Control of alkali species in gasification systems: Final report

Abstract

Gas-phase alkali metal compounds contribute to fouling, slagging, corrosion, and agglomeration problems in energy conversion facilities. One mitigation strategy applicable at high temperature is to pass the gas stream through a fixed bed of sorbent or getter material, which preferentially adsorbs alkali via physical adsorption or chemisorption. This report presents results of an experimental investigation of high-temperature (600° to 700°C) alkali removal from a hot filtered gasifier product gas stream using a packed bed of sorbent material. Two getter materials, activated bauxite and emathlite, were tested at two levels of space time (0.7 and 1.5 s) by using two interchangeable reactors of different internal diameters (51 and 76 mm). The effect of getter particle size was also investigated.The experimental apparatus consisted of a bench-scale fluidized bed gasifier, a hot ceramic filter unit, and a packed bed alkali getter reactor. Banagrass, a candidate crop for tropical and subtropical dedicated feedstock supply systems, was used as fuel. Banagrass's high K and Cl content facilitates experimental measurement of these trace elements at levels well above analytical detection limits. Potassium, Na, and Cl concentrations in the gas stream exiting the filter were 28, 11, and 1,300 ppmw, respectively, with total alkali to Cl ratios of ~0.03, indicating that Cl is present in forms other than alkali chlorides. Potassium and Na removal efficiencies by the getter beds were >99% and >92%, respectively, for all tests, with no apparent differences attributable to the variables tested: getter material, space time, or getter particle size. The highest concentrations of K and Na in the getter materials were measured in the sorbent recovered from the top 5 cm (inlet) of the packed bed. Average alkali concentrations of 4.6 mg K g -1 sorbent and 1.1 mg Na g -1 sorbent were measured in the 51-mm diameter reactor tests. Alkali concentrations in the top 5 cm of the 76-mm diameter reactor were lower because of the greater sorbent material volume.Element balances around the getter reactor for K, Na, and Cl were generally poor because of the small analyte masses, compared to the total getter mass in the reactor. Element balances around the gasifier/ceramic filter/getter reactor system were generally acceptable. Carbon balances ranged from 93% to 103% of complete closure, with carbon conversion efficiencies on the order of 93%. Balances for K, which were present as ~1% of fuel mass, varied from 75% to 91% of closure. Balances for Cl (~0.6% of fuel mass) showed a wider range, from 63% to 93% of full closure. Sodium was present as ~0.1% of fuel mass, and elemental balances were in the range of 63% to 150%.i . Distribution of the Na mass present in the input fuel among 62 the outputs; gasifier bed, filter char, getter material, and gas phase. Table 1. vi LIST OF TABLESComposition of alkali getter materials reported in the literature. 6 Table 2. Thermodynamic equilibrium prediction of phase distribut...

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