Combustion in bubbling fluidised bed with bed material of limestone to reduce the biomass ash agglomeration and sintering

Fuel 85 (2006) 2081–2092 www.fuelfirst.com

Combustion in bubbling fluidised bed with bed material of limestone to reduce the biomass ash agglomeration and sintering M.J. Ferna´ndez Llorente a,*, R. Escalada Cuadrado a, J.M. Murillo Laplaza a, J.E. Carrasco Garcı´a b a

b

Centro de Investigaciones Energe´ticas, Medioambientales y Tecnolo´gicas (CIEMAT), CEDER, Der-Biomass-Solid Biofuels, Ctra. N-111, km 206, 42290 Lubia, Soria, Spain Centro de Investigaciones Energe´ticas, Medioambientales y Tecnolo´gicas (CIEMAT), Avda. Complutense 22, 28040 Madrid, Spain Received 9 November 2005; received in revised form 6 March 2006; accepted 16 March 2006 Available online 21 April 2006

Abstract A bed material of limestone was used in order to reduce/eliminate the tendency for bed material agglomeration and sintering that normally occurs in plants that operate with the traditional silica bed material. Combustion tests were carried out in a bubbling fluidised bed (BFB) combustion pilot plant (1 MWth). Mass balances of the inorganic elements and ash characterisation with respect to bed agglomeration, fouling and emissions were performed in the BFB combustion pilot plant. Limestone bed material with particle sizes between 0.25 and 2 mm, corresponding to a mean fluidisation velocity of 1.2 m/s and at a mean bed temperature of 775 C, were chosen. It has been successfully proven that the limestone bed material eliminates the bed agglomeration. The calcium particles, which escape from the limestone bed material and are adhered on heat exchangers, reduce the sintering of ash deposits on the tubes.  2006 Elsevier Ltd. All rights reserved. Keywords: Agglomeration; Sintering; Limestone

1. Introduction Lignocellulosic biomass is considered a renewable and CO2 neutral energy resource, with a high potential for utilisation in the future for purposes of heat and electricity generation. Increased use of this biomass could have socially beneficial consequences like employment in forest thinning and in cultivation of biomass energy crops. However, slagging and fouling occur in combustion processes, in general, with herbaceous biomasses and some agro-industrial biomasses which have a high content in alkaline elements in their ashes [1–4]. The alkaline compounds are responsible for the partial melting of the ashes at temperatures as low as 700 C. As a result, the ash *

Corresponding author. Tel.: +34 975 281013; fax: +34 975 281051. E-mail address: [email protected] (M.J. Ferna´ndez Llorente). 0016-2361/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.03.018

deposits on the heat exchangers are sintered, decreasing the thermal efficiency of the power plant and obstructing considerably their cleaning. In the worst cases, it leads to mechanical failure of the heat exchangers [5]. A bubbling fluidised bed (BFB) combustor may also be subject to bed agglomeration which ultimately results in thermal plant shutdown [6–9]. Co-combustion and the addition of chemical materials (additives) and/or alternative bed materials for BFB combustion seem to be practical and cheap treatments to reduce the sintering caused by alkaline compounds. The effect of these treatments is to increase the melting point of the biomass ashes. Several authors [2,10–12] have proposed and tested different additives: kaolin, dolomite, limestone, lime, alumina, etc. However, several problems related to its efficacy exist such as the following reported by Miles: the addition of limestone in a BFB combustor burning a mixture of wood and agricultural prunings does

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Nomenclature BIO biomass ash obtained in laboratory at 550 C IB initial bed material FB final bed material (FB) EC ash of the economiser bin MC ash of the multicyclone bin STACK ash of stack sampling (filter and probe)

not prevent troublesome deposits in the convection passes of the boiler [2]. Bed materials for BFB combustion proposed by Werther [11] to replace the traditional silica bed material, which tend to form unwanted low melting point silicates or eutectics [1–3,5–8], are: dolomite, magnesite, ferric oxide, alumina and feldspar. In this paper, limestone was compared with silica utilised as bed materials in a BFB combustion pilot plant (1 MWth) to try to eliminate or reduce the bed agglomeration and sintering tendencies related to combustion of biomass. Mass balances of ash and its elements, and physical–chemical characterisation of the bed materials, deposits and the ashes of the input and the output streams of BFB combustion pilot plant were performed to estimate formation mechanisms of agglomerates and sinters, as well as to obtain a better knowledge of the ash behaviour in the BFB combustion pilot plant. 2. Materials and methods 2.1. Biomasses tested Three biomass fuels were considered in this paper: • Brassica ‘‘Brassica carinata’’: Brassica is a species similar to rape and is one of the most important sources of vegetable oil in the world. Recently, due to the high biomass yield observed in some varieties, there is also interest in developing this species as a new source of solid biofuel crops [13]. • Thistle ‘‘Cynara cardunculus’’: The whole plant was collected. Thistle could have an important future application as an energy crop due to its adaptation to semiarid climatic conditions, e.g. Mediterranean areas [14]. • Almond shell: This biofuel was considered as an example of an agro-industrial biomass that is derived from the food processing industry in many countries.

2.2. Limestone bed material In order to replace the silica bed material, several materials were tested in the laboratory, for the purpose of eliminating or at least reducing the bed agglomeration, slagging

FOUL ash deposited on water cooled tubes placed in the combustor exit streams (fouling area) EC1 ash deposited on tubes placed in the first part of the economiser EC2 ash deposited on tubes placed in the second part of the economiser

and fouling [15]. Materials tested such as lime (CaO) or calcined dolomite (CaO–MgO) were proved very efficient in reducing the sintering. Thus, limestone (CaCO3) was chosen as material to be used as bed in the BFB combustor, considering that CaCO3 is de-carbonated to produce CaO during the combustion process. For each biomass studied, two combustion tests were carried out. The first one with 200 kg of silica and the second one with 200 kg of limestone. Both materials were dried previously. The granular silica and limestone were obtained from Spanish quarries. Silica came from Pra´dena (Segovia) and limestone came from Golmayo (Soria). The assumptions made in this work for selecting the particle sizes of the bed materials were that all the limestone or silica particles contained in the bed were fluidised, taking into account the minimum fluidisation velocity, and that no particle of limestone or silica was elutriated from the bed to the freeboard, according to the terminal velocity. The minimum fluidisation velocity is calculated by means of the following equation [16]: ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 K2 1 K2 Re ¼ þ Ar  ð1Þ K1 2K 1 2K 1 where Re is the Reynolds number and Ar is the Archimedes number. K1 and K2 depend on the porosity and sphericity. Wen and Yu (1996) were the first to note that K1 and K2 stayed nearly constant for different kinds of particles. For fine particles the values recommended by Wen and Yu [17] are K2/2K1 = 33.7 and 1/K1 = 0.0408. The terminal velocity is calculated using the following equation [16]: " #1 18 2:335  1:744/  ut ¼ þ ð2Þ 2 0:5 ðd p Þ ðd p Þ where the non-dimensional gas velocity ðut Þ and the nondimensional particle size ðd p Þ are introduced, / is the sphericity. The ut and d p are functions of the gas velocity (ut) and particle size (dp), respectively, according to ut

ut ¼ h

q2 lðqs qÞg

dp ¼ h

i13

ð3Þ

i13

ð4Þ

d p

qðqs qÞg l2

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where g is the acceleration of gravity, q is the gas density, l is the gas viscosity and qs is the particle density.

version efficiency in the BFB combustion pilot plant was higher than 98%.

2.3. BFB combustion pilot plant and combustion conditions

2.4. Samplings

A scheme of the BFB combustion pilot plant, where the biomass combustion tests were made, is shown in Fig. 1. The biomass supply system basically consisted of a conical bin and a screw conveyor with a variable speed. The biomass discharge, around 200 kg/h, was 30 cm above the distributor with a bed height in shutdown around 30 cm (200 kg of bed material). The furnace was cylindrical in shape, with an inner diameter of 1.1 m and a height of 4 m. The primary air (60% of the total air) was distributed through multiple nozzles on a plate. The bed temperature during the combustion tests was maintained to 775 C ± 75 C. The mean fluidisation velocity during the tests was 1.2 m/s. The duration of each combustion test was approximately 8 h under steady state conditions, with a start up around 2 h. Water cooled tubes were installed in the exit of the combustor (fouling zone) to take samples of the ash deposited on the tubes. The average gas temperatures in that zone was 800 C. An economiser was installed after the combustor, where the flue gases and particles flowed downwards internally through an array of tubes during the first part of the economiser (gas temperature in the range of 800– 350 C), and in the opposite direction during the second part of the economiser (gas temperature between 350 and 200 C). The ash cleaning system consisted of a multicyclone. The gas and residual particles flow was forced towards the stack (average gas temperature of 150 C) by a fan installed after the multicyclone. A continuous analyser was utilised to measure the emission levels of CO, CO2, NO, NOx and SO2. A continuous zirconium oxide cell meter was used for the O2 measurement. The air excess was around 60% in all combustion tests. The carbon con-

In order to obtain data to carry out mass balances of the inorganic elements with relevance for bed agglomeration, fouling and emissions, the following materials have been sampled in the atmospheric BFB combustion pilot plant:

3 4

5

• Biomass (BIO). Several gross samples, more than seven samples, were collected in each one of the combustion tests. Moisture content was carried out in each biomass sample. After this process, all biomass samples were mixed, generating the laboratory sample. • Initial bed material (IB) and final bed material (FB). The sampling of the bed material was carried out before and after each combustion test. • Ash from the economiser bin (EC) and the multicyclone bin (MC). One ash sample was taken after each combustion test. • Stack emissions (STACK). Stack sampling was carried out under isokinetic conditions using US EPA Method 29: ‘‘Determination of Metals Emissions from Stationary Sources’’, which was adapted to capture chlorine and sulphur gases. A stack sample is withdrawn isokinetically from the stack, with particle emissions collected in the probe and on a heated filter, and gaseous emissions collected in solutions. The measurements of weight were carried out in all input and output materials and ashes, including ash of stack sampling. To approach the physical and chemical processes involved in the formation of ash deposits, the following ash layers were sampled (Fig. 1): • Deposits on water cooled tubes placed in the combustor exit streams (FOUL). Point 3 (Fig. 1). • Deposits on tubes placed in the first part of the economiser (EC1) and the second part of the economiser (EC2). Points 4 and 5 (Fig. 1), respectively.

6

BFBC

2.5. Analytical methods

2 Bin

7

Secondary air

Multicyclone

1

Primary air

Economiser Stack

Fig. 1. Scheme of the atmospheric bubbling fluidised bed combustion pilot plant. 1: Bed material, 2: main heat exchanger, 3: water cooled tubes placed in fouling area (FOUL), 4: first heat exchanger of economiser (EC1), 5: second heat exchanger of economiser (EC2), 6: outlets for continuous analysers of gases, 7: stack sampling system.

Laboratory samples of biomass, initial and final bed, economiser ash, multicyclone ash and deposits were obtained by an adequate pre-treatment consisting of dividing, drying, homogenisation and grinding. Analyses of calorific value, volatile matter, ash at 550 C, moisture, bulk density, distribution of particle size, chlorine and ultimate analysis (C, H, N, S) were carried out for biomass samples by internal procedures, which are mainly based on ASTM norms for wood, refuse derived fuels and coal. Chlorine analysis was also carried out by ionic chromatography after bomb combustion for the almond biomass.

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Ash samples collected in the pilot plant and biomass ash at 550 C, were digested in a microwave furnace, and inorganic elements such as silicon, calcium, magnesium, potassium, sodium, sulphur, phosphorous, iron and aluminium were analysed by atomic emission spectrophotometry (ICP) using a THERMO JARRELL ASH simultaneous spectrometer. Particle size, chlorine and ultimate analysis were also carried out for the ash samples following ASTM norms for refuse derived fuels and coal. The ash samples used to determine organic carbon were previously treated in a silver capsule with a HCl acid solution to liberate the CO2. Next, the residue was dried and introduced into the elemental analyser (CARLO ERBA). The inorganic carbon was calculated to be the difference between total carbon and organic carbon. Classical titration, ionic chromatography and atomic emission spectrophotometry (ICP) were carried out to determine the levels of chloride, chlorine and sulphate contained in the solutions of the stack sampling train. Dust X-ray diffraction was utilised to determine the inorganic compounds in the ashes, using a PHILIPS X’PERT-MPD diffractometer. Identification of compounds was made using the diffractometer software with the JCPDS database as a source of reference data. The ash fusibility was based on the changes in shape detected during the heating of a cylindrical pellet of ash (produced at 550 C) from room temperature to 1400 C in an atmosphere of air. The instrument utilised was an optical heating microscope (LEICA). The characteristic temperatures measured were: initial deformation, sphere, hemisphere and fluid, following DIN norms 51730-1998 and DIN 51730-1994. The last norm was only followed to determine the fluid temperature, which is established when the height of the ash pellet is at 1/3 of the initial height. The particle density or the envelope density of the bed material particles was determined by means of a pycnometer of solid materials (MICROMERITICS). The envelope density is the mass of an object divided by its volume, including pores and small cavities in that volume. 3. Results and discussion 3.1. Biomass characterisation The physical and chemical characterisation of the three biomass fuels studied is summarised in Table 1. Almond shell shows a very low ash content (0.94 wt.%) compared with the other two herbaceous biomasses, while the carbon content in almond shell is higher than in brassica and thistle. As a consequence, the calorific value is higher in the almond shell than in the herbaceous biomasses. Brassica has a high relative sulphur (0.49 wt.%) content. The concentration of chlorine (0.78 wt.%) and sodium (0.69 wt.% calculated on biomass sample) in the thistle is high in relation with other solid biofuels. Brassica and thistle were ground to pass a 70 mm sieve. The particle size of thistle and brassica is not possible to

Table 1 Physical and chemical characterisation of the biomasses tested in the BFB combustion pilot plant Unit

Brassica

Thistle

Almond shell

Moisture Bulk density Particle density Ash Volatile Carbon Hydrogen Nitrogen Sulphur Chlorine GCV NCV

wt.% w.b. kg(wet)/m3 kg(wet)/m3 wt.% d.b. wt.% d.b. wt.% d.b. wt.% d.b. wt.% d.b. wt.% d.b. wt.% d.b. MJ/kg d.b. MJ/kg d.b.

7.4 92 Nt 7.7 76.2 46.3 6.1 0.70 0.49 0.41 18.5 17.2

10.1 220 Nt 8.9 76.2 44.6 6.2 0.76 0.17 0.78 17.4 16.1

13.7 400 1010 0.94 81.1 49.7 6.3 0.26 0.01 0.0037 20.0 18.7

Sieve hole

Accu. fraction Distribution of particle size

32+16 16+8 8+4 4+2 2 Arith. mean value

wt.% wt.% wt.% wt.% wt.% mm

Ash composition Al2O3 CaO Fe2O3 K2O MgO Na2O P2O5 SiO2

wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.%

Fusibility temperatures Initial C Sphere C Hemisphere C Fluid C

d.b. d.b. d.b. d.b. d.b. d.b. d.b. d.b.

Nt Nt Nt Nt Nt <70 mma

Nt Nt Nt Nt Nt <70 mma

10.82 86.30 95.49 99.29 100 18.4

1.3 25 0.53 16 2.2 0.72 3.8 8.1

2.3 29 0.87 9.1 3.3 7.7 1.4 12

0.49 16 0.27 31 2.6 0.49 2.4 3.5

1280 Nd Nd >1400

>1400 >1400 >1400 >1400

740 760 860 >1400

GCV, gross calorific value; NCV, net calorific value; Nt, not tested; Nd, not detected. a It was ground to pass a 70 mm sieve.

measure properly due to its morphology filiform and the agglomeration of their fibrous threads. This agglomeration increases while thistle and brassica are being sieved. The initial deformation temperature (IDT) in an oxidising atmosphere has been used to compare the temperature of biomass ash sintering obtained in laboratory with the experimental agglomeration results in a fluidised bed combustor [9]. In this context, almond shell, which has a high potassium content (31 wt.%) in its ash, shows the lowest IDT of the three biomasses considered in this study. Consequently, almond shell could cause agglomeration and sintering problems at temperatures as low as 740 C in the combustion tests performed in the BFB combustion pilot plant. 3.2. Particle size of the bed materials The minimum fluidisation velocity (umf) and terminal velocity (ut) are calculated considering the equations from

M.J. Ferna´ndez Llorente et al. / Fuel 85 (2006) 2081–2092

velocity of the particles (ut). The plot of the ut as a function of the particle size is shown in Fig. 3. With regard to that figure, limestone and silica particle sizes less than 0.22 mm will be elutriated at mean fluidisation velocity of 1.2 m/s. This means that limestone particles calcined to lime with sizes below 0.35 mm will be elutriated towards the freeboard. The lime particles could have a positive effect to reduce the formation of hard sinters. According to the mentioned above, the range of particle sizes is selected between 0.25 and 2 mm for both silica and limestone in the combustion tests. The fluidodynamic particle sizes of the initial and final bed materials in the BFB combustion tests are shown in Table 2. The size of the silica particles tends to increase from the initial bed to final bed. In contrast, the limestone particles tend to reduce in size during the combustion tests. This last effect could be a consequence of the erosion of the bed particles that occurs in the bubbling fluidised bed. The subsequent carryover of these fine calcium particles could also have a beneficial effect to reduce the sintering in the heat exchangers.

1 to 4, and comments described in Section 2.2. Air density and air viscosity are obtained from the bibliography at the mean bed temperature of 775 C. The particle density determined by the pycnometer is 2650 kg/m3 for the granular silica and 2590 kg/m3 for granular limestone. A limestone sample was totally calcined in a muffle furnace, producing lime with a particle density of 1190 kg/m3. The plot of the umf as a function of the particle size is shown in Fig. 2. In contrast to lime, limestone and silica show near plots due to their similar particle densities. Limestone and silica particles with particle sizes around 2.0 mm will fluidise at a mean fluidisation velocity of 1.2 m/s. Consequently, particle sizes of maximum 2.0 mm are utilised for silica and limestone bed materials in ulterior combustion tests. The sphericity of the silica and limestone is obtained from the bibliography [16] and with the help of images obtained using scanning electron microscopy [15]. Accordingly the sphericity is estimated to be 0.86 (round sand) for both, silica and limestone. Variations in the order of one tenth do not produce significant differences in the terminal

SILICA

2085

LIMESTONE

LIME

3.00 2.50

Umf (m/s)

2.00 1.50 1.00 0.50 0.00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Particle size (mm)

Fig. 2. Minimum fluidisation velocity (Umf) as a function of the particle size and bed material, calculated at 775 C and atmospheric pressure.

SILICA

LIMESTONE

LIME

3.00 2.50

Ut (m/s)

2.00 1.50 1.00 0.50 0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Particle size (mm)

Fig. 3. Terminal velocity (Ut) as a function of the particle size and bed material, calculated at 775 C and atmospheric pressure.

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Table 2 Mean particle size of the initial and final bed materials in the BFB combustion tests Brassica

Thistle

Almond shell

Silica Limestone Silica Limestone Silica Limestone Particle size of initial bed Particle size of final bed

0.618 0.532

0.524 0.599

0.524 0.624

0.709 0.542

0.688 0.445

Nt

tests with limestone bed material offer, in general, better results than the combustion tests with silica material. Therefore, limestone bed material reduces the ash sintering in the deposits on the heat exchangers in comparison to silica bed material. The cause of this behaviour will be mainly studied from view point of its ash composition in Section 3.4.

0.604

3.4. Ash analyses

Nt, not tested.

3.3. Bed agglomeration and sintering The sintering degree of the agglomerates found in the bed materials and in the ash deposited on the tubes, as determined by applying visual and ash disintegration assessment of the deposits and agglomerates, is shown in Table 3. The combustion tests using thistle and almond shell generate agglomerates with silica as bed material. Round agglomerates are formed near the biomass feeding point in thistle combustion test, probably due to higher temperature at this point of the silica bed. In the case of almond shells, homogeneous agglomerates formed in a narrow film on top of the silica bed, which caused poor fluidisation and shutdown of the combustion test 2 h after its start up, as a consequence of higher residence time of the almond shell in this area with respect to other biofuels with a lower density and higher porosity such as herbaceous biofuels (Fig. 4). Agglomerates are not observed when limestone is used as bed material, indicating the positive effect of this material to reduce or even avoid the ash agglomeration. As the gas temperature in EC2 is too low to produce sinters, only ash as dust is observed. The ash deposits on the front and back position of the FOUL tube show different sintering behaviour. Baxter comments that mass and heat transfer are much faster on the windward vs. leeward side of the individual tubes of an industrial bubbling fluidised bed combustor. This produces composition changes, i.e. an increase of K, Cl and S in the front side with respect to the back side of the individual tubes, as well as structure changes of the deposits as a function of the position around of the tube [5]. According to the data of the sintering degree of the ash deposited on the tubes shown in Table 3, the combustion

The results of the ash composition for brassica in combustion tests with silica and limestone bed materials are shown in Figs. 5 and 6, respectively. Most of sums of elements expressed as oxides plus chlorine are near 95 wt.%. The content of iron is low (below 3% as Fe2O3) in all samples, except in some deposits’ samples. The highest iron value was found in brassica with silica bed material in the FOUL sample (6.5%). In order to eliminate the effect of the iron resulting from the corrosion of the metal tubes the ash compositions are iron-free. The organic carbon is negligible in the ash deposits and other ash samples. Finally, the ash compositions are normalised at 100 wt.% to improve comparison among ash samples. Silicon is recovered in the economiser bin (EC) of the combustion test with silica bed material (Fig. 5) in a proportion higher than with limestone bed material (Fig. 6). In contrast, calcium is more recovered with limestone than with silica bed material. These results corroborate the carryover of some particles from the bed to the freeboard and other parts of the pilot plant such as the economiser bin. The ashes of the stack emission for both bed materials show a very important increase in potassium, chlorine and sulphur. XR diffraction of the stack ash for brassica combustion test with limestone bed material (Table 4) shows that these elements are mainly forming KCl (sylvite) and K2SO4 (arkanite). Elements such as K, S and Cl are usually enriched in the ash deposits of FOUL, EC1 and EC2 when they are compared with ash biomass (BIO), according to Fig. 5. Sylvite and arkanite are the major alkaline compounds in these ash deposits. The major proportion of the phases found in the final bed, bins, deposits and stack ash correspond to the phases of the compounds found in the biomass (BIO) fed to the pilot plant and in the initial bed material such as the above mentioned KCl and

Table 3 Degree of sintering in the agglomerates found in the final bed material and in the ash deposited on the different tubes of the BFB combustion pilot plant Location

Brassica

Thistle

Almond shell

Silica

Limestone

Silica

Limestone

Silica

Limestone

FB

Agglomerates

Nd

Nd

Weak sinter

Nd

Weak sinter

Nd

FOUL

Frontside Backside

Weak sinter Dust

Dust Dust

Weak sinter Dust

Weak sinter Dust

Dust Dust

Dust Dust

Weak sinter Dust

Dust Dust

Weak sinter Dust

Dust Dust

Dust Dust

Dust Dust

EC1 EC2 Nd, not detected.

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Fig. 4. Photographs of the final bed materials: (a1) thistle combustion test with silica bed, (a2) thistle combustion test with limestone bed, (b1) almond shell combustion test with silica bed (a portion), (b2) almond shell combustion test with limestone bed.

Fig. 5. Analyses of elements in the ash samples obtained from the brassica combustion test with silica bed material. Symbols are defined in Section 2.3.

K2SO4, and others such as CaCO3, Ca(OH)2, Ca5(PO4) OH, MgO and SiO2. Calcium sulphates are found in EC, MC, FOUL, EC1 and EC2, which are formed by chemical reaction among O2, SO2 and CaO (solid). This chemical

reaction is widely utilised for eliminating sulphur dioxide of the emissions in coal combustion by means of limestone. Thistle combustion test with limestone as bed material (Fig. 7 and Table 5) presents the same general comments

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Fig. 6. Analyses of elements in the ash samples obtained from the brassica combustion test with limestone bed material. Symbols are defined in Section 2.3.

Table 4 Major (M) and minor (X) compounds detected by XR diffraction in the brassica combustion test with limestone bed material Phases

BIO

IB

FB

EC

MC

STACK

FOUL

EC1

EC2

CaCO3 Ca(OH)2 CaO CaSO4 CaSO4,0.5H2O Ca5(PO4)3OH KCl K2SO4 K3Na(SO4)2 K2Ca2(SO4)3 MgO SiO2 KAlSi3O8 Ca(Mg,Al)(Si,Al)2O6

M

M M X

M M M

M M M

M M M M

X

M X

X M X M X

M X M

X M M X

M X X M X X M M

M X X M X X M M

X X X

X X

X X

X X

X X M

X X X X

X X M X

X X X

X X X

X M X

X M

M X M X X X X X

X

Symbols are defined in Section 2.3.

as above mentioned related to elements and compounds as a function of the ash sample location. Thistle possesses a high content of potassium as KCl and K2Ca(CO3)2 and even sodium in the form of NaCl and K3Na(SO4)2. Alkaline compounds are directly condensed on the cool surfaces of the metal tubes, or they can also condense on other particles which are subsequently deposited on the tube surface by means of thermophoresis (fine particles, below 10 lm) and inertial impaction (ground particles) [18]. The alkaline compounds contained in the thistle such as KCl, NaCl and K3Na(SO4)2 are melted in the bed and freeboard and they are the main solid compounds emitted by the stack. Fairchildite (K2Ca(CO3)2) does not appear in any ash sample collected and depicted in Table 5, consequently, it can react to form other compounds and/or not to be detected by XR diffraction.

The initial bed of limestone contains CaCO3 and Ca(OH)2 as major phases and CaO as a minor phase in the BFB combustion tests of brassica (Table 4) and thistle (Table 5). In the brassica combustion test, the initial bed of limestone was recovered of a previous combustion test burning the same biomass to verify the behaviour of the limestone bed material at longer duration of the test. This is the cause of detecting other minor phases in the initial bed of the brassica test (Table 4). The final beds of limestone contain silicates but they are compounds with high melting point such as Ca2SiO4 of 2130 C (Table 5), and moreover, are found as minor compounds. Other reactions applied to the limestone bed material as well as the ash deposits are de-carbonation of CaCO3, carbonation of CaO, de-hydration of Ca(OH)2 and hydration of CaO, all these reactions are a function of the ash temperature,

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Fig. 7. Analyses of elements in the ash samples obtained from the thistle combustion test with limestone bed material. Symbols are defined in Section 2.3.

Table 5 Major (M) and minor (X) compounds detected by XR diffraction in the thistle combustion test with limestone bed material Phases

BIO

IB

FB

EC

MC

STACK

FOUL

EC1

EC2

CaCO3 Ca(OH)2 CaO CaSO4 Ca5(PO4)3OH KCl K2SO4 K2Ca(CO3)2 K3Na(SO4)2 MgO NaCl SiO2 Ca2SiO4

M M

M M X

M M M

M M M X X X

M M M X X X

X

M M X X X M

M M M X X X

M M M X X X

X X X M X

X X X M X

X X M X X

X X X X X

X X X X X

X M

X

M

X X X X M M

X X X

M M

Symbols are defined in Section 2.3.

i.e., location of the ash sample in the BFB combustion pilot plant and transversal position within the ash layer deposited on the heat exchangers. The results obtained in the almond shell combustion test with limestone as bed material are shown in Fig. 8. The collected ash samples contain a very large amount of calcium. An explanation of this fact is that the contribution of the almond shell ash on the ash deposits is reduced due to the low ash content of the almond shell (0.94 wt.%) when it is compared with the influence of the carryover of calcium particles that escape from the bed material. The de-carbonation degree of the limestone to lime is high in the limestone bed material, although it is not total because there is a small percentage of CO2 in the final bed for the three biomass considered (Figs. 6–8).

An explanation of the good behaviour of the limestone bed material compared with the silica bed material is due to the dilution effect which is caused by adsorption of alkaline salts on the surface of the pores of the material [10]. Considering that lime (CaO) and portlandite (Ca(OH)2) are more porous than limestone [19], and taking into account the hypothesis offered by Steenari [10], then, the mechanism to reduce ash sintering based on the dilution of the biomass ash is consistent in this regard. It is important to note that the content of calcium in the ash deposits (FOUL, EC1 and EC2) is higher in the combustion test with limestone bed material than with silica bed material (Figs. 5 and 6). In this context, the increase of calcium concentration and the corresponding lime and portlandite compounds can be the cause of the reduction of ash sintering on the heat exchangers.

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Fig. 8. Analyses of elements in the ash samples obtained from the almond shell combustion test with limestone bed material. Symbols are defined in Section 2.3.

3.5. Mass balances In order to reach data for analysing the behaviour of the inorganic elements in a BFB combustor, mass balances of the ash and its inorganic elements are calculated. The input streams in the BFB combustion pilot plant are the biofuel feed and the initial bed, and the output streams are the final

bed, economiser ashes, multicyclone ashes and the stack emissions (particles and gases). The amounts and recoveries of the mass balances for brassica and thistle combustion tests with limestone bed material are shown in Tables 6 and 7, respectively. These combustion tests are considered as representative examples of all combustion tests commented in this paper, i.e., these

Table 6 Mass balance of ash and major and minor elements in the brassica combustion test with limestone bed material Biomass

Initial bed

Final bed

149

200

205 59

13 4

13 4

23 7

27

Ca (g dry) Ca (wt.%)

28,139

69,451

67,255 69

3926 4

4648 5

587 1

22

K (g dry) K (wt.%)

20,211

8424

14,117 49

1184 4

991 3

8202 29

14

Mg (g dry) Mg (wt.%)

2027

1406

2097 61

261 8

275 8

35 1

22

Na (g dry) Na (wt.%)

829

445

686 54

68 5

55 4

350 27

9

P (g dry) P (wt.%)

2585

754

1972 59

381 11

356 11

63 2

17

S (g dry) S (wt.%)

9834

3371

6187 47

527 4

590 4

4371 33

12

Cl (g dry) Cl (wt.%)

8229

1720

1681 17

557 6

317 3

4427 44

30

Si (g dry) Si (wt.%)

5877

9584

14,080 91

689 4

408 3

67 0

1

Al (g dry) Al (wt.%)

1044

1581

2028 77

133 5

103 4

5 0

14

Ash (kg dry) Ash (wt.%)

Comments on the calculations are offered in Section 3.2.

Economiser

Multicyclone

Stack

Closure

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Table 7 Mass balance of ash and major and minor elements in the thistle combustion test with limestone bed material Biomass

Initial bed

Final bed

158

200

178 50

19 5

8 2

23 6

36

Ca (g dry) Ca (wt.%)

32,371

74,010

72,749 68

5910 6

2267 2

220 0

24

K (g dry) K (wt.%)

11,965

123

2491 21

838 7

324 3

5804 48

22

Mg (g dry) Mg (wt.%)

3154

800

2426 61

353 9

164 4

20 1

25

Na (g dry) Na (wt.%)

8984

57

4208 47

526 6

216 2

2214 24

21

P (g dry) P (wt.%)

960

0

469 49

122 13

55 6

15 2

31

S (g dry) S (wt.%)

3011

777

1688 45

273 7

124 3

1682 44

1

Cl (g dry) Cl (wt.%)

13,814

0

1709 12

1022 7

272 2

8315 60

18

Si (g dry) Si (wt.%)

8891

1819

5379 50

1010 9

448 4

66 1

36

Al (g dry) Al (wt.%)

1891

236

804 38

215 10

117 6

20 1

46

Ash (kg dry) Ash (wt.%)

Economiser

Multicyclone

Stack

Closure

Comments on the calculations are offered in Section 3.2.

mass balance results are very similar to the combustion tests with silica bed material [15]. The closures in some elements performed for the almond shell mass balances with limestone bed are negative due to the contamination of ashes retained on the walls of the pilot plant and main heat exchanger from the previous combustion test, thistle with limestone bed. In the almond shell test only 18 kg of ash were fed to the combustor compared with 158 kg of the thistle test. Relative element and ash removals in different parts of the pilot plant and closure are calculated taking into account the corresponding total input streams as a base of calculation. The closure is calculated according to the following equation: Closure ¼

ðinput streams  output streamsÞ  100 input streams

As is shown in Tables 6 and 7, the bed material is a sink of biomass ash and its elements, especially the less volatile elements such as Si, Al, Ca, Mg and P. Other elements with a more volatile character such as K and Na are often retained in the bed material with recoveries higher than 45 wt.%. Even elements considered to be volatile (S and Cl) are retained in the limestone bed material in a high quantity with recoveries higher than 40 wt.% and 10 wt.% for sulphur and chlorine, respectively. This is a consequence of the adhesion or capture to the bed material particles of the fraction of the alkaline compounds nonvolatilised or released as was commented in Section 3.4. Chlorine and sulphur are the elements most emitted by the stack of all elements studied in this paper, followed by

potassium and sodium, and finally, the rest of the elements: P, Ca or Mg, Al and Si. Consequently, the following tendency is noticed: the higher the recovery of an element in the bed is, the less the emission of that element by the stack. These results for the elements also are in agreement with what is described in Section 3.4 concerning the compositions of the ash during its transport from the bed to the stack. The ash closure (27 wt.% and 36 wt.% for brassica and thistle tests, respectively) is higher than the sum of the ashes recovered in the economiser and multicyclone bins together with the ash emitted by the stack, indicating that a large amount of ashes are retained within the BFB combustion pilot plant. By recalculating the closure, and considering a total loss of CO2 from the initial limestone to the gas emissions, then, the closure decreases, to 22 wt.% and 26 wt.% for brassica and thistle tests, respectively, although this value is still high. The retention of these ashes within the pilot plant is due, in general, to ground particles that impact on the heat exchangers placed into the freeboard as well as to ground particles deposited by gravity in some conduits of the pilot plant; and to fine particles and gases that are mainly adhered to cool surfaces by thermophoresis and condensation, respectively. 4. Conclusions • Limestone bed material, with a selected range of particles between 0.25 and 2 mm for a fluidisation velocity of 1.2 m/s, has been successfully proved to eliminate the ash agglomeration with biomasses of thistle and

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almond shell which produce agglomerates in BFB combustor tests with silica as bed material under the same operational conditions. • Combustion tests with limestone as bed material reduce the ash sintering in the deposits on the heat exchangers in comparison to silica bed material. • No important chemical reactions are detected in the limestone bed material in relation with the bed agglomeration. Consequently, an explanation of the good behaviour of the limestone bed material compared with the silica bed material is due to the dilution effect which is caused by adsorption of alkaline salts on the surface of the lime and portlandite pores. This explanation can also be applied to ash deposits, considering the higher particle flow of CaCO3, CaO and Ca(OH)2 that escape to the heat exchangers from the limestone bed material, compared with silica bed material. • The limestone bed material is a sink of lignocellulosic biomass ash and its elements, even with elements considered to be volatile (sulphur and chlorine).

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