Biomass gasification based on calcium looping sorption enhanced reforming (CLSER) has the advantages of generating syngas with high purity hydrogen and simultaneously capturing CO2 in process. This study first aims to develop a novel synthetic CaO-based sorbent, addressing its tar reduction potential and mechanical strength along with cyclic carbonation reactivity. The novel sorbent was synthesized by integrating CaO with iron catalyst and an inert support based on a two-step sol–gel method. Comprehensive properties of the novel sorbent including chemical components, effects on biomass pyrolysis and tar reduction, cyclic CO2 capture reactivity, morphology, and mechanical strength were examined by using various methods and facilities. The characterized chemical and physical properties were also compared with pure CaO and two referenced synthetic sorbents. Results showed that a novel sorbent (Ca–Fe–Al) consisting of CaO, iron oxide (Fe2O3), and mayenite (Ca12Al14O33) was successfully synthesized. Evolutions of tar species and CO2 during wheat-straw pyrolysis were found to be lowered in the presence of Ca–Fe–Al as determined by thermogravimetric Fourier transform infrared (TG-FTIR). The enhanced evolutions of light gases such as CO, CH4, and CO2 at temperatures higher than 580 °C examined by thermogravimetry-mass spectrometry (TG-MS) indicated the catalysis effects of Ca–Fe–Al on biomass tar cracking and char decomposition. Different from CaO and other synthetic sorbents, Ca–Fe–Al sorbent showed increasing carbonation capacity and reactivity with growing cycle numbers. After being hydrated, the cyclic carbonation performance of Ca–Fe–Al sorbent became superior to other sorbents. Mechanical strength of Ca–Fe–Al sorbent was also greater than that of CaO and was more suitable for long term storage.

•Higher H2S and COS yields could be obtained by the presence of CaSO4 at high temperature.•CaSO4 could reduce tar sulfur at high temperatures.•High proportions of CaSO4 could promote the decomposition of organic sulfur in the char at high temperatures.•A significant increase of sulfide sulfur was observed with the addition of CaSO4 above 600 °C.The present paper was devoted to investigate the sulfur transformation during pyrolysis of the mixture of coal and CaSO4 in a fixed bed reactor at a temperature range of 500–800 °C. The results indicated that the presence of CaSO4 could promote the evolution of H2S and COS at high temperature, which should be due to the higher decomposition rate of organic sulfur and the reactions between CaSO4 and pyrolysis products, e.g., H2 and CO. Moreover, when the blending ratio(CaSO4/coal) exceeded 20%, CaSO4 could greatly promote the decomposition of organic sulfur at 800 °C. The mechanism of organic sulfur decrease due to CaSO4 was proposed. In comparison of raw coal, sulfide sulfur was significantly increased with the addition of CaSO4 above 600 °C, which should be mainly due to CaS formation through CaSO4 decomposition.

In this study, the sulfur behavior of coal and ash were investigated in the coal staged conversion process coupling coal pyrolysis and residue char combustion. The mixture of XLT lignite and its ash with high sulfur content were pyrolyzed first, and then the pyrolyzed ash and the char underwent combustion in a fixed bed reactor, respectively. The presence of coal ash was found to decrease the total sulfur and organic sulfur retained in the char at whole pyrolysis temperatures. The mechanism of the organic sulfur decrease at different temperatures was discussed. The carbon and pyrolyzed gas during XLT lignite pyrolysis could greatly facilitate the ash sulfur (CaSO4) decomposition. SO2 evolution from pyrolyzed ash combustion was far higher than that from char combustion. Both FeSx and CaS due to ash sulfur (CaSO4) decomposition played an important part in SO2 release during pyrolyzed ash combustion. Higher SO2 release during pyrolyzed ash oxidization was observed at higher combustion temperatures, and the possible reason was that elevating temperatures could enhance SO2 release due to CaS oxidization.

•The coal strongly enhanced CaSO4 decomposition during pyrolysis.•The solid–solid reaction mechanisms could provide a better explanation for CaSO4 decomposition during pyrolysis.•The inherent minerals of coal could promote CaSO4 decomposition.•CaSO4 decomposition rate was significantly decreased with the increase of coal particle size.Coal with different proportions of CaSO4 was pyrolyzed in a fixed-bed reactor in a temperature range of 500–800 °C to study the mechanism of CaSO4 decomposition during coal pyrolysis. The results showed that the presence of coal could greatly promote CaSO4 decomposition at high temperatures, and 87% CaSO4 was decomposed at 800 °C during Xiaolongtan lignite pyrolysis, which was mainly attributed to the solid–solid reaction (2C + CaSO4 → CaS + 2CO2). The effects of inherent minerals, coal type, holding time and coal particle size on CaSO4 decomposition were also discussed. By comparing the CaSO4 decomposition rate between raw coal and demineralized coal, it could be concluded that the inherent minerals could greatly enhance CaSO4 decomposition at high temperatures, which was also be proved by the results of thermogravimetric analysis (TGA). CaSO4 decomposition rate presented a significant decrease with the increase of coal particle size. Four coals used in this study could all promote CaSO4 decomposition, but the promotion effect differed among those four coals which may be attributed to the difference in inherent minerals and the reactivity of coal.

The kinetic study of coal char gasification at elevated pressure is very limited and much less than that at atmospheric pressure, especially in the mixture of H2O, CO2, H2, and CO. The inhibition effect of H2 and CO and a suitable reaction model are both needed to be studied under pressured conditions. A Langmuir–Hinshelwood (L-H) type model has been widely adopted to describe coal char gasification at atmospheric pressure. But its applicability to pressured conditions is questionable. In this paper, the experiments of coal char gasification in the mixture of H2O, CO2, H2 and CO were carried out using a modified pressured thermogravimetric analyzer (PTGA) system at 0.5 MPa and within 1148–1198 K. Experimental results indicate that the gasification reaction rates of H2O and CO2 under pressured conditions are much higher than those at atmospheric pressure, and the inhibition effects of H2 and CO are also stronger. The kinetic parameters in L-H model were determined from pressured pure H2O and CO2 gasification (N2 as diluent). And the applicability of L-H model to pressured conditions was verified. It is shown that the L-H models based on common or separate active sites assumptions could not give satisfactory predictions to experimental data. Finally, we proposed a modified L-H model. This model only needs several extra experiments to calculate the modification factor, and can describe char gasification in the mixture of H2O, CO2, H2 and CO under pressured conditions very well.

•Thermodynamic simulation of coal-CaO–H2O and coal-H2O system was performed.•A typical Chinese bituminous coal was steam gasified in a pressurized fluidized bed at a pressure of 4 bar.•H2-rich syngas production was promoted with the presence of CaO.•H2 production was enhanced with the increase of operating pressure, temperature and [H2O][C].•Solid residues were analyzed with SEM/EDX and XRD.Steam gasification of a typical Chinese bituminous coal for hydrogen production in a lab-scale pressurized bubbling fluidized bed with CaO as CO2 sorbent was performed over a pressure range of ambient pressure to 4 bar. The compositions of the product gases were analyzed and correlated to the gasification operating variables that affecting H2 production, such as pressure (P), mole ratio of steam to carbon ([H2O]/[C]), mole ratio of CaO to carbon ([CaO]/[C]) and temperature (T). The experimental results indicated that the H2 concentration was enhanced by raising the temperature, pressure and [H2O]/[C] under the circumstances we observed. With the presence of CaO sorbent, CO2 in the production gas was absorbed and converted to solid CaCO3, thus shifting the steam reforming of hydrocarbons and water gas shift reaction beyond the equilibrium restrictions and enhancing the H2 concentration. H2 concentration was up to 78 vol% (dry basis) under a condition of 750 °C, 4 bar, [Ca]/[C] = 1 and [H2O]/[C] = 2, while CO2 (2.7 vol%) was almost in-situ captured by the CaO sorbent. This study demonstrated that CaO could be used as a substantially excellent CO2 sorbent for the pressurized steam gasification of bituminous coal. For the gasification process with the presence of CaO, H2-rich syngas was yielded at far lower temperatures and pressures in comparison to the commercialized coal gasification technologies. SEM/EDX and gas sorption analyses of solid residues sampled after the gasification showed that the pore structure of the sorbent was recovered after the steam gasification process, which was attributed to the formation of Ca(OH)2. Additionally, a coal-CaO–H2O system was simulated with using Aspen Plus software. Calculation results showed that higher temperatures and pressures favor the H2 production within a certain range.

The calcium looping process is one of the most promising approaches for CO2 capture, which is based on the cyclic carbonation/calcination reactions of Ca-based sorbent. However, the sorbent suffers from an unavoidable deactivation of CO2 capture capacity and durability during the cyclic CO2 capture process. Separate steam hydration after calcination is valid to regenerate the reactivity of spent sorbent. In this study, the effects of hydration temperature, steam concentration, and hydration frequency on the sorbent reactivity during 10 carbonation/calcination cycles were investigated using a pressurized thermogravimetric analyzer with reagent-grade CaCO3 used as a precursor under atmospheric pressure. The morphology changes of spent sorbent after calcination under various conditions were observed by a scanning electron microscope. The results revealed that the reactivity and durability of the spent sorbent is significantly recovered by separate hydration after calcination. In addition, the enhancement was more pronounced at lower hydration temperatures and higher steam concentrations. Separate hydration after every calcination performed far better than the low-frequency hydration hydrated just once or every 3 cycles. In comparison to other steam reactivation strategies, such as the steam addition during the carbonation and calcination process, separate steam hydration after calcination has shown excellent reactivation performance.

The mechanisms of char gasification in the mixture of H2O and CO2 are not clear, and some problems are yet to be solved. The Langmuir–Hinshelwood (L–H) model of char gasification in the mixture of H2O and CO2 and the inhibition effect between char–H2O and char–CO2 reactions are two controversial issues among these problems. This paper presented new data to elucidate these two issues. Experiments were carried out at atmospheric pressure using a modified thermogravimetric analyzer (TGA) system at various reactant partial pressures and within a temperature range of 1173–1273 K. The kinetic parameters in the L–H model were determined from pure H2O and CO2 gasification (N2 as a diluent). The experimental results showed that the L–H model based on common active site assumption is applicable to describe the experimental data and the pressure is not the reason leading to different results in validity experiments of common or separate active site assumptions. Including H2 and CO in the reactant gas does not change the reaction mechanisms from common active sites to separate active sites either. It was also found that the reaction rate first decreases and then increases as the CO2 concentration increases at a fixed H2O concentration, while the reaction rate continuously increases as the H2O concentration increases at a fixed CO2 concentration. This means that the char–H2O reaction is inhibited by the char–CO2 reaction. Finally, the specific surface area tests of char samples at 20% carbon conversion confirm the common active site assumption.

The effects of the pressure and temperature on the fusibility of coal ash during combustion and gasification were investigated. Experimentation was conducted using a high pressure thermogravimetric analyzer (HPTGA) apparatus. In order to observe the conversion of minerals with changing temperature and pressure in different atmospheres, the resulting ash samples were analyzed using an X-ray diffractometry (XRD) analyzer. In addition, the quantitative XRD analyses of ash samples and a field emission scanning electron microscope (FSEM), together with X-ray energy dispersive spectroscopy (EDS) were employed to verify the detailed mechanisms of ash fusion. The results indicated that temperature and type of atmosphere had a dominant effect on the ash fusion characteristics while the effect of pressure was somewhat more complicated, depending on the temperature and atmosphere being experienced by the ash. The high-temperature minerals such as mullite were formed with increasing temperature under both combustion and gasification atmospheres. However, in gasification atmosphere, there were more fluxing minerals and feldspar minerals present, such as muscovite, anhydrite and K-feldspar, decreasing the fusion temperature. The effect of pressure on the ash fusibility showed different behavior at different temperatures. At 900 °C, the decompositions of low-temperature minerals, such as muscovite, anhydrite and oldhamite, were suppressed with increasing pressure, resulting in a decrease in the fusion temperatures. On the other hand, at 1000 °C, the low-temperature minerals transformed into high-temperature minerals such as mullite and sanidine with rising pressure. However, the presence of fluxing minerals and the melting of iron-containing minerals resulted in the lowering of the fusion temperatures.Highlights► Temperature effect on ash fusion characteristics strongly depends on atmospheres. ► High temperature minerals form in combustion, increasing ash fusion temperatures. ► Low-melting eutectics form in gasification, decreasing the ash fusion temperatures. ► Pressure influences mineral transformation by affecting reactions between minerals. ► Increasing pressure accelerates the formation of high-temperature minerals.

To determine the ash characteristics during fluidized bed combustion and gasification purposes, the investigation of the impacts of chemical composition of Jincheng coal ash on the sintering temperature was conducted. A series of experiments on the sintering behavior at 0.5 MPa was performed using the pressurized pressure-drop technique in the combustion and gasification atmospheres. Meanwhile, the mineral transformations of sintered ash pellets were observed using X-ray diffractometer (XRD) analyzer to better understand the experimental results. In addition, quantitative XRD and field emission scanning electron microscope/energy dispersive X-ray spectrometer (FE-SEM/EDS) analyses of ash samples were used for clarifying the detailed ash melting mechanism. These results show that the addition of Fe2O3 can obviously reduce the sintering temperatures under gasification atmospheres, and only affect a little the sintering temperature under combustion atmosphere. This may be due to the presence of iron-bearing minerals, which will react with other ash compositions to produce low-melting-point eutectics. The FE-SEM/EDS analyses of ash samples with Fe2O3 additive show consistent results with the XRD measurements. The CaO and Na2O can reduce the sintering temperatures under both the combustion and gasification atmospheres. This can be also contributed to the formation of low-melting-point eutectics, decreasing the sintering temperature. Moreover, the fluxing minerals, such as magnetite, anhydrite, muscovite, albite and nepheline, contribute mostly to the reduction of the sintering temperature while the feldspar minerals, such as anorthite, gehlenite and sanidine, can react with other minerals to produce low-melting-point eutectics, and thereby reduce the sintering temperatures.

This paper presents the experimental results of CaO sorption enhanced anaerobic gasification of biomass in a self-design bubbling fluidized bed reactor, aiming to investigate the influences of operation variables such as CaO to carbon mole ratio (CaO/C), H2O to carbon mole ratio (H2O/C) and reaction temperature (T) on hydrogen (H2) production. Results showed that, over the ranges examined in this study (CaO/C: 0–2; H2O/C: 1.2–2.18, T: 489–740 °C), the increase of CaO/C, H2O/C and T were all favorable for promoting the H2 production. The investigated operation variables presented different influences on the H2 production under fluidized bed conditions from those obtained in thermodynamic equilibrium analysis or fixed bed experiments. The comparison with previous studies on fluidized bed biomass gasification reveals that this method has the advantage of being capable to produce a syngas with high H2 concentration and low CO2 concentration.

The interest in utilization of biomass by CO2 sorbent enhanced gasification is increasing due to the concerns about global warming and the wish to produce high purity hydrogen. Pyrolysis in the presence of abundant CaO additives is a fundamental step which determines primary product distribution, composition and properties. In this paper, pure wheat-straw pyrolysis and CaO catalyzed pyrolysis (loading various amounts of CaO additives) at different heating rates were compared using the thermogravimetric Fourier transform infrared (TG-FTIR) analysis. Results showed that CaO additives affected pyrolysis. In the main mass loss stage, the mass loss and maximum mass loss rate decreased with increasing amount of CaO additives. The temperature at the maximum mass loss rate shifted to lower value. CaO additives could not only absorb the released CO2 but also restrain the CO and CH4 yields and meanwhile catalyze the tar reduction reactions. Calculated activation energies at different heating rates were generally in close agreement with that of a previous study and the average activation energy was lowered. Unlike pure wheat-straw pyrolysis, a second mass loss stage mainly caused by calcium carbonate decomposition appeared in CaO catalyzed pyrolysis. The evolution of CO was found to be enhanced by CaO additives in this stage. The total mass loss decreased in the presence of CaO additives. CaO additives can play the roles of both CO2 sorbent and tar reduction catalyst in sorbent enhanced biomass gasification.

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.

This paper presents a study on measuring rotation speed of moving glass beads with an average diameter of 500 μm in a pilot-scale circulating fluidized bed (CFB) riser with a high-speed digital imaging system. Two methods have been developed to calculate particle rotation speed from the particle images. The first method consists of a fully automated algorithm based on cross-correlation of gray distribution of particle images for particles whose rotation axes are (nearly) perpendicular to the imaging plane, and the second method calculates the speed of particle rotation by identifying its rotation axis using two or more characteristic points on its surface. The reliability of the two methods is verified by using a small sphere with known speed and direction of rotation. The first method is shown to be capable of measuring accurately the rotation speed for the particle with a rotation axis (nearly) perpendicular to the imaging plane and filtering off other particles using an appropriate threshold of correlation coefficient. The second method is shown to be capable of yielding both the speed and direction of particle rotation, with a measurement error of less than 10%. Results of both methods on real glass beads in a CFB riser are compared against each other.