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waste heat recovery system

Oct 29, 2014

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marine engine waste heat recovery system

Waste Heat Boilers and Systems WASTE HEAT BOILERS & SYSTEMS OBJECTIVE Upon completion of this lesson, students should be able to: 1. Know the reasons for waste heat recovery from the diesel engine. 2. Know the various layouts for such a WHR plant. 3. Know how to start, stop and operate the EGE safely. 4. Explain how to control the steam evaporation rate from the EG boiler. 5. Explain the reason for soot fire and hydrogen fire 6. Know the problems associated with dry running of the EG boiler. 7. Know how to combat EG boiler fire and control it. LESSON OVERVIEW In a diesel engine vessel the steam and electricity requirement at sea could be met by heat recovery from the engine exhaust gases using a waste heat recovery (WHR) boiler. The lesson discusses various such arrangements including the design and management issues associated with such a plant. REFERENCES Morton, Thomas D, Steam Engineering Knowledge for Marine Engineers (1994), Thomas Reed Publications Flanagan, G T H, Marine Boilers Question & Answers (2002) Milton, J H and Leach, Roy M, Marine Steam Boilers (1995)

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Waste Heat Boilers and Systems INTRODUCTIONThe propulsion diesel engine converts the chemical energy of fuel to heat energy by combustion and that energy is then converted to work in -rotating the crankshaft and propel the ship. When considering a heat balance diagram which, by way of example, is shown in Fig. 1 for a nominally rated highly efficient engine version 6S60MC-C (or 6S60ME-C), operating on 80% SMCR (80% of specified maximum continuous rating), the most attractive waste heat source is the exhaust gas heat. Approximately one fourth of the fuel energy comes out as exhaust gas heat. Even though the exhaust gas temperature the last 25 years has decreased about 130C, from approx. 375C to approx. 245C (ISO), as a result of the obtained higher efficiency of diesel engines, exhaust gas boilers are installed on almost all merchant ships of today.

Fig 1 Heat balance for a diesel engine

TYPES OF WASTE HEAT BOILERS The various W.H.R. Boilers can be broadly divided into two groups, one with natural' and the other with 'forced' circulation.I

Natural Circulation Boiler The Natural Circulation Units are normally low capacity units (maximum 2-5 t/h) with a Tank type design. Tubular designs are also possible, but these must be

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Waste Heat Boilers and Systems positioned at the same elevated Engine Room position together with the Auxiliary boilers. In many Tank boiler installations, a Composite design is used. Here, different tube banks are allocated for the oil firing and the exhaust gas flow. This enables simultaneous firing on oil as well as running on exhaust gas. The composite marine steam boiler is a combination of an oil-fired steam boiler and an exhaust gas economizer. When the diesel engine is at full load the fuel oil burner only starts if the steam demand exceeds the steam production achieved from the diesel engines exhaust gases. Smoke tube type

Fig 2 Mission OC Boiler

Fig 3 Smoke tube boiler

Aalborg MISSION OC is a 5 tons/hour vertical composite boiler with an exhaust gas section consisting of smoke tubes. The cylindrical shell surrounds the smoke tubes, the furnace, the steam space and the convective section consisting of pintube elements. The boiler pressure part is made of well-proven mild steel with elevated temperature properties. Forced Circulation Boiler The Forced Circulation Units are more popular and possess better heat transfer potential. Extreme flexibility in their positioning inside the engine room (E.R.) is possible due to the compact size and force circulation through the boilers. They can either be of the Coil Type or the Tubular Type. The Coil type has problem in cleaning and maintenance. The Tubular design is more common to-day because of better thermal efficiency, easier cleaning & repair ability and flexibility to control steam generation.

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Waste Heat Boilers and Systems The design of the W.H.R. Unit depends mainly on the available thermal energy at the boiler inlet and may range from a simple tank type unit to a multi-bank tubular design. The exhaust gasTubular type normally used for the production of Water tube boiler system

Fig 4 Miura Exhaust Gas Economiser external view

Fig 5 Miura Exhaust Gas Economiser- Internal view

saturated steam needed for heating services is shown in Fig. 6. This is a simple, single-pressure steam system in which the exhaust gas boiler consists solely of an evaporator section. The feed water is pumped directly to the oil-fired boiler which is used as a common steam drum for the oil-fired boiler and the exhaust gas boiler. Separate steam drums may also be used, so that one boiler can be run if the other should malfunction. Because of its simplicity and low capital cost, this system is widely used and is often entirely adequate when the steam production is viewed as a means of meeting the steam demand for heating services on the ship. The loss of water from the exhaust gas boiler is in the magnitude of about 1% of the steam production.

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Waste Heat Boilers and Systems

Fig 6 Normal exhaust gas boiler system for steam production Single pressure steamsystem with evaporator section only

A larger demand for steam in a Tanker or Specialized cargo vessel may require the use of a high capacity auxiliary water tube boiler. A dedicated W.H.R. Unit is also more likely. A big and powerful engine will mean a lot of waste energy. This could then be used to drive a Turbo-Alternator to supply electrical energy and even aid propulsion through appropriate gearing arrangement.

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Waste Heat Boilers and Systems

170 oC

130oC 180oC 165oC 220oC 165oC

Feed water

15oC

250oC

165oC

Fig 7 Special exhaust gas boiler system with turbo generator for electricity production

Fig 8 T/Q diagram

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Waste Heat Boilers and Systems

Pinch point A temperature/heat transfer diagram, a so-called T/Q diagram, illustrates the characteristic temperature course through the exhaust gas boiler. As an example valid for the special exhaust gas boiler system shown in Fig. 7, a T/Q diagram is shown in Fig. 8. The utilization efficiency of an exhaust gas boiler is characterized by its pinch point. The pinch point is the lowest temperature difference between the exhaust gas and the saturated steam, i.e. the temperature difference between the exhaust gas leaving the evaporator section and the saturated steam, see the T/Q diagram in Fig. 8. Normally, the steam pressure will be above 7 bar abs. (6 barg) and often equal to 8bar abs. (7 barg), corresponding to a minimum evaporation temperature of 165C. According to the T/Q diagram the gas outlet temperature, even for a boiler with feed water preheater section, will therefore not be lower than about 165C, when 20C or above is used as the pinch point. In principle, the pinch point may be considered a measure of how extensive and how efficient the heat utilisation of the exhaust gas boiler is. Corrosive action of Sulphur & uptake temperature The exhaust gases enter the superheater at a temperature of 250 C . As they pass through evaporator section their temperature drops to 180 C and then to about 170C at outlet from preheater. However, if any metal surfaces in the uptakes fall below the 'dew point' temperature at which the water vapour in the exhaust gases condense, then any sulphur dioxide or trioxide present in the exhaust gases, formed by the combustion of any sulphur in the fuel, will be absorbed by the water. This results in the formation of acidic deposits on any metal surfaces below 170C. This acid can cause corrosive attack on mild steel surfaces at temperatures below 138C, and on cast iron at temperatures below 115C. Feed water temperature The feed temperatures are also important as the metal temperatures involved are a function of these feed temperatures, being in general some 5C higher. It is thus important to maintain these minimum temperature values under all steaming conditions. If the normal feed temperature entering the economiser is about 140C there is an allowable margin of some 15-20C, but the feed temperature should never be allowed to fall below the 115C limit under steaming conditions; if it does, heavy fouling can occur in addition to the corrosive attack, due to soot deposits building up on the acid dew forming on the cool metal surfaces. Thus some means should be provided to maintain the feed temperatures above these minimum values; for example, by supplying extra steam to a deaerator in a system where one of these is fitted. These points are vital as the gas temperature leaving the economiser, provided it is above the dew point temperature, has little to do with fouling or corrosion as long as the metal temperatures are kept above the minimum value quoted. The water temperature within the economiser should not be allowed to exceed a temperature 30C below the corresponding evaporation temperature; this prevents the formation of steam within the economiser.

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A DOUBLE EVAPORATION SYSTEM: CONTAINING WATER TUBE AND TUBULAR WASTE HEAT BOILER The above system was designed to safeguard the aux. w.t. boiler tubes from early failures. In tankers a w.t. boiler/tubular waste heat boiler utilising an open feed system may develop frequent tube rupture problems due to contamination from air, dissolved salts and rust particles. In the double evaporation system, the water tube boiler is part of the 'primary' circuit and steam produced at the w.t. boiler is fed to a submerged coil in the Steam/Steam Generator that is p