An Economic Analysis of VPSA Oxygen Plant（6,000Nm3/h, 100%）for BF Enriched-Oxygen Combustion Supporting in Steel Mills

Vacuum pressure swing adsorption units are used to produce oxygen at purities between 80%-94%. The technical principle is to separate the oxygen in the air utilizing the different selectivity of adsorbents for oxygen and nitrogen. Units using this type of non-cryogenic oxygen production process may be referred to as VPSA (Vacuum Pressure Swing Adsorption), VSA (Vacuum Swing Adsorption) or PVSA (Pressure-Vacuum Swing Adsorption) systems. The vacuum pressure swing adsorption technology mentioned in this article is all represented by VPSA.

The process of VPSA oxygen generation technology has been relatively mature after its development for decades. Transformation from the original axial flow-bed adsorber with calcium-based adsorbent to radial adsorption vessel with lithium-based adsorbent has been achieved. The maximum capacity of double-vessel plants now reaches 6,000Nm3/h for 100% oxygen product. Those plants are particularly suitable to supply oxygen as a supporting equipment of blast furnaces or electric furnaces to iron & steel mills due to its relatively low cost, high safety, short construction period and easy maintenance. At the same time, for power sectors, easy start-up and swift load adjustment of VPSA technology enjoys big advantages in regulating the overall oxygen demand, reducing oxygen release, and further lowering production costs. In summary, VPSA technology is the first choice for iron and steel enterprises to supplement oxygen in the case that nitrogen is sufficiently provided by cryogenic air separation.

In 2008, Anshan Baode Iron and Steel Co., Ltd. concluded a purchase agreement of VPSA oxygen plant(7,500 Nm3/h, 80%) with Beijing Peking University Pioneer Technology Co., Ltd. for an iron-making blast furnace. The unit was successfully put into production and has showed stable operation till now. The blast furnace delivers an oxygen enrichment rate of 2.38%, a production growth by about 8.5%, a coal injection ratio of 145 kg/ton of iron, and a reduction in coke ratio by 5%. In August 2017 and September 2018, VPSA oxygen unit with a capacity of 7,500 Nm3/h and purity of 80%（6,000Nm3/h, 100%）was equipped for the blast furnace successively while the oxygen enrichment rate of the blast furnace went up to 4.38%, and the coke ratio was reduced by 10%, thus impressive economic benefits were created.

In recent years, VPSA oxygen generation technology has undergone rapid development in China, showing obvious advantages displayed as follows:

First of all, the reliability and long service life of VPSA oxygen production technology stands the test of time. The VPSA oxygen unit that Anshan Baode Iron and Steel Co., Ltd. applied in 2008 has not been weakened so far, and its energy consumption has remained at the original level.

Secondly, the VPSA oxygen production technology only requires power equipment including a blower, a vacuum pump, an electric motor and pneumatic valves, thus no additional staff is required cause the cost-effective components are relatively easy to maintain.

Thirdly, high security is insured. The VPSA oxygen plant injects enriched oxygen before the blower without pressurization to the blast furnace. With relatively commodious operating environment, hidden security risks are avoided and management is simplified.

The two sets of VPSA oxygen generators around ten years ago have undergone configuration adjustments and other major changes as seen from the actual operation. Specific parameters are shown in the table below:

With its ten-odd years of rapid development, VPSA oxygen production technology has been significantly improved in its configuration and process. According to the analysis of the above table, its process improvement is reflected by:

1. Reduction in the Number of Rotary Equipment

Compared with previous plants of the same capacity, it contains much less power equipment, electric motors and valves, significantly reducing the maintenance workload and indirectly, the cost of labor and maintenance as well. Due to the improved performance of the adsorbent, the capacity per ton has been increased by over 20%. In addition, the axial absorption vessel was transformed to radial-flow vessel, so the entire internal space structure has undergone an overall change. The manufacturing of absorbent and adsorbers has stood on top internationally and all the optimization results in the simplified configuration selection and less input.

2. Reduction in Failure Rate and Increase in Service Life

In the entire unit, the normal operation of valves is an important condition to ensure the continuous running of the system while valves are the main parts which get into failures easily. The core maintenance work for the VPSA oxygen plant produced in 2008 lie in valves. Generally, the sealing rings need to be replaced once a year on average. The number of valves in oxygen plants manufactured in 2017 was reduced by 30% with the caliber increased, greatly cutting the overall maintenance down. The last but not the least is the further reduction in failure rate. Meanwhile, improved material of the sealing rings prolonged the replacement frequency of valves to once approx 18 months on average ( i.e., Although the times running sealing rings is doubled, the absorption cycle is shortened and the on-off times rises).

3. Reduction in Power Consumption

Power consumption is the most important indicator to measure the advanced performance of VPSA oxygen generators. With the adoption of new absorbent and radial adsorber, the power consumption has been decreased by 10%. 1Nm3 of pure oxygen can save about 0.04 kW/h of electricity with the calculation based on 6,000Nm3/h. With the assumed annual operating duration of 8,400 hours, a substantial cost saving of approx $29 million may be achieved.

4. Remote Service

The proposition of "intelligent manufacturing" has promoted the integration of China's advanced manufacturing technology with informatization. Remote data analysis system, Pioneer Cloud Service, is a new practical function loaded on the oxygen plant. The running data is transmitted to the client through the specific system in real time, then the system compares those data with standard curve under normal operating conditions and presents a visualized result report. In case of any abnormality, prompt alarm will be raised to direct the operators to conduct adjustment and maintenance, further strengthening its stability, avoiding unknown risks and eliminating hidden trouble as soon as possible.

5. Others

The reduction is also achieved in absorption vessels and covering area as well as absorbent. The loading adjustment goes easier and faster.

In short, vacuum pressure swing adsorption oxygen production technology has the advantages of low power consumption, high reliability, easy operation & maintenance, rapid loading adjustment, etc. therefore, its application scope will be largely expanded. A full reference to VPSA technological applications by steelworks in various countries helps China's steel industry to better lower production costs and improve enterprise benefits, thereby consolidating and increasing the market competitiveness of steel companies.