The solution of DCS control system of blast furnace ironmaking plant
I.Technological process of blast furnace
A blast furnace is composed of feedingsystem, blast furnace body, dust removal system, hot blast stove system, gaspressure station, blower system and coal injection system of blast furnace, inwhich the feeding system includes under-tank and hoisting system and the hotblast stove also includes steam humidification and oxygen enrichment system.
Parameters including top temperature ofhot blast stove, air for combustion furnace, flow pressure of gas, pressureequalizing of temperature and cold wind and gas temperature will be detectedand ratio of combustion will be controlled; the oxygen enrichment system shallinclude the detection of flow, pressure and temperature of oxygenenrichment system, as well as the oxygen control and safety interlocking;detection of air, forward and temperature of gas pre-heater, pressure andpressure difference; detection of cold wind pressure, flow and temperature andcontrol of hot wind temperature.
Based on production requirements ofblast furnace, the hot blast stove will keep sending hot wind to blast furnaceduring production and will automatically switch to another burning hot stove incase the temperature of current stove reaches the lower limit; the replacedstove will burn again and when it reaches the upper limit or certaintemperature and its temperature increase is slow, it will be used as standbystove if it’s not needed to supply air; there are four such stoves which havecircular operation of burning – stewing furnace – supply air and they can bothbe automatically or manually operated by operators.
Other auxiliary systems of blast furnacemay include blower station and coal injection system and can be added to blastfurnace system or become independent system according to user or field conditions.
a. Blower station, which shall havecertain wind pressure, is used to provide stable cold wind for production ofblast furnace to satisfy the oxygen required by smelting. Blowers is classifiedto electric blower and turbo blower according to drag mode; the blowers shallhave constant air volume to guarantee stability of temperature and gas flow inblast furnace and smooth operation of blast furnace; the large fluctuation ofair volume may directly affect feeding speed and further affect the heatbalance of hearth and reduce the iron production volume. The blower shall beoperated under constant wind pressure when replacing hot blast stove and thewind shall be increased or decreased according to changes of furnace.
b. Coal injection system, which iscomposed of powder preparing part and injection part, can be helpful inpromoting coefficient and quality of blast furnace. In the technologicalprocess, the raw coals will be sent to raw coal warehouse through crane andsent to belt of coal machine through valve; the raw coal will enter coal millthrough belt and be processed into qualified powders. In the meantime, one endof coal mill will inject the hot wind from heating furnace to dry the coalpowder, while the other end will extract wind through fan of coal powder toform negative pressure in order to extract the fine coal powder and hot wind intocloth bag for dedusting; the cloth bag will have the coal powder recycled tocoal powder warehouse and sent to blast furnace for burning throughintermediate tank, injection tank and injection pipe. It mainly controls inletand outlet temperature, load and front negative pressure of coal mill, as wellas injection quantity and pressure of injection tank. Inlet temperature of coalmill can be adjusted through flow ratio control, that is to adjust the mixingratio between high-temperature flue gas of heating furnace and waste gasproduced by hot blast stove. Cascade can be formed by taking inlet temperatureof coal mill as vice ring and wind and powder temperature at outlet as mainring since it’s difficult to control wind and powder temperature at outlet ofcoal mill. As the core part of whole powder preparing system, the coal millwill have its load automatically controlled by adjusting the coal feedingquantity, which is regulated by adjusting the speed of coal feeding beltthrough frequency converter. Negative pressure control exists only when powderis prepared with intermediate speed. The front negative pressure control ofcoal mill is designed as single circuit and the air volume is used asmoderating variable. The injection is designed as cascade loop and thesecondary air volume is used for adjusting injection volume. The pressurecontrol of injection tank is designed as single circuit.
II.Plan of control system
The control plan of DCS control systemis introduced by taking 450M3 blast furnace of certain iron mill as example.
According to the process set-up, ourplan includes the following three systems: blast furnace body (includingfeeding and dedusting), hot blast stove, auxiliary facilities; the auxiliaryfacilities include blower station, injection coal and gas pressure station, toppressure power generation (TRT) and circulating water can be added properly asan option; each independent system is connected through communication to useand review the data.
1. The master control room of blastfurnace is generally installed in the duty room of furnace superintendent anprovided with 1# and 2# control station, in which the 1# station is used tocollect and control the process parameters of blast furnace, includingdedusting of cloth bag and furnace body, temperature at each layer of hearth,hot wind temperature and pressure, flow of cold wind, furnace top pressure,cross type temperature measuring and detection of technical parameters of bellsystem and the automatic control furnace top pressure and mixer selector valveshall be implemented. Action of belts, vibrating screen, gate, rolling plate,stock rod, feeding trolley, stock distributing device, small bell, big bell,pressure equalizing and spreading equipment are controlled by 2# station according to feeding program ofcontroller to realize the weighing compensation of feeding and the logiccontrol of under-tank process equipment, hoisting equipment and furnace topequipment.
The dedusting system can control andexecute back blowing, box, terminal valve of raw and clean gas of box andsolenoid valve on back-blowing air bag; back-blowing control is generallyimplemented by nitrogen pulse and the control methods include back-blowing accordingto pressure difference and time; the blowing is implemented offline, whichmeans the terminal valve is closed before back-blowing; the terminal valveshall be closed during temporary damping down and it shall have normaloperation in case of sudden power failure of dedusting system when the blastfurnace is operating normally.
The automation control plans anddrawings for under-tank system, hoisting and feeding system of blast furnaceshall be prepared by professional electrical personnel. The feeding systemmeans the whole procedure of feeding materials from under-tank to furnace topand sending materials (sinter and coke) in the furnace; the materials shall befilled up by two skip cars according to production requirements and lifted tofurnace top by hoist of skip cars; the materials will be evenly spread in thefurnace through material chute after they passe upper seal valve, materialstorage valve, throttle valve and lower seal valve. As the key program infeeding system, the material batch program is used to determine the loadingsequence of mineral and coke in each batch of materials and used as thematerial feeding method of under-tank weight hopper. The program control systemincludes A, B, C and D.
There are four material batch programs,each of them is provided with N trolley positions and each bath of materialscan be composed of N trolleys as the maximum; all kinds of material feedingmethods can be used in any of the N positions of each material batch program. Thematerial bath cycle is determined according to A, B, C and D.
The combination of four differentbatches constitutes a large circulation. The material batch cycle program has10 positions, each of them can select any of the four material batches andallow additional coke of cycle program and it shall be restored to the normalmaterial batch cycle program before disconnection. The batching process is usedto determine the composition of materials in each trolley, namely controlling ofactions of under-tank stock bin. When there are no empty trolleys according tomaterial list and the material batch pointer points at coke, then the lefttrolley moves to the bottom, the left choke hopper becomes full, the delay timeis reached and the signal of “left hopper selected” is sent since there is nodistinguish about left and right of coke and it is the same for the righttrolley; if the material bath pointer points at mine, the left trolley moves tothe bottom, the left mine trolley becomes full, the delay time is reached andthe signal of “left mine hopper selected” is sent since the types of mines aredifferent and there are distinguishes between left and right, if the righttrolley moves to the bottom, the signal of empty trolley will be sent and it isthe same for the right side. The left and right stock rod will detect andcontrol the distribution of stock distributing device.
The detection of furnace body includesthe temperature detection of furnace foundation, bottom, hearth, body andthroat and the control includes furnace top pressure and furnace top pressureequalizing control. Control of furnace top pressure is implemented by reducingvalve set, which is driven by control command outputted by DCS according tologic sequence to adjust the gas flow which passes reducing valve in order tostabilize the furnace top pressure. The reducing valve set is composed of 3-5butterfly valves, each of them can be divided into manual valve, automaticvalve, range valve and emergency drain valve according to functions; theautomatic valve receives conditioning signals of DCS while the manual valve ismanually operated by operator. The pressure equalizing at furnace top isrealized by controlling open/close of electric control valve and seal valvethrough control signals outputted by DCS in order to adjust the pressure inweighing hopper to send materials into blast furnace smoothly.
2. Large furnace is generally providedwith four hot blast stoves. DCS system can have logic control to combustion,air supply and damping down of four hot blast stoves and switch furnaces inwhole system and switch state of furnace by controlling the open/close ofcontrol valve.