Tomasz CHMIELNIAK, Józef POPOWICZ ? Institute for Chemical Processing of Coal (IChPW), Zabrze, Poland
Please cite as: CHEMIK 2013, 67, 5, 415?422
The reasons for the development of fuel gasification technologies in power engineering and chemical industry that has been observed in recent years are high efficiency of the gasification process, the possibility of a multi-purpose use of process gas in both energy production and chemical synthesis, as well as relatively small negative impact of those technologies on the natural environment. Besides the intensive development of fuel gasification technologies, fluidised bed gasification is drawing an increasing interest. Process temperatures lower than in entrained-flow reactors (below the temperature of ash melting) influence the reduction in investment and operation costs and improve the reliability and availability of the discussed technology. Moreover, fluidised bed reactors are considered to be an attractive alternative for gasification process in entrained-flow reactors due to their high performance, moderate oxygen and steam requirements, and high fuel flexibility .
A long-term trend for developing gasification technologies for fuel includes using CO2 as the gasifying agent. This can be achieved by the Boudouard-Bell reaction, whose product is carbon oxide, which besides hydrogen, is the fundamental component of syngas. Carbon dioxide introduced to the reaction system acts as the carrier of carbon and oxygen, which results in reduced oxygen consumption, improved efficiency of the gasification process and reduced relative CO2 emission . The technology of fluidised bed gasification of fuels in the CFB (circulating fluidised bed) reactor in CO2atmosphere has been developed at the Institute for Chemical Processing of Coal.
Coal gasification in CO2 atmosphere
Effective use of carbon dioxide in fuel gasification requires the fulfilment of a few fundamental conditions. The process should take place at suitably high temperatures, preferably at 800?950°C, at which the reversible Boudouard-Bell reaction (3) is shifted towards the formation of carbon oxide (Fig. 1) . The pressure increase in the reaction system is in favour of the course of the reversible reaction ? decomposition of carbon oxide, and the considerable conversion degree of carbon dioxide can be achieved only at high temperatures of gasification (exceeding 1000°C).
The efficiency of Boudouard-Bell reaction depends mainly on the kinetic conditions of the process, that is, the contact time of reagents (char and carbon dioxide), process temperature and pressure, and the conditions for heat and mass exchange between the solid phase and the gaseous phase. Contrary to the thermodynamic conditions, the process pressure favours the increase in the rate and degree of conversion of char from CO2 in the Boudouard-Bell reaction (Fig. 2) [3, 4].
Apart from temperature and pressure, the carbonization degree and the applied catalysts, including the catalytic effect of a mineral substance of the gasified fuel can also influence the reaction course of coal gasification in the CO2 atmosphere . As coal is a complex substance, it is difficult to determine unequivocally the impact of the carbonization degree of fuel on coal behaviour during the gasification process with carbon dioxide. Low-carbonated coal is generally more reactive which is probably caused by the presence of functional groups containing oxygen and a high degree of dispergation of a mineral substance capable of catalysing the process . Employing catalysts can lower the energy required for activating the gasification reaction which, in practice, can reduce the process temperature and simultaneously maintain high efficiency of that process. Metals belonging to group 8 in the periodic table as well as alkali and alkali-earth metals belong to the substances that catalyse the gasification process of coal.
The results of thermodynamic calculations indicate that the gasification process with carbon dioxide as the gasifying agent can affect the process yield and improve its efficiency as well as reduce relative CO2 emission to the atmosphere. Figure 3 shows the results of thermodynamic calculations for C gasification in the O2/CO2 atmosphere. The introduction of carbon dioxide to the gasification reactor increases the conversion degree of C in comparison to the typical gasification system (without introducing carbon dioxide to the reaction system), and at the same time results in the increased content of carbon oxide in syngas (Fig. 3) .
In practice, the application of coal gasification in the CO2 atmosphere requires a technological system that ensures effective course of this process regarding its application in industry. Gasification technology in the fluidised bed seems to be the optimal solution as, despite lower process temperatures, this technology provides more favourable conditions for the Boudouard-Bell reaction than other entrained-flow technologies. This relates to, among other things, a high concentration of a reactive char and relatively long residence times of the solid phase (carbon, char) in the reaction space of the apparatus (Tab. 1).
The application of a circulating fluidised bed (CFB) reactor seems to be particularly attractive as it provides the favourable conditions for mass and heat exchange during the gasification reaction in the CO2 atmosphere. Coal gasification in the CFB reactor is conducted at temperatures of 800?950°C with carbon oxide and carbon dioxide, and alternaltively with steam. Gas and char are produced during the reactions of degasification and gasification of solid fuel. Char is separated from gas in the reactor dedusting system. A reactive char being a constant heat-transfer carrier is returned to the reactor and partially removed from it, whereas gas is passed for purification and further processing. Recirculation of char creates the conditions of high concentration of the solid phase (char and coal) which is well mixed in gas stream. On its surface, a reactive char present in the CFB reactor provides an opportunity for conversion of CO2 to carbon oxide ? the fundamental component of syngas.
While heating coal particles in the reactor, drying and degasification (pyrolysis) process occur on their surface and in their internal structure. A degree of coal degasification depends on the heating rate of its particles and their residence time in the bed. The heating rate of coal particles depends on the conditions for heat and mass exchange between the gaseous and solid phases, and their residence time in the bed depends on aerodynamics of the fluidised bed. Pyrolysis results in releasing volatile elements from carbon, that is, simple gases (e.g. H2, CH4, CO, CO2, H2O, H2S, NH3), and aliphatic and aromatic hydrocarbons (C2H6, C6H6, tar) which undergo secondary homogeneous reactions of thermal cracking, semi-combustion and reforming processes in the reaction medium. Char is also formed which in the presence of an oxidiser (O2+CO2+H2O) is partially gasified and consequently, gas containing mainly CO, H2 and CO2 is formed. The following parallel heterogeneous reactions of char gasification occur:
Those reactions are accompanied by a considerable release of heat for heating raw coal, evaporating moisture, degasifiying coal and heating reaction products. An original, semi-empirical, pseudo-equilibrium numerical model of the CFB reactor in a steady state was developed to determine the process parameters of output streams of the coal gasification reactor using CO2, that is, temperature and composition of the raw process gas and char at known process parameters of input streams (coal, oxidiser, steam). For the purpose of simplifying the model, the process of drying raw coal was assumed to occur immediately in the area of its dosing to the reactor. From physical perspective, this process was considered to be a balance process. It was also assumed that degasification (pyrolysis) of carbon particles is a kinetic process occurring at a constant rate of particle heating under non-isothermal conditions, uniformly in the whole volume of particles. This process was described using the Ściążko model [6, 7]. Moreover, gasification process of char particles was regarded to occur on the solid surface in equilibrium mode under isothermal conditions.
The calculations  based on the developed numerical model of the CFB reactor for gasification of coal from ZG ?Janina? with 12% moisture content at a temperature of 900°C and under the pressure of 1.6 MPa using 95% pure oxygen in the amount of 0.40 kg/kg of coal with CO2 additive in the amount of 0.65 kg/kg of coal indicate that a 75% conversion degree of C in fuel supplied to the reactor can be expected. The yield of products of the gasification process of coal from ZG ?Janina? in the CO2 atmosphere is 0.23 kg/kg of coal for char and 1.84 kg/kg of coal for raw gas. Estimated contents of raw gas components are presented in Table 2.
Regarding the process, the effective use of CO2 for coal gasification, besides the conditions for the gasification process that are favourable for the Boudouard-Bell reaction, requires also a suitable configuration of the whole technological installation for syngas production for the needs of energy sector or chemical industry. Particularly significant aspects that have to be analysed are the issues related to the method of handling char formed in that process, the integration of carbon dioxide circulations and the optimisation of installations for purifying process gases formed in the stream system. A selection of adequate technological nodes and their optimum configuration can lead to high efficiency of the production process at the minimised environmental impact, particularly regarding the reduced relative CO2 emission.
A concept of technological configuration of the installation for fluidised bed gasification with CO2
A two-product feature distinguishes the technology of fluidised bed gasification of coal from the gasification technology in jet reactors. Gas and a solid product ? char, are the main products of fluidised bed gasification of coal. Process gas contains H2, CO, CO2 and CH4, and also smaller amounts of light aliphatic and aromatic hydrocarbons, tar and such pollutants as H2S, COS, NH3 and HCN. In addition to ash, char also contains a relatively large amount of C and smaller amounts of such elements as O, H, N and S. The application of products of fluidised bed gasification of coal for chemicals or energy production requires a series of unit operations, that is, cooling, purification and conversion to prepare gas and char for the final process of their processing (combustion or chemical synthesis). For fluidised bed gasification of coal, the configuration of syngas production installation for chemicals and/or energy production is characterised by two technological lines: gas ?pathway? and char ?pathway? (Fig. 4).
Process gas from coal gasification, after its cooling, clean-up and conversion, is directed to methanol, hydrogen or energy production unit, whereas char is transferred directly to the combustion unit. Char ?pathway? is only related to the production of electric power, and gas ?pathway? includes various technological variants:
? energy variant ? production of electric power
? chemical variant ? production of hydrogen or methanol
? mixed variant (polygeneration system) ? production of electric power and either hydrogen or methanol.
Considering the whole technology of fluidised bed gasification of coal (both technological lines), generally the energy variant or polygeneration variant is used.
In the energy variant, process gas is combusted in a gas turbine, and combustion gas from the gas turbine produces steam in the recovery boiler. Steam is also generated in the coal gasification reactor unit, process gas cooling and conversion unit, and char combustion unit. Electric power is generated in gas and steam turbine generators. Such a unit produces electric power and heat in the form of hot water. Heat can be also produced in heat exchangers supplied with steam from steam turbine bleeding or with steam generated in the recovery boiler (usually the final receiving stage for heat in combustion gas pathway).
The polygeneration variant intended for hydrogen production provides two possible solutions. If production capacity of hydrogen is maximized, then the total amount of purified process gas is sent to the hydrogen separation unit. Following the process of hydrogen separation, tail gas of low calorific value is combusted in the char combustion unit associated with the recovery boiler and the steam turbine. This solution does not include the gas turbine. If hydrogen and electric power are produced simultaneously (in the gas ?pathway?), then process gas stream is divided into two parts. One part of the stream is sent directly to the gas turbine, and the second one is directed to the hydrogen separation unit. Tail gas from the hydrogen separation unit is passed to the char combustion unit or to the gas turbine.
The polygeneration variant intended for methanol production provides two possible solutions. If production capacity of methanol is maximized, then the whole stream of process gas is sent to the methanol synthesis unit (one through reactors or gas recycle reactors). Unconverted gas from the synthesis installation is directed to the gas turbine (configuration in a series) or the char combustion unit. If methanol and electric power are produced simultaneously (parallel configuration), then the gas turbine and the installation of methanol synthesis are simultaneously fed with the process gas in a specific ratio resulting from the demand on electric power or methanol. In this case, unconverted gas from the synthesis installation is also directed to the gas turbine or the char combustion unit.
In comparison to the power generation system, the polygeneration system is characterised by high efficiency in the consumption of fuel primary energy and by lower emission of carbon dioxide to the atmosphere at simultaneously low investment and operational costs. The technological configuration of the syngas production installation for chemicals and/or power energy production depends largely on the technological, economic and legal attitudes towards CO2 emission. In accordance with EU directives, that installation should at least satisfy the requirement of ?CCS ready? which restrains the above mentioned variant solutions for the installation configuration.
Oxy-combustion, that is, char combustion in the mixture of O2 and CO2 is an optimal solution in the char ?pathway?. Polluted carbon dioxide (containing >95% CO2) is the process product which, after its purification (mainly from SO2) and compression, can be directly stored underground. By using oxy-combustion of char, in comparison to its typical combustion, the process of separating CO2 from N2 and O2, which is expensive as an investment and during operation, can be avoided, and moreover the energy efficiency of the applied technology is not reduced. In the energy and polygeneration variants with the production of hydrogen from process gas, after its cleaning up and conversion in the WGS system, carbon dioxide of the purity level meeting the requirements for the criteria for its underground storage is released (in Selexol or Rectisol units). Hydrogen gas is also the end product of such a configuration. It is sent to the PSA unit to obtain pure hydrogen or to be combusted in the gas turbine without emitting greenhouse gases to the atmosphere (CO2).
For the polygeneration variant intended for methanol production, it can be problematic to meet the ?CCS ready? requirement for the installation in the process gas ?pathway?. Process gases from coal gasification and unreacted gas after methanol synthesis contain mainly CO and H2. Gas combustion in the gas turbine supplied with air would lead to the production of, among other things, carbon dioxide and necessitate its separation from N2 ? an expensive process from technological and operational points of view. In this case, oxy-combustion (in O2 and CO2 atmoshpere) of this gas in the gas turbine seems to be a reasonable solution which is a subject of conceptual discussions in the specialist literature. The combustion of unreacted gas after methanol synthesis, in the char oxy-combustion unit is also possible.
Summary and conclusions
Coal gasification is an attractive method of the efficient production of energy and gas for chemical synthesis. Besides the intensive development of gasification technologies in the entrainedflow reactors, also the gasification technologies occurring in the fluidised bed are drawing an increasing interest. A long-term trend for developing gasification technologies for fuel includes using CO2 as the gasifying agent.
Coal gasification in the CFB (circulating fluidised bed) with CO2, which is developed at the Institute for Chemical Processing of Coal (IChPW) can serve as an example of a technology of fluidised bed coal gasification. This concept is interesting because carbon dioxide is used as the gasifying agent which transports carbon (C) and oxygen to the system, and consequently improves the process efficiency (increased chemical enthalpy flux of process gas, reduced consumption of oxygen) and reduces relative CO2 emission.
Gas and a solid product ? char, are the main products of the process of fluidised bed gasification of coal in the CO2 atmosphere. Process gas contains H2, CO, CO2 and CH4, and also smaller amounts of light aliphatic and aromatic hydrocarbons, tar and such pollutants as H2S, COS, NH3and HCN. Char, besides ash, contains also a relatively large amount of C.
Using the products of fluidised bed gasification of coal for chemicals or energy production requires the application of a series of unit operations, that is, cooling, clean-up and conversion to prepare gas and char for the final process of their processing (combustion or chemical synthesis).
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?This paper was elaborated as part of the Research Task No. 3 financed by the National Centre for Research and Development [NCBiR] under the Agreement No. SP/E/3/7708/10?.
Tomasz CHMIELNIAK ? Ph.D., Eng., graduated from the Faculty of Environmental Engineering in 1992 and obtained an academic degree of doctor at the Faculty of Chemistry at the Silesian University of Technology in Gliwice in 1998. Since 2004, he has been a manager of the Centre for Laboratory Research at the Institute for Chemical Processing of Coal in Zabrze. He is an expert in chemical engineering and environmental protection. He is involved in the research and consultancy activities in the field of gas dedusting and purification processes and the technologies for thermal processing of solid fuels, biomass and waste, and in particular gasification and pyrolysis. He is an author and a co-author of more than twenty scientific papers and patents. e-mail: firstname.lastname@example.org; phone: +48 605 364 493
Józef POPOWICZ ? M.Sc., graduated from the Faculty of Chemistry at the Silesian University of Technology in 1982. He works at the Institute for Chemical Processing of Coal in Zabrze. He is an expert in chemical engineering and waste management. He is involved in the research, design and consultancy activities in the field of thermal and chemical processing of solid fuels, biomass and waste as well as purification and conversion of process gases. He is a co-author of several papers and patents. e-mail: email@example.com; phone: +48 32 271 00 41