Energy & Fuels 1995,9, 855-865
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Direct Observation of Alkali Vapor Release during Biomass Combustion and Gasification. 1. Application of Molecular BeamMass Spectrometry to Switchgrass Combustion David C. Dayton,* Richard J. French, and Thomas A. Milne Industrial Technologies Division, The National Renewable Energy Laboratory, 161 7 Cole Boulevard, Golden, Colorado 80401-3393 Received March 6, 1995@
Electricity from biomass and biomass-derived fuels has become an attractive and viable alternative energy source. Alkali metal release during biomass combustion can cause significant problems in terms of severe fouling and slagging of heat transfer surfaces in boilers thus reducing efficiency, and in the worst case, leading to unscheduled plant shutdown. Future biomass to electricity facilities will benefit from increased efficiencies by incorporating integrated gasification combined cycle systems that use biomass combustion gases to directly drive an aeroderivative turbine. These systems will have even lower tolerances for alkali vapor release because accelerated erosion and corrosion of turbine blades results in shorter turbine lifetimes. One solution t o the fouling and slagging problem is t o develop methods of hot gas cleanup to reduce the amount of alkali vapor to acceptable levels. A detailed understanding of the mechanisms of alkali metal release during biomass combustion as well as identifying alkali metal containing vapors and how the vapors lead to fouling and slagging could greatly benefit the development of hot gas cleanup technology. This paper demonstrates the application of molecular beadmass spectrometry to the study of alkali metal speciation and release during switchgrass combustion. We have successfully used this experimental technique to identify alkali metal containing species released during the combustion of switchgrass at four different conditions: 1100 "C in He/Oz(20%), 800 "C in He/02(20%), 1100 "C in He/02(5%), and 1100 "C in He/Oz(lO%)/steam(20%). These conditions were chosen to study the effect of temperature, oxygen concentration, and excess steam on alkali metal release and speciation. Initial feedstock composition is the most significant factor which affects the amount and species of alkali metal released during biomass combustion. The switchgrass sample screened in the present study is high in both alkali metal (potassium) and chlorine. As a result, the predominant alkali metal containing species released during switchgrass combustion is potassium chloride. Varying the combustion condition affects the amount of alkali metal released by a factor of 2 or less. Adding excess steam to the combustion environment tends to shift the form of alkali metal release from the alkali chloride to the hydroxide.
Introduction A large majority of the electric power in this country is generated by fossil fuel (oil, coal, or natural gas) combustion.l Interest in converting biomass to electricity has increased, however, as the benefits of biomass power continue t o be realized. Biomass is a domestic energy source and a strong biomass power industry would reduce the United States dependence on foreign oil imports for power generation. Biomass also offers the advantage of being a renewable and sustainable energy resource. Dedicated energy crops planted and harvested to supply a nearby biomass power generating facility would make better use of idle farmlands and reduce the rate of accumulation of greenhouse gases because the carbon dioxide emitted during biomass combustion is taken in during crop growth. Clearly, an increase in the output of biomass power could have significant environmental, economic, and social implications. e Abstract
published in Advance ACS Abstracts, August 1, 1995. (1)Larson, E. D. Technology for Electricity and Fuels From Biomass, Annu. Rev. Energy Environ. 1993 18, 567-630.
Current technology used in biomass power facilities consists of biomass combustion t o generate steam that is expanded through a steam turbine to produce electricity. A large fraction of the electricity produced from biomass is generated by the pulp and paper and wood products industries and not commercially available. This electricity is cogenerated with heat and both are directed back into the process. Commercial biomass power facilities rely on the availability of low-, zero-, or negative-cost feedstocks because conversion efficiencies tend to be low in comparison t o coal-fired plants.2 As a result, the feedstocks used in biomass facilities are mainly waste materials. Waste-wood fired systems account for the majority of the capacity with small contributions from agricultural wastes and landfill gas combustion fa~ilities.~ The low efficiencies and feedstock cost and availability limit the size of current biomass electric power plants typically to the 5-50 M W , range,3 considerably smaller than coal-fired plants. (2) Williams, R. H.; Larson E. D. Advanced Gasification-Based Biomass Power Generation. In Renewable Energy: Sources for Fuels and Electricity; Johansson, T. B., Kelly H., Reddy, A. K. N., Williams, R. H., Eds.; Island Press: Washington, D.C., 1992; Chapter 17.
0887-0624l95I2509-0855$09.~~l00 1995 American Chemical Society
866 Energy & Fuels, Vol. 9, No. 5, 1995
More efficient fuel-handling systems and improved conversion efficiencies would benefit the biomass power industry. To address these issues, next generation biomass power facilities are being designed as integrated combustion or gasification systems including direct-fired aeroderivative gas turbines for power generation in conjunction with dedicated feedstock supply systems. Dedicated feedstock systems are projected t o provide low-cost energy crops for power generation. The potential for higher generation efficiencies with the advanced turbine systems should also provide lower electricity costs compared t o current steam turbine systems and ease the burden on the feedstock supplies. One of the challenges that limits the use of biomass in commercial power generating facilities is fouling and slagging (ash deposition). The relatively high alkali metal content of biomass is a primary concern for biomass facilities because substantial alkali metal release during biomass combustion accelerates the fouling and slagging of heat transfer surfaces in industrial boilers. The primary gas phase alkali metal containing species released during biomass combustion are potassium salts: chlorides, hydroxides, and sulfates. The high chlorine content of some biomass (usually much higher than coal) also raises concerns about corrosion of these surface^.^ Alkali metal containing vapors can lead to substantial and unpredictable problems that reduce the efficiency of power generation and often require unscheduled plant shutdowns to remove
deposit^.^ Ash deposition is not unique to biomass combustion systems. In fact, most of the available information concerning ash deposition comes from investigating the fate of alkali metals and mineral matter during coal c o m b ~ s t i o n . ~Studies ,~ have shown that ash deposition from coal combustion involves a number of complex and simultaneous processes such as the vaporization, condensation, fragmentation, and agglomeration of mineral matter. Several mechanisms have been developed which encompass these processes and attempt to explain the details of ash deposition during coal The behavior of alkali metals during coal combustion is coupled to the overall mineral matter present in the coal sample. The total alkali content of coal is not necessarily an indication of the alkali metal containing vapors that would be released during combustion or (3)Bain, R. L.; Overend, R. P. Biomass Electric Technologies: Status and Future Development. In Advances in Solar Energy: An Annual Review of Research and Development, Volume 7; Boer, K. W., Ed.; American Solar Energy Society: New York, 1992; Chapter 11. (4) Thielsch, H.; Cone, F. M. Remedies for WTEs Most Vexing O&M Problem: Tube Failure. Solid Waste Technol. 1994,JanlFeb, 32-39. ( 5 ) Turnbull, J. H. Use of Biomass in Electric Power Generation: The California Experience Biomass Bioenergy 1993,4, 75-84. (6) Bryers, R. W. Ash Deposits and Corrosion Due to Impurities in Combustion Gases; Hemisphere Publishing Corp.: New York, 1978. (7) Reid, W. T. Coal Ash-Its Effect on Combustion Systems. In Chemistry of Coal Utilization; Elliot, M. A,, Ed; Wiley: New York, 1981; Chapter 21. (8)Baxter, L. L. Ash Deposition During Biomass and Coal Combustion: A Mechanistic Approach. Biomass Bioenergy 1993,4,85-102. (9) Haynes, B. S.; Neville, M.; Quann, R. J.; Sarofim, A. F. Factors Governing the Surface Enrichment of Fly Ash in Volatile Trace Species. J . Colloid Interface Sci. 1982,87, 266. (10) Helbe, J. J.; Sarofim, A. F. Factors Determining the Primary Particle Size of Flame-Generated Inorganic Aerosols. J . Colloid Interface Sci. 1982,128, 348. (11)Wendt, J. 0. L.; Peterson, T. W. Fixation of Alkali and Trace Metals in High Temperature Co-Current Entrained Flow Gasifiers and Combustors: Proc. -Fifth Annu. Contractors Meeting Contaminant Control Coal-Derived Gas Streams. DOE/METC-85-6025(DE85 013703), 1986.
Dayton et al.
gasification. Both alkali metal vapors and condensed species have important roles in deposit formation. On the basis of investigations of ash deposition from coal combustion, one would hope to gain some insights into the nature and formation of deposits produced during biomass combustion. Clearly, it is important to know the form and amounts of alkali metal in biomass to anticipate the types of alkali metal transport to be expected and the forms of alkali metal containing species that will be transported. Coal and biomass, however, are dissimilar fuels and the ash deposition mechanisms developed for coal combustion may need to be modified for biomass combustion. In general, biomass tends to have less sulfur, fured carbon, and fuelbound nitrogen compared t o coal. Biomass also has a higher oxygen content and as a result is more reactive than coal. The ash content of biomass is typically less than coal, however, it has a different elemental composition and the amount of ash widely varies in different forms of biomass. Coal ash tends to be composed of mineralogical material while biomass ash tends to reflect the inorganic material required for plant growth. As a result of these fundamental differences, operators of power plants firing either coal or biomass often encounter different operational difficulties and the hot gas cleanup technologies used in the coal-fired industry may not be applicable to biomass systems. Considerable effort has gone into developing accurate and effective methods for determining the composition of biomass feedstocks and how to relate this composition to deposits that form in industrial power generating facilities.12 Effective hot gas cleanup technology would benefit from a detailed understanding of the mechanisms of alkali metal release during biomass combustion and gasification as well as identifying the alkali metal vapor species that contribute t o fouling and slagging. Our approach is to directly sample the hot gases liberated from the combustion of small quantities of biomass in a variable temperature quartz tube reactor employing molecular beam sampling/mass spectrometry (MBMS)t o monitor the combustion event. This MBMS technique has several advantages over the various techniques that have been developed to measure alkali metal in coal combustion systems.13-15 The previous techniques can be distinguished as extractive and insitu detection methods which often require long sampling intervals providing average measurements of total alkali metal concentrations. The MBMS technique, however, can be used to sample directly and continuously the combustion gases, in realtime, and speciate the detected alkali metal vapors. (The MBMS technique has also previously been used to study coal combustion in flames.16) The present study demonstrates the (12) Miles, T. R. Alkali Deposits; Periodic Reports of the Alkali Deposits Investigation; Thomas R. Miles, Consulting Design Engineers: Portland, OR, 1993. (13)Lee, S. H. D.; Teats, F. G, Swift, W. M. Alkali-Vapor Emission from PFBC of Illinois Coals. Combust. Sci. and Technol. 1992,86,327336. (14) Haas, W. J.; Eckels, D. E. Fiber Optic Alkali Metal Sampling. In Proceedings of the Seventh Annual Coal-Fueled Heat Engines and Gas Stream Cleanup Systems Contractors Review Meeting. Webb, H. A., Rebos, N. F., Fothari, V. P., Bedick, R. C., Eds.; NTIS: Springfield, VA, 1990; DOEMETC-90-6110, (DE90 000480). (15) Hensel, J . P.; Goff, D. R.; Logan, R. G.; Pineault, R.; Romanosky, Jr., R. R.; Wachter, J. K. On-Line, Real-Time Alkali Monitor For Process Stream Analysis. Rev. Scz. Instrum. 1987,58, 1647. (16) Greene, F. T.; O’Donnell, J. E. Investigation of Mechanisms of Ash Deposit Formation From Low-Rank Coal Combustion;” Final Report DOE/FC/10287-2416, 1987.
Alkali Vapor Release during Biomass Combustion
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Table 1. Proximate, Ultimate, and Ash Analysis Results for the Switchgrass Sample
proximate (%) moisture ash volatile fured carbon total ultimate (wt %) moisture carbon hydrogen nitrogen sulfur ash oxygenu total chlorineb ash (wt %) Si Al Na K Ca Mg
P
as received
dry basis
8.16 4.22 72.73 14.89 100
4.59 79.19 16.22 100
8.16 43.04 5.37 0.53 0.10 4.22 38.58 100 0.46
46.86 5.84 0.58 0.11 4.59 42.02 100 0.50
0.94 0.03 0.007 0.989 0.223 0.117 0.284
1.04 0.033 0.008 1.09 0.246 0.129 0.313
n-OmY
JME~MS
0.00
0.00
uOxygen by difference. bNot usually reported as part of the ultimate analysis.
successful application of the MBMS technique to identi^ the alkali metal containing species released during switchgrass combustion. Switchgrass was chosen for initial study because it has been targeted as an energy crop for use in dedicated feedstock supply systems. Several other biomass fuels, including 10 real-world feedstocks studied in the MilesBaxter effort,12 have been investigated using the MBMS technique. The results of these studies will be the subject of a future publication.
Experimental Apparatus The release of alkali vapor species during switchgrass combustion was monitored and studied using a direct sampling, molecular beadmass spectrometer (MBMS) system. The MBMS system is ideally suited for studying the high-temperature, ambient pressure environments encountered during the present studies. The integrity of the sampled high-temperature combustion gases is preserved during the free-jet expansion since chemical reactions are effectively quenched and condensation is inhibited. The nonequilibrium nature of the free jet expansion and the subsequent formation of a molecular beam allows reactive and condensable species to remain in the gas phase at temperatures far below their condensation point for long periods of time in comparison to reaction rates. Comprehensive detection of all gas phase species of interest is possible using the mass spectrometer and facilities for pattern matching and tandem MS are available to aid in deciphering complex mass spectra. This apparatus has been described in the literature17-19so only the salient points will be discussed in relation to the present study. The switchgrass sample (Panicum uirgutum L. variety Alamo, grown near Stephensville, TX) used in the present study was ground in a mill to +20/-80 mesh. The proximate, ultimate, and ash analyses for this switchgrass are presented (17)Soltys,M. N.; Milne, T. A. The SERI High-pressure, Mass Spectrometric Sampling System; Solar Energy Research Institute: Golden, CO, Sept 1982; SERI/TR-622-1172. (18)Evans, R. J.; Milne, T. A. Molecular Characterization of the Pyrolysis of Biomass: I. Fundamentals. Energy Fuels 1987, 1 , 123127. (19) Evans, R. J.; Milne, T. A. Molecular Characterization of the Pyrolysis of Biomass: 11. Applications. Energy Fuels 1987, 1 , 311319.
Digital Readout /
Sample Boat
Figure 1. Schematic representation of the high temperature reactor used to study alkali metal release during switchgrass combustion. 1
I
40
35
1200
I
I
I
I
I
I
30
25
20
15
10
5
1100 1000
900
G L 800 700
2i 600 t-. 500 400
300
0
Distance From the Orifice (cm)
Figure 2. Hot gas temperature profiles measured in the quartz tube reactor at the two furnace temperature settings. in Table 1. Small (20-50 mg) switchgrass samples were combusted in a tubular quartz reactor (18 mm 0.d. and 16 mm i.d.) which was placed into a standard two zone, electric clamshell furnace as depicted in Figure 1. The heated zone in the furnace was 30 cm long. For the experimentsdiscussed below, the furnace temperature was set at either 1100 or 800 "C. A profile of the hot gas temperature inside the reactor at these two furnace settings is shown in Figure 2. Temperature was measured with a type-K (chromel-alumel) thermocouple at 1 cm intervals while traversing the thermocouple in the forward and reverse directions. The dip in the profiles near the middle of the reactor corresponds to where the two heating elements are physically joined. Switchgrasssamples were loaded into hemicapsular quartz boats of such a size that approximately 40 mg of ground, loosely packed, material filled the boat. The boats were then placed on the end of a 6 mm diameter (0.25 in.) piece of stainless steel (or quartz) tubing which was inserted into the furnace through a 18-mm (0.75 in.) brass Swagelok tee. A type-K thermocouple surrounded by a 0.5 mm (0.020 in.) diameter inconel sheath was inserted through the stainless steel tubing such that the junction was close to the edge of the sample boat. The actual boat temperature and flame temperature were not measured. However, the hot gas temperature in the vicinity of the sample boat could be monitored. A mixture of helium and oxygen ( 5 , 10, or 20%) flowed through the reactor from back to front at a total gas flow rate of 4.4 L(STP)/min. Under these conditions the combustion gases have a residence time of about 0.1 s in the reactor before reaching the sampling orifice. When appropriate, steam was added to the atmosphere inside the reactor by injecting water into the rear of the furnace through a needle fed by a syringe pump. Stainless steel shot (0.125 in. diameter) was packed around the tip of the needle to increase the surface area for water evaporation. This provided a steady flow of 20 vol % steam.
858 Energy & Fuels, Vol. 9, No. 5, 1995
Dayton et al.
i:H
I200 I loo 1000
2.20
*
