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Effect of Fuel Type and Deposition Surface Temperature on the Growth and Structure of an Ash Deposit Collected during Co-firing of Coal with Sewage Sludge and Sawdust† Tomasz Kupka,* Krzysztof Zaja˛c, and Roman Weber Institute of Energy Process Engineering and Fuel Technology, Clausthal UniVersity of Technology, Agricolastrasse 4, Clausthal-Zellerfeld 38678, Germany ReceiVed NoVember 10, 2008. ReVised Manuscript ReceiVed January 21, 2009
Blends of a South African bituminous “Middleburg” coal, a municipal sewage sludge, and a sawdust have been fired in the slagging reactor to examine the effect of the added fuel on the slagging propensity of the mixtures. Two kinds of deposition probes have been used, uncooled ceramic probes and air-cooled metal probes, to examine the influence of the deposition surface temperature on both the deposit growth and its structure. The initial stages of slagging (140 min of sampling) have been investigated in two temperature ranges: a high-temperature range of 1100-1300 °C and a low-temperature range of 550-700 °C. Laboratory ash (created in the laboratory furnace), ash sampled on the deposition probes, and ash collected in the cyclone have been analyzed using the X-ray fluorescence technique. Additionally, the electron probe microanalysis (EPMA) of the embedded resin deposit probes have been performed. Using this technique, the thickness, structure, porosity, and chemical composition in different layers of the deposit have been determined and evaluated as a function of the fuel type and the deposition surface temperature. Distinct differences in structures of the deposits collected using the uncooled ceramic probes and air-cooled steal probes have been observed. Glassy, easily molten deposits collected on uncooled ceramic deposition probes are characteristic for co-firing of municipal sewage sludge with coal. Porous, sintered (not molten), but easily removable deposits of the same fuel blend have been collected on the air-cooled metal deposition probes. The addition of sawdust does not negatively influence the deposition behavior. Loose, easy removable deposits have been sampled on aircooled metal deposition probes during co-firing of coal-sawdust blends. The mass of the deposit sampled at lower deposition surface temperatures (550-700 °C) was always larger than the mass sampled at higher surface temperatures (1100-1300 °C).
1. Introduction Unknown ash behavior and ash-related operational problems, such as the effects of the co-fired fuel on slagging and fouling in the system, are challenges in co-firing technology. Careless co-firing of difficult alternative fuels could lead to a reduction of boiler reliability and availability and to unscheduled plant shut-downs. These ash-related problems are for the boiler operators of vital importance.1-10 The goal of this work is to investigate how both fuel type and surface temperature of the deposition probe influence the † Impacts of Fuel Quality on Power Generation and Environment. * To whom correspondence should be addressed. Telephone: +49 (0) 532372-2297. Fax: +49 (0) 5323-72-3155. E-mail:
[email protected]. (1) Raask, E. Mineral Impurities in Coal Combustion: BehaViour, Problems, and Remedial Measures; Hemisphere Publication Corporation: New York, 1985; p 484. (2) Bryers, R. W. Fireside slagging, fouling and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 1996, 22 (1), 29–120. (3) Wall, T. F.; Juniper, L.; Lowe, A. State-of-the-art review of ash behaviour in coal fired furnaces. Australian Coal Association Research Program (ACARP) Project C9055, 2001. (4) Rushdi, A.; Sharma, A.; Gupta, R. An experimental study of the effect of coal blending on ash deposition. Fuel 2004, 83, 495–506. (5) Leithner, R.; Bozic, O.; Hoppe, A.; Neuroth, M. Einfluss vom Verschlackung und Verschmutzung auf Kohleauswahl, Dampferzeugeruslegung und -betrieb. Erfahrung und Simulation, Verbrennung und Feuerungen, 21 Deutcher Flammentag, VDI Berichte 1750, 2003; pp 11-22. (6) Tomeczek, J.; Palugniok, H.; Ochman, J. Modelling of deposits formation on heating tubes in pulverised coal boilers. Fuel 2004, 83, 213– 221.
slag formation mechanism during co-combustion of pulverized coal with municipal sewage sludge and sawdust in pulverized form. A high-volatile bituminous coal from South Africa of a relatively low slagging propensity has been chosen as a basic fuel. The objective is to examine to what extent and through which mechanisms the slagging propensity of this coal is altered when it is co-combusted with a sewage sludge and a sawdust. In particular, the deposit growth rate, the chemical composition, and the physical structure of the ash deposits are investigated as a function of different fuel blends and different surface temperatures of the deposition probe. The basic research tools include the slagging rig used for deposit collection, the fuel and ash chemical analyses, as well as analytical methods for determination of the deposit properties. The collected deposits are also analyzed using an electron probe microanalyzer (7) Mueller, Ch.; Selenius, M.; Theis, M.; Skrifvars, B.-J.; Backman, R.; Hupa, M.; Tran, H. Deposition behaviour of molten alkali-rich fly ashesdevelopment of submodel for CFD applications. Proc. Combust. Inst. 2005, 30, 2991–2998. (8) Zbogar, A.; Frandsen, F.; Jensen, P. A.; Glarborg, P. Heat transfer in ash deposits: A modelling tool-box. Prog. Energy Combust. Sci. 2005, 31 (1), 1–51. (9) Heinzel, T.; Siegle, V.; Spliethoff, H.; Hein, K. R. G. Investigation of slagging in pulverized fuel co-combustion of biomass and coal at a pilotscale test facility. Fuel Process. Technol. 1998, 54 (1-3), 109–125. (10) Wee, H. L.; Wu, H.; Zhang, D.; French, D. The effect of combustion conditions on mineral matter transformation and ash deposition in a utility boiler fired with a sub-bituminous coal. Proc. Combust. Inst. 2005, 30 (2), 2981–2989.
10.1021/ef800976y CCC: $40.75 2009 American Chemical Society Published on Web 03/06/2009
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Figure 1. Schematic diagram of the slagging reactor.
(EPMA). The work presented in this paper is a part of our ongoing research on slagging and fouling in power station boilers.11,12 2. Experimental Rig The slagging test rig, shown in Figure 1, has been used. The essential parts of the rig are the fuel feeding system, the burner, the vertical reactor, and the ash-collecting facility. The pulverized fuel K-Tron feeder, not shown in Figure 1, is used to provide a constant fuel flow rate to the reactor system. The feeding rates can be adjusted in the range of 1-7 kg/h and are controlled/maintained within a 4% inaccuracy margin. The fuel is brought into an intermediate storage hopper, which has a maximum weighing capacity of 11 kg. A screw feeder at the bottom of the hopper gently moves the bulk material to the large throat and then into the discharge screws. A twin screw system is used because it is particularly useful for handling difficult and hardly flowing fuels, such as sewage sludge. The pulverized fuel is transported pneumatically to the burner (1) in an air stream coming from an air injector. The experimental burner (1) provides a wide range of operating conditions. The burner nominal fuel input is 50 kW, and it can be fired with a gaseous or solid fuel or both. If required, the secondary air can be swirled using a movable block swirl generator. The secondary air stream can be preheated to a maximum temperature of around 400 °C. An important feature of this burner is that its aerodynamics is well-known. The burner (11) Kupka, T.; Mancini, M.; Irmer, M.; Weber, R. Investigation of ash deposit formation during co-firing of coal with sewage-sludge, saw-dust and refuse derived fuel. Fuel 2008, 87, 2824–2837. (12) Weber, R.; Kupka, T.; Zaja˛c, K. Jet flames of a refuse derived fuel. Combust. Flame 2009. DOI: 10.1016/j.combustflame.2008.12.011.
exit velocities, velocity profiles, and turbulence have been measured using a laser Doppler anemometry for a number of inlet swirl numbers, and this information is essential for scientific measurements. In this work, a natural gas is used during the start-up procedure only. The burner generates stable flames of pulverized fuels with and without swirl. The reactor (2) consists of a vertical cylindrical combustion chamber that is fired from the top using the axisymmetrically installed burner. It is divided in a high-temperature, 2.2 m long radiative section of 0.3 m in diameter, made of a refractory material, and a low-temperature, 1.8 m long convective section of 0.25 m in diameter, which is made of a carbon steel. The radiative section is equipped with four heating elements to compensate for heat losses. The elements allow for setting up a required temperature profile along the radiative section. The heating elements have a thermal input of 8.8 kW each and withstand temperatures up to 1600 °C. The convective 1.8 m long section is water-cooled using three independent jackets, which allow for co-current, counter-current, and cross-flow cooling installations. The main purpose of the convective section is to cool the offgas to temperatures below 1000 °C. A direct quench system (3) is also installed to cool the offgas to about 180 °C by means of pressurized air. The cooling is necessary to avoid exceeding operating temperatures of downstream equipment. Fly ash is collected in the offgas system in a cyclone (4), which is installed in the cold offgas stream, and in a filter (5), located just downstream of the cyclone, to remove fine particles. Because all above units cause pressure drops in the offgas system, a vacuum unit (6) is required to drive the flow through all reactor parts.
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Table 1. Properties of the Five Fuels Used in the Experiment, General Conditions of the Investigation, and Typical Gas and Particle Properties at Three Measurement Ports fuel/blend
100% coal
Moisture (as fired) Ash Volatile matter C (as fired) H S N Cl O LCV
% % % % % % % % % MJ/kg
softening temperature hemisphere temperature fluid temperature test duration air ratio fuel thermal input fuel feed rate air flow rate air preheated temperature wall temperature
5% sewage sludge
15% sewage sludge
5% sawdust
Proximate and ultimate analysis, calorific value 4.93 5.17 5.59 10.64 12.49 15.70 31.14 33.28 37.00 68.67 65.42 59.77 4.66 4.68 4.70 0.83 0.87 0.94 1.78 2.05 2.52 0.00 0.01 0.02 8.49 9.32 10.76 26.54 25.32 23.21
15% sawdust
5.09 9.88 34.69 66.97 4.77 0.77 1.66 0.00 10.86 25.78
5.38 8.48 41.23 63.85 4.98 0.66 1.43 0.00 15.22 24.38
Ash Fusion Behavior of the Laboratory Ash According to the German Standard DIN 51730 °C 1240 1200 1170 1270 °C 1350 1290 1230 1350 °C 1370 1300 1250 1360
1230 1360 1380
min kW kg/h N m3/h °C °C
140 1.2 15 2.03 17.39 250 1200
General Conditions of Investigation 140 1.2 15 2.13 17.42 250 1200
140 1.2 15 2.33 17.50 250 1200
140 1.2 15 2.09 17.39 250 1200
140 1.2 15 2.22 17.40 250 1200
Gas and Particle Properties in the Slagging Reactor port 1 port 2 port 3 port 1 port 2 port 3 port 1 port 2 port 3 port 1 port 2 port 3 port 1 port 2 port 3 gas temperature oxygen mole fraction particle residence time carbon in ash (dry) particle burnout
°C 1300 0.057 s 0.3 % 59.9 % 81.1
1280 0.039 0.6 16.6 97.5
1200 0.036 1.8 6.9 99.1
1335 0.044 0.3 43.6 88.3
1270 0.034 0.6 21.1 95.9
1190 0.032 1.8 6.43 99.0
Three openings, which are marked in Figure 1 as ports 1-3, are used for the measurements. At each port, a radial distribution of the temperature is measured using a suction pyrometer. The O2, CO, and CO2 concentrations are measured using a gas sampling probe and a set of gas analysers. Another, specially designed, sampling probe allows for the measurements of carbon burnout. Two 22 m in diameter deposition probes are inserted at ports 2 and 3 (see section 3). 3. Experimental Procedure The South African bituminous high-volatile (Middleburg) coal of a relatively low slagging propensity has been chosen as the basic fuel. Coal/sawdust and coal/sewage-sludge mixtures have been milled together and four different blends have been prepared. Altogether, five fuels have been investigated: a pure coal (Middleburg coal) and four blends (5 and 15% of sewage-sludge content and 5 and 15% of sawdust content by thermal input). Ultimate and proximate analysis, calorific values, and ash fusibility temperatures of the fuel blends are given in Table 1, while Figure 2 shows the particle size distribution. Each fuel blend has been fired at 15 kW thermal input with an identical burner setup (see Table 1). After reaching steady-state conditions in the reactor, the temperature, gas composition, and char burnout have been measured at several radial locations at the three measurement ports. The residence times quoted in Table 1 have been obtained by performing computational fluid dynamics (CFD) simulations of trajectories of individual coal particles and subsequent averaging of the obtained residence times over a large number of particles. Thus, the residence time figures quoted in Table 1 correspond to the mean residence time of coal particles. Two kinds of deposition probes have been used: an uncooled ceramic probe (the composition of the ceramic: 60% Al2O3, 35% SiO2, and 3% K2O) and an air-cooled steel probe (X5CrNi18-10). The following experimental procedure has been followed. An uncooled ceramic deposition probe (Figure 3) has been inserted through port 2, and the mass of the deposits has been recorded. After completion of
1350 0.053 0.3 34.3 89.5
1260 0.038 0.6 12.8 97.1
1180 0.036 1.8 2.68 99.5
1310 0.058 0.3 53.1 86.8
1270 0.042 0.6 17.1 97.6
1190 0.038 1.8 4.8 99.4
1270 0.045 0.3 53.9 88.5
1240 0.037 0.6 18.4 97.8
1180 0.037 1.8 6.68 99.3
the deposit collection at port 2, a new ceramic deposition probe has been inserted through port 3 and the process has been repeated. The same procedure has been carried out using air-cooled metal deposition probes (Figure 3). After 140 min of deposit collection, the probes have been carefully removed and cooled. Then, each probe has been embedded in resin, sectioned, polished, and analyzed using an EPMA. The remaining ash from the deposition probes has been removed form the probe surface and analyzed for the chemical composition with X-ray analysis. In Table 1, the temperature, oxygen concentration, and char burnout measured at ports 1-3 are given to indicate typical conditions prevailing in the reactor.
4. Surface Temperature of the Deposition Probe The surface temperature of the uncooled ceramic deposition probe is the same as the gas temperature: at port 2, it is in the
Figure 2. Size distribution of the fuel blends.
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Figure 3. Cartoon drawing of (1) the ceramic uncooled deposition probe and (2) the air-cooled metal probe.
min, the surface temperatures of the deposition probes inserted in port 3 were measured to be in the range of 450-500 °C (see Figure 4). 5. Results and Discussion
Figure 4. Surface temperature of air-cooled metal deposition probes inserted in ports 2 and 3.
range of 1250-1300 °C, while at port 3, it is in the range of 1150-1200 °C. The surface temperature of the air-cooled metal deposition probe has been registered during the experiments using a thermocouple (Figure 3) and is shown in Figure 4 as a function of the time for all fuels investigated. At the beginning of the deposit sampling in port 2, the surface temperature was measured to be in the range of 650-750 °C. During the experiment, the ash deposit thickness increased, resulting in an increase of the heat-transfer resistance and the decrease of the surface temperature. At the end of the experiments after approximately 140 min, the measured surface temperatures in port 2 were in the range of 500-550 °C. In port 3, the flue gas temperatures were lower than in port 2 (see Table 1), which was reflected in the measured surface temperatures of the aircooled metal deposition probe inserted in port 3. The measured surface temperatures of the deposition probe in port 3 at the beginning of the ash sampling were approximately 650 °C. At the end of the deposit sampling after 140
5.1. Influence of the Fuel Type and Surface Temperature on the Deposit Growth. Parts a-e of Figure 5 show the mass of the deposit (divided by the deposition area) as a function of the time for all of the fuel blends tested, where the deposition area of π × 0.022 m × 0.3 m ) 0.0207 m2 remains constant in all of the experiments performed (the deposition area is π times larger that the projected area). During the 140 min lasting experiments, the deposit grows linearly with time as shown in Figure 5. Industrial-scale deposits grow up to a certain thickness; then the deposition rate decreases; and after a certain time, no further growth is observed. Sometimes, dependent upon both the deposit properties and the conditions at the deposition surface, the mass of the deposit may either decrease or so-called “stable stage of slagging” is achieved. The latter stage occurs when the amount of ash deposited on the surface does not change any more with time. Thus, our observations are valid for initial stages of slagging only. Figure 5e shows the deposition rate calculated after 140 min of the slag collection. The deposition rates at port 2 are substantially larger than at port 3. As shown in Figure 5e, the deposition rate at port 2 strongly increases with the sewagesludge content. An addition of 5% sludge doubles the slagging rate in port 2, while an increase to 15% triples it in the same port. In port 3, the aerodynamic and temperature conditions differed from the conditions in port 2 (a more even radial velocity profile and a lower gas temperature), which could be observed on the deposition rate. Indeed, the addition of sewage sludge increased the deposition rate but not to such a degree as in port 2. The sawdust seems to act as a cleansing agent. Its addition, the coal decreases the deposition rate by around 20%, with a relatively weak dependence upon dust content.
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Figure 5. Measured deposition rates: (a) deposition growth rate as a function of the time in port 2 for the air-cooled metal deposition probe, (b) deposition growth rate as a function of the time in port 2 for the ceramic deposition probe, (c) deposition growth rate as a function of the time in port 3 for the air-cooled metal deposition probe, (d) deposition growth rate as a function of the time in port 3 for the ceramic deposition probe, and (e) deposition rates after 140 min of the experiments in ports 2 and 3.
An interesting observation can be made by a comparison of the deposit growth on uncooled ceramic and air-cooled metal deposition probes. The mass of the deposit sampled on aircooled metal probes was, after 140 min of sampling, always larger than the mass sampled on the uncooled ceramic probes. The reasons for this effect are different surface temperatures of the deposition probe. First, the ash sampled in higher temperature conditions (surface temperature in the range of 1100-1300 °C) agglomerated and sintered faster than the ash sampled in lower temperatures (surface temperature in the range of 500-700 °C). Second, a clear influence of the surface temperature on the loss on ignition measured in the ash deposit was observed. The amount of unburned combustibles increased when the probe surface temperature was lowered. It was measured to be in the range between 0% for ash deposits sampled on the uncooled ceramic probe in port 2 and 3.0% for ash deposits sampled on the aircooled metal probe in port 3. Additionally, a clear correlation between the surface temperature and the sulfur and chlorine content of the ash deposit is visible, as shown in Table 2 (Xray analysis) and Table 3 (EPMA analysis). The sulfur (SO3) and chlorine (Cl) contents increase with the decreased surface
temperature. Thus, the increase of combustible matter, sulfur, and chlorine contents with decreasing surface temperature explain the observed dependence of the deposition rate upon surface temperature. 5.2. Influence of the Fuel Type and Surface Temperature on the Chemical and Physical Structure of the Ash Deposit. After 140 min of deposit collection, the deposition probes have been carefully removed and cooled. Then, they have been embedded in resin, sectioned, polished, and analyzed using an EPMA. The remaining ash from the deposition probes have been removed from the probe surface and analyzed for chemical composition with X-ray analysis. Table 2 shows the chemical analysis of the deposits. The deposit of the pure Middleburg coal contains mainly silica and alumina oxides (up to 70%), with around 13% of calcium oxide. The addition of the sewage sludge results in a decrease in the acid components (SiO2 and Al2O3), an increase in the iron oxide (Fe2O3) content, and the substantial increase of phosphor oxides (P2O5). The high contents of both Fe2O3 and P2O5 are responsible for this increase. Pure sewage sludge ash produced
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Kupka et al. Table 2. X-ray Analysis of the Deposit Ash MnO (%)
MgO (%)
CaO (%)
Na2O (%)
K2O (%)
P2O5 (%)
SO3 (%)
0.1 0.1 0.1 0.1 0.2
3.3 3.5 3.4 3.7 3.6
10.4 11.3 12.4 11.4 11.9
0.8 0.8 0.9 0.8 0.7
0.8 1.1 1.3 1.0 1.2
1.5 5.7 10.3 1.7 1.5
0.6 0.2 0.5 1.5 0.7
Ash Sampled on the Deposition Probe during the Combustion Trials (Port 100% Middleburg coal-ceramic deposition probe 39.4 28.4 1.1 10.9 0.1 2.7 100% Middleburg coal-metal deposition probe 36.8 27.2 1.1 8.1 0.1 3.0 5% sewage sludge-metal deposition probea 33.4 22.6 1.1 11.6 0.2 2.9 30.8 18.1 0.9 13.3 0.2 2.8 15% sewage sludge-metal deposition probea 5% sawdust-metal deposition probe 36.4 27.3 1.2 8.9 0.1 2.9 5% sawdust-ceramic deposition probe 39.5 28.9 1.1 9.0 0.1 2.9 15% sawdust-ceramic deposition probe 38.9 27.7 1.1 10.2 0.2 2.8 15% sawdust-metal deposition probe 35.5 25.9 1.1 11.3 0.2 2.8
2) 13.6 13.3 14.4 13.8 13.2 13.4 13.2 12.8
0.5 0.6 0.8 0.9 0.6 0.5 0.6 0.6
1.3 0.7 1.1 1.3 0.8 1.4 1.5 0.9
1.4 1.6 9.1 15.7 1.6 1.5 1.5 1.6
0.0 1.9 0.0 0.1 1.7 0.0 0.0 1.9
Ash Sampled on the Deposition Probe during the Combustion Trials (Port 100% Middleburg coal-ceramic deposition probe 37.9 26.1 1.0 11.5 0.1 3.1 100% Middleburg coal-metal deposition probe 37.3 28.2 1.2 6.5 0.1 2.7 5% sewage sludge-metal deposition probea 33.7 23.2 1.0 9.8 0.1 2.8 15% sewage sludge-metal deposition probea 30.4 18.0 0.9 12.8 0.2 2.7 5% sawdust-metal deposition probe 35.4 26.4 1.1 7.4 0.1 2.8 15% sawdust-ceramic deposition probe 38.1 26.9 1.0 10.1 0.2 3.0 15% sawdust-metal deposition probe 36.0 26.6 1.1 7.6 0.1 2.8
3) 14.2 11.3 13.3 13.5 13.1 13.4 12.9
0.4 0.8 0.7 0.9 0.7 0.4 0.6
1.5 0.8 1.0 1.3 0.9 1.7 1.0
1.4 1.8 7.4 14.9 1.7 1.4 1.6
0.5 1.6 1.9 0.1 3.6 0.8 3.4
SiO2 (%)
sample 100% Middleburg coal 5% sewage sludge 15% sewage sludge 5% sawdust 15% sawdust
34.3 31.6 30.0 35.9 36.1
Al2O3 (%)
TiO2 (%)
Fe2O3 (%)
Ash Sampled in the Cyclone 27.3 1.5 4.3 24.6 1.4 6.7 22.4 1.3 9.9 28.6 1.6 4.2 28.7 1.6 4.6
a The sewage sludge deposits sampled on ceramic deposition probes were molten. After the cooling of the deposition probe, it was not possible to remove the ash from the probe surface. Therefore, for sewage sludge blends, only deposition ash from metal probes is analyzed.
Table 3. EPMA Analysis of the Ash Deposit Chemistry for 15% Sewage Sludge and 15% Sawdust Deposits Sampled on a Ceramic Deposition Probe without Cooling and an Air-Cooled Metal Deposition Probe in Port 2a sampling probe type uncooled ceramic air-cooled metal uncooled ceramic air-cooled metal a
fuel 15% 15% 15% 15%
sewage sludge sewage sludge sawdust sawdust
SiO2 (%) Al2O3 (%) TiO2 (%) Fe2O3 (%) MgO (%) CaO (%) Na2O (%) K2O (%) P2O5 (%) SO3 (%) CI (%) 33.6 32.0 40.6 38.0
24.1 20.9 32.6 28.7
0.9 1.0 1.0 1.1
9.9 10.8 6.6 6.3
3.0 3.1 1.9 2.4
10.4 11.6 10.5 9.1
0.6 1.1 0.9 0.8
2.3 1.4 2.1 0.8
15.5 18.4 0.0 0.0
0.0 0.0 3.3 10.6
0.0 0.2 0.7 2.0
The average chemical composition was obtained by scanning of the ash deposit.
in a laboratory muffle furnace at 815 °C has 25.4% P2O5 and 16.8% Fe2O3. Pure coal ash produced at the same conditions has 1.7% P2O5 and 5.8% Fe2O3. Adding the sawdust into the coal causes a small decrease of the acid elements and an increase of base elements. Generally, the fly ash composition does not change significantly by adding up to 15% sawdust. No clear dependence between the surface temperature variations in a range of 600-1300 °C and the content of main ash deposit elements for the investigated fuels is observed. The sum of contents of the main elements (SiO2 + Al2O3 + TiO2 + Fe2O3 + CaO + MgO + P2O5) measured with X-ray analysis is in a range between 88 and 98%. The sum of the remaining elements (Na2O + K2O + SO3) measured with X-ray analysis is in the range of 1.8-5.0%. Thus, only a very small amount of alkali metals Na2O and K2O have been detected in the sampled deposits. No clear tendency between surface temperature and the contents of sodium and potassium (Na2O + K2O) is visible, because the differences are often smaller than the accuracy of the measurement. It has to be remarked that the lowest surface temperature of approximately 500 °C was measured at the end of the slag sampling, when a deposit layer was already formed. The outer surface temperature of the deposit was surely higher than the temperature at the probe surface. Such high temperatures of the deposit surface prevent condensation of alkali compounds. There is a clear tendency between the surface temperature and the sulfur content of the ash deposit; the sulfur (SO3) content increases with the decreased surface temperature. This behavior is confirmed by both the X-ray and EPMA analyses and can be related to the condensation of sulfates at lower temperatures or bonding of SO2 from the gas phase to, e.g., CaO in the ash deposit. This process is more intensive at lower temperatures,
as already observed.1 The chlorine content (Cl) in the ash deposit shows similar behavior as a function of the surface temperature (see Table 3). A very interesting observation was the high amount of phosphor oxide (P2O5) fractions in the deposits of sewage-sludge blends. This was observed not only in the low-temperature (550-700 °C) deposits but also at higher surface temperatures (1100-1300 °C). The high phosphor content is due to phosphate and calcium phosphate minerals, which are often components of sewage-sludge fuels. These compounds form different eutectics during the combustion process, which are stable at relatively high temperatures of around 1300 °C. To clarify this point, some extra work is needed. Figure 6 shows photos of the ash deposits collected after 140 min for the pure coal, two sewage-sludge blends (5 and 15%), and two sawdust blends (5 and 15%). The effect of the sewagesludge addition is visible. At higher surface and gas temperatures, the sludge deposits sampled on the ceramic deposition probe are molten. A layer of molten deposit flows on the probe surface inserted in port 2 at 15% sewage-sludge load. It was not possible to remove completely the sludge deposits after cooling the probe to ambient temperature. Porous, sintered (not molten), but easily removable deposits of the same fuel blend have been collected at lower surface temperatures on the aircooled metal deposition probe. Observations of deposits of sawdust blends (Figure 6) indicate that the deposit structure does not change significantly when sawdust is added. This is because the softening point of the Middleburg coal ash is high. The sawdust deposits remain solid (not molten), and they are characterized by a loose structure. There seems to be no substantial (visible) differences between the structure of pure coal deposits (Figure 6) and coal/sawdust
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Figure 6. Photographs of the deposits after 140 min of deposit sampling in ports 2 and 3.
deposits. Loose, easy removable deposits have been sampled on an air-cooled metal deposition probe during co-firing of coal-sawdust blends. Higher surface temperatures are responsible for collecting more sintered deposits on ceramic deposition probes during co-firing of the sawdust blends. Figure 7 shows the EPMA images of the deposits. Table 3 includes EPMA analysis of the ash deposit chemistry for 15% sewage-sludge and 15% sawdust deposits sampled on a ceramic deposition probe without cooling and air-cooled metal deposition probe in port 2. The average chemical composition was obtained by scanning the ash deposit. At port 2 on the ceramic deposition probe (surface temperature of about 1300 °C), a very homogeneous, glassy, and completely molten, 600 µm thick 15% sewage-sludge deposit layer is clearly visible. As noticed above, the high content of phosphor oxide P2O5 (15-18%) is evident. Neither carbon nor sulfur has been detected, as shown in Table 3. Sporadically, 20-30 µm iron oxide particles occur in this molten deposition layer (iron particles have a white color). The pyrite has been completely oxidized. The black spots that are visible in the deposit layer are artifacts of the imageproducing process and are to be disregarded. A very sharp and distinct boundary between the ceramics and the slag layer is visible with no traces of any alkali metal sublayer. There seems to be no reactions taking place between the ceramics and the deposit. Altogether, the observed deposit structure is rather
unusual and substantially different from typical deposits on metal surfaces.4 This is certainly related to the high temperature of 1300 °C prevailing at this deposition probe throughout the deposit collection process. At this temperature, no condensation of alkali vapor is possible and the deposit melts. At port 3, the 15% sewage sludge deposit layer sampled on a ceramic deposition probe (surface temperature of about 1200 °C) is 300 µm thick and its structure is still glassy and molten but less homogeneous than at port 2. Within this molten layer, some crystal structures begin to form. The black spots embedded in the layer are voids; the size of the largest and smallest voids are 80 and 20 µm, respectively. Particles containing iron or particles containing alumina and silica oxides are embedded in the molten layer. The black area above the deposit layer is to be disregarded because it shows the resin used. The presence of both the crystal structures and the voids is an indication that the deposit temperature of 1200 °C is not high enough to melt all of the components. Thus, the deposit layer for 15% sewage sludge at port 3 is less fluid than that of port 2. The 15% sewage-sludge deposits sampled at lower surface temperatures on the air-cooled metal deposition probes (surface temperature in a range of 500-700 °C) have different structures. In port 2, the thickness of the deposit is larger than 2.5 mm, which is a maximum value of the measurable thickness range of the EPMA method. Therefore, only a part of the deposit
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Energy & Fuels, Vol. 23, 2009
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probe have a similar structure; they are also porous and inhomogeneous. Because of lower temperatures, the sintering and agglomeration processes do not proceed as rapidly as on the ceramic deposition probe. Consequently, the low-temperature deposits are thicker and less sintered than the high-temperature ones. 6. Conclusions
Figure 7. EPMA image of 15% sewage sludge and 15% sawdust deposits sampled on a ceramic deposition probe without cooling and air-cooled metal deposition probe.
thickness is shown in Figure 7. The deposit in port 2 is porous and inhomogeneous. Because of the higher temperature of the outer surface of the deposit, some agglomerated and sintered, and therefore, larger particles (300-400 µm) are visible in the upper part of the deposit layer. The 15% sewage-sludge deposit sampled on an air-cooled metal deposition probe in port 3 is about 2.2 mm thick. It also has a porous and inhomogeneous structure. In the upper part of the deposit layer, some agglomerations of bigger particles (100-200 µm) are also visible. However, these agglomerates are not as large as those in port 2 because the temperature in port 3 is lower. Figure 7 also shows the EPMA images of the 15% coal-sawdust deposits. The deposits sampled on the ceramic deposition probe are indeed very different to 15% sewage-sludge deposits sampled on the same ceramic deposition probe. They are porous and rather inhomogeneous (the dark area above the ceramic probe is the resin and should not be confused with the molten sewage-sludge deposits). Particles in the 20-200 µm size range, which contain mainly alumina, silica, and calcium oxides, form the porous 840 µm 15% sawdust deposit at port 2; occasionally, iron oxide particles as large as 600 µm are present (iron particles have a white color). The deposit layer at port 3 is 220 µm thick and incorporates smaller particles, typically in the size range of 10-60 µm. The deposit structure presented in Figure 7 is indeed similar to the deposits previously observed4 for coal blends. The 15% sawdust deposits sampled at lower surface temperatures on an air-cooled metal deposition
Blends of a South African bituminous “Middleburg” coal, a sewage sludge, and a sawdust have been fired in the slagging reactor to examine the effect of the added fuel on the slagging propensity of the mixtures. Uncooled ceramic deposition probes and air-cooled steel probes have been used to investigate the initial stages of slagging in the 1200-1300 °C temperature range and in the 550-700 °C temperature range. The deposition rates and the deposit structure have been a strong function of both the fuel blends and the deposition surface temperatures. The following has been concluded: (a) The parent coal has formed light and easily removable deposits on both the ceramic probe of high surface temperatures (1100-1300 °C) and the air-cooled metal probe of 550-700 °C surface temperature. However, the addition of even a relatively small amount of the sewage sludge alters both the deposit structure and the deposition rate. (b) Glassy, compact, and easily molten deposits are characteristic for deposit formation on surfaces with high temperatures (1100-1300 °C) during co-firing of sewage sludge with coal. The initially formed deposits have become rapidly molten, and they begin to flow on the surface of the deposition probes particularly when the sewage-sludge fraction exceeds 15% by thermal input and the gas flow temperature exceeds the ash melting point. Porous, sintered (not molten), but easily removable deposits of the same fuel blend have been collected at lower surface temperatures (550-700 °C) on the air-cooled metal deposition probe. (c) The addition of the sawdust to the coal seems to have a positive influence on the deposition behavior and deposit structure of the mixture (a slight decrease in the deposition rate even at higher temperatures). Loose and easily removable deposits are formed by firing of coal/sawdust blends. (d) The mass of the deposit sampled at lower surface temperatures (550-700 °C) on air-cooled metal probes was always larger than the mass sampled at higher surface temperatures (1100-1300 °C) on the uncooled ceramic deposition probes. The ash sampled at higher temperatures agglomerated and sintered faster than the ash sampled at lower temperatures. (e) For the investigated sewage-sludge and sawdust blends, the variation of the surface temperature in the 500-1300 °C temperature range does not seem to influence substantially the chemical composition of the deposit formed. However, the content of unburned combustibles, sulfur, and chlorine decreased when the deposit surface temperature increased. Acknowledgment. The work has been performed within the European Commission (EC) Marie Curie INSPIRE Network (MRTN-CT-2005-019296). The authors thank the EC for financial support. EF800976Y
