3D numerical modelling of turbulent biogas combustion in a newly generated 10 KW burner

doi:10.1016/j.joei.2016.10.004

Journal of the Energy Institute

Volume 91, Issue 1, February 2018, Pages 87-99

https://doi.org/10.1016/j.joei.2016.10.004 Get rights and content

Highlights

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The newly generated burner has been described in detail.
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The PDF/Mixture Fraction combustion model, standard k-Ɛ turbulence model and P-1 radiation model have been used.
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Temperature and emission distributions of the biogases have been obtained.
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The effects of the preheated air and H₂S amount in the biogases have been investigated.

Abstract

This study concentrates on the 3D numerical modelling of combustion of different biogases in a generated burner and combustor. The main goal of this study is to investigate the combustion characteristics (such as temperature and emissions) of biogases through a combustor due to depletion of natural gas. Moreover, the effect of the preheated air on flame temperatures of biogases have been studied in the present study. Finally, the effect of H₂S amount in biogas on SO₂ emissions has been investigated within these predictions. The numerical modelling of turbulent diffusion flames has been performed by using the standard k–ε model of turbulent flow, the PDF/Mixture Fraction combustion model and P-1 radiation model in the combustor. A CFD code has been used for all predictions. Temperature gradients have been determined on axial and radial directions for better understanding combustion characteristics of biogases. Modelling has been studied for thermal power of 10 kW and excess air ratio of λ = 1.2 for each biogas combustion. The first finding is that combustion of biogases is possible via the newly generated burner. Moreover, the results show that the one of biogas is very close to methane in terms of temperature distributions in the combustor due to including high amount of methane compared to other biogases. It is also concluded that the flame temperatures of biogases increase with preheating the combustion air as expected. It is finally revealed that SO₂ emissions increase as amount of H₂S in biogas is increased through the combustor.

Introduction

Although fossil fuels are still used in power generation systems, buildings and elsewhere, these have two problems. First, fossil fuels are restricted and are not equally distributed all over the World. Scientists estimate that coal, crude oil and natural gas will be rapidly depleted in the near future. Second, fossil fuels are responsible for global warming. Because, CO₂ is majorly released when fossil fuel are burned in any system. Because, there are aforementioned problems, the scientists try to find new energy resources such as solar energy, wind energy, hydrogen energy, biomass and the others.

The growing interest in the use of biogas has promoted nowadays. Biogas arising from effluent, municipal solid waste, residuals, gasification of biomass are very appropriate for power generation, heating systems and elsewhere. Biogas consists chiefly of methane and carbondioxide as well as it has a trace amount of nitrogen, hydrogen sulphur and oxygen depending on production or gasification methods [1]. Methane in biogas has high amount of energy as major component of natural gas. However, the other components are non-combustible.

Biogas combustion has to be investigated for better understanding of combustion characteristics of biogases. There are some studies about combustions of biogas or blending fuels in the literature. Adouane et al. [2] have conducted an experimental study related to reduction of NO_X emissions arising from fuel-bound nitrogen. They conclude that ammonia in the fuel drastically affects NO_X formation. Bhoi and Channiwala [3] have experimentally investigated emission characteristics and axial flame temperature distributions of producer gas including high amount of nitrogen by varying thermal inputs and equivalance ratios. The results show that the maximum temperature emerges in flame front for this gas. Hosseini et al. [4] have performed combustion characteristics of biogas under flameless combustion conditions. Hosseini et al. estimate that biogas is not appropriate for any furnaces due to its low calorific value (LCV). Huynh and Kong [5] have determined NOx emissions coming from syngas combustion obtained three different biomass feedstock under different conditions. It can be concluded that highest NOx levels form under syngas combustion obtained seed corn as the seed corn has the highest nitrogen content. Leung and Wierzba [6] have investigated the effect of hydrogen addition on stability limits of non-premixed biogas flames. It is demonstrated that hydrogen addition on the enhancement of biogas highly affects.

İlbaş and Karyeyen [7] have modelled the combustion characteristics of coal gases including high amount of hydrogen or nitrogen depending on gasification or carbonization methods. They reveal that the coke oven gas is very suitable in combustion systems in terms of combustion performances (e.g. temperature values). Sethuraman et al. [8] have focused on the effects of nitrogen content in biomass feedstock on the producer gas composition and NOx emissions. They have found that there are relationships between nitrogen in biomass, ammonia in the producer gas, and NOx emissions in the flue gas. Somehsaraei et al. [9] have examined the fuel flexibility and performance analysis of biogas in micro gas turbines. When minor modifications to fuel valves and compressor were taken place, They have claimed that these modifications were assumed to allow engine operation with the simulated biogas composition. Chen and Zheng [10] have predicted hydrogen-enriched biogas MILD oxy-fuel conditions. It is found that biogas flame can be sustained under the MILD oxy-fuel combustion. Nikpey et al. [11] have investigated the impact of biogas and natural gas on combustion performance and emissions in a micro gas turbine. The results indicate that there is almost no change in electrical efficiency comparing with the natural gas fired case. Selim et al. [12] have studied effect of CO₂ and N₂ concentration during H₂S combustion. They have found that injection of CO₂ highly affected temperature values. Mordaunt and Pierce [13] have designed a combustion device and investigated the effects of CO₂ on combustion of CH₄ and emissions. Hosseini and Wahid [14] have studied combustion characteristics of biogas under hydrogen-enriched combustion conditions. The results show that NO_X formation increases as the flame temperature increases. Lafay et al. [15] have compared stability combustion domains, flame structures and Dynamics between methane and biogas flames. They have revealed that laminar flame speed strongly depends on fuel composition. Jahangirian et al. [16] have examined chemical and thermal influences of biogas CO₂ content. It is demonstrated that the presence of CO₂ in the fuel highly affects NO_X emissions (reducing). Hosseini and Wahid [17] have modelled biogas under flameless combustion conditions in a tangential burner. The results indicate that the temperature uniformity in the tangential flameless burner is more than coaxial configurations.

Some studies related to methane combustions were also reviewed as the methane combustion has been also modelled in the present study in order to validate the modellings. Feyz et al. [18], for example, have researched the effect of recess length on the combustion performance of methane flame. They have observed that the flame temperature increase as the recess length is increased. Saqr et al. [19] have modelled effect of free stream turbulence on NO_X formation of methane flames. They have reported that change in free stream turbulence affects NO_X formation level considerably. In addition to these studies, there is a study regarding combustion of low calorific value gas by authors. İlbaş and Karyeyen [20] have also studied combustion behaviours of low calorific value coal gases by enriching hydrogen in order to improve their combustion characteristics in that study. They have concluded that hydrogen addition highly affects the flame temperature of the low calorific value coal gases.

Although there are some aforementioned studies related to biogas or blending fuels combustion, there needs to be extremely studied combustion characteristics of biogas. Because, there are a lot of problems concerning combustion of biogas such as flame stability, flame propogation, flame temperature and emissions. However, the authors think that the newly generated of burner can contribute flame stability and relative high flame temperature of biogas because it has radial fuel inlets and angular and straight air inlets. These inlets can provide better fuel-air mixture and as a result of this, better combustion performance. If they are all to be considered, this study focuses on combustion performances and emission characteristics of biogases including different gas components by means of the newly generated of burner under thermal power of 10 kW and excess air ratio of 1.2.

Section snippets

Descriptions of burner and model combustor

3D CFD modelling has been performed by using the newly generated diffusion flame of burner in order to model combustions of methane (as baseline fuel) and biogases in the present study. This burner is shown in Fig. 1a. A and B in Fig. 1a represent the fuel and air inlets, respectively. It has also turbulators with 15° angles in the side of air stream in order to provide flame stabilization. Moreover, the fuel outlet of this burner has radial inlets for better mixing of air and fuel can be seen

Experimental setup

In the experimental part of the present study, temperature values have been measured on some axial and radial locations through the combustor in order to validate the modellings. The schematic view of the combustion system is shown in Fig. 3.

A general view of the combustion chamber is illustrated in Fig. 4. The length and diameter of the combustion chamber are fixed at 100 cm and 40 cm, respectively. The combustion chamber consists of a sight glass made of tempered glass and five measuring

Mesh independence

Mesh independence is very important situation in numerical studies as excessive mesh structure needs advanced computer technology, long computational time and etc. For this reason, mesh independence can be taken into account before predictions. The methane has been selected as baseline fuel in order to determine fine mesh structure. Grid structures including 311812, 447198, 613947, 893508, 1523435 elements have been tested in the predictions. The influence of the mesh refinement on the axial

Conclusions

Numerical investigation of combustion characteristics of biogases has been performed via the newly generated burner in the present study. The effects of the preheated combustion air on the flame temperatures and H₂S amount in biogases on SO₂ emissons have also been studied within these predictions.

It has been found out that the predictions are in good agreement with the measurements when The PDF/Mixture Fraction combustion model, the P-1 radiation model and the k-Ɛ standard turbulence model

Acknowledgements

The authors gratefully acknowledge to TÜBITAK (Project Number: 114M668) and Gazi University (Project Number: 07/2016-01) for their technical and financial supports.

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Citation Excerpt :
Instead, individual component concentrations for species of interest are derived from the predicted mixture fraction distribution. The interaction of turbulence and chemistry is accounted for with the help of a probability density function (PDF) [29]. Simple schemes for pressure–velocity coupling, Presto for pressure, Second-Order Upwind for momentum, and turbulent kinetic energy have also been selected.
H₂ – CH₄ mixture fuels can be promising for reducing carbon-based emissions. However, because of higher pollutant emission (such as NO_X) problems during hydrogen combustion, a new combustion method can be favorable. Colorless distributed combustion (CDC) is emerging here. CDC enables ultra-low pollutant emissions along with reduced flame instabilities, combustion noise, improved combustion efficiency, etc. Considering those benefits, methane and the hydrogen-enriched methane (60% CH₄ – 40% H₂, 50% CH₄ – 50% H₂, 40% CH₄ – 60% H₂) fuels have been consumed using a cyclonic burner providing more residence time at an equivalence ratio of 0.83 under distributed regime. For the modelings, Reynolds Stress Model (RSM) turbulence model, the assumed-shape with β-function Probability Density Function combustion model, and P-1 radiation model have been selected. To seek CDC, the oxygen concentration in the oxidizer was reduced with N₂ or CO₂ diluent from 21% O₂ to 13% O₂ at an interval of 2%. The air and the fuel temperatures were kept constant at 300 K. Besides, for seeking high-temperature air combustion (HiTAC) conditions the oxidizer temperature was changed to 600 K to simulate flue gas recirculation. The results showed that the temperature distributions changed to be more uniform considerably with a decrease in oxygen concentration for all cases. CDC also provided a considerable decrease in NO_X and a favorable reduction in CO at a certain oxygen concentration. It has been concluded that CO₂ as the diluent was more effective for reducing temperature levels and NO_X levels due to its greater heat capacity.

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3D numerical modelling of turbulent biogas combustion in a newly generated 10 KW burner

Highlights

Abstract

Introduction

Section snippets

Descriptions of burner and model combustor

Experimental setup

Mesh independence

Conclusions

Acknowledgements

Appl. Therm. Eng.

Biomass Bioenergy

Energy Convers. Manag.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Appl. Energy

Appl. Energy

Fuel

Renew. Sustain. Energy Rev.

Appl. Therm. Eng.

Int. J. Heat Mass Transf

Int. J. Hydrogen Energy