| In recent years,heating area has rapidly increased,and many district heating systems have faced two main problems such as shortages in energy supply and lower heat capacity of the primary heating network in northern China.The large scale of district heating systems consumes many million tons of coal,and it results in atmospheric environment pollution.On the other hand,there is a great deal of low-temperature industrial waste heat in the northern China.In winter,the heat output of cogenerations is relatively large due to its operation mode about electricity production determined by heat output.In view of this,this paper puts forward a new low temperature industrial waste heat district heating system based on compression heat exchangers.The compression heat exchangers can reduce the temperature of the primary heating network to 20°C step by step,and its application is not restricted by the supply water temperature of the primary heating network.The compression heat exchangers are significantly different from the absorption heat exchangers and the ejection heat exchangers.The lower return water temperature of the primary heat network can not only effectively recycle low-temperature industrial waste heat,but also greatly increase the economic heat transmission distance of the primary heat network.Thus the new heat exchangers can help to solve the bottleneck of low-temperature industrial waste heat transmission over long distance.In this paper,the exhausted heat of the blast furnace slag flashing water from a steel plant in Changchun is used as a low-temperature industrial waste heat resource.The mass flow water rate of the slag flashing water of the selected steel mill is 936t/h,and its outlet temperature is about 80°C.In this paper,the thermodynamic model of the compression heat exchangers is firstly established and verified by experimental data.Secondly,configuration of the heating substation and heating station of the heating system are optimized,and four industrial waste heat district heating systems are proposed.In order to compare the advantages and disadvantages of different schemes,the four heating systems is analyzed from the perspective of thermodynamics,and evaluated by the product exergy efficiency,system SCOP,waste heat recovery rate,economic benefits,and environmental benefits.The results studied indicate that the industrial waste heat district heating system with a mid-high temperature heat pumps and compression heat exchangers(Scheme 2)and conventional industrial waste heat district heating system with compression heat exchangers(Scheme 3)have higher rate of waste heat utilization about 60%;the scheme 3 has higher thermodynamic performance.When the heat transmission distance of the primary heating network reaches60km,the annual product efficiency of the scheme 3 is about 52.79%,and its SCOP approaches to 10,which is much higher than the other three schemes.Scheme 3 is suitable for industrial waste heat district heating system with heat transmission over long distance.For the waste heat price about30?/GJ and the heat transmission distance of the primary heating network about 20 km,and the heating cost(about 46.96?/GJ)of the scheme3 is lower than that for conventional district heating systems based on coal-fired boilers.And its investment recovery period(about 9 years)of the scheme is lower than the benchmark investment period of district heating.The scheme 3 can recover industrial waste water by about 576,000 GJ which accounts for 95.4%of total heat output.Compared with conventional district heating system based on coal-fired boilers,the scheme 3 can save 15.9 kg ce/m~2 per year.Therefore,the industrial waste heat district heating system based on compression heat exchangers can effectively recycle and use low-temperature industrial waste heat resource,and it is characterized by higher thermodynamic performance,better economic benefit,and larger potential of energy-saving and emission-reducing.This paper could provide new ideas for industrial waste heat district heating and prevention and control of atmospheric pollution in northern China. |
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