Use of exhaust gases on ships
The use of exhaust gas on ships has been advanced in several phases over the past 50 years , based on rising fuel prices and taking into account investment costs , interest and personnel costs.
introduction
With the commissioning of the first motor ships between 1910 and 1914, shipping entered a development phase that has not yet been completed. The fuel utilization of around 30%, twice as good as the steam engine, the space gained by eliminating the steam boiler and coal bunker, the quick readiness for operation, significantly fewer machine personnel and the simple, clean fuel takeover were convincing. In the meantime, the steam boilers on larger ships also burned oil, the coal trimmers and many heaters were also saved on steamers. The annoyance of the passengers on deck by smoke and ash from the chimney was over. Diesel engines were also used for large and powerful passenger ships. One example is the Monte class of the Hamburg-Süd . Behind these engines were exhaust gas boilers that supplied steam for air conditioning and hot water generation, but also to drive many auxiliary machines such as compressors and pumps.
Progress was made only slowly in the use of diesel engines, however, until the end of the process, steam turbines served to drive fast Atlantik liners. Although the diesel engine used the fuel about twice as well as the steam turbine , it did not finally gain acceptance until the early 1950s. The steam boilers could meanwhile burn poorer and cheaper viscous heating oils, whereas the diesels needed the better and more expensive diesel oil.
Steam for fuel preheating
Global success was made possible in the 1950s when the two-stroke diesel engines, which were meanwhile as high as a house, could also be operated with cheap “heavy fuel”. Additional systems (filters, separators ) for cleaning and heating the heavy fuel oil up to 140 ° C were necessary. Now the diesel engines conquered the cargo shipping, the performance of the engines continued to increase. In 1950 it was a maximum of 15,000 kW per engine, and 50 years later, at the turn of the millennium, almost 75,000 kW were achieved. The largest and most powerful new cargo ships , tankers , bulkers and container ships with diesel engines are now also being built. The engine systems allow a fuel utilization of up to 50%. The waste heat utilization of the exhaust gas for steam generation for fuel preheating, living space heating, air conditioning and drinking water production provide an additional 15-20%.
High oil prices force the use of exhaust gases
The auxiliary diesel engines for generating electrical energy are now mainly operated with heavy fuel oils. Based on rising fuel prices and taking into account investment costs, interest and personnel costs, the use of waste heat has been advanced several times over the past 50 years. In these phases, special topics such as design, optimization, heating surface contamination and the regulation were examined and the self-regulation behavior was optimized with the sliding pressure . This means that the steam production, which is dependent on the diesel engine, can be adapted to requirements using simple means.
When fuel prices fell, however, the use of waste heat was also reduced again because the investments no longer “paid off”. With larger motor outputs, an electrical energy extraction is useful, for this purpose different circuits were implemented in the past. The considerable oil price increases triggered by the Iraq war in 2003 increased the price of heavy oil from 60–100 $ / t by 2008 to 280–350 $ / t. This has led the shipowners and shipyards to consider using exhaust gas and steam turbine generators again for new buildings.
Exhaust gas boiler
The exhaust gas boilers for generating heating steam are usually designed in the pressure range of 5–9 bar, primarily as smoke tube boilers. Water-tube boilers are also widely used, whereas finned-tube boilers only play a subordinate role due to the pollution problems caused by heavy oil operation. A new development in steam boiler construction, the use of surface-structured tubes, is currently being discussed in specialist circles. You could reduce the sizes and weights of flue gas and auxiliary boilers and also reduce investment costs. An unsolved question here is the contamination of the heating surfaces, which causes major problems when using finned tube boilers. The heat transfer takes place predominantly by convection, the radiation component is low. Depending on the type of flue gas routing, a distinction is made between flue tube and water tube boilers.
Smoke tube boiler
In the smoke tube boilers, the engine exhaust gases are fed through vertically arranged tubes and give off part of the exhaust gas energy through the tube material to the water surrounding the tubes. The ratio of the water mass to the hourly generated steam mass flow is around 5–10 t water / t steam. This results in a large storage mass with great insensitivity to changes in the power supply on the exhaust side and the power decrease by the steam consumer.
Water tube boiler
In the case of water tube boilers (e.g. La Mont boiler), the water is guided through the horizontally arranged smooth or, more rarely, finned tubes, the exhaust gas flow is directed perpendicular to them. The distribution and collector pipes for the pipes connected in parallel are routed vertically or horizontally, depending on the design, and end in the evaporation drum.
The exhaust gas boilers downstream of the main engine must not exceed the exhaust gas pressure losses specified by the engine manufacturers (100 - 300 mm water column ). Due to the high sulfur content of fuels used today, the flue gas temperature after the boiler should not fall below the dew point temperature (reference values 150 - 180 ° C). Therefore, the lower exhaust gas temperature of 180 - 200 ° C is used in the design.
Complete system of the steam cycle
The exhaust gases are conducted from the engine via the exhaust gas turbocharger through the exhaust gas boiler (sketched here as a water tube boiler) and here transfer part of the exhaust gas energy into the circulating water of the exhaust gas boiler. In the closed steam-condensate system, the water in the generator circuit is circulated by a centrifugal pump , so that the energy is transported from the exhaust gas boiler to the auxiliary boiler. The oil-fired auxiliary boiler thus serves as an evaporation tank and steam collector for the exhaust gas boiler during operation at sea. In the area and port operation, it generates the required heating steam with the oil furnace. In sea operation, the auxiliary boiler is switched on or off depending on the steam pressure and thus ensures a continuous supply of the heating steam system in automatic ship operation. This means that there is only limited redundancy.
The consumer circuit serves the heating steam consumers, whose steam demand is set depending on the environmental and operating conditions. The preheaters and tank heaters transfer the enthalpy of vaporization to the medium to be heated. The steam traps downstream of the steam consumers cause a complete release of the enthalpy of evaporation, as only the condensate can flow through. If the amount of steam generated in the exhaust gas boiler exceeds the demand, the excess amount of steam is deposited in an excess condenser. The steam mass flow through the excess condenser is set with the excess steam valve, which is controlled by the excess regulator depending on the operating pressure. This regulates the upper steam pressure in normal operation. The condensate from the excess condenser and from the auxiliary steam consumers flows through a condenser and condensate pump into the feed water tank. From here, the feed water is pumped back into the auxiliary boiler with the feed pump, depending on the auxiliary boiler water level.
Influence of the engine on the amount of steam generated
The influence of the main engine has to be considered from a constructive, operational and environmental point of view. The increases in thermal efficiency achieved in the past resulted in a reduction in the exhaust gas temperature and the specific exhaust gas mass flow. The data of an exhaust gas boiler are plotted against the related engine power (two-stroke engine) in characteristic curves. These values, the exhaust gas temperatures before and after the exhaust gas boiler and the exhaust gas mass flow, only apply to stationary operating points.
The environmental influences act on the system under consideration through the air and water temperatures. Low air and sea water temperatures increase the losses in bunkers and fuel tanks, and the falling exhaust gas temperature reduces the steam mass flow generated in the exhaust gas boiler.
In the future, new regulations will require, in addition to internal engine measures, exhaust gas aftertreatment, which will reduce the amount of steam generated. The SCR catalysts (Selective Catalyt Reduction) to lower the NO x values and the desulphurisation systems to reduce the sulfur dioxide lower the usable exhaust gas temperature.
Heating steam consumer
The consistent increase in the overall efficiency of the ship's propulsion system has led, especially in the recent past, in many ships to supplying all heat consumers with waste heat energy as far as possible. Most of the heat consumers are heated by steam (less often thermal oil or electrical energy), a few by cooling water from the high-temperature system.
To determine the total steam requirement as the basis for the design of the exhaust gas and auxiliary boiler, a steam balance is drawn up, in which all steam consumers are taken into account. The steam balance contains the demand of the auxiliary steam consumers depending on the operating mode:
- Sea operation, stationary continuous operation of the main engine
- Port operations, no main engine operation
- Territorial operation, variable main engine operation
Reduction of the heating steam requirement
In order to reduce the fuel costs of the overall system, passive measures to reduce heating steam consumption were implemented
- Elevated bunkers instead of double-floor bunkers
- better insulation
- Use of cooling water for bunker heating
Electrical energy generation from the exhaust gas heat
Some of the steam on some ships is also used to generate electricity through the use of steam turbines. Exhaust gas turbines connected in parallel to the exhaust gas turbocharger are used for further exhaust gas utilization in order to generate electrical energy. At the service speed of the ships, the steam turbines driven by the steam from the exhaust gas boiler and the exhaust gas utility turbines driven by the exhaust gas generate far more electricity than is required. Therefore, combined shaft generators and electric motors are arranged in the shaft train on these ships , which deliver excess electric power to the propeller. Such systems can be found, for example, on the container ships of the Emma Mærsk class .
The operational influences result from the fuel prices and the respective capacities of the ship's hold. Depending on this, the shipowner or charterer selects the speed when designing the ships that has the greatest influence on the engine power and thus the exhaust gas parameters. This relationship can be seen from the flue gas boiler characteristics. Partial load operation can increase the contamination of the heating surfaces and reduce the heat transfer, resulting in less steam generation. This has a negative effect on the engine through the increased back pressure.
literature
- J. Jung: Use of heat and exhaust gases and cooling water in diesel systems. In: Yearbook of the Shipbuilding Society . Volume 53, 1959, pp. 284-296.
- O. Geisler, M. Gietzelt, W. Tischler: Efficiency of exhaust gas boiler systems. = Hansa special issue. April 1976.
- An-Thai Nguyen: Contribution to the design of exhaust gas boilers for diesel engine systems with special consideration of the heating surface contamination Dissertation. TU Hamburg-Harburg.
- K.-H. High-rise: dynamic behavior of auxiliary steam systems in ship operation. (= Progress reports VDI. Series 12; traffic engineering, vehicle technology. No. 75). Dissertation. VDI-Verlag, Düsseldorf 1986, ISBN 3-18-147512-2 .
- G. Großmann, C. Hadler: Bunker heating by engine cooling water. In: Yearbook of the Shipbuilding Society. Volume 75, p. 219.
- U. Janssen, K.-H. High-rise: long-term measurements of auxiliary steam consumption. In: Hansa. No. 14, 1984.
- M. Rupp: Energy use of diesel exhaust gases to generate electrical on-board network energy with a power turbine generator unit. In: Yearbook of the Shipbuilding Society. Volume 86, p. 187.
- F. Deichmann, K.-H. High-rise building: Experience with the electrical energy supply on diesel-powered merchant ships with turbo-generator and auxiliary diesel. In: Hansa. No. 19/20, 1989.
- K.-H. Hochhaus, O. Jacobsen: Use of exhaust gas on motor ships to generate electrical energy. In: Schiffsingenieur Journal. 3/2006.
- W. Oehlers: The Diesel Engines, Past Present and Future. In: Engine & Environment. MAN B&W Diesel AG, Augsburg 2006.