Minghou Xu

Modeling of homogeneous mercury speciation using detailed chemical kinetics

Abstract Homogeneous mercury speciation in combustion-generated flue gases was modeled by a detailed kinetic model consisting of 107 reactions and 30 species. This kinetic model includes the oxidation and chlorination of key flue-gas components, as well as six mercury reactions involving HgO with new reaction rate constants calculated neither from experimental data nor by estimated, which was commonly used by other investigators before, but directly from transition state theory (TST). An important and previously unrecognized pathway of homogeneous Hg oxidation mechanism including Hg reactions involving HgO was proposed. Among those reactions involving HgO, the progress of reaction HgO + HCl → HgCl + OH is HgO + HCl → TS 1( HgClOH )→ M ( HgClOH )→ TS 2( HgClOH )→ HgCl + OH , in which the controlling step is HgO + HCl → TS 1( HgClOH )→ M ( HgClOH ). The progress of reaction HgO + HOCl → HgCl + HO 2 is HgO + HOCl → M ( HgClOOH )→ TS ( HgClOOH )→ HgCl + HO 2 , in which the controlling step is M ( HgClOOH )→ TS ( HgClOOH )→ HgCl + HO 2 . Four other reactions are one-step, with no intermediates formed. The performance of the model was assessed through comparisons with experimental data conducted by three different groups. The comparison shows that model calculations were in agreement with only one set of all the three groups experimental data. The deviation occurs due to the absence of accurate rate constants of existing mechanism, the adding of reactions involving HgO, as well as the exclusion of heterogeneous Hg oxidation mechanism. Analyses by quantum chemistry and sensitivity simulations illustrated that the pathway Hg + ClO = HgO + Cl is more significant than some of the key reactions in the kinetic mechanism proposed in the literature, which indicates the necessity of including reactions involving HgO in the mercury speciation kinetic mechanism. Studies on the effects of oxygen show that O 2 weakly promotes homogeneous Hg oxidation, especially under the condition of low Cl 2 concentration. In all cases, 1.5–6.0% of the mercury is predicted to be present as HgO.Keywords: Mercury speciation; Reaction mechanism; Quantum chemistry; Kinetic modeling

Comparison of sulphur retention by coal ash in different types of combustors

During coal combustion, a part of the total sulphur in coal will be retained as solid compounds in ash. In the present paper, the retention of sulphur by ash of typical Chinese coal used for electric power generation was correlated with the contents of alkaline elements relative to sulphur in the parent coal. Comparison was performed among the capabilities of sulphur retention by coal ashes in laboratory ashing procedure, pulverised coal-fired combustor and fluidised bed combustor (FBC). It was shown that calcium plays a dominant role in sulphur retention of laboratory-prepared ash and the sulphur retention percentage increases with the increase of Ca/S molar ratio of the parent coal, while the contributions of other elements are limited. The sulphur retention behaviour in pulverised coal-fired combustor is significantly different from that in laboratory ashing procedure. The contribution of calcium is reduced markedly, while the roles of the other alkaline elements are obviously enhanced. Using the literature data from foreign coals, it was found that the sulphur retention in FBC is comparable with that in laboratory ashing procedure. It was suggested from the results obtained in this study that the sulphur retention of laboratory-prepared ash can be applied to fluidised bed combustion because the behaviour of calcium is similar to that of lime in FBC.

Simulation of the gas temperature deviation in large-scale tangential coal fired utility boilers

Abstract For tangential coal fired boilers, the gas temperature deviation at the exit of the furnace will occur when boiler capacity becomes large. In this paper, the flow, heat transfer and combustion process in a boiler of 300 MWe unit at Shiheng Power Plant, Shandong, China and a boiler of 600 MWe unit at Shidongkou Power Plant, Shanghai are simulated. The causes and decrease ways of the gas temperature deviation are investigated. It is found that the gas temperature deviation results from the after twirl and the platen superheater. Decreasing the diameter of tangential circle formed by secondary air flow can result in the decrease of temperature deviation to a certain extent. Although the after twirl can be decreased by opposing the tangential direction of some secondary air flows, the flow field in the furnace might be disturbed, and the temperature deviation might be increased, if the momentum and the angle of the opposite secondary air flow are not appropriate.

Modeling of a front wall fired utility boiler for different operating conditions

Numerical modeling of a front wall pulverised coal fired utility boiler is presented in this paper. Comparison between the predictions and measurements in the boiler is addressed. The accordance of the calculation results with the experimental data at several operating conditions validates the models and algorithm employed in the computation. The need to improve burner or boiler design with the objective of producing NOx abatement while avoiding some negative side effects is also the motivation for the prediction of furnace performance for different boiler operating conditions. The predicted results also lay a foundation for the boiler operating expert system.

Synthesis of CaO-Based Sorbents for CO2 Capture by a Spray-Drying Technique

Highly effective and durable CO(2) sorbents were synthesized with different calcium and support precursors using a spray-drying technique. It was found that spray-drying could be a useful technique for producing sorbents with enhanced cyclic performance, especially when d-gluconic acids of calcium and magnesium were used. Seven sorbents were synthesized with five calcium precursors and three inert solid precursors, and the sorbent made from calcium d-gluconte monohydrate and magnesium d-gluconate hydrate with 75 wt % CaO content achieved a high CO(2) sorption capacity of 0.46 g of CO(2)/g of calcined sorbent at the 44th cycle of carbonation and calcination.

Emissions of organic hazardous air pollutants during Chinese coal combustion

This paper provides a general investigation of the emissions of organic hazardous air pollutants (HAPs) during the combustion of several typical Chinese coals. First, the distribution of four types of HAP, i.e., aliphatics, cyclic hydrocarbons, monoaromatic compounds and PAHs, in the CH2Cl2 extracts of six Chinese coals were studied and the influences of the extractive times and coal varieties were also evaluated. Second, the partitioning of these HAPs in the flue gas during coal combustion in a small-scale reactor were investigated, depending on oven temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) and coal varieties. The behaviors of HAP in the combustion flue gas were compared with those in the CH2Cl2 extracts. Finally, combustion was conducted at given conditions in two laboratory-scale reactors: a fluidized bed and a fixed bed. Two coals (Shengmu bituminous coal and Xunhuan anthracite coal) and one coke were considered in this case. The HAP partitioning both in flue gases and in ashes were evaluated and compared between the two combustors.

Preparation of Novel Li4SiO4 Sorbents with Superior Performance at Low CO2 Concentration

This work produced Li4 SiO4 sorbents through an impregnated-suspension method to overcome its typical poor performance at low CO2 concentrations. A SiO2 colloidal solution and two different organic lithium precursors were selected. A bulgy surface morphology (and thus, the significantly enlarged reacting surface area) was obtained for Li4 SiO4 , which contributed to the high absorption capacity. As a result, the capacity in cyclic tests at 15 vol % CO2 was approximately 8 times higher than conventional Li4 SiO4 prepared through a solid-state reaction. The phenomenon of a progressively increasing capacity (i.e., sustainable usage) was observed over the 40 cycles investigated, and this increasing trend continued to the last cycle. Correspondingly, over the course of the multicycle absorption/ desorption processes, the sorbents evolve from lacking porosity to having a high number of micron-sized pores.

A study on the reaction mechanism and kinetic of mercury oxidation by chlorine species

Ab initio MP2 method was performed to study the reaction mechanism and kinetic of mercury oxidation by chlorine species during the process of coal combustion. Seven channels were detected by calculations, the geometric configurations of reactants, products, intermediates and transition states were optimized and energies were computed at QCISD(T)/SDD level. Besides, we used the classic transition state theory to compute the rate constants of the seven channels. Compared with the experimental values, the results show that the rate constants are in good agreement with experimental datum. So our study is believable.

Alkali‐Doped Lithium Orthosilicate Sorbents for Carbon Dioxide Capture

New alkali-doped (Na2 CO3 and K2 CO3 ) Li4 SiO4 sorbents with excellent performance at low CO2 concentrations were synthesized. We speculate that alkali doping breaks the orderly arrangement of the Li4 SiO4 crystals, hence increasing its specific surface area and the number of pores. It was shown that 10 wt % Na2 CO3 and 5 wt % K2 CO3 are the optimal additive ratios for doped sorbents to attain the highest conversions. Moreover, under 15 vol % CO2 , the doped sorbents present clearly faster absorption rates and exhibit stable cyclic durability with impressive conversions of about 90 %, at least 20 % higher than that of non-doped Li4 SiO4 . The attained conversions are also 10 % higher than the reported highest conversion of 80 % on doped Li4 SiO4 . The performance of Li4 SiO4 is believed to be enhanced by the eutectic melt, and it is the first time that the existence of eutectic Li/Na or Li/K carbonate on doped sorbents when absorbing CO2 at high temperature is confirmed; this was done using systematical analysis combining differential scanning calorimetry with in situ powder X-ray diffraction.

Multi-Monte Carlo method for particle coagulation: description and validation

In the interest of decreasing computation cost and increasing computation precision of Monte Carlo method for general dynamics equation (GDE), a new multi-Monte Carlo (MMC) method for particle coagulation is prompted, which has characteristic of time-driven, constant-number and constant-volume Monte Carlo technique. The paper has described detailedly the scheme of MMC method, including the setting of time step, the choice of coagulation partner, the judgment the occurrence of coagulation event, and the consequential treatment of particle coagulation event. MMC method is validated by five special coagulation cases: (1) constant coagulation kernel of monodisperse particles; (2) constant coagulation kernel of exponential polydisperse particle distribution; (3) linear coagulation kernel of exponential polydisperse particle distribution; (4) quadratic coagulation kernel of exponential polydisperse particle distribution; (5) Brownian coagulation kernel of log-normal polydisperse particles in the continuum regime. The simulation results of MMC method for GDE agree with analytical solution well, and its computation cost is low enough to apply engineering computation and general scientific quantitative analysis.