Prospective contributions of biomass pyrolysis to China's 2050 carbon reduction and renewable energy goals.

Recognizing that bioenergy with carbon capture and storage (BECCS) may still take years to mature, this study focuses on another photosynthesis-based, negative-carbon technology that is readier to implement in China: biomass intermediate pyrolysis poly-generation (BIPP). Here we find that a BIPP system can be profitable without subsidies, while its national deployment could contribute to a 61% reduction of carbon emissions per unit of gross domestic product in 2030 compared to 2005 and result additionally in a reduction in air pollutant emissions. With 73% of national crop residues used between 2020 and 2030, the cumulative greenhouse gas (GHG) reduction could reach up to 8620 Mt CO2-eq by 2050, contributing 13-31% of the global GHG emission reduction goal for BECCS, and nearly 4555 Mt more than that projected for BECCS alone in China. Thus, China's BIPP deployment could have an important influence on achieving both national and global GHG emissions reduction targets.

Photocatalytic removal of elemental mercury via Ce-doped TiO2 catalyst coupling with a novel optical fiber monolith reactor.

Reduction of mercury emission from coal combustion is a serious task for public health and environmental societies. Herein, Ce-doped TiO2 (Ce/TiO2) catalyst coupling with a novel optical fiber monolith reactor was applied to efficiently remove elemental mercury (Hg0) from coal-fired flue gas. Under the optimal operation condition (i.e., 1.5 mW/cm2 UV light, 90 °C), above 95% of Hg0 removal efficiency was attained over the optical fiber monolith reactor coating with 3.40 g/m2 Ce/TiO2 catalyst. The effects of flue gas compositions on Hg0 removal performance were clarified systematically. Gaseous O2 replenished the surface oxygen, hence maintaining the production of free radicals and promoting the removal of Hg0. SO2, HCl, and NO inhibited Hg0 removal in the absence of O2 due to the competitive adsorption and consumption of free radicals. However, SO2 and HCl significantly enhanced Hg0 removal with the participation of O2, while NO exhibited obviously inhibitory effect even with the assistance of O2. H2O also decreased the Hg0 oxidation capacity owing to the competitive adsorption and reduction of HgO. The optical fiber monolith reactor exhibited much superior Hg0 removal capacity than the powder reactor. Utilization of Ce/TiO2 catalyst coupling with an optical fiber monolith reactor provides a cost-effective method for removing Hg0 from coal-fired flue gas.

Fate of Mercury in Volatiles and Char during in Situ Gasification Chemical-Looping Combustion of Coal.

Mercury emission is an important issue during in-situ gasification chemical-looping combustion ( iG-CLC) of coal. This work focused on experimentally "isolating" two elementary subprocesses (coal pyrolysis and char gasification) during iG-CLC of coal, identifying mercury distribution within the two subprocesses, and examining the effects of a hematite oxygen carrier (OC) on the mercury fate. The mercury measurement accuracy was carefully ensured by comparing online measurements (by a VM 3000 instrument) and benchmark measurements (by the standard Ontario Hydro Method, ASTM D6784) as well as repeated tests (10 times for each case). The mercury mass balance was 115% for the entire iG-CLC. A total of 44.7% of the mercury was released as the gas phase form within the coal pyrolysis process at a typical CLC operation temperature (950 °C), whereas 13.4% was released during the char gasification process. The release rate and amount of mercury were minimally affected by the presence of OC; however, the OC promoted the conversion of Hg0(g) to Hg2+(g). Only a small amount of mercury was absorbed by the OC and transported into the air reactor along with carbon residue, released as Hg0(g) and Hg2+(g) or remained in the OC and coal ash as particulate mercury.

Experimental study on the stability of the ClHgSO3- in desulfurization wastewater.

Wet flue gas desulfurization technologies have received much concern for their superior performance on co-controlling the acid gases and mercury. However, high concentrations of mercury-containing desulfurization wastewater, which discharge from wet flue gas desulfurization system regularly, have received researchers' attention since it might generate the risk of secondary pollution. In this paper, the species of mercuric complexes in the desulfurization wastewater was investigated. It speculated that ClHgSO3- might determine the residual rate of Hg2+ in the desulfurization wastewater. Besides, the stability of ClHgSO3- on the condition of various wastewater features was also evaluated. The experiment revealed that the high temperature and high pH level promoted the decomposition of ClHgSO3-. SO32- could restrain the decomposition of ClHgSO3- gently; the Hg2+ residual rate was determined by the new mercury complexes which compounded by Hg2+ and SO32-. The decrease of SO42- and increase of Ca2+ concentrations could also stimulate the stability of ClHgSO3- in wastewater. Cu2+ and Fe2+ disturbed the stability of complexes for their catalysis and reduction activities. The study proposed that the ClHgSO3- probably decomposes and releases Hg0 in two pathways. Furthermore, changes of the water's features could disturb the balance of Hg2+-Cl--SO32- systems, which might stimulate the decomposition of ClHgSO3.

Influence of carbonation under oxy-fuel combustion flue gas on the leachability of heavy metals in MSWI fly ash.

Due to the high cost of pure CO2, carbonation of MSWI fly ash has not been fully developed. It is essential to select a kind of reaction gas with rich CO2 instead of pure CO2. The CO2 uptake and leaching toxicity of heavy metals in three typical types of municipal solid waste incinerator (MSWI) fly ash were investigated with simulated oxy-fuel combustion flue gas under different reaction temperatures, which was compared with both pure CO2 and simulated air combustion flue gas. The CO2 uptake under simulated oxy-fuel combustion flue gas were similar to that of pure CO2. The leaching concentration of heavy metals in all MSWI fly ash samples, especially in ash from Changzhou, China (CZ), decreased after carbonation. Specifically, the leached Pb concentration of the CZ MSWI fly ash decreased 92% under oxy-fuel combustion flue gas, 95% under pure CO2 atmosphere and 84% under the air combustion flue gas. After carbonation, the leaching concentration of Pb was below the Chinese legal limit. The leaching concentration of Zn from CZ sample decreased 69% under oxy-fuel combustion flue gas, which of Cu, As, Cr and Hg decreased 25%, 33%, 11% and 21%, respectively. In the other two samples of Xuzhou, China (XZ) and Wuhan, China (WH), the leaching characteristics of heavy metals were similar to the CZ sample. The speciation of heavy metals was largely changed from the exchangeable to carbonated fraction because of the carbonation reaction under simulated oxy-fuel combustion flue gas. After carbonation reaction, most of heavy metals bound in carbonates became more stable and leached less. Therefore, oxy-fuel combustion flue gas could be a low-cost source for carbonation of MSWI fly ash.

Integrated removal of NO and mercury from coal combustion flue gas using manganese oxides supported on TiO2.

A catalyst composed of manganese oxides supported on titania (MnOx/TiO2) synthesized by a sol-gel method was selected to remove nitric oxide and mercury jointly at a relatively low temperature in simulated flue gas from coal-fired power plants. The physico-chemical characteristics of catalysts were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses, etc. The effects of Mn loading, reaction temperature and individual flue gas components on denitration and Hg0 removal were examined. The results indicated that the optimal Mn/Ti molar ratio was 0.8 and the best working temperature was 240°C for NO conversion. O2 and a proper ratio of [NH3]/[NO] are essential for the denitration reaction. Both NO conversion and Hg0 removal efficiency could reach more than 80% when NO and Hg0 were removed simultaneously using Mn0.8Ti at 240°C. Hg0 removal efficiency slightly declined as the Mn content increased in the catalysts. The reaction temperature had no significant effect on Hg0 removal efficiency. O2 and HCl had a promotional effect on Hg0 removal. SO2 and NH3 were observed to weaken Hg0 removal because of competitive adsorption. NO first facilitated Hg0 removal and then had an inhibiting effect as NO concentration increased without O2, and it exhibited weak inhibition of Hg0 removal efficiency in the presence of O2. The oxidation of Hg0 on MnOx/TiO2 follows the Mars-Maessen and Langmuir-Hinshelwood mechanisms.

Dynamic Exergy Method for Evaluating the Control and Operation of Oxy-Combustion Boiler Island Systems.

Exergy-based methods are widely applied to assess the performance of energy conversion systems; however, these methods mainly focus on a certain steady-state and have limited applications for evaluating the control impacts on system operation. To dynamically obtain the thermodynamic behavior and reveal the influences of control structures, layers and loops, on system energy performance, a dynamic exergy method is developed, improved, and applied to a complex oxy-combustion boiler island system for the first time. The three most common operating scenarios are studied, and the results show that the flow rate change process leads to less energy consumption than oxygen purity and air in-leakage change processes. The variation of oxygen purity produces the largest impact on system operation, and the operating parameter sensitivity is not affected by the presence of process control. The control system saves energy during flow rate and oxygen purity change processes, while it consumes energy during the air in-leakage change process. More attention should be paid to the oxygen purity change because it requires the largest control cost. In the control system, the supervisory control layer requires the greatest energy consumption and the largest control cost to maintain operating targets, while the steam control loops cause the main energy consumption.

Relation between leaching characteristics of heavy metals and physical properties of fly ashes from typical municipal solid waste incinerators.

Due to the alkalinity and high concentration of potentially hazardous heavy metals, fly ash from a municipal solid waste (MSW) incinerator is classified as hazardous waste, which should be of particular concern. Physical and chemical characterizations of the contrasted fly ashes were investigated to explore the relation between leaching characteristics of heavy metals and physical properties of fly ashes. The results showed that CaClOH, NaCl, Ca(OH)2, KCl and SiO2 were primary mineral compositions in the MSWI fly ashes, and the particle size distribution of fly ash ranged between 10 µm and 300 µm. The smaller the particle size distribution of fly ash, the larger the BET-specific surface area, which was beneficial to the leaching of heavy metals. As a result of various pores, it easily accumulated heavy metals as well. The leaching tests exhibited a high leachability of heavy metals and the leaching concentration of Pb in almost all of the fly ash samples went far beyond the Standard for Pollution Control on the Landfill Site of Municipal Solid Waste. Thereupon, it is necessary to establish proper disposal systems and management strategies for environmental protection based on the characteristics of MSW incineration (MSWI) fly ash in China.

Mechanisms of Elemental Mercury Transformation on α-Fe2O3(001) Surface from Experimental and Theoretical Study: Influences of HCl, O2, and SO2.

The reaction mechanisms of a mixture gas of HCl, O2, and SO2 in Hg0 adsorption on α-Fe2O3(001) surface are clarified by a group of adsorption experiments and theoretical calculations based on the density functional theory. The role of O2 in removing Hg0 is greatly influenced by the reaction temperature, meanwhile, the O atom coverage could affect the adsorption performance of Hg0. The dissociated O2 competes with the active sites of Cl species on Fe surface at low temperature, however, at medium temperature HCl and O2 could simultaneously facilitate the Hg0 transformation. Combined with the theoretical calculations, the role of SO2 and the probable pathways in removing Hg0 are discussed. Lower concentration of SO2 as well as HCl could dissociate on α-Fe2O3(001) surface, and the intermediates combine with gaseous Hg0, forming mercury-sulfur, mercury-chlorine compounds, and so forth. In addition, the different concentrations of SO2 are also discussed, and the corresponding X-ray photoelectron spectroscopy analysis on contrasted samples is conducted to research the morphological characterization, providing a reliable basis for judging the probable pathways of Hg0 transformation.

Mercury Removal by Magnetic Biochar Derived from Simultaneous Activation and Magnetization of Sawdust.

Novel magnetic biochars (MBC) were prepared by one-step pyrolysis of FeCl3-laden biomass and employed for Hg0 removal in simulated combustion flue gas. The sample characterization indicated that highly dispersed Fe3O4 particles could be deposited on the MBC surface. Both enhanced surface area and excellent magnetization property were obtained. With the activation of FeCl3, more oxygen-rich functional groups were formed on the MBC, especially the C═O group. The MBC exhibited far greater Hg0 removal performance compared to the nonmagnetic biochar (NMBC) under N2 + 4% O2 atmosphere in a wide reaction temperature window (120-250 °C). The optimal pyrolysis temperature for the preparation of MBC is 600 °C, and the best FeCl3/biomass impregnation mass ratio is 1.5 g/g. At the optimal temperature (120 °C), the Fe1.5MBC600 was superior in both Hg0 adsorption capacity and adsorption rate to a commercial brominated activated carbon (Br-AC) used for mercury removal in power plants. The mechanism of Hg0 removal was proposed, and there are two types of active adsorption/oxidation sites for Hg0: Fe3O4 and oxygen-rich functional groups. The role of Fe3O4 in Hg0 removal was attributed to the Fe3+(t) coordination and lattice oxygen. The C═O group could act as act as electron acceptors, facilitating the electron transfer for Hg0 oxidation.

DFT and Experimental Study on the Mechanism of Elemental Mercury Capture in the Presence of HCl on α-Fe2O3(001).

To investigate the mechanism of Hg(0) adsorption on the α-Fe2O3(001) surface in the presence of HCl, which is considered to be beneficial for Hg(0) removal, theoretical calculations based on density functional theory as well as corresponding experiments are carried out. HCl adsorption is first performed on the α-Fe2O3(001) surface, and the Hg(0) adsorption on HCl-adsorbed α-Fe2O3(001) surface is subsequently researched, demonstrating that HCl dissociates on the surface of α-Fe2O3, improving the Hg(0) adsorption reactivity. With further chlorination of the α-Fe2O3(001) surface, FeCl3 can be achieved and the adsorption energy of Hg(0) on the FeCl3 surface reaches -104.2 kJ/mol, representing strong chemisorption. Meanwhile, a group of designed experiments, including Hg(0) adsorption on HCl-preadsorbed α-Fe2O3 as well as the coadsorption of both gaseous components, are respectively performed to explore the pathways of Hg(0) transformation. Combining computational and experimental results together, the Eley-Rideal mechanism with HCl preadsorption can be determined. In addition, subsequent X-ray photoelectron spectroscopy analysis verifies the appearance of Cl species and oxidized mercury, exhibiting the consistency with experiments.

Synthesis, characterization and enhanced photocatalytic CO2 reduction activity of graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets.

It is known that the combination of TiO2 and graphene and the control of TiO2 crystal facets are both effective routes to improve the photocatalytic performance of TiO2. Here, we report the synthesis and the photocatalytic CO2 reduction performance of graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets (G/TiO2-001/101). The combination of TiO2 and graphene enhanced the crystallinity of TiO2 single nanocrystals and obviously improved their dispersion on graphene. The "surface heterojunction" formed by the coexposed {001} and {101} facets can promote the spatial separation of photogenerated electrons and holes toward different facets and the supports of graphene can further enhance the separation through accelerated electron migration from TiO2 to graphene. The G/TiO2-001/101 exhibited high photocatalytic CO2-reduction activity with a maximum CO yield reaching 70.8 µmol g(-1) h(-1). The enhanced photocatalytic activity of the composites can be attributed to their high surface area, good dispersion of TiO2 nanoparticles, and effective separation of excited charges due to the synergy of graphene supports and the co-exposure of {001} and {101} facets.

Mercury Adsorption and Oxidation over Cobalt Oxide Loaded Magnetospheres Catalyst from Fly Ash in Oxyfuel Combustion Flue Gas.

Cobalt oxide loaded magnetospheres catalyst from fly ash (Co-MF catalyst) showed good mercury removal capacity and recyclability under air combustion flue gas in our previous study. In this work, the Hg(0) removal behaviors as well as the involved reactions mechanism were investigated in oxyfuel combustion conditions. Further, the recyclability of Co-MF catalyst in oxyfuel combustion atmosphere was also evaluated. The results showed that the Hg(0) removal efficiency in oxyfuel combustion conditions was relative high compared to that in air combustion conditions. The presence of enriched CO2 (70%) in oxyfuel combustion atmosphere assisted the mercury oxidation due to the oxidation of function group of C-O formed from CO2. Under both atmospheres, the mercury removal efficiency decreased with the addition of SO2, NO, and H2O. However, the enriched CO2 in oxyfuel combustion atmosphere could somewhat weaken the inhibition of SO2, NO, and H2O. The multiple capture-regeneration cycles demonstrated that the Co-MF catalyst also present good regeneration performance in oxyfuel combustion atmosphere.

Regenerable cobalt oxide loaded magnetosphere catalyst from fly ash for mercury removal in coal combustion flue gas.

To remove Hg(0) in coal combustion flue gas and eliminate secondary mercury pollution of the spent catalyst, a new regenerable magnetic catalyst based on cobalt oxide loaded magnetospheres from fly ash (Co-MF) was developed. The catalyst, with an optimal loading of 5.8% cobalt species, attained approximately 95% Hg(0) removal efficiency at 150 °C under simulated flue gas atmosphere. O2 could enhance the Hg(0) removal activity of magnetospheres catalyst via the Mars-Maessen mechanism. SO2 displayed an inhibitive effect on Hg(0) removal capacity. NO with lower concentration could promote the Hg(0) removal efficiency. However, when increasing the NO concentration to 300 ppm, a slightly inhibitive effect of NO was observed. In the presence of 10 ppm of HCl, greater than 95.5% Hg(0) removal efficiency was attained, which was attributed to the formation of active chlorine species on the surface. H2O presented a seriously inhibitive effect on Hg(0) removal efficiency. Repeated oxidation-regeneration cycles demonstrated that the spent Co-MF catalyst could be regenerated effectively via thermally treated at 400 °C for 2 h.

Sulfur evolution in chemical looping combustion of coal with MnFe2O4 oxygen carrier.

Chemical looping combustion (CLC) of coal has gained increasing attention as a novel combustion technology for its advantages in CO2 capture. Sulfur evolution from coal causes great harm from either the CLC operational or environmental perspective. In this research, a combined MnFe2O4 oxygen carrier (OC) was synthesized and its reaction with a typical Chinese high sulfur coal, Liuzhi (LZ) bituminous coal, was performed in a thermogravimetric analyzer (TGA)-Fourier transform infrared (FT-IR) spectrometer. Evolution of sulfur species during reaction of LZ coal with MnFe2O4 OC was systematically investigated through experimental means combined with thermodynamic simulation. TGA-FTIR analysis of the LZ reaction with MnFe2O4 indicated MnFe2O4 exhibited the desired superior reactivity compared to the single reference oxides Mn3O4 or Fe2O3, and SO2 produced was mainly related to oxidization of H2S by MnFe2O4. Experimental analysis of the LZ coal reaction with MnFe2O4, including X-ray diffraction and X-ray photoelectron spectroscopy analysis, verified that the main reduced counterparts of MnFe2O4 were Fe3O4 and MnO, in good agreement with the related thermodynamic simulation. The obtained MnO was beneficial to stabilize the reduced MnFe2O4 and avoid serious sintering, although the oxygen in MnO was not fully utilized. Meanwhile, most sulfur present in LZ coal was converted to solid MnS during LZ reaction with MnFe2O4, which was further oxidized to MnSO4. Finally, the formation of both MnS and such manganese silicates as Mn2SiO4 and MnSiO3 should be addressed to ensure the full regeneration of the reduced MnFe2O4.

Generalized second-order slip boundary condition for nonequilibrium gas flows.

It is a challenging task to model nonequilibrium gas flows within a continuum-fluid framework. Recently some extended hydrodynamic models in the Navier-Stokes formulation have been developed for such flows. A key problem in the application of such models is that suitable boundary conditions must be specified. In the present work, a generalized second-order slip boundary condition is developed in which an effective mean-free path considering the wall effect is used. By combining this slip scheme with certain extended Navier-Stokes constitutive relation models, we obtained a method for nonequilibrium gas flows with solid boundaries. The method is applied to several rarefied gas flows involving planar or curved walls, including the Kramers' problem, the planar Poiseuille flow, the cylindrical Couette flow, and the low speed flow over a sphere. The results show that the proposed method is able to give satisfied predictions, indicating the good potential of the method for nonequilibrium flows.

Electrospun metal oxide-TiO2 nanofibers for elemental mercury removal from flue gas.

Nanofibers prepared by an electrospinning method were used to remove elemental mercury (Hg(0)) from simulated coal combustion flue gas. The nanofibers composed of different metal oxides (MO(x)) including CuO, In(2)O(3), V(2)O(5), WO(3) and Ag(2)O supported on TiO(2) have been characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersing X-ray (EDX) and UV-vis spectra. The average diameters of these nanofibers were about 200nm. Compared to pure TiO(2), the UV-vis absorption intensity for MO(x)-TiO(2) increased significantly and the absorption bandwidth also expanded, especially for Ag(2)O-TiO(2) and V(2)O(5)-TiO(2). Hg(0) oxidation efficiencies over the MO(x)-TiO(2) nanofibers were tested under dark, visible light (vis) irradiation and UV irradiation, respectively. The results showed that WO(3) doped TiO(2) exhibited the highest Hg(0) removal efficiency of 100% under UV irradiation. Doping V(2)O(5) into TiO(2) enhanced Hg(0) removal efficiency greatly from 6% to 63% under visible light irradiation. Ag(2)O doped TiO(2) showed a steady Hg(0) removal efficiency of around 95% without any light due to the formation of silver amalgam. An extended experiment with 8 Hg(0) removal cycles showed that the MO(x)-TiO(2) nanofibers were stable for removing Hg(0) from flue gas. Factors responsible for the enhanced photocatalytic activities of the MO(x)-TiO(2) nanofibers were also discussed.

Chequerboard effects on spurious currents in the lattice Boltzmann equation for two-phase flows.

Spurious currents near an interface between different phases are a common undesirable feature of the lattice Boltzmann equation (LBE) method for two-phase systems. In this paper, we show that the spurious currents of a kinetic theory-based LBE have a significant dependence on the parity of the grid number of the underlying lattice, which can be attributed to the chequerboard effect. A technique that uses a Lax-Wendroff streaming is proposed to overcome this anomaly, and its performance is verified numerically.

Force imbalance in lattice Boltzmann equation for two-phase flows.

The capability of modeling and simulating complex interfacial dynamics of multiphase flows has been recognized as one of the main advantages of the lattice Boltzmann equation (LBE). A basic feature of two-phase LBE models, i.e., the force balance condition at the discrete lattice level of LBE, is investigated in this work. An explicit force-balance formulation is derived for a flat interface by analyzing the two-dimensional nine-velocity (D2Q9) two-phase LBE model without invoking the Chapman-Enskog expansion. The result suggests that generally the balance between the interaction force and the pressure does not hold exactly on the discrete lattice due to numerical errors. It is also shown that such force imbalance can lead to some artificial velocities in the vicinity of phase interface.

High temperature capture of CO2 on lithium-based sorbents from rice husk ash.

Highly efficient Li(4)SiO(4) (lithium orthosilicate)-based sorbents for CO(2) capture at high temperature, was developed using waste materials (rice husk ash). Two treated rice husk ash (RHA) samples (RHA1 and RHA2) were prepared and calcined at 800°C in the presence of Li(2)CO(3). Pure Li(4)SiO(4) and RHA-based sorbents were characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy, nitrogen adsorption, and thermogravimetry. CO(2) sorption was tested through 15 carbonation/calcination cycles in a fixed bed reactor. The metals of RHA were doped with Li(4)SiO(4) resulting to inhibited growth of the particles and increased pore volume and surface area. Thermal analyses indicated a much better CO(2) absorption in Li(4)SiO(4)-based sorbent prepared from RHA1 (higher metal content sample) because the activation energies for the chemisorption process and diffusion process were smaller than that of pure Li(4)SiO(4). RHA1-based sorbent also maintained higher capacities during the multiple cycles.