Emissions to air .1 Particulate dust

Dust is emitted from soda ash production in limited quantities, arising from the following steps:

handling of mineral raw materials (coke, limestone) as diffuse sources

limestone burning in kilns, but in limited quantities or during abnormal operation only, since all the process kiln gas is collected prior to a washing-cooling step and thereafter is used in the carbonation stage, whilst the excess kiln gas (which can optionally pass through a gas washing step), is either vented to the atmosphere or partially used in the production of sodium bicarbonate

handling of soda ash and densification of light soda ash (hydration and dehydration) to produce dense soda ash

during the handling of these products.

It is common to use bag filters or wet scrubbers which significantly reduce the levels of dust emitted to the atmosphere.

The dust emitted is around 0.10 – 0.15 kg of dust/t soda ash, and for a plant with the annual capacity of 500 kt per year, this represents a typical quantity of 50 – 75 t/year.

The composition of dust relates to the material handled, namely:

C from coke

CaCO3, Al2O3 and SiO2 from limestone (and sand and clays present in limestone) CaO from burned lime

Na2CO3 and NaHCO3 from the production and transport of soda ash and sodium bicarbonate.

The high volume of gas flows that require treatment often require very large pieces of specific equipment. A number of abatement techniques are used depending on the characteristics of streams to be treated – refer to the BREF on CWW.

For dry gas streams, such as those encountered in product handling (conveying) and the storage section of a soda ash plant, dust abatement techniques normally achieve figures below 50 mg/Nm³, and with modern equipment (e.g. bag filters) below 20 mg/Nm³, in accordance with national environmental regulations in some Member States of the EU (e.g. TA Luft 2002).

For wet streams, dust abatement techniques normally achieve figures below 200 mg/Nm³ and with modern equipment (e.g. wet scrubber) may be expected to achieve figures below 50 mg/Nm³.

Specific emissions of 0.014 kg dust from all sources per tonne of soda ash are quoted by a German plant in Rheinberg, whilst the range of dust emissions from different sources is reported between 1.2 – 25 mg/Nm³ [45, UBA - Germany, 2001].

The Dutch plant in Delfzijl reaches values: <10 mg/Nm³ for dust of soda ash and limestone, and

<5 mg/Nm³ for CaO dust. Only two sources, scrubbers of the light and dense soda dedusting, comply with a higher limit value of <25 mg/Nm³ [85, EIPPCB, 2004-2005].

To minimise dust emissions, raw materials and products should be optimally handled, e.g. by encapsulating storage and transport facilities, equipping vents with bag filters, etc.

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During CaCO3 burning to CaO in the lime kilns, CO and CO2 are produced from the combustion of coke and decomposition of limestone. The Solvay process needs an excess of CO2 above that stoichiometrically required. Some of the excess is required to compensate for non-ideal absorption of CO2 in the carbonation towers.

Carbonating towers also have an outlet for the discharge of gases that have not reacted in the process. This gas is cleaned with brine in a washer to recover NH3 and possibly H2S, if present, and to recycle these components back into the process, while CO2, CO and other inert gases pass out into the atmosphere (stream GO2 in Figure 2.2).

Further excess CO2 may be beneficially used in the production of sodium bicarbonate. Any surplus CO2 is vented as kiln gas to the atmosphere (GO1).

The amount of CO2 vented to the atmosphere from a standalone soda ash process is in the range of 200 to 300 kgCO2/t soda ash. The split of losses to the atmosphere depends on the detailed plant configuration.

CO gas is virtually inert throughout the process. All CO produced must therefore be vented to the atmosphere either at the kilns or through the carbonation tower at the outlet of gas scrubbers.

CO generation is in the range of 4 to 20 kg CO/t soda ash, depending on the conversion of CO to CO2 during the limestone calcination step.

When released into the atmosphere, the CO is converted by natural processes into CO2. Furthermore, provided the dispersion of CO and CO2 is adequate and the stack responds to the normal dispersion rules, no local impacts on the environment or on health are expected or experienced.

2.3.3.3 Nitrogen oxides

NOX are produced inside the kiln by the oxidation of nitrogen contained in the air used in the combustion process. Since the temperature inside the kiln is moderate (up to 1100 °C), the formation of NOX is rather limited.

Specific emission value of 0.193 kg NO2/t soda ash was reported for the soda ash plant in Rheinberg, Germany [45, UBA - Germany, 2001].

Measurements in some plants indicate concentration after gas washing of less than 500 mg NOX/Nm³ of the outlet gas. Concentrations in the range of 240 – 290 mg NO2/m³ at the outlet of gas scrubbing section, and below 300 mg NO2/m³ of the outlet gas from the lime kiln, were reported for the soda ash plant in Rheinberg, Germany [45, UBA - Germany, 2001].

The NOX emission limit values of <200 mg/Nm³ are achieved by the soda ash plant in Delfzijl, the Netherlands.

2.3.3.4 Sulphur oxides

SOX are produced by the oxidation of compounds containing sulphur in the limestone and coke.

The formation of SOX is limited both due to the low sulphur content in fuels used in limestone burning and some autopurification reactions in the lime kilns (see Section 2.3.8.1). Furthermore, SOX in the kiln gas sent to the process are absorbed.

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58 August 2007 Anorganische Grundchemikalien – Feststoffe und andere - A specific emission value of 0.0003 kg SO2/t soda ash and the concentration of 2.5 mg SO2/m³ of the outlet gas were reported for the soda ash plant in Rheinberg, Germany [45, UBA - Germany, 2001].

It should be noted, however, that such emission values are not achievable by any plant on a regular basis, because it depends on the raw materials and equipment used, the location of the sampling, and other factors [85, EIPPCB, 2004-2005].

2.3.3.5 Ammonia

The main atmospheric emissions containing ammonia originate from the bicarbonate precipitation and the filtration stages of the process:

from the precipitation of bicarbonate in carbonation towers after cleaning in tower washers from the filtration of bicarbonate, after cleaning in filter washers

in addition, there are a number of diffuse losses of ammonia from filters, bicarbonate conveyors and from the handling and processing of the distillation effluent.

The emissions fluctuate and can be explained by:

performance of the stripping columns and operating parameters control (height, steam injection, temperature control, monitoring of outlet concentrations)

disturbances in the mother liquor feed (flowrate, composition).

Because of the above-mentioned fluctuations, the spread of reported emitted gaseous loads can be very large. The total emitted gaseous load is in a typical range of 0.6 to 1.5 kg NH3/t soda ash from the production unit, representing a release into the environment of 300 to 750 t/year for a 500 kt/year soda ash unit.

Emissions as low as 0.06 kg NH3/t soda ash have been reported for individual source emissions by one plant in Germany in 1996 [45, UBA - Germany, 2001].

The typical concentration is around 30 – 40 mg/Nm³, but much higher values can be encountered (>100 mg/Nm³) [33, CEFIC-ESAPA, 2004].

For point source emissions only, one plant reported five measurements of between 19 and 40 mg/Nm³ in 1996 [45, UBA - Germany, 2001], this being largely in compliance with an ELV of 50 mg/Nm³ set in TA Luft 2002 for ammonia emissions of existing soda ash plants in Germany (the ELV for ammonia in the German TA Luft is 30 mg/Nm³) [85, EIPPCB, 2004-2005].

This wide range of ammonia concentration results from a number of international variations in regulatory requirements and equipment availability. Considering the high turnover in the process (550 kg NH3/t soda ash), the loss rate of ammonia in the process is, therefore, very low (generally much less than 0.5 %).

2.3.3.6 Hydrogen sulphide

In some plants H2S may be added as a corrosion inhibitor, in the form of sodium hydrogen sulphide. Emission sources are from the tower gas washers and H2S is typically controlled at maximum emission levels of 5 to 15 mg/Nm³ of the outlet gas.

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