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A guide to waste heat recovery – Part Two

World Cement,

This is part two of a three-part article written by Peter Beleznay, Heatcatcher,for World Cement’s August issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch upon-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the August 2015 issue here

Integration challenges

The main design considerations when integrating a WHR system are:

Minimising risk to the production process

The design must have a fail-safe operation, capable of full isolation from the existing process, when the WHR system is not operational. By positioning the heat exchanger of the recovery system in a bypass duct, and using an automated control system to operate the main control dampers, diverting the exhaust gas between the WHR duct and the original duct, the production process is independent from the operation of the WHR system.

Dust loading from the exhaust gas

The amount of dust present in the exhaust gas can cause fouling and clogging issues if it collects on the tubes of the heat exchanger. This can cause increased pressure drop for the induced draft fan to overcome and reduce the efficiency of the heat exchanger. The design of the heat exchanger is key to creating a reliable system with a long operational life. A heat exchanger tube cleaning system, suitable for the site specific characteristics of the dust, is essential. Dust from the clinker cooler exhaust and the preheater exhaust has different characteristics. Generally, the nature of the clinker cooler dust is hard and abrasive, whereas kiln dust is fine, soft and sticky. The chemical composition of these gases are also vastly different. Therefore the design for recovering waste heat from these streams must be treated differently.

Pressure drop across the heat recovery system

The installation of an exhaust gas heat exchanger and additional duct work into the process increases the pressure in the system. An increased system pressure requires more fan power to overcome. The amount of electrical power consumed by the fan as a result of increased system pressure (albeit handling a reduced flow rate due to the air being cooler) needs careful design consideration. Minimising the pressure drop is key to avoiding the extra project costs of upgrading the Induced Draft (ID) fan. The use of Computational Fluid Dynamics (CFD) software packages to optimise the geometry of the heat exchanger design and bypass ductwork is an essential tool to minimising the additional pressure drop.

Abrasion/corrosion of heat exchangers, condensation of flue gas

A further challenge is the reduced operational lifetime of the heat exchangers, due to the corrosiveness of exhaust gas and abrasive nature of the dust. Abrasion can be controlled by reducing the gas velocity across the heat exchanger, and with the use of a de-duster device before the heat exchanger.

Corrosion can be limited by keeping the heat exchanger temperature constantly above the dew point so that the formation of acid solutions are reduced. This can be achieved by balancing the electricity generation output according to the available thermal power in the exhaust gas flow. Using additional heating units can reduce condensation on the heat exchanger while the plant is in shutdown. The materials chosen for the heat exchanger are also key to extending its operational life in a harsh environment.

Every plant is different due to the nature of the product, the fuel used, and the type of kiln design. The use of best available techniques to simulate the dust loading conditions, flow and temperature profiles are key to designing the heat exchanger, in order to provide the longest possible operational life and best efficiency. Therefore a WHR system design needs to consider all the best available technologies and techniques to optimise cost, operational life time, reliability and overall system efficiency.

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