Optimisation of ammonia injection for an efficient nitric oxide reduction

Transcription

1 Energy and Sustainability II 481 Optimisation of ammonia injection for an efficient nitric oxide reduction S. Ogriseck & G. P. Galindo Vanegas Infraserv GmbH & Co. Hoechst KG, Frankfurt am Main, Germany Abstract Dry flue gas cleaning systems using activated coke as a catalyst reduce sulphur dioxide and nitric oxide emissions. The sulphur dioxide reduction in this system has an efficiency of over 98%. Therefore, the main purpose is the optimisation of the NOx reduction process by means of ammonia injection optimisation. Such a flue gas cleaning system is currently in operation at the Industriepark Hoechst site in Germany. This paper shows the analysis of the ammonia load on activated coke and denitrification experiments in laboratory scale under variation of different parameters. The results of the ammonia loading tests show a relation between temperature and loading capacity. The tests were driven for temperatures between 110 and 469 C. The denitrification tests pointed out that the highest efficiency is obtained with the activated coke loaded at the highest temperature at 469 C. Keywords: ammonia, activated coke, nitric oxides, denitrification, dry flue gas cleaning, adsorption. 1 Introduction The maximum emission limits for nitric oxides (NOx) in Germany are set in the Ordinance on Large Combustion Plants and Gas Turbine Plants, 13th BImSchV. For power plants with a thermal input of more than 100 MW, the NOx emission limit value is of 200 mg/m³ (STP) dry basis measured as NO2. A new ordinance for the assurance of air quality requirements (37th BImSchV) has been proposed. One of the focus points of this regulation is to reduce the amount of emitted nitrogen oxides and for this reason sets up a new limit for new plants, starting operation on the 31st of December 2012, to 100 mg/m³ (STP). The decrease in the emission limit allowance for existing plants is very likely to happen due to doi: /esu090441

2 482 Energy and Sustainability II the everyday increasing efforts of the governments to protect and improve the environment. This work will expose the evaluation of the possibilities to optimise the existing flue gas cleaning system at the combined heat and power plant (CHP) at Infraserv Hoechst, Germany to ensure a more efficient NOx emissions reduction without a complete change in the process. 2 Description of the power plant Infraserv Hoechst is the operator and service provider of the Industriepark Hoechst in Frankfurt am Main, Germany, one of Europe's largest production and research sites. As part of its wide range of services Infraserv Hoechst is in charged of the production, purchase, management and distribution of electrical energy, heat and process steam. In site, Infraserv Hoechst operates a combined heat and power plant fueled with hard coal and natural gas. Flue gas to the stack Flue gas from the boilers Classifier to Firing Desorber Fuel Burner Activated coke SO2 Rich gas 3 to Rich gas cleaning 4 Water 2 Air 5 Nitrogen Injection cooler Ammonia Adsorber 1 Sieve 1 First Stage 2 Second Stage 3 Heating Zone 4 Degassing Zone 5 Cooling Zone AC to by out Figure 1: Modified BF/Uhde Process at Infraserv Hoechst. The CHP plant has a thermal capacity of 750 MW and an electricity capacity of 160 MW. The plant consists of four boilers with a steam capacity of 830 t/h together. The control of the emissions of nitrogen oxides in the plant is specially achieved by the employment of primary measures. The secondary measures for the control of nitrogen oxides emissions are not a main use. The flue gas from the two coal boilers containing predominantly nitrogen and sulphur oxides is cleaned after combustion by means of a simultaneous desulphurisation/

3 Energy and Sustainability II 483 denitrification dry flue gas cleaning system based on the adsorption and catalytic reaction with adsorbed ammonia on activated coke (AC), operating under the principles of the Bergbau-Forschung-Uhde process [1], Figure 1. According to the mass balance of the plant, the losses of the ammonia that is injected on the activated coke for the catalytic reduction of NOx in the system are considerable. This is why the main focus on the efforts directed to increase the efficiency of the system were set on the optimisation of the ammonia injection. The flue gas cleaning system in the plant was designed for a simultaneous desulphurisation and denitrification of the flue gas produced during combustion of coal. In the past, the direct injection of ammonia was stopped due to the clogging of the system caused by the formation of ammonium chloride. Today, only primary measures are applied in the combustion to ensure the emission limits of 200 mg/m 3 (STP), dry basis. The initial design of the Bergbau- Forschung-Uhde process was modified in such a way that the ammonia is injected in the system to load the activated coke before entering the adsorber unit. NH 3 loaded activated coke Flue gas from Boilers Adsorber NH 3 slip to stack Desorber NH 3 slip Rich Gas Cleaning Ammonia input Figure 2: Ammonia flow. 3 Current ammonia injection The current situation of the ammonia mass balance, Figure 2, was analysed and also the losses as an ammonia slip with the flue gas or with the rich gas. The reduction of the ammonia losses should be executed by the optimisation of the ammonia injection. This optimisation should, on the other hand, ensure a better adsorption of the ammonia on activated coke with the aim to both increase the denitrification capacity and decrease the ammonia losses of the system.

4 484 Energy and Sustainability II The total mass balance showed that 60% of the ammonia fed is being lost part as ammonia slip (27%) and part with the rich gas (33%). Only 40% of the ammonia is being used for the reduction of the nitric oxide. Longer residence times at the NH3 injection point increase the probability of the adsorption and reaction of the ammonia with the nitric oxide and could avoid the ammonia slip to the rich gas. This way the residence time can increase the efficiency for the nitric oxide reduction, as is the case of this system. The influence of the residence time has already been discussed by [2]. 4 Experimental investigations of ammonia load and denitrification 4.1 General overview In order to evaluate possible process modifications for the above mentioned optimisation, the analysis of the system and the information obtained through the literature research were used to set a plan for laboratory scale tests. The experimental work done in a laboratory test plant was divided into two groups. The first group was the ammonia loading test, focused on the evaluation of the ways to optimise the ammonia loading. The optimisation of ammonia loading was tested using two variables: the activated coke type (different sulphur content) and the loading temperature. The second group was the denitrification test which aim was the determination of the nitric oxide reduction efficiency with some of the previously loaded activated cokes. Two samples of activated coke, one before and one after the desorber, were extracted from the plant. An additional activated coke mixture sample was produced. This sample was composed of 50% vol of activated carbon before desorber and 50% vol of activated coke after desorber and represents a point in between the points before and after desorber. A reference gas for the tests was set according to the real operating conditions of the flue gas cleaning system at Infraserv Hoechst. In the case of the denitrification tests, the flue gas composition entering the denitrification stage of the flue gas cleaning plant had to be measured. The mass of loaded ammonia was calculated by the breakthrough time. As a way to verify the results of the calculation of the mass of ammonia adsorbed on the activated coke, a test under the method for the determination of the Kjedahl- Nitrogen was done on the samples. The Kjedahl-Nitrogen under the ISO 1126 [3] is a measurement of the total nitrogen content as: ammonium nitrogen, nitrate nitrogen, nitrite nitrogen and organical bounded nitrogen. The samples where tested under this method to determine the amount of nitrogen on them and its increase or decrease after treatment, as a reference to determine the amount of ammonia physisorbed or chemisorbed. A sample taken from each of the tests after the loading was finished (that means 10 minutes after breakthrough time) was analysed under this method and as a result the total Kjedahl-nitrogen contents were obtained.

5 Energy and Sustainability II Test results of ammonia load The ammonia loading tests (see Figure 3) proposed that the highest ammonia loading was obtained for the activated coke before desorber, compared to all of the other samples, even those loaded at high temperatures. The results of the ammonia loading tests show a relation between temperature and loading capacity. The tests were driven for temperatures between 114 and 469 C. Calculated with the breakthrough time this relation shows a decrease in the ammonia loading capacity for the temperatures between 114 and 250 C and a posterior increase for temperatures between 250 and 469 C, Figure 3. 1,6 1,6 1,4 1,4 Loading (% ma NH3) 1,2 1,0 0,8 0,6 0,4 Highest Efficiency Highest Quantity 1,2 1,0 0,8 0,6 0,4 Kjedahl-Nitrogen (% ma) 0,2 0,2 0,0 0, Loading Temperature ( C) AC after desorber (breakthrough) AC before desorber (Kjedahl-N) Polynomial (AC after desorber (Kjedahl-N)) AC mixture (Kjedahl-N) AC after desorber (Kjedahl-N) Polynomial (AC after desorber) Figure 3: Ammonia loading and Kjedahl-Nitrogen results. The increase in the load capacity of ammonia on activated coke until 250 C (on line calculated with breakthrough time) can be explained with dominant physisorption which state that the increase in temperature decreases the capacity of the adsorbent (activated coke) to trap the adsorptive (ammonia), explained also in the fact that adsorption is an exothermic process. A similar result was obtained by Rodriguez [4] on studies done for the ammonia adsorption in a fixed bed of activated carbon for temperatures between 40 and 120 C. It was found that with increasing testing temperature the ammonia breakthrough time was smaller. According to the literature (e.g. [5]) the behaviour of increase at temperatures above 250 C is caused by the temperature-activated chemisorption of the ammonia on the activated coke. The Kjedahl-Nitrogen tests confirmed that the highest ammonia load is achieved at the highest temperatures, Figure 3. Regarding the activated coke type variable it was determined that the activated coke sample before desorber had a higher ammonia loading capacity

6 486 Energy and Sustainability II than the sample after desorber and this was explained with the existence of already adsorbed species that can react with the ammonia and delay the breakthrough time. The comparison of the two polynomials in the above figure results in the determination of a region of highest efficiency and a point of highest quantity. The highest efficiency region represents the temperature at which, the relation between the amount of nitrogen introduced to the amount of ammonia used is highest. The highest loading, referred as highest quantity, was in any case achieved at the highest temperatures. 4.3 Test results of denitrification The denitrification process works by the reaction of the nitric oxide with ammonia in the presence of oxygen, as expressed in Equation (1). 4 NO + 4 NH3 + O2 4 N2 + 6 H2O H R = -1628,36 kj/mol (1) This equation has been referred in the literature to represent the reaction happening on the surface of the activated coke when this last is used as catalyst. The comparison between the different denitrification tests will be focused on the tendency of its efficiency for the removal of NO from the flue gas. The efficiency was calculated as the relation between the exit concentration and the concentration after 300 minutes (c300), that is c/c300. The results of this analysis are shown in Figure 4. 1,0 0,9 0,8 0,7 0,6 c/c 300 0,5 0,4 0,3 0,2 0,1 0, Time (min) AC after desorber withot NH3 loading AC before desorber, NH3 load at 116 C AC mixture, NH3 load at 116 C AC after desorber, NH3 load at 115 C AC after desorber, NH3 load at 469 C D-ABD-O2ausfall Figure 4: Denitrification results. The denitrification tests pointed out that the highest efficiency is obtained with the activated coke loaded at the highest temperature at 469 C. This result expresses a higher nitric oxide reduction efficiency than that of the activated coke test representing the current situation, ammonia loading at 110 C, Figure 3.

7 Energy and Sustainability II 487 For the test with AC mixture and NH3 load at 116 C the dotted section represents an oxygen stoppage, caused by a technical failure. The oxygen concentration is a factor affecting the reduction of NO with ammonia on carbonaceous adsorbents. It has been probed that the oxygen content in the gas stream affects the behaviour of this reduction. In literature it is found that in the absence of oxygen, the NO conversion through reduction with ammonia on activated carbon is only 15% [6]. These effects gain importance for contents of less that 3% of oxygen. The sudden increase in the c/c300 ratio in Figure 4 shows a reduction in the efficiency caused by the absence of the oxygen. Once the technical problem was corrected and the oxygen was normally flowing, the efficiency increased again and the gradient of the curve was decreased. 5 Summary The combined loading and denitrification conclusions suggest that the injection of ammonia at temperatures over 350 C will result in the optimisation of the process. The increase in the efficiency of a flue gas cleaning system is in every case important to permanently reduce the influence of pollutants on the environment. The investigations at Infraserv Hoechst make a contribution to improve the dry flue gas cleaning process for desulphurisation and denitrification under the use of activated coke. The results of the work represent that the efficiency of ammonia adsorption on activated coke and subsequent denitrification can be enhanced with some process changes. It is planned, that with further investigations the founded results can be verified and a technical concept for the realisation can be finalised. References [1] Richter, E., BF/Uhde/Mitsui-Active Coke Process for Simultaneous SO2- and NOx-Removal, in Sulphur Dioxide and Nitrogen Oxides in Industrial Waste Gases: Emission, Legislation and Abatement, Springer, [2] Knoblauch, K., Richter, E., Jüntgen, H., Application of Active Coke in Processes of SO2- and NOx-removal from Flue Gases, Fuel, Vol. 60, pp , [3] International Norm ISO 11261:1995. Soil Quality Determination of Total Nitrogen, Modified Kjedahl Method, [4] Rodrigues, C. C.; et al., Ammonia Adsorption in a Fixed Bed of Activated Carbon. Biore-source Technology, Vol. 98, pp , [5] Guo, J., et al.: Adsorption of NH3 onto Activated Carbon Prepared from Palm Shells Impregnated with H2SO4, Journal of Colloid and Interface Science, Vol. 281, pp , [6] Jüntgen, H., Kühl, H., Richter, E., Katalytische Reaktionen an Aktivkoks und Aktivkohle zur Entfernung von SO2 und NOx aus Rauchgasen, DECHEMA- Congress, Frankfurt am Main, February 1987.