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Research on the technical renovation of intermittent coal gasification in Chinese nitrogen plants: The processes and equipment used for coal gasification in China’s nitrogen production enterprises mostly follow those introduced from the United States in the 1930s and from the former Soviet Union in the 1960s. Each cycle in water gas production is divided into six processes: air supply, steam purging, top blowing, bottom blowing, secondary top blowing, and steam purging. The blowing air passes through the dust collector at the outlet of the generator, where some of the dust in the gas is removed, before entering the combustion chamber where secondary air is added for combustion. The combustion gases then proceed to the waste heat boiler via an upward pipe; their temperature drops to around 250°C, after which they are sent to the chimney and discharged after further dust removal by a dust collector. The upward-flowing gas passes through the dust collector at the generator outlet, where some dust is removed, before entering the combustion chamber. It then goes through the rising pipe into the waste heat boiler, where its temperature drops to around 250°C. From the lower part of the waste heat boiler, it enters the scrubber via a three-way gas valve; by this point its temperature has dropped to around 40°C, and it then enters the gas holder. The downstream gas enters the scrubber through the three-way gas valve, and after its temperature drops to around 40°C, it enters the gas inlet tank. This process has many shortcomings, resulting in high energy consumption for gas production in medium-nitrogen enterprises. Over the past 30 years, many companies have carried out extensive technical upgrades to the equipment used in the gas generation process. Such as furnace diameter expansion upgrades, gas generator jacket modifications, grate replacements, combustion chamber expansion and heat storage capacity enhancements, scrubber upgrades, as well as pipeline and valve modifications. At the same time, optimized operation methods were also proposed. These efforts have accumulated rich experience in technical upgrades for the medium-nitrogen gas generation process, and have also made significant contributions to the stable operation of the equipment as well as to energy savings and consumption reduction. However, at present, the gas production energy consumption of medium-sized nitrogen enterprises is still 10% to 20% higher compared to that of advanced small nitrogen enterprises. The reason for this is, firstly, the design of the combustion chamber per furnace is unreasonable; since there is no combustion assistance system in the chamber, it is required that the safe combustion temperature of the blowing air be no less than 650°C, otherwise it cannot operate properly. In most medium-nitrogen enterprises, due to the differences in the raw materials used for gas production, it is not possible to maintain the temperature of the blowing gas above 650°C, resulting in unstable operation and high costs. The second is the gas sensible heat recovery system; due to the high design pressure of the waste heat boiler, the sensible heat in the gas cannot be fully recovered, resulting in energy waste and increased water consumption by the scrubber. The Handan Steel Fertilizer Plant, Jinan Fertilizer Plant, and Lunan Fertilizer Plant have changed the original process of using one combustion chamber per furnace for burning the blowing gas, to a new process in which multiple furnaces share one combustion chamber for centralized burning of the blowing gas. The energy consumption for gas production in the modified process has been reduced by more than 10%. Here, we introduce the concentrated waste heat recovery of the blowing gas and the sensible heat recovery of the upward and downward gas streams. 1. Blowing gas centralized waste heat recovery process: The blowing gas centralized waste heat recovery method involves feeding the blowing gas from two or more gas generation furnaces into a single combustion furnace, where it burns safely and stably with the assistance of the gas from the flare system. When designing centralized waste heat recovery systems for blast air in medium-nitrogen enterprises, the following principles should be followed: ① Ensure safe and stable operation, that is, the system should be able to operate safely and stably even when the raw materials used for gas production or the operating parameters are changed ； ②Ensure the steam generated is utilized in a rational and effective manner ； ③The heat generated by combustion is fully recovered ； ④Long system operational life ； ⑤Environmental protection principles ； ⑥Principle of economy. The main equipment for the hot air blowing concentrated waste heat recovery process is as follows. 1.1 Combustion Furnace: The combustion furnace must be capable of maintaining a temperature that allows for the combustion of flammable gases at low concentrations, and it must have proper air supply; otherwise, it cannot operate stably and explosions of varying severity may occur. It is also required to achieve its own thermal equilibrium, have low heat loss, a high heat storage capacity, as well as sufficient combustion space and residence time. There are currently two types of combustion furnaces that operate successfully; one is the top-firing regenerative combustion furnace, in which the number of layers and the quantity of grid bricks play an important role in its stable operation. Another type is the medium-combustion furnace, which eliminates all grid bricks and adopts zoned combustion, namely: a mixed combustion zone, a high-temperature combustion zone, and a coal powder combustion zone. The cyclone-type coal powder combustion zone located at the lower part of the furnace allows over 90% of the coal powder, tar, volatiles, etc., carried by the blast air to participate in combustion, thereby creating a \"reverse temperature gradient\" – that is, the temperature at the furnace outlet is higher than that at the upper part. 1.2 High-temperature Air Preheater: The installation of a high-temperature air preheater enhances the adaptability of the blowair recovery system for gas production using different feedstocks. Air preheaters need to be heat-resistant, with sufficient thermal expansion taken into account. The GWKY type high-temperature air preheater uses corrugated stainless steel seamless tubes; the corrugations disrupt the surface boundary layer during heat transfer, resulting in advantages such as a high heat transfer coefficient and excellent surface condensation performance. This technology can increase the service life of the equipment by 3 to 5 times compared to ordinary stainless steel heat exchangers. Expansion joints are installed on both sides of the equipment, which prevents damage to it caused by thermal expansion and contraction. 1.3 Boilers: In nitrogen-related enterprises, the blowair systems use medium-pressure waste heat boilers; the steam generated by these boilers is fed into the steam network for power generation. 1.4 Soft water heater and first air preheater: Both of these devices are low-temperature preheating and recovery units; when choosing them, the main consideration is low-temperature corrosion resistance. Heat pipe heat exchange equipment is generally chosen; firstly, by taking advantage of the heat distribution characteristics of heat pipes, the wall temperature is designed to be above the dew point, thereby preventing dew point corrosion ； Secondly, in heat pipe heat exchange, both fluids flow outside the tube; the gas side can be finned to enhance heat transfer. 2. Upstream and downstream gas sensible heat recovery processes: At present, the waste heat boilers used in the sensible heat recovery processes of gas generation furnaces with a diameter of over ￠3000 mm for medium-nitrogen fertilizers are mostly Semir-Solvay-type waste heat boilers introduced from the United States. Depending on the diameter of the combustion furnace, the heat exchange area of these waste heat boilers ranges from 480, 588, 600, 675 to 900 m2. The operating pressure of this waste heat boiler is generally 0.8–2.5 MPa, with a designed gas outlet temperature of 250°C. The tube bundle of this waste heat boiler is connected together by tube sheets fixed at both ends; during operation, the process gas flows through the tube side, while water and steam flow through the shell side. This device has the following issues in practical applications. (1) Since the process gas contains a large amount of dust, and the gas flow area in the waste heat boiler is small with a high gas flow velocity, the erosive kinetic energy of the dust is high; thus, erosion wear is one of the main causes of equipment damage and leakage. (2) This waste heat boiler has a tube-type structure, with the gas and water separated by a layer of tube walls. The wall temperature is close to the temperature of water, and it is relatively low. When the tube wall temperature is below the dew point, sulfides in the gas cause severe corrosion of the tubes. To avoid dew point corrosion, the operating pressure of the waste heat boiler had to be increased. The waste heat boiler operates at high pressure, with high temperatures of the water and gas in the shell side, which results in a high outlet temperature of the process gas and a high temperature upon entry into the gas washing tower. On the one hand, the waste heat cannot be properly recovered; on the other hand, this results in higher water and electricity consumption for gas washing. Some manufacturers have now modified their original single-furnace combustion process using blast air to a process with centralized heat recovery for that blast air. For sensible heat recovery, the original waste heat boiler is still used; this boiler handles only the upward-flowing gas, with the inlet gas temperature at around 350°C and the outlet temperature at 180°C. This process has two drawbacks: first, the temperature of the gas exiting the waste heat boiler at the upper section is high, and the residual heat in the gas coming out at the lower section, around 260°C, is not recovered, resulting in energy waste and increased water consumption for gas washing ； Second, the waste heat boiler cannot operate stably over long periods of time. Practice has shown that heat-tube waste heat boilers, when used in this process system, feature stable operation and a long service life. The methods include a process in which one gas generation furnace is equipped with one heat pipe waste heat boiler, and multiple gas generation furnaces share one waste heat boiler. The process and equipment for multiple gas generation furnaces to share one waste boiler are described in detail below. 2.1 Process flow for multiple gas generation furnaces sharing one waste boiler The process flow is shown in Figure 1, and the flow of gases at each stage is as follows. http://www.zaoqiwang.com/upload/060524844555656.jpg Air blowing: Air enters from the bottom of the furnace, exits from the upper part of the gas generator and enters the dust collector, before being sent to the air blowing recovery system. Upward gas flow: It enters from the bottom of the furnace and exits from the top, passing successively through a dust collector, a safety water seal, and a waste heat boiler (where the temperature of the gas in the upper and lower sections is reduced to 140°C); it then goes through a gas washing tower to have its temperature lowered to 40°C before reaching the gas holder. Downstream gas: It enters from the upper part of the furnace and exits from the lower part, passing successively through a safety water seal, a waste heat boiler (where the temperature of the gas in the upper and lower sections is reduced to 140°C), and a gas scrubbing tower to be cooled to 40°C before reaching the gas holder. This heat pipe combined waste heat boiler is a key device in the system for waste heat recovery, achieving high efficiency and low consumption while ensuring the proper operation of the equipment. 2.2 Heat pipe combined waste heat boiler 2.2.1 Structure of the heat pipe combined waste heat boiler The RLG type heat pipe combined waste heat boiler is a combined device of gas-to-gas heat exchangers and gas-to-liquid heat exchangers. The upper section is a gas-steam type heat exchanger, namely a heat pipe steam superheater, while the lower section is a gas-liquid type heat exchanger, namely a heat pipe waste heat boiler. The two structures are basically identical, both consisting of an inner cylinder, an outer cylinder, and a heat pipe. The heat pipes are radially distributed on the inner tube, dividing it into two sections; the inserted section is a smooth pipe, while the section of the heat pipe outside the inner tube is equipped with fins. 2.2.2 Working principle of the RLG heat pipe combined with a waste heat boiler: The upward and downward flowing gas enters from the gas inlet at the top of the outer cylinder, passes through the space between the inner and outer cylinders, and exits from the gas outlet at the bottom of the outer cylinder. Steam enters from the bottom of the upper inner cylinder and exits from the steam outlet at the top of the upper inner cylinder. Soft water enters through the soft water inlet at the bottom of the lower inner cylinder, and exits through the soft water outlet at the upper part of the lower inner cylinder. Gas flows between the inner and outer sleeves, transferring heat to the heat pipes; these heat pipes then transfer the heat to the steam and soft water in the inner sleeve at an extremely high speed. 2.2.3 Characteristics of the RLG heat pipe combined waste heat boiler: (1) High heat exchange efficiency – The gas flowing upward and downward at temperatures of 300–400°C passes through the heat pipe combined waste heat boiler, resulting in its temperature being reduced to around 140°C. At the same time, the low-temperature saturated steam passing through the heat pipes in combination with the upper section of the waste heat boiler (heat pipe steam superheater) is superheated from 120°C to 200–220°C. (2) Low resistance and erosion resistance. The radial distribution of the heat pipe ensures a sufficiently large flow area; taking the RLG—1300 type heat pipe combined waste heat boiler as an example, its minimum flow cross-sectional area is 2 m2, which is equivalent to the flow area of a DN1600 pipe. The mass flow rate of gas is less than 3 kg/(m·s), resulting in an extremely low kinetic energy due to dust erosion. This solves the design challenge of dust erosion and wear-induced equipment damage. Due to the low velocity of the coal gas stream, the gas resistance of the equipment is very low; the flow resistance is usually no more than 200 Pa. (3) Corrosion-resistant and resistant to dust accumulation. When the lengths of the heating and condensing sections of a heat pipe are different, a certain amount of heat is transferred by the heat pipe; as a result, the heat flux densities on the surfaces of its heating and condensing sections vary. Applying this principle allows the heat pipe wall temperature to be adjusted so as to stay above the gas dew point. On the one hand, the heat pipe controls dew point corrosion ； On the other hand, the ash that settles on the surface of the heat pipe is dry ash; this dry ash remains on the surface of the heat pipe only temporarily and does not accumulate there for long, meaning that ash buildup is unlikely to occur. (4) Stable operation and long service life: With individual heat tubes in operation, if one or some of the heat tubes are damaged, the two fluids will not mix, and this will not affect the operation of the equipment. It features a long continuous operation cycle for the equipment. (5) Good pressure resistance: The device has a circular cross-section, which provides good pressure resistance; thus, even when the gas pressure changes sharply, it does not deform and this prevents any impact on the heat exchange efficiency. (6) Gas generation process suitable for various raw materials. It can be used for waste heat recovery in gas generation processes involving lump coal, coke, coal balls, coal rods, etc. 2.2.4 Economic benefits of enterprises adopting heat pipe combined with waste boilers for exhaust gas recovery The process using a combined heat pipe waste heat boiler enables the temperature of the upward-flowing gas to drop from around 350°C and that of the downward-flowing gas from around 260°C to 140°C. Compared to the original process (which only recovered the upward waste heat to 180°C), 150 kg more steam is produced per ton of ammonia, and 45 tons of water are saved in the scrubber. The direct economic benefit is 12.35 yuan. A company with an annual production capacity of 100 kt of synthetic ammonia generates direct economic benefits of 1.235 million yuan per year.