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Urban Waste (Municipal Solid Waste—MSW) to Energy
Published in Sheila Devasahayam, Kim Dowling, Manoj K. Mahapatra, Sustainability in the Mineral and Energy Sectors, 2016
Moshfiqur Rahman, Deepak Pudasainee, Rajender Gupta
NOx can be controlled using a low NOx burner, controlling fuel–air mixing rates, or by installing a selective catalytic reactor (SCR) and selective noncatalytic reactor (SNCR). SCRs have been widely used in coal-fired power plants and incinerators because of their higher efficiency (80%–90%). In the SCR process, ammonia (NH3) or urea reduces NOx to N2 and H2O. Metal oxide catalysts such as titanium dioxide-supported vanadium pentoxide (TiO2/V2O5) are used in the SCR process. SNCR can reduce NOx emissions by 30%–50%. To achieve similar NOx reductions, the SNCR process requires 3–4 times more reagent (ammonia or urea) than the SCR process (IEA, 2014).
Air pollution, regulation and management
Published in Andrew Farmer, Managing environmental pollution, 2002
Nitrogen oxides can be also be cleaned from exhaust gases. There are two principal means of achieving this – selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). With SCR, a metal catalyst is used to reduce the nitrogen oxides to ammonia. It needs to operate between 200 and 400°C and can cope with a variety of other conditions (e.g. the presence of particulates). It can achieve an 80–90 per cent reduction in emissions. SNCR involves the injection of ammonia or urea into the flue gases to reduce chemically the nitrogen oxides. These reactions will only operate at high temperatures (900–1,100°C), so the technique is less versatile than SCR. SNCR can achieve a reduction in emissions of 70 per cent, although in practice lower figures are usually found.
Waste-to-Energy Combustion
Published in D. Yogi Goswami, Frank Kreith, Energy Conversion, 2017
Charles O. Velzy, Leonard M. Grillo
Selective noncatalytic reduction (SNCR) appears to be the most practical method of reducing NOx emissions for most municipal waste combustors. SNCR involves the use of ammonia to reduce NOx to nitrogen and water. The SNCR reaction occurs at a temperature of 1600°F–2100°F. At lower temperatures, a catalyst is required to promote the reaction (selective catalytic reduction, or SCR). SCR is not used on municipal waste combustors. Tests conducted at a municipal waste combustor demonstrated that NOx emission levels of 150 ppmv (45%–55% reduction) can be achieved with SNCR.4
Experimental study on influencing factors of NOx reduction by combining air staging and reagent injection
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2019
Degui Bi, Zhongxiao Zhang, Zhixiang Zhu, Xinwei Guo, Hao Bai
Lots of experimental and theoretical studies have been done focused on several technical approaches for NOx reduction by injecting NH3 reagent. A schematic diagram of some technologies is given in Figure 1. In SNCR, the reagent is injected to the furnace downstream to reduce NOx and the excess oxygen is required (Javed, Irfan, and Gibbs 2007; Yang, Yu, and Zhang et al. 2016). The reduction reaction is limited for the narrow temperature window, in the range of 1123 and 1373 K (see Figure 1). In advanced reburning (AR) (Hao, Wei, and Ping et al. 2014; Lu et al. 2016), the reagent is injected to downstream of the combustion zone, following the reburning fuel injection, some amount of O2 is helpful for reduction and the optimal temperature window depends on local conditions (the concentrations of O2, CO, NH3) (see Figure 1). Injection reagent combined with air staging means that the NH3 reagent was injected into the primary fuel-rich combustion zone (see Figure 1), under the conditions of a reducing atmosphere and high temperatures. In this study, the key factors, including the primary stoichiometric ratio (SR1), temperature and normalized stoichiometric ratio (NSR) on NOx reduction by injecting reagent combined with air staging were investigated mainly on a bench-scale combustion test system, which is important for the optimization and design of new low NOx combustion technology.
A simplified simulation of the reaction mechanism of NOx formation and non-catalytic reduction
Published in Combustion Science and Technology, 2018
The technology of SNCR is based on dosing chemical reducing agents in a suitable reaction zone in the boiler. During the SNCR process NOx are reduced in the presence of reducing agents without the presence of a catalyst. The reduction reaction rates of NOx are very susceptible to flue gas temperature. The appropriate temperature at which NOx reduction reactions take place with an optimum conversion is called temperature window (Nath and Cholakov, 2009; Oliva et al., 2000). Depending on the reagent and SNCR operation conditions the optimal temperature window is usually between 850°C and 1100 °C at 0 vol% O2 concentration (Bi et al., 2016; Lee et al., 2008; Zandaryaa et al., 2001). The reducing agent for SNCR can be ammonia water (NH4OH), urea (NH2)2CO or cyanuric acid (HNCO)3 also called isocyanuric acid (2,4,6-trihydroxy-1,3,5-triazine) (Bae et al., 2006; Baleta et al., 2016; Daood et al., 2013; Mansha et al., 2016). Overall the reactions taking place during the SNCR process are expressed in the general Equations (2–8) (Dvořák et al., 2010; Kitto and Stuktz, 2005):
Adjoint-based optimization in the development of low-emission industrial boilers
Published in Engineering Optimization, 2021
Georgios Kanellis, Antti Oksanen, Jukka Konttinen
Usually, those boilers are supplied with a Selective Non-Catalytic Reduction (SNCR) system, which is a post combustion system for NO abatement. The SNCR system consists of several nozzles which inject NH into the flue gas in order to react with NO, the main products being harmless N and water. It is well known that NO reduction is possible only in a confined temperature range between 800 and 1100C, the so-called ‘temperature window’. Temperatures that are lower relative to the window stop the SNCR reactions, while higher temperatures oxidize the injected ammonia to NO and even higher NO emissions can be observed. The efficiency of the SNCR process varies between 30 and 80% and depends on several factors including temperature, NO level, the mixing of NH into the flue gas, NH/NO molar ratio and residence time (Radojevic 1998). In addition, the presence of CO in the flue gas shifts the temperature window down to lower temperatures (Saario, Ylitalo, and Oksanen 2008). From the above, it is obvious that the SNCR system should be finely tuned in order to reduce NO in the flue gases effectively. Otherwise, if the mixing between NH and NO is not proper and/or if the injected NH follows a path outside the optimum temperature window, not only will SNCR fail to reduce NO to the desired level, but an appreciable amount of NH, an equally harmful substance, will also be released into the environment (ammonia slip).