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Coking

I.Brief introduction to coking

As the fundamental energy source inChina, coal can provide heat through combustion and produce different chemical productsthrough comprehensive processing and utilization. The automation in recovery ofcoking and chemical products plays an important role in comprehensiveutilization of coal resources in China.

The gas generated when producingmetallurgical coke, ammonium sulfate and crude benzene can also be fullyutilized. Safe production and improvement of atmospheric environmental qualityare the primary targets since the coking plant has poor production conditionsand the operators are exposed to inflammable, explosive and toxic environment.The automatic control in coal blending, gas collecting tube pressure, pressureof oven gas header pipe and main pipe and coke pushing is quite important forensuring safety production, strengthening products quality, reducing productioncost, improving production efficiency, reducing pollution to atmosphericenvironment, creating good economic benefits and making decisions and guidanceefficiently in coordination with production managers and enterprise leaders.

The coke automation control which canexpand scale and realize deep processing has the highest priority since thesmall-scale and outdated heap coking fails to catch up with modern industrialdevelopment along with the increasing environmental awareness and environmentalprotection standards. The process control has been developed from pneumaticinstruments, electrodynamic instruments, multiloop controller and distributedcontrol system (DCS), which has become the trend in process control system ofcoking industry.

II. Brief introduction to technological process

The main processing method in cokingindustry is to heat coal in high temperature (950-1050℃) to produce coke and recover thechemical products produced in the coking process. The coke products can be usedas fuels for blast furnace process or used for steel forging, nonferrous metalmetallurgy and producing water gas (it can produce producer gas for preparingsynthesis ammonia or produce calcium carbide). By recovering and processingchemical products generated in the coking process, the tar oil, ammonia,naphthalene, crude benzene, hydrogen sulfide, hydrogen cyanide and pure cokeoven gas can be extracted.

A coking plant is generally composed ofcoal preparation workshop, coking workshop, recovery (coke production) workshopand public engineering part. The coal preparation workshop is mainly used tostart and stop equipment and realize the logic interlock of DCS. The cokingworkshop, gas recovery workshop and public engineering part are controlled byDCS.

The technologicalprocesses for coal preparation workshop and coking workshop is as follows:

III. Typical control plan

Coke oven heating control system and gascollecting tube pressure control are the most difficult control points in DCSsystem of coking plant; the control plan in main control loops of Hollysys willbe explained as follows:

1. Fuzzy control system for oven heating

As the most complex thermal equipment inthermal kiln and the control object which has complex structure, large timedelay, high nonlinearity and multi-factor coupling, the coke oven may produce alot of black smokes, pollute the environment and affect the coke quality andfurnace’s life if it’s not well controlled and the traditional PID and cascadecontrol technology are ineffective. By using multivariable fuzzy control anddynamic neural network modeling technology, fully considering of historicalinformation and adjusting the gas flow and suction at coke and machinemeasuring flue, Hollysys can well control longitudinal temperature of coke andmachine, achieve stability and evenness of longitudinal temperature in thewhole furnace, realize automatic control of coke oven temperature and havebetter control effects than manual operation and control of imported DCS. Theadvanced control adopted can limit the fluctuation range of averagelongitudinal temperature at ±5℃,save 3% heating gas in coke oven (when coke oven gas is used as heating gas),ensure coke quality, save a lot of energy sources, prolong furnace’s life andreduce the environmental pollution.

2. Gas collecting tube pressure controlof coke oven

As thecritical parameter in coking production, the stability of controlling gascollecting tube pressure of coke oven will affect the yield and quality of gas;high pressure of gas collecting tube will increase pressure in coking chamber,lead to smoke of coke oven, make equipment unclear in close distance, dischargea lot of coke oven gas into air and pollute the environment; low pressure ofgas collecting tube will cause suction of a lot of air, negative pressure incoking chamber, send air to coking chamber, cause combustion of partialchemical components in coking room, increase CO2 and N2 in gas, reducecalorific value of gas and coke oven’s life.

The gascollecting tube pressure is difficult to be controlled due to a lot ofinfluencing factors:

a. Gasyield and quality. Higher gas yield will lead to lower pressure and vice versa;it is also affected by coal quality; the high quality coal and stabilizedblower and gas load can realize good automatic control effects, while thepoor-quality coal and big changes of blower pressure can result to vibrationand impede the automatic control of system.

b. For gasconveying system which is shared by multiple coke ovens, the fluctuation of gascollecting tube pressure of one coke oven can affect the fluctuation of headerpipe and the automatic adjustment loops will have mutual effects. Actually, thegas collecting tube pressure control system of coke oven is a multivariablesystem which has severe coupling, nonlinearity, time-variant characteristicsand disturbance changes and cannot be satisfied by general PID adjustment andcascade control.

c. Coalcharging has big effects on gas collecting tube.

d.Revolving speed adjustment of blower affects pressure of gas collecting tube.

e. Changesof gas consumption amount of users also have influences.

f.Influences from exchange of gas switch.

g. As one of the control mechanisms, thegate valve of blower has severe nonlinearity, big lag and cannot be satisfiedby the combination of ordinary servo amplifier and performing structure.

3.Pressure control of crude gas collecting tube: it is controlled to ensure stablepressure of coke oven. The openness of turning plate on crude gas pipeline isautomatically adjusted according to pressure changes of gas collecting tube inorder to stabilize the pressure of gas collecting tube. High-pressure coke ovenwill increase pressure in coking chamber and enhanced the possibilities of gasleakage; low-pressure coke oven will cause negative pressure in coking chamber,send air to coking chamber and cause combustion of partial chemical componentsin coking room, increase CO2 and N2 in gas, reduce calorific value of gas andcoke oven’s life.

4. Controlof primary gas flow: it is implemented to maintain coke oven temperature above 1250-1350℃. Theopenness of turning plate on the main gas pipeline is automatically adjustedaccording to changes of gas flow to stabilize primary gas flow and maintaincoke oven temperature. When coking coal properties are stable, the changes ofheating temperature will affect the quality and productivity of cokingproducts. When gas properties are stable, the coke oven heating can be ensuredby controlling gas flow in combustion chamber.

5.Pressure control of separation flue

It iscontrolled to ensure stable suction of flue, reach reasonable coefficient ofexcess air, reduce heat loss and improve thermal efficiency. The openness ofturning plate in flue is automatically adjusted according to pressure changesof separation flue to stabilize the pressure of separation flue.

6.Automatic control of suction pressure of blower: it is to ensure stable suctionof gas in order to keep stable pressure of gas collecting tube and stable pressurein subsequent section of blower. The control in different blowers can berealized by adjusting revolving speed of blower, inlet butterfly valve of fanor controlling fan bypass. The pressure signals can be taken from inlet ofprimary cooler, introduced to DCS and outputted to 4-20mAsignal by DCS to complete automatic control; manual controller can be providedas the backup means of DCS.

7. Gastemperature control after primary cooler: it is to stabilize the outlettemperature of primary cooler. High outlet temperature will increasenaphthalene content in gas and affect stable operation of washing tower, whilelow outlet temperature will cause condensing of gas, increase resistance ofprimary cooler, consumption of cooling water and energy. By adjusting thecooling water flow at lower segment of primary cooer, the gas which passesprimary cooler can be reduced to 21-22℃ in orderto control temperature.

8. Liquidlevel control of upper condensate tank: it is to avoid overflow of condensatetank. The overflow of condensate will be harmful to environment and difficultto be cleaned by conventional methods since it contains light tar oil andammonia water. The control is realized by adjusting the control valve on crudegas pipeline in front of primary cooler.

9. Bycontrolling pressure of saturated steam, the control of insulation box ofelectric coke catcher can ensure that the insulation box temperature is notlower than 90℃ and avoid condensation at surface of porcelaininsulator and damaging.

10.Pressure control of high-pressure ammonia water: only stable pressure ofhigh-pressure ammonia water can generate stable suction in ascending tube,intake the gas generated during coal charging into gas collecting tube in orderto improve operation environment at furnace top and realize smokeless coalcharging. The control can be realized by controlling revolving speed of frequencymodulation motor of high-pressure ammonia water pump.

11.Monitoring of blower unit: it will monitor lubricating oil pressure andtemperature, revolving speed of unit, shaft vibration and displacement, bearingtemperature of unit and motor stator temperature.