A detailed experimental analysis of air–steam gasification in a dual fired downdraft biomass gasifier enabling hydrogen enrichment in the producer gas

Energy 187 (2019) 115937

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A detailed experimental analysis of airesteam gasification in a dual fired downdraft biomass gasifier enabling hydrogen enrichment in the producer gas Narasimhan Kodanda Ram a, b, *, Nameirakpam Rajesh Singh a, Perumal Raman b, Atul Kumar a, Priyanka Kaushal c a b c

Department of Energy and Environment, TERI School of Advanced Studies, Plot No. 10 Institutional Area, Vasant Kunj, New Delhi, 110 070, India Renewable Energy Technology Division, The Energy and Resources Institute, IHC Complex, Lodhi Road, New Delhi, 110 003, India Centre for Rural Development and Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 June 2019 Received in revised form 1 August 2019 Accepted 11 August 2019 Available online 16 August 2019

The use of ambient air as an oxidizing agent in the biomass gasification process is well understood. Ambient air contains 79% nitrogen (N2) and 21% Oxygen (O2) by volume. Consequently, producer gas generated through air gasification comprises of non-combustible gases around 62% (on a volume basis). As a result, the heating value of the producer gas is low, which results in low adiabatic flame temperature (AFT), low AFT results in reduced efficiency and higher specific fuel consumption (SFC) when the producer gas (PG) is used in internal combustion (IC) engines. This study is focused on the reduction of the non-combustible fraction in producer gas, thereby to increase the heating value of the producer gas through the air- steam gasification. The results of the experimental study are presented in details. A detailed mass and energy balance analysis conducted. A maximum of 27.24% (by volume) hydrogen achieved at equivalence number (EN) 1.54 suitable for bio-hydrogen production. Whereas EN 1.5e2.2 is more suitable for power generation applications since maximum higher heating value (HHV) occurs in this range, i.e., 6.33 MJ Nm
3. The enrichment of producer gas resulted in an increase of HHV by 44%. The cold gas efficiency is 86e87%. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Biomass Airesteam gasification Producer gas Hydrogen enrichment Syngas Steam to biomass ratio Equivalence ratio

1. Introduction Biomass gasification is a partial oxidation process; that converts the solid fuel into gaseous fuel. The gas generated through partial oxidation process is called producer gas or syngas. The standard constituents of producer gas reported on volume basis is hydrogen (H2) 16%e17%, carbon monoxide (CO) 17%e18%, methane (CH4) 2%e3%, nitrogen (N2) 45e50% and carbon dioxide (CO2) 10%e12% [1e4]. The biomass gasifiers are classified based on the movement of fuel and gas inside the reactor viz. updraft (counter-current), downdraft (co-current), and cross draft [1,5e7]. Producer gas generated from the gasifier is at a higher temperature. It contains impurities in the form of tar and dust, which will vary, based on the

* Corresponding author. Department of Energy and Environment, TERI School of Advanced Studies, Plot No. 10 Institutional Area, Vasant Kunj, New Delhi, 110 070, India. E-mail addresses: [email protected], [email protected] (N.K. Ram). https://doi.org/10.1016/j.energy.2019.115937 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

type and design of the reactor. For power generation, through internal combustion (IC) engines, producer gas with low impurities is preferred [8,9]. The downdraft gasifier is suitable for such cases, due to less impurity in the producer gas [10e12]. Predominantly, ambient air is used as an oxidizing agent in biomass gasification [13e15]. The literature presents different designs of biomass gasifier operated with different feed stalks as fuel [2,16,17]. The producer gas thus generated is used for a variety of end-use applications like) Thermal applications in the industries to meet the process heat requirements; replacing the diesel and furnace oil ii) Power generation applications for replacing the fossil fuel. Mostly modified compression ignition (CI) engine or spark ignition (SI) engines are used to run on producer gas. The efficiency of the modified SI engine-using producer gas under full load conditions is around 23e25% [1,6]. The efficiency of the modified CI engine with producer gas was reported as 30% [1,6,18]. The efficiencies of the producer gas-based generators are on the lower side in comparison with IC engines fueled with diesel, petrol, and natural gas [19]. Engine efficiency is a function of compression ratio. Fuel intake and

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N.K. Ram et al. / Energy 187 (2019) 115937

the adiabatic flame temperature influence the compression ratio, which is again a function of the higher heating value of the air-fuel mixture. Hence, there is a need to increase the higher heating value of the producer gas, which in turn increases the energy density, adiabatic flame temperature (AFT), and the overall efficiency.

gasification. He concluded that High-temperature steam gasification (HTSG) is the chosen to be better technology for hydrogen
-vis solar-assisted steam gasification. However, enrichment vis-a HTSG has lower overall efficiency. Results of several published articles were referred for identification of gaps and to understand the status of the steam gasification technologies and to identify the gaps and challenges which need to be improved. Different type of reactors and the results obtained are presented in Table 1. As presented in Table 1, studies on steam gasification conducted on fluidised bed reactors reported high hydrogen content in the producer gas ranging from 43% to 57.4% (on a volume basis). From Table 1, it may be noted that the optimum steam to biomass ratio (SBR) reported is in the range of 1e4.7. Increase in SBR was observed to increase the hydrogen production as increased steam supply did not result in temperature drop in the reactor due to external/indirect heating. However, the reported cold gas efficiencies were in the range of 42e66%. Similarly, studies conducted on fluidized bed reactor; using airsteam as oxidizing agent reported high hydrogen content in the producer gas, which is in the range of 18.6e49.1. The variation in SBR ranged from 0.2 to 4.7. The high concentration of hydrogen can be attributed to the size of the feedstock and indirect external heating, which provide the ideal conditions for reduction reactions. Different studies have also been conducted on fixed bed reactors with different approaches. An updraft gasifier integrated with pyrolysis, which yielded enriched syngas with a hydrogen content of 64% (on a volume basis), along with external heating of the reactor [33]. Another study by Ref. [35] conducted on an updraft gasifier with a porous ceramic reformer produced syngas with a hydrogen

1.1. Gap analysis and novelty of the present study Since three decades, researchers are investigating a variety of oxidizing agents such as oxygen, oxy-steam, air-steam, and steam for biomass gasification. Oxygen as gasification agent results in the production of a high concentration of combustible components in producer gas since oxygen is free from nitrogen [20,21]. Oxy-gasification result in; increase in the higher heating value of the producer gas; thereby increasing the energy density and AFT. It also increases the cost of power generation, as it would require a continuous supply of oxygen [22]. Studies conducted on steam gasification make use of superheated steam to prevent sharp fall in reactor temperature which hinders Water-gas shift reaction (WGSR) and Water gas reaction (WGR) and consequently hydrogen enrichment in the producer gas. Also, external heating is employed to prevent fall in reactor temperature and for better control of the reactor temperature. However, the generation of superheated steam and external heating increases the energy input to the system. Provision of external heating decreases the overall efficiency of the system. Duan et al. used molten blast furnace slag as a heat carrier in the steam gasification of coal to obtain hydrogen-enriched syngas [23]. Sepe et al. [24] conducted a thermodynamic analysis of solarassisted steam gasification and high-temperature steam

Table 1 Different type of reactors and comparison of the results. Ref [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [7] [36] [37] [38] [38] [39] [40] [41] a b c d e

Reactor design/Reactor Tempt.  C 

FB Externally heated/750 C FB Externally heated/1000  C FB Externally heated FB Externally heated/800  C FB Externally heated FB Externally heated/727  C FB Externally heated/800  C FB Externally heated/800  C Integrated gasifier and pyrolysis Externally heated/950  C Counter flow Self-heated Updraft with a porous ceramic reformer Externally heated/850  C Updraft Pre-heater. Heated up to 900  C lab scale Externally heated/900  C Downdraft Preheated/900  C Downdraft Self-heated Downdraft Self-heated Downdraft Self-heated Downdraft Self-heated Downdraft Self-heated

Molar fraction. ASTR Air steam ratio. a Molar fraction of steam in the feed gas. (dry, inert free gas). HHV.

Feedstock/Oxidizing agent Rice husk/Steam Air-steam Wood residue Steam Coir pith Steam a-cellulose Air-steam Wood pellets Air-steam Pine sawdust Air-steam Sawdust Steam Coconut shell Air-steam Diary biomass Air-steam Pine sawdust Steam Wood pellets Air-steam Olive oil waste Air-steam Eucalyptus wood chips Air-steam Eucalyptus Air-steam Eucalyptus Oxy-steam dry casuarina wood Oxy-steam dry casuarina wood Oxy-steam Pine wood blocks/Oxy-steam

ER

SBR

CO

H2 a

CO2 a

CH4 a

a

HHV (MJ Nm
3)

CGE

Gas Yield

11.6e

66.0

1.1

na na na 0.2 0.27 0.19 0.22 0 0.24

1 1 1 4.5 1 0.28 2.7 4.7 1.95

22.7 27a 18.0 15.0 8.3 11.5 37.73d 7.3d 21.0

48.9 49.1a 51.0 43.0 18.6 16.2 32.10d 57.4d 64.0

22.2 23a 16.0 41.0 27.4 18.6 18.55d 30.2d 6.0

6.2 1a 12.0 1.0 3.0 5.9 7.46d 4.1d 9.0

1.53

0.7b

11.9

18.6

16.1

0.2

0

2.53

18.0

53.0

20.0

7.0

12.2

24.4

21.8

16.1

1.7

8.9

c

10.8 7.2 5.3

13.0

0.52

na

1.2

21a

65a

12a

4a

0.12

0.17

18.9

22.3

16.2

2.0

6.2

0.409

0.476

12.2

19.4

na

1.6

4.2

0.431

0.495

23.2

34.3

na

3.5

7.9

0.33

0.28

18.04

16.9

18.06

1.61

0.18

1

24.1

0.24

0.61

38.66

45.8 28.58

24.0 d

24.45

5.0 d

42.0 2.2 1.6

na

d

1.3

1.8

na

na

8.6 d

6.01

11.1

1.5

N.K. Ram et al. / Energy 187 (2019) 115937

content of 53%. However, the studies conducted on updraft gasifier reactors (i.e., which do not have any external heating system) produce syngas with significantly lower hydrogen content in comparison with the above studies; 18.6% (on a volume basis) on a study by Ref. [33] and 21.8% (on a volume basis) by Ref. [7]. The studies conducted on downdraft gasifier using oxy-steam gasification yielded producer gas with high hydrogen content and high HHV. A study by Sandeep et al. [40] reported hydrogen content of 45.8% with an HHV of 8.6 MJ Nm
3 at SBR of 1. It should be noted that the steam supplied was superheated to 873 K. A different study conducted by Lv et al. [41] also reported producer gas with LHV of 11.1 MJ Nm
3 at SBR of 0.61. Ngamchompoo et al. applied Hightemperature air gasification (HTAG) technology to preheat air and steam (150  C) to 900  C in a downdraft reactor [37]. The maximum hydrogen content achieved was 22.3% (on a volume basis) with HHV of 6.2 MJ Nm
3. However, it requires external heating to reach the required high temperature, which increases the cost. De Sales [38] conducted a study downdraft gasifier (on a prototype of capacity 8-12 h
1.) using saturated steam. The maximum hydrogen content in the producer gas reported in this study is 19% on a volume basis and the CGE of 68%. Sharma et al. [42] also conducted a study on air-steam gasification in a downdraft reactor (bench scale of capacity 1.5e3 kg h
1) with the supply of saturated steam (at 150  C). Jarungthammachote et al. conducted a study on a twostage downdraft gasifier (on a prototype of capacity 6e8 kg h
1) using saturated steam at 150  C [39]. The maximum hydrogen content in the producer gas reported in this study is 16.94% on a volume basis. Studies conducted in downdraft gasifier shows hydrogen composition in producer gas in the range of 16.94e22.34% (on a volume basis).

1.2. Aim and objectives of the study The main aim of the article is to increase the HHV of the producer gas by increasing the combustible gas components through the airesteam gasification and reduction in non-combustible components in gas (like Nitrogen). The conventional air gasification system produces the gas with more than 50% of nonecombustible gas components. The non-combustible gases occupy a large volume of the cylinder when used in IC engines. Presence of the non-combustible components in producer gas results in de-rating of the design capacity of the IC engines to the

3

order of 30%. Typically, the power de-rating reported in Refs. [43e45] is at the tune of 17%e20% depending on the producer gas quality. In case of thermal applications, the nonecombustible components of the producer gas does not contribute to the heating value, but it absorbs a significant portion of the energy produced during the process of combustion. Hence, the noncombustible fractions of producer gas have a substantial impact on adiabatic flame temperature (AFT). In thermal application, furnace temperature, heat transfer, and efficiency are linked to AFT. Reduction in non-combustible fraction of producer gas will lead to an increase in the heating value of the producer gas, increase in AFT, and better efficiency. Hence, optimizing the parameters concerned to air-steam gasification to maximize its heating value by reduction of nonecombustible fraction is selected as the main aim of the project. The finding of the study will enhance the airesteam gasification process of biomass, instead of the air gasification process. The improvement in gas quality through air-steam gasification can benefit the industries, which requires thermal energy for process heating. The biomass gasifier based electricity production schemes can be benefited with the use of good quality producer gas, obtained from the airesteam gasification. The parameters considered for the analysis of hydrogen enrichment in the producer gas are as listed below: I. II. III. IV. V.

Reactor temperature, Equivalence ratio (ER), Steam to biomass ratio (SBR), Equivalence number (EN), Gas composition, higher heating value (HHV) and gas yield (GY), VI. The energy density (Ed) and adiabatic flame temperature (AFT), VII. Cold gas efficiency (CGE),

2. Experimental setup The experimental setup consists of a dual fired downdraft (DFD) gasifier, heat exchanger, blower and flare line with gas sampling port. The details of the experimental set-up are shown in Fig. 1.

Fig. 1. The experimental setup.

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N.K. Ram et al. / Energy 187 (2019) 115937

2.1. Dual-fired downdraft gasifier reactor

3.2. Airflow rate

The gasifier reactor is a dual fired type wherein the oxidizing agent is supplied in two different zones of the reactor, as shown in Fig. 1. This paper presents the experimental study conducted on dual fired downdraft gasifier using air-steam as an oxidizing agent to enrich the hydrogen content in the producer gas. As shown in Fig. 1, the dual fired downdraft biomass gasifier consists of four parts; these include (i) Fuel hopper (ii) Reactor-I (drying and pyrolysis zone), (iii) Reactor-II (oxidation and reduction zone) and (iv) Ash pit/gas outlet. The fuel hopper is used to feed fuel into the gasifier by opening a lid placed over it. The DFD gasifier has two reactors, i.e., vertically mounted one on the top of another as shown in Fig. 1. Accordingly, the reactor-I is placed on top of the reactoreII. This arrangement ensures the free flow of biomass from the fuel hopper to reactoreI. The DFD gasifier has a continuous and free flow (gravity flow) of hot charcoal from the reactoreI to the reactoreII. Hence, there will not be a situation where reactoreII runs out of carbon for the reduction process. The ash is removed periodically through a vibrating grate arrangement from the reactoreII. The reactoreI is a cylindrical shell of the internal diameter of 0.50 m and a height of 0.50 m. It is made of mild steel and provided with a double layer of high-temperature insulation lining. The lining helps to prevent heat losses and maintain high-temperature level inside the reactor. The insulation layer is provided to withstand a temperature of up to 1600  C, and the conductivity of the insulation layer used is in the range of 8e15 Wm
1K
1. The thickness of each insulation layer is 0.05 m reducing the inner diameter of the reactor to 0.30 m. ReactoreI was provided with a header supplying both air and steam. This header was connected to a blower (forced draft) and a steam generator. Control valves are in place to control the flow of air and steam according to the requirements. A hot-wire anemometer was used to measure the velocity and to estimate the airflow rate. The Reactor-II is installed underneath the reactoreI. ReactoreII is also provided with a refractory lining and consists of an air supply header, which connects to four air supply nozzles.

The velocity of air supply is measured with a duly calibrated hot wire anemometer (Airflow TA2 anemometer) to estimate the mass flow rate of air supplied by the centrifugal blower. The airflow rate was calculated as per Eq. (1).

ma ¼ Ac  va  ra

(1)

3.3. Fuel consumption rate The operation of the biomass gasifier is in batch mode, i.e., the gasifier is refueled every 3 h to maintain a continuous supply of biomass. To ensure the consistency in the feed rates; fuelwood was refilled till the brim of the hopper at every batch. The weight of fuel supplied in every batch is carefully measured using a digital balance. The fuel consumption rate is estimated using Eq. (2).

mf ¼

Wf tf

! (2)

3.4. Equivalence ratio (ER) The equivalence ratio is one of the critical operating parameters to optimize the performance of the gasifier. It is the representative value of the actual air supplied for gasification of a unit mass of fuelwood. The equivalence ratio is the ratio of air supplied for gasification of fuelwood and the stoichiometric air requirement. The equivalence ratio can be estimated using Eq. (3).

! ma mf

!

ER ¼

Actual

(3)

ma mf

2.2. Heat exchanger

stoichiometric

The temperature of the producer-gas coming out of the reactorII was about 500  C, which is allowed to pass through a shell and tube heat exchanger. The heat exchanger is a counter flow type with the multi-pass flow for both the streams. In the heat-exchanger air is passed through the shell and hot gas is allowed to pass through the tubes, and the temperature of the air was raised to 220  C. Higher air temperature avoids condensation of the steam in the pipelines and maintains higher reactor temperature. 3. Methodology The selected parameter, which influences the gas quality during air-steam gasification, was monitored during the experiments. 3.1. Reactor temperature measurement A “k” type thermocouple is used to measure the temperature inside the reactor. The reactor temperature is measured diagonally across the reactor, through the nozzles from both the ends. The reactor temperature is also an indicator for initiation of steam injection and to maintain the reactor temperature at steady-state conditions.

3.5. Steam to biomass ratio Steam to biomass ratio is one of the critical parameters which control the reactor temperature and the gas quality. It is essential to study and optimize steam to biomass ratio (SBR). SBR is estimated using Eq. (4).

SBR ¼

ms mf

! (4)

The water supply to the boiler is measured using the water flow meter mounted on the water supply pipe at the inlet to the boiler. The water flow is measured continuously during the operation, and the difference between the initial and final reading gives the mass of steam supplied (Ms) to the reactor for a particular duration (ts) of operation. The mass of steam supplied to the reactor is estimated using the Eq. (5).

Ms ¼ ða
bÞ The steam flow rate ms is estimated using Eq. (6).

(5)

N.K. Ram et al. / Energy 187 (2019) 115937

 ms ¼

Ms ts

 (6)

3.6. Gas analysis The gas composition is analyzed with an online gas analyzer (MODEL: ONG -AMBESTECH- 2011). The online gas analyzer consists of consists three different sensors to analyse the various components of the producer gas they are i) Non-Dispersive InfraRed (NDIR) detectors to measure the concentration of carbon oxides (CO and CO2). The Non-Dispersive Infra-Red (NDIR) detectors work on the principle of absorption of light of the gas under test. ii) Thermal conductivity sensor to measure the concentration of H2 and CH4 measures. The thermal conductivity gas analyzer works on the thermal conductivity principle. iii) Electrochemical oxygen detector works on the fuel cell principle used to measure the concentration of O2 in the producer. 3.7. Estimation of higher heating value (HHV) The higher heating value of the producer gas is monitored along with the variations in other critical parameters such as reactor temperature, SBR, and ER. The constituents of the producer gas sample were analyzed using an online gas analyzer. The higher heating value of the gas (HHV) is estimated based on the analysis of the gas composition in the producer gas. The HHV of producer gas is estimated using Eq. (7).

HHV ¼

X

Xi  HVi

(7)

3.8. Estimation of energy density The energy density of the “air-producer gas” mixture and its importance is reported in Ref. [17]. The energy density of the “airproducer gas” mixture (air and producer gas) can be defined as the ratio between the energy content of the “air- producer gas” mixture per unit volume (MJ m
3). Here the quantity of air in the “air þ producer” mixture is the amount of stoichiometric air required per unit volume of the PG. The energy density of the “airfuel” mixture is estimated using Eq. (8). Here the energy density refers to the energy content of the fuel mixture (air þ gas) for a unit volume in MJ. HVg refers to the higher heating value of the gas in (MJ.Nm
3). Vg and Va represent the volume of gas and air, respectively in Nm3.

Ed ¼

HVg Vg þ Va

5

gasification process. Increase in ER will result in high reactor temperature, but it also will contribute to high nonecombustible gaseous components (CO2 and N2) presence in producer gas (PG). Increase in SBR will increase hydrogen production. Also, the increase in SBR will tend to reduce the reactor temperature, which will reduce the hydrogen content and the overall gas quality. It is important to strike a balance between these two parameters of ER and SBR. Hence, a new parameter named Equivalence number (EN) is defined as the ratio of ER and SBR. Optimized ratio of EN is arrived based on the optimized ER and SBR. EN is a single influencing factor, which can be used effectively in design and scale-up of air steam gasification reactors. The equivalence number is estimated using Eq. (9).

EN ¼

ER SBR

(9)

3.10. Estimation of adiabatic flame temperature The adiabatic flame temperature (AFT) is an essential factor which influences the expansion ratio of the air-gas fuel mixture, which is an essential factor which controls the efficiency of the internal combustion engine (ICE). The adiabatic flame temperature is estimated using Eq. (10).

Hreactants ¼ Hproducts

(10)

3.11. Gas yield (GY) There are three key inputs to the gasifier system, namely fuelwood, air, and the steam. The estimation of fuelwood, air, and the steam input to the system is discussed in sections 3.3, 3.2, and 3.5, respectively. In order to measure the gas output from the system, a venturi meter is installed in the gas pipeline. The gas flow rate through the venturi meter is estimated using Eq. (11).

C  3600  rpg qffiffiffiffiffiffiffiffiffiffiffiffiffiffi A0 Q¼ 4 1
b

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2gDHru

ra

(11)

The gas yield is defined as the percentage/fraction of gas products converted from the biomass is estimated from Eq. (12).

GY ¼

Q mf

! (12)

(8) 3.12. Mass balance

3.9. Equivalence number Steam gasification is mainly controlled by two parameters, which is equivalence ratio (ER- air supplied for gasification of a unit mass of biomass) and steam to biomass ratio (SBR). When the ER increases the reactor temperature along with an increase in CO2. When the SBR is increased, there will be a drop in reactor temperature, and it is well established that high reactor temperature is essential to sustain an increase in hydrogen production. Equivalence ratio (ER) and steam to biomass ratio (SBR) are two key parameters, which influence the hydrogen enrichment in air steam

Mass Balance analysis is performed for the experimental results by accounting for all the inflows and outflows in the system. The gasifier is loaded with the biomass every 3 h. There is a reference line in the gasifier hopper, which acts as a datum for filling up the gasifier to a standard capacity. The charcoal is collected after the experiment and measured. Air supply and steam supply rate is monitored during the experiment. Condensates from the gas cooling line were collected and measured. The gas flow rate was measured using a venture meter. The total mass input to the gasifier is estimated using the Eq. (13).

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N.K. Ram et al. / Energy 187 (2019) 115937

Iw ¼ Ww þ Aw þ Sw þ Ma þ Mw

(13)

The total mass of fuel supplied to the reactor during the experiment is estimated using the Eq. (14).

ðWw Þ ¼

n X

ðWi Þ

(14)

i¼1

m   X Aj

(15)

j¼1

The total mass of steam supplied to the reactor is estimated by using the Eq. (16).

ðSw Þ ¼

m=n X

ðSk Þ

(16)

k¼1

The moisture content in the air is estimated using the Eq. (17).

ðMa Þ ¼ AG  H

GL ¼ GW  Cv  ðT3
T0 Þ

(26)

The sensible heat gained by the air supplied for gasification is estimated using the Eq. (27)

AG ¼ Aw  Cva  ðT2
T0 Þ

The total mass of air supplied to the reactor is estimated by using the Eq. (15).

ðAw Þ ¼

The sensible heat carried away by hot producer gas is estimated using the Eq. (26)

(17)

(27)

The heat carried away by char and ash content is estimated using Eq. (28)

Ec ¼ Mc  6200 calkg
1

(28)

The unaccounted portion of the energy balance analysis is estimated by using Eq. (29)

UE ¼ IE
ðEG þ GL þ EC Þ

(29)

The cold gas efficiency (CGE) is discussed by Ref. [46]. In the present study, it is estimated by using the Eq. (30)

Gw  HHVg CGE ¼ ðww  H  HVw Þ þ hfg

!  100

(30)

The moisture content in wood is estimated using the Eq. (18).

Mw ¼ MCð%massÞ  Mw

(18)

The total weight of the output products from the gasification process is estimated using the Eq. (19).

Om ¼ Gw þ Rw þ Dw þ Uw þ Cw

(19)

The total mass of producer gas (Gw) produced during the test was estimated using the Eq. (20).

Gw ¼

m   X Gj

(20)

j¼1

The total quantity of dust carried away by the producer gas was estimated using the Eq. (21).

Dw ¼ Dc  Gw

(21)

The unaccounted portion of the mass balance analysis was estimated using the Eq. (22).

Uw ¼ Im
ðGw þ Rw þ Dw Þ

(22)

3.13. Energy balance Energy balance analysis is performed to assess the performance of the gasifier. The analysis has been carried out by estimating the total Input and output energy. The input energy to the system is estimated using the Eq. (23).

Input energy: IE ¼ Ww  Cvw þ AG þ ES

(23)

The output energy to the system is estimated using the Eq. (24).

Output energy: OE ¼ GL þ EG þ EC þ UE

(24)

The higher heating value of the producer gas is estimated using the equation Eq. (25),

EG ¼ GW  CVG

(25)

4. Results and discussion The experimental analysis of the air- steam gasification was performed. Detailed studies on the critical influencing parameters (discussed in section 1.2) were conducted to meet the objective. The selected parameters were optimized, to reduce the noncombustible fraction of the producer gas, with an emphasis on maximizing the higher heating value through hydrogen enrichment. The analysis of the experimental results is presented in the subsequent sections. The steam injection process was carried out when the reactor had reached the threshold temperature, i.e., 900  C. Temperature drop in the reactoreI, and reactoreII was noticed after the steam injection. However, it was observed that in due course of time, the reactor temperature was restored to 900  C. During the experiment, the reactor-I & reactoreII temperature was maintained in the range of 900  Ce1200  C, which is a desirable condition for air-steam gasification. The temperature of reactor-I and reactor-II influences the gas components since the reaction rate is dependent on the temperature. In the air-steam gasification process, both exothermic, as well as endothermic reactions co-occur. The details of the reactions occurring in air-steam gasification are provided in Eq. (31)e(37). The reactor temperature is a critical operating parameter, which influences the constituents of the producer gas in the air-steam gasification process. According to Le Chatelier's principle, high temperature encourages reactants in exothermic reactions (31e32).

Boudouard reaction : C þ CO2 42CO þ 172kJmol
1

(31)

Water gas reaction : C þ H2 O4CO þ H2 þ 131:5kJmol
1

(32)

Water gas shift reaction : CO þ H2 O4CO2 þ H2
41kJmol
1 (33) Methanation reaction : C þ 2H2 4CH4
74:8kJmol
1

(34)

N.K. Ram et al. / Energy 187 (2019) 115937

C þ O2 4CO
246:4kJmol
1

(35)

C þ O2 4CO2
408:8kJmol
1

(36)

2H2 þ O2 42H2 O
484kJmol
1

(37)

4.1. Proximate and ultimate analysis of fuelwood Fuelwood available in the market was used for experimental purpose. The thin pruning of the fuelwood of dimensions 25 mme60 mm thick was used. The samples of the fuelwood used in the experiment were analyzed. The results of the proximate and ultimate analysis are presented in Table 2 The results of the ultimate analysis of the fuelwood is used for estimating the higher heating value of the biomass and the same is used in the energy balance analysis. 4.2. Stabilisation period to attain steady-state conditions During the cold start conditions, the dual-fired reactor of the downdraft gasifier was operated on the air gasification mode (i.e., without steam injection). Once the reactor reached the steady-state

Table 2 Proximate analysis and Ultimate analysis of fuelwood. Components

Unit

Percentage

Proximate analysis Moisture Ash Volatile matter Fixed carbon

Mass Mass Mass Mass

fraction fraction fraction fraction

9.8 1.5 68.8 19.8

Carbon Hydrogen Oxygen Nitrogen

Mass Mass Mass Mass

fraction fraction fraction fraction

48.1 9.2 41.3 1.4

Heating value (HHV)

Kcal kg
1/MJ kg
1

Ultimate analysis

4400/18.4

7

condition in air gasification mode; then the gasifier operation was shifted to air-steam gasification mode by injecting the saturated steam at 150  C, inside the pyrolysis zone of the reactoreI along with the preheated air in the temperature range 180e220  C. It was observed that the reactor-I temperature drops on steam injection. The radial temperature profile measured across the reactor-I is presented in Fig. 2. The variation in the temperature profile across the nozzle surface of the reactor-I is presented. During air gasification, it was observed that the reactor-I temperature at the periphery of the reactor and center is at 1050  C and 1015  C respectively. After steam injection, the temperature drop across the reactor-I was noticed. The temperature near the periphery of the reactor-I drops to 794  C. The temperature at the core of the reactor drops to 450  C. It was observed that the temperature of the reactor was gradually increased along with time. The steady-state conditions were achieved after 2 h (at SBR 0.21) of steam injection when the reactor-I attains 900  C. On continuous operation, the temperature of the reactor-I further increases to 1125  C at the center and the periphery; it is increased to 915  C. Radial temperature profile across the nozzles in reactor-II is shown in Fig. 3. From Fig. 3, It can be noticed that after steam injection, the radial temperature distribution of the reactor-II too drops. The temperature profile after steam injection inside the reactor-II shows that the radial temperature of the reactor-II was dropped as low as 731  C, whereas the same was at 1109  C before steam injection. It was observed that after 2 h, the reactor-II temperature reaches 1000  C at the center. Correspondingly, the temperature was around 915  C and 830  C on the left and right edges of the Reactor-I. It may be noted that the temperature stabilisation time is same for both the reactors. Both of the reactors take about 2 h (at SBR 0.21) to reach the steady-state conditions after injecting the steam along with the preheated air, i.e., 180  Ce200  C. However, SBR and ER influence the duration to attain the steady-state conditions of the reactors. The effect of SBR and ER on duration to attain the steady-state conditions is discussed in detail in the subsequent sections.

4.3. Analysis of the temperature profile of reactor-II along with time The time required to attain steady-state conditions is discussed in this section. Initially, the gasifier is operated on air gasification

Fig. 2. Radial temperature profile across the nozzles in reactor-I.

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N.K. Ram et al. / Energy 187 (2019) 115937

Fig. 3. Radial temperature profile across the nozzles in reactor-II.

mode. Once the maximum temperature is achieved, i.e., 900  C in the reactor-II, steam (at 150  C) is injected through the nozzles provided in the reactor-I. A temperature profile in the reactor-II for different values of EN is shown in Fig. 4. It can be observed that the

drop-in temperature of reactor-II is a function of SBR. At low EN 0.80 (i.e., at high SBR) the temperature of reactor-II dropped from 900  C to less than 700  C. It is because of the high mass flow rate of steam, leading to a more significant fall in reactor

Fig. 4. The temperature profile of reactor-II with time at different EN.

N.K. Ram et al. / Energy 187 (2019) 115937

temperature, and longer duration is required to restore the temperatures. On the contrary at EN 2.87 and 4.61 (i.e., at low SBR) the temperature drop in the reactor-II is only 50  C. It may be noted that high EN corresponds to low SBR; hence, the temperature drop is relatively lower, thereby time required to restore to steady-state conditions is also shorter, when compared to high SBR conditions. From the results obtained, it was observed that EN have a greater influence on the duration to achieve steady-state conditions. Low EN values are not suitable for attaining high temperature inside the reactor. The EN of 1.54 and above, which presents an increasing trend of temperature along with the time (in both of the reactors). The time-based temperature profile of reactor-I, reactor-II, hot air and the gas outlet temperature for the optimum conditions is shown in Fig. 5. It is important to note that Water-gas reaction is highly endothermic and occurs at high temperature, i.e. above 900  C whereas water gas shift reaction is an exothermic reaction and occurs at lower temperature, i.e. between 700 and 800  C. The key factors influencing the reactor temperature are ER, SBR, the temperature of the hot air supplied for gasification, and the temperature of the steam.  When the steam flow rate is increased (high SBR, which corresponds to low EN, i.e. lower than 1.5) water gas reaction and water gas shift reaction are enabled. The drop in reactor

9

temperature is on account of two factors. 1) Since the temperature of the saturated steam (150  C) and the gasifier is maintained at temperature above 900  C, addition of more steam at high SBR would result in a significant drop in reactor temperature. 2) the endothermic nature of the water gas reaction would further reduce the reactor temperature below 900  C and when the reactor stabilises below 900  C, it provides favourable conditions for water gas shift reactions thereby increasing the CO2 and H2 content in the producer gas.  When the steam flow rate is maintained in the medium range (i.e., EN 1.5 to 2.2), the drop in reactor temperature on account of steam injection would be lower, and the reactor stabilises above 900  C and the temperature increases above 1000  C with duration as presented in Fig. 4. Boudard reaction and water-gas reaction are dominant; this becomes favourable conditions for water gas reactions, thereby increasing the CO and H2 content in the producer gas.  When the steam flow rate is very low (i.e., EN greater than 2.2) the drop in reactor temperature on account of steam injection is negligible since the negligible quantity of steam is injected and the reactor stabilises above 900  C in a shorter duration. However, low steam flow rates resulting in non-availability of H2O, which decreases the probability of occurrence of water gas shift and water-gas reactions, thereby decreasing hydrogen production.

4.4. Influence of equivalence number on gas composition, AFT, energy density

Fig. 5. Reactor temperature profile for optimum condition.

4.4.1. Influence of EN on reactor temperature Maintaining a high reactor temperature is critical for higher hydrogen production as it enhances reduction reactions. ER and SBR have a divergent effect on reactor temperature. There is a need for a parameter, which captured the combined effect of ER and SBR on both reactor-I and reactor-II temperature. Equivalence number was defined and studied to optimize the performance parameters of the gasifier. Variation of reactor-II temperature with reference to EN is presented in Fig. 6. The trend of reactor-II temperature with EN is in line with the previous discussions in section (4.3 and 4.4.) showing an increasing trend. There is a corresponding increase in temperature of the reactoreII with a decrease in SBR as presented

Fig. 6. Variation in reactor-II temperature with EN (with emphasis on variation in SBR).

10

N.K. Ram et al. / Energy 187 (2019) 115937

in Fig. 6.

4.4.2. Influence of EN on hydrogen content, gas yield, and HHV Low EN corresponds to high SBR, which leads to more significant temperature drop as presented in Fig. 6. It can be observed that maximum temperature attained inside the reactor-II is when EN is in the range of 1.5e2.2, which can be classified as an optimum

range of EN. Variation in H2 and HHV with variation in EN is shown in Fig. 7. As observed in Fig. 7, H2 content shows an increasing trend with a maximum of 27.24% (on a volume basis) at EN 1.54. CO and CO2 variation with EN are presented in Fig. 8. CO also shows an increase in this band, and CO2 exhibits the opposite trend. Increase in temperature provides thermodynamically favourable conditions for reductions reactions, which facilitate hydrogen production.

Fig. 7. Variation in hydrogen content in the producer (vol %), higher heating value and gas yield variation in EN.

Fig. 8. Variation in carbon monoxide, carbon dioxide content in the producer (vol %), and Higher heating value with variation in EN.

N.K. Ram et al. / Energy 187 (2019) 115937

Water-gas reaction and boudouard reaction are enhanced in this temperature zone. Occurrence of Water-gas reaction and boudouard reaction is explained the increase in H2 and CO content in this EN band. CO2 decreases due to boudouard reaction. It can be observed that the temperature of reactor-II is maintained at high temperature (above 1000  C) with further increase in EN. High EN corresponds to low SBR; due to the low volume of steam supplied to the reactor, high temperature of the reactoreII is achieved after 2e3 h of operation. As observed in Figs. 7 and 8, there is an increase in H2 and a decrease in CO2 with an increase in EN, i.e., from EN 0 to 2.2. At high EN, i.e., greater than 2.2H2 and CO2 content decreases while CO increases. Water-gas reaction and boudouard reaction are favoured at a higher temperature and likely to occur; however, due to less steam being supplied, water gas reaction will be retarded. Water-gas reaction and boudouard reaction explains the decrease in H2. The increase in CO can be explained by boudouard reaction, which produces CO at the expense of CO2. As observed in Fig. 7, there is an increase in H2 and a decrease in CO2 with an increase in EN, i.e., from EN 0 to 2.2. It explains that the increase in a combustible fraction in this range, as presented in Fig. 9. At high EN, i.e., greater than 2.2 H2 and CO2 content decreases and, CO increases. It can be observed from Fig. 9 that the combustible fraction of the producer gas is increased from 34 to 43% when the EN is increased from 0.85 to 2.2. An increment of 26% in the combustible fraction of the producer gas was achieved when compared to the baseline values. 4.4.3. Influence of EN on combustible and nonecombustible gas components Due to the increase of combustible gases fraction in the producer gas, there is a net increase in the HHV of the producer gas, as indicated in Fig. 7, when EN is 1.5e2.2. The HHV of the producer gas decreases when the EN value is beyond 2.2. The increment in the HHV of the producer gas increases the volumetric efficiency, which in turn increased the energy density (Ed) of the air-fuel mixture when used in IC engines. The analysis of Ed with variation in EN reveals that Ed exhibits the same trend as an HHV with EN. There is a net increase in Ed due

11

to air-steam gasification in the optimum EN range of 1.5e2.2. It can be explained based on improvement in HHV of the producer gas in the optimized EN range due to increase in H2 and reduction in the non-combustible fraction of the producer gas. 4.4.4. Influence of EN on energy density and AFT In the case of internal combustion (IC) engines, the amount of energy released during the power stroke is directly proportional to the amount of energy input during the intake stroke. The volume of fuel feed for each intake stroke is constant, i.e. governed by the bore and stroke of the piston and cylinder/suction volume available in the cylinder. Hence, it becomes important to have an increased energy density to match the required energy within the available cylinder volume. Also, the non-combustible components present in the producer gas carry away the energy released by the combustible components and reduce the adiabatic flame temperature [47]. Hence, the energy density of the fuel mixture is considered as one of the critical parameters. The energy released for a given volume of fuel is directly proportional to its energy density [48]. When the gaseous fuel, i.e. NG, is used in IC Engine, the air-fuel mixture have the energy density of 3.0 MJ Nm
3 [19]. When the producer gas is used in the IC engine, the energy density of the air-gas mixture is 2.59 MJ Nm
3 [19]. The primary purpose of considering the energy density of producer gas is to denote the importance to increase the combustible gaseous components and reduction of non-combustible components (like N2). Variation of AFT with EN is shown in Fig. 10. It may be noted that AFT reaches the maximum value of 2083 K when the EN is 1.54, where maximum H2 production was observed. In the optimized range of EN, there is an increase in AFT of the air-fuel mixture due to the reduction of non-combustible fraction in the producer gas. However, it also may be noted that there is a further increase in EN increases the non-combustible fraction, which leads to a reduction in AFT of the air-fuel mixture. In order to summarise the discussions, for maximum hydrogen production, the optimized EN was found to be 1.54. However, optimization of hydrogen production does not lead to optimization of HHV due to the corresponding increase in CO2 and decrease in

Fig. 9. Variation of the combustible and non-combustible fraction with variation in EN.

12

N.K. Ram et al. / Energy 187 (2019) 115937

Fig. 10. Variation of Ed and AFT with variation in EN.

CO, which influence the higher heating value of the producer gas. So, optimization of EN for air-steam gasification can be classified based on end-use applications. For hydrogen production, EN 1.54 is optimum, and for power generation application, EN in the range of 1.5e2.2 was found to be more suitable. This is because of combustible fraction, HHV, and energy density was found to be optimum when EN is in the range of 1.5e2.2. 4.4.5. Influence of EN on cold gas efficiency Cold gas efficiency shows an increasing trend with EN with a maximum of 87% when the EN is in the range of 1.5e2.2 as presented in Fig. 11. An increasing trend of cold gas efficiency is due to the increase in HHV with an increase in H2 content and combustible

fraction in the producer gas. With further increase in EN, CGE exhibits a downward trend. CGE reaches a maximum of 87% in the EN range of 1.5e2.2. Several studies have reported different cold gas efficiency achieved in air gasification in a downdraft gasifier. Cold gas efficiency of 69 ± 6% has been reported [6]. Studies on similar two-stage downdraft gasifier have reported the efficiency of 65e67% [49]. The values reported are generally in the range of 60e70% [1,18,19,50] the cold gas efficiency of 88% has been reported by Ref. [51]. The results of the present study are compared with published results of cold gas efficiency of air-steam gasification. In studies conducted on different gasifying agents [38] reported cold gas

Fig. 11. Cold gas efficiency vs. EN.

N.K. Ram et al. / Energy 187 (2019) 115937

efficiency as 70e75% for three different gasifying agents: air, AirSteam, and Oxygen-steam. A much lower value of 60% was reported by Ref. [52] using Oxygen-steam in an updraft gasifier. Comparison with the available results of cold gas efficiency of the present study is higher by11e26% than the reported values.

4.5. Energy balance and mass balance analysis 4.5.1. Energy balance A detailed energy balance analysis was carried out. Energy balance analysis was carried out to understand the energy flow through different components of input and output. The energy input for steam production is by using a biomass gasifier turbo stove. The biomass consumption rate for production of a kg of steam (at 150  C) is observed as a kg per kg steam. The input energy to the gasifier through steam (latent and sensible heat) is 12 MJ, which is 2.7% of the energy input by fuelwood. Hence the energy supply for production of steam is a small fraction of the total energy input by the biomass. It may be noted that the producer gas runs the IC engine (20 kWe) provides the flue gas at 450  C. The energy available from the flue gas of the engine is about 25 MJ. Hence the energy required for steam generation can be very well managed within the energy available from the flue gas without dependence on the external energy supply. In the case of external energy input for steam, generation will reduce the cold gas efficiency by 2.74%. In the present study, the CGE of the system is 83.5% in case energy consumed for steam production, i.e., energy input from the external source. The CGE is 86.3% when engine exhaust heat is used for steam generation. The waste heat recycling of the energy within the system increases the CGE. A Sankey diagram of energy flow in the system is shown in Fig. 12. The energy input from fuelwood represents 97.26% (461 MJ) of the total input energy input. The energy input from steam is 2.74% (13 MJ). The components of the output energy consist of the higher heating value of the producer gas, the heat loss in the condensate and ash with dust particulates. It may be noted that from Fig. 12 that the producer gas consists of 86.29% (409 MJ) of the input energy. Heat loss through the ash with dust particle is 1.27% (6 MJ) of the input energy. A portion of the sensible heat in the hot producer gas is recycled in the gasifier through a heat exchanger. The fraction of the recycled energy components is 4.43% (21 MJ). The unaccounted portion of the energy balance is 9.07% (43 MJ).

13

4.5.2. Mass balance analysis Mass balance is a valuable tool to understand the material flow and the effective conversion of biomass into producer gas. The mass flow analysis also indicates the conversion efficiency, data validation of experimental results, and scope for improvement. The different components in the mass flow analysis were estimated using equations (13)e(22). A Sankey diagram representing the mass flow of different input and output materials are shown in Fig. 13. The input materials comprise of biomass, air, and steam. From Fig. 13, It may be noted that 50 kg h
1 of air is supplied, which comprises 62.5% of the total input material. Biomass fuel input rate is 25 kg h
1, which comprises of 31.25% of the total input. The steam flow rate to the system is 5 kg h
1, which comprising of 6.25% of the total input. The optimal SBR obtained in the current study is closer to the optimal SBR of 0.26 reported by Ref. [39] and SBR 0.17e0.34 reported by Ref. [37]. These studies were conducted on air-steam gasification in a downdraft reactor. The output materials comprise of producer gas, char, dust, and condensate. The flow rate input material is estimated at 80 kg h
1. The producer, gas flow rate, is 72.44 kg h
1. Hence, the mass conversion efficiency of the biomass gasifier system is 90.5% (Biomass to producer gas). The gas yield is 2.80 Nm3 kg
1. The char and dust collected comprise of 1% (0.8 kg h
1) which is much lower than 3.5% reported by Ref. [53] and 3e5% reported by Ref. [54] it may be noted that the char removal rate influences the gasification efficiency. With the efficient ash removal system, 90.5% of biomass to gas conversion efficiency could be achieved. The unaccounted portion of the mass balance analysis is 8.5%. The unaccounted components include the non-collectible fraction of tar, dust particles and condensates. The unaccounted portion is a 3.5% higher than ±5% reported by Ref. [42] but is 2.5% lower than the average

Fig. 13. Results of mass balance analysis.

Fig. 12. Sankey diagram for energy balance analysis.

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N.K. Ram et al. / Energy 187 (2019) 115937

mass balance reported by Ref. [55]. 5. Conclusions The experimental study on dual fired downdraft gasifier using air-steam as an oxidizing agent was conducted. The key findings of the study are; the reactor temperature drops on steam injection; the ratio of ER and SBR has a significant impact in attaining the steady-state condition. The equivalence number (EN), which is a ratio of ER to SBR, captures the combined effect. The impact of EN on hydrogen enrichment was studied and optimized. Maximum H2 of 27.24% (volume basis) in the producer gas was observed when EN was 1.54. A 51% increment in hydrogen content was achieved when compared with the baseline value of H2 content of 18% in case of air gasification. However, it should be noted that due to fall in CO content and increase in CO2, maximum hydrogen production does not translate to maximum HHV, Ed of producer gas. Hence, EN is optimized for air-steam gasification based on end-use applications. For hydrogen production, EN 1.54 and EN in the range of 1.5e2.2 suitable for power generation applications. The optimum condition for the enrichment of the combustible fraction in the producer gas was observed when the EN was in the range of 1.5e2.2. The combustible fraction increased from 35 to 43%, thereby decreasing the non-combustible fraction from 65 to 57%. The maximum HHV of the producer gas in air-steam gasification was 6.33 MJ Nm
3, which is 44% higher than the baseline value, i.e., in comparison with HHV of producer gas using air as an oxidizing agent reported in the literature. The optimum EN for HHV and Ed was found to be in the range of 1.5e2.2. The maximum gas yield was 2.85 Nm3 kg
1, which has the maximum adiabatic flame temperature of 2090 K at EN in the range of 1.5e2.2. AFT is 16% higher than the reported value of AFT (1800 K) of producer gas using air gasification. Energy balance analysis presents a CGE of 86.29% of the total input energy to output energy. Declaration of interests None. Acknowledgments The authors are grateful to the Petroleum Conservation Research Association for extending the funding support. The authors would like to express their sincere thanks and gratitude to Dr. Ashvini Kumar Sr. Director Renewable Energy Technologies division, Dr Ajay Mathur Director General TERI, Mr Amit Kumar, Mentor, Department of Energy and Environment, TERI University and Dr Leena Srivastava Vice Chancellor, TERI University, for their continuous encouragement and motivation. Finally, the authors would like to thank the technical team for all the support extended in conducting the experiments.

CPA

specific heat of Air ðkJkg
1 K
1 Þ

Cvw

heating value of the fuelwood ðMJkg
1 Þ

CPG Cpi

specific heat of producer gas ðkJkg
1 K
1 Þ specific heat of each of the products of combustion

Cvf Cw

ðCalmol
1 K
1 Þ the energy content of the fuel ðMJÞ total condensate from the producer gas collected at the end of the experiment ðkgÞ

g GL Gw

energy density in ðMJNm
3 Þ chemical energy of the producer gas ðMJÞ heat carried away by char and ash through the grate ðMJÞ acceleration due to gravity ðm s
2 Þ sensible heat of hot producer gas ðMJÞ quantity of gas produced ðkgÞ

GY H Hproducts Hreactants DH HHVg

gas yield ðNm3 kg
1 Þ humidity ðkg m
3 Þ enthalpy of products ðMJÞ enthalpy of reactants ðMJÞ difference of level in manometer (m) heating value of the gas used in airefuel mixture ðMJÞ

HHV IE LHVi

higher heating value ðMJNm
3 Þ total Input Energy ðMJÞ lower the heating value of each component of the gas

Ed EG| EC

ðMJNm
3 Þ ma

amount of air supplied for gasification of air ðkghr
1 Þ

mf

fuel consumption rate ðkghr
1 Þ

msf msa

fuel consumption (stoichiometric) ðkghr
1 Þ stoichiometric air required (for complete combustion) ðkghr
1 Þ

ms Ms OE

the mass flow rate of steam ðkghr
1 Þ mass of steam supplied to the reactor ðkgÞ total output Energy ðMJÞ

Q Rw SW tf ts UE UEL

gas flow rate ðkg h
1 Þ ash and char falling through the grate collected at the end of the experiment total weight of steam supplied for gasification ðkgÞ duration of one batch of gasifier operation ðhÞ duration of boiler operation ðhrÞ unaccounted energy quantity of unaccounted for elements

Vf

volume of the fuel in the air-fuel mixture ðNm3 Þ

Va va Vg

volume of air in the air-fuel mixture ðNm3 Þ velocity of air across the pipeline ðm s
1 Þ Volume of the gas ðN m
3 Þ

Va Wf WMW

stoichiometric air required for V(g) ðN m
3 Þ weight of fuel supplied in one batch ðkgÞ weight of the moisture in wood supplied for gasification ðkgÞ weight of the moisture in air supplied for gasification ðkgÞ total fuelwood supplied for gasification ðkgÞ volumetric percentage of each component of the gas

Nomenclatures WMA Ac AG AW A0 a b C Ccond

cross-sectional area of pipeline ðm2 Þ sensible heat gained by air fed for gasification from the heat exchanger ðMJÞ total weight of air supplied for gasification ðkgÞ area of the throat ðm2 Þ initial reading in the water flow meter to the boiler (l) final reading in the water flow meter at the end of steam injection (l) discharge coefficient of the venturi meter condensate collected by gas cooling equipment ðkgÞ

Ww Xi

Greek letters density of air ðkg m
3 Þ density of manometer fluid (water) ðkg m
3 Þ

ra rw rpg b

density of producer gas ðkgNm
3 Þ ratio of throat to inlet area

N.K. Ram et al. / Energy 187 (2019) 115937

Abbreviations AFT adiabatic flame temperature CGE cold gas efficiency CI engine compression ignition engine CNG compressed natural gas Ed energy density EN equivalence number ER equivalence ratio IE heat value HP horsepower HTAG High-temperature air gasification HWA hotwire anemometer IC engine internal combustion engine PG producer gas SBR steam to biomass ratio SI engine spark ignition engine WG water gas reaction WGS water gas shift reaction

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