NUMERICAL ANALYSIS OF NOx FORMATION IN CO2 DILUTED BIOGAS PREMIXED COMBUSTION

A further investigation of premixed biogas combustion towards the NOx formation is presented in this study. The purpose of the simulation is to determine the addition of CO2 in biogas fuel to the combustion behavior of premixed biogas on NOx formation, and to determine the occurrence of NOx in the pre-mixed biogas combustion. In this study, the Counterflow Premixed Flame class is used where this class is based on the One Dim class which is the basis for simulations with a 1-dimensional domain. The Counterflow Premixed Flame class uses an axisymmetric stagnation flow domain which has been written based on the equations. Cantera uses Newton’s method to solve them. Completion is carried out in two stages. The first stage is to solve the solution using the equilibrium at each z coordinate point that has been determined. Many estimation starting points are determined from the start of the program. The second stage is the recalculation process at each point and then subdivided to get a smoother solution. The premixed excess CO2 biogas fuel and air combustion analyzed using a 1-dimensional numerical study. The diluted CO2 mass fraction ranged between 0–40 %. The CH4/CO2/air volume flow rate was maintained in ±L/min. The analysis implements the 1-D Counter Flow approach. Two counterflow nozzles were 20mm in diameter and the flame stagnation point at 10 mm. The results show that NOx mass fraction formed only on a fuel-lean mixture of CH4/CO2/air and its values decreased along with CO2 added. The addition of CO2 could reduce the NO species mass fraction down to 18 %, and NO2 reduction down to 7 %. This is mainly caused by a decreasing heat release rate of NO+N↔N2+O, N+O2↔NO+O, N+OH↔NO+H, and N+CO2↔NO+CO reactions. The N+CO2↔NO+CO reaction increased as CO2 was added but its values were not as much as the decline of three other reactions.


Introduction
Human consumptive behavior towards energy use has increased in recent decades. This is motivated by technological developments and of course an increase in the number of human populations, however, the large level of energy consumption is not matched by the amount of supply of energy sources that will eventually run out. Therefore an alternative energy source is needed as a substitute for non-renewable energy sources [1][2][3].
One of the alternative energy sources currently being developed is biogas [2]. Biogas is defined as a gas released by organic materials (such as livestock manure, human dung, straw, husks, and leaves of sorted vegetables) which are fermented or undergo a methanation process [4]. Biogas is a gas mixture produced by methanogenic bacteria that occurs in materials that can decompose naturally under anaerobic conditions [5,6].
Biogas is a gas produced by anaerobic activity or fermentation of organic materials which can be degraded naturally. Materials for making biogas can be found easily in the environment around us. Organic waste is the main material for making biogas. Substrates containing high lignin, cellulose, and hemicellulose require special handling before the anaerobic digestion process. So that from the formation of biogas et two benefits at the same time, namely obtaining biogas as fuel and reducing the volume of waste optimally [4,7].
The composition of the gas produced by the formation of biogas varies depending on the source of the material used. The amount of methane gas content in the anaerobic digestion process Energy is one of the important criteria as a benchmark for selecting the main material for making biogas. The higher the content of methane gas formed from the anaerobic digestion process, the better the potential for biogas to be used as fuel [8,9]. The content of biogas itself includes CH 4 (50 %-70 %), CO 2 (30 %-40 %), H 2 S (0 %-10 %), H 2 O (0.3 %), N 2 (1 %-2 %), H 2 (5 %-10 %) and other gases [10]. The component that has the potential as fuel is CH 4 because the calorific value contained in it is quite large. The largest content in biogas after CH 4 is CO 2 [11]. The existence of CO 2 itself is a detrimental thing. This is due to its characteristics which can reduce the calorific value contained in the biogas. The high specific heating value of CO 2 gas will make some of the heat from combustion absorbed by CO 2 gas. The dissolved CO 2 content in the fuel will decrease the rate of the combustion reaction which results in the biogas combustion process requiring a longer time [12].
Counterflow burner installation is an installation that is very suitable for studying combustion characteristics. This is because this installation produces a simple flame in which the flame front and the shadow area of the flame are visible [13]. Counterflow burners use two flows in a pipe in opposite directions. If the two flows have the same flow rate, it will form an almost flat flame [14].
Numerical simulation is one solution to solve the combustion problem. With the increasing number of previous experimental studies that can be used as a reference, making today's numerical simulations more robust and the results of the calculations more accurate. The development of technology in the field of computerization also makes numerical simulation-based research easier. In general, numerical simulation can provide analysis results that can be considered in a relatively fast time and at a much more affordable cost when compared to experimental based research.

Materials and methods
The anaerobic process in conventional biogas reactor produces CO 2 gas which affects the biogas combustion characteristic. This study aims to investigate the CO 2 roles in NO x production of conventional biogas especially in premixed combustion. In order to analyze the combustion characteristic, this study focusing in the chemical reaction chain that contribute to NO x formation. Thus, the 1D analysis in counterflow reacting flow are used in this study. The study focuses in the detailed investigation based on the flame characteristic especially in chemical reaction analysis. However, this study only limited to a steady 1dimensional analysis.
The 1D analysis is only performed on one side of the opposite flow. There are two coordinates, namely the r (radial) coordinate and the z (axial) coordinate. Where the time of the calculation engineering was carried out so that all the existing parameters were only affected by the z coordinate. Each calculation will produce the values of combustion parameters such as species mass fraction (x), temperature, axial velocity (u), and radial velocity (v). These parameters are calculated at each node along the z coordinate direction flow path (starting from the nozzle to the point of stagnation. The number of nodes in each calculation varies depending on the number of reactions that occur from the mixture provided.
Before doing numerical calculations using Cantera 2.2.1, several calculations are required. The gas specifications used 1in the calculation is from the GRI-Mech 3.0 database. By using predetermined conditions, namely a pressure of 1 atm and inlet temperature of 300 °K, gas specifications are shown in Table 1.

Energy
In this study, the CounterflowPremixedFlame class is used where this class is based on the OneDim class which is the basis for simulations with a 1-Dimensional domain. The Counter-flowPremixedFlame class uses an axisymmetric stagnation flow domain which has been written based on the equations.

Fig. 1. 1-Dimensional premixed flame nodes
Cantera uses Newton's method to solve them. Completion is carried out in two stages. The first stage is to solve the solution using the equilibrium at each z coordinate point that has been determined. Many estimation starting points are determined from the start of the program. The second stage is the recalculation process at each point and then subdivided to get a smoother solution.
The data collection process is carried out in several stages.
To simulate the combustion of a counterflow premixed flame in all variations, the following steps are carried out: a) the process of making 1 main line of coding for the counterflow premixed flame simulation. The coding master file is saved in .py form to be operated in the IDLE Python 3.4.3 window; b) calculation of input data for the entire specified variation; c) in various types of reactants CH 4 /CO 2 /air. Specified oxidation; d) determined xCO 2 ; e) enter the parameters of reactant composition, mass flux, and combustion conditions according to the calculation data (b) into the variables that have been prepared; f) enter the numerical calculation criteria parameter i.e.: -relative tolerance for steady conditions = 1.0×10 -7 ; -absolute tolerance of steady-state = 1.0×10 -13 ; -relative tolerance of the stepping process time = 1.0×10 -7 ; -absolute tolerance of time stepping process = 1.0×10 -11 ; -ratio = 3, slope = 0.1, curve = 0.2 and prune = 0.02; g) enter the name of the .csv file to be used as the container for the calculation results; h) the process of running the program in the Python Shell window. If there is convergence in the iteration process, the procedure (g) is repeated by changing the absolute tolerance value and the relatively steady-state = 1.0×10 -20 . If there is still an error, then a round of 3 digits behind the comma is carried out on the mass flux value in and out of the flow stage (f). If there are no errors, the procedure continues to step (k); i) the calculation operation was successfull, the calculation result data has been saved in the step (h) .csv file. The output in the Python Shell window is stored in a .txt file with the same file name as the .csv file; j) steps (f)-(k) were repeated for variations of x CO2 10 %, 20 %, 30 % and 40 %; k) steps (e)-(k) are repeated for another variation of x Oxidators ; l) steps (d)-(m) are repeated for various types of CH 4 /CO 2 /O 2 reactants; m) the data obtained from each program run are sorted based on the data from the dependent variable that is sought and graphed for further analysis. Energy Formulation of the problem: 1. How is the effect of adding CO 2 to biogas fuel on the combustion behavior of pre-mixed biogas on NO x formation? 2. How does NO x occur in pre-mixed biogas combustion? Fig. 2 shown that the temperature decrease caused by the addition of CO 2 is greater at the equivalent ratio > 1. This means that the effect of CO 2 is more significant on combustion with a rich fuel mixture. The smallest temperature difference is 10,698 °K is obtained at the equivalent ratio of 0.5, while the largest difference is 28.64°K is obtained at the equivalent ratio of 1.68.

Fig. 2. The temperature difference in each of equivalence ratio
It can be seen that the temperature decrease caused by the addition of CO 2 is greater at the equivalent ratio > 1. This means that the effect of CO 2 is more significant on combustion with a rich fuel mixture.
In fuel-lean mixture combustion, the effect of adding CO 2 is not too significant due to the abundance of oxidizing species in the reactant mixture. When observed from a collision theory point of view, this makes the possibility of CH 4 molecules to collide with O 2 very large. So that the role of CO 2 as a diluent that inhibits or reduces the number of successful collisions between CH 4 and O 2 is not too large. As the number of oxidizers decreases at a greater equivalent ratio, the presence of CO 2 on the reactant side can reduce the number of collisions. With the presence of N 2 species in the CH 4 /CO 2 /air reactant, the possible number of collisions between CH 4 and O 2 . CO 2 influence towards the formation of harmful species NO x (NO 2 and NO) has been reviewed on the overall equivalent ratio variations. The data taken was the peak mass fraction of species of NO and NO 2 along with z coordinates. The results can be seen in Fig. 3.   Fig. 3. NO mass fraction in each equivalence ratio NO species can be formed when combustion reached high temperature and the unburned O species for pairs are presented. As shown in Fig. 2, the highest mass fraction of NO species was 3,059×10 -3 gained at 0.85 equivalent ratio variations with the additions of CO 2 0 %. In rich-fuel mixture regions, there is no species formation of NO presented. This is due to the lack of O species supply.

Energy
The O 2 species was completely reacted with the fuel species at nearly the stoichiometric. The decline of NO mass fraction in the lean fuel mixture was because of some reaction that converts NO into species NO 2 . This can be seen in Fig. 3 that on each additional CO 2 impairment the largest decrement of NO mass fraction occurred at an equivalent ratio of 0.85. A further analysis was conducted at this equivalence ratio by analyzing the heat release rate of some reaction that produces NO species. Fig. 4, a shows the heat release rate from the reaction of NO+N↔N 2 +O at each of the z coordinates. It is seen that the reaction NO+N+O↔N 2 has the largest heat release rate of 2026.3 W/m 3 with the addition of CO 2 0 %. The heat release rate had a positive value indicates the presence of heat absorption due to the dissociation of N 2 . Unlike the three other reactions, the reaction NO+N+O↔N 2 takes place almost in every z coordinates. There is a gap in the peak value of the reaction about 234.34 W/m 3 between 0-40 % CO 2 . This reaction has a high activation energy of about 318 kJ/mole, due to the triple bond of the N 2 molecule [15]. So along with the decreasing temperature due to the addition of CO 2 , the amount of energy which can be used for this reaction progresses becomes less.

Energy
The reaction of N+O 2 ↔NO+O and N+OH reaction↔NO+H have the same tendency, both reactions showed decreasing values in any additional CO 2 . These reactions are releasing heat that was characterized by the negative value of the heat release rate. The heat release rate of the reaction of N+O 2 ↔NO+O worth -10 331 W/m 3 , whereas the N+OH↔NO+H reaction worth -12 637 W/m 3 . The differences between the heat release rate without the addition of CO 2 and with the addition of CO 2 40 % on the reaction of N+O 2 ↔NO+O gained about 1489.5 W/m 3 , while the N+OH↔NO+H reaction was 2223.5 W/m 3 . Although it has a large amount, the occurrence of both reactions in the overall range of z coordinates was quite narrow.
The reaction of N+CO 2 ↔NO+CO had the value of the heat release rate of -463.92 W/m 3 highest at the addition of 40 % CO 2 . There was an increase in the heat release rate of this reaction when the amount of CO 2 added was 40 % that generated about 100.6 W/m 3 indifference. Unlike the three previous reactions, the heat release rate of N+CO 2 ↔NO+CO getting greater along with the increase of CO 2 . This happened because of CO 2 species is one of the reactant species required for this reaction to take place. Among those four reactions, three of these reactions decreased in any addition of CO 2 . Only the N+CO 2 ↔NO+CO reaction is increased with the addition of CO 2 . Even so, the amount of increase gained by the reaction N+CO 2 ↔NO+CO was not very significant when compared with the decline of three other reactions. So from this analysis it can be concluded that the addition of CO2 will reduce the formation of NO species which can be identified by decreasing the rate of heat release in several reactions that form NO species, as shown in Fig. 5.   Fig. 5. NO 2 mass fraction in each equivalence ratio As seen in Fig. 5 NO 2 species formed when the equivalent ratio was less than 1. Although the highest temperature gained near stoichiometric, the fewer species NO 2 is formed due to the depleted availability of O 2 species because the species has already reacted with fuel. In the fuel-rich mixture zone, there was no formation of NO 2 . The highest NO 2 mass fraction was 3.07×10 -6 at an equivalent ratio of 0.62 where the maximum temperature at this point was 1702.2 °K. At the equivalent ratio of 0.56 and 0.5 there was a decline in NO 2 mass fraction, this is due to the lower temperature gained at those equivalence ratio that was about 1588.6 °K and 1486 °K. The following reactions could form NO 2 species: To determine the effect of CO 2 on this reaction, Fig. 6 shows a further investigation of the equivalent ratio of 0.62. Analysis was carried out on each addition of CO 2 .
As shown in Fig. 6, NO+HO 2 ↔NO 2 +OH reaction was releasing the heat in which the highest heat release rate was -3605.09 W/m 3 at the variation without the addition of CO 2 . Along with the addition of CO 2 , the heat release rate continues to decrease until it reached -2509 W/m 3 at the addition of 40 % CO 2 . From the reaction's equation, it can be seen that the formation of NO 2 species is highly dependent on the availability of NO species. So the reaction NO+N+O↔N 2 outline a significant role in the formation of a whole species of harmful NO x . Energy Fig. 6. The heat release rate of NO+HO 2 ↔NO 2 +OH reaction at the equivalence ratio of 0.62 in each point From the results of this study the NO x species formation are occurs varies in the stoichiometric composition. In the nearly stoichiometric condition (equivalent ratio = 0.85), the highest NO mass fraction gained 3,0598×10 -3 without the CO 2 addition. Meanwhile the 40 % CO 2 dilution could reduce the NO mass fraction down to 2,5235×10 -3 . The NO 2 species highest formation gained in equivalence ratio of 0.25 with the NO 2 mass fraction of 3.07×10 -6 without the CO 2 . By adding 40 % CO 2 the NO 2 mass fraction decreases to 2.84×10 -6 . The CO 2 dilution was less significant in other equivalence ratio.
Some further investigation still needs to be conducted especially on the NO x formation at the specific time. By implementing transient analysis, the evolution of the NO x formation can be investigated thoroughly. In order to achieve the ideal premixed biogas combustion with less dangerous gas species production and enhance the conventional biogas premixed combustion efficiency.

Conclusion
1. The decreasing temperature caused by CO 2 addition that more significant on the fuel-rich side. CO 2 does not only decrease the temperature by absorbing heat but also has the potential to make changes in its chemical reactions path. CO 2 dilution could decrease the heat release rate of several reaction that partaking the role in NO and NO 2 species formation.
2. NO x mass fraction formed only on a fuel-lean mixture of CH 4 /CO 2 /air and its values decreased along with CO 2 added. The addition of CO 2 could reduce the NO species mass fraction down to 18 %, and NO 2 reduction down to 7 %. This is caused by a decreasing heat release rate of NO+N↔N 2 +O, N+O 2 ↔NO+O, N+OH↔NO+H, and N+CO 2 ↔NO+CO reactions. The N+CO 2 ↔NO+CO reaction increased as CO 2 was added but its values were not as much as the decline of three other reactions.