Material Flow, Energy Flow and Energy Flow Network in Iron and Steel Enterprise

The research on material flow, energy flow and the relationship between them will benefit the construction of a resource saving and environmentally friendly enterprise. Recently, more and more researchers are paying their attentions to the study of material flow and energy flow of iron and steel enterprise. The modern iron & steel joint enterprise is a complicated iron-coal chemical system, and the steel production process is, in essence, the dynamic operation of the materials flow under the drive and effect from energy flow following the definite rules (Yin, 2000, 2007, 2008). Lu proposed the e-p analysis method (Lu et al., 2000) and the standard material flow diagram for blast furnace – converter process (Dai & Lu, 2004) which is used to analyze the Fe resource efficiency. Based on the standard process energy consumption, the influence of material flow of iron and steel enterprise on its energy consumption was studied (Chen et al., 2002). The relationship between materials flow and energy flow was established (Cai et al., 2006) and they were expressed by the item of energy consumption per ton of steel (Cai et al., 2008).


Introduction
Based on the theory of Metallurgical Process Engineering (Yin, 2011) and methodology of Systems Energy Conversion (Lu & Cai, 2010), the physical essence of the steel production process is re-observed, and the steel production system is decomposed into two parts: materials transportation process and energy transportation process.
The research on material flow, energy flow and the relationship between them will benefit the construction of a resource saving and environmentally friendly enterprise. Recently, more and more researchers are paying their attentions to the study of material flow and energy flow of iron and steel enterprise. The modern iron & steel joint enterprise is a complicated iron-coal chemical system, and the steel production process is, in essence, the dynamic operation of the materials flow under the drive and effect from energy flow following the definite rules (Yin, 2000(Yin, , 2007(Yin, , 2008. Lu proposed the e-p analysis method  and the standard material flow diagram for blast furnace -converter process (Dai & Lu, 2004) which is used to analyze the Fe resource efficiency. Based on the standard process energy consumption, the influence of material flow of iron and steel enterprise on its energy consumption was studied (Chen et al., 2002). The relationship between materials flow and energy flow was established (Cai et al., 2006) and they were expressed by the item of energy consumption per ton of steel (Cai et al., 2008).
In the following sections material flow and energy flow model of an iron and steel enterprise will be developed. In each model the flow of a unit, a process and an enterprise will be represented in order. Network is the running type of energy flows. Then, the operating behaviour and effectiveness of energy flow network and the interacting mechanism among several energy flows are analyzed in order to find out the new direction of energy conservation in an iron and steel industry enterprise. And finally, the conclusions on the research will be given.

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of processes related to the production of iron and steel, such as coke making, sintering, pelletizing, iron making, steel making, and steel rolling. For iron making process, it consists of several blast furnaces with different volume. These blast furnaces are the units in iron making process. So, the units' material flow structure, flow rate, flow direction is varied. They together determine the structure, flow rate, flow direction and flow indices of the whole material flow of an enterprise. To analyze the material flow of an enterprise, the material flow model of a unit should be established.

Material flow model of a unit
For iron and steel enterprise, five material flows shown in Fig. 1 may emerge in j-th unit of ith process, described as unit (i, j). The flow rates of five material flows are average values of a statistical period (one year, one month, one day, etc.). Actually, flow rates of all material flows through a unit changes when time is changed. So, flow rate of material flow is the assembly average of the material flow rate per unit time. i. Material flows from the anterior producing unit ( 1 j i F  ). They are products of (i-1)-th process served as materials of unit (i, j). e.g., hot metal of blast furnace flowed into a converter. . e.g., the iron dust emitted from a sintering machine.

From environment
v. Material flows to the followed producing unit ( j i F ). They are qualified products of unit (i, j). e.g., hot metal from iron making process. Based on 1 ton of steel, the input-output relationship model for unit (i, j) is developed as

 
Therefore, the parameters and evaluating index of material flow of unit (i, j) are listed as i. Waste rate ( j i w ). It is defined as the wastes produced by per unit of qualified product of unit (i, j) and is expressed as ii. Recycle rate of waste ( j wi r  ). It is defined as the rate of recycled wastes to total wastes produced and is expressed as From the view of reducing resource consumption, it is the demanded material flow per unit of product that is the most concerned for one producing unit. Thus, metal yield ratio of unit (i, j) is taken for evaluating the material flow of a producing unit.
iii. Metal yield ratio ( j i R ). It is defined as the output of products per unit of material and is expressed as It can be found from Eq. (9) that the metal yield ratio is related with the waste rate. To increase the metal yield ratio, waste rate of this unit should be reduced.

Material flow model of a process
Material flows through a producing process are shown in Fig. 2. Process i is consisted of k i units. Then, the material flow of process i is the sum of material flows of the k i units.
So the input-output relationship model for process i is developed as

Material Flow, Energy Flow and Energy Flow Network in Iron and Steel Enterprise 247
Likely, the parameters of material flow of process i are waste rate ( i w ) and waste recycle rate ( wi r  ); and metal yield ratio of process i ( i R ) is taken for evaluating the material flow. They are expressed respectively as From the view of reducing resource consumption, it is the demanded material flow per unit of product that is the most concerned for one process. To obtain a smaller demand of natural resource, it is better if waste is less for iron and steel enterprise. As for the waste produced, it is best to be recycled to reduce the consumption of natural resource. i.e., the objective of material flow recycling is to obtain a smaller i w and a larger wi r  mentioned in Eqs. (12) and (13).

Material flow model of an enterprise
Based on the material flow model of a process, it is considered that the iron and steel enterprise is composed of n processes. And the material model of an enterprise is shown in Fig. 3.  Fig. 3 is the output of process i in statistical period and is expressed as The material flow input-output balance of enterprise is ii. Recycled material flow per ton of steel.
iii. Emitted material flow per ton of steel.
iv. Waste rate of an enterprise.

 
v. Waste recycle rate of an enterprise.
Resource efficiency is the output of final product per unit natural resource used. It is the evaluating index for material flow of an iron and steel enterprise with the expression as Considering the waste rate and waste recycle rate of an enterprise, Eq. (22) can be rewritten as It can be seen from Eq. (23) that resource efficiency (R) will be raised by reducing waste rate of an enterprise (w), increasing waste recycle rate of an enterprise (r), and/or increasing external material flow per ton of steel (  ). To increase the resource efficiency of an iron and steel enterprise, for one thing the waste output of every process should be reduced, for another the formed waste should be utmost recycled and reused.

Energy flow
The energy flow is formed when the energy runs along the route of conversion, usage, recovery and emission. Similar as material flow, energy flow model of a unit will be established, based on which energy flow model of a process or an enterprise are developed.

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Material Flow, Energy Flow and Energy Flow Network in Iron and Steel Enterprise 249

Energy flow model of a unit
For iron and steel enterprise, six energy flows shown in Fig. 4 may emerge in j-th unit of k-th energy conversion process, described as unit (k, j). The flow rates of six material flows are average values of a statistical period (one year, one month, one day, etc.). Actually, flow rates of all energy flows through a unit changes when time is changed. So, flow rate of energy flow is the assembly average of the energy flow rate per unit time.
By-product energy where, Based on 1 ton of steel, the energy flow input-output relationship for unit (k, j) is Considering energy product is energy carrier, energy value of energy product is selected as the evaluating index of energy flow of unit (k, j). Its expression is is the sum of energy flows consumed in this unit,   ζ C j C j kk gg    is the sum of energy flows recycled in this unit.
It can be found from Eq. (31) that the energy value of energy product is determined by energy flows consumed and recycled.

Energy flow model of a process
Energy flows through a producing process are shown in Fig. 5. Process k is consisted of m k units. Then, the energy flow of process k is the sum of energy flows of the m k units.
where , k j  is the product weights of each energy conversion unit with the expression as where 0 C j k F  is the output of energy products of unit (k, j) and 0 C k F  is the output of energy products of process k.
The energy flow input-output relationship model for process k is developed as Energy value of energy product of a process is also the evaluating index of energy flow of energy conversion process, which is expressed as is the sum of energy flows consumed in process k,   ζ CC kk gg   is the sum of energy flows recycled in process k.

Energy flow model of an enterprise
It has several energy conversion ways for any type of energy. Each conversion needs several transportation and energy conversion devices. The energy flow model of an enterprise is shown in Fig. 6. Energy media k has l types of conversion. Anyway, the final output energy is k g after several conversions from the original energy k g .
As for j-th energy conversion way of k-th energy media, supplied energy , The energy conversion efficiency of media k is where , k j  is the ratio of energy input of j-th energy conversion way to total energy input, expressed as ,, kj kj k gg   It is indicated from Eq. (40) that some measures must be taken to improve the conversion efficiency and reduce the energy value of an energy product for an iron and steel enterprise. They are: increasing the weight factors of the energy conversion modes whose efficiency is higher; reducing the amount of the conversion units, optimizing the transportation mode of energy, and shortening the distance of transportation.

Energy flow network
Energy flow is dynamically operates in energy flow network following some rules. The energy flow network of iron and steel enterprise is composed of energy origin nodes (such as blast furnace, coke oven, converter), connectors (such as gas pipe), intermediate buffers (such as gas holder), and energy terminal nodes (such as sintering machine, blast furnace, heating furnace, boiler). The energy flow network of modern iron and steel enterprise is shown in Fig. 7.
According to the input-output feature of energy flow network, the energy flow network of iron and steel industry is divided into six regions. It includes energy transfer region (C), energy usage region (U), waste heat recycling region (R), surplus energy buffer region (B), energy storing region (S) and city energy system (CES).
In Fig. 7, the direction of arrow represents the flow direction of energy flow, parameters above flow lines represent the flow rates or allocation proportion of energy flows, and parameters below flow lines represent the temperatures or energy values of energy flows. C F and C G represent fuel conversion and power generation, respectively. U H , U M and U L represent high-grade, medium-grade and low-grade energy consumers, respectively. R H , R P and R C represent heat recovery, power recovery and combined heat and power recovery, respectively. B C and B D represent centralized and distributed energy buffers, respectively. t is the temperatures of energy flows, according to which the energy values of energy flows can be gotten.  in Fig. 7 is the allocation proportion of energy flow with where 0  and 0  represent the ratios of energy flow converted ( 0 G ) and unconverted ( 0 G ) to external energy flow ( 0 G ), respectively; CF  and CG  represent the ratios of energy flow of fuel conversion part ( CF G ) and power generation part ( CG G ) to converted energy flow ( 0 G ), respectively; and so on.  Systems energy efficiency is the evaluating index for energy flow network of iron and steel enterprise. It is defined as the output of steel product per unit of net energy input and is expressed as where g is the energy input per ton of steel.
Eq. (44) It can be achieved that from Eqs. (44) to (49) that systems energy efficiency of an iron and steel enterprise is related with energy conversion efficiency, energy usage efficiency, waste heat recovery efficiency and energy transportation efficiency.
With the continuous improvement of the technology and method of energy conservation, the energy conservation potential of the energy flow network of future steel enterprise lies in imbalance: the quantitative imbalance between the energy supply and energy demand of device, the qualitative imbalance of energy gradation between energy flow inlet and outlet, and the dynamic imbalance between the generation and consumption of secondary energy. So, the dynamic method of imbalance must be used to recognize the running regularity of energy flows, such as gas, oxygen and steam. And the imbalance between energy generation and utilization should be scientifically plan, design and schedule to seek for a new approach from non-equilibrium state to orderly structure. i.e., more attentions should be paid to energy conversion efficiency, energy usage efficiency, waste heat recovery efficiency and energy transportation efficiency.

Conclusion
Based on the five material flows including those from the anterior producing unit or process and those recycled through a unit or process, the material flow model for a production unit, a process and an enterprise are constructed in this chapter. The influence that the material flow of a process has on the metal yield ratio is described, and so is the influence of the parameters on the resource efficiency of an enterprise. For iron and steel enterprise, the produced waste material should be recycled to the process itself or others to reduce the consumption of natural resource.
In this chapter, energy flow through an energy conversion unit or a process, such as the converted energy and the energy of the product, is discomposed into six parts, and the energy flow model for a conversion unit, a process, and an enterprise are established. The influence that the flux of each energy flow, the largening extent of equipments and the recovery of residual heat (or energy) have on the energy value of the product is analyzed.
It is proposed in this chapter that the energy flow network is divided into six regions according to their functions: energy conversion region, energy usage region, waste heat recycling region, surplus energy buffer region, energy storing region and city energy system. The energy flow network model is established for evaluating the comprehensive energy level with the index of system energy efficiency.
Directions of next research include: (i) The forecasting of energy generation and energy consumption; (ii) The optimized allocation of surplus energy; and (iii) The optimized connection between energy flow network of an iron and steel enterprise and city energy system.