Design and application of carbon monoxide incinerator control system based on PLC

0 Introduction

There are two forms of catalytic cracking regeneration process: complete regeneration and incomplete regeneration. For the incomplete regeneration process, the flue gas contains 3% to 10% carbon monoxide, and its recovery and utilization is an important issue for saving energy and protecting the environment. For the complete regeneration process, due to the limitations of heat balance and regeneration equipment, it is often necessary to transform the regeneration facilities, and the equipment investment is relatively large. In addition, the heavy oil catalytic cracking feed contains a high level of precious metals (such as platinum, rhodium, etc.), which causes catalyst failure during production and operation, and the loss of combustion aid is also large. Therefore, the catalytic cracking regeneration process often adopts an incomplete regeneration process, equipped with subsequent devices to remove carbon monoxide gas. Many refineries are equipped with carbon monoxide waste heat boilers, supplemented by gas combustion, to recover the physical sensible heat and chemical energy of C0 high-temperature regeneration flue gas, and at the same time eliminate the pollution of CO and other harmful gases in the regeneration flue gas to the atmosphere.

At present, the control systems of CO incinerators and waste heat boilers use imported modular distributed control systems (DCS) and sequential control systems (SCS), and the equipment is relatively expensive. In addition, due to intellectual property protection and technical communication issues, once there is a problem with the equipment, on-site technicians are unable to promptly judge and deal with abnormal phenomena, and CO and other harmful gases in the regenerated flue gas cannot be completely burned or exploded, causing the waste heat boiler tubes of the downstream device to overheat and the quality of superheated steam to decrease. Serious accidents. In response to the above situation, an independent PLC-based incineration automatic control system was studied and designed. The design is simple and easy, with low equipment cost and maintenance costs, reducing dependence on foreign technology and greatly improving production safety and reliability.

1 Overview of the incinerator process

The CO incinerator is a cylindrical upright structure, with the lower part being the fuel combustion chamber, and the lower middle part being the mixing chamber for the catalytic regeneration flue gas and the secondary air. After the catalytic regeneration flue gas enters the incinerator, it is fully mixed with the high-temperature flue gas after combustion, so that the temperature of the catalytic regeneration flue gas reaches the ignition point (about 850°C), and the CO is adiabatically burned in the incinerator. The high-temperature flue gas generated after incineration enters the waste heat boiler system to generate 3.82 MPa, 450°C medium-pressure superheated steam.

The combustion chamber of the incinerator adopts a circular air intake and air intake system. The air intake pipe is arranged in a circular manner outside the combustion chamber. Several air intake/air intake ports are evenly distributed around the combustion chamber wall, so that the gas (fuel) is radially sprayed into the combustion chamber from all sides, forming a vortex in the combustion chamber, so that the fuel is fully and completely burned. The mixing chamber adopts the same design concept. The regenerated flue gas in the middle and lower parts enters the incinerator tangentially through multiple radial circular holes. The secondary air supply system is to intake air along the periphery of the flue gas nozzle to ensure that the regenerated flue gas is fully and strongly mixed with the high-temperature flue gas at the combustion chamber outlet, so that the regenerated flue gas burns adiabatically in the incinerator.

The burner adopts oil-gas combined burner. The oil-gas pipe is a set structure, with an oil nozzle in the middle and a gas nozzle in the outer sleeve. The oil nozzle is a pressure steam atomized oil. The gas nozzle adopts 0.3-0.5 MPa high-pressure gas. It can burn oil or gas alone or a mixture of oil and gas. In this case, gas is the main burner. If there is an abnormal situation such as insufficient gas, fuel oil will be burned.

2 Composition of the incinerator control system

According to the combustion process of the incinerator, to ensure the safe operation of the device and to consider saving investment, the principle process control diagram of the incineration system is shown in Figure 1.

2.1 Control mechanism

CO can burn quickly and completely decompose into harmless gas under the appropriate high temperature and sufficient oxygen environment, but it may not be completely decomposed under the condition of lower temperature or insufficient oxygen. Therefore, the temperature and oxygen supplement of the incinerator are the key to control. The CO incinerator first uses the burner to increase the temperature of the combustion chamber furnace, and uses the fan to make the combustion chamber furnace sufficient with oxygen. Then, the PLC starts the gas or fuel control unit according to the predetermined program according to the temperature range value detected by the thermocouple and other temperature control equipment, and sends the gas or fuel into the combustion chamber for full combustion. The high-temperature flue gas from the combustion chamber is fully mixed with the catalytic cracking regeneration flue gas, causing the CO flue gas to ignite and burn completely. During the CO combustion process, heat energy will be released, which can further increase the temperature of the mixing chamber. At this time, the PLC can control the switch of the burner flame nozzle to control the temperature, so that the temperature of the mixing chamber is within an appropriate range with the secondary air provided by the fan unit, which not only ensures the safety of the mixing chamber but also ensures that harmful gases such as CO in the regeneration flue gas are fully and completely burned.

2.2 Monitoring objects

(1) Incinerator body: including combustion chamber and mixing chamber. The key control point in the process is the temperature of the incinerator, including combustion chamber temperature, mixing chamber temperature, incinerator outlet temperature, etc.

(2) Burner unit: The burner unit includes a burner body, an ignition device and a flame monitoring device. The burner body adopts a diffusion structure and is equipped with a swirl stabilizer to enhance the combustion effect. This burner is equipped with a total of six nozzles arranged in a ring, and any one or more nozzles can be opened according to process requirements. The ignition device adopts a telescopic direct ignition method and uses a high-energy ignition device with signal feedback. It consists of a telescopic ignition gun, an igniter and a valve group. The flame detection device is composed of a flame monitor for ultraviolet light detection, a signal processor, a control component and a cooling valve group, etc., which is used to monitor the burner flame.

(3) Fuel control system: divided into gas control unit and fuel control unit. The gas control unit includes self-operated pressure regulating valve, electric quick closing valve, electromagnetic relief valve, low-pressure steam purge electromagnetic valve, gas pneumatic regulating valve, flame arrester and gas high and low pressure switches and other instruments; the fuel control unit includes self-operated pressure regulating valve, electromagnetic quick closing valve, low-pressure steam purge electromagnetic valve, fuel pneumatic regulating valve, flame arrester and fuel high and low pressure switches and other equipment. The valve instruments are all explosion-proof to ensure safety. PLC adjusts the opening of the gas regulating valve and the fuel valve group according to the temperature of the combustion chamber and mixing chamber of the incinerator to adjust the fuel amount and secondary air supply of the burner, and finally corrects the temperature of the mixing chamber.

(4) Primary and secondary combustion air units: They consist of a blower, electric damper, air filter and air duct, etc. They provide suitable combustion air for the burner and cooling and purge for equipment such as flame detectors, and provide interlocking signals for the PLC.

(5) The safety protection unit consists of fuel high and low pressure alarm switches, shutdown purge valve group, leakage detection components and host computer DCS emergency control components.

(6) The operation control cabinet is used for on-site ignition, operation and shutdown operations, fire detection processor, on-site display alarm, fan automatic control system and connection with the host computer DCS and on-site instruments, etc.

2.3 Control system

SIMATIC S7-300 PLC is selected as the core control device because it has the characteristics of high reliability, strong anti-interference ability, complete hardware support, and convenient maintenance. It communicates with the host computer DCS, accepts the command of the host computer, and transmits the working conditions of the combustion system to the host computer truthfully. It accepts various analog, digital and switch signals downward, and at the same time, controls the burner unit, fuel control system, primary and secondary air supply units and various on-site equipment. In order to realize the automatic control function of the combustion system, the software and hardware parts of the control system are specially designed. Its hardware includes detection instruments, controllable valve groups, ignition devices, flame monitoring components, fan control components, safety protection devices and isolation units in the control cabinet, as well as PLC control center. Various on-site detection devices send monitoring signals to the PLC control center through input and output isolation units for logical operations and corresponding control adjustments.

PLC can accept operation instructions from the combustion site operators and the host computer DCS at the same time, and monitor the operation process of the combustion system. In order to ensure the safety, reliability and maintenance and transformation of the combustion system, PLC adopts redundant configuration. As a relatively independent subsystem of the entire waste heat boiler automatic control, the combustion control system can be controlled by the waste heat boiler DCS control center while completing its own functions. The control system uses SIEMENS Step 7 V5.2 software platform to design and complete the control program. The PLC system control program can complete the control of the corresponding hardware equipment while automatically performing safety interlock checks, confirming the safety conditions of the equipment when it is started and during operation, and automatically performing safety self-locking and protection.

3 Incinerator Control System Design

3.1 Combustion process and its control

When the main equipment operating conditions and upstream and downstream processes require the incinerator to be started, the combustion process block diagram is shown in Figure 2. When the control system is started, the fan blows the furnace to clean the original gas in the furnace to prevent residual combustible gas in the furnace from causing explosion and damaging the combustion chamber during ignition, and at the same time, to provide the combustion chamber with sufficient oxygen.

According to the process requirements, in order to ensure that the CO in the regenerated flue gas can be completely incinerated. The minimum temperature of the combustion chamber set by the control system is 1000℃, and the minimum temperature of the mixing chamber is 850℃. According to the amount of combustion gas and its components, the theoretical primary ventilation volume required for its combustion is determined to ensure that the combustion-supporting gas can be fully burned in the combustion chamber; the secondary air volume should be adjusted based on the parameters such as the combustion chamber temperature on the basis of the adjustment of the primary air in place to ensure that CO is completely burned at high temperature. During operation, reasonable furnace air dynamic conditions and combustion conditions should be organized through the adjustment of the total air volume and the reasonable allocation of primary and secondary air. When the combustion chamber temperature is lower than 1000℃, multiple nozzles must be ignited or the fuel system must be turned on to accelerate the temperature rise; when the furnace temperature reaches 1100℃, for the safety of the furnace and energy saving, some nozzles or fuel systems can be closed to maintain the combustion chamber temperature.

3.2 Function Implementation

In order to meet the economical operation and safe production of CO incinerator, the incineration control system not only has the functions of program control, automatic load adjustment and safety protection, but also all status indications, alarms and controls can be automatically realized on PLC. For this purpose, several subroutines are designed to realize the functions of automatic ignition, pipeline safety automatic purge, leakage detection, automatic fire extinguishing and fuel replenishment.

(1) Automatic ignition: The ignition process is fully automated. When the ignition conditions are fully met, the operator clicks the start ignition button or starts the ignition button on the PLC monitoring screen. The ignition subroutine is enabled and the corresponding equipment completes the corresponding action. The fan starts running, the purge solenoid valve BW1, the electric valves BV1 and BV2 are opened, and after 5 minutes of purge pipeline, the electric valves BV1 and BV2 are closed, the ignition solenoid valves GV1 and GV2 are opened, the ignition device BE continues to ignite for 5 seconds to establish the flame, the gas electric valves BV1 and BV2 are opened, and after 5 seconds, the ignition solenoid valves GV1 and GV2 are closed, the regulating valve TY1 adjusts the flame, and finally the flame monitor UV monitors the combustion. If there is no fire detection signal, all gas valves are immediately closed and an "ignition failure" alarm is issued at the same time.

(2) Automatic pipeline purging: Before ignition, after ignition failure, after shutdown or during operation, the combustion chamber, fuel gas pipeline and valve group need to be purged to effectively remove the combustible gas that may accumulate in the furnace, pipe valves and flue to prevent safety accidents.

(3) Leakage detection: Whether the fuel gas pipeline connection, electric quick-closing valve and relief valve leak is related to the safety of combustion station equipment and operators. Before the cold start of the combustion station, after the combustion chamber, pipeline and valve group are purged, leakage detection must be carried out. This subroutine is divided into the regulating valve TY1 leakage subroutine, the electric valve BV1 leakage subroutine, the electric valve BV2 leakage subroutine, the solenoid valve GW1 leakage subroutine, etc.

(4) Automatic fire extinguishing: When the fuel signal or the host computer DCS sends a shutdown signal, the incinerator automatically extinguishes the fire and enters the standby state or shutdown state. At the same time, the PLC control cabinet will sound an audible and visual alarm and indicate the shutdown reasons as follows: fan failure, fuel gas high and low pressure alarm, flame monitor no flame signal, control valve failure,

etc.

The design scheme has been applied by a petrochemical company in Northeast China and proved that the PLC-based carbon monoxide incinerator control system can meet the technical indicators of carbon monoxide incineration and can operate stably for a long time under harsh on-site environments. After acceptance by the environmental protection department, the exhaust gas emitted by the incinerator fully meets the national emission standards. The control system has a low failure rate and high cost performance, fully demonstrating its strong adaptability and high reliability. It has certain promotion value for the application of small and medium-sized combustion control systems.