Elsevier

Catalysis Today

Volume 117, Issue 4, 15 October 2006, Pages 447-453
Catalysis Today

Fuel-rich catalytic combustion of methane in zero emissions power generation processes

https://doi.org/10.1016/j.cattod.2006.06.010Get rights and content

Abstract

A novel catalytic combustion concept for zero emissions power generation has been investigated. Catalysts consisting of Rh supported on ZrO2, Ce-ZrO2 or α-Al2O3 were prepared and tested under fuel-rich conditions, i.e. for catalytic partial oxidation (CPO) of methane. The experiments were performed in a subscale gas-turbine reactor operating at 5 bar with exhaust gas-diluted feed mixtures.

The catalyst support material was found to influence the light-off temperature significantly, which increased in the following order Rh/Ce-ZrO2 < Rh/ZrO2 < Rh/α-Al2O3. The Rh loading, however, only had a minor influence. The high activity of Rh/Ce-ZrO2 is probably related to the high dispersion of Rh on Ce-ZrO2 and the high oxygen mobility of this support compared to pure ZrO2. The formation of hydrogen was also found to increase over the catalyst containing ceria in the support material.

Introduction

An increasing concern regarding environmental pollution during the last years has resulted in stricter emission regulations for NOx, SOx and greenhouse gases. Due to these, and potentially more demanding future restrictions, there is a need to improve existing power generation processes. The advanced zero emission power (AZEP) concept enables NOx elimination as well as cost reduction of CO2 separation compared to conventional techniques (tail-end-capture) [1]. These targets can be achieved by (i) combusting natural gas in pure oxygen produced by a mixed conductive membrane (MCM) in which O2 is separated from air and (ii) dilution of the fuel/oxygen mixture with combustion products, i.e. water and carbon dioxide. The AZEP concept is described in Fig. 1.

The high amounts of steam and carbon dioxide present in the AZEP process, i.e. about 50 and 25%, respectively, render homogeneous ignition of the fuel mixture particularly challenging. On the other hand, catalytic combustion is an attractive alternative. The commonly used catalysts for methane combustion in lean air mixtures are based on palladium. However, several studies have shown that water severely inhibits the activity of palladium [2], [3], [4], while other materials like rhodium could be less sensitive. Therefore, a two-stage process has been investigated, wherein catalytic partial oxidation (CPO) of fuel-rich methane/oxygen mixtures over Rh is used to stabilize a subsequent overall fuel-lean homogeneous combustion zone. Syngas, H2 and CO, is catalytically produced under fuel-rich conditions resulting in hydrogen stabilized flame combustion. The advantages of the fuel-rich approach in terms of light-off and hydrogen selectivity have been reported earlier [5] for CH4/air mixtures without exhaust gas dilution in the same subscale gas-turbine reactor as the one used in the present investigation. For the catalytic systems used in the fuel-rich first stage of the AZEP process, low light-off temperatures and high syngas selectivities are required. Rh-based catalysts have shown the required properties yet in great variation with respect to support materials, noble metal loading, etc. [6].

In this study, catalysts for the first fuel-rich step of this process were investigated. Various supported rhodium catalysts have been prepared and tested in a high-pressure subscale gas-turbine test rig under AZEP conditions. The influence of support material, oxygen-to-fuel ratio and Rh loading on the methane conversion and outlet gas composition was studied.

Section snippets

Experimental

Supported rhodium catalysts were prepared by the incipient wetness impregnation technique using Rh(NO3)3 as metal precursor. The support materials had previously been calcined at 1073 K for 5 h. The only exception was alumina, which was calcined at 1373 K for 10 h to obtain the α-Al2O3 phase. The impregnated powders (0.5–2 wt% noble metal loading) were dried at 383 K and calcined at 873 K for 5 h before coating on FeCrAlloy metal. The coated FeCrAlloy was assembled to a fully coated honeycomb structure

Catalyst characterization

The characteristics of the Rh catalysts are presented in Table 1. The BET surface area for the Rh/ZrO2 catalysts was about 20 m2/g whereas a higher value of 44 m2/g could be measured for Rh/Ce-ZrO2. The lowest surface area was detected for Rh/α-Al2O3 (6 m2/g). The metal dispersion values calculated from H2 chemisorption experiments varied in the range 17–54%. The temperature programmed reduction experiments show that rhodium oxide (Rh2O3) is completely reduced at T < 473 K for all catalysts. The

Discussion

For the catalytic stage of the AZEP process described above it is important to achieve ignition temperatures in the range 723–823 K as this is the temperature of the inlet gas mixture and, therefore, preheaters could be avoided. The ignition temperature can be higher than that of conventional catalytic combustor systems since the recirculation of hot exhaust gas preheats the reactant mixture. Furthermore, the production of hydrogen is of importance for stabilizing the following homogeneous

Conclusions

The results presented here show that a hybrid combustor design consisting of a fuel-rich catalytic stage followed by H2-stabilized homogeneous combustion is a promising combustion method for the AZEP concept. Supported rhodium catalysts are suitable for the light-off stage in the exhaust gas-diluted reaction mixtures that are present in the AZEP configuration.

The support material had a significant effect on the light-off temperature, which increased in the following order Rh/Ce-ZrO2 < Rh/ZrO2 < 

Acknowledgements

Financial support from the European Commission and the Swiss Government through the AZEP project, contract no. ENK5-CT-2001-00514, is gratefully acknowledged.

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