Thermal DeNOx Process Research Articles

Chemical mechanism of ammonia-methanol combustion and chemical reaction kinetics analysis for different methanol blends

As a carbon-free fuel, ammonia is considered as an attractive alternative fuel for internal combustion engines. However, ammonia has the problems of high ignition temperature and NOx emissions while using in engines. Methanol as liquid fuel is easy to store and transport. Ammonia-methanol blended fuel is probably-one of the most appropriate methods to solve the problem. However, there are few studies of chemical mechanism for the combustion of ammonia/methanol dual fuel. Moreover, it is important to study ammonia-methanol combustion by chemical reaction path analysis. In this paper, a chemical reaction mechanism of ammonia-methanol blends with 60 species and 399 reactions was developed. This mechanism can accurately predict the laminar burning velocity (LBV) and ignition delay time (IDT) in a wide range of equivalent ratios (Φ) and methanol blends. By using this chemical reaction mechanism, the effects of methanol addition on ammonia combustion and emissions at different equivalent ratios were numerically studied. Results showed that the addition of methanol significantly improves the chemical reactivity of ammonia, and blending a small amount of methanol can greatly reduce the IDT. However, the accumulation of radical HO2 caused by ammonia production enhances the fuel NOx path of NH2 → H2NO, which, together with high temperature and oxygen enrichment, leads to a NOx peak of 12500 ppm at equivalent ratio of 0.8. However, with the increase of equivalent ratio, NOx emission will decrease, which is only 2500 ppm at equivalent ratio of 1.4. Due to the existence of cross reaction of NH2 + CH3OH = NH3 + CH2OH and NH2 + CH3OH = NH3 + CH3O between ammonia and methanol, NH2 is difficult to participate in the thermal DeNOx process in the premixed combustion process, resulting in high NOx emissions. In addition, it is also found that after equivalent ratio of 1.1, H will replace OH as the most important chain propagation carrier.

Stability and emission characteristics of ammonia-air flames in a lean-lean fuel staging tangential injection combustor

The present work suggests the stability and emission characteristics of an ammonia-air tubular flame in a fuel staging tangential injection combustor. Owing to the development of a large inner recirculation zone induced by a high swirling motion, the ammonia tubular flame exhibited wide operational flexibility at an ambient temperature condition. With ammonia fuel staging, we confirmed that the Thermal DeNOx process significantly mitigates NO emissions when the exit temperature of the primary region is close to the threshold for active NO reduction. In this study, fuel multi-stage technology was used for pure ammonia-air combustion, and the result effectively challenges the existing paradigm of the NO reduction strategy: Rich-Quench-Lean staging configuration. Thus, our results demonstrate the potential use of ammonia as a fuel in a dry low-NOx system.

A Comparative Study of NOx Emission Characteristics in a Fuel Staging and Air Staging Combustor Fueled with Partially Cracked Ammonia

Recently, ammonia is emerging as a potential source of energy in power generation and industrial sectors. One of the main concerns with ammonia combustion is the large amount of NO emission. Air staging is a conventional method of reducing NO emission which is similar to the Rich-Burn, Quick-Mix, Lean-Burn (RQL) concept. In air-staged combustion, a major reduction of NO emission is based on the near zero NO emission at fuel-rich combustion of NH3/Air mixture. A secondary air stream is injected for the oxidation of unburned hydrogen and NHx. On the other hand, in fuel-staged combustion, NO emission is reduced by splitting NH3 injection, which promotes the thermal DeNOx process. In this study, NOx emission characteristics of air-staged and fuel-staged combustion of partially cracked ammonia mixture are numerically investigated. First, the combustion system is modeled by a chemical reactor network of a perfectly stirred reactor and plug flow reactor with a detailed chemistry mechanism. Then, the effects of ammonia cracking, residence time, and staging scheme on NOx emission are numerically analyzed. Finally, the limitations and optimal conditions of each staging scheme are discussed.

Investigation on the potential of using carbon-free ammonia in large two-stroke marine engines by dual-fuel combustion strategy

Ammonia (NH3) as a carbon-free fuel is gaining much attention in the shipping industry for its enormous potential of global decarbonization, which is expected to achieve large-scale applications in marine engines by dual-fuel (DF) combustion technology. As such, this study was conducted to explore the emission reduction potential of NH3/diesel DF combustion strategy via low-pressure gas injection in large low-speed two-stroke marine engines using computational fluid dynamics (CFD) modeling coupled with chemical kinetics. An established in-house CFD platform, KIVA-CHEMKIN-CDAC, was employed to perform engine simulations, while an DF combustion mechanism was constructed in this work to mimic the oxidation behaviors of NH3 and diesel as well as the emissions formation. It was found that NH3 admission exhibits a significant inhibiting effect on the autoignition of pilot fuel. Increasing ammonia substitution ratio (ASR) will prolong the ignition delay, resulting in high intensity of premixed combustion. Generally, NH3/diesel DF combustion mode shows two-stage heat release shape. The first stage is characterized by the premixed combustion of diesel–NH3–air mixtures, whereas the dominant combustion regime of the second stage highly depends on the NH3 concentration in the premixed charge. In the very lean NH3-air mixtures, the second stage corresponds to the mixing-controlled diffusion combustion phase; but for the richer NH3-air mixtures, it could be controlled by the turbulent flame propagation. NOx emission decreases when the ASR does not exceed 40%, otherwise increases significantly. The former is probably due to the Thermal DeNOx process dominated by NH2 + NO = N2 + H2O, while the latter is due to the fuel-bound nitrogen. As expected, CO2 emission is reduced monotonically for the same total fuel energy with the increase of ASR, confirming that the utilization of zero-carbon fuel is the most direct means to reduce CO2 emissions. Moreover, there is a trade-off relationship between NOx and N2O emissions. This is because N2O formation usually occurs at lower temperatures (i.e., T < 1400 K). Furthermore, advancing the pilot-fuel injection timing could reduce the unburned NH3 and N2O emissions. Therefore, optimization of injection timing can achieve lower emissions while maintain higher efficiency.

Effects of ammonia energy fraction and diesel injection timing on combustion and emissions of an ammonia/diesel dual-fuel engine

Ammonia has been receiving increasing interest as a hydrogen carrier and carbon-free fuel to tackle the issue of greenhouse gas (GHG) emissions from transportation. In this study, an ammonia/diesel dual-fuel (ADDF) engine is experimentally and numerically investigated, with focus on its feasibility to reduce GHG emissions while achieving a diesel-like efficiency. A single-cylinder, heavy-duty diesel engine is used to investigate the effect of ammonia energy fraction and start of diesel injection (SODI) timing on the combustion performance and emissions of the ADDF engine. Results revealed that due to the low flame speed of ammonia, increasing the ammonia energy fraction decreased the thermal efficiency of the ADDF combustion mode compared to the diesel-only combustion mode. Increasing ammonia energy fraction from 0 to 40% reduced the nitrogen oxides (NOx) emissions by 58.8% at a given SODI due to the effect of the thermal DeNOx process. However, increasing the ammonia energy fraction at a given SODI increased the nitrous oxide (N2O) emissions, which offsets the benefit of lower intrinsic carbon dioxide (CO2) emissions of ADDF combustion and resulted in a higher GHG emission compared to diesel-only combustion. Advancing SODI helped reduce the N2O and overall GHG emissions while achieving a diesel-like thermal efficiency in ADDF combustion mode. The lowest GHG emissions of ADDF combustion achieved by advancing the SODI were 12% lower compared to those of diesel-only combustion. The thermal efficiency of ADDF combustion mode at the optimum point of GHG emissions (i.e., ITE = 37.85%) was slightly lower than that of the diesel-only combustion mode (i.e., ITE = 38.53%). This, however, comes with a benefit of 10% and 20% reduction in NOx and CO emissions, respectively. The unburned ammonia concentration is high (i.e., about 4445 ppm) in the exhaust flow. The reduction of ammonia emissions in the exhaust flow should be further investigated in the future.

Detailed Modeling of the Effects of K/Na Additives on the Thermal DeNOx Process

A reduced mechanism simplified from a detailed chemical kinetics mechanism containing N/H/O/K/Na elements was developed and validated in this paper. When the reduced mechanism was integrated into computational fluid dynamics (CFD) software, the effect of potassium and sodium additives on the selective non-catalytic reduction (SNCR) thermal DeNOx process was simulated. The simulation results were compared to those of experiments under different oxygen concentrations, normalized stoichiometric ratios (NSRs) of the N agent/NO, and alkali metal additive concentrations within the temperature range from 1023 to 1523 K, and the simulation results coincided qualitatively with those of the experiment in an entrained flow reactor. The alkali metal additives did not change the effects of the oxygen concentration and NSR on the SNCR process: a conversion temperature point exists at about 1173–1223 K; below the conversion temperature point, a higher oxygen concentration can promote the effect of SNCR, while above the conversion temperature point, the efficiency will be reduced; and a higher NSR is beneficial for NO reduction, but its effect becomes less obvious with the increase of the reducing agent. The alkali metal additives extend the “temperature window” toward a lower temperature by about 50–100 K with more OH and NH2 radical production, and the effect of K additives is less obvious than that of Na. However, the promoting effect of the K additive cannot be well-simulated because of the lack of a suitable mechanism. A K chemistry mechanism should be optimized on the basis of its effect on the SNCR process. The K or Na concentration almost has no influence on the effect of alkali metal additives on the thermal DeNOx process when the K or Na concentration is beyond a certain value.

Effects of ammonia substitution on combustion stability limits and NOx emissions of premixed hydrogen–air flames

The combustion stability limits and nitrogen oxide (NOx) emissions of burner-stabilized premixed flames of ammonia (NH3)-substituted hydrogen (H2)–air mixtures at normal temperature and pressure are studied to evaluate the potential of partial NH3 substitution to improve the safety of H2 use. The effects of NH3 substitution, nitrogen (N2) coflow and mixture injection velocity on the stability limits and NOx emissions of NH3–H2–air flames are experimentally determined. Results show a reduction of stability limits with NH3 substitution and coflow, supporting the potential of NH3 as a carbon-free, green additive in H2–air flames and indicating a different tendency from that for no coflow condition. The NOx emission index is almost constant even with enhanced NH3 substitution, though the absolute value of NOx emissions increases in general. At fuel-rich conditions, the NOx emission index decreases with increasing mixture injection velocity and the existence of coflow. The thermal deNOx process in the post-flame region is involved in reducing NOx emissions for the fuel-rich flames.

Influences of O 2 concentration on NO reduction and N 2O formation in thermal deNOx process

Flow reactor experiment and chemical kinetic modeling were carried out to evaluate the influences of oxygen concentration on NO reduction and N 2O formation during thermal deNOx process for temperatures of 1073–1473 K. Increasing oxygen concentration in the reactant gas from 0.3 to 9.3% not only reduces the NO reduction potential and N 2O emission, but also lowers and widens the “temperature window” for NO conversion and N 2O formation. Trace amount of impurity O 2 leads to the lowest NO emission at a high temperature of 1373 K with low N 2O emission. Test reveals that high molar ratio NH 3/NO suppresses the reverse effect of oxygen and brings out high percentage of NO reduction at leaner environment. Chemical kinetic simulations using 4 detailed mechanisms display differing trends in the NO and N 2O dependence on O 2 and temperature. Rota 2000 model gives the best description of our experiments, but some assumptions made in this mechanism are not fully realistic. This study provides new experimental data on the less noticed N 2O, and raises a question about the understanding of the detailed elementary chemistry on the thermal deNOx process. Further work is still required for reasonable kinetic explanations.

20415 高温窒素プラズマによるディーゼル排ガスの浄化(OS3 エンジン燃焼と排気浄化)

A technique for removing the NOx contained in combustion exhaust gas has been developed by using radicals produced in the mixing arc plasma. By feeding the diesel exhaust from engine into (N_2+diesel oil) arc plasma torch, diesel oil and N_2 radicals such as NH, HC, N, H are produced, and these radicals are mixed in a reactor with the exhaust gas. The NOx decreases as a result of the formation of the innocuous products N_2, H_2O and CO_2 if O_2 is contained in the gas. Under present experimental conditions, diesel oil reacts as reduction agency such as the thermal De-NOx process, the oxidation of diesel oil are not expected. In the case of t O_2 rich gas, NOx increases with increasing input power to plasma. The effects of input power, Diesel oil/NOx, treated gas flow rate and the distance from nozzle exit to exhaust pipe L/d on the NOx removal rate are investigated experimentally.

Flow reactor study of the effect of pressure on the thermal de-NOx process

The effect of pressure on the thermal de-NOx reaction was studied in a flow reactor capable of operating at pressures up to 50 bar. Hot combustion products were supplied to the reactor by a burner operating with a lean, premixed CH4-air mixture. Nitric oxide, ammonia and nitrogen were injected into the combustion products at the throat of a converging-diverging nozzle through radially opposed jets oriented perpendicular to the flow to provide rapid mixing. Nearly isothermal conditions exist downstream of the mixing region. Typical axial temperature variations are ±5 K. Axial profiles of NO, NH3, NO2, and N2O mole fractions were measured in the reactor at pressures of 1, 2, 5, and 10 bar for temperatures of 1200 K and 1240 K using extractive sampling techniques. Initial injected mole fractions of NO and NH3 corresponded to 250 ppm and 500 ppm, respectively, in the combustion products, and the O2 mole fraction was near 3%. At a given temperature, increasing pressure resulted in a modest decrease in the rate of NO removal, with little effect on the amount of NO2 and N2O formed. Measured NO2 and N2O mole fractions were less than 20 ppm for all conditions of this study. To allow comparison of the experimental data with predictions, a model was developed that couples the entrainment and mixing processes in the flow reactor with a detailed thermal de-NOx reaction mechanism. To improve agreement between measured and calculated species profiles, particularly at elevated pressures, the reaction mechanism was optimized by systematic variation of key kinetic parameters within reported uncertainties. This optimization reduced the overall rate and branching fraction of the NH2 + NO → products reaction to values consistent with recent experimental studies of this reaction and provided a modest improvement in agreement between the calculated and measured species profiles.

Chemical Kinetic Analysis of the Thermal DeNOx Process at High Reactant Concentration

Fossil fuel combustion is an important source of NOx, thus making the control of NOx emissions a primary issue in almost all the developed countries. This paper discusses a post-combustion process for reducing NOx emissions, Thermal DeNOx. After introducing the general problem of post-combustion technologies for NOx abatement, the relevance of developing reliable kinetic models for a real-size plant design is discussed. Subsequently, some experimental findings are summarized together with the main issues arising from the chemical kinetics of the process. Finally, the influence of some operating parameters, with particular reference to the overall pressure (or concentration), is discussed, showing that there are no kinetic limitations to the efficiency of the process.

Experimental Study and Kinetic Modelling of Nitric Oxide Reduction with Ammonia

An important part of a general simulation model of the Thermal DeNOx process is the detailed kinetic mechanism used to represent all the interconnecting reaction paths. Unfortunately, the location and dimension of the operating window investigated experimentally to validate a detailed kinetic mechanism strongly influences the reliability of its predictions, and explains the large differences occasionally found among the predictions of different mechanisms. Therefore, it is important to produce new sets of data kinetically significant to investigate the behavior of these models in different conditions, with particular reference to well-stirred conditions that are usually not considered in the literature. This paper presents the results of some experiments carried out in a laboratory jet-stirred reactor at atmospheric pressure and in the range 950-1450 K. Such experimental results enlarge the operating window available for model validation since they involve well-stirred conditions and the presence of both carbon monoxide and water. Then, they have been used, together with other experiments previously presented in the literature, to validate a detailed kinetic mechanism. Such a mechanism has been proved to be able to predict experiments performed not only in plug-flow conditions but also in well-stirred one.

Modeling the thermal De-NOx process: Closing in on a final solution

We have extended our previous kinetic model for the Thermal De-NOx Process to include recent information about the branching fraction α1(T) of the NH2 + NO reaction, NH2 + NO ⟷ N2 + H2O(R1a) NH2 + NO ⟷ NNH + OH,(R1b) where α1(T) = k1b/(k1a + k1b). The values of the rate coefficients k1a(T) and k1b(T) used in the model are k1a = 2.77 × 1020 T−2.65 exp(−1258/RT) cm3/mole-sec. and k1b = 2.29 × 1010 T0.425 exp(814/RT) cm3/mole-sec. These two rate coefficients combine to give values of the total rate coefficient and branching fraction that are in excellent agreement with experiments over a wide range of temperatures, 300 K < T < 1400 K for k1 = k1a + k1a and 300 K < T < 2000 K for α1(T). The smaller values of α1(T) in the Thermal De-NOx temperature window, 1100 K < T < 1400 K, used in the model allow us to reduce the value of the NNH lifetime to 1.5 × 10−8 sec, which is consistent both with the upper limit set by experiment and with the best theoretical predictions. We compare the model predictions with the experiments of Kasuya et al. and discuss the results in some detail. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 757–765, 1999

Modeling the thermal De‐NOx process: Closing in on a final solution

We have extended our previous kinetic model for the Thermal De-NOx Process to include recent information about the branching fraction α1(T) of the NH2 + NO reaction, NH2 + NO ⟷ N2 + H2O (R1a) NH2 + NO ⟷ NNH + OH, (R1b)where α1(T) = k1b/(k1a + k1b). The values of the rate coefficients k1a(T) and k1b(T) used in the model are k1a = 2.77 × 1020 T−2.65 exp(−1258/RT) cm3/mole-sec. and k1b = 2.29 × 1010 T0.425 exp(814/RT) cm3/mole-sec. These two rate coefficients combine to give values of the total rate coefficient and branching fraction that are in excellent agreement with experiments over a wide range of temperatures, 300 K < T < 1400 K for k1 = k1a + k1a and 300 K < T < 2000 K for α1(T). The smaller values of α1(T) in the Thermal De-NOx temperature window, 1100 K < T < 1400 K, used in the model allow us to reduce the value of the NNH lifetime to 1.5 × 10−8 sec, which is consistent both with the upper limit set by experiment and with the best theoretical predictions. We compare the model predictions with the experiments of Kasuya et al. and discuss the results in some detail. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 757–765, 1999

The barrier height for decomposition of HN2

The barrier height and exothermicity for the HN2→H+N2 reaction are predicted by high level ab initio quantum mechanical methods. The classical barrier is predicted to be 10.0±1.0 kcal/mol, and the reaction exothermicity is predicted to be 3.8±0.5 kcal/mol. The importance of these parameters to the thermal De–NOx process is discussed. The apparent conflict between the theoretical potential energy surface for HN2 and recent kinetic modeling studies is seen to persist.

A kinetic model for the reduction of nitric oxide by hydrazine

A chemical kinetic model is developed for the non-catalytic reduction of nitric oxide (NO) by hydrazine (N2H4). Since the reduction of NO was observed in an experimental reaction with N2H4, the hydrazine has been suggested as a new reductant of NO in addition to the conventional ammonia, urea and isocyanic acid. In the proposed kinetic model, a set of fifty one chemical reactions that includes the various branching reactions of N2H4 to NH2 and the wellknown reaction NO+NH2→N2+H2O is simultaneously considered with the usual partial equilibrium assumptions. The NO reduction is estimated to occur at a temperature range between 700K and 1400 K, which is wider and lower than in the conventional Thermal DeNOx process. The maximum amount of the reduced NO is slightly less than that in the Thermal DeNOx. The effects of the other input parameter on the NO reduction rate also discussed.

“Clean burning” low flame temperature solid gun propellants

Abstract : In an attempt to eliminate nitrogen oxides (NOx's) from the combustion and pyrolysis products of solid propellants, an investigation into the use of 'De-NOx' agents was performed. The agents selected were all compounds that thermally decompose to generate amines; participation of amines in the thermal De-NOx process results in the reduction of nitrogen oxides to nitrogen. Of all the compounds screened, it was found that urea was the best suited for use as a De-N0x agent in solid gun propellants. Use of - resulted in a significant decrease in NOx production. Furthermore, urea is thermally stable up to a temperature of 130 deg C, and is compatible with nitrocellulose-based formulations. Compared to neat JA2 (a double-base propellant), a JA2/urea formulation containing 10 weight-percent urea generates 60% less NOx. Calculations indicate that the incorporation of 4% urea results in a 200 K decrease in flame temperature, and a decrease in impetus and velocity of approximately 4% and 2%, respectively.

Pressure effects on the thermal de-NOx process

The effect of pressure on the thermal de-NO x process has been investigated in flow reactor experiments. The experiments were performed at pressures from 1 to 10 bar and temperatures ranging from 925 to 1375 K. The inlet O 2 level was varied from 1000 ppm to 10%, while NH 3 and NO were maintained at 1000 and 500 ppm, respectively. At the highest pressure, CO was added to shift the regime for NO reduction to lower temperatures. The results show that the pressure affects the location and the width of the temperature window for NO reduction. As the pressure is increased, both the lower and the higher boundaries for the process are shifted toward higher temperatures. Overall, the temperature range for NO reduction appears to be slightly broadened. The shift in chemistry toward higher temperatures is most pronounced at low oxygen concentrations. High oxygen levels act to counterbalance the inhibiting effect of the pressure but also cause a slight decrease in the NO reduction potential. The results are consistent with recent atmospheric pressure experiments of thermal de-NO x covering a wide range of reactant partial pressures. Comparisons of the experimental data with the recent chemical kinetic model of Miller and Glarborg show satisfactory agreement. The implications of the results for application of thermal de-NO x in high-pressure systems, such as pressurized fluidized bed combustion and gas turbines, are discussed.

Kinetics of the gas‐phase reaction between nitric oxide, ammonia and oxygen

AbstractFor the purpose of developing a simple and quantitative description of the rate of reactions occurring in the Thermal DeNOx process, the kinetics of the homogeneous reaction between NO and NH3 with excess of O2 has been studied in an isothermal plug flow reactor.Temperature windows of NO reduction were observed with the optimum temperature decreasing with increasing residence time. The degree of reduction of NO increases with residence time at lower temperatures but is sensitive to residence time only at very short residence times at higher temperatures. The degree of conversion of NH3 increases with both temperature and residence time.A simple model has been proposed to describe the reaction rates:rNO = kax [NH3] ‐ kr [NH3] [NO]r =‐kax [NH3]‐ kr [NH3] [NO]The two rate constants in the model were estimated based on the experimental data obtained in the present plug flow reactor at about 4% (vol.) oxygen:kax = (2.21 ± 0.33) × 1014 exp [‐(38160 ± 170)/T], I/skr = (2.45 ± 0.49) × 1014 exp [‐(29400 ± 250)/T], m3mol. sIt is verified that the kinetic model can give a satisfactory prediction of the experimental results under different conditions.

A reexamination of the RapreNO x process

The recently proposed RapreNO x process involves the injection of HNCO into combustion effluents in order to reduce their NO content via what was initially assumed to be a homogeneous gas-phase reaction. Although the process is thus similar to the Thermal DeNO x process it had the apparent advantage of operating at temperatures as low as 450°C, as contrasted with the 700°C minimum temperature of Thermal DeNOx. Further, the Thermal DeNOx process had the disadvantage that downstream of the reaction zone unreacted ammonia could react with SO 3 to form NH 4HSO 4 and cause fouling problems, a difficulty thart using HNCO would seem to avoid. In this reexamination of the RapreNO x process we show that the process involves three different modes of NO reduction. The first is catalytic; NO reductions at temperatures significantly below 700°C are found not to occur in the absence of catalytic surfaces. While noncatalytic NO reductions were found to occur at 700 °C in the presence of wet CO, NH 3 will also reduce NO at these conditions. Modeling calculations indicate that for this mode of reaction HNCO reduces NO via a complex chain reaction mechanism very similar to that involved in the Thermal DeNOx process. Within this reaction mechanism the interconversion of HNCO and NH 3 is sufficiently rapid that the amounts of HNCO and NH 3 left at the end of reaction are roughly independent of whether one starts with HNCO or NH 3. The modeling calculations also indicate the existence of a third mode of NO reduction by HNCO. In this third mode OH formed by thermal dissociation of H 2O reacts with HNCO to form the NCO radical, which reduces NO to N 2O. This third mode of NO reduction is predicted to occur at temperatures substantially higher than those used in the Thermal DeNOx process and it is thus potentially suitable for limiting NO x emissions from gas turbines, but the N 2O produced in this mode may be a significant disadvantage.