Energy saving technical renovation case of compressed air system
1. Compressed air is one of the widely used power sources in the industrial field, but obtaining high-quality compressed air that meets basic requirements requires significant energy consumption. In most production-oriented enterprises, the energy consumption of compressed air accounts for 10% to 40% of the total electricity consumption, while in the shipbuilding industry it reaches 50%. At the same time, compressed air is also a high-quality and expensive power source among the four major power sources (heat, electricity, hydraulic, and pneumatic).
2. Background and preliminary work of the project: With the vigorous development of China's manufacturing industry in the past 30 years, energy consumption and environmental pollution have become increasingly serious, as well as overcapacity and vicious competition, leading to enormous pressure on enterprises to save energy and reduce emissions, while also bringing new impetus to the promotion of high-efficiency energy-saving technologies. Saman Machinery (Shanghai) Co., Ltd.'s Energy saving Technology Department has timely launched an energy-saving renovation project for compressed air systems. In response to the two problems of high energy consumption and large emission leakage in Tangshan Iron and Steel's compressed air system, preliminary research and evaluation work has been carried out, and corresponding research and development plans have been formulated: firstly, to achieve high and low pressure separation. According to actual needs, the exhaust pressure of the air compressor is divided into 6 bar and 4.5 bar, and corresponding three-stage compression and two-stage compression are adopted. This can save 10% to 15% of the compression power wasted by the virtual high exhaust pressure; The second is to eliminate high-energy consumption micro heating dryers and adopt a new type of dryer with compressed air system waste heat utilization and isobaric cycle regeneration zero emission technology. This is expected to reduce the operating costs of the original micro heating dryer by more than 80%; The third is to renovate the condensate discharge valve of the compressor and post-treatment purification equipment, install a new type of zero gas consumption automatic drainage valve, and prevent leakage and dripping in the station building. The gas loss of the latter two items can reach 20% to 30% of the output gas volume of the air compressor. Gas loss not only directly leads to energy loss, but also drives the air compressor and auxiliary equipment to increase the number of units or operating time, which in turn affects maintenance and daily consumption costs.
3. Transformation ideas and measures
3.1 Optimize the compressed air system process to achieve high/low pressure and dry/wet gas separation, dividing the system into three types of gas supply systems. High pressure moisture system: 0.55 MPa supply gas for continuous casting of No. 3 and No. 4 steel, supplied from the hot plate air compressor station; Low pressure moisture system: 0.45 MPa, supplying the remaining moisture gas demand, supplied by three newly added low-pressure centrifugal air compressors; Dry gas system: 0.6 MPa, supplied to meet the gas demand of the cold plate plant, supplied from the cold plate air compressor station; Supply all other dry gas needs from the 3200 air compressor station. Three new two-stage low-pressure centrifuges will be installed at the cold plate air compression station, and a new low-pressure wet gas main will be arranged from the cold plate air compression station to the gas consumption points of the first steel, second steel, and ironmaking departments. The low-pressure dry gas system will use the original wet gas main to supply gas, and four new waste heat regeneration dryers will be configured at the cold plate air compression station. Four air compressors at the 3200 air compression station will be equipped with zero emission waste heat in the air drying machine. Each independent gas supply system adopts one-way crossover regulation. Crossflow regulating valves are installed from the high-pressure dry gas system to the low-pressure dry gas system, and from the low-pressure dry gas system to the low-pressure wet gas system, all of which achieve precise upstream pressure control to achieve balanced load control of the high-pressure dry gas system, low-pressure wet gas system, and low-pressure dry gas system units, reduce or eliminate the occurrence of unit discharge, and optimize the system's regulation capability. Due to the higher air pressure in the fan room of the ironmaking department, the lubricating oil station of the long material department's high line, and the high line packaging machine compared to the renovated dry air pressure, booster devices are installed separately to meet the local high-pressure gas demand.
3.2 Optimizing Compressed Air Drying Process
Dismantling and replacing old with new significantly reduces the energy consumption of the original micro heating dryer. During the air compression process, more than 70% of the mechanical energy is converted into heat energy and removed from the system by the interstage and end coolers. However, centrifuges are oil-free machines, and their final compression heat can be utilized by dryers as regenerative energy. Although this heat can only meet about 70% of the energy required for the regeneration process of adsorption dryers, if the energy supplementation measures are appropriate, a lower dew point can still be obtained. If the blown cold air can be recycled again, it can save more than 90% of operating costs compared to the original generation (no heat, micro heat regeneration dryer). After research and bidding, this project chooses a zero gas consumption, low dew point waste heat regeneration adsorption dryer. This process is extended and promoted from the patented technology of natural gas dehydration equipment, adding secondary cooling at the inlet end, which can adapt to various working conditions (lower exhaust temperature, higher ambient temperature), and the pressure dew point can reach -40 ° C. A high-pressure circulating fan is used to perform secondary high-temperature dry gas regeneration on the adsorbent in the upper part of the tower. The adsorbent can be cooled with high flow and fully recovered for circulation, with power consumption accounting for only 1% to 1.5% of the input power of the air compressor and zero regeneration gas consumption.
3.3 Optimize the condensate discharge process
Adopting a new type of automatic condensate remover, it has the characteristics of high reliability, low maintenance rate, and no leakage of compressed air. It can timely and reliably discharge the condensate, greatly improving the corrosion of the air compressor and ancillary equipment by the condensate, avoiding the impact and damage of the gas equipment caused by the condensate flowing into the lower stage. When there is a problem, an alarm signal is issued for easy monitoring, timely detection of problems, and avoiding unnecessary losses. It is also equipped with a slag remover, which will not block the drain and can remove impurities of 5-8 mm.
4. Economic and social benefits
4.1 Economic benefits
(1) Low pressure system energy-saving: average gas volume of 36000 Nm3/h; Annual operation of 8000 hours; Optimization of gas electricity ratio of 0.032 kW · h/m3 (low voltage single unit 0.083 kW · h/m3; single unit before renovation 0.115 kW · h/m3). Reduce energy consumption by 9216000 kW · h annually; Equivalent electricity cost: 5.16 million yuan.
(2) Energy saving renovation of drying machines: All drying machines at the hot plate station and the end of the second steel plant have been shut down (with a regeneration gas consumption of 4500 m3/h), while all cold plate stations and 3200 stations have been upgraded to waste heat regeneration drying machines (reducing regeneration gas consumption by 10500 m3/h). Reduce the gas loss in the regeneration process of the dryer: 15000 m3/h; The gas to electricity ratio is calculated at 0.12 W · h/m3; Reduce system energy consumption by 1440000 kW · h annually; Equivalent electricity cost: 8.06 million yuan.
(3) Energy saving condensate management: Increase the specific power of individual units by a total of 68 points, with an expected solar term of 3000 Nm3/h. The gas electricity ratio is calculated at 0.12 kW · h/m3, resulting in an annual reduction of energy consumption of 2880000 kW · h; Equivalent electricity cost: 1.61 million yuan.
The total energy-saving benefits of the above three theories result in an annual energy consumption reduction of 26.496 million kW · h; Equivalent to electricity cost savings of 14.83 million yuan.
4.2 Social benefits
Since the implementation of the project, the control accuracy and compressed air utilization rate have been improved, the system operation is stable and reliable, the monitoring operation is flexible and convenient, avoiding frequent start and stop times, greatly saving operation time, and reducing the labor intensity of operators, thus improving production efficiency. According to 1 kWh of electricity equivalent to 0.123 kg of standard coal, the annual electricity savings of this project are approximately 3260 tons of standard coal equivalent. Eliminated the original second-generation compressor heat dryer. At the same time, in 2016, Handan Iron and Steel, which also belongs to Hebei Iron and Steel Group, began to purchase the same type of compression heat zero gas consumption dryer equipment. Tangshan Iron and Steel's series of actions have also affected many private steel mills in Hebei and peers across the country.
5.1 Throughout the entire process of energy-saving technological transformation, the first step is to change the concept and mindset, such as the taste and values of energy, specifically in terms of the professional equivalence between gas and electricity. In this project, the importance of solar terms is given due to the consensus reached with domestic and foreign peers through research and communication that electricity accounts for 40% of the cost of oil-free dry compressed air; The second is the business model. This project adopts the Energy Performance Contracting (EMC) method, which is a typical example in the energy-saving and technological transformation of air compression stations in the steel industry. It effectively mobilizes the potential and enthusiasm of users, EMC, and equipment suppliers, and obtains sustainable energy-saving space for users with high-end products and excellent engineering; At the same time, the project also marks the third change in the industry, where procurement bids are all won at high prices. This move is also due to the learning and implementation of Product Life Cycle Cost Analysis (LCC), which maximizes energy-saving benefits; The fourth change point is to abandon the traditional practice of state-owned enterprises not being allowed to dismantle equipment before the depreciation period expires, and directly replace the high energy consuming first generation micro heating dryer with the fourth generation waste heat regeneration zero gas consumption dryer.
5.2 Year on year recognition of waste heat regeneration dryer
The original micro heating dryer consumes both electricity and gas. After on-site testing, the gas consumption ratio reached 15%, and in summer it was nearly 20% or more. The test results are consistent with the current revision of the compressed air dryer product standard. The types of air compressors used in steel mills are mostly oil-free centrifuges. Many cases have previously used compression heat dryers, but after research, it was found that the failure rate is close to 50% (such as the stainless steel plant of Tangshan Iron and Steel), and the non-compliance rate is over 80% (the pressure dew point is generally higher than -20 ℃, and the waste heat regeneration dryer of the double cooler type is even higher than zero benefit; the regeneration gas consumption is significantly greater than 6%, and the instantaneous flow rate is over 12%). This phenomenon is more serious in cold northern regions. At present, the waste heat dryers in multiple air compression stations in the industry have undergone or are undergoing secondary renovation and have paid a high price. This is because compression heat belongs to low-grade energy, especially in the past 10 years, due to technological progress, the exhaust temperature of centrifugal air compressors has decreased from 120-140 ℃ to 80-120 ℃. However, this has also had a negative impact on adsorption dryers that utilize compression heat, greatly reducing their space for utilizing waste heat. That is, the compression heat provided by the air compressor can only meet about 70% of the regeneration energy required by adsorption dryers, and the supplementation of the remaining regeneration energy has become a technical research and development challenge.
5.3 Supplementary Explanation
The gas loss in the benefit calculation of this project is only based on electricity costs, but the actual expenses should also include fixed asset depreciation, financial expenses, management expenses, maintenance expenses, and daily consumption expenses. That is to say, electricity costs only account for about 40% of the actual cost of oil-free dry compressed air. Calculated based on the heat value, 1 kWh of electricity is equivalent to 0.123 kg of standard coal. However, no one would think that the cost saved by saving 1 kWh of electricity is the cost of coal, because the cost of 1 kWh of electricity (at 0.6 yuan) is 8 times that of 0.123 kg of standard coal (at 600 yuan/t). If calculated based on the electricity bill accounting for 50% of the gas cost, the actual cost generated by the dryer during the solar term should be 8.06 million yuan, which equals 16.12 million yuan per year. However, even if calculated based solely on the electricity bill of 8.06 million yuan, the investment payback period is 8-10 months, which is rare among many energy-saving and technological renovation projects currently.
Another noteworthy feature of this project is the application of remote monitoring technology in the dryer. Professional engineers from Saman Machinery (Shanghai) Co., Ltd. not only participated in the operation monitoring, fault prediction, fault analysis, and troubleshooting of the equipment, but also benefited greatly from its design and development personnel. Through the Internet plus technology, the on-site operation data and cumulative operation curve are obtained, which can effectively judge and verify the design accuracy and the matching of various subsystems and components, and truly integrate into the era of big wisdom, big data and big systems.
6 Conclusion
Efficiency, energy conservation, emission reduction, and haze reduction are currently important national policies in China, and economic transformation, industrial upgrading, and product replacement are the top priorities for enterprises. Enterprises of all sizes should comply with national policies in order to survive and develop in the new normal and remain invincible.
