Heat recovery from a boiler exhaust to pre-heat air to a spray dryer

Abstract

This demonstration project at BIP Chemicals Ltd, is one of a package of projects being funded by the Energy Efficiency Office of the Department of Energy, under the Energy Efficiency Demonstration Scheme. It involves the recovery of waste heat to pre-heat spray dryer inlet air. It is estimated that the adoption of this technology within the UK will lead to energy savings worth £1.5M/year by 1990.

The project at BIP demonstrates the use of a heat pipe exchanger to recover the heat from a boiler exhaust in order to pre-heat the inlet air. Other projects in the package are at Clayton Aniline Ltd and ABM chemicals Ltd. In these the heat is recovered from the spray dyrer exhaust using a run-around coil and a glass tube heat exchanger respectively.

The performance of the system at BIP was independently monitored by the Harwell Laboratory, and this report details the performance of the exchanger and the resulting energy and cost savings at the spray dryer.

The total cost of the installation was £22,000, of which £5500 was funded by the Energy Efficiency Office. The heat recovery system has resulted in a 27% reduction in the specific energy consumption of the spray dryer. For BIP in 1983, this was worth £11,300/year giving a simple payback period of 1.9 years.

The savings associated with this type of heat recovery system are dependent upon a number of factors including:

• 1.

  (i) The quantity of available flue gas—at the maximum flue gas flow-rate, the savings to BIP would be worth £14,500/year, giving a project payback of 1.5 years;

• 2.

  (ii) The frequency of operation of the spray dryer—BIP operated their dryer for 3677 h in 1983. Assuming a notional 6000 h, the savings, at maximum flue gas availability would be worth £22,000 with a payback of one year;

• 3.

  (iii) The mode of operation—BIP installed the exchanger in a co-current flow mode. In the more efficient counter-current orientation, savings worth £31,000 could have been achieved giving a payback period of just eight months. BIP installed the exchanger in a cocurrent mode because of the simplicity of installation.

A number of operational factors associated with the heat exchanger have been identified in the course of this project:

• 1.

  (i) It is essential to ensure that the heat pipe heat exchanger cannot be subjected to temperatures in excess of the design limit. During the course of this project this led to tube fracture which has necessitated the replacement of the exchanger unit:

• 2.

  (ii) To enable good efficiency to be maintained, it is essential that provision be made to allow for routine cleaning of the heat exchanger.

In addition, to maximise the cost savings, dryer operation should be as efficient as possible. A number of areas found worthy of investigation in order further to improve performance on this particular dryer are follows:

• 1.

  (i) Leakage through the explosion relief doors;

• 2.

  (ii) Excessive start-up and shut-down periods;

• 3.

  (iii) Inefficient scheduling of dryer operation.

These areas have now been improved with significant further operational savings.

This project has shown that the use of a separate boiler exhaust, even if located some 60 m from the dryer, is a practical and cost-effective means of improving the efficiency of a spray dryer.