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A Brief Discussion on the Operation of Small Gas Stoves Author/Source: Pei Hesheng, Zhao Yanjun (Raw Material Gas Plant, Anyang Chemical Industry Group Company, Anyang 455133, Henan) Date: 2009-2-6 -------------------------------------------------------------------------------- 1. Proper Control of the Bed Height A proper bed height is necessary to ensure sufficient contact time between the gasifying agent and the material layer, as well as to guarantee even distribution of the fuel across the cross-section of the furnace. Therefore, the fuel layer needs to have a certain height, which is generally maintained within the range of 1700–1900 mm. Among these: the height of the ash layer is 250–300 mm, the height of the oxidation layer is 400–450 mm, with a temperature of 1000–1200 ℃; the height of the reduction layer is 500–550 mm, at a temperature of 800–1000 ℃; the height of the dry distillation layer is 250–300 mm, at a temperature of 550–800 ℃; and the height of the drying layer is 250–300 mm, at a temperature of 450–550 ℃. When the coal fed into the furnace has large particle sizes and a high moisture content, and a moderate gasification strength is required, the total height of the material layer should be kept higher; otherwise, it is better to keep it lower. When using low-quality coal as the feedstock for gasification, the layer thickness should not be too high; the total height of the layer should be kept between 1500 and 1700 mm (measured from the top of the grate) ； If coal with better strength (coke) is mixed into the coal, the layer thickness can be increased by 100–200 mm accordingly. Operators generally determine the height of the material layer based on the height of the empty space below it; therefore, it is essential to measure the height of that empty space once per hour. By controlling the feeding time according to the rate at which the material layer decreases, stability of the material layer can be maintained. In daily operations, some operators find that the layer of material is too high and thus increase the speed of the rod feeder sharply, or they find it too low and reduce that speed drastically. These are all highly incorrect methods of operation; not only is it difficult to control the height of the material layer, but it also increases coal consumption. We should follow this principle: the amount of material added must be determined based on the downward speed of the material layer, in order to maintain a stable height for it. 2 Determine the appropriate blowing load. The O2 in the primary air should react completely with carbon in the oxidation layer to form CO2 and CO, and the height and temperature of the oxidation layer must meet the requirements mentioned earlier. To achieve this, the pressure of the primary air needs to be adjusted based on the furnace diameter and the gasification intensity. The downstream pressure during blowing is generally kept within the range of 15–25 kPa. The determination of the blowing load takes into account the quality of the raw coal, as well as process conditions and equipment status. In principle, 0.95–1.05 m3 of air is required to produce 1 m3 of semi-water gas; the lower end of this range is used when process control is effective or high-quality coal is utilized, while the higher end is applied when low-quality coal is used or the process is suboptimal. In situations where there are large differences between day and night temperatures, resulting in significant changes in air density, the output of blowers varies under the same conditions, and the amount of air entering the furnace per unit time differs greatly, it is important to adjust the air flow rate promptly. Once a setting has been determined, it is best not to change it arbitrarily. For low-quality coal, the blowing time should be short and not too long. If the blowing time is too long, low-quality coal tends to break down rapidly at high temperatures, increasing the resistance of the bed layer; it is advisable to keep the blowing percentage below 20%. Due to the requirement for a short blowing time, yet in order to increase the gasification intensity, it is necessary to use strong winds for a short duration in order to meet the high-temperature requirements inside the gas furnace; it is best to choose a blowing intensity of around 4000 m3/(m2·h), as this can effectively reduce the blowing time. 3. Adjust the amounts of steam used for upper and lower blowing appropriately. Once the blowing load is determined, the amount of steam used must correspond to it; in other words, the heat released during blowing must be balanced with the heat absorbed during steam production. The ratio of steam consumption and time for upper and lower blowing is primarily determined by the blowing volume, the resistance of the material layer, the heat storage capacity of the medium, and the condition of the equipment. Generally, the steam consumption remains fixed, with only the upper blowing time allowing for adjustment based on the process conditions. Based on daily operational experience, it is appropriate to control the opening degrees of the upper and lower steam injection handwheel valves for the φ2.65m small gas furnace at 9–11 turns and 11–13 turns respectively; under these conditions, the steam consumption for upper and lower injection is within the ranges of 4–6 t/h and 6–8 t/h respectively. The steam decomposition rate of low-quality coal is lower than that of high-quality coal; as a result, the CO2 content in the water gas is higher. If the CO2 content in the mixture produced from high-quality coal is 5%–6%, it can be kept at 6%–7% for low-quality coal. When low-quality coal is burned, too low CO2 levels can cause abnormalities such as the formation of large lumps inside the furnace, affecting the stable operation of the gas furnace. For top-blown CO2 in a single furnace, it should be controlled within 6%–7%, while for bottom-blown CO2, it should be kept between 5%–6%. Therefore, when burning low-quality coal, it is appropriate to increase the opening degree of the steam inlet and outlet handwheel valves slightly. 4. Control the upper and lower temperatures of the furnace. The upper temperature of a gas generator refers to the temperature at the gas outlet at the upper part of the furnace. Excessively high upstream temperature not only results in too much sensible heat being carried away by the gas, increasing fuel consumption, but it can also lead to slag formation on the walls surrounding the furnace if not handled carefully ； If the upward temperature is too low, it indicates that the temperature in the gasification zone is low; this could be due to insufficient air supply to the furnace, excessive steam supply from below, or a downward shift of the gasification zone. All these factors directly affect the amount of gas produced by the gasifier as well as the quality of the gas. For ordinary low-grain coal, it is appropriate to maintain an upward temperature of 250–300°C ； For low-quality coal, due to its tendency to break easily, the upward temperature should be kept as low as possible, ideally below 300°C. If the temperature at the top of the furnace is too high, a situation known as \"porridge in the furnace\" can occur. That is, when the furnace lid is opened to examine the interior, no lumped coal can be seen; instead, everything is in the form of fine powder, which is why it is called \"porridge in the furnace\". If the downward temperature of the gas generator is too high, it indicates that the gasification layer has moved downward; this not only affects the amount of gas produced by the gas generator but can also damage equipment such as the grates and ash trays ； If the temperature at the downward flow is too low, it indicates that the gasification layer moves upward, which in turn affects the amount of gas produced. Additionally, a low temperature at the downward flow leads to incomplete combustion, resulting in a high carbon content in the slag and increased fuel consumption. This not only affects the gasification efficiency but also increases the CH4 content in the gas. For ordinary low-grain coal, it is appropriate to control the downward temperature at 200–280°C. During routine operations, some operators noticed an increase in the upward temperature, so they increased the feed rate ； Or the upward temperature can be reduced by decreasing the feed rate; in other words, the feed rate is used to control the upward temperature. These are all incorrect methods of operation, as increasing or decreasing the feed rate will inevitably cause changes in the layer height. If the speed of the grate machine is then used to control this layer height, it will lead to a vicious cycle that will gradually deteriorate the conditions inside the furnace. The control of upper and lower temperatures is primarily achieved through proper distribution of the blowing time, upper and lower blowing periods, as well as by adjusting the volume of air and steam used. Therefore, appropriate process parameters should be selected for stable control, and adjustments should be made in a timely manner based on temperature changes, to ensure that both the upper and lower temperatures remain within suitable ranges. 5. Identify problems and address them promptly. For example, if it is detected during operation that the temperatures on the upper and lower sides are too high or that the rate at which the carbon layer descends is slow, then the machine should be stopped safely. The round door on the second floor should be opened to check for any large obstructions; if such obstructions exist, they must be removed manually. Additionally, the blowing time should be reduced by 1–2 seconds or the upper blowing time increased by 1–2 seconds ； If there is no large blockage and red carbon or red slag is present on both sides, the speed of the rod feeder should be reduced appropriately during operation, along with shortening the feeding time, to allow the carbon layer to recover slowly. For another example, what should be done if it is found that the carbon layer is too low during operation? It depends on whether the temperatures at the upper and lower levels are stable. If they are stable, with normal temperature differences and normal temperatures inside the gray box, then the feeding rate should be increased to gradually raise the layer of material. At this stage, since the carbon layer is low, the resistance to air flow is small and the capacity to consume raw materials is high; therefore, it is not advisable to reduce the speed of the furnace strip machine ； If the upward temperature is high with large temperature differences, while the downward temperature is low with small temperature differences, then the speed of the furnace rod machine should be increased, along with the feeding rate, in order to gradually raise the carbon layer. In short, the ability to identify and resolve problems in a timely manner during operation is a necessary condition for determining whether an operator is qualified to perform such tasks. 6. Make rational use of the optimization microcomputer to achieve cost reduction and efficiency improvement. At present, we are using the \"Yifeng Brand\" furnace condition optimization control system (LYK-3Ⅷ), produced by Hunan Yifeng Optimization Control Technology Co., Ltd. (hereinafter referred to as the optimization microcomputer). The optimization microcontroller analyzes the current heat storage condition inside the furnace in real time, based on the dynamic changes in the temperatures at the upper and lower levels, and thereby controls the timing of air supply from these areas online. This approach aims to increase the thickness of the gasification layer under various process conditions, so as to enhance its heat storage capacity. Only after the heat storage capacity of the gasification layer is enhanced can the air supply volume be increased effectively, thereby effectively raising the gas production load. Once the optimization microcomputer finds an optimal state, the parameters enter a stable phase; when external process conditions change, the parameters make automatic adjustments to adapt, thereby ensuring high stability of the furnace operation. However, since the quality of coal used for gas production cannot remain stable over the long term, if operations are not carried out carefully and the coal quality deteriorates, leading to deficiencies in the carbon layer or arbitrary changes in process conditions, then the level of gas production will decline. Therefore, the operator should strictly control the speed of the rod machine to avoid exceeding the limits, and pay attention to monitoring the rate at which the carbon layer decreases in order to maintain stability in its thickness. Careful and accurate operation is required; it is essential to have a clear understanding of factors such as the steam pressure entering the furnace, the air pressure from the fans, the base values for blowing air, the base values for upper blowing, the degree of opening of the handwheels used for upper and lower blowing in each furnace, the rotation speed of the grate mechanism, the control values for the rising and falling temperatures, the height of the empty layer, the time required to remove ash, and the properties of the semi-water gas. In this way, through the careful operation of the operators, the optimal state identified by the optimization microcomputer can remain relatively stable, thereby enhancing the benefits that come from using such a microcomputer in the company. With stable furnace conditions over time, increases in production and reductions in consumption will be evident. Thus, the production goal of \"using an optimization control system to produce high-quality small-grained coal (briquetted coal) and reducing fertilizer costs\" can certainly be achieved. Conclusion: This article provides a brief overview of the daily operation of the φ2.65m small gas furnace. Due to a lack of experience, the views presented are rather superficial; I hope that colleagues and experts will offer their valuable suggestions to provide better references for the improvement and refinement of gas production processes. This post was last edited by jensse on 2009-3-30 16:07]