ARTICLE pubs.acs.org/EF
Gasification of Fuel Cane Bagasse in a Downdraft Gasifier: Influence of Lignocellulosic Composition and Fuel Particle Size on Syngas Composition and Yield Galip Akay*,†,‡ and C. Andrea Jordan† †
Process Intensification and Miniaturization Centre, School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom ‡ ITI Energy Limited, Advanced Manufacturing Park, Brunel Way, Rotherham S60 5WG, United Kingdom ABSTRACT: Gasification of fuel cane bagasse briquettes and pellets was carried out in a novel 50 kWe downdraft autothermal gasifier to evaluate the most suitable densified form for use in downdraft gasifiers. The results showed that the calorific value of the syngas produced from the briquettes was low, ranging from 1.95 to 3.12 MJ/m3n, and ballooning of the briquettes accompanied by bridging of the fuel repeatedly occurred in the pyrolysis and throat zones. In contrast, gasification of pelletized bagasse under similar operating conditions produced syngas ranging in calorific value from 4.90 to 5.94 MJ/m3n, the syngas yield increased by 65%, and bridging was non-existent. Investigation of the lignocellulose composition of the bagasse found that this high-fiber cane has a high lignin/cellulose ratio with a typical lignin content of 33 ( 4%. Scanning electron microscopy studies of the partially pyrolyzed and expanded briquettes showed that extensive porous structures had developed during pyrolysis. Because cellulose devolatilizes at a faster rate than lignin, this suggested that, during pyrolysis, the rapid production of volatile gases within the briquettes resulted in ballooning and cracking of these structures. This increase in volume coupled with the reduced mass of the briquette resulted in reduced bulk density, which restricted the capacity of the fuel bed to flow under gravity and ultimately led to bridging in the reactor, poor syngas composition, and low yield.
1. INTRODUCTION A rapidly increasing global demand for primary energy coupled with declining fossil fuel reserves has fueled a resurgence in the development of economically viable sources of renewable energy. Biomass, which currently provides 10% of global primary energy demand, is the sole form of renewable energy that can replace fossil fuels in all energy markets for the production of heat, electricity, and fuels for transport.1 One biomass under investigation is fuel cane bagasse (FCB), the residue from a new energy crop known as “Fuel Cane”, which was developed in Barbados for use in electricity generation. The term “Fuel Cane” represents a group of high-fiber canes produced by genetic hybridization of wild Saccharum spontaneum clones and commercial clones of sugar cane. Several thermochemical conversion technologies can be used for the production of energy from biomass. However, gasification is recognized as the most appropriate option because it offers higher efficiencies compared to combustion or pyrolysis.2 Turn et al.3 reported that, unlike combustion, which captures less than 30% of the energy in sugar cane, gasification can capture between 75 and 80% of the energy in this biomass. Gasification is defined as the thermochemical conversion of a solid organic material into a combustible gas by partial oxidation at temperatures greater than 500 °C. The gasifying agent can be air, oxygen, steam, or carbon dioxide, and the gas produced is known as product gas, producer gas, or syngas. It consists mainly of hydrogen, carbon monoxide, methane, carbon dioxide, and nitrogen.3 During gasification, devolatilization, the first step in thermochemical conversion of biomass, occurs, whereby lignin, r 2011 American Chemical Society
cellulose, and hemicelluloses, which are the primary components of biomass, undergo depolymerization to low-molecular-weight volatiles and gases.4 Lignin is a highly cross-linked polymer of methoxy- and phenoxy-substituted phenyl propane units,5 whereas cellulose is a complex polymer of glucose and hemicelluloses are complex polymers of various sugar units and exhibit extensive chain branching. According to Gani and Naruse6 and Lv et al.,7 owing to the extensive occurrence of phenyl rings in lignin, although devolatilization begins to occur around 235 °C, it is extremely slow and continues even at 900 °C, whereas pyrolysis of cellulose occurs rapidly and is completed at 400 °C. The rate of devolatilization is dependent upon the rate of heating and particle size. In a downdraft throated gasifier operating on low bulk density fuel, such as FCB, densified fuel particles are required as the fuel moves through the gasifier under the effect of gravity because there are few if any internal moving parts. It is necessary, therefore, that the densified particles be sized to facilitate free flow through the narrow throat and also to permit heating at an optimum rate for rapid depolymerization of lignin and cellulose. Gasification is a series of concurrent and parallel reactions, and therefore, the rate of devolatilization impacts the thermochemical processes occurring in the gasifier and can significantly affect the yield and composition of the syngas.8,9 Researchers at Newcastle University have studied the gasification of a variety of biomass types in small pilot-scale gasifiers Received: November 4, 2010 Revised: April 2, 2011 Published: April 11, 2011 2274
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Figure 1. Newcastle University 50 kWe downdraft gasifier system.
(50 kWe),1013 which were subsequently scaled up to industrial systems (1 MWe).1416 However, the effect of biomass composition and fuel structure were not investigated in these studies. Although there are several laboratory-scale studies on the differences in the devolatilization rate of lignin and cellulose during pyrolysis and their effect on the products of pyrolysis, data on the impact of devolatilization of lignocellulose on syngas production, composition, and carbon conversion efficiency during gasification in small-scale industrial gasification systems (800 °C, because of the occurrence of pyrolytic gasification. An average superficial velocity of 1.1 ( 0.3 m/s was observed during gasification, showing that pyrolysis should occur rapidly in this gasifier. It can, therefore, be expected that, in high lignin content fuels undergoing thermal conversion in this gasifier, the larger the densified particle, the slower the rate of devolatilization of lignin because of the low temperatures near the core of the fuel particles. A specific gasification rate expresses the rate of fuel consumption per unit reactor area. Tiangco et al.38 observed that, in downdraft gasifiers, cold gas efficiency increased as the specific gasification rate increased up to 200 kg m2 h1 and, subsequently, declined with a further increase in the gasification rate. Similarly, Hsi et al.39 found when characterizing an air-blown downdraft gasifier that the LHV of the syngas produced 2281
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Table 3. Comparison of Composition of Syngas from Gasification of Briquetted Bagasse, Briquetted þ Fibrous Bagasse, and Pelletized Bagasse briquetted þ fibrous bagassea
briquetted bagassea syngas composition (mol %)
a
pelletized bagassea
A1
A2
A3
B1
B2
B3
C1
C2
C3 13.12
H2
8.91
6.58
6.30
9.89
9.06
10.84
10.17
12.18
CO2
14.38
13.69
15.76
10.31
10.50
10.77
15.12
15.8
11.31
O2 CH4
3.79 1.83
1.11 0.00
0.85 1.05
3.06 2.30
3.00 2.56
3.95 2.66
4.10 2.83
0.91 3.42
1.21 3.54
CO
10.88
13.10
10.75
11.90
13.57
12.78
16.33
17.2
17.3
LHV (MJ/m3n)
3.10
2.45
2.50
3.52
3.74
3.88
4.90
5.60
5.94
Moisture content = 9 wt %.
increased with an increasing specific gasification rate up to ∼268 kg m2 h1. The specific gasification rates obtained varied from 86.6 to 301.9 kg m2 h1. A specific gasification rate of 150 ( 7 kg m2 h1 was obtained in this study, which shows that, under the observed gasification conditions, a low rate of fuel conversion to syngas occurred during gasification. This is further substantiated by the low cold gas efficiency values obtained during gasification. Given that the briquette size was within the upper limit recommended for the throat diameter of this gasifier, rapid pyrolysis followed by high calorific value syngas production and low char and ash production should have occurred during gasification of the briquettes. A comparison of the mean syngas composition generated during selected experiments with briquetted bagasse, briquetted and fibrous bagasse, and the pelletized bagasse is outlined in Table 3.
4. CONCLUSION Gasification of the three densified forms of FCB in this preliminary study indicates that selection of the most appropriate fuel particle size for gasification must also be informed by the relative amount of lignin to cellulose in the biomass. The formation of a highly reactive char as evidenced by the SEM micrographs, the increase in syngas yield and calorific value, and the absence of bridging in the reactor with use of the pelletized bagasse provide empirical evidence of the role of lignocellulose in the overall rate of devolatilization of fuel particles. The higher the ratio of lignin/ cellulose, the slower the overall rate of devolatilization; therefore, smaller densified fuel particle sizes are necessary for optimization of syngas production and composition in small-scale fixed-bed gasifiers. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ44(0)-191-222-7276. Fax: þ44(0)-191-222-5292. E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge that this work was supported by the U.K. Engineering and Physical Sciences Research Council (EPSRC), EU FP7 Integrated Project (COPIRIDE). Andrea Jordan was supported for her Ph.D. studies by a National Development Scholarship from the Government of Barbados and a research grant from the Barbados Light and Power Company Limited,
which also supplied FCB for the experiments. We are grateful for all of the support received.
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