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Reduction of air emissions at kraft pulp mills


Behaviour of nitrogen during kraft pulping of different wood raw materials was investigated in detail, in order to search for possible methods to affect nitrogen dissolution from chips to cooking liquor. The main overall conclusion here is that there is very little that could be done for the NOx emissions during pulping. The only approach might based on separate treatment of nitrogen-rich impregnation liquors and nitrogen-poor cooking liquors. It still remains unsolved how such treatments could be arranged at the mills without violating normal recovery operations.
Andritz has delivered or several novel air systems (vertical air systems) for kraft pulp mill recovery boilers, or has constructed totally new recovery boilers equipped with these systems. The novel systems make it possible to significantly reduce NOx emissions. For this project, a set of experiments was conducted at one of these recovery boilers, to find out the main parameters that affect NOx emissions and that also affect nitrogen incorporation into smelt (leaving the recovery boilers). As a result, there is now increase information available on the role of the novel vertical air systems in reducing NOx emissions at kraft pulp mills. The results are highly promising and have attracted a lot of interest. At the moment, there are new recovery boilers under construction that will be equipped with this system.
Behaviour of nitrogen during black liquor was investigated in detail during evaporation of black liquors. The results indicate that ammonia and other volatile nitrogen compounds are readily released into the condensates at early evaporation stages. There is also some formation of additional ammonia at higher temperatures during later evaporation stages.
Ammonia is and formed during green and white liquor processing and is easily released from the alkaline solutions of the kraft chemical recovery cycle. The ammonia is derived from cyanate, which is formed during black liquor combustion. The kinetics of ammonia fromation from cyanate was studied in detail, and several mill-specific features were also discovered. The nitrogen released as ammonia in the causticising area is significant and usually not collected from the vent gases. Minimising the venting from the green liquor tanks could ensure that the majority of ammonia from causticising is released in the slaker and causticiser vent gases, which could easily be treated as one stream. Minimising venting from the white liquor clarifier and storage tanks will keep the ammonia in the white liquor until it reaches the digesters. It will then end up in the condensates and can be handled in a proper way. The vent gas emissions from causticising can be controlled either by collecting the most important gas streams or by scrubbing these streams with an acidic solution. This would of course require a two-stage scrubbing system on the slaker for example where the alkaline scrubber captures reduced sulphur compounds, but it is relatively ineffective for ammonia.
One of the key outcomes of the project was the creation of nitrogen balances for kraft pulp mills, covering all fibreline and recovery processes. The balances were created for total nitrogen, although different main species (ammonia, cyanate) were also taken into account. The creation of these balances, and the current knowledge on the principle of their creation for any pulp mill, is of great importance when considering different options how the mills can reduce their overall NOx and ammonia emissions. Once such balances have been created and the main flows of nitrogen established, it is possible to start to locate the most effective ways or process modifications how the nitrogen flows and emission rates can be reduced. An important part of the balances and NOx reduction strategies will be to establish nitrogen contents in all the streams that are either combusted or incinerated, and to direct these streams to the combustion sites that are most feasible in specific cases. The creation of the balances and the effect of process modifications take benefits from the use of the simulation model, which is also one of the main results of the project.
Studies on the fate and behaviour of nitrogen and nitrogen compounds in all kraft pulping process streams are possible only if reliable and sound analytical methods are available. During the course of this project, it was necessary to refine further or to develop certain new analytical methods. The novel or refined methods now include: methgod for total nitrogen in black liquors (KCL), method for volatile nitrogen and sulphur compounds in rectified methanol (KCL), and method for cyanate in recovery liquors. The use of these methods is essential when nitrogen balances in kraft pulp mills are created and when the success of NOx reduction measures is verified. They have been used during this project and in other applications, and are expected to have important future applications as well.
The conversion of black liquor nitrogen to a volatile form (preferably ammonia) during evaporation would decrease nitrogen content in the as fired liquors. This would naturally be beneficial for the NOx emissions from the recovery boilers. The results of our studies, from both laboratory-scale investigations and mill-scale measurements indicate, however, that under normal operation conditions there is only very little generation of ammonia during the evaporation (at higher temperatures). Therefore, the increase could be achieved only by high-temperature treatments or by other, apparently catalytic processes. The latter approach could not be now used as no ideas on suitable catalysts or related processes could be presented. The potential use of high-temperature treatments (from 170 to 300 °C) was now investigated in detail, using different black liquors and the treatment times up to 24 hours. There remained no doubt that such treatments can be applied for the reduction of the nitrogen contents in the liquors, but the required conditions are so drastic that currently no big chances can be foreseen for these processes. Typically, several hours were required at 200 °C to remove some 30–50% of liquor nitrogen, and no recovery cycles can yet be constructed for such requirements. In the future, there may be different policies that facilitate this approach. If so, there is now a substantial amount of basic information available for planning the new processes. That would naturally require some modifications for the handling of the nitrogen- and sulphur-containing condensates from the heat treatments.
The chemistry of nitrogen in recovery boilers (especially the formation of NOx and smelt nitrogen) has been studied for many years by several teams or groups globally, but the complexity of the reactions has restricted some parts of the studies. During this project, further studies on nitrogen chemistry were conducted in the following areas: kinetic calculations, the effect of extra streams (malodorous gases, biosludge) on NOx formation, and the chemistry of nitrogen in char and smelt. The results can be used to plan air-staging systems that reduce NOx formation, and they can also be used to design combustion of some biosludge and molodorous gases in recovery boilers, without increasing NOx formation. The results have, when used together with all other existing information, wide application potential in pulp industry. In certain areas (such as kinetic calcluations), certain further studies are still required.