A Non-Wetting Packed Bed Gas Scrubber

Abstract

SummaryPresent integrated gasification combined cycle (IGCC) systems demonstrate high system efficiency and impressive environmental performance, giving them an edge over conventional pulverised fuel power stations. A key area in the development of IGCCs is hot fuel gas clean-up (HGCU). Fuel gas cleaning at elevated temperatures reduces thermal efficiency losses associated with gas quenching in conventional cold gas cleaning methods. Current hot gas desulphurisation techniques focus on the use of regenerable metal oxide sorbents, however the long-term sorbent performance issues have yet to be fully addressed. A fresh and radical approach may provide the key to overcoming the inherent limitations associated with metal oxide sorbents. A molten tin irrigated packed bed scrubber adopted in this research project is one such innovative way forward in HGCU. The hot scrubber offers the prospect of a multicomponent clean-up device.High-temperature sulphur removal takes place via absorption of H 2 S (and COS) into molten tin whilst discrete molten tin droplets and rivulets on the packing surface act as solid particulate collectors.The primary aim of this research project was to investigate the workings of a small-scale roomtemperature packed bed scrubber operating under non-wetting flow conditions analogous to the molten tin irrigated scrubber. Water irrigation of low surface energy packings simulated the nonwetting flow of liquid metals. The air-water analogue of the liquid metal scrubber provided the platform for hydrodynamics (flow visualisation, flooding and liquid holdup), particulate removal and mass transfer studies under non-wetting flow conditions. The performance of a small air lift for water circulation through the column was also investigated. These cold studies offered insight into the operation and performance of the liquid metal hot scrubber.Prior to the cold gas scrubber studies, preliminary small-scale gasification tests on petroleum coke samples were performed to investigate the effect of molten tin on H 2 S in the product fuel gas. The tests provided actual experimental evidence of the possibility of sulphur removal by molten tin in a gasification environment.It was shown that the maximum possible size of a liquid droplet hanging from a non-wetting spherical solid surface could be predicted from the liquid surface tension and density based on force balance. The mobility of static holdup in a non-wettable packed bed has been demonstrated, this being due to the tendency for the liquid to form discrete droplets rather than spreading films. Existing flooding and liquid holdup correlations that hold for conventional wettable packed beds were shown to be inadequate where non-wetting systems were concerned. Summary hence alternative methods applicable to the latter were sought. The introduction of a non-wetting tendency factor based on the ratio of the solid critical surface tension to the liquid surface tension, enabled the flooding capacities of non-wetting systems including those of this study to be predict...

Liquid Tin Irrigated Packed Bed for Hot Fuel Gas Desulfurization

“…Figure 1 shows the countercurrent packed bed scrubber used in this study. It was originally designed by Chang 6 and has gone through a series of modifications. The rig consisted essentially of an absorber column (height 23 cm, internal diameter 8 cm) placed in a semicylindrical heated compartment (ceramic radiant heaters) covered by insulation materials (Kaowool board and blankets).…”

“…High-purity alumina has excellent resistance to liquid metals and to reducing atmospheres at elevated temperatures. Details on the packing 6 are summarized in Table 3.…”

“…However, a lower than desirable level of sulfur removal (80−90%) requires an additional desulfurization process . The current hot gas desulfurization technologies focus on the development of regenerable metal oxide sorbents, ,, to be used in fixed, moving, or fluidized bed configurations. Metal oxides investigated in recent studies include Ca based, Zn/Ti based, and Zn and Cu based .…”

“…For the proposed nonwetting flow scrubber, previous work by Chang , presented the results of cold tests using water conducted on nonwetting packed beds of polyethylene and waxed glass balls, and applied these data to the design of a lab-scale scrubber for liquid tin.…”