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3.1 APPLIED COMMON TECHNIQUES

3.1.10 Water use and techniques to reduce emissions to water

3.1.10.4 Waste water from flue-gas treatment systems

All wet-type flue-gas cleaning systems produce waste water that, due to the fuel and materials used, contains metals among other components. One of the main sources of waste water in this context is the wet limestone scrubber that is used in a large number of combustion plants for desulphurisation of the flue-gas, although this water volume can be reduced by using fuels with a lower chlorine content and by designing the absorber to operate at a higher chloride concentration. This results in a reduction in the purge to the waste water treatment plant, which in turn reduces emissions to water. An example of a conventional waste water treatment system is presented in Figure 3.6, but it is noted that there are many different types of systems, depending on the different national regulations, the type of fuel and site-specific factors.

The pH value of FGD waste water is increased in order to precipitate metals. This is generally achieved using either lime milk or caustic soda, causing the formation of metal hydroxides. By adding flocculants (iron(III) chloride), flakes are formed. The addition of coagulation aids (polyelectrolytes) allows the agglomeration of individual flakes, so that a greater flake formation ensues. The sludge is then pre-sedimented, drained and disposed of or co-combusted in the case of slag tap combustion. Part of the ‘thin’ sludge is recycled to the flocculation stage where the sludge particles serve as initial crystallisation nuclei promoting more rapid flocculation.

The treated waste water from the pre-sedimentation stage can be supplied to a baffle plate thickener for further sedimentation. The suspended micro-particles deposit on the inclined baffle plates. The sludge falling off the plates is gathered at the lower point of the baffle plate thickener and can also be recycled. The cleaned waste water is fed to the drain via the overflow of the baffle plate thickener, provided the regulatory limit values are met. In addition, if the pH value is required to be between 6 and 9.5, the water is neutralised. Although it is generally not necessary, the content of ammonia in the waste water may lead to it first being fed to an ammonia stripping plant before it is discharged to the drains. In some processes, e.g. with a higher input of Hg from the co-combustion of wastes, it is customary to also add (organic) sulphide after the addition of lime milk, thereby precipitating the metals as sulphides, which is more effective than using hydroxide. The disadvantage is that the metal sulphides (greater quantities) need to be disposed of, as by co-combusting these residues the sulphur would be released as sulphur dioxide and Hg would be released again.

Various plants treat FGD waste waters differently. While some of them use flocculants and flocculation aids for example, others use only flocculation auxiliaries and organic sulphide. There are, however, also operators who use flocculants, flocculation auxiliaries and organic sulphide.

In the example shown in Figure 3.6, FGD waste water is pre-neutralised in an agitator with the aid of lime slurry. The pH is further increased by additional dosing of lime slurry in the second reactor. Initial flocculation and settling of heavy metal hydroxides occur in the circular concentration reactor tank. Polyelectrolytic solution is fed into the supply line to the concentration reactor tank, in order to avoid repulsion between hydroxide particles and to accelerate sedimentation.

The treated water, with a pH of 6 to 9, may be transferred from the upper zone of the circular concentration reactor tank to the main water inlet. If the pH is above 9, it is corrected with an acid additive, e.g. hydrochloric acid. Part of the slurry withdrawn from the concentration tank is fed as contact slurry to support flocculation in the first agitator. This slurry acts as an accelerator for the precipitation of the hydroxides. Most of the slurry from the agitator is temporarily stored in a slurry container, dewatered in a filter press and finally stored in a bunker prior to disposal.

Source: [ 123, Eurelectric 2001 ]

Figure 3.6: FGD waste water treatment plant

Two-stage precipitation processes (see Figure 3.7) are widespread in FGD waste water treatment. Waste water from FGD first reaches an oxidation stage, in which conversion is generally accomplished with sodium hypochlorite (NaOCl), particularly mercury dissolved into Hg(II). This is followed by the gypsum desaturation stage. Here, by the addition of calcium

hydroxide (Ca (OH)2), the pH of the waste water is raised and iron(III) chloride (FeCl3) may be

added for flocculation. With the addition of a flocculant, a sedimenting sludge is formed, which is then deposited in a first sedimentation stage. A portion of this sludge from the sedimentation

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is recirculated to improve sedimentation capability. This step can, for example, be a circular clarifier whose efficiency can be increased if necessary by the incorporation of lamellae. The deposited slurry (about 99 % of the total sludge accumulation) consists mainly of calcium sulphate and can therefore be used further as a resource.

The clear effluent of the first sedimentation stage then enters the metal removal stage. Here, by the addition of organic sulphides, the metals are precipitated as sulphides, and the pH may be further increased by the addition of calcium hydroxide.

The sedimentation capability of the metal sulphides is improved by contact between the sludge and the polymers. In the second stage of the two-stage procedure only a small amount of sludge is accumulated (about 1 %), which must be disposed of. The same construction of both sedimentation stages offers the advantage that the system can also be operated as a single stage, for example during times of revision. Another advantage of a two-stage procedure is that the gypsum sludge and the mercury sludge accumulate separately.

Source: [ 169, TWG 2006 LCP BREF 2003 ]

Figure 3.7: Two-stage waste water treatment plant

ZLD (zero liquid discharge) is a combination of techniques that results in no waste water discharges. Depending on plant-specific conditions, ZLD may be achieved for different waste water streams and by using different combinations of techniques. After the neutralisation and sedimentation unit (pH adjustment, ferric co-precipitation, flocculation, clarification, etc.), a Softening-Evaporation-Crystallisation (SEC) system can be installed. The products of this system are high-quality water, to be recycled, and salts, to be disposed of. Evaporation allows plants to recover clean water for reuse, thereby reducing water usage. A few plants worldwide

use evaporation, including larger plants (e.g. Plants 211/212 (418+433 MWth) and Plant 253

(1420 MWth)). However, evaporation is energy-intensive, which may offset the environmental

benefits. For new FGD applications, the design can be optimised for the ZLD concept, taking into account the possible additional cost and decrease in energy efficiency, by reducing the FGD purge flow rate.

Figure 3.8 and Figure 3.9 show that, for sulphate and mercury emissions to water for example, there is differentiation between plants operating with a wet abatement system for air pollutants (wet FGD, FG condenser) and those operating without.

Source: [ 3, LCP TWG 2012 ]

Figure 3.8: Yearly sulphate concentrations in direct emissions to water – Comparison of plants with and without a wet abatement system for air pollutants

Source: [ 3, LCP TWG 2012 ]

Figure 3.9: Yearly mercury concentrations in direct emissions to water – Comparison of plants with and without a wet abatement system for air pollutants

The reported average levels of the direct emissions to water for each category of plant (i.e. those fitted with a wet abatement system for air pollutants and those without) from a set of plants operated in Europe are given in Table 3.3. In this table, no distinction is made concerning the type of fuel burnt or the type of combustion plant. The ranges include emissions from coal-fired

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plants (which represent the majority of plants fitted with wet abatement systems), biomass-fired plants, and gaseous- or liquid-fuel-fired plants for those not fitted with such abatement systems. The plants themselves are boilers, gas turbines or engines. The concentrations of pollutants emitted by plants fitted with a wet abatement system are generally higher, especially for metals.

Table 3.3: Yearly emissions to water

Parameters

Unit Flue-gas treatment system used

mg/l With wet abatement Without wet abatement As < 0.048 < 0.028 Sb < 0.0051 < 0.02 Pb < 0.1 < 0.1 Cr < 0.083 < 0.08 Co < 0.005 < 0.008 Cu < 0.06 < 0.13 Ni < 0.05 < 0.06 Mn < 0.237 < 0.35 V < 0.015 < 0.037 Cd < 0.01 < 0.4 TI < 0.034 0.001 Fe < 3.85 < 2.4 Hg < 0.004 < 0.0015 Zn 0.47 < 0.34 F < 15.2 < 9.9 Cl < 18 250 < 5525 TOC < 34.8 < 37.4 Total suspended solids (TSS) < 41 < 126 Total P < 2 < 1.89 Sulphate as SO4 < 1704 < 1135 Sulphide as S < 0.3 < 0.89 Sulphite as SO3 4.8 < 5 Total N 0.7–303 < 73.5 AOX < 0.95 < 0.225 THC < 1.5 < 7 Source:[ 3, LCP TWG 2012 ]