Combined techniques to prevent and/or reduce SO X and NO X emissions

emissions

Combined SOX/NOX abatement techniques have been developed with the aim of replacing

conventional FGD/SCR techniques. Some of the combined SOX/NOX abatement techniques

have only been applied in a very small number of units or exist more or less only as demonstration plants and have not yet found a market penetration for commercial (cost) reasons.

Each of these techniques employs a unique chemical reaction to remove SOX and NOX

simultaneously. The development of combined techniques has been triggered by a major problem of conventional SCR followed by the FGD technique, which is related to the oxidation

of SO2 in the SCR reactor. Usually 0.2–2 % of the SO2 is oxidised to SO3. This has various

effects on the flue-gas cleaning system. For low-sulphur coal for instance, SO3 may improve the

removal efficiency of a cold-side ESP. However, SO3 usually increases the deposits and

corrosion in the air preheater and gas-gas heat exchanger.

Combined SOX/NOX abatement techniques can generally be divided into the following

 solid adsorption/regeneration (desorption);

 gas/solid catalytic operation;

 electron beam irradiation;

 alkali injection;

 wet scrubbing.

Within these categories several processes are still under development, whereas other techniques are already commercially available and in operation in a number of plants.

3.1.5.1 Solid adsorption/regeneration

This type of process employs a solid sorbent or catalyst, which adsorbs or reacts with SOX and

NOX in the flue-gas. The sorbent or catalyst is regenerated for reuse. Sulphur or nitrogen species

are liberated from the sorbent in the regeneration step, which generally requires a high temperature or reducing the gas for a sufficient residence time. The recovered sulphur species are processed, for example in a ‘Claus’ plant, to produce elemental sulphur, a saleable by-

product. The nitrogen species are decomposed into N2 and water by injection of ammonia or by

recycling to the boiler. In other processes such as the activated carbon process, copper oxide, zinc oxide and magnesium oxide-vermiculite are involved in solid adsorption/regeneration.

3.1.5.1.1 Activated carbon process

As activated carbon has a very large specific surface area, it has been widely used as an air cleaning and waste water treatment agent since the nineteenth century. It has also long been

known that activated carbon adsorbs SO2, oxygen and water to produce sulphuric acid.

Simultaneous SO2 and NOX removal becomes possible by adding ammonia.

The flue-gas from the boiler is first dedusted, passed through a heat exchanger where heat is extracted for activated carbon regeneration, and then cooled in a water pre-scrubber. The gas enters the first stage of the activated carbon (dry porous charcoal) bed at a temperature of 90– 150 ºC. The sulphur dioxide reacts with oxygen and water vapour in the flue-gases (through catalytic oxidation) to form sulphuric acid, which is adsorbed on the activated carbon.

Prior to entering the second-stage adsorber, ammonia is injected into the flue-gases in a mixing chamber. Nitrogen oxides react catalytically with the ammonia in the second stage to form

nitrogen gas (N2) and water. The cleaned flue-gases and liberated nitrogen and moisture pass to

the stack for discharge. The reduction process takes place in an adsorber, where the activated carbon pellets are transported from the top to the bottom in the form of a moving bed. The gas flows across the layers, first entering the lowest part of the bed.

The sulphur-laden activated carbon passes to a regenerator where desorption is performed thermally, by indirect heating using heat extracted earlier from the flue-gases, at a temperature of about 400–450 ºC. Carbon dust is removed and make-up pellets added, prior to recycling

them back to the absorber. As a result of the regeneration, enriched SO2 gas is generated from

the desorber. The enriched gas is converted, using a Claus or another process, to elemental sulphur, or sulphuric acid that can be sold as a by-product. Figure 3.4 shows a schematic diagram of the activated carbon process.

Large Combustion Plants 113

Source: [ 148, CIEMAT 2000 ]

Figure 3.4 The activated carbon process

3.1.5.1.2 Other solid adsorption/regenerative processes

Other processes such as the copper/zinc oxide process are still being developed and are therefore not discussed further in this chapter.

3.1.5.2 Gas/solid catalytic processes

This type of process employs catalytic reactions such as oxidation, hydrogenation or SCR. Elemental sulphur is recovered as a by-product. Waste water treatment is not required. WSA-

SNOX (Wet gas sulphuric acid with integrated selective catalytic reduction DeNOX step),

DeSONOX and SNRB are included in this category.

The WSA-SNOX process employs two catalysts sequentially to remove NOX by SCR and to

oxidise SO2 to SO3, condensing the latter to sulphuric acid for sale. About 95 % of the sulphur

and nitrogen oxides in the flue-gas can be removed. The process produces no waste water or

waste products, nor does it consume any chemical, apart from ammonia for NOX control.

In the DeSONOX process, flue-gases are first passed through an ESP to remove particulates,

followed by ammonia injection and SCR. The gases are then cooled by preheating combustion air, and reheating the fully treated flue-gases prior to release to the atmosphere. The temperature of the flue-gas is thus reduced to approximately 140 ºC, which enables the catalytic oxidation of

SO2 to SO3 and its subsequent condensation to sulphuric acid (70 %). The latter step is

accomplished in a recirculating acid tower. The flue-gases are finally directed through a wet electrostatic mist precipitator and are reheated prior to release.

In the SOX-NOX-Rox Box

TM

process (SNRB), a dry sorbent such as lime or sodium bicarbonate is injected into the flue-gas upstream of a specially designed filter arrangement. This process

combines the removal of SO2, NOX and dust in one unit, i.e. a high-temperature catalytic

ceramic or bag filter. The process requires less space than conventional flue-gas cleaning

technology. The SNRB process aims to remove up to 90 % of the SO2 and NOX and at least

99 % of the dust, but no information is available about whether this process is actually applied to a large combustion plant. Therefore, no information on the general performance of the SNRB process is given.