The Fate of Char Nitrogen in Black Liquor Combustion—Cyanate Formation and Decomposition

Abstract

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.

Replies to reviewer 1's comments

“…The PM 2.5 filter extraction and subsequent instrumental analysis methods used here are the same as those described in Xie et al (2017a). Briefly, a 4-6 cm 2 piece of each filter was pre-spiked with 25 µL of 10 ng µL −1 nitrophenol-d4 (internal standard, IS) and extracted ultrasonically in 3-5 mL of methanol twice (15 min each).…”

“…1, respectively. The Q-ToF MS/MS spectra of standard and surrogate compounds used in this work are obtained from Xie et al (2017a) and provided in Fig. S2 for comparison.…”

“…The compounds corresponding to each NAC formula (including isomers) were quantified individually and added together for the calculation of mass contribution (%) to organic matter (OM µg m −3 ) in each sample. The quality assurance and con- trol (QA/QC) procedures applied for NAC quantification were provided in Xie et al (2017a). Field blank and background samples were free of contamination for NACs.…”

“…These results suggest that NACs are important BrC chromophores, but their composition and structures are less certain for BB aerosols. Nitrophenols, nitrocatechols, and methyl nitrocatechols (including isomers) are commonly observed in BB aerosols (Iinuma et al, 2010;Claeys et al, 2012;Lin et al, 2016Lin et al, , 2017 and are also generated from the photooxidation of benzene, toluene, and mcreosol in the presence of NO x (Iinuma et al, 2010;Lin et al, 2015;Xie et al, 2017a). As such, other NAC structures specific to BB are needed to represent BB BrC chromophores.…”

Combustion Characteristics of Plant Chemical Polyol Waste Liquor in a Pilot Water-Cooled Incinerator

“…At high temperatures, the conversion of elemental nitrogen into NO in the smelt increases first and then decreases with the increase of CO 2 concentration in the atmosphere, and the conversion rate is the highest under the condition of 13% CO 2 + 87% N 2 ; cyanate in 13% CO 2 + 87% N 2 atmosphere and at 800 • C is very stable, but gradually decomposes at 900 • C; the atmosphere is essential for the decomposition of cyanate. The decomposition rate of cyanate at 900 [33].…”