Gasification reactivity and synergistic effect of conventional and microwave pyrolysis derived algae chars in CO2 atmosphere

This research focuses on the isothermal and non-isothermal CO₂ gasification of an algal (Chlorella) char prepared via two different thermal processing systems, i.e. conventional and microwave-assisted pyrolysis. It was found that chars prepared via microwave irradiation showed higher CO₂ gasification reactivity than that of chars prepared via the conventional method. Meanwhile, the activation energy of microwave char was found to be 127.89 kJ/mol, which was 46.3 kJ/mol lower than that of conventional char, indicating improved reactivity of microwave char. The systematic characterisation of both conventional and microwave chars shows that the higher reactivity of microwave char could be attributed to its large BET surface area, low crystalline index and high active sites. In addition, it was found that microwave heating contributed to high reactivity of chars through generating large amount of primary char, the formation of hot spot and high specific surface area and pore volume. Results of co-gasification under isothermal conditions revealed the existence of greater synergistic effects between coal char and microwave algae char than those of coal char and conventional algae char. Furthermore, based on the relative Rs (average gasification rate), a novel index proposed to quantify the interactions in co-gasification process, Australian coal char/microwave assisted char blend experienced 10% higher interactions compared to Australian coal char/conventional assisted char blend.     

Impacts of char structure evolution and inherent alkali and alkaline earth metallic species catalysis on reactivity during the coal char gasification with CO2/H2O

In this work, we studied the effects of char structural evolution and alkali and alkaline earth metallic species (AAEMs) catalysis on the reactivity during the char gasification with CO₂, H₂O, and their mixture. The gasified chars with different carbon conversion levels were prepared, and their physicochemical structures were characterized via nitrogen adsorption and FT‐Raman techniques. The concentrations of AAEMs in different modes were obtained by the sequential chemical extraction method. The reactivities of the raw and gasified chars were analyzed by the thermogravimetric analysis. The gasification atmospheres had varied effects on the physicochemical structure of coal char. The gasified char obtained in the CO₂ atmosphere had a lower aromatic condensation degree compared with that obtained in the H₂O atmosphere, irrespective of the temperature. The impact of the atmospheres on the specific surface area of the char varied with the temperature because H₂O and CO₂ have different routes of development of pore structure with coal char. A large specific surface area facilitates the exposure and dispersion of more AAEMs on the surface of the channel, which is conducive to their contact with the gasification agent to play the catalytic role. Thus, the reactivity of the gasified char is well correlated with its specific surface area at different gasification temperatures. In the absence of AAEMs, the chemical structure of coal char becomes the dominant factor affecting the reactivity.     

Steam gasification of Greek lignite and its chars by co-feeding CO2 toward syngas production with an adjustable H2/CO ratio

Low-rank lignite is among the most abundant and cheap fossil fuels, linked, however, to serious environmental implications when employed as feedstock in conventional thermoelectric power plants. Hence, toward a low-carbon energy transition, the role of coal in world's energy mix should be reconsidered. In this regard, coal gasification for synthesis gas generation and consequently through its upgrade to a variety of value-added chemicals and fuels constitutes a promising alternative. Herein, we thoroughly explored for a first time the steam gasification reactivity of Greek Lignite (LG) and its derived chars obtained by raw LG thermal treatment at 300, 500 and 800 °C. Moreover, the impact of CO₂ addition on H₂O gasifying agent mixtures was also investigated. Both the pristine and char samples were fully characterized by various physicochemical techniques to gain insight into possible structure-gasification relationships. The highest syngas yield was obtained for chars derived after LG thermal treatment at 800 °C, due mainly to their high content in fixed carbon, improved textural properties and high alkali index. Steam gasification of lignite and char samples led to H₂-rich syngas mixtures with a H₂/CO ratio of approximately 3.8. However, upon co-feeding CO₂ and H₂O, the H₂/CO ratio can be suitably adjusted for several potential downstream processes.     

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.     

Direct production of hydrogen-enriched syngas by calcium-catalyzed steam gasification of Shengli lignite/chars: Structural evolution

Calcium has been proved to be efficient for the catalytic steam gasification of lignite. In this study, we studied the steam gasification of lignite with Ca(OH)₂ added by wet impregnation for to produce hydrogen. The effect of calcium on the microstructural evolution and char gasification behavior in the presence of steam were studied. The distribution of the gaseous products was examined by using fixed-bed micro reactor equipment for steam gasification. Partially gasified chars obtained at different temperatures (400, 500, 600, and 700 °C) in the gasification process were characterized by scanning electron microscopy coupled with energy dispersive spectroscopy, ¹³C nuclear magnetic resonance, Fourier-transform infrared spectroscopy, and Raman spectroscopy. The results suggested that a “Ca-carboxylate-like structure” was formed due to the addition of calcium to lignite. This Ca-carboxylate-like structure increased the thermal stability of the original carboxylic structure in lignite. As a result, the calcium-added coal retained more oxygen-containing structures, formed more defects, and possessed significantly lower aromatic structures due to the formed Ca-complexes during steam gasification. Therefore, it can be inferred that the Ca-complexes in the lignite char increased the gasification reactivity of char.     

生物质半焦气化的反应动力学

利用热重分析仪研究了CO2气氛下的生物质半焦的反应性。研究发现，所研究的4种生物质半焦都表现出了相同的反应性趋势。其反应性随着转化率的增加而增加。这可能是由于生物质焦样中的碱金属含量，尤其是钾的含量较高的原因。对比生物质气化反应动力学参数研究表明，4种焦样的气化行为可以用收缩核模型来描述，并求出了4种生物质焦样的反应动力学参数。在不同的CO2分压下进行了花生壳焦样的反应性实验研究，发现焦样的反应性正比于反应气体浓度，求出了花生壳焦样的反应动力学方程式。     

Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis

The CO₂ gasification reactivity of pine sawdust chars (PS char) obtained from the different high-temperature pyrolysis is studied based on non-isothermal thermogravimetric method. Results show that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473. Under the effect of high-temperature pyrolysis, the surface structure of biomass char is gradually destroyed and the pore structure parameters of specific surface area, total pore volume and average pore diameter increase. By means of the N₂ adsorption-desorption isotherms, it is seen that biomass char has more micro- and mesoporous at higher pyrolysis temperature. Besides, the PS char-1073 mostly has rich closed cylinder pores and parallel plate pores, and the PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. An increase of pyrolysis temperature contributes to the development of porosity and improves diffusion path, which promotes the gasification reactivity. But, its effect on the decline of active site hinders the gasification reactivity. What's more, the kinetic model of distributed activation energy model (DAEM) is applied to calculate activation energy and pre-exponential factor with the integral and differential methods. The calculation results of integral method is more accurate and precise because the differential method is more sensitive than integral method for experimental noise. There is a compensation effect in the CO₂ gasification process.     

Physicochemical structure characteristics and intrinsic reactivity of demineralized coal char rapidly pyrolyzed at elevated pressure

This study aims to investigate the effect of pyrolysis pressure on the physical and chemical structure characteristics and reactivity of subbituminous demineralized coal char. The pyrolysis experiments were studied under different pressures using a pressurized drop tube reactor (PDTR). Structural properties of coal chars were investigated by the application of scanning electron microscopy (SEM), nitrogen adsorption analyzer, automatic mercury porosimeter, and Raman spectroscopy, respectively. The Random Pore Model was used to determine kinetic parameters and intrinsic reactivity of chars. The specific pore volume of chars pyrolyzed at 900–1000 °C increased first and then decreased with pyrolysis pressure increasing, and the maximum value of the specific pore volume of chars occurred at 1.0 MPa. The degree of graphitization of chars deepened with the increase of temperature or pressure. Intrinsic activation energy of char-O₂ was within the range of 126–134 kJ/mol. The intrinsic reactivity of char-O₂ reaction showed strong correlation the Raman parameters with the change of pyrolysis conditions, and it suggested that the intrinsic reactivity of char-O₂ reaction was mainly affected by aromatic ring structures rather than pore structures.     

Combustion Characteristics and Kinetics of Partial Gasified Coal Char

Abstract

Combustion characteristics and kinetics of partial gasified coal char (PGC-char) were investigated in this article. Two-stage distributed activation energy model (TS-DAEM) and simple model were used to model the combustion kinetics. The experimental results showed that the combustion characteristics of PGC-chars varied with coal types and percentage of gasification. Modeling results showed that TS-DAEM could suitably describe the combustion of PGC-char. The activation energy and preexponential factor increased with the increasing of PGC-char gasification percentage, but the percent of high reactivity part decreased. This indicated that the difficulty reactive part in the partial gasified coal char became more and more with the gasification conversion. However, simple model wasn't fit to describe the combustion of PGC-char, especially for PGC-chars with obvious second peak in combustion DTG curves.     

Rapid pyrolysis and CO2 gasification of anthracite at high temperature

Anthracite could be burnt efficiently at high temperature utilizing oxy-coal technology. To clarify the effects of temperature and atmosphere on char porosity characteristics, char morphology, fuel-N conversion, and reducing products release, rapid pyrolysis and CO₂ gasification of anthracite was carried out in a high temperature entrained-flow reactor to simulate the condition in a pulverized coal furnace. Developed pore structure was formed in the gasification chars, which could be contributed to charCO₂ reaction at high temperatures. More mesopores were formed in internal carbon skeleton and retained against collapse and coalescent for gasification chars than pyrolysis chars. Compared with pyrolysis char, smoother and denser surface was observed in gasification char with the irregular bulges disappeared due to the destruction of external carbon skeleton. Char-N could be oxidized to NO in CO₂ atmosphere and then reduced to N₂ by (CN) on the char surface. Char-N release was greatly promoted due to gasification reaction along with poly-condensation at high temperature; and the preact release of char-N would result in a larger portion of NO_x reduction in the following reduction zone with the oxygen-staging combustion technology compared with that in air-staging combustion. Complementally, homogeneous reduction in NO_x emission would play a minor effect for anthracite in oxy-coal combustion because of the deficiency of CH₄ and HCN, especially at high temperature.     

Experimental simulate on hydrogen production of different coals in underground coal gasification

Research on hydrogen production from coal gasification is mainly focused on the formation of CO and H₂ from coal and water vapor in high-temperature environments. However, in the process of underground coal gasification, the water gas shift reaction of low-temperature steam will absorb a lot of heat, which makes it difficult to maintain the combustion of coal seams in the process of underground coal gasification. In order to obtain high-quality hydrogen, a pure oxygen-steam gasification process is used to improve the gasification efficiency. And as the gasification surface continues to recede, the drying, pyrolysis, gasification and combustion reactions of underground coal seams gradually occur. Direct coal gasification can't truly reflect the process of underground coal gasification. In order to simulate the hydrogen production laws of different coal types in the underground gasification process realistically, a two-step gasification process (pyrolysis of coal followed by gasification of the char) was proposed to process coal to produce hydrogen-rich gas. First, the effects of temperature and coal rank on product distribution were studied in the pyrolysis process. Then, the coal char at the final pyrolysis temperature of 900 °C was gasified with pure oxygen-steam. The results showed that, the hydrogen production of the three coal chars increased with the increase of temperature during the pyrolysis process, the hydrogen release from Inner Mongolia lignite and Xinjiang long flame coal have the same trend, and the bimodality is obvious. The hydrogen release in the first stage mainly comes from the dehydrogenation of the fat side chain, and the hydrogen release in the second stage mainly comes from the polycondensation reaction in the later stage of pyrolysis, and the pyrolysis process of coal contributes 15.81%–43.33% of hydrogen, as the coal rank increases, the hydrogen production rate gradually decreases. In the gasification process, the release of hydrogen mainly comes from the water gas shift reaction, the hydrogen output is mainly affected by the quality and carbon content of coal char. With the increase of coal rank, the hydrogen output gradually increases, mainly due to the increasing of coal coke yield and carbon content, The gasification process of coal char contributes 56.67–84.19% of hydrogen, in contrast, coal char gasification provides more hydrogen. The total effective gas output of the three coal chars is 0.53–0.81 m³/kg, the hydrogen output is 0.3–0.43 m³/kg, and the percentage of hydrogen is 53.08–56.60%. This study shows that two-step gasification under the condition of pure oxygen-steam gasification agent is an efficient energy process for hydrogen production from underground coal gasification.     

Evolution of pore structure and fractal characteristics of coal char during coal gasification

This study classifies the evolutionary properties of coal char pore structure which occur during coal gasification. CO₂ gasification of various coal samples was carried out in a fixed bed reactor. The resulting chars were analysed using N₂ isothermal adsorption/desorption and scanning electron microscopy (SEM), coupled with fractal theory. Analytical results indicate that the pore structure of coal char underwent micropore evolving, enlarging and overlapping, while more mesopores and macropores developed with continued gasification. The surface area of coal char increased to its maximum value when carbon conversion reached approximately 50%. Fractal calculation results showed that two types of fractal structures associated with the coal char surface and pore structure underwent stereome development and elapsing. However, the evolutionary properties were unique for different coal samples. High rank coal had a complex spatial structure with more micro-pores, whereas lower rank coal had a much flatter spatial structure.     

Kinetics of CO2 gasification of biomass char in granulated blast furnace slag

The CO₂ gasification reactions of biomass char in granulated BFS (blast furnace slag) were isothermally investigated using a thermogravimetric analyzer with the temperature ranging from 1173 K to 1323 K. The effects of temperature, biomass type and granulated BFS on the kinetic characterizations of CO₂ gasification of biomass char were illuminated. The kinetic mechanism models and parameters were obtained through a novel two-step calculation method. The results indicated that the CO₂ gasification reactivity of biomass char as conversion and gasification index increased with the increase of temperature and it could be promoted through granulated BFS. The CO₂ gasification reactivity of CS (cornstalk) char with lower alkali index was lower than that of PS (peanut shell) char. The A₄ model (Avrami-Erofeev (m = 4) model) and A₃ model (Avrami-Erofeev (m = 3) model) were demonstrated as the best appropriate models for CO₂ gasification of CS char and PS char, respectively. The gasification activation energy of CS char ranging from 155.08 to 160.48 kJ/mol was higher than that of PS char whether with or without granulated BFS. Granulated BFS could decrease the activation energy of CO₂ gasification of char at any biomass type.     

部分气化煤焦燃烧动力学的活化能分布模型研究

运用两段DAEM模型分析了部分气化煤焦的燃烧动力学．结果表明，两段DAEM模型能准确地描述部分气化煤焦的燃烧行为这是因为部分气化煤焦随着气化率的增加，其燃烧曲线的二次峰越来越显著，而两段DAEM模型正好从理论上考虑了部分气化煤焦燃烧曲线的二次峰特性．同时由两段DAEM模型计算结果发现，随着3种煤焦气化率的增加，其活化能与指前因子增加，代表易反应物质量m的减小，这说明了实验与理论结果一致，随着气化反应的进行，难反应物质的比例增加．     

Fuel conversion characteristics of black liquor and pyrolysis oil mixtures: Efficient gasification with inherent catalyst

Alkali metals inherent in black liquor (BL) have strong catalytic activity during gasification. A catalytic co-gasification process based on BL with pyrolysis oil (PO) has the potential to be a part of efficient and fuel-flexible biofuel production systems. The objective of the paper is to investigate how adding PO into BL alters fuel conversion under gasification conditions. First, the conversion times of single fuel droplet were observed in a flat flame burner under different conditions. Fuel conversion times of PO/BL mixtures were significantly lower than PO and comparable to BL. Initial droplet size (300–1500 μm) was the main variable affecting devolatilization, indicating control by external heat transfer. Char oxidation was affected by droplet size and the surrounding gas composition. Then, the intrinsic reactivity of char gasification was measured in an isothermal thermogravimetric analyser at T = 993–1133 K under the flow of CO₂–N₂ mixtures. All the BL-based samples (100% BL, 20% PO/80% BL, and 30% PO/70% BL on mass basis) showed very high char conversion. Conversion rate of char gasification for PO/BL mixtures was comparable to that of pure BL although the fraction of alkali metal in char decreased because of mixing. The reactivities of BL and BL/PO chars were higher than the literature values for solid biomass and coal chars by several orders of magnitude. The combined results suggest that fuel mixtures containing up to 30% of PO on mass basis may be feasible in existing BL gasification technology.     

An adaptive neuro–fuzzy inference model for prediction of coal char conversion in CO2 gasification

Today, in industry, the gasification of coal as an important energy source is an interesting perspective. In this study, the application of adaptive neuro–fuzzy inference system (ANFIS) technique for estimation of char conversion in CO₂ gasification is investigated. The main variables affecting char conversion are particle size, reaction time, and reaction temperature, which are chosen as input variables of the proposed model. Experimental data which are gathered from the literature are applied for training, testing, and validation of developed ANFIS model. The results reveal the exact estimation of char conversions with the corresponding experimental values with the regression coefficients (R²) greater than 0.99.     

Kinetics and mechanism of CO2 gasification of coal catalyzed by Na2CO3, FeCO3 and Na2CO3–FeCO3

The purpose is to develop a mild catalytic CO₂-gasification technology that can promote CO₂ utilization and reduce cost in air separation systems with improving system efficiency and obtaining desirable gaseous products. In this study, the influence of Na, Fe and their composite catalysts on the structure and gasification reactivity of chars derived from pyrolysis of Powder River Basin (PRB) coal was investigated. The results showed that a strong positive synergistic effect between Na and Fe catalyst in the gasification process was observed, the catalytic activity of the added catalysts was in order of: 4% Na > 3% Na–1%Fe > 2% Na-2% Fe > 1% Na-3% Fe > 1% Na-2% Fe > 4% Fe > raw coal. The catalysts inhibited the growth of the aromatic ring structure and enriched the generation of O-containing functional groups. Compared to Fe, the Na-based catalyst could easily diffuse into inner pores of coal char, forming C–O–Na structure and thus increasing the gasification reactivity of chars. In addition, due to the formation of inert material between SiO₂ and Na, the catalytic activity of Na catalysts was significantly decrease at the late stage of char conversion. Comparatively, the Fe-based catalysts showed better stability life. Moreover, it was found that the activation energy for CO₂-gasification of PRB coal can be decreased by 50% due to the addition of Na catalyst.     

Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation

A new process integrating a circulating fluidized bed (CFB) reactor and an entrained bed reactor was proposed for gasification of preheated coal. The CFB reactor as a preheater was successfully used in clean coal combustion. In this study, gasification of preheated coal was tested in a bench-scale test rig, which consisted of a CFB preheater and a down flow bed (DFB) gasifier. The effects of operating parameters of the preheater and gasifier were revealed via thermodynamic equilibrium calculations. A stable preheating process was obtained in the CFB preheater at the O₂/C molar ratio of 0.31 and higher gasification reactivity was gained in preheated char owing to the improvement in intrinsic reactivity, specific surface area and total pore volume. Effective gasification of preheated char was achieved in the DFB gasifier at 1100 °C and the total O₂/C molar ratio of 0.67, meanwhile the CO + H₂ yield and carbon conversion increased. Thermodynamic equilibrium calculations revealed when the gasification reaction rates varied little above 1100 °C and the same carbon conversion was achieve in gasifier, lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O₂ demand.     

Catalytic effect of sodium components on the microstructure and steam gasification of demineralized Shengli lignite char

The catalytic effect of sodium on the demineralized Shengli (SL⁺) lignite char microstructure and the performance of steam gasification were studied. Various sodium compounds including NaNO₃, CH₃COONa, Na₂CO₃ and NaOH were loaded on the demineralized coal samples, respectively, and the steam gasification was tested on the fix-bed reactor. The char samples were characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and FT-IR spectroscopy. Experimental results showed that sodium hydroxide loaded samples exhibited the highest gasification reactivity among the coal samples prepared. With the increase of the alkalinity of sodium compounds, the carbon crystallite structure tended to be disordered. In the process of pyrolysis, the introduction of sodium species promoted the ring-opening and polycondensation process of the chemicals in the coal samples. The possible reaction mechanism might be inferred that the sodium ions may replace the hydrogen ions in the oxygen-containing functional groups to form sodium phenolate intermediate, which may be critical for the catalytic effect of sodium species during gasification. It was speculated that the ring-opening of the condensation aromatic nucleus was the rate-limiting step in the whole process of gasification.     

Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells

The anode reactions of solid oxide electrolyte based carbon fuel cells (SO-CFCs) are explored by comparing the electrochemical behaviors of SO-CFCs under varying anode carrier gas flow rates (FAr) and at different contact modes. The electrochemical performance of four raw carbon fuels, including a graphitic carbon (GC), two coals (lignite CF and anthracite YQ) and an activated carbon (AC), and their chars is tested to investigate the influence of carbon fuel properties on the cell performance. The results show that CO electro-oxidation and C-CO₂ gasification were main anode reactions. The direct carbon electro-oxidation is insignificant under high F_Ar. Polarization performance of the chars under high F_Ar was similar with that of 5–10% CO. It is also concluded that the cell performance is greatly dependent on the carbon fuel gasification reactivity with CO₂. Thermal pretreated AC displays the best durability performance for its stable and moderate CO₂ gasification rate. Additionally, the coal ash does not affect the cell performance significantly.