chang Xu

Development of coal combustion pollution control for SO2 and NOx in China

Abstract Pollution control of coal combustion in China is a very urgent task. New low-NO x combustion and flue gas desulfurization (FGD) techniques suitable for China should be researched and developed. These techniques should be comparably effective, but have low investments, operating cost and water consumption, so that they can be widely used in China. To optimize the ecology of the coal-energy system and find new measures to rectify large areas of saline-alkali soil and deserts in China is important.

Evolution of pore fractal dimensions for burning porous chars

The evolution of pore fractal dimensions for three Chinese chars was investigated. The pores of porous chars can be classified into macro-, micro- and transition-pores based on their fractal dimensions. The radius ranges of the macro-, micro- and transition-pores may be different for various chars. For different conversions in the char combustion process, the fractal dimensions of the micro-pores for different chars are almost the same and the fractal dimensions of macro-pores for the same kind of chars are roughly unchanged. The effects of fractal structure on combustion rate are also discussed.

DRIFTS study of ammonia activation over CaO and sulfated CaO for NO reduction by NH3.

CaO catalyzes NH(3) oxidation, while sulfated CaO catalyzes NO reduction by NH(3) in the presence of O(2), and the adsorption and transformation of ammonia over CaO and sulfated CaO has been investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to understand their catalytic mechanism. It has been found that ammonia is first adsorbed over Lewis or Brönsted acid sites, and later undergoes hydrogen abstraction giving rise to either NH(2) amide or NH imide intermediates. The intermediates react with NO or lattice O to produce N(2) or NO. Comparing the DRIFTS of NH(3) adsorption over CaO and sulfated CaO, it is obvious that ammonia adsorbed over CaO is activated mainly in NH form apt to react with surface oxygen to produce NO, while ammonia adsorbed over sulfated CaO is activated mainly in NH(2) form apt to reduce NO. The DRIFTS results agree with experimental data and explain the catalytic mechanisms of CaO and sulfated CaO.

Study of the effect of fly ash on desulfurization by lime

Abstract Fly ash was used as an additive, and the effect of it on desulfurization by lime at 400–800°C was studied experimentally. The conversion rate of fly ash–lime to calcium sulfate was found to be greater than that when using lime alone. One reason is that fly ash can act as a support medium for lime, thereby increasing its dispersion and extending its effective reaction surface area. Fly ash with a high iron or its oxides is likely to have some ability to absorb particles of lime on its surface.

FRACTAL GENERATION OF CHAR PORES THROUGH RANDOM WALK

A method based on the random walk is proposed to generate three-dimensional pore models. The pore shapes of the models appear irregular, which is similar to most of natural porous media. This method can generate a pore model with an arbitrary given porosity and an arbitrary fractal dimension. With the combination of any porosity and fractal dimension, the generated pore models can be potentially used in simulating enormous kinds of porous media.

The influence of high-temperature crystallite growth and petrography of pulverized char on combustion characteristics

Abstract In this paper, Yangquan anthracite was separated into three different coal lithotypes in terms of density. The effect of high-temperature crystallite growth on char combustion of coal is characterized. It shows that there is large difference between the combustion characteristics of the two lithotypes of coal (one is

NUMERICAL SIMULATIONS OF GAS DIFFUSION AND REACTION IN FRACTAL PORES DURING CHAR COMBUSTION

Abstract Classic molecular gas dynamics and fractal pore models are used to simulate gas diffusion and reaction within char pores in combustion. Through numerical simulations it is found that the char combustion model based on Thiele modulus can be well used in regular pores, but is not well used in real char particles that have fractal pore structures. When oxygen concentrations are the same in different char pores, which is often happened in initial stage of combustion, fractal dimensions of pores will affect char combustion rates in exponential form. These numerical simulations can partly explain the previous experimental results.

Impact of Flue Gas Species and Temperature on Mercury Oxidation

Abstract Systematic experimental research has been conducted in a fix-bed reactor system to determine the impact of coal-fired flue gas species and temperature on mercury oxidation. This work focuses on the temperatures range of 200°C to 800°C to demonstrate that temperature is a critical factor for the effect of the gas components on the mercury oxidation process. Among the investigated gases, hydrogen chloride is essential for oxidizing the elemental mercury. Nitrogen oxide was also found to have a positive correlation with the mercury oxidation when hydrogen chloride was present. Sulfur dioxide can either promote or inhibit the oxidation depending on the conditions; however, when nitrogen oxide is also present, sulfur dioxide has a negative impact. Ammonia exhibits an strong inhibitory effect. Several plausible mercury oxidation pathways are suggested in this paper.

Experimental study of the mechanisms of steam reactivation of unreacted calcium-based sorbent for Flue Gas Desulphurisation

This paper reports an experimental investigation of the mechanisms of steam reactivation of spent sorbent for flue gas desulphurisation. Sulphated lime samples were reactivated by using steam to increase the conversion rate of sorbent for flue gas desulphurisation. Samples were characterised using mercury porosimeter, XRD, and SEM. Reactivation temperatures were in the range of 200-800°C. Retention times were 5, 10, and 20 min. For the conditions within this work, it was found that sorbent particles broke down when reactivated at 200°C, and migration of trapped CaO outward was the main mechanism when reactivation temperature was greater than 300°C. [Received: April 9, 2007; Accepted: October 2, 2007]

INTERACTION BETWEEN SO2 FROM FLUE GAS AND SORBENT PARTICLES IN DRY FGD PROCESSES

Abstract Among the technologies to control SO 2 emission from coal-fired boilers, the dry flue gas desulphurization (FGD) method, with appropriate modifications, has been identified as a candidate for realizing high SO 2 removal efficiency to meet both technical and economic requirements, and for making the best quality byproduct gypsum as a useful additive for improving alkali soil. Among the possible modifications two major factors have been selected for study: (1) favorable chemical reaction kinetics at elevated temperatures and the sorbent characteristics; (2) enhanced diffusion of SO 2 to the surface and within the pores of sorbent particles that are closely related to gas-solid two-phase flow patterns caused by flue gas and sorbent particles in the reactor. To achieve an ideal pore structure, a sorbent was prepared through hydration reaction by mixing lime and fly ash collected from bag house of power plants to form a slurry, which was first dewatered and then dried. The dry sorbent was found capable of rapid conversion of 70% of its calcium content at 700 o C, reaching a desulphurization efficiency of over 90% at a Ca/S ratio of 1.3. Experiments confirmed that the diffusion effect of SO 2 is an important factor and that gas-solid two-phase flow plays a key role to mixing and contact between SO 2 and sorbent particles. For designing the FDG reactor, a new theoretical drag model was developed by combination of CFD with the Energy Minimization Multi-Scale (EMMS) theory for dense fluidization systems. This new drag model was first verified by comparing calculated and measured drag values, and was then implemented in simulation of gas-solid two-phase flow in two circulating fluidized beds with different sizes and flow parameters. One riser has diameter and height of 0.15m×3m and another one 0.2m×14.2m. Their superficial gas velocities are 4 and 5.2m·s −1 , respectively, and the circulating rate 53 and 489 kg·(m −2 ·s −1 ). FCC particles were used in both cases. The results show that not only the static pressure drop along the riser height, but also radial distributions of particle volume fraction have been very well predicted in comparison with experiments. The new drag model is expected to shed more light on the further improvement of SO 2 diffusion to solid sorbent and optimization of reactor structure.