BIOMASS PRETREATMENT FOR GASIFICATION. Keywords: Biomass pretreatment, gasification, alkali and alkaline metals removal

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1 Presented at the 8 th International Symposium on Gas Cleaning at High Temperatures August 23-25, Taiyuan, Shanxi, China BIOMASS PRETREATMENT FOR GASIFICATION Hong Cui, Scott Q Turn, Thai Tran Hawaii Natural Energy Institute, University of Hawaii at Manoa 1680 East-West Rd, POST 109, Honolulu, HI ABSTRACT: Fresh biomass can be used as fuel to reduce net CO 2 emissions, but may require processing to improve fuel qualities prior to use in thermal conversion facilities. Processing fresh banagrass, a fast-growing tropical plant, by mechanically removing juice and water-soluble elements using a screw press is explored. Treatments were found effective in reducing ash content including alkali and alkaline earth metals, as well as chlorine. Fuel analysis indicated that the treated banagrass had lower ash content, improved heating value, higher ash deformation temperatures, and higher volatile matter to fixed carbon ratios, than the untreated material. The elemental composition of the residual liquids from the treatment process was determined. From this preliminary analysis, the liquids appear suitable for irrigation or use as fertilizer. Keywords: Biomass pretreatment, gasification, alkali and alkaline metals removal INTRODUCTION The production of fuels, power, and chemicals from biomass is of increasing importance in addressing issues of global warming and sustainability. Thermochemical conversion systems will play an important role in large scale implementation of biomass based systems both as a primary platform using biomass directly and as a secondary conversion system to handle residue streams from biochemical processes. Fuel properties of biomass materials vary and ash content and composition are of particular importance to thermochemical conversion systems. Most minerals that exist naturally in biomass are the same as those in coal but with varied proportions. Alkali and alkaline earth metals in biomass plants, principally potassium (K), sodium (Na) and calcium (Ca), are commonly higher in concentration than that in coal. These elements can vaporize at peak temperatures typical of thermal conversion facilities and react with other inorganic constituents, silica (Si), sulfur (S), and chlorine (Cl). Downstream heat recovery reduces product temperatures, resulting in condensation of inorganic compounds that cause deposits, fouling, and corrosion [1-4]. In the case of gasification, syngas generated from biomass without effective gas conditioning can produce adverse effects on downstream system components, such as gas turbines, solid oxide fuel cells (SOFC) [5], or catalysts for fuel or chemical synthesis. According to current gas turbine specification [6, 7, 8], alkali metals (Na and K) are limited to concentrations of 0.1 ppm or 50 ppb in the fuel gas for the protection of gas turbine hardware. More strict tolerance was reported and cited as <10 ppb for the alkali and HCl concentration when syngas was used for liquid fuels production by Fischer-Tropsch synthesis [9]. Methods to control elements of concern in thermochemical systems include fuel pre-processing, removing harmful elements prior to fuel utilization. For herbaceous fuels, mechanical dewatering and washing or leaching by water [3,10] may be used. This technique was used in previous lab scale tests with small samples [3] and a batch-process pressing system to treat banagrass (Pennisetum purpureum), a fast growing tropical grass. Results found it to be effective as a way to reduce ash content and remove most water soluble metals including alkali and alkaline earth metals from the fuel. The treatment of banagrass, for example, resulted in reductions of ash (45%), potassium (90%), chlorine (98%), sulfur (55%), sodium (68%), phosphorus (72%), and magnesium 1

2 (68%). The washing or leaching with water also proved effective in removing these elements from wheat straw and rice straw [10]. A screw press provides a continuous process for mechanically dewater fresh biomass that can often have moisture contents as high as 75% wet basis. Juices expressed from the fresh plant material include both intra- and extra-cellular fluids and water-soluble elements. The resulting solid material can be rehydrated with water to attain additional element removal. This paper explores fuel treatment processes and investigates the effects of screw press operating parameters on element partitioning between process streams. Tests were performed using banagrass, a fast-growing tropical grass with potential for use as a dedicated biomass feedstock. EXPERIMENTAL Banagrass was harvested at the University of Hawaii's, Poamoho Research Station, in Waialua, Oahu, in February, For each treatment, approximately 15 kg of fresh banagrass were shredded to ~6 mm particles. A compact screw press (Model CP-4, Vincent Corporation, Tampa, FL, USA) was used to express liquids from the chopped material. The screw press operates at a constant rpm and is equipped with a pneumatically loaded cone that applies back pressure to the screw discharge. Pressure in the pneumatic cylinder can be adjusted from 0 to 6 atm, however practical upper limits depend on the characteristics of the material being processed. Four treatments were conducted with different back pressure settings on the screw discharge. The four treatments were designated as P0, P20, P40 and P60, corresponding to the compressed air inlet pressures on the pneumatic cylinder of 0, 1.4, 2.8, and 4.2 atm (0, 20, 40, and 60 psig), respectively. The press generated liquid and solid process streams and both were collected and samples removed for analysis. After an initial pressing, the material was leached. The pressed solids were loaded in a bucket with walls made from 100 mesh screen and immersed in a barrel filled with enough tap water to submerge all of the material, a fuel to water ratio of ~6.5. Leaching and draining free leachate from the material was completed in 5 min. Previous work found that longer contact times did not result in improved leaching efficiency [3]. Once drained of free leachate, the leached solid samples were subjected to a second dewatering treatment with the screw press. For each treatment, the initial and final screw press processes were conducted with the same pressure applied to the screw discharge. The mass of all input and output process streams to the pressing and leaching steps were weighed and recorded so that yields and mass balances could be calculated. The processing and generated samples were schematically shown in Fig. 1. Both solid and liquid samples were analyzed to obtain information on fuel characteristics and process efficiency. Solid samples were subjected to elemental and fuel analysis by Hazen Research, Inc. (Golden, Co) Liquid samples were analyzed to determine concentrations of K +, Na +, Ca 2+, and Cl - using ions selective electrodes (ISE) (Cole-Parmer, Vernon Hills, IL. Cat. No for K + ; Cat. No for Na + ; Cat. No for Ca 2+ and Cat. No for Cl - ). 2

3 Fresh Banagrass Bana Chopping P0 P20 P40 P60 P0-L1 P0-S1 P20-L1 P20-S1 P40-L1 P40-S1 P60-L1 P60-S1 P0-L2 P0-S2 P20-L2 P20-S2 P40-L2 P40-S2 P60-L2 P60-S2 P0-L3 P0-S3 P20-L3 P20-S3 P40-L3 P40-S3 P60-L3 P60-S3 First press Varied back-pressures Collection of Liquid and solid samples Leaching Collection of Liquid and solid samples Second press Collection of Liquid and solid samples Fig. 1. Schematic of fuel pretreatment process and samples generated from banagrass. Material streams are identified by screw press discharge pressure setting (P#) and liquid (L#) or solid (S#). RESULTS and DISCUSSION Initial Banagrass Characteristics The as-harvested fuel properties and ash analysis for chopped banagrass are presented in the Table 1. The moisture content of the harvested material was low (60.5%) compared to more typical values of ~70% [3]. Ash content of the material was 4.6% and the three dominant ash species were Si, K, and Cl, accounting for more than 75% of the ash mass (as oxide). Table 1 Fuel property and ash analysis for freshly harvested banagrass. Moisture (%) 60.5 Ash Analysis (% dry basis) Proximate Analysis (% dry basis) SiO Ash 4.61 Al 2 O Volatiles TiO Fixed Carbon 15.9 Fe 2 O HHV (MJ/kg) 18.4 CaO 5.79 Ultimate Analysis (% dry basis) MgO 3.24 C Na 2 O 1.27 H 5.69 K 2 O 27.3 O (by difference) P 2 O N 0.37 SO S 0.03 Cl Cl 0.71 CO Ash 4.61 Undetermined 2.54 Products in the Liquid juice and pressed solid were the two product streams from the screw press. Table 2 lists the juice yields (wt. %, based upon the input mass) and the moisture contents of the solids produced by the four treatments. The results shows that the yields of liquid samples increased with increasing the back pressure, accordingly the moisture contents of resulting solid samples were reduced. Note 3

4 that the solid samples resulting from the second press step have higher moisture contents than the samples generated from the first press step. The rinse step fully hydrated the solid material above the 60% moisture of the freshly harvested banagrass and the data indicate that the final moisture content is dependent on the inlet moisture content. Tables 3 and 4 list the mass balance and element mass balances for each treatment step. Results show that all treatment steps had good mass balance between 0.9 and 1.0, indicating that more than 90% of products were recovered. Element mass balances were calculated at the first pressing and again at the leaching and second pressing. The data vary depending on the element and treatment conditions. Overall, the element mass balances are in the range of 0.63 to 0.84 for K, 0.68 to 0.98 for Na, 0.66 to 0.91 for Ca, 0.75 to 1.05 for Cl. Table 2 Yields of liquid juice and moisture contents of pressed solid samples Yield & Moisture, % P0- P20- P40- P60- L S1 (moisture content) L S3 (moisture content) Table 3 Mass balance (output mass divided by input mass) for each treatment step (see Fig. 1). Operating First Rinse Second parameters P P P P Table 4 Elemental mass balance in each step Operating Pressures Treatment First P0 (0 atm) P20 (1.4 atm) P40 (2.8 atm) P60 (4.2 atm) Rinse + Second First Rinse + Second First Rinse + Second First Rinse + Second K Na Ca Cl Removal of ash, K, Ca, Na, Cl Water soluble elements can be removed by the screw press in the expressed liquid juices and this removal results in reduced ash content in the fuel. Ash contents were reduced from 25% to 40% in the first pressing compared with the original ash content of banagrass. The pressed solid samples have a significant reduction in ash contents, as high as 67.9%, following leaching and second pressing. The other water soluble elements, such as K, Na, Ca, Cl also have reduced elemental contents as shown in Table 5. Under the most severe operating conditions (P60), reductions in elemental mass for K, Na, Ca, and Cl were determined to be 90.5%, 72.7%, 56.9%, and 95.4%, respectively. 4

5 Table 5 Removal efficiency (fraction of initial mass removed) of ash, K, Ca, Na, and Cl under varied operating parameters Operating parameter P0 (0 atm) P20 (1.4 atm) P40 (2.8 atm) P60 (4.2 atm) Treatments 1 st 1 st + Rinse +2 nd 1 st 1 st + Rinse +2 nd 1 st 1 st + Rinse +2 nd 1 st 1 st + Rinse +2 nd Ash K Na Ca Cl Fig. 2 shows the distribution of elements K, Na, Ca and Cl in the first expressed liquids, free leachate, and second expressed liquid. Results show that most removable ions are extracted from the first pressing and rinse step. It seems rinse step improving the ions removal from the solid samples, especially for the ions of Na and Ca. The second pressing accounts for ~20% of the total mass removed for each element. P0-L1 P0-L2 P0-L3 P20-L1 P20-L2 P20-L3 100% 100% 80% 80% Distribution 60% 40% Distribution 60% 40% 20% 20% 0% K Na Ca Cl 0% K Na Ca Cl P40-L1 P40-L2 P40-L3 P60-L1 P60-L2 P60-L3 100% 100% 80% 80% Distribution 60% 40% Distribution 60% 40% 20% 20% 0% K Na Ca Cl 0% K Na Ca Cl Fig. 2. Distribution elements of K, Na, Ca, and Cl in the liquid samples Effects of Pretreatments on the Fuel Qualities Pressed banagrass samples were characterized for proximate and ultimate analysis, major ash components, ash deformation temperatures, and heating value. Tables 6, 7 and 8 present these analytical results. Treated banagrass samples have lower ash contents, lower sulfur contents, and lower Cl contents compared with untreated banagrass, as shown in Table 6. It is also apparent that treatments 5

6 that included leaching and a second pressing result in a higher ratio of volatile matter to fixed carbon as shown in Fig. 3. Ash fusion temperatures were altered for treated banagrass. All S3 samples have higher ash fusion temperatures under both reducing and oxidizing atmospheres, indicating that the treatments resulting in a more refractory material. The detailed ash analysis data is presented in Table 8. Table 6 Ultimate and proximate analyses for banagrass and treated banagrass samples Sample i.d. Ultimate analysis, %, dry Proximate analysis, %, dry C H N S O Cl Ash Volatile Fixed C HHV (MJ/kg) Unprocessed P0-S P0-S P20-S P20-S P40-S P40-S P60-S P60-S Table 7 Ash fusion temperatures for banagrass and treated banagrass samples Ash Fusion Temperature (Oxidizing Atm.), C Ash Fusion Temperature (Reducing Atm.), C Sample ID Initial Softening Hemispherical Fluid Initial Softening Hemispherical Fluid Unprocessed P0-S P0-S3 >1482 >1482 P20-S P20-S > >1482 P40-S P40-S > >1482 P60-S P60-S > >1482 Table 8 Ash analysis for banagrass and treated banagrass samples, % Sample ID SiO 2 Al 2 O 3 TiO 2 FeO 3 CaO MgO Na 2 O K 2 O P 2 O 5 SO 3 Cl CO 2 unknown Unprocessed P0-S P0-S P20-S P20-S P40-S P40-S P60-S P60-S

7 10 8 VM/FC Bana P0-S1 P0-S3 P20-S1 P20-S3 P40-S1 P40-S3 P60-S1 P60-S3 Fig. 3. Mass ratio of volatile matter (VM) and fixed carbon (FC) for the banagrass samples Juice and Leachate for Land Application If pressing and leaching techniques are to be used in a process for improving fuel properties, then utilization or disposal of the expressed liquids and free leachate must be considered. Table 9 presents extended elemental analysis results for the liquid samples generated with air pressure of 2.8 atm (40 psig) on the pneumatic cylinder loading the screw discharge. The results indicate that the liquid streams could be used as fertilizer because of the rich potassium (K) and phosphorus (P) content. However, the potential and long term effects of land application of the residual liquid streams on soil and water systems should be determined prior to implementation. Table 9 Elemental analysis for the liquid samples generated from the P40 fuel treatment; (units in mg L -1 ). * Elements Al Sb As Ba Be B Cd Ca Cl Cr Co Cu F Ge Fe Pb Det. Limit Tap H 2 O 0.6 <0.5 <0.02 <0.2 <0.1 <2 < <0.2 <0.05 <0.05 <20 < <0.3 P40-L1 2.7 <0.5 <.02 3 <.1 < <20 < <0.3 P40-L2 0.4 <0.5 < <0.1 < <0.2 < <20 < <0.3 P40-L3 1.4 <0.5 < <0.1 < <20 < <0.3 Elements Mg Mn Hg Mo Ni P K Se Si Na Sr S Ti V Zn Det. Limit Tap H 2 O 12 <0.05 <0.001 <0.5 <0.05 < <0.005 < <0.2 <50 <1 < P40-L <.001 < <1 <0.2 5 P40-L <0.001 < < <50 <1 < P40-L <.001 < < <50 <1 < * : all samples were analyzed by Hazen Research, Inc. CONCLUSION Mechanical dewatering and leaching with water were applied as a fuel treatment and demonstrated to be beneficial in upgrading fuel properties. In this paper, the efficiencies of removal of ash, potassium (K), sodium (Na), calcium (Ca) and chlorine (Cl) were investigated under varied 7

8 screw press operating parameters. Overall mass and elemental mass balances were determined for the tests. Results show that ash contents were reduced from 25% to 40% in the first pressing compared to the original ash content, and had a maximum reduction of 68% following water rinsing and a second pressing. Under the most severe operating conditions, removal rates of K, Na, Ca, and Cl were determined to be 90.5%, 72.7%, 58.8, and 95.4%, respectively. Moreover, fuel analysis for the treated banagrass samples demonstrated that the screw press and water rinse had beneficially upgraded the biomass fuel. The biomass treatments had lower ash, sulfur, and Cl contents compared with untreated banagrass. Preliminary analysis indicates that liquid juice samples and leachate water can be used for land application, but concerns about the decay of the organic materials must be addressed. Acknowledgements: This work was supported by the U.S. Department of Energy under Award No. DE-FG36-08GO References: 1. Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J., The fate of inorganic constituents of biomass in fluidized bed gasification. Fuel 1998, 77, (3), Turn, S. Q., Chemical Equilibrium Prediction of Potassium, Sodium, and Chlorine Concentrations in the Product Gas from Biomass Gasification. Ind. Eng. Chem. Res. 2007, 46, (26), Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M., Removal of inorganic constituents of biomass feedstocks by mechanical dewatering and leaching. Biomass Bioenergy 1997, 12, (4), Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Hiraki, T. T.; Zhou, J.; Masutani, S. M., An Experimental Investigation of Alkali Removal from Biomass Producer Gas Using a Fixed Bed of Solid Sorbent. Ind. Eng. Chem. Res. 2001, 40, (8), Cayan, F. N.; Zhi, M.; Pakalapati, S. R.; Celik, I.; Wu, N.; Gemmen, R., Effects of coal syngas impurities on anodes of solid oxide fuel cells. J. Power Sources 2008, 185, (2), Kurkela, E.; Stahlberg, P.; Laatikainen-Luntama, J. Pressurized fluidized-bed gasification experiments with wood, peat and coal at VTT in Part 2. Experiences from peat and coal gasification and hot gas filtration; Valtion Teknillinen Tutkimuskeskus,Espoo,Finland.: 1995; p 74 pp. 7. Salo, K.; Mojtahedi, W., Fate of alkali and trace metals in biomass gasification. Biomass and Bioenergy 1998, 15, (3), Hanaoka, T.; Minowa, T.; Miyamoto, A.; Edashige, Y., Effect of chemical property of waste biomass on air-steam gasification. Journal of the Japan Institute of Energy 2005, 84, (12), Tijmensen, M. J. A.; Faaij, A. P. C.; Hamelinck, C. N.; van Hardeveld, M. R. M., Exploration of the possibilities for production of Fischer Tropsch liquids and power via biomass gasification. Biomass and Bioenergy 2002, 23, (2), Jenkins, B. M.; Bakker, R. R.; Wei, J. B., On the properties of washed straw. Biomass and Bioenergy 1996, 10, (4),