DOWNDRAFT GASIFICATION OF SUGAR CANE BAGASSE Murat Do

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Biofuels and Biomass

PROCESS INTENSIFICATION AND MINIATURIZATION IN GASIFICATION TECHNOLOGY: DOWNDRAFT GASIFICATION OF SUGAR CANE BAGASSE Murat DOGRU, and AHMET ERDEM Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03460 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 9, 2019

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Energy & Fuels

PROCESS INTENSIFICATION AND MINIATURIZATION IN GASIFICATION TECHNOLOGY: DOWNDRAFT GASIFICATION OF SUGAR CANE BAGASSE Murat Dogru1,2,*, Ahmet Erdem2 1

School of Chemical Engineering and Advanced Materials, University of Newcastle, UK

2

Environmental Engineering Department, Gebze Technical University, Kocaeli, Turkey

Corresponding author: Murat Dogru* Address: Department of Environmental Engineering, Gebze Technical University, Kocaeli, Turkey E-mail: [email protected] Tel: +90 505 517 15 38

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ABSTRACT On a global scale, the quantity of biomass waste is steadily increasing, particularly in densely populated urban areas. The electricity and heat energy generation from biomass gasification is viable sustainable process and substitutes fossil fuels, that can decrease greenhouse gas emissions in particular reduce carbon dioxide gas. In this work, sugar cane bagasse as a biomass fuel was considered for combustible gas generation by utilising an intensified downdraft-throated gasifier. The study showed that sugar cane bagasse has a calorific value of around 17.7MJ/kg and densified in the form of briquettes which is found to be an appropriate feedstock for gasification in a downdraft gasifier generating an average of 2.05 Nm3 of produced gas per kilogram of bagasse with a calorific value between 3.30 to 4.56 MJ/Nm3. Biomass feed rates for the gasifier varied between 2.87 to 7.44 kg/hr was utilised in the experiments. The produced combustible gas mixtures were determined as CO, H2, CH4, C2H2, C2H4 and C2H6 which contained 25% to 27% V/V of the entire produced gas. The volume of produced synthetic gas (syngas) ranged from 2.05 to 2.16 Nm3/kg of bagasse gasified. Above 99% transformation of solid carbon in the bagasse to product gas was accomplished by utilising the intensive gasification system however, it is required to apply a gas clean-up process to mitigate against any possible harmful impacts. Keywords: Gasification, Downdraft Gasifier, Biomass, Sugar cane bagasse, Syngas


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Energy & Fuels

1. INTRODUCTION The potential power in biomass feedstock might be realised either by using direct combustion, or by upgrading into more usable and beneficial products such as produced gas, fuel oil and higher valuable products for utilisation in the industry for chemical production or clean energy generation [1]. Unlike direct combustion where thermal oxidation is substantially completed in a single process, gasification transforms the intrinsic chemical energy of the carbon in the biomass feedstock into a combustible produced gas [2]. The produced gas could be utilised in High Temperature Solid Oxide Fuel Cells (SOFC), Internal Combustion Engines (ICE) or in gas turbines for both electricity generation and heat production. In the thermo-chemical decomposition of lignocellulosic compounds, Sulphur and Nitrogen are very low, so are the emissions of SOx and NOx [3]. Furthermore, biomass materials have a zero CO2 balance within a few years. In this sense, utilisation of biomass in energy (power) generation should be considered as one of the most effective use of these feedstocks. One source of biomass is sugar cane, the world’s largest agricultural crop [4]. Sugar cane is cultivated in 127 countries in both the tropics and subtropics with a global production in 2001/2 of 1341 million tons of cane. Sugar cane bagasse is the main byproduct for the sugar cane industry. Sugarcane bagasse is the residue that acquired from the extraction of the sugarcane stalks sucrose extract. It consists of approximately 50% cellulose, 25% hemicellulose and 25% lignin. Due to its high amount of presence, it can provide as an optimal substrate for microbial converts as well as energy transformation via thermal processes for the production of fuels and value-added products. In sugar cane industry, generally bagasse is incinerated inefficiently in the boilers for heat production, 3 ACS Paragon Plus Environment

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that causes particulate, NOX and SOx emissions due to the inadequate conditions of incineration. Energy production is provided from steam generation by combustion; however, the challenge of this transform is associated to the net electrical efficiency, which is quite low (around 10 up to 20%) as compared to the gasification system, that can have a performance as high as 67 to 80% [5]. Bagasse can further be utilized for the manufacture of biofuel (ethanol) by fermentation however, considerable capacities are demanded for transforming bagasse to ethanol. Chemical energy yield from bagasse is attractive since it is a sustainable source of energy and is carbon neutral because the gasification of sugarcane provides the same output of CO2 as it absorbs during its growth [6]. Sugar cane is a C4 plant with photosynthetic metabolism type. This implies that it provides acids with 4 carbon atoms, releasing the plant to consume higher CO2 which can generate more substance than the other plants [7]. Sugar cane residues such as bagasse and cane trash (cane tops and leaves) are now being studied as important sources of biomass for use in the production of renewable energy by gasification method. The characterization of sugar cane bagasse properties is important for gasification system design. Fluidised bed gasifier was utilised and experiments were carried out to measure the effects of various system specifications such as gasification temperature, equivalence ratio, gasification steam/biomass ratio from sugarcane bagasse [8]. The most considerable properties of any biomass that determined to impact the gasifier operations are the size of feedstock, water content and structure, bulk density and absolute density, chemical contents (i.e. ultimate analysis and proximate analysis) and the gross calorific value. In an investigation, gasification characteristics of sugarcane bagasse are examined using Thermogravimetric Analysis (TGA)-Mass


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Energy & Fuels

Spectrometry (MS) method and kinetic analysis. The results indicate that the gas discharge and char yield during gasification is considerably altered based on the heating rates [9]. Pre-processing performs a vastly role in the gasification process of sugar cane bagasse when utilising the downdraft gasification process and remains a key bottleneck in the productive utilisation of bagasse for power generation [10]. The bagasse used in this study was obtained from Barbados where 55000 tons of sugar cane are harvested annually. Approximately 33 tons of bagasse is produced per 100 tons of cane in Barbados which results in an estimated 17160 tons of bagasse which is used for the onsite production of steam and power at each of the two sugar factories during the sugar cane harvesting season. A portion of the bagasse is stored until the next year to provide fuel for startup operations of the plant in the cane-harvesting season. Cane trash (sugar cane tops and leaves) is left in the fields to provide soil cover as well nutrients for soil fertilization. Estimates for Barbados indicate that if only 25% of the cane trash were left in the fields the quantity of cane derived biomass available for usage would be doubled. This study will focus on the production of power and heat from the gasification of sugar cane bagasse in an intensified and miniaturized fixed bed throated downdraft gasifier. The objectives of the study are to: (i) Evaluate the performance of the downdraft gasifier using sugar cane bagasse as a fuel to proof of the concept. (ii) Determine the composition of gas and calorific value of the produced gas formed by gasification of bagasse in a throated downdraft gasifier. (iii) Derive energy and mass balance for the gasifier system. 5 ACS Paragon Plus Environment

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(iv) Assess the feasibility of operating an internal combustion engine or gas turbine on the product gas from the thermal conversion of bagasse in the gasifier. Propose the type of gas cleaning equipment, which will be required, based on the particulate matter content, particle size distribution, fly ash melting temperature, tar and alkali metal content of the product gas.

2. EXPERIMENTAL SYSTEM AND METHOD 2.1. Experimental System The intensive throated downdraft gasifier based on a conical cylinder reaction chamber, with a throat over the grate at the lower section. Biomass is fed near the upper section of the gasifier, and moves by gravity down through the reactor. Four distinct reaction zones can be named from top to bottom as drying, pyrolysis, flaming pyrolysis (partial-oxidation) and gasification zones. Figure 1 and Figure 2 illustrates schematic diagram and the typical temperature profile through the gasifier and the reaction zones. The whole system which consists of a gasifier, hot gas-water vortex scrubber and vacuum and booster fan is illustrated in Figure 3. The experimental system and the method are also explained in detail in the recent and past works [11-14]. In the top drying section of the gasifier, thermal energy transferred by conduction and convection within the reactor upward to the new feedstock load. During this process the heat of the fuel load increases whilst the water content of the fuel decreases. The height of the drying section is 10 cm and the temperature is varied between 70 to 200oC in this zone.


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In the pyrolysis section of the gasifier, the heat of the feedstock increases to the level where the volatile components are released. This process occurs by utilising the heat discharged by the combustion of the pyrolytic components at the lower section. The height of the pyrolysis zone is 17 cm and the pyrolysis temperature is variable between 350 to 500oC. In the oxidation section of the reactor, limited oxidising agent (air) is supplied via air-jet nozzles to the charred fuel. Not only is solid material entering this zone, but also the gaseous pyrolysis product such as tars. The heat energy captured in the other zones is provided by the exothermic reactions in this stage. The heavier hydrocarbon molecules burn in the gas-phase to produce CO2 with H2O at about 1200oC. Due to the highly exothermic tar combustion, heavier hydrocarbons (tars) are thermally cracked to achieve uniform temperatures within the gasifier at the throat level where limited oxygen in air is supplied through air jets. Unlike other fixed bed gasifiers such as updraft, this unique feature makes the downdraft throated gasifier suitable for power generation from biomass as product gas contains very low level (50-100mg/m3) tars. The height of the oxidation section is 12 cm and the temperature is as high as 1000-1200oC.


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Figure 1. Imitation of the intensified downdraft gasifier with throat In the reduction zone with a high availability of heat energy, endothermic reactions take place among the char particles and the pyrolytic gases, producing predominantly CO and H2, along with CH4. The char in the reduction zone reacts with the incandescent CO2 which should be within a temperature range of 900-1000oC, and shrinks as it gives up carbon molecules to the CO2 to become CO. H2O evolved from raw biomass at the upper drying zone, in form of steam in reduction zone is cracked at the throat into hydrogen and this joins the CO to become the fuel gas. The continuous shrinkage and consumption of the char from the outlet grate up through the throat and oxidation zone, enables the flow of fuel to take place almost under 8 ACS Paragon Plus Environment

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Energy & Fuels

the

influence

of

gravity.

Wrongly

designed

gasifiers

or

improper

fuel

utilisation create fuel flow complications in the reaction chamber and these are the main reasons for many issues of the past trials. The product gas exits the reactor at a temperature of around 200 to 300oC. It is necessary to cool and clean tar and particulate laden hot gas mixture since the clean-up system must be produced with regard to the final utilisation of the product gas. It should be free from tar, water vapor and particulates and also must be cooled for gas engine applications to generate power. The produced gas was treated via a vortex water scrubber unit which is shown schematically in Figure 4. In the water spray process, the soluble inorganic materials such as ash particulates and some compounds like organic tar in the product gas are stripped off through the water film from the jet nozzles. The film created via highpressure water through the tight orifice in the scrubber produces millions of teeny water particles (mist) in order to increase contacting surface area between gas and water. The higher surface contacting between gas and liquid creates mass transfer of water-soluble mixtures among the phases. Additionally, the fast water chilling produces precipitation of heavier hydrocarbons and separating those from the produced gas. The scrubbed gas then exits the high-pressure spray process through the water film from the upper part with a temperature around 25-45oC depending on the inlet gas and cooling water temperature.


Energy & Fuels

L

Fuel hopper Drying zone

T - drying

H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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T - pyrolysis

Pyrolysis zone

D

T - throat

Oxidation zone

Air Supply d

Ash pit

1200

900

600

300

Reduction zone

25 C

Figure 2. Layout drawing of the throated gasifier reactor with operating temperatures. (Dimensions: Height (H)=810mm; Diameter of the reactor (D)=450mm; Diameter of upper section (L)=305mm; Throat (d)=135mm)

A gas circulation fan (suction fan) is utilised to create the vacuum effect, which would be exerted by an engine coupled to the gasifier so as to pull the product gas out of the gasifier, through the scrubber and up the stack where it is burnt in a flare stack burner.

2.2 Experimental Method The experimental method for operating the gasifier can be split into four section and is described below: 1) Commissioning that covers whole operating procedure necessary to reach a steadystate condition, which the final gas property is suitable for the engine operation.


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Energy & Fuels

Measured weight of sugar cane bagasse in the form of densified logs are loaded through the hopper from the gasifier top to set level in the reactor, and form then the product gas suction booster fan with the vortex scrubber circulation water pump are started. The biomass is set alight through air jets from the oxidation zone which corresponds to incoming air section of the gasifier.

2) The gasifier system reaches the steady state around 6-8 minutes after initial ignition. Beyond this point, the temperature of the oxidation zone reaches around 1100-1200oC and the product gas is ignited in the gas burner. During the steady-state function of the gasifier, the collected data were the feedstock rate, product gas components, heavy hydrocarbons with particulates and water, pressure and temperature profile. Temperature data was collected and saved with an analogue to digital converter each 15 seconds for air intake, drying, pyrolysis, oxidation zones and water scrubber exit. Pressure data was also recorded from the reactor gas outlet pipe and water scrubber exit. These are illustrated in Figure 4. The produced gas flowrate was measured by an air flow rotameter placed after the vacuum blower and tar with particulates and water in the produced gas were measured from the sample of gas received at the gasifier reactor outlet and the vortex scrubber outlet.


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SP3

T5

T3

Ps

FUEL

FLARE STACK

T2 Water Spray

T4

T1

Pg

FLAME TRAP GAS ROTAMETER

SP1

GAS GATE VALVE AIR T6 GAS

ASH

GASIFIER

WATER-VORTEX SCRUBBER

WATER CIRCULATION PUMP

GAS BOOSTER FAN

TAR TRAP

1 2 3 4 5 6 7

Figure 3. Process flow layout diagram of the experimental rig (T1: reactor throat temperature, T2: gasifier outlet temperature, T3: drying section temperature, T4: reactor gas exit temperature, T5: water exit temperature, Pg: pressure drop across the gasifier, Ps: pressure point at the scrubber exit, SP1: product gas sample collection at the reactor exit, SP2: product gas sample collection at the scrubber exit)


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Energy & Fuels

3) End of the experimental run, stopping the whole process comprises several important steps in order to shut down the gasification system safely. In an orderly sequence, reactor air intake valves, water pump, suction blower switch off and the product gas valve is closed. However, remaining product gas is flared by keeping the gas igniter switch on for further few minutes until all the gas is discharged from the flare stuck.

4) To obtain the data for the mass balance calculations, sample collection includes all the procedure necessary to save char remains, ash, tars with particulates and condensate. When the gasifier temperature is dropped, char and ash samples collected from the ash box and weights are recorded. In the last step, the fuel hopper of the reactor was removed and then all remaining sugar cane bagasse were vigilantly collected and weighted. The average biomass fuel feeding rate to the gasifier was estimated by means of dividing the whole sugar cane bagasse gasified to the total operation period during that particular experiment.

3. RESULTS WITH DISCUSSIONS 3.1. Properties of sugar cane bagasse The suitability of a biomass as a fuel for gasification must be established based on an evaluation of its physicochemical characteristics prior to conducting the experimental gasification runs. An assessment of the physicochemical characteristics of bagasse was made from the results of a series of analyses; these are outlined in Table 1.


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Figure 4. Schematic of the vortex scrubber. The physical features determine the fuel size, bulk and absolute densities. The proximate analyses reveal the water content, volatile matter, fixed carbon and ash contents. The partial ultimate analyses determined hydrogen, carbon, nitrogen, oxygen, sulfur and chlorine contents of the sugar cane bagasse. The data generated by the ultimate analysis is in the range of that reported by several authors including Suarez [15] and Gabra [16]. The small differences in elemental composition between this data and that of other workers is very likely to be due to the differences in genetic composition of the various sugar cane varieties which gives rise to a difference in the % fiber content of the sugar cane.


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Energy & Fuels

Proximate Analysis Moisture (%wt)

6.4

Ultimate Analysis 45.16 ± 0.14

Carbon

Fixed carbon (%wt)

13.00 ± 2.85

Hydrogen

5.66 ± 0.01

Volatile matter (%wt)

39.89 ± 4.30

Oxygen

48.68 ± 0.06

Ash (dry basis) %wt

1.51 ± 0.32

Nitrogen

0.42 ± 0.06

HHV (dry fuel)(MJ/kg)

18.14 ± 1.16

Sulphur

0.02 ± 0.002

LHV (dry fuel (MJ/kg)

17.70 ± 1.16

Chlorine

0.06 ± 0.02

Physical Properties Size (mm)

65 x 40 x 40

Bulk Density (kg/m3)

642.05

Absolute Density (kg/m3)

678.89

Table 1. Chemical and Physical characteristics of sugar cane bagasse

The Higher Heating Value (HHV) based on the ultimate analyses was determined by utilising both the Institute of Gas Technology (IGT) method [17] and that by Channiwala [18]. IGT Method: HHV = 1323H + 341C + 68S – 15.3Ash – 120 (O+N) Channiwala: HHV = 1.1783H +0.3491C + 0.1005S – 0.1034O – 0.0211Ash– 0.0151N The HHV of bagasse was determined experimentally to be 18.14MJ/kg (standard deviation 1.16) and theoretically calculated as 17.03MJ/kg (standard deviation 0.46). 15 ACS Paragon Plus Environment

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These numbers are also in agreement with that of other researchers including Gabra et al., 2001 and Suárez et al. 2000. To design the thermo-chemical conversion processes, using lower heating value rather than higher heating value of a fuel in the calculations are better in terms of that omits the water heat of vaporization and the water content of the biomass as these do not add any value to the calorific value of the feedstock. Based on the HHV, the LHV of bagasse was calculated using the relationship: LHV (dry basis) = HHV (dry basis) – 2.44(9H) Typical biomass fuels for gasification have LHVs of approximately 15-17%; wood waste, which has been the traditional fuel for biomass gasification, has a LHV in the range 17.8-20.8 %. Based on the LHV value obtained as 16.90MJ/kg for bagasse it was determined that this fuel was suitable for gasification in terms of its equivalent calorific value to wood. The biomass water content highly influences both the operational parameters of the reactor and the quality of the produced gas. High amount of water in fuel lowers the operating temperature of the gasifier and this result higher hydrocarbons as form of heavy tars in the final product gas leaving the gasifier. The water content restricts for gasifier feedstocks and determine the type of reactor design utilised. High moisture contents are accepted for updraft gasifiers however, the maximum level possible for a downdraft gasifier is normally accepted to be approximately 40% dry basis [17]. Under normal circumstances, water content of most biomass changes between 11% to 18% dry basis, thus widely accepted as very appropriate and suitable for the gas engine systems [1]. The moisture content of the air dried sugar cane bagasse was around 6 to 7%. 16 ACS Paragon Plus Environment

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Energy & Fuels

The bulk and absolute densities of feedstock is significant for system design for the storage and handling [1]. Feedstock with higher bulk density shall demand lesser gasifier vessel for a certain re-fueling period. However, lower bulk density biomass occasionally creates inadequate movement under the gravity which results less gas calorific value and high char burn out in the reaction zone [19]. The bulk densities of sugar cane bagasse are greater than woodchips (250.28 kg/m3) [1], and the trials proved that there was less char combustion occurred in the gasification section. In order to miniaturize the gasifier size and hence to increase the fuel load capacity of the gasifier, the fuel is densified in the form of briquettes. Figure 5 illustrates the briquetted biomass (70mm diameter): sugar cane bagasse obtained from Barbados utilized in this study. In general terms, briquettes are produced under high compression of the bagasse using standard equipment of the shelf. Densification of biomass can reduce the size of the fuel occupational space in the gasifier up to 12 fold and this represents approximately 1/3 reduction of total size of the gasifier reactor. Biomass densification has several advantages. These are; gasifier reactor size reduction, ease of fuel handling and prevent dust exposure, uniform size of briquettes allows uniform fuel flow by gravity and also uniform briquettes creates uniform void space in the reactor which prevents channeling around the throat zone.


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Figure 5. Briquetted sugar cane bagasse Higher volatile content of feedstock in general rises tar quantity in the produced gas and significant amount of tar must be separated prior to feeding the product gas to an engine generator [14-16]. Sugar cane bagasse has smaller amount of volatile content than woodchip (64.79%) [1], resulting in high gas quality and lower tar content.

The ash content of sugar cane bagasse is also found to be low so it could be removed from the reactor less frequently and continuously, without disruption of the nonintermittent generation of high quality product gas. Low amount of ash in bagasse is also useful property to prevent possible clinker formation in the gasifier due to high operating temperature in gasifier. Clinker formation can create channeling and/or bridging around throat region.


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Energy & Fuels

3.2. Gasification Results Wet product gas compositions and other mass and energy balance data with their derivatives obtained from sugar cane bagasse gasification are listed in Table 2.

Parameter

Fuel feed rate (kg/h) Air intake (kg/h) Equivalence ratio Wet Gas Composition (% vol.) H2 N2 CO CH4 CO2 C2H2 C2H4 C2H6 Mass flow rate of tar (kg/hr) LHV wet gas (MJ/Nm3) Mass Balances (%) C H O N

Gasification Runs 1 2 7.44 16,37 0.34

4.21 5.89 0.22

3 4.43 7.09 0.25

4 2.87 4.88 0.26

5 3.98 6.37 0.25

10.13 60.55 8.46 2.03 16.11 0.03 0.46 0.00 0.057 3.30

9.47 70.24 5.66 0.28 12.09 0.00 0.03 0.00 0.024 1.95

13.40 56.89 15.56 1.54 9.98 0.00 0.03 0.03 0.055 4.10

10.86 58.48 16.45 1.54 10.19 0.00 0.15 0.00 0.058 3.95

11.20 58.48 12.16 3.57 11.36 0.17 0.52 0.00 0.086 4.56

70.68 58.22 56.09 129.67

45.71 43.85 41.82 161.76

123.82 91.55 93.90 128.09

150.75 102.59 93.90 128.09

138.16 119.69 79.64 125.52

Table 2. Gasification mass and energy balance data

Gasifier operated at the highest feed rate during run-1 and relatively high amount of air injection shifted the reaction from gasification to combustion in the oxidation zone and this has caused CO oxidation to CO2. During run 2 the gasifier was operated at a low airflow rate, as briquetted fuel was limited. In addition, bridging occurred for a short period during the run. It is possible that the combination of these conditions resulted in the low carbon conversion.


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3.3. Product Gas Composition and Calorific Value As shown in Table 2, the wet fuel rates varied between 2.87-7.44 kg/hr in these runs. The produced combustible gas mixtures are CO, H2, CH4, C2H2, C2H4 and C2H6, and approximately 25-27 % of the total produced gas. The calorific value of the wet produced gas from bagasse gasification ranged from 3.30 – 4.56 MJ/Nm3. The volume of wet gas produced ranged from 2.05-2.16 Nm3/kg of bagasse gasified. The product gas burnt at the open flare stack with a blue flame and was able to sustain the flame in the absence of the pilot burner for the remainder of the run (1½ hours). This can be clearly explained that gasifier responded well with the briquetted fuel without showing any sign of bridging during the operation of the gasifier. The gas samples collected every half-hour showed only small changes in gas composition and exemplified the steady nature of the flame. It is illustrated in Figure 6. This also suggests good fuel flow characteristics within the reactor, which is expected from densified fuel.


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18 16 14 % Composition

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Energy & Fuels

12 10 8 6 4 2 0 00:15:00 H2

CH4

00:35:00 CO

01:05:10

01:40:10

Time (hr:min:sec) CO2 C2H2

02:10:00 C2H4

C2H6

Figure 6. Variability in product gas composition.

Figure 7 illustrates the high variability in gas composition over a 1 ½ hour period, here the bridge was cleared approximately one hour after it developed by agitating the fuel bed with external impact to gasifier outer surface by means of a motorized vibrator. Immediately after the bridge developed the flame height decreased at the stack outlet and eventually production of combustible gas stopped as was seen by the disappearance of the flame which turned in smoke. Within seconds of agitating the fuel bed combustible gas was produced as this was evident from the redevelopment of a strong royal blue flame.


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% Composition

Energy & Fuels

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18 16 14 12 10 8 6 4 2 0 00:35:00

01:05:10

01:40:10

Time (hr:min:sec) H2

CH4

CO

C2H2

C2H4

C2H6

CO2

Figure 7. Variability in product gas composition during bridging The rapid decrease in carbon monoxide during the development of bridging followed by the increase in carbon monoxide and the associated decrease in carbon dioxide after the fuel bed has been agitated is clearly illustrated in Figure 7. Bagasse is very similar in chemical composition to wood and that wood has been the traditional fuel in biomass gasification, a comparison of the average percentage composition of the component gases for gasification of both types of biomass using the downdraft gasifier was carried out. The comparison was done using the same product gas flow rate for wood and bagasse, the results are shown in Figure 8. The results showed that there is an insignificant difference in the percentage composition of the main components of the product gas from the gasification of these two types of biomass using a downdraft gasifier. Woodchips and sugarcane bagasse [19] gasification comparison consequences were illustrated in Table 3.


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Energy & Fuels

Sugar Cane Bagasse

Woodchips [19]

Fuel feed rate (kg/h)

3.98

0.65

%(V/V) ↓ / Equivalence ratio →

0.25

0.45

H2

11.20

5.90

N2

58.48

N/A

CH4

3.57

0.38

CO

12.16

11.50

CO2

11.36

5.40

C 2H 2

0.17

N/A

0.52

N/A

5.56

2.50

C 2H 4 3

CV (MJ/m )

Table 3. Sugar cane bagasse vs Woodchips gasification comparison

3.4 Gasifier Temperature Profile The temperature profile in the drying, pyrolysis, throat and gasifier exit shown in Figure 10. was observed for both of the gasification runs using briquetted bagasse. The graph clearly shows the swift rise in heat, which occurs after the throat has been lit, and the relatively steady temperatures in each of the thermal conversion zones. This figure is also clearly illustrated the short start up period of the gasifier limited to 6 minutes.


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70 60 50 40 % Composition 30 20 10 0

N2

H2

CH4

CO

CO2

C2H2

C2H4

C2H6

Gas Component bagasse (left)

wood (right)

Figure 8. Comparison between product gas composition from bagasse and wood gasification

3.5 Tar Content A high percentage of volatile matter in feedstock can increase the tar amount of the produced gas since tars are formed from the condensable organic fraction of the devolatilized biomass. The bagasse investigated had an average volatile matter content of 74.30 wt% (dry basis). It is lower than to that of wood, which has an average volatile matter content of 82 wt% (dry basis) [20]. The gravimetric analysis using the U-tubes established that the tar and particulate matter content of the product gas ranged from 2.63–5.00 g/Nm3 at the gasifier exit before gas cleanup (Table 4). This represents a percentage of 0.13–0.50 wt% of the product gas. This procedure measured both tar and particulate since both contaminants are entrained in the product gas and will condense on the U tubes. Therefore, particulate constitute a


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Energy & Fuels

portion of the ‘tar fraction’ using this method of analysis. The concentration of tar alone was not determined at this study.

Tar + Particulates

Gasification

Gas Flow Rate

Volume of product gas

Run

3

(Nm /hr)

3

(Nm /kg)

(g/Nm3)

1

17.14

2.16

3.35

2

9.24

2.05

2.63

3

17.14

3.40

3.20

4

13.11

4.00

3.83

5

17.14

3.80

5.00

Table 4. Particulate and Tar amounts of the product gas

3.6. Mass and Energy Balance Results and Analyses One critical aspect in the evaluation of the potential for power production from bagasse using the throated downdraft gasifier is the energy and mass balance of the process. The energy and mass balance on the reactor provides a quantitative measure of the efficiency for conversion of fuel to product gas and ultimately to electricity using this particular type of gasifier. Mass and energy balances for a specific type of fuel will vary


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

from one gasifier to another as the thermodynamic equilibria and reaction kinetics of the three main reactions in gasification vary depending on the gasifier operating conditions. The mass balance analysis on the gasifier requires an evaluation of the inputs to and outputs from the gasifier. From the calculations, achieving 100% closure is not easy to obtain and the results illustrate the complications of acquiring this data. However, the mass balance closure for 5 runs was found to be average 81.5% which represents a reasonable figure for the initial proof of concept assessment of bagasse gasification study trials. Mass and energy balance diagram for run 5 is given in Figure 9.

Figure 9. Mass and energy balance diagram for gasifier run 5


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Page 27 of 31

1200 1000 Temperature oC

800 600 400 200 01:44:40

01:38:30

01:32:20

01:26:10

01:20:00

01:13:50

01:07:40

01:01:30

00:55:20

00:49:10

00:43:00

00:36:50

00:30:40

00:24:30

00:18:20

00:12:10

0 00:06:00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Time (hr:min:sec) Pyrolysis °C

Drying °C

Gasifier exit °C

Throat °C

Figure 10. Temperature stability in the thermal conversion zones

4. CONCLUSION It is proved that gasification of sugar cane bagasse for clean green energy generation is a possibility by using miniaturized and intensified throated downdraft fixed bed gasifier with an appropriately designed grate and with a required amount of oxidation agent (air) circulation in the reactor bed. The experiments illustrated that the calorific value of the wet combustible produced gas mixture (3.30-4.56MJ/Nm3) from sugarcane bagasse gasification is convenient for a gas engine application to generate electricity and to produce heat. The


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

obtained calorific value of the gas is also greater than the minimum value of 2.5 MJ/Nm3 required by gas turbine manufacturers. Gasifier produced gas generation up to 4 Nm3 per kg of sugar cane bagasse with a calorific value of 4.56 MJ/Nm3 is successfully achieved during the trials. Gasifier reactor operated in excess of 1000oC at the throat zone over the period of an hour at a stable condition. The high temperature operation of the gasifier resulted low tar yield and high gas production. It is important to note that a minor clinker issue over the grate section in the reactor at higher fuel rates. It is recommended that slightly agitating the grate section and hence reducing operating temperature of the gasifier by means of steam injection with air will overcome this problem.


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Energy & Fuels

REFERENCES (1) Dogru M. Fixed-bed gasification of biomass. (2000). PhD Thesis, University of Newcastle, United Kingdom. (2) McKendry P. (2002).

Energy production from biomass (part 3): Gasification

Technologies. Bioresource Technology 83; 55-63. (3) Mathieu P. and Dubuisson R. (2002). Performance analysis of a biomass gasifier. Energy Conversion and Management 43; 1291-1299. (4) Garcia-Perez M, Chaala A, Yang J, Roy C. Co-pyrolysis of sugarcane bagasse with petroleum residue. Part 1: Thermogravimetric analysis. Fuel 80 (2001), 1245-1258 (5) Asadullah M. Barriers of commercial power generation using biomass gasification gas: a review. Renew Sustain Energy Rev 29 (2014) 201–15 (6) I.I. Ahmed, A.K. Gupta “Sugarcane bagasse gasification: Global reaction mechanism of syngas evolution” Applied Energy 91 (2012) 75–81 (7) R.F. Sage, M.M. Peixoto, T.L. Sage, Photosynthesis in sugarcane, Sugarcane Physiol. Biochem. Funct. Biol, John Wiley & Sons Ltd, (2013) 121–154 (8) Abanti Sahoo, Deo Karan Ram” Gasifier performance and energy analysis for fluidized bed gasification of sugarcane bagasse” Energy 90 (2015) 1420-1425 (9) Kandasamy Jayaraman, Iskender Gokalp, Sebastien Petrusa, Veronica Belandria, Stephane Bostyn “Energy recovery analysis from sugar cane bagasse pyrolysis and gasification using thermogravimetry, mass spectrometry and kinetic models” Journal of Analytical and Applied Pyrolysis 132 (2018) 225-236 29 ACS Paragon Plus Environment

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(10) Anthony Anukam, Sampson Mamphweli, Prashant Reddy, Edson Meyer, Omobola Okoh “Preprocessing of sugarcane bagasse for gasification in a downdraft biomass gasifier system: A comprehensive review” Renewable and Sustainable Energy Reviews 66 (2016) 775–801 (11) Dogru M, Howarth CR, Akay G, Keskinler B, Malik AA. Gasifiaction of hazelnut shells in a downdraft gasifier. Energy 27 (2002) 415-427 (12) Calkan OF, Dogru M, Akay G, Midilli A, Howarth CR. Clean Bio-Fuel productıon from sewage sludge for future applications of fuell cells. First International Symposium on Process Intensification and Miniaturisation in Biological, Chemical, Environmental and Energy Conversıon Technologies. Newcastle, UK (2003). (13) Akram M, Dogru M, Akay G. Gasification of Municipal Solid Waste (MSW) a Move Towards a Sustainable World. First International Symposium on Process Intensification and Miniaturisation in Biological, Chemical, Environmental and Energy Conversıon Technologies. Newcastle, UK (2003). (14) Dogru, M., Akay, G., Howarth, C., Keskinler, B., Ling, M., Malik, A. (2002b) Olive pips – A potential valuable biomass source of clean combustible gas using an intensive gasifier. 13th International Symposium on Transport Phenomena, July 14-18, 2002 Vancouver Canada (15) Suárez J., Luengo C, Felfli F, Bezzon G, Beatón P. Thermochemical properties of Cuban biomass. Energy Sources 22 (2000), 851-857.


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DOWNDRAFT GASIFICATION OF SUGAR CANE BAGASSE Murat Do

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