Qinhui Wang

Heavy metal characterization of circulating fluidized bed derived biomass ash.

Although the direct combustion of biomass for energy that applies circulating fluidized bed (CFB) technology is steadily expanding worldwide, only few studies have conducted an environmental assessment of biomass ash thus far. Therefore, this study aims to integrate information on the environmental effects of biomass ash. We investigated the concentration of heavy metal in biomass ash samples (bottom ash, cyclone ash, and filter ash) derived from a CFB boiler that combusted agricultural and forest residues at a biomass power plant (2×12 MW) in China. Ash samples were gathered for the digestion and leaching test. The heavy metal content in the solution and the leachate was studied via an inductively coupled plasma-mass spectrometer and a Malvern Mastersizer 2000 mercury analyzer. Measurements for the chemical composition, particle size distribution, and the surface morphology were carried out. Most of the metals in cyclone ash particles were enriched, whereas Ti and Hg were enriched in filter ash. Residence time contributed most to heavy metal enrichment. Under HJ/T 300 conditions, the heavy metals showed serious leaching characteristics. Under EN 12457-2 conditions, leaching behavior was hardly detected.

A mathematical model for a circulating fluidized bed (CFB) boiler

In developing a mathematical model for a CFB boiler we use earlier work. Our model includes mathematical descriptions of the underlying physical and chemical processes. It has been applied to simulation of a 12 MW CFB boiler. The calculations agree well with test results.

Development of a new external heat exchanger for a circulating fluidized bed boiler

An external heat exchanger (EHE) is believed to be a viable way to solve the heat transfer surface arrangement and furnace temperature control to scale-up of circulating fluidized bed (CFB) boilers. A new concept of EHE is developed in this paper. The flow of hot solid particles diverted from the standpipe is controlled by pneumatic (aeration airflows of recycle device or EHE), not the mechanical valve that is in popular use in many CFB boilers. The major feature of this EHE system is that it avoids the use of the mechanical valve that causes heavy erosion and high cost. An in-series connection between the EHE and the recycle device (i.e. the recycle device is on top of the EHE) is proposed. Experiments completed in cold test rigs show that the solid mass flow rate diverted from the standpipe to the EHE or the loop seal may be controlled well by adjusting the fluidizing air of the overflow part of the loop seal, the fluidized air of the fluidized chambers of the EHE and the aeration airflow to the standpipe. In addition, the new EHE has been validated successfully in a 12 MWe CFB boiler.

EFFECT OF CHEMICAL COMPOSITION ON SINTERING BEHAVIOR OF JINCHENG COAL ASH UNDER GASIFICATION ATMOSPHERE

In order to obtain detailed understanding of coal ash sintering behavior, a series of experiments have been done on the effect of Jincheng coal ash composition on sintering temperature under gasification atmosphere by the pressure-drop technique. The mineral transformations within its sintering temperature range are simulated by the thermodynamic equilibrium calculation module of FactSage to better understand the experiment results. These results show that the sintering temperature decreases initially, and then rises with increasing SiO2. On the whole, the sintering temperatures decrease with increase in Fe2O3, CaO, Na2O, and K2O. Their effects are quite similar, but the degrees are different. As MgO increases, the sintering temperature decreases. However when MgO is increased above a certain limit, the effect of MgO on the sintering temperature is not obvious.

Hydration Reactivation of CaO-Based Sorbent for Cyclic Calcination-Carbonation Reactions

Cyclic calcination-carbonation (CC) reactions have important applications, such as carbon dioxide (CO2) capture from power plant flue gases and zero emission systems for hydrogen production. A critical challenge is the severe activity loss of CaO-based sorbent with increasing cycle numbers. In present study, A Thermax 500 high pressure thermogravimetric analyzer (pTGA) and the Scanning Electron Microscopy (SEM) technology were used to examine hydration reactivation on CaO-based sorbents. Two hydration methods, liquid hydration and steam hydration, were both investigated. It was found that CaO sorbents calcined under lower CO2 partial pressurelose activity moreslowly with cycle number. Both the two hydration methods efficiently improved sorbent activity during cyclic CC reactions. The mean values of activity increase for liquid hydration and steam hydration after 6 cycles were ∼22% and ∼27% respectively. SEM images showed that the hydrated CaO particles both exhibited surface area and porosity more favorable for CO2 diffusion through the CaCO3 product layer. It seemed that hydration frequency could affect sorbent cyclic performance and a relatively high frequency should be beneficial. Moreover, comparison with previous study confirmed that hydration reactivation couldbe efficient at a wide range of condition. Results of the present study demonstrate that hydration is a promising method toimprove long performance of CaO sorbents in cyclic CC reactions, which provide consultations for process designing in flue gases CO2 capture and zero emissions systems.

Investigation of the evolution behavior of light tar during bituminous coal pyrolysis in a fluidized bed reactor

The evolution behavior of the light tar fraction of tar during large bituminous coal particle pyrolysis in a fluidized bed reactor at 500―900 °C was investigated to optimize the industrial process. The coal residence time was varied from 30 s to 150 s. The crude tar obtained was distilled and separated into the light fraction and the heavy fraction through an evaporator under conditions of 280 °C and 104 Pa. The light tar was subjected to GC-MS analysis, and the 300 main species in each spectrum were determined and analyzed. The experimental results showed that the evolution behavior of the light tar fraction and the heavy tar fraction presented spectacularly different variation trends with pyrolysis temperature and coal residence time. The molecular weight and number of rings per aromatic com-pound molecule contained in the light tar fraction increased with increasing pyrolysis temperature because of poly-merization. It was also observed that the amount of methyl substituents decreased with an increasing number of rings per molecule of poly-aromatic ring compounds. The chain aliphatic compound evolution was suppressed and under-went heterocyclization with increasing temperature.

Model prediction of the operating behavior of a circulating fluidized bed boiler

An improved mathematical model for a circulating fluidized bed (CFB) boiler based on the model developed earlier by the authors was applied to simulate the operation of a 12 MW CFB boiler. The influences of the excess air ratio, primary air ratio, coal particle size distribution, coal properties (ash content and volatile content) and Ca/S ratio on the boiler operation were analyzed. The results showed that the model simulation may be applied to the optimum design and economic operation of the CFB boiler.

Ash deposition behavior of a high-alkali coal in circulating fluidized bed combustion at different bed temperatures and the effect of kaolin

High alkali and alkali earth metals (AAEMs) content in coal causes severe slagging and fouling during combustion in a boiler. In this study, the ash deposition behavior of a high-alkali coal at different bed temperatures and the effect of kaolin were investigated in a 30 kW circulating fluidized bed (CFB) test system using an ash slagging probe and deposition probe. The results show that the ash deposition tendency increases with the bed temperature. The condensation of Na2SO4 is an important inducement for slag formation in the furnace. The melting or partial melting of slags is attributed to Na–Fe–Ca eutectics. At 920 °C, Na2SO4 will react with CaSO4 to form the low-melting compound of Na2SO4–CaSO4. The deposited ash on the convection-heating surface consists of granular particles. On the windward side, the layered-structure ash deposits, i.e. the inner and outer layers, are formed at the bed temperature of 920 °C but are absent at lower temperatures (820 °C and 870 °C). The formation of the inner layer consists of fine particles (<2 μm) and is closely related to Na2SO4. The size of the deposited ash in the outer layer is larger than 10 μm, while that on the leeward side is less than 10 μm. By adding kaolin in the coal, the slags are replaced by loose particles due to the absorption reactions between kaolin and alkali metals. The ash deposition tendency is improved and the optimal result is achieved when kaolin is added at an addition ratio of 3%.

A study on the preparation of pitch-based high-strength columnar activated carbon and mechanism of phenol adsorption from aqueous solution

Coal tar pitch was ground into powder and hydroformed with high pressure. After pre-oxidation, the pitch was activated by CO2 at high temperature. The effects of different preparation conditions on the yield, pore structure and phenol adsorption capacity of activated carbon were investigated, and activated carbon prepared under suitable conditions had good adsorption performance. A pore volume of 1–10 nm is the main absorption structure according to the analysis of pore size distribution and phenol adsorption capacity. The activated carbon showed high mechanical strength through compressive strength tests. Graphite nanocrystals around 5 nm were observed in the TEM images, and it illustrates that grain refinement results in the high strength. These nanocrystal stacked structures are easier to make and enlarge pores by activation than graphite layer stacked structures. Surface functional groups are considered not to be the active sites of phenol adsorption as suggested by the results of FTIR and Boehms titration, and acidic oxygen-containing functional groups are harmful to phenol adsorption, which happen to be removed in the reductive preparation atmosphere. The donor–acceptor complex mechanism can be ruled out, and the π–π interactions are considered the most likely mechanism. The Langmuir and Redlich–Peterson models are better fitted to the adsorption isotherms. Adsorption kinetics fit the intraparticle diffusion model best. Comparison of different activated carbons shows that suitable pore size is important for phenol adsorption. Thermodynamic parameters demonstrate that the adsorption process is spontaneous and exothermic, and the entropy increases. Pitch-based high-strength columnar activated carbon is an effective and low cost adsorbent for phenol wastewater treatment. This carbon nanocrystal material also provides a new direction for catalyst carriers.

Effect of Chemical Compositions on Ash Fusibility Characterization of a Jincheng Anthracite during Combustion and Gasification

The ash melting temperature of coal ash has an important effect during the fluidized bed combustion and gasification process, which affects the slagging and deposition characteristics of the boiler. Experiments on the effects of chemical components on the ash fusion behaviors have been completed on the ash fusion temperatures (AFTs) analyzer under typical gasification and combustion atmospheres. Meanwhile, calculations on the variation of minerals in ash with ash composition were conducted using the FactSage software. The results indicated that the AFTs under gasification were a little higher than those under the combustion atmosphere. On increasing the Fe2O3, CaO, and Na2O contents under the combustion and gasification atmospheres, the four temperatures deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) decreased dramatically and the generation and transformation of minerals occurred. The iron-containing minerals, such as hercynite and fayalite, formed with increase in the content of Fe2O3; the Ca-bearing feldspar minerals, like gehlenite and anorthite, started appearing on increasing the CaO content, and the Na-containing feldspar minerals, like carnegieite, were detected as the Na2O was increased. These three minerals can form low-temperature eutectics, decreasing the fusion temperature.