Liquid heaters and vaporizers – Industrial – Waste heat
Reexamination Certificate
2001-06-25
2002-10-29
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Industrial
Waste heat
C122S00400R, C165S104180
Reexamination Certificate
active
06470834
ABSTRACT:
The invention relates to a method for utilising the heat in the flue gas from incinerator plants, such as, e.g., steam boiler plants and hot water boiler plants with varying loads while simultaneously removing condensable elements in the flue gas and substantially reducing the flue gas's corrosive activity The method may be employed in all types of incinerator plant where a combustion zone produces hot flue gases which are cooled in one or more heat exchangers which transfer the flue gas's exploitable heat into a liquid or gas. The method is also well-suited to incinerator plants with considerable load variations which require the amount of fuel to be adjusted to keep pace with the heat requirement.
BACKGROUND AND THE PRIOR ART
Hot flue gases from incinerator plants which are fired by solid fuels such as biomass, household rubbish, wood chips, industrial waste or hydrocarbons in solid or liquid form, often contain condensable components with high boiling points. These elements will condense on surfaces which have a lower temperature than their evaporation point, thus forming deposits. These elements are often called slag formers, and exist in solid and liquid form. The deposits are always heat-insulating and usually corrosive. In addition, the flue gases contain other components which are often corrosive and/or erosive.
In incinerator plants which are fired by biomass, household rubbish or industrial waste, melted salts constitute a particularly problematic condensate. The salts will condense on surfaces which have a lower temperature than the evaporation point. In conventional boiler plants the hot flue gases will be passed to heat exchangers, such as, e.g., preheaters, evaporators and superheaters which produce water vapour, where the gases exchange their heat content for another heat-transferring medium (water or steam). In such plants the salts will be deposited on heat exchanger surfaces and other walls which are sufficiently cold. As time goes by the deposits become so thick that they will create a problem, since the deposits substantially reduce the heat transfer from the flue gas to the other heat-transferring medium in the heat exchangers. In some cases the situation may also arise that the deposits become so large that they physically block the passage of the flue gases. Regular stoppages are therefore necessary in order to clean deposits from the relatively cold parts of the plant and the heat exchangers. An additional factor is that in conventional plants, the plants have had to be overdimensioned in order to increase the cooling capacity to counteract the reduction in the heat transfer resulting from the deposits, and in some cases expensive purification plants have had to be provided. Furthermore, a reduction in the heat transfer will have the result that the flue gases are not cooled to the same extent, thus reducing the plant's fuel economy and the discharge of polluting gases will be greater.
In addition to this, the salts in particular, but also other components in the flue gases have a corrosive effect on the walls of the plant and the heat exchangers. This entails a further need for cleaning and maintenance and not infrequently considerable repair costs for these steam boiler plants and/or hot water boiler plants. Another consequence of reduced heat transfer (cooling) as a result of the build-up of deposits is that the temperature of the gases increases. A local rise in temperature in corrosive environments often leads to increased repair costs and to the choice of more corrosion-resistant and thus more expensive materials in the plant. Another effect of the flue gases' rise in temperature is that the composition is changed. This may lead to problems with undesirable gases such as, e.g., NO
X
.
The production in a heating plant or thermal power plant has to be regulated in step with the consumption of hot water and/or steam. In the case of supplies to the processing industry in particular the load variations can be substantial. In conventional incinerator plants the heat exchangers are usually fixed installations with a constant surface, with the result that large sections of the plants receive almost constant capacity. When the cooling capacity is not proportional with the amount of fuel supplied, the flue gas's temperature will normally fall when the power is reduced. As already mentioned, variations in the flue gases' temperature are undesirable since they result in an alteration in combustion conditions and thereby variations in the degree of combustion and discharge of polluting gases. This means that conventional plants are lacking in flexibility with regard to variations in load.
Due to the increasingly stringent environmental requirements in recent times the combustion is performed with such a low level of solid soot particles and other impurities that many of the fixed fuel fractions are directly sublimated into the gas phase. Under the given conditions, these gases may condense into very small crystals or form an amorphous dust. This dust lies like a relatively thin but effectively heat-insulating layer on walls and surfaces in the plant. This increases the need for further cleaning of the plant. Examples are known of modern boilers for ships where the need to stop production in order to clean this dust layer has increased by a factor of 15 to 20-fold.
The use is known from Norwegian patent application no. 971603 from ABB Fläkt AB of an adsorbent in powder form for condensable components in hot industrial waste gas for cleaning condensate from the surfaces of a boiler. The adsorbent is mixed with the waste gas stream before the boiler and is carried along with the gas stream. When the gas reaches the cold walls the condensable components will be condensed on the walls of the cooler and the adsorbent will adsorb the condensate, thereby cleaning the walls of the cooler. This method. however, will not work for salt deposits as they are too hard. Condensable salt deposits should be stopped before they reach the cold surfaces in the plant.
U.S. Pat. No. 4,702,818 from Oyamoto et al. concerns a method for recovering heat from a hot gas containing tar substances. The hot gas is first passed through a layer of hot solid particles in a jet cooler, thus forming a fluidised bed where the particles are carried along with the gas stream and cooled together with the gas by means of a plate heat exchanger, with the result that a first portion of the tar substances is absorbed on the solid particles. In the next stage the gas is passed through a tar cooler where the gas is sprayed with tar drops which absorb a second portion of the tar substances in the gas before the gas is cooled in a third stage where it is sprayed by fine tar drops which absorb the last portion of the tar substances,
THE OBJECT OF THE INVENTION
It is an object of the present invention to provide a method which utilises the heat from flue gases flowing out of an incinerator plant and which reduces and/or eliminates the above-mentioned disadvantages.
It is also an object of the present invention to provide embodiments of steam boiler plants and/or hot water boiler plants for implementing the method which are flexible with regard to varying loads.
DESCRIPTION OF THE INVENTION
The objects of the present invention are fulfilled by a method and plants as indicated in the attached patent claims, and which are described in detail below.
The present invention is based on the idea that the above-mentioned drawbacks of corrosive activity and deposits of condensable components in the flue gases can be eliminated and/or substantially reduced by separating the hot flue gases from the heat-transferring of the heat exchangers to another heat-transferring medium. This may be achieved, e.g., by the hot flue gases giving up heat to a cold particulate material which is transported away from the zone where it comes into contact with the flue gases to a zone where it comes into contact with one or more heat exchangers where the material's heat is transferred to another heat-tran
Abel Christian D.
Wilson Gregory
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