Yufeng Duan

Comparison of mercury removal characteristic between fabric filter and electrostatic precipitators of coal-fired power plants

Abstract By applying the Ontario Hydro Method (OHM) and the US EPA standard methods, the sampling of mercury in the flue gas before and after fabric filter (FF)/electrostatic precipitator (ESP) located at five coal-fired power plants was carried out on site and various mercury speciation such as Hg 0 , Hg 2+ , and Hg P in flue gas were analyzed. The solid samples, such as coal, bottom ash, and FF/ESP ash, were analyzed by DMA 80 based on EPA Method 7473. According to mercury balance, the mercury speciation and its distribution in different locations downstream the flue gas were obtained. The mercury removal efficiency of coal-fired power plants 1 and 2 installed FF is higher than 80% and 20%, respectively, while that of plants 3, 4, and 5 installed ESP is around 6%, 20%, and 4%, respectively. It can be concluded that the performance of mercury removal in FF is better than ESP, and then various factors have important effects on the mercury removal.

Experimental study on mercury release behavior and speciation during pyrolysis of two different coals

Abstract Mercury release behavior and speciation during pyrolysis of Datong and Baorixile coals were studied in a temperature-programmed tube furnace under N 2 atmosphere. The results showed that mercury emission from coal pyrolysis significantly depended on temperature. The ratio of Hg release in coal increased dramatically with increasing temperatures, and more than 92% mercury was released at 600°C. Elemental mercury was the dominant species in the gaseous mercury during coal pyrolysis. The percentage of Hg 0 of Datong coal increased first, and then decreased, while it reduced gradually for Baorixile coal. Both the rate of Hg release and the percentage of Hg 0 rose finitely through the time ranging from 0 to 60 min, and Baorixile coal displayed more significant increase than Datong coal. Higher heating rate promoted the mercury release in coal and enhanced the proportion of Hg 2+ in gas; especially for Baorixile coal it reached 19.5%, being apparently higher than that for Datong coal.

Adsorption equilibrium, kinetics and mechanism studies of mercury on coal-fired fly ash

Fly ash samples were collected from the electrostatic precipitator (ESP) of a 600MW pulverized coal boiler firing Zhungeer bituminous coal in China to evaluate and explore its mercury adsorption capacity and mechanism. Samples characterization was conducted to feature their morphologies correlated to mercury content, and experimental studies on mercury adsorption in a fixed-bed apparatus were carried out to further verify its mercury adsorption availability. Based on the experimental data, adsorption isotherm was modeled with Langmuir, Freundlich, and Temkin equations. Adsorption kinetic analysis was also performed. The results show that mercury content of fly ash samples is associated with particle size, unburned carbon content and functional groups of Al-O/Si-O or Si-O-Si/Si-O-Al tetrahedron on fly ash. Increase of initial mercury concentration is beneficial to promote mercury adsorption due to the enhancement of mercury diffusion force onto the fly ash surface, mercury intraparticle diffusion rate and initial mercury adsorption rate. Fly ash with medium size displays better mercury adsorption capacity. Smaller particle size results in higher specific surface area, but brings about low specific surface area utilization rate for mercury adsorption. Freundlich isotherm equation presents better fitting result, indicating that fly ash surface is non-uniform. Mercury adsorption on fly ash at 120°C is mainly physisorption enhanced by chemisorption with ΔG at −36.73 kJ/mol. The pseudo-first-order kinetic model can describe the adsorption process more accurately and predict mercury adsorption capacity of fly ash preferably, showing that mercury adsorption on fly ash surface in fixed-bed is controlled dominantly by external mass transfer.

Coal Gasification Characteristics in a 2MWth Second-Generation PFB Gasifier

Coal gasification process and equipment feasibility research were 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.© 2005 ASME

Comparion of Mercury Emissions Between Circulating Fluidized Bed Boiler and Pulverized Coal Boiler

Mercury emissions between a circulating fluidized bed (CFB) utility boiler and two pulverized coal (PC) boilers equipped with electrostatic precipitators (ESP) were in situ measured and compared. The standard Ontario Hydro Method (OHM) was used to sample the flue gas before and after the ESP. Various mercury speciations such as Hg0, Hg2+ and Hgp in flue gas and total mercury in fly ashes were analyzed. The results showed that the mercury removal rate of the CFB boiler is nearly 100%; the mercury emission in stack is only 0.028 g/h. However, the mercury removal rates of the two PC boilers are 27.56% and 33.59% respectively, the mercury emissions in stack are 0.80 and 51.78 g/h respectively. It concluded that components of the ESP fly ashes especially their unburnt carbons have remarkable influence on mercury capture. Pore configurations of fine fly ash particles have non-ignored impacts on mercury emissions.

Effect of Surface Chemistry and Structure of Sludge Particles on Their Co-slurrying Ability with Petroleum Coke

Abstract Gasification and combustion performance of petroleum coke–water slurry (PCWS) can be enhanced by the addition of sludge. In this study, we have mixed three types of sludge with petroleum coke to form petroleum coke–sludge slurry (PCSS) and analyzed the effects of surface chemistry and structure of the sludge samples on the rheological properties of PCSS. Results show that the surface of petroleum coke particles is strongly hydrophobic, while the surface of sludge particles is hydrophilic. The sludge particles primarily consist of oxide and mineral salts, while petroleum coke particles mainly consist of organic carbon. Modification of the sludge changes the distribution of mineral components on the surface of sludge. In addition, the surface area, pore structure, and size of sludge tend to increase after modification. Sludge shows strong shear-thinning behavior, while the rheological properties of PCWS change from shear-thickening to shear-thinning behavior with excessive addition of sludge more than 6 wt.%. The suitable mass concentration for industrial application of PCSS is determined to be 62–64 wt.% for the sludge addition of 10–15 wt.%. Increasing the amount of sludge may enhance the yield stress and improve the static stability of PCSS, and sludge might be the most effective stabilizer used in PCWS.

Mercury Emission and Removal of a 135MW CFB Utility Boiler

To evaluate characteristic of the mercury emission and removal from a circulating fluidized bed (CFB) boiler, a representative 135 MW CFB utility boiler was selected to take the onsite measurement of mercury concentrations in feeding coal, bottom ash, fly ash and flue gas using the US EPA recommended Ontario Hydro Method (OHM). The results show that particulate mercury is of majority in flue gas of the CFB boiler. Mercury removal rate of the electrostatic precipitator (ESP) reaches 98%. Mercury emission concentration in stack is only 0.062μg/Nm3, and the mass proportion of mercury in bottom ash is less than 1%. It was found that the fly ashes were highly adsorptive to flue gas mercury because of its higher unburned carbon content. Adsorption effect is related to carbon pore structural properties of fly ash and temperature of flue gas. However mercury adsorption capacity by fly ash can not be improved any more when unburned carbon content in fly ash increases further.

Investigation of mercury adsorption and cyclic mercury retention over MnOx/γ-Al2O3 sorbent

In this study mercury sorbent based on manganese oxides impregnated γ-alumina was synthesized. Mercury retention characteristics were investigated by mercury speciation and thermal desorption experiments. No gaseous mercuric oxide was observed in mercury speciation experiments with a mercury mass balance ratio of 89.11%. Maximum mercury desorption peak at 480 °C indicated that mercury was adsorbed in the form of mercuric oxide. Three cycles of mercury retention were tested with different thermal treatment in-between to evaluate the cyclic performance. Changes in surface phase and manganese chemistry before and after thermal treatment were characterized by XRD and XPS. Deterioration in mercury retention capacity was observed after thermal desorption at 500 °C, which was interpreted with reduced initial adsorption rate calculated by Pseudo-second order kinetic model. XPS studies suggested that atomic ratios of Mn4+/(Mn4++Mn3+) decreased from 73.2% to 32.3% and 33.9% after thermal desorption in N2 and air, respectively. The reduction of MnO2 to Mn2O3 was irreversible thus the mercury retention capacity could not be restored by thermal desorption at high temperatures. Spent sorbents that were reactivated at 200 °C in air without thermal desorption at 500 °C possessed considerable cycling performance for mercury retention due to the preserved Mn4+.

The Migration and Transformation of Heavy Metals in Sewage Sludge during Hydrothermal Carbonization Combined with Combustion

The migration and transformation behaviors of heavy metals (HMs), including Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb, during the hydrothermal carbonization (HTC) of sewage sludge (SS) were investigated. The immobilization of HMs during the combustion of solid residual (SR) produced from HTC of SS was also analyzed. With increasing HTC temperature and residence time, the majority of HMs (except As) accumulated in the SR. The residual rate of As in the SR decreased from 73.95% to 56.74% when the residence time was increased from 1h to 3h and reduced significantly from 73.95% to 37.48% when the temperature increased from 220°C to 280°C, implying that numerous arsenic compounds dissolved into liquid phase products. Although the HTC process has a positive influence on the transformation of HMs from weakly bound fractions to the more stable fractions, the exchangeable and reducible fractions of Mn, Zn, As, and Cd in the SR were still high. In addition, the leached amounts of Zn and As were high (14.61 and 6.16 mg/kg, respectively) and showed a high leaching risk to the environment. An increase in HTC temperature and residence time led to an increase of the residual rate of HMs in the combustion residual of SR, implying that the HTC process promotes the stabilization of HMs in the combustion process.

Effects of the Types and Addition Amounts of Sludge on the True Rheological Properties of Petroleum Coke Slurry Flowing in Pipelines

Abstract Effects of the types and addition amounts of sludge on the true rheological properties of petroleum coke water/sludge slurry (PCWS/PCSS) flowing in pipelines were investigated using the Tikhonov regularization method. Results show that PCWS of 59.8 wt% changes from pseudo-plastic fluid to dilatant fluid as the shear rate increases. However, PCWS of 63.4 wt% is a very complex fluid: shear thinning at low shear rate, followed by shear thickening over a critical shear rate, and a subsequent shear thinning at high shear rate. Rheological properties of PCWS have a significant change after the sludge was added. PCSS changes from dilatant fluid to pseudo-plastic fluid when the addition amounts of sewage sludge ascend to 10 wt%. Petroleum coke particles are trapped by the stable “network” structures, which are formed by flocculent sludge particles. The wall slip velocity of PCSS is higher than that of PCWS with the sludge amount increases, which is beneficial for pipe transportation. In addition, the wall slip velocity of PCSS containing sewage or petrochemical sludge increases with the wall shear rate increases, but the slip velocity of PCSS containing paper mill sludge first increases and then decreases.