Ash Properties of Alternative Biomass - ACS Publications - American

Energy & Fuels 2009, 23, 1965–1976

1965

Ash Properties of Alternative Biomass Joaquı´n Capablo,† Peter Arendt Jensen,*,⊥ Kim Hougaard Pedersen,‡,⊥ Klaus Hjuler,‡,⊥ Lars Nikolaisen,§ Rainer Backman,| and Flemming Frandsen‡ Fluid Mechanics Group/LITEC-CSIC (Spanish Council for Scientific Research), UniVersity of Zaragoza, Maria de Luna 10, 50018 Zaragoza, Spain, CHEC Research Centre, Department of Chemical Engineering, Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark, Danish Technological Institute, KongsVang Alle´ 29, DK-8000 Aarhus C, Denmark, and Energiteknik och Termisk Processkemi, Umeå UniVersitet, SE-90187 Umeå, Sweden ReceiVed October 7, 2008. ReVised Manuscript ReceiVed February 2, 2009

The ash behavior during suspension firing of 12 alternative solid biofuels, such as pectin waste, mash from a beer brewery, or waste from cigarette production have been studied and compared to wood and straw ash behavior. Laboratory suspension firing tests were performed on an entrained flow reactor and a swirl burner test rig, with special emphasis on the formation of fly ash and ash deposit. Thermodynamic equilibrium calculations were performed to support the interpretation of the experiments. To generalize the results of the combustion tests, the fuels are classified according to fuel ash analysis into three main groups depending upon their ash content of silica, alkali metal, and calcium and magnesium. To further detail the biomass classification, the relative molar ratio of Cl, S, and P to alkali were included. The study has led to knowledge on biomass fuel ash composition influence on ash transformation, ash deposit flux, and deposit chlorine content when biomass fuels are applied for suspension combustion.

Biomass is used as a renewable solid fuel on power plants to provide CO2-neutral electricity and heat, where mainly wood and, in some cases, also straw have been applied in recent years.1-5 To make optimal use of all available biomass resources possible, other types of biomass should be made available for power production. Biowaste types from industry are rarely used for power production, mainly because only limited knowledge is available on their combustion properties. Such industrial biomass wastes could be, for example, pectin waste, mash from a beer brewery, or waste from cigarette production. One of the main problems of substituting fossil fuels by biomass in power-plant boilers is the biomass ash properties. Biomass fuels usually have a high content of alkali metals, which, together with other mineral components of the ash, give rise to severe ash deposition, thereby reducing the heat transfer

and inducing increased boiler tube corrosion.6-8 Previous experiments have shown that the elemental composition of the fuel ashes strongly influence the ash deposit formation process.9-12 Biomass combustion has mainly been performed in grate boilers; however, wood and straw are used as fuels in suspension-fired power-plant boilers;1 therefore, some experience regarding the behavior of the wood and straw ash in suspension-fired boilers is available. The objective of the present study has been to test the combustion properties of alternative solid biofuels, with special emphasis on the formation of fly ash and ash deposits. If industrial biomasses are to be used as fuels, it would be an advantage to have information on the ash behavior relative to the more well-known straw and wood fuels. In this study, 12 industrial biomasses are compared to straw and wood fuels. The fuels are classified according to fuel ash analyses, and experiments are performed on laboratory reactors that can simulate

* To whom correspondence should be addressed. Telephone: +45-45252849. Fax: +45-4588-2258. E-mail: [email protected]. † University of Zaragoza. ‡ Technical University of Denmark. § Danish Technological Institute. | Umeå Universitet. ⊥ Current address: FLSmidth A/S, Vigerslev Allé 77, DK-2500 Valby. (1) Frandsen, F. J. Utilizing biomass and waste for power productionsA decade of contributing to the understanding, interpretation and analysis of deposits and corrosion products. Fuel 2005, 84 (10), 1277–1294. (2) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hørlyck, S.; Karlsson, A. Influence of deposit formation on corrosion at a straw-fired boiler. Fuel Process. Technol. 2000, 64 (1-3), 189–209. (3) Jensen, P. A.; Frandsen, F. J.; Hansen, J.; Dam-Johansen, K.; Henriksen, N.; Ho¨rlyck, S. SEM investigation of superheater deposits from biomass-fired boilers. Energy Fuels 2004, 18, 378–384. (4) Nielsen, H. P. Deposit and high-temperature corrosion in biomassfired boilers. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1998. (5) Wieck-Hansen, K.; Overgaard, P.; Larsen, O. H. Cofiring coal and straw in a 150 MWe power boiler experiences. Biomass Bioenergy 2000, 19 (6), 395–409.

(6) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The behavior of inorganic material in biomass-fired power boilers: Field and laboratory experiences. Fuel Process. Technol. 1998, 54 (1-3), 47–78. (7) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Combustion properties of biomass. Fuel Process. Technol. 1998, 54 (13), 17–46. (8) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Boiler deposits from firing biomass fuels. Biomass Bioenergy 1996, 10 (2-3), 125–138. (9) Aho, M.; Silvennoinen, J. Preventing chlorine deposition on heat transfer surfaces with aluminium-silicon rich biomass residue and additive. Fuel 2004, 83 (10), 1299–1305. (10) Davidsson, K. O.; Steenari, B. M.; Eskilsson, D. Kaolin addition during biomass combustion in a 35 MW circulating fluidized-bed boiler. Energy Fuels 2007, 21, 1959–1966. (11) Tobiasen, L.; Skytte, R.; Pedersen, L. S.; Pedersen, S. T.; Lindberg, M. A. Deposit characteristic after injection of additives to a Danish strawfired suspension boiler. Fuel Process. Technol. 2007, 88, 1108-–1117. (12) Zheng, Y.; Jensen, P. A.; Jensen, A. D.; Sander, B.; Junker, H. Ash transformation during co-firing coal and straw. Fuel 2007, 86 (7-8), 1008–1020.

1. Introduction

10.1021/ef8008426 CCC: $40.75  2009 American Chemical Society Published on Web 03/30/2009

1966 Energy & Fuels, Vol. 23, 2009

Capablo et al.

Table 1. Proximate and Ultimate Analyses of Fuels and Additives (wt %, on an As Received Basis) M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 A4 A5 M14 M15 M16 M17 M18 M19 M20 M21 M22 M23 M15B M23B TSa TS1b TWa TW1b a

component

water

ash

C

H

O

N

Cl

S

pectin waste grain screen mash from beer brewery empty fruit bunch (EFB) shea waste carrageenan waste olive waste cigar waste cigarette waste coffee waste grain screening/coffee kariten wood bark kaolin limestone 75% M3 + 25% M5 75% M6 + 25% M5 50% M3 + 50% M5 25% M4 + 75% M13 25% M2 + 75% M13 96% M4 + 4% Kao 95% M5 + 5% CaCO3 96% M10 + 4% Kao 15% M8 + 85% M10 75% M1 + 25% M7 50% beet + 50% potato 75% pectin + 25% potato typical straw typical straw 1 typical wood typical wood 1

12.0 10.3 11.4 7.0 13.0 12.5 15.5 10.4 9.1 11.3 14.2 0.4 5.7 1.5 5.0 11.8 12.6 12.2 6.0 6.9 6.8 12.6 10.9 11.2 12.9 10.0 11.8 14.0 6.5 45.0 6.7

1.2 9.0 3.3 4.6 4.8 8.6 8.1 32.1 20.6 5.7 7.0 0.6 2.8 98.5 95.0 3.7 7.7 4.1 3.3 4.4 8.4 9.3 9.4 9.7 2.9 3.8 1.4 3.9 7.3 0.6 1.0

42.3 39.9 44.5 43.8 43.3 39.2 42.9 29.5 34.6 43.7 40.8 83.9 47.8

5.5 5.2 6.0 5.7 4.6 5.0 4.8 3.6 4.3 5.7 5.2 11.9 5.9

38.0 34.1 31.6 37.2 31.6 33.6 27.3 20.7 27.8 30.9 30.4 3.0 37.6

0.9 1.1 2.9 1.3 2.3 0.3 1.1 2.4 2.3 2.2 2.2 0.1 0.2

0.03 0.25 0.01 0.35 0.07 0.26 0.24 0.90 1.00 0.39 0.04 0.05 0.02

0.09 0.15 0.22 0.13 0.24 0.69 0.13 0.39 0.30 0.14 0.19 0.04 0.02

44.2 40.2 43.9 46.8 45.8 42.0 41.1 42.0 41.6 42.5 42.3 41.9 40.9 42.1 27.5 50.2

5.7 4.9 5.3 5.9 5.7 5.5 4.4 5.5 5.4 5.3 5.5 5.4 5.1 6.1 3.2 6.3

31.6 33.1 31.6 37.5 36.7 35.7 30.0 29.7 29.4 35.3 33.8 36.1

2.8 0.8 2.6 0.5 0.4 1.2 2.2 2.1 2.2 0.9 1.8 1.2 0.6 1.1 0.2 0.13

0.03 0.21 0.04 0.10 0.08 0.34 0.06 0.37 0.47 0.09 0.01 0.03 0.34 0.51 0.01 0.01

0.23 0.58 0.23 0.05 0.06 0.12 0.13 0.13 0.18 0.10 0.10 0.10 0.13 0.20 0.03 0.10

36.1 42.4

b

Data on TS and TW were obtained from ref 16. Data on TS1 and TW1 were obtained from refs 18 and 19, respectively.

the combustion process in suspension-fired power-plant boilers. Both the industrial biomass fuels and some fuel blends were tested. The analyses carried out for each fuel included chemical analysis of fuels and fuels ashes, and for selected fuels, chemical equilibrium calculations and test in an entrained flow reactor and in a swirl burner furnace were performed. 2. Fuel Composition Analysis and Fuel Classification A total of 25 fuels are investigated in different ways in this paper. The proximate and ultimate analyses of the fuels and additives used are shown in Table 1, which also include the typical composition of straw and wood (TS and TW). Fuels denominated M1-M12 include waste materials from agriculture and industrial processes, and M14-M23 are blends generated by mixing M1-M12 biomasses or by the addition of some additives (M13, wood bark; A4, kaolin; or A5, limestone). M15B and M23B are the result of mixing potato with beet and pectin, respectively. The blend proportions are shown in Table 2. The main criterion followed to prepare the blends is based on the knowledge that the addition of inorganic compounds or inorganic additives, which are based on elements, such as Si, Al, Ca, P, or S, in some cases, can reduce problems with deposit formation of chlorine- and alkali-rich fuels.9-12 The criterion for the mixing of the single fuel blends is discussed later in this section. The results presented in Table 1 originate from analysis on pulverized samples. Ash contents was determined by ashing at 550 °C, ultimate analysis of C, H, N, and S according to ISO/TS 12902, and oxygen by difference. The water content in all of the fuels is in the range of 5-15 wt %, except for M12 (0.4 wt %) and typical wood (45 wt %). The ash content is generally near the levels known from straw, except for M8 and M9, which contain a higher ash content, >20 wt %, and M1, M12, and M23B, fuels with a lower ash content, 0.30 wt %) and some others (M12, M17, and M18) have low sulfur contents ( 0.5], a

Ash Properties of AlternatiVe Biomass

Energy & Fuels, Vol. 23, 2009 1969

the A type varies significantly within the whole range of the boundaries seen on the triangular diagram in Figure 1. To test the influence of the fuel ash composition on deposit and fly ash properties, combustion tests were performed in an entrained flow reactor and on a small 30 kW swirl burner. In Table 4, the fuels applied for the combustion tests are marked. 3. Entrained Flow Reactor

Figure 2. Sketch of the entrained flow reactor.

Figure 3. Sketch of the swirl burner furnace.

relatively high Cl [Cl/(Na + K) > 0.25], and a relatively high sulfur content [S/(Na + K) > 0.5] in bold. This fuel classification is used later in this paper when ash deposit formation experiments are discussed. The fuel mixtures appear in the triangular diagram in points of the lines that joins their raw biomasses. Fuel mixing was performed to investigate if alkali- and chlorine-rich fuels could be mixed with less troublesome fuels, and thereby a manageable fuel with respect to deposit formation and corrosion could be obtained. The following fuel mixtures were provided: (i) M14, M16, M23, and M23B were made by mixing alkali-rich M5, M7, and potato waste with P-rich M3 and M1; (ii) M15 and M15B were made by mixing K-rich M5 and potato waste with sulfur-rich M6, M3, and beet; (iii) M17 was made by mixing Cl-rich M4 with wood M13; (iv) M18 was made by mixing Si-rich M2 with wood M13; (v) M19 and M21 were made by mixing kaolin with K-rich M4 and M10; (vi) M20 was made by adding CaCO3 to K-rich M5; and (vii) M22 was made by mixing the ash-, Cl-, and Si-rich M8 with the Cl-rich M10. It can be of interest to comment that, while fuel type ash B and C are more or less in the same range, the composition of

Combustion experiments with selected fuels were conducted in an entrained flow reactor, which is designed to simulate the environment in suspension-fired boilers.12 The complete facility includes equipment for data acquisition, gas supply, fuel particle feeding, gas preheating, and controlled extraction of gas and particles. A schematic diagram of the reactor is shown in Figure 2.12 The fuel feeding system (not shown in Figure 2) consists of a gravimetric screw feeder and a slightly inclined vibrator that helps to smooth out fluctuations in the fuel mass flow. The pulverized fuel is fed into the reactor by a central water-cooled air injection probe that passes through the gas preheater. The fuel and the preheated air are then mixed in the top of the reactor. The reactor has an internal length of 2 m and an internal diameter of 79 mm and can be electrically heated to a maximum temperature of 1500 °C. The reactor is especially well-suited to study heterogeneous reactions at high temperature and short residence times. The flow reactor setup is mounted with equipment to facilitate ash sampling and deposit probe measurements. A refractory lined bottom chamber is placed at the reactor tube exit. The bottom chamber turns the flue gas 90° and directs it onto an air-cooled vertical deposit tube probe, with an outer diameter of 10 mm. A hot water-cooled extraction probe is mounted in the bottom chamber, whereby a representative sample of the flue gas and fly ash particles can be extracted. The exit slit from the bottom chamber has a size of 4 × 8 cm. Large ash particles are separated from the extracted flue gas sample in a cyclone, with a cut size diameter of 2.5 µm. A heated paper filter is used to collect particles that pass the cyclone (the filter collects all ash particles above 0.2 µm). The flue gas is analyzed for its concentration of CO, CO2, O2, NO, and SO2. Generally, stable experimental conditions with an exit flue gas oxygen content of 6-8 vol % were obtained. A cup placed in the bottom chamber is used to collect the ash that is separated from the flue gas in the 90° bend. To prevent the flue gas temperature from decreasing too much at the distance between the reactor exit and the deposit probe, a small propane burner can be used to heat the gas in the bottom chamber. The burner is used during the experiments to ensure a flue gas temperature of approximately 800 °C near the deposit probe. An air-cooled deposit probe with a diameter of 10 mm is mounted in front of the exit slit. The outer metal tube of the deposit probe can be removed; therefore, the deposit (13) Jensen, P. A.; Sørensen, L. H.; Hu, G.; Holm, J. K.; Frandsen, F.; Henriksen, U. B. Combustion experiments with biomass fuels and additives in a suspension fired entrained flow reactorsTest of Ca and P rich additives used to minimize deposition and corrosion. Department of Chemical Engineering, Technical University of Denmark (DTU), CHEC Report 0504, 2005. (14) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Transformation and release to gas phase of Cl, K and S during combustion of annual biomass. Energy Fuels 2004, 18, 1385–1399. ¨ hman, M.; Bostro¨m, D.; Nordin, A. Effect of kaolin and limestone (15) O addition on slag formation during combustion of wood fuels. Energy Fuels 2004, 18, 1370–1376. (16) Sander, B. Properties of danish biofuels and the requirements for power production. Biomass Bioenergy 1997, 12 (3), 177–183.


Capablo et al.

Figure 4. Distribution of potassium as a function of the temperature for M1. Table 5. Summary of Selected Results of the Conducted Equilibrium Calculations fuel

type

TS1b A

K phase distribution relative melt K2O in ash (%) T (°C) gas % liquid % solid % in ash %a 28.4

(high Cl) M3

A (high S and P) M14 A (high P) TW1c B M1

1.8 17.9 13

M5

B (high S) B (high S) B (high S) C

51

M7

C

36

M4

C

31

M6 M15

(high Cl)

5.1 6.3 13.3

700

34

0

66

9.7

1000

28

34

38

17.6

700 1000 700 1000 700

0 1 0 2 10

0 0 0 0 0

100 99 100 98 90

0.0 0.0 0.0 0.4 1.3

1000 700 1000 700 1000 700 1000 700 1000 700 1000 700

85 2 30 5 38 3 17 3 15 3 17 5

0 0 50 95 62 26 78 0 0 17 0 35

15 98 20 0 0 71 5 97 85 80 83 60

11.1 0.1 4.1 6.3 6.3 3.9 12.6 1.5 7.7 7.2 6.1 12.4

1000

25

18

57

13.3

speciation of potassium 28% KOH (g), 28% K2SO4 (s2), 20% K2Si2O5 (s3), 15% KCl (s), 3% KCl (g), 3% (KCl)2 (g), 2% KAlSi2O6 (s2), 1% K2Si4O9 (s) 26% K2SO4 (s2), 28% KCl (g), 34% K2Si4O9 (l), 10% K2Si2O5 (s3), 2% KAlSi2O6 (s2) 50% K2SO4 (s), 50% KPO3 (s) 100% KPO3 (s) 70% K2SO4 (s), 30% KPO3 (s) 41% K4P2O7 (s), 34% K3PO4 (s), 23% K slag (s), 2% KCl (g) 63% K2Ca2(CO3)3 (s), 25% K2SO4 (s), 9% KCl (g), 2% K2CO3 (s), 1% KOH (g) 10% K2SO4 (g), 10% KCl (g), 15% K2SO4 (s), 65% KOH (g) 75% K2SO4 (s), 28% KAlSi2O6 (s), 2% KCl (g) 50% K2SO4 (l), 20% K slag, 28% KCl (g), 2% KOH (g) 70% K2SO4 (l), 25% KCl (l), 5% KCl (g) 62% K2SO4 (l), 36% KCl (g), 2% KOH (g) 71% K2SO4 (s), 16% K2SO4 (l), 10% KCl (l), 3% KCl (g) 78% K2SO4 (l), 17% KCl, 5% K2Ca2(SO4)3 (s) 30% K3PO4 (s), 29% K2SO4 (s), 26% K slag, 12% K2CO3 (s), 3% KCl (g) 30% K3PO4 (s), 29% K2SO4 (s), 26% K slag, 10% KOH (g), 5% KCl (g) 45% K slag (s), 27% K3PO4 (s), 8% K2SO4 (s), 8% KCl (l), 6% KCl (g) 43% K slag (s), 27% K3PO4 (s), 13% K2SO4 (s), 10% KCl (g), 7% KOH (g) 42% K slag, 18% K2SO4 (l), 15% K3PO4 (s), 15% KCl (l), 5% KCl (g), 5% K2SO4 (s) 45% K slag, 18% K2SO4 (l), 12% K3PO4 (s), 25% KCl (g)

Relative melt in ash is calculated by (melt + gas at T (°C)) × K2O in ash. b Data on TS1 were obtained from ref 18. c Data on TW1 were obtained from ref 19. a

Figure 5. Measured NO emission as a function of the fuel nitrogen content.

can be weighed and stored intact for later analysis. The probe is cooled by preheated air to obtain surface temperatures, measured with thermocouples, of approximately 550 °C, which is a relevant temperature for superheater coils in biomass boilers.

Figure 6. Measured SO2 emission as a function of the fuel sulfur content.

During the experiments, 760 g of fuel per hour is feed to the reactor. The deposit probe is exposed to the ash containing flue gas in 1.5 h, except in the experiment with M1 fuel, where the probe is inserted 3 h to ensure a sufficient amount of deposit. At the end of an experiment, the probe is withdrawn and the


Figure 7. SEM picture of an ash sample (M21) from the filter.

fuel feeding is stopped. Samples of deposit, fly ash, aerosols, and bottom ash are stored and labeled for later analysis. The diagnostic of the single experiment includes the following measurements: (i) total mass of deposit and collection of deposit for later analysis, (ii) total collected amount of large fly ash fraction (cyclone ash, above 2.5 µm), aerosols (below 2.5 µm), and bottom ash, (iii) determination of the ash composition of the deposit, (iv) determination of the water-soluble K, Cl, and S content of deposit, aerosols (filter ash), and fly ash (cyclone ash), and (v) online gas concentration measurements of SO2, O2, CO, NO, and CO2. A carbon burnout of above 99% was obtained for all of the tested fuels. In two cases (M5 and M7), high CO emissions (>1150 ppm) were observed, which we believe was caused by unstable fuel feeding. It can be seen that those fuels also have a high water content, which may have caused the feeding problems. 4. Swirl Burner Furnace Combustion experiments with selected fuels are performed in a swirl burner furnace, shown in Figure 3, with the aim to determine the characteristics of the produced fly ash. The experimental setup consists of a particle and gas feeding system, a furnace chamber where the swirl burner is mounted on the top, a particle collection system, and a gas- and temperature-measuring system. To maintain a stable flame, natural gas is co-fired with the solid fuel, with a typical input of solid fuel of 2.5 kg/h. The typical input power of gas was 20 kW, and the typical input power of biomass was 10 kW. The solid fuel is fed with the maximum possible rate, and the natural gas is adjusted to achieve an outlet O2 concentration of approximately 4 vol %. The temperature is measured by thermocouples placed in 2 ports, respectively, 400 and 1750 mm from the swirl burner. Temperatures of approximately 1200 and 800 °C were measured. The flue gas is analyzed for CO2, O2, CO, and NO. The particle sampling flow is approximately 40% of the total flue gas production. The fly ashes are collected in three fractions. Particles are collected from the bottom of the sampling probe line, in a cyclone, and by a metal filter (see Figure 3). To minimize the fuels from contamination of each other, the whole sampling line is dismantled and cleaned with pressurized air when shifting to a new fuel. 5. Chemical Equilibrium Calculations By equilibrium calculations, the stable chemical species and physical phases of the elements included are determined as a


function of the temperature, pressure, and total composition of ¨ hman et al.15 have shown that a good the system considered. O agreement between equilibrium calculations and the appearance of stable phases determined by X-ray diffraction can be obtained for biomass ashes. The calculations are performed by minimization of the total Gibb’s free energy of the system under a mass balance constraint. This type of calculation has some important limitations. Equilibrium calculations are based on the assumption that all elements are available for reaction (perfect mixing), kinetic limitations are ignored, and also all phases are considered ideal. Equilibrium calculations were performed in the temperature range from 500 to 1600 °C on the fuels M1, M3-M7, M14, and M15 at 1 bar. For most fuels, the distribution of potassium, sodium, calcium, sulfur, and chlorine is calculated. As an example of the results, the partitioning of potassium from 500 to 1600 °C in the case of M1 (pectin waste) is shown in Figure 4. The M1 fuel ash has a high content of Ca (22 wt %), Si (12 wt %), and S (7 wt %) but only a moderate K (4 wt %) content. In Figure 4, it can be seen that potassium mainly appears as solid sulfate and silicate below 820 °C. Between 820 and 1000 °C, most potassium appears as melted potassium sulfate and as K2O in slag. Above 1000 °C, significant amounts of KOH and KCl gas appear and some K2O in slag is present. Using equilibrium calculations on biomass, there are some limitations regarding the interpretation with respect to practical combustion. The calculations may predict the presence of a slag rich in alkali metals and silicon. This glass slag does not melt at a precise temperature but experiences a gradual decrease in viscosity with an increasing temperature, making it difficult to define at which temperature the slag is melted. The thermodynamic data of the K-Ca-P system is very limited, and the calculations on this system may not be precise.20 Anyhow, with some precaution, the equilibrium calculations provide insights into the behavior of biomass ash in boilers. A summary of the main results of the calculations is shown in Table 5. Potassium is often an abundant element in biomass ash components with relatively low melting temperature and ash components with high gas-phase concentration at low temperature. The interpretation of the data has therefore focused on the fate of potassium-containing species. The most severe problems with deposit formation appear with ashes that, to a high degree, are present in the gas and liquid phase at relatively low temperature. Table 5 is therefore shown as the amount of potassium predicted to be present in gas, liquid, and solid phases at 700 and 1000 °C, as well as the distribution of potassium between different species. The temperatures 700 and 1000 °C were chosen to represent typical flue gas temperatures in the first part of the convective heat-transfer section of biomass boilers. The calculated “melt fraction” in the ash, shown in Table 5, is based on a sum of melt and gas-phase potassium times the potassium oxide in the ash. This value may provide a first impression of how troublesome a biomass ash may be. In comparison to the results, previous equilibrium calculations with typical straw (TS1) and wood (TW1) are included in Table 5.18,19 The calculations predicted the following general species distribution. (i) Type A fuels (TS1, M3, and M14): a typical straw biomass TS1 forms a large amount of melt and gas-phase alkali (17) Jensen, P. A.; Frandsen, F.; Backmann, R.; Nikolaisen, L.; Hjuler, K.; Busk, J. Fuels for CO2 reduction in power plants. Final report of the DTU part of the PSO Eltra project 5075, 2005. (18) Ma, X.; Jensen, P. A.; Jensen, A.; Frandsen, F.; Lin, W. Chemical equilibrium sensitivity analysis for combustion of straw. CHEC, Department of Chemical Engineering, Technical University of Denmark (DTU), 2003. (19) van Lith, S. Combustion of wood. CHEC, Department of Chemical Engineering, Technical University of Denmark (DTU), 2007.


Capablo et al.

Figure 8. SEM images with corresponding EDX analyses for the ashes collected from the swirl burner furnace tests.

chlorine at relatively low temperatures. The straw ash does also contain a high content of K, and therefore, relatively high amounts of melt are formed. In type A fuels with phosphaterich ashes (M3 and M14), stable solid species containing both K and P are predicted to be formed. However, the thermodynamic data for phosphate species are somewhat uncertain.20 (ii) Type B fuels (TW1, M1, M6, and M15): the B type biomasses resemble typical Ca-rich wood biomass. For wood TW1, at 1000 °C, a high content (85%) of potassium is present in the gas phase mainly as potassium hydroxide. The biomasses M1, M6, and M15 resemble the wood composition but with a higher sulfur content. Also, observed here is relatively small amounts of solid-phase potassium; however, a lot of melted potassium sulfate appears. In these biomasses, a reasonably large fraction

of alkali metal is melted, but because the fuels have a low alkali content, the total amount of melt in the ash is limited. Increased Si content in this biomass class decreases the melt fraction of the ash. (iii) Type C fuels (M4, M5, and M7): type C ash is defined as having high a K or Na content and relatively low Si and Ca contents. In the cases also with low chlorine content (M5 and M7), most alkali is present as solids in K slag, potassium sulfate, and potassium phosphate (K3PO4), even up (20) Sandstrom, M.; Bostrom, D.; Nordin, A. Phase of relevance for ash formation during thermal processing of biomass and sludgessReview of thermodynamic data, phase transitions and crystal structure in the system CaO-K2O-P2O5. Second World Biomass Conference, Rome, Italy, 2004; paper V3B63.



Figure 9. SEM images with corresponding EDX analyses for the ashes collected from the swirl burner furnace tests. Table 6. Fraction of Fly Ash Particles in Different Size Classes and Mean Particle Size of Above 1 µm Classa M14 M15 M18 M21 M22 M15B M23B

% AE

%L

dp L (µm)

16 19 18 18 20 18 17

84 81 82 82 80 82 83

15 12 14 11 12 11 14

a AE, aerosol (d < 1 µm); L, large particles (d > 1 µm); d , average p p p particle size.

to 1000 °C. When a larger amount of chlorine is present (M4), the melt fraction increases. 6. Emission of NO and SO2 In Figure 5, the NO emission from the EFR and the swirl burner furnace tests as a function of fuel nitrogen content is shown. An increased NO emission with increased fuel nitrogen content is observed in both kind of tests.5 The values for the EFR test are probably higher because of a faster mixing of fuel and air in that reactor. It can also be seen that the relative conversion of fuel nitrogen to NO decreases with an increasing fuel nitrogen content. In Figure 6, the SO2 emission produced in the EFR tests as a function of fuel sulfur content is shown. A general tendency of increased SO2 emission with increased fuel sulfur content

Figure 10. Deposit flux as a function of the fuel ash content. BDF9 and ADF3 are straw fuels. BDF5 and BDF13 are wood fuels.

can be seen but, in this case, with much more scattering of the data. Of the tested fuels, M1, M3, M6, and M15 are relatively sulfur-rich compared to the alkali metal content [S/(Na + K) from 0.7 to 3.7], which could explain the relatively high SO2 emission by a limited amount of available alkali metal to bind the sulfur in the ash. In the case of M14, rich in phosphor, the phosphor probably binds to the alkali metals and sulfur is emitted. In the cases of M7 and M5, a lot of reactive potassium is present, and because the contents of chlorine, silicon, and phosphor are low, the potassium mainly binds to sulfur in the ash. In general, it is seen that a high amount of reactive alkali metal gives rise to a large fraction of the sulfur being present in the ash. For M15, a high fraction of sulfur in the ash is observed as well, in this case probably caused by a high Ca


Capablo et al.

content. It is known that species containing Ca and Mg react with sulfur and thereby contain sulfur in the ash. However, the results presented in Figure 6 indicate that, for the biomasses, a large content of reactive potassium (maybe present as gaseous KOH in the furnace) is most efficient to react with the fuel sulfur.

Cl-rich M10 with kaolin (alumina silicate). Large particles 1-10 µm, composed of Si, Al, Ca, and K, and sub-micrometer particles, composed of K, S and Cl, probably potassium sulfate and chloride, are observed in the fly ash. Nonconverted kaolin particles are not seen. On the other hand, KCl is not completely converted to alkaline alumina silicates. (v) M22 is a mix of the alkali- and Cl-rich M10 with the Si- and Ca-rich M8. The EDX measurements indicate that the ash mainly constitutes 1-7 µm Si, Mg, Ca, and K particles covered with a layer of K, Cl, and S, probably alkali chloride and sulfate. (vi) M15B contains a high content of Ca and K in the fly ash. Ca particles sometimes covered with alkali salts rich in K and S, probably potassium sulfate (and maybe carbonates), are observed in this fly ash. (vii) M23B ash is rich in Ca, K, and P. Large particles up to 20 µm containing Ca, Si, Al, and Mg, smaller (2 µm) particles rich in K and Cl, probably KCl, and sub-micrometer particles of K and S, probably potassium sulfate, are observed. In Table 6, the results of Malvern particle size measurements on the swirl burner fly ashes are shown. For the larger fly ash particles, mean sizes from 11 to 15 µm were seen. These results agree reasonably well with the results obtained by SEM.

7. Fly Ash Properties

8. Deposit Properties

Scanning electron microscopy (SEM) images, energydispersive X-ray (EDX), and Malvern particle size analyses were performed on the ashes collected from the swirl burner furnace. Two distinct kind of particles have usually been observed: a small type (aerosols) with particle size < 1 µm, with an average particle size of 300 nm, and composed of S, Cl, K, and P salts, sometimes attached to the surface of the another type, coarse particles with an particle size of typically 2-20 µm, composed mainly of Si, Ca, Mg, K, and Al. Figure 7 shows a SEM image, where both kinds of particles can be observed. In Figures 8 and 9, examples of SEM images and corresponding results of EDX analysis are shown. By SEM, it has been observed that the coarse particles are spherical, which means that they have been molten, and are often associated with silicon. Non-spherical particles have usually a great proportion of calcium. In the aerosol mode, it has been observed that the sulfur-rich particles are generally smaller than the chlorine-rich ones. On the basis of the SEM and EDX measurements on the fly ash samples from the swirl burner, the following can be concluded regarding the individual fuels: (i) M14 is a mix of the alkali-rich M5 with the Si- and P-rich M3. The EDX measurements indicate that the ash mainly constitutes 1-15 µm probably silica-rich particles covered with a layer composed mainly of K, P, and Cl. Even the chemical analysis of the ash showed a high content of Si (29.2 wt % SiO2), while the EDX analysis showed only a low Si content. We interpretate this as a result of condensation of volatile salts on the surface of the Si-rich particles. The volatile ash species are probably alkali phosphate, some alkali chloride, and possibly alkali carbonate (C is not detected by the SEM-EDX). (ii) M15 is a fuel rich in alkali from M5 and in Ca and S from M6. The SEM images reveal that there are many 1-10 µm spherical particles with smaller material attached. The smaller material was analyzed by EDX and is mainly composed of S, K, and Ca; therefore, sulfates probably appear. (iii) M18 ash is composed mainly of K and Si (from M2) and Ca (from M13). Spherical 1-10 µm particles with smaller white particles attached are observed. Two different kind of large particles, one spherical type, composed of Si, Ca, Al, and K and another irregular type, composed mainly of Si and Ca, are observed. (iv) M21 is a mix of the alkali- and

The combustion experiments conducted on the entrained flow reactor included deposit measurements and a fly ash collection. In Figure 10, the deposit probe flux as a function of the fuel ash content is shown. The data from this investigation are compared to some wood and straw data from a previous investigation (ADF3, BDF5, BDF9, and BDF13).13 As expected, an increased deposit flux with an increased ash content is observed, but some scattering appears. The measured deposit formation fluxes from 20 to 400 g h-1 m-2 are high compared to deposit fluxes measured in straw-fired grate boilers (typically 20-100 g h-1 m-2).21 The main reason is the high amount of fly ash formed during suspension combustion compared to the grate combustion process. It is remarkable that there is not a clear difference with respect to deposit formation tendency whether it is a type A, B, or C fuel. Also, even though M1, M3, and M14 are predicted by equilibrium calculations to cause a low amount of melt in the ash, relatively high deposit fluxes are measured. It may be that some phosphor- and silica-rich components that are predicted to be solid at low temperature in reality form a melted slag. However, as seen in Figure 10, the fuels with a high content of Cl [Cl/(Na + K) > 0.25] (BDF13, M4, ADF3, and M6), a high content of S [S/(Na + K) > 0.5] (M1, M3, M6, and M15), and a high content of P [P/(Na + K) > 0.5] (M14 and M3) all give a relatively high deposit flux compared to the fuel ash content. This indicates that, when the alkali appears as a salt, a higher deposit formation appears. It can be seen in Figure 10 that all fuels with more than 3.5% ash gave rise to deposit formation fluxes above 100 g h-1 m-2. The only exception is BDF9, which for unknown reasons gave a relatively low flux. The amount of chlorine in a deposit has a large influence on the corrosion potential1-4 of the deposits. In Figure 11, the deposit chlorine content is shown as a function of fuel chlorine content and some scattering can be observed. The deviations can be explained on the basis of fuel compositions. In both M14 and M3, no chlorine was observed in the deposit and the measuring points can be seen in the bottom of the left side in

Figure 11. Deposit chlorine content as a function of the fuel chlorine content.

(21) Jensen, P. A.; Zhou, H.; Frandsen, F.; Hansen, J. Ash deposits removal in biomass power plant boilers. Proceedings of the 15th European Biomass Conference and Exhibition, Berlin, Germany, 2007; pp 14061411.



Table 7. Comparison of Composition Analysis on Fuel Ash Deposits and Fly Ashes wt % of dry sample

K

Cl

S

Na

P

Si

Ca

Mg

Al

Fe

molar ratio (Na + K)/(2S + Cl)

M1 M1 M1 M1 M1 M1

fuel ash deposit ash deposit ash (water soluble) deposit ash rel. water cyclone ash (water soluble) filter ash (water soluble)

4,2 6.5 5.3 82 2.8 9.6

2.4 1.2 1.2 100 2.3 9.6

7.1 5.3 5.1 96 0.7 4.9

5.1 2.9 1.7 59 1.0 6.6

4.4 4.1 0.0 0 0.0

11.7 13.0

22.9 22.0

0.8 0.7

0.9 0.7

2.2 2.2

0.6 0.8 0.6

M3 M3 M3 M3 M3 M3


1.5 2.0 0.6 30 1.4 6.9

0.3 0.0 0.0

6.8 0.0 0.0 0.3 0.7

14.4 14.0 1.4 10 0.6

17.8 19.0

0.9 0.5

0.4 0.4 0.1 25 0.1 1.5

M4 M4 M4 M4 M4 M4


25.7 23.0 16.0 70 10.0 43.0

7.7 10.0 11.0 110 6.1 24.6

2.9 1.8 1.9 106 1.1 6.3

2.1 1.1 0.4 36 0.6 2.2

3.8 1.9 0.3 15 0.7

14.0 17.0

M5 M5 M5 M5 M5 M5


42.3 30.0 25.0 83 18.0

1.5 3.6 3.9 108 1.9

5.1 4.1 4.2 102 3.4

0.2 0.2 0.2 71 0.2

3.4 2.9 0.7 23 1.0

5.1 8.2

M6 M6 M6 M6 M6 M6


5.2 3.6 2.9 81 3.0 24.7

3.0 2.2 2.1 95 2.4 19.9

8.1 7.3 2.1 29 2.0 12.8

3.8 2.3 1.5 65 1.6 13.1

0.6 0.8 0.0 0 0.0

6.1 6.8

M7 M7 M7 M7 M7 M7


29.9 24.0 18.0 75 15.0

3.0 4.4 4.8 109 5.0

1.6 1.3 1.3 100 2.0

0.4 0.6 0.3 56 0.6

2.1 2.2 0.0 0 0.0

7.9 9.2

M14 M14 M14 M14 M14 M14


14.8 13.0 4.1 32 7.8 36.7

0.7 0.0 0.0

0.4 0.4 0.1 31 0.7 1.2

10.8 11.0 1.2 11 0.6

13.6 13.0

2.2 11.4

6.1 0.9 0.9 104 2.0 2.6

M15 M15 M15 M15 M15 M15


11.0 7.1 6.1 86 7.3

2.8 2.1 2.5 119 2.7

7.5 6.4 3.5 55 3.4

3.2 1.7 1.0 56 1.4

1.0 1.2 0.0 0 0.0

5.9 6.1

Figure 11. M3 and M14 are the most phosphorus-rich fuels; therefore, the KCl has reacted with the phosphor, and HCl is released to the gas phase. It is seen in Figure 11 that the type B biomasses do generally contain relatively small amounts of chlorine in the deposits compared to the fuel ash chlorine contents. The type B biomass are characterized by high Ca and Mg contents, but we do not believe that this should in itself cause low deposit chlorine contents. A more reasonable explanation is that M1, M15, and M6 do have sulfur-rich ashes that can bind a large fraction of alkali as sulfate, whereby Cl is released to the gas phase as HCl. The relatively low deposit chlorine content of BDF13 can presently not be explained on the basis of fuel analysis. In Table 7, comparisons of composition analysis on fuel ashes, deposits, and fly ash from the entrained flow reactor experiments are shown. Generally, it can be seen that the fuel ash and the deposit chemical compositions are reasonably similar. Looking at the fraction of water-soluble potassium in the deposits, it is observed to be in the range from 70 to 86% in the experiments with M1, M4-M7, and M15. This strongly indicates that, in those experiments, potassium in the deposits mainly appears as KCl and K2SO4. Only in experiments with M3 and M14 (the fuels with high phosphor contents), the water-soluble fraction of potassium in the deposit is down to 30%, and in those cases, the potassium is mainly

1.1 0.9 6.1 7.3

5.3 6.3

0.1 0.1

0.5 0.7

0.1

1.0 4.1 5.9 5.3

2.1 1.7

1.0 1.1

1.6 2.0

1.9 1.6 1.0 1.2 1.1

3.4 4.1

3.6 4.3

0.8 1.3

0.6 1.0

3.0 2.2 1.7 1.8

22.2 27.0

4.5 5.6

1.3 1.5

0.8 1.2

0.5 0.4 0.7 0.8 0.9

7.9 9.3

4.3 5.4

1.3 1.8

1.5 2.1

4.3 3.1 2.2 1.5

5.2 7.0

4.7 6.2

0.3 0.6

0.5 1.1

1.0 6.2 1.9 1.2 2.1

19.2 25.0

4.3 5.6

1.2 1.6

0.8 1.3

0.8 0.6 0.7 0.9

present as phosphate. In the following, some specific comments on the deposit compositions are provided, supported by Figure 11 and Table 7. Group A (M3, M13, M14, BDF9, and ADF3): For M13, BDF9, and ADF3, the chlorine content in the deposit is higher than in the fuel ash. The large amount of potassium has the capability to bind chlorine in the deposit. In M3 and M14, the deposit contains no chlorine or sulfur and the alkali in the deposit mainly appears as phosphate, in agreement with the high phosphor content of the ash of these fuels. Group B (M1, M6, and M15): The deposit has a lower chlorine content compared to the fuel chlorine content. Probably some chlorine is released from the deposit by on site sulfation. Group C (M4, M5, and M7): As for fuels in group A, for M4, M5, and M7 the chlorine content in the deposit is higher than in the fuel ash. The large amount of potassium has the capability to bind chlorine in the deposit. 9. Conclusions Chemical analysis, combustion experiments, and thermodynamic equilibrium were performed on alternative biomasses to obtain tools, whereby the fuel ash and deposit formation


properties can be determined relative to wood and straw. The investigated fuels were classified into three groups depending upon their contents of the main ash-forming elements (SiO2, Na2O + K2O, and CaO + MgO). To obtain an understanding of the ash chemistry behavior, this classification was supplemented with information on the amount of the elements Cl, S, and P relative to the alkali content of the fuel ashes. No clear difference with respect to deposit formation flux was found whether it is a type A (Si rich), B (Mg and Ca rich) or C (alkali rich) fuel. It is observed that the relative molar ratio of Cl, S, and P to alkali does have an influence on the speciation of the alkali metals. No chlorine was observed in the deposits with phosphorus-rich fuels. The chlorine content in deposit increases as the molar ratio of S/(Na + K) in the fuel ashes decreases. On the basis of results from this study, the 12 alternative biomass fuels analyzed can be classified with respect to their probable deposit formation flux and chlorine content of the deposit. Generally, it is observed that a high ash content gives rise to an increased deposit formation flux (see Figure 8), and a simple prediction of the deposit flux level can be based on the fuel ash content of the biomasses. Thereby, the fuels can be divided into three deposit formation flux levels: (A) Fuels M8 (cigar waste) and M9 (cigarette waste) with an ash level above 10 wt % are predicted to cause a superheater deposit flux above 400 g h-1 m-2. (B) Fuels M2 (grain screen), M4 (empty fruit bunch), M5 (shea waste), M6 (carrageenan waste), M7 (olive waste), M10 (coffee waste), and M11 (grain screening) with an ash level between 3.5 and 10 wt % are predicted to cause a superheater deposit flux similar to straws between 100 and 400 g h-1 m-2. (C) Fuels M1 (pectin waste), M3 (grain screen), and M12 (kariten) with an ash level below 3.5 wt % are predicted to cause a superheater deposit flux similar to woods below 100 g h-1 m-2.

Capablo et al.

Generally, deposits with high chlorine contents (above 3 wt %) could probably appear in deposits from all biomasses that have an ash chlorine content above 0.5 wt % and, relative to alkali, have a low sulfur molar ratio [(S/K + Na) < 0.5] and a low phosphorus ratio [(P/(K + Na) < 0.25]. This study indicates that the biomasses M2, M4, M5, M7, M8, M9, M10, and M12 will form a deposit with chlorine contents above 3 wt %. Entrained flow reactor experiments with mixed fuels and fuels and additives were very limited in this study; therefore, the conclusions with respect to additives are limited. The results indicate that mixing a phosphorus-rich fuel with a fuel rich in K and Cl can lead to a significant reduction in deposit chlorine content. By SEM and Malvern particle size analyses of swirl burner biomass fly ashes, two distinct kinds of particles have usually been observed: a small type with a particle size < 1 µm, with an average particle size of 300 nm, composed of sulfur, chlorine, potassium, and phosphor salts, sometimes attached to the surface of the other coarse type, with an average particle size of 10-15 µm, composed mainly of silicon and potassium if spherical or calcium if irregular. Acknowledgment. This study is financially supported by Energinet.dk through contract 1996 and 2006-1-6356. This work is part of the Combustion and Harmful Emission Control (CHEC) research program, which is funded a.o. by the Technical University of Denmark, the Danish Technical Research Council, the European Union, the Nordic Energy Research, Dong Energy A/S, Vattenfall A.B., F L Smidth A/S, and Public Service Obligation funds from Energinet.dk and the Danish Energy Research program. Financial support for J. Capablo during his Ph.D. studies was provided by the FPU programme of the Spanish Ministry of Education and Science. EF8008426

Ash Properties of Alternative Biomass - ACS Publications - American

Recommend Documents