Thread Content
Technical renovation of φ2600 mm fixed-bed gasification process Author/Source: Zhang Xiaoxia (Shanxi Chemical Engineering Design Institute, Dayuan, Shanxi 030024) Date: 10-7-2008 Since the 1970s, coal gasification technology has developed rapidly. The gasification technologies used in nitrogen fertilizer companies mainly include four processes: atmospheric pressure fixed-bed batch gasifier (UGI), Texaco, End, and ash fusion. Among them, the UGI furnace is a type of furnace widely used in small and medium-sized nitrogen fertilizer plants in China, accounting for over 70% of the total ammonia synthesis capacity to date. The gas production section is the main unit for the synthesis of ammonia and methanol; therefore, reducing energy consumption in gas production and achieving clean conversion have always been key objectives in its energy-saving renovation efforts. In recent years, in order to improve the gasification efficiency of gasifiers and the gas output per unit reactor, as well as to achieve clean production, a series of targeted technical upgrades have been carried out in the design of gasification processes, with certain results achieved. 1 Process and flow configuration The atmospheric pressure fixed-bed batch gas production process mainly consists of 5 stages: purging stage, upper blowing gas production stage, lower blowing gas production stage, secondary upper blowing stage, and air purging stage. These 5 stages form a gas generation cycle, with each cycle lasting 120s to 150s. In practical operation, each enterprise determines the proportion of time allocated to each of the 5 stages in each cycle based on the properties of the raw coal, the equipment available, and its own operational experience. After years of improvement, the process configuration has been changed from one waste heat boiler and one gas washing tower per furnace to a system in which dust removal is carried out for each furnace, while waste heat recovery, cleaning, and cooling are handled centrally for multiple furnaces. Typically, 4 or 8 furnaces are used as one set in a waste heat recovery system; such a configuration enables the continuous operation of the equipment following the gasification furnace, while also saving on equipment investment and land use, reducing the amount of cooling water required, and allowing for a more compact layout of the equipment. The process flow includes the addition of top and bottom nitrogen injection techniques: injectors are used on the steam inlet pipes to introduce air into the gasification furnace via steam; the air pipeline is connected to the main air supply line, with a diameter of DN200, while the nozzle pipe has a diameter of DN80. Steam is ejected from the nozzle to form a high-speed jet, and the suction effect generated draws air into the jet and into the furnace as well. The use of top and bottom nitrogen injection techniques can shorten the blowing time and extend the gas production time. Moreover, the addition of air during the gas production phase helps to stabilize the temperature of the gasification layer, which is beneficial for increasing both the gas output and the quality of the gas. The accompanying blast gas recovery process involves the mixed combustion of blast gas, air, and combustion aid in a combustion furnace. The high-temperature flue gas generated as a result undergoes heat exchange with a waste heat boiler to produce superheated steam, which is then supplied to the chemical processing section. The residual heat from the flue gas is used to heat air and soft water, thereby enabling self-sufficiency in steam for the chemical processing section, saving energy and achieving the conversion of \"two types of coal into one type of coal\". At the same time, it reduces the emission of pollutants in the blast gas, resulting in improved environmental benefits. Compared to the original process, a high-temperature air preheater has been added; the air from the air preheater, which is currently at 180°C, is heated to around 450°C and then mixed with the blast air before entering the combustion furnace, thereby improving the combustion conditions within the furnace. A water mist dust removal device is installed before the release of the flue gas after combustion, to further reduce environmental pollution. 2 Technical Improvements 2.1 Equipment Improvements 2.1.1 Blowers To increase the gas production per unit of the gasification furnace, it is necessary to increase the air flow rate, reduce the blowing time, extend the gas production time, and minimize heat loss. In recent years, C700 blowers have been commonly used for gasification, with one blower serving four gasification furnaces, which provides ample flexibility to achieve a \"high-pressure short-duration blowing\" or \"low-pressure long-duration blowing\" approach in the process. Accordingly, the air inlet pipe was changed from φ600mm to φ700mm, and a hydraulic seat valve was used in place of the original hydraulic gate valve; when used together with a safety butterfly valve, this extended the operational life of the valves and reduced the need for maintenance. 2.1.2 Automatic coal feeder: By using an automatic coal feeder, coal can be added once per cycle without shutting down the furnace. The amount of coal added is small and its distribution is even, which overcomes the disadvantages associated with manual coal feeding – such as heat loss due to frequent opening of the furnace mouth, as well as fluctuations in furnace temperature caused by adding too much coal at once. This improves the steam decomposition rate, reduces the consumption of raw coal, extends the effective gas production time, and increases the gas output of the gasifier. Compared to manual coal loading, automatic coal loaders reduce the workload of operators, prevent coal ash pollution, and improve the working environment. 2.1.3 Cyclone dust collector: The dust collector was originally lined with fire-resistant cement, but this lining would often peel off over time, leading to increased equipment wear and heat loss. Replacing it with wear-resistant cast iron can significantly increase the service life and dust removal efficiency of the dust collector. Generally, primary dust removal equipment is used for the upward-flowing gas stream. However, to ensure the service life of the waste heat boiler and the gas scrubber and to improve heat transfer efficiency, it is recommended to add another stage of dust removal equipment before the waste heat boiler. This approach not only reduces the dust content in the cooling water used in the gas scrubber but also simplifies the treatment of circulating water. 2.1.4 Waste Heat Recuperators: Waste heat recuperators have evolved from the traditional water-tube or fire-tube boilers to the current heat-pipe boilers, resulting in a significant improvement in their heat exchange efficiency and service life. A heat pipe is an efficient heat transfer element that utilizes the heat absorption during boiling and heat release during condensation of the fluid inside a sealed tube to conduct heat. Heat exchangers composed of finned heat pipes feature high heat transfer efficiency, a compact structure, and low fluid resistance; the normal operation of the equipment is not affected even if one or a few heat pipes are damaged. Furthermore, the wall temperature can also be controlled by adjusting the heat exchange areas at the hot and cold ends, thereby preventing dew point corrosion. 2.1.5 Scrubber: Instead of using spray nozzles throughout the tower, the upper part now features packing while the lower part uses spray nozzles for spraying. Although the gas resistance increases compared to the spray-nozzle-only design, the use of packing significantly improves heat and mass transfer efficiency, allowing the temperature of the gas exiting the tower to be maintained at around 40°C. Ceramic structured packing is commonly used as the filler, but due to its drawbacks of being prone to clogging and breaking, some manufacturers have begun to use metal grid packing instead. The mass and heat transfer efficiency of metal grid fillers is superior to that of ceramic fillers, which allows for **reducing the diameter of the scrubber tower, saving on equipment costs, and minimizing the required floor space. 2.1.6 Combustion Furnace The combustion furnace features a conical roof design; this design increases the combustion space and residence time by more than 20% compared to furnaces with a flat roof and the same diameter and cylinder height, and it also results in a load that is more than 20% higher. Installing a lining in the combustion furnace not only ensures the long-term safe operation of the equipment, but also increases the temperature inside the combustion furnace as well as the temperature of the flue gases exiting it. This design incorporates pre-arranged expansion joints in the lining to prevent cracks in the lining during operation ； The expansion joints are filled with aluminum silicate fiber material, preventing heat from escaping from the joints to the outside of the furnace, thereby improving thermal insulation and safety. By adopting a new arrangement of regenerative bricks, it is possible to ensure that the combustible gas and the oxidizing gas (air) come into contact with each other repeatedly, thereby facilitating ignition and complete combustion, eliminating ash accumulation that could cause blockages, and achieving efficient operation over extended periods of time. 2.2 Improvements in pipeline configuration and control technologies 1) The upward gas pipelines are lined with refractory bricks or heat-resistant cast iron, which reduces the erosion of the pipe walls by the upward flowing gas (blowing air), and also minimizes heat loss. 2) The temperature of the blast air pipeline is around 450°C, while the temperature of the semi-coke gas pipeline before it enters the waste heat recovery unit is around 300°C. Therefore, taking into account the thermal expansion of the pipelines, expansion joints should be installed at appropriate locations after the piping is completed, as determined through calculations. 3) Increase the steam decomposition rate. Currently, most gasification units use superheated steam for gas production; the temperature of the superheated steam entering the furnace is around 220°C, with a pressure of approximately 0.1 MPa. The steam inlet pipes have been changed from φ219mm to φ273mm to ensure an adequate flow of steam. One φ219mm pipe is installed on each of the upper and lower steam injection pipes, which are arranged around the upper and lower rings of the furnace body; additionally, 4 DN100 pipes are used to introduce steam radially into the furnace body, thereby enabling a more even distribution of steam within it. 3 Optimizing operating conditions: In addition to process modifications, optimizing operating conditions and improving furnace management are also important factors for saving energy and reducing consumption in the gas processing section. Currently, gasification production control typically makes use of DCS control systems. Based on program control for individual furnaces while taking into account the operating conditions of each furnace, new technologies such as comprehensive optimization control systems for gasification production and systems for monitoring and optimizing furnace conditions to automate coke addition are employed. Optimization measures such as automatic adjustment of steam input and the hydrogen-to-nitrogen ratio are also used to achieve stable and high production levels in the gasification furnaces, along with reduced energy consumption. The instrumentation on the pipelines is optimized, and the status of gasification production is monitored by tracking parameters such as temperature, pressure, and flow rate. The composition of the blowing gas and semi-water gas is analyzed every half hour in order to control the operation of the gasification furnace and the waste heat recovery unit, allowing for the timely identification and resolution of any issues. 4 Conclusion: Over the past 40 years, the fixed-bed batch gas production process has continuously worked to overcome its own shortcomings and has undertaken various technical improvements, achieving good results to date. In terms of energy conservation, process improvements have increased the carbon conversion rate, enabling steam self-sufficiency ； In terms of environmental protection, the blowing air is no longer discharged directly; instead, its latent heat is recovered, and the resulting steam, coal ash, and slag are fed into a fluidized bed boiler for combustion. The water used in the gas generation process is circulated in a closed loop, thereby achieving zero emissions.