1008
Energy & Fuels 2006, 20, 1008-1014
Investigation of Ash Deposition Rates for a Suite of Biomass Fuels and Fuel Blends Shrinivas S. Lokare, J. David Dunaway, David Moulton, Douglas Rogers, Dale R. Tree, and Larry L. Baxter*,† Chemical Engineering Department, Brigham Young UniVersity, ProVo, Utah 84602 ReceiVed December 8, 2005
This investigation details the effects of the fuel constituents through systematic experimental and theoretical analysis of a suite of fuels with widely varying inorganic contents and compositions. The experiments were carried out in the multifuel flow reactor (MFR) at Brigham Young University. Fuels included straw, grain screenings, sawdust, sunflower shells, sugar beat pulp, and shea nutshells. Out of the six major fuels, two base fuels (straw and grain screenings) were mixed with the other four fuels in 1:1 ratio by weight. The data from a series of experiments quantitatively illustrated the effects of fuel properties and fuel interactions on ash deposition rate. In the deposition rate measurement investigation, the predicted results from a simple model of ash deposition assuming “no interactions” between fuels were compared with the measured results to demonstrate the fuel interactions. A significant decrease in ash deposition rate was observed for fuels blends in comparison with that of pure fuels (straw and grain screening). The decrease in deposition rate was attributed to the fuel interactions.
Introduction Coal, a major fossil fuel, represents the most substantial energy reserve in the United States and contributes more than half of the electric power generation in many developed countries. Important challenges to coal combustion include deposition and corrosion in boilers, pollutant emissions, and greenhouse gas emissions to the atmosphere.1-5 Carbon dioxide and NOx are two of the most problematic products of coalfired power plants. Progress has been made to reduce NOx emissions through low NOx burner design and prudent fuel selection, and catalysts can be used to achieve acceptable NOx control where necessary, but technologies to reduce CO2 from coal combustion are yet to be proven. Biomass is a renewable fuel that can be used in existing combustion facilities and provides an energy source with essentially zero (95% CO2 closure) CO2 emission.6 It has been shown that biomass combustion closes the CO2 cycle when combined with the photosynthesis process during plant growth. With this inherent advantage of biomass fuels, technologies are developed for coalbiomass co-firing and dedicated biomass combustion as a potential solution for greenhouse gas formation.7-12 * Corresponding author. E-mail:
[email protected]. † Current address: 350 CB, BYU, Provo, UT 84602. (1) Baxter, L. L. Ash Deposit Formation and Deposit Properties: A ComprehensiVe Summary of Research Conducted at Sandia’s Combustion Research Facility; SAND2000-8253; Sandia National Laboratory: Livermore: CA, 2000. (2) Chou, M.-I. M.; Lytle, J. M.; Kung, S. C.; Ho, K. K. Fuel Process. Technol. 2000, 64, 167-176. (3) Doane, E. P.; Abbott, M. F. Proceedings of the 57th Annual American Power Conference; Chicago, IL, 1995; pp 1231-1241. (4) Gibb, W. H. J. Inst. Energy 1986 (December), 206-212. (5) Osborn, G. A. Fuel 1992, 71, 131-141. (6) Mann, M. K.; Spath, P. L. The Net CO2 Emissions and Energy Balances of Biomass and Coal-Fired Power Systems; National Renewable Energy Laboratory: Golden, CO, 1999. (7) Green, A.; Wagner, J.; Green, B.; Van Ravenswaay, H.; Clauson, D.; Schwartz, J.; Yurchisin, T.; Rockwood, D.; Prine, G.; Mislevy, P.; Jenkins, F.; Gaffney, S. Biomass 1989, 20, 249-262.
Unfortunately, the use of biomass in combustion systems has introduced added operating difficulties mainly related to ash deposition and corrosion. Because biomass sources vary, so do the ash content and composition of these fuels.11,13-17 The alkali salts present in biomass ashes play an important role in deposition due to their ability to bind fly ash particles by sintering. The outcomes of coal-biomass co-firing have been the motivation for investigating how the wide variation in ash content and composition of biofuels can be used to reduce ash deposition rates.18-23 (8) Benestad, C. Biomass 1990, 22, 329-342. (9) Green, J. H. Energy Eng. 1994, 91, 18-28. (10) Bain, R. L.; Overend, R. P.; Craig, K. R. Fuel Process. Technol. 1998, 54, 1-16. (11) Demirbas, A. Prog. Energy Combust. Sci. 2004, 30, 219-230. (12) Robinson, A. L.; Rhodes, J. S.; Keith, D. W. EnV. Sci. Technol. 2003, 37, 5081-5089. (13) Kandpal, J. B.; Madan, M. Energy Sources 1996, 18, 767-771. (14) Quantification of Deposit Formation Rates as a Function of Operating Conditions and Fraction of Biomass Fuel Used in a ConVerted pc Boiler (100 MW); Nordin, A., Skrifvars, B., Eds.; Plenum Press: New York, 1996. (15) Characterization of Biomass Ashes; Skrifvars, B., Hupa, M., Moilanen, A., Lundqvist, R., Eds.; Plenum Press: New York, 1996. (16) Jenkins, B. M.; Miles, T. R., Jr.; Baxter, L. L.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17-46. (17) Kaufmann, H.; Nussbaumer, Th.; Baxter, L.; Yang, N. Fuel Process. Technol. 2000, 79, 141-151. (18) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energy Combust. Sci. 2001, 27, 171-214. (19) Alekhnovich, A. N.; Bogomolov, V. V.; Artem’eva, N. V. Therm. Eng. 2001, 48, 113-120. (20) Robinson, A.; Junker, H.; Buckley, S. G.; Sclippa, G.; Baxter, L. L. Twenty-SeVenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 1351-1359. (21) Hughes, E. E.; Tillman, D. A. Fuel Process. Technol. 1998, 54, 127-142. (22) Hein, K. R. G.; Bemtgen, J. M. Fuel Process. Technol. 1998, 54, 159-169. (23) Deposit Formation during the Co-Combustion of Coal Biomass Blends; Hein, K. R. G., Heinzel, T., Kicherer, A., Spliethoff, H., Eds.; Plenum Press: New York, 1996.
10.1021/ef050410e CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006
Ash Deposition Rates for Biomass Fuels and Fuel Blends
Energy & Fuels, Vol. 20, No. 3, 2006 1009
Deposits on heat transfer surfaces are formed by ash and/or char particles and condensable gases. These ash particles are formed by various mechanisms. Some mechanisms are simple (e.g., fuel particle undergoing combustion to produce a single or multiple fly ash particles), whereas some are as complex as evaporation from the fuel, disintegration from the fuel by chemical inorganic reaction, convective disintegration due to organic reactions or rapid devolatilization, and chemical transformation in flue gas.17,24 Specifically, the factors that play a key role in deposit formation are the alkali content of biofuels, fluid dynamics, gas and heat transfer surface temperatures, and surface interactions and reaction. Apart from these characteristics, sintering is a very important aspect of the ash deposition process, which fuses two solid particles together at high temperature. Ash fusion temperature is an indicator of the possibility of sintering for various solid fuels. Biofuels exhibit relatively low ash fusion temperatures and therefore tend to undergo sintering to a higher extent.23,25 Ash deposition leads to declining boiler efficiency and capacity, and deposits can grow to the extent that they often bridge across superheater tubes and tube bundles. This results in premature shutdowns of the facilities causing significant losses. This decline in efficiency appears through the heat transfer resistance created by ash deposits. In combustion systems, the thermal energy transfer from flame to exterior surfaces of ash deposits is by radiative and convective heat transfer. Heat transfer through ash deposits and tube material is by conduction and then by convection to the inside fluid stream. Among the three modes of heat transfer, radiation and conduction are affected greatly by corresponding transport properties of the deposits, such as thermal conductivity, emissivity, deposit strength (characterized by shedding index), etc. Ash deposition rate is proposed to be controlled by the summation of five processes: inertial impaction (I), eddy impaction (E), thermophoresis (T), condensation (C), and chemical reaction (R),1 as described in Eq 1.
dm )I‚G+E+T+C+R dt
(1)
Inertial impaction is simply the process of particles impacting on heat transfer surface due to their momentum, and this process constitutes a major part of the ash deposition mechanism. The ratio of particles impacting a surface to total particles flowing across the projected surface area is called the impaction efficiency, η. The product of particle flux, projected surface area, and impaction efficiency, η is I. The ratio of attached particles to total particles impacting the surface is called the capture efficiency, G. The product of I and G becomes the impacted particles that remain attached. Eddy impaction involves fine ash particles that have been entrained in turbulent eddies. Since turbulent eddies are difficult to describe, this process is little understood. Thermophoresis is the process of particle motion in gas flow due to local temperature gradients. Thermophoretic forces on a particle may be induced either by temperature gradient in surrounding gas or by temperature the gradient in the particle itself. Condensation is the mechanism by which vapors are collected on cooled heat transfer surfaces. If the partial pressure of a vapor at the relatively cold temperature of heat transfer surface exceeds the vapor pressure, the vapor will condense on the surface. However, the vapor pressure of a condensing salt depends on its concentration in (24) Baxter, L. L. Biomass Bioenergy 1993, 4, 85-102. (25) Dayton, D. C.; Milne, T. A. Appl. AdV. Technol. Ash-Relat. Probl. Boilers [Proc. Eng. Found. Conf.] 1996, 161-179.
Figure 1. Multifuel flow reactor.
the gas phase; therefore, condensation of alkali salts is also governed by the mass fraction of salts in fuel. Heterogeneous chemical reactions (e.g., sulfation, alkali absorption, and oxidation) between the gas and deposits or deposit surfaces can also add or remove mass.5,26-28 Several other mechanisms of deposit growth are electrostatic interactions, photophoresis, and Brownian motion; however, there is not enough evidence to suggest that these mechanisms are significant contributors to deposit formation. Experimental Section A premixed, 10 kWth, down-fired, MFR shown in Figure 1 was used to investigate deposition rates. The MFR consists of eight refractory lined circular metal sections connected coaxially. All sections except one are 275-mm long, and all sections have an outside diameter of 365 mm. The refractory is 115-mm thick lined inside a 5-mm thick metal wall, leaving an inside diameter of 120 mm. Section 1 (the top section) contains a window for visual observation of the flame. Section 3 (the third section from the top) is 145-mm long and contains methane injectors used to preheat the reactor to temperatures above 1000 °C. The preheat methane is then shut off during testing. Section 7 contains a portal for inserting the ash sampling probe 1.93 m below the burner surface. Thermocouples placed flush with the inside refractory measure the wall surface temperature profile vertically along the reactor. Fuel and air enter the MFR through a water-cooled burner located at the top of the reactor. Solid fuel feeds volumetrically from an auger feeder located above the reactor. The mass of fuel in the auger bin is weighed continuously, allowing a measurement of fuel feed rate. The fuel is transported to the reactor through a single premixed line consisting of both primary and secondary air. Manual controls located near the MFR meter air, methane, fuel flow rates, and pressure. Flue gases exit the final section of the reactor into a cyclone scrubber located below the reactor, which removes large particulate and SOx before the gases vent to the atmosphere. (26) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78. (27) Dayton, D. C.; Belle-Oudry, D.; Nordin, A. Energy Fuels 1999, 13, 1203-1211. (28) Liu, K.; Wei, X.; Li, D.; Pan, W.-P., Riley, J. T.; Riga, A. Energy Fuels 2000, 14, 963-972.
1010 Energy & Fuels, Vol. 20, No. 3, 2006
Lokare et al.
Figure 2. Deposit collection probe design.
Figure 4. Particle size distribution of biofuels. Table 2. Ash Composition of Fuels (as Received)
Figure 3. Biomass fuel suite and blending configuration.
mass %
Temperature-regulated probes as shown in Figure 2 were used to collect deposits and determine both rate and mechanistic information. A 12.7-mm diameter and 120-mm-long sleeve was fit over the air-cooled probe and was used to collect the deposit. The sleeve consisted of two pieces, each 60-mm long. The two sleeve pieces were connected by male and female relief cuts to improve thermal contact. A groove was milled into the first sleeve section which was attached permanently to the probe surface through a single solder point. Ash deposited on the attached half of the sleeve was scraped into a small container for weighing and future wet chemistry ash analysis. The second sleeve section slid off the probe and was removed. This removable sleeve was weighed before and after deposition to obtain the mass of the deposit. Experimental conditions were selected to simulate those common of superheater environments in commercial combustors. Gas temperature in the superheater section of commercial facilities exceeded 1000 °C, but in the MFR the maximum gas temperatures achieved burning biomass were 900-1000°C with a nominal residence time of approximately 1 s. The probe diameter was selected to best approximate Stokes and Mach numbers of industrial boiler tubes and particles. The probe temperature was kept at 500 °C by regulating the flow of cooling air to simulate superheater tube temperatures in industrial boilers that range from 450 to 550 °C. A suite of six biomass fuels incorporating a wide range of organic and inorganic components was used to investigate blending effects on deposition rate. The fuels included straw, grain screenings, sawdust, shea nut shells, sugar beet pulp, and sunflower shells. Some of the fuel properties are listed in Tables 1 and 2. All of the fuels were prepared and shipped from Denmark. The test matrix included measured deposition rates on each of the six pure fuels and eight fuel blends as shown in Figure 3. The fuel blends were selected by mixing 50% by mass of the four fuels initially expected to have the lowest deposition rates (sawdust, shea nut shells, sugar beet pulp, and sunflower shells) with 50% by mass of the two fuels expected to have the highest ash deposition rates (grain screenings and straw). The test matrix therefore consisted of 14 points, 6 pure fuels, and 8 fuel blends.
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 P2O5 Cl other sum
straw
sawdust
grain screenings
sunflower shells
shea nut shells
52 0.6 1.1 9.2 1.8 0.3 21.9 4 3.2 5.6 0.3
6.4 2.9 0.9 45.3 9.8 3.2 20.6 2.8 2.6 0.2 5.3
34.4 2.2 2.6 15.8 3.9 1.9 19.2 5.1 11.6 2.8 0.5
1.1 0.5 0.9 16 13.1 < 0.2 45.1 11.7 10.1 1.2 0.3
6.6 1.7 2.4 6.4 7.9 0.4 53.3 10.4 9.3 1.4 0.2
100
100
100
100
100
Particle size distributions of all fuels were determined by screening the fuels. Shea nutshells were screened, but their size is not reported because the particle size was 100 µm or less (approaching the lower limit of screen size range, 0.045 mm). The particle size distributions are important for predicting the size of ash particles and the tendency to impact surfaces. Almost all of the fuels analyzed had particle size density functions that were bimodal as shown in Figure 4 with a dominant mode in the 400600 µm range and a secondary mode near 100 µm. Exceptions include sugar beet pulp, which has a larger mode near 100 µm and a smaller mode between 400 and 600, and sawdust, which has a smaller mode at 200 µm. Using the fuel particle size distributions, we predicted ash particle size distribution by assuming the ash fraction of a single fuel particle collapsed into an ash particle. The results are shown in Figure 5. The average specific gravity for the ash particles was assumed to be 2.2. Although the initial particle size of the fuels was fairly uniform, because the ash composition varied among fuels, the ash particle size distribution is different for all fuels except straw and grain screenings and ranges from 30 to 120 µm. Sawdust is expected to have the smallest ash particles due to low ash content, while straw and grain screenings will have the largest ash particles. Data Analysis. A model of ash deposition based on inertial impaction as the dominant contributor to deposition1 was used in an attempt to describe the deposition of the tested biofuels and fuel
Table 1. Fuel Analysis of Biofuels (as Received, Ash-Free) % by weight
straw
sawdust
grain screenings
sunflower shells
sugar beet pulp
shea nut shells
moisture C H O N S
11.0 43.9 5.9 38.57 0.5 0.13
11.0 44.9 5.8 38.1 0.2
