Introduction to specialized control technologies in the gas production process of intermittent fixed-bed gasifiers

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Preface: The gas production mode using intermittent, fixed-bed gasifiers has been in use since the inception of in-house nitrogen fertilizer production. The control technologies associated with it have evolved from nothing to something, from manual to automatic, and from basic to advanced – it can be said that \"needs\" led to their development. Since this type of gas production model is almost unique to our country, the control technologies required are specialized ones that cannot be imported, nor are there any precedents to follow. Over the years, thanks to the efforts of researchers and developers dedicated to this technology, significant achievements have been made. All control technologies in gas production have now been improved (including specialized control technologies). This has enabled the characteristics of modern control to be reflected in our gas production process, achieving the transition from manual control to fully automated control ； By means of intelligent adjustment and optimization, parameters that were previously uncontrollable can now be precisely controlled. By adopting these technologies, enterprises not only achieve fully automated continuous production as a result of such optimizations, but they also gain benefits and enhance their competitiveness. 2 Introduction to Special Control Technologies in the Gas Production Using Intermittent Fixed-Bed Gasifiers 2.1 Achieving Automatic Coal Feeding and Control of Carbon Layer Height and Coal Feeding Amount Automatic coal feeding control is a system that replaces manual coal feeding with automatic feeding ； Carbon layer height control is a control technique aimed at ensuring that the actual carbon layer height matches the requirements specified in the process ； Coal addition control is the regulation of the amount of coal added in each gas production cycle to meet the gas generation requirements. Here, an electromechanically integrated automatic coal feeder is first used to carry out automatic coal feeding. Subsequently, based on the automatically measured height of the coal layer – a device for measuring the coal layer height has been developed by us and successfully applied in gas production facilities – as well as the temperature measured in the gasification zone (incidentally, we can now automatically measure the temperature in the gasification zone) and the production load, the coal feeding time for each gas production cycle is automatically adjusted, thereby automatically controlling the height of the coal layer and the amount of coal required per cycle. The specific control system block diagram is shown in Figure 1: 2.2 Implementing automatic ash discharge as well as control over the rotation speed of the furnace tray and the amount of ash discharged. Automatic ash discharge control transforms manual operation into automatic ash discharge ； The rotation speed control of the furnace tray can also be manual or automatic; naturally, modern control systems use the automatic option ； Ash discharge control is a mechanism for achieving material balance in production; it deals with the question of how much coal should be added and how much ash needs to be discharged during production. Here, first, an mechatronic automatic ash discharge device (which we have successfully developed and produced) is used to carry out automatic ash discharge (that is, ash discharge without shutting down the furnace). Based on the temperature of the gasification layer, the temperature of the furnace bed, the temperature of the ash bin, as well as the coefficient determined from the measured coking rate, the rotation speed of the furnace bed is automatically controlled, thereby regulating the amount of ash discharged. At present, although automatic control of ash discharge has not been achieved, the adjustable speed of the furnace tray can still be used to control the amount of ash discharged. The block diagram of the control system is shown in Figure 2: 2.3 Implementation of feedforward compensation and regulation for the steam pressure entering the furnace. This is a steam pressure stabilization control system, where the process variable to be controlled is the steam pressure entering the furnace (the value required by the process), and the control parameter is the steam flow rate. The goal is to maintain the steam pressure entering the furnace at the value required by the process at all times, which is highly beneficial for stabilizing the furnace conditions. The block diagram of the control system is shown in Figure 3. The underlying control principle is that fluctuations in steam pressure are caused by the opening and closing of the upper or lower blowing valves; that is, when the valve opens, the steam pressure drops sharply, and when the valve closes, the steam pressure rises sharply. General tuning methods are insufficient to stabilize the controlled parameters for such control objects. Since we know that the operation of the up and down blowing valves is determined by signals sent from the DCS system, when we send signals to control these valves, we transmit those signals to the steam pressure control valve one or two seconds in advance, allowing it to act accordingly in advance. This is feedforward control; subsequent PI control is then used to keep the steam pressure stable at the value required by the process. The control system flow is shown in Figure 4: 2.4 Implementation of open-loop proportional control for the steam flow entering the furnace. This is a proportional control system, where the variable to be controlled is the temperature of the gasification zone in the gas generator. The control parameter is the steam flow rate after pressure stabilization (the opening degree of the upper and lower blow control butterfly valves), so that the steam flow entering the furnace remains in a constant proportion to the temperature of the gasification zone, thereby increasing the steam decomposition rate. In this way, energy savings are achieved, the gas composition is optimized, and fluctuations in furnace conditions (the temperature of the gasification layer) are reduced, which in turn facilitates optimal gas production. The regulation principle is shown in Figure 5; the control system flow is shown in Figure 6. 2.5 Automatic adjustment of the hydrogen-to-nitrogen ratio for ammonia synthesis: This is a special type of control system, where the element to be regulated has a long \"pure lag time\" ； There are many interfering factors ； The system has three main characteristics: strong integral property. In simple terms, it means that the gas produced in the gas generation section takes at least about 1 hour to reach the point where it can be used in synthesis, which reflects a long time lag ； The gas must go through processes such as desulfurization, reforming, and compression before it can be used in synthesis; this naturally leads to many interfering factors, as all of these processes affect the quality and quantity of semi-water gas ； In the process of ammonia synthesis, semi-water gas needs to be circulated. If, after the first cycle, the proportion of hydrogen is higher than that of nitrogen, then in the second cycle the hydrogen level will be even higher than that of nitrogen, and equilibrium cannot be achieved (this is a manifestation of strong integral behavior), until the gas is vented. This not only wastes semi-water gas but also increases parameters such as the pressure in the synthesis system, leading to energy waste. Therefore, the self-regulation of the hydrogen-to-nitrogen ratio is particularly important in ammonia synthesis production. Based on the above characteristics, following the principles of fuzzy control theory and adaptive control, and by applying neural network theory, the hydrogen-to-nitrogen ratio control scheme is endowed with self-learning, self-organization, self-analysis, self-judgment, and adaptive capabilities. (The control system block diagram is shown in Figure 7; the control rules are adjusted in a timely manner to account for the dynamic changes during the production process, and different control methods are employed to address the deviations caused by various conditions and disturbances, thereby achieving stability in the H2/N2 ratio.) Currently, with our technology, the average pass rate for the hydrogen-to-nitrogen ratio is ≥98%. 2.6 Implementing intelligent optimization control for furnace conditions 2.6.1 Prerequisites for furnace condition optimization control Control without measurements is blind control. First, create the necessary conditions: First, for the measurement of the actual height of the coal layer, an automatic measuring device is used to periodically determine the actual height of the coal layer, thereby providing accurate signals regarding this height for the optimal control of furnace operation (as shown in Figure 8). The automatic carbon layer height measurement device we have developed ensures that the control system can still receive accurate signals regarding the carbon layer height, even when in manual mode. Secondly, the method for measuring the temperature of the ash bin involves dividing the top of the ash hopper into four equal sections. The thermocouple used to measure the ash bin temperature is installed in the center of one of those sections, on the side opposite to the ash hopper and close to the furnace body; it should be positioned vertically or at a slight inward angle to facilitate maintenance. The insertion depth should be 10–15 cm. The two thermocouples used for measuring the ash bin temperature must be symmetrical, with the same insertion depth (as shown in Figure 9). Third, the method for measuring the temperature of the gasification layer involves using special thermocouples (whose protective sleeves have high-temperature and wear-resistant properties). Based on the typical position of the carbon layer, temperatures at three points—the upper end (dry layer), the middle part (gasification layer), and the lower end (ash layer) of the gasification layer—are measured vertically from top to bottom on the furnace body above one of the ash hoppers. To fully obtain the temperature distribution parameters of the gasification layer (as shown in Figure 10). T1 – Temperature of the drying layer; T2 – Temperature of the vaporization layer; T3 – Temperature of the ash layer. It is also important to obtain values for other parameters such as the upward temperature, downward temperature, temperature in the empty space, grate temperature, steam pressure, air pressure, top pressure of the furnace, bottom pressure, instantaneous flow rate of steam entering the furnace, as well as the concentrations of CO and CO2. The availability of these values satisfies the prerequisites for achieving optimal control of the furnace operation. 2.6.2 Specific methods for optimizing furnace operation: We adhere to the principle of \"one stability and two balances\" in gas production in order to achieve the best operating conditions and maximum efficiency. First, stability: that is, the position of the vaporization layer, its thickness, and its temperature must remain stable. Analysis shows that the factors affecting them are: the steam pressure entering the furnace; an increase or decrease in this pressure does not have a proportional effect on the increases or decreases in the temperatures above and below the furnace. This can cause unreasonable movement of the vaporization layer position. The steam flow rate into the furnace: if this flow rate changes when steam is introduced from above or below, it will inevitably affect the position of the gasification layer. If the layer moves upward at a time when it shouldn’t, it will lead to an increase in carbon buildup; if it doesn’t move at a time when it should, the gasification layer will become thinner, resulting in waste of steam and effective gas production time, as well as affecting the heat accumulation within the furnace. The frequent starting and stopping of backup boilers can cause instability in the steam produced by the other boilers, affecting the temperatures both above and below the boilers as well as the position of the gasification zone. Based on the above analysis, our control system adjusts the upper and lower blowing times according to the steam pressure during upper and lower blowing, as well as the difference in temperature between these two processes. This helps to keep the temperature of the gasification layer relatively stable, minimize the sensible heat loss inside the furnace, maximize the amount of heat retained, improve the effective utilization rate of carbon, and reduce the consumption of white coal. In short, proper adjustment of the upper and lower blowing times enables stability to be achieved. Secondly, there are two types of balances: one is the material balance, that is, the balance between coal addition and ash discharge. This balance may seem simple, but it plays a very important role in optimal control. Here, the coal feeding time and the rotation speed of the furnace bed are in balanced correspondence ； The dry layer and the ash layer are in dynamic equilibrium. We use the three temperature measurement points at the location of the gasification layer as the main operational control parameters, while using the upward or idle temperature and the carbon layer height as secondary parameters, allowing the control system to regulate the coal feeding time ； Using the three temperature measurement points at the location of the gasification layer as the main operational control parameters, and the temperature at the lower section or in the ash bin as secondary parameters, the control system is used to regulate the rotation speed of the furnace rod machine. The purpose of the above controls is to keep the position of the gasification layer constant. Secondly, in material balance, there is also an upper balance, which is the coal feeding balance, and a lower balance, which is the ash discharge balance. ①When the carbon layer height remains constant and the load increases along with the rise in the temperature of the drying layer, the control system increases the coal feeding time; conversely, it reduces the coal feeding time ； ②If the temperature of the ash layer decreases, increase the rotation speed of the furnace tray; otherwise, decrease it. It is necessary here to ensure a balance between coal addition and ash generation, as well as a balance between ash generation and ash discharge. This also ensures stability in the carbon layer height, the ash layer thickness, the gasification layer condition, and the drying layer status, ultimately minimizing coal consumption and achieving optimal gas production. The other is heat balance; in gas production, when the position of the gasification zone remains relatively stable, changes in the temperatures at the upper and lower levels mainly reflect an increase or decrease in the heat within the furnace. When the heat release from blowing air and the heat absorption in gas production reach equilibrium, the temperatures at the upper and lower levels remain stable and parallel to each other ； If the heat released during blowing increases while the heat absorbed in gas production does not increase, then the heat stored inside the furnace increases, and both the upper and lower temperatures will rise steadily; otherwise, they will fall. For example, due to the temperature difference between day and night, the wind speed can be 5% higher at night than during the day. If the heat absorbed during gas production remains constant, then the heat released by wind at night is greater than that released during the day, and the heat storage capacity of the furnace is higher at night than during the day. In this way, if operations are carried out according to night-time standards, then during the day the furnace operates at reduced capacity, resulting in waste of steam and brown coal ； If operated according to daytime standards, the furnace conditions are likely to deteriorate at night. Based on the above analysis, and taking into account the fluctuations in steam as well, we determine an optimal furnace operating temperature (which serves as the set temperature) that represents the sum of the upward temperature and the downward temperature. If the upstream temperature plus the downstream temperature is greater than the set temperature, the control system automatically reduces the corresponding blowing time ； Conversely, increase the corresponding blowing time. This automatically controls the heat balance inside the furnace, thereby ensuring stable and economical operation of the gas generator under optimal control. To achieve high gas production, high-quality gas, and low consumption. 2.6.3 Examples of intelligent optimization control: Ningxia Fengyou Fertilizer Co., Ltd., which uses coal from Ningxia, has adopted the various specialized control technologies mentioned above, especially the intelligent optimization technology for furnace operation. As a result, the furnace operation has become stable (as shown in Figure 11, the curve showing the key parameters of Fengyou Company’s operations), enabling optimal production levels to be achieved. Today, with seven gas generation furnaces, each furnace can produce over 65 tons of synthetic ammonia per day on average ； The coal consumption per ton of ammonia has decreased by more than 20% compared to the past. At the same time, gas production achieves fully automated continuous production. 3 Conclusion Looking at these specialized control technologies for gas production, it is clear that they were not achieved easily; many of them resulted from decades of research and development by engineering professionals ； Secondly, the application of these specialized control technologies can indeed solve the problems that arise in production, increase output, reduce environmental pollution, and enhance both the economic benefits of enterprises and those of society as a whole. Currently, in an era of technological advancement, these mature specialized control technologies should be employed in the gas production using intermittent fixed-bed gasifiers; this is a key aspect for measuring the level of sophistication of gas production technology. Don’t assume that just by having advanced DCS control technology, all such specialized control techniques can be implemented; it depends on whether they have mature specialized control technologies available. Otherwise, if such a system is implemented, it will only address the control aspects related to gas production; as a result, it’s very likely that specialized control technologies for gas production will never be developed. Therefore, companies that intend to adopt a gas production control system must be cautious. Once such a system is in place, these specialized control measures must be put into practice to improve efficiency, so that gas production, although having Chinese characteristics, can reach world-class standards. In other words, the manifestation of modern industry in developed countries is full automation and precise parameter control. Now, we can say that for the gas production using intermittent fixed-bed gasifiers, if these specialized control technologies can be effectively implemented, then all the characteristics of modern control will be present, meaning such systems will reach world-class standards.