Chemical fractionation tests on South African coal sources to obtain species-specific information on ash fusion temperatures (AFT)

Coals from the different sources used by Sasol vary substantially in terms of chemical and physical properties and directly relates to gasifier behavior. Due to the large variation in coal properties from various sources, detailed coal and feedstock characteristics are essential to predict gasification performance when a specific coal source is to be gasified. One property that specifically gives detail information on the suitability of a coal source [Alpern B, Nahuys J, Martinez L. Mineral matter in ashy and non-washable coals—its influence on chemical properties. Commun Serv Geol Portugal 1984; 299–317.] for gasification purposes is the ash fusion temperature (AFT). The AFT of a coal source indicates the extent to which ash agglomeration and ash clinkering are likely to occur within the gasifier. Ash clinkering inside the gasifier can cause channel burning, pressure drop problems and unstable gasifier operation. The principle aim of this paper is to obtain mineral species-specific information on ash properties and the specific affect on AFT. Chemical fractionation treatment resulted in coals having different mineral properties that can be used to explain the affect of specific minerals on the AFT of coal. The highest concentration and species of minerals were removed from the coal by acid leaching (HCl and HNO₃) where Al, Ca, Mg, Na and Fe were removed in high concentrations from the coal. An interesting finding in the ash composition of the coal after leaching was that the SO₃ concentration decreased from >2 mass % in the original coal sources to <0.3 mass % after the acid leaching. The AFT of coal after leaching increased to >1600 °C. Based on the 95% confidence intervals depicted the following components can be highlighted as having a statistical significant effect on the AFT: Al₂O₃, Fe₂O₃, CaO, MgO, P₂O₅ and SiO₂–Al₂O₃ combination. When mineral ratio was used, the best correlation coefficient (R) with AFT was obtained with the dolomite ratio. This is in agreement with the results obtained from the correlations between the AFT and the ash composition where CaO and MgO resulted in the best correlation with AFT. Although the correlation (R) of 0.81 is fairly similar to that of the individual correlations with CaO and MgO, the dolomite ratio also includes Fe, Na and K, which can have mineral interaction with the Ca and Mg and thus may be included in the ratio. Results presented in this paper again highlights the fact and confirmed work from other researchers [Slegeir WA, Singletary JH, Kohut JF. Application of a microcomputor to the determination of coal ash fusibility characteristics. J Coal Quality 7: 248–54.] that ash composition (elemental analyses) on its own does not explain AFT behavior or commercial performance of coal accurately.     

Identification of reaction zones in a commercial Sasol-Lurgi fixed bed dry bottom gasifier operating on North Dakota lignite

The Sasol-Lurgi fixed bed dry bottom gasification technology has the biggest market share in the world with 101 gasifiers in operation. To be able to further improve the technology and also to optimise the operating plants, it is important that the fundamentals of the process are understood. The main objective of this study was to determine the reaction zones occurring in the Sasol-Lurgi fixed bed dry bottom (S-L FBDB) gasifier operating on North Dakota lignite. A Turn-Out sampling method and subsequent chemical analyses of the gasifier fuel bed samples was used to determine the reaction zones occurring in the commercial MK IV, S-L FBDB gasifier operating on North Dakota lignite. The reaction zones were further compared with the same reactor operating on bituminous coal.Based on the results obtained from this study it was found that about two thirds of the gasifier volume was used for drying and de-volatilising the lignite thus leaving only about a third of the reactor volume for gasification and combustion. Nonetheless, due to the high reactivity of the lignite, the char was consumed within a third of the remaining gasifier volume. Clear overlaps between the reaction zones were observed in the gasifiers thus confirming the gradual transition from one reaction zone to another as reported in literature. Due to the high moisture content in the lignite, the pyrolysis zone in the gasifiers operating on North Dakota lignite occurred lower/deeper in the gasifier fuel bed as compared to the same gasifier operating on South African bituminous coal from the Highveld coalfield. All the other reaction zones in the gasifier operating on bituminous coal were also higher in the bed compared to the lignite operation. This can therefore explain the higher gas outlet temperatures for the S-L FBDB gasifiers operating on higher rank coals when compared to the gasifiers operating on lignite. The fact that the entire reactor volume was utilized for drying, de-volatilisation, gasification and combustion with carbon conversion of >98% makes the S-L FBDB gasifier very suitable for lignite gasification.     

Coal char temperature profile estimation using optical reflectance for a commercial-scale Sasol-Lurgi FBDB gasifier

In the Sasol-Lurgi fixed-bed dry-bottom (FBDB) gasifier the temperature in the combustion zone should not exceed the melting point of the ash-forming minerals, causing them to melt/flow and agglomerate. Sintering of ash particles is considered desirable in Sasol-Lurgi FBDB gasification, since it promotes easy gas flow, whereas clinkering creates channeling and localized “hot spots”, leading to unstable gasifier operation. Due to the counter-current mode of operation, hot ash exchanges heat with the cold incoming agent (steam and oxygen), while at the same time hot raw gas exchanges heat with cold incoming coal. This results in the ash and raw gas leaving the gasifier at relatively low temperatures compared to other types of gasifiers, which improves the thermal efficiency and lowers the steam consumption.Vitrinite reflectance analyses were performed on a range of Sasol-Lurgi MK IV commercial-scale gasifier turn-out samples, applying ISO standards 7404-5. Average temperature profile measurements of the solid particles, successfully revealed the temperature range occurring within the various zones of the gasifier. The average (mean) temperature ranged from ca. 400 °C up to 850 °C within the pyrolysis region. In this region of the gasifier, the particle surface temperature and peak temperature showed visible evidence of heat transfer limitations occurring through lump coal when compared to the mean particle temperature. This provides some evidence of the complex radial and localized behaviour occurring within the averaged axial sample slices. In the oxidizing and combustion regimes, exothermic conditions prevail and heat transfer differences across the particles are minimized. A characteristic spike, indicative of an increase in temperature, was found in the sample taken directly above the ash-grate, seeming to indicate that agent distribution through the nozzles positioned just above the grate is not uniform, resulting in localized oxygen concentration increases with subsequent “hot-spots” and channel-burning occurring. Homogenization of the ash bed could help to optimize the agent distribution within the reactor.The surface temperature profile of the gasifier solids was thus found to be in reasonable agreement with literature, albeit that different coal types and temperature profile estimation methods were utilized.     

An understanding of the behaviour of a number of element phases impacting on a commercial-scale Sasol-Lurgi FBDB gasifier

Chemical properties of coal which impact on gasification performance relate to those processes which do effect a change in chemical constitution, these in turn may lead to changes in physical properties such as particle size distribution and surface area of the coal. Turn-out samples obtained from a commercial-scale Sasol-Lurgi fixed-bed dry bottom (FBDB) gasifier were characterized to understand and interpret the internal chemical property behaviour and are discussed in relation to the residual C, H, N, S and O distribution profiles obtained. Thermodynamic equilibrium simulation of the organic and inorganic speciation behaviour occurring within a fixed-bed gasifier was modelled using the Fact-Sage simulation package, and used to support the measured ultimate analysis profile data obtained.The measured gasifier ultimate analysis profiles provided good insight into understanding the development of aromaticity of the char, expressed by the carbon:hydrogen ratio calculated on a mass basis. Equilibrium compositional profiles calculated for C, H, N, S and O provided discernment regarding the speciation and partitioning behaviour occurring within the fixed-bed-reactor. Fact-Sage thermodynamic equilibrium modeling of the gasifier related to the ultimate analysis results, was found to be useful in identifying an oxygen scavenging effect created by the mineral transformation behaviour occurring during reduction. It was found that oxygen-containing species such as Mg₂Al₄Si₅O₁₈ (corderite) and Fe₂Al₄Si₅O₁₈ (ferro-corderite) form within the reduction zone. It would appear that mineral composition is a more fundamental property than merely ash content in the gasification process, when viewed on an oxygen consumption basis.     

Pipe reactor gasification studies of a south african bituminous coal blend. Part 1 - Carbon and volatile matter behaviour as function of feed coal particle size reduction

The Sasol-Lurgi fixed-bed dry-bottom (FBDB) MKIV gasifiers are proven to be robust as far as acceptable coal properties are concerned, in particular its ability to accommodate a range of particle size distributions (PSD) fractions. Over the years, the findings from a number of studies conducted at Sasol have played a key role in the optimization of the Sasol-Lurgi gasifiers as far as the limited amount of coal preparation by crushing and screening is concerned. The continued optimization efforts by Sasol over many years have led to a robust and reliable gasification technology for coal conversion, and more improvements are envisaged for the near future.In this study, gasification profiles inside real coal beds were investigated experimentally using a pilot scale combustor unit (pipe reactor), where the top size of the coal blend was systematically reduced from 75 mm, 53 mm and 37.5 mm. The pilot scale combustor has an inside diameter of 400 mm, is approximately 3 m long and the combustion rate is controlled by regulating the oxygen/nitrogen ratio of the gas feed. Ash is not removed continuously, so the combustion front moves upwards through the coal bed with time, resulting in a temperature gradient across the bed. The combustion process can be stopped at any point in time by removing all of the oxygen from the feed gas (i.e. quenching with nitrogen). The combustor was constructed so that it can be tilted onto its side and opened up like a coffin to allow sample taking and visual inspection of the combustion profile. In this case, equivalent sized slices were taken across the length of the reactor bed contents and the samples were analysed for PSD, proximate analysis, ultimate analysis, Fisher assay and coal char CO₂ reactivity. This paper focuses on the coal property transformational behaviour (as characterized by the proximate analysis and Fischer tar results) through packed coal beds of different feed coal size distributions.The proximate analysis results showed clear reaction zone profiles to be occurring within the pipe reactor, i.e. drying, pyrolysis, reduction and combustion (ash bed) zones, in agreement with the SL-FBDB MKIV commercial-scale findings. It was found that a decrease in feed coal particle size resulted in better heat transfer across the particles with ensuing faster volatile matter and tar evolution.     

Study on the ash fusion temperatures of coal and sewage sludge mixtures

The coal, sewage sludge, water and chemical additives are milled to produce coal-sludge slurry as a substitute for coal-water slurry in entrained-flow gasification, co-gasification of coal and sewages sludge can be achieved. The ash fusion temperature is an important factor on the entrained-flow gasifier operation. In this study, the ash fusion temperatures (DT, ST, HT and FT) of three kinds of coals (A, B and C), two kinds of sewage sludges (W1 and W2) and series of coal-sewage blends were determined, and the mineral composition during the ash melting process was analyzed by X-ray diffraction (XRD). The results showed that the ash fusion temperatures of most coal-sewage blends are lower than those of the coals and sewage sludges. The ashes have different mineral composition at different temperature during the heating process. It was found that the mineral composition of AW1 blend ash is located in the low-temperature eutectic region of the ternary phase diagram of SiO₂-Al₂O₃-CaO. The minerals found in BW1 blend ash are almost the same as those in B coal ash. Kyanite is detected in CW1 blend ash, which results in the ash fusion temperatures of CW1 blend ash higher than those of C coal. We found that sodium mineral matters are formed because of NaOH added to W2, which can reduce the ash fusion temperature of coal-sewage blends.     

煤灰熔融黏温特性及对气流床气化的适应性

以21个中国典型煤样为研究对象,根据煤灰中CaO和Fe₂O₃含量,将之分为低钙低铁类、中钙中铁类、中钙高铁类、高钙低铁类、高钙高铁类等类别。利用高温黏度计测量煤灰熔渣黏温特性,并利用计算软件FactSage对煤灰熔融状态进行热力学平衡计算,研究了液相熔渣及固体矿物质结晶与熔渣黏度的关系,分析整理了煤灰最初硅铝比(SiO₂/Al₂O₃)、固体结晶物以及液相熔渣组成3个因素对煤灰熔融特性和熔渣黏温特性的影响,为根据煤灰组分分析来预测不同煤的熔渣黏温特性及对气流床气化的适应性提供了一个简单而实用的判断方法。对气流床气化液态排渣的适应性从高到低依次为:中钙高铁类、高钙低铁类、中钙中铁类、低钙低铁类和高钙高铁类。     

Trace element behaviour in the Sasol-Lurgi MK IV FBDB gasifier. Part 1 - The volatile elements: Hg, As, Se, Cd and Pb

Coal-fired power and heat production are the largest single source of Hg in the atmosphere, and in March 2005, the US-EPA ruled regarding Hg reduction from coal utilization in the USA. Appropriate Hg pollution control of technology, as well as reductions in the uses of Hg and coal-containing Hg can readily reduce the releases of Hg from coal utilities. Integrated multi-pollutant (SOx, NOx, particulate matter and Hg) control technologies may be a cost-effective approach. Prior to considering mitigation technologies, it is necessary to understand the quantity of mercury in the feed coal, its mode of occurrence (i.e. mineral or organic associations), its partitioning behaviour during the process, and the volume and species in which it is being emitted via stacks. These factors have all been investigated up to the point of release for the Sasol gasification and steam-raising plants, including other trace elements.The focus of this paper is to discuss the more recent environmental research developments by Sasol, where trace element simulation and validation of model predictions have been undertaken for the Sasol-Lurgi gasification process operating on lump coal. Fact-Sage thermodynamic equilibrium modeling was used to simulate the trace elements: Hg, As, Se, Cd and Pb gas phase and ash phase partitioning and speciation behaviour occurring in a fixed-bed pressurised gasifier. A Sasol-Lurgi Mark IV (MK IV) fixed-bed dry bottom (FBDB) gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, findings being used to validate the modeled results. This paper will focus on the behaviour of the volatile Class I trace elements: Hg, As, Se, Cd and Pb within the Sasol-Lurgi MK IV FBDB gasifier as function of coal quality. This study excludes the downstream gas cleaning partitioning and speciation behaviour of these elements, which will form the basis of a future paper.Good agreement between model-predictions and measurements have been attained in this study, with the exception of As. Hg, Cd, Pb, As and Se were all found to be highly volatile, partitioning into the gas phase. Hg was found to be the most volatile element during fixed-bed gasification and is present in the gas phase in the form of elemental Hg (g). As, Se, Cd and Pb have lower volatilities when compared to Hg, and they vary in an order: Hg > Se > Cd > Pb > As. Speciation predictions showed that: Hg, AsH₃, H₂Se, PbSe, Cd, CdS, and PbS/Pb/PbCl, species could potentially exist in the raw gas phase.     

煤质对气流床煤气化的影响研究进展

对煤质与气流床气化炉的关系进行了论述。介绍了水煤浆气化炉与干煤粉气化炉的各自技术特点,分别讨论了煤阶、成浆性、发热量、工业分析、元素分析、灰熔融性、渣的流动性、熔渣腐蚀及煤灰沉积对气流床煤气化选型、设计、操作条件及气化性能的影响。分析了两类气化炉对不同煤性质的适应性并给出了典型的煤质适用范围。总结了工业应用针对不同煤质要求的解决方案并回顾了相应领域的研发进展。在分析讨论的基础上对煤质与气流床气化炉的选择、应用与进一步研发方向提供了建议。     

Reactions between sodium and kaolin during gasification of a low-rank coal

The experimental results from the reactions between a low-rank coal containing sodium either in the form of ion-exchanged carboxylate or physical impregnation with NaCl, with kaolin during pyrolysis in nitrogen and gasification with steam or carbon dioxide in conditions corresponding to atmospheric FBG are presented. Sodium and kaolin react to form sodium aluminosilicates. Thermodynamic equilibrium predictions have indicated the formation of either a solid sodium aluminosilicate nepheline, Na₂O·Al₂O₃·2SiO₂ or liquid albite, Na₂O·Al₂O₃·6SiO₂ during coal gasification. The reaction between kaolin and either forms of sodium present in the coal is more effective in steam than in nitrogen or carbon dioxide. This results principally in the preservation of meta-kaolinite hexagonal crystal structure and the formation of a high melting point sodium aluminosilicate nepheline, Na₂O·Al₂O₃·2SiO₂ or its polymorph carnegietite. It appears that the catalytic effect of carboxylate sodium has been reduced by the binding of sodium readily through the reaction with kaolin, resulting in lower char conversion. During gasification with steam at least 20 wt.% of sodium present in coal as sodium chloride reacted into insoluble aluminosilicates. No sodium silicate or liquid albite formation was detected in any of the samples. It can be expected that during gasification of coal containing sodium in the presence of kaolin, the formation of solid aluminosilicates, principally nepheline, should help to avoid the formation of liquid silicates and the problems associated with agglomeration and potentially defluidisation of a fluidised bed.     

Effect of Al2O3/CaO on the melting and mineral transformation of Ningdong coal ash

Coal ash melting characteristics has a direct impact on the smooth operation of entrained gasifier. Mineral conversion of coal ash is very significant to be investigated, because the mineral can affect the melting temperature and viscosity under high temperature conditions. In this paper, the effects of different Al₂O₃/CaO on the mineral conversion, melting temperature and viscosity of Ningdong coal ash are studied by the combination of experiment and simulation. The trend of melting temperature decreases firstly and rises with increasing Al₂O₃/CaO. The ash melting point reached to the lowest when the ratio is 1.23. XRD and Factsage software are used to analyze crystallization behavior of samples. The results show that the content of anorthite, albite and corundum increases and subsequently decreases, while the content of mullite decreases firstly and then rises with increasing Al₂O₃/CaO. High content with CaO can contribute to form albite and anorthite of low-melting. Besides, high content with Al₂O₃ can tend to produce mullite of high-melting. The results of experimental and simulation are basically the same, which lays a foundation for the melting characteristics of Ningdong coal ash and can provide technical support for the smooth operation of the entrained-gasifier.     

Characterization of sintered coal fly ashes

Çan, Çatala?z?, Seyitömer and Af?in-Elbistan thermal power plant fly ashes were used to investigate the sintering behavior of fly ashes. For this purpose, coal fly ash samples were sintered to form ceramic materials without the addition of any inorganic additives or organic binders. In sample preparation, 1.5 g of fly ash was mixed in a mortar with water. Fly ash samples were uniaxially pressed at 40 MPa to achieve a reasonable strength. The powder compacts were sintered in air. X-ray diffraction analysis revealed that quartz (SiO₂), mullite (Al₆Si₂O₁₃), anorthite (CaAl₂Si₂O₈), gehlenite (Ca₂Al₂SiO₇) and wollastonite (CaSiO₃) phases occurred in the sintered samples. Scanning electron microscopy investigations were conducted on the sintered coal fly ash samples to investigate the microstructural evolution of the samples. Different crystalline structures were observed in the sintered samples. The sintered samples were obtained having high density, low water adsorption and porosity values. Higher Al₂O₃ + SiO₂ contents caused to better properties in the sintered materials.     

Numerical simulation of coal gasification in entrained flow coal gasifier

This paper presents modeling of a coal gasification reaction, and prediction of gasification performance for an entrained flow coal gasifier. The purposes of this study are to develop an evaluation technique for design and performance optimization of coal gasifiers using a numerical simulation technique, and to confirm the validity of the model. The coal gasification model suggested in this paper is composed of a pyrolysis model, char gasification model, and gas phase reaction model. A numerical simulation with the coal gasification model is performed on the CRIEPI 2 tons/day (T/D) research scale coal gasifier. Influence of the air ratio on gasification performance, such as a per pass carbon conversion efficiency, amount of product char, a heating value of the product gas, and cold gas efficiency is presented with regard to the 2 T/D gasifier. Gas temperature distribution and product gas composition are also presented. A comparison between the calculation and experimental data shows that most features of the gasification performance were identified accurately by the numerical simulation, confirming the validity of the current model.     

Experimental investigations of the effect of coal type and coal burner with different oxygen supply angles on gasification characteristics

The IGCC (Integrated gasification combined cycle) is known as one of the coal fueled power generating technologies with the highest efficiency and lowest emissions. To achieve the required higher efficiency and lower emission performance, Korea's 300 MW IGCC RDD&D (Research Development, Demonstration and Dissemination) project was launched in December 2006 under the leadership of the Korea Electric Power Corporation (KEPCO), with the support of the Korea Ministry of Knowledge Economy. Our research group, the KEPCO Research Institute, set up a coal gasifier for the pilot test and conducted many experiments for the parametric study in this project.Our group focused its research on the effect of different types of coal and oxygen supply angles on the optimum O₂/coal ratio for gasification. Through this study we found that using higher oxygen content coal reduced the optimum O₂/coal ratio for gasification. However, there was no apparent relationship between the oxygen supply angle and the optimum O₂/coal ratio. And, two types of coal burners having different oxygen supply angles were used to conduct the feasibility study on a variable angle burner for the coal gasifier.     

Experiments on the effect of the pressure on the mineral transformation of coal ash under the different reaction atmosphere

This paper investigated the effect of the pressures, reaction atmospheres and coal ash species on the ash fusibility with high-pressure thermogravimetric analysis (PTGA) apparatus and X-ray diffraction (XRD) analysis. Each specimen analyzed by XRD was observed for the mineral conversion and formation of new minerals with the pressures under different atmospheres. These results indicate that the pressure restrains the transformation and decomposition of minerals. Many low-temperature minerals are still present under the elevated pressure. The different reaction atmospheres have different effects on the formation of coal ash minerals. Under the N₂ atmosphere, the present microcline may decrease the melting temperature of coal ash. And later, it transforms into sanidine at high pressure; thus, the melting temperature of coal ash may increase. Under the CO₂ atmosphere, the minerals such as microcline, lomonitite, geothite and illite are still present with the increase in pressure; this may reduce the melting temperature. While under the H₂O atmosphere, there are magnetite and anorthoclase, which may produce the low-temperature eutectics decreasing the melting temperature. The coal ash abundance in basic oxides or higher SiO₂, Fe₂O₃, K₂O and Na₂O has lower melting temperature. While the ash sample with more SiO₂ and Al₂O₃ and less Fe₂O₃ and basic oxides may lead to higher melting temperature.     

Ash deposition characteristics of Moolarben coal and its blends during coal combustion

We report a systematic and comprehensive laboratory investigation of the ash deposition behavior of Moolarben (MO) coal, which has recently begun to be imported into Korea. Ash deposition experiments were conducted in a drop tube reactor, and a water-cooled ash deposit probe was inserted into the reactor to affix the ash. The tests were conducted using five types of single coals (two bituminous and three sub-bituminous, including MO coal) and blended coals (bituminous coal blended with sub-bituminous coal). Two indices represent ash deposition behavior: capture efficiency and energy-based growth rate. A thermomechanical analysis evaluated the melting behavior of the resulting ash deposits. The MO coal had the least ash deposition of the single coals due to its high melting temperature, indicated by high ash silica content. Indonesian sub-bituminous coals formed larger ash deposits and were sticky at low temperatures due to relatively high alkali content. However, blends with MO coal had greater ash deposition than blends with other bituminous coals. This non-additive behavior of MO coal blends is likely due to interactions between ash particles. Coals with higher silica content more effectively retain alkali species, resulting in lower melting temperatures and larger ash deposits. Therefore, we recommend that when blending in a boiler, silica-rich coals (SiO₂>80%, SiO₂/Al₂O₃> 5) should be blended with relatively low-alkali coals (Na₂O+K₂O<3%), and the blending ratio of the silica-rich coals indicates less than 10%, which can safely operate the boiler.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part IX. Effects of coal ash and externally loaded-Na on fuel-N conversion during the reforming of coal and biomass in steam

Ash interacts strongly with char and volatiles in a gasifier, especially in a fluidised-bed gasifier. This study aims to investigate the effects of ash or ash-forming species on the conversion of fuel-N during gasification. A Victorian (Loy Yang) brown coal and a sugar cane trash were gasified in two novel fluidised-bed/fixed-bed reactors where the interactions of ash with char and/or volatiles could be selectively investigated. Our results show that the interaction of ash with char and/or volatiles could lead to increases in the yield of NH₃ and decreases in the yield of HCN although the increases were not always matched exactly by the decreases. Loading NaCl or Na₂CO₃ into the brown coal was also found to affect the formation of HCN and NH₃ during gasification. In addition to the possible catalytic hydrolysis of HCN into NH₃ particularly at high temperatures, two other causes were identified for the changes in the HCN and NH₃ yields. It is believed that some ash species could migrate into the char matrix to affect the local availability of H radicals or to catalyse the formation of NH₃ selectively. The interactions of ash (or Na loaded into the coal) with volatiles could enhance the formation of soot-N, which would be gasified favourably to form NH₃.     

Pressure-drop predictions in a fixed-bed coal gasifier

In the Sasol Synfuels plant in Secunda, Sasol-Lurgi fixed-bed dry-bottom gasifiers are used for the conversion of low-grade bituminous coals to synthesis gas (syngas). The gasifiers are fed with lump coal having a particle size in the range from 5 to 100 mm. Operating experience shows that the average particle size and particle-size distribution (PSD) of feed coal, char and ash influence the pressure drop across the bed and the gas-flow distribution within the bed. These hydrodynamic phenomena are responsible for stable gasifier operation and for the quality and production rate of the syngas. The counter-current operation produces four characteristic zones in the gasifier, namely, drying, de-volatilization, reduction and combustion. The physical properties of the solids (i.e. average particle size, PSD, sphericity and density) are different in each of these zones. Similarly, the chemical composition of the syngas, its properties (temperature, density and viscosity) and superficial velocity vary along the height of the bed. The most popular equation used to estimate the pressure drop in packed beds is that proposed by Ergun. The Ergun equation gives good predictions for non-reacting, isothermal packed beds made of uniformly sized, spherical or nearly spherical particles. In the case of fixed-bed gasifiers, predictions by the Ergun equation based on the average or inlet values of bed and gas flow parameters are unsatisfactory because the bed structure and gas flow vary significantly in the different reaction zones. In this study, the Ergun equation is applied to each reaction zone separately. The total pressure drop across the bed is then calculated as the sum of pressure drops in all zones. It is shown that the total pressure drop obtained this way agrees better with the measured result.     

Fusibility and flow properties of coal ash and slag

We studied the fusibility and flow properties of laboratory ash and Shell gasifier slag from the same coal sample (Chinese Huainan coal). The physical properties of ash and slag were analyzed with X-ray fluorescence, X-ray diffraction, and scanning electron microscopy. The fusion temperature and the experiment viscosity were measured for the ash and slag with the addition of fluxing CaO. Ash and slag have properties that were approximated by the SiO₂-Al₂O₃-‘FeO’-CaO system. The computer software package FactSage was used to predict the proportion of solid phase and the mineral phase formed as a function of the composition and the temperature of the SiO₂-Al₂O₃-‘FeO’-CaO system. The results show that the fusion temperatures and the temperature of critical viscosity (T_cv) of ash are always higher than that of slag. Also, the viscosity of ash is always higher than that of slag at the slag tapping temperature range of 1400-1500 °C, and the hysteresis between the heating and cooling cycles for ash is more obvious than that of slag because of different physical properties. The fusion temperature and T_cv of ash and slag decrease with increasing CaO content, whereas those values increase rapidly with CaO content higher than 35%. Also, the sensitivity of the viscosity of the ash and slag with temperature decreases with increasing CaO content because the sensitivity of the phase equilibria of in the SiO₂-Al₂O₃-‘FeO’-CaO system to temperature excursions decreases with increasing CaO content.