Coal-methanol Plant Automation Solutions
I.Introduction
Methanol is not only an important chemical rawmaterial, but also clean fuel. It is the need for national energy security andrapid development of chemical industry to develop large coal-methanolproduction, and process it into olefins and alternative fuels and replace oilby coal. With the improvement of coal gasification technology, methanolsynthesis technology and equipment and machinery processing technology, theaverage production scale of methanol production plant has reach 2,000 ~ 3,000t/ d, with the maximum scale of 7,000t / d. The raw materials for producingmethanol include coal, coke, natural gas, oil and liquid hydrocarbons (naphtha,vacuum residue) and industrial exhaust containing H2/CO or CO-CO2. Methanolproduction with different raw materials, scales and operations may havedifferent technology options. Because China has “rich coal, poor oil and lessgas”, coal to methanol will replace natural gasto methanol and become the primary means of methanol production.
II.Introduction of Process Flow
Coal-methanol production process mainlyconsists of feed gas preparation, change and decarbonization, gas purification,gas compression, synthesis of methanol, crude methanol distillation and otherprocesses related to safety and environmental protection (such as spent catalystrecycling, water treatment). Feed gas preparation methods mainly consists ofcoal gasification method (TEXACO water-coal-slurry gasification, SHELL drycoal gasification, GSP dry coal gasification, Lurgi broken coalgasification and UGI pressure gasification), natural gas conversion method,coke oven gas method; feed gas processing, refining and the production ofcompression process are similar with the traditional production process ofsynthetic ammonia; methanol synthesis process consists of: ICI low pressuremethanol synthesis process, Lurgi low pressure methanol synthesis process,TEC’s new reactor and liquid-phase methanol synthesis process moving towardsmaturity. See Figure 1 for the typical process to produce synthesis gas withcoal through gas gasification, and synthesize methanol with synthesis gas underthe conditions of copper-based catalyst.
Crude gas with high CO content is madefrom coal and oxygen from air separation, CO is converted to H2through high temperature to realize the hydrogen-carbon ratio required bymethanol synthesis, excess CO2 and sulfide are removed through purificationprocess to obtain methanol synthesis gas. Because the carbon in coal-methanolproduction is more than hydrogen, the hydrogen must be recovered from synthesispurge gas to reduce coal consumption and energy consumption. The recoveredhydrogen and purified gas form the synthesis gas required by methanol, that is,(H2-CO2) / (CO+CO2)＝2.00~2.05,crude methanol is produced through compression, synthesis and other processes,and product methanol is produced through distillation process. The gasificationand synthesis in the eight processes are two decisive technologies; and airseparation, conversion, purification, compression, hydrogen recovery, distillationare all common processes, which are basically consistent with the technologiesin related industries (such as metallurgy, synthetic ammonia industry).
III. Main ControlScheme
3.1 Control scheme for Texaco gasifier
Texaco gasifier controls mainly include conventionalcontrol, sequence control and safety instrumented systems, andsafety instrumented system is the control difficulty and focus.
The conventional control of Texaco gasifieris mainly based on simple adjustment, and some measurements have certaincharacteristics. For example, gasifier temperature measurement and levelmeasurement in chilled room are related to security, the control is mainly toensure the security, so it has a certain particularity. There are multipletemperature measurement couples, 3 liquid level gauges are often equipped forlevel measurement, and different measurement values can be chosen as SP valuein control.
Oxygen-coal ratio and load lifting adjustmentis a more complex loop in control.
3.2 Control scheme for SHELL gasifier
3.2.1Oxygen-coal ratio control system
Median selection of pulverized coal flow—thecontrol of pulverized coal flow is carried out through using coal dust controlvalve or changing the pressure of materials issuing tank. Median selectionloop for pulverized coal flow, and themedian is taken after selecting two in three of pulverized coal flow for PIDadjustment to control the pressure of material issuing tank or the opening ofcoal dust control valve.
Compensation and purity correction of oxygenflow—the oxygen flow into the furnace is a key factor to affect gasifiertemperature, and the accuracy of oxygen flow rate is particularly important.The temperature and pressure compensation of oxygen flow is set, thecompensation flow and oxygen purity obtained from compensation calculation arecorrected, and oxygen flow is ultimately got for control.
Oxygen-coal ratio control—oxygen-coalratio control ensures the stability of oxygen-coal ratio withstandard ratio function and proportion calculation of internal instruments,oxygen flow OSP is calculated through multiplier with the given value ofoxygen-coal ratio as the remote setting of oxygen loop. Pulverized coal flow iscalculated through multiplier after taking the inverse as the remote setting ofcoal dust single-parameter loop, so as to realize interactive control. Coaldust flow changes, automatic control is carried out throughoxygen-coal ratio, oxygen flow is calculated through measured coaldust flow, and oxygen self-adjusting valve action is controlled through PIDregulating the output value.
Oxygen flow changes, automatic control iscarried out through oxygen-coal ratio to calculate thecorresponding amount of coal dust, coal dust control valve is controlledthrough the output value adjusted by PID, so as to make pulverized coal flowchange according to oxygen-coal ratio.
3.2.2 Gasifierload control
Gasifier load control is given, in order toprevent too large gasifier load, speed limiter is set, so as to limit the loadchanges per minute within a certain range. In order to prevent excessiveoxygen, high and low selectors are set. High selector is set at pulverized coalloop, the measured amountof coal dust is compared with the given amount of coaldust, and higher one is taken as the remote given final value of coal dustloop. Low selector is set at oxygen loop, the measured amount ofcoal dust is compared with the given amountof coal dust, and higher one is taken as the given value of oxygen loop.
3.3 Methanol synthesis process control program
The control of methanol synthesis part ismainly based on simple control, including the pressure of purgegas after primary pressure reduction, pressure of purge gas aftersecondary pressure reduction, by-product medium pressure steampressure at exiting section, flash tank level adjustment, methanol separatorlevel, alcohol washing tower level adjustment, manualoperation of purge gas at the bottom of alcohol washing tower, manual remotesynthesis operation at entering section, manual remote recycle gas operation atexiting section, crude methanol buffer tank level adjustment, reformed gastemperature at the outlet of waste heat boiler, water at the outlet of boilerwater feeding pre-heater, drum operating pressure, warming gas pressure, etc.
Complex loop control is mainly composed ofboiler level three-impulse control. The inner loop is to adjust the feed waterflow at section into boiler, outer loop is to adjust drum level, andfeed-forward is by-product medium pressure steam flow at exiting section.