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Automation solutions for the coal-to-methanol (large-scale plants) industry: In the petrochemical sector, Zhejiang University Zhongkong possesses strong technical expertise and excellent track records in applications; it is particularly capable of providing mature automation solutions and implementation services for various plants, especially in terms of production control technologies for basic chemical raw materials. In the process control of large-scale coal-to-methanol production, Zhejiang University Zhongkong can provide comprehensive automatic control solutions and successful application cases covering all production stages from gasification and raw gas purification to methanol synthesis and distillation, as well as utility systems such as air separation and water treatment. The typical process flow of a large-scale coal-to-methanol plant is shown in Figure 1. Methanol plants share many similarities with ammonia synthesis plants; therefore, for information on the control of the conversion and purification stages, reference can be made to the \"Zhejiang University Zhongkong Solutions for Large-Scale Ammonia Synthesis Plants\". This article focuses on the control strategies for processes such as coal gasification and methanol synthesis. 1. Control strategies for Texaco gasifiers The control of Texaco gasifiers primarily includes conventional control, sequential control, and the SIS safety instrument system, with the SIS safety instrument system representing the main challenge and focus. (1) Normal control The normal control of the Texaco gasifier relies on simple adjustments; the multiple measurement methods are primarily used to ensure safety, such as for measuring the temperature of the gasifier and the liquid level in the quench chamber. Different measurement values can be used as SP values during control. The oxygen-coal ratio and load regulation are relatively complex loops in control. The control block diagram is shown in Figure 2: (2) Sequential Control The control of the slag collection hopper at the lower part of the gasifier’s quench chamber is essentially a sequential control system, involving processes such as pressurization, slag collection, depressurization, and slag discharge; it is an alternating pressure control system. (3) SIS Safety Instrumented System The safety system of the gasifier consists of two main components: sequential control and safety interlocks. The functions of these two parts are independent of each other, but the priority of the safety interlock function is higher than that of sequential control. At any time, whenever abnormal conditions arise, the safety interlock function will take over control of the device from the sequential control function. The gasifier safety system is a comprehensive set of safety automatic operation and monitoring systems that cover everything from startup and operation to shutdown. During the operation of the gasification furnace, with each step carried out, whether manually or automatically, the safety system continuously monitors the position of the valves that are crucial to the production process, and automatically adjusts the key process parameters to ensure that the unit operates in the most safe manner under normal or specified conditions. If the valve is not in its correct position or remains outside the specified operating parameters even after automatic adjustment, the system will alert the operator; in severe cases, this will trigger a shutdown mechanism to bring about a stoppage. Once the safety system issues a stop signal, the system’s stopping action will proceed automatically to completion, regardless of whether it has returned to normal operation during that process; only after the entire action is finished and the system has recovered can it be restarted to operate again. The gasifier SIS system includes sequential control (start-up sequence, shutdown sequence, purging sequence) and gasifier safety interlocks. a. Startup sequence of the gasifier: The task is to ensure the safe and coordinated startup of the gasification unit. Before proceeding with the startup sequence, check whether the relevant conditions are met: the vaporizer should have been preheated to the startup temperature ； The process gas nozzle has been replaced ； All devices in the apparatus that are not subject to the security system have been activated ； Systems such as graywater, burner cooling water, process condensate, etc., as well as the oil supply system, have established normal circulation ； The pressure in the nitrogen tank is higher than the minimum pressure required for operation ； The oxygen and steam pressures are normal; the manual cut-off valves for steam and oxygen have been opened. After completing the relevant steps, oxygen and steam venting is established by adjusting the opening degrees of the oxygen and steam control valves, so that the flow rates of water-coal slurry and oxygen reach the set values for startup. Once the conditions for starting the machine are met and the stabilization time exceeds 30 seconds, the start button can be pressed to proceed with the startup sequence. All subsequent steps, up to the pressurization of the gasifier, are carried out automatically, without the need for operator intervention. In the startup sequence, if the execution time of any sequence step that does not require operator intervention exceeds the predetermined time limit, the startup fails and the shutdown sequence is initiated. The DCS alarm indicates that startup has failed, and it also shows the reason for the failure. b. Gasifier shutdown sequence: At any time, when the shutdown trigger conditions are met or the manual shutdown button is pressed, the shutdown sequence is initiated to shut down the gasification unit. The parking sequence is unidirectional and irreversible; once initiated, it proceeds until completion, even if the conditions that caused the vaporization unit to shut down have returned to normal during the process. Since parking can occur under any circumstances, the position of each valve at the moment of parking is not fixed. Therefore, the first step in the parking sequence involves not only shutting off the material fed into the furnace but also setting the open/closed positions of each interlock control valve. c. Purging sequence: Purging is divided into two parts – purging of the oxygen pipeline and purging of the water-coal slurry pipeline. The principle to be followed is to purge the oxygen pipeline first and then the water-coal slurry pipeline; also, purging of the water-coal slurry pipeline should be stopped before stopping the purging of the oxygen pipeline. During the purging of the water-coal slurry pipeline, purging of the oxygen pipeline must continue to prevent the backflow of water-coal slurry into the oxygen pipeline. The purging can be repeated multiple times as needed. d. Safety interlock function: The purpose of the safety interlock function is to continuously monitor the operating conditions of the device; when an unsafe situation arises, it triggers an action to shut down the device safely, thereby preventing damage to the equipment or injury to personnel. During the process of driving the material feeder, strict time requirements apply to the relevant actions. The safety system is equipped with a \"watchdog\" timer; if the predetermined action is not completed within the specified time, it triggers the shutdown of the vaporization unit. 2. Control strategies for the SHELL gasifier The control of the SHELL gasifier primarily includes online control of the coal grinding system, process interlocks for the gasification step, control by the SIS safety instrument system (whose requirements are similar to those of the Texaco gasifier, so they will not be repeated here), as well as various complex control systems. (1) Online control of the coal grinding system a. System interlock shutdown The system shutdown interlock will be activated under the following process conditions and operations: extremely low coal feeding flow from the coal weighing feeder, stoppage of the mill motor, or activation of the shutdown button. After the parking interlock is activated, the shutdown proceeds automatically according to the following steps: the belt conveyor stops, the coal weighing feeder stops, and the mill motor stops. b. Interlock conditions for the coal weighing feeder: system interlock shutdown, low motor speed of the coal weighing feeder. (2) Process interlocks for the gasification step a.. Sequencing for starting up and shutting down gasification Outlet pressure of the carbon scrubbing tower below 0.5 MPa, system initialization, system reset, coal powder delivery, oxygen timeout. After coal is fed, after a 7-second delay, the oxygen flow timer starts counting for 40 seconds; once 40 seconds have passed, the oxygen vent valve closes while the oxygen cut-off valve opens. If the valve positions are not correct, the gasification furnace will shut down via interlock. Including system initialization, system reset, coal feeding during the operation of the coal grinding system, and interlock shutdown triggers for the gasifier. c. Lock hopper PLC logic program The control of the lock hopper involves processes such as \"pressure release – cleaning – slag discharge – pressure filling – slag collection\". It involves interlocks for the slag water process, slag pool agitator interlocks, carbon scrubber feed pump interlocks, ash water pump interlocks, etc. (3) Explanation of some complex control systems a. Oxygen-coal ratio control system Selection of the median value for pulverized coal flow – The flow rate of pulverized coal is controlled either by using a pulverized coal control valve or by adjusting the pressure in the feed tank. A median value selection circuit has been designed for this purpose; after selecting two values out of three for the pulverized coal flow rate, the median value is used to perform PID control on the pressure in the feed tank or on the opening degree of the pulverized coal control valve. Compensation for oxygen flow rate and purity correction – The oxygen flow rate into the furnace is a key factor affecting the temperature of the gasification furnace; therefore, the accuracy of this flow rate is particularly important. Temperature and pressure compensations are applied to the oxygen flow rate, and the compensated flow rate obtained through these calculations is then adjusted based on the oxygen purity, so as to determine the final oxygen flow rate to be used for control. Oxygen-coal ratio control – This control mechanism makes use of standard proportional functions and proportional calculations carried out by internal instruments to ensure stability in the oxygen-coal ratio. The value of the oxygen-coal ratio is used to calculate the oxygen flow rate OSP through a multiplier; this value serves as the remote setpoint for the oxygen circuit. By taking the reciprocal of this value and using another multiplier, the coal powder flow rate is calculated, which then acts as the remote setpoint for the coal powder control circuit, thereby enabling interactive control. When the coal powder flow rate changes, automatic control is implemented via the oxygen-to-coal ratio; the oxygen flow rate is calculated based on the measured coal powder flow rate, and the output value from PID control is used to regulate the operation of the oxygen self-regulating valve. As the oxygen flow rate changes, automatic control based on the oxygen-coal ratio is used to calculate the corresponding amount of coal powder; the output value obtained through PID control is then used to regulate the coal powder control valve, so that the flow rate of coal powder adjusts according to the oxygen-coal ratio. b. Gasifier load control The gasifier load control is determined as follows: to prevent the load on the gasifier from becoming too high, a speed limiter is installed to keep the minute-by-minute change in load within a certain range. To prevent excessive oxygen, high and low selectors are installed. A high selector is placed in the pulverized coal circuit, where the measured amount of pulverized coal is compared with the amount specified by the load; the higher value is used as the final value for remote control in that circuit. A low selector is installed in the oxygen circuit, where the measured amount of pulverized coal is compared with the amount specified by the load; the lower value is used as the set value for that circuit. When the load is increased, the coal powder flow rate exceeds the load set value, and this is detected by the high-selective detector; as a result, the oxygen-to-coal ratio is adjusted and the oxygen flow rate increases. When the load is decreased, the coal powder flow rate falls below the load set value, and this is detected by the low-selective detector; in this case, the oxygen flow rate is reduced first, and then, through control of the oxygen-to-coal ratio, the coal powder flow rate also decreases. c. Explanation of the lock-hopper logic system: The PLC system mode for the lock-hopper includes operation, slag collection, and slag discharge modes. It involves valves such as the lockhopper safety valve, lockhopper inlet valve, lockhopper outlet valve, lockhopper circulation pump inlet valve, lockhopper circulation pump control valve, lockhopper flushing water valve, lockhopper cleaning valve, lockhopper pressure reducing valve, lockhopper pressure charging valve, lockhopper overflow valve, slag pool overflow valve, lockhopper low liquid level switch, lockhopper circulation pump operation status, lockhopper circulation pump shutdown status, lockhopper flushing tank liquid level, lockhopper low pressure switch, lockhopper high pressure switch, locks hopper and quenching chamber control valves, quenching chamber liquid level, as well as various on-site monitoring signals. 3. Methanol synthesis control strategies The control in the methanol synthesis section is primarily based on simple control methods, including the pressure after the first depressurization of the vent gas, the pressure after the second depressurization of the vent gas, the pressure of the medium-pressure steam generated as a by-product in the output section, the regulation of the liquid level in the flash tank, the liquid level in the methanol separator, the regulation of the liquid level in the alcohol washing tower, manual control of the vent gas from the bottom of the alcohol washing tower, remote manual control of the synthesis process in the input section, remote manual control of the recycle gas in the output section, the regulation of the liquid level in the crude methanol buffer tank, the temperature of the converted gas at the outlet of the waste heat boiler, the water temperature at the outlet of the boiler feedwater preheater, the operating pressure of the drum, and the pressure of the gas used for heating. Complex loop control mainly includes 3-pulse control for boiler level. The inner loop regulates the feedwater flow rate to the boiler section, the outer loop controls the drum level, and the feedforward element is the flow rate of the medium-pressure steam generated in the output section. The safety interlock involved in the synthesis section is such that it activates when the drum liquid level is too low (below the specified value), thereby shutting down the feed gas compressor and the syngas compressor. By the end of 2005, Zhejiang University Zhongkong had provided high-quality products and services to over 3,000 users in the process industry worldwide, spanning sectors such as petrochemicals, chemicals, metallurgy, power generation, building materials, brewing, and papermaking. It had completed more than 4,000 engineering projects, including a large number of typical and distinctive projects. Zhejiang University Zhongkong looks forward to working together with more users in the coal-to-methanol industry to improve the automation level and economic efficiency of the entire sector.