Ind. Eng. Chem. Res. 1997, 36, 2547-2557
2547
Reactions of Nitrate Salts with Ammonia in Supercritical Water Philip C. Dell’Orco,*,†,‡ Earnest F. Gloyna,‡ and Steven J. Buelow§ Los Alamos National Laboratory, Mail Stops C920 and J567, Los Alamos, New Mexico 87545, and Department of Environmental and Water Resources Engineering, The University of Texas at Austin, Austin, Texas 78712
Reactions involving nitrate salts and ammonia were investigated in supercritical water at temperatures from 450 to 530 °C and pressures near 300 bar. Reaction products included nitrite, nitrogen gas, and nitrous oxide. Observed reaction rates and product distributions provided evidence for a free-radical reaction mechanism with NO2, NO, and NH2• as the primary reactive species at supercritical conditions. In the proposed elementary mechanism, the rate-limiting reaction step was determined to be the hydrolysis of MNO3 species, which resulted in the formation of nitric acid and subsequently NO2. A simple second-order reaction model was used to represent the data. In developing an empirical kinetic model, nitrate and nitrite were lumped as an NOx- reactant. Empirical kinetic parameters were developed for four MNOx/NH3 reacting systems, assuming first orders in both NH3 and NOx-. Observed MNOx/NH3 reaction rates and mechanisms suggest immediately a practical significance of these reactions for nitrogen control strategies in supercritical water oxidation processes. Introduction Supercritical water oxidation (SCWO) has been proposed as a method for the conversion of hazardous organic compounds to benign byproducts. Bench- and pilot-scale studies have demonstrated the technology on individual organic compounds (Crain et al., 1993), waste simulants (Bramlette et al., 1991), and actual wastes (Shanableh and Gloyna, 1991). Cited advantages to incineration include no NOx production, the complete conversion of organics at relatively low temperatures and residence times, and cost effectiveness (McBrayer, 1995). Although organic destruction has been rigorously studied in supercritical water reactors, little emphasis has been placed on reactions of other elements common to waste streams. Nitrogen, in particular, is present in many waste streams of interest. Ammonia has been reported as a primary constituent or stable reaction intermediate in several waste streams targeted for treatment with SCWO. Shanableh and Gloyna (1991) and Tongdhamachart (1990), using SCWO to treat industrial and municipal sludges, reported ammonia as a refractory reaction product at temperatures of 400-500 °C. Studies of the SCWO of metabolic wastes for space missions have also shown ammonia as an oxidation-resistant reaction product, remaining primarily unreacted at temperatures up to 650 °C (Takahashi et al., 1989; Hong et al., 1987). In a more basic study, ammonia was relatively unreactive with oxygen in supercritical water environments, at 248 bar and temperatures of 600-700 °C (Webley et al., 1991). Conversions of less than 20% were observed at residence times of less than 20 s at these conditions, using stoichiometric oxygen. In addition to ammonia, nitrate and nitrite have also been reported as reaction products in the hydrothermal treatment of energetic materials (TNT, RDX, HMX) (Harradine et al., 1993). Wastes from nuclear produc* Author to whom correspondence is addressed. Phone: (505)-667-7854. Fax: (505)-667-0500. E-mail:
[email protected]. † Los Alamos National Laboratory, Mail Stop C920. ‡ The University of Texas at Austin. § Los Alamos National Laboratory, Mail Stop J567. S0888-5885(96)00589-1 CCC: $14.00
tion activities (such as Hanford tank wastes) contain high amounts of nitrate and nitrite. These nitrates are also thermally stable, except in the presence of an appropriate reducing agent (Foy et al., 1995; Cox et al., 1994). Hydrothermal treatment has been proposed as a method for treating such nitrate-containing wastes (Robinson et al., 1993). Due to low regulatory limits for the discharge of ammonia and nitrates to receiving streams, treatment of wastes using SCWO may be unable to meet nitrogen treatment requirements without using prohibitively high temperatures or additional treatment. One possible method for converting ammonia or nitrate to relatively benign byproducts is suggested by off-gas treatment processes for incineration and energy production. Technologies such as thermal DeNOx and the urea acidic process use ammonia or NHx free radicals to reduce gaseous NOx species to molecular nitrogen and nitrous oxide (Lyon, 1987; Lasalle et al., 1992). Other abatement technologies use metal oxide catalysts to promote reaction efficiency between NOx and NHx species (Wong and Nobe, 1986; Baurle et al., 1978). In fact, NHx-NOx reactions have been demonstrated to occur in near-critical water. Reactions of ammonium chloride and sodium nitrate at 300 bar and 350-380 °C were found to produce primarily nitrogen gas and nitrous oxide, with near complete reaction occurring at 380 °C (Dell’Orco et al., 1993). These air pollution control strategies and initial near-critical water results prompted the current study. This research was conducted to determine the potential utility of NH3/MNO3 reactions as a waste treatment strategy in SCWO processes. It was hoped that such reactions would result in the chemical conversion of NH3/MNO3 to more benign chemical forms (i.e., N2), at reasonable SCWO operating temperatures (
