Coal Char Gasification Research Articles

Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Transformations of sulfur

K2CO3-catalyzed char gasification with steam was carried out on a laboratory fix-bed reactor and it featured in the preparation of char from a high-sulfur coal by the prior addition of Ca(OH)2 to the coal. This work was focused on the clarification of sulfur transformations during the coal pyrolysis and the char gasification. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy was employed to determine quantitatively the forms of sulfur in coal and their changes in process. Results showed that the addition of Ca(OH)2 to coal promoted the decomposition of both pyrite and organic sulfur during the charring. In the K2CO3-catalyzed char gasification, the calcium additive enabled more sulfur to be retained eventually as K2SO4 rather than CaSO4. The formation of K2SO4 would be conducive to the recycled use of potassium catalyst.

상용모사기를 이용한 로토석탄의 분할유동층 가스화기 가스화 특성 모사

본 연구에서는 상용모사기를 이용하여 분할유동층 가스화기에서 로토석탄의 가스화 특성 모사를 수행하였다. 분할 유동층 가스화기는 가스화영역에서 일어나는 연소반응과 가스화반응(발열반응과 흡열반응)을 각각 다른 영역에서 일어날 수 있도록 반응기 내부를 분할한 가스화기이다. 분할유동층 가스화기의 주요 개념은 가스화에 요구되는 열을 연소영역에서 생성된 열을 이용하여 공급하는 것으로 가스화기 내부에서의 부분 연소를 억제하고, 격벽을 통한 열전달과 열매체의 이동을 통해 공급하는 것이다. 분할유동층 가스화기 모델은 열분해, 촤 가스화, 타르/오일 가스화, 촤 연소반응으로 4개의 영역을 가지도록 구현하였다. 열분해의 경우, 대상 석탄을 반응온도, 반응가스, 석탄주입량을 변화시켜 실험을 수행하여 실험데이터로부터 correlation 모델을 작성하였다. 가스화는 Gibbs free energy를 최소화하는 모델을 이용하고 촤 연소영역은 combustion 모델을 이용하였다. 분할유동층 가스화기 모사결과를 비교하기 위해 우선 단일영역 가스화기 모사를 수행하였다. 단일영역 가스화기의 경우 석탄열분해 반응기와 석탄가스화 반응기 두 개로 구성되며 반응모델은 분할유동층 가스화기와 일치한다. 분할유동층 가스화기 모사 결과, 냉가스효율은 84.4%로 단일영역 가스화기와 유사한 결과를 얻었으며 합성가스의 조성은 <TEX>$H_2$</TEX>와 <TEX>$CH_4$</TEX>이 다소 증가하고 CO와 <TEX>$CO_2$</TEX>가 다소 감소한 것을 확인하였다. 모델 검증을 위해 10건의 단일영역 가스화 실험에 대하여 모사를 수행하였다. 모사를 통해 얻어진 합성가스의 조성은 CO, <TEX>$CO_2$</TEX>, <TEX>$CH_4$</TEX>의 경우 실험결과와 모사결과가 거의 일치하는 반면 <TEX>$H_2$</TEX>의 경우 모사결과가 실험값과 비교하여 다소 높은 값을 갖는 것을 확인하였으나 경향은 실험값과 유사함을 확인하였다. 탄소전환율의 경우, 모델결과가 실험값과 비교하여 높은 전환율을 보이는 것을 알 수 있으며 이는 모사에 사용된 가스화 모델이 평형반응기로 반응기에서의 체류시간과 접촉시간이 실제 실험과 차이가 있기 때문으로 파악된다. In this study, we used a commercial simulator to investigate the gasification characteristics of Roto coal in the partitioned fluidized-bed gasifier, which consists of 4 parts such as coal pyrolysis, char gasification, tar/oil gasification and char combustion. The heating medium was exchanged between the combustion part and the gasification part in order to supply the energy needed for pyrolysis and gasification. The correlation model from experimental data in relation to the reaction temperatures, the reaction gases and the coal feed rates was derived for the coal pyrolysis. The equilibrium model was used for the gasification and the combustion model for the char combustion. In order to compare the reaction behavior of the partitioned fluidized-bed gasifier, the single-bed gasifier was also simulated. The cold gas efficiency of both partitioned fluidized-bed gasifier and single-bed gasifier was almost the same. The <TEX>$H_2$</TEX> and <TEX>$CH_4$</TEX> contents of the syngas in the partitioned fluidized-bed gasifier slightly increased and the CO and <TEX>$CO_2$</TEX> contents slightly decreased, compared with the singlebed gasifier. In order to verify the model, ten cases of the single-bed gasification experiment have been simulated. The contents of CO, <TEX>$CO_2$</TEX>, <TEX>$CH_4$</TEX> in the syngas from the simulation corresponded with the experimental data while those of <TEX>$H_2$</TEX> was slightly higher than experimental data, but the tendency of <TEX>$H_2$</TEX> content in the syngas was similar to the experiments. In the coal conversion, the simulation results were higher than the experiments since equilibrium model was used for the gasification so that the residence time and contact time in the model is different from the experiments.

Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Influences of calcium species

Experiments were carried out to investigate the K2CO3-catalyzed gasification performances for varying chars prepared from four coals with prior addition of calcium additive and without calcium additive. Three calcium species, Ca(OH)2, Ca(Ac)2 or Ca(CH3COO)2, and CaCO3, were used in the study. Experiments were also conducted in an attempt to achieve the understanding of the effect of calcium additive on the potassium deactivation, as well as the interactive effect between potassium, calcium and char. It was found that each calcium additive acted as a deterrent to the potassium deactivation and thus promoted the catalytic gasification, and the magnitudes of promotion varied depending on coal and calcium species. Ca(OH)2 and Ca(Ac)2 were more effective to inhibit the potassium deactivation than CaCO3. Furthermore, potassium together with calcium showed a mineral-unrelated synergy towards the char gasification. A bimetallic carbonate, K2Ca(CO3)2, which formed even when the chars prepared from coal with either Ca(OH)2 or Ca(Ac)2 was subsequently mixed with K2CO3, was likely to contribute to the synergistic effect.

Modeling of coal char gasification in coexistence of CO2 and H2O considering sharing of active sites

In a coal gasifier, CO2 gasification and H2O gasification occur at the same time. Many researchers have constructed CO2 gasification model and H2O gasification model separately. Only a few models explain the competition between CO2 gasification and H2O gasification. These models are divided into two types according to the properties of the active sites. Some models assume that CO2 gasification and H2O gasification occur at separate active sites, whereas others assume that all of the active sites are shared and are common to both CO2 and H2O gasification. There are some coals whose gasification reaction rates, with CO2 and H2O in coexistence, cannot be described by the conventional models. In this study, a Langmuir–Hinshelwood type gasification model was modified to explain the char gasification by CO2 and H2O. In the proposed model, CO2 gasification and H2O gasification partially share active sites. Coal chars, which were prepared through pyrolysis using a drop tube furnace at ambient pressure and 1673K, were gasified with CO2 and H2O using a pressurized drop tube furnace (PDTF) and a thermogravimetric analyzer (TGA) at various temperatures. The proposed model was found to describe the gasification reaction rates of the experiments more accurately than the conventional models.

Gravity separation and its effect on CO2 gasification

The CO2-blown integrated coal gasification combined cycle (IGCC) is a promising electric power generation technology that will reduce CO2 emission, due to its high efficiency. Recent studies have found that base metals improve the coal char gasification in cases when they can interact with the char matrix at an atomic level. In this paper, we clarified the effect of heavy medium separation, which is one of the most popular coal cleaning technologies, on the CO2 char gasification. Before sink and float test and thermogravimetric analysis, Datong Coal from China was ground to below 20 mesh, which is a size usually used for a coal cleaning process. The float and sink test revealed that the highest separation efficiency would be obtained at a specific gravity of 1.45. The ash content would be reduced dramatically from 10.2% to ca. 4% and about 85% of combustibles would be recovered. The chars derived from different specific gravity fractions showed different gasification reactivity; the char from heavier fraction has gasification reactivity higher than the lighter fraction-derived char. This tendency agrees with the ash content; the heavier fraction has a higher ash content. Removal of ash from well-ordered holes in Fusinite and Semi-fusinite in Inertinite might increase the gasification reactivity.

No.47 鉄系触媒を用いるチャーのガス化とタールモデル化合物の分解(研究発表)

No.47 鉄系触媒を用いるチャーのガス化とタールモデル化合物の分解(研究発表)

Synthesis gas production from catalytic gasification of waste biomass using nickel-loaded brown coal char

This paper presents an experimental research concerning the catalytic gasification of waste biomass to synthesis gas using Ni-loaded brown coal char (Ni/BCC). The attention is focused on the catalytic conditions for enhancing the synthesis gas production, improving its composition and extending the catalyst lifetime. The aim is achieved by means of the characterization of Ni/BCC and the product gas analysis, through gasification experiments of a woody biomass and an animal-waste-derived biomass both in a fixed bed reactor and a fluidized bed reactor at various catalytic temperatures and steam/biomass–carbon (S/C) ratios. Effects of the pyrolysis temperature and the steam treatment on the nickel crystallite size are analyzed and used as the reference of choosing optimum gasification conditions. With brown coal char as support material, Ni/BCC is found as an excellent catalyst even at low temperature of about 650°C and shows a good resistance ability of coke formation, compared to non-catalyst and Ni/Al2O3. More than 25h continuous operation is also performed in a 1kg/h Internally Circulating Fluidized-bed Gasifier (ICFG) to assess the lifetime of Ni/BCC. Smaller S/C ratio at the appropriate temperature is suggested to lower the coal char gasification, thereby reducing the degradation of the support material.

CFD modeling of oxy-coal combustion in circulating fluidized bed

Based on the Computational Fluid Dynamics (CFD) method, the previously established and validated 2-dimensional model was employed to predict the oxy-coal combustion processes in a 50 kW circulating fluidized bed (CFB) at Southeast University, China. The simulated processes of coal conversion in our CFB combustion system included particle drying, dry coal devolatilization, volatile combustion, char combustion, char gasification and SO 2 emission. The effects of combustion atmospheres, including air (21% O 2/79% N 2) and oxygen/recycled flue gas (O 2/RFG) atmosphere with different O 2 concentration (from 21% to 40%), were investigated on distributions of solid volume fraction, temperature and gas concentration in the riser. During combustion of Xuzhou bituminous coal in O 2/RFG modes with different O 2 concentrations referred to above, about 70% volume fraction of CO 2 were achieved in the wet flue gas with more than 25% volume fraction of H 2O. The rates of char gasification by CO 2 and H 2O in 30% O 2/70% RFG atmosphere were obviously higher than that in air. However, compared to air combustion, CO concentration in the dense zone in 30% O 2/70% RFG atmosphere was lower because high concentration of H 2O accelerated CO combustion. Both indirect and direct desulfurization mechanisms were considered for sulfur retention by limestone and the corresponding overall reaction rates were calculated statistically. With increase in O 2 inlet concentration gaseous pollutant SO 2 was enriched.

Kinetic Parameters and Reactivity for the Steam Gasification of Coal Chars Obtained under Different Pyrolysis Temperatures and Pressures

The effect of devolatilization conditions (pressure and temperature) on the steam gasification kinetics and reactivity of coal chars was investigated. For this purpose, five different rank coals were selected: a lignite (LT), two high-volatile bituminous coals (DT and SA), a low-volatile bituminous coal (DI), and a semianthracite (HV). The coal chars were obtained by devolatilization in a novel pressurized fixed-bed reactor at temperatures of 800, 900, and 1000 °C and pressures of 1, 5, 10, and 20 atm. The reactivity of the chars was studied by isothermal thermogravimetric analysis in a kinetically controlled regime at different temperatures under a steam atmosphere. The reactive behavior of the chars was evaluated by means of the random pore model (RPM), which satisfactorily described the steam gasification of the coal chars. The gasification apparent reaction rate increased as the devolatilization pressure increased, whereas this parameter decreased as the devolatilization temperature increased.

Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 2: Mathematical modelling and model validation

In gasification of biomass, coal and blended biomass and coal, there are two steps including an initial pyrolysis process followed by gasification of solid char. The latter process is a slow process and thus dominates the whole gasification. In our previous paper ( Xu et al., in press), the differences between steam gasification of biomass chars and that of coal chars have experimentally been investigated and the results show that these differences are mainly due to the difference in microstructures of these two fuels. In this work, a mathematical model of char gasification is developed based on reaction kinetics and gas transportation of both the producer gas and the gasification agent (steam). The model also includes mass conservation equations for each of the gas components and solid carbon involved in the gasification process. This has resulted in a set of highly nonlinear differential equations which have been solved using a numerical technique to predict gas production rate, gas compositions and carbon consumption rate during the gasification. The developed mathematical model is validated using experimental results reported in previous paper ( Xu et al., in press), and close agreement between the simulation results and the experimental values have been observed. From the modelling, it has been confirmed that the char gasification is mainly determined by the characteristics of char matrix including the exposed surface area and the micro-pore size. The former determines intrinsic reaction rate and the latter influences the intra-particle mass transportation. Biomass char has more amorphous structure, thus the intrinsic reaction rate is enhanced. For coal char, the larger pore size enables the high transport rate of the gasification agent (water vapour) into the char particles but the resultant gases have higher resistance to transfer through compact clusters. For simulation of the blended biomass and coal, the blend properties were determined based on the blend proportion of each fuel. The close agreement between the simulation results and experimental data suggests that the approach in this work can adequately quantify the gasification kinetics and the gas composition.

Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 1: Experimental investigation

Biomass and coal are important solid fuels for generation of hydrogen-rich syngas from steam gasification. In this work, experiments were performed in a bench-scale gasifier to investigate the effect of coal-to-biomass ratio and the reaction kinetics for gasification of chars of biomass, coal and coal–biomass blends. In the gasification of these chars, steam was used as the gasification agent, while nitrogen was used as a gas carrier. The gasification temperature was controlled at 850, 900 and 950 °C. Gas produced was analysed using a micro-GC from which carbon conversion rate was also determined. From the experiments, it is found that the coal and biomass chars have different gasification characteristics and the overall reaction rate decreases with an increase in the ratio of coal–to-biomass. The microstructure of the coal char and biomass char was examined using scanning electronic microscopy (SEM), and it was found that the biomass char is more amorphous, whereas the coal char has larger pore size. The former enhances the intrinsic reaction rate and the latter influences the intra particle mass transportation. The difference in mass transfer of the gasification agent into the char particles between the two fuels is dominant in the char gasification.

10.2478/s11814-009-0168-9

Two sized fractions (<75 μm and 150–250 μm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500–900 °C. Gasification of coal chars with excess carbon dioxide was then performed at 900–1,100 °C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conversion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher R s values than higher rank Lampang subbituminous B coals. Smaller chars from both coals had higher R s values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various pyrolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.

Coal gasification in CO2 atmosphere and its kinetics since 1948: A brief review

Numerous coal gasification studies have been found in the literature those employed various kinds of gasifying agents such as steam and carbon dioxide. These studies are featured with wide variations in the parametric conditions and the usage of equipments. Steam is frequently employed as a gasifying agent, however, in several studies carbon dioxide has also been used as a gasifying agent either pure or in combination with other gasifying agents (H2O, O2, CO, H2). This paper is a brief review of the coal gasification with CO2 as a diluent. Different factors were studied over the coal gasification with CO2 such as coal rank, pressure, temperature, gas composition, catalyst and the minerals present inside the coal, heating rate, particle size, and diverse reactor types. It also deals with the application of the gas–solid models developed in the literature and the combustion and gasification mechanisms for O2/CO2 streams. Moreover, it reviews the kinetics and the reaction rate equations (Arrhenius and Langmuir–Hinshelwood types) for coal-char gasification both in the reaction kinetic control region (low temperature) and the diffusion control region (high temperature) and at both low and high pressures.

Two-dimensional computational fluid dynamics simulation of coal combustion in a circulating fluidized bed combustor

A computational fluid dynamics (CFD) approach was applied to simulate the air-coal two-phase flow and combustion characteristics in a 50 kW circulating fluidized bed (CFB) combustor. Eulerian–Granular multiphase model with a drag coefficient correction based on the extended energy-minimization multi-scale (EMMS/matrix) model was used to study the gas–solid hydrodynamics. One energy conservation equation was applied to the mixture of gases and solids, considering heat conduction, heat convection and heat sources from chemical reactions. Reactions during coal combustion included moisture evaporation, dry coal devolatilization, volatile combustion, char combustion and char gasification. The model predicted the main features of the complex gas–solid flow, including the cluster formation of the solid phase along the walls, the flow structure of upward flow in the core and downward flow in the annular region. The voidage and temperature profiles in the furnace and the concentrations of gas components from the riser were validated with experimental data. Distributions of reaction rates in the riser were also obtained. This indicates a promising way to simulate the coal combustion in CFB to help understanding the combustion mechanism and designing new combustion technologies.

Pressurized chemical-looping combustion of coal with an iron ore-based oxygen carrier

Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO2. Most of the previous investigations on CLC of solid fuels were conducted under atmospheric pressure. A pressurized CLC combined cycle (PCLC-CC) system is proposed as a promising coal combustion technology with potential higher system efficiency, higher fuel conversion, and lower cost for CO2 sequestration. In this study pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor. CVRD iron ore particles were exposed alternately to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87.2% steam/N2 mixture and oxidation with 5% O2 in N2 at 970 °C. The operating pressure was varied between 0.1 MPa and 0.6 MPa. First, control experiments of steam coal gasification over quartz sand were performed. H2 and CO2 are the major components of the gasification products, and the operating pressure influences the gas composition. Higher concentrations of CO2 and lower fractions of CO, CH4, and H2 during the reduction process with CVRD iron ore was achieved under higher pressures. The effects of pressure on the coal gasification rate in the presence of the oxygen carrier were different for pyrolysis and char gasification. The pressurized condition suppresses the initial coal pyrolysis process while it also enhances coal char gasification and reduction with iron ore in steam, and thus improves the overall reaction rate of CLC. The oxidation rates and variation of oxygen carrier conversion are higher at elevated pressures reflecting higher reduction level in the previous reduction period. Scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM–EDX) analyses show that particles become porous after experiments but maintain structure and size after several cycles. Agglomeration was not observed in this study. An EDX analysis demonstrates that there is very little coal ash deposited on the oxygen carrier particles but no appreciable crystalline phases change as verified by X-ray diffraction (XRD) analysis. Overall, the limited pressurized CLC experiments carried out in the present work suggest that PCLC of coal is promising and further investigations are necessary.

Coal gasification studies applied to H 2 production

The kinetics of the gasification of an Argentinean coal char with carbon dioxide has been studied by thermogravimetry between 1073 and 1223 K. The starting temperature for the reaction of the char with carbon dioxide is determined at about 873 K. The influence of gaseous flow rate, sample mass, temperature and carbon dioxide partial pressure in the reaction rate is analyzed. The conditions under which the progress of reaction is controlled by chemical reaction are established. In those conditions, an activation energy of 45.6 kcal mol −1 was obtained with an isoconversional method. Regarding the effect of partial pressure of CO 2, the gasifications were found to be of first order reaction for CO 2 for partial pressures of CO 2 up to 61 kPa. In addition, a Langmuir–Hinshelwood type equation fits the experimental data for whole range of pressures to a certain extent.

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O 2 and CO 2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O 2 balanced with argon and pure CO 2. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO 2 at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O 2 also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O 2 and pure CO 2) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.

Enhanced catalysis of K 2CO 3 for steam gasification of coal char by using Ca(OH) 2 in char preparation

A novel approach has been proposed for mitigating the potassium deactivation in the K 2CO 3-catalyzed steam gasification of coal char by addition of Ca(OH) 2 in the char preparation. It was experimentally found that the Ca(OH) 2-added char had higher reactivity for the catalytic gasification than the raw char. Ca(OH) 2 played a role in suppressing the interactions of K 2CO 3 with acidic minerals in coal during the gasification and also probably in forming more active oxygenated intermediate on the char surface. The distribution of gaseous products was examined during the catalytic gasification. An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the catalytic gasification.

A Study on the Applicability of Kinetic Models for Shenfu Coal Char Gasification with CO2 at Elevated Temperatures

In this paper, measurements of the CO2 gasification kinetics for two types of Shenfu coal chars, which were respectively prepared by slow and rapid pyrolysis at temperatures of 950 °C and 1,400 °C, were performed by an isothermal thermo-gravimetric analysis under ambient pressure and elevated temperature conditions. Simultaneously, the applicability of the kinetic model for the CO2 gasification reaction of Shenfu coal chars was discussed. The results showed: (i) the shrinking un-reacted core model was not appropriate to describe the gasification reaction process of Shenfu coal chars with CO2 in the whole experimental temperature range; (ii) at the relatively low temperatures, the modified volumetric model was as good as the random pore model to simulate the CO2 gasification reaction of Shenfu coal chars, while at the elevated temperatures, the modified volumetric model was superior to the random pore model for this process; (iii) the integral expression of the modified volumetric model was more favorable than the differential expression of that for fitting the experimental data. Moreover, by simply introducing a function: A = A★exp(ft), it was found that the extensive model of the modified volumetric model could make much better predictions than the modified volumetric model. It was recommended as a convenient empirical model for comprehensive simulation of Shenfu coal char gasification with under conditions close to those of entrained flow gasification.

CO2 gasification of Thai coal chars: Kinetics and reactivity studies

Two sized fractions (<75 µm and 150-250 µm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500-900 o C. Gasification of coal chars with excess carbon dioxide was then performed at 900-1,100 o C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conver- sion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher Rs values than higher rank Lampang subbitumi- nous B coals. Smaller chars from both coals had higher Rs values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various py- rolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.