Application of VPSA Oxygen Generation Technology in Ladle Heating

In 2020, China's crude steel, pig iron and steel production were 1,053 million tons, 887.52 million tons and 1,324.89 million tons respectively, representing year-on-year increases of 5.2%, 4.3% and 7.7%. The crude steel production accounted for 58% of the world's total output. From 2011 to 2020, China’s manufacturing industries and crude steel output annually climbed by 7.9% and 5.1% on average, supporting the high-quality development of China's economy. Although China's overall energy consumption per ton of steel decreased from 920 kgce/t in 2000 to 567 kgce/t in 2017, the energy consumption of the steel industry still accounts for 20~25% of that of the total industrial sectors and 15% of the national total value. The carbon emissions of China's steel industry account for 15% of the total, making it the largest carbon emitter among the 31 categories of the manufacturing industries.

As global environmental and climate issues become more and more prominent in recent years, serious challenges have been posed to worldwide steel and metallurgical enterprises on the energy-saving and environmentally friendly production. Many enterprises in China are exploring new energy-saving and eco-friendly production measures in the context of the "dual carbon target" of "carbon peak and carbon neutral". A large number of theoretical studies and industrial tests have been carried out on oxygen-enriched and oxyfuel combustion, raising the concern about the sources and the economy of oxygen for enterprises.

1. VPSA Oxygen Generation Technology

1.1 Process Description

VPSA is an oxygen generation process via pressurized adsorption and vacuum desorption. Depending on the adsorption ability of the oxygen molecular sieve to different gases in the air, N2 in the air is adsorbed by pressurization to generate O2. The adsorbent is regenerated after the vacuum desorption of N2, thus realizing the stable supply of oxygen. When the compressed air passes through the adsorbent (zeolite molecular sieve), a large amount of N2 is captured and adsorbed while oxygen molecules are separated from nitrogen molecules. When the pressure drops, the N2 adsorbed by the zeolite molecular sieve is released in order to regenerate the molecular sieve. In practical operation, since the adsorbent’s adsorption capacity for Ar and O2 are basically the same, the purity of O2 collected by the VPSA process is lower than 95% with Ar and N2 not being fully adsorbed.

1.2 Technical Flow

Vacuum Pressure Swing Adsorption (VPSA) is composed of a blower, a vacuum pump, switching valves, adsorption towers and buffer tanks. Compressed air by Roots blower is fed to the adsorption vessel after removing dust particles by filters at the entrance zone. The adsorbers are loaded with adsorbent. H2O, CO2 and a small amount of other gas components are adsorbed at first and N2 is adsorbed by the oxygen molecular sieve, whereas O2 (including Ar), as a non-adsorbed component, flows out of the outlet at the bottom of the adsorbers into the buffer vessels as the product gas.

When the adsorbent is fully saturated with N2, the switching valves activate the vacuum pump to evacuate the adsorber (in the opposite direction of adsorption) and the adsorbed H2O, CO2, N2 and other gases are vented to the atmosphere to regenerate the adsorbent.

To summarize the above, the air is sent to the radial adsorption vessels under pressure through the filters before the blower, and the two radial adsorbers work alternately to complete the adsorption and desorption cycle. O2 generated enters into the buffer tanks to stabilize the pressure, thus forming a stable and low-pressure O2 external supply.

1.3 Technical Advantages

The wide application of VPSA oxygen generation in the metallurgical industry benefits from its unique technical and economic advantages over cryogenic air separation process.

(1) simple and stable process, fewer supporting and moving equipments, lower operation and maintenance cost.

(2) independently integrated complete set, flexible operation and production, 50~100% load regulation to adapt to production fluctuations, fast start-up and shutdown of no more than 30 min.

(3) smaller footprint, lower investment. Flow rate of 2,000-15,000Nm3/h can better meet the needs of different processes. In China's industrial sectors, large-scale VPSA oxygen units spanning from 20,000Nm3/h to 50,000Nm3/h have already been commonly applied.

(4) Low-pressure O2 better matches the low-pressure oxygen demand for combustion in most metallurgical enterprises and saves the energy consumption of compressors used for high-pressure O2 at the same time.

(5) Lower oxygen production cost. Natural gas can be saved by oxygen-enriched or oxyfuel combustion technologies to reduce the production cost of the enterprise effectively. Its cost of about ¥0.2～0.3/Nm3 is far lower than that of traditional cryogenic air separation of ¥0.5/Nm3.

2. Application Case

A steel company in China reformed its 3 ladle heating systems (120t). The former natural gas combustion supported by air was upgraded to oxyfuel combustion, i.e., the natural gas is assisted by 91% oxygen gas supplied by a VPSA oxygen unit. The oxygen capacity of the VPSA oxygen generation system is 800Nm3/h with a purity of ≥91% taking into consideration the simultaneous usage of 3 ladle heaters and the available residual O2 replenishment in the steel mill.

3. Main Renovations

(1) Burner and Combustion System Renovation

The burner uses high-speed natural gas and oxygen nozzles. Graded oxygen supply is adopted, including central natural gas nozzle, central primary oxygen supply and eccentric secondary oxygen supply. The burner is integrally mounted with a heating capacity of 2 MW, a rated natural gas flow rate of 200Nm3/h and a calorific value of 33,440 kJ/Nm3.

The original combustion system was remodeled by removing the original air blower and piping system to build a new set of oxygen supply control valves containing flow regulating valves and emergency shut-off valves to ensure the safety of oxygen supply.

(2) Automatic Control Upgrades

The original control system is upgraded to realize the emergency warning, automatic cut-off or heating based on the natural gas and oxygen volume ratio control.

(3) Other Modifications

By modifying the size and load requirements of the burner, the structure of the ladle cover, the winch hoisting mechanism, the refractory inside the ladle cover and the rotating arms were renovated accordingly.

4. Effect Analysis

4.1 Energy Saving and Emission Reduction

The average natural gas consumption of a 120t ladle before transformation is 227Nm3/h. After the upgrading, it decreased to 131.6Nm3/h, saving on average 95.4Nm3/h (42%), and 315.84Nm3/h of O2 consumption was concurrently saved. Considering the usage frequency of per ladle heating equipment as 6,000h annually, 572,400Nm3 natural gas could be saved per year. Moreover, taking the calorific value of natural gas as 33,440 kJ/m3, the user can save 654.1 tce/a and reduce 21,124.4 t/a CO emissions, i.e., 0.3Nm3 of natural gas can be saved for 1Nm3 O2 introduced into the ladle heater, and 1.96 kg of CO2 emissions can be reduced.

4.2 Economic Benefits

Supplied by pipelines, the average price of its natural gas is ¥3.5/Nm3 for the whole year, while oxygen supplied by the VPSA-O2 plant is only ¥0.4/Nm3 in consideration of operation, maintenance and personnel costs in contrast. The oxygen generated by VPSA technology is ≥91%. To fully combust natural gas, the oxygen content is controlled at about 3%, and the ratio of oxygen to natural gas is maintained at 2.4. Therefore, the average O2 consumption after the modification is 131.6 m3/h×2.4=315.84 m3/h.

Without considering other factors, the oxygen cost of a heater=315.84×6,000×0.4=¥758,016/a, natural gas saved=95.4×6,000×3.5=¥2,003,400/a, and the direct economic benefits of a heater after renovation=natural gas cost saved-oxygen cost=¥1,245,000/a.

5. Conclusion

(1) VPSA oxygen production has unique technical advantages over the cryogenic process and is more adaptable to the demands of low oxygen consumption with variable loads in the metallurgical industry. It is conducive to reducing the costs and risks of investment, construction land, operation and maintenance, etc., and helps the users optimize and upgrade the existing production process.

(2) With the total cost of VPSA oxygen production of about ¥0.4/Nm3, after applying oxyfuel combustion, 1Nm3 of oxygen saves 0.3Nm3 of natural gas and cut about 1.96 kg of CO2 emissions. Comprehensively, an annual production cost of about ¥1,245,000 could be saved, resulting in significant economic and environmental benefits.

(3) At present, the air-to-gas ratio in the heater is 2.4, and residual oxygen content in the flue gas is measured to be around 3%. In view of the combustion rate and flame propagation speed of oxygen-enriched combustion, the oxygen-to-fuel ratio still has potential for further optimization.

(4) The article analyzes the co-production of VPSA oxygen equipment and ladle heating system, and VPSA oxygen generation technology is also widely applied in other processes such as blast furnace oxygen-enriched combustion and electric furnace steelmaking. It is of interest to improve efficiency and profits for the users.