sequential control of cascading hearth staged

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action of the rams are pneumatically powered and controlled by a PLC unit. ... tools. 3). Wide range of application and diversity– capable of running ..... center). Power line. Signal line. Air line. LEGEND. S9. Figure 6. Pneumatic and PLC control ...
SEQUENTIAL CONTROL OF CASCADING HEARTH STAGED COMBUSTOR USING PROGRAMMABLE LOGIC CONTROLLERS Wan Ahmad Najmi b. Wan Mohamed Fakulti Kejuruteraan Mekanikal Universiti Teknologi MARA (UiTM) 40450 Shah Alam Abstract This project focuses on the design of an automated control program for a cascading hearth staged combustor for solid fuel. The combustor consists of a primary combustion chamber, secondary combustion chamber, and a feeder system. Charging rams are the mechanism for fuel feed, and the action of the rams are pneumatically powered and controlled by a PLC unit. The type of PLC unit used is the Siemens Micro-Win S700 microprocessor. The control program is developed by determining the feed process requirements, actuator sequences, listing the variables, describing the step displacement diagram, and programming using the ladder diagram tool. The control of the charging ram feeder incorporates safety procedures and was successfully tested in actual combustion experiments. It is found capable of operating the combustion system effectively according to the process requirements. Keywords: Programmable Logic Controller, Staged combustion, Automated control. 1.

Introduction

A cascading hearth staged combustor is a combustion system for solid wastes. It utilizes the concept of cascading hearths for primary stage solids combustion, with a secondary chamber for furthering gaseous combustion. A cleaner emission is expected from this process. The handling and control of solid wastes throughout the combustor is important. Weaknesses or failures in the design and control of materials handling systems can greatly reduce the efficiency of the combustion system. As solid wastes differ physically in terms of bulk density, shape, size, viscosity, strength etc., most combustion systems are built with material handling facilities to service a specific type of waste, such as a front-end loader, a

concrete pit and bridge crane system. Rubber belts, screw feeders and vibrating conveyors are used widely to transport raw and shredded solid wastes [1]. To select and design a suitable material handling system requires full understanding of the overall process of the combustion system in order to attain maximum reliability. A suitable conveyor for the cascading hearth system is the charging ram mechanism, where batch feeding and fuel displacement is required in the combustion process. Most solid waste combustion system utilizes automated control system for overall process control to increase combustion efficiency and minimize manual power. There are mechanical control devices of common use such as

gears, timers and servomotors. Relays are used in electro-mechanical control devices, such as solenoid valves for hydraulic and pneumatic applications. A more advantageous control device is the Programmable Logic Controller, which is fast replacing applications that uses four or more relay controls.

need for rewiring as found in relay circuits. It is an important capability as the newly developed combustion system will undergo many and different combustion processes to determine the systems optimum performance or best process for a particular solid fuel. Other advantages offered are: 1)

1.1

High efficiency and adaptability - the use of PLC over hard-wired relay circuits ensures easy process modification, time saving and cost effective. Compact ability – micro PLCs are small in size and light, reducing the space requirements for control system tools. Wide range of application and diversity– capable of running programs, storing data, easily modified program, with ability to adjust to process malfunctions. Economical – very suitable when four or more relays are required.

Programmable Logic Controllers

Programmable Logic Controllers, or PLC, is a digital electronic device with internal memory programming and processing capabilities, similar to a computer. It is capable to execute specific functions such as logic, relay, timer, counter, and arithmetic control, by digital signals of input and output from a certain process. It was first assembled by General Electric in 1969 as a substitute for electromechanical relay controllers [2]. To form a PLC control system, the basic equipment required includes a PLC Central Processing Unit, a personal computer, programming software, and a communications cable. PLC is selected as the control tool for the cascading hearth system as it provides an efficient and easy programming interface, capable of converting from one sequence or process to another without the

Solid fuel from container

2)

3)

4)

2.

Design Of Cascading Hearth Combustor

The cascading hearth combustor system consists of six basic component systems, illustrated in Figure 1.

Secondary Combustion Chamber Exhaust

Pneumatic circuit Feed mechanism

Programmable Logic Controller unit

Primary Combustion Chamber

Air feed lines

Figure 1. Schematic of the cascading hearth combustor

The combustion system has the capability to handle a maximum of 40 kg/hour of solid fuel feed, and handling bulk solids of different shapes, sizes and characteristics. The fuel is fed in periodic batches, and due to the estimated variations in process sequence, PLC controlled and pneumatically driven conveying system was installed. A cascading hearth system with underfire air feed is to be applied for higher combustion efficiency in the primary combustion chamber. Secondary combustion process takes place in the swirl combustor. The combustion system is designed to closely comply with BS3813: Part 1 (1964). 2.1

Primary Combustion Chamber

The Primary Combustion Chamber (referred as PCC) is the main component system of the incinerator. Volumes of solid wastes will be moved through, ignited, and reduced to smaller particles by combustion chemical reactions in the PCC, while producing a number of combustion end products such as organic and inorganic particulate matter, combustible as well as carry-over gases, and ash. The main parameters for the designed primary chamber is as following:

m3,

Refractory wall thickness: 0.1 m, Overall internal volume : 0.338 m3, Effective combustion volume : 0.264 Estimated weight : 620 kg.

2.2

Secondary Combustion Chamber

The secondary combustion chamber operates on the principal of swirl operation. Swirl is used extensively in combustion chambers as a mean of controlling flame size, shape, stability and combustion intensity. The principle of swirl flow

operation in a secondary combustion process is where the mixture of air and combustible gases from primary combustion swirls along the circular walls of the secondary chamber. Burning gases travels back towards the burner bringing heat energy and reactive species to promote ignition in the entering fuel-mixture [3]. The secondary stage swirl combustor is designed with a swirl number of 1.57, providing an effective combustion volume of 0.00628m3. 2.3

Feeder System

A charging ram feeding system is a suitable conveyor for the cascading hearth. The solid fuels may come in different sizes, shapes, flow ability, and bulk density, but a properly designed charging ram system would be capable to cope without sacrificing its efficiency. The key to it is to develop a reliable feed process control system that can be modified with ease for each waste fuel combustion requirements. Therefore, the charging ram is driven by a pneumatic cylinder system, with the application of Programmable Logic Controller (PLC) as the control system. The conveyor system consists of 7 main component units; the charging rams, ram chambers, gate, cascading hearths, pneumatic actuator circuit, the PLC, and support frame structure, as shown in Figure 2. Three charging rams are used, the first ram is the feeding ram with the purpose to load new solid fuels from the inlet feed chamber to the hearth. The second and third rams function to discharge burning fuels on the first and second hearths respectively to the lower hearths, as well as acting to mix and distribute the fuels during combustion.

NOTES: 1. Drawing not to scale. 2. All units in mm.

Gate DAL 20X200

Ram A

Inlet Feed Chamber Ram B

FDA 50x350

refractory hearth 1

underfire air chamber

FDA 50x400

Ram C refractory hearth 2

FDA 50x400

underfire air chamber

underfire air pipe

Figure 2: Feeder system arrangement 2.4

Cascading Hearth

The cascading hearth is a modification of the traveling grate concept, and also referred to as “stepped hearths”. It is operated with a charging ram mechanism that conveys the solid fuel from a container through the primary chamber to the ash residue discharge. Figure 3 shows the fuel conveying process in a cascading hearth system as designed for the combustor system. It consists of two effective cascading hearths. Three charging rams are utilized to convey the fuel to the ash residue discharge. The discharge from the first hearth is actuated by the ram and tumbles onto a second hearth, thereby mixing, breaking and redistributing the fuel bed on the second stage. This redistribution is then repeated, and capable of increasing combustion efficiency as a more complete burnout of the fuel is achieved.

3. Development Program

Of

Control

The elements in the control system are represented by symbols and signal flow that indicate the function of the element. Sensors give input signals for the charging ram. Magnetic switch sensors are more suitable than roller type switches as it can be positioned on the cylinder actuator body. Roller type switches are unsuitable as it requires placement along the path of the cylinder rod stroke, whereas the rams’ path goes through the hearths where combustion takes place. Magnetic sensors, in the other hand, are contact less sensing element, detecting the position of the cylinder piston as it moves inside the cylinder casing, and sends electrical signal pulses to the processing element. Signal processing elements are the heart of the circuit system as it generates, redirects or cancels signals depending on the signal inputs received. The need to simplify the circuit can be achieved by using a single computerized logic processor element that incorporates all the necessary functions, such as the Programmable Logic Controller. Signals that is generated by the PLC is sent to the final control element for acquiring the work output. Directional

solenoid control valves converts the electrical pulse signals from the PLC to

1

Pneumatic cylinder FDA 50 x 400 ram chamber

pneumatic signals by controlling the air valve passages.

Solid fuel (load) Second ram action on First Hearth (H1)

ram advance stroke 2

cylinder rod

ball valve

ram surface

underfire air pipe continuously burning fuel

3

new layer of fuel ram advance stroke

core layer of burning fuel

Third ram action on Second Hearth (H2)

4

Figure 3. Fuel conveying process in the cascading hearth combustor The actuators are the working elements. The first ram is driven by a double acting cylinder of 50mm bore and 350mm stroke length (FDA50x350), the second and third by FDA50x400 cylinders, and the gate by a smaller DAL20x200 cylinder.

application that supports the 32-bit Windows 95, Windows 98, and Windows NT environments. This program offers different editor choices for creating control programs, which includes the Statement List (STL), Ladder Logic (LAD), or Function Block Diagram (FBD) editor.

3.1 The Siemens S7-200 Micro PLC (Siemens 1999)

3.2 Process Requirements

The Siemens S7-200 series is a line of micro PLCs that can control a variety of automation applications. The compact design, expandability, low cost and powerful instruction set of the S7-200 Micro PLC makes a perfect solution for controlling small applications. A programming software is required to give task instructions to the PLCs CPU. The recommended software is the STEP 7Micro/Win 32; a Windows based software

The objective of the application is to effectively move solid waste fuels, starting from the fuel inlet chamber towards the end of the staged combustion process (ash container). The fuel type is palm oil shell. Fuel combustion in the PCC is required to be operated in starved air mode that leads to fuel gasification. This will maintain primary chamber temperatures at lower levels. However, a good gasification condition can only be achieved by careful control of air/fuel ratio, increasing fuel core

temperatures, minimize existence of flame on the fuel bed surface, as well as suitable fuel distribution on the combustion hearths.

well as to mix and distribute fuels efficiently to promote gasification. The feeding system of the combustion system consists of four actuator cylinders. Three are for fuel displacement purposes, and the other for controlling the gate movement. For programming, the first ram actuator is termed cylinder A, the second ram controlled by cylinder B, and for the third ram actuator, cylinder C. The gate actuator is termed cylinder G. The sequence of the operation cycle based on the process definition can be visualized as in Figure 4.

From the process requirements, the sequential control of the rams is temperature and time dependent. For the current stage, the scope is limited to programming based on the time variable. There are three charging rams and a gate to operate, and the required actions are basically to feed palm oil shells from container to the first hearth, convey it from the first hearth onto the second hearth and then towards the ash disposal section, as

Start

g1

G+

a1

A+

c1/2

C+

a0

A-

c0

C-

g0

G-

B+

c1

C+

b1

c0

C-

Figure 4. Sequence of actuator operation

bo

B-

The sequence control diagram only shows the sequence of signals and commands without properly establishing the time intervals between commands. Another important tool to visualize the whole cylinder process is the step displacement diagram. 3.3

Step Displacement Diagram

A step displacement diagram is a simple principle to effectively describe a sequence of a number of cylinder movements. The method used is as described by J.P. Hasebrink and R. Kobler [4]. Two horizontal lines are drawn for each cylinder. The upper line representing the fully extended (max stroke) position, the lower the retracted position (original). In equal intervals, vertical lines are drawn in to mark the cycle steps. A step is one cylinder movement, sometimes with two or even more simultaneous strokes. Figure 5 shows the step displacement diagram that describes the cylinders movements during the designed feed process, for one cycle.

4.

Testing And Evaluation

Several combustion experiments were carried out. For PLC programming, these experiments are important to find the suitable time intervals between cylinder movements. To support the experiments, a manual control program was written for putting the pneumatic cylinders into action. This program enables the operator to actuate the respective cylinders by pushbutton switches whenever the need for feeding, stirring, or mixing arises. It allows a wide range of timing sequence to be tested. The automated control program is tested in a 1.0 kg/fuel feed combustion experiment. It is divided into two phases – fuel feed for ignition phase and gasification phase. The major differences of the two phases are the new fuel feed intervals and number of discharges. For the ignition phase, new solid fuels are fed at 5 minutes

intervals, ending at the 5th discharge stroke. The 5th discharge signals the start of the gasification phase fuel feed, at faster feed rates. The gasification phase feeds are conducted at approximately 4 minute intervals. After each new fuel feed to the first hearth on both phases, the 2nd ram (cylinder B) advances slightly to mix and push the new fuels to the center of the hearth. This action promotes the formation of colder fuel layers stacked on top of the hot core. At the end of the ignition phase, the 2nd ram fully advances to discharge the burning fuels onto the second hearth below. During gasification phase, fuels on the first hearth are discharged to the second hearth at approximately 8 minute intervals. The 3rd ram is also programmed to slightly advance at half its maximum stroke after each new fuel feeds to the second hearth. The total combustion time on the second hearth should be adequate to ensure complete incineration of the fuel particles, and is controlled by timer T47. When all fuels are discharged off the second hearth towards the ash collector section, the entire program resets and the whole feed cycle restarts from the ignition phase. The program is initiated by pushing the “start” push button on the control panel. For safety, pushing the “stop” button at any time during the process will immediately terminate all signals and concurring commands, and turns off the main power command. 10 timers are utilized in the program, mostly to allow for a time gap between commands. These On-Delay Timers are of 100ms resolutions; therefore each count of the timer value is a multiple of the time base. As an example, a count of 3000 in T37 represents an actual time of 300 seconds, or 5 minutes. For counters, CountUp counters are used to keep track of the ignition phase feed strokes (C1), the gasification phase feed strokes (C3), and the

total 3rd ram half-strokes (C2) that signals the end of the entire cycle. The control circuit is shown in Figure 6. 4.1

Test Analysis

Preliminary tests carried out focuses on correct circuit connections, signal responses from magnetic sensors, smooth compressed air flow, and ram-cylinder alignment. Later stages concentrate on smooth ram action especially concerning plate surface friction and roller-guide efficiency, suitable charging speeds for each ram, and component modification. The suitable ram charging speeds is in the range of 230 mm/s to 270 mm/s, with a maximum required thrust force of 77 N. Problems in retracting rams are sometimes encountered due to low compressed air pressure supply (minimum 2 bar) and the blockage in the ram guide channel by large fuel particles. The safety commands incorporated in the program is useful to force rams to retract back to its original position when it is jammed at mid-stroke. Figure 7 shows the overall temperature profile of palm shell combustion at 1.0 kg/fuel feed. PCC temperatures are maintained lower than 600oC at all times (shown by T1, T2 and T3). This is due to the success in controlling flame propagation on the fuel bed by effective method of fuel conveying throughout the PCC. Good gasification conditions were promoted by the periodic mixing actions of the charging rams, where these actions successfully piles or stack colder fuel layers on top of the hot fuel core. The temperature of the fuel core (T5) is shown to steadily increase, achieving very high temperatures as high as 1215oC in the 59th minute. It is observed that the thickness of the smoke increases as higher core temperatures is achieved. Figure 8 compares the fuel core temperatures at constant air flow on the second hearth with and without periodic fuel mixing by the third charging ram. The graph proves that

good fuel distribution on the combustion hearth increases achievable core temperatures, and consequently, gasification rates. This is caused by the adequate exposure of particle surface area for combustion reactions to react effectively [5].

5.

Conclusions

Programmable Logic Controllers is a suitable processing element in the sequential control of palm oil shell distribution in a cascading hearth staged combustor. Steps for programming the automated control program for the charging ram mechanism consists of defining the process requirements, establishing the sequence of the actuator operation, describing the step displacement diagram, and conduct combustion tests to find the suitable time variables for optimum performance of the combustion process. The testing of automated feed control in the combustion experiment of 1.0 kg/fuel feed shows that it is capable in adequately control charging ram strokes. Minor difficulties in retracting the charging rams can be overcome by incorporating safety commands in the program to manually retract the rams by push button. Finally, sequential control of a cascading hearth staged combustor using Programmable Logic Controllers shows the capability to meet the demands of the combustion process requirements, especially regarding the need for effective fuel distribution to promote gasification.

Acknowledgements The author would like to thank the Prof Dr Farid Nasir Hj. Ani of the Faculty of Mechanical Engineering, UTM Skudai, and IRPA research grant by Ministry of Science, Technology and Environment for their contributions to this project.

References [1].

[2]. [3].

Mason, J.S and Woodcock, C.R., Bulk solids handling: Introduction to the Practice and Technology, Chapman and Hall, New York, 1987. Otter, J.D., Programmable Logic Controllers, Prentice-Hall Inc., New Jersey, 1998. Bowen, P.J., et.al, Characterization of industrial swirl burners for efficient combustion of low calorific value

INITIAL COMBUSTION PHASE

[4].

[5].

gases, The Institute of Energy’s 2nd International Conference on Combustion and Emissions Control, London UK, pp. 239-248, 1995. Hasebrink, J.P. and Kobler, R., Fundamentals of Pneumatic Control Engineering : 4th edition, Festo Didactic KG, Germany, 1989. Niessen, W., Combustion and Marcel Incineration Processes, Dekker, Inc, 2nd Edition, New York, 1995.

GASIFICATION PHASE

GATE

CYL. A

CYL. B

CYL. C

Figure 5. Step displacement diagram for feed actuator sequence

S8

S1

FIRST RAM CYLINDER

3/2 way Solenoid Valve Y1

S2

SECOND RAM CYLINDER

Y2

S3

3/2 way Solenoid Valve

S4

THIRD RAM CYLINDER

Y3

Y4

S5

S9

S7

S6

5/3 way Solenoid Valve (closed center)

Y5

GATE CYLINDER

3/2 way Solenoid Valve

START S1 S2 S3 S4 S5 S6

S7 S8 S9

LEGEND Air line Signal line Power line

Figure 6. Pneumatic and PLC control circuit layout

T1- 200mm above first hearth T2- 200mm above second hearth T3- exhaust of Primary Chamber T4- exit throat of Secondary Chamber T5- center of second hearth surface Figure 7. Temperature profile of combustion process at 1.0 kg/fuel feed

Figure 8. Temperature of fuel core on second hearth (T5) during gasification at constant airflow rate (0.227 m3/min)