Scientific Journals
Zeszyty Naukowe
Maritime University of Szczecin
Akademia Morska w Szczecinie
2011, 28(100) z. 1 pp. 27–33 2011, 28(100) z. 1 s. 27–33
Degrading mechanisms of selective catalytic reduction
systems
Mechanizmy degradacji systemów selektywnej katalitycznej
redukcji (SCR)
Jerzy Herdzik
Gdynia Maritime University Akademia Morska w Gdyni
81-225 Gdynia, ul. Morska 81/87, e-mail: georgher@am.gdynia.pl
Key words: emission control, tier 3, selective catalytic reduction, degrading mechanisms of SCR Abstract
It was presented problems of fulfillment the IMO tier 3 requirements. The proposition of using selective catalytic reduction was very interesting, but provided to many maintenance problems due to the degrading mechanisms of that systems. Some of those mechanisms were presented. It provided to degradation of those systems and worsening their emission reduction possibilities. The certificate of EIAPP may not tell the truth. It needs periodical surveys (minimum one per year) for checking the validity of issued EIAPP certificate. A SCR system would be activated only on restricted areas of environmental emission control due to maintenance costs.
Słowa kluczowe: kontrola emisji spalin, wymogi tier 3, selektywna katalityczna redukcja (SCR),
mechani-zmy degradacji systemów SCR
Abstrakt
W referacie przedstawiono problemy spełnienia wymagań Międzynarodowej Organizacji Morskiej (IMO) odnośnie wymagań ograniczenia emisji toksycznych składników spalin z silników okrętowych. Propozycja użycia metod selektywnej katalitycznej redukcji (SCR) jest bardzo interesująca, ale prowadzi do wielu pro-blemów w eksploatacji z powodu mechanizmów, które degradują systemy SCR. Niektóre z nich zaprezento-wano w referacie. Degradacja systemów SCR powoduje pogorszenie parametrów ich pracy oraz spadek zdol-ności do zmniejszania emisji szkodliwych substancji w spalinach. Certyfikat EIAPP może nie potwierdzać rzeczywistych osiągów systemów SCR. Wymagać to będzie okresowych przeglądów potwierdzających speł-nianie wymagań. Z powodu dużych kosztów eksploatacji systemów SCR będą one wykorzystywane jedynie na obszarach, na których zachodzi konieczność ich użycia, natomiast poza tymi obszarami będą one wyłącza-ne. Pracować będą na systemach omijających systemy SCR.
Introduction
An introduction to emissions from combustion engines is given, with focus of the real pollutants as NOx, SOx, CO, HC and PM. CO2 has not been
defined as a real pollutant as it’s not itself toxic, but still is a major concern due to its contribution to the greenhouse effect. The real pollutants only repre-sents about 0.3–0.6% of the total emissions from a combustion engine.
Emissions from ships have been of common interest for several years (marine transport in
Europe produced about 4% of total emission), and various methods for emission reduction have been developed in recent years through research and development from engine manufacturers and specialized companies. This is due to international and national authorities’ respond to concern over air pollution. Emissions from marine Diesel engines consist of nitrogen, oxygen, carbon dioxide and water vapour. Smaller amount of carbon monoxide, oxides of sulphur and nitrogen and particulate material are also present, and the ones which should be reduced due to their harmful effect. An example
Jerzy Herdzik
of emission from engine process was shown in figure 1.
Fig. 1. Emission from engine process for two stroke low speed engine [1]
Rys. 1. Emisja z procesu spalania w wolnoobrotowym silniku dwusuwowym [1]
The amount of SOx produced is equal to the
sulphur content of the fuel burned, and mostly com-prise sulphur dioxide with some sulphur trioxide. Thus, the combustion process cannot control the amount of SOx produced. Sulphur oxides have an
unpleasant odour and are the number one contri-butor to acid rain, and once emitted SOx can be
carried over a large area in the atmosphere before it’s deposited in lakes and streams, reducing their alkalinity [1, 2]. SOx can be reduced as mentioned
easily with burning bunkers with low sulphur content. For example with sulphur content of about 3 per cent in the fuel, this leads to 64 kg of SOx per
ton fuel burned. If this is reduced to 1 per cent, the emitted SOx is about 21 kg per ton burned, which is
a significant reduction. This is the best way of SOx
reduction: no sulphur in burned fuel, no sulphur oxides in exhaust gas, and no maintenance pro-blems with SCR systems. Some methods for after treatment of the exhaust gas in order to reduce SOx
emissions exist, where the most commonly used is a scrubber. When a SCR system is installed with the purpose of reducing NOx, the sulphur content of
the fuel is important in order to reduce the downgrading of the catalyst, and for regulation of the system.
Selective Catalytic Reduction for nitrogen oxides
Nitrogen oxides (NOx) are generated during the
combustion from nitrogen and oxygen at high temperatures in the cylinder. NOx are of special
interest of authorities due their possible carcino-genic effect, contribution to photochemical smog formation over cities and acid rain. The major influences of the formation of NOx in combustion
engines are the temperature and concentration of
oxygen in the combustion. Also the residence time of the combustion plays a role, thus high tempera-ture and long residence time will increase the amount of NOx. This is the reason for why low
speed two stroke engines generate a larger amount of NOx, than medium speed four stroke engines.
The possibilities of NOx emission reduction was
presented in figure 2.
Particle Material Emission
Particle Emissions can be divided into three main components: Soot, Soluble Organic Fraction (SOF) and Inorganic Fraction (IF). Most particulate material results from incomplete combustion of the hydrocarbons in the fuel and lube oil [1, 3]. More than 50 per cent of the total PM emissions are soot, which is the visible black smoke. PM are particles as small as 10 nm, and 90 per cent of the total particulate materials are smaller than 1 μm [3]. The formation process of PM is dependent of many factors:
the combustion and expansion process, fuel quality (sulphur and ash content), lubrication oil quality and consumption, combustion temperature,
exhaust gas cooling.
In recent years, the harmful effect of particulate matter has been a topic for discussion. However, it is now documented that inhaling of these particles may be a cause to premature death, asthma and lung cancer, and other cardiovascular issues. This is one of the reasons for the recent growth in research about how to reduce these emissions.
The soot fraction of PM is the visible smoke in the exhaust and is made up of carbonaceous material originating from the fuel and lube oil. Soot formation takes place in a Diesel combustion pro-cess between about 1000 K and 2800 K, and pressure about 5–10 MPa. The time available for the particles to form is within milliseconds, and the growth can be separated into two stages. The first stage is for the particles to form, and these particles are very small (less than 2 nm in diameter). The second stage is where the particle grows, which in-cludes surface growth, coagulation and aggregation. The formation process of particle material was shown in figure 3. The particle materials may be the cause of increasing the pressure drop on the SCR filter.
Degrading mechanisms of SCR systems Selective Catalytic Reduction (SCR) is a system that reduces the NOx in the exhaust gas into
harm-less nitrogen (N2) and water (H2O). This is done by
HEAT ENGINE PROCESS WORK AIR FUEL LUBE 8.5 kg/kWh 175 g/kWh 1 g/kWh 21% O2 79% N2 97% HC 3% S 97% HC 2.5% Ca 0.5% S EXH. GAS 13.0% O2 75.8% N2 5.6% CO2 5.35% H2O 1500 ppm NOx 600 ppm SOx 60 ppm CO 180 ppm HC 120 mg/Nm3 Part
adding a reduction agent as urea or ammonia to the exhaust flow. This system is placed under the post-treatment category of reduction measures for NOx
and works independent of the combustion process. Depending on different engine types and parame-ters, the SCR unit in general will have reduction
efficiency of more than 90%. The efficiency are directly linked to the amount of urea added to the exhaust gas flow, thus, in theory the system are capable of reducing all NOx, however, then with
high risk of a considerable amount of ammonia slipping through the system. For urea, the
stoichio-Category Measure Technology
Pre-treatment Pre-treatment Pre-treatment Substitute fuel Combustion Scavenging Water injection into cylinder Water addition Water addition Emission de-NOx Methanol LNG Emulsified fuel
Fuel injection timing retard Lean combustion Rich Combustion
Pre-chamber type combustion Fuel valve nozzle spec. modification
Scavenging air cooling
Water mixture (independence valve) High pressure of fuel injection
Water mixture (mixed valve) Water mixture into suction air De oxidised furnace Exchange gas recirculation Selective catalytic reduction Catalytic decomposition Fig. 2. NOx emission control alternatives [4]
Rys. 2. Możliwości ograniczenia emisji NOx [4]
Fig. 3 Formation process of PM [3] Rys. 3. Proces tworzenia cząstek stałych [3]
Nucleation Surface growth Agglomeration Adsorption and condensation Hydrocarbons Dehydrogenation Oxidation Dehydrogenation Oxidation Dehydrogenation Oxidation Dilu ti on tu nn el Cy li nd er Ti m e
Jerzy Herdzik
metric reaction that takes place in the reactor is as follows: 4NO + 2(NH2)2CO + O2 = 4N2 + 4H2O + 2CO2 (1) where: NO – nitrogen oxide, (NH2)2CO – urea, O2 – oxygen, N2 – nitrogen, H2O – water, CO2 – carbon dioxide.
The most preferred reduction agent is ammonia in the form of a 40% urea solution of urea in water as urea is a stable and safe to store onboard, as opposed to ammonia. The urea is hydrolyzed to ammonia NH3, as the urea evaporates moisture with
the heat of the exhaust gas. The reaction is as follows:
(NH2)2CO + H2O = 2NH3 + CO2 (2)
A Selective Catalytic Reduction system represents a large additional investment and some additional operating cost. The operating cost in-cludes maintenance and cost of urea. Approxima-tely the capital cost is 30–50 €/kW and operational expenditure 5–8 €/kWh [5]. A layout of SCR system was presented in figure 4. The catalyst is located in the exhaust system after the mixing of the exhaust gas and the urea solution. The reduction reaction with catalyst was presented in figure 5. The sulphur content will also have an influence on the temperature needed for the reaction to take
place. The needed temperature will increase rapidly with the sulphur content.
The needed minimum temperature at SCR inlet to avoid ammonia sulphate formation was pre-sented in figure 6. For fuel below 0.5% of sulphur, the needed temperature is below 290C and in-creases due to sulphur content up to 340C. If the temperature is too high, the ammonia (urea solu-tion) will burn rather than react with the nitrogen oxides, and if it is too low the reaction rate will be too low. This will result in a condensation of am-monia sulphate and amam-monia bisulphate that will destroy the catalyst [4].
Fig. 5. SCR system – reaction with catalyst [6]
Rys. 5. System SCR – reakcja w obecności katalizatora [6]
The pressure drop will increase across the catalyst and eventually block the flow.
Fig. 4. SCR system layout for two stroke engine [5]
Fig. 6. Needed minimum temperature at SCR inlet to avoid ammonia sulphate formation [4]
Rys. 6. Wymagana minimalna temperatura na wlocie do sys-temu SCR zapobiegająca tworzeniu się siarczanu amonu [4]
A bypass valve will always be installed to bypass the whole SCR system if needed, or in case of emergency. As the reduction rate of the SCR system makes it by far the most effective system on the market today we tend to see that the vessels with SCR installed, mostly are vessels with special emission limits requirements. The most common use of the system for these vessels is to activate the SCR when needed due to special requirements and shut it down when no special regulations are present. This will reduce the operational costs of urea consumption and reduce degrading of SCR system and will be normal maintenance procedure [4]. So, the bypass valve on SCR system will be still opened, only will be shut when should be needed due to local area regulations.
The soot blower is an important function of the SCR system. The soot blower contributes to maintaining the catalyst throughput holes clean from soot and other deposits. At the catalyst cabinet there are installed air jets, which inject air at high pressure into the catalyst. The cleanness of the catalyst is of importance to its efficiency, and therefore the soot blower is indispensable. How-ever, there is still little knowledge about the effect of the soot blowing when it comes to emissions of particles, but this is a problem for solving.
The control unit is the SCR system’s brain and consists of a process computer and an ammonia dosing unit. The computer takes input data from different measuring units in the system and from the engine. This data is again processed and gives input to the dosing unit that regulates the amount of ammonia fed into the exhaust gas flow. The most important input parameter fed into the control unit is engine load. The correspondence between engine load and NOx are measured in the engine testbed.
Based on these results, the process computer
controls the ammonia feed rate. Too low feed rate results in poor NOx conversion, too high in
ammonia slipping through the catalyst referred to as slip. Usually, there are no continuous feed of NOx
reduction data fed into the control unit during operation. The other parameters that are read by the control unit is the inlet and outlet temperatures of the catalyst [4].
Temperature as an indicator of degrading SCR systems
It was done a test with the engine operating on “white Diesel”, which is Diesel with 30% water added, resulting in a mass factor of water at 23.1% [4]. Water addition is one of the primary methods for NOx reduction and involves lowering the temperatures in combustion with the heat capacity of the water “stealing” heat during the compression phase. The rule of thumb is that one percent water equals one percent reduction in NOx emitted. The effects of NOx reduction was presented in table 1 with using of 7.5 dm3/h of urea in SCR in two directions (downstream and upstream).
Table 1. The effects of NOx reduction on MGO and “white
Diesel” at 50% load with 7.5 dm3/h of urea [4]
Tabela 1. Efektywność redukcji NOx w silniku zasilanym
lekkim olejem napędowym oraz „białym dislem” przy obcią-żeniu silnika 50% oraz dawką mocznika 7,5 dm3/h [4]
Parameter Marine gas oil MGO
Marine gas oil with 30%
of water NOx (SCR downstream) [ppm] 973 852
NOx (SCR upstream) [ppm] 73 7.2
NOx reduction [%] 93 99
The outlet temperature of the catalyst is higher than inlet. The increase in temperature over the catalyst can be explained by the exothermic reac-tion that takes place in the catalyst. As the reacreac-tion takes place at the catalyst surface, we find reason to believe at this point that the heat is exchanged from the catalyst wall to the exhaust gas. The probability for degrading of the catalyst due to thermal strain in ceramic material is limited [4].
It was seen the connection between catalyst activity and temperatures. It was assumed that the activity of NOx reduction is reduced when they had
deposits in the catalyst and it was varied the activity. It was decided to vary the feed rate of urea. After calibrating the urea control system, the feed rate was 7.5 dm3/h at 50% load. The feed rate could
be adjusted by changing a k-factor that for 7.5 dm3/h is 100%, and they adjusted it by setting the
Jerzy Herdzik Table 2. k-factor of urea at normal operation and with bleeding
compressor at 50% load [4]
Tabela 2. Współczynnik k dla określenia dawkowania mocznika w czasie normalnej pracy sprężarki i z pogorszonym chłodzeniem powietrza doładowującego przy obciążeniu silni-ka od 50% do 110% [4] Normal operation Bleeding compressor Normal operation Bleeding compressor 100% 100% 70% 60% 95% 90% 60% 50% 90% 80% 50% – 80% 70% 110% 126%
The effects of NOx reduction depending on urea
feed rate was presented in figure 7. It must be seen that the bigger urea feed rate provides to better reduction (but it may be cause of overdosing the urea into the catalyst) and the reduction efficiency at the beginning and the end of tests are almost the same.
For the temperatures, if they are connected to the activity, we should according to the hypothesis see a fall in Tcenter together with the fall in urea feed
rate as the NOx reduction rate declined accordingly.
As shown in figure 8, this is not the case as Tcenter is
close to constant during the feed rate drop.
Earlier test indicates that the SCR system shall
have a NOx reduction efficiency of about 90%, and
the last test gave a reduction of about 70%. This is far from the desired results. The theory is that we should be able to reach a reduction efficiency of about 95%. The reduction efficiency depends on the amount of urea introduced and the exhaust gas temperature. The catalyst (based on TiO2, vanadium
and tungsten) will have highest efficiency with temperatures higher than 340C.
EIAPP Certificate
The EIAPP Certificate is issued by the authori-ties. The “Technical file” is needed to obtain an International Air Pollution Prevention certificate. It needs to provide a Statement of Compliance (SOC) from an authorized classification society and NOx measurement report, showing that the engine
complies with the new NOx limits (according to
IMO regulations: tier 1, tier 2 or tier 3) [6, 7, 8]. The NOx measurement report specifies:
engine speed and torque for power calculations; fuel consumption for calculations according to
ISO 3046-1;
temperatures of charge air, exhaust gas, cooling water and lubricating oil;
a) b)
Fig. 7. Urea feed rate and NOx reduction efficiency (activity) [4]: a) normal operation (λ = 3.1), b) bleeding compressor (λ = 2.98)
Rys. 7. Efektywność redukcji NOx (aktywność katalizatora) w zależności od ilości podawanego mocznika i współczynnika nadmiaru
powietrza (stanu technicznego sprężarki) [4]: a) praca normalna (λ = 3,1), b) brak chłodzenia powietrza doładowującego (λ = 2,98)
a) b)
Fig. 8. Temperatures logged and SCR efficiency [4]: a) beginning, b) end
Rys. 8. Mierzone temperatury na dolocie, w środku i wylocie z katalizatora oraz sprawność redukcji NOx przez system SCR [4]: a) na początku badań, b) na końcu badań
pressures and humidity in the charge air system; exhaust gas composition with regard to NOx,
CO, CO2 and O2;
the measurements will be performed in accor-dance with the Technical Code for a number of different load conditions corresponding to the application category [8].
The emission measurements ought to meet the IMO Annex VI of Marpol 73/78 Regulations for the Prevention of Air Pollution from Ships, NOx
Technical Code, ISO 8178-4. This code demands certified equipment, accurate calibrations, correct measurement procedures, and skilled personnel [9]. Conclusions
The SO2 to SO3 conversion rate is one of the key
elements that set the basis for degrading of the catalyst. The SO3 conversion rate is mostly
dependent on sulphur content in the fuel and amount of vanadium in the catalyst’s honeycombs.
The temperature variations in the catalyst could be significant enough to cause fatigue in of the honeycombs due to thermal load. To verify ΔT (temperature difference between inlet and outlet of the catalyst wall) as a reliable indicator, we must focus further on the development of T2 as we get
deposits in the catalyst and reduced activity, where it should decrease with decreased activity.
The activity and formation of deposits have a positive development with increased temperature.
A condensation of ammonia sulphate and ammonia bisulphate may destroy the catalyst. To avoid the blocking and to prolong the period of
operation, the SCR system would be used only when it was needed.
References
1. WOODYARD D.: Pounder’s Diesel Engines and Gas Turbines. B&H Ltd, 9th Edition, 2010.
2. ME_GI Dual Fuel MAN B&W Engines – A Technical, Operational and Cost-effective Solution for Ships Fuelled by Gas. MAN Diesel & Turbo, Copenhagen 2010.
3. HEYWOOD J.B.: Internal Combustion Engine Fundamentals. McGraw-Hill, 1988.
4. SELAS M.: Exhaust Gas Cleaning with Selective Catalytic Reduction. Master Thesis, Trondheim, Norway 2010. 5. SCHMID H.: Weisser G. Marine Technologies for Reduced
Emissions. Conference on Green Ship Technology, Amsterdam 2005.
6. Tier III Compliance – Low Speed Engines. MAN Diesel & Turbo, Copenhagen 2010.
7. How to obtain the IMO NOx Emission Certificate. MAN B&W, Copenhagen 2010.
8. EIAPP Certificate. Wartsila Corporation, Vaasa Finland, 2004.
9. Advanced Emissions Monitoring for the Maritime Industry, ET Marine, London 2010.
Other
10. Exhaust Gas Emission Control Today and Tomorrow. Technical Report, MAN B&W, Copenhagen 2009.
The paper was published by financial supporting of West Pomeranian Province
