Gaseous Emissions and Combustion Efficiency Analysis of Hydrogen- Diesel Dual Fuel Engine under Fuel-Lean Condition

Exhaust gas emissions from diesel engine combustion using alternative fuel may change in their quantities that can affect exhaust gas after-treatment devices and environmental ambient. This study presents theoretical analysis of combustion generated emissions and efficiency of hydrogen-diesel duel fuel in fuel-lean condition. A chemical equilibrium method by minimizing Gibbs free energy is employed to estimate exhaust gas products from diesel and hydrogen-diesel mode combustion. The combustion products, e.g., unburned hydrocarbons (CH4), hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO) are comparatively investigated, based upon similar specific energy input. Subsequently, the obtained combustible products (CH4, H2 and CO) are used to calculate combustion efficiency, based upon chemical energy left in waste exhaust gases. The main findings are associated with the reduction in CO2 corresponding to the increase in combustion efficiency in hydrogen-diesel combustion mode, depending on relative air-to-fuel ratios. Meanwhile, the CH4, H2 and CO contents in the flue gas increase in the operating conditions used.


INTRODUCTION
In present, diesel engines are applied in many works such as industrial, agriculture and transportation. Some advantages comparing to spark ignition (gasoline) engine are mainly to offer better fuel conversion efficiency. Moreover, diesel engine exhibits in fuel flexibility under various operating conditions, superior engine durability, higher thermal efficiency and lower fuel consumption.
However, the diesel engines also have a few disadvantages. Firstly, the exhaust gas consists of hydrogen (H 2 ), carbon monoxide (CO), unburned hydrocarbons (uHCs), excess oxygen (O 2 ) and carbon dioxide (CO 2 ). The latter is considered as a major source of greenhouse effect. Secondly, the lack of fossil fuel makes it expensive in the diesel fuel price. From these reasons, diesel engine vehicles, used in industry and transportation have involved environmental problems and need remediation.
An alternative energy such as hydrogen is becoming a choice among others due to its positive effects. It contains no carbon and has higher energy density when compared to fossil diesel as well as its global availability. Moreover, a trend of increasing use of hydrogen is prospectively observed towards the future.
Hydrogen can be found in chemical compounds, e.g., natural gas, coal, biomass, water, as it does not exist by itself on Earth (Williams, 1980). Nowadays, the most hydrogen production is by reforming natural gas process (Pohl, 1995). In addition, pure hydrogen from water electrolysis is produced by fossil fuel burning generated electricity (Marrero-Alfonso et al., 2007), hydroelectric, wind energy and solar energy (Khaselev and Turner, 1998). Among such renewable resources, biomass conversion is also a technique usually used for hydrogen production (Cortright et al., 2002). Padro and Lau (2000) listed some other advanced techniques for hydrogen generation.
A number of research works apply hydrogen for reducing the fossil fuels. Tomita et al. (2001) and Tsolakis et al. (2005) presented their experimental findings on exhaust emission effects from dual fuel diesel engine operation using H 2 . Kumar et al. (2003)  From the aforementioned reasons, there are yet some aspects of using hydrogen as dual fuel with fossil diesel in terms of theoretical analysis. Therefore, the main aim of this study is to analyze the combustion generated exhaust gas emissions and efficiency of hydrogen-diesel duel fuel mode.

MATERIALS AND METHODS
The combustion of hydrogen-diesel dual fuel was studied through the chemical equilibrium method. The concept of minimization of Gibbs free energy combined with the energy balance was used. The amount of added hydrogen was varied to study its effect on the combustion process. Moreover, the result of changing relative air/fuel ratio was also observed. In every case, the compositions of exhaust gas, which are CO 2 , CO, H 2 O, H 2 , N 2 , O 2 and HC represented by CH 4 and the combustion temperature were found. Then, the combustion efficiency was evaluated. However, the calculations in all cases were carried out based on similar total energy input.

Combustion Reaction of Hydrocarbon Fuel
For diesel fuel with the empirical formula CH y , the composition of the unburned and burned gas fractions can be theoretically calculated using a stoichiometric combustion equation, showing in Equation 1: In such circumstances the combustion can also be in fuel-rich or fuel-lean condition. Therefore, the relative air/fuel ratio (λ) defined in Equation 2 are used: where, (A/F) actual is air-to-fuel ratio on mass basis used in real combustion and (A/F) stoichiometric is chemically correct or theoretical proportion of air and fuel on mass basis (Chuepeng and Komintarachat, 2010).

Chemical Equilibrium Analysis
The where, λ is relative air/fuel ratio. From Equation 1, it can be observed that, for lean fuel combustion, the major gas species in exhaust gas are CO 2 , H 2 O, excess O 2 and N 2 . The amount of each gas species can be obtained by using mole balance concept. However, in the real combustion process, CO and H 2 can also be found in significant amount in the exhaust gas. Considering CO, H 2 and incomplete cracked hydrocarbon represented by CH 4 , the simple mole balance method cannot find the number of mole of each gas species. To evaluate the final product in complex chemical reaction, like combustion process, the chemical equilibrium method is often used. This method is based on the concept that at equilibrium state, the total Gibbs free energy of the system has minimum value. For the multi-reaction, single phase system, the total Gibbs free energy (G t ) can be expressed as Equation 4: where, g i and n i are the chemical potential and the number of mole of specie i, respectively. The problem is to find the set of which minimizes the total Gibbs free energy of system. In combustion system, the ideal gas assumption can be applied because of high temperature. Thus, the total Gibbs free energy for ideal gas system can be calculated by Equation 5: where, is the standard Gibbs free of formation of species i and R is the universal gas constant. To find the minimum value of G t , an optimization method, called Lagrange multipliers, is used. The mass balance of each chemical element is carried out and it is set as the constraint of this problem. The obtained final equation is as Equation 6: where, a ik is the number of atom of the k th element in a mole of the i th species. The values of ∏ k represent the Lagrange multipliers. The desired solution can be achieved be solving equation 6 with constraint equations. In this problem, the mass balance equations of chemical elements found in reactants were applied as constraint equations.
To predict the combustion temperature, the energy balance equation is formed as Equation 7: In Equation 7, the energy loss from combustion system is assumed as zero. r H and p H represent the enthalpies of reactants and products, respectively. The combustion temperature, T p is implicitly obtained from the enthalpy of product mixture. The step of calculation can be shown as in Fig. 1.

Combustion Efficiency
Generally, there are still combustible species left in exhaust gas i.e., CO, H 2 and unburned hydrocarbon. The higher amounts of these species reflect combustion inefficiency. The combustion efficiency (η c ) can be calculated using Equation 8: where, x i are the mass fractions of CO, H 2 and CH 4 (obtained from chemical equilibrium analysis), Q HVi are the lower heating values of each species and the subscripts a and f denote air and fuel, respectively.

Combustion Temperature
The temperature variation of the diesel-H 2 duel fuel combustion under fuel lean conditions is shown in Fig.  2. It has seen that the temperatures tend to reduce with relative air-to-fuel ratios. However, there is only a subtle increase in the temperature when hydrogen is added to the combustion at constant relative air-to-fuel ratio.

Exhaust Gas Emissions
The combustion generated exhaust gas variations, i.e., H 2 , CO, unburned hydrocarbon (CH 4 ) and CO 2 are shown in Fig. 3-6, respectively. When hydrogen added to the engine increases, the amounts of H 2 , CO and CH 4 in the exhaust gas are increased, depending on relative air-to-fuel ratios. Meanwhile, the CO 2 composition in the exhaust gas is reduced when the increasing hydrogen is added to the engine.

Combustion Efficiency
The combustion efficiencies on the basis of energy left in unburned combustible products were calculated and are compared in Fig. 7. It has seen that almost all the efficiencies of the combustion are subtle increased when hydrogen is added, depending on the relative airto-fuel ratios.

DISCUSSION
In Fig. 2 the combustion temperature strongly depends on the fuel composition in the mixture to be combusted. As the relative air-to-fuel ratios increase, more air is induced in to the combustion chamber, resulting in lesser fuel proportion for burning and thus, the reduced combustion temperature. Even the calculation was accomplished on the equivalent energy basis, the hydrogen added up to 10 molar percentage as a dual fuel Science Publications AJAS operation can generate the increase in temperature within 5 K as seen in the magnified section in Fig. 2.
In Fig. 3-5, at a constant relative air-to-fuel ratio, the hydrogen addition to the engine results in CO, H 2 and CH 4 left in the exhaust gas due to incomplete combustion even in global fuel lean condition. At lower relative air-to-fuel ratios in Fig. 3, the lesser amount of the air is induced to the engine while hydrogen is also added to the engine. This results in greater proportion of H 2 in the exhaust gas composition. This effect is also applied to Fig. 4 and 5 for the CO and CH 4 , respectively. Figure 6 shows, at a constant relative air-to-fuel ratio that the reduction of CO 2 when increasing in H 2 addition is as a result of lesser amount of diesel that was substituted by hydrogen is combusted. At lower relative air-to-fuel ratios, the lesser amount of the air is induced to the engine during hydrogen addition. This results in greater reduction proportion of CO 2 in the exhaust gas composition.
It is to note in Fig. 7 that the combustion efficiency change is apparently at lower relative air-to-fuel ratios while showing only subtle amount within 0.03% over the relative air-to-fuel ratios investigated.

CONCLUSION
The combustion of hydrogen-diesel dual fuel is analyzed using a chemical equilibrium model of Gibbs free energy minimization and energy balance to first determine combustion temperatures. The subsequent combustion temperatures and chemical elements are input to the solver to obtain gas-phase species i.e., CO 2 , CO, H 2 O, H 2 , N 2 , O 2 and CH 4 . Combustible species (CO, H 2 and CH 4 ) in the exhaust gases are combustion inefficiency. It is found that CO 2 was reduced while CO, H 2 and CH 4 increased. H 2 addition increases combustion efficiency in majority while maintaining the efficiency in very fuel-lean conditions.