A Novel Biomass Supported Na2CO3 System for Flue Gas Desulfurization

Chinese Chemical Letters Vol. 13, No.9, pp 903 - 906, 2002903 .http://www .imm.ac.cn/journal/ccl/htmlA Novel Biomass Supported Na2CO3 System for Flue GasDesulfurizationHong Shan SHANG, Yuan KOU*, Fan YANGCollege of Chemistry & Molecular Engineering, Peking University, Beijing 100871.Abstract: The breakthrough and stoichiometric SO2 adsorption efficiencies of a biomass supportedNa,CO3 system (80 wt %Na,CO3/straw) have reached 48.9 % and 80.6 % respectively at adesulfurization temperature of 80°C.Keywords: Straw, adsorption fficiency, SO2.Studies on recyclable solid sorbents'b in dry desulfurization processes have been thesubject of much recent attention. For example, in the NOXSO and Powder-ParticleFluidized Bed (PPFB) processest-, Na2CO/Y-Al2O3 has been employed as a regenerativesorbent for SO2 in order to realize the subsequent transformation of SO2 at operating hightemperatures. However, the outlook for actual application of the processes in currentlyoperating power stations is not promising due mainly to the complex processes involvedhigh cost of the sorbents and high energy consumption. Therefore, a cheap, easilyobtainable and disposable sorbent seems to be a solution for the development of a highlyefficient, industrially acceptable desulfurization system. Utilization of biomaterials orabandoned biomaterials (BIOM), of which the major component is cellulose (C&HoO3)n,is a creative new approach to the problem7. The BIOM includes straw, rice husks andstallks, dried branches and leaves, waste papers and similar materials.The wheat straw employed as the BIOM in this work was composed of 40.50 % C,5.58 % H and 45.26 % 0 based on elemental analysis (Elementar Vario EL, Germany).The typical preparation procedure for an 80 wt % Na,CO/straw mixture was shown inliterature'The reference sorbent Na2CO3/y-Al2O3 was prepared according to thliterature?.The SO2 removal experiment was carried out under atmosphere pressure at70~300°C, using a fixed-bed quartz reactor, in which the sorbent (0.33 g) was supportedon a quartz frit of medium porosity. The flow rate was 40 mL/min. The simulatedflue gas contained 1960 ppm SO2 with N2 as the balance gas. An FT-IR spectrometer(Vector 22, Bruker) with an on-line cell was used to monitor the SO2 concentrations*E-mail: yuankou@ pku.edu.cn中国煤化工MHCNMH G.904Hong Shan SHANG et al.before and after the reactor. The adsorption efficiency η (breakthrough/stoichiometric&9/saturation) of the sorbent is given by η= f,/ f。X 100 %, where f。is the amount in molsof Na2CO3 in the sorbent and f, is the amount in mols ofSO2 adsorbed by the sorbent. The Na/S ratio is given by r= 2 f。1 f，and istheoretically equal to 2.Figure 1 Desulfurization curves of sorbents at a temperature of 80°C2500200015001000500C5101520Time/hours(a) Na,CO,， (b) 80 wt % Na2CO/y-Al2O3, (c) 20 wt % Na2CO3 /1γ-Al2O3, (d) 80 wt %Na,CO,/straw. The adsorption efficiencies (A) for curve d have shown in the Figure, in which,ABreak. = Breakthrough efficiency, Asmie. = Stoichiometric efficiency, Asau = Saturation efficiency.Table 1 Result of several sorbents on removal of SO2 at temperature 80°CBreakthroughSaturationStoichiometricEfficiencyTimeNa/SEffciencymol %hours_mol %8.623.27.60.9024.28.2624.08.3437.21.102.12.172.6748.95.805.12.3580.6Figure 1 and Table 1 show a comparison of Na2CO,/straw, Na2CO3/Y-Al2O3 andpure Na2CO3 for removal of SO2 at a temperature of 80°C. The breakthrough andstoichiometric adsorption efficiencies of 80 wt % Na2CO,/straw are 48.9 % and 80.6 %,whilst that of 80 wt % Na,CO}/Y-Al2O3 are 7.6 % and 24.0 %, respectively. The activityof the 80 wt % Na2CO;/straw is also significantly higher than 20 wt % Na2CO3/Y-Al2O,which is a typical sorbent employed in PPFB processes2. As shown in Table 1, thebreakthrough adsorption efficiency of 80 wt % Na2CO3/straw is 11.7 percent higher thanthe 20 wt % Na2CO3/Y-Al2O3, whilst the stoichiometric efficiency is 5.7 percent higher.In adition, with respect to saturation of the sorbents, the efficiency of 80 wt %Na2CO,/straw is significantly higher than that of the 80 wt % Na2CO/Y-Al2O3, and is .almost equal to 20 wt % Na2CO/Y-AI2O3It is noteworthy that the higher saturationeffciency of 20 wt % Na2CO/Y-Al2O3 compared with that of中国煤化工TYHCNMH G.A Novel Biomass Supported Na2CO3 System for Flue Gas Desulfurization 905mainly due to the adsorption properties of the γ-Al2O3 support2- 10.11. In brief, the datashows that the adsorption efficiency and Na/S of breakthrough and stoichiometricadsorption of the 80 wt % Na2CO3/straw are the best of the samples investigated. XPSanalysis demonstrated that the main product in each is Na2SO3.Figure2 Effect of termperature on SO2 removal for sorbent 80 wt %Na2CO/straw.1008(6(4C50 100 150 200 250 300 350Temperature/CBreakthrough efficiency，Stoichiometric efficiencyFigure 3 Curves of TGA and DTA of wheat straw.1300.0t 10-0.5 t←.0 |→g -1.5-2.0↓20200400 600800 1000Temperature/9CFigure 2 shows the effect of temperature on SO2 removal for 80 wt % Na2CO/straw.It can be seen that the breakthrough adsorption efficiency changes between 45.1 % and48.9 % whilst the stoichiometric adsorption efficiency changes between 76.9 % and 82.1% as the temperature increases from 70 to 300°C.In order to further investigate the properties of the straw, TGA and DTA werecarried out. The results are shown in Figure 3. The TGA weight loss curve showsthat no chemical change occurs below 200°C. When the temperature rises above 200°C,the straw starts to carbonize and two stages of weight loss appear in the regions 250 ~中国煤化工MHCNMH G.906Hong Shan SHANG et al.350°C and 350 ~ 650°C. The DTA curve shows two distinct peaks between 273 ~ -407°C and 407 ~ 510°C respectively.Both are exothermic oxidation processes,suggesting that the straw is a potential reducing agent in the temperature range 200 ~510°C.It should be noted that the reduction properties of cellulose-based materials havebeen reported previously. Examples include reduction of NO, bivalent copper andhexavalent chromium by cellulose- based BIOM12-14. Based on previous work and thisstudy, a more efficient and complete flue gas desulfurization (FGD) process can beproposed. The key steps in the proposed process are the enrichment of SO2 and thesubsequent reduction of SO2 toS. The reactions are described schematically below:Na,CO/BIOM + SO2- > Na2SO/BIOM + CO2CHoO3+ 6SO2-→ 6S + 6CO2+ 5H,OIt can be seen from the above reactions that SO2 is first enriched by the BIOMsystem and then selectively reduced to elemental S by BIOM under oxygen freeconditions. The Na2CO3 is capable of being recycled. In that case, suitable catalystwill be needed, and further studies are being carried out in this laboratory.In conclusion, the sorbent Na2CO3 supported on straw can efficiently adsorb SO2 at80C, and the desulfurization process can be operated over a large temperature range.In addition, the straw is a possible promising reducing agent for SO2 reduction.References1.W. T. Ma, J. L. Hasbeck, Environ. Prog.. 1993, 12, 163.2.G. Xu, G. Luo, H. Akamatsu, K. Kato, Ind. Eng. Chem. Res, 2000, 39, 2190.3.0. C. Snip, M. Wood, R. Korbee, J. C. Schouten, Chem. Eng. Sci, 1996, 51, 2021.4.S. G. Deng, Y. s. Lin, Ind. Eng. Chem. Res, 1996, 35, 1429-1437.5.J. H. A. Kiel, w. Prins, Appl. 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