Comparison of Cellulose Decomposition by Microwave Plasma and Radio Frequency Plasma ()
Biomass conversion by plasma has the advantage of mainly producing gaseous products, H2, CO and CO2. Though the thermal plasma has been used for this conversion, the plasma temperature is too high to be unfit for the conversion biomass. The temperature of cold plasma, however, is lower under 3000 K. It expects to be adequate for biomass conversion. Cold plasma can be obtained with irradiation microwave (2.45 GHz) or radio frequency (13.5 MHz) under reduce gas pressure. Therefore, in present study, the effective decomposition of cellulose by microwave plasma (MWP) and radio frequency plasma (RFP) is examined. The conversion of cellulose by MWP (XMWP) is higher than that by RFP (XRFP), irrespective of the reaction time. XMWP and XRFP reach 92.8 wt% at 10 min and 68.1 wt% at 30 min. The maximum yield of gaseous products (Ygas) by MWP is 85.1 wt% at 10 min, higher by 23.2 wt% than Ygas by RFP at 30 min. The amount of H2 and CO obtained by MWP is 18.0 mmol/g and 23.5 mmol/g, it is larger than that obtained by RFP. Comparing the relationship between conversion and yield, Ygas of MWP is slightly higher than that of RFP under X of 60 wt%, and both Ygas is almost same over 60 wt%. The amount of H2 and CO obtained by MWP is larger by 9.3 mmol/g and 9.6 mmol/g than that obtained by RFP. C, H and O element in cellulose is mainly distributed to H2 and CO by MWP. RFP mainly distributes H and O element to the other gases without H2 and CO. In addition, a large amount of C element is remains in the residue. Those results is found that MWP was more suitable for cellulose gasification than RFP, since MWP can highly convert C, H and O element to H2 and CO by higher energy of microwave frequency in comparison with radio frequency.
1. Introduction
Recently, renewable energy is noticed again following reasons: 1) soaring the price of petroleum; 2) the uncertain supply of petroleum; 3) a crisis at the Fukushima No. 1 nuclear power plant. Biomass is a renewable resource by the photosynthesis of H2O and CO2, and biomass widely exists distribution on the earth. The abundance of unused biomass is exists in the field of agriculture, forestry and livestock industry. If those resources convert to the energy resource, for example hydrogen for fuel cell, it will become an expecting technique.
Plasma is considered to be the 4th state of matter, it is gas including ion, electron and neutral particle. Plasma is roughly classified in thermal plasma and cold plasma. Because thermal plasma is equilibrium state of the temperature of electron, ion and neutral particle, the temperature of thermal plasma becomes as high as 3000 K ~ 10000 K. Therefore, plasma method has the advantage of the possible abundance production of H2 and CO from biomass, and gaseous products obtained by plasma contain low tar [1]. Katou et al. reported that the tar yield was low and H2, CO and CO2 were obtained when biomass was converted by DC arc plasma, which is thermal plasma [2]. But the temperature of thermal plasma is too high to be unfit for the energy efficient conversion of biomass.
Cold plasma is non-equilibrium state of the temperature of electron, ion and neutral particle. The electron temperature is only higher than ion and neutral particle temperature. Therefore, plasma temperature is under 3000 K, which might be adequate for the conversion of biomass to gaseous products. Tang et al. studied that the gasification of rice straw by RF plasma (RFP), which was obtained under 3 - 8 kPa at 1600 - 2000 W of power by the irradiation of radio frequency (13.56 MHz). In their results, the yield of gaseous products reached at 66% and H2, CO, CH4 and CO2 was mainly obtained [3].
It has been studied in our laboratory that the effective conversion of coal (Yallourn coal and Taiheiyo coal) [4-6] and biomass (cellulose, lignin, saw dust, sugar cane bagasse) [7-9] by microwave plasma (MWP) are performed under reduce pressure by microwave (2.45 GHz), generated by a for lower electric power (300W) than the above mentioned works. When cellulose was used as a raw material, the yield of gaseous products (Ygas) reached at 79 wt% and H2 and CO was mainly obtained [7]. Com-paring those results with that obtained at same temperature of MWP by conventional pyrolysis, Ygas obtained by MWP is higher by 67.7 wt% than that obtained by conventional pyrolysis. Mainly CO2 was obtained at 620 K, CO and CO2 were obtained in the range from 1000 K - 1050 K, and H2, CO and CO2 were obtained over 1100K by conventional pyrolysis [7].
Both cold plasma methods can give the high yield of gaseous products and produce mainly H2 and CO. The difference between microwave plasma and RF plasma is the frequency of electromagnetic wave for obtained plasma. As the difference of energy is depend on the frequency of electromagnetic, the plasma density, the electron state and the activated species might be different between MWP and RFP. It is considered that this affects the decomposition of biomass.
In present study, the decomposition of cellulose by MWP and RFP was compared under Ar at irradiation power of 300 W.
2. Method of Gasification by Plasma
2.1 Raw Material
Cellulose (Merck Co., 200 mesh under), which is main composition of biomass, was used as raw material. The proximate and elemental analysis of cellulose was summarized in Table 1. Cellulose was dried under argon at 110˚C for 3 hours.
2.2. Plasma Apparatus
2.2.1. Plasma Reactor with Radio Frequency
Figure 1(a) shows the radio frequency plasma appa
Table 1. Proximate and elemental analysis of cellulose.
ratus. A low-pressure flow reactor was used with a quartz glass reaction tube (12 mm of internal diameter and 600 mm of length). Cellulose was packed in the middle of the vertical reaction tube. The radio frequency (RF) was introduced at 13.56 GHz from two ring copper electrode to produce a plasma in the reaction tube. The space of between two electrodes was 17 mm.
2.2.2. Plasma Reactor with Microwave
Figure 1(b) shows the microwave plasma apparatus. A low-pressure flow reactor was used with a quartz glass reaction tube (25 mm of internal diameter and 800 mm of length). Cellulose was packed in the middle of the vertical reaction tube. The microwaves (MW) were introduced at 2.45 GHz from the side of the tube via a wave guide to produce a plasma in the reaction tube.