Experience in modifying steam double-effect lithium bromide absorption chillers

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Experience in the modification of steam double-effect lithium bromide absorption chillers Chu Kejia, Pan Yuqin, Zhu Bo, Hao Gang (Tengzhou Shenglong Coal Coking Co., Ltd., Tengzhou 277519). Our company uses steam double-effect lithium bromide absorption chillers, with technical specifications as follows: cooling capacity of 4070 kW； Low-temperature water outlet temperature: 16℃ ； Low-temperature water flow rate: 500 m3/h; cooling water inlet temperature: 32℃ ； Cooling water flow rate: 765 m3/h; steam pressure: 0.6 MPa; steam consumption: 4620 kg/h. The steam used by the refrigeration units is supplied by Shengyuan Thermal Power Plant of Zaozhuang Mining Group; the steam pressure ranges from 0.3 to 0.4 MPa, and the price of the steam is 88 yuan per ton. The steam cost required for 2 refrigerators per day is approximately 20,000 yuan; assuming they operate for 4 months per year, the steam cost alone amounts to 2.4 million yuan. Due to insufficient steam pressure, the cooling effect is not adequate, making it difficult to meet the production requirements. The vent gas generated during the production of methanol from coke oven gas is residual gas, with a calorific value of 10.5 MJ/m3, and it is completely released for combustion. To make full use of waste heat resources and save steam. In May 2007, the company converted the steam double-effect lithium bromide absorption chiller unit into a direct-fired lithium bromide absorption chiller unit, and it was successfully commissioned in June of the same year. 1 Renovation Plan (1) Renovation objectives. The transformation goal is to convert the steam double-effect lithium bromide absorption chiller unit into a direct-fired lithium bromide absorption chiller unit. Except for the replacement of the evaporator, the rest of the original unit remains unchanged. Configure another split-type burner, with a pretreatment device installed in front of the burner. (2) Supply of gas source. Direct-fired engines primarily burn off-gases, and the supply pressure needs to be maintained at 6–10 kPa. To ensure that the direct-fired engine continues to operate without interruption during maintenance of the methanol plant, an additional backup coke oven gas pipeline has been installed. Both types of gas sources need to be processed through a filtration device. (3) Composition analysis of the gas source. The volume composition of the flared gas is: CO2 5.55%, O2 0%, CO 4.25%, H2 73.51%, CH4 5.06%, N2 9.49%, with a calorific value of 10.5 MJ/m3. The volume composition of coke oven gas is: CO2 2.82%, O2 0.44%, CO 9.44%, H2 59.83%, CH4 21.1%, N2 3.84%, CnHm 2.46%, with a calorific value of 17.05 MJ/m3. (4) Rated gas flow rate per unit. The rated volume of the bleed gas is 1160 m3/h, while the rated volume of coke oven gas is 700 m3/h. The pre-treatment device for the burner is required to have a surplus gas delivery capacity of over 2.5%. (5) Turbine combustor parameters. The furnace diameter is 900 mm, the length is 3590 mm, the combustion pressure in the furnace is 1.3 kPa, and the elevation of the burner installation center is 1000 mm. (6) Direct combustion engine ignition. Gas turbines are ignited with coke oven gas and then burned using flare gas. When the amount of vent gas is insufficient, it can be manually switched to burning coke oven gas, but mixing coke oven gas with vent gas for combustion is not allowed. (7) Safety. A backflow prevention water seal is installed in front of the valve assembly; upper and lower limits for extinguishing can be set based on the gas pressure, allowing the valve assembly to open and close automatically according to these values. 2 Modification steps: (1) Drain the lithium bromide solution from the original unit completely, and purge it with nitrogen for protection. (2) Remove the evaporator and condensate heat exchanger of the unit. (3) Install a gas-type evaporator and connect the piping. (4) Install the burner door and gas burner. (5) Install a gas water seal, gas heater, gas filter, pneumatic valve assembly, fan, and corresponding pipelines. (6) Install gas heaters, steam pipelines, pneumatic valve assemblies, compressed air pipelines, and gas exhaust pipelines. (7) Perform leak testing on the entire unit as well as on the adapter pipes. (8) Install the chimney. (9) Replace the control program in the unit control system. (10) Lithium bromide solution is added to the unit. (11) Bleed gas is introduced into the unit and burner systems for combustion, and adjustments are made. The process after the refrigeration unit modification is shown in Figure 1. Figure 1: Process flow after the refrigeration unit modification 3. Effect comparison: Our company has two refrigeration units, A and B; only unit B has been modified. After the modification, chillers A and B are used simultaneously. The operation results show that the average cooling temperature of refrigerator A is 25°C, with the lowest outlet water temperature being 21°C; however, this occurs less frequently due to the high steam pressure (approximately 0.5 MPa). The maximum temperature was 27.8°C, due to insufficient steam pressure (about 0.3 MPa). The average outlet temperature of the B chiller is 21.6°C, with a minimum outlet temperature of 19°C and a maximum outlet temperature of 25°C. The design requires the chilled water temperature to be 16°C. To address this, the burner temperature can be increased by increasing the amount of bleed air. The quality of the cooling water in our plant is quite hard; if the water temperature is too low, scale will accumulate on the copper heat exchange tubes, causing blockages. Therefore, the outlet water temperature is kept between 20 and 22°C. 4 Conclusion By converting steam double-effect lithium bromide absorption chillers into direct-fired lithium bromide absorption chillers, and using the off-gases from methanol synthesis instead of steam to produce low-temperature water, more than 2 million yuan in steam costs can be saved each year, thereby effectively reducing production costs. Production practice has shown that the modification of the refrigeration machine was successful, achieving the desired results. It resolved the issue of the refrigeration machine failing to operate properly due to insufficient steam, thereby ensuring the smooth progress of production during the summer.