Improvement of part-load performance of gas turbine by adjusting compressor inlet air temperature and IGV opening

Abstract

A novel adjusting method for improving gas turbine (GT) efficiency and surge margin (SM) under part-load conditions is proposed. This method adopts the inlet air heating technology, which uses the waste heat of low-grade heat source and the inlet guide vane (IGV) opening adjustment. Moreover, the regulation rules of the compressor inlet air temperature and the IGV opening are studied comprehensively to optimize GT performance. A model and calculation method for an equilibrium running line is adopted based on the characteristic curves of the compressor and turbine. The equilibrium running lines calculated through the calculation method involve three part-load conditions and three IGV openings with different inlet air temperatures. The results show that there is an optimal matching relationship between IGV opening and inlet air temperature. For the best GT performance of a given load, the IGV could be adjusted according to inlet air temperature. In addition, inlet air heating has a considerable potential for the improvement of part-load performance of GT due to the increase in compressor efficiency, combustion efficiency, and turbine efficiency as well as turbine inlet temperature, when inlet air temperature is lower than the optimal value with different IGV openings. Further, when the IGV is in a full opening state and an optimal inlet air temperature is achieved by using the inlet air heating technology, GT efficiency and SM can be obviously higher than other IGV openings. The IGV can be left unadjusted, even when the load is as low as 50%. These findings indicate that inlet air heating has a great potential to replace the IGV to regulate load because GT efficiency and SM can be remarkably improved, which is different from the traditional viewpoints.

Graphical abstract

Keywords

inlet air temperature / inlet guide vane (IGV) opening / part-load / equilibrium running line / gas turbine (GT) efficiency / surge margin (SM)

1 Introduction

In the field of gas turbine (GT) power generation, many GT plants in the world, especially in China, frequently operate in part-load conditions. This may be due to the fact that GT power plants mainly undergo peak load regulation by power grids and are subject to the operation regulation mode of GTs. The traditional method is to turn down the inlet guide vane (IGV) to decrease inlet mass flow for reducing the load. If the load must be further reduced, it shall be realized through fuel regulation. This scheme not only markedly increases extra flow loss but also decreases GT efficiency resulted from the low turbine inlet temperature, and then substantially affects the operating economy of GT units. Therefore, an analysis for improving GT performance under part-load conditions is necessary. Many studies were widely conducted on the improvement of part-load performance in GTs. Especially, the IGV control was extensively studied and used to improve the part-load performance of heavy-duty GTs [1] and their combined cycles [2]. He et al. [3] investigated the GT part-load performance run with different compositions of syngas and the impacts of the variable IGV. Kim et al. [4–7] proposed models to analyze the relationship between design performance and part-load performance of GTs and their combined cycle plants. The use of variable IGV (VIGV) and variable guide vane (VGVs) was suggested for improving the part-load performance of GTs and stationary combined cycles. In addition, VGVs and variable area nozzle (VANs) were also adopted for achieving higher part-load efficiency of GTs [8–10].

It is well known that compressor inlet air temperature has been extensively valued as an important factor that affects the output and efficiency of GTs and their combined cycles [11,12]. Especially in summer, the power output and efficiency of GT decrease significantly due to the rise in ambient temperature [13]. Many GT power plants frequently use the inlet air cooling technology to improve GT performance in the world [14–16]. Compressor inlet air temperature is influenced by the seasonal variation effects or the inlet air cooling technology, which are widely investigated. Another type of the inlet air temperature adjusting technology, the inlet air heating technology, were studied. To the authors’ knowledge, the earliest research on inlet air heating technology, inlet bleed heating (IBH), was proposed by GE Corporation [17]. IBH adopted the exhaust air of the compressor to heat the inlet air, which could lead to the degradation of GT efficiency. In addition, some studies focused on the positive influence of the inlet air heating technology on the combined-cycle efficiency under part-load condition. Miroslav and Otto [18] adopted a dry condenser as the waste heat source for heating compressor inlet air, which could achieve fuel savings under part-load conditions. Then, less fuel needs to be consumed in GTs for the power output of the requested combined cycle. The average electric efficiency increases from 44% to 45% at the given load in condensing mode. Yang et al. [19] proposed a novel method to improve part-load performance for the combined cycle by adopting the compressor inlet air preheating technology. The heating source is from the exhaust flue gas which comes from the heat recovery steam generator (HRSG). The HEAT-T3-T4 regulation can reduce the fuel consumption rate by increasing the compressor outlet air temperature to achieve a better combine cycle efficiency and exergy efficiency. Nevertheless, few researchers paid enough attention to the positive effect of inlet air heating technology on GT performance. Recently, Liu et al. [20] noticed that the inlet air heating technology can also effectively improve GT efficiency by using the waste heat source, which includes the exhaust gas of a combined- or single-cycle GT. During heating process, inlet air temperature approaches the optimal temperature to make the running point move toward the design condition to improve GT efficiency. Therefore, inlet air temperature can be adjusted by cooling or heating technologies.

However, few studies have been conducted to investigate the correlation of adjusting the inlet air temperature and the IGV opening for part-load condition. In China, a new inlet air heating system was recently used in a GE9171E power plant in Tianjin municipality. The heating resource was generated by the waste heat of a combined-cycle heat recover steam generator (HRSG). This inlet air heating system is combined with IGV opening regulation to improve GT efficiency while keeping part-load constant. Some operating data from the said GT power plant indicates when the load rate (the operating power quantity divided by the baseload) is constant, the increase of ambient air temperature can improve GT efficiency, such as an efficiency improvement of 0.6% at a 60% load and 0.13% at a 85% load. Meanwhile, the change of inlet air temperature corresponds to the small adjustment of the IGV opening.

This paper focuses on the correlation of the inlet air heating technology and the IGV opening adjustment for part-load of a heavy-duty GT operating on its own, and discovers that there is an optimal matching relationship between the IGV opening and the inlet air temperature. Both the inlet air heating technology and the IGV opening adjustment are used to improve GT performance. More importantly, the inlet air heating technology has a great potential to replace IGV to regulate load because GT performance can be remarkably improved, and this finding is different from the traditional viewpoints.

This paper first demonstrates the model and calculation method adopted in the study. It then mainly deals with the calculation and analysis of equilibrium running lines in the compressor and turbine characteristic curve. After that, it presents the detailed analysis of performance parameters of the equilibrium running points, focusing on the change of GT efficiency and surge margin (SM) with different IGV openings under part-load conditions. Finally, it gives the conclusion.

2 Model and calculation method

The research object of this paper is a heavy-duty GT with a single-shaft structure, which is composed of a compressor with an IGV, a combustion chamber, and a turbine. In practical conditions, a compressor consists of a number of stages, and the cooling air flow is extracted from the compressor inter-stages and the compressor exit [21]. In this paper, the turbine and the compressor are simplified, as shown in Fig. 1. The simplified modeling is generally adopted for GT performance analysis to reduce the required computational load.

The primary characteristic of component matching is represented by power balance, i.e., the power output of power components is equal to the sum of the output power and the power consumed by the driven components. Component matching also includes the speed balance of coaxial parts and the corresponding internal mass flow and pressure balance. Flack [22] calculated and analyzed the single-shaft GT performance by using a parameter-cycle method based on the GT characteristic curves and component-matching conditions. Plis and Rusinowski [23] presented a simulation model of PG9171E GT which contains partial models to calculate energy assessment indicators and non-measured operating parameters. In the present paper, the thermal-cycle calculation program is initially made and the compressor model, the combustion chamber model, and the turbine model are considered collectively. In addition, Qusai et al. [24] established a thermodynamic simulation model of the main components of a single-shaft GT, and developed a calculation method of steady-state performance for analyzing the distance between the equilibrium running line and the vibration kick boundary of the compressor. Pontus et al. [25,26] discussed the effect of fuel composition on GT performance by analyzing the change in the equilibrium running line. Therefore, the equilibrium running line is widely used in the study of GT performance. However, studies are lacking in the comprehensive analysis of relational variations of GT performance and the compressor inlet air temperature relative to the equilibrium running line.

The off-design performance for part-load was modeled using Matlab [27,28]. The GT model was established by the mass balance, the energy balance, and stoichiometric equations. The ambient air was in ISO condition (15°C, 1.013 bar, and a 60% relative humidity). The fuel lower heating value (LHV) was assumed as 47229 kJ/ kg for all calculations. The GT rotational speed was stable at 3000 r/min.

2.1 Modeling of compressor

The modeled compressor is equipped with an IGV for the regulation of the compressed inlet air mass flow rate. The inlet conditions like air mass flow and pressure, and inlet pressure loss in the air filter are used as input data. The reference ambient condition is the ISO condition. The simulation model of an axial compressor includes generalized compressor maps for different IGVs. The pressure ratio map describes the relationship among the air mass flow rate m_c, the pressure ratio π_c and the speed N_c. In addition, the internal efficiency map shows the relationship among the air mass flow rate, the compressor internal efficiency η_c and the speed. The details of generalized compressor maps are given in Section 3.2. The maps are composed of the following similarity parameters: corrected air mass flow rate, corrected compressor speed relative pressure ratio, and isentropic efficiency. Furthermore, the corrected air mass flow rate m_Ccorr and the corrected compressor speed N_Ccorr of the compressor are adopted to calculate the running line, as expressed in Eqs. (1) and (2).

(1){{\displaystyle m}}_{\mathrm{C}\mathrm{orr}}={\displaystyle \frac{{\displaystyle \frac{{{\displaystyle m}}_{\mathrm{c}}\cdot \sqrt{{{\displaystyle T}}_{\mathrm{1}}}}{{{\displaystyle P}}_{\mathrm{1}}}}}{{{\displaystyle \left({\displaystyle \frac{{{\displaystyle m}}_{\mathrm{c}}\cdot \sqrt{{{\displaystyle T}}_{\mathrm{1}}}}{{{\displaystyle P}}_{\mathrm{1}}}}\right)}}_{\mathrm{rated}}}},

(2){{\displaystyle N}}_{\mathrm{C}\mathrm{orr}}={\displaystyle \frac{{\displaystyle \frac{n}{\sqrt{{{\displaystyle T}}_{\mathrm{1}}}}}}{{{\displaystyle \left({\displaystyle \frac{n}{\sqrt{{{\displaystyle T}}_{\mathrm{1}}}}}\right)}}_{\mathrm{rated}}}},

where m_c is the compressed air mass flow rate, kg/s; T₁ is the compressor inlet air temperature, K; P₁ is the compressor inlet air pressure, kPa; n is the rotational speed, r/min; and the index “rated” refers to rated parameters.

2.2 Combustion chamber modeling

The thermodynamic characteristic of the combustion chamber was simulated. In general, the GT thermal-cycle calculation model could simplify the model of the combustion chamber with a constant combustion efficiency. But it is not very close to reality. In fact, the combustion efficiency changes with the working conditions [29], which has some influence on the GT efficiency. According to Ref. [30], the efficiency was assumed to be a constant value of 0.99 in the combustion chambers under design condition. The model of the combustion chamber was established by mass balance equations, energy balance equations, and stoichiometric equations. For a GT combustor, when chemical kinetics is the limiting factor in combustor performance, Lefebvre [31,32] introduced a combustor loading parameter θ, which correlates well with combustion efficiency, and is defined as

(3)\theta ={\displaystyle \frac{{{\displaystyle P}}_{\mathrm{2}}^{\mathrm{1.75}}{{\displaystyle A}}_{\mathrm{r}\mathrm{e}\mathrm{f}}H\cdot {\mathrm{e}}^{\frac{{{\displaystyle T}}_{\mathrm{2}}}{b}}}{{{\displaystyle m}}_{\mathrm{2}}}},

where P₂ is the combustor inlet pressure, psi; T₂ is the combustor inlet temperature, ⁰R; A_ref is the combustor reference maximum cross-sectional area, in²; H is the combustor height, in; and m₂ is the combustor inlet mass flow rate, lb/s. Note that the dimensions of these parameters in Eq. (3) are in English units. The dependence of the reaction rate parameter b on the primary zone equivalence ratio \varphi is estimated by Herbert [33] to be

(4)

These expressions are plotted in Fig. 2, which are obtained from Ref. [34]. The abscissa is the reaction rate parameter (\theta parameter) and the ordinate is the combustion efficiency.

2.3 Modeling of GT

A cooling model was invoked through the turbine model, and the mixing of cooling air and mainstream gas was assumed to have an effect on the turbine inlet temperature and pressure, whose calculation formulas are

(5){\displaystyle \frac{{{\displaystyle m}}_{\mathrm{g}}\cdot h({{\displaystyle f}}_{\mathrm{g}},{{\displaystyle T}}_{\mathrm{g}})+{{\displaystyle m}}_{\mathrm{c}}\cdot h(\mathrm{0},{{\displaystyle T}}_{\mathrm{c}\mathrm{l}})}{{{\displaystyle m}}_{\mathrm{g},\mathrm{c}\mathrm{l}}}}=h({{\displaystyle f}}_{\mathrm{g},\mathrm{c}\mathrm{l}},{{\displaystyle T}}_{\mathrm{g},\mathrm{c}\mathrm{l}}),

(6){\displaystyle \frac{\mathrm{\Delta }{{\displaystyle P}}_{\mathrm{g}}}{{{\displaystyle p}}_{\mathrm{g}}}}\approx -K{\displaystyle \frac{{{\displaystyle m}}_{\mathrm{c}\mathrm{l}}}{{{\displaystyle m}}_{\mathrm{g}}}},

where h(f_g,T_g)is the gas enthalpy, KJ/kg; f_g is the gas fuel–air ratio; T_g is the gas temperature, K; h(0,T_cl) is the air cooling enthalpy, KJ/kg; T_cl is the air cooling temperature, K; m_g,cl is the hybrid flow mass, kg/s; f_g,cl is the hybrid fuel-air ratio; T_g,cl is the hybrid temperature, K; ∆P_g is the hybrid pressure loss, kPa; p_g is the gas pressure, kPa; K is an empirical constant; m_cl is the air cooling flow, kg/s; and m_g is the gas flow, kg/s.

In calculating hybrid pressure loss, the compressible gas hybrid model of Hartsel [35] was used to compute the change in the time-delay pressure of the cooling air and mainstream gas mixture. Previously, Horlock et al. [36] and Jonsson et al. [37] simplified the model and directly related pressure loss to relative cooling air mass flow. Jonsson et al. [37] suggested that the value range of parameter K must be set between 0.15 and 0.5. In the present paper, the K value is set to 0.325.

The simulation model of the turbine includes mass balance, energy balance, and a generalized turbine map which describes the relationship among the gas mass flow rate m_T, the expansion ratio π_T, the speed N_T and the turbine efficiency η_T. The details of the generalized turbine map are given in Section 3.2. In terms of the limitation of turbine gas mass flow, if the load condition reaches the boundary, the expansion ratio is equal to the critical expansion ratio to calculate the turbine efficiency and the mass flow. The generalized turbine map is expressed by similarity parameters: the corrected air mass flow rate m_Tcorr and the corrected turbine speed N_Tcorr, as expressed in Eqs. (7) and (8).

(7){{\displaystyle m}}_{\mathrm{T}\mathrm{c}\mathrm{orr}}={\displaystyle \frac{{\displaystyle \frac{{{\displaystyle m}}_{\mathrm{T}}\cdot \sqrt{{{\displaystyle T}}_{\mathrm{4}}}}{{{\displaystyle P}}_{\mathrm{4}}}}}{{{\displaystyle \left({\displaystyle \frac{{{\displaystyle m}}_{\mathrm{T}}\cdot \sqrt{{{\displaystyle T}}_{\mathrm{4}}}}{{{\displaystyle P}}_{\mathrm{4}}}}\right)}}_{\mathrm{rated}}}},

(8){{\displaystyle N}}_{\mathrm{T}\mathrm{c}\mathrm{orr}}={\displaystyle \frac{{\displaystyle \frac{n}{\sqrt{{{\displaystyle T}}_{\mathrm{4}}}}}}{{{\displaystyle \left({\displaystyle \frac{n}{\sqrt{{{\displaystyle T}}_{\mathrm{4}}}}}\right)}}_{\mathrm{rated}}}},

where m_T is compressed air mass flow rate, kg/s; T₄ is turbine inlet temperature, K; and P₄ is the turbine inlet pressure, kPa; and the index “rated” refers to rated parameters

A calculation method for the equilibrium running line of a single-shaft GT under specific load conditions is established, as illustrated in Fig. 3. With this calculation method, the positions of the equilibrium running points of all components and their corresponding characteristic lines can be determined. The turbine power W_T and the compressor power W_C can be calculated. Iterative calculations are conducted in accordance with the imbalances between GT power and output power W and between compressor inlet air mass flow and turbine mass flow to obtain the equilibrium running point. The equal load equilibrium running line can be obtained by connecting the equilibrium running points of the same load. In the present paper, a model and calculation method for the equilibrium running line based on the characteristic curves and a gas hybrid model [35] are used to analyze the relationship between GT performance and inlet air temperature under specific load conditions.

3 Model verification and analysis of equilibrium running lines

3.1 Simulation verification

The simulation models of the compressor, the combustion chamber, the cooling air system, and the turbine, consist of the heavy-duty GT model. The input data are as follows: air ambient parameters and fuel parameters into the combustion chambers. The design condition of the PG9171E GT is listed in Table 1. In Ref. [23], simulation calculations were performed for 38 sets of measured data. Besides, one set of measured data, which is near design condition, is selected to validate the models. In the present paper, the formulas of turbine mass flow and efficiency in Ref. [23] are adopted to get the design point for validating the models. As listed in Table 1, the calculated GT power output of PG9171E unit is 125.83 MW which agrees well with the data of the design condition. The results of air thermal parameters at the compressor outlet and flue gas temperature at the turbine outlet also confirm the high quality of the model prediction.

3.2 Analysis of equilibrium running lines

The inlet air temperature frequently changes in the actual operation, thereby significantly influencing GT performance. For the comprehensive analysis of inlet air heating on GT performance, a wide range of temperature (from −15°C to 62°C) is selected. Figure 4 plots the compressor characteristic curve, where the abscissa is the relative converted flow (the ratio of the actual flow to the rated flow), the ordinate is the relative pressure ratio (the ratio of the actual pressure ratio to the rated pressure ratio), and the dotted line is the surge line. The reduced speed lines and compressor efficiency lines that correspond to inlet air temperature are calculated by the corresponding reduced speed and relatively reduced flow rate to obtain the running lines for simulation calculation. The designed point (DP) is also shown in Fig. 4(a). The relative pressure ratio and corrected mass flow rate are both 1.0, and the compressor efficiency and turbine efficiency values are 0.88 and 0.92, respectively.

Moreover, IGV control is frequently required in engineering practice to change GT performance. Therefore, three IGV openings are selected, i.e., IGV= 0°, IGV= 10°, and IGV= 17°, which correspond to 100%, 71%, and 53% openings. The performance changes of certain loads are compared and analyzed, and the equilibrium running lines of these part-loads can be obtained by using the calculation method. The part-loads of 90%, 70%, and 50% are selected. The calculated equilibrium running points are described in the compressor characteristic curve and turbine characteristic curve, as illustrated in Figs. 4 and 5.

In Fig. 4(a), at IGV= 0°, with the rise in inlet air temperature, the location of the equilibrium running point moves to the left, thereby indicating a decrease in the inlet mass flow at a certain load. Taking the 90% load as an example, from −15°C to 17°C, the relative pressure ratio gradually reduces, but the change range is minimal; Above 17°C, the relative pressure ratio rises, but the change range is large, thus denoting that the rise in inlet air temperature evidently influences the relative pressure ratio. However, by comparing the low-temperature and high-temperature environments, the variation of the relative pressure ratio is different because the variation of the compressor efficiency is distinct and can directly be obtained by a compressor characteristic curve. From −15°C to 17°C, the compressor efficiency rises gradually, but it gradually reduces above 17°C. Furthermore, with the rise in inlet air temperature, the equilibrium running point moves toward the surge line. In Fig. 4(b), at IGV= 10°, with the increase in inlet air temperature, the relative pressure ratio of 90% load gradually rises; at 70% and 50% loads, the relative pressure ratio is nearly the same and then rises, but the change range is large. The compressor efficiency of 90%, 70%, and 50% load changes significantly. At IGV= 17°, Figs. 4(c) and 4(b) demonstrate the same trend of relative pressure ratio and compressor efficiency. Nevertheless, the point is close to the surge line when the inlet air temperature is 15°C, 27°C, and 38°C at a 90%, 70%, and 50% load, correspondingly.

In summary, when the inlet air temperature rises under part-load conditions, the inlet mass flow decreases significantly, the relative pressure ratio and the compressor efficiency change evidently, and the point moves toward the surge line. Especially when the IGV opening decreases, the SM is easy to be influenced by the rise in inlet air temperature.

In Fig. 5(a), at IGV= 0°, with the rise in inlet air temperature, the location of the equilibrium running point moves to the left, thus denoting the rise in turbine inlet temperature. The variation of the relative expansion ratio is the same as that of the relative pressure ratio. The turbine efficiency varies with the rise in inlet air temperature. Moreover, the turbine efficiency increases gradually from −15°C to 15°C and then changes slightly at 90% and 70% loads; from 15°C to 62°C, the turbine efficiency increases gradually at a 50% load. At IGV= 10°, Fig. 5(b) exhibits the same trend as the turbine inlet temperature, the relative expansion ratio, and the turbine efficiency. At IGV= 17°, Fig. 5(c) displays the turbine inlet temperature and the fact that the turbine efficiency increases gradually with the rise in inlet air temperature under part-load conditions.

In summary, the turbine is in the off-design condition in the part-load operation mode. When the compressor inlet air temperature increases, the turbine inlet temperature increases, as shown in Fig. 10(a). The turbine inlet temperature gradually moves to the temperature of the design condition to improve the aerodynamic performance of the turbine, which is conducive to the improvement of the turbine efficiency. A low load leads to an evident change in the turbine efficiency with the same IGV opening. In addition, the variation of the expansion ratio is consistent with that of the compressor pressure ratio at a certain load.

4 Analysis of the performance parameter of the equilibrium running point

4.1 Comparative analysis of GT efficiency under different part-load conditions

Based on the calculation method for the equilibrium running line, the GT efficiency at each point is obtained, and the optimal inlet air temperature corresponding to the highest GT efficiency can be obtained at 90%, 70%, 50% loads, as presented in Fig. 6. The results show that an optimal inlet air temperature exists for a certain part-load and IGV opening. A higher IGV opening denotes a higher optimal inlet air temperature at a certain load. The inlet air heating technology has a considerable potential to improve GT efficiency when the inlet temperature is lower than the optimal value with different IGV openings. Compared with the same inlet air temperature, a low IGV opening denotes a high GT efficiency in the low-temperature region. A high IGV opening denotes a high GT efficiency in the high-temperature region. Moreover, under different inlet air temperature conditions, an optimal IGV opening must be determined to maximize the GT efficiency, as shown in Fig. 7.

In addition, the optimal inlet temperature and the highest GT efficiency at different IGV openings and different part-load conditions are compared and analyzed, as summarized in Tables 2 and 3. In Table 2, the optimal inlet temperature is reduced when the IGV opening is turned down at a certain load, and the influence of the IGV opening on the optimal inlet temperature is evident. Compared with the same IGV opening, the optimal inlet temperature increases obviously with the decrease of part-load. Besides, the influence of load change on the optimal inlet temperature is also apparent.

In Table 3, it is clearly seen that when the IGV is in the full-opening state and the optimal inlet temperature is achieved by using the inlet air heating method, GT efficiency can be higher than other IGV openings. According to the comparative results, the maximum values of GT efficiency is 39.16% at the 90% load, 36.43% at the 70% load, and 31.17% at the 50% load. Then, the IGV can be left unadjusted, even when the load is as low as 50%. When the IGV opening is dropped from IGV= 0° to IGV= 17°, the highest GT efficiency decreases gradually at a certain load with the difference of approximately 0.8% at the 90% load, 1.1% at the 70% load, and 0.8% at the 50% load. By turning down the IGV opening, the throat area of the IGV passage reduces, and the inlet air flow reduces, too. The positive incidence angle of the IGV blade becomes larger to worsen the compressor inlet velocity triangle, leading to a local flow separation and a degradation of the compressor performance. Due to the relationship of component matching, the combustion performance and turbine performance worsens. Then GT efficiency decreases. When the optimal inlet air temperature is achieved by using the inlet air heating technology, the influence of the IGV opening on GT efficiency is large, which indicates that both the inlet air heating technology and the IGV opening adjustment are important on GT efficiency. Compared with the same IGV opening, the highest GT efficiency decreases gradually with the decrease of part-load, and the impact of load change on GT efficiency is significant with the difference of approximately 5%.

As can be seen from Tables 2 and 3, it is beneficial to increase the IGV opening with a higher optimal inlet air temperature in order to improve GT efficiency. Moreover, the IGV opening should be kept in the full-opening state if possible and only the inlet air temperature should be adjusted. Therefore, inlet air heating has the potential to replace IGV regulation to achieve a higher GT efficiency. Moreover, although the part-load is as low as 50%, the IGV can still be unadjusted by using only the inlet air heating technology, that is, the highest GT efficiency can be obtained at IGV= 0°, which is the full-opening state.

In reality, the possible problem is that the maximum inlet air heating temperature cannot reach the optimal inlet air temperature or the inlet air heating speed cannot meet the peak regulation requirements. In this case, the IGV must be properly turned down in time for improving GT efficiency. Figure 7 presents the optimal IGV angles corresponding to inlet air temperatures and the corresponding optimal GT efficiencies are calculated at the given part-load. Therefore, there is an optimal matching relationship between the IGV opening and the inlet air temperature. For the best GT performance of a given load, the IGV could be adjusted according to the inlet air temperature. Further, the inlet air temperature adjusting technology, especially the inlet air heating technology, has a great potential to replace IGV to regulate load, because IGV can be kept in the full opening state for the optimal inlet air temperature.

The theoretical cause analysis shows that the operating condition of GT deviates from the design point when GT is operating at part-load. The inlet air temperature adjustment technology will make the GT operating point close to the design point of each component. The inlet air heating technology is useful in a certain low temperature range which is lower than the optimal inlet air temperature, so as to improve the GT efficiency. In terms of the limitation of inlet air temperature adjustment, IGV regulation is combined to reach the condition close to the best operating condition.

4.2 Impact analysis of performance parameters on GT efficiency under 50% load

The compressor efficiency, the combustion efficiency, and the turbine efficiency of the ideal cycle of a GT is constant and the increase of inlet air temperature will lead to the increase of the compressor specific power. However, as shown in Figs. 4 and 5, the compressor efficiency and the turbine efficiency are changed by using the inlet air heating technology in the actual cycle and the performance parameters of compressor and turbine have an obvious effect on GT efficiency. A quantitative analysis is conducted to analyze the performance of each equilibrium running point based on Figs. 4 and 5. Taking the 50% load as an example, Figs. 8–10 present the changes of compressor efficiency, relative pressure ratio, combustion efficiency, turbine inlet temperature, and turbine efficiency relative to the inlet air temperatures and the IGV openings, respectively.

Figure 8(a) illustrates the compressor efficiencies at IGV= 0°, IGV= 10°, and IGV= 17°, in which efficiency initially rises and then reduces with the rise in inlet air temperature. The highest compressor efficiency is 86.8% at 27°C, 86.7% at 15°C, and 86.2% at 0°C. The optimal inlet temperature gradually decreases with the IGV opening, thus indicating that a larger opening leads to a higher compressor efficiency by increasing the inlet temperature. In Fig. 8(b), the relative pressure ratio at IGV= 0°, IGV= 10°, and IGV= 17° initially decreases and then rises with the inlet air temperature. The lowest relative pressure ratio is 0.86 at 27°C and IGV= 0°, 0.85 at 0°C and IGV= 10°, and 0.81 at 0°C and IGV= 17°. In summary, the relative pressure ratio of the various IGV openings changes differently in low- and high-temperature zones. When the IGV opening is constant, the volume flow is fixed. If the inlet air temperature of GT is promoted, the inlet air density will be reduced, thus reducing the inlet air mass flow, as shown in Fig. 8(c). As is known, with the increase of compressor inlet air temperature, the compressor specific power increases obviously, if the compressor efficiency is assumed ideally to be constant. However, the compressor efficiency changes along with the inlet air temperature in this paper, as shown in Fig. 8(a). Then the variation of the compressor specific power with the inlet air temperature is obtained, as shown in Fig. 8(d). The compressor specific power changes slightly at low-temperature zones and greatly in high-temperature zones.

As described in Section 2.2, the combustion efficiency changes with the operating conditions. Based on the fact that the primary zone equivalence ratio \varphi of the research heavy-duty GT studied in this paper is 0.7, the reaction rate parameter b of Eq. (4)is calculated as 392. Combined with the combustion chamber size and parameter P₂, T₂, and m₂ of each equilibrium running point, the \theta parameter of Eq. (1) is obtained, which is shown in the red solid line in Fig. 2. The correlation values of the combustion efficiency at each equilibrium running point is shown in Fig. 9. The combustion efficiency gradually increases with the rise of the inlet air temperature at a constant IGV opening. Compared with different IGV openings, the combustion efficiency changes differently between low- and high-temperature zones. In summary, the increase of inlet air temperature can be conducive to the improvement of combustion efficiency.

Figure 10(a) depicts a gradual rise in turbine inlet temperature. The turbine inlet temperatures are 950.98°C at − 15°C and 1071.1°C at 38°C and IGV= 0°. An average rise of 10°C in the inlet air temperature requires a rise of 22°C in the turbine inlet temperature. The turbine inlet temperatures of the different IGV openings do not exceed the maximum temperature limit of the turbine inlet temperature, which is provided by the design manufacturer. Figure 10(b) demonstrates that the turbine efficiency gradually increases at different IGV openings. Compared with the same inlet air temperature conditions, the turbine efficiency is nearly the same at different IGV openings. Compared with the same IGV opening, the increase in the inlet air temperature evidently influences the turbine efficiency.

The influences of the inlet air temperature and IGV opening (e.g., relative pressure ratio, the compressor efficiency, the combustion efficiency, the turbine efficiency, and the turbine inlet temperature) on GT performance are studied based on Figs. 4 and 5. The improvement of GT efficiency from −15°C to 38°C is caused by the rise of the compressor efficiency, the combustion efficiency, and the turbine efficiency as well as the turbine inlet temperature at IGV= 0°. However, GT efficiency decreases beyond 38°C, because the rise of the pressure ratio, the combustion efficiency, the turbine efficiency, and the turbine inlet temperature cannot compensate for the negative impact caused by the evident decrease in the compressor efficiency and the rise in the compressor specific power despite their increases.

At IGV= 10°, GT efficiency is improved from −15°C to 27°C, which corresponds to the gradual increase in the compressor efficiency, the combustion efficiency, the turbine inlet temperature, and the turbine efficiency. Above 27°C, the decline can be attributed to the obvious decrease in the compressor efficiency and the rise in the compressor specific power.

At IGV= 17°, the rise in GT efficiency from −15°C to 22°C can be attributed to the rise in the relative pressure ratio, the combustion efficiency, the turbine inlet temperature, and the turbine efficiency. Above 22°C, the decline can be attributed to the apparent decrease in the compressor efficiency and the increase in the compressor specific power.

In summary, the inlet air temperature and the IGV opening significantly influence the performance parameters. The veriations of the compressor efficiency, the combustion efficiency, and the turbine efficiency are almost consistent with the variation of GT efficiency with the inlet air temperature. Meanwhile, the turbine inlet temperature also increases gradually. The findings on the improvement of GT efficiency caused by the inlet air heating technology and the IGV regulation under part-load conditions can be explained by the improvement of the compressor efficiency, the combustion efficiency, the turbine efficiency, and the rise in the turbine inlet temperature.

4.3 Comparative analysis of SM under different part-load conditions

In a GT, the index of the compressor surge performance is called SM. Surge boundary is important for compressor characteristic curves in measuring whether the compressor has surged. If the GT performance is advanced, and the stable working range is wide, the SM must be large, and the compressor operating point is far from the surge boundary. Generally, the SM of the compressor must be maintained at more than 10%. The SM is calculated as

(9)\mathrm{S}\mathrm{M}=\left({\displaystyle \frac{{{\displaystyle \pi }}_s/{{\displaystyle m}}_s}{{{\displaystyle \pi }}_{\mathrm{0}}/{{\displaystyle m}}_s}}-\mathrm{1}\right)\cdot \mathrm{100}\%,

where π₀ and m₀ are the relative pressure ratio and the converted flow rate of the operating point, correspondingly, and π_s and m_s are the relative pressure ratio and the converted flow rate of the operating point along the equivalent speed line corresponding to the operating point on the surge boundary.

The SM at each equilibrium running point is calculated, and the SM on the equilibrium running line that varies with different IGV openings and inlet air temperatures is analyzed, as illustrated in Fig. 11. The equilibrium running point indicated by the black arrow in Fig. 11 is of the highest GT efficiency. For the 90% load, with the rise in the inlet air temperature, the SM initially increases and then decreases at IGV= 0°, and decrease gradually at IGV= 10° and IGV= 17°. Corresponding to the highest GT efficiency condition, the SM is 20.54% at IGV= 0°, 11.78% at IGV= 10°, and 6.2% at IGV= 17°. For the 70% load, with the rise in the inlet air temperature, the SM initially increases and then decreases at IGV= 0° and IGV= 10°. Corresponding to the highest GT efficiency condition, the SM is 22.87% at IGV= 0°, 13.98% at IGV= 10°, and 7.2% at IGV= 17°. For the 50% load, with the rise in the inlet air temperature, the SM of the three IGV openings initially increases and then decreases. With the drop of the IGV opening, the SM gradually decreases. Corresponding to the highest GT efficiency condition, the SM is 27.1% at IGV= 0°, 21.87% at IGV= 10°, and 14.39% at IGV= 17°. When the IGV opening degree drops from IGV= 0° to IGV= 10°, the SM decreases by nearly 9% under the highest GT efficiency condition.

In summary, the drop of the IGV opening can obviously reduce the SM at a certain load. At IGV= 17°, the SM is lower than 10% at the 90% and 70% load under the highest GT efficiency condition. Therefore, at IGV= 0° and the optimal inlet air temperature is achieved by using the inlet air heating technology, a higher SM can be obtained compared with other IGV openings. When the GT is operating under part-load conditions, each component performance is in off-design condition. In this paper, the inlet air density is reduced by using the inlet air heating technology. To keep the load operation constant, the IGV opening is forced to turn up and then the positive incidence angle of the IGV blade becomes smaller to improve the compressor inlet velocity triangle. As a result, the aerodynamic performance of the rear stages of the compressor can be improved, and the SM can be improved.

In reality, if the maximum inlet heating temperature cannot reach the optimal inlet temperature or the inlet heating speed cannot meet the peak regulation requirements, the IGV should be properly turned down in time for improving GT efficiency and the SM must satisfy the safety requirements. As shown in Fig. 11, the inlet air heating technology has the potential to replace IGV regulation to achieve a higher SM in steady-state operation. Moreover, although the part-load is as low as 50%, the IGV can still be unadjusted by using only the inlet air heating technology, that is, a higher SM can be obtained at IGV= 0°, which is the full-opening state.

4.4 Impact analysis of the pressure loss connected to inlet air heating on GT efficiency

After discussing the change of GT performance caused by the novel inlet air heating method, the impact analysis of the inlet pressure loss connected to inlet air heating is also performed. In the previous discussion, it is assumed that the compressor inlet pressure loss is ignorable. But it is not very close to the reality. In this paper, a retrofitted GT cycle using a compressor inlet heat exchanger with a 1kPa inlet pressure loss is simulated and the effect of inlet air pressure loss on GT efficiency is discussed at different IGV openings and part-loads.

The simulation results show that the compressor inlet air pressure loss of 1kPa could decrease a power output of about 2% and a GT efficiency of about 0.3% (absolute value). When the combustion outlet temperature T₃* is adjusted for cycle load control, the GT efficiency is improved with an increase of about 0.1% (absolute value). The inlet pressure loss results are still lower than the results assuming that the compressor inlet pressure loss is ignorable at the given load, and the variation of GT efficiency with the inlet air temperature is not affected, as shown in Fig. 12. The solid line represents the result assuming that the compressor inlet pressure loss is ignorable and the dotted line represents the inlet pressure loss. Therefore, the results of each equilibrium running point are obtained by using the inlet air heating technology. The influence of the inlet pressure loss connected to the inlet air heating technology has been considered at all operating points. Although the GT efficiency is reduced and the deviation magnitude is relatively small due to the inlet pressure loss, the influence on GT efficiency is not changed. To more accurately analyze the effect of the inlet air heating technology on GT performance, the influence of the inlet pressure loss should be considered.

In summary, the inlet air heating technology causes the inlet air pressure loss, which will weaken the performance improvement, but the effect is not obvious. Meanwhile, some cost consequence exist associated with the installation of such a system, including an inlet heat exchanger, pipes, valves, and other components. The inlet air pressure loss connected to the inlet heat exchanger has already existed. In addition, little capital investment is required to install the necessary pipes and valves to reverse the flow through the inlet heat exchanger from the cold side to the hot side. The double inlet air temperature adjusting technology, including inlet air cooling technology and inlet air heating technology, can be used in GT plants according to the actual operating condition.

5 Conclusions

A novel adjusting method for the improvement of GT efficiency and SM under part-load conditions is proposed. This method uses the inlet air heating technology, which uses the waste heat as the heating resource, and the IGV opening as the adjustment. A calculation method is adopted for the equilibrium running lines of the compressor and the turbine. Three part-load conditions, i.e., 90%, 70%, and 50% loads, and three IGV openings, i.e., IGV= 0°, IGV= 10°, and IGV= 17° are selected. By analyzing the variation of the combined results of these main performance parameters with the inlet air temperature, the causes of the effectual mechanisms of the inlet air heating technology and the IGV opening adjustment on GT performance are comprehensively discussed. The following conclusions are derived:

For a given part-load, there exists an optimal matching relationship between IGV opening and inlet air temperature. A higher IGV opening or a lower load denotes a higher optimal inlet temperature.

When the IGV opening is constant and the inlet air temperature is lower than the optimal value, as the inlet air temperature decreases, the GT efficiency decreases. At this time, the inlet air heating technology can remarkably improve the GT efficiency.

The improvement of the GT performance by adjusting inlet air temperature and IGV opening under part-load conditions can be explained by the fact that the inlet air temperature approaches the optimal temperature to make the running point move toward the design condition of each component. Then the compressor efficiency, the combustion efficiency, and the turbine efficiency are improved. Meanwhile, the turbine inlet temperature also rises gradually.

When the IGV is in the full opening state and the inlet air temperature is optimal, the GT efficiency and the SM can be obviously higher than those at other IGV openings. The IGV can be left unadjusted, even when the load is as low as 50%.

In summary, for the best GT performance of a given load, the IGV should be adjusted according to the inlet air temperature. Furthermore, the inlet air temperature adjusting technology, especially the inlet air heating technology, has a great potential to replace IGV to regulate load, because IGV can be kept in the full opening state for the optimal inlet air temperature.

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