346 Nordic Pulp and Paper Research Journal Vol 16 no. 4/2001 Keywords: Black liquor, Nitrogen oxide, Single droplet combus- tion, Measurement, Volatiles, Char SUMMARY: Nitrogen oxide formation in single droplet com- bustion of black liquors was measured under different combus- tion conditions (droplet size 5–45 mg, temperature 800–900˚C, O 2 -concentration 3–21%). The NO formation in relation to the carbon conversion is presented. The amounts of NO formed during pyrolysis (pyrolysis-NO), i.e. from the burning of volatile nitrogen species, and during char burning stage (char-NO) are reported. Under all conditions studied, the NO formation followed the carbon conversion up to about 40–50%, after which the NO for- mation stopped, and began again only when about 90% of the carbon had been converted. The relative amount of pyrolysis-NO varied with droplet size at 900˚C and with oxygen concentration at 800˚C. Possible mechanisms for these effects are suggested. The char-NO originated mostly in the oxidation of the inorganic residue left after the burnout of the char. The amount of char-NO depended on temperature: more NO was formed at 800˚ than at 900˚C. The implications of the findings for NO x emissions in black liquor burning in recovery boilers are discussed. ADDRESS OF THE AUTHORS: Åbo Akademi University (ÅAU), Process Chemistry Group, Combustion and Materials Research, Biskopsgatan 8, FIN-20500 Åbo, Finland Nitrogen oxide emissions from recovery boilers originate mainly in the oxidation of liquor-bound nitrogen. Owing to the relatively low temperatures in the recovery furnace as compared, for example, with pulverized coal firing, the contribution from the fixation of molecular nitrogen in the combustion air is minor. (NCASI 1992; Nichols et al. 1993; Adams et al. 1993; Nichols, Lien 1993; Aho et al. 1994a,b). The pathways for the liquor-bound nitrogen in black liquor combustion, as suggested by Forssén et al. (1997), are presented in Fig. 1. During devolatilization, black liquor nitrogen divides between volatiles (N pyrolysis ) and char (N char ) and the volatili- zed and char nitrogen contribute to the nitrogen oxide emission through very different pathways. The contribu- tion of the char nitrogen has been suggested to be less important because most of it remains in the inorganic salt residue (smelt) and leaves the boiler with the smelt. Thus, the distribution of the liquor nitrogen between volatiles and char would influence the amount of the nitrogen oxide emission. This distribution, i.e. the yields of pyrolysis and char nitrogen, has been found to be liquor specific (Aho et al. 1994a,b; Martin 1995; Forssen et al. 1999) and depen- dent on temperature (Aho et al. 1994a; Forssen et al. 1997; Iisa et al. 1995; Martin 1995). In single droplet pyrolysis tests, the distribution has proven to be relatively constant over a wide temperature range, between about 500˚ and 900˚C. On the basis of chemical analysis, Forssén et al. (1997) have reported the char nitrogen yields in pyrolysis of 300 seconds to vary between 20–30% of the initial nitrogen in the liquor, depending on the liquor. Later, how- ever, they found some discrepancies between the amounts of NO formed during char combustion and the results of nitrogen analysis, suggesting uncertainty in the nitrogen analyses and the analyzed values lower than the actual (Forssen et al. 1999). In the pyrolysis experiments in a laminar entrained flow reactor with a residence time of 0.85 seconds carried out by Iisa et al. (1995), the char nitrogen yield was at a constant level, of about 50% at temperatures between 800˚ and 1 000˚C. In general, the devolatilization of fuels such as coal and wood leads in the end to simple cyanide and amine species such as HCN and NH 3 . In addition, some molecular nitro- gen (N 2 ) is formed during the pyrolysis stage. No HCN has been reported in black liquor pyrolysis (Aho et al. 1994a; Iisa et al. 1995). However, in those studies a slow heating rate and long residence time have been used, which in stu- dies on other fuels than black liquor have been shown to decrease the amount of HCN in the pyrolysis gas (Bas- silakis et al. 1993; Leppälahti, Koljonen 1995). In addi- tion, the measurements for HCN have been indirect. Thus, these results cannot be taken to exclude the possibility of the presence of HCN in black liquor pyrolysis. Interestingly, some NO has been detected (Aho et al. 1994; Iisa et al. 1995; Martin 1995). The formation mechanisms for the NO are unknown, however, and it must be conclu- ded that the distribution of the volatile nitrogen between different nitrogen species under different conditions and for different liquors is not well known. Yet such knowled- ge would be invaluable in understanding the NO formation mechanisms in black liquor burning. The gas phase conditions under which the volatilized nitrogen species are burned (i.e. oxidized) determine the final split between the products, NO and N 2 . Although the reaction mechanisms of the volatile nitrogen species in gas phase are complex, the present level of understanding is reasonably good: the main reaction paths have been identi- fied and the accuracy of the reaction rate constants is good (Kilpinen et al. 1999). Thus, the trends in the formation of Nitrogen oxide formation in black liquor single droplet combustion Maritta Kymäläinen, Mikael Forssén, Pia Kilpinen and Mikko Hupa, Åbo Akademi University, Turku, Finland Fig. 1. The fuel nitrogen pathways in black liquor combustion as sug- gested by Forssén et al. (1997). Brought to you by | Göteborg University - University of Gothenburg Authenticated Download Date | 8/26/19 3:42 PM