A pilot-scale fireside deposit study of co-firing Cynara with two coals in a fluidised bed

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Fuel 87 (2008) 58–69 www.fuelfirst.com

A pilot-scale fireside deposit study of co-firing Cynara with two coals in a fluidised bed Martti Aho

a,*

, Antonia Gil b, Raili Taipale a, Pasi Vainikka a, Hannu Vesala b

a

a VTT, P.O. Box 1603, FIN-40101 Jyva¨skyla¨, Finland CIRCE, Universidad de Zaragoza, Marı´a de Luna, 3 50018 Zaragoza, Spain

Received 25 September 2006; received in revised form 20 March 2007; accepted 21 March 2007 Available online 4 May 2007

Abstract Costs of biofuel production from energy crops can be reduced by applying the crop residues in heat and power production. Perennial herbaceous crops like Cynara cardunculus L. are challenging fuels because they tend to have high ash and chlorine contents. Coals, however, are often rich in aluminium silicates and sulphur, and co-firing of these biofuels with coal could be expected to reduce operational problems. In addition, CO2 emissions are lower than during coal firing alone. Blends of Cynara and two coals, South African bituminous and Spanish sub-bituminous coal, were combusted in a 20 kW bubbling bed pilot reactor to ascertain the ability of the coals to reduce operational problems by alkali capture. The Cynara fuel sample contained almost 2 wt% chlorine. The South African coal was rich in kaolinite capable of capturing alkalies from chlorides to produce alkali aluminium silicate and HCl. The Spanish coal was rich in sulphur (mostly present as FeS2), and produced high concentrations of SO2 that partially oxidised to SO3. The SO3 can capture alkalies from chlorides by sulphation. Up to 30% Cynara, on energy basis, could be co-fired with Spanish coal without operational problems, whereas the same percentage of Cynara with South African coal led to strong Cl deposition. Co-firing of Cynara with both coals resulted in high HCl emissions (up to 1500 mg/Nm3 in 6% O2). In addition, co-firing of the Spanish coal led to very high SO2 emissions (up to about 16,000 mg/N m3 in 6% O2). Thus, a power plant capable of firing such blends must be equipped with flue gas cleaning equipment for effective SO2 and HCl capture in the flue gas channel after the superheaters, or else the quality of the Cynara must be markedly improved by changing the harvesting technology and fertilisers, which could be major sources of high ash and chlorine content in the fuel.  2007 Elsevier Ltd. All rights reserved. Keywords: Fluidised bed; Co-firing; Corrosion; Cynara cardunculus

1. Introduction The use of energy crops for co-production of biofuel, heat and power is a promising way to reduce net CO2 emissions in the atmosphere and exploit local renewable energy sources as fuels. At present, poor availability and high cost make the use of energy crops in biofuel-based energy production uneconomical. Fuel flexibility can decrease operational risks and, at the same time, probably reduce the operating costs, increasing efficiency of the total chain. Cynara (Cynara cardunculus L.) is a potential low-cost energy crop for the *

Corresponding author. Tel.: +358 40 558 6945; fax: +358 20 722 2597. E-mail address: martti.aho@vtt.fi (M. Aho).

0016-2361/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.03.046

production of biodiesel. It is an herbaceous perennial plant, well adapted to dry areas, and capable of producing 15–25 tonnes of biomass per hectare and year [1]. Seeds comprise 8–11 wt% of the plant, and the oil content is 25 wt% (dry) at average [2,3]. Cynara oil resembles sunflower oil in composition, and it can be used in biodiesel production [3,4]. Although the oil yield per hectare is lower than that of sunflower, the rest of the biomass can be applied in heat and power production or as a raw material for paper pulp production [4–7]. Such an extended use of Cynara would enable oil production at competitive costs [2]. Despite its energy potential, Cynara has not yet been developed for heat and power applications. Its use as a solid fuel has been tested in EU projects investigating

M. Aho et al. / Fuel 87 (2008) 58–69

59

Nomenclature BFB bubbling fluidised bed CYN Cynara d.f. dry fuel DR dilution ratio ELPI electrical low pressure impactor ESP electrostatic precipitator FTIR Fourier transform infrared spectroscopy IC-ICAP Ion chromatography-inductively coupled argon plasma i.d. inner dimensions LPI low pressure impactor MBM meat and bone meal

pyrolysis, grate and fluidised bed combustion and gasification at laboratory scale ([8–14]) and in crop development projects in Mediterranean countries such as Spain, Italy and Greece ([15–17]). Herbaceous biomasses, like cereal straws, contain silicon (0.5–15 wt% d.f), potassium (1–2 wt% d.f.), calcium (0.1–5 wt% d.f.), sulphur (0.1–0.5 wt% d.f.) and chlorine (0.2–2 wt% d.f.) as their main inorganic components, although in young plants potassium content may be as much as 5 wt% d.f, and in mature leaves calcium concentration may exceed 10 wt% d.f. [18]. A rapid metabolism and different organic structure cause herbaceous biomasses, in general, to have higher ash content than biomass fuels such as wood. In addition, the ash content of herbaceous biomass may depend on the harvesting period and on the harvesting technique employed [19]. Another source of minerals is soil contamination, which may be enhanced by heavy rainfall, dry and windy conditions releasing dust, and a harvesting process that also collects some soil [18]. Further, soil type influences the mineral composition of the biomass ash: clay soils tend to produce biomass ashes with higher silica content than sand soils. According to field results, chlorine content of herbaceous biomass correlates strongly with the KCl fertiliser dosage. Use of chlorine-free fertilisers could thus be an effective method to reduce the Cl content of biomass. A similar benefit could be achieved by delaying the harvest whenever possible, to allow chlorine to be leached out by rain [20]. Ash content of Cynara varies between 4% and 17% on dry basis, depending on the harvesting method [21–23]. High heating value (HHV) varies correspondingly, in the range of 15–18 MJ/kg of dry biomass. The ash composition of herbaceous biomasses may also vary with the part of the plant. Stems have the lowest and leaves highest ash content. This behaviour is detected in Cynara biomass, where nearly 5% ash content in weight has been found in stem and as much as 11% in leaf [21,22]. The seed head (capitula) of the Cynara plant has the highest ash content, about 15%, but this will not cause problems since the seeds are harvested separately for oil production.

MFC mass flow control RDF refuse-derived fuel SAC South African coal SEM-EDS scanning electron microscopydispersive spectrometer SPA spanish coal Tgas temperature of flue gases Tsurface temperature at probe surface w.s. water soluble XRD X-ray diffraction XRF X-ray fluorescence

energy

During combustion, fuel-bound chlorine is partly released to gas phase to HCl and alkali chlorides, mainly potassium chloride (KCl) and sodium chloride (NaCl) [24]. Alkali salts and Cl in the fly ash lower ash melting temperatures and increase ash stickiness, strengthening deposition on the superheaters. Chlorine-rich deposits tend to be highly corrosive to superheaters at metal temperatures higher than 450 C [24], which are usual in steam power plants. With its high chlorine and alkali contents, Cynara can be expected to cause severe operational problems and shutdowns if burned alone in power plants. High concentrations of Si, K and Ca can lead to slagging and agglomeration [25]. Operational problems originating in the high Cl concentration of Cynara may be solved with the aid of protective elements naturally present in coals. At the same time, cofiring Cynara with coal would significantly reduce the CO2 emissions of coal-fired power plants, without large investments. Many types of biomass (herbaceous, woody and animal wastes and municipal solid wastes) could be combined with a wide range of coals, and combusted in a variety of boilers [26,27]. Several co-firing projects are currently underway in the EU. Clean wood waste is an excellent fuel with low ash and alkali concentrations, and several commercial-scale co-firing demonstration tests have been completed without deposition problems up to 10% biomass on energy basis [26–28]. According to Danish tests, straw (an herbaceous biomass) can be co-fired with coal up to 20% on energy basis without severe deposition or corrosion problems [29]. Fluidised bed combustion is a good technology for burning low-grade fuels. Uniform bed temperatures, long residence time of particles and good mixing of bed material result in high mass and heat transfer coefficients, leading to high combustion efficiencies with low emissions. Fluidised bed boilers enable the use of a wide variety of fuels. In our earlier studies [30–32], large portions of Cl-rich biomass (meat and bone meal (MBM) and refuse-derived fuel (RDF)) have been co-fired with selected coals without operational problems. The sulphur and aluminosilicates

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M. Aho et al. / Fuel 87 (2008) 58–69

present in coal are able to capture alkalies from alkali chlorides and release HCl, preventing Cl condensation on superheaters as alkali chlorides. HCl does not bring chlorine to the deposits. The key reactions are sulphation (Eq. (1)) and alkali aluminium silicate formation (Eq. (2)): 2MCl + SO2 + 1/2O2 + H2 O ! M2 SO4 + 2HCl

ð1Þ

where M is Na or K, and Al2 O3  2SiO2 + 2MCl + H2 O ! M2 O  Al2 O3  2SiO2 + 2HCl ð2Þ These reaction mechanisms were experimentally confirmed under bubbling fluidised bed (BFB) and circulating fluidised bed (CFB) conditions [30,33]. Clear differences were found in the protecting powers of coals. The alkali capture appeared to be dominated by reaction 2. Increased kaolinite (Al2Si2O5(OH)4) and decreased alkali contents in the coals improved the alkali capture power and allowed larger contents of Cl-rich biomass in co-firing without Cl deposition [30]. The purpose of this work was to find means to prevent chlorine deposition during combustion of a demanding biomass (Cynara) with help of the protective elements present in coals. Combustion experiments were carried out in a 20 kW BFB reactor. Advanced analytics were used (leaching and XRD analysis of fuels and elemental analysis of fly ash collected at the impactor stages). Use of impactors required an advanced sampling method to minimise changes in the flue gas and in the fine fly ash particles in the sampling line. Deposits were collected on two probes simulating conditions at different superheater zones in a boiler furnace and analysed at several locations by SEMEDS. 2. Experimental 2.1. Fuels and their composition The fuels were South African bituminous coal (SAC), Spanish sub-bituminous coal (SPA) and Spanish pelletized C. cardunculus L. (CYN). The Cynara sample, composed of stems and side branches from the plantation of Encı´n (Guadalajara, Spain), was mechanically harvested [21,22], stored and pelletized. The coals were milled and sieved to particle size less than 1 mm, and Cynara was crushed to particle size less than 5 mm. The main results of the fuel analyses are shown in Table 1, where critical molar ratios of the elements have also been presented. High Ca/S ratio (>3) can indicate effective autocapture of SO2, High S/Cl ratio (>4) can indicate effective sulphation (e.g. [1,24]), high Al/Cl ratio can indicate effective alkali aluminium silicate formation (e.g. [2]) if a significant portion of aluminium is present in active kaolinite [30,31]. (Na + K)/Cl > 1 indicates excess of alkalies to form alkali chlorides. Cynara sample was strongly enriched with chlorine (Table 1). Although the KCl fertiliser is assumed to be the main Cl source, halophytic behaviour has been

Table 1 Fuel analyses of Cynara and the coals Parameter Proximate analysis, wt% Moisture, as received Volatile matter, dry solids Ash at 550 C, dry solids Ash at 815 C, dry solids

Cynara

Spanish coal

South African coal

12.4 63.8

21.3 31.0

5.2 26.7

19.0

33.9

16.1

17.5

32.7

15.4

Ultimate analyses, wt%, dry solids (wt% Carbon 39.4 (47.8) Hydrogen 4.7 (5.7) Nitrogen 0.98 (1.19) Sulphur 0.16 (0.19) Chlorine 1.76 (2.13) Potassium 2.16 Sodium 1.67 Calcium 2.16 Iron 0.25 Aluminium 0.45 Silicon 2.52 Ca/S (molar) 10.8 S/Cl (molar) 0.10 Al/Cl (molar) 0.34 (Na+K)/Cl (molar) 2.6 Heating value, dry solids HHV (MJ/kg) 15.57 LHV (MJ/kg) 14.54

in dry ashless solids) 45.8 (68.1) 70.4 (83.2) 3.1 (4.7) 4.0 (4.7) 0.61 (0.91) 1.6 (1.9) 8.0 (11.9) 0.59 (0.70) 0.01 (0.015) 0.03 (0.04) 0.45 0.15 0.06 0.04 1.30 1.20 3.70 0.68 4.8 2.8 6.00 3.30 0.13 1.6 880 22 630 120 50 7 18.88 18.21

28.3 27.44

reported and some Cl absorption from soil is also probable [34,35]. Good adaptation in semiarid lands allows the plant to absorb a diversity of salts from soil. Chlorine creates problems for example during combustion of straw with about 0.5 wt% Cl in dry fuel [36]. Molar ratio (Na + K)/Cl is typically 1 for biomass. Therefore, there is excess of alkalies for reactions with chlorine to produce vaporisable alkali chlorides, capable of reacting with bed material and causing strong chlorine deposition on superheaters if not destroyed before the superheater zone. Ash fusibility tests were performed on Cynara according to ASTM D1857-03 in reducing atmosphere. Like straw and other herbaceous biomasses, Cynara had rather low ash melting temperatures compared to typical coals [37]: the initial deformation temperature for Cynara ash was 1060 C, while the hemisphere and fluid temperatures were 1130 C and 1180 C, respectively, all measured under inert conditions. As expected, Cynara, as biomass, contained much more volatile material than the coals. Of the two coals, Spanish coal was more heavily enriched with minerals (as much as 33 wt%), which lowered the heating value markedly. The high ash content will lead to high rates of fly ash flow. And it also led to rapid increase of bed mass and change in the bed composition. The ash composition of Cynara differed sharply from that of the coals. The aluminium silicates made up about one-third of the mass of ash (calculated from Table 1), and the portion of risky elements (Na + K + Cl) was

M. Aho et al. / Fuel 87 (2008) 58–69

5.6 wt% d.f. In contrast, the coals were rich in aluminium silicate, and the sum concentration of risky compounds (Na + K + Cl) was only 0.5 wt% d.f. for SPA and 0.22 wt% d.f. for SAC. Spanish coal has high sulphur content and its Ca/S ratio is 1 indicating weak autocapture of SO2 during combustion. Sulphation of alkali chlorides will occur effectively if the fuel-S is oxidised to SO3 in significant amounts in the furnace [42]. Instead, Ca/S ratio of Cynara is high (10.5) predicting strong autocapture of SO2 and potential to capture also SO2 produced from other fuel components in blends. 2.2. Combustion experiments An electrically stabilised 20 kW bubbling fluidised bed (BFB) was used for the research, (see Fig. 1) [38]. Inner

61

dimensions (i.d.) of the bed area are as follows: height 0.55 m and diameter 0.16 m. This zone was joined to the freeboard, height 3.5 m and diameter 0.23 m (i.d.). The electric heaters controlling the wall temperature are shown in Fig. 1. The lowest one surrounds the bed. Secondary air inlet is shown in Fig. 1 and the tertiary air inlet used was the lowest of the three optional inlets shown in the figure. The bed material was sand of particle size 0.1–0.6 mm, mean diameter 0.33 mm and composition (wt%) Na2O 3.0, K2O 2.3, MgO 0.59, CaO 2.3, Al2O3 11.8, Fe 1.4 and SiO2 77.5. Mean gas velocity in the reactor was about 0.5 m/s, which meant a total residence time of 7–8 s. Air staging was kept constant (prim./sec./tert. 50:30:20). Table 2 describes the composition of the blends in the combustion experiments. The percentage values are based on energy contribution. The molar ratio of (Na + K)/Cl

Fig. 1. Experimental test rig.

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M. Aho et al. / Fuel 87 (2008) 58–69

Table 2 Bubbling fluidised bed combustion tests Test

Code

% en.b. % m.b. CYN

% en.b. % m.b. SAC

1

20CYN-SAC

2

30CYN-SAC

3

50CYN-SAC

4

100SPA

20.0 34.3 29.9 47.1 49.1 66.8 0

80.0, 0.0 65.7 70.1 52.9 50.9 33.2 0

5

10CYN-SPA

6

20CYN-SPA

7

30CYN-SPA

9.3 10.1 16.9 18.2 28.7 30.6

0 0 0

% en.b. % m.b. SPA

Ca/S molar

S/Cl molar

Al/Cl molar

(Na + K)/Cl molar

0

2.77

0.78

4.12

2.70

0

3.42

0.51

2.63

2.65

0

4.87

0.28

1.36

2.60

100 100 90.7 89.9 83.1 81.8 71.3 69.4

0.13

880

630

0.15

43

31

4.9

0.18

22

16

3.8

0.22

11

8

3.2

50

CYN, Cynara; SAC, South African coal; SPA, Spanish coal; en.b., energy basis; m.b. mass basis.

2.3. Analyses of fuels, flue gases, fly ashes and deposits Proximate, ultimate and major ash constituents were analysed by X-ray fluorescence (XRF) from the ashes obtained at 550 C. Some key minerals were determined semi-quantitatively by X-ray diffraction (XRD). Sulphur was determined by ASTM D 4239 and chlorine by ASTM D 4208 or Ion Chromatography (IC-ICAP). Dry flue gas was analysed (for O2, CO2, CO, NO, SO2) with standard on-line analysers, and wet and hot (180 C) flue gas (for CO2, CO, NO, NO2, CH4, SO2, H2O, HCl) by Fourier Transform Infrared Spectroscopy (FTIR). Deposits were analysed by SEM-EDS. 2.4. Sampling technology Two air-cooled deposit sampling probes were placed in the test rig. The first, with a surface temperature of

8 7

Residence time [s]

6 5

20CYN-SAC 30CYN-SAC

4

50CYN-SAC 3 2 1 0 600

700

800

900

1000

Temperature [ºC] 8 7 6

Residence time [s]

was 2.6–50 indicating excess of alkalies in relation to chlorine in all the blends (to form alkali chlorides). Ca/S molar ratio indicated SO2 autocapture potential in CYN-SAC blends and lack of this potential in CYN-SPA blends. Bed temperature was kept relatively low (<860 C, Fig. 2) and the oxygen concentration of the fluidising air was lowered to 15 vol%, simulating flue gas recirculation, to prevent bed agglomeration. The freeboard temperature varied and typically the hottest areas were close to the secondary and tertiary air inlets (Fig. 2). In most cases, temperature profiles were typical for the BFB boiler. In two cases (20% and 30% Cynara with Spanish lignite) a temperature drop was measured just above the bed. Since the low temperature prevailed only briefly, however (<0.5 s, Fig. 2) we suggest that in these cases, too, the conditions simulated rather well a full-scale BFB. The upper half of the freeboard of the BFB reactor was designed as a wellcontrolled zone for various samplings and observations, and the residence time in that zone is thus relatively high (Figs. 1 and 2).

5

100SPA 10CYN-SPA

4

20CYN-SPA 30CYN-SPA

3 2 1 0 600

700

800

900

1000

Temperature [ºC]

Fig. 2. Temperature distributions vs. residence time in the experiments. For the codes see Table 2.

500 ± 10 C, was placed in the upper part of the freeboard, where the gas temperature was 900 ± 10 C. The second, at lower temperature (Tsurface = 420 ± 10 C), was placed in the flue gas channel at Tgas = 660–670 C. Total deposit collection time was 3 h. Deposits were sampled at

M. Aho et al. / Fuel 87 (2008) 58–69

63

Dilution air

Ejector ELPI

Cyclone

T 800-1000°C

VTT diluter

Ejector Cyclone

MFC LPI Flue gas

Dilution N2

FTIR & CO2

Fig. 3. Scheme of particle sampling with the LPI and ELPI. (MFC: mass flow control).

three probe locations: wind side, 50 from wind side, and lee side. Samples of fine fly ash (<4 lm) were collected and particle size distributions were measured with low pressure and electrical low pressure impactors (LPI and ELPI). A scheme of the sampling arrangement is shown in Fig. 3. Sampling flow rates were 10 l per min. The measurements were made at two BFB locations: at the hotter deposit probe and the cooler deposit probe. A two-stage dilution system was used for LPI with a dilution ratio (DR) of 30, and a three-stage dilution system was used for ELPI (DR 330). A 10-lm cut-size cyclone was installed in both measurement lines and used as a cut-off for larger particulates. The primary dilution was made with nitrogen gas, and other dilutions with dry air. The primary diluter (DR 10) was VTT’s technology especially developed for hot conditions (up to 1200 C). The aim was to maintain constant sampling conditions during the dilution process. The dilution ratio was controlled with on-line gas analysers.

temperature. Finally, the washed filtration residue was shaken for 3 h at 70 C with hydrochloric acid in an ultrasonic bath. The concentrations of the separate elements in the solutions were determined by AAS or ICP spectroscopy. In addition, the portion of the undissolved residue of each element was calculated from the difference between the total concentrations (determined by XRF after careful ashing at 550 C) and the soluble concentrations. The contents of water soluble alkalies and chlorine can be used to estimate the content of alkali chlorides among the alkali compounds. The insoluble part of the alkalies may have originated from alkali aluminium silicates. Calcium carbonate, which effectively binds SO2, is soluble in 1 M HCl.

2.5. Chemical fractionation analyses

According to the X-ray diffraction results, kaolinite (Al2Si2O5(OH)4) was the dominant aluminium silicate compound in the SAC coal, representing about 10 wt% of the ash and corresponding to about 80% of the sum of Al2O3 and SiO2 (calculated from Table 1). Kaolinite can effectively destroy alkali chlorides via the reaction of Eq. (2). Kaolinite content in the Spanish lignite was about 7 wt% and corresponded to about one-third of the sum of Al2O3 and SiO2. Silicon was also present in the form of SiO2 in the Spanish lignite. Since the lignite ash contained some potassium, a small portion of the aluminium silicate may have been present in inactive form as potassium aluminium silicate. In addition, the Spanish lignite

Chemical fractionation of the solid fuels separates different types of inorganic matter according to the solubility in solvents of different strengths [39]. This is a relatively inexpensive technique for obtaining information on chemical compounds of the key elements in a fuel. In order of strength the solvents were water <1 M ammonium acetate < 1 M HCl. The volume of the solvents was 0.1 dm3. In the experiments, 0.005 kg homogenised fuel (ground to <1 mm particle size) was shaken for 16 h with distilled water at room temperature. The washed filter cake was then stirred for 16 h with ammonium acetate, also at room

3. Results and discussion 3.1. Chemical compounds in the fuels

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M. Aho et al. / Fuel 87 (2008) 58–69

was rich in iron and sulphur. According to X-ray diffraction analysis, most of the iron and sulphur was present as pyrite (FeS2). 3.2. Solubility of key elements Chlorine (86%), 70% of sulphur, 77% of potassium and 58% of sodium were in water soluble form in Cynara (Table 3). The value of the molar ratio (Na + K)/ Cl = 2.6 suggests excess of alkalies in relation to chlorine to produce alkali chlorides but also sulphur can compete with chlorine to produce alkali sulphates. Taking also sulphur into account one can write the molar ratio as follows: (Na + K)/(Cl + 2S). The value of this ratio (2.1) shows excess of alkalies both for S and Cl. One can re-calculate this ratio on the basis of water soluble portions of Na, K, Cl and S: (Naw.s.+Kw.s.)/(Clw.s. + 2Sw.s.) = 1.7. The value >1 suggests still excess of alkalies in CYN in relation to Cl (and also to S) predicting the presence of high concentrations of alkali chlorides which could easily be vaporised in the boiler furnace [36,40]. Temperature of the burning fuel can be 50–250 higher than the sand temperature in the bed [41] leading to effective vaporisation of alkali chlorides from the fuel during the combustion. Almost all aluminium and silicon, in turn, were in highly insoluble form, suggesting the presence of aluminium silicates. In contrast to the Cynara ash, which contained about 40 wt% of water soluble minerals and salts, the ashes of Spanish lignite, SPA and South African coal, SAC contained less than 5 wt% of water soluble minerals (Table 3). The only water soluble fractions were found (at low content) among sulphur and calcium compounds. Almost all potassium, for example, was insoluble (Table 3) suggesting its presence in alkali aluminium silicates. On these basis,

the portion of aluminium silicates inactivated by alkalies was less than one-fifth of the total mass of aluminium silicates, which suggests that most of the insoluble aluminium and silicon was present in alkali-free aluminium silicate (e.q. kaolinite). This suggestion is in good agreement with the high content of kaolinite in the aluminium silicates measured by XRD for SAC. In the case of SPA, the concentration of kaolinite was clearly lower than the concentration of alkali-free aluminium and silicon, and the Al and Si may be present in significant amounts in other alkali-free silicates than kaolinite (indeed, SiO2 was detected by XRD). Some aluminium may further be present in other compounds than silicates. Pyrite (FeS2), detected by XRD in large amounts in SAC, is insoluble. The poor solubility of S and Fe supports this finding (Table 3). Sulphur present in the fuel as pyrite can destroy alkali chlorides because pyrite is oxidised fast at FB temperatures. The dominant reaction path is probably through the formation or iron (III) oxide, as follows [42]: 2FeS2 + 5.5O2 ! Fe2 O3 + 4SO2

ð3Þ

Part of the SO2 may then be oxidised to SO3 enabling the reaction of Eq. (1). 3.3. Flue gas composition 3.3.1. Comparison of FTIR and online measurements Carbon monoxide concentration in the flue gas is an indicator of the degree of burnout of combustion gases and assists in comparing the burnout conditions in the pilot BFB and full-scale BFB boilers. Burnout of a fuel is dependent on temperature distribution, residence time, staging and mixing (both affecting O2 distribution). CO concentrations were lower than 220 ppm (calculated against 6 vol% O2) with all blends except 30% Cynara SAC, where the

Table 3 Results from leaching analysis (concentrations as wt% dry fuel) Al

Ca

Fe

K

Na

P

S

Si

Cl

Mg

CYN Total (XRF) H2O Acetate HCl Rest (calc.)

0.45 0.004 0.0079 0.06 0.38

2.16 0.28 1.01 0.39 0.48

0.25 0.0076 0.0068 0.14 0.096

2.16 1.67 0.38 0.24 0

1.67 0.96 0.19 0.066 0.45

0.18 0.034 0.035 0.026 0.085

0.16 0.095 0.019 0.015 0.031

2.52 0.042 0.089 0.15 2.24

1.76 1.51 0.189 0 0.061

0.56 0.12 0.14 0.05 0.25

SPA Total (XRF) H2O Acetate HCl Rest (calc.)

4.8 0.001 0.0096 0.16 4.63

1.3 0.37 0.62 0.093 0.217

3.7 0.0003 0.0019 0.75 2.95

0.45 0.012 0.011 0.017 0.41

0.06 0.009 0.007 0.0032 0.0412

0.03 0.0003 0.0005 0.017 0.0122

7.99 0.43 0.053 0.041 7.47

6.0 0.0042 0.024 0.13 5.84

0.01 0 0.01 0 0

0.27 0.075 0.060 0.011 0.124

SAC Total (XRF) H2O Acetate HCl Rest (calc.)

2.8 0 0 0.039 2.76

1.2 0.074 0.78 0.29 0.052

0.68 0 0 0.37 0.31

0.15 0 0.001 0.003 0.146

0.04 0.002 0.003 0.002 0.033

0.11 0 0 0.055 0.055

0.59 0.061 0.002 0.006 0.52

3.3 0 0 0.023 3.28

0.007 ? ? ? ?

0.18 0.014 0.129 0.057 0

XRF= analysed from the dry fuel with XRF; Rest (calc) = difference between the total concentration and the sum leached by each solvent; ? = too low concentration to be determined.

M. Aho et al. / Fuel 87 (2008) 58–69

online analyser

CO emissions, ppm

500 400 300 200 100 0

30CYN-SAC

online analyser

50CYN-SAC

FTIR

160

CO emissions, ppm

140 120 100 80 60 40 20 0

100SPA

10CYN-SPA

20CYN-SPA

100% conversion

SO2 emissions, ppm

350 300 250 200 150 100 50 0 20CYN-SAC

b

30CYN-SAC

FTIR

50CYN-SAC

100% conversion

9000 8000 7000 6000 5000 4000 3000 2000 1000 0 100SPA

10CYN-SPA

20CYN-SPA

30CYN-SPA

Fig. 5. SO2 in flue gases: (a) Cynara with South African coal and (b) Cynara with Spanish coal.

50 CYN SAC blend it was only about 50%, suggesting the presence of large mass flows of alkali chlorides in the furnace. 3.4. Concentration of key elements in fine fly ash

FTIR

600

20CYN-SAC

FTIR 400

SO2 emissions, ppm

values were somewhat higher than normal in BFB boilers (Fig. 4). The existence of a cool region just above the bed during combustion of 20% and 30% Cynara in Spanish coal blends (see Fig. 2) did not result in high CO emissions. SO2 availability plays a key role in sulphation since part of the SO2 will be oxidised to SO3 (capable to fast sulphation) [40]. Oxidation of SO2 to SO3 has been reported to depend on molar ratios in the following way: [SO2] · [O2]1/2 [43]. Direct sulphation by SO2 is not possible because all fast reaction mechanisms to produce alkali sulphates require SO3 [40]. As shown in Table 2, Ca/S molar ratios for CYN-SAC blends are greater than 2, owing to the high concentration of reactive calcium in Cynara (Table 3) and the low sulphur concentration in SAC (Table 1). As a consequence, CaSO4 formation is effective and the conversion of coal-S to SO2 is low, resulting in poor SO2 availability (Fig. 5a). In contrast, Spanish coal is pyrite-S rich. Even when blended with Cynara it produces Ca/S molar ratios lower than 0.3, causing the release of huge concentrations of SO2 (Fig. 5b). Differences between the calculated maximum HCl concentrations (with 100% conversion of fuel-Cl to HCl) and the measured HCl concentrations allow an estimate of the mass flows of other Cl compounds than HCl (Fig. 6). We suggest that, during combustion of these blends, chlorine is mostly present as vaporised alkali chlorides which tend to condense on the superheater. The conversion of fuel-Cl to HCl was close to 100% with all blends of Cynara and SPA, indicating very low concentrations of alkali chlorides. However, the conversion of fuel-Cl to HCl was much less than 100% with the 20 CYN SAC blend, and with the

65

30CYN-SPA

Fig. 4. CO in flue gases: (a) Cynara with South African coal and (b) Cynara with Spanish coal.

Figs. 7 and 8 show the chlorine distribution, in mg/N m3, in the fly ash of sizes finer than 4.1 lm, measured by LPI at the same position as the 500 C deposit probe and the 420 C deposit probe, respectively. As expected from the high conversion of fuel-Cl to HCl, no chlorine was found in the finest fly ash of the CYN-SPA blends at either probes. Therefore, those columns are not shown in Figs. 7 and 8. Chlorine concentration was high in the fine fly ash of the blends of 30% and 50% Cynara with SAC. About 90% of the Cl in the fly ash of the 20 CYN SAC blend was measured in particles smaller than < 0.26 lm, while the corresponding portions with the blends containing 30% and 50% Cynara in SAC were about 80% and about 65%. This aerosol fraction was probably formed from vaporised alkali chlorides after condensation in the sampling line. Figs. 9 and 10 show the sulphur concentration distribution, at the same locations as for Figs. 7 and 8. Sulphation (Eq. (1)) of alkali chlorides was probably highly effective with all the blends of Cynara and Spanish coal, and high mass flows of sulphur could be expected to appear in the finest fly ash because sulphation occurs in the gas phase

66

M. Aho et al. / Fuel 87 (2008) 58–69 FTIR

450

100% conversion

1.61-4.02 µ m 0.64-1.61 µ m 0.26-0.64 µ m 0.1-0.26 µ m 0.03-0.1 µ m

1400

400 350

1000

300

800

Cl mg/Nm3

HCl emissions, ppm

1200

600 400 200

250 200 150

0 20CYN-SAC

b

30 CYN-SAC

FTIR

100

50CYN-SAC

50

100% conversion

700

0 20CYN-S AC

HCl emissions, ppm

600

50CYN-S AC

Fig. 8. Chlorine concentrations in the finest fly ash at the 420 C deposit probe.

500 400 300 200

120 1.61-4.02 µ m 0.64-1.61 µ m 0.26-0.64 µ m 0.1-0.26 µ m 0.03-0.1 µ m

100 100

0

10CYN-SPA

20CYN-SPA

30CYN-SPA

Fig. 6. HCl in flue gases: (a) Cynara South African coal blends and (b) Cynara Spanish coal blends.

80

S mg/Nm3

100SPA

450 400 350

60

40

1.61-4.02 µ m 0.64-1.61 µ m 0.26-0.64 µ m 0.1-0.26 µ m 0.03-0.1 µ m

20

300

Cl mg/Nm3

30CYN-S AC

0 20CYNSAC

250

30CYNSAC

50CYNSAC

100-SPA

10CYNSPA

20CYNSPA

30CYNSPA

Fig. 9. Sulphur concentration in the finest fly ash at the 500 C probe.

200 150 100

120

50 0 20CYN-S AC

30CYN-S AC

100

50CYN-S AC

Fig. 7. Chlorine concentrations in the finest fly ash at the position of the 500 C deposit probe.

80

S mg/Nm3

presumably producing aerosolic sulphates first. Indeed, high concentration of sulphur was measured in the fly ash <4.1 lm. More sulphur was present in the fine fly ash at the 420 C probe than the 500 C probe suggesting continuous sulphation. The portion of sulphur in particles <0.26 lm tended to increase with the percentage of Cynara, suggesting increasing mass flows of alkali sulphates in the finest fly ash. With Spanish lignite alone, where the sulphation is insignificant (due to lack of Cl), the portion

1.61-4.02 µ m 0.64-1.61 µ m 0.26-0.64 µ m 0.1-0.26 µ m 0.03-0.1 µ m

60

40

20

0 20CYNSAC

30CYNSAC

50CYNSAC

100-SPA

10CYNSPA

20CYNSPA

30CYNSPA

Fig. 10. Sulphur concentration in the finest fly ash at the 420 C probe.

M. Aho et al. / Fuel 87 (2008) 58–69

If present in the fly ash in significant concentration, alkali chlorides may produce high Cl concentrations at lee and side positions of deposits [30]. They can also increase deposition rate by lowering the melting temperature of the fly ash, which makes it sticky. According to earlier studies, about 15 wt% melt in the fly ash initiates growth of a tacky deposit, and about 75 wt% melt terminates deposit growth [44]. According to thermodynamic equilibrium calculations, the start temperature for ash melting decreases sharply with Cl concentration [45]. Rates of deposition 500 C probe (in the zone where the gas temperature was about 900 C) were clearly higher with the CYN-SAC than the CYN-SPA blends, and there was a strong increase in deposition when the percentage of Cynara was increased from 30% to 50% (Fig. 11). No comparable increase in deposition rate with the percentage of Cynara was seen in the CYN-Spanish coal blends. Deposit from the CYN-SPA blends contained <1 wt% Cl at all positions (no columns shown in Fig. 12). This result is in agreement with predictions based on the fuelCl conversions to HCl and mass flows of chlorine in the fine fly ash. In contrast to this, strong Cl deposition was found with blends of 30 and 50% Cynara-SAC. The relatively low mass flow of Cl in the finest fly ash allowed very little Cl deposition from the 20 CYN-SAC blend (Fig. 12). Sulphur concentrations were lower in deposits of the CYN-SAC than the CYN-SPA blends (Fig. 13), but the differences were not large enough to account for the huge differences in Cl deposition.

25

% Cl in deposit probe

3.5. Deposition on probes

30

20

500W 500S 500L 420W 420S 420L

15

10

5

0 20CYN-SAC

30CYN-SAC

50CYN-SAC

Fig. 12. Cl concentration in deposits: W, wind; L, lee side; S, side position, about 50 up from W. (Cl concentrations were <1 wt% at all positions with blends containing Spanish lignite.).

14

12

10

wt% S

of S in the largest fraction measured (1.61–4.02 lm) was high at the position of the 500 C probe. Sulphur concentration in the fine fly ash was low for the Cynara-SAC blends, and almost all sulphur was present in particles >0.26 lm.

67

500W 500S 500L 420W 420S 420L

8

6

4

2

0 20CYN-SAC 30CYN-SAC 50CYN-SAC

100SPA

10CYN-SPA 20CYN-SPA 30CYN-SPA

Fig. 13. S concentration in deposits. For definition of W,S and L, see Fig. 12.

It is noteworthy that similar Al concentrations were found in deposits of the CYN-SPA blends and those of the 20 CYN-SAC blend, where Cl deposition was prevented (Fig. 14). This finding was unexpected, since the

500 500 ºC probe 420 ºC probe

30

25

20

300

wt% Al2O3

deposition, g/m2-h

400

500W 500S 500L 420W 420S 420L

200

15

10 100

5

0

0 20CYNSAC

30CYNSAC

50CYNSAC

100SPA

10CYNSPA

20CYNSPA

30CYNSPA

Fig. 11. Rate of deposit formation on the 500 C and 420 C probes.

20CYN-SAC 30CYN-SAC 50CYN-SAC 100SPA 10CYN-SPA 20CYN-SPA 30CYN-SPA

Fig. 14. Al2O3 as concentration in deposits. For definition of W, S and L, see Fig. 12.

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M. Aho et al. / Fuel 87 (2008) 58–69

content of aluminium silicate in reactive form (as kaolinite, Al2Si2O5(OH)4), was clearly higher in SAC than in SPA. Deposition of Al was weaker with the blends of 30% and 50% CYN in SAC due to dilution by alkali chlorides. The mass flow of kaolinite was too weak to prevent strong Cl deposition during the combustion. 4. Discussion We suggest that alkali chlorides are formed from Cynara during combustion in all the conducted experiments due to the excess of alkalies as chlorine carriers: (Na + K)/ (Cl + 2S) = 2.1 and (Naw.s.+Kw.s.)/(Clw.s. + 2Sw.s.) = 1.7 in CYN. However, one can claim that for blends of CYN-SPA alkali chlorides are not formed because of the low values of the following molar ratios (Na + K)/ (Cl + 2S) < 0.15 and Ca/S < 0.25 (Table 2). Anyway, the alkali chlorides were destroyed during all combustion tests of CYN-SPA blends with CYN percentages up to 30% on energy basis. S/Cl2 molar ratios were 11–43 and Al/Cl molar ratios 8–31 in these tests. In spite of the high excess of Al in relation to Cl, we suggest that, in these tests, sulphation (Eq. (1)) strongly dominated over alkali aluminium silicate formation (Eq. (2)) as alkali capture mechanism because of the high measured concentration of SO2 (Fig. 5), the high measured mass flow of sulphur in the fine fly ash (Figs. 9 and 10) and the low value for ratio of kaolinite/ (SiO2 + Al2O3) in SPA. SPA is very rich in pyrite (FeS2), which easily decomposes to SO2 in oxidising atmosphere [42], producing SO2 concentrations close to 16,000 mg/ N m3. A significant quantity of this high-SO2 gas probably oxidised to SO3 effectively sulphating the alkali chlorides in the flue gases [40,43]. With CYN-SAC blends, autocapture of SO2 with the reactive calcium in CYN was effective (Fig. 5) (Ca/S varied between 2.8 and 4.9, Table 2), and SO2 concentrations measured for the CYN-SAC blends were two magnitudes lower than the SO2 concentrations measured for the CYN-SPA blends (Fig. 5). Molar ratio of (Na + K)/ (Cl + 2S) was >1 for all the CYN-SAC blends and the concentration of available sulphur is much lower than its total concentration. Also the mass flow of sulphur in the fine fly ash (as a sulphation product) was low (Figs. 9 and 10). In consequence, we suggest that for CYN-SAC blends alkali chlorides are formed during the combustion and destroyed mainly by alkali aluminium silicate formation (Eq. (2)) [30–32]. This mechanism was effective up to 20% CYN, but its protective power was insufficient at higher CYN percentages. According to our earlier studies, refuse-derived fuel containing 0.6 wt% Cl can be burnt safely with SAC up to 60% content [31]. Cynara with this or lower Cl content might be burned in as high proportion with SAC. Addition of a high percentage of Cynara to coal would lead to a marked decrease in CO2 emissions. Clearly, then, there would be substantial benefit in reducing the ash content in Cynara by measures in the field of by pre-treatment. The reduced

fly ash flow could be cleaned with low-cost gas cleaning devices, and ash disposal costs would also be reduced. 5. Conclusions The Cynara sample of this study has high chlorine and alkali contents, and if it was burned alone, the deposition of alkali chlorides would damage superheaters through high temperature corrosion. Burning of Cynara alone in fluidised bed boilers cannot be recommended. Co-combustion of Cynara with two different coals was studied experimentally in pilot scale to evaluate and solve the operational problems. The results suggest that BFB co-combustion of Cynara in small portions (up to 10% on energy basis) with South African coal is safe. At this level, SO2 emissions are so low that there would be no need for limestone addition, and HCl emissions would be less than 400 mg/N m3. At present, there are no limits on HCl emissions from coalbiomass fired power plants, and a traditional ESP would be sufficient for flue gas cleaning. From the environmental point of view, such HCl emissions would nevertheless be relatively high and investments should be made to improve flue gas cleaning. The SPA coal produces very high concentrations of SO2, which need to be captured by limestone in the flue gas line after the superheaters. Addition of limestone to the furnace would lead to strong reduction in SO2 concentration, and probably result in Cl deposition. Attempts should be made to reduce the Cl content in Cynara before combustion by applying low-chlorine fertilisers and modifying the harvesting technology. A larger percentage of CYN could then be included in CYN-SAC blends. As another option, small portions of SPA (20–30% on energy basis) might then be sufficient to prevent Cl deposition from CYN without too high SO2 emission reduction costs. Acknowledgements This work is part of the EU VI FP Biocard project ‘‘Global Process to Improve Cynara cardunculus Exploitation for Energy Applications’’. We wish to thank to the ˚ bo Akademi for the deposit Process Chemistry Group of A analysis, and Endesa Generacio´n (Spain) and Universidad Polite´cnica de Madrid (Spain) for the coal and Cynara samples. References [1] Ferna´ndez J. Cardoon. In: El Bassam N, editor. Energy plant species. James & James; 1998. [2] Ferna´ndez J, Curt MD. Low-cost biodiesel from Cynara oil. In: Proceedings of the second world conference on biomass for energy, industry and climate protection; 2004. p. 109–12. [3] Ferna´ndez J, Curt MD. State of the art of Cynara cardunculus L. as an energy crop. In: Proceedings of the 14th European biomass conference; 2005. p. 22–7.

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