ARTICLE pubs.acs.org/EF
Experimental Study of Biogas Combustion in a Two-Layer Packed Bed Burner Huaibin Gao, Zhiguo Qu,* Wenquan Tao, Yaling He, and Jian Zhou Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education (MOE), Energy and Power Engineering School, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China ABSTRACT: Biogas is a promising low-calorific fuel composed of 5080% CH4 and 2050% CO2 and provides numerous economic and environmental benefits. In this study, biogas combustion in a two-layer porous burner packed with spherical alumina beads was experimentally investigated to provide guidelines for biogas burner design. The equivalence ratios varied from 0.75 to 0.95 at four different CO2 content values (25, 30, 35, and 40%). The flame stability limits at various equivalence ratios and CO2 concentrations were obtained. The range of biogas equivalence ratios for stable flame was at a higher level than the range for pure CH4. The flame tended to move from the upstream to the downstream region as the flame speed increased, resulting in high exhaust gas temperatures. The effects of the biogas CO2 ratio on the flame temperature, pressure drop of reaction flow, and radiation efficiency were also examined in detail. The NOx, CO, and HC emissions and the flame speed at the four CO2 concentrations were also analyzed. The NOx concentration was below 12 ppm for both biogas and pure methane. The CO and HC emissions for biogas and pure CH4 in the sphere-packed bed were higher than pure CH4 in a foam burner at lower flame speeds (S < 40 cm/s). However, the emissions were almost identical and maintained constant values at higher flame speeds (S > 40 cm/s) for the two different kinds of burners.
1. INTRODUCTION Combustion in porous-media burners is a relatively new technique that is characterized by high flame speeds, high efficiencies, high power densities, large dynamic ranges, and low NOx and CO emissions. In recent decades, many studies have been conducted on porous media burners, which have shown a number of advantages over conventional burners. Weinberg1 presented the idea of adiabatic combustion, also known as enthalpy combustion, in the 1970s. Takeno and Sato2 proposed inserting a highly conductive, porous solid into a flame to produce a postflame enthalpy to aid in preheating fresh mixtures. Echigo et al.3 theoretically and experimentally investigated combustion in porous media. Their results showed that porous media combustion extends combustion flame limits and could be applied for low-calorific gaseous combustion. Gaseous fuels include highcalorific gaseous fuels, such as methane, propane, and hydrogen, and low-calorific gaseous fuels, such as biogas, liquefied petroleum gas (LPG), and coke oven gas (COG). Most previous studies on combustion in porous media focused on high-calorific gaseous fuels. Low-calorific gaseous fuel combustion has demonstrated inherent problems related to both flame stability and combustion efficiency and has received much attention in recent years. Porous burners can be classified into two groups, the singlelayer type and the multi-layer type. Sathe et al.4 experimentally studied the stability and heat-transfer characteristics of lean premixed CH4/air flames embedded in a porous layer to understand the operating behavior of single-layer porous burners. Akbari et al.5 numerically investigated premixed methane/air combustion in a single-layer porous burner and found that the stable performance range of the burner is extended when the equivalence ratio increases. Zhdanok et al.6 conducted ultra-lean super-adiabatic combustion and obtained CO2 and water from r 2011 American Chemical Society
the complete oxidation of hydrocarbon fuel. Two-layer porous burners have been proposed for flame control. Hsu et al.,7 Zhou et al.,8 Mathis and Ellzey,9 and Vogel et al.10 reported that flames can be efficiently stabilized at or near the interface between two ceramic blocks of different porosities. Kulkarni and Peck11 and Barra et al.12 further indicated that the upstream section of a two-section porous burner should have low conductivity, a low volumetric heat-transfer coefficient, low porosity and short length, and a high radiative extinction coefficient, whereas the downstream section should have high conductivity, a high volumetric heattransfer coefficient, and an intermediate radiative extinction coefficient. The performance of two-layer porous burners in terms of their operating conditions, pollutant emission, and radiation efficiency has been extensively investigated. Ellzey and co-workers1315 conducted experiments, analyzed two-section porous burners, and showed that the stable operating range increases with equivalence ratio and that gas-phase dispersion was only important at higher equivalence ratios. Additionally, they found that the dominant mode of heat recirculation depends upon the equivalence ratio and flame speed. Low pollutant emissions were consistently found in previous investigations.1618 Khanna et al.16,17 indicated that CO and NOx emissions increased with flame speed. However, Marbach et al.18 found that the NOx concentration was weakly dependent upon flow velocity or the inert porous media pore size. They claimed that the NOx and CO concentrations increased with increasing equivalence ratios. Sathe et al.19 investigated the radiant outputs and efficiencies in a porous burner using a one-dimensional model and showed Received: April 2, 2011 Revised: June 2, 2011 Published: June 03, 2011 2887
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Figure 2. Schematic of the porous burner.
Figure 1. Schematic of the experimental setup.
Table 1. Composition of the Ceramic Balls element
that, for a maximum radiant output, the optical depth should be around 10 and the flame should be stabilized near the center of the porous medium. In a later study, Malico and Pereira20 numerically investigated combustion in inert porous media using two-dimensional models and reported that the predicted temperature profile was not in agreement with the available experimental values when radiation was neglected. The heat feedback of combustion enthalpy by porous media makes it possible to sustain combustion using low-calorie fuels. Wood et al.21 reviewed related studies and indicated that porous burners were capable of burning low-calorie fuels and lean fuel/ air mixtures without flame, potentially allowing for the exploitation of current wasted energy resources. However, few studies concerning porous media combustion with low-calorie gases have been conducted. Cho et al.22 experimentally investigated combustion characteristics of LPG and COG in a metallic fiber mat. They concluded that the range of surface loads for radiantmode combustion and the surface load for the peak surface temperature are mainly dependent upon the flame speed of the fuel used. Al-Hamamre et al.23 numerically and experimentally investigated the combustion behavior of low-calorie gaseous mixtures emitted by landfills and pyrolytic processes in silicon carbide (SiC) and Al2O3 porous structures. The inert species in the gas mixture was found to reduce the adiabatic flame temperature and burning velocity depending upon its heat capacity. Wood et al.24 examined a pilot-scale burner filled with a porous bed of alumina saddles for mine ventilation gas. They determined that the stable operating range of the burner at natural gas concentrations was as low as 2.3 vol %, with transient combustion at concentrations down to 1.1 vol %. Francisco et al.25 investigated the combustion of hydrogen-rich gaseous fuels with low calorie values in a porous burner and indicated that the laminar flame speed increased with an increasing H2 content in the mixture, resulting in larger stability limits. Zheng et al.26 experimentally investigated the combustion characteristics of low-calorie gas in SiC foam and showed that the combustion wave propagation velocity increased with an increasing inlet velocity and a decreasing equivalence ratio of the premixed gases. As a low-calorie fuel, biogas, which typically consists of 5080% CH4 and 2050% CO2, can be obtained from the
content (%) g92
AL2O3
e7
SiO2 Fe2O3
e0.1
CaO þ MgO þ others
e2
anaerobic breakdown of organic matter from numerous sources, such as landfills, sewage treatment plants, and anaerobic digesters. Biogas can act as a promising alternative fuel by substituting considerable amounts of fossil fuels to provide economic and environmental benefits. However, biogas combustion in porous media is a field that remains relatively unexplored. The general objective of the present study was to experimentally investigate the combustion of biogas in a porous burner consisting of two layers of alumina balls with diameters of 3.0 and 8.0 mm. In particular, the effects of the biogas CO2 ratio on the flame stability limits, flame temperature profile, pressure drops, radiation efficiency, and pollutant emissions were examined.
2. EXPERIMENTAL SECTION 2.1. Experimental Setup and Techniques. Figure 1 shows the experimental setup, which consisted of the supply system for fuel/air/ CO2, the porous burner, and the measuring and data acquisition system. Both CH4 (99.8%) and CO2 (99.995%) were stored in high-pressure bottles to flow through the pressure-reducing valve. The volumetric flow rates were regulated by two mass flow controllers (SierraC10Smart-Trak) to satisfy the required equivalence ratio before entering the mixing chamber. The air, which was supplied from a compressor connected to an air storage tank for pressure stabilization, passed through a filter and was controlled by another mass flow controller (SierraC100Smart-Trak). The gaseous fuels and air were fed into a static mixer to ensure that the fuelair mixture was spatially uniform and to maintain a constant equivalence ratio during the duration of the experiment. The mixture was then introduced into the porous medium burner. The porous burner was built of a 5 mm thick corundum tube with an internal diameter of 50 mm, as shown in Figure 2. A 30 mm thick Al2O3 foam plate with 60 pores per inch (PPI) was located at the chamber inlet to ensure a homogeneous mixture and to prevent flame flashback. Smaller alumina spheres with 3 mm diameters were located on the Al2O3 foam plate, while large ones with 8 mm diameters filled the top layer (ε ≈ 0.4). 2888
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Table 2. Summary Chart of Burners from Previous Studies Compared to the Present Study upstream
downstream
PPC (foams) or fuel
PPC (foam) or
equivalence ratio
material and its porosity
diameter (packed spheres, mm)
thickness (mm)
diameter (packed spheres, mm)
thickness (mm)
references and data type
0.60.70
Al2O3, ε = 0.4
3.0 mm
4.0
8.0 mm
4.0
present study, experimental
biogas
0.750.95
Al2O3, ε = 0.4
3.0 mm
4.0
8.0 mm
4.0
present study, experimental
methane
0.600.80
PSZ, ε = 0.85
25.6 PPC
3.5
3.9 PPC
2.55
Khanna et al.,16,17 experimental
methane
0.550.80
PSZ, ε = 0.85
25.6 PPC
3.5
3.9 PPC
2.55
Barra et al.,12 numerical
methane
0.550.70
YZA, ε = 0.85
23.6 PPC
5.08
3.9 PPC
5.08
Smucker et al.,15 experimental
methane
Table 3. Uncertainty for Measured Variables variable
formula
parameters EB = (1.26 °C, ETh = (8.5 °C
temperature
uncertainty 8.59 °C
ET = EB2 þ ETh2)1/2 ∂S/∂V_ = 1/A = 0.0509 cm2 ∂S/∂A = V_ max(1/A2) = 3.56 cm1 s1
flame speed
_ CH 2 þ (∂V_ )CO 2)1/2 = 1.04 L/min = 17.33 cm3/s ∂V_ = ((∂V_ )air2 þ (∂V) 4 2 _V max = 82.48 L/min = 1374.67 cm3/s ES = [((∂S/∂V_ )∂V_ )2 þ((∂S/∂A)δA)2]1/2/S ∂V_ air = 1.0 L/min, ∂V_ air-max = 75.0 L/min ∂V_ CH4 = 1.0 L/min, V_ CH4-max = 5.61 L/min ∂V_ CO = 1.0 L/min, V_ CO -max = 1.87 L/min 2
1.4%
2
δA = 0.04 cm2, Smax = 70 cm/s ∂ϕ/∂V_ CH = 9.52(1/V_ air-max) = 0.127 min/L 4
equivalence ratio
Eϕ = [(∂ϕ/∂V_ CH4)∂V_ CH4)2 þ (∂ϕ/∂V_ air)∂V_ a)2 þ (∂ϕ/∂V_ CO2)∂V_ CO2)2]1/2/ϕ
mole fraction
ER = [(∂R/∂V_ CH4)∂V_ CH4) þ (∂R/∂V_ CO )∂V_ CO )2]1/2/R 2
2
2
∂ϕ/∂V_ air = 9.52 (V_ CH4-max þ V_ CO2-max)(1/V_ air-max2) = 0.127 102 min/L ∂ϕ/∂V_ CO2 = 9.52(1/V_ a) = 0.127 min/L V_ air-max = 75.0 L/min, ϕmax = 0.95 ∂R/∂V_ CH4 = V_ CO2-max/(V_ CO2-max þ V_ CH4-max)2 = 3.34 102 min/L ∂R/∂V_ CO = V_ CH -max/(V_ CO -max þ V_ CH -max)2 = 0.10 min/L 2
4
2
4
2.3%
2.7%
Rmax = 0.40
pressure drop
1.5 Pa
emission
1 ppm
The thickness of the 3 mm ball layer and that of the 8 mm ball layer were each 50 mm. It should be noted that the Al2O3 foam plate supported the porous media and was used as a structure to prevent flashback and that the 3 mm packed bed of spheres was a component of the burner, which was not for preventing flashback. Hence, the burner could be considered as a two-layer burner. Table 1 shows the composition of the alumina balls as measured by X-ray fluorescence (XRF) and atomic absorption spectroscopy (AAS). The burner was covered by a Kaowool high-temperature insulation material layer with a thickness of 45 mm to minimize heat loss. The temperature distribution in the packed bed was measured at 12.5 mm intervals by nine B-type thermocouples located in the centerline of the tube along the flow direction. The local thermal non-equilibrium existed because of the thermal conductivity difference between the solid balls and the hot gas. However, the temperature for the solids and the gas could not be obtained separately in the present study. The measured temperature distribution could only be based on the local thermal equilibrium assumption, which should be understood as a mean temperature between the gas and the solid phases. This method has been applied in previous experimental studies.25,27,28 The pressure drop for the gas flowing through the burner was tested by a Rosemount 3051 pressure transducer. All signals for the thermocouples and pressure transducer were recorded using an Agilent data acquisition system. A stainless-steel
probe was used to sample the combustion products at the top of the burner, and the NOx, CO, and HC concentrations in the exhaust gases were measured using a TESTO 350 Pro-analyzer. 2.2. Experimental Procedure. The flame was initially ignited near a stoichiometric ratio at the burner top. The flame front reversely penetrated the porous media, reaching the small-diameter Al2O3 ball layer. The dilution gas (CO2) was then added to the fuel, and the operating parameters were adjusted to the desired test conditions. The flame was considered stable when the temperature fluctuation remained within 10 K for at least 30 min. Data on pressure drops, emissions, flame structure temperatures, and flame locations were obtained. The equivalence ratio was fixed at a constant value by simultaneously adjusting the volumetric flow rates of CH4, air, and CO2. The lower stability limit was designated as the flow velocity at which the flame front reached the position of thermocouple TC8 (seen in Figure 2); below this velocity, a flashback occurred. The mass flow rate of the fuel mixture was then adjusted in small increments until the flame floated on the burner surface, and the corresponding velocity was defined as the upper flame stability. The value of the equivalence ratio was then changed, and the above procedure was repeated to obtain the flame stability at a different concentration. The maximum equivalence ratio used was limited by the thermal resistance of the porous media. In the current study, the ceramic 2889
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from the mass flow controllers. When a flame is stable, the flame speed S is equal to the mean flow velocity. The flame speed S and the radiation efficiency ηrad, which is an important factor for surface burner design, were introduced by Francisco et al.25 and are defined as S¼
V_ CH4 þ V_ CO2 þ V_ air V_ ¼ A A
ηrad ¼ εσðTsurf 4 Tsurr 4 Þ=ðV_ CH4 LHVÞ
Figure 3. Comparison of flame stability limits for pure CH4 at different equivalence ratios with previous studies. balls consisted of Al2O3, which can sustain an extreme temperature of 1700 °C; thus, the maximum temperature allowed was set at 1650 °C. 2.3. Parameter Definitions. The global combustion reaction of the biogas in the porous burner is expressed as ð1 RÞCH4 þ
1þϕ ðO2 þ 3:76N2 Þ ϕ
þ RCO2 f CO2 þ 2ð1 RÞH2 O þ 7:52ð1 RÞN2 1ϕ ð1 RÞðO2 þ 3:76N2 Þ þ ϕ
ð1Þ
where ϕ and R are the equivalence ratio and the CO2 mole fraction, respectively, which are defined in eqs 2 and 3 as ϕ¼
V_ CH4 þ V_ CO2 ðmfu =mair Þactu ¼ 9:52 ðmfu =mair Þstoic V_ air
ð2Þ
nðCO2 Þ V_ CO2 ¼ nðCO2 þ CH4 Þ V_ CH4 þ V_ CO2
ð3Þ
R¼
In eqs 2 and 3, mfu and mair are the mass flow rates of the biogas and air, respectively, and V_ CH4 V_ CO2, and V_ air are the respective volumetric flow rates of CH4, CO2, and air at standard conditions, which were obtained
ð4Þ ð5Þ
where V_ is the total volumetric flow rate of the biogas and air and A is the inner cross-sectional area of the burner. The various CO2 mole fractions can be obtained by adjusting the volumetric flow rates of CH4 and CO2 under a fixed equivalence ratio. In the present study, the equivalence ratio ranged from 0.75 to 0.95 and the CO2 content R in the fuel ranged from 25 to 40%. For comparison, the fuel type, test conditions, porous burner configuration, and porous media materials of previous studies12,1517 with two-layer burners for pure methane are summarized in Table 2. 2.4. Uncertainty Analysis. An uncertainty analysis was conducted for the temperature measurement, flame speed, equivalence ratio, and CO2 mole fraction using the root-sum-squares (RSS) method by Moffat.29 The temperature uncertainty includes that of the temperature terminal block of data acquisition and thermocouple. The uncertainty for the terminal block, EB, was 1.26 °C, and the uncertainty of the B-type thermocouple, ETh, was 8.5 °C in the temperature measurement range of 8001700 °C. Thus, the total uncertainty of the temperature measurement system was ET = 8.59 °C. The uncertainty for the burner diameter was 0.05 mm. Thus, the uncertainty for the cross-sectional area was 0.04 cm2. The uncertainty for the three volumetric flow rates of V_ CH4, V_ CO2, and V_ air was 0.1, 0.1, and 1.0 L/min, respectively. Therefore, the uncertainty of the total volumetric flow rate was 1.04 L/min. Thus, the uncertainty of the flame speed was determined to be 1.4%. Similarly, the uncertainty for the equivalence ratio Eϕ and that of the CO2 mole fraction ER were 2.3 and 2.7% based on the maximum volumetric flow rates of V_ CH4 (5.61 L/min), V_ CO2 (1.87 L/min), and V_ air (75.0 L/min). The pressure drop and the emission measurements had an uncertainty of 1.5 Pa and 1 ppm according to the manufacturer. More details of the experiment uncertainties are provided in Table 3.
3. RESULTS AND DISCUSSION Flame stability limits are important factors that determine the general combustion performance of a burner, and the effect of CO2 concentrations in biogas on the flame stability limits is vital. The flame stability limits of pure CH4 for an equivalence ratio range of 0.600.70 are shown in Figure 3a, in which the mean flow velocity as a function of the equivalence ratio, ϕ, is plotted. The continuous lines represent the upper and lower stability limits. The diagram can be divided into different regions: the blow off region (above the upper stability limit), the stable region, and the flashback region (below the lower stability limit). The stable operating ranges for the methane/air mixture in the present study were compared to the numerical results by Barra et al.12 and the experimental data by Kkanna17 for a two-section foam burner of partially stabilized zirconia (PSZ) (ε ≈ 0.85) in Figure 3b. The pore density was 25.6 and 3.9 pores per centimeter for the upstream and downstream sections, respectively. Both the results from the present study and results from previous studies12,17 showed similar trends. The stable operating range extended and shifted to larger values when the equivalence ratio increased. The present results were lower than the computational results by Barra et al.12 This discrepancy was because radial heat losses were not included in the calculations 2890
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Figure 4. Flame stability limits for biogas at four CO2 concentrations.
in the study by Barra et al.12 Furthermore, the present results were higher than the experimental results by Khanna et al.17 because of the lower porosity of packed spheres (ε ≈ 0.4) in the present study. The flame stability limits for biogas at four CO2 concentrations are shown in Figure 4. In comparison to CH4, the range of biogas equivalence ratios for stable flame moved to a higher level and the biogas stable flame speed was lower. The equivalence ratio of pure CH4 for both the upper and lower limits was in the range of 0.550.70; in comparison, the range of biogas for both the upper and lower limits moved to 0.750.95 and 0.80.95 at CO2 concentrations of 2535 and 40%, respectively. The upper stability limits and the stable flame region decreased with increasing CO2 concentrations. For example, the maximum flame speeds were 60, 50, 35, and 25 cm/s for CO2 contents of 25, 30, 35, and 40%, respectively, at an equivalence ratio of ϕ = 0.9. These results indicated that more CH4 was available in the biogas at a given equivalence ratio and a higher flame speed was necessary. Figures 5 and 6 display the measured temperature profiles along the centerline for pure CH4 at ϕ = 0.65 and the biogas at ϕ = 0.9 with different CO2 mole fractions. The flame location can be identified as the position where the temperature reaches the highest values. The flame speeds and temperatures increased with a decreasing CO2 content. However, the overall shape of the temperature profile did not change significantly with variations in the CO2 concentration. At a fixed CO2
Figure 5. Temperature profiles with pure CH4 for ϕ = 0.65 at different mean flow velocities.
concentration, the flame location moved from the interface to the downstream region as the flame speed increased, leading to a high exhaust gas temperature and significant heat loss. At a fixed flow velocity in the flame stability range, the flame location also moved to the downstream region with an increasing CO2 concentration. 2891
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Figure 6. Measured centerline temperature profiles for biogas at a constant equivalence ratio of ϕ = 0.9.
Figure 7. Maximum flame temperature as a function of the equivalence ratio for biogas.
Figure 8. Flame temperature as a function of the mean flow velocity for different CO2 fractions (ϕ = 0.9).
Aside from the equivalence ratios and velocities, the CO2 concentration is undoubtedly one of the most important factors in biogas combustion in porous media. Figure 7 shows the effects of the equivalence ratio on the maximum flame temperature at various CO2 concentrations. The maximum flame temperature increased with increasing equivalence ratios or decreasing CO2 concentrations because of relatively lower heat loss at higher CH4 contents.
Figure 8 shows the flame temperatures as a function of the mean flow velocity for different CO2 contents in the biogas at ϕ = 0.9. The flame temperature increased with an increasing flame speed or a decreasing CO2 concentration in the biogas. For a given CO2 mole fraction, the temperature increased significantly at low velocities and the increasing trend became slight at high velocities. The maximum flame temperature did not specifically 2892
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Figure 9. Radiation efficiency as a function of the mean flow velocity for biogas (ϕ = 0.9).
Figure 10. Pressure drop as a function of the mean flow velocity for different CO2 fractions (ϕ = 0.9).
correspond to the maximum flame speed at higher CO2 concentrations, indicating that the flame location moves downstream near the outlet with an increasing flame speed, where the heat loss was more significant. Hence, the temperature of a high velocity flame located near the outlet may be lower than that of the flame located inside the burner with a low velocity. The influence of velocity on radiation efficiencies at various CO2 concentrations is shown in Figure 9. Hayashi et al.30 studied the influence of downstream surrounding temperatures on the performance of a two-layer foam burner and reported that the burner radiative heat loss was relatively lower when the burner radiated to a hot environment, which led to higher temperatures and pollutant emissions. The surrounding temperature in the present study ranged from 15 to 30 °C during the experimental process. In the calculation of the radiation efficiency, the surrounding temperature was considered to be the local concrete measured temperature. However, the influence of surrounding temperature variation on the radiation efficiency was mild and negligible because the surrounding temperature was significantly lower than the burner surface temperature. In Figure 9, the higher velocities yielded higher radiation efficiencies for a given CO2 ratio. In the radiation efficiency definition for eq 5, the volumetric flow rate increased linearly with an increasing velocity, whereas the surface temperature increased exponentially.
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Figure 11. NOx emissions as a function of the mean flow velocity for different CO2 fractions (ϕ = 0.9).
This result demonstrates that the surface temperature explicitly affects the radiation efficiency. The effect of the CO2 concentration on the radiation efficiency was insensitive compared to the flow velocity. The radiation efficiency varied from 14 to 28% for the biogas at different CO2 concentrations. The pressure drop of the reaction flow for various CO2 concentrations at an equivalence ratio of 0.9 is shown in Figure 10. The pressure drop of the cold flow for the fuel mixture was also plotted as a reference case. The pressure drop of the reaction flow was higher than that of the cold flow, and the pressure drop increased with an increasing CO2 concentration because the gas density decreased with an increasing velocity as it flowed through the reaction zone, resulting in a significant pressure drop. Smucker et al.15 and Xiong et al.31 also indicated that the pressure drop increased with an increasing velocity, and this trend became weaker at higher flame velocities. Meanwhile, the pressure drop dramatically increased at low flame velocities and increased only slightly or even decreased with an increasing flow velocity at higher CO2 mole fractions because the flame was located further downstream and the distance traveled by the expanded reaction flow was shortened. NOx, CO, and HC emissions with an equivalence ratio of 0.9, as measured by the TESTO 350 Pro-analyzer, are shown in Figures 1113. The NOx emissions at different CO2 mole fraction dilutions as a function of velocity are shown in Figure 11. The NOx emissions for pure methane in the present study were compared to the results by Smucker et al.15 and Khanna et al.16 Although the equivalence ratio for biogas was higher than pure methane, the NOx concentration generally increased with an increasing inlet velocity and was below 12 ppm for both biogas and pure methane. The NOx emission was reduced with an increase in the CO2 diluent fraction because of the lower power input and NOx formation mechanism. The formation rate of NOx, which is strongly dependent upon the concentration of N2 and O2, is proportional to [O2]0.5[N2]. Both N2 and O2 concentrations increased in the flame zone with a decreasing CO2 concentration, thus producing higher NOx emissions. The effect of the flame speed on the NOx emission is dependent upon the CO2 concentration. The NOx emission significantly increased as the velocity increased at a higher CO2 diluent fraction and slightly increased at a lower CO2 diluent fraction. Figures 12 and 13 show the effect of the flow velocity on CO and HC emissions for biogas at the four CO2 concentrations 2893
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concentration levels. Furthermore, the effect of the flame speed on CO and HC emissions was also dependent upon the CO2 concentration. At a low CO2 dilution content (2530%), the CO and HC emissions slightly decreased with increasing velocity. In contrast, the CO and HC emissions at a high CO2 dilution content (3540%) sharply decreased with increasing velocity. The above trend is associated with the flame temperature at various velocities, as indicated in Figure 8. At high CO2 dilution contents, increasing flame temperatures were evident with an increasing velocity, resulting in enhanced CO and HC burning. Conversely, the flame temperature increment was mild with an increasing velocity at a low CO2 dilution content, resulting in unchanged burning quantities of CO and HC and almost constant final CO and HC concentrations.
Figure 12. CO emissions as a function of the mean flow velocity for different CO2 fractions (ϕ = 0.9).
Figure 13. HC emissions as a function of the mean flow velocity for different CO2 fractions (ϕ = 0.9).
(ϕ = 0.9). The CO and HC emissions for pure methane in the present study were compared to the results by Smucker et al.15 and Khanna et al.16 The CO and HC emissions for methane in the foam burner were independent of the flame speed. Comparatively, the CO and HC emissions of pure methane and biogas apparently decreased with an increasing flame speed in the packed sphere bed and were higher than that of pure methane in the foam burner at low flame speeds (S < 40 cm/s). At high flame speeds (S > 40 cm/s), the CO and HC emissions of both biogas and pure methane were almost identical and maintained a lower constant value. The formations of CO and HC were affected by inhomogeneous mixing. The porosity of the packed sphere (ε ≈ 0.4) in the present study was lower than the porosity of the foam used by Khanna et al.16 and Smucker et al.15 (ε ≈ 0.85). The ceramic beads have a near point of contact with neighboring beads, while the tortuous path network in the foam structure exhibits good interconnectivity. Hence, radial mixing was limited for packed beads compared to foam, resulting in higher CO and HC emissions at low velocities. However, the mixing homogeneity discrepancy became less evident for the two structures at high velocities, leading to similar lower CO and HC
4. CONCLUSION Biogas combustion at different CO2 contents in a two-layer packed bed burner was investigated. The flame stability limits for the biogas at various CO2 dilution concentrations were obtained. The stable flame speed for the biogas was lower than that of pure CH4, and the biogas equivalence ratio range for stable flame moved to a higher level. The maximum flame temperature increased with an increasing equivalent ratio. With an increasing flame speed, the flame location moved downstream. The maximum flame temperature likely did not correspond to the maximum flame speed when the flame location was near the outlet of the burner. With an increasing CO2 concentration, both the stable flame region and the flame temperature decreased. The radiation efficiency was mainly related to the flow velocity, and CO2 concentrations were observed to have a mild effect on the radiation efficiency. The pressure drop of the reaction flow was higher than that of the cold flow and increased with an increasing CO2 concentration. Additionally, the pressure drop dramatically increased at a low flame speed and slightly increased or even decreased with an increasing flow velocity at higher CO2 mole fractions. The effect of velocity on NOx, CO, and HC emissions was dependent upon CO2 and was more significant at lower flow velocities. In comparison to pure methane combustion in a twolayer foam burner, the NOx concentration was below 12 ppm for both biogas and pure methane. The CO and HC emissions for biogas and pure CH4 in a sphere packed bed were higher than pure CH4 in a foam burner at lower flame speeds (S < 40 cm/s) and were almost identical at higher flame speeds (S > 40 cm/s). ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Key Projects of Fundamental Research and Development of China (973 Project; 2011CB610306) and the National Natural Science Foundation of China (50806057). ’ NOMENCLATURE A = inner cross-sectional area of the burner (m2) Amax = maximum inner cross-sectional area of the burner (m2) EB = uncertainty of the terminal block (°C) ES = uncertainty of the flame speed (cm/s) 2894
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Energy & Fuels ET = uncertainty of the temperature measurement system (°C) ETh = uncertainty of the thermocouples (°C) ER = uncertainty of the CO2 mole fraction in the biogas Eϕ = uncertainty of the equivalence ratio mair = mass flow rate of air (kg/s) mfu = mass flow rate of biogas (kg/s) (mfu/mair)actu = actual mass fuelair ratio (mfu/mair)stoic = stoichiometric mass fuelair ratio LHV = low heating value of biogas (J/m3) n(CO2) = moles of CO2 n(CO2 þ CH4) = moles of biogas S = flame speed (cm/s) Smax = maximum flame speed (cm/s) TC = temperature of the centerline Tsurf = surface temperature (K) Tsurr = surrounding temperature (K) V_ = total volumetric flow rate of biogas and air (m3/s) V_ max = maximum total volumetric flow rate of biogas and air (m3/s) V_ air = volumetric flow rate of air (m3/s) V_ air-max = maximum volumetric flow rate of air (m3/s) V_ CH4 = volumetric flow rate of CH4 (m3/s) V_ CH4-max = maximum volumetric flow rate of CH4 (m3/s) V_ CO2 = volumetric flow rate of CO2 (m3/s) V_ CO2-max = maximum volumetric flow rate of CO2 (m3/s) R = CO2 mole fraction in the biogas Rmax = maximum CO2 mole fraction in the biogas ε = porous matrix surface emissivity ϕ = equivalence ratio ϕmax = maximum equivalence ratio ηrad = radiation efficiency (%) σ = StefanBoltzmann constant (W m2 K4)
ARTICLE
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