Cleaning contaminated soil at Point Cook

In 2012, Enviropacific was appointed as a remediation contractor for the clean-up of the former fire training area at RAAF Base Williams, Point Cook. Due to firefighting and other activities, the underlying soil and groundwater of the area were contaminated with organic hydrocarbons, providing risks to the environment and to human health.

Enviropacific installed and commissioned a direct thermal desorption (DTD) plant, with a soil processing line from the USA. The line included a desorber to dry the soil and drive off organic contaminants and an oxidiser to render the contaminants suitable for discharge to the atmosphere. Each item was supplied with a burner which was capable of an output exceeding 50 GJ/h.

Hurll Nu-Way was awarded the contract to design, manufacture and supply the gas trains and the burner management panels for each of the burners. It also installed the gas trains and was responsible for liaising with Energy Safe Victoria over system approval and carrying out the burner commissioning and start-ups.

One of the difficulties in the construction of the plant was its location - near the coast of Port Philip Bay, opened to harsh weather conditions and under an active airfield. The plant’s components therefore had to be rugged and reliable, so Hurll Nu-Way supplied the equipment with the following features:

• Filters and pressure regulating equipment with overpressure shut-offs (OPSOs). Independent pilot gas rate regulators with OPSOs were installed in the pilot lines to ensure reliable pilot ignition on start-up.
• Pneumatically operated, double-block, safety shut-off valves were included in each gas train, with a valve proving system to check their effectiveness prior to each start-up and after each shutdown.
• Self-checking ultraviolet flame detectors were fitted to each burner to operate in conjunction with the panel-mounted Siemens LGK ignition and flame detection programmers in a system designed to operate 24 hours a day, six days a week.
• Control of the firing rate of each burner was performed through motorised linked valves - a butterfly valve in the air line and a square port valve in the gas line assembled in tandem and positioned by a Honeywell motor.
• The linked valves option controls the burners at a fixed air/gas ratio and so are less flexible than alternative schemes using independent actuators, but offer reliable and repeatable control.

The burner systems are monitored and controlled by the plant’s PLC control system, but the burners’ ignition and safety system is hardwired as required by current Australian codes.

Enviropacific incorporated safety features beyond those required by a strict interpretation of the Australian Code. Hurll Nu-Way assisted in interlocking two burners by the overall control system. Although the burner systems are independent of each other, the overall control ensures that the oxidiser burner is started first and its chamber reaches a preset operating temperature before the desorber burner system can be started.

Products of combustion together with water vapour and volatiles from the soil are drawn from the desorber and through the oxidiser. The temperature in the oxidiser chamber is maintained between a minimum of 7500°C and a maximum of 11,000°C, with excess oxygen present in the process stream to ensure the total destruction of the contaminants. The pressure inside the processing line is maintained below the atmospheric pressure to prevent any leakage of the process stream to the atmosphere.

Exhaust gases being delivered to the stack are continuously monitored for oxygen, carbon monoxide, NO_x and SO_x to ensure complete destruction is achieved. NATA registered testing was regularly carried out to analyse the gases for a comprehensive suite of parameters, including VOCs, SVOCs such as dioxins and difurans, metals, acid gases and particulate matter.

The plant was constructed allowing for the possible relocation in the future; the gas trains and panels were designed to allow their transport by road to new sites, while the burners, currently running on LNG, can operate with natural gas, LPG or oil. The plant is currently remediating 20-25 tonnes of soil per hour, with circa 60,000 tonnes of contaminated soils and 18 ML of contaminated groundwater successfully treated and re-used on-site.