Elsevier

Fuel

Volume 235, 1 January 2019, Pages 810-821
Fuel

Full Length Article
Experimental and computational investigation of the laminar burning velocity of hydrogen-enriched biogas

https://doi.org/10.1016/j.fuel.2018.08.068Get rights and content

Abstract

Experimental investigation of the adiabatic laminar burning velocity (LBV) of mixtures representative of biogas and hydrogen-enriched biogas was carried out using heat flux method (HFM). To prepare samples of biogas for testing, pure methane (CH4) was diluted with 5–50 percent carbon dioxide (CO2). Thereafter, some of the biogas samples were enriched with either 20% or 40% hydrogen (H2). The combined effect of dilution with CO2 and enrichment with H2 on the LBV of CH4 at different equivalence ratios were studied experimentally using HFM and computationally using ANSYS Chemkin-Pro® with GRI Mech. 3.0 and San Diego reaction mechanisms. The experimental results indicate that dilution with CO2 reduces the LBV of richer mixtures more significantly than that of leaner or stoichiometric mixtures. However, when the biogas samples were enriched with H2, a significant rise in the LBV was observed for fuels with higher content of CO2. Sensitivity analyses of mass flow rate using GRI Mech. 3.0 revealed that with increased CO2 concentration, the sensitivity coefficient of H + O2 ↔ O + OH (R38) increases significantly and that of OH + CO ↔ H + CO2 (R99) decreases slightly. The sensitivity coefficients of H + O2 + H2O ↔ HO2 + H2O (R35) and H + CH3(+M) ↔ CH4(+M) (R52) are largest for fuel with the maximum percentage of CO2. Further, with an increased H2 concentration in biogas mixtures, the sensitivity coefficient of H + O2 ↔ O + OH (R38) increases, and that of OH + CO ↔ H + CO2 (R99) decreases. Additionally, the sensitivity coefficients of reactions H + O2 + H2O ↔ HO2 + H2O (R35) and H + CH3(+M) ↔ CH4(+M) (R52) increases with increased H2 concentration. The combined effect of CO2 and H2 on the rate of consumption of CO via R99 for all tested fuels having CH4:CO2 from 1:1 to 4:1 was predicted computationally. The rate of consumption of CO was least for CH4:CO2 = 1:1 and the peak value of the rate of consumption increases with decreasing CO2 concentration irrespective of H2 concentration in the fuel.

Introduction

According to the World Energy Council, the World’s per capita primary energy demand will peak before 2030 [1]. At the same time, combustion devices will remain one of the most significant sources of environmental pollution. Majority of the conventional fuels (e.g., diesel, petrol, etc.) used currently have high global warming potential (GWP) due to their long carbon cycles and emit high exhaust emissions. The greatest challenge for researchers is to limit the global average temperature rise below 2 °C [2] and develop sustainable alternative sources of energy to reduce greenhouse gas emissions [3].

An important candidate for non-conventional cleaner fuel technology is hydrogen (H2) among biogas, natural gas, producer gas and others. These gaseous fuels emit less pollutants and readily form homogenous mixtures in contrast to conventional liquid fuels [4]. Biogases that are produced by the biochemical decomposition of biomass (e.g., animal dung, agro-wastes, and municipal solid wastes), are primarily a mixture of 45–60% CH4, 40–55% CO2 along with some trace amounts of H2S, N2, CO, O2, and water vapor [5]. Biogas has a low heating value due to its high CO2 content. The maximum LBV of biogas (60% CH4 - 30% CO2 - 0.18% CO - 0.18% H2) in the air at ambient temperature is about 25 cm/s, while that of liquefied petroleum gas (30% C3H8 - 70% C4H10), which has LBV of 38.25 cm/s and of H2 is 275 cm/s [4]. At 1 bar, 15 °C the LHV of biogas, LPG, natural gas, and H2 with above concentrations are 17 MJ/kg, 45.7 MJ/kg, 50 MJ/kg and 120 MJ/kg respectively [4]. Besides the fact that the biogas has a low calorific value, it still finds applications in cooking, refrigeration, electricity generation, transportation, and lighting especially in the Indian context due to the large cattle population [6]. Further, biogas may be used to substitute diesel in diesel engines, saving up to 70–80% diesel [6]. On the other hand, H2 as a fuel has the following properties to its advantage: low density, high flammability, high mass-based calorific value, high reactivity and zero emissions. H2 not only improves extinction and flammability limits of lean hydrocarbon-air mixtures but also enhances the combustion efficiency of lean and low calorific value fuels. Due to these characteristics, H2 has significant potential as a clean energy carrier for a broad range of applications including power generation and transportation.

For premixed combustion, important characteristics are LBV, adiabatic flame temperature (AFT), ignition delay, flammability limits and quenching distance. The primary objective of this paper is to study the effects of CO2 dilution and H2 enrichment on the LBV and flame structure of CH4-air mixtures, specifically when it is diluted and enriched simultaneously.

The study by Kishore et al. [5] shows that the LBV of CH4 is reduced more by CO2 dilution than N2 dilution. Additionally, the peak LBV for CH4-air mixtures occur near the stoichiometric condition when diluted with 20%, 40% or 60% CO2 by volume. Mohammed et al. [7], Xu et al. [8] and Gascoin et al. [9] reported that the replacement of N2 with CO2 decreases OH concentration, peak flame temperature and NO concentration in emissions due to the high specific heat of CO2. Various studies [10], [11], [12], [13], [14], [50] reported that the presence of CO2 in any fuel tries to capture H radicals via reaction CO + OH ↔ CO2 + H, which decreases the reaction rate, the flame temperature, and the LBV. Jahangirian and Engeda [15] concluded that for a fixed hydrocarbon content (i.e., CH4 = 50%), replacing CO2 with N2 and H2O led to an increase in H radical concentration and observable NO emissions. Park et al. [17] observed that the peak burning velocity shifted towards stoichiometry (equivalence ratio, ϕ = 1) with an increased CO2 concentration in the fuel mixture. These observations were similar to Chan et al. [16] and Yang et al. [18]. The probes of Lee et al. [19] and Hu et al. [20] reported that with increased CO2 dilution in different gaseous fuels, the reduction in LBV and temperature was due to dilution, thermal, chemical and radiation effects. H2 enrichment effects on CH4 and biogas fuels were also analyzed in previous studies [21], [22], [23], [24], [25], [26], [27], [28], [31], [49]. According to these, the heat release rate, flammability, flame stability, flame temperature, reaction energy, LBV, and active radicals concentration (H, O, and OH) increases due to H2 enrichment. However, the CO emission decreases and peak LBV shifts towards the rich mixture side. Wei et al. [25] numerically studied the chemical kinetics of the premixed laminar biogas flames in the presence of H2 using GRI Mech. 3.0 [43]. They observed that the CO2 dilution reduces the global heat release rate due to the dilution, thermal and chemical effects. However, the addition of H2 in biogas improves the global heat release rate significantly. Zhen et al. [26] studied the stability and thermal emission characteristics of the biogas-H2 mixture using Bunsen burner method (for 400 ≤ Re ≤ 800 and 0.8 ≤ ϕ ≤ 1.2) and observed that the LBV of biogas-air mixtures increases monotonically with H2 addition. Further the peak LBV shifts from ϕ = 1 to ϕ = 1.2. Xie et al. [32], [33] reported that the key free radicals like H and OH play an important role in chemical kinetics of the diluted fuels. Pizzuti et al. [34] conducted a detailed review on LBV and flammability limits of biogas and concluded that when biogas is enriched with fuels like H2, natural gas and propane, the lean flammability limit and LBV increases. Cardona et al. [29] measured the LBV of biogas (66% CH4 - 34% CO2) and a biogas/C3H8/H2 mixture (33% CH4 - 17% CO2 - 40% C3H8 - 10% H2) with normal and oxygen-enriched air at 0.828 atm and 298 K for varying equivalence ratios. They observed that the LBV of CH4 and biogas/C3H8/H2 mixture differ by about 10%, thus making them interchangeable. Mameri and Tabet [30] found that by enriching biogas diffusion flames with H2, the mixture becomes more reactive leading to increase in reactive radical concentrations, heating value and flame temperature.

Porpatham et al. [35] experimentally examined an SI engine fueled by H2-enriched biogas (5%, 10%, and 15%), and reported that even small quantities of H2 addition to biogas may enhance the thermal efficiency and the power output of the engine with an appreciable reduction in the hydrocarbon emissions. Donohoe et al. [36] reported that an increase in temperature and pressure results in a decrease in ignition delay for H2-enriched CH4 and natural gas at atmospheric and high pressures.

Abundant literature is available on LBV measurements of CO2 diluted CH4 using different measurement techniques. However, not much data is available for mixtures when CH4 is diluted and enriched simultaneously with CO2 and H2 respectively. The literature [12], [28], [29], [30], [31], [35], [49] suggests that the combustion characteristics of low calorific value fuels like biogas can be significantly improved by enriching them with some high-grade fuels like H2, as the fuels with low energy content acquire some of the combustion properties of H2 when suitably enriched. In the present work, an attempt has been made to improve the combustion characteristics of biogas fuels by enriching them with H2. Also, the flame structure of CO2-diluted and H2-enriched CH4-air mixtures are studied in detail. For experimental work, the heat flux method due to de Goey and co-workers [37], [38], [39], [40], [41], is used for measuring the adiabatic laminar burning velocity of multicomponent gaseous fuel mixtures at 1 bar and 298 K.

Section snippets

Experimental and computational details

For experimental investigations, a flat flame burner similar to de Goey and co-workers [37], [38], [39], [40], [41] and Kishore et al. [5] was used. The setup was developed by our group, details of which can be referred from [5], [56], [57], is used for experimental work. However, a brief description is presented here. The setup consists of a circular; honeycomb holed, brass burner plate of thickness 2 mm and diameter 30 mm. The number of holes in the burner plate is approximately 1519 with

Results and discussion

The compositions for the experimental and computational studies conducted in the present work are shown in Table 1.

The experiments conducted on heat flux set-ups have uncertainty in LBV within 0.5–1 cm/s [45]. In this paper, the heat flux setup was validated with results obtained for CH4-air mixtures at 1 bar and 298 ± 1 K. The results of the present experiments for CH4-air mixtures compare well with earlier studies [5], [21], [40], [54] and computational predictions of GRI Mech. 3.0 [43] (Fig.

Chemical kinetics and sensitivity analysis

To get an insight into the important chemical reactions, and to analyze the chemical effects of CO2 dilution on CH4-air and of H2 enrichment on biogas-air mixtures, the flame structure was studied numerically, and sensitivity analyses were carried out using ANSYS Chemkin-Pro® with GRI Mech. 3.0 at 1 bar and 298 K. The sensitivity analyses were conducted with respect to mass flow rate, a quantity that is directly proportional to LBV. For sensitivity analysis, only the most dominant reactions

Conclusions

In this paper, the LBV of CH4 in the presence of CO2 as a diluent was measured using the heat flux method. The computational results using ANSYS Chemkin- Pro® with the San Diego and GRI Mech. 3.0 were compared with measured values. Experiments were conducted on different biogas mixtures (i.e., CH4 diluted with 5% to 50% CO2) at 298 ± 1 K and 1 bar at varying equivalence ratios. Fuel samples with 20%, 30%, 40% and 50% CO2 were enriched with 20% and 40% H2. To get an insight into the important

Acknowledgement

Vinod Kumar Yadav is grateful to G L Bajaj Institute of Technology and Management, Greater Noida (India) for sanctioning study leave for carrying out this research.

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