Energetic and Environmental analysis

ENERGETIC AND EXERGETIC ANALYSIS OF

COGENERATION SYSTEMS AND STEAM NETWORKS

The steam network of a process plant is a system which generates and distributes steam to different kinds of end consumer. Steam can be used as a source for heating, consumed as a chemical or used to generate electric or mechanical power. The production of steam generally requires the consumption of fuel, demineralized water, electric energy and refrigeration water. In order to reduce the operation cost of this network, it is useful to have a tool to simulate and optimize steam networks such as program XV, developed in the Chemical Engineering Department at the URV.

First, a standard data file is made which includes all the data and parameters of the units present in the network. Program XV can simulate most conventional equipment that is to be found in a steam network of a process plant, such as:

Cogeneration systems consisting of a gas turbine and a heat recovery boiler.
Different types of headers:
- Steam headers
- Condensate headers
- Boiling water headers
Conventional boilers.
Waste heat boilers.
Steam turbines.
Steam compressors.
Steam consumers.
Steam condensers.
Throttle valves.
Flashs vessels.
Deaerator.
Electric motors.

In a first step, the program runs a simulation model which reproduces the performance of the network, and is used as a starting point to optimize this network. The optimization module solves the linearized equation system made up of material and energy balances by using a sequential linear programming algorithm (SLP). The results show the features of all the streams of the network (flow rates, enthalpies, pressures, temperatures, fuel consumption, and so on).

A subroutine predicts the thermodynamic properties of steam and water by using adjusted mathematical correlations. This subroutine gives the value of a property as a function of other variables such as pressure, temperature, vapor quality, enthalpy or entropy, depending on the state of water (subcooled liquid, saturated liquid or vapor, or superheated steam).

The model used in this program can optimize any usual steam network structure at three levels:

1-Optimization of flow rates and enthalpies

2-Optimization of pressure in steam headers

3-Optimization of the steam plant structure: use of a steam turbine or an electric motor, or shut down of boilers.

To perform the optimization, a linearized model is used in which the operation costs of the steam system are included in the objective function. Equipment capital costs are not taken into account because the program is expected to be used to optimize the performance of existing plants. The objective function also includes the economic income generated by selling the electricity generated by the plant.

The exergetic analysis, based on the second thermodynamic principle, takes into account both energy quantity and quality. Introducing the concept of exergy, internal and external irreversibilities of an energy production system are calculated, and the losses generated in each piece of equipment analyzed. One of the applications of this type of analysis is the distribution of production costs among the products (electricity and steam) based on their exergetic content

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