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ISSN : 1225-4517(Print)
ISSN : 2287-3503(Online)
Journal of Environmental Science International Vol.27 No.10 pp.809-820
DOI : https://doi.org/10.5322/JESI.2018.27.10.809

# Effect of Hydrocarbon Additives on SNCR DeNOx Characteristics under Oxidizing Diesel Exhaust Gas Conditions

Changmo Nam*
Division of Health and Science, Yeungnam University College
E-mail: cmnam@ync.ac.kr
*Corresponding author: Changmo Nam, Division of Health and Science, Yeungnam University College, Daegu 42415, Korea
Tel: +82-53-650-9284
28/05/2018 26/06/2018 06/07/2018

## Abstract

DeNOx experiments for the effects of hydrocarbon additives on diesel SNCR process were conducted under oxidizing diesel exhaust conditions. A diesel-fueled combustion system was set up to simulate the actual cylinder and head, exhaust pipe and combustion products, where the reducing agent NH3 and C2H6/diesel fuel additives were separately or simultaneously injected into the exhaust pipe, used as the SNCR flow reactor. A wide range of air/fuel ratios (A/F=20~40) were maintained, based on engine speeds where an initial NOx level was 530 ppm and the molar ratios (β=NH3/NOx) ranged between 1.0~2.0, together with adjusting the amounts of hydrocarbon additives. Temperature windows were normally formed in the range of 1200~1350K, which were shifted downwards by 50~100K with injecting C2H6/diesel fuel additives. About 50~68% NOx reduction was possible with the above molar ratios (β) at the optimum flow #1 (Tin=1260K). Injecting a small amount of C2H6 or diesel fuel (γ=hydrocarbon/NOx) gave the promising results, particularly in the lower exhaust temperatures, by contributing to the sufficient production of active radicals (OH/O/HO2/H) for NOx reduction. Unfortunately, the addition of hydrocarbons increased the concentrations of byproducts such as CO, UHC, N2O and NO2, and their emission levels are discussed. Among them, Injecting diesel fuel together with the primary reductant seems to be more encouraging for practical reason and could be suggested as an alternative SNCR DeNOx strategy under diesel exhaust systems, following further optimization of chemicals used for lower emission levels of byproducts.

Yeungnam University

## 1. Introduction

Recently, more NOx emissions from diesel engines made by a major commercial car company have been a great issue worldwide during all normal driving than in the laboratory or testing station. This violates not only each country’s stringent emission standard, but also may cause a serious health concern and environmental problems (Palash et al., 2013, U.S. EPA, 2018). Although diesel engines have various advantages like fuel economy, high engine efficiency (~52%) and lower emissions of CO/CO2 species, the efforts to decrease diesel NOx emissions have been little successful since the current state of the art three-way catalysts (noble metals) for gasoline engines hardly work particularly under oxygen-rich exhaust gas conditions (Nakatsuji et al., 2008; Toops, et al., 2010). Thus, many investigations into diesel NOx reduction have been conducted first of all using the Selective Catalytic Reduction (SCR) process by NH3/urea or by hydrocarbon agents in search of so called “lean NOx catalysts.” Especially, the fact that NOx could be reduced by hydrocarbons over Cu loaded/metal oxide catalysts has brought much attention over the possibility of diesel NOx reduction, but the technology is still far away from commercial production (Heimrich and Deviney, 1993; Anderson, et al., 1997; Yang, et al., 2011).

## 2. Experimental

To investigate the effects of hydrocarbon additives on the diesel SNCR process under oxidizing exhaust gas conditions, a diesel-fueled, combustion-driven flow reactor was set up to simulate as closely as possible gas compositions, temperature profiles (gas quenching rates), residence times and aerodynamics of a diesel engine cylinder and exhaust pipe. The detailed experimental system was previously described by Nam and Gibbs(2012), based on a Perkins 4.236 single cylinder diesel engine (965 cm3 capacity), and a schematic diagram is illustrated in Fig. 1. The flow reactor consisted of a combustor, firing diesel fuel and the simulated cylinder (76 mm ID and 213 mm long, stainless steel tube) which was bolted onto an engine cylinder head/exhaust manifold (cut from a real engine) and finally an exhaust pipe of 2 m long tube (70 mm ID). The exhaust pipe insulated with kaowool fibre included 4 sampling ports, which allowed gas samplings at different residence times along the pipe. And the primary reducing agent (NH3) was first directly injected into the inlet of the exhaust pipe (Injector 2), used as the SNCR flow reactor. Then, C2H6 or diesel fuel additive was further introduced simultaneously or separately together with NH3 to investigate the effects of hydrocarbon additives on NOx reduction characteristics both in diesel combustion products and in low temperature exhaust conditions. Three different airflow rates were used such as 1000, 800 and 600 L/min, which corresponded to engine speeds of 3000 rpm, 2000 rpm and 1200 rpm, respectively, based on the above single cylinder (Perkins 4.236). And fuel flow rates normally varied in the range of 0.0832~0.0368 L/min (69~30 g/min) that are equivalent to air/fuel ratios (AFR) of 20~45 (ϕ =0.72~0.32), close to actual diesel operating conditions. At a typical air/fuel ratio of 27.5, the residence time of the exhaust pipe section reached about 50 msec, while the simulated cylinder only provided ~20 msec, varying with air flow rate and temperature. The gas concentration measurements (NOx, N2O, CO, CO2, O2 and UHC) were achieved using the on-line gas analyzing systems, directly connected into the exhaust pipe section. Meanwhile, NH3 slip measurements were separately carried out by a wet chemical method, using a diluted acid solution. NOx concentrations were continuously measured by a chemiluminescent analyser (Rotork Model 440), while other instruments for emission concentrations were described in previous study (Nam and Gibbs, 2012). Typical initial levels of combustion products at the inlet of the exhaust pipe for all 8 flows are presented in Table 1. The initial NOx concentrations for all flows were set to ~530 ppm. For instance, CO/UHC levels (flow #1) were maintained around <482/180 ppm, but these gases continuously oxidized to the end of exhaust pipe, eventually remaining very low, less than 30/4 ppm respectively, while those of flow #6 reached high about 1250/365 ppm due to the lower temperatures. As O2 level increases from 3.7 to 12.8%, CO2 concentration inversely decreases from 11.31 to 6.37% according to air/fuel ratios. Typical temperature profiles along the exhaust pipe section are shown in Fig. 2, where the initial gas temperatures for all flows covering a wide range of engine operating conditions were maintained in the range of 1000~1350 K for possible DeNOx experiments. Particularly, the gas quenching rates (temperature gradients) of 1430 to -3260 K/sec were developed along the exhaust pipe for all flows due to natural cooling. Gas temperatures were measured by shielded thermocouples (type K) and corrected for radiation losses (20~40 K).

## 3. Results and Discussion

### 3.1. DeNOx characteristics without additive

The selective noncatalytic reduction (SNCR) of NOx was investigated under diesel combustion conditions, by directly injecting NH3 into the exhaust pipe, used as the SNCR flow reactor. The initial NOx level was set to ~530 ppm, changing the molar ratio (β=NH3/NOx) from 1.0 to 2.0, when air flow rate was normally maintained at 1000 L/min. The effect of temperature on NOx reduction without additives is shown in Fig. 3 in which O2 levels varied from 3.7 to 12.8% at each flow according to the air/fuel ratios. About 50~68% NOx reduction occurred as the molar ratio was changed from 1.0 to 2.0 at the optimum flow #1 (Tin=~1260 K) and O2 level of 6.6%. Temperature windows were formed in the range of 1200~1350 K, not much different from previous laboratory and power plant conditions (Stohr et al., 1997; Quang Dao et al., 2009). NOx reduction tends to decrease with decreasing injection temperatures or with increasing above the optimum flow #1. As molar ratio increases, the extent of NOx reduction at the higher temperatures turned out to be much better than those at the lower temperatures. When temperature further decreased, much closer to the typical exhaust temperature (~1000 K), only 8% NOx reduction was obtained at a molar ratio of 1.5. First of all, the SNCR process proved to be highly dependent on optimum injection temperature even in diesel conditions, particularly requiring uniform and isothermal conditions, which are not available, although many other parameters such as residence time, initial NOx levels and various combustion products were simultaneously involved. On the other hand, the above sufficient diesel combustion products (CO/UHC levels and a wide range of O2, as in Table 1) were not significantly affecting NOx reduction potential in three major reaction components (NH3/NO/O2), implying that the key reaction factor should be still the temperature.

From kinetic calculations (Miller and Bowman, 1989; Nam and Gibbs, 2002; An et al., 2015), NH3 is mostly initiated by active OH radical among many species (OH/O/H/HO2) to produce NH2 (amidogen) intermediate in oxidizing diesel exhaust conditions as follows;

$NH 3 + OH = NH 2 + H 2 O$
(R1)

Then, NH2 species effectively reacts with NO by the key reactions (R2, R3) below among complex chain reactions, leading to proper NO reduction paths.

$NH 2 + NO = N 2 + H 2 O$
(R2)

$NH 2 + NO = NNH + OH$
(R3)

Meanwhile, over the higher temperatures (>1260 K), NH2 radicals rather react with active oxidants (O, OH) than NO to produce NHi and HNO species, which further oxidize and decompose to produce NO, following so called NH3 oxidation paths. So, the competition and balance between NO reduction and NH3 oxidation is maintained throughout, depending on the injection temperature, as in experimental results.

Another big challenge is that the temperature gradients (-2240 K/sec at flow #1, Fig. 2) are also quite substantial both axially and radially along the exhaust pipe, which must be one of the main reasons for low NOx reductions. Ostberg and Dam-Johansen (1994) reported that when the axial temperature gradient was -1000 K/sec in SNCR experiments in a plug flow reactor, it had a significant impact on the temperature window shifting upwards and developed into a little lower NOx reduction. This implies that direct injection of NH3 or other reducing agents into the exhaust pipe would not probably be a suitable DeNOx strategy in mobile or stationary diesel engines, because of the lower temperatures and even severe temperature gradients along the exhaust pipe. Therefore, an alternative strategy by adopting hydrocarbon additives was implemented to investigate possible increase of NOx reduction and further emission trends, particularly in the exhaust temperatures.

C2H6 was injected simultaneously with the primary reducing agent (NH3) into the inlet of the exhaust pipe, using a molar (mass) ratio (ϒ=C2H6/NOx) of 0.2 to 0.5 and 1.0, when the value of β=NH3/NOx was set to a constant value of 1.5.

The influence of C2H6 additive on NOx reduction is shown in Fig. 4. The first interesting thing is that the addition of C2H6 with the range of ϒ=0.2~1.0 mostly shifted the temperature windows downwards. With the molar ratio of ϒ=0.2, the temperature window was shifted downwards by about 50~100 K with little change of shape where the optimum temperature was centered at around 1150~1200 K from 1260 K. However, further increases of ϒ from 0.2 to 0.5 and 1.0 caused further shifts of the temperature windows at higher temperatures (1150~1300 K), but had little effect at lower temperatures (1000~1100 K).

The second, prominent feature is the change of NOx reduction potential caused by additive. At a molar ratio of ϒ=0.2, the degree of NOx reduction is similarly maintained, showing about 60% NOx reduction, compared with those of NH3 alone (no additive). Any further increase of ϒ from 0.2 to0.5~1.0 caused the NOx reduction to diminish significantly up to 40% at the higher temperatures. In the lower exhaust temperatures, more NOx reduction efficiencies (reaching about 15~25%) were surprisingly achieved by the injection of C2H6, which is compared with about 8% NOx reduction potential (NH3 only). According to the previous results (Duo et al., 1990; Yao et al., 2017), C2H6 addition caused temperature windows to be shifted downwards by about 150~ 200 K, mostly under laboratory and oil fired boiler (1MJ/sec) conditions (negligible CO/UHC and <3% O2). The present results show that the shift of temperature windows and the extent of NOx reduction were less effective, due to the severe temperature loss along the exhaust pipe and sufficient diesel combustion products (CO/UHC and O2). Nevertheless, the most encouraging finding is that less amount of C2H6 (ϒ=0.2) gave better NOx reduction potential, reaching almost as good level as with no additive.

The hydrocarbons are ignited and broken into their fragments in the presence of excess O2 than NH3 because of their lower activation energies (40~55 kcal/mole), compared with 62 kcal/mole for the NH3+O2 reaction (Niu et al., 2010; Raj et al., 2011). The addition of hydrocarbons prompts more active radical production even at lower temperatures, hence shifting the temperature window downwards, based on the following general reactions (R4~R6).(R5)

$C n H 2n+2 + O 2 = C n H 2n+1 + HO 2$
(R4)

$Eact= 40 ∼ 55 kcal/mol =xCO + yH 2 O$
(R5)

(R6)

In these reactions, alkyl radicals further decompose mainly via dissociation reactions or via reactions with molecular O2. In case of C2H6, alkyl radicals such as C2H5, C2H3, and HCO are dissociated into highly reactive H and HO2 radicals, which would again lead to further formation of sufficient OH/H/O radicals. The existence of these radicals not only shifts the temperature window downwards, but also accelerates NO reaction with NH3 in the presence of O2 to increase NOx reduction potential, particularly in the lower exhaust temperatures.

The injection of diesel fuel as an additive for diesel SNCR process will be much more practical than any other hydrocarbons (CH4, C2H6, C3H8 and CO) investigated, due to the ease of use and transport safety to actual diesel engines.

### 3.4. Emission Levels of CO, N2O and NO2

Fig. 6(A, B) shows the emissions of CO and N2O before and after injecting C2H6 and diesel fuel as additives for the whole flows (#A to #6). Simultaneous measurements were carried out for these emissions. UHC emission levels were negligible, almost a factor of 10 smaller than CO levels for all flows except #6 and not depicted due to complexity.

First, without additive (NH3 only, β=1.5), maximum N2O emission (~23 ppm) peaks around 1260 K (flow #1), which is consistent with maximum NOx reduction flow. At higher temperatures (1300~1350 K), N2O emissions tend to decrease slightly by decomposition, where less NOx reduction was also achieved. At lower temperatures (1000~1100 K), N2O production sharply decreased to ~8 ppm even at flow #2 (1210 K), despite substantial NOx reduction occurred at the same flow.

When C2H6 was added together with NH3 (Fig. 6A), N2O emissions were almost doubled (45~50 ppm) with increasing C2H6 amounts (ϒ=0.2~1.0) at and above the optimum temperature, despite the adverse effect on the NOx reduction (Fig. 4). However, at the optimum temperature of 1210 K, N2O emissions fall and continue to decrease as the temperature goes down further.

The temperature characteristics of N2O emissions with diesel fuel (Fig. 6B) are quite different from those of C2H6. Injection of diesel fuel triggered temperatures a little high by about 20~40℃, depending on the mass ratios (diesel/NOx), so that less N2O production occurred at the higher temperatures due to decomposition reaction. However, at lower temperatures, unlike C2H6, a substantial amount of N2O (20~65 ppm at 1100K) was still produced, depending on the mass ratios of diesel fuel.

Secondly, the CO emissions with no additives at the higher temperatures (1200~1350 K) usually remained quite low at the end of exhaust pipe because of very efficient combustion (Fig. 6A). As temperatures go down, the CO emission levels become significant, and increased up to ~500 ppm at flow #4 (1100 K), and again rose up to about 1600 ppm due to the low combustion temperatures (1000 K). With C2H6 injection at the molar ratios (ϒ =C2H6/NOx) of 0.2 and 0.5, CO emissions were slightly increased by about 100~300 ppm at the lower temperature flows, but at the higher temperatures, the extent of increase was negligible. As diesel fuel was injected, two different aspects emerged according to the temperature (Fig. 6B). At higher temperatures, CO emissions were further decreased by additional combustion. However, at the lower temperatures (1000~1100 K of flow #6~#4), CO emission levels were substantially increased by almost two or three times those of no additive because of incomplete combustion and higher carbon content of diesel fuel, compared to C2H6.

Table 2 shows a comparison of “carbon content” between C2H6 and diesel fuel for the injected quantities. The carbon content of diesel fuel was 6~9 times higher than C2H6 according to the mass ratios. Therefore, more CO and N2O emissions were observed with diesel fuel, although more NOx reductions were achieved at the same conditions.

Fig. 7 shows NO2 increase, resulting from the NO/NH3/O2 reactions with/without the injection of C2H6 and diesel fuel, as observed in Fig. 6. At higher temperatures (1150~1200 K), actually lower parts of temperature windows for NOx reduction, the NO2 emissions were less than 2% (<10 ppm), and not affected by either existing combustion products (CO/UHC) or by further injecting C2H6/diesel fuel additives. However, as temperature was lowered to 1040~1000 K (flows #5 and #6), the NO2 emissions sharply increased and reached ~50 ppm at 1000 K without additives. Injecting C2H6 or diesel fuel caused the NO2 emissions to increase substantially from ~50 ppm to about 80~95 ppm at Tin=1000 K where for diesel fuel (ϒ=3.2), more NO2 was produced due to its higher carbon content.

The oxidation of CO and N2O/NO2 formation in a SNCR process including diesel combustion products can be explained via the following reactions below (Glarborg et al., 1994; Yao et al., 2017);

$CO + OH = CO 2 + H$
(R7)

And

$NH + NO = N 2 O + H$
(R8)

$NCO + NO = N 2 O + CO$
(R9)

And

$NO + HO 2 = NO 2 + OH$
(R10)

The injection of NH3 for NO reduction retards the main CO oxidation reaction rate (R7), resulting in the increase of CO concentration because many chain reactions compete for OH radicals, and particularly the reaction (R1) is very rapid, compared with reaction (R7). The major sensitive reactions for N2O formation are (R8) and (R9), while the conversion of NO to NO2 is mainly dominated by the reaction R10 at the lower temperatures. Increasing C2H6 and diesel fuel additives would increase the concentrations of not only active radicals (OH/O/HO2/H), leading to further NOx reduction particularly at lower temperatures, but also of nitro-carbonaceous species (NCO, NH, HCN and CN) and hydrocarbon fragments (CxHy), which then eventually contribute to the increase of N2O and NO2 byproducts and CO emissions as well through complex chain reactions.

## 4. Conclusions

After all, the injection of C2H6 and diesel fuel into the exhaust pipe is believed to have promoted the production of OH/O/HO2/H radicals in N/H/O/ hydrocarbon reactions which certainly contributed to further NOx reduction at lower temperatures. In addition, the existence of higher CO/UHC levels would accelerate the rate of NOx reduction in short residence times (~50 msec), but decrease the selectivity for the reaction of NH3 with NOx, resulting in overall slightly lower NOx reduction, and contribute to higher CO/N2O/NO2 emission levels.

Nevertheless, Injecting a small amount of diesel fuel together with a primary reducing agent (NH3) seems to be more encouraging for practical reason and could be suggested as an alternative SNCR DeNOx strategy under diesel exhaust systems, following further strict optimization of chemicals used for lower emission levels of byproducts.

## Acknowledgments

The author would like to thank Professor B. M. Gibbs at the University of Leeds for experimental works and fruitful discussions. This research was supported by the Yeungnam University College research grants in 2017.

## Figure

A schematic diagram of a diesel-fueled, combustion-driven flow reactor for SNCR DeNOx experiments.

Typical temperature profiles of all flows along the exhaust pipe (reactor) positions.

Temperature characteristics of NOx reductions through direct injection of NH3 into the inlet of the exhaust pipe under various molar ratios (β =1.0~2.0) without hydrocarbon additive.

C2H6 effects as additive on NOx reduction characteristics with the simultaneous NH3 (β=1.5) injection into the exhaust inlet; 530 ppm NOx.

Diesel fuel effects as additive on NOx reduction characteristics with the simultaneous NH3 (β=1.5) injection into the exhaust inlet; 530 ppm NOx.

Variations of CO and N2O emissions by injecting C2H6 (A) and diesel fuel (B) as additives with the simultaneous NH3 (β=1.5) injection into the exhaust inlet; 530 ppm NOx.

NO2 emission increase by injecting C2H6 and diesel fuel as additives with the simultaneous NH3 (β=1.5) injection into the exhaust inlet; 530 ppm NOx.

## Table

Initial levels of combustion products at the inlet of the exhaust pipe section for all flows in SNCR DeNOx experiments

Comparison of carbon content between C2H6 and diesel fuel additives injected

## Reference

1. An, H. , Yang, W. , Li, J. , Zhou, D. , 2015, Modeling analysis of urea direct injection on the NOx emission reduction of biodiesel fueled diesel engines , Ener. Conv. Manage., 101, 442-449.
2. Anderson, J. A. , Marquez-Alvarez, C. , Lopez-Munoz, M. J. , Guerrero-Ruiz, A. , 1997, Reduction of NOx in C3H6/air mixtures over Cu/Al2O3 catalysts , Appl. Catal. B, 14, 189-202.
3. Capener, L. , 2008, Advanced overfire air/SNCR and sorbent injection system , Power Eng., 112, 192-198.
4. Duo, W. , Dam-Johansen, K. , Ostergaard, K. , 1990, Widening the temperature range of the Thermal DeNOx process; an experimental investigation , Proc. Combust. Inst., 23, 297-303.
5. Glarborg, P. , Dam-Johansen, K. , Miller, J. A. , Kee, R. J. , Coltrin, M. E. , 1994, Modeling the Thermal DeNOx process in flow reactors , Inter. J. Chem. Kinet., 26, 421-4213.
6. Heimrich, M. J. , Deviney, M. L. , 1993, Lean Nox catalysts evaluation and characterization, SAE paper930736.
7. Jodal, M. , Neilsen, C. , Hulgaard, T. , Ostergaard, K. , 1990, Pilot-scale experiments with NH3 and urea as reductants in SNCR of NO , Proc. Combust. Inst., 23, 237-243.
8. Lyon, R. K. , 1987, Thermal DeNOx controlling nitrogen oxides emissions by a noncatalytic process , Environ. Sci. Tech., 21(3), 231-236.
9. Miller, J. A. , Bowman, C. T. , 1989, Mechanism and modeling of nitrogen chemistry in combustion , Prog. Ener. Combust. Sci., 15, 287-338.
10. Miyamoto, N. , Ogawa, H. , Wang, J. , Shudo, T. , Yamazaki, K. , 1995, Diesel NOx reduction with ammonium deoxidizing agents directly injected into the cylinder , Int. J. Vehi. Desi., 16(1), 71-79.
11. Nakatsuji, T. , Yamaguchi, T. , Sato, N. , Ohno, H. , 2008, A selective NOx reduction on Rh-based catalysts in lean conditions using CO as a main reductant , Appl. Catal.B, 85, 61-70.
12. Nam, C. M. , Gibbs, B. M. , 2002, Application of the Thermal DeNOx process to diesel engine DeNOx: an experimental and kinetic modeling study , FUEL, 81, 1359-1367.
13. Nam, C. M. , Gibbs, B. M. , 2012, SNCR application to diesel engine DeNOx under combustion-driven flowreactor conditions , J. Environ. Sci., 21(7), 769-778.
14. Niu, S. , Han, K. , Lu, C. , 2010, Experimental study on the effect of urea and additive injection for controlling NOx emissions , Environ. Eng. Sci., 27, 47-53.
15. Ostberg, M. , Dam-Johansen, K. , 1994, Empirical modeling of the SNCR of NO: comparison with large-scale experiments and detailed kinetic modeling , Chem. Eng. Sci., 49(12), 1897-1904.
16. Palash, S. , Masjuki, H. , Kalam, M. , Masum, B. , Sanjid, A. , Abedin, M. , 2013, State of the art of NOx mitigation technologies and their effects on the performance and emission characteristics of biodiesel-fueled compression ignition engines , Ener. Conv. Manage., 76, 400-420.
17. Quang Dao, D. , Gasnot, L. , Marschallek, K. , Bakali, A. , Pauwels, J. F. , 2009, Experimental study of NO removal by gas reburning and selective noncatalytic reduction using ammonia in a lab-scale reactor , Ener. Fuels, 24, 1696-1703.
18. Raj, A. , Zainuddin, Z. , Sander, M. , Kraft, M. , 2011, A mechanistic study on the simultaneous elimination of soot and NO from engine exhaust , Carbon, 49, 1516-1531.
19. Srivastava, R. K. , Hall, R. E. , Khan, S. , Culligan, K. , Lani, B. W. , 2005, NOx emission control options for coal-fired electric utility boilers , J. Air & Waste Manage. Assoc., 55, 1367-1388.
20. Stohr, M. , Schutz, M. , Kruger, H. , 1997, Status of and experience with NOx reduction in coal-fired powerplants , Proc. Inst. Mech. Eng., 211, 27-41.
21. Toops, T. , Nguyen, K. , Foster, A. , Bunting, B. , Ottinger, N. , Pihl, J. , Hagaman, E. , Jiao, J. , 2010, Deactivation of accelerated engine-aged and field-aged Fe-zeolite SCR catalysts , Catal. Today, 151, 257-265.
22. U.S. Environmental Protection Agency (EPA), 2018, http://www.epa.gov.
23. Vedharaj, S. , Vallinayagam, R. , Yang, W. , Saravanan, C. , Chou, S. , Chua, K. , Lee, P. , 2014, Reduction of harmful emissions from a diesel engine fueled by kapok methyl ester using combined coating and SNCR technology , Ener. Conv. Manage., 79, 581-589.
24. Weijuan, Y. , Zhijun, Z. , Junhu, Z. , Hongkun, L. , Jianzhong, L. , Kefa, C. , 2009, Application of hybrid coal reburning/SNCR processes for NOx reduction in a coal-fired boiler , Environ. Eng. Sci., 26, 311-318.
25. Xu, B. , Tian, H. , Yang, J. , Sun, D. , Cai, S. , 2011, A system of selective non catalytic reduction of NOx for dieselengine , Adv. Mater. Res., Vols. 201-203, 643-646.
26. Yang, S. , Wang, C. , Li, J. , Ma, L. , Chang, H. , 2011, Low temperature selective catalytic reduction of NO with NH3 over Mn-Fe spinel: performance, mechanism and kinetic study , Appl. Catal. B, 110, 71-80.
27. Yao, T. , Duan, Y. , Yang, Z. , Li, Y. , Wang, L. , Zhu, C. , Zhou, Q. , Xhang, J. , She, M. , Lie, M. , 2017, Experimental characterization of enhanced SNCR process with carbonaceous gas additives , Chemosphere, 177, 149-156.