Index of /rozprawy2/11468

Pełen tekst

(1)AGH University of Science and Technology in Kraków. Faculty of Energy and Fuels DEPARTMENT OF FUEL TECHNOLOGY. Dissertation Name and surname:. Katarzyna ŚPIEWAK. Field of study:. CHEMICAL TECHNOLOGY. Topic of dissertation:. The influence of catalytic additives on pressurised coal gasification Topic of dissertation in Polish:. Wpływ dodatków katalitycznych na proces ciśnieniowego zgazowania węgla. Mark:. ………………………………………. Supervisor:. dr hab. Stanisław Porada Co-supervisor:. dr inż. Grzegorz Czerski Kraków, 2019.

(2) Acknowledgements. I would like to especially thank. my supervisor Stanisław Porada and my co-supervisor Grzegorz Czerski,. as well as all people who help in the creation of this work..

(3) Table of Contents Introduction .............................................................................................................................. 6 1.. Coal gasification process................................................................................................... 9 1.1.. Fundamentals of the coal gasification process ........................................................................ 9. 1.1.1.. Thermodynamics of gasification reactions.................................................................... 11. 1.1.2.. Gasifiers for coal gasification........................................................................................ 14. 1.1.2.. Underground coal gasification (UCG) .......................................................................... 17. 1.1.2.. Coals for gasification process........................................................................................ 19. 1.2.. Development of gasification technology ............................................................................... 21. 1.2.1.. 2.. Current coal gasification technologies .......................................................................... 22. 1.3.. Application of syngas from coal gasification ........................................................................ 26. 1.4.. Challenges of coal gasification technology ........................................................................... 30. Kinetics of coal gasification process .............................................................................. 32 2.1.. Experimental methods for studying gasification process kinetics ....................................... 33. 2.1.1.. Techniques for examination of the gasification process ............................................... 33. 2.1.2.. Methods of process conduction ..................................................................................... 35. 2.2.. 3.. 4.. Calculation methods for studying gasification process kinetics........................................... 37. 2.2.1.. Model fitting methods .................................................................................................. 37. 2.2.2.. Isoconversional Methods (Model-Free methods)......................................................... 38. 2.3.. Models for description coal gasification reactions ............................................................... 39. 2.4.. Factors affected the kinetics of coal gasification ................................................................. 41. 2.4.1.. Properties of feedstock ................................................................................................. 42. 2.4.2.. Conditions of the gasification process.......................................................................... 46. 2.4.3.. Conditions of pyrolysis ................................................................................................ 50. 2.4.4.. Inhibiting effect of gaseous products ............................................................................ 52. Catalytic coal gasification process ................................................................................. 53 3.1.. Type of catalysts used ........................................................................................................... 53. 3.2.. Mechanism of the catalytic coal gasification process ........................................................... 56. 3.3.. Methods of catalysts addition ................................................................................................ 60. 3.4.. Catalysts loading ................................................................................................................... 61. 3.5.. Effect of catalysts on the kinetics of the gasification process ............................................... 63. 3.6.. Catalytic gasification technologies........................................................................................ 64. Thesis, objectives and scope of the work ....................................................................... 66 4.1.. Thesis .................................................................................................................................... 66.

(4) 4.2.. 5.. Methodology of the research .......................................................................................... 69 5.1.. Characteristic of coal ..................................................................................................... 69. 5.1.2.. Characteristic of coal samples with catalysts ................................................................ 72. Description of laboratory equipment ............................................................................ 80. 5.2.2.. Measurements conditions .............................................................................................. 81. 6.1.. Formation rate of gasification products................................................................................ 87. 6.2.. Progress of CO and H2 formation reactions ......................................................................... 91. 6.3.. Yields and compositions of the resulting gas ....................................................................... 92. 6.4.. Carbon conversion degree .................................................................................................... 94. 6.5.. Summary .............................................................................................................................. 96. Assessment of ‘Janina’ coal gasification with one˗component catalysts .................... 98 7.1.. Formation rate of gasification products................................................................................ 98. 7.2.. Progress of CO and H2 formation reactions ....................................................................... 103. 7.3.. Yield and composition of resulting gas .............................................................................. 107. 7.4.. Carbon conversion degree .................................................................................................. 114. 7.5.. Kinetic parameters.............................................................................................................. 120. 7.5.1.. Kinetic parameters of CO and H2 formation reaction ................................................. 121. 7.5.2.. Kinetic parameters of carbon conversion reaction ...................................................... 129. Summary ............................................................................................................................. 135. Effect of composite catalysts on ‘Janina’ coal gasification process .......................... 137 8.1.. Formation rate of gasification products............................................................................. 137. 8.2.. Progress of CO and H2 formation reactions ....................................................................... 141. 8.3.. Yields of gaseous products ................................................................................................. 144. 8.4.. Carbon conversion degree .................................................................................................. 150. 8.5.. Kinetic parameters.............................................................................................................. 155. 8.5.1.. Kinetic parameters of CO and H2 formation reaction ................................................. 156. 8.5.2.. Kinetic parameters of the carbon conversion reaction ................................................ 161. 8.6.. 9.. Methodology of calculation of parameters describing the gasification process .................... 83. Evaluation of legitimacy of catalysts utilisation in the ‘Janina’ coal gasification process .............................................................................................................................. 87. 7.6.. 8.. Laboratory equipment for gasification measurements .......................................................... 79. 5.2.1.. 5.3.. 7.. Material ................................................................................................................................. 69. 5.1.1.. 5.2.. 6.. Objectives and scope of the work......................................................................................... 67. Summary ............................................................................................................................ 164. Comparison of one-component catalysts with composite catalysts .......................... 166 9.1.. Halt-times of H2 and CO formation reactions .................................................................... 167.

(5) 9.2.. Yields of gaseous components .......................................................................................... 168. 9.3.. Reactivity indicators ........................................................................................................... 171. 9.4.. Kinetic parameters.............................................................................................................. 172. 9.5.. Summary ............................................................................................................................ 176. 10. Conclusion ...................................................................................................................... 177 List of Figures ....................................................................................................................... 181 List of Tables ......................................................................................................................... 184 References.............................................................................................................................. 186.

(6) Introduction The global, intense demand growth for energy and chemicals makes many industries confronting with the challenge of effectively addressing these needs in a manner consistent with environmental standards. Coal-based technologies undoubtedly fit into this frame. Coal is mainly associated with the power industry since in 2017 38.1% of global, and 21.1% of the E.U. power generation was based on this fossil fuel. In regions abundant in coal deposits this share was even higher, e.g. 79% in Poland [1]. However, due to volatility in oil and natural gas prices coal is also used as feedstock for chemical production [2]. The vast and varied use of coal is caused by its availability, abundance, and affordability - reserves of coal and lignite amount to 705 Gtce (billion tonnes of coal equivalent) (see Fig. 1.1) and are more favourably distributed than those of oil and natural gas [3]. As a result, reserves of coal are sufficient to meet 114 years of global production, while resources of other fossil fuels are depleting much faster (oil reserves are sufficient to meet 50.7 years, gas reserves to meet 52.8 years) [4] (data from 2017).. Fig. 1. Global reserves of hard coal and lignite [3]. The largest producer and consumer of coal is China, but coal is also appreciated by the United States, India, Australia and the European Union [4]. However, given global warming, identified with greenhouse gases emissions from the combustion of fossil fuel (in particular coal), a policy has been launched to reduce CO2 emissions into the atmosphere. The first, major step in this direction was the Kyoto Protocol adopted in 1997 [5] that did not apply to the largest emitter – China [6]. The Kyoto Protocol was replaced by Paris 6.

(7) Agreement accepted in 2015 and ratified so far by 169 parties (including China emitting 27.6% of total emissions in 2017) [6]. The agreement will enter into force at the latest in 2020, and its primary goal is to restrict global average temperature increase to less than 2, and if possible to 1.5°C compared to the pre-industrial age as a result of joint, solidarity actions [7]. To achieve these goals, the EU implies a move away from an economy driven by fossil fuels, especially coal. However, renewable energy is not able to meet current needs, especially as it requires reliable backup from conventional energy sources [8]. Moreover, decarbonisation will increase energy prices, unemployment and energy dependence of the E.U. countries. Thus, the elimination of global dependence on coal is currently impossible. Therefore, it is necessary to develop so-called clean coal technologies (CCTs), i.e. methods that enable efficient and environmentally friendly use of coal. CCTs are designed to improve the efficiency of coal extraction, treatment, processing and utilisation and to increase the acceptability of these processes from the point of view of environmental impact. One of these technologies is gasification, i.e. process of converting solid fuel into gas under high temperature, usually high pressure and the atmosphere of gasifying agent [9]. The gas produced consists mainly of carbon monoxide and hydrogen (so-called syngas) and is used in chemical industry, power industry and for the production of liquid or gaseous fuels [10]. Gasification is a mature and safe technology that has great potential to become one of the most efficient coal conversion processes since it offers a plethora of advantages, such as: o feedstock. flexibility:. from. anthracite. to. low-quality. brown. coals,. alone. or in combination, o facilitated impurities removal: almost entire removal of CO2, SOx, NOx, and trace contaminants such as Hg, Ar, Cd from syngas is possible, o facilitated carbon dioxide removal: carbon dioxide capture and sequestration is easier/cheaper than in traditional coal combustion systems, o product flexibility: a variety of commodities can be produced from syngas, including substrates for chemicals, power and fuels production, o efficiency of electricity production: power plant based on syngas are characterised by higher thermal efficiency and reduced water consumption than a conventional coal-fired plant, o diversification of fuels: production of liquid or gaseous fuels from coal reduce energy dependency from other countries, 7.

(8) o easy disposal of process waste – waste such as slag may be used in, e.g. cement industry. The fact that the gasification is a well-known technology is a good starting point for its development, modernisation and coping with problems emerging during operation of gasifiers. However, the implementation of new, modernised gasification technologies is associated with high investment costs. Thus, the process has to be very efficient to make the construction of new plants cost-effective [10]. For this reason, researches are carried out on the appropriate selection of coal type, type of gasifier and process conditions [9]. Moreover, the process improvement through the addition of catalysts is widely analysed, especially in case of gasifiers operating at relatively low temperatures [11]. Catalysts have the capacity of increasing process rate and carbon conversion degree, lowering process temperature and shortening its duration, which is highly advantageous from an economic point of view [11]. An essential feature of catalysts is also their selectivity, which enables the production of gas with the desired composition that meets requirements for further use [11]. Based on the available knowledge in the field of catalytic coal gasification, cheap and efficient catalysts can be pre-selected. These are mainly alkali and alkaline earth metals, which have a good chance of being used on an industrial scale [12]. However, the choice of catalyst is problematic since, in addition to its type, the catalyst loading and dispersion degree are also relevant [12]. Thus, the addition of catalyst intensifies the complexity of the gasification, and to thoroughly understand the process, studies of its kinetics should be carried out. Considering the above, the influence of one-component as well as composite catalysts based on potassium, sodium and calcium in various amounts on the steam gasification of ‘Janina’. coal. was. investigated.. The. measurements. were. carried. out. using. the thermovolumetric method under isothermal conditions and elevated pressure. On their basis the following were determined: i) kinetics of the formation reactions of main gas components (CO and H2) as well as the conversion reaction; ii) composition and yield of the resulting gas; and iii) reactivity indicators, including maximum carbon conversion degree. As a result, it was possible to determine the impact of individual catalysts on the gasification process, their selectivity as well as to define the most efficient catalyst for pressurised steam gasification of ‘Janina’ coal. The performed research was aimed at contributing to. the. development. of. gasification. process. and not liquidation of environment-friendly coal-based technologies. 8. to. ensure flowering.

(9) 1. Coal gasification process The foundations of the gasification process were formed in the early 1800s during the production of ‘town gas’ (through heating of coal without oxygen) for residual/industrial heating and lighting [13]. However, to increase coal fraction converted into gas, in the mid1800s process with a gasifying agent was developed (the first patent for gasifier was obtained by LURGI in 1887) [13]. Since then gasification technology was developing, and the syngas has found new applications. In the first half of the twentieth century, syngas was used in the chemical industry for the production of methane by catalytic hydrogenation of carbon monoxide, whereas in 1923 for the synthesis of liquid hydrocarbons (Fisher-Tropsh synthesis) [14]. These developments initiated further research into gasification process until natural gas replaced manufactured syngas. However, interest in the gasification process has returned due to both the scarcity as well as high prices of natural gas and oil. Currently, it is enjoying a renaissance since it enables the production of many valuable products in an environmentally friendly way. Thus, the gasification has grown from a simple conversion process to advanced, multi-product technology of today and tomorrow.. 1.1.. Fundamentals of the coal gasification process. Gasification is a thermo-chemical process of converting coal into gas. Under high temperatures and with gasifying agents such as steam, carbon dioxide, hydrogen or their mixtures the coal molecules break apart, initiating series of heterogeneous and homogeneous reactions accompanied by mass and heat exchange processes. As a result syngas, i.e. a mixture of gaseous components, as well as mineral residue in the form of ash or slag (depending on process conditions), are formed. Moreover, at low operating temperatures, other by-products such as condensable volatile and liquid tar substances may also be created [15]. The composition of syngas depends on the gasifying agent used; however, the typical composition is within the range presented in Table 1.1.. 9.

(10) Table 1.1. Typical gas composition from coal gasification [16]. Component (vol %) CO 30-60 H2 25-30 CO2 5-30 CH4 0-5 H2O 2-30 H2S 0.2-1 COS 0-0.1 NH3 + HCN 0-0.3. The heat required for gasification and to maintain an operating temperature in gasifier may come from partial combustion of vapours, combustible gas and char with a controlled amount of oxidiser (autothermal process) or from external sources (allothermal process). In industrial conditions, an autothermal process is carried out, the successive steps of which are shown in Figure 1.1.. DRYING wet coal = dry coal + H2O. H E A T. COMBUSTION REACTIONS. tar, CH4, H2, CO2, CO, H2O, CnHn. CO2, CO, H2O. C + H2O(g) = GASIFICATION CO(g) + H2(g) REACTIONS. CH4, H2, CO2, CO, H2O. Fig. 1.1. Successive steps of the autothermal gasification process.. 10. SYNGAS. PYROLYSIS dry coal = pyrolytic products + char char. H2O.

(11) In. laboratory conditions. the. allothermal. process. is. usually carried. out,. that can be divided into the following stages: drying, pyrolysis and char gasification. In the first stage that occurs at 100-200°C, the moisture evaporates from the coal. Further heating to about 300°C causes a release of CO2 and light hydrocarbons adsorbed on the coal surface. Subsequently, rapid and relatively short pyrolysis stage, i.e. release of volatiles takes place. Due to the complex structure of coal, the pyrolysis stage is difficult to describe with chemical reactions; however, certain characteristic transformations can be distinguished. At 300-500°C so-called primary pyrolysis occurs, i.e. an intensive decomposition of organic matter, as a result of which char is formed, and the following are excreted: aromatic hydrocarbons that condense to low-temperature tar, pyrogenetic water, unsaturated and paraffinic hydrocarbons, methane and its homologs, carbon monoxide and dioxide. Above 500°C secondary. pyrolysis. occurs,. namely. further. thermal. decomposition. accompanied. by an ordering of the char structure as well as excretion of the remaining part of tar and light gases, such as H2, CH4, CO, H2S. At temperatures higher than 750°C (typically>1000°C) the last, slowest gasification stage takes place, during which char is converted as a result of reactions with the oxidising agent [10, 14]. The most important reaction during steam gasification process is a carbon-steam reaction, whereas during gasification in an atmosphere of carbon dioxide - Boudouard reaction [10]: Carbon-steam gasification reaction. Bouduard reaction. Cs + H2O ↔ C(O) + H2. (1.1). Cs + CO2 ↔ C(O) + CO. (1.3). C(O) → Cs + CO. (1.2). C(O) → Cs + CO. (1.4). Where: Cs represents a potential active site that may bind oxygen-containing gases (steam, CO2). During the process the dissociated molecules of these gases are adsorbed on the carbon surface, resulting in the formation of C(O) complexes and release of molecules of hydrogen (1.1) or carbon monoxide (1.3). The adsorbed gases create adsorption layers, which after decomposition form CO molecules and new active sites (1.2, 1.4) [17]. Thus, gasification is a complex process that results in gaseous products such as CO, H2, CH4, CO2, liquid and tar substances as well as coke/slag. 1.1.1.. Thermodynamics of gasification reactions. During the gasification stage, reactions between the char and gasifying agent are accompanied by many homo- and heterogeneous reactions. The most important gasification reactions that should be considered from a thermodynamics point of view are [15]: 11.

(12) Cs + H2O ↔ CO + H2. ΔH°298K = 131 kJ∙mol−1*. (1.5). Cs + CO2 ↔ 2 CO. ΔH°298K = 172 kJ∙mol−1*. (1.6). Cs + 2 H2 ↔ CH4. ΔH°298K = −75 kJ∙mol−1*. (1.7). CO + H2O ↔ CO2 + H2. ΔH°298K = −42 kJ∙mol−1. (1.8). *. enthalpy of C corresponds to the enthalpy of graphite Gasification reactions are reversible, and their direction is subjected to the constraints. of thermodynamic equilibrium. According to Le Chatelier's principle, as the temperature rises, the equilibrium of the endothermic reactions (1.5) and (1.6) is shifted towards the products, whereas the exothermic reactions (1.7) and (1.8) towards the substrates. In turn, increase in pressure does not affect the equilibrium position of the reaction (1.8), shifts the equilibrium of reactions (1.5) and (1.6) towards substrates, while reaction (1.7) towards the products, as can be seen in Table 1.2. Table 1.2. Effect of process conditions on the equilibrium position of gasification reactions. Reaction Cs + H2O ↔ CO + H2 Cs + CO2 ↔ 2 CO Cs + 2 H2 ↔ CH4 CO + H2O ↔ CO2 + H2. Conditions ↑Temperature ↑Pressure ↑Temperature ↑Pressure ↑Temperature ↑Pressure ↑Temperature ↑Pressure. Direction of reaction → ← → ← ← → ← =. In summary, high temperatures promote the formation of CO and H2 but counteract the formation of CH4, whereas high pressure has the opposite effect on their formation. Thus, process conditions need to be chosen carefully. The preferred temperatures for C-H2O reaction (1.5) are those above 830°C, where the equilibrium is entirely on the side of the products. The Boudouard reaction (1.6) is especially significant at temperatures above 730°C, while methanation reaction (1.7) proceed at above 630°C in the presence of catalysts. The equilibrium of water–gas shift reaction (1.8) is shifted to the products at low temperatures, therefore is usually conducted in a separate reactor at temperatures 200–250°C (low-temperature shift) or 310-450oC (high-temperature shift) with the use of catalysts [18]. The effect of pressure on the composition of the syngas from coal gasification at 700°C is presented in Figure 1.2. As can be seen, at industrially preferred pressures the concentrations of CO and H2 decreased, while the content of other components grew compared to lower 12.

(13) pressures. However, according to Highman [10], above temperatures of 1200°C, the syngas composition shows little changes as a function of pressure.. Fig. 1.2. Effect of pressure on syngas composition from steam/air coal gasification at 700°C [19]. It should be kept in mind that commercial gasification process is conducted under pressure mainly for economics reason since it provides a reduction in capital costs (through increased single-unit capacities) and in operating costs of gas purification (through increased partial pressure). In term of temperature, properly selected conditions need to ensure process efficiency, economics as well as consider ash fusion behaviour. The ash melting point is crucial since it determines the limiting temperature of the process (upper or lower, depending on the type of gasifiers) [20], and it depends on the ash composition and the process atmosphere, as can be seen in Figure 1.3. [21].. Fig. 1.3. Influence of ash composition on melting temperature in an oxidising atmosphere tB(o) and half reducing atmosphere tB(or) [21]. 13.

(14) 1.1.2. Gasifiers for coal gasification The coal gasification is conducted in reactors called gasifiers. There are many criteria by which coal gasifiers can be classified, as can be seen in Table 1.3. Table 1.3. Classification of gasifiers [22]. Criteria of classification Generation Method of heat supply Type of bed Direction of fuel flow through the gasifier Direction of gas flow through relative to coal Form of removed mineral matter Method of coal supply Number of feedstock dosage stages Construction of gasifier wall Method of gas cooling Gasifying agent. Division I II III old technologies dominant technologies developing technologies Autothermal Allothermal Moving bed Fluidised bed Entrained bed Upwards. Downwards. Co-current. Countercurrent. Ash. Agglomerate. Slag. Dry. Slurry. Single-stage. Two-stage. Refractory linings Heat recovery in exchangers Oxygen. Water screen. Water screen + slag layer. Direct water cooling. Chemical. Air. Steam. Carbon dioxide. The division due to the type of bed describes the gasifiers in the most comprehensive way. According to this criterion, three types of gasifiers can be distinguished: 1. Moving-bed gasifiers Moving-bed gasifiers are based on the oldest and the most mature Lurgi technology. There are two basic types of moving-bed gasifiers: i) with slag removal; ii) with unmelted ash removal. The process is carried out at a pressure of 3-10 MPa, while the temperature in the combustion zone reaches 1500-1800°C and 1300°C for gasifier with slag and ash removal, respectively. Coal crushed to 5-80 mm enters the top of the gasifier and is sequentially preheated, dried, pyrolysed, gasified and combusted while moving towards the grate. A mixture of steam with air or oxygen is usually entered from the bottom; thus the produced gas is cooled by the incoming, permeable feed to 400-500°C. Coal residence times range from 15 to 60 min (at high pressure and with steam/O2 mixture) up to several hours (at atmospheric pressure and with steam/air mixture). The most significant disadvantages of these gasifiers are high-temperature gradients in the bed and the presence 14.

(15) of tar, phenol and higher hydrocarbons in the produced gas [23]. The temperature profile in moving-bed gasifier is shown in Figure 1.4.. Fig. 1.4. Temperature profile in moving-bed gasifier [24]. 2. Fluidised-bed gasifiers Fluidised-bed gasifiers are the least used gasification reactors. Crushed coal (0.5-6 mm to even 0.5 μm for circulating fluidised-bed) is introduced at the bottom of the gasifier into an upward flow of gasifying agent that enable to keep the particles suspended above the grate. The bed is usually formed by sand, coke, sorbent and ash. The coal remains in the reactor between 10-100 s, but this time can be extended depending on the amount of feedstock. The fluidised-bed gasifiers work at relatively low temperatures (900-1050°C), which prevent the melting of ash but results in a low carbon conversion degree. Thus, the char formed during the process is often separated and used in the combustion units or recycled to the gasifier. The most important advantages of these gasifiers are maintaining a uniform distribution of temperature inside the reaction zone, high flexibility (can operate at various loads and variable fuel parameters), a possibility of using sulphated coals (sulphur is removed by sorbent) and lower costs of construction materials [23]. The temperature profile in fluidised-bed gasifier is shown in Figure 1.5.. 15.

(16) Fig. 1.5. Temperature profile in fluidised-bed gasifier [24]. As the flow rate of gasifying agent increases, the bed passes gradually through the bubbling, turbulent and fast state up to pneumatic transport. Different states of fluidised material became the basis for distinguishing several types of fluidised-bed gasifiers [25]: o. Bubbling Fluidised Bed – bubbles filled with flowing gas and a small amount of solid material are formed within the bed. The obtained good mixing of ingredients is conducive to the process but preclude separation of fly ash from unreacted char. Thus, both components are extracted together from the reactor resulting in lower efficiency, similarly as free diffusion of oxygen from bubbles to emulsion phase [25].. o. Circulating Fluidised Bed – refers to the fast state when the velocity of the fluidising gas is higher than the velocity of free grain falling. In this solution recirculation loops are used that allow separation of unreacted char and its recycling to the gasifier, increasing the fuel conversion degree. Substantial advantages are also low amounts of hydrocarbons in the raw gas and the possibility of using various coals as a feedstock. In turn, ash agglomeration and corrosion are considered the main disadvantages [26].. o. Transport Reactor – can be considered as a separate type of gasifiers, but the similarity to fluidised-bed reactors allows qualifying them into this group. The high flow rate of gas ensures good mixing and heat exchange as well as blowing away all grains from the reactor. The transport reactors consist of two sections: i) mixing zone where recirculating char, ash, sorbent and gasifying agent are supplied. In this zone, fuel is combusted which is accompanied by heat generation and coal degasification. ii) rising zone where raw coal is supplied. Coal is heated through a flow of hot gases and particles from the mixing zone, and the reactions of char gasification, steam reforming of methane or water gas shift reaction take place [26]. 16.

(17) 3. Entrained-bed gasifiers Entrained-bed gasifiers are the most widely used and the most advanced type of gasifiers. It is caused by high degrees of coal conversion with a short residence time (seconds) as well as flexibility of coal used. However, this technology requires pulverisation of coal (below 0.1 mm), which is subjected to the gasifier in a dry or wet state in the stream of gasifying agent. The gasification reactions take place in a dense cloud of fine particles. The entrained-bed gasifiers work at high temperatures (1200-1600°C) and pressures (2˗8 MPa); thus ash is discharged in the form of slag. The high temperature favours the formation of gas deprived of tar and oils; however, high-efficiency cooling systems are required. Moreover, due to the low concentration of suspended dust, it is critical to control the coal/oxidant ratio within narrow limits to maintain a stable flame close to the injector tip [23]. The temperature profile in entrained-bed gasifier is shown in Figure 1.6.. Fig. 1.6. Temperature profile in entrained-bed gasifier [24].. 1.1.2. Underground coal gasification (UCG) A natural geological formation containing unmined coal can be considered a specific type of gasifier. Underground coal gasification is much more complicated than above-ground gasification since this is combined extraction and conversion process for the production of synthetic gas from an in-situ coal seam. The first experiments at underground gasification were carried out in England in 1912. However, the substantial research and development programs on large-scale UCG technology started in the 1930s in the former Soviet Union, and there had been in commercial operation for over 50 years. Recently, the UCG is gaining renewed interest, what is reflected by an increase in the number of pilot plants throughout the world [27]. The general concept of UCG is shown in Figure 1.7. 17.

(18) Fig. 1.7. Scheme of underground coal gasification [27]. The coal is ignited by an electric coil or by gas firing. The compressed gasification agents are fed into the coal seam through drilled injection wells which trigger and controls in situ sub-stoichiometric. combustion. process. (combustion. zone).. Then,. along. with the direction of the gasifying agent flow, the reduction and dry distillation and pyrolysis zones appear, ensuring continuous gasification reactions (each zone is characterised by different temperatures, pressures, and gas compositions). As a result, a syngas consisting mostly of CO, H2, and CH4 is produced, extracted by production wells and processed for further use. Other products of the process are H2S, As, Hg, Pb, and ash [27]. In order to conduct UCG the proper preparation of a coal seam is necessary. Two methods may be used for this purpose [28]: 1) shaft methods - use coal mine galleries and shafts to transport gasification reagents and products. It is the first technique used in the UCG, currently only employed in a closed coal mine, 2) shaftless methods - employ directional drilling techniques. In-seam boreholes for oxidant injection and product collection are created using drilling and completion technology. Lately, most of the research is focused on these methods. The significant advantage of UCG is the possibility of using coal resources that are unrecoverable through conventional extraction. It is estimated that only 15-20% of the total coal resources can be recovered by mining; thus the implementation of UCG would substantially extend the lifetime of resources. Moreover, UCG eliminates many of the challenges connected with standard mining practices (elevated risk to people, severe changes in landscapes or high costs of machinery). Finally, feasibility studies point out that it is technically possible to conduct UCG-CCS (underground coal gasification - carbon 18.

(19) capture and storage) operation. The CO2 may be stored within voids created during underground gasification, which may reduce its emission and limit the need for storage sites identification. Despite these advantages, there are substantial uncertainties concerning the technological challenges and environmental risks of UCG, which inhibits the development and implementation of this technology [29]. 1.1.2. Coals for gasification process Almost any carbon material such as coal, oil, char, biomass or waste can be subjected to gasification. However, as can be seen in Figure 1.8., among the operating, under construction and planned gasification plants, coal is by far the most frequently used feedstock.. Fig. 1.8. Primary feedstocks for gasification [30]. Such strong domination of coal is caused by its price and energy density as well as by versatility of gasification plants designed for coal (e.g. petcoke can be gasified in plants for coal). Oil and gas are more expensive than coal, while biomass and waste are characterised by low energy density, thus by a high cost of feedstock supplies. Therefore, coal is and will be the primary feedstock for the gasification process, which is particularly crucial for the development of gasification technology in countries with large deposits of poor-quality coal. Nevertheless, the choice of coal is usually the least modifiable factor from the geographical, economic and political point of view. Thus, it is necessary to adapt the gasification technology to the available coal [31]. The requirements for coal intended for gasification in various gasifiers are presented below.. 19.

(20) 1. Moving-bed gasifier In order to achieve stable and efficient operation of the gasifier, it is necessary to avoid pressure drops by ensuring mass and heat transfer through the coal bed. The bed permeability depends on coal properties, such as [31, 32]: o. Caking properties - while heating of caking coals they turn into a plastic state, melt and sinter, resulting in larger particles. To avoid it, low caking coal or a blend of caking and no-caking coal should be gasified.. o. Ash properties  Ash fusion temperature (AFT) - low AFT may result in the formation of fused ash and clinkers in the ash bed that cause channel burning and pressure drop. Since the presence of Ca, Fe and Na in the ash result in lower AFT, the contents of these elements have to be limited by, for example, coals blending.  Ash content – coals with ash content up to 30% can be gasified in moving-bed gasifier.. 2. Fluidised-bed gasifiers [31, 33] The most important properties of coal intended for gasification in fluidised-bed gasifiers are: o. Reactivity - coal has to be sufficiently reactive to undergo strongly endothermic reactions especially that these gasifiers work at relatively low temperatures. Thus, reactivity determines the suitability of coal for the process.. o. Free swelling index (index describing ability of coal to swelling during heating) - high free swelling index indicates that particles will swell and may combine to form agglomerates. To avoid this phenomenon coal with a low swelling index or blend of swelling and non-swelling coals should be gasified.. o. Sulphur content – there are no restrictions on sulphur content since it can be almost entirely retained in bed by use of sorbents.. o. Ash properties  Ash fusion temperature – cannot be too low because softening of mineral matter components can lead to agglomeration and uneven fluidisation. To avoid these problems, the coals characterised by AFT higher than operating temperature (>1000°C) should be used.  Ash content – can be high, especially in Bubbling Fluidised Bed gasifiers.. 20.

(21) 3. Entrained-bed gasifiers [31, 34] The most important properties of coal intended for gasification in entrained-bed gasifiers are: o. Grindability - coal has to be pulverised to a particle size below 0.1 mm, thus has to have high Hardgrove Grindability Index (HGI), indicating its ability to pulverisation.. o. Slurryability - ability to create a stable mixture with water is essential for coals supplied in the form of a slurry. Such properties have, in particular, bituminous coals with low content of inherent moisture and hydrophobic nature.. o. Reactivity – any coal can be handled in these gasifiers since high coal fragmentation and temperature ensure a high degree of carbon conversion. Nevertheless, reactive coals can be gasified at a lower temperature.. o. Ash/Slag properties  Ash content – coals with high ash content are not recommended since it leads to increased slag production.  Ash composition – content of some component, such as SiO2, CaO or iron oxides that can penetrate the refractory materials and accelerate their deterioration, should be as low as possible.  Ash fusion temperature/Slag viscosity – the AFT should be lower than the operating temperature to ensure its flow and discharge from the gasifier as molten slag. Moreover, the viscosity of slag has to be sufficiently low (about 25-15 Pas). In order to lower the AFT and viscosity, the blend of coal feedstock with flux (limestone) or with coal characterised by low AFT should be used.. o. Sulphur/chlorine content – the tolerance of gasifiers to corrosive H2S or chlorine that poisons catalyst depends on the operating conditions and resistance of the material used in the cooling, cleaning and tapping systems.. 1.2. Development of gasification technology The development of gasification technology on the basis of thermal power in the produced syngas from 1970 to 2022 is presented in Figure 1.9. The first explicit peak was noted after the oil crisis, followed by the second oil crisis in 1979-1982 caused by the Iranian revolution. The post-crisis lowering of oil prices caused most of the innovative projects from that period were abandoned, and new ones were not planned. Only recently, since 2010, there has been a noticeable increase in thermal power of syngas from gasification 21.

(22) process, which was caused by the protection of the natural environment as well as search for new, efficient and sustainable technologies of coal processing.. Fig. 1.9. Cumulative capacity of synthesis gas over the years with a probable increase [30]. The returning interest in gasification technology is also reflected in the number of plants that are under construction and planned to implement. According to the latest database [30], in 2018 there were 451 plants in the world equipped with 1074 gasifiers with a total capacity of 206 GWth. Currently, 129 plants with a capacity of 108 GWth are under construction and forecasts assume that by 2022 there will be another 135 plants with a capacity of 115 GWth. As can be seen, plans assume the construction of installations with higher thermal capacity than before, which proves the dynamic development of gasification. For many years, South Africa and the U.S. were the leaders in the application of gasification technologies, but this situation has changed. Currently, mainly due to the dynamic development of gasification in China, the capacity of gasifiers in Asia is higher than this of the rest of the world put together. From other parts of the world, South Africa and Qatar play a role in syngas production [30]. 1.2.1. Current coal gasification technologies Development of the gasification process results in numerous commercial technologies, as can be seen in Figure 1.10.. 22.

(23) Coal Gasification Technology. Fluidised-Bed. Entrained-Bed. Slurry feed. Moving-Bed. HTW. Dry feed SHELL. Dry Ash Gasifiers. Slagging Gasifiers. Lurgi. BGL. U-Gas AFB. Prenflo Siemens BHEL GE. GreatPoint Energy E-Gas IDGCC ECUST KBR MHI. Fig. 1.10. Commercial coal gasification technologies. Currently most common used coal gasification technologies include: o Moving-bed technologies, such as: . Lurgi Fixed-Bed Dry Bottom Technology. The Lurgi process was originated in Germany in 1927. The counter-current, pressurised reactor is fed with highly reactive coals and a mixture of oxygen/steam, while ash is discharged in dry-state. The process is carried out at a pressure of 3 MPa, at temperature 1000-1300°C, and carbon conversion reaches up to 98%. Currently, the Lurgi gasifier is used in 6 gasification plants for production of, among others, SNG (Beulah, North Dakota), Fischer-Tropsch fuels (Secunda, South Africa), ammonia (Shanxi, China), and power (IGCC power plant in Vřesova, Czech Republic) [25, 35]. . British Gas Lurgi Technology (BGL). The development of BGL slagging moving-bed gasifier started in 1953 in Holten, Germany. It is targeted to low-reactive coal; thus the operation conditions are severe - about 23.

(24) 1800°C and 2.4 MPa. A mixture of oxygen/steam is used as a gasifying agent, and the carbon conversion is above 99%. Currently, several BGL plants are in operation, e.g. in Westfield, Scotland; Freiberg, Germany; and Yunnan, China. Moreover, BGL technology is involved in several Chinese projects and is intended to be used for ammonia production in India [25]. o Fluidised-bed technologies, such as: . High-Temperature Winkler Technology (HTW). The Rheinbraun AG Company from Germany started work on upgrading the Winkler fluid-bed process in the mid-1970s, and between 1978-1992 several improved installations were operating. Since then, only one new installation has been launched (Japan). The HTW gasifier works at a pressure of 1.5-2.7 MPa, at temperature range 900-1000°C and is adapted for reactive coals. A mixture of steam with air or O2 is given in two streams (under the grate and above the bed) to increase temperature and decompose the tar. Such high temperature and recirculation of unconverted char ensure almost complete carbon conversion. An increasing interest in this technology from Australia, India or the Chech Republic has been currently reported, especially that a new HTWplus gasifier was developed [25, 36]. . Utility-Gas Gasifier (U-Gas). The U-gas bubbling fluidised-bed gasifier, developed by the Chicago Gas Technology Institute, is designed for different types of coal (including low-rank coals). A mixture of steam and oxygen or air is fed into the reactor on two levels to create a hot zone in the bed. The temperature is maintained at 840-1100°C and the pressure between 0.3-3 MPa. The raw gas is cooled and purified in cyclones, whereas unconverted char is recycled to the reactor to improve carbon conversion up to 97.8%. Numerous projects based on this technology are under development, e.g. in China or the U.S. [25]. o Entrained-bed technologies, such as: . Shell coal gasification process (SHELL). The Shell process is based on the pressurised version of the Koppers-Totzek gasifier, and the first demo plant was launched in Hamburg (1978-1982). The technology uses an updraft entrained-flow gasifier in which dry-fed coal is partially oxidised and gasified in an atmosphere of steam and O2. The raw syngas is cooled by mixing with cool recycled gas and by water screen. The process proceeds at 3.3 MPa and 1400-1700°C. The gasifier is adapted to bituminous coals and achieves a high carbon conversion (99.8%). Worldwide, over 50 Shell gasifiers are in operation (mainly in China for ammonia and methanol synthesis), and new projects are under development, e.g. in Vietnam [24, 25]. 24.

(25) . General Electric Energy Technology (GE) (previous Texaco). The GE technology goes back to the 1950s and since then has been intensively developed. GE single-stage downdraft entrained flow gasifier is fed by the coal-water slurry and O2. The process is performed at 1260-1480°C and 3-7 MPa. The raw syngas containing liquid slag flows down where is cooled by water quench or radiant syngas cooler. The granulated slag and unconverted char are collected and discharged or, in case of char, recycled to slurry preparation. Both high- and low-rank coals can be subjected to gasification achieving conversion ~ 99.8%. Since 1993, the process was licensed at least 27 times, mainly on the Chinese market for chemical production. Other installations are in the planning phase [24, 25]. . Siemens Fuel Gasification Technology. The Siemens technology was created in 1977 in Freiberg. This single-stage downdraft entrained-flow gasifier is divided into two parts: i) upper part, where gasification of dry coal in O2/steam stream takes place; ii) bottom part, where the gas is cooled through a water quench. The reaction chamber features a cooling screen, along which the liquid slag flows down. The temperature in the gasifier is in the range 1300-1800°C and pressures 2.0-4.0 MPa. The various coals, from bituminous to low-rank coals, can be gasified while maintaining high carbon conversion (99.8%). Currently, five Siemens gasifiers using coal are in operation (Ningxia, China); several other projects are under development in China [24, 25]. . E-Gas Technology. The development of the E-Gas gasifier was started in 1973 by the Dow Chemical Company. The two-stage updraft entrained flow gasifier uses coal-water slurry that can be prepared from various types of coal. Coal (~75%), recycled char and O2 are fed at the bottom of the reactor (I stage) where the temperature reaches about 1300-1450°C, and pressure about 2.9 MPa. In the upper II stage, the rest of slurry is sprayed into the stream of hot gas and is cooled through chemical quench. The dust- and char-loaded syngas flows through the convective cooler, then is purified and the solid particles are recycled to the I stage. Currently, only one E-Gas gasifier is in service worldwide (at the Wabash River IGCC power plant in Terra Haute). However, several projects have been announced, e.g. construction of SNG facility in Korean [24, 25]. . East China University of Science and Technology Gasifiers (ECUST) The ECUST in Shanghai started development of entrained-flow gasifier in 1996. and the first coal-water-slurry, single-stage downflow gasifier was launched in Dezhou in 2004. The gasifier features several slurry burners, therefore is called an opposed multiple 25.

(26) burner gasifier. The raw syngas is cooled with special bubble distributor device and purified in a cyclone, jet mixer and water scrubber. The main operating parameters are pressure: 2.0-2.5 MPa; temperature: 1300-1400°C; coal conversion: 98-99%. As a result of development, a dry-feeding system was designed. In 2013, 29 ECUST gasifiers were operating in China [24, 25]. The development of gasification by technology is presented in Figure 1.11.. Fig. 1.11. Development of coal gasification by technology [30]. As can be seen, GE followed by Shell; thus technologies based on entrained-bed gasifiers, have the dominant share in gas production. Moreover, considerable numbers of projects under construction are based on these technologies. It is worth paying attention to the ECUST technology, which for the first time appeared in the official databases in 2010, currently is in the 4th place in terms of the generated capacity of the syngas, and by 2022 will be in the 2nd place. The ECUST is not the only rapidly developing gasification technology on the Chinese market. According to Higman [30], the upcoming Chinese technologies such as SEDIN Dry Bottom Gasification (moving-bed gasifier), Multicomponent Slurry Gasifier (MCSG, entrained-bed gasifier) and HT-L (Hangtian Lu Gasifier, entrainedbed gasifier) have a significant contribution to total thermal gas capacity and their role in gasification market will grow.. 1.3. Application of syngas from coal gasification Syngas from coal gasification can be turned into a wide range of product from chemicals, through liquid fuels, to clean fuel gas and electricity, as shown in Figure 1.12. 26.

(27) Fig. 1.12. Directions of synthesis gas utilisation [16]. Syngas to be used in a particular technology has to meet requirements concerning composition, especially in the case of chemical syntheses. The most important is a high total share of H2 + CO with the appropriate H2/CO ratio, as well as low content of nitrogen, hydrocarbons and pollutants (in particular sulphur, which adversely affects the catalysts used in chemical syntheses) [37]. The required H2/CO ratios for individual syngas applications are presented in Table 1.4. Table 1.4. The H2/CO ratios for individual syngas applications [2]. Product requirement H2/CO Hydrogen production Maximum Ammonia Maximum Methanol synthesis 2.4-3.0 Fischer-Tropsch synthesis approximately 2.5 Synthetic natural gas (SNG) approximately 3 Integrated gasification combined cycled (IGCC) Undefined IGCC with CO2 removal Maximum. The most important directions of syngas application are productions of chemicals, synfuels and power [2]. o Chemicals Typically, the chemical industry uses gasification to produce hydrogen, methanol as well as ammonia, which is the foundation of nitrogen-based fertilisers. . Hydrogen - the estimated total world hydrogen production by gasification is over 500.000 Nm3/h. The market for this gas is exceptionally diversified – from petroleum 27.

(28) refiners, through fuel cells, to the food industry. Depending on hydrogen application the demands on quality are different. Many hydrocracking processes accept H2 purity of 98% that can be obtained through the appropriate treatment of raw synthesis gas consisting of tar and volatiles removal, desulfurization, traditional shift (Water Gas Shift Reaction), CO2 removal, methanation route and final removal of CO and water. In the case of higher purities requirements, the pressure swing adsorption (PSA) is used as a final step of purification [2, 10]. . Methanol - over 30% of the world’s methanol is currently produced by gasification. First, a syngas with the proper H2/CO is obtained by the water gas shift reaction. Then, in a gas-phase process at high pressures of 5-10 MPa and temperature of 250°C, syngas reacts on the surface of catalysts (Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3) to produce methanol, according to reaction (1.9). In these conditions, the reaction is characterised by high selectivity (>99.8%) [2, 10]. ΔH°298K = - 91 kJ∙mol−1. CO + 2 H2 ↔ CH3OH . (1.9). Ammonia - about 25% of the world's ammonia is currently produced by gasification. The synthesis gas used for the NH3 production has to be processed (similarly as in case of hydrogen production) to obtain the maximum hydrogen content. Then, ammonia synthesis is conducted at high pressure of 20-40 MPa and temperatures between 400˗650°C over Fe-catalyst, according to reaction (1.10): N2 + 3 H2 ↔ 2 NH3. ΔH°298K = - 92 kJ∙mol−1. (1.10). Most ammonia plants are built in conjunction with urea plants. This solution enables direct use of the CO2 from the NH3 plant for urea production [2, 10]. o. Synfuels The gasification process is used to convert coal into another fuel, such as liquid fuel. or synthetic natural gas (SNG). . Liquid fuels – production of liquids fuels takes place via Fischer-Tropsch (FT) synthesis. The FT process has been utilised in South Africa for decades for production of motor gasoline and diesel fuel. The syngas has to be enriched with hydrogen and then is subjected to (1.11) reaction that occurs over an iron-containing catalyst at temperatures of about 220-270°C or 320-340°C (higher temperatures favour production of gasoline fraction, lower of diesel fraction) and pressures of 2-3 MPa. CO + 2 H2 ↔ –[CH2]– + H2O. (1.11). 28.

(29) A competing technology to the FT synthesis is methanol to gasoline (MTG) process, where the syngas is converted to methanol, while methanol to gasoline by reacting over catalysts. Commercial projects of MTG plant are currently under development in China and the U.S. [2, 10]. . Synthetic natural gas - currently, the only commercial plant for production of SNG from coal is in operation in Beulah, North Dakota. Before the process, partial removal of CO and CO2 from raw gas is required. Subsequently, methane is synthesised by the reaction of carbon oxides (mono and dioxide) with hydrogen over a nickel catalyst, according to reactions (1.12, 1.13) [2, 10]: CO + 3 H2 → CH4 + H2O. ΔH°298K = - 210 kJ∙mol−1. (1.12). CO2 + 4 H2 → CH4 + 2 H2O. ΔH°298K = - 165 kJ∙mol−1. (1.13). o. Power generation An. Integrated Gasification-Combined Cycle (IGCC). power plant. merges. the gasification process with a ‘combined cycle’ power block (consisting of one or more gas turbines and a steam turbine). In the case of IGCC plant without CO2 removal the clean syngas is combusted in highly efficient gas turbines to produce electricity. For IGCC plants that include carbon dioxide capture, syngas before combustion is processed in a catalytic WGSR reactor to convert CO into CO2, which is then separated via physical absorption method. The gas turbines are adapted to combust syngas with high hydrogen content. The excess heat from the gas turbines and gasification process is then captured, converted into steam, and sent to a steam turbine to produce additional electricity [10].. As can be seen in Figure 1.13., over 50% of the syngas produced in the operating gasification plants are used for chemical syntheses, followed by synfuels production and electric power generation. It is mainly due to the numerous installations operating in China. for. the. production. of. methanol. and valuable. downstream. products. (Methanol˗to˗Olefins, Methanol-to-Propylene) with dozens more in construction or planning. Thus, syngas will remain mainly used as a substrate for the chemical industry; however, the planned SNG plants in China will raise the gaseous fuels into the second largest category.. 29.

(30) Fig. 1.13. End-use application of syngas [30].. 1.4. Challenges of coal gasification technology The coal gasification may be at the heart of clean coal technology if current technical and economic challenges will be addressed, such as [38]: o. Lifetime of refractories One of the main challenges is the short lifespan of refractories (used to line and protect. the inside of a gasifier), which is caused by severe conditions during gasification. Currently, refractories have a lifespan of 12 to 16 months, and each relining of a gasifier involves costs (approximately $1 million) and downtime (3 to 6 weeks). o. Perception Poor public-perception and understanding of gasification technology is a significant. barrier to some investments. Social concerns regard especially the use of coal, considered as ‘dirty’ fuel, as an energy source. Thus, educative programs about gasification, for both investors and the general public are necessary. o. Permitting Lack of clear and comprehensive government vision regarding energy resources. exploitation (in many developed countries) or lack of affirmative policies (in a developing country) is a severe challenge. Moreover, in countries where policies are open to such technologies, complex permitting issues are a problem (licensing process lasts longer, is more expensive and more complex than for a coal combustion plant). This process needs to be streamlined to facilitate the launch of new gasification plants. 30.

(31) o. Economic and Financial Challenges Gasification is intimidating because of its costs. This problem is even more significant. in many developing countries, where the costs include the building of coal conversion plants, but also, e.g. coal transportation infrastructure. To reduce this risk the capital and operating cost has to be lowered by a combination of improved efficiency, integration and other technological advances. o. Fuels market The discovery and intended exploitation of other fuels, as well as fluctuation of their. prices, make coal conversion investments less attractive (e.g., in some Africa’s developing countries, the massive offshore natural gas deposits stopped development of coal gasification projects. In turn, in North America shale gas caused a significant drop in the energy price, which made the coal-based generation of power unattractive again). o. High demand for resources Huge resources of water, as well as other substrates, are consumed during. the gasification process for the chemical industry. The availability of hydrological water supplies can be then a serious limitation on the implementation of gasification technologies, mainly in arid regions. Reduction of process water requirements is very challenging, but, e.g. saline water may be recovered, desalinated and use in the gasification process.. 31.

(32) 2.. Kinetics of coal gasification process Two approaches to the problem of the coal gasification process are distinguished:. thermodynamic (equilibrium models) and kinetic (kinetic models). Kinetics of coal gasification is the study of process rate, the goal of which is obtaining reliable methods of reaction rate parameterisation. Moreover, kinetics analysis can provide information useful in the interpretation of catalytic phenomena as well as design, modelling and optimisation of gasifiers. However, due to the complex heterogeneous reactions, kinetics of coal gasification is not well known. Additionally, depending on the temperature at which the gasification is carried out, the stage that determines the process rate is: i) kinetics of chemical reaction (up to 1000°C); ii) internal diffusion; and iii) external diffusion (above 1600°C) [39]. The concept of the kinetics of heterogeneous reactions evolved from principles of homogenous kinetic. The general kinetic equation of any heterogeneous reaction can be represented by equation (2.1) [40]: 𝑅𝑎𝑡𝑒 =. 𝑑𝛼 𝑑𝑡. = 𝑘(𝑇, 𝐶𝑔 )𝑓(𝛼). (2.1). Where: 𝛼 is a conversion degree, t is time, k(T, Cg) is rate constant dependent on temperature T and reagent concentration C, and f(𝛼) is a differential form of reaction model that accounts for. physical. or. chemical. changes. occurring. with. the progress. of the reaction.. If the concentration of reagent is constant during the reaction, the dependence of the reaction rate on the temperature can be expressed by the Arrhenius equation (2.2): 𝐸𝑎. 𝑘(𝑇) = 𝐴𝑒 −𝑅𝑇. (2.2). Where: A is a pre-exponential factor, Ea is activation energy, T is temperature, R is the gas constant. Substitution of Eq. (2.2) into (2.1) gives Eq. (2.3), which after integration takes the form of Eq. (2.4). 𝑑𝛼 𝑑𝑡. 𝐸𝑎. = 𝐴𝑒 −𝑅𝑇 𝑓(𝛼) 𝐸𝑎. 𝑔(𝛼) = 𝐴𝑒 −𝑅𝑇 𝑡 Where: 𝑔(𝛼) is the integral form of the reaction model. 32. (2.3) (2.4).

(33) 2.1. Experimental methods for studying gasification process kinetics Various instrumental techniques that allow carrying out the process under various conditions are used to obtain data for kinetics analysis of the gasification process. 2.1.1. Techniques for examination of the gasification process Generally, any analytical method that measures reactant loss or product generation can be converted to ∝-time (or temperature) plots, thus can be used to examine the kinetics of coal gasification process [40]. The most common experimental techniques employed for this purpose are thermogravimetry and thermovolumetry, which involve the continuous measurement of a change in physical properties, such as weight and volume, during the process. o. Thermogravimetry Thermogravimetry (TG) is a frequently used technique for analysing the kinetics. of the gasification process, in which the changes in sample mass are monitored as a function of time or temperature. During the measurements, the fuel sample is placed in a ceramic crucible and put onto the sample holder situated in an oven. Experiments can be carried out in pure gases or mixtures as well as under elevated pressure. The low temperature of measurements (usually 700-1000°C) ensures that the process rate is determined by the chemical reaction (this is so-called the Regime I). The rate of gasification reaction is defined by conversion fraction ranges from 0 to 1 that is a measure of reaction progress [41]. The main advantages of the thermogravimetric method are: . possibility of choosing a wide range of precisely defined operating conditions,. . possibility of evaluation of a milligram-scale sample,. . high accuracy,. . simplicity of measurements,. . lack of extraordinary expenditures (time or personnel) to modify the experimental facility or change spare parts.. The essential disadvantages include: . limitation to Regime I conditions (higher temperatures accelerate the chemical reaction, reactant gas are consumed faster than being transported to the char surface 33.

(34) and, as a result, a concentration gradient is developed within the fixed bed. Consequently, the obtained kinetic data are not reliable), . limitation in heating rate to 20-30 K/min (as a result, pyrolysis is carried out at low heating rate, which does not correspond to the industrial conditions applications),. . generally non-homogeneous materials and liquid fuels cannot be tested,. . selectivity of competitive reactions cannot be analyzed,. . only global kinetics may be estimated.. Based on the TGA measurements it is possible to: o evaluate coal reactivity, o identify changes in the nature of the occurring transformations (e.g. pyrolysis, gasification), o determine changes in carbon conversion degree, o calculate global kinetic parameters, i.e. activation energy and pre-exponential factor of individual process stages and entire gasification process. Besides, TGA analysers can be combined with the appropriate devices (such as gas chromatograph, mass spectrometer); thus the process can be analysed in more detail [41]. o. Thermovolumetry Termovolumetry is a method that allows determining the kinetics of the gasification. process based on the analysis of the gas generated in the process. The measurement system consists of a reactor (with moving, fluidised or entrained-bed) to which the coal sample and the gasifying agent are provided. Inside the reactor, chemical reactions occur and syngas is formed, which after drying, cleaning and decompression (in case of pressurised measurement) is subjected to analysis. For this purpose, gas chromatographs equipped with appropriate detectors or automatic analysers that continuously examine the gas composition are used. The advantages and disadvantages of this technique are primarily related to the type of reactor used. The thermovolumetry in itself is very useful because it creates great possibilities, i.e. the measurements take less time and allow for more detailed analysis of process kinetics. From changes in concentrations of gaseous products, it is possible to: o assess the reactivity of coal, o analyse changes in the rate of formation of individual gas components,. 34.

(35) o determine the composition of the gas formed in the process at any moment of its duration, o designate yield of gas generated in the process, o define the changes in the nature of transformations that take place (e.g. pyrolysis, gasification), o determine changes in the carbon conversion degree (based on the analysis of all gaseous components containing carbon), o designate the progress of the formation reaction of individual gas components, o calculate the kinetic parameters (activation energy and pre-exponential factor) of formation reactions of individual gas components formed during gasification as well as global kinetic parameters of the process. The fact that thermovolumetry is based on the analysis of the resulting gas is fundamental while assessing the selectivity of the catalysts used in the gasification process. While comparing the two methods mentioned above, it should be emphasised that thermogravimetry is a more commonly used technique to assess the kinetics of the gasification process and is characterised by higher accuracy than thermovolumetry. However, thermovolumetry allows for more comprehensive process analysis, since besides results obtained from TGA measurements, it provides detailed information about the resulting gas. Therefore, to assess the kinetics of catalytic coal gasification, the thermovolumetry is a more suitable method as it allows the analysis of the impact of various factors (including catalyst) on the process rate as well as the yield and composition of the resulting gas. 2.1.2. Methods of process conduction The experimental method for studying the gasification process may be generally grouped into two categories: isothermal and non-isothermal [40]. o. Isothermal methods Isothermal methods are based on the law rate equation (2.3) and consist of maintaining. coal samples at several constant temperatures. Consequently, a set of 𝛼–time points at each temperature is obtained. The disadvantage of this approach is that the coal sample requires time to reach the temperature of the process. Thus, in the initial stage of the measurement non-isothermal heating takes place, in which coal sample undergoes transformations that may. 35.

(36) affect the kinetics analysis. This issue is important since under isothermal conditions the gasification rate is the highest at the beginning of the process. o. Non-isothermal methods Non-isothermal methods involve heating of coal samples at one or more heating rates. (usually linear) and produce 𝛼–temperature data. The link between temperature and time is provided by the heating rate 𝛽 = dT/dt which remains constant throughout the experiment. The non-isothermal measurements can be expressed as: 𝑑𝛼 𝑑𝑇. =. 𝑑𝛼 𝑑𝑡. ∙. 𝑑𝑡. (2.5). 𝑑𝑇. Substituting Eq. (2.3) into (2.5) gives a differential form of non-isothermal rate law: 𝑑𝛼 𝑑𝑇. 𝐸𝑎. 𝐴. = 𝛽 𝑒 −𝑅𝑇 𝑓(𝛼). (2.6). The integral form of Eq. (2.6) is presented by Eq. (2.7), and has no analytical solution [42]. 𝑇. 𝐴. 𝐸𝑎. 𝑔(𝛼) = 𝛽 ∫0 𝑒 −𝑅𝑇 𝑑𝑇. (2.7). To transform this integral to a more general form the integration variable can be redefined 𝐸. as 𝑥 = 𝑅𝑇𝑎 , and as a result, the temperature integral takes the form of Eq. (2.8): 𝑔(𝛼) =. 𝐴𝐸𝑎 𝛽𝑅. ∞ 𝑒 −𝑥. ∫𝑥. 𝑥2. 𝑑𝑥 =. 𝐴𝐸𝑎 𝛽𝑅. 𝑝(𝑥). (2.8). Where: p(x) is the exponential integral that may be evaluated through approximation. The most common approximations used are: . Doyle approximation based on the assumption that logp(x) is linear for x over a short range of x values (28-50) [43]. 𝑙𝑜𝑔𝑝(𝑥) ≈ −2.315 − 0.4567𝑥. . (2.9). Senum-Yang nonlinear approximation that is accurate over a wide range of x values [44]: p(x) ≅. exp(−x). x3 +18x2 +86x+96. x2. x4 +20x3 +120x2 +240x+120. Senum-Yang approximation is considered more precise than Doyle approximation.. 36. (2.10).

(37) Non-isothermal methods appear to be of more relevance because they are less cumbersome and yield more useful data with less experimentation. Also, non-isothermal conditions are nearer to the real conditions existing in industrial practice and may shorten the duration of the tests. However, there are disadvantages such as severe computational difficulties associated with the kinetic analysis.. 2.2.. Calculation methods for studying gasification process kinetics. Methods to analyse kinetic data (both from isothermal and non-isothermal measurements) can be divided into two groups: modelistic and model-free methods [40]. 2.2.1.. Model fitting methods. The model fitting methods are based on fitting of different models to the experimental data and choosing of model characterised by the best statistical fit to calculate the activation energy (Ea) and pre-exponential factor (A). o. Isothermal model-fitting method (conventional method) involves [40]: 1) fits that determine the rate constant of the model that best fits the experimental data, according to 𝑔(𝛼) = 𝑘𝑡, 2) fits that determine the kinetics parameters, according to the Arrhenius equation.. o. Non-isothermal model-fitting methods Numerous model fitting methods allow obtaining the kinetic parameters from. non˗isothermal data. However, it was proved that the sole use of these methods is not correct because they assume constant values of A, Ea and model as well as involve fitting based on a single heating rate that is not sufficient [45]. The most frequent methods used are: . Direct differential method. The method is based on taking the logarithm of differential non-isothermal rate law that gives: 𝑙𝑛. 𝑑𝛼/𝑑𝑇 𝑓(𝛼). 𝐴. 𝐸. = 𝑙𝑛 𝛽 − 𝑅𝑇𝑎. (2.11). Plotting the left side versus 1/T allows to determine Ea and A from the slope and intercept, respectively [46].. 37.

(38) . Coats–Redfern Method. The method is based on the integral form of the non-isothermal rate law and utilisation of asymptotic series expansion for approximating p(x). As a result, Eq. (2.12) is obtained: 𝑙𝑛. 𝑔(𝛼) 𝑇2. 2𝑅𝑇𝑒𝑥𝑝. 𝐴𝑅. = ln (𝛽𝐸 [1 − (. 𝐸𝑎. 𝑎. 𝐸. )]) − 𝑅𝑇𝑎. (2.12). Plotting the left side of the equation versus 1/T gives Ea and A from the slope and intercept, respectively [47]. 2.2.2.. Isoconversional Methods (Model-Free methods). Isoconversional methods are so-called model-free methods since allow to calculate the activation energy without modelistic assumptions. Isoconversional methods are based on an evaluation of Ea at specific conversion degree (the same for several curves at different temperature conditions). As a result, the isoconversional plot (Ea vs ∝) is created. In the case of pre-exponential factor (A) the modelistic assumption is required; thus model-free methods usually report only Ea [40]. o. Isothermal Isoconversional Methods . Standard Isoconversional Method. This method requires taking the logarithm of the isothermal rate law and rearranging it to obtain: 𝐴. 𝐸. −𝑙𝑛𝑡 = 𝑙𝑛 (𝑔(𝛼)) − 𝑅𝑇𝑎. (2.13). A plot of -lnt versus 1/T for each ∝ gives Ea from the slope for that ∝, regardless of the model used [40, 48]. . Friedman’s Isoconversional Method. It is a differential method, according to which the logarithm of the isothermal rate law provides: 𝑑𝛼. 𝐸. ln ( 𝑑𝑡 ) = (𝑙𝑛𝐴𝑓(𝛼)) − 𝑅𝑇𝑎. (2.14). A plot of ln(dα/dt) versus 1/T at each ∝ gives Ea from the slope for that ∝, regardless of the model used [40, 49].. 38.

(39) o. Nonisothermal Isoconversional Methods . Ozawa, Flynn, and Wall (OFW) Method. The OFW method is derived from the common logarithm of the non-isothermal rate law, resulting in Eq. (2.15) [50]. 𝑙𝑜𝑔𝑔(𝛼) = 𝑙𝑜𝑔. 𝐴𝐸𝑎 𝛽𝑅. + 𝑙𝑜𝑔𝑝(𝑥). (2.15). Substituting the Doley approximation and rearranging provides Eq. (2.16). A plot of ln𝛽 versus 1/T at each 𝛼 yields Ea from the slope for that 𝛼, regardless of the model used. 𝐴𝐸. 𝐸. 𝑎 𝑙𝑜𝑔𝛽 = 𝑙𝑜𝑔 𝑔(𝛼)𝑅 − 2.315 − 0.475 𝑅𝑇𝑎. . (2.16). Modified Coats–Redfern Method. The method was developed by rearranging isoconversional Coats-Redfern method to Eq. (2.17): 𝐴𝑅(𝑛(1−𝛼)𝑛−1 𝑚 ). 𝛽. 𝑙𝑛 𝑇 2 = 𝑙𝑛 (. 𝐸𝑎. 𝑚. 𝐸. ) − 𝑅𝑇𝑎. (2.17). 𝑚. A plot of ln𝛽/T2 versus 1/T at each 𝛼 yields Ea from the slope for that 𝛼, regardless of the model used [51].. 2.3. Models for description coal gasification reactions The rate of heterogeneous reactions may depend on numerous factors, such as a rate of nuclei formation, interface advance, diffusion or geometrical shape of solid particles. Therefore, there are many models describing heterogeneous gas-solid reactions [40]. In practice, the following single-step models (i.e. models based on the assumption that adsorption and desorption are not limiting factors) are most commonly used to describe both, catalytic and non-catalytic gasification reactions [52]: o. Volumetric model The volumetric model is considered as the easiest model, which assumes that. the gasification. reaction. proceeds. homogeneously. throughout. the. entire. volume. of grain/elemental coal particle as well as that the specific surface area decreases linearly as the degree of conversion increases. The kinetic equations in differential and integral form are as follow: 𝑑𝛼 𝑑𝑡. = 𝑘𝑉𝑀 (1 − 𝛼) 39. (2.18).