By Peter Burrowes, Bruce Bartel and Levent Takmaz
The Green Bay Wastewater Treatment Plant (WWTP) is owned and operated by NEW Water and is designed to treat 113 million litres of wastewater per day from surrounding communities. It uses fluid bed incineration to reduce the volume of digested biosolids generated during the wastewater treatment process. The facility went through a major solids handling upgrade. This included two anaerobic digesters, coupled with a combined heat and power (CHP) cogeneration system and a new cold wind-box fluid bed incineration system. This replaced the existing multiple hearth furnaces.
A sludge dryer was installed upstream of the fluid bed incineration system, increasing the dryness of digested sludge in order to achieve autogenous combustion with no fossil fuel consumption.
Incineration basics and air pollution controls
The reactor features a cold wind-box, refractory arch dome and teardrop shape freeboard. It is designed to burn 46 dry tonnes per day autogenously with “zero” fuel consumption.
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Sludge has a heat value of 5,833 kcal/kg, based on volatile, 64.9% volatile and 39% total solids at reactor inlet. A sludge dryer is installed upstream of the fluid bed incinerator to increase sludge dryness from about 21% to 39%. This eliminates auxiliary fuel consumption during normal operation.
A hot oil economizer is installed downstream from the reactor to produce 200°C oil. After this, a wet scrubber equipped with a quench section, cooling tray and a multiple venturi section is installed to remove particulate and acid gas from the flue gas.
A wet electrostatic precipitator (WESP) is installed after the scrubber to polish clean flue gas before it enters the demister. Here, free water droplets are removed. The flue gas temperature is increased above the dew point through a hot oil heat exchanger installed after the demister.
Clean flue gas from the hot oil heat exchanger is passed through a fixed carbon bed adsorber for mercury, dioxin and furan removal. Caustic is injected into the scrubber tray section to meet limits for hydrogen chloride (HCl) and sulphur dioxide (SO2).
A caustic recirculation system is employed to minimize the caustic consumption during normal operation. A pH analyzer is installed on the return line of the caustic recirculation system to monitor the pH level to adjust the amount of caustic injection.
Ammonia injection is part of the overall design for NOx removal. There are six ammonia injection guns installed on the reactor freeboard. In addition, there are three injection guns installed on the hot gas duct to inject ammonia at 19% concentration to meet the NOx emission limit.
A low NOx preheat burner is used during cold start up. Ash slurry from the scrubber is discharged to an ash tank and then transferred by ash pumps to a lagoon.
Commissioning and performance testing
Table 1 shows the overall stack emission test results from the new incinerator. It meets the emission limits for a new incinerator, including mercury. Average mercury emissions during the two-stack testing were 0.0002 mg/dscm. It also shows the dioxin and furan emissions for both units.
The level of cadmium (Cd) emissions stayed below 0.00007 mg/dscm and 0.0002 mg/dscm during the first and second stack tests, respectively, being significantly lower than the emission limit of 0.0011 mg/dscm.
Overall, dioxin and furan emissions based on total mass basis stayed at 0.003 ng/dscm during the first stack testing. Dioxin and furan emissions were below 0.0007 ng/dscm during the second stack testing, meeting the requirement of 0.013 ng/dscm for both tests. Fixed carbon bed mercury removal system is the main reason for meeting the dioxin and furan emissions.
Dioxin and furan emissions based on toxic equivalency basis stayed at 0.0026 ng/dscm during the first stack testing. Dioxin and furan emissions were below 0.0019 ng/dscm during the second stack testing, meeting the emission limit of 0.0044 ng/dscm during both stack tests.
The new incinerator had an average of 1.2 ppmvd and 0.37 ppmvd carbon monoxide (CO) emissions during the first and second stack testing, respectively. This is significantly lower than the emission limit of 27 ppmvd. Low CO emissions are the result of high combustion efficiency inside the fluid bed and having seven seconds of freeboard residence time inside the reactor. This resulted in complete combustion.
Stack test results showed lower than 0.1 ppmvd hydrogen chloride (HCl) emissions during the first stack testing. Average HCl emissions were lower than 0.07 during the second stack testing. The incinerator met the limit on HCl emissions during both tests. Caustic injection system resulted in low HCl emissions.
Average mercury (Hg) emissions in the stack were 0.0002 mg/dscm. The fixed carbon bed adsorber demonstrated great performance on removing mercury from the flue gas.
Average oxides of nitrogen (NOx) emissions stayed at 12 ppmvd during the first stack testing, meeting the emission limit of 30 ppmvd. The incinerator was able to meet the NOx emissions limit without using the ammonia injection system during the first stack test.
Currently, the facility is still meeting emission limits on NOx without using the ammonia injection system. Average NOx emissions during the second stack testing in May 2019 were 2.8 ppmvd.
| Pollutant | Units (7% O2) | MACT | TEST RESULT (Oct 17 – 18, 2018) | TEST RESULT (May 1 – 2, 2019) |
|---|---|---|---|---|
| Cd <0.00007 | mg/dscm | 0.0011 | <0.00007 | <0.0002 |
| CDD/CDF TMB | ng/dscm | 0.013 | 0.003 | <0.0007 |
| CDD/CDF TEQ | ng/dscm | 0.0044 | 0.0026 | <0.0019 |
| CO | ppmvd | 27 | 1.2 | 0.37 |
| HCl | ppmvd | 0.24 | <0.1 | <0.07 |
| Hg | mg/dscm | 0.001 | 0.0002 | <0.0002 |
| NOx | ppmvd | 30 | 12 | 12.8 |
| Opacity | % | 0 | 0 | 0 |
| Pb | mg/dscm | 0.00062 | 0.00049 | 0.00053 |
| PM | mg/dscm | 9.6 | <0.6 | <0.6 |
| SO2 | ppmvd | 5.3 | 0.1 | 3.4 |
Table 1: NEW Water incinerator stack emission test results.
Visual observations showed zero opacity in line with emission limits. Average lead (Pb) emissions during the first stack testing were 0.00049 mg/dscm. This is lower than the emission limit of 0.00062 mg/dscm. Results of the second stack testing indicated that average lead emissions were 0.00053 mg/dscm. Particulate emissions (PM) stayed below 0.6 mg/dscm during both stack testing. Average sulphur dioxide (SO2) emissions were 0.1 and 3.4 ppmvd during the first and second stack testing, respectively.
Peter Burrowes is with Jacobs Engineering. Email: peter.burrowes@jacobs.com.
Bruce Bartel is with NEW Water.
Email: bbartel@newwater.com.
Levent Takmaz is with SUEZ WTS. Email: levent.takmaz@suez.com
*References are available upon request.
Read the full article in ES&E Magazine’s April/May 2020 issue below.
