07-10-2016, 02:21 PM
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Abstract
This paper outlines the various stages of operation involved in the conversion of manually operated boiler in controlling combustion towards a fully automated boiler. The Air preheater and Economizer helps in this process. And the paper mainly focuses on introducing an automatic boiler combustion control in a boiler using plc in order to improvise the combustion efficiency. Thus the pressure in the boiler is constantly monitored and controlled ,maintained in the plant. The automation is further enhanced by constant monitoring the pressure value using SCADA screen which is connected to the PLC by means of communication cable. By means of tag values set to various variable in SCADA the entire process is controlled. This paper will be proved to be very efficient in increasing the combustion efficiency practically as the need for automation grows day by day.
Power plants require continuous monitoring and inspection at frequent intervals. There are possibilities of errors in measuring the pressure at various stages manually by human workers.Thus this paper takes a sincere attempt to explain the advantages in this industry will face by implementing automatic boiler combustion control in order to improve the combustion efficiency. The boiler control which is the most important part of any power plant, and its automation is the precise effort of this paper. In order to automate a power plant and minimize human intervention, there is a need to develop a SCADA (Supervisory Control and Data Acquisition) system that monitors the plant and helps reduce the errors caused by humans. While the SCADA is used to monitor the system, PLC (Programmable Logic Controller) is also used for the internal storage of instruction for the implementing function such as logic, sequencing, timing, counting and arithmetic to control through digital or analog input/output modules various types of machines processes.
INTRODUCTION : Process automation involves using computer technology andsoftware engineering to help power plants and factories in industries as diverse as paper, mining and cement operate more efficiently and safely. Process automation simplifies this with the help of sensors at thousands of spots around the plant that collect data on temperatures, pressures, flows and so on. The information is stored and analyzed on a computer and the entire plant and each piece of production equipment can be monitored on a large screen in a control room. Plant operating settings are then automatically adjusted to achieve the optimum production. Plant operators can manually override the process automation systems when necessary optimum production. Steam is produced byevaporation of water, which is a relatively cheap and plentiful commodity in most parts of the world.
BOILER: Boiler is a closed vessel in which the heat produced by thecombustion of fuel is transferred to water for its conversion into steam at the desired temperature and pressure. Boileraccessories are Feed pump ,Injector, Economizer , Air pre-heater, Super heater , Steam separator. A boiler is a closed vessel in which water or other fluid is heated.
Boiler efficiency = (Eout – Efw)/Efuel
Eout : amount of energy in the steam or hot water
Efw : amount of energy in feedwater
Efuel : amount of energy in fuel
CONTROL PARAMETERS IN BOILER:
A. Level Control: Steam Drum level, De-aerator level and hot well level
B. Pressure Control:Force draft pressure, Induced draft pressure, Steam drum pressure, Deaerator pressure, Turbine inlet steam pressure, balanced draft pressure
C. Flow Control:Air flow, Steam flow, Water flow
D. Temperature Control: Deaerator temperature, Steam drum temperature, overrbed boiler temperature, Turbine inlet steam temperature, Flue gas temperature.
E. Fuel control: Level of the fuel
ADVANTAGES:
Higher combustion efficiency.
Higher sulphur retention degree.
Better limestone utilization.
Lower emission level of NOx and SOx.
More economic produces 75 – 100 T/hr of steam .
Capacity range is about 400-500 MW & that of FBC boiler is10-300 MW.
No. of feeding point less, because lateral mixing is done by high velocity fluidizing air.
EXISTING PROCESS:
The efficiency of the boiler depends on the ability of the burner system to provide the proper air to fuel mixture through out the firing rate day in and day out.Therefore the air,fuel, pressure and oxygen control is essential for a complete combustion.In this paper we are going to monitor air and fuel rate for controlling the pressure rate.the boiler used here is AFBC therefore uniform heating is done. Coal is crushed to a size of 1 – 10 mm depending on the rank of coal.
The atmospheric air, which acts as both the fluidization air and combustion air.The velocity of fluidising air is ranges from 1.2 to 3.7 m /sec.Almost all AFBC/ bubbling bed boilers use in-bed evaporator for extracting the heat from the bed to maintain the bed temperature.
The bed depth is 0.9 m to 1.5 m deep.The process is brough out by supplying air through the forced draft(FD),which is a wild stream and uncontrollable.The fuel is supplied by measuring the rate using the sensor. Here a PID controller is used which gets one value from the lookup table another value is given through the sensor. Now the controller is regulates the process and provide a signal to the flow transmitter which transmits the signal to the valve. Now the valve is adjusted and supplies the fuel to the furnace. For feeding fuel, sorbents like limestone usually two methods are followed one is under bed pneumatic feeding and over-bed feeding. In under Bed Pneumatic Feeding, the coal, it is crushed to 1-6 mm sizeand pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor. In Over-Bed Feedin the crushed coal, 6-10 mm size is conveyed from coal bunker to a spreader by a screw conveyor.The spreader distributes the coal over the surface of the bed uniformly. This type of fuel feeding system accepts over size
fuel also and eliminates transport lines, when compared to under-bed feeding system.Water is supplied to the steam drum.Here 60% steam is always maintained in order to balance the pressure.The air is preheated and supplied to the fludising bed.In order to maintain the boiler pressure apressur gauge is used to monitor. In boiler drum level at steady loads. A two element control system utilizes a level transmitter and the amount of steam flow from the boiler to maintain control of the boiler drum water level. A two element control system provides some compensation for variable loads. It does not adequately correct for the expansion of water within a boiler due to the decreased boiler pressure that occurs when a large amount of steam is required, or for the contraction of heated water in a boiler due to the addition of cold feedwater. The output from the boiler i.e, the steam outputs and the level of water is given to transmitters. The output of transmitter is given to the controller which act as level indicator controller is given to the converter which will open or close the valve and the water will be drained out or filled according to required steam.Drum Level Control Systems are used extensively throughout the process industries and the Utilities to control the level of boiling water contained in boiler drums on process plant and help provide a constant supply of steam.If the level is too high, flooding of steam purification equipment can occur.If the level is too low, reduction in efficiency of the treatment and recirculation function.Pressure can also build to dangerous levels.A drum level control system tightly controls the level whatever the disturbances, level change, increase/decrease of steam demand, feedwater flow variations.It Provide controls to always maintain the boiler drum level within the boiler manufacturer's specifications under all operating conditions. Three types of control systems used in boiler plants are single element, two element and three element.A single element control system utilizes just a level transmitter to maintain control of the boiler drum water level. Use a single element control system only for boilers operating at steady loads. A two element control system utilizes a level transmitter and the amount of steam flow from the boiler to maintain control of the boiler drum water level. A two element control system provides some compensation for variable loads. It does not adequately correct for the expansion of water within a boiler due to the decreased boiler pressure that occurs when a large amount of steam is required, or for the contraction of heated water in a boiler due to the addition of cold feedwater. A three element control system utilizes a level transmitter, the amount of steam flow from the boiler and the amount of water into the boiler to maintain control of the boiler drum water level. The steam flow from the boiler is usually compensated for pressure and temperature. A three element control system corrects the problems associated with a two element control system and provides the best method of compensating for variable loads.A pressure gauge is situated near the boiler for recording the pressure.As per the boiler constraint the pressure value should not exceed above240wc in the gauge.If it occurs the operators should adjust the air and fuel value in the lookuptable.By changing the air-fuel value in lookup table manually thepressure is adjusted below 240wc.
PROPOSED SYSTEM:
Steam boilers are usually designed to work at high pressures in order to reduce their physical size. Operating them at lower pressures can result in reduced output and 'carryover' of boiler water. It is, therefore, usual to generate steam at higher pressure.
Steam at high pressure has a relatively higher density, which means that a pipe of a given size can carry a greater mass of steam at high pressure, than at low pressure. It is usually preferable to distribute steam at high pressure as this allows smaller pipes to be used throughout most of the distribution system.Lower condensing pressures at the point of use tend to save energy. Reduced pressure will lower the temperature of the downstream pipework and reduce standing losses, and also reduce the amount of flash steam generated when condensate from drain traps is discharging into vented condensate collecting tanks.
It is worth noting that if condensate is continuously dumped to waste, perhaps because of the risk of contamination, less energy will be lost if the condensing pressure is lower.Because steam pressure and temperature are related, control of pressure can be used to control temperature in some processes. This fact is recognised in the control of sterilisers and autoclaves, and is also used to control surface temperatures on contact dryers, such as those found in papermaking and corrugator machines. Pressure control is also the basis of temperature control in heat exchangers.Many plants use steam at different pressures. A 'stage' system where high-pressure condensate from one process is flashed to steam for use in another part of the process is usually employed to save energy. It may be necessary to maintain continuity of supply in the low pressure system at times when not enough flash steam is being generated. A pressure reducing valve is ideally suited for this purpose.Therefore pressure transmitter is essential for automatic control of pressure in a boiler.Therefore this paper replaces the pressur gauge in the existing system as pressure transmitter.This because to improve the steamfuel ratio above 5 and also to increase overall efficiency of the boiler.Automation in the boiler is very important for accuracy.On placing the pressure transmitter, it senses the pressure in the boiler and maintains 240wc pressure in the boiler.If the pressure is reduced or increased automatically a setpoint of 240wc will be given through the lookup table.the pressure transmitter automatically transmit the signal and adjust the pressure in the boiler.Thus the human intervention in the exisisting system by the usage of the pressure gauge is completely eliminated by this proposed system
SCADA DESIGNING:
SCADA stands for Supervisory Control and Data Acquisition. As the name indicates, it is not a full control system, but rather focuses on the supervisor level .It is used to monitor and control plant or equipment. The control may be automatic or initiated by operator commands. The data acquisition is accomplished firstly by the RTU’s scanning the field inputs connected to the RTU (it may be also called a PLC – programmable logic controller.). This is usually at a fast rate. The central host will scan the PTU’s (usually at a slower rate). The data is processed to detect alarm conditions, and if an alarm is present,it will be displayed on special alarm list.A SCADA system consists of a number of components . The RTU’s. Remote telemetry or terminal units. The central SCADA master system. Field Instrumentation The SCADA RTU is a (hopefully) small ruggedized computer, which provides intelligence in the field, and allows the central SCADA master to communicate with the field instruments. It is a stand-alone data acquisition and control unit. Its function is to control process equipment at the remote site, acquire data from the equipment, and transfer the data back to the central SCADA system.
CONCLUSION:
The most important aspect of any power plant is the boiler control. Several techniques can be implemented to control the boiler in power plant. The method that has to be used relies on varied objectives like superior quality, increased efficiency, high profit and other such points depending upon the purpose of the company that implies it. With the prime objective of catering to these necessities and the needs of the industrial sector, significance has been given here to automation.Here replacing the pressure transmitter made ease way of controlling pressure.In this paper SCADA designing is performed using PLC devices. Further it can be implemented using DCS.here also the usage of feedwater can be purified The future work deals with thepurification of water to the boiler and the air circulation for the boiler to burn the fuel using same automation technique.