Simulation and Optimization of the Power Station Coal-Fired Logistics System Based on Witness Simulation Software

Yabin Li 1, Rong Li 2

1School of Mechanical Engineering, North China Electric Power University, Baoding , HeBei, 071003, China

2Information and Network Management Center, North China Electric Power University, Baoding , HeBei, 071003, China

lybhnjz@163.com

Abstract

Based on analyzing characteristics and relations of the power station coal-fired logistics system, including the choice of coal suppliers, the railroad transport system, the power station stockpile system, the power station coal transfer system and human resources, by Witness software the thesis sets up a simulation and optimization model of fuel coal logistics system and detailedly analyzes one example. The model results not only macroscopically image bottleneck points and resources availability, but define optima separate stage ordering goods batches and stockpile quantities. The thesis is powerful and significant for thermal power station to establish scientific reasonable logistics system strategies, reduce the cost and increase competition ability.

1. Introduction

According to statistics, the velocity of national electric

power increase is very quick in the last few years. In the total output of electrical energy, the thermal power occupies about 80%. Among them, the cost of a thermal power station coal-fired logistics system in China, including the purchase of coal, transport and storage et al.., is often accounted for 60% to 70% of the cost of power generation. Under the condition of guarantying coal-fired supply, how to cause the least coal-fired logistics system expense, is the primary mission and goal of the coal-fired logistics system administration [1].

In recent years there are also some literatures on transport and stockpile models of coal-fired thermal power stations. Literature [2] has theoretically merely studied the power station coal-fired logistics system's characteristic, and not given the substantive solution. Literature [3] has studied coal purchase and transportation questions on Taiwan Electricity companies, and established the transportation plan of many suppliers selection and the stockpile control mix integer project model. Literature [4] has studied the thermal power station coal-fired

stockpile optimization strategy, and established the dynamic project model of order time and order quantity. Literature [5] has studied an integrated coal transportation and stockpile model under the condition of rail direct transportation.

On the one hand, about the above-mentioned documents conditions are limitedly considered, and the study of human resources is also lack in thermal power station coal-fired transportation and stockpile problems; on the other hand, it is crucial not to realize the simulation of the power station coal-fired logistics system, so it is very difficult to find what time and what position bottlenecks occur in the power station coal-fired logistics system and thus it is absent to promptly and effectively improve the characteristics of the power station coal-fired logistics system.

Therefore, this paper, according to the premise of coal boiler requirements, on the basis of the comprehensive analysis of coal-fired rail transportation and stockpile that are two important links of logistics system, major studies, sets up and simulates by Witness2006 software the multi-cycle logistics system of coal-fired thermal power stations. And by demonstrating the entire process of its logistics systems, the model makes users clearly understand its logistics problems in the process and adjust the model parameters in a timely. And the model makes the total costs of the logistics system as the optimal goal, which includes coal purchase cost, transportation cost, stockpile cost, capital cost, and applies Witness software optimization modules for optimization, so as to implement the best value of the power station coal-fired logistics system.

2. Analysis of the power station coal-fired logistics system

The power station coal-fired logistics system has three

major components, namely, suppliers’ selection, railroad transport systems and power stations.

2008 Workshop on Power Electronics and Intelligent Transportation System

978-0-7695-3342-1/08 $25.00 © 2008 IEEE

DOI 10.1109/PEITS.2008.103

394

2.1. Suppliers’ selection 2.1.1. Guarantying coal supplies. When coal production capacity or supply bottlenecks encounters blocked, it is easy for the supply chain to trigger coal-shortage crisis. Therefore, power enterprises can unite with coal enterprises to handle an ore for reaching the purpose participating in competition on the headstream, and also can sign the middle-long supply and demand contract with coal enterprises under strategic partnership, which can cut down incomplete coal risk, coal purchase cost, and build healthy developed economic supply chaining [6]. 2.1.2. Ensuring coal qualities. Because the fired coal has the strict request to the indexes, when power stations select suppliers they must consider qualities and quantities of coals in the lowest purchase cost, as well as after mixed, to be able to achieve the coal-fired requests. 2.2. Railway transport systems

More than 80 % of power station coals need railway

transports, and the vast majority of coal rail transports are used to rail direct transport. Of course, this way has certain requirements on the number of direct cargo trains, the consignor loading capacities, the rail carrier capacities, economic freight weights, plant unloading capacities and stockpile system conditions.

2.3. Power stations 2.3.1. stockpile systems. In the coal-fired logistics system, the plant stockpile system is essential aspect, which is closely related to security, stability and economic operation. It is decided by the factors that are power station demanding fuel quantities per day, security stockpile, ordering cycle, delayed arrival time, the ability unloading capacities and so on. [7].

2.3.2. Factory coal-fired conveying systems. It is mainly composed of dropping coal machines, belt conveyers, screening machines, measurement machines, sampling machines, transportation machinery and so on. This system mainly researches unloading coals machinery related to unloading ability and transport machinery related to the boiler load.

2.3.3. Human resources. In this study of the logistics system, the human resources mainly are for checking and unloading coals, machinery service and conveying coals. And, it is very important for quality testers to check coals qualities and quantities, which not only relates the round turn economic interest, but also relates the accuracy of coal-fired consumption computation used for electricity generation. And the magnitudes of moisture contents of

coals arriving to factories are affective to coal-fired qualities and quantities [8].

3. Modeling principle by Witness

Based on the thermal power station coal-fired logistics

system, the modeling principle is: ①power stations may select suppliers according to suppliers' quote, freight, coal composition and delayed payment, et al.; ② the quantities and the time of supplying fired coals depend on power stations’ demands and the minimum cost principle; ③ coals transport systems should be consistent with the requirements of railway system and ensure the economy transport weigh and transport efficiency; ④ power station stockpile system should ensure that the optimal stockpile costs, and the use of human resources, as well as the scheduling of electrical equipments should meet maximize efficiency.

4. Constructing models 4.1. Parameters definition

The model input parameters: Q(k) as the demand for power station at stage k, t; E as the proportion of a supplier’ supplies occupying total demands (top limit),%; Ci(k) as order cost of coal i at stage k ,yuan/time; Bi(k) as unit railway direct transport costs of coal i at stage k, yuan/t; Pi(k) as unit purchase price of coal i at stage k, yuan/t; G(k) as power station unit storage costs at stage k, yuan/t; S as stockpile capacity of a power station, t; Li as lower limit of railway direct transport capacity, t; S(k) as safety stockpile quantities of a power station at stage k, t ; Ui as the arriving rate of supplier i providing coals ,%; U as the minimum acceptable arriving rate of a power station,% ;R as the discount rate,%; Ti as the delayed period of a power station paying for coal i, d; Xij as index j value of coal i,%; Yj as a boiler requesting coal index j value,%; W(k) as the wages of power station workers at stage k, yuan/month.

The model output parameters: Zi(k)as the quantities of ordering coals from suppliers i at stage k, t, V(k) as the stockpile of power station fired coals at initial stage k, t; Costi(k) as power station coals purchase total costs and transport total costs from supplier i at stage k; Stock(k) as power station stockpile costs at stage k; Cost as the total cost of thermal power station coal-fired logistics system. 4.2. Assumption of constraint conditions and determination of objective functions

Construction of the model based on the following

assumptions: (1) In order to maintain consistent with the ordering

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fired coal plan, assume the research cycle of the model for one year, namely, designing a one-year coal-fired subscription plan. And the research phases of the model are monthly divided into 12 stages, and the specific research object is one day; (2) In order to ensure the security and stability of coal-fired supplies, the level of a power station depending on each supplier is not more than 50%, and each supplier provide only one type of coal, namely:

)(5.0)( kQkZ i ∗≤ ,i=1,2,…,n，k=1,2,…,12 (1) (3) Assuming that applying the rail direct transport

mode, trains have minimum and maximum capacity constraints; (4) In order to the actual situation of unequal paths from suppliers to power stations, and to ensure the accessibility of the model, the model utilize different transport costs per km to express unequal paths, namely, under the conditions of different fired coals with unequal transport costs per km and the coal-fired invariable unit transport cost per km at every stage , the paths in the model show the equal distance from different suppliers to the same power station;

(5) In general, the coal enterprises pay for the freight in advance, and after coals arrive to the plant the power station together pays for purchase costs and the freight. Therefore, in order to reflect capital time value on the impact of the coal-fired cost, the model introduce a discount rate and a power station delayed payment period for coal i to accurately reflect the true value cost, and the delayed period is different with different suppliers;

(6) Ordered coals arrive in the same stage [9]; (7) The stockpile is the quantities of the beginning of

each stage (month, day), and unit storage cost is related to storage quantities and stages, and there are the largest stockpile and safety stock restrictions, namely:

∑=

≤+n

ii SkVkZ

1)()( , ki,∀ (2)

∑=

≥−+=+n

ii kSkQkZkVkV

1)()()()()1( , ki,∀ (3)

(8) The coals-mixed quality requirements of the boiler mainly reflect coal-fired caloric power, the volatile content, moisture content, sulfur content and ash content, and so on. These indexes must be controlled to set limits. When the raw coal arrives at the power station, there is the lowest request regarding the coal arriving percentage. After quality testers checkout indexes and quantities, it can be arranged by the dropping coal personnel to unload the coal to the coal field, namely:

)()()(1

kQYkZX ji

n

iij ∗≥≤∗∑

=

， kji ,,∀ (4)

)()(1

kQUkZUn

iii ∗≥∗∑

=, ki ,∀ (5)

The objective function is set to the minimum total cost of the logistics system, namely:

iTiiiii R)(k))/(1B(k)(P(k)Z(k)C(k)Cost ++∗+= (6)

(k))/2 +V(k+1)+V(k)Z((k)Stock(k)=Gn

1ii∑

=

∗ (7)

∑ ∑= =

∗++=12

1 1][

k

n

ii W(k) labors Stock(k) (k) CostCost

ki,∀ (8) Total cost of the logistics system consists of three parts,

namely, coal-fired purchase and transportation costs, stockpile costs and labor costs. 4.3. Construction of simulation and optimization models

Figure 1. Simulation and optimization models of the

power station coal-fired logistics system Based on assumptions, applying Witness software

constructs the model as shown in Figure 1. In the Model, the suppliers may have more than one, and there is a coal-loading machine and a loading buffer zone corresponding to one supplier. Under assumptions (4), corresponding to the power station there are a number of trains and one circular direct railway (track 1 and track 2). The model also orderly sets up the power station buffer to unload coals, three quality testers, two coal-whippers, the power station stock, one dig and transport coal machine and nine operators, as well as one boiler and so on.

The coal supplies in the model depend on the needs of power stations, with ton as coal units and hours as simulation time; the adjusted cycle of coal-unloading and coal-mining machines are one time per 100,000 time work, and the adjusted time is 24-hour long. And maintenance cycle meets NEGEXP (20,1) distribution, and repairing time meets POISSON (12,2) distribution; a worker works eight hours under the condition of three shifts of work system, and so on.

The model applies optimizer 4.3 module for optimization. The model makes assumption conditions as restrictions, ‘cost’ as the objective function and Simulated Annealing method as algorithms.

5. Case study 5.1. Parameters assignment

Aimed directly at South China thermal power station A

coal-fired logistics condition, this paper applies the model

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of simulation and optimization. The plant purchase coals from three coal suppliers, with the procurement period of 12 months and one month as a stage( k =12).

Initially stockpile V(0) = 105000 t, E=0.5, Ci (k) = 900 Yuan, G(k) = 0.4 Yuan, S = 420000 t, Li =1200t, S(k)=80000 t, R=0.02, Ti=10 days, U1=0.98, U2=0.97, U3=0.97, U=0.97, W(k)=5,000 Yuan, 3 quantity testers, 9 operators and other variables as shown in table 1.

5.2 Analysis

After the system operation, the simulation results can macroscopically show the bottlenecks of the logistics system operation and the information of resource utilization efficiency at all aspects, which can be used for targetedly repeatedly adjusting logistics system resource allocation to ensure the final maximize resource utilization, and thereby minimize logistics costs. After the optimization, optimization results of the procurement strategies, the stock quantities and the total cost as shown in table 2.

Table 1. Parameters of the power station coal-fired logistics system

k Time B1 B2 B3 P1 P2 P3 Q

1 2 3 4 5 6 7 8 9

10 11 12

0-720 720-1440

1440-2160 2160-2880 2880-3600 3600-4320 4320-5040 5040-5760 5760-6480 6480-7200 7200-7920 7920-8640

91.35 91.35 89.25 89.25 87.15 89.25 89.25 89.25 86.1 86.1 90.3

91.35

96.6 96.6 94.5 94.5 92.4 94.5 94.5 94.5 90.3 90.3 95.55 96.6

94.5 94.5 92.4 92.4 90.3 92.4 92.4 92.4 89.25 89.25 93.45 94.5

388.5 367.5 367.5 378

388.5 399 399

388.5 378

367.5 378

388.5

393.75 362.25 372.75

378 393.75

399 399

393.75 378

372.75 378

388.5

393.75 367.5 378

383.25 393.75

399 393.75 388.5

383.25 372.75

378 388.5

202860 210168 273168 275436 277200 293832 315000 357084 319284 268884 252000 231084

j X1 X2 X3 Y

1* 2*

0.25 0.02

0.35 0.03

0.3 0.04

0.3 0.03

Note:1*-volatilize content ；2*-sulfur content

Table 2. Optimized results of the power station coal-fired logistics system k Z1（k） Z2（k） Z3（k） V（k） Cost(Million) 1 2 3 4 5 6 7 8 9

10 11 12

101430 105084 136584 137718 138600

0 115332 178542 159642 134442 126000

0

0 105084 136584 137718

0 0 0 0

159642 134442 96642

0

80430 105084 136584 94332 138600

0 157500 178542

0 134442 126000

0

84000 189084 325668 420000 420000 126168 84000 84000 84000 218442 315084 84000

1200.293

6. Conclusions Based on the analysis of characteristics of the thermal power station coal-fired logistics system, applying witness simulation software builds the thermal power station coal-fired multi-cycle logistics system model under the condition of a rail direct transport. Objectives is the minimum total cost of the logistics system , including coal purchase costs, transportation costs, stockpile costs, capital costs. Logistics system take full account of the various constraints relying on the needs of a coal-fired power station, including suppliers selection, railway

transport capacity constraints, transport quality constraints, stockpile capacity constraints, security stock constraints, boiler requirements for coal qualities, as well as blending ratio, et al.. Through simulation and optimization of the model, optima separate stage ordering goods batches and stockpile quantities can be defined, which provide fundamental strategies to formulate scientific and rational logistics system. The analysis of the practical sample through the model further explains the construction process of the model and verifies the validity and the feasibility of the model. Acknowledgement

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It is a project supported by North China Electric

Power University scientific research fund.

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