Cyclone separation in a supercritical water circulating fluidized bed reactor for coallbiomass gasification： Structural design and numerical analysis

A new concept of a supercritical water （SCW） circulating fiuidized bed reactor is proposed to produce hydrogen from coal/biomass gasification. The cyclone is a key component of the reactor system, in this paper, cyclones with a single circular inlet （SCI） or a double circular inlet （DCI） were designed to adapt to the supercritical conditions. We evaluated the separation performance of the two cyclones using numerical simulations. A three-dimensional Reynolds stress model was used to simulate the turbulent flow of the fluid, and a stochastic Lagrangian model was used to simulate the particle motion. The flow fields of both cyclones were three-dimensionally unsteady and similar to those of traditional gas-solid cyclones. Secondary circulation phenomena were discovered and their influence on particle separation was estimated. Analyzing the distribution of the turbulence kinetic energy revealed that the most intensive turbulence existed in the zone near the vortex finder while the flow in the central part was relatively stable. The particle concentration distribution was non-uniform because of centrifugal forces. The distribution area can be divided into three parts according to the motion of the particles. In addition, the separation efficiency of both cyclones increased with the inlet SCaN velocity. Because of its perturbance flow, the DCI separator had higher separation efficiency than the SCI separator under comparable simulations. However, this was at the expense of a higher pressure drop across the cyclone.

Numerical comparison of two modes of gas-solid riser operation： Fluid catalytic cracking vs CFB combustor

Two modes of gas-solid riser operation, i.e., fluid catalytic cracking （FCC） and circulating fluidized bed combustor （CFBC）, have been recognized in literature; particularly in the understanding of choking phenomena. This work compares these two modes of operation through computational fluid dynamics （CFD） simulation. In CFD simulations, the different operations are represented by fixing appropriate boundary conditions： solids flux or solids inventory. It is found that the FCC and CFBC modes generally have the same dependence of solids flux on the mean solids volume fraction or solids inventory. However, during the choking transition, the FCC mode of operation needs more time to reach a steady state; thus the FCC system may have insufficient time to respond to valve adjustments or flow state change, leading to the choking. The difference between FCC and CFBC systems is more pronounced for the systems with longer risers. A more detailed investigation of these two modes of riser operation may require a three-dimensional full loop simulation with dynamic valve adjustment.

Waste to energy(WTE)in China:from latecomer to front runner

This paper discusses the 2000-2018 evolution of energy and metals recovery from urban wastes in the European Union and China.As a result of the zero-landfilling directive,in twenty years the European Union tripled its recycling rate(11%-30%)and its composting rate(6%-17%),doubled its WTE rate(14%-28%)and more than halved its landfilling(64%-25%).At the beginning of this century,the rapidly growing cities of China were literally surrounded by landfills.Therefore,the national government instituted policies,such as a credit of US$30 per MWh of WTE(waste to energy)electricity that resulted in the construction,by 2020,of 510 WTE plants with an annual WTE capacity of 193 million tons.In comparison,the European Union(EU)WTE capacity is 96 million tons and the USA has remained static at about 27 million tons,i.e.,10%of its post-recycling MSW(municipal solid waste),with the other 90%being landfilled.In the first decade of this century,two WTE technologies,moving grate and circulating fluid bed were developed in China at about the same rate.However,since 2010,the moving grate technology has become dominant and the WTE plants are built functionally and esthetically comparable to and U.S.plants.