Circulating fluidized bed gasification of low rank coal: Influence of O<Subscript>2</Subscript>/C molar ratio on gasification performance and sulphur transformation

To promote the utilization efficiency of coal resources, and to assist with the control of sulphur during gasification and/or downstream processes, it is essential to gain basic knowledge of sulphur transformation associated with gasification performance. In this research we investigated the influence of O₂/C molar ratio both on gasification performance and sulphur transformation of a low rank coal, and the sulphur transformation mechanism was also discussed. Experiments were performed in a circulating fluidized bed gasifier with O₂/C molar ratio ranging from 0.39 to 0.78 mol/mol. The results showed that increasing the O₂/C molar ratio from 0.39 to 0.78 mol/mol can increase carbon conversion from 57.65% to 91.92%, and increase sulphur release ratio from 29.66% to 63.11%. The increase of O₂/C molar ratio favors the formation of H₂S, and also favors the retained sulphur transforming to more stable forms. Due to the reducing conditions of coal gasification, H₂S is the main form of the released sulphur, which could be formed by decomposition of pyrite and by secondary reactions. Bottom char shows lower sulphur content than fly ash, and mainly exist as sulphates. X-ray photoelectron spectroscopy (XPS) measurements also show that the intensity of pyrite declines and the intensity of sulphates increases for fly ash and bottom char, and the change is more obvious for bottom char. During CFB gasification process, bigger char particles circulate in the system and have longer residence time for further reaction, which favors the release of sulphur species and can enhance the retained sulphur transforming to more stable forms.     

Effect of the type of gasifying agent on gas composition in a bubbling fluidized bed reactor

It is commonly accepted that gasification of coal has a high potential for a more sustainable and clean way of coal utilization. In recent years, research and development in coal gasification areas are mainly focused on the synthetic raw gas production, raw gas cleaning and, utilization of synthesis gas for different areas such as electricity, liquid fuels and chemicals productions within the concept of poly-generation applications. The most important parameter in the design phase of the gasification process is the quality of the synthetic raw gas that depends on various parameters such as gasifier reactor itself, type of gasification agent and operational conditions. In this work, coal gasification has been investigated in a laboratory scale atmospheric pressure bubbling fluidized bed reactor, with a focus on the influence of the gasification agents on the gas composition in the synthesis raw gas. Several tests were performed at continuous coal feeding of several kg/h. Gas quality (contents in H₂, CO, CO₂, CH₄, O₂) was analyzed by using online gas analyzer through experiments. Coal was crushed to a size below 1 mm. It was found that the gas produced through experiments had a maximum energy content of 5.28 MJ/Nm³ at a bed temperature of approximately 800 °C, with the equivalence ratio at 0.23 based on air as a gasification agent for the coal feedstock. Furthermore, with the addition of steam, the yield of hydrogen increases in the synthesis gas with respect to the water–gas shift reaction. It was also found that the gas produced through experiments had a maximum energy content of 9.21 MJ/Nm³ at a bed temperature range of approximately 800–950 °C, with the equivalence ratio at 0.21 based on steam and oxygen mixtures as gasification agents for the coal feedstock. The influence of gasification agents, operational conditions of gasifier, etc. on the quality of synthetic raw gas, gas production efficiency of gasifier and coal conversion ratio are discussed in details.     

Hydrogen-rich gas from catalytic steam gasification of eucalyptus using nickel-loaded Thai brown coal char catalyst

Hydrogen gas production from eucalyptus by catalytic steam gasification was carried out in an atmospheric pressure of two-stage fixed bed. The gasifier was operated with the temperature range of 500–650 °C and steam partial pressure of 16, 30 and 45 kPa; nickel-loaded Thai brown coal char was used as a catalyst. The yields and compositions of the gasification products depend on the operating conditions, especially, the reaction temperature and the steam. The yield of H₂ increased at elevated temperatures, from 26.94 to 46.68%, while that of CO dramatically decreased, from 70.21 to 37.71 mol%. The highest H₂ yield, 46.68%, was obtained at the final gasifying temperature of 650 °C. Eucalyptus catalytic steam gasification indicated that the maximum H₂/CO ratio reached 1.24 at the gasification temperature of 650 °C and the steam partial pressure of 30 kPa. It can be concluded that eucalyptus is appropriate for synthesis gas production from eucalyptus volatiles by catalytic steam gasification while using nickel-loaded brown coal char as a catalyst.     

Circulating Fluidized Bed Gasification of Low Rank Coal:Influence of O_2/C Molar Ratio on Gasification Performance and Sulphur Transformation

To promote the utilization efficiency of coal resources,and to assist with the control of sulphur during gasification and/or downstream processes,it is essential to gain basic knowledge of sulphur transformation associated with gasification performance.In this research we investigated the influence of O_2/C molar ratio both on gasification performance and sulphur transformation of a low rank coal,and the sulphur transformation mechanism was also discussed.Experiments were performed in a circulating fluidized bed gasifier with O_2/C molar ratio ranging from 0.39 to 0.78 mol/mol.The results showed that increasing the O_2/C molar ratio from 0.39 to 0.78 mol/mol can increase carbon conversion from 57.65%to 91.92%,and increase sulphur release ratio from 29.66%to63.11%.The increase of O_2/C molar ratio favors the formation of H_2S,and also favors the retained sulphur transforming to more stable forms.Due to the reducing conditions of coal gasification,H_2S is the main form of the released sulphur,which could be formed by decomposition of pyrite and by secondary reactions.Bottom char shows lower sulphur content than fly ash,and mainly exist as sulphates.X-ray photoelectron spectroscopy(XPS)measurements also show that the intensity of pyrite declines and the intensity of sulphates increases for fly ash and bottom char,and the change is more obvious for bottom char.During CFB gasification process,bigger char particles circulate in the system and have longer residence time for further reaction,which favors the release of sulphur species and can enhance the retained sulphur transforming to more stable forms.     

Regulation of chemical properties of ash and kinetic model construction of Lu’an coal gasification

The objective of this study was to enhance the suitability of Lu’an coal for gasification in large entrained-flow gasifiers currently used by the Lu’an Group Mining Company in its 1.8 million ton per annum coal-based oil synthesis demonstration project. The effect of coal blending and flux addition on the ash fusion temperature (AFT) and gasification reactivity was investigated. CaO, Fe₂O₃, and MgO decreased the AFT of Lu’an coal by 150°C, 73°C, and 68°C, respectively, by a flux addition of up to 7%. Within the range of the experimental investigation, the AFT of Lu’an coal decreased by 3°C for each 1% of Shenmu coal addition. The gradual reduction of mullite and the formation of fayalite and hessonite in blended coal ash decreased the AFT. The addition of a fluxing agent significantly increased the reaction activity of the char, with Fe₂O₃ exhibiting the largest catalytic effect on char gasification. Blending with Shenmu char significantly increased the gasification reactivity. The random pore model best describes the gasification process of Lu’an char, and a kinetic equation for the process was developed on the basis of this model.     

High-temperature air and steam gasification of densified biofuels

An experimental study was carried out to investigate gasification of densified biofuels using highly preheated air and steam as a gasifying agent. Preheat of air and steam is realised by means of the newly developed high-cycle regenerative air/steam preheater. Use of highly preheated feed gas provides additional energy into the gasification process, which enhances the thermal decomposition of the gasified solids. For the same type of feedstock the operating parameters, temperature, composition and amount of gasifying agent, were varied over a wide range. Results of experiments conducted in a high-temperature air/steam fixed bed updraft gasifier show the capability of this technology of maximising the gaseous product yield as a result of the high heating rates involved, and the efficient tar reduction. Increase of the feed gas temperature reduces production of tars, soot and char residue as well as increases heating value of the dry fuel gas produced. Overall, it has been seen that the yield and the lower heating value of the dry fuel gas increase with increasing temperature.     

Characteristics of microalgae gasification through chemical looping in the presence of steam

As a novel gasification technology, chemical looping gasification (CLG) was considered as a promising technology in solid fuel gasification. In this work, CLG was applied into microalgae, and the characteristics of syngas production and oxygen carrier in the presence of steam were obtained through experiments in a fixed bed reactor. The results showed that the partial oxidation of oxygen carrier improved the gasification efficiency from 61.65% to 81.64%, with the combustible gas yield of 1.05 Nm³/kg, and this promotion effect mainly occurred at char gasification stage. Also, an optimal Fe₂O₃/C molar ratio of 0.25 was determined for the maximum gasification efficiency. 800 °C was needed for the gasification efficiency over 70%, but excess temperature caused the formation of dense layer on oxygen carrier particle surface. Steam as gasification agent promoted syngas production, but excess steam decreased the gasification efficiency. Steam also enhanced the hydrogen production by the conversion of Fe/FeO into Fe₃O₄, avoiding the intensive reduction of oxygen carrier. The Fe₂O₃ oxygen carrier maintained a good reactivity in 10th cycle while used for microalgae CLG. The results indicated that CLG provided a potential route for producing combustible gas from microalgae.     

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO₂ is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO₂ capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH₄ concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO₂ capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO₂ capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.     

Syngas production by gasification of aquatic biomass with CO2/O2 and simultaneous removal of H2S and COS using char obtained in the gasification

Applicability of gulfweed as feedstock for a biomass-to-liquid (BTL) process was studied for both production of gas with high syngas (CO + H₂) content via gasification of gulfweed and removal of gaseous impurities using char obtained in the gasification. Gulfweed as aqueous biomass was gasified with He/CO₂/O₂ using a downdraft fixed-bed gasifier at ambient pressure and 900 °C at equivalence ratios (ER) of 0.1–0.3. The syngas content increased while the conversion to gas on a carbon basis decreased with decreasing ER. At an ER of 0.1 and He/CO₂/O₂ = 0/85/15%, the syngas content was maximized at 67.6% and conversion to gas on a carbon basis was 94.2%. The behavior of the desulfurization using char obtained during the gasification process at ER = 0.1 and He/CO₂/O₂ = 0/85/15% was investigated using a downdraft fixed-bed reactor at 250–550 °C under 3 atmospheres (H₂S/N₂, COS/N₂, and a mixture of gases composed of CO, CO₂, H₂, N₂, CH₄, H₂S, COS, and steam). The char had a higher COS removal capacity at 350 °C than commercial activated carbon because (Ca,Mg)S crystals were formed during desulfurization. The char simultaneously removed H₂S and COS from the mixture of gases at 450 °C more efficiently than did activated carbon. These results support this novel BTL process consisting of gasification of gulfweed with CO₂/O₂ and dry gas cleaning using self-supplied bed material.     

Physicochemical properties and gasification reactivity of the ultrafine semi-char derived from a bench-scale fluidized bed gasifier

Zhundong coalfield is the largest intact coalfield worldwide and fluidized bed gasification has been considered as a promising way to achieve its clean and efficient utilization.The purpose of this study is to investigate the physieochemical properties and gasification reactivity of the ultrafine semi-char,derived from a bench-scale fluidized bed gasifier,using Zhundong coal as fuel.The results obtained are as follows.In comparison to the raw coal,the carbon and ash content of the semi-char increase after partial gasification,but the ash fusion temperatures of them show no significant difference.Particularly,76.53％ of the sodium in the feed coal has released to the gas phase after fluidized bed gasification.The chemical compositions of the semi-char are closely related to its particle size,attributable to the distinctly different natures of diverse elements.The semi-char exhibits a higher graphitization degree,higher BET surface area,and richer meso-and macropores,which results in superior gasification reactivity than the coal char.The chemical reactivity of the semi-char is significantly improved by an increased gasification temperature,which suggests the necessity of regasification of the semi-char at a higher temperature.Consequently,it will be considered feasible that these carbons in the semi-char from fluidized bed gasifiers are reclaimed and reused for the gasification process.     

煤的气化反应活性对常压固定床气化的影响

简要介绍了煤的气化反应活性及其影响因素,提出了根据煤的气化反应活性界定气化用煤灰熔融性温度指标的方法,并就煤的气化反应活性对常压固定床气化炉内氧化层、还原层、干馏层、干燥层及炉出煤气温度的影响进行了系统分析。     

Experimental assessment of woody biomass gasification in a hybridized solar powered reactor featuring direct and indirect heating modes

Solar thermochemical gasification is an opportunity for the production of sustainable fuels from carbonaceous resources including biomass. Substituting conventional gasification processes by solar-driven technologies may enable cleaner production of H₂-rich syngas while saving feedstock resources and alleviating CO₂ emissions. This work addresses hybrid solar-autothermal gasification of mm-sized beech wood particles in a lab-scale 1.5 kW_th spouted-bed reactor. Hybridization under reduced solar power input was performed by injecting oxygen and additional biomass inside the gasifier for complementary heat supply. Increasing O₂:C molar ratios (in the range 0.14–0.58) allowed to heat the reactor cavity and walls progressively, while gradually impairing the reactor performance with an increase of the syngas CO₂ content and a decrease of the reactor cold gas efficiency (CGE). Gasification with mixed H₂O and O₂ was then assessed at thermodynamic equilibrium and global trends were validated experimentally, showing that control of H₂:CO ratio was compatible with in-situ combustion. The impact of reaction temperature (1200–1300 °C) and heating mode (direct or indirect) was experimentally studied during both allothermal and hybrid gasification. Higher H₂ and CO yields were achieved at high temperatures (1300 °C) under direct reactor heating. Hybridization was able to counterbalance a 40% drop of the nominal solar power input, and the measured CGE reached 0.82, versus values higher than 1 during allothermal gasification.     

Experimental study on the key factors affecting the gasification performance between different biomass: Compare citrus peel with pine sawdust

This work aims to reveal the advantages of citrus peel gasification and investigate the key factors affecting gasification performance. The gasification performance of citrus peel and pine sawdust are compared in a fixed bed reactor, and the reactivity and properties of biochar were investigated. The results showed that the H₂ yield and carbon conversion efficiency of citrus peel gasification were 34.35 mol/kg_biomass and 66.30%, respectively, which were higher than those of pine sawdust. Due to the high reactivity of citrus peel char, it only takes 100 min for the citrus peel to complete the gasification reaction, which is significantly faster than pine sawdust. Although the specific surface area of citrus peel char is lower than that of pine sawdust char, both the low degree of graphitization and the high catalytic index (2426.96) are favorable for the conversion of char, which ultimately lead to the high reactivity of citrus peel char.     

Reduction of primary tar vapor from biomass by hot char particles in fixed bed gasification

This study investigated the reduction of primary tar vapor from biomass pyrolysis over a bed of hot char particles, focusing on the effect of different operating conditions and char properties. The char samples were prepared from wood, paddy straw, palm kernel shell, and activated carbon. The primary tar was produced from fir wood by pyrolysis at 500 °C and passed through a reactor filled with char particles with different lengths and temperatures.The tar cracking reactions became active above 700 °C, and the presence of hot char particles promoted more tar reduction compared with thermal cracking alone. The mass yield of the primary tar was reduced from 24.8% by pyrolysis to 13.7% by thermal cracking at 800 °C, and further to 7.7% by hot char particles in a reactor volume of 1.48 cm³/g_wood. In terms of carbon yield, these values correspond to 32.1%, 19.9% and 11.8%, respectively. The tar with smaller molecular weights was quickly decomposed to gases, whereas the heavy tar was resistant to cracking, even when the reactor volume was increased to 6.90 cm³/g_wood. The tar cracking behaviors were similar for four char types despite differences in microscopic surface areas, pore-size distributions, and inorganic contents. The results suggest that creating a tar-cracking zone using char particles situated between the pyrolysis and gasification zones could be helpful in converting the primary tar vapor in a downdraft fixed-bed gasifier, but the degree of conversion is not high enough to eliminate tar issues completely.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

Kinetic characteristics of in-situ char-steam gasification following pyrolysis of a demineralized coal

This study aims to examine the char-steam reactions in-situ, following the pyrolysis process of a demineralized coal in a micro fluidized bed reactor, with particular focuses on gas release and its kinetics characteristics. The main experimental variables were temperatures (925 °C?1075 °C) and steam concentrations (15%–35% H₂O), and the combination of pyrolysis and subsequent gasification in one experiment was achieved switching the atmosphere from pure argon to steam and argon mixture. The results indicate that when temperature was higher than 975 °C, the absolute carbon conversion rate during the char gasification could easily reach 100%. When temperature was 1025 °C and 1075 °C, the carbon conversion rate changed little with steam concentration increasing from 25% to 35%. The activation energy calculated from shrinking core model and random pore model was all between 186 and 194 kJ/mol, and the fitting accuracy of shrinking core model was higher than that of the random pore model in this study. The char reactivity from demineralized coal pyrolysis gradually worsened with decreasing temperature and steam partial pressure. The range of reaction order of steam gasification was 0.49–0.61. Compared to raw coal, the progress of water gas shift reaction (CO + H₂O ? CO₂ + H₂) was hindered during the steam gasification of char obtained from the demineralized coal pyrolysis. Meanwhile, the gas content from the char gasification after the demineralized coal pyrolysis showed a low sensitivity to the change in temperature.     

Lime enhanced biomass gasification. Energy penalty reduction by solids preheating in the calciner

Hydrogen is expected to be one of the most important energy carriers in the future. Gasification process may be used to produce hydrogen when joined with carbon capture technologies. Furthermore, the combination of biomass gasification and carbon capture presents a significant technical potential in net negative greenhouse gas emissions. Lime enhanced biomass gasification process makes use of CaO as a high temperature CO₂ carrier between the steam biomass gasifier and an oxy-fired regenerator. Important energy penalties derive from the temperature difference between the reactors (around 250–300 °C). A cyclonic preheater similar to those used in the cement industry may improve the energetic efficiency of the process if the particles entering the regenerator reactor are heated up by the gas leaving this reactor. A lime enhanced biomass gasification system was modelled and simulated. A cyclonic preheater was included to evaluate the improvement. Results show an increase of the gasification chemical efficiency and a reduction of the energy consumption in the regenerator.     

Coal-gasification kinetics derived from pyrolysis in a fluidized-bed reactor

Coal pyrolysis and gasification reactions were carried out in a fluidized-bed reactor (0.1 m i.d. by 1.6 m height) over a temperature range from 1023 to 1173 K at atmospheric pressure. The overall gasification kinetics for the steam–char and oxygen–char reactions were determined in a thermobalance reactor. The compositions of the product gases from the coal-gasification reactions are 30–40% H₂, 23–28% CO, 27–35% CO₂ and 6–9% CH₄ with heating values of 2000–3750 kJ m⁻³. The heating value increases with increasing temperature and steam/coal ratio but decreases with increasing air/coal ratio. Our kinetic data derived from the two-phase theory on coal gasification in a thermobalance reactor and coal pyrolysis in a fluidized bed may be used to predict the product-gas compositions.     

Calcium looping gasification for high-concentration hydrogen production with CO2 capture in a novel compact fluidized bed: Simulation and operation requirements

The coal/CaO/steam gasification system is one of the clean coal technologies being developed for hydrogen production with inherent carbon dioxide separation. A novel reactor configuration for the system is proposed in this paper. It consists of three major counterparts: a gasifier, a riser and a regenerator. A regenerable calcium-based sorbent CaO is used to remove carbon dioxide. In the gasifier, the coal-steam gasification reaction occurs with in situ carbon dioxide removal by carbonation reaction. The removal of carbon dioxide favors the gasification and water-shift reaction equilibrium and enables the production of a hydrogen-rich gas stream. CaO is regenerated in the regenerator by burning the unreacted char with oxygen, and a pure stream of carbon dioxide is separated after a cyclone. The regenerated CaO then flows into the riser above the gasifier, and removes the carbon dioxide in the outlet gases from the gasifier and drives the water-gas shift reaction forward, further improving the hydrogen purity. In this work, the feasibility and optimum process conditions of the proposed system were described. The hydrogen purity can reach 96 vol% at a steam flow 80 mol/s and CaO recycle rate 30 mol/s when the carbon conversion rate is 0.50. Increasing the steam flow and CaO recycle rate can enhance the hydrogen yield and purity. With the rise of operation pressure from 1 bar to 10 bar, the hydrogen yield and purity decrease and methane yield increases. High pressure leads to higher calcination temperature. At 10 bar, the temperature for CaCO₃ decomposition is approximately 1100 °C, at such temperature, the sorbent is easy to deactivate. The appropriate temperatures in the gasifier and the riser are 700 and 600 °C, respectively. An analysis of heat integration is conducted. The maximum carbon conversion rate is ∼0.65. A hydrogen production efficiency of 58.5% is obtained at a carbon conversion rate 0.50, steam flow 60 mol/s and CaO recycle rate 30 mol/s, with a hydrogen purity of 93.7 vol%.     

The effect of air preheating in a biomass CFB gasifier using ASPEN Plus simulation

In the context of climate change, efficiency and energy security, biomass gasification is likely to play an important role. Circulating fluidised bed (CFB) technology was selected for the current study. The objective of this research is to develop a computer model of a CFB biomass gasifier that can predict gasifier performance under various operating conditions. An original model was developed using ASPEN Plus. The model is based on Gibbs free energy minimisation. The restricted equilibrium method was used to calibrate it against experimental data. This was achieved by specifying the temperature approach for the gasification reactions. The model predicts syn-gas composition, conversion efficiency and heating values in good agreement with experimental data. Operating parameters were varied over a wide range. Parameters such as equivalence ratio (ER), temperature, air preheating, biomass moisture and steam injection were found to influence syn-gas composition, heating value, and conversion efficiency. The results indicate an ER and temperature range over which hydrogen (H₂) and carbon monoxide (CO) are maximised, which in turn ensures a high heating value and cold gas efficiency (CGE). Gas heating value was found to decrease with ER. Air preheating increases H₂ and CO production, which increases gas heating value and CGE. Air preheating is more effective at low ERs. A critical air temperature exists after which additional preheating has little influence. Steam has better reactivity than fuel bound moisture. Increasing moisture degrades performance therefore the input fuel should be pre-dried. Steam injection should be employed if a H₂ rich syn-gas is desired.