Application case 17: 900m3blast furnace Design of automatic control system of combustion technology

1 Overview

When hot blast stove automatic combusts, it is doing intelligent optimization control on combustion process by the control device and the host computer, to make combustion process on the best. There are two optimal meaning, one is that when combustion, the air/gas is the most reasonable, so that the fuel can be best utilized, and make furnace inside have sufficient saving heat; two is that the whole combustion process is homogenization, improving the efficiency of the hot blast stove, prolonging the furnace life. Therefore, the economic benefits of automatic combustion are mainly reflected in: the material can be saved; the hot air temperature can be rising; improving the service life of furnace.

Optimal control is that under the set constraint condition, choosing one or several indicators (such as the top temperature, exhaust gas oxygen content, thermal efficiency and so on) as the objective function, and then according to the control model determined by the actual calculation or experience to control the combustion, make process maximum in or close to the state of the target function.

The process key of numerical simulation is to establish the control model in accordance with practical production, and the basis of establishing the model is the calculation of heat balance of hot blast stove, the control model not only can make the ratio (air, gas) the most reasonable, also can control the burning rate, make the input flow (gas) best, greatly improving the efficiency.

2 Combustion control system design

Burning control process shown in Figure 2 to 1, the whole control process consists of three functions:①proportional of flue gas oxygen content feedbacks and controls air/gas ratio (model 1);②the vault temperature control model (model 2); the exhaust temperature control model (model 3). The principles and functions of the control process are as follows.

1)  Proportional of flue gas oxygen content feedbacks and controls air/gas ratio

The control model is cascade control system which uses proportional monitor as a simple monitor, and uses the feedback and the control of flue gas oxygen content as a detailed monitor. In the control process, the oxygen content in flue gas is a signal, fixed gas volume, to obtain the best air/gas ratio adjusting air quantity. The target value of oxygen content in flue gas is confirmed according to the production experience, after beginning combustion，when the model reaches normal state，it will be put into using. In order to shorten the heating time of the vault, burning as soon as possible into the best, at the beginning, the operator need preset a proper air/gas ratio on the basis of experience, to shorten the feedback control time, this value can also be work out through the computer, make the air/coal gas ratio at the end of the upper furnace feedback control adding a constant as the initial value of the burning oven.

In order to reduce the valve action times, procedures are set as following: when the flue gas residual oxygen is in the optimal range, stopping feedback control for several minutes and then start again, this procedure may depend on the gas composition is stable or not to cancel or use. When manual adjusting the gas quantity, this model temporarily interrupt the execution, priority in adjustment proportion, when the gas ratio reaches the adjustment proportion value, model automatic resumes execution. Delay can also be used to resume the execution.

The proportional controller has proportional tracking function.

Flue gas sampling device is a special design, make accurate sampling, and is not affected by the cold air valve leakage.

 2) The vault temperature control model

When the beginning of combustion ends, vault temperature goes up to the target vault temperature, it goes into the heat storage period of regenerator. In order to ensure heat storage, protect vault masonry, the vault temperature control model needs to be started (which is the key module in the model).

The combustion control of hot blast stove is to control the blast furnace, coke oven gas flow, air flow and the ratio of air flow. In the vault temperature control model, the task of gas flow control is to ensure that the hot blast stove can store enough heat during burning, to meet the needs of the next blowing wind, and the vault temperature controlled in the vicinity of the target vault temperature. So the gas flow input (V`) is related to the vault temperature (T0), the mixed gas calorific value (H0), the cold air flow in the next time(Fc), cold air temperature (Tc), hot air temperature (Th) and the hot blast stove thermal efficiency (f) and other factors. That is:

V`=G (T0, H0, Fc, Tc, Th, f) --------------- (1)

It is difficult to accurate calculate interpretation of V`, because of many complex heat balance calculation, and the basic calculation needs hot blast stove system set many detection points, if detection points is a lot, not only the investment will be large, product maintenance also will be difficult, and when some detection point is wrong, the application of model often will be influence. So the design was simplifies when building model.

Substantially Cold temperature is made to order, therefore; when the blast furnace is normally product, the sum is substantially constant. If the sum is changed in the actual production, and then affects the air sending time of the hot blast stove, even affects the gas of heating ramp, regardless of hot blast stove uses open-loop control or closed-loop control, the changes can eliminate all of the influence by adjusting combustion flow and time by operator. Therefore, this model will be treated the sum as a constant. So, the (1) can change to the vault temperature control type in which the vault temperature control model is prominent, and the most effective means of controlling the vault temperature is controlling gas calorific value. The model selects the method of regulating coke oven gas amount to control the gas calorific value, obtains the vault temperature empty box model 1, when using transfer, it can be expressed as follows:

Vn=V（n-1）--------------------------(2)

In the formula:

Vn----------- the n moment of the coke oven gas flow,

V(n-1)---------- (n-1) time of coke oven gas flow,

T(n-1)---------- (n-1) moments of the vault temperature

α----------model factor

The model factor α is the function about gas calorific value function H and the heat efficiency of furnace F. Because the model kills the vault temperature control, f on the impact of a is limited to the process of flue gas from the burner exit flow to the vault, in the process flue gas flow has high velocity, time is short, it can be approximated as adiabatic process, therefore, the main factors affecting the A is the H, means: a is varies with the change of H. The design is gas calorific value on-line analyzer, cannot obtain any a value in any moment, so the value should be changed with the actual H as the manual correction. In the actual production, the heating value of the gas often fluctuate, but a could not make frequent adjustments, which will affect the accuracy of model control, the vault temperature fluctuation increases in the vicinity, but in normal circumstances, time effect is not significant, and the vault temperature allows a certain fluctuation. When the fluctuation value exceeds the vault temperature superior limit, alarm device alarms, the operator can timely treat.

The initial setting value of A can be approximate accounted out according to the gas calorific calculated by the combustion temperature.

Modeled on the principle of model 1, vault temperature control model 2 could be launched, as following:

Mn=M（n-1）--------------------------(3)

In the formula:

Mn≥Mo

Mn--------n moments of air gas ratio;

M(n-1)-------- (n-1) moments of air gas ratio

b ----------Factor model

M0- the air gas ratio of ensuring to complete combustion

Factor model b is similar to a, not in the discussion again.

M0is the set value, setting flue gas oxygen not less than the optimum value of coal residue oxygen content g, the corresponding air/gas ratio of g is M0, when Ma<M0,follow Ma=M0to execute.

When using the control model II, the operator can choose the control model 1 or 2, when using the control model 1, control model I can continue to implement, but when the gas volume changes, can make proportional regulation execution priority to the flue gas oxygen content feedback fine turning, and then fine turning feedback after sometime. The proportional value Ma=M(n-1)+ Δ M, when the gas flow rate (Va-Vn-1) is relatively small, cannot perform the proportional regulator, direct using feedback control. When VA≤ △V (△V is the set lower limit), should directly turn the control model into the control model 2 to doing process control.

Starting from the energy use point of view, we should try our best to use the control model 1, but due to coke oven gas and blast furnace gas in the gas main pipe mixed rather than in branch pipe, when the two hot blast furnace combust together, for reducing the mutual interference of the direct combustion process, we can choose that one furnace uses the control model 1, the other furnace uses the model control 2.

Usage control model 2, model I automatically stops the execution of control.

 (3) Exhaust gas temperature control model

With the regenerator checker brick (especially the upper) continuous heating, gas and the lattice brick Wensu time is reduced, the heat exchange conditions is getting worse. In order to ensure furnace grate support does not exceed the safe temperature of using, and continue to expand the storage of checker bricks, ensure the combustion time, so later the burning furnace need carry on the management to the exhaust gas temperature, designed to gradually reducing combustion flow to manage the waste gas temperature.

When entering the waste combustion temperature management period, with the formula to control the temperature of the exhaust gas.

Va=Vn-1× r

In the formula:

Va- -------Mixed gas flow mgn/h in n moment

Vn-1------------ mixed gas flow of mgn/h in (n-1) moments

r ----------- Gas flow rate of decline, as the setting value

The r value is set based on field experience, gas flow reduced several times to complete burning furnace, if the control model I is still running, when the gas flow rate changes, oxygen content in flue gas feedback control should give priority to proportional controller implementation, after some time, feedback tuning, ratio Ma=M(n-1), before the end of burning furnace, firstly automatic control model 1 should be turned off.