LIGHTING UP THE AFTERBURNERS

Client: Bitumen Equipment
Project: Downer Works Hamilton Asphalt Plant

• Improved plant efficiency
• Improved plant performance
• Energy savings
• Operator friendly PLC interface

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Background

Downer Work’s Hamilton asphalt plant is a Bitumen Equipment designed and built plant. Bitumen Equipment has manufactured most of NZ’s new asphalt plants over the last 20 years. A vital part of these plants is the automation that allows the continuous manufacturing process to produce high quality mix. N2P Controls’ senior engineers, Peter Crane and Peter Taylor, were the brains behind these plants’ automation. Peter Crane has over 30 years of experience in asphalt manufacturing and, with Shane Sayers from Bitumen Equipment, are widely regarded as the preeminent asphalt manufacturing experts within NZ.

A critical part of the continuous asphalt manufacturing process is the drying of the aggregate and mixing of the bitumen. Original plants used parallel flow, where aggregate was introduced at the same end of the drying drum as the burner. The burner runs at temperatures close to 1,000 ̊C in order to dry the aggregate. The aggregate is typically heated to approximately 160 ̊C before leaving the drying drum. In the parallel flow configuration the exhaust air leaves the drum at approximately 200 ̊C and contains particulate. The particulate is passed through a wet scrubber and the water from the scrubber goes into a sediment pond. The removal of the sediment from the pond involved considerable costs. Consequently the design of the asphalt plants was changed to counter flow where the aggregate is introduced to the drying drum from the opposite end of the burner. This, and the introduction of bag houses to remove particulate instead of wet scrubbing, greatly improved the efficiency of these asphalt plants.

Problem

A side effect of this improved heat transfer was that the air leaving the drum was below its dew point. This caused excess moisture in the bag house. To overcome this, the main burner heated the mix to a higher temperature to ensure the gases leaving the drum were above its dew point. This resulted in a lot of energy being lost with the mix leaving the drum hotter than it required to be. Bitumen Equipment’s owner, Shane Sayers, had a mechanical solution for Downer Works to improve the plant’s efficiency further, but just needed a cost effective method to manage the automation of the plant.

With N2P Controls’ experience in heavy industry, especially asphalt, N2P Controls were the logical partner to Bitumen Equipment in helping with the automation of this afterburner. Coupled with N2P Controls’ purchasing power and low overheads, they were able to offer a cost competitive solution.

Solution

It occurred to Shane that heating just the air and particulate as it left the drum would require significantly less energy than heating the entire mix to reach the desired drum exit temperature.

With N2P Controls assistance, a design was completed. This included having a temperature probe at the exit of the drum. A small afterburner was installed at the end of the drum that heated the exhaust air and associated particulate to a set temperature. The burner now modulates its combustion to achieve the desired set temperature. N2P Controls used a small PLC with an operator friendly HMI to allow the asphalt plant operator to select his exhaust temperature for the air entering the bag house.

The PLC then controlled the afterburner to meet this exhaust temperature. The PLC was programmed to have interlocks to ensure no superheated air entered the bag house and also had standard safety features to lock out the afterburner until the main burner had gone through its purge process and started.