Ash Deposit Analysis of the Convective Section of a Pilot-Scale

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Ash Deposit Analysis of the Convective Section of a Pilot-Scale Combustor Firing Two Different Sub-bituminous Coals Manuel García Pérez,*,† Andrew R. Fry,‡ Esa Vakkilainen,† and Kevin J. Whitty‡ †

Lappeenranta University of Technology, Post Office Box 20, 53851 Lappeenranta, Finland Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112, United States

‡

ABSTRACT: This work presents detailed measurements of fouling deposits of a 100 kW (nominal thermal input) pilot-scale combustor operated over different tests with different combustion settings. After the tests, the heat exchangers of the convective section were removed for deposit examination. The results offer information regarding the fouling behavior during periods that lasted more than 20 h, starting from clean tubes. These long periods yielded valuable information about the bulk deposit buildup and final shapes as well as the effects of the oscillating flow patterns that occur in the staggered-plate tube banks. Special consideration was given to the deposit thickness profile around the tubes, final deposit shapes, and deposit mechanisms. Two different sub-bituminous coal test campaigns have been considered in this study. Typical bulky deposits were observed for a Wyoming coal (first test), where the deposits were about 4−6 mm in the tube winds and about 1 mm in the lees. These results were significantly different for an Indonesian coal (second test), where, in most of heat exchange surfaces, agglomerations caused by direct vapor condensation on the surface were observed, especially at gas temperatures below 470 °C. Combinations of very different deposition mechanisms (inertial impaction, vapor/aerosol condensation, and thermophoresis) were observed in the fouled material.

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shapes within the context of the complicated oscillating flow patterns that occur in the heat exchangers of industrial boilers.12,13 Because of the physical difficulty of sampling the ash-laden flue gas with probes of a geometry similar to a bank of tubes, only very few studies9 are concerned with the effect that the different tube arrangements can have during long periods of deposition. This study aims to report the final features, shapes, and sizes of the fouled layer of heat exchanger tubes after long (around 20 h) periods of actual fouling growth, starting with new, clean tubes. As such, the study captures both initial and long-term fouling behavior. A 100 kW (nominal thermal input) pilot-scale combustor was run to perform two different tests, each firing a low-rank coal with a different origin. The heat exchangers of the convective section of this combustor were extracted after each test for deposit examination. The chemical properties of the deposits have been the target of numerous studies found in the literature, some of which have already been referenced. As a novelty, this study focuses more on the final shapes of these deposits and aims to explain them. A comparison of the amount and shape of deposit around the perimeter of individual tubes and among different tubes is presented. The resulting deposit profile is a consequence of the fluid pattern across the tubes and cannot be captured properly with the often used approach of one-tube probe sampling. In addition, the deposit compositions are briefly analyzed. Some empirical ash deposition investigations have been complemented with numerical models,14,15 with the aim to explain the implications of the flow patterns on the fouling and slagging. Similarly, a

INTRODUCTION The ash deposition on the cold surfaces of industrial boilers represents one of the major technical challenges in their operation and maintenance. Whether the ash is melted as a result of radiative heat (slagging) or not (fouling), its excessive deposition is the most typical reason for unplanned shutdowns.1 The low thermal conductivity2−4 of the deposits coating the tubes entails a strong heat transfer resistance, penalizing the overall boiler performance and increasing the temperature of the flue gases at the stack. In addition, the deposits lead to corrosion issues and potential plugging of the flow area. As a result of these challenges, as a common practice, industrial boilers are designed with safety factors aiming to oversize their performance in the case of unpredicted uncertainties of operation, often entailing penalties in overall efficiencies and increased investment costs.5 These reasons constitute a motivation and need for a deep comprehension of the fouling and slagging nature, elimination (e.g., sootblowing and thermal shock), and predicting tools (modeling). Informative work has been carried out regarding deposit analysis and its chemical composition found under different firing conditions.2,6,7 These earlier studies typically focused on the collected ash in, e.g., precipitators, bottom ash collectors, or bag houses. Other researchers have investigated the ash deposition on special probes during periods of time varying from 1 min8 to longer periods of the order of 1 h.6,9−11 These samples might be sufficiently long to capture representative deposits for an analysis of the physical properties and chemistry. Moreover, these tests may also yield valuable information about the initial ash deposition rates6 and help to understand the deposition mechanisms10 and corrosion implications. Unfortunately, little information is obtained about the advanced fouled layer growth rate and/or the deposit final © XXXX American Chemical Society

Received: July 8, 2016 Revised: September 18, 2016

A

DOI: 10.1021/acs.energyfuels.6b01662 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. (Left) Sketch of the combustor and (right) photograph.

Figure 2. (Left) Geometry of a plate and (right) photograph of a complete heat exchanger.

computational fluid dynamics (CFD) model is implemented for the second test presented in this work, with emphasis on the effects of the flow features on thermophoretical deposition.

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This combustor has been used in previous research,10,11,16,17 which has focused on the burner and combustion zone. The present work aims to study the deposits in the convective heat exchangers, and hence, the descriptions given here focus on the convective section with more detail. A detailed description of the whole unit can be found in the references given, especially in ref 16. Although the whole unit is refractory-lined as a result of its relatively small size compared to industrial boilers, the heat losses throughout the reactor walls may be significant in the combustor, especially in the near-flame region. In the convective section, which is the area of interest of this work, the heat losses are of the order of 230 W, while the heat removed by water is approximately 6.5 kW. Those heat loses

MATERIALS AND METHODS

Pilot-Scale Coal Combustor. The combustor is located at the Industrial Combustion and Gasification Research Facility of the University of Utah in Salt Lake City, UT. It was designed as a 100 kW oxy-combustor, although typically operates at 30−50 kW. In the tests described here, it was air-fired with a target thermal input of 36 kW. The unit consists of two main parts: the combustion zone and the convective heat transfer zone, which has water-cooled heat exchangers inside. B

DOI: 10.1021/acs.energyfuels.6b01662 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

The temperature difference between the tube outer wall and the flue gas is a key parameter for the thermophoretical driving force.18,19 Thermophoresis is understood to be a significant deposition mechanism, especially for the fine fraction of the particles that travel through the convective section. Particle Size Distribution Measurements. Large particles (with a diameter of about 20 μm or larger) resulting from combustion are caught in the ash trap at the bottom of the vertical section of the furnace. Most of the fine fraction of the particulate turns the corner and travels toward the heat exchangers. The particle size distribution is measured in real time right before the turn in the beginning of the convective section. The hardware used for this measurement consists of a water-cooled nitrogen-quenched isokinetic sampling probe, a nitrogen gas cylinder, a flow meter, clean compressed air, an orifice, and a dilution manifold, to which are connected a TSI model 3080 scanning mobility particle sizer (SMPS) capable of measuring particles in a range of 0.014−0.67 μm20 and a TSI model 3321 aerodynamic particle sizer (APS) capable of measuring particles over a size range of 0.53−20 μm.21 This sampling system uses a 400:1 two-stage dilution ratio (nitrogen and cleaned air) to prevent interaction and transformation of particles downstream of the sampling location. Deposit Thickness Measurement. To determine the thickness profile of the deposits at different locations, the material deposited on a longitudinal section of the tubes was carefully cleaned off. Hence, a part of the tube was naked, and the other part remained coated with the deposit. By careful analysis of photographs of the tubes, the deposit thickness could be determined by measuring its width in the photo (in pixels) and comparing it to the tube outer diameter of the cleaned section of the tube, which is known to be 12.7 mm. Figure 3 shows this

are, nevertheless, accounted for when calculating and reporting the data and firing conditions relevant for this work. Burner, Combustion, and Radiative Section. The unit uses a tworegister custom-made burner, which consists of an inner pipe, which carries the primary air and coal in an air/coal mass ratio of 1.8. The rest of the air (secondary air) is injected through an outer annulus with swirl vanes to induce turbulence and mixing. The burner feeds pulverized coal at variable rates. The coal feed rates in this study will be detailed in the Combustion Parameters section. The burner fires downward into the combustion chamber, which has a shape of a vertical cylinder of 61.0 cm diameter and 1.14 m height. After the main combustion chamber, the product gases flow down through a circular radiant section of 26.7 cm diameter. A cone connects the combustion chamber to the radiative section. Convective Section and Location of Heat Exchangers. After the radiative section of the combustor, the gases experience a 90° turn at the bottom to enter the second part, the horizontal convective section, where several water-cooled heat exchangers are located. This convective zone has a square cross-section of 15.2 cm per side. After this section, there is a bag house, a scrubber, and the exhaust stack. As noted in Figure 1, several water-cooled heat exchangers can be placed in the horizontal section of the combustor. There are a total of eight slots for these exchangers, although during the tests in this study, only four exchangers were placed in slots 2, 3, 4, and 5 (the slots are numbered according to the flow itinerary; Figure 1). In this work, only the first two heat exchangers, located in slots 2 and 3, were studied. The first slot was left empty (just sealed) to favor a fully developed flow before the gas entered the cooling section. From this point forward, in the text, the heat exchanger located in slot 2 will be referred to as “the first heat exchanger” or “HE1” and the heat exchanger located in slot 3 will be referred to as “the second heat exchanger” or “HE2”, consistently with the path of the flue gas. Description of Heat Exchangers. All of the heat exchangers are identical and consist of five plates of bent pipes staggered in such a way that the tubes are in a triangular pitch pattern of 60° to one another. Each plate consists of one tube with four passes. The geometry of the plates is detailed in the left panel of Figure 2. The tubes are water-cooled and have an outer diameter of 12.7 mm, and the tube wall thickness is 1.24 mm. The transverse separation between two consecutive plates is 2.86 cm, i.e., 2.25 times the tube diameter. The longitudinal spacing between two consecutive tube passes is 7.62 cm, 6 times the diameter. The refractory lining spans 17.5 cm from the headers of the pipes, making the pipes tangent to the refractory in their middle bend. One of these heat exchangers can be seen in the right panel of Figure 2. The plate in the center of the heat exchanger will be referred to as the central plate. The two (one at each side) outermost plates are referred to here as edge plates. Finally, the plates that are located between the central plate and an edge plate will be referred as interior plates. The four passes of each plate will be numbered from 1 to 4 according to the flue gas flow, similar to what has been done to distinguish between the two heat exchangers. A thermocouple was positioned to read the temperature of the flue gas upstream of the first heat exchanger. Another thermocouple was placed to log the temperature downstream of the second heat exchanger. Those thermocouples were placed at 5 cm from the refractory surface. All thermocouples in this study were K-type. Measurements for the water side were also taken. The water flow rate of each exchanger was registered with the use of totalizers. The water inlet and outlet temperatures were also monitored with thermocouples, with the aim to calculate the global heat flux to the water in the heat exchangers. However, because the water temperature rise was small (about 2−4 °C), the measuring error was relatively significant to the total temperature difference. Therefore, the heat flux to the heat exchangers was calculated by means of energy balances on the flue gas side, accounting for the previously mentioned heat leaks. This heat to the water was used to calculate the evolution of the heat transfer performance (in units of power per tube area and per wall−gas temperature difference), which is expected to decrease with the time as the tube surfaces become fouled.

Figure 3. Sketch showing the procedure for measuring the deposit thickness through a photograph. In particular, this photograph corresponds to the fouled surface of the third pass of the edge plate of the second heat exchanger after the first coal test. The deposit thickness can be estimated by multiplying the diameter of the tube (12.7 mm) by the deposit/tube diameter pixel ratio. process. It was possible, by taking photographs from different adequate angles, to outline the profile of the deposits around the circumference of the tube. A photo taken perpendicular to the tube axial direction and to the flow direction shows the wind and lee deposit thicknesses. Photographs that were taken in parallel to the flow gas direction were useful to determine the deposits in the sides (i.e., at 90° over the tube circumference from the lee or from the wind).

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COMBUSTION PARAMETERS Coal Analysis. Two main types of coal from different origins were used in this study. As better detailed in the following section, two different combustion test campaigns were performed, each firing a different kind of coal. A coal mined in Wyoming, U.S.A., was fired in test 1, whereas an Indonesian coal was used in test 2. The tests consisted of two C

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Energy & Fuels Table 1. Proximate (As Received) and Ultimate (Dry Basis) Analyses of Fired Coalsa proximate analysis

a

ultimate analysis

wt %

moisture

ash

volatile

fixed carbon

C

H

O

N

S

ash

HHV (MJ/kg)

raw Wyoming coal dried Wyoming coal raw Indonesian coal dried Indonesian coal

29.97 9.46 30.08 8.08

5.32 6.70 1.94 2.47

31.28 40.58 34.16 45.00

33.43 43.26 33.82 44.45

66.79 68.18 68.78 69.30

4.98 5.00 5.17 5.06

19.07 17.61 22.30 21.80

0.97 1.25 0.84 1.02

0.59 0.56 0.14 0.13

7.60 7.40 2.77 2.69

19.01 24.81 19.14 25.39

The heating value (as received) is also provided.

Table 2. Ignited Ash Elemental Analysis wt %

SiO2

CaO

Al2O3

SO3

MgO

Fe2O3

Na2O

TiO2

P2O5

BaO

SrO

K2O

MnO

undetermined

raw Wyoming dried Wyoming raw Indonesian dried Indonesian

30.67 31.42 7.51 8.96

24.51 25.39 45.92 43.53

16.11 15.33 5.71 5.94

13.64 13.04 8.48 9.32

6.06 6.16 7.66 7.54

5.54 5.00 21.01 18.69

1.98 1.96 0.14 0.20

1.29 1.53 0.40 0.32

0.84 0.88 0.06 0.16

0.48 0.29 0.64 0.63

0.36 0.35 0.42 0.41

0.35 0.31 0.09 0.08

0.03 0.02 0.60 0.59

−1.86 −1.68 1.36 3.63

Table 3. Main Test Conditions test

period

coal

ṁ f (g/s)

period duration

q̇1 (L/s)

q̇2 (L/s)

Twater(°C)

1

1 2 1 2 3

Wyoming raw Wyoming dried Indonesian dried Indonesian raw Indonesian raw

1.52 1.46 1.57 1.45 1.57

10 h and 16 min 9 h and 3 min 9 h and 2 min 3 h and 15 min 11 h and 49 min

0.479 0.391 0.460 0.456 0.460

0.207 0.144 0.485 0.483 0.487

25.7 22.6 22.7 23.2 23.9

2

Table 4. Values of Relevant Fouling Parameters (Highest, Time-Averaged, Lowest) magnitude

test 1, period 1

test 1, period 2

test 2, period 1

test 2, period 1

test 2, period 3

T1 (°C) T2 (°C) T3 (°C) v1 (m/s) v2 (m/s) v3 (m/s) ΔT1 (°C) ΔT2 (°C)

(688, 664, 638) (502, 483, 409) (360, 336, 248) (2.28, 1.99, 1.52) (1.86, 1.55, 1.13) (1.52, 1.25, 0.86) (662, 636, 613) (466, 431, 376)

(692, 681, 638) (533, 521, 409) (392, 388, 248) (2.31, 2.20, 1.52) (1.92, 1.84, 1.13) (1.59, 1.50, 0.86) (667, 653, 613) (507, 493, 375)

(697, 685, 626) (495, 486, 402) (337, 329, 242) (2.49, 2.34, 1.84) (1.97, 1.84, 1.35) (1.57, 1.45, 1.02) (672, 658, 601) (470, 452, 378)

(696, 665, 652) (506, 473, 461) (350, 314, 311) (2.14, 2.08, 2.04) (1.72, 1.65, 1.62) (1.38, 1.31, 1.29) (671, 644, 626) (481, 450, 437)

(676, 664, 655) (495, 482, 460) (346, 334, 307) (2.23, 2.16, 2.09) (1.80, 1.73, 1.65) (1.45, 1.39, 1.31) (650, 638, 629) (470, 456, 434)

or three different periods with slightly different firing rates and with different coal pretreatment processes. Those pretreatment processes consisted mainly on a drying of the coal. The properties of the coals used are shown in Table 1. The detailed ignited ash composition is given in Table 2. Test Description and Main Firing Conditions. As mentioned earlier, the tests consisted of two or three different periods where the fired coal (whether raw or pre-dried) was switched and with slightly different firing rates. At the end of each test, the two heat exchangers were extracted for deposit examination and substituted with two clean heat exchangers. The combustor was fired with natural gas between the tests to maintain hot conditions, so that it would be warm at the beginning of the following test, in an attempt to minimize startup-related unsteady conditions. The global features of the two tests and their periods are summarized in Table 3. In Table 3, ṁ f stands for the mass flow rate of fuel, q̇1 and q̇2 are the volumetric flow rates of water through the heat exchangers, and Twater is the temperature of the inlet water for the heat exchangers (which is the same for both exchangers). All of these magnitudes were not exactly constant because they experienced slight variations and fluctuations typical from boiler operation; the values shown in Table 3 have been timeaveraged.

Measured and Calculated Magnitudes. Table 4 contains the value of those variables (directly measured or indirectly computed from other measurements) that is relevant for a proper determination of the fouling conditions. All of these parameters were not strictly constant with the time and were subject to small fluctuations typical of combustion operation, especially at the beginning of each period as a result of unsteady effects. Although these fluctuations were not long (a duration of 1 or 2 min is a very short time compared to the duration of a test) nor significant, the highest, time-averaged, and lowest values for each magnitude are provided. In Table 4, T1, T2, and T3 are the flue gas temperatures upstream of HE1, between both heat exchangers, and downstream of HE2, respectively. Similarly, v1, v2, and v3 correspond to the flue gas velocities at the same locations. ΔT1 and ΔT2 stand for the temperature differences between the upstream gas flow temperature and the average tube outer wall temperature for each heat exchanger. These ΔT values were calculated from the water temperature inside the tubes, assuming that the thin tube walls offer no significant resistance to the heat conduction. This temperature difference is known to be a key parameter for the thermophoretical driving force, as mentioned earlier. D

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Figure 4. Particle distribution measured at the 90° turn (upstream of the first heat exchanger) with the SMPS/APS.

Figure 5. Temporal evolution of the global heat transfer coefficient (combining HE1 and HE2) for each test. The green dots indicate the change of a period (see Table 3), where relatively short perturbations occur as a consequence of dynamic unsteady effects.

Particle Distribution. The results of the measurements of the SMPS/APS devices are shown in Figure 4. For all coals, most of the particles were found in the range of 1−10 μm. Coarser particles are not expected to follow with the flow after the 90° turn. Instead, they become caught in the ash trap. Other Specifications. The excess air in the combustion was set to match a 3% O2 concentration in the dry flue gases at the exit of the combustor. On the basis of the carbon content in the ash, it was determined that the combustion of the fuel was over 99% complete. The combustor was operated at an absolute pressure of 85 kPa, corresponding to the atmospheric pressure in Salt Lake City.

the slope of these trends with the time is possibly due to the fact that, as the deposit layer grows, the temperature difference between the gas and the outermost layer of deposit decreases, resulting in a weaker thermophoretical force. This behavior is typical in situations where the thermophoresis is significant.18,22,23 By comparison of the results of both tests, it can be observed that the evolution (reduction) of the heat transfer coefficient is similar, although the first test displayed consistently a lower heat transfer coefficient by about 15 W m−2 K−1. Because the water flows fast through the cooling tubes and the tube walls are very thin, the deposit layer and outer convection are expected to be the most important heat transfer resistances. Hence, they should determine the shape of these charts. Because that difference (of about 15 W −2 K−1) between the tests was consistent already from the beginning of the tests (which start with clean tubes for both cases), it must be caused only by parameters affecting the outer convection and not by those parameters related to the deposits. It is thus assumed that the difference relates to the somewhat different operating conditions for the two coals and the properties of the flue gas

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RESULTS AND DISCUSSION Heat Transfer. The global heat transfer coefficient is calculated by dividing the heat transfer rate to the water by the total tube area (which is 0.1638 m2, combining HE1 and HE2) and by the logarithmic temperature difference. The evolution of this variable is shown in Figure 5. As expected, a clearly descending trend in heat transfer was found as a result of the effects of the deposits. The reduction in E

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Figure 6. Deposits of the heat exchangers after test 1. The gas flow comes from left to right in the images: (left) heat exchanger 1 and (right) heat exchanger 2.

(e.g., thermal conductivity, gas composition, density, and velocity). It is worth noting that this difference in the heat transfer between the tests remained essentially constant over time. As shown in the next section, the first test resulted in a much larger volumetric amount of deposits when compared to the second test. Because the decrease on the heat transfer performance was similar for both cases, it appears that the deposits of the second tests (the Indonesian coal) had a significantly lower thermal conductivity than the deposits of the coal from Wyoming. Test 1 (Wyoming Coal) Deposits. The deposits of the heat exchangers after the first test are shown in Figure 6. The first heat exchanger was accidentally bumped into after the extraction and before examining the deposits. As a consequence, most of the deposits of the wind of all its tubes fell off. The deposits on the sides (i.e., at 90° around the circumference perimeter from the wind or from the lee) and on the lee of the tubes remained. This made it impossible to obtain further information about the wind deposit of the first heat exchanger for this test. The measured deposit thicknesses, calculated as described in the Deposit Thickness Measurement section, are summarized in Tables 5 and 6. From those results, the approximated shapes

a consequence, it happened twice that, while attempting to clean half a tube pass prior to photography, the whole material fell off, even though the best possible care was taken. The thickness information was lost for the last pass of the interior plates of HE1 and for the second pass of the interior plates of HE2. The interior plates were the hardest plates to access for manual cleaning. As the flue gas travels through the heat exchange section and cools off, it faces a weaker thermophoretical driving force (the temperature gradient toward the tube). In the first heat exchanger, the deposits on the lee sides of the tubes decreased slightly, supporting the idea that lee side deposits are influenced mainly by thermophoresis. However, in the second heat exchanger, the deposits did not decrease along the flue gas path but, instead, became somewhat uniform on the lee and side positions. A possible explanation for this is that the gas reduces its velocity as it cools (and its density rises), increasing the influence of thermophoresis to deposit particles in these lee areas.19 The deposits on the wind of the tubes were significantly larger than those on the trailing edges, suggesting that inertial impaction may occur even for some small particles and at relatively low velocities. The deposits on the wind sides of the tubes in the second heat exchanger increase, especially for the edge plates. This can be due to its proximity to the lateral refractory of the combustor; the edge plates are submerged into the fluid boundary layer with a flow pattern somewhat different from that of the central plate. The global overview of these results suggests that a combination of thermophoresis and inertial impaction is causing these observations. Inertial impaction is more relevant in the first passes because the flow velocity is larger. When the flow decreases its velocity as a consequence of cooling, the particles reduce their Stokes number and have it easier to avoid any obstacle (tubes) along with the flow.1 Consequently, inertial impaction fouling weakens in the direction of the flow, favoring slightly the significance of thermophoretical deposition of small particles.

Table 5. Measured Deposit Thickness for the First Heat Exchanger, in Millimetersa plate

first pass

second pass

third pass

fourth pass

central interior edge

(1.0, 2.8) (1.2, 2.6) (1.1, 2.4)

(1.2, 1.9) (1.3, 2.0) (0.8, 1.1)

(1.2, 1.4) (1.1, 1.6) (1.3, 2.3)

(0.9, 3.0) (1.6, 1.9)

a

The pair of values in each table entry represents the deposit at different tube angles, as (lateral side, lee). The deposits of the fourth pass of the interior plate fell off before they could be measured. The deposits of the winds of all tubes also fell off before they could be measured.

of the fouled material are sketched in Figure 7 for the second heat exchanger. The deposit was very powdery and delicate. As

Table 6. Measured Deposit Thickness for the Second Heat Exchanger, in Millimetersa plate

first pass

second pass

third pass

fourth pass

central interior edge

(3.5, 1.4, 1.1) (6.0, 0.8, 1.5) (3.0, 1.1, 2.1)

(3.5, 0.9, 0.9)

(4.3, 0.6, 1.2) (3.3, 0.9, 1.0) (3.8, 0.9, 1.0)

(4.2, 1.5, 1.0) (7.9, 0.8, 1.1) (6.7, 1.2, 1.8)

(4.2, 1.0, 1.2)

a

The values in each table entry represents the deposit at different tube angles, as (wind, lateral side, lee). The deposits of the second pass of the interior plate fell off before they could be measured. F

DOI: 10.1021/acs.energyfuels.6b01662 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. Qualitative sketch of the approximate deposit shapes for a cross-section of the second heat exchanger. This figure has been built from the data of Table 6. The distances between the tubes are not on scale; they have been reduced in the presentation for the sake of space. A polar grid has been placed in each tube for deposit measurement. The radial resolution of the grid is 2 mm. The angular resolution of the grid is π/8 rad.

Figure 8. Deposits of the heat exchangers after test 2: (left) first heat exchanger, (middle) passes 1 and 2 of the second heat exchanger, and (right) passes 3 and 4 of the second heat exchanger. The gas flow comes from left to right in the images. Note how different the deposits are from the fourth pass of the first heat exchanger.

Test 2 (Indonesian Coal) Deposits. The deposits formed during the second test vary significantly from the deposits measured in the first test, as observed by contrasting Figure 8 with Figures 6 and 3. In the first two passes of HE1 (where the flue gas temperature and velocity were higher), the deposit growth was somewhat similar to what was seen after the first test (the deposit grew as a bulk). On the other hand, in the last two passes of the first heat exchanger and along the whole second heat exchanger, the deposits showed a different structure. Scattered and relatively large (of 1 mm or even larger) buildups had appeared, especially away from the lateral sides of the tubes. A larger concentration/agglomeration of these buildups could be seen in the tube bends, especially in the interior part of the turn. Deeper within these deposits (i.e., closer to the tube surface) and also in all those areas of the tube surface free from these formations, there was a thin, powdery layer of brown fine particles (diameter of