Economical optimization of steam data for recovery boilers

Transcription

1 Economical optimization of steam data for recovery boilers Johan Jansson Master s thesis, 30 hp Master of science in energy engineering, 300 hp Spring term 2017 EN1715

2 Johan Jansson Abstract Pulp and paper mills are high power consuming industries. Pulp and integrated mills produce power via steam turbines in recovery boilers. Due to high power prices and the fact that biomass combusted in the recovery boiler is considered as green energy, there is today a desire to always increase the power generation when investing in new recovery boilers. In order to increase power output from the steam turbine the steam data (i.e temperature and pressure) needs to be increased. With higher steam temperature follows a higher risk of corrosion due to the non process element potassium in the boiler fuel. The uncertainties of high temperature corrosion and the unpredictable environment in the furnace makes it difficult to design recovery boilers. This results in higher investment cost and could lead to less profit for the mill buying the boiler. The question then stands whether the revenue obtained from the higher power generation, is higher than the investment made for the upgrade in order to produce the higher steam data over a certain time. And more specifically what steam data will be the most economical, when comparing revenue from power generation with investment cost? In this study, together with ÅF Industry AB, four boilers with different steam data (Boiler A: 38.5 bar, 450 C; Boiler B: 92 bar, 480 C; Boiler C: 106 bar, 500 C; Boiler D: 115 bar, 515 C) were compared. The boilers were compared for four potassium levels: 1.0wt%, 1.5wt%, 2.5wt%, 3.5wt%. And two values of power were used, 300 SEK/MWh and 700 SEK/MWh. The marginal differences between the boilers were: the amount of material used in the superheaters in order to produce different steam data; the type of material used in the superheaters and the furnace; whether an ash-treatment system was needed (in order to purge potassium from the process); the turbines and generators; whether a feed water demineralization equipment was needed; the yearly cost for make-up chemicals (due to usage of an ash-treatment system) and the amount of power generated. The boilers investment cost and net yearly revenue were compared in order to determine the marginal payoff in years. The most economical choice of boiler for the different potassium levels for 300 SEK/MWh: 1.0wt%, Boiler D; 1.5wt%, Boiler C; 2.5wt%, Boiler B; 3.5wt%, Boiler D (A). And for 700 SEK/MWh: 1.0wt%, Boiler D; 1.5wt%, Boiler C; 2.5wt%, Boiler D (B); 3.5wt%, Boiler D. The conclusion in this thesis was that the deciding factor is whether the boiler is in need of an ash-treatment system. Higher steam data is preferable as long as ash-treatment can be avoided. However, when comparing two boilers with ash-treatment the one with higher steam data is more feasible. Low steam data, such as boiler A, will never be feasible, regardless of potassium level and value of power. i

3 Johan Jansson Preface This thesis covers 30 credits and has been carried out in cooperation with ÅF Industry AB during the spring term of The work is the final part of the Master of science program in energy engineering at Umeå University and the Department of applied physics and electronics. First I would like to thank my supervisors at ÅF Kajsa Fougner, Sten Lundborg and Kristian Rosenqvist for the all the guidance and help around my work. I would also like to thank my supervisor at Umeå University Markus Broström for all the support. Further, I wish to thank the suppliers and pulp and paper mills who have helped me make this thesis possible. Finally I would like to thank everyone at ÅF who made my time at ÅF great. ii

4 Contents 1 Introduction Background Ash Materials Heat absorption Simulation tools Problem Objective Methods Simulation tools Scenario/Case Simulation Evaluation of thermocouple data Boiler design Summary of boilers Power generation Result and discussion 31 4 Conclusion 38 5 Future work 39 References 40 Appendices A Superheater dimensions for Boiler A and B B Superheater dimensions for Boiler C and D C Material distribution along superheater tubes i i ii iii

5 1 Introduction Johan Jansson 1 Introduction 1.1 Background According to Energimyndigheten [1] the Swedish industry accounted for 38% of the country s total energy consumption in Of these 38%, more than half is consumed by the pulp and paper industry. Despite the pulp and paper process high level of energy consumption, it is important to mention the power generation in kraft pulp mills and integrated mills. During the recycling process of the white liquor in the mills recovery boiler, high pressure steam is produced and via a back pressure turbine power is generated. Due to high power prices and the fact that biomass combusted in the recovery boiler is considered as green energy, there is today a desire to always increase the power generation when investing in new recovery boilers. In order to have a high power generation, the steam pressure to the back pressure turbine needs to be high. In order to produce high steam pressure the steam temperature needs to be high. With increased steam temperature follows less cooling from the tubes in the convective heat transfer section. With higher temperature follows corrosive characteristics from alkali salts in the inorganic residues and condensed dust formed during combustion of black liquor. This can cause serious material damage on boiler components in high temperature regions when critical temperatures are reached. The high temperature regions of the boiler refers mainly to the superheaters due to the high temperatures of the steam inside the superheater tubes. The hot steam provides less material cooling and thus the critical temperatures, where corrosive characteristics begin, can be reached on the outside of the superheater tubes where the alkali salts exists. There are ways to reduce the risk of corrosion. A common alternative is by choosing corrosion resistant materials for the high temperature regions. Thus the high temperatures needed can be reached in order to generate more power. If it is not enough, a more modern approach is to remove the NPE s from the process with an ash-treatment system. A system can treat the ash with different techniques in order to separate NPE s from process elements. Thus in order to increase steam temperature, measures needs to be taken to prevent damaging boiler components. These measures comes at a high price and will increase the investment cost. Aside from the measures to control the corrosion risk in the boiler there are more changes that needs to be implemented which increases the cost of a new boiler with increased power generation. Measures such as higher capacity back pressure turbine, material thickness of all water and steam 1

6 1 Introduction Johan Jansson carrying components, etc. With higher power generation a higher revenue will be obtained, or saved money when less purchased power is needed. The question then stands whether the revenue obtained from the higher power generation, is higher than the investment made for the upgrade in order to produce the higher steam data over a certain time. And more specifically what steam data (i.e temperature and pressure) will be the most economical, when comparing revenue from power generation with investment cost? In the kraft pulping process wood chips are cooked with an aqueous solution containing NaOH and Na 2 S as active cooking chemicals, the solution is commonly called white liquor. The purpose of white liquor is to separate the wood fibers from the the organic constituents such as hemicellulose, cellulose and lignin. After cooking, the expended cooking chemicals and organic residues, called black liquor, are washed away from the kraft pulp. The black liquor is then evaporated in order to increase its dry solids before it continues to the recovery boiler [2]. The recovery boiler s main function is to serve as a chemical reactor where the expended cooking chemicals in the black liquor are to be recovered in its correct chemical composition. The purpose is to remove the organic compounds, reduce all sulfur as Na 2 S and forward it with the retrieved Na 2 CO 3 (which will be causticized later in the process to NaOH). In the process of recovering the chemicals it also functions as a steam boiler producing pressurized steam from the combustion of the organic residues in the black liquor [3][4][5]. 1.2 Ash In the black liquor there are process elements such as sodium (Na) and sulfur (S) due to the active cooking chemicals being NaOH and Na 2 S. The sodium content is relatively high and usually is around 20%. The sulfur content varies a lot depending on geographic location, but is in the region of 3-6% [6]. With raw material follows substances called non-process elements (NPE) such as potassium (K), chlorine (Cl). Cl can also enter the process via make-up chemicals and possibly in pulp bleaching filtrates. As with the sulfur the amount of Cl entering the process varies with location; however, the amount entering the process varies considerably if the raw material has been seaborn or not. The chlorine content can vary between 0.1 to 5%, while the potassium content is typically 1 to 3%. Another big factor influencing the concentrations of non-process elements is the degree of closure of the mill s chemical recovery cycle [7]. Thus the composition of black liquor varies a lot between mills due to disparity "in/between" raw material(s) and chemical 2

7 1 Introduction Johan Jansson used in the process [4][6]. This can be observed in Hupa s et al. s [8] comparison between liquors in Tab.(1). Table 1 Composition of studied liquors, given as weight % in the dry solids content [8]. Compound (wt%) Mill C H Na K S Cl N O* A B C D E F G H *oxygen by balance The four elements Na, S, K and Cl are all ash-forming elements. Together they typically make up 30% of the black liquor [4]. The majority of the ash-forming elements fall down to the furnace bed. However, some of the ash-forming elements follow the flue gas as carryovers (inorganic residues) or as fume (dust which later condenses i.e condensed dust). Carryovers are large particles of black liquor from the black liquor gun s spray or char from the char bed at the bottom of the furnace, there is also carbon present here. Fume is when the ash-forming elements evaporates into sub-micron particles. The fume consists of sodium and potassium compounds [9]. When the flue gases passes the tube section in the upper part of the boiler, i.e the convective heat section, the ash-forming elements can get stuck and form deposits on the tubes. These deposits consist of more than 99 wt% water-soluble alkali compounds. The alkali compounds exists as Na 2 S, NaCl, Na 2 CO 3, Na 2 SO 4 and the equivalent potassium salts, where Na 2 CO 3 and Na 2 SO 4 are the most occurring compounds [6][7]. First melting temperature - FMT Alkali compounds are corrosive at high temperatures. As alkali compounds melt their corrosive characteristics increases. The first melting temperature (FMT) is the temperature where deposits start to melt. The temperature is usually around C for alkali deposits in recovery boilers [10]. The complete melting tem- 3

8 1 Introduction Johan Jansson perature refers to the temperature where the material is in complete liquid phase [7]. Pure alkali salts have high melting temperatures, and the FMT is the same as the complete melting temperature. E.g Na 2 SO 4 melts at 840 C and NaCl melts at 800 C. For a mixture of these compounds, 90 mole% Na 2 SO 4 10 mole% NaCl, the FMT has decreased to 628 C while the mixtures complete melting temperature is above 850 C [10]. This is due to the alkali salts being eutectic mixtures [10][7]. Two other important temperatures are the sticky temperature and the radical deformation temperature. The sticky temperature is when about 15 to 20% of the material is liquid phase, that is when the material becomes sticky enough to get stuck and start to accumulate in the convective heat section. The radical deformation temperature is when the material consists of about 70% liquid phase. Above this point the material will consists of too much liquid to stick to surfaces, instead it will run off. Thus, between these temperatures accumulation of deposits will occur in the convective heat transfer sections [7]. The effect of the temperature reaching above the sticky temperature is that the accumulation of deposits can lead to plugging of the convective heat transfer section [11]. Thus, plugging of the boiler is an indication of high levels of NPE in the flue gas deposits. The important temperatures varies depending on the composition of the deposits. Cl and K have the largest impact on the temperatures [11]. When discussing superheater corrosion, the FMT is the most discussed and important temperature. The superheater tubes are the part of the boiler which contains the highest steam temperatures. These temperatures can in some cases be as high as the FMT. If the temperature of the steam is the same as the FMT, the deposits on the outside of the tube will start to melt [7][10][12]. Adams et al. [7] states that, due to the fact that most deposits contain some Cl the major effect on the FMT arises from the concentration of K. In Adam et al. s book, they show how the FMT decreases from 620 C to 520 C when the K concentration increases from 0 to 10 mole% K/(Na+K) [7]. In Fig.(1), Tran et al. [10] shows the correlation between the FMT and K content of 150 deposits and precipitator dust samples. There is a distinct correlation where the FMT decreases with increasing K content. They state that the FMT decreases from 610 C to 520 C when the K content increases from 0 to 8 mole% K/(Na+K) [10]. 4

9 1 Introduction Johan Jansson Figure 1 "Effect of potassium on the FMT of boiler deposits and precipitator dust", adapted from [10]. According to Skrifvars et al. s [13] research, only a small amount of Cl present in the deposits will lower the FMT and also trigger corrosion below the FMT. They also show that Cl with K will lower the FMT substantially, and that the presence of melt in the salt deposit increases the corrosion considerably. When Cl is not present in the deposit there is no corrosion, despite the fact that K and Na is present [14]. According to Skrifvars et al. [13], there is an increasing interest in increasing the power output of recovery boilers. One way of doing so is to increase the steam temperature and thus the pressure. The majority of the recovery boilers today run at steam temperatures around C; however, there are boilers running at +500 C [15]. As mentioned before, FMT of alkali salts containing Cl and K is between C. With a steam temperature of 500 C the steel temperature on the outside of the superheater tubes is somewhat higher. This suggests that steam temperatures above 500 C could very likely result in a corrosive environment in the boiler [13]. 5

10 1 Introduction Johan Jansson Removal of potassium and chloride As mentioned before Cl and K enriches in the flue gas dust due to their ashforming property and thus get stuck in the convective heat transfer section. The dust which does not get stuck continues to the electrostatic precipitator (ESP). The dust passes through the ESP where a high potential forces the dust particles to get stuck. The dust is collected and then solved before being re-circulated to the black liquor before the recovery boiler [3]. Most of the carryovers get get stuck in the first part of the convective heat transfer section, i.e the superheaters. This results in less carbonates in the ESP dust, whilst the small fume particles gets through and ends up in the ESP dust. However, thanks to sootblowers in the heat transfer section, some of the stuck deposits can be knocked off and collected via ash hoppers at the bottom. The sootblowers usually exists at the boiler bank, economizers and superheaters (in the flue gas direction) [7]. One way of dealing with corrosion, due to the high levels of K and Cl, is to remove the NPE from the process. Because of their ash-forming property they are enriched in the ESP dust and boiler ash [16]. Due to enrichment of the NPE, pulp mills have been purging small amounts of ash regularly in order to control the levels of Cl and K. However, the purging results in losses of the important process elements such as sodium, sulfate and carbonate. To balance these losses more make-up chemicals must be added which is an expensive cost for the mill. Efforts of purging NPE from the process has therefore been aimed towards purifying the ash/esp dust (later referred to as ash) [16][17]. Commercially-available today there are four K and Cl removal techniques: leaching, evaporation/crystallization, freeze crystallization, and ion exchange [18]. Leaching, evaporation/crystallization and freeze crystallization all uses the same principle, where the alkali sulfates and alkali chlorides in the ash have different solubility. The main compounds in the ash are (as before): Na 2 SO 4, Na 2 CO 3, NaCl, K 2 SO 4, K 2 CO 3 and KCl [16]. The leaching process mixes the ash with water (and sometimes recycled leachate) to a slurry. Due to a common ion effect, solutions of mixed salts with NaCl present (preferably NaCl-saturated) lowers the solubility of Na 2 SO 4 considerably. With the right amount of water/leachate (important that not too much water is added) NaCl and KCl will dissolve while the insoluble Na 2 SO 4 stays intact. With a filter or centrifuge the solids can be separated from the solution which is enriched with Cl and K. The solution is purged from the process but a portion is returned to the earlier stage as leachate [16][19]. A problem with the process occurs when Na 2 CO 3 also has a tendency to dissolve under these conditions. The unnecessary loss of sodium can be an expensive cost. A solution is to add sulfuric acid in order 6

11 1 Introduction Johan Jansson to convert Na 2 CO 3 to Na 2 SO 4. However, this could lead to an increase of the sulfidity which may not be wanted. The risk of corrosion also increases and may require corrosion resistant materials, thus increasing the cost for the leaching plant [20][16]. As Cl exists only as NaCl or KCl the removal of Cl works well. However, K can exist as K 2 SO 4 or 3K 2 SO 4 Na 2 SO 4. These elements has the same insoluble tendency as Na 2 SO 4. Thus, if the K level gets to high the only way to lower it is to purge the ESP dust [16]. The evaporation/re-crystallization process dissolves the ash completely in water and/or recycled process condensate. It then evaporates the solution where, as before, the solubility difference between the compounds makes the Na 2 SO 4 crystallize first. The rest of the process is as mentioned for the leaching process [16]. However, in this process the crystals generated are larger than the crystals generated in the leaching process. Thus, it is easier to separate the solids from the solution in the evaporation/re-crystallization process. [18]. There are today three different commercially available evaporation/re-crystallization process: HPD CRPTM (Chloride Removal Process), Andritz ARC (Ash Re-Crystallization process) and Eka Chemicals PDR (Precipitator Dust Recovery). They all use steam-driven evaporators and the same technique but with different evaporation designs [16][18]. Mitsubishi Heavy Industries (MHI) have installed six freeze crystallization process in Japan [16]. The freeze crystallization process is very similar to the evaporation/recrystallization process but decreases the temperature instead of increasing it. The process mixes water with the ash at a temperature around C in order to dissolve K 2 SO 4 and NaCl completely. The slurry then continues to the precipitation tank where ice is added to decrease the temperature to 15 C. The Na 2 SO 4 starts to precipitates as Na 2 SO 4 10H 2 O and forms crystals in the size of 100µm. The solids can then be purged in an decanter or centrifug. The MHI freeze crystallization systems have a reported efficiency of 90% for Cl and 75% for K with 70% sodium recovery [21]. The last Cl and K removal process, ion exchange, uses a totally different technique. PDP (Precipitator Dust Purification) is an ion-exchange treatment system. It uses a resin with anion- and cation-exchange groups in order to adsorb the NPE from a solution of ash and water that has been mixed in an earlier stage. NaCl is the only monovalent anion in the ashes and will thus be adsorb, while Na 2 SO 4 and Na 2 CO 3 remain in the solution. According to Ferreiera et al. [22] the solution is percolated through a fixed bed packed with the resins. The NaCl is adsorbed while the alkali sulfates and carbonates continues through untouched. In order to release the NaCl from the resins they are rinsed with water in the opposite direction, and thus gets regenerated. 7

12 1 Introduction Johan Jansson This makes the ion-exchange process recover most of the important alkali compounds and with a high removal of NaCl. However, the removal of K is very low due to the high concentration of Na compared to the low concentration of K. Thus it is not a preferable system for K removal. Another drawback is that the system recycles the process compounds Na 2 SO 4 and Na 2 CO 3 as a solution instead of as solids. This could be a problem if the mill does not have any spare capacity in the black liquor evaporators [16][18]. At Sodahuskonferensen 2016, Foan at NORAM Engineering [23] talked about a PDP Cl and K removal system with efficiencies up to: 80-97% Cl removal; 80% K removal; 90-96% CO 3 recovery; 90-96% SO 4 recovery and 90-96% Na recovery. 1.3 Materials One way of dealing with high temperature corrosion is to choose material that is more resistant to corrosion for the high temperature regions, mainly the superheaters. The material needs to be resistant to the oxidation of the surfaces,as was mentioned before. According to Sharp [12] and Adams et al [7], commonly used superheater tube materials are: T11 (1% Cr), T22 (2.25% Cr 1% Cr), Type 347 stainless steel (19% Cr), and Alloy 310 (25% Cr). In the order of least to most corrosion resistant. However, the most corrosive resistant materials could suffer from stress-corrosion cracking from the inside if the boiler water gets contaminated, whilst the less corrosive resistant material does not [12][7]. A solution is to use composite tubes. For example a composite tube can consist of an outer layer of Alloy 310 while the inner layer is of T22 in order to get the corrosion resistant property against the deposits and the stress-corrosion resistant property on the inside [12]. Skrifvars et al [13] performed laboratory-scale corrosion studies with six common superheater materials. In Tab.(2) the materials are listed with their composition in order of increasing corrosion resistance. 8

13 1 Introduction Johan Jansson Table 2 "The compositions of the steels used in the corrosion tests", adapted from [13]. # Steel grade Chemical composition of tested steels (nominal), weight C Si Mn Cr Ni Mo Others 1 10CrMo < T N=0.05 Al=0.03 Nb= Esshete Nb= V= Sanicro 28 <0.02 <0.60 < Cu=1.0 5 HR11N <0.03 <0.60 < N= Sanicro 38 <0.03 <0.50 < Bal. 8.5 Nb=3.5 Fe=3.0 From Tab.(2) its obvious to see that materials with higher corrosion resistance have more content of the compounds Ni (nickel), Cr (chromium and Mo (molybdenum). The last three are Ni based special stainless steels while 10CrMo9-10 is a low alloyed steel; T91, a high alloy steel and Esshete 1250, a austenitic steel stainless steel. Thus, more alloyed steel increases corrosion resistance [13]. 1.4 Heat absorption In order to produce high pressure steam from water there are different stages in the boiler to exchange the heat from the hot flue gas to the water. The water is preheated, vaporized and superheated. The heat exchanging stages consists of: economizers, the water is preheated; boiler bank, the water is vaporized to steam (which also occurs in the furnace tubes); superheaters, the steam is heated from saturated steam to superheated steam (i.e a higher temperature). Depending on the size of the heat exchanging areas the heat from the flue gas will be distributed different. The hot flue gas produced from the furnace bed at the bottom of the boiler first enters the superheaters. Steam flows through the superheater tubes in order to be superheated. The flue gas, which has lost some of its heat, then enters the boiler bank where the feed water is vaporized. Finally the flue gas enters the economizers where feed water is preheated. With a large superheater surface area the steam will be heated to a higher temperature due to the large heat exchanging area, thus the steam will absorb more of the flue gas heat. With a smaller superheater surface area less flue gas heat will be absorbed by the steam in the superheaters. Thus, the flue gas leaving the superheaters will be hotter, containing more energy. This energy will instead be absorbed in the the boiler bank and economizers due to the higher temperature difference. The result is that 9

14 1 Introduction Johan Jansson more water will be vaporized in the boiler bank, and the water will be preheated to a higher temperature in the economizers, thus vaporize more easily in the boiler bank. At the end there will be a higher steam production instead of a higher steam temperature, when comparing small and large superheater areas. A superheater stage consists of multiple (usually between 20-40) panels separated with a transverse pitch of a couple of 100 mm, in order to make a flue gas path through the superheaters. Every stage has an incoming and outgoing distribution header. The distribution header s role is to mix the steam between the superheater panels. From the incoming distribution header there are (usually) three to five tubes entering the furnace through the roof of the furnace. The tubes run in parallel through a number of bends before leaving the furnace through the roof into the outgoing distribution header. There is a small longitudinal pitch between the tubes going in parallel, separating the tubes in order to make more space for the flue gas to. A schematic figure of two superheater panels with the distribution headers is presented in Fig.(2a) (a) Two superheater stages seen from the side presenting the steam path from the distribution headers (D) through the furnace. (b) A 3D figure of a superheater stage with the first and last panels. For visualization the panels in between have been removed. Figure 2 Schematic figures of superheater stage and panels. A schematic cross section figure for a superheater stage is presented in Fig.(2b). 10

15 1 Introduction Johan Jansson Where only the first and last superheater panels are shown in order to give a fundamental representation of the stage. 1.5 Simulation tools WinGEMS is a modular based energy and mass balance simulation program, created by Pacific Simulation. The program is oriented for the pulp and paper industry to simulate parts of processes or whole processes. WinFURN is an extended version of WinGEMS more oriented for combustion energy and mass calculations. WinGEMS can for example be used when a new pulp mill is to be built. The energy and mass balance can be simulated for the whole mill in order to check that the concept is correct. 1.6 Problem The uncertainties of high temperature corrosion and the unpredictable environment in the furnace makes it difficult to design recovery boilers. When higher steam data is desired it puts even greater demands on the design. This results in higher investment cost and could lead to less profit for the mill buying the boiler. 1.7 Objective The main objective with this thesis is to carry out an economical optimization of material and components cost by conducting a theoretical study of its influence of steam data and potassium level. This includes a comparison of investment cost, annual make-up cost and revenue made from power generation for recovery boilers producing different steam data. The study aims at investigating the different choices of steam data a mill stands ahead, when planning to invest in a new recovery boiler. The study consisted of: Setting up a scenario Designing of superheaters and simulation of the recovery boilers Determine the FMT for different potassium levels Material(s) and component(s) design of the recovery boilers Determine the power generation 11

16 1 Introduction Johan Jansson Collection of price estimates for components and materials Evaluation of the investment cost and the revenue from power generated (between the recovery boilers) The study also includes collection of thermocouple data from the superheaters of two recover boilers. The data are to be evaluated in order to determine how the steam temperature fluctuate between superheater panels due to fluctuations in the furnace. 12

17 2 Methods Johan Jansson 2 Methods 2.1 Simulation tools WinFURN The modular based program means that blocks and streams are used to symbolize the processes parts. The blocks performs energy or/and mass operations with the streams as inputs and outputs. The streams connects the blocks with each other and transport the flows of media symbolizing the real life flows in the process. The blocks can have one or several inputs and outputs of streams. The operations are made iteratively to converge to a solution. In WinFURN the blocks can for example represent: superheaters, economisers, boiler bank and the whole black liquor chemistry. The inputs for WinFURN when simulating a recovery boiler consists of: the black liquor composition, feed water, combustion air and external sootblowing steam (as in this study). The output consist of: flue gas, high pressured steam and smelt. The input streams have to be defined with: temperature, pressure, mass or volume flow, specific heat, etc. For example a block can symbolize a heat exchanger, a combustion taking place or mixing of streams. Thus, the blocks needs to be defined with constants, parameters or calculation if needed. When a block is used as a heat exchanger, such as a superheater, the block needs to be defined with heat exchanger area, "thermal conductivity", transverse and longitudinal pitch between the tubes and panels, etc. WinFURN also considers flue gas velocities through the boiler for heat transfer calculations. 13

18 2 Methods Johan Jansson FMT and materials As mentioned in the background, Skrifvars et al. s [13] states that corrosion can occur below the FMT for superheater materials. However, with higher alloy the better the material withstands the corrosiveness. This is also stated by Enestam et al. [24]. They show in their study about, corrosion prediction, material selection and optimization of steam parameters for biomass and waste boilers, how the maximum allowed material temperature for low and high alloyed materials differ from each other for different flue gas temperatures. This means that when selecting materials for the superheaters, the corrosion that can occur below the FMT must be taken into account. Thus, there must be a margin between the highest allowed material temperature and the FMT for the specific material. ÅF provided a function to determine the FMT, where the FMT is dependent of the mole fraction K/(Na+K) [mol%] in the fly ash. The mole fraction in the fly ash can be derived from the potassium and sodium levels in wt% in the virgin black liquor, together with their molar masses and an enrichment factor. The function is presented in Fig.(3) FMT [ C] K/(Na+K) [mol%] Figure 3 Function of the FMT dependent of the mole fraction K/(Na+K) [mol%] of potassium and sodium in the fly ash. From ÅFs experience studying a number of recovery boiler configurations (from suppliers) they have seen different margins for different materials between the 14

19 2 Methods Johan Jansson highest allowed material temperature and the FMT. This follows the research from the literature that higher alloyed materials, can withstand the corrosion better than the low alloyed material. The margin for four different non alloyed and alloyed materials can be seen in Tab.(3). Table 3 Suggested temperature margin between material and the FMT for different alloyed materials. Type of material Required margin between tube material and the FMT ( C) Carbon steel 60 Low alloyed 30 High alloyed 20 Composite tubes, Higher alloyed outside 10 Due to the fact that the superheater materials are continuously cooled from the steam on the inside, steam data affects the material temperature on the outside of the tubes. When selecting superheater materials it is more logic to express a margin between the FMT and the highest allowed steam temperature for the materials. Assuming the temperature increase across the tube wall, including deposits, from the steam temperature to be T=5 C. A supplier assumed maximum deviation between the hottest superheater tube and the average steam temperature from that same superheater stage to be T=30 C. This deviation occurs due to fluctuations of the flue gas in the furnace. Adding up the margins, the minimum margin between the FMT and the average steam temperature from a superheater for the four materials is presented in Tab.(4). Table 4 Minimum margin between steam temperature and the FMT for the different alloyed materials carrying the steam. Type of material Minimum margin between average steam temperature and the FMT ( C) Carbon steel 95 Low alloyed 65 High alloyed 55 Composite tubes, Higher alloyed outside 45 Thus, when selecting materials for a superheater (depending on steam data and the FMT) these are the margins deciding what material to put where. This study focuses on the economical part, which means that as long as a cheaper material fulfill the stated minimum margin, that is the material to use. 15

20 2 Methods Johan Jansson 2.2 Scenario/Case First of all a scenario needed to be set up. A pulp mill is investing in a new recovery boiler. The recovery boiler needs to combust 2500 tons of black liquor dry solids every 24 hours (tds/24h). The mill in the selected scenario also has a power boiler at constant maximum load. The steam produced is fed to a back pressure turbine where power is generated. In the back pressure turbine the steam is reduced to low pressure (LP) steam at 4.5 bar and medium pressure (MP) steam at 12 bar which is then fed to the mills processes. The steam is also reduced to 30 bar for sootblowing steam in the boilers. Excess heat in the form of LP steam is fed to a condense turbine. This is feasible as the efficiency for power production is higher for a back pressure turbine than for a condense turbine. The heat input with liquor and biofuel is in this mill constant. Therefore net power generation from back pressure and condense turbine is affected by e.g. the selected steam data, which is studied here. The steam data to be studied were selected in a manner to include the highest and lowest steam data from recovery boilers existing today, also two intermediate steam data were included. Thus, in order for the study to be as comprehensive as possible. The steam data were then chosen with the help from ÅF s many years of experience working with recovery boilers. The steam data to be studied are presented in Tab.(5) together with the steam flow. The steam flow was determined with ÅF s heat balance program for a selected normal set of boiler operation parameters. Table 5 Steam data to be examined in the thesis. Boiler Pressure (bar) Temperature ( C) Steam flow (ton/h) A B C D As mentioned in the theory, with increased steam temperature the amount of produced steam decreases, assuming constant heat uptake in the boiler. However, in boiler A that is not the case. This is due to the fact that boiler A only requires softened water while boiler B, C and D requires demineralized water. The required quality of feed water is depending on the boiler pressure. With high pressure (such as boiler B, C and D) dissolved solids in the feed water can damage the 16

21 2 Methods Johan Jansson boiler s water and steam circulation system. In order to remove dissolved solids the feed water is demineralized. With low steam pressure (such as boiler A) the risk of damaging the boiler due to dissolved solids is less, thus there is no need for demineralized feed water. However, over time dissolved solids start accumulate in the process and could lead to damaging of the boiler. In order to not accumulate dissolved solids in the process, a high continuously blowdown of feed water from the steam drum will ensure an adequate boiler water quality. This means that boiler A is not in need of the demineralization equipment as boiler B, C and D are, but will have a blowdown of 12% of its feed water. This will keep the cost down even further. Due to the many differences between recovery boilers made today, recovery boilers are always tailor made for the buyer depending on the amount and quality of black liquor that is to be combusted; the desired production of steam; the desired power generation and depending on the budget. In order to carry out a thesis study about this subject the boilers to be compared all had the same combustion properties (amount and quality of the black liquor), parameters (such as feed water temperature, combustion air temperature and amount) and size of the boiler. Also the flue gas temperature exiting the economisers of the boilers was kept the same. This means all boilers have the same amount of energy absorbed in the boiler. In order to carry out a study as general as possible the properties and parameters for the boilers were chosen in a manner to be as overall normal and "intermediate" as already existing boilers/as existing boilers today. Thanks to ÅFs experience working with recovery boilers they provided a general model and black liquor for the simulations. A schematic figure of the boilers seen from the side is presented in Fig.(4) and the basic data for the boilers are listed below. Boiler load, 2500 tds/24h Higher heating value (HHV) of the black liquor, 13 MJ/kgDS Flue gas temperature, 197 C Dry solids (DS), 72% DS 30 C steam cooling in all boilers 3% O 2 in flue gas 17

22 2 Methods Johan Jansson Figure 4 Schematic picture of the boiler with dimensions. As mentioned in the background of the thesis the amount of potassium can substantially affect the corrosiveness of the flue gas deposits by lowering the FMT. This together with the steam temperature affect(s) the choice of material used in the boiler. It could also lead to the need of an ash-treatment process to keep the potassium level down. Thus, for this study the boilers were studied with four different virgin black liquor potassium levels. The potassium levels chosen for this study are presented in Tab.(6) together with the corresponding FMT. 18

23 2 Methods Johan Jansson Table 6 The potassium level for the chosen virgin black liquors and the correspondingly FMTs of their ash. Potassium level (wt%) FMT ( C) The FMTs were calculated by determine the molar fraction K/(Na+K) [mol%] with their molar masses and an enrichment factor and then use the function shown in Fig.(3) in the theory section. In order to keep the scenarios at a reasonable amount the assumption was made that the sodium content was kept at a constant level of 20 wt%. From the literature survey it could be stated that 20 wt% represents a normal sodium level for a virgin black liquor. With the case stated above the following components are to be designed and cost evaluated: Superheater materials, due to different steam qualities Furnace materials, due to the different boiler pressures Wall thickness of boiler tubes, due to different steam qualities Ash-treatment, due to different steam temperatures and potassium levels Turbine and generator, due to different steam productions Power consumption for the feed water pump, due to different steam pressures 19

24 2 Methods Johan Jansson 2.3 Simulation The model that ÅF established for this study was used in WinFURN in order to produce the steam data desired. Four simulations were made, one for each steam data. This was done by altering the heating surface for the boilers. A number of real recovery boiler superheaters were studied in order to use existing superheater configurations. The total superheater surface area of the boilers were: 5836m 2, 7919m 2, 9503m 2 and 10354m 2 for boiler A, B, C and D respectively. The superheater dimensions can be seen in appendix A and B. In some of the cases the economisers area needed alteration to obtain the right steam data and right amount of steam. However, it was concluded that the small changes in area of the economizers would not affect the cost noticeable. In Fig.(5) a simple schematic figure of the boilers superheaters is presented. The figure shows the number of superheater stages and approximate positions of the stages. 20

25 2 Methods Johan Jansson (a) Boiler A (b) Boiler B (c) Boiler C (d) Boiler D Figure 5 A simple schematic picture of the superheaters position and size for the different boilers. From the simulations a steam temperature profile along the superheater tubes could be obtained, that include the inlet and outlet steam temperature for each stage. The temperature profile could then be used to determine the temperature increase across each superheater stage. Together with the length for each tube 21

26 2 Methods Johan Jansson the temperature increase per meter tube could be determined. The temperature profile, temperature increase, tube length and temperature increase per meter superheater tube for each boiler and superheater stage is presented in Tab.(7). Table 7 Temperature profile for each boiler s superheater stages. Boiler A B C D Superheater IA IB II IA IB II III IA IB II III IV IA IB II III IV T in ( C) T out ( C) T ( C) Tube length (m) T/meter Evaluation of thermocouple data As mentioned before, a supplier assumed maximum deviation of 30 C between the hottest superheater tube and the average steam temperature from that same superheater stage, is used when determine the minimum margin between steam temperature and FMT. This 30 C deviation is due to the fact that the temperature varies between the tubes and the panels in the same superheater stage. Today superheaters are usually equipped with thermocouples monitoring the temperature of the tube material. A thermocouple is placed on the outside of a tube below the outgoing distribution header. In Fig.(6) the positions of where thermocouples can be placed is shown. Figure 6 A schematic figure for a superheater stage and where thermocouples can be positioned. 22

27 2 Methods Johan Jansson The thermocouples are usually placed on a number of panels i each superheater stage. In order for these thermocouples to work properly the need to be insulated against the tube due to the colder surroundings above the furnace roof. When properly insulated the thermocouple is monitoring the material temperature, which should be almost the same as the steam temperature inside that tube. Thus, if there are fluctuations between the tubes and/or the panels this can be observed. This study included collection of thermocouple data from two recovery boilers. The temperature data from the thermocouples were compared with the average steam from the superheaters in order to investigate whether the assumed 30 C is reasonable or not. From one mill the data were from 24 hours during normal boiler load. The data were taken from a period after the boiler had been water washed. This would imply that the tubes were free from deposits, and should experience a hotter material temperature compared to when the tubes are covered with deposits. The data were taken with one hour intervals. The average outgoing steam temperature was subtracted from the highest observed thermocouple temperature from the last superheater stage for every hour. The majority of the temperature differences turned out to be negative and thus suggest that the thermocouples are not insulated against the tubes, and thus are cooled by the surroundings. However, the highest observed temperature difference was +10 C, and With proper insulated thermocouples this temperature difference would likely be higher. From the other mill the data were from three hours during normal boiler load. The data were taken with one minute intervals. These thermocouples were known to be insulated against the tubes. The same procedure as above was done for the last, second last and third last superheater stage. The highest observed temperature difference from the last, second last and third last superheater stage was 13 C, 16 C and 14 C respectively. Thus, the data indicates that the fluctuations in steam temperature between the tubes is considerably lower than the assumed 30 C. 23

28 2 Methods Johan Jansson 2.5 Boiler design Ash-treatment First of all the boilers steam temperatures were compared with the FMTs of the different potassium levels. This was done to see whether the boilers could be run without any removal of potassium. In order to establish this, the smallest minimum margin between the average steam temperature and the FMT from Tab.(4) (that is 45 C) was used. Which meant the most corrosion resistant material was used in order to see if ash-treatment could be avoided. By taking Tab.(6) and subtracting 45 C from the FMTs, the maximum allowed average steam temperatures that were obtained are presented in Tab.(8). Table 8 Maximum allowed steam temperature for the potassium levels to be examined, considering the highest corrosion resistant material. Potassium level (wt%) FMT ( C) Maximum allowed average steam temperature ( C) By taking the maximum allowed average steam temperatures obtained above and compare it with the steam temperatures from the boilers from Tab.(5), it could be decided whether potassium removal is necessary. In Tab.(9) the result is presented. Table 9 Potassium removal needed or not. Potassium level (wt%) A B C D 1.0 No No No No 1.5 No No No Yes 2.5 No No Yes Yes 3.5 No Yes Yes Yes Thus, boiler B, C and D are in need of potassium removal for some of the cases. Boiler A does not need any removal of potassium due to the steam temperature of 450 C does not exceed the maximum allowed average steam temperature in Tab.(8). By taking boiler B, C and D s steam temperatures and adding the minimum margin of 45 C, the lowest FMT they can withstand was obtained. By using the function from Fig.(3) together with the molar masses of potassium and 24

29 2 Methods Johan Jansson sodium and an enrichment factor, the potassium levels of the virgin black liquors for these lowest possible FMTs were obtained. The potassium levels of the virgin black liquors are presented in Tab.(10) Table 10 The lowest possible FMTs for the steam data presented, and the correspondingly potassium levels for the FMTs. Boiler Steam temperature ( C) Lowest possible FMT ( C) Potassium level (wt%) B C D With the obtained potassium levels inserted in Tab.(9), Tab.(11) was obtained, which presents the highest possible potassium levels for the boilers in need of potassium removal. Table 11 Representation of the needed potassium reduction for the boilers. Potassium level (wt%) A B C D 1.0 x x x x 1.5 x x x 1.0wt% 2.5 x x 1.5wt% 1.0wt% 3.5 x 2.8wt% 1.5wt% 1.0wt% As mentioned in the introduction there are different types of ash-treatment systems. From the literature the most common and commercially available is the ash leaching system, which also has the lowest investment cost. Based on this, the ash leaching system has been chosen as a basis for cost estimates in this study. The ash leaching system chosen is a single stage with no adding of sulfuric acid. The system applies to an ordinary mill with a recovery boiler with a MCR of 2500tDS/24h. Together with ÅF simplified mass balances have been made for the removal of potassium in order to obtain the sodium loss for the different cases. The sodium losses for the six potassium removal cases are presented in Tab.(12), as tons of Na 2 SO 4 make-up per year. 25

30 2 Methods Johan Jansson Table 12 Amount of sodium make-up due to losses from the different potassium removals. Potassium level reduction Sodium make-up (ton/year) 1.5wt% 1.0wt% wt% 1.0wt% wt% 1.0wt% wt% 1.5wt% wt% 1.5wt% wt% 2.8wt% 5745 Pressure difference, Turbine/generator and Feed water demineralization One of the objectives was to study and determine how the investment cost for the pressurized vessels changes between the steam data. That includes: steam drum, distribution headers, main downcomers and all water and steam carrying components for the boilers circulation system. From this investigation also the tube thickness for the furnace and superheater tubes were to be estimated. Despite consulting with suppliers and studying recovery boiler configurations, an estimation for how the thickness of the components change could not be determined. This means that the tube dimensions used in the estimation of investment cost were chosen by studying a number of existing superheaters and furnaces with similar steam pressures as the steam pressures used in this thesis. With higher steam data the power output from turbine and generator will increase. This also implies a higher investment cost for turbine and generator. Turbine and generator suppliers have been consulted with in order to estimate the increase in investment cost for turbine and generator for the boilers with higher steam data. For the feed water demineralization equiptment an older configuration was used from ÅF s archive. It was scaled to date and size accordingly, where 1.25 was used as inflation rate and 0.7 for scaling factor. New capacity New investment cost = ( Known capacity ) Known investment cost 26