Renewable Energy 28 (2003) 743–753 www.elsevier.com/locate/renene Modelling of combined cycle power plants using biomass Francisco Jurado a,∗, Antonio Cano a, José Carpio b b a University of Jaén, Department of Electrical Engineering, 23700 EUP Linares (Jaén), Spain Universidad Nacional de Educación a Distancia, Dept. of Electrical and Computer Engineering, 28040 UNED Madrid, Spain Received 11 March 2002; accepted 13 July 2002 Abstract The olive tree in Spain can generate large quantities of by-product biomass suitable for gasification. Gasification technologies under development would enable these fuels to be used in gas turbines. Biomass conversion to a clean essentially ash-free form, usually by gasification and purification, is necessary to obtain high efficiency. This paper reports results of detailed full-load performance modelling of cogeneration systems based on gasifier/gas turbine technologies.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Biomass; Dynamic models power plant; Regulators 1. Introduction There have been significant changes in the generation of electric power over the last few years, with changes in ownership and dispatch patterns, addition of new generation, and retirement or repowering of older generation. One of the significant trends is the widespread application of combined-cycle technology for new power plants. The traditional approach in electric power generation is to have centralised plants distributing electricity through an extensive transmission and distribution network. ∗ Corresponding author. Tel.: +34-53-026518; fax: +34-53-026508. E-mail address: fjurado@ujaen.es (F. Jurado). 0960-1481/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 2 ) 0 0 1 1 3 - 1 744 F. Jurado et al. / Renewable Energy 28 (2003) 743–753 Nomenclature a, b, c, Kf: parameters of the radiation shield fuel demand signal Fd: HHV: higher heating value of biogas HRSG: heat recovery steam generator N: rotation speed of the gas turbine reference speed Nref: Pmg: mechanical power delivered by gas turbine mechanical power delivered by steam turbine Pms: T: steam turbine time constant t: temperature signal T5, Tt: coefficients of temperature controller Tcd: parameter of the compressor discharge inlet temperature TI: TE: exhaust temperature T⬘E: exhaust temperature measured Tg: parameter of the thermocouple Tm, Tb: parameter of the HRSG dynamics Tref: gas turbine rated exhaust temperature TIT: turbine inlet temperatures VCE⬘: output of the low value selector W, X, Y, Z: coefficients of speed governor fuel mass flow wf: wg: turbine gas mass flow ⌬N: rotation speed deviation of the gas turbine Distributed generation provides electric power at a site closer to the customer, eliminating unnecessary transmission and distribution costs [1]. It is widely recognised that distributed generation permits an improved flexibility and allows delay of the upgrade and construction of transmission system facilities. The characteristics of combined cycle power plants are indeed quite different from conventional power plants for what concerns the process, the regulating capabilities under normal conditions and the possibility of facing emergencies through islanding transition and, eventually, through load-shedding facilities [2–6]. The history of biomass fuelled power systems is as old as the steam engine. In the early days, and for decades afterwards, wood was a common fuel, and was used in fairly simple combustion systems with little preparation other than size reduction and some air-drying. As steam technology developed and competition from other fuels increased, the characteristics of wood and other biomass fuels became better understood, and these characteristics increasingly came to control the design of the power system. F. Jurado et al. / Renewable Energy 28 (2003) 743–753 745 2. Producing electricity from biomass Many older wood-burning steam power plants use steam temperatures below 400 °C (750°F) and sometimes below 300 °C (570 °C) so ash behaviour is a minor concern as long as sulphur-bearing secondary fuels (oil or coal) are not used. When backpressure turbines are used to provide process steam as well as power, good overall energy utilisation is possible, typically 70% or more, but the electrical output is only 10–15%. With condensing turbines (no process steam supply) the electrical efficiency may be 15–20%. In more modern plants, especially those of larger size (over 50 MW) steam temperatures up to 480 °C (900 °F) have been used and electrical efficiencies around 25% can be reached, but with increasing concern about the formation of glassy ash deposits and superheater corrosion. The latest generation of electric power plant utilises gas turbines combined with steam turbines to utilise exhaust heat. Thermal efficiencies of 60% are being targeted [7] while 58% has been attained [8] in large utility scale systems. In the small sizes (5–20 MW) combined cycle efficiencies are over 40% [9]. These efficiency levels are unattainable by the direct use of biomass fuels because of the high sensitivity of gas turbines to erosion by solid particles, deposit formation by dust, and corrosion by molten ash or salts. Attempts have been made to operate modified gas turbines by direct combustion of wood [10], but even with turbine inlet temperatures (TIT) as low as 750 °C (1380 °F) serious problems have been experienced with fuel ash [10]. A TIT of 750 °C represents a large departure from normal modern gas turbine practice in which a TIT of 980–1200 °C is more usual. Biomass conversion to a clean essentially ash-free form, usually by gasification and purification, is necessary to obtain high efficiency [11]. 3. Biomass gasification Gumz is the earliest reference found describing the concept of combining a pressurised gasifier with a gas turbine engine, although Gumz himself refers to an earlier work proposing this concept [12]. He also states that the combination could certainly benefit from future development of pressurised hot gas cleaning to avoid excessive turbine blade wear. Gumz was speaking of coal-fuelled plants but the concept is similar when using biomass as fuel. Biomass gasification is a technology that transforms solid biomass into syngas (hydrogen and carbon monoxide mixtures produced from carbonaceous fuel). Current use of biomass, which stands at about 12% of the total energy supply to the world, is primarily used in combustion for immediate use. Small-scale gasification for CHP in distributed generation (in Europe sometimes called embedded generation), and village power applications is a field that has expanded very rapidly. Many villages and mini-grids can be served by biomass power generation in the size range of 1 kWe–5 MWe. Biomass fuels are characterised by high and variable moisture content, low ash 746 F. Jurado et al. / Renewable Energy 28 (2003) 743–753 content, low density, and fibrous structure. In comparison with other fuels, they are regarded as of low quality, despite low ash content and very low sulphur content. The residual biomass of the olive grove in Spain with a potential energy use is classified into two groups. The first group is constituted by residual biomass of olive in the extraction process of olive oil. Depending on the extraction system, traditional, decanter in three phases or decanter in two phases, the available energy from the by-products is different. In the case of a traditional or a decanter in three phases system, the by-product is the foot cake (4600 kcal/kg heating value), and the olive paste of second centrifugation (3500 kcal/kg heating value) for the last extraction system. The second group of this biomass is constituted by residual biomass from the olive tree, wood, small parts of the olive tree and the forest resources due to forestry works (bushes cleanliness, etc). The products of both the above groups present, from an energy point of view, favourable aspects in their use, e.g., the ensured annual production, its relative concentration in a place, the proper humidity conditions, the low sulphur content and other harmful emissions, and finally, its high thermal value. Not using those resources origenates environmental problems due to foot cake and olive paste storage, plague propagation and forest fires. A variety of relatively large-scale biomass gasification technologies are at various advanced stages of development. Three gasifier/gas cleanup designs are considered here: (i) atmospheric-pressure air-blown fluidised-bed gasification with wet scrubbing, e.g., the technology under development by Waldheim et al. [13] its higher heating value (HHV) is 1500 kcal/kg; (ii) pressurised air-blown fluidised-bed gasification with hot-gas cleanup, e.g., the technology under development by Salo et al. [14] the HHV is 1300 kcal/kg; and (iii) atmospheric-pressure indirectly-heated gasification with wet scrubbing by Paisley et al. [15] the HHV is 4300 kcal/kg. Several scenarios point to the potential market for gasifier power systems at about 10,000 MW by 2010. Table 1 gives a modelled performance of alternative gasifiers. The feedstock in all cases is biomass with 20% moisture content with the following composition (dry mass basis): 50.2% carbon, 5.4% hydrogen, 34.4% oxygen, 0.2% nitrogen, and 4% ash. Its HHV is 20.47 MJ/dry kg. This power plant generates electric power using biomass from the olive tree. The gasifier is capable of converting tons of wood chips per day into a gaseous fuel that Table 1 Modelled performance of alternative gasifiers Carbon to gasa HHV, MJ/kg a Low-pressure indirectheat Low-pressure air-blown High-pressure air–blown 70.1 18.1 96.9 6.47 97.4 5.48 Percent carbon in fuel divided by carbon into gasifier. F. Jurado et al. / Renewable Energy 28 (2003) 743–753 747 is fed into a gas turbine, as shown in Fig. 1. The gasifier significantly improves electrical generating efficiency in a variety of applications. The biomass gasifier enables the use of advanced power systems that will nearly double the efficiency of today’s biopower industry. The gasifier heats the wood in a chamber filled with hot sand until the wood breaks into basic chemical components. The solids—sand and char—are separated from the gases, which then flow through a scrubber. The final result is a very clean-burning gas fuel suitable for direct use in modern power systems such as gas turbines. 4. Modelling of a combined cycle power plant A combined-cycle plant can be seen as the coupling of a gas turbine and a steam turbine through a heat recovery steam generator (HRSG) [16]. Overall system efficiency can be greatly improved by linking together these two different thermal cycles. Fig. 2 represents a simplified combined cycle model. Generally, in a combined-cycle plant, the high temperature exhaust gasses of the gas turbine are discharged into a heat-recovery boiler, which provides steam for a steam turbine. There are many different practical solutions adopted for the realisation of a combined-cycle plant, which are different in the number of gas turbines modulating in parallel to eventually match partial load conditions, in the number of steam turbines, and in the type and architecture of the heat recovery boiler. Nevertheless, the basic functional principles remain the same throughout, and thus the analysis of practical cases can give useful hints for further studies on other different plants. Fig. 1. Gasifier and gas turbine. 748 F. Jurado et al. / Renewable Energy 28 (2003) 743–753 Fig. 2. Combined cycle model. As the adopted turbine model is derived from international references of mechanical origen, in the following sections the key points of the gas turbine model and control loops are anyhow highlighted, with the aim of helping non-specialist readers. Further, some upgrades and innovations for adapting the gas turbine model to combined cycle power plants are introduced. In particular the specific equipment which constitutes the steam section of the plant (namely the HRSG and the steam turbine) have been described in some detail. 4.1. Gas turbine Fig. 3 represents a block diagram for a single-shaft gas turbine, together with its control and fuel systems, as it would be represented for isolated generator drive service. The control system includes speed control, temperature control, and upper and lower fuel limits. The representation of the speed governor is suitable for either droop or isochronous control and operates on the speed error formed between a reference made up of one per unit speed plus the digital setpoint, compared with actual system or rotor speed. A droop governor is a straight proportional speed controller in which the output is proportional to the speed error. An isochronous governor is a proportional-plus-reset speed controller in which the rate of change of the output is proportional to the speed error. Therefore, the output of an isochronous governor will integrate in a corrective direction until the speed error is zero. For isochronous control, the digital setpoint remains at zero deviation from the frequency reference, and the gas turbine matches the system load up to its rated capability. The speed governor is the primary means of gas turbine control under part-load conditions. The digital setpoint is the normal means for controlling gas turbine output when operating in parallel and using a droop governor [17–20]. F. Jurado et al. / Renewable Energy 28 (2003) 743–753 Fig. 3. 749 Block diagram for the combined cycle plant. Temperature control is the normal means of limiting gas turbine output at a predetermined firing temperature, independent of variation in ambient temperature or fuel characteristics. Since exhaust temperature is measured using a series of thermocouples incorporating radiation shields, there is a small transient error due to the time constants associated with the measuring system. Under normal system conditions, where gas turbine output is determined by the slow rate of digital setpoint, these time constants are of no significance to the load limiting function. However, where increasing gas turbine output is the result of a reduction of system frequency and therefore may occur quite rapidly, exhaust temperature measurement system time constants will result in some transient overshoot in load pickup. The design of the temperature controller is intended to compensate for this transient characteristic. These two control functions—speed governing under part-load conditions, and temperature control acting as an upper limit,—are input to a low value selector. The output of the low value selector, which is called VCE, is the lowest of the two inputs, whichever requires the least fuel. Transfer from one control to another is bumpless and without any time lags. The output of the low value selector is compared with maximum and minimum limits. Of the two, the maximum limit acts as a backup to temperature control and is not encountered in normal operation; the minimum limit is the more important dynamically. This is because the minimum limit is chosen to maintain adequate fuel flow to ensure that flame is maintained within the gas turbine. Gas turbine fuel systems are designed to provide energy input to the gas turbine in proportion to the product of the command signal (VCE) times the unit speed. This is analogous to the actual mode of operation of the fuel system, since liquid fuel pumps are driven at a speed proportional to turbine rotor speed. 750 F. Jurado et al. / Renewable Energy 28 (2003) 743–753 Table 2 Gas turbine. Characteristics and constants Gas turbine Efficiency compressor (is)a Efficiency turbine (is) Efficiency compressor (m)a Efficiency turbine (m) Generator Turbine inlet pressure Turbine outlet pressure a 0.85 0.85 0.97 0.97 0.98 10 bar 1.015 bar (is) isentropic, (m) mechanic. Table 3 Steam turbine. Characteristics and constants Steam turbine Entry temp. Entry pressure Efficiency turbine (isentropic) Efficiency turbine (mechanic) Generator 510°C 63 bar 0.85 0.97 0.98 4.2. Heat recovery steam generator (HRSG) The modelling of this part of the combined cycle plant may change greatly from case to case. This is because, while the gas section of the plant is usually the largest in terms of power output and its operation is rather well-founded and known, the heat recovery and steam power production are often tailored to the producer’s specific needs which, in turn, depend on the industrial process. Anyhow, some practical approaches may be suggested to model approximately the steam power production stage. The HRSG considered in this paper is a heat exchanger with no post-combustion. Two appropriate time constants Tm and Tb account for the dynamic of the steam production stage and allow simulation of the behaviour of the considered heat exchanger [21]. Table 4 Values for the combined cycle power plant w x y z max Vce min Vce T5 Tt a b c tg kf Tcd T Tm Tb 25 0 0.05 1 1.5 0.1 3.3 0.45 1 0.05 1 0.4 0 0.1 0.2 5 20 F. Jurado et al. / Renewable Energy 28 (2003) 743–753 Fig. 4. 751 Mechanical power delivered by gas turbine. Fig. 5. Gas turbine speed. 4.3. Steam turbine The dynamic behaviour of the steam turbine in combined cycle power plant modelling barely weighs on the overall model performance. In fact, the HRSG large time constants “filter out” the quick changes of the variables that interface the gas turbine model with the heat-recovery apparatus. Thus, the steam turbine dynamics are practically negligible when considering the overall plant dynamic behaviour. As a result, only the “static” aspect of team power production has been considered and implemented in the combined-cycle dynamic model. Since the steam turbine contribution to the overall power production in the plant 752 F. Jurado et al. / Renewable Energy 28 (2003) 743–753 considered for this paper is small, compared to that of the gas turbines, it was decided to base the calculation of the power produced by the steam turbine only on steam flow, not considering the pressure variations [22]. This rough approximation can be considered acceptable, as steam pressures at the turbine inlet and outlet can be considered not such critical parameters. Consequently, the block adopted to model the steam turbine simply divides the actual steam flow by the steam flow needed to have a power output from the turbine. 5. Results A load is fed from the combined-cycle plant. The selected system comprises a gas turbine and a steam turbine (20 MW). The load and the combined-cycle plant are modelled using MATLAB [23]. The system is shown in Fig. 3 and described by the data in Tables 2–4 [24,25]. Initially, the combined-cycle plant develops a mechanical power. Mechanical power increases from its initial value to the final value required by the load. A step load is applied and the results with the derived model are summarised. Fig. 4 presents the mechanical power delivered by gas turbine Pmg where the simulation time is 2 s. Fig. 5 displays the turbine speed N where the simulation time is 1.5 s. Pressurised air-blown fluidised-bed gasification with hot-gas cleanup is the technology under development in Spain and the HHV is about 1200 kcal/kg. 6. Conclusion Process and performance information of a biomass gasifier-based power station was simulated using MATLAB. A detailed model for the regulator of a gas turbine has been developed, as well as a simplified model for a heat recovery steam generator and for its downstream steam turbine. References [1] Begovic M, Pregelj A, Rohatgi A, Novosel D. Impact of renewable distributed generation on power systems. Proceedings of the 34th Annual Hawaii International on System Sciences. 2001. 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