Energy consumption analysis and energy-saving approaches in the gas generation section of ammonia synthesis production

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Energy Consumption Analysis and Energy-Saving Approaches in the Gas Generation Section of Ammonia Synthesis Production Author/Source: Sun Huatian (Shandong Mingshui Dahuagroup) Date: 2008-12-5 The gas generation section is the part of ammonia synthesis production where the most energy is consumed, and it also generates the greatest amount of waste heat. Striving to reduce the consumption of raw materials, steam, and electricity in gas production, as well as ensuring effective recovery and utilization of waste heat, are effective ways to lower the overall energy consumption and production costs associated with ammonia synthesis. The theoretical energy consumption for producing one ton of synthetic ammonia is 22.78×106 KJ, but in reality, even in some more advanced companies, the actual energy consumption is much higher than this value. Therefore, both from a theoretical perspective and in practice, there is significant room for further reducing the energy consumption in ammonia synthesis. Fully tapping into the potential for cost reduction and striving to lower production costs are key to the survival and development of the small-scale nitrogen fertilizer industry. Energy conservation issues affect all processes in the plant; this article provides specific details on how the gas production section can improve carbon utilization, reduce steam consumption, and lower electricity usage. 1 Ways to improve fuel efficiency: The amount of carbon that is converted into the composition of semi-water gas during the gasification process of fuel is referred to as effective consumption. In actual gasification processes, a large amount of carbon is also consumed in other forms, such as the carbon burned during the blowing process, the unburned carbon in the ash, and the carbon contained in the dust particles carried away by the gases. In fact, the amount of carbon converted into semi-water gas is only a part of the raw material consumption in the entire gas production process, and this ratio represents the effective utilization rate of carbon. In production, it is desirable to have a higher proportion of total consumption that is utilized effectively; this requires efforts to improve fuel efficiency and to minimize carbon losses in other forms. During the gasification process, the carbon that moves into semi-water gas exists in the forms of carbon monoxide and carbon dioxide; the formation of methane is an unwanted side reaction. If the type of gasification feedstock and the gasification conditions are determined, the total amount of carbon monoxide and carbon dioxide in the semi-water gas will also be a fixed value accordingly. Based on the current level of gas production, the carbon monoxide content in semi-water gas is generally in the range of 28% to 31%, while the carbon dioxide content ranges from 7% to 8%. If V_half represents the amount of semi-water gas consumed to produce one ton of ammonia, and the total content of carbon monoxide and carbon dioxide in semi-water gas is 30% + 7.5% = 37.5%, then the amount of carbon transferred to the semi-water gas per ton of ammonia produced is: 0.375×12/22.4V_half = 0.2009V_half (kg/tNH3). Converting this to standard coal gives: 0.2009V_half ÷ 0.84 = 0.239V_half (kg/tNH3). Here, 0.375 represents the percentage composition of CO + CO2 in semi-water gas ； 12——Atomic weight of carbon ； 22.4 —— At standard conditions, volume per thousand moles (Nm3) V_half —— Volume of ammonia-ammonium hydrosulfide consumed per ton (Nm3) ； 0.84 —— the carbon content per ton of standard coal. If 3200 Nm3 of semi-water gas is consumed per ton of ammonia, then under conditions of a CO content of 30% and a CO2 content of 7.5% (with CH4 excluded), the theoretical carbon consumption per ton of ammonia is: 0.239 × 3200 = 764.8 kg of standard coal. In actual production, not all of the carbon consumed is converted into semi-water gas. If the effective utilization rate of carbon is 65%, the standard coal consumption per ton of ammonia is: 764.8÷0.65=1176.6(kg). When the carbon utilization rate increases to 70%, the standard coal consumption per ton of ammonia becomes: 764.8÷0.7=1092.6(kg). An increase in the effective carbon utilization rate from 65% to 70% results in a reduction of 84 kg of standard coal per ton of ammonia. It is clear that striving to improve the carbon utilization rate is the main way to reduce consumption. It is also key to improving a company’s economic efficiency. For a company that produces 100,000 tons of synthetic ammonia per year, if the carbon utilization rate in the gas production process increases from 65% to 70%, the annual savings amount to: 10×0.084×800=672 (10,000 yuan per year). Here, 10 represents the annual output of synthetic ammonia ； 0.084——tons of ammonia equate to standard coal saved, per ton ； 800 —— at the current price per ton of standard coal fed into the furnace, in yuan. To improve the effective utilization rate of carbon, the following tasks need to be carried out ； (1) Improve blowing efficiency. The purpose of blowing is to raise the temperature of the gasification layer and store heat to create conditions for the gas production process. Blowing efficiency is the ratio of the heat stored in the fuel layer to the calorific value of the fuel consumed. Its meaning can be expressed by the following formula: E_blowing = 100(Q_reverse – Q_air) / Q_combustion. Where: E_blowing – the efficiency% during the blowing phase ； Q_combustion – the calorific value (KJ) of the fuel consumed during the blowing phase ； Q_reverse – Heat released during the reaction when blowing air (kJ) ； Q_air – Heat carried away by the blowing air (kJ). Obviously, to improve the blowing efficiency (E_blowing), it is necessary to strive to increase Q_reverse and reduce Q_gas and Q_combustion. As can be seen from the chemical reactions during the blowing stage, the reaction heat Q_rev released per thousand moles of carbon consumed in this stage is related to the contents of carbon monoxide and carbon dioxide in the resulting products. The heat released upon the production of 1,000 moles of carbon dioxide is 393.51×103 KJ, while only 110.52×103 KJ of heat is released upon the production of 1,000 moles of carbon monoxide; the heat released in the latter case is 28.1% of that released in the former case. Therefore, it is very important to maintain the temperature of the gasification zone within an appropriate range, increase the air flow rate (without causing the carbon layer to be overturned), and reduce the carbon monoxide content in the purge gas. An increase in the carbon monoxide content in the blowing gas (the average CO content in the blowing gas should be less than 6.0%) or an excessively high temperature of the blowing gas leads to a corresponding increase in Q gas, thereby reducing the efficiency of the blowing process. In actual production, it is inevitable that the carbon monoxide content in the blowing gas increases over time as the blowing duration prolongs. Lowering the temperature of the gasification layer can reduce the reaction of carbon dioxide being reduced to carbon monoxide. However, a low furnace temperature results in poor quality of the gas produced, a low rate of steam decomposition, and undecomposed steam carrying away a large amount of heat from the furnace – all of which are detrimental to reducing coal consumption and improving the gasification efficiency of the gasifier. Clearly, the requirements for the temperature of the gasification layer in the blowing stage and the gas production process are contradictory. To maintain a high temperature in the gasification zone while reducing the carbon monoxide content in the blowing air, it is possible to increase the fan pressure and airflow, as well as reduce the percentage of blowing air used. However, excessive wind pressure and airflow lead to two problems: first, the amount of material carried away during the blowing phase increases significantly; second, the carbon layer inside the furnace is prone to being overturned, making it difficult to maintain stable operation over a long period. On the other hand, too low wind pressure and air volume lead to a reduced gasification efficiency of the gas stove; the slow air flow facilitates the reduction of carbon dioxide to carbon monoxide, resulting in lower blowing efficiency. This increases the amount of carbon consumed during the blowing phase and reduces the efficiency of carbon utilization. Therefore, by considering factors such as the properties and particle size of the fuel used in various plants, selecting blowers with appropriate wind pressure and flow rates as well as circulation times and blowing percentages, and controlling the appropriate height of the carbon layer as well as the temperature, thickness, and position of the gasification layer, it is possible to reduce heat losses across all aspects. This is one of the main ways to improve the utilization rate of carbon and reduce the consumption of coal. Based on theoretical analysis and practical exploration, for the Ф2600mm series of gas burners, the blowing intensity should be controlled at 3800–5000 Nm3/m2h (the upper limit is used for high-quality raw materials, while the lower limit is applied to low-quality ones). For a F2600mm gas stove, the blowing efficiency is generally less than 60%; when the temperatures of the air exiting from the top and bottom of the stove are high, it drops to around 50%. To store enough heat in the carbon layer for gas production, it is often necessary to burn fuel whose calorific value is twice that amount of heat. Therefore, the heat in the carbon layer should be used as much as possible for gas production; it is more reasonable to use any heat to superheat the steam or air fed into the furnace to supply heat for the gas production reaction, rather than using that heat to generate steam. In other words, any heat removed from the vaporization layer, although it can be recovered by being used to generate steam, is still uneconomical due to the low efficiency of heat accumulation in the vaporization layer. Through calculations, when air is blown, the temperature on the furnace drops from 450°C to 250°C, and the blowing efficiency can be increased by about 6%. To achieve optimal blowing efficiency, first, maintain the temperature of the gasification layer within an appropriate range and select an appropriate space velocity, while striving to minimize the carbon monoxide content in the blowing gas. Secondly, provided that the height-to-diameter ratio of the gas furnace permits it, it is necessary to control the appropriate heights of the carbon layer, gasification layer, and ash layer, while maintaining a relatively high thickness for the upper preheating layer (as allowed by the fan’s capacity). This helps to increase the heat storage capacity of the fuel layer in the gas furnace, thereby creating favorable conditions for improving its gasification efficiency. During the blowing stage, the reaction between carbon and oxygen is a combustion reaction. Experiments have shown that at temperatures above 800°C, this reaction proceeds almost irreversibly from left to right, and it is diffusion-controlled. Therefore, at the temperatures typical of gas generator operation, the diffusion rate of oxygen is the main controlling factor for the overall reaction rate of carbon dioxide formation. Studies on the reaction between carbon and oxygen show that this reaction is kinetically controlled below 775°C. Above 900°C, it is under diffusion control, and between these two temperatures, it can be considered to be in a transition zone. Based on the characteristics of the gasification process in fixed-bed gasifiers, it is believed that a combustion reaction occurs first between carbon and oxygen, and then the resulting CO2 undergoes a reduction reaction with the carbon atoms located in the upper part of the gasification zone. It is generally believed that the reaction rate between carbon and carbon dioxide is much slower than the combustion rate of carbon. Below 2000°C, it is essentially kinetically controlled, and the reaction rate is also considered to be first order with respect to CO2. Based on the characteristics of the reaction during the blowing process, controlling an appropriate blowing intensity and gasification layer temperature is of great significance for improving blowing efficiency and reducing the consumption of raw coal and steam. (2) Improving gas production efficiency: The efficiency E of gas production refers to the ratio of the calorific value Q_gas of the semi-water gas obtained to the sum of the calorific value Q_fuel of the fuel consumed during gasification, the heat added by the gasifying agent (steam) Q_vap, and the usable heat stored in the fuel layer during the blowing stage Q_utilizable. E_gas_production = Q_gas / (Q_combustion + Q_vaporization + Q_utilized) × 100%
Q_utilized = Q_reverse + Q_gas + Q_loss
Q_reverse – the heat absorbed by the gas production reaction, in kJ/mol ； Q_gas – Heat carried away by water gas and unreacted steam, kJ/mol ； Q loss – heat loss due to jackets, etc., kJ ； Q utilization – heat stored in the fuel layer by blowing air, KJ. As can be seen from the above equation, to improve the gas production efficiency, it is necessary to increase Q_gas, that is, to raise the amount of gas produced per unit volume as well as the content of the effective components in water gas, namely carbon monoxide and hydrogen. During the gas production process, given a constant amount of heat consumed by combustion (Q_combustion) and the heat brought in by the vaporized reagent (Q_vaporization), improving the efficiency of gas production means increasing the effective utilization rate of the heat Q_available stored within the fuel layer during blowing. The heat Q stored in the fuel layer during blowing that can be utilized should equal the heat absorbed during the gasification reaction, plus the heat carried away by the water gas and the undecomposed water vapor after the reaction, along with the heat losses. In other words, since the heat loss due to the jacket remains constant, improving the gas production efficiency involves controlling the temperature of the gasification layer at an appropriate level, increasing the rate of steam decomposition, reducing the temperatures at the top and bottom of the furnace, minimizing heat loss, and thereby enhancing both the quantity and quality of water gas. The reaction between carbon and steam remains very slow within the temperature range of 400°C to 1000°C; therefore, it is kinetically controlled. Above 1100°C, the reaction rate increases rapidly and becomes diffusion-controlled. When the reaction between water vapor and carbon reaches equilibrium at high temperatures, the amount of residual water vapor is low; in other words, the decomposition rate of water vapor is high, resulting in higher concentrations of H2 and CO in the water gas. At the same temperature, as pressure increases, the concentrations of H2O, CO2, and CH4 in the gas increase, while the concentrations of H2 and CO decrease. Therefore, to produce water gas with high CO and H2 contents, from an equilibrium perspective, it should be carried out at low pressure and high temperature. Based on practical experience in intermittent fixed-bed gas production, when using metallurgical coke with high activity as the feedstock, increasing the rate at which the gasifying agent is fed into the furnace at the same temperature can enhance the gasification intensity without affecting the quality of the gas (the CO content in the gas does not decrease). When using low-activity anthracite, increasing the feed rate of the gasifying agent at the same temperature results in a rapid decline in gas quality and gasification efficiency. In particular, as the temperature inside the furnace drops slightly, the gas quality and gasification efficiency drop significantly immediately; this indicates that the reaction rate for the former is likely in the diffusion or transition zone, while it is in the kinetically controlled zone for the latter. Therefore, controlling a higher gasification layer temperature and a lower flow rate of the gasifying agent during gas production is an important way to improve gas production efficiency, gas quality, and steam decomposition rate. (3) Reducing the carbon return rate in slag. The ratio of carbon content to ash content in slag is called the carbon return rate. The high carbon reversion rate in the ash is usually caused by defects in the leak-proof devices at the bottom of the gas furnace, unreasonable operating procedures, or poor gasification within the furnace. Although carbon returned with a larger particle size can be recycled, it has already carried away some heat, resulting in certain heat loss ； The small particles of carbonized material in the ash are difficult to recover and are disposed of along with the ash; although they can be reused, their value for recovery is low. Therefore, striving to reduce the combustible content in ash is an important step in improving carbon utilization efficiency. The main reasons for the high combustible content in the ash include the improper selection or installation of the leak-proof device at the bottom of the gas stove, or damage to this device during use; such issues lead to uneven ash discharge, as well as carbon leakage and collapse, resulting in a high rate of carbon return ； Improper feeding methods result in an uneven distribution of height across the carbon layer, both around its edges and in its center; this leads to uneven distribution of the gasifying agent. As a consequence, localized overheating occurs within the furnace, causing scorching and caking, as well as an unbalanced thickness of the ash layer, with uneven ash discharge on both sides ； The percentage of upper/lower blowing or the amount of steam used for upper/lower blowing is not properly set ； The particle size of the raw material is too large, the temperature in the gasification layer is low, resulting in incomplete combustion ； The ash discharge rate of the grate machine is not compatible with the rate at which ash is generated after the fuel is vaporized, resulting in an excessive amount of ash being discharged ； Incomplete ash removal leads to the accumulation of ash in the ash box, causing the layer of ash and slag inside the gas stove to rise locally, which results in damage to the gasification layer ； Improper control of the furnace temperature leads to scarring and caking, resulting in uneven distribution of the gasifying agent and poor gasification; or if the furnace temperature is too low, excessive steam is used and the fuel does not react completely before being discharged. In short, there are multiple factors contributing to the high carbon return rate. In daily operations, it is necessary to identify the main factors responsible for a high rate of carbon reversion based on specific circumstances, and to take appropriate corrective measures to reduce this rate to below 15%; ideally, it should be around 10%. (4) Reduce emissions from blowing air and gas generation; increasing wind speed can decrease the likelihood of carbon dioxide reducing carbon monoxide. But as the wind speed increases, the amount of entrained material inevitably increases. Therefore, the wind speed should be chosen appropriately, following the principles of minimizing the amount of material carried away and preventing the carbon layer from being blown over. To reduce carryover, the following measures can be adopted in production: select an appropriate fan; for gas furnaces of the Ф2600mm series, the fan capacity can range from 450 m3/min to 550 m3/min, with an air pressure within the range of 25 kPa to 28 kPa ； Based on the properties of the raw materials, determine the appropriate air volume and control the carbon layer height suitable for the fan ； Choose fuels with good mechanical strength and thermal stability ； Strictly control the processing quality of the fuel fed into the furnace to reduce the amount of powder entering it ； Strive for uniform resistance of the material layer ； A properly designed grate should have strong slag discharge and crushing capabilities, ensure even distribution of the gasifying agent, and reduce the amount of slag carried away by the downward airflow ； Ensure proper control of the ash quality, keeping the slag content in the ash at over 65%, and reducing the amount of fine ash in it ； Select an appropriate gasification intensity; when the gas stove is under heavy load and the flow rate of the gasifying agent is high, more material is carried away. This is especially true during bottom-blown gasification, where a large amount of steam is used at a high flow rate, resulting in an increased amount of material being carried away ； Take appropriate measures to improve the upward air exhaust method and enhance the dust removal efficiency inside the furnace. (5) Reducing heat loss: To reduce heat loss, it is first necessary to select an appropriate carbon layer height that suits the fan (generally, the carbon layer height from the top of the wind cap upwards should be between 1800 mm and 2400 mm). Secondly, it is important to choose suitable operating procedures and gasification conditions so that the furnace maintains an optimal heat storage state, that is, by keeping the temperature of the gasification zone high while keeping the temperatures at the top and bottom of the furnace at lower levels. At present, manufacturers that use an automatic and continuous carbon addition method and employ reasonable control strategies for parameters such as the carbon layer height, the position of the gasification zone, the thickness of the ash layer, and the load on the gas furnace have achieved a total gas outlet temperature at the top and bottom of the furnace of less than 460°C. The ammonia consumption per ton of feedstock entering the furnace is kept within the range of 1100 kg to 1150 kg, which represents a high level of efficiency. If the vaporization layer moves upward or locally upward, it will inevitably lead to an increase in the temperature at the top of the furnace, as well as an increased amount of heat carried away by the airflow. There is significant heat loss during the reaction, resulting in a decrease in vaporization efficiency. Improper control of the gasification layer position, as well as an excessively thin or uneven ash layer, inevitably lead to an increase in the gas temperature at the furnace bottom, resulting in greater heat loss and a reduction in gasification efficiency. Furthermore, excessive steam consumption is also an important factor contributing to heat loss. The excessive amount of steam used results in a large quantity of unreacted steam carrying away heat from the furnace, thereby lowering the temperature in the gasification zone. This not only reduces the gasification efficiency but also wastes steam; furthermore, it increases the thermal load on the gas washing tower, causing its outlet temperature to rise, which in turn affects the effective gas injection volume in subsequent processes. 2 Ways to Reduce Steam Consumption In the ammonia synthesis production system, the steam consumption for gas production accounts for over 65% of the total steam usage in the entire plant; therefore, reducing the steam consumption quota is one of the key tasks for the gas production section. The steam consumption in the gas production section can be calculated using the following formula: QT = V_half × H2% / 100 × 100 / Q × 18 / 22.4. Here, QT represents the steam consumption required for gas production, in kg per ton of (NH3) ； V half – Standard consumption of semi-water gas in Nm3/t(NH3) ； H2 —— H2 content in semi-water gas, % ； Q — Average steam decomposition rate during gas production, %. From the above equation, it can be seen that the steam consumption in the gas generation section is proportional to the consumption of semi-water gas ； It is inversely proportional to the steam decomposition rate. With the same consumption quota for semi-water gas, the amount of steam used in the gas production section varies significantly due to differences in the steam decomposition rate. For example: the consumption rate of semi-water gas is 3200 Nm3 per ton of ammonia. When semi-water gas contains 40% hydrogen and the steam decomposition rate is 40%, the steam consumption is: QT = 3200 × 40/100 × 100/40 × 18/22.4 = 2571. If the steam decomposition rate increases to 50%, then the steam consumption becomes: QT = 3200 × 40/100 × 100/50 × 18/22.4 = 2057. From these calculations, it can be seen that an increase in the steam decomposition rate from 40% to 50% results in a savings of 514 kg of steam per ton of ammonia. For a plant producing 100,000 tons of synthetic ammonia per year, with a steam price of 100 yuan per ton, the annual savings amount to: 10×0.514×100 = 5.14 million yuan per year. It can be seen that increasing the steam decomposition rate is the main way to reduce steam consumption, and it is also the key to improving a company’s economic efficiency. During the gas production process in intermittent gasifiers, the reaction between carbon and water vapor is essentially under kinetic control; maintaining a higher furnace temperature can increase the reaction rate. In the gasification process, the main factors affecting the steam decomposition rate are: first, the temperature and thickness of the gasification zone. II. Steam flow rate during gas generation. III. Volume of the space above and below the furnace. IV. Steam temperature entering the furnace. Theoretical discussions show that the vaporization efficiency in the actual vaporization process decreases as the amount of steam used increases. The main reason is the excessive influence of kinetic control, which leads to incomplete reactions. When the amount of steam used increases, the endothermic reaction intensifies, causing the temperature of the gasification layer to drop; this in turn results in a decrease in the steam decomposition rate. This is one of the key reasons for the increased steam consumption. In daily production, the amount of steam supplied to the furnace should be controlled based on the temperature of the gasification zone; the steam flow rate should be reduced as much as possible and kept stable, in order to increase the contact time between the red-hot carbon and the steam and thereby improve the steam decomposition rate. Using superheated steam for gas production is an effective measure to improve the steam decomposition rate and gas production efficiency. Superheated steam not only prevents water from being carried in with the steam entering the furnace, but also reduces fluctuations in furnace temperature by increasing the heat content of the steam entering the furnace. According to the actual measurement values from some manufacturers, when superheated steam at a temperature of over 200°C is used for gas production, the steam decomposition rate can increase by 5%, while the steam consumption can be reduced by around 250 kg/t(NH3). 3 Ways to Reduce Power Consumption: The power consumption in the gas generation section is mainly due to the use of air blowers. There are two main ways to reduce the power consumption of air blowers: first, select blowers with appropriate parameters (air volume, air pressure), and determine scientifically the proper ratio between the number of blowers and the number of gas stoves. Second, reduce the system resistance during the blowing phase. The following is explained from two aspects: One of the energy-saving measures is to select appropriate air blowers that match a reasonable number of gas stoves. Currently, for ammonia synthesis plants, the fans used in Ф2600mm series gas furnaces range from D400 to D700 (or C400 to C700), with air pressures varying between 25 kPa and 30 kPa. From manufacturer practices to theoretical calculations, for those manufacturers that manage various process parameters well, the air consumption per ton of ammonia during the blowing stage ranges from 1900 Nm3 to 2300 Nm3. Assuming a consumption of 3200 Nm3/h of semi-water gas per ton of ammonia, the ratio of air to semi-water gas is between 0.594:1 and 0.719:1. Assuming that the gasification intensity of a furnace with a diameter of 2600 mm is 1320 Nm3, which means it produces 7000 Nm3 of semi-water gas per hour, then the air consumption per hour ranges from 4158 Nm3/h to 5033 Nm3/h. Assuming the ratio of air to semi-water gas is 0.65, 3200 Nm3 of semi-water gas is required per ton of ammonia produced, while the amount of air needed per ton of ammonia is 0.65×3200 = 2080 Nm3. The Ф2600 furnace can produce 2.19 tons of NH3 per hour; therefore, the air volume required for blowing air is 2.19×2080 = 4555 Nm3/h. In practice, the fan delivers an air volume of 24000 Nm3/h, with a circulation time of 150 seconds and a blowing percentage of 20%, which means 30 seconds: 24000/3600×30×(3600/150) = 1800 Nm3/h. Through calculations, for the Ф2600mm gas furnace, with a daily gas production volume sufficient to produce 2.19×24=52 t of NH3 per day, a fan with an actual air flow rate of 24,000 Nm3/h is sufficient to meet the production requirements. It is understood that in most production systems, one blower is used to supply air to four furnaces. Under normal conditions, the blowing phase overlaps with the nitrogen injection process in the upper furnace; therefore, blowers of the D450–D550 type with an air pressure of 25 kPa–28 kPa can meet the requirements of the Ф2600mm gas furnace at its economic operating load. If the fan’s wind pressure and airflow are too high, the gas stove will often tip over; as a result, throttle control of the airflow is necessary using inlet or outlet valves. Whether throttling is done at the inlet or the outlet, it represents a waste of efficiency and leads to increased power consumption. Second power-saving measure: Minimize the system resistance during the blowing phase to fully utilize the maximum efficiency of the blower. Reducing system resistance is the main measure to lower the power consumption of the blower. During operation, the actual air volume delivered by the fan does not reach the rated value. Apart from the fact that the fan’s design parameters are not suitable for the actual gasification conditions, this is mainly due to high resistance in the area between the fan’s inlet and the air-gas outlet. The resistance in the system consists of two parts. —Part of it is the resistance from the equipment and the carbon layer of the gas stove, and the other part is the resistance from the pipes and valves. The process pipelines in fertilizer plants are divided into high-pressure and low-pressure sections; the gas production system operates at low pressure, while copper washing and synthesis take place at high pressure. In the past, various factories did not treat the piping systems of these two types differently; they all emphasized that the pipeline layout should be arranged in a \"horizontal and vertical\" manner. Some manufacturers do not fully understand the special significance of reducing the resistance in the pipelines of the gas generation system. When dealing with the low-pressure system pipelines during the blowing phase of gas stoves, it is not necessary to follow the same strict planning principles as those applied to high-pressure pipelines; instead, emphasis should be placed on reducing the resistance in the pipeline system and valves. Otherwise, not only will energy consumption be high, but more importantly, it will affect the operating characteristics of the fan. Excessively high system resistance during the blowing phase has two serious consequences. On one hand, it limits the increase in air volume; as shown in Figure 1, the air volume delivered by the fan decreases from Q2 to Q1 due to the increased system resistance. On the other hand, it increases power consumption. The pipeline loss ΔP can be converted into power loss N using the following formula: N_resistance_loss = QsΔP/resistance_loss / 102η (Kw), where η is the efficiency of the fan (typically around 75% for ordinary fans) ； N resistance loss – conversion of different resistances into power consumption ； Qs —— the volume of air per second corresponding to the pressure loss P, in m3/s ； 102——Unit conversion constant (1kw=102kg·m/s). The table below shows the equivalent electrical power consumption N at different P resistance losses. When the system resistance is low, fans with a lower air pressure can be used. For example, the overall system resistance decreased by 3 kPa after measures were taken, allowing the air pressure delivered by the fan to drop from 28 kPa to 25 kPa. Reducing system resistance and selecting air blowers that meet the requirements of the factory’s processing processes is an important measure for saving electricity. 4 Relationship between main process parameters and consumption: The quality of control over the main process parameters in the gas generation section has a significant impact on ammonia production volume and consumption levels. To draw attention, this section illustrates it through some simple calculations. 4.1 Hydrogen-to-nitrogen ratio The hydrogen-to-nitrogen ratio in semi-water gas is an important control parameter in ammonia synthesis. The equilibrium concentration of ammonia is highest when the volume ratio of hydrogen to nitrogen is 3:1. If either hydrogen or nitrogen is in excess, the percentage of the hydrogen and nitrogen gases that participate in the reaction decreases in the total gas volume. Under normal circumstances, hydrogen and nitrogen account for about 80% of the gases in the synthesis tower. If the inert gases and ammonia present in the recycled gas are excluded, and the remaining hydrogen and nitrogen are used in a synthesis reaction in a 3:1 ratio, then hydrogen should make up 75% of it, while nitrogen accounts for 25%. If there is a 3% excess of hydrogen, the hydrogen content in the hydrogen-nitrogen mixture will be 78% while the nitrogen content will be 22%. Of this hydrogen, only 66% reacts with the 22% nitrogen; thus, the hydrogen and nitrogen gases that participate in the reaction account for 88% of the total gas volume. If the nitrogen content in the hydrogen-nitrogen mixture is 3% too high, its composition becomes 28% nitrogen and 72% hydrogen; of this 72% hydrogen, only 24% can combine with nitrogen, resulting in 96% of hydrogen and nitrogen participating in the reaction. Obviously, an excess of nitrogen is much better than an excess of hydrogen. Furthermore, in terms of the reaction rate of ammonia synthesis, under non-equilibrium conditions, appropriately increasing the nitrogen partial pressure is beneficial for the rate at which nitrogen is adsorbed by the catalyst, as the active adsorption of nitrogen is the controlling step in the ammonia synthesis process. A hydrogen-to-nitrogen ratio slightly below \"3\" can increase the partial pressure of nitrogen in the gas, allowing more nitrogen to diffuse to the catalyst surface and increasing the chances of adsorption, thereby raising the synthesis rate. Due to the increased synthesis rate, the electrical consumption of the compressor also decreases accordingly. A too high hydrogen-to-nitrogen ratio is highly detrimental to the synthesis reaction. For example, when the cycle gas consists of 75% hydrogen, 20% inert gas, and 5% nitrogen, 5 parts of nitrogen can only react with 15% of the hydrogen; in other words, only 20% of the components participate in the reaction, while the remaining 80% does not take part in it. As a result, the synthesis rate decreases significantly, the synthesis pressure rises, and the amount of gas vented increases. This not only leads to the loss of large amounts of hydrogen, affecting ammonia production, but also causes an increase in various consumption metrics. Therefore, it is appropriate to maintain a hydrogen-to-nitrogen ratio in the cycle gas between 2.4 and 2.8. However, hydrogen and nitrogen gases are combined in a 3:1 ratio to form ammonia, so the hydrogen-to-nitrogen ratio of the supplementary gas should still be “3”. 4.2 Effective gas content: Effective gases generally refer to carbon monoxide and hydrogen in semi-water gas. With a constant gas consumption per ton of ammonia, the amount of semi-water gas consumed per ton of ammonia is directly related to the effective gas content. The theoretical utilization rate of semi-water gas (n) can be expressed by the following formula: where CO2_half, O2_half, and CO_half represent the volume percentages of each gas in semi-water gas ； X——Transformation rate ； α —— Regenerated gas recovery rate. Assume that the components in semi-water gas are: CO2, 7.8% ； O2, 0.3% ； CO, 30% ； H2, 40% ； N2, 21.5% oxygen conversion rate; X is 95%, and the regenerated gas recovery rate α is 90%. Therefore: if 2900 Nm3 of refined gas is required per ton of ammonia, then the amount of semi-water gas needed per ton of ammonia is: 2900/0.9115 = 3182 Nm3. Additionally, if the composition of semi-water gas is CO2 at 12.3%, ； O2, 0.7% ； CO, 26% ； H2, 40% ； N2, 21%. The conversion rate X is 90%, and α=0 for the unrecycled regenerated gas. Substituting this value into the formula gives the theoretical utilization rate of semi-water gas. Therefore, the amount of semi-water gas required per ton of ammonia is: 2900/0.83 = 3494 Nm3. In the latter case, 312 Nm3 more semi-water gas is needed per ton of ammonia. It can be seen that changes in the effective components of semi-water gas have a significant impact on its consumption. 4.3 Oxygen content: Oxygen in semi-water gas is a highly harmful gas. It enters the conversion section where it undergoes redox reactions with the catalyst; two volumes of effective components are consumed for every volume of oxygen used, and a large amount of heat is released. If the conversion process is not carried out promptly, the catalyst can be damaged. To prevent an increase in the temperature of the catalyst layer, increasing the amount of steam used can lead to resulfidation of the low-conversion catalyst, thereby affecting its activity. Therefore, it is important to reduce the oxygen content in semi-water gas in order to ensure safe production and minimize consumption. The reaction of oxygen with the shift catalyst is as follows: As can be seen from the equation, one mole of oxygen consumes two moles of carbon monoxide, releasing heat equal to thirteen times that generated in the shift reaction. The 0.1% (on a dry basis) of oxygen present in semi-water gas, under normal steam conditions, generates enough heat in the shift reactor to raise the temperature of the gas by 6°C to 7°C; therefore, an excess of steam must be added to carry away this heat. It is calculated that for every 0.1% increase in the oxygen content in semi-water gas, about 100 kg more steam is required per ton of ammonia produced. If one ton of bituminous coal can generate seven tons of steam, then a plant that produces 100,000 tons of synthetic ammonia per year will consume an additional 100,000×0.1÷7 = 1,429 tons of coal per year. At a cost of 450 yuan per ton of coal, the annual loss in revenue amounts to 1,429×450 = 642,800 yuan per year. The theoretical calculation of the oxygen content in semi-water gas and the amount of useful component carbon monoxide consumed during the conversion process: The amount of useful component (CO) consumed per ton of ammonia is: 3200×0.001×2=6.4 Nm3 per ton of ammonia. 6.4×100000=640000 Nm3 per year. If 2000 Nm3 of useful component are consumed per ton of ammonia, then the amount of ammonia that can be produced annually is: 640000/2000=320 tons. In the above formula, 3200 Nm3 represents the volume of semi-water gas consumed per ton of ammonia, in Nm3 ； 2000 Nm3 is the amount of hydrogen required per ton of ammonia, in Nm3. One volume of carbon monoxide can produce the same volume of hydrogen in the conversion process. Furthermore, when the oxygen content in semi-water gas reaches a certain level, an explosive gas is formed, posing a direct threat to safe production. Therefore, taking effective measures to strive to reduce the oxygen content in semi-water gas is an important task in gas production. Measures to control and reduce the oxygen content in semi-water gas: Maintain an appropriate distribution in the gasification layer to ensure uniform gasification, and avoid phenomena such as the overturning of the carbon layer or wind tunnels ； Ensure that the safety interlock between the downstream gas valve and the blowing valve is in good working condition and sensitive, and maintain the downstream gas valve to be airtight with no leaks ； The top-blown nitrogen addition valve uses a dual-valve design to ensure no leakage when closed ； Reduce the \"dead zone\" at the bottom of the furnace (the distance between the downward valve and the blowing valve from the furnace bottom), and adopt steam cleaning measures to eliminate the air accumulated in this dead zone, thereby preventing air from being pushed into the gas holder during downward blowing ； Ensure that the safety interlock between the blowout valve and the safety baffle is sensitive and functional ； For manufacturers that use bottom-blown nitrogen injection, it is essential to ensure that the bottom-blown nitrogen injection valve does not leak ； Moreover, there is sufficient steam purging time after the nitrogen injection from below is completed. 4.4 Methane content: Methane is an inert gas in ammonia synthesis; it does not poison the catalyst nor participate in the synthesis reaction, but it is difficult to remove. The gradual accumulation of methane in the synthesis system reduces the partial pressures of hydrogen and nitrogen, thereby decreasing the ammonia synthesis rate. Additionally, since it does not participate in the reactions, it carries away heat from within the tower as it passes through it, causing the catalyst temperature to drop; this also results in the compressor and circulator having to perform unnecessary work. To achieve system equilibrium, forced venting is carried out, resulting in the simultaneous loss of some hydrogen and nitrogen gases. The amount of gas released during the production of one ton of ammonia cycle gas can be calculated using the following formula: V_release = V_replacement × ι_replacement ÷ ι_release. Here, V_release represents the volume of gas released per ton of ammonia, in Nm3 ； V supplement – volume of the ammonia-tonnage supplement mixture, Nm3 ； Addition — Addition of inert gas content in the gas, % ； ίVenting – percentage of inert gas content in the atmosphere. In fact, since a portion of the inert gas is dissolved in liquid ammonia (about one-third), the actual volume of gas released is lower than the value calculated using the formula above. As can be seen from the above equation, when the make-up gas volume is fixed, the amount of recycle gas vented per ton of ammonia is proportional to the content of inert gases in the make-up gas. The inert gas added to the gas stream comes from semi-water gas; assuming that the inert gas content in the recycled gas remains at 15%, the volume of gas required per ton of ammonia is 2900 Nm3. When the inert gas content in the added gas is 1%, the volume of gas released per ton of ammonia is: 2900×1%÷15% = 193.3 Nm3. If the content of inert gas in the makeup air is increased to 1.5%, then the vent volume will be: 2900×1.5%÷15% = 290 Nm3. That is, 96.7 Nm3 of recycled gas can be released per ton of ammonia produced. It is evident that it is very meaningful to strive to reduce the methane content in semi-water gas (the inert gas components in the recycle gas are mainly methane and argon). The methane content in semi-water gas mainly depends on the volatility of the fuel, with the furnace temperature being a secondary factor. The methane generation reaction is as follows: Methane formation is an exothermic reaction that results in a decrease in volume; reducing pressure and increasing temperature helps to suppress the formation of methane. 4.5 Air humidity The atmosphere is a mixture of dry air and water vapor, and this mixture is known as moist air. The moisture content in humid air has a certain impact on the operation and consumption of the gas generation process. In moist air, the amount of water vapor contained per unit mass of dry air is called the moisture content or absolute humidity, commonly referred to simply as humidity. At a constant total pressure, the ratio of the water vapor partial pressure in air to the saturated vapor pressure of water at the same temperature is called relative humidity. Relative humidity indicates the degree of unsaturation of the air. The humidity of the air is generally within the range of 30% to 80%, varying depending on the region, temperature, season, and weather conditions. Example 1: In summer, the temperature is 30°C. According to the table, when the humidity of the air is 80%, the water vapor content per Nm3 of moist air is 0.02432 kg ； Assuming that 2300 Nm3 of air is required per ton of ammonia to be blown through, the amount of water vapor introduced is: 2300×0.02432÷18 = 3.11×103 mol. The reaction between steam and carbon in the gasification layer is as follows: Therefore, the heat absorbed by the introduced water vapor is: 3.11×103×131.3 = 408.343×103 KJ. Assuming a lower heating value of standard coal of 29.19×103 KJ/kg, the additional amount of coal required is: 408.3/29.19×84/72 = 16.32. Here, 72 represents the fixed carbon content of the actual fuel ； 84 represents the fixed carbon of standard coal. When the blowing efficiency is 55%, the actual additional amount of coal required is: 16.32/0.55 = 29.67. Example 2: In winter, the temperature is 0°C and the air humidity is 50%; according to tables, the water vapor content in humid air is 0.0049 kg/m3. Therefore, 2300 Nm3 × 0.0049 × 50% = 5.6. It can be estimated that at 0°C in winter with a humidity of 50%, the amount of water vapor present is approximately 10% of that in summer at 30°C with a humidity of 80%. In a plant that produces 300 tons of ammonia, the high temperatures and high humidity in summer result in a consumption of about 5 additional tons of coal per day compared to when the air temperatures and humidity are lower in winter. 5 The relationship between the gas furnace load (the percentage of air supply) and the fuel coal and steam consumption. The gas furnace load refers to the production capacity of the gas furnace (Nm3/h or tons of ammonia per furnace per day). Due to the differences in furnace types, the load level is often determined by the gasification intensity (i.e., the output per unit area in Nm3/m2h). During normal operation, when the blowing rate remains constant, the load of the gas stove is generally adjusted by extending or shortening the blowing time. Appropriately increasing the gasification intensity and raising the production load per furnace in order to reduce the number of gasifiers in operation and improve the overall heat utilization rate in the gas production process is the goal that gas production workers are striving to achieve. However, choosing an excessively high single furnace load and a prolonged blowing time is, as demonstrated by both theoretical analysis and practical experience, not conducive to reducing the amount of raw coal and steam used. Let: The air blower’s airflow be 24,000 Nm3/h ； The cycle time is 150s ； 2000 kg of steam per ton of ammonia consumed ； 3200 Nm3 of semi-water gas is consumed per ton of ammonia ； The volume ratio of the blowing air to semi-water gas is 0.65:1 ； The porosity of the fuel layer is 0.3 ； The temperature in the gasification zone is on average 1100°C, with the specific density of steam taken as 0.25 kg/m3 ； 2.6—F2600 type gas stove ； 23—(24-1), considering that one cycle of slag removal and cleaning equipment must be available per hour. Based on the above conditions, the following two operation schemes are compared: Blowing – Upward blowing, downward blowing, double upward blowing, and cleaning. Scheme (1): 100%, 25, 24, 39, 8, 4; time: 150 seconds; values: 37.5, 36, 58.5, 12, 6. Scheme (2): 100%, 20, 26.5, 41.5, 8, 4; time: 150 seconds; values: 30, 40, 62, 12, 6. The steam required for semi-water gas production is: 2000 kg/3200 Nm3 = 0.625 kg/Nm3. The volume of air used for blowing is: 24000 Nm3/3600 seconds = 6.67 Nm3/second. The amount of steam needed per unit volume of air is: 0.625/0.65 = 0.962 kg/m3. The total amount of steam required for upward blowing + downward blowing + double upward blowing is: For Scheme (1): 37.5 × 6.66 × 0.962 = 240.5 kg/cycle; for Scheme (2): 30 × 6.66 × 0.962 = 192 kg/cycle. Gas production volume: For Scheme (1): 37.5 × 6.66 ÷ 0.65 × 23 = 8845 Nm3/h; for Scheme (2): 30 × 6.66 ÷ 0.65 × 23 = 7076 Nm3/h. Comparison of steam flow rates during gas production: For Scheme (1), the time per cycle for gas production is: 36 + 58.5 + 12 = 106.5 seconds. The steam flow rate in the vaporization layer during this period is: ÷ (1.32 × 3.14 × 0.3) = 5.66 m/s. For Scheme (2), the time per cycle for gas production is: 40 + 62 + 12 = 114 seconds. The steam flow rate in the vaporization layer during this period is: ÷ (1.32 × 3.14 × 0.3) = 4.23 m/s. Rough calculations show that Scheme (1) has the advantage of a higher vaporization intensity, but its disadvantage is that the fast steam flow rates in the upper and lower areas of the vaporization layer result in a lower rate of steam decomposition. With a constant average temperature, this leads to increased steam consumption. Unreacted water vapor carries away heat from the vaporization layer, reducing the effective utilization of heat there and thus increasing the consumption of fuel coal and steam. With a constant air-blowing rate for the gas stove, the fluctuations in furnace temperature depending on the length of the air-blowing time are shown in Figure 2. Note: Figure 2 illustrates the comparison of furnace temperature fluctuations when the cycle time is set at 150 s, with the air-blowing times being 40 s and 30 s respectively. In summary, selecting an appropriate operating load for the gas stove is one of the important measures to reduce fuel coal and steam consumption. Practice has shown that when the blower is reasonably matched with other gasification conditions, it is more economical to keep the blowing percentage at around 20%. 6 Relationship between cycle time and coal consumption The so-called cycle time refers to the total time of all stages in one cycle. The range of temperature fluctuations within the gas stove over one cycle varies depending on the length of the circulation time. For ease of comparison, the 180s and 120s cycle patterns are compared, as shown in Figure 3: With a constant load on the gas furnace, a shorter cycle results in less fluctuation in furnace temperature, which contributes to the stability of the gas furnace. This is one of the theoretical bases for the \"three highs – short cycle\" operating method that has been advocated over the years. However, the short cycle is merely a relative concept; choosing a cycle time that is too short also has some disadvantages. Below is a brief analysis of cycle times of 120 seconds and 150 seconds: In recent years, the diameter of the gas furnaces used in small nitrogen fertilizer production facilities has gradually increased from Ф1980mm to Ф2260mm, Ф2400mm, Ф2600mm, Ф2650mm, and Ф2800mm. To meet the requirements of a high height-to-diameter ratio, the height of the furnace body has been gradually increased from 4025 mm to 4500 mm–5200 mm, or even higher. The inner diameter of the upward pipeline has also been increased from Ф500mm to 700mm–900mm. The downcomer pipe is enlarged from Ф400mm to 600mm–700mm, and all other auxiliary equipment is correspondingly thickened and enlarged. This helps to reduce system resistance and increase gasification efficiency; however, it also has certain drawbacks. One of these is the dead zone at the top of the furnace, namely the space between the surface of the carbon layer on the furnace and the top cover of the furnace, which is filled with steam when gas is blown downward in this area. The space between the upward pipeline and the upward valve also serves as a reservoir for the steam blown downward (as the pressure at the top of the furnace is higher during downward blowing than during upward blowing). When switching to secondary upward blowing, this steam is forced into the gas scrubber where it is condensed and wasted. After the bottom pipe is thickened, the \"dead zone\" also increases accordingly; the steam remaining during the top-blowing phase is forced into the gas scrubber during bottom-blowing, thereby increasing the thermal load on the gas scrubber and leading to steam waste. Secondly, the expanded downward pipeline leads to the space of the downward valve pipeline equipment (the downward dust collector), which increases the amount of air retained during the blowing and upward nitrogen injection phases. If the time required to purge the steam from the furnace after upward nitrogen injection is too short, or if the location where the upward steam pipeline enters the furnace is not chosen appropriately, it becomes impossible to effectively remove the air remaining in the space below the furnace. When the process moves to the downward blowing phase, this air is forced into the gas holder, resulting in a higher oxygen content in the semi-water gas. This, in turn, leads to losses of useful components in the conversion section as well as an increase in steam consumption. When the cycle time is 150 s, the gas stove operates 24 cycles per hour ； At a cycle time of 120 seconds, the gas stove operates 30 cycles per hour. Compared to 150s, 120s has 6 more cycles per hour, resulting in an increase in steam waste from the upper and lower dead zone volumes across those 6 cycles, as well as losses due to the increased oxygen content in the gas. Furthermore, in a system using a combined waste heat boiler, it takes some time (usually 3–4 seconds) for the main gas valve and the blast air recovery valve (or chimney valve) to be switched into position. Since the pressure outside the main gas valve is higher than the pressure at the blast air recovery valve, the useful gas can flow back into the blast air recovery system or be released through the chimney valve during this switching process (the amount of waste depends on the speed of valve operation and the pressure difference), which increases the likelihood of six instances of gas waste. The third drawback is that the number of times the hydraulic valve opens and closes increases by six, reducing its service life. At the same time, due to pressure differences at various stages, it takes some time to reach the appropriate operating pressure during the conversion process, which increases the idling time of the gas stove. It is therefore believed that while a short cycle time has the advantage of minimal fluctuations in furnace temperature, the waste resulting from it, as mentioned above, cannot be ignored. In short, after enlarging the diameter and height of the gas stove as well as thickening the pipeline equipment, it is not advisable to choose a too short circulation time. A cycle time of 150 s is appropriate for high-quality fuel, while it can be appropriately shortened for lower-quality materials. In summary, there are many factors that influence and a wide range of aspects involved in reducing the consumption of coal, steam, and electricity as raw materials for gas production; in practical manufacturing, these issues need to be addressed through the following approaches. (1) Select a scientific and reasonable process flow and equipment for design. (2) Explore the development of gas stove operation procedures and gasification conditions suitable for the characteristics of this enterprise. (3) Establish a strict quality management system for raw materials entering the furnace. (4) Take effective measures to minimize the heat loss (latent and sensible heat) carried away by the blowing air. (5) Control an appropriate thickness of the ash layer, striving to increase the slagging rate of the ash and reduce its carbon content. (6) Strive to minimize the carryout of gases in the upward and downward flows. (7) Select an appropriate carbon layer height, strive to reduce the temperatures of the gas flowing upward and downward, minimize heat loss, and improve the effective utilization rate of carbon. (8) Select appropriate cycle time, gasification layer temperature, and steam input volume to achieve a more economical steam decomposition rate. (9) Improve the quality of semi-water gas and reduce the consumption of semi-water gas per ton of ammonia produced. (10) Adopt new processes and equipment, continuously optimize process conditions and carry out technical upgrades of equipment to extend its service life. (11) Strengthen employee training to continuously improve the professional competence of gas production staff. (12) Establish a scientific and reasonable management and evaluation system for gas production to fully motivate employees and ensure the safe, stable, and economical operation of the gas furnaces.