OVERCOMING HIGH-TEMPERATURE-CORROSION

Transcription

1 CONSEIL INTERNATIONAL DES MACHINES A COMBUSTION INTERNATIONAL COUNCIL ON COMBUSTION ENGINES OVERCOMING HIGH-TEMPERATURE-CORROSION Pinheiro Pereira, Senior Engineer, Jomar Enercaima Cogeneration Diesel Producao de Energia S. A., Apartado 5104, Freixiero, Perafita, Portugal Tel: ; Fax: PPereira@Jomar.pt Arnim Marschewski, Technical Service Manager, Octel Deutschland GmbH, Thiesstr. 61, Herne, Germany Tel: ; Fax: Marschewski@Octel.de ABSTRACT In modern, high-load 4-stroke engines the exhaust gas temperatures before T/C easily reach C. In this temperature range vanadium (V) and sodium (Na) ashes can appear in liquid form. In the T/C they solidify and build up very hard deposits which can attack the material surface, commonly called High-Temperature-Corrosion (HTC). A known method to suppress the effects of vanadium/sodium ashes and to influence their properties after combustion is the use of additives. Experience with the use of additives in Heavy Fuel Oil (HFO) in a Diesel power plant with a 4- stroke, 400 mm bore engine is presented. INTRODUCTION Operating diesel engines on Heavy Fuel Oil can cause problems in the exhaust system due to deposits and corrosion. Normally, deposits are containing mainly carbon and ashes whereas corrosion is based on an attack of sulphuric acid. For modern 4-stroke engines with an increasing temperature level in the exhaust gas system, another problem is more and more appearing, the so called High-Temperature-Corrosion which occurs due to liquid vanadium and sodium ashes. It is well known that some 30 years ago valves and valve seats were attacked by this effect. Since then, improved engine design and better quality materials of the engine components have been the solution. Today, the place of corrosion is very often the turbo chargers. Besides the material quality also of the T/C components, an alternative way to overcome HTC is the use of a fuel additive. This report may provide information and give an overview about the chemical way to solve this particular problem. CIMAC 802 Congress 2001, Hamburg

2 SCIENTIFIC CHEMICAL BACKGROUND Description When burning heavy fuels that contain ashes and sulphur in Diesel engines, the exhaust gases cause considerable corrosion on the metal surfaces they come into contact with. The degree of corrosion is depending on the composition of the fuel and the local exhaust gas temperatures. Vanadium and sodium, being a common component of HFO, are mainly responsible for corrosion at high temperatures. The vanadium and sodium contents in HFO can reach ratios up to 600 ppm V and 200 ppm Na [1]. Chemical Reaction During the combustion, vanadium and sodium oxidise to V 2 O 5 and Na 2 O. In combination with sulphur, they are able to form different types of sodium-vanadyl-vanadates. Some of those have properties such as low melting points of oxygen transitions, which are of decisive influence on the corrosion. Chemical Composition Melting Points ( C) V 2 O Na 2 O. V 2 O Na 2 O. V 2 O Na 2 O. V 2 O 4. 5V 2 O Na 2 O. V 2 O 4. 11V 2 O Na 2 SO The behaviour of the compounds under melting conditions is especially important. An example: crystallise Na 2 O. 6V 2 O 5 Na 2 O. V 2 O 4. 5V 2 O 5 + ½ O 2 melt This shows that a liberation of atomic oxygen takes place at the moment of solidification and this oxygen will then attack the metal surfaces. The Power Plant FIELD EXPERIENCE The Jomar power plant in Portugal is producing electrical energy for an industrial plant and for the public network. The genset went in operation in The system: Engine Sulzer 8 ZA 40 S Power 5,600 BHP T/C ABB VTR Generator GEC-Alsthom RK Elect. Output 6,970 kva Fuel Fueloleo No. 4 de Petrogal Operational Problems After starting up the system into operation in 1994, very soon T/C problems due to deposits on the nozzle ring and turbine blades appeared. With the normal cleaning methods it was impossible to maintain a Time Between Overhauls (TBO) of longer than 4 months, or approx. 3,000 hours, of operation. This caused high maintenance efforts, increased costs and very low time between overhauls and did never reach the expected operational conditions. At that time, the cleaning intervals of the T/C were: Water washing: Nut shell cleaning: every 48 hours every 24 hours The plant was operating in the guarantee phase, so that also Sulzer in Winterthur and ABB in Baden were searching for a solution to overcome the existing problems. One discussed possibility was testing a fuel oil additive. The decision was made to test the combustion catalyst PLUTOcen FW from Octel Deutschland GmbH, an additive that contains ferrocene as the chemical working agent, i. e. active substance. The problems were described as carbon deposits on turbine blades as well as on the nozzle ring and it was expected that a combustion catalyst would bring a positive influence with the aim to increase the time between overhauls to a normal and acceptable level. Furthermore, a reduction of the cleaning intervals was expected. CIMAC 803 Congress 2001, Hamburg

3 Results of Combustion Improver The test was started in September 1996 after a complete overhaul of the system, where the turbo charger, nozzle ring and turbine had been cleaned and also the bearings were changed. The necessary dosing unit for the additive had been installed by Jomar. The additive dosing rate was set to 50 ppm of the active substance ferrocene for the first six weeks and was reduced to 25 ppm for the rest of the time. The test was finished after 4 months (approx. 3,000 running hours) and a T/C overhaul was carried out by ABB, Portugal. During this inspection the nozzle ring and turbine blades showed a great amount of hard deposits, looking similar to those that appeared before the application of the additive. Therefore, the expected result to achieve a clean T/C was not realised. The deposits were then analysed in a chemical laboratory. The results showed only a very small amount of carbon which was a proof, however, that the application of the additive had the expected influence on the carbon deposits. Besides the small amount of carbon, the vanadium and sodium contents of the deposits were very high. Also calcium was detected which is a normal ash from the burned cylinder lubrication oil. Compared with the fuel composition and the temperature level before T/C ( C), all the results showed that the reason for the turbo charger problems were not carbon based, but occurred due to temperature corrosion caused by vanadium and sodium. The analytical results showed that a different additive composition was necessary to overcome the problems that were caused by the analysed deposits. Development of a Special Additive To provide a solution for this problem, first of all a literature research was carried out in February Based on the gathered information hereof, the development of a new product started. The basic idea was to implement two different metals into the existing vanadium and sodium compositions [2],[3], in order to change the eutectic relations of the ashes [4]. The result was expected to be a tremendous increase of the melting temperatures of the compounds, a lot higher than the exhaust gas temperatures of the engine themselves. By doing this, it was assumed that it must be possible to suppress the high-temperaturecorrosion by keeping the ashes crystalline and therefore be removable by normal cleaning procedures. The result of this development was the product PLUTOcen FW-M which is containing a magnesium compound in addition to the iron-organic compound ferrocene. The dosing rate has been adjusted to the particular vanadium content of the HFO used at the power station. However, not the standard stoichiometric relation as known from the literature was used, because a synergetic effect between the iron and the magnesium has been found which made it possible to use much lower dosing ratios. Additionally, a completely new dosing system for the additive was developed. Second Trial Results After finishing the development work at the end of 1997, a new test run was started in Portugal with the newly developed additive to finally combat the existing high-temperature-corrosion. During the test run, very stable operation conditions for the complete system were observed. No unplanned overhaul for the turbo charger had been necessary. At the first inspection in August 1998 (after approx. 8,000 running hours) all turbo charger parts, in particular the nozzle ring and the turbine blades, were reported to be in a very good condition. Also, the analytical results of the remaining small amount of deposits showed a variation of the vanadium and sodium relations. Compared with the eutectic diagram, these ashes had a much higher melting point. After cleaning the turbo charger parts, no corrosion marks that are typical for HTC were found on the material surfaces of the T/C. The effects of the additive were still visible in August 2000, when the trial was successfully finished after approx. 17,000 running hours of using PLUTOcen FW-M (see Fig. 1). Since the additive has been applied, the cleaning intervals of the T/C are: Water washing: 1-2 times per week Nut shell cleaning: not at all These intervals show the expected and required effects of the additive to combat the hightemperature-corrosion by shifting the melting point of the V / Na ashes to a level which is higher than the exhaust gas temperatures before T/C (as can be seen in Fig. 2). Thereby, also the goal to decrease maintenance, reduce costs, increase the TBO and improve the overall operational reliability of the system was achieved. CIMAC 804 Congress 2001, Hamburg

4 Side Line Observations A special interest has been to judge whether there is an additional influence to any other parts of the system that came in contact with the fuel. Therefore, the fuel system, the fuel pump, the injectors and also pistons and liners were inspected. One reason has been to check if there was an additional wear due to the additional metal content in the HFO coming from the additive. All investigated parts were in a very good condition without any evidence for higher wear rates. Especially the conditions of pistons and cylinder liners showed a positive effect of the additive. CONCLUSIONS All the results of this long term test at the Jomar power station show that it is possible to overcome the problems based on high-temperature-corrosion from vanadium and sodium by the use of an especially developed and tailored additive. Everything that was expected by the laboratory simulations and the theoretical background from the literature has been achieved and found as result in this field test. The operational condition of the system is back on a normal level since the additive has been applied and no additional wear and tear to other parts of the engine could be observed. ACKNOWLEDGEMENTS The authors would like to express their appreciation and gratitude to the Jomar Enercaima Power Station in Perafita, Portugal for their very good co-operation. Furthermore, the valuable assistance and co-operation of Sulzer Portugal in Lisbon and ABB Portugal in Oporto has been greatly appreciated. REFERENCES [1] BLUDSZUWEIT ET AL. Sodium in the Intake Air, Project Paper, MET GmbH, Rostock, 1999 [2] FIEDLER, GROTH, HESSE Investigations on high-temperature corrosion on exhaust valves and on combustion of high-viscosity residual fuels in Medium-Speed Four-Stroke diesel engines, CIMAC Paper D-69, Helsinki, 1981 [3] JEDLICKA Werkstoffschädigung durch Hochtemperaturkorrosion bei Dieselmotoren, VDI-Bericht Nr. 235, Augsburg, 1975 [4] HESSE Mischkraftstoffe Motorverschleiß bei Verbrennung von Mischkraftstoffen, Institut für Kolbenmaschinen der Universität Hannover, Hannover, 1980 CONTACT INFORMATION BHP HFO HTC kva Na ppm T/C TBO V NOMENCLATURE Break Horse Power Heavy Fuel Oil High-Temperature-Corrosion Kilo Volt Ampere Sodium Parts per million Turbo Charger Time Between Overhauls Vanadium PINHEIRO PEREIRA Senior Engineer Jomar Cogeneration Diesel Enercaima Producao de Energia, S. A. Freixeiro, Apartado 5104 P-4460 Perafita Portugal Phone: Fax: ppereira@jomar.pt ARNIM MARSCHEWSKI Service Manager Technical Service Octel Deutschland GmbH Thiesstr. 61 D Herne Germany Phone: Fax: marschewski@octel.de CIMAC 805 Congress 2001, Hamburg

5 Before field test, August 1996 After field test, August 2000 Figure 1: Comparison of turbine blades before and after the field test in Portugal (after approx. 17,000 running hours with PLUTOcen FW-M) CIMAC 806 Congress 2001, Hamburg

6 Temperature before T/C: C 1996 Na: 32,4 mol% V: 68,6 mol% 1998 Na: 16,8 mol% V: 83,2 mol% Figure 2: Eutectic Diagram (V / Na ashes) CIMAC 807 Congress 2001, Hamburg