WO1998017947A1

WO1998017947A1 - Waste heat boiler

Info

Publication number: WO1998017947A1
Application number: PCT/FI1997/000639
Authority: WO
Inventors: Pasi Aaltonen
Original assignee: Aalborg Industries Oy
Priority date: 1996-10-24
Filing date: 1997-10-23
Publication date: 1998-04-30
Also published as: AU4783397A; FI964280A; FI101736B; FI101736B1; FI964280A0

Abstract

In this disclosure is shown a natural circulation type waste heat boiler (1), preferably a steam boiler, which recovers the thermal energy in exhaust gases from a Diesel engine or the like, and the output of which is variable. The output is controlled by moving the water level (37) in the water tube constructions of convection part (31). The change in boiler output is due to the different heat transfer characteristics of water and steam, even though the quantity of exhaust gas flowing in the convection part (31) remains unchanged. The exhaust flow velocity is chosen such that it is sufficiently high for keeping the boiler clean. The valve (4) and/or the pump (5) controls the feed water flow (3) to the steam cylinder (2), wherein a guide plate (11) is provided on the partition (10) for augmenting the heat exchange between steam (9) and feed water (7). The steam cylinder (2) and the lower end of the convection part (31) are directly connected by means of at least one drain pipe (30). Flooding of the waste heat boiler (1) is prevented by means of a high water level control (16) provided in the lower part of the steam cylinder (2).

Description

WASTE HEAT BOILER

The invention relates to a device recovering thermal energy from the exhaust gases produced by a diesel engine or a corresponding engine, to a free-circulation exhaust gas boiler, and to its power regulation system. The exhaust gas boiler according to the invention preferably acts as a steam boiler wherein liquid is converted to vapor. The exhaust gas boiler has a convection part coupled to a steam cylinder, a frame, a feed-water system, a main steam line, the requisite valves, as well as safety and control devices.

The thermal energy contained in the exhaust gases of diesel engines has conventionally been recovered for many purposes. The most common use has been the recovery of energy for heating purposes. Fuel costs can be lowered substantially, for example on ships, by mounting exhaust gas boilers at a point after the principal engines. As the ship moves the exhaust gas boilers produce all the necessary steam used on the ship. An inexpensive exhaust gas boiler system will pay for itself very rapidly also in connection with auxiliary engines, especially if the ship remains long in port.

The number of diesel power stations used on land has increased almost explosively. A diesel power station is often a superior option for difficult conditions such as islands, jungle, wilderness, etc. From the exhaust gases of diesel power stations the heat has been recovered by using exhaust gas boilers. In the largest plants there have even been used systems in which the steam is superheated and directed to the turbines, thus increasing the efficiency ratio of the plant in the production of electricity. Thermal energy obtained from exhaust gases has also been used for the heating of housing and other premises. It is true that, for example, in the tropics the need for heat is small, and complicated and expensive forced-circulation exhaust gas boilers are nor suitable for these purposes. Furthermore, complicated and technically highly advanced regulation systems have not functioned faultlessly in severe conditions and with often deficient operating experience.

Power regulation in exhaust gas boilers has most commonly been effected by using bypass regulation, i.e. a bypass duct by means of which a portion of the exhaust gases is caused to bypass the heat exchange surfaces of the boiler. This arrangement requires a separate regulating damper, or separate regulating dampers, by means of which flows are directed. Since the dampers are operated in hot conditions, they have to be made sturdy and there have been problems with their bearings. Furthermore, their regulating and operating reliability has not been of the desired standard.

Another commonly used regulating method has been condensation regulation, wherein excess steam is condensed by means of water-cooled or air-cooled condensers. This system is quite expensive as regards its apparatus, and on the other hand it also consumes energy; for example, the electricity consumption of the air-cooler blowers may be significant.

Diesel engines using heavy fuel oil as the fuel require heating power for the preheating of the oil. In plants equipped with boilers this can be attended to by using a separate heat exchanger. In diesel power stations which do not have recovery of heat from exhaust gases or any other boiler plant, electric resistors are commonly used for preheating the oil. In terms of apparatus, heating with electricity is simple. In terms of the total energy economy the system is poor, since there is unused thermal energy available and electricity produced with low efficiency is expensive for heating purposes. A sufficiently simple and inexpensive apparatus for replacing electric preheating of oil has been lacking from the market. The object of the invention according to the present application is to provide an apparatus which is sufficiently simple to replace the electric oil preheaters of diesel engines and the like, and which also generates energy for other uses, for example for heating. The apparatus according to the invention does not reduce the generation of electricity by the diesel power station but exploits waste energy.

The invention is based on directing all of the exhaust gases of a diesel engine or the like through an exhaust gas boiler, the rate and velocity of the flow depending on the running step of the engine. The power of the exhaust gas boiler is regulated by regulating the feed-water rate by means of a valve and/or a pump in the feed-water line, the water rate and water level varying in the convection part. The regulability depends on the different heat transfer properties of the vapor and liquid phases.

More precisely, the invention is characterized in what is stated in the characterizing clauses of the claims below.

For the power regulation of the exhaust gas boiler, the power is adjusted to the desired level by varying the quantity of water and the water level inside the preferably vertical pipes of the convection part on the basis of the different heat transfer properties of the liquid and vapor phases. The water level in the convection part is regulated by means of a pump and/or a valve mounted in the feed-water pipe. A surface alarm situated in the lower section of the steam cylinder or the upper section of the downcomer prevents the overfilling of the convection part and the exhaust gas boiler with water. The pump mounted in the feed water pipe may work continuously, in which case the regulating is preferably carried out by means of a valve on the pressure side of the pump, either by continuous regulation or by open/shut regulation. In adjusting the output and operation of the pump, the regulating properties can be im- proved by adjustment of the valve. In the simplest case the pump is on/off regulated and the valve is a back valve. The valve may also be of the self-actuated type, being controlled directly by means of the pressure, temperature or the like. The downcomer or downcomers can be replaced with a steam cylinder extending to the level of the lower section of the convection part, the steam cylinder in this case also serving as the feed- water container and slowing down the regulation but increasing the water space. The feed water can be injected into the steam cylinder in drop form in order to improve direct heat exchange with steam. Various guide baffles and/or pipes may be used with the aim of lengthening the retention time of the feed water and of arranging the feed water into a film-like shape or the like, in which case the superheated steam will be condensed and the feed water will be heated with maximum efficiency. The output and power of the exhaust gas boiler are regulated by means of a pressostat, a thermostat or a sensor measuring some other quantity. The pipes of the convection part may be ribbed pipes or smooth pipes, and they may be replaced entirely or in part by sheet structures. The exhaust gases flow in their entirety, unregulated, through the convection part. The amount of feed water and the power of the exhaust gas boiler can also be regulated by signals obtained from the targets of use and/or operating devices. If, for example, a power station or a ship has several diesel engines, the exhaust gas boilers can be connected to a common steam cylinder, and the separate exhaust gas boilers can be regulated by means of sensors mounted in the common steam cylinder.

The invention provides significant advantages.

The exhaust gas boiler is simple and inexpensive in structure. The boiler has no moving parts, which means low maintenance costs. Power regulation is very simple. The exhaust gas boiler according to the invention is highly suitable for severe and demanding operating conditions owing to its reliability in operation. The need for sweeping soot from the convection part is reduced, since the boilers can be dimensioned for the maximum exhaust gas flow velocities, thus improving their self- cleaning. Usability is also improved by the reduction of the risk of soot fires. By the use of an exhaust gas boiler according to the invention and the steam produced by it for preheating the oil or for some other electricity-consuming purpose, the overall efficiency ratio of the plant is improved, since expensive electric energy is replaced by surplus heat. The formation of boiler scale can be reduced by using a vertical pipe structure in the convection part. Owing to the simple regulating method, the number of spare parts is small, and they are easily available all over the world. In problem situations the exhaust gas boiler can easily also be run manually.

The invention is described below with reference to the accompanying drawings .

Figure 1 depicts a front elevation one embodiment of the exhaust gas boiler schematically, in cross-section.

Figure 2 depicts a side elevation of the exhaust gas boiler according to Figure 1, partly in cross-section.

Figure 3 depicts schematically a flow diagram in which the exhaust gas boiler is coupled to the preheating heat exchanger for diesel engine oil.

In Figures 1 and 2, the storage container for the steam generated by the exhaust gas boiler is a steam cylinder 2. The feed-water flow 3 from a water treatment apparatus is regulated by means of a valve 4 and a pump 5. A feed-water pipe 6 directs the feed water 7 through the mantle 8 of the steam cylinder 2. The feed water 7 condenses the steam 9. Guide baffles 11, attached to a partition 10 inside the steam cylinder 2, lengthen the retention time of the feed water 7 in the steam flow 13 entering via an opening 12. A pressostat 14 regulates, for example, by an electric signal, the valve 4 and the pump 5 via a wire 15. A limiter 16 of the upper water level of the feed water 7 transmits a signal via a wire 17 and closes the valve 4 and/or stops the pump 5. A safety valve 18 in the steam cylinder 2 releases excess pressure from the exhaust gas boiler 1, and an underpressure guard 19 prevents the formation of underpressure upon the cooling of the exhaust gas boiler 1. A pressure gauge 20 indicates the steam 9 pressure prevailing in the steam cylinder 2. The steam flow 21 produced by the exhaust gas boiler 1 is regulated by a main steam valve 22 mounted in the main steam pipe 23. The mantle 8 of the steam cylinder 2 may extend as indicated by the dotted line 24, thus increasing the water space for feed water 7.

A downcomer 30 interconnects the lower section of the steam cylinder 2 and the lower distributor pipes 32 of the convection part 31, the pipes 32 distributing the feed water 7 to the vertical pipes 33. The heat exchange properties on the exhaust gas side of the convection part 31 are improved by means of ribs 34. Upper collector pipes 35 direct the steam generated in the convection part via a connecting steam pipe 36 to the steam cylinder 2. The power of the exhaust gas boiler 1 is regulated by means of the water level 37. The flow of the feed water 7 to the convection part 31 is indicated by arrows 38 and the upper limit for the level of the feed water 7 by arrow 39.

The convection part 31 is attached to bear, by mediation of lower and upper support plates 51 and 52, against the frame 50 of the exhaust gas boiler 1. The exhaust gas flow 53 in its entirety runs through the convection part 31 via inlet and outlet openings 54 and 55. The exhaust gas boiler 1 is attached to the exhaust gas duct by means of flanges 56 and 57. Cylinders 58 and 59 connect the flanges 56 and 57 to the ends 60 and 61 of the frame 50. The convection part 31 of the exhaust gas boiler 1 can be inspected and cleaned via hatches 62.

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Claims

1. An exhaust gas boiler (1) recovering thermal energy from the exhaust gases of a diesel engine or the like, preferably a steam boiler the power of which can be regulated, the operating mode of which free-circulation, and which comprises a steam cylinder (2) acting as the steam space; a convection part (31) acting as the vaporizer, mounted in the vertical direction so as to have its upper section at a point lower than the steam cylinder (2); at least one downcomer (30) interconnecting the lower section of the convection part (31) and the steam cylinder (2); a main steam pipe (23); and a feed-water pipe (6), characterized in that

- in the feed-water pipe (6) connected to the steam cylinder (2) there are mounted regulating devices, a valve (4), a pump (5) and/or the like, by means of which the water rate and the water level (37) in the convection part (31) are regulated;

- the preferably vertically disposed pipes (33), or the like, of the convection part (31) are on the inside partly filled with water, and thus by changing the water level (37) in the convection part (31) the power of the exhaust gas boiler (1) is regulated on the basis of the different heat transfer properties of water, a water-steam mixture and steam; and

- in the lower section of the steam cylinder ( 2 ) or the upper section of the downcomer (30) there is fitted an upper level limiter (17) which prevents overfilling of the convection part (31).

2. An exhaust gas boiler (1) according to Claim 1, characterized in that the valve ( 4 ) in the feed-water pipe ( 6 ) is manual and/or continuously regulated and/or of the open-shut type.

3. An exhaust gas boiler (1) according to Claims 1-2, characterized in that the power of the pump ( 5 ) mounted in the feed-water line is regulable, preferably regulable on the basis of the rotation speed and/or of the on/off type.

4. An exhaust gas boiler according to Claims 1-3, characterized in that the lower section (24) of the steam cylinder (2) extends to the level of the lower section of the convection part (31) and is filled with feed water (7) in accordance with the power of the exhaust gas boiler (1).

5. An exhaust gas boiler (1) according to Claims 1-4, characterized in that the feed-water pipe ( 6 ) is connected to the steam cylinder (2) at a point close to the opening (12) of the connecting steam pipe (36), that to the partition wall (10) of the upper section of the steam cylinder (2) there is attached at least one whole or perforated baffle (11) in an upwardly oblique disposition, or that the feed-water pipe (6) is extended to the inside of the steam cylinder (2) by means of a pipe arrangement which increases the heat exchange surface and the retention time of the feed water ( 7 ) in the steam cylinder (2) in order to condense the superheated steam (9) and to heat the feed water ( 7 ) .

6. An exhaust gas boiler (1) according to Claims 1-5, characterized in that the pump (5) and/or valve (4) mounted in the feed-water pipe (6) is controlled by means of a pressostat (14), a thermostat (78) or some corresponding sensor, or that the valve (4) is self-actuated, controlled directly by the pressure, temperature, or the like.

7. An exhaust gas boiler (1) according to Claims 1-6, characterized in that the pipes (33) of the convection part (31) are ribbed pipes and/or smooth pipes, or the pipes are replaced by a sheet structure.

8. An exhaust gas boiler (1) according to Claims 1-7, characterized in that the exhaust gas flow (53) running through the exhaust gas boiler (1) travels in its entirety through the convection part (31).

9. An exhaust gas boiler (1) according to Claims 1-8, characterized in that the sensor, thermostat (78) or the like of the operating device is fitted to regulate the feed-water flow (3).

10. An exhaust gas boiler (1) according to Claims 1-9, characterized in that a plurality of exhaust gas boilers are coupled to a common steam cylinder (2), which, alone and/or together with operating devices, controls the separate exhaust gas boilers ( 1) .