Technical Engineering for Catalytic Reduction of Nitrous Oxide Emissions
Abstract — Production of nitric acid is the major emission source of the greenhouse gas nitrous oxide N2O. High temperature catalytic decomposition of N2O by installing a secondary catalyst has been applied at a nitric acid plant in Devnya, Bulgaria. A reconstruction of the ammonia burning reactors was done in august 2012 in order to increase the thickness of the secondary catalyst layer from 28 mm to 60 mm. Nitrous oxide concentration has been monitored for 5 year period – from 2010 to 2014. Monitoring results indicate that the effectiveness of N2O catalytic decomposition depends on the thickness of the secondary catalyst layer - increasing the layer thickness twice leads to over 60 % reduction of the maximum registered and annual average concentrations of N2O. Increasing the thickness of the secondary catalyst layer enhances the effectiveness of high temperature N2O decomposition, which is essential for the parties to the United Nations framework convention on climate change and the Kyoto protocol regarding the fulfillment of their quantitative commitments for greenhouse gas emission reduction.
Nitric acid production is a major emission source of the greenhouse gas nitrous oxide N2O [1]–[4]. Effective reduction of N2O emissions from nitric acid plants is essential for the parties to the Kyoto Protocol, as they have committed themselves to reduce the overall greenhouse gas emissions with at least 5 % below their 1990 level. The European Union and its member states have made a mutual commitment for 20 % reduction of greenhouse gas emissions compared to their 1990 level by 2020 [5] and 30 % reduction provided that other carbon intensive economics commit themselves into comparable emission reduction [6]-[8].
II. LITERATURE REVIEW
Most nitric acid plants are based on the Oswald process
where N2O is formed in ammonia burning reactors as undesired by-product of ammonia catalytic oxidation [9], [10]. Generally, N2O emission abatement techniques at nitric acid production can be divided into 4 groups: primary measures,
secondary measures, tertiary measures and sequential (end-of-pipe) techniques [11]–[14].
Primary measures affect the formation of N2O during the catalytic oxidisation of ammonia. Some primary measures like modifying the geometry of the platinum catalysts can lead to a higher conversion of ammonia to NO and/or 30 – 50% reductions in the formation of N2O [9], [11]. Secondary measures are taken with regard to the process gas stream, produced in the process from the oxidisation catalyst, to the absorption tower. Homogeneous decomposition and high temperature catalytic decomposition are common secondary measures that can lead to 80 – 90% N2O emission reduction [15]-[29]. Tertiary measures like low temperature catalytic decomposition, selective catalytic reduction SCR and non selective catalytic reduction NSCR are taken in the process that takes place between the absorption tower and the expansion turbine. Applying tertiary measures can reduce N2O emission up to 98 – 99% [11], [14]. Sequential (End-of-Pipe) measures include some of the techniques described as tertiary measures but placed behind the expansion turbine. End-of-Pipe measures can perform 80 -97% N2O emission reduction [10], [13].
High temperature selective catalytic reduction of N2O is a secondary measure that provides decomposition of N2O right after its formation to nitrogen and oxygen at high temperature (800 – 9500C). Catalytic N2O reduction is based on the introduction of a suitable decomposing catalyst in the reactor chamber where oxidation of ammonia to nitrogen monoxide NO takes place and N2O and nitrogen are formed as undesired by-products of the reaction. The effectiveness of N2O catalytic reduction depends on various factors like the type of the decomposing catalyst, the duration of its operating period, the thickness of the catalyst layer and the specific technological conditions of nitric acid production process [16], [19], [21], [30].
The present research aim is to define the effect of increasing the thickness of the secondary N2O decomposing catalyst layer upon the achieved level of N2O emission reduction. For that reason, a comparison is done between the results of N2O emissions continuous monitoring at a nitric acid plant in Devnya, Bulgaria before and after a reconstruction of ammonia burning reactors was done in order to increase the thickness of the secondary catalyst layer.
Maya Stefanova and Rozalina Chuturkova
DOI: 10.5176/2251-3701_3.2.131
GSTF Journal of Engineering Technology (JET) Vol.3 No.2, July 2015
A catalytic reduction of N2O emissions by installing a
secondary non-platinum N2O decomposing catalyst is applied at a nitric acid plant in Devnya, Bulgaria [31]. It is a dual pressure plant consisting of four parallel ammonia burning reactors with nominal production capacity 1100 tons nitric acid per day. The secondary decomposing catalyst is filled in a catalyst basket that is mounted right under the platinum gauze
in order to support both the platinum gauze and the secondary catalyst layer. The catalyst layer is equally disturbed directly on top of the support grid of each reactor separated on top and bottom by steel screens (Fig. 1). All segments are sewn together to avoid gas bypass. The N2O reduction catalyst is made of precious metal coated ceramic pellets. Each pellet consists of 20 % CuO, 16% ZnO and AL2O3 plus promoters. The thickness of the catalytic layer in the basket is approximately 28 mm. The pressure drop over the catalyst is not significant and is normally less than 2.5 mbar.
N2O decomposing Catalyst
Pt gauzes
NH3+N2/O2(air)
NOx+N2 +H2O
Depiction of principle
Figure 1. Catalytic decomposition technology for N2O emission reduction
GSTF Journal of Engineering Technology (JET) Vol.3 No.2, July 2015
In August 2012 a reconstruction of the ammonia burning reactors was done in order to mount a deeper catalyst basket that increases the catalyst layer thickness to 60 mm. The structure of the new catalyst basket consists of assembly beams, placed on new brackets welded to the main reactor flange. Openings have been made on the 0°, 90°, 180°, 270° axes in the ribs of the load-bearing beams, making it possible to insert thermocouples for temperature measurement under the catalytic layer. The basket structure is mounted on supporting arms placed around its circumference and welded to the main flange. In the central part of the basket, beams rest upon 2 columns fastened to the load-bearing structure of the steam super heater.
Segments of the honeycomb grid are placed on the main beams. A supporting gauze 1 mesh/cm made of 2 mm diameter wire is placed on the honeycomb grid. This mesh is fastened at points to the honeycomb grid by means of kanthal wire hooks with a diameter of 2 mm. Next, fine ø 0.25 mm wire gauze with 7 mesh/cm (49 mesh/cm2) is placed on top. The purpose of this mesh is to protect the catalyst against overflow. After this mesh is installed, a screen is installed and fastened to the main flange by means of screws and nuts. A segmented seal is placed under the screen. After the installation of platinum catalytic gauzes, the segmented seal is replaced. In order to seal the basket to protect against movement of the catalyst, a 500 mm wide strip of fine mesh lining is placed around its circumference (applied in segments of length 2000 mm, with an overlap of 200 mm).
Key information on technical parameters of ammonia burning reactors and secondary catalyst layer before and after reconstruction is presented in Table 1.
TABLE I. PARAMETERS OF THE PLANT REACTORS BEFORE AND AFTER
RECONSTRUCTION
Item Value
Burner diameter 4254 mm
Basket diameter 4254 mm
Inner diameter of the reactor flange
4244 mm
Oxidation temperature 835 – 8750C
Working reactor pressure 3.3 – 3.5 bar abs.
Nominal height of the secondary catalyst layer
28 mm before reconstruction
60 mm after reconstruction
Catalyst bulk density 1100 kg/m3
Pressure drop in catalytic layer
2.5 mbar before reconstruction
5.5 mbar after reconstruction
A continuous monitoring of N2O emissions is done by on-line measurement of the tail gas parameters (N2O concentration, flow rate, temperature, absolute pressure and oxygen content). The effect of increasing the catalyst layer thickness upon the achieved N2O emission reduction has been studied within the present research by analysing results
from N2O emission monitoring before and after the reconstruction of ammonia burning reactors. The 5-year monitoring period lasts from 2010 to 2014. Monitoring results for the period January 2010 – July 2012 indicate N2O concentration with 28 mm thickness of the secondary catalyst layer. Monitoring results for the period August 2012 – December 2014 indicate N2O emissions after the reconstruction when the secondary catalyst layer thickness was increased over 2 times. Analysis results provide reasonable conclusions on the dependence between the thickness of the secondary catalyst layer and the achieved level of N2O emission reduction.
IV. RESULTS AND DISCUSSION
N2O emission levels for the monitoring period 2010 – 2014
(N2O concentration in tail gas from nitric acid plant, mg/Nm3) are indicated on Fig. 2 – 6.
Figure 2. N2O emission levels in 2010
Figure 3. N2O emission levels in 2011
GSTF Journal of Engineering Technology (JET) Vol.3 No.2, July 2015
As indicated on Fig. 2, N2O emissions in 2010 lightly increase as N2O concentration vary from 310.7 mg/Nm3 at the beginning of the year to 375.9 mg/Nm3 at its end. In the middle of the year, there is an emission drop to 185.3 mg/Nm3 due to planned maintenance of the plant for platinum loss recovery. Usually higher N2O concentrations are typical for the period right before annual plant shut down for platinum loss recovery compared to N2O concentrations right after plant start with recovered platinum gauzes. The annual average N2O concentration in 2010 is 391.65 mg/Nm3 while the peak value is 515.6 mg/Nm3 registered in December 2010.
N2O emissions in 2011 (Fig. 3) are considerably higher than emissions in 2010 – at the beginning of the year N2O concentration of 376.6 mg/Nm3 is registered and it increases more than twice to 802.6 mg/Nm3 by June just before the plant maintenance. An emission drop to 167.6 mg/Nm3 is registered right after the platinum loss recovery in August, and this N2O concentration is 4.8 times lower than the peak value for 2011. Afterwards N2O concentration lightly increases to 438.1 mg/Nm3 by the end of the year. The annual average N2O concentration in 2011 is 472.6 mg/Nm3, which is 1.2 times higher than the annual average N2O concentration in 2010.
Fig. 4 indicates N2O concentration in 2012. At the beginning of the year, a concentration of 440.2 mg/Nm3 is registered and it increases 1.8 times to 784.3 mg/Nm3 by June just before the plant maintenance for platinum loss recovery. Along with the maintenance in the summer of 2012 a reconstruction of the ammonia burning reactors was done and the thickness of the secondary catalyst layer was doubled from 28 mm to 60 mm. Right after plant start in August a considerable emission drop to 68.6 mg/Nm3 is registered. N2O concentration increases slightly to 94.8 mg/Nm3 by the end of the year and the lowest value 54.5 mg/Nm3 is registered in August 2012 being the minimum N2O concentration for the entire monitoring period. The average N2O concentration for the period January 2012 – June 2012 is 534.99 mg/Nm3 while the average N2O concentration for the period August 2012 – December 2012 is 89.07 mg/Nm3, which is 6 times lower.
N2O emissions in 2013 start at 95.1 mg/Nm3 and the lowest value for the year is 92.6 mg/Nm3 registered in January. Fig. 5 indicates a slight increase of N2O concentration to 242.1 mg/Nm3 by the end of the year and a peak value of 322.3 mg/Nm3 is registered in September. The annual average N2O concentration in 2013 is 200.51 mg/Nm3 and it is lower than the annual average concentrations in 2010, 2011 and 2012.
In 2014 N2O emissions are higher during the first half of the year (Fig. 6) starting at 241.6 mg/Nm3 and reaching 351.6 mg/Nm3 in May right before plant maintenance for platinum loss recovery. The peak value for the year is 463.2 mg/Nm3 registered in March. After the plant start in July with recovered platinum gauzes an emission drop to 136 mg/Nm3 is registered. N2O concentration slightly decreases to 86.8 mg/Nm3 by the end of the year and the lowest value is 56.5 mg/Nm3 registered in July. The annual average N2O concentration in 2014 is 188.59 mg/Nm3 and it is the lowest annual average concentration for the entire monitoring period (Fig. 7).
GSTF Journal of Engineering Technology (JET) Vol.3 No.2, July 2015
Figure 7. Annual average N2O concentration for the monitoring period
Analysis results indicate that the reconstruction of the ammonia burning reactors in order to double the thickness of the secondary catalyst layer is a technical engineering solution that enhances the environment protection effect of high temperature N2O decomposition. The average N2O concentration for the period January 2010 – June 2012 is 451.83 mg/Nm3 with 28 mm thickness of the secondary catalyst layer. The average N2O concentration for the period August 2012 – December 2014 is 175.70 mg/Nm3 with 60 mm thickness of the secondary catalyst layer. Increasing the thickness of the secondary catalyst layer twice leads to 2.5 times lower average N2O concentration and enhances the effectiveness of N2O catalytic decomposition thus giving valuable contribution to greenhouse gas emission reduction and global warming prevention.
V. CONCLUSION
Nitrous oxide N2O is a greenhouse gas with global warming potential 298 times higher than carbon dioxide CO2 thus becoming one of the main reasons for global warming effect. Nitric acid production is a major industrial source of N2O emissions. From 1 January 2013 onwards N2O emissions from nitric acid production are included in the scope of the European Union emission trading scheme as a climate change combat tool [32]. Achieving effective N2O emission reduction at nitric acid plants is a priority task for the parties to the Kyoto Protocol regarding the fulfillment of their respective quantitative commitments. Introducing a secondary decomposing catalyst in ammonia burning reactors is a measure for N2O emission reduction and its effectiveness depends on the technological process conditions, the type of the secondary catalyst, the duration of its operating period, etc. According to the present research results, the thickness of the secondary catalyst layer affects considerably N2O emissions. After the reconstruction of the ammonia burning reactors in August 2012 the thickness of the secondary catalyst layer is doubled which leads to 59.8 % reduction of the maximum registered N2O concentration and 60.1 % reduction of the annual average N2O concentration for the monitoring period.
Analytical results from the present research provide a reasonable conclusion that increasing the thickness of the secondary catalyst layer another 1.5 times to 90 mm may lead to additional 45 % reduction of N2O emissions and that is a
subject of further scientific research. Effective N2O emission reduction at nitric acid production is a contribution to combatting the adverse changes of the climate system and a step towards accomplishing the ambitious aim of the European Union for 50 % reduction of global greenhouse gas emissions by 2050 compared to their 1990 level.
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AUTHORS’ PROFILE
Maya Stefanova is a post-graduate at the Department of Ecology and Environmental Protection, Technical University Varna, Bulgaria. She is doing her PhD degree in Greenhouse Gas Emission Reduction, focusing on catalytic reduction of nitrous oxide emissions from nitric acid production. Since 2011, she has published several research articles in scientific journals. Total Impact factor: 8.164.
Rozalina Chuturkova is an Associate Professor at the Department of Ecology and Environmental Protection, Technical University Varna, Bulgaria since 2002. She did her PhD degree in Toxicology in 1985 at the National center of hygiene, medical ecology and nutrition, Sofia. Currently she is a lecturer on Air Pollution and Waste Gas Treatment, Management and Quality of Ambient Air, Industrial Impact upon the Environment, Ecotoxicology. She has participated in multiple environmental projects at Technical University Varna and one joint venture project along with University of Cincinnati. R. Chuturkova has published over 100 research articles and has patented 2 innovations and several standards. Total Impact factor: 11.409, h-index: 2.
GSTF Journal of Engineering Technology (JET) Vol.3 No.2, July 2015
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