Description

The purpose of the micro-turbine LIBPF process model is to simulate a stationary recuperated, single-shaft gas turbine-based CHP (combined heat and power) system from natural gas.

The model quantitatively reproduces the nominal performance (nominal power 30 kW_el) and part-load behavior of a commercial unit available from a leading vendor (Capstone Turbine Corporation, “Technical Reference - Capstone Model C30 Performance” 410004 Rev. D, April 2006).

Process flow diagram

The system is fed with:

• Ambient air (S01) at ISO conditions: 15 °C, 60% relative humidity (1 % v/v water content) and 101325 Pa (standard sea level pressure)

• High Pressure (4 bar) Natural Gas (S02): 99% CH₄, 1% N₂.

All streams are gas-phase.

The ambient air (S01) is fed to the compressor unit C, the compressed air (S07) can be optionally partially routed to an external user of compressed air (S10, not used in this configuration) or proceed to the cold side of the recuperator (HX) where it is preheated (S09), then admixed to an optional compressed hot air recovery (S03, not used in this configuration) before proceeding (S04) to the the burner (RX) where the natural gas (S02) is added and burned completely (CO, H₂ and CH₄ total combustion to H₂O and CO₂).

The hot, pressurized gases (S05) from the burner are fed to the turbine expander (T). The turbine exhaust (S06) proceeds to the hot side of the recuperator (HX) before leaving the system.

Operating conditions

The operating conditions at the nominal point are:

• air in 38.802009 kmol/h

• methane in 0.5226831 kmol/h

• max P = 4 bar

• discharge pressure 1.01325 bar

• burner deltaP = 100.0 mbar

• HX.deltaPhot = HX.deltaPcold = 50 hPa

• C.η_M = 99% (mechanical efficiency)

• C.η_E = 100% (electrical efficiency)

• T.η_M = 99% (mechanical efficiency)

• T.η_E = 100% (electrical efficiency)

The matching of the model to the vendor-provided nominal key performance parameters:

• exhaust temperature 275 °C

• Turbine Inlet Temperature (TIT) 1144 K

• net electrical efficiency 26 %

are performed by:

1. Matching vendor claimed exhaust temperature by tuning the HX.UA;

2. Matching vendor claimed TIT by tuning the T.θ;

3. Matching vendor claimed net efficiency by tuning the C.θ (this is performed manually).

The result of the matching is:

• HX.UA = 1428.7 W/K

• T.θ = 71.9% (thermodynamic isentropic efficiency)

• C.θ = 85% (thermodynamic isentropic efficiency).

Variable load modelling

For accurate pressure driven modelling at part-load, the relationship between the rotation frequency, pressure and flow should be included, based on the characteristic curves of the turbine and compressor.

This model implements some simple relationships based on adimensional or reduced quantities from A. Capetti, Motori termici , UTET, 1964, I ed. to qualitatively represent the behaviour, based on the following assumptions:

• geometries do not change and load control is achieved by changing the frequency;

• the operating point of the compressor should be kept clear of the stonewall and surge limits, and move along an optimal curve

• the corrected mass flows are linear functions of the frequency of both the compressor and the turbine;

• the turbine will operate at stonewall (incipient choked flow), so that the mass flow does not depend on the pressure ratio;

• the TIT is constant;

• the thermodynamic (isentropic) efficiency of the compressor will not change significantly, whereas the thermodynamic efficiency of the turbine varies as a certain function of the reduced frequency

The fresh-air flow is adjusted to keep the actual compressor corrected mass flow equal to the should-be value, and the fuel is adjusted to keep the TIT.

Results

The part-load behavior can be calculated by acting on the compressor set pressure from the nominal value of 4 bar down to 1.65 bar (corresponding to an electrical power of 2 kW). The predicted net efficiency compares well to the vendor data:

The predicted exhaust temperature on the other hand deviates from the vendor data:

indicating that the assumption that the TIT is kept constant by the control system may not apply.

The model user can also freely change the inlet conditions, the fuel composition (fuels containing CO and H₂ can be handled), the unit pressure drops and reaction conversions in the burner.