Effects on a 50 MWth Circulating Fluidized-Bed Boiler Co

Article pubs.acs.org/EF

Effects on a 50 MWth Circulating Fluidized-Bed Boiler Co-firing Animal Waste, Sludge, Residue Wood, Peat, and Forest Fuels H. Hagman,* R. Backman, and D. Boström Department of Applied Physics and Electronics, Thermochemical Energy Conversion Laboratory, Umeå University, SE-901 87 Umeå, Sweden ABSTRACT: This work is a part of an effort to maximize the operational safety of a 50 MWth circulating fluidized-bed (CFB) boiler located in Perstorp, Sweden, co-firing animal waste, peat, waste wood, forest residues, and industrial sludge. An increase in the CFB boiler availability reduces the use of expensive fossil fuel (oil) in backup boilers during operational problems of the CFB boiler. The work includes a thorough mapping and analysis of the failure and preventive maintenance statistics, together with elemental analysis of boiler ash and deposits, flue gas, and fuel fractions. Correlations between boiler parameters and boiler availability are sought, and recommendations regarding boiler design and operation are made. An explicit description of the boiler is made to allow for the use of presented material as future reference material. It was observed that the failure frequency is especially high where (1) rapid chloride-rich windward deposit buildup is combined with (2) high construction material temperature and (3) windward soot blowing. In areas where one of these factors was absent, a more moderate material loss could be seen. The flue gas average elemental composition can be regarded as close to constant as it flows through the series of heat exchangers. Thus, the significant differences in deposit buildup of different flue gas cross-sections cannot be a result of changed average flue gas composition. The areas of the steam tubes suffering from rapid material loss are also exposed to high deposit rates. Downstream of a well-defined temperature threshold in the secondary superheater, neither material loss nor substantial deposit buildup could be seen. Tube deposits are dominated by Na, S, Ca, K, and P, but only Na, K, and S are enriched in the windward tube deposits relative to the fly ash bulk composition. The temperature of the flue gas is the major parameter governing the rate of deposit buildup in the boiler heat exchangers. Of the fuel nitrogen, 95 wt % leaves the process as N2(g). Fuel mix ash content analysis via a separate ashing of different fuel fractions by heating to 550°C does not reflect the ash content of the fuel mix correctly. The soot blowing angle of attack on the deposits should be regarded in areas with rapid deposit growth when boilers and soot blowers are designed to allow for efficient tube cleaning. The use of heterogeneous fuel in the boiler creates strong variations in fuel, flue gas, and particle composition and makes it increasingly important to have online measurements to be able to understand and control the furnace chemistry. The filter ash in the flue gas baghouse filter effectively sorbs HCl(g) and NH3(g) from the flue gas already without the addition of sorbents. Online flue gas measurement to control the furnace chemistry must therefore be installed upstream of the filter to enable accurate control. Also, a significantly larger filtration area can be installed in the baghouse filters with a slight increase in cost, to allow for efficient use of the ash as free of cost sorbent and lowered emission levels. Scanning electron microscopy analysis of the flue gas deposits shows that no pieces of ground bone, sand particles, or other relatively large flue gas particles contribute directly to the deposit buildup. White crystals rich in N and Cl, most likely ammonium chloride, precipitate downstream of the flue gas filter. The precipitation interferes with the dust emission measurement and forces a reduced usage of waste-derived fuels because of the exceedance of environmental limits. More expensive forest fuels are used to replace waste-derived fuels, resulting in a higher fuel cost. The present work regards a 50 MWth circulating fluidizedbed (CFB) boiler located in Perstorp, Sweden, co-firing animal waste, peat, waste wood, forest fuels, and industrial sludge. The fuel mix contains high amounts of phosphorus, calcium, silicon, sulfur, chlorine, nitrogen, potassium, and sodium, which makes it complex and problematic. Rapid deposit formation and corrosion rates have resulted in frequent and severe operational problems that reduce the availability of the boiler several hundred hours per year. Extensive work has been performed to study local mechanisms and limited chains of correlation in FB boilers. Attempts to compile broader and more complete models of

1. INTRODUCTION As the energy and raw material prices increase and environmental issues become more and more important, the interest in using cost-effective waste-derived fuels is also increasing. The Europe-wide ban on feeding meat and bone meal1 resulted in large amounts of animal waste that needed to be destructed. Incineration in fluidized-bed (FB) boilers was shown to be a suitable alternative. Animal waste can be fed by the concept of Biomal, where crushed and ground animal waste slurry is pumped into the boiler furnace for combustion. Hence, no energy is spent on upgrading the animal waste to, for example, meat and bone meal before use of its energy content. The introduction of waste-derived fuels is often problematic; frequently, bed agglomeration, increased rates of corrosion, and/or deposit buildup occur, decreasing the availability of boilers and resulting in an increased use of fossil fuel in backup boilers. © 2013 American Chemical Society

Received: March 14, 2013 Revised: July 9, 2013 Published: July 12, 2013 6146

dx.doi.org/10.1021/ef4004522 | Energy Fuels 2013, 27, 6146−6158

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2. EXPERIMENTAL SECTION

understanding is yet limited in number. However, several interesting studies of relevance for the present work have been carried out on a 12 MW semi-full-scale CFB boiler owned by Chalmers University of Technology, Gothenburg, Sweden. One of the studies2 describes the impact of the ash chemistry of wood, coal, and peat on slagging, fouling, and bed agglomeration in the boiler. Although the evaluated fuel mixes lacked significant amounts of phosphorus, the role of phosphorus in combination with calcium was discussed. In 2006−2007, a Swedish Thermal Engineering Research Institute (Värmeforsk) program was reported focusing on the cocombustion of biomass and alkali-related operational problems associated with waste-derived and biomass fuels. The effects of different bed materials and additives when co-firing residue wood, forest fuel, wheat straw, reed canary grass, industrial hemp, and sewage sludge in different combinations were evaluated.3 Recently, two studies with high relevance to the present work were carried out.4 The fate of phosphorus and alkali species was studied, co-firing wood chips and coal with phosphorus-rich rapeseed. The impact of the chemical composition of the fuel on the chemical fractioning between bottom and fly ash (FA), deposit probe, and flue gas was clarified. The study included in situ alkali chloride monitoring (IACM).5 Tests with and without the addition of limestone were performed. The fuel mix in the Perstorp 50 MWth CFB boiler differs in several aspects from the previously published studies: Animal waste dominates as the source of phosphorus and calcium and is also a main contributor of alkali metals (especially sodium), nitrogen, and chlorine, together with residue wood fractions and industrial wastewater treatment sludge. Peat is a main contributor to sulfur, iron, silicon, and aluminum in the fuel mix. The forest fuel makes a considerable contribution to the fuel mix content of silicon, potassium, and aluminum. The present paper is the first of two and renders a thorough mapping and analysis of failure and preventive maintenance statistics, together with elemental analysis of boiler ash and deposits, flue gas, and fuel fractions. The primary areas of interest are defined by the boiler failure and preventive maintenance statistics, maximizing the industrial relevance of this project. An explicit description of the boiler and ash material is made to allow for the use of presented material as future reference material. Correlations between the process parameters and boiler failures are sought, and some recommendations regarding boiler design and operation are made. The present paper will be followed by a second paper (10.1021/ef400542h), where a deeper analysis of the fuel ash system to determine the main chemical species will be formed and the overall ash transformation reactions throughout the combustion process will be carried out. The second paper (10.1021/ef400542h) also addresses availability issues in terms of the chemical interaction between flue gas particles and construction material, fuel feed control strategies, and an analysis and use of the literature review. In all, the work aims at maximizing the operational safety of the Perstorp CFB boiler, minimizing the use of fossil fuel (oil) in backup boilers during operational problems. This will in part be performed by making recommendations regarding boiler design and operation (present paper) and formulating a rough fuel mix evaluation method (in the second paper; 10.1021/ef400542h) based on the findings presented in the two papers.

2.1. Description of the CFB Boiler. The boiler consists of an 18.5 m high furnace with a rectangular base of 3 × 6.5 m (see Figure 1).

Figure 1. Overview of furnace, cyclones, and main heat-transfer areas: primary air nozzles (PANs), secondary air nozzles (SANs), sand seals (SSs), vortex finders (VFs), cyclone exits (CEs), ternary superheater (TSH), secondary superheater (SSH), primary superheater (PSH), upper vaporizer (UV), lower vaporizer (LV), economizers 4, 3, 2, 1, and 0 (EcoX), and lower bend (LB) ©Metso Power. Combustion air is forced through primary air nozzles (PANs) at the bottom of the furnace. At 0.1 m above the PANs, recirculated flue gas is forced through nozzles in the furnace walls. At 2.5 m above the PANs, secondary air is added through secondary air nozzles (SANs) positioned in the boiler walls, and the conditions in the furnace volume above become globally oxidizing. Between the two air stages, fuel, bed material, additives, and recycled bed material are added. In the top of the furnace, flue gas exits through two rectangular ports and enters two parallel cyclones. The mean cross-sectional flue gas velocity varies between 3.8 and 6.5 m/s in the furnace, resulting in a furnace residential time between 2.5 and 4.2 s calculated from the SANs to the furnace exit. The mean velocity of the flue gas is 4.9 m/s. The temperature of the flue gas in the furnace is mainly kept within the interval of 860−920°C, keeping an average temperature of around 890°C. Gasified ammonia is injected into the flue gas in the furnace exit for the reduction of nitrogen oxides (NOx). The flue gas passes through 6147


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scrubber water to keep the pH at a constant level and enable simultaneous absorption of NH3, SO2, and HCl. The flue gas that exits the scrubber is analyzed for nitrogen oxides (NOx), SO2, CO, NH3, H2O, HF, and HCl. 2.2. Failures and Maintenance Tasks. The screening was made by studies of failure reports, historical boiler process data, and interviewing personnel as well as studying photographs of failures in the boiler from a period of 5.5 years. Diagrams were compiled that display the occurrences of major incidents and maintenance activities. Events that forced a shutdown of the boiler were counted as failures, while measures that were preventive and crucial for continued boiler operation were counted as preventive maintenance. 2.3. Fuel. Of the around 20 different fractions of fuel (Table 3), bed material, and additives fed to the furnace of the Perstorp CFB boiler, 13 lacked reliable information regarding chemical compositions. Samples of these 13 fractions were collected (measures were taken to ensure representative samples). For the heterogeneous and large fuel fractions of residue wood and forest residues, a total of 4 L of sample material was collected at two separate occasions. The more homogeneous fractions were sampled at one occasion with a sample volume of 2 L. The samples were sent to an accredited laboratory, where the samples were homogenized in a grinder and analyzed mainly by inductively coupled plasma−atomic emission spectroscopy (ICP− AES) and inductively coupled plasma−mass spectrometry (ICP−MS) according to United States Environmental Protection Agency (U.S. EPA) Methods 200.7 and 200.8, respectively. To make the data more manageable, the fuels were divided into subgroups and main groups. The main groups, called fuel types, are illustrated in Figure 2. The expression fuel types will be used in the

the cyclone exits (CEs) together with particles and enters the top section of the vertical draft containing the main heat-transfer areas. The walls of the furnace and cyclones are cooled, and the inner walls are protected with brick and injection mold compound. The metallic vortex finders (VFs) in the two parallel cyclones are neither cooled nor protected by brick or injection mold compound. The vertical draft has a cross-sectional flow area of approximately 2.1 × 6.4 m2 and contains tertiary superheater (TSH), secondary superheater (SSH), primary superheater (PSH), upper vaporizer (UV), lower vaporizer (LV), and economizers 4, 3, 2, and 1 (Eco4, Eco3, Eco2, and Eco1). A fifth economizer (Eco0) is positioned in the flue gas duct that connects the vertical draft with the particle removal system. All heat exchangers in the vertical draft use the countercurrent principle, except for the TSH, which is of cocurrent type to protect the superheater tube material from overheating and has horizontally oriented tubes. The boiler tube dimensions and construction material are specified in Table 1. The final temperature and pressure of the

Table 1. Construction Material and Tube Dimension of the Main Fireside Components (for Steel-Grade Compositions, See for Example the Work by Lai11) position

construction material

furnace tubes furnace brick cyclone and draft walls cyclone VFs TSH and SSH PSH UV, LV, and economizers

alloy steel 1330 SiC compound alloy steel 1330 alloy steel 253MA alloy steel T22 alloy steel T18 alloy steel 1330

outer diameter/wall thickness (mm) 60.3/5.6 60.3/4.5 1300/14.7 (average) 33.7/5 33.7/4.5 33.7/4

steam exiting the lower part of the TSH is 465°C and 65 bar, respectively. The temperature of the boiler feedwater entering the lower part of Eco0 is 125°C. The TSH, SSH, PSH, UV, and LV each have a centered rotary soot blower to the left of the access hatches (Figure 1), with nozzles positioned about 0.4 m above the top row tubes of each heat exchanger. Soot blowers in the economizers are non-rotating with a multi-nozzle setup directed downstream, only cleaning the heat exchanger below each blower. The draft containing the heat-transfer areas are followed by a particle removal system consisting of a pre-separation stage of multicyclones (MCs) followed by a fabric filter (FF). The 288 cyclones in the batteries have approximately a length of 1 m and a diameter of 0.2 m at their widest point. The partially cleaned flue gas is led to the FF for further cleaning, and the separated coarse FA is collected in hoppers positioned beneath the MCs. The FF has a total area of 3034 m2 and consists of vertically hanging filter hoses 5.8 m long with a diameter of 0.15 m. When a certain pressure drop is reached over the hoses, they are cleaned with pressurized air from the clean gas side of the filter and the formed ash cake is forced to release from the filter surface and falls down in a hopper below. Ash from the hoppers below the MCs and FF is fluidized and transported by pressurized air to a shared FA silo. The particle removal system and flue gas ducts in Perstorp suffer heavily from dew point corrosion. Atmospheric air entering the system through the failing steel walls results in a dilution of the flue gas. Approximately 30% of the gas flow downstream of the particle removal system is made out of air that is forced through the failing walls because of the subatmospheric pressure of the flue gas system. The flue gas filter is followed by a flue gas fan, after which the flue gas is split in two separate flows; 10−15% of the total volume flow (on average) is led back to the bottom of the furnace via a flue gas recirculation fan, while the remaining part of the gas is led into the wet gas scrubber. The latter stream is analyzed for particles, NH3, H2O, volatile organic compounds, and CO. Caustic solution is added to the

Figure 2. Fuel mass (wet and dry) added to the furnace during the 9 month period, divided into fuel types and additives. analysis and discussion of the fuel mix. A more detailed description of the fuel type residue wood is given in Table 3 in the Results. Chemical quantification of 36 elements was made on each fuel fraction. The chemical composition of the overall fuel mix was calculated as an average from the fuel analysis results and the fuel delivery reports, for a period of 9 months (when calculating mean mass flow of the process for this period, an operational time of 6000 h is used). All elements making up more than 0.05 wt % of the total fuel mix, Al, Ca, C, Cl, Fe, H, K, Mg, N, Na, O, P, S, Si, and Zn, were considered in the fuel mix and ash mass balances, whereas Mn, Ti, As, Ba, Be, Cd, Co, Cr, Cu, Hg, Mo, Nb, Ni, Pb, Sc, Sn, Sr, V, W, Y, and Zr were excluded. The water and ash contents and calorimetric value of the fuels were also quantified. In the fuel mix and mass balance calculations, Al, Ca, Cl, Fe, K, Mg, Na, P, S, Si, Ti, and Zn (and when specified also N) are counted as the ash-forming elements of the process. The ash content of each fuel fraction was quantified from the fuel samples by an accredited laboratory, by heating in atmospheric air to a temperature of 550°C (according to standard SS-EN 14775). This ash content quantification together with the fuel mass data was used to calculate a rough estimate of the total amount of ash. 6148


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2.4. Flue Gas. The flue gas composition was measured with Fourier transform infrared spectroscopy (FTIR) at two positions in the boiler, before and after the flue gas filter, to quantify the flue gas composition from the boiler and the sorption of gaseous flue gas species in the FF. The oxygen level of the flue gas was also measured with an electrochemical sensor in the same sample points, and the flue gas composition data were normalized to 11% O2 dry gas (dg). The measurement was made during a total time of 31 h, during which a fuel mass of approximately 600 tons was fired. From the chemical composition of the fuel and the amount of combustion air used during boiler operation, the maximum possible level of the flue gas components was calculated, assuming that all potentially gaseous elements are in their most dominating gaseous form. The calculated maximum levels are compared to the actual gas concentrations to assess the distribution between gas and solidified matter. 2.5. Ashes, Deposits, and Temperatures. With the failure screening of the boiler as background, photo documentation was made and samples were collected during maintenance stops. The following positions were sampled and documented: bottom ash, PANs, CEs, TSH, SSH, PSH, UV, LV, Eco4, LB (Figure 1), MCs, FF, FA, and particle analyzer. Deposit samples were collected from the windward side of the horizontally positioned top row tubes of each heat exchanger. The bottom ash sample was taken from the conveyor belt, transporting the hot bottom ash from the ash fractioner to the container, before the shutdown sequence was initiated to ensure a sample not influenced by the shutdown. FF and MC samples were collected from the hoppers of each unit, and FA was collected from the silo used for storage of MC and FF ash. All samples, except FF, were ground in an agate mortar. FF was not ground because it mainly consists of fine particles. The coarse ash particle samples bottom ash, SSs, MCs, and FA were also further ground in a vibration mill with alumina (Al2O3) jars. All samples were mounted at carbon tapes, and the mean chemical composition of the samples was analyzed with a scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) system [Philips XL30 ESEM with an energy-dispersive analysis of X-rays (EDAX) detector]. Three subareas of 1.5 mm2 were selected for quantification of the chemical composition of each prepared sample. The mean value and sample standard deviation were calculated for each sample. When the samples were ground, a qualitative evaluation was made of the relative hardness of each sample. The size distribution of the bottom ash was determined by sieving. The MC and FF ash size distributions were determined with a Master Seizer 2000 laser diffractometer at the YKI (Institute of Surface Chemistry). The flue gas temperature was measured in the top and bed of the furnace, between PSH and UV, between Eco1 and Eco0, between Eco0 and MC, and at the particle analyzer. The temperatures of the flue gas in other positions are calculated from the temperature gradient known from the temperature measurements of the flue gas and the water and steam temperatures entering and exiting the heat exchangers. Because of the general lack of temperature measurement of boiler construction material, the fireside surface temperature of the boiler construction material is assessed from gas and water temperatures and aspects of heat transfer. 2.6. Mass Balance. Bottom ash and FA were weighted separately as transported by trailers from the site. The weight data together with composition data from SEM/EDAX were used to calculate the mass of each element in the two major ash fractions. The mass of accumulated ash and deposits was assessed from the amount removed with a vacuum truck used to clean the boiler during maintenance shutdowns. Online volume flow measurements and pressure data combined with emission data recorded with FTIR are used to calculate the mass of elements occurring in the gas phase. The elemental mass contribution of the fuel mix is described in section 2.3. The quantification of each stream elemental contribution was then used to create an elemental mass balance of the process.

3. RESULTS 3.1. Failures and Maintenance Tasks. The screening of the CFB failures and maintenance tasks is summarized in Figure 3. A short description of the most frequently occurring events is given in Table 2.

Figure 3. Events of importance during a period of 5.5 years.

On the basis of the failure screening (Figure 3), the TSH, SSH, PSH, UV, LV, and Eco4 are identified as the primary areas of interest, because of the large and negative impact failures that these components have on the boiler availability. Isolated incidents and incidents that can be explained as normal wear will not be further discussed. Thus, the furnace, Eco3, Eco2, Eco1, Eco0, and scrubber will not be subjects for indepth studies. Rapidly failing VFs in the main cyclones is a reoccurring problem, but because no sampling was possible within the frame of this work, they are principally excluded from further analysis. 3.2. Fuel. The fuel fractions and additives shown in Table 3 are summarized to the wet mass of the seven fuel types considered in this work. The amount of dewatered wastewater treatment sludge added to the CFB furnace (Table 3) is uncertain because the excess water is removed after the sludge flow meter. This uncertainty has a 1−3 vol % effect on the calculated moisture content of the flue gas and an effect of the elemental contribution of the sludge. The bark pellet is mainly used when problems arise in the biofuel conveyor system or when problems arise with a low temperature in the furnace because of low-quality fuel or rapid variations in boiler load. The animal waste, sludge, residue wood, limestone, ammonia, and adhesive residue along with bed sand are fed continuously to the furnace and can be controlled separately (Figure 4). Peat and forest fuel, on the other hand, share the same stoker, which results in a heterogeneous/intermittent feed from these stokers where the composition can vary from a quite homogeneous mixture of the two types of fuel to only peat or only forest fuel. The Biomal makes up for 13% of the energy content (lower heating value), while it contributes to as much as 36 wt % of the ash-forming elements. The ash-forming elements selected for further analysis make up 3952 tons (N excluded), of the total of 3975 tons identified, added during the 6000 h of operation. The 23 tons of ash-forming elements not represented in the selected data set consists of approximately of 18 tons of barium, 2 tons 6149


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Table 2. Representative Failures and Preventive Maintenance Tasks Performed position

failures

preventive maintenance tasks/improvements

furnace cyclones TSH SSH

failing masonry and ash fractioner failure of the 253MA VFs extreme deposit buildup tube failures primarily near the soot blower and in the top tube rows

PSH UV LV Eco4 Eco0 particle removal system scrubber

tube failures near the soot blowers damages and failures of tubes near tube supports and soot blowers damages and failures of tubes near tube supports and soot blowers erosion damages on the tube bends near the draft wall failure of the soot blower heavy corrosion damage forces chamber shutdown and repairs (can mainly be performed during boiler operation) collapse of the metal filler material

improvements of masonry and injection mold compound frequent replacements of VFs manual cleaning of deposits frequent replacement of damaged tubes and installation of protection shields replacement of weakened tubes welding of cavities and installation of protection shields welding of crevices and installation of protection shields massive and frequent welding repairs none repair of corrosion damages, improvements of the insulation, and replacement of seals and bellows cleaning of the scrubber water distribution system and installation of a quench

Table 3. Fuels Fractions and Additives Added to the Furnace in the Perstorp Boiler in Tons (Wet Mass) during a Period of Approximately 6000 h of Operation residue wood

sludge

residue wood

4039

residue wood fuel mix 1a residue wood fuel mix 2a

wet mass

15037

17094

peat 1

5195

16349

peat 1

23156

12163

animal waste

forest fuel

peat 1

37481

wastewater treatment sludge

peat wood chips

25679

forest residues (mainly bark of spruce) bark pellets

43447

additives

industrial waste

bed sand

248 waste-activated carbon

121

30191

limestone

6

960

ammonia

171 ammonia from wastewater 98 adhesive residues plastic fiber residues 517

56829

Biomal

29792

1038 174 1339

a

Typically consists of 54 wt % wood, 8 wt % plastic, 16 wt % paper, 4 wt % textile, 16 wt % not defined combustibles, 1 wt % inert inorganic, and 1 wt % metal.

Figure 4. Relative contribution of fuel to the furnace shown as the contribution to the total wet and dry mass, energy content (lower heating value), and ash content.

of copper, 2 tons of chromium, 1 ton of arsenic, and a number of trace elements. The excluded elements make up less than 0.6 wt % of the fuel ash-forming elements and are not further discussed. In general, the different fuel types differ largely with respect to the composition of ash-forming elements (Figure 5). Typical for the Biomal is high amounts of Ca, P, K, N, and Na, and sludge has a high concentration of Cl, N, Na, and S. The composition of the residue wood fractions varies considerably between each fraction, especially regarding Ca, Cl, Na, and Si. The residue wood contains primarily high amounts of Al, N, and Si. Bark has high Ca, K, Mg, and Si contents, with a noticeably low amount of N. Peat primarily contains high amounts of Al, Fe, N, S, and Si.

Figure 5. Molar contribution to the ash-forming elements from the different fuel types and additives.

The element contributing to ash in the fuel mix is dominated by metals (55 mol %), with 53 mol % Ca, 13 mol % Na, 13 mol % Al, 7 mol % Fe, 6 mol % K, 6 mol % Mg, 2 mol % Ti, and 500 μm indicates that the fractioner also works in terms of removing coarse particles from the bed. The large fraction of finer particles in the interval of 100−300 μm is interpreted as a statistical phenomenon; because of the design, the bottom ash fractioners are progressively more efficient in terms of recycling efficiency (to the furnace) as the particles becomes smaller. Thus, the most likely explanation of the high concentration of “fine” particles in the bottom ash is that the concentration of particles 6153


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accumulate to a larger extent, while the chunks also limit the effect of soot blowing. After 3−4 months of operation at a high boiler load, the accumulation of ash and chunks of deposit has blocked the flue gas passage through the heat exchanger to such an extent that gas and particles are forced to reroute to the narrow space between draft walls and heat exchanger. This results in high velocity flow of flue gas containing solid particles that erodes the tubes near the walls, leading to severe damage. Periodical shutdowns to enable cleaning of the boiler is essential to prevent heavy erosion and tube failures. A more effective cleaning of the TSH and SSH would release deposits in smaller chunks, leading to fewer problems in the economizers downstream. The results from the failure screening makes it apparent that vertical drafts with superheaters (TSH and SSH) and, thus, the main areas of deposit buildup, positioned in the top of the draft above heat exchangers with more narrowly spaced tubes (economizers), are likely to be problematic when fuels primarily rich in K, Na, S, and Cl will be used. 3.6.2. Fuel. The fuel mix consists of fuel types with large differences in chemical composition (Figure 5). Thus, a change of the feed ratio between these types may affect the overall chemical composition of the process significantly. When the fuel mix mean composition is compared to the composition of specific major fuel types, one can see how each fuel type chemically influences the fuel mix. However, a large portion of the fuel ash is more or less inert; caution is therefore of uttermost importance during such assessments. According to Piotrowska et al.,4b the deposit buildup varies linearly with the amount of alkali chlorides in the flue gas when the gas enters the superheaters. If this applies also to the Perstorp CFB boiler, it should mean that residue wood, animal waste, and sludge are the most likely fuel types contributing to deposit buildup (Figure 5). The European waste incineration directive (2000/76/EC) dictates that the combustion temperature has to be kept above 850°C during at least 2 s when waste-derived fuels are used. In practice, this means that the temperature in the furnace must be kept significantly higher as a margin for fluctuations in boiler load and fuel quality. In the case of the Perstorp furnace, the temperature is mainly kept within the interval of 860−920°C, keeping an average temperature of around 890°C. A higher temperature increases the risk of deposit growth and boiler material corrosion. The solid fuel fractions are delivered by trailers and dumped into stokers, mainly without mixing. Peat and forest fuel share the same stoker, which results in a varying and intermittent feed of each fuel type, creating strong variations in fuel, flue gas, and particle composition in the boiler. The intermittent feed of peat for example results in a strong variation of the sulfur content in the flue gas, and the fluctuating feed of forest residues results in variation in the amount of potassium. The estimation of the total amount of ash based on the fuelashing procedure at 550°C and fuel mass data results in an overestimation of around 25 mass % compared to the actual weight data. The deviation reveals difficulties to obtain correct ash content quantifications relevant for real combustion environments with standardized ashing procedures. Ca, Mg, and Mn, for example, are elements occurring mainly as oxalates and carbonates in biomass fuel.9 Ashing at 550°C is not enough to decompose all carbonates but enough to initiate the formation of new relatively stable carbonates from ash-forming elements (for example, K and Na) not originally occurring as carbonates in the fuel. This can result in a sizable over-

In the lower part of the TSH, upper and lower part of SSH, PSH, and upper part of UV and LV, the material is negatively affected and damages near the soot blowers have led to numerous tube failures. There is a strong correlation between the deposit buildup rate and flue gas temperature (Figure 9) and between the corrosion rate and material temperature of the boiler tubes. To avoid exaggerated tube cleaning, resulting in premature tube failure, an individual tuning of each soot blower is needed. In positions where there are both an insufficient cleaning effect and tube damages, a configuration with multiple soot blowers with a lower cleaning pressure or alternative cleaning methods should be considered. Another difficulty with rotary soot blowing is that the deposit growth and deposit hardness are much higher on the windward and upper side of the tubes. Thus, a correctly adjusted cleaning effect for the heat exchanger downstream means tube damages in the lower part of the heat exchanger upstream. This could be solved by placing the soot blower closer to the heat exchanger downstream. An incorrect tuning of the soot blowers in combination with a poor heat exchanger design can lead to tube failures even at moderate flue gas and material temperatures, as seen near the tube supports in UV and LV. The soot blower angle of attack should also be considered. The rapid deposit growth on the windward side of the TSH and SSH tubes creates deposits pointing toward the flue gas and soot blowing steam. The poor angle of attack between soot blowing steam and deposit results in only a small torque contributing to the release of the tube deposits. If the angle of attack of the soot blowing instead would be normal to the surface of the pointy deposit, the torque would increase and a more effective cleaning of the tubes would be accomplished, without any net increase of the cleaning force. Thus, tube wear could be reduced and boiler efficiency could be increased. Investigations performed earlier6 also pointed out that the direction of soot blowing steam relative the deposit among other parameters was of great importance for deposit release. Zbogar et al.7 later made a useful summary of the mechanisms governing deposit release in general. The concentration of Cl peaks in the superheater deposits (Figure 10), but only the lowermost three tube rows of a total of 16 in the TSH and the uppermost four tube rows of 32 in the SSH suffer from high corrosion rates. Salmenoja8 concluded that tube material temperatures of 460°C or above could lead to significant loss of material in superheater applications with chlorides present in the flue gas. He also concluded that the lower threshold temperature for pure Fe to volatilize in chlorine-rich environments is around 450°C, meaning that, below this temperature, no severe material loss via volatilization can occur for Fe-based alloys. These threshold temperatures seem to apply fairly well in the Perstorp CFB boiler, where low alloy tubes (T22) are used in the TSH and SSH. Further, this means that only part of the superheaters needs to be built from corrosion-resistant and expensive material; thus, the cost for corrosion-resistant heat exchangers in waste-derived fuel boilers can be reduced. The rapid deposit buildup coincides with the high corrosion rates in the SSH and virtually ceases downstream of the fourth tube row in the SSH. When chunks of deposit created primarily in the TSH and to some extent in the SSH fall down through the widely spaced superheaters and vaporizers, they stick in the narrowly spaced economizers (Figure 8). The chunks of deposit create stagnation points, allowing for relatively fine ash particles to 6154


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3.6.4. Ashes, Deposits, and Temperature. Approximately 60 tons of ash and additives was accumulated in the boiler during the 9 months of operation, of which around 40 tons is accumulated between CEs and MCs. A rough estimation, where this 40 ton accumulation is compared to the mass of the main contributors to the deposit buildup (Na, K, S, Ca, Si, and P) passing through this section of the boiler (Figure 12), gives at hand that a maximum of 6 mass % of K, Na, and S accumulates on surfaces or in the LB and is removed during maintenance stops. In other words, the flue gas average elemental composition can be regarded as close to constant between the positions CEs and MCs, and thus, significant differences in deposit buildup of different flue gas cross-sections cannot be a result of a varying flue gas elemental composition. A near constant flue gas composition through the draft, together with the fact that the rapid buildup in TSH also continues when the deposits become thicker and the surface temperature of the deposit increases, indicates that the high flue gas temperature is the main factor causing the more rapid deposit buildup in the TSH and not the temperature of the tubes. However, for the SSH, where samples were taken from both non-protected tubes and tube shields (which have a noticeably higher surface temperature), it was indicated that the temperature of the tube/shield affects the composition, hardness, and strength of the deposit (Table 5) and, therefore, probably also the deposit resistance to soot blowing. The flue gas velocity increases from a maximum of 6.5 m/s in the furnace to over 30 m/s when the flue gas leaves the cyclones through the cyclone VFs. This initially high velocity gas and particle flow, in combination with the 180° bend in the CEs and draft when exiting the VFs, forces the condensed particles toward the roof and back wall of the draft. This effect is clearly shown in Figure 7, where the deposit is much thicker against the back wall of the TSH. The effect could be used as an advantage if the soot blower with its relatively local effect is placed over the main area of deposit buildup and can soot with high pressure and/or frequency without damaging the SH, where lesser cleaning is needed. Between the particle removal system and the wet gas scrubber, dust emission is measured. The flue gas normally contains 2−5 mg/Nm3 particles at 11% O2 dg, although problematic particle levels as high as 15−25 mg/Nm3 at 11% O2 dg have been observed. These levels exceed the allowed levels for co-combustion, forcing a reduced use of cheap wastederived fuels, resulting in higher fuel costs. Samples of the flue gas particles downstream of the particle removal system have been analyzed. The result shows that the dominating part of these particles has approximately the same composition as the white crystals sublimed on the relatively cool surfaces on the inside of the flue gas ducts, mainly consisting of Cl and N. The heavy corrosion damages on the flue gas duct and particle removal system force the surrounding air to mix with the flue gas, cooling it enough for the white crystals to sublime, creating problems with the dust measurement and the emission directives. 3.6.4.1. Chemical Composition. The tube deposits are dominated by Na, S, Ca, K, and P (Figure 10). Cl is also present in the deposits but in relatively small amounts, with the highest concentration in the SSH deposit. The deposit buildup is most rapid in the TSH (Figure 9), where the Cl content is considerably lower. The concentration of Na, K, and S on the other hand is higher in TSH than in SSH.

estimation of the fuel ash content at the ashing temperature of 550°C. Obernberger et al. investigated the ash content of densified wood and bark at 550°C according to SS 187171 and at 815°C according to DIN 51719. The conclusion was that the amount of 550°C ash was between 15 and 32 mass % higher than the 815°C ash and that the difference was a combined result from the exaggerated carbonate contents in the 550°C ash and the vaporization of alkali metals, chlorides, and sulfur in the 815°C ash.10 When ashing the separate fuel fractions, the lack of possible reactants for potentially volatile species containing P, N, Cl, and S can result in a significant loss of elements. In a real fuel mix, these potentially volatile elements can react to form solids. Thus, co-combustion can lead to a different amount of ash and ash composition compared to when each fuel fraction in a fuel mix is ashed or combusted separately. Table 4 shows that the moisture content of the sampled fuel is lower than the actual fuel moisture content. This might be due to the fact that the fuel samples were collected too close to the surface in the open fuel stokers, where the fuel is exposed to wind and sunlight. It might also be an effect of the fact that dryer and lighter fuel particles tend to end up closer to the fuel pile surface. However, the mass balance regarding the ashforming elements (Figure 12) sums up well, confirming an adequate fuel sampling. 3.6.3. Flue Gas. Because of the heterogeneous fuel mix, online measurements are required to control the combustion environment and secure the quality of the fuel over time. A manually controlled addition of limestone to the furnace has frequently resulted in low SO2 levels and rapid material loss in the SH. When the addition of limestone was reduced from a constant feed of approximately 70 to 40 kg/h, the corrosion rates in the SH were heavily reduced. However, the reduction of limestone also led to frequently exaggerated levels of SO2 in the flue gases leaving the furnace, which likely has worsened the problem with dew-point corrosion in flue gas ducts and increased the expensive use of NaOH in the wet gas scrubber. The filter ash effectively sorbs HCl and NH3 in the flue gas (Figure 6). The flue gas measurement must therefore be positioned upstream of the FF to enable accurate control of the furnace chemistry, even if no sorbent is added. The reduction in the FF ash is of great importance when designing flue gas cleaning equipment. An increased filtration area lowers the filtration velocity and reduces the pressure drop exponentially, thus allowing for more filter ash on the filtration surface. This result in a more effective use of ash and/or sorbent. A recent implementation of a modern FF in Perstorp shows that an increase in the filter area from 3200 to 4070 m2 increased the cost of the project to only 1.5%. When the size of the baghouse filter is increased, the wall surfaces become more susceptible to dew point corrosion, wherefore filter designs with wellcontrolled surface temperatures become increasingly important. The ratio (measured HCl in the gas phase before the flue gas filter)/(calculated maximum HCl from fuel), values shown in Table 4, might be used to make qualitative assessment of the fuel mix and combustion environment. A ratio near 1 indicates that nearly all Cl added with the fuel is prohibited from forming deposits and ash, making accelerated deposit buildup and corrosion in the superheaters less likely. Whereas a ratio of ≪1 indicates that Cl occurs to a large extent in ash, making superheater deposit buildup and high-temperature corrosion feasible. 6155


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leeward soot blowing. As a result, the material loss is heavily reduced. When the flue gas (and particles) reaches the top row tubes of the PSH, it has a temperature of approximately 570°C and the tube material has a temperature of approximately 430°C. At such low temperatures, the deposit is too small to have any severe impact on the boiler operation. 3.6.4.2. Particle Size. A considerable amount of particles as large as 100−200 μm originating from bone, limestone, and sand is present downstream of the main cyclones and can be found in the FA, but virtually none of these particles has been found in the flue gas deposits during the SEM analysis. This shows that these particles do not contribute to the deposit buildup. On the contrary, they might have an erosive effect on the buildup caused by smaller particles. The MC and FF size distributions in Table 6 likely differ from the size distribution of ash from continuous operation, as a result of a decreased amount of waste-derived fuels in the fuel mix before boiler shutdown, affecting the samples taken in the units. The “composite” FA sample collected from the FA silo is more adherent to a representative fuel mix and shows that the MC and FF samples have lower alkali and chloride contents compared to continuous operation FA. The same effect that can be seen on the ash samples collected in FF and MC during maintenance stop compared to the FA sample might also apply to thin tube deposits (Figure 9). 3.6.5. Mass Balance. The accumulation of ash and deposits in the boiler is equivalent to approximately 1 wt % of the total amount of ash-forming elements passing through the process. This means that the deviation between the mass of elements entering and leaving the process (Figure 12) cannot be explained fully by the accumulation. One probable explanation of the deviations is that the fuel, flue gas, and/or ash compositions of the sampled matter are not fully representative for the full period of 9 months. Thus, an error is created when the composition data are extrapolated with the mass of fuel, flue gas, and ash for the whole period. With regard to the gaseous species, part of the error can also be a result of deviations of the industrial-scale flow meters. Of the 1700 tons of N entering the process with the fuel (fuel nitrogen), only 70 tons is identified in the ash or flue gas particles. This means that 95 wt % of the fuel nitrogen can be assumed to exit the process as elemental N2 with the flue gas. Of the amount of N identified in the FA (Figure 12), approximately 6 tons can be explained by the NH3(g) sorbed in the flue gas filter ash (Figure 6).

The FA fraction contains the majority of the elements contributing to the deposit buildup (Figure 12). An enrichment factor can therefore be calculated, as the ratio of the elemental molar concentration of each deposit divided by the molar elemental composition of the FA. All compounds occurring in the deposits in high amounts are not necessarily “sticky” themselves but might have been caught in other sticky materials or have been deposited as a result of direct impaction. Elements present in the deposits can also be a result of secondary reactions between deposits, flue gas, and/or tube material. The high enrichment of K, Na, and S in the deposits (typically 4, 3, and 4 times compared to the FA concentration, respectively) largely hides the variation of the other elements in Figure 10. A normalized data set excluding these elements was therefore generated. N was also excluded from this data set because of the large uncertainty in the analysis (Figure 10). The normalized data highlight the variations and enrichment of the other elements regarded in the system analysis (Figure 13).

Figure 13. Enrichment relative to the normalized FA composition, excluding Na, K, S, O, and N.

The occurrence of these are generally quite constant compared to the levels shown in Figure 10, which implies that the occurrence of K, Na, and S governs the overall composition of the boiler tube deposits. The relative enrichment calculated from the normalized data set (Figure 13), together with the deposit thickness in Figure 9, shows that the iron concentration increases in the deposits with a decreasing deposit thickness. This is probably an effect of the iron oxide scaling together with thin deposits. From Figures 9 and 10, the amount of each specific element in the top row deposits of different heat exchangers could be assessed. For example, the Cl concentration of the SSH deposit is twice as high (1.1 mol %) compared to the Cl concentration in the TSH deposit (0.5 mol %). On the other hand, the accumulation of Cl in the TSH deposit is yet twice as high as the accumulation in the SSH because of the significantly larger deposit buildup. The failure frequency is especially high in the SSH, where rapid chloride-rich windward deposit buildup is combined with high tube material temperature and windward soot blowing. In areas where one of these factors is removed, a more moderate material loss can be seen. For example, the lower tube rows in the TSH have practically the same material temperature and flue gas temperature and windward deposit buildup as the SSH, but the lower tubes of the TSH have

4. CONCLUSIONS AND RECOMMENDATIONS The failure screening, chemical composition, deposit growth, and construction material temperature of the boiler show that the failure frequency is especially high in the SSH, where (1) rapid chloride-rich windward deposit buildup is combined with (2) high construction material temperature and (3) windward soot blowing. In areas where one of these factors is removed, a more moderate material loss can be seen. The failure screening also shows that the majority of the tube failures are associated with soot blowing, also where the material temperature and/or deposit buildup rate are not critical. To avoid excessive tube wear, the tube cleaning must be designed and tuned individually to adapt to the conditions in each position. Further, rotating soot blowers should be placed closer to the downstream heat exchanger than to the heat exchanger upstream, to limit the impact on the leeward side tubes and 6156


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enrichment of K, Na, and S in the heat exchanger tube deposits can therefore be considered as the governing factor of the overall composition of the tube deposits. The temperature of the flue gas is the major parameter controlling the rate of deposit buildup in the boiler heat exchangers. Also, the tube temperature affects the deposit hardness and strength and, thus, likely its resistance to soot blowing. SEM analysis shows that no pieces of ground bone, sand particles, or other relatively large flue gas particles contribute directly to the deposit buildup. White crystals rich in N and Cl, most likely ammonium chloride, precipitate downstream of the FF because of thermal bridges to the surroundings and leakage of atmospheric air into the flue gas. The precipitation results in an increased level of corrosion in the flue gas ducts and clean gas chambers. The precipitation of these particles also interferes with the dust emission measurement and forces a reduced usage of wastederived fuels because of the exceedance of environmental limits, resulting in a higher fuel cost.

avoid tube wear, where almost no deposits are protecting the tube. In the superheaters suffering from high corrosion rates, only limited sections are affected. Thus, only sections of the superheaters need to be built in more corrosion-resistant and expensive material. The areas suffering from rapid material loss are also subject to high deposit rates. In the SSH, neither material loss nor substantial deposit buildup can be seen downstream of a well-defined flue gas temperature limit. The flue gas average elemental composition can be regarded as a constant as it flows through the series of heat exchangers (between the positions CEs and MCs). Thus, the significant differences in deposit buildup of different flue gas cross-sections cannot be a result of a changing flue gas elemental composition. Of the 1700 tons of fuel nitrogen and NH3 additive, 70 tons is identified as part of the ash or gaseous flue gas particles other than N2. Thus, 95 wt % exits the process as N2(g). Fuel mix ash content analysis via a separate ashing of different fuel fractions by heating to 550°C may not reflect the ash content of the fuel mix correctly. At 550°C, carbonates originating from the biomass do not decompose completely and new carbonates can be formed. Also, a possible lack of reactants in each fuel fraction when ashed separately can lead to volatilization of major ash-forming elements compared to the “real” process where different fuel fractions are mixed. The soot blowing angle of attack should be regarded in areas of rapid deposit growth, especially in the ternary and secondary superheaters, where the size and height of the deposit itself can be used as leverage to maximize the internal stress in the deposit−tube interface for effective release of deposits. The frequent tube failures in Eco4 were partly an effect of a rapid deposit buildup in TSH and SSH because of lumps of deposit that fall down and block the path between the soot blower and heat exchanger and limit the blowers cleaning effect. The lumps also effect the flow distribution of the flue gas and accelerate the accumulation of ash in the heat exchanger. Boilers built with vertical drafts and the main areas of deposit buildup (superheaters) positioned above heat exchangers with more narrowly spaced tubes are a problematic design when fuels rich with K, Na, S, and Cl are to be used. Analysis of flue gas and deposits shows that the intermittent feed of fuel types and use of heterogeneous fuels create strong variations in fuel, flue gas, and particle composition in the boiler over time. This represents a problem when attempting to optimize the operation of the boiler and makes it increasingly important to have online measurements to control the fuel quality and flue gas composition to reduce high-temperature corrosion and severe deposit buildup in the superheaters. The filter ash in the flue gas filter effectively sorbs HCl(g) and NH3(g) from the flue gas already without the addition of sorbents. Therefore, the flue gas composition must be measured upstream of the FF to make it possible to accurately control the furnace chemistry. A significantly larger filtration area can be installed with a slight increase in cost, to allow for a considerable thicker ash cake on the filter textile and a more efficient use of the ash as a free-of-cost sorbent. The ratio [measured HCl(g) upstream of the flue gas filter]/ [calculated maximum HCl(g) from fuel] can be used to make qualitative assessment of a fuel mix. A ratio near 1 indicates that nearly all Cl added to the fuel is kept from deposits and ash, thus making problem-free operation more likely. Tube deposits are dominated by Na, S, Ca, K, and P, but only Na, K, and S are enriched in the windward tube deposits relative to the FA bulk composition. The strong presence and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The financial support given by Perstorp Speciality Chemicals AB and the National (Swedish) Strategic Research Program Bio4Energy is gratefully acknowledged. The cooperation and support given by friends and colleagues at Thermochemical Energy Conversion Laboratory, Umeå University, and Perstorp Speciality Chemicals AB are deeply appreciated.

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