Reliability straight down the line.

Transcription

1 PROMOTING ON-SITE POWER AND COGENERATION WORLDWIDE INDEPENDENT POWER MAY Reliability straight down the line.

2 NOx reduction system for diesel engines today & tomorrow NOx reduction systems on engines are becoming mandatory in many regions around the world. Fortunately, technical developments have increased efficiency and reduced complexity and cost of the DeNOx system, reports Henrik Trolle Jacobsen of Haldor Topsøe A/S. Nitrogen oxides, NO and NO2 (commonly referred to as NOx), are hazardous pollutants formed in combustion processes. Human health and crops are seriously affected by NOx emissions and the resulting external cost for the society is as high as 12 /kg NOx emitted to the atmosphere (Mike Holland et al. 2005, cba.org/assets/marginal_damage_03-05.pdf). Diesel engines' share of total emitted NOx constitutes up to 44% in some regions and the percentage is increasing as emissions from other sources are being cut down rapidly. NOx emission from a diesel engine is typically 12 kg/mwh but can reach up to 18 kg/mwh. In comparison, the NOx emissions from a coal-fired power plant is typically around 2 kg/mwh electric power before NOx reduction equipment and kg/mwh on the majority of plants in the developed world who has applied reduction equipment. Naturally, environmental regulations are becoming increasingly strict as it is realised that NOx emissions from engines contribute significant damage to human health and considerable cost to society and this forces independent power producers to look for effective solutions to cut their NOx emissions. Fortunately, several technical solutions have been developed over the past 30 years and one technology in particular has prevailed as being effective and reliable without inducing any noticeable fuel penalty - the Selective Catalytic Reduction DeNOx system, the SCR. The SCR process is based on the reaction of NOx with ammonia over a catalyst to form harmless elemental nitrogen and Figure 2. Data from an IMO Genset SCR Test cycle Figure 3: Zoom in on data in the period when low is reduced in Fig. 2. water vapour. The reduction of the NOx takes place by injecting an ammonia source as reducing agent in the form of anhydrous ammonia, aqueous ammonia or an aqueous solution of urea into the exhaust gas. Urea solutions are preferred for most engine applications because it, contrary to NH3, is harmless, easy to handle and available everywhere in the world. As shown in Figure 1 (on page 23), the urea solution is injected upstream of the catalyst. Urea will decompose to ammonia (NH3) in the hot gas and subsequently the NH3/exhaust gas mixture passes through the catalyst, where the NOx is converted. The NOx, which primarily is present as NO and only to a minor extent as NO2, is converted according to the following reaction schemes: 4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O NO + NO2 + 2 NH3 -> 2 N2 + 3 H2O As can be seen from the schematics, the conversion of NOx does not create any secondary pollution as the products formed are only nitrogen and water vapour, which are already present in the atmosphere in large amounts. W ORLDWIDE I NDEPENDENT P OWER MAY

3 The NOx/NH3 reaction will take place without assistance from a catalyst at temperatures above 800 C but by using a catalyst the reaction can take place at temperatures between 170 C- 575 C with the most efficient temperature range being from 360 C C. The urea consumption is 1.6 kg of 40% urea-water solution per kg of NOx removed. In addition to the reduction of NOx, up to 35% of particulate matter (PM) and up to 90% of hydrocarbons is removed from the exhaust gas when passing through the catalyst. For both 2-stroke and 4-stroke diesel engines the SCR reactor can, in principle, be installed either before or after the turbocharger. However, the optimum catalyst operating temperature and potential problems with high concentrations of SO3 and NH3 at low temperatures will normally dictate that the reactor be installed before the turbocharger on 2- stroke low-speed engines and after the turbocharger on 4-stroke medium and high-speed engines. When choosing the catalyst for an engine application it is important to select a catalyst which has resistance to sulphur and other potential catalyst poisons in the fuel originating from the fuel and lubrication oil. Furthermore, attention should be addressed to the ability to endure vibrations and thermal shocks. Some power producers will also be able to benefit from choosing a catalyst with low mass that enables rapid start-up of the system. Especially for 2-stroke engines the SCR position before the turbocharger stresses the requirement for a catalyst installation with a low thermal capacity to avoid cold high-density gas harming the turbocharger. Several types of SCR catalysts are available on the market and most of them are based on vanadium and titanium oxides as the main active components. The catalysts are presented in four different forms: 1) Pellet-type catalysts where the active components are compressed to pellets that are placed in a perforated basket in the SCR reactor. The pellet types have superior DeNOx capabilities at temperatures below 170 C but the installation cannot tolerate any particles in the exhaust gas. 2) Plate-type catalysts made from a supporting structure of a wire mesh or metal plate on which the active components have been rolled. The plates are stacked in boxes which are placed in the SCR reactor. The plate-type catalyst has relatively low activity which is why it requires a large reactor, but it is believed to have a better ability to endure extreme dust loads. 3) Extruded catalyst blocks with a honeycomb structure, i.e. a geometry with many parallel channels through which the exhaust gas can pass. The extruded catalyst types are cheap to manufacture but suffer from high mass and fragility due to the dense and rigid structure of the material. 4) Corrugated honeycomb-type catalysts based on glass-fibre reinforced titania material in a corrugated structure which is homogeneously impregnated with active components, followed by a hightemperature calcination process. This catalyst type is characterised by having a high porosity, giving a correspondingly light product with a high surface area where the DeNOx process can take place. Furthermore, the core fibre structure ensures the capability to handle mechanical and thermal stress. All the SCR catalyst types are manufactured several places worldwide by various independent companies. The catalyst is built into a reactor and can often replace the engine silencer as the catalyst has an impressive sound attenuation capability. As seen in Figure 1 [see page 23] the SCR system as a whole is a relatively simple technology. Only the catalyst is a highly sophisticated product. Despite the overall simplicity of the SCR system several issues need to be considered in the design. It is necessary to evaluate the importance of both emitted excessive NH3, NH3 slip, and DeNOx % as there is a Honeycomb catalyst material. trade off between these two parameters. Low slip and high DeNOx % can be achieved simultaneously but this will require increased catalyst volume. To ensure a high level of NOx reduction and a low NH3 slip it is also important that the NH3 is well mixed into the exhaust gas before the catalyst, as the NOx and NH3 molecules need to meet for the reaction to take place. This requires knowledge about urea spray nozzles for homogeneous injection and considerations regarding the gas behaviour in the overall system layout. In many cases adequate NH3 mixing can only be achieved by use of a stationary mixer in the duct between the urea injection point and the SCR reactor. If the exhaust gas and NH3 are not sufficiently mixed more catalyst will be required to meet the required DeNOx efficiency and the system will eventually let too much NOx and NH3 pass untreated. To ensure correct dosing of urea the SCR system is equipped with a control system. There are different ways to design the control system and the choice of design is based on the following parameters: Required DeNOx efficiency, typical operation mode of the engine, the required documentation to the authorities and the maintenance schedule accepted by the operator. 20 M AY 2011 WORLDWIDE I NDEPENDENT P OWER

4 DIGITAL AIRLESS MULTIPOINT SCR SYSTEMS FOR POWER PLANT APPLICATIONS Digital Dosing System Ensures Precise, Dynamic Response to Engine Load Variations Airless TwinJet Nozzles Eliminate Need for Compressed Air and Mixer Compact Catalyst with Remarkable Noise Attenuation Replaces Traditional Silencer Advanced Control Software Ensures Optimal System Performance Up to 95% NOx-reduction Potential for 6-9% Saving in Fuel and CO 2 Emission Highly Corrosion Resistant Technology Tested by Leading Engine Manufacturers Unique Patent Pending Technology. PCT/DK2010/ Easy and Low Cost Installation Low Weight and Highly Compact System Robust System with Low Maintenance Requirements Remarkable Noise Attenuation Chosen and Implemented by the Royal Danish Navy DANSK TEKNOLOGI Østre Teglværksvej Allerød, Denmark ktl@dansk-teknologi.dk

5 G4100 NOx/O2 Analyzing System Cost-Effective Emission Control Green Instruments A/S offers a practical and direct in-situ gas analyzer for measuring NOx/O2 concentrations in the hot, wet gas. Key features: Latest zirconia sensor technology Low maintenance and low cost of ownership No sample lines, sampling systems, or converters Type approved Perfecting Sensible Technology SCR DeNOx x Systems Eco-NOx SCR systems s can satisfy most of NOx - CO O - particles removal requirements from m the exhaust gas generated by endothermic engines fed with different oils and gases. Eco-NOx basiically include the following equipments, as well as a complete set of instrum ments : The reagent Dosing Un nit with Control Box mounted on sk kid, The reagent Mixing Unit (duct) equipped with proprieta ary injection Lances, Nozzles and Turbulator, T The Static Reactor (h horizontal or vertical) fits with an n appropriate Horizontal Eco-NOx x system for exhaust gas volume of DeNOx an nd/or CatOx catalysts*) disposed d on various cleaning of 1 MW W MAN engine (palm oil) layers, plus a suitable soot-blowing s system if needed. *) Ecospray uses Haldor Top psøe tungsten trioxide (WO3) and va vanadium pentoxide (V2O5) DeNOx and a palladium oxide CatOx. Vertical Eco-NOx system for 1 MW Cummins engine e (rape-seed oil) durrbulator Ecospray Techn nologies s.r.l. Via Circonvalla azione, Alzano S. (AL) - ITALY pray.eu Lances exibits at Power--Gen EU 2011 Stand 4F15 - Milan, M June 7-9 Flow dŝɛƚƌŝďƶƚžƌɛ

6 In recent years, new NOx sensors, typically zirconia based, have become available for larger engines via the automotive industry. These sensors cost only a fraction of the price of traditional sensors based on sample extraction. This has opened up new designs of affordable control systems. Four different principles for control systems are illustrated in the boxes in Figure 1 and briefly described here below: 1). Feed forward only solution is preferred when the requested DeNOx efficiency is less strict or a relatively high slip of untreated NH3 can be accepted. As a part of the commissioning the engine's NOx production is recorded at different loads and RPMs and in some cases parameters such as fuel consumption and shaft torque are also recorded. The NOx flow at different loads is organised in an engine map which will contain a plot providing NOx flow as a function of engine load. The engine map and the feed-forward signal from engine are used by the control system to calculate the correct amount of urea to be injected into the exhaust gas stream. The advantage of this control solution is that it requires no investment and no maintenance in association with NOx sensors. However, this solution will not be able to adjust to engine wear and changes in ambient conditions that can both lead to variation in the NOx flow coming from the engine and it will not ensure that the urea is always fully utilised as it has to be designed with a slight overdosing to ensure the requested DeNOx efficiency is achieved at all times. 2). Feed forward with feedback adjustment enables precise dosing of urea under all loads and ambient conditions. It will also enable the detection of malfunctions - and some systems are selflearning, ensuring increasingly faster and more precise response to load changes even when system performance changes due to e.g. fouling of charge air cooler or reduced catalyst activity due to ageing. Furthermore, this control system is able to continue operation even if one of the signals is lost. This design is regarded by many as being the optimal solution for Figure 1: SCR system layout with different control designs. efficient high-performing systems. 3). and 4). Stand alone systems are control systems that are able to work without a signal from the engine and are based on either NOX conversion or NOx emission requirements. If not handled manually, all control systems are equipped with a temperature sensor which will detect when the gas is hot enough to start the DeNOx process. The capability of the control system is shown in Figure 2 which is a real-time plot of a recently completed DeNOx system test. This test was done with stand-alone NOx conversion control system, design 3. The figure shows a full day IMO test cycle for a genset application why it is done with fixed RPM. The figure includes values for engine load, NOx in and out values, NOx conversion degree (DeNOx %) and NH3 slip. The results in Figures 2 and 3 [see page 20] are from a SCR system designed for 10 ppm NH3 slip and 80% NOx conversion. It appears that the slip never exceeds 4 ppm during the test cycle. The design values will be met at the end if the catalyst lifetime which is typically 5 years for a well-designed system using high quality catalyst. The SCR system's ability to response to load changes depends not only on the control system but also on the design of the catalyst implemented in the system. Modern low-density catalysts are able achieve the required temperature for operation very fast after engine start-up and to respond immediately to variations in NOx and NH3 due to changes in engine load. Such smooth adoption to a load change can be seen in Figure 3, which gives a zoom-in on the data in Figure 2 in the period when the load is changed from 100% to 75%. It can be seen that within about a minute the DeNOx efficiency has returned to the 80% set point. The ammonia slip is lower after the change to 75% engine load as the load put on the catalyst is correspondingly reduced. Older systems have a tendency to emit excessive NH3 and NOx during load changes but modern catalyst types with lower mass reacts faster to changes in operating conditions and enable rapid system start-up and achievement of stable performance. SCR systems are by many engine power producers seen as an undesired additional investment that requires maintenance and induces a cost for consumables. It is important to notice that besides the ability to significantly reduce hazardous emissions some power producers will be able to increase engine efficiency by up to 5% by engine optimisation to maximum electrical output. Such tuning will normally lead to increased NOx formation but this effect can be disregarded with an SCR applied. Others are even able to utilise the temperature increase of 2-3 degrees across the catalyst in a down-stream turbocharger, turbine or boiler. The SCR thereby both improves economy and contributes to an overall reduced carbon footprint. The SCR available today is a matured and well-tested technology for cleaning engine exhaust with thousands of W ORLDWIDE I NDEPENDENT P OWER MAY

7 installations on and offshore. Due to the present worldwide focus on NOx reduction, resources are invested in improving the technology further in order to make the SCR system even more compact, integrate in the silencer and improve the ability to eliminate PM, CO and hydrocarbons in one unit. Currently, organisations such as CEFIC1, IACCSEA2 and IMO3 are developing standards which will help engine operators select systems which comply with regulations in their specific area of operation and help urea-solution and equipment suppliers to offer standardised products. The majority of the few SCR problems which have been experienced can be attributed to the use of urea of poor quality. The standardisation is furthermore expected to facilitate more competition in the urea industry which is why the costs of the SCR consumables are expected to be reduced in the years to come. About Haldor Topsøe Haldor Topsøe is one of the world's largest manufacturers of SCR DeNOx catalyst and supplied the world's first engine SCR DeNOx system in 1988 on the main engine on the bulk carrier M/V Pacific Success. Today, more than 150 larger engines and 130,000 on and off-road heavy vehicles use clean exhaust systems with DeNOx catalyst from Topsøe. Topsøe's primary business focus is on the development and sales of superior DeNOx catalysts and process know-how. Henrik Trolle Jacobsen "We have supplied SCR systems for engines since 1988, however, it is within the last five years the technology for this segment has taken the biggest steps ahead. The driver has been the huge market demand for DeNOx systems for automotive applications which has lead to development of very fast reacting compact catalyst types and effective integrated systems that can be supplied at a very low cost." - Henrik Trolle Jacobsen, Sales Manager for Engine Applications in Haldor Topsøe's Air Pollution Control Group. The bulk carrier M/V Pacific Success. However, today the cost of removing the NOx from a diesel engine is already only a fraction of the costs imposed on our society if the NOx is emitted to the atmosphere. Notes: 1. Cefic - European Chemical Industry Council 2. International Association for Catalytic Control of Ship Emissions to Air, IACCSEA 3. International Maritime Organisation WIP Internet link 24 M AY 2011 WORLDWIDE I NDEPENDENT P OWER