ERK5-1999-00020 Development of Selective Catalytic Oxidation "SCO" Technology and Other High Temperature Ammonia Removal Processes for Gasification Power Plant

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Chemical Conversion : EESD (Energy, Environment and Sustainable Development) : Electricity

Proposal No:	ERK5-1999-00020
Date Prepared:	February 2004
Source:	European Bio-Energy Projects (EUR 20808)

Introduction

During gasification of solid fuels, the fuel nitrogen is released into the hot gasification gas primarily as ammonia (NH₃), hydrogen cyanide (HCN), organic compounds (tar-N) or as molecular nitrogen. The predominant compound in gasification is usually ammonia. Consequently, when this gas is combusted, large amounts of NO_x can be formed from the reactive fuel bound nitrogen compounds.

In gasification applications, NO_x emission can be reduced or almost totally eliminated by reducing the amount of the fixed nitrogen compounds in the gas. Even partial reduction of the amount of fixed nitrogen compounds in the fuel gas will relieve the demands for the burner and lower the resulting emission.

If the fixed nitrogen (and tar) compounds could be decomposed at high temperature in the fuel gas, the formation of liquid waste streams could also be avoided. Together with efficient burners, this technology could eliminate the need for the costly post-combustion flue gas cleaning technologies such as SCR.

Activities

The project was performed by three laboratories (VTT, Leeds University, Åbo Akademi) and two companies (Foster Wheeler Energia Oy and Energi E2) which were focused on the SCO. The work on nickel catalysts was performed by the Universidad Complutense de Madrid.

The project was focused on developing two main technologies for ammonia removal: 1) Nickel catalysts to decompose NH₃ in gasification gas at high temperature and 2) Selective Catalytic Oxidation process (SCO).

The work was divided into the following main topics:

• Improving the properties of the SCO catalysts
• Testing the SCO process with real gases
• Studying the reaction mechanisms behind the SCO
• Testing nickel monolith catalysts with real gases
• Evaluating the economic feasibility of the both processes in relevant applications
• Studying NO_x formation in gasification cocombustion applications.

Results

• Better SCO catalysts were found than were originally expected. Metal/alumina catalysts with varying properties were prepared and their applicability for the SCO process was characterised. The most promising catalyst identified was Cu/alumina. In addition, a ZrO₂ catalyst was found to be suitable for simultaneous tar and ammonia decomposition, giving relatively high conversions for both impurities. The SCO process was also tested in real gasification gas atmospheres. The results indicated that the ZrO₂ catalyst activity with real gases was comparable to lab scale results.
• A feasible operating window for nickel catalysts was identified. These catalysts were tested in product gas lines of small-scale gasifiers. A reactor model for nickel monolith was developed so that the effects of various operational conditions and the behaviour of the monolith could be studied. The most important operational parameters that affected achievable ammonia and tar conversions were studied. These include air partitioning within a gasifier/catalytic reformer system, H₂O/C ratio and catalyst inlet temperature. The monolith catalyst was more challenging to operate than was foreseen. However, a narrow operating window was successfully found, where over 90% ammonia conversions could be achieved.
• Comparisons of the SCO and nickel-monolith systems to filtering only and wet cleaning methods were made for a case where an atmospheric-pressure CFB gasifier was connected to an existing coal/peat-fired boiler. The evaluation indicated that from an economical point of view all the studied catalytic gas cleaning concepts were very close to each other. Thus, the selection can be fully based on technical feasibility and on the required level of gas cleaning. All catalytic gas cleaning concepts are naturally more expensive than the reference case based on filtration only.
• Directly applicable results were obtained from the CFD studies considering the Lahti Kymijärvi Power Plant18. These modelling studies gave valuable information about the means that could be applied to minimise NO_x-emissions from the plant. It also improved understanding of where, why, and how NO_x is formed in the boiler. This information can be used in the design of new plants and modifications of similar boilers elsewhere.

Impact and exploitation

Typical applications, in which the new ammonia removal technology could be used, are simplified biomass IGCC plants and various other plants, where hot gasification gas is burned. Recently, considerable interest has been expressed towards co-combustion plants among many power companies.

The aim of these plants is to use some local, renewable fuel, gasify it and feed the gas to the larger boiler in order to replace part of the main fuel (usually coal or oil). Even in these applications the NO_x level in flue gases may increase due to NH₃ in gasification gas. It is clear that the applicability of the NH₃ removal technologies of this project is not limited to biomass applications. They can also be used in connection with other gasification plants, regardless of the fuel.