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Energy 211 (2020) 118187

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Energy

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Air-Steam gasification of lignite in a fixed bed gasifier: Influence ofsteam to lignite ratio on performance of downdraft gasifier

Darshit S. Upadhyay , Krunal R. Panchal , Anil Kumar V Sakhiya , Rajesh N. Patel *

Institute of Technology, Nirma University, Ahmedabad, Gujarat, 382481, India

a r t i c l e i n f o

Article history:Received 19 April 2020Received in revised form11 June 2020Accepted 19 June 2020Available online 29 July 2020

Keywords:Air-steam gasificationDowndraft gasifierLigniteHydrogenTarParticulate matter

* Corresponding author.E-mail addresses: rajeshnpatel1969@yahoo

(R.N. Patel).

https://doi.org/10.1016/j.energy.2020.1181870360-5442/© 2020 Elsevier Ltd. All rights reserved.

a b s t r a c t

This work aims to identify the optimum Steam to Lignite ratio, w/w (SLR) to achieve higher H2 yield andlower tar yield in the producer gas. Experiments were carried out in a 10kWe atmospheric pressuredowndraft gasifier. Low-rank high ash lignite (22e25 mm) was used as a feedstock to investigate theeffect of six different SLRs (0, 0.06, 0.14, 0.18, 0.24, 0.30 and 0.48). The producer gas Lower Heating Value(LHV) and Cold Gas Efficiency (CGE) were found in the range of 4.96 MJ Nm�3e5.62 MJ Nm�3 and 70.6%e81%, respectively for different SLR. The optimal SLR was identified to be 0.24, having lower specific fuelconsumption (1.437 kg kWh�1), lower tar content (112.28 mg Nm�3), lower Particulate Matter (PM)(27.34 mg Nm�3), higher LHV (5.62 MJ Nm�3) and higher CGE (81%). H2 yield and H2/CO ratio improvedby 34.7% and 52%, respectively whereas tar yield reduced by 78.31% at 0.24 SLR compared to air gasi-fication. The mass balance, exergy analysis, heat loss analysis were also carried out for this study. Thestudy concludes that the 0.24 SLR offered better results among the selected ratios to achieve higher H2

yield and lower tar yield in the producer gas.© 2020 Elsevier Ltd. All rights reserved.

1. Introduction

India has one of the fastest-growing economies having annualgrowth in Gross Domestic Product (GDP) as 7% in the year 2019 [1].The rapidly growing economy directly affects the energy demand ofthe country. The electricity consumption of India has kept growingby the rate of 2.3% every year and had reached to about 934 milliontons of oil equivalent (Mtoe) in the year of 2017e18 [2]. Coal is themajor source of primary energy for a developing country like India.The power generating capacity of India by coal/lignite fired powerplants was 205.13 GW in the year 2018e19 [3]. Although India haslarge reserves of coal, there is still a shortcoming in meeting energyneeds due to a lack of availability of high-rank coal. India importedabout 235.24 million metric tonnes of coal in the year of 2018e19[4]. One of the major drawbacks of using coal-fired power plants isincreasing CO2 and other harmful emissions. Hence the currenttechnological demand is to mitigate the problem of low-rank coalusage and rise in the pollution levels [5].

India has large reserves of low-rank lignite coal. The lignite coalis a porous, lightweight and brownish material that has high ash,

.com, rnp@nirmauni.ac.in

moisture, and sulfur contents. This makes combustion of lignitehighly susceptible to clinker-formation, fouling and other harmfulsubstances [6]. Hence, this type of coal does not find wide appli-cability in commercial-scale applications. Gasification is one suchthermochemical process, which is suitable to generate electricityfrom any carbon-based solid fuel in an environmentally friendlymanner [7]. In recent years, vast advancement to make this processfeasible and commercially viable has been made from small-scaleindustrial setup to large Integrated Gasification Combined Cycle(IGCC) power plants [8]. In the gasification process, the feedstock isoxidized in a sub-stoichiometric oxidizing medium to generate gas,rich in H2 and CO, called producer gas. This producer gas can beused to generate electricity by combusting it inside Internal Com-bustion Engines (I$C.) coupled with generators, inside gas turbines,or inside high-temperature fuel cells. Portability of such a plant canplay a key role in tackling the rural electrification in India, throughestablishing standalone gasifier systems coupled with the I.C. En-gine and generator system at remote locations. The gasificationprocess mainly depends on parameters such as feedstock contents,gasifying medium, and operational temperature. Oxygen, air, steamor a combination of all has been used as a gasifying medium in thebiomass gasification process [9]. Air gasification is the simplest,cheapest, and the most widely used route as air is abundant innature [10].

D.S. Upadhyay et al. / Energy 211 (2020) 1181872

A comparison of gasifier reactors is based on the main fivecriteria: technology, use of material, energy, environment, andeconomy. There are categorized as fixed bed gasifiers (downdraft,updraft, and cross-draft), fluidized bed gasifiers (bubbling fluidizedbed, circulating fluidized bed), entrained flow gasifier etc. The mainadvantage of updraft gasifiers is the low producer gas temperatureon the outlet side. But the higher tar yield of the updraft gasifier is amajor drawback. The cross-draft gasifiers, on the other hand, arevery compact and light in weight, but it can handle only low ashfeedstock [11]. In the fluidized bed gasifiers, the bed is preparedusing pulverized fuel. The air is forced through the bottom grate,whichmakes the bed be in a fluidized state. Themain advantages ofthe fluidized bed gasifiers are efficiency and flexibility with thetype of fuels. But the complex design and operations make it lessattractive [12].

The downdraft is able to generate gas with low tar yield and canbe operated with the fuels with higher ash content. The downdraftgasifier is also simple and robust technology as compared to afluidized bed reactor [13]. The residence time of fuel is morecompared to other reactors. Hence, solid to gas conversion is more.Moreover, less pressure drop occurs in a downdraft reactor where ahigh-pressure drop occurs in a fluidized bed reactor. Downdraft ismore suitable for small scale applications for off-grid electricitygeneration in the rural area, whereas fluidized bed technology issuitable for medium/large scale applications. Operational diffi-culties are more in fluidized reactors. In terms of economy, theoperating cost of the downdraft gasifier is low compared to thefluidized bed [13].

There are several articles available for air-steam gasificationwith different biomass but limited literature is available for ligniteair-steam gasification. Vimal et al. [14] carried out the coal gasifi-cation with air to study the effect of different particle sizes on theproducer gas quality. The optimum particle size was found to be22e25 mm and the heating value of the producer gas was found tobe 4.17 MJ Nm�3. The downside of air gasification was the highconcentration of N2 (56%), which diluted and degraded the pro-ducer gas quality. Excess N2 in producer gas possesses high risk ofproducing NOx (engine application). It requires additional effort onN2 removal techniques [15]. To overcome this problem, variousresearchers have suggested to add steam as an oxidizer in thegasifier reactor [16,17]. It has been observed that steam injectionimproves the H2 yield. Moreover, it has the potential to diminishthe gas contaminants such as tar, particulate matter (PM), etc.

Meng et al. [18] studied the effect of different gasifying agentssuch as air, air-steam, and oxygen-steam on sawdust gasification ina fluidized bed reactor. It was observed that hydrogen yield(13.52%e24.48%) increased in air-steam gasification as compared toair gasification (3.41%e8.73%). The addition of steam provides moreH2 and O2 resulting in hydrogen rich syngas [19]. Wang et al. [20]studied the steam gasification properties and reaction kinetics ofthree different coal such as lignite, sub-bituminous coal, andanthracite. The steam gasification reactions showed that the pro-ducer gas from low-rank coal (lignite) had a high H2/COmolar ratio,while the producer gas from sub-bituminous coal and anthracitehad relatively high CO2 and CO content.

Young et al. [21] carried out gasification of dried sewage sludgewith steam/oxygen as the gasifying medium and found the H2 yieldof about 52.06%. Cristina et al. [22] carried out the steam gasifica-tion of Eucalyptus spp. in a two-stage downdraft gasifier. They re-ported that H2 content increased in the producer gas and tarcontent decreased from 418.95 mg Nm�3 to 91.41 mg Nm�3 whensaturated steam was injected into the gasifier reactor. Ram et al.[23] studied the air-steam gasification of fuelwood in the duel fireddowndraft gasifier for hydrogen enrichment in producer gas. By theaddition of steam, hydrogen yield initially increased up to 27%, and

then after it was decreased. The excess steam addition reducedreactor temperature and H2 decreases. Moreover, the higher heat-ing value obtained from air-steam gasification was 6.33 MJ Nm�3.Ocampo et al. [24] studied the gasification characteristics ofColombian coal in a fluidized bed reactor at different steam to coalratio at atmospheric pressure and found improvement in cold gasefficiency, gas heating value, and carbon conversion efficiency. Leeet al. [25] analyzed the gasification process of Australian coal in thefluidized bed gasifier in the presence of air-steam mixture at at-mospheric pressure. The result revealed that the cold gas efficiencyand gas heating value increased with an increase in steam to coalratio.

Despite of different advantages of steam gasification, the same isnot popular especially for small and medium scale gasificationsystems. This may be due to the fact that the cost is involved in therequired steam production. However, the same is viablewith poweror process industries where excess steam is available at no addi-tional cost. For large gasifier setups, the required steam could begenerated from the utilities such as water jackets surrounding thecombustion zone, etc making process viable.

In the present work, air-stream gasification of low-rankedlignite is conducted in a pilot-scale 10kWe atmospheric pressuredowndraft gasifier. An attempt is made to evaluate the effect ofSteam to Lignite Ratio (SLR) on gasifier performance parameterssuch as gasification temperature, producer gas composition, gasheating value, cold gas efficiency, tar, and particulate matter, etc.Exergy efficiency, heat loss analysis, and mass balance is also pre-sented based on the experimental results.

2. Experimental

2.1. Materials and methods

Lignite was selected as a feedstock for the experiments in thedowndraft gasifier. Lignite was procured from Panandhro lignitemine, Gujarat state, India. The particle size of lignite was optimizedfor the experimental setup and was found in the range of22 mme25 mm [14]. The proximate analysis (Test method: IS 1350(Part I)-1984, Instrument: Leco TGA 701), ultimate analysis (Drybasis, Instrument: Leco ThuSpec CHNS), bulk density (Test method:IS 7190e1974) and calorific value (Test method: IS 1350 (Part II)-1970, Instrument: Leco AC-350 Bomb Calorimeter) of the lignitewere carried out and results are shown in Table 1.

2.2. Experimental setup

A schematic diagram of the gasifier setup is as shown in Fig. 1. A10kWe, pilot scale, atmospheric pressure downdraft gasifier wasused for carrying out the experiments. The main body (2)comprised of a gasifier reactor mounted over the reduction zone.The detailed design of the pilot-scale gasifier setup can be found inthe authors’ previous work [14,26e28]. The top cover was providedto facilitate the dumping of the feedstock into the reactor and alsoto maintain an airtight seal during the experiments. A vibratingmotor was mounted on the side of the main body of the reactor toallow the smooth down flow of the feedstock, char, and ash,avoiding any bridging and channeling of the feedstock. Two nozzleswere provided on the side of the reactor jacket to facilitate thesupply of air and steam into the combustion zone. The gas wascooled and cleaned using a setup comprised of a water tank, watercirculating centrifugal pump (0.5 HP) with convergent water jetnozzle, awet scrubber, sawdust filled surge tank and bag type fabricfilter as shown in Fig. 1 (9, 10, 11, 12, 14 and 15). This unit alsoprovided the required suction mechanism to suck the gasifyingmedium (air) inside the gasifier reactor.

Table 1Characterization of lignite.

Proximate Analysis Ultimate Analysis

Moisture (%) 4.04 C (%) 38.9Volatile (%) 45.38 H (%) 4.63Ash (%) 27.27 N (%) 1.49Fixed Carbona (%) 23.31 S (%) 8.25

Oa (%) 42.69Lower Heating Value (MJ kg�1) 16.84 Bulk Density (kg m�3) 776

a By difference.

Fig. 1. A detailed layout of the downdraft gasifier system.

D.S. Upadhyay et al. / Energy 211 (2020) 118187 3

For generating steam, a 6 kW, 12 L capacity, steam generator(Lucky Engineering Pvt. Ltd.) was used. A pressure transmitter witha digital pressure controller (Danfoss MBS 1900) and temperaturecontroller were used to maintain steam pressure at 1 bar (±1%) andtemperature at 114 �C (±1 �C), respectively. Additional six heaters(0.5 kW capacity each) were installed over the length of the con-necting pipe between a steam generator and a gasifier reactor toensure the quality of the steam before injecting into a gasifierreactor. Pipe connectionwas completely insulated with cerawool torestrict heat loss.

For measuring the Particulate Matters (PM 2.5) concentration,an assembly of glass fiber filter paper (47 mm diameter; Axivamake), placed inside the SS304 PM filter holder along with a

vacuum pump was used as shown in Fig. 1 (16). The water vaporcontent in the producer gas condensed due to the lower pressureinside the filter assembly. To overcome this problem, a mica heater(with Arduino Nano controller) was wounded over the filter as-sembly to maintain the temperature of the gas inside the filterholder at 110 �C. For measuring tar concentration in the producergas, a shell, and tube type gas condenser, receiver flask, vacuumpump, and cold water (5e6 �C) tank with water circulating pumparrangement as shown in Fig. 1 (17 and 18).

2.3. Instrumentation

Temperatures of the different zones inside the gasifier reactor

Fig. 2. Temperature profiles of combustion and reduction zone at different SLR.

D.S. Upadhyay et al. / Energy 211 (2020) 1181874

were measured with K type (Chromel - Alumel) thermocouples.The airflow rate was measured with a hot-wire anemometer with adata logger (Fluke make Amprobe TMA-21HW). The steam massflow rate and gas flow rate were determined by the gravimetricmethod and an orifice meter with a U tube manometer, respec-tively. The concentrations of the tar and Particulate Matter (PM 2.5)were measured as per the guideline by the Ministry of New andRenewable Energy (MNRE), Government of India [29]. The gasconcentrations were measured with a gas chromatograph (Shi-madzu 2010). This instrument was operated on a micro-thermalconductivity detector (mTCD) and equipped with a Shin Carbon ST100/120 micro packed column. The surface temperature wasmeasured using thermal imager (Fluke TIA759HZ) and infra-redtemperature gun (TESTO 835-T2).

2.4. Experimental procedure

The gasifier reactor was initially filled with lignite and thereduction zone was filled with activated charcoal to absorb mois-ture and CO2 from the producer gas. Nozzles provided on thegasifier reactor were used to supply air and steam inside thereactor. The airflow rate was controlled by actuation of suctionmechanism. The suction mechanism was operated by the watercirculating pump (as shown by (12) in Fig. 1). The water circulatingpump forced the water through the water jet nozzle (as shown by(12) in Fig. 1), which generated the adverse pressure drop on thegasifier side. This pressure drop resulted in air suction from the airnozzle. Controlling the water flow rate through the water jet nozzlecan control the airflow rate. The lignite flow rate (lignite con-sumption rate) was controlled by controlling the airflow rate to thecombustion zone; the increase/decrease in airflow rate can in-crease/decrease the lignite consumption rate. A flame torch wasused to feed the flame into the gasifier reactor from the air nozzles.The heat released in the combustion zone diffused to the otherzones. Initially, only the air was allowed into the reactor. Once thepyrolysis temperature reached 150 �C, the steam was allowed in-side the reactor. The steam was injected through the nozzles pro-vided on the gasifier jacket (as shown by (8) in Fig. 1). The Steam toLignite Ratio (SLR) for the current experiments was chosen be-tween 0 and 0.4, as suggested in the literature [30,31]. The regulatorvalve was used to control the flow rate of steam. The de-volatilization of the feedstock was initiated inside the pyrolysiszone which mainly formed non-condensable gases and the tar. Thenon-condensable gases, along with the combustion products,started reacting on the surface of the charcoal present in thereduction zone at a higher temperature. The gasification reactionshelped to generate the combustible gas constituents such as H2, CO,and CH4 along with some traces of sulphide, oxides, and otherheavier hydrocarbons. The wet scrubber separated water and pro-ducer gas through centrifugal action. Heavy pollutants werecaptured by water in a wet scrubber. After that, producer gas wassupplied to a sawdust-filled surge tank and a bag type fabric filter todiminish moisture and solid impurities. This process improved theenergy density of the producer gas by removing the moisturecontent. The flame test was carried out at the chimney section toensure the quality of the producer gas. All the experiments wererepeated three times to check the repeatability of the study.

A fixed mass of lignite was dumped into the gasifier reactor foreach experiment, and the initial height of the lignite bed from thetop cover was measured for four different locations. After theexperiment was conducted, the height of the lignite bed was againmeasured. Using the difference in the initial and final height as wellas the bulk density of the lignite (given in Table 1), the ligniteconsumption rate in kg h�1 was calculated. This procedure is alsoexplained in authors’ previous work [14].

For measuring the tar and particulate matter (PM) concentra-tions in the producer gas, the gas from a fabric filter assembly wassucked by a vacuum pump. Around 3 Nm3 of producer gas wasallowed to pass through this assembly to measure these pollutantsin the producer gas. Initially, this gas entered the PM assemblywhere the filter was placed in the SS304 filter holder. Despite micaheater assembly, some tar content was presented in the filter as-sembly along with PM. To separate tar content from a filter paper, afilter was washed with acetone. After washing a filter paper, theratio of the mass difference between the final and initial conditionsof a filter paper was noted. The ratio of the mass difference to theproducer gas volume supplied to the filter assembly was calculated.It is PM concentration in the producer gas (mg Nm�3). The coldwater was circulated from the tube side, and the producer gas wassupplied from the shell side of the glass condenser. A receiver flaskwas attached to the shell side to collect the condensed tar. Apartfrom that, tar was also collected from filter paper, filter holder,connecting pipes and condenser by rinsing with acetone. Thecollected tar was heated in a solvent recovery chamber and allowedto settle. The ratio of the mass of the tar to the producer gas volumesupplied to this assembly was calculated. It is tar concentration inthe producer gas (mg Nm�3). The detailed experimental method-ology can also be found in the authors’ previous work [27,28].

3. Results and discussion

Experiments were carried out with different Steam to LigniteRatios (SLR) ranging from 0, 0.06, 0.14, 0.18, 0.24, 0.30 and 0.48 toevaluate the gasifier performance in terms of temperature, fuelconsumption, air-steam flow rate, producer gas calorific value, coldgas efficiency, tar, and PM. Thermal analysis was also carried out toevaluate the exergy efficiency and heat loss in the air-steam gasi-fication process. The results for the same are discussed in thefollowing sections.

3.1. Temperature profile

The steady-state temperature of combustion and reduction zonein the air-steam gasification system concerning different Steam toLignite Ratio (SLR) is as shown in Fig. 2. A bell-shaped curve was

D.S. Upadhyay et al. / Energy 211 (2020) 118187 5

followed by the temperature profiles of both the zones. The steady-state temperatures, for all the zones, were found to be increasingwith SLR till 0.24 SLR. The results indicate that the increase in theSLR leads to higher H2 and CO2 (up to 0.24 SLR), which can beexplained by the water gas shift reaction [32]. The water-gas shiftreaction (CO þ H2O 4 CO2 þ H2 � 41 kJ mol�1) is an exothermicreversible reaction. Apart from that, endothermic reactions such asBoudouard reaction (C þ CO2 4 2CO þ 172 kJ mol�1), water gasreaction (C þ H2O 4 CO þ H2 þ 131 kJ mol�1), steam forming re-action (CH4 þ H2O 4 CO þ 3H2 þ 206 kJ mol�1), etc. also occurduring the gasification process. The extent of water gas shift reac-tion is dominating compared to other endothermic reactions until0.24 SLR, which results in higher zone temperatures. That indicatesthe thermodynamic equilibrium shifts toward the forward direc-tion, at a given SLR. An increase in temperature leads to higherhydrogen yield resulting from the steam reforming and cracking ofhydrocarbons. However, a further increase in the SLR (above 0.24)causes the decline of reaction rate of steam reforming reactions. Athigher SLR, a large amount of steam was fed to the gasifier reactor.More carbonwas needed to be burnt out in the combustion zone toheat up and break down the steam. It is responsible for decreasingthe temperature and carbon conversion in the gasifier, and hencereducing H2 yield, as mentioned in the literature [33]. Moreover,the extent of endothermic reactions would dominate, resulting inlower zone temperatures [34]. Temperatures of combustion andreduction zone were found in the range of 1070 Ke1134 K and625 Ke763 K, respectively for all SLR.

3.2. Fuel consumption, airflow rate, specific fuel consumption

Fuel consumption, airflow rate, and Specific Fuel Consumption(SFC) with different SLR are shown in Fig. 3. Air-steam gasificationexperiments were carried out for constant Equivalent Ratio (ER).Type and amount of oxidizers play a significant role in fuel con-sumption. As SLR increased, the airflow rate reduced, and the steamflow rate increased. It was observed that fuel consumption reducedas SLR increased. This is due to the reduction in the amount ofoxidizer with increase in SLR. Fuel consumption and airflow ratewere found in the range of 5.52 kg h�1 -8.75 kg h�1 and 11.34 kg h�1

e 18.12 kg h�1, respectively. SFCs were calculated as per the formula

Fig. 3. Airflow rate, fuel consumption, and specific fuel consumption at different SLR.

given by Karagiannidis [35]. SFCs were found in the range of 1.48 kgkWh�1 - 1.65 kg kWh�1 which are in line with the literature [35].SFC was obtained minimum at 0.24 SLR.

3.3. Gas composition

The steam gasification of lignite may be broadly classified intothree stages: 1) Pyrolysis of the lignite, 2) Combustion of the py-rolysis products in the presence of steam, and 3) Gasification of theproduct gases [36]. Gasification temperature and SLR are theimportant parameters which govern the steam gasification re-actions. Various reactions such as Boudouard, water-gas shift,water-gas, methanation, and steam reforming are taking placeduring the air-steam gasification. The sub-stoichiometric combus-tion of lignite in the presence of steam produces CO, CO2, H2O, H2and other by-products such as tar, phenol, etc. These products arethen reacted in the reduction (gasification) zone in the presence ofactivated char resulted in the producer gas as the output.

The producer gas compositions (in % volume) obtained duringthe air-steam gasification are shown in Fig. 4. The concentration ofH2, CO, CO2, CH4 of the producer gas were found 24.05%, 2.19%,13.6%, and 1.8%, respectively for air gasification (at 0 SLR). Thepresence of steam inside the reactor enhances the steam reformingand water gas shift reaction along with water gas reaction [12]. Thewater-gas shift reaction is thermodynamically favourable at atemperature below 1100 K [34,37]. The gasification temperatureincreases with an increase in SLR till 0.24 SLR and decreases withfurther increase in SLR as shown in Fig. 2. This higher temperaturefavours the conversion of CO and steam to CO2 and hence, increasesin CO2 and H2 is observed till 0.24 SLR. Also, the rate of metha-nation reaction decreases with an increase in temperature [34].These reactions are responsible for the production of H2 and CO2and the ingestion of CO and CH4 with the increase in the SLR till0.24, which is in line with the literature [38]. The maximum H2content (32.2%) was found in the producer gas at 0.24 SLR.

There was a deviation perceived in the trend followed by theconcentrations of the producer gas beyond 0.24 SLR. It wasobserved that H2 concentrations reduced abruptly due to excessivesteam injected in the gasifier reactor after 0.24 SLR. It would reduce

Fig. 4. Concentrations of the producer gas at different SLR.

Fig. 6. Gas flow rate and Lower Heating Value at different SLR.

D.S. Upadhyay et al. / Energy 211 (2020) 1181876

the residence time of steam in the reactor and made gasifier re-actions less effective. It was also observed that the amount of watervapor in the producer gas was marginally higher at higher SLRcompared to lower SLR conditions. Moreover, at a higher temper-ature (around 1100 Ke1200 K), reversewater gas shift reactionmayoccur easily. It may be responsible for the higher consumption of COin the reaction and increment of CO2 concentration [39]. Apart fromthis, two major influences occurred instantaneously when highersteam was injected into the gasifier reactor, 1) additional steamcontent shifts the equilibrium constant for water gas shift reaction,and 2) steam reforming reaction in the direction of the productside. However, this effect would slow the combustion process tosome extent. The unreacted steam particles carried with producergas would also be responsible for reducing reduction zone tem-perature. The concentrations of H2, CO2, CO and CH4 in the producergas were found in the range of 24.05%e32.4%, 13.6%e16.58%,10.82%e12.19%, and 1.57%e1.8%, respectively for all selected SLR.

The total combustible gas (CO, CH4, and H2) in the present studywas ranging from 38.04% to 44.78% in the producer gas. As theconcentration of CH4 was very less (around 2%), the concentrationof CO þ H2 was the major combustible component in the producergas. It can be seen from Fig. 5 that the H2/CO ratio increased from2.21 to 2.42 when SLR increased from 0 to 0.24. After that, this ratiodecreased from 2.42 to 2.22. CO/CO2 ratio is helpful to understandthe rate of gasification reactions and the conversion of hydrocar-bons of the feedstock into combustible gases. From Fig. 5, it can beseen that as SLR increases till 0.24, the CO/CO2 ratio decreases from0.9 to 0.65. This may be due to the higher consumption of CO due toa rise in gasification temperature. As the SLR increases from 0.24,the CO/CO2 ratio was found increasing from 0.65 to 0.83. At a highertemperature, Boudouard reaction dominates over water gas shiftreaction which leads to higher CO concentration. The air-steamgasification improved the yield of H2 by 34.7% compared to airgasification (0 SLR). The H2/CO ratio was also improved by 52%compared to air gasification.

3.4. The gas flow rate and lower heating value

Fig. 6 illustrates the gas flow rate and lower heating value (LHV)of the producer gas at different SLR. The gas flow rate depends onthe consumption of fuel and oxidizers utilized during the

Fig. 5. Ratios of H2/CO, CO þ H2 and CO/CO2 at different SLR.

gasification process. It was observed that fuel and air consumptiondecreased with an increase in steam flow rate leads to a diminutionof the producer gas (20.95 kg h�1 to 13.62 kg h�1). As the SLRincreased from 0 to 0.24, LHV of the producer gas also increaseddue to the higher H2 yield. The lower heating value of the producergas was calculated by the formula given by TB Reed et al. [40]. LHVof the producer gas was found in the range of 4.98 MJ Nm�3 to5.62 MJ Nm�3 for all selected SLRs. LHV of the producer gas forsteam gasification was increased by 12.85% (at 0.24 SLR) comparedto air gasification (at 0 SLR).

3.5. Cold gas efficiency and carbon conversion efficiency

The gasifier performance is generally assessed by cold gas effi-ciency or hot gas efficiency. CGE parameter is essential when theproducer gas is to be employed to the I. C. Engine or gas turbine forelectricity generation. CGE can be calculated by the ratio of energycontent of the producer gas (Gas Flow Rate * LHV) to the energystored in the feedstock (FC * CV of the feedstock). CGE was calcu-lated and found in the range of 70.6%e78.9%. It was observed thatthe addition of the steam in the combustion zone of a gasifierreactor is valuable in terms of CGE. The highest 78.9% CGE wasfound at 0.24 SLR among all selected SLR. Carbon conversion effi-ciency (CCE) was calculated as per the formula given by Sharmaet al. [41]. CCE for different SLR was found in the range of 90.97%e96.64%. It was noticed that the CCE was improved by injectingsteam to the gasifier reactor, as shown in Fig. 7. It may be due to thehigher tar and char conversion into the producer gas. Carbon in thefeedstock was utilized in a much better way by steam gasificationcompared to air gasification.

3.6. Tar and particulate matter

Tar and Particulate matter (PM) in the producer gas weremeasured by an in-house facility as described earlier. Tar and PMwere measured at the downstream of the gasifier system after thefabric filter arrangement, as shown in Fig. 1. The gasification tem-perature was played a substantial role in the reduction of tar andPM in the producer gas. Tar and PM were calculated and found inthe range of 112.28 mg Nm�3 to 517.64 mg Nm�3 and 27.34 mg

Fig. 7. Cold gas efficiency and carbon conversion efficiency at different SLR.

D.S. Upadhyay et al. / Energy 211 (2020) 118187 7

Nm�3 to 92 mg Nm�3, respectively as shown in Fig. 8. Injection ofsteam into the combustion zone offered good results to diminishtar and PM in the producer gas. It may be due to the conversion ofthe tar into the producer gas, which is in line with the literature[38].

Fig. 8. Tar and PM concentrations in the producer gas at different SLR.

Table 2The compositions of tar without and with steam gasification of lignite.

Sr. No. Retention time Compound

1 2.524 Toluene2 3.539 H2O, CH3C]O,C3H7, carbonyl compound3 4.776 C4H7,C3H7, Cyclohexanone, Ether4 19.095 Amide5 26.01 Ester

The tar was obtained from the tar measuring systemwas dilutedwith acetone. After the filtration unit, the tar from the product gaswas examined using Gas Chromatography-Mass Spectrometry (GC/MS). Auto system XL with Turbomass mode was used in Perki-nElmer make GC/MS. For tar analysis, the PE-5MS (30 m) capillarycolumn with H2 carrier gas was used. The carrier gas flow-rate andsample injection volume were kept 1 ml min�1 and 1 � 10�6 L,respectively. The pre-oven temperature holds initial isothermal at75 �C for 5 min, then accelerating the heating rate up to 280 �C by10 �C/min. For better outcomes, the analysis was carried out after30 min residence time.

The composition analysis of coal tar with or without using steamby GC/MS is compared in Table 2. The reaction between feedstock(hydrocarbons) and steam becomes severe at higher temperatures[39]. The complex components and aromatic hydrocarbons of tardecrease with the residence time and convert into aliphatic andsmall hydrocarbon compounds as shown in Fig. 9.

As per Table 2, the maximum concentration of ethers was ob-tained which are sparingly soluble in steam. Thus, the hydratedether percentage increases in steam gasification of lignite. Alongwith it, the percentage of aldehyde carbonyl compounds is more inlignite with steam feedstock because of its resolution with steam.Similarly, in the esterification process, the product formed with thesteam gasification is less in comparison without steam. Toluene, astable aromatic element is usually formed at high-temperaturesteam gasification, responsible for higher risk during downstreamoperation in the gasifier system. As per Table 2, data reveals that theconcentrations of aliphatic hydrocarbon compounds are increasedby 30.80% whereas aromatic hydrocarbons are decreased by16.89%. It is due to the fact that lignite feedstock initially de-composes into aromatic elements (primary tar compounds) and ata higher temperature, they decompose to a large number ofoxygen-containing hydrocarbon (secondary tar components) [42].This effect is responsible for increasing H2 concentration in theproducer gas as suggested in the literature [43e45]. The aboveinvestigations conclude that the steam gasification process is theeco-friendly and productive procedure of gasification of poorcarbonaceous feedstock also.

3.7. Thermal analysis

Mass, exergy, and heat loss analysis were carried out in thepresent study. Detailed methodology is also available in the au-thors’ previous work [46,47].

3.7.1. Mass balanceThe mass balance was carried out to understand the input and

output mass of the gasification system. The input products fed tothe gasifier were feedstock (lignite) and oxidizers (air and steam).The output products from the gasifier were dry gas, char, tar, ash,and water. The following equation signifies the conservation ofmass in the gasification process:

Lignite feedstock (w/w) Lignite feedstock with steam (w/w)

13.31 9.2112 14.7129.88 42.7132.28 33.378.92 6.32

Fig. 9. GC/MS analysis of tar without and with steam gasification of lignite.

D.S. Upadhyay et al. / Energy 211 (2020) 1181878

mfuel þ mair þ msteam ¼ mgas þmchar þmtar þmash þmwater

where mfuel, mair , msteam, mgas, mchar , mtar , mashand mwater showsthe mass flow rate of fuel, air, steam, dry gas, char, tar, ash, andwater, respectively. The tar yield obtained during the gasificationprocess was very less; hence, it was neglected from the mass bal-ance analysis. Mass Balance Closure (MBC) was found in the range

Table 3Mass Balance at different SLR.

SLR Inputs (kg h�1) O

Fuel Steam Air Total D

0.00 8.75 0 18.12 26.87 20.06 8.15 0.49 17.01 25.65 10.14 7.68 1.04 16.00 24.72 10.18 7.15 1.32 15.00 23.47 10.24 6.75 1.60 14.15 22.50 10.30 6.11 1.82 12.78 20.71 10.48 5.39 2.60 11.34 19.33 1

of 0.94e0.97. However, the non-closure of mass balance was due tothe errors in measurement and some unaccounted losses/errors.Mass balance closure, which is the ratio of total output to inputmass, is shown in Table 3.

3.7.2. Exergy balanceExergy analysis of air-steam gasification with lignite as a feed-

stock was also carried out in the present study. An exergy analysis

utputs (kg h�1) MBC ratio

ry gas Char Water Ash Total

0.95 1.20 1.34 1.89 25.41 0.949.62 1.08 1.69 1.75 23.52 0.948.55 0.86 1.97 1.69 23.12 0.937.24 0.60 2.66 1.61 21.85 0.945.92 0.52 3.21 1.58 20.93 0.944.45 0.41 3.26 1.47 19.83 0.953.72 0.38 3.32 1.41 18.83 0.97

D.S. Upadhyay et al. / Energy 211 (2020) 118187 9

of the system shows a thermodynamic performance of the system.Exergy balance of gasification can be written as (Eq. (1)):

S Fin ¼SFout þ I (1)

where, Fin and Fout denote the input and output exergy, and Idenotes the irreversibility induced during the conversion process.Exergy for in any gas components is the sum of thermo-chemicaland thermo-physical exergy. Thermo-physical exergy representsthe change in enthalpy of the specific gas component from thereference state to the specified pressure and temperature. Chemicalexergy of the mixture is the standard chemical exergy mixing of allthe constituents and the loss in entropy due to the blending ofdifferent species of gases [48]. Exergy efficiency (Eq. (2)) is definedas the ratio of the exergy of the producer gas to a summation of theexergy of the feedstock and gasification mediums:

h ex ¼ Fgas

Flignite þ Fairþsteam(2)

In the present study, the air and steam mixture was used asgasification mediums. The air was supplied to the gasifier reactor atatmospheric pressure. Due to its negligent exergy, it was neglected[49]. The physical exergy is generally dependent on gasificationtemperature, so it was highest at 0.24 SLR. The chemical exergy isdepended on the producer gas concentration. From Fig. 10, it isobserved that the exergy efficiency decreases with the increment inSLR. It is because of the fact that increases in the steam flow ratelead to the higher exergy of the gasifyingmedium. Exergy efficiencywas calculated and found in the range of 45.09%e70.71% for all SLR.

3.7.3. Heat loss analysisDuring the gasification process, heat loss occurred from the

surface of a gasifier reactor. Heat transfer generally occurs throughthe mode of conduction, convection, and radiation. For the presentanalysis, conductive heat transfer was neglected, as the only heatloss occurring through this mode would be heat loss to the groundand other smaller sections. Hence, convective and radiative losseswere considered in the present analysis. Heat loss to the sur-roundings directly depends on the surface temperature of thereactor. The analysis was considered by dividing the peripheral

Fig. 10. Exergy Analysis at different SLR.

surface of the reactor into distinct zones, and heat loss analysis wascarried out for each zone individually.

Convective heat transfer coefficient and characteristic length arecalculated using Eq. (3) and Eq. (4), respectively.

h¼ðNu*kÞLc

(3)

Lc ¼As

p(4)

where As is surface area and p is the perimeter of a gasifier zonesurface. For flow over a cylinder, the correlation for the Nusseltnumber (Nu) is expressed as follows (Eq. (5)) [50]:

Nu¼

8>>>>>>>>><>>>>>>>>>:

0:825þ 0:387*ðRaÞ162641þ

�0:492Pr

� 916

375

827

9>>>>>>>>>=>>>>>>>>>;

(5)

The expression for the Rayleigh number and coefficient of vol-ume expansion is given by Eq. (6) and Eq. (7), respectively.

Ra¼ g*b*ðTs � T∞Þ*L3cy2

*Pr (6)

b¼ 1Tf

(7)

where Ra is Rayleigh number, Gr is Grashof number, g is the grav-itational acceleration (m/s2), b is coefficient of volume expansion, Tsis the surface temperature (�C), T∞ is free stream temperature, Lc isthe characteristic length of geometry (m), and y is the kinematicviscosity of the fluid (m s�2), Tf is the average temperature betweenthe ambient and surface of the gasifier reactor.

The correlations to find these properties, developed in AspenHYSYS (V9) at different temperatures, are as follow (Eq. (8-10))[51]:

mhN:s:m�2

i¼�36425 *108T þ18148 *105

�(8)

Cp

�kJkgk

�¼�99088 * e0:0002T

�(9)

K�Wmk

�¼�70744 *10�5 * T þ2423 *10�2

�(10)

The radiation heat loss (Qradiation) was found using the Stefan-Boltzmann law of thermal radiation. Fig. 11 shows the tempera-ture distribution over the peripheral surface of the reactor. It wasobserved that the highest surface temperature was found near acombustion zone. It is due to the fact that the surface temperaturehas a direct relationship with the gasification temperature. As thecombustion zone is the major source of heat in the gasifier reactor,the surface temperature was also found higher compared to otherzones. Convective heat transfer and radiative heat transfer werefound in the range of 0.462 kWe0.833 kWand 0.258 kWe0.651 kWrespectively. The highest surface temperature was achieved at 0.24SLR. Maximum convection and radiation heat loss were found (atSLR 0.24) 0.833 kWand 0.651 kW, respectively. It was also observedthat the heat loss by convection heat loss was higher than radiation

Fig. 11. Thermal image of the downdraft gasifier reactor.

Fig. 12. Convective and radiative loss from a gasifier surface.

D.S. Upadhyay et al. / Energy 211 (2020) 11818710

heat transfer as shown in Fig. 12. The details of heat loss analysiscan also be found in the authors’ previous work [47].

4. Conclusions

Steam gasification has the potential to improve H2 yield in theproducer gas and at the same time, diminish the concentration oftar and particulate matter (PM). With this objective, six differentSteam to Lignite Ratio (0e0.48) selected for the experimentationwith low-rank lignite feedstock. Air-steam gasification of lignitewas carried out on a pilot-scale 10kWe downdraft gasifier.Following are the significant observations from the present work:

1 Fuel consumption, specific fuel consumption, and gas flow ratewere calculated and found in the range of 5.39 kg h�1 to

8.75 kg h�1,1.437 kg kWh�1e1.65 kg kWh�1 and 13.72 kg h�1 to20.95 kg h�1, respectively for different SLR.

2 H2 yield and H2/CO ratio in producer gas were improved by34.7% and 52% (at 0.24 SLR), respectively compared to air gasi-fication (at 0 SLR). LHV of the producer gas and CGE were foundin the range of 4.96 MJ Nm�3e 5.62 MJ Nm�3 and 70.6% - 81%,respectively.

3 Tar and PM were remained in the range of 112.28 mg Nm�3 to517.64 mg Nm�3 and 27.34 mg Nm�3 to 92 mg Nm�3,respectively.

4 The concentrations of aliphatic hydrocarbon compounds wereincreased by 30.80% whereas aromatic hydrocarbons aredecreased by 16.89%.

5 Mass balance closure and exergy efficiency remained in therange of 0.93e0.97 and 45.09%e70.71%, respectively whereasradiation and convection heat losses were found in the range of0.462 kWe0.833 kW and 0.258 kWe0.651 kW, respectively forall SLR.

6 0.24 SLR was offered better performance as per higher LHV, CGEand lower tar and PM concentration in the producer gas.

The higher tar conversion and the higher yield of H2 make theair steam gasification superior over simple air gasification. It is alsoan important fact that the generation of steam requires an addi-tional initial and/or running cost. However, by integrating theprocess in such away that generation of required steam is producedfrom waste heat may have potential energy benefits.

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Authors wish to confirm that there are no known conflicts ofinterest associated with this publication.

Authors confirm that the manuscript has been read andapproved by all named authors and that there are no other personswho satisfied the criteria for authorship but are not listed. Authorsfurther confirm that the order of authors listed in the manuscripthas been approved by all of us.

Authors confirm that we have given due consideration to theprotection of intellectual property associated with this work andthat there are no impediments to publication, including the timingof publication, with respect to intellectual property. In so doingauthors confirm that authors have followed the regulations of ourinstitutions concerning intellectual property.

Authors understand that the Corresponding Author is the solecontact for the Editorial process (including Editorial Manager anddirect communications with the office). He is responsible forcommunicating with the other authors about progress, sub-missions of revisions and final approval of proofs. Authors confirmthat we have provided a current, correct email address which isaccessible by the Corresponding Author and which has beenconfigured to accept email from rajeshnpatel1969@yahoo.com,rnp@nirmauni.ac.in.

CRediT authorship contribution statement

Darshit S. Upadhyay: Conceptualization, Methodology, Inves-tigation, Writing - original draft. Krunal R. Panchal: Formal anal-ysis, Validation, Data curation, Writing - original draft. Anil KumarV Sakhiya: Formal analysis, Validation, Data curation, Writing -original draft. Rajesh N. Patel: Writing - review & editing, Visual-ization, Supervision.

Declaration of competing interest

The authors declare that they have no known competing

D.S. Upadhyay et al. / Energy 211 (2020) 118187 11

financial interests or personal relationships that could haveappeared to influence the work reported in this paper.

Acknowledgment

Authors have sincerely valued the Department of Science andTechnology, New Delhi (Project No: SR/S3/MERC-0114/2010) andNirma University, Ahmedabad (Project No: NU/Ph.D./MRP/IT-ME/16e17/851) for the financial assistance. The authors are verythankful to Dr. Femina Patel, Dr. Niraj Shah, Dr. Amita Chaudhary,Mr. Karan Patel, Mr. Savan Patel, and Ms. Gayatri Barad for helpingduring experiments and gas chromatography analysis.

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