Processing Rice Straw and Pennisetum by Microwave-Induced Torrefaction
Date Issued
2011
Date
2011
Author(s)
Chen, Wei-Rong
Abstract
Torrefaction is a mild pyrolysis process which used to improve the properties of the biomass. Microwave-induced torrefaction can minimize the disadvantages of conventional torrefaction methods such as low heating rate, long processing time and severe temperature gradient between central and surface of biomass. Microwave irradiation is single-mode from the industrial microwave device. Two agricultural residues, rice straw and pennisetum were torrefied under different microwave power in nitrogen ambient. In this study, microwave power level was set between 150W~350W. Different operating conditions, including microwave power, processing time and temperature, were undertaken to examine optimization of the process for the biomass. After torrefaction, solid product would be investigated its caloric value, O/C ratio, proximate analysis, elemental analysis, mass reduction, energy distribution of three phase products and energy yield.
The caloric value of solid product was enhanced 30~34% in the condition that rice straw was torrefied under 300 W for 25 minutes and 350 W for 10~15 minutes. With regards to pennisetum the caloric value was enhanced 55~56% in the condition that pennisetum was torrefied under 300 W for 25 minutes and 350 W for 10 minutes. In Comparison with conventional torrefaction, microwave-induced torrefaction could shorten processing time significantly, and make reaction go completely within 25 minutes.
The energy yield of rice straw and pennisetum which were torrefied under 150W for 10 minuntes could reach 84.9% and 79.6% respectively. When the biomass was heated to 250℃ and 300℃ under 350 W, the results indicated that mass yield and energy yield of rice straw and pennisetum under 250℃ were similar to those under 300℃. The mass yield and energy yield of rice straw at 250℃ was 43.7% and 52.9% separately, and at 300℃was 42.9% and 54.7% separately；pennisetum at 250℃ was 42.9% and 59.8% separately, and at 300℃ was 41.6% and 56.9% separately. Therefore, temperature had less correlation to mass yield and energy yield under high level microwave power. By contract, processing time and microwave power were the key parameters of the reaction.
Elemental analysis showed that carbon content of solid product increased with the increase of microwave power. However, oxygen content decreased with the increase of microwave power and processing time, and hydrogen content had slight decrease. Overall, O/C ratio and H/C ratio decreased with the increase of microwave power and processing time. The size of torrefied char was smaller and more uniform with the increase of microwave power.
The majority of gas products were CH4, CO2, CO and H2, with a few C2H6 and C3H8. Under the 150 W-10 min operating condition, The percentage of CO content of rice straw and pennisetum were 54.91% and 43.07%, and H2 19.59% and 28.31%. Comparing with conventional method, microwave -induced method could reduce greenhouse gas CO2. The major compounds in liquid products were Phenol、Benzofuran, 2,3-dihydro-、Phenol, 4-ethyl- and Phenol, 4-methyl-.
In conclusion, the heating rate of microwave-induced torrefaction was faster than conventional torrefaction and could easily reach high temperature. This method could save time and energy. The solid products provided stability and hydrophobic characteristic for longer storage, and had a tendency to increase the energy density of solid products.
Subjects
microwave-induced torrefaction
agricultural residues
torrefied char calorific value
energy yield
Type
thesis
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