Development and application of waste heat recovery technology for industrial low temperature circulating water - Shandong Jiugang Tisco Steel Co., Ltd.

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Developmentandapplicationofwasteheatrecoverytechnologyforindustriallowtemperaturecirculatingwater-ShandongJiugangTiscoSteelCo.,Ltd.

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Development and application of waste heat recovery technology for industrial low temperature circulating water

Development and application of waste heat recovery technology for industrial low temperature circulating water

Iron and steel enterprises have abundant low-temperature circulating water waste heat resources, but the utilization rate is low, and heat pump technology has great potential in this regard. This article introduces the characteristics of * heat pump technology and the highlights of circulating water waste heat recovery. It explains the use of * heat pump technology in Application and effect of recycling industrial circulating water waste heat when heating in cold areas..

Key words: circulating water waste heat * heat pump technology energy saving

1 Introduction

In 2017, the total amount of waste heat resources in China’s steel industry was 8.44 GJ/t steel, accounting for 37% of the comparable energy consumption per ton of steel. The sensible heat carried by cooling water in the steel industry was 1.24 GJ/t steel, accounting for 15% of the total waste heat %. At present, most companies use an open circulating cooling water system for cooling. The water temperature is about 15-35 ℃. The waste heat recovery rate of circulating water is very low, only about 1.9%. A lot of low-temperature waste heat is wasted. Use winter heating. There are two forms of steam, one is steam for hot water exchange, and the other is the problem of large waste of steam pressure and heat loss. At present, metallurgical enterprises rarely use the technology of heat pump recovery of circulating water waste heat in winter. .This technology can not only greatly improve the utilization efficiency of primary energy, but also has the characteristics of zero pollutant emission. Therefore, the development and application of high-efficiency heat pump energy-saving technology is of great significance for heating in winter in northern regions or utilizing low-temperature waste heat in the industry.

2 Classification and application of heat pump technology

2.1 Classification of heat pump technology

Heat pump technology is based on the reverse Carnot cycle principle. Depending on the driving force, heat pumps can be divided into compression heat pumps and absorption heat pumps. Compression heat pumps are mainly composed of an evaporator, a compressor, a condenser and an expansion valve. By making the working fluid continuous Complete the thermal cycle of evaporation (absorbing heat)-compression-condensing (releasing heat)-throttling-re-evaporation, and the low temperature transfers the heat of the heat source to the heat user. The absorption heat pump is mainly composed of heat accumulator, absorber, condenser, evaporation It uses the gas-liquid equilibrium characteristics of the solution composed of two substances with different boiling points to work [1].].

According to the heat source medium of the heat pump, it can be divided into air source heat pump and water source heat pump. Add a cold area to the return pipe heat exchanger, etc.), so it is the most widely used equipment in heat pump technology at present; water source heat pump uses hot water as the heat source, because the heat source temperature of water source heat pump is generally 15~35 ° C, the whole It is basically stable over the years, and its heating and cooling coefficients can reach 3.5-4.5, which is about 30% higher than the traditional air source heat pump [2].

2.2 Application of heat pump technology

In recent years, water source heat pump technology has developed rapidly in China, and the market has become increasingly active. The low-grade heat source of low-temperature circulating water waste heat can be used as a water source heat pump unit, and the technology of large-scale * heat pump units has become the key and has broad prospects. *Application of centrifugal water source heat pump technology in recycling waste heat of circulating water.

3 Highlights of the combination of circulating water and heat pump technology

Currently, heat pumps are mainly used in urban life. The heat source is mainly low-level heat energy, such as groundwater, and the temperature of the hot water after heat exchange is 50°C. It is difficult to guarantee heating in cold areas. It needs to be combined with a low-temperature heat source to meet the Demand for heating hot water temperature in cold areas [3]. The highlight of heating with circulating water combined with heat pump technology.

3.1 Eliminate the limitations of heat pump technology

The heat pump does not need to absorb groundwater, avoids the influence of groundwater resources and sediment content, and does not need to deepen the installation pipeline and the installation shaft of the heat pump, saving the initial investment. Because the circulating water has good quality and continuity, the closed-loop operation is stable .

Since the circulating water is relatively clean and there is no corrosion problem, it is not easy to reduce the heat transfer effect; if the heat pump needs to be cleaned, it is relatively easy [1].

3.2 Solve the problems existing in the operation of the circulating water station

Utilize the waste heat of circulating water to reduce the evaporation of circulating water station, that is, reduce the heat emission into the atmosphere and reduce the greenhouse effect.

Reduce circulating water inlet temperature. The circulating water temperature does not need to be cooled by cooling towers or coolers, but is sent directly to the circulating water system, thus saving the power of the circulating water pump.

3.3 Overcome the heat loss of the original steam heating system

The steam pipe network of the original heating system is long, and the heat preservation effect is not good; at the same time, the original steam and hot water heat exchange is like a shell-and-tube heat exchanger, plate heat exchanger and other equipments are aging, and the efficiency is too low, resulting in The heat loss rate of the original system is very large. The heat pump technology completely overcomes the heat loss of the original heating system.

4*Technical characteristics of heat pump

The performance of the heat pump unit can be improved by improving the performance of each component of the heating system or improving the system circulation. COP refers to the heating performance index (abbreviation of performance), that is, the ratio of heating capacity to input power. That is, how to use low The input power to generate high heat is the key to improving the COP performance index. At present, the heating performance index COP of heat pumps is generally 3.5-4.5. Using a heat pump and optimizing operating parameters can increase the COP performance index by more than 30%.

4.1* Centrifugal two-stage compressed air supplement and enthalpy heat pump technology

The use of two-stage compression cycle and economizer can greatly improve heating capacity and energy consumption without increasing heat. Therefore, the system performance is significantly improved, and the energy saving effect is significant. The flow and pressure enthalpy lines of the quasi-two-stage compression cycle system Figure, throttling is performed before the economizer, as shown in Figure 1. The high temperature and high pressure refrigerant vapor discharged from the compressor enters the condenser to release heat, and then enters the condenser. The economizer flashes after throttling. The generated medium The pressurized vapor enters the intermediate supplementary air hole, and the remaining working fluid enters the evaporation after throttling and decompression. The evaporator absorbs heat and evaporates in the evaporator, and is sucked by the suction port of the compressor. After being compressed to a certain pressure, it is combined with the The refrigerant sucked in by the intermediate intake is mixed, then further compressed and discharged from the compressor to complete a cycle (4).

Two-stage compressed air supplementary circulation is the best choice. When cooling, it can increase the circulation efficiency of the system by 5%; when heating, it can increase the circulation efficiency of the system by about 8%, with higher energy efficiency and higher stability. as shown in picture 2.

 

Fig.1 Two-stage compressed air supplementary system with increasing enthalpyFig.2 System period Ph

4.2 Compressor full state aerodynamic design technology

The traditional compressor design method is based on the rated operating point, while the * centrifugal heat pump is designed for operating conditions up to 75% of the operating point and then extended to 50% and 100% of the operating conditions.W-type three-element full-length vane enclosed Impeller, low concentration vane diffuser 360° annular air inlet and other pneumatic components. In the normal operating range, the adiabatic efficiency of the compressor can reach more than 85% [5].

4.3 Technologies to reduce heat transfer temperature difference

The heat transfer temperature difference is an important factor that causes the actual heat pump cycle to deviate from the ideal reverse Carnot cycle. It can be seen from equation (17) that the larger the heat transfer temperature difference, the greater the irreversible loss, and the greater the deviation from the ideal cycle: En = ∫12dQ(T1-T2)/(T1T2) Ta × when the logarithmic average temperature difference decreases by 1°C, the irreversibility of the heat exchanger decreases by 0.01l. Therefore, an appropriate heat transfer temperature difference is selected such that The evaporation temperature should not be too low, and the condensation temperature should not be too high, so as to improve the system performance index [4].

5*Application of heat pump technology in recycling low temperature waste heat of industrial circulating water in heating

Taking the heating of the steel plant area as an example, * the centrifugal heat pump technology is used to remove the low temperature waste heat of the steel company. The circulating water is used to replace the original steam heating; at the same time, the heat pump operating parameters are optimized during the operation of the heating system to reduce the energy efficiency index COP The index increased to above 5.5.

5.1 The scheme of heat pump technology to recover waste heat of circulating water system

The circulating water of the rolling mill reaches the heat pump system through the switching valve, and then the heat in the circulating water is extracted through the evaporator of the heat pump and transported to the condenser, and the heat is transferred to the heating and heating network system in the condenser, and the circulating water system and the first heat pump are combined The up-and-coming heating method technology is realized. The generated heated hot water is sent to each heating user by the hot water pump through the hot water pipe network of the factory, as shown in Figure 3.

 

5.2 Optimizing the operation of the heat pump system

5.2.1 Optimize heat pump hot water temperature difference and improve COP performance index

In the centrifugal heat pump unit during operation, the circulating flow of hot water is fixed, and the heating capacity of the unit is adjusted by the temperature difference of the hot water. The COP performance index is improved by optimizing the control of the hot water temperature difference of the heat pump. According to the actual demand Qs and The hot water flow m calculates the heat load, and calculates the temperature difference between the feed water and the return water. The calculation formula: Qs = m × c × (t1-t2) = m × c × △t, where c is the specific heat capacity KJ / Kg° C, Δt is the temperature difference between feed water and return water, and for the heat pump unit, this temperature difference is 5–7 °C. As the temperature difference of the outlet hot water increases, the COP performance index decreases (see Figure 4).

5.2.2 Optimizing the heat pump hot water outlet temperature and improving the COP performance index

Reduce hot water temperature. The existing heat pump unit is a two-stage compression system. During the initial and final stages of heating, the temperature of the hot water provided by the system is controlled at 40-45°C, reducing the required heat load and optimizing the effluent temperature parameters, and dividing the heat pump into two-stage compression and optimizing it into one-stage compression can reduce the power consumption of the heat pump system, meet the on-site heat load demand, and improve the heat pump performance index COP (see Figure 5).

 

Fig.4 Hot water temperature difference performance curveFig.5 Hot water outlet temperature performance curve

5.2.3 Actual Operation Analysis of COP Performance Index

COP = Qs / P Qs Power consumption of the heating load P system (measured according to the table), the following data is based on an average heating period of 4 months. Qs = m × c × (t1-t2), where M is the amount of hot water supplied, c is the specific heat capacity KJ/Kg °C, t1 and t2 are the supply and return temperatures °C. The system power P is the total power consumption of the heat pump system, by meter Qs = m × c × (t1-t2) = 31974.3MJ P = 5778MJ COP = Qs/P = 5.53 Measured by the above optimization operation, the COP performance index is consistent with the traditional one, which is an increase of more than 38.25% compared with this index 4.0.

6 Operational effect and profit analysis

6.1 Operation effect and economic benefit

After the project is implemented, all heating is provided by the heat pump system. When the heating temperature reaches the design temperature of 55°C, all original steam must be stopped to achieve the desired effect. The goal of replacing steam heating while reducing the evaporation of circulating water and carbon dioxide and nitrogen oxide emissions, which have significant economic and environmental benefits.

The steam saving is 62.25t/h, the heating cycle is 4 months (2880h), the annual steam saving is 523,497.6GJ, the steam price is calculated at 45 yuan/GJ, and the annual steam saving benefit is 23.557 million yuan. The annual power consumption of the heat pump heating system is 4.69 million The cost of electricity is 2.495 million yuan. After deducting the annual maintenance fee and annual depreciation fee of 2.41 million yuan, the annual saving is 18.65 million yuan. The investment recovery period of this project is 1.2 years.

6.2 Energy saving and environmental protection benefits

The project saves 523,497.6GJ of steam per year, and the annual energy saving after conversion is 17,862 tons of standard coal.

Burning about 2.6 tons of carbon dioxide, about 24 kilograms of sulfur dioxide and about 7 kilograms of nitrogen oxides per ton of standard coal, it can reduce the emission of 46,441 tons of carbon dioxide, about 430 tons of sulfur dioxide and 126 tons of nitrogen monoxide per year, greatly improving the life around surroundings.

7 Summary

The first heat pump that recovers the waste heat of circulating water can be used in the northern heating mode to realize the cascade utilization of energy and realize the cascade utilization of energy. The saving effect is significant; at the same time, it can reduce the amount of circulating water, evaporation loss and environmental impact. The heat pollution of the heat pump has good thermal performance and social benefits. The operating parameters of the heat pump are optimized to make the COP performance index of the heat pump reach 5.5 or more, which is at the leading domestic level and has the value of promoting the low-temperature waste heat recovery technology in the same industry.

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