Kinetic analysis of the role of selective NOx recirculation in reducing NOx emissions from a hydrogen engine
Introduction
The global consumption of marketed energy is projected to increase by 48% between 2012 and 2040 [1]. A major fraction of the energy is provided by fossil fuels, which are depleting fast due to their rapid consumption. Since the combustion of fossil fuels produces CO2 which is considered to be a major greenhouse gas, the researchers are focusing on renewable and cleaner fuels. Hydrogen, due to its various characteristics such as low cost per unit energy produced, high auto-ignition temperature, high burning velocity, and low ignition energy, has caught the researchers’ attention [2]. H2 is considered to be a clean fuel as its combustion generates only water and the formation of any carbon-based pollutants is minimal. However, because of the high temperatures generated within the engine, the thermal fixation of N2 can result in the formation of NOx.
Various modeling studies have been reported on the combustion of H2 and the associated NOx formation. Kinetic models consisting of various species and reactions are available in the literature [3], [4], [5], [6], [7], [8]. These kinetic models have been validated against experimental data from shock tubes, flow reactors, and/or laminar premixed flames. Additionally, some of the models have been used to study the combustion of H2 in internal combustion engines [9], [10]. The formation of NOx and the various mechanisms contributing to its formation have been studied by various researchers [11], [12], [13]. Some of the NOx formation and reduction studies have been specifically performed with H2 as the fuel [7], [14].
Various methods have been used to reduce the engine-out NOx emissions some of which include injection of water, charge dilution, throttling, and modification of operating parameters such as spark timing and fuel injection timing [15], [16], [17], [18], [19]. Lee et al. [20] showed that the variation of valve timing and lean boosting can be used to achieve low amounts of NOx. The reburning of NOx using a fuel has been studied as a potential method for NOx reduction [21], [22]. In order to elucidate the kinetics of the reburn chemistry, modeling as well as experimental studies have been performed [23], [24], [25]. Glarborg et al. [23] showed that NO catalyzes the removal of hydrogen atom under fuel-rich conditions and obtained a rate constant for the H + NO + N2 reaction. Exhaust gas recirculation (EGR) is another NOx reduction method which makes use of the combustion products for dilution and has been widely used for various fuels including gasoline and diesel. The combustion products from the previous engine cycle are recycled back and mixed with the incoming fresh fuel-air mixture in the present cycle. The recycled gases not only decrease the relative in-cylinder oxygen concentration but also increase the specific heat of the intake gaseous mixture [26]. This reduces the in-cylinder temperature which helps in the reduction of thermal NOx.
Many researchers have used EGR for the reduction of NOx emissions [26], [27], [28], [29], [30], [31], [32]. However, excessive use of EGR can result in a reduction in the power obtained. Selective NOx recirculation (SNR) is one of the alternative methods where NOx is selectively recirculated into the engine at concentrations higher than those with the conventional EGR. A sorbent material is used to adsorb the NOx generated, which is then desorbed and recycled back to the engine. Sorbent technologies Inc. developed a sorbent bed made of a special type of sulfur-tolerant carbon granule material [33]. The bed was used downstream of a Cummins engine and experiments were performed to study the amount of NOx adsorbed at various temperatures. Monticelli et al. [34] studied the potential of Y-Zeolites for NOx adsorption and desorption using a synthetic exhaust gas with the temperature and composition representative of a lean-burn internal combustion engine. They carried out NOx adsorption at low temperatures followed by its desorption at higher temperatures. It was observed that at temperatures above 100 °C, NO2 can be selectively desorbed from the synthetic exhaust gas using the ion-exchanged Y-zeolites. Clark et al. [35] studied the effect of recirculated NOx on its decomposition within the engine cylinder for lean-burn natural gas engines. The NOx decomposition rate was shown to be affected by the air-fuel ratio, injected NO quantity, EGR amount, and the engine operating points. They developed a homogenous batch reactor model to study the effect of operating conditions on the reduction in NOx. NOx decomposition rates were predicted to be low for lean conditions and high for rich conditions. However, a complete engine model was not used and calculations were performed for fixed temperatures wherein the predictions were done for time periods obtained from the duration of normal combustion. Krutzsch et al. [36] studied the combustion as well as the NO decomposition mechanism during SNR application by experimental as well as theoretical techniques for two diesel engines and one gasoline engine. It was reported that the NO conversion efficiencies of up to 90% were achievable for a gasoline engine under rich or stoichiometric conditions whereas less than 20% conversion was obtained under lean conditions. The reaction paths by which the decomposition of NO occurs were also identified and analyzed by performing numerical simulations. Tissera et al. [37] studied the performance of SNR for lean-burn natural gas-based engines. It was reported that the engine load, amount of NO injected and the air-fuel ratio significantly affected the NO conversion rate while the engine speed did not affect the NO conversion rate if the amount of injected NO was kept constant.
Most of the studies with either SNR or injection of NOx justify the reduction in the net amount of NOx by the reaction of NOx with the fuel, e.g., it was reported by Clark et al. [35] that the additional fuel injected creates a secondary re-burn zone, in which the NOx formed in the main combustion zone gets converted to nitrogen. Moreover, it was assumed in their modeling studies that NO did not react once the conversion of the fuel (CH4) reached 98%. They also noted that NO decomposition in a gasoline engine cannot be directly extended to a natural gas engine because of the low reactivity of natural gas. Similarly, Krutzsch et al. [36] proposed that the reduction in NOx is primarily attributed to the chemical reaction between NO and ketenyl radicals, given by . Glarborg et al. [24] also studied the reduction of NO with C1 and C2 hydrocarbons in a flow reactor and their modeling studies showed that NO is predominantly reduced by HCCO + NO reaction when using natural gas or C2 hydrocarbons as the reburn fuel. However, we have shown that even if NOx does not interact with the fuel, a net reduction in NOx can still be achieved. Moreover, to the best of our knowledge, the injection of NOx into a H2 engine and its impact on the net NOx emissions has not been reported elsewhere.
This work focuses on elucidating the reasons for the net reduction in NOx upon injection of NOx during the intake stroke of a H2 engine and finding the conditions which favor the net reduction. A model is developed based on the coupling of basic conservation equations and the NOx formation kinetics and is used to predict the effect of injected NO on the emission characteristics of the engine. The temporal profiles of in-cylinder pressure, temperature, and H2O concentration are analyzed. It is shown that for the conditions of our study, these variables remain nearly unchanged for various amounts of injected NO and hence are not responsible for the reduction in the amount of NO formed during the power stroke. It is shown that even if the chemistry of NOx reactions with the fuel is not considered, the amount of ‘fresh’ NO generated decreases with an increase in the amount of NO injected. It is also shown that a reduction in NOx generation takes place at high equivalence ratios () and significantly lower reduction is predicted at lower equivalence ratios. These observations are related to the attainment of near-equilibrium conditions, which occur only at high equivalence ratios.
Section snippets
Description of the setup
The engine simulated in the present work is a single-cylinder spark-ignition four-stroke engine with a displacement volume of 394 cc [38]. The engine cylinder is equipped with annular fins for the effective heat transfer and cooling of the gaseous mixture inside the engine. Pure hydrogen is used as a fuel and atmospheric air is used as a coolant. A fuel injector designed for hydrogen is used for the fuel injection. The details of the engine including the various geometric parameters, heat
Results and discussion
Using the parameters given in Table 1, the model equations were solved to obtain the variation of in-cylinder variables such as concentration, temperature, and pressure with respect to time. The simulations were performed for various equivalence ratios between 0.4 and 0.9 at a fixed engine speed of 1200 rpm. Equivalence ratio is defined as the ratio of actual fuel-air ratio and the stoichiometric fuel-air ratio, such that a value less than one indicates that the amount of air is more than the
Conclusions
A model based on fundamental conservation equations is used to study the effect of NO injected into a spark-ignited hydrogen engine on its further formation. The simulations are performed for varying amounts of NO injected during the intake stroke for the equivalence ratios in the range of 0.4–0.9. For an equivalence ratio of 0.65, an increase in the amount of NO injected during the intake stroke results in a reduction in the net amount of NO generated during the power stroke. However, the
Acknowledgements
This work was supported by the Ministry of New and Renewable Energy (MNRE) (Sanction Order No. 103/242/2015-NT).
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Present address: JSW Centre, Bandra Kurla Complex, Bandra East, Mumbai, Maharashtra 400051, India.