Detailed process model description for cogeneration plants

Introduction

Intended audience

The present document is oriented to:

Model developers
Model users
System Integrators.

Scope

The scope of the present document is to describe the capabilities of the Cogeneration demo, for the process modeling of stationary energy cogeneration power plants (Combined Heat and Power plants, CHP) based on micro-turbines.

Prerequisites

basic knowledge of process engineering and / or of thermoelectric energy generation processes.
know about the different roles involved with the lifecycle of solutions developed using the LIBPF™ enabling technology, see LIBPF™ Technology Introduction

Cogeneration kernel

In LIBPF™ one kernel can support many process models, each as a different flowsheet type.

All the process models supported by a kernel share the same list of components and can use all LIBPF™ embedded types plus the custom types registered by the kernel itself.

Type list

The Cogeneration kernel registers the following models, based on the built-in LIBPF™ FlowSheet type:

Type	Description	options
MicroTurbineC30	30 kW micro-turbine power generation unit matching C30
MicroTurbineOpen	as MicroTurbineC30 but with hot air inlet and compressed air outlet
MicroTurbine	parametrized micro-turbine power generation unit	model: turbine model
C30	30 kW micro-turbine power generation unit matching C30	model = C30
C65	65 kW micro-turbine power generation unit matching C65	model = C65
C200	200 kW micro-turbine power generation unit matching C200	model = C200
C1000	1 MW micro-turbine power generation unit matching C1000
Burner	Combustion of hydrocarbons, hydrogen and carbon monoxide	unit", false,
DeltaT	Compute the adiabatic combustion temperature
DeltaH	Compute the lower heat of combustion
InternalCombustionEngine	Internal Combustion Engine
Cogeneration	Combined Heat and Power with configurable turbine	turbine, default = C30
CogenerationC30	CHP with C30 turbine	turbine = C30
CogenerationC65	CHP with C65 turbine	turbine = C65
CogenerationC200	CHP with C200 turbine	turbine = C200
CogenerationC1000	CHP with C1000 turbine	turbine = C1000

Component list

The fluids to be processed are broken down in their constituents and represented as a mixture of basic components.

The components are defined using built-in LIBPF™ basic types.

More precisely the Cogeneration kernel defines the following component list (click on the component type to jump to the reference documentation for the component):

Type	Name	Description
purecomps::water	water	standard model for water
purecomps::N2	N2	standard model for nitrogen
purecomps::O2	O2	standard model for oxygen
purecomps::methane	H2	standard model for methane
purecomps::CO	CO	standard model for carbon monoxide
purecomps::CO2	CO2	standard model for carbon dioxide
purecomps::H2	H2	standard model for hydrogen

Process descriptions and schemes

Microturbine

The microturbine models represent stationary, single-shaft gas turbines operated according to a recuperated Brayon cycle, with different electric power outputs: 30 kW, 65 kW, 200 kW and 1000 kW (obtained assembling five 200 kW units in parallel).

The model quantitatively reproduces the nominal performance 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).

The following process flow diagram should clarify the process.

Process Flow Diagram

Stream list

The streams are defined using built-in LIBPF™ basic types.

Click on the type to jump to the reference documentation for the stream.

Type	Name	Description	From	To	options
StreamVapor	S01	Air feed	source out	C in
StreamVapor	S02	Fuel feed	source out	RX in
StreamVapor	S03	Turbine Inlet	RX out	T in
StreamVapor	S04	Turbine Outlet	T out	HX	hotin
StreamVapor	S05	Compressed Air and HX cold inlet	C out	HX	coldin
StreamVapor	S06	HX cold outlet and RX burner inlet	HX	coldout	RX in
StreamVapor	S07	Recuperator exhaust	HX	hotout	sink in

Unit list

The unit operations are defined using built-in LIBPF™ basic types and the Burner type provided by the kernel.

Click on the type to jump to the reference documentation for the unit operation.

Type	Name	Description
Compressor	T	Turbine
Compressor	C	Compressor
Burner	RX	Burner
Exchanger	HX	Turbine recuperator

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 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 (S05) proceeds to the cold side of the recuperator (HX) where it is preheated (S06), then proceeds 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 (S03) from the burner are fed to the turbine expander (T). The turbine exhaust (S04) 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.ηM = 99% (mechanical efficiency)
C.ηE = 100% (electrical efficiency)
T.ηM = 99% (mechanical efficiency)
T.ηE = 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:

Matching vendor claimed exhaust temperature by tuning the HX.UA;
Matching vendor claimed TIT by tuning the T.θ;
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 modeling

For accurate pressure driven modeling 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 behavior, 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:

alt result1

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

alt result2

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.

Model options

This model has no options.