Classifications

• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10B—DESTRUCTIVE DISTILLATION OF CARBONACEOUS MATERIALS FOR PRODUCTION OF GAS, COKE, TAR, OR SIMILAR MATERIALS
  • C10B57/00—Other carbonising or coking processes; Features of destructive distillation processes in general
  • C10B57/08—Non-mechanical pretreatment of the charge, e.g. desulfurization
  • C10B57/10—Drying
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B02—CRUSHING, PULVERISING, OR DISINTEGRATING; PREPARATORY TREATMENT OF GRAIN FOR MILLING
  • B02C—CRUSHING, PULVERISING, OR DISINTEGRATING IN GENERAL; MILLING GRAIN
  • B02C23/00—Auxiliary methods or auxiliary devices or accessories specially adapted for crushing or disintegrating not provided for in preceding groups or not specially adapted to apparatus covered by a single preceding group
  • B02C23/04—Safety devices
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B02—CRUSHING, PULVERISING, OR DISINTEGRATING; PREPARATORY TREATMENT OF GRAIN FOR MILLING
  • B02C—CRUSHING, PULVERISING, OR DISINTEGRATING IN GENERAL; MILLING GRAIN
  • B02C23/00—Auxiliary methods or auxiliary devices or accessories specially adapted for crushing or disintegrating not provided for in preceding groups or not specially adapted to apparatus covered by a single preceding group
  • B02C23/18—Adding fluid, other than for crushing or disintegrating by fluid energy
  • B02C23/24—Passing gas through crushing or disintegrating zone
  • B02C23/34—Passing gas through crushing or disintegrating zone gas being recirculated to crushing or disintegrating zone
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B5/00—Making pig-iron in the blast furnace
  • C21B5/001—Injecting additional fuel or reducing agents
  • C21B5/003—Injection of pulverulent coal
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B17/00—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement
  • F26B17/10—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by fluid currents, e.g. issuing from a nozzle, e.g. pneumatic, flash, vortex or entrainment dryers
  • F26B17/101—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by fluid currents, e.g. issuing from a nozzle, e.g. pneumatic, flash, vortex or entrainment dryers the drying enclosure having the shape of one or a plurality of shafts or ducts, e.g. with substantially straight and vertical axis
  • F26B17/103—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by fluid currents, e.g. issuing from a nozzle, e.g. pneumatic, flash, vortex or entrainment dryers the drying enclosure having the shape of one or a plurality of shafts or ducts, e.g. with substantially straight and vertical axis with specific material feeding arrangements, e.g. combined with disintegrating means
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B21/00—Arrangements or duct systems, e.g. in combination with pallet boxes, for supplying and controlling air or gases for drying solid materials or objects
  • F26B21/06—Controlling, e.g. regulating, parameters of gas supply

Definitions

• pulverized coal In the metallurgical industry, pulverized coal is generally injected as combustible into blast furnaces. It is important, in order to ensure good functioning of the blast furnace, that the pulverized coal is of good quality, i.e. that the pulverized coal has the right consistence, size and humidity level.
• the pulverized coal is generally produced in a grinding and drying installation, wherein raw coal is ground in a pulverizer and dried to the right humidity level before the resulting pulverized coal is fed to a hopper for storage or direct use in a blast furnace. It is known to subject the freshly ground coal to a stream of hot gas so as to dry the pulverized coal.
• the pulverized coal can e.g.
• the pulverized coal is then separated from the gas and fed to the hopper.
• Part of the gas is recirculated and heated before it is reintroduced into the pulverizer .
• it is important to monitor the oxygen level in the drying gas generally downstream of the filter. If the oxygen level becomes too high, the combination of drying gas and pulverized coal may become an explosive mixture with potentially dangerous consequences.
• exhaust gasses are extracted from the drying gas and fresh air is injected.
• the oxygen level in the drying gas is monitored and, if the measured oxygen level is found to be too high, the amount of fresh air introduced into the drying gas in the recirculation line is reduced. This allows lowering the oxygen level in the drying gas.
• the reduction of the amount of fresh air introduced into the drying gas may not be enough to sufficiently reduce the oxygen level. Indeed, once the amount of fresh air introduced into the drying gas is reduced to zero, i.e. no more fresh air is introduced, the oxygen level may in such circumstances still be too high. In order to avoid any damage to the installation it may then be necessary to shut down the grinding and drying installation. Such a shut down not only leads to a loss of production, but also to extra costs relating to the replacement or conditioning of the drying gas.
• the object of the present invention is to provide an improved method for producing pulverized coal, which does not present the drawbacks of the prior art methods. This object is achieved by a method as claimed in claim 1.
• the present invention proposes a method for pro- ducing pulverized coal, the method comprising the steps of:
• drying gas preferably an inert gas
• the oxygen level in the drying gas is determined during a grinding cycle wherein heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer and if, during the grinding cycle, the determined oxygen level is higher than a predetermined oxygen threshold, water is injected into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.
• the injection of water into the drying gas during the grinding cycle allows increasing the overall volume of the drying gas, thereby reducing the relative oxygen volume. The water injection therefore allows reducing the oxygen level to an acceptable level and thereby avoids any damage to the installation or the need to shut down the grinding and drying installation.
• the method further comprises injecting, in the recirculation line, fresh air into the drying gas wherein, if the determined oxygen level is higher than the predetermined oxygen level thresh- old, the volume of fresh air injected into the drying gas is reduced.
• the method comprises first reducing the volume of fresh air injected into the drying gas, and then, if the volume of fresh air injected reaches zero and the oxygen level is still higher than the predetermined oxygen threshold, injecting water into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.
• the predetermined oxygen threshold is chosen to be between 0 and 14 volume %, preferably between 5 and 12 volume %.
• the method com- prises the further steps of determining an exit temperature of the mixture of drying gas and pulverized coal exiting the pulverizer; and controlling the exit temperature by controlling a volume of water injected into the heated drying gas before feeding it into the pulverizer.
• the temperature of the drying gas entering the pulverizer can be adjusted rapidly so as to take into account temperature differences occurring due to raw coal with different levels of humidity being introduced into the pulverizer. It is thereby possible to maintain the temperature of the drying gas exiting the pulverizer, hereafter referred to as exit temperature, as constant as possible.
• the present aspect is of particular advantage during a startup phase of the installation, wherein the method comprises a startup cycle wherein heated drying gas is fed through the pulverizer without introducing raw coal, the exit temperature being kept below a first temperature threshold, and a grinding cycle wherein heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer, the exit temperature being kept at a preferred working temperature.
• the method comprises:
• heating said drying gas to a temperature above the first temperature threshold and injecting a volume of water into the heated drying gas, the volume of water being calculated so as to reduce the temperature of the heated drying gas to obtain an exit temperature below the first temperature threshold;
• drying gas is generally fed through the installation before raw coal is introduced into the pulverizer. This allows the individual components to be heated to the desired working temperature.
• the drying gas which may be heated to a temperature above the maximum tolerated exit temperature, can be cooled down again so that the temperature downstream of the pulverizer does not exceed the first temperature threshold.
• the volume of water injected into the heated drying gas can be determined based on the exit temperature. Alternatively, the volume of water injected into the heated drying gas can be determined based on a pressure drop measured across the pulverizer. It is not excluded to use other measurements, alone or in combination, to determine the volume of water to be injected into the heated drying gas.
• the method comprises the further steps of reducing the heating of the drying gas; and reducing the volume of water injected into the heated drying gas to maintain the desired exit temperature.
• This allows reduc- ing consumption of energy once the installation is running.
• the importance of the overheating and subsequent cooling of the drying gas is particularly important during the startup phase of the installation, wherein it allows providing a buffer to compensate for the drop in temperature occurring when the introduction of raw coal is started. Once the installation is running, only smaller temperature drops might occur and the buffer can be reduced.
• the method may also comprise continuous monitoring of the exit temperature and comparing the measured exit temperature to a maximum temperature, wherein, if the measured exit temperature exceeds the maximum temperature, the volume of water injected into the heated drying gas is increased. This allows using the water injection means used for general process control, to be used for emergency cooling also.
• Fig.1 shows a schematic representation of a grinding and drying installation used for carrying out the method according to the present invention.
• Figure 1 shows a grinding and drying installation for producing pulverized coal using the method according to the present invention.
• the heated drying gas is carried from the hot gas generator 26 to the pulverizer 20 via a conduit 28.
• pulverized coal is entrained.
• a mixture of pulverized coal and drying gas is carried from the pulverizer 20, via a conduit 32, to a filter 34, where the pulverized coal is again removed from the drying gas and fed to a pulverized coal collector 36, ready further use.
• the drying gas exiting the filter 34 is fed to a recirculation line 38 for feeding it back to the hot gas generator 26.
• the recirculation line 38 comprises fan means 40 for circulat- ing the drying gas through the installation.
• the fan means 40 may be located upstream or downstream of a line 42, e.g. a stack, which is used to extract part of the drying gas from the recirculation line 38.
• the recirculation line 38 further comprises gas injection means 44 for injecting fresh air and/or hot gas into the recirculation line 38.
• the injected fresh air and/or hot gas is mixed with the recycled drying gas.
• the injected fresh air allows reducing the due point of the drying gas and the injected hot gas is used to improve the thermal balance of the grinding and drying circuit.
• the oxygen level in the drying gas can be reduced by injecting water into the drying gas by means of the water injection means 46.
• the water injection means 46 can be instructed to increase the volume of water injected into the drying gas, thereby reducing the oxygen level downstream of the filter 34.
• the oxygen level is first reduced by the conventional method of reducing the volume of fresh air injected into the dying gas by the gas injection means 44 and if this is not sufficient, the oxygen level is then further reduced by increasing the volume of water injected into the drying gas by the water injection means 46.
• Another function of the water injection means 46 may be to help regulate the temperature of the drying gas at the exit of the pulverizer 20.
• the drying gas is heated to a predefined temperature in the hot gas generator 26 and fed through the pulverizer 20.
• the temperature of the drying gas is reduced in the pulverizer 20 as the heat from the drying gas is used to dry the pulverized coal.
• the level of humidity of the raw coal determines the temperature loss of the drying gas.
• the temperature of the mixture of pulverized coal and drying gas exiting the pulverizer 20, hereafter referred to as the exit temperature is monitored, e.g. by means of a temperature sensor 48.
• the temperature of the drying gas entering the pulverizer needs to be controlled, which is generally achieved by controlling the output power of the burner 27 of the hot gas generator 26.
• this process has a relatively slow response time, meaning that once the installation has determined that the exit temperature is too high or too low and the burner 27 has been made to react in consequence, some time passes before the exit temperature reaches the correct exit temperature again.
• the drying gas 27 is set to heat the drying gas well above the desired exit temperature.
• the heated drying gas is then subjected to controlled cooling by injecting water into the heated drying gas through the water injection means 46, whereby the drying gas is cooled so that the desired exit temperature can be achieved.
• the exit temperature suddenly drops well below the desired exit temperature.
• the amount of water injected into the drying gas by the water injection means 46 is reduced. The heated drying gas is hence cooled less and the desired exit temperature can be kept stable.
• the water injection means 46 is also used for an emergency cooling.
• the method may comprise continuous monitoring of the exit temperature and comparing the measured exit temperature to a maximum temperature. When the measured exit temperature exceeds the maximum temperature, the water injection means 46 is instructed to increasing the volume of water injected into the heated drying gas, thereby reducing the temperature of the drying gas entering the pulverizer 20 and consequently also the temperature of the drying gas exiting the pulverizer 20.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Food Science & Technology (AREA)
• General Engineering & Computer Science (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Manufacturing & Machinery (AREA)
• Metallurgy (AREA)
• Solid Fuels And Fuel-Associated Substances (AREA)
• Drying Of Solid Materials (AREA)
• Disintegrating Or Milling (AREA)
• Coke Industry (AREA)

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• 2008
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