low hydrogen sulfide exit partial pressures are required. Also, it must regenerate nearly ideally to maintain activity over numerous cycles. Furthermore, regeneration must yield a sulfur product which is economically recoverable directly or indirectly. In response to stability difficulties to formulate single and binary metal oxide sorbents, effort is increasingly being directed towards incorporation of an inert component into sorbent formulation as witnessed by the various Zn-titanates. The role of the inert component is primarily to increase pore structure integrity while stabilizing the active metal oxide against reduction. The inert solid may also be used as a porous monolith for impregnated active metals. Mn-based sorbents are resistant to reduction to the metal in most coal gas atmospheres. However, their pore structure requires further investigation, as this may determine desulfurization kinetics due to intra-particle transport resistances. Objective The objective of this project is to develop a pellet formulation which is capable of achieving low sulfur partial pressures and a high capacity for sulfur, of being loaded from a hot fuel gas and which is readily regenerable. Furthermore the pellet must be strong for potential use in a fluidized bed and regenerable over many cycles of loading and regeneration. Regeneration should be in air or oxygen-depleted air to produce a high-concentration of sulfur dioxide. Approach Pellet Composition Exploratory investigation of Mn-based sorbents began with the consideration of the following parameters of feed materials and preparation techniques. Composition variables were; • Manganese source, • Substrate composition, • Mn to substrate molar ratio, • Non-volatile binder wt %, • Porosity enhancer composition, • Porosity enhancement wt%. The manganese sources were chosen from a commercially available MnCO3 and a pyrolusite ore. Substrate were chosen based on thermodynamic equilibrium between the mixed metal oxide sorbent (MnO.MexOy) and hydrogen sulfide (H2S). 350 400 450 500 550 600 650 700 Temperature C 0 25 50 75 100 125 150 175 200 E q u il ib ri u m H yd ro ge n S u lf id e p p m v Equilibrium Composition at 1 atm. 100 mol Shell Gas 10 mol Metal Sorbent MnSiO3 Mn2SiO4 MnAl2O4 MnTiO3 Mn2TiO4 4MnO.TiO2 MnO CO = 64 m ol% H2 = 27 CO2 = 2 H2O = 2 H2S = 0.5 N2 = 4.5 (bal.) Maximum Allowable Figure 1. Equilibrium composition for Mn-based sorbents with various substrates in a simplified Shell type fuel gas. Figure 1. is a thermodynamic analysis of the hydrogen sulfide levels which can theoretically be produced for a series of manganese oxide phases. The molar ratio of Mn to substrate was varied in an attempt to optimize this parameter. Only bentonite was used as a non-volatile binder with 0,2, or 5% by weight added to various formulations. In an effort to increase the pore volume and surface area of the indurated pellet, volatile components were added to initial pellet mixtures. Finally, the amount of porosity enhancer was varied for a selected number of formulations. Pellet Preparation Feed powders were hand mixed and pelletized in a balling wheel forming "green" pellets. Green pellets were air dried for one day than dried to a constant weight at 100 oC. Dry pellets were calcined for four hours at 350oC. Immediately after calcination, pellets were placed in a high-temperature furnace (preheated to 500oC), where the temperature was ramped up for two hours until final induration temperature was achieved. Due to the variety of pellet compositions, it was decided to restrict the induration length to two hours, at the designated temperature, for all formulations. To date over 50 induration campaigns have been conducted for the fifteen present formulations (approximately four campaigns per formulation). The indurated pellet size was formulated in the range of 1-3 mm. Characterization Several methods of physical and chemical characterization were employed. Crush strength was the first parameter measured for all freshly indurated sorbent. Sorbent with requisite strength was then reduced and sulfided in a thermogravimetric analyzer to determine reaction evolution and fractional conversion. The test conditions for reduction/sulfidation are reported in Table 1. Sample mass 200-800 mg Reduction duration: T > 550oC T < 550 oC 30 min 60 min Sulfidation duration 120 min Average pellet diameter 1-3 mm Gas composition and flow rate 1 L/min H2 H2S concentration 30,000 ppmv Pressure 1 atm Table 1. Conditions of TGA reduction/sulfidation experiments for all formulations. Each TGA test sample consisted of three pellets. The sulfided pellets were also regenerated in the TGA. Regeneration conditions are described in Table 2. Sample mass 200-800 mg Regeneration Temperature 900 oC Regeneration duration 60 min Average pellet diameter 1-3 mm Gas composition and flow rate 1 L/min Air Pressure 1 atm Table 2. Conditions of TGA regeneration experiments for all formulations. A limited number of formulations were subjected to mercury porosimetry for pore structure characterization. This testing is currently limited to freshly indurated sorbent. Also, chemical analysis of unreacted and reacted sorbent was conducted to corroborate TGA data. Sorbent S capacities' (based upon initial formulation and chemical analysis) are listed in Table 3. Formula Sulfur Capacity Sg / 100g sorbent Formula Sulfur Capacity Sg / 100g sorbent A1-0 28.8 (29.1) C4-5 28.2 (27.9) A1-2 28.1 (27.8) C5-2 31.4 (33.4) A2-2 29.8 (29.1) C5-5 30.1 (30.7) C4-2 29.3 (27.3) C6-2 25.5 (31.7) C4-2A 29.3 (27.5) C7-2 23.0 (21.6) C4-2D 29.3 (27.8) C8-0 25.9 (28.3) C4-2M 29.3 (*) C9-2 32.6 (34.1) C10-2 29.3 (27.6) Table 3. Formula designations and sulfur capacities based on; initial formulation and chemical analysis ( ).*Chemical analysis not available. Formulation designations are described by listing sequentially; a letter, a number, a dash, a second number, a second dash, and finally a third number (ex. C6-2-1100). The first letter corresponds to the manganese source (C for MnCO3, and A for MnO2-ore). The first number refers to the molar ratio of Mn to substrate and the substrate composition (1,2,4,5,7 for alundum, 6,8, 9 for titania, and 10 for bauxite). The second number is the weight percent of bentonite binder. Note, some C4-2 formulations have letters following the weight percent bentonite. These letters refers to porosity promoters added, i.e., C4-2x (A for activated, D for dextrin, and M for MoO3). The most promising sorbents are to be further tested in an ambient pressure fixed-bed reactor. This will allow operational parameters of; space velocity, temperature, and gas composition, to be varied to determine sorbent performance during sulfidation and regeneration. Primarily, steady-state H2S concentrations and breakthrough times are being measured. Also, regeneration SO2 concentrations and breakthrough times are measured. Characterization of fixed-bed pellets includes crush strength, sulfur analysis, porosimetry, and scanning electron microscopy. The reactor consists of a 1" inner diameter tube and bed outlet thermocouple. Figure 2. is a schematic of the reactor system. 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