Abstract 10TH ISEC Osnabrueck Sept 2001

During the last decade extensive research has been made at Lund University, Sweden, on a lean premix (prevaporize) combustion concept with burnt gas recirculation and a metallic flame holder. From this concept a new lean premixed natural gas combustion chamber with internal combustion gas recirculation (CGR) has been developed for the V160/SOLO 161 Stirling engines. This combustor has ultra-low emission levels, comparable to those of catalytic combustion. At the start of the current project the combustor was ready to be adapted for production, with expected market introduction in 2001. The Lund combustion chamber was modified to investigate the impact of air-fuel ratio and combustion gas recirculation rate on emissions and controllability of the combustion system, and on pressure losses in the combustion chamber. Different start-up strategies as well as different fuel-gas control valves were tried in order to find well-working control routines/parameters. The combustion chamber was redesigned using the gained knowledge, making it easy to manufacture while giving it maximum life expectancy and durability. The SOLO 161 Stirling engine’s control system was adapted to the new combustion system. Emissions of the final combustion system were measured and found to be close to the design goal values. Combustor function and reliability has proved to be very good. Introduction To find a place for Stirling engines on today’s market for heat and power generation, it will be necessary to have combustion systems that produce the least amount possible of harmful emissions. The fact that modern gasoline engines produce less than 25 ppm (after the catalytic converter) of both nitrogen oxides (NOx) and hydrocarbon emissions (HC), only stresses the importance of Stirling combustor design. Also, the use of well-designed combustion chambers with low emissions will increase overall system performance. High emission levels means poor combustion efficiency, and combustion gas flow patterns in the combustion chamber have a considerable impact on heater performance. A combustor for a Stirling engine is characterised by the preheating of the inlet air to high temperatures, typically 500-600oC, which is needed for reasons of efficiency. This makes the Stirling combustor quite different from most other combustors. Its closest relatives can be said to be the gas turbine combustors, which due to the air compression and in some cases recuperators are also working with hot inlet air. The formation of nitrogen oxides (NOx) is highly temperature dependent (figure 1). With inlet air that is preheated to high temperatures, measures have to be taken to avoid high flame temperatures with accompanying high NOx emission levels. The following strategies have been used in trying to get low NOx combustion: Rich burn – Quick quench – Lean burn Lean premixed burn Recirculation of burnt gases (EGR, CGR) Catalytic combustion Using a three-way catalyst Figure 1 Temperature influence on NO formation rate [Ref 1]. Rich burn – Quick quench – Lean burn means that first the fuel is burned in a rich mixture, then the combustion is quenched, heat is removed from the partly burnt mixture, extra air is added and then the combustion is continued in a lean mixture. This method has been tried in Stirling combustors by United Stirling, but results were not satisfactory and there was a problem with soot formation. Tests in gas turbines have given a 50% NOx reduction compared to traditional diffusion combustors. Lean premixed burn means that the combustion is taking place in a mixture with excess air compared to what is consumed by the combustion. The excess air acts as a bulk gas absorbing heat from the flame during the combustion. The drawback is that the extra air has to pass through the preheater, lowering preheater efficiency (or demanding a bigger preheater). Also, the excess air increases oxygen concentrations in the combustion chamber, affecting the oxidizing rate of nitrogen. Premixed combustion demands some sort of flame stabilizing device (e.g. a swirler or flame holder), or the flame will propagate backwards against the fuel outlet, where the flame will turn into a diffusion flame. The use of excess air will increase combustor pressure losses. Recirculation of burnt gases works the same way as does lean premixed burn, but in this case it is the recirculated burnt gases that act as bulk flow. As the burnt gases are mostly inert, this means that the recirculation will decrease oxygen concentrations inside the combustion chamber. There are two main methods of recirculation, EGR and CGR. EGR (Exhaust Gas Recirculation) means that exhaust gases (cooled in the preheater) are recirculated. CGR (Combustion Gas Recirculation) means that hot combustion gases are recirculated from after the heater, inside the combustor. The use of EGR will decrease preheater efficiency, and both EGR and CGR will in varying degrees increase combustor pressure losses. Catalytic combustion means that the combustion takes place in a catalytically active surface, where the oxygen and fuel molecules are bound to the surface, split, and recombined to form combustion products by the catalytically active material (e.g. platinum, palladium, rhodium) in the surface. The released heat then has to be removed from the material by convection, conduction and/or radiation. Catalytic combustion is developed in gas turbine combustors. Promising tests with catalytic combustion on the surface of Stirling heaters have been made at Lund University [Refs 2, 3]. Using a three-way catalyst means reducing the emissions only after they are already formed. For NOx reduction to be possible the combustion has to be fuel-rich or stoichiometric. For oxidization of CO and HC to be possible the combustion has to be stoichiometric or lean. E.g., for the catalyst to be effective in lowering all three target emission levels, the combustor has to be run at stoichiometric conditions. Also the catalyst has to reach light-off temperature to work, typically 400-500oC, which means that it cannot be used at the exhaust pipe of a Stirling engine because of its low exhaust temperature. However, a catalyst can be mounted inside the Stirling combustor downstream of the Stirling heater, where the combustion gas has a temperature of 700-900oC. Laboratory tests at Lund University Initial tests along different development lines were made on the Lund University United Stirling V160F laboratory engine, onto which an experimental combustion chamber was fitted (figure 2). The design of the combustion chamber makes it possible to replace different parts inside it. The combustor chamber is at one end equipped with a big quartz glass window, allowing optical access to the flame and flame holder. Using the knowledge gained in these tests, a prototype combustion chamber was designed by Intersol and mounted on a SOLO 161 Stirling engine unit supplied by SOLO Kleinmotoren. The unit was installed in the Lund University engine laboratory, and was equipped with a solar application control system to made it possible to run the engine independent of what combustor control system was used. Subsequent tests were carried out on the SOLO 161 engine, and the SOLO engine control system was adapted to the new combustion system. stirling engine heater flame holder