The main gaseous nitrogen emissions from a Kraft pulp mill are in the form of NO x and NH 3 . In black liquor combustion roughly 60% of the black liquor nitrogen is released during the devolatilization phase and finally forms N 2 or NO. The remaining 40% is retained in the char and forms sodium cyanate during char conversion. A part of the sodium cyanate decomposes in the lower furnace, while the rest exits the recovery boiler with the smelt. Sodium cyanate decomposes to NH 3 in green and white liquor, which can contribute to the overall nitrogen emissions of a pulp mill. The purpose of this work is to better understand the formation and decomposition of cyanate during thermal conversion of black liquor. The formation and stability of cyanate was studied during thermal conversion of black liquor droplets in different gas compositions, and the formation was also studied by controlled char gasification in CO 2 atmosphere. The decomposition was studied by exposing laboratory made black liquor smelt to various gas atmospheres at different temperatures. The results show that gas atmosphere and temperature play a role in how much cyanate is formed as well as the rate of decomposition. The share of black liquor nitrogen converted to cyanate is clearly higher if the gas composition contains CO 2 or O 2 mixed with N 2 . Less cyanate is formed if the surrounding atmosphere is pure N 2 gas or contains water vapor. The formed smelt cyanate is stable at 800 °C in atmospheres containing only CO 2 and N 2 , whereas it decomposes slowly at 900 °C in atmospheres containing only CO 2 and N 2 . The cyanate in smelt decomposes quickly in oxygen and water vapor containing atmosphere compared to pure N 2 atmosphere. Also, importantly, cyanate leads to very little NO formation (less than 5% of the original black liquor N) in all of the conditions tested. This information will be utilized in the future to develop a simplified char-N model for implementation in computational fluid dynamics (CFD) with the ultimate goal of finding new ways of further lowering NO emissions from recovery boilers without downstream NO x reduction technologies.

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Kraft black liquor is a by-product of the Kraft chemical pulping process. It is burned in a special boiler, called a Kraft recovery boiler, to recover energy and chemicals. One proposed concept is the mixing of biomass and black liquor for co-combustion in the recovery boiler. This laboratory scale study was done as a first step in evaluating this concept by comparing the combustion behavior of different levels of biomass addition and the fate of the fuel nitrogen. A unique feature of black liquor combustion is that up to 40% of the fuel nitrogen can be found in the molten salts after combustion, as sodium cyanate. This study found that part of the biomass nitrogen can also be found in the ash from the biomass–black liquor mixtures. Combustion experiments were carried out with black liquor–biomass mixtures made with wood or bark of up to 50% biomass. Single droplets were burned in a laboratory furnace at 900 °C and 10% O2 and at 1100 °C and 3.3% O2. Cyanate formation was studied by pyrolysis and gasification of individual droplets at 800 °C in 13% CO2/87% N2. The laboratory tests showed that there is an increase in combustion times, mostly the char burning time and an increase in NO formation with increased biomass addition. Cyanate formation tests indicated that black liquor promotes the conversion of biomass nitrogen to cyanate.

The object of this work was to determine whether inorganic catalysts would react with char nitrogen to form cyanate during catalytic gasification. The formation of volatile nitrogen compounds during gasification was not studied. Gasification studies with black liquor, a wood-based biomass fuel from the pulping process, have shown that organic char nitrogen is converted into the inorganic compound cyanate during char conversion. The purpose of this work was to determine whether cyanate formation could be seen during catalytic gasification of other fuels loaded with different alkali metal and alkalineearth metal salts with varying amounts, thus also gaining a better understanding of cyanate formation in black liquor conversion. The catalyst-loaded fuels were pyrolyzed for 10 s to form a char and then gasified at different temperatures in a 13% CO 2 in N 2 atmosphere. The nitrogen contents of the untreated fuel samples, pyrolyzed fuel samples, and catalyst-loaded fuel samples after pyrolysis and gasification were analyzed with two different methods. The results showed that more nitrogen is volatilized from biomass samples than from fossil fuel samples and also that the catalyst loading has no clear effect on the amount of nitrogen remaining in the chars after pyrolysis. The residues remaining after gasification were analyzed for cyanate with ion chromatography. The alkali metal salts K 2 CO 3 , Na 2 CO 3 , and K 2 SO 4 showed catalytic activity in conversion of char nitrogen to cyanate, while KCl and CaCO 3 did not show any catalytic activity toward cyanate formation. Less char nitrogen was converted to cyanate for coal than for peat and bark during gasification. This indicates that the form of the organically bound nitrogen in the fuel has an impact on cyanate formation.

At many mills, biosludge, which has a high nitrogen content, is added to black liquor and burned in kraft recovery boilers. The aim of this work was to determine the fate of biosludge nitrogen in the high solids black liquor concentrators and in the recovery boiler. Specifically, does biosludge addition result in higher nitric oxide (NO) and cyanate formation during black liquor combustion? To obtain this information, samples were collected from the chemical recovery cycle of a Finnish kraft pulp mill along with relevant process data. Laboratory combustion experiments clearly showed an increase in NO formation for the mill black liquor with biosludge, but no clear increase in nitrogen oxide emissions was detected in the recovery boiler after biosludge addition. Analysis of the green liquor samples from the dissolving tank showed a significant increase in nitrogen exiting the recovery boiler as cyanate. This finding was supported by laboratory tests studying cyanate formation. The increased cyanate results in increased ammonia formation in the recausticizing cycle, which can lead to higher NO emissions, as seen in the noncondensible gas incinerator at the mill.