SPECIATION OF ALKALI METALS IN BIOMASS COMBUSTION AND GASIFICATION PAVANKUMAR BAJRANG SONWANE HENG BAN, COMMITTEE CHAIR THOMAS K. GALE PETER M.

Transcription

1 SECIATION OF ALKALI METALS IN BIOMASS COMBUSTION AND GASIFICATION by AVANKUMAR BAJRANG SONWANE HENG BAN, COMMITTEE CHAIR THOMAS K. GALE ETER M. WALSH A THESIS Submitted to the graduate faculty of The University of Alabama at Birmingham, in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering BIRMINGHAM, ALABAMA 2006

2 SECIATION OF ALKALI METALS IN BIOMASS COMBUSTION AND GASIFICATION SONWANE AVANKUMAR BAJRANG ABSTRACT Electricity from biomass and biomass derived fuels has become an attractive and viable alternative energy source. Alkali metals, mainly sodium and potassium, together with other ash forming inorganic components in biomass, increase fouling, slagging, and high temperature corrosion of heat transfer surfaces in boilers thus reduces efficiency during biomass combustion. Future biomass-to-electricity facilities will benefit from increased efficiencies, by incorporating integrated gasification combined cycle systems that use biomass syngas directly in gas turbine. These systems will have even lower tolerances for alkali vapor release, because accelerated erosion and corrosion of turbine blades results in shorter turbine life. One solution to the fouling and slagging problem is to develop methods of hot gas cleanup to reduce the amount of alkali vapor. A detailed understanding of the mechanism of alkali metals release during biomass gasification could greatly benefit the development of hot gas cleanup technology. In this study, thermodynamic equilibrium predictions were made of the distribution and mode of occurrence of gaseous chlorine and alkali metals of three types of biomass (corn stover, switch grass, and wheat straw) in combustion and gasification processes. The influence of temperature, pressure, and air-fuel ratio was also evaluated. Results show that the percent stoichiometric air has limited influence on the speciation of chlorine and potassium during combustion. However, the influence of the percent ii

3 stoichiometric air is significant during gasification. Increasing percent stoichiometric air enhances the formation of vapor HCl and KOH as well as reduction in vapor KCl and K 2 Cl 2. In biomass combustion and gasification, increasing pressure increases vapor HCl and K 2 Cl 2 and reduces the amount of vapor KCl and KOH. At higher temperatures (>1100K), the gaseous alkali species increased greatly. iii

4 ACKNOWLEDGMENTS I would like to take this opportunity to thank those people without whom this research effort wouldn t have been possible. First and foremost I would like to thank my parents for their immense support and encouragement during my higher studies. Their love has always been a source of succor throughout. I will always be indebted to them for the sacrifice they have made for me. I am grateful to my advisor, Dr. Heng Ban for guiding me through this often grueling world of research. This valuable feedback helped me shape my research skills. I would also like to thank my committee members, Dr. Thomas Gale and Dr. eter Walsh for their precious advice and efforts that greatly contributed to this research work. My friends here at University of Alabama at Birmingham and back home in India have helped me in all possible ways. I will always be indebted to them. iv

5 TABLE OF CONTENTS age ABSTRACT... ii ACKNOWLEDGMENTS... iv LIST OF TABLES... vi LIST OF FIGURES... vii CHATER 1. INTRODUCTION AND BACKGROUND Goals and Objectives Overall goal Objectives Scope of the work METHOD AND ROCEDURE RESULTS AND DISCUSSION Model Validation Results and Discussion CONCLUSION...40 LIST OF REFERENCES...42 v

6 LIST OF TABLES Table age 1. Biomass composition analysis...13 vi

7 LIST OF FIGURES Figure age 1-a. otassium speciation for corn stover combustion ( = 1.2; = 0.1 Ma) b. otassium speciation for switch grass combustion ( = 1.2; = 0.1 Ma) c. otassium speciation for wheat straw combustion ( = 1.2; = 0.1 Ma) a. Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for corn stover ( = 1.2, 1.5, 1.8; = 0.1 Ma) b. Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for switch grass ( = 1.2, 1.5, 1.8; = 0.1 Ma) c. Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for wheat straw ( = 1.2, 1.5, 1.8; = 0.1 Ma) a. Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for corn stover ( = 0.2, 0.5, 0.8; = 0.1 Ma) b. Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for switch grass ( = 0.2, 0.5, 0.8; = 0.1 Ma) c. Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficient for wheat straw ( = 0.2, 0.5, 0.8; = 0.1 Ma) a. Speciation of potassium and chlorine in combustion with various pressures for corn stover ( = 1.2; = 0.1, 0.5, 1.0 Ma)...27 vii

8 4-b. Speciation of potassium and chlorine in combustion with various pressures for switch grass ( = 1.2; = 0.1, 0.5, 1.0 Ma) c. Speciation of potassium and chlorine in combustion with various pressures for wheat straw ( = 1.2; = 0.1, 0.5, 1.0 Ma) a. Speciation of potassium and chlorine in gasification with various pressures for corn stover ( = 0.5; = 0.1, 0.5, 1.0 Ma) b. Speciation of potassium and chlorine in gasification with various pressures for switch grass ( = 0.5; = 0.1, 0.5, 1.0 Ma) c. Speciation of potassium and chlorine in gasification with various pressures for wheat straw ( = 0.5; = 0.1, 0.5, 1.0 Ma) a. Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for corn stover ( = 0.2, 0.5, 0.8; = 5.0 Ma) b. Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for switch grass ( = 0.2, 0.5, 0.8; = 5.0 Ma) c. Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for wheat straw ( = 0.2, 0.5, 0.8; = 5.0 Ma) a. Speciation of potassium and chlorine in oxygen blown gasification with various pressures for corn stover ( = 0.5; = 3.0, 5.0, 7.0 Ma) b. Speciation of potassium and chlorine in oxygen blown gasification with various pressures for switch grass ( = 0.5; = 3.0, 5.0, 7.0 Ma) c. Speciation of potassium and chlorine in oxygen blown gasification with various pressures for wheat straw ( = 0.5; = 3.0, 5.0, 7.0 Ma)...39 viii

9 CHATER 1 INTRODUCTION AND BACKGROUND Biomass is an organic matter- wood, agricultural crop, animal wastes- that can be used as an energy source. Currently, the major source of biomass consists of residues from forestry and agriculture, industry and domestic wastes, and sludges. While these residue fuels provide an important initial feedstock for the bio-energy industry, large scale energy production from biomass is still heavily dependant upon energy crops such as sugar cane and switch grass. Approximately 13% of world energy demand is supported by biomass fuels while biomass constitutes 7% of the primary energy source in the United States. Biomass has some environmental advantages too over fossil fuels. Biomass contains very small amounts of sulfur and nitrogen, therefore a biomass power plant emits very little sulfur dioxide and nitrogen dioxide, which are major source of acid rain. Due to the growing concern about future energy sources and the need to limit CO 2 emissions, biomass and biomass-derived fuels have become an attractive and viable 1

10 alternative energy source for heat and power production. Biomass is a renewable energy source and the use of biomass as fuel makes zero net contribution to the global CO 2 level. However, biomass typically has a high content of potassium ( wt%), chlorine ( wt%), and silicon ( wt%), as well as minor amounts of Ca, Mg, Al, Na, Fe, S, and [1, 2] ( The mass fractions are expressed as the elements, not their oxides). The primary gas phase alkali metals released during biomass combustion are potassium salts: chlorides, hydroxides and sulfates. The substantial alkali metal release during biomass combustion accelerates fouling and slagging on heat transfer surfaces in industrial boilers. The high chlorine content in some biomass also raises a concern about corrosion of these surfaces. Integrated gasification combined cycle (IGCC), with the combination of a steam cycle and a gas turbine cycle, has the advantage of high overall efficiency and low emissions. Unfortunately, the impact and condensation of alkali vapors from the syngas can reduce the lifetime of gas turbine blades used in IGCC systems, because of high-temperature corrosion [3]. The presence of alkali metals in combustion and gasification systems may also cause other problems, with a potential negative effect on the overall efficiency. The function of fluidized bed gasifiers may be deteriorated by agglomeration of the bed material particles, due to low-melting eutectic alkali salt mixtures. The sticky particles also cause plugging of barrier filters in hot gas clean-up systems. Condensation of alkali metal compounds on heat-exchange surfaces requires costly plant shutdowns for the removal of deposits. One solution to alkali metal deposition is to develop methods in hot gas cleanup systems to reduce the amount of alkali vapor. A detailed understanding of the mechanisms of alkali metal speciation 2

11 during biomass gasification could greatly benefit the development of hot gas clean-up technology. Dayton et. al. [6] investigated the effect of coal minerals on chlorine and alkali metal formation during biomass and coal co-combustion. Along with the experiments, Dayton et.al. [6] also performed the equilibrium calculations. The equilibrium calculations for red oak (low Cl biomass) showed that about 70% of the potassium was predicted to form sanidine (KAlSi 3 O 8 ), and only about 8% of the potassium was predicted to form KCl(g) and KCl(s), where (g) denotes the gas phase vapor and (s) for solid phase. For the flue gas produced from the co-combustion of red oak and coal, most of the potassium (85% to100%) was predicted to form sanidine (KAlSi 3 O 8 ). The results of equilibrium calculations for wheat straw (moderate Cl biomass) indicated that 30% of the potassium was predicted to form KCl(g), KOH(g), and K 2 SO 4 (g) and the remaining 70% of the potassium was predicted to form potassium silicates and K 2 SO 4 (s,l), where (l) denotes liquid phase. In case of wheat straw and Kentucky coal co-combustion flue gas, more than 95% of the potassium was predicted to form sanidine (KAlSi 3 O 8 ). Similar results were obtained from predictions of wheat straw and ittsburgh coal co-combustion flue gas, except the alkali metal capture (in solid phase) was less efficient, because silicon and aluminum contents in ittsburgh coal are less than in Kentucky coal. The equilibrium calculations for Imperial wheat straw (high Cl biomass) predicted that 70% of the potassium was in gaseous compounds. For the equilibrium calculations of Imperial wheat straw co-combustion with coal, 50-80% of the potassium was predicted to form sanidine (KAlSi 3 O 8 ). The equilibrium calculation results indicated that an increase in silicon and 3

12 aluminum contents increases the retention of alkali metals in the condensed phase (solid or liquid)[6]. Dayton et. al. [3] studied the effect of oxygen and steam concentration on the speciation of alkali metals in flue gas by using equilibrium calculations. The equilibrium calculations showed that a reduction in oxygen concentration increased the formation of KCl and KOH. Excess steam increased the formation of HCl. Excess steam also increased the formation of KOH and decreased the KCl formation through the conversion of KCl to KOH [3]. Neilson et. al. [9] performed the equilibrium calculations to investigate the effect of sulfur on the speciation of alkali metals in flue gas. The equilibrium calculations for wheat straw showed that potassium was present as KCl(s), K 2 SO 4 (s) and K 2 SiO 3 (s) at lower temperatures, whereas at higher temperatures the gaseous KCl(g) and KOH(g) were the thermodynamically stable species. Formation of K 2 SO 4 (g) was predicted to be in the temperature range of 1000 o C to 1300 o C. The equilibrium calculation predicted KCl(g) was the most stable form of chlorine and potassium at temperatures above 600 o C. The increase in sulfur at high temperature condition was found to reduce the formation of KCl(g) and KOH(g) and increase the formation of K 2 SO 4 (g). This may be due to an increase in sulfation of KCl(g) and KOH(g) to K 2 SO 4 (g) at high temperature [9]. 4

13 Jensen et. al. [7] investigated the effect of potassium (K), chlorine (Cl) and silicon (Si) on the formation of potassium and chlorine species through the equilibrium calculations under combustion conditions. The equilibrium calculations were performed for four different cases: Case I (K: 0.209%, Cl: 0.08 % and Si: 0.444%), Case II (K: 0.209%, Cl: 0.08%, Si: 0.05%), Case III (K: 0.209%, Cl: 0.08%, Si: 0.0%), and Case IV (K: 0.209%, Cl: 0.0%, Si=0.0%). The relative distribution of potassium predicted by equilibrium calculations was summarized as: for Case I (38% as KCl and 62% as K 2 SiO 3 ), for Case II (38% as KCl, 48% as K 2 SiO 3 and 14% as K 2 CO 3 ), for Case III (38% as KCl, and 62% as K 2 CO 3 ), and for Case IV (100% as K 2 CO 3 ). The equilibrium calculations showed that silicon and chlorine have a major effect on the formation of potassium species. The predicted amount of K 2 SiO 3 was highest in Case I due to higher content of silicon. Wei et. al. [10] performed the equilibrium calculations to investigate the chlorinealkali- minerals interactions during co-combustion of coal and straw. The equilibrium results for flue gas produced from the co-combustion of hard coal and less than 50% straw showed that most of the potassium was predicted to form KAlSi 2 O 6 (s) and less than 10% of the potassium was predicted to form KCl(g) and KOH(g). An increase in the straw fraction reduced KAlSi 2 O 6 (s) and increased KCl(g), because aluminum contents in straw are less. For the flue gas produced from the combustion of pure straw, KCl(g) was predicted to be the main species and more K 2 Si 4 O 9 (l) was formed. The potassium behavior in the flue gas produced from the co-combustion of brown coal and straw was very different from the potassium behavior in the flue gas produced from the co- 5

14 combustion of hard coal and straw. Most of the potassium was predicted to form as KCl(g), KOH(g), and K 2 SO 4 (g) because calcium(ca) and magnesium(mg) contents in brown coal are higher than in hard coal[10]. Most of calcium(ca) and magnesium(mg) react with aluminum(al), reducing aluminum(al) available for formation of condensed potassium species. Glazer et. al. [11] investigated the effect of fuel composition on the formation of chlorine and alkali metals in biomass combustion through equilibrium calculations. The results of chemical equilibrium modeling showed that for the flue gas produced from the combustion of wheat Marius and Maize (high silica biomass), most of the potassium was predicted to form silica-based compounds. For the flue gas produced from the combustion of Brasica carinata (high sulfur biomass), 40% of the potassium was predicated to form potassium sulfate. The equilibrium results indicated that the fuel composition had significant effect on potassium variation. Wei et. al. [4] performed equilibrium calculations to investigate the effect of pressure and air-fuel ratio on the behavior of gaseous chlorine and alkali metals for the flue gas and syngas produced from biomass combustion and gasification. The equilibrium calculation showed that an increase in air excess coefficient enhanced the formation of HCl(g) and KOH(g) as well as reduced the formation of KCl(g) and K(g). In biomass combustion or straw gasification, an increase in pressure enhanced the formation of 6

15 HCl(g) and reduced the amount of KCl(g), NaCl(g), or NaOH(g) formed at high temperature [4]. The equilibrium calculations by Dayton et. al. [6] shows that chlorine(cl), silicon (Si) and aluminum(al) has a major effect on the speciation of alkali metals in flue gas. Dayton et. al.[6] observed that an increase in silicon and aluminum contents increases the retention of alkali metals in condensed phase such as alkali silicates and alkali aluminosilicates [6]. Glazer et. al.[11] and Jensen et. al. [7] also showed that fuels with high Si content forms silicon-based compounds, thus reducing the formation of gas-phase alkali metals, such as KCl(g) and KOH(g). Neilson et. al.[9] found that an increase in sulfur content increases the formation of K 2 SO 4 (g), while decreasing the formation of KCl(g) and KOH(g) through sulfation reaction at high temperature. Similar findings were observed by Glazer et. al. [11]. Although the effect of major ash forming elements (Al, Si) on alkali and chlorine distribution has been studied [3, 6, 7, 9, and 11], other mineral elements (Ca, Mg,, Ti, and Mn) were not considered in the equilibrium calculations. The effect of Si and Al on the speciation of chlorine and alkali metals in flue gas was individually investigated [3, 6, 7, 9, and 11]. The presence of Ca, Mg, Ti, and Mn in flue gas may affect the retention of alkali metals by Si and Al. The equilibrium calculation by Wei et. al.[10] showed that a higher amount of Ca and Mg in flue gas reduces the retention of alkali metals in the condensed phase and increases the formation of alkali metals in the vapor phase. Since Ca and Mg react with most of the Al and form Ca 2 Al 2 O 6 (s), CaAl 2 O 4 (s), and MgAl 2 O 4 (s) 7

16 and very little Al is available to form condensed alkali components. This finding shows the necessity of considering other mineral elements such as Ca, Mg, and in the equilibrium calculation while obtaining the effect of Al and Si on distribution of chlorine and alkali metals in flue gas. Except for the work of the Wei et. al. [10,12], few publications involving equilibrium studies showed the effect of Al and Si on the speciation of chlorine and alkali metals, while considering the influence of Ca, Mg, Ti, and Mn. Most of the studies were related to co-combustion of coal and European biomass such as Danish straw, wheat Marius and Maize, and Swedish wood. Only a few studies have analyzed the influence of the minerals on the distribution of chlorine and alkali metals in flue gas for United States biomass, such as Switch grass and wheat straw. Switch grass, Corn stover, and wheat straw biomass are major agricultural residue and have the potential to supply a significant portion of America s energy needs. In addition, there is limited information on the speciation of alkali metals in biomass syngas produced. The aim of this project was to produce accurate thermodynamic equilibrium predications of potassium speciation in flue gas and syngas for corn stover, switch grass, and wheat straw under pressurized conditions, considering its interaction with the following elements: Al, Si, K, Na, Ca, Mg, S, Cl, and. 8

17 1.1 Goals and Objectives Overall goal The overall goal of this project was to develop an equilibrium model, based upon the NASA CEA code [24], to predict the effect of biomass composition on the speciation of chlorine and alkali metals in flue gas and syngas. In addition, the effect of air-fuel ratio and pressure on chlorine and alkali metal speciation was also evaluated. A detailed understanding of alkali metal speciation in syngas could greatly benefit the development of hot gas cleanup technology Objectives To develop an equilibrium model to predict the formation and distribution of chlorine and alkali metals for three kind of biomass (corn stover, switch grass and wheat straw) in flue gas and syngas. To use an equilibrium model to observe the effect of air-fuel ratio (through the coefficient ()), on the speciation of chlorine and alkali metals in flue gas and syngas. To use an equilibrium model to observe the effect of pressure on the speciation of chlorine and alkali metals in flue gas and syngas. 9

18 1.1.3 Scope of the work All of the thermodynamic equilibrium calculations were performed for the range of temperature (T): K, air-fuel ratio (): 0-1.8, and pressure (): Ma. For oxygen blown gasification system, the equilibrium calculations were performed for the range of pressure (): Ma. The equilibrium calculations were performed for corn stover, switch grass, and wheat straw considering the most of relevant elements: Al, C, Ca, Cl, H, K, Mg, N, Na, O,, S and Si. 10

19 CHATER 2 METHOD AND ROCEDURE Thermodynamic equilibrium calculations were performed using the Chemical Equilibrium Analysis (CEA) software, originally developed by NASA [24]. The stable chemical species and their physical phases were determined as a function of temperature, pressure and total composition of the system. The calculations were performed by minimization of the total Gibb s free energy for the system under a mass balance constraint. Equilibrium calculations were based on the assumption that all elements were available for reaction and kinetic limitations were ignored, the gas phase was considered ideal, and all condensed phases were assumed to be pure [24]. This simplified approach may not be an accurate representation of reality for processes that are kinetically or transport process controlled. Regardless, the purpose of the equilibrium calculations in this study was to give the equilibrium distribution of species and the interactions among the various compounds. 11

20 In this study, 462 species, 318 gas (e.g. KCl(g), HCl(g), K 2 Cl 2 (g)) and 144 condensed (e.g. K 2 Si 2 O 5 (l), K 2 Si 2 O 5 (c)) species and related thermodynamic data were used to predict the equilibrium gas and condensed-phase composition. The calculations were performed under a given initial pressure, temperature, and moles of the following elements: Al, C, Ca, Cl, H, K, Mg, N, Na, O,, S and Si. Three types of biomass (corn stover, switch grass, and wheat straw) were used. These biomasses are major agricultural residue and have the potential to supply a significant portion of America s energy needs. While corn is currently the most widely used energy crop, switch grasses are likely to become popular in the future. In Table 1, the analysis of these biomass compositions is listed [3, 8]. 12

21 Table 1. Biomass composition analysis [3, 8]. Chemical analysis (wt. %) Corn Stover Switch grass Wheat straw roximate (as received) Moisture Ash Volatile matter Fixed Carbon Ultimate ( dry) C H N S O Cl Ash Ash Si Al Na K Ca Mg

22 CHATER 3 RESULTS AND DISCUSSION 3.1 Model Validation The current equilibrium model was used to reproduce the speciation of chlorine and alkali metal results for Danish straw and Swedish wood, published by Wei et. al. [4] for validation. The results produced by the current equilibrium model matched the published results [4] perfectly. Such good agreement indicates that the current model can be used to observe the speciation of alkali metals the United States biomass combustion and gasification. 3.2 Results and Discussion Figure 1 (a,b,c) shows the equilibrium results of potassium speciation for a flue gas composition based on corn stover, switch grass, and wheat straw combustion at 20% excess air (or percent stoichiometric at = 1.2) and 0.1 Ma pressure (1 atm). At high temperatures >1400 K (2060 F), potassium is mainly found as KCl(g) and KOH(g) in the 14

23 gas phase and K 2 Si 2 O 5 (l) in the condensed phase. Due to the higher content of chlorine in switch grass, chlorides are the predominant form of potassium predicted to be in the vapor phase. While in the case of corn stover and wheat straw, K 2 Si 2 O 5 (l) is predicted to be the most important species, because of a higher silicon content. Liquid K 2 Si 2 O 5 is more prone to stick to furnace walls, which may cause slagging and fouling in the furnace. The ratio of KOH to KCl in the gas phase for wheat straw was predicted to be greater, because of a lower Cl content. At low temperatures (800 K (980 F)-1400 K (2060 F)), the main species predicted were K 2 Cl 2 (g), K 2 Si 2 O 5 (c) and KCl(c) for switch grass and K 2 Cl 2 (g), K 2 Si 2 O 5 (c), K 2 SO 4 (c) and KCl(c) for wheat straw. The KCl dimmer (K 2 Cl 2 ) has a small peak at about 1000 K( 1340 F), because KCl dimmer is thermodynamically stable in the temperature range (900 K(1160 F)-1200 K(1700 F)) (Figure 1). The increase of condensed KCl at low temperatures correlated with the decrease in the gas-phase KCl. Because of the higher content of silicon and lower content of chlorine in the wheat straw, potassium sulfate and silicate were more dominant. The alkali and chlorine speciation predictions for corn stover and wheat straw were similar due to approximately similar biomass composition, except for the presence of K 2 SO 4 (c) at lower temperatures (< 900 K (1160 F)). 15

24 Figure 1(a). otassium speciation for corn stover combustion ( = 1.2; = 0.1 Ma). 16

25 70% 60% KCl(s) K 2 Si 2 O 5 (s) KCl(g) 50% KOH(g) otassium 40% 30% 20% K 2 Si 2 O 5 (l) 10% K 2 Cl 2 (g) 0% T(K) Figure 1(b). otassium speciation for switch grass combustion ( = 1.2; = 0.1 Ma). 17

26 90% 80% K 2 Si 2 O 5 (s) 70% K 2 Si 2 O 5 (l) KOH(g) 60% otassium 50% 40% 30% KCl(s) KCl(g) 20% 10% K 2 SO 4 (s) K 2 Cl 2 (g) 0% T(K) Figure 1(c). otassium speciation for wheat straw combustion ( = 1.2; = 0.1 Ma). Figure 2 (a, b, c) shows the speciation of chlorine and potassium in combustion with various percent stoichiometric air coefficients. The quantity shown on the y-axis in these and subsequent figures is moles of species per 100 g of dry fuel. The predicted formation of HCl(g) increased significantly in the temperature range of 800K (980 F) to1000 K(1340 F). Gaseous HCl(g) formation peaked at 1000 K (1340 F). The predicted amount of HCl(g) starts reducing, thereby KCl(g) begins to form and gradually increases in the temperature range of 1000 K(1340 F) to 1800 K(2780 F) Under the combustion conditions (Figure 2-a,b,c), the percent stoichiometric air coefficient has less significant influence on the speciation of chlorine or potassium. 18

27 Increasing the percent stoichiometric air coefficient reduced the formation of HCl(g) and increased the formation of KCl(g) Mole K 2 Cl 2 (g) KOH(g) KCl(g) HCl(g) T(k) T (K) Figure 2 (a). Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for corn stover ( = 1.2, 1.5, 1.8; = 0.1 Ma). 19

28 KCl(g) Mole KOH(g) HCl(g) K 2 Cl 2 (g) T T(k) (K) Figure 2 (b). Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for switch grass ( = 1.2, 1.5, 1.8; = 0.1 Ma). 20

29 Mole KOH(g) K 2 Cl 2 (g) KCl(g) HCl(g) T(k) T (K) Figure 2 (c). Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for wheat straw ( = 1.2, 1.5, 1.8; = 0.1 Ma). Figure 3(a, b, c) illustrates the speciation of chlorine and potassium in syngas with various percent stoichiometric air coefficients. The percent stoichiometric air coefficient has significant influence on the amount of potassium and chlorine formation under gasification conditions [Figure 3-a, b, c] compared to combustion condition [Figure 2-a, b, c]. For gasification conditions, increasing the percent stoichiometric air coefficient increases the formation of HCl(g) and KOH(g) and reduces the formation of KCl(g). Under gasification conditions, increasing the percent stoichiometric air coefficient increases the concentration of H 2 O and OH radical in the syngas. The conversion of KCl(g) to HCl(g) and KOH(g) is likely to occur through the following reaction [4], 21

30 KCl(g) + H 2 O = KOH(g) + HCl(g) (1) The increase of KOH(g) formation with an increasing percent stoichiometric air coefficient might occur through the following reaction [4], K(g) + OH = KOH(g) (2) Figure 4 (a, b, c) describes the effect of pressure on the speciation of chlorine and potassium in combustion gases. At lower temperatures (800 K (980 F) to1100 K (1520 F)), increasing pressure postpones the formation of HCl(g) and K 2 Cl 2 (g). At higher temperatures (>1100 K (1520 F)), the increase in pressure increases the formation of HCl(g) and K 2 Cl 2 (g) and decreases the formation of KCl(g) and KOH(g). At higher temperatures (>1400 K (2060 F)), the effect of pressure is more significant on the formation of KOH(g) in the case of corn stover, wheat straw however the effect of pressure is more significant on the formation of KOH(g) and KCl(g) for switch grass. At lower temperatures (< 1400 K (2060 F), the pressure effect is more profound on the formation of HCl(g) and KCl(g). Figure 5 (a, b, c) describes the effect of pressure on the speciation of chlorine and potassium in syngas. ressure has more significant effect on potassium speciation in syngas (Figure 5-a, b, c) than in flue gas (Figure 4-a, b, c). At lower temperatures (800 K 22

31 (980 F) K (1520 F)), increasing pressure postpones the formation of HCl(g). At higher temperatures (>1100 K (1520 F)), increasing pressure increases the formation of HCl(g) and K 2 Cl 2 (g) and decreases the formation of KCl(g) and KOH(g). The pressure has a significant effect on the formation of HCl(g), KCl(g), and KOH(g) at both high temperatures (>1400 K (2060 F)) and low temperatures (<1400 K (2060 F)). 23

32 Mole KOH(g) KCl(g) HCl(g) K 2 Cl 2 (g) T(k) T (K) Figure 3(a). Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for corn stover ( = 0.2, 0.5, 0.8; = 0.1 Ma). 24

33 KCl(g) Mole HCl(g) KOH(g) K 2 Cl 2 (g) T(k) T (K) Figure 3 (b). Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for switch grass ( = 0.2, 0.5, 0.8; = 0.1 Ma). 25

34 KOH(g) Mole KCl(g) K 2 Cl 2 (g) HCl(g) T T(k) (K) Figure 3 (c). Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for wheat straw ( = 0.2, 0.5, 0.8; = 0.1 Ma). 26

35 Mole K 2 Cl 2 (g) KOH(g) KCl(g) HCl(g) T T(k) (K) Figure 4 (a). Speciation of potassium and chlorine in combustion with various pressures for corn stover ( = 1.2; = 0.1, 0.5, 1.0 Ma). 27

36 KCl(g) Mole KOH(g) HCl(g) K 2 Cl 2 (g) T (K) T(k) Figure 4 (b). Speciation of potassium and chlorine in combustion with various pressures for switch grass ( = 1.2; = 0.1, 0.5, 1.0 Ma). 28

37 Mole K 2 Cl 2(g) KOH(g) KCl(g) HCl(g) T(K) Figure 4 (c). Speciation of potassium and chlorine in combustion with various pressures for wheat straw ( = 1.2; = 0.1, 0.5, 1.0 Ma). 29

38 Mole KCl(g) KOH(g) HCl(g) K 2 Cl 2 (g) T T(k) (K) Figure 5 (a). Speciation of potassium and chlorine in gasification with various pressures for corn stover ( = 0.5; = 0.1, 0.5, 1.0 Ma). 30

39 KCl(g) Mole HCl(g) KOH(g) K 2 Cl 2 (g T T(k) (K) Figure 5 (b). Speciation of potassium and chlorine in gasification with various pressures for switch grass ( = 0.5; = 0.1, 0.5, 1.0 Ma). 31

40 Mole KCl(g) KOH(g) K 2 Cl 2(g) HCl(g) T T(k) (K) Figure 5 (c). Speciation of potassium and chlorine in gasification with various pressures for wheat straw ( = 0.5; = 0.1, 0.5, 1.0 Ma). The above results show that thermodynamic equilibrium calculations can be used to predict the speciation of chlorine and alkali metals in combustion and gasification processes operating at pressure range ( Ma) and in temperature range ( K). As most of the gasification plants are oxygen blown and work at elevated pressure range ( Ma), the current study was expanded for prediction of speciation of potassium and chlorine in oxygen blown gasification system working at pressure range ( Ma). Figure 6(a, b, c) illustrates the speciation of chlorine and potassium in syngas with various percent stoichiometric oxygen coefficients. The percent stoichiometric oxygen coefficient has significant influence on the amount of potassium and chlorine formation 32

41 under oxygen blown gasification condition. An increase in percent stoichiometric oxygen coefficient increases the formation of HCl(g) and KOH(g) and reduces the formation of KCl(g). Figure 7(a, b, c) the effect of pressure on the speciation of chlorine and potassium in oxygen blown gasification system. At lower temperatures (800 K-1000 K), increasing pressure postpones the formation of HCl(g). At higher temperatures (>1300 K), increasing pressure increases the formation of HCl(g) and K 2 Cl 2 (g) and decreases the formation of KCl(g) and KOH(g). 33

42 Mole KCl(g) HCl(g) K 2 Cl 2 (g) KOH(g) T (K) Figure 6 (a). Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for corn stover ( = 0.2, 0.5, 0.8; = 5.0 Ma). 34

43 KCl(g) Mole HCl(g) K 2 Cl 2 (g) KOH(g) T (K) Figure 6 (b). Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for switch grass ( = 0.2, 0.5, 0.8; = 5.0 Ma). 35

44 Mole KCl(g) HCl(g) K 2 Cl 2 (g) KOH(g) T (K) Figure 6 (c). Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for wheat straw ( = 0.2, 0.5, 0.8; = 5.0 Ma). 36

45 Mole KCl(g) HCl(g) KOH(g) K 2 Cl 2 (g) T (K) Figure 7 (a). Speciation of potassium and chlorine in oxygen blown gasification with various pressures for corn stover ( = 0.5; = 3.0, 5.0, 7.0 Ma). 37

46 KCl(g) Mole HCl(g) K KOH(g 2 Cl 2 (g) T(K) Figure 7 (b). Speciation of potassium and chlorine in oxygen blown gasification with various pressures for switch grass ( = 0.5; = 3.0, 5.0, 7.0 Ma). 38

47 Mole KCl(g) HCl(g) KOH(g) K Cl 2 (g) T (K) Figure 7 (c). Speciation of potassium and chlorine in oxygen blown gasification with various pressures for wheat straw ( = 0.5; = 3.0, 5.0, 7.0 Ma). 39

48 CHATER 4 CONCLUSION The chemical equilibrium calculations performed for three types of biomass (corn stover, switch grass, and wheat straw) in this study identified equilibrium chlorine and potassium species in combustion and gasification product gases. The speciation of chlorine and alkali species is affected by the composition of biomass. At high temperatures (> 1400 K (2060 F)), most of potassium forms as KCl(g) in high chlorine switch grass derived flue-gas while K 2 Si 2 O 5 (l) is the predominant species formed in corn stover and wheat straw derived flue-gas. At lower temperatures, because of the higher content of silicon and lower content of chlorine in straw, potassium sulphate and silicate become dominant. The distribution of chlorine and potassium is influenced by pressure and percent stiochiometric air. Under combustion conditions, the percent stiochiometric air coefficient only has a limited influence on the speciation of chlorine and potassium. Increasing percent stiochiometric air coefficient reduces HCl(g) and increases the formation of KCl(g) in high temperature range. 40

49 Compared to the results in combustion, the percent stiochiometric air coefficient has significant influence on the formation of chlorine and potassium during biomass gasification condition. At higher temperatures (>1100 K (1520 F)), increasing percent stiochiometric air coefficient increases formation of HCl(g) and KOH(g) and reduces KCl(g) formation. During biomass combustion and gasification, increasing pressure increases HCl(g) formation and reduces the formation of KCl(g) and KOH(g) in the high temperature (>1100 K (1520 F)) range. The effect of pressure on the formation of HCl(g) and KCl(g) is more significant in syngas as compared to flue gas. 41

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