Ind. Eng. Chem. Res. 2005, 44, 241-249
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APPLIED CHEMISTRY Selective Removal of Hydrogen Sulfide from Gaseous Streams Using a Zinc-Based Sorbent Jose´ M. Sa´ nchez,* Esperanza Ruiz, and Jesu ´ s Otero Centro de Investigaciones Energe´ ticas, Medioambientales y Tecnolo´ gicas (CIEMAT), Avda. Complutense 22, 28040 Madrid, Spain
A sorbent that was developed by Phillips Petroleum Company, now ConocoPhillips Company, has demonstrated very high and stable reactivity under numerous fixed-bed testing regimes in simulated gases. This sorbent, under the tradename of Z-Sorb III, has been determined to be suitable for sulfur removal applications in a wide range of temperatures (250-650 °C), pressures (2-20 atm), and gas space velocities (3500-10 000 h-1). The suitability of the system has been evaluated according to the following criteria: operational conditions (temperature, pressure), sulfur retention, sulfur capacity, side reactions, mechanical stability, and regeneration features. The execution of the experimental work program has been conducted in a high-temperature, high-pressure bench-scale plant that operates with synthetic gases. The influence of fuel gas components, temperature, pressure, gas hourly space velocity, gas type, and cyclic testing on sulfur sorption has been studied. Introduction The removal of hydrogen sulfide (H2S) from fluid streams can be desirable for a variety of reasons. If the fluid stream is to be released as a waste stream, the removal of sulfur from the fluid stream may be necessary to meet the sulfur emissions requirements that have been set by various air pollution control authorities. Such requirements are generally in the range of ∼10-500 ppmv of sulfur in the fluid stream. If the fluid stream is to be burned as a fuel (for instance, in the case of a gasification gas fed into a gas turbine), the removal of sulfur from the fluid stream may be necessary to prevent environmental pollution. If the fluid stream is to be processed further, removal of the sulfur is often needed to prevent the poisoning of sulfursensitive catalysts or to satisfy other process requirements, such as equipment tolerance. Future energy generation processes must be environmentally acceptable and must operate efficiently. One of these promising new technologies under commercialization is the gasification of solid fuels integrated in a combined cycle (IGCC) to generate electricity by means of a gas turbine and a steam turbine. Although IGCC systems show higher efficiency than conventional power generation concepts, there can be still a significant emission of pollutants, such as carbon dioxide (CO2), nitrogen oxides (NO, NO2), and sulfur dioxide (SO2) into the atmosphere. Lately, CO2 has received great attention, because of its contribution to the greenhouse effect. One way to reduce total CO2 emissions is to increase the efficiency of power plants. Moreover, because most of commercial-scale IGCC plants operating worldwide are demonstration projects, they have been expensive, * To whom correspondence should be addressed. E-mail:
[email protected].
in terms of capital costs and electricity generation costs. One potential route to improving thermal efficienciess and, therefore, decreasing CO2 emissions and reducing costssis to integrate hot gas chemical cleanup to purify the coal-derived gas streams. Particularly, the removal of H2S from fuel gas (the main pollutant) is regarded as an efficient and cost-effective method of meeting SOx emission standards, compared to desulfurization of the flue gas. Particularly, in the case of advanced power generation cycles, practically all of them rely on the use of gassolid absorption processes for the removal of sulfurous gases at high temperatures and pressures. Hot gas desulfurization is accomplished with the use of in-bed sorbents fed with the coal, by sorbents injected into the gas phase, or by gas percolation through an external sorbent bed.1 Several metal oxides, mixed metal oxides, and supported metal oxides have been identified as thermodynamically feasible candidates to be used for high-temperature sulfur removal.2 A sorbent must satisfy many requirements to be successful:3 (i) high sulfur retention (it must remove 95%-99% of the H2S, depending on the application, gas turbine, or fuel cell, and on whether it is intended for bulk desulfurization or as a polishing stage); (ii) high sulfur capacity and regeneration efficiency, to reduce the quantity of sorbent that is required and the size of the equipment; (iii) mechanical stability, to minimize the loss of sorbent; and (iv) chemical stability, to maintain high sulfur capture during repeated sulfidation reactions with minimum unwanted side reactions. The external adsorption bed processes, which are in the most advanced stages, are those based on the zinc titanate or zinc oxide sorbents.4 Among these, a zinc oxide-nickel oxide sorbent named Z-Sorb (produced by Phillips Petroleum and originally developed for H2S
10.1021/ie0497902 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004
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Figure 1. Schematic of CIEMAT’s Hot Gas Desulfurization Plant.
removal from Claus plant tail gas) has been extensively tested.5 Several advanced versions of this sorbent have been developed, with features such as resistance to sintering in high-temperature steam, low light-off temperature, and high attrition resistance.6 In this work, a systematic study to understand the effect of gas components, temperature, gas space velocity, pressure, and H2S concentration on the performance of the Z-Sorb III sorbent was conducted under simplified conditions, and the results of this study are discussed in this paper. In addition, the performance of the sorbent in the desulfurization of synthetic gases that are representative of an oxygen-blown gasifier has been studied. Experimental Approach Test Rig. The removal of H2S with regenerable sorbents was evaluated in CIEMAT’s high-temperature, high-pressure (HTHP) bench-scale sorbent test facility; a schematic of this facility is shown in Figure 1. The plant can treat up to 20 Nm3/h of a gas mixture simulating the composition of the flue gases from different processes, such as combustion or gasification gases. It is designed to operate at a maximum temperature of 973 K and a pressure of 30 bar. The main components of the desulfurization facility are (1) the gas delivery system, (2) the reactor assembly, (3) the data acquisition and process control unit, (4) the gas analysis system, and (5) the reactor off-gas venting system. A battery of 12 mass flow controllers (MFCs), which are capable of operation at pressures up to 80 atm, controls the flow rate and composition of simulated gases, using bottled gases for CO, H2, CO2, N2, H2S, and air. A positive displacement pump (PP) feeds deionized water to a boiler (EH2) and a superheater (EH3) to generate steam. The delivery system can generate simulated coal gases that are representative of all types of gasifiers. Gases are heated to 673 and 973 K, respectively, in a preheater (EH1) and a superheater (EH3) that are connected in series. Each temperature is controlled by means of separate temperature controllers (TIC1-TIC4). The reactor (R) is constructed from Incoloy 800 H. It has a height of 1 m and an internal diameter of 80 mm. Inside the reactor, the sorbent is contained in a remov-
able cage. The bottom of this cage is a distributor plate of R-alumina. Fixed- and fluidized-bed implementations are possible. The reactor is housed inside a fourzone furnace equipped with separate temperature controllers (TIC5A-TIC5D) for each zone and the furnace can heat the reactor up to 973 K. The reactor temperature is monitored at different heights, bed inlets, free boards, and bed outlets, using 11 Type K thermocouples (TIA21-TIA31). The reactor exit gas is cooled using an air heat exchanger (HE1). Sulfur condensates and water are removed in a water-refrigerated vessel (V1). Two 50-µm filters (F2A, F2B) downstream of the condenser capture particles from the sulfidation and regeneration lines upstream of the pressure control valve (PCV). This valve precisely controls pressure. A differential pressure transducer (PAC01) across the reactor detects any plugging. Furthermore, this differential pressure module can be used to ascertain good-quality fluidization in the reactor, when a fluidized bed is used. Gases downstream of the pressure control valve are cooled to ambient temperature in a water heat exchanger (HE2). The sulfidation exit gases, which contain toxic CO and H2S and regeneration off-gases, are properly disposed. First, gases pass through an active carbon bed, and then a high-powered blower is used to dilute the gases before emitting them into the atmosphere. Slipstreams of the gas (CG0-CG4) are diverted to the gas analysis system, which consists of a gas chromatograph that is equipped with two detectors: a flame photometric detector (FPD) and a thermal conductivity detector (TCD). Chromatographic data are collected and analyzed by Hewlett-Packard acquisition software. A data-logging module developed by ICP (CSIC, Spain) is used to control and monitor the main process parameters. It has 64 separate channels. Each channel accepts one input. The input can be a voltage signal (0-5 V), a direct thermocouple input, or a current signal (mA). The channels are used to monitor flow rates from the MFCs, temperatures, and pressure. The output from the PLC (48 signals) is connected to a personal computer via a RS232 communication cable. The ADQUIR software is a real-time data acquisition program that controls the inflow and outflow of information from the various channels. It is an integrated, very user-friendly, software program for data acquisition, process control, and monitoring. Sorbent Characteristics. Selection of the sorbent (Z-Sorb III) was based on the expected activity, capacity, stability, and selectivity under the applied test conditions, as well as on information obtained from literature. It is a zinc-based sorbent, supported on a proprietary matrix that is designed to provide stability and prolong the life of the sorbent. According to the Material Safety Data Sheet, it contains
