CONTROLLING RAPPING CYCLE

Abstract: A method and apparatus for controlling rapping of heat exchanging surfaces based on the heat transfer coefficient of the exchanger systems.

Description: The present invention relates to a method for controlling rapping of heat exchanging surfaces of an indirect heat transfer zone according to the first part of claim 1. Such a method is known from US-A-4 466 383. Conventional systems for removing dust or scale deposited on heat exchanger surfaces in furnaces, boilers, etc., include soot blowing, mechanical rappers, and cleaning bodies, such as brushes, pigs or the like, passed through cooling tubes. Use of rappers to remove deposits is typically done based on a preselected cycle and frequency and with a preselected force. However, maintaining the effectiveness of heat exchanger systems requires optimizing the removal of deposits to minimize the additional heat transfer resistance attributable to the equilibrium thickness of deposits on heat exchanging surfaces, which deposits can accumulate under changing conditions. The present invention is directed towards optimizing the removal of deposits from heat exchanging surfaces in systems involving partial vaporization of water at the boiling point. The primary purpose of the present invention relates to controlling rapping of heat exchanging surfaces of an indirect heat transfer zone having fouling deposits thereon. In particular, this invention relates to controlling rapping of heat exchanging surfaces of an indirect heat transfer zone having fouling deposits, such as ash and soot, thereon within a synthesis gas system. Generation of synthesis gas occurs by partially combusting hydrocarbon fuel, such as coal, at relatively high temperatures in the range of about 700 °C to about 1800 °C and at a pressure range of from about 1 to 200 bar in the presence of oxygen or oxygen-containing gases in a gasifier. Oxygen-containing gases include air, oxygen enriched air, and oxygen optionally diluted with steam, carbon dioxide and/or nitrogen. The coal, fluidized and conveyed with a gas such as nitrogen, is discharged as fluidized fuel particles from a feed vessel apparatus, in communication with at least one burner associated with the gasifier. Typically, a gasifier will have burners in diametrically opposing positions. Generally, the burners have their discharge ends positioned to introduce the resulting flame and the agents of combustion into the gasifier. Hot raw synthesis gas is quenched, usually with recycle synthesis gas, upon leaving the gasifier and passes to an indirect heat exchanger zone, said zone having diverse one- or two-phase heat transfer sections where boiler feed water is heated to the boiling point, vaporized and/or steam is superheated. The zone supplies dry superheated steam to a steam turbine, which drives an electrical generator. Of particular importance in the economic production of synthesis gas is the optimization of heat transfer of the zone. Various factors substantially affect the heat transfer of the heat exchanger zone. In particular, fouling caused by the deposition of solids, fly ash and soot contained in the synthesis gas, on the heat transfer surfaces adversely affect the heat transfer of heat exchanger zone. It is desirable to remove these deposits by rapping in a controlled manner which takes into account that fouling deposits can accumulate in each section of the zone at different rates because of differences in conditions which occur in the sections of the zone. The invention therefore provides a method for controlling rapping of heat exchanging surfaces of an indirect heat transfer zone having fouling deposits thereon within a synthesis gas system comprising the steps of: (a) feeding particulate solids and oxygen-containing gas into a reactor, (b) partially oxidizing the solids at an elevated temperature within the reactor, (c) producing product gas within the reactor, (d) passing the product gas from the reactor to a heat exchanging zone in gas flow communication with the reactor, the zone including at least one section adapted to generate superheated steam, and a lower temperature heat exchanging section, (e) removing heat from the product gas in the heat exchanging zone by indirect heat exchange with a heat transfer using cooling system of steam and/or water, said zone comprising a plurality of sections at least one of which is a one- or two-phase heat transfer section, and in which sections, fouling deposits accumulate on the surfaces thereof the various sections at different rates because of different conditions; characterized by (f) determining the overall heat transfer coefficient of the heat transfer surfaces, including any fouling deposits thereon for each section of the zone, said determining includes determining mass flow rates of the product gas and cooling system within the heat exchanging zone, determining temperatures of the product gas and cooling system within the heat exchanging zone, and determining heat fluxes of the product gas and cooling system either directly on the product gas side or on the coolant side within the heat exchanging zone, (g) determining the relative change of the overall heat transfer coefficient due to the change of the thickness of the fouling deposits for each section as a function of time, (h) comparing the relative change of the overall heat transfer coefficient from (f) of each section with a preselected reference section, said reference section being the section of least fouling which is rapped based on its current overall heat transfer coefficient as compared to its initial overall heat transfer coefficient; (i) removing the fouling deposits from each section of the zone using rapping means, the rapping means having separate and independently controllable rapping parameters for each section of the zone, and (k) adjusting the rapping parameters for each section of said zone, the adjusting (1) including one or more of adjusting a time interval between rapping of individual rappers in a section of individual rappers, (2) adjusting rapping force, (3) adjusting the number of strikes of an individual rapper in its cycle, (4) adjusting the time interval for rapping and individual rapper and (5) adjusting the time interval between complete rapping cycle of rappers in said section. In this manner, the rapping of the heat exchanging zone is optimized and the operation of the heat exchanging zone can be performed more efficiently. Advantageously, the rapping is done on line while the heat-exchanger zone is operating as such. The method of the invention can also include the additional feature of rapping each section of the heat exchanger zone in an adjusted sequential cycle which includes rapping of the other sections of the zone based on the changes in the overall heat transfer coefficient due to the change of the thickness of the fouling deposits of each section compared to the other sections to optimize the rapping of the heat exchange zone, which can result in the optimization operation of the heat exchanging zone. The present invention utilizes a combination of heat transfer measurements in conjunction with process instrumentation to determine the overall heat transfer coefficient of each section of a one-phase or a two-phase, i.e., liquid and/or gas, indirect heat exchanging zone. In one embodiment of this present invention, the high (synthesis) gas temperature and gas composition prohibit accurate monitoring of heat transfer on the side being cooled above about 550°C to about 750 °C by means of thermocouples. The present invention uses means other than by direct measurement of gas temperatures to determine the overall heat transfer coefficient from the quality of the steam-water mixtures of a two-phase heat exchanging zone such as by gamma ray densitometer, in these areas. Additionally, the present invention permits controlling of the rapping of heat exchanging surfaces to remove fouling deposits therefrom. Controlling rapping is preferred to rapping based on a preselected cycle and frequency. Rapping too frequently can cause structural fatigue of the heat exchanging system. Also, when deposits are too thin, there is not enough internal force (i.e., not enough mass) to facilitate dislodging of deposits. Rapping too infrequently can make the deposits more difficult to remove because of sintering of the unremoved deposits caused by the high operating temperatures of the coal gasification process. Another advantage of the present invention is the ability to separately and independently control rapping means for removing the fouling deposits from each section of the heat exchanging zone. Advantageously, the means for removing deposits are operated sequentially beginning with the section closest to the reactor, and moving in the direction of synthesis gas flow. Another advantage of the present invention is the ability to calculate the relative change of overall heat transfer coefficient of the heat transfer surfaces, including any fouling deposits thereon, for each section of the heat exchanging zone which adversely affects heat transfer. A further advantage of the present invention is the capability of minimizing deposits on heat exchanging surfaces, while the heat exchanger is on line, which results in extended run lengths of gas cooling, e.g., in a coal gasification process, since significant fouling of the heat exchanger zone could otherwise require shutdown of the process to remove the fouling deposits. Although in one embodiment the invention is described hereinafter primarily with reference to cooling gas resulting from the gasification of pulverized coal, the method and apparatus according to the invention are also suitable for other finely divided solid fuels which could be partially combusted in a gasifier, such as lignite, anthracite, bituminous, brown coal, soot, petroleum coke, and the like. Advantageously, the size of solid carbonaceous fuel is such that 90 percent by weight of the fuel has a particle size smaller than No. 6 mesh (A.S.T.M.). It is remarked that US-A-4,466,383 discloses a boiler cleaning optimization with fouling rate identification and in particular economic optimization of efficiency versus sootblowing. However, the efficiency measurement has not been specified at all. Further, EP-A-0,254,379 discloses rapping means for removing deposits in a boiler system. The invention will now be described by way of example in more detail with reference to the accompanying drawings, in which: Fig. 1 illustrates an advantageous embodiment of the present invention for optimizing rapping of heat exchange surfaces in a synthesis gas system; and Fig. 2 illustrates an advantageous embodiment of the apparatus for measuring the overall heat transfer coefficient of deposits within a bundle in heat exchanging section, as applied in the present invention. The drawings are of a schematic process flow type in which auxiliary equipment, such as pumps, compressors, cleaning devices, etc., are not shown. All values are merely exemplary or calculated. Referring to Fig. 1, an apparatus for controlling rapping of heat exchanging surfaces having fouling deposits thereon, e.g., within a synthesis gas system, includes feeding particulate coal 11 and an oxygen-containing gas 12 into a gasifier 13. The coal is partially oxidized at elevated temperatures within the gasifier 13. A raw synthesis gas 20 is produced within the gasifier 13 having a temperature of from about 1100 °C to about 1700 °C. The raw synthesis gas is passed from the gasifier 13 to a heat exchanging zone in gas flow communication with the gasifier 13. The zone can include the following major sections: a quench section 14 in which recycle synthesis gas is injected at Q for cooling; an open duct section 15; and the superheater, evaporator and economizer sections, 17, 18, and 19, respectively. Each of sections 17, 18, and 19 can be subdivided into minor sections 21. Heat is removed from the synthesis gas 20 in the heat exchanging zone by indirect heat exchange whereby a one- or two-phase circulating cooling system comprising steam and/or water, in some cases at a temperature of from above about 650 °C to about 900 °C and under various conditions. In some parts of the heat exchanging zone, the circulating coolant is contained in passages embedded in the surfaces 22 of the walls of the sections 15 or 21. Additional circulating coolant can be contained in cylindrical bundles in the surfaces 22 within a section 21 of the heat exchange zone. The overall heat transfer coefficient of the heat transfer surfaces, including any fouling deposits, for each section of the zone is determined by measuring the mass flow rates, temperatures, and heat fluxes of the synthesis gas and heat transfer cooling system within the various sections of said zone using units 23-29. Units 23-29 contain the instruments, such as flow meters, thermocouples, and gamma densitometers, needed to measure the flow rates, temperatures, steam quality, etc., and transmit the signals to the processor-controller 30. The units 23-29 represent the conglomeration of these devices. The units are shown one unit per section of the heat exchanging zone. However, it should be understood that even more than one unit per conventional heat exchanger section of the zone can be needed, although not shown. The number of units and type of devices depends on the configuration of the heat exchanger section and the coolant phase flow. Fig. 2, to be described later, is a more detailed description of a unit operating to determine the overall heat transfer resistance of a conventional heat exchange section with heat removal by partial evaporation of the coolant. In this case, a densitometer is used to determine the degree of vaporization of the coolant, and thereby determine the heat flux in that section. In other cases where the coolant phase does not change as it passes through the section, the temperature difference of the entering and leaving coolant is sufficient to determine the heat flux. Another problem occurs in the quench and duct zones, where it is not possible to utilize thermocouples to determine the change in synthesis gas temperatures. In this case the gas temperatures at various heat exchanger section locations are calculated from the heat fluxes determined from the coolant system measurements, since the heat gained by the cooling system in this section is substantially identical to the heat lost from the synthesis gas in the same section. It is difficult to measure heat flux in those sections where heat is removed by partial vaporization of liquid coolant, since there is little temperature change on the water-steam side of the cooling medium. However, a device for measuring the relative liquid and vapor fractions from gamma ray absorption can be used to measure the heat flux based on the different gamma ray absorption of vapor and liquid. For example, steam absorbs gamma rays much less effectively than water. The temperature of the (synthesis) gas being cooled can then be determined based on the fact that the heat gained by the steam/water cooling system is substantially identical to the heat lost from the (synthesis) gas being cooled. The above-mentioned measurements can be transmitted to a processor-controller 30 via signals 23A-29A, and manipulated to yield the overall heat transfer coefficient of each individual section of the heat exchanger zone. The heat transfer coefficient (U) for a section A is generally calculated based on the following relationships. Where T = temperature F = mass flow rate G = synthesis gas W = coolant (water and/or steam) H = hot end C = cold end A = heat exchanger section area (m² ) (Heat Flux) = (FG) * (Gas Heat Capacity) * (TGH-TGC)/A, kJ/(hr)(m² ) where FG = Mass Flow of Synthesis Gas (kg/hr) TGH, TGC are temperatures at the hot and cold ends, respectively. Similarly, (Heat Flux) = (FW) * (V) * (λ)/A (evaporating part only) where (FW) = Mass Flow of Coolant (kg/hr) V = Mass fraction vaporized λ = Latent heat of vaporization (kJ/kg) also, DTH ≡ TGH - TWH DTC ≡ TGC - TWC being the temperature differences between the synthesis gas and coolant at the hot and cold ends, respectively, and (MTD) = (DTH-DTC) ln (DTH/DTC) (logarithmic mean temperature difference) so U = (Heat Flux) (MTD) where U = Overall Heat Transfer Coefficient kJ/(hr * m² * °C) The overall heat transfer coefficients and the relative change therein as a function of time for each section are thus continuously calculated by the processor-controller. Changes in the overall heat transfer coefficients within a section may be due to differences in the thickness of the fouling deposits, which is the process variable we are attempting to minimize in the heat exchanging zone by manipulating the rapping variables. However, the overall heat transfer coefficients also change due to gas flow variations, including mass flow, temperature, pressure and composition. Some sections of the heat exchange zone incur only negligible heat transfer resistance due to fouling, hence almost any rapping sequence maintains them close to their initial performance. This makes it possible to discount the effect of gas flow variations upon the other heat transfer sections by forming the ratio of the other sections to such a section which does not change much due to fouling, and can be considered a reference section. The open duct section is useful as such a reference section. Referring to Fig. 2, an apparatus for measuring the overall heat transfer coefficient of deposits for two evaporation sections 21 of an indirect heat exchanging zone includes processor-controller 30, which determines the overall heat transfer coefficient of the heat transfer surfaces, including any fouling deposits thereon, for each section and the relative change therein collectively of the zone. A cooling medium (e.g., steam or water) is passed via line 53 into a (venturi) flow meter 54 or the like to determine the mass flow of the medium and then is contacted with a thermocouple 55 or the like to determine the inlet temperature TWC of the medium and then through the inlet of heat exchanging section 21 where it comes into indirect heat exchange with hot synthesis gas and some or all of the remaining liquid of the two-phase cooling medium is converted into additional vapor. Cooling medium is removed from the section 21 via outlet line 57 and is then subjected to gamma ray detection with a densitometer 58 or the like for measuring the ratio of liquid and vapor fractions in the cooling medium needed to determine the outlet heat content of the medium. The medium is held in drum 60 where any steam is let off at line 59, the pressure is determined by a pressure device 61 and the mass flow rate is determined by flow meter device 62. The liquid coolant medium passes via line 63 into pump 64 for recycle via line 53. Signals 54A, 55A, 58A, 61A and 62A, respectively, from devices 54, 55, 58, 61, and 62, respectively, are transmitted to processor-controller 30. Similar means 65, 66, and 68 to determine the flow rates, temperatures, and the fraction of the cooling medium vaporized and to pass the signals 65A, 66A and 68A to the processor-controller are provided for other sections. A combined set of these means for measuring the cooling medium and the hot sythesis gas correspond to a single unit of the type previously broadly described as unit 23 or the like. Conventional systems optimizing indirect heat exchanger zone cleaning are usually based on observing the temperature of the synthesis gas exiting the heat exchanging zone. However, this does not account for the effects of changing conditions in the gasifier, which affect the velocity of the gas, gas composition, temperature and pressure and the like, which affect each section of a conventional heat exchanging zone. Hence, to account for these multiple effects not associated with fouling deposits, it is necessary to calculate the overall heat transfer coefficient for each section of the heat exchanging zone. The relative change in overall heat transfer coefficient of the heat transfer surfaces, including any fouling deposits thereon, for each section is determined as a function of time by the processor-controller 30. The process-controller 30 compares the relative change of the overall heat transfer coefficient of a section with a preselected reference section. The fouling deposits such as flyash and soot are removed using conventional rapping means, such as a mechanical rappers 40, 44 and 48-50, acoustical horns, or in any other manner well known to the art, in particular based on signals 40A, 44A and 48A-50A received from the processor-controller 30. Since the heat exchanging zone includes sections of different geometries, average temperature, flow velocities and water-side phase regimes (i.e., vapor superheating, partial vaporization, and liquid phase heating), it is expected that each section could have a different deposition rate. Therefore, it is desirable to have the rappers arranged having separate and independently controllable rapping parameters for each section of the zone controllable via processor-controller 30. The parameters include a time interval between rapping cycles between individual rappers in a section, rapping force, number of strikes of a rapper, rapping frequency of an individual rapper in its own cycle, time interval for rapping an individual rapper and time interval between complete rapping cycles of rappers in a section. In the present invention, the separation of the particulate deposit from the impacted heat transfer surface requires a rapping force which is sufficient to overcome the adhesion between the deposit and the heat transfer surface, as well as any elastic force which may exist in a well formed, continuous layer of deposit. In addition, the force must be small enough not to cause structural fatigue over the intended service life of the heat transfer surface. When an impact force is applied to a heat transfer surface, the surface vibrates in all of its normal modes, each mode having a different frequency and standing wave shape. Generally, the lower frequency modes have larger displacement maxima while the higher frequency have larger acceleration maxima. If the force is applied on a line of zero response for a particular mode, that mode will be very ineffectively excited. If the force is applied near the location of maximum response, that mode is effectively excited. When the structure is large and the force is small, the motion may decay rapidly with distance from the source, so that multiple excitation locations are necessary for effective cleaning motion. The present invention provides a means for determining the effects of vibration frequencies and mode shapes and rapper timing, forces, phases, locations, and numbers on both structural reliability and cleaning performance. Although the system is shown in Fig. 1 in its distributed form as discrete components, it would be readily understood by those skilled in the art that these components could be combined into a single unit or otherwise implemented as may be most convenient for the particular application at hand.

Claims: A method for controlling rapping of heat exchanging surfaces of an indirect heat transfer zone having fouling deposits thereon within a synthesis gas system comprising the steps of: (a) feeding particulate solids (11) and oxygen-containing gas (12) into a reactor (13), (b) partially oxidizing the solids at an elevated temperature within the reactor (13), (c) producing product gas (20) within the reactor (13), (d) passing the product gas from the reactor (13) to a heat exchanging zone (14, 15, 17, 18, 19, 21) in gas flow communication with the reactor (13), the zone including at least one section adapted to generate superheated steam, and a lower temperature heat exchanging section, (e) removing heat from the product gas (20) in the heat exchanging zone (14, 15, 17, 18, 19, 21) by indirect heat exchange with a heat transfer using cooling system of steam and/or water, said zone comprising a plurality of sections (17, 18, 19, 21) at least one of which is a one- or two-phase heat transfer section, and in which sections, fouling deposits accumulate on the surfaces thereof the various sections (17, 18, 19, 21) at different rates because of different conditions; characterized by (f) determining the overall heat transfer coefficient of the heat transfer surfaces, including any fouling deposits thereon for each section (17, 18, 19, 21) of the zone, said determining includes determining mass flow rates of the product gas and cooling system within the heat exchanging zone, determining temperatures of the product gas and cooling system within the heat exchanging zone, and determining heat fluxes of the product gas and cooling system either directly on the product gas side or on the coolant side within the heat exchanging zone, (g) determining the relative change of the overall heat transfer coefficient due to the change of the thickness of the fouling deposits for each section as a function of time, (h) comparing the relative change of the overall heat transfer coefficient from (f) of each section with a preselected reference section, said reference section being the section of least fouling which is rapped based on its current overall heat transfer coefficient as compared to its initial overall heat transfer coefficient; (i) removing the fouling deposits from each section of the zone using rapping means (40, 44, 48, 49, 50), the rapping (40, 44, 48, 49, 50) means having separate and independently controllable rapping parameters for each section of the zone, and (k) adjusting the rapping parameters for each section of said zone, the adjusting including one or more of (1) adjusting a time interval between rapping of individual rappers in a section of individual rappers, (2) adjusting rapping force, (3) adjusting the number of strikes of an individual rapper in its cycle, (4) adjusting the time interval for rapping and individual rapper and (5) adjusting the time interval between complete rapping cycle of rappers in said section. The method as claimed in claim 1 characterized in that said synthesis gas is produced by operating said reactor (13) at a temperature of from about 1100 °C to about 1700 °C. The method as claimed in claim 1 or 2 characterized in that said synthesis gas from said reactor (13) is passed to a heat exchanging zone and includes passing said gas through a quench section (14), an open duct section (15), superheater section (17), evaporator section (18) and economizer section (19). The method as claimed in any one of claims 1-3 characterized in that removing heat from said synthesis gas includes operating said at least one section of cooling zone of said system at a temperature of from about 650 °C to about 900 °C. The method as claimed in any one of claims 1-4 characterized in that the overall heat transfer coefficient of a two-phase heat transfer section used to cool gas at above about 550 °C to about 750 °C is determined using a gamma-ray densitometer (58) to determine the quality of the steam-water two-phase mixture.

Inventors: DOERING, EGON LORENZ, HARENSLAK, GERD, RUSSEL, PAUL FELIX, SEGERSTROM, CLIFFORD CHARLES, STIL, JACOB HENDRIK, VAN KESSEL, MATHEUS MARIA
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: F28G7
IPC codes: F28G9, F28G15, F28G7, F28G1, C10J3, F22B37