The article presents an analysis of the methods and impact of the use of waste heat on the efficiency of the ship’s main boilers, based on the literature, the ship’s technical documentation, and the results of operational tests. The literature mainly deals with the development of boiler designs, increasing steam parameters and materials used for their construction in order to increase the efficiency of marine turbo steam power plants. The advisability of using waste heat in exhaust gases to heat boiler feed water and air to raise combustion temperature has also been discussed [ 2 10 ]. Boiler manufacturers’ company materials [ 11 13 ] and vessel technical documentation were used to obtain data on the achieved temperatures of heated air supplied to the boiler furnace chamber. The methodology for determining the effect of exhaust gas heat recovery on exhaust loss and the effect of air temperature on combustion temperature was developed based on [ 5 15 ].
It is worthwhile to note that until 2005 all LNG tankers were equipped with a turbo steam drive, which is the only one that makes it possible to use naturally evaporating gas as a fuel in main boilers. The main drawback of turbo steam power plants is their low efficiency, in the range of 36–38%. Increasing the efficiency of ship turbo steam power plants resulted in increasing the parameters of steam produced in the main boilers. In the years 1995–2010, the pressure of steam produced in ships’ main boilers was ranging between 6.0 and 6.8 MPa, and steam’s superheating temperature was 515–525 °C [ 2 3 ]. The improvement of material engineering allowed for the construction of experimental ship turbo steam power plants where main boilers produced steam with a pressure of 10–12 MPa, and a superheating temperature of 560 °C [ 4 6 ]. Apart from increasing the values of steam parameters, using steam inter-stage superheating, carnotization, and the use of waste heat, new boiler designs have been introduced to increase the speed of natural circulation [ 2 ]. These procedures allowed for achieving the efficiency of ship turbo steam power plants in the range of 39–41% [ 7 ].
Turbo steam power plants were popular in 1960–2005 on ships of high load bearing capacity and high demand for drive power in the range between 20 and 50 MW (tankers, passenger ships, LNG tankers). Such powers were achievable only by steam turbines due to the low power of diesel engines produced at that time. The development of ship construction of diesel engines by increasing their power, introducing dual fuel engines, as well as using systems for the deep utilization of waste heat [ 1 ], made it possible for the diesel power plants to work more efficiently, up to 50%. This resulted in decreasing the number of ships with steam drive being built.
The choice of the utilization method of the waste heat of the boiler exhaust gas depends on the applied other methods of increasing the efficiency of the turbo steam power plant. In the case when carnotization is used in the engine room (heating of the feed water with the use of bleed steam from turbines), the boiler feed water temperature reaches high values (in the range 190–230 °C, see Table 1 ). One water heater is used to heat it up to the saturation temperature for the pressure and combustion air heater ( Figure 2 b). If no carnotization is used in the turbo steam power plant, the temperature of the boiler feed water is 110–140 °C, and in order to heat it up to the saturation temperature, more heat is required, which requires the use of multi-stage feed water heaters. Meanwhile, for air heating heat exchangers supplied with saturated steam are used ( Figure 2 a).
In the case of MB boilers made by Mitsubishi, two methods of exhaust gas waste heat utilization have been proposed. The first method ( Figure 2 a) includes using waste heat to heat water feeding the boiler in a multistage water heater. For the purpose of heating the air, a heat exchanger fed with saturated steam of the pressure from 0.6 to 2.0 MPa were assembled [ 5 7 ]. The second method ( Figure 2 b) consists in the dividing the waste heat for the purpose of heating both the water feeding the boiler and the air needed in the combustion process.
In order to recover waste heat, additional heat exchangers are installed in the boiler flue gas outlet system in the form of feed water heaters and air heaters. The use of waste heat will result in cooling down the flue gas to the temperature acceptable from the point of view of low-temperature corrosion (130–150 °C), which will allow it to obtain the outlet loss value in the range of 5–6% ( Figure 1 ).
The highest calculated value of the exhaust loss was 18% for an exhaust gas temperature of 400 °C ( Figure 1 ). This loss value occurs when the waste heat resources of the flue gas are not used. The minimum value of this loss of 5% occurs with the maximum use of waste heat while cooling the flue gas to a temperature of 130 °C, which does not cause low-temperature corrosion yet [ 2 4 ].
The calculations were performed after assuming the constant values of t e n v , CO 2 , and W d , so the calculated values of the outlet loss S out depended only on the changes in the temperature tout of the exhaust gases leaving any main boiler. The calculations were made on the assumption that the flue gas temperature tout was changed at the boiler inlet in the range from 130 °C to 400 °C.
The greatest resource of waste heat is included in exhaust emissions coming out from the boiler, whose temperature affects the boiler’s outlet loss value. On the basis of calculation methods presented in the literature [ 4 9 ] we determined the values of outlet loss depending on exhaust emissions temperature.
Water heaters with water evaporation—the water is heated to the boiling point and it partially evaporates. The degree of wet steam dryness is low and practically amounts to x = 0.02 ÷ 0.10 [ 7 16 ]. Higher degrees of dryness are not used, because when x ≥ 0.15, there will be a sharp increase in resistance and flow disturbances. Therefore, these types of heaters require precise hydrodynamic calculations.
Water heaters without water evaporation—the water is heated to a temperature of 15–30 °C lower than the saturation temperature. Their construction is based on a bundle of steel pipes, often finned, rolled into coils, and welded at their ends into collectors. The whole as one assembly of modular structure is placed in the chimney chute. The modular structure facilitates access to the heater during renovation works.
Theoretically, in combustion water heaters supplying boilers, the water should be heated to the saturation temperature, namely 280–324 °C for the relevant working pressures of 6.3–12.0 MPa. In practice, depending on the amount of waste heat contained in the exhaust emissions, two types of water heaters are used:
As can be seen in Figure 3 , in the ship’s main boilers, where operating pressures in the range of 6.3–12.0 MPa are used, 28–40% of the total amount of heat generated during fuel combustion is used to heat water to the saturation temperature in the boiler’s combustion chamber. The use of a combustion water heater will significantly reduce this amount of heat or bring it to zero when the temperature of water heating reaches the saturation temperature. As a result, the amount of fuel burned is reduced while maintaining a constant amount and parameters of the steam produced, or increasing the amount of steam produced while maintaining a constant amount of fuel burned. In both cases, there is also an increase in boiler efficiency.
The heat exchange process in a steam boiler consists of three stages: water heating from the supply temperature to the saturation temperature for a given operating pressure, water evaporation, and steam superheating. Based on the literature [ 1 9 ], the percentage of heat required to heat water to the saturation temperature was determined and presented in Figure 3 in relation to the total heat required for the above-mentioned three stages of the heat exchange process.
The main elements of this type of heater are ( Figure 7 ): a rotating drum with an installed package of thin corrugated sheets with a thickness of about 0.5 mm, and an electric motor which, through a mechanical transmission, gives the drum a rotational speed in the range of 1–2 rev/min. Half of the sheet packet is in the flue gas stream receiving heat from it, and then, due to the rotational movement, the sheet packet is moved into the air flow channel, where the air washing around the sheet packet is heated. One of the most important operational advantages of rotary heaters is their high resistance to low-temperature corrosion. When burning fuels with a high sulphur content of 3–3.5%, it was observed that they were practically non-corrosive [ 7 ]. Another important advantage is the self-cleaning of the heater. The heat exchange surfaces are washed alternately with exhaust gases and air of different velocities, flow directions, and temperatures. These factors make it much harder for contaminants to settle.
Based on the methodologies of calculating the combustion temperature in the boiler presented in the literature [ 9 ], the increase in combustion temperature depending on the air temperature was calculated. The results of the calculations allowed for drawing Figure 6
The heated air will bring additional heat into the combustion chamber. This results in an increase in the theoretical combustion and flue gas temperature and an increase in flue gas enthalpy.
In all main boilers installed on the ships with turbo steam power plants, exhaust waste heat is used for heating feed water and the air necessary for the combustion process. In order to assess the temperature of the heated air, temperature of feed water before the diesel heater, as well as boilers’ efficiency, Table 2 has been drawn.
The standard parameters of steam produced in modern main boilers are a working pressure of 6.0–6.6 MPa and a steam superheating temperature of 510–515 °C. The manufacturers of main boilers strive to maximize these parameters and use secondary steam superheating in order to increase the efficiency of turbo steam power plants. As a result, the MBR and UTR-II boilers (operating pressure 10.0–12.0 MPa, steam superheating temperature 560–565 °C) have been introduced to the market. This is primarily due to the high market competitiveness of diesel combustion engines (mainly dual-fuel) for ship propulsion systems.
Modern main boilers are characterized by very high efficiency, exceeding 88%. Such high efficiencies are achieved by the utilization of waste heat for heating the feed water and air. A barrier to further utilization of heat contained in exhaust gas is the threat of aggressive exhaust components reaching the dew point and the occurrence of low-temperature corrosion. The highest recorded efficiencies are in the range between 90.0 and 91.3% ( Table 2 ). The ESD IV boiler additionally uses the external boiler walls to preheat the air, thereby reducing the amount of heat radiating outside and allowing achievement of the highest efficiency of 91.3%. The lowest efficiencies are observed for the following types of boilers: MB-E, MBR, and UME, which are equipped with multi-section feed water heaters only. At nominal loads, the design of these heaters does not allow the exhaust gases to cool below a temperature of 170 °C. Therefore, the efficiency of this group of boilers is 88.5%. Boiler manufacturers provide their efficiency values at nominal efficiency and when using 100% liquid fuel with a gross calorific value (Wd = 42.500 kJ/kg).
The manufacturers of the main boilers report the feed water temperature at the inlet to the internal boiler heater. For boilers equipped with multi-section feed water heaters, the inlet water temperature is quite low and amounts to about 1400 °C (e.g., MB-E, UME, and MBR boilers). Boilers equipped with a single-section heater require preliminary heating of the water to a temperature of about 200 °C (e.g., MB, UM, and V2M9 boilers).
The heaters constructed without an internal boiler feed water heater need to heat the water to the highest possible temperature in the exchangers supplied by steam from the main drive turbines. Usually, the water is heated to a temperature of about 200 °C (e.g., V2M8 type boiler).
The temperature of the heated air supplying the furnace chamber, as well as the water temperature, is very diverse and depends mainly on the structure of the waste heat utilization section. The main boilers that use steam heaters to heat the air are able to increase the temperature to about 130 °C (these are boilers equipped with a multi-section water heater which reduces the exhaust gas temperature at the outlet to a minimum, i.e., MB-E, UME, and MBR types). Boilers equipped with a rotary air heater increase the temperature to about 150 °C (cooperating with a single-section water heater, e.g., UM boiler). The design of the waste heat utilization section of the V2M8 boiler consists solely of a rotary air heater. This allows the air to be heated up to 290 °C (with a nominal amount of steam produced).