Environmental Science & Engineering

Evaluating the performance of new fluid bed incinerators

By: E. Ferguson, B. Dobson, K. Dangtran and L. Takmaz

The Duffin Creek Water Pollution Control Plant (WPCP) in Pickering, Ontario, is jointly owned by the Regional Municipalities of York and Durham. Both regions are experiencing significant population growth, which necessitated increasing rated plant capacity to 630 million litres per day (ML/d) through several process expansion projects.

Two fluid bed thermal oxidizer units were built back in the late 1970s by GL&V. Two additional reactors built by SUEZ were recently added to provide the facility with a firm solids processing capability of 270 dry tonnes per day. Identical to the original systems, the new oxidizers are also equipped with two waste heat recovery systems producing superheated steam to drive two steam turbines coupled with two fluidization air blowers. Unit No. 4 fluidization air blower is connected to an electric motor (447 kW) and a steam turbine through a clutch mechanism. Unit No. 3 fluidization air blower is connected to only a steam turbine. A clutch mechanism between the fluidization air blower and the steam turbine enables the switching from turbine to electric motor during normal operation.

Once both units are operating at full capacity, the fluidization air blowers are driven by steam turbines during steady state operation. This unique design should be a template for future incineration systems. Each fluid bed unit has the capacity to incinerate 105 metric dry ton per day (MDTPD) total solids. Each fluid bed unit is equipped with a dedicated heat recovery system employing a primary heat exchanger and waste heat recovery boiler. Primary heat exchanger (shell and tube design) is used to preheat fluidization air to minimize the auxiliary fuel usage during steady state operation. Each unit was designed to be autogeneous with a sludge feedstock composition of 68% volatile, 28% total solids and 5,560 kcal/kg sludge heat value based on volatile. Based on the feedback from the plant, both units are operating autogeneously even with a sludge feedstock content as low as 24%.

Flue gas from the fluid bed reactor is passed through a primary heat exchanger and then discharged into a waste heat boiler to generate superheated steam. From waste heat boiler, flue gas is sent to a wet scrubber to remove particulate and acid gas (SO2, HCl). The wet scrubber is not equipped with a caustic injection system.

Due to more stringent air requirements, a Kombisorbon mercury removal system is installed downstream of the multi-venturi wet scrubber to remove mercury, dioxins and furans. It includes a conditioner having a droplet separator to remove free water droplets from the clean flue gas discharged from the wet scrubber. The conditioner also has a heat exchanger to increase the saturated flue gas temperature about 20°C above the dew point temperature, to prevent moisture formation inside the fixed carbon bed adsorber.

Clean flue gas discharged from the wet scrubber flows through the cold side of the conditioner heat exchanger and the steam from the low pressure steam header is used as the heating medium for the hot side of the conditioner heat exchanger. Clean flue gas from the conditioner heat exchanger is sent to the Kombisorbon adsorber to remove mercury, dioxins and furans.

There is also an ID Fan installed before the stack to maintain vacuum conditions inside the waste heat boiler to prevent any potential flue gas leak into the building. The adsorber has three layers, with the first layer filled with regular carbon and the following two filled with activated carbon.

Operators can take carbon samples from the adsorber during normal operation and test the samples for mercury loading to determine the current condition of activated carbon. It is estimated that every two or three years, carbon bed material will need to be replaced. The actual life expectancy of the carbon is based on the mercury-loading rate at each facility. However, the Duffin Creek units do not have sufficient operating time on them at present to provide a definitive life expectancy.

Commissioning and performance testing

The Duffin Creek fluid bed incinerators went through start-up, commissioning and performance testing in 2013 and 2014. Unit No. 4 passed the performance testing in June 2013. Unit No. 3 passed the performance testing in February 2014. During the performance testing, sludge, ash and water samples were collected and analyzed.

The successful operation at Duffin Creek WPCP has shown that the improved thermal oxidizer design, incorporating enhanced air pollution control and energy recovery systems to reduce the operational expenditures, is an economical, environmentally friendly and cost-effective solution for sludge disposal.

Duffin Creek WPCP fluid bed units have satisfied the sludge disposal needs of the plant, emission requirements of the Ministry of the Environment and Climate Change, and the future growth needs of the Regions of York and Durham.

E. Ferguson is with York Region. B. Dobson is with the Duffin Creek Water Pollution Control Plant. K. Dangtran and L. Takmaz are with SUEZ. This article appears in ES&E Magazine’s October 2017 issue.

Source: https://esemag.com/air-pollution/evaluating-the-performance-of-new-fluid-bed-incinerators/