Improved combustion stability of biogas at different CO2 concentrations using inhomogeneous partially premixed stratified flames

0 Introduction

With fossil fuels increasing in exhaust emissions [1],countries worldwide are pursuing development of renewable fuels that can replace fossil fuels [2].The early replacement of fossil fuels is conducive to solving the harmful emissions caused by fossil fuel combustion and the adverse effects of global warming as soon as possible [3,4].The selection of alternative fuels depends on the availability of resources,environmental impact,and applicability to replacement of fossil fuels.In addition to renewable energy sources that can be directly used,such as wind power and photovoltaic resources,biogas produced from organic wastes,such as manure and straw,is a type of renewable carbon-containing biomass gas with low pollution,wide distribution,and high comprehensive benefits.Biogas consists of 50%-80%CH4,20%-40% CO2,0-5% N2,less than 1% H2,less than 0.4% O2 and 0.1%-3% H2S,etc.After pollutants are removed by chemical adsorption,biological pretreatment,or other processes,they can be used as renewable energy sources [5].The utilization of biogas includes a gas supply by producing hydrogen and methane,heat supply by biogas combustion,and electric power supply through thermoelectric units.Organic wastes,such as agricultural waste,can be effectively treated in the process of biogas utilization.Biogas combustion emits fewer greenhouse gases.In addition,the biogas residue and slurry obtained as additional products can also be used as biogas fertilizer to feed back to agricultural production.Therefore,it is considered an environmentally friendly renewable fuel with great development potential.This is conducive to the development of renewable energy sources and low-carbon economies.Recently,the use of biogas has rapidly increased worldwide.Based on an analysis report on the scale and share of the biogas market [6],the development speed of biogas worldwide from 2023 to 2028 shows that Europe is developing the fastest.According to the latest “Research Report on Global Biogas Upgrading Market”,it is estimated that the global biogas upgrading market will reach US$1004.9 million in 2029,and the CAGR will be 24.2% in the next few years.At present,based on a research report on the market status and future development trend of biogas power generation industry in China [7],the largest biogas market in the world is China,accounting for more than 30% of the market share,followed by the United States and Germany,which together account for approximately 45% of the market share.Sweden began to use biogas as automobile fuel 20 years ago,and now it has been widely used in public transport,such as buses and trains,accounting for approximately 55% of the gas fuel of vehicles.As biogas resources account for a large proportion of renewable energy,the United States began experimenting with biogas fuel cells instead of traditional internal combustion engines to generate electric power.Japan’s biogas fuel cell industry is at the forefront in the world.

Biogas power generation projects have certain economic benefits and investment feasibility,and can greatly reduce carbon emissions,making them the focus of promotion and research in certain countries.In recent years,the installed capacity of biogas power generation has shown a steady upward trend,with the installed capacity increasing by 0.6 times from 13.13 GW in 2012 to 21.4 GW in 2021 [8].By 2021,the installed capacity of biogas power generation accounted for 15% of the total installed capacity of biomass energy in the world.The biogas power generation in Germany has increased from 32 billion kWh in 2017,accounting for nearly 6% of the total power generation in Germany,to 76 billion kWh by 2020,accounting for 17%of the total power generation in the country.By 2022,the installed biogas power generation capacity has reached 2.7 million kW,accounting for 73% of the biomass power generation capacity.

Because its biogas resources account for a large proportion of renewable energy,the United States has conducted experiments to replace traditional internal combustion engines with biogas fuel cells,and its installed capacity for biogas power generation in 2022 is approximately 2.25 million kW.At present,the scale of biogas projects in China is gradually expanding;in 2022,the installed capacity of biogas power generation is approximately 1.22 million kW,accounting for approximately 2.9% of biomass power generation.Wresta [9] analyzed the economic aspects of biogas power generation on small-scale farms and believed that government policy support could encourage users to better execute biogas projects.Technical and economic analyses of biogas projects at different scales,such as large-scale biogas power generation projects and small-scale biogas projects,have been studied by relevant scholars [10].Abdullah believes that biogas power-generation projects in Cairo,Egypt have played a positive role in reducing energy consumption,energy costs,and carbon dioxide emissions[11].CO2 in biogas is non-flammable,and its content is as high as 25%-50%,which not only reduces the quality and combustion performance of biogas but also increases its compression cost,resulting in low utilization rate,narrow application range,and poor stability of biogas.Therefore,some of the main challenges in biogas applications are its combustion performance,stability,and energy density.Several research groups [12-26] have addressed biogas combustion performance,efficiency,stability,and emissions in different applications,and have studied the effects of CO2 concentration on combustion characteristics.

As an inert gas,CO2 increases the heat capacity of the fuel and decreases the biogas heating value,flame propagation speed [13,15-17],and flame combustion temperature [17].Furthermore,increasing the CO2 concentration reduces the main reactions of the radicals OH,H,O,and CH3 [17].The combustion efficiency [21]and performance of several combustion systems [19,20,22-26] are negatively affected by the increase in CO2 concentration.These effects reduce flame stability [23].In contrast,increasing the CO2 concentration decreases the NOx level [12-14] and soot concentration [18],as expected because of the lower flame temperature.The application of biogas in several combustion systems to replace fossil fuels is the goal of future energy development and one of the strategic planning goals for net zero emissions.Therefore,to solve these problems,this study adopted different technologies to enhance the combustion efficiency and stability of biogas.Other technologies can also be used to enhance combustion stability,such as H2 enrichment[22],reducing CO2 concentration [26],applying Moderate or Intense Low oxygen Dilution (MILD) combustion[27-29],applying plasma-assisted combustion [12],and modifying combustion systems [14].During the last few decades,our research has aimed to develop new burners[30,31] for the highly stabilized combustion of gaseous fuels based on simple technology to control the level of mixture inhomogeneity.The data showed that the combustion stability was affected by the level of mixture inhomogeneity within a mode of inhomogeneous partially premixed stratified (IPPS) combustion.Higher combustion stability can be achieved at a certain level of mixture inhomogeneity between the fully premixed (FP) and nonpremixed (NP) modes [32,33].Masri and his research team have also developed similar technology [34,35] for IPPS mode and obtained the same results.The concentric flow burner of Meares and Masri [34],concentric flow conical nozzle (CFCN) burner of Mansour [30],concentric flow slot burner (CFSB) of Mansour et al.[31],and concentric flow burner of Lee et al.[36] controlled the mixture fraction inhomogeneity by mixing the air and fuel streams within a mixing chamber prior to the nozzle exit.The level of mixture inhomogeneity is controlled by changing the mixing chamber length (L).The conditions for maximum combustion stability can be achieved at a certain length L.Thus,this simple technology can be implemented in many combustion systems without modifying their designs.Applying the IPPS mode with this simple technology to control the level of mixture inhomogeneity in lower heating value fuels requires detailed investigation.Accordingly,this study aimed to use this technology in the combustion of biogas to achieve conditions of higher combustion stability within the IPPS mode of combustion.

Creating the IPPS mode in combustion systems leads to a mixture fraction distribution of lean and rich pockets,and hence,the generation of multi-reaction zones.The rich reaction zone likely creates a radical pool capable of supporting a lean reaction zone.Thus,the IPPS mode can enhance the combustion stability without modifying the combustion system design.In addition,the conical nozzle of Mansour’s burner [30] and pilot flame in Meares and Masri’s burner [34] significantly enhance the combustion stability.Furthermore,the air co-flow in the CFCN burner[30,37] improves the combustion stability by creating a recirculation zone at the conical nozzle exit.As discussed above,biogas combustion is less stable than the combustion of fossil fuels.Consequently,they cannot replace fossil fuels,without enhancing their combustion stability.Fischer and Jiang [21] showed that the kinetic effects of CO2 on biogas are complex,nonlinear,and counterintuitive.Addition of CO2 to methane reduces the production of OH radicals and combustion temperature.Accordingly,creating an environment with a higher production rate of radicals should improve the combustion stability and efficiency.The IPPS mode in the piloted concentric,CFCN,and CFSB burners can provide a suitable environment for the highly stable flames of gaseous fuels.

Accordingly,we used the newly developed CFSB [31]to create highly turbulent planar IPPS flames.A mixture of natural gas and CO2 at different concentrations was used to simulate biogas fuel in the current study.The effects of the level of mixture inhomogeneity,turbulence,overall jet equivalence ratio,air-to-fuel velocity,and momentum ratios were investigated for different CO2 concentrations up to 40%.A stability map and flame structure based on temperature measurements are presented and discussed to explore the conditions with the highest combustion stability at higher CO2 concentrations.

1 Experimental details

To achieve the aim of the current study,we applied the technology of the IPPS mode for a more stable and efficient combustion of biogas.The newly developed CFSB by Mansour et al.[31] was used in this study involving biogas at different CO2 concentrations.The aim of CFSB is to create highly turbulent planar IPPS flames with a well-controlled mechanism for the degree of mixture inhomogeneity.Early measurements in a CFCN burner [34,35,38] showed a considerable improvement in the combustion stability at a certain level of mixture inhomogeneity of natural gas and propane.Similar results were recently obtained for CFSB [33,39].Accordingly,the biogas combustion stability should be improved in CFSB.In this section,the design of the CFSB burner and experimental techniques are explained.

1.1 Concentric flow slot burner (CFSB)

The CFSB burner was operated in IPPS mode.This mode covers a wide range of fully premixed and nonpremixed combustion modes,where the mixture is classified based on the range of mixture fraction fluctuations and mean mixture stoichiometry [40].The CFSB burner design and flame sample photo with 30% CO2 concentration are illustrated in Fig.1.The burner consists of an inner rectangular cross-section slot,6 mm wide and 60 mm long,surrounded by two outer slots of equal rectangular cross-sectional areas,1 mm wide and 60 mm long.Fuel flowed through the inner slot,whereas air flowed through the outer slots.The air and fuel streams are then mixed in a mixing chamber with a rectangular cross-section and length L,called the mixing length.The level of mixture inhomogeneity was controlled by varying the mixing length L,as shown in Fig.1.At L=0,that is,without the mixing chamber,the flames are considered non-premixed;however,the mixing length L should be larger than 30 times the burner hydraulic diameter for fully premixed flames.The nozzle exit rectangular cross section is 10 mm wide and 60 mm long,and its hydraulic diameter,D,is equal to 17.14 mm.The mixing chambers at the top of the burner are replaced with chambers of different lengths.The measurements were performed at different values of L/D,as listed below,to study the effect of the level of mixture inhomogeneity on the flame structure and stability.

Fig.1 Burner (a) 3-D drawing,(b) longitudinal crosssection side view,(c) enlarged nozzle top view A-A,(d) sample flame side view photo,and (e) its front view with 30% CO2 concentration

1.2 Operating conditions

The biogas fuel was simulated using a mixture of natural gas,NG,and CO2 at different ratios,as listed in Table 1.A reference fuel of natural gas with 0% CO2,was also selected.The natural gas composition NG,is [41] 95.56%CH4,0.84% CO2,3.52% H2O,0.01% H2,0.03% N2,and 0.04% O2,by mass.Compressed CO2 bottle with purity of 99.9% was used.The operating conditions for the stability measurements are listed in Table 1,whereas the selected flames for the temperature measurements are listed in Table 2.The NG and CO2 flow rates were controlled using fineneedle valves and measured using volumetric flow meter sensors.Honeywell HAF300slm sensors were used for the air and CO2 lines (accuracy of ±0.5% at 0-14% of full scale and ±3.5% at 14%-100% of full scale).Sensirion SFM3000 sensors were used for the NG line with accuracy of ±0.05%of the full scale.An Arduino Uno connected to a laptop was used to collect signals from all the sensors.The MATLAB code was used to calculate the fuel and air velocities,Reynolds number,equivalence ratio,and momentum during the experiments.

Table 1 Operating parameters and conditions

Table 2 Flame operating conditions for temperature measurements

2 Temperature measurement technique

The thermal flame structure was investigated based on temperature measurements by using a B-type thermocouple fixed to a 3D transverse mechanism to cover the longitudinal,axial,and vertical directions.The thermocouple wires had a diameter of 100 mm.The thermocouple was moved in steps of 0.5 mm along the slot width at the middle of the length of the slot.The B-type thermocouple wires were made of platinum with 30% rhodium as the positive terminal and platinum with 6% rhodium as the negative terminal.Both terminals are connected at a junction with a diameter of 300 mm.The maximum radiation errors in the temperature measurements were calculated and found to be less than 2.5%.

3 Flame images

The flames at a premixing ratio of L/D=15 were captured from the front and side views at different CO2 concentrations,as shown in Fig.2.All the flames were adjusted at the same Reynolds number of 4000 and an equivalence ratio of 2.The flame height decreases from left to right as the CO2 concentration increases.In addition,the blue color of the flame fades and becomes lighter when the CO2 concentration is increased in the biogas mixture,which is clear from the flame side view shots.This observation supports the results obtained by Ilminnafik et al.[42],as the flame of biogas used before purification (higher CO2 content) achieved a lighter blue color than that obtained from purified biogas (lower content).This is because the presence of CO2 leads to frequent collisions with the fuel molecules,which in turn decreases the number of ions formed;thus,the color of the flame becomes lighter [42].

Fig.2 Front (top photos) and side views (bottom photos)flame images at 0%,10%,20%,30%,and 40% CO2 concentrations from left to right,respectively.The flames are adjusted at Re=4000 and Φ=2.0

Fig.3 Temperature profiles at L/D=7 (a-e) and at L/D=15 (f-j)

4 Flame thermal structure

The flame thermal structure is discussed in this section based on fine-wire thermocouple temperature measurements.The temperature profiles were measured in ten stable flames,as listed in Table 2,for two different L/D values at the same equivalence ratio and Reynolds number.The burner loads Q (kW) for the investigated flames are listed in Table 2.The measurements were performed at three vertical locations above the nozzle exit and one near the nozzle exit at Z=5 mm.This should provide the boundary conditions at the nozzle exit.The two downstream positions at Z=30 mm and 60 mm along the burner on the X-axis as shown in Fig.1.A were selected to discuss the effects of different operating conditions on the flame structure.Temperature scans were performed in the middle of the slot length,as shown in Fig.1.A across the flame with a step of 0.5 mm to resolve the flame structure.

The temperature profiles across the flames are shown in Figs.3(a-e) below for L/D=7 and in Figs.3(f-j) for L/D=15 for all CO2 concentrations.The flames were symmetrically planar;the profiles are only illustrated on one side.The temperature increases from the lower position at Z=5 mm to the higher vertical positions at Z=30 and 60 mm.A cold core was observed at Z=5 mm,where the two planar flames were separated.In addition,at Z=30 mm,for the case of L/D=15 and 10% CO2,a lower temperature at the centerline was observed indicating the tip of the cold core between the two planar flames.At higher vertical positions,the planar flames merged.In all the cases,the flame thermal width increased along the vertical distance Z.The flame width data are illustrated in more detail in the following paragraph.The effect of CO2 concentration on the maximum flame temperature is illustrated in Fig.4 for L/D=7 and L/D=15.The maximum temperatures versus the vertical position Z for all CO2 concentrations are illustrated in Figs.4a and 4c for L/D =7 and L/D =15,respectively.The maximum temperatures at the three different positions are plotted versus the CO2 concentration in Figs.4b and 4d for L/D=7 and L/D=15,respectively.The data show that the temperature decreased with increase in CO2 concentration for both L/D cases,as expected,because of the decrease in combustion energy,as shown in Fig.4b below.The maximum temperature increased along height Z at both L/D and CO2 concentrations.These trends are expected for both vertical location Z and CO2 concentration x%.

Fig.4 Maximum temperature along the vertical height Z for (a) L/D=7,and (c) L/D=15;and maximum temperature for different CO2 concentrations for (b) L/D=7,and (d) L/D=15

The flame width (FW) is defined as the distance between the temperature points at the half-maximum temperature on both sides of the flame.The flame width versus height Z data for both cases at L/D=7 and 15 and different CO2 concentrations are shown in Fig.5.FW increased along the vertical distance Z for all L/D and CO2 concentrations.FW decreased slightly with increasing CO2 concentration.Figure 2 shows the reduction in the flame length with increase in CO2 concentration.This means that the flame size is decreased by increasing the CO2 concentration,as expected,because of the decrease in the combustion energy,as discussed later.

Fig.5 Effect of CO2 concentration on the flame width,FW

5 Stability characteristics of biogas in the CFSB

The stability characteristics of biogas in the CFSB were determined at different CO2 concentrations and equivalence ratios (Refer Table 1).To this end,the effects of the level of mixture inhomogeneity,turbulence level,mixture equivalence ratio,and velocity and momentum ratios between the air and fuel streams on the combustion stability of biogas were investigated in detail.The experiments covered a wide range of operating parameters to identify the optimal stability conditions for biogas.In addition,the lean combustion conditions were investigated.Vr is defined as follows:

where Vair is the air velocity,VF is the fuel velocity,Qair is the air volumetric flow rate,Aair is the air cross-sectional area,QF is the total fuel volumetric flow rate,and AF is the fuel cross-sectional area.The fuel volume flow rate is the sum of the natural gas,QNG,andvolumetric flow rates.The momentum ratio I is defined as follows:

where (AF)st is the stoichiometric air-to-fuel ratio by mass and Φ is the overall equivalence ratio of the jet.The momentum ratio,I,in Eq.3,is directly proportional to the velocity ratio Vr and inversely proportional to the equivalence ratio Φ.

The flame blow-off/blowout limits were investigated to determine the stability limit.The blow-off limit is defined as the flame extinction from the attached condition,whereas the flame blowout is defined as the flame extinction from the lifted condition.The fuel flow rate was fixed at 10,20,30,40,and 60 standard L/min to cover a wider range of turbulence,and the air flow rate was then increased to approach flame extinction.This limit was precisely determined and the measurements were repeated three times to ensure repeatability.The recorded flow-rate data were used to calculate the operating parameters.The data are illustrated in the following paragraphs,covering the effects of CO2 concentration,air-to-fuel velocity,momentum ratios,and level mixture inhomogeneity (for different values of L/D)at different levels of turbulence.

5.1 Effect of CO2 concentration

The effect of increasing the CO2 concentration on biogas flame stability in the CFSB burner was investigated by varying the CO2 concentration.Fig.6a shows the stability data at different CO2 concentrations at L/D=9.In addition,the reference pure natural gas,NG,i.e.,0%CO2,stability data are also illustrated for comparison.The flame extinction Reynolds number Re was plotted against the overall equivalence ratio Φ.Increasing the CO2 concentration leads to a decrease in the biogas forced ignition [43],flammability limits [26,44,45],adiabatic flame temperature [17],and laminar burning velocity [12,46].Xiang et al.[17] concluded that increasing the CO2 concentration leads to a reduction in the net rates of the main reactions of the radicals CH3,OH,H,and O.The heating value of the biogas was calculated at different CO2 concentrations (x) and presented in Fig.6b.It decreased significantly with increasing CO2.These effects limit the practical applications of biogas in combustion systems.The data in Fig.6a shows that increasing CO2 concentration decreases the stability limit.Our recent study [39] on another slot burner with different aspect ratios showed a similar trend in the extinction limit,with fuel in the outer slots and air in the inner slot.This trend can be partially attributed to a decrease in the released combustion energy of the fuel.The Wobbe index (WI) is used to characterize the effects of the combustion energy and interchangeability of fuel in industrial burners.WI is defined as:

Fig.6 (a) The stability limit at L/D=9 for different CO2 concentrations,(b) the HHV and WI of the fuel at different CO2 concentrations,and (c) the normalized Reynolds number,ReN,vs the equivalence ratio at different CO2 concentrations

where HHV is the higher heating value of the fuel and γ is the fuel specific gravity,defined as the ratio between the fuel density and the air density at standard pressure and temperature.Figure 6b shows variations in the higher heating value and WI for different concentrations of CO2.The WI and HHV decreased with increase in CO2 concentration.The WI follows a polynomial fit given by Eq.4 as:

Liu and Sanderson [47] investigated the effects of energy on the combustion process in a combustion system.The effect was clear on the combustion performance,NOx and CO emissions.Increasing the CO2 concentration led to a reduction in NOx and CO,while the temperature decreased.In the present study,to investigate the correlation between the stability limit and combustion energy using the Wobbe index,the Reynolds number is normalized (ReN) as follows:

whereandare the Wobbe indices of the reference fuel,NG,and the biogas at x% of CO2 concentration,respectively.The values of x are listed in Table 1.The data for the normalized Reynolds number,ReN,are shown in Fig.6c.The partial effect of the combustion energy is now clear,where the data points converge compared to the original data in Fig.6a.Thus,the stability at this level of mixture inhomogeneity partially depends on the fuel Wobbe Index.However,the scattering of the data in Fig.6c may be attributed to other physical and chemical parameters,such as flame speed,flammability limits,ignition energy,and reaction rates of the radicals.The straight lines in Figs.6a and 6c represent the boundaries of the data in the range of CO2 concentrations between 0 and 40%.The normalized data in Fig.6c show a constant slope of the illustrated boundaries compared to the diverging nature in Fig.6a.The effect of combustion energy on flame stability was partial at a mixing level of L/D=9.However,this trend was not observed for all the levels of mixture inhomogeneity at different L/D ratios,as discussed later.Figure 6(a) The stability limit at L/D=9 for different CO2 concentrations,(b) the HHV and WI of the fuel at different CO2 concentrations,and (c) the normalized Reynolds number,ReN,vs the equivalence ratio at different CO2 concentrations

5.2 Effect of the air-to-fuel velocity and momentum ratios on the flame stability

The mixing process between two streams of different gases is significantly affected by their velocity ratio [48,49].Therefore,the momentum ratio also affects the mixing process and level of mixture inhomogeneity.In this study,the air-to-fuel velocity and momentum ratio were calculated using Eqs.1-3.The data are illustrated in Figs.7-9 for different CO2 concentrations and L/D ratios.The stability was reduced almost linearly by increasing the velocity ratio,Vr,and momentum ratio,I,for all CO2 concentrations and L/D ratios.At higher values of Vr and I,mixing is likely to improve,and a less-mixed inhomogeneous structure is expected.Thus,the mixture approaches full premixing at higher Vr and I,and the reaction follows premixed-like flame propagation with an expected lower radical production rate,and consequently,less stability.The combustion process in modern internal combustion engines follows the IPPS mode with fuel injection to improve engine stability and performance compared to the traditional premixed mode using a carburetor for mixing air and fuel [50].Similar combustion stability enhancement can be achieved in practical combustion systems by adjusting the level of mixture inhomogeneity.Mukhopadhyay and Abraham [50]showed that the level of stratification significantly affected autoignition in internal combustion engines,consequently improving flame initiation and enhancing combustion stability.The mixture fraction gradient and flow field structure are highly effective parameters for the combustion of highly turbulent flames in several combustion systems.The inverse trend between the combustion stability and both the velocity and momentum ratios can be used as a guide for designing combustion systems to achieve the best conditions for highly stabilized flames.

The effect of combustion energy on the relationship between stability and air-to-fuel velocity ratio is also investigated in this section.Figure 8 shows the normalized Reynolds number,ReN,versus Vr.In general,the data points converge at the normalized Reynolds number,as illustrated in Fig.8,compared with the Reynolds number in Fig.7.This indicates that stability is partially affected by the combustion energy.However,this effect was prominent in the case of L/D=7,and was partially effective for other values of L/D.The interactions between the different flame parameters,for example combustion energy,flame speed,flammability limits,chemical reactions,and rate of radical production,vary according to the level of mixture inhomogeneity.The optimum conditions for highly stable biogas combustion can be achieved with a certain level of mixture inhomogeneity.More detailed discussions are presented in the following section to study the effect of mixture inhomogeneity on burner stability by varying L/D.

Fig.7 Stability plots,Re versus Vr,at different levels of mixture inhomogeneity,by varying L/D ratios,and using different CO2 concentrations

Fig.8 Stability plots,ReN versus Vr,at different levels of mixture inhomogeneity,by varying L/D ratios,and using different CO2 concentrations

Fig.9 Stability plots,Re versus I,at different levels of mixture inhomogeneity,by varying L/D,and using different CO2 concentrations

5.3 Effect of the level of mixture inhomogeneity on the flame stability

The mixing field structure is one of the main parameters affecting the flame stability and combustion process.Our previous study [32] demonstrated that the combustion process in an IPPS environment is more stable than that in fully premixed and non-premixed environments.Our previous studies [51,52] focused on the relationship between the mixing field structure and the combustion stability.The correlation between mixture inhomogeneity and combustion stability is clear [51].Optimal stability can be achieved at a certain level of mixture inhomogeneity as concluded by Mansour et al.[51] and Meares et.al [35].This can be explained as follows.The IPPS mode usually leads to multi-reaction zones within the flames owing to the existence of lean and rich pockets.This likely increased the production rates of radicals that maintained the flame stable,even within an overall lean mixture.The rich side of the flame produced sufficient heat and radicals to stabilize the lean side.Similar explanations have been introduced by Mukhopadhyay and Abraham [49].

The mixing field structure in the CFSB was affected by varying the normalized mixing length,L/D,and air-to-fuel velocity ratio,Vr.Therefore,to decouple both effects and focus on the L/D effect,the data are plotted in Fig.10 for constant values of Vr.The data are also shown for different CO2 concentrations.For all CO2 concentrations,there was a clear peak at L/D=5 for maximum stability,and stability was regained at L/D=15.These results are similar to those of our previous studies [51] and those obtained by Masri et al.[34,35].

Fig.10 Stability plots,Re versus L/D,at different values of Vr and different CO2 concentrations

Accordingly,the stability plots of the CFSB using biogas at different CO2 concentrations for each L/D case are shown in Fig.11,where the Reynolds number at flame extinction is illustrated versus the overall equivalence ratio of jet Φ,similar to the plot in Fig.6 at L/D=9.The effect of CO2 concentration on flame stability was significant for L/D=1,3,7,9,and 12.In contrast,the data at L/D=5 and L/D=15,where the combustion stability is higher,show little reduction in biogas stability with increasing CO2 concentrations.In addition,lean combustion could be achieved at high CO2 concentrations.This is a significant achievement,as biogas combustion stability can be improved in practical combustion systems by controlling the mixing field structure without many changes in the system design.Designers of industrial burners and combustion systems for biogas fuel can then modify the mixing technique to achieve the optimum combustion stability.The higher combustion stability of biogas at different CO2 concentrations may be attributed to the IPPS mode with specific conditions of mixture inhomogeneity,which led to a multi-reaction zone structure,where the radicals generated from the rich side supported the reaction at the lean side of the flame.Combustion instability problems in both the premixed and non-premixed modes for biogas fuels can be avoided.More detailed measurements and modeling of the radical concentrations in these flames are required to better understand the optimum conditions for stability enhancement.Thus,improving the stability of biogas combustion as a renewable fuel can be achieved to support its implementation in several practical combustion systems to replace fossil fuels.

Fig.11 Stability plots,Re versus Φ,at different levels of mixture inhomogeneity,by varying L/D ratios,and using different CO2 concentrations

6 Discussion on the prospect of biogas power generation and utilization

With the increase in biogas production,biogas can be used as a clean fuel and energy storage medium.Combined with the biogas combustion technology proposed in this paper,the stability of biogas in the combustion process is greatly improved,so that its energy loss in the energy conversion process is reduced,which is beneficial for improving the mobility of system energy and reducing the system cost while satisfying the system requirements.With the continuous development of clean and low-carbon energy power generation,clean energy,represented by wind power and solar energy,has been effectively utilized in certain areas.Combined with the characteristics of biogas,which is easy to store and has good regulation performance,biogas with strong controllability can be used to generate electricity to balance the fluctuation and random output of wind power and photovoltaic power;thus,the electric power and electric quantity of wind and photovoltaic power generation in power systems can achieve self-balance in the region,which is conducive to reducing the consumption and peak-shaving costs of the power system and promoting the installed capacity of new energy power generation,such as wind and photovoltaic resources.

In addition,a regional electricity-gas-heat comprehensive energy system can be constructed using the gas supply,heat supply,and power supply capacity of biogas,which can be converted into electricity and heat energy through energy conversion equipment to satisfy the electricity,heat,and gas load requirements of users.In the process of energy conversion,surplus wind power and photovoltaic resources can be used to provide heat for the biogas digester using the thermodynamic effect of the biogas digester;thus,the biogas digester can be maintained at the optimum temperature and converted into biogas [53],which is easy to store,and the biogas output of the biogas digester can be further improved.In [54],photovoltaic heat collection systems and waste heat recovery systems were used as heating sources for anaerobic fermentation heating systems in large-scale biogas power generation.Through system parameter calculations and example analysis,it was proven that this heating system can reduce the heat loss of photovoltaic and power generation equipment and improve the economy of the entire biogas project.

Wind power,photovoltaics,and biogas resources are complementary.Reference [55] proved that renewable energy is the key to the sustainable provision of energy power in comprehensive energy systems.The combination of energy storage system and biogas system can compensate for the shortage of wind power and photovoltaic output.However,if we rely too much on biogas power generation as the power source,the entire system will generate more power generation costs because of increased biogas power generation.Therefore,in the comprehensive energy system,when the wind power and photovoltaic generations cannot satisfy the power demand,biogas is used to generate electricity and provide the power source for the system.Owing to the slow response time of the biogas power generation system,the system needs reasonable coordination of wind power,photovoltaics,and biogas to ensure an uninterrupted power supply for the entire system.

Various technical measures have been implemented to reduce coal consumption in thermal power units.In Europe,biomass mixed combustion power generation is more common way of utilization.With regard to the energy utilization efficiency,the mode of co-burning biomass in large thermal power units is the highest,and the investment cost is relatively low.Because this power generation method mainly uses an internal combustion engine or gas turbine to generate electricity,the power generation efficiency of such generators does not depend on the installed scale;therefore,the installed scale can be flexibly selected according to the local biomass resources.Compared with pure thermal power generation,biogas combined with coal-fired power generation has a higher power generation efficiency,and biogas can be used to replace the ignition and combustionsupporting oil of thermal power units.

7 Conclusions

Biogas is a promising renewable fuel that is produced from organic waste.In several combustion systems,this fuel is attractive as a renewable alternative to fossil fuels.However,their application is limited by the reduction in combustion stability owing to the large amount of CO2.This study aimed to apply inhomogeneous partially premixed stratified (IPPS) technology to a newly developed concentric flow slot burner (CFSB) for biogas combustion to achieve a higher level of combustion stability at different CO2 concentrations.

The stability and temperature measurements were conducted at CO2 concentrations up to 40%.Mixture inhomogeneity was controlled using a mixing chamber with a variable length L to achieve the highest stability conditions.The mixing mechanism in the CFSB burner is relatively simple and satisfies our target of controlling the level of mixture inhomogeneity within the IPPS mode between non-premixed and fully premixed conditions.

The flame stability is partially affected by the fuel combustion energy,represented by the Wobbe index(WI),at a certain level of mixture inhomogeneity.This effect varied according to the level inhomogeneity of the mixtures.Thus,the biogas combustion stability depends partially on the fuel combustion energy and the level of mixture inhomogeneity.The mixture fraction distribution and inhomogeneity depend on the mixing length L and airto-fuel velocity ratio,Vr.The air-to-fuel velocity Vr and momentum I significantly affected the combustion stability of both natural gas and biogas.The combustion stability,and values of Vr and I were inversely proportional.This effect is due to the variation in the mixture inhomogeneity caused by varying Vr and I.

One of the main conclusions of this study is the possible control of the mixture inhomogeneity to achieve the highest stability of biogas combustion at a certain mixing length,L/D=5,where the flame stability is not significantly affected by the CO2 concentration.Biogas combustion has improved significantly over the last few decades by several research groups.However,the current achievements should encourage the application of biogas in many existing combustion systems by controlling mixture inhomogeneity without major design modifications.Future application of this technology requires the development of air and fuel feeds in the current combustion systems to achieve the required level of mixture inhomogeneity.

As expected,the maximum flame temperature decreased with increasing CO2 concentration because of the decrease in fuel combustion energy.The flame width increased slightly along the vertical height and was not significantly affected by the CO2 concentration at different mixing levels.The flame height decreases with increasing CO2 concentration.

The proposed future research plan is implementing the IPPS technology using biogas in practical combustion systems,e.g.industrial burners,gas turbine combustors,and internal combustion systems.This should satisfy the main goal of this research by spreading the application of biogas as a promising renewable fuel in practical combustion systems to replace fossil fuels.

Acronym

CFCN Concentric flow conical nozzle

CFSB Concentric flow slot burner

FP Fully premixed

IPPS Inhomogeneous partially premixed stratified

MILD Moderate or intense low oxygen dilution

NG Natural gas

NP Non-premixed

Nomenclature

Aair Nozzle area of the air stream,m2

AF Nozzle area of the fuel stream,m2

(AF)st Stoichiometric air-to-fuel ratio

D Nozzle hydraulic diameter,m

FW Flame width at the temperature half maximum,mm

HHV Higher heating value of the fuel,kJ/kg

I The air-to-fuel momentum ratio

L Mixing length of the mixing chamber,m

moair air mass flow rate,kg/sec

moF fuel flow rate,kg/sec

Q burner load,kW

Qair air volume flow rate,kg/sec

QF fuel volume flow rate,kg/sec

Re Reynolds number

ReN Normalized Reynolds number

Vair The air stream velocity at the nozzle exit,m/sec

VF The fuel stream velocity at the nozzle exit,m/sec

Vr The air-to-fuel velocity ratio

WI Wobbe Index

x % of CO2 in the fuel

X distance across the x-coordinate,mm

Z height above the nozzle,mm

Symbols

Φ Equivalence ratio

ρair air density,kg/m3

ρF fuel density,kg/m3

Acknowledgments

This work was funded by the American University in Cairo research grants (Project number SSE-MENG-M.M.-FY18-FY19-FY20-RG (1-18) -2017 -Nov-11-17-52-02).

Declaration of Competing Interest

We declare that we have no conflict of interest.