Speciation and distribution of alkali,alkali earth metals and major ash forming elements during gasification of fuel cane bagasse

Gasification of fuel cane bagasse, the waste residue from fuel cane, a hybrid of wild and commercial clones of sugar cane, was carried out in a novel 50 kWe air-blown autothermal downdraft gasifier. The speciation and distribution of alkali, alkali earth metals and major ash forming elements during gasification were investigated to evaluate the extent of volatilisation of these elements into the syngas and to determine the likely impact on syngas fuelled solid oxide fuel cell systems. Also assessed was the potential for defluidisation of the fuel bed due to agglomerate and deposit formation. Chemical fractionation studies showed that 30% of the potassium was captured by aluminosilicates and was retained in the ash, thereby reducing the alkali loading in the syngas and that more than 50% of the alkali earth metals were released to the syngas. In contrast, although the major ash forming elements were transformed from acid insoluble to acid soluble forms during gasification they remained hard bound in the ash and less than 30% of each one was released into the gas phase. The composition of clinkers and agglomerates produced during gasification was investigated by SEM-EDX and XRD which confirmed the presence of the eutectic systems KAlSi₂O₆–SiO₂, KAlSi₂O₆–CaMgSi₂O₆–SiO₂ and CaMgSi₂O₆–NaAlSi₃O₈. A preliminary model of the distribution behaviour during gasification of the ash forming elements has been developed.     

Mineral reaction and morphology change during gasification of coal in CO2 at elevated temperatures

Studies of the gasification of char in CO₂ at elevated temperatures are necessary for the development of IGCC technology. Experiments at high heating rates and elevated temperatures revealed that the temperature dependence of gasification reactivity was very different for low compared with high temperature ranges. To elucidate these mechanisms, the reaction of mineral matter and the change in morphology during gasification of a char at elevated temperatures were examined by char characterisation. CO₂ gasification experiments showed a large difference in gasification rate for chars prepared at higher temperatures compared to those prepared at lower temperatures. Changes in char particle morphology and mineral matter during gasification are also quite different. At higher carbonisation temperatures, mineral reactions during pyrolysis, which occurs in addition to ash fusion, appear to be one of the factors accounting for these differences. Certainly, a change of mechanism is involved. Graphite enrichment may also contribute to the decrease in char reactivity.     

Sulfur transfers from pyrolysis and gasification of direct liquefaction residue of Shenhua coal

The sulfur transformation during pyrolysis and gasification of Shenhua direct liquefaction residue was studied and the release of H₂S and COS during the process was examined. For comparison, the sulfur transfer of Shenhua coal during pyrolysis and that of pyrolyzed char during gasification were also studied. The residue was pyrolyzed at 10 °C /min to 950 °C. During pyrolysis about 33.6% of sulfur was removed from the residue, among which 32.1% was formed H₂S in gas and 1.5% was transferred into tar, 66.4% of the sulfur was remained in residue char. Compared with coal, the residue has generated more H₂S due to presence of Fe_1−xS which was enriched in residue during liquefaction process. There is a few COS produced at 400-500 °C during pyrolysis of coal, but it was not detected form pyrolysis of the residue. During CO₂ gasification, compared with pyrolysis and steam gasification, there are more COS and less H₂S formation, because CO could react with sulfide to form COS. During steam gasification only H₂S was produced and no COS detected, because H₂ has stronger reducibility to form H₂S than CO. After steam gasification no sulfur was detected in the gasification residue. The XRD patterns show after steam gasification, only Fe₃O₄ is remained in the gasification residue. This indicates that the catalyst added during the liquefaction of coal completely reacted with steam, resulting in the formation of H₂ and Fe₃O₄.     

Effects of Fe- and Ca-based additives on NO emission during gasification of N-containing model compound under different atmospheres

The effects of iron and calcium compounds on NO emission during CZC (carbazole char) gasification under different O₂ concentrations have been investigated in a quartz tube fixed bed reactor. Temperature-programmed gasification was preformed to determine the influence of O₂ concentration and catalytic effects of iron and calcium on NO emission. The results show that the O₂ concentration strongly influences NO emission during CZC gasification; suitable O₂ concentration could largely reduce NO emission, but high or low O₂ concentration is unfavorable to reduce NO pollution. Iron catalysts increase NO emission during CZC gasification in O₂ concentration ranging from 20.9 to 1.2%, and their catalytic effects are strongly depended on O₂ concentration. The order of their catalytic effects is Fe(AC)₂>FeCl₂≈FeCl₃. Different kinds of iron compounds show different catalytic activities for their different dispersion on CZC and their different decomposition tendency to form active sites. Calcium compounds show different effects on NO emission depending on O₂ concentration. Ca(AC)₂ and CaCl₂ increase NO emission under high O₂ concentration (such as 20.9%), but they decrease NO under low O₂ concentration (such as 1.2%). When CZC gasification under 4.8% O₂-Ar, Ca(AC)₂ decreases NO emission, but CaCl₂ increases NO emission.     

Distribution of volatile sulphur containing products during fixed bed pyrolysis and gasification of coals

Three sub-bituminous and two bituminous coals from Western Canada were used to study the evolution of H₂S, COS and SO₂ during the pyrolysis and gasification processes in a fixed bed reactor. For all types of coals, most of H₂S and SO₂ were released during the devolatilization stage. COS was formed only during the gasification stage in the presence of CO₂. The mineral matter of coal may have played a role during the gasification stage. Some observations made during this latter stage in CO₂ and/or steam were interpreted in terms of the equilibrium effects.     

Enhanced catalysis of K2CO3 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 K₂CO₃-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 K₂CO₃ 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.     

Effect of pressure on gasification reactivity of three Chinese coals with different ranks

The gasification reactivities of three kinds of different coal ranks (Huolinhe lignite, Shenmu bituminous coal, and Jincheng anthracite) with CO₂ and H₂O was carried out on a self-made pressurized fixed-bed reactor at increased pressures (up to 1.0 MPa). The physicochemical characteristics of the chars at various levels of carbon conversion were studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), and BET surface area. Results show that the char gasification reactivity increases with increasing partial pressure. The gasification reaction is controlled by pore diffusion, the rate decreases with increasing total system pressure, and under chemical kinetic control there is no pressure dependence. In general, gasification rates decrease for coals of progressively higher rank. The experimental results could be well described by the shrinking core model for three chars during steam and CO₂ gasification. The values of reaction order n with steam were 0.49, 0.46, 0.43, respectively. Meanwhile, the values of reaction order n with CO₂ were 0.31, 0.28, 0.26, respectively. With the coal rank increasing, the pressure order m is higher, the activation energies increase slightly with steam, and the activation energy with CO₂ increases noticeably. As the carbon conversion increases, the degree of graphitization is enhanced. The surface area of the gasified char increases rapidly with the progress of gasification and peaks at about 40% of char gasification.     

Prediction of the behaviors of H2S and HCl during gasification of selected residual biomass fuels by equilibrium calculation

H₂S and HCl released during biomass gasification can decrease the performance of high-temperature fuel cells in an Integrated Biomass Gasification Fuel Cell power-generating system. In this study, the behaviors of such poisonous gases during the gasification of different biomass fuels at various temperatures ranging from 673 to 1473 K were predicted using an equilibrium calculation approach. The predictions showed not only a difference in emission behaviors of HCl and H₂S among the biomass fuels, but also a low HCl emission (below 10 ppmv) for a few of the fuels at any temperature. In addition, the influence of biomass metal composition and gasification temperature on emission behavior was investigated by analyzing the distribution of chlorine and sulfur compounds and the phase diagram of selected elements such as silicon and aluminum. Finally, we suggest that the addition of a potassium-rich biomass to a potassium-poor biomass has the potential to reduce the HCl emission during gasification and then to maintain the HCl concentration in gas phase below the tolerance concentration of the fuel cells.     

Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst

Effects of steam (H₂O) and carbon dioxide (CO₂) pretreatments on methane (CH₄) decomposition and carbon gasification over doped-ceria supported nickel catalysts have been studied from 400 to 500 °C. The doped ceria employed were gadolinia-doped ceria and samaria-doped ceria. Results indicate that a drastic increase of both H₂O and CO₂ dissociation activities occurs as the temperature increases from 450 to 500 °C. The formation of the surface hydroxyl species during H₂O treatment inhibits the followed CH₄ decomposition. CO but no CO₂ was formed during CH₄ reaction after H₂O treatment. Carbon deposition during CH₄ decomposition is quite large but can be removed via gasification with afterward CO₂ treatment. However, some of the deposited carbon species is in a form which can not be removed with CO₂ treatment but can be removed with O₂ treatment. And, higher values of the oxygen-ion conductivity and the density of the surface oxygen vacancies lead to higher activities for all dissociation and decomposition reactions.     

基于赤铁矿载氧体的煤化学链燃烧试验

化学链燃烧是一种具有CO₂内分离特性的燃烧方式。以赤铁矿为载氧体,在1 kW_th级串行流化床上进行了煤化学链燃烧试验。讨论了燃料反应器温度对气体产物组分的影响;比较了各反应参数对煤气化效率、煤气化产物的转化效率及碳捕集效率的影响情况,分析了煤中硫的排放问题。试验结果表明：温度由900℃升高到985℃,燃料反应器中CO体积份额逐渐增加,CO₂体积份额逐渐减小,空气反应器中CO₂浓度呈线性下降。燃料反应器温度的升高促进煤气化效率及碳捕集效率大大提高。载氧体量和系统负荷是煤气化产物转化效率的主要影响因素,载氧体量的增加和负荷的增加分别会使煤气化产物转化效率提高和下降。燃料反应器中的硫主要以SO₂形式存在于燃料反应器,随温度的升高,SO₂浓度由515×10^-6逐渐增加到562×10^-6相似文献   

CO2 and steam gasification of a grapefruit skin char

A kinetic study on the gasification of carbonised grapefruit (Citrus Aurantium) skin with CO₂ and with steam is presented. The chars from this agricultural waste show a comparatively high reactivity, which can be mostly attributed to the catalytic effect of the inorganic matter. The ash content of the carbonised substrate used in this work falls around 15% (db) potassium being the main metallic constituent. The reactivity for both, CO₂ and steam gasification, increases at increasing conversion and also does the reactivity per unit surface area, consistently with the aforementioned catalytic effect. Lowering the ash content of the char by acid washing leads to a decrease of reactivity thus confirming the catalytic activity of the inorganic matter present in the starting material. Saturation of this catalytic effect was not detected within the conversion range investigated covering in most cases up to 0.85-0.9. Apparent activation energy values within the range of 200-250 kJ/mol have been obtained for CO₂ gasification whereas the values obtained for steam gasification fall mostly between 130 and 170 kJ/mol. These values become comparable with the reported in the literature for other carbonaceous raw materials including chars from biomass residues and coals under chemical control conditions.     

Gasification of woody biomass char with CO2: The catalytic effects of K and Ca species on char gasification reactivity

The effects of alkali and alkaline earth metals such as potassium (K) and calcium (Ca) on CO₂ gasification reactivity of Japanese cypress (hinoki) char under various temperatures (1123-1223 K) and CO₂ concentration (20-80 vol.%) were studied using thermal gravimetric analysis. The presence of K and Ca compounds in char improved the reactivity of hinoki char for CO₂ gasification catalytically. It was also confirmed that K and Ca compounds can be supported on char to exhibit an enhanced catalytic effect during CO₂ gasification of K-char and Ca-char. The char gasification rate increased with the increase of CO₂ concentration at higher temperatures (1173-1223 K), however at lower temperature (1123 K) the gasification rate decreased at 80% CO₂. The retardation of char gasification rate at higher CO₂ concentration is caused by the inhibition effect of CO: CO is disproportionated on alkali metal catalysts to CO₂ and carbon, and affected the CO₂ gasification rate. The dependence of char gasification rate on reaction temperature yielded a straight line in an Arrhenius-type plot which indicated that there was no significant change in the gasification mechanism in the temperature range of 1123-1223 K.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam

The purpose of this study is to investigate the major factors influencing the Na-catalysed and non-catalysed gasification reactivity of a Victorian brown coal in steam. An acid-washed (H-form) sample and a Na-exchanged (Na-form) sample prepared from the same Loy Yang brown coal were gasified in 15% steam in a novel two-stage fluidised-bed/fixed-bed reactor. All C-containing species in the gasification product gas were converted into CO₂ that was monitored with a mass spectrometer continuously to determine the in situ gasification reactivity. While the volatile-char interactions were responsible for the volatilisation of Na when the coal was continuously fed into the reactor, the physical entrainment by gas of agglomerated Na-containing crystalline species (likely to be Na₂CO₃ or Na₂O) from char surface was the main mechanism for the loss of Na during char gasification. The Raman spectroscopy of char showed the preferential release of smaller aromatic ring system to be more significant during the non-catalysed char gasification than the Na-catalysed gasification. The dispersion of Na in char appeared to deteriorate with the enrichment of large aromatic ring systems in char, greatly affecting the char gasification reactivity. The char gasification reactivity showed a maximum with increasing conversion with the maximum to shift towards lower conversion with increasing temperature. Increasing temperature does not always lead to increases in the in situ char gasification reactivity.     

Pyrolysis/gasification of agricultural residues by carbon dioxide in the presence of different additives: influence of variables

Pyrolysis/gasification of grape and olive bagasse by CO₂ under different experimental conditions has been studied. Variables investigated were particle size, temperature, type and concentration of additive and chemical washing with sulfuric and phosphoric acid solutions. Compounds like H₂, CH₄, CO and methanol, acetone, furfuryl alcohol, furfural, naphthalene, phenol and o-cresol were identified as components of gas and liquid fractions obtained from pyrolysis/gasification processes. Particle size had no influence, while temperature was a significant variable yielding increases of fixed carbon and gas content. In most of cases, a temperature between 600 to 700°C lead to a maximum liquid production. The principal additive used was ZnCl₂, concentration of this salt exerted a positive effect on hydrogen production, about 5 to 8 times higher than that obtained in the absence of additive. As far as structural characteristics of activated carbon are concerned, the increase of temperature, ZnCl₂ and acid solution concentrations (during chemical washing) lead to an increase of the specific surface area.     

Entrained-flow dry-bottom gasification of high-ash coals in coal-water slurries

It was shown that the effective use of dry ash removal during entrained-flow gasification of coal-water slurries consists in simplification of the ash storage system and utilization of coal ash, a decrease in the coal demand, a reduction in the atmospheric emissions of noxious substances and particulate matter, and abandonment of the discharge of water used for ash slurry. According to the results of gasification of coal-water slurries (5–10 μm) in a pilot oxygen-blow unit at a carbon conversion of >91%, synthesis gas containing 28.5% CO, 32.5% H₂, 8.2% CO₂, 1.5% CH₄, the rest being nitrogen, was obtained. The fly ash in its chemical composition, particle size, and density meets the requirements of the European standard EN 450 as a cement additive for concrete manufacture.     

CO2 gasification of carbon catalysed by alkali metals: Reactivity and mechanism

A systematic study of the catalytic activity of alkali metal carbonates on the CO₂ gasification of activated carbon revealed the following order: Li < Na < K < Rb < Cs. Outgassing in an inert gas results in a pronounced activity decrease for Cs, whereas the other alkali metals show a slight increase. The activated carbon itself is unaffected. Apparent activation energies for the CO₂ gasification are also changed by outgassing and decrease from Li to Cs. Upon outgassing of the samples, CO₂ and CO are released in five distinguishable temperature regions, arising from decomposition of surface complexes and carbonate-like species, gasification phenomena and reduction of oxidic species. Outgassing patterns of all alkali metals are quite similar. During alkali-metal-catalysed gasification in CO₂ two types of oxidic species are present: surface bonded -OM species of high stability and oxidic species having less interaction with the carbon surface.     

Gasification of Microalgae Using Supercritical Water and the Potential of Effluent Recycling

The gasification of microalgae in supercritical water was investigated in this work. The product gas contained mainly H₂, CO₂, CH₄, and C₂H₆. Operation at high temperatures and lower biomass concentrations resulted in the highest carbon gasification efficiency and the lowest total organic carbon levels in the residual water. Due to its content of inorganic nutrients, the residual water was applied as cultivation medium for microalgae. However, algal growth in the untreated residual water was inhibited by the existence of potentially toxic substances evolved from gasification. Upon treatment by activated carbon filtration and ultraviolet light degradation, these substances were eliminated and cultivation in the residual water was possible. The major fraction of inorganic residues from gasification was recovered by means of water purging, increasing the potential of nutrient recycling for cultivation.     

On the high‐gasification rate of Brazilian manganese ore in chemical‐looping combustion (CLC) for solid fuels

The high rate of char gasification observed when using a Brazilian manganese ore as compared to ilmenite is investigated in a batch fluidized‐bed reactor. Experiments were carried out at 970°C using petroleum coke, coal and wood char as fuel with a 50% H₂O in N₂ as fluidizing gas. A manufactured manganese oxygen carrier was also used, however, which presented a slower char conversion rate than the manganese ore. It is concluded that decrease in H₂ inhibition and oxygen release are unlikely to be the main responsible mechanisms for the ore's unexpected gasification rate. The ore was also mixed in different ratios with ilmenite and it was observed that the presence of even small amounts of ore in the bed resulted in increased gasification rate. Thus, the high‐gasification rate for the manganese ore could be due to a contribution from the impurities in the ore by catalyzing the gasification reaction. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4346–4354, 2013     

Intrinsic char reactivity of plastic waste (PET) during CO2 gasification

Char reactivity has a strong influence on the gasification process, since char gasification is the slowest step in the process. A sample of waste PET was devolatilised in a vertical quartz reactor and the resulting char was partially gasified under a CO₂ atmosphere at 925 °C in order to obtain samples with different degrees of conversion. The reactivity of the char in CO₂ was determined by isothermal thermogravimetric analysis at different temperatures in a kinetically controlled regime and its reactive behaviour was evaluated by means of the random pore model (RPM). The texture of the char was characterised by means of N₂ and CO₂ adsorption isotherms. The results did not reveal any variation in char reactivity during conversion, whereas the micropore surface area was affected during the gasification process. It was found that the intrinsic reaction rate of the char can be satisfactorily calculated by normalizing the reaction rate by the narrow micropore surface area calculated from the CO₂ adsorption isotherms. It can be concluded therefore that the surface area available for the gasification process is the area corresponding to the narrow microporosity.     

Effect of experimental conditions on co-gasification of coal,biomass and plastics wastes with air/steam mixtures in a fluidized bed system

The effect of temperature and of gasification medium was studied, using only air, only steam and mixtures of both as gasification medium, with the aim of optimising co-gasification of coal and wastes. The rise in gasification temperature promoted hydrocarbons further reactions, leading to a decrease in tars and hydrocarbons contents and an increase in H₂ release. Increasing temperature, from 750 to 890 °C, during gasification of a mixture with 60% (w/w) of coal, 20% of pine and 20% of PE wastes, led to a decrease in methane and other hydrocarbons concentration of about 30 and 63%, respectively, whilst hydrogen concentration increased around 70%. Hydrocarbons contents decrease was also achieved by increasing air flow rate, because partial combustion caused by oxygen decreased tars and gaseous hydrocarbons, with even a decrease in heating requirements. However, the presence of air is disadvantageous, because it decreases the higher heating value of the gasification gas, due to nitrogen diluting effect. The rise of steam flow rate has proven to be advantageous, because reforming reactions were favoured, thus hydrocarbons concentrations decreased and hydrogen release increased.