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Energy & Fuels 2002, 16, 343-355

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Pilot-Scale Investigation of the Influence of Coal-Biomass Cofiring on Ash Deposition Allen L. Robinson,*,† Helle Junker,‡ and Larry L. Baxter§ Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550

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Received June 14, 2001. Revised Manuscript Received December 8, 2001

Cofiring biomass with coal is a promising short-term option for reducing the net CO2 emissions from existing coal-fired power plants. This paper examines the effects of cofiring biomass and coal on ash deposition under conditions representative of those found in the superheater region of pulverized-coal boilers. Experiments were conducted with blends of eight different fuelssthree types of bituminous coal, sub-bituminous coal, two types of straw, switchgrass, and wood. For each fuel, reference tests of unblended fuel establish a baseline against which to compare the results from the cofiring tests. The deposition rates for the cofire blends are between the measured deposition rates of the unblended fuels. Therefore, blending straw with coal reduces the high deposition rates observed while firing unblended straw, and cofiring coal with wood results in slightly lower deposition rates than those that occur while firing unblended coal. The primary interaction between the biomass and coal during cofiring is the reaction of the sulfur from the coal with the alkali species from the biomass. This sulfation reduces the stickiness of the deposit, which substantially reduces the deposition rate of the coal-straw blends in comparison to expectations based on the performance of the unblended fuels. Sulfation also reduces the chlorine content of the deposits, potentially reducing the corrosion potential of the deposits. A scaling parameter is proposed to estimate the deposit chlorine content on the basis of the properties of the cofire blend; the ratio of fuel-S to available alkali must be in excess of 5 times the S-to-alkali stoichiometric ratio to eliminate chlorine from the deposit. The results demonstrate that cofiring can mitigate some of the fouling difficulties associated with combustion of high-fouling biofuels.

Introduction Cofiring biomass with coal is a near term option to reduce CO2 emissions from existing coal-fired utility boilers and to increase utilization of biomass energy.1,2 Cofiring refers to the replacement of a fraction (typically less than 15% on an energy basis) of the coal used in an existing power plant with biomass, which in this context refers to wood residues, agricultural residues, and energy crops. Cofiring reduces the net CO2 emissions when the biomass is used in a sustainable fashion. In practice, some fossil energy resources are consumed in biomass cultivation, transportation, and processing. The feasibility of coal-biomass cofiring depends, in part, on its impact on ash deposition. Ash deposits form from fly ash, inorganic vapors, and some gas species that deposit or react through a variety of mechanisms.3 Ash deposition can have important impacts on boiler operations, boiler efficiency, corrosion of heat transfer surfaces, and fly ash utilization. There is a vast literature on the ash deposition characteristics of coal; more * Corresponding author. † Currently at the Department of Mechanical Engineering and the Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA 15213. Telephone: (412) 268-3657. Fax: (412) 268-3348. E-mail: [email protected]. ‡ Currently at Tech-wise, Kraftvaerksvej 53, DK-7000 Fredericia, Denmark. § Currently at the Department of Chemical Engineering, 350 Clyde Building, Brigham Young University, Provo, UT, 84601. (1) Hughes, E. Biomass Bioenergy 2000, 19, 457-465. (2) Tillman, D. A. Biomass Bioenergy 2000, 19, 363-364. (3) Baxter, L. L. Biomass Bioenergy 1993, 4, 85-102.

limited data are available on the ash deposition characteristics of biomass fuels. In comparison, relatively little information on the impact of coal-biomass cofiring on ash deposition has been published. Coal-biomass cofiring has the potential to create ash deposition problems because of the differences in the inorganic composition (high alkali levels) and mode of occurrence of inorganic species (mostly mobile forms) in biomass compared to many coals.3-5 These differences may result in enhanced fouling and slagging when cofiring in pulverized coal boilers, particularly when cofiring coal with agricultural residues or herbaceous materials. Of particular interest is the impact of interactions between the coal and the biomass on ash deposition. Results from commercial scale cofiring demonstrations indicate that cofiring coal with clean wood wastes does not create ash deposition problems.6 Clean wood residues are excellent fuels with low ash and alkali levels,4 but the potential supply of wood residues for cofiring applications is limited. Agricultural residues such as straw represent a significant fraction of biomass resources potentially available for cofiring.7 Agricultural (4) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17-46. (5) Baxter, L. L.; Miles, T. R.; Miles, T. R. J.; Jenkins, B. M.; Dayton, D. C.; Milne, T. A.; Bryers, R. W.; Oden, L. L. Alkali deposits found in biomass boilers: The behavior of inorganic material in biomass-fired boilers - Field and laboratory experiences; Rep # SAND96-8225 Vol II.; Sandia National Laboratories, 1996. (6) Tillman, D. A. Biomass Bioenergy 2000, 19, 365-384.

10.1021/ef010128h CCC: $22.00 © 2002 American Chemical Society Published on Web 02/07/2002

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Energy & Fuels, Vol. 16, No. 2, 2002

Robinson et al.

Table 1. Results of Chemical Analysis of the Coal and Biomass Fuelsa RO

IS

DS

SG

BT

P8

EK

SA

moisture (% mass, as rcvd) 3.41 4.91 proximate analysis (% mass, dry fuel basis) ash 1.31 15.35 volatile 83.71 68.18 fixed C 14.97 16.47

4.69

8.08

22.31

1.52

1.11

10.5

7.37 75.64 17.00

8.28 72.85 18.88

7.22 43.20 49.58

7.73 39.49 52.78

7.59 38.15 54.26

16.46

HHV (as rcvd) BTU/lb

6420

7405

ultimate analysis (% mass, dry fuel basis) carbon 49.47 40.94 hydrogen 5.57 4.79 nitrogen 0.09 0.88 sulfur 0.06 0.42 ash (S- & Cl-free) 1.31 14.69 chlorine 0.00 1.87 oxygen (diff) 43.50 36.41

8023

43.24 5.14 0.78 0.21 6.58 0.57 43.48

44.04 6.56 0.82 0.12 8.20 0.11 40.16

70.37 7.91 1.01 0.56 7.23 0.09 12.83

78.24 5.21 1.41 2.84 7.35 0.11 4.84

77.21 5.34 1.60 0.84 7.46

ash composition (% mass, ash basis) SiO2 44.73 Al2O3 9.41 TiO2 0.56 Fe2O3 7.81 CaO 16.20 MgO 1.25 0.77 Na2O K2O 9.31 P2O5 1.15 SO3 1.41 Cl 0.58 CO2 2.79 total 95.97

41.51 1.00 0.00 0.72 8.13 1.99 0.57 31.95 4.42 3.28 7.42 0.15 101.15

58.15 2.30 0.26 11.51 11.18 5.44 0.38 9.06 4.00 3.04

43.30 18.60 1.50 5.50 22.80 4.70 1.80 0.50 1.30 4.11

42.83 22.11 0.98 20.06 5.73 1.12 1.06 1.21 0.47 4.93

51.75 34.49 1.53 5.20 1.97 0.63 0.34 1.42 1.76 0.35

60.8 19.2 0.68 7.9 1.9 1.9 0.84 2.3 0.21 2.35 0.08

105.32

104.11

100.50

99.44

98.16

38.68 1.36 0.09 0.57 4.32 1.66 13.50 20.50 3.07 4.29 14.50 0.12 102.66

7002

9254

14032

13642

7.55

10749 67.29 4.53 1.33 1.03 16.06 0.01 9.75

Key: RO ) Red Oak wood; IS ) Imperial Wheat straw; DS ) Danish Wheat straw; SG ) Switchgrass; BT ) Black Thunder coal; P8 ) Pittsburgh No. 8 coal; EK ) Eastern Kentucky coal; SA ) South American coal. a

residues and many herbaceous fuels often have high alkali and chlorine levels creating deposition concerns; for example, straw-fired boilers often experience severe slagging, fouling, and corrosion problems.4,5,8,9 Denmark has significant experience cofiring straw with coal at commercial scale.10-13 The commercial scale tests indicate that coal-straw cofiring causes increased ash deposition, corrosion, and fouling of selective catalytic reduction (SCR) catalyst compared to firing coal alone. These problems appeared to be manageable for up to a 20% straw share (by energy). In addition, cofiring straw with coal may reduce some of the problems associated with corrosion and deposition compared to combustion of straw alone.11,14 This paper presents results from a pilot-scale investigation of the ash deposition characteristics of blends (7) Walsh, M. E.; Perlacka, R. L.; Turhollowa, A.; Ugarteb, D. d. l. T.; Beckerc, D. A.; Grahama, R. L.; Slinsky, S. E.; Rayb, D. E. Biomass Feedstock Availability in the United States: 1999 State Level Analysis, http://bioenergy.ornl.gov/resourcedata/index.html, accessed 2000. (8) Nielsen, H.; Frandsen, F.; Dam-Johansen, K.; Baxter, L. Prog. Energy Combust. Sci. 2000, 26, 283-298. (9) Jensen, P. A.; Stenholm, M.; Hald, P. Energy Fuels 1997, 11, 1048-1055. (10) Pedersen, L. S.; Nielsen, H. P.; Kiil, S.; Hansen, L. A.; Damjohanesn, K.; Kildsig, F.; Christensen, J.; Jespersen, P. Fuel 1996, 75, 1584-1590. (11) Frandsen, F. J.; Nielsen, H. P.; Jensen, P. A.; Hansen, L. A.; Livbjerg, H.; Dam-Johansen, K.; Hansen, P. F. B.; Andersen, K. H.; Sørensen, H. S.; Larsen, O. H.; Sander, B.; Henriksen, N.; Simonsen, P. In Impact of Mineral Impurities in Solid Fuel Combustion; Gupta, R., Wall, T., Baxter, L., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 271-283. (12) Wieck-Hansen, K.; Overgaard, P.; Larsen, O. Biomass Bioenergy 2000, 19, 395-409. (13) Andersen, K. H.; Frandsen, F. J.; Hansen, P. F. B.; WieckHansen, K.; Rasmussen, I.; Overgaard, P.; Dam-Johansen, K. Energy Fuels 2000, 14, 765-780. (14) Robinson, A. L.; Junker, H.; Buckley, S. G.; Sclippa, G.; Baxter, L. L. In Twenty-Seventh Symposium (International) on Combustion; Combustion Institute: Boulder, CO, 1998; pp 1351-1359.

of four coals with four different biomass fuels. The coals represent a range of common utility and industrial fuels. The biomass samples include both herbaceous and wood fuels and are representative of typical biomass residues likely to be available in quantities of interest to pulverized-coal-fired systems. Experiments were conducted to determine the deposition rate and deposit chemical composition for blends of these fuels. A scanning electron microscope (SEM) was used to examine the deposit microstructure. The cofiring results are compared to the behavior of the unblended fuel samples to identify interactions between the coal and biomass that affect the deposition characteristics of the blend. To help explain the observed interactions, the data are compared to thermochemical equilibrium calculations. Preliminary results from this investigation have previously been reported.14,15 Methods Fuel Selection and Preparation. This paper examines the deposition behavior of blends of four types of coal, Eastern Kentucky (EK), Pittsburgh No. 8 (P8), Black Thunder (BT), and South American (SA), with four different biomass fuels, Red Oak wood (RO), Imperial (American) Wheat straw (IS), Danish Wheat straw (DS), and Switchgrass (SG). Several samples of each fuel were analyzed in order to characterize the variability of fuel properties. Table 1 lists average fuel properties determined from standard proximate, ultimate, heating value, and ash chemistry analyses. All of the coal samples are common utility and industrial fuels whose properties are representative of (15) Junker, H. Cofiring Biomass and Coal: Plant Comparisons and Experimental Investigation of Deposit Formation. Ph.D. dissertation, Aalborg University, Denmark, 1997.

Coal-Biomass Cofiring and Ash Deposition

Energy & Fuels, Vol. 16, No. 2, 2002 345

Table 2. Coal-Biomass Blends Considered by This Study

a

coal

biomass

blends (%coal/%biomass on an energy basis)

Eastern Kentucky Eastern Kentucky Eastern Kentucky Pittsburgh No. 8 Pittsburgh No. 8 Pittsburgh No. 8 Pittsburgh No. 8 Black Thunder South American

Imperial Wheat straw Danish Wheat straw Red Oak wood Imperial Wheat straw Danish Wheat straw Red Oak wood Switchgrass Red Oak wood Danish Wheat straw

97.5/2.5; 95/10;a 90/10; 85/15;a 80/20; 75/25; 50/50 85/15a 85/15;a 75/25a 85/15a 85/15a 85/15a 85/15 95/5; 85/15; 75/25; 50/50; 25/75 85/15a

Blends for which deposit composition was measured.

a wide range of typical U. S. coals. Pittsburgh No. 8 is a high-sulfur bituminous coal with a low ash fusion temperature; Eastern Kentucky is a modest-sulfur bituminous coal with a high ash fusion temperature; Black Thunder is a low-sulfur western sub-bituminous coal; the South American coal is high-volatile bituminous coal. The biomass samples were chosen to represent typical biomass residues likely to be available in quantities of interest to pulverized-coal-fired systems. These samples also span the wide range of ash contents and ash properties found in common biomass fuels.4 Red Oak wood is a typical hardwood fuel, with low ash content high in calcium. Straw is a common herbaceous agricultural waste; both of the samples examined here have moderate ash contents with significant alkali and chlorine levels. The switchgrass sample is an herbaceous energy crop with ash properties falling between that of the straws and the Red Oak. The South American coal and the Danish Wheat straw samples are the same as those used in commercial-scale Danish cofiring experiments.13 The coal samples were fired in pulverized form, commercial grind, 70% through 200 mesh. Samples of the biomass fuel were processed using a Thomas-Wiley laboratory mill (model no. 4). For these experiments, the biomass samples were milled finely enough to pass through a 1-mm mesh. Coal-biomass blends are prepared prior to an experiment by mixing samples of a pulverized coal and a milled biomass in correct proportion. Table 2 lists all of the blends considered in this paper. A blend ratio of 85% coal and 15% biomass on an energy basis represents an upper end of realistic blends for commercial scale cofiring. On a mass basis this mixture contains roughly 25-30% biomass because of the almost factor of 2 difference in heating value between coal and biomass. Unless otherwise indicated, all references to blend ratio in this paper refer to blends on an energy basis. For each fuel, a reference test of unblended fuel is performed to establish a baseline for assessing the impact of cofiring on deposition. Experimental Setup and Procedure. Ash deposits from different blends of biomass and coal were collected using Sandia National Laboratories Multifuel Combustor (MFC). A schematic diagram of the MFC is shown in Figure 1. The MFC is a pilot-scale, 4.2-m-high, downfired, turbulent flow reactor that simulates gas temperature and composition histories experienced by particles in commercial combustion systems. A typical solid fuel feed rate is 3.5 kg/h, which corresponds to a heat input of 30 kW for a bituminous coal. The combustor has a circular cross section with an ID of 15 cm. Electric heaters are used to control the temperature of

Figure 1. Schematic of the MFC.

the walls of the combustor. A more detailed discussion of the MFC is available in the literature.16 Solid fuel (coal, biomass, or a blend) is injected into the MFC pneumatically via a water-cooled lance inserted through the side of the furnace. For these experiments, the fuel was injected at the top of the combustor just below the natural gas burner, ∼4.2 m above the test section. The residence time of a fuel particle in the combustor is approximately 1 s, which is comparable to the residence time available for combustion of particles in commercial pulverized coal (pc) boilers. The natural gas preheat burner was operated during these experiments to create a vitiated air stream. The fuel blend was fired for approximately 1 h before starting to collect deposits in order to ensure steadystate operations. A constant combustion air flow rate was used for these experiments in order to create consistent gas and particle velocities across the set of experiments. The solid fuel feed rate was adjusted to maintain an oxygen level in the combustion products of 3 vol % (dry basis) at the reactor exit. The oxygen level was monitored by continuously sampling combustion products from a port 0.6 m above the reactor exit, passing them through a (16) Baxter, L. L. Combust. Flame 1992, 90, 174-184.

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heated filter and a heated line, and then analyzing them using a paramagnetic O2 analyzer. For these experiments, the MFC exit gas temperature was maintained at 1000 °C by adjusting the reactor wall heaters. Ash deposits are collected on an air-cooled, stainless steel probe placed in the test section of the MFC (see Figure 1). This probe is designed to simulate a superheater tube of a commercial boiler. The surface temperature of the probe is measured using two type-K thermocouples embedded into the outside of the probe wall. A constant probe surface temperature of 540 °C is maintained during the experiment by adjusting the flow rate of the cooling air. The outside diameter of the deposition probe (1.6 cm) is selected to match the Stokes number found in typical boilers. We choose to match the Stokes number because inertial impaction contributes the majority of the deposit mass in commercial boilers. Matching Stokes numbers ensures that the size distribution of fly ash particles striking the deposition probe in the MFC by inertial impaction is the same as that hitting a superheater tube in a commercial boiler. We cannot match both the Stokes and Reynolds numbers found in typical boilers because the gas and particle velocities through the test section of the MFC are roughly a factor of 4 smaller than typical convective pass velocities (∼5 m/s in the MFC versus ∼20 m/s in a power plant). The Reynolds number of the deposition probe used in this study is roughly a factor of 10 smaller than that of a typical superheater tube, which results in lower convective heat transfer to the deposition probe in the MFC compared to a typical superheater tube. In all cases, we match the surface temperature of the deposition probe to that found in a utility boiler by adjusting the cooling air flow rate through the probe. Deposits are collected for a 1-h period. A test period of this length ensures that enough ash is deposited on the probe for chemical analysis, but is short enough to minimize the chance of part of the deposit falling off the probe. A limited number of 4-h tests were conducted to examine the influence of experiment duration on deposit composition. At the end of each experiment the sample probe is carefully removed from the reactor test section, and the deposits are weighed and photographed. For a subset of the experiments a portion of the sample is scraped off the probe for standard analyses to determine the bulk elemental composition of the deposit (Si, Al, Ti, Fe, Ca, Mg, Na, K, P, and S by Inductively Coupled Plasma-Atomic Emission Spectroscopy; Cl by ion chromatography, and C using a Carlo Erba analyzer). Table 2 indicates the blend ratios for which deposit composition measurements were made. Duplicate analyses were performed on all of the deposit samples except for the unblended wood and wood-coal deposits because of the limited amount of deposit material. SEM analyses were performed on selected deposits to examine deposit microstructure and chemical composition. For SEM analysis, a removable section of the probe with an undisturbed sample of the deposit was impregnated in epoxy. The impregnated samples were sectioned and polished using Buehler automatic lapping oil no. 60-32-50-128 as the lubricating fluid to minimize dissolving of chlorides or sulfates.

Robinson et al.

Data Analysis. Ash Deposition Rate. The ash deposition rate, DR (grams of deposit per gram of fuel), is defined as

DR )

Ma 0.13Mf

(1)

where Ma is the mass of the ash deposit, Mf is the mass of the fuel fired, and 0.13 is the fraction of the crosssectional area of the MFC occupied by the probe. Particle Collection Efficiency. The particle collection efficiency is a more convenient parameter than deposition rate to compare the ash deposition characteristics of different fuels because it accounts for differences in fuel ash content. The particle collection efficiency, β, is defined as

β)

DR Xa

(2)

where Xa is the mass fraction of ash in the fuel on an as-received basis (Table 1). The particle collection efficiency varies between 0 and 1sa value of 0 indicates that no ash is collected on the probe; a value of 1 indicates that the probe collects 100% of the ash in the area swept by the probe. Enrichment Factor. Results of deposit chemical composition are presented in terms of an enrichment factor. The enrichment factor is defined as

EFi )

Xd,i Xa,i

(3)

where Xd,i is the mass fraction of element i in the deposit on a carbon-free basis, and Xa,i is the mass fraction of element i in the fuel ash (Table 1 contains fuel ash content on an oxide basis). An enrichment fraction greater than 1 indicates enrichment of an element in the deposit relative to the fuel ash (the mass fraction of an element in the deposit is greater than it is in the fuel ash); an enrichment fraction of 1 indicates that the mass fraction of an element in the deposit and the fuel ash is the same; and an enrichment fraction less than 1 indicates depletion of an element in the deposit relative to the fuel ash (the mass fraction of an element in the deposit is less than it is in the fuel ash). Experimental Variability and Uncertainty. To identify the impacts of cofiring biomass and coal on ash deposition, the observed changes in deposition characteristics must be greater than the experimental uncertainty. Of particular importance is the uncertainty of the deposition characteristics of the unblended fuels because they are used as reference points to evaluate blend performance. Experimental uncertainty was examined in two ways. First, we compared results from repeated experiments conducted under nominally identical conditions. Second, we estimated values by propagating the uncertainty of the individual measurements (e.g., deposit mass) using standard error analysis procedures.17 Comparing results from repeated experiments conducted under identical conditions provides an overall (17) Taylor, J. R. An introduction to error analysis: The study of uncertainties in physical measurements, 2nd ed.; University Science Books: Sausalito, CA, 1997.


estimate of the experimental uncertainty. Variations between identical experiments are primarily caused by the complexity of solid fuel combustion and ash deposition in the MFC and the inherent variability of the fuel properties. The uncertainties of the individual instruments, such as the balance used to measure the deposit mass, are generally a small contributor to the overall uncertainty except for the unblended wood experiments. The instrumental uncertainties are significant for the unblended wood experiments because only a small amount of deposit material was collected (∼10 mg). Data from repeated experiments are only available for a limited set of fuel combinations because of the expense and scale of MFC operations. Repeated experiments were conducted using unblended Pittsburgh No. 8 coal, unblended wood, unblended Danish Wheat straw, and four different cofire blends. An experiment was typically only repeated once, but in a few cases experiments were repeated as many as four times. To quantify variability we define the relative variability as the standard deviation divided by the average determined from a group of experiments using the same fuel blend. The relative variability in the deposition rate calculated from repeated experiments involving unblended coal or coal-wood blends is less than 10%. The relative variability in the deposition rate calculated from repeated experiments involving unblended Danish Wheat straw or coal-straw blends ranged from 2% to 19%. The relative variability in the deposition rate calculated from repeated unblended wood experiments is large (63%) because an extremely small deposit is formed while firing unblended wood. However, the large relative variability in the deposition rate of unblended wood has little effect on our ability to draw conclusions from the results of the wood tests because on an absolute basis this variability is quite small. The relative variability in deposit chemical composition typically ranged between 10% and 20%. The relative variability of refractory species such as SiO2 or Al2O3 that contribute a large fraction of the deposit mass is less than 5%. Volatile species such as alkali and Cl exhibit more variability. The largest variability was observed for Cl in deposits formed while firing 85% Eastern Kentucky coal and 15% Imperial Wheat straw. For this blend the relative variability of Cl was 33% based on the analyses of 4 different deposits. The analytical procedures used to determine deposit composition do not appear to be a significant source of the experimental variability. We estimated the precision of these procedures by comparing results from duplicate analysis performed on the same deposit. For species present at levels greater than 1% of deposit mass, the average relative deviation between duplicate analyses on the same deposit was less 1% for Si, Ca, Na, and S; less than 2% for Al, Fe, and K; and less than 5% for Cl and C. These values are significantly smaller than the previously discussed variations in deposit composition between repeated experiments, and are comparable or larger than uncertainties estimated from analyses of standard reference materials performed for quality assurance and quality control purposes. Variation in fuel properties may be an important contributor to the observed experimental variability. The variability in fuel properties was determined by


comparing results from repeated fuel analyses. The maximum relative variability in the ash content of the coal and biomass samples was 4% and 12%, respectively. In terms of elemental composition of the fuel ash, Si is the least variable (relative variability of less than 5%) and Cl and alkali species are the most variable (relative variability of less than 19%) when considering elements present at levels greater than 1% of fuel ash on a mass basis. The available information suggests that relative differences in deposition rate and deposit composition greater than (20% are significant. This figure bounds the majority of the observed experimental variability and provides a benchmark for comparing experiments. This value is also greater than the uncertainty determined by propagating the uncertainties of the individual measurements using standard error analysis procedures (except for the case of unblended wood). In general, our conclusions are based on relative differences that are much larger than (20%. All conclusions are based on the results from a large number of experiments. For example, conclusions regarding changes in deposit composition caused by cofiring straw and coal are based on the chemical analyses of 25 different deposits (17 from coal-straw blends, and 8 from unblended fuels). It is particularly important that the coal-straw experiments involve blends of 5 different fuels (3 coal and 2 straw), because this reduces the possibility of drawing incorrect conclusions due to an erroneous reference test of unblended fuel. No conclusions are based on the results from a single experiment, or blends of only two fuels. Results and Discussion This section discusses the ash deposition characteristics of cofire blends fired in the MFC. The experiments were conducted under the same conditions to facilitate intercomparisons among the results for the different fuels and blends. First, data from reference tests of unblended fuels are presented to establish baseline deposition characteristics for each fuel. The results from the experiments of coal-biomass blends are then examined to identify interactions between the coal and biomass that significantly impact ash deposition. Statements regarding the performance of a cofire blends are based on comparisons with the unblended fuels; whether the blend has superior or inferior performance depends on which fuel the blend is being compared to. For example, a coal-straw blend has improved ash deposition characteristics relative to unblended straw, but worse characteristics relative to unblended coal. Unblended Fuels. Pictures of deposits formed while firing Eastern Kentucky coal, Red Oak wood, and Imperial Wheat straw are shown in Figure 2 to illustrate the strikingly different appearance of the deposits formed while firing the different fuels. These differences are due to the wide variation in ash contents, fly ash size distribution, ash composition, and mobility of the inorganic material of the different fuels. All of the coal deposits are similar in appearance to the Eastern Kentucky coal deposit shown in Figure 2a. Over a 1-h period, Eastern Kentucky coal forms a relatively small deposit with a triangular structure on the leading edge of the probe. The triangular shape

348


Robinson et al.

Figure 2. Pictures of deposits formed while firing unblended fuel (a, b, c) and cofire blends (d, e, f). Each of the pictures has a slightly different scale; the diameter of the probe in each picture is 1.6 cm.

indicates that the coal fly ash particles are likely to bounce upon impact. The coal deposits have a powdery structure and could be easily blown off the probe. The most notable feature of the picture of the Red Oak wood deposit shown in Figure 2b is the lack of deposit. Closer examination reveals a fume-like deposit with only a few impacted particles on the upstream side of the probe. Shiny, bare metal can be seen on the downstream side of the probe. The Imperial Wheat straw deposit (Figure 2c) is significantly larger than the Eastern Kentucky coal and Red Oak wood deposits. This straw deposit has an ∼1cm-thick layer of impacted particles with a columnar, boxy structure on the upstream side of the probe. The columnar structure is due to the stickiness of the straw fly ash particles. The dark color of the particles is not due to unburned carbon; the carbon content of the Imperial Wheat straw deposit was 0.6% on a mass basis. A thick white layer can clearly be seen on the sides and bottom of the probe. This white material is alkali chloride salts that have condensed onto the cool probe surface. During the initial stages of deposit formation this layer covers the entire probe. The deposits formed while firing Danish Wheat straw have a similar appearance and size to that of the Imperial Wheat straw deposit shown in Figure 2c. The particles in the straw deposits are bonded together creating an interconnected structure, which forms large flakes of material when the deposit is removed from the probe. Deposition Rate. Figure 3 presents the ash deposition rates (g-deposit/kg-fuel) measured while firing unblended fuels. The deposition rates for the coals are relatively similars1.3, 2.0, 2.2, and 4.5 g of deposit per

Figure 3. Deposition rate as a function of particle collection efficiency for the tests conducted while firing unblended fuels. Note that results are plotted on a log-log scale. Key: RO ) Red Oak wood; SG ) Switchgrass; DS ) Danish Wheat straw; IS ) Imperial Wheat straw. The error bars represent an estimate of the experimental uncertainty.

kg of fuel for Black Thunder coal, Pittsburgh No. 8 coal, Eastern Kentucky coal, and South American coal, respectively. Almost a factor of 800 separates the ash deposition rates of the biomass fuels with Red Oak wood and Imperial Wheat straw the two extremes. The deposition rate of Red Oak wood, 0.04 g of deposit per kg of fuel, is almost 2 orders of magnitude less than that of all the coals. The very low deposition rate of the wood fuel suggests that it is unlikely to cause ash deposition


problems, which is consistent with previous experience cofiring coal with clean wood wastes.6 The ash deposition rate for both straw samples is almost an order of magnitude greater than that of the coals. The high deposition rate of the straw samples is consistent with previous investigations that report severe slagging, fouling, and corrosion problems in straw-fired boilers.4,5,8,9 The deposition rate of the switchgrass fuel (0.4 g of deposit per kg of fuel) is lower than that of the coals. Ash deposition rate is a useful parameter for assessing overall fouling potential of a fuel, but particle collection efficiency provides more insight into the inherent deposition characteristics of the different fuels because it accounts for variations in fuel ash content. Figure 3 also plots the particle collection efficiencies for the unblended fuels. The variation in particle collection efficiency among the coals is only 30%, which indicates that the coal samples have similar deposition characteristics and that the differences in deposition rate are mainly due to differences in coal ash content. A factor of 65 separates the particle collection efficiencies of the different biomass samples, which reflects the significant differences in deposition characteristics of the biomass samples. The trends in particle collection efficiency are similar to that of deposition rate; Red Oak wood has by far the lowest collection efficiency, whereas the two straw samples have the highest. We attribute the factor of 65 that separates the particle collection efficiencies of the biomass fuels to a combination of chemistry and particle size effects. The high alkali content of the straws results in stickier particles and softer ash deposits, which increases the probability that particles that strike the deposit or probe will stick. Alkali materials have low melting points when not chemically combined with other materials and uniformly lower softening and melting points of silicaand aluminum-based materialssthe melting points of silica, potassium silicate, and potassium hydroxide are approximately 1700, 980, and 380 °C, respectively, depending on crystal structure. In addition, a typical straw fly ash particle is much larger than a wood fly ash particle from fuel particles of the same initial size because of the straw’s much higher ash content and the presence of large silica grains in the straw. The larger particle size increases the impaction rates of straw fly ash compared to that of wood fly ash because inertial impaction efficiencies on probes depend on the square of the particle diameter.18 Deposit Chemical Composition. Results from the chemical analyses of deposits collected during the coal and straw reference tests are shown in Figure 4. For discussion the results are presented in three groups: nonvolatile species: Si, Al, Fe, and Ca (Figure 4a); semivolatile species: Na and K (Figure 4b); and volatile species: S and Cl (Figure 4c). First we discuss the composition of the coal deposits, followed by a discussion of the biomass deposits. Coal Deposits. Examining the results in Figure 4 for the deposits formed while firing Eastern Kentucky and Pittsburgh No. 8 coal indicates an increase in iron concentration, and a decrease in calcium, sulfur, and chlorine concentration in the deposits relative to the fuel ash. Within experimental uncertainty, the concentration (18) Baxter, L. L.; DeSollar, R. W. Fuel 1993, 72, 1411-1418.


Figure 4. Deposit compositions reported as enrichment factors for the unblended coal and straw reference tests: (a) Si, Al, Fe, and Ca (nonvolatile species); (b) Na and K (semivolatile species); and (c) S and Cl (volatile species). Key: EK ) Eastern Kentucky coal; P8 ) Pittsburgh No. 8 coal; DS ) Danish Wheat straw; IS ) Imperial Wheat straw. The error bars represent an estimate of the experimental uncertainty.

of the four other species is essentially the same in both the deposit and the fuel ash. Note that the enrichment factors for elements present at low levels such as sodium in Eastern Kentucky coal are often highly uncertain. Deposits formed while firing Pittsburgh No. 8, a relatively high iron-content coal, have the largest enrichment of iron. This is not due to the iron content per se but to the form of iron in this coal. Iron in Pittsburgh No. 8 coal appears dominantly in the form of pyrite. The enrichment of iron in deposits from pyriterich fuels is a common observation during coal combustion.19 The decrease in deposit sulfur concentration is due to the much higher temperatures that the coal particles are exposed to during combustion than the temperatures particles are exposed to during ash analysis. For ash analysis, a sample is heated to a temperature between 700 and 800 °C; at these temperatures, a moderate amount of sulfur remains in the ash. However, at combustion temperatures greater than 1200 °C, essentially all of the sulfur-containing inorganic species (19) Raask, E. Mineral impurities in coal combustion: Behavior, problems, and remedial measures; Hemisphere Publishing Corporation: Washington, DC, 1985.

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in coal (primarily sulfates and pyrites) decompose to form SO2, and none of the sulfur in the fuel passes unreacted through the combustor to reach the deposit. At the temperature of the deposit/probe, some of the gasphase SO2 reacts with alkaline earth and, to a lesser extent, alkali oxides and salts to form sulfates. The net effect is a reduction of the concentration of sulfur in the ash deposit relative to the S content of ash determined by a standard ash analysis. The large reduction in the chlorine content of the deposit relative to the fuel ash indicates that the chlorine in the coal occurs in a mobile form, which is released fairly early in the combustion process. The fate of chlorine is discussed in detail in subsequent sections of this paper. None of these coals is considered to be a high-chlorine coal. Biomass Deposits. Results from the chemical analyses of the deposits collected during the straw reference tests are also shown in Figure 4. Figure 4a indicates that the silicon and calcium levels in the ash deposits are greater than that in the fuel ash for both straw fuels. The cause of the large enrichment of iron in the Danish Wheat straw deposit is unknown, and is likely an artifact. The fuel ash of Danish Wheat straw has very low iron levels (Table 1). Figure 4b shows that there are significant changes in deposit alkali levels relative to fuel ash for the biomass fuels. The potassium levels in both straw deposits are lower than those of the fuel ash; the sodium levels in the Imperial Wheat straw deposit are lower than in the fuel ash. Low fuel sodium levels result in a large uncertainty in the enrichment factor for sodium in Danish Wheat straw deposit. The depletion of alkali species in the straw deposits relative to the fuel ash indicates that these species are mobile and enter the gas phase during the combustion process, which is consistent with results from chemical fractionation analysis.5 For short-duration tests such as these, gas-phase alkali primarily contributes to an ash deposit by condensation reactions. Additionally, alkali species can react with silicon to form alkali silicates. This reaction can occur between residual alkali in the fly ash particles (condensed-phase reaction) or between gas-phase, alkali-bearing species and condensed-phase silica or silicates. Alkali silicates have relatively low melting temperatures, which increases the probability that a particle impacting the probe surface will stick. The formation of alkali silicates may explain the enhancement in the silicon concentration shown in Figure 4a in the ash deposits formed while firing straw. Figure 4c shows substantial reduction of the chlorine and sulfur concentration in the ash deposits formed while firing straw. As in the case with the coals, we suspect that the reduced sulfur concentration in the straw ash deposits relative to the fuel ash is due to the difference between the combustion and ashing temperatures. Most of the chlorine and all of the sulfur are expected to enter the gas phase during combustion. The chlorine detected in the ash deposit formed while firing straw is probably due to formation of gas-phase KCl or NaCl and subsequent condensation on the probe, which is consistent with the white layer in the picture of the Imperial Wheat straw deposit shown in Figure 2a. In addition to condensing, these salts can also nucleate to

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Figure 5. Measured particle collection efficiency for blends of Eastern Kentucky coal and Imperial Wheat straw as a function of ash origin (fraction of ash from biomass). The error bars represent an estimate of the experimental uncertainty.

form submicron aerosol particles that can be transported to the probe surface by thermophoresis. Cofire Blends. Pictures of deposits formed while cofiring Eastern Kentucky coal with Imperial Wheat straw and with Red Oak wood are also shown in Figure 2. The appearance of the coal-wood deposit is very similar to that of the unblended coal. After 1 h, the overall appearance of the deposit formed while firing the straw-coal blend is more similar to that of the unblended coal than that of the unblended straw, but the cofire deposit is substantially larger and has a more boxy structure than the unblended coal deposit. A layer of white fume can also be seen on the downstream side of the straw-coal cofired deposit suggesting an alkali chloride layer, similar, but to a significantly lesser extent, than that formed while firing unblended straw (Figure 2c). The picture of the Eastern Kentucky coal and Imperial Wheat straw deposit collected for 4 h provides some insight into the evolution of the deposit with time. Comparing the pictures of the 1- and 4-h deposits indicates that the height of the coal-straw deposit continues to increase with time, resulting in the formation of a relatively narrow deposit wall along the upstream side of the probe. The approximately 3 cm height of the 4-h deposit underscores the internal strength of the deposits formed while firing the coalstraw blends. Deposition Rate. Figure 5 presents particle collection efficiencies measured while cofiring blends of Eastern Kentucky coal and Imperial Wheat straw plotted as a function of ash origin (fraction of ash in the cofire blend that comes from the biomass). To provide reference points for evaluating the performance of the cofire blends, Figure 5 also shows the collection efficiencies measured while firing unblended Eastern Kentucky coal and Imperial Wheat straw. The data indicate that the particle collection efficiencies for all of the cofire blends fall between the observed behaviors of the unblended fuels. This finding is true for both particle collection efficiency and ash deposition rate for all of the blends considered in this study. Therefore, blending straw with coal creates a fuel blend with a lower collection efficiency than an unblended straw, but with a higher collection efficiency than an unblended coal. Similarly, cofiring coal with wood results in slightly lower particle collec-


tion efficiencies than those that occur while firing unblended coal; however, the overall deposition rates of the coal-wood blends are only slightly lower than that of the unblended coal because of the very low ash content of the Red Oak wood sample. To identify interactions between the coal and the biomass when cofiring, we linearly interpolate between the particle collection efficiencies measured during the reference tests (100% coal or 100% biomass). Ash origin is used as the independent variable for the interpolation because it should be a better indicator for the blend particle collection efficiency than the energy or mass fraction of the blend. The straight line drawn in Figure 5 indicates the set of interpolated values. For the simplest casesno interactions (or offsetting interactions) between the coal and biomasssthe particle collection efficiency measured while cofiring would fall on this line, and the particle collection efficiency is accurately interpolated for a cofire blend on the basis of the blend ratio and the results of the reference tests. Deviations from this line indicate that interactions between the coal and biomass affect the particle collection efficiencies. Comparing the measured deposition rates to the interpolated values in Figure 5 indicates that interactions occur while cofiring Eastern Kentucky coal and Imperial Wheat straw that reduce the measured particle collection efficiency of the coal-straw blends relative to expectations based on the performance of the unblended fuels. This reduction is greater than the expected experimental uncertainty. Although Figure 5 is useful for assessing interactions between two fuels for a given set of combustion conditions, this approach has two major drawbacks. First, similar diagrams must be drawn to examine particle collection efficiencies at different combustion conditions and for different fuel pairs. Second, interpretation of results in Figure 5 is highly dependent on the uncertainty of the two reference tests used to define the interpolated behavior. A parity diagram, such as the one shown in Figure 6, overcomes both of these limitations by combining results from tests involving all of the different fuel combinations into one plot. This parity diagram plots interpolated versus measured particle collection efficiency for all of the cofire blends; each point requires three experimental measurements (two reference tests to establish the interpolated behavior and the blended fuel test to establish the measured behavior). Again, the interpolant is the fraction of the ash in the cofire blend contributed by the biomass fuel. Since the interpolated values shown in Figure 6 are calculated using results from reference tests of 8 different unblended fuels, a parity diagram reduces the chance of erroneous conclusions due to problems with a single reference test. Dashed lines are drawn in Figure 5 to illustrate how a parity diagram is constructed. For each blend ratio there is both a measured and an interpolated value. The vertical dashed line in Figure 5 indicates a blend for which straw contributes ∼80% of the total ashsa blend of 50% Eastern Kentucky coal and 50% Imperial Wheat straw on an energy basis. The horizontal dashed lines mark the interpolated and measured particle collection efficiencies for this blend, 17.6% and 10.1%, respectively. Pairs of measured and interpolated values are similarly


Figure 6. Particle collection efficiency measured while firing different fuel blends. As discussed in the text, the interpolated values are determined by linearly interpolating between the results from the tests of the unblended fuels. Points below the 1:1 parity line indicate decreased deposition relative to the tests of unblended fuels. The dashed lines are an estimate of the uncertainty due experimental variability ((20%).

determined for all of the experiments and plotted against each other to construct a parity diagram such as the one shown in Figure 6. For each coal-biomass blend there is only one interpolated value, but potentially more than one measured value depending on the number of experiments performed with that particular coal-biomass blend. For the data shown in Figure 5, this situation applies to the blend that is ∼40% ash from straw. A parity diagram is interpreted by comparing the plotted values to the parity (diagonal) line. Points that fall on the parity line indicate that no significant net interaction is observed between the fuels; i.e., the fuels behave as if they were combusted in isolation with the products mixed before making the measurement. Deviations from the parity line indicate interactions between the coal and biomass that affect deposition. Using this framework, the deposition behavior of a wide range of fuel pairs and combustion conditions can be efficiently examined to help identify systematic trends. A parity diagram combining all of the particle collection efficiency data for all of the cofire blends is shown in Figure 6. The dashed lines indicate an estimate of the experimental uncertainty ((20%). All of the blends fall on or below the parity line. Points below the line indicate that the cofire blends have a lower deposition rate than would be expected on the basis of the behavior of the unblended fuels. This reduction appears particularly significant for the coal-straw blends. The measured collection efficiencies for the coal-straw blends are on average 37% lower than the interpolated values, a difference significantly larger than the expected experimental uncertainty ((20%). It is unlikely that this reduction is an artifact due to problems with the references tests of unblended coal and straw used to determine the interpolated values because these values

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are based on 8 different reference tests using 5 different fuels (3 coal and 2 straw). This reduction is discussed further after presentation of the results from the measurements of deposit composition. Deposit Chemical Composition. This section focuses on the chemical composition of the coal-straw deposits. The composition of the coal-wood deposits are the same as that of the unblended coal deposit (within experimental uncertainty), except for an enhancement in the calcium levels of the cofire deposit relative to the unblended coal deposit. Parity diagrams are shown in Figure 7 to identify interactions between the coal and straw that affect deposit composition while cofiring. Figure 7a presents data for the nonvolatile species Si, Al, Fe, and Ca; Figure 7b presents data for the semi-volatile species Na and K; and Figure 7c presents data for the volatile species S and Cl. Points below the 1:1 parity line indicate that the cofire deposit is depleted in an element; points above the 1:1 parity line indicate that the cofire deposit is enriched in an element. This enrichment or depletion is relative to the expected cofire deposit composition determined by linearly interpolating between the results of the reference tests based on blend ratio. The dashed lines shown in Figure 7 are an estimate of the uncertainty due experimental variability ((20%). Again, the large number reference tests (8) used to calculate the interpolated values shown in Figure 7 makes it unlikely that problems from a single reference test will bias interpretation of the results. Figure 7a shows that within experimental uncertainty the Al, Fe, Ca, and Si levels of all the deposits formed while cofiring fall on the parity line indicating that the deposit composition of these nonvolatile species is not affected by cofiring. These species behave as if the fuels were combusted in isolation, and then the deposits mixed before analysis. Figure 7b indicates that the deposit Na varies linearly with ash composition; while there is some enhancement in the K levels in some deposits. The deposit Cl and S data shown in Figure 7c reveal the major changes in deposit composition due to cofiring. On average there is a factor of 7 enhancement in the S level of the cofire deposit relative to expectations based on linearly interpolating between the results from the tests of unblended fuel. This enhancement is significantly greater than the uncertainty in deposit composition due to experimental variability. There is a significant reduction in the Cl content for several of the deposits. The deposits formed while firing 85% Eastern Kentucky coal and 15% Imperial Wheat straw show little change or a slight enhancement of the Cl levels. The reduction in deposit Cl and enhancement of deposit S is consistent with data from commercial-scale coalstraw cofiring tests.11,13 Coal-Biomass Interactions. For the fuels examined here significant interactions occur when cofiring coal and straw. These interactions change the deposit chemical composition, and reduce the particle collection efficiency and ash deposition rate. The fate of the alkali species is the key to understanding the coal-straw interactions. The reduction of deposit Cl and enhancement of deposit S shown in Figure 7c indicates that alkali chlorides present in

Robinson et al.

Figure 7. Deposit composition measured while cofiring different coal-straw blends: (a) Si, Al, Fe, and Ca (nonvolatile species); (b) Na and K (semi-volatile species); and (c) S and Cl (volatile species). As discussed in the text, the interpolated values are determined by linearly interpolating between the results from the tests of the unblended fuels. The solid diagonal lines are the 1:1 parity lines. The dashed lines are an estimate of the uncertainty due experimental variability ((20%).

deposits formed while firing pure straw are transformed into alkali sulfates when S-containing coal is added to the system. SEM analysis of the deposit immediately adjacent to the probe surface provides additional insight into this transformation. Figure 8 shows pairs of SEM


Figure 8. SEM images of a deposit formed while firing 100% Imperial Wheat straw and a deposit formed while firing a blend of 85% Eastern Kentucky coal and 15% Imperial Wheat straw by energy. Sets of paired images are shown: (a) secondary electron image, (b) chlorine, (c) potassium, and (d) sulfur. The column of images on the left is from the deposit formed while firing pure Imperial Wheat straw and the column on the right is from the deposit formed while firing the coalstraw blend. The probe can clearly be seen in the bottom of image (a).

micrographs from deposits formed while firing Imperial Wheat straw and a blend of Eastern Kentucky coal and Imperial Wheat straw. The probe surface can be seen at the bottom of the images. The Imperial Wheat straw deposit contains a thick K and Cl layer immediately adjacent to the probe surface, indicating the condensation of alkali chlorides on the relatively cool probe surface during the initial stages of deposit formation. Little evidence can be found of Cl in the Eastern Kentucky coal-Imperial Wheat straw deposit, but a clear K and S layer adjacent to the probe surface. This


layer could have been formed by K2SO4 directly depositing on the probe or by deposition of KCl followed by sulfation to release the Cl and create a K2SO4 layer. The transformation of alkali chlorides released during straw combustion into alkali sulfates is consistent with the changes in particle collection efficiency in the coalstraw blends shown in Figures 5 and 6. This transformation decreases the stickiness of the fly ash and deposit material formed from the straw; the melting temperatures of KOH, KCl, and K2SO4 are 380, 790, and 1100 °C, respectively. The alkali released from the straw will also react with silicates from the coal to create alkali silicates and alkali-aluminum silicates, which will increase the stickiness of the coal fly ash relative to firing unblended coal. However, Figures 5 and 6 show that the overall effect of the interactions is to reduce particle collection efficiency, which suggests that, under the conditions of these experiments, changes in stickiness due to sulfation are more significant than those due to the formation of silicates. Thermochemical equilibrium can be used to better understand the interactions between coal and straw. Andersen et al. used an equilibrium model to predict the composition of deposits formed from combustion of unblended Danish Wheat straw, unblended South American coal, and a blend of 81.7% South American coal 18.3% Danish Wheat straw on an energy basis.13 Their calculations include a large number of species containing C, H, O, N, S, Cl, Al, Si, K, and Ca, and assume that the gas-phase species are ideal and that condensedphase species are pure. The deposit composition data for both unblended straw and coal-straw blends presented here are largely consistent with thermochemical equilibrium calculations of Andersen et al.13 For the case of unblended straw combustion, we observe significant deposit Cl levels, which is consistent with equilibrium predictions that condensed-phase KCl forms during unblended Danish Wheat straw combustion. The equilibrium model also predicts that during Danish Wheat straw combustion, potassium (the only alkali species considered by the model) partitions between the condensed-phase species KCl, K2SO4, and K2OSiO2 with K2OSiO2 as the dominant form, at typical deposit temperatures.13 Our composition data suggests that more than 70% of the alkali on a molar basis in unblended straw deposits is not in the form of chlorides or sulfates, but presumably in the form of silicates. For the case of a blend of Danish Wheat straw and South American coal, we observe little deposit Cl (less than 0.1% by mass). The equilibrium model predicts that during combustion of Danish Wheat straw and South American coal all of the potassium forms condensed-phase K2SO4 and that all of the Cl occurs as gas-phase HCl at typical deposit temperatures.13 Thermochemical equilibrium suggests that there should be a relationship between deposit Cl and the potential for forming alkali sulfates. When present in a deposit Cl occurs as an alkali chloride; however, sulfates are the thermodynamically favored form of alkali species at typical deposit temperatures. Therefore, if the coal provides sufficient sulfur to react with all of the available alkali then there will be no condensedphase Cl, assuming that the system achieves thermo-

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Figure 9. Relationship of measured deposit chlorine content to fuel sulfur and alkali properties showing how deposit chlorine decreases as the potential for sulfation of alkali chlorides increases. The error bars represent an estimate of the experimental uncertainty.

dynamic equilibrium. Figure 9 provides an experimental evaluation of this relationship by plotting the deposit Cl as a function of the ratio of twice the fuel S to available fuel alkali for a variety of fuels. This scaling parameter is chosen because a value of unity is the stoichiometric ratio of fuel sulfur to alkali required to convert all of the available alkali to sulfate and to release all of the Cl from a deposit as gas-phase HCl. Values greater than unity indicate excess S in the system, conditions under which equilibrium indicates there should be negligible deposit Cl. Available alkali refers to the fraction of the alkali that enters the gas phase and is available for chemical reaction during combustion. We estimate availability using a chemical fractionation procedure, which distinguishes between different types of inorganic material according to its solubility in a series of increasingly aggressive solvents (water, 1 M ammonium acetate, and 1 M hydrochloric acid).5 Alkali materials that are soluble in the two least aggressive solvents (water and ammonium acetate) are mobile and likely vaporize or readily react with other material during combustion. We refer to this fraction of the fuel alkali as available alkali. The material that is not soluble or only soluble in hydrochloric acid (typically carbonates, oxides, sulfides, and silicates) is unlikely to volatilize during combustion or interact with other particles. This chemical fractionation procedure defines 50% of the Na and 90% of K in the straw samples as available. Figure 9 presents the measured deposit Cl data as a function of the fuel-S-to-available-alkali scaling parameter. For values greater than 5, the Cl content of an ash deposit is very low; between 1 and 5, some of the deposits have significant Cl whereas others have negligible Cl. The deposits in this range with significant Cl are formed from firing coal-Imperial Wheat straw blends. Imperial Wheat straw has the highest Cl levels of any of the fuels considered by this study (Table 1). The presence of Cl in deposits formed from blends whose

Robinson et al.

scaling parameter is greater than 1 indicates kinetics or mass transfer limitations prevent the system from reaching equilibrium. Therefore, simply blending fuels to simply exceed the stoichiometric ratio of fuel S to available alkali will not ensure low Cl levels in the deposits. The results in Figure 9 indicate that the value of the proposed fuel-S-to-available-alkali scaling parameter must be at least 5 to avoid significant deposit Cl levels. This finding is somewhat unexpected because the stoichiometric fuel-S-to-available-alkali ratio should represent an upper limit of the amount of sulfur required to completely sulfate the available alkali. In actual deposits, a significant fraction of the alkali likely occurs as alkali silicates and alkali-aluminum silicates that are unlikely to sulfate. The molar ratios of the chlorine and sulfur to alkali (Na and K) in the cofired deposits indicate that the majority of the deposit alkali does not occur as a chloride or sulfate, but presumably as a silicate. The presence of alkali silicates should reduce the amount of sulfur required to sulfate all of the alkali chlorides, alkali hydroxides, and other reactive alkali species. Incomplete mixing is unlikely to be the cause of the failure to completely sulfate all of the available alkali in these experiments. The blends were carefully mixed before firing, and therefore, the combustion products inside the MFC should be as well mixed as physically possible. The conditions in a commercial boiler will probably not be as well mixed as the conditions of these experiments; especially when the biomass is fired through a separate burner. For situations with incomplete mixing, the fuel-S-to-available-alkali ratios will probably need to be even greater than 5 to avoid deposit Cl. Corrosion. High temperature corrosion can be a major problem for biomass boilers operating on highCl fuels, especially at elevated steam temperatures (greater than around 490 °C).8 Although corrosion experiments were not conducted as part of this study, the reduction in deposit Cl observed when cofiring straw with coal suggests that cofiring Cl containing biomass fuel with a S-containing coal may reduce the potential Cl-based corrosion. The data indicate that sulfur from the coal reacts with alkali chlorides from the straw to create alkali sulfates and release Cl to the gas phase (primarily as HCl). The effect of these reactions is to reduce and potentially eliminate condensed-phase chlorides from deposits. If the sulfation reactions occur before the alkali chlorides deposit on heat transfer surfaces, then Cl will never reach the deposit. The presence of an alkali sulfate layer adjacent to the probe surface (Figure 8) suggests that during coal-straw cofiring some alkali chlorides may deposit on the probe surface where they then react with sulfur from the coal. Under these conditions, Cl species are released into the gas phase within the deposit. These species will most likely diffuse and convect away from the heat transfer surface reducing the overall Cl content of the deposit. They could also react with the metal surface to accelerate corrosion.20 The role of gas-phase Cl species and alkali chlorides in Cl-based corrosion is still uncertain. (20) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K. Energy Fuels 1999, 13, 1114-1121.


Several long-term full-scale cofiring tests with high-Cl straw fuels have reported no difference or only slightly higher corrosion rates when cofiring straw with coal compared to firing unblended coal, which suggests cofiring may mitigate corrosion problems associated with high-Cl biofuels.11,12 The absence of chlorine in the deposits does not necessarily imply that corrosion is not an issue, as it still leaves corrosion by sulfates as a potential problem. In particular, alkali trisulfates are known to be aggressive on heat transfer surfaces.21 The deposits formed while cofiring coal and straw have a factor of 7 enhancement of sulfur relative to expectations based on the deposits of the unblended fuels. This enhancement occurs because the alkali from the straw reacts with the sulfur from the coal creating condensed-phase alkali sulfatesswithout the alkali this sulfur would likely have been released from the stack as SO2. Although the presence of alkali from the biomass enhances sulfur levels within the deposit, one cannot assume that the alkali level in the fuel determines the deposit S level even though alkali sulfate is the thermodynamically favored form of alkali at deposit temperatures. The molar ratios of alkali, sulfur, and chlorine of the coalstraw deposits data suggests that the majority of deposit alkali occurs as silicates, not chlorides and sulfates. Alkali-silicates or alkali-aluminum-silicates are unlikely to sulfate. Corrosion of surfaces by alkali sulfates can usually be managed by maintaining low surface temperatures, preventing occurrences of locally reducing conditions, and proper soot-blowing. Conclusions Pilot-scale experiments were performed using blends of four different types of biomass fuels and three different types of coal to examine the effect of cofiring on deposition rate and deposit composition. The ash deposition rates and particle collection efficiencies for the coal-biomass blends fall between the observed behaviors of the unblended fuels. Therefore, coal-straw blends have a lower deposition rate than unblended straw, but higher than unblended coal. Similarly, cofiring coal with wood results in slightly lower particle collection efficiencies than those that occur while firing unblended coal; however, the deposition characteristics of the coal-wood blends are largely determined by the properties of the coal because of the extremely low ash content of the wood sample. For the straw-coal blends there was a substantial reduction in the particle collection efficiency compared to expectations based on the performance of the unblended fuels. This reduction is attributed to interactions between the alkali chlorides from the straw and the sulfur from the coal, which reduce the stickiness of the fly ash and deposit material. This reduction results in the coal-straw blends having significantly improved deposition characteristics relative to the unblended (21) Harb, J. N.; Smith, E. E. Prog. Energy Combust. Sci. 1990, 16, 169-190.


straw. Therefore cofiring high alkali biomass fuels with S-containing coal may result in a fuel blend with more acceptable deposition characteristics than the unblended biomass. The sulfation of alkali chlorides is the major interaction that occurs within the cofire deposits. The coal is the source for the S; the biomass (especially straw) supplies the alkali species and the chlorine. High alkali and Cl levels are often found in agricultural residues or herbaceous materials.4 The effect of sulfation is to reduce the chlorine content of the deposits, which may reduce the potential for Cl-based corrosion. A scaling parameter of the ratio of twice the fuel S to available alkali provides some insight into the fuel mixtures that will avoid significant deposit Cl levels. When this parameter is greater than 1, no Cl should be present within the deposit at thermodynamic equilibrium. Under the conditions of these experiments, a fuel blend must have a ratio of twice the fuel S to available alkali in excess of 5 to eliminate chlorine from the deposit. The discussion presented in this paper is based on relatively short deposition tests at modest deposit temperatures. Reactions that lead to sintered and unmanageable deposits include reactions of alkali with silica to form alkali silicate.5 There is ample alkali and silica in these fuels for such reactions to occur. Such reactions are generally slow, perhaps too slow to significantly affect the measured results over the time of these experiments. Also, increases in deposit temperature would increase the reaction rate significantly and could lead to considerably different results. Andersen et al. compares results from short duration probe depositions collected during full-scale coal-straw cofiring tests with mature superheater deposits.13 The severe ash deposition characteristics of the two straw samples considered in this study underscore the critical role of alkali and chlorine levels in fouling and slagging. The alkali and chlorine levels in biomass fuels often depend on the agricultural practices, and can vary between fuels identified with the same name. For example, the switchgrass sample considered here exhibited surprisingly little deposition problems, in contrast to the high fouling potential of more aggressively cultivated switchgrass samples which behave more like the straw samples considered here.5 Therefore, careful consideration of fuel properties must be made, in particular fuel alkali and chlorine levels, when extending the results of this study to other fuels. Acknowledgment. This work was sponsored by the U.S. DOE Federal Energy Technology Center, Advanced Research and Technology Development Coal Utilization program, and the U.S. DOE Office of Energy Efficiency and Renewable Energy’s Biomass Power Program. H. Junker acknowledges the financial support of ELSAM, The Danish Academy of Technical Sciences (ATV) and Tech-wise. EF010128H

Pilot-Scale Investigation of the Influence of ... - ACS Publications

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