Changfu You

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.

Effect of humidity on negative corona discharge of electrostatic precipitators

The effect of humidity on the negative corona discharge of electrostatic precipitators was experimentally and numerically studied. The current-voltage characteristics of a wire-plate electrostatic precipitator was measured over a range of temperatures and relative humidities. The inception voltage and electric field were determined based on the streamer theory. A numerical method was used to study the influence of humidity on the distributions of electric field and charge density in an electrostatic precipitator. The results showed that the inception voltage and electric field decreased with the increase of humidity. The corona current was significantly affected by the humidity. A modified Peek formula was proposed with a correction to the humidity. The corona currents calculated with the modified Peek formula were in acceptable agreement with the experimental results. The distributions of electric field and charge density were affected by the humidity and applied voltage.

Phase Doppler anemometry measurements and analysis of turbulence modulation in dilute gas–solid two-phase shear flows

Flow velocities of a dilute gas-solid two-phase flow in a vertical sudden expansion were measured using phase Doppler anemometry to study the behaviour of the turbulence modulation for the stronger shear for various particle mass loadings, inlet Reynolds numbers and particle diameters. The measurements show that the particles changed the gas turbulence by elongation of the entire gas flow field in the downstream direction, which displaced the axial profile of the section-averaged fluctuation velocity in comparison with that of the single-phase flow, and by either the particle inertia reducing the local turbulence or the wake eddy effects enhancing the turbulence. Both mechanisms resulted in an apparent turbulence modulation, which has not been referred to in the related literature, and have led to an ambiguous understanding of turbulence modulation. The elongation and inlet effects should be eliminated to estimate whether the gas turbulence was really modified. The linear relationship between the gas mean velocity gradient and the root-mean-square fluctuation velocity, which was found to be similar to that in single-phase flows, gradually disappeared as the flow developed and the shear intensity reduced. The linear relationship also varied with different conditions. Specifically, the turbulence modulation was enhanced by higher particle mass loadings and the linear relationship disappeared with increasing particle mass loading. This linearity can perhaps be regarded as a criterion for determining the effect of stronger turbulence modulation.

Biochar Effect on Water Evaporation and Hydraulic Conductivity in Sandy Soil

Abstract Biochar, as a kind of soil amendment, has important effects on soil water retention. In this research, 4 different kinds of biochars were used to investigate their influences on hydraulic properties and water evaporation in a sandy soil from Hebei Province, China. Biochar had strong absorption ability in the sandy soil. The ratio of water content in the biochar to that in the sandy soil was less than the corresponding ratio of porosity. Because of the different hydraulic properties between the sandy soil and the biochar, the saturated hydraulic conductivity of the sandy soil gradually decreased with the increasing biochar addition. The biochar with larger pore volume and average pore diameter had better water retention. More water was retained in the sandy soil when the biochar was added in a single layer, but not when the biochar was uniformly mixed with soil. Particle size of the added biochar had a significant influence on the hydraulic properties of the mixture of sand and biochar. Grinding the biochar into powder destroyed the pore structure, which simultaneously reduced the water absorption ability and hydraulic conductivity of the biochar. For this reason, adding biochar powder to the sandy soil would not decrease the water evaporation loss of the soil itself.

Adhesive carrier particles for rapidly hydrated sorbent for moderate-temperature dry flue gas desulfurization.

A rapidly hydrated sorbent for moderate-temperature dry flue gas desulfurization was prepared by rapidly hydrating adhesive carrier particles and lime. The circulation ash from a circulating fluidized bed boiler and chain boiler ash, both of which have rough surfaces with large specific surface areas and specific pore volumes, can improve the adhesion, abrasion resistance, and desulfurization characteristics of rapidly hydrated sorbent when used as the adhesive carrier particles. The adhesion ability of sorbent made from circulation ash is 67.4% higher than that of the existing rapidly hydrated sorbent made from fly ash, the abrasion ratio is 76.2% lower, and desulfurization ability is 14.1% higher. For sorbent made from chain boiler ash, the adhesion ability is increased by 74.7%, the desulfurization ability is increased by 30.3%, and abrasion ratio is decreased by 52.4%. The abrasion ratios of the sorbent made from circulation ash having various average diameters were all about 9%, and their desulfurization abilities were similar (approximately 150 mg/g).

Experimental and numerical investigation on the pyrolysis of single coarse lignite particles

This paper reports on the mathematical modeling of the pyrolysis of single coarse lignite particles using a kinetics model coupled with a heat transfer model. The parallel reaction kinetics model of the lignite pyrolysis makes no assumptions about the activation energy distribution and the conversion of sub-reactions. The pyrolysis kinetics parameters were obtained on the basis of experimental data from thermogravimetric analysis (TGA) tests. The heat transfer model includes diffusive, convective and radiative heat transfer modes. The experimental investigations were carried out for single lignite particles in an electrically heated reactor. Measurements of the temperature and mass loss were performed during the pyrolysis in a nitrogen atmosphere. The model predictions for the temperature and mass loss histories agree well with the experimental data, verifying that the mathematical model accurately evaluates the pyrolysis of lignite particles. The effects of temperature and particle size on the pyrolysis time and final residual mass fraction were evaluated using the numerical model.

Experimental study on the reuse of spent rapidly hydrated sorbent for circulating fluidized bed flue gas desulfurization.

Rapidly hydrated sorbent, prepared by rapidly hydrating adhesive carrier particles and lime, is a highly effective sorbent for moderate temperature circulating fluidized bed flue gas desulfurization (CFB-FGD) process. The residence time of fine calcium-containing particles in CFB reactors increases by adhering on the surface of larger adhesive carrier particles, which contributes to higher sorbent calcium conversion ratio. The circulation ash of CFB boilers (α-adhesive carrier particles) and the spent sorbent (β and γ-adhesive carrier particles) were used as adhesive carrier particles for producing the rapidly hydrated sorbent. Particle physical characteristic analysis, abrasion characteristics in fluidized bed and desulfurization characteristics in TGA and CFB-FGD systems were investigated for various types of rapidly hydrated sorbent (α, β, and γ-sorbent). The adhesion ability of γ-sorbent was 50.1% higher than that of α-sorbent. The abrasion ratio of β and γ-sorbent was 16.7% lower than that of α-sorbent. The desulfurization abilities of the three sorbent in TGA were almost same. The desulfurization efficiency in the CFB-FGD system was up to 95% at the bed temperature of 750 °C for the β-sorbent.

Fictitious domain method for fully resolved reacting gas–solid flow simulation

Fully resolved simulation (FRS) for gas–solid multiphase flow considers solid objects as finite sized regions in flow fields and their behaviours are predicted by solving equations in both fluid and solid regions directly. Fixed mesh numerical methods, such as fictitious domain method, are preferred in solving FRS problems and have been widely researched. However, for reacting gas–solid flows no suitable fictitious domain numerical method has been developed. This work presents a new fictitious domain finite element method for FRS of reacting particulate flows. Low Mach number reacting flow governing equations are solved sequentially on a regular background mesh. Particles are immersed in the mesh and driven by their surface forces and torques integrated on immersed interfaces. Additional treatments on energy and surface reactions are developed. Several numerical test cases validated the method and a burning carbon particles array falling simulation proved the capability for solving moving reacting particle cluster problems.

High-order ALE method for the Navier–Stokes equations on a moving hybrid unstructured mesh using flux reconstruction method

The flux reconstruction (FR) formulation can unify several popular discontinuous basis high-order methods for fluid dynamics, including the discontinuous Galerkin method, in a simple, efficient form. An arbitrary Lagrangian–Eulerian (ALE) extension to the high-order FR scheme is developed here for moving mesh fluid flow problems. The ALE Navier–Stokes equations are derived by introducing a grid velocity. The conservation law are spatially discretised on hybrid unstructured meshes using Huynh’s scheme (Huynh 2007) on anisotropic elements (quadrilaterals) and using Correction Procedure via Reconstruction scheme on isotropic elements (triangles). The temporal discretisation uses both explicit and implicit treatments. The mesh movement is described by node positions given as a time series, instead of an analytical formula. The geometric conservation law is tested using free stream preservation problem. An isentropic vortex propagation test case is performed to show the high-order accuracy of the developed method on both moving and fixed hybrid meshes. Flow around an oscillating cylinder shows the capability of the method to solve moving boundary viscous flow problems, with the numeric method further verified by comparison of the result on a smoothly deforming mesh and a rigid moving mesh.

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.