The Simulation of Biogas Combustion in Top Open Burner

The top open furnace is commonly used for the heating processes. The efficiency of the top open furnace is low due to the amount of heat wasted through the top open. This study was using ANSYS Fluent, a two-dimensional computational fluid dynamic (CFD) model that was used to build the top open furnace. Exhaust gas recirculation (EGR) was installed in the boiler in order to collect the exhaust gas and reuse it to dilute the oxygen supply. To check the combustion of low calorific gas (LCV) gas, which is biogas composed of 60% methane and 40% carbon dioxide by mass fraction, the computational study started with a standard combustion with EGR. Medium-sized mesh is used for the smoothing grid. This mesh uses inflation to enable the size of the cells and the stack element. The findings of the numerical sensitivity test show that the boundary conditions along the combustion chamber wall can affect the flame temperatures. The MILD regime was reached using biogas fuels when the right parameters were used.


INTRODUCTION
Today, fossil fuels are widely used to meet energy needs.By 2042, the world's energy demand is expected to be over 18 billion tons of oil equivalent, with fossil fuel combustion accounting for 80% of it.To meet increasing energy demands, combustion is expected to be the most important source of energy [1][2][3].These fuels have two significant flaws.The first is that fossil fuels are finite by nature and will likely run out soon.The second is that carbon-rich fuels release carbon dioxide during combustion, which has negative effects on the environment.The utilisation of clean and renewable energy sources, such as green energy, hydrogen energy, biofuel, or biogas, is therefore a research priority for scientists.Fuels made from biomass also play a significant role as energy carriers.Especially in rural regions, biogas or biohydrogen, a type of clean and renewable energy, can effectively replace conventional energy sources (fossil fuels, oil, etc.), which also have environmental drawbacks and deplete more quickly [4][5][6][7].
A national and industrial scale of biogas implementation has been accomplished, making it a promising renewable energy source [8].In order to meet the Sustainable Development Goals (SDGs), this work offers a preliminary evaluation of the function and contribution of biogas as a sustainable source of energy.It is stated that 12 out of the 17 SDGs have been demonstrated to directly affect and be involved with biogas.The capacity of biogas to boost renewable energy, lessen climate change, enhance waste management, and create jobs are its main contributions.
For meeting a portion of the energy requirement, biogas offers a very alluring solution.The proper operation of biogas systems can benefit consumers and communities in numerous ways, protecting resources and the environment [1].
Biogas produced from garbage, waste, and energy products will be significant in this area in the future.A flexible renewable energy source, biogas can also be used to fuel automobiles.It can be utilised to generate electricity and heat in place of fossil fuels.Biogas with a high methane content (biomethane) can take the place of natural gas as a raw material for the synthesis of chemicals and other compounds.Compared to other methods of producing bioenergy, biogas has a number of advantages.The method, which employs locally accessible resources to significantly lower greenhouse gas emissions compared to fossil fuels, is ranked as one of the most effective energies and environmentally benign technologies for producing bioenergy [9][10][11].
In 2007, European biogas energy production increased by more than 20% annually to 6 million tonnes of oil equivalents (Mtoe).Due to the rapid expansion of agricultural biogas plants on farms, Germany has surpassed all other countries in the world in terms of its production of biogas.On German farms, there were about 4,000 agricultural biogas production units operating by the end of 2008.Energy crops make up million tonnes of biomass that might be anaerobically digested annually within the agricultural sector of the European Union (EU) [12].The most crucial co-substrates in the EU are energy crops, which have the most potential.The useable biogas potential of organic wastes and energy crops in Germany is depicted in Figure 1 [13].If energy crops are grown on 2 million hectares (11% of agricultural area), they might produce more than 50% of the biogas that could be produced.More than 80% of the potential feedstocks originate in agriculture, including harvesting residues and animal dung [14,15].In this study, numerical modelling for biogas combustion in a furnace model was carried out using computational fluid dynamics (CFD).A two-dimensional CFD model and ANSYS Fluent software were used to construct the open-top furnace.There is a spike in interest in increasing energy efficiency as a result of the need to address environmental pollution (emissions) and energy sustainability (fuel depletion).Excellent thermal efficiency combustion technologies and biogas (renewable) fuels are long-term options.

METHODOLOGY
CFD is a method of numerically solving a fluid flow-related physical event using computational power after modelling it mathematically.The most popular technology for developing remedies for liquid-related or non-solid liquids is now computational fluid dynamics (CFD).In CFD analysis software, studies of water flow are conducted using physical characteristics including velocity, pressure, temperature, density, and viscosity [16,17].One method for designing and running the simulation experiment virtually without having to physically construct the model is CFD.Building a model and then repeating the procedure until the desired outcome is achieved is quite expensive.This technique can be carried out utilising commercial software and CFD modelling, which is significantly less expensive than making physical 2 models.Numerous engineering challenges, including those involving gas turbines, industrial furnaces, boilers, internal combustion engines, flameless combustor technology, and other engineering applications, have successfully been simulated using CFD [18,19].
To minimise errors, boundary conditions must be precisely stated.The choice of boundary conditions has a significant impact on the outcome of CFD simulations.An air and fuel supply pipe were used, and a velocity inlet boundary condition was used.The exhaust outlet on top of the burner was defined by an outlet vent boundary condition.There was a no-slip wall barrier.
The boundary condition for velocity at the combustion chamber wall was implemented using a typical wall function.The wall's temperature was set to 300 K, and the exhaust outlet boundary condition employed an exhaust fan with zero-gauge pressure.To verify that the furnace wall boundary conditions were equivalent to the experimental ones, they were placed at a constant temperature (300 K) [20][21][22].
Estimating the primary combustion qualities of biogas constituents based on their chemical composition is useful for understanding a variety of phenomena that occur during combustion processes and conducting comparative exchangeability analyses.This is accomplished through the use of numerical simulation and computation tools.Below is a balanced equation (1) for methane combustion: Biogas is a concoction of several gases.The volume parts xvi that are numerically equal to the molar parts xi are used to express the composition of biogas.Calculating the mixture M's molecular weight as below (Eq.2). ( where m is the mixture's weight in kilogrammes; n is the quantity of each component; xi is the component's molar fraction; and Mi is the component's molar weight. Calculation for the mixture's r universal gas constant is as below (Eq.3). ( where: Rm-molar gas constant; M -molar weight of the mixture.The mass part of individual gas components σi is calculated using equation (4) from xi value below. ( where: mi -weight of the component (kg); m -weight of the mixture (kg)

Heating Power of Biogas
The biogas heating power, Qn, based on 1 kg of the mixture is calculated: (5) The heating power of biogas Qn, based on 1 m 3 of mixture at basic conditions is calculated: where: σi -mass part of particular component; Qni -heating power of the component.

Density of Biogas
The density of biogas ρBG is calculated by the equation ( 7) of state for an ideal gas as below.(7) where: v -specific volume of the mixture, T -thermodynamic temperature.
In terms of relative density, the density of biogas is determined by dividing the gas density (BG) by the density of the surrounding air (a) under the following basic conditions (Eq.8) below.

Simulation for Geometry Design
The Design Modeller programme in ANSYS Workbench was used to draw the furnace.The combustion chamber seen in Figure 1 was initially created using a CFD simulation of an enclosed combustion chamber similar to that in [23,24].

Preliminary Model
To check the combustion of low calorific gas (LCV) gas, which is biogas composed of 60% methane and 40% carbon dioxide by mass fraction, the computational study started with a standard combustion with EGR.Model meshing is the most crucial step in the CFD process.The mesh is crucial, because it's known that the snare size affects how delicately the simulation results come out.The result of meshing is a grid of cells or primitives on which all of the fluid inflow equations need to be solved.The "cost of the simulations" is the sum of the processing time and data stores, and the size of the grid will have a considerable impact on both.The speed of the confluence and the delicateness of the outcome will both be greatly impacted by the grid.While narrow grids will provide a low speed of confluence with high delicacy, coarse grids (many cells) will produce a high speed of confluence but most likely low delicacy.Large numbers of cells, often several hundred thousand grid cells, are required to solve industrial CFD problems.Figure 2 depicts the simulation's meshing in use [23].In order to allow the stack element and the cells to size in a distinctive manner that is frequently only allowed at their boundaries, inflation must be utilised in this mesh.Close to noslip barriers, the normal gradient can be trapped to create an expanding layer.Only by using thin elements and few, large elements can this be accomplished.5 layers, a 20% growth rate, and a maximum thickness of 0.002 m.

RESULTS AND DISCUSSIONS
The MILD combustion technique can reduce pollutants while improving thermal efficiency.Although MILD combustion has had a lot of success, it still requires further in-depth investigation due to the necessity for open-ended furnaces.Using an open-ended furnace with an enclosure wall, the exhaust gas was collected and used in this study's Exhaust Gas Recirculation (EGR).To dilute the oxygen before it reacts with the fuel and raises the temperature of the reactant, the EGR pumps some of the exhaust gas back into the combustion chamber.This system is an open-ended furnace because some of the exhaust gas can flow out and be used as external EGR.Numerical modelling for MILD combustion using CFD was done in a furnace.The MILD furnace was drawn using the design modeller tool in ANSYS Workbench.A CFD simulation of a simple enclosed combustion chamber, like the one shown in Figure 1 above, served as the initial step in the construction of the combustion chamber.To test the burning of LCV gas (biogas), which is composed of 60% methane and 40% carbon dioxide, 65% methane and 35% carbon dioxide, and 70% methane and 30% carbon dioxide by mass fraction, the computational work began with regular combustion with EGR.Through a fuel delivery pipe with a 17 cm wide, the fuel reaches the combustion chamber at a speed of 0.003 m/s.Through a gas supply pipe downstream of the chamber, air was infused at 0.09 m/s.EGR functions by returning some exhaust gas to the combustion chamber.
The basic goal of EGR is to directly heat the mixture by heating the hot flue gas, which will dilute the oxygen in the combustion chamber.Numerical modelling was used to analyze the performance of a recently constructed combustion chamber.The CFD setups and setup are the same for the other furnace designs.As illustrated in Table 3 [25], the furnace configuration in use includes clearly defined inlets for the delivery of fuel as well as for boundary conditions.The flame temperatures are one of the principal discoveries and the primary sign that the MILD combustion regime was attained.The results of the temperature distribution simulation are shown in Figures 3, 4, and 5, respectively, using 60% CH4, 40% CO2, 65% CH4, and 70% CH4, 10 30% CO2.The narrow temperature range demonstrates that MILD combustion was accomplished for the synthetic air simulation with oxygen mass fractions ranging from 3% to 21%.For each air input, the air stream velocity was maintained at 0.09 m/s.The simulations were run for the situation of atmospheric air with a mass fraction of 10% oxygen.Figures 6, 7, and 8 demonstrate the magnitude of velocity.The air entering the engine is moving at the fastest speed, 0.09 metres per second, into the EGR pipe and the combustion chamber.The outcome demonstrates that the chamber temperature is considerably influenced by the exhaust opening while the air inlet velocity has little effect on it.For an air entrance velocity of 0.09 m/s, all maximum and average chamber temperatures as well as air mixing temperatures are practically similar [27].
In the temperature contour cases of Figures 4 and 5, when the biogas composition is 65% methane and 35% carbon dioxide, followed by 70% methane and 30% carbon dioxide, there is a very slight increase in temperature.Since different biogas compositions have been used, a slight increase in velocity can be detected.The greatest speed recorded in Figure 6

Figure 1 :
Figure 1: Basic enclosed combustion furnace chamber with top open.

Table 3 :
Typical data of furnace.

Table 4 :
Flame temperature and MILD condition.
is 0.092621 metres per second.The maximum velocity for Figures7 and 8is 0.092630 m/s and 0.092638 m/s, respectively.