Combustion and Heat Release Characteristics of Biogas Under Hydrogen- and Oxygen-Enriched Condition

energies

Article Combustion and Heat Release Characteristics of Biogas under Hydrogen- and Oxygen-Enriched Condition

Jun Li 1, Hongyu Huang 2,*, Huhetaoli 2, Yugo Osaka 3, Yu Bai 2, Noriyuki Kobayashi 1,* and Yong Chen 2

1 Department of Chemical Engineering, Nagoya University, Nagoya, Aichi 464-8603, Japan; [email protected] 2 Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; [email protected] (H.); [email protected] (Y.B.); [email protected] (Y.C.) 3 Faculty of Mechanical Engineering, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan; [email protected] * Correspondence: [email protected] (H.H.); [email protected] (N.K.); Tel.: +86-20-870-48394 (H.H.); +81-52-789-5428 (N.K.)

Received: 10 May 2017; Accepted: 20 July 2017; Published: 13 August 2017

Abstract: Combustion and heat release characteristics of biogas non-premixed flames under various hydrogen-enriched and oxygen-enriched conditions were investigated through chemical kinetics simulation using detailed chemical mechanisms. The heat release rates, chemical reaction rates, and molar fraction of all species of biogas at various methane contents (35.3–58.7%, mass fraction), hydrogen addition ratios (10–50%), and oxygen enrichment levels (21–35%) were calculated considering the GRI 3.0 mechanism and P1 radiation model. Results showed that the net reaction rate of biogas increases with increasing hydrogen addition ratio and oxygen levels, leading to a higher net heat release rate of biogas flame. Meanwhile, flame length was shortened with the increase in hydrogen addition ratio and oxygen levels. The formation of free radicals, such as H, O, and OH, are enhanced with increase in hydrogen addition ratio and oxygen levels. Higher reaction rates of exothermic elementary reactions, especially those with OH free radical are increased, are beneficial to the improvement in combustion and heat release characteristics of biogas in practical applications.

Keywords: biogas; heat release; methane content; hydrogen addition; oxygen enriched

1. Introduction With continuous depletion of fossil fuels and worsening pollution, research and development of renewable energy sources have attracted considerable interest [1–7]. Biogas can be produced using an anaerobic digester of biomass or biodegradable organic wastes. Compositions of biogas vary with the feedstock and the fermentation process [1–3]. The main compositions of biogas are methane (CH4) and carbon dioxide (CO2), as well as a small amount of water (H2O), nitrogen (N2), and hydrogen (H2)[3–5]. As an advantageous energy source, biogas is widely considered as a suitable fuel for heating facilities, power generation, and vehicles [8–15]. However, biogas possesses a relatively lower heating value (about 3000–6000 kcal·m−3) and lower heat release rate than conventional fuels, which limits the application of biogas in combustion devices. Somehsaraei et al. [16] developed a steady state thermodynamic model for biogas application in 100 kW micro gas turbines. They found that mass flow and pressure ratio in the micro gas turbines are lower than those of the natural gas–fueled turbines. Furthermore, the total electrical efficiency decreases with decreasing CH4 content in biogas fuel, and the use of biogas negatively affects heat recovery. A three-dimensional computational fluid dynamic study on biogas flameless mode was developed by Hosseini et al. [17]. Although increasing preheated temperature positively affects the reduction of fuel consumption, a fuel with

Energies 2017, 10, 1200; doi:10.3390/en10081200 www.mdpi.com/journal/energies Energies 2017, 10, 1200 2 of 11 high calorific value is necessary to preheat the furnace prior to the application of biogas combustion in the combustion furnace. The flameless combustion temperature of a biogas furnace is lower than traditional combustion throughout a combustion chamber. Several experimental and numerical studies have explored the fuel characteristics of biogas. In such studies, high-quality fuel such as H2 [18–22] is blended with biogas to improve the relatively poor combustion characteristics of biogas. Zhen et al. [18] experimentally investigated the effect of H2 addition on the characteristics of a biogas diffusion flame. They found that H2 addition significantly improved the stability of the biogas flame. The flame temperature increased and visible flame length reduced with increasing H2 addition ratios. Wei et al. [19] numerically investigated the effects of equivalence ratio, H2, and CO2 on the heat release characteristics of the premixed laminar biogas-hydrogen flames. The results showed that OH + H2 ⇔ H + H2O was major endothermic reaction for biogas-hydrogen premixed flames. The total heat release was enhanced evidently with H2 addition. Non-premixed are widely applied in the industrial process, while the heat release rate of biogas flame is low, which obstructs the application of biogas in practical application. Therefore, the improvement of heat release characteristics of biogas flame at various condition are necessary. The above results show that numerous studies have investigated the combustion characteristics of biogas premixed flame. Nevertheless, understanding the non-premixed combustion of a fuel in practical application is necessary. To utilize biogas in internal combustion engines, the unburnt composition CO2 or N2 is expected to be excluded from biogas. Therefore, mechanical biological treatment is needed to upgrade raw biogas and increase CH4 content in biogas. The high cost of such treatment process is unfavorable to the application of biogas as a fuel. Therefore, the direct application of raw biogas is more economic for engines or heat supply. The current research proposes biogas combustion under hydrogen-enriched and oxygen-enriched conditions as potential methods to improve biogas combustion. The non-premixed flames of biogas under various hydrogen-enriched and oxygen-enriched conditions are studied, and the combustion and heat release characteristics of biogas non-premixed flames are numerically investigated. A non-premixed combustion model of biogas is established. The GRI 3.0 mechanism with 53 species and 325 elementary chemical reactions is employed, and the P1 radiation model is considered. All elementary chemical reaction rates and heat release rates are programmed. To enhance the combustion and heat release characteristics, hydrogen addition and oxygen enriched combustion are selected for biogas combustion in future practical application. The mass contents of CH4 in biogas, hydrogen addition ratios, and oxygen enrichment levels are ranged from 35.3% to 58.7%, 0% to 50%, and 21% to 35%, respectively.

2. Numerical Simulations A typical gaseous combustion cylindrical chamber with a diameter of 200 mm (r) and 600 mm length (z) is employed in this research, as shown in Figure1. The fuel inlet is located at a central nozzle with a radius of 6.8 mm, and air inlet is surrounded with a fuel inlet with a radius of 100 mm. Three kinds of biogas fuel are applied in this research. The H2 addition ratio to fuel (XH2 , based on low heat value) and O2-enriched level (ΩO2 ) are deﬁned as following equations:

VH2 × LHVH2 XH2 = (1) VCH × LHVCH + VH ×LHV 4 4 2 H2

VO2 ΩO2 = (2) VO2 + VN2 where VCH4 , VH2 , VO2 , and VN2 are the ﬂow rates of CH4,H2,O2, and N2 respectively; LHVH2 and

LHVCH4 are the low heat values of H2 and CH4. The inlet fuel and oxidizer conditions at various H2-enriched condition and O2-enriched condition are shown in Table1. The inlet power is kept constant under all calculation conditions. The mass contents of CH4 in biogas are 35.3%, 45.9%, and Energies 2017, 10, 1200 3 of 11 Energies 2017, 10, 1200 3 of 11

58.7%,namely namely BG-1, BG-1, BG-2, BG-2, and andBG-3, BG-3, respectively. respectively. H2 addition H2 addition ratio to ratiobiogas to is biogas arranged is arranged from 10% from to 50%, 10% to and O2 in the oxidizer is from 21% to 35%. 50%, and O2 in the oxidizer is from 21% to 35%.

FigureFigure 1. 1.Geometry Geometry of ofthe the combustioncombustion chamber chamber (not (not to toscale). scale).

TableTable 1. Composition 1. Composition of of biogas biogas ﬂames flamesat at HH22-enriched and and O O2-enriched2-enriched conditions. conditions.

Items CH4/L·min−1 CO2/L·min−1 H2/L·min−1 O2/L·min−1 Air/L·min−1 Power/kW Items CH /L·min−1 CO /L·min−1 H /L·min−1 O /L·min−1 Air/L·min−1 Power/kW BG-1 4 1.829 2 1.220 2 - 2 - 18.340 1 BG-1BG-2 1.8291.829 1.220 0.784 - - - - 18.340 18.340 1 1 BG-2BG-3 1.8291.829 0.784 0.457 - - - - 18.340 18.340 1 1 BG-3 1.829 0.457 - - 18.340 1 H2 10% 1.647 1.098 0.602 - 18.014 1 H2 10% 1.647 1.098 0.602 - 18.014 1 H2 30% 1.281 0.854 1.805 - 17.361 1 H2 30% 1.281 0.854 1.805 - 17.361 1 H2 50% 0.915 0.610 3.008 - 16.709 1 H2 50% 0.915 0.610 3.008 - 16.709 1 O2 25% 1.829 1.220 - 0.780 14.626 1 O2 25% 1.829 1.220 - 0.780 14.626 1 O2 30%O2 30% 1.829 1.829 1.220 1.220 - - 1.463 1.463 11.376 11.376 1 1 O2 35%O2 35% 1.829 1.829 1.220 1.220 - - 1.950 1.950 9.054 9.054 1 1

The flow mass, chemical species, momentum, and energy equation based on Navier–Stokes equationsThe flow is mass, expressed chemical in Equation species, (3): momentum, and energy equation based on Navier–Stokes equations is expressed in Equation (3): + = Γ + (3) δ(ρY) → + div(ρVY) = div(ΓYgradY) + SY (3) where , , , Γ , and δ tare density, general dependent variable, velocity vector, associated transport→ coefficient of Y, and the source term. Thermal radiation is programmed to the source term whereof ρthe, Y ,mixtureV, ΓY, andenthalpy.SY are In density, this research, general dependentP1 radiation variable, developed velocity by Grosshandler vector, associated of National transport coefficientInstitute of ofY Standards, and the and source Technology term. Thermal (NIST) [23] radiation was selected. is programmed The gas radiation to the was source considered term by of the mixturethe determination enthalpy. In of this Planck research, mean P1absorption radiation coef developedficient. Gas byradiation Grosshandler contributed of Nationalto the gases Institute of of StandardsCH4, CO2, andCO, and Technology H2O. The mean (NIST) absorption [23] was coefficient selected. of The these gas gases radiation () can be was fitted considered as functions by the of temperature, which are summarized as Equations (4)–(6): determination of Planck mean absorption coefficient. Gas radiation contributed to the gases of CH4, Mean absorption coefficient of CH4: CO2, CO, and H2O. The mean absorption coefficient of these gases (αi) can be fitted as functions of temperature, which are summarized = + as Equations+ (4)–(6):+ + + (4)

MeanMean absorption absorption coefﬁcient coefficient of of CH CO4:2 and H2O: 1000 1000 2 10003 4 1000 5 1000 = + αi = a0 ++ a1T + a2T++a3T ++a4T + a5T + (5) (4)

MeanMean absorption absorption coefﬁcient coefficient of of CO CO:2 and H2O:

= + + + + (6) 1000 1000 2 1000 3 1000 4 1000 5 α = a + a +a + 4a 2 + a 2 + a (5) Thei values0 of1 constantsT 2 to T for CH , 3CO ,T CO, and H4 O areT shown in5 TableT 2. The boundary wall is treated as a gray heat sink with emissivity of 0.8, and the wall temperature is Mean absorption coefﬁcient of CO:

αi = a0 + T{a1 + T[ a2 + T(a3 + a4T)] } (6) Energies 2017, 10, 1200 4 of 11

The values of constants a0 to a5 for CH4, CO2, CO, and H2O are shown in Table2. The boundary wall is treated as a gray heat sink with emissivity of 0.8, and the wall temperature is maintained at 300 K through water cooling. The total absorption coefficient of radiating gas mixture is calculated as the sum of the absorption coefficients of CH4, CO2, CO, and H2O in the flame. All transport equations with radiation are solved by CFD software PHOENICS 2014 (Cham-Japan, Tokyo, Japan) [24]. The velocity and pressure coupling are treated by a SIMPLE method algorithm [25]. The GRI 3.0 mechanism with 53 species and 325 reactions [26] is applied to this research. The transport and thermal properties of all species are calculated by GRI 3.0 database and CHEMKIN codes. As a key parameter in fuel combustion, the heat release rates of all elementary reactions are defined and programed. Taking reaction R52 as example, we can calculate the heat release rate by following equations:

R52 : H + CH3(+M) ⇔ CH4(+M)

= + − · −1 ∆hi=52 hH hCH3 hCH4 ,J mol (7)

qi=52 = ∆hi=52 × ki=52 (8) 325 qnet = ∑ qi (9) i=1 −1 where hj is formation enthalpy of specie j (J·mol ); ∆hi, ki, and qi are enthalpy change of reaction i (J·mol−1), the reaction rate of reaction i (mol·cm−3·s−1), and heat release of reaction i (J·cm−3·s−1), −3 −1 respectively. Furthermore, qnet indicates the total heat release rate of all reactions (mol·cm ·s ).

Table 2. Absorption coefﬁcient of a0 to a5 for CH4, CO, CO2, and H2O.

Coefﬁcient CO2 H2O CH4 CO (T < 750 K) CO (T > 750 K)

a0 18.741 −0.23093 6.6334 4.7869 10.09 −3 a1 −121.31 −1.1239 −3.5686 × 10 −0.06953 −0.01183 −8 −4 −6 a2 273.5 9.4153 1.6682 × 10 2.95775 × 10 4.7753 × 10 −10 −7 −10 a3 −194.05 −2.9988 2.5611 × 10 −4.25732 × 10 −5.87209 × 10 −14 −10 −14 a4 56.31 0.51382 −2.6558 × 10 2.02894× 10 −2.5334 × 10 −5 a5 −5.8169 −1.884 × 10 0 - -

3. Discussion Figure2 shows several typical measured outlet temperatures of BG-2/air flames with various H2 addition ratios and O2-enriched levels at the same input power of 1.0 kW and equivalence ratio of 0.95; the simulation results are shown for comparison. The outlet temperature was measured by a K-type thermocouple on a same combustion chamber and burner size with simulation, which was described in our previous study [27]. The error of experimental temperature at all H2 addition ratios and O2-eneriched levels are less than 15%. Furthermore, the deviation of simulation and experimental results are below 11%. Therefore, the simulation results show good agreement with experimental data at all H2 addition ratios and O2-enriched levels, thereby confirming that the GRI 3.0 mechanism provides an acceptable and repeatable result for biogas combustion under H2-enriched and O2-enriched conditions. The effect of CH4 content in biogas fuel on the combustion temperature distribution is shown in Figure3. The maximum combustion temperature increases with increasing CH 4 content in biogas fuel. Furthermore, the flame length through the z-direction decreases at a higher CH4 content condition, and flame width through the r-direction also shrinks with increasing CH4 content in biogas fuel. These findings are attributed to the following explanations: (a) the reaction rate of elementary reactions in biogas fuel flames increases with increasing CH4 content in biogas fuel as shown in Figure4, and is usually related to the flame structure; (b) the heat loss to heat up unburnt CO2 in biogas fuel gas decreases with higher CH4 content in biogas fuel; and (c) the total volume of combustion gas (including Energies 2017, 10, 1200 5 of 11 fuel gas and air) decreases when CH content in biogas fuel increases, leading to higher temperature EnergiesEnergies 20172017,, 1010,, 12001200 4 55 ofof 1111 Energies 2017, 10, 1200 5 of 11 and heat release rate under higher CH4 content condition. The maximum heat release rate increased −3 −1 · · −3 −1 4 bymaximummaximum about 19.3% heatheat releaserelease (from 362 raterate to increasedincreased 432 J cm byby aboutabouts ) with 19.3%19.3% increasing (from(from 362362 CH toto 4324432content J·cmJ·cm−3·s·s in−1) biogas) withwith increasingincreasing from BG-1 CHCH to4 maximum heat release rate increased by about 19.3% (from 362 to 432 J·cm−3·s−1) with increasing CH4 BG-3,content leading in biogas to a strongerfrom BG-1 heat to releaseBG-3, leading rate flame to a with stronger shorter heat flame release lengths rate atflame higher with CH shortercontent flame in contentcontent in biogas in biogas from from BG-1 BG-1 to to BG-3, BG-3, leading leading to a strongerstronger heat heat release release rate rate flame flame with with shorter shorter4 flame flame biogaslengths fuel. at higher CH4 content in biogas fuel. lengthslengths at higher at higher CH CH4 content4 content in in biogas biogas fuel. fuel.

Figure 2. Comparison of predication temperature (line) and measurement temperature (plot) at the Figure 2. Comparison of predication temperature (line) and measurementtemperature (plot) at the Figureoutlet 2. ofComparison BG-2/air flames of withpredication various Htemperature2 addition ratios (line) (black and line measurement) and O2 enrichment temperature levels (red (plot) line ).at the outletFigure of2. BG-2/airComparison ﬂames of predication with various temperature H2 addition (line) ratios and (measurementblack line) and temperature O2 enrichment (plot) levelsat the 2 2 (outletredoutlet line ofof ). BG-2/airBG-2/air flamesflames withwith variousvarious HH2 additionaddition ratiosratios ((blackblack lineline)) andand OO2 enrichmentenrichment levelslevels ((redred lineline).).

Figure 3. Effect of CH4 content in biogas fuel on combustion temperature distribution.

Figure 3. Effect of CH44content in biogas fuel on combustion temperature distribution. FigureFigure 3.3. EffectEffect ofof CHCH4 contentcontent inin biogasbiogas fuelfuel onon combcombustionustion temperaturetemperature distribution.distribution.

Figure 4. Effect of CH4 content in biogas fuel on net heat release rate distribution.

The effects of H2 addition ratios and O2 enrichment levels in BG-2/air flame on combustion temperature distributions are shown in Figure 5. The maximum combustion temperature in flames Figure 4. Effect of CH4 content in biogas fuel on net heat release rate distribution. 4 FigureFigure 4.4. EffectEffect ofof CHCH4 contentcontent inin biogasbiogas fuelfuel onon netnet heatheat releaserelease raterate distribution.distribution.

2 2 TheThe effectseffects ofof HH2 additionaddition ratiosratios andand OO2 enrichmentenrichment levelslevels inin BG-2/airBG-2/air flameflame onon combustioncombustion temperaturetemperature distributionsdistributions areare shownshown inin FigureFigure 5.5. TheThe maximummaximum combustioncombustion temperaturetemperature inin flamesflames

Energies 2017, 10, 1200 6 of 11

The effects of H addition ratios and O enrichment levels in BG-2/air flame on combustion Energies 2017,, 10,, 12001200 2 2 6 of 11 temperature distributions are shown in Figure5. The maximum combustion temperature in flames increasedincreasedincreased by byby approximately approximatelyapproximately 10.8% 10.8%10.8% fromfromfrom 180318031803 tototo 199819981998 KKK asasas HHH22 additionaddition ratioratio variedvaried varied fromfrom from 0%0% 0% toto to 50%,50%, 50%, andand was was slightly slightly higher higher by by about about 11.2% 11.2% fromfrom 18031803 toto 20052005 KK asas OO222 enrichmentenrichment levelslevels increasedincreased increased fromfrom from 21%21% to to 35%. 35%. Conversely, Conversely, the the maximum maximum heat heat release release rate rate in in flames flames increased increased by by 3.69 3.69 times times from from 362 362 to −3 −31 −1 −3−3 −−1 1336toto 13361336 J·cm J·cmJ·cm·s−3·s·s−at1 atat H H2Haddition22 additionaddition conditions conditionsconditions and andand only onlyonly 2.29 2.292.29 times timestimes from fromfrom 362 362362 to toto 829 829829 J· cm J·cmJ·cm−·3s·s·s−1 underunder OO 222 additionaddition conditions, conditions, asas shownshown in Figure 66.. This diff differenceerence is is also also related related to to thethe the shapeshape shape andand and structurestructure structure ofof ofbiogas biogas flames, flames, which which is is important important in in the the direct direct application application of of biogas biogas for for heating. heating. Specifically, Specifically, the the positionposition of of maximummaximum heatheat release rate rate varying varying with with H H22 additionaddition2 addition andand and OO22 Oenrichmentenrichment2 enrichment isis different.different. is different. AtAt Atthethe the HH22 H additionaddition2 addition condition,condition, condition, thethethe fuelfuel fuel compositioncomposition composition atat fuelfuel atfuel inletinlet inlet variesvaries varies withwith with HH22 additionaddition H2 addition rate,rate, rate,thethe fuelfuel the fuelcombustion combustion velocity velocity enhances enhances because because of ofhigh high reactivity reactivity (more (more OH OH and and H H radicals radicals with with higher higher formationformationformation rate raterate because becausebecause of ofof high highhigh burning burningburning velocity velocityvelocity [ [28–30][28–30]28–30] andandand combustioncombustioncombustion temperaturetemperature of of H H22))) andand and highhigh high mobilitymobility of of H H2 22[ 28[28–30].[28–30].–30]. Therefore,Therefore,Therefore, thethethe maximummaximummaximum heatheat releaserelease rate rate at at thethe HH2 additionaddition conditionsconditions isis is locatedlocatedlocated at atat center centercenter of ofof the thethe flame. flame.flame. At AtAt the thethe O OO222 enrichmentenrichmentenrichment condition,condition, the the fuel fuel compositioncomposition keepskeeps keeps constant,constant, constant, 2 andand the the air air composition composition at outsideat outside of fuel of inletfuel inlet varies varies with Owith2 enrichment. O2 enrichment.enrichment. The fuel TheThe combustion fuelfuel combustioncombustion velocity alsovelocity improves also improves because of because the higher of the collision higher rate collis ofionion oxidizer raterate ofof with oxidizeroxidizer fuel nearwithwith flame fuelfuel nearnear edges. flameflame Therefore, edges.edges. Therefore, the maximum heat release rate locates near the flame edges at O2 enriched condition. theTherefore, maximum the heat maximum release heat rate release locates rate near locates the flame near edges the flame at O2 edgesenriched at O condition.2 enriched condition.

FigureFigure 5. 5. Effects EffectsEffects ofofof H HH222 additionadditionaddition ratioratioratio (((aa))) andandand OOO222 enrichmentenrichmentenrichment levelslevels ((b)) inin BG-2/airBG-2/airBG-2/air flameflame ﬂame onon on thethe the distributiondistribution of of combustion combustion temperature. temperature.

Figure 6. Effects of H addition ratio (a) and O enrichment levels (b) in BG-2/air ﬂame on the Figure 6. EffectsEffects ofof HH222 additionaddition ratioratio ((a)) andand OO222 enrichmentenrichment levelslevels ((b)) inin BG-2/airBG-2/air flameflame onon thethe distributiondistribution of of the the heat heat release release rate. rate.

ToTo obtain obtain anan in-depthin-depth understanding understanding of of heat heat release release characteristics characteristics of biogas of biogas flames flames under under H22-- Henriched2-enriched and and O22-enriched-enriched O2-enriched conditions,conditions, conditions, thethe netnet the heatheat net heatreleaserelease release raterate distributiondistribution rate distribution atat centerlinecenterline at centerline alongalong thethe along z-- thedirectionz-direction is shown is shown in Figure in Figure 7. The7. The maximum maximum net netheat heat release release rate rate at centerline at centerline is enhanced is enhanced by −−−33 −−−11 1 byapproximately approximately two two orders orders of of magnitude magnitude (from (from 0.155 toto 120.0120.0 J·J·cmcm ·s·s·s )) )whenwhen when HH H22 2additionadditionaddition ratioratio ratio increasesincreasesincreases from fromfrom 0% 0%0% to toto 50%; 50%;50%; and andand the thethe maximum maximummaximum netnetnet heatheatheat releasereleaserelease rateraterate atat centerlinecenterline isis enhancedenhanced byby by aboutabout about −−33−3−−11 −1 oneone order order of of magnitude magnitude (from(from 0.155 to 3.66 3.66 J·cm J·cm ·s·s ·s)) whenwhen) when thethe theOO22 additionaddition O2 addition ratioratio ratio increasesincreases increases fromfrom from21%21% 21%toto 35%.35%. to 35%. TheThe Theenhancementenhancement enhancement ofof thethe of heatheat the heatreleaserelease release raterate cancan rate leadlead can leadtoto aa changechange to a change inin flameflame in flame structurestructure structure andand heatheat and a heatrelease release characteristics characteristics in the in theflame flame reaction reaction zone. zone. Therefore, Therefore, average average temperature temperature Ta waswasTa was defineddefined defined toto explore the effect of H22-enriched-enriched andand OO22-enriched-enriched conditionsconditions onon flameflame kernel.kernel. Taa isis basedbased onon thethe barycenter of heat release rate as follows:

Energies 2017, 10, 1200 7 of 11

to explore the effect of H2-enriched and O2-enriched conditions on ﬂame kernel. Ta is based on the barycenterEnergies of 2017 heat, 10 release, 1200 rate as follows: 7 of 11

z R d 0 qTdz Ta== (10) (10) Rz d 0 qdz

(a) (b)

Figure 7. Heat release rate of biogas flames under H2-enriched (a) and O2-enriched (b) conditions Figure 7. Heat release rate of biogas ﬂames under H -enriched (a) and O -enriched (b) conditions along the centerline. 2 2 along the centerline.

Figure 8 shows the average temperature Ta together with flame height of peak heat release rate at various H2 addition ratios and O2 enrichment levels. As shown in Figure 8, the average temperature Figure8 shows the average temperature Ta together with flame height of peak heat release Ta shows a gradual decrease with the increasing H2 addition ratio and O2 enrichment levels. For the rate at various H addition ratios and O enrichment levels. As shown in Figure8, the average biogas flame2 at same input power of 12 kW and equivalence ratio of 0.95, the value of average temperature T shows a gradual decrease with the increasing H addition ratio and O enrichment temperaturea Ta increases from 1215 to 1864 K, nearly yielding an 2enhancement of 53.4%, when2 H2 levels. Foraddition the biogasratio varied flame from at 0% same to 50%. input In contrast, power the of flame 1 kW height and of equivalence peak heat release ratio rate of decreases 0.95, the value of averageby approximately temperature 54.5%Ta increases from 19.8 from to 9.0 1215 cm when to 1864 the H K,2 addition nearly yieldingratio increases an enhancement from 0% to 50%. of 53.4%, The enhancement of average temperature Ta and reduction of flame height of maximum heat release when H2 addition ratio varied from 0% to 50%. In contrast, the flame height of peak heat release rate rate resulted from the high mobility and high reactivity of H2, leading to a higher reaction rate of decreases by approximately 54.5% from 19.8 to 9.0 cm when the H2 addition ratio increases from 0% to biogas flame under H2-enriched condition as shown in Figure 9a. By contrast, the value of average 50%. The enhancement of average temperature Ta and reduction of flame height of maximum heat temperature Ta increased from 1215 to 1428 K (~17.5% increase) and the flame height of peak heat release raterelease resulted rate decreases from the from high 19.8 mobility to 10.9 cm and (~44.9% high decrease) reactivity when of H O2,2 leadingenrichment to alevels higher increased reaction rate of biogasfrom flame 21% under to 35%. H With2-enriched the enrichment condition of O as2 in shown combustion in Figure air, the9 a.reaction By contrast, rate can thebe improved value of as average temperatureshownT ina increasedFigure 9b. fromFurthermore, 1215 to unnecessary 1428 K (~17.5% sensible increase) heat to heat and up the the flame unburnt height N2 gas of also peak heat decreases with increasing O2 enrichment levels. As shown in Figure 9, the net reaction rate of biogas release rate decreases from 19.8 to 10.9 cm (~44.9% decrease) when O2 enrichment levels increased flames under the H2-enriched condition markedly increases as H2 addition ratio increases from 0% to from 21% to 35%. With the enrichment of O2 in combustion air, the reaction rate can be improved 50% and lower increase is observed under O2-enriched condition when O2 levels increases from 21% as shownto 35%. in Figure To clarify9b. the Furthermore, main elementary unnecessary reaction in biogas sensible flame, heat we to compared heat up the the typical unburnt elementary N 2 gas also decreasesexothermic with increasing reaction rate O2 ofenrichment biogas flame levels. along Asthe showncenterline in at Figure H2 addition9, the ratio net reactionof 0% and rate 50% of in biogas flames underFigure the10. The H2-enriched formation of condition free radicals, markedly such as increasesH, O, and asOH, H are2 addition enhanced ratio with increasesthe increase from in 0% to 2 ⇔ 2 2 50% andhydrogen lower increase addition isratios; observed in particular, under R83: O2-enriched OH + H condition H + H O markedly when O increases2 levels as increases H is added from 21% to 35%.to To biogas clarify flame, the main similar elementary results have reaction been report in biogased for the flame, heat werelease compared rate of premixed the typical laminar elementary biogas-H2 flame by Wei et al. [19] and CH4/air premixed flames with H2 addition by Hu et al. [31]. exothermic reaction rate of biogas flame along the centerline at H2 addition ratio of 0% and 50% in Furthermore, the typical elementary exothermic reaction rates of R83: OH + H2 ⇔ H + H2O; Figure 10. The formation of free radicals, such as H, O, and OH, are enhanced with the increase R37: H + O2 ⇔ O + OH are important to enhance biogas combustion. The elementary reaction of R83 in hydrogen(H2 as additionreactant) is ratios; higher than in particular, R37 (O2 as reactant). R83: OH Therefore, + H2 ⇔ HH2 addition + H2O has markedly a drastic effect increases on heat as H2 is added torelease biogas rate flame, (and reaction similar rate) results enhancement have been than reported O2 enrichment for the heatas shown release in Figures rate of 6 premixed and 7. The laminar enhancement reaction rate can lead to structure and shape changes in biogas flame. The flame biogas-H2 flame by Wei et al. [19] and CH4/air premixed flames with H2 addition by Hu et al. [31]. thickness along centerline under H2-enriched conditions and O2-enriched conditions are shown in Furthermore, the typical elementary exothermic reaction rates of R83: OH + H2 ⇔ H + H2O; R37: Figure 11. The definition of flame thickness can be expressed as follows: H + O2 ⇔ O + OH are important to enhance biogas combustion. The elementary reaction of R83 (H2 as reactant) is higher than R37 (O2 as reactant). Therefore, H2 addition has a drastic effect on heat release rate (and reaction rate) enhancement than O2 enrichment as shown in Figures6 and7. The enhancement reaction rate can lead to structure and shape changes in biogas flame. The flame thickness along centerline under H2-enriched conditions and O2-enriched conditions are shown in Figure 11. The definition of flame thickness can be expressed as follows: Energies 2017, 10, 1200 8 of 11

Energies 2017, 10, 1200 8 of 11 Tmax − T0 Energies 2017, 10, 1200 DF = 8 of 11 (11) (dT/dz)max Energies 2017, 10, 1200 − 8 of 11 = (11) ⁄ T T − (dT dz) where max and 0 are the maximum temperature = and unburned temperature respectively, (11)/ max ⁄ − is thewhere peak gradient and in temperature are the maximum profile temperature along the centerline. and unburned As showntemperature in Figure respectively, 11, the flame = (11) ⁄ ⁄ thicknesswhere decreases is andthe with peak the are gradient increasing the maximum in temperature H2 addition temperature profile ratio along and the Ounburned2 centerline.enrichment temperature As levels.shown Thisinrespectively, Figure decrease 11, can the flame⁄ thickness decreases with the increasing H2 addition ratio and O2 enrichment levels. This be attributedwhere to the is and burningthe peak are gradient velocity the maximum in enhancement temperature temperature ofprofile biogas along and flames theunburned centerline. under Htemperature2 As-enriched shown inrespectively, and Figure O2-enriched 11, decreasethe flame⁄ can thickness be is theattributed peak decreases gradient to the with burning in thetemperature increasing velocity profile enhancement H2 addition along the ratioof centerline.biogas and Oflames2 enrichment As shownunder Hin levels.2 -enrichedFigure This 11, conditions. Therefore, a flame with enhanced heat release rate is formed under H2-enriched and anddecreasethe Oflame2-enriched can thickness be attributedconditions. decreases to Therefore, the with burning the a increasing flame velocity with enhancementH enhanced2 addition heat ratio of biogasrelease and Oflames rate2 enrichment is underformed H levels.under2-enriched ThisH2- O2-enriched conditions, which are beneficial to the practical application of biogas as a fuel. enrichedanddecrease O2-enriched and can O be2-enriched attributedconditions. conditions, to Therefore, the burning which a flame velocityare beneficial with enhancement enhanced to the practical heat of biogas release application flames rate is underofformed biogas H under2 -enrichedas a fuel. H2- enrichedand O2-enriched and O2-enriched conditions. conditions, Therefore, which a flame are beneficial with enhanced to the practical heat release application rate is formedof biogas under as a fuel. H2- enriched and O2-enriched conditions, which are beneficial to the practical application of biogas as a fuel.

(a) (b) (a) (b) Figure 8. Effects of H2 addition ratio (a) and O2 enrichment levels (b) in BG-2/air flame on the peak Figure 8. Effects of H2 addition(a) ratio (a) and O2 enrichment levels (b) in BG-2/air(b) ﬂame on the peak heatFigure release 8. Effects height of and H2 averageaddition temperature ratio (a) and T Oa. 2 enrichment levels (b) in BG-2/air flame on the peak heat release height and average temperature Ta. heatFigure release 8. Effects height of and H2 additionaverage temperatureratio (a) and TOa.2 enrichment levels (b) in BG-2/air flame on the peak

heat release height and average temperature Ta.

(a) (b) (a) (b) Figure 9. Net reaction rate of biogas flame along centerline at various H2 addition ratios (a) and O2 (a) (b) enrichmentFigure 9. Net levels reaction (b). rate of biogas flame along centerline at various H2 addition ratios (a) and O2 Figure 9. Net reaction rate of biogas ﬂame along centerline at various H2 addition ratios (a) and O2 enrichmentFigure 9. Net levels reaction (b). rate of biogas flame along centerline at various H2 addition ratios (a) and O2 enrichment levels (b). enrichment levels (b).

(a) (b)

Figure 10. Comparison of(a typical) elementary reaction exothermic reaction rates(b) of biogas flame along (a) (b) theFigure centerline 10. Comparison at H2 addition of typical ratio elementaryof 0% (a) and reaction 50% (b ex). othermic reaction rates of biogas flame along

theFigure centerline 10. Comparison at H2 addition of typical ratio elementaryof 0% (a) and reaction 50% (b ex). othermic reaction rates of biogas flame along Figure 10. Comparison of typical elementary reaction exothermic reaction rates of biogas ﬂame along the centerline at H2 addition ratio of 0% (a) and 50% (b). the centerline at H2 addition ratio of 0% (a) and 50% (b). Energies 2017, 10, 1200 9 of 11 Energies 2017, 10, 1200 9 of 11

Figure 11. Flame thickness along the centerline at various H2 addition ratios (black line) and O2 Figure 11. Flame thickness along the centerline at various H2 addition ratios (black line) and O2 enrichmentenrichment levels ( red line ).

4. Conclusions 4. Conclusions The combustion and heat release characteristics of biogas non-premixed flames under various The combustion and heat release characteristics of biogas non-premixed flames under various hydrogen-enriched and oxygen-enriched conditions were investigated utilizing chemical kinetics hydrogen-enriched and oxygen-enriched conditions were investigated utilizing chemical kinetics simulation and the detailed chemical mechanism. The results are summarized as follows. simulation and the detailed chemical mechanism. The results are summarized as follows. (1) The net reaction rate of biogas increases with increasing hydrogen addition ratio and oxygen (1) levels,The net leading reaction to a rate higher of biogas net heat increases release withrate of increasing biogas flame; hydrogen addition ratio and oxygen (2) Thelevels, formation leading of to free a higher radicals, net such heat releaseas H, O, rate and of particularly biogas flame; OH, are enhanced with the increase (2) inThe hydrogen formation addition of free ratio radicals, and such oxygen as H, levels; O, and particularly OH, are enhanced with the increase in hydrogen addition ratio and oxygen levels; (3) Flames with enhanced heat release rates are formed under H2-enriched and O2-enriched (3) conditions.Flames with Therefore, enhanced H2-enriched heat release and ratesO2-enriched are formed combustion under is H beneficial2-enriched to the and improvement O2-enriched ofconditions. combustion Therefore, and heat H release2-enriched characteristics and O2-enriched of biogas combustion in practical is beneficial application. to the improvement of combustion and heat release characteristics of biogas in practical application. Acknowledgments: This research is partially supported by the “Knowledge Hub Aichi” Priority Research Project from Aichi Prefectural Government, Japan. Acknowledgments: This research is partially supported by the “Knowledge Hub Aichi” Priority Research Project Authorfrom Aichi Contributions: Prefectural Government, Jun Li contributed Japan. to the experiment and simulation, data analysis and paper writing; HongyuAuthor Contributions:Huang, Huhetaoli,Jun LiYu contributed Bai, and Yong to the Chen experiment contributed and to simulation, experimental data design, analysis and and Yugo paper Osaka writing; and NoriyukiHongyu Huang,Kobayashi Huhetaoli, contributed Yu Bai, to modeling and Yong selection Chen contributed and programing to experimental instruction. design, and Yugo Osaka and Noriyuki Kobayashi contributed to modeling selection and programing instruction. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. Nomenclature Nomenclature

H2 addition ratio to fuel (based on low heat value) XH2 H2 addition ratio to fuel (based on low heat value) 2 −1 Flow rate of H (m·s ) −1 VH Flow rate of H2 (m·s ) 2 4 −1 Flow rate of CH (m·s ) −1 VCH Flow rate of CH4 (m·s ) 4 Low heat value of H2 (kJ·mol−1) −1 LHVH Low heat value of H2 (kJ·mol ) 2 Low heat value of CH4 (kJ·mol−1) −1 LHVCH4 Low heat value of CH4 (kJ·mol ) O2-enriched level (-) Ω O -enriched level (-) O2 Flow2 rate of O2 (m·s−1) −1 V Flow rate of O (m−·1s ) O2 Flow rate of N2 (m·s2 ) −1 −3 · VN 2 DensityFlow rate(kg·m of N) 2 (m s ) −3 ρ GeneralDensity dependent (kg·m ) variable (-) Y VelocityGeneral vector dependent (m·s−1) variable (-) −1 YΓ associatedVelocity vectortransport (m· coefficients ) of Y (-) ΓY Theassociated source term transport (-) coefﬁcient of Y (-) SY MeanThe absorption source term coefficient (-) of species i (-) −1 αℎi FormationMean absorption enthalpy coefﬁcient of specie j of(J·mol species) i (-) −1 ∆ℎ Enthalpy change of reaction i (J·mol )

Energies 2017, 10, 1200 10 of 11

−1 hj Formation enthalpy of specie j (J·mol ) −1 ∆hi Enthalpy change of reaction i (J·mol ) −3 −1 ki The reaction rate of reaction i (mol·cm ·s ) −3 −1 qi Heat release rate of reaction i (J·cm ·s ) −3 −1 qnet Total heat release rate of all reaction (J·cm ·s ) DF Flame thickness (cm) Tmax The maximum temperature (K) T0 The unburned temperature (K) −1 (dT/dz)max The peak gradient in temperature (K·cm )

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