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  Block heat exchanger assembly for dust-containing flue gases and method of operating the sameUSPTO Application #: 20060194159Title: Block heat exchanger assembly for dust-containing flue gases and method of operating the same Abstract: A method of operating a heat exchanger assembly for combustion devices which operate by using condensing boiler technology and produce dust-containing flue gases, e.g. from the combustion of biomass, reduces the formation of dust deposits in flue gas channels of the heat exchanger. A heat exchanger assembly suitable for operation in the presence of dust-containing flue gases is also provided. (end of abstract) Agent: Lerner Greenberg Stemer LLP - Hollywood, FL, US Inventors: Marcus Franz, Gerhard Kalterherberg, Jurgen Kunzel, Peter Pollmann, Soren Gotz USPTO Applicaton #: 20060194159 - Class: 431011000 (USPTO) Related Patent Categories: Combustion, Process Of Combustion Or Burner Operation, Heating Feed The Patent Description & Claims data below is from USPTO Patent Application 20060194159. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION FIELD OF THE INVENTION [0001] The invention relates to methods of operating block heat exchanger assemblies for combustion devices which operate by using the condensing boiler technology and produce dust-containing flue gases. The invention also relates to block heat exchanger assemblies suitable for operation in the presence of dust-containing flue gases. [0002] In condensing boiler technology, the flue gases to be taken off from a combustion device are cooled in a heat exchanger to below to the dew point, so that at least partial condensation of the water vapor present in them is brought about and the heat withdrawn in the process is utilized by transferring it to a low-temperature heat transfer medium, e.g. the return stream of a heating circuit, or to the feed water for a hot water source. Stainless steel heat exchangers are usually used for condensation. [0003] The condensate includes water and possibly further water-soluble-flue gas constituents, e.g. oxides of sulfur (SO.sub.2, SO.sub.3), which form corrosive acids in water. The composition of the condensate and its corrosive action depend very greatly on the fuel being used. While the combustion of desulfurized natural gas is relatively unproblematical, high concentrations of SO.sub.2 and SO.sub.3 are formed in the flue gas in the combustion of heating oil or biomass. In the case of heating oil having a sulfur content of 1,000 ppm, considerable corrosion problems occur in metallic heat exchangers, according to information from the manufacturers. [0004] A further problem is the relatively high chlorine content of biomass fuels such as straw, cereal, wood chips, wood pellets or firewood. This, too, leads to corrosion of metallic condensing boilers. For this reason, the reduction of the flue gas temperature to below the dew point and the utilization of the heat of condensation is frequently not carried out in the case of these fuels. [0005] Corrosion-resistant heat exchangers are necessary, to be able to exploit the advantages of the condensing boiler technology even in the case of devices operated by using biomass or fuels having a high sulfur content. A heat exchanger block composed of plates of graphite is known from German Utility Model 296 04 521 U1. Channel systems for two media are disposed on different planes within the block. The channels for the gaseous medium which gives off heat are formed by grooves inserted into the flat sides of the plates, and ridges remain between them. The channels for the second medium to be heated (hereinafter referred to as the heat transfer medium) are formed by holes penetrating through the plates. The thickness of the plates is chosen in such a way that the two channel systems located on different planes are separated by a thin material barrier which offers little hindrance to heat transfer but has a thickness which is sufficient to separate the two channel systems from one another in a fluid-tight manner. The channels of the two groups can be disposed parallel or perpendicular to one another, depending on whether the media are to flow in countercurrent or cocurrent or in cross-current. [0006] A heat exchanger block includes at least two superposed plates, at least one of the adjoining flat sides of which is provided with channel-forming grooves. Since a plurality of such pairs of plates can be laid side by side and on top of one another, many combinations are possible. This modular construction allows specific matching of the heat exchanger to various requirements. The plates are joined in a fluid-tight manner through the use of adhesives or through the use of gaskets and tensioning or clamping anchors. [0007] In these heat exchangers, the channels for the gaseous medium are preferably configured in such a way that a high ratio of heat exchange surface to channel volume is achieved. However, it has to be noted that the free flow cross section has to be large enough to ensure outflow of the gases as a result of the chimney draft. One suitable structure includes channels in the form of grooves having a high ratio of depth to width. [0008] However, the effectiveness of those measures for improving heat transfer and flow conditions is impaired if solid deposits are formed on the walls of the gas channels. That problem occurs in particular in the case of biomass, e.g. combustion devices operated by using wood chips or pellets, since their offgases have a high dust and ash content. The fouling of the heat exchange surface leads to an increase in the heat transfer resistance and consequently to a reduction in the rate of heat transfer. At the same time, the deposition of dust inevitably leads to significant pressure drop, since the free cross section of the gas channels is reduced. [0009] In a published article in the journal "Sonne, Wind und Warme" from the year 2002 (Number 03/2002, pp. 50-59), a necessary development time of 10 years was predicted for the utilization of condensing boiler technology for wood-fired boilers because of the difficulties associated with the high ash content. In a market review published in the same journal, the engineering parameters and operating properties of various wood-fired systems available on the market were compared and the necessity of regular cleaning of the flue gas heat exchanger to remove fly ash was emphasized. That has to be carried out manually 15-29 times per heating period for one third of the apparatuses tested, while other manufacturers offer an automatic cleaning facility for the heat exchanger, but that increases the price of the heating system. [0010] The publication "Erprobung der Brennwerttechnik in hauslichen Holzhackschnitzelfeuerungen mit Sekundarwarmetauscher"" (issued by the Technologie-und Forderzentrum im Kompetenzzentrum fur Nachwachsende Rohstoffe Straubing 2004, pp. 21-23) also addresses the problem of the dust content of the flue gas. An experimental device included a boiler fired with wood chips and a shell-and-tube heat exchanger having a plurality of horizontal bundles of tubes composed of a special ceramic having a high thermal conductivity, disposed next to and above one another. Flue gas and outflowing condensate were conveyed in cocurrent within the shell-and-tube heat exchanger. [0011] Studies were carried out to determine whether or not the dust content of the offgas and the deposition of dust on the heat exchanger tubes could be reduced by spraying of fresh water (quenching) into the flue gas stream immediately before it enters the heat exchanger. However, both objectives were not able to be achieved, and the dust emission in quench operation was actually higher. The configuration of the heat exchanger tubes above one another was intended to prevent the dust from depositing on the heat exchanger tubes and blocking the spaces between the tubes. However, it was found that it could not be achieved even through the use of quench operation in the case of the flue gases laden with a large amount of dust from the combustion of wood chips. [0012] Deposition of dust in shell-and-tube heat exchangers is less problematical than in the block heat exchangers known from German Utility Model 296 04 521 U1, since in those heat exchangers the milled channels become unavailable for flue gas transport and thus for heat transfer over their entire length in the case of blockage, so that the heat transfer area decreases drastically. On the other hand, the compact construction of the block heat exchangers made up of plates as described in German Utility Model 296 04 521 U1 is a great advantage over shell-and-tube heat exchangers. SUMMARY OF INVENTION [0013] It is accordingly an object of the invention to provide a block heat exchanger assembly for a combustion device, which operates by using condensing boiler technology that produces dust-containing flue gases, and a method for operating the block heat exchanger assembly, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensure that walls of gas channels required for heat transfer and condensate formation are reliably kept free. [0014] With the foregoing and other objects in view there is provided, in accordance with the invention, a method of operating a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The method comprises causing flue gas and condensate to flow in countercurrent in a heat exchanger block. [0015] With the objects of the invention in view, there is also provided a method of operating a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The method comprises spraying condensate from a neutralization and collection vessel and/or fresh water, continuously or at intervals, into hot flue gas, at or immediately before entry into a heat exchanger block. [0016] With the objects of the invention in view, there is additionally provided a method of operating a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The method comprises causing flue gas and condensate to flow in cocurrent. Condensate from a neutralization and collection vessel and/or fresh water are/is sprayed into hot flue gas at or immediately before entry into a heat exchanger block, in an amount causing a flue gas stream to be saturated with water vapor. [0017] In accordance with another mode of the invention, the heat exchanger block is made of graphite, and clarified condensate is used without neutralization for moistening the flue gases. [0018] The method of the invention simultaneously increases the condensate formation per unit area and ensures that the maximum calorific value of the fuels is utilized to the greatest possible extent. The fuel usage is reduced due to the improved utilization of the maximum calorific value according to the invention. [0019] With the objects of the invention in view, there is also provided a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The assembly comprises a heat exchanger block having a flue gas inlet in a flue gas inlet region. A flue gas line has an entry into the flue gas inlet region of the heat exchanger block. A neutralization and collection vessel is provided for condensate with overflow. A nozzle is disposed at the flue gas inlet region of the heat exchanger block or at the flue gas line immediately before the entry into the heat exchanger block. A line connects the neutralization and collection vessel to the nozzle. A pump conveys the condensate from the neutralization and collection vessel through the line to the nozzle. [0020] With the objects of the invention in view, there is additionally provided a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The assembly comprises a heat exchanger block being made of graphite and having a flue gas inlet in a flue gas inlet region. A flue gas line has an entry into the flue gas inlet region of the heat exchanger block. A collection vessel is provided for condensate having an overflow connected to a neutralization and purification vessel. A nozzle is disposed at the flue gas inlet region of the heat exchanger block or at the flue gas line immediately before the entry into the heat exchanger block. A line connects the neutralization and collection vessel with the nozzle. A pump conveys the condensate from the collection vessel through the line to the nozzle. [0021] In accordance with another feature of the invention, the heat exchanger block includes flue gas channels having walls provided with a coating reducing adhesion of dust particles and having a thickness of from 30 to 500 .mu.m. The coating includes a fluoropolymer or a dust-repellent paint or varnish. Continue reading... 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