Article pubs.acs.org/EF
Industrial Trials of High-Temperature Selective Noncatalytic Reduction Injected in the Primary Combustion Zone in a 50 MWe Tangentially Firing Pulverized-Coal Boiler for Deeper NOx Reduction Degui Bi, Jian Zhang, Zhongxiao Zhang,* Yanyan Rong, Pujie Yue, Zhenhua Fu, and Xinqiang Ji School of Environment and Construction Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China ABSTRACT: This paper aims to demonstrate an industrial retrofit of NOx reduction through use of a new technology, hightemperature selective noncatalytic reduction (abbr. HTSNCR), in a 50 MWe tangentially firing boiler of pulverized coal. Over the last several years, the characteristics of HTSNCR were investigated in the bench-scale experiments, as presenting the temperature window of SNCR can extend to a higher temperature range in the absence of oxygen. Therefore, HTSNCR provides a promising option of greater NOx reduction via injecting urea solution or ammonia into the primary combustion zone specialized with oxygen of nearly zero in furnace. These retrofit experiments in this paper successfully showed the ability of HTSNCR. NOx emission of 68 mg/Nm3 was finally achieved through use of the optimum hybrid application of HTSNCR, SNCR, and OFA. The overall reduction efficiency is approximately 90%, in which 17% is devoted by HTSNCR. The main factors in HTSNCR were studied extensively, including (1) primary stoichiometric ratio (SR1) of air staging, (2) normalized stoichiometric ratio (NSR) of reagent quantity injected, (3) allocation of injectors, e.g., on the corners or at the middle of side walls, (4) ammonia slip brought about by HTSNCR or SNCR, (5) optimized hybrid configuration about SNCR and HTSNCR. On the basis of the optimum setting of the above factors, two key features of HTSNCR employed in a tangentially firing furnace were obtained. First, there is a critical minimum value of NOx emission in the relationship of NOx emission versus NSR1 of HTSNCR. More NSR1 beyond the critical value, i.e., more reagent quantity injected, results in more NOx formation. Secondarily, the injection of reagent near the corners is beneficial to reach higher NOx reduction rather than that injected from the side walls, due to the aerodynamics in the tangentially firing furnace.
1. INSTRUCTION The air pollution emitted from coal-fired power plants makes contributions to environmental pollution problems. The control of pollutant emissions is a worldwide concern when the utilization of pulverized coal continues in electricity generation. With an increasing public demand for reduced pollution, many countries have lowered the NOx emission limits in flue gases. In the European Union, the NOx emission is dropped to 200 mg/m3 at 6% O2 for power plants after 2016.1 In China, the most rigid enforcement of pollutant emission standards for thermal power plants was issued in 2011,2 in which NOx emission is claimed to control below 100 mg/Nm3 at 6% O2 for all power plants after 2014. Therefore, more/deeper reduction of NOx emission from burning pulverized coal still continues to be a challenge in many boiler facilities with different types and powers. Many industrial retrofits relate to the deployment of hybrid integration of various existing technologies, such as low-NOx burners, air staging, reburning, selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR), and so on, in order to satisfy the rigid criterion of environmental protection. Combining low-NOx burners and air staging, the NOx reduction efficiency is generally approximately 40−60%. Hence, SNCR or SCR is commonly necessary in the hybrid application to reach higher NOx removal.3−5 The SNCR is a conceptually simple process of NOx emission control.6 At temperatures between 850 and 1175 °C, a reducing reagent such as ammonia or urea solution is injected and thoroughly mixed in a flue gas stream containing NOx. The © 2016 American Chemical Society
reagent selectively reduces NOx in spite of the presence of excess oxygen generally required in the furnace. The technology is attractive due to its simplicity, catalyst-free system and hence free of associated problems, ease of installation on existing plants, as well as applicability to all types of stationary-fired equipment. However, the NOx reduction efficiency of SNCR is confined due to its narrow temperature window. Below 800 °C, the NOx reduction reactions are too slow to give any reduction and most of the injected NH3 remains unreacted. At high temperatures, NH3 tends to oxidize to form NO. Therefore, the temperature window plays an important role for the effective operating of SNCR. The reactive temperature windows have been investigated on the basis of a kinetic mechanism on NH3/NO/O2. And relative test data are listed in Table 1, including some NH3-related reagents. (1) Ammonia: Lyon7 revealed the temperature window of SNCR is between 800 and 1070 °C using ammonia at a normal stoichiometric ratio (NSR, i.e., NH3/NO molar ratio) of 1.7 with an oxygen concentration of 2.0%. The optimum temperature is 950 °C. More similar data were summarized in ref 6. (2) Urea solution: Regarding urea solution of 15% w/v tested in a flow reactor combustion facility, the NOx reduction temperature range showed 900−1220 °C for the range of NSR of 0.25−2.0.8 It testified urea solution suits a Received: August 5, 2016 Revised: October 18, 2016 Published: October 21, 2016 10858
DOI: 10.1021/acs.energyfuels.6b01955 Energy Fuels 2016, 30, 10858−10867
Article
Energy & Fuels Table 1. Studies on Temperature Windows of SNCR no.
temperature window widening
1 2 3 4 5 6
baseline
7
shifting to higher temperature (i.e., hightemperature SNCR)
shifting to lower temperature
reduction reagent ammonia urea ammonia ammonia ammonia urea ammonia
other additive none none CO H2 CH4 hydrazine hydrate solution none
8
ammonia
none
9 10
urea urea
none none
O2 conc.
temperature window (°C)
optimum temperature (°C)
ref
2.0% 3.0% excess 4.0% excess
800−1170 900−1220 600−880 650−850 740−1100 500−650
950 1050−1100 750 720 830 550
7 8 9 10, 11 12 13
0.01%
>900
1100
14
1.1% 9.3% 1% 5% 0% 0% 0.5% 1%
>800 800−1200 >750 >750 >800 >950 >750 750−1290
1000 950 1000 950 1100 1200 1000 950
15 16 17
NOx reduction is widened and shifted to higher temperature, increasing about 150 °C rather than those of excess O2 (1.1− 9.3%). That phenomenon of high-temperature SNCR (abbr. HTSNCR) has also been proved in recent experimental and modeling studies.15−17 Regarding SNCR without any catalytic additive,15,16 the temperature windows are affected slightly as oxygen beyond 1.0%. However, under fuel-rich conditions, the optimum temperatures shift to higher values, 1100−1200 °C. The shifting toward higher temperature is because the regeneration of O and OH radicals via reaction H + O2 = OH + O cannot take place without O2. Consequently, the conversion from NH3 to NH2 is very limited,15 including the reactions NH3 + O = NH2 + OH and NH3 + OH = NH2 + H2O. As the results, NO reduction reactions, such as NH2 + NO = N2 + H2O, NH2 + NO = NNH + OH, and NNH + NO = N2 + HNO, are all limited at lower temperature
