Emissions to the Atmosphere

Typically, the combustion processes in the recovery or the bark boiler or lime kiln

Result in the emission of particulate matter. Such emission may be controlled

Using effective scrubbers or electrostatic precipitators.

During the pulping process, volatile organic compounds (VOC = volatile

Organic carbon) are generated. The typical odor of the kraft process is caused by

Mercaptans. These can be released either during the blow (discharge) of the digester

Or during brownstock washing or evaporation of the black liquor. All gaseous

Emissions must be collected and sent to a combustion unit. These malodorous

emissions are described as “total reduced sulfur” (TRS) and expressed as hydrogen

Sulfide (H2S). These sulfur compounds are removed by oxidation to sulfur

Dioxide. During the combustion process, nitrogen is oxidized to a mixture of nitrogen

Oxides; they are described as NOx. The amount of sulfur dioxide or nitrogen

Oxides within the gaseous effluent must not exceed certain threshold values that

Vary slightly different depending upon the type of combustion unit and the fuel

Material. The emission sources, the emissions and the interrelation between the

Different process steps are shown schematically in Fig. 10.1.

The emissions of all these compounds are controlled using either BAT or AMT.

This could be achieved either by combustion or by washing in scrubbers with suitable

Liquids. The combustion itself must be controlled to avoid the emission of

Carbon monoxide or incomplete incineration. Likewise, nitrogen oxide and sulfur

Dioxide emission must be controlled. Examples of the emissions permitted from

The recovery boiler and the lime kiln, the most important parts of the process, are

Listed in Tab. 10.1.

Emissions to the Atmosphere

Wood handling

Malodorous

Compounds

Volatile organic

Compounds

Bark boiler

Cooking

Pulp

Washing

Pulp

Screening

Bleaching pulp drying

Chlorine

Compounds

Particles

Particles,

SO2, NOx

Lime kiln

Particles, TRS

SO2, NOx

Evaporation

Recovery

Boiler

Recausticizing

Bleaching

Chemical

Preparation

Chlorine

Compounds

Malodorous

Compounds

Malodorous

Compounds

Particles, TRS

SO2, NOx

Particles, SO2,

NOx

Malodorous

Compounds

Tanks

Auxiliary

Boiler(s)

(oil, gas, etc)

Fig. 10.1 Sources and emissions to the atmosphere from kraft pulp mills [2].

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