SPECIATION OF ALKALI METALS IN BIOMASS COMBUSTION AND GASIFICATION

by

PAVANKUMAR BAJRANG SONWANE

HENG BAN, COMMITTEE CHAIR

THOMAS K. GALE

PETER 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

SPECIATION OF ALKALI METALS IN BIOMASS COMBUSTION AND GASIFICATION

SONWANE PAVANKUMAR 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

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

K2Cl2. In biomass combustion and gasification, increasing pressure increases vapor HCl

and K2Cl2 and reduces the amount of vapor KCl and KOH. At higher temperatures

(>1100K), the gaseous alkali species increased greatly.

iii

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. Peter 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

TABLE OF CONTENTS

Page

ABSTRACT........................................................................................................................ ii

ACKNOWLEDGMENTS ................................................................................................. iv

LIST OF TABLES............................................................................................................. vi

LIST OF FIGURES .......................................................................................................... vii

CHAPTER

1. INTRODUCTION AND BACKGROUND ...........................................................1

1.1. Goals and Objectives ........................................................................................9

1.1.1. Overall goal............................................................................................9

1.1.2. Objectives ..............................................................................................9

1.1.3. Scope of the work ................................................................................10

2. METHOD AND PROCEDURE...........................................................................11

3. RESULTS AND DISCUSSION...........................................................................14

3.1. Model Validation ............................................................................................14

3.2. Results and Discussion ...................................................................................14

4. CONCLUSION.....................................................................................................40

LIST OF REFERENCES..........................................................................................42

v

LIST OF TABLES

Table Page

1. Biomass composition analysis ..................................................................................13

vi

LIST OF FIGURES

Figure Page

1-a. Potassium speciation for corn stover combustion (λ = 1.2; P = 0.1 MPa)................16

1-b. Potassium speciation for switch grass combustion (λ = 1.2; P = 0.1 MPa)..............17

1-c. Potassium speciation for wheat straw combustion (λ = 1.2; P = 0.1 MPa) ..............18

2-a. Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for corn stover (λ = 1.2, 1.5, 1.8; P = 0.1 MPa). ......19

2-b. Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for switch grass (λ = 1.2, 1.5, 1.8; P = 0.1 MPa). ....20

2-c. Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for wheat straw (λ = 1.2, 1.5, 1.8; P = 0.1 MPa). .....21

3-a. Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for corn stover (λ = 0.2, 0.5, 0.8; P = 0.1 MPa). ......24

3-b. Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for switch grass (λ = 0.2, 0.5, 0.8; P = 0.1 MPa). ....25

3-c. Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficient for wheat straw (λ = 0.2, 0.5, 0.8; P = 0.1 MPa). ......26

4-a. Speciation of potassium and chlorine in combustion with various pressures for corn stover (λ = 1.2; P = 0.1, 0.5, 1.0 MPa)......................................................................27

vii

4-b. Speciation of potassium and chlorine in combustion with various pressures for switch grass (λ = 1.2; P = 0.1, 0.5, 1.0 MPa)............................................................28

4-c. Speciation of potassium and chlorine in combustion with various pressures for wheat straw (λ = 1.2; P = 0.1, 0.5, 1.0 MPa) ............................................................29

5-a. Speciation of potassium and chlorine in gasification with various pressures for corn stover (λ = 0.5; P = 0.1, 0.5, 1.0 MPa)......................................................................30

5-b. Speciation of potassium and chlorine in gasification with various pressures for switch grass (λ = 0.5; P = 0.1, 0.5, 1.0 MPa)............................................................31

5-c. Speciation of potassium and chlorine in gasification with various pressures for wheat straw (λ = 0.5; P = 0.1, 0.5, 1.0 MPa) ............................................................32

6-a. Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for corn stover (λ = 0.2, 0.5, 0.8; P = 5.0 MPa)..........................................................................................................................34

6-b. Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for switch grass (λ = 0.2, 0.5, 0.8; P = 5.0 MPa)....................................................................................................................35

6-c. Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for wheat straw (λ = 0.2, 0.5, 0.8; P = 5.0 MPa)....................................................................................................................36

7-a. Speciation of potassium and chlorine in oxygen blown gasification with various pressures for corn stover (λ = 0.5; P = 3.0, 5.0, 7.0 MPa) ........................................37

7-b. Speciation of potassium and chlorine in oxygen blown gasification with various pressures for switch grass (λ = 0.5; P = 3.0, 5.0, 7.0 MPa) ......................................38

7-c. Speciation of potassium and chlorine in oxygen blown gasification with various pressures for wheat straw (λ = 0.5; P = 3.0, 5.0, 7.0 MPa).......................................39

viii

CHAPTER 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 CO2

emissions, biomass and biomass-derived fuels have become an attractive and viable

1

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 CO2 level.

However, biomass typically has a high content of potassium (0.5-1.3 wt%), chlorine (0.2-

0.7 wt%), and silicon (0.3-1.0 wt%), as well as minor amounts of Ca, Mg, Al, Na, Fe, S,

and P [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

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 (KAlSi3O8), 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 (KAlSi3O8). 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 K2SO4(g) and the remaining

70% of the potassium was predicted to form potassium silicates and K2SO4(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 (KAlSi3O8). Similar

results were obtained from predictions of wheat straw and Pittsburgh coal co-combustion

flue gas, except the alkali metal capture (in solid phase) was less efficient, because silicon

and aluminum contents in Pittsburgh 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

(KAlSi3O8). The equilibrium calculation results indicated that an increase in silicon and

3

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), K2SO4(s) and K2SiO3(s) at

lower temperatures, whereas at higher temperatures the gaseous KCl(g) and KOH(g)

were the thermodynamically stable species. Formation of K2SO4(g) was predicted to be

in the temperature range of 1000oC to 1300oC. The equilibrium calculation predicted

KCl(g) was the most stable form of chlorine and potassium at temperatures above 600oC.

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 K2SO4(g). This may be due to an

increase in sulfation of KCl(g) and KOH(g) to K2SO4(g) at high temperature [9].

4

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

K2SiO3), for Case II (38% as KCl, 48% as K2SiO3 and 14% as K2CO3), for Case III (38%

as KCl, and 62% as K2CO3), and for Case IV (100% as K2CO3). The equilibrium

calculations showed that silicon and chlorine have a major effect on the formation of

potassium species. The predicted amount of K2SiO3 was highest in Case I due to higher

content of silicon.

Wei et. al. [10] performed the equilibrium calculations to investigate the chlorine-

alkali- 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 KAlSi2O6(s) and less than

10% of the potassium was predicted to form KCl(g) and KOH(g). An increase in the

straw fraction reduced KAlSi2O6(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 K2Si4O9(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

combustion of hard coal and straw. Most of the potassium was predicted to form as

KCl(g), KOH(g), and K2SO4(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

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 K2SO4(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, P, 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 Ca2Al2O6(s), CaAl2O4(s), and MgAl2O4(s)

7

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 P 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, P

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 P.

8

1.1 Goals and Objectives

1.1.1 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.

1.1.2 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 (λ)), 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

1.1.3 Scope of the work

All of the thermodynamic equilibrium calculations were performed for the range

of temperature (T): 800-1800 K, air-fuel ratio (λ): 0-1.8, and pressure (P): 0.1-1.0

MPa. For oxygen blown gasification system, the equilibrium calculations were

performed for the range of pressure (P): 3.0-7.0 MPa. 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, P, S and Si.

10

CHAPTER 2

METHOD AND PROCEDURE

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

In this study, 462 species, 318 gas (e.g. KCl(g), HCl(g), K2Cl2(g)) and 144

condensed (e.g. K2Si2O5(l), K2Si2O5(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, P, 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

Table 1. Biomass composition analysis [3, 8].

Chemical analysis (wt. %) Corn Stover Switch grass Wheat straw

Proximate (as received)

Moisture 6.06 8.16 6.5

Ash 4.75 4.22 5.7

Volatile matter 75.96 72.73 70.87

Fixed Carbon 13.23 14.89 16.92

Ultimate ( dry)

C 46.82 46.86 46.1

H 5.73 5.84 5.6

N 0.66 0.58 0.5

S 0.11 0.11 0.08

O 41.62 42.02 41.7

Cl 0.26 0.5 0.288

Ash 5.1 4.59 6.1

Ash

Si 1.2 0.94 1.49

Al 0.05 0.03 2.3

Na 0.01 0.007 0.024

K 1.08 0.989 0.958

Ca 0.29 0.223 0.331

Mg 0.18 0.117 0.068

P 0.18 0.284 -

13

CHAPTER 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 λ = 1.2) and 0.1 MPa pressure (1 atm). At high

temperatures >1400 K (2060°F), potassium is mainly found as KCl(g) and KOH(g) in the

14

gas phase and K2Si2O5(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, K2Si2O5(l) is predicted to

be the most important species, because of a higher silicon content. Liquid K2Si2O5 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 K2Cl2(g), K2Si2O5(c) and KCl(c) for switch

grass and K2Cl2(g), K2Si2O5(c), K2SO4(c) and KCl(c) for wheat straw. The KCl dimmer

(K2Cl2) 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 K2SO4(c)

at lower temperatures (< 900 K (1160°F)).

15

Figure 1(a). Potassium speciation for corn stover combustion (λ = 1.2; P = 0.1 MPa).

16

0%

10%

20%

30%

40%

50%

60%

70%

800 1000 1200 1400 1600 1800

T(K)

Pot

assi

um

KCl(g)

KOH(g)

K2Si2O5(l)

KCl(s)

K2Cl2(g)

K2Si2O5(s)

Figure 1(b). Potassium speciation for switch grass combustion (λ = 1.2; P = 0.1 MPa).

17

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

800 1000 1200 1400 1600 1800

T(K)

Pot

assi

umKCl(g)

KOH(g)K2Si2O5(l)

KCl(s)

K2Cl2(g)

K2Si2O5(s)

K2SO4(s)

Figure 1(c). Potassium speciation for wheat straw combustion (λ = 1.2; P = 0.1 MPa).

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

Increasing the percent stoichiometric air coefficient reduced the formation of HCl(g) and

increased the formation of KCl(g).

0

0.005

0.01

0.015

0.02

0.025

800 1000 1200 1400 1600 1800

T(k)

Mol

e

KCl(g)

KOH(g)

λ

λ

HCl(g)λλK2Cl2(g)

T (K)

Figure 2 (a). Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for corn stover (λ = 1.2, 1.5, 1.8; P = 0.1 MPa).

19

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

800 1000 1200 1400 1600 1800

T(k)

Mol

e

K2Cl2(g)

KCl(g)

KOH(g)HCl(g)

λ

λ

λ

λ

λ

T (K)

Figure 2 (b). Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for switch grass (λ = 1.2, 1.5, 1.8; P = 0.1 MPa).

20

0

0.005

0.01

0.015

0.02

0.025

800 1000 1200 1400 1600 1800

T(k)

Mol

e KOH(g)

KCl(g)

HCl(g)K2Cl2(g)

λ

λ

λ

T (K)

Figure 2 (c). Speciation of potassium and chlorine in combustion with various percent stoichiometric air coefficients for wheat straw (λ = 1.2, 1.5, 1.8; P = 0.1 MPa).

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 H2O 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

KCl(g) + H2O = 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 K2Cl2(g). At higher

temperatures (>1100 K (1520°F)), the increase in pressure increases the formation of

HCl(g) and K2Cl2(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. Pressure 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

(980°F) -1100 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 K2Cl2(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

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

800 1000 1200 1400 1600 1800

T(k)

Mol

e

HCl(g)K2Cl2(g)

KCl(g)

KOH(g)

λ

λ

λ

T (K)

Figure 3(a). Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for corn stover (λ = 0.2, 0.5, 0.8; P = 0.1 MPa).

24

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

800 1000 1200 1400 1600 1800

T(k)

Mol

e

K2Cl2(g)

KCl(g)

KOH(g)

HCl(g)

λ

λ

λ

T (K)

Figure 3 (b). Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for switch grass (λ = 0.2, 0.5, 0.8; P = 0.1 MPa).

25

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

800 1000 1200 1400 1600 1800

T(k)

Mol

e

KOH(g)

KCl(g)

HCl(g)K2Cl2(g)

λ

λ

λ

T (K)

Figure 3 (c). Speciation of potassium and chlorine in gasification with various percent stoichiometric air coefficients for wheat straw (λ = 0.2, 0.5, 0.8; P = 0.1 MPa).

26

0.000

0.005

0.010

0.015

0.020

0.025

800 1000 1200 1400 1600 1800

T(k)

Mol

e

HCl(g)

P

K2Cl2(g)

KCl(g)

KOH(g)

P

P

T (K)

Figure 4 (a). Speciation of potassium and chlorine in combustion with various pressures for corn stover (λ = 1.2; P = 0.1, 0.5, 1.0 MPa).

27

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

800 1000 1200 1400 1600 1800

T(k)

Mol

eKOH(g)

KCl(g)

HCl(g)

K2Cl2(g)

P

P

P

P

T (K)

Figure 4 (b). Speciation of potassium and chlorine in combustion with various pressures for switch grass (λ = 1.2; P = 0.1, 0.5, 1.0 MPa).

28

0

0.005

0.01

0.015

0.02

0.025

800 1000 1200 1400 1600 1800

T(K)

Mol

eKOH(g)

KCl(g)

HCl(g)K2Cl2(g)

PP

P

Figure 4 (c). Speciation of potassium and chlorine in combustion with various pressures for wheat straw (λ = 1.2; P = 0.1, 0.5, 1.0 MPa).

29

0

0.002

0.004

0.006

0.008

0.01

0.012

800 1000 1200 1400 1600 1800

T(k)

Mol

e

HCl(g)

K2Cl2(g)

KCl(g)

KOH(g)PP

P

T (K)

Figure 5 (a). Speciation of potassium and chlorine in gasification with various pressures for corn stover (λ = 0.5; P = 0.1, 0.5, 1.0 MPa).

30

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

800 1000 1200 1400 1600 1800

T(k)

Mol

e

K2Cl2(g

KCl(g)

KOH(g)

HCl(g)

P

PP

)

T (K)

Figure 5 (b). Speciation of potassium and chlorine in gasification with various pressures for switch grass (λ = 0.5; P = 0.1, 0.5, 1.0 MPa).

31

0

0.002

0.004

0.006

0.008

0.01

0.012

800 1000 1200 1400 1600 1800

T(k)

Mol

e

KOH(g)

KCl(g)

HCl(g)

K2Cl2(g)

PP

P

T (K)

Figure 5 (c). Speciation of potassium and chlorine in gasification with various pressures for wheat straw (λ = 0.5; P = 0.1, 0.5, 1.0 MPa).

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 (0.1-1.0 MPa) and in temperature range (800-1800

K). As most of the gasification plants are oxygen blown and work at elevated pressure

range (3.0-7.0 MPa), the current study was expanded for prediction of speciation of

potassium and chlorine in oxygen blown gasification system working at pressure range

(3.0-7.0 MPa).

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

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 K2Cl2(g) and decreases the

formation of KCl(g) and KOH(g).

33

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

0.005

800 1000 1200 1400 1600 1800

T (K)

Mol

e

KCl(g)

HCl(g)

KOH(g)K2Cl2(g) λ

λ

λ

λ

Figure 6 (a). Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for corn stover (λ = 0.2, 0.5, 0.8; P = 5.0 MPa).

34

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

800 1000 1200 1400 1600 1800

T (K)

Mol

e

KCl(g)

HCl(g)

K2Cl2(g)KOH(g) λ

λ

λ

λ

Figure 6 (b). Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for switch grass (λ = 0.2, 0.5, 0.8; P = 5.0 MPa).

35

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

800 1000 1200 1400 1600 1800

T (K)

Mol

e

K2Cl2(g)

KCl(g)

KOH(g)

HCl(g)

λ

λ

λ

λ

Figure 6 (c). Speciation of potassium and chlorine in oxygen blown gasification with various percent stoichiometric oxygen coefficients for wheat straw (λ = 0.2, 0.5, 0.8; P = 5.0 MPa).

36

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

800 1000 1200 1400 1600 1800

T (K)

Mol

eKCl(g)

HCl(g)

KOH(g)K2Cl2(g)P

P

P

P

Figure 7 (a). Speciation of potassium and chlorine in oxygen blown gasification with various pressures for corn stover (λ = 0.5; P = 3.0, 5.0, 7.0 MPa).

37

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

800 1000 1200 1400 1600 1800

T(K)

Mol

e

P

HCl(g)

KOH(gK2Cl2(g)

KCl(g)

P

P

P

Figure 7 (b). Speciation of potassium and chlorine in oxygen blown gasification with various pressures for switch grass (λ = 0.5; P = 3.0, 5.0, 7.0 MPa).

38

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

800 1000 1200 1400 1600 1800

T (K)

Mol

e

K2Cl2(g)

KCl(g)

KOH(g)

HCl(g)

PP

P

P

Figure 7 (c). Speciation of potassium and chlorine in oxygen blown gasification with various pressures for wheat straw (λ = 0.5; P = 3.0, 5.0, 7.0 MPa).

39

CHAPTER 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 K2Si2O5(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

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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44