Effect of additives on sintering of lignites during CFB combustion

Effect of additives on sintering of lignites during CFB combustion

Applied Thermal Engineering 67 (2014) 480e488 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.c...

1MB Sizes 2 Downloads 40 Views

Applied Thermal Engineering 67 (2014) 480e488

Contents lists available at ScienceDirect

Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Effect of additives on sintering of lignites during CFB combustion P. Selvakumaran a, *, A. Lawerence b, A.K. Bakthavatsalam c a

Process & Captive Power Systems, Bharat Heavy Electricals Limited, Tiruchirapalli 620 014, Tamil Nadu, India Plant Laboratory, Bharat Heavy Electricals Limited, Tiruchirapalli 620 014, Tamil Nadu, India c National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India b

h i g h l i g h t s  Rapid sintering of Lignite causes blocking and choking in CFB Combustion.  Lignite sintering is characterized using the heating microscope.  Sintering examined for six lignite samples, one bituminous coal and one anthracite.  Silica or calcium additives to sample ashes reduce the rate and quantum of shrinkage.  Field trial with silica additive successfully demonstrated in operating CFB unit.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 December 2013 Accepted 16 March 2014 Available online 8 April 2014

Fossil fuel lignites are used widely for power generation employing CFB combustion. In CFBC, agglomeration due to sintering, contributes to loose deposit formation. Rapid sintering causes choking and blocking, which lead to shut down of the unit. A breakthrough has been achieved by averting the chocking and blocking inside the CFB steam generator by shifting the sintering pattern of ash, either to a lower or to a higher temperature, from the CFB operating range of 800e900  C. Lignite sintering is characterized using the heating microscope. The addition of chemical modifiers to the fuel, alters the ash chemistry, and reduces the rate and the quantum of shrinkage. Sintering is examined for eight samples; six Indian lignite samples, one southern hemisphere bituminous coal and anthracite. The effects of adding silica, calcium and alumina on the sintering tendency and the efficacy in reducing that tendency are brought out. With an economic quantity of 5e10% by weight of the selected additive, which are available in abundance at relatively cheap cost, peak sintering temperatures could be either increased by 30  C or lowered by 60  C. Reductions up to 30% in the rate of shrinkage (%/ C) and 10% in the quantum of shrinkage could be achieved. Thus, the sintering start and end temperatures could be managed well and the operating temperature of CFB maintained. This novel technical research was demonstrated on utility scale lignite fired 2  125 MWe CFB steam generators wherein the sintering issue and the blocking of dense phase at cyclone outlet faced were successfully resolved with the addition of silica additive and the units started operating successfully with high availability now. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Lignite Sintering CFBC Silica Calcium Additives

1. Backdrop/introduction World, with growing energy consumption, is further constrained in utilizing its potential energy resources in an economically and environmentally sustainable manner. The per capita consumption of electricity and the Gross Domestic Product (GDP) growth have a direct relation. Energy intensity in developing * Corresponding author. Tel.: þ91 431 2574068. E-mail addresses: [email protected], (P. Selvakumaran). http://dx.doi.org/10.1016/j.applthermaleng.2014.03.031 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.

[email protected]

countries is comparatively more than the developed world, and the gap between supply and demand is ever increasing. The demand for all forms of energy is expected to increase substantially in the foreseeable future. Coal would continue to be a major energy source due to its availability. Lignite, when available locally, is emerging as an economic fuel for power generation, provided the SO2 emission could be controlled. Circulating Fluidised Bed Combustion (CFBC) technology is applied considering the impurities, moisture, ash and sulfur content and wide variations in fuel quality. Slagging and fouling, leading to ash deposition, are the major problems experienced in pulverized fuel (PF) boilers.

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

Agglomeration of bed particles is a primary operational issue in CFB boilers, employing bed material and fuel ash as binary system. The interaction and the coalescence of bed particles and ash shrinkage (sintering) are considered to be the principal sources of agglomeration. Particle-to-particle sinter bonding usually results in shrinkage. Choking and blocking in fuel path is another peculiar operational problem experienced worldwide in CFB boilers firing pet-coke and some low rank coals, especially with bio fuels. Lignite mineralogy influences combustion behavior. Agglomeration, clogging and blockage are experienced due to sintering of some lignite at the lower temperature regime of 800  Ce900  C in which the CFB boilers operate. At this low temperature range, the extensive knowledge built for pulverized coal combustion with respect to slagging, fouling and high temperature corrosion are not applicable [1]. In CFB boilers, ash sintering contributes to the deposit formation in the cyclone, return leg and post cyclone flue gas channel [1]. In some operating units, rapid sintering led to heavy agglomerate formation, and finally inhibited circulation in dense phase areas such as seal pot, and in the second pass [2]. For avoiding such problems, it is required to understand the sintering behavior before the fuel is used [1,2]. The present research aims to develop a method of shifting the sintering pattern of ash to either to a lower temperature or to a higher temperature and at the same time modify the rate of shrinkage (sintering) of ash for reducing the agglomeration, and choke formation (blocking inside fuel/gas path) resulting from combustion of lignite in CFBC. The ash particles generated from the lignite will be in close contact with the bed material inside the furnace with an operating temperature range of 800  Ce900  C. Lignite comprises combustible and non-combustible portions. The non-combustible portion comprises different forms of moisture, volatile gases and minerals. The minerals originally present in the lignite, gets converted into ash during combustion. The ash consists of oxides of Si, Al, Fe, Ti, Ca, Mg, Na, K, S and other trace elements [1,2]. If the ash generated during combustion and ash chemistry are related to a specified range of oxide composition, shrinkage and sintering results in. Depending on the chemistry and the chemical constituents of ash, the ash particles undergo varying magnitude of agglomeration and sintering leading to choke formation in the fuel path. Once choke formation in the path of the furnace components reaches the critical level, the boiler has to be shutdown to clear the choking. This research focuses on the addition of chemical modifiers to fuel which would alter the ash chemistry and reduce the rate and the quantum of shrinkage thereby reducing sintering and averting the choking. The effects of the addition of different chemical materials on the sintering tendency of typical low grade lignite ashes and the efficacy of the additives tested in reducing that tendency are studied and compared with high rank bituminous and anthracite coals. 2. Review of additives in combustion 2.1. Additives to bio mass Biomass is the third largest primary energy resource in the world, after coal and oil. Llorente et al. studied the effects of addition of chemical materials on sintering of biomass ash [3]. Despite, being a renewable energy resource, the large scale use of bio mass for generation of electricity and heat is limited by sintering and slagging of bio fuels. Strege et al. reported the coutilization of biomass in the existing coal and lignite combustion systems which led to serious problems such as slagging, fouling, and clinker formation [4]. Low melting point potassium in bio

481

mass-hazelnut shell and silicon compounds present in lignite might be the reason for reduced sintering temperature [5]. Bartels et al. reviewed agglomeration in fluidized beds [6]. In fluidized bed systems, accumulation of such agglomerates might lead to the loss of fluidization (de-fluidization) and unscheduled shutdown of the plant. The slagging and the fouling are the underlying cause for operational difficulties and increased maintenance. Agglomerates were formed during operation at several hundred degrees below the initial deformation temperatures of coal ash tested by American Society for Testing Materials (ASTM) procedure [6]. Mroczek et al. investigated the effect of halloysite additive on biomass firing [7]. Sintering, agglomeration and slagging, fouling are managed by special designs such as protective cladding and air installations and other operational methods that include dilution of biomass properties by co firing mixtures, and by higher excess air, with a consequent increase in costs [7]. There are techniques through which the melting point of biomass ash can be raised to overcome the deposition problems. Towards the elimination or reduction of sintering, agglomeration and slagging, fouling, various process treatments were tried. In order to increase the melting point of the biomass ash, additives were added to the fuel [3]. Werther et al., indicated that the materials which increased the softening temperature of the biomass ash to temperatures higher than those normally encountered in boiler furnaces include: alumina (Al2O3), calcium oxide (CaO), magnesium oxide (MgO), dolomite (CaCO3$MgCO3), magnesite and kaolin [8]. In the case of bed agglomeration of fluidized bed combustors, several materials were proposed and tested as alternatives to the traditional silica bed material. As reported by Bartels et al. these included: silica sand, feldspar, dolomite, magnesite, iron oxide (Fe2O3), alumino silicates, including kaolin, calcium oxide (CaO), magnesium oxide (MgO), and alumina [6]. Besides, reducing the sintering, agglomeration and slagging, fouling, an ideal additive needs to be cheap and should be easy to handle and not significantly affect the fuel combustion behavior and ash utilization. Kaolinite addition to the biomass combustion process forms high-temperature melting (>1400  C) alkali-aluminum-silicates, eliminating liquid-phase reactions. Thereby, the high temperature corrosion rate is decreased. Halloysite (Al4(OH)8/Si4O10  10H2O) is a rare alumino-silicate clay mineral of low hardness, high specific surface area resulting from the nano-tubular structure and high temperature resistance. Halloysite’s high reactivity is a consequence of phase changes, occurring above 550  C allowing the formation of high melting compounds with alkali metals. Since halloysite is an alumino silicate containing mainly Si, Al and admixtures of Fe and Ti, it is expected to decrease the slagging and fouling tendencies in biomass combustion. The addition of halloysite suppresses the formation of mycotoxins and improves the storage and handling of biomass [7]. Wu et al. took up the minimization of ash related problems during biomass combustion through additives that can convert the vaporized inorganic species to less harmful forms. They categorized the additives as Ale Si-based, S-based, P-based, and Ca-based. A typical example of the AleSi based additives is kaolinite, proven to be effective in minimizing the gaseous alkali concentration in flue gas through the formation of alkali-alumino-silicates with higher melting temperature. Other AleSi-based additives such as bauxite, emathlite, bentonite, clay, quartz and coal ash also showed some extent of decreasing the effect on gaseous alkali concentrations [9]. For minimizing the ash related problems in biomass combustion, the additives which have a pronounced decreasing effect on the harmful alkali chlorides in the flue gas are desirable. For biomass with large chlorine and alkali content, the utilization of Cabased additives such as Ca(OH)2 or CaCO3 could even increase the concentration of alkali chlorides/fine particles in the flue gas.

482

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

However, for biomass rich in P and alkalis, a significant decreasing effect on the fine particle formation could be achieved by using Cabased additives [9]. Schmitt et al. reported that the ash melting behavior and the associated emissions were influenced by the wood/straw ratio and by the mineral additives. Calcium hydroxide and kaolin, when added, elevated the ash melting temperature. Sintering and high dust emissions resulted in, due to the high potassium, sulfur and chlorine contents in the bio fuel. While wood showed high levels of calcium, straw showed higher potassium levels. Potassium lowers the melting point of ash. A part of the potassium reacted in the presence of silicon to potassium silicate (K2Si4O9), which was present at temperatures of 800  Ce1000  C in molten form. Ca- and Alcontaining additives were added to the raw biomass before pelletizing the woodestraw. Ca-containing additives (e.g. limestone, dolomite) are widely used in such combustion applications. Pure calcium oxide has melting point of about 2570  Ce2580  C. The melting temperature of ashes was elevated when calcium is added. Consequently, the building of potassium silicates was prevented, which had low ash melting temperatures. Al-containing additives, however, transform potassium silicates to potassium aluminum silicates, (e.g. lucite) causing an increase in ash melting temperature [10]. Kaolinite is the most efficient chemical additive to reduce the sintering of the biomasses which were studied. Dolomite, limestone and alumina offer poor results. Silica is not very efficient to reduce biomass sintering under the tested conditions [3]. The use of halloysite as an additive increased the level of ash sintering temperature for all agricultural biomasses tested [7]. 2.2. Additives to coal ash Ersoy et al., found that, the sintering behavior of fly ash depended on its chemical and mineralogical composition and physical properties, i.e. particle size, shape, and thermal treatment conditions [11]. The chemical and the mineralogical components and the physical properties of fly ashes varied depending on the coal type (anthracite, bituminous coal, lignite and sub-bituminous coals) and the combustion conditions of pulverized coal {dry combustion (1100e1400  C)/high temperature combustion (1500  Ce1700  C)/fluidized bed combustion (<900  C)} and the fly ash collection operations (cyclone, electrostatic precipitation, bag filters) [11]. The studies on sintering behavior of fly ash without chemical additive reported that the fly ash was bloated at high temperatures (1200  C). The bulk density and the bonding strength of the fly ash samples sintered between 900  Ce1050  C were enhanced as particle size decreased. Shrinkage curve of the fly ash sintered between 600  C and 1200  C, showed two distinct densification stages; the first one operated in the 800  Ce1070  C range and the second one at temperatures higher than 1180  C. Between the two stages, the densification remained constant. The densification process observed beyond 1180  C was controlled by liquid phase. Sheng et al. studied the effect of Na2O addition (w10 wt %) on vitrification of fly ash at 1200  C and obtained glass products of good chemical durability to immobilize the heavy metals [12]. Satapathy pointed out that the bulk density of the sintered fly ash at 1255  C was increased whereas its linear shrinkage remained constant with the additions of zirconia (ZrO2) up to 25 wt% to fly ash [13]. With BaCO3 addition, the decomposition of CaSO4 was shifted to higher temperatures due to the formation of BaSO4 phase. Fly-ash without additive was bloated completely when sintered at 1150  C due to the thermal decomposition of anhydrite. BaCO3 additions at levels of 5e10 wt% to fly ash, inhibited the bloating and swelling due to the phase transformation from anhydrite (CaSO4) to barite (BaSO4) during sintering. BaCO3

addition had a significant positive effect on the sintering of fly ash at 1150  C, whereas it indicated a negative effect on the sintering at 1100  C [11]. Mu et al. reported that fly ash particles were mostly spherical in shape and ranged from less than 1 mme100 mm, with a low apparent density of typically between 530 and1260 kg/m3. The chemical composition of fly ash is similar to clay, with the total amount of SiO2 and Al2O3 reaching 70%e90%. Shale could be used as a substitute for clay in sintered fly ash, due to the similar physical and chemical properties [14]. Kong et al. studied the increased addition of CaCO3 and found that the slag viscosity changes were due to different processes of solid phase formation [15]. Fourier transform infrared (FTIR) spectrum indicated that Ca2þ led to the breakage of polymerized SieOeSi into SieO, and the increasing Ca2þ in slag resulted in a decrease of viscosity above liquidus temperature. The effects of addition of limestone on Ash Fusion Temperatures (AFT) of coal ash were studied by experiments and by thermodynamic calculations. Hurst et al. used the ternary equilibrium phase diagrams to study the fluxing effect of CaCO3 on Australian coal ashes [16]. Song et al. examined the effect of CaO as pure compound on the AFTs of coal ash with Factsage [17]. Beyond 35%, the AFTs increased quickly with further addition of CaO. As the Si/Al ratio in coal ashes varied, the corresponding minimum AFTs were obtained with different CaO percentages as additive [18]. Since AFTs could also increase with the addition of excess limestone, the proper selection of flux amount was important. Seggiani provided a method to predict the effect of adding minerals such as CaO on AFTs [19]. Sintering temperature of the mix for shale and fly ash was lower than that of pure fly ash. Compared with pure fly ash, the microscopic evidence supported the hypothesis that the sintering mix for shale and fly ash produced a liquid phase at lower temperature [14]. 3. Experimental procedure laboratory Eight Samples e six lignite samples collected from India and one bituminous coal from southern hemisphere and one high rank coal anthracite e were considered for the present study of sintering propensity of the fuels for CFBC boiler applications. The samples of fuels were prepared in accordance with ASTM eD 2013. The asreceived solid fuels were crushed to pass a number 4 sieve (4.75 mm). The samples were then air-dried until the loss in weight is not more than 0.1% per hour. The air-dried samples were again crushed to pass a number 72 mesh (212 micron). The minus 72 mesh samples were used for the analyses of proximate, ultimate, and calorific value (Table 1). An adequate quantity of ash of each fuel was generated as per proximate analyses at 750  C for analyses of chemical composition, ash fusion temperature and for sintering measurement. The proximate, ultimate and gross calorific values, of the samples were carried out, using TGA 701 proximate Analyzer (LECO), Elemental analyzer Vario EL III and PARR Isoperibol Bomb Calorimeter respectively. The chemical composition of ashes was carried out by ICP- AES, Perkin Elmer. Sintering measurement and ash fusion tests were carried out by Leica Heating microscope. (Leica Microsystems, Wetzlar, Germany). For sintering and ash fusion measurement, ash pellets (specimen) of shape (truncated cone of 4 mm height, diameters of 3 mm e bottom and 1.5 mm e top) were prepared by uni-axial pressing of ash powder in a die without using any binder. The ash pellet was then air-dried. The ash pellet was introduced at room temperature into the furnace of the microscope. As per the heating program, the furnace was heated from ambient to 1000  C at a constant heating rate of 10  C per min. and then further heated to 1400  C with a reduced heating rate of 8  C per minute. The sintering and fusion test runs were performed in air. The images of the ash pellet were

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

483

Table 1 Proximate, ultimate, chemical composition of ash and ash fusion temperatures of samples. Sample ID

Sample 1 lignite

Sample 2 lignite

Proximate Analysis (wt % on air dried basis) Moisture 9.4 13.9 Volatile Matter 36.5 39.8 Ash 27.1 10.9 Fixed Carbon 27.0 35.4 GCV Cal/g 4150 5100 Ultimate(wt % on air dried basis) Carbon 43.2 52.9 Hydrogen 3.2 4.1 Nitrogen 0.8 0.7 Sulfur 0.7 1.0 Chemical composition of Ash (wt %) SiO2 46.4 45.4 Al2O3 38.6 36.9 Fe2O3 1.7 0.8 TiO2 0.9 0.5 CaO 6.3 8.0 MgO 1.9 2.5 Na2O 0.7 2.4 K2O 0.1 0.1 SO3 2.1 3.1 Ash Fusion Temperatures  C (Oxidizing Atmosphere) Deformation T1 >1450 1320 Softening T 2 >1450 >1400 Hemisphere T 3 >1450 >1400 Fusion T 4 >1450 >1400 Additive trials scheme Sample plus þ10% Ca þ10% Ca additive additive

Sample 3 Bit coal

Sample 4 lignite

Sample 5 A&B lignite

Sample 6 Lignite

Sample7 A&B Lignite

Sample 8 anthracite coal

9.1 40.1 11.6 39.2 6230

13.5 38.4 9.6 38.5 5520

13.2 35.2 26.2 25.4 3860

11.1 33.8 36.4 18.7 3160

7.9 35.7 24.6 31.8 4550

2.0 3.8 4.7 89.5 8140

63.7 4.5 1.1 0.6

56.5 3.5 1.2 0.3

39.8 3.2 1.0 1.0

32.6 2.9 1.2 0.7

47.4 3.8 1.3 0.5

89.9 1.1 0.8 0.4

52.6 33.9 4.6 1.6 3.3 0.6 1.6 0.6 0.7

45.9 29.7 1.5 3.3 14.4 4.1 0.4 0.3 0.0

41.6 39.5 2.3 0.9 9.5 2.5 0.2 0.3 2.8

52.8 35.4 1.9 0.8 3.9 1.1 0.4 0.2 2.5

43.7 39.1 1.0 0.8 6.1 2.1 2.3 0.1 0.2

49.2 35.6 6.5 1.8 1.6 1.2 0.4 3.2 0.0

>1450 >1450 >1450 >1450

1306 1319 1374 1381

1410 1425 >1450 >1450

>1450 >1450 >1450 >1450

>1400 >1400 >1400 >1400

>1450 >1450 >1450 >1450

þ10% Ca additive

þ20% Ca additive

þ5% Alumin or þ5% Silica

þ5% silica

þ5% silica or þ10% silica

þ1% silica

continuously captured using camera at pre-selected time intervals (10 s), during the whole heating process. The dimensional changes of the ash pellet namely height, corner angle, area and shape factor were also measured from the stored images of the specimen by the built-in software ePicture Analysis Software -supplied by Leica Heating microscope. The area shrinkage % ¼(A0AT)/Ao  100 where Ao and AT are the initial projected area of the ash pellet and the area after each interval of temperature “T” respectively were calculated from room temperature to final temperature of each run. Two test runs were carried out for each lignite sample with accuracy and reproducibility as per DIN 51730 (1998-4). To analyze the effect of chemical additives, sample ash was generated from the mixtures of fuel (95%, 90%, 80%) by weight and calcium additive (5%, 10% and 20%) by weight respectively, (Table 2) at 750  C as per standard procedure ASTM D-3174:04. The sintering pattern of four sample fuel ashes generated out of combustion of respective fuels (sample-1esample-4) and the corresponding four samples (sample-1esample-4) mixed with calcium based additive

Table 2 Chemical composition of additivesa. Composition

Calcium based additive % by weight

Silica based additive % by weight

Silica (SiO2) Aluminum Oxide (Al2O3) Titanium Oxide (TiO2) Iron Oxide (Fe2O3) Calcium Oxide (CaO) Magnesium Oxide (MgO) Sodium Oxide (Na2O) Potassium Oxide (K2O) Barium Oxide BaO Strontium Oxide SrO Loss on Ignition at 900  C

0e10 0e5 0e2 0e10 35e56 0e4 0e1 0e1 0e1 0e2 30e44

93.9 1.2 0.3 0.6 0.5 0.2 0.5 0.2 Nil Nil 0.6

a

Patented by BHEL/India.

mixtures were studied (Table 3). Similarly, the addition of silica additive (Table 2) in proportions of 5% and 10% by wt were made to fuel of 95% and 90% respectively and ashed at 750  C as per standard procedure ASTM D-3174-04. The sintering pattern of four samples of fuel ashes generated out of combustion of respective fuels (sample-5B to sample-8) and the corresponding four samples (sample-5B to sample-8) mixed with silica based additive mixtures were studied (Table 3). 4. Results and discussions The results of proximate and ultimate analyses, calorific value, chemical composition of ashes and ash fusion temperatures of all the fuel samples are shown in Table 1. The review of the sintering profile of the coal ashes revealed that the start temperature of sintering of the ash of the fuels could be anywhere between 800  C to 1100  C depending on the ash chemistry. For some fuel ashes, the start temperature of sintering might go beyond 1100  C as well [1]. In addition to exhibiting the “start temperature” of sintering within the operating temperature range of CFBC boiler systems (800  Ce 900  C) sample-2 showed a “significant percentage” of the sintering with a narrow derivative peak (Fig. 1) within the above operating temperature range. The firing such fuels resulted in agglomeration and choking in CFB boilers. In the field, choking phenomena in dense phase locations of the CFBC boilers were observed, where the temperature exceeded just 10  Ce20  C more than the sintering start temperature of 860  C for sample-2. The sintering effect of such fuels with narrow peak, and the consequent chocking in dense phase areas might easily be overcome by selecting a suitable temperature regime for combustion on either side of the rate peak, thereby avoiding the critical temperature range, (depending on the positioning of the peak), which allows efficient CFB operating regime. Hence, to overcome the above issue, either the operating temperature range of the boiler needs to be changed or the

484

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

Table 3 Sintering characteristics data of fuel ashes. Sintering characteristics data of fuel ashes with calcium based additive Fuel Ash Sample

Sample Sample Sample Sample

1 2 3 4

Lignite Lignite Bit Coal Lignite

Without additive

With Calcium based additive

Start of sintering  C

End of sintering  C

Total shrinkage %

Rate of shrinkage%/ C

Additive percent %

Start of sintering  C

End of sintering  C

Total shrinkage %

Rate of shrinkage%/ C

850 850 900 930

1050 1000 1200 1200

40 42 32 35

0.2 0.28 0.11 0.23

10 10 5 20

820 810 800 820

920 860 920 920

37 32 30 32

0.37 0.64 0.25 0.32

Sintering characteristics data of fuel ashes with Silica based additive Fuel Ash Sample

Sample Sample Sample Sample Sample

5B Lignite 6 Lignite 7A Lignite 7B Lignite 8 Anthracite

Without additive

With Silica based additive

Start of sintering  C

End of sintering  C

Total shrinkage %

Rate of shrinkage%/ C

Additive percent %

Start of sintering  C

End of sintering  C

Total shrinkage %

Rate of shrinkage%/ C

850 870 870 870 900

1050 1100 1050 1050 1100

38 35 40 40 17

0.19 0.15 0.22 0.22 0.09

5 5 5 10 1

900 900 900 900 970

1100 1100 1070 1100 1100

28 17 26 30 10

0.14 0.09 0.15 0.15 0.08

sintering profile of ash could be modified using a chemical additive. If need be, both techniques are required to be adopted. Sintering profiles with broader peak i.e. profiles extending over a wide temperature range could really pose a problem in selecting the operating regime parameters. Such fuels (for eg. sample-3-Fig. 1), call for injection of additives to alter the sintering characteristics. 4.1. Calcium based additive* for lowering the “start of sintering” temperatures The addition of suitable chemicals as additive to modify the rate of sintering and the quantum of shrinkage provides flexibility in the selection of suitable combustion operating temperature regime, which would reduce the choking at critical locations of the flue gas and fuel path. Hence, the effect of different proportions of calcium additive was taken up for study to change the sintering pattern of ashes which otherwise caused either slow or rapid sintering in CFB boiler operating temperature regime of 800  Ce900  C. The ashes generated from the fuels by standard ASTM D-317404 at 750  C were used to prepare ash pellets for sintering measurements. As the ash pellet was subjected to further heat treatment in the heating microscope, the ash pellet tended to start shrinking around 850  Ce900  C and sintered gradually, either slowly or quickly based on ash constituents, as the temperature increased. The sintering pattern of the ash specimen was illustrated by the “area shrinkage” profile and the maximum shrinkage reached around 950  Ce1100  C. Depending on the ash chemistry, the “rate of ash shrinkage” and the “total shrinkage percentage” compared to the initial ash pellet specimen prepared at ambient temperature varied for different fuel ashes (Table 3). The area shrinkage pattern for fuel ashes (sample-1esample-4) are shown in Fig 1. The sintering of ash “sample-1” and “sample-2” starts around 850  C, whereas ash sample-4 started at 950  C. For ash sample-1 and sample-2, the sintering pattern exhibited the “start temperature of sintering” falling within the operating temperature range of CFB and further it was observed that the significant percentage of sintering (30e35%) also fell within the above operating temperature range itself. Hence, it was clearly seen that the firing of fuel sample-1 and sample-2 would result in agglomeration and choking formation. The composition of the calcium based chemical additive is furnished in Table 2. The ash pellets with calcium additive mixture were also subjected to similar heat treatment and the sintering profiles were

observed. The changes in sintering phenomenon of the ash with and without calcium additive are shown for all the fuel ashes samples 1 to 4 in Fig. 1. The sintering data obtained from the profile viz, start of sintering, end of sintering, % of sintering and rate of shrinkage for the ashes both with additive and without additive are shown in Table 3. The shrinkage profiles with calcium based additive clearly indicated that the start of sintering was shifted to lower temperatures. For “ashes with additive” sample-2 and sample-3, the start of sintering was shifted to lower temperatures of 810  C and 800  C respectively compared to 850  C and 900  C for the ashes without calcium based additive. For the ashes sample-1 and sample-4 also, it was observed that the “start of sintering” got shifted to lower temperatures with calcium based additive. The total quantum of shrinkages was also reduced for “ashes with calcium based additive” and the mean rate of shrinkage (% shrinkage/  C) was also observed to be increased two times for some of the ashes with additive compared to “ashes without calcium based additive”. The maximum rate of shrinkage was also increased with additive (Fig-1) as can be seen from the height of the peak (dA/dT curves). The addition of calcium based additive in sufficient amounts to the fuel increased the rate of sintering during combustion. Such additives could shift the sintering pattern to a lower temperature as well. The shrinkage almost came to an end at the lower temperature as compared to “end temperature of sintering” for fuel ashes without additive. It is clear that the method of mixing additives with fuels would modify the sintering pattern of ash. The shifting of sintering pattern to a lower temperature aids boiler operator/designer to select a suitable combustion temperature range, so that sintering/shrinkage related choking can be averted. For example: for fuels sample-1 and sample-4, the combustion temperature range of 850  Ce920  C with additive, would complete the sintering in the combustor itself and thereby minimize the choking possibilities downstream in the dense phase areas of the fuel and flue gas path. Since the sintering pattern of sample-3 without the additive showed “a wide temperature range”, it is not feasible to select a suitable temperature range for minimizing the shrinkage/sintering without any additive. This innovative method provides a technique for completing a major portion of sintering shrinkage of difficult fuels within the combustor furnace operating temperature range by using additives, so that the further sintering shrinkage and the related choking are minimized or averted in the flue gas path downstream.

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

485

Fig. 1. Area Shrinkage and Rate of Shrinkage (Sintering Profile) curves of samples with Calcium/Alumina based additives.

The effect of calcium additive on the sintering pattern is further confirmed with different samples of lignite, bituminous coal and anthracite (Fig. 1). Calcium as a flux [16,18] causes both chemical and dilution effects resulting in the shift of shrinkage profile to the lower temperature range and the quantum of sintering (total shrinkage %) gets reduced in proportion to the additive quantum added. 4.2. Silica based additive* for increasing the “start of sintering” temperatures Earlier research has developed a correlation and had shown that the ratio of silica to alumina and the sum total of % silica, % alumina by weight in the ash are the guiding factors for classifying the lignite ash for sintering phenomena [1]. Silica as an additive would change the silica alumina ratio in the mineral matter associated with the fuel. It follows in lieu of calcium based additive, silica based additive with fuel could be prepared to alter the ash chemistry. With silica additive, the sintering profile shifts to a higher temperature and the reduction in quantum of total shrinkage also takes place. The shift to a higher temperature along with the reduction in total shrinkage percentage indicates that agglomeration gets reduced and thereby sintering of fuel ash is reduced when

the silica additive is added to the lignite before becoming ash. The reduction in sintering phenomenon of the ash with and without the addition of silica based additive is shown for the fuel ashes (samples-5B, 6, 7 A&B, and 8) in Fig. 2. The sintering data obtained from the profile viz., start of sintering, end of sintering, % of sintering and the rate of shrinkage for the ashes both with additive and without additive are shown in Table 3. The composition of the silica additive is tabulated in Table 2. The sintering of ash sample-5B started at 850  C and it shrunk by 30% by 950  C. The sintering of ash sample-8 started at about 900  C and shrunk by 17% as it reached 1100  C. For the sample-7B ash shrinkage started at 870  C and 30% shrinkage took place between 870 and 950  C, and further 10% shrinkage took place as the sample reached 1100  C. For sample-6 ash, shrinkage started at 870  C and 35% shrinkage took place between 870  C to 1100  C. With the addition of silica additive “sintering start” temperatures were delayed by 50  C for sample-5B (850  Ce900  C) and 30  C for sample-7B (870  Ce900  C) and the quantum of shrinkage were reduced by 10% for sample-5B (from 38% to 28% shrinkage) and by 10% for sample-7B. For samples 5B & 7 (A&B) “sintering start temperatures” fell within the CFB operating temperature regime, (800  Ce900  C) and the addition of 5% silica based chemical

486

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

Fig. 2. Area Shrinkage and Rate of Shrinkage (Sintering profile) curves of samples with Silica based additive.

modifier (sample-7A) was seen quite effective in the field/operating CFB units 2  125 MWe with local lignite. The sintering of ash sample-8 (anthracite) started at about 900  C and it shrunk by 17% as the ash reached 1100  C. For fuel ash sample8 with the addition of silica additive the “sintering start temperature” was delayed by 70  C from 900  C to 970  C and shrinkage quantum came down by 7% from 17% shrinkage to 10% shrinkage. For sample-6 ash shrinkage started at 870  C and 35% shrinkage was noticed between 870  C to 1100  C. For fuel ash sample-6 the addition of 5% silica additive influenced the “sintering start temperature” by 30  C from 870  C to 900  C and the total shrinkage came down to 17%. In all the samples 5B, 6, 7 A&B, and 8 the reduction in the rate of shrinkage %/ C also could be observed up to a maximum of 30%. 4.3. Effect of alumina as additive Alumina is an inert material up to 1600  C. To differentiate the chemical impact of calcium and silica from mass dilution effects, alumina is selected as an additive and added to sample-5A. The shrinkage pattern- area shrinkage and rate of shrinkage of lignite with an addition of 5% by wt alumina is shown in Fig. 1 for sample-

5A lignite. There was no temperature shift in the “start of sintering” and no change in the “rate of sintering”; but it results in some reduction in the quantum of sintering due to the mass dilution effect. The above phenomenon indicates that with alumina as additive, only dilution effect is predominant, whereas with chemical additives (silica based/calcium based) both chemical and dilution effects occur with pronounced chemical reaction effect. 4.4. Wide sintering profile control with additives The effect of the silica based additive and the effect of calcium based additive on the area shrinkage and the rate of shrinkage (sintering profile) are superimposed in Fig. 4 for sample 7B and sample-2 (samples of lignite from the same mine location). The operation of large CFB power stations involves firing large quantities of fuel 100e150 tons per hour for a 125 MWe CFB boiler and the addition of modifiers beyond 10% by wt is not an attractive economic proposal. Fig. 4 shows the “peak sintering temperature” could be either lowered (calcium additive) or increased (silica additive) and the effect with 10% by wt of modifiers studied on sample-2 lignite is 60  C reduction with calcium additive and 30  C increase with silica additive for sample-7B.

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

487

Fig. 3. Cyclone opening and refractory cavity above seal pot in CFB boiler e choked by sintered ash and clean view with addition of 5% silica additive to lignite.

4.5. Field trials The sintering of samples -5B, 6 and 7 A&B is correlated well with the ash chemistry and it has been linked to the silica/alumina ratio and the quantum of oxides of silicon and aluminum [1]. The firing of this lignite resulted in clogging and blockage during the initial operation of the CFB boilers (Fig. 3) at Barsingsar CFB 2  125 MWe in Rajasthan State, India. After confirming the sintering characteristics of the lignite through several laboratory experiments with samples, field trials and demonstrations were taken at this site to modify the ash chemistry by mixing silica additive with lignite in the operating 2  125 MWe CFB units. The quantum of additive must be compatible with the quantum of ash present in the lignite so that the ratio of critical elements could be effectively altered to the desired value in an operating power plant associated with the mine quality variations. During the field trials, additive feed rate change was provided for, to accommodate the variations in the ash content of the lignite. The additives were mixed and fed to the “fuel conveyor” from an additive bunker, before entering the CFB boiler. Thus the fuel with the additive mixed, was fed into the seal pot return leg in the operating CFB unit. With 5% silica addition, the operation of the plant was made quite stable without cyclone choking (Fig. 3). The sintering issue

and the blocking of dense phase at cyclone outlet faced were successfully resolved with the addition of silica additive and the units started operating successfully with high availability. The process with additive is patented by M/s Bharat Heavy Electricals Ltd, India. 5. Conclusions A novel method that will ensure, by adding suitable chemical modifiers in sufficient amounts to the solid fuels, viz., lignite with high sintering characteristics that will effectively reduce the agglomeration and sintering during combustion is developed. Such additives effectively dilute and chemically react which shift the sintering pattern to either a higher temperature (silica based) or a lower temperature (calcium based), and also reduce the rate and the quantum of sintering, resulting in reduced agglomeration and sintering shrinkage that avert chocking in critical locations of CFB boiler. The selected samples of lignite cover a wide range from India and one bituminous coal from abroad and one higher rank (ASTM) coal anthracite. The additive may either be added along with the fuel in the required proportion before fuel preparation in a device so that the fuel plus additive mixture enters the combustor through the fuel feed system or be added into the combustor directly from a separate bunker, where the additive is stored and fed at a suitable rate corresponding to the fuel feed rate to attain the desired fuel and additive proportion. An ideal additive is also expected to be cheap and easy to handle and should not significantly affect the fuel combustion behavior and ash utilization. The additives demonstrated, herein, both silica and calcium are available in plenty for large scale industrial applications economically. With 20% by wt calcium based additive, the peak sintering temperature could be lowered by 140  C (Fig. 1 sample-4). The addition of modifiers beyond 10% in large scale CFB utility application may not be an attractive proposal from an economical point of view. The “peak sintering temperature” could be either lowered (calcium additive) or increased (silica additive) and the effect with 10% by wt of modifiers studied on sample-2 lignite is 60  C reduction with calcium additive and 30  C increase with silica additive for sample-7B (Fig. 4) providing a solution for overcoming agglomeration induced choking and blocking in fuel gas path with high sintering lignites in CFB. Acknowledgements

Fig. 4. Area Shrinkage and Rate of Shrinkage (Sintering profile) curves of samples with 10% Calcium or 10% Silica based additives.

The authors thank the Management of BHEL for the opportunity to present our views through this paper. The views expressed in this article are that of the authors and not necessarily that of BHEL.

488

P. Selvakumaran et al. / Applied Thermal Engineering 67 (2014) 480e488

Patents * details Title 1: A method for modifying the sintering pattern of ash and optimizing combustion temperature range for reducing agglomeration and choking formation resulting from combustion of solid fuels in fuel firing systems- Application No: 217/KOL/2014- PR Number: 130144TP-Dt: 20/02/2014. Title 2: A method for reducing the agglomeration, sintering, shrinkage and choking formation resulting from combustion of solid fuels in fuel firing systems- Application No: 1109/KOL/2011 PR Number: 110051TP–Dt: 24-08-2011–*Patented by BHEL-India/ Agent: L.S.Davar & Co.

References [1] P. Selvakumaran, A. Lawerence, M. Lakshminarasimhan, A.K. Bakthavatsalam, Mineralogical influence of mining intrusions in CFB combustion of Indian lignite-Springer Open, Int. J. Energy Environ. Eng. 4 (2013) 34. [2] A. Lawrence, V. Ilayaperumal, K.P. Dhandapani, S.V. Srinivasan, M. Muthukrishnan, S. Sundararajan, A novel technique for characterizing sintering propensity of low rank fuels for CFBC boilers, Fuel 109 (Jan 2013) 211e216. [3] M.J. Fernandez Llorente, P. Dıaz Arocas, L. Gutierrez Nebot, J.E. Carrasco Garcı, The effect of the addition of chemical materials on the sinteringof biomass ash, Fuel 87 (2008) 2651e2658. [4] Joshua R. Strege, Christopher J. Zygarlicke, Bruce C. Folkedahl, Donald P. McCollor, SCR deactivation in a full scale co. fired utility boiler, Fuel 87 (2008) 1341e1347. [5] H. Haykiri-Acma, S. Yaman, S. Kucukbayrak, Effect of biomass on temperatures of sintering and initial deformation of lignite ash, Fuel 89 (2010) 3063e 3068. [6] Malte Bartels, Weigang Lin, John Nijenhuis, Freek Kapteijn, J. Ruud van Ommen, Agglomeration in fluidized beds at high temperatures: mechanisms, detection and prevention, Prog. Energy Combust. Sci. 34 (2008) 633e666. [7] Kazmierz Mroczek, Sylwester Kalisz, Marek Pronobis, Józef So1tys, The effect of halloysite additive on operation of boilers firing agricultural bio mass, Fuel Process. Technol. 92 (2011) 845e855. [8] J. Werther, M. Saenger, E.-U. Hartge, T. Ogada, Z. Siagi, Combustion of agricultural residues, Prog. Energy Combust. Sci. 26 (2000) 1e27. [9] Hao Wu, Peter Glarborg, Flemming Jappe Frandsen, Kim Dam-Johansen, Peter Arendt Jensen, Dust-firing of straw and additives: ash chemistry and deposition behavior, Energy Fuels 25 (2011) 2862e2873.

[10] Verena E.M. Schmitt, Martin Kaltschmitt, Effect of straw proportion and Caand Al-containing additives on ash composition and sintering of woodestraw pellets, Fuel 109 (2013) 551e558. [11] B. Ersoy, T. Kavas, A. Evcin, S. Baspınar, A. Sarıısık, G. Önce, The effect of BaCO3 addition on the sintering behavior of lignite coal fly ash, Fuel 87 (2008) 2563e 2571. [12] Jiawei Sheng, Bill X. Huang, Jian Zhang, Heng Zhang, Jinyou Sheng, Suwen Yu, Miji Zhang, Production of glass from coal fly ash, Fuel 82 (2003) 181e185. [13] L.N. Satapathy, The physical, thermal and phase identification studies of zirconia- fly ash material, Ceram. Int. 24 (1998) 199e203. [14] Song Mu, Bao-guo Ma, Geert De Schutter, Xiang-guo Li, Yao-cheng Wang, Shou-wei Jian, Effect of shale addition on properties of sintered fly ash, Constr. Build. Mater. 25 (2011) 617e622. [15] Lingxue Kong, Jin Bai, Zongqing Bai, Zhenxing Guo, Wen Li, Effects of CaCO3 on slag flow properties at high temperatures, Fuel 109 (2013) 76e85. [16] H.J. Hurst, F. Novak, J.H. Patterson, Phase diagram approach to the fluxing effect of additions of CaCO3 on Australian coal ashes, Energy Fuels 10 (1996) 1215e1219. [17] Wen J. Song, Li H. Tang, Xue D. Zhu, Yong Q. Wu, Zi B. Zhu, Shuntarou Koyama, Effect of coal ash composition on ash fusion temperatures, Energy Fuels 24 (2010) 182e189. [18] H.J. Hurst, F. Novak, J.H. Patterson, Viscosity measurements and empirical predictions for fluxed Australian bituminous coal ashes, Fuel 78 (1999) 1831e 1840. [19] M. Seggiani, Empirical correlations of the ash fusion temperatures and temperature of critical viscosity for coal and biomass ashes, Fuel 78 (1999) 1121e 1125.

Glossary AFT: ash fusion temperature ASTM: American Society for Testing Materials Al2O3: aluminum oxide BFBC: bubbling fluidized bed combustion CaCO3: calcium carbonate CaO: calcium oxide CaSO4: calcium sulfate CFBC: circulating fluidized bed combustion FBC: fluidized bed combustion GDP: gross domestic product MWe: mega watt electrical NOx: nitrous oxides PFA: pulverized fly ash SO2: sulfur di oxide SiO2: silicon di oxide TGA: thermo gravimetric analysis XRD: X-ray diffraction