Plasma Gasification Analysis of Low Rank South African Coals

TABLE OF CONTENTS

ACKNOWLEDGEMENTS

ABSTRACT

CHAPTER 1: INTRODUCTION
1.1 BACKGROUND
1.2 AIMS AND OBJECTIVES

CHAPTER 2: LITERATURE REVIEW
2.1 LOW GRADE COAL
2.1.1 COAL SOURCING
2.1.2 EFFECTS OF LOW GRAD COAL GASIFICATION
2.2 PLASMA GASIFICATION IN SOUTH AFRICA
2.3 TERNARY DIAGRAM
2.4 STOICHIOMETRIC REGION AND REACTION LINES
2.5 CARBON BOUNDARY
2.6 THERMALLY BALANCED LINE

CHAPTER 3: IDEALIZED CHO DIAGRAM
3.1 COAL CHARACTERIZATION
3.2 STOICHIOMETRIC OPERATING REGION

CHAPTER 4: EQUILIBRIUM AND MINIMIZATION OF GIBBS FREE ENERGY
4.1 THERMALLY BALANCED REACTIONS
4.2 GIBB'S FREE ENERGY MINIMIZATION
4.3 CARBON BOUNDARY
CHAPTER 5: REPRESENTATION OF EXPERIMENTAL DATA
5.1 PROCESS DESCRIPTION
5.2 EXPERIMENTAL DATA
5.2.1 EXPERIMENTAL RUN AT 1600 K
5.2.2 EXPERIMENTAL RUN AT 700 K
5.2.3 FINAL RESULT AND DISCUSSION

CHAPTER 6: CONCLUSION

REFERENCES

LIST OF TABLES

TABLE 3-1: COAL CHARACTERIZATION

TABLE 3-2: MOL FRACTION OF COAL

TABLE 3-3: BOND EQUIVALENCE OF REACTION PRODUCTS

TABLE 3-4: BOND EQUIVALENCE OF REACTIONS

TABLE 4-1: GIBBS FREE ENERGY OF FORMATION VS TEMPERATURE

TABLE 4-2: GIBBS FREE ENERGY TREND LINE EQUATIONS

TABLE 4-3: MODEL RESULTS AT 700 K AND 1 BAR

TABLE 4-4: MINIMIZATION SUMMATION RESULT AT 700 K AND 1 BAR

TABLE 4-5: CARBON BOUNDARY DATA POINTS AT 700 K AND 1 BAR

TABLE 4-6: CARBON BOUNDARY DATA POINTS AT 1600 K AND 1 BAR

TABLE 5-1: SYNGAS COMPOSITION

TABLE 5-2: SYNGAS COMPOSITION AT 700 K

LIST OF FIGURES

FIGURE 2-1: MAJOR COAL FIELDS

FIGURE 2-2: BOND EQUIVALENT DIAGRAM

FIGURE 2-3: STAR OF GASIFICATION

FIGURE 2-4: OPTIMUM OPERATING REGION FOR SAWDUST GASIFICATION

FIGURE 3-1: STOICHIOMETRIC REGION

FIGURE 4-1: THERMALLY BALANCED LINE

FIGURE 4-2: GIBBS FREE ENERGY OF FORMATION VS TEMPERATURE

FIGURE 4-3: CARBON BOUNDARIES

FIGURE 5-1: PLASMA GASIFIER PILOT PLANT

FIGURE 5-2: GAS CHROMATOGRAPHY SAMPLE ANALYSIS

FIGURE 5-3: GAS CHROMATOGRAPHY SAMPLE ANALYSIS AT 700 K

FIGURE 5-4: BE DIAGRAM WITH EXPERIMENTAL RESULT

ACKNOWLEDGEMENTS

Firstly, I would like to thank Dr. S Kauchali for his belief in the completion of My Master’s degree. His expertise and supervision has guided me throughout my work. I am very thankful to him for giving me this golden opportunity to work beside him. His continuous support and inspiration helped me during my thesis work.

This thesis would not be in this shape if not because of my co-supervisor, Dr L Van Dyk. I am most grateful for the time she spent providing insightful critique and guidance when it was most necessary.

Finally, I would like to thank my family for all their love and support from the beginning of time, and their unrelenting faith in my abilities.

ABSTRACT

Plasma systems are designed to consistently exhibit much lower environmental levels of both air emissions and slag toxicity than other competing technologies. However, correct operating conditions of the plasma process can lead to an increased production rate and a more efficient process.

A key feature of this work is to be able to predict the feed requirements and output compositions of a plasma gasification system using low grade South African coal. Furthermore, a graphical technique is proposed here that will represent regions in which gasification is feasible.

A sample of low grade coal was analysed and used as basis. A theoretical model was developed that incorporates thqe Gibb’s free energy minimization method and a bond equivalent diagram to predict the syngas composition as well as feed reactant requirements for optimal operation of a plasma gasifier. Experiments were conducted to validate the model at these optimum conditions.

To prevent carbon deposition, at a temperature of 1600 K and a pressure of 1 bar, the minimum required steam to carbon ratio is 1H2O : 0.28 C. The minimum steam to oxygen ratio is 1H2O : 1O2. The syngas analysis shows a composition of: 45.2 % H2; 37.6 % CO; 8.5 % CH4; 8.4 % CO2 and 0.3 % N2.

Plasma gasification plays a vital role in the mitigation of environmental concerns due to CO2 production. Gasification processes incorporate many reactions that are fairly complex to analyse making their design difficult to interpret. In this work, it is shown that the overall changes and product distributions, from a plasma gasification system, are severely limited by consideration of mass and energy balances only. The product distributions are constrained by basic gasification reactions and further limited to a plane or line by considering thermally balanced operations.

It is shown, for low grade coal, that a sensible gasification operation occurs within a confined region where the product distributions can be determined without considerations to reactor kinetics, reactor design and operation. This analysis is an indispensable tool for process design and flowsheet development using gasification as an intermediate step.

CHAPTER 1: INTRODUCTION

1.1 Background

South Africa has a wide array of various coal grades for export and domestic use. There are existing mines, as well as new reserves being explored and developed. According to Gupta & Katalambula (2009), low-grade coals are usually those that are low in specific energy because of high moisture and/or ash content, or produce high emissions of concern. The types of low-grade coals are usually lignites or sub-bituminous coals. There is a growing need of using these low-grade coals because of higher quest for power generation and elimination of environmental concerns during storage. In general, the direct use of the low-grade coals by combustion or incineration results in: higher costs of emissions reduction; or in lower efficiency and, consequently, higher greenhouse gas emissions (Gupta & Katalambula, 2009). Industrial companies look towards gasification as a means of reducing green house gas concerns as well as provide high efficiencies of carbon conversion to produce electricity or synthetic fuels.

Gasification is an environmentally friendly technique to transform any carbon based raw material into usable energy without burning it (Kolev and Georgiev, 1987). There are many kinds of gasification technologies all over the world, such as fixed bed gasification, fluidized bed gasification, entrained flow gasification and plasma assisted gasification.

A concern regarding the conventional (standard) gasification methods is that in the present carbon-constrained environment, there is a need to upgrade these coals in terms of low moisture, ash, and/or other trace elements. Therefore, drying is required to reduce moisture and cleaning the coal to reduce mineral content of coal and related harmful constituents such as sulfur and mercury. On the other hand plasma gasification eliminates the need for coal cleaning and drying processes. According to Kolev and Georgiev (1987), high temperature plasma gasification is the gasification of matter in an oxygen-starved environment to decompose matter into its basic molecular structure. Electricity is fed to a torch with two electrodes creating an arc. Inert gas is passed through the arc which heats up the gas to internal temperatures as high as 13,800 oC. Furthermore, in comparison to the conventional method of gasification, plasma gasification may provide higher concentrations of hydrogen which, in turn, may result in a more viable option to generate electricity or produce fuel.

One of the key advantages of plasma gasification is the flexibility of feedstock types such as waste, biomass and coals that it can be convert into valuable fuels. With landfill availability declining, power companies look towards plasma gasification not only for the waste reduction, but also to continue supplying heat, electricity and hydrogen or fuel gases. This method of gasification requires fewer utilities to run the process and is able to produce high efficiencies from coal of high ash, sulphur and moisture content (Kolev and Georgiev, 1987). Size reduction is not usually required therefore eliminates the use of grinding and milling as well as sorting and separation.

The objective of a gasification process is to convert any carbon based raw material into a fuel gas or synthesis gas composed of CO and H2 and not to completely combust any of the raw materials.

Plasma systems are designed to consistently exhibit much lower environmental levels of both air emissions and slag toxicity than other competing technologies. However, correct operating conditions of the plasma process can lead to an increased production rate and a more efficient process. Understanding what these operating conditions are, depends on the use of the synthetic gas produced, i.e. electricity or liquid fuel production.

A key feature of this work is to be able to predict the output compositions of a plasma gasification system using low grade South African coal. Furthermore, a graphical technique is proposed here that will represent regions in which gasification is feasible. This method will prove to be an indispensable tool for further downstream process design.

1.2 Aims and Objectives

The purpose of this study is to determine a feasible confined region (on the CHO diagram) to operate a plasma gasifier using low grade South African coal. Moreover, product distributions can be determined without reactor kinetics and reactor design considerations. This analysis will be an essential tool for the plasma gasification process design development. In order to achieve this, the following will be investigated:

a. To characterize a low grade coal sample by determining its ultimate analysis and calorific value

b. To estimate the outcome of the gasification process on an idealised basis by:
- A CHO diagram to define bonding capability as well as reaction representation.
- Determine the important reactions and Stoichiometric operating region for gasification using steam and /or oxygen

c. To incorporate energy considerations by

- Identifying important exothermic and endothermic reactions
- pairing reactions to obtain thermally balanced operation

d. To incorporate equilibrium considerations by

- minimisation of Gibb’s free Energy
- Plotting the carbon boundary

e. To utilize the coal sample in a pilot scale plasma gasifier to provide a representation of experimental data to compare the theoretical findings.

CHAPTER 2: LITERATURE REVIEW

2.1 Low grade coal

Since low grade coal is of high ash and moisture content, power companies are hesitant to utilize these coals due to the added costs of drying and the cleaning processes. Power companies would rather pursue the more expensive but greater power producing fuels. Consequently a large amount of lower ranked coal remains dormant or is discarded. Therefore, processes such as gasification, is a promising development in the field of low grade coal to power and fuel production.

2.1.1 Coal Sourcing

According to Pinheiro (1999), nineteen (19) coal fields are recognized in South Africa covering a surface area in excess of 9.7 million ha. Several more are developed in Swaziland, Botswana and numerous others in surrounding countries to the north, east and west of Southern Africa.

It is also shown by Pinheiro (1999) that Bituminous (low grade) coal is the predominant coal resource in the country and that this type of coal forms the core of South African resources and reserves. These coals are present in the central and northern regions. In general the coal becomes higher in rank from west to east across the country. Figure 2-1 below shows a map of South African major coal fields.

COAL FIELDS OF THE REPUBLIC OF SOUH AFRICA

2.1.2 Effects of low grad coal gasification

The general methods used for the analysis of coal namely the proximate and ultimate analysis have been standardized by all major standards institutions e.g. ASTM, ISO and others. The proximate analysis is an essential tool that provides an initial indication of the coal quality and type. The analysis determines the fixed carbon, ash, volatile matter and moisture.

Although the proximate analysis provides indicative information about the coal, it is also important to have an ultimate analysis for gasification purposes, since it provides data on elemental composition of the hydro-carbonaceous part of the coal. Elemental or ultimate analysis encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulphur, oxygen, ash as well as moisture.

The worlds total coal consumption is approximately 3724 MMtoe/y (toe is a unit of energy, tonnes of oil equivalent, approximately equal to 42 GJ of energy) (BP, 2011). Of this, South Africa accounts for a portion of the total consumption. The three Sasol plants in South Africa account for one-third of the world’s gasification capacity. Based on Operating plants, Africa and the Middle East is now the leading region in the world for gasification capacity.

Higman and v.d. Burgt, (2008) explains that coal is classified in terms of its rank, which increase from brown coal to anthracite. It is stated that brown coal, lignite and sub-bituminous coal are classified as low rank coals, whereas higher rank coals such as anthracite are known as hard coals.

2.2 Plasma gasification in South Africa

The South African Nuclear Energy Corporation (Necsa) is the nuclear research centre of South Africa. The main functions of Necsa are to undertake and promote research and development in the field of nuclear energy and related technologies. In the 1980s Necsa ventured into the plasma field and started using plasma technology to convert mineral oxides into fluorides in a one-step process. Necsa also developed plasma technology for other industrial applications, including:

- Chemical vapour deposition (CVD) on glass and other surfaces
- Mineral beneficiation (thermal dissociation of zircon)
- Plasmachemical conversion (recovery of HF and nano-SiO2 from SiF4, manufacture of HF from fluorspar)
- Nanoparticle manufacture (SiO2, TiO2, ZrO2, etc)
- Plasma-based manufacture of fluorocarbons (eg C3F6 (Hexafluoropropylene), c-C4F8 (Octafluorocyclobutane) and C2F4 (Tetrafluoroethylene) from CF4 and carbon
- Waste destruction

The major applications of plasma technology currently in South Africa are found in mineral treatment and metal alloy manufacturing. Plasmas are also used by local industries for welding, the production of ozone, lighting, surface treatment and silicon etching.

Waste treatment by a plasma process is a relatively new application in South Africa. This is, however, a well-established technology worldwide and can be evaluated alongside incineration and pyrolysis as a possible solution for waste treatment.

One application that is currently being explored by Necsa SOC, in collaboration with the Centre of Material and Process Synthesis (COMPS) at the University of the Witwatersrand, is plasma waste treatment for energy production. Such a system will consist of a refractory lined, high-temperature reactor fitted with a plasma torch, a quench probe and a waste feeder. This combination of equipment is known as the gasifier. The purpose of the gasifier is to convert general waste into syngas (CO/H2). When compared to the system for gas cleaning and fuel production, the gasifier is actually a small part of the whole process. The concentrated heat and the superior temperature of the plasma decrease the physical size significantly when compared to conventional gasification units (J van der Walt, 2012).

Although relatively new and not well-known in South Africa, many useful and proven applications have already been demonstrated for plasma technology, which offers several specific advantages when compared with conventional technologies. These advantages include superior temperature, concentrated heat source, smaller footprint and a controlled (even inert) reactor atmosphere.

2.3 Ternary diagram

A method of representing conversion processes is by the use of a bond equivalent diagram (BED), otherwise known as a ternary CHO (Carbon- Hydrogen-Oxygen) diagram (Battaerd & Evans, 1979). The ternary diagram is also known as the Gibbs triangle where each corner of the equilateral triangle represents a pure component. Points within the triangle represent ternary mixtures of the three substances based on its molar composition. Coal generally consists of C, H, O, and may be represented by CxHyOz. Where x, y, z are the atomic fractions respectively, ignoring other elements such as N, S, Cl and ash. For example, to obtain the bond equivalent fraction, the contribution by carbon is 4(x), hydrogen is 1(y) and oxygen is 2(z), which is normalized for each species. Thus CH4 is represented by:

This places the point midway between C and H. Similarly CO2 and H2O are midway between C-O and H-O respectively. CO is a third between C-O. Figure 2-2 below shows the graphical representation of the constituent bond equivalence.

An important result of the CHO diagram is the fact that individual species, mixtures of species as well as process can be represented on the BE diagram.

In the work of Battaerd and Evans (1979), a coal to liquids process is graphically represented on the CHO diagram making the interpretation of such processes easier.

2.4 Stoichiometric region and reaction lines

Yoon et al (1979) use a CHO diagram to determine the feasible region of a moving bed gasifier. Wei (1979) shows that any coal gasification process can be constrained to a region, by stoichiometry, and further to a line or plane by energy considerations. Thus complex coal gasification reaction schemes can be readily interpreted before the consideration of thermodynamic equilibrium, kinetics, reactor design and operation. The basic gasification reaction chemical formulae are shown below (Mountoris et al, 2005):

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The diagram below illustrates the star of gasification. This is the typical gasification reaction lines in which the stoichiometric region of operation may be found.

2.5 Carbon boundary

Another factor for optimum gasification operation is the influence of temperature on carbon deposition. This is also known as carbon boundary. As defined by Mountoris et al (2005), the carbon boundary is a line drawn for a group of points where marginally no solid carbon is present in the heterogeneous equilibrium with the gaseous products. This means that there is a tendency for unreacted carbon in the product stream which results in low carbon conversion.

The carbon boundary can be estimated by developing an equilibrium model that minimizes the Gibbs free energy of the system (Li et al 2011). Mountoris et al (2005), states that in the case where no solid carbon remains in the plasma equilibrium state, only two independent reactions need to be considered. These reactions are:

Similarly, equilibrium reactions will be derived for the low grade coal gasification used for this work.

In addition Mountoris et al (2005) states that for soot formation to occur, a third methane decomposition reaction needs to be considered. However since this work looks to developing a model that best predicts zero carbon formation and 100% conversion, the methane decomposition reaction will be excluded.

Visagie (2008) shows the affect that reaction kinetics has on composition. It is stated that detailed kinetic experiments were conducted to determine the relevance of reaction kinetics over a range of coal bed gasifiers. The findings were compared to a kinetic free model. The research shows that the product gas temperature and composition remains unaffected despite changes in the chemical process. The kinetic free model compared well with the kinetic model, therefore reaction kinetics will be excluded from this research.

2.6 Thermally balanced line

In an ideal situation and under adiabatic conditions, without heat loss or added heat, the system balances the endothermic reactions with the exothermic reactions (Kauchali, 2012). This is known as thermally balanced operation and is required to obtain higher thermal efficiencies of gasification processes. By considering the carbon boundary line, the thermally balanced line a region of optimum operation may be obtained as shown Figure 2-4 below for sawdust biomass gasification (Kauchali, 2012).

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Figure 2-4: Optimum Operating Region for Sawdust Gasification

The shaded area indicates the region of optimum operation. For standard gasification, products that emerge below the thermally balanced line are considered to be close to gasification temperature or on the hot side, while products plotted above the line or on the cold side, implies that an external heat source is required. According to Wei (1979) for coal gasification processes, most operations occur below the thermally balanced line. Kauchali, (2012) shows that the reason gasification processes occur below the line is due to a combination of compensation for heat losses as well as methanation in real gasification systems. In this work it is envisaged that a plasma gasifier would operate above the thermally balanced line. Furthermore, such a diagram may provide insight for plasma assisted gasification where electricity input may be minimized for more economic operations.

CHAPTER 3: IDEALIZED CHO DIAGRAM

3.1 Coal characterization

It is noted in the work of Wei (1979) that coal is represented by elemental carbon (C) for which the gasification operating region was identified. In this work it is intended that the complete CHO compositions are used to represent the coal. The coal will be characterized in terms of its ultimate analysis for calculation. The low grade coal used for this research was sourced and by the CSIR,(2009). The Table 3-1 below shows the results of the coal characterization. The ultimate analysis from here is used to determine the CHO fractions. All analysis is done on an air dry basis and values are in volume percentages.

Table 3-1: Coal characterization

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The formula which represents the ultimate analysis of the coal can now be determined. This calculation is shown in the Table 3-2 below. Hence it is shown that the formula for the coal is CH0.932O0.099. Carbon is represented as 1 as this will allow for easy plotting on the BE diagram showing the x-y axis as O-H respectively. The coal can now be represented on the Bond Equivalent diagram which is shown in the next section.

Table 3-2: Mol fraction of coal

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3.2 Stoichiometric operating region

In an important follow on work, by Wei (1979), it was shown that any coal gasification process can be constrained to a region, by stoichiometry, and further to a line or plane by energy considerations. Thus complex coal gasification reaction schemes can be interpreted readily before the consideration of thermodynamic equilibrium, kinetics, reactor design and operation.

The gasification reactions will be balanced to represent ideal coal gasifier reactions. These reactions will be plotted on the bond equivalent diagram to determine the stoichiometric region of operation, as shown in section 2.4. Since the bond equivalent method will be used to calculate the molar ratios, the lever arm rule is not applicable as it would be in the traditional mole fraction CHO diagram.

By using the method outlined in section 2.3, the coal can be represented on the BE diagram as follows: C = 0.780; H = 0.182; O = 0.039.

The products from the standard gasification reactions shown in section 2.4 are also determined using the BE method and plotted on the BE diagram. Table 3-3 below shows the bond equivalence for the reaction products.

Table 3-3: Bond equivalence of reaction products

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These points are plotted on the BE diagram and a line is drawn between the reactants of each reaction; and the products of each reaction. Where the lines intersect is known as the ideal stoichiometric region. Figure 3-1 below shows the ideal stoichiometric region.

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Figure 3-1: Stoichiometric region

It is noted that these reactions, which form part of the extreme boundary, span all sensible gasification products within the region.

These reactions are chosen on the initial premise that no product should contain any reactant, hence any reaction that forms steam (or oxygen) is automatically rejected. On the low side of the region (r10 and r11) is most likely oxygen (or air) gasification, and on the upper end (r8 andr9) is commonly steam gasification. Also, operation of a gasification system to the left of r8 and r10 implies that the feed contains more coal than steam and oxygen, which inherently implies that unreacted coal (carbon) should be expected at the exit of the reactor. Similarly, operating to the right of r9 and r11 implies that the feed contains more steam/oxygen which will leave the gasifier unreacted, implying non-optimal usage of steam/oxygen. It is in this context that it is implied that sensible gasification occurs within the region.

To confirm the correctness of the intersection points, the stoichiometric balanced equations can be used:

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By Finding the mol fractions of the products and using the BE method, the following bond equivalents are calculated, as shown in Table 3-4. These results accurately match the intersection points on the BE diagram.

Table 3-4: Bond equivalence of reactions

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CHAPTER 4: EQUILIBRIUM AND MINIMIZATION OF GIBBS FREE ENERGY

4.1 Thermally balanced reactions

An exothermic reaction occurs when the temperature of a system increases due to the evolution of heat. This heat is released into the surroundings, resulting in an overall negative quantity for the heat of reaction (qrxn < 0 ). An endothermic reaction occurs when the temperature of an isolated system decreases while the surroundings of a non-isolated system gains heat. Endothermic reactions result in an overall positive heat of reaction (qrxn > 0).

Exothermic and endothermic reactions cause energy level differences and therefore differences in enthalpy (AH), the sum of all potential and kinetic energies. AH is determined by the system, not the surrounding environment in a reaction. A system that releases heat to the surroundings, an exothermic reaction, has a negative AH by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system.

When the gasifier is run under adiabatic conditions, without heat loss or added heat, the system balances the exothermic reactions with the endothermic reactions. Some important reactions considered are:

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ChemPack was used to determine the heats of reaction for each of the above reactions. The two exothermic reactions (r13 and r14) can be used to balance the two endothermic reactions (r12 and r15). Balancing the reactions simply means that the heat of reaction becomes 0 kJ/mol and the inlet temperature is equal to the exit temperature (Kauchali, 2012). The mechanism for balancing the equation is as follows:

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The bond equivalents from r16 and r17 can now be determined and plotted on the BE ternary diagram. See Figure 4-1 below.

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Figure 4-1: Thermally balanced line

From the figure above, the thermally balanced line is shown for the linear combination of endothermic and exothermic reactions. For this case, no methane is formed. When methane is not produced, any thermally balanced process can be obtained by the linear combination of the two balanced basis reactions. Below r16-r17, the products emerge hotter, while above the line they are colder.

Furthermore, point at r16 is preferred under low H2O/O2 ratios while the point at r17 would be preferred for high H2O/O2 ratios. According to Wei (1979) for coal gasification processes, most operations occur below the thermally balanced line and on the hot side. The reason is a combination of compensation for heat losses as well as methanation in real gasification systems. Operating in the colder section is an indication of external heat sources used to drive the endothermic reactions or coal with higher hydrogen content than the one used for this analysis.

4.2 Gibb’s free energy minimization

Overall the Gibbs free energy minimization method is an essential tool for determining product parameters. However, I order to explain Gibb’s free energy, chemical equilibrium must be expanded further, as it is the basis to the Gibbs formation. Chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction (Lower, nd). It is at equilibrium that a chemical reaction is most stable.

A general description of Gibb’s free energy is a quantitative measure of the favorability of a given reaction in terms of its difference( AGj that is (or would be) effected by proceeding with the reaction. When the calculated energy of the process indicates that AG is negative, it means that the reaction will be favoured and will release energy. The energy released equals the maximum amount of work that can be performed as a result of the chemical reaction. In contrast, if conditions indicated a positive AG, then energy, in the form of work, would have to be added to the reacting system for the reaction to occur (Lower, 2010).

Chemical equilibrium calculations have traditionally been made through the use of equilibrium constants of known reactions, a procedure still useful for simple problems. However, when the equilibrium composition is determined by a number of simultaneous reactions, the computations required become complex and tedious. A more direct and general method for solving these complicated problems is the direct minimization of the Gibbs function of the system (Y Lwin, 2000).

In order to determine an accurate analysis of the plasma gasification process, a Gibbs free energy model is developed. For the development of this model approach, the number of independent reactions has to be determined by applying the phase rule. In the case where no solid carbon remains in the equilibrium state, only two independent reactions need to be considered for the equilibrium equations. In the case of some remaining solid carbon, i.e. soot, in the gasification products, three independent reactions have to be considered in the equilibrium calculations. However, since we are assuming complete combustion for this study, only two equations are used. Any unreacted carbon will be proof that an additional reaction is needed.

Two examples from the literature relevant to the gasification process are shown here. Zainal et al, (2001) have selected one reaction resulting from the combination of the Primary water gas shift reaction; heterogeneous water gas shift reaction and the hydrogenating gasification as the main gasification reactions, while Schuster et al, (2001) have selected the gas shift reaction along with the methane decomposition reaction. According to the thermodynamic theory of independent reaction selection, there is no significant difference between the above reported modeling efforts.

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Modeling the above method would be necessary provided that AG° data is unavailable. Alternatively, AG° can be determined provided there is known data for each compound or element in the reaction. The following data was extracted from kauchali (2010). The Table 4-1 below shows the AG° data of each compound at various temperatures.

Table 4-1: Gibbs free energy of formation vs temperature

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Below is the plot of given Gibbs free energy of formation data for the components at various temperatures shown in the table above. Trend lines were fitted to the data so that the Gibbs free energy of formation values could be calculated for all components at any temperature.

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Figure 4-2: Gibbs free energy of formation vs temperature

The Figure 4-2 above shows the graphical representation of the Gibbs free energy of formation at various temperatures of each component. Trend line equations were extracted for each compound is given in the table below. Where the x axis is temperature (T) and the y axis is AG°(T)

Table 4-2: Gibbs free energy trend line equations

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4.3 Carbon boundary

Li et a/.(2001) have studied the effect of temperature and pressure on carbon formation in gasification systems. It was identified that it is common for carbon to partially gasify and, due to kinetic limitations, solid carbon does not achieve equilibrium. Furthermore, the carbon boundary, under thermodynamic limits, may be represented on the CHO diagram as isotherms at constant pressure.

Operating a gasification process within the carbon boundary indicates that there is a propensity for unreacted carbon to occur in the product stream. This results in low carbon conversions with some carbon remaining in the ash. Moreover, it is desirable to operate in a carbon-free region.

It is just before equilibrium that carbon deposition will occur and is the reason why equilibrium is incorporated into the model. Since the equilibrium constant (K) is a function of Gibb’s free energy, K is a determining factor of minimizing AG and finding the carbon boundary. The equilibrium constant is a ratio of the concentration of the products to the concentration of the reactants. If the K value is less than one the reaction will move to the left and if the K value is greater than one the reaction will move to the right. From thermodynamic principles it is shown that:

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In order to determine the carbon boundary, the Gibbs minimization can now be used. The analysed coal was used as feed input as well as a constant temperature of 700 K and 1 bar for the first carbon boundary. For the second carbon boundary, 1600 K at 1 bar was used. The temperature selection was based on the experimental results, which will be discussed further in the next section.

An example of the model calculation at 700 K is shown in the table below. These results are subject to a few constraints and conditions:

- Mass balance - the number of moles of each element in the reactants must be equal to the number of moles of each element in the product
- The equilibrium constants in equation 25 and 26 must be equal to equation 23.
- The fugacity term in the model is 1.

Adding the above constraints to the solver function in excel and setting the objective function to “min” whilst changing the initial moles, ni of all components in the product, results in the following:

Table 4-3: Model results at 700 K and 1 bar

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Table 4-4: Minimization summation result at 700 K and 1 bar

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By using the above inputs and constraints, then minimizing AG, we find the equilibrium mole fractions. This can now be used to determine the bond equivalence at 700K at 1 bar and can be plotted on the BE ternary diagram. To further plot points on the BE diagram to extrapolate a carbon boundary, the

oxygen bond equivalent composition is fixed between 0 and 0.5. Solver is then run to minimize Gibb’s free energy and determine the hydrogen bond equivalent at constant temperature and pressure of 700 K and 1 bar respectively. The following results were obtained.

Table 4-5: Carbon boundary data points at 700 K and 1 bar

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Similarly the results for 1600 K and 1 bar pressure, results in the following data.

Table 4-6: Carbon boundary data points at 1600 K and 1 bar

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The following Figure 4-3 shows the trend lines of the data from the above two tables which represent the carbon boundaries for both 700 K and 1600 K.

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Figure 4-3: Carbon boundaries

The figure above shows the complete theoretical representation of low grade coal plasma gasification using a particular feed material. The shaded region is now the region of optimal operability (steam to carbon and steam to oxygen ratio). The area below the carbon represents the region in which carbon deposition occurs. As can be seen in Figure 4-3, the thermally balanced line lies intersects the 1600 K isotherm.

The minimum steam to carbon ratio required can be determined by drawing a line from the oxygen node through the point of intersection between the thermally balanced line and the isothermal carbon boundary and to the line drawn between the coal feed and H2O. This gives the minimum steam to carbon ratio required while the maximum steam to carbon ratio can be determined by drawing a line from the oxygen node through the end point of the thermally balanced line (which lies outside the region of carbon deposition) and a line drawn between the coal feed and H2O. Operating at any ratio between the min and max will eliminate the occurrence of carbon deposition.

Similarly, the minimum steam to oxygen ratio required can be determined by drawing a line from the coal feed through the point of intersection between the thermally balanced line and the isothermal carbon boundary and to the line drawn between O and H2O. This gives the minimum steam to oxygen ratio required while the maximum steam to oxygen ratio can be determined by drawing a line from the coal feed to the end point of the thermally balanced line (which lies outside the region of carbon deposition) and a line drawn between O and H2O. Operation at any ratio between the min and max will eliminate the occurrence of carbon deposition.

CHAPTER 5: REPRESENTATION OF EXPERIMENTAL DATA

The Nuclear Energy Corporation of South Africa (NECSA) plasma technology group specializes in high temperature and plasma chemistry for the development of advanced plasma systems and applications such as the gasification and reduction of the volume of a variety of waste materials.

NECSA provides a small scale plasma gasifier in the laboratory specifically for plasma experiments and gasification analysis. The main components of the plasma gasification unit are:

- The plasma generater with an electric power unit
- The plasma reactor
- Purification unit and;
- Gas cooling unit

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5.1 Process description

The plasma system used for this work is schematically presented in Figure 5-1. A 30 kW direct current power supply provides power to a water-cooled non- transferred arc plasma torch. The plasma is started with argon at 99% purity.

Steam is used as the main source to generate the plasma flame due to its non­linear dependence of its enthalpy on temperature as a result of its dissociation at high temperatures to produce elemental oxygen, hydrogen and hydrogen gas, which also reacts actively with the organic mass of coal.

Oxygen in the form of air is stoichiometrically used as the initial plasma source in order to carry out the release of carbon monoxide (CO).

Once power is supplied between the electrodes, an electric arc is created and electric current flows from one electrode to the other (anode and cathode). By passing pressurized air through the arc creates the plasma tail flame, a few minutes later the air was replaced by steam. Once the plasma flame is controlled (temperature and pressure and steam flow rate) the reactor is allowed to warm up for 20 minutes, then Argon is used as the carrier gas to transfer the coal into the reactor.

Once the coal is broken down into its elemental state, the syngas produced is stripped from particulates by a purifier and then cooled by the use of a gas cooler or heat exchanger. The cooled gas is then analysed.

5.2 Experimental data

5.2.1 Experimental run at 1600 K

The coal was milled to a semi fine powder and the experiment was carried out within the following limits and conditions:

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In order to determine the above input requirements, refer to Figure 4-3. As described in section 2.5, the steam to carbon ratio is determined by drawing a line between the oxygen node and the point of intersection of the thermally balanced line and the carbon boundary, to the line drawn between the coal and steam (H2O).

By using the Pythagorus method (x^[2] + y^[2] = z^[2] ), this indicates that the distance from the coal feed to this point is approximately 0.122 and the distance from the point to the steam node is 0.438. It can therefore be shown that the steam to carbon ratio is given by 1C : 3.57H2O.

The steam to oxygen ratio is determined by drawing a line between the coal feed to the point of intersection of the thermally balanced line and the carbon boundary, to the line drawn between steam and oxygen. Again, by using Pythagorus, this indicates that the distance from the steam node to this point is 0.35 making it equidistant. This implies that the steam to oxygen ratio is 1H2O : IO2.

It was given that the feed rate to the plasma gasifier is between 0.3 - 0.6 g/s, this was used as the basis of calculation.

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Once the syngas was produced, a sample was taken and analysed by the use of a gas chromatograph. A gas chromatograph is a chemical analysis instrument for separating chemicals in a complex sample. The coil inside the tube of the gas chromatograph is lined with an absorbent. The syngas sample is introduced and Argon again is used as the carrier gas at a specific rate. Since the syngas is composed of compounds of different physical and chemical properties, the time each component in the syngas reaches the detector is different. This detection is transferred to a digital signal on a computer screen where a peak is shown at different times for each constituent of the syngas sample. The Figure 5-2 below shows the results of the syngas sample.

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Figure 5-2: Gas chromatography sample analysis

The figure above shows the amplitude vs time that the components of the syngas pass the detector. Numbers 1 - 4 represent H2; CO2; CH4; and CO respectively. The peak height and width is directly proportional to the molar composition of each component in the syngas. It can be seen from Figure 5-2 that Hydrogen gas (1) has the highest peak and a comparative half width, which gives hydrogen the highest composition. The results from the gas chromotograph show the following compositions:

Table 5-1: Syngas composition

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5.2.2 Experimental run at 700 K

For this experiment, all the operating conditions remained the same as that of the 1600 K experiment, except the temperature. The temperature of this experiment was changed to 700 K. The procedure from above was followed and the results are shown below.

Table 5-2: Syngas composition at 700 K

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5.2.3 Final result and discussion

The bond equivalence for the syngas at 700 K and 1600 K can now be determined and plotted on the BE diagram. See Figure 5-4 below.

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Figure 5-4: BE diagram with experimental result

Point A in Figure 5-4 above shows the bond equivalence of syngas based on the experimental data taken from the syngas analysis. Since the plasma gasifier operating conditions were predetermined based on the minimum steam to carbon ratio; minimum steam to oxygen ratio; temperature and pressure, it was expected that the experimental data point would lie on the 1600 K carbon boundary and that there would be zero carbon deposition.

Point A lies on the 1600 K isotherm which implies that the model is accurate. However, doing a full mass balance over the plasma gasifier shows that from a 50 g sample of coal, analysis of the elemental composition of the slag revealed that there was a 1.4 % carbon content. This indicates that the total carbon conversion is approximately 98.6 % with a small amount of carbon dioxide produced. Thus making the process environmentally viable. The margin of error between the model and experimental data is most likely due to significant figures used throughout the model; human error during the experiment and minor unforeseen reactions within the gasifier. It is also noted that realistically it is virtually impossible to achieve 100% carbon conversion.

Thus, for the 700 K carbon boundary, the entire thermally balanced line lies below the carbon boundary. Therefore, a plasma gasifier operating at 700K will always experience carbon deposition, regardless of the steam to carbon and steam to oxygen ratios. The results from Figure 5-3 and Table 5-2 are represented as a point (B) on the BE diagram. The theoretical diagram indicates that at 700 K, it is expected that point B lies below the carbon boundary and above the thermally balanced line. Since the temperature is low, this is an indication that additional heat is required. Given that the operating parameters are the same as the previous experiment, it is expected that the steam requirements are in excess and the oxygen requirements are limited. This is therefore, the reason for carbon deposition to occur since the limited oxygen does not provide complete carbon conversion.

The data shown throughout this study indicates that for: a given quantity of coal and its ultimate analysis; specific known gasification reactions; and required operating temperature and pressure, the model developed for this study can be used to provide a fairly accurate prediction of the operating conditions and parameters.

CHAPTER 6: CONCLUSION

Since gasification provides high carbon conversion by partial combustion to produce a synthesis gas with little environmental concern, makes it a viable technology. The results within this study indicate that plasma gasification of low grade coal can produce almost complete carbon conversion with little environmental effect, whilst other methods may produce a greater concern in terms of environment effects. The outcome from this study however, shows that in order to achieve such a result of near complete carbon conversion and low environmental concern, the oxygen and steam requirements are high, thus increasing cost of feed resources and electricity.

While gasification systems are complex, the important reactions are represented by basis reactions that span the stoichiometric region of operation on a CHO or BE diagram. The operation of thermally balanced gasification systems, is further represented by a line within the stoichiometric region. It is verified from the pilot plasma gasification plant data for gasification of low grade coal, that the operation, at 1600 K occurs within the stoichiometric region and on the hot-side of the thermally balanced line. Since the 700 K isotherm (carbon boundary) does not intersect the thermally balanced line, temperatures at 700 K and less, will always result in unreacted carbon. It is therefore required to operate this system at temperatures above 700 K at the correct steam to carbon and steam to oxygen ratios.

The analysis in this study thus enables the determination of outputs from low grade coal gasification which can further be used to design downstream processes.

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