Design, fabrication, and performance evaluation of a novel biomass-gasification-based hot water generation system

Energy 185 (2019) 148e157

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Design, fabrication, and performance evaluation of a novel biomass-gasification-based hot water generation system Sunil a, Rahul Sinha a, Bathina Chaitanya a, Birendra Kumar Rajan a, Anurag Agarwal b, Ajay D. Thakur c, **, Rishi Raj a, * a b c

Thermal and Fluid Transport Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Patna, Bihar, 801103, India New Leaf Dynamic Technologies Pvt. Ltd., New Delhi, 110019, India Department of Physics, Indian Institute of Technology Patna, Bihar, 801103, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 January 2019 Received in revised form 21 June 2019 Accepted 30 June 2019 Available online 2 July 2019

Design and development of a novel 22kWth hot water generation system comprising a biomassgasification unit in the core and an integrated fire-tube water heat exchanger in the annulus is reported. Producer gas from the gasifier is combusted in a burner and the resulting hot flue gases are routed back through the helical tube to heat the water contained within the annular shell. This allows recovery of the thermal energy typically lost from the outer surface of a conventional gasification system. The simple design improvisation leads to a doubling of the overall efficiency (z48 %) compared to a standard system with physically separated gasifier and heat exchanger units (z24 %). The developed system holds promise in domestic and industrial water heating applications. Overall, this integrated design presents an efficient and environment-friendly waste-to-value concept and proposes a meaningful use of the underutilized 36 EJ/annum potential of biomass from crop residue globally. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Biomass Gasification Hot water generator Heat recovery Helical coil heat exchanger

1. Introduction Meeting energy requirements associated with domestic, agricultural, industrial, and infrastructural requirements in a sustainable manner is key to the development of the any nation. Annual global primary energy demand has increased from z300 to 550 EJ in last four decades, primarily due to rapid industrialization in developing countries such as China, India, South Africa, Brazil, and Nigeria, among others [1]. The industrial sector alone is responsible for 28 % of this global demand [1]. Process industries which majorly require low grade thermal energy (Table 1) utilize a significant share of the total energy consumption within the industrial sector. These industries majorly rely on fossil fuels (Table 2) such as coal, diesel, natural gas, and electrical heating (mostly from fossil fuels). Such industries contribute to 79 % of the total CO2 emissions and cause severe anthropogenic effect [2]. In this regard, renewable and clean energy sources such as solar and wind have received attention by several small and

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A.D. Thakur), [email protected] (R. Raj). https://doi.org/10.1016/j.energy.2019.06.186 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

medium scale industries. However, relatively high investment costs, large installation area requirements, long payback times, and associated intermittency limit their practical implementation. Conversely, agricultural biomass is a cheap and readily accessible source of energy which may fulfil the requirements of lowgrade thermal energy in various industries. Despite multiple competitive uses such as cattle feeding, mulching, and decomposing [11,12], a sizeable amount of energy from agricultural residue (36 EJ/year) is being wasted through open field burning of crop residue (FBCR) in countries such as India, China, Philippines, and Thailand, among others [13e17]. The inefficient supply chain management, inept conversion technologies, and low selling profits are the major practical limitations which force farmers to choose farm burning as an easy and a quick solution to get rid of farm residue. Eventually, FBCR induced emissions pose severe risks such as air pollution, poor visibility from resulting smog leads to road accidents, chronic respiratory diseases, and premature mortality [12]. Furthermore, heat from residue burning increases the soil temperature, results in death of farmer friendly microorganisms such as bacteria, fungi and earthworms, and damages the biodiversity [12,14]. In light of the above discussion, the dual problem of low-grade industrial energy demand and FBCR may be addressed in a single

Sunil et al. / Energy 185 (2019) 148e157 Table 1 Hot water requirements in various industries. Application

Temperature Range (
C)

Process

Beverages [3]

Washing, Sterilization Pasteurization Paper [3] Bleaching Boiler feed water Cold Storage [4] Adsorption/Desorption Dairy and food Industry [5] Pasteurization Sterilization Leather [6] Waste drying Pharmaceutical [7] Heating Power Plant [8] Biomass drying Tea Industry [9] Washing Textile [3,10] Bleaching, Dyeing Cooking, Drying

60e80 60e70 130e150 60e90 70e85 74 120 110 20e120 90 80 80 60e80

shot by channelizing the waste agricultural biomass as energy source in target process industries. In this regard, biomass gasification, which is the conversion of solid biomass fuel into a combustible producer gas through a sequence of thermo-chemical reactions under oxygen deficient environment, may be a suitable alternative [28e30]. However, usage of biomass gasification for thermal application is relatively less explored [29e35]. Low energy density and random particle size are the main challenges pertaining to the usage of agricultural biomass as feedstock in gasification process. Fluffy or fine grain feedstock may lead to flow problems and high pressure drop across the gasifier [36]. Conversely, large size biomass feed may create start-up problems and poor-quality gas. It is reported that pelletization, briquetting, and chopping process can be used to address such issues [37e39]. In this work, systematic experiments performed in order to develop a novel gasifier-based hot water generation (HWG) system with high thermal efficiency are reported. While a typical biomass gasification-based HWG (BGHWG) system with physically separated gasifier and heat exchanger units reduces emission significantly in comparison to the traditional direct combustion based HWG system [40,41], the high surface heat loss from the BGHWG implies comparable/relatively lower efficiency in comparison to the traditional direct combustion-based systems (DCHWG). This issue is addressed by developing a novel design of an integrated gasifier and hot water generation (IGHWG) system wherein the heat exchanger in the annulus surrounds the gasifier in the core. Experiments are performed to show that such a simple strategy to recover surface heat losses improves the overall thermal efficiency

149

of the IGHWG (48 %) significantly in comparison to BGHWG (24 %). The integrated design proposed in this work wherein the gasification process addresses emission concerns and the core-annular geometry allows efficiency improvements is believed to have the potential to make this waste-to-value concept viable. Such efforts may not only reduce burden on our conventional energy resources, will also go a long way to address the environmental concerns, particularly in primarily agrarian-based economies such as India, China, and, Brazil, among others [42]. 2. Experimental setup 2.1. Testing platform A commercially available off-the-shelf adsorption-based refrigeration system (Make: New Leaf Dynamic Technologies, Model: GreenCHILL™; Specifications are provided in Table 3) is employed as the testing platform (Fig. 1) for the various versions of hot water generators discussed later in this study (BGHWG and IGHWG). GreenCHILL™ is a double-bed adsorption refrigeration system with ammonia-solid adsorption matrix as the working pair. During the charging process (hot bed, process a-b), water flowing at mass flow rate of 0:67 kg =s and inlet temperature Thw;in of 120
C (3 bar Table 3 Adsorption-refrigeration system specifications and requirements. Specifications Type Adsorption pair No. of adsorption beds Refrigeration capacity COP Electronic control Half cycle time Expansion valve Requirements Thermal energy Inlet temperature Outlet temperature Mass flow rate of water (m_ w ) Water pressure Electrical power requirement

Vapour adsorption refrigeration system NH3/Solid adsorption matrix 2 z3 TR=11 kW z0:50 Solenoid valves, Motors (with PLC programming) 20 min Automatic type z22 kWth Thw;in ¼ 120±5
C (Hot bed), Tcw;in ¼ 25±5
C (Cold bed) Thw;out ¼ 112±5
C (Hot bed), Tcw;out ¼ 32±5
C (Cold bed) 0:67 kg =s (each bed) 3 bar (absolute) Within 1 kW (for all accessories)

Table 2 Current hot water generation solutions in various industries. Application Industry

Process

Heat Source

Challenges

Chemical [18]

Chemical processes, District heating system Chemical processes

Oil, biomass, industrial waste, and electricity Natural gas, hard coal, and heating oil (Back up) Solar thermal

Emissions from direct combustion of non-renewables and industrial waste.

Chemical, Pharmaceutical [19] Food, Textile, AirConditioning [20] Food [21] Food [22]

Drying, Pasteurizing, Washing

Baking, Washing, Refrigeration Fuel oil, electricity

Iron, Steel [24] Paper and Pulp [25]

Blanching, Drying, Sterilizing, Pasteurizing General Model (100e160
C) 100 kW demand Manufacturing Chemical processing

Textile [26]

Heating and cooling

General Model [23]

Textile [27]

Non-woven fabric @ 250


C

Electricity Solar thermal Coke, heavy oil, and electricity Oil, biomass, industrial waste (Black Liquor) Heavy fuel oil Solar thermal

Direct combustion of non-renewables. Technically feasible, but not financially viable due to payback period of 17 years. Electricity and fuel oil-based boiler associated withCO2 emissions of 0.36 kgCO2 /kWh of energy consumption. Electricity based equipment and non-uniform heating. Not feasible for short period. Future fuel price hike may justify the existing model. High PAH (Polycyclic Aromatic Hydrocarbon) emissions. Direct combustion of black liquor leads to high CO2 emission. Heavy fuel oil replaced by natural gas (A non-renewable energy source). Payback period is 5e8 years. Saved 40e50% natural gas, however, have long payback period of 6 years.

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Fig. 1. Schematic of the testing platform (adsorption-refrigeration system) used for evaluating the performance of various versions of hot water generator designs discussed in this study.

absolute pressure) provides the heat to desorb ammonia and rejuvenate the adsorbent bed. The water exits the bed at Thw;out of 112
C and is routed back to the HWG. Desorbed ammonia vapour is next routed through a condenser (process b-c) wherein it is condensed by a cold-water loop via the cooling tower. The liquid ammonia is then throttled through an expansion valve to the evaporator section placed inside the cold room (process c-d-e). The superheated ammonia from the evaporator is then routed back to the second bed (referred to as the cold bed, process e-f) wherein it is adsorbed on to the adsorbent matrix. The heat released from the exothermic adsorption process is carried away by another coldwater loop circulating from the cooling tower. Complete charging of one bed and simultaneous discharging of the other bed takes approximately 20 min, after which the valves V1 and V4 are closed and valves V2 and V3 are opened to switch the processes in the beds and start a new cycle. 2.2. Gasifier design The design and fabrication of the air-blown downdraft gasifier is explained in this section. The gasifier primarily comprises of three main parts: (a) biomass feeding mechanism including a hopper and the biomass shaking mechanism, (b) the reactor/gasifier core, and (c) the ash removal mechanism with a grate (Fig. 2). An overall thermal efficiency hoverall of 50% is assumed as the design criterion for 22 kWth hot water generation system. Accordingly, biomass consumption rate m_ bm of z9kg=hr is calculated using the heating value of biomass as HVbm ¼ 18 MJ=kg [43,44].

m_ bm ¼

HWGcapacity  3600 hoverall  HVbm

(1)

Next, the inner diameter of the gasifier core/reactor is closely related with the hearth load capacity (Bs ) of the gasifier. Here, Bs (biomass based) is defined as the amount of biomass (in kg =hr) consumed per unit cross-sectional (measured in cm2 ) area of the reactor. In this regard, various studies in literature suggest the typical value of Bs in the range of 0.01e0.024 kg =cm2  hr[45e47]. Accordingly, reactor diameter is estimated to be 213e 330 mm as

shown below.

dreactor

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi HWGcapacity  3600  4 ¼ hoverall  HVbm  Bs  p

(2)

Furthermore, Venselaar et al. [46] suggested that 300 mm is the optimal diameter for a typical throat-less downdraft gasifier where biomass consumption rate is nearly 10 kg =hr. Consequently, for steady operation of the gasifier, 300 mm inner diameter of the gasifier core is selected as the design value in addition to a height of 1100 mm to accommodate both, the reactor and the ash removal system. A batch type hopper (Fig. 2a) is designed by considering random packing fraction RPF ¼ 0:5 for frustum shaped hopper [48], half cone angle q ¼ 20
[49] for smooth sliding of biomass through the conical section, height h z1 m, reactor diameter d ¼ 0:3 m (Fig. 2b), outer diameter Dz1 m and average biomass density rbm ¼ 550 kg=m3 . Accordingly, the hopper can accommodate z70  80 kg of biomass, which is sufficient to run the system for z 7  8 hours. Further, a biomass shaking mechanism (Fig. 2c) is provided between the hopper and the reactor to ensure a steady supply of feedstock into the reactor core. A rotating grate (Fig. 2d) is designed to ensure continuous disposal of ash from the bed. Since the average temperature inside the gasifier reactor is typically of the order of 700 ± 100
C [35,50e53], a refractory insulation is required to prevent the direct contact of combustion chamber with the metal surface. Accordingly, a refractory which can withstand up to 1450
C with a thermal conductivity of 0:5 W =m  K, and density of 2650 kg =m3 is selected. Furthermore, the outer surface temperature of the metal casing (i.e. exposed to ambient) may be high enough for accidents and heat losses despite the refractory insulation. Hence, rockwool insulation (thermal conductivity of 0:05 W =m  K, and density of 48 kg =m3 ) which can withstand temperatures up to 700
C, is considered for further calculations. Heat transfer analysis of the gasifier reactor surface with various thickness of refractory (used to insulate reactor) and rockwool (used to minimize surface heat losses) is performed using the

Sunil et al. / Energy 185 (2019) 148e157

151

Fig. 2. Design of the downdraft gasifier comprising: (a) a hopper, (b) the gasifier core, (c) the biomass shaking mechanism, and (d) the grate.

thermal resistance network shown in Fig. 3. Here, nodes A represents the gasifier core/reactor surface, B and C are the carbon steel inner and outer surfaces, respectively, and D is the outer surface of the rockwool insulation layer. Accordingly, the surface heat loss (Q_ Þ, the temperature of carbon steel ðT Þ, and the weight of reC

loss

fractory ðMref Þ are estimated using Eqs. (3)e(5) shown below,

TA  TD ln

rB rA

2pkAB

ln

þ 2pk L

rC rB

BC

ln

þ 2pk L

rD rC

CD L

¼

TD  T∞ 1 hair AD þsεAD ðT 2D þT 2∞ ÞðTD þT∞ Þ

¼ Q_ loss

(3)

2

3 rB rC ln ln 6 rA rB 7 _ þ þ TA TC ¼ 4 5Q 2pkAB L 2pkBC L loss

(4)

  Mref ¼ p r 2B  r 2A L  rref

(5)

where rA ¼ 150 mm is the diameter of the gasifier core and rB ; rC ; rD are the outer radius of refractory, the outer radius of carbon steel, and the outer radius of rockwool insulation, respectively. In our calculations, the average temperature of gasifier inner surface TA (Fig. 3) is assumed as 700
C. Moreover, the thermal conductivities of the refractory, carbon steel, and rockwool insulation are assumed as kAB ¼ 0:5W=m  K, kBC ¼ 35W=m  K and kCD ¼ 0:05W=m  K respectively. L ¼ 1:1 m is the height of the gasifier reactor, hair is the heat transfer coefficient of air and it is estimated as 7:23 ±0:47 W=m2  K using the Churchill and Chu correlation [54]. AD is the outer curved surface area (m2 ) of the rockwool insulation layer, s ¼ 5:67  108 W=m2  K4 is the Stefan Boltzmann constant, and, ε ¼ 0:1 [55,56] is the emissivity of the thin aluminium

Fig. 3. Heat transfer path with thermal resistance network in BGHWG with insulation.

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sheet metal used for covering the rockwool insulation. Furthermore, it should be understood that the aluminium sheet metal will not be required if carbon steel is directly exposed to the ambient, i.e., no rockwool insulation is used. Accordingly carbon steel emissivity ε ¼ 0:3 [54] is considered and respective surface heat loss, outer surface and carbon steel temperatures are shown in Fig. 4. In our design process, the three major operational constraints are: (i) surface heat losses should be minimal ðQ_  4400 W; i:e: z10 % of energy inputÞ [32e35], (ii) the loss

temperature of the carbon steel should not exceed those based on material considerations (TC  400
C) [57], and, (iii) the overall weight of the refractory layer should be reasonable (Mref  300 kg). A careful look at Fig. 4 suggests that the feasible region identified as per the constraints (Fig. 4aec) would imply refractory thickness and insulation thickness in the range of z40  85 mm; and z0  8 mm, respectively. Hence, a refractory thickness of 80 mm ðMref ¼ 278 kgÞ without any rock-wool insulation (point X in Fig. 4) was selected as the design value. Considering hair ¼ 7:23± 0:47 W= m2  k and T∞ ¼ 30±10
C, Q_ and T were accordingly estiloss

C

mated to be 3:824±0:7 kW, and 227±33
C respectively. Accordingly, all parts of the gasifier were fabricated from carbon steel and shown in Fig. 5a. A 40 W blower (Delta, BFB1224GH) was used to supply air for the gasification process and the resulting producer

gas was combusted in a naturally aspirated burner (Fig. 5b). The hot flue gas routed to a typical/traditional fire tube heat exchanger (Fig. 5b) for hot water generation. 2.3. Water heater design Water heater consists of two concentric cylindrical shells where the inner shell (an outer diameter of 616 mm and a height of 1350 mm) and the outer shell (an outer diameter of 736 mm and height of 850 mm) are assembled as shown in Fig. 5b. A radial gap of 52 mm is provided between the shells. Furthermore, inner shell consists of a producer gas combustion chamber (height of 500 mm) with a burner located underneath. A fire tube heat exchanger with flue gas in the tube and water in the shell is welded above the combustion chamber. It consists of 33 straight tubes (outer diameter of 60:3 mm, thickness of 4:5 mm, and the height 800 mm) contained within the inner shell. A chimney with an outer diameter of 176 mm and height of 1600 mm is attached above the heat exchanger with the help of a suitable cone and flanges. Water level indicator and pressure gauges are mounted on the system. In addition, a pressure relief valve (PRV) is also provided to maintain the working pressure of water (3 bar) as per the requirement of adsorption-refrigeration system and ensure safe operations.

Fig. 4. Variation of (a) the surface heat loss, (b) carbon steel temperature, (c) the weight of the refractory, and (d) the outer surface temperature of BGHWG plotted as a function of refractory thickness and rockwool insulation thickness. The color bars below x-axis label values represent a zoomed in view of the zero insulation thickness data (sudden change due to change in emissivity discussed earlier). Selected design values (point X) imply Q_ loss ¼ 3824 ±686 W, TC ¼ 227±33
C, and Mref ¼ 278 kg. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Sunil et al. / Energy 185 (2019) 148e157

153

Fig. 5. Biomass gasification based HWG (BGHWG) with physically separated gasifier and water heater units. (a) Gasifier, (b) water heater, and (c) the working model.

3. Testing and performance evaluation 3.1. Biomass gasification based HWG (BGHWG) The experiment with BGHWG starts with lighting up the gasifier after a measured amount of feedstock (wood chips of average size of 40 mm  50 mm  30 mm) is fed into the reactor via the hopper. Once the hopper (Fig. 2a) is completely filled with biomass, top opening is closed with the lid and a water seal to prevent leakage. Ash collecting bin, which also acts as a seal to prevent the leakage of gas from the bottom of the gasifier, is also filled with water. Substoichiometric air is supplied using a blower (Delta, BFB1224GH) via an air-ring (Fig. 5a). Combustible producer gas is realized at the burner access within 5e10 min and lighted up with a torch. Consequently, the flue gas is passed through the heat exchanger, resulting in the rise of water temperature. Biomass shaking mechanism (Fig. 2c) and grate (Fig. 2d) are operated manually every z15  20 min to avoid any channelling and bridging problems. The outside surface temperature of the reactor and flue gas exit temperature are measured at regular intervals using an infrared thermometer (HTC™, MT-6). When the temperature of the water in the shell reaches 120
C, hot water is circulated to the adsorptionrefrigeration system to initiate the cooling of the room (cold room). This experiment is continued up to 3  5 hours to achieve the desired cooling temperature (7
C in this case) depending upon the ambient/starting temperature in the cold room. The hopper is refilled every z3  5 hours for unimpeded continuous runs. Biomass consumption is noted to estimate the overall thermal efficiency. Here, the overall thermal efficiency hth is defined as the ratio of thermal energy gained by water to the product of fuel consumption rate m_ bm and its calorific value HVbm :

hoverall ¼

  m_ w  Cp  Thw;out  Thw; in m_ bm  HVbm

(6)

where Thw;in ¼ 112
C and Thw;out ¼ 120
C are the inlet and outlet temperatures of the hot water from hot water generator (Fig. 5) respectively, m_ w ¼ 0:67 kg=s is the mass flow rate of water,

and, Cp ¼ 4:24 kJ=kg  K is the specific heat of water. At the end of the experiment, ash and residue are collected from the ash chamber (Fig. 5a). The experiments are repeated multiple times to ensure that the average rate of biomass consumption is estimated accurately and is repeatable over longer runs. Using this strategy, the overall thermal efficiency is estimated as 24 ± 0:6 % based on the average biomass consumption m_ bm of 18 ± 0:5 kg=hr. This value is comparable to those reported by Tippayawong et al., 2010 [58] who experimentally investigated the performance of a downdraft gasifier in cashew processing industry using a physically separated gasifier and heating system similar to BGHWG. According to their study, physically separated gasifier and heating system demonstrated an overall thermal efficiency of z20 % with cashew nut shells as the feedstock. Hence, it is clearly seen that the performance of BGHWG does not meet the design values set earlier ðhoverall ¼ 50 %Þ. In order to identify the reasons for the same, the performance of this system is quickly compared with a traditional direct combustion based hot water generator (DCHWG, Figure A1). In order to ensure a fair comparison, DCHWG also uses the same heat exchanger design (water heater in Fig. 5b) as BGHWG. Bottom half of BGHWG is used as the combustion chamber for open burning in DCHWG. A grate is located underneath which divides the whole chamber into two parts, i.e., the lower part serves as the ash collection chamber while the upper part is used as the combustion chamber. Detailed design, testing procedure, and performance evaluation of such a traditional HWG is discussed in appendix A.1. Interestingly, the overall thermal efficiency of the DCHWG (27±1:3 %) was little higher/comparable to the BGHWG. While the BGHWG addresses the problem of emission, such low thermal efficiency and the additional cost associated with gasification unit may clearly not appeal the end users. Furthermore, the overall efficiency of a system which may use biomass gasification-based power generation technology to electrically power a water heating system is also estimated (appendix A.2). These calculations show that associated intermediate energy conversation steps (thermal to electrical and then back to thermal) result in a very low

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overall efficiency (19 %) of such a system, again making it less practical (additional cost associated with gasifier, engine, and electrical generator) in comparison to the traditional HWG which relies on direct combustion for producing heat. While regulatory pushes towards taxing or limiting the use of direct combustion based traditional HWGs is an alternative in the long term, it is also important to improve the design of BGHWG to enhance the thermal efficiency of the gasification based direct water heating systems to make it viable for practical applications. In this regard, various sources of losses which result in such low efficiency of the BGHWG are identified. A very high flue gas exits temperature Tfg; out of z380±20
C recorded experimentally acz11:2 kW (please refer counts for a substantial heat loss Q_ fg; loss

to appendix A.3 for further details on this calculation) estimated as follows:

Q_

fg; loss

  ¼ m_ fg  Cp; fg  Tfg; out  Thw; out

(7)

where m_ fg (kg =s) is mass flow rate of flue gas, Cp;fg ðkJ=kg  KÞ is specific heat of flue gas and Tfg;out is exit temperature of flue gas, and Thw;out ¼ 120
C is the hot water temperature. Cp; fg is estimated as 1:2 kJ =kg  K by assuming elemental composition of flue gas and m_ fg is calculated to be129 kg =hr using mass balance. While the gasifier outer surface temperature TC of 225±10
C (without rockwool insulation) matches with the design value of 227 ± 33
C, it is nonetheless a source of surface heat loss Q_ z 3:8 kW (Fig. 4). loss

In addition to this combined heat loss of z15 kW from BGHWG, a substantial amount of energy losses which are typically difficult to quantify may be attributed to open burning of producer gas in the naturally aspirated burner (Fig. 5b), sensible heat loss of producer gas (due to separate gasifier and HWG unit in Fig. 5a and b), and

heat losses from water seals and hopper. 3.2. Integrated gasifier and HWG (IGHWG) In light of the above discussion, a novel design of an integrated gasifier HWG (IGHWG) which incorporates the coupled design of a gasifier in the core and a water shell with helical coil heat exchanger in the annulus is presented in Fig. 6. Helical coil heat exchangers allow 16  43 % higher heat transfer coefficient (due to secondary flows from centrifugal force) in comparison to the typical straight tube heat exchangers [59]. However, it is important to optimize such systems since the increased pressure drops in helical coils increases the power consumption of the blower [60]. The IGHWG system (see Fig. 6) includes a blower1 similar to BGHWG to supply air to the gasifier2 and the burner3. The producer gas outlet pipe4 is connected to the horizontal premixed chamber5 which is further connected to the burner6 (inner diameter of 154 mm and length of 500 mm). Premixed burner facilitates thorough mixing of fuel with the air and increases the efficiency of burner in comparison to a naturally aspirated burners used in BGHWG. Burner6 is further connected to a helical coil heat exchanger7 (fabricated with a tube of 77 mm outer diameter), which is placed in the annular region of water shell9 (z180 litre capacity and water shell thickness 132 mm). The flue gas travels through the helical coil7 and exits from the chimney8 connected on the top. After several trials based on thermal requirements of z22 kWth and pressure drop limitations of the blower, 2.5 turns of helix with 600 mm of helix diameter were chosen as the design value. Furthermore, the water shell includes a water jacket10 around the burner which is connected to a recirculation pipe11. The recirculation pipe facilitates thorough mixing of water in the shell and the jacket. In addition, rockwool insulation

Fig. 6. Gasification based HWG with integrated gasifier and water heater units. (a) Front view, (b) top view, and (c) perspective view.

Sunil et al. / Energy 185 (2019) 148e157

(80 mm thickness) is provided around the water shell to impede any surface heat losses. Apart from the novel integrated design, some additional electronics and instrumentation such as automatic producer gas ignition system, and temperature and pressure sensors are incorporated to ensure convenient operation. Water seals at various locations are provided to prevent gas leakage. The schematic and the working prototype of IGHWG is shown in Fig. 7. Testing procedure of this version is same as that of BGHWG and multiple experiments are performed to ensure reliable estimation of average biomass consumption rate. Biomass consumption rate m_ bm of IGHWG is recorded to be 9± 0:5 kg=hr, i.e., an improved thermal efficiency hoverall of 48± 2:6 %. Since the gasifier design is the same as BGHWG, this improvement in overall thermal efficiency can primarily be attributed to the heat recovery strategy due to the integrated design. This saving in addition to the confined/controlled burning in case of IGHWG (open burning in BGHWG) is responsible for the significant reduction in biomass fuel consumption. Apart from improved efficiency, IGHWG (Fig. 7) is also compact and significantly lighter in comparison to BGHWG with physically separated gasifier and hot water generator units (Fig. 5). A comparison of the performance/ specifications of the two versions of HWGs is listed in Table 4. 4. Prospect and challenges The novel design of the hot water generation system proposed in this work promises clean energy from biomass. IGHWG has the potential to minimize the dependence of small and medium scale industries on grid power/conventional fuels. For example, farm/ dairy produce worth a whopping 6 billion USD are wasted annually due to the lack of cold storage facilities [61] and limited/intermittent grid connectivity in many rural parts of India. In this regard, hot water from IGHWG can be used to power a typical adsorption/ absorption refrigeration system (also used as the test bed in this work) and address the concerns with the wastage of dairy/farm produce. Conversely, a simple extension of such refrigeration system can be conceived to develop an atmospheric water harvesting system for the water stressed regions [62]. Large scale commercialization of such technologies will empower the farmers by creating an economic value for the otherwise wasted crop residue. Moreover, such systems also have a potential for job creation in

155

Table 4 Performance comparison between BGHWG (Fig. 5) and IGHWG (Fig. 7). Parameters

BGHWG (m2 )

System footprint Total weight of system (kg) (approx.) Outer surface area (m2 ) Outer surface temperature (
C) Exhaust flue gas temperature, Tfg;out (
C) Biomass consumption rate, m_ bm (kg=hr) Overall thermal Efficiency, hoverall (%)

IGHWG

1:54

1:2

1200 1:63

700 2:29

225±10 380 ± 10 18 ± 0:5 24±0:6

31±5 240 ± 10 9 ± 0:5 48±2:6

rural and off-grid vicinities of primarily agrarian economies such as India. Apart from refrigeration application, this system has great relevance for other processes such as space heating, dehumidification, tanning, pasteurizing, sterilizing, bleaching and drying. Here, space heating covers all the possibilities of home heating in cold countries, timber seasoning, tea drying, among others [63]. The implementation of IGHWG for such processes needs a critical investigation of corresponding temperatures and thermal loads. In general, thermal capacity of small and medium scale process industries ranges from few kW to MW with a temperature range of 40  150
C[3e7]. The aforementioned capacity and temperature range falls well within the operational range of downdraft gasifier (10 kW  1 MW) [36,64]. Suitable modifications in the gasifier size, and the heat exchanger and the burner designs can facilitate scale up of the hot water generator for higher heat demand applications. In addition, multi-system installation where a group of IGHWG may work together in parallel depending upon load fluctuations can be implemented for large capacity process industries. Furthermore, this system can also act as natural gas-producer gas hybrid system wherever natural gas is available [65e68]. Conversely, the mixture of natural gas and producer gas can also be used in the confined burner to heat the water in the helical coil heat exchanger [69e71]. Apart from integration with conventional resources such as natural gas, it can also be combined with the solar thermal technology to develop a hybrid system. While discussing the potential, it is also important to acknowledge the immediate challenges facing this technology. For example, biomass gasification technology is still not well publicized and is under development. In the absence of strict implementation

Fig. 7. (a) Schematic of IGHWG with an adsorption-refrigeration system (b) Working setup of IGHWG in the Department of Mechanical Engineering at the Indian Institute of Technology Patna, Bihar, India.

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of government regulations, it is very difficult to compete with well acquainted, less expensive and traditional uncontrolled direct combustion-based technology. Moreover, easy accessibility and affordable price of conventional fuels (coal, compressed natural gas, and, liquefy petroleum gas) discourages the end users to use environment friendly biomass in the absence of suitable policies. Hence, it is important to sensitize people regarding the advantages of biomass over conventional fossil fuels and develop a proper supply chain mechanism to encourage the usage. Furthermore, agricultural residue which has low energy density, high moisture content (more than 20%), and random particle size poses the problem of supply (high transportation and storage cost) and feed (bridging and channelling problem) in gasification process [38,39]. In this regard, biomass gasification needs the support from certain energy intensive processes such as drying, chopping and pelletization. Agricultural residue can be transported to nearby pelletization plants (within the radius of 40e50 km) and betterquality pellets can be produced. Such energy intensive processes and transportation results in greenhouse gas emissions. In this regard, mobile pelletizers [72] or stationary farm pelletizers can be used on fields where residue are decentralized and far from pelletization plants. Such an approach will drop the transportation and storage costs in addition to the reduction in greenhouse gas emissions. Apart from the aforementioned solutions, introduction of renewable energy resource for pelletization and drying may be considered as an option. Finally, it is important to plan the disposal of waste such as solid char, ash, and tar generated from the biomass gasification process. While char and tar may further be gasified to produce more energy and liquid fuels, biomass ash may be utilized as building materials, soil amendment and fertilizer, and adsorbent [73e75]. Problem of ash clogging and clinker formation are also a concern with some high ash content biomass fuels such as rice husk. Various techniques such as air staging [44], catalytic tar cracking [76,77] and improved grate design may be incorporated to address such issues regarding tar and ash clogging. 5. Conclusion In summary, the design and development process of a first of its kind gasifier based hot water generation system with thermal capacity of 22 kWth is reported. The typical heat losses during gasification process are recovered by covering gasifier unit in the core with a fire-tube water heat exchanger in the annulus. This integrated system with confined burning of producer gas not only reduces emissions, but also demonstrates a very high overall thermal efficiency of z48 % in comparison to 20  24 % for conventional systems with physically separated gasifier and hot water generator units. Overall, this integrated design presents an efficient and environment friendly waste-to-value concept with large potential in domestic and industrial water heating applications. Moreover, current innovation is believed to have implications in various industrial sectors such as textile, dairy, paper and pulp, and pharmaceutical as well as in domestic use such as space heating, and hot water generation. Acknowledgement We gratefully acknowledge funding support from Science and Engineering Research Board (SERB), DST, Government of India, through project # IMP/2018/000321 of IMPRINT-IIA scheme. We would also like to acknowledge the Ministry of Human Resource Development (MHRD) and Department of Science and Technology (DST) for financial support through the Uchhatar Avishkar Yojana

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