The ultimate objective is to develop technically feasible methods for the control of air emissions, particularly nitrogen oxide emissions, at kraft pulp mills. This will be achieved by detailed investigations on the formation, distribution, and behaviour of different nitrogen compounds during the main sub-processes of kraft pulping and recovery of pulping chemicals. It will then be investigated how the distribution and behaviour of the nitrogen compounds can be affected by modifying conditions during cooking, black liquor evaporation and combustion, smelt dissolving, and white liquor preparation. Only with all that type of information it will be possible
1) to create overall millwide nitrogen balances;
2) to evaluate the role of each sub-process as a source of nitrogen chemicals;
3) to evaluate the potential of each sub-process modification as a means to control the emissions; and
4) to compare different emission control methods and strategies.
As a result of the project, there is a deeper understanding on the formation and handling of the NOx and other nitrogenous emissions at kraft pulp mills. Especially important is that there is also now a deeper understanding on the entire pathways of wood nitrogen (proteins) from chips to the emission compounds (NOx and ammonia) in each sub-process of the fibreline and recovery systems.
In more detail, 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; there are no raw material-specific differences. 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 early evaporation stages. There is only some 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 applications. 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 thus also found in rectified methanol.
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 strong potential for minimising the 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 vent gases is of great importance for controlling the millwide NOx balances. The experience has shown that the recovery boilers offer the best option for the low NOx combustion of these ammonia-containing streams.
13. If necessary, it is also possible to remove ammonia from stripper off-gases, methanol or vent gases before combustion. 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.
Funding SchemeCSC - Cost-sharing contracts
20500 Turku / Abo