Chemical looping combustion (CLC) is a promising and efficient technology for sewage sludge (SS) combustion with carbon capture. The CLC reactor configuration is crucial to the intimate contact between the solid phase and the gas phase species. This work proposed a novel CLC unit with a two-stage fuel reactor. Different from the conventional CLC unit with a single-stage bubbling bed as fuel reactor, two-stage fuel reactor design can make gas phase and solid phase be in adequate contact and achieve gas flow redistribution in the fuel reactor. On this unit, both cold and hot experiments were conducted. The gas-solid flow characteristics were studied on the cold model, and the system was successfully commissioned and in stable operation. No gas leakage occurred between reactors. In the hot experiments, SS from the municipal wastewater treatment plant was chosen as solid fuel and hematite with a size range of 0.3–0.45 mm was used as oxygen carrier. To thoroughly evaluate the performance of this noval CLC system, the combustion compensation efficiency and carbon supplementation efficiency were proposed. The results showed that the two-stage fuel reactor design was beneficial to improving the carbon conversion efficiency of the whole system. The carbon conversion efficiency, carbon capture efficiency, and combustion compensation efficiency increased within the temperature range of 800–900 °C, while the carbon supplementation efficiency decreased at high temperature. Besides, the effect of SS feed rate was also investigated. High SS feed rate resulted in lower carbon conversion efficiency due to the large bubbles produced during the SS gasification process. At last, the phase characteristics of fresh oxygen carrier and oxygen carrier extracted from the both fuel reactors were detected. Fe3O4 was the main reduced phase of hematite in the FR.

Hematite has become a promising oxygen carrier (OC) due to its abundance and low cost for chemical looping hydrogen generation (CLHG). However, the poor redox reactivity, low yield, and purity of hydrogen are the main issues using hematite as an OC. In order to solve the problems, a synthesized OC based on iron ore comodified by copper and potassium was first proposed for CLHG to increase the reduction reactivity and hydrogen yield. Experiments were conducted in a batched fluidized bed reactor to evaluate the performance of the synthesized OC. The results demonstrated that the adding of potassium and copper elevated the reduction reactivity. The reduction reactivity was in the sequence of 5Fe1.67Cu10K > 5Fe1Cu10K > 5Fe0.625Cu10K > 5Fe1Cu5K > 5Fe1Cu0K > hematite. As compared with hematite, the oxygen transport conversion increased 70.11% using 5Fe1.67Cu10K. The reduction reactivity enhancement was attributed to the self-diffusion and pores formation via adding potassium as well as the high reactivity and oxygen transport conversion of copper loading. K2Fe4O7 and CuFe2O4 were detected in the synthesized OCs by XRD analysis, which were active phases for reduction. Moreover, the high oxygen transport conversion and reactivity revealed the deep reduction of iron oxides. The hydrogen yield increased 2.1 times on account of the existence of potassium and copper. Meanwhile, the hydrogen production rate was improved. Additionally, 850 °C was suitable for CLHG in consideration of the reaction rate and the low melting point of the additive. The hydrogen purity was up to 99.9%, indicating that copper and potassium play significantly synergistic roles on suppressing carbon deposition. Therefore, the synthesized oxygen carrier based on iron ore comodified by potassium and copper was suitable for CLHG.

•Hematite-CuO interaction for the direct mixing method of Fe-Cu bimetallic oxygen carrier.•Effect of Cu in oxygen carrier on reactivity and stability in structure.•Cyclic characteristics of oxygen carriers in fluidized reactor and Cu loss with cycle.•The distribution of Cu and exist form in the oxygen carriers.Cu-Fe bimetallic oxygen carrier is a promising candidate for chemical looping combustion, but the cost of manufacture is a significant limit for its application. In this study, the bimetallic oxygen carrier was prepared using hematite and CuO simply through direct mixing method with low cost. The cyclic combustion performance and effects of temperature were investigated in a batch fluidized bed. Cycle experiments with gas fuel indicated that the bimetallic oxygen carrier possessed increasing reactivity with cycle, which could be ascribed to improved porous structure. However, some agglomeration occurred when the content of Cu reached 20% in weight. Besides, the cyclic operation with coal showed that the bimetallic oxygen carriers with Cu ratios of 5% and 10% possessed good long term operation characteristics and the oxygen carrier with 10% Cu performed better. The conversion of coal increased with the reaction temperature and higher temperature, above 900 °C, was needed for fast conversion of coal. Additionally, the manufactured oxygen carriers were characterized. In the bimetallic oxygen carriers, Cu distributed uniformly on the surface of particle but the content of Cu decreased with cycles caused by attrition. Besides, the bimetallic oxygen carrier possessed a stable chemical composition. Overall, the Cu-Fe bimetallic oxygen carrier prepared by direct mixing method is a competitive oxygen carrier, which deserves more attention.

Co-combustion of sewage sludge (SS) and Zhundong (ZD) coal can be an attractive way of disposing SS and using ZD coal. Chemical looping combustion (CLC) technology is an alternative solution applied to this co-combustion with capturing carbon dioxide and minimizing gaseous pollutants, especially NOx. The performance of chemical looping co-combustion of SS and ZD coal (SZ) was investigated in a 1 kWth continuous CLC reactor using natural hematite as the oxygen carrier in this work. The influences of the temperature in the fuel reactor (800–930 °C) were studied. The carbon conversion efficiency, carbonaceous gas conversion efficiency, and carbon capture efficiency for SZ increased with the increase of the FR temperature. In comparison to dewatered SS, SZ could obtain higher carbonaceous gas conversion efficiency among the temperature range. Although SZ obtained lower carbon conversion efficiency and carbon capture efficiency, both efficiencies of SZ reached an appropriate value at a high temperature and were higher than these for single ZD coal. The carbonaceous gas conversion efficiency and carbon conversion efficiency of SZ reached 91.3 and 86.5%, respectively, at 930 °C. After 5 h of operation, the reacted oxygen carrier showed similar reactivity compared to the fresh oxygen carrier, indicating that the hematite oxygen carrier possessed good long-term reactivity during the co-combustion process. Besides, although ZD coal had a higher content of alkali metal, sodium, there was no melting on the hematite and ash agglomeration occurred during continuous long time operation, which could be ascribed to the reduction of the sodium content in the ash and the generation of alkali metal compounds with high-temperature melting points, sodium aluminosilicate and sodium pyrophosphate aluminum.

Chemical looping hydrogen generation (CLHG) consists of an oxidation process, a reduction process, and a hydrogen generation process. Achieving deep reduction of the oxygen carrier is the challenge for the CLHG process. In this paper, experiments on CLHG using K-modified iron ore as an oxygen carrier and CO as a fuel were carried out in a laboratory-scale fluidized bed reactor. A high temperature improved the reduction reactivity. However, at the same reduction condition, a higher temperature did not improve the hydrogen generation process, which means that a higher temperature mainly benefited the reduction process and then elevated hydrogen generation in a CLHG process. Adding KNO3 improved the rate of reduction and hydrogen generation. With the KNO3 loading in iron ore increasing from 0 to 10%, not only the carbon conversion but also the hydrogen production was accelerated. A high KNO3 loading in iron ore can also maintain longer reaction time. The 10% K-modified iron ore could decrease carbon deposition. The scanning electron microscopy analysis and the cycling experiments indicated that adding K could keep the porous structure of the oxygen carrier and the K-modified iron ore was a stable catalyst in the CLHG process.

Chemical looping combustion (CLC) has been as a viable and efficient alternative for coal combustion with CO2 inherent separation. Iron ore is a promising candidate for CLC of coal due to its low cost. Due to a limitation of particle size of the iron ore used in the fluidized bed, the fines for the iron ore after sieving with a particle size of <0.075 mm were reused for reactivation in the present work. The calcium aluminate cement as a very cheap material was used to combine the fines to produce a cement-supported oxygen carrier. The CLC performance with the developed oxygen carriers was experimentally investigated using Chinese traditional anthracite as fuel. In this study, the components of Fe2O3 and cement were optimized to obtain a better reactivity in the coal CLC process. Further, two additives (Ni and K) were added as catalysts during the oxygen carrier preparation process in order to improve the coal conversion rate and gasification rate. The experimental results suggested that compared with the single iron-based oxygen carriers, the oxygen carriers in the presence of additives demonstrated a high carbon conversion rate, gasification rate, and greater oxygen usage. The effect of reducing temperature (850–950 °C) and reducing/oxidizing cycles (20 cycles) on the reaction performance of the oxygen carrier samples developed in this study were evaluated. Also, the attrition behavior during multiple cycles was investigated. The detailed catalytic mechanism under the effect of additives was further discussed.

This study evaluates the biomass-to-synthetic natural gas (SNG) using calcium looping gasification (CLG) with CaO sorbent (CLG-SNG) via thermochemical methods. The CLG-SNG process consists of three steps in sequence: steam gasification in situ CO2 capture using CaO sorbents, gas cleaning, and methanation. The concept of interconnected fluidized beds was adopted for repeated carbonation/calcination cycles of CaO sorbents in the gasification unit. A process simulation was conducted based on the chemical equilibrium method using Aspen Plus. Then, the effects of some key variables on the thermodynamic performances, such as the gas composition, yield of SNG (YSNG), cold gas efficiency (ηcold), the overall energy efficiency (η), exergy efficiency (ψ) of the process, and the unit power consumption (WSNG) were investigated. The variables include CaO-to-biomass ratio (Ca/B) in the range of 0.7–1, steam-to-biomass ratio (S/B) in the range of 0.1–1.5, and gasification temperature (tG) in the range of 600–700 °C. At Ca/B = 0.83, i.e., a stoichiometric number of SN = 1, the CH4 content in SNG and WSNG each reach the maximum while the YSNG reaches the minimum. With S/B increasing from 0.1 to 1.5, CH4 content in SNG gradually decreases while WSNG shows an increasing tendency. YSNG, ηcold, η, and ψ reach the maximum at S/B = 0.6 (i.e., when the gasifier reach the heat equilibrium). Generally, lower tG values are favorable for the thermodynamic performances (mainly YSNG and ηcold) of the CLG-SNG process. The optimal performances demonstrate that the CLG-SNG process has a strong competitiveness, compared to the traditional SNG production process.

Chemical-looping combustion (CLC) is an emerging combustion technology that can be used to meet the demand of energy production with inherent separation of CO2. Because of the presence of sulfur contaminants in fossil fuels, the gaseous products of sulfur species and the interactions between these sulfur contaminants and the oxygen carrier are significant concerns in chemical-looping combustion. Experiments on chemical-looping combustion of a sulfur-containing gaseous fuel with iron ore as the oxygen carrier were performed by thermogravimetric analysis and Fourier transform infrared (TGA–FTIR) spectroscopy. The effects of reaction atmosphere (N2 and CO2), H2S concentration, and pressure on the reactivity of iron ore in the presence of H2S were investigated. The evolution of gaseous sulfur species was also investigated in both N2 and CO2 atmospheres. With a higher concentration of H2S in the gaseous fuel, the weight loss was slower, and a weight gain was even observed due to the sulfidation of iron ore. Sulfidation of iron ore was observed in both N2 and CO2 atmospheres, and elevated pressure contributed to a higher sulfidation rate. Compared with N2 atmosphere, CO2 atmosphere gave higher concentrations of COS and an initial SO2 peak but a lower concentration of CS2. Furthermore, the effect of sulfidation on the structure of the iron ore was investigated in a fluidized bed, and scanning electron microscopy with energy-dispersive analysis by X-rays (SEM-EDAX) and X-ray diffraction (XRD) were utilized to characterize the iron ore. The sulfidation of iron ore caused a decrease in both surface area and pore volume, and the porous surface of the oxygen carrier became smoother and almost imperforate. FeS was the only iron sulfide observed during the sulfidation process. The vulcanized iron ore could be regenerated with air calcination treatment, and the addition of H2O in the gaseous fuel could prevent the sulfidation of iron ore. The addition of CaO in the oxygen carrier was effective in mitigating the sulfidation and reducing the emission of gaseous sulfur species.

CaSO4 as a promising oxygen carrier has a larger oxygen transport capacity. However, the formation of CaO in the CaSO4 reduction process by side reactions can cause sulfur species evolution and decrease its reactivity. An innovative method was investigated by means of adding hematite together with the CaSO4 to be combined oxygen carriers in CLC of coal. The objective was to decrease sulfur species evolution and increase the coal conversion. Experiments were performed in a batch fluidized-bed reactor at atmospheric pressure. Ten reduction/oxidation cycles confirmed the occurrences of side reactions toward sulfur species evolution, although the CaSO4 oxygen carrier showed an increasing reactivity in the initial cycles. With the addition of hematite, the gaseous sulfur species evolution was remarkably decreased in both reduction and oxidation processes during the cycles, while the coal conversion and CO2 capture were enhanced. The extensive physical and chemical properties of the oxygen carrier were characterized by X-ray diffractometer (XRD) analysis and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (SEM-EDX) analysis to detect the mechanism. It was not the desulfurization capacity of iron oxide that lowered sulfur species emission but the suppression of the side reactions by adding hematite.

In this work, a novel Ca-enhanced hematite oxygen carrier was developed for chemical looping combustion of coal. Calcium aluminate cement was used to bind hematite particles to enhance its mechanical strength and improve its chemical properties. An anthracite coal was used as the feed stock. Coal gasification and combustion tests at 900 °C were carried out with the silica sands particles, calcium aluminate cement particles, pure hematite, and the combined Ca-enhanced hematite oxygen carrier particles in a fluidized-bed reactor at atmospheric pressure. It was found that, by the presence of the contents of CaO, Al2O3, as well as SiO2, a material of Ca2Al2SiO7 with high melting point was produced. This Ca2Al2SiO7 material was confirmed to improve the mechanical strength for the newly developed oxygen carriers. Also, during the initial reduction process, the coal gasification rate was improved by nearly twice with the addition of the calcium aluminate cement. The physical and chemical properties for the combined oxygen carrier were characterized by X-ray powder diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy and measurements of BET surface area. Multiple cycles tests confirmed that a stable reactivity for the newly developed oxygen carrier. The presence of Ca2Al2SiO7 prevented the sintering process during multiple cycles.

Chemical-looping combustion (CLC) using a ShenHua bituminous coal and an HuaiBei anthracite as fuel was investigated in 1 kWth interconnected fluidized beds under continuous operation. The evaluation of hematite oxygen carrier in coal CLC was performed. Meanwhile, the fate of fuel-N in the CLC process was investigated. According to a 20 h continuous operation with the natural hematite at a fuel reactor temperature of 970 °C, results showed that the CO2 capture efficiency for ShenHua bituminous coal was 82%, whereas the one for HuaiBei anthracite was 65% due to a low gasification rate for the anthracite. With the fuel reactor temperature range of 880–970 °C, there were neither hydrocarbons heavier than CH4 nor tars in the exit of the fuel reactor. For both of the two coals used, the CO2 fraction in the flue carbonaceous gas of the fuel reactor reached 92% at a fuel reactor temperature of 970 °C, indicating a high reactivity of the hematite as an oxygen carrier. Under the continuous operation, the inert material of SiO2 in the hematite was difficult to react with the active phases of Fe2O3 or Fe3O4, resulting in the alleviated sintering degree of the oxygen carrier. The mass loss rate of the hematite was about 0.12 wt.%/h. There was no nitrogen oxides evolution in the exit gas of the fuel reactor with the fuel reactor temperature range of 880 °C–970 °C. N2 was the sole product of the nitrogen transfer of fuel-N in the fuel reactor. A high fuel reactor temperature produced more fuel-N to N2. Along with residual char circulating to the air reactor, there was some nitrogen oxides formation in the air reactor. The NO concentration was mostly influenced by the amount of char coming into the air reactor, which was less at a high fuel reactor temperature. Enough residence time for the fuel in the fuel reactor should be ensured with respect to eliminate NO formation in the air reactor, and a carbon stripper is better to be employed and developed in the future design and operation of CLC plant.Highlights► Hematite oxygen carrier showed a good behavior in coal CLC for CO2 capture. ► N2 was the sole product of fuel-N transfer in the fuel reactor. ► There were neither hydrocarbons heavier than CH4 nor tars in the exit of the fuel reactor. ► A high fuel reactor temperature produced more fuel-N to N2. ► The extent of carbon conversion in the fuel reactor determined the conversion of fuel-N to N2.

To modify the temperature mismatch between coal gasification and CO2 capture in one gasifer, a novel technique route of coal gasification with CO2 capture via three-stage interconnected fluidized beds (ICFB) was proposed. The three-stage ICFB consisted of a gasifier, an adsorber, and a calcinator, which separated the coal gasification, CO2 capture, and sorbents regeneration. Also, it could keep the three processes performing at the rational reaction temperatures. A process simulation was constructed on the basis of the chemical and phase equilibrium method using Aspen Plus, and the effects of the adsorption temperature and the steam/coal ratio on adsorption products composition, CO2 capture efficiency, H2 yield, and carbon conversion ratio were investigated. The results indicate the adsorption temperature range 600–630 °C and the steam/coal mass ratio range 2.1–2.7 are the possible optimal reaction conditions. The modified process shows an optimal coal gasification with increasing H2 concentration of 32.60% and capturing CO2 efficiency of 85.17%, compared with the coal gasification with CO2 capture in one gasifier.

Fast pyrolysis of cellulose, xylan, and lignin was experimentally conducted between 350 and 650 °C in a tube furnace, and the effect of temperature on pyrolysis products (char, noncondensable gas, and bio-oil) was investigated. The yields of char, noncondensable gas, and bio-oil were quantified using gas chromatography and gas chromatography with mass spectrometry. The noncondensable gas mainly consists of CO, CO2, CH4, and H2. The bio-oil includes acids, ketones, aldehydes, esters, benzenes, alcohols, alkenes, phenols, alkanels, carbohydrates, etc. The results show that cellulose is the principal source of carbohydrates and phenols are the basis of the bio-oil from lignin, while the bio-oil from xylan mainly consists of acids, ketones, aldehydes, and phenols. The char yields for the three components decrease with an increase in temperature, and the gas yields and bio-oil yields increase with an increase in temperature, reach a maximum at a certain temperature, and then decrease after that temperature. The maximum bio-oil yields for cellulose, xylan, and lignin are 65, 53, and 40%, respectively; and their corresponding temperatures are 400, 450, and 500 °C, respectively. To investigate a relationship between biomass and three major components (hemicellulose, cellulose, and lignin), the pyrolysis of three typical biomass samples (rice straw, corn stalk, and peanut vine) was also studied, and the additivity law is adopted to predict the product components of biomass pyrolysis based on the content of hemicelluloses, cellulose, and lignin. The results show that the additivity law can predict reasonably the trend of product yields of biomass samples from their composition of hemicelluloses, cellulose, and lignin.

Estimation of chemical exergy of biomass is one of the basic steps in performance analysis and optimization of biomass conversion systems. A practical method for estimating specific chemical exergy of biomass on dry basis (db) from basic analysis data was developed on Szargut’s reference environment model. The method is based on exergy and entropy equations of reaction, Gibbs free energy relations, a modified estimation of the standard entropy of organic matter in biomass and an assumption about original state of inorganic matter-forming elements in biomass. The method was applied to 86 varieties of biomass, and the statistical results indicate that specific chemical exergy of dry biomass varies in the interval of 11.5–24.2 MJ·kg–1, which is always slightly larger than the higher heat value (HHV) (db). Owing to the relative very small value, the influence of inorganic matter in the form of chemical exergies of ash and oxygen reacting with inorganic matter, and the entropy change in ash formation can be neglected. The average ratio of specific chemical exergy to HHV is 1.047 for dry biomass. Consequently, specific chemical exergy of dry biomass can be conveniently estimated from ultimate analysis data plus ash content (in wt %, db) or HHV.

Chemical-looping Combustion (CLC) has been proposed as an energy-efficient combustion method for in situ capture of CO2. Kinetic model for parallel reactions of the CaSO4 oxygen carrier with CO in a CLC process is explored in this paper. Tests on an isothermal reaction were carried out in a Thermogravimetric Analyzer coupled with Fourier Transform Infrared spectrum (TGA-FTIR), and the instantaneous evolutions of SO2 and COS were monitored by the FTIR quantitative analysis. The reaction temperature was varied between 850 and 1050 °C, while 5-28% CO concentrations were utilized. The experiments showed that the reduction of CaSO4 by CO was a complex process, with the products of either sole CaS or both CaS and CaO depending on the reaction temperature as well as the concentration of CO reactant. The parallel reactions of CaSO4 with CO were investigated in terms of the selectivity based on the nucleation and growth model under isothermal conditions. According to the fitting results, the nucleation and growth model fit well the conversion−time data, and some of the kinetic parameters were obtained.

Chemical looping combustion (CLC) is a new innovative technology with inherent separation of CO2 without energy penalty. Experiments on chemical looping combustion of biomass/coal were conducted in a 1 kWth continuous reactor, and an Australia iron ore was selected as oxygen carrier. Both biomass/coal mixture and biomass were used as fuels. The effect of temperature on gas composition of both the fuel reactor and the air reactor, conversion efficiency of carbonaceous gases, carbon capture efficiency, and oxide oxygen fraction was investigated. An increase in the fuel reactor temperature produced a higher CO2 concentration in the fuel reactor for biomass/coal mixture, whereas it produced a lower one for pure biomass. CO concentration in the fuel reactor increased in both fuel conditions. Due to the poor oxygen transport capacity and the thermodynamic constraint of the iron ore conversion from Fe2O3 to Fe3O4, a higher temperature would contribute to decreasing the conversion efficiency of carbonaceous gases for both biomass and biomass/coal mixture. Both carbon capture efficiency and oxide oxygen fraction were enhanced with increasing the fuel reactor temperature, and the deviation between them was caused by the combustible carbonaceous gases in the fuel reactor. Both the fresh and the used oxygen carrier particles were characterized. X-ray diffraction (XRD) results indicated that the iron ore as oxygen carrier possesses a good regenerable ability in the CLC process. This is attributed to the existence of quartz in the iron ore particles and its sintering inhibition. Reactions between SiO2 and Fe3O4 may occur at a high temperature under a reducing condition. Scanning electron microscope (SEM) analysis showed that as a consequence of accumulative effect of redox reaction and thermal stress, the used oxygen carrier particles obtained a porous structure facilitating the gas−solid reactions. Energy dispersive X-ray (EDX) results demonstrated the deposition of alkali metals on the particle surface of oxygen carrier during the CLC process of biomass. Blending biomass with coal and adding some additives might be effective measures to reduce the potential negative influence of biomass ash on oxygen carrier.

The catalytic performances of natural Ca-based catalysts (dolomite and limestone) and synthetic Ca-based catalysts, used for improving biomass steam gasification, were fully investigated in a circulating spout-fluid bed reactor. In comparison to the catalytic role in biomass gasification, the synthetic Ca-based catalyst, 20% CaO/Al2O3 (20CaAl), displayed a better catalytic effect on the biomass carbon conversion reaction and tar reforming reaction than natural Ca-based catalysts, but the latter played a better role in the reforming reaction of light hydrocarbon because it contained small quantities of iron oxides. For the two types of Ca-based catalysts, the optimal operating temperature was about 860 °C to upgrade the quality of gas product and increase H2 yield. The catalytic activity of synthetic Ca-based catalysts was visibly improved with the increment of CaO loading, and the preferential range of CaO loading was 12.5−20%. In comparison to the lifetime, 20CaAl (the synthetic catalyst) displayed better stability in the catalytic role than natural Ca-based catalysts.

Biomass gasification consists of two procedures: pyrolysis and char gasification. To better understand the influences of additives on the biomass gasification with steam, the two procedures were studied in part 1 and part 2, respectively. This paper is the part 1 of this series, where we focus on the influences of additives on the pyrolysis procedure of biomass gasification in a fixed-bed reactor. The additives used for this paper were alkali metal carbonates (Na2CO3 and K2CO3), alkaline earth metal oxides (CaO and MgO), transition metals (Ni and Fe2O3), natural ores (dolomite, olivine, and sepiolite) and clays (kaoline and diatomite). The results demonstrated that alkali metal carbonates (AMC), alkaline earth metal oxides (AEMO), dolomite, and sepiolite mainly increased the yields of permanent gases (H2, CO2, etc.) and improved the quality of gaseous product by promoting the decomposition reactions of tar and light hydrocarbon (CnHm) and the gasification reaction of char; transition metals (TM) and olivine primarily enhanced reforming reactions of tar and CnHm to raise the yields of permanent gases (H2, CO2, etc.) and upgrade the quality of gaseous product; clays played few roles in the conversion reactions of tar and CnHm. For AEMO, Fe2O3, dolomite, and olivine, these additives also display roles in improving the water-gas shift reaction to further raise the yields of H2 and CO2. In addition, the pyrolysis procedure is further improved by the acts of these additives with increasing the temperature. On the basis of the properties of additives, TM should be selected as one portion of active ingredients of the composite catalyst, which could be used for promoting steam gasification of biomass to produce high-quality gas and increasing the overall efficiency of biomass gasification, to upgrade the quality of gaseous product; AMC or AEMO should be used as the promoter to enhance the steam gasification of carbon/char deposited on the catalyst.

Chemical-looping combustion of biomass was carried out in a 10 kWth reactor with iron oxide as an oxygen carrier. A total 30 h of test was achieved with the same batch of iron oxide oxygen carrier. The effect of the fuel reactor temperature on gas composition of the fuel reactor and the air reactor, the proportion of biomass carbon reacting in the fuel reactor, and the conversion of biomass carbon to CO2 in the fuel reactor was experimentally investigated. The results showed that the CO production from biomass gasification with CO2 was more temperature dependent than the CO oxidation with iron oxide in the fuel reactor, and an increase in the fuel reactor temperature produced a higher increase for the CO production from biomass gasification than for the oxidation of CO by iron oxide. Although the conversion of biomass carbon to CO2 in the fuel reactor decreased with the increase of the fuel reactor temperature, there was a substantial increase in the proportion of biomass carbon reacting in the fuel reactor. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to characterize fresh and reacted oxygen carrier particles. The results showed that the transformation of Fe2O3 to Fe3O4 is the favored step in the process of iron oxide reduction with biomass syngas. The low reactivity of reacted oxygen carrier was mainly ascribed to the sintering grains on the particle surface. To restrain the surface sintering of oxygen carrier particles, an intensive oxidization of reduced oxygen carrier with air in the air reactor should be avoided in the process of oxygen carrier regeneration, and air staging should be adopted for the oxidization of reduced oxygen carrier with air in the air reactor.

Chemical-looping combustion is an indirect combustion technology with inherent separation of the greenhouse gas CO2. The feasibility of using NiO as an oxygen carrier during chemical-looping combustion of coal has been investigated experimentally at 800−960 °C in the present work. The experiments were carried out in a fluidized bed, where the steam acted as the gasification−fluidization medium. Coal gasification and the reaction of oxygen carrier with the water gas take place simultaneously in the reactor. The oxygen carrier particles exhibit high reactivity above 900 °C, and the dry basis concentration of CO2 in the exit gas of the reactor is nearly 95%. The flue gas composition as a function of the reactor temperature and cyclic reduction number is discussed. At 800−960 °C, the dry basis concentration of CO2 in the flue gas presents a monotonously increasing trend, whereas the dry basis concentration of CO, H2, and CH4 decreases monotonously. The concentrations of CO2, CO, H2, and CH4 in the flue gas as a function of cyclic reduction number present a para-curve characteristic at 900 °C. With the increase of cyclic reduction number, the dry basis concentration of CO2 decreases remarkably, while the dry basis concentrations of CO, H2, and CH4 increase rapidly. Moreover, the peak value of H2 concentration is less than that of CO. The performance of the NiO-based oxygen carriers was also evaluated using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues of oxygen carrier. The results indicate that NiO is one of the suitable oxygen carriers for chemical-looping combustion of coal.

Chemical looping combustion is the indirect combustion by use of oxygen carrier. It can be used for CO2 capture in power generating processes. In this paper, chemical looping combustion of coal in interconnected fluidized beds with inherent separation of CO2 is proposed. It consists of a high velocity fluidized bed as an air reactor in which oxygen carrier is oxidized, a cyclone, and a bubbling fluidized bed as a fuel reactor in which oxygen carrier is reduced by direct and indirect reactions with coal. The air reactor is connected to the fuel reactor through the cyclone. To raise the high carbon conversion efficiency and separate oxygen carrier particle from ash, coal slurry instead of coal particle is introduced into the bottom of the bubbling fluidized bed. Coal gasification and the reduction of oxygen carrier with the water gas take place simultaneously in the fuel reactor. The flue gas from the fuel reactor is CO2 and water. Almost pure CO2 could be obtained after the condensation of water. The reduced oxygen carrier is then returned back to the air reactor, where it is oxidized with air. Thermodynamics analysis indicates that NiO/Ni oxygen carrier is the optimal one for chemical looping combustion of coal. Simulation of the processes for chemical looping combustion of coal, including coal gasification and reduction of oxygen carrier, is carried out with Aspen Plus software. The effects of air reactor temperature, fuel reactor temperature, and ratio of water to coal on the composition of fuel gas, recirculation of oxygen carrier particles, etc., are discussed. Some useful results are achieved. The suitable temperature of air reactor should be between 1050–1150°C and the optimal temperature of the fuel reactor be between 900–950°C.

Chemical-looping combustion (CLC) has been suggested as an energy-efficient method for the capture of the greenhouse gas carbon dioxide from combustion. The reactivity of using Fe2O3 as an oxygen carrier during CLC of coal has been investigated experimentally at 800–950°C. The experiments were carried out in a fluidized bed, where the steam acted as the gasification-fluidization medium. The reactivity of Fe2O3 as a function of the reactor temperature, reaction time, and cyclic reduction number was discussed. The reactivity of Fe2O3 oxygen carriers was enhanced as temperature increased at 800–950°C. Moreover, the time of chemical reaction control between the oxygen carrier and coal gasification products decreased with increased reaction temperature. When the reaction temperature was above 900°C, the rate of carbon to form CO2 was higher than 90%; however, it was lower than 75% below 850°C. At 900°C, the dry basis concentration of CO2 decreased with increased cyclic reduction period, while that of CO and CH4 increased. Moreover, the value of the CO concentration was less than that of CH4. The performance of the reacted Fe2O3-based oxygen carriers was also evaluated using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues of oxygen carrier. The results show that Fe2O3-based oxygen carriers are only reduced to Fe3O4. With the increase of cyclic reduction period, the oxygen carrier sinters gradually.

Experiments on chemical-looping combustion of coal were conducted in a 1 kWth interconnected fluidized-bed reactor using a natural hematite as an oxygen carrier and Shenhua bituminous coal and Huaibei anthracite as fuel. An evaluation of a hematite oxygen carrier decorated with NiO by mechanical mixing and impregnation methods was also performed. The results indicate that coal gasification rate is a time-limiting step that is employed in the chemical-looping combustion of coal, and the coal type has a great impact on the CO2 capture efficiency. When the fuel reactor temperature is 970°C and using hematite as an oxygen carrier, the CO2 capture efficiency for Shenhua bituminous coal is 81.7%, whereas the one for Huaibei anthracite is 65%. Hematite shows stable reactivity during a process of long-term operation, and it should be a good candidate as an oxygen carrier for the chemical-looping combustion of coal. It is an effective way of improving the reactivity of the hematite oxygen carrier as well as the CO2 capture efficiency by mechanical mixing with the NiO/Al2O3 oxygen carrier. Impregnating NiO on hematite exhibits a negative effect of the reaction performance between the product of coal gasification and the oxygen carrier due to the poor microstructure after calcination. Further investigations on the method and process of impregnation should be conducted.

Using H2 as a reactant gas, the reaction characterization of hematite oxygen carrier was investigated in a thermogravimetric analyzer (TGA). The solid reduction products were characterized by XRD (X-ray diffraction) and SEM-EDS (Scanning Electron Microscope-Energy Dispersive Spectrometer). The results show that there is a maximum reaction rate when the conversion is 0.11. In this stage, Fe2O3 as an active phase was converted to Fe3O4 and the reduction reaction was easy to happen. Then, there is a decrease of reaction rate. When the conversion of hematite oxygen carrier was 0.178, the solid reduction products were composed of Fe3O4 and FeO. When the conversion is 0.477, Fe3O4, FeO and Fe were all found in the reduction products. SEM-EDS results show that the grains at the surface of hematite oxygen carrier are gathered and grown up, and the particle volume shrinkage and sintering effect are observed, especially in the region rich in Fe element. However, a relative stable structure was seen in the region rich in SiO2 or Al2O3 contents. Further, compared with the results based on interconnected beds, the sintering of hematite was suppressed due to a good inert material distribution, which kept a good long-term reactivity of hematite oxygen carrier.