Process steam generation based on a stroke engine cogeneration unit

Transcription

1 Process steam generation based on a stroke engine cogeneration unit J. POSPISIL, M. LISY, J. FIEDLER, L. CHROBOCZEK Faculty of Mechanical Engineering Brno University of Technology Technicka 2896/2, Brno CZECH REPUBLIC Abstract: This contribution presents the analysis of possible utilizing of a small cogeneration unit for effective production of electricity, heat and process steam. A cogeneration unit (CHP) offers more effective utilizing of primary energy sources in comparison with separate production of electricity and heat. The studied poly-generation system consists from a cogeneration unit based on a stroke engine technology connected with a low-pressure steam generator. Low- pressure steam is led to a compressor for achievement of required steam pressure. Natural gas is considered as the most common primary energy source utilized in small-scale cogeneration units. The presented analysis respects technical parameters of a nowadays cogeneration units and synthesizes technical requirements for production of hot water, electricity and process steam. This paper focuses on environmental benefit and economical assessment of presented ply-generation technology applicable in small- and mid-size technological processes. Key-Words: poly-generation, cogeneration, heat, electricity, steam generation 1 Introduction Poly-generation of different kinds of useful energy is the nowadays trend of sophisticated power production systems. Poly-generation offers significant advantages in comparison with separate energy production (Fig. 1). Electricity is commonly produced in power plants, where a primary chemical energy of fuel is transformed to electricity with the average efficiency about 32 %. The rest of the primary energy leaves the power plants in form of a waste heat. On the second side, the heat energy for heating of water is produced commonly in boilers with the efficiency close to 90 %. Process steam required by technology can be produced in different ways. In this study, a gas heated steam generator is considered as the most common equipment for small production of process steam. The efficiency of a steam generator depends on stem parameters (pressure, temperature). We considered production of saturated steam on absolute pressure level 1.2 atm for quantification of the steam generator efficiency. The corresponding efficiency of the gas heated steam generator is 71 %, it means ratio heat energy and primary energy of fuel. Poly-generation systems generally enable to produce different kinds of energy and occasionally some products (bioethanol, hydrogen, etc.). Main idea of poly-generation is to decrease amount of waste energy rejected without utilizing. Poly-generation systems can be formed by different energy production systems and technological processes. Generally, poly-generation systems have to provide two or more useful outputs in form of energy or other commercial products [1]. Poly-generation significantly increases effectiveness of primary energy sources utilizing and decreases production of CO 2. Small poly-generation units represent effective production of energy close to a position of consumption. A short connection between the position of the generation and the position of consumption is an important advantage that enables to utilize majority of the generated energy. A combined production of electricity, heat and steam offers great potential in energy systems supplying different technological processes. The most common type of poly-generation systems are co-generation (combined heat and power) and tri-generation (combined heat, power and cool). The tri-generation consists from a cogeneration unit connected with a convenient absorption cycle for production of chill water. Hot water or steam serves as a convenient source of a driving heat for a cooling absorption cycle. The terminal cooling capacity of the absorption system depends on the temperature and quantity of the driving heat from a cogeneration unit. Another possible application of poly-generation is simultaneous production of electricity, hot water and process steam. This is very frequent requirement of galvanic coating industry, where a process steam serves for keeping required temperature of galvanic baths and significant amount of electricity is supplied for carrying out of the galvanic process. This paper focuses on comparison of different types of small-scale poly-generation systems with steam ISSN: ISBN:

2 generation based on a stroke engine cogeneration technology. Parametrical studies were carried out for expression of an influence of different technical parameters on the entire effectiveness of poly-generation systems. Utilizing of poly-generation requires simultaneity in consumption of all kinds of produced energy. In a different way, a control system has to decrease power output of the poly-generation unit or the superfluous energy must be accumulated or rejected. technology for small- and mid-size applications. A steam generator is frequently heated by natural gas with regard to low investment and good operation parameters. It is significant to mention that necessary and an expensive part of a steam generator is a water purifier.. Figure 2: Cogeneration unit with separate production Figure 1: Separate energy production 2 Poly-generation systems This section focuses on a particular effective energy supply in process industry. Utilizing of two different poly-generation systems is considered for production of power, hot water and process steam. First, a cogeneration unit and a gas heated steam generator create a poly-generation system for required energy production. Second, a cogeneration unit with successive low-pressure steam generator (heated by hot water from cogeneration unit) is considered. Primary energy saving and major energy flows were compared against a separate production mentioned in the previous section. 2.1 Cogeneration unit with separate production A cogeneration unit with separate production represents the most common application of polygeneration in process industry (see Fig. 2). A cogeneration unit is commonly based on stroke engine Cogeneration unit A cogeneration represents the most common type of poly-generation. Different types of stroke engines are used at cogeneration units and their power efficiency vary in range 20 % to 50 %. The technical parameters of the particular cogeneration unit considered in this study are as follows: power production efficiency 32%, heat production efficiency 59 % (with a sufficient temperature level 90 C), 9 % of energy is taken out from the system by thermal losses. The Fig. 3 shows the real energy fluxes in the considered cogeneration unit. Fig. 3 Energy fluxes in the stroke engine cogeneration unit ISSN: ISBN:

3 2.1.2 Environmental benefit The saving of a primary energy in the poly-generation system with a cogeneration unit and separate production can be expressed as 1 E HW H E + H S W H S Qsave = + +, (1) ηpp ηhp ηsg ηchp ηsg where E is generated electricity, H W is produced heat transferred into hot water, H S is produced heat transferred into steam, η PP is the efficiency of a power plant, η HP is the efficiency of a heating plant, η SG is the efficiency of a steam generator and η CHP is the efficiency of a cogeneration plant. The value of the primary energy saving depends on amount of produced energy. This is not convenient for a clear comparison of different poly-generation systems. The comparative saving of primary energy Q CR presents a better comparative criterion independent on quantity of produced energy. 1 Qsave QCR = (2) E HW η + PP ηhp The fig. 4 shows obtained relation between the comparative saving of primary energy Q CR and the power to heat ratio of a cogeneration unit [2]. The power to heat ratio e COP is parameter expressing the quality of energy transformation processes and the entire technological efficiency of a poly-generation assembly. Ep e COP = (3) H p Where E p is a produced electricity output and H p is heat output to a heating system. Fig. 4 The relationship between the comparative saving of primary energy Q CR and the power to heat ratio of a cogeneration unit 2.2 Cogeneration with sequential production Another high-effective poly-generation system is formed by connection of a cogeneration unit with a low-pressure steam generator driven by heat from the cogeneration unit (see Fig. 5). The generated steam pressure level depends directly on temperature of the hot water leaving a cogeneration unit (commonly 90 C). A low-pressure steam is insufficient for majority of process applications. So, an additional compression of the low-pressure vapor is engaged for increasing of the pressure level on the required value. The compression process consumes Fig. 5: Cogeneration with sequential production significant amount of power. This amount of power must be taken from power production of a cogeneration unit. This decreases the entire net power production of a polygeneration system. In Fig. 6, compression of saturated vapor is drawn in T-s diagram. An initial state of liquid water for steam generation is labeled as the point 1. A direct steam generator changes the saturated liquid 1 to the saturated vapor 5. An indirect steam generation utilizing heat from a cogeneration unit starts in the saturated liquid point 1 too, but on the low-pressure level. (The position of the points 1 is same for both systems due to overlapping of the low- and the high-pressure isobars with the saturated liquide curve.) The saturated liquid 1 is changed to the saturated steam 2 by heat from a cogeneration unit. ISSN: ISBN:

4 Figure 6: Vapor compression expressed in the Temperature-entropy diagram By a subsequent compression of the saturated steam 2, we obtain superheated form on a high-pressure level. The line 2-3 indicates an isentropic compression and the line 2-4 indicates a real compression. The vapor leaving a compressor 4 is on the same pressure line as the required saturated steam 5. Temperature in the point 4 is significantly higher in comparison with steam in the point 5. This over-heating results from utilizing of a compressor and it must be considered for correct expression of primary energy savings. Significant consumption of compression power requires detail analysis of all energy flows in the system, for correct assessment its effectiveness. In this study, we compare a cogeneration with sequential production of steam against separate production of power, heat and process steam Environmental benefit The saving of a primary energy in a poly-generation system with based on a cogeneration unit with sequential production is expressed as E H H ( E + E ) + ( H + H ) 2 W S K W S1 Q = + +, save η η η η PP HP SG CHP (4) where E is generated electricity, H W is produced heat for heating of hot water, H S is heat necessary for direct steam generation, H S1 is heat utilized in a low-pressure steam generator, η PP is the efficiency of a power plant, η HP is the efficiency of a heating plant, η SG is the efficiency of a steam generator and η CHP is the efficiency of a cogeneration unit. In the same way as in the previous sections, the comparative saving of primary energy Q CR (2) is used for quantification of environmental and economical benefits of poly-generation in comparison with separate production of electricity, heat and process steam. Basic parameter values used for the carried out theoretical studies were: the efficiency of a power plant η PP = 0.32, the efficiency of a heating plant η HP = 0.9, the efficiency of a direct steam generator η SG = 0.71, the entire efficiency of a cogeneration unit η CHP = 0.9 and the electricity efficiency of a cogeneration unit η CHPe = The first study was focused on expression of the relationship between the comparative saving of primary energy Q CR and the electricity efficiency of a cogeneration unit η CHPe, see Fig. 7. The obtained results show that increase of the power efficiency of cogeneration unit causes increase of the comparative saving of primary energy. The second derivation of this relationship reaches positive values. It implies higher importance of power efficiency increase at range of the highest tested power efficiencies of a cogeneration unit. Figure 7: The relationship between the comparative saving of primary energy Q CR and the electricity efficiency of a cogeneration unit A compression work is another parameter significantly influencing effectiveness of the studied poly-generation system. A compression work rate depends on thermodynamic quality of a compressor, frequently expressed as the compressor efficiency. Comparative saving QCR [1] 0,43 0,41 0,39 0,37 0,35 0,33 0,31 0,29 0,27 0,25 0 0,2 0,4 0,6 0,8 1 Compressor efficiency [1] Figure 8: The relationship between the comparative saving of primary energy Q CR and the compressor efficiency ISSN: ISBN:

5 The compressor efficiency varies in wide range of values depending on type of the compressor. The Fig. 8 shows the relationship between the comparative saving of primary energy Q CR and the compressor efficiency. The comparative saving increases with increase of the compressor efficiency in all range of tested values. The steepest increase was obtained for low value of compressor efficiency (< 40 %). It is impossible to reach compression efficiency close to 100 %. Real compressors commonly reach efficiency in the range 40 % to 80 %. Reasonable discussion can be led on influence of the high pressure level value on the comparative saving of energy. The Fig. 9 shows a trend in decrease of the comparative saving of primary energy with increase of the high pressure level value. High-effective polygeneration systems must minimize the steam pressure increase by compression. Comparative saving QCR [1] 0,45 0,4 0,35 0,3 0,25 0,2 0,15 0,1 0, High pressure level [MPa] Figure 9: The relationship between the comparative saving of primary energy Q CR and the steam highpressure level steam compression process, namely the compressor efficiency and steam pressure increase. With respect to mentioned relationships, the system offers another application for sophisticated energy supply systems. References: [1] Osterreicher D., Pol O., Concerto Inititiative and Polygeneration, Proceedings of the 1 st European Conference on Polygeneration, Vol.1, No.1, 2007, pp [2] POSPÍŠIL, J.; FIEDLER, J., Applicability of Trigeneration Energy Production for Air-conditioning Systems in Czech Republic, WSEAS Transactions on Mathematics, Vol. 1, 2007, pp [3] Petchers, Neil, Combined heating, cooling & power handbook, Lilburn: The Fairmont Press, Inc [4] Campanari S., Boncompagni L., Macchi E., Microturbines and Trigeneration: Optimalization Strategies and Multiple Engine Configuration Effects, ASME Transaction, Vol. 126, 2004 [5] Minciuc E., Corre O., Athanasovici V., Tazerout M., Fuel saving and CO2 emissions for tri-generation systems, Applied Thermal Engineering 23, pp , Conclusion This contribution presents the analysis of possible utilizing of the ply-generation system formed by a stroke engine cogeneration unit, a low pressure steam generator and a compressor. This system enables simultaneous production of power, heat and process steam. The polygeneration system was compared with separate production of power, heat and process steam. The presented analyses were carried out for parameters required by galvanic coated process. An environmental benefit of the poly-generation process was quantified by expression of the comparative saving of primary energy. Influence of the electricity efficiency of a cogeneration unit, the compressor efficiency and the steam high-pressure level was tested and analysed. The poly-generation system based on a stroke engine cogeneration unit offers more effective utilizing of primary energy sources in comparison with separate production of electricity and heat. The entire effectiveness of this system is strongly influenced by ISSN: ISBN: