The influence of the operating conditions in ammonia burner on the effectiveness of the catalyst for a nitrous oxide decomposition

Nitrous oxide (N2O) is a greenhouse gas, whose emission into the atmosphere is subject to increasingly stringent emission standards. One of the significant sources of this gas emission are the nitric acid plants. Nitrous oxide is formed as a by-product in the process of catalytic ammonia oxidation. It does not undergo any chemical transformations in the whole technological line of nitric acid manufacturing and is fully emitted into the atmosphere. Therefore, the use of an effective method of N2O emission abatement is highly reguired [1]. One of such methods is the catalytic decomposition of nitrous oxide from a process gas stream at a high temperature. In this case, the layer of deN2O catalyst is installed in an ammonia burner, directly beneath the catalytic gauzes [2]. The catalyst, which is to be used in ammonia burner, should have a high activity, selectivity and stability under the reaction conditions. A big chalange is to ensure a high mechanical strength and high abrassion resistance of the catalyst extrudates, as well as its long lifetime under the extremely difficult operating conditions (high temperature, possibility of local catalyst overheating, high water vapor pressure in the process gas stream). New Chemical Syntheses Instiute has developed its own Fe2O3-Al2O3 catalyst (PKR-2), which is active in HT-deN2O process. The catalyst works stably in many industrial installations and allows to reduce N2O emission to the desired level. Nevertheless, the investigations on the improvement of the catalyst functional properties are still carried out. They are focused on two practical aspects: modification of the catalyst manufacturing technology and determination of the influence of ammonia burner operating conditions. The studies, described below are related to the determination of the catalyst resistance to the deactivation (thermal and time-related).


Introduction
Nitrous oxide (N 2 O) is a greenhouse gas, whose emission into the atmosphere is subject to increasingly stringent emission standards. One of the significant sources of this gas emission are the nitric acid plants. Nitrous oxide is formed as a by-product in the process of catalytic ammonia oxidation. It does not undergo any chemical transformations in the whole technological line of nitric acid manufacturing and is fully emitted into the atmosphere. Therefore, the use of an effective method of N 2 O emission abatement is highly reguired [1]. One of such methods is the catalytic decomposition of nitrous oxide from a process gas stream at a high temperature. In this case, the layer of deN 2 O catalyst is installed in an ammonia burner, directly beneath the catalytic gauzes [2].
The catalyst, which is to be used in ammonia burner, should have a high activity, selectivity and stability under the reaction conditions. A big chalange is to ensure a high mechanical strength and high abrassion resistance of the catalyst extrudates, as well as its long lifetime under the extremely difficult operating conditions (high temperature, possibility of local catalyst overheating, high water vapor pressure in the process gas stream).
New Chemical Syntheses Instiute has developed its own Fe 2 O 3 -Al 2 O 3 catalyst (PKR-2), which is active in HT-deN 2 O process. The catalyst works stably in many industrial installations and allows to reduce N 2 O emission to the desired level. Nevertheless, the investigations on the improvement of the catalyst functional properties are still carried out. They are focused on two practical aspects: modification of the catalyst manufacturing technology and determination of the influence of ammonia burner operating conditions.
The studies, described below are related to the determination of the catalyst resistance to the deactivation (thermal and time-related).

Experimental
The catalyst activity was tested in a pilot ammonia oxidation plant in the flow of a real nitrous gases mixture. Parallel to the studies of PKR-2 catalyst activity and selectivity, the changes in its structure were also investigated. In the adopted research metodology, the activity of the catalyst samples with different "thermal history" was compared with the fresh catalyst sample (reference sample).
All measurements were performed under the identical operating conditions: p=5 bar, T=890 o C, V mix,inlet =56 Nm 3 /h. The run time of the test in the pilot plant was 120 h. During the measurements, the same volume of deN 2 O catalyst bed was used, as well as the same catalytic gauzes package. The diameter of ammonia burner was 100 mm. The catalyst activity was determined on the basis of the difference of N 2 O concentration in nitrous gases stream, measured downstream of Pt-Rh catalytic gauzes and after a secondary catalyst layer, at the outlet of the reactor. For the activity tests, the following PKR-2 catalyst samples were selected: P1fresh (reference) catalyst calcined at a temperature of 500 o C, P2catalyst after 100 days of work in an industrial installation, 3catalyst after 400 days of work in an industrial installation, 4catalyst overheated at a temperature of 1100 o C, 5catalyst overheated at a temperature of 1400 o C.
In Fig. 1, the activity of tested PKR-2 catalyst samples was compared with the reference sample activity. The PKR-2 catalyst after shaping and calcination at a temperature of 500 o C (P0) is a mesoporous material with an average pore diameter of 20 nm and specific surface area of 50 m 2 /g. In case of the catalyst, subjected to a high temperature of 890 o C (P1), the sintering process is observed. In the catalyst structure disappear the mesopores and increases the average pore diameter, while a total pore volume does not change. Moreover, a thermal shrinkage of the catalyst extrudates, 6-percent weight loss of the catalyst bed and lowering of a specific surface area is observed. Longer operation of the catalyst in ammonia burner does not cause any futher changes in its structure (samples P2 and P3).
Exposure of the extrudates to a temperature higher than the operating temperature in ammonia burner (1100 o C (P4) and 1400 o C (P5), e.g. due to the local catalyst overheating, causes further changes in the catalyst structure, due to the greater progress of sintering process. In Table  1, the physicochemical parameters for different PKR-2 catalyst samples are given. where: S -surface area, Vtotal pore volume, V mesop.volume of mesopores, d poreaverage pore diameter , P-porosity.

Conclusion
The significant changes in the PKR-2 catalyst structure are observed in the initial period of its operation in the ammonia burner. The sintering of the catalyst extrudates causes the decrease of catalyst specific surface area, as well as disappearance of mesopores and appearance of macropores in its structure. Calcination of the catalyst at a temperature of 500 o C does not allow for the complete stabilization of its structure. Exposure of the catalyst to a higher temperature results in a weight loss of the catalyst bed and leads to a reduction of the catalyst bed volume.
During the catalyst work, its gradual sintering is observed, accompanied by an increase in the pore diameter. Despite the changes in the catalyst structure, its operation at a temperature of 890 o C in the ammonia burner does not cause any negative changes in its activity. The catalyst retains its initial activity, even after 13 months of work in the industrial plant.
Overheating of the catalyst (1100C) causes further sintering of the extrudates. At a higher temperature, the porosity and a total pore volume of the catalyst are decreased, but despite this, only a slight lowering of its activity is observed. Its significant overheating above this temperature (1400C) causes the complete destruction of the catalyst structure and leads to its permanent deactivation.