Due to the special structure

in furnace of a two-speed fluidized bed boiler

rapid change of mass concentration of particles in air-solid two phase flow between the dense-phase region and rare-phase region enhances the heat transfer intensity on the surface of heat exchangers

but it also aggravates the erosion of the metal heating surface. In order to resolve this contradictory

cold state numerical simulation on air-solid two-phase flow in a 75 t/h two-speed circulating fluidized bed boiler firing fuels with high proportion of coal slime is carried out

through using the Euler-Lagrangian algorithm. The flow velocity and mass concentration of the air and solid particles in the furnace are calculated by simulation. Moreover

the influence of cross section ratio of the rare-phase region area to that of the dense-phase region area in the furnace on wall erosion and convective heat transfer is investigated

under the same boundary conditions

and the optimal furnace structural parameters are obtained. The results show that

both the wear ratio and convective heat transfer coefficient decreases with the increasing furnace cross section ratio. Considering the combined effects of furnace wall surface wear and convective heat transfer

the suggestion that the furnace cross section ratio should be within the range of 82.5% to 92.5% is presented. Additionally

this suggestion is applied in boiler structure optimization of a power plant

and after the retrofitting

the boiler runs well.