Due to the increasing shortage of oil resources, my country's coal chemical industry has entered a rapid development stage, and a variety of coal gasification processes have coexisted. Among the many new processes, the SCGP pulverized coal gasification process developed by Shell in the Netherlands has been applied in 17 large-scale coal chemical plants in China in less than 10 years, but problems have also followed.
The carbon monoxide (CO) content in the raw gas produced by the Shell pulverized coal gasification process is as high as 60%, which not only increases the CO conversion load of the sulfur-resistant conversion system, but also may cause the occurrence of highly exothermic methanation side reactions, causing the catalyst bed to overheat. Therefore, how to control and inhibit the methanation side reactions of the first conversion furnace has become the key to whether the Shell pulverized coal gasification process can be successfully used for synthetic ammonia or methanol production.
The new low-water/gas sulfur-resistant conversion process for pulverized coal gasification, jointly developed by Qingdao Lianxin Chemical Co., Ltd., Guangxi Liuzhou Chemical Industry Co., Ltd., and Sinopec Ningbo Engineering Co., Ltd., has successfully solved this problem and opened up a new conversion process path for the production of synthesis gas using pulverized coal gasification with stable production and operation, energy saving and significant environmental protection effects. This technology passed the appraisal organized by the China Petroleum and Chemical Industry Association this month.
The imported device encountered "methanization" obstruction
Previously, in order to avoid the methanization side reaction in the conversion section of the Shell pulverized coal gasification process, the design department mostly recommended the use of high-water/gas conversion process. Therefore, among the 17 Shell pulverized coal gasification units that have been signed in my country, 15 units have selected the high-water/gas sulfur-resistant conversion process. Although high water/gas can inhibit the methanization side reaction, due to the high CO concentration and water/gas, the driving force of the reaction is large. As long as there is a small change in the amount of the catalyst in the first stage, it will significantly affect the depth of the reaction and then affect the hot spot temperature of the bed, resulting in poor control of the bed temperature.
Practice has proved that in the early stage of some units, due to low load or large catalyst loading surplus, the first reactor has overheating phenomenon. In order to reduce the temperature of the reactor, the water/gas of the first reactor is increased to above 1.3. This not only causes a great waste of energy consumption, but also because the first stage catalyst operates under the harsh conditions of high water/gas and high temperature, the catalyst has desulfurization phenomenon, which affects the normal production of the unit.
Successful development of new process for low water/gas shift
Qingdao Lianxin Chemical Co., Ltd. has been committed to the development of new sulfur-resistant shift process. As early as the beginning of 2002, the company began to pay attention to the introduction of Shell's new pulverized coal gasification process in my country, and carried out the development and research of supporting new sulfur-resistant shift process, focusing on the "methanation" side reaction that people are most concerned about. Through the study of water/gas, bed hot spot temperature and space velocity and catalyst performance, the researchers found out the law that affects the degree of methanation side reaction on cobalt-molybdenum-based catalysts, as well as the temperature at which the methanation side reaction is activated under low water/gas conditions.
After that, they conducted in-depth research on how to segment the shift reaction and how to control the depth of the first furnace reaction. Through kinetic calculations and verification tests on the QDB-04 catalyst, they found two ways to control the depth of the reaction. On this basis, Lianxin Chemical developed a new process for Shell pulverized coal gasification with low water/gas and sulfur-resistant shift. This process makes full use of the low water content of the raw gas in the Shell waste boiler process. By controlling the water/gas in the process gas, the depth of the shift reaction in the first reactor is controlled, and then the hot spot temperature of the bed is controlled, so that the high-concentration CO is partially shifted under the condition of low hot spot temperature, that is, under the condition that no methanogenic side reaction occurs, so that the high-CO concentration Shell pulverized coal gasification new process can be implemented under low water/gas stable and mild process conditions.
The first industrial application was a complete success
After the new process was successfully developed, it did not immediately attract the attention of the design department and manufacturers due to the lack of industrial application results and people's excessive concerns about the overheating of the methanogenic side reaction. Most manufacturers who choose the Shell pulverized coal gasification process still use the high water/gas shift process.
Later, China Petrochemical Corporation Ningbo Engineering Co., Ltd. designed and recommended a low water/gas process to Guangxi Liuzhou Chemical Co., Ltd. in response to the steam shortage of the company. Guangxi Liuzhou Chemical Co., Ltd. conducted in-depth research on the pulverized coal gasification-low water/gas sulfur-resistant shift process and conducted many in-depth and detailed exchanges and discussions with the technical staff of Lianxin Chemical on the methanation problem in the first reactor. It was decided to take the lead in using the low water/gas shift process in the shift unit of the synthetic ammonia transformation project, and all use the QDB type sulfur-resistant shift catalyst produced by Qingdao Lianxin Chemical Co., Ltd. So far, Shell's pulverized coal gasification low water/gas sulfur-resistant shift new process has its first industrial user in China, and it is also the first industrial user in the world. So far, the new process has been successfully operated for 14 months in the shift unit of the second synthetic ammonia plant of Guangxi Liuzhou Chemical Co., Ltd. The results of industrial application show that the QDB-04 catalyst produced by Qingdao Lianxin Chemical Co., Ltd. can ideally control the reaction depth and hot spot temperature of the first stage furnace by controlling the water/gas and bed inlet temperature of the reaction. The maximum water/gas value of the first and second stage reactions does not exceed 0.28, the inlet temperature does not exceed 250℃, the hot spot temperature of the bed is 360℃～380℃, the CO index distribution of each stage outlet is reasonable, the final CO outlet is less than 0.60%, and no methanogenic side reaction occurs. The device has been running for more than a year, not only the device operation is stable but also the energy saving effect is significant. It is an ideal process for producing methanol or synthetic ammonia using high-concentration CO process gas.
