A Gas Scheduling Optimization Model for Steel Enterprises

Regarding the scheduling problems of steel enterprises, this research designs the gas scheduling optimization model according to the rules and priorities. Considering different features and the process changes of the gas unit in the process of actual production, the calculation model of process state and gas consumption soft measurement together with the rules of scheduling optimization is proposed to provide the dispatchers with real-time gas using status of each process, then help them to timely schedule and reduce the gas volume fluctuations. In the meantime, operation forewarning and alarm functions are provided to avoid the abnormal situation in the scheduling, which has brought about very good application effect in the actual scheduling and ensures the safety of the gas pipe network system and the production stability.


Introduction
Steel industry is one of the energy-intensive industries which shares 10% total energy consumption of the national economy and the energy cost of steel enterprises takes about a third of the production cost [1].High energy consumption not only increases the cost of steel products but brings about much pollution and emission.In practice, due to the changes of factors such as the production and maintenance schedule, gas production and consumption keep changing.Gas system exhibits imbalance in a short time which results in low gas utilization rate.The aforementioned problems need to be solved by optimizing the gas system scheduling.
Recently, researches about the energy optimization model of steel enterprises have been carried out extensively.Ref. [2] described the relationship between the various units of gas pipeline network as the form of matrix then modeled and solved the problem with partial least squares regression method.Ref. [3][4][5] proposed the gas supply and demand forecast model of steel enterprises and realized the gas tank safety control and integrated power generation benefit maximization.
Although the above algorithm is theoretically reasonable and the simulation results are satisfactory, the instantaneity and effectiveness of are not enough for the gas scheduling.In this research, according to the characteristics of gas pipeline network system and the actual scheduling process in steel enterprises, the generation and consumption of gas are scientifically and effectively estimated and the by-product gas system is modeled and analyzed.Then the gas scheduling optimization model is proposed according to the rules and priorities.The proposed model is of great significant in reducing the gas loss and balance of the whole gas system.

Problem description
The by-product gas system of steel enterprises is mainly composed of blast furnace gas and converter gas system.The gas system is illustrated in Figure 1: The basic data of energy control system of steel enterprises is relatively consummate, telephone scheduling has been replaced by centralized scheduling.However, balance scheduling operation is still dependent on the experience of scheduling staff.Gas scheduling is based on the expertise of scheduling staff which is highly influenced by human intervention besides the following problems: (1) When the process system state changes, the gas consumption variation is very larger.The gas will exhibit large fluctuation and loss if the scheduling center is not informed timely.
(2) At present, the measured value of some gas meter different from the actual value.For examples: the gas consumption power plant boiler is not accurate, the gas generation meter of blast furnace is not accurate in small scale range.
(3) When there is a large fluctuation of gas, the extra blast furnace gas is usually released to maintain the pipe network pressure.For examples: re-blowing of blast furnace after maintenance will cause large amount of gas, maintenance of rolling accident will decrease the gas consumption sharply.
(4) There are no operation forewarning and alarm functions in existing gas scheduling.For example: when there is gas release while the power plant boilers capable to consume the gas and the boiler is running normally, the boiler load should be raised to reduce gas release.

Gas scheduling optimization design
According to the features of steel enterprises' gas pipe network, the gas scheduling optimization is proposed based on the energy control system, including: (1) Process state calculation By the calculation of process states including blast furnace, cowper stove, steel rolling furnace, sintering, thermal power plant, etc., the switching state of the main gas units are judged so that the scheduling staff can see each process state in real time.
(2) Gas soft measurement When the process states change, the gas consumption of the current states can be estimated according to the features of the gas system.Then the fluctuation quantity of gas consumption caused by process states change can be calculated.
(3) Building up the gas scheduling optimization model based on the rules According to the effective scheduling rules, the fluctuating range and the consumption amount of gas are determined.
Then the gas can be scheduled according to the priorities.

Process states calculation
The processes related to the gas production and consumption, including sintering, blast furnace, converter, steel rolling furnace, etc., are calculated.When the states change, the scheduling staff will be informed by warning so that the real-time process states can be taken into account for gas scheduling optimization.Take the sintering process as an example, the state logic is shown in Figure 3 where: n Q is the gas consumption of the sintering furnace in normal state, 3 / Nm h ; p Q is the gas consumption of the sintering furnace in preservation state, 3 / Nm h .The state calculation logic of the sintering ignition furnace is described below: (1) Blowing out: the gas consumption of sintering furnace is below the setting value (such as 100 m3/h) or the average firing temperature of sintering furnace is below the setting value (such as 400 ˚C), and the temperature rate is 0 m/min.
(2) Preservation: the average ignition temperature of ignition furnace is above the setting value (such as 400 ˚C), and the temperature rate is 0 m/min.
(3) Ignition: the temperature rate of sintering furnace is above the setting value and the average ignition temperature of ignition furnace is above the setting value (such as 900 ˚C).

Gas soft measurement
The measurement accuracy of gas meter is bad considering small range measurement and the impurities and soft measurement should be carried out to calculate and correct the gas production.At the same time, when the process states change, gas production and the gas consumption of power plant boiler can be calculated according to the correlations between the processes' data, so that the scheduling staff can be aware of the pipeline network fluctuating quantity.When the deviation between estimated and actual value is intolerant, warning will be given to remind the scheduling staff to optimize the scheduling of gas consumption.Take the gas consumption of power plant boiler as an example, the algorithm model of gas soft measurement is introduced as below.
Regarding the gas consumption calculation of single boiler, the boiler heat absorption is first calculated according to the main steam enthalpy rise and gas flow, then the boiler heat release can be obtained from the calorific value of gas.
Finally, the gas consumption per hour can be calculated based on the heat efficiency of boiler: is the calorific value of gas ( 3 / KJ m ) which can be obtained from the calorific value analyzer.

Function description of scheduling based on rules
In this research, gas scheduling algorithm is designed based on the rules, which includes:

Scheduling priority division
In order to further utilize the gas resource and satisfy the needs of actual production, each unit of the gas system is simplified and the units are divided into three categories according to the applications as shown in Table 1: Table 1.Categories of the users.

Categories Contents
Type 1 Users must be ensured in main production.
Type 2 Users with small correlation between main production.Set the scheduling priority for each type of user.According to the priority ranking, gas supply of some users are suggested to be cut off upon gas shortage.The definition of priority can be given by the user.The priorities are represented by integers, the greater value means that the user is less important and the adjustment is faster, which will be adjusted to relieve the gas pipeline pressure when the gas is not balanced.

Scheduling rules description
The type 3 users are first adjusted upon gas fluctuation, type 1 and 2 users will be adjusted when the adjustment range of type 3 is beyond.Specific scheduling principles depend on the fluctuation quality, unit of action, state of the consumption user, and the adjustment ability of the unit user, the purpose of adjustment is the between production and consumption of gas.Negative fluctuation quality means gas oversupply and the consumption of type 3 users should be increased to balance the gas supply.If the fluctuation quality is in the adjustment range of a certain type of users, the users of high priority value will be adjusted.When the fluctuation quality is beyond the adjustment range of a certain type of users the adjustment will minimum the number of adjusted users.For gas oversupply, the gas consumption of type 3 users will be adjusted to the maximum value and the extra gas will be released.For gas undersupply, the gas consumption of type 3 users will be adjusted to the minimum value and some users will be cut off according to the priorities.

Scheduling function process
The gas medium of the new district includes blast furnace gas (BFG) and Linz-Donawitz process gas (LDG).The specific algorithm process depends on the gas medium as illustrated in Figure 4: The proposed gas scheduling optimization functions take the different characteristics and technical requirements of various gas units of steel enterprises in practical production process in account.The rule-based gas scheduling optimization model has been established.Algorithm models of Process state calculation and gas consumption soft measurement have been proposed to deal with specific scheduling problems.Based on the rules of scheduling optimization, the scheduling staff are timely informed about the gas consumption state of various processes then schedule gas in time and reduce fluctuation quality.The model can effectively avoid the abnormal situations by forewarning and improve the safety of the gas tank and the stability of production, which has obtained well application effect in practice.This model is of great theoretical and practical significance in reducing gas release and realizing the balance of the whole gas system.

Fig. 3 .
Fig. 3. Sintering state logic.The sintering ignition furnace have three operation states including ignition, preservation, and blowing out.The dynamic model of gas consumption is shown below: