1. Process Introduction
This boiler system primarily powers a 1000m³ blast furnace of a steel company by burning blast furnace gas and coke oven gas, and also provides seasonal industrial heating. The boiler mainly consists of a gas (blast furnace gas, coke oven gas) system, furnace body, convective heating surfaces (steam drum and cooling walls, superheaters I and II, economizers I and II, air preheaters I and II), igniter, and forced and induced draft equipment.
Based on their functions, the systems can be roughly divided into several subsystems: steam and water system, air and flue gas system, combustion system, de-temperature and pressure reduction system, and utility system.
This control system mainly controls the production process of the boiler and related auxiliary equipment to ensure that it meets the requirements of the process, including steam temperature (450℃), pressure (3.82MPa), flow rate (130t/h), and purity (superheated steam).
1.1 Steam-Water System The steam-water system supplies deoxygenated water for boiler protection and steam generation. The resulting superheated steam is sent to the turbine for expansion and work, or, after desuperheating and pressure reduction, is used for heating. Deoxygenated water from the deaerator feedwater system is regulated and sent to the I and II economizers for preheating. It is then sent to the boiler drum and the boiler cooling walls connected to the drum. There, it is heated by the high-temperature flue gas generated by boiler combustion to produce unsaturated steam. This unsaturated steam passes through the I-stage superheater and its steam header, and is desuperheated by a spray desuperheater. After passing through the II-stage superheater and its steam header, it becomes saturated superheated steam, which is then sent to the steam header. Part of this superheated steam is sent to the turbine for expansion and work, part enters the desuperheating and pressure reduction system, and part supplies feedwater to the deaerator-driven feedwater pump.
1.2 The air (cold air) of the flue gas system is purified and sent to the I and II air preheaters by blowers No. 1 and No. 2 for preheating into hot air. The hot air is sent to the hot air burner to mix and burn with the gas. The blast furnace gas and coke oven gas are sent to the combustion nozzles through the blast furnace gas pipeline and coke oven gas pipeline to mix and burn with the hot air, generating high-temperature flue gas. This heats the deoxygenated water in the boiler drum to make it unsaturated steam. Then, the high-temperature flue gas passes through the I superheater, II superheater, II economizer, II air preheater, I economizer, and I air preheater in sequence to heat the unsaturated steam into high-temperature and high-pressure saturated steam. This steam preheats the deoxygenated water sent to the boiler drum and the air sent to the boiler furnace. Finally, it is led to the chimney by the induced draft fan for discharge.
1.3 Combustion System: Blast furnace gas is supplied externally and divided into four streams, each entering one of the four corners of the boiler (four combustion nozzles per corner) to participate in combustion. Hot air, mixed with blast furnace gas, also enters one of the four corners of the boiler (four combustion nozzles per corner) to participate in combustion. Coke oven gas is supplied externally and divided into four streams, each entering one of the four corners of the boiler (two combustion nozzles per corner) to participate in combustion. Under normal circumstances, the fuel is blast furnace gas; coke oven gas is only used during ignition and otherwise serves as a safety gas (to protect the boiler from low furnace temperatures during combustion).
During combustion, thermocouples and flame detectors are used to monitor furnace temperature changes. The boiler furnace temperature is regulated by adjusting the ratio of blast furnace gas, coke oven gas, and air (the fuel ratio is generally 100% blast furnace gas, but can also be 80%-90% blast furnace gas plus 20%-10% coke oven gas, or 50% coke oven gas). The furnace temperature is maintained at approximately 1100±10℃ throughout the combustion process.
1.4 Desuperheating and Pressure Reduction and Utility System Most of the superheated steam generated by this boiler is sent to the steam turbine to supply air to the blast furnace. A portion of the remainder is sent to the medium-temperature and medium-pressure connecting pipe, and another portion is sent to the No. 1 and No. 2 desuperheaters and pressure reducers. After being desuperheated and pressure reduced by industrial water, it becomes low-temperature and low-pressure steam. A portion of this steam is sent to the plant area for heating, another portion is sent to the deaerator through the heating steam header, and a portion is used to supply water to the deaerator steam-driven feedwater pump.
2. System Configuration 2.1 DCS System The Distributed Control System (DCS) consists of a host system and a slave system. The host system uses an industrial control computer and Siemens WinCC configuration software to complete the real-time display, storage, alarm processing, printing, and control parameter setting of field data. The slave system consists of Siemens PLCs and is connected to field devices. Communication between the host and slave systems uses Ethernet, with a maximum transmission rate of 10-100 Mbit/s, fully meeting the requirements for real-time data monitoring. The automatic control system uses S7 400 series PLC hardware to form the basic automation system, employs WINCC V6.0 monitoring software, STEP 7 V5.3 programming software, and Windows 2000 as the system platform interface, forming the computer operating system and enabling human-machine communication.
2.2 System Configuration Diagram
Figure 1. Composition of DCS System
3. Control Functions 3.1 Combustion Control The boiler's steam supply is designed to meet the operating load requirements of the steam turbine and the need for low-pressure steam from the blower station. Changes in the turbine and external steam supply loads will affect the boiler steam pressure. Maintaining stable steam pressure will ensure sufficient steam volume. Therefore, the purpose of automatic boiler combustion control is to stabilize the steam header pressure through automatic combustion, thereby meeting the steam requirements of the turbine and external steam supply.
Because the boiler's combustion system to steam supply system is a relatively complex thermodynamic process, during operation it will be affected by: changes in steam load caused by changes in turbine operating conditions and changes in steam load caused by external steam supply (referred to as external disturbances); changes in steam volume caused by changes in the boiler's internal heat load, such as fuel calorific value and fuel type (referred to as internal disturbances); the time from the start of fuel changes to the establishment of heat load in the furnace (referred to as the inertia of the combustion equipment); and the boiler's heat absorption and release capacity when the combustion conditions remain unchanged under external disturbances (referred to as the boiler's heat storage capacity). Therefore, the automatic combustion program should have the ability to resist interference in order to achieve smooth automatic regulation.
To achieve economical combustion in the boiler, the forced draft and induced draft volumes should be adjusted simultaneously with fuel or load regulation. Boilers may use the following fuel ratios: 100% blast furnace gas (used during normal operation), 80-90% blast furnace gas plus 20-10% coke oven gas, or 50% blast furnace gas plus 50% coke oven gas. When the boiler load changes significantly, automatic regulation can be achieved by switching on and off the electric regulating valves on the gas pipelines of one or more blast furnace gas burners and the electric regulating valves on the hot air pipelines.
3.2 The boiler drum water level control adopts a three-impulse regulation, that is, the main feedwater valve is adjusted according to the feedwater flow rate, boiler drum water level, and steam flow rate to ensure the stability of the boiler drum water level. It is a feedforward-feedback cascade regulation loop, and the block diagram is as follows: In the boiler feedwater system, the boiler provides two feedwater regulating valves. The DN150 regulating valve is the main regulating valve, used during normal and high load operation; a DN100 regulating valve is installed on the bypass pipe, used during low load, and also serves as a backup valve for the main regulating valve. In automatic feedwater mode, only one of the regulating valves is allowed to automatically regulate the feedwater. At this time, the other regulating valve can be manually fed water. Before the program is activated, the operator needs to pre-select which regulating valve to activate automatically. If the setting is not set this time, the previous setting will be executed.
Figure 2. Structural diagram of the steam drum water level control system
The liquid level is adjusted by PID1 and the output is added to the steam flow rate. The result is used as the set point for PID2. PID2 adjusts the deviation between this set point and the water supply flow rate, and the output drives the actuator to adjust the water supply valve.
The steam drum liquid level is the primary controlled variable, the feedwater flow rate is the secondary controlled variable, and the steam flow rate is the feedforward variable. When the steam drum liquid level rises, the output of PID1 decreases, which in turn decreases the output of the adder, causing the feedwater valve to close slightly and thus reducing the feedwater flow rate. When the steam drum load increases, i.e., the steam flow rate increases, the output of the adder increases, which causes the feedwater valve to open wider and thus increases the feedwater flow rate.
When the steam load suddenly increases and a "false level" occurs, PID1, being a reaction-acting PID controller, decreases its output, meaning X1 in the adder decreases. Conversely, due to the increased load, X2 in the adder increases, resulting in a relatively small change in the adder's output. After a short time, the pressure inside the steam drum returns to equilibrium, the "false level" disappears, and the level begins to drop due to increased evaporation. PID1 then increases its output, leading to an increase in feedwater flow until the steam drum level returns to the given position.
3.3 Furnace Negative Pressure Regulation Automatic furnace negative pressure control maintains a slightly negative pressure in the furnace between -20 and 10 Pa by adjusting the opening of the induced draft fan inlet damper, ensuring safe boiler combustion. When two induced draft fans are running simultaneously, they should be adjusted in parallel or one fan should be fixed to adjust the other. The option to adjust in parallel or fixed (1 for fixed adjustment of 2, 0 for fixed adjustment of 1) can be selected on the screen. High and low furnace negative pressure alarms are provided.
3.4 Automatic Control of Boiler Air Supply The purpose of automatic air supply control is to automatically supply an appropriate air volume when the fuel is burned in the furnace, ensuring economical combustion of the boiler. The main controlled parameters are gas pressure and air supply pressure, thereby achieving the highest boiler thermal efficiency. The oxygen content of the flue gas is used as a correction factor for the total air volume. The air supply pressure is adjusted by regulating the damper opening of the blower. When two blowers are running simultaneously, the inlet dampers should be adjusted in parallel (or one should be fixed while the other is adjusted). The option to adjust in parallel or fixed 1 and adjust 2 can be selected on the screen (1 for fixed 1 and adjust 2, 0 for fixed 2 and adjust 1).
3.5 Automatic regulation of boiler superheated steam temperature (automatic regulation of desuperheating water)
The boiler superheated steam temperature regulation employs a self-made condensate spray desuperheating device. Automatic superheated steam temperature regulation adjusts the opening of the desuperheating water regulating valve based on the steam temperature at the collector and desuperheater outlet, controlling the desuperheating water flow to ensure the steam temperature in the collector is maintained within the range of 430-450℃. When the steam temperature at the collector outlet decreases, the automatic temperature regulation system automatically reduces the desuperheating water flow; as the steam temperature rises, the desuperheating water flow increases to ensure stable steam temperature at the collector outlet. Conversely, it reduces the desuperheating water flow to prevent large temperature fluctuations. The spray desuperheating system uses two feedwater regulating valves provided by the boiler. The DN50 regulating valve is the main regulating valve, used during normal operation; a DN50 regulating valve is installed on the bypass pipe as a backup for the main regulating valve. In automatic feedwater mode, only one valve is allowed to automatically regulate the feedwater, while the other is on standby. Before the program is activated, the operator needs to pre-select which regulating valve will be automatically activated. If the setting is not completed this time, the previous setting will be followed. A DN150 regulating valve is installed after the main feedwater regulating valve. The opening of this regulating valve is adjusted according to the required condensate flow. Cascade regulation is adopted. The steam outlet temperature is regulated by PID1 and output as the setpoint of PID2 (desuperheater outlet temperature regulation). PID2 adjusts the deviation between this setpoint and the desuperheater outlet temperature, and the output drives the actuator to regulate the desuperheating water regulating valve.
When the measured steam temperature at the outlet of the steam collector is high, the output of PID1 increases, and the desuperheating water regulating valve opens wider, increasing the desuperheating water flow; conversely, the valve opening decreases, reducing the desuperheating water flow.
When disturbances occur (primarily caused by changes in flue gas flow and temperature, steam flow and temperature at the desuperheater inlet, and desuperheating water pressure), the first effect is a change in the steam temperature at the desuperheater outlet. A higher temperature necessitates an increase in the desuperheating water flow, resulting in rapid regulation. The impact on the steam temperature at the steam collector outlet is relatively small, thus improving regulation quality. The block diagram is as follows:
Figure 3. Structural diagram of the automatic temperature regulation and control system for superheated steam.
4. Monitoring Functions : The screen displays the temperature, pressure, and flow distribution of various parts of the boiler, collected data, historical trends, alarm flashing screens, and allows for the opening and operation of various valves and equipment. It also allows for the operation and adjustment of gas and combustion air regulating valves, seamless automatic switching between automatic and soft/manual/hard manual adjustment of various systems, and the operation and adjustment of each regulating valve, as well as the dynamic display of various data. The main screen is shown below:
Main Menu: Complete system login, select work system, and enter the main screen.
Main screen: Displays the entire process flow of the boiler and related key parameter values, alarm flashing, and function buttons to switch to other screens. Sub-screens: Screens for each control system, including function keys for parameter settings, bar graphs, control flowcharts, alarm records, related information; historical trends, and related PID parameter settings, etc.
Alarm screen: According to process requirements, when the process value exceeds the upper and lower alarm limits, an alarm is issued, and the alarm occurrence time, alarm value, alarm level, and alarm point are displayed on the alarm screen. Operators can perform functions such as alarm confirmation and alarm information filtering on the alarm screen.
Report printing: Reports can be formatted in any way and can be printed with all input and output parameters. Additionally, the monitoring station is managed with multiple security levels, each with different access permissions to prevent infringement or accidental operation.
5. Application Results After adopting the DCS system and related control processes, the operation becomes significantly more convenient for operators, requiring only mouse clicks. The entire boiler's operating status is clearly displayed on the computer screen. In practical applications, the main advantages of using DCS and related control technologies include: improved energy utilization, ensuring efficient and safe system operation; stable outlet water temperature, improving comfort; and rapid heating. From a control performance perspective, adjustments are timely, overshoot is minimal, fluctuations are small, and operation is stable.
From the perspective of energy conservation and consumption reduction, the production line has improved system reliability by using industrial process optimization automatic control technology. Compared with boilers of the same type, it reduces downtime by about 200 hours per year. With a boiler power of 130 tons/hour and steam costing 70 yuan/ton, it can save more than 1.8 million yuan per year, bringing considerable economic benefits to the enterprise.
The innovations of this paper lie in proposing a distributed gas combustion boiler control system. This system fully utilizes the rich hardware and software resources of a PC to achieve a user-friendly human-machine interface, and communicates with a PLC via an industrial Ethernet structure to acquire and process data from the boiler site in a timely manner, achieving optimal boiler combustion conditions and energy conservation. It fully integrates instrumentation and electrical systems, with the PLC and DCS jointly handling control functions, realizing a comprehensive EIC (Engineering, Procurement, and Control) integrated system. This system features advanced control concepts, stable operation, safety, reliability, and energy efficiency. The boiler control is achieved using a SIMATIC S7-400 series programmable controller, where the hardware can be configured via software. The software programming hierarchy is clear, on-site debugging is convenient, and its powerful communication capabilities allow for the integration of various distributed monitoring and management systems.
