Magnetic γ-Fe2O3-Loaded Attapulgite Sorbent for Hg0 Removal in Coal-Fired Flue Gas

A magnetically recoverable composite mercury removal sorbent was produced by introducing magnetic γ-Fe2O3 into attapulgite (ATT) (xFe1ATT) via the co-precipitation method and used to remove Hg0 in the simulated coal-fired power plant flue gas. The as-prepared 0.5Fe1ATT sorbent was characterized by X-ray diffraction, Brunauer–Emmett–Teller, transmission electron microscopy, vibrating sample magnetometer, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy analyses. The results showed that the Hg0 removal performance of the composite of γ-Fe2O3 and ATT was significantly promoted in comparison to pure γ-Fe2O3 and ATT individually. A relatively high magnetization value and good Hg0 removal performance were obtained by the sample of 0.5Fe1ATT. O2 could enhance Hg0 removal activity via the Mars–Maessen mechanism. NO displayed a significant promotion effect on Hg0 removal as a result of the formation of active species, such as NO2 and NO+. SO2 inhibited the removal of Hg0 as a result of its competition adsorption against Hg0 for the active sites and the sulfation of the sorbent. However, the introduction of NO could obviously alleviate the adverse effect of SO2 on the Hg0 removal capability. H2O showed a prohibitive effect on Hg0 removal as a result of its competition with Hg0 for the active sites. The findings of this study are of fundamental importance to the development of efficient and economic magnetic mercury sorbents for Hg0 removal from coal-fired boiler flue gases

Dynamic experimental investigation on the volatilization behavior of lead and cadmium in the simulated Municipal Solid Waste (MSW) influenced by sulfur compounds during incineration

In China, coal, often with a high level of sulfur, is always mixed with municipal solid waste (MSW) in waste incineration plants due to the low heating value and high moisture content of MSW. The influence of sulfur compounds on the volatilization of heavy metals in MSW is of great concern for China’s waste incineration plants. In this study, the continuous dynamic volatilization process of Pb and Cd is investigated by adding different forms of sulfur compounds, elemental sulfur (S), and sodium sulfate (Na2SO4), to the simulated MSW in a laboratory incinerator, at both 1 and 3 wt %, respectively. The experimental results show that the added S begins to affect the volatilization of Pb and Cd at about 700 °C; adding S can lead up to 49.6% reduction in the volatilization of Pb, as the produced sulfur dioxide is promoting the formation of a condensed sulfate phase, and part of Pb is fixed in the form of PbS in the bottom ash. But for Cd, adding S causes up to 15.9% increase in its volatilization as S seizes part of O2 in the air, which is conducive to forming the reducing atmosphere. In the reducing atmosphere, CdO can be easily reduced to Cd, which volatilizes more easily than CdO at high temperatures. In fact, in the reducing atmosphere, the volatilization of Cd far outweighs the volatilization of Pb at 700–800 °C. On the other hand, adding Na2SO4 almost has no influence on the volatilization of lead and cadmium below 900 °C

STUDY ON POLLUTANTS EMISSION CHARACTERISTIC OF COAL GASIFICATION IN A FLUIDIZED BED TEST RIG

ABSTRACT This paper presents the results of coal gasification in a fluidized bed test rig of Xuzhou bituminous coal. The diameter of the fluidized bed combustor is 0.1m and the height is 4.22m. The bed temperature is maintained by a method of high temperature flue gas interline heating to overcome high heat losses associated with a oil burner. Test results are reported for variations in the bed temperature, air to coal, steam to coal and Ca to S ratio and their influence on gas yields and desulphurization efficiency. The distribution of polycyclic aromatic hydrocarbons (PAHs) and heavy metal trace elements into the char and syngas are also presented. The molar contents for CH 4 and H 2 in the coal syngas are found to decrease with increasing air to coal feed ratio from 2.5 to 5, while the content of CO shows little variation. Increasing the steam to coal feed ratio from 0.4 to 0.65 results in all three of the main gas components measured to form a local maximum content at a steam/coal feed ratio of 0.55. The efficiency of desulphurization improves as the ratio of Ca to S, air to coal and the bed temperature are increased, while decreasing with increasing steam to coal feed ratios. The volatile trace element species in decreasing order of relative mass ratio released into the gas phase are Hg, Se, As, Co, Cr, Cd, Cu, and Zn. Besides Hg, Se, and As, for all other trace heavy metals the majority of their mass distribution remains within the char with the proportion contained within char always greater than their combined yields in the coal syngas and slag. The total PAHs in the coal syngas is greater than that contained in the original coal and this indicates that PAHs are formed during the coal gasification process. Keywords: coal, gasification, fluidized bed, desulphurization, trace element, PAHs INTRODUCTION Advanced Pressurized Fluidized Bed CombustionCombined Cycle (APFBC-CC) is a promising technology for the cleaner and more efficient generation of electricity from coal. Coal partial gasification is a key step in the APFBC cycle. The majority of the sulphur present in coal changes into hydrogen sulfide under a reducing partial gasification atmosphere. At the same time, trace elements like As, Cd, Co, Cr, Cu, Mn, Mg, Ni, Hg, Pb, V, Se, Sr and Zn and PAHs in the coal gasification redistribute themselves into the slag, fly ash and coal gas phases. Xiangling Ho

Desulfurization using limestone during sludge incineration in a fluidized bed furnace: Increased risk of particulate matter and heavy metal emissions

Incineration of sludge can be an effective method to minimise waste whilst producing useful heat. However, incineration can cause secondary pollution issues due to the emission of SO2, therefore a set of experiments of sludge incineration in a bubble bed furnace were conducted with limestone addition to study desulfurization of sludge incineration flue gas. As expected, over 93% emission of SO2 was reduced with limestone addition, and that of CO and NOx were increased and decreased respectively when the fuel feeding rate raised. The distribution of fly ash was also increased by raising the fuel feeding rate due to increasing fragmentation of the ash. However, distributions of PM2.5 and heavy metals in submicron particles have dramatically increased with limestone desulfurization. The mechanism was revealed by SEM and EDS statistical analysis, indicating that the reaction between aluminosilicate and calcium made particles agglomerate and eutectic mixtures form, these larger ash particles were found to divide between collection as cyclone ash and fragmentation into finer particles that bypassed the cyclone. Those fine particles provided more surface area for heavy metal condensation. Furthermore, it was found that the reaction mechanism for semi-volatile metals involved them being released from the sludge and forming PM1 particles due to the vaporization-condensation mechanism, leading to higher emission of PM1 and distribution of heavy metals in PM1. Thus, it should be considered that there may actually be higher emission risks of PM and heavy metal emissions when aiming to desulfurize a flue gas using Ca-based minerals in certain circumstance

COAL GASIFICATION CHARACTERISTICS IN A 2MWth SECOND-GENERATION PFB GASIFIER

ABSTRACT Coal gasification process and equipment feasibility research w ere carried out in a 2 MW thermal input pressurized spout-fluid bed pilot-scale gasifier and a long-time-run test was performed to study the effects of operating parameters on coal partial gasification behaviors. The test results have demonstrated the feasibility of the gasifier to provide suitable fuel gas and residual char for downstream system of 2G PFBC-CC. The concentration of methane decreased at higher gasification temperature due to the secondary cracking of methane while the carbon conversion increased, and the concentration of hydrogen increased with an increase of steam flow rate. The main experimental results were compared with those of pilot-scale facilities in the world

Green production of a novel sorbent from kaolin for capturing gaseous PbCl2 in a furnace

The pollution of semi-volatile heavy metals is one of the key environmental risks for municipal solid waste incineration, and in-situ adsorption of metals within the furnace by mineral sorbents such as kaolin has been demonstrated as a promising emission control method. To lessen the consumption of sorbent, a novel material of amorphous silicate was produced from kaolin through pressurised hydrothermal treatment. Its performance of gaseous PbCl2 capture was tested in a fixed bed furnace and compared with unmodified kaolin and metakaolin. With increasing temperature, the adsorption rates for all sorbents declined due to higher saturated vapour pressure, while the partitions of residual form lead increased which indicated higher stability of heavy metals in the sorbent because of melting effect. The new sorbent with a larger surface area and reformed structure presented 26% more adsorption efficiency than raw kaolin at 900 °C, and increasing the modification pressure improved these properties. Additionally, the production of this high-temperature sorbent was relatively inexpensive, required little thermal energy and no chemicals to produce and no waste effluent was generated, thus being much cleaner than other modification methods

Modelling of redox flow battery electrode processes at a range of length scales : a review

In this article, the different approaches reported in the literature for modelling electrode processes in redox flow batteries (RFBs) are reviewed. RFB models vary widely in terms of computational complexity, research scalability and accuracy of predictions. Development of RFB models have been quite slow in the past, but in recent years researchers have reported on a range of modelling approaches for RFB system optimisation. Flow and transport processes, and their influence on electron transfer kinetics, play an important role in the performance of RFBs. Macro-scale modelling, typically based on a continuum approach for porous electrode modelling, have been used to investigate current distribution, to optimise cell design and to support techno-economic analyses. Microscale models have also been developed to investigate the transport properties within porous electrode materials. These microscale models exploit experimental tomographic techniques to characterise three-dimensional structures of different electrode materials. New insights into the effect of the electrode structure on transport processes are being provided from these new approaches. Modelling flow, transport, electrical and electrochemical processes within the electrode structure is a developing area of research, and there are significant variations in the model requirements for different redox systems, in particular for multiphase chemistries (gas–liquid, solid–liquid, etc.) and for aqueous and non-aqueous solvents. Further development is essential to better understand the kinetic and mass transport phenomena in the porous electrodes, and multiscale approaches are also needed to enable optimisation across the relevent length scales

Prediction of Rice Husk Gasification on Fluidized Bed Gasifier Based on Aspen Plus

A biomass gasification model was developed using Aspen Plus based on the Gibbs free energy minimization method. This model aims to predict and analyze the biomass gasification process using the blocks of the RGibbs reactor and the RYield reactor. The model was modified by the incomplete equilibrium of the RGibbs reactor to match the real processes that take place in a rice husk gasifier. The model was verified and validated, and the effects of gasification temperature, gasification pressure, and equivalence ratio (ER) on the gas component composition, gas yield, and gasification efficiency were studied on the basis of the Aspen Plus simulation. An increasing gasification temperature was shown to be conducive to the concentrations of H2 and CO, and gas yield and gasification efficiency reached peaks of 2.09 m3/kg and 83.56%, respectively, at 700 °C. Pressurized conditions were conducive to the formation of CH4 and rapidly increased the calorific value of syngas as the gasification pressure increased from 0.1 to 5 MPa. In addition, the optimal ER for gasification is approximately 0.3, when the concentrations of H2 and CO and the gasification efficiency reach peaks of 23.65%, 24.93% and 85.92%, respectively

Investigation of elemental mercury removal from coal-fired flue gas over MIL101-Cr

In this work, the MIL101-Cr sorbent with a large BET surface area was prepared and used to remove Hg0 from the simulated coal-fired boiler flue gas. The chemical and physical properties of the prepared sorbent were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). A range of experiments was conducted in a fixed-bed reactor to investigate the effects of reaction temperature, Hg0 inlet concentration, gas hourly space velocity (GHSV) and flue gas composition on the Hg0 removal for the prepared sorbent. The mechanisms and kinetics of the Hg0 adsorption were also studied. The results showed that the MIL101-Cr sorbent achieved the Hg0 removal efficiency of more than 85% for 4 h at 200 oC under the condition of a relatively high Hg0 inlet concentration (203 μg/m3) and large GHSV (8 105 h-1). The O2 in the flue gas was found to be beneficial to Hg0 removal. The NO in the flue gas favoured Hg0 removal both in the presence and absence of O2. The SO2 in the flue gas notably inhibited Hg0 adsorption in the absence of O2, whereas a low concentration of SO2 slightly inhibited Hg0 removal in the presence of O2. However, high concentrations of SO2 in the flue gas still significantly weakened the Hg0 removal ability even in the presence of O2 due to the competitive adsorption of SO2 with Hg0 on the sorbent and the sulfation of the sorbent. A simultaneous presence of O2 and NO in the flue gas could overcome the adverse impact of SO2 on the Hg0 adsorption. The H2O in the flue gas could have a minor influence on Hg0 removal as a result of the competitive adsorptions between Hg0 and H2O. The XPS analysis indicated that the surface Cr3+, oxygen species and C=O group in MIL101-Cr acted as the active adsorption/oxidation sites for Hg0. The Hg0 removal by MIL101-Cr belonged to chemisorption and could be described by the pseudo-second-order model. The equilibrium adsorption capacity calculated for the sorbent amounted to 25656 μg/g at 200 oC, which indicated that MIL101-Cr could be used as a promising sorbent to remove Hg0 from coal-fired boiler flue gases

EFFECTS OF SOLID ADDITIVES ON THE CONTROL OF TRACE ELEMENTS DURING COAL GASIFICATION

ABSTRACT Based on the Modified Geo-chemical Enrichment Factor (MGEF), the contents of As, Cd, Co, Cr, Cu, Mn, Hg, Pb, V, Se, Sr, Zn in coal and coal char were analyzed by using Hydride Generation-Atomic Fluorescence Spectrometry (HG-AFS) and Inductively Couple Plasma-atomic Emission Spectroscopy (ICP-AES). Limestone, dolomite and sodium carbonate were studied to control trace elements during coal gasification. Different additives show different performances in the control of trace elements. The adsorbing capacity of coal char to all of trace elements enhances when coal is mixed with limestone and dolomite. Chemical adsorption and physical adsorption of lime, which is decomposition product of limestone under high gasification temperature, are both important for As, Co, Cr, Se and Zn. The effects of limestone on Cd, Cu, Hg, Pb, V and Sr are merely caused by physical adsorption of CaO and the adsorbing capacity to Cd, Cu, V is much stronger than those to Hg, Pb, Sr. Dolomite has stronger adsorbing capacity to most of elements (except Cu, Se, Sr) than limestone. Addition of Na 2 CO 3 decreases the MGEFs of As, Cd, Cr, Pb and Se while increases the MGEFs of Zn in coal char. Na 2 CO 3 has little effect on the MGEFs of Co, Cu, Hg, V and Sr in coal char