The Enhancement of Energy Utilizing Efficiency by Using Low-Temperature Grade Heat of The
THE ENHANCEMENT OF ENERGY UTILIZING EFFICIENCY BY USING LOW-TEMPERATURE GRADE HEAT OF THE STEEL INDUSTRY
LEE, Yong-Kuk; PARK, Heung-Soo; CHANG, Rae-Woong
ENVIRONMENTAL AND ENERGY DIVISION, RIST
Pohang, Korea
the 17th World Energy Congress held in Houston, Texas, USA in September 1998
1. Introduction
Effective utilization of process-consumed energy in any industry not only solidifies the competitiveness between companies through the reduction of energy/production costs, but meets the environmental regulations and contributes to the reduction of environmental cost. When it comes to the steel industry, one of the representative massive energy consumption industries, whose energy efficiency is lower than 60%, it is understood that the development of technology for effective energy utilization is one type of the research that should be steadily carried out.
Methods for the recovery and reuse of exhaust heat have advantages over the development of the new economical technology and the rationalization of heat management of facilities in the following ways : easier in practical application and more effective in energy saving in comparison with investment money. Therefore, this has been regarded as one of the most important technologies of energy saving, and the investment in facilities and R&D have been continuously done. Development of the technology of continuous recovery and reuse of exhaust heat and widened application made the traditional recovery and utilization technology of the exhaust heat such as preheating the combustion air, production of the process steam, or power generation using high pressure steam come to supply limit. Moreover, heat sources themselves became dispersed and lower grade.
Considering the characteristics of present exhaust heat sources and utilization conditions and the practical applicability of exhaust heat recovery technology, it is necessary to develop new technology which utilizes the dispersed low temperature grade exhaust heat sources and makes it easier to transport and reuse them. Much dispute over the recovery methods of low temperature grade heat may arise, but it is widely accepted that recovering it as a form of electricity through driving force of a machine and preheating Blast Furnace Gas (BFG) by recovering the exhaust heat from the boiler are the most desirable methods.
It is known that the most effective heat engine, or power cycle utilizing the low temperature grade heat that can convert it to electricity, is the Organic Rankine Cycle System(ORCS)[1]. This power cycle using hydro-carbonic organic material as a working fluid is a system which generates electricity by using the turning force of the shaft in the turbine driven by high pressure steam emitted from the boiler. In general, water can be used as a working fluid for high-temperature heat sources. However, for low temperature grade heat sources, for the sake of economic feasibility, it is desirable to use organic fluid, which has a lower boiling point and higher vapor pressure than that of water.
As BFG has a lower heating value compared with other gaseous fuels, stored latent heat amount is much larger with preheating. So it is possible to effectively reuse the recovered heat at low preheating temperature. Especially in the power generation boiler in which BFG is used as a main fuel, when recovering the latent heat of exhaust gas and preheating it up to 120, 35% improvement in boiler efficiency and the cost recovery of the first investment within a year are generally expected.
2. Status of waste heat in domestic industries
The types of waste heat sources in domestic industries are exhaust gas, exhaust steam and exhaust water. Waste heat has a wide range of temperature and has a number of sources such as in the steel, ceramic and chemical industries. The waste heat in the iron and steel industry is mainly generated from the reheating furnace, sintering plant and hot stove and its temperature range is 2001000.
The energy sources in the integrated iron and steel industry consist of over 90% coal, a little electric power and petroleum. The iron-making processes which produce steel manufactured from iron ore are composed of coal carbonization, sintering, reduction smelting, refining, casting & rolling, and heat treatment etc. The majority of these processes work at high temperatures, over 1000, which makes a great deal of waste heat. The quantity of this waste heat amounts to 451011kcal a year and about 66 % of that is very difficult to recover (such as the sensible heats of coke oven gas, sintered ore and slag). The remaining 34% can be recovered. But only 15%, 71011 kcal, of the total waste heat is being reused and the rest, i.e. 19%, 8.71011 kcal is exhausted due to problems of facility. Most of these types of waste heat are low-temperature grade waste heat less than 350. Fig.1 shows the status of waste heat and quantity at the Pohang Iron and Steel Co in Korea.
In the iron-making division, about 70% - 1,600,000kcal/T-iron of the total energy consumption of the iron and steel industry is consumed. About 64% of total input energy is used as available heat and the rest is exhausted as unrecovered state. In the steel-making division only about 2.8% of the total energy is consumed, but energy quantity that it is dealing with is enormous because pig iron's heat quantity is 9% of the total energy. For example, exhaust gas heat occupies 5% of the total energy in the decarbonizing process, the main process of the steel-making, and about 35% of it is effused as unrecovered. The next step after the steel-making process, the rolling process, occupies about 16% of the total energy consumption. And exhaust heat at 700 900 is released from the reheating furnace where slab is heated or cooled. The exhaust heat is recovered as preheating the combustion air and the gaseous fuel, local area heating, etc., but a vast amount of the exhaust heat produced from this process is still unrecovered.
Exhaust gases in the ceramic industry are flue gas and kiln exhaust gas at 150 450. In the chemical industry exhaust gases are latent heat from processes, sensible exhaust gas heat, and exhaust vapor at 60 400. In other industries - such as the textile industry, the paper industry, and the food industry - the waste heat sources are exhaust gas and vapor, condensed water at 50 400.
As reviewed in the previous paragraphs, heat recovery technology is a very urgent field of research. Heat recovery technology, especially from low-temperature grade heat sources, will be the last target in that research field. Further, it is desirable to recover the waste heat as electricity BFG preheating source.
3. Heat recovery with organic rankine cycle
The basic principles of the ORC (Organic Rankine Cycle) are similar to those of the conventional Rankine cycle. But the major difference is that the working fluid in the ORC is an organic fluid which has a lower boiling point and a higher vapor pressure than that of water, which improves the total performance or cycle efficiency[2]. Steam produced from the evaporator turns the turbine stage, which makes it possible to transform axial power into electric energy. This cycle shows characteristics similar to the ideal Carnot cycle. The Carnot cycle, which is made up of 2 isothermal processes and 2 isentropic processes between 2 separate heat sources, cannot be realized because of the following restrictions : (1) Heat transfer between the heat exchanger and the heat source without any temperature difference is required, but that is not possible. (2) Up to now a fluid machine which can compress reversibly the working fluid at 2-phase flow has not been invented. Therefore, the real objective of the research is focused on the development of a highly efficient cycle similar to the Carnot cycle, represented by the Rankine cycle. Operation of the Rankine cycle is depicted in Fig.2, and the process descriptions are as follows :
1-2 : The process of driving the turbine by high pressure steam after passing the mist separator
2-3 : The regenerative process which preheats condensed working fluid by high temperature working fluid after passing the turbine.
3-5 : The process of condensing high temperature working fluid after passing the regenerator.
5-6 : The process of compressing working fluid after passing the condenser by the pump.
6-8 : The process of preheating liquefied working fluid during this process.
8-9 : The process of evaporating the preheated working fluid at saturated state by the evaporator.
9-1 : The process of separating the mist in high pressure steam by the mist separator.
Efficiency calculation in this cycle is shown below mathematically. The maximum work (Wmax) when it is applied to the situation of receiving heat Q at T and releasing heat at T0 is
Wmax = Q(T-T0)/T
The maximum work here, Wmax, is exergy which means the maximum available work, and it also means the potential energy which can be converted to axial power from the waste heat source. Another method to calculate the system efficiency is described below. Q1 is the heat input to the waste heat source(boiler) and We is the electric output of the system. Thus, the overall system efficiency, ET0, is
ET0= We /Q1
Boiler efficiency EB is defined as
EB = QB / Q1
where QB is the sum of the total energy of preheating, evaporating and superheating the working fluid. Turbine efficiency and generator efficiency are defined as follows :
ET = WT / WAD
EG = WE /WT
where WAD is the adiabatic expansion work by the steam at the turbine stage, and WT the turbine output.
Efficiency on the T-S diagram, i.e. the diagram efficiency, is generally accepted as thermal efficiency by the 1st thermodynamic law and is defined as
EI = WAD / QB
As described above, the total system efficiency, ET0, can be obtained from the multiplication of each compartment, i.e.
ET0 = E R EB ET E G
The Rankine cycle is a device to produce driving force by transforming the working fluid at a low temperature into steam at high temperature and high pressure. It is recommended to use a hydrocarbonic organic fluid having a higher saturation vapor pressure and a lower boiling point than that of water if you want to recover waste heat at low-temperature grade as power. Organic fluid also has a lower specific volume than that of water, which makes it possible to minimize the system dimension and recover the waste heat economically.
Unlike working fluids for the conventional Rankine Cycle system, the working fluid for ORCS should have a lower boiling point and a higher vapor pressure than that of water (such as Freon or toluene). Selection of organic fluid depends on the characteristics of the system, but some general restrictions in the selection of working fluids are as follows :
- High recovery efficiency of power
- Low costs of installation, equipment, maintenance, and operation
- Keeping the proper vapor pressure in the gamut of usage
- High critical temperature for safe elevation of evaporating temperature
- High adiabatic enthalpy drop for obtaining large power output
- Low latent heat for lowering heat loss
- High specific heat for enlarging heat capacity
- High chemical and thermal stability
- No combustability for the protection of explosion
- Self-lubricity for the smooth rotation of the turbine and the pump
- Low viscosity for lowering the fluid friction in piping lines
- Low cost and easy purchasing
- Coexistency with engineering material
In consideration of our experience and experimental data, HCFC-123 and HCFC 225a/b are chosen to be working fluids for our system. These show the appropriate properties and behavior within the restrictions stated above.
4. Heat recovery with separate type heat pipe system
The heat pipe is a kind of heat transfer equipment which transports the latent heat of a working fluid charged within a closed tube from phase formation (evaporation / condensation). Working fluid undergoes phase transformation within the heat pipe and circulates between the evaporation side and the condensation side. Evaporated vapor moves from the evaporation side to the condensation side by pressure gradient, while the condensate returns to the condensation side by the capillary force of the wick within the heat pipe or the gravitational force or the centrifugal force. The former type heat pipe is generally referred to as the heat pipe, and the latter type of heat pipe, having no wick and using the gravitational force, is known as the thermosyphon. Wick type heat pipe can operate well regardless of the positions of the evaporation side and the condensation side, but the evaporation side should be placed at higher position than the condensation side for the thermosyphon.
Fig.3 shows several types of thermosyphon[3]. Thermosyphons are classified as either open type thermosyphon or the closed type (due to the open-endedness of working fluid). Or they are categorized due to the flow directions of vapor and condensation liquid : the counter current thermosyphon and the co-current one. The separate type thermosyphon within which the evaporation side and the condensation side are separate and connected to each other by the evaporation/condensation tube is a co-current thermosyphon where the evaporated vapor and condensation liquid flows in the same direction. There are an increasing number of examples of application nowadays, because this type of the thermosyphon shows some merits, such as ease in manufacturing and application, and higher heat transfer performance[4].
A design program was made to find the optimum design condition by the repeated calculations on the thermal equations. The calculation procedure to do the thermal design is shown below.
- Calculate the entrance and exit temperatures using the basic design conditions and the thermal equilibrium equations.
- Determine the arrangement of the heat exchangers.
- Determine the number of the heat exchangers per heat transfer loop.
- Determine the pitch and type of the fin, and calculate the fin efficiency.
- Calculate the flow rates of exhaust gas and BFG.
- Calculate the heat transfer quantity, the exit temperature, the physical properties of exhaust gas and BFG for each heat transfer loop.
- Calculate the heat transfer quantity and the exit temperature for the overall heat transfer loop.
- Calculate the overall pressure drop.
The final design values are obtained by the repeated calculations on the system balances with the changes in design parameters, the exit temperature, the BFG temperature, the pressure drop amount, the size of a heat exchanger, the number of heat exchangers to meet the given conditions.
Heat from the heating fluid is transferred to the subject fluid in the following paths in the separate type heat pipe system ; convective heat transfer between the outer surface of the evaporator and the heating fluid, conductive heat transfer within the evaporator tube, convective heat transfer(evaporation) between the inner surface of the evaporator and working fluid, heat transport by the produced vapor, convective heat transfer (condensation) between the inner surface of the condenser and the vapor, conductive heat transfer within the condenser tube, and convective heat transfer between the outer surface of the condenser and the subject fluid.
The total heat exchanger system consists of 6 heat transfer loops, and each loop is constructed as shown in Fig.5. Heat exchange tubes for the evaporator and the condenser are connected to the upper and lower headers. Produced vapor and condensation moves back and forth between the evaporator and the condenser through the connection tube equipped within the central part of the header. 44 heat exchange tubes of the evaporator and 24 heat exchange tubes of the condenser in the axial direction are connected to the header in 3 rows.
The header, drawn in Fig.5, connects the heat exchange tubes which belong to one heat transfer loop, and moreover the upper header of the evaporator prevents droplets of working fluid from flowing into the condenser through the vapor connection tube. 2 phase-flow of vapor/liquid within the evaporator tube of the heat pipe can be operable at normal condition. The volume of the fluid is increased 1.5 to 3 times larger than its initial volume. Therefore, the upper header should contain a plenum room for the expansion of the working fluid. The designed volume of the upper header of the evaporator was 1.5 times larger than the inner volume of the evaporator for this system.
Fig.6 shows the reduction amount of the fuel in variation with the preheating temperature. Reduction amount depends on the BFG consumption amount, so the larger the boiler, the larger the fuel reduction effect. Fuel reduction amount of 5,000Nm3/hr can be achievable when the consumption rate of BFG is 90,000Nm3/hr and the preheating temperature is 120oC.