Complex Modeling of a Waste Heat Boiler

Introduction

Waste heat boilers are a sophisticated piece of equipment important for recovering heat and in turn protecting the environment. Waste heat boilers are needed during the operation of facilities in the energy sector such as gas turbine plants and diesel engines, as well as in metallurgy and other industries where excessive heat of high temperature up to 1,000 degrees form during the technological processes. Waste heat boilers are used to recover excess heat energy, as well as to increase the overall efficiency of the cycle. Another feature of waste-heat boilers used at these installations is to protect the environment – by disposing of harmful emissions.

This article discusses the accurate modeling of these sophisticated waste heat boilers. We will consider the simulation of a Heat Recovery Steam Generator (HRSG), which is used in a combined steam-gas cycle for utilizing the outgoing heat from a gas turbine plant and generating superheated steam, using the programs thermal-fluid network approach and complexes of optimization.

The HRSG has four main heat exchangers: cast-iron economizer, boiling type steel economizer, evaporator with separator, and superheater.

On the one side of the HRSG, feed water is supplied from the cycle, and on another side, hot gas is supplied from the gas turbine in the process of operation.  The water is preheated and goes to the steel economizer where the boiling process begins in the tubes. After the process in the economizers, the water goes to the shell side of the evaporator, where its active boiling occurs. In the separator, the steam-water mixture is divided into saturated steam and overflow. Saturated steam is sent to the superheater, where superheated steam is formed and goes to the steam turbine cylinder. Overflow water returns to the steam formation. An induced-draft fan is used for gas circulation and removal in the HRSG. The HRSG model also has a spray attemperator for steam cooling. The operation principle of desuperheater is the following: feed water is taken from the economizer and goes to the superheater section, passes to superheated steam flow through nozzles, finely divided water droplets mix, heat up and evaporate and as a result, the steam is cooled.

HRSG Flows Direction
Picture 1 – HRSG Flows Direction
Different Approaches

The boiling process takes place in both the steel economizer tubes and in the shell side of the evaporator while the HRSG is operating. As a result, the two-phase flow is formed. Boiling leads to the intensification of heat exchange processes, changes in the flow structure and produces bubbles, which must be accurately taken into account during the simulation. Thermal-fluid network software AxSTREAM NET™ was used to determine the hydraulic resistance of heat exchangers and the recovery boiler as a whole, as well as simulating the processes of phase transition. In addition, the software allows taking into account convective and radiant heat exchange. These complex methods allow users to determine all the necessary parameters of gas and water and accurately simulate the heat transfer coefficients.

HRSG Scheme in AxSTREAM NET
Picture 2 – HRSG Scheme in AxSTREAM NET™

It should be noted that AxSTREAM NET™ could be used for the detailed modeling of each HRSG component and for non-interval modeling depending on the task required. What are the differences between these two approaches? Let’s figure out!

The Interval Method vs Non-Interval Method

We have used both of these approaches in the modeling of HRSG. The interval method was used to simulate a cast-iron economizer. In this case, the number of pipes was divided into 7 bundles of 95 tubes each, and the length of the pipes was split in half. The non-interval method was used for simulations of a steel economizer of boiling type. In this case, the heat exchanger was modeled by two elements – a pipe element for modeling the resistance of the tube bundle – it specified the total number of heat exchanger tubes and the element for modeling pipe flow resistance in the shell side. It should be noted that the interval method allows the user individually to perform simulation of various schemes depending on the task, as well as to obtain more accurate results in the modeled installation.

Complex Modeling

Nowadays, it is necessary to apply an integrated approach to solve any engineering problems which evaluate the work of the entire cycle, where installations are used, and to conduct an accurate analysis of their interaction. Thus, we consider the analysis of the influence and interaction of the turbine installation parameters and HRSG in various operating modes, which depend on environmental parameters.

To analyze the impact of environmental parameters on HRSG values, we additionally used software systems developed by SoftInWay. This provides a modern approach to cycle analysis implementation and calculation at the same time all components of the scheme. Thus, the 0D software for thermodynamics cycle analysis and calculation (AxCYCLE™) and integration and optimization software (AxSTREAM ION™) were used for automatically matching gas turbine cycle calculations from different regimes in AxCYCLE™ with the flow rate calculations, temperatures and heat transfer coefficients in AxSTREAM NET ™.

Execution Process via AxSTREAM ION
Picture 3 – Execution Process via AxSTREAM ION™

It is no secret that usually GT’s typically do not work on the design modes (Tair=15 C;P=0.1031 MPa;Humidity=60%-ISO-2314) when compared to a steam turbine. Outside air parameters are constantly changing. As a result, the main characteristics of the GT-cycle is changed such as electric power, efficiency, gas parameters at the turbine outlet.

Obviously, the efficiency of a gas turbine cycle grows with a drop in outside air temperature, which affects the entire thermodynamic cycle. As a result, the amount of generated steam decreases with increasing ambient temperature, and generated steam temperature increases. Moreover, the gas flow rate from the GT and HRSG resistance increases with reducing ambient temperature.

HRSG Parameters Dependency from Air Inlet Temperatures
Picture 4 – HRSG Parameters Dependency from Air Inlet Temperatures
Conclusions

Thus, we were able to perform complex optimization tasks for flow analysis calculation and parameter interactions in sophisticated systems (such as combined gas-steam cycles) via AxSTREAM ION™, and provide an opportunity to use different modeling approaches which reduces the time of scheme analysis (very important for every engineer 🙂 ). If you need more information about complex modeling, please feel free to contact the SoftInWay team at info@softinway.com.

One thought on “Complex Modeling of a Waste Heat Boiler

  1. accurate modeling of waste heat boiler was explained so well…
    very good concept….
    grate work done…….
    tanks for sharing and keep going on……..

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