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Behaviour of gaseous chlorine and alkali metals during biomass


Fuel 84 (2005) 841–848 www.fuelrst.com

Behaviour of gaseous chlorine and alkali metals during biomass thermal utilisation
Xiaolin Weia, Uwe Schnellb,*, Klaus R.G. Heinb
b a Institute of Mechanics, Chinese Academy of Sciences (CAS), Bei Si Huan Xi Lu.15, Beijing 100080, People's Republic of China Institute of Process Engineering and Power Plant Technology (IVD), University of Stuttgart, Pfaffenwaldring 23, D-70569 Stuttgart, Germany

Received 26 August 2004; received in revised form 30 November 2004; accepted 30 November 2004 Available online 15 December 2004

Abstract The behaviour of gaseous chlorine and alkali metals of three sorts of biomass (Danish straw, Swedish wood, and sewage sludge) in combustion or gasication is investigated by the chemical equilibrium calculating tool. The ranges of temperature, air-to-fuel ratio, and pressure are varied widely in the calculations (TZ400–1800 K, lZ0–1.8, and PZ0.1–2.0 MPa). Results show that the air excess coefcient only has less signicant inuence on the release of gaseous chlorine and potassium or sodium during combustion. However, in biomass gasication, the inuence of the air excess coefcient is very signicant. Increasing air excess coefcient enhances the release of HCl(g), KOH(g), or NaOH(g) as well as it reduces the formation of KCl(g), NaCl(g), K(g), or Na(g). In biomass combustion or straw and sludge gasication, increasing pressure enhances the release of HCl(g) and reduces the amount of KCl(g), NaCl(g), KCl(g), or NaOH(g) at high temperatures. However, during wood gasication, the pressure enhances the formation of KOH(g) and KCl(g) and reduces the release of K(g) and HCl(g) at high temperatures. During wood and sewage sludge pyrolysis, nitrogen addition enhances the formation of KCN(g) and NaCN(g) and reduces the release of K(g) and Na(g). Kaolin addition in straw combustion may enhance the formation of potassium aluminosilicate in ash and signicantly reduces the release of KCl(g) and KOH(g) and increases the formation of HCl(g). q 2004 Elsevier Ltd. All rights reserved.
Keywords: Chlorine; Alkali metals; Behaviour; Biomass utilization

1. Introduction The thermal utilisation of biomass can contribute to the reduction of CO2 emissions. Compared with coal, biomass has a high amount of potassium, chlorine, and silicon as well as minor amounts of Ca, Mg, Al, Fe, Na, and S, etc. During combustion or gasication (i.e. air–fuel ratio lO1 or !1) of biomass, signicant amounts of chlorine and alkali metals are released into the gas phase, such as HCl(g), KCl(g), KOH(g), and NaCl(g), etc. They are very harmful in terms of causing fouling, slagging, and high temperature corrosion in the furnace [1–10]. For direct red combined cycles, the alkali species in the gas phase can result in serious corrosive problems at gas turbines [11–13]. Hydrogen chloride may
* Corresponding author. Tel.: C49 711 685 3574; fax: C49 711 685 3491. E-mail addresses: xlwei@imech.ac.cn (X. Wei), schnell@ivd.unistuttgart.de (U. Schnell). 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.11.022

also tend to produce unacceptably high emissions of HCl and dioxins [14–20]. In addition, HCl, SO2, and released alkali species are likely to form aerosols in the ue gas [13,21–23]. It is found that the major ash forming elements (Al, Si) and the composition of gas phase (O2, N2, and H2O, etc.) have signicant inuence on the behaviour of chlorine and alkali metals [24–31]. In fact, it may be affected by many factors, such as composition of the fuel, combustion or gasication conditions, and low or high pressure. However, few studies have been carried out to analyse the inuence of fuel minerals on this behaviour considering all of the mineral elements (Al, Si, K, Na, Ca, Mg, Fe, Ti, Mn, S, Cl, and P, etc.). In addition, there are very limited results in this eld for biomass gasication under pressurized conditions. The aim of this work is to investigate the release of chlorine and alkali metals for three kinds of biomass during combustion or gasication under pressurized conditions by the equilibrium calculating tool FactSage (Table 1 shows

842 Table 1 Pulverized biofuel composition Chemical analysis (%) Moisture Volatile dry Ash dry Fixed C dry C dry N dry S dry H dry O (diff.) dry Cl dry LHV (MJ/kg) dry
a

X. Wei et al. / Fuel 84 (2005) 841–848

Danish straw 11.71 71.32 13.42 15.26 41.43 1.09 0.10 4.18 39.25 0.53 16.01

Swedish wood 7.80 84.10 0.20 15.70 49.57 0.07 0.06 6.05 44.04 0.01 18.52

Sewage sludge 5.86 48.45 48.06 3.65 25.77 3.02 0.81 4.31 17.99 0.04 10.72

Ash analysis (%) dry Al2O3 CaO Fe2O3 K2O MgO Na2O SiO2 SO3 TiO2 P2O5 MnO2

Danish straw 0.36 4.34 0.26 14.79 0.87 0.35 68.59 1.93 0.02 1.93 0.02a

Swedish wood 4.69 30.49 2.67 9.46 5.93 2.10 18.90 4.04 0.98 2.37 4.61

Sewage sludge 10.34 16.04 17.83 0.93 2.05 2.16 26.52 2.83 0.75 15.40 0.02a

Data estimated.

the composition of biomass). The ranges of temperature, air–fuel ratio, and pressure are varied widely, e.g. TZ 400–1800 K, lZ0–1.8, and PZ0.1–2.0 MPa.

2. Calculating method The equilibrium analysis software FactSage was used to determine thermodynamic stable chemical and physical forms in the chemical system. When the parameters, such as the elementary composition of the fuel and air, temperature and pressure have been entered, FactSage will search the species including these elements from the database. In this paper, 611 species (143 gas, 94 liquid, and 374 solid species) are selected to conduct the thermodynamic equilibrium calculation for the chemical system including the elements C, H, O, N, S, Cl, Si, P, Ca, K, Na, Mg, Al, Fe, Ti, and Mn between 800 and 1800 K. Table 2 gives the moles of biomass composition based on 1000 kg fuels for equilibrium calculation. Although the equilibrium analysis is a powerful tool to predict the stable species during the chemical process, there are some disadvantages of this method applied to the combustion case [32]. Either the temperature must be high enough or the species residence time should be long enough to reach the thermodynamic equilibrium. The difculty will arise when the results of the equilibrium calculation are compared with those of real combustion systems. In general, the chemical equilibrium analysis may be used to give the equilibrium distribution of elements and the reaction mechanism of various species.

K2SO4(g), KOH(g), and K(g), etc. And the others may be retained in ash, forming potassium silicate, aluminosilicate or sulphate. During the cooling process, the gaseous potassium may condense on the coarse y ash as KCl(s) or K2SO4(s). Some of gaseous potassium directly forms aerosols or ne y ash because of condensation, sulfation and carbonization. A part of these aerosols may also attach on the coarse y ash. The enrichment of potassium, sulphur and chlorine in ash is very harmful because of causing fouling, slagging, and high temperature corrosion in the furnace. In addition, if aerosols are not collected in the ash separator, they may cause air pollution. Fig. 2 shows the equilibrium results of potassium behaviour for Danish straw and Swedish wood combustion. At high temperatures (O1100 K), the main potassium containing species are K2Si4O9(liq), KCl(g), and KOH(g) for straw combustion, and KCl(g), K2SO4(g), and KOH(g) for wood combustion. Because of the higher content of

Table 2 Moles of biomass composition based on 1000 kg fuels (lZ0) Fuels Main elements C H O N S Cl Minor elements Si P Ca K Na Mg Al Fe Ti Mn Danish straw 30,457 49,612 30,737 686.91 27.54 132.00 1194.10 28.45 80.96 328.49 11.81 22.58 7.39 3.41 0.26 1.90 Swedish wood 38,055 63,997 29,737 46.07 17.26 2.60 5.35 0.57 9.24 3.41 1.15 2.50 1.56 0.57 0.21 1.43 Sewage sludge 20,200 46,758 24,335 2029.30 237.85 10.62 1879.80 924.20 1218.20 84.10 296.87 216.61 863.88 951.06 39.98 7.75

3. Results and discussion Fig. 1 describes the transformation of alkali metals (e.g. potassium) during biomass thermal utilisation. In biomass gasication or combustion, some of potassium and chlorine will be released into gas, such as HCl(g), KCl(g), (KCl)2(g),

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Fig. 1. Transformation of potassium in biomass thermal utilization.

silicon in straw, a higher amount of K2Si4O9(liq) is formed, which may cause slagging and fouling in the furnace. The gaseous potassium species, e.g. KCl(g) and KOH(g), may form aerosols in the gas cooling process. At midtemperatures (800–1100 K), the main species are potassium silicate and sulphate (i.e. K2Si4O9(s,s2) and K2SO4(s2)) for straw combustion. Obviously, K2Si4O9(s,s2) is transformed by the liquid silicate K2Si4O9(liq) under high temperature. For wood combustion, the main species are potassium aluminosilicate and sulphate (i.e. KAlSi2O6(s,s2) and K2SO4(s2)). At low temperatures (400–900 K), the main species are potassium chloride and sulphate (i.e. KCl(s), K2SO4(s), and K2Si2O5(s)) for straw combustion. Because of the higher content of chlorine than sulphur in straw, a higher amount of chloride is formed than sulphate. For wood combustion, the main species are K2SO4(s) and KAl(SO4)2(s). Because of the relatively higher content of sulphur than chlorine in wood, potassium sulphate and aluminosilicate becomes important in ash.

Fig. 3 gives the release behaviour of chlorine and alkali metals in combustion with various air excess coefcients. For straw combustion in Fig. 3(a), increasing temperature from 800 to 1000 K signicantly increases the amount of released HCl(g). This indicates that the gaseous HCl concentration attains maximum at 1000 K. The reason of this might be that most of chlorine in gaseous HCl comes from decomposing KCl(s) in the mid-temperature range. Then increasing temperature from 1000 to 1800 K reduces the amount of released HCl(g), thereby KCl(g) begins to form and gradually increases. At high temperatures (O 1400 K), a small amount of KOH(g) also occurs. For wood combustion in Fig. 3(b), all of the chlorine is released as HCl(g) in the temperature range of 800–1000 K. With increasing temperature, KCl(g) and K2SO4(g) begins to form and causes HCl(g) to decrease. At 1350 K, KCl(g) attains maximum and HCl(g) attains minimum. In addition, from 1200 K, the release of KOH(g) signicantly increases. Because of the low content of chlorine in wood

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Fig. 2. Potassium behaviour during Danish straw and Swedish wood combustion (lZ1.2, PZ0.1 MPa).

and relatively high content of hydrogen (meaning that more hydroxyl radical may be produced), increasing temperature from 1300 to 1800 K might induce the conversion of KCl(g) to KOH(g) via the reaction: KClg C H2 O0 KOHg C HClg (1)

Therefore, at 1350 K, KCl(g) begins to reduce and HCl(g) and KOH(g) to increase. Due to the higher sodium content than potassium in sewage sludge, Fig. 3(c) shows the release behaviour of sodium and chlorine in sewage sludge combustion. Similar with the results in Fig. 3(b), all of the chlorine is released as HCl(g) in the temperature range of 800–1200 K. Then increasing temperature from 1200 to 1800 K reduces the amount of released HCl(g), whereas NaCl(g) begins to form and gradually increases. At high temperature (O1500 K), NaOH(g) also occurs and signicantly increases with increasing temperature. Under combustion conditions shown in Fig. 3, the air excess coefcient (l) has less signicant inuence on the released amount of chlorine and potassium or sodium. For straw or sludge (except wood) combustion in Fig. 3(a) or (c), increasing air excess coefcient reduces the release of HCl(g) and increases the amount of KCl(g) or NaCl(g) in the high temperature range. Obviously, a lot of alkali metals may release from the condensed phase (e.g. liquid alkali silicate or solid aluminosilicate) and produce gaseous alkali chloride at high temperatures. Increasing the oxygen concentration in the ue gas might promote the reaction (K* denotes the radical potassium from

Fig. 3. Release of chlorine and alkali metals in combustion with various air excess coefcients.

the condensed phase): K HClg O0 KClg OH (2)

This increases the release of KCl(g). In addition, KOH(g) or NaOH(g) also increase with increasing air excess coefcient because of higher oxygen concentration in the gas phase. Fig. 4 gives the release behaviour of chlorine and alkali metals in gasication with various air excess coefcients. Compared with Fig. 3, the air excess coefcient has signicant inuence on the released amount of chlorine, potassium or sodium under gasication conditions. For straw gasication in Fig. 4(a), increasing air excess coefcient from 0.2 to 0.8 increases the release of HCl(g) and reduces the formation of KCl(g). For wood gasication in Fig. 4(b), KCl(s) directly vaporizes as KCl(g) in the temperature range of

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For sludge gasication, due to the higher sodium content than potassium, Fig. 4(c) shows the release behaviour of sodium and chlorine. Because the content of silicon and aluminium is high in sludge, sodium may retain in ash as aluminosilicate and is only likely to release at high temperatures (O1300 K). Increasing air excess coefcient increases the release of HCl(g) and NaOH(g) and reduces the formation of NaCl(g) and Na(g). This might be explained by the reactions: NaClg C H2 O0 NaOHg C HClg Nag C OHg0 NaOHg (4) (5)

Fig. 5 describes the effect of pressure on the release of chlorine and alkali metals during biomass combustion. For straw combustion in Fig. 5(a), at mid-temperatures

Fig. 4. Release of chlorine and alkali metals in gasication with various air excess coefcients.

800–1100 K and KCl(g) attains maximum at 1100 K. Increasing temperature from 1000 to 1800 K enhances the release of KOH(g) and K(g). HCl(g) begins to occur at high temperatures (O1100 K) and increases sharply with temperature. At high temperatures (O1100 K), increasing air excess coefcient increases the release of HCl(g) and KOH(g) and reduces the formation of KCl(g) and K(g). Under gasication conditions, increasing air excess coefcient increases the concentration of H2O and radical OH in the gas, and the reaction (1) is likely to promote the conversion of KCl(g) to HCl(g) and KOH(g). With increasing air excess coefcient, the decrease of K(g) and increase of KOH(g) might be explained by the reaction: Kg C OH0 KOHg (3)
Fig. 5. Release of chlorine and alkali metals in combustion with various pressures.

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(800–1100 K), increasing pressure postpones the release of HCl(g), but the maximum amount of HCl(g) increases. At high temperatures (O1100 K), increasing pressure signicantly enhances the release of HCl(g), and thus reduces the formation of KCl(g) and KOH(g). For wood combustion in Fig. 5(b), it is interesting that the pressure only has an inuence in the temperature range between 1100 and 1500 K, thereby the pressure enhances the release of HCl(g) and decreases the formation of KCl(g) and KOH(g). For sludge combustion in Fig. 5(c), the inuence of pressure only occurs in the temperature range above 1300 K. Similar with the results of straw and wood combustion, the pressure enhances the release of HCl(g) and reduces the formation of NaCl(g) and NaOH(g). Fig. 6 gives the effect of pressure on the release of chlorine and alkali metals during biomass gasication.

Results of straw or sludge gasication in Fig. 6(a) or (c) are similar with those in Fig. 5(a) or (c). At high temperatures (O1200 K), increasing pressure enhances the release of HCl(g), and thus reduces the formation of KCl(g), NaCl(g), KOH(g), or NaOH(g). In addition, the release of K(g) or Na(g) also decreases with increasing pressure. For wood gasication in Fig. 6(b), at mid-temperatures (800–1200 K), the pressure reduces the formation of KCl(g) and KOH(g). However, at high temperatures (O1200 K), the pressure enhances the formation of KOH(g) and KCl(g) and reduces the release of K(g) and HCl(g). Figs. 7 and 8 indicate the effect of additional nitrogen on the release of chlorine and alkali metals during wood and sewage sludge pyrolysis. In the pyrolysis reactor, high temperature nitrogen is often used to heat the biomass and carry the pyrolysis gas, and thus it may have inuence on the release of chlorine and alkali metals. At high temperatures (O1200 K), Fig. 7 shows that nitrogen enhances the release of HCl(g) and reduces the amount of KCl(g) and NaCl(g). Increasing temperature signicantly enhances the release of K(g) and Na(g). Nitrogen addition reduces the release of K(g) and Na(g), but causes a sharp increase of KCN(g) and NaCN(g). Due to the limited content of chlorine in sludge, Fig. 8 shows that nitrogen has less signicant inuence on the release of HCl(g), KCl(g), and NaCl(g). However, at very high temperatures (O1500 K), the amount of K(g), Na(g), KCN(g), and NaCN(g) signicantly increases during

Fig. 6. Release of chlorine and alkali metals in gasication with various pressures.

Fig. 7. Release of alkali metals during pyrolysis of Swedish wood in nitrogen environment. Solid line denotes the condition without additional nitrogen; dashed line denotes the condition with additional nitrogen.

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4. Conclusions The chemical equilibrium calculation was used to investigate the release of chlorine and alkali metals during three sorts of biomass combustion or gasication under atmospheric or pressurized conditions. Results show that the main alkali containing species are K2Si4O9(liq), KCl(g), and KOH(g) in straw combustion, and KCl(g), K2SO4(g), and KOH(g) in wood combustion, as well as NaCl(g) and NaOH(g) in sewage sludge combustion. Under combustion conditions, the air excess coefcient only has a limited inuence on the release of chlorine and potassium or sodium. For straw or sludge combustion, increasing air excess coefcient reduces the release of HCl(g) and increases the formation of KCl(g) or NaCl(g) in the high temperature range. Compared with the results in combustion, the air excess coefcient has signicant inuence on the release of chlorine, potassium or sodium during biomass gasication. At high temperatures (O1100 K), increasing air excess coefcient increases the release of HCl(g), KOH(g) or NaOH(g) as well as it reduces the formation of KCl(g), NaCl(g), K(g) or Na(g). Increasing air excess coefcient increases the concentration of H2O and radical OH in the gas, and this is likely to promote the following reactions:
Fig. 8. Release of alkali metals during pyrolysis of sewage sludge in nitrogen environment. Solid line denotes the condition without additional nitrogen; dashed line denotes the condition with additional nitrogen.

KClg C H2 O0 KOHg C HClg Kg C OH0 KOHg

(6) (7)

sludge pyrolysis. Nitrogen addition also enhances the formation of KCN(g) and NaCN(g) and reduces the release of K(g) and Na(g). In Fig. 9, kaolin is mixed with biomass to investigate the inuence of aluminosilicate addition on the release of chlorine and alkali metals during straw combustion. In the uidized bed reactor, kaolin may be contacted very well with biomass particle and thus lead to the decrease of the release of alkali metals [8]. Obviously, kaolin may enhance the formation of potassium aluminosilicate and retain potassium in ash. Increasing kaolin dosage signicantly reduces the formation of KCl(g) and KOH(g) and increases the release of HCl(g).

During biomass combustion, the pressure may enhance the release of HCl(g) and reduces the formation of KCl(g), NaCl(g), KCl(g), and NaOH(g) at high temperatures. During straw or sludge gasication, the pressure also enhances the release of HCl(g) and reduces the formation of KCl(g), NaCl(g), KCl(g), and NaOH(g) as well as K(g) and Na(g) at high temperatures. During wood gasication, at mid-temperatures (800–1200 K), the pressure reduces the formation of KCl(g) and KOH(g). However, the pressure enhances the formation of KOH(g) and KCl(g) and reduces the release of K(g) and HCl(g) at high temperatures (O1200 K). During wood and sewage sludge pyrolysis, nitrogen addition enhances the formation of KCN(g) and NaCN(g) and reduces the release of K(g) and Na(g). Kaolin addition in straw combustion may enhance the formation of potassium aluminosilicate in ash. Increasing kaolin dosage signicantly reduces the formation of KCl(g) and KOH(g) and increases the release of HCl(g).

Acknowledgements This work was conducted at IVD, University of Stuttgart and nalized at IMech, CAS. Financial support by the Alexander von Humboldt foundation and Chinese Natural Science Foundation (No. 50376068) is gratefully acknowl¨ edged. The authors would also like to thank Jorg Maier for his help on this work.

Fig. 9. Effect of kaolin addition on the behaviour of chlorine and alkali metals in combustion.

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