Jun Xiao

Analysis of reactivity of Fe-based oxygen carrier with coal during chemical-looping combustion

Abstract 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 Fe 2 O 3 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 Fe 2 O 3 as a function of the reactor temperature, reaction time, and cyclic reduction number was discussed. The reactivity of Fe 2 O 3 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 CO 2 was higher than 90%; however, it was lower than 75% below 850°C. At 900°C, the dry basis concentration of CO 2 decreased with increased cyclic reduction period, while that of CO and CH 4 increased. Moreover, the value of the CO concentration was less than that of CH 4 . The performance of the reacted Fe 2 O 3 -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 Fe 2 O 3 -based oxygen carriers are only reduced to Fe 3 O 4 . With the increase of cyclic reduction period, the oxygen carrier sinters gradually.

Simulation and energy performance assessment of CO2 removal from crude synthetic natural gas via physical absorption process

The paper presents an energy performance assessment of CO2 removal for crude synthetic natural gas (SNG) upgrade by Selexol absorption process. A simplified process simulation of the Selexol process concerning power requirement and separation performance was developed. The assessment indicates that less pressure difference between crude SNG and absorption pressure favors the energy performance of CO2 removal process. When both crude SNG and absorption pressures are 20 bar, CO2 removal process has the best energy performance. The optimal specific power consumption of the CO2 removal process is 566 kJ/kgCO2. The sensitivity analysis shows that the CO2 removal efficiency would significantly influence the total power consumption of the removal process, as well as higher heating value (HHV) and CO2 content in SNG. However, the specific power consumption excluding crude SNG and SNG compressions changes little with the variance of CO2 removal efficiency. If by-product CO2 is compressed for CO2 capture, the process would turn into a CO2-sink for the atmosphere. Correspondingly, an increase of 281 kJ/kgCO2 in specific power consumption is required for compressing the separated CO2.

Experimental investigation of hematite oxygen carrier decorated with NiO for chemical looping combustion of coal

Abstract Experiments on chemical-looping combustion of coal were conducted in a 1 kW th 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 CO 2 capture efficiency. When the fuel reactor temperature is 970°C and using hematite as an oxygen carrier, the CO 2 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 CO 2 capture efficiency by mechanical mixing with the NiO/Al 2 O 3 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.

Characterization of hematite oxygen carrier in chemical-looping combustion at high reduction temperature

Abstract 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.

Estimation of Specific Enthalpy and Exergy of Biomass and Coal Ash

Ash is one of the most studied but poorest understood characteristics. Based on the latest summary on chemical ash composition of varieties of biomass and coal, specific enthalpy, physical, and chemical exergy of biomass and coal ash were calculated then studied statistically. Three correlations were derived for estimating specific enthalpy, physical, and chemical exergy of ash, respectively. Then the ratios of the energy and exergy losses with discharged ash indicate that ash should be taken into account in an accurate exergy analysis of a process that simultaneously involves fuels with high ash content and a high exhaust gas temperature.

Simulation of Bio-syngas Production from Biomass Gasification via Pressurized Interconnected Fluidized Beds

Bio-syngas production from biomass gasification via pressurized interconnected fluidized beds was described. The interconnected fluidized beds technology separates the gasification and combustion processes of biomass, and the heat is transferred from combustor to gasifier by bed materials, while extra heat needed in gasification process is provided by additional biomass burning in the combustor. The simulation of the whole process was carried out with Aspen Plus software. The effects of gasification temperature (T g), gasification pressure (p g) and steam to biomass ratio (S/B) on bio-syngas production were studied. The results showed that gasification temperature, gasification pressure, and S/B had great influences on the bio-syngas composition and to achieve high carbon conversion and yield of high-quality bio-syngas, the suitable gasification temperature is around 750 °C, and the gasification pressure and S/B could not too high.

Integrated Study on Syngas-to-Synthetic Natural Gas (SNG) Process

A simulation of syngas-to-synthetic natural gas (SNG) process is presented. It mainly consists of the modeling of methanation process via a fluidized bed reactor and CO2 removal via Selexol absorption process. The effects of methanation temperature and pressure on the composition, yield and higher heating value (HHV) of SNG, as well as exergy efficiency of the process were investigated. The results indicate that the methanation temperature with a range of 300 °C to 350 °C and methation pressure with a range of 2.5 bar to 15 bar are recommended for the syngas-to-SNG process. The CO2 removal efficiency should be carefully determined to make the composition of SNG meet the relevant technical requirement. The syngas-to-SNG process with heat recovery has high exergy efficiency, which varies from 90.9% to 94.5%. There is less potential for improving the exergy efficiency of the process.

A techno-economic assessment of SNG production from agriculture residuals in China

ABSTRACT Biomass provides an attractive option for satisfying the demand of the rapid gas consumption growth. A process of BioSNG production via interconnected fluidized beds as gasification system and fluidized bed methanation reactor was modeled using Aspen Plus. The process performances and economic cost of BioSNG production from rice straw, wheat straw, corns stalk, and cotton straw were performed and compared. The exergy efficiency of BioSNG production from the four varieties of biomass ranges from 54.8% to 65.6%. With assumption on price of biomass including transportation of 250 RMB/t, the unit cost of BioSNG is equivalent to that of coal-based SNG. However, sensitivity analysis indicates that the unit cost of BioSNG linearly increases with the increase of biomass price. The exergy efficiency and unit cost results also strongly suggest that the CO2 produced during BioSNG production should be captured rather than treated as waste.