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Reduction of air emissions at kraft pulp mills

Kraft pulp mills produce air emissions, particularly nitrogen oxide emissions. This project will develop a deeper understanding of the formation and handling of NOx and other nitrogenous emissions at these mills. There will be detailed investigations of the formation, distribution and behaviour of different nitrogen compounds during the main sub-processes of kraft pulping and recovery of pulping chemicals. The effect of the distribution and behaviour of the nitrogen compounds during cooking will also be investigated, along with black liquor evaporation and combustion, smelt dissolving, and white liquor preparation.

The main objective of the project was to develop technically feasible methods for reducing nitrogen oxide (NOx) emissions at kraft pulp mills, particularly from the recovery boilers. An additional objective was to ensure that all other air emissions simultaneously remained at low levels.

Progress to Date
The work was completed according to plan and the expected achievements were reached. The pathways of nitrogen from wood chips to emission compounds (nitrogen oxide and ammonia) were followed through all the kraft pulping and recovery processes. Changes in the process conditions affecting the nitrogen behaviour and how this can be used to control nitrogenous emissions were also investigated. The results illustrate the important role of ammonia as an emission compound from the causticisation line, and the principal role of the recovery boiler as the nitrogen oxide source. Proposals, indications and well-established technologies to reduce or handle these emissions have been made.
Collecting the above information made it possible to:
1. create overall mill-wide nitrogen balances
2. evaluate the role of each sub-process as a source of nitrogen chemicals
3. evaluate the potential of each sub-process modification as a means to control the emissions
4. compare different emission control methods and strategies.
This project has attracted a lot of interest from pulp and paper companies, particularly in Finland and Sweden, where most of this work was conducted.

The results show that:
1. The nitrogen content of the wood raw material is typically 0.05-0.15% (by weight).
2. A substantial part of the nitrogen is dissolved from the wood chips into the cooking liquor at early pulping stages; no raw material-specific differences exist. The pulps going to bleaching contain only traces of nitrogen.
3. The cooking conditions have only marginal effects on the release of nitrogen from chips.
4. Approximately 10-15% of wood nitrogen is converted into ammonia during pulping.
5. Ammonia and other volatile nitrogen compounds are readily transferred to the condensates at the early evaporation stages. There is only a small amount of additional ammonia formation during later evaporation stages.
6. The extent of nitrogen removal during evaporation can be increased by high-temperature black liquor treatments. The required conditions are, however, probably too drastic for full mill-scale application. The increased formation of volatile sulphur compounds in such treatments also needs to be taken into account.
7. Ammonia and other volatile nitrogen compounds are stripped off from foul condensates, and are found in rectified methanol as well.
8. Nitrogen entering the recovery boiler with the black liquor is converted during combustion to gaseous N2, NO, and cyanate (OCN) in the smelt. The combustion conditions can be controlled to increase nitrogen incorporation into smelt, which offers certain opportunities to force more nitrogen as ammonia to different gaseous streams (for separate handling or combustion).
9. During combustion, carefully designed air-staging conditions and novel vertical-air systems have a strong potential for minimising NOx emissions.
10. Cyanate, originating from the smelt, forms ammonia during green liquor handling and white liquor preparation. The reaction kinetics shows significant mill-to-mill differences.
11. Some ammonia is removed from the green and white liquors with the vent gases, but most of it returns to cooking with the white liquor. Ammonia emissions from the causticisation lines may correspond to one third of all nitrogen emissions from the recovery cycles.
12. Proper handling of all non-condensable gases (NCGs), stripper-off gases or rectified methanol, and causticisation of vent gases is of great importance for controlling mill-wide NOx balances. Experience has demonstrated that the recovery boilers offer the best option for low NOx combustion of these ammonia-containing streams.
13. It is also possible to remove ammonia from stripper-off gases, methanol or vent gases before combustion, if necessary. The removed ammonia could be used, for example, in selective catalytic-reduction systems at recovery boilers, or as a nutrient (ammonium sulphate or phosphate) at the activated sludge plants.


Scientist responsible for the project

Tekniikantie 2 Box 70
02151 Espoo
Finland - FI

Phone: +358 9437 1519
Fax: +358 9464 305


Project ID QLRT-1999-01105
Organisation Oy Keskuslaboratorio - Centrallaboratorium Ab
Area 5.3.2
Start date 01 January 2000
Duration (months) 36
Total cost 1 358 310 €
Total EC contribution   740 465 €
Status Completed
Web address of the project

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