Project Topics
Get Complete Project Material File(s) Now! »
Nyköping heat demand
The heat demand of Nyköping is illustrated in table 6. This demand can be partly satisfied by SSAB in Oxelösund [11], [20].
Nyköping heat demand when heat from SSAB is not available
The heat from the SSAB plant in Oxelösund could be unavailable for shorter or longer periods. Even then the heat pump in Oxelösund should be able to cover Nyköpings heat demand using sea water as a source.
Feasibility study and potential for heat pump utilization
A feasibility study on waste heat sources of SSAB Oxelösund iron & steel process has been performed and the possibility of heat pump utilization is considered. The results are summarized in table 7 and most possibilities of a low temperature waste heat recovery using heat pumps are described in this table.
It would be possible to use a low temperature heat source, also in an absorption heat pump, if a high temperature gas or other high temperature heat source was available. The main alternative is to use a cooling tower with low temperature heat recovered using with heat pump.
Why using the cooling tower?
Regarding to table 7 there are limitations to recover waste heat from many sources. Gases are mostly polluted and difficult to recover because of erosion, high cost and complexity. Also, Flue gas from combustion of BFG/COG in coking ovens are emitted to atmosphere from three 70 m height stacks and are situated 800 m distance from the Blast furnace. Flue gases from combustion of BFG/COG in hot stoves are emitted to the atmosphere from two stacks at 400 m distance from the blast furnace. Flue gases from the combustion of COG in six heat treatment furnaces are emitted to the atmosphere from three stacks with 2 km distance from the blast furnace.
LD gases, have a high temperature and would be a good source, however the process duration is just 20 (min/h), thus there is a need for a storage to achieve a continuous heat flow. On the other hand it is a good complementary heat flow to other processes.
Some other processes like quenching coke and cooling LD gas, is used in other parts of the plant. Sprits cooling is a mixture of steam and air cooling and therefore difficult to recover. Also, slag and steel furnace cooling are other alternatives but difficult to recover in a process. In general, there is little distance between the cooling tower and the blast furnace thus it would be a good source for heat recovery. This will also help reducing the cooling tower’s maintenance costs.
When the cooling tower clogs it is difficult to maintain the efficiency and it also requires a lot of chemicals to avoid legionella. This tower also needs to be partly rebuilt, if its life length is to be prolonged. Electricity within SSAB plant is not so expensive and could help to improve the heat pump economy if that is allowed by the tax laws. If steel production will cease for a longer or shorter period the heat pump could use sea water as source.
In general, to cover the heat demand, high temperature industrial heat pumps can be used as low temperature waste heat recovery from the blast furnace. Sweet and clean water is coming from Nyköpingsån (a river) is used by the blast furnace and is also used in some other internal systems. The water delivered from the blast furnace to the cooling tower is today fluctuating between 40°C to 50°C and the return water temperature fluctuates between 30°C to 40°C. The water flow rate in summer and winter respectively is 1500 (m³/h) and 2000 (m³/h). Moreover, the cooling tower is running about 8100 hours per year (not working in July). July can be covered using sea water as heat source.
Maybe at a later stage also other higher temperature sources can help improving the efficiency of the concept.
Heat source (cooling tower) specification
Table 8 illustrated heat source and heat sink temperature with flow rates in summer and in winter. Because of fluctuating water temperatures in the cooling tower, a representative average temperature has been considered [11].
The high temperature industrial heat pump
Industrial heat pumps
Their possible temperature range has increased significantly during the last decades. Industrial heat pumps temperature range is today up to over 100 °C and with power capacities ranging from maybe of 50 kW and many MW. This is achieved mainly by the development of new refrigerants.
Industrial heat pumps are implemented for many purpose such as waste heat recovery, air conditioning in industry , district heating, steam production and many other applications. The industrial heat pumps are of course designed to meet their specific needs and the specific conditions. As the conditions differ, the serial length of production is smaller than for e.g. domestic heat pumps. The energy consumption in the industry- in the household- and the service sector are rather equal, figure 2. Industrial heat pumps can sometimes have the following advantages compared to residential heat pumps:
– Higher COP due to a lower temperature span
– Lower investment cost due to a low distance between heat source and heat sink
– Higher duty factor 6000 h/year or more
– Simultaneously use being both heat source and heat sink
– Ability to use cheap waste heat in industry reducing total usage of primary energy and cost
-Although IHPs thus often have advantages compared to residential heat pumps, the lack of experience, lack of consult IHP-experience in industry causes a lower amount of IHP installations rather than residential heat pumps. Industrial heat pump temperature range is sometimes divided to three levels [5],[21]:
– Medium temperature
– High temperature
– Very high temperature
Industrial heat pump applications in general
Industrial heat pumps are able to recover waste heat in industry and make it usable for other industry processes. They depended on matching heat sink and heat source temperatures and capacities. The higher the temperature lift the higher the pressure ratio in the compressor and the lower the COP. In situations with a high temperature lift it is better to use multistage heat pumps.
Table 9 suggests many processes in industry where high temperature heat pumps can be used. The type of heat pump is indicated according to the temperature range [21], [22].
Heat pump principle
Mechanical heat pumps, frequently used in industry, using the common refrigeration cycle compressing and expanding a refrigerant thereby absorbing heat from a source and releasing heat to a heat sink. This type of heat pumps has four main parts:
– Evaporator
– Compressor
– Condenser
– Expansion valve
Heat is delivered from a waste heat source to a refrigerant in the evaporator. A good heat source has a steady and high temperature when needed. The refrigerant is compressed in the compressor and the temperature is increased. The refrigerant, with a higher temperature is then enters the condenser where the heat is delivered to the sink, whereby the refrigerant is liquefied, figure 10. After that the liquid refrigerant goes to the expansion device where the refrigerant is expanded and cooled down. Usually the expansion device is just a valve. The added energy needed for the compressor to compress the refrigerant is equal to the difference between the heat given to the sink and the heat absorbed from the source. Finally, the refrigerant is expanded through the expansion valve from the condenser to the evaporator. The circuit is closed. This the general principle for all kind of heat pumps. The COPheating = useful heat to sink / used compressor electricity.
The heat pump efficiency is often measured using the Coefficient Of Performance (COPh). This is for heating the ratio between useful heat given to the sink and the compressor’s energy consumption. When cooling, COPc is defined as the heat absorbed from the sink, divided by the electricity consumed by the compressor. According to the second law of thermodynamics, a higher temperature difference between heat source and heat sink, will decrease the COP [6].
Evaporator and condenser
The evaporator is used to transfer heat from the heat source to the refrigerant. The condenser is used to transfer heat from the refrigerant to the heat sink. The refrigerant changes from liquid to gas in the evaporator and in the opposite direction in the condenser. Thereby absorbing or exuding latent heat. The pressures in the evaporator and condenser depend on the boiling curve of the refrigerant. The heat transfer is proportional to the product of the heat exchange area and the heat transfer coefficient. Shell & tube heat exchangers are mostly used in industry for both evaporators and condensers. When using shell and tube evaporators with water there is always a risk of freezing which must be avoided.
In a shell and tube condenser the condensing normally takes part outside the tubes. The refrigerant can also be boiling outside the tubes in an evaporator (normally it is inside). It is much easier to clean the tubes on the inside when the source is polluted. However the volume outside the tubes is normally larger than inside the tubes so the total filling tends to get larger using shell and tube heat exchangers this way. The dimensioning of the condenser and evaporator is an optimization problem. Large surfaces give a high COP but also have a high investment cost. In the specific case of SSAB in Oxelösund another type of evaporator is suggested.
Concerning the two types of shell and tube evaporators with flow inside or outside the tubes. When refrigerant flows inside the tubes and is evaporated and superheated (dry expansion), there is less risk of oil accruing in the evaporator and the refrigerant charge is smaller. In the second case when brine flows inside the tubes and refrigerant boils outside, the oil in the refrigerant must be returned by skimming it of the boiling surface, heating it up and returning it to the suction line at a point where it can reach the compressor. Turbo compressors leak very little oil into the refrigerant (50 ppm). Thus the return of oil can be done even manually with long time intervals. In this later geometry the evaporation does require any following superheating which enhances the COP.
Presently the water (heat source) is originally coming from Nyköpingsån, (a river) which is sweet and is then cleaned in the SSAB plant before it is used in the cooling tower. Thus when using only this water as a heat source a horizontal shell and tube evaporator with refrigerant inside the tube could be recommended. However if also sea water should be used – other forms of heat exchangers would be better.
In shell and tube condenser, the refrigerant is condensed outside the tubes and water (the heat sink) flows inside the pipes. A typical horizontal shell and tube condenser is shown in figure 11. It is possible to sub-cool the liquid in a shell and tube heat exchanger slightly, if the inlet tubes from the sink are first passing through a liquid pool of refrigerant at the bottom [23], [24]. Often subcooling is however performed in a special heat exchanger after the condenser.*
Compressor
Both the pressure and temperature of the working fluid increases in the compressor. Some compressor types can accept a wet inlet – a small fraction of liquid entrained in the gas. Other compressor types require a dry inlet (only pure gas). Most compressors require some oil-lubrication. Oil free types are much more expensive. The oil is entrained in the refrigerant. Turbo compressors require only a small amount of oil, but cannot accept drops, whereas screw compressors require a lot of oil and can accept drops. Compressors are classified as dynamic compression or positive displacement compressors. Both these two compressor types have several subsystems implemented and yield different characteristic data (figure 13).
In positive displacement compressors, like reciprocating- and screw compressors the fluid pressure increases due to reduction of its gas volume. A built in pressure ratio of the compressor can be a result of the physical design. The compressor should then be used around this built in pressure ratio. Reciprocating compressors do not have a built in pressure ratio. Normally it is also possible to achieve higher temperature lift with positive displacement, than with dynamic compressors, though the volumetric flow rate is normally lower than for dynamic compressors. Reciprocating compressors are like combustion engines working with valves and often piston rings [23]. The pressure is pulsating. A tank on the pressure side can smooth out this and a larger rotating balancing mass or a flywheel can smooth out vibrations. Positive displacement compressors often need more maintenance, than dynamic compressors due to wear and tear.
Screw compressor can achieve a high pressure ratio and rather high volumetric flow rate. Then for very high capacities many parallel screw compressor would be needed. Screw compressors have advantages compared to both dynamic and reciprocating compressors, but are not proper for very high capacities, and their necessary auxiliary components are expensive.
They are also able to work in high temperature machines. Some of them can vary their built in pressure ratio and most of them can easily vary their capacity using a built in sliding piston or the rotational speed. Normally oil is used for sealant and inner lubrication. They are also less sensitive to and wet compression and are often most cost effective when the shaft power is less than around 1 MW [23].
In different screw compressors, the volume (and pressure-) ratio can thus either be fixed or adjusted. Compressors with a fixed volume ratio are less energy efficient outside this ratio but have lower capital and maintenance costs and higher durability than variable volume ratio compressors have. Figure 12 illustrates side view of screw compressor [23].
Table of contents :
1 Introduction
1.1 Background
1.2 Aims and objectives
1.3 Methodology
2 Waste heat recovery in Iron and Steel industry
2.1 Waste heat recovery in the US and Sweden
2.2 SSAB recovery potential for heat pump utilization
2.3 Nyköping and Oxelösund heat demand
2.3.1 Oxelösund heat demand
2.3.2 Nyköping heat demand
2.3.3 Nyköping heat demand when heat from SSAB is not available
2.4 Feasibility study and potential for heat pump utilization
2.4.1 Why using the cooling tower?
2.4.2 Heat source (cooling tower) specification
3 The high temperature industrial heat pump
3.1 Industrial heat pumps
3.2 Industrial heat pump applications in general
3.3 Heat pump principle
3.3.1 Evaporator and condenser
3.3.2 Compressor
3.3.3 The expansion valve
3.4 Many types of industrial heat pumps..
3.5 A high temperature heat pump for SSAB
4 Refrigerant
4.1 Environmental indicators:
4.2 Types of the refrigerants:
4.2.1 CFCs
4.2.2 HCFCs (hydrochlorofluorocarbons)
4.2.3 HFCs (hydrofluorocarbons):
4.2.4 Blends
4.2.5 Natural working fluids
4.2.6 Ammonia (NH3)
4.2.7 CO2
4.2.8 Water
4.2.9 Hydrocarbons
4.3 How to choose the proper refrigerant – compilation
4.4 Refrigerant for high temperature heat pump
4.5 Refrigerants suitable for District heating heat pumps
4.6 Result
5 Simulation
5.1 Modeling description
5.2 EES Modeling, of Oxelösund’s heat demand:
5.2.1 Modeling Flow
5.2.2 Heat Pump formulas and relations
5.2.3 P-h and T-s diagram
5.2.4 Simulation results
5.3 Modeling, of Oxelösund-Nyköping – also considering that waste heat could be unavailable.
5.3.1 Heat Pump formulas and relations
5.3.2 P-h and T-s diagram
5.3.3 Simulation result
6 Environment and CO2 emissions
7 Cost
8 Results and discussion
9 Conclusion
Bibliography
10 Appendix
10.1 Appendix A
10.2 Appendix B
10.3 Appendix C
