Bio-Oil Production From Lignocellulosic Biomass Using Fast Pyrolysis in A Fluidized-Bed Reactor
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Date
2015-05-22
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Abstract
This study focused on bio-oil production from lignocellulosic biomass using fast pyrolysis in a fluidized-bed reactor as a substitute or replacement energy source to reduce the dependence on imported and expensive fossil fuels for transportation and stationary engines applications. Seven major lignocellulosic biomass, mostly agricultural crop residues (i.e. corn cobs and its straw, rice straw and its husk, sugarcane bagasse and cocoa pod husk) and wood processing residue (Triplochiton scleroxylon sawdust) were characterized using proximate, ultimate, chemical and x-ray fluorescence analyses. Fast pyrolysis was carried out in a pilot-scale fluidized-bed reactor to investigate the effect of temperature on product distribution, i.e. bio-oil, char and non-condensable gas yields. This was followed by characterization of the bio-oils obtained for fuel properties using physical and ultimate analyses. Based on the results obtained, a scheme was proposed for bio-oil production. The proximate analysis gave moisture contents (MCs) of 9.15 and 10.29 %wt. respectively for the corn stalk and cocoa pod husk and high volatile matter contents of 79.48, 80.48 and 80.72 %wt. respectively for the sugarcane bagasse, corn cobs and wawa sawdust. The XRF analysis gave high silica contents of 16.30 and 22.38%wt. for the rice husk and its straw respectively. Ultimate analysis gave carbon contents of 42.65, 43.32 and 43.87 %wt. respectively for the corn stalk and its cobs as well as cocoa pod husk and oxygen contents of 59.00 and 66.57 %wt. respectively for the rice husk and its straw. Wawa sawdust, sugarcane bagasse and corn residues performed well in both the proximate and ultimate analyses, giving carbon contents of 44. 0 %wt and volatile content of 75.0 – 80.74 %wt and fixed carbon content of 7.60 – 12.01 %wt., while wawa sawdust, sugarcane bagasse and cocoa pod husk gave the lowest ash contents during the XRF analysis. Sulphur levels were, however, below 1.0 %wt. for all the biomass samples, while nitrogen levels were 1.49 and 2.23 %wt. respectively for the corn stalk and cocoa pod husks. Chemical analysis gave cellulose contents of 43.82 and 44.40 %wt. respectively for the corn stalk and wawa sawdust, hemicellulose content of 46.59 %wt. for the corn cobs and high lignin content of 41.08 %wt. for the rice husk. The highest HHV of 18.61 MJkg-1 was recorded for the wawa sawdust. During the biomass fast pyrolysis, bio-oil yields increased from 53.0 to 60.0 %wt. for the rice straw as the temperature increased from 400 to 500 oC and then reduced to 50.3 %wt. at 700 oC. The non-condensable gas yields increased from 8.2 to 32.5 %wt. while bio-char yields decreased from 38.6 to 17.2 %wt. as the temperature increased from 400 to 700 oC. Similar trends in product distribution were obtained for other biomass samples. Maximum bio-oil yields ranged from 53.0 to 66.0 %wt. respectively at 550 oC for the cocoa pod husk and corn stalk. The high yield of bio-oil at high temperatures was due to primary
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reactions, such as decomposition of cellulose and hemicellulose which resulted in the bio-oil production. At temperatures above 500oC, chemical reactions involving mainly the decomposition of lignin and secondary reactions involving the breakdown of the bio-oils resulted in high yields of bio-char. Characterization of the bio-oils obtained gave a pH ranging from 2.1 to 3.2 respectively for the wawa sawdust and corn stalk bio-oils, compared to a pH of 5.6 for light petroleum diesel oil. The high pH of the bio-oils makes them highly acidic, and thus corrosive, which is a problem. Carbon contents were 41.70 and 65.68 %wt. respectively for bio-oils obtained from the rice husk and its straw, lower than those of fossil diesel fuels. The highest MC of 26.7 %wt. was recorded for the cocoa pod husk bio-oils compared to very low MCs in light petroleum and heavy fuel oils. The HHVs, which ranged from 16.80 to 23.30 MJkg-1 respectively for the corn cobs and sugarcane bagasse bio-oils were lower than those of light petroleum oil (40.0 MJkg-1) and heavy fuel oil (40.0 MJkg-1). These results indicate that the bio-oils obtained had lower fuel properties than fossil fuels. The low sulphur and nitrogen levels indicate, however, that low SO2 and NO2 would be expected to be emitted during combustion. The bio-oils can, however, be upgraded by hydrodeoxygenation which, reduces the oxygen content and thus the acidity. Of all the biomass samples characterized, the corn stalk, corn cob, wawa sawdust and sugarcane bagasse samples performed the best for bio-oil production due to their high carbon, cellulose and volatile matter contents as well as HHVs and produced the highest bio-oil yields. The cocoa pod husk and rice husk, however, performed the least due probably to high silica, low carbon and cellulose contents. Slight differences in maximum bio-oil yields obtained and trends of the other product yields may be due to differences in proximate, ultimate and chemical compositions of the biomass samples. This work would have significant economic and environmental benefits since it would reduce the dependence on imported and expensive fossil fuels for transportation and stationary engine applications. The proposed scheme could result in cost-effective bio-oil production at both the national and international levels at a low production cost since lignocellulosic biomass is cheap and fast pyrolysis is also a cheap process.
Description
Thesis Submitted to the Department of Wood Science and Technology, Kwame Nkrumah University Ofscience and Technology in Partial Fulfilment of the Requirements for
the Degree
of
DOCTOR OF PHILOSOPHY