Energy & Fuels 1996, 10, 293-298
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Inorganic Compounds in Biomass Feedstocks. 1. Effect on the Quality of Fast Pyrolysis Oils F. A. Agblevor* and S. Besler National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401 Received October 2, 1995. Revised Manuscript Received December 19, 1995X
Inorganic compoundssespecially potassium, calcium, sodium, silicon, phosphorus, and chlorinesare the main constituents of the ash in biomass feedstocks. The concentrations of ash range from less than 1% in softwoods to 15% in herbaceous biomass and agricultural residues. During biomass pyrolysis, these inorganics, especially potassium and calcium, catalyze biomass decomposition and char-forming reactions. Chars formed during these reactions invariably end up in the biomass pyrolysis oils (biocrude oils) as suspended submicron particles. The presence of high concentrations of submicron char particles in biocrude oils will make them problematic for combustion in steam boilers, diesel engines, and turbine operations because of the potential release of the ash and alkali metals during combustion. We experimented with sequential cold filtering of biocrude oils using filters of varying pore size and this revealed that most of the ash and alkali metals detected in biocrude oils are trapped in the chars. Leaching studies conducted on the chars suspended in the oils showed no leaching of alkali metals from the chars into the oils. Cold filtration of the oils dissolved in acetone was ineffective in reducing the alkali metals content to acceptable levels, but hot gas filtration of the pyrolysis vapors reduced the alkali metals content below 10 ppm. Our data suggest that hot gas filtration can potentially reduce the ash and alkali metals content of the biocrude oils to acceptable levels so they can be used as turbine, diesel engine, or boiler fuels.
Introduction The production of fuels and chemicals from biomass resources is one of the major areas of renewable energy research at the National Renewable Energy Laboratory (NREL). The potential of using biocrude oils for large scale power production is an attractive option because the oils are produced from renewable resources, they are highly oxygenated and consequently will reduce carbon monoxide emission, and they are easier to handle, transport, and store compared to the raw biomass. Additionally, biocrude oils are potential refinery feedstocks. However, biocrude oils, like any other fuel in their developmental stage, have several unique problems that must be solved before they can have any significant impact on fuel supply. These problems include high acidity, high viscosity, long-term storage, high water content, and high inorganics content. The presence of alkali metals in biocrude oils has been postulated as a potential source of fouling, hot corrosion, and erosion of turbine blading in power generation systems, as well as fouling in steam boiler tubes. In the marine environment, serious corrosion of marine turbine blades has been attributed to alkali metals. In the presence of sulfur oxides produced during fuel combustion, alkali metals form sulfates which initiate hot corrosion of turbine blading made from highly alloyed steels and superalloys.1-6 This is particularly severe for gas turbines operating at gas inlet temperatures greater than 650 °C. Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Moses, C. A.; Bernstein, H. Impact of the Use of Biomass-Derived Fuels in the Gas Turbines for Power Generation. NREL/TP-430-6085; UC Category: 247, DE94000261. National Renewable Energy Laboratory, Golden, CO, 1994. X
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Gas turbine operators have found that slag-forming compounds in petroleum derived oils can cause corrosion and deposits. Corrosion can result from vanadium reacting with sodium and potassium to form a low melting eutectic (mp 566 °C), but this corrosion can be reduced by adding magnesium compounds. However, the presence of lead in the fuel can reduce the beneficial effects of the magnesium compounds and, additionally, lead can cause corrosion. Calcium, on the other hand, does not cause corrosion but can form deposits that are very difficult to remove from turbine blading.1 For gas turbines using solid biomass fuel, it was discovered that if the gas inlet temperatures were below 800 °C, ash deposited on the turbine blading, which was extremely difficult to remove.7 Because of the inherent inorganic materials in biomass feedstocks, biocrude oils produced from the fast pyrolysis of biomass are contaminated with high levels of inorganic compounds. However, the mode of transport of inorganic material into the biocrude oils was not known. In this paper, we discuss the origin of alkali (2) Elliot, J. F. Chemistry of Hot Corrosion. In Solid State Chemistry of Energy Conversion and Storage; Adv. Chem. Ser. 163; Goodenough, J. B., Whittingham, M. S., Eds.; American Chemical Society, Washington, DC, 1977; pp 225-238. (3) Raymond, L. Future Fuels for Automotive Gas Turbines. In Gas Turbines for Autos and Trucks; Society of Automotive Engineers: Warrendale, PA, 1981; pp 297-312. (4) Lay, K. W. Ash in Gas Turbines Burning Magnesium-Treated Residual Fuel. Am. Soc. Mech. Eng, Pap. 1974, 73-WA/CD-3. (5) Lee, S. Y.; Young, W. E.; Vermes, G. Evaluation of Additives for Prevention of High Temperature Corrosion of Superalloys in Gas Turbines. Am. Soc. Mech. Eng. Pap. 1973, 73-GT-1. (6) Stoeckly, E. E. Marinization of the General Electric LM 1500 Gas Turbine. Preprint 65-GTP-20, ASME Gas Turbine Conference, Washington, D. C., February-March, 1965, pp 1-11. (7) Hamrick, J. T. Biomass-Fueled Gas Turbines. In Clean Energy from Waste and Coal; ACS Symp. Ser. 515; Khan, M. R., Ed; American Chemical Society: Washington, DC, 1992; p 78.
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metals in biocrude oils, and a potential method for producing alkali metal-free biocrude oils. In part 2 of this series, we discuss the structure of the pyrolysis chars and how they influence the properties of the biocrude oils. Experimental Section Materials. Switchgrass (Panicum virgatum L.), the “Alamo” variety, was used for these studies. This material was used in studies of the influence of storage on the composition of biocrude oils.8 The switchgrass was obtained from a sixyear-old stand located 7 km south of Stephenville, Texas, and was not stored before being used for these studies. The feedstock was milled in a Wiley mill (Model 4) until all the material passed through a 1-mm screen. The Sauter mean diameter of the particles was 340 µm and the moisture content of the materials prior to pyrolysis was 5%. The moisture content of the feedstock was very low because of the high altitude and low humidity in Golden, Colorado, site of the pyrolysis experiments. Fluidized Bed Pyrolysis of Biomass Feedstocks. To study the mode of transport of alkali metals, calcium, and other inorganics into the biocrude oil, switchgrass was pyrolyzed in a bench-scale fluidized bed reactor. A detailed description of the fluidized bed reactor has been published elsewhere.8 The pyrolysis temperature of the fluidized sand bed was 500 °C and the fluidizing gas was nitrogen. The apparent pyrolysis vapor residence time in the free volume of the reactor was less than 0.4 s. The pyrolysis vapors exiting the reactor were passed through a cyclone separator to separate the char and any entrained attrited sand from the pyrolysis vapors. The clean pyrolysis vapors were condensed in a condensation train that consisted of a chilled water condenser, an ice/salt condenser, an electrostatic precipitator, and a coalescing filter. The gaseous products were measured by a dry test meter. Cold Filtration of Biocrude Oils. The biocrude oils from the condensers were recovered by washing the condensers with acetone. The acetone solution of the oils were made up to a total volume of 5 L. A fraction (3 × 200 mL) of the acetone solution of the oil was not processed further and was retained for comparison with other fractions (each 3 × 200 mL) that were filtered sequentially through filters of different pore size (40-60, 10-15, 4.5-5.0, 1.0, and 0.7 µm). The solvent was removed from the acetone soluble oils by rotary vacuum evaporation (61.3 kPa and 40 °C), and the samples were sent to Huffman Laboratory, Golden, Colorado, for ash, potassium, sodium, calcium, silicon, chlorine, and phosphorus analysis. Leaching Studies of Biocrude Oils. To ascertain whether inorganic materials especially potassium and other alkali metals leached from the char into the oil phase, a 5 L sample of the acetone solution of the biocrude oil was filtered through a 40-60 µm filter and then stored in a cold room (10 °C) for 1, 2, 7, 15, and 30 days. At the end of each storage period, 3 × 200 mL samples were taken and filtered through a 10-15 µm filter. The solvent was removed by vacuum rotary evaporation as above. These samples were also analyzed by the same laboratory for the same elements as above. Hot Gas Filtration of Biomass Pyrolysis Vapors. In the conventional method of producing biocrude oils in a fluidized bed reactor as described in ref 8, pyrolysis vapors exiting the fluidized bed reactor pass through a cyclone separator to remove char and any entrained attrited sand. The clean hot vapors are then condensed in a series of condensers. In the hot gas filtration method, the cyclone separator is replaced by a 2-µm cylindrical 316 stainless steel filter: 50.8 mm (2.0 in) outside diameter, 254 mm (10 in) long, enclosed in 63.5 mm (2.5 in) diameter by 292 mm (11.5 in) high stainless (8) Agblevor, F. A.; Besler, S.; Wiselogel, A. E. Fast Pyrolysis of Stored Biomass Feedstocks. Energy Fuels 1995, 9, 635-640.
Agblevor and Besler Table 1. Composition of Switchgrass Feedstock element
composition
element
composition
carbon (%) hydrogen (%) oxygen (%) nitrogen (%) ash (%) chlorine (%)
45.13 6.05 42.77 0.56 4.92 0.68
calcium (%) potassium (%) sodium (ppm) phosphorus (%) sulfur (%) HHV (MJ/kg)
0.24 0.85 31 0.08 0.10 18.8
steel housing. The volume of the filter is such that the residence time of the pyrolysis vapors in this chamber was estimated at 1 s. The filter was equipped with three K-thermocouples: one at the entrance of the filter, the second on the surface of the filter, and the third where the vapors exit. The filter was heated with a heating tape between 300 and 400 °C. This range of temperatures (300-400 °C) was investigated to determine which was the most suitable for the effective operation of the filter without condensation or excessive loss of the pyrolysis vapors through cracking and polymerization. The pressure drop across the filter was measured by a pressure gauge. The pyrolysis vapors and chars exiting the fluidized bed reactor passed through the hot filter before condensing in the train of condensers described above. The condensed oils were recovered by acetone washing and rotary vacuum evaporation and analyzed for inorganic material content by the same laboratory as described above. Char Particle Size Analysis of Biocrude Oils. To understand the role of char particles in the quality of biocrude oils, the biocrude oils were dissolved in acetone and the char particle size distribution in the oils was measured by a Malvern diffraction instrument. This instrument, which works on laser diffraction principles, can measure particle sizes directly from 0.5 to 500 µm. Below the lower limit, the particle sizes are extrapolated using defined mathematical formulas that are part of the routine of the instrument. Particles size distributions were measured for both stored and fresh biocrude oils filtered through a 10-15 µm filter.
Results Biocrude Oils Obtained by Cyclone Separation of Char. The elemental composition and trace inorganic content of the switchgrass feedstock is shown in Table 1. Clearly, the ash content of this feedstock is very high compared to woody biomass feedstocks. The nitrogen and chlorine contents are also very high partly because of the relatively high fertilization of the switchgrass compared to the woody feedstocks and also because the switchgrass feedstock included leaves as well as stems. The nitrogen and ash contents of the leaves are usually severalfold that of the stems9 and the switchgrass is fertilized with potassium chloride. Table 2 shows the material balance for the pyrolysis products obtained from the fluidized bed reactor. The total liquid yields (61%) are lower than those obtained for woody biomass feedstocks (68%) pyrolyzed under similar conditions.10 The liquids were brownish and viscous and had a moisture content of 12% as determined by Karl Fisher titration. (9) Johnson, D. K.; Ashley, P. A.; Deutch, S. P.; Davis, M. F.; Fennell, J. S.; Wiselogel, A. Compositional Variability in Herbaceous Energy Crops. In Proceedings, Second Biomass Conference of the Americas: Energy, Environment, Agriculture, and Industry; NREL/CP-200-8098, DE95009230; National Renewable Energy Laboratory: Golden, CO, 1995; pp 267-277. (10) Agblevor, F. A., Besler-Guran, S.; Wiselogel, A. E. Plant Variability and Bio-Oil Properties. In Proceedings, Second Biomass Conference of the Americas: Energy, Environment, Agriculture, and Industry; NREL/CP-200-8098, DE95009230; National Renewable Energy Laboratory: Golden, CO, 1995; pp 1099-1109.
Inorganic Compounds in Biomass Feedstocks
Energy & Fuels, Vol. 10, No. 2, 1996 295
Table 2. Product Yields and Material Balances for the Fluidized Bed Pyrolysis of Switchgrass Feedstock Using a Cyclone Separator and Hot Gas Filter cyclone separated
pyrolysis temp (°C) char yield (%) total liquid yield (%) gas yield (%) mass closure (%) press. drop across filter (cm of H2O) hot gas filter temp (°C)
hot gas filter separated
run 1
run 2
run 3
run 4
500 18.8 60.2 13.6 92.6
500 19.0 63.1 12.6 94.7
500 18.3 62.0 23.0 103.3 23-38
500 19.1 56.7 20.4 96.2 10-13
380-390
355-365
Although both switchgrass and woody biocrude oils appear brownish and viscous, there are some differences in their physical properties. Switchgrass biocrude oils were stickier than wood biocrude oils and tended to stick more tenaciously to the glass condensers. Wood biocrude oils were easily washed from the glassware with acetone after each run, whereas switchgrass biocrude oils did not readily dissolve in acetone when dry, but readily dissolved in an alcoholic potassium hydroxide solution. The pH of the switchgrass biocrude oil was 2.4-3.0, which is similar to those obtained for woody biocrude oils.8 Table 3 shows the inorganic elements and ash contents of the filtered and unfiltered oils. The ash content, and consequently the inorganic elements content of the unfiltered oil, were higher than those of the filtered oils. The ash content of the filtered oils were very low, but that for the unfiltered oil was relatively high (0.45%) but constituted only 0.27% of the total ash in the original biomass feedstock. Cold filtration had a significant impact on the concentrations of inorganic elements in the oils (see Figure 1 and Table 3). As the filter size decreased, the concentrations of potassium, calcium, and other inorganic elements in the biocrude oil decreased until the 4.5-5.0-µm filter, after which the concentrations leveled off or approached an asymptote. The only exception to this trend in inorganic element content was sodium that showed a steady increase, probably because some sodium leached from the glass containers used for collecting and storing the oils. The leveling off of the potassium and calcium concentrations could be attributed to either submicron char particles in the oil or inorganics leaching from the char particles into to the oil phase. However, it appears that the chars leaching into the oil phase did not contribute to the alkali metals content of the oils as shown by the detailed discussion under the leaching section. Char Products from Cyclone-Separated Process. The char was defined as the carbonaceous solid residue left after the pyrolysis (including the ash). This material was insoluble in common solvents such as acetone, alcohols, etc. In our system, char was recovered from three sources: solid residue left in the reactor with the silica sand (3-4% of the total char); solid residue collected from the char pot after separation by the cyclone separator (94-95% of the total char); and acetone-insoluble residue recovered from the filtration of the biocrude oil (2-3% of the total char). During filtration of the biocrude oil dissolved in acetone, significant amounts of char were recovered
from 40-60 and 10-15 µm filters (2-3% of the total char). Smaller filter sizes did not separate any significant amounts of char. The total char yield from the switchgrass (see Table 2) was significantly higher than from woody biomass on both ash-free and whole biomass basis because the high ash and protein content of the original biomass feedstock, switchgrass or wood, tend to promote char forming reactions during pyrolysis.8 The ash contents of the chars recovered from the biocrude oil filtration ranged from 9 to 15% while the ash content of the char from the cyclone and reactor was 24.9 ( 0.2%. The ash content of the char from the reactor and char pot was very close to the theoretical value assuming all the initial ash in the feedstock was sequestered in the char (Table 4). The ash content of the feedstock was 4.9% and hence the expected ash content for the char yield of 19.0% was 25.9%. However, the material balance of the ash from the unfiltered oil, plus those from the reactor and char pot, accounted for 97.5% of the total ash initially present in the feedstock. This clearly shows that almost all the ash initially present in the feedstock is sequestered in the char during the pyrolysis process. Leaching of Biocrude Oils during Storage. Ash and inorganic elements analysis for the stored biocrude oil samples revealed some surprising but interesting results. The ash contents of the biocrude oils did not show any trend with storage time because all the values were below 0.05% which was the detection limit of the analytical method used. The potassium content of the oils decreased with storage time and gradually leveled off after the seventh day of storage in the cold room and did not show any significant variation up to 30 days of storage (see Figure 2). The calcium concentration showed a similar trend. The sodium content of the biocrude oil did not follow any definite trend. The chlorine levels were all below the detection limit of the analytical method (
