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Boiler Energy Efficiency Measures

A boiler in one form or another will be found on every type of ship. Where the main machinery is steam powered, one or more large watertube boilers will be fitted to produce steam at very high temperatures and pressures. On a diesel main machinery vessel, a smaller (usually firetube type) boiler will be fitted to provide steam for the various ship services. Even within the two basic design types, watertube and firetube, a variety of designs and variations exist.

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Boiler energy efficiency measures

Figure (i) shows typical energy efficiency characteristics of a boiler that is normally specified by boiler manufacturer. As can be seen, the boiler efficiency is a factor of its load.

In operation, the efficiency tends to be lower than the above design values. There are a number of major areas that need to be managed in order to keep the auxiliary boiler at its highest energy efficiency levels as signified by Figure (i).

boiler-efficiency-characteristics
Fig (i): Boiler efficiency characteristics

Fouling of surface

The boiler main function is to generate steam at correct pressure and temperature and with best energy efficiency. Optimal energy efficiency means optimal transfer of fuel energy to boiling water via various boiler pipes and heating surfaces. Aspects that could lead to a reduced rate of this heat transfer include:

The above will normally translate into a less heat transfer from gas and more heat retention by the exhaust gases as they leave the boiler. Thus high boiler outlet exhaust gas temperature could be a good indication of such fouled conditions. To remedy the case, maintenance practices should include boiler’s soot blowing, de-scaling, good water, combustion adjustments (to reduce soot formation) and son on. For this purpose, the heat transfer areas of the boiler must be monitored.

The soot blowing of the boiler must be done regularly as build-up of soot acts like an insulator and reduces the heat transfer rate. The same goes for the buildup of scale in the water tubes. The stack temperature must be monitored regularly and any increase in it means that heat recovery is not optimum. High increases of exhaust gas temperature beyond those experienced after the last cleaning would indicating build-up of fouling and would require another cleaning action.

Optimum hot well temperature and blow-down levels

There is a hot well (see Figure (v)) that collects all the condensates from steam system end-users plus where water treatment and cleaning may take place. It is from hot-well that the feed water is supplied to the boiler. Hot well temperature must be maintained at temperature specified by manufacturers. A low temperature (e.g. below 80-85 C) will cause colder feed water to enter the boiler thus increasing the fuel cost due to the need for more heating for evaporation.

An overheated hot well may cause evaporation of water at the suction of feed pump (e.g. cavitation) and cause vapor lock in the feed pump and loss of suction. For heat retention in the hot well to keep temperature higher, heat losses due to poor insulation can be reduced. Also, control of make-up water is important as excessive need for make-up water will be indicative of leak in the steam system as well as more heating for make-up water that is normally at low temperature.

The blow down of the boiler is required for controlling the amount of dissolved solids as a result of evaporation and impurity of make-up water or addition of other chemical. Blow down must be calculated and done after measuring the level of dissolved water. In some cases, the engineers blow down the boiler excessively, thus not only loose hot water, but also increase the need for make-up water and make-up water generation.

Excessive combustion air

In order to burn the fuel, air needs to be supplied to the boiler. The excess air unused in the combustion gets heated and then discharged through the chimney. This is waste of energy. Thus, any excess air that is not needed for combustion will cause energy loss as it will take away heat from boiler and discharge to the atmosphere, thus normally should be avoided.

Boilers normally have certain amount of optimal excess air and the air input must be adjusted to this level. It signifies a balance between combustion efficiency and amount of air supplied. Excessive “excess air” is identified in the form of either high O2 concentration or low concentration of CO2 in the boiler exhaust gas. These two parameters thus need to be monitored as part of controlling boiler excess air thus its energy efficiency. Figure (ii) shows the boiler efficiency as a function of CO2 concentration. As can be seen, it is desirable to maximise the CO2 concentration in the exhaust gas for best efficiency. As indicated, the optimum level would normally be specified by the manufacturer.

boiler-efficiency-comparison
Fig (ii): Boiler efficiency as a function of CO2 level in the exhaust gas



Boiler efficiency and load factor

Like any other devices, the boiler energy efficiency is a factor of its load factor. Figure (i) and also Figure (iii) show typical of such efficiencies.

boiler-efficiency-load-factor
Figure (iii) – Impact of boiler load factor on efficiency

Accordingly, for this boiler the efficiency is highest at certain point and then drop off with changes in load. Thus boiler load management could be considered as one aspect of energy efficiency. Figure (iii) shows that at 70% load, the efficiency is 80% and at 30% load, the efficiency is 63%; a significant drop.

Operating the boiler at low load is thus inefficient. Avoiding low load boiler operation will depend on ship type, number of boilers and where the steam is used. Generally, if there are two auxiliary boilers on a ship, they must not run in parallel if one can supply the whole steam demand; unless safety issues dictates the need for such a parallel operation case.

boiler-fuel-consumption
Fig (iv): Boiler fuel consumption

Boilers and Steam System

The steam system plays a major role in energy efficiency of certain ship types (such as steam driven LNG ships) and a medium role in ships such as oil tankers carrying liquid cargo that require cargo heating or there is a need for cargo transfer using steam driven pumps but also need to generate Inert Gas for cargo tank cleaning, purging or tank top ups. Figure (iv) shows typical level of fuel use in boilers as compared to main and auxiliary engines for a VLCC vessel.

Overview of a ship’s steam system

In commercial ships, the steam system normally includes the following equipment:
  1. Auxiliary boilers
  2. Exhaust gas economisers
As the names imply, the exhaust gas economiser is a waste heat recovery system that recovers heat from exhaust of main or auxiliary engines and thus does not use fuel. The more the second system is used, the less will be a need for use of the auxiliary boilers, thus good maintenance and operating conditions of exhaust gas economiser should always be regarded as part of energy saving in the steam system. Figure (v) shows a typical steam system for a ship.

auxiliary-boiler-steam-system
Fig (v): Auxiliary boiler steam system

Figure (v) – A typical auxiliary boiler steam system configuration

For the sake of presenting the energy efficiency case, the ship-board steam system will be divided into the following parts:

Figure (vi) shows a typical arrangement for such components.

shipboard-steam-system
Fig (vi): shipboard steam system

Figure (vi) – Overall ship-board steam system [Alfa Laval]



Summarized below marine boiler detail Info pages:

  1. Requirement for various boiler types - water tube boilers and more
    2. The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.....

  2. Fire tube boilers working principle and operational procedure
    3. The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed. The name 'tank boiler is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler are also in use....

  3. Exhaust Gas Boilers And Economisers working procedure
    4. The use of exhaust gases from diesel main propulsion engines to generate steam is a means of heat energy recovery and improved plant efficiency.The auxiliary steam installation provided in modern diesel powered tankers usually uses an exhaust gas heat exchanger at the base of the funnel and one or perhaps two watertube boilers .....

  4. The use of boiler mountings
    5. Watertube boilers, because of their smaller water content in relation to their steam raising capacity, require certain additional mountings: Automatic feed water regulator. Fitted in the feed line prior to the main check valve, this device is essential to ensure the correct water level in.the boiler during all load conditions. Boilers with a high evaporation rate will use a multiple-element feed water control system ....

  5. Purity of boiler feedwater & feed water pump
    6. Most 'pure' water will contain some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer, which can result in local overheating and failure of the tubes. Other salts remain in solution and may produce acids which will attack the metal of the boiler. An excess of alkaline salts in a boiler, together with the effects of operating stresses, will produce a condition known as 'caustic cracking'. This is actual cracking of the metal which may lead to serious failure.....

  6. The steam-to-steam generator working principle and operational procedure
    7. Steam-to-steam generators produce low-pressure saturated steam for domestic and other services. They are used in conjunction with watertube boilers to provide a secondary steam circuit which avoids any possible contamination of the primary-circuit feedwater. The arrangement may be horizontal or vertical with coils within the shell which heat the feedwater.....

  7. How to control combustion in a marine boiler
    8. The essential requirement for a combustion control system is to correctly proportion the quantities of air and fuel being burnt. This will ensure complete combustion, a minimum of excess air and acceptable exhaust gases. The control system must therefore measure the flow rates of fuel oil and air in order to correctly regulate their proportions.....

  8. Safe boiler operation - Preparations & raising steam
    9. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water.....

  9. Fuel oil burning process - various design burners
    10. Marine boilers currently burn residual low-grade fuels. This fuel isstored in double-bottom tanks from which it is drawn by a transfer pump up to settling tanks. Here any water in the fuel may settle out and be drained away.

  10. Boiler arrangement - combustion process - supply of air
    11. Combustion is the burning of fuel in air in order to release heat energy. For complete and efficient combustion the correct quantities of fuel and air must be supplied to the furnace and ignited. About 14 times as much air as fuel is required for complete combustion....

  11. Ordinary spring-loaded safety valve and improved high-lift safety valve for a marine boiler
    12. Safety valves are fitted in pairs, usually on a single valve chest. Each valve must be able to release all the steam the boiler can produce without the pressure rising by more than 10% over a set period.....

  12. Correct working level for marine boilers - use of water level gauges
    13. The water level gauge provides a visible indication of the water level in the boiler in the region of the correct working level.

  13. How to maintain water level in a marine boiler ?
    14. A modern high-pressure, high-temperature watertube boiler holds a small quantity of water and produces large quantities of steam. Very careful control of the drum water level is therefore necessary. The reactions of steam and water in the drum are complicated and require a control system based on a number of measured elements......

  14. Safety precautions for working with marine boiler
    15. All boiler controls, regulators, alarms and trips must be tested regularly in accordance with the applicable Planned Maintenance System and maker’s recommendations. Each test is to be recorded with the signature of the Engineer Officer who conducted the test....



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