Simulation of Inner-Engine NO

Abstract

Hydrogen is an ideal engine fuel. Pure hydrogen engines do not produce CO and HC emissions but face the high NOx emission problem. Inner-engine control and outer-engine control are two ways to decrease the NO_x emission. Outer-engine control mainly reduce NO_x emission through selective catalytic reduction （SCR）， which has been well studied. However， there are few studies on NO_x emission control of pure hydrogen engines through inner-engine control. In this paper， the closed homogeneous reactor （CHR） in Chemkin Pro was used to simulate the main inner-engine NO_x emission control in pure hydrogen engines. The results show that single exhaust gas recirculation （EGR） decreases NO_x emission by 45.3% at an EGR ratio of 20%， indicating that the NO_x emission is not significantly reduced. However， EGR plus lean-burn decreases NO_x emission by 96.31% at a λ of 1.4 and an EGR ratio of 20%， achieving ultra-low NO_x emission of pure hydrogen engines. Compared with single EGR and EGR plus lean-burn， SNCR are better for NO_x emission control. A NH₃ ratio of only 10% can decrease NO_x emission by 96.32% on pure hydrogen engines， while a NH₃ ratio of 15% can achieve zero NO_x emission on pure hydrogen engines without a large λ value and EGR ratio. However， it is necessary to accurately control the NH₃ ratio in the cylinder， otherwise it is easy to produce residual NH₃ which can pollute the environment.

Keywords

pure hydrogen engine; NO_x emissions; lean-burn; selective non-catalytic reduction (SNCR); exhaust gas recirculation (EGR)

The massive consumption of fossil energy has brought severe pollution problem^［1-2］. Seeking clean and efficient renewable energy could solve the pollution problem and alleviate energy crisis^［3-4］. Hydrogen is a kind of renewable fuel whose only combustion product is water， which will not cause any damage to the environment^［5-6］. Pure hydrogen on engines can almost completely remove CO， CO₂， unburned HC emissions， and output higher power than pure gasoline. But NO_x emission is the main disadvantage of pure hydrogen engine^［7］. In order to solve the problem of NO_x emission， the exhaust gas recirculation （EGR） technology， the EGR plus lean-burn technology， the selective non-catalytic reduction （SNCR） technology， and the selective catalytic reduction （SCR） technology， are currently the main technical means to reduce NO_x emission^［8-10］. Outer-engine control mainly reduces NO_x emission through SCR， which has been well studied. The EGR technology， the EGR plus lean-burn technology， and the SNCR technology are the main means of inner-engine NO_x emission control at present. However， there are few studies on NO_x emission control of pure hydrogen engines through inner-engine control. Therefore， this paper simulated and compared three main inner-engine NO_x emission control means of pure hydrogen engines， providing theoretical basis for the choice of technical means on inner-engine to reduce NO_x emission of pure hydrogen engines.

1 Simulation setup and procedure

The simulation software used in this simulation was Chemkin Pro， and the model was closed homogeneous reactor （CHR）. The H₂ combustion mechanism used in this simulation was the detailed mechanism of hydrogen combustion， the NO_x generation mechanism used the improved version of Zeldovitch-mechanism， and the NO_x desorption mechanism is provided by Golovitchev^［11-13］. All the chemical reaction mechanisms in the simulation were verified by extensive experiments， and the experimental results could match the simulation accurately. This simulation simulated three inner-engine NO_x emission control technical means， EGR， lean-burn plus EGR and SNCR. In this experiment， five λ values （1， 1.1， 1.2， 1.3， 1.4）， five EGR ratios （0， 5%， 10%， 15%， 20%） and five NH₃ ratios （0， 5%， 10%， 15%， 20%） were set. The EGR ratio is defined in Equation （1）， and the NH₃ ratio is defined in Equation （2）. Tab.1 shows the initial conditions for the closed homogeneous reactor. V_x represented the volume of x in the following equations.

Tab.1  Initial conditions for closed homogeneous reactor

Parameters	Values
Simulation time/s	0.04
Initial temperature/K	1000
Initial pressure/MPa	0.1
Fuel mixture （.vol）	$\phi \left({\mathrm{H}}_2\right)=100\mathrm{\%}$
Oxidizer mixture （.vol）	$\phi \left({\mathrm{O}}_2\right)=21\mathrm{\%}$； $\phi \left({\mathrm{N}}_2\right)=79\mathrm{\%}$
Added species	H2O； N2； NH3
Excess air ratio	1， 1.1， 1.2， 1.3， 1.4

$$\mathrm{E}\mathrm{G}\mathrm{R}ratio=\frac{V_{\mathrm{E}\mathrm{G}\mathrm{R}}}{V_{\mathrm{E}\mathrm{G}\mathrm{R}}+V_{\mathrm{A}\mathrm{I}\mathrm{R}}+V_{{\mathrm{H}}_{\mathrm{2}}}}$$

（1）

$$\mathrm{N}{\mathrm{H}}_{\mathrm{3}}ratio=\frac{V_{\mathrm{N}{\mathrm{H}}_{\mathrm{3}}}}{V_{\mathrm{N}{\mathrm{H}}_{\mathrm{3}}}+V_{\mathrm{A}\mathrm{I}\mathrm{R}}+V_{{\mathrm{H}}_{\mathrm{2}}}}$$

（2）

2 Results and discussion

2.1　Effect of EGR on NO_x emission

Fig.1 shows the effect of EGR on total NO_x production rate and NO_x emission. As can be seen from Fig.1， with the increase of the EGR ratio， the peak value of the total NO_x production rate and the total NO_x emission decrease. When the EGR ratio increases from 0% to 20%， the peak value of the total NO_x production rate decreases by 38.52%， 63.93%， 80.33%， and 89.34%， respectively. The total NO_x emissions decrease by 11.42%， 22.82%， 34.18%， and 45.30%. The NO_x formation conditions are high temperature， oxygen enrichment， and high temperature duration. On the one hand， the increase of the EGR ratio reduces the temperature in the cylinder， and at the same time， the increase of the EGR rate also dilutes the concentration of N₂ and O₂， and reduces the NO_x generation rate. Therefore， the use of the EGR technology can effectively reduce NO_x emission generated during hydrogen combustion.

  

  

Fig.1  Effect of EGR on total NO_x production rate and NO_x emission

2.2　Effect of EGR plus lean-burn on NO_x emission

The EGR ratio was kept at 20% and the excess air ratio was increased from 1 to 1.4 to observe the effect of EGR plus lean-burn on NO_x emission. Fig.2 shows the effect of EGR plus lean-burn on total NO_x production rate and NO_x emission. It can be observed that the peak value of the total NO_x production rate decreased continuously with the increase of λ， but the NO_x emission increases first and then decreases when λ increases. At a λ of 1.1， the NO_x emission is the highest. Although lean-burn could lower the combustion temperature， a larger λ leads to an increase of O₂ and creates favorable conditions for the oxygen enrichment， which is conducive to the generation of NO_x. The increase in oxygen results in the fact that the combination of an EGR ratio of 20% with a λ of less than 1.3 cannot effectively reduce the NO_x emission. The NO_x emission can be reduced by 96.31% when λ is 1.4 and the EGR ratio is 20%.

  

  

Fig.2  Effect of EGR plus lean-burn on total NO_x production rate and NO_x emission

2.3　Effect of NH₃ on NO_x emission

Fig.3 shows the effect of NH₃ on total NO_x production rate and NO_x emission. It can be observed that both the positive or negative peak value of the total NO_x production rate decrease continuously with the NH₃ ratio increasing， but the NO_x emission decreases with the NH₃ ratio increasing.

  

  

Fig.3  Effect of NH₃ on total NO_x production rate and NO_x emission

In addition， a larger NH₃ ratio would lead to a later peak value of the total NO_x production rate. When the NH₃ ratio is 10%， the NO_x emission decreases by 96.32% than without NH₃ addition while when the NH₃ ratio is large than 15%， pure hydrogen engines could achieve no NO_x emission. However， a large NH₃ ratio is not recommended， because a large amount of residual NH₃ will overflow， causing serious pollution to the environment.

3 Conclusions

This paper used the CHR module in CHEMKIN Pro to simulate the three main means of inner-engine NO_x emission control on pure hydrogen engines， providing theoretical guidance for pure hydrogen engines to choose NO_x purification means. The main results are as follows：

（1） EGR reduces the NO_x emission generated during hydrogen combustion. The NO_x emission is reduced only by 45.3% when the EGR ratio is 20%. Therefore，to achieve ultra-low NO_x emission using pure hydrogen engines by adopting the EGR technology， a larger EGR ratio or EGR in combination with external purification should be used.

（2） Compared with single EGR， EGR plus lean-burn is more efficient in reducing the NO_x emission by using pure hydrogen engines. Pure hydrogen engines need a large EGR ratio and λ value. The NO_x emission can be reduced by 96.31% when λ is 1.4 and the EGR ratio is 20%， achieving ultra-low NO_x emission of pure hydrogen engines. To control NO_x emissions by using pure hydrogen engine and EGR plus lean-burn， the engine condition monitoring should be strengthened to avoid misfire because of the large EGR ratio and λ value.

（3） Compared with EGR， and EGR plus lean-burn， SNCR is better in inner-engine NO_x emission control， because it requires only a NH₃ ratio of 10% to achieve ultra-low NO_x emissions on pure hydrogen engines. A NH₃ ratio a 15% can make pure hydrogen engines achieve zero NO_x emission. SNCR avoids pure hydrogen engines having to operate under a large λ and EGR ratio to decrease NO_x emission， avoiding the power loss of pure hydrogen engines， making combustion more stable. In the control of NO_x emission of pure hydrogen engines， SNCR should be the main technical means to be adopted， and zero emission can be achieved when the proportion of NH₃ is controlled reasonably.