Energy Fuels 2010, 24, 2127–2132 Published on Web 01/21/2010
: DOI:10.1021/ef901491a
Study on Deposits on the Surface, Upstream, and Downstream of Bag Filters in a 12 MW Biomass-Fired Boiler Yanqing Niu, Houzhang Tan,* Xuebin Wang, Zhengning Liu, Yang Liu, and Tongmo Xu State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 710049 Shaanxi, China Received December 8, 2009. Revised Manuscript Received January 12, 2010
Deposits on the surface, upstream, and downstream of bag filters in a 12 MW biomass-fired grate furnace in China were collected, sampled, and analyzed by X-ray fluorescence (XRF) and X-ray powder diffractometry (XRD). Major elements in the surface and upstream deposits were Si, Ca, K, Cl, S, and Na; the share of Al, P, Mg, and Fe was relatively poor. In comparison to the upstream deposits, the high concentrations of K, Na, Cl, and S in the surface deposits were indicative of the formation of submicrometer particles in the boiler, which were subsequently captured on the bag filters. However, high concentrations of Si, Al, and Ca in the upstream deposits could trap chloride via the formation of aluminosilicate and calcium chloride. In addition, the relatively lower contents of K, Na, Cl, and S in the upstream deposits went against the formation of sub-micrometer particles and led to upstream deposits that were more like an aggregate of fly ash and unlike the surface deposits, which were as hard as stone and adhered to the surface of bag filters when flue gas passed through the bag filters. XRD analysis further proved the above results. The intensity of sylvine and halite phases in surface deposits were significantly higher than those in the upstream deposits, and the intensitise of quartz and berlinite phases in the surface deposits were obviously lower than those in the upstream deposits. Higher contents of sylvine and halite as well as lower contents of quartz and berlinite in the surface were indicative of the formation of sintered ash in the boiler. Then, it adhered to the surface of the bag filters. At the exit of bag filters, a mysterious substance evinced as NH4Cl by XRD was generated by the recombination reaction of NH3 (g) and HCl(g) at the outlet temperature of bag filters. The purity of NH4Cl was almost 100%.
transfer, reduce boiler efficiency, and even cause superheater tube explosions.3-5 To prevent the ash-related problems discussed above, numerous studies have been performed. The deposits on fireside surfaces and superheater tubes are mainly caused by sub-micrometer particles containing mostly potassium and chlorine.6 KCl has been found in superheater deposits.7 In biomass, potassium is the dominant alkali element. It is essential to understand how potassium is transported from the fuel into deposits on the surface of superheater tubes and the furnace. It has been found that chlorine facilitates alkali release from biofuels by forming gaseous potassium chloride,8 which is stable at combustion temperatures. Potassium chloride then condenses on particles in the flue gas or on surfaces, and potassium may form hydroxide without chlorine.9 The ash deposits of biomass are closely correlated with the contents of Si and alkali elements in biofuels.1 Vapor-phase alkali promotes the formation of deposits on tube surfaces at high temperatures, while Si combined with Al can trap the alkali before it can react to form sticky deposits. Similarly,
1. Introduction With the depletion of fossil fuels and increasingly serious environmental problems associated with fossil-fuel combustion, an increasing growth of sustainable energy production is being made globally. Biomass is a sufficiently renewable and CO2-neutral source of energy. Its use has attracted worldwide attention. In China, 50 million tons of biomass will be used annually by 2020.1 To achieve the goal of renewable energy sources taking up to 12% of the total energy consumption by 2010, the European Union will exploit approximately 1.3 billion tons of biomass.2 However, combustion of biomass faces several challenges, such as rapid buildup of unmanageable deposits because of the nature of the ash-forming elements in the biomass. Such deposits degrade the aerodynamic flow field, deteriorate the efficiency of burning, inhibit heat
*To whom correspondence should be addressed. E-mail: tanhouzhang@ yahoo.cn. (1) Xiong, S. J.; Burvall, J.; Orberg, H.; Kalen, G.; Thyrel, M.; Ohman, M.; Bostrom, D. Slagging characteristics during combustion of corn stovers with and without kaolin and calcite. Energy Fuels 2008, 22 (5), 3465–3470. (2) European Environment Agency (EEA). How much biomass can Europe use without harming the environment? EEA Briefing 2005, 02, 4. (3) Aho, M.; Silvennoinen, J. Preventing chlorine deposition on heat transfer surfaces with aluminium-silicon rich biomass residue and additive. Fuel 2004, 83 (10), 1299–1305. (4) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Transformation and release to the gas phase of Cl, K, and S during combustion of annual biomass. Energy Fuels 2004, 18 (5), 1385–1399. (5) Szemmelveisz, K.; Szucs, I.; Palotas, A. B.; Winkler, L.; Eddings, E. G. Examination of the combustion conditions of herbaceous biomass. Fuel Process. Technol. 2009, 90 (6), 839–847. r 2010 American Chemical Society
(6) Johansson, L. S.; Leckner, B.; Tullin, C.; Amand, L. E.; Davidsson, K. Properties of particles in the fly ash of a biofuel-fired circulating fluidized bed (CFB) boiler. Energy Fuels 2008, 22 (5), 3005–3015. (7) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Horlyck, S.; Karlsson, A. Influence of deposit formation on corrosion at a straw-fired boiler. Fuel Process. Technol. 2000, 64 (1-3), 189–209. (8) Olsson, J. G.; Jaglid, U.; Pettersson, J. B. C.; Hald, P. Alkali metal emission during pyrolysis of biomass. Energy Fuels 1997, 11 (4), 779– 784. (9) Westberg, H. M.; Bystrom, M.; Leckner, B. Distribution of potassium, chlorine, and sulfur between solid and vapor phases during combustion of wood chips and coal. Energy Fuels 2003, 17 (1), 18–28.
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: DOI:10.1021/ef901491a
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Figure 1. Deposits on the surface, downstream, and upstream of big filters.
when there are higher amounts of Si and Al in the ash, alkali also can be removed from the vapor-phase postcombustion. Thus, all of these things are related to the formation of vaporphase alkali in the boiler. In addition, the content of these elements depends upon plant species and growing conditions, such as soil type, fertilization, etc.1,4 Several authors have proposed using various kinds of mineral additives to solve ash-related problems to some extent. Kaolin addition can significantly enhance the operation of boiler with biofuels by reducing superheater deposition, corrosion, and slagging.6,10-12 The slagging quantities could decrease by 50 and 67% with kaolin and calcite addition, respectively.1 The addition of sulfur and chlorine to wood promotes the formation of submicrometer particles, leading to deposition of potassium sulfate and chloride.13,14 In addition, some researchers devote
attention to co-combustion of two different types of biomass or biomass with other fuel, such as sewage sludge and refuse.15-17 Adding pulp sludge to pine bark and agricultural waste could prevent deposits on heat-transfer surface efficiently. Meanwhile, the efficiency is in positive correlation with the concentrations of aluminum and silicon in ash.15 Other researchers value the effects of dewatering and leaching17 in removing alkali and, thus, reducing slagging and fouling. Although some aspects of the ash-related problems have been studied and some precautions have also been attempted, previous research has mainly been conducted in the laboratory, in which there are inevitable differences in comparison to the real situation in a boiler. Additionally, previous studies have mainly focused on superheater and furnace deposits. Few data are available with respect to filters in the boiler. In fact, the deposition on filters is significantly serious. The main objective in the present work was therefore to determine deposit characteristics on the surface, downstream, and upstream of bag filters.
(10) Jensen, P. A.; Stenholm, M.; Hald, P. Deposition investigation in straw-fired boilers. Energy Fuels 1997, 11 (5), 1048–1055. (11) Davidsson, K. O.; Steenari, B. M.; Eskilsson, D. Kaolin addition during biomass combustion in a 35 MW circulating fluidized-bed boiler. Energy Fuels 2007, 21 (4), 1959–1966. (12) Davidsson, K. O.; Amand, L. E.; Steenari, B. M.; Elled, A. L.; Eskilsson, D.; Leckner, B. Countermeasures against alkali-related problems during combustion of biomass in a circulating fluidized bed boiler. Chem. Eng. Sci. 2008, 63 (21), 5314–5329. (13) Davidsson, K. O.; Amand, L. E.; Leckner, B.; Kovacevik, B.; Svane, M.; Hagstrom, M.; Pettersson, J. B. C.; Pettersson, J.; Asteman, H.; Svensson, J. E.; Johansson, L. G. Potassium, chlorine, and sulfur in ash, particles, deposits, and corrosion during wood combustion in a circulating fluidized-bed boiler. Energy Fuels 2007, 21 (1), 71–81. (14) Strand, M. Particulate and CO emissions from a moving-grate boiler fired with sulfur-doped woody fuel. Energy Fuels 2007, 21, 3653– 3659.
(15) Khan, A. A.; Aho, M.; de Jong, W.; Vainikka, P.; Jansens, P. J.; Spliethoff, H. Scale-up study on combustibility and emission formation with two biomass fuels (B quality wood and pepper plant residue) under BFB conditions. Biomass Bioenergy 2008, 32 (12), 1311–1321. (16) Pettersson, A.; Zevenhoven, M.; Steenari, B. M.; Amand, L. E. Application of chemical fractionation methods for characterisation of biofuels, waste derived fuels and CFB co-combustion fly ashes. Fuel 2008, 87 (15-16), 3183–3193. (17) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M. Removal of inorganic constituents of biomass feedstocks by mechanical dewatering and leaching. Biomass Bioenergy 1997, 12 (4), 241–252.
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Figure 2. Schematic diagram of the boiler system. Table 1. Temperatures Up- and Downstream of Bag Filters load (MW)
position
average temperature (°C)
10
upstream downstream upstream downstream
135 129 135 110
9
Table 2. Analysis Results of Cotton Stalka component
content (wt %)
oxides
content (wt %)
moisture ash volatiles fixed carbon Cad Had Oad Nad Sad Clad
2.63 4.22 72.61 20.54 45.86 5.53 40.96 0.61 0.19 0.44
SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O
13.74 4.03 1.16 25.15 10.99 0.21 5.75 6.98 25.40 6.58
a
Figure 3. Content of all elements in deposits on the surface and upstream of bag filters.
third stage, fourth stage, and secondary and primary superheaters in turn, and then it passed through the economizer and air preheater in the furnace. At last, it passed through the bag filters and discharged into the air. In addition, both secondary and primary superheaters contained two stages. The temperatures up- and downstream of the bag filters were measured under 9 and 10 MW load, respectively (Table 1). Average temperatures up- and downstream of the bag filters were 135 and 110 °C under 9 MW load, respectively. Corresponding temperatures were 135 and 129 °C under 10 MW load, respectively. The biomass fuels used in the power plant were isolated cotton stalks. The corresponding ultimate and proximate analyses as well as ash composition are listed in Table 2. As seen from Table 2, the content of Cl, K, Na, Mg, and Ca in ash was higher than that in wood ash and most of straw biomass ash, and the content of Al was relatively lower.10,18
ad = air-dried basis.
2. Experimental Section 2.1. Experimental Methods. X-ray fluorescence (XRF) (S4Pioneer, Bruker Co., Germany) was used for elemental determinations. The main crystalline compounds in the samples were identified by qualitative X-ray powder diffractometry (XRD) using a D/max2400X powder diffractometer (Japan) with the characteristic Cu-KR radiation. Operating conditions were 40 kV and 100 A. Peak identification was performed by a comparison to standards from the JADE5 software package. The reproducibility of the experiments was acceptable, and the experiments were carried out twice. 2.2. Sample. The deposition on bag filters was significantly serious, as seen in Figure 1b, which shows parts of the bag filters (Figure 1a). Before the experiments, the slagging deposits in the boiler were cleaned thoroughly. After 2 weeks of running time, deposits on the surface, upstream, and downstream of bag filters (panels b-d of Figure 1) were collected, sampled, and analyzed by XRF and XRD. The locations of the sample collection were addressed in the schematic diagram of the boiler system (Figure 2). The samples used in the XRF and XRD analyses were obtained from a 12 MW biomass-fired grate furnace in China. It was a typical M-type nature circulation boiler, which contained four-stage superheaters. Flue gas passed through the
3. Results 3.1. Upstream of the Bag Filters. Upstream deposits were just like an accumulation of fly ash (Figure 1d). The contents of all of the elements were quantitatively analyzed by XRF. Results are illustrated in Figure 3. It could be seen that the major elements in the upstream deposits were Si, Ca, K, Cl, S, Al, and Na; the share of P, Mg, and Fe was relatively poor. (18) Kaufmann, H.; Nussbaumer, T.; Baxter, L.; Yang, N. Deposit formation on a single cylinder during combustion of herbaceous biomass. Fuel 2000, 79 (2), 141–151.
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Figure 4. XRD analyzed results of deposits on the surface and upstream of bag filters.
On the basis of XRF, a further study on the compounds in the deposits was conducted with XRD, and corresponding XRD analysis results are shown in Figure 4. From Figure 4, it was observed that the major compounds in the upstream deposits were sylvine (2θ = 28.347°, 40.528°, 50.188°, 58.643°, and 66.393°), quartz (2θ = 20.859° and 26.640°), halite (2θ = 31.692°, 45.449°, and 56.477°), and berlinite, whose peaks were coincident with quartz. In addition, there were some minor components, such as calcite (2θ = 29.363°), huntite (2θ = 31.554°), anhydrite (2θ = 25.405°), and potassium aluminum silicate (2θ = 44.186°). Of course, some unidentified compounds were also likely to be present in the deposits, such as leucite, calcium magnesium silicates, etc.1 3.2. Deposits on the Surface of Bag Filters. Deposits on the surface of bag filters were as hard as stone. The contents of all elements in the deposits on the surface of bag filters were quantitatively determined by XRF (Figure 3). It could be seen that the major elements were Si, Ca, K, Cl, S, and Na, and the share of P, Al, Mg, and Fe was relatively minor. In comparison to the upstream deposits, the surface deposits contained more K, Na, Cl, and S. However, the contents of Si, Al, Ca, Fe, and Mg were relatively lower, and the content of P was almost the same. From Figure 4, it was observed that the phases determined by XRD were basically in agreement with those in the upstream deposits, except for the intensity of some compounds. Additionally, huntite, which existed in upstream deposits, disappeared, but magnesium calcite (2θ = 29.379°) was identified. 3.3. Downstream of the Bag Filters. At the exit of the bag filters, a mysterious white water-soluble substance (Figure 1c), with a salty and slightly bitter flavor as well as a pungent odor, was observed. The amount of the mysterious substance was huge, and the thickness of the deposit could reach 10-15 mm (Figure 5). The crystalline compound of the downstream sample was identified by XRD, and the result was illustrated in Figure 6. It could be seen that the major crystalline phase was NH4Cl, which was detected at 2θ = 23.121°, 32.859°, 40.460°,
Figure 5. Thickness of downstream deposits.
47.042°, 52,961°, 58.459°, 68.581°, and 73.363°. The peaks of the sample were well-consistent with standard peaks, and the purity of the sample was almost 100%.19 4. Discussion From Figure 3, it was observed that surface deposits contained higher concentrations of K, Na, Cl, and S. However, the concentrations of Si, Al, Ca, and Mg were relatively lower. Higher K, Na, Cl, and S and lower Si, Al, Ca, and Mg concentrations would be the main reason of the surface deposits, which were as hard as stone and adhered to the surface of bag filters. Previous studies showed that chloride (slyvine and halite) promoted the formation of sub-micrometer particles, leading to deposition.3,6,13,14,20-22 Cl combined with (19) Vainshtein, B. K. Akad. Nauk SSSR 1956, 12 (18). (20) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. Fly ash formation and deposition during fluidized bed combustion of willow. J. Aerosol Sci. 1998, 29 (4), 445–459. (21) Lillieblad, L.; Szpila, A.; Strand, M.; Pagels, J.; Rupar-Gadd, K.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Boiler operation influence on the emissions of submicrometer-sized particles and polycyclic aromatic hydrocarbons from biomass-fired grate boilers. Energy Fuels 2004, 18 (2), 410–417.
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of NH4þ and Cl- is not successfully understood presently, and additional investigations are required. When XRD analysis results in Figure 4 were compared, major compounds in the upstream and surface deposits were sylvine, quartz, berlinite, and halite. Additionally, there were some minor components, such as calcite, huntite, magnesium calcite, anhydrite, and potassium aluminum silicate. Although each of these components existed in both upstream and surface deposits, the intensities were different. It could be seen that the intensities of quartz and berlinite in the surface deposits were significantly lower than those in the upstream deposits and the intensities of sylvine and halite in the surface deposits were significantly higher than those in the upstream deposits. Higher contents of sylvine and halite as well as lower contents of quartz and berlinite in the surface deposits were indicative of the formation of sintered ash in the boiler, which then adhered to the surface of the bag filters when the flue gas passed through the bag filters. It was as hard as stone. This was in agreement with a previous study, in which aluminum and silicon in ash could prevent deposits on the heat-transfer surface efficiently.15 Besides, carbonate and potassium aluminum silicate were identified similar to the previous studies.1,24
Figure 6. XRD analyzed results of downstream deposits of bag filters.
alkali metal in fly ash could reduce the melting temperature and increase stickiness, which could lead to extensive deposition.22 The addition of sulfur and chlorine to biofuels could promote the formation of sub-micrometer particles, leading to the deposition of potassium sulfate and chloride.13,14 High concentrations of K, Na, Cl, and S in the surface deposits were indicative of the formation of sub-micrometer particles in the boiler, which were subsequently captured on the bag filters when flue gas passed through bag filters. In comparison to surface deposits, the concentrations of Si, Al, Ca, and Mg in the upstream deposits were higher, while the concentrations of K, Na, Cl, and S were relatively lower. High concentrations of Si and Al (according to eq 1) could trap alkali species via the formation of aluminosilicate and hydrogen chloride, which could reduce the formation of sub-micrometer alkali halide particles (KCl and NaCl) or vapor-phase alkali. Meanwhile, Ca could trap hydrogen chloride (according to eq 2) by a direct reaction to form calcium chloride and, thus, prevent the reformation of alkali halide particles. Therefore, the deposits just aggregated on the bag filters when the flue gas passed through the bag filters. Al2 O3 3 2SiO2 þ 2MCl þ H2 O f M2 O 3 Al2 O3 3 2SiO2 þ 2HCl
5. Conclusion Biomass is a sufficiently renewable and CO2-neutral source of energy. Its use has attracted worldwide attention. However, some ash-related problems, such as deposition and slagging, hinder efficient and economically viable large-scale use of biofuels. Keeping this in view, it was very crucial to study the deposit characteristics. Deposits on the surface, upstream, and downstream of bag filters in a 12 MW biomass-fired grate furnace in China were collected, sampled, and analyzed by XRF and XRD. The results showed that major elements in the surface and upstream deposits were Si, Ca, K, Cl, S, and Na. The share of Al, P, Mg, and Fe was relatively poor. In comparison to the upstream deposits, the surface deposits contained higher concentrations of K, Na, Cl, and S and lower concentrations of Si, Al, Ca, and Mg. The higher concentrations of K, Na, Cl, and S were the result of the formation of sub-micrometer particles in the furnace, which then deposited and adhered to the surface of the bag filters when the flue gas passed through the bag filters. However, high concentrations of Si, Al, and Ca in the upstream deposits could trap chloride via the formation of aluminosilicate and calcium chloride in the furnace. In addition, the concentrations of K, Na, Cl, and S in the upstream deposits were relatively lower. All of this could prevent the formation of sub-micrometer particles in the furnace and lead to the upstream deposits, which were like an aggregate of fly ash and were unlike the surface deposits, which were as hard as stone and adhered to the surface of the bag filters when the flue gas passed through the bag filters. XRD analysis results showed that major compounds in the upstream and surface deposits were sylvine, quartz, berlinite, and halite, as well as some minor components, such as calcite, huntite, magnesium calcite, anhydrite, and potassium aluminum silicate. The intensities of sylvine and halite in surface deposits were significantly higher than those in the upstream deposits, and the intensities of quartz and berlinite in surface deposits were obviously lower than those in the upstream
ð1Þ
where M is K, Na, etc. CaOðsÞ þ 2HClðgÞ f CaCl2ðsÞ þ H2 OðgÞ
ð2Þ
For identification of NH4Cl, as a mysterious substance that appeared at the exit of bag filters, its physicochemical properties may be of significance. With an increasing temperature, NH4Cl significantly evaporates at 100 °C and decomposes into NH3 (g) and HCl(g) at 337.8 °C (eq 3).23 Then, NH3 (g) and HCl(g) recombine to form the small particles of NH4Cl at low temperatures (eq 4), which was very difficult to dissolve in water again. The corresponding reactions are as follows: ð3Þ NH4 Cl ¼ NH3ðgÞ þ HClðgÞ NH3ðgÞ þ HClðgÞ ¼ NH4 ClðsÞ
ð4Þ
In the exit of bag filters, the temperature was just 110-129 °C (Table 1). Therefore, gaseous NH3 (g) and HCl(g) could form NH4Cl via eq 4 at this temperature. Unfortunately, the source (22) Aho, M.; Ferrer, E. Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass. Fuel 2005, 84 (2-3), 201–212. (23) http://baike.baidu.com/view/527693.htm#1.
(24) Osan, J.; Alfoldy, B.; Torok, S.; Van Grieken, R. Characterisation of wood combustion particles using electron probe microanalysis. Atmos. Environ. 2002, 36 (13), 2207–2214.
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deposits. Higher contents of sylvine and halite as well as lower contents of quartz and berlinite in the surface deposits might have been because of the formation of sintered ash in the furnace, which then adhered on the surface of bag filters when the flue gas passed through bag filters. At the exit of the bag filters, the mysterious substance identified as NH4Cl by XRD was generated when NH3 (g)
reacted with HCl(g) at the outlet temperature of bag filters. The purity of NH4Cl was almost 100%. Acknowledgment. The present work was supported by the National Nature Science Foundation of China (50976086). The authors also thank the biomass-fired power plant of Bachu, China.
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