Research On The Gas-solid Behavior In Parallel External Loops Of Large-scale CFB Boiler

The scaling up of CFB boiler would improve the thermal efficiency,which make the CFB boiler much more competitive compared with PC boiler,and the scaling up of CFB boiler has attracted increasing attention.The large-scale CFB boiler has multiple parallel external loops,which could keep high separation efficiency with high flue gas flux and uniform distribution of solids concentration at the furnace bottom.The gas-solid flow in the parallel external loops is quite different from that in the single external loop.The gas-solid flow in the multiple parallel cyclones has the characterization of mal-distribution,and the gas-solid distribution in parallel cyclones is always non-uniform.The factors influencing the non-uniform distribution of gas-solid flow are complex,which mainly includes the boiler structures and the characterization of cyclone pressure drop.The parallel external loops normally has single blower to supply the aeration flux at the parallel standpipes bottom,and the gas-solid flow among parallel external loops would act and react upon each other by the aeration flux.Besides,the pressure drop across the horizontal leg of loop-seal is quite high,and the stable gas-solid flow in horizontal leg is difficult especially with high solids circulation rate.The engineering experiences have shown the gas-solid flow in parallel external loops would be unstable at certain condition,which brings negative impact on the performance of boiler.To solve the two questions in parallel external loop,the research conducted in this paper includes:(1)The non-uniform gas-solid flow in parallel cyclones of CFB boiler was experimentally investigated.The investigated boilers are the 300 MW CFB boiler and 600 MW CFB boiler,which has M-shaped and H-shaped structure for parallel cyclones.The characterization of the non-uniform gas-solid flow in the two boilers were found.(2)The influence of characterization of cyclone pressure drop on the maldistribution was theoretically analyzed,the mechanism to limit the non-uniform solids flow through parallel cyclones was proposed.(3)The unstable gas-solid flow in the CFB with a pair of external loops was experimentally investigated,and the factor leading to the unstable gas-solid flow was found.(4)The characterization of horizontal leg to drive solid circulation was proposed,and the material balance considering the capacity of external loop to drive solids circulation is used to investigate the factors influencing the unstable gas-solid flow in parallel external loops of CFB boilerThe solids circulation rate would not just influence the non-uniform gas-solid flow in parallel cyclones,but also influence the unstable gas-solid flow in parallel external loops.This paper proposed a pressure balance model to analyze the influence of riser height on the solids circulation rate and the axial solids concentration distribution in the riser.The pressure balance model is the research foundation to investigate the gas-solid flow in parallel external loops.