Lunbo Duan

Effect of SO2 and steam on CO2 capture performance of biomass-templated calcium aluminate pellets

Four types of synthetic sorbents were developed for high-temperature post-combustion calcium looping CO2 capture using Longcal limestone. Pellets were prepared with: lime and cement (LC); lime and flour (LF); lime, cement and flour (LCF); and lime, cement and flour doped with seawater (LCFSW). Flour was used as a templating material. All samples underwent 20 cycles in a TGA under two different calcination conditions. Moreover, the prepared sorbents were tested for 10 carbonation/calcination cycles in a 68 mm-internal-diameter bubbling fluidized bed (BFB) in three environments: with no sulphur and no steam; in the presence of sulphur; and with steam. When compared to limestone, all the synthetic sorbents exhibited enhanced CO2 capture performance in the BFB experiments, with the exception of the sample doped with seawater. In the BFB tests, the addition of cement binder during the pelletisation process resulted in the increase of CO2 capture capacity from 0.08 g CO2 per g sorbent (LF) to 0.15 g CO2 per g sorbent (LCF) by the 10th cycle. The CO2 uptake in the presence of SO2 dramatically declined by the 10th cycle; for example, from 0.22 g CO2 per g sorbent to 0.05 g CO2 per g sorbent in the case of the untemplated material (LC). However, as expected all samples showed improved performance in the presence of steam, and the decay of reactivity during the cycles was less pronounced. Nevertheless, in the BFB environment, the templated pellets showed poorer CO2 capture performance. This is presumably because of material loss due to attrition under the FB conditions. By contrast, the templated materials performed better than untemplated materials under TGA conditions. This indicates that the reduction of attrition is critical when employing templated materials in realistic systems with FB reactors.

Solid–gaseous phase transformation of elemental contaminants during the gasification of biomass

Disposal of plant biomass removed from heavy metal contaminated land via gasification achieves significant volume reduction and can recover energy. However, these biomass often contain high concentrations of heavy metals leading to hot-corrosion of gasification facilities and toxic gaseous emissions. Therefore, it is of significant interest to gain a further understanding of the solid-gas phase transition of metal(loid)s during gasification. Detailed elemental analyses (C, H, O, N and key metal/metalloid elements) were performed on five plant species collected from a contaminated site. Using multi-phase equilibria modelling software (MTDATA), the analytical data allows modelling of the solid/gas transformation of metal(loid)s during gasification. Thermodynamic modelling based on chemical equilibrium calculations was carried out in this study to predict the fate of metal(loid) elements during typical gasification conditions and to show how these are influenced by metal(loid) composition in the biomass and operational conditions. As, Cd, Zn and Pb tend to transform to their gaseous forms at relatively low temperatures (<1000°C). Ni, Cu, Mn and Co converts to gaseous forms within the typical gasification temperature range of 1000-1200°C. Whereas Cr, Al, Fe and Mg remain in solid phase at higher temperatures (>1200°C). Simulation of pressurised gasification conditions shows that higher pressures increase the temperature at which solid-to-gaseous phase transformations takes place.

HCl removal performance of Mg-stabilized carbide slag from carbonation/calcination cycles for CO2 capture

Mg-stabilized carbide slag (MSCS) was fabricated with carbide slag, magnesium nitrate hydrate and a by-product of biodiesel from transesterification by combustion, and was used as a CO2 sorbent in calcium looping cycles. The cycled MSCS containing a ratio of CaO to MgO of 80 : 20 from the calcium looping cycles (i.e. carbonation/calcination cycles) for CO2 capture was subsequently used as an HCl sorbent. The HCl capture performance of the cycled MSCS which had experienced repetitive CO2 capture cycles using calcium looping was investigated in a triple fixed-bed reactor. The reaction products of the cycled MSCS after HCl absorption are CaClOH, CaO, and MgO. MgO is an inert support. The cycled MSCS reaches the highest HCl capture capacity at 750 °C. The number of CO2 capture cycles increases the effect of chlorination temperature on the HCl capture capacity of the cycled MSCS. The HCl capture capacity of the cycled MSCS drops slightly with the number of CO2 capture cycles. The HCl capture capacity of the MSCS which has undergone 10 CO2 capture cycles can retain 0.21 g HCl/g sorbent, which is about 1.7 times as high as that of the carbide slag. The presence of CO2 leads to a reduction in the HCl capture capacity of the cycled MSCS. MSCS can maintain a more stable microstructure due to the presence of MgO during the repetitive CO2 capture cycles. The cycled MSCS from the CO2 capture cycles exhibits a more porous structure than the cycled carbide slag, especially in the pore size range of 2–10 nm in diameter, which benefits HCl capture. Therefore, the cycled MSCS from CO2 capture cycles using calcium looping appears promising to remove HCl.

Modeling and Coupling Particle Scale Heat Transfer with DEM through Heat Transfer Mechanisms

The detailed heat transfer mechanisms particle interior, gas film around particles, gas gap between contact surfaces, and rough surface are considered to model heat transfer between particles. The validation of the heat transfer model is accomplished and the predicted results show good agreement with other experiments. From the quantitative comparison of four heat transfer paths, it is revealed that the heat transfer through gas gap and rough surface could be neglected for a particle diameter larger than 2 mm. Furthermore, the detailed heat transfer model is coupled with the discrete element method (DEM) to calculate macro effective thermal conductivity (ETC) of fixed beds, and the accuracy and applicability is verified by comparing with other estimated and experimental results. The influence of particle diameter, density, specific thermal capacity, and thermal conductivity on ETC is investigated. Results show that the proposed heat transfer model provides an effective and accurate way to couple with DEM in the particle system.

Effect of kaolinite additive on formation of PM2.5 under O2/CO2 atmosphere during coal combustion

The influence of kaolinite additive on the emission characteristics of PM2.5 (particulates with aerodynamic diameter less than 2.5μm) was studied with a tube furnace. The combustion tests were carried out at 1123 K under O2/CO2 atmosphere. The PM2.5 generated from coal combustion was collected and analyzed with an Electrical Low Pressure Impactor (ELPI). The results indicate that kaolinite is an important factor for the formation of PM2.5 during coal combustion under O2/CO2 atmosphere. The number and mass concentrations of PM1 diminish, but those of PM1–2.5 enhance slightly after kaolinite is added. The size distributions of PM2.5 are similar, which display two peaks around 0.2 μm and 2.0 μm, respectively. With increasing the weight ratio of kaolinite, the concentrations of S, Pb, Cu, Na and K decrease. The submicron-size ash particles smaller than 0.317 μm are formed via nucleation of vaporized ash components. The supermicron-size ash particles are formed by coagulation and coalescence of the submicron-size ash, and fragmentation and coalescence of mineral matter.

Effects of Air Pollution Control Devices on the Chlorine Emission from 410 t/h Circulating Fluidized Bed Boilers Co-firing Petroleum Coke and Coal

The emission characteristics of Cl were investigated on the basis of the field experiments at three 410 t/h circulating fluidized bed boilers co-firing petroleum coke and coal. All of the boilers were equipped with advanced air pollutant control devices (APCDs) to meet the ultralow emission requirement, such as selective non-catalytic reduction, electrostatic precipitator (ESP) or fabric filters (FFs), and wet flue gas desulfurization (WFGD). Gaseous Cl was sampled simultaneously at both the inlet and outlet of the ESP or FFs and at the outlet of the WFGDs based on U.S. Environmental Protection Agency Method 26A. Feed fuel, limestone, bottom ash, ESP or FF ash, limestone slurry, flush water, wastewater, and gypsum were also sampled to determine Cl in each stream. Results showed that more than 86.1% of total chlorine was emitted into the flue gas in the gaseous form (e.g., HCl and Cl2). HCl was the dominant species in the flue gas at the outlet of boilers, accounting for 85.1–88.0%. The removal efficiencie...

Experimental on Fly Ash Recirculation with Bottom Feeding to Improve the Performance of a Circulating Fluidized Bed Boiler Co-burning Coal Sludge

With the aim of reducing carbon content in fly ash, fly ash recirculation with bottom feeding (FARBF) technology was applied to a 75 t/h Circulating Fluidized Bed (CFB) boiler burning mixture of coal and coal sludge. And industrial experiments were carried out to investigate the influence of FARBF technology on the combustion performance and pollutant emission characteristics of the CFB boiler. Results show that as the recirculation rate of fly ash increases, the CFB dense bed temperature decreases while the furnace outlet temperature increases, and the temperature distribution in the furnace becomes uniform. Compared with the conditions without fly ash recirculation, the combustion efficiency increases from 92 to 95% when the recirculation rate increases to 8 t/h, and the desulfurization efficiency also increases significantly. As the recirculation rate increases, the emissions of NO and CO decrease, but the particulate emission increases. The present study indicates that FARBF technology can improve the combustion performance and desulfurization efficiency for the CFB boilers burning coal sludge, and this can bring large economical and environmental benefits in China.

Trace Elements Partitioning During Coal Combustion in Fluidized Bed Under O2/CO2 Atmosphere

Experiments were conducted to investigate the effects of temperature and O2/CO2 atmosphere on trace elements (Cr, Mn, Co, Ni, Cd, Pb, Hg, As, Se) partitioning during combustion of Xuzhou bituminous coal in a 6 kWth fluidized bed. Inductively coupled plasma mass spectrometry (ICP-MS) and atomic fluorescence spectrometry (AFS) were used to determine trace elements contents in raw coal, bottom ash, fly ash and flue gas. The results indicate that with bed temperature increase, the relative enrichment of all the trace elements except Cr in bottom ash decreases suggesting that their volatility is enhanced. The relative enrichments of hardly volatile elements, like Cr and Mn in fly ash increase with bed temperature increase while those of partially volatile and highly volatile elements in fly ash are opposite. The relative enrichments of trace elements except Cr and Mn in fly ash are higher than those in bottom ash. Increasing bed temperature promotes elements like As, Se and Hg to migrate to vapor phase, Mn to migrate to fly ash and Cr to migrate to both bottom ash and fly ash. 21%O2/79%CO2 atmosphere improves the volatility of Cr, Mn, Co, Se and their migration to fly ash, while restrains the volatility of As, Ni, Pb. It has little effect on the volatility of Hg but improves its migration to fly ash.