Wastewater Treatment

Incineration is a method of treating waste which involves the combustion of the organic substances found in waste materials. The solid mass of the original waste is reduced by around 80 to 85%, while the volume is reduced by between 95 and 96%. While incineration does not totally replace the process of landfilling, it does reduce the amount of waste to be disposed of considerably.

Not only this, but incineration has definitive benefits when used to treat more specialist types of waste such as clinical or hazardous waste, where the high temperatures can destroy potentially dangerous toxins and pathogens.

When burning waste however, pollutants are created. These pollutants are emitted alongside the flue gas and depending on the composition of the waste being incinerated, smaller quantities of CO, NOx, HCl, HF, HBr, PCDD/F, SO2, VOCs, PCBs or heavy metal compounds are either formed or remain following incineration. Most countries regulate these emissions and have laws in place to protect the environment.

Of particular note is HCl, which must be properly managed not only due to potential environmental damage, but also for its risk to plant equipment. Wastes often contain chlorinated organic compounds or chlorides and in municipal waste, around 50% of these chlorides come from PVC. During the process of incineration, the organic component of these compounds is destroyed and any chlorine present is converted into HCl.

The following types of plants are typically affected by this issue:

  • Waste incinerators such as those handling hazardous, municipal and sewage sludge type waste.
  • Plants that use waste for co-incineration, for example power plants, cement plants or biomass plants.

Techniques for the Reduction of Acid GasesHCl, SO2 and HF are usually cleaned from the flue-gas using alkaline reagents. The following processes are applied:

  • Dry processes: A dry sorption agent (such as sodium bicarbonate or lime) is added to the flow of flue-gas. The product of the reaction is also dry.
  • Semi-wet processes: These processes are sometimes referred to as semi-dry. Here, the sorption agent is a suspension (such as a slurry) or an aqueous solution (such as lime milk). The water solution evaporates, leaving behind dry reaction products. Any residue can be recirculated to improve utilization of the reagent. Another variation of this technique is fly-dry processes. Here an injection of water gives fast gas cooling, and reagent at the filter inlet.
  • Wet processes: This process involves the flue-gas flow being fed into hydrogen peroxide, water and/or a washing solution containing part of the reagent (for example sodium hydroxide solution). The reaction product is aqueous.

Motivation

The ideal outcome for these processes is optimum control of the scrubber plant, with maximum efficiency which allows compliance with relevant environment regulations; in particular the monitoring of plant conditions and minimal use of the reagent. Additionally, gypsum may be produced for sale.

Schematics of the flue gas desulfurization in a power plant

Schematics of the flue gas desulfurization in a power plant

Task: SO2 and HCl Scrubber ProcessTwo key parameters are required for the scrubbing of acid gas:

  • SO2, HCl and H2O before scrubber to control the treatment process
  • SO2, HCl and H2O after scrubber to monitor efficiency

The control process is derived from these two values. Additionally, oxygen levels may be monitored and measured as a means of detecting leakages.

Typical measuring ranges before scrubber:

  • SO2: 0 to 200 / 1000 / 3000 mg/m3
  • HCl: 0 to 500 / 2000 / 5000 mg/m3 x H2O: 0 to 10 / 20 / 30 / 40 vol %
  • O2: 0 to 10 / 25 vol %

Typical measuring ranges after scrubber:

  • SO2: 0 to 75 / 300 mg/m3
  • HCl: 0 to 15 / 100 mg/m3
  • H2O: 0 to 10 / 20 / 30 / 40 vol %
  • O2: 0 to 10 / 25 vol %

ABB Alternative Solutions: ACX with LS25, ACF5000

ABB offers a range of measurement solutions which can be utilized within these processes, each offering improved efficiency and economic security.

ACX

Using ACX alongside LS25 and connected via an ethernet connection provides an efficient solution for both downstream and upstream measurement.

The ACX system itself provides a complete solution for the extractive continuous gas analysis, and can be totally externally controlled. On the inside, reliable analyzers such as the Advance Optima series can work with proven components for sample conditioning.

Not only this, but due to a standardized design, the ACX system is especially easy to maintain. Comprehensive digital communication allows the system to be maintained remotely from anywhere in the world using the AnalyzeIT Explorer package.

LS25

The LS25 is an in-situ laser analyzer which is able to selectively measure water and HCI concentration. Operating in accordance with the single-line spectroscopy principle, a single absorption line is selected from the gas which Is then measured in the almost infrared spectral range, at which there is no cross sensitivity from other gases. The laser scans the absorption line, and the receiver opposite detects any absorption caused by the sample gas. The gas concentration is then calculated from this.

Key advantages of the LS25 include:

  • The system offers a rapid response time (T90).
  • In-situ measurement without specific sample handling – this would require a far more complex system for hydrogen chloride.
  • Minimum maintenance due to the lack of specific sample handling components.
  • The ability to provide a cross-stack averaged concentration.
  • Humidity or maximum dust loads do not impair or prevent cross-stack measurement.

ACF5000

An alternative solution for upstream and downstream measurement, ACF5000 combines the benefits of an infrared spectrometer using Fourier transformations, along with the recognized technology of ZrO2 analyzer modules. This system does not require frequent calibration and the inclusion of a high resolution FTIR spectrometer offers stable and sensitive selective infrared measurement of the active gas molecules. Like the LS25 system, it is possible to remotely maintain and control the ACF5000 with the AnalyzeIT Explorer.

The ACF5000 is preferable if:

  • High dust loads or the need for a back-purge option mean that using an in-situ technology is not viable.
  • There is a requirement to measure components other than HCl and SO2. The FTIR technology allows the addition of further components such as HF and NH3.
  • Rather than a mix of extractive and in-situ, a consistent extractive solution is preferred or required.

ABB solutions

ABB solutions

Typical set-up for a SO2 and HCl scrubber

Typical set-up for a SO2 and HCl scrubber

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.

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