With the treatment of flue gas desulphurization wastewater becoming mandatory in coal-fired plants, Prakash Govindan and Zhifeng Fan make the case for carrier gas extraction as a cost-effective and performance-boosting solution.
Flue gas desulphurization (FGD) wastewaters are produced at coal-fired power plants in increasing quantities as the regulation on air emissions is tightened worldwide.
Low cost and environmentally favourable reuse of this wastewater stream has become an important topic with the respective national and local regulatory bodies stipulating minimum treatment levels and standards.
Traditional technologies, which are otherwise used for concentration of saline streams, fail the economic and performance benchmark that needs to be met.
Carrier gas extraction (CGE) technology, which was specifically developed to handle high levels of contamination and variability in feed waters, is optimal for this application and offers the lowest cost solution within the required performance.
Source and constituents
Coal-based power plants generate over a third of the planet’s electricity. The combustion of coal in these facilities produces a flue gas that is emitted to the atmosphere.
Many power plants are required to remove SOx emissions from the flue gas using FGD systems. The leading FGD technology used globally is wet scrubbing (85 per cent of the installations in the US and 90 per cent of the installations in China).
Commonly, three kinds of scrubbers are used for wet scrubbing – venturi, packed, and spray scrubbers – and entail injection of alkaline scrubbing agents into the scrubber.
Typically, the agent is limestone (i.e., calcium carbonate), quick lime and caustic soda.
For example, when limestone reacts with SOx in the reducing conditions of the absorber, sulphur dioxide (the major component of SOx) is converted into sulfite, and a slurry rich in calcium sulfite is produced. In forced oxidation FGD systems, an oxidation reactor is used to convert calcium sulfite slurry to calcium sulfate (gypsum).
From the blowdown and dewatering processes of the slurry, the FGD wastewater stream is created. The composition of coal and limestone primarily affects the composition of the wastewater. Other parameters which have a smaller effect are the type of scrubber and the dewatering system used. Coal contributes chlorides, fluorides and sulfate to the wastewater. Because of the metallurgy used in the scrubbers it is typical to purge the wastewater before the chlorides exceed 12,000 mg/L. The level to which chlorides are tolerated by the metallurgy of the scrubber determines the amount of wastewater generated. Use of better metallurgy can help reduce the amount of wastewater by not purging until a chlorides level of up to 35,000 mg/L is reached.
Coal also adds trace metals, including arsenic, mercury, selenium, boron, cadmium and zinc. Limestone could contribute iron and aluminium to the FGD wastewater.
Varying water quality
Defining a standard composition of FGD wastewater across different power plants is tricky because there is no consensus in the industry on where (in the process train) the sample for measuring the composition has to be collected, and the design of the process train downstream of the scrubber itself changes from one facility to another. For example, some facilities may employ primary and secondary hydro cyclones to maximize the capture of solids before gypsum dewatering.
It is also common for a plant to change coal and limestone suppliers so the wastewater constituents will change over time during operation of the FGD system. Unlike other industrial wastewater treatment fields, FGD wastewater samples for a specific plant are likely not available for testing before the plant is designed, built and commissioned. Additionally, during the operation of the coal-fired power plant, there might be periods when the plant is not run at full capacity and the SOx levels and FGD water quality and volumes can vary.
Complicating matters further, the plant’s wastewater treatment system must be flexible to handle these varying inputs yet produce a treated stream that meets the plant’s wastewater discharge permit requirements. All traditional treatment technologies fail because they cannot handle these requirements as they were developed for sea water or other applications where the feed water quality and volumes remain fairly constant.
Invented at Massachusetts Institute of Technology, CGE is a novel method of desalinating high salinity water streams using a carrier gas. The technique was specifically developed to handle high contamination wastewaters at varying volumetric rates and quality. Over 50 patent families cover various innovations which make CGE an economical technology for treating FGD wastewaters to high recovery rates and with high levels of influent water variability.
CGE is a desalination process that mimics the rain cycle. It uses a carrier gas as a medium to desalinate saline streams by using a humidification-dehumidification configuration. CGE consists of two main unit operations: humidifier and dehumidifier (see Figure 1).
Both the humidifier and the dehumidifier are direct contact heat and mass exchange devices. The humidifier is a packed bed device wherein a heated water stream (<90à‚°C) is introduced in the form of droplets and the carrier gas (which is typically ambient air) is introduced at the bottom of the device in a counter-current configuration.
The carrier gas comes in direct contact with the saline droplets and there is evaporation from the surface of the droplet into the carrier gas stream. Hence, as the carrier gas rises through the device, it accumulates increasing amounts of pure water vapour from the saline stream. The concentrate, which retains all of the dissolved salts, exits the humidifier, is diluted with feed wastewater, is preheated with heat from the dehumidifier, is heated using a source of energy (like solar heaters) and is recirculated back into the humidifier.
The dehumidifier is a multi-stage bubble column device. In this device, the air-vapour mixture from the humidifier is sparged through several shallow layers of fresh water in successively cooler stages. As a result, small bubbles are formed and the vapour condenses from the surface of the bubbles into these shallow pools. As this fresh water is generated it also picks up the heat of condensation, which is transferred back to the feed water in the preheating heat exchanger.