Gasification of Biomass as a Source of Hydrogen Rich Syngas Production

Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2005
Istanbul, Turkey, 13-15 July 2005
Gasification of Biomass as a Source of Hydrogen Rich Syngas Production
M. Dogru1, G. Akay1, A. Midilli2, O.F. Calkan1 and C.R. Howarth1
1School of Chemical Engineering & Advanced Materials, University of Newcastle, NE1 7RU, UK
2Mechanical Engineering Department, Nigde University, 51100, Nigde, Turkey
Tel/Fax: + 44 191 222 7276/5292 e-mail: murat.dogru@newcastle.ac.uk
ABSTRACT
The proposed paper is relevant to the development of advanced small fuel processing units for decentralized hydrogen production. It investigates a novel-integrated-intensified sustainable gasification technology to produce hydrogen rich clean syngas to be used in electricity generation via internal combustion engine but which can also be used in high temperature fuel cells and catalytic conversion to bio-ethanol.
Gasification is central to the establishment of biomass based sustainable energy technology, irrespective of the route (thermal or biological) taken for the production of the primary target fuels (namely hydrogen and bio-ethanol) and electricity. To make hydrogen and bio-ethanol productions economically viable, socially and environmentally acceptable and technologically reliable, these processes and gasification must be integrated. Sustainability of such an integrated technology can be achieved through the application of the fast emerging Process Intensification and Miniaturization (PIM) technology which addresses all the tenets of sustainability, including reduction in capital and operating cost reduction, drastic reduction of plant size, inherent safety and reliability as well as the elimination of parasitic side reactions thus reducing the environmental impact.
Gasification is the thermal decomposition of solid fuel to a combustible gas, or synthesis gas (syngas) rich in carbon monoxide and hydrogen. By using limited amount of oxidant (pure oxygen, oxygen enriched air or steam) a partial oxidation (gasification) will take place. There are several types of gasifiers available; while fixed bed gasifiers have electrical energy output of up to a few MWe, fluidised bed gasifiers are suitable for multi-MWe operations. Fluidised bed gasifiers are suitable large capacity systems while fixed bed gasifiers have relatively low capacity which is however more suitable for de-centralized power generation up to ca. 20MWe.
The objective of this study is to operate a well instrumented responsive 5kWe fixed-bed gasifier system to generate hydrogen rich combustible gas and power from biomass in particular to hazel nutshells.
For this purpose, an experimental study is conducted using a pilot scale 5 kWe downdraft gasifier. Initial results suggested that hydrogen production via the downdraft gasification reactor is around 2.4 kg from 100 kg of nutshells and the volume by volume percentage of hydrogen gas is found to be between 11% to 14% which represents substantial amount of hydrogen production from a dedicated biomass feedstock.
The proposed paper will deliver and present all the results of this research study at the University of Newcastle in United Kingdom.
Keywords: Hydrogen, biomass, gasification, nut shell, downdraft gasifier.
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1. INTRODUCTION
Conversion of waste to hydrogen has the environmental advantage of decreasing the number of future landfill sites needed with a contaminant decrease in the associated air and water pollution issues [1-3]. Although being one of the universe’s most abundant elements hydrogen is present in the atmosphere only in concentrations of less than one part per million. Hydrogen can be considered as substitute for gasoline as 9.5 kg of hydrogen produces energy equivalent to that produced by 25 kg of gasoline [4-6]. Most of hydrogen does not exist in free-state in nature, and must be, therefore, produced from hydrogen containing compounds [7]. However, hydrogen can be also derived via gasification from biomass such as hazel nutshells. Hazelnut shell can, thus, be assumed as one of the most important types of biomass for gasification, as being an abundant and important agricultural and commercial material in Turkey, Spain, Italy and USA. At present, two third of the world hazelnut production is from Turkey. Forestry operations produce around 250 thousand tons of hazelnut shells per year (equivalent to 4.63x109 MJ) in the Black-Sea region of Turkey. It is equivalent to one-third of hazelnut shells produced by America, Italy and Spain respectively [2, 5, 8]. Hazelnut shells have only been used in simple combustors and/or boilers for heat recovery for a long time. It is interesting if hazelnut shells can be converted to hydrogen rich syngas for clean energy supply by applying downdraft gasification technique. Downdraft gasification of hazelnut shells can be identified as a possible system for producing renewable hydrogen energy [9]. The most significant properties of hazelnut shells that are known to influence the gasification process for the production of hydrogen gas are moisture content and chemical composition. Therefore, the primary focus of this study is the potential of hydrogen production rate via downdraft air gasification technique of hazelnut shells. If all locally available hazelnut shells were utilized in air-blown-downdraft gasifier, the production of hydrogen gas could be almost 6000 ton per year. Thus, this amount of hydrogen gas would represent about 675000 GJ of energy output.
2. MATERIAL AND METHOD
2.1 Material
The experimental system utilized for the test runs is shown in Figure 1 consists of a downdraft gasifier, packed bed scrubber, filter box, gas blower and a pilot gas flare stack [9]. Downdraft gasifier has four reaction zones, which are drying, pyrolysis, partial oxidation and reduction zones from top to bottom of the gasifier respectively. In drying zone, hazelnut shells descend into the gasifier and moisture is released by evaporation utilizing the heat generated in the zones below. In pyrolysis zone, the irreversible thermal degradation of dried hazelnut shells descended from the drying zone takes place using the thermal energy released and radiated by the partial oxidation of the pyrolysis products. In oxidation zone, the volatile products of pyrolysis are partially oxidised in exothermic reactions resulting in a rapid rise of temperature up to 1200oC in the throat region. The heat generated is used to drive the drying and pyrolysis of the feed and the gasification reactions. In the reduction zone as often referred as gasification zone, the remaining shell chars are converted to the produced gas by reaction with the hot gases from the upper zones. The gases are reduced to form a greater proportion of hydrogen and carbon monoxide by means of water shift gas reactions. While the producer gas leaving the gasifier at temperatures between 400ºC and 500ºC, the gases absorb and collect some dust, pyrolytic products (tar) and water vapour.
In the gas treatment section, the packed bed scrubber was made of stainless steel because of corrosive condensate content of the acid gases. It consists of water tank to re-circulate the spray water and a cooling tower packed with sprays. After gas cooling and scrubbing, the cleaned produced gas is further micro-filtered by a vertical filter box.
The filter box has two layers, including wood chips in the upper section and charcoal at the bottom respectively. The contaminated filter media can be recycled as fuel for the gasifier. In order to prevent excessive pressure drop over the box filter, the wood chips and charcoal are thoroughly
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sieved to remove any fines before installation. Remaining tar and condensate from the produced gas are collected at the base of the box filter.
Figure 1: Schematic figure of the experimental set-up (1. Temperature measurement of drying and pyrolysis zone, 2. Temperature measurement of oxidation zone, 3. Oxidation zone, 4. Reduction zone, 5. Grate, 6. Produced gas outlet temperature measurement, 7. Sampling gas, 8. Pressure measurement of the produced gas, 9. Wastewater outlet, 10. Fresh water inlet, 11. Tar collection chamber, 12. Wood chip, 13. Charcoal, 14. Rotameter, 15. Tar and dust trap, 16. Steal table, 17. Hydrogen gas analyzer)
2.2 Method
For this study, four types of hazelnut shell (Tombul, Palaz, Badem and Sivri) from Trabzon, Turkey transported to Newcastle, UK and were utilised as biomass feedstock in the downdraft gasifier. Total eleven test runs were performed for hydrogen production rate analyses from hazelnut shells.
During the test runs, hazelnut shells were initially weighed and loaded into the hopper. Then, the gas booster fan and the circulation pump of water scrubber were switched on. The fuel was ignited on the grate using solid fuel igniters. The feed rate of wet hazelnut shells, the flow rates of produced wet and dry gas, the flow rates of hydrogen gas, combustible gas and ash were experimentally recorded during the test runs. Temperatures were measured in the drying, pyrolysis and oxidation zones and pressure drops were measured at the gasifier, scrubber and filter box outlet by an Analog to Digital converter at every 15 seconds. The flow rate of the producer gas was measured by a gas flow meter settled at the outlet of circulation fan. Gas samples were taken using the sample bottles from the outlets of the gasifier and filter box for analysis using gas chromatography (GC Shimatsu 8A) with dual columns (chromosorp 101 and molecular sieve) and a thermal conductivity detector. CO, CH4, CO2, O2, N2, C2H4 and C3H4 were determined utilizing helium as the carrier gas. H2 was also determined using argon as the carrier gas. Determination of three-carbon to six-carbon hydrocarbons can be carried out using higher oven temperatures. The
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U-tube apparatus was used to collect tar and condensate and to clean the gas samples for GC analysis. The U-tube apparatus basically consisted of a stainless steel sampling mouth (5 mm diameter) and three U-tubes (pyrex). The three "U" tubes were connected in series into an ice bath container for trapping tars, particulates and moisture in the sampled gas. The first trap (U1) contained smaller diameter glass tubes to provide a large contact surface area with the produced gas. The second trap (U2) also contained silica gel and the third trap (U3) contained glass-wool.
A gas sampling bottle with two Teflon taps was used to collect the producer gas samples. A differential pressured mercury column manometer (Gallencamp) and a rotameter (MFG Fischer 10L/min) were placed between the sampling bottle and the vacuum pump (AEI type BS 2406, 0.25 hp). The quantity of the trapped tar and condensate were measured by weighing the U-tubes.
3. RESULTS AND DUSCUSSION
The results from this experimental investigation were analysed and illustrated for the conversion rate of hazelnut shells into hydrogen gas. Moreover, the pressure drops were measured across the gasification system due to the fact that produced combustible gas mixture containing hydrogen gas was efficiently utilized in an internal combustion engine. The pressure drops were between 2.43-3.78 mmHg at the gasifier outlet and 2.84-3.88 mmHg at the water scrubber outlet and between 2.99-3.93 mmHg at the filter box outlet. Total pressure drop of the whole gasification system was additionally determined and found to be between 7.96 and 11.48 mmHg.
Figure 2 shows the variations of hydrogen gas flow rates and combustible gas mixture versus the flow rate of produced dry gas. The produced gas (or humid producer gas) contains the combustible and non-combustible gases, moisture, tar and dust. The produced dry gas has around 40% of combustible gases with an average 3.66 to 5.44 MJ/m3 of gross calorific value. Gas analyses showed that the produced dry gases comprised of H2, CO, N2, CO2, CH4, C2H4 and C3H4 and the combustible fraction of these are H2, CO, CH4, C2H4 and C3H4.
The flow rate of produced gas (kg/h)The flow rate of hydrogen gas (kg/h)The flow rate of combustible gas (kg/h)00.511.522.533.5400.020.040.060.080.10.12345678910111213hydrogen gascombustible gas
Figure 2: The Variations of the flow rate of hydrogen gas and combustible gas versus the flow rate of produced dry gas
As shown in Fig. 2, the flow rates of the produced dry gas were between 3.75-11.34 kg/h. However, the amounts of combustible fraction of the producer gas changed between 0.417-2.486
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kg/h. Flow rates of hydrogen and combustible gas increased almost linearly up til 8.58 kg/h of the flow rate of total produced dry gas. It can be said that, by increasing of the produced dry gas, the composition of combustible gas contains less hydrogen gas and however more CH4, CO, C2H4 and C3H4 gases after reaching 8.58 kg/h of the produced dry gas.
Figure 3 shows the variations of the flow rate of hydrogen gas and the temperature at oxidation zone versus air fuel ratios. As shown in Fig. 3, the air to fuel ratios were adjusted between 1.37-1.64 m3/kg. The flow rate of hydrogen gas and the temperatures of oxidation zone decreased with an increasing trend of air to fuel ratio arising from the more air entering the reactor bed than feed rate of wet hazelnut shells. Accordingly, it was observed that hydrogen gas was produced in between 1.44 and 1.52 m3/kg of air fuel ratios by applying the air-blown downdraft gasification technique. As one might be expected, the production of hydrogen gas went down with the increase of air fuel ratio. This can be explained as above the sub-stoichiometric air addition into the gasifier caused excessive char burning in the oxidation zone resulting combustion rather than gasification. Temperature decrease at the same time is the evidence of this movement from gasification towards combustion. Therefore it is concluded that lower air to fuel ratios are found to be increasing effect on hydrogen gas production. Additionally, the limits of ash (0.015-0.059 kg/h), tar (0.0058-0.0104 kg/h after scrubbing) and condensate (0.05-0.11 kg/h after scrubbing) should be also taken into consideration to investigate the regular production of hydrogen production based on air fuel ratios. Air/fuel (m3/kg)The flow rate of hydrogen gas (kg/h)Temperature of oxidation zone (oC)020040060080010001200140000.020.040.060.080.10.120.140.161.341.381.421.461.51.541.581.621.66hydrogen gastemperature
Figure 3: Variations of the flow rate of hydrogen gas and temperature of oxidation zone versus air fuel ratio
Figure 4 illustrates the variation of the flow rate of hydrogen gas and hydrogen gas percentage in producer gas versus the feed rate of wet hazelnut shell. As shown in Fig. 4, the hydrogen gas percentages in the produced dry gas changed between 11-15%. The flow rate of hydrogen gas increased with the feed rate of wet hazelnut shells and however, hydrogen gas percentages in the produced dry gas decreased. It is clear that the other combustible gases: CO, CH4, C2H4 and C3H4 were more produced at the conditions where hydrogen gas percentage was lower than the others. This can only be explained with water-gas shift reaction (CO2 + H2 = CO + H2O ΔH298K = -41.2kJ/mol). At high flow-rates which caused temperature to decrease in the gasifier and reaction shifted to right producing more CO than H2. Highest H2 production is found at around 3kg/hr feed-rate. It can be consequently said that the hydrogen yield in the downdraft reactor is almost 2.4 kg/h of hydrogen from 100 kg of hazelnut shells and the percentage of hydrogen gas (between 11.11-14.77%) is the second highest among all combustible gases.
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Feed rate of wet hazelnut shell (kg/h)The flow rate of hydrogen gas (kg/h)Percentage of hydrogen gas in produced gas (%)1011121314151600.020.040.060.080.10.12123456percentage of hydrogen gasflow rate of hydrogen
Figure 4. Variations of the flow rate of hydrogen gas and the percentage of hydrogen gas in produced gas versus the feed rate of wet hazelnut shell
Table 1 presents the conversion ratios of hazelnut shells to hydrogen gas via air-blown-downdraft gasification technique. In Table 1, Hpdg defines the hydrogen gas percentage in produced dry gas; Hhs, hydrogen gas from hazelnut shells; Hpr, hydrogen gas production ratio; Gpdghs, produced dry gas from hazelnut shells.
Table 1. Conversion ratios of hazelnut shells into hydrogen
Run
Hpdg
(%)
Hhs
(kg/h)
Hpr
(%)
Gpdghs
(kg/h)
1
14.77
0.042
2.42
3.75
2
14.62
0.051
2.37
4.56
3
14.77
0.061
2.31
5.40
4
14.77
0.067
2.53
5.85
5
14.12
0.081
2.19
7.38
6
12.67
0.081
1.99
8.36
7
13.13
0.085
2.11
8.33
8
11.86
0.083
1.85
9.27
9
13.83
0.097
2.06
9.06
10
11.33
0.087
1.76
10.25
11
11.11
0.094
1.74
11.34
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Dogru, Akay, Midilli, Calkan and Howarth
The production percentage of hydrogen gas via gasification from hazelnut shells was estimated between 1.74-2.53% of wet hazelnut shells.
Total hydrogen gas production from 250 thousand tons per annum of hazelnut shells in Turkey, can be approximately estimated for each year and is shown in Table 2.
Table 2. Annually average hydrogen production from hazelnut shells using downdraft air gasification technique
Hazelnut shell production
250000 ton
The production of dry producer gas
525000 ton
The production of combustible gas
131250 ton
The production of hydrogen gas
6000 ton
The production of hydrogen energy
675000 GJ
Power generation using hydrogen
21.4 MWe
Annual production of wet hazelnut shells in Turkey is 250000 ton. From Table 2, following relations are found:
• the average ratio of the produced gas to biomass is 2.1 (kg dry gas/kg wet hazelnut shell),
• the average production ratio of combustible gas to biomass is 0.525 (kg combustible gas/kg wet hazelnut shell),
• the average production ratio of hydrogen gas to biomass is 0.024 (kg hydrogen gas/kg wet hazelnut shell),
• the average production ratio of hydrogen energy is 2.7 MJ/kg,
4. CONCLUSION
Hazelnut shells are the most common and widely available biomass feedstock to recover energy in middle and east black-sea region of Turkey. If all hazelnut shells were utilized in air-blown-downdraft gasifier, the production of hydrogen gas could be almost 6000 ton per annum. Thus, this amount of hydrogen gas would represent about 675000 GJ of energy output. In this work, the conversion possibility of hazelnut shells to hydrogen gas was experimentally investigated using a pilot scale downdraft gasifier. The following conclusion may be basically drawn:
• The production of hydrogen gas could be efficiently produced from a downdraft air gasifier between 1.44 and 1.50 m3/kg of air to fuel ratios. The hydrogen yield obtained in the downdraft reactor was almost 2.4 kg/h of hydrogen from 100 kg of wet hazelnut shells.
Consequently, hazelnut shells can make an important contribution concerning energy to the economy of the region where nuts shell residues are abundant, and even they can be used as a renewable material for hydrogen production. However, the authors emphasize that annually
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hydrogen production capacity from hazelnut shells may be conducted as 6000 tones in Turkey. Using this amount of hydrogen, one can say that it is possible to run 1000 of today's prototype hydrogen-fuelled cars to travel 32500 kilometres each annually.
5. ACKNOWLEDGEMENTS
We are grateful to the UK Engineering and Physical Sciences Research Council (EPSRC), UK and MEB, Turkey for their support.
REFERENCES
1. P.H. Wallman, C.B. Thorsness, J.D. Winter, Hydrogen production from wastes, Energy 23(4), 271-278 (1998).
2. A. Midilli, H. Olgun, P. Rzayev and T. Ayhan, Solar hydrogen production from hazelnut shells, Int J Hydrogen Energy 25, 723-732 (2000).
3. T.N. Veziroglu, F. Barbir, Hydrogen: the wonder fuel, Int J Hydrogen Energy 17, 391-404 (1992).
4. A. Demirbas, M.M. Kucuk, Kinetic study on the pyrolysis of hazelnut shell. Cellulose Chemistry and Technology 28, 85-94 (1994).
5. A. Midilli, M. Dogru, C.R. Howarth, T. Ayhan, Hydrogen production from hazelnut shell using air-blown downdraft gasification technique. Int J Hydrogen Energy 26, 29-37 (2001).
6. Akay G. and Dogru M. “Process Intensification and Miniaturization in Biological, Chemical, Environmental and Energy Conversion Technologies”, Akay G. and Dogru M. (eds.), Docqwise publishers, ISBN 0-9545956-0-2, United Kingdom, 2003.
7. A. Midilli, M. Dogru, G. Akay, Hydrogen Energy and Applications in This Millennium, Proceedings of The 4th National Clean Energy Symposium 2, 733-745 (2002), Istanbul, Turkey.
8. M. Dogru, C.R. Howarth, G. Akay, B. Keskinler, A.A. Malik, Gasification of hazelnut shells in a downdraft gasifier, Energy 27(5), 415-427 (2002).
9. M. Dogru, Fixed-bed Gasification of Biomass, PhD Thesis, University of Newcastle, United Kingdom, 2000.
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