Elsevier

Journal of Power Sources

Volume 159, Issue 2, 22 September 2006, Pages 995-1004
Journal of Power Sources

Removal of sulphur-containing odorants from fuel gases for fuel cell-based combined heat and power applications

https://doi.org/10.1016/j.jpowsour.2005.11.100Get rights and content

Abstract

Natural gas (NG) and liquefied petroleum gas (LPG) are important potential feedstocks for the production of hydrogen for fuel cell-based (e.g. proton exchange membrane fuel cells (PEMFC) or solid oxide fuel Cells (SOFC) combined heat and power (CHP) applications. To prevent detrimental effects on the (electro)catalysts in fuel cell-based combined heat and power installations (FC-CHP), sulphur removal from the feedstock is mandatory. An experimental bench-marking study of adsorbents has identified several candidates for the removal of sulphur containing odorants at low temperature. Among these adsorbents a new material has been discovered that offers an economically attractive means to remove TetraHydroThiophene (THT), the main European odorant, from natural gas at ambient temperature. The material is environmentally benign, easy to use and possesses good activity (residual sulphur levels below 20 ppbv) and capacity for the common odorant THT in natural gas. When compared to state-of-the-art metal-promoted active carbon the new material has a THT uptake capacity that is up to 10 times larger, depending on temperature and pressure. Promoted versions of the new material have shown potential for the removal of THT at higher temperatures and/or for the removal of other odorants such as mercaptans from natural gas or from LPG.

Introduction

Natural gas (NG) and liquefied petroleum gas (LPG) are important feedstocks for the production of hydrogen for fuel cell-based (e.g. proton exchange membrane fuel cells (PEMFC) or solid oxide fuel cells (SOFC) combined heat and power (CHP) applications. In densely populated areas, natural gas is a widely applied fuel for residential applications. For less inhabited areas and for leisure applications, LPG is the fuel of choice. LPG consists of a mixture of propane (s) and butane (s) in various ratios, depending on country and season. It is a versatile fuel that is used widely in de-centralised applications like heating (residential, industrial, agriculture, leisure, etc.). This broad range of applications is important for the development of the market for LPG-fuelled and fuel cell-based power and CHP applications.

A typical fuel cell-based CHP application may consist of several unit process steps [1], [2], [3] such as fuel pre-treatment (e.g. cleaning and preheating), catalytic partial oxidation or (steam or autothermal) reforming, high and/or low temperature water–gas shift, preferential CO oxidation (PROX), fuel cell and off-gas treatment in the afterburner. Fig. 1 presents a schematic drawing of a natural gas fuelled PEMFC-based CHP installation.

Natural gas and LPG contain sulphur components, either naturally occurring, or added deliberately as odorant. In cases where the fuel gas is to be used for residential purposes, naturally occurring sulphur species are first removed before adding (organo) sulphur compounds to odorise the otherwise odourless gas. This is a legislated practice in order to be able to detect the gas by its scent in case of leaks. Typical sulphur containing odorants are TetraHydroThiophene (THT), mercaptans, like tertiary butyl mercaptan (TBM) and ethyl mercaptan (EM), and organic sulphides such as dimethylsulphide (DMS) or mixtures thereof. In The Netherlands, and in most of Europe, THT is used to odorise natural gas. In the United States and Japan, natural gas is commonly odorised with mercaptans and sulphides or with mixtures thereof. LPG is often odorised with ethyl mercaptan (EM). These sulphur containing components present in the fuel processor fuel, are likely to be converted to H2S in the fuel processor section of the hydrogen generating system.

To prevent detrimental effects on the catalysts in a typical fuel cell-based combined heat and power installation, sulphur removal from the feedstock is mandatory. The sulphur removal task for LPG is more challenging than it is for natural gas. Propane, butane and heavier hydrocarbons are potential competitors for the sulphur compounds for adsorption sites, decreasing the adsorption capacity. Sulphur levels in LPG can be much higher when compared to natural gas; in the US levels can be as high as 120 ppmw. In Europe levels are generally less than 50 ppmw (EC legislation), depending on country of use and origin of the LPG.

From literature an ambiguous picture emerges on the sulphur tolerance of typical fuel processing catalysts. In general it seems that – due to the high operating temperature – low amounts of sulphur (typically below 10 ppm) do not seem to be a problem for platinum group metal based partial oxidation [4], autothermal reforming [5], or steam reforming [6] catalysts. This means that if sulphur removal is allowed to occur after CPO or autothermal reforming, the gas stream will be at elevated temperatures, which could enhance the opportunities for removing sulphur. While high temperature FeCr-based shift catalysts are not particularly susceptible to low levels of sulphur, low temperature CuZn-based shift catalysts are easily poisoned by even very low levels of H2S [7]. Information on the sulphur tolerance of alternative (e.g. noble metal based) shift catalysts or on the susceptibility of catalysts for the preferential oxidation of carbon monoxide is scarce. Considering the composition of PrOx catalysts and new generation WGS catalysts, it seems reasonable to assume that they will be poisoned by low levels of sulphur as well.

For the PEMFC it can be stated that sulphur compounds such as H2S already at (sub) ppm levels lead to a significant degree of coverage on fuel cell anodes and that this coverage will poison its capability for oxidizing hydrogen, thus leading to lower fuel cell performance in terms of power output and cell efficiency. For PEMFC-based CHP installations the temperature level in the whole system is the lowest in the PEMFC itself and due to its catalytic composition, the influence of H2S is expected to be largest of the PEMFC. Uribe and Zawodzinski of Los Alamos National Laboratory assessed the effect of fuel impurities on PEM fuel cell performance [8] using a PEMFC with a Pt/C anode. They found that concentrations as low as 0.2 ppmv H2S adversely affect the performance of the fuel cell.

The effect appears to be cumulative and causes severe deterioration of the fuel cells’ performance. Regardless H2S concentration and running time, replacing the contaminated fuel stream with pure H2 does not allow any recovery.

With respect to SOFC applications, Cunningham et al. [9] determined the sulphur tolerance of a reforming catalyst and two fuel cell anode formulations of the Rolls-Royce Integrated Planar Solid Oxide Fuel Cell. Both the reforming catalyst and the SOFC anodes suffered rapid and severe deactivation with H2S concentrations similar to the sulphur content of UK natural gas (16 mg m−3) of a mixture of ethyl mercaptan and diethyl sulphide. The susceptibility increases with sulphur concentration, but the safe concentrations of sulphur that result in little or no deactivation are far lower than the concentrations typically found in natural gas throughout Europe. To keep degradation due to sulphur poisoning at an economically sensible level, sulphur concentrations on ppbv level may be required at the fuel cell inlet.

As discussed above, the susceptibility of both fuel processing as fuel cell (anode) materials towards sulphur poisoning shows a clear need for desulphurisation of the fuel to be included in the fuel cell system. Hydro-desulphurisation (HDS) of the (organic) sulphur compounds, followed by scavenging the resulting H2S on ZnO is the state of the art industrial sulphur removal technology [7]. However, HDS is not an attractive option for fuel cell systems, especially for relatively small-scale systems, since it requires high pressure, hydrogen, elevated temperatures and a complex (thermal) integration into the system. In addition, commercial – elevated temperature – adsorbents for sulphur contain heavy metals – often toxic, – requiring specific handling procedures according to guidelines for the use of hazardous substances. An ambient absorption would be the preferred technology in view of system simplicity and costs. Although heavy metal activated low-temperature adsorbents are used, a wide choice exists in relatively harmless materials without toxic metals. Porous adsorbents like active carbons, zeolites or molecular sieves are candidates for ambient temperature sulphur removal technology [10]. Unfortunately, the use of low temperature adsorbents can be quite cumbersome and costly because of their relatively low capacity for sulphur. A large amount of adsorbent material may preclude its use as a sulphur filter for a CHP installation, because of size, weight and economic (material costs) constraints. Another drawback can be the adsorptive accumulation of toxic organic compounds from the fuel gas matrix (e.g. aromatics). Although the choice for a specific desulphurisation technology depends on many factors, the large scale aimed application of small-scale micro-combined heat and power primarily asks for a cost-effective desulphurisation technology that is simple to use without significant environmental objections. Within this scope this article describes the results of an extensive screening and testing programme of several low temperature adsorbents for their potential to desulphurise natural gas and LPG for fuel cell-based CHP applications.

Section snippets

Experimental assessment sulphur tolerance of the PEMFC

The effect of 0.2 and 2 ppm H2S in H2 on the performance of a PEMFC with a 1:1 Pt:Ru containing anode has been investigated. Taking into account the dilution in the fuel processor by air, steam and expansion by conversion, a gas feed containing 5 ppmv of THT would lead to reformate containing approximately 1 and 2 ppm of H2S when entering the fuel cell assuming no sulphur is scavenged in the fuel processor itself. The PEMFC voltage is monitored at a constant current density of 0.4 A cm−2 upon

Sulphur tolerance of the PEMFC

Fig. 4 shows that exposing the fuel cell to the 2 ppm H2S containing anode gas leads to an – almost instantaneous – and drastic decrease in performance. In about 65 h the performance of the fuel cell dropped to 50% of the original value. Recovery of the performance with pure hydrogen appeared to be impossible, while exposing the anodes to air during several minutes caused a significant restoration of the original performance (>90%). Anode gas containing 0.2 ppm H2S does not lead to such a drastic

Conclusions

To prevent detrimental effects on the catalysts in fuel cell-based power and CHP applications, sulphur removal from the feedstock is mandatory. From experimental bench-marking studies of adsorbents, several candidates for the removal of sulphur containing odorants at low temperature have been identified.

Among these adsorbents a new material has been discovered that offers a economically attractive means to remove the main European odorant THT from natural gas at ambient temperature. The

Acknowledgement

Ing. Michiel P. de Heer and Jurriaan Boon of ECN are gratefully acknowledged for their experimental support.

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