For modern recovery boilers, which are designed for low NOx emissions, the conversion rate of N2 to NOx can be lower than 15-20%. The recovery boiler operating conditions such as excess air, furnace temperature and air staging have a direct effect on the emission rates [1]. Selective non-catalytic NOx reduction (SNCR) has been used in power boilers to reduce NOx emissions. This technology was borrowed for a series of tests on a 2000 tds/d recovery boiler built in 2001 by Metso Power, equipped with quaternary air [2]. The SNCR technology injects ammonia into the flue gas stream at a certain temperature range to reduce NOx into N2 gas. Critical factors are good mixing and sufficient residence time at the right temperature. At too high a temperature, the ammonia oxidizes to NOx whereas, at low temperatures, the reactions are too slow and ammonia leaves the boiler un-reacted (ammonia slip).
A series of tests were carried out with ammonia injections, and the gas and deposit compositions were monitored extensively. The injection took place through lances, at three different injection levels, with air atomized spray nozzles. Carrier air was provided around each lance at the screen level to improve penetration in the furnace. At the lowest level, the quaternary air was used as carrier air. Long term tests at the quaternary level were carried out at 62% load while, at the injection level below the screen tubes, tests were conducted at 74% load or higher.
During the trials, FTIR was used for monitoring the emission composition. The flue gas temperature at the injection position was also measured. Deposit probes were used in the superheater and economizer areas, with temperatures controlled at 450, 250 and 150 C. Based on the data obtained, a significant reduction in NOx emission was achieved (30-50%). For other gaseous compounds, the CO emission did not change much, although it was difficult to operate the boiler under exactly controlled conditions. Other species, such as SO2, HCl, HF and N2O were not clearly impacted by the addition of ammonia. In nine out of twelve tests the reduction in NO was followed by a reduction in NO2. Only in one case was there a significant increase in NO2 which was due to very low levels of NO2 in both cases, i.e., with and without the SNCR turned on. In this study, the ammonia slip was typically less than 20 ppm. A slip much higher than this cannot be tolerated, for environmental and economical reasons.
CFD simulations showed that the penetration of the SNCR jets into the recovery boiler gas steam was poor, indicating that a limited number of injection ports were not adequate to ensure an even distribution. This suggests that significant improvements to the injection system can be made.
The compositions of the ESP ash and deposits collected on the probes showed that ammonia injection had little impact on the properties of the ash. Thermodynamic calculations confirmed that ammonium salts would not form in the superheater region. The formation of liquid NH4HSO4 in the cooler part of the flue gas channel, for boilers with high SO2 emission, is a possibility. However, due to the significant amount of sodium present, most of the liquid salt will be in the form of NaHSO4 and the effect of NH4HSO4 is expected to be negligible.
References:
1. Distribution of Black Liquor Nitrogen Between Smelt, NOx and Flue Gases in Recovery Boilers, K. Saviharju et al, TAPPI 2007 Chemical Recovery Conference, Quebec City, QC, Canada.
2. Effect of Ammonia Injection on Black Liquor Recovery Boiler NOx Emissions and Ash Chemistry, M. Lundberg et al, TAPPI Engineering, Pulping & Environmental Conference, Aug. 24-27, 2008, Portland, Oregon, USA.
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