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Summary of Energy-Saving Technical Upgrades to the Gas Generation System in Our Company / Author/Source: Lu Ping, Meng Qingshun (Jiangsu Hengsheng Fertilizer Co., Ltd., Xinyi 221400) Date: 12-25-2007 1. Upgrades to the gas generation system: Jiangsu Hengsheng Fertilizer Co., Ltd. has 21 fixed-bed gas generation furnaces in its system; 8 of these were upgraded in 2005, and the remaining 13 furnaces were phased out in June 2007. 1.1 Main Equipment (1) Gas generation furnace: A φ2800 mm gas generation furnace is used; it has a jacketed design, and the slag is removed through mechanical drive of the furnace grates. (2) Wash tower: φ5800 mm, total height 23610 mm, equipped with ceramic ring packing; the packing is installed in two sections, each section being 4500 mm high. (3) Air blower: A C600-1.30 type high-pressure, high-efficiency blower is selected, with an air volume of 36,000 m3/h, an air pressure of 30 kPa, a motor power of 440 kW, and a voltage of 6 kV. (4) A heat pipe waste heat boiler (φ2600 mm, H=15100 mm) is used to recover the sensible heat of water gas, producing low-pressure steam at 0.3 MPa as a by-product. (5) Water curtain dust collector: φ1200 mm/φ2400 mm, H=35552 mm. (6) Cyclone dust collector: φ1800 mm, H＝9250 mm. 1.2 Energy-saving modifications (1) Increase furnace diameter: all changed to φ2800 mm. During the renovation, the upward outlet pipes of each furnace were enlarged to DN700 mm and extended to the top surface of the third-floor floor slab, thereby increasing the available space. This facilitates operations with thicker carbon layers and reduces gas production resistance, resulting in an **increase in the gas output per furnace**. (2) Selection of the furnace bottom: After the expansion of the gas generation furnace’s furnace chamber, the amount of coal added to the furnace increases, which raises the operating resistance at the furnace bottom. To ensure normal operation, the furnace bottom mechanism has been changed to a rolling chassis, which results in a lower friction coefficient during transmission and reduces the operational load on the furnace rod machine. As a result, the continuous operation cycle of the furnace bottom is nearly doubled, creating favorable conditions for stable and high-volume gas production. (3) Adoption of new grates: All grates are replaced with hexagonal grates. This grate features even air distribution, a large ventilation area, enhanced slag discharge and removal capabilities (the discharged ash and slag are all below 150 mm), and complete combustion (with a 5% reduction in residual carbon content). (4) Modification of the blowing system: The optimal operating condition for the gas generator is to raise the furnace temperature in a short time, thereby storing sufficient heat for gas production. This requires the fans to have high wind speeds and large air volumes per unit of time; therefore, all fans were replaced with C600-1.30 type fans, which also enabled one fan to serve four gas production furnaces. To further increase the blowing intensity inside the furnace, all the air inlet pipes and valves were replaced from DN500 mm to DN600 mm, reducing the resistance from nearly 2750 Pa to 1050 Pa. (5) Reform of the gas generation system: Superheated steam is used for gas production, and measures such as simplifying the process flow, enlarging pipe diameters, and reducing elbows have been taken. Furthermore, reducing the water seal height of the gas scrubber to around 80 mm **decreased** the gas production resistance, thereby increasing the gas output per furnace in the gas generator. (6) Use of an automatic carbon addition system: Under the same operating conditions, instead of shutting down the furnace every 30–40 minutes to add carbon once (with a shutdown time of 1.5 minutes per instance, resulting in 40 shutdowns per day), carbon is added every 2 minutes without shutting down the furnace. This saves 60 minutes per day in carbon addition time, and it also helps maintain a stable and higher average temperature in the gas generator, thereby increasing the gas production per furnace. (7) Reform of the gas generator oil pressure system: Replace the oil pressure pump with a capacity of 50 L/min with one having a capacity of 80 L/min ； The valve station manifold with a φ10 mm diameter and the solenoid valves have been replaced with ones having a φ12 mm diameter ； Valves such as the blow-off valve, downflow valve, and upper nitrogen injection valve are controlled by dual solenoid valves. This **accelerates the opening and closing speed of each hydraulic valve; even for the large DN600 mm valves, the time required for opening and closing is less than 3.5 seconds. As a result, the percentage of time spent on opening and closing the valves during a cycle is reduced, thereby increasing the time available for gas production. 2 Eliminate small, scattered boilers and achieve combined heat and power generation. The original boilers were of the SHFC15-2.45/400 and SHC20-2.45/400 types, with a combustion efficiency of only 50%–60%, resulting in energy waste. Therefore, 3 ultra-high pressure circulating fluidized bed (CFB) boilers with a capacity of 75 t/h each were selected, along with 2 extraction steam turbines of 15 MW each, and 2 generator sets of 18 MW each. Approximately 100 kt/a of anthracite powder cannot be utilized by the gas generator in the 300 kt/a ammonia synthesis system, and the amount of slag and fly ash emitted by the gas generator is about 134.8 kt/a; these two types of waste together total around 250 kt/a. Therefore, the slag-firing CFB boiler technology promoted by the China Fertilizer Industry Association is adopted, using the already discarded gas generation furnace slag and anthracite powder as fuel for the CFB boiler. Due to the intense turbulent mixing in the bed and the cyclic combustion of the fuel, CFB boilers increase the fuel combustion time; their combustion efficiency can reach 97.08%, while the thermal efficiency of the boilers is over 90%. This results in a savings of nearly 100 kt of coal per year, with direct economic benefits in terms of energy savings amounting to 26 million yuan, as well as a reduction in SO2 emissions. 3 A 50 t/h waste gas mixed combustion furnace is used; this furnace of type waste gas fluidized mixed combustion for gas production enables the blowing air generated during the gas production process, gas production slag, and fine dust from dust collectors to be combined with some pulverized coal and coal gangue, as well as synthetic purge gas, and to undergo fluidized combustion within this furnace, thereby producing high-temperature steam. Part of the generated medium-temperature and medium-pressure steam is used to drive the compressor via backpressure; the low-pressure steam resulting from this process is utilized for gas production and methanol distillation. A small amount of medium-temperature and medium-pressure steam, after being cooled and depressurized, is used for shift reaction and other processes, thereby enabling multiple uses from a single furnace. The process flow is shown in Figure 1. file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image1.jpg 3.1 Main configuration parameters (1) Materials for combustion: gas generation purge air, gas generation slag, gas generation fine ash, high-sulfur coal, coal gangue, wet carbon removal flash vapor, copper washing regenerated gas, syngas (not necessarily required), and reverse exhaust gas from pressure swing adsorption, etc. (2) Mixed waste combustion furnace: φ8528 mm×20000 mm. (3) Modular dust collector: Dimensions are φ6520 mm×19800 mm. (4) 1 set of tunnel kiln-type waste heat boiler: Q=50 t/h, P=3.82 MPa, T=450 ℃ (including steam superheater, boiler, economizer, air preheater, and furnace cooler). (5) Key parameters: combustion temperature of 950–1050 °C, flue gas temperature ≤150 °C, cross-sectional flow velocity in the mixed combustion furnace ≤4 m/s, system resistance ≤2500 Pa, and furnace pressure of –10 to –50 Pa. 3.2 Energy-saving effects: (1) With the use of this device, the blast air that used to be discharged into the atmosphere and the air released throughout the company is thoroughly recycled, turning waste into resources; this reduces pollution of the atmosphere and helps protect the environment. (2) In the process of recovering gas-making blowing air using a three-waste fluidized mixed combustion furnace, coal is used as the ignition source, allowing for less or no use of synthetic ignition gas; thus, the recovery of gas-making blowing air is not affected by synthetic factors, which saves hydrogen and semi-water gas and enables an increase in ammonia production by 3% to 5%. (3) By using the three-waste fluidized mixed combustion furnace, the company can achieve self-sufficiency in steam, thus realizing the goals of replacing two furnaces with one for urea production and replacing two types of coal with one. (4) It solved the problem of comprehensive treatment of waste gas, waste residues, and coal ash generated in gas production, thus protecting the environment. (5) Multiple uses per furnace: by shutting down boilers with high energy consumption, energy is saved, efficiency is improved, and investment costs are reduced by 50%. (6) Under the same operating conditions, using a turbine to drive the compressor can improve energy efficiency by about 10% compared to using electric power for driving it. 4 A production line for autoclaved fly ash bricks was established. Gas generators, thermoelectric circulating fluidized bed boilers, and waste fluidized mixed combustion furnaces generate large amounts of waste such as coal sludge, coal ash, and slag; therefore, in June 2007, an investment of 12 million yuan was made to set up a production line capable of manufacturing 50 million bricks per year. As a substitute for clay bricks, autoclaved fly ash bricks are in line with current **industrial policies; they save land resources and enable the reuse of waste, resulting in significant economic and social benefits. An annual net profit of 3 million yuan can be achieved. The production process flow of fly ash bricks is shown in Figure 2. file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image2.jpg The ash produced by thermoelectric circulating fluidized bed boilers has strong self-hardening properties, and can be used for manufacturing bricks, road filling, and as a cement admixture. The company uses all the ash it generates as raw material for brick manufacturing, thus creating a closed-loop industry chain for the reuse of ash and similar materials across the company (see Figure 3). file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image3.jpg 5 Upgrades to wastewater treatment systems for achieving zero wastewater discharge 5.1 Upgrading the wastewater treatment systems used in gas production and desulfurization processes As production scales continue to expand, the existing wastewater treatment systems are no longer sufficient to meet the requirements of production; the turbidity of the water in these systems is excessively high, which leads to blockages in the system’s pipes, the fillers in cooling towers, as well as the nozzles and sprayers of various equipment, making it impossible to achieve zero wastewater discharge. To this end, the company collaborated with Xuzhou Water Treatment Research Institute to carry out an expansion and renovation of the gas generation and desulfurization wastewater treatment systems using micro-vortex tray clarifiers. After the project was put into operation, the sewage treatment capacity reached 3,000 m3/h. After more than two years of operation, it has met the production requirements while also saving water resources. 5.1.1 Revised process: The wastewater from the gas generation and desulfurization units first enters a horizontal flow sedimentation tank. In the sedimentation tank, larger dust particles settle down and are sent to the thickening tank by a sludge pump; the concentrated sludge is then removed from the system, while the clear water returns to the sedimentation tank. The precipitated hot water is pumped by a hot water pump into a micro-vortex tray clarifier; simultaneously, a flocculant and a coagulant aid are added at the inlet and outlet of the hot water pump, respectively. In the micro-vortex tray clarifier, the vast majority of particles are flocculated and settled, then sent to the concentration tank. The clear water flows into the cooling tower under the force of static pressure, and the water after cooling is pumped by a cooling water pump to various locations where it is used in gas production and desulfurization processes. The modified process is shown in Figure 4. file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image4.jpg 5.1.2 Main equipment specifications and structure: (1) Micro-vortex tray clarifier: Specifications: φ22000 mm×13500 mm; concrete structure. It is divided into the first, second, and third reaction chambers, and is equipped with grid plates and inclined tube structures. (2) Cooling towers: made of fiberglass, with a water treatment capacity of 1000 m3/h; there are 3 such towers in total, and the temperature difference for water cooling is >25 ℃. (3) Water pumps: 350s-16 type circulating hot water pump, 350s-26 type circulating cold water pump. (4) Square tanks: horizontal flow sedimentation tank 20 m×30 m, thickening tank 10 m×20 m, clear water tank 10 m×10 m. 5.1.3 Economic benefits (1) Using micro-vortex tray clarifiers to treat gas generation wastewater can reduce the frequency of replacement for pipes, cooling tower fillers, equipment nozzles, and spray nozzles, saving 325,000 yuan per year. (2) Water conservation: The system’s water replenishment volume has been reduced from 12.5 t/h before the renovation to 2.5 t/h. Based on 300 days per year and a cost of 1.5 yuan per ton of water, this results in annual savings of 108,000 yuan. (3) Savings in treatment chemicals: The cost of treatment chemicals for the sewage treatment system has been reduced from 0.065 yuan/t to 0.040 yuan/t. Based on a treatment capacity of 3000 m3/h, this results in annual savings of 540,000 yuan in chemical costs. (4) Electricity cost savings: The outlet water temperature of the system is 3 °C lower than before the renovation, resulting in an annual reduction in electricity consumption for the cooling tower fans of approximately 530,000 kW·h. With an electricity price of 0.25 yuan per (kW·h), the annual savings in electricity costs amount to 132,500 yuan. In summary, this system saves 1.1055 million yuan per year. 5.2 Comprehensive Treatment of Circulating Water On June 18, 2006, a circulating water cooling and treatment system was established at an investment of approximately 38 million yuan, covering systems for urea circulating water, synthetic ammonia circulating water, compressor circulating water, thermal power plant circulating water, desulfurization circulating water, gas production circulating water, as well as a wastewater recycling and treatment station. Pressurized water return is achieved, reducing losses such as water leakage and splashing, and conserving water resources. All circulating water systems have been put into operation, and all process condensate water is recycled; thus, the entire company has successfully achieved zero wastewater discharge (see Figure 5). file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image5.jpg 6 Conclusion Through energy-saving and emission-reduction measures, the three types of waste generated in the company’s ammonia synthesis process are utilized effectively; this represents a model of circular economy that helps save energy, improve the environment, and ensure the most efficient use of resources, thereby reducing resource input and converting waste into reusable resources ； Achieve win-win development in social, economic, and environmental aspects.